Pelletizing of Iron Ores - Kurt Meyer

Kurt Meyer

Pelletizing

of Iron Ores

Kurt Meyer

PeIletizing of Iron Ores

W i t h 146 F i g u r e s

1980 Springer-Verlag Berlin Heidelberg NewYork Verlag Stahleisen m.b.H. Düsseldorf

Professor Dr. Phil. Dr. Ing. Dipl.-Chem. Kurt Meyer Peter-Böhler-Straße 22 6000 Frankfurt/M.

ISBN 3-540-1021.5-9 Springer-Verlag B e r l i n H e i d e l b e r g N e w Y o r k ISBN 0-387-10215-9 Springer-Verlag N e w Y o r k Heidelberg Berlin ISBN 3-514-00246-0 Verlag Stahleisen m b H Düsseldorf

Library of Congress Cataloging in Publication Data: Meyer, Kurt, 1911-. Pelletizing of iron ores. Bibliography; p. Includes index. 1. Pelletizing (Ore-dressing). 2. Iron ores. I. Title. T N 535.M47. 622'.341. 80-23891. This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under § 54 of the G e r m a n Copyright Law where copies are made for other than private use, a fee is payable to the publisher, the amount of the fee to be determined by agreement with the publisher. © by Springer-Verlag Berlin, Heidelberg, and Verlag Stahleisen mbH, Düsseldorf, 1980 Printed in Germany. The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting, printing and bookbinding: Druckerei K. Triltsch, Würzburg 2060/3020-543210

Preface

After the Second World War, there was in m a n y countries a great backlog d e m a n d in nearly all branches of life, which resulted i. a. in a production facilities h a d to be replaced, reconstructed and extended. This was a good opportunity of revising the p r o d u c t i o n concept of the iron and steel industry and introducing innovations where it was possible and promising. This renewal covered i. a. two i m p o r t a n t sectors. O n the one hand, the dimensions of the production units a n d auxiliary e q u i p m e n t were extended to such a degree that considerably higher capacities were achieved. A n interesting example is the extension of the blast furnace volume and hearth diameters u p to 15 m at a pig iron production of a b o u t 10,000 tons per day. In addition, remarkable progress was m a d e by improving existing process parameters and introducing new technologies. S o m e of these innovations had already been formerly known, but not yet applied. A few examples of such developments in connection with ore p r e p a r a t i o n and particularly with the agglomeration technology are given below:

1. Physical Preparation of Blast Furnace Burden by Crushing, and Classification of Constituents

Screening

Already in the thirties, it was known to m a n y experts that it is advisable to classify the blast furnace b u r d e n b e f o r e it is used 2 ). However, this idea was not consistently realized until a b o u t 1950. F r o m this date onwards, raw ores, coke and other b u r d e n constituents were crushed, screened and supplied to the blast furnace in a narrow size range. Increasing amounts of fine ores emerged f r o m this operation and r e q u i r e d a strong extension of sinter plant capacity, as shown in Figures 9 and 10, chapter 1. As a result of these measures, by using a physically p r e p a r e d b u r d e n with rising sinter portions, the gas permeability of the blast f u r n a c e b u r d e n greatly improved the pig iron capacity increased and the coke c o n s u m p t i o n decreased.

worldwide

VI

Preface

2. Thermal

Separation

of Volatile Ballast Materials from Raw

Materials

A f u r t h e r i m p r o v e m e n t of the blast f u r n a c e o p e r a t i o n was a c h i e v e d b y r e m o v a l of volatile ballast m a t e r i a l , such as H 2 O or C O 2 , f r o m r a w ores d u r i n g sintering or pelletizing b e f o r e they enter t h e blast f u r n a c e . S u c h a n o r e p r e p a r a t i o n is of p a r t i c u l a r i m p o r t a n c e w h e n M i n e t t e or l i m o n i t e ores h a v e to b e treated. T h e r e are plants in w h i c h the blast f u r n a c e b u r d e n consists of 100% sinter.

3. Mechanical Beneficiation of Ores by of Mineral Ballast Materials

Separation

By m e c h a n i c a l b e n e f i c i a t i o n , it is possible to r e m o v e a great p o r t i o n of m i n e r a l ballast c o m p o n e n t s f r o m the iron ores b e f o r e they enter the blast f u r n a c e . B u t t h e concentrates so p r o d u c e d are very f i n e - g r a i n e d , a n d agg l o m e r a t i o n is t h u s necessary. T h i s a g g l o m e r a t i o n is achieved by sintering and pelletizing. T h e pelletizing process was i n t r o d u c e d o n a n i n d u s t r i a l scale as a new a g g l o m e r a t i o n m e t h o d a n d a n alternative to sintering.

4. Change of Chemical

Composition

of Ores During

Agglomeration

T h e possibility of m o d i f y i n g the chemical c o m p o s i t i o n of the ores b y using c o r r e s p o n d i n g additives d u r i n g a g g l o m e r a t i o n leads to the desired c h a n g e or i m p r o v e m e n t of metallurgical p r o p e r t i e s of the agglomerates. F r o m this p r o c e d u r e resulted the p r o d u c t i o n of basic or even over-basic agglomerates, as a l r e a d y p r o p o s e d in 1938 2 ). T h e d e v e l o p m e n t of the agg l o m e r a t i o n technology in the f o r m of sintering or pelletizing is closely connected with the p r o m o t i o n of the blast f u r n a c e technology. Pellets o f f e r a d d i t i o n a l a d v a n t a g e s d u e to t h e i r good transportability. W i t h the i n t r o d u c t i o n of pellets into the world m a r k e t , the h o p e of m a n y metallurgists — to p r o d u c e steel f r o m ores by direct r e d u c t i o n a n d bypass the blast f u r n a c e — c a m e closer to realisation. T h e increasing a n d successful efforts to i m p r o v e the direct r e d u c t i o n processes w e r e o n e of the m o s t i m p o r t a n t consequences of the newly developed pelletizing technology. Acknowledgement T h e a u t h o r very m u c h t h a n k s the m a n a g e m e n t of L u r g i C h e m i e u n d H ü t t e n technik G m b H * for t h e p e r m i s s i o n to use test results a n d o t h e r relevant k n o w l e d g e w h i c h h a d h i t h e r t o not b e e n p u b l i s h e d as well as for p r o v i d i n g t h e facilities r e q u i r e d for t h e p r e p a r a t i o n of this b o o k . * Frankfurt/Main, Federal Republic of Germany

Preface

VII

H e also t h a n k s his colleagues in F r a n k f u r t as well as those of the s u b s i diary c o m p a n i e s in G r e a t Britain, C a n a d a , J a p a n , S w e d e n , T h e U n i t e d States, S o u t h A f r i c a and A u s t r a l i a for t h e i r assistance in c o m p i l i n g d a t a , in p e r f o r m i n g t h e necessary a d d i t i o n a l tests, in e l a b o r a t i n g d r a w i n g s a n d d i a grams, in t r a n s l a t i n g t h e text a n d in revising t h e translation. F u r t h e r m o r e , the a u t h o r expresses his g r a t i t u d e to o t h e r sources f o r the kind disclosure of interesting a n d m o s t recent i n f o r m a t i o n s p r i m a r i l y to the representatives of L K A B ( S w e d e n ) , H o o g o v e n s I j m u i d e n ( N e t h e r lands), H a n n a M i n i n g C o m p a n y , C l e v e l a n d ( O h i o ) , P i c k a n d s M a t h e r a n d Company, Cleveland (Ohio), Studiengesellschaft für Eisenerzaufbereitung (Federal R e p u b l i c of G e r m a n y ) , Verein D e u t s c h e r Eisenhüttenleute ( F e d eral R e p u b l i c of G e r m a n y ) , a n d Institut f ü r E i s e n h ü t t e n k u n d e d e r R h e i nisch-Westfälischen T e c h n i s c h e n H o c h s c h u l e A a c h e n ( F e d e r a l R e p u b l i c of G e r m a n y ) . F r a n k f u r t / M a i n , S e p t e m b e r 1980

Kurt Meyer

Contents

Introduction

1

1 1.1 1.1.1 1.1.2 1.2 1.2.1 1.2.2 1.2.3 1.2.3.1 1.2.3.2 1.2.3.3 1.3 1.4

Definition and Development of Pelletizing Process . . Definition Differentiation Against O t h e r Iron O r e Agglomerates . Principal Process Steps for the P r o d u c t i o n of Pellets . D e v e l o p m e n t of Pelletizing Process . First Phase, Alternative to Sintering Second Phase, Pellets f r o m Concentrates T h i r d Phase, Pellets f r o m O r e s T h e Two-Stage G r a n u l a t i o n of the Sinter Mix . . . . Pellet Sintering Mixed Firing M e t h o d Pelletizing, a Contribution to Ore P r e p a r a t i o n . . . . Sites of Pelletizing Plants a n d Transportability of Pellets

3 3 3 4 5 6 7 9 11 13 13 15 20

2 2.1 2.1.1 2.1.2 2.1.2.1 2.1.2.2 2.1.2.3 2.2 2.2.1 2.2.1.1 2.2.1.2 2.2.1.2.1 2.2.1.2.2 2.2.1.3 2.2.1.4 2.2.2 2.2.2.1

Fundamentals of Pelletizing Bonding Mechanisms for G r e e n Ball F o r m a t i o n I m p o r t a n t Bonding Factors Ball F o r m a t i o n Alternatives Compacting M e t h o d Green Ball F o r m a t i o n M e c h a n i s m of Ball F o r m a t i o n Induration of G r e e n Balls Drying of G r e e n Balls Drying Procedure of Individual Balls Drying of Pellets in a L a y e r Unidirectional Drying U p - D r a u g h t - D o w n - D r a u g h t Drying D r y Pellet Strength Shock Resistance Pellet Firing Bonding by Change of the Crystalline Structure

23 24 24 24 25 25 26 29 29 30 33 34 35 35 37 37 39

. . .

.

. . .

X 2.2.2.1.1

Contents Crystal C h a n g e D u r i n g the I n d u r a t i o n of Pellets f r o m Magnetite Concentrate Crystal C h a n g e D u r i n g I n d u r a t i o n of H e m a t i t e Pellets T h e R e a c t i o n of S l a g - F o r m i n g C o m p o n e n t s C o o l i n g of I n d u r a t e d Pellets

40 41 43 45

3.1 3.1.1 3.1.1.1 3.1.1.1.1 3.1.1.1.2 3.1.1.1.3 3.1.1.1.4 3.1.2 3.1.3 3.1.4 3.1.4.1 3.1.4.2 3.1.4.3 3.1.4.4 3.1.4.5 3.2 3.2.1 3.2.1.1 3.2.1.2 3.2.1.3 3.2.1.4 3.2.1.5 3.2.1.6 3.2.1.7 3.2.2 3.2.2.1 3.2.3 3.2.3.1

Raw Materials and Their Preparation for Pellet Production R a w Materials Iron-Bearing Materials N a t u r a l Iron Ores Magnetite Hematite W e a t h e r e d Ores Limonite Beneficiation Products S e c o n d a r y R a w Materials Binders and Additives Binders Additives Bentonite Lime Compounds Other Additives P r e p a r a t i o n of R a w M a t e r i a l s for Pelletizing . . . . Separation Washing Gravity Separation Flotation Magnetic Separation Magnetizing Roasting Electrostatic S e p a r a t i o n P r o p o r t i o n of D i f f e r e n t O r e s in Pellet P r o d u c t i o n . . Physical P r o p e r t i e s of Fine-grained Iron Ores . . . . Size D i s t r i b u t i o n - S p e c i f i c S u r f a c e A r e a Grinding Dewatering

47 47 47 47 48 49 49 50 51 52 53 53 53 53 55 56 56 57 57 57 58 58 59 60 60 62 62 64 66

4 4.1 4.2 4.3 4.3.1

The Pelletizing Laboratory and its Tasks A p p l i c a t i o n R a n g e of L a b o r a t o r i e s T h e T a s k s of a L a b o r a t o r y R a w M a t e r i a l P r e p a r a t i o n a n d Pellet P r o d u c t i o n R a w Material Preparation

68 68 69 70 70

2.2.2.1.2 2.2.2.2 2.2.3 3

. . .

Contents 4.3.2 4.3.3 4.3.4 4.4 4.4.1 4.4.2 4.4.2.1 4.4.2.1.1 4.4.2.1.2 4.4.2.1.3 4.4.2.1.4 4.4.3 4.4.4 4.5 4.5.1 4.5.2 4.5.2.1 4.5.2.2 4.5.2.3 4.5.2.4 4.5.3 4.6 4.6.1 4.6.1.1 4.6.1.2 4.6.1.3 4.6.2 4.6.2.1 4.6.2.1.1 4.6.2.1.2 4.6.2.1.2.1 4.6.2.1.2.2 4.6.2.1.2.3 4.6.2.1.2.4 4.6.2.1.2.5 4.6.2.1.3 4.6.2.1.3.1 4.6.2.1.3.2 4.6.2.1.3.3 4.6.2.1.3.4 4.6.2.1.4

Grinding. G r i n d i n g E q u i p m e n t a n d G r i n d i n g Energy Filtration M i x P r e p a r a t i o n for Ball F o r m a t i o n Mix Preparation G r e e n Ball F o r m a t i o n G r e e n Ball F o r m a t i o n a n d T e s t i n g M e t h o d s Pellet M o i s t u r e D e t e r m i n a t i o n C r u s h i n g Strength Drop Number D r o p Resistance Capacity Determination Bulk D e n s i t y G r e e n Ball I n d u r a t i o n F u r n a c e s f o r O r i e n t i n g Tests Stationary Pot G r a t e f o r P r i n c i p a l Tests P o t G r a t e w i t h Side W a l l s a n d H e a r t h L a y e r . . . . Pot G r a t e w i t h C o r r u g a t e d S i d e W a l l s C o n t r o l S c h e m e of Pot G r a t e Tests Movable Pot Grate Pilot Plants T h e P r o p e r t i e s of I n d u r a t e d Pellets a n d T h e i r Testing M e t h o d s T h e Physical P r o p e r t i e s C r u s h i n g Strength T u m b l e r Resistance Mcroporosity B e h a v i o u r of I n d u r a t e d Pellets D u r i n g R e d u c t i o n . Testing M e t h o d s for R e d u c t i o n M e c h a n i c a l Strength E x a m i n a t i o n of F i r e d Pellets f o r Blast F u r n a c e Operation L o w - T e m p e r a t u r e D i s i n t e g r a t i o n Test (Static Test) . L o w - T e m p e r a t u r e D i s i n t e g r a t i o n Test ( D y n a m i c Test) Swelling T e s t Reduction under Load T e s t ( R u L ) Other Testing Methods E x a m i n a t i o n of F i r e d Pellets f o r the D i r e c t R e d u c t i o n L o w - T e m p e r a t u r e D i s i n t e g r a t i o n Test ( D y n a m i c ) . . . Swelling T e s t Sticking Test ( R M C ) D i r e c t R e d u c t i o n D i s i n t e g r a t i o n Stability T e s t ( D R D S ) Present State of Testing M e t h o d s

XI 73 75 76 77 77 77 79 79 80 82 82 82 83 83 83 84 85 85 86 86 89

.

. .

.

89 89 90 90 91 91 93 93 93 93 94 94 94 95 96 96 96 96 96 98

X 5 5.1 5.1.1 5.1.1.1 5.2 5.2.1 5.2.1.1 5.2.1.2 5.3 5.3.1 5.3.1.1 5.3.1.1.1 5.3.1.1.2 5.3.1.1.3 5.3.1.1.4 5.3.1.1.5 5.3.1.2 5.3.1.2.1 5.3.1.2.1.1 5.3.1.2.1.2 5.3.1.2.1.3 5.3.1.2.2 5.3.1.2.3 5.3.1.2.4 5.3.1.3 5.3.1.4 5.3.1.5 5.3.1.6 5.3.1.7 5.3.1.7.1 5.3.1.7.2

Contents Process Influencing Factors F a c t o r s Influencing Ball F o r m a t i o n G r a n u l o m e t r i c P r o p e r t i e s of R a w M a t e r i a l s G r a i n Size, Size D i s t r i b u t i o n a n d Specific S u r f a c e A r e a I n f l u e n c e of W a t e r A d d i t i o n on G r e e n Ball F o r m a t i o n O p t i m u m Moisture Content O p t i m u m M o i s t u r e C o n t e n t a n d Specific S u r f a c e A r e a O p t i m u m M o i s t u r e C o n t e n t a n d S u r f a c e C o n d i t i o n of O r e Particles I n f l u e n c e of Binders a n d A d d i t i v e s F a c t o r s for l m p r o v i n g the M e c h a n i c a l P r o p e r t i e s . . . B e n t o n i t e as B i n d e r I n f l u e n c e of Bentonite o n G r e e n Pellet Strength a n d DropResistance I n f l u e n c e of Bentonite on D r y Pellet Strength . . . . I n f l u e n c e of Bentonite o n C r u s h i n g Strength a n d A b r a s i o n Resistance of F i r e d Pellets D i f f e r e n t Bentonite T y p e s I n f l u e n c e of Bentonite on t h e C h e m i c a l C o m p o s i t i o n of Pellets I n f l u e n c e of Alkaline Earth C o m p o u n d s T h e I n f l u e n c e of C a l c i u m O x i d e [ C a O ] a n d C a l c i u m Hydroxide [Ca(OH)2] I n f l u e n c e of C a l c i u m H y d r o x i d e [ C a ( O H ) 2 ] on G r e e n Pellet Strength a n d D r o p Resistance I n f l u e n c e of [ C a ( O H ) 2 ] o n D r y Pellet Strength . . . . I n f l u e n c e of [ C a ( O H ) 2 ] o n C r u s h i n g Strength, T u m b l i n g R e s i s t a n c e a n d Porosity of I n d u r a t e d - P e l l e t s I n f l u e n c e of C a l c i u m C a r b o n a t e [ C a C O 3 ] a n d D o l o m i t e [ ( C a 1 M g ) C O 3 ] o n the Strength of I n d u r a t e d Pellets . . I n f l u e n c e of a M i x t u r e of D i f f e r e n t A d d i t i v e s o n Pellet Properties I n f l u e n c e of C a l c i u m C h l o r i d e [CaCl 2 ] on Pellet Properties I n f l u e n c e of Alkali C o m p o u n d s I n f l u e n c e of Ores with G o o d B o n d i n g P r o p e r t i e s . . . B e h a v i o u r of O r e M i x t u r e s I n f l u e n c e of Oxidized a n d P r e r e d u c e d R e t u r n F i n e s . . I n f l u e n c e of S p o n g e Iron o n Pellet P r o p e r t i e s I n f l u e n c e of S p o n g e Iron on G r e e n a n d D r y Pellet Strength I n f l u e n c e of S p o n g e Iron o n the Strength of I n d u r a t e d Pellets

99 99 100 100 105 105 106 107 109 110 110 110 112 112 113 114 115 116 116 118 121 121 125 126 127 127 128 131 132 133 133

Contents 5.3.1.8 5.3.1.9 5.3.1.10 5.3.1.11 5.4 5.4.1 5.4.1.1 5.4.1.1.1 5.4.1.1.2 5.4.1.2 5.4.1.3 5.4.2 5.4.2.1 5.4.3 5.4.3.1 5.4.3.2 5.4.3.3 5.4.3.4 5.4.4 5.4.5 5.4.6 5.4.6.1 5.4.6.2 6 6.1 6.1.1 6.1.2 6.1.2.1 6.1.2.2 6.1.2.3 6.1.2.3.1 6.1.2.3.2 6.1.2.3.3 6.1.2.4

I n f l u e n c e of Inplant F i n e s o n Pellet P r o p e r t i e s . . . . I n f l u e n c e of O r g a n i c B i n d e r s I n f l u e n c e of Coal A d d i t i o n Summarizing Considerations I n f l u e n c e of T h e r m a l T r e a t m e n t o n Pellet P r o p e r t i e s . F a c t o r s I n f l u e n c i n g G r e e n Ball D r y i n g T e m p e r a t u r e of D r y i n g G a s e s I n f l u e n c e of D r y i n g G a s T e m p e r a t u r e o n D r y i n g T i m e I n f l u e n c e of T e m p e r a t u r e o n S h o c k R e s i s t a n c e of Pellets During Drying I n f l u e n c e of Velocity of D r y i n g G a s F l o w o n D r y i n g Degree C h a n g e of Pellet Strength D u r i n g D r y i n g P r e h e a t i n g of D r i e d Pellets C h a n g e of W e i g h t a n d Strength D u r i n g D r y i n g a n d P r e h e a t i n g of G r e e n Balls F i r i n g a n d Cooling of Pellets H e a t - H a r d e n i n g of M a g n e t i t e G r e e n Pellets Influence of F i r i n g T e m p e r a t u r e a n d Basic A d d i t i v e s o n t h e Strength of Pellets f r o m M a g n e t i t e O r e s F i r i n g of H e m a t i t e G r e e n Pellets Influence of T e m p e r a t u r e a n d Basic A d d i t i v e s o n H e m a t i t e Pellet Q u a l i t y R e a c t i o n s of A d d i t i v e s w i t h I r o n O x i d e s a n d G a n g u e Constituents T h e r m a l D i s s o c i a t i o n of H e m a t i t e in Pellets S c h e m e of T h e r m a l T r e a t m e n t H e a t i n g P a t t e r n for Pellets f r o m M a g n e t i t e F i r i n g P a t t e r n f o r Pellets f r o m O t h e r O r e s Behaviour of Indurated Pellets During Reduction . . . C h a n g e of Pellet S t r u c t u r e D u r i n g R e d u c t i o n . . . . Reduction Mechanisms Structural C h a n g e D u r i n g R e d u c t i o n V o l u m e V a r i a t i o n b y Crystal C h a n g e Structural C h a n g e b y R e a c t i o n of G a n g u e C o m p o n e n t s with Iron O x i d e s a n d w i t h E a c h O t h e r I n f l u e n c e of A d d i t i v e s o n P e l l e t Swelling P r o p e r t i e s of A c i d Pellets w i t h a Basicity of Less t h a n 0.1 P r o p e r t i e s of Pellets w i t h a Basicity of 0.1 to 0.6 . . P r o p e r t i e s of Pellets w i t h a Basicity of H i g h e r t h a n 0.7 . I n f l u e n c e of G a n g u e B o n d s o n Pellet S t r u c t u r e at T e m p e r atures of 4 0 0 - 6 0 0 0 C U n d e r R e d u c i n g A t m o s p h e r e .

XIII 135 136 138 139 140 140" 141 141 142 142 143 145 145 147 148 150 151 152 153 154 155 155 157 159 162 163 165 165 169 170 174 174 176 176

XIV 6.1.2.5 6.2 6.3 7 7.1 7.1.1 7.1.2 7.2

Contents I n f l u e n c e of G a n g u e B o n d s at A b o u t 1 0 0 0 0 C o n Pellet Structure Under Reducing Atmosphere 178 B e h a v i o u r of I n d u r a t e d Pellets Consisting of M a g n e t i t e and Wüstite During Reduction 178 Conclusions 180

7.4

Special Processes for Pellet Production Pellet H a r d e n i n g b y U s i n g Binders G r a n g c o l d Process C O B O a n d M T U Process C h l o r i d i z i n g Volatilization of N o n - F e r r o u s M e t a l O x i d e s a n d Pellet P r o d u c t i o n C h l o r i d i z i n g Volatilization a n d Pelletizing in the S h a f t Furnace C h l o r i d i z i n g Volatilization a n d Pellet P r o d u c t i o n in a Rotary Kiln R e c o v e r y of V a n a d i u m P e n t o x i d e f r o m V a n a d i u m B e a r i n g Iron Ores T h e r m a l T r e a t m e n t of V a n a d i u m - B e a r i n g Pellets in a Shaft Furnace . . . T h e r m a l T r e a t m e n t of V a n a d i u m - B e a r i n g Pellets A c c o r d i n g to the G r a t e - K i l n Process D e a r s e n i f i c a t i o n and Pelletizing of I r o n Ores . . . .

8 8.1 8.2 8.2.1

Balling Equipment 192 H o m o g e n e i t y of C o m p o n e n t s for Pelletizing M i x t u r e s 192 D e m a n d s o n t h e M o d e of O p e r a t i o n of Balling U n i t s 193 Ball F o r m a t i o n a n d O p e r a t i o n of t h e Principal Pelletizing U n i t s

8.2.2 8.2.2.1

Balling D r u m T h e P r i n c i p a l D r u m C o m p o n e n t P a r t s a n d the Balling Operation . . . T h e M a i n C o m p o n e n t Parts . . . O p e r a t i o n of the Balling D r u m I n f l u e n c i n g F a c t o r s for G r e e n Ball F o r m a t i o n in a D r u m D r u m Rotating Speed L e n g t h a n d Tilt A n g l e of D r u m Balling D r u m C a p a c i t y Balling Disc T h e Principal C o m p o n e n t Parts a n d D i s c O p e r a t i o n . T h e M a i n C o m p o n e n t Parts O p e r a t i o n of the Balling D i s c I n f l u e n c i n g F a c t o r s of G r e e n Ball F o r m a t i o n o n a D i s c

7.2.1 7.2.2 7.3 7.3.1 7.3.2

8.2.2.1.1 8.2.2.1.2 8.2.2.2 8.2.2.2.1 8.2.2.2.2 8.2.2.3 8.2.3 8.2.3.1 8.2.3.1.1 8.2.3.1.2 8.2.3.2

181 181 181 182 183 184 186 188 189 190 190

196 196 196 197 198 198 199 201 202 202 202 203 204

Contents 8.2.3.2.1 8.2.3.2.2 8.2.3.2.3 8.2.3.2.4 8.2.3.3 8.2.4 8.2.5 8.2.6 8.2.6.1 8.2.6.2 8.2.6.3 8.2.6.4 8.3 8.3.1 8.3.2 8.3.3 9 9.1 9.1.1 9.1.2 9.1.2.1 9.1.2.2 9.1.2.3 9.1.2.4 9.1.3 9.2 9.2.1 9.2.2 9.2.3 9.2.4 9.2.5 9.2.6 9.3 9.3.1 9.3.2 9.3.3 9.3.4 9.3.5 9.3.5.1

XV

D i s c Slope a n d R i m H e i g h t 205 Disc Rotating Speed 206 R e s i d e n c e T i m e of M a t e r i a l in t h e D i s c 206 DiscDiameter 207 Balling D i s c C a p a c i t y 207 C o m p a r i s o n B e t w e e n Balling D r u m a n d Balling D i s c . . 208 C o m p a r i s o n of V i b r a t i n g a n d R o l l e r Screens 209 O t h e r Balling Systems 210 Balling C o n e 210 Mix Granulator 210 Vibrating Trough 211 Eccentrically M o v i n g U n i t 211 Handling and Feeding Devices 211 Roller C o n v e y o r 212 R o l l e r Screen 213 R o l l i n g Belt C o n v e y o r 213 H e a t Treatment Systems Shaft Furnace . . . Shaft Furnace Types Process Stages C h a r g i n g of G r e e n Balls to t h e F u r n a c e Drying, Preheating and Firing C o o l i n g of Pellets Heat Consumption Furnace Dimensions, Capacity and Market Position . The Grate-Kiln Combination T h e Travelling G r a t e a n d its F u n c t i o n s T h e R o t a r y K i l n a n d its F u n c t i o n s T h e Cooler Heat Consumption C u r r e n t State and M a r k e t P o s i t i o n of t h e Allis C h a l m e r s G r a t e - K i l n Process O t h e r G r a t e - K i l n Processes Travelling G r a t e Systems ' A p p l i c a t i o n of Travelling G r a t e s to T h e r m a l T r e a t m e n t of Pellets General Features U p - d r a u g h t I n d u r a t i o n Process f o r S p e c u l a r H e m a t i t e Travelling G r a t e Process A c c o r d i n g to A r t h u r G . M c K e e and C o m p a n y L u r g i - D r a v o Travelling G r a t e Process I m p o r t a n t Process F e a t u r e s

215 216 217 218 218 219 221 221 221 223 224 225 226 226 227 228 229 230 231 232 233 235 235

XVI 9.3.5.2 9.3.5.2.1

Contents

9.5.4

A p p l i c a t i o n of the Process o n an I n d u s t r i a l Scale . . C h a r g i n g of G r e e n Balls to I n d u r a t i o n G r a t e a n d the M o d e of O p e r a t i o n • Firing Pattern and Heat Consumption C a p a c i t y , Flexibility a n d M a r k e t S i t u a t i o n O t h e r H e a t T r e a t m e n t Systems Circular Indurating Furnace " H e a t F a s t " Process " A n n u l a r F u r n a c e " of H u n t i n g t o n - H e b e r l e i n C o m p a r i s o n of I m p o r t a n t Pelletizing Systems . . . . C h a n g e of O r e Basis P r o d u c t i o n F i g u r e s p e r U n i t of V a r i o u s I n d u r a t i n g Systems P r o p o r t i o n of V a r i o u s F i r i n g Systems in the W o r l d Pellet Production Cost C o m p a r i s o n .

245 246

10 10.1 10.2 10.2.1 10.2.2 10.2.3 10.2.4

Plant Layout and Process Control Plant Layout . Process Control . . . . D i s t r i b u t i o n a n d P r o p o r t i o n i n g of M a t e r i a l F l o w . . . P r o p o r t i o n i n g of Solid C o m p o n e n t s a n d W a t e r . . . F o r m a t i o n a n d T r a n s p o r t of G r e e n Pellets G r e e n Pellet C h a r g i n g

247 247 247 249 249 249 249

10.2.5 10.3

T h e r m a l T r e a t m e n t of G r e e n Balls D e v e l o p m e n t a n d T r e n d s of F u r t h e r C o n t r o l V a r i a n t s

250 250

11 11.1 11.2 11.3 11.4 11.5

Pellets in the Blast Furnace Burden 251 I n f l u e n c e of M e c h a n i c a l P r o p e r t i e s . . . 251 I n f l u e n c e of C h e m i c a l C o m p o s i t i o n 252 M e t h o d s of Pellet C h a r g i n g to the Blast F u r n a c e . . 253 C o m p a r i s o n of Pellets a n d Sinter 253 Pellet P r o p o r t i o n in the Blast F u r n a c e B u r d e n . . . . 255

12

The Utilization of Pellets in Direct Reduction Plants

13 13.1 13.1.1 13.1.1.1 13.1.1.2 13.1.1.3 13.1.1.4

S o m e Theoretical Considerations G r e e n Ball F o r m a t i o n B o n d s B e t w e e n W a t e r a n d F i n e - G r a i n e d Particles B o n d i n g b y L i q u i d Bridges B o n d i n g F o r c e s in T r a n s i t i o n R a n g e B o n d i n g F o r c e s in C a p i l l a r y R a n g e O v e r s a t u r a t i o n with W a t e r

9.3.5.2.2 9.3.5.2.3 9.4 9.4.1 9.4.2 9.4.3 9.5 9.5.1 9.5.2 9.5.3

236 237 238 239 241 242 242 243 243 244 244

. 257 259 262 . . 263 263 264 264 265

Contents 13.1.2 13.1.3 13.1.4 13.1.4.1 13.1.4.2 13.1.4.3 13.2 13.2.1 13.2.2 13.2.2.1 13.2.2.1.1 13.2.2.1.2 13.2.2.1.3 13.2.2.2 13.2.2.2.1 13.2.2.2.2 13.2.3 13.2.4 13.2.4.1 13.2.4.2 13.2.5 13.2.5.1 13.2.5.2 13.3 13.3.1 13.3.2

-

XVII

I n f l u e n c e of G r a n u l o m e t r i c P r o p e r t i e s o n G r e e n Pellet Strength I n f l u e n c e of Rolling F o r c e s D u r i n g M o v e m e n t of G r e e n Pellets D e s i g n a n d O p e r a t i o n of Balling D i s c a n d Balling D r u m D e s i g n a n d D i m e n s i o n s of Balling D i s c R o t a t i n g S p e e d a n d D i s c Slope Balling D r u m a n d its D e s i g n D a t a T h e r m a l T r e a t m e n t of G r e e n Pellets H e a t T r a n s f e r in a Pellet C h a r g e o n the T r a v e l l i n g G r a t e or in t h e S h a f t F u r n a c e H e a t T r a n s f e r of G r e e n Pellets b y C o n v e c t i o n a n d G a s Flow Gas Flow R e s i s t a n c e of a C h a r g e of U n i f o r m Pellet Size to G a s Flow R e s i s t a n c e of Pellet C h a r g 6 of D i f f e r e n t Pellet Size to Gas Flow R e s i s t a n c e of Pellet C h a r g e a n d T r a v e l l i n g G r a t e to G a s Flow Heat Transfer by Convection H e a t T r a n s f e r to the Pellet C h a r g e o n t h e T r a v e l l i n g Grate H e a t T r a n s f e r to the Pellet C h a r g e in t h e S h a f t F u r n a c e . H e a t T r a n s f e r to the Pellet C h a r g e in t h e R o t a r y K i l n . . H e a t T r a n s f e r by R a d i a t i o n . .. . . . Gas Radiation Kiln Lining Radiation Heat Conduction H e a t C o n d u c t i o n Inside t h e Pellet . . . . . H e a t C o n d u c t i o n b y P o i n t s of C o n t a c t G r e e n Ball D r y i n g D r y i n g of a n l n d i v i d u a l Pellet D r y i n g of a Pellet C h a r g e

275 275 275 276 277 277 278 278 279 279 280 282

Final Remarks

286

References Subject Index

: .

265 266 267 267 267 268 269 269 270 270 271 271 272 273

288 .

298

Introduction

In contrast to sintering, for which the down-draft sintering m e t h o d is only succesfully employed, pellets are today indurated according to three methods: in the shaft furnace, in grate-kilns a n d on travelling grates. T h e greatly varying properties of ores resulting f r o m their origin, genesis, shape, crystal f o r m and chemical composition are to be taken into account for ore p r e p a r a t i o n and pelletizing in order to p r o d u c e at any time pellets of u n i f o r m and good quality. N o w a d a y s , measures are known, by which the differences in the ore properties can be compensated. However, the corresponding parameters have to be variable and selected according to the nature of ores involved. In practice, this means that the design of new plants or the conversion of existing ones to other ore types cannot b e based on generalized programmes. In each particular case, it is practically unavoidable to find out the o p t i m u m p a r a m e t e r s by the p e r f o r m a n c e of tests. This is a decisive factor for the concept and composition of this book, which virtually relies on experimental knowledge and comprises the following chapters: — T h e most important development phases for pelletizing and the underlying causes are described in Chapter 1. — T h e f u n d a m e n t a l s for successful green ball f o r m a t i o n and induration are described in Chapter 2. — Chapter 3 deals with the utilizable ores and additives as well as their preparation for processing into pellets. — In Chapter 4 adequately e q u i p p e d laboratories and the testing of pellet quality according to different test standards are described. — The efficiency and kind of the individual influencing factors are to be found out in a d e q u a t e tests for obtaining a u n i f o r m pellet quality. T h e s e considerations are the subject of Chapter 5. — The decisive quality criterion is the b e h a v i o u r of pellets during reduction, Chapter 6. Experiments and considerations in connection with oxygen removal are described although despite m a n y efforts some question have not yet been fully clarified.

2

Introduction

— Special processes for pellet i n d u r a t i o n a n d t h e possibilities of utilizing u n u s u a l r a w m a t e r i a l s are dealt with in Chapter 7. — A survey of t h e m a i n e q u i p m e n t a n d f u r n a c e s used is given in Chapters 8 and 9. A t t h e s a m e time, these chapters s h o w the i m p o r t a n c e of the c o o p e r a t i o n of process engineers, m e c h a n i c a l engineers a n d designers for ensuring a successful process application. — F u r t h e r m o r e , s o m e c o m m e n t s on plant design and a u t o m a t i c process control are m a d e . A direct conversion of the collected d a t a to o t h e r conditions s h o u l d be c o n s i d e r e d carefully to avoid m i s i n t e r p r e t a t i o n , Chapter 10. — T h e b e h a v i o u r of pellets in the blast f u r n a c e or in direct r e d u c t i o n plants is very essential for j u s t i f y i n g the a p p l i c a t i o n of the pelletizing process a n d its f u t u r e i m p o r t a n c e , Chapters 11 and 12. — In o r d e r not to i n t e r r u p t the continuity of the v a r i o u s c h a p t e r s by theoretical or m a t h e m a t i c a l considerations, s o m e interesting f o r m u l a e a n d e q u a t i o n s are c o m p i l e d in Chapter 13 a n d s i m u l t a n e o u s l y references are m a d e to relevant literature. This b o o k will n e i t h e r be a m a n u a l for the construction of i n d u s t r i a l pelletizing plants nor gives it precise instructions for pellet p r o d u c t i o n since the u n d e r l y i n g conditions considerably d i f f e r d u e to the great variety of r a w m a t e r i a l characteristics. T h e p u r p o s e of this b o o k is r a t h e r to investigate a n d to describe t h e possibilities a n d m e t h o d s to p r o d u c e pellets of good a n d u n i f o r m quality irrespective of t h e varying p r o p e r t i e s of r a w materials. T h e existing a n d ever increasing a m o u n t of literature was considered ins o f a r as it w a s u s e f u l f o r the c o n f i r m a t i o n a n d s u p p o r t of relevant theories, p a r a m e t e r s a n d correlations.

1 Definition and Development of Pelletizing Process

1.1 Definition Pellets are balls p r o d u c e d f r o m concentrates and natural iron ores of different mineralogical and chemical composition with some remarkable properties such as: — uniform size distribution within a m a i n range of 9—15 m m diameter — high and even porosity of 2 5 - 3 0 % — high iron content of m o r e than 63% iron — practically no loss on ignition or volatiles — uniform mineralogical composition in the f o r m of an easily reducible hematite or hematite-bearing c o m p o u n d s — high and u n i f o r m mechanical strength — low tendency to abrasion and good behaviour during transportation — sufficient mechanical strength even at thermal stress under reducing atmosphere.

1.1.1 D i f f e r e n t a t i o n A g a i n s t O t h e r Iron O r e A g g l o m e r a t e s T h e simplest and earliest process for agglomerating fine-grained raw materials is briquetting. Fine-grained iron ores, for example, are pressed into briquettes with the addition of some water or another binder u n d e r high mechanical pressure. These briquettes m a y undergo direct f u r t h e r treatment or thermal processing before their use. A l t h o u g h their metallurgical behaviour in melting or reduction furnaces is very good, the iron ore briquetting could not m a k e h e a d w a y since t h e processing costs are relatively high and, above all, the production capacity of briquetting units is limited when c o m p a r e d with the enormous quantities of fine ores or The briquetting process is still utilized to agglomerate small quantities of dust or other circulating materials. This process has, of late, acquired growing importance for briquetting of fine-grained sponge iron.

concentrates

4

I

Definition and Development of Pelletizing Process

Fig. 1. Comparison of Briquettes, Pellets and Sinter

T h e second a n d presently m o s t i m p o r t a n t a g g l o m e r a t i o n process is down-draught sintering. It d i f f e r s f r o m pelletizing by v a r i o u s characteristics, s u c h as: — f e e d of c o a r s e r - g r a i n e d ore particles u p to a d i a m e t e r of 8 m m — coke breeze as m a i n energy source — heating u p of the g r a n u l a t e d m i x to slightly a b o v e the s o f t e n i n g temperature — the final p r o d u c t consists of a spongy sinter cake, partly m o l t e n , w h i c h by crushing, g r i n d i n g a n d screening, is b r o u g h t to the necessary grain size of 5 - 3 0 o r 5 - 5 0 m m . Fig. 1 shows the d i f f e r e n t o u t e r s h a p e of the a g g l o m e r a t e s p r o d u c e d according to the t h r e e processes.

1 . 1 . 2 P r i n c i p a l P r o c e s s S t e p s f o r the P r o d u c t i o n o f P e l l e t s T h e first stage is the f o r m a t i o n of green balls. F i n e - g r a i n e d iron ores having a d e q u a t e size d i s t r i b u t i o n are rolled with the a d d i t i o n of a wetting liquid, usually water, in suitable devices such as d r u m s or discs. In this way, wet balls are f o r m e d , the so-called green pellets. D u r i n g the ball f o r m a t i o n , it is also possible to use other a d d i t i v e s for i m p r o v i n g t h e properties of green a n d fired pellets, e.g. b e n t o n i t e , a n d for c h a n g i n g t h e metallurgical p r o p e r t i e s of t h e i n d u r a t e d pellets, e.g. l i m e s t o n e or d o lomite. In a second step the green pellets are dried a n d i n d u r a t e d to o b t a i n the typical f e a t u r e s of pellets. T h i s is achieved, in m o s t cases, by c a r e f u l heating u n d e r oxidizing a t m o s p h e r e to just below the s o f t e n i n g p o i n t of the

1.2 Development of Pelletizing Process

5

ores used. D u r i n g this heating, not only the crystalline s t r u c t u r e is c h a n g e d b u t also o t h e r b o n d s a p p e a r , such as reactions b e t w e e n s l a g - f o r m i n g constituents — b o t h b e t w e e n each o t h e r a n d with i r o n oxides. T h e hot pellets are carefully cooled in o r d e r to m a i n t a i n as far as possible the resulting crystalline s t r u c t u r e s a n d o t h e r b o n d s as well as to avoid tension cracks. G r e e n pellets can also be i n d u r a t e d by h y d r a u l i c a l l y acting binders, e.g. cement or c a l c i u m h y d r o x i d e , possibly b y using s t e a m u n d e r h i g h pressure. H o w e v e r , s o m e p r o p e r t i e s of such pellets d i f f e r f r o m t h e r m a l l y i n d u r a t e d pellets (see C h a p . 7).

1.2 Development of Pelletizing Process Iron ore agglomerates, be they b r i q u e t t e s , sinter or pellets, a r e not t h e final products. T h e y are f o r m e d f r o m such f i n e - g r a i n e d iron ores which, in this physical shape, cannot be utilized and serve as a n i n t e r m e d i a t e p r o d u c t on the way f r o m the ore m i n e to t h e blast f u r n a c e o r direct r e d u c t i o n plant. T h e sole p u r p o s e of a g g l o m e r a t e p r o d u c t i o n is to k e e p t h e cost p r i c e of pig iron or steel at the lowest level. F o r m a n y years until a b o u t the t u r n of the century, the iron ores charged to blast furnaces h a d been crushed a n d partly classified either at the m i n e or at t h e i r o n a n d steel works. In this case l u m p ores w e r e p r e f e r r e d a l t h o u g h small p o r t i o n s of fine ores could be tolerated. As a result t h e fines w h i c h were not utilised f o r m e d c o n t i n u o u s l y growing d u m p s with n o e c o n o m i c use. T h e y c o u l d only be e m p l o y e d to a limited extent in the blast f u r n a c e since they d e c r e a s e d t h e gas p e r m e ability of the blast f u r n a c e b u r d e n in an i r r e g u l a r m a n n e r a n d d i s t u r b e d the blast f u r n a c e o p e r a t i o n . M o r e o v e r , a great p a r t of these fines was b l o w n o u t of the blast f u r n a c e and h a d to be recovered as flue dust. T h e s e d u s t q u a n t i t i e s r e p r e s e n t e d a considerable i r o n v a l u e which, like t h e u n u s e d fine ore d u m p s , was lost; this was of lesser i m p o r t a n c e in countries w i t h g r e a t iron reserves t h a n in those with small iron reserves. T h e a m o u n t of the a c c u m u l a t i n g d u s t d e p e n d s largely o n the ore type t r e a t e d . In t h e case of M i n e t t e or o t h e r ores with a h i g h loss of ignition, it is s u b s t a n t i a l l y g r e a t e r t h a n in the case of high-grade, dense ores with a small loss o n ignition. Possibilities w e r e e x a m i n e d a n d tests to a g g l o m e r a t e t h e f l u e dust b y sintering or briquetting a n d to recycle it to the blast f u r n a c e were started at a p p r o x i m a t e l y t h e t u r n of the c e n t u r y in v a r i o u s i n d u s t r i a l i s e d countries a l t h o u g h with d i f f e r i n g intensities. C o u n t r i e s w i t h i m p o r t a n t i r o n reserves were less interested in this a g g l o m e r a t i o n . T h e y c o n s i d e r e d sintering as a

6

1 Definition and Development of Pelletizing Process

"necessary evil"1sobrescrito).T h e situation was q u i t e d i f f e r e n t in countries with small ore reserves. H e r e the d e v e l o p m e n t of t h e sinter process c o n t i n u e d intensively a n d not only flue dust b u t also o t h e r i r o n - b e a r i n g s e c o n d a r y r a w materials s u c h as pyrite cinders, mill scale or red m u d were of great interest. T h e sintering of fines, o b t a i n e d d u r i n g t h e crushing a n d screening of unclassified l u m p ores, was also gaining significance. At a b o u t t h e s a m e t i m e , s o m e researchers w e r e looking for an alternative process to sintering, especially in areas in w h i c h very fine ores o r concentrates were available. This was t h e b e g i n n i n g of the pelletizing process.

1 . 2 . 1 F i r s t P h a s e , A l t e r n a t i v e to S i n t e r i n g T h e d i f f e r e n t stages of d e v e l o p m e n t , progress a n d s p e e d of i n t r o d u c t i o n of the sintering process, especially if very fine i r o n ores were to b e t r e a t e d , led to considerations for i m p r o v i n g t h e process a n d finally for d e v e l o p i n g a n alternative to sintering, n a m e l y pelletizing. A b o v e all, countries such as S w e d e n or G e r m a n y 2 ) , w h i c h in the very early days h a d a l r e a d y b e e n c o m p e l l e d to give p a r t i c u l a r a t t e n t i o n to sintering, h a d to solve the p r o b l e m of processing increasing a m o u n t s of very fine concentrates. U p o n using m a j o r p o r t i o n s of such fines in the sinter mix, limits o n the specific p r o d u c t i o n capacity of sinter plants b e c a m e evident. T h i s b r o u g h t a b o u t , first in Sweden, the d e v e l o p m e n t of pelletizing. C o n c e n t r a t e s were n o longer a d d e d to the sinter mix b u t were s e p a r a t e l y f o r m e d into balls with the a d d i t i o n of water a n d t h e n i n d u r a t e d by using b i n d e r s or in a t h e r m a l way. S u c h a p a t e n t h a d a l r e a d y b e e n g r a n t e d in 1912 u n d e r No. 35 124 to the S w e d e A. G . Andersson. U n fortunately, n o f u r t h e r details or metallurgical results w e r e p u b l i s h e d 3 ). A l m o s t s i m u l t a n e o u s l y , similar research a n d d e v e l o p m e n t w o r k was started in G e r m a n y . In 1913, a G e r m a n p a t e n t N o . 289606 was g r a n t e d to the inventor C. A. Brackelsberg. T h i s patent protects a process according to w h i c h ore fines were m i x e d with w a t e r or binders, f o r m e d into balls a n d i n d u r a t e d at low t e m p e r a t u r e s . N o c o n s e q u e n c e s resulted f r o m the S w e d i s h p a t e n t w h e r e a s Brackelsberg k n o w l e d g e t h a t t h e pellets ( r e f e r r e d to as "GEROELL" d e r i v a t e d f r o m rolling) could b e m o r e quickly r e d u c e d than l u m p ore or sinter m a d e of the s a m e raw m a t e r i a l . In the course of this w o r k , a pilot p l a n t 5 ) with a capacity of 120 tons per d a y "GEROELL" was built in 1926 for K r u p p at the R h e i n h a u s e n steel plant. T h i s plant was reconstructed in 1935, and a l r e a d y s h o w e d essential f e a t u r e s of the pelletizing process. T h i s pilot p l a n t was d i s m a n t l e d in 1937 to m a k e available the area r e q u i r e d for the construction of a large sinter plant.

c o n t i n u e d his w o r k

1.2 Development of Pelletizing Process

7

In this way, t h e first d e v e l o p m e n t p h a s e c a m e to a n a b r u p t end. T h e pelletizing was forgotten. Sintering s p r e a d as t h e only i m p o r t a n t a g g l o m eration process t h r o u g h o u t the entire world. T h e pelletizing k n o w - h o w was practically lost until the w a y was p a v e d for t h e second p h a s e , especially in t h e U S A a n d again in S w e d e n . 1.2.2 Second Phase, Pellets from Concentrates T h e second p h a s e was initiated b y the p r o b l e m of securing t h e ore supply f r o m the L a k e S u p e r i o r region, especially f r o m the M e s a b i R a n g e . F r o m there, m a n y iron a n d steel w o r k s in t h e U n i t e d States h a d h i t h e r t o been s u p p l i e d with h i g h - g r a d e l u m p ores a n d c o a r s e - g r a i n e d c o n c e n t r a t e s with a n i r o n content of 50% a n d m o r e , d e m a n d i n g n o f u r t h e r t r e a t m e n t . D u r i n g a n d at the end of the S e c o n d W o r l d W a r , t h e reserves of such h i g h - g r a d e ores were o n the decline so t h a t o t h e r sources h a d to b e o p e n e d up. O n e of the richest deposits in t h e M e s a b i R a n g e c o n t a i n e d large ore reserves, the w e l l - k n o w n " t a c o n i t e s " w h i c h h a v e a. low i r o n content, a b o u t 30% total iron, a l m o s t exclusively in t h e f o r m of m a g n e t i t e . These taconites are m e c h a n i c a l l y very h a r d . T o l i b e r a t e t h e m a g n e t i t e , very finely d i s s e m i n a t e d t h r o u g h the ore, a very f i n e g r i n d i n g was necessary which, a f t e r m a g n e t i t e s e p a r a t i o n , y i e l d e d concentrates with more t h a n 85% fines m i n u s 325 m e s h (0.044 m m ) . A typical analysis of such a concentrate, h a v i n g b e e n t r e a t e d in the p l a n t of Reserve M i n i n g C o m p . , Silver Bay M i n n e s o t a , is s h o w n in T a b l e 1 6 ) . Table 1. Chemical composition and grain structure of magnetite concentrate in Chemical analysis, dry

Fe SiO2 Al2O3 CaO MgO Mn P S TiO 2 Fe 2+ Moisture, % H 2 O of Pellet Feed

Structure %

mesh

wt-%

Cumulative wt-%

65.5 7.8 0.5 0.5 0.6 0.25 0.032 0.003 0.10 21.75 10.00

+ 100 + 150 + 200 + 270 + 325 + 400 + 500 -500

0.1 0.6 1.5 4.0 3.5 9.0 8.4 72.9

0.1 0.7 2.2 6.2 9.7 18.7 27.1 100.0

Specific Surface Area 1700 cm 2 /g

pellet

plan

8

1 Definition and Development of Pelletizing Process

T h e high fine o r e p o r t i o n of a b o v e 96% m i n u s 325 m e s h is t o o fine-grained for efficient sintering d u e to t h e low p e r m e a b i l i t y of the sinter mix. A r o u n d 1943 intensive d e v e l o p m e n t of the pelletizing process f o r taconite concentrates was started u n d e r g u i d a n c e of a n d at the MINES EXPERIMENTAL STATION OF THE UNIVERSITY OF MINNESOTA.

W h e n this d e v e l o p m e n t work b e c a m e k n o w n in Sweden, the J e r n k o n toret (Swedish iron and steel institute) in S t o c k h o l m f o u n d e d a c o m m i t t e e in 1946 to a p p l y concentrates 7 ), a historical curiosity in r e m e m b r a n c e of the p a t e n t s p e c i f i c a t i o n by Andersson. T h e work of t h e Swedish researchers, u n d e r the direction of M a g n u s Tiegerschiöld, soon led to the construction of several small industrial plants. T h e pellets p r o d u c e d in these plants were, inter alia, used for direct r e d u c t i o n of iron ores according to the W i b e r g process with r e m a r k a b l e success 8 ). T h i s was a first indication that pellets were particularly s u i t a b l e for direct reduction and it was a decisive i m p u l s e for the f u r t h e r d e v e l o p m e n t of direct r e d u c t i o n processes. A t first, pellet p r o d u c t i o n a n d f u r t h e r d e v e l o p m e n t of c o r r e s p o n d i n g processes were limited to the two a b o v e regions w h e r e particularly f a v o u r a b l e c o n d i t i o n s prevailed. T h e concentrates p r o d u c e d there were so fine-grained that they could be formed into green pellets without further grinding. Even if pelletizing initiated a n interesting d e v e l o p m e n t , o t h e r alternative processes for the agg l o m e r a t i o n of taconite concentrates were also tested. At almost the s a m e t i m e that the first m a j o r p u b l i c a t i o n on the d e v e l o p m e n t progress of the new pelletizing process was m a d e in 1944 9 ), t h e Oliver M i n i n g Division of U. S. Steel C o r p o r a t i o n in Extaca, Virginia, M i n n e s o t a d e c i d e d to d e v e l o p f u r t h e r , on a large scale, a n o t h e r a g g l o m e r a t i o n m e t h o d , the so-called " n o d u l i z i n g p r o c e s s " for t h e p r o d u c t i o n of n o d u l e s f r o m fine-grained taconite c o n c e n t r a t e s in a d u f f coal fired rotary k i l n 1 0 ) . At the s a m e site a sintering plant was built a l t h o u g h n o r m a l l y such plants h a d h i t h e r t o b e e n erected n e a r t h e blast furnaces. A f t e r a p e r i o d of initial troubles, b o t h plants w e r e s t a r t e d u p a n d o p e r a t e d . H o w e v e r , a f t e r a few years, they were s h u t down. T h e p r o d u c t i o n of pellets — w h o s e qualities were increasingly gaining recognition — a d v a n c e d so r a p i d l y t h a t f r o m t h e n o n w a r d s pelletizing plants were exclusively built. In these plants, the ever-growing quantities of concentrates p r o d u c e d could be processed successfully so t h a t the original goal of also o p e n i n g u p deposits with a low iron content was fully achieved. A r o u n d 1955, the second p h a s e of the pelletizing process d e v e l o p m e n t was t e r m i n a t e d with the s t a r t - u p of the two large pelletizing plants of R e s e r v e M i n i n g Co. a n d Erie M i n i n g Co. with a n a n n u a l capacity of 12 million tons. T h e successful d e v e l o p m e n t of t h e d i f f e r e n t pelletizing processes in the M e s a b i R a n g e is the result of intensive c o o p e r a t i o n between big mining companies, e.g. Erie and R e s e r v e M i n i n g Co. with science, the U n i v e r s i t y of M i n n e s o t a as well as i m p o r t a n t engineering com-

1.2 Development of Pelletizing Process

9

panies, contractors a n d suppliers. T h e latter c o u l d partly use m a c h i n e r y , e q u i p m e n t a n d structures a l r e a d y k n o w n a n d a p p r o v e d . T w o f i r i n g systems f o r t h e i n d u r a t i o n of green pellets were e m p l o y e d o n a large-scale. Erie M i n i n g used the s h a f t f u r n a c e11sobrescrito),a n d R e s e r v e M i n i n g a m o d i f i e d sintering m a c h i n e 6). In Sweden, the s h a f t f u r n a c e was exclusively used d u r i n g this p e r i o d . W i t h o u t t h e i n t e n t i o n or c o m p u l s i o n of securing the f u t u r e ore s u p p l y f r o m the M e s a b i R a n g e , d e v e l o p m e n t of t h e pelletizing process w o u l d p r o b a b l y not h a v e b e e n started at this location.

1.2.3 Third P h a s e , Pellets from O r e s T h e close correlations b e t w e e n sintering a n d pelletizing b e c a m e explicit d u r i n g b o t h the first a n d second d e v e l o p m e n t phases. Since there was n o c o m p u l s i o n at that t i m e to d e v e l o p f u r t h e r the i n g e n i o u s k n o w l e d g e of Andersson a n d Brackelsberg, the first p r o m i s i n g steps to pelletizing w e r e soon forgotten. In the second p h a s e of d e v e l o p m e n t t h e sinter process was exclusively c o n f r o n t e d w i t h ever-increasing q u a n t i t i e s of very f i n e - g r a i n e d magnetite concentrates, so that t h e p r o d u c t i o n c a p a c i t y was closely limited. T h e e n o r m o u s q u a n t i t i e s of concentrates a n d the necessity of f i n d i n g a s o l u t i o n to the p r o b l e m gave the i m p e t u s to d e v e l o p pelletizing u p to a process a p p l i c a b l e to industrial o p e r a t i o n . A t t h a t t i m e , n e i t h e r in t h e U S A n o r in S w e d e n was t h e r e a n e e d for pelletizing o t h e r ores t h a n concentrates. T h e third p h a s e started f r o m q u i t e a n o t h e r situation. Various newly d e v e l o p e d steps f o r i m p r o v i n g the sinter process w e r e m o d i f i e d a n d succesfully c o m b i n e d to f o r m t h e basis f o r a new pelletizing process v a r i a n t for iron ores a n d mixtures. T h i s process was d e v e l o p e d in parallel to the k n o w l e d g e derived f r o m the pelletizing of concentrates. T h e sintering of i r o n ores h a d a very d i f f e r e n t i m p o r t a n c e in v a r i o u s countries according to the availability of ore reserves. C o n s e q u e n t l y , the readiness to give this process a n a d e q u a t e p o s i t i o n w i t h i n t h e c o n t e x t of pig i r o n production in conjunction with the o p e r a t i o n of the blast f u r n a c e d i f f e r e d greatly. T h i s was a p p a r e n t at the e n d of t h e F i r s t W o r l d W a r as well as at the beginning of a n d d u r i n g the Second W o r l d W a r . T h e necessity also to p r o d u c e pig i r o n f r o m l o w - g r a d e f i n e - g r a i n e d ores resulted in i n t e n s i f i e d d e v e l o p m e n t w o r k in t h e sintering field 12 ). T h i s d e v e l o p m e n t w o r k r e f e r r e d b o t h to process p a r a m e t e r s a n d constructional i m p r o v e m e n t s of the necessary m a c h i n e r y a n d e q u i p m e n t . T h e m o s t recent d e v e l o p m e n t progress in sintering is d e s c r i b e d in a b o o k b y F. C a p p e l and H . B. W e n d e b o r n 2 ) . A g a i n a f t e r the S e c o n d W o r l d W a r in a varied r a w m a t e r i a l s i t u a t i o n h i g h e s t p r o d u c t i o n rates of the sinter

10

1 Definition and Development of Pelletizing Process

Fig. 2. Influence of particle size on sinter productivity

m a c h i n e at m i n i m u m f u e l c o n s u m p t i o n for the p r o d u c t i o n of a g g l o m e r a t e were the c o n d i t i o n s to be fulfilled. F l o t a t i o n p y r i t e cinders a n d S w e d i s h concentrates constituted growing portions of t h e ore mix. T h e g r e a t e r fineness resulted in a r e d u c e d specific o u t p u t (in tons s i n t e r / m 2 a r e a in 24 hours) as is clearly d e m o n s t r a t e d in Fig. 2. W i t h a rising p r o p o r t i o n of fines —0.2 m m a c o n s i d e r a b l e o u t p u t decrease was observed 13 ). F r o m this resulted the d e m a n d to k e e p the fines p o r t i o n a p p r o x i m a t e l y —0.2 m m as low as possible. F o r example, in G e r m a n y it was expected that the p o r t i o n —0.125 m m should not exceed 10% 2 ). T h e fine ores at present available o n the world m a r k e t generally contain a m u c h h i g h e r percentage of fines. A c c o r d i n g to experience, a c o r r e s p o n d i n g d e c r e a s e of o u t p u t of sinter plants in o p e r a t i o n t h r o u g h o u t t h e world w o u l d b e unavoidable. F o r t h e owners of such sinter plants s u p p l i e d with ores f r o m m i n e s all over t h e world a n d not f r o m their o w n mines, this w o u l d m e a n h i g h e r p r i m e costs for sinter a n d thus for pig iron. T h e designers a n d suppliers of such sinter plants were also c o n f r o n t e d with the a b o v e p r o b l e m . It was obvious that this u n a v o i d a b l e deficiency of fine ores h a d to be compensated for. T h e s e p a r a t i o n of fines f r o m the sinter mix could be theoretically conceivable ( b u t d i f f i c u l t to realise) as is d e m o n s t r a t e d b y t h e test described below 14 ). A sinter m i x consisting of 11 d i f f e r e n t ore c o m p o n e n t s with a grain size p o r t i o n of a b o u t 40% —0.5 m m was sintered at a capacity of X t o n s / m 2 p e r d a y a f t e r c a r e f u l p r e p a r a t i o n . T h e s a m e ore was screened at 0.5 m m a n d only the coarser f r a c t i o n was sintered. In this case, the capacity rose by a b o u t 35% c o m p a r e d to the first test. A second m e t h o d of c o m p e n s a t i n g the r e d u c e d capacity to be expected w o u l d be the e n l a r g e m e n t of the suction a r e a of the sinter strand. H o w -

1.2 Development of Pelletizing Process

11

ever, a sinter strand with a suction area designed for specific ores would not be sufficiently flexible to compensate for the capacity fluctuations resulting f r o m varying o r e m i x t u r e s a n d the c o r r e s p o n d i n g l y d i f f e r e n t fines portions. N o r would the s e p a r a t i o n of the very fine particles or t h e extension of the suction a r e a b e c o n s i d e r e d s u i t a b l e m e a s u r e s to solve this p r o b l e m . 1.2.3.1 The Two-Stage Granulation of the Sinter M i x Finally, t h e i n t r o d u c t i o n of a n e w process stage b r o u g h t t h e expected solution. A f t e r the d e t r i m e n t a l effect t h a t excessive fines p o r t i o n h a s on the sinter o u t p u t b e c a m e k n o w n , t h e sinter m i x p e r m e a b i l i t y was improved in intensive l a b o r a t o r y tests b y f u r t h e r t r e a t m e n t of the prepared sinter mix. In these tests t h e p r e p a r e d m i x was c o n v e y e d to a second m i x e r in w h i c h it was a d d i t i o n a l l y rolled. T h i s process v a r i a n t is k n o w n as two-stage granulation (rerolling) a n d n o w f o r m s p a r t of m o s t of m o d e r n sinter plants in o p e r a t i o n t h r o u g h o u t t h e world. By rerolling, the very f i n e particles a d h e r e to coarser particles. T h e ore m i x now contains small balls a n d o r e particles with a size d i s t r i b u t i o n of a b o u t 0 . 5 - 8 m m . Micro-pellets are f o r m e d as can be seen f r o m the c o m p a r i s o n s h o w n in Fig. 3. A n o r m a l sinter m i x a n d a sinter m i x subjected to this a f t e r - t r e a t m e n t s h o w distinct d i f f e r e n c e s in the size distribution a n d s h a p e . T h e rerolling a p p a r a t u s , e.g. a d r u m , is e q u i p p e d in a d i f f e r e n t way to a normal m i x i n g d r u m . It is practically identical to a balling d r u m . T h e rolling effect a n d the d i f f e r e n t sintering t i m e f o r s o m e h i g h - g r a d e iron ores w i t h good sintering p r o p e r t i e s are s h o w n in s o m e tests d e s c r i b e d

Fig. 3. Piles of sinter mix with and without re-rolling

12

1 Definition and Development of Pelletizing Process

Fig. 4. Influence of re-rolling of a Venezuelan ore feed on sinter plant capacity

below w h i c h w e r e carried out with a n d w i t h o u t portions of f i n e - g r a i n e d flue d u s t 1 5 ) . V e n e z u e l a n f i n e ores ( m i x A) a r e m i x e d with coke breeze, return fines a n d w a t e r a n d are sintered at a p r o d u c t i o n rate of 48 t / m 2 / 2 4 h. D u r i n g rerolling, only a very slight o u t p u t increase is observed. By the a d d i t i o n of 10% flue dust ( m i x B), the o u t p u t d r o p s to a b o u t 38 tons. By rerolling over 5 minutes the o u t p u t is re-increased to t h e p r e v i o u s value (see Fig. 4). A substantially l o w e r specific o u t p u t ( a b o u t 22 t / m 2 / 2 4 h) is o b t a i n e d with a n o t h e r ore m i x h a v i n g a high p r o p o r t i o n of grains - 0 . 2 m m . If this m i x is rerolled b e f o r e sintering, t h e o u t p u t can be g r a d u a l l y raised to m o r e t h a n 30 tons, w h i c h represents an increase of almost 38% (Fig. 5).

Fig. 5. Influence of re-rolling of a sinter mix (magnetite concentrate, flue dust, pyrite cinders) on sinter plant capacity

1.2 Development of Pelletizing Process

13

By this s i m p l e process variant, w h i c h is at p r e s e n t a p p l i e d t h r o u g h o u t the world, it is possible to i m p r o v e the sinter mix p e r m e a b i l i t y and thus the sinter process in such a way t h a t t o d a y p o r t i o n s of m o r e t h a n 10% fines - 0 . 2 m m c a n b e used. T h e result of this i m p r o v e d sintering m e t h o d was that in industrialised countries p u r c h a s i n g a high p e r c e n t a g e of e x t r a n e o u s ores (Japan, G r e a t Britain, G e r m a n y ) , t h e r e was b u t little incentive to introduce the pelletizing process. Nevertheless, the p r o d u c t i o n of m i c r o - p e l l e t s was t h e first step t o w a r d s pelletizing in these countries. T h i s m e t h o d of p r o d u c i n g micro-pellets, virtually d e v e l o p e d in G e r m a n y , was, in o n e p a r t i c u l a r case, a d o p t e d to 100% flotation pyrite cinders w i t h o u t the expected success. 1.2.3.2 Pellet Sintering T h e sinter process was then m o d i f i e d in such a way t h a t not the total m i x but only the fine r a w m a t e r i a l w a s f o r m e d into balls of 3 - 6 m m d i a m e t e r w h i c h were m i x e d w i t h r e t u r n fines a n d coke breeze a n d sintered. T h i s process h a s b e c o m e k n o w n as pellet sintering a n d is a n int e r m e d i a t e process b e t w e e n n o r m a l sintering a n d pelletizing. T h e final p r o d u c t is a sinter c a k e in w h i c h the pellet s t r u c t u r e of the raw mix c a n still be recognised in the finished sinter p r o d u c t . S u c h a pelletizing p l a n t was o p e r a t e d for several years by C O M I N C O in Trail, C a n a d a . S i m i l a r results were o b t a i n e d in an u p - d r a u g h t pelletizing p l a n t erected b y Cleveland Cliffs Iron C o m p a n y n e a r I s h p e m i n g , M i c h i g a n , called " E a g l e Mill" p l a n t 1 6 ) . In this p l a n t the total h e a t r e q u i r e d was s u p p l i e d in a solid state. T h e final p r o d u c t o b t a i n e d consisted largely of sinter l u m p s a n d not, as expected, of pellets. H o w e v e r , n e i t h e r of t h e process variants was f u r t h e r d e v e l o p e d . T h e s e plants are n o longer in o p e r a t i o n . In m a n y countries w h i c h exclusively used the sinter process for i r o n ore a g g l o m e r a t i o n , a n o t h e r p r o b l e m arose, n a m e l y to s u p p l y the necessary solid, lean fuel, m a i n l y coke breeze. T h e r e was a scarcity of this f u e l in certain countries b e c a u s e d u e to t h e c o n s i d e r a b l y increasing n u m b e r of sinter plants b e i n g built, t h e necessary a m o u n t s of c o k e breeze, c h i e f l y originating f r o m coke screening, w e r e n o longer s u f f i c i e n t to m e e t the r e q u i r e m e n t s . It was necessary to c r u s h h i g h - g r a d e , e x p e n s i v e m e t a l l u r g i cal coke for sintering. T h e search for c h e a p r e p l a c e m e n t fuel led to a n increased use of blast f u r n a c e gas f r o m w h i c h a n o t h e r process variant of the n o r m a l d o w n - d r a u g h t sintering, n a m e l y the mixed firing method, was developed. 1.2.3.3 Mixed Firing Method N o r m a l l y , t h e coke b r e e z e o n the s u r f a c e of the sinter b e d is ignited by the c o m b u s t i o n of f l u e gas in an i g n i t i o n h o o d . T h e ignition h o o d length

14

1 Definition and Development of Pelletizing Process

Fig. 6. Influence of mixed firing on sintering of iron ores

and the d u r a t i o n of the i n f l u e n c e of the h o t c o m b u s t i o n gases is accordingly limited. T h e heat a m o u n t d e v e l o p i n g inside the ignition h o o d is bed. W h e n a d o p t i n g the m i x e d firing m e t h o d , the ignition h o o d is substantially extended in o r d e r t h a t a h i g h e r gas v o l u m e can be b u r n e d a n d greater h e a t quantities c a n be sucked i n t o the sinter bed. W i t h such e x t e n d e d ignition h o o d s it is possible to d e c r e a s e the coke c o n s u m p t i o n remarkably. F l u e gas can be replaced by o t h e r h e a t sources, such as hot air of a b o u t 8 0 0 0 C , oil or natural gas, according to t h e i r availability. C o k e b r e e z e c a n n o t be r e p l a c e d by hot gas to a n u n l i m i t e d extent, w i t h o u t i m p a i r i n g the plant o u t p u t and sinter strength, as s h o w n in Fig. 6. A n a d v a n t a g e of this m e t h o d is t h e better reducibility with increasing p o r t i o n of h o t g a s 1 7 ) .

insignificant

1.3 Pelletizing, a Contribution to Ore Preparation

15

T h e m i x e d firing m e t h o d is n o w a d a y s o f t e n a d o p t e d p a r t i c u l a r l y in countries w h e r e coke breeze is expensive. O n c e it h a d b e e n possible to p r o d u c e micro-pellets f r o m fine-grained m i x t u r e s by rolling of balls of 3—6 m m for pellet sintering, it was not very d i f f i c u l t to p r o d u c e green pellets of a g r e a t e r d i a m e t e r a n d of a u n i f o r m , close size r a n g e ( 9 - 1 5 m m ) . However, it was not yet possible to i n d u r a t e t h e s e pellets exclusively w i t h coke breeze or by a d o p t i n g the m i x e d firing m e t h o d in o r d e r to m e e t the pertinent r e q u i r e m e n t s . A consistent f u r t h e r d e v e l o p m e n t of this f i r i n g technology by the exclusive use of gas o r oil l e a d s to the a p p l i c a t i o n of t h e pelletizing process. T h i s firing technology a p p l i c a b l e to the firing of g r e e n pellets f r o m iron ores was a d o p t e d inter alia to c e r a m i c mass, glass constituents, b u r n i n g of l i m e s t o n e 18 ). T h e three d e v e l o p m e n t p h a s e s of pelletizing are s u m m a r i z e d below: First Phase — A l t e r n a t i v e to Sintering: 1910—1927. Besides t h e sintering process u n d e r d e v e l o p m e n t , a n alternative b e c o m e s a p p a r e n t , to a g g l o m erate very fine-grained ores by pelletizing w i t h o u t any c o n s e q u e n c e s ensuing. Second Phase - Pellets f r o m C o n c e n t r a t e s : 1 9 4 0 - 1 9 5 5 . P r o d u c t i o n of very fine-grained concentrates increasing c o n s i d e r a b l y in s o m e regions, such as in the M e s a b i R a n g e / U S A , c o m p e l s d e v e l o p m e n t of t h e pelletizing process as a n alternative to sintering. T h e state of d e v e l o p m e n t at t h a t t i m e did not yet allow for the use of h i g h p o r t i o n s of fines in t h e ore mix. Third Phase - Pellets f r o m Ores: 1948 u p to t h e p r e s e n t d a t e . In v a r i o u s countries t h e sinter process is f u r t h e r d e v e l o p e d intensively to a d a p t it to the varying s u p p l y of ores of d i f f e r e n t fineness. A s a result of this d e v e l o p m e n t work, the a p p l i c a t i o n r a n g e of t h e pelletizing process was also e x t e n d e d , b e y o n d the use of c o n c e n t r a t e s as only c o m p o n e n t s , to o t h e r ores a n d , m o r e o v e r , with c o n s i d e r a b l e success to o r e mixtures, see i t e m 9.3.5.

1.3 Pelletizing, a Contribution to Ore Preparation F o r e c o n o m i c reasons, the t r e a t m e n t of i r o n ores in t h e blast f u r n a c e or in direct r e d u c t i o n plants is nowadays n o longer possible w i t h o u t intensive ore preparation. Even if i n d i v i d u a l process stages involve h i g h p r i m e costs, these are accepted, p r o v i d e d t h a t t h e total p r o d u c t i o n costs of pig iron or s p o n g e iron can, in this way, b e k e p t at lowest level. T h e p u r p o s e of i r o n

16

1 Definition and Development of Pelletizing Process

o r e p r e p a r a t i o n is the q u a l i t a t i v e i m p r o v e m e n t of d i f f e r e n t f e a t u r e s of r a w materials: (a) M e c h a n i c a l crushing, grinding, screening, classification. (b) Physical s e p a r a t i o n of various m i n e r a l constituents for the elimination of g a n g u e f r o m the lean ores a n d p r e p a r a t i o n of concentrates with high i r o n content. (c) T h e r m a l or chemical t r e a t m e n t for the elimination of volatile energyConsuming constituents such as H 2 O , C O 2 , S O 4 , S or conversion of h e m a t i t e to magnetite. (d) Metallurgical c h a n g e by basic additives w h i c h decrease the energy c o n s u m p t i o n of the succeeding process stages. In all f o u r steps pelletizing plays a significant role, such as: — Agglomeration of the finest ore particles or concentrates [(a) and (b)] — Volatilisation of components, such as H2O, CO2, SO4, S [(c)] — Changing to the chemical composition by basic additives. T h e entire c o m p l e x of p r e s e n t - d a y iron o r e p r e p a r a t i o n is s h o w n in Fig. 7. Since the significance of "physical mixing" is recognised t h r o u g h o u t the world and a c c e p t e d , even iron-rich ores are c r u s h e d to a m a x i m u m l u m p size, for the blast f u r n a c e to approx. 30—50 m m , for direct r e d u c t i o n to a p p r o x . 2 0 - 3 0 m m . T h e e l i m i n a t e d fine ore c a n either b e sintered or a f t e r f u r t h e r fine g r i n d i n g pelletized. L o w - g r a d e ores with an iron content of less t h a n 50% are u p g r a d e d , the solid g a n g u e constituents s e p a r a t e d a n d in so

Fig. 7. Alternatives for iron ore preparation

1.3 Pelletizing, a Contribution to Ore Preparation

17

Fig. 8. Size distribution of pellets

doing the iron content in the c o n c e n t r a t e i n c r e a s e d . A c c o r d i n g to its fineness the c o n c e n t r a t e c a n be pelletized i m m e d i a t e l y or following f u r t h e r grinding. In certain p r o p o r t i o n s concentrates c a n also b e i n c o r p o r a t e d in sinter mixes. D u r i n g t h e r m a l t r e a t m e n t the s e p a r a t i o n of volatile c o m ponents takes place in both pelletizing a n d sintering. Because of their d e f i n e d p r o p e r t i e s , high iron content t o g e t h e r w i t h u n i f o r m size d i s t r i b u tion and close size r a n g e pellets are a n i m p o r t a n t c h a r g e constituent f o r blast f u r n a c e s a n d direct r e d u c t i o n plants. T h e size d i s t r i b u t i o n , s h o w n in Fig. 8, as a n a v e r a g e of t h r e e c o m m e r c i a l s a m p l e s 1 9 ) is r e m a r k a b l e . T h e c o n d i t i o n s are very strict: 8 0 - 9 0 % of t h e pellets s h o u l d h a v e a d i a m e t e r of 9 - 1 5 m m , t h e m a j o r p a r t of w h i c h a d i a m e t e r of 9—12 m m . T h e c h a r g e s h o u l d , if possible, c o n t a i n n e i t h e r fines —5 m m nor pellets of a d i a m e t e r e x c e e d i n g 25 m m . T h i s is also a n i m p o r t a n t size range for use with o t h e r i r o n - b e a r i n g c o m p o n e n t s in, f o r example, the blast f u r n a c e . T h e f e e d s h o u l d consists of grains 5 to m a x . 50 m m , p r e f e r a b l y 30 m m ; Fig. 9 s c h e m a t i c a l l y s h o w s the various o p e r a tions l e a d i n g to this d e s i r e d size distribution: c r u s h i n g of coarse ores a n d sinter cake, screening a n d s e p a r a t i o n i n t o 3 fractions. T h e oversize is recycled to t h e g r i n d i n g plant, the product of 5-50 mm or 30 mm goes into the blast furnace a n d t h e u n d e r s i z e is c o n v e y e d to a g g l o m e r a t i o n , e i t h e r sintering or pelletizing plant. T h e undersize also includes concentrates, pyrite cinders, mill scale, flue d u s t a n d possibly additives. Pellets, c r u s h e d sinter a n d classified raw ore constitute, e i t h e r a l o n e or in the f o r m of mixtures, the blast f u r n a c e b u r d e n .

18

1 Definition and Development of Pelletizing Process

Fig. 9. Scheme of preparation and size distribution of blast furnace feed

Besides sinter, still the m o s t i m p o r t a n t t y p e of a g g l o m e r a t e s f o r blast f u r n a c e b u r d e n s , pellet portions are, c o m p a r e d to l u m p ores, of g r o w i n g i m p o r t a n c e f o r p i g i r o n a n d sponge iron p r o d u c t i o n . In Fig. 10 the develo p m e n t of p i g i r o n p r o d u c t i o n f r o m 1960 to 1978 is c o m p a r e d with sinter a n d pellet p r o d u c t i o n . F r o m 1974 the curves f o r sinter a n d pig i r o n decline, t h e latter c a u s e d b y e c o n o m i c reasons. T h e pellet curve r e m a i n s steep u p to a n e s t i m a t e d average pellet p o r t i o n of 25% in the blast furnace burden. In the pellet p r o d u c t i o n curve, Fig. 10, the a m o u n t of pellets as f e e d m a terial for direct reduction processes is i n c l u d e d b u t the p o r t i o n is r e m a r k a bly h i g h e r by u p to 100%. In s h a f t f u r n a c e s efforts are m a d e to replace a p a r t of the pellets by c h e a p e r , carefully selected and screened l u m p ores. A m i x t u r e of 55—75% pellets a n d 45—25% l u m p ores h a s recently p r o v e d to b e suitable f o r o p t i m u m productivity, as s h o w n in Fig. 11.

1.3 Pelletizing, a Contribution to Ore Preparation

19

Fig. 10. World production of pig iron, sinter and pellets

Fig. 11. Influence of a mixture of pellets and sized lump ores on direct reduction shaft furnace capacity

20

1 Definition and Development of Pelletizing Process

1.4 Sites of Pelletizing Plants and Transportability of Pellets

As already m e n t i o n e d above, pellet p r o d u c t i o n a n d the d e v e l o p m e n t of a p p r o p r i a t e processes is directly connected with t h e b e n e f i c i a t i o n of lowg r a d e iron ores - originally chiefly m a g n e t i t e s to p r o d u c e (very fine-grained) In the first place, it was necessary to convert these concentrates into agglomerates r e a d y f o r the blast f u r n a c e . At t h e s a m e t i m e the q u e s t i o n arose r e g a r d i n g t h e o p t i m u m site of such a pelletizing plant. T h e very with long a n d cold winters because they freeze d u r i n g this t i m e a n d can neither be l o a d e d nor transported. T h e s e p r o b l e m s were solved by erecting the pelletizing p l a n t in the close vicinity of the b e n e f i c i a t i o n p l a n t a n d thus practically at the ore mine. Pellets are resistant to the influences of storage and winter conditions. T h e y w i t h s t a n d long t r a n s p o r t a t i o n routes with several t r a n s - s h i p m e n t s better t h a n l u m p ores o r even sinter. D u r i n g the first years, pellets were p r o d u c e d , m a i n l y in the U S A , to supply blast f u r n a c e s w i t h i n a controlled m a r k e t . T h e i r t r a n s p o r t a b i l i t y was accepted b u t n o special controls were instituted. O n l y a f t e r m a j o r pellet q u a n t i t i e s h a d a p p e a r e d o n the world m a r k e t a n d h a d r e a c h e d o t h e r blast furnaces, was greater i m p o r t a n c e attached to the b e h a v i o u r of pellets d u r i n g transport. T h u s , the p r e p a r a t i o n of a large-scale test (1963), r u n o n a b o u t 150,000 tons pellets f r o m the Reserve M i n i n g C o m p a n y in a blast f u r n a c e of t h e F e d e r a l R e p u b l i c of G e r m a n y , also included, e.g. t h e study of the a b r a s i o n b e h a v i o u r , c o m p r e s s i o n strength a n d variation of pellet size 2 0 ). F r o m P o r t Silver Bay to the works p o r t s i t u a t e d on the river R h i n e , t h e pellets were subjected to six trans-shipments, a w a t e r t r a n s p o r t a n d a n 18 m o n t h storage period at the p o r t a r e a of A m s t e r d a m d u r i n g two winters. T h e fines p o r t i o n —6 m m , c o n t a i n e d in t h e pellets, was used as a reference value. This fines p o r t i o n was at L a k e S u p e r i o r 8.0% a n d increased d u r i n g t r a n s p o r t a t i o n via the P o r t of A m s t e r d a m to t h e D u i s b u r g - H u c k i n g e n Works, a n d a f t e r screening b e f o r e t h e blast f u r n a c e to a b o u t 18%. T h e p o r t i o n of 10—15 m m d i a m e t e r varied f r o m 85% to 82% a n d , a f t e r screening, rose again to 88% at the blast f u r n a c e . In this way, p r o o f was f u r n i s h e d that pellets c a n be t r a n s p o r t e d satisfactorily. T h i s large-scale test — successfully carried out in a n i n d e p e n d e n t steel plant with high pellet p r o p o r t i o n s in the blast f u r n a c e b u r d e n - b e c a m e k n o w n publicly a n d aroused increased interest in these n e w a g g l o m e r a t e s also at those works w h e r e pellets h a d h i t h e r t o not b e e n used at all.

concentrates.

fine-grained,

1.4 Sites of Pelletizing Plants and Transportability of Pellets

21

Table 2. Countries leading in iron ore production and export 113 ) Countries

106 t/year 1976

1966 Production Africa Liberiaª Rep. South Africa America Brazil* Venezuela ª Canada* USA Asia Indiaª Australiaª Europe France ª Great Britain Swedenª USSRª

Export

Production

Export

17.0 6.8

16.6 3.1

35.0 15.7

20.8 5.0

23.3 17.9 36.6 91.6

11.8 17.0 31.2 9.9

70.0 23.0 56.0 81.2

47.3 15.6 44.5 2.9

26.8 11.0

13.4 2.0

42.6 93.1

24.0 81.1

18.2

45.5 4.6 30.5 239.0

55.7 13.9 28.0 160.2

-

22.3 26.1

15.8 —

22.0 43.1

* Leading Exporters Pellet p r o d u c t i o n was initially always linked with a b e n e f i c i a t i o n plant, usually located at the m i n e w h e r e s u f f i c i e n t q u a n t i t i e s of very f i n e - g r a i n e d concentrates were available w i t h o u t a n y i n t e r m e d i a t e t r e a t m e n t . T h e increasing w o r l d - w i d e d e m a n d for h i g h - g r a d e i r o n ores led to the discovery of n e w deposits with i r o n ores of d i f f e r e n t m i n e r a l o g i c a l a n d chemical c o m p o s i t i o n . T h e i r quality c o u l d also be i m p r o v e d by b e n e f i c i a tion as in the case of M a r c o n a in P e r u . H o w e v e r , m a n y of these deposits were located in countries with little d o m e s t i c d e m a n d f o r pellets so t h a t their e x p l o i t a t i o n could only be secured by e x p o r t a t i o n . S o m e countries h a v e a d a p t e d themselves p a r t i c u l a r l y to this s i t u a t i o n so that a c o n s i d e r a b l e c h a n g e of ore s u p p l i e r s h a s o c c u r r e d d u r i n g t h e last ten years, as is s h o w n in T a b l e 2 f o r the years 1 9 6 6 - 1 9 7 5 . Firstly, it tpy or 50% c o m p a r e d to 1966. In s o m e instances t h e r e were even m o r e i m p o r t a n t c h a n g e s as, f o r e x a m p l e , in A u s t r a l i a a n d Brazil. A f u r t h e r r e m a r k a b l e p h e n o m e n o n is the p r o d u c t i o n d e c r e a s e of l o w - g r a d e ore in F r a n c e ( m i n e t t e ) a n d in G r e a t B r i t a i n ( h o m e ores). T h e s e countries n o w i m p o r t the c o r r e s p o n d i n g iron units in t h e f o r m of h i g h - g r a d e ores. O t h e r countries, s u c h as J a p a n a n d the F e d e r a l R e p u b l i c of G e r m a n y as well as nearly all o t h e r E. E. C. countries m e e t t h e i r r e q u i r e m e n t s a l m o s t exclusively by i m p o r t s .

indicates

22

1 Definition and Development of Pelletizing Process Table 3. Screen analysis of exported fine ores 2 )

Screen Undersize in mm -11.2 - 8 - 5.6 - 4 - 2.8 - 2 - 1.4 - 1 - 0.71 - 0.5 - 0.355 - 0.25 - 0.18 - 0.125 - 0.09 - 0.063 - 0.045

Liberia (Bomi Hill)

Mano River washed

Kiruna B Hamersley CVRD CVRD weight % (cumuCommon Sinter lative) Fines Feed B

15.5 22 29 35.5 42 49 56.5 64 72 80.5 89.5

13.5 24 35 44 53.5 61 68 75 80.5 85 88.5

9 18.5 27 35 41.5 47 51.5 55 58.5 63 67.5 73.5 80

1.0 11 24.5 35 43.5 49 54.5 60.5 65 69.5 75 79 84.5 87.5 90.5 92.5

21.5 34 42 47 51.5 54 55.5 57.5 59.5 61 62.6 64.5 67 70 75

3 10 19 26.5 32 36 40.5 45 49 52 57 63 74 85.5

Nimba

8 16 24 31 35.5 39 42.5 46 50.5 55 68 73.5 80.5 84.5

T h e fine ores to be s h i p p e d f r o m p o r t to c u s t o m e r f r e q u e n t l y c o n t a i n a very high a n d d i f f e r e n t p o r t i o n of fines as is a p p a r e n t f r o m s o m e e x a m p l e s given in T a b l e 3. T r a n s p o r t a t i o n difficulties as a result of d u s t arising d u r i n g loading, t r a n s - s h i p m e n t a n d u n l o a d i n g of such ores as well as difficulties d u r i n g sintering led to the p r o d u c t i o n of pellets at the p o r t a n d to the e x p o r t a t i o n of pellets instead of fine ores. T h i s was the beginning of the construction of new pelletizing plants in s h i p p i n g ports, e.g. C V R D , T u b a r a o / B r a z i l , H a m e r s l e y , P o r t D a m p i e r / A u s t r a l i a , M a r c o n a , San concentrates s u p p l i e d f r o m various mines; they can be d e s i g n a t e d as pelletizing plants located at the port. A f t e r pelletizing technology h a d b e e n sufficiently d e v e l o p e d , b l e n d e d ore pellets consisting of varying ore types were also p r o d u c e d 21 ). A s a result of this it b e c a m e possible to build pelletizing plants in direct connection with the blast f u r n a c e . Only this latter c o m b i n a t i o n w o u l d h a v e the a d v a n t a g e of being a b l e to pelletize very f i n e - g r a i n e d ores a n d concentrates as well as waste oxides a n d thus release the sinter plant. A typical e x a m p l e of such a c o m b i n a t i o n is the pelletizing p l a n t of t h e K o n i n k l i j k e N e d e r l a n d s c h e H o o g o v e n s en S t a a l f a b r i e k e n N . V. in I j m u i d e n / N e t h e r l a n d s 22 ), where, besides the pellet plant (3.5 million tpy) t h r e e sinter plants, altogether h a v i n g the s a m e capacity, are in o p e r a t i o n . F u r t h e r plants are o p e r a t i n g at the J a p a n e s e Iron & Steel W o r k s K a w a s a k i - C h i b a , K o b e - N a k a h a m a a n d K o b e - K a k o gawa. This t h i r d t y p e of p l a n t can b e d e s i g n a t e d as pelletizing plant located near the blast furnace.

Nicolas/Peru,

Wab

2 Fundamentals of Pelletizing

Pellets differ f r o m l u m p ore and, to a certain extent, also f r o m sinter by several properties which are p r e d e t e r m i n e d and definable. Despite the great variety of raw materials used, the pellets p r o d u c e d must have the same properties which are j u d g e d in accordance with internationally accepted standards. D u e to their importance and great variety, the properties d e m a n d e d f r o m pellets are specified u n d e r item 1.1. According to today's technology, nearly all iron ores with a correspondingly high iron content can be pelletized: magnetite, hematite, limonite and their concentrates as well as purposely prepared mixtures, waste oxides, pyrite cinders or pertinent by-products f r o m other industries. To obtain the required properties and taking t h e great variety of raw materials into account, a d e q u a t e p r o d u c t i o n m e t h o d s are to be adopted which are described and discussed below: Three process stages are - Stage 1: R a w material - Stage 2: F o r m a t i o n of - Stage 3: Induration of

involved to produce the pellet f r o m raw material: preparation green balls green balls.

Successful pellet production calls for an o p t i m u m efficiency and harmony between all three process steps with the preceding stage highly influencing the subsequent one. An error m a d e in the preceding stage can only be corrected to a limited extent in the subsequent process stages. Even during induration, no first-class pellet can be p r o d u c e d f r o m a defective green ball. T h e purpose of the green ball f o r m a t i o n is to obtain balls of the desired size range and having a mechanical strength which enables them to be safely transported f r o m the balling e q u i p m e n t to the induration facilities. D u r i n g the f o r m a t i o n of green balls f r o m solid fine-grained particles, many different forces co-act, which are designated as bonding mechanisms.

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2 Fundamentals of Pelletizing

2.1 Bonding Mechanisms for Green Ball Formation At a n early stage, b o t h e x p e r i m e n t a l a n d theoretical considerations were m a d e with r e g a r d to the b o n d i n g m e c h a n i s m s u n d e r l y i n g the green ball f o r m a t i o n . S o m e of the theories d e v e l o p e d in this c o n n e c t i o n were a l r e a d y k n o w n f r o m o t h e r processes, e.g. those of the fertilizer industry, f o o d - s t u f f s industry, p h a r m a c e u t i c a l and r e f r a c t o r y industries. S i m i l a r b o n d i n g m e c h anisms also play a great p a r t in the g r a n u l a t i o n a n d c r u m b l i n g of the sinter mix. T h e r a p i d progress of pelletizing technology was achieved particularly by a l m o s t simultaneous e x p e r i m e n t a l a n d theoretical progress in contrast to o t h e r process d e v e l o p m e n t s , e.g. in the blast f u r n a c e technology.

2.1.1 Important Bonding Factors T h e decisive factors for the green ball f o r m a t i o n a n d green properties can b e d i v i d e d into the following g r o u p s 23 ):

ball

(a) physical forces, such as van der Waals', m a g n e t i c or electrostatic forces (b) surface-dependent factors, such as particle size, particle size d i s t r i b u tion, particle s h a p e a n d crystalline structure (c) material-dependent factors, such as wettability, a b s o r p t i v e capacity d u e to p o r o u s structure, availability of swelling c o m p o n e n t s , c h e m i c a l p r o p e r t i e s in p r i m a r y ores or b y - p r o d u c t s a f t e r p r e v i o u s t r e a t m e n t (d) capillary forces and surface tension d u r i n g t h e a d d i t i o n of l i q u i d binders, s u c h as w a t e r or others. S o m e of these factors, mainly the raw m a t e r i a l - d e p e n d e n t factors are not variable. H o w e v e r , they influence the green ball f o r m a t i o n to a great extent. O t h e r forces also acting on the green ball p r o p e r t i e s are variable. By utilizing such forces, the raw materials to be pelletized can be a d a p t e d to the relevant r e q u i r e m e n t s . V a r i a b l e factors are, for e x a m p l e the q u a n t i t y of wetting agent a d d e d , the particle fineness and shape, the balling e q u i p m e n t used for green ball f o r m a t i o n , the forces arising in such e q u i p m e n t as well as the m o v e m e n t of raw materials in these units.

2.1.2 Ball Formation Alternatives G r e e n balls c a n be f o r m e d in d i f f e r e n t ways. In each p a r t i c u l a r case, the various b o n d i n g m e c h a n i s m s act at d i f f e r e n t intensity.

2.1 Bonding Mechanisms for Green Ball Formation

25

2.1.2.1 Compacting Method T h e solids are pressed into b r i q u e t t e s only by the a p p l i c a t i o n of h i g h mechanical forces. S h a r p - e d g e d b r i q u e t t e s m a y be r o u n d e d off b y rerolling. T h e i n d i v i d u a l grains are held t o g e t h e r b y a d h e s i v e a n d cohesive forces which, in turn, d e t e r m i n e the m e c h a n i c a l strength of the b r i q u e t t e s . T h e solid particles, m i x e d either with or w i t h o u t b i n d e r s are p a c k e d r a n d o m l y w i t h o u t any relative m o v e m e n t . T h e location of the v a r i o u s grain faces to each o t h e r is at r a n d o m — it does n o t h a v e to b e t h e o p t i m u m l o c a t i o n for the f o r m a t i o n of the greatest a d h e s i v e forces. T h e c o m p a c t i n g t e c h n i q u e - well k n o w n f o r b r i q u e t t i n g a n d t a b l e t c o m pressing — is n o w a d a y s a p p l i e d to o r e p r e p a r a t i o n only in specific cases. 2.1.2.2 Green Ball Formation Besides the solid phase, a liquid p h a s e is, in each case, r e q u i r e d f o r green ball f o r m a t i o n . T h e i n t e r f a c e forces arising h a v e a cohesive effect o n the solid particles, liquid a n d air. T h e s e i n t e r f a c e forces consist, on the one h a n d , of t h e s u r f a c e tension of the b i n d e r , usually w a t e r , a n d o n t h e other, of capillary forces d e v e l o p i n g in the l i q u i d b r i d g e s b e t w e e n t h e ind i v i d u a l o r e particle faces, the surfaces of w h i c h are of concave s h a p e . U n d e r these conditions, a certain tensile strength occurs. T h e forces resulting f r o m the s u r f a c e tension f o r m a concave l i q u i d s u r f a c e w h e r e b y compression strength b e c o m e s active. F o r the f o r m a t i o n of green balls f r o m solid particles a n d l i q u i d , t h e r e are two possibilities: — T h e solid particles are w i t h o u t any active relative movement. They are mixed in s u i t a b l e e q u i p m e n t , so-called m i x g r a n u l a t o r s , by t h e r o t a t i o n of the vessel a n d the m i x i n g devices a r r a n g e d inside w i t h o u t h a v i n g to p e r f o r m their own rolling m o v e m e n t as in t h e case of a d r u m . A t the s a m e time, they are p u s h e d together by a p u s h i n g m o v e m e n t and slight m e c h a n i c a l forces a n d are c o m p a c t e d . T h e o r e particles are b r o u g h t to such a f a v o u r a b l e position to e a c h o t h e r t h a t the a g g l o m e r a t e f o r m s a tight packing. C a p i l l a r y a n d s u r f a c e forces co-act with a d h e s i v e f o r c e s 2 4 ) . T h e liquid b r i d g e s a n d surfaces b e t w e e n the i n d i v i d u a l o r e particles f o r m automatically. In most cases the relevant b a l l i n g facilities o p e r a t e intermittently. which are n o w a d a y s processed in pelletizing plants. H o w e v e r , in m a n y industries with l o w e r t h r o u g h p u t , t h e y are used with great success. — In the s e c o n d case, t h e ball f o r m a t i o n is m a i n l y a c h i e v e d by rolling of s o l i d s / l i q u i d m i x in well-known b a l l i n g units, s u c h as d r u m s o r discs. T h e green ball f o r m a t i o n is similar to snow-ball g r o w t h by layering. T h e l i q u i d surface is f r e e l y m o v a b l e w h i c h is a d v a n t a g e o u s for t h e c o n t a c t b e t w e e n the ore particles a n d the f o r m a t i o n of capillary bridges.

T h e y are le

26

2 Fundamentals of Pelletizing 2.1.2.3 Mechanism of Ball Formation

T h e feed m a t e r i a l m a y consist, a c c o r d i n g to its p r e p a r a t i o n , of either a c o n g l o m e r a t e of dry grains or a w e t filter cake. In the o n e case, t h e ores were s u b j e c t e d to dry grinding, in t h e other, to wet grinding. M o s t concentrates are a v a i l a b l e as filter cake. If d r y solid particles c o m e i n t o contact with water, the ore s u r f a c e is wetted. T h e ore particle is coated with a w a t e r film, as is schematically s h o w n in Fig. 12, p h a s e A. In m a n y places, t h e wet particles t o u c h each other. D u e to the s u r f a c e tension of the water film, l i q u i d bridges are f o r m e d , p h a s e B. A s a result of the m o v e m e n t of t h e particles inside the balling u n i t and of the c o m b i n a t i o n of i n d i v i d u a l w a t e r droplets, each c o n t a i n i n g o n e or several o r e grains, the first a g g l o m e r a t e s are f o r m e d , p h a s e C. In the interior of the loose agglomerate, the first l i q u i d bridges a p p e a r a m o n g a large n u m b e r of voids still existing. T h e s e l i q u i d bridges hold the particles t o g e t h e r as in a network. L o o s e balls are f o r m e d . W i t h the f u r t h e r s u p p l y of w a t e r the a g g l o m e r a t e s condense. M o r e a n d m o r e w a t e r is layered in the interior a n d the a g g l o m e r a t e s b e c o m e m o r e dense, p h a s e D . A t this stage of green ball f o r m a t i o n the capillary forces of the i n d i v i d u a l l i q u i d bridges are essentially active. T h e o p t i m u m of this b a l l - f o r m a t i o n p h a s e is a t t a i n e d w h e n all p o r e s inside the balls are filled w i t h l i q u i d b u t the latter does not yet u n i f o r m l y coat the w h o l e agglomerate, p h a s e E. T h e effect of the capillary forces is clearly s h o w n by I l m o n i a n d T i g e r s c h i o l d in Fig. 13 24sobrescrito). ing the ore particles together. T h e f i n a l stage is exceeded, w h e n the solid particles are fully coated with a water film. N o w , t h e s u r f a c e t e n s i o n of the w a t e r droplets

Fig. 12. Influence of water addition on green ball formation

Concave

2.1 Bonding Mechanisms for Green Ball Formation

27

containing the solid particles b e c o m e s fully active, Fig. 12, p h a s e F , a n d the effect of t h e capillary forces d r o p s s h a r p l y , see Fig. 135. Besides this effect, t h e rolling m o v e m e n t of the grains a n d t h e m o v e m e n t or s h i f t i n g of the particles relative to e a c h o t h e r plays a n i m p o r t a n t role too. T h e y increase a d h e s i o n b y t h e g r e a t n u m b e r of contact points at a s i m u l t a n e o u s c o m p r e s s i o n s t r e n g t h d u e to the load of t h e rolling m a t e r i a l .

Fig. 13. Influence of capillary forces on bonding mechanism

A p a r t f r o m the c o m p a c t i n g a n d s o l i d i f i c a t i o n of a g g l o m e r a t e s d u e to pressure a n d m o v e m e n t , these factors m a y also h a v e a n e g a t i v e influence. T h e y d a m a g e m e c h a n i c a l l y w e a k granules w h i c h h a v e not yet a t t a i n e d a sufficient a d h e s i v e strength. T h e d a m a g e resulting is that the w e a k g r e e n balls m a y either c r u m b l e into m i n o r f r a g m e n t s or d i s i n t e g r a t e into even finer particles. D u r i n g g r e e n ball f o r m a t i o n , these f r a c t i o n s m a y b e l a y e r e d onto m o i s t stable g r e e n balls and b e i n c o r p o r a t e d into the latter. Besides t h e ideal ball f o r m a t i o n f r o m f i n e - g r a i n e d i n d i v i d u a l solid particles initially described, v a r i o u s o t h e r possibilities exist m o r e or less simultaneously in practical o p e r a t i o n . T h i s also a p p l i e s to loose a g g l o m e r ates as in the case of wet filter cake. A c c o r d i n g to Sastry and Fuerstenau 2 5 ) the following m e t h o d s , as s h o w n in Fig. 14, can b e a d o p t e d : A) Layering of very fine particles to others a n d t h u s f o r m a t i o n of a n agglomerate. B) C o n g l o m e r a t i n g of s m a l l e r balls a l r e a d y existing resulting f r o m relative m o t i o n a n d a certain pressure. C) L a y e r i n g o n and i n c o r p o r a t i o n of m i n o r f r a g m e n t s f r o m d a m a g e d green balls into existing s o u n d ones. D) I n c o r p o r a t i o n of f i n e - g r a i n e d a b r a d e d m a t e r i a l f r o m w e a k pellets into the s u r f a c e of stronger pellets.

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2 Fundamentals of Pelletizing

Fig. 14. Alternatives for green pellet formation

D u r i n g green ball p r o d u c t i o n , their f o r m a t i o n p r o c e e d s parallel to the disintegration of a certain n u m b e r of balls. O n l y such balls w h i c h can withstand the d i v i d i n g or destructive forces d u r i n g rolling survive. A selection of t h e best balls takes place. T h e contest of the constructive a n d destructive forces favours the f o r m a t i o n of astonishingly u n i f o r m , d e n s e and stable g r e e n balls 2 3 ). As set f o r t h u n d e r i t e m 2.1.1, v a r i o u s factors m a y co-act d u r i n g g r e e n ball f o r m a t i o n , either d e p e n d e n t l y o n r a w m a t e r i a l s or i n d e p e n d e n t l y thereof. M u c h d e v e l o p m e n t a n d research w o r k has b e e n p e r f o r m e d to investigate the raw m a t e r i a l i n d e p e n d e n t factors. In this connection, s o m e of the researchers w h o h a v e p l a y e d a n i m p o r t a n t role are m e n t i o n e d 2 6 ) . T h e t h e o r y a n d the m a t h e m a t i c a l f o r m u l a resulting t h e r e f r o m are d e a l t with u n d e r item 13.1. T h e s e investigations are based, for the most part, o n the use of exactly d e f i n e d particle shapes, such as balls or on specific r a w materials, such as quartz, l i m e s t o n e or glass. T h e laws discovered can t h u s only serve as a g u i d e since each ore — even of the s a m e size — h a s its typical size d i s t r i b u t i o n or particle shape. T h i s again requires an i n d i v i d u a l investigation of each ore or m i x t u r e .

2.2 Induration of Green Balls

29

2.2 Induration of Green Balls In a very few cases it h a s b e e n possible to use g r e e n balls directly in a metallurgical process d e s p i t e their low m e c h a n i c a l strength. Pellets m u s t have a substantially h i g h e r strength p r i m a r i l y to w i t h s t a n d their t r a n s p o r tation a n d the stresses occurring in m e t a l l u r g i c a l o p e r a t i o n s . Such a strength can b e achieved by t h e r m a l t r e a t m e n t u n d e r a controlled a t m o s p h e r e or b y utilizing h y d r a u l i c a l l y - a c t i n g binders. T h e m o s t w i d e l y - s p r e a d t h e r m a l g r e e n ball i n d u r a t i o n o n an i n d u s t r i a l scale, n a m e l y f i r i n g to directly b e l o w the s o f t e n i n g p o i n t of the respective ore, is carried o u t in two stages: drying of the balls f o l l o w e d b y h e a t i n g a n d firing. Just as for green ball f o r m a t i o n , o p t i m u m c o n d i t i o n s m u s t also b e a i m e d at f o r d r y i n g to ensure m a x i m u m a c h i e v a b l e final properties. T h e r e f o r e , the green ball strength m u s t n o t d e t e r i o r a t e d u r i n g drying. H o w e v e r , s h o u l d this be the case d u e to the ore p r o p e r t i e s , a d e q u a t e action s h o u l d b e taken. 2.2.1 Drying of Green B a l l s D u r i n g drying, t h e m o i s t u r e c o n t a i n e d in t h e green balls is e v a p o r a t e d by w a r m gases. In this connection, it s h o u l d b e noted t h a t the water m a y be c o n t a i n e d in the green balls in a variety of c o m b i n a t i o n s : (a) T h e r e is w a t e r in the interstices b e t w e e n t h e particles, i.e. in the p o r e s a n d capillaries. (b) In the case of a p o r o u s ore, w a t e r m a y also be in the pores of t h e i n d i v i d u a l o r e grain. (c) W a t e r m a y be chemically combined as h y d r a t e , e.g. in l i m o n i t e or in o t h e r m i n e r a l s a c c o m p a n y i n g the iron oxide. (d) W a t e r m a y b e incorporated into such b i n d e r s w h i c h t e n d t o w a r d s t h e f o r m a t i o n of gels, e.g. clays or b e n t o n i t e . (e) W a t e r m a y be chemically combined as hydrate in such b i n d e r s w h i c h leads to h y d r a t e f o r m a t i o n , e.g. C a ( O H ) 2 , M g ( O H ) 2 . ( f ) W a t e r m a y be present as crystal components in existing or a d d e d salts. T h e m o i s t u r e content in the green ball m a y t h u s be c o m p o s e d of water, contained in t h e ore, or b y such b e i n g a d d e d d u r i n g pelletizing. Only p a r t of the w a t e r contained in the green balls e v a p o r a t e s at 100 ° C . Salts a n d h y d r a t e c o m b i n a t i o n s loose their w a t e r at h i g h e r t e m p e r a t u r e s only. T h e d r y i n g speed, t h e r e f o r e , m u s t be a d a p t e d to the d i f f e r e n t c o m binations to a v o i d a w e a k e n i n g of t h e pellet s t r u c t u r e d u e to cracks o r splintering of f r a g m e n t s . D u r i n g drying of a pellet layer or a pellet b e d at a certain d e p t h , t w o o p e r a t i o n s can be observed. D r y i n g starts in the individual ball a n d u n i f o r m l y p r o c e e d s in the pellet layer.

30

2 Fundamentals of Pelletizing 2.2.1.1 Drying Procedure of Individual Balls

This drying c o m m e n c e s w h e n h e a t e d air or c o m b u s t i o n gases flow over wet green balls. In this connection, t h e gas t e m p e r a t u r e , d e w p o i n t , q u a n t i t y and drying velocity play a n i m p o r t a n t role. T h e m o i s t u r e contained in the green pellets starts to e v a p o r a t e evenly on the entire surface. A f t e r this e v a p o r a t i o n , water f r o m the pellet interior is e m i t t e d by capillary forces to the surface. As long as this p r o c e d u r e continues, t h e drying velocity r e m a i n s constant. H o w e v e r , if the drying velocity on t h e surface is greater t h a n the w a t e r e m i s s i o n f r o m the pellet interior, the drying f r o n t travels d o w n into the pellet interior. T h e water v a p o u r thus arising has to cover a n ever increasing distance t h r o u g h the capillaries w h i c h are a l r e a d y d r i e d out until it reaches the pellet s u r f a c e f r o m w h e r e it can escape t o g e t h e r with the air flow. D u r i n g this drying p h a s e , t h e drying velocity is no longer constant b u t decreases. As soon as t h e capillary w a t e r evaporates, the drying p r o c e d u r e is t e r m i n a t e d . H o w e v e r , if the pellet contains hygroscopic or c h e m i c a l l y - c o m b i n e d water, t h e drying p r o c e d u r e only c o n t i n u e s w h e n the t e m p e r a t u r e of the drying m e d i u m is high e n o u g h to dissociate t h e c o m p o u n d s . T h e velocity of this third drying p h a s e is again lower. T h e d r y i n g p r o c e d u r e of such a g r e e n ball, w h i c h also contains hygroscopic water, was described b y K r i s c h e r a n d Jaeschke 27) in a curve s h o w n in Fig. 15. O n the ordinate, the drying velocity is plotted. It is m e a s u r e d b y the e v a p o r a t e d w a t e r a m o u n t , r e f e r r e d to t h e drying surface, d u r i n g a given period. evaporated water amount . k, . , , g = in k g / m ² . h s u r f a c e of m a t e r i a l to be d r i e d x unit of t i m e O n the abscissa, the t i m e is plotted w h i c h is n e e d e d f o r the v a r i o u s drying phases. T h e r e are t h r e e d i f f e r e n t drying velocity ranges w h i c h are i n d i c a t e d by t h r e e b r e a k s of the curve: Range I: T h e water evaporates at constant velocity on the entire pellet surface. T h i s p r o c e d u r e continues as long as h u m i d i t y f r o m the pellet interior is emitted at a s u f f i c i e n t speed by capillary forces or d i f f u s i o n to the ball surface. T h e constant drying velocity is i n f l u e n c e d by the following factors: — T e m p e r a t u r e a n d m o i s t u r e content ( d e w point) of t h e drying gas, — V o l u m e a n d speed of the drying gas w h i c h flow, p e r unit of t i m e , over the ball surface, — S u r f a c e of ball. This r a n g e I is t h u s characterized by the evaporation of surface water. Range II: If the m o i s t u r e f r o m the interior of the pellet is e m i t t e d to the surface m o r e slowly t h a n water e v a p o r a t e s there, t h e e v a p o r a t i o n level

2.2 Induration of Green Balls

31

Fig. 15. Drying stages of moist material

(drying level) moves f u r t h e r towards the pellet core. T h e w a t e r e v a p o r a t e s inside the pellet, the d i f f u s i o n d i s t a n c e to b e covered by w a t e r v a p o u r becomes longer a n d the drying velocity d i m i n i s h e s . It is n o w d e t e r m i n e d by the : - D i f f u s i o n resistance - D e p t h of d r y i n g level below the ball s u r f a c e - D i f f u s i o n index expressed in m / h . R a n g e II is c h a r a c t e r i z e d by the evaporation of capillary water f r o m t h e pellet interior. A f t e r this stage, the pellets are n o r m a l l y dry. Range III: If the pellet contains m o i s t u r e o t h e r t h a n s u r f a c e a n d capillary water, the d r y i n g p r o c e d u r e c o n t i n u e s u n d e r o t h e r conditions, In this case, the t e m p e r a t u r e of d r y i n g gas is i m p o r t a n t for the dissociation of the w a t e r c o m p o u n d s . Besides t h e l o n g e r d i f f u s i o n distance, this dissociation has to b e considered a n d c o n s e q u e n t l y the d r y i n g velocity decreases even m o r e . It is virtually d e t e r m i n e d by the o p e r a t i o n s p r o ceeding in t h e pellet core a n d by the a m b i a n c e of t h e pellet. In R a n g e III, the evaporation of the hygroscopic or chemically combined water m a i n l y takes place. T h e t e r m " m o i s t u r e " is not u n i f o r m l y d e f i n e d in literature. O n e version refers to the w a t e r a m o u n t of the dry substance. In practice, the wet substance is f r e q u e n t l y used as r e f e r e n c e v a l u e o n w h i c h the d a t a given below are also b a s e d . M o i s t u r e is t h u s d e f i n e d a c c o r d i n g to t h e following formula:

32

2 Fundamentals of Pelletizing

T h e drying p r o c e d u r e d e t e r m i n e d with one i n d i v i d u a l pellet on a a n d 0.7% S i O 2 , as well as 1.2% loss o n ignition, was pelletized with 0.8% W y o m i n g b e n t o n i t e a n d 8.2% w a t e r and slowly d r i e d in a laboratory-scale drying oven at a t e m p e r a t u r e of 400 ° C u n d e r air flow.

Fig. 16. Drying of an individual green pellet

T h e left o r d i n a t e shows the r e m a i n i n g water content of the pellet as well as t h e m o i s t u r e d e c r e a s e in percent while the right o r d i n a t e indicates the loss of weight. U p to a m o i s t u r e loss of a b o u t 67% d u r i n g a drying p e r i o d of 4.5 mins. — p l o t t e d on the abscissa — the drying curve p r o c e e d s linearly. T h e drying velocity u p to this m o i s t u r e c o n t e n t is constant. A b o u t % of the water e v a p o r a t e s f r o m the pellet surface. F r o m n o w on, the drying proceeds m o r e slowly and asymptotically until the residual m o i s t u r e content of t h e pellet core has e v a p o r a t e d . By v o l u m e a n d t e m p e r a t u r e of t h e drying gas the h e a t s u p p l y f o r w a t e r e v a p o r a t i o n has to be adapted to t h e relevant h e a t c o n s u m p t i o n in such a way that d u r i n g t h e capillary m o v e m e n t of w a t e r and d i f f u s i o n of water v a p o u r , the relative position of the i n d i v i d u a l ore grains to each o t h e r is not changed. If the h e a t s u p p l y a n d thus the f o r m a t i o n of water v a p o u r p r o c e e d too quickly, cracking or even d e c r e p i t a t i o n m a y occur (shock t e m p e r a t u r e ) .

thermoscale

2.2 Induration of Green Balls

33

T h e material-dependent p r o p e r t i e s of t h e ore, t h e g r a i n size, size distribution, d e n s e or loose p a c k i n g of grains play such a n i m p o r t a n t role that it is a d v i s a b l e to f i n d o u t the drying c o n d i t i o n s b y tests with the ore to be pelletized. A l t h o u g h the drying p r o c e d u r e of the i n d i v i d u a l pellet can still b e s u r v e y e d t h e d r y i n g conditions of g r e e n balls in layers are m o r e complicated. In industial plants, g r e e n pellets are always t h e r m a l l y t r e a t e d in layers of a d i f f e r e n t d e p t h , either in s h a f t f u r n a c e s , o n travelling grates or in gratekiln plants. T h e b e h a v i o u r of g r e e n balls in layers of d i f f e r e n t thickness is t h u s of interest f o r all t h r e e processes. In view of t h e great n u m b e r of factors a f f e c t i n g the d r y i n g technology, it is difficult to describe precisely each of these factors. O n the o t h e r h a n d , it is possible to ascertain q u a l i t a t i v e d e p e n d e n c i e s . F o r this p u r p o s e , exp e r i m e n t s f u r n i s h , for the present, m o r e reliable results t h a n m a t h e m a t i c a l calculations do. 2.2.1.2 Drying of Pellets in a Layer T h e r e f o r e , it is w o r t h w h i l e to look i n t o this m a t t e r m o r e closely. T h e e q u i p m e n t a n d m e a s u r e m e n t s r e q u i r e d for t h e d r y i n g a r e d e s c r i b e d u n d e r items 4 . 5 - 4 . 5 . 2 . 3 . D u r i n g drying, the drying gases m a y be s u c k e d t h r o u g h t h e pellet layer in a d o w n w a r d direction. T h i s process is called down-draught drying. H o w ever, the gas flow m a y also be f o r c e d in a n u p w a r d d i r e c t i o n t h r o u g h the pellet bed. In this case, a n up-draught drying is involved. A c o m b i n a t i o n of the a b o v e m e t h o d s is also possible a n d is, in practice, adopted. W h e n t h e d r y i n g air flows t h r o u g h the first pellet layer, it absorbs a certain m o i s t u r e v o l u m e f r o m t h e pellets u p to their s a t u r a t i o n ( d e w point). D u r i n g this o p e r a t i o n , t h e d r y i n g air cools f r o m its inlet t e m p e r a ture d o w n to its e v a p o r a t i o n t e m p e r a t u r e . This w a t e r - v a p o u r s a t u r a t e d air consecutively i m p i n g e s o n t o wet a n d cold pellet layers. O n this occasion, p a r t of t h e w a t e r a b s o r b e d b y the first pellet layer is c o n d e n s e d out a n d this c o n d e n s a t i o n h e a t is t r a n s f e r r e d to the second pellet layer w h i c h is t h u s also h e a t e d to e v a p o r a t i o n t e m perature. T h e energy r e q u i r e d for the e v a p o r a t i o n of t h e w a t e r f r o m pellets of the directly a d j a c e n t layers is s u p p l i e d f r o m the i n c o m i n g h o t d r y i n g gases. In this case the gas again cools d o w n to e v a p o r a t i o n t e m p e r a t u r e as long as water is to be e v a p o r a t e d . H o w e v e r , in t h e course of this i n t e r p l a y b e t w e e n h e a t a n d m a t e r i a l exchange, a n over-wetting m a y occur so t h a t a w e a k e n i n g of the pellet structure a n d h e n c e a decrease of pellet strength m a y t e m p o r a r i l y t a k e

34

2 Fundamentals of Pelletizing

place. D u r i n g d o w n - d r a u g h t drying, the l o w e r pellet layers can be w e a k e n e d a n d c o m p r e s s e d in such a way t h a t the pellet b e d m a y b e c o m e i m p e r m e a b l e to gas. D u r i n g u p - d r a u g h t drying, the d r y i n g air flows t h r o u g h the pellet b e d in a n u p w a r d direction. O n this occasion, w a t e r droplets m a y c o n d e n s e o n t h e s u r f a c e of the pellet bed. A l t h o u g h t h e d r y i n g s h o u l d b e carefully carried out in o n e single direction to save energy, a c o m b i n e d u p - d r a u g h t a n d d o w n - d r a u g h t d r y i n g is f r e q u e n t l y a p p l i e d . In this way, it is possible to render the lower layers m o r e resistant to pressure. S o m e tests 2 8 ) are described b e l o w in w h i c h uni-directional d r y i n g as well as the u p - d r a u g h t a n d d o w n - d r a u g h t drying with c h a n g e in d i r e c t i o n were qualitatively tested. T h e pellet d i a m e t e r was b e t w e e n 10 a n d 12 m m and the m o i s t u r e content of g r e e n balls 6.50% H 2 O . 2.2.1.2.1 Unidirectional Drying. T o avoid a w e a k e n i n g of t h e pellet structure, the d r y i n g gas t e m p e r a t u r e is k e p t b e l o w the shock t e m p e r a t u r e at the beginning of the drying process. T h e t e m p e r a t u r e g r a d i e n t of d r y i n g is shown in Fig. 17 by way of e x a m p l e of u p - d r a u g h t drying. F o r this purpose, a pot grate as r e p r e s e n t e d in Fig. 39 was used. T h e c h a r g e consisted of 10cm h e a r t h layer and 30 cm pellet bed. T h e drying was achieved with hot air of 240—330 ° C . T h e t e m p e r a t u r e g r a d i e n t was m e a s u r e d with 7 t h e r m o c o u p l e s . First, p a r t of the h e a t a m o u n t is c o n s u m e d f o r the h e a t i n g of grate a n d h e a r t h layer. T h e first curve shows the t e m p e r a t u r e g r a d i e n t a f t e r a drying t i m e of 11/2m i n u t e s (curve I); a b o u t 25% of the layer has b e e n dried. A f t e r 5 m i n u t e s a b o u t % of the pellets are dry (curve II). T h e pellet t o p layer still shows an e v a p o r a t i o n t e m p e r a t u r e of approx. 42 0 C a l t h o u g h the b o t t o m

Fig. 17. Updraught drying of a green pellet layer

2.2 Induration of Green Balls

35

layer has a l r e a d y a t e m p e r a t u r e of a r o u n d 310 0 C . W i t h a f u r t h e r s u p p l y of drying air, t h e m o i s t u r e level m o v e s u p w a r d s . In the case of the s h a f t f u r n a c e , t h e d r y i n g z o n e is k e p t at a specific furnace level b e c a u s e the dried pellets m o v i n g d o w n w a r d s , are r e p l a c e d f r o m a b o v e by wet pellets. 2.2.1.2.2 Up-Draught — Down-Draught Drying. A f t e r 5 m i n u t e s the u p draught drying was t e r m i n a t e d in this d e m o n s t r a t i o n test and replaced by down-draught drying. T h e recorded t e m p e r a t u r e gradient is shown in Fig. 18. Curve I indicates the end of u p - d r a u g h t , curves II a n d III represent t h e d o w n - d r a u g h t p r o c e d u r e a f t e r 1/2 a n d 3 m i n u t e s , c o r r e s p o n d i n g to a total drying of 8 m i n u t e s . T h i s p e r i o d was selected for a p a r t i c u l a r p r o j e c t f o r which a longer total d r y i n g t i m e o n t h e pellet i n d u r a t i n g m a c h i n e was n o t available.

Fig. 18. Up- and down-draught drying of a green pellet layer

As can b e seen f r o m this d i a g r a m , a b o u t 15% of t h e pellet b e d in the m i d d l e of the layer h a d not yet b e e n d r i e d a f t e r 8 m i n u t e s a l t h o u g h t h e drying air h a d a l r e a d y r e a c h e d a t e m p e r a t u r e of 450 0 C . P a r t of t h e l o w e r layers was t e m p o r a r i l y slightly cooler (curves II a n d III). 2.2.1.3 Dry Pellet Strength Like g r e e n pellets, dry pellets h a v e a s h o r t life w h i c h o f t e n is only a m a t t e r of m i n u t e s . H o w e v e r , d u r i n g this t i m e , a m i n i m u m d r y pellet strength is necessary to ensure t h a t the pellets w i t h s t a n d , for e x a m p l e , t h e m o v e m e n t of the c h a r g e d u r i n g its m i g r a t i o n t h r o u g h t h e s h a f t f u r n a c e , without d a m a g e . In other b u r n i n g systems, for e x a m p l e in the case of a travelling grate, a m i n i m u m dry pellet strength is also n e e d e d so that t h e

36

2 Fundamentals of Pelletizing

pellets w i t h s t a n d the load of the layers located a b o v e or the pressure of gases flowing t h r o u g h the charge. T h e m e c h a n i c a l strength of green pellets is chiefly d e p e n d e n t o n the superficial tension a n d capillary tensile forces of t h e wetting agent w h e r e a s other b o n d i n g m e c h a n i s m s are of great i m p o r t a n c e for the dry pellets. Even if, a c c o r d i n g to R u m p f 2 3 ) , n o bridges exist b e t w e e n the i n d i v i d u a l solid particles, a d h e s i o n a n d tensile forces m a y b e c o m e active. M o l e c u l a r a d h e s i o n forces are always effective w h e n t h e distance b e t w e e n the i n d i v i d u a l grains is sufficiently short. V a n de W a a l s ' forces are also to be considered as possible b o n d i n g forces. Electrostatic forces m a y , in p r i n ciple, b e acting if particles with a positive a n d n e g a t i v e l o a d p a r t i c i p a t e — t h o u g h it be to a very small extent - in the g r a n u l a t e f o r m a t i o n . T h e s e physical factors, b e i n g i n d e p e n d e n t of the specific materials, b e c o m e active b u t their n a t u r a l b o n d i n g force is i n s u f f i c i e n t for m a n y ores (e.g. h i g h - g r a d e m a g n e t i t e o r h e m a t i t e concentrates) to attain the strength of green pellets. In the case of s u c h ores, b i n d e r s h a v e to be a d d e d . A few o t h e r ores a l r e a d y contain c o m p o n e n t s , s i m i l a r to binders, w h i c h enable a s u f f i c i e n t pellet dry strength to b e o b t a i n e d . T a b l e 4 shows the g r e e n a n d dry strength of pellets p r o d u c e d f r o m different ores, see items 3.1.4 a n d 5.3. T h e dry strength of pellets 1 - 3 (concentrates) is i n s u f f i c i e n t while t h a t of ore 4 is satisfactory. T h e dry strength of pellets 1—3 can be i m p r o v e d by t h e a d d i t i o n of bentonite. In practical o p e r a t i o n , a dry pellet strength of a b o u t 10 N e w t o n is r e q u i r e d . T h e s e values can be o b t a i n e d either directly or b y the a d d i t i o n of binders. T h e gel-containing constituents in the ore or in the a d d e d b i n d e r s — seemingly soluble in water or reacting with w a t e r — concentrate in the ever shrinking l i q u i d bridges d u r i n g drying. T h e s e processes can be c o m p a r e d with the solidification of d r y i n g l o a m or clay. T h e d e h y d r a t i o n of t h e gels is a c c o m p a n i e d by a s h r i n k a g e w h e r e b y the a d h e s i o n forces are increased. Table 4. Compressive strength of green and dry pellets from various ore types No.

Types of ore

Compressive Strength in N/Pellet without binder

with 0.5% bentonite

with 1.0% bentonite

wet

dry

wet

dry

wet

dry

Magnetite Concentrate Artif Magnetite Concentrate

11

7

12

20

13

33

11

4

10

23

3

Spec. Hematite

12

4

10

20

4

Earthy Hematite

14

30

16

70

1 2

-

11 -

-

35 -

2.2 Induration of Green Balls

37

R u m p f 23) even designates this p h e n o m e n o n as " m o r t a r b r i d g e s " . if, n o w a d a y s , f o r metallurgical or ecological reasons, t h e direct a d d i t i o n of salts has b e e n a b a n d o n e d , the f o r m a t i o n of s u c h b r i d g e s m a y , n e v e r t h e less, occur w h e n ores are pelletized w i t h s e a water.

Crystallizing

2.2.1.4 S h o c k Resistance A p a r t f r o m m a n y influences w h i c h are c o n n e c t e d with the n a t u r e of each i n d i v i d u a l ore, the initial temperature of t h e d r y i n g gases is of decisive i m p o r t a n c e for t h e dry pellet strength. T h e h e a t s u p p l y has to b e so controlled t h a t the arising water v a p o u r a n d t h e air inclusions still existing in t h e p o r e s can escape or e x p a n d f r o m t h e pellet cores t h r o u g h the capillaries w i t h o u t any ensuing over-pressure. A n o v e r p r e s s u r e w o u l d w e a k e n the pellet structure and, thus, its s t r e n g t h b y the f o r m a t i o n of cracks or b y splintering of f r a g m e n t s 3 0 ) . T h e a d m i s s i b l e d r y i n g gas t e m perature, at w h i c h the pellet structure is n o t yet d a m a g e d , is called " s h o c k t e m p e r a t u r e " , s o m e t h i n g w h i c h was a l r e a d y f o u n d o u t very early, e.g. by T i g e r s c h i o l d 7 ) . T h e shock t e m p e r a t u r e can b e raised b y the a d d i t i o n of binders, see i t e m 5.4.1.1.2, Fig. 72. Since a sufficiently h i g h dry strength can b e o b t a i n e d with all types of ores b y using s u i t a b l e binders, it is still i m p o r t a n t to a d j u s t or, better, to d e t e r m i n e t h e d r y i n g velocity in tests in such a w a y t h a t the f o r m a t i o n of cracks is a v o i d e d a n d the c o n g l o m e r a t e s , f o r m e d d u r i n g green pellet p r o d u c t i o n , are n o t d a m a g e d . G r e e n ball f o r m a t i o n a n d pellet d r y i n g are essential p r e p a r a t o r y steps leading to the p r o d u c t i o n of pellets, r e a d y f o r r e d u c t i o n . H o w e v e r , t h e decisive s t e p is the t h e r m a l t r e a t m e n t by controlled h e a t i n g of a pellet bed.

2.2.2 Pellet Firing T h e last processing step b e f o r e utilizable pellets are o b t a i n e d , is the t h e r m a l i n d u r a t i o n of d r i e d pellets. T h e i n d u r a t i o n i m p a r t s such metallurgical t r e a t m e n t , p r o v i d e d t h e p r e c e d i n g steps were m a d e u n d e r o p t i m u m conditions. D u r i n g the first years, w h e n p r o d u c t i o n was c a r r i e d o u t o n a s m a l l e r scale a n d the p o r t i o n of pellets in t h e blast f u r n a c e b u r d e n was low, few features were s u f f i c i e n t for the d e t e r m i n a t i o n of the pellet quality. W i t h rising pellet q u a n t i t i e s , it was possible a n d necessary to s t u d y m o r e precisely the i n f l u e n c e of the pellets o n the blast f u r n a c e o p e r a t i o n . A s a result, the q u a l i t y specifications h a v e b e c o m e m o r e rigorous. In a d d i t i o n , special c o n d i t i o n s h a d to be c o n s i d e r e d for the use of pellets in the direct

characteristics

"salt b r i d g e

38

.

2 Fundamentals of Pelletizing

r e d u c t i o n processes. N o t only the s o u n d t r a n s p o r t a b i l i t y b u t p r i m a r i l y the b e h a v i o u r of pellets d u r i n g the d i f f e r e n t r e d u c t i o n stages was given particular attention. T o m e e t the m o r e severe r e q u i r e m e n t s , v a r i o u s process p a r a m e t e r s are to be taken into account, such as: (a) Influence of metallurgically-effective a d d i t i v e s (b) H a r m o n i z a t i o n of h e a t i n g velocity a n d firing p e r i o d (c) O b s e r v a t i o n of close t e m p e r a t u r e ranges a n d precise control of the firing t e m p e r a t u r e (d) E l a b o r a t i o n of h e a t i n g p r o g r a m m e s especially a d a p t e d to specific ores or ore mixtures. In connection with these factors, the q u e s t i o n arises r e g a r d i n g the t y p e a n d i m p o r t a n c e of efficient b o n d i n g m e c h a n i s m s w h i c h h a v e to d i f f e r f r o m those for the green a n d dry pellet strength. In n u m e r o u s tests carried out p r i m a r i l y with pellets f r o m industrial plants, it was f o u n d t h a t two t h e r m a l b o n d i n g types are decisive for the q u a l i t y of i n d u r a t e d pellets: (a) C h a n g e of the crystalline structure d u r i n g f i r i n g either b y crystal t r a n s f o r m a t i o n a n d g r o w t h u p o n o x i d a t i o n of m a g n e t i t e to h e m a t i t e or by crystal g r o w t h w h e n h e m a t i t e is used only. (b) T h e r e a c t i o n of slag-forming constituents which are either p r e s e n t as g a n g u e in ores or concentrates or a d d e d b e f o r e the ball f o r m a t i o n , such as bentonite, q u a r t z or basic additives. Basic c o m p o n e n t s react w i t h acid c o m p o n e n t s a n d - u n d e r certain conditions — also with i r o n oxides. Since the ores or concentrates to be pelletized always have a certain g a n g u e c o n t e n t a n d since additives are used to a n increasing extent, b o t h b o n d i n g m e c h a n i s m s s i m u l t a n e o u s l y occur u n d e r n o r m a l o p e r a t i n g c o n d i tions. A d d i t i v e s are used because it is i n t e n d e d t h e r e b y to i m p r o v e the pellet quality. T h e i m p o r t a n c e a n d effect of the additives are in close relation with the o r e to be treated, see items 3.1.4 a n d 5.3. T h e t h e r m a l t r e a t m e n t by h e a t s u p p l y with hot c o m b u s t i o n gases proceeds in two stages. T h e first stage, the preheating stage, directly follows the t e m p e r a t u r e s of the drying zone. A c c o r d i n g to the ore type involved, t h e t e m p e r a t u r e is raised f r o m 3 0 0 - 3 5 0 0 C to 1 2 5 0 - 1 3 4 0 0 C by a c o n t i n u o u s a n d controlled h e a t supply. In the second, the final induration stage, the pellet c h a r g e is h e a t e d to an o p t i m u m t e m p e r a t u r e for each ore type, a n d this t e m p e r a t u r e is m a i n tained for a controlled period. T h e said t e m p e r a t u r e is to be b e l o w t h e m e l t i n g t e m p e r a t u r e b u t within the reactivity r a n g e of g a n g u e c o m p o n e n t s and additives. In general, the i n d u r a t i o n is a c h i e v e d u n d e r a d e f i n e d , oxidizing a t m o s p h e r e . D u r i n g p r e h e a t i n g , various reactions p r o c e e d in parallel a n d successively according to the c o m p o n e n t c o m p o s i t i o n . — E v a p o r a t i o n of crystal w a t e r — D e c o m p o s i t i o n of hydrates, carbonates, s u l p h a t e s

2.2 Induration of Green Balls

39

— Roasting of s u l p h u r f r o m s u l p h i d e c o m p o n e n t s — Conversion of iron oxides f r o m siderite, l i m o n i t e , pyrite as well as mainly m a g n e t i t e into the highest o x i d a t i o n stage, the h e m a t i t e . T h e p r e h e a t i n g velocity depends, inter alia, on the c o m p o s i t i o n a n d quantity of the c o m b i n a t i o n s to b e d e c o m p o s e d a n d also on the o x i d a t i o n velocity r e q u i r e d for magnetite. T h e b o n d i n g types decisive for o b t a i n i n g the d e s i r e d pellet c h a r a c t e r istics finally arise in the i n d u r a t i o n zone. 2.2.2.1 Bonding by Change of the Crystalline Structure As a rule, t h e ores or concentrates to be pelletized c o n t a i n a c e r t a i n percentage of g a n g u e or additives w h i c h — even at m i n i m u m p o r t i o n s — react with each o t h e r by the f o r m a t i o n of i n t e r g r a n u l a r m e l t i n g phases. A s transport m e d i u m , these p h a s e s m a y play a n i m p o r t a n t p a r t for the growth of iron o x i d e crystals. H o w e v e r , t h e b e h a v i o u r of p u r e iron o x i d e phases s h o u l d b e c o n s i d e r e d first. T h e conglomerates, arising d u r i n g the f o r m a t i o n of g r e e n pellets a n d their drying, are stabilized by t h e r m a l t r e a t m e n t a n d are f u r t h e r consolidated. T h e c o n s o l i d a t i o n is achieved by energy s u p p l y d u r i n g p r e h e a t ing and i n d u r a t i o n . T h e solids react with each o t h e r in c o n f o r m i t y w i t h natural laws. T h e reaction p r o c e e d s b e l o w t h e m e l t i n g t e m p e r a t u r e while new solid p h a s e s are f o r m e d . S u c h reactions of fine crystalline a n d pulverous m a t e r i a l s d e v e l o p in d e p e n d e n c e o n the energy s u p p l i e d d u r i n g the t e m p e r a t u r e rise in various stages: — First, a n activation occurs in the crystal lattice w h i c h can b e recognized in the interfaces. A r e - a r r a n g e m e n t of the m o l e c u l e s takes place. T h e particles d r a w closer t o g e t h e r w i t h strength b e g i n n i n g to increase. — D u r i n g the f u r t h e r t e m p e r a t u r e rise, the m o b i l i t y of t h e ions in t h e lattice increases. A lattice d i f f u s i o n occurs w h i c h leads to r e - a r r a n g e ments, vacancies are o c c u p i e d a n d inclusions are e l i m i n a t e d . — T h e dislocations intensify and involve a d j a c e n t crystals in the d i f fusion. — Crystal b r i d g e s are f o r m e d b e t w e e n the i n d i v i d u a l ore grains. T h e s e bridges lead to a consolidation of t h e c o n g l o m e r a t e . — A r e - a r r a n g e m e n t of the crystal lattice a n d a re-crystallization occur. T h e b r i d g e f o r m a t i o n changes to a b r i d g e b o n d i n g . T h e re-crystallization intensifies t h e latter. — A f u r t h e r energy s u p p l y causes the r o u n d i n g of the grains. T h e p o r e s b e t w e e n the o r e grains b e c o m e smaller. T h e latest p h a s e w o u l d be m e l t i n g which, h o w e v e r , has to b e a v o i d e d because, in this case, i n d i v i d u a l pellets w o u l d n o longer exist. T h i s r e a c t i o n d e v e l o p m e n t is w e l l - k n o w n f r o m solid-state c h e m i s t r y a n d physics. It d e p e n d s on m a n y factors, with the genesis of the ores b e i n g of a

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certain i m p o r t a n c e . D u e to their small crystal size, fine crystalline ores of s e d i m e n t a r y origin are m o r e reactive t h a n c o a r s e crystalline ores w h i c h r e q u i r e h i g h e r t e m p e r a t u r e s or m o r e t i m e for crystal growth. T h e incentive of t h e crystal g r o w t h a n d c o n s o l i d a t i o n r e a c t i o n in a solid state consists of the f r e e energy of a system. O n account of the t e n d e n c y to a thermodynamic e q u i l i b r i u m , the very f i n e particles with the greatest interface energy lie on greater particles w h i l e the energy p o t e n t i a l diminishes. At a s u f f i c i e n t reaction t i m e a n d a d e q u a t e t e m p e r a t u r e as well as at a f u r t h e r decrease of the interface energy, these particles w o u l d d r a w near to a spherical shape. T h e c o n s o l i d a t i o n of t h e c o n g l o m e r a t e s is exclusively caused by recrystallization a n d crystal growth. T h e o x i d a t i o n of m a g n e t i t e d u r i n g heating is a special case i n s o f a r as the o x i d a t i o n energy is a d d i t i o n a l l y available a n d f r o m a crystallographic viewpoint, a crystal t r a n s f o r m a t i o n f r o m the cubic m a g n e t i t e to the h e x a g o n a l h e m a t i t e lattice takes place first. T h e s e newly f o r m e d very fine h e m a t i t e crystals n o w start to g r o w intensively a c c o r d i n g to the f u r t h e r t e m p e r a t u r e rise. 2.2.2.1.1 Crystal Change During the Induration of Pellets from Magnetite Concentrate. T h e s t r u c t u r e of a g r e e n ball is d e t e r m i n e d by its shape, size and location of m a g n e t i t e grains to each other, in a similar way to t h a t

Fig. 19. Grain structure of green pellets from magnetite concentrate. 1 cm = 0.025 mm Fig. 20. End of oxidation, crystal growth and first bridging. Temperature 1150 0 C, 1 cm = 0.025 mm

2.2 Induration of Green Balls

41

Fig. 21. Intensified bridging, crystal growth. Temperature 1250 °C, 1cm=0.025mm Fig. 22. Crystal growth after further temperature rise to 1320 °C, 1 cm = 0.025 mm

shown in Fig. 19 3 1 ). T h e concentrate i n d i c a t e d in this p l a t e contains a b o u t 71% iron a n d less t h a n 1% gangue. D u r i n g h e a t i n g u n d e r oxidizing a t m o s p h e r e , t h e m a g n e t i t e oxidizes i n t o h e m a t i t e at a s i m u l t a n e o u s conversion of t h e c u b i c m a g n e t i t e into t h e hexagonal h e m a t i t e lattice. A c c o r d i n g to the genesis of m a g n e t i t e , t h e oxidation starts at a b o u t 3 0 0 - 6 0 0 0 C a n d is to be t e r m i n a t e d at a t e m p e r a t u r e of 1 1 0 0 0 C . It begins on the crystal a n d g r a i n surfaces, as represented in Fig. 20. T h e First very small crystals s i m u l t a n e o u s l y f o r m first bridges b e t w e e n the grains. At a t e m p e r a t u r e of 1150 ° C , the o x i d a tion is f i n i s h e d a n d all the m a g n e t i t e s h o u l d be c o n v e r t e d into h e m a t i t e . A t the grain edges, n u m e r o u s small new crystals h a v e f o r m e d , which s i m u l taneously intensify the f o r m a t i o n of b r i d g e s b e t w e e n t h e i n d i v i d u a l grains. Already at a t e m p e r a t u r e of 1250 0 C (Fig. 21), t h e b r i d g e s have intensified a n d the re-crystallization h a s b r o u g h t a b o u t a g r o w t h a n d t h e r o u n d i n g - o f f of crystals. T h e o p t i m u m strength has b e e n r e a c h e d . F u r t h e r heating w o u l d result in f u r t h e r crystal g r o w t h a n d r o u n d i n g off at t e m peratures of a b o u t 1320 ° C , Fig. 22, b u t dissociation of h e m a t i t e w o u l d start. 2.2.2.1.2 Crystal Change During Induration of Hematite Pellets. H e m a t i t e ores a n d c o n c e n t r a t e s a r e b e i n g used to a n ever increasing extent for pellet production. H e m a t i t e is present, a c c o r d i n g to its genesis, in the f o r m of

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Fig. 23. Crystal structure in a hematite pellet at 1200 0 C, 1 cm = 0.025 mm Fig. 24. First bridging via fine-grained hematite crystals at 1300 °C, 1 cm = 0.025 mm

Fig. 25. Crystal bridges and strong recrystallization at 1350 °C, 1 cm = 0.025 mm

2.2 Induration of Green Balls

43

m a n y crystalline variants as set forth u n d e r item 3.1.1.1. D u r i n g t h e induration the strength increase is only a c h i e v e d — in contrast to m a g n e t i t e pellets - t h r o u g h crystal growth a n d recrystallization. T h i s g r o w t h c a n only be m e a s u r e d at t e m p e r a t u r e s of a b o v e 1200 0 C . U p to this t e m p e r a ture, the individual ore grains and the pellet structure m a i n t a i n their original shape as shown in Fig. 23. T h e various grains lie side by side without any bond. Only at higher t e m p e r a t u r e s of 1300 ° C the small ore particles f o r m the first crystal bridges, Fig. 24, and recrystallization can be observed at 1350 0 C as is a p p a r e n t f r o m Fig. 25. Fig. 24 also shows h o w i m p o r t a n t t h e presence of sufficiently fine-grained o r e particles is for crystal g r o w t h a n d thus for creating pellet strength. T h e r e is a direct r e l a t i o n s h i p b e t w e e n grain size a n d c r u s h i n g strength of pellets 3 2 ). T o e n s u r e t h a t a s u f f i c i e n t f o r m a t i o n of b r i d g e s occurs in the h e m a t i t e , a longer i n d u r a t i o n t i m e at a n o p t i m u m i n d u r a t i o n t e m p e r a t u r e is necessary. T h i s is reflecting the capacity of i n d u s t r i a l pellet i n d u r a t i o n plants. F o r e x a m p l e , in travelling grate plants, 28 —30 t of pellets can be p r o d u c e d f r o m m a g n e t i t e per m 2 suction a r e a a n d d a y , b u t only 2 2 - 2 5 t of pellets f r o m h e m a t i t e . T h e theoretically possible acceleration of crystal g r o w t h by h i g h e r induration t e m p e r a t u r e s c a n n o t be realised 3 3 ). A t t e m p e r a t u r e s a b o v e 1350 0 C h e m a t i t e starts to dissociate into m a g n e t i t e a n d oxygen, resulting in a w e a k e n i n g of pellet strength. 2.2.2.2 The Reaction of Slag-Forming Components Usually, h i g h - g r a d e iron ores a n d c o n c e n t r a t e s a r e used for pelletizing only. T h e r e f o r e , t h e g a n g u e p o r t i o n , mostly consisting of acid c o m ponents, is c o r r e s p o n d i n g l y low. In spite of a great variety of ore types the pellets h a v e to b e of u n i f o r m m e c h a n i c a l a n d metallurgical condition. It was investigated early w h i c h m e t h o d s s h o u l d be a d o p t e d to equalize any d i f f e r e n t ore properties. A very e f f i c i e n t m e t h o d p r o v e d to be the a d d i t i o n of certain substances such as basic e a r t h y c o m p o u n d s , silicates, q u a r t z , b e n t o n i t e a n d others. I n o r g a n i c additives, or such w h i c h d o not volatilize d u r i n g i n d u r a t i o n , p a r t i c i p a t e in the reactions. T h e y c a n react either w i t h the g a n g u e constituents or with the iron oxides. W h e n they react w i t h t h e g a n g u e , they c a n f o r m p a r t of the i n t e r g r a n u l a r m e l t i n g phase which, besides the crystalline phase, c o n t r i b u t e s to pellet solidification a l t h o u g h this p h a s e is qualitatively insignificant c o m p a r e d to the crystalline p h a s e . T h i s can be seen f r o m the p h a s e triangle, Fig. 26, w h i c h is c o m p o s e d of the p r i n c i p a l basic constituents such as C a O / M g O , t h e acid constituents S i O 2 / A l 2 O 3 a n d h e m a t i t e (Fe 2 O 3 ). T h e analyses of s o m e well-known pellet types are plotted o n the p h a s e d i a g r a m . T h e d o m i nating role of t h e iron o x i d e p h a s e is a p p a r e n t . A l t h o u g h the

intergranular

p

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2 Fundamentals of Pelletizing

Fig. 26. Iron oxide corner in phase triangle F e 2 O 3 - C a O - S i O 2 composition of various pellet types

for the crystal g r o w t h p r i m a r i l y of h e m a t i t e . F o r example, C a O reacts with F e 2 O 3 at a s i m u l t a n e o u s f o r m a t i o n of d i f f e r e n t calcium ferrites which substantially accelerate the crystal g r o w t h p a r t i c u l a r l y at t e m p e r a tures of a b o v e 1250 ° C . T h i s is d u e to the fact t h a t these ferrites m e l t i n g at lower t e m p e r a t u r e s speed u p the d i f f u s i o n of i n d i v i d u a l crystal ions in such a way that the crystal growth p r o c e e d s m o r e r a p i d l y t h a n in crystal structures f r e e f r o m any melting p h a s e . Even low C a O a d d i t i o n s are sufficient to p r o d u c e this effect, as is s h o w n in Fig. 27 3 4 ). A f t e r the a d d i t i o n of o n e or two percent C a O , a concentrate w i t h 99.1% m a g n e t i t e is pelletized a n d i n d u r a t e d at d i f f e r e n t t e m p e r a t u r e s . A n a d d i t i o n of as low as 1% C a O is sufficient to m a k e the h e m a t i t e crystals grow m o r e intensively t h a n t h e s a m p l e free of l i m e f r o m t e m p e r a t u r e s of 1300 0 C onward. SiO 2 in the f o r m of q u a r t z does not r e a c t with h e m a t i t e d u r i n g i n d u r a t i o n . SiO 2 in t h e f o r m of reactive silicates, w h i c h are partially or completely converted to glassy or crystalline c o m p o u n d s can be c o m b i n e d with ore grains to f o r m silicate bridges. As m e n t i o n e d at the beginning of this section, c e r t a i n additives are of great i m p o r t a n c e for pellet p r o d u c t i o n . N o w a d a y s , in m a n y pelletizing

2.2 Induration of Green Balls

45

Fig. 27. Influence of CaO and firing temperature on crystal formation of hematite plants, additives are increasingly used w h i c h serve not only f o r the original p u r p o s e to i m p r o v e the pellet strength, b u t go b e y o n d that. In a similar way, as in the p r o d u c t i o n of self-fluxing sinter, the additives are selected in such a way t h a t the acid g a n g u e c o m p o n e n t s a r e n e u t r a l i z e d to i m p r o v e to a large extent the slag formation in succeeding melting units or reduction furnaces. T h e t y p e a n d q u a n t i t y of these a d d i t i v e s d e p e n d o n the analysis of ores to b e t r e a t e d (see item 5.3).

2.2.3 C o o l i n g of Indurated Pellets T h e r e are t w o reasons w h y the pellet cooling is a n i m p o r t a n t step in t h e final pellet p r o d u c t i o n . O n the o n e h a n d , the h o t pellets h a v e a c e r t a i n h e a t c o n t e n t which, with a view to a f a v o u r a b l e h e a t balance, s h o u l d be recycled, if possible w i t h small losses, to the h e a t i n g process. In all pellet firing systems presently a p p l i e d on a n i n d u s t r i a l scale, c o r r e s p o n d i n g technological a n d constructional m e a s u r e s are taken.

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H o w e v e r , it is also i m p o r t a n t that the crystalline a n d glassy c o m p o u n d s arising during induration are not d a m a g e d by a too rapid cooling. Relevant tests have d e m o n s t r a t e d 1 8 ) that pellets, w h i c h were c a r e f u l l y cooled by air to 300 0 C , will not be d a m a g e d if their f u r t h e r cooling is a c h i e v e d in water. H o w e v e r , d u r i n g cooling pellets h a v i n g a t e m p e r a t u r e of 600 0 C , almost at 50% c r u m b l e d into smaller f r a g m e n t s . O t h e r tests yielded s i m i l a r results. In these tests, it was f o u n d that the c r u s h i n g strength of pellets h a v i n g b e e n q u e n c h e d in w a t e r at a t e m p e r a t u r e of a b o u t 700 0 C d i m i n i s h e d to less t h a n half of their initial c r u s h i n g strength. C a r e f u l pellet cooling for m a i n t a i n i n g pellet strength in c o n j u n c t i o n w i t h extensive h e a t recovery are d e m a n d s which m u s t be c o n s i d e r e d for the d i m e n s i o n i n g of the cooling units. If, finally, the various processing steps f r o m the raw ore to the finished pellets are r e v i e w e d , it is f o u n d , that several b o n d i n g m e c h a n i s m s are responsible for t h e f o r m a t i o n of s o u n d pellets. — Liquid bridges b e t w e e n the i n d i v i d u a l grains of the c o n g l o m e r a t e to be pelletized are responsible for the strength of the green balls d u r i n g their f o r m a t i o n . — Solid bridges in t h e f o r m of " m o r t a r b o n d s " arising t h r o u g h gels, hydrates a n d inter-facial forces are r e s p o n s i b l e f o r the strength of dry pellets. — T h e final strength of indurated pellets is a g a i n d u e to solid bridges arising by crystal change, crystal growth a n d sintering of the interg r a n u l a r m e l t i n g phases.

3 Raw Materials and Their Preparation for Pellet Production

3.1 Raw Materials According to their intended use for pelletizing and their chemical composition, there are two different groups of raw materials. O n e of t h e m consists of the iron-bearing minerals and represents the pellet matrix. T h e other one comprises mostly raw materials low in or free f r o m iron, which are exclusively used for: — facilitating the pellet production (binders) — improving the physical-mechanical quality (binders a n d additives) — modifying the metallurgical properties of pellets (additives).

3 . 1 . 1 Iron-Bearing M a t e r i a l s The by f a r greatest quantity of iron-bearing materials consists of natural ores either in the f o r m of high-grade ores or concentrates originating f r o m the beneficiation of low-grade ores. W h e n high-grade ores are used, the iron content of the pellets is mostly higher t h a n 65%. T h e concentrates generally contain more t h a n 64% iron. Occasionally, small quantities of high-grade secondary materials such as calcines f r o m pyrite roasting plants are used. S o m e of these secondary materials m a y contain other impurities either in the f o r m of detrimental constituents to be separated such as phosphorus, non-ferrous metals or utilizable components, e.g. v a n a d i u m or copper. F o r the separation of such impurities, special process variants are to be applied which are described in C h a p . 7. 3.1.1.1 Natural Iron Ores Natural high-grade iron ores and concentrates complying with t h e demands m a d e on the chemical composition of pellets are forming the basis of the pellet feed. N a t u r a l ores with low iron contents which cannot be upgraded, such as iron silicates, are insignificant f o r pellet production. The iron ores which are nowadays industrially utilized originate primarily

48

3 Raw Materials and Their Preparation for Pellet Production

f r o m highly m e t a m o r p h i c , initially s e d i m e n t a r y deposits with the p r e c a m b r i a n era d o m i n a t i n g as geological f o r m a t i o n . A h i g h p o r t i o n of ores f r o m these deposits originates f r o m w e a t h e r i n g zones in w h i c h the iron content was enriched. A c c o r d i n g to the c l i m a t i c zone in w h i c h the w e a t h e r i n g took place, a l u m i n a m i n e r a l s such as b a u x i t e m a t e r i a l s or lateritic w e a t h e r e d p r o d u c t s p r e d o m i n a t e as g a n g u e constituents of the iron ores in tropic zones. In cooler zones, silicate minerals or q u a r t z are d o m i n a t i n g . In the original parts of these deposits, the q u a r t z - b a n d e d iron ores occur as itabirite, locally d e s i g n a t e d also as taconite or jaspilite, p r e d o m i n a n t l y in t h e f o r m of m a g n e t i t e ore. O t h e r deposits in t h e contain t h e o r e in two d i f f e r e n t forms: as c a r b o n a t e a n d m a g n e t i t e , a n d also in t h e f o r m of pyrite as accessory mineral. T h e great q u a n t i t i e s of geologically y o u n g e r s e d i m e n t a r y deposits consisting of cretaceous a n d d o g g e r f o r m a t i o n s are of great i m p o r t a n c e for m a n y g e o g r a p h i c a l l y l i m i t e d areas b u t not as o r e supplies to pelletizing plants. T h e p r i n c i p a l i r o n m i n e r a l s used f o r pellet p r o d u c t i o n are described below: 3.1.1.1.1 Magnetite. M a g n e t i t e is the m a i n i r o n - b e a r i n g m i n e r a l in unw e a t h e r e d a n d n o n - o x i d i z e d m e t a m o r p h i c s e d i m e n t a r y deposits or in r e p l a c e m e n t o r e deposits in the m a g m a t i c area. T h e i r o n contents of these ores range f r o m 20 to 50% for itabirites a n d u p to 65% in the m a g m a t i c deposits. Its c h e m i c a l c o m p o s i t i o n is F e 3 O 4 , f r e q u e n t l y also d e s i g n a t e d as F e O F e 2 O 3 w i t h a theoretical iron content of 72.4%. A r e p l a c e m e n t of the bivalent iron by m a n g a n e s e or c a l c i u m ions, of t h e trivalent iron by a l u m i n i u m ions can be observed in ore r e p l a c e m e n t deposits. In deposits with a high f o r m a t i o n t e m p e r a t u r e , T i O 2 contents are f o u n d w h i c h are mostly p r e s e n t in the f o r m o f segregated i l m e n i t e l a m e l l a e w i t h i n the m a g n e t i t e crystals. V a n a d i u m - p e n t o x i d e i n c o r p o r a t e d into m a g n e t i t e lattices, occurs in m i n e r a l i z e d g a b b r o massifs mostly together with t i t a n i u m . O t h e r u n d e s i r a b l e a c c o m p a n y i n g m i n e r a l s are a p a t i t e ( N o r t h e r n S w e d e n ) or metal s u l p h i d e s (Peru, M a r c o n a ) . M a g n e t i t e , being of the spinel type, crystallizes as d o u b l e oxide with t h e iron o c c u r r i n g in bivalent f o r m as F e O a n d in trivalent f o r m as F e 2 O 3 . T h e p r e d o m i n a n t crystal s h a p e is the o c t a h e d r o n or the r h o m b i c d o d e c a h e d r o n . In general, m a g n e t i t e ores are b e n e f i c i a t e d to a c h i e v e a c h e m i c a l c o m p o s i t i o n s u i t a b l e for smelting. As the b e s t - k n o w n process, m a g n e t i c s e p a r a t i o n is a p p l i e d a n d s o m e t i m e s , in a d d i t i o n , f l o t a t i o n is also used. D u r i n g pellet i n d u r a t i o n , the m a g n e t i t e oxidizes to h e m a t i t e . D u r i n g this reaction a b o u t 498 kJ p e r kg m a g n e t i t e are l i b e r a t e d which, as a d d i t i o n a l energy, positively i n f l u e n c e the i n d u r a t i o n process.

perimagmatic

3.1 Raw Materials

49

3.1.1.1.2 Hematite. H e m a t i t e is t h e iron ore w h i c h occurs m o s t f r e q u e n t l y and in the greatest quantities. A c c o r d i n g to its genesis, it is a v a i l a b l e in coarse to m e d i u m crystalline structure d e s i g n a t e d as iron glance or specular i r o n ore, in fine crystalline s t r u c t u r e o r in e a r t h y c o n d i t i o n as earthy h e m a t i t e . Its c h e m i c a l c o m p o s i t i o n is F e 2 O 3 w i t h a theoretical i r o n content of 70.0%. H e m a t i t e is also the final o x i d a t i o n stage of i n d u r a t e d pellets. F r o m a crystallographical viewpoint, it p e r t a i n s to t h e c o r u n d u m g r o u p and p r e d o m i n a n t l y crystallizes in the h e x a g o n a l system with a g r e a t n u m b e r of s h a p e s in d i f f e r e n t c o m b i n a t i o n s u p to m i c a c e o u s scales. D u e to weathering of m a g n e t i t e to h e m a t i t e , the c u b i c crystalline s h a p e is o f t e n m a i n t a i n e d . T h i s type of h e m a t i t e is k n o w n as m a r t i t e . T h e p r i m a r y itabirites mostly c o n t a i n 3 0 - 4 5 % i r o n w i t h q u a r t z a n d silicates as g a n g u e . M o r e i m p o r t a n t are the w e a t h e r i n g zones of i t a b i r i t e deposits in w h i c h the iron is e n r i c h e d to contents of 50 u p to a l m o s t 70%. T h e iron c o n t e n t is m a i n l y present as specular i r o n ore or e a r t h y h e m a t i t e , in lower p e r c e n t a g e s also as m a g n e t i t e or m a r t i t e a n d — d e p e n d e n t on t h e weathering d e g r e e — as iron h y d r o x i d e ( " l i m o n i t e " , F e O O H ) . T h e m a j o r part of the i r o n ores used at p r e s e n t o r i g i n a t e s f r o m tropical a n d subtropical z o n e s a n d r e p r e s e n t s lateritic w e a t h e r i n g p r o d u c t s . T h e caps of such lateritic o r e deposits generally consist of a h a r d o r e layer of u p to 20 m d e p t h w i t h h i g h i r o n a n d a l u m i n a contents. S o f t to brittle fine i r o n ores (mostly i r o n glance) with iron contents d i m i n i s h i n g w i t h increasing d e p t h are s i t u a t e d u n d e r n e a t h . T h e h e m a t i t e ores a n d their concentrates seldom c o n t a i n metallurgically d e t r i m e n t a l constituents except f o r the u n f a v o u r a b l e r a t i o of a l u m i n a to silica in s o m e ores. 3.1.1.1.3 Weathered Ores. T h e s e ores deserve s o m e a d d i t i o n a l lines since, quantitatively, they are of very great i m p o r t a n c e as f e e d m a t e r i a l s f o r pelletizing plants. T h e best-known deposits are at present situated in C a n a d a , West Africa, Brazil, Venezuela, India and W e s t e r n Australia. In all cases, the original m a t e r i a l s consist of itabirite deposits of the P r e c a m b r i a n E r a with m a g n e t i t e a n d iron glance as basic minerals. By w e a t h e r i n g , g a n g u e minerals, q u a r t z a n d silicates are d e c o m p o s e d a n d r e m o v e d to a g r e a t e r extent d u e to t h e i r h i g h e r solubility w h e r e b y the i r o n c o n t e n t in t h e residual ore is increased. T h e g a n g u e content is locally d e c r e a s e d to less than 1% e.g. in s o m e parts of M i n a s G e r a i s / B r a z i l . T h e iron m i n e r a l s are also s u b j e c t e d to a t m o s p h e r i c i n f l u e n c e s a n d are thus partly c o n v e r t e d to a h i g h degree. A typical f e a t u r e of the w e a t h e r i n g deposits is t h e i r heterogeneous c o m p o s i t i o n a n d their c o n s i d e r a b l e variations at shortest distances. T h e r a t i o of the i n d i v i d u a l i r o n m i n e r a l s to each other such as s p e c u l a r i r o n ore, e a r t h y h e m a t i t e , l i m o n i t e a n d m a g n e t i t e changes w i t h i n w i d e limits. Pelletizing plants f o r s u c h ores can only b e operated with g r e a t difficulties, unless they are p r e c e d e d b y h o m o g e n i z i n g

50

3 Raw Materials and Their Preparation for Pellet Production

systems. T h e very close i n t e r g r o w t h of a l u m i n a m i n e r a l s a n d l i m o n i t e is typical for the w e a t h e r e d lateritic ores. 3.1.1.1.4 Limonite. L i m o n i t e can be c o n s i d e r e d as a typical e x a m p l e of w e a t h e r e d ore. T h i s s o m e w h a t inexact collective t e r m covers t h e iron hyd r o x i d e m i n e r a l s . Mineralogically, t h e y occur in the m o d i f i c a t i o n s " n e e d l e i r o n - o r e " a n d " p y r r h o s i d e r i t e " w h o s e s t r u c t u r a l f o r m u l a is F e O O H . O t h e r mineralogical designations are " g e o t h i t e " or " l e p i d o c r o c i t e " . T h e interm e d i a t e g r a d e s with u n d e f i n e d h y d r a t i o n d e g r e e are also specified as " h y d r o h e m a t i t e " . Iron h y d r o x i d e is the basic m i n e r a l of nearly all s e d i m e n t a r y as well as m e t a m o r p h i c deposits. E c o n o m i c a l l y i m p o r t a n t limonite deposits originate f r o m w e a t h e r i n g zones of the i t a b i r i t e a n d siderite deposits as well as f r o m deposits with oolitic a n d f r a g m e n t a l i r o n ores (Mesozoic Era). In the f o r m e r itabirite deposits l i m o n i t e deposits occur t o g e t h e r with s p e c u l a r i r o n ore a n d e a r t h y h e m a t i t e . D u r i n g t h e pellet i n d u r a t i o n process, l i m o n i t e is c o n v e r t e d i n t o trivalent iron oxide, F e 2 O 3 . This process is e n d o t h e r m i c a n d t h e r e f o r e increases a d d i t i o n a l l y the h e a t c o n s u m p t i o n . A f t e r drying a n d p r e h e a t i n g , l i m o n i t e leaves a very f l u f f y p o r o u s s t r u c t u r e w h i c h calls for a r a t h e r l o n g i n d u r a t i o n t i m e at high temperatures. P r i m a r y l i m o n i t e ores s o m e t i m e s c o n t a i n p h o s p h o r u s at a p e r c e n t a g e necessitating special s t e e l m a k i n g processes such as T h o m a s a n d L D A C processes b e c a u s e s e p a r a t i o n of the p h o s p h o r o u s m i n e r a l s b y m i n i n g m e t h o d s is very d i f f i c u l t or only possible to a l i m i t e d extent (Minette) d u e to t h e i r d i s s e m i n a t i o n with the iron minerals. At present l i m o n i t e is a n i m p o r t a n t ore c o n s t i t u e n t in several pelletizing plants, e.g. R o b e River, W e s t e r n A u s t r a l i a , Sidor, V e n e z u e l a . F r o m all ores used for pelletizing, h e m a t i t e is the final o x i d a t i o n stage a f t e r i n d u r a t i o n , as s h o w n in T a b l e 5. T a b l e 6 gives s o m e d a t a of ores n o w a d a y s utilized w i t h o u t p r e v i o u s beneficiation. Table 5. Conversion of iron compounds into hematite during induration Types of compounds

Hematite Martite Magnetite Goethite/Limonite (Hydro-Hematite) Siderite Pyrite a

After intermediate roasting

Formula

% Iron content before Induration

after Induration

Fe 2 O 3

70

70

Fe 3 O 4 FeO (OH) (Fe 2 O 3 • H 2 O) FeCO 3 FeS 2

72.4 62.9

70 70

48.3 46.6

70a 70ª

3.1 Raw Materials

51

Table.6. Type, chemical analysis and specific surface area of rich iron ores directly used in pellet plants Types of ores Spec. surface area cm 2 /g

Fe tot.

CVRD Brazil

Hematite 1600- 1800

66

Samarco / Brazil

Hematite 1500- 1700

66.5/ 67.5

Hamersley/ West-Austr.

Hematite/ Limonite 1200-2700

63.4 68.7

Robe River WestAustralia

Limonite

Chowgule/ Goa India Sidor/ Venezuela

Plants

Al 2 O 3

CaO

MgO L.O.I.

2.7

1.0

0.2

0.2

2.5/ 3.6

0.4/ 0.7

0.1

0.2

4.5/ 1.2

2.6/ 1.0

0.4

0.1

57.5

n. d.

3.0

Hematite/ Limonite

63/64

1.5

1.5

Hematite/ Limonite 3400

62.9

1.6

1.2

Fe" SiO 2

in % about -

-

0.7

—

1.0

3.51.0 9.5

4.0

6.0

3.1.2 Beneflciation Products Ores h a v i n g a metallurgically u n s u i t a b l e c o m p o s i t i o n u n d e r g o b e n e f i c i a tion processes b e f o r e pelletizing in w h i c h t h e u n d e s i r a b l e constituents are largely s e p a r a t e d . T h e concentrates to b e pelletized are o f t e n s u f f i c i e n t l y fine-grained f o r g r e e n ball f o r m a t i o n as in t h e case of m a g n e t i t e . A p a r t f r o m t h e g a n g u e , the contents of d e l e t e r i o u s substances such as p h o s p h o r u s , arsenic, t i t a n i u m , c h r o m i u m , chlorides, f l u o r i d e s a n d n o n f e r r o u s metals, w h i c h are partly d e t r i m e n t a l d u r i n g s m e l t i n g a n d p a r t l y affect adversely t h e q u a l i t y of f i n i s h e d p r o d u c t s , s h o u l d b e d e c r e a s e d b y beneficiation. Mineralogically, the beneficiation products mostly c o n f o r m — as far as the iron m i n e r a l s are c o n c e r n e d — to the r a w ores. P a r t i c u l a r reference s h o u l d b e m a d e to the artificial m a g n e t i t e arising f r o m l i m o n i t e or h e m a t i t e - c o n t a i n i n g ores b y roasting u n d e r r e d u c i n g c o n d i t i o n s a n d which is s e p a r a t e d as m a g n e t i t e b y low-intensity m a g n e t i c s e p a r a t i o n a n d occasionally by flotation. O w i n g to its h i g h p o r o s i t y a n d crystalline structure, its reactivity to oxygen is very high. T h e b e g i n n i n g of its o x i d a tion is m a r k e d l y lower t h a n t h a t of n a t u r a l m a g n e t i t e . T h e i r o n c o n t e n t of concentrates, irrespective of t h e process a c c o r d i n g to which they a r e o b t a i n e d , is at least 64%, w i t h f e w exceptions.

52

3 Raw Materials and Their Preparation for Pellet Production 3.1.3 Secondary Raw MateriaIs

In s o m e cases, a p a r t f r o m iron ores a n d their concentrates, m a t e r i a l s are pelletized alone or m i x e d with ores o r i g i n a t i n g f r o m o t h e r t h e r m a l or chemical processes. Such materials cover pyrite cinders, leaching residues a n d inplant fines. T h e cinders are p r o d u c e d d u r i n g the roasting of iron sulphides, such as pyrite (FeS 2 ) or p y r r h o t i t e (FeS). T h e i r use is l i m i t e d d u e to the o f t e n existing n o n - f e r r o u s m e t a l ( C u , P b , Zn), or a r s e n i c impurities. H o w e v e r , in s o m e countries, t h e r e are exceptions. Pelletizing plants for these ore types are u n d e r construction or in o p e r a t i o n in J a p a n , in the U n i t e d States, C a n a d a , Italy a n d R u m a n i a . D u r i n g t h e roasting of iron s u l p h i d e s ( o x i d a t i o n of s u l p h u r to gaseous SO 2 ), the i r o n is oxidized to either F e 2 O 3 or F e 3 O 4 according to the a d j u s t e d roasting a t m o s p h e r e . In these cases, highly p o r o u s oxides are involved. R e s i d u a l s u l p h u r contents of 0 . 5 - 2 % are usual. R e s i d u e s f r o m the l e a c h i n g of c o p p e r or nickel-bearing s u l p h i d e s such as chalcopyrite or pentlandite, w h i c h are roasted b e f o r e leaching, are also used as ores, f o r e x a m p l e artificial m a g n e t i t e concentrates f r o m t h e Inco p l a n t at C o p p e r Cliff, O n t a r i o , C a n a d a . In pelletizing plants located in iron a n d steel works, n o w a d a y s a t t e m p t s are m a d e to a d d in-plant fines to the ore m i x to be pelletized. In this case, materials are involved w h i c h are either o b t a i n e d as waste materials with a h i g h iron c o n t e n t or are tolerated as basic materials. T h e s e m a t e r i a l s consist of B O F dust, mill scale, blast f u r n a c e d u s t a n d slag a n d B O F slag. E x a m p l e s of analyses are given in T a b l e 7. T h e dusts h a v e a h i g h fineness degree at a great specific s u r f a c e area w h e r e a s slag a n d mill scale h a v e to be g r o u n d b e f o r e pelletizing. T h e presence of mill scale calls for special attention. T h e oil a d h e r i n g m a y adversely a f f e c t the pelletizing process a n d is to b e c a r e f u l l y c o n s i d e r e d d u r i n g the d e d u s t i n g of the pelletizing p l a n t waste gases.

Table 7. Chemical composition of in-plant fines62sobrescrito) Fe total

SiO2

Al 2 O 3

CaO

MgO

MnO

S

2.7 14.8 0.2 6.4 1.9 0.8

1.1 41.8 5.2 42.5 3.2 0.6

1.2 4.3 0.6 3.5 0.3 0.9

1.2 0.9 1.5 4.2 1.3 0.1

0.4 1.5 0.1 0.6 0.1 0.02

in % about Blast furnace dust Blast furnace slag BOF dust BOF slag Mill scale Pellet undersize

34.8 0.3 64.1 12.0 64.0 64.8

10.2 34.6 1.8 20.5 3.0 4.3

3.1 Raw Materials

53

3 . 1 . 4 Binders and Additives 3.1.4.1 Binders Binders, in c o n j u n c t i o n with the f i n e l y - g r o u n d ore particles, serve to improve the p r o p e r t i e s of pellets in wet, d r i e d or i n d u r a t e d c o n d i t i o n . T h e most i m p o r t a n t b i n d i n g reagents, n a m e l y w a t e r a n d iron o r e particles a r e not c o n s i d e r e d h e r e since they will b e discussed in c h a p t e r 5. D u r i n g t h e d e v e l o p m e n t of the pelletizing process, a great n u m b e r of o r g a n i c a n d However, n o w a d a y s only bentonite, slaked lime, l i m e s t o n e a n d d o l o m i t e are used. B i n d e r s w h i c h are deleterious or w h i c h give off p o l l u t a n t s to t h e waste gas of pellet plants are n o longer t a k e n i n t o c o n s i d e r a t i o n . A l t h o u g h some b i n d e r s are s i m u l t a n e o u s l y used as additives, b e n t o n i t e serves as a binder only.

3.1.4.2 Additives Additives are p r o v i d e d for m o d i f y i n g t h e c h e m i c a l c o m p o s i t i o n of pellets, especially their s l a g - f o r m i n g constituents. S o m e of t h e m h a v e binding p r o p e r t i e s too: (a) L i m e a n d l i m e - m a g n e s i u m c o m p o u n d s , (b) Recycling m a t e r i a l s , (c) Ores with a p a r t i c u l a r l y high b o n d i n g ability, (d) Siliciferous substances such as q u a r t z , q u a r t z i t e o r rocks w i t h a h i g h q u a r t z content. Siliceous m a t e r i a l is s o m e t i m e s utilized if the g a n g u e c o n t e n t of the o r e to be pelletized is t o o low for o b t a i n i n g a n i n t e r g r a n u l a r p h a s e . T h u s , in s o m e b e n e f i c a t i o n plants the s e p a r a t i o n is, d u e to the e l i m i n a t i o n of m e t a l sulphides ( M a r c o n a ) or p h o s p h o r o u s m i n e r a l s ( N o r t h e r n S w e d e n ) , accomplished to such a d e g r e e that, s u b s e q u e n t l y , the i n s u f f i c i e n t g a n g u e content in t h e c o n c e n t r a t e has to be increased b y a d d i t i o n of g r o u n d silica. Besides b i n d e r s a n d additives, g r o u n d pellet r e t u r n fines (fines f r o m the screening of i n d u r a t e d pellets) or g r o u n d sponge iron c a n b e a d d e d to t h e material to b e pelletized. R e t u r n fines s h o w a n inert b e h a v i o u r w h e r e a s sponge iron i m p r o v e s the quality a n d raises the p r o d u c t i v i t y (see items 5.3.1.6-8). 3.1.4.3 Bentonite In this case, a clayey rock is involved o r i g i n a t i n g as a result o f w e a t h e r i n g f r o m glassy, m a g m a t i c masses a n d o c c u r r i n g in cretaceous a n d tertiary layers. T h e p r i n c i p a l m i n e r a l c o m p o n e n t is m o n t m o r i l l o n i t e . Besides this m a i n m i n e r a l , small q u a n t i t i e s of q u a r t z , m i c a , f e l d s p a r a n d

inorganic

subs

54

3 Raw Materials and Their Preparation for Pellet Production

kaoline are present. As concerns m o n t m o r i l l o n i t e , t h e following structural f o r m u l a is given:

It has a lattice s t r u c t u r e a r r a n g e d in layers w h i c h is c a p a b l e of a b s o r b i n g great w a t e r q u a n t i t i e s b e t w e e n the i n d i v i d u a l layers; this is r e p r e s e n t e d by n H 2 O in the f o r m u l a . O n such occasions the distances b e t w e e n the lattice layers increase considerably. T h e m i n e r a l swells. T h i s swelling p r o p e r t y a n d the high t h i x o t r o p i c b e h a v i o u r are the m o s t i m p o r t a n t characteristics for its b o n d i n g capacity. T h e t e t r a h e d r a l crystalline s t r u c t u r e of silica can i n c o r p o r a t e c a l c i u m a n d m a g n e s i u m cations in a n e x c h a n g e a b l e m a n n e r with the C a ions f r e q u e n t l y b e i n g r e p l a c e d b y N a ions. T h e s e ions are responsible for w a t e r b o n d i n g in the i n t e r m e d i a t e a r e a b e t w e e n lattice layers. T h e r e are two large g r o u p s , the calcium bentonites, p r e d o m i n a n t l y occurring in the M e d i t e r r a n e a n area, a n d the m o r e active s o d i u m bentonites in the U n i t e d States. O n e of the b e n t o n i t e s w i t h the highest swelling capacity is the W y o m i n g bentonite. It is possible also to i n t r o d u c e s o d i u m ions into the calcium b e n t o n i t e a n d such a b e n t o n i t e is t h e n designated as activated bentonite. T h e swelling capacity is c h a r a c t e r i z e d by the Enslin value. It is a typical coefficient f o r t h e swelling b e h a v i o u r of the m o n t m o r i l l o n i t e g r o u p with intercrystalline swelling p r o p e r t y . T h e water a b s o r p t i o n of a p r e d e t e r m i n e d b e n t o n i t e q u a n t i t y is a s c e r t a i n e d at intervals of 30 m i n u t e s u p to 24 h o u r s altogether. T h e Enslin value is calculated a c c o r d i n g to the f o r m u l a : Ew

-

Ew = Enslin value in % V = a b s o r b e d w a t e r q u a n t i t y in c m 3 P = bentonite q u a n t i t y in g. T h e bentonite index indicating t h e swelling capacity and t h e emptying index characterizing the viscosity of b e n t o n i t e suspensions are used as f u r t h e r data. T h e r e is a certain p r o p o r t i o n a l i t y b e t w e e n these three indices. T h e c h e m i c a l c o m p o s i t i o n of s o m e bentonites is s h o w n in T a b l e 8 35 ). W i t h o u t t a k i n g the interstitial water into account, the typical c o m ponents are as follows 36 ): SiO 2 66.7%, A l 2 O 3 28.3%, H 2 O 5%. Bentonite b e n e f i c i a t i o n mostly consists of c r u s h i n g or extrusion followed b y drying f r o m a n average n a t u r a l m o i s t u r e c o n t e n t of 30% ( 2 5 - 5 0 % ) to 7—8%. T o m a i n t a i n the b e n t o n i t e activity, t h e d r y i n g t e m p e r a t u r e s h o u l d

3.1 Raw Materials

55

Table 8. Specifications of various Bentonite types Chemical analysis

A Ca.-B.

B Ca.-B.

D C act.ª B. act. B.

E act. B.

F Na.-B.

H Na.-B.

I act.ª B.

Fe ges. SiO2 Al2O3 CaO MgO Na 2 O K2O L.O.I.

14.02 42.70 17.40 1.79 2.74 0.04 0.11 11.83

0.67 56.80 3.37 1.34 26.37 0.25 0.44 9.88

1.42 72.76 11.72 1.51 3.06 2.14 0.70 5.38

4.47 56.56 19.10 1.79 3.39 2.31 1.70 7.84

1.14 72.08 13.96 1.00 2.12 3.00 0.58 4.88

2.70 60.50 21.18 1.48 2.24 2.25 0.62 6.10

2.55 60.64 21.70 1.12 2.58 2.56 0.44 5.80

4.20 56.90 19.52 1.06 3.26 3.20 0.99 7.48

Spec. surface area cm 2 /g Permeability methods

6502

4340

8480

7056

7350

2900

3380

4610

200

280

480

610

620

800

900

900

Enslin values in % a

act. = activated

not exceed 150 ° C . S u b s e q u e n t l y or d u r i n g drying, the b e n t o n i t e is g r o u n d to at least 9 0 % - 0 . 0 4 4 m m c o r r e s p o n d i n g to a s p e c i f i c s u r f a c e a r e a of u p to 8000 cm 2 /g. T h e t r a n s p o r t a t i o n is a c h i e v e d in closed c o n t a i n e r s such as tank cars or bags. 3.1.4.4 Lime Compounds Limestone, C a C O 3 , a n d h y d r o x i d e , C a ( O H ) 2 , are p r e f e r a b l y used. In nature, l i m e s t o n e is s o m e t i m e s c o n t a m i n a t e d b y q u a r t z or silicates such as clay. L i m e s t o n e is s e d i m e n t a r y s t r a t i f i e d rock or of c o m p a c t coralline nature. T h e l i m e m i n e r a l consists mostly of calcite a n d m o r e s e l d o m of aragonite. Both d i f f e r by their crystal f o r m . T h e i r c h e m i c a l c o m p o s i t i o n is C a C O 3 . T h e y are b e n e f i c i a t e d b y crushing, classification, w a s h i n g a n d grinding, if necessary. Excessively c o n t a m i n a t e d l i m e s t o n e a n d especially limestone w i t h a h i g h clay c o n t e n t ( m a r l ) is not u s e d f o r pelletizing. Before l i m e s t o n e is a d d e d to t h e o r e to b e pelletized, it is g r o u n d to a fineness c o r r e s p o n d i n g to a s p e c i f i c s u r f a c e a r e a of a b o u t 2 5 0 0 - 4 0 0 0 c m 2 / g . T h e dry g r i n d i n g m e t h o d is usually a d o p t e d . I n m o s t cases, roller mills o r c o m p a r a b l e mill types are used. L i m e s t o n e is m a i n l y e m p l o y e d f o r m o d i f y i n g the pellet g a n g u e basicity.

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3 Raw Materials and Their Preparation for Pellet Production

As a b i n d e r , calcium h y d r a t e C a ( O H ) 2 is p r e f e r r e d , w h i c h is p r o d u c e d f r o m C a O b y slaking with water. C a l c i u m o x i d e is o b t a i n e d by calcination of limestone, C a C O 3 , at a t e m p e r a t u r e of a b o v e 900 ° C in several r e a c t o r types such as s h a f t furnaces, rotary kilns or f l u i d b e d reactors. T h e selection of the r e a c t o r type d e p e n d s o n the size a n d t e n d e n c y to d e g r a d a t i o n of C a C O 3 d u r i n g h e a t t r e a t m e n t . T h e oxide is continuously converted to h y d r o x i d e at a slightly overstoichiometrical water a m o u n t in q u e n c h i n g units s i m i l a r to m i x i n g screws a n d is t h e n g r o u n d in o r d e r to pulverize any r e m a i n i n g coarse-grained constituents. As in the case of bentonite, t r a n s p o r t is c a r r i e d out in closed containers. L i m e h y d r a t e is b o t h b i n d e r and basic additive. Dolomite or d o l o m i t e containing l i m e s t o n e is occasionally used as f u r t h e r basic additive. D o l o m i t e is a m i x e d crystal i s o m o r p h o u s to calcite. Its f o r m u l a is (Ca, M g ) C O 3 a n d it is p r e p a r e d like limestone. D u r i n g pellet i n d u r a t i o n , t h e basic a d d i t i v e s react first with the " a c i d " g a n g u e constituents w h e r e b y a neutral or basic m a t r i x b e t w e e n the iron oxide grains is f o r m e d . 3.1.4.5 Other Additives If, a p a r t f r o m pellet i n d u r a t i o n , o t h e r reactions are to be carried out, c o r r e s p o n d i n g reagents can b e used. Calcium chloride ( C a Cl 2 ) is zinc of the pellet mix. S o d i u m c a r b o n a t e or s o d i u m s u l p h a t e is a d d e d if v a n a d i u m is to be dissolved as s o d i u m v a n a d a t e f r o m v a n a d i u m - c o n t a i n ing ores, see C h a p . 7.

3.2 Preparation of Raw Materials for Pelletizing M a n y of the n a t u r a l ores, s e c o n d a r y raw m a t e r i a l s , a d d i t i v e s or b i n d e r s are naturally not s u i t a b l e for direct processing into pellets. In these cases, a p r e p a r a t i o n a d a p t e d to the raw materials involved is r e q u i r e d . This refers to the - increase of iron content in the l o w - g r a d e r a w ore, - s e p a r a t i o n of u n d e s i r a b l e constituents, - g r i n d i n g to t h e fineness r e q u i r e d for pelletizing. T h e m e t h o d s a d o p t e d for this p u r p o s e a r e described in the following items as f a r as t h e y are of i m p o r t a n c e in c o n j u n c t i o n with pellet p r o d u c tion. In this connection, it is also r e c o m m e n d e d t h a t the facilities r e q u i r e d for the p e r f o r m a n c e of a d e q u a t e tests (see i t e m 4.2) s h o u l d be p r o v i d e d in the pelletizing l a b o r a t o r y .

employed

3.2 Preparation of Raw Materials for Pelletizing 3.2.1

57

Separation

S e p a r a t i o n m e t h o d s cover all b e n e f i c i a t i o n processes w h i c h are necessary for o b t a i n i n g a s u i t a b l e c h e m i c a l - m i n e r a l o g i c a l c o m p o s i t i o n of t h e materials to b e pelletized. D e p e n d i n g o n the p r o p e r t i e s of the ores, several methods are a v a i l a b l e f o r this p u r p o s e : 3.2.1.1 Washing W a s h i n g is m a i n l y used to d e c r e a s e the p o r t i o n of clayey constituents in high-grade lateritic ores. Ores can b e s u b j e c t e d to the w a s h i n g process as screened n a t u r a l fine ores or a f t e r a d d i t i o n a l grinding. By washing, t h e clayey constituents of the w e a t h e r e d ores are slurried w i t h w a t e r a n d t h e dissolved particles a r e s e p a r a t e d . In this case very fine-grained a l u m i n a c o m p o u n d s as well as s o m e i r o n m i n e r a l s , p r e p o n d e r a n t l y l i m o n i t e , go i n t o the tailings. F o r this reason, the c o n c e n t r a t e always has a h i g h e r ratio of magnetite or s p e c u l a r i t e to l i m o n i t e t h a n t h e r a w ore. F r o m a physical viewpoint, this w a s h i n g process is a classification process ( s e p a r a t i o n according to g r a i n size). D u e to t h e h i g h e r specific gravity of the o r e minerals c o m p a r e d with that of t h e g a n g u e , a n a d d i t i o n a l u p - g r a d i n g effect occurs. T h e e q u i p m e n t used f o r w a s h i n g consists of w a s h i n g t r o u g h s or w a s h i n g d r u m s , s p i r a l classifiers, h y d r o - c y c l o n e s a n d t u m b l e mills. T h e iron c o n c e n t r a t i o n is n o t very high. B u t the iron losses are c o n s i d e r a b l e in the very fine tailing particles. Fig. 33, i t e m 4.2.1 s h o w s a b e n e f i c i a t i o n flow-sheet f o r a n e a r t h y , l i m o n i t e c o n t a i n i n g ore. 3.2.1.2 Gravity Separation T h e gravity m e t h o d s are based on d i f f e r e n c e s in s p e c i f i c gravity between g a n g u e (2.8—3 g / c m 3 ) a n d iron m i n e r a l s ( 4 . 5 - 5 . 1 g / c m 3 ) . T h e s e m e t h o d s are a d o p t e d for fine-grained ores w i t h s p e c u l a r h e m a t i t e o r magnetite as i r o n - b e a r i n g m a t e r i a l a n d q u a r t z - s i l i c a t e g a n g u e . B e f o r e separation, t h e ores are g r o u n d to such a d e g r e e t h a t the i r o n - c o n t a i n i n g constituents are m e c h a n i c a l l y l i b e r a t e d f r o m the g a n g u e . T h e o p t i m u m size range is b e t w e e n 1.5 a n d 0.1 m m . In the finer size range, the i r o n recovery d r o p s c o n s i d e r a b l y . T h e m o s t i m p o r t a n t unit w h i c h is o f t e n used on a n i n d u s t r i a l scale is the spiral s e p a r a t o r , k n o w n as Humphrey's Spiral, which h a s a n a c c e p t a b l e t h r o u g h p u t . O t h e r a p p a r a t u s , h a v i n g in p r i n c i p l e the s a m e or even a b e t t e r s e p a r a tion efficiency such as s h a k i n g tables are h a r d l y used d u e to their g r e a t space r e q u i r e m e n t s a n d insufficient t h r o u g h p u t . In f u t u r e , t h e cone s e p a r a t o r a c c o r d i n g to R e i c h e r t , w h i c h w i t h i n a s h o r t t i m e has g a i n e d a d o m i n a t i n g p o s i t i o n f o r p r e c o n c e n t r a t i n g h e a v y m i n e r a l sands, m a y p a r t l y s u p p l a n t t h e spiral s e p a r a t o r . H u m p h r e y ' s spirals are used for e x a m p l e in

58

3 Raw Materials and Their Preparation for Pellet Production

the Carol L a k e a n d W a b u s h L a k e plants p r o d u c i n g concentrates with a n iron content of a b o v e 63%. 3.2.1.3 Flotation This process w h i c h has p r o v e d to b e s u i t a b l e f o r the c o n c e n t r a t i o n of m a n y s u l p h i d e ores has also gained a certain i m p o r t a n c e for oxidized iron ores w i t h i n t h e last 30 years. T h e s e p a r a t i o n is a c h i e v e d o n the basis of the d i f f e r e n t physical-chemical s u r f a c e p r o p e r t i e s of the i n d i v i d u a l m i n e r a l constituents, particularly o n the basis of their w e t t a b i l i t y by water. T h e ore surface p r o p e r t i e s are i n f l u e n c e d by several reagents which, owing to their polar — n o n p o l a r structure, are surface-active. O t h e r reagents are e m ployed to p r o v i d e f a v o u r a b l e conditions in t h e circulating l i q u i d phase. Such reagents are d e s i g n a t e d as m o d i f i e r s . T h e w a t e r - r e p e l l e n t particles a n d those d i f f i c u l t to wet a c c u m u l a t e on the air b u b b l e s p r o d u c e d in the ore pulp. T h e y are d i s c h a r g e d by f o a m f o r m e d by these a n d s e p a r a t e d f r o m the p u l p surface. In m a n y cases, a r e a g e n t f a v o u r i n g the f o a m f o r m a tion is necessary. T h e w e t t a b l e grains of the g a n g u e r e m a i n in the p u l p . W h e n this s e p a r a t i n g m e t h o d is a d o p t e d , it is u n d e r s t o o d that the m i n e r a l particles s e p a r a t e d b y flotation m u s t n o t exceed a d e f i n i t e u p p e r grain size d i a m e t e r w h i c h is of the o r d e r of 0.1 m m a n d d e p e n d e n t o n the specific weight. T h e o p t i m u m size r a n g e s e e m s to be 0 . 0 1 - 0 . 0 6 m m . Initially, the f l o t a t i o n m e t h o d was a d o p t e d in t w o plants in N o r t h A m e r i c a for the direct c o n c e n t r a t i o n of itabirite s p e c u l a r iron ore in which concentrates with a b o u t 65% i r o n were p r o d u c e d f r o m r a w o r e with a b o u t 50% iron. In o n e of the two plants, t h e f l o t a t i o n was t e m p o r a r i l y carried out at a b o u t 45 0 C w h e n concentrates with a n i r o n content of 6 8 - 6 9 % were r e q u i r e d . In this way, a m o r e intensive i n f l u e n c e of the reagents a n d a m o r e efficient s e p a r a t i o n were a c h i e v e d d u e to a lower viscosity of t h e liquid phase. In the m e a n t i m e , t h e flotation m e t h o d is a d o p t e d to a m u c h greater extent for the s e p a r a t i o n of residual g a n g u e f r o m concentrates and of m e t a l s u l p h i d e s or p h o s p h o r o u s m i n e r a l s f r o m concentrates a l r e a d y p r o d u c e d otherwise. In the case of this " i n v e r s e f l o t a t i o n " , the ironb e a r i n g m i n e r a l s are thus h y d r o p h i l i z e d or " d e p r e s s e d " a n d the r e m a i n i n g g a n g u e particles are h y d r o p h o b i z e d a n d float in the f o a m . T h e concentrates p r o d u c e d by inverse or indirect flotation c o n t a i n 6 3 - 6 7 % iron. T h e iron losses in the tailings are mostly b e l o w 10%. 3.2.1.4 Magnetic Separation A c c o r d i n g to the m a g n e t i c p r o p e r t i e s of the i r o n - b e a r i n g minerals, two d i f f e r e n t m a g n e t i c s e p a r a t o r types are used (see i t e m 4.3.1). T h e lowintensity magnetic separator is e m p l o y e d for h i g h l y m a g n e t i c ores such as

3.2 Preparation of Raw Materials for Pelletizing

59

magnetite. A c c o r d i n g to the necessary l i b e r a t i o n g r a i n size, the s e p a r a t i o n is carried out in d r y m a n n e r ( a b o v e 2 m m g r a i n size) o r in wet m a n n e r when a finer g r i n d i n g is r e q u i r e d . D r u m s e p a r a t o r s w i t h p e r m a n e n t magnets are utilized as separators. T h e size of recently d e v e l o p e d units is 3000 m m w i d t h a n d 1200 m m d i a m e t e r at a c a p a c i t y of 250 m 3 / h f e e d pulp containing 20—50 weight % solids. T h e Alnico m a g n e t s f o r m e r l y used are at p r e s e n t b e i n g s u p p l a n t e d t o a high degree by ferrite magnets. T h e m a g n e t i c i n d u c t i o n (field intensity) m e a s u r e d on t h e d r u m s u r f a c e or at a distance of 50 m m is 0 . 0 4 - 0 . 2 tesla. In m a n y cases, m a g n e t i t e ores a r e so finely g r o u n d f o r l i b e r a t i o n that t h e concentrates p r o d u c e d a l r e a d y have the grain size r e q u i r e d for pelletizing. T h e low-intensity wet m a g n e t i c s e p a r a t i o n allows a h i g h i r o n c o n c e n t r a t i o n to 65—72% at a recovery of 95—97% r e f e r r e d to the highly m a g n e t i c o r e portion. In s o m e cases, it is possible to u p g r a d e m a r t i t e ores with a h i g h residual m a g n e t i t e content by using d r u m s of a s t r o n g e r field intensity. Dry high-intensity magnetic separators w i t h a field intensity of u p to 2.4 tesla are s o m e t i m e s used for w e a k l y m a g n e t i c ores, p r i m a r i l y for t h o s e with a m e a n size r a n g e of 0.05—2 m m . U n d e r f a v o u r a b l e conditions, a concentration to 6 5 - 6 6 % iron is possible. A n i m p o r t a n t d e v e l o p m e n t of the past twenty years is the h i g h - i n t e n s i t y wet magnetic separator. It permits the u p g r a d i n g of relatively finely g r o u n d s p e c u l a r i r o n ores in units having a capacity of 1 0 0 - 1 5 0 t / h . As r e g a r d s the fineness of the ores to be treated, its a p p l i c a t i o n range is partly identical w i t h t h a t of flotation. In s o m e cases the f l o t a t i o n c o n c e n t r a t e processing p r o b l e m s can t h u s b e obviated w h e n this s e p a r a t o r t y p e is used. U p to now, this s e p a r a t o r t y p e is being largely e m p l o y e d for t h e b e n e f i c i a t i o n of Brazilian l o w - g r a d e itabirite w e a t h e r e d ores. 3.2.1.5 Magnetizing Roasting Even at a very early stage, a t t e m p t s w e r e m a d e to b e n e f i c i a t e h e m a t i t e bearing ores by converting the h e m a t i t e first to m a g n e t i t e by r e d u c t i o n and by s e p a r a t i n g the m a g n e t i t e f r o m the g a n g u e in m a g n e t i c s e p a r a t o r s . Shaft furnaces, rotary kilns a n d f l u i d - b e d roasters can b e used f o r reduction. A c c o r d i n g to the density a n d p o r o s i t y of t h e o r e t o b e t r e a t e d , the ore is c h a r g e d at a grain size of u p to 20 m m . G a s e o u s , l i q u i d a n d solid fuels are u s a b l e as h e a t i n g a n d r e d u c t i o n energy. Solid fuels can b e employed w h e n they a r e sufficiently reactive, e.g. lignite. D u e to t h e relatively h i g h f u e l costs, this process is p r e f e r a b l y u s e d w h e n c h e a p energy is a v a i l a b l e or in c o n j u n c t i o n w i t h o t h e r processes. U p to the fifties, a p l a n t for the t r e a t m e n t of d o g g e r ores with a t h r o u g h p u t of 10000 t p d 37) was in o p e r a t i o n in t h e Salzgitter district i n G e r m a n y . Large-sized plants are presently o p e r a t i n g in the Soviet U n i o n and Australia. In a p l a n t for the processing of n i c k e l - c o n t a i n i n g ores,

60

3 Raw Materials and Their Preparation for Pellet Production

o p e r a t e d in C a n a d a , artificial m a g n e t i t e is o b t a i n e d as is also the case in a plant for the r o a s t i n g of pyrite cinders in Italy. Large-sized p i l o t plants were o p e r a t e d b y Oliver M i n i n g a n d H a n n a M i n i n g Co. in t h e M e s a b i range f o r several years. H o w e v e r , o w i n g to excessive costs, n o i n d u s t r i a l plants w e r e erected in this area. A c c o r d i n g to this process, very h i g h - g r a d e concentrates with a n i r o n content of u p to 70% could be p r o d u c e d . O n a c c o u n t of its h i g h reactivity, the artificial m a g n e t i t e a l r e a d y oxidizes at very low t e m p e r a t u r e s of a r o u n d 300 ° C. T h e fact that it reacts very quickly m u s t b e considered d u r i n g h e a t i n g a n d o x i d a t i o n of g r e e n pellets f r o m artificial m a g n e t i t e . 3.2.1.6 Electrostatic Separation In the m a g n e t i c field, m i n e r a l m i x t u r e s m a y s h o w a d i f f e r e n t b e h a v i o u r a c c o r d i n g to t h e i r m a g n e t i z a b i l i t y so that t h e s e p a r a t i o n of m a g n e t i c and n o n - m a g n e t i c solids is based on this p r o p e r t y . S i m i l a r t e n d e n c i e s can also be observed in a n electrically-loaded field w h e n electrically c o n d u c t i v e a n d n o n - c o n d u c t i v e m i n e r a l m i x t u r e s are i n t r o d u c e d . By c h a r g i n g the m i n e r a l surfaces a n d s u b s e q u e n t l y discharging t h e c o n d u c t i v e particles, m i g r a t i o n of the c o n d u c t o r s in one d i r e c t i o n a n d of t h e n o n - c o n d u c t o r s in the o p p o s i t e d i r e c t i o n can be achieved in a n electric field. C o n d u c t o r s and n o n - c o n d u c t o r s c a n t h u s b e s e p a r a t e d f r o m each other. T h i s p r i n c i p l e is also a p p l i c a b l e to i r o n - c o n t a i n i n g m i n e r a l s of d i f f e r e n t conductivity or to a m i x t u r e of t h e s a m e with o t h e r i r o n - f r e e m i n e r a l s . This process is particularly s u i t a b l e for s e c o n d a r y cleaning of m a g n e t i c concentrates w h i c h are c o n t a m i n a t e d by i r o n - c o n t a i n i n g m i n e r a l s , such as h o r n b l e n d e , g a r n e t a n d m i c a or f o r s e c o n d a r y cleaning of gravimetrically p r o d u c e d concentrates. A typical e x a m p l e is t h e W a b u s h L a k e b e n e f i c i a t i o n plant. In this plant, spiral concentrates are s u b j e c t e d to secondary electrostatic c l e a n i n g a f t e r p r e v i o u s drying. A l s o in the case of m a g n e t i c concentrates, as f o r e x a m p l e in a p l a n t b u i l t in N o r t h e r n Sweden, a s e c o n d a r y electrostatic cleaning is carried out. T a b l e 9 shows the results of t h e principal b e n e f i c i a t i o n processes, analyses of r a w ores a n d their concentrates. A c c o r d i n g to this table, several suitable processes are available in w h i c h a high iron c o n t e n t of the b e n e f i c i a t i o n p r o d u c t s is o b t a i n e d . A second i m p o r t a n t task of pellet p r e p a r a t i o n is to ensure a s u i t a b l e size d i s t r i b u t i o n of t h e r a w materials f o r ball f o r m a t i o n . 3.2.1.7 Proportion of Different Ores in Pellet Production U p to a b o u t 1962, m a g n e t i t e h a d a l m o s t exclusively b e e n processed i n t o pellets and this p r i m a r i l y in the M e s a b i R a n g e . O n l y f r o m t h a t d a t e o n

Table 9. Chemical composition of pellets and concentrates produced from crude ores according to various beneficiation methods Methods of Benefication Types of Ores Plant/Location

Stage of Beneficiation

Analysis * Fe total

SiO2

Al 2 O 3

CaO

MgO

L.O.I.

2.4 1.8 1.9

2.7 2.0 2.2

n.d. n.d. 0.5

n.d. n.d. n.d.

4.0 3.7 -

n.d. 0.34 0.11

bivalent

Crude Concentr. Pellet

62.7 64.2 66.0

Gravimetry Hematite (Lac Jeanine Canada)

Crude Spiral Concentr. Pellet

38.0 64.93 65.05

n.d. n.d.

40.5 5.62 5.64

n.d. 0.21 0.30

n.d. 0.20 0.20

n.d. 0.18 0.17

Flotation and flocculation Hematite (Tilden Mine USA)

Crude Concentr. Pellet

36.0 66.0 65.0

n.d.

45.0 5.0 5-7

n.d.

n.d.

n.d.

0.7

n.d.

n.d.

Magnetic separation (Low-intensity) Magnetite

Crude Concentr. Pellet

42.1 69.4 66.8

12.5 22.1 0.5

14.7 1.1 1.0

4.4 0.5 0.5

2.8 0.5 0.5

2.3 0.1 0.0

Magnetizing roasting Low-intensity separation Jaspilite (Whyalla Australia)

Crude Concentr. Pellet

36.4 66.9 65.0

n.d. n.d. 0.35

n.d. n.d.

0.26

Wet magnetic separation High-intensity Hematite (CVRD Brasil)

Crude Concentr. Pellet

1.0 45.9 n.d. 20.2 5.6 0.8 5.4 1.0 0.7 56.4 0.2 15.4 1.3 68.3 0.2 0.6 0.7 Used in mixture with other ores

n.d. n.d.

n.d. n.d.

1.8 1.2

Gravimetry and

Crude Concentr. I

36.0 61.4

n.d. n.d.

n.d. n.d.

n.d. n.d.

n.d. n.d.

n.d. n.d.

Electrostatic separation Magnetite/ Hematite (Wabush Mine Canada)

Concentr. II Pellet

66.3 65.4

n.d. n.d.

n.d. 0.27

n.d. 0.1

n.d. 0.1

n.d.

41.4 5.8 2.8 2.97

0.3

61

* Analysis of pellets incl. additives or binders

1.8 1.9

3.2 Preparation of Raw Materials for Pelletizing

Washing Hematite/ Limonite (Goa India)

62

3 Raw Materials and Their Preparation for Pellet Production

were o t h e r ores increasingly used for pellet p r o d u c t i o n , such as h e m a t i t e concentrates ( M a r q u e t t e R a n g e , U S A ) , m a g n e t i t e - h e m a t i t e m i x t u r e s ( U S A a n d C a n a d a ) or h e m a t i t e containing h i g h - g r a d e ores (Brazil, Australia). L i m o n i t e - c o n t a i n i n g h e m a t i t e was successfully pelletized on an industrial scale even at a later date, f r o m a b o u t 1966. ( G o a , R o b e River, W e s t e r n Australia.) Table 10. Proportion of main ore types used in pellet plants in 1978 Ore types Magnetite Hematite Magnetite/Hematite Hematite/Limonite

106 tons/year

%

156.0 62.0 47.0 28.0

53.4 21.1 16.0 9.5

293.0

100.0

O r e m i x t u r e s were treated as late as a b o u t 1 9 6 8 - 7 0 . F o r this p u r p o s e , the technological c o n d i t i o n s h a d first to be e l a b o r a t e d . T a b l e 10 gives a survey of the ore supply for pellet plants t h r o u g h o u t t h e world. It also contains a l r e a d y the figures for the plants w h o s e s t a r t - u p is s c h e d u l e d for 1978. F u r t h e r m o r e , this table represents the percentages of t h e v a r i o u s ore types b e i n g used for pellet p r o d u c t i o n u p to this date.

3 . 2 . 2 Physical Properties of Fine-Grained Iron Ores T h e physical p r o p e r t i e s of fine-grained i r o n ores a r e t h e m o s t influencing factors for green ball f o r m a t i o n , see items 2.1, 4.3.1, a n d 5.1.1. S o m e of t h e m are variable, others not. By controlling a n d i n f l u e n c i n g t h e v a r i a b l e factors t h e pellet quality can b e i m p r o v e d . Such v a r i a b l e factors are t h e following: — grain size a n d size d i s t r i b u t i o n — grain s h a p e — s u r f a c e c o n d i t i o n (e.g. p e a k — to — valley height, specific s u r f a c e area) — porosity — p o r e types. 3.2.2.1 Size Distribution - Specific Surface Area T h e r e is n o d e f i n i t e i n f o r m a t i o n available o n the i d e a l physical p r o p erties for g r e e n ball f o r m a t i o n . In practical o p e r a t i o n , a suitable size d i s t r i b u t i o n is empirically ascertained. S u b s e q u e n t l y , o n e a t t e m p t s to m a k e this size d i s t r i b u t i o n r e p r o d u c i b l e by a precise description. W i t h c e r t a i n

3.2 Preparation of Raw Materials for Pelletizing

63

limitations this leads to the desired success. A n i m m e n s e d i f f i c u l t y is t h a t the ore to b e pelletized u n d e r g o e s f l u c t u a t i o n s d u e to its r a w m a t e r i a l d e p e n d e n t properties. In m a n y cases, h o m o g e n i z i n g processes are at present a p p l i e d to e q u a l i z e the ore p r o p e r t i e s to m a x i m u m extent. A f u r t h e r p r o b l e m is caused by t h e d i f f i c u l t y in p e r f o r m i n g a n a c c u r a t e m e a s u r e m e n t of size distribution. T h i s is c o m p r e h e n s i b l e b e c a u s e t h e m a j o r part of t h e grains of a pelletizable g r o u n d p r o d u c t is b e l o w t h e usual size r a n g e of test screens. T o avoid a n excessive a m o u n t of w o r k a n d time, only the coarse g r a i n size p o r t i o n d o w n to 0.037 or 0.045 m m is d e t e r m i n e d b y d r y or wet screening, a n d the p o r t i o n b e l o w this size r a n g e which m a y cover 6 5 - 9 5 % by w e i g h t is a s c e r t a i n e d b y a d o p t i n g a n o t h e r method. A r a t h e r s u i t a b l e criterion for this p u r p o s e is t h e d e t e r m i n a t i o n of t h e specific s u r f a c e a r e a (see i t e m 4.3.2.). H o w e v e r , in this case also, t h e m e a s u r e m e n t p r o b l e m s h a v e to be t a k e n into account. T h e usual m e t h o d s are those in w h i c h the p e r m e a b i l i t y of a s a m p l e p r e p a r e d according to exact specifications is m e a s u r e d . By c o m p a r i s o n with s t a n d a r d samples, the size of the specific s u r f a c e area of the s a m p l e is i n f e r r e d a n d this size is indicated in c m 2 / c m 3 or c m 2 / g or m o r e s e l d o m in m 2 / k g . A c o m p a r i s o n of different m e a s u r i n g m e t h o d s , particularly w h e n the gas a d s o r p t i o n the m e a s u r e d values w h i c h m a y be u p to m u l t i p l e v a l u e s of o t h e r methods. H o w e v e r , in practical o p e r a t i o n , it h a s p r o v e d t h a t s i m p l e p e r m e a b i l i t y m e a s u r e m e n t s , which c a n be q u i c k l y c a r r i e d out, can b e standardized to such a n extent t h a t their r e p r o d u c i b i l i t y is s u f f i c i e n t f o r process control. A c c o r d i n g to experience the s p e c i f i c s u r f a c e a r e a a l m o s t exclusively d e p e n d s on the p r o p o r t i o n of particles, p r i m a r i l y of t h o s e a r o u n d 0.001 m m .

Fig. 28. Grinding energy consumption versus specific surface

processes

(e.g. B. E

64

3 Raw Materials and Their Preparation for Pellet Production

O r e " g r i n d i n g " s h o u l d be carried o u t as " s u r f a c e g r i n d i n g " in contrast to " l i b e r a t i o n g r i n d i n g " used for u p g r a d i n g . T h e values f o u n d o u t by m e a s u r i n g t h e specific s u r f a c e are d e p e n d e n t o n t h e t y p e of iron m i n e r a l s . T h e usual values for specular iron ores are 1 6 0 0 - 1 8 0 0 c m 2 / g a n d 1600—2300 c m 2 / g f o r m a g n e t i t e concentrates. If l i m o n i t e p o r t i o n s a r e Pyrite cinders are of a similar o r d e r of m a g n i t u d e . H o w e v e r , the m e a s u r e d values for m a g n e t i t e a n d specular i r o n ore are d e p e n d e n t on the s u r f a c e shape. M a g n e t i t e tends to disintegrate into c u b i c particles d u r i n g g r i n d i n g while specular i r o n ore o f t e n yields flat, m i c a - l i k e f r a g m e n t s . In t h e case of specular iron ore, the s u r f a c e values m e a s u r e d a c c o r d i n g to the p e r m e a bility m e t h o d thus d i f f e r m u c h m o r e f r o m t h e real s u r f a c e t h a n f o r magnetite. T h e size of the specific s u r f a c e a r e a highly influences t h e grinding energy c o n s u m p t i o n as can b e seen f r o m Fig. 28.

3.2.3

Grinding

Concentrates f r o m d i s s e m i n a t e d m a g n e t i t e ores are, in m o s t cases, already o b t a i n e d at a grain size s u i t a b l e f o r pelletizing w h e r e a s in o t h e r cases this g r a i n size has to be achieved b y grinding. T h e g r i n d i n g units used consist of t u m b l e mills with a g r i n d i n g m e d i a c h a r g e c o m p o s e d of steel balls, except for s o m e cases in w h i c h g r i n d i n g rods or ore f r a g m e n t s ( " p e b b l e s " ) are used. N o r m a l l y , t h e ore is f e d to the mills as a p u l p with a solids c o n t e n t of 60—80% by w e i g h t (wet grinding). N o w a d a y s t h e dry grinding method is a d o p t e d for a b o u t 1 5 - 2 0 % of the capacity of p l a n t s u n d e r construction and in o p e r a t i o n . T h i s g r i n d i n g process is a p p l i e d if wet g r o u n d o r e c a n n o t be d e w a t e r e d or not sufficiently by v a c u u m filtration. Particularly in t h o s e cases in w h i c h p u r c h a s e d ores p r e d o m i n a t e the d r y g r i n d i n g m e t h o d is r e c o m m e n d e d to ensure a flexible ore supply. T h e grinding can be a c h i e v e d in an o p e n circuit (single passage t h r o u g h the mill) or in a closed circuit. In the latter case, hydrocyclones (wet grinding) or air classifiers (dry grinding) are a l m o s t exclusively u s e d f o r classification of mill discharge. T h e decision as to w h e t h e r the closedcircuit or o p e n - c i r c u i t g r i n d i n g m e t h o d is to b e a d o p t e d is t a k e n empirically o n the basis of grinding a n d pelletizing tests. Figs. 29 a n d 30 d i a g r a m m a t i c a l l y illustrate t h e d i f f e r e n c e s b e t w e e n t h e two systems. In most cases, the a m o u n t of energy r e q u i r e d for g r i n d i n g is, r e f e r r i n g to the s a m e ore type, t h e lowest f o r wet closed-circuit g r i n d i n g a n d the h i g h e s t for dry open-circuit grinding. T h e ratio of these values to each o t h e r differs f r o m o n e ore type to the other. N o d e f i n i t e rule can be given. A s regards the capital costs, the wet o p e n - c i r c u i t g r i n d i n g is the c h e a p e s t while the dry closed — circuit g r i n d i n g necessitates t h e greatest expense.

included

3.2 Preparation of Raw Materials for Pelletizing

Figs. 29/30. Wet grinding in open and closed circuit

Fig. 31. Two alternatives for dry grinding

65

66

3 Raw Materials and Their Preparation for Pellet Production

H o w e v e r , w h e n considering the total g r i n d i n g costs, i n c l u d i n g c a p i t a l investment, t h e d r y g r i n d i n g m e t h o d , Fig. 31, is m o r e s u i t a b l e for e a r t h y h e m a t i t e ores since, in this case, the w e a r costs are substantially lower d u e to the lower c o n s u m p t i o n of ball p e b b l e s a n d mill lining t h a n f o r wet grinding. In this way, the costs for o r e drying, w h i c h is carried out b e f o r e or d u r i n g grinding, are m o r e t h a n c o m p e n s a t e d .

3.2.3.1 Dewatering W e t g r o u n d ores m u s t be d e w a t e r e d b e f o r e pelletizing in such a way that their m o i s t u r e content is identical, with, or slightly below, t h a t of the moisture r e q u i r e d for g r e e n ball f o r m a t i o n . T h e c o r r e s p o n d i n g v a l u e is a b o u t 8 . 5 - 1 0 % moisture, according to ore t y p e a n d g r i n d i n g fineness. Ores w h i c h are wet g r o u n d in closed-circuit a r e first o b t a i n e d as a p u l p with a b o u t 1 5 - 2 0 % solids b y weight. In circular thickeners, the solids content is raised to a b o u t 6 0 - 6 5 % b y weight. R o t a r y v a c u u m filters, p r e d o m i n a n t l y disc-type filters are u s e d for f u r t h e r d e w a t e r i n g to ensure a m a x i m u m c a p a c i t y at a m i n i m u m space. Before the o r e p u l p is f e d to the filter, it is a d j u s t e d to an empirically ascertained o p t i m u m density. T h i s density c o r r e s p o n d s to a solids content of a b o u t 5 0 - 5 5 % by weight for m a g n e t i t e a n d 5 5 - 6 5 % for s p e c u l a r iron ore. Filter discs m a d e f r o m metal, h a r d rubber, or plastic, have a f a b r i c layer as filter m e d i u m . U p to now, m o n o f i l e plastic f a b r i c s h a v e p r e v a i l e d . N o w a d a y s , relatively dense multifiles a n d p a r t l y felt-like plastic f a b r i c s are increasingly p r e f e r r e d . T h e filters c o m p r i s i n g u p to 12 discs h a v e a m a x i m u m suction area of 100 m 2 at a disc d i a m e t e r of a b o u t 3 m. L a r g e r filters with a m a x i m u m d i a m e t e r of 5.3 m h a v e m e a n w h i l e been d e v e l o p e d and will possibly be used in c o m m e r c i a l plants in the n e a r f u t u r e . A v a c u u m of a b o u t 85%, r e f e r r e d to a t m o s p h e r i c pressure, is necessary for a sufficient dewatering. T h i s v a c u u m is p r o d u c e d in water-ring or rotaryp i s t o n - t y p e - p u m p s . A capacity of a b o u t 1 4 0 - 2 0 0 m 3 / h air is r e q u i r e d p e r m 2 filter area. In the case of iron ore, t h e v a c u u m filter capacity is 0.4—1.5 t / h p e r m 2 filter area. S o m e ores c a n n o t be sufficiently d e w a t e r e d by f i l t r a t i o n for ball f o r m a tion d u e to their mineralogical c o m p o s i t i o n . In s o m e f o r m e r plants, the filter cake is, t h e r e f o r e , subjected to drying in drum dryers. T h i s has detrimental c o n s e q u e n c e s for the pellet q u a l i t y b e c a u s e t h e d r u m d i s c h a r g e consists a l m o s t completely of micro-pellets of a b o u t 0 . 5 - 3 m m f r o m w h i c h perfect green balls can h a r d l y b e p r o d u c e d . F o r this reason, in s o m e m o d e r n plants the filter cake f o r m e d on the filter disc is h e a t e d to a h i g h e r t e m p e r a t u r e by using s t e a m instead of air. T h e decrease of water viscosity resulting t h e r e f r o m is sufficient to i m p r o v e satisfactorily the filtration. In

3.2 Preparation of Raw Materials for Pelletizing

67

principle, t h e s a m e e f f e c t could also b e a c h i e v e d b y a c o r r e s p o n d i n g t e m p e r a t u r e increase of the p u l p b u t the relevant h e a t c o n s u m p t i o n w o u l d be higher. Finally, t h e e n d e a v o u r s to i m p r o v e f i l t r a t i o n by filter aids s h o u l d b e referred to. F o r m e r a t t e m p t s failed b e c a u s e t h e reagents k n o w n at t h a t time adversely a f f e c t e d the green ball f o r m a t i o n . T o d a y the c h e m i c a l industry s e e m s to h a v e reagents w h i c h d o not s h o w such u n d e s i r a b l e secondary effects so that a n alternative to s t e a m filtration m a y soon b e available.

4 The Pelletizing Laboratory and its Tasks

The f u n d a m e n t a l d e m a n d for indurated pellets is their uniformly good quality irrespective of the raw material f r o m which they were p r o d u c e d , of the p u r p o s e for which they are used and of the system according to which they are made. D u e to different raw material properties it is necessary to ensure in tests the required pellet quality. This can be done in laboratories adequately equipped. Above all, in the design of new pelletizing plants it is imperative to convert the results of tests o b t a i n e d in the laboratories into technological and constructional basic data. Combined with sufficient experience available f r o m industrial plants, one can establish reliable and convertible data in the laboratory.

4.1 Application Range of Laboratories Laboratories in which the stages starting f r o m the ore up to the testing of indurated pellets can be investigated are i m p o r t a n t for: (a) Pelletizing Plant Constructors. F o r engineering companies building pelletizing plants, a well-equipped pelletizing research center is imperative. Careful test and research work is important f o r a reliable plant design and for the guarantees regarding quality, capacity a n d energy consumption. (b) Pellet Producers. T h e greatest n u m b e r of pelletizing plants is located near the ore deposit. However, it is seldom that an orebody is so homogeneous that the original design data r e m a i n s valid forever. W h e n the orebody undergoes any changes (particularly hematite-limonite possible benefication and for the induration process. These parameters can be f o u n d out b e f o r e h a n d in laboratory tests so that necessary process variations can be quickly m a d e at a constant quality without great production losses. Such investigations are also important if the pellet quality has to be changed f r o m for example blast furnace to direct

ores) other technological

4.2 The Tasks of a Laboratory

69

reduction pellets f o r w h i c h o t h e r m e t a l l u r g i c a l p r o p e r t i e s are o f t e n d e m a n d e d . S u c h l a b o r a t o r y tests are of p a r t i c u l a r i m p o r t a n c e for pelletizing plants in w h i c h m i x e d ores are treated. (c) Pellet Consumers. P u r c h a s e r s of pellets are p r i m a r i l y the blast f u r n a c e and the direct r e d u c t i o n p l a n t o p e r a t o r s . T h e y o f t e n h a v e l a b o r a t o r i e s in which mostly i n d u r a t e d pellets are tested. (d) Universities, Mining Schools as well as often have only specialized e q u i p m e n t . T h e and e q u i p p e d a c c o r d i n g to and in c o n f o r m i t y required. T h e institutions r e f e r r e d to u n d e r a plete scope of relevant facilities.

Independent and Public laboratories can be adapted w i t h the p u r p o s e of the tests a n d b s h o u l d h a v e the c o m -

4.2 The Tasks of a Laboratory F r o m the processing of the ore u p to the p r o d u c t i o n of t h e f i n i s h e d i n d u r a t e d pellet, the following f u n c t i o n s are in general to b e fulfilled: — Choice of s u i t a b l e b e n e f i c a t i o n processes f o r d i f f e r e n t ores — D e t e r m i n a t i o n of the o p t i m u m g r i n d i n g of all r a w m a t e r i a l s — Choice a n d use of b i n d e r s or metallurgically e f f i c i e n t a d d i t i v e s — O p t i m u m g r e e n pellet c o m p o s i t i o n — E l a b o r a t i o n of s u i t a b l e firing p a t t e r n s f o r d e t e r m i n a t i o n of t h e necessary pellet q u a l i t y — D i m e n s i o n i n g of the m o s t i m p o r t a n t process units — D a t a on t h e energy c o n s u m p t i o n to b e expected. Essential p r e r e q u i s i t e s to the d e t e r m i n a t i o n of all the necessary d a t a are: (a) the use of representative r a w materials. A c c o r d i n g to the p u r p o s e of tests, s o m e h u n d r e d s of k i l o g r a m s to several tons of testing m a t e r i a l m a y be r e q u i r e d ; (b) f u r t h e r m o r e , such e q u i p m e n t , facilities a n d f u r n a c e s w h i c h allow a conversion of t h e results thus a s c e r t a i n e d t o i n d u s t r i a l plants are to be used. Fig. 32 s h o w s a testing p r o g r a m m e a c c o r d i n g to w h i c h v a r i o u s iron ores and additives c a n be processed u p to t h e finished i n d u r a t e d pellet. A c cording to the p r e p a r a t i o n stage of the ore, s o m e process steps m a y be omitted. A b o v e all, for the design a n d construction of a c o m p l e t e p l a n t beginning w i t h t h e r a w o r e a n d e n d i n g with the f i n i s h e d pellet, it is advisable to i n c o r p o r a t e the b e n e f i c i a t i o n , p a r t i c u l a r l y t h e fine g r i n d i n g , into the l a b o r a t o r y cycle.

Research

Institution

70

4 The Pelletizing Laboratory and its Tasks

Fig. 32. Process flow sheet from ore to burned pellets

4.3 Raw Material Preparation and Pellet Production 4.3.1 Raw Material Preparation Analogously to the c o m m e n t s given u n d e r i t e m 3.2, a n a d e q u a t e l y e q u i p p e d pelletizing l a b o r a t o r y s h o u l d also i n c l u d e the most i m p o r t a n t facilities a n d e q u i p m e n t for the b e n e f i c i a t i o n of ores. (a) Washing of Ores. F i n e l y d i s s e m i n a t e d ores s h o u l d be g r o u n d to m i n u s 0.5 m m . In a p r e l i m i n a r y test, a small q u a n t i t y of ore is slurried in an agitating t a n k a n d its suspension b e h a v i o u r is e x a m i n e d . A f t e r a certain settling time, t h e suspension is s e p a r a t e d f r o m t h e s e d i m e n t e d solids. A f t e r weighing a n d c h e m i c a l analysis of the s e p a r a t e d solids portion, a m e t a l b a l a n c e is to b e established. In semi-industrial tests for t h e p r o d u c t i o n of m a j o r concentrate a m o u n t s w a s h i n g a p p a r a t u s of types w h i c h are generally used in i n d u s t r i a l plants, such as w a s h i n g d r u m s , spiral classifiers, wet screens a n d hydrocyclones are utilised. A f l o w s h e e t b a s e d o n a test with a n earthy, l i m o n i t e - c o n t a i n i n g h e m a t i t e f r o m G o a is described in Fig. 33. In this case the iron c o n c e n t r a t i o n effect was of s e c o n d a r y interest while t h e decrease of the a l u m i n a content was m o r e i m p o r t a n t .

r 71 3 RawMaterialsandTheirPreparation for Pellet Production

Fig. 33. Flow sheet and material balance in a pilot-scale washing plant for earthy hematite

(b) Gravity Methods. T h e characteristics of t h e ores are firstly a s c e r t a i n e d with only m i n o r quantities. By h e a v y - m e d i a s e p a r a t i o n in o r g a n i c l i q u i d s of a higher density t h a n water, the g a n g u e is s e p a r a t e d f r o m the ironbearing m a t e r i a l . T h e density of the h e a v y m e d i a — generally a r o u n d 2.8 g / c m 3 f o r i r o n ores - is a d j u s t e d to o p t i m u m selectivity. T h e s e laboratory tests are to be c o n f i r m e d by e x p e r i m e n t s on a s e m i - i n d u s t r i a l scale. S u i t a b l e e q u i p m e n t for such tests is a v a i l a b l e o n the m a r k e t . (c) Flotation. D u r i n g f l o t a t i o n the ore s u r f a c e is, in d i f f e r e n t ways, altered by using s p e c i f i c chemicals. T h e effect of i n d i v i d u a l c h e m i c a l s is s t u d i e d . Since, in c o n t r a s t to m a n y physical b e n e f i c i a t i o n processes, t h e t i m e f a c t o r plays a certain role d u r i n g flotation, the next testing p h a s e is c a r r i e d o u t in a continuously o p e r a t i n g pilot plant. O n this occasion, not only the t i m e factor but also t h e i n f l u e n c e of m i d d l i n g s in internal circuits o n the process are investigated. S u c h tests on a larger scale a r e the p r e r e q u i s i t e s to t h e eleboration of a suitable flowsheet. W i t h a v i e w to a g o o d filtrability of t h e concentrates a n d , if necessary, a n efficient w e t t a b i l i t y of the g r a i n s u r f a c e s d u r i n g pellet p r o d u c t i o n , the choice of reagents is of i m p o r t a n c e . (d) Magnetic Separation. A c c o r d i n g to their b e h a v i o u r in a m a g n e t i c field, the minerals c a n be d i v i d e d into t h r e e classes, n a m e l y highly-, weakly-, and n o n - m a g n e t i c minerals. S o m e m i n e r a l s i m p o r t a n t f o r pelletizing a r e

72

4 The Pelletizing Laboratory and its Tasks

specified b e l o w 3 8 ) , with the b e h a v i o u r of i r o n in a m a g n e t i c field serving as a criterion: iron magnetite titanomagnetite pyrrhotite siderite hematite limonite quartz dolomite calcite

= 100 = 40.2 = 24.7 6.7 = = 1.8 = 1.3 = 0.8 = 0.37 0.22 = = 0.03

highly magnetic

weakly magnetic

non magnetic

In c o n f o r m i t y w i t h these properties, low-intensity m a g n e t i c s e p a r a t o r s are used for h i g h l y m a g n e t i c ores a n d h i g h - i n t e n s i t y m a g n e t i c s e p a r a t o r s for weakly m a g n e t i c ores. F o r the d e t e r m i n a t i o n of t h e l i b e r a t i o n size, the ores are g r o u n d to d i f f e r e n t fineness. T h e D a v i s t u b e , w h i c h a l r e a d y with a few g r a m s of m a t e r i a l gives representative results a n d g u i d e values for f u r t h e r tests o n a larger scale, is suitable for highly magnetic ores. Such tests are carried out with e q u i p m e n t having a capacity of a b o u t 1— 3 t / h o u r . Mostly d r u m s e p a r a t o r s are used. T h e s e separators are n o r m a l l y e q u i p p e d with p e r m a n e n t m a g n e t i c systems w h o s e field intensity, m e a s u r e d o n the m a g n e t surface, is a b o u t 0.2 Tesla. A d r u m s e p a r a t o r is used for dry s e p a r a t i o n while a l i f t o u t s e p a r a t o r is e m p l o y e d f o r w e t s e p a r a t i o n . Both types c a n b e a r r a n g e d in a mass f l o w circuit so that t h e d a t a thus o b t a i n e d is a l r e a d y valid for the design of industrial plants. S u b s e q u e n t l y , r e p r e s e n t a t i v e pelletizing tests are r u n o n the m a j o r c o n c e n t r a t e quantities t h u s p r o d u c e d . Fig. 34 shows, for e x a m p l e , the b e n e f i c i a t i o n flowsheet f o r a m a g n e t i t e ore. H o w e v e r , if weakly m a g n e t i c ores ( h e m a t i t e s , limonites) are to be u p g r a d e d , substantially stronger m a g n e t i c fields of u p to 2.5 Tesla are necessary. T h e s e separators are, t h e r e f o r e , called high-intensity magnetic separators. In this case, a test with small a m o u n t s of m a t e r i a l c a n yield first d a t a o n the field intensity r e q u i r e d . F o r the p r e p a r a t i o n of flowsheets, a p p a r a t u s o n a semi-industrial scale with a t h r o u g h p u t of several h u n d r e d kilograms p e r h o u r are used. In the event of d i s s e m i n a t e d ores with an a d e q u a t e g r i n d i n g degree, wet s e p a r a t o r s are m o s t suitable. D r y s e p a r a tors are e m p l o y e d for m o r e coarsely i n t e r g r o w n ores. (c) Magnetizing Roasting. L e a n h e m a t i t e b e a r i n g ores are r e d u c e d to m a g n e t i t e u n d e r a weakly r e d u c i n g a t m o s p h e r e . T h i s process calls for

73 3 RawMaterialsandTheirPreparation for Pellet Production

Fig. 34. Alternative routes for wet magnetic separation of magnetite ore in a pilot plant

relatively h i g h processing costs so that it is only used rarely o n a n industrial scale. Intermittently o p e r a t i n g small r o t a r y kilns or f l u i d - b e d reactors are e m p l o y e d for the p e r f o r m a n c e of o r i e n t i n g tests in w h i c h the principal characteristics are to be d e t e r m i n e d . B e f o r e t h e construction of an industrial p l a n t , pilot plants are generally erected a n d o p e r a t e d o v e r a period of at least several m o n t h s . ( f ) Electrostatics. If this process is t a k e n i n t o c o n s i d e r a t i o n , t h e b e h a v i o u r of the m i n e r a l s in an electric field is first tested w i t h small q u a n t i t i e s . W i t h this process, the c o r r e s p o n d i n g p a r a m e t e r s m u s t b e closely o b s e r v e d . T h e o p e r a t i n g c o n d i t i o n s m u s t only vary w i t h i n very n a r r o w limits, t h e y have to be precisely ascertained in p r e v i o u s tests. It is also possible to condition t h e o r e grain s u r f a c e in o r d e r to increase t h e d i f f e r e n c e s in t h e conductivity of t h e v a r i o u s minerals.

4.3.2

Grinding

T h e m o s t i m p o r t a n t p r e r e q u i s i t e to green ball f o r m a t i o n is a s u f f i c i e n t fineness a n d a s u i t a b l e size d i s t r i b u t i o n of t h e raw m a t e r i a l s . W i t h t h e exception of concentrates, w h i c h o f t e n are a l r e a d y o b t a i n e d in a s u f f i c i e n t fineness f r o m t h e b e n e f i c i a t i o n p l a n t , n a t u r a l ores, coarser c o n c e n t r a t e s

74

4 The Pelletizing Laboratory and its Tasks

and additives h a v e to be g r o u n d or r e - g r o u n d . T h e g r i n d i n g tests carried out in the l a b o r a t o r y h a v e virtually two p u r p o s e s : (a) P r e p a r a t i o n of g r i n d i n g s a m p l e s with a r e p r e s e n t a t i v e size r a n g e for the p e r f o r m a n c e of pelletizing tests. (b) D e t e r m i n a t i o n of t h e m o s t e c o n o m i c m e t h o d t a k i n g the capital investment, energy c o n s u m p t i o n a n d w e a r i n t o c o n s i d e r a t i o n with the d e m a n d f o r h i g h - g r a d e pellets b e i n g of greatest i m p o r t a n c e . T h e r e are f o u r variants for the p e r f o r m a n c e of fine grinding: — O p e n - c i r c u i t d r y g r i n d i n g in ball mills, - Closed-circuit dry grinding, - O p e n - c i r c u i t w e t grinding, — Closed-circuit wet g r i n d i n g (see item 3.2.2.2). In all cases p r e c r u s h i n g to m i n u s 6 m m is r e c o m m e n d a b l e , F i g u r e s 29 a n d 30. G r i n d i n g r o d s or p e b b l e s m a y also be u s e d instead of grinding balls. Pebbles are o r e f r a g m e n t s h a v i n g a d i a m e t e r of 4 0 - 8 0 m m . In the case of e a r t h y h e m a t i t e a n d p a r t i c u l a r l y limonite, only the dry grinding m e t h o d s h o u l d be a d o p t e d b e c a u s e a f t e r wet grinding, these ores cannot be sufficiently d e w a t e r e d by f i l t r a t i o n f o r green ball f o r m a t i o n . A n a d e q u a t e l y e q u i p p e d l a b o r a t o r y s h o u l d c o m p r i s e sufficiently d i m e n s i o n e d grinding test facilities f o r this p u r p o s e . T h i s particularly applies to pelletizing p l a n t constructors. In o r d e r to l i m i t the scope of test w o r k accordingly it is advisable to r u n pellet firing tests in parallel to the g r i n d i n g tests since o f t e n a f t e r a few tests p r e l i m i n a r y decisions can a l r e a d y be t a k e n . M o r e o v e r , the fineness has, of course, to b e d e t e r m i n e d in all g r i n d i n g tests. T w o m e t h o d s h a v e p r o v e d to b e successful b u t it is r e c o m m e n d e d to ascertain the fineness according to b o t h m e t h o d s : (a) Screen Analyses. N o r m a l l y , t h e sreening is achieved in a dry m a n n e r . In this connection, a screen cut of 0.045 m m = 325 m e s h has b e c o m e usual. In principle, the screen cut m a y also b e lower, d o w n to 0.032 m m . In this regard, the use of air-blown screens has p r o v e d to be suitable. A f t e r dry grinding, s o m e ores t e n d to f o r m loose agglomerates. In such cases, the wet screening m e t h o d is p r e f e r a b l e . A screen analysis is reliable for the particle size r a n g e b e l o w 0.02 m m . F o r the precise d e t e r m i n a t i o n of these very fine fractions in c o n n e c t i o n with the g r i n d i n g degree, it is a d d i t i o n a l l y necessary to ascertain t h e specific s u r f a c e a r e a (see i t e m 3.2.2.1). (b) Surface Determination. F o r this p u r p o s e , the m e t h o d according to Blaine, k n o w n f r o m t h e c e m e n t industry, was first t a k e n over a n d is n o w a d a y s a d o p t e d for pelletizing also. In s o m e laboratories, the Svenson m e t h o d is p r e f e r r e d .

75 3 RawMaterialsandTheirPreparation for Pellet Production N e i t h e r m e t h o d is q u i t e i n d e p e n d e n t of h u m a n errors, i. e. of t h e p e r s o n charged w i t h t h e p e r f o r m a n c e of t h e p e r t i n e n t test. In this respect, the m o r e m e c h a n i s e d s u r f a c e d e t e r m i n a t i o n with the Fisher-Subsieve-Sizer is less sensitive. E a c h m e t h o d yields r e p r o d u c i b l e results. H o w e v e r , t h e a b s o l u t e values o b t a i n e d , i n d i c a t e d in c m 2 / g o r in cm 2 /cm 3 , are not always identical. Before the p e r f o r m a n c e of screen analyses a n d s u r f a c e d e t e r m i n a t i o n s , a preparatory t r e a t m e n t of the s a m p l e s f o r the d e s t r u c t i o n of pseudo-agglomerates previous r e p u l p i n g in alcohol f o r dissolving t h e a g g l o m e r a t e s has also proved to b e s u c c e s s f u l 3 9 ) .

4 . 3 . 3 G r i n d i n g E q u i p m e n t and G r i n d i n g E n e r g y T o o b t a i n a size r a n g e r e q u i r e d for practical o p e r a t i o n in the g r i n d ing tests, it is r e c o m m e n d e d to use mills w i t h a m i n i m u m d i a m e t e r of 0.6 m w h e r e the r a t i o of length to d i a m e t e r s h o u l d b e a b o u t 1.5:1. F u r t h e r m o r e , c o n t i n u o u s grinding is to be given p r e f e r e n c e . In the case of closed-circuit grinding, care s h o u l d b e t a k e n t h a t the classifiers b e sufficiently d i m e n s i o n e d since o f t e n t h e y only e n s u r e a r e p r e s e n t a t i v e classification f r o m a c e r t a i n capacity o n w a r d w h i c h m a y involve the use of correspondingly greater mills. T h i s d e t a i l s h o u l d b e p a r t i c u l a r l y c o n s i d e r e d for wet g r i n d i n g with hydrocyclones as classifiers. T h e p e r f o r m a n c e of large-scale tests in mills with e.g. 1.0 m d i a m e t e r at a t h r o u g h p u t of 1—3 t / h is r e c o m m e n d e d for ascertaining the exact energy consumption. T h e t h r o u g h p u t is d e p e n d e n t o n the grindability of the ore a n d of the r e q u i r e d fineness. Consequently a larger ore quantity of u p to 5 0 - 1 0 0 t is r e c o m m e n d e d for such tests. In the event of a closed circuit, it takes a b o u t 3 hours for wet g r i n d i n g a n d a b o u t 4 h o u r s for dry g r i n d i n g until the system is in balance. N o r m a l l y , several tests h a v e to be r u n to o b t a i n d e p e n d a b l e results. T h e specific energy c o n s u m p t i o n for g r i n d i n g c a n b e d e t e r m i n e d by: M e a s u r i n g the t o r q u e at the s h a f t b e f o r e t h e m a i n drive p i n i o n or stopping t h e mill r o t a t i o n by using a p r o n y b r a k e or d e t e r m i n i n g t h e active p o w e r c o n s u m p t i o n o f the mill m o t o r . Such m e a s u r e m e n t s especially t h e t o r q u e m e t e r i n g yield very r e l i a b l e data for d i m e n s i o n i n g t h e g r i n d i n g e q u i p m e n t a n d f o r the d e t e r m i n a t i o n of the energy c o n s u m p t i o n . T a b l e 11 s h o w s test results which w e r e o b t a i n e d d u r i n g g r i n d i n g tests in large-scale p i l o t mills. In these tests, t h e o r e was g r o u n d in various g r i n d i n g systems to t h e same fineness. C o n s e q u e n t l y , the q u a l i t y of g r e e n a n d i n d u r a t e d pellets is roughly identical w i t h i n t h e scope of test accuracy.

by g

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4 The Pelletizing Laboratory and its Tasks

Table 11. Influence of various grinding methods on grinding energy and properties of hematite pellets Wet grinding

Dry grinding

Ball mill

Pebble mill

Circuit

Circuit

1. Specific surface area (cm 2 /g) 2. Grinding energy (Kwh/t) 3. Wet compression strength (N/pellet) 4. Drop number at 46 cm distance 5. Green pellet moisture content (%) 6. Dry compr. strength (N/pellet) 7. Compression strength of fired pellets (N/pellet) 8. Tumbler index 0.6 mm (%) 9. Swelling rates (JIS) (%) 10. Specific capacity of a travelling grate (t/m 2 24 h)

Ball mill

open

closed

1600

1600

1600

14.5 7.0

16.5 7.0

21.0 8.0

15.5 8.0

3.5 8.2

4.4 8.2

3.6 7.4

3.6 7.5

9.0

9.0

9.0

open

closed

1600

1600

18.0 7.0 4.0 8.5 14

14

3000

3500

3200

3600

3400

3.5 15 24

3.2 14 25

3.4 16 23

3.5 14 26.5

4.3 21 24

As regards t h e energy c o n s u m p t i o n , closed-circuit wet g r i n d i n g yields the lowest values. In the case of closed-circuit dry grinding, the energy c o n s u m p t i o n is almost the s a m e 40 ). In s o m e cases, t h e specific p o w e r c o n s u m p t i o n for g r i n d i n g m a y be d e t e r m i n e d with a certain precision by a d o p t i n g the B o n d m e t h o d 4 1 ) . But often, in the case of s u r f a c e grinding, this m e t h o d is not precise e n o u g h .

4.3.4

Filtration

W e t g r i n d i n g is f o l l o w e d by filtration in all cases in o r d e r to achieve a m o i s t u r e content of t h e filter cake w h i c h is at or b e t t e r b e l o w the o p t i m u m green pellet m o i s t u r e content. As concerns t h e filter cake m o i s t u r e c o n t e n t to be attained, small-scale tests with filter plates give indications. H o w e v e r , continuous f i l t r a t i o n test's with the c o r r e s p o n d i n g filter type, such as disc or d r u m - t y p e filters are r e c o m m e n d e d f o r d i m e n s i o n i n g the filter plant.

4.4 Mix Preparation for Ball Formation

77

In the event that the filter cake is difficult to dewater, hot steam filtration may be a d v a n t a g e o u s . A t h e r m a l s e c o n d a r y d r y i n g is not only expensive but also o f t e n leads, p r i m a r i l y w h e n d r u m - t y p e dryers are used, to u n desirable a g g l o m e r a t e s w h i c h m a y b r i n g a b o u t g r e a t d i f f i c u l t i e s d u r i n g green ball f o r m a t i o n . T h e s e a g g l o m e r a t e s h i n d e r : (a) the d e s i r e d close size range of green pellets f r o m b e i n g a c h i e v e d ; (b) a h o m o g e n e o u s m i x t u r e , p a r t i c u l a r l y w i t h a d d i t i v e s a f t e r drying. A f u r t h e r possibility f o r the s u b s e q u e n t r e g u l a t i o n of t h e filter c a k e moisture content is the use of d r y i n g additives. In this connection, t h e highly w a t e r - b o n d i n g b e n t o n i t e has, in p a r t i c u l a r , a c o n s i d e r a b l e i n f l u e n c e as has also c a l c i u m o x i d e ( C a O ) w h e n special m e a s u r e s are taken.

4.4 Mix Preparation for Ball Formation 4.4.1 M i x Preparation All f i n e - g r a i n e d c o m p o n e n t s for the pellet m i x , b i n d e r s or a d d i t i v e s a r e previously m i x e d intensively. A n o p t i m u m m i x i n g is, of course, a c h i e v e d during the c o m m o n g r i n d i n g of raw materials. H o w e v e r , possibly existing great differences in the grindability of ores o r a d d i t i v e s m u s t be considered. In the case of wet grinding, n o b e n t o n i t e c a n b e a d d e d since t h e swelling of b e n t o n i t e w o u l d adversely a f f e c t t h e filtrability. T o e n s u r e a h o m o g e n e o u s m i x , d i f f e r e n t m i x e r types are used. Small-sized experimental units of s o m e of these types exist. In l a b o r a t o r i e s b a t c h - t y p e positive mixers h a v e p r o v e d to be suitable. Since they o p e r a t e i n t e r m i t tently, the charge c o m p o n e n t s and m i x i n g t i m e c a n be precisely controlled. In these mixers, a p r e - w e t t i n g of the c o m p l e t e m i x to a m o i s t u r e c o n t e n t being a b o u t 1—2% b e l o w the o p t i m u m pellet h u m i d i t y is also achieved. T h e testing m a t e r i a l to be pelletized is t h e n to b e continuously and uniformly c h a r g e d to t h e balling e q u i p m e n t p r e f e r a b l y b y using a v a r i a b l e speed belt. In the event of m i x t u r e s t e n d i n g to the f o r m a t i o n of l u m p s , it is r e c o m m e n d e d to insert a f l u f f e r for f l u f f i n g any u n d e s i r a b l e c o n g l o m e r a t e s before t h e mix is f e d to the disc.

4.4.2 Green Ball F o r m a t i o n T h e balling d r u m a n d the balling disc h a v e f o r m a n y years p r o v e d to b e equally s u i t a b l e f o r g r e e n ball f o r m a t i o n . L ö f g r e n 4 2 ) c o m p a r e d the quality of green balls p r o d u c e d f r o m

Malmberget

magnetite co

78

4 The Pelletizing Laboratory and its Tasks

plants a n d d e t e c t e d practically n o d i f f e r e n c e s . W h e n c h o o s i n g a balling unit for the l a b o r a t o r y , in m o s t cases the disc w a s selected. H o w e v e r , this decision has n o t h i n g to d o with the choice of t h e unit to be used in the industrial p l a n t to b e designed. A m a i n r e a s o n for the p r e f e r e n c e of a disc is its easy handling. Discs o p e r a t e practically w i t h o u t circulating undersize load or w i t h only a small a m o u n t . C o n s e q u e n t l y , n o recirculation of undersize is r e q u i r e d , a n d the disc q u i c k l y comes into balance, i.e. the q u a n t i t y of pelletizing m a t e r i a l s u p p l i e d c o r r e s p o n d s to t h e s a m e a m o u n t of g r e e n balls discharged. W i t h i n a s h o r t e r t i m e a n d with s m a l l e r quantities of testing m a t e r i a l satisfactory results are o b t a i n e d m o r e q u i c k l y in the disc t h a n in the d r u m . T h e q u e s t i o n r e g a r d i n g the convertibility of laboratory test results to a n industrial plant is positively answered by o p e r a -

Table 12. Comparison of green pellet properties produced in discs on pilot and industrial scale Equipment

DiscsNo. 2

DiscsNo. 3

Pilotdisc

Airplanetire

Size(diametrom) Spec. surface area (cm 2 /g) Green-strength (N/pellet) Drop number (46 cm) Moisture (% H 2 O) Size (mm diametro)

6 1500 7.8 5.8 9.3 12-15

6 1500 7.3 5.1 9.3 12-15

1.0 1500 7.6 4.6 8.9 12-15

_ 1500 7.0 2.6 7.9 12-15

tional values of i n d u s t r i a l plants, as can be seen f r o m T a b l e 12. T h e results o b t a i n e d in a disc h a v i n g a d i a m e t e r of o n e m e t e r are c o m p a r a b l e with those r e a c h e d in discs with a d i a m e t e r of six meters. C o n s e q u e n t l y , it is j u s t i f i e d to use a balling disc of smaller d i a m e t e r f o r t h e work p e r f o r m e d in the l a b o r a t o r y . T h e last c o l u m n of T a b l e 12 gives the results o b t a i n e d with a n o t h e r unit, the so-called " a i r p l a n e - t i r e " . T h i s s i m p l e b a l l i n g unit consists of an a d e q u a t e l y s u p p o r t e d , rotating tire. Testing m a t e r i a l is intermittently i n t r o d u c e d into t h e interior of the tire a n d g r e e n balls are p r o d u c e d in batches. T h e tire p r i m a r i l y serves for d e t e r m i n i n g the green ball q u a l i t y while it is not s u i t a b l e for capacity d e t e r m i n a t i o n . T h e green ball f o r m a t i o n in a n e x p e r i m e n t a l disc serves for t h e determ i n a t i o n of: — o p t i m u m m o i s t u r e content — conditions for t h e f o r m a t i o n of green balls of d i f f e r e n t d i a m e t e r — possible capacity in d e p e n d e n c e on the s h a p e a n d fineness of grains and o t h e r ore characteristics.

4.4 Mix Preparation for Ball Formation

79

Fig. 35. Experimental disc and pre-mixer In o r d e r t h a t t h e e x p e r i m e n t a l disc can f u l f i l these f u n c t i o n s , it m u s t meet the f o l l o w i n g r e q u i r e m e n t s : - disc d i a m e t e r : 6 0 - 8 0 c m - rim height: a b o u t 20 c m (to be v a r i e d ) - disc slope: a d j u s t a b l e b e t w e e n 45—55° - rotating speed: a d j u s t a b l e b e t w e e n 10—18 R. P. M . - a d j u s t a b l e l o c a t i o n of ore a n d w a t e r supply. T h e m a t e r i a l s u p p l y is to be a c h i e v e d steadily a n d evenly s p r e a d . Fig. 35 s h o w s such a n e x p e r i m e n t a l disc a n d a p r e m i x e r a r r a n g e d beside. 4.4.2.1 Green Ball Formation and Testing Methods G r e e n balls cannot be stored d u r i n g their t r a n s p o r t a t i o n f r o m t h e b a l l i n g to the i n d u r a t i n g unit d u e to their low m e c h a n i c a l strength. T h e pelletizing p l a n t constructors intend, t h e r e f o r e , to k e e p the transp o r t a t i o n r o u t e as s h o r t as possible with a m i n i m u m n u m b e r of t r a n s f e r points a n d low d r o p heights. In m o d e r n travelling grate plants, for e x a m p l e , the t r a n s p o r t a t i o n t i m e s are b e t w e e n t w o a n d m a x i m u m f o u r m i n u t e s w i t h a b o u t f o u r t o five transfer points a n d d r o p heights of 60 to a m a x i m u m of 100 cm. T h e testing m e t h o d s a d o p t e d at p r e s e n t were e s t a b l i s h e d a c c o r d i n g to t h e s e practical r e q u i r e m e n t s . 4.4.2.1.1 Pellet Moisture Determination. A b o u t 100 g of g r e e n balls are dried in a d r y i n g oven at a t e m p e r a t u r e of 105—110 ° C .

80

4 The Pelletizing Laboratory and its Tasks

4.4.2.1.2 Crushing Strength. A c e r t a i n m i n i m u m c r u s h i n g strength is necessary in o r d e r t h a t the pellets c a n w i t h s t a n d the c o m p r e s s i o n load in the pellet b e d o n a belt conveyor, drying grate, i n d u r a t i n g grate, or in a shaft furnace. T h e a v e r a g e c r u s h i n g strength of green a n d d r i e d pellets is controlled by compressing at least 10 pellets b e t w e e n parallel steel plates u p to their breaking. T h e m e a n value of the tested pellets gives their crushing strength. In a s i m p l e m a n n e r , this test is c a r r i e d o u t o n a p l a t f o r m b a l a n c e with weight i n d i c a t i o n b y m e a n s of a p o i n t e r . T h e pellet to be tested is p l a c e d on the lower steel p l a t e of the b a l a n c e a n d is g r a d u a l l y c o m p r e s s e d with a steel plate while t h e p o i n t e r position is observed. T h e pellet b r e a k a g e is indicated by t h e j u m p i n g back of t h e pointer. T h e m a x i m u m w e i g h t l o a d observed c o r r e s p o n d s to the c r u s h i n g strength m e a s u r e d in N / p e l l e t . Often, m e c h a n i c a l l y o p e r a t i n g testing units are r e c o m m e n d e d . If t h e crushing strengths ascertained with a p l a t f o r m b a l a n c e are c o m p a r e d w i t h the results of a m e c h a n i z e d test press, t h e r e are practically n o d i f f e r e n c e s for pellets of t h e s a m e d i a m e t e r , as c a n b e seen f r o m the following data: Crushing strength: G r e e n Pellet

N/pellet D r i e d Pellet

H a n d press

11.1

40.6

A u t o m a t i c press

11.6

46.9

This m e t h o d of d e t e r m i n i n g t h e crushing strength is scientifically case with a pellet. A p o i n t load, r e p r e s e n t e d b y a p o i n t crushing strength, is also not correct for the spherical s u r f a c e of a pellet. H o w e v e r , the m e a s u r e m e n t s result in practically c o m p a r a b l e values as r e f e r e n c e values if the e x a m i n a t i o n is carried out with pellets of a p p r o x i m a t e l y the s a m e diameter. C o n s e q u e n t l y , small pellets m u s t yield a lower strength t h a n those of a g r e a t e r d i a m e t e r 4 3 ) . C o n t r o l m e a s u r e m e n t s with the s a m e earthy h e m a t i t e as described a b o v e c o n f i r m e d this a s s u m p t i o n . Pellets having a d i a m e t e r of 10 m m , 12 m m a n d 15 m m yielded t h e strength specified b e l o w b o t h for green a n d dry pellets:

Pellet diameter in m m G r e e n pellets N / P Dry pellets N / P

10

12

15

8.0

9.3

11.1

33.6

39.0

40.6

incorrect

since p r e s

4.4 Mix Preparation for Ball Formation

81

The pellets m u s t w i t h s t a n d a f u r t h e r l o a d d u r i n g t h e d r o p s in the course of their t r a n s p o r t a t i o n . T w o values a r e a s c e r t a i n e d for these drops. 4.4.2.1.3 Drop N u m b e r . T h e d r o p n u m b e r indicates h o w o f t e n green balls can be d r o p p e d f r o m a height of 46 cm = 18" b e f o r e they show p e r ceptible cracks or c r u m b l e . T e n green balls are i n d i v i d u a l l y d r o p p e d o n to a steel plate. T h e n u m b e r of d r o p s is d e t e r m i n e d for each ball. T h e arithmetical a v e r a g e values of the c r u m b l i n g b e h a v i o u r of the ten balls yield the d r o p n u m b e r . A c c o r d i n g to experience, the m i n i m u m value is four. As a result, a pellet is to w i t h s t a n d , w i t h o u t any d a m a g e , f o u r d r o p s f r o m a h e i g h t of 46 cm. C o m p a r a t i v e values a r e given in T a b l e 13. If the necessary values are not achieved w i t h the ore alone, b i n d e r s are a d d e d . T h e ball d i a m e t e r is also to be i n d i c a t e d for the d e t e r m i n a t i o n of the d r o p n u m b e r b e c a u s e , in this case, also correlations exist. 4.4.2.1.4 Drop Resistance. This testing m e t h o d is only occassionally a d o p t e d today. By using this m e t h o d , t h e c r u s h i n g strength of green balls is f o u n d o u t a f t e r t h r e e d r o p s f r o m d i f f e r e n t heights. T h i s value is a n indication of t h e a d m i s s i b l e h e i g h t d i f f e r e n c e at v a r i o u s t r a n s f e r p o i n t s during g r e e n ball t r a n s p o r t a t i o n . A b o u t 7 N e w t o n are r e q u i r e d as m i n i m u m strength for t h r e e d r o p s f r o m a h e i g h t of 46 cm. Desired v a l u e s for t h e m e c h a n i c a l p r o p e r t i e s of green, dry a n d i n d u r a ted pellets h a v e to be ascertained in l a b o r a t o r y tests, 4 . 6 . 1 - 4 . 6 . 1 . 2 a n d shown in t a b l e 13, first c o l u m n . F o r c o m p a r i s o n , m e a s u r e d values of

4.4.3 Capacity Determination A l t h o u g h pellet p l a n t constructors h a v e b e e n g a i n i n g experience in industrial plants with d i f f e r e n t ores for several years, it is r e c o m m e n d e d t h a t capacity d e t e r m i n a t i o n s be carried out on an e x p e r i m e n t a l balling disc. T h e m a x i m u m capacity c a n be recognized by the d e c r e a s i n g quality, particularly b y a n extension of the size r a n g e of the pellets p r o d u c e d . Optically too, the capacity l i m i t c a n be recognized b y a n increasing pellet s u r f a c e roughness. F r o m the c o n s i d e r a b l e n u m b e r of p u b l i c a t i o n s dealing w i t h t h e q u e s t i o n r e g a r d i n g t h e d i m e n s i o n i n g of pelletizing plants, the article issued by B h r a n y 44) is to b e noted. H e has set f o r t h a t h e o r y which p e r m i t s the design of industrial balling discs o n the basis of laboratory e x p e r i m e n t s .

pellets

82

Table 13. Mechanical properties of pellets, desired and obtained in industrial plants Desired values

Results in commercial plants (examples) Magnetite

Hematite

Limonite Hematite

Mixture of ores 2% CaCO 3 Lime stone

4100

Additives

Variable

Bentonite 0.7%

Lime hydrate 1% ( C a / O H ) 2

Grinding

Variable 10 Minimum 4 10

72 1900 11 7 33

98 1900 13 5 15

0.5% Bentonite 3.0% CaCO 3 Lime stone 67 2630 16 7 32

2000

3100

3600

2800

-0.045 m m in % spec. surface area (cm 2 /g) Green pellets compression strength (N/pellet) Drop numbers at 46 cm distance Dry pellets compression strength (N/pellet) Indurated pellets compression strength (N/pellet) Tumbler index (% + 6.3 mm) Abrasion index (%-0.5 mm)

94 5

97.5 2.2

97.0 2.8

95 4.2

58 2200 16 10 39

96 3.5

4 The Pelletizing Laboratory and its Tasks

Test specifications according to standard methods

4.5 Green Ball Induration

83

4.4.4 Bulk Density T h e d e t e r m i n a t i o n of b u l k densities of ores, c o n c e n t r a t e s a n d a d d i t i v e s is i m p o r t a n t f o r the d i m e n s i o n i n g of bins a n d h a n d l i n g e q u i p m e n t . In addition, this d e t e r m i n a t i o n gives i n d i c a t i o n s of the c a p a c i t y of ball f o r m i n g units, w h i c h is decisively i n f l u e n c e d b y the b u l k densities.

4.5 Green Ball Induration T h e i n d u r a t i o n of g r e e n balls m e a n s t h e i r t h e r m a l t r e a t m e n t in t h e following stages: - D r y i n g of g r e e n balls - H e a t i n g of d r i e d pellets and o x i d a t i o n of m a g n e t i t e pellets u p to induration temperature - F i r i n g at i n d u r a t i o n t e m p e r a t u r e - Cooling of i n d u r a t e d pellets. D e p e n d i n g o n the i n t e n d e d p u r p o s e , the v a r i o u s trials are c a r r i e d o u t with testing m a t e r i a l s in f u r n a c e s h a v i n g a d i f f e r e n t t h e r m a l capacity a n d volume. O r i e n t i n g tests are r u n o n q u a n t i t i e s of i n d i v i d u a l pellets of u p to one k i l o g r a m . Tests p e r f o r m e d with q u a n t i t i e s of u p to 80 kg are p e r f o r m e d in p o t grates, w h i c h serve to: - ascertain t h e quality of the pellets p r o d u c e d as a d e p e n d e n c e o n the r a w material properties, - d e t e r m i n e the oxygen r e q u i r e d for the m o s t c o m p l e t e o x i d a t i o n of m a g n e t i t e to h e m a t i t e , - d e t e r m i n e t h e t e m p e r a t u r e p r o f i l e f o r the ore types involved, - d e t e r m i n e the capacity of e.g. travelling g r a t e plants expressed in tons of pellets p r o d u c e d per s q . m . g r a t e a r e a a n d day, - o b t a i n d a t a f o r gas f l o w a n d h e a t i n g balances.

4.5.1 Furnaces for Orienting T e s t s (a) In a n orienting test the shock sensitivity d u r i n g g r e e n ball d r y i n g is d e t e r m i n e d . F o r this p u r p o s e , shock tests a r e c a r r i e d o u t in a m u f f l e f u r n a c e w h i c h c a n b e h e a t e d f r o m 100 0 C to 1000 ° C . T e n wet pellets are, for e x a m p l e , i n t r o d u c e d into such p r e h e a t e d m u f f l e s to f i n d o u t t h e n u m b e r of pellets w h i c h b u r s t at a p r e d e t e r m i n e d t e m p e r a t u r e . T h e admissible shock t e m p e r a t u r e is t h a t at w h i c h all pellets r e m a i n u n d a m a g e d . M o r e precise m e a s u r e m e n t s are m a d e in a pellet layer t h r o u g h which gas is flowing. F o r this p u r p o s e , pellets a r e c h a r g e d in a b a s k e t to a t u b e f u r n a c e as s h o w n in Fig. 36.

84

4 The Pelletizing Laboratory and its Tasks

Fig. 36. Tube furnace for drying tests with green balls in a gas stream

(b) O r i e n t i n g pellet induration tests are r u n in conventional laboratory furnaces, e.g. m u f f l e furnaces, w h i c h can b e h e a t e d u p to a t e m p e r a t u r e of a b o u t 1350 ° C . In these tests, small pellet q u a n t i t i e s are used m a i n l y to ascertain the o p t i m u m firing t e m p e r a t u r e .

4.5.2 Stationary P o t Grate for Principal T e s t s T h e a b o v e two f u r n a c e types can be f o u n d in every metallurgical l a b o r a t o r y w h i l e the principal tests are p e r f o r m e d in special e q u i p m e n t developed for pellet firing, t h e so-called pot grate. In these p o t grates, representative tests f o r firing a c c o r d i n g to t h e travelling grate, grate-kiln and shaft f u r n a c e process can be carried out. Irrespective of special pelletizing processes this p o t grate is o f t e n used f o r general investigations also those of a scientific nature. T h e drying, p r e h e a t i n g , firing a n d cooling are achieved in the s a m e pot grate with the process gases being sucked or forced t h r o u g h the pellet layer. A c c o r d i n g to t h e firing system a p p l i e d , the b e d height varies b e t w e e n 20 c m for the grate-kiln process a n d a b o u t 40 cm for the travelling grate process.

4.5 Green Ball Induration

85

T h e pot g r a t e was d e v e l o p e d o n t h e basis of d o w n - d r a u g h t sintering technology. It was t h e n a d a p t e d to t h e special t h e r m a l c o n d i t i o n s of pellet firing. In p a r t i c u l a r , the so-called side wall effect initially c a u s e d t r o u b l e s in the course of t h e process. This effect b r o u g h t a b o u t two p h e n o m e n a : (a) T h e gas f l o w is faster along t h e wall t h a n in the pellet bed. A s a result, the m e a s u r e d waste gas v o l u m e is g r e a t e r t h a n in industrial plants. (b) T h e h e a t i n g gases in the zone n e a r t h e wall give off a c o n s i d e r a b l e part of t h e i r h e a t energy to the wall. In this w a y , t h e pellets placed o n t h e wall a r e a are not h e a t e d to the necessary t e m p e r a t u r e a n d their q u a l i t y is thus c o r r e s p o n d i n g l y lower. T w o possibilities for a v o i d i n g or d i m i n i s h i n g this effect are used in practical o p e r a t i o n . 4.5.2.1 P o t Grate with Side Walls and Hearth Layer F i r e d pellets of a s m a l l e r d i a m e t e r t h a n t h a t of g r e e n balls or screened fired pellet f r a g m e n t s are p l a c e d b e t w e e n t h e s t a t i o n a r y side wall a n d t h e green ball f e e d . T h i s i n t e r m e d i a t e layer r e p r e s e n t s a n insulating z o n e which d a m s h e a t convection a n d holds off t h e h o t gases f r o m the stationary s m o o t h side wall. T h i s side wall m a t e r i a l can be s e p a r a t e d by simple screening f r o m t h e fired pellets a f t e r t e r m i n a t i o n of the test. T h e m a i n p o r t i o n of the s a m p l e can b e u s e d for q u a l i t y d e t e r m i n a t i o n . Since the p o t grate b o t t o m is c o m p o s e d of metallic g r a t e bars, it n e e d s to be p r o t e c t e d against o v e r h e a t i n g . This is a c h i e v e d with a layer of i n d u r a t e d pellets of 6 - 1 0 cm thickness. T h e green pellet b e d is thus p r o t e c t e d by a " r e f r a c t o r y e n v e l o p e " consisting of t h e s a m e m a t e r i a l as t h e pellets. S m a l l e r pellet s a m p l e s f r o m d i f f e r e n t o r e types could also be e m b e d d e d in o n e or several baskets of stainless steel into the m i d d l e of the g r e e n pellet layer. A f t e r t e r m i n a t i o n of the test, these baskets can be s e p a r a t e l y w i t h d r a w n . Fig. 37 shows t h e section of s u c h a p o t g r a t e with side wall p r o t e c t i n g zone, h e a r t h layer a n d pellet filling. 4.5.2.2 P o t Grate with Corrugated Side Walls A t t e m p t s w e r e also m a d e to a v o i d a i r - i n - l e a k a g e b y using a r e f r a c t o r y brick lining of a n inside c o r r u g a t e d s h a p e . In this case it is r e c o m m e n d e d that any f i n i s h e d pellets s h o u l d be p r e f e r a b l y w i t h d r a w n f r o m t h e m i d d l e of t h e b e d or f o r relevant tests b a s k e t s b e e m p l o y e d . T h e section of s u c h a pot grate is r e p r e s e n t e d in Fig. 3 8 45). In b o t h cases h e a r t h layer c a n be used. I n s t e a d of side layer or r e f r a c t o r y r a m m i n g m a s s a newly d e v e l o p e d r e f r a c t o r y fiber m a t is n o w a d a y s u s e d w h i c h is resistant to t e m p e r a t u r e s u p to 1600 0 C . T h i s m a t e r i a l has t h e following characteristics: — high t e m p e r a t u r e resistance — low h e a t c a p a c i t y — good i n s u l a t i n g effect

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Figs. 37/38. Alternative pot grate systems for heat treatment of pellets

4.5.2.3 Control Scheme of P o t Grate Tests In the firing tests, d i f f e r e n t test conditions can b e v a r i e d a n d checked, such as: — air flow direction, drying m e t h o d — gas volume, suction and pressure in the w i n d b o x — preheating rate and temperature profile — fuel type: oil or gas — heating gas a t m o s p h e r e by using a d d i t i o n a l oxygen, if necessary. A b o v e all, t e m p e r a t u r e control is decisive for the q u a l i t y of f i r e d pellets a n d process d e v e l o p m e n t so that in all i m p o r t a n t locations t h e r m o couples are installed. T h e side view of such an e q u i p p e d stationary pot grate as it is f r e q u e n t l y e m p l o y e d for pellet f i r i n g is s h o w n in Fig. 39. 4.5.2.4 Movable P o t Grate In the pot grate, the consecutive process stages d e v e l o p in a stationary state in contrast to the travelling grate in w h i c h the i n d i v i d u a l process steps p r o c e e d locally a n d separately f r o m e a c h other. W h e n the necessary flow direction of the process gases is reversed, t h e test conditions are i m p a i r e d in the stationary p o t grate by the fact t h a t the ancillary e q u i p m e n t connected with t h e p o t grate, such as w i n d b o x a n d firing h o o d , c a n n o t b e b r o u g h t so quickly to t h e t e m p e r a t u r e s r e q u i r e d for the process. T h i s has d e t r i m e n t a l consequences p r i m a r i l y at the e n d of the firing process. D u r i n g the change-over to cooling, the air is first p r e h e a t e d on the hot walls of the wind box a n d then passes in p r e h e a t e d condition to

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Fig. 39. Temperature control in pot grate tests the pellet charge. However, in a n industrial plant, cool air can i m m e d i a t e l y flow t h r o u g h the pellet bed. Also d u r i n g p r e h e a t i n g , d e l a y s w h i c h a r e caused by t h e h e a t c o n s u m p t i o n of t h e b u r n e r h o o d b r i c k w o r k m u s t b e considered. L e a r n i n g f r o m this experience o n e p r o c e e d e d to r e n d e r t h e p o t grate m o v a b l e . It is, as in a n industrial plant, b r o u g h t i n t o a n o t h e r p o s i t i o n during each of the v a r i o u s process stages. F i g . 40 s h o w s t h e s c h e m e f o r such a pot g r a t e w h i c h is m o v e d f r o m t h e r i g h t side to t h e left side d u r i n g the heating o p e r a t i o n . In position I, g r e e n balls are f e d to t h e p o t grate. In position II, t h e u p d r a u g h t d r y i n g is a c h i e v e d w h i l e in p o s i t i o n III the final drying, p r e h e a t i n g , m a g n e t i d e o x i d a t i o n , f i r i n g a n d a f t e r - f i r i n g are acc o m p l i s h e d . In position IV, the h o t c h a r g e is cooled w i t h a n u p d r a u g h t air flow. T h e p o t grate is e m p t i e d in position V. T h e a r r a n g e m e n t of the m e a s u r i n g device, p a r t i c u l a r l y t h a t for m e a s u r ing the t e m p e r a t u r e in the pellet layer c o r r e s p o n d s to the s c h e m e s h o w n in Fig. 39. Fig. 41 is a p h o t o g r a p h of a l a b o r a t o r y e q u i p m e n t with v a r i o u s pots, h e a t i n g device a n d i n s t r u m e n t panel. F r o m t h e t h r e e pots s h o w n in

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Fig. 40. Movable pot grate system for heat treatment tests the plate, the p o t in the centre serves f o r testing, the two others are stand-by pots. T h e d a t a logged d u r i n g the m e a s u r e m e n t is a u t o m a t i c a l l y recorded. T h e m e a s u r i n g i n s t r u m e n t s can b e directly connected to a computer. All d a t a is logged, stored a n d is at any t i m e a v a i l a b l e for r e n e w e d evaluations. T h e s e d a t a can also b e directly utilized t h r o u g h t h e c o m p u t e r for the design of industrial plants.

Fig. 41. Photograph of a movable pot grate system

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4.5.3 Pilot Plants R e f e r e n c e is m a d e to T a b l e 13 w h i c h s h o w s t h e m i n i m u m d e m a n d s o n pellet p r o p e r t i e s e m p i r i c a l l y a s c e r t a i n e d a n d the values m e a s u r e d in industrial plants, w h i c h were originally e s t a b l i s h e d o n t h e basis of test results o b t a i n e d in l a b o r a t o r i e s . In this way, p r o o f h a s b e e n f u r n i s h e d that it is possible to design a n d construct pelletizing plants o n t h e basis of laboratory results. In s o m e cases, it will, nevertheless, b e a d v i s a b l e to operate a pilot p l a n t h a v i n g a capacity of 1 0 - 2 0 t p d pellets. T h e p u r p o s e of such a pilot plant is: - to p r o v i d e m a j o r q u a n t i t i e s of pellets for s p e c i f i c r e d u c t i o n tests in pilot plants of d i f f e r e n t direct r e d u c t i o n systems, - to test n e w f i r i n g systems, - to investigate new process variants f o r trying o u t d i f f e r e n t fuels, e. g. coal a l o n e or m i x e d w i t h gaseous or l i q u i d f u e l . H o w e v e r , results f r o m pilot plants are n o l o n g e r n e e d e d f o r the design of conventional plants.

4.6 The Properties of Indurated Pellets and Their Testing Methods F r o m t h e i n d u r a t e d pellets such p r o p e r t i e s are d e m a n d e d w h i c h a r e necessary f o r their b e h a v i o u r d u r i n g t r a n s p o r t a t i o n (physical p r o p e r t i e s ) and p r i m a r i l y d u r i n g metallurgical t r e a t m e n t e i t h e r in t h e blast f u r n a c e or in direct r e d u c t i o n plants (metallurgical p r o p e r t i e s ) . T h e pellet q u a l i t y is evaluated b y a d o p t i n g a p p r o p r i a t e testing m e t h o d s , w h i c h w e r e d e v e l o p e d f r o m experience g a i n e d m a i n l y in i n d u s t r i a l plants. A t t e m p t s a r e successively being m a d e to develop and introduce international standards, called ISO S t a n d a r d s . T h e s e s t a n d a r d s are e l a b o r a t e d a n d p r o p o s e d by the International O r g a n i z a t i o n f o r S t a n d a r d i z a t i o n . C o u n t r i e s w i t h a h i g h pellet i m p o r t rate, such as J a p a n , are using in parallel, t h e i r o w n testing m e t h o d s . L a b o r a t o r i e s of pellet exporters s h o u l d be e q u i p p e d for the use of all i m portant testing m e t h o d s .

4.6.1 The Physical Properties Testing m e t h o d s w e r e initially t a k e n over f r o m t h o s e used for o t h e r products, as for e x a m p l e f r o m the c e m e n t i n d u s t r y f o r testing t h e c r u s h i n g strength as well as f r o m coking plants f o r testing t h e t u m b l i n g b e h a v i o u r . At an early stage W. E. M a r s h a l 4 6 ) p o i n t e d o u t the i m p o r t a n c e of the

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m e c h a n i c a l strength of pellets in the blast f u r n a c e . H e f o u n d clear correlations b e t w e e n the q u a n t i t y of f l u e d u s t a n d the t u m b l i n g b e h a v i o u r of pellets c h a r g e d .

4.6.1.1 Crushing Strength, According to I S O T C 1 0 2 / S C 3 D P 4700 It is a p p a r e n t f r o m practical o p e r a t i o n , t h a t the pellet c o n s u m e r s d e m a n d a m i n i m u m strength of a n average s a m p l e of a b o u t 2000..N/pellet. In o r d e r to d e t e r m i n e the crushing strength, one i n d i v i d u a l pellet is placed b e t w e e n t w o steel plates in a similar m a n n e r as for g r e e n balls a n d the c o m p r e s s i o n load is increased according to a specified t i m e schedule. Pellets of an o p t i m u m r o u n d n e s s a n d most u n i f o r m d i a m e t e r are selected to ensure a n even p o i n t load. A m i n i m u m of t e n pellets is used for each test. In the case of travelling grate plants, the e x a m i n a t i o n is carried o u t for the u p p e r , m i d d l e a n d b o t t o m layer of pellets. T h e average strength of ten i n d i v i d u a l pellets is t h e n r e g a r d e d as c r u s h i n g strength. N o w a d a y s , v a r i o u s testing m e t h o d s are a p p l i e d : (a) H a n d press: It is o p e r a t e d by h a n d t h e p o w e r b e i n g h y d r a u l i c a l l y transmitted. (b) H y d r a u l i c press w i t h m o t o r i z e d drive. (c) Electrically o p e r a t e d press with a w e i g h t p l a c e d o n a m o v a b l e lever arm.

4.6.1.2 Tumbler Resistance A l t h o u g h the a b r a s i o n test according to I S O S t a n d a r d 3271 1975 E has been s t a n d a r d i z e d internationally, a pelletizing l a b o r a t o r y s h o u l d c o m p r i s e facilities w h i c h will allow d i f f e r e n t e x a m i n a t i o n s to be m a d e according to the customer's wish. T h e r e exists in parallel to ISO, the A S T M ( A m e r i c a n Society f o r Testing a n d Materials) as well as the JIS ( T u m b l e T e s t J a p a n e s e Industrial Standards). T h e I S O Test, a c c o r d i n g to ISO S t a n d a r d 3271 1975 E is, for e x a m p l e , carried out as follows: F r o m a r e p r e s e n t a t i v e s a m p l e , 15 kg of pellets with a grain size of a b o v e 8 m m are i n t r o d u c e d i n t o a d r u m having the following dimensions: diameter = 1 m length = 0.5 m T w o lifters, each 5 cm high, are located inside the d r u m w h i c h is r o t a t e d for eight m i n u t e s at a s p e e d of 25 r p m , i.e. 200 revolutions. S u b s e q u e n t l y , the pellets are screened a n d the fractions + 6.3 m m a n d - 0.5 m m are ascertained. T h e p e r c e n t a g e of the s e p a r a t e d fractions in p r o p o r t i o n to t h e feed weight is the value of the T u m b l e r I n d e x ( + 6.3 m m ) a n d A b r a s i o n Index ( - 0 . 5 m m ) . All values s h o u l d be c h e c k e d by repetition of t h e test.

4.6 The Properties of Indurated Pellets and Their Testing Methods Since for each m e a s u r e m e n t ISO d e v e l o p e d a m e t h o d f o r A c o m p a r i s o n of physical ed pellets b e t w e e n desired given in T a b l e 13.

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a relatively large pellet a m o u n t is c o n s u m e d , a 3 kg pellet charge. a n d m e c h a n i c a l q u a l i t y of green a n d i n d u r a t a n d m e a s u r e d values in i n d u s t r i a l plants is

4.6.1.3 Microporosity N o w a d a y s , n o great i m p o r t a n c e is a t t a c h e d to t h e d e t e r m i n a t i o n of t h e microporosity, w h i c h was t e m p o r a r i l y c o n s i d e r e d as i m p o r t a n t for t h e evaluation of t h e r e d u c t i o n velocity in pellets. A c c o r d i n g to experience, this velocity is in practice always so h i g h t h a t even p o r o s i t y v a r i a t i o n s between a b o u t 1 0 - 3 0 % h a r d l y cause great d i f f e r e n c e s a n d the m a j o r p a r t of pellet q u a l i t i e s is w i t h i n t h e u p p e r p a r t of this range. O n l y pellets of a very glassy s t r u c t u r e can be m o r e dense b u t such pellets a r e exceptions. T h e porosity (P) is d e t e r m i n e d by m e a s u r i n g the t r u e specific d e n s i t y (D) with b e n z e n e or k e r o s e n e in a p y c n o m e t e r . T h e a p p a r e n t specific density is m e a s u r e d w i t h m e r c u r y a c c o r d i n g to the r e p l a c e m e n t m e t h o d . T h e porosity is

4 . 6 . 2 Behaviour of Indurated P e l l e t s D u r i n g R e d u c t i o n T h e d e m a n d s h i t h e r t o m a d e on t h e pellet q u a l i t y exclusively r e f e r r e d t o the m e c h a n i c a l b e h a v i o u r of pellets d u r i n g s t o r a g e a n d t r a n s p o r t a t i o n . W i t h the b e g i n n i n g of reduction, a d d i t i o n a l p r o p e r t i e s are r e q u i r e d such as: — good r e d u c i b i l i t y w h i c h c o r r e s p o n d s to a h i g h t h r o u g h p u t a n d a n e c o n o m i c energy c o n s u m p t i o n , — low t e n d e n c y to swelling, c o r r e s p o n d i n g l y g o o d gas p e r m e a b i l i t y of pellet charge, — good m e c h a n i c a l resistance in all r e d u c t i o n stages a t low a n d h i g h r e d u c t i o n t e m p e r a t u r e , w h i c h m e a n s little d e g r a d a t i o n , little a b r a s i o n , m i n i m u m strength a n d good gas p e r m e a b i l i t y , — low t e n d e n c y to sticking u n d e r o p e r a t i n g conditions, n o cluster f o r m a tion a n d n o u n e v e n gas p e r m e a b i l i t y , — softening a n d m e l t i n g b e h a v i o u r in a c c o r d a n c e with o t h e r blast f u r n a c e burden components. T h e necessity of k n o w i n g these p r o p e r t i e s b e f o r e the pellets are used in the r e d u c t i o n f u r n a c e s was recognized in g o o d time. C o n s e q u e n t l y , m e t h o d s f o r testing t h e blast f u r n a c e c h a r g e c o m p o n e n t s were d e v e l o p e d accordingly. S o m e of t h e m were t a k e n o v e r u n c h a n g e d or, in s o m e cases,

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in a m o d i f i e d m a n n e r for direct reduction. F r o m early k n o w l e d g e of the b e h a v i o u r of n a t u r a l l u m p ores, sinter a n d pellets, it is essential to influence the c h a r g e in such a w a y t h a t d u r i n g r e d u c t i o n n o d i f f i c u l t y occurs. In the case of natural l u m p ores, this is not possible. T h e i r p r o p e r t i e s are d e t e r m i n e d by n a t u r e . T h e s i t u a t i o n is d i f f e r e n t f o r sinter and pellets. By t h e selection of a d e q u a t e a d d i t i v e s a n d by a v a r i a b l e firing pattern, the b e h a v i o u r of pellets d u r i n g r e d u c t i o n can be largely i n f l u e n c e d in advance. As a result, the testing m e t h o d s a r e of increasing i m p o r t a n c e . T h e y shall b e p r a c t i c a b l e in a laboratory, m o s t simply a p p l i c a b l e a n d imitate the effect of i m p o r t a n t p a r a m e t e r s . H o w e v e r , they c a n n o t s i m u l a t e , in all details, the r e d u c t i o n p r o c e d u r e n e i t h e r in the blast f u r n a c e nor in the direct r e d u c t i o n plant, b u t a similar c o r r e l a t i o n b e t w e e n the process in an industrial p l a n t a n d the l a b o r a t o r y investigations shall be ensured. T h e great i m p o r t a n c e of such testing p r o c e d u r e s c a n be recognized by their large n u m b e r n o w a d a y s b e i n g a p p l i e d . A n a d e q u a t e l y e q u i p p e d pelletizing l a b o r a t o r y should, t h e r e f o r e , c o m p r i s e a m a x i m u m n u m b e r of the principal testing facilities. In view of t h e w o r l d w i d e interest t h e establishing of internationally valid a n d recognised testing m e t h o d s is intensively p u s h e d f o r w a r d . T h e ISO S t a n d a r d s are right n o w receiving m o r e a n d m o r e attention, but the JIS, J a p a n e s e Industrial Standards, m u s t b e considered if pellets are supplied to J a p a n . L K A B in S w e d e n as an i m p o r t a n t pellet a n d ore supplier, are a t t e m p t i n g , t o g e t h e r with international organisations, such as t h e British a n d G e r m a n Steel Industry, to develop a d e q u a t e standards. H o w e v e r , the e n d e a v o u r s of all o r g a n i s a t i o n s are increasingly concentrated o n i n t e r n a t i o n a l c o o p e r a t i o n 4 7 ) . Special testing m e t h o d s still exist in p l a n t c o m p l e x e s w h e r e pellets are p r o d u c e d for internal blast furnaces. T h e r e d u c t i o n conditions in blast f u r n a c e s are, in the first p h a s e , very similar to those of s h a f t f u r n a c e r e d u c t i o n processes if the d i f f e r e n c e s in the r e d u c i n g gas a t m o s p h e r e are neglected. T h e t e m p e r a t u r e p r o f i l e is also similar at the b e g i n n i n g b u t changes at a b o u t 8 0 0 ° - 1 0 0 0 0 C . In this t e m p e r a t u r e range, the oxygen r e m o v a l d u r i n g direct r e d u c t i o n ends. Sponge iron is cooled and available for m e l t i n g to steel in o t h e r f u r n a c e s . F o r this p u r p o s e , it is necessary that sponge i r o n has a certain m e c h a n i c a l strength a n d a g o o d t u m b l i n g b e h a v i o u r . T h e final r e d u c t i o n in the blast f u r n a c e p r o c e e d s differently. T h e partially r e d u c e d m a t e r i a l r e m a i n s in t h e s a m e f u r n a c e a n d is h e a t e d to higher t e m p e r a t u r e s u p to the l i q u i d phase. In this phase, the g a n g u e separates as slag f r o m the metallic i r o n in t h e f o r m of liquid pig iron. In this case, the pellets m u s t h a v e a s o f t e n i n g a n d m e l t i n g b e h a v i o u r which a d a p t s to the p r o p e r t i e s of the o t h e r b u r d e n c o m p o n e n t s . In direct r e d u c t i o n , a s o f t e n i n g of t h e c h a r g e is, in any case, to be a v o i d e d d u r i n g reduction. C o n s e q u e n t l y , g r e a t value is a t t a c h e d to t h e

4.6 The Properties of Indurated Pellets and Their Testing Methods

93

knowledge of the s o f t e n i n g b e h a v i o u r (sticking). A c c o r d i n g to the d i f f e r e n t d e m a n d s m a d e o n the c h a r g e p r o p e r t i e s , tests are c a r r i e d o u t by a d o p t i n g various m e t h o d s . 4.6.2.1 Testing Methods for Reduction These m e t h o d s serve to find out t h e b e h a v i o u r of pellets d u r i n g r e d u c tion c o m p a r e d with n a t u r a l l u m p ores a n d classified sinter. 4.6.2.1.1 Mechanical Strength. S u f f i c i e n t m e c h a n i c a l strength of f i r e d pellets is a n i m p o r t a n t p r e r e q u i s i t e to t h e i r o p t i m u m b e h a v i o u r d u r i n g t r a n s p o r t a t i o n a n d reduction. T h e c o r r e s p o n d i n g testing m e t h o d s a r e described u n d e r i t e m 4.6.1. 4.6.2.1.2 Examination of Fired Pellets for Blast Furnace Operation. D u r i n g r e d u c t i o n , highly oxidized pellets of g o o d q u a l i t y pass t h r o u g h several process stages w h i l e a structural c h a n g e occurs. It is i m p e r a t i v e that in these p h a s e s t h e pellet quality is m a i n t a i n e d as f a r as possible. S u c h changes m a y a l r e a d y occur at a relatively low t e m p e r a t u r e of 500 ° to 600 ° C resulting in a certain d e g r a d a t i o n a n d t h u s i m p a i r i n g the gas p e r m e a b i l i t y of t h e pellet charge. At rising t e m p e r a t u r e s of u p to 1000 ° C , t h e pellet v o l u m e m a y increase beyond the initial v o l u m e . This v o l u m e increase, k n o w n as "swelling", decreases t h e voids in the c h a r g e accordingly a n d t h u s i m p e d e s the gas flow. W i t h swelling, t h e s t r e n g t h m a y d i m i n i s h so t h a t t h e pellets m a y even c r u m b l e w h e r e b y the gas p e r m e a b i l i t y is f u r t h e r i m p a i r e d . A c c o r d i n g to the c h e m i c a l c o m p o s i t i o n of the pellets, they start to d e f o r m plastically at a relatively early stage a n d can t h u s also i m p e d e t h e gas flow. V a r i o u s m e t h o d s are a d o p t e d f o r e x a m i n a t i o n of pellet q u a l i t y d u r i n g the reduction stages c o n f o r m i n g to existing s t a n d a r d s . H o w e v e r , s o m e p e l l e t consumers are still using other, s i m i l a r m e t h o d s . Mr. R. L i n d e r 4 8 ) , as o n e of the first researchers d e v e l o p e d a rotary kiln m e t h o d in w h i c h , d u r i n g t h e kiln r o t a t i o n , t h e b e h a v i o u r of ores, sinter or pellets is s t u d i e d u n d e r a c h a n g i n g a t m o s p h e r e in the p r e s e n c e of coke of a size roughly t h e s a m e as t h a t of pellets a n d at a t e m p e r a t u r e r a n g i n g f r o m a m b i e n t t e m p e r a t u r e to 1000 ° C . T h e a c c u m u l a t i n g fines p o r t i o n , reduction d e g r e e a n d c r u s h i n g strength of the d i s c h a r g e w e r e ascertained. This m e t h o d w a s a p p l i e d for several years. In s o m e cases t h e rotary kiln p r o p o s e d b y L i n d e r is in a similar f o r m used f o r d y n a m i c testing m e t h o d s . 4.6.2.1.2.1 Low-Temperature Disintegration Test (Static Test). [ISO/TC 102/SC 3 (Sect. 98) July 76 341 E Fourth Draft Proposal ISO/DP 4696]. T h e p u r p o s e of this test is the d e t e r m i n a t i o n of d e g r a d a t i o n u n d e r c o n d i -

94

4 The Pelletizing Laboratory and its Tasks

tions as they prevail in the u p p e r p a r t of t h e blast f u r n a c e . Pellets screened at 10—12.5 m m r e m a i n for one h o u r at a t e m p e r a t u r e of 500 0 C u n d e r a weakly r e d u c i n g a t m o s p h e r e in a vertical t u b e f u r n a c e . A f t e r c a r e f u l cooling u n d e r a nitrogen a t m o s p h e r e , the kiln d i s c h a r g e u n d e r g o e s a special t u m b l e test according to I S O / T C 2 4 - 7 5 a n d is t h e n screened. A s a result, the r e d u c t i o n disintegration d e g r e e is i n d i c a t e d by the p e r c e n t a g e of fractions plus 6.3 m m , plus 3.15 m m a n d m i n u s 0.5 m m . 4.6.2.1.2.2 Low-Temperature Disintegration Test (Dynamic Test). [ISO/TC 102/SC 3 (Section 87) 286 E April 1974]. U n d e r e q u a l conditions, t h e r e d u c t i o n is carried o u t in a rotary kiln (similar to t h a t used b y L i n d e r ) with t h e disintegration b e h a v i o u r d u r i n g r o t a t i o n b e i n g tested. A f t e r cooling u n d e r a n i t r o g e n a t m o s p h e r e to a m b i e n t t e m p e r a t u r e , the d i s c h a r g e is then screened as in the previous test. Fig. 42 shows t h e s c h e m e of such a testing unit c o n f o r m i n g to the ISO S t a n d a r d s .

Fig. 42. Externally heated rotary kiln for low-temperature breakdown test (LTBT) according to ISO TC102/SC3 286 E (1974)

4.6.2.1.2.3 Swelling Test. [ISO/TC 102/SC 3 (Section 89) 288 E August 1974]. T h e p u r p o s e of this test is l i m i t e d to t h e study of the swelling b e h a v i o u r of pellets. T h e y are p l a c e d in a t u b e f u r n a c e so t h a t each pellet has, a f t e r swelling, s u f f i c i e n t space w i t h o u t c o m i n g in contact with o t h e r pellets. T h e t e m p e r a t u r e is 1000 0 C ± 10 ° C . T h e a t m o s p h e r e consists of 40% C O and 60% N 2 . T h e test is achieved by f o u r single d e t e r m i n a t i o n s at: 15, 40, 75 a n d 120 m i n u t e s with a r e d u c t i o n of 2 5 - 8 0 % b e i n g attained. T h e v o l u m e of a certain n u m b e r of pellets is m e a s u r e d b e f o r e a n d a f t e r the test. 4.6.2.1.2.4 Reduction under Load Test (RuL). [ISO/TC 102/SC 3 430 E]. T h e p u r p o s e of this test is the control of the gas p e r m e a b i l i t y of a pellet b e d of 1.2 kg ( 1 0 - 1 2 . 5 m m d i a m e t e r ) at a t e m p e r a t u r e of 1050 0 C with 40% C O , 60% N 2 a n d at a pressure of 0.5 b a r ( d a N / c m 2 ) . T h e d i f f e r e n t i a l pressure a b o v e a n d b e l o w t h e c h a r g e is c o n t i n u o u s l y m e a s u r e d at a p r e d e t e r m i n e d gas v o l u m e p e r unit of time. T h i s p r e s s u r e

4.6 The Properties of Indurated Pellets and Their Testing Methods

1 Compressed air to pressure

95

cylinder

causing a Ioad on pellet sample 2 Reduction gas inlet 3 Reduction gas outlet 4 Temperature recorder 5 Al 2 O 3 balls 6 Pellet sample 7 Pressure drop 8 Balance for weight loss 9 Recorder for pressure drop and weight loss 10

Fig. 43. Equipment for reduction test under load (RUL) ISO TC102/SC3 430E (1978)

changes w h e n t h e gas p e r m e a b i l i t y varies. If t h e pellets are d e f o r m e d or disintegrate u n d e r test conditions, the p e r m e a b i l i t y of the b e d decreases a n d the d i f f e r e n t i a l pressure rises. A p a r t f r o m the d i f f e r e n t i a l pressure, the pellet w e i g h t is p e r m a n e n t l y checked. In this way, t h e oxygen r e m o v a l is measured. Fig. 43 d i a g r a m m a t i c a l l y illustrates e q u i p m e n t d e s i g n e d as p e r the ISO S t a n d a r d . 4.6.2.1.2.5 Other Testing Methods. As a l r e a d y m e n t i o n e d at t h e b e g i n n i n g of this chapter, still o t h e r testing m e t h o d s are used by pellet c o n s u m e r s . This particularly a p p l i e s to the J a p a n e s e steel i n d u s t r y b e c a u s e it i m p o r t s great pellet q u a n t i t i e s f r o m d i f f e r e n t countries such as A u s t r a l i a , Chile, India, Brazil a n d P e r u . - S t a n d a r d JiS M 8713 - 1972. M e t h o d for M e a s u r i n g the R e d u c i b i l i t y of Iron Ores serves f o r testing the reducibility, r e d u c t i o n velocity a n d strength of r e d u c e d pellets. T h i s s t a n d a r d was d e v e l o p e d f r o m d i f f e r e n t versions of the " G a k u s h i n " m e t h o d w h i c h w a s a d o p t e d at the b e g i n n i n g of the sixties. - S t a n d a r d JiS M 8715 - 1968. M e t h o d f o r M e a s u r i n g the Swelling Index is used f o r testing t h e swelling b e h a v i o u r of pellets. - T h e I S O S t a n d a r d T C 1 0 2 / S C 3 357 E of t h e d r a f t p r o p o s a l I S O / D P 4695 " I r o n O r e s D e t e r m i n a t i o n of R e l a t i v e R e d u c i b i l i t y " , is s o m e t i m e s a p p l i e d additionally.

Indicator for v

96

4 The Pelletizing Laboratory and its Tasks

4.6.2.1.3 Examination of Pellets for the Direct Reduction. In p r i n c i p l e , similar conditions a p p l y to direct r e d u c t i o n processes as prevail in t h e t o p part of the blast f u r n a c e . H o w e v e r , the gas c o m p o s i t i o n is d i f f e r e n t a n d the r e d u c t i o n potential is greater. M o r e o v e r , t h e pellets s h o u l d not s o f t e n p r e m a t u r e l y (sticking) a n d f o r m l u m p s b u t a r e to be d i s c h a r g e d as ind i v i d u a l pellets. A f t e r r e d u c t i o n a m i n i m u m strength is r e q u i r e d for their t r a n s p o r t a t i o n . A l t h o u g h several industrial plants h a v e a l r e a d y b e e n successfully o p e r a t ing for s o m e years, n o generally valid s t a n d a r d s , s i m i l a r to the ISO S t a n d a r d s h a v e as yet b e e n established. Nevertheless, it is a l r e a d y k n o w n which pellet p r o p e r t i e s are most i m p o r t a n t . 4.6.2.1.3.1 Low-Temperature Disintegration Test (Dynamic). Standard I S O / T C 102/SC 3 (Section 87) 286 E A p r i l 1974 (item 4.7.1.2.2) is used for this test at a t e m p e r a t u r e of 600 ° C . 4.6.2.1.3.2 Swelling Test. It is suggested to p e r f o r m swelling tests s i m i l a r to I S O / T C 102/SC 3 (Section 89) 288 E A u g u s t 1974 at o t h e r gas c o m p o s i tions. In the case of the H o j a l a t a y L a m i n a Process t h e s a m p l e is t r e a t e d for 30, 60 and 90 minutes at a t e m p e r a t u r e of 1000 0 C with a reduction gas containing 74% H 2 , 13% C O , 8% C O 2 a n d 5% N 2 . A f t e r cooling, the v o l u m e increase, r e d u c t i o n efficiency, crushing strength a n d m e t a l l i z a t i o n degree are m e a s u r e d , see item 4.6.2.1.2.3.

4.6.2.1.3.3 Sticking Test. [ISO/TC 102/SC 3/429 E, RMC = "Reducibility s t u d i e d at d i f f e r e n t t e m p e r a t u r e s f r o m 760 ° C to 870 0 C a n d at a gas c o m p o s i t i o n of 55% H 2 , 35% C O , 6% C O 2 a n d 4% C H 4 . A f t e r cooling the f u r n a c e discharge is screened at 25, 20, 16 m m a n d the p e r c e n t a g e + 16 m m is considered as sticking criterion.

4.6.2.1.3.4 Direct Reduction Disintegration Stability Test (DRDS). [ISO/ TC 102/SC 3/428 E]. D u r i n g direct r e d u c t i o n w i t h gaseous r e d u c t a n t s it is i m p o r t a n t to d e t e r m i n e the stability of i r o n ores a n d pellets. This d e t e r m i n a t i o n h a s to be carried out in a r o t a r y kiln u n d e r t h e following conditions: - t e m p e r a t u r e : 750 0 C or 800 0 C , - gas composition: 55% H 2 , 35% C O , 6% C O 2 a n d 4% C H 4 , - time: 3 hours. A f t e r c a r e f u l cooling, the kiln discharge is screened at + 6.3 m m a n d - 0.5 m m . S u b s e q u e n t l y the crushing strength of the particles + 6.3 m m is ascertained. A t u m b l e test is p e r f o r m e d with t h e f r a c t i o n + 6.3 m m . A f t e r

Metallisation

Table 14. Desired properties of reduced pellets, values obtainable in industrial plantsª Values

Minimum requirement Blast furnace

Low-temperature disintegration (dyn.) ISO TC 102 SC 3 sect. 87 286 E 500 ºC blast furnace 600 0 C direct reduction

Free swelling ISO TC 102 SC3 sect. 89 288E 1000 0 C Swelling index Compression strength Degree of reduction (depending on given time) Rul test Reduction under load ISO TC 102/SC 3 430 E, R = Reducibility Pressure drop 80% red. deg. (differential pressure) Bed height at 80% reduction in % of the initial height a b

in % +6.3 mm - 0 . 5 mm + 6.3 mm in % - 0 . 5 mm

+ 80 -20%

% N/pellet %

-20% ca. 450 60%

Results obtainable

Gaseous(direct)bsobrescrito reduction

+ 80 - 10

-10 and -5

not required - 16% HyL not required

10-15% 450 70-75% aprox.

(dR)/(dt) . 40

0.8% min

mbar

2 mbar

not required

1 - 1.1%/min 1 mbar

4.6 The Properties of Indurated Pellets and Their Testing Methods

Test specifications according to standard methods

In Connection with table 13 Depending on special Reduction Process 97

98

4 The Pelletizing Laboratory and its Tasks

t u m b l i n g , the tested m a t e r i a l is screened at + 6.3 m m a n d — 0.5 m m . Finally, t h e m e t a l l i z a t i o n d e g r e e is d e f i n e d . In this connection r e f e r e n c e is m a d e to a n article issued b y D . K a n e d o a n d his colleagues w h i c h was e l e b o r a t e d for s h a f t f u r n a c e processes in the central r e s e a r c h l a b o r a t o r y of K o b e Steel Ltd., K o b e , J a p a n 4 9 ) . 4.6.2.1.4 Present State of Testing Methods. A l t h o u g h several testing m e t h o d s h a v e b e e n finally e l a b o r a t e d , s o m e others are still u n d e r p r e p a r a t i o n . In this connection it s h o u l d b e n o t e d t h a t all these m e t h o d s m i g h t b e i m p r o v e d in f u t u r e . T a b l e 14 shows the test s t a n d a r d s w h i c h n o w a d a y s are p r e f e r a b l y utilized.

5 Process-Influencing Factors

D u e to the n a t u r e and often varying properties of the ores it is difficult to predict the pellet quality. Therefore, it is of great importance to have necessary instruments at h a n d to compensate for these variations. T h e r e are m a n y factors available for influencing t h e pellet quality and for securing good properties of indurated pellets irrespective of the ore nature. T h e various factors can be classified into two groups: (a) T h e first group is raw material d e p e n d e n t and not variable. T h e y consist mainly of the ore nature, e.g. magnetite, h e m a t i t e or others, and of the crystalline f o r m in which they appear. If they have a positive influence, they ensure a good pellet quality. If they have a negative influence, attempts must be m a d e to counteract this effect by other factors. (b) T h e second g r o u p is i n d e p e n d e n t of raw material properties and variable. T h e y allow an efficient correction of negative factors of the first group and contribute to a uniformly good pellet quality. Some of t h e factors affect chiefly the green ball f o r m a t i o n whereas others influence the thermal treatment. These factors mostly react in c o m b i n a t i o n with others. Most important are the factors of the second group, which will be dealt with below.

5.1 Factors Influencing Green Ball Formation Evenly f o r m e d green balls serve as a basis for the mechanical strength and other properties of the indurated pellets. Sufficient fineness and surface properties of the ores and the o p t i m u m quantity of water are the principal factors for green ball formation. F o r most ores, the addition of a binder is, nevertheless, unavoidable in o r d e r to ensure the necessary strength of green balls. Three of these factors play an important role: — granulometric properties — o p t i m u m moisture content — efficiency of binders.

100

-5

Process-Influencing Factors

H o w e v e r , t h e i n f l u e n c e of one of the factors w i t h o u t the others is insufficient, b u t their s i m u l t a n e o u s co-action is a d v a n t a g e o u s . 5.1.1 Granulometric Properties of R a w M a t e r i a l s This t e r m covers all p r o p e r t i e s in c o n n e c t i o n w i t h t h e ore grain s h a p e , such as: grain size, size distribution, s h a p e a n d f o r m of s u r f a c e of the various grains, f o r e x a m p l e crystalline f o r m , rugosity, porosity a n d , in this conjunction, wettability a n d a d s o r p t i v e capacity as well as p o r e v o l u m e in the conglomerates. A f u r t h e r i m p o r t a n t f a c t o r is the so-called specific s u r f a c e a r e a of grains. It gives s o m e i n f o r m a t i o n a b o u t the fines p o r t i o n in the lower r a n g e of 5 microns — w h i c h is d i f f i c u l t to ascertain by screen analysis — as well as a b o u t the porosity a n d rugosity of the particle surface. T h e grain size is f o u n d o u t b y screen analysis w i t h t h e f r a c t i o n ( - 0 , 0 4 5 m m ) ( - 3 2 5 mesh) playing a decisive role. T h e specific s u r f a c e (see i t e m 4.1.1) is d e f i n e d as the ratio of s u r f a c e to w e i g h t or v o l u m e in cm2/g or c m 2 / c m 3 . 5.1.1.1 Grain Size, Size Distribution and Specific Surface Area O u t of the great n u m b e r of factors, the g r a i n size a n d size d i s t r i b u t i o n play an i m p o r t a n t part. As early as 1912/13 3 / 4 t h e fine-grained N o r w e g i a n a n d Swedish m a g n e t i t e concentrates gave the incentive f o r t h e first tests for t h e d e v e l o p m e n t of the pelletizing process. F o r the f o r m a t i o n of pellets t h e decisive value was f o u n d to be t h e p o r t i o n - 0 . 0 4 5 m m w h i c h is generally used as a criterion for the fineness of grains. In most cases, particularly for the m a g n e t i c s e p a r a t i o n of m a g n e t i t e , t h e liberation fineness is sufficient for direct g r e e n ball f o r m a t i o n . F o r m a g netites c o n t a i n i n g impurities, f u r t h e r g r i n d i n g m a y be r e q u i r e d , e.g. p h o s p h o r o u s c o m p o u n d s ( N o r t h e r n Sweden) o r s u l p h i d e s ( M a r c o n a ) f o r their s e p a r a t i o n . F o r s o m e r a w ores, it is possible to reach a l r e a d y the i n t e n d e d iron oxide c o n c e n t r a t i o n at a coarser grinding degree, e.g. in spiral concentrates. H o w e v e r , f u r t h e r grinding is n e e d e d for their pelletization. T o save grinding costs, t h e m a t e r i a l is only g r o u n d until t h e size d i s t r i b u t i o n req u i r e d for o p t i m u m green ball f o r m a t i o n a n d q u a l i t y of i n d u r a t e d pellets has been r e a c h e d . N o w a d a y s , large quantities of h i g h - g r a d e i r o n ores are a l r e a d y p r o cessed into pellets w i t h o u t previous b e n e f i c i a t i o n . In such cases, the grinding is only u n d e r t a k e n for green ball f o r m a t i o n , a n d only u p to t h e fineness r e q u i r e d . As far as grain size is c o n c e r n e d , the r a w m a t e r i a l s to be pelletized can b e classified into t h r e e categories: (a) D u r i n g b e n e f i c i a t i o n , the raw ore is a l r e a d y so finely g r o u n d t h a t the concentrate can be directly f o r m e d into g r e e n balls.

5.1 Factors Influencing Green Ball Formation

101

Table 15. Chemical composition and screen analysis of pellet feed from various ore sources I Magnetite concentrate Vitafors (Sweden)

II Spiral concentrate Wabush (Canada)

III Hematite crude ore - CVRD Tubarao (Brazil)

6 6 . 9 - 69.3 0 . 3 - 1.6 2 . 4 - 1.6 n.d. n.d. n.d. n.d.

68.5 0.2 1.3 0.5 0.1 0.1 0.3

Chemical analysis Values in % (about) Fe tot. Fe" SiO2 Al 2 O 3 CaO MgO L.OJ.

68.9 21.7 3.8 0.47 0.15 0.27 —

Screen analysis mm

+1 + 0.5 +0.25 + 0.125 + 0.063 +0.045 -0.045 Spec. surface area cm 2 /g

I

II

Delivered as concentrate

Coarse concentrate

III Pellet feed

~1.765

50.2 -

Pellet feed

14.9 6.4 7.0

7.0 42.8 13.9 8.3 77.8

Crude fines

11.8 5.2 83.0 ~1.920

32.6 12.4 26.7

0.7 6.0 6.7 86.6 1.820

(b) D u r i n g b e n e f i c i a t i o n , a coarser-grained c o n c e n t r a t e is o b t a i n e d . (c) F i n e ores or m i x t u r e s h a v e n a t u r a l l y such a h i g h i r o n content t h a t their b e n e f i c i a t i o n is s u p e r f l u o u s . T h e raw o r e has t h e n to be g r o u n d . T h e relevant costs are to t h e expense of pelletizing. T a b l e 15 shows s o m e d a t a of d i f f e r e n t raw m a t e r i a l s w h i c h c o r r e s p o n d to the a b o v e d e s c r i p t i o n and m a y serve as a n e x a m p l e of the r a w materials t r e a t e d in a great n u m b e r of pelletizing plants. D u e to m a n y test results a n d d a t a f r o m industrial plants, it is k n o w n t h a t a h i g h p o r t i o n o f

Further

g r i n d i n g is r e q u i

102

5 Process-Influencing Factors

Fig. 44. Influence of particle size and specific surface area on compressive strength of green and indurated hematite pellets

Fig. 45. Influence of particle size on compressive strength and tumble resistance of indurated pellets

5.1 Factors Influencing Green Ball Formation

103

very fine particles is a decisive f a c t o r to a c h i e v e the o p t i m u m effect of the capillary a d h e s i o n forces. Fig. 44 50 ) d e m o n s t r a t e s the i n f l u e n c e of t h e fine p a r t i c l e s on c o m pression strength f o r green a n d i n d u r a t e d pellets. Curves I a n d II c o n f i r m t h e increase of strength with rising portions of fines. A b o v e a c e r t a i n fineness the curves b e c o m e steeper. F r o m this results that besides the size d i s t r i b u t i o n o t h e r factors such as specific surface are active. C u r v e III shows a l i n e a r c o r r e l a t i o n b e t w e e n specific surface a n d pellet strength. C u r v e s I a n d III are b a s e d o n t h e s a m e o r e mixture. T h e specific s u r f a c e provides m o r e reliable i n f o r m a t i o n a b o u t the pelletizability of fine-grained ores. As c a n b e seen f r o m c u r v e IV, t h e strength of i n d u r a t e d pellets is also d e p e n d e n t o n the fineness a n d s p e c i f i c s u r f a c e of ore. T h e i n t e r d e p e n d e n c e b e t w e e n curve II for g r e e n balls a n d curve IV f o r i n d u r a t e d pellets p r o d u c e d f r o m the s a m e o r e m i x t u r e is o b v i o u s . T h e decisive i n f l u e n c e of the rising fines p o r t i o n o n t h e strength of fired pellets a n d their close r e l a t i o n to the a b r a s i o n b e h a v i o u r are s h o w n in Fig. 45. A f t e r the c o r r e l a t i o n b e t w e e n pellet q u a l i t y a n d fines p o r t i o n h a d b e e n d e t e r m i n e d in l a b o r a t o r i e s a n d pilot plants, t h e q u e s t i o n r e g a r d i n g the results o b t a i n e d in i n d u s t r i a l plants was of interest. F o r this p u r p o s e , a n d as far as it c o u l d be used, p e r t i n e n t d a t a f r o m 16 i n d u s t r i a l plants, w h i c h were c o m m i s s i o n e d d u r i n g a p e r i o d f r o m 1956 to 1978, was c o m p i l e d . In those plants still o p e r a t i n g , pellets f r o m d i f f e r e n t r a w m a t e r i a l s a r e produced. F i v e plants are b a s e d o n m a g n e t i t e - f o u r o n h e m a t i t e — in t h r e e plants a m a g n e t i t e - h e m a t i t e m i x t u r e is treated, in t h r e e plants a h e m a t i t e l i m o n i t e m i x t u r e is processed — a n d t h e o p e r a t i o n of one p l a n t is b a s e d o n a m i x t u r e of ores — p u r c h a s e d o n the w o r l d m a r k e t . T h e results are (curve 1) a n d t h e specific s u r f a c e area (curve 2) are p l o t t e d . I n p a r t B the green ball strength (curve 3) a n d in p a r t C ( c u r v e 4) t h e s t r e n g t h of f i r e d pellets are r e p r e s e n t e d . T h e vertical c o l u m n s c o r r e s p o n d to v a r i o u s g r o u p s of ores used: Group Group Group Group

I: II: III: IV:

magnetites hematites magnetite-hematite mixtures hematite-limonite mixtures

a n d an ore m i x c o m p r i s i n g v a r i o u s constituents, V. A c o m p a r i s o n of the t w o curves in p a r t A shows s o m e interesting d i f ferences. A t first, the relative u n i f o r m i t y of t h e specific s u r f a c e area of t h e g r o u n d ores b e t w e e n 1 5 0 0 - 2 0 0 0 c m 2 / g is striking. O n t h e o t h e r h a n d , the f r a c t i o n - 0 . 0 4 5 m m u n d e r g o e s m u c h g r e a t e r variations ( 7 0 - 9 5 % ) . T h e

indicated

104

5 Process-Influencing Factors

reversion of the values for ores N o . 13 ( G o a ) a n d N o . 14 (Venezuela) containing relatively small fine grain portions at h i g h specific s u r f a c e a r e a is r e m a r k a b l e . T h e green ball strengths r e c o r d e d in part B are m o r e in parallel with the values of the c o r r e s p o n d i n g specific s u r f a c e a r e a t h a n with those of fraction —0.045 m m . This w o u l d b e a c o n f i r m a t i o n of the correlations f o u n d in p r e v i o u s tests. T h e relatively w i d e variations of fired pellet strength (part C) are d e p e n d e n t on the p r e v a i l i n g o p e r a t i n g conditions, a n d particularly o n the use of additives and t h e m a r k e t s i t u a t i o n so t h a t they d o not h a v e to tally absolutely with the values of p a r t s A a n d B. Fig. 46 also allows a s t a t e m e n t on the t e c h n o l o g y of the ore p r e p a r a tion. A c c o r d i n g to this technique, t h e ore can b e g r o u n d to such a d e g r e e that a largely u n i f o r m specific s u r f a c e a r e a is o b t a i n e d d e s p i t e d i f f e r e n t r a w ore properties. H o w e v e r , the r e c o r d e d details are r a t h e r of a q u a l i t a tive n a t u r e t h a n of a q u a n t i t a t i v e one. T h e r e f o r e , it will always b e necessary to carry o u t g r i n d i n g tests f o r each p a r t i c u l a r o r e to be pelletized to avoid unnecessary g r i n d i n g expenses. F o r this reason, o n e s h o u l d i n d i c a t e the efforts o f t e n m a d e to p r o d u c e suitable pellets by a coarser

Fig. 46. Properties of pellets from 16 pellet plants comparison between grain size, specific surface area and compressive strength

grinding

5.2 Influence of Water Addition on Green Ball Formation

( 105

3.2.2) to the o r e type involved s h o u l d b e p o i n t e d out. C o n s i d e r a b l e e x p e n d i t u r e was m a d e to d e f i n e the i n f l u e n c e s of t h e p r i n c i p a l factors in m a t h e m a t i c a l terms. C o r r e s p o n d i n g r e m a r k s are m a d e u n d e r i t e m 13.1.

5.2 Influence of Water Addition on Green Ball Formation D u r i n g g r e e n ball f o r m a t i o n , t h e effect of w a t e r as a b i n d e r is of decisive i m p o r t a n c e . P r o v i d e d t h a t s u f f i c i e n t g r a n u l o m e t r i c p r o p e r t i e s are ensured, it largely d e p e n d s o n t h e a m o u n t of w a t e r a d d e d w h e t h e r g r e e n pellets can b e f o r m e d . T h e ore fines or concentrates to b e pelletized m a y a l r e a d y h a v e a certain m o i s t u r e c o n t e n t a c c o r d i n g to their p r e v i o u s p r e p a r a t i o n . If the ore originates f r o m dry g r i n d i n g , it is a v a i l a b l e as a dry c o n g l o m e r a t e . In the case of wet s e p a r a t i o n or w e t g r i n d i n g , a filter c a k e is o b t a i n e d w h i c h , a c c o r d i n g to the o r e n a t u r e , m a y b e m o r e or less moist. This results in d i f f e r e n t starting conditions f o r ball f o r m a t i o n r e f e r r e d to the o p t i m u m m o i s t u r e c o n t e n t r e q u i r e d , n a m e l y : (a) T h e g r a i n m i x is dry. T h e full a m o u n t of w a t e r r e q u i r e d is a d d e d . (b) T h e filter cake does not yet c o n t a i n the o p t i m u m a m o u n t of water. T h e lacking q u a n t i t y of water is a d d e d . (c) T h e filter c a k e has the o p t i m u m m o i s t u r e content. (d) T h e filter c a k e is t o o wet. It has either to b e p r e d r i e d or a drying a d d i tive m a y b e used.

5.2.1 Optimum Moisture Content T h e i n f l u e n c e of the granulometric properties o n the g r e e n ball strength c a n only b e c o m e effective with a n optimum water content. T h e s e are t h e two decisive factors f o r green ball formation. T h e green ball q u a l i t y c a n be i n f l u e n c e d by f u r t h e r factors. T h e i r e f f e c t o f t e n overlaps so m u c h that it is h a r d l y possible to i d e n t i f y t h e role of each i n d i v i d u a l factor. T h u s , t h e q u e s t i o n r e g a r d i n g the o p t i m u m m o i s t u r e c o n t e n t c a n n o t b e clearly d e f i n e d . It is d e s i r e d to use a m i n i m u m w a t e r a d d i t i o n d u e to t h e h e a t r e q u i r e d f o r e v a p o r a t i o n a n d owing to t h e s p e c i f i c capacity of the firing e q u i p m e n t . P a r t i c u l a r a t t e n t i o n s h o u l d be given to two i m p o r t a n t green ball p r o p e r ties: high c o m p r e s s i o n strength a n d d r o p resistance w h i c h are c o m p l e m e n t a r y . A h i g h c r u s h i n g strength is a c h i e v e d at a slightly lower w a t e r a d d i t i o n , b u t b e t t e r d r o p n u m b e r s with pellets of a h i g h e r m o i s t u r e con-

106

5 Process-Influencing Factors

tent. T h e green pellet g r o w t h f r o m i n d i v i d u a l grains or c o n g l o m e r a t e s proceeds, as s h o w n in Fig. 12, t h r o u g h d i f f e r e n t p h a s e s u n d e r t h e influence of a rising water a d d i t i o n a n d rolling m o v e m e n t of m a t e r i a l , item 2.1.2.3. 5.2.1.1 Optimum Moisture Content and Specific Surface Area The correlation of o p t i m u m m o i s t u r e c o n t e n t a n d the g r a n u l o m e t r i c p r o p e r t i e s of a specific o r e is s h o w n by d a t a t a k e n f r o m records 28 ) in Fig. 47 A a n d B. G r e e n balls were p r o d u c e d f r o m a m a g n e t i t e c o n c e n t r a t e with a n iron content of 68.0% w h i c h was g r o u n d to d i f f e r e n t fineness a n d c o r r e s p o n d i n g specific s u r f a c e areas b e t w e e n 1100 a n d 3370 cm 2 /g. T h e w a t e r content was varied u p to the o p t i m u m range. C r u s h i n g strength a n d d r o p resistance were ascertained in d e p e n d e n c e o n t h e specific s u r f a c e a n d water content. Fig. 47 shows m a x i m u m strength at d i f f e r e n t m o i s t u r e contents a n d v a r y i n g specific s u r f a c e areas. T h e g r e e n ball strength decreases at values below a n d a b o v e o p t i m u m m o i s t u r e content w h e r e a s

Fig. 47. Combined effect of moisture content and specific surface area on green pellet properties

5.2 Influence of Water Addition on Green Ball Formation

(

107

the d r o p resistance f u r t h e r increases at h i g h e r values, see Fig. 47 B. D u r i n g green ball t r a n s p o r t a t i o n , the d r o p resistance is of g r e a t e r i m p o r t a n c e t h a n t h e c r u s h i n g strength. In practical o p e r a t i o n , it can t h e r e fore be o b s e r v e d that the g r e e n balls a r e slightly o v e r w e t t e d . T h e o p t i m u m m o i s t u r e content s h o u l d b e d e t e r m i n e d for each p a r t i c u l a r ore. D r y g r o u n d m a t e r i a l s h o u l d b e p r e w e t t e d to i m p r o v e t h e balling o p e r a t i o n as is s h o w n in Fig. 48. Pellets of 12 m m d i a m e t e r are p r o d u c e d f r o m a m a g n e t i t e c o n c e n t r a t e with a d i f f e r e n t initial m o i s t u r e c o n t e n t a n d

Fig. 48. Influence of original moisture content in concentrate on pelletizing disc efficiency the balling c a p a c i t y is ascertained at o p t i m u m m o i s t u r e content. T h i s capacity is the lowest f o r a dry ore a n d r e a c h e s its m a x i m u m v a l u e at optim u m m o i s t u r e c o n t e n t of i n c o m i n g g r o u n d m a t e r i a l or filter cake. T h e Filtration t e c h n o l o g y a f t e r w e t g r i n d i n g is n o w a d a y s d e v e l o p e d to such a d e g r e e that t h e filter c a k e can b e s u p p l i e d at o p t i m u m m o i s t u r e content.

5.2.1.2 Optimum Moisture Content and Surface Condition of Ore Particles Besides t h e g r a i n size a n d specific s u r f a c e a r e a the s h a p e a n d s u r f a c e conditions of t h e i n d i v i d u a l o r e grains play a n i m p o r t a n t role in g r e e n ball f o r m a t i o n . T h e g r e e n ball p o r o s i t y is i n f l u e n c e d by the q u a n t i t y a n d type of c o n n e c t i n g bridges d u e to the s u r f a c e of the g r a i n structure. A substantial c o n t r i b u t i o n to g r e e n ball f o r m a t i o n is m a d e by t h e wettability of the grains. G l o s s y crystal surfaces h a v e a lower a b s o r p t i o n power t h a n r o u g h s u r f a c e a n d m i c r o - p o r e s . F r o m a g o o d wettability of t h e g r a i n surface, a h i g h e r m o i s t u r e c o n s u m p t i o n results. T a b l e 16 shows d a t a of s o m e ores w i t h d i f f e r e n t wettability in c o n j u n c t i o n with t h e i r specific s u r f a c e area, o p t i m u m m o i s t u r e content a n d c r u s h i n g strength.

108

5 Process-Influencing Factors

Table 16. Moisture content for optimum green pellet strength of different ore types No.

Type of ore

%

Degree of grinding

about

spec. % -0.045 mm surface area/cm 2 /g

Fe

%

H2O

Crushing strength pellet wet

dry

Pellet 0

1

Magnetite

68.3

70

2000

7.1

11

7

1 2 - 15

2

Hematite

66

90

1900

9.0

13

3

12-15

3

Earthy Hematite Artificial Magnetite from Pyritecinders Flotation Pyritecinders

65

74

2050

10.5

14

30

12-15

69

90

1900

15.0

11

4

abt. 20

61

60

2800

14.2

23

45

18-20

4

5

In the case of ore 1, the green balls consist of cubical m a g n e t i t e crystals with glossy f r a c t u r e surfaces, the w a t e r c o n s u m p t i o n is relatively low. S i m i l a r r e m a r k s a p p l y to ore 2, pellets f r o m s p e c u l a r i t e crystals. A n e a r t h y h e m a t i t e , r a w m a t e r i a l N o . 3, h a s a h i g h e r water c o n s u m p t i o n . T h e wettability of this o r e is better t h a n that of the magnetites. T h e t w o ores, 4 a n d 5, artificial m a g n e t i t e and flotation pyrite cinders n e e d a h i g h e r w a t e r a d d i t i o n for g r e e n ball f o r m a t i o n . Both r a w m a t e r i a l s o r i g i n a t e f r o m roasting processes — in the case of one raw m a t e r i a l , oxygen a n d in t h e other, s u l p h u r was r e m o v e d . P a r t of the w a t e r is a b s o r b e d b y t h e m i c r o pores of the i n d i v i d u a l ore grains. T h i s w a t e r d o e s not directly c o n t r i b u t e to the f o r m a t i o n of capillary forces w h i c h is d e m o n s t r a t e d by t h e w a t e r c o n s u m p t i o n of 15% for artificial m a g n e t i t e a l t h o u g h the green a n d dry pellet strength is r o u g h l y identical with t h a t of n a t u r a l m a g n e t i t e . T h e flotation pyrite c i n d e r s s h o w a slightly d i f f e r e n t b e h a v i o u r . T h e g r e e n a n d dry pellet strength is higher. A p a r t f r o m t h e g r e a t specific s u r f a c e a r e a , b o n d i n g by salt bridges m a y constitute a f a v o u r a b l e factor. T h e i n f l u e n c e of the g r a n u l o m e t r i c p r o p e r t i e s of ores in c o m b i n a t i o n with water f o r g r e e n ball f o r m a t i o n is very o f t e n i n s u f f i c i e n t since only few ores can b e f o r m e d into s u i t a b l e g r e e n balls w i t h o u t a d d i t i o n a l binders. Only ores a l r e a d y c o n t a i n i n g n a t u r a l constituents w h i c h , f o r instance, tend to gel f o r m a t i o n c a n b e f o r m e d into green balls. In practical operation, a d d i t i v e s are n o r m a l l y used, see T a b l e 13.

5.3 Influence of Binders and Additives

109

5.3 Influence of Binders and Additives Andersson a l r e a d y c o n s i d e r e d it very a d v a n t a g e o u s to use water a n d a d d i t i o n a l l y b i n d e r s f o r ball f o r m a t i o n . T h u s , h e m e n t i o n e d in the third of his p a t e n t c l a i m s 3 ), molasses, s u l p h i t e waste l i q u o r , s u l p h a t e s , chlorides and slaked lime. B r a c k e l s b e r g 4 ) used s o l u b l e s o d i u m silicates as special h a r d e n i n g agent w h i c h b r o u g h t a b o u t s o l i d i f y i n g salt b r i d g e s d u r i n g drying a n d h e a t i n g of pellets. It has t h u s b e e n p r o v e d that, d u r i n g t h e first d e v e l o p m e n t p h a s e of the pelletizing process, b i n d e r s a l r e a d y p l a y e d a n i m p o r t a n t p a r t b e s i d e s water. D u r i n g t h e second d e v e l o p m e n t p h a s e in w h i c h m a i n l y concentrates were treated, b i n d e r s were a l r e a d y tested in t h e very first experiments a n d in p a r t i c u l a r b e n t o n i t e p l a y e d a n i m p o r t a n t part. D u r i n g t h e third d e v e l o p m e n t p h a s e in w h i c h concentrates, n a t u r a l ores a n d m i x t u r e s were pelletized, t h e r e was a greater n u m b e r of possibilities f o r i m p r o v i n g the pellet properties. In case of m i x t u r e s a c o m b i n a t i o n of the p r o p e r t i e s of various ores m a y be a d v a n t a g e o u s for b o n d i n g purposes. N o r m a l l y this b o n d i n g is a c h i e v e d by using a d d i t i v e s w h i c h c a n be so selected that t h e y c h a n g e the c h e m i c a l c o m p o s i t i o n of the pellets c o m p a r e d to t h a t of c r u d e o r e to i m p r o v e the m e t a l l u r g i c a l b e h a v i o u r of the i n d u r a t e d pellets. T h e p u r p o s e , efficiency a n d n a t u r e of a d d i t i v e s used m a y vary in t h e following m a n n e r : (a) T h e y i m p r o v e only the m e c h a n i c a l - p h y s i c a l p r o p e r t i e s of green, dry a n d fired pellets. (b) In a d d i t i o n , chemical c o m p o s i t i o n a n d m e t a l l u r g i c a l p r o p e r t i e s are influenced. (c) T h e additives serve only for m e t a l l u r g i c a l purposes. (d) T h e a d d i t i v e s m a y be l i q u i d , solid, s o l u b l e or insoluble. (e) T h e y can react with water either b y h y d r a t e , salt b r i d g e s or gel f o r m a tion with a s i m u l t a n e o u s i n f l u e n c i n g of m o i s t u r e content. (f) T h e y can e v a p o r a t e , b u r n or volatilize d u r i n g h e a t t r e a t m e n t . (g) T h e y can also f o r m c o m p o u n d s w i t h the ore constituents, b o t h with g a n g u e a n d i r o n oxides. (h) T h e y can p r o v e a d v a n t a g e o u s o r d e t r i m e n t a l f r o m a m e t a l l u r g i c a l v i e w p o i n t a c c o r d i n g to their c h e m i c a l c o m p o s i t i o n . T h e type a n d q u a n t i t y of additives d e p e n d on t h e p u r p o s e for w h i c h they are used. T h e y are classified as follows: (a) A d d i t i v e s are only used for i m p r o v i n g g r e e n ball f o r m a t i o n as well as green, dry a n d i n d u r a t e d pellet p r o p e r t i e s . T h e a m o u n t of additives is l i m i t e d to this p u r p o s e a n d is i n d e p e n d e n t of t h e c h e m i c a l structure of t h e pellet feed. T h e s e a d d i t i v e s are d e s i g n a t e d as binders. T h e i r p r o t o t y p e is bentonite.

110

5 Process-Influencing Factors

(b) A d d i t i v e s m a y react as binders, a c c o r d i n g to (a) b u t they m a y also be used for influencing the c h e m i c a l c o m p o s i t i o n of t h e pellet feed such as l i m e hydrate. T h e y can b e d e s i g n a t e d as b i n d e r s or additives. (c) A d d i t i v e s have n o i n f l u e n c e o n the b o n d i n g m e c h a n i s m s . T h e y m u s t be p r e p a r e d like other constituents of t h e pellet feed. T h e i r m a i n p u r p o s e is, in principle, the c h a n g e of t h e c h e m i c a l c o m p o s i t i o n of the i n d u r a t e d pellets. A typical e x a m p l e is l i m e s t o n e or d o l o m i t e w h i c h are d e s i g n a t e d as additives only. T h e effect of a great n u m b e r of a d d i t i v e s is c o m p i l e d in a w i d e - s p r e a d , extensive literature. T h e most f r e q u e n t l y used a n d m o s t efficient additives are discussed below.

5 . 3 . 1 F a c t o r s f o r I m p r o v i n g the M e c h a n i c a l P r o p e r t i e s T h e life s p a n of green balls a n d the t r a n s p o r t a t i o n r o u t e f r o m t h e pelletizing to t h e Firing unit are relatively short. A c c o r d i n g to the a r r a n g e m e n t of the e q u i p m e n t involved, s o m e t r a n s f e r points are u n a v o i d a b l e in this r o u t e w h e r e the pellets d r o p f r o m a h i g h e r to a lower level. O n account of the low d r o p resistance of green balls, design engineers e n d e a v o u r in pelletizing plants to r e d u c e the t r a n s f e r points to a m i n i m u m a n d to p r o v i d e small d r o p heights. In m o d e r n travelling grate plants, for e x a m p l e , the t r a n s p o r t a t i o n t i m e is 2 - 4 m i n u t e s a n d the pellets m a y d r o p f o u r or five times f r o m a h e i g h t of 60 to a m a x i m u m of 100 cm. O n the basis of such experience, c o r r e s p o n d i n g testing m e t h o d s f o r pellet q u a l i t y were d e v e l o p e d a n d described in c h a p t e r 4. If the desired values for g r e e n or dry pellets p r o d u c e d f r o m d i f f e r e n t pelletizing r a w materials are not reached, the use of a binder is i m p e r a t i v e . 5.3.1.1 Bentonite as Binder Bentonite, described as a m i n e r a l u n d e r i t e m 3.2 h a s s o m e r e m a r k a b l e properties. It c o m b i n e s with water and f o r m s a gel at a b o u t five to six times its weight with its v o l u m e increasing to a p p r o x i m a t e l y tenfold. Bentonite envelops the ore grains together w i t h w a t e r a n d can t h u s only b e c o m e active in connection with water. D u e to its high w a t e r b o n d i n g power, it can also b r i n g a b o u t an a p p a r e n t drying of filter cake w h i c h is too w e t w i t h o u t v a r i a t i o n of the total m o i s t u r e content. 5.3.1.1.1 Influence of Bentonite on Green Pellet Strength and Drop Resistance. T h e effect of b e n t o n i t e is s t u d i e d b e l o w on the basis of the influence of the g r a n u l o m e t r i c properties a n d o p t i m u m water a d d i t i o n .

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H e r e too, only g u i d e lines can be given f o r d i f f e r e n t o r e types. By way of example, for a h e m a t i t e w i t h a specific s u r f a c e a r e a of 1700 c m 2 / g a n d a magnetite with a specific surface a r e a of 2100 c m 2 / g , t h e i n f l u e n c e of d i f f e r e n t b e n t o n i t e a d d i t i o n s on the p r o p e r t i e s of green a n d dry pellets at o p t i m u m m o i s t u r e c o n t e n t was investigated 52 ). T h e result is s h o w n in Fig. 49. T h e g r e e n pellet strength changes only insignificantly ( C u r v e s I), while the d r y strength is highly i n f l u e n c e d ( C u r v e s II).

Fig. 49. Influence of bentonite on green and dry pellet compressive strength

Since b e n t o n i t e is o n e of the m o s t f r e q u e n t l y used binders, values f r o m industrial plants t r e a t i n g very d i f f e r e n t ores w i t h a n d w i t h o u t a b o u t 0.7% b e n t o n i t e a d d i t i o n a r e s h o w n in Fig. 50, A, B, C. F o r green balls p r o d u c e d f r o m m a g n e t i t e (M), artificial m a g n e t i t e (A. M . ) , crystalline h e m a t i t e ( H ) , earthy h e m a t i t e (EA. H.) a n d an ore m i x consisting of v a r i o u s c o m p o nents, the following d a t a are c o m p a r e d : green pellet strength A drop numbers B and dry pellet strength C. N o n o t a b l e i n f l u e n c e o n the green pellet strength was observed. (a) T h e plasticity is considerably i m p r o v e d . All ores h a v e h i g h e r d r o p n u m b e r s w h e n b e n t o n i t e is a d d e d . (b) T h e d r o p indices thus o b t a i n e d are s u f f i c i e n t b o t h f o r f u r t h e r transp o r t a t i o n a n d for d i f f e r e n t t r a n s f e r points.

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Fig. 50. Influence of 0.7% bentonite on properties of green and dry pellets from various ore types

5.3.1.1.2 Influence of Bentonite on Dry Pellet Strength. As a l r e a d y shown in C u r v e s II of Fig. 49, the use of b e n t o n i t e has a great i n f l u e n c e o n dry pellet strength. T h e bentonite, finely d i s t r i b u t e d in the water, concentrates d u r i n g drying particularly in the residual l i q u i d , especially at the contact points a n d on the contact surfaces of t h e o r e grains. D u r i n g final evaporation of water, t h e gels concentrated t h e r e dry u p a n d f o r m solid m o r t a r b r i d g e s the influence of w h i c h can be recognized b y the increased dry pellet strength. T h e i n f l u e n c e of gels is also clearly s h o w n in Fig. 50, B, a n d C with the pellets f r o m earthy h e m a t i t e a n d ore m i x reacting most intensively. T h o s e o p e r a t i o n s p r o c e e d i n g d u r i n g drying also h a v e an influence on t h e shock temperature. A t a b e n t o n i t e a d d i t i o n of 0.7% the a m o u n t of gelatinously c o m b i n e d w a t e r m a y b e u p to 2.5 — t h r e e times the b e n t o n i t e weight. T h i s is a b o u t o n e t h i r d of t h e total m o i s t u r e content w h i c h only escapes slowly in the t h i r d drying p h a s e , as s h o w n in Fig. 15. In this way, the drying p r o c e e d s g r a d u a l l y a n d not a b r u p t l y - a n d at a h i g h e r drying t e m p e r a t u r e . In all pelletizing plants, the drying and h e a t i n g cycle is a d j u s t e d to a g r a d u a l h e a t i n g velocity to avoid the negative influence occurring at a n excessive h e a t i n g velocity. 5.3.1.1.3 Influence of Bentonite on Crushing Strength and Abrasion Resistance of Fired Pellets. T h e c r u s h i n g strength of fired pellets is chiefly

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a t t r i b u t a b l e to crystallization processes a n d reactions of g a n g u e constituents. F o r h i g h - g r a d e concentrates, crystallization processes a n d for ores with a m a j o r g a n g u e content, glassy slag f o r m a t i o n s a d d i t i o n a l l y p l a y an i m p o r t a n t role. N e i t h e r in the one n o r in t h e o t h e r case are t h e b e n t o n i t e a d d i t i o n s of decisive i m p o r t a n c e . In c o n n e c t i o n w i t h investigations on the o p t i m u m fines portion, pellets w i t h a n d w i t h o u t 0.62% b e n t o n i t e a d d i t i o n were i n d u r a t e d 53 ). T h e result is s h o w n in F i g . 51. T h e strength increase

Fig. 51. Combined effect of 0.62% bentonite and grain size on compressive strength and abrasion resistance of indurated pellets from hematite at 1300 0 C

at a rising fineness d e g r e e is obvious. H o w e v e r , the b e n t o n i t e a d d i t i o n is of m i n o r i m p o r t a n c e . T h e effect o n t h e p e l l e t t r a n s p o r t a t i o n is m o r e f a v o u r a b l e since t h e t u m b l i n g b e h a v i o u r is p a r t i c u l a r l y i m p r o v e d at a coarser g r a i n size ( C u r v e s I a a n d II a). D u r i n g m i c r o g r a p h studies, a slightly stronger bridge f o r m a t i o n b e t w e e n the finest ore particles - 0 . 0 2 m m was observed w h e n b e n t o n i t e w a s a d d e d ; this i m p r o v e s t h e a b r a s i o n behaviour. 5.3.1.1.4 Different Bentonite Types. T h e m o s t i m p o r t a n t b e n t o n i t e p r o p erty is its swelling capacity, see i t e m 3.1.4.3. T h e swelling c a p a c i t y d i f f e r s f o r various b e n t o n i t e types so that its suitability as a b i n d e r h a s to b e e x a m i n e d b e f o r e t h e b e n t o n i t e t y p e is selected. Fig. 52 shows the g r e e n a n d dry pellet strength as well as t h e d r o p resistance of pellets p r o d u c e d f r o m a m a g n e t i t e - h e m a t i t e m i x w i t h 65.3% i r o n content, 6.0% SiO 2 a n d a n

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Fig. 52. Influence of various bentonite types on green and dry pellet properties

a d d i t i o n of 0.7% bentonite, according to K o r t m a n n a n d M a i 3 5 ) , with 7 d i f f e r e n t b e n t o n i t e qualities ( A - I ) h a v i n g b e e n u s e d a n d a r r a n g e d in conf o r m i t y with their rising swelling p r o p e r t y (see also T a b l e 8, i t e m 3.1.4.3). C e r t a i n variations occur in the pellet h u m i d i t y (I). T h e green pellet strength practically does n o t c h a n g e (IV) while t h e d r o p indices (III) a n d dry pellet strength (II) rise w i t h t h e a d d i t i o n of b e n t o n i t e of a h i g h e r swelling capacity. 5.3.1.1.5 Influence of Bentonite on the Chemical Composition of Pellets. In m a n y ores a n d concentrates to b e pelletized, t h e acid constituents SiO 2 (and Al 2 O 3 ), are p r e p o n d e r a n t in t h e g a n g u e . D u r i n g t h e s u b s e q u e n t metallurgical t r e a t m e n t , they have to b e n e u t r a l i z e d by b a s i c c o m p o n e n t s w h i c h can also b e achieved d u r i n g pelletizing. B e n t o n i t e consists of a b o u t 6 0 - 7 0 % SiO 2 a n d of 1 5 - 2 0 % A l 2 O 3 . A c c o r d i n g to t h e q u a n t i t y of b e n t o n i t e a d d e d , the p o r t i o n of acid constituents w h i c h are to b e c o m p e n s a t e d rises p r o p o r t i o n a t e l y . In the case of ores or concentrates with a high i r o n and low g a n g u e content, t h e p e r c e n t a g e of acid constituents o r i g i n a t i n g f r o m b e n t o n i t e is m u c h h i g h e r t h a n for ores or concentrates w i t h a h i g h g a n g u e content as is s h o w n by way of e x a m p l e in Fig. 53. 0.5, 1.0 a n d 2.0% b e n t o n i t e is a d d e d to t h r e e concentrates with 1.7, 4.4 a n d 7.1% SiO 2 and A l 2 O 3 . T h e acid g a n g u e p o r t i o n t h e n rises b y 100% for t h e c o n c e n t r a t e w i t h the highest i r o n content a n d by as low as

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Fig. 53. Influence of bentonite on acid gangue components 22% for c o n c e n t r a t e with a h i g h g a n g u e content. F r o m a m e t a l l u r g i c a l chemical view-point, b e n t o n i t e is, in a n y case, a d e t r i m e n t a l constituent the scorification of w h i c h is a cost increasing factor. T h e r e f o r e , it is advisable to k e e p the b e n t o n i t e rate as low as possible, w h i c h is also d o n e in practice. In t h e m a j o r p a r t of t h e plants, the b e n t o n i t e rate is b e l o w 0.7% and mostly below 0.5%, irrespective of the ore n a t u r e . 5.3.1.2 Influence of AIkaline Earth Compounds As a c o n s e q u e n c e of the f o r e g o i n g those a d d i t i v e s are m o r e a d v a n t a geous w h i c h b o t h i m p r o v e the m e c h a n i c a l p r o p e r t i e s of pellets a n d h a v e a positive m e t a l l u r g i c a l effect. S u c h additives are, in a n y case, b a s i c minerals, e.g. c a l c i u m c o m p o u n d s w h i c h react with t h e a c i d g a n g u e constituents d u r i n g i n d u r a t i o n . T h e y can be u s e d in d i f f e r e n t f o r m s , n a m e l y as: (a) (b) (c) (d) (e)

calcium calcium calcium calcium calcium

oxide hydroxide chloride carbonate magnesium carbonate (dolomite)

CaO Ca (OH)2 C a Cl 2 Ca C O 3 (Ca, M g ) C O 3

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A c c o r d i n g to their properties, these c o m p o u n d s m a y , as binders, i n f l u ence the green a n d dry pellet strength as well as the d r o p resistance of pellets. A t a s u f f i c i e n t firing t e m p e r a t u r e , they b r i n g a b o u t , in m o s t cases, an i m p r o v e m e n t in final pellet strength. T h e first two c o m p o u n d s c a n b e used as binders. T h e c o m p o u n d s N o s . 4 a n d 5 virtually serve as b a s i c additives for n e u t r a l i z a t i o n of acid constituents in the o r e a n d are a d a p t e d to it. T h e y only h a v e little influence on g r e e n ball f o r m a t i o n . C a l c i u m chloride is only e m p l o y e d in special cases if it is necessary to volatilize i m p u r i t i e s of n o n - f e r r o u s metals, such as c o p p e r , lead, zinc. T h e use of lime c o m p o u n d s (see item 3.2.2) has b e e n k n o w n for pelletizing a l m o s t as long as that of b e n t o n i t e . 5.3.1.2.1 The Influence of Calcium Oxide [CaO] and Calcium Hydroxide [Ca(OH) 2 ]. If, d u r i n g moistening, C a O is a d d e d , it reacts with w a t e r a n d calcium h y d r o x i d e thus f o r m e d b e c o m e s effective as a b i n d e r . A c c o r d i n g to the e q u a t i o n C a O + H 2 O = C a ( O H ) 2 , a s t o i c h i o m e t r i c w a t e r a m o u n t is c o m b i n e d . T h e r e a c t i o n is highly e x o t h e r m i c . D u r i n g h y d r a t i o n , the volume expands threefold. D u e to t h e c o n s i d e r a b l e v o l u m e increase, the slaking process has to b e completely t e r m i n a t e d a n d the h y d r o x i d e be t h o r o u g h l y m i x e d w i t h ore b e f o r e green balls are f o r m e d . If this is not the case, the b u r n t l i m e is only slaked d u r i n g g r e e n ball f o r m a t i o n which, at t h e b e g i n n i n g of the drying process at the latest, inevitably causes a local v o l u m e increase a n d d a m a g e to the pellet structure. T o avoid such difficulties, only h y d r o x i d e is e m p l o y e d in practical o p e r a t i o n which is r e a d y f o r use w i t h o u t a n y p r e treatment. P u b l i s h e d values o n the i n f l u e n c e of C a O t h u s r e f e r t o t h e effect of C a ( O H ) 2 . In the course of h y d r a t i o n a n d s i m u l t a n e o u s v o l u m e increase, a very great specific s u r f a c e a r e a is o b t a i n e d w h i c h m a y b e 10.000 c m 2 / g a n d over according to t h e a d j u s t a b l e conditions. A h i g h e r water a m o u n t a d h e r e s to this big s u r f a c e t h a n c o r r e s p o n d s to the stoichiometry of h y d r a t e f o r m a t i o n . T h i s excessive w a t e r in c o n j u n c t i o n with the great s u r f a c e even i m p a r t s the c h a r a c t e r of a h y d r o g e l to the hydroxide; the colloidal p r o p e r t i e s of the h y d r o g e l i m p r o v e t h e plasticity of the ore m i x a n d , a b o v e all, s t r e n g t h e n t h e b o n d i n g m e c h a n i s m s d u r i n g drying. If C a ( O H ) 2 is chiefly a d d e d to i m p r o v e the m e c h a n i c a l physical properties, the c o r r e s p o n d i n g a m o u n t c a n be low a n d i n d e p e n dent of the acid g a n g u e constituents of ore. 5.3.1.2.1.1 Influence of Calcium Hydroxide [ C a ( O H ) 2 ] on Green Pellet Strength and Drop Resistance. D u r i n g the firing of pellets to the t e m p e r a ture r e q u i r e d , all t h e a f o r e s a i d lime c o m p o u n d s are dissociated to C a O which, at firing t e m p e r a t u r e s , reacts with the acid g a n g u e or with F e 2 O 3 . T h e lime a d d i t i o n has, in any case, a n e u t r a l i z i n g effect d u r i n g slag

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f o r m a t i o n . L i m e c o m p o u n d s are n o r m a l l y c h e a p e r as additives t h a n bentonite even if t h e y are less effective. T h e r e f o r e , it is w o r t h w h i l e to ascertain the i n f l u e n c e of C a ( O H ) 2 o n the physical p r o p e r t i e s of pellets which is illustrated in the following e x a m p l e 5 4 / 2 8 ): A n I t a b i r a - t y p e h e m a t i t e was used a n d m i x e d with slaked lime b o t h having the f o l l o w i n g characteristics: Itabira ore F e tot. F" SiO 2 Al 2 O 3 CaO MgO

= 64.5% 0.3% 4.4% = n. d. 0.3% 0.1%

Lime hydrate Ca(OH)2 94.4% CaCO3 2.7% CaO 1.0% H2O (Al 2 O 3 + F e 2 O 3 ) 1.7% spec. s u r f a c e a p p r o x . 1 0 0 0 0 c m 2 / g

At first, the i n f l u e n c e of the great specific s u r f a c e a r e a of l i m e h y d r a t e on the mix to b e pelletized was ascertained. F o r this p u r p o s e , o r e of t h r e e d i f f e r e n t sizes was used. T h e result is s h o w n in Fig. 54. T h e s p e c i f i c surface area of the mix composed of ore and lime h y d r a t e does not greatly

Fig. 54. Influence of Ca(OH) 2 on specific surface area of ore

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Fig. 55. Combined effect of specific surface area and Ca(OH) 2 on compressive strength of green pellets

vary a c c o r d i n g to the m i x i n g rule. At g r e a t e r fineness degrees, a certain grain size increase d e p e n d i n g o n ore p r o p e r t y m a y occur, d u e to a c e r t a i n coagulation. By t h e a d d i t i o n of very f i n e - g r a i n e d m a t e r i a l , like h y d r a t e d lime, t h e i m p r o v e d specific s u r f a c e a r e a of the mix c o u l d allow t h e use of ores w i t h coarser fineness. In all t h e s u b s e q u e n t tests, 0.5, 3.5 a n d 7% C a ( O H ) 2 were used. A trial with 0.5 b e n t o n i t e a d d i t i o n was p e r f o r m e d f o r c o m p a r i s o n . T h e pellet size was u n i f o r m l y k e p t b e t w e e n 10 and 12 m m d i a m e t e r . T h e i n f l u e n c e o n the g r e e n pellet strength is shown in Fig. 55. As expected, t h e influence of b e n t o n i t e is insignificant. T h e strength of green pellets p r o d u c e d f r o m coarse-grained ores (740—1120 c m 2 / g — C u r v e s I, II) r e m a i n s at or below 10 N / p e l l e t even at h i g h e r a m o u n t s of C a ( O H ) 2 additions. O n l y w h e n the ore h a s a higher B l a i n e i n d e x of 1720 c m 2 / g the h y d r a t e a d d i t i o n b e c o m e s effective. C a ( O H ) 2 has a positive i n f l u e n c e on the drop resistance, Fig. 56, similarly to the m o r e efficient b e n t o n i t e . 5.3.1.2.1.2 Influence of [ C a ( O H ) 2 ] on Dry Pellet Strength. A good b o n d i n g p o w e r of h y d r a t e d l i m e in dry pellets is s h o w n in Fig. 57. T h e

Fig. 56. Influence of Ca(OH) 2 on drop resistance of green pellets

Fig. 57. Influence of Ca(OH) 2 on compressive strength of dry pellets

Fig. 58. Influence of Ca(OH) 2 on compressive strength of indurated pellets

Fig. 59. Influence of Ca(OH) 2 on tumble resistance of indurated pellets

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121

Fig. 60. Influence of Ca(OH) 2 on porosity of indurated pellets b o n d i n g p o w e r of b e n t o n i t e is h i g h e r a n d c o m p a r a b l e to a b o u t 2% l i m e a d d i t i o n . Since, as a p p e a r s f r o m the results, the i n f l u e n c e of l i m e h y d r a t e should m a i n l y b e c o n s i d e r e d in c o n n e c t i o n w i t h the s p e c i f i c s u r f a c e a r e a of ore, the o b s e r v a t i o n s m a d e w i t h hematite o r e a r e also valid f o r pellets f r o m magnetite concentrates. 5.3.1.2.1.3 Influence of [Ca (OH) 2 ] on Crushing Strength, Tumbling Resistance and Porosity of Indurated Pellets. T h e i n f l u e n c e of c a l c i u m h y d r a t e o n the final strength of f i r e d pellets is r e m a r k a b l e , Fig. 58. E v e n t h e m o r e coarsely g r o u n d ore a l r e a d y shows strengths of m o r e t h a n 2000 N / p e l l e t with a n a d d i t i o n of 0.5% C a ( O H ) 2 . T h e s e strengths rise u p to a n a d d i t i o n of 5%. A t a h i g h e r a d d i t i o n of 7%, the strength d i m i n i s h e s . It m a y b e t h a t a glassy s t r u c t u r e is f o r m e d w h i c h is i n d i c a t e d b y t h e relatively low porosity of these pellets, as s h o w n in Fig. 60. T h e a b r a s i o n values i m p r o v e with rising h y d r a t e a d d i t i o n as can b e seen in Fig. 59. T h e porosity of f i r e d pellets p r o d u c e d f r o m t h e h y d r a t e a d d i t i o n s . H o w e v e r , the o r e with the g r e a t e r specific s u r f a c e a r e a , C u r v e III, reacts m o r e intensively a n d the p o r o s i t y decreases, but its reducibility is still sufficient.

5.3.1.2.2 Influence of Calcium Carbonate (CaCO 3 ) and D o l o m i t e [(Ca, Mg)CO 3 ] on the Strength of Indurated Pellets. A s d e s c r i b e d u n d e r item 3.2, c a r b o n a t e is a natural mineral which is insoluble in water and is,

coarse-grained

122

5 Process-Influencing Factors

therefore, ineffective as a b i n d e r d u r i n g g r e e n ball f o r m a t i o n . L i m e s t o n e is used r a t h e r for neutralizing the acid gangue. C o n s e q u e n t l y , n o g r e a t influence on the green a n d dry pellet strength is to b e expected. L i m e s t o n e is liable to the s a m e conditions f o r g r e e n ball f o r m a t i o n as the ore itself. It should h a v e at least the s a m e fineness to e n s u r e t h a t a f t e r t h e c a r b o n a t e has previously b e e n dissociated c a l c i u m o x i d e c o m p l e t e l y reacts with g a n g u e a n d h e m a t i t e . F r e e C a O s h o u l d not b e present in the i n d u r a t e d pellets b e c a u s e , a f t e r a certain t i m e , it h y d r a t e s a n d then causes a w e a k e n i n g of the pellet structure, see Fig. 62. Since l i m e s t o n e is ineffective as a b i n d e r f o r i m p r o v i n g the g r e e n ball p r o p e r t i e s , only its i n f l u e n c e on fired pellets will be d e s c r i b e d 5 5 , 2 8 ) . A n a c i d m a g n e t i t e c o n c e n t r a t e was pelletized with very p u r e limestone. T a b l e 17 shows the c h e m i c a l c o m p o s i t i o n a n d size distribution. Table 17. Chemical composition and screen analysis of a magnetite concentrate and limestone for special pelletizing tests Concentrate in % (about) Fe total SiO2 Al 2 O 3 CaO CO 2

63.5 8.4 0.5 0.1 -

Limestone in % (about) 1.0 -

55.0 43.2

Screen analysis + 0.1 mm + 0.045 mm -0.045 mm

9.3 9.9 80.8

21.0 70.5 8.5

D u r i n g the f o r m a t i o n of green balls h a v i n g a d i a m e t e r of a b o u t 15 m m , the l i m e a d d i t i o n was varied b e t w e e n 0 a n d 12% a n d t h e firing t e m p e r a t u r e b e t w e e n 1150 and 1300 0 C . T h e green ball strength was practically not i n f l u e n c e d . In the case of dry pellets, only small d i f f e r e n c e s were f o u n d . F o r c o m p a r i s o n the o p t i m u m i n d u r a t i n g t e m p e r a t u r e was first ascertained w i t h o u t l i m e s t o n e a d d i t i o n as s h o w n in Fig. 61. F o r this concentrate, a m a x i m u m strength was o b t a i n e d at 1250 ° C w h i c h results f r o m its chemical composition. S u b s e q u e n t l y , the i n f l u e n c e of a rising a m o u n t of l i m e s t o n e a d d i t i o n was ascertained at 1150 0 C a n d 1200 0 C first of all directly a f t e r firing a n d a g a i n a f t e r a storage of 6 weeks in a protected r o o m , Fig. 62. At the h i g h e r firing t e m p e r a t u r e (1200 0 C ) a substantially h i g h e r strength was o b t a i n e d w h i c h r e a c h e d its m a x i m u m at

5.3 Influence of Binders and Additives

123

Fig. 61. Influence of firing temperature on compressive strength of pellets from magnetite concentrate without binders

Fig. 62. Influence of limestone and firing temperature on compressive strength of indurated pellets from magnetite concentrate, before and after 6 weeks of storage

124

5 Process-Influencing Factors

Fig. 63. Combined effect of limestone and firing temperature on porosity of indurated pellets from magnetite concentrate

a C a C O 3 a d d i t i o n of a b o u t 8% while at 1150 ºC the m a x i m u m was at 6% C a C O 3 . A f t e r a storage of 6 weeks, the strengths of b o t h s a m p l e s considerably d i m i n i s h e d . W h i t e spots of lime h y d r a t e were o b s e r v e d in the pellets w h i c h d i d originate f r o m not scorified f r e e C a O . T h e screen analysis of limestone showed s o m e portions of too coarse particles. T h e m a x i m u m strength which, for all s a m p l e s was a r o u n d 2500 N / p e l l e t , is r e m a r k a b l e a n d indicates the great influence of slag b o n d . As can be seen f r o m Fig. 63, t h e porosity of fired pellets is highly d e p e n d e n t o n t h e i r C a O content. U p to a certain C a O a m o u n t it decreases a n d the m i c r o structure b e c o m e s m o r e dense. At a higher C a O a d d i t i o n , it rises again, a fact which can be explained b y t h e f o r m a t i o n of a g r e a t e r n u m b e r of m a c r o - p o r e s , as can be d e m o n s t r a t e d by m i c r o g r a p h s . In each case, the porosity was sufficiently h i g h so that it w o u l d not adversely affect the b e h a v i o u r d u r i n g reduction. M e r k l i n and D e V a n e y observed similar results with a n o t h e r m a g n e t i t e a n d increasing a d d i t i o n s of l i m e s t o n e or dolomite. In view of the great e c o n o m i c i m p o r t a n c e of l i m e s t o n e a d d i t i o n , m a n y researchers have dealt with this p r o b l e m 56), c h i e f l y in c o n j u n c t i o n with the p r o d u c t i o n of self-fluxing pellets. T h e m a i n r e a s o n f o r the a d d i t i o n of l i m e s t o n e or d o l o m i t e is the p r o d u c t i o n of pellets with a rising p o r t i o n of basic constituents a n d t h u s with increasing basicity.

5.3 Influence of Binders and Additives

125

5.3.1.2.3 Influence of a Mixture of Different Additives on Pellet Properties. In tests for the p r o d u c t i o n of self-fluxing pellets f r o m a m a g n e t i t e concentrate with 71.3% F e , 0.7% SiO 2 , 0.1% A l 2 O 3 , 0.38% C a O a n d 0.14% M g O , increasing a n d d i f f e r e n t q u a n t i t i e s of f i n e l y - g r o u n d limestone, d o l o m i t e a n d f o r a t i m e sand, for raising t h e total slag p o r t i o n w e r e a d d e d . Inter alia, t h e m e c h a n i c a l pellet p r o p e r t i e s w e r e ascertained. D u e to the low b o n d i n g p o w e r of the a l k a l i n e e a r t h c a r b o n a t e s in g r e e n a n d dry pellets, b e n t o n i t e was a d d e d . T h e results of s o m e of these tests are c o m p i l e d in T a b l e 18. T h e c o n c e n t r a t e h a d a s p e c i f i c s u r f a c e a r e a of 1810 c m 2 / g a n d the pellet d i a m e t e r was a d j u s t e d to 9—15 m m . As was to be expected, the green pellet strength d i d not d i f f e r m a r k e d l y . As concerns the dry pellet strength a n d d r o p resistance, only t h e b e n t o n i t e i n f l u e n c e was perceptible. T h e strengths of f i r e d pellets w e r e q u i t e d i f f e r e n t . Pellets without l i m e a n d d o l o m i t e a d d i t i o n s h o w e d the lowest value (test 1). A h i g h e r l i m e a d d i t i o n resulted in a h i g h e r strength (tests 2 - 5 a n d 7). W h e n limestone was r e p l a c e d by d o l o m i t e alone (tests 6 a n d 8), the c r u s h i n g strengths w e r e slightly lower. T h e m a i n p u r p o s e of this test series was to

Table 18. Influence of additives on properties of indurated pellets from magnetite 1

2

3

0.6

0.36

0.6 1.35 1.35 1.0 0.90

0.6 3.7 3.7 2.7 0.94

3.7 3.7 2.7 0.94

0.40

1.0

1.1

N/pellet

13

16

Times

6.2

N/pellet N/pellet

Test specification Additives Bentonite Limestone Dolomite Quarzite Basicity CaO/SiO 2 Basicity CaO + MgO / SiO 2 + Al 2 O 3 Green pellet Compression strength Green pellet drop number Dry pellet strength Fired pellet strength

% % % %

-

4

-

5

6

7

0.6 8.1

0.6 -

0.6 0.9 0.9

-

-

-

2.7 1.18

7.4 2.7 0.75

1.1

1.1

11

11

6.8

4.6

33

33

2630

3410

-

-

8

0.6 4.1

1.0

0.5

1.1

1.1

2.0

17

13

11

14

3.6

5.0

7.8

3.6

4.6

18

8.0

30

38

18

29

3830

3500

3830

3340

3480

3440

* Grain size -0.045 mm = 85.8%; Spec. surface = 1810 cm 2 /g; Pellet size = 9 - 1 5 mm; Moisture = 8.4-8.9% H 2 O, optimum firing temperature.

126

5 Process-Influencing Factors

indicate t h e i n f l u e n c e of the m e c h a n i c a l p r o p e r t i e s of green, dry a n d fired pellets b y m i x i n g the pellet feed with various additives. Similarly as with m a g n e t i t e , the a d d i t i o n of basic c a r b o n a t e s a n d mixtures of o t h e r additives also has an e f f e c t o n hematite ore. T h e pertinent results are s h o w n in T a b l e 19. As always, b e n t o n i t e increases the dry pellet strength m o r e intensively t h a n d o e s c a l c i u m h y d r o x i d e . A c o m b i n a t i o n with l i m e s t o n e results even at a lower t e m p e r a t u r e in a high strength of f i r e d pellets. M o r e o v e r , it is k n o w n t h a t ores with a lower fines p o r t i o n can also be processed into s u i t a b l e pellets. T h i s k n o w l e d g e is utilized in m a n y industrial plants w h e r e m i x t u r e s of b e n t o n i t e , l i m e h y d r a t e or c a r b o n a t e s are used. As r e g a r d s t h e correlations b e t w e e n basic additives a n d o p t i m u m firing t e m p e r a t u r e , r e f e r e n c e s h o u l d b e m a d e to items 5.4.3.1.2 and 6.1.2.3.5 for the effect of a d d i t i v e s o n t h e pellet behaviour during reduction. Table 19. Influence of additives on properties of pellets from hematite orea Test No.

1

2

3

Firing temp. in 0 C Additives

1300 Ca(OH) 2 0.5%

1300 Bentonite 0.5%

1280 Bentonite 0.5% Limestone 4.5%

Compression strength in N/pellet (about) Green Dry Fired Abrasion in % 0.5 mm a

11 7 4250 4.6

14 46 2300 6.8

14 32 5240 3.5

Fe = 66.2%; SiO2 = 3.3%; CaO = 1.0%

5.3.1.2.4 Influence of Calcium Chloride [CaCl 2 ] on Pellet Properties. If additives are e m p l o y e d to i m p r o v e the pellet p r o p e r t i e s any i n h e r e n t u n a v o i d a b l e d e t r i m e n t a l influences s h o u l d b e r e d u c e d to a m i n i m u m . W h e n calcium c h l o r i d e is used, the basic c o n s t i t u e n t has, at c o r r e s p o n d i n g t e m p e r a t u r e s , the s a m e effect as o t h e r l i m e c o m p o u n d s . H o w e v e r , c h l o r i n e liberated d u r i n g firing m a y cause corrosion. Nevertheless, s u c h an additive can still be successfully e m p l o y e d if, b y a c h e m i c a l r e a c t i o n b e t w e e n chlorine a n d i m p u r i t i e s in the o r e s u c h as n o n - f e r r o u s metals, c o p p e r , lead a n d zinc are to be r e m o v e d or possibly recovered as v a l u a b l e substances. N o w a d a y s , calcium c h l o r i d e is used in several industrial plants, a b o v e all for processing pyrite cinders c o n t a i n i n g n o n - f e r r o u s metals. R e l e v a n t details are described u n d e r i t e m 7.2.

5.3 Influence of Binders and Additives

127

5 . 3 . 1 3 Influence of Alkali Compounds F o r a s i m i l a r r e a s o n alkali salts are also e m p l o y e d d u r i n g pelletizing, e.g. w h e n v a n a d i u m - c o n t a i n i n g ores are t r e a t e d . A l k a l i f o r m s a soluble v a n a d i u m salt. T h i s can be dissolved f r o m t h e pellet. T h e i n f l u e n c e of alkali salts is t h u s discussed u n d e r i t e m 7.3. T h e d e t r i m e n t a l effect of alkali salts p a r t i c u l a r l y d u r i n g r e d u c t i o n of pellets h a s b e e n k n o w n f o r a long t i m e a n d is t h e subject of m a n y studies, see item 6.1.2.3. T h i s effect is so e n o r m o u s t h a t every e n d e a v o u r is m a d e to e l i m i n a t e alkali salts to t h e m a x i m u m possible extent f r o m the m i x to b e pelletized. C o n c e n t r a t e s b e n e f i c i a t e d with sea w a t e r are s o m e t i m e s r e w a s h e d with water c o n t a i n i n g a low p e r c e n t a g e of salt as, f o r e x a m p l e , in the case of M a r c o n a concentrates. 5.3.1.4 Influence of Ores with Good Bonding Properties T h e g r i n d i n g expenses for o b t a i n i n g the fineness r e q u i r e d for pelletizing represent an i m p o r t a n t cost f a c t o r in pellet p r o d u c t i o n . T h i s fineness largely d e p e n d s o n the p r o p e r t i e s of t h e v a r i o u s ores. Crystalline o r e s d e m a n d a h i g h e r fineness d e g r e e t h a n those of a n e a r t h y character, as is shown in T a b l e 16. C o n s e q u e n t l y , o n e c a n c o n c e i v a b l y i m p r o v e t h e pelletizability of a crystalline o r e b y m i x i n g with a n o t h e r o r e h a v i n g a greater b o n d i n g p o w e r . Such c o m b i n a t i o n is possible, for instance in plants treating d i f f e r e n t ores. F o r the d e s i g n of a pelletizing p l a n t at a N o r t h Sea port, this possibility was once s t u d i e d . In this case, I t a b i r a h e m a t i t e was used, see i t e m 5.3.1.2.1.1. A n e a r t h y D o g g e r ore w i t h a r o u g h , p o r o u s s u r f a c e a n d a fines p o r t i o n of 36% b e l o w 0.045 m m was used as a d d i t i v e ; its specific s u r f a c e a r e a was 2400 c m 2 / g . Its c h e m i c a l analysis s h o w e d the following a p p r o x i m a t e values: F e tot. SiO 2 CaO Al 2 O 3 MgO Loss on ignition

=22-34% =11-20% = 8-12% = 6 - 8% = 1 - 2% =20-22%.

F o r m a n y years, t h e a b o v e ore h a d b e e n processed in G e r m a n blast furnaces. T h e I t a b i r a h e m a t i t e h a d b e e n g r o u n d to d i f f e r e n t fineness degrees b e t w e e n 740 c m 2 / g a n d 1720 c m 2 / g specific s u r f a c e area. 5 a n d 10% of e a r t h y o r e was a d d e d to the coarser h e m a t i t e 28 ). T h e results t h u s o b t a i n e d are given in T a b l e 20. F r o m the a d d i t i o n of 5 or 10% of e a r t h y ore, satisfactory pellet p r o p e r t i e s (tests N o s . 3 a n d 4) were a l r e a d y o b t a i n e d with a relatively coarse g r i n d i n g of crystalline ore. T h e a d d i t i o n

128

5 Process-Influencing Factors

Table 20. Influence of earthy iron ore as additive on properties of hematite pellets Mixture

Test conditions

Rich hematite ore (Brasil) 0.045 mm in % Specific surface area (cm 2 /g) Earthy low-grade ore specific surface area (cmVg) Addition of earthy ore to Hematite in %

1

2

3

4

100% 67 1720

100% 30 740

95% 30 740 830

90% 30 740 910

-

-

5

10

9 28 2400 8 6.8

4 13 1100 2 28.3

11 38 2500 9 10.5

15 56 3720 11 7.7

Pellet quality Compression strength in N/pellet Green Dry Fired Drop number of green pellets Abrasion index of fired pellets -0.5 mm in %

of a n e a r t h y o r e f r o m C e r r o Bolivar (Venezuela) t o the crystalline K i r u n a m a g n e t i t e concentrate exerts a similar influence. 5.3.1.5 Behaviour of Ore Mixtures A f t e r d e m o n s t r a t i n g t h a t the b e h a v i o u r of ores d u r i n g pelletizing can b e i m p r o v e d b y t h e a d d i t i o n of a n o t h e r o r e — w h i c h need not be the rule — the q u e s t i o n soon presents itself r e g a r d i n g t h e b e h a v i o u r of several o r e m i x t u r e s d u r i n g pelletizing and the p r o p e r t i e s of such pellets. T h e treatm e n t of o r e m i x t u r e s b e c a m e acute w h e n t h e construction of pelletizing plants in t h e vicinity of blast f u r n a c e s s i m i l a r to sinter plants was considered. In such a case, it was necessary to treat ores of d i f f e r e n t origin and m o r phology in the f o r m of varying mixtures, a n d nevertheless to p r o d u c e pellets of u n i f o r m quality. C o u n t r i e s covering their r a w ore d e m a n d mainly by i m p o r t s , such as G r e a t Britain, the E u r o p e a n c o n t i n e n t a n d a b o v e all J a p a n are particularly interested in the construction of such pelletizing plants. T h e o p e r a t i o n of these plants w h i c h m a y b e installed in the vicinity of sinter plants already existing o f f e r s t h e a d v a n t a g e t h a t t h e ores can be processed, according to their c o m p o s i t i o n , e i t h e r in a sinter or pelletizing plant. T h e s i m u l t a n e o u s availability of b o t h a g g l o m e r a t i o n systems allows t h e a p p l i c a t i o n of a m o r e flexible i m p o r t strategy. F i r s t

5.3 Influence of Binders and Additives

129

investigations w e r e carried o u t at a n early d a t e the p u r p o s e of w h i c h s o m e t i m e s was the t r e a t m e n t of d o m e s t i c ores 57). S y s t e m a t i c investigations grinding, w e r e r e p o r t e d in 1962 14 ). A t t h e suggestion of a G e r m a n steel plant, studies r e g a r d i n g t h e erection of a pelletizing p l a n t o n the basis of i m p o r t e d ores in the E u r o p e a n c o n t i n e n t w e r e started a r o u n d 1962 21 ). F o r this p u r p o s e , seven ores a n d c o n c e n t r a t e s a v a i l a b l e in t h e E u r o p e a n m a r k e t w e r e chosen. T h e pelletizing p r o p e r t i e s of s o m e of these r a w materials w e r e a l r e a d y k n o w n , those of others were not. O r e s especially p r e p a r e d f o r sinter plants (sinter f e e d ) were partly involved. T h e o r e types, h e m a t i t e a n d m a g n e t i t e , t h e c h e m i c a l analysis a n d t h e size p o r t i o n m i n u s 0.045 m m as well as the p o r t i o n s of the v a r i o u s o r e m i x constituents are c o m p i l e d in T a b l e 21. T h e a m o u n t of b i v a l e n t i r o n i n d i c a t e s t h e e n d e a v o u r to m a i n t a i n a u n i f o r m p o r t i o n of m a g n e t i t e in the mix. Similarly as for i n d i v i d u a l ores, the p o r t i o n of fines m i n u s 0.045 m m was at first v a r i e d a n d the green balls of 9 - 1 5 m m w e r e fired at 1300 ° C , see Fig. 64. T h e b e h a v i o u r of m i x t u r e s is s i m i l a r to t h a t of i n d i v i d u a l ores, see Figs. 44, 45, 51, s o m e t i m e s even better, see T a b l e 20. T h e results of tests r u n on m i x 2, T a b l e 21, were not s h o w n in t h e a b o v e figures, since only a n insufficient n u m b e r of m e a s u r e d values w a s available. H o w e v e r , t h e s e figures c o r r e s p o n d with the o t h e r values.

Table 21. Chemical composition and size of various ores, concentrates and mixtures for pellet feed Ores of various origin

Type Size -0.045 mm

% Fe

% Portions in mixture I

Sydvaranger Mano river Brazil fines Marcona (sinter feed) Krivoj rog Kiruna Bomi hill

Ma Hb H M M M M

57.6 1.7 26.7 2.6 73.0 41.7 2.4

II

III

IV

6 18 18 18 9 18 13

3 9 27 9 9 37 6

4 13 40 13 7 13 10

12 6 47 8

100

100

100

100

67.5 14.1 4.1 0.3

65.4 9.4 4.9 0.2

67.0 15.4 3.6 0.2

66.3 54.7 66.5 63.0 63.3 71.9 65.6

3 12

Chemical composition of mixtures Fe tot Fe" SiO2 CaO b

a M = Magnetite H = Hematite

64.5 12.4 5.6 0.3

with o r e m i x t u r e

130

5 Process-Influencing Factors

Fig. 64. Influence of particle size on properties of indurated pellets from ore mixtures

F o r c o m p a r i s o n , C u r v e V was a d d i t i o n a l l y i n t r o d u c e d f r o m Fig. 51 ( C u r v e I). T h e d i f f e r e n c e in c r u s h i n g strength b e t w e e n C u r v e V ( h e m a t i t e ) a n d C u r v e s I, IV, (mixtures) is not significant. In view of t h e relatively low slag p o r t i o n , the f o r m a t i o n of crystals is m o r e i m p o r t a n t . Basic a d d i t i v e s also h a v e a f a v o u r a b l e i n f l u e n c e o n mixtures, as can be seen f r o m C u r v e s III, IlIa, IIIb. Finally, it can be stated, that ore mixtures with s u f f i c i e n t grain size d i s t r i b u t i o n or with additives can b e f o r m e d into pellets of g o o d quality, similarly as for i n d i v i d u a l ore types w i t h o u t a n y difficulties. T h e mix is g r o u n d to a b o u t 67% m i n u s 0.045 m m at a specific s u r f a c e a r e a of approx. 2600 c m 2 / g . Less t h a n 0.5% b e n t o n i t e is u s e d as b i n d e r . T h e pellets p r o d u c e d m e e t the r e q u i r e m e n t s of t h e blast f u r n a c e o p e r a t i o n . T h e k n o w l e d g e thus a c q u i r e d in the l a b o r a t o r y tests was realized in a c o m m e r c i a l p l a n t at t h e I j m u i d e n steel p l a n t in H o l l a n d . T h e r e , a pellet plant with a n a n n u a l capacity of 3.5 m i l l i o n tons b a s e d o n i m p o r t e d o r e s 5 8 ) has b e e n o p e r a t i n g since 1970 b e s i d e a sinter p l a n t of e q u a l

5.3 Influence of Binders and Additives

131

capacity for the p r o d u c t i o n of a highly b a s i c sinter. T h e c o m b i n a t i o n of t h e two a g g l o m e r a t i o n plants can b e c o n s i d e r e d as a typical e x a m p l e f o r a n o p t i m u m p r e p a r a t i o n of i m p o r t e d ores f o r blast f u r n a c e o p e r a t i o n . B r a g a r d a n d M a t t h i e u 59 ) r e p o r t e d o n the c o n s t r u c t i o n a n d o p e r a t i o n of a small pellet p l a n t in Belgium with a n a n n u a l c a p a c i t y of 0.5 million tons of basic pellets f r o m m i x t u r e s of calcareous m i n e t t e , S w e d i s h c o n c e n t r a t e a n d V e n e z u e l a n h e m a t i t e in d i f f e r e n t p r o p o r t i o n s . D u e to the g o o d b o n d i n g p o w e r of m i n e t t e , n o e x t r a n e o u s b i n d e r s were necessary. H e r e also the q u a l i t y of the pellets p r o d u c e d m e t t h e r e q u i r e m e n t s of the blast f u r n a c e o p e r a t i o n . T h e c o n s i d e r a t i o n s l e a d i n g to t h e c o n s t r u c t i o n of pellet plants are very interesting a l t h o u g h at b o t h works sinter plants w e r e already in operation. In both cases, the extension of the agglomeration capacity was u n d e r discussion. T h e decision was t a k e n in favour of the erection of a pellet plant d u e to the greater flexibility w h i c h was expected, and also achieved, with regard to ore i m p o r t a n d the e q u a l l y h i g h d e m a n d s m a d e b y the blast f u r n a c e o p e r a t i o n o n the q u a l i t y of a g g l o m e r a t e s . In J a p a n , the conditions f o r the construction of pelletizing plants n e a r blast f u r n a c e s are also f a v o u r a b l e , a fact w h i c h is c o n f i r m e d by the erection of several plants with a n a n n u a l capacity of a b o u t 6 m i l l i o n tons of pellets 60 ). H e r e , as well as in G r e a t Britain, f u r t h e r plants w i t h a n a n n u a l capacity of 3 million a n d 5 million tons are u n d e r c o n s t r u c t i o n or will shortly b e started up. In this way, it has b e e n d e m o n s t r a t e d that b a s e d o n o r e m i x t u r e s pelletizing plants c a n be built a d v a n t a g e o u s l y n e a r the blast f u r n a c e s .

5.3.1.6 Influence of Oxidized and Prereduced Return Fines T h e question arises w h e t h e r a n i n f l u e n c i n g factor b o t h i m p o r t a n t a n d necessary for d o w n - d r a u g h t sintering, n a m e l y the use of r e t u r n fines, m i g h t also b e of interest for pelletizing. D u r i n g sintering, a h i g h p o r t i o n of 40—100% b y w e i g h t of the sinter m i x is c o n t i n u o u s l y c i r c u l a t e d as r e t u r n fines. T h e s e r e t u r n fines virtually consist of f i n e - g r a i n e d sinter o b t a i n e d d u r i n g d i f f e r e n t process a n d t r a n s p o r t phases. T h e c o n d i t i o n s f o r pellet p r o d u c t i o n are q u i t e d i f f e r e n t . Pellets consist of i n d i v i d u a l balls of p r e scribed d i m e n s i o n s . T h e a m o u n t of f i n e - g r a i n e d a b r a s i v e m a t e r i a l arising d u r i n g t r a n s p o r t of i n d u r a t e d pellets is c o r r e s p o n d i n g l y low a n d h a r d l y exceeds 3% in c o m m e r c i a l plants. T h i s low q u a n t i t y is r e t u r n e d , n o r m a l l y a f t e r s e p a r a t e i n t e r m e d i a t e g r i n d i n g , to the circulating pellet l o a d w i t h o u t p r o d u c i n g a special effect. F r o m the process-technological viewpoint, t h e r e is thus in contrast to sintering n o r e a s o n to attach g r e a t i m p o r t a n c e to the return fines p r o b l e m . Nevertheless, H . W. G u d e n a u a n d W. W e n zel61oudois)investigated the i n f l u e n c e of g r e a t e r r e t u r n fines a m o u n t s on t h e p r o p e r t i e s of i n d u r a t e d pellets p r o d u c e d f r o m d i f f e r e n t ores with t h e

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return fines n o r m a l l y b e i n g p r e s e n t as oxidized fines. Tests with u p to 35% p r e r e d u c e d r e t u r n fines were also carried out. T h e results of such tests r u n on h e m a t i t e o r e with 64% iron a n d with a rising r e t u r n fines q u a n t i t y having a fineness d e g r e e of 40% - 0 . 0 4 5 m m at o p t i m u m f i r i n g t e m p e r a t u r e are s h o w n in Fig. 65. C u r v e I only s h o w s a slight i n f l u e n c e of oxidized r e t u r n fines a d d i t i o n w h i l e C u r v e II indicates a s u b s t a n t i a l i m p r o v e m e n t of c r u s h i n g strength w h e n p r e r e d u c e d return fines are used.

Fig. 65. Influence of oxidized and reduced return fines on compressive strength of indurated pellets

D u r i n g c a r e f u l firing a n d with a s u f f i c i e n t o x y g e n content of b u r n i n g gas, r e o x i d a t i o n a n d recrystallization of the crystalline structure occurs. U n d e r e q u a l oxidizing conditions, p r e r e d u c e d return fines could be replaced by s p o n g e iron with t h e total i r o n b e i n g finally present as h e m a t i t e in the pellet. H o w e v e r , it w o u l d also be i m a g i n a b l e to allow the metallic i r o n of s p o n g e iron to react directly with iron oxides u n d e r a controlled, n o n - o x i d i z i n g a t m o s p h e r e . 5.3.1.7 Influence of Sponge Iron on Pellet Properties This q u e s t i o n was t h o r o u g h l y s t u d i e d by t h e a u t h o r 6 2 , 2 8 ) . H i g h g r a d e specially p r e p a r e d concentrates f r o m h e m a t i t e a n d m a g n e t i t e with iron contents of 68.9% a n d 70%, a n d SiO 2 values of 0.68 a n d 0.1% were g r o u n d to 2040 a n d 2280 c m 2 / g specific s u r f a c e area. Increasing quantities of s p o n g e iron with iron contents of 96—98% at a specific surface area of 6 0 0 - 7 0 0 c m V g were a d d e d d u r i n g green ball f o r m a t i o n . Pellets of 10—12 m m d i a m e t e r w e r e p r o d u c e d .

5.3 Influence of Binders and Additives

133

5 3 . 1 . 7 . 1 Influence of Sponge Iron on Green and Dry Pellet Strength. It would h a v e b e e n possible that s p o n g e i r o n could r e a c t with t h e m o i s t e n ing w a t e r d u r i n g green ball f o r m a t i o n . T h i s would h a v e resulted in a loss of metallic i r o n by corrosion. H o w e v e r , the s p o n g e iron used, w h i c h was p r o d u c e d f r o m the c o n c e n t r a t e involved at t e m p e r a t u r e s of 1 1 5 0 - 1 2 0 0 ° C , showed a low reactivity against w a t e r d u r i n g t h e s h o r t t i m e of contact. In a d d i t i o n , t h e s p o n g e iron h a d a relatively coarse g r a i n size with a specific s u r f a c e a r e a as low as a b o u t 6 0 0 - 7 0 0 c m 2 / g so t h a t a f t e r pellet drying n o c h a n g e of t h e a m o u n t of m e t a l l i c i r o n previously a d d e d was observed. T h e strength of g r e e n balls f o r m e d f r o m m a g n e t i t e c o n c e n t r a t e a n d sponge iron w i t h a n d w i t h o u t b e n t o n i t e a d d i t i o n was c o m p a r e d . T h e result is s h o w n in Fig. 66. W i t h o u t b e n t o n i t e , t h e g r e e n a n d d r y pellet

Fig. 66. Effect of sponge iron and bentonite on compressive strength of green and dry pellets

strength is very low irrespective of w h e t h e r s p o n g e i r o n is a d d e d or not. T h e rising rate of s p o n g e iron a d d e d even decreases t h e strength, p r o b a b l y d u e to the coarser grain size resulting f r o m s p o n g e i r o n a d d i t i o n . O n l y b e n t o n i t e raises t h e strength. T h e m e a s u r e m e n t s m a d e with h e m a t i t e concentrate were similar, b u t lower values were o b t a i n e d . T h e tests did n o t s h o w any r e a c t i o n of s p o n g e iron. T h u s , t h e r e was n o d a n g e r of m e t a l l i c iron b e i n g lost in this p h a s e of pellet p r o d u c t i o n . 5 3 . 1 . 7 . 2 Influence of Sponge Iron on the Strength of Indurated Pellets. T h e p u r p o s e of these tests was to p r o d u c e pellets low in g a n g u e with a

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5 Process-Influencing Factors

Fig. 67. Effect of sponge iron on compressive strength of indurated hematite pellets

high iron content by s p o n g e iron a d d i t i o n in o r d e r to avoid the d i s i n t e g r a tion w h i c h w o u l d otherwise b e expected d u r i n g r e d u c t i o n . T h e m e t a l l i c iron of sponge iron was to react with F e 2 O 3 and F e 3 O 4 to f o r m lower oxides while the new crystal combinations a n d crystalline structures arising were to bring a b o u t the pellet strength. T o avoid undesirable reactions b e t w e e n oxidizing heating gas a t m o s p h e r e and metallic iron, t h e basic tests were carried out with nitrogen at temperatures between 950 a n d 1150 0 C and a n i n d u r a tion t i m e of 1 5 - 4 0 minutes. T h e t e m p e r a t u r e of 1100 ° C was f o u n d to b e resents the influence of a rising sponge iron portion on the crushing strength, C u r v e I, in contrast to a decreasing pellet strength d u r i n g r e d u c t i o n of fired pellets w i t h o u t s p o n g e iron a d d i t i o n by using r e d u c i n g gas, C u r v e II. W h e n a d d i n g approx. 10 parts of s p o n g e iron, the h e m a t i t e p h a s e is converted to magnetite, a n d a crushing strength of 2200 N / p e l l e t is a l r e a d y achieved with n o f u r t h e r c h a n g e at h i g h e r s p o n g e iron rates. T h e reaction of metallic iron with oxides is a l r e a d y t e r m i n a t e d at 1100 0 C a f t e r 20 minutes. Metallic iron similarly reacts with m a g n e t i t e pellets h a v i n g a strength of almost 3500 N / p e l l e t , Fig. 68. T h e t w o Figs. 67 a n d 68 indicate a n interesting possibility for s u b s t a n tially i m p r o v i n g the q u a l i t y of i n d u r a t e d pellets w h i c h was p a r t i c u l a r l y observed d u r i n g their reduction, as described u n d e r i t e m 6.2.

sufficient

5.3 Influence of Binders and Additives

135

Fig. 68. Influence of sponge iron on compressive strength of pellets from magnetite concentrate

5 3 . 1 . 8 Influence of Inplant Fines on Pellet Properties Since a g r o w i n g n u m b e r of pelletizing p l a n t s are erected w i t h i n a steel plant c o m p l e x , it is possible to i n c o r p o r a t e a p a r t of t h e i n p l a n t fines d a i l y a c c u m u l a t i n g at the iron a n d steel w o r k s i n t o t h e pellet f e e d . T h i s refers, for e x a m p l e , to d u s t a n d slurries f r o m gas c l e a n i n g installations of blast oxygen f u r n a c e s a n d blast furnaces, mill scale, c a l c a r e o u s B. O. F. slag as well as pellet fines screened out b e f o r e e n t e r i n g the blast f u r n a c e . A b o v e all, p r o d u c t s w i t h a high lime content are interesting as additives. In a pelletizing p l a n t in Mexico, the use of such i n p l a n t fines in t h e m i x is envisaged 63 ). M a g n e t i t e concentrates, h e m a t i t e ore a n d i n p l a n t fines a r e used as i r o n - b e a r i n g materials. L i m e c o m p o u n d s are a d d e d in o r d e r to p r o d u c e basic pellets with a basicity of m o r e t h a n one. T h e recycled m a t e r i a l s consist of six d i f f e r e n t constituents w h i c h h a v e to be g r o u n d to the r e q u i r e d fineness b e f o r e they a r e m i x e d . T h e c h e m i c a l analysis of t h e recycled m a t e r i a l s is s h o w n in T a b l e 22. O u t of a large n u m b e r of tests carried out f o r d e s i g n work, t h e b e h a v i o u r of t h r e e m i x t u r e s is d e s c r i b e d below with t h e basicity h a v i n g b e e n u n i f o r m l y a d j u s t e d to 1.3. T h e c o m position of m i x t u r e s is given in T a b l e 23.

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Table 22. Mixture and chemical composition of inplant fines as components of Fe tot. SiO 2

Blast furnace dust Blast furnace slag BOF dust BOF slag Mill scale PeUet

Al 2 O 3

CaO

MgO

MnO

S

pellet

feed 63 )

Composition in %

34.8

10.2

2.7

1.1

1.2

1.2

0.4

3.7

0.3

34.6

14.8

41.8

4.3

0.9

1.5

40.4

64.1 12.0 64.0 64.8

1.8 20.5 3.0 4.3

0.2 6.4 1.9 0.8

5.2 42.5 3.2 0.6

0.6 3.5 0.3 0.9

1.5 4.2 1.3 0.1

0.1 0.6 0.1 0.02

5.2 21.1 8.5 21.1

Granulometric properties of mixture -0.045 mm = 55.6%; Spec. surf. area cmVg 2200 Table 23. Mixture of pellet feed: ores, inplant fines, limestone and hydrated lime 63) II

III

95.5

82.4

-

-

65.9 16.5 16.1

I Magnetite Hematite Inplant fines Limestone Lime hydrate

16.6

-

3.5 1.0

-

-

1.0

1.5

T h e m i x t u r e s w e r e u n i f o r m l y g r o u n d to 68—71% m i n u s 0.045 m m a n d a specific s u r f a c e a r e a of a b o u t 1 6 8 0 - 1 7 5 0 c m V g . T h e i n c o r p o r a t e d recycled nor on d r y or i n d u r a t e d pellets (Fig. 69). T h e a b r a s i o n resistance of all pellets h a s to b e r e g a r d e d as very g o o d . T o s u m up, it c a n be stated that, a f t e r an a d e q u a t e p r e p a r a t i o n , recycled materials can b e a d d e d to the pellet m i x w i t h o u t i m p a i r i n g the pellet quality - p r o v i d e d that they d o not c o n t a i n a n y d e t r i m e n t a l m e t a l l u r g i c a l constituents. This is a n i m p o r t a n t c o n t r i b u t i o n to the i n t e g r a t i o n of pelletizing plants into the blast f u r n a c e area. 5.3.1.9 Influence of Organic Binders I n o r g a n i c or m i n e r a l i c b i n d e r s a d d i t i o n a l l y c h a n g e t h e c h e m i c a l c o m position of t h e i n d u r a t e d pellets a n d m a y i n f l u e n c e t h e m e t a l l u r g i c a l

materials have no n

5.3 Influence of Binders and Additives

137

Fig. 69. Influence of in-plant fines on pellet properties

b e h a v i o u r d u r i n g f u r t h e r t r e a t m e n t . O n the o t h e r h a n d , o r g a n i c b i n d e r s b u r n or volatilize at t h e latest d u r i n g pellet firing. C o n s e q u e n t l y , they d o not c h a n g e t h e c h e m i c a l c o m p o s i t i o n of the pellets. A t a very early stage organic b i n d e r s were a l r e a d y tested at least o n a l a b o r a t o r y scale. In this connection, r e f e r e n c e s h o u l d be m a d e to the a f o r e - m e n t i o n e d p a t e n t of Andersson3). D u e to the c o m p a r a t i v e l y high prices of such substances, t h e r e was a n d is, for the m o s t p a r t , interest only in waste p r o d u c t s of o t h e r industries, such as b i t u m e n a n d s p e n t s u l p h i t e liquor. In view of t h e c o n s i d e r a b l e q u a n t i t i e s w h i c h can possibly be a d d e d d u r i n g pelletizing of i r o n ores, t h e mineral oil i n d u s t r y is interested in r e n d e r i n g such p r o d u c t s attractive 64 ). Since the efficiency of o r g a n i c b i n d e r s is l i m i t e d by the t e m p e r a t u r e , any i n f l u e n c e o n the final strength of i n d u r a t e d pellets is h a r d l y to b e expected. S o m e of such types of b i n d e r s are m e n t i o n e d below: Starch is a f r e q u e n t l y tested b i n d e r , w h i c h was a d d e d in q u a n t i t i e s of 0 . 4 5 - 2 . 3 k g / t c o n c e n t r a t e by Erie M i n i n g C o m p a n y d u r i n g pelletizing of magnetite. A p a r t f r o m a n i m p r o v e m e n t of plasticity, starch p r i m a r i l y increased t h e dry pellet strength 65 ).

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5 Process-Influencing Factors

Spent sulphite liquor, o b t a i n e d as a b y - p r o d u c t in the p u l p a n d p a p e r industry, is a p r o d u c t which is relatively c h e a p in s o m e places. Spent sulphite l i q u o r c o n t r i b u t e d to a n i m p r o v e m e n t of t h e green a n d d r y pellet strength d u r i n g pelletizing of Sierra L e o n e concentrate. A f t e r firing, no f u r t h e r i n f l u e n c e was observed. Bitumen, a b y - p r o d u c t a c c u m u l a t i n g d u r i n g m i n e r a l oil processing, is often used as e m u l s i o n which also i m p r o v e s the green a n d dry pellet strength. 5.3.1.10 Influence of Coal Addition In the past, the s h a f t f u r n a c e , as b u r n i n g unit, p l a y e d an i m p o r t a n t role 6 6 ). H o w e v e r , it was difficult to ensure u n i f o r m h e a t i n g of pellet f e e d over the w h o l e f u r n a c e section. T o achieve this goal, 1.0—1.5% a n t h r a c i t e was a d d e d to the concentrate or t h e green balls were c o a t e d with finelyg r o u n d coal. T h i s m e t h o d was previously a d o p t e d in all s h a f t f u r n a c e plants a n d also initially, even at lower coal rates, in s o m e travelling g r a t e plants. In t h e course of the years, the i n c o r p o r a t i o n of solid f u e l into green balls was a b a n d o n e d because t h e difficulties d u r i n g i n d u r a t i o n and a b o v e all the varying q u a l i t y of i n d u r a t e d pellets e x p e r i e n c e d at t h a t t i m e o u t w e i g h e d the expected advantages. N o w a d a y s , the shortage and increasing prices of fuel oil suggest the use of coal again. T w o possibilities were investigated, t h e a d d i t i o n of coal to fine particles d u r i n g balling a n d the b u r n i n g of coal in s e p a r a t e b u r n i n g c h a m b e r s . D e t a i l e d studies of coal a d d i t i o n are d e s c r i b e d by B. H ü t t e r 6 7 ) . U p to 2% of coals of d i f f e r e n t reactivity was a d d e d to five d i f f e r e n t ores ground to the fineness r e q u i r e d f o r green ball f o r m a t i o n . T h e pellet d i a m e t e r was u n i f o r m l y 1 0 - 1 2 m m . N e i t h e r t h e g r e e n nor the d r i e d pellet strength c h a n g e d as c o m p a r e d with that of pellets w i t h o u t coal a d d i t i o n . H o w e v e r , the c r u s h i n g strength of i n d u r a t e d pellets decreases w i t h coal a d d i t i o n of m o r e t h a n 0.5% for m a g n e t i t e pellets a n d the resistance to abrasion d i m i n i s h e s at a coal a d d i t i o n of a b o v e 1%. O n the o t h e r h a n d , it was possible to raise considerably t h e capacity of the pelletizing travelling grate for h e m a t i t e pellets. T h e r e f o r e , the negative a n d positive influences are to be o u t w e i g h e d for each p a r t i c u l a r case. Fig. 70 shows the influence of increasing coal a d d i t i o n s on h e m a t i t e and m a g n e t i t e pellets. In the case of h e m a t i t e , the strength rises u p to a b o u t 0.5% coal a d d i t i o n . T h e strength t h e n decreases b u t is still s u f f i c i e n t u p to 1.5% coal a d d i t i o n . This influence is explained by partial r e d u c t i o n of h e m a t i t e to m a g n e t i t e a n d s u b s e q u e n t i n c o m p l e t e r e o x i d a t i o n of m a g n e tite to h e m a t i t e . H i g h e r coal a d d i t i o n s m a y even result in a f u r t h e r r e d u c t i o n towards the f o r m a t i o n of w u s t i t e which, w i t h i n t h e t i m e available, does not reoxidize a g a i n a n d f o r m s f a y a l i t e with the silica

5.3 Influence of Binders and Additives

139

Fig. 70. Influence of coal addition on pellet strength and productivity of travelling grate

present. A t present, the pelletizing p l a n t at I j m u i d e n is b e i n g o p e r a t e d with low a d d i t i o n of g r o u n d coke w h i c h allows a r e d u c t i o n of n a t u r a l gas c o n s u m p t i o n w i t h o u t i m p a i r i n g the pellet q u a l i t y . O t h e r travelling grate plants are using m e a n w h i l e also coal a d d i t i o n successfully. D e v e l o p m e n t w o r k was recently started to b u r n t h e coal in s u i t a b l e c o m b u s t i o n c h a m b e r s a n d to i n t r o d u c e hot c o m b u s t i o n gases exclusively into the b u r n e r h o o d . A d e t a i l e d d e s c r i p t i o n of this interesting a p p l i c a t i o n of firing technology to pelletizing plants is given in t h e Skillings M i n i n g Review68). 5.3.1.11 Summarizing Considerations Binders were a l r e a d y used in the first pelletizing tests. W a t e r - s o l u b l e salts were p r e f e r r e d w h i c h f o r m salt bridges d u r i n g d r y i n g thus i m p r o v i n g the m e c h a n i c a l strength of pellets. N o w a d a y s , salts are used in few special cases only. Binders a n d a d d i t i v e s are at p r e s e n t selected a c c o r d i n g to t h e following aspects: (a) I m p r o v e m e n t of green ball p r o p e r t i e s , a b o v e all better plasticity f o r increasing the d r o p resistance d u r i n g t r a n s p o r t a t i o n . (b) Increase of d r y pellet strength f o r t h e s u b s e q u e n t h e a t t r e a t m e n t stages.

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(c) I m p r o v e m e n t of i n d u r a t e d pellet properties, in principle, their b e h a v iour d u r i n g reduction. (d) Binders s h o u l d be c h e a p a n d not adversely a f f e c t i n g the c h e m i c a l c o m p o s i t i o n of pellets a n d o p e r a t i o n . (e) N o w a d a y s , alkaline e a r t h c o m p o u n d s of d i f f e r e n t m i x t u r e s are preferred. O n e a t t e m p t s to k e e p t h e b e n t o n i t e a d d i t i o n at a m i n i m u m . In all plants at present in o p e r a t i o n , b i n d e r s a n d a d d i t i v e s are used. In the m e a n t i m e , a s e p a r a t e industry has d e v e l o p e d for p r o v i d i n g a d d i t i v e s and binders.

5.4 Influence of Thermal Treatment on Pellet Properties T h e considerations relating to the f u n d a m e n t a l s of pellet p r o d u c t i o n (item 2.2) a l r e a d y d e m o n s t r a t e d t h e great i m p o r t a n c e of t h e r m a l treatm e n t f o r the final pellet quality. F u r t h e r m o r e m o s t b i n d e r s a n d additives only b e c o m e fully effective in c o n j u n c t i o n with h e a t i n g a n d firing. T h e t r e a t m e n t c a n b e d i v i d e d into t h r e e ( t h e r m a l ) phases: (a) D r y i n g at 2 5 0 - 4 0 0 0 C (b) P r e h e a t i n g u p to 1 1 0 0 - 1 2 0 0 0 C (c) Firing at 1 2 0 0 - 1 3 5 0 0 C . T h e t i m e a n d velocity of the necessary t e m p e r a t u r e rise d e p e n d o n the properties of the i n d i v i d u a l ores and mixtures. T h e i m p o r t a n c e of the drying p h a s e a n d its bases were a l r e a d y e x p l a i n e d u n d e r item 2.2.1. In this connection, the various factors were i n d i c a t e d w h i c h b e c o m e effective and i n f l u e n c e t h e pellet quality. T h e principal factors are d e s c r i b e d below. In view of the c o m p l e x i t y of this p r o c e d u r e , m a t h e m a t i c a l c o n s i d e r a t i o n s h a v e a d d i t i o n a l l y b e e n m a d e f o r interested r e a d e r s a n d set f o r t h u n d e r item 13.3.

5.4.1 F a c t o r s Influencing Green Ball Drying F o r clarification of the drying p r o c e d u r e , it a p p e a r e d a d v i s a b l e - to consider f u r t h e r aspects 6 9 ). T h e most i m p o r t a n t f a c t o r i n f l u e n c i n g t h e drying p r o c e d u r e is the drying m e d i u m : d r y i n g air o r drying gas. V a r i a b l e factors are t e m p e r a t u r e , velocity of gas flow, a n d drying time. T h e y not only d e t e r m i n e the pellet q u a l i t y b u t also i n f l u e n c e the t i m e of total thermal t r e a t m e n t and thus, the b u r n i n g unit c a p a c i t y , see Figs. 83 a n d 84. T h e s e t h r e e factors are i n t e r d e p e n d e n t . T h e e f f e c t of o n e s h o u l d only b e considered in c o n j u n c t i o n with others. F o r the relevant tests a t u b e

5.4 Influence of Thermal Treatment on Pellet Properties

141

f u r n a c e , s h o w n in Fig. 36 28 ), item 4.5.1, w a s used. Pellets of 1 0 - 1 2 m m d i a m e t e r p r o d u c e d f r o m a m a g n e t i t e - h e m a t i t e c o n c e n t r a t e with 67.5% iron and o p t i m u m size d i s t r i b u t i o n were tested. T h e b e d h e i g h t a p p l i e d in the tests was 6 cm. T h e criterion for t h e d r y i n g progress is the d r y i n g degree"letragregatau"w h i c h can b e r e p r e s e n t e d b y the f o l l o w i n g f o r m u l a :

W h e n"letragregatau"reaches t h e v a l u e one, t h e d r y i n g is t e r m i n a t e d . T h e following p a r a m e t e r s were varied. 5.4.1.1 Temperature of Drying Gases 5.4.1.1.1 Influence of Drying Gas Temperature on Drying Time. T h e greater the t e m p e r a t u r e d i f f e r e n c e b e t w e e n d r y i n g m e d i u m a n d green ball feed, the shorter is the p e r i o d r e q u i r e d f o r drying. T h e t e m p e r a t u r e of the m e d i u m s h o u l d t h e r e f o r e be as h i g h as possible. T h e t e m p e r a t u r e influence at a p r e d e t e r m i n e d velocity of flow of d r y i n g gas is s h o w n in Fig. 71. F o r reasons set forth in Figs. 15 a n d 16, i t e m 2.2.1 t h e drying velocity is not in parallel with the t e m p e r a t u r e . F r o m a t e m p e r a t u r e of a b o u t 250 0 C , the d r y i n g slows d o w n m o r e a n d m o r e . T h e t i m e for w a t e r e v a p o r a t i o n a n d s t e a m d i f f u s i o n in t h e pellets a r e l i m i t i n g factors. T h e drying t e m p e r a t u r e is f u r t h e r limited by a n o t h e r pellet p r o p e r t y , viz. t h e shock b e h a v i o u r of g r e e n balls.

Fig. 71. Effect of drying gas temperature on drying time

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5.4.1.1.2 Influence of Temperature on Shock Resistance of Pellets During Drying. A s i n d i c a t e d u n d e r item 2.2.1.4, the shock t e m p e r a t u r e is of decisive i m p o r t a n c e for the pellet quality. T h e shock b e h a v i o u r of each i n d i v i d u a l o r e is a limiting factor f o r the drying t e m p e r a t u r e a n d d r y i n g speed. It is virtually d e t e r m i n e d b y the t y p e of ore, g r a n u l o m e t r i c properties of ore, porosity of the green balls a n d binders. O w i n g to the effect of drying t e m p e r a t u r e on the drying t i m e , o n e a t t e m p t s to i m p r o v e the shock t e m p e r a t u r e possibly by the a d d i t i o n of b i n d e r s as e x p l a i n e d in the following test 2 8 ). T h e shock t e m p e r a t u r e of g r e e n balls of 10—12 m m

Fig. 72. Influence of drying gas temperature on shock behaviour of pellets with and without additives

d i a m e t e r p r o d u c e d f r o m Itabira b l u e dust with 69.3% iron was a s c e r t a i n e d with a n d w i t h o u t t h e a d d i t i o n of b e n t o n i t e a n d lime hydrate. T h e results o b t a i n e d a r e s h o w n in Fig. 72. D e p e n d i n g on the green ball s t r u c t u r e the m a x i m u m p e r m i s s i b l e shock t e m p e r a t u r e of 240 ° C w i t h o u t b i n d e r s c a n be raised u p to 600 0 C with bentonite and u p to 440 0 C with l i m e h y d r a t e a d d i t i o n b e f o r e the pellets start to b e d a m a g e d . T h e result refers to this specific ore, b u t can be r e g a r d e d as typical f o r the drying p r o c e d u r e a n d shock t e m p e r a t u r e of green balls f r o m o t h e r ores. 5.4.1.2 Influence of Velocity of Drying Gas Flow on Drying Degree T h e velocity of flow is m e a s u r e d in m e t e r s p e r second at 0 0 C a n d 1.0 b a r r e f e r r e d to the f r e e sectional area of the f u r n a c e as W 0 = N m / s .

5.4 Influence of Thermal Treatment on Pellet Properties

143

T h i s velocity of gas flow is a n i m p o r t a n t f a c t o r f o r t h e d i m e n s i o n i n g of fans in t h e d r y i n g zone a n d f o r t h e size of r e a c t i o n s u r f a c e of travelling grates or t h e v o l u m e of s h a f t f u r n a c e s or rotary kilns. O p t i m u m d e s i g n d a t a are o b t a i n e d by pot grate tests. T h r e e d i f f e r e n t air velocities of W o = 0 . 0 4 5 , 0.27 a n d 1.29 m / s w e r e a p p l i e d . T h e latter value is a l r e a d y w i t h i n t h e r a n g e of velocities occurring in a travelling grate pelletizing plant. Fig. 73 very clearly shows t h e i n f l u e n c e of air velocity. A t a velocity of 1.29 m / s the d r y i n g is a l r e a d y t e r m i n a t e d a f t e r five m i n u t e s while t h e d r y i n g takes 14 m i n u t e s at a n air velocity of 0.27 m / s . C o n s e q u e n t l y , a n o p t i m u m velocity of f l o w is to b e a i m e d at.

Fig. 73. Effect of drying gas velocity and time on dryng stage

5.4.1.3 Change of Pellet Strength During Drying T h e basic c o n d i t i o n s for the g r e e n a n d d r y pellet strength are very different. T h e c r u s h i n g strength of green balls is chiefly d e t e r m i n e d by t h e capillary forces of water. It reaches its m a x i m u m value w h e n the l i q u i d filling d e g r e e in t h e capillaries is a b o u t 0 . 8 - 0 . 9 . D u r i n g drying, t h e l i q u i d b o n d s are r e d u c e d a n d g r a d u a l l y r e p l a c e d b y others. In t h e case of d r y pellets, solid b o n d s are virtually effective b e t w e e n the ore particles. It is i m p o r t a n t t h a t the pellets retain s u f f i c i e n t strength even in this interm e d i a t e stage to w i t h s t a n d the m e c h a n i c a l stress b o t h in t h e s h a f t f u r n a c e a n d on the travelling grate. D. F. B a l l 6 9 ) s t u d i e d the strength of m a g n e t i t e a n d h e m a t i t e pellets with a n d w i t h o u t t h e a d d i t i o n of b e n t o n i t e a n d iron s u l p h a t e d u r i n g drying. A t t h e b e g i n n i n g of m o i s t u r e removal, he

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observed a slight strength increase and, shortly b e f o r e the final drying, a m i n i m u m strength. G u d e n a u , Wenzel u n d Izgiz 7 0 ) f o u n d a n i n t e r m e d i a t e strength increase d u r i n g drying for d i f f e r e n t ores, a n d this strength increase was f a r a b o v e the green a n d dry pellet strength. F r o m this it follows t h a t the capillary forces b e c o m e m o r e effective d u r i n g d r y i n g t h a n d u r i n g m o i s t e n i n g a n d green ball f o r m a t i o n . If drying is achieved in a pellet layer, t h e strength indices vary according to the progress of the drying f r o n t , so t h a t the strength is slightly d i f f e r e n t in the various pellet layers. In a test 2 8 ) r u n o n pellets of 1 0 - 1 2 m m d i a m e t e r a n d with a m o i s t u r e c o n t e n t of 7.5%, the d r y i n g progress a n d the c o r r e s p o n d i n g pellet strength w e r e investigated in t w o layers each of 6 c m height. T o ensure t h a t a m e a s u r a b l e d r y pellet strength can b e achieved, 0.7% bentonite was a d d e d to the mix. T h e variation of the m o i s t u r e content of the b e d as well as the strength of t h e u p p e r a n d lower layer were ascertained. T h e m e a s u r e m e n t s were m a d e every m i n u t e . T h e results o b t a i n e d are s h o w n in Fig. 74. A g r e a t e r t o p layer strength as o p p o s e d to b o t t o m layer strength can be clearly o b s e r v e d which m e a n s t h a t the pellet strength varies with the m o i s t u r e c o n t e n t d u r i n g drying. T o w a r d s the e n d of the test, t h e strength curves a p p r o a c h each other. T h e o b s e r v a t i o n m a d e with a 12 cm b e d height can be a n a l o g o u s l y a p p l i e d to g r e a t e r bed heights. As can b e seen f r o m Fig. 18, i t e m 2.2.1, it m a y h a p p e n t h a t the drying zone is a l r e a d y o v e r l a p p e d by the p r e h e a t i n g zone which directly follows. As is a p p a r e n t f r o m T a b l e 4, i t e m 2.2.1, t h e

Fig. 74. Difference of compressive strength in two layers of a green pellet bed during drying

5.4 Influence of Thermal Treatment on Pellet Properties

145

drying strength of pellets w i t h o u t b i n d e r a d d i t i o n is o f t e n i n s u f f i c i e n t t o m e e t the m e c h a n i c a l r e q u i r e m e n t s . Since n o w a d a y s practically all pelletizing plants a r e o p e r a t e d with b i n d e r s as is d e s c r i b e d u n d e r item 5.3, a sufficient d r y pellet strength is e n s u r e d in i n d u s t r i a l plants.

5.4.2 Preheating of Dried Pellets T h e p r e h e a t i n g zone is the t h e r m a l link b e t w e e n t h e d r y i n g and firing of pellets. It r a n g e s f r o m 3 0 0 / 4 0 0 0 C u p to 1200 0 C . A s a l r e a d y i n d i c a t e d u n d e r i t e m 2.2.2, d i f f e r e n t reactions m a y d e v e l o p s i m u l t a n e o u s l y o r consecutively d u r i n g p r e h e a t i n g . T h e s e reactions such as d e c o m p o s i t i o n of hydrates, c a r b o n a t e s or s u l p h a t e s as well as roasting of s u l p h i d i c constituents m a y be i m p o r t a n t b o t h for t h e final pellet q u a l i t y a n d for t h e capacity of pelletizing plants. Last b u t n o t least all i r o n oxides b e s i d e s h e m a t i t e s are c o n v e r t e d into the highest o x i d a t i o n stage, n a m e l y h e m a t i t e . T h e p r e h e a t i n g velocity in this zone m u s t b e h a r m o n i z e d with the c o m p o u n d s to b e d e c o m p o s e d a n d oxidized. T h e o x i d a t i o n of m a g n e t i t e is m o s t i m p o r t a n t since it has a decisive influence o n b o t h the m e c h a n i c a l strength a n d t h e b e h a v i o u r d u r i n g reduction. D u r i n g m a g n e t i t e o x i d a t i o n , the b i v a l e n t i r o n is c o n v e r t e d to trivalent iron (see i t e m 5.4.3.1). T h e progress of o x i d a t i o n is n o r m a l l y analytically a s c e r t a i n e d t h r o u g h the r e m a i n i n g p a r t of b i v a l e n t iron. A f t e r c o m p l e t e o x i d a t i o n , bivalent iron s h o u l d n o longer be present. H o w e v e r , it is also possible to analyse t h e progress of o x i d a t i o n t h r o u g h the c h a n g e of weight d u r i n g m a g n e t i t e p r e h e a t i n g u n d e r o x i d i z i n g a t m o s p h e r e 71 ). 5.4.2.1 Change of Weight and Strength During Drying and Preheating of Green Balls D u r i n g drying, the w e i g h t of g r e e n balls d i m i n i s h e s b y the q u a n t i t y of moistening w a t e r e v a p o r a t e d until a constant w e i g h t is r e a c h e d . Ores with d e c o m p o s a b l e c o m p o u n d s c o n t a i n i n g h y d r a t i o n or crystallization w a t e r o r c a r b o n a t e s vary their w e i g h t in a c c o r d a n c e w i t h the n e w c o m p o u n d s t h u s arising. F o r instance, in the case of m a g n e t i t e pellets, t h e weight again rises w h e n t h e o x i d a t i o n starts. H e r e a n d also in t h e o t h e r c o m p o u n d s , a constant w e i g h t is only a c h i e v e d w h e n t h e i r o n oxides h a v e finally b e e n c o n v e r t e d into h e m a t i t e . A t t h e s a m e time, a d i f f e r e n t increase of pellet strength m a y occur d u r i n g d r y i n g a n d , a b o v e all, d u r i n g heating. T h e d i f f e r e n t b e h a v i o u r of m a g n e t i t e a n d h e m a t i t e green balls was c o m p a r a b l y investigated 28) a n d the results o b t a i n e d are s h o w n in Fig. 75. T h e p r i n c i p a l test d a t a w e r e as follows: M a g n e t i t e pellets w i t h 71.3% iron a n d 6.8% water, pellet d i a m e t e r : 10—12 m m , s p e c i f i c s u r f a c e a r e a of concentrate: 1810 c m 2 / g .

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Fig. 75. Combined effect of drying gas temperature and time on green pellet weight and compressive strength

H e m a t i t e pellets with 64.6% iron, 7.1% water, pellet d i a m e t e r 1 0 - 1 2 m m , specific s u r f a c e area: 1845 c m V g , pellet b e d height: 6 c m , w e i g h t of wet pellets: 300 g, initial t e m p e r a t u r e of h e a t i n g air: 300 ° C , relative h e a t i n g air velocity: 0.09 m / s e c . T h e m e a s u r e m e n t s were m a d e in the t u b e f u r n a c e represented in Fig. 36, item 4.5.1. T h e low air velocity was intentionally chosen in o r d e r to avoid too rapid a heating and to extend the heating procedure. Both ore samples h a d practically n o loss o n ignition. In this way, the drying a n d h e a t i n g could b e c a r r i e d out speedily. Curve I of Fig. 75 shows t h e average t e m p e r a t u r e g r a d i e n t in t h e pellet layer. A f t e r the d r y i n g of m a g n e t i t e pellets (Curve I I ) h a d b e e n t e r m i n a t e d after 20 m i n u t e s , t h e m i n i m u m w e i g h t h a d b e e n r e a c h e d - the o x i d a t i o n started at 400 0 C with w e i g h t increase. T h e o x i d a t i o n is t e r m i n a t e d at a b o u t 1000 0 C a n d constant weight. D u r i n g oxidation, the c r u s h i n g strength c o n t i n u o u s l y rose a n d r e a c h e d a b o u t 1100 N / p e l l e t at 11000C (Curve IV). T h e arising h e m a t i t e crystals (see Fig. 20) f o r m e d t h e first crystal c o m p o u n d s .

5.4 Influence of Thermal Treatment on Pellet Properties

147

Fig. 76. Effect of rising gas temperature on pellet strength, supplement to Fig. 75

T h e c o r r e s p o n d i n g curves for h e m a t i t e pellets d e v e l o p q u i t e d i f f e r e n t l y . A f t e r drying, a constant weight is o b t a i n e d w h i c h is m a i n t a i n e d d u r i n g i n d u r a t i o n ( C u r v e III). In the case of h e m a t i t e , a b r i d g e f o r m a t i o n occurs only at h i g h e r t e m p e r a t u r e s so that u p to 1100 0 C a low c r u s h i n g strength of a b o u t 200 N / p e l l e t is observed, (Curve V). T h e d i f f e r e n t strength d e v e l o p m e n t in curves IV a n d V shows t h a t t h e h e m a t i t e p r o d u c e d f r o m oxidized m a g n e t i t e h a s a m u c h h i g h e r reactivity a n d that crystal g r o w t h a l r e a d y occurs at lower t e m p e r a t u r e s , as is f u r t h e r a p p a r e n t f r o m t h e c o m p l e m e n t a r y curves IV a n d V in F i g . 76.

5.4.3 Firing and C o o l i n g of Pellets D u r i n g firing as well as cooling, those pellet p r o p e r t i e s s h o u l d be o b t a i n e d w h i c h are r e q u i r e d for t h e i r f u r t h e r t r e a t m e n t . T h e firing directly follows the p r e h e a t i n g . A c c o r d i n g to the o r e type, q u a n t i t y a n d c o m position of a d d i t i v e s , it is carried o u t at t e m p e r a t u r e s u p to 1250 0 C a n d 1350 0 C u n d e r a n oxidizing a t m o s p h e r e . A certain m i n i m u m t e m p e r a t u r e has to be a t t a i n e d in o r d e r to e n a b l e the necessary crystal transf o r m a t i o n s a n d t h e reactions of o x i d i c g a n g u e constituents. T h e m a x i m u m t e m p e r a t u r e is l i m i t e d by the b e g i n n i n g dissociation of h e m a t i t e a n d sticking of pellets b e c a u s e of s o f t e n i n g of constituents. T h e o p t i m u m firing t e m p e r a t u r e a n d cycle in c o n j u n c t i o n with the c o r r e s p o n d i n g f i r i n g p e r i o d is the decisive influencing factor f o r pellet i n d u r a t i o n . Practically all the o t h e r i n f l u e n c i n g factors e i t h e r g r a n u l o m e t r i c

properties

or t h e v a r i o u s

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t h e r m a l t r e a t m e n t . Besides t h e strength increasing reactions others o c c u r in parallel w h i c h h a v e partly b e g u n a l r e a d y in the p r e h e a t i n g zone, s u c h as d e c o m p o s i t i o n of carbonates' or s u l p h a t e s a n d the rest of h y d r a t e s . E x o t h e r m i c reactions too, mainly the o x i d a t i o n of m a g n e t i t e , m a y p l a y a certain role at the b e g i n n i n g of the firing zone. It is necessary to control these partly o p p o s e d reactions by t h e speed of h e a t s u p p l y so t h a t a decrease of pellet quality is a v o i d e d . T h e s m a l l e r the n u m b e r of i n f l u encing t h e r m a l factors to be a d a p t e d to each other, the s i m p l e r is t h e control of p r e h e a t i n g and firing t e m p e r a t u r e . F r o m this c o m e s the a t t e m p t to use, if possible, one single o u t e r h e a t i n g source, such as oil or gas, b u t not s i m u l t a n e o u s l y coal. T h e h e a t s u p p l y h a s to be so p r o p o r t i o n e d t h a t the t e m p e r a t u r e steadily rises. T o o slow a h e a t i n g has n o n e g a t i v e influence o n the pellet quality b u t the firing t i m e is e x t e n d e d w h i c h results in a capacity decrease of firing units. T o o r a p i d a h e a t i n g can b r i n g a b o u t a heat a c c u m u l a t i o n in the pellet s u r f a c e w h i c h causes o v e r h e a t i n g a n d , thus, a q u a l i t y decrease. T h e greatest a d d i t i o n a l i n f l u e n c i n g factor is the o x i d a t i o n of m a g n e t i t e which has to be i n c o r p o r a t e d into the firing p a t t e r n d u r i n g p r e h e a t i n g a n d firing. 5.4.3.1 Heat-Hardening of Magnetite Green Pellets M a g n e t i t e is almost exclusively processed as concentrate. T h e concentrate can originate either f r o m n a t u r a l m a g n e t i t e - c o n t a i n i n g ores or

Fig. 77. Oxidation of natural and artificial magnetite during thermal treatment under oxidizing athmosphere

5.4 Influence of Thermal Treatment on Pellet Properties

149

f r o m other ores as a result of m a g n e t i z i n g r o a s t i n g f o l l o w e d b y m a g n e t i c separation. T h e artificial m a g n e t i t e h a s a m u c h h i g h e r reactivity t h a n natural m a g n e t i t e . T h i s is p a r t i c u l a r l y d e m o n s t r a t e d by t h e o x i d a t i o n to h e m a t i t e , w h i c h in the case of artificial m a g n e t i t e a l r e a d y starts at lower t e m p e r a t u r e s a n d p r o c e e d s m o r e quickly t h a n with n a t u r a l m a g n e t i t e . Fig. 77 shows the c o n s i d e r a b l e d i f f e r e n c e . F o r e x a m p l e , t h e o x i d a t i o n d e g r e e of artificial m a g n e t i t e is at 400 0 C n e a r l y identical w i t h that of natural m a g n e t i t e at 1000 ° C . D u r i n g this o x i d a t i o n , m a g n e t i t e is c o n v e r t e d to h e m a t i t e . T h e m e c h a nism of this o x i d a t i o n was the s u b j e c t of intensive investigations 72) a n d does not yet a p p e a r to be fully clarified. T h e following f a c t o r s are of i m p o r t a n c e d u r i n g oxidation: t e m p e r a t u r e , gas flow, velocity a n d oxygen partial pressure of h e a t i n g gases, grain s h a p e a n d grain size of concentrates. T h e porosity of dry pellets plays a c e r t a i n role. A c c o r d i n g to the present state of k n o w l e d g e , the o x i d a t i o n p r o c e e d s in t w o s u b s e q u e n t partial p h a s e s to y(letragregag a m a ) F e 2 O 3 . T h e first p h a s e p r o c e e d s m u c h m o r e q u i c k l y t h a n t h e second. T h e o x i d a t i o n is a n e x o t h e r m i c r e a c t i o n a c c o r d i n g to the f o r m u l a : 2 F e 3 O 4 +1/2O 2 = 3 F e 2 O 3 - Q T h e Q value is a p p r o x i m a t e l y 260 k J / M o l e . T h i s a d d i t i o n a l h e a t source has to be c o n s i d e r e d a n d u s e d accordingly d u r i n g p r e h e a t i n g a n d firing. D u e to o x i d a t i o n heat, t h e pellet core is h e a t e d to a h i g h e r t e m p e r a t u r e t h a n the pellet surface. T h i s involves the risk of a m o r e intensive sintering or even m e l t i n g of pellet core a n d d u e to s h r i n k a g e its s e p a r a t i o n f r o m the pellet shell w h i c h results in a c o n s i d e r a b l e pellet q u a l i t y i m p a i r m e n t 73). T h e a d a p t a t i o n of o x i d a t i o n velocity to firing velocity h a s a positive effect d u r i n g firing of m a g n e t i t e pellets. A t a c o n s t a n t pellet q u a l i t y , the h e a t c o n s u m p t i o n , firing unit c a p a c i t y a n d firing t e m p e r a t u r e are m o r e f a v o u r a b l e t h a n d u r i n g firing of h e m a t i t e pellets. D u r i n g the o x i d a t i o n of m a g n e t i t e to h e m a t i t e , first h e m a t i t e crystals are already f o r m e d at low t e m p e r a t u r e s o n t h e edges a n d s u r f a c e s of o r e grains a n d crystals. T h e s e crystals s i m u l t a n e o u s l y cause the f o r m a t i o n of the first b r i d g e s as s h o w n in Fig. 20, i t e m 2.2.2.1.1. At 1150 0 C m a g n e t i t e pellets a l r e a d y reach a strength of 1100 N / p e l l e t while the h e m a t i t e pellets only have a strength of a b o u t 200 N / p e l l e t . W h e n the f i r i n g t e m p e r a t u r e is f u r t h e r raised, t h e strength increases, as can b e seen f r o m Fig. 76. T h i s figure is to b e c o n s i d e r e d as a s u p p l e m e n t to Fig. 75. C u r v e IV shows t h e f u r t h e r strength d e v e l o p m e n t of m a g n e t i t e pellets a n d C u r v e V that of h e m a t i t e pellets. T h e c o r r e s p o n d i n g v a r i a t i o n of crystalline structure, s h o w n in Figs. 21 and 24 of i t e m 2.2.2.1.1/2 s h o u l d b e noted. Callender 73) s t u d i e d the i n f l u e n c e of the p r e h e a t i n g a n d f i r i n g t e m p e r a t u r e o n pellets p r o d u c e d f r o m artificial m a g n e t i t e c o n c e n t r a t e with a n d

150

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w i t h o u t additives. T h e crushing strength of pellets p r o d u c e d w i t h o u t additives rises u p to a t e m p e r a t u r e of 1300 0 C . T h e i n f l u e n c e of 1% l i m e h y d r a t e was clearly p e r c e p t i b l e b y t e m p e r a t u r e decrease. 5.4.3.2 Influence

of Firing Temperature and Basic Additives Strength of Pellets from Magnetite Ores

on

the

A c c o r d i n g to the chemical c o m p o s i t i o n of o r e or concentrates a n d the i n t e n d e d use of pellets, the basic additives a r e varied. Q u a n t i t y a n d type of additives d e t e r m i n e the o p t i m u m firing t e m p e r a t u r e . I n t e r d e p e n d e n c e b e t w e e n these two factors a n d t i m e can be o b s e r v e d f r o m Figs. 61 a n d 62 of item 5.3.1.2.2. W i t h o u t additives, a specific m a x i m u m strength is achieved, Fig. 61. T h i s strength can b e substantially increased with t h e use of additives, Fig. 62. F o r a n e q u a l pellet strength, it s h o u l d b e possible to lower the firing t e m p e r a t u r e accordingly w h e n b a s i c additives are used. T h e result of such test series 28) is r e p r e s e n t e d in Fig. 78. T w o m a g n e t i t e concentrates with p o r t i o n s of 10% SiO 2 a n d 1% SiO 2 were fired with d i f f e r e n t a m o u n t s of basic additives at such t e m p e r a t u r e s w h i c h are permissible f o r the highest pellet strength without additives. T h e concentrate with a high g a n g u e c o n t e n t (Curve I) h a d r e a c h e d its o p t i m u m strength of 3500 N / p e l l e t at a t e m p e r a t u r e of 1250 ° C , w h e r e a s the concentrate low in g a n g u e h a d a c h i e v e d its

Fig. 78. Effect of additives on induration temperature of pellets from magnetite at equal strength

maximum

5.4 Influence of Thermal Treatment on Pellet Properties

151

additives b o t h c o n c e n t r a t e s c a n b e i n d u r a t e d at m u c h lower t e m p e r a t u r e s for o b t a i n i n g c o n s t a n t strength. C a ( O H ) 2 ( C u r v e I I ) or C a C O 3 (Curve I) have a similar influence. Artificial m a g n e t i t e w i t h 64.3% F e a n d 9% SiO 2 reacts m o r e sensitively to l i m e h y d r a t e a d d i t i o n (Curve III). 5.4.3.3 Firing of Hematite Green Pellets In the case of m a g n e t i t e a n d n a t u r a l h e m a t i t e , t h e pellet strength is virtually d e t e r m i n e d b y crystal m o d i f i c a t i o n s . In contrast to pellets f r o m m a g n e t i t e c o n c e n t r a t e s the s o l i d i f i c a t i o n of h e m a t i t e pellets has to be

Fig. 79. Influence of firing temperature on grain growth in hematite pellets

achieved by crystal g r o w t h in o n e step only. T h i s r e q u i r e s a higher i n d u r a t i o n t e m p e r a t u r e a n d longer f i r i n g t i m e as a l r e a d y e x p l a i n e d u n d e r item 2.2.2. T h e i m p o r t a n c e of the t e m p e r a t u r e f o r the g r o w t h of crystals f r o m h e m a t i t e pellets with 99.5% F e 2 O 3 is s h o w n in Fig. 79 32 ). O n l y f r o m 1300 0 C o n w a r d was a n o t a b l e c h a n g e of particle size o b s e r v e d . In parallel to this growth a n d b r i d g e f o r m a t i o n , the strength increases with rising t e m p e r a t u r e s , as is a p p a r e n t f r o m C u r v e V of Fig. 76 r e f e r r i n g to pellets p r o d u c e d f r o m n a t u r a l h e m a t i t e ores. O n l y at t e m p e r a t u r e s a b o v e 1250 ° C is an a p p r e c i a b l e increase of c r u s h i n g strength obvious, w h i c h reaches its o p t i m u m value at 1320 0 C . In this c o n n e c t i o n the visible crystal changes on m i c r o g r a p h s N o s . 2 4 / 2 5 of item 2.2.2.1.2 w h i c h occurred at rising t e m p e r a t u r e s are i n f o r m a t i v e . T h e crystal g r o w t h is not only d e p e n d e n t on the t e m p e r a t u r e b u t also on the time, as s h o w n in Fig. 80 34 ). A t a firing t e m p e r a t u r e of 1250 ° C , 30 m i n u t e s are n e e d e d in o r d e r to o b t a i n a specific crystal size, b u t only five m i n u t e s at a t e m p e r a t u r e of 1350 0 C .

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5 Process-Influencing Factors 5.4.3.4 Influence of Temperature and Basic Additives on Hematite Pellet Quality

Basic constituents reacting with h e m a t i t e s h o u l d also h a v e a magnetite, a similar i n f l u e n c e is s h o w n in Fig. 78.

temperature-decreasing

Fig. 80. Crystal growth as function of firing temperature and time

Fig. 81. Influence of additives on firing temperature of hematite pellets of equal strength

effect as

5.4 Influence of Thermal Treatment on Pellet Properties

153

5 . 4 . 4 R e a c t i o n s o f A d d i t i v e s with I r o n O x i d e s a n d Gangue Constituents D u r i n g the t h e r m a l t r e a t m e n t of pellets c o n t a i n i n g basic additives, t h e latter should f o r m c o m p o u n d s with t h e g a n g u e m i n e r a l s or iron oxides at a p p r o p r i a t e firing t e m p e r a t u r e . D u r i n g i n d u r a t i o n , p r i m a r i l y F e 2 O 3 , C a O and SiO 2 react w i t h e a c h o t h e r with the iron p o r t i o n greatly d o m i n a t i n g , Fig. 26, item 2.2.2.2. T h e iron oxides F e 2 O 3 , F e 3 O 4 a n d F e O i n f l u e n c e the t y p e of the n e w arising c o m p o u n d s . In view of the short firing p e r i o d at o p t i m u m t e m p e r a t u r e s , balances a c c o r d i n g to the p h a s e t h e o r y can hardly be expected. Nevertheless, t h e s t u d y of the correlations b e t w e e n the i n d i v i d u a l constituents s h o w n in the e q u i l i b r i u m d i a g r a m indicates the direction in w h i c h the mineralogical c o m p o u n d s to b e e x p e c t e d m a y develop. T h e kinetics of the f o r m a t i o n of i n d i v i d u a l p h a s e s d u e to reaction in a solid p h a s e play a n i m p o r t a n t role. T h e e q u i l i b r i u m relations of the three constituents F e 2 O 3 - C a O - S i O 2 , w h i c h m a y also be F o r a better e x p l i c a t i o n of these correlations, it is a d v i s a b l e to consider the system F e 2 O 3 - S i O 2 , C a O - SiO 2 a n d F e 2 O 3 - C a O . T h e low reactivity of SiO 2 w i t h F e 2 O 3 is k n o w n 75). A t a d e q u a t e f i r i n g t e m p e r a t u r e s , F e 2 O 3 reacts, a c c o r d i n g to t h e C a O concentrations, b y t h e f o r m a t i o n of l i m e ferrites 7 6 / 7 7 ), s u c h as: C a O • 2 Fe2O3 calcium diferrite C a O • F e 2 O 3 m o n o c a l c i u m ferrite 2 C a O • Fe2O3 dicalcium ferrite of w h i c h C a O • F e 2 O 3 is m o s t interesting f o r pellet p r o d u c t i o n . C a O a n d SiO 2 a l r e a d y r e a c t at 1000 0 C in a solid state to f o r m orthosilicate, C a O • SiO2. W h e n all t h e t h r e e constituents react u n d e r a n oxidizing a t m o s p h e r e , a glassy slag p h a s e is virtually f o r m e d u p to a basicity ( C a O : SiO 2 ) of 0.8 f r o m which it is d i f f i c u l t to i d e n t i f y clearly a d e f i n e d m i n e r a l o g i c a l p h a s e , such as C a O . SiO 2 31)- A p a r t f r o m t h e m a t r i x of F e 2 O 3 , the glassy p h a s e contains C a O - SiO 2 a n d varying q u a n t i t i e s of F e 2 O 3 . A s soon as the system a p p r o a c h e s r e d u c i n g a t m o s p h e r e , i r o n - c o n t a i n i n g olivines with d i f f e r e n t m e l t i n g points are f o r m e d 78 ), as set f o r t h u n d e r i t e m 6.1.2.3, a n d the following items, with t h e pellet p r o p e r t i e s b e i n g highly i n f l u e n c e d d u r i n g r e d u c t i o n . A t a basicity h i g h e r t h a n 0.8, l i m e ferrites m a y b e additionally p r e s e n t w h i c h h a v e a positive effect o n the r e d u c t i o n behaviour.

influenced

154

5 Process-Influencing Factors 5 . 4 . 5 T h e r m a l D i s s o c i a t i o n of H e m a t i t e in P e l l e t s

A f t e r the usual firing, the iron oxides s h o u l d always b e p r e s e n t as h e m a t i t e in the i n d u r a t e d pellets. T h e following c o n s i d e r a t i o n s t h u s r e f e r to all pellet qualities. T h e e q u i l i b r i u m of the reaction: 2F e 3 O 4 + 1 / 2O 2 = 3 F e 2 O 3 shifts to t h e right u p to a t e m p e r a t u r e of a b o u t 1400 0 C . H o w e v e r , at a higher t e m p e r a t u r e , a dissociation of h e m a t i t e (see i t e m 2.2.2.1.2) begins. T h e e q u i l i b r i u m again shifts to the left c a u s i n g once m o r e the f o r m a t i o n of Fe 3 O 4 a n d , thus, an i m p a i r m e n t of the pellet quality. T h i s h e m a t i t e dissociation starts at lower t e m p e r a t u r e s if the pellets c o n t a i n b a s i c additives. A c c o r d i n g to the q u a n t i t y of these additives, the dissociation t e m p e r a t u r e lowers a n d the m a g n e t i t e p o r t i o n in the pellet rises, as s h o w n in Fig. 82 34 ). By m a g n e t o m e t r i c m e a s u r e m e n t s , t h e F e 3 O 4 p o r t i o n in the pellet was ascertained at d i f f e r e n t t e m p e r a t u r e s a n d with d i f f e r e n t additives. W i t h o u t additives, t h e h e m a t i t e pellet practically r e m a i n s w i t h o u t m a g n e t i t e p o r t i o n even at a t e m p e r a t u r e of 1400 0 C ( C u r v e I). A n increasing C a O content in pellets f a v o u r s t h e h e m a t i t e dissociation (Curves II, III a n d IV). T h i s p h e n o m e n o n is e x p l a i n e d by a t e m p e r a t u r e rise inside the pellets, d u e to t h e e x o t h e r m i c h e a t of f o r m a t i o n of c a l c i u m ferrites a n d t h e i r a d d i t i o n a l m e l t i n g h e a t w h i c h brings a b o u t an overheating in the pellet core to t e m p e r a t u r e s e x c e e d i n g 1400 0 C a n d t h u s h e m a t i t e dissociation. T h e e x o t h e r m i c h e a t c a u s e d by the f o r m a t i o n of ferrite m a y lead to a n o v e r h e a t i n g a n d s o f t e n i n g of the pellet core a n d to

Fig. 82. Effect of additives on dissociation temperature of hematite in pellets

5.4 Influence of Thermal Treatment on Pellet Properties

155

the f o r m a t i o n of a glassy structure w h e r e b y r e o x i d a t i o n d u r i n g cooling is h i n d e r e d . T h e dissociation t e n d e n c y of C a O - c o n t a i n i n g pellets c a n only be r e d u c e d b y a d r a s t i c lowering of firing t e m p e r a t u r e . A s r e g a r d s pellets p r o d u c e d in i n d u s t r i a l plants, the q u e s t i o n r e g a r d i n g h e m a t i t e dissociation always arises so t h a t g r e a t a t t e n t i o n is given to the u p p e r limit of firing t e m p e r a t u r e a n d the necessary firing t i m e in each pelletizing plant. In r e l e v a n t investigations, it h a s not yet b e e n possible to clarify all q u e s t i o n s since various a u t h o r s are still of d i f f e r e n t o p i n i o n concerning t e m p e r a t u r e a n d dissociation 79 ).

5.4.6 S c h e m e of Thermal Treatment All p h a s e s of t h e r m a l t r e a t m e n t are c o n s e c u t i v e irrespective of the f i r i n g system a p p l i e d . In this connection, v a r i o u s i n f l u e n c i n g factors s h o u l d b e a d a p t e d to each other: (a) T h e temperature is d i f f e r e n t for each r e a c t i o n , (b) The time is n e e d e d for the d e v e l o p m e n t of t h e i n d i v i d u a l reactions, (c) The heat supply, by m e a n s of h e a t i n g gases, p r o v i d e s t h e v a r i o u s stages with the h e a t energy r e q u i r e d . It is c o n t r o l l e d b y the h e a t i n g gas flow: T h e c o m b i n a t i o n of these three process p a r a m e t e r s r e p r e s e n t s the firing p a t t e r n a c c o r d i n g to w h i c h the o p t i m u m pellet q u a l i t y a n d the b u r n i n g capacity can b e p r e d e t e r m i n e d in the l a b o r a t o r y irrespective of the type of raw m a t e r i a l s used. A c c o r d i n g to t h e characteristics of ores, b i n d e r s a n d additives, firing p a t t e r n s h a v e b e e n d e v e l o p e d w h i c h p r o c e e d d i f f e r e n t l y in the various process stages. F o r the principal ore properties, typical firing patterns are d e s c r i b e d b e l o w by way of e x a m p l e of the travelling g r a t e pelletizing system 28) since in this system t h e t e m p o r a l a n d local t h e r m a l treatment p h a s e s can b e clearly o v e r l o o k e d . In principle, the succession in t h e r m a l steps is s i m i l a r in o t h e r firing systems.

5.4.6.1 Heating Pattern for Pellets from Magnetite T h e tests for e l a b o r a t i o n of the f i r i n g p a t t e r n c a n b e carried o u t in t h e pot grate a n d m e a s u r i n g devices d e s c r i b e d u n d e r i t e m 4.5. A c c o r d i n g to the process stage involved, the d i r e c t i o n of flow of the process gases is changed. U p - d r a u g h t a n d d o w n - d r a u g h t is a p p l i e d f o r drying, d o w n d r a u g h t for p r e h e a t i n g , firing a n d a f t e r - f i r i n g a n d a g a i n u p - d r a u g h t for cooling. T h e t e m p e r a t u r e s a b o v e the pellet b e d a r e m e a s u r e d (T 1 ) as well as those b e t w e e n pellet b e d a n d h e a r t h layer (T 3 ), in the m i d d l e of t h e b e d (T 2 ) a n d in t h e w i n d b o x (T 6 ). Pressure, s u c t i o n a n d gas v o l u m e s a r e also recorded. A typical firing p a t t e r n f o r m a g n e t i t e pellets of 1 0 - 1 5 m m d i a m e t e r is s h o w n in Fig. 83.

156

5 Process-Influencing Factors

Side wall layer a n d h e a r t h layer were used. T h e tests were p e r f o r m e d in the e q u i p m e n t s h o w n in Figs. 39 a n d 40, item 4.5. T e m p e r a t u r e a n d pressure are i n d i c a t e d on the ordinates, t i m e on t h e abscissa. T h e d r y i n g is achieved b e l o w t h e shock t e m p e r a t u r e e.g. of a b o u t 300 0 C . D u r i n g heating, the o x i d a t i o n of m a g n e t i t e with the s i m u l t a n e o u s b e g i n n i n g of the relatively s h o r t firing t i m e at i n d u r a t i o n t e m p e r a t u r e s of a b o u t 1300 0 C is sufficient. As t h e h e a t f r o n t p r o c e e d s in t h e pellet b e d , the b o t t o m layers are h e a t e d accordingly, as is seen f r o m rising t e m p e r a t u r e s T 2 a n d T 3 . A f t e r a certain firing p e r i o d controlled by the i n d i v i d u a l t e m p e r a t u r e m e a s u r e m e n t s the h e a t s u p p l y is s t o p p e d . T h e sensible h e a t f r o m the u p p e r layer is n o w sucked into the layers u n d e r n e a t h . In the case of m a g n e t i t e pellets, it m a y serve to oxidize any still r e m a i n i n g m a g n e t i t e a n d , generally, to t e r m i n a t e the crystal change. T h e s u c c e e d i n g cooling is achieved at constant cooling air pressure. T h e firing progress is also controlled by t h e t e m p e r a t u r e c h a n g e of waste gas inside the w i n d b o x (T 6 ). T h e pressures or suctions are a d j u s t e d to the p e r t i n e n t process stage in o r d e r to control the gas a n d h e a t flow. T h u s , the p r e s s u r e a p p l i e d for u p - d r a u g h t d r y i n g a n d cooling is relatively high since, here, no d a m a g e to t h e pellet s t r u c t u r e is to be f e a r e d w h e r e a s

Fig. 83. Pattern of thermal treatment for pellets from magnetite concentrate

5.4 Influence of Thermal Treatment on Pellet Properties

157

d u r i n g d o w n - d r a u g h t d r y i n g a n d p r e h e a t i n g low pressure d i f f e r e n c e s a r e maintained. Even t h o u g h this firing p a t t e r n is typical f o r m a g n e t i t e pellets — longer p r e h e a t i n g time, shorter firing p e r i o d - the p r i n c i p l e is also a p p l i c a b l e to pellets with o t h e r c h e m i c a l compositions. T h e v a r i o u s process stages a n d periods can be a d a p t e d to each o t h e r a c c o r d i n g to the ore types involved as well as the n a t u r e a n d q u a n t i t y of the a d d i t i v e s used. 5.4.6.2 Firing Pattern for Pellets from Other Ores F o r the following ore types, firing p a t t e r n s h a v e b e e n d e v e l o p e d by c o m p u t e r calculation, (Fig. 84):

Fig. 84. Pattern of thermal treatment of pellets from various ores, relationship between time and induration temperature

158

5 Process-Influencing Factors

(A) m a g n e t i t e c o n c e n t r a t e (B) m a g n e t i t e c o n c e n t r a t e + 9% C a C O 3 (0.64 basicity) (C) h e m a t i t e o r e (itabirite) ( D ) h e m a t i t e - l i m o n i t e mix. T h e d i f f e r e n t p e r i o d s of t h e r m a l t r e a t m e n t w e r e e q u a l i z e d to 100% for all pellets. In this way, the various process stages c a n be directly c o m p a r e d with each other. T h e firing t e m p e r a t u r e s ( o r d i n a t e ) , the p e r i o d s in m i n u t e s r e q u i r e d for the i n d i v i d u a l process stages a n d the p e r c e n t a g e s were plotted o n the d i a g r a m . T h e various process stages are m a r k e d by 1 - 6 . The drying p e r i o d s 1 a n d 2 resulted f r o m t h e p e r t i n e n t m o i s t u r e content of. g r e e n balls. D u r i n g p r e h e a t i n g 3, c o n s i d e r a b l e d i f f e r e n c e s w e r e f o u n d . T h e t w o m a g n e t i t e pellet types A a n d B n e e d a relatively long p r e h e a t i n g t i m e because the o x i d a t i o n to h e m a t i t e takes place s i m u l t a n e o u s l y ; the d i f f e r ence b e t w e e n A a n d B results a d d i t i o n a l l y f r o m t h e c a r b o n a t e dissociation, C u r v e B. T h e two h e m a t i t e curves, C a n d D , are steeper since in this case only the p r e h e a t i n g of h e m a t i t e crystals is a c h i e v e d a n d n o r e a c t i o n takes place. Nevertheless, t h e r e is a small d i f f e r e n c e b e t w e e n C a n d D since a d d i tionally in t h e case of D , a small a m o u n t of c o m b i n e d w a t e r f r o m the limonite p o r t i o n is to b e liberated. M a r k e d d i f f e r e n c e s exist d u r i n g pellet firing. T h e m a g n e t i t e pellets A necessitate shorter firing p e r i o d s since, d u r i n g o x i d a t i o n , the s o l i d i f i c a t i o n has a l r e a d y started intensively in the p r e h e a t i n g p h a s e . In t h e case of B, the positive i n f l u e n c e of the basic additives, d u e to t e m p e r a t u r e decrease, is still perceptible. F o r h e m a t i t e pellets, t h e firing p e r i o d s a r e c o n s i d e r a b l y longer in o r d e r to ensure h e m a t i t e crystal g r o w t h . F o r this p u r p o s e , h i g h e r firing t e m p e r a t u r e s are also r e q u i r e d . In the after-firing zone, n o m a r k e d d i f f e r e n c e s occur. D u r i n g cooling, t h e r e are h a r d l y any d i f f e r e n c e s . In this case, u n i f o r m cooling of h e m a t i t e pellets p r o d u c e d f r o m all ores d u r i n g firing is achieved. If the real firing p e r i o d s on the abscissa are c o m p a r e d w i t h each other, very c o n s i d e r a b l e d i f f e r e n c e s of 3 3 - 4 1 m i n u t e s are observed. T h e s e differences are decisive for d e t e r m i n i n g t h e t h r o u g h p u t of t h e v a r i o u s firing units d u r i n g i n d u r a t i o n of d i f f e r e n t ores.

6 Behaviour of Indurated Pellets During Reduction

Since it is possible to p r o d u c e pellets of u n i f o r m quality by — suitable preparation of raw materials — use of quality-improving additives and optimum firing technique, the goal which was aimed at to m a k e a satisfactory reduction charge f r o m fine-grained, high-grade iron ores or concentrates has been achieved. Good reducibility and sufficient mechanical strength during reduction are the most i m p o r t a n t properties d e m a n d e d f r o m pellets. D u e to the high iron content, the uniformly high porosity and easy reducibility of hematite, as well as d u e to the spherical shape of pellets, a good behaviour in the blast furnace can be expected. This expectation was confirmed as early as 1916 by C. Brackelsberg 4 ) who p r o d u c e d pellets f r o m Sydvaranger magnetite concentrate with water glass as binder. H e c o m p a r e d t h e m with sinter and briquettes p r o d u c e d f r o m the s a m e raw material and f o u n d a better reducibility of pellets. To a greater extent, it was the mechanical strength during reduction which d e m a n d e d special attention f r o m the date when m a j o r pellet quantities were available f o r industrial processing. As was soon apparent, different d e m a n d s resulted f r o m the intended purpose: - In the blast furnace, the most important d e m a n d was the stability and sufficient gas permeability of the whole charge d u r i n g its passage through all reduction phases, especially in the u p p e r part, despite a long retention time under a slightly reducing atmosphere at a relatively low t e m p e r a t u r e of 500—700 °C, a n increasing reduction potential, a rising pressure a n d rising temperatures up to the molten phase in the lower level of the blast furnaces. — During direct reduction, pellets are completely reduced to sponge iron in the solid state. They must maintain their s h a p e and sufficient strength for their f u r t h e r transport to melting furnaces. Initially, opinion was that a high mechanical strength of indurated pellets would be sufficient for their b e h a v i o u r during reduction. This opinion was a b o v e all maintained in the U S A where particularly favourable conditions prevailed. In the USA, the pellets were mainly produced and tested in pilot plants or laboratories all belonging to the

160

6 Behaviour of Indurated Pellets During Reduction

Fig. 85. Effect of pellets in blast furnace operation •

s a m e concerns, as is reported by W. E. M a r s h a l l 4 6 ) . H e f o u n d t h a t t h e m e c h a n i c a l strength of i n d u r a t e d pellets, especially their g o o d a b r a s i o n b e h a v i o u r , was the essential criterion for the increasing success a n d w i d e s p r e a d use of pellets in the blast f u r n a c e , as is a p p a r e n t f r o m Fig. 85. Pellets with a h i g h e r resistance to a b r a s i o n ( C u r v e I) allow a h i g h e r portion of pellets in the b u r d e n . F r o m this results a lower flue d u s t a c c u m u l a t i o n ( C u r v e II) a n d , owing to the b e t t e r gas p e r m e a b i l i t y of the charge, a n increase in p r o d u c t i o n ( C u r v e III). T h e s e First f a v o u r a b l e operational results o b t a i n e d in blast f u r n a c e s of the relevant concerns were c o n f i r m e d in a large-scale test r u n on 150,000 t pellets f r o m t h e p l a n t of Reserve M i n i n g C o m p a n y in a blast f u r n a c e o u t s i d e t h e U S A 20 ). A high m e c h a n i c a l cold crushing strength of pellets was a f t e r w a r d s - a n d is u p to the p r e s e n t day - the m a i n criterion f o r the b e h a v i o u r of pellets f r o m the M e s a b i R a n g e in the blast f u r n a c e s of U S A . A l m o s t s i m u l t a n e o u s l y with the U S A , first o p e r a t i o n a l experience was also m a d e w i t h pellets in Sweden. H o w e v e r , f r o m the conditions p r e v a i l ing there resulted two f u n d a m e n t a l l y d i f f e r e n t starting positions c o m p a r e d with t h e U S A : — In S w e d e n , the pellets serve, besides for export, a l m o s t exclusively as charge f o r direct reduction furnaces o p e r a t i n g according to t h e W i b e r g process, as r e p o r t e d by S t a h l h e d 8 ).

161 6 Behaviour of Indurated Pellets During Reduction Table 24. Analysis and basicity of pellets from two types of magnetite concentrates with high and low gangue content Concentrates

Fe total Fe"

SiO 2

Al 2 O 3

CaO

MgO

Gangue content in %

Basicity C a O + MgO SiO 2 +Al 2 O 3

Reserve mining Malmberget

63.3

1.8

8.0

0.5

0.4

0.6

9.5

about 0.12

68.5

n.b.

0.7

0.4

0.5

0.25

1.85

about 0.7

— In a d d i t i o n — a n d this was the second g r e a t d i f f e r e n c e — t h e S w e d i s h M a l m b e r g e t pellets h a d a n o t h e r c h e m i c a l c o m p o s i t i o n as can be s e e n f r o m the f o l l o w i n g T a b l e 24. C o n s e q u e n t l y , t h e t w o kinds of pellets d i f f e r c o n s i d e r a b l y b y their iron content, slag a m o u n t a n d basicity. Less f a v o u r a b l e e x p e r i e n c e was m a d e with the M a l m b e r g e t pellets, low in gangue, d u r i n g r e d u c t i o n . D e s p i t e h i g h m e c h a n i c a l strength of i n d u r a t e d pellets, a c o n s i d e r a b l e v o l u m e increase a n d c r u s h i n g strength decrease were observed. T i g e r s c h i ö l d a l r e a d y r e p o r t e d on this p h e n o m e n o n of " s w e l l i n g " d u r i n g r e d u c t i o n of s u c h pellets with a h i g h iron content. H e investigated this p r o b l e m t o g e t h e r with his colleagues. It was f o u n d that t h e v o l u m e o f these pellets i n c r e a s e d by u p to 60%. A t the s a m e time, it could also b e d e m o n s t r a t e d t h a t additives c a n decisively i n f l u e n c e this b e h a v i o u r w i t h o u t f i n d i n g e x h a u s t i v e explications for this. Even by the a d d i t i o n of 1% C a O , the v o l u m e increase could be l o w e r e d to 8% of the initial volume. O n t h e o t h e r h a n d , the a d d i t i o n of 0.25% N a C l i n t e n s i f i e d swelling u p to 110%. A check test r u n on M a l m b e r g e t pellets with 68.5% iron in a blast f u r n a c e yielded, contrary to the s t a t e m e n t m a d e by M a r s h a l l f o r M e s a b i R a n g e pellets, n o positive results 80 ). O p i n i o n w a s that the h o t c r u s h i n g strength of M a l m b e r g e t pellets was i n s u f f i c i e n t t o w i t h s t a n d reduction. T h e r e a s o n f o r this b e h a v i o u r was not yet k n o w n at t h a t time. T h e s e two controversial results o b t a i n e d with M a l m b e r g e t and R e s e r v e M i n i n g pellets were, f o r a long t i m e , the s u b j e c t of discussions p r i m a r i l y b e t w e e n experts and pellet consumers. T h e " f a i l u r e " encountered with the as only small q u a n t i t i e s of such pellets low in g a n g u e w e r e available at that t i m e f o r processing in t h e blast f u r n a c e . In Sweden, h o w e v e r , the question of pellet swelling, a n d especially the cause of it, was t h o r o u g h l y investigated by J. O. E d s t r ö m 81 ). T h e discussion of the b e h a v i o u r of pellets low in g a n g u e received a fresh i m p e t u s w h e n , in 1963/64, m a j o r q u a n t i t i e s of the first M a r c o n a pel-

Malmberget

pe

162

6 Behaviour of Indurated Pellets During Reduction

lets with 68.9% iron were available p r i m a r i l y f o r J a p a n e s e blast f u r n a c e s . T h e surprisingly negative initial b e h a v i o u r of M a r c o n a pellets, d e s p i t e a high m e c h a n i c a l strength, was the subject of intensive investigations particularly in J a p a n , w h i c h was reflected in n u m e r o u s p u b l i c a t i o n s 82). Such investigations were c o n t i n u e d especially in those places w h e r e pellet consumers h a d to cover their d e m a n d in the w o r l d m a r k e t . O w i n g to the s u p p l y situation in the U S A with pellets of t h e wellknown c o m p o s i t i o n with high acid g a n g u e content, swelling was only of secondary i m p o r t a n c e . T h e r e was no d i r e c t c o m p u l s i o n to deal so t h o r o u g h l y as in o t h e r places with the p r o b l e m of swelling w h i c h was c o n f i r m e d b y M. C h a n g 83) a n d his colleagues in 1967 by test results with a great n u m b e r of pellet types originating f r o m N o r t h A m e r i c a .

6.1 Change of Pellet Structure During Reduction T w o i m p o r t a n t basic theories were f o u n d at a very early stage w h i c h , in principle, are also valid today, viz.: (a) Insufficiently-fired pellets with a low m e c h a n i c a l initial strength swell a n d disintegrate d u r i n g r e d u c t i o n irrespective of the ore type a n d a d d i t i v e s used. (b) Pellets b o t h high a n d low in g a n g u e h a v e a d i f f e r e n t b e h a v i o u r d u r i n g reduction. Pellets with a h i g h g a n g u e c o n t e n t are m o r e resistant to swelling a n d disintegration, w h e r e a s pellets w i t h a low g a n g u e c o n t e n t swell a n d disintegrate intensively if n o c o u n t e r a c t i o n is taken. D e s p i t e n u m e r o u s investigations, the reasons f o r this pellet b e h a v i o u r d u r i n g reduction h a v e not yet been fully clarified. Nevertheless, p a r a m e t e r s are n o w a d a y s k n o w n according to w h i c h pellets can be p r o d u c e d f r o m d i f f e r e n t r a w materials a n d h a v e the q u a l i t y r e q u i r e d f o r the blast f u r n a c e and direct r e d u c t i o n plant. T h i s knowledge, at first empirically a c q u i r e d , was also t h e basis on w h i c h test m e t h o d s a d o p t e d o n a n industrial scale could b e d e v e l o p e d . T h e i m p o r t a n c e of satisfactory m e c h a n i c a l pellet p r o p e r t i e s b e f o r e r e d u c t i o n is d e m o n s t r a t e d by a test in w h i c h pellets of h i g h - g r a d e h e m a t i t e ore with 65.9% Fe, 2.9% SiO 2 a n d 5% C a ( O H ) 2 a d d i t i o n h a d been fired at t e m p e r a t u r e s b e t w e e n 1000 0 C a n d 1300 0 C u n d e r o t h e r w i s e equal conditions. T h e i n d i v i d u a l s a m p l e s w e r e subjected to the r e l e v a n t r e d u c t i o n test m e t h o d s , and the results are s h o w n in Fig. 86 28). A d e f i n i t e d e p e n d e n c e of crushing strength o n firing t e m p e r a t u r e is a p p a r e n t . T h e crushing strength of r e d u c e d pellets shows t h e s a m e t e n d e n c y ( C u r v e s I and II). D e s p i t e the high lime a d d i t i o n , the pellets disintegrate a l m o s t completely d u r i n g r e d u c t i o n ( C u r v e III). T h e p o r t i o n of f r a g m e n t s a n d

6.1 Change of Pellet Structure During Reduction

163

Fig. 86. Influence of oxide pellet strength on pellet properties after reduction pellets plus 6 m m also decreases and the v o l u m e increase (swelling) rises continuously ( C u r v e IV). T h i s k n o w l e d g e is generally valid a n d c o n f i r m e d by n u m e r o u s passages in literature. T h e following c o n s i d e r a t i o n s are t h e r e f o r e b a s e d only o n indurated pellets having satisfactory mechanical properties. 6.1.1 Reduction M e c h a n i s m s A n extensive survey of the kinetics of i r o n o x i d e r e d u c t i o n b y r e d u c i n g gases is given by L. v. B o g d a n d y a n d H . J. Engell 8 4 ). T h e r e d u c t i o n

As regards kinetics, the r e d u c t i o n is a series of consecutively p r o c e e d i n g individual reactions w h i c h can be i n f l u e n c e d b y physical a n d c h e m i c a l factors. Such factors m a y b e connected w i t h the r a w m a t e r i a l a n d with t h e p r o p e r t i e s of r e d u c t i o n gas. F a c t o r s d e p e n d e n t o n the raw material are, f o r instance: C h e m i c a l a n d m i n e r a l o g i c a l c o m p o s i t i o n , o x i d a t i o n d e g r e e of iron, mechanical s t r e n g t h b e f o r e reduction, pellet porosity as well as gas p e r m e a b i l i t y of t h e charge. As concerns t h e reducing gas the following f a c t o r s are i m p o r t a n t : R e d u c t i o n potential, gas v o l u m e , gas p r e s s u r e , gas velocity a n d t e m perature.

proceeds

a c c o r d i n g to the

6 Behaviour of Indurated Pellets During Reduction

164

Of these, the latter influences the reaction in b o t h t h e gaseous a n d solid phases. F o r e a c h i n d i v i d u a l step, t h e r e is a specific e q u i l i b r i u m p o s i t i o n which can b e varied by c h a n g i n g the c o n c e n t r a t i o n of t h e reactants. In this connection, two m e c h a n i s m s p l a y a decisive role, namely: (a) T h e p h a s e b o r d e r reaction o n the p h a s e b o r d e r b e t w e e n solids a n d gas (b) T h e solids r e a c t i o n on the b o r d e r a n d w i t h i n the solid reactants b y d i f f u s i o n of metallic iron into the zones w i t h a h i g h e r oxygen content. This r e a c t i o n is only d e p e n d e n t o n the i r o n c o n c e n t r a t i o n 85). T h e r e a c t i o n itself p r o c e e d s in t h r e e steps: (a) O x y g e n r e m o v a l o n the o x i d e grain s u r f a c e u p to metallic i r o n f r o m the wustite p h a s e (b) D i f f u s i o n of metallic iron towards t h e oxides to b e r e d u c e d (c) C o n v e r s i o n of t h e h i g h e r oxides at their g r a i n limits into lower oxides. T h e following f o r m u l a e describe these reactions:

(a) Wustite (b) Wiistite is r e s u p p l i e d by m e t a l l i c iron f r o m m a g n e t i t e

(c) A c c o r d i n g to the supply of metallic iron, h e m a t i t e is c o n v e r t e d

to

magnetite.

Fig. 87. Influence of temperature and time on reaction between metallic iron and iron oxides

6.1 Change of Pellet Structure During Reduction

165

T h e conversion is s h o w n b y way of d i f f e r e n t r e a c t i o n e x a m p l e s , Fig. 87 86 ). M e t a l l i c iron was m i x e d a n d h e a t e d b o t h w i t h m a g n e t i t e a n d h e m a t i t e u n d e r v a c u u m at d i f f e r e n t t e m p e r a t u r e s a n d f o r various p e r i o d s . T h e reaction progress was c h e c k e d b y the rising p o r t i o n of bivalent iron. T h e b e g i n n i n g of the r e a c t i o n at a relatively low t e m p e r a t u r e s h o u l d b e noted. W h e n starting w i t h m a g n e t i t e , t h e r e a c t i o n p r o c e e d s m o r e quickly t h a n with h e m a t i t e . T h i s is d u e to the d i f f e r e n t crystalline s t r u c t u r e of b o t h iron oxides.

6.1.2 Structural Change During Reduction T h e c o m p o n e n t s to b e r e d u c e d react d i f f e r e n t l y according to t h e prevailing conditions. As a l r e a d y i n d i c a t e d u n d e r i t e m 6.1.1, m a n y factors m a y i n f l u e n c e t h e reaction. T h u s , s o m e a p p a r e n t l y s o m e w h a t c o n t r a dictory results w e r e p u b l i s h e d b e c a u s e the u n d e r l y i n g c o n d i t i o n s h a d not b e e n sufficiently k n o w n or considered. It m a y also h a p p e n t h a t s o m e results are i n f l u e n c e d b y factors w h i c h w e r e not k n o w n . T o be able to i n t e r p r e t results c o m p a r a b l y , all given c o n d i t i o n s m u s t be c o n s i d e r e d exactly. As a n d w h e n new e x p e r i e n c e is g a i n e d , t h e test m e t h o d s b e c o m e m o r e a n d m o r e s o p h i s t i c a t e d a n d t a i l o r e d to o p e r a t i n g conditions. 6.1.2.1 Volume Variation by Crystal Change In c o n j u n c t i o n with d i f f i c u l t i e s e n c o u n t e r e d d u r i n g the o p e r a t i o n of W i b e r g f u r n a c e s with pellets low in g a n g u e , E d s t r ö m 81 ) first a t t e m p t e d to f i n d out w h e t h e r a n d to w h a t extent as well as at w h i c h d e g r e e of oxygen r e m o v a l a v o l u m e c h a n g e occurs. F o r this p u r p o s e , he r e d u c e d a c u b e of p u r e h e m a t i t e o r e with a d e f i n e d e d g e l e n g t h at a t e m p e r a t u r e of 1000 0 C b y using C O . T h e r e d u c t i o n p o t e n t i a l was v a r i e d by m i x i n g with C O 2 in o r d e r to a d j u s t s p e c i f i c stages of oxygen r e m o v a l . E d s t r ö m ' s m e a s u r i n g d a t a are c o m p a r a t i v e l y as well as d i a g r a m m a t i c a l l y s h o w n in Fig. 88. T h e r e d u c t i o n stages of i r o n with d i m i n i s h i n g o x i d a t i o n d e g r e e u p to m e t a l l i c iron are s h o w n on the abscissa. T h e v o l u m e variations m e a s u r e d at each stage are r e p r e s e n t e d o n the o r d i n a t e w i t h t h e v o l u m e b e i n g b a s e d on t h e h e m a t i t e c u b e as 100% a n d the v a r i a t i o n in p e r c e n t b e i n g i n d i c a t e d . D u r i n g r e d u c t i o n of h e m a t i t e , a v o l u m e increase, in a n y case, takes p l a c e (Curves 1 - 4 ) . A f t e r a close e x a m i n a t i o n , it was f o u n d t h a t the m o s t intensive swelling occurs d u r i n g t h e c o n v e r s i o n of h e m a t i t e to m a g n e t i t e (Curves 1 a n d 2). H o w e v e r , it c o n t i n u e s u p to the f o r m a t i o n of w ü s t i t e ( C u r v e s 1 - 3 ) , a n d a c c o r d i n g to the p r e v a i l i n g r e d u c t i o n conditions, u p to metallic i r o n ( C u r v e 4). Except f o r t h e c o n d i t i o n s r e f e r r i n g to the latter curve, a v o l u m e s h r i n k a g e is o b s e r v e d at 1, 2 a n d 3 as soon as the first metallic iron h a s b e e n f o r m e d f r o m the w ü s t i t e p h a s e . T h e v o l u m e

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Fig. 88. Influence of reduction degree on volume change of a hematite cube

v a r i a t i o n p r o c e e d s q u i t e differently d u r i n g r e d u c t i o n of a m a g n e t i t e c u b e ( C u r v e 5). In this case, a s h r i n k a g e i m m e d i a t e l y occurs f r o m w h i c h it c a n be d e d u c e d t h a t d u r i n g m a g n e t i t e reduction, n o v o l u m e increase takes place. T h e v o l u m e increase is explained by E d s t r ö m as the result of a rising gas pressure in the pellet core. It occurs at a h i g h r e d u c t i o n velocity w h e n the arising gases of C O 2 a n d H 2 O are m o r e q u i c k l y f o r m e d t h a n they can o u t w a r d l y d i f f u s e t h r o u g h the pores. But the r e d u c t i o n velocity is d e p e n d e n t on v a r i o u s factors, such as d i f f e r e n t p o r e s t r u c t u r e of d i f f e r e n t s h a p e of h e m a t i t e crystals. In o t h e r tests 87 ), a varying crushing strength decrease was f o u n d d u r i n g r e d u c t i o n of pellet types with iron contents of 64 to 68%. T h e c r u s h i n g strength d e c r e a s e m o r e or less reaches a m i n i m u m v a l u e for all s a m p l e s in the wüstite phase. W h e n those values are c o m b i n e d with the v o l u m e increase o b s e r v e d by E d s t r ö m , the curves s h o w n in Fig. 89 are o b t a i n e d . T h e iron o x i d e s with decreasing oxygen content, d o w n to m e t a l l i c iron, are r e p r e s e n t e d on the abscissa. C u r v e 1 shows the v o l u m e v a r i a t i o n a n d

6.1 Change of Pellet Structure During Reduction

167

Fig. 89. Relationship between volume change and compressive strength during oxygen removal

curve 2 the c o r r e s p o n d i n g c r u s h i n g s t r e n g t h of the pellets in p e r c e n t d u r i n g the i n d i v i d u a l r e d u c t i o n phases. Based on an initial strength of u n r e d u c e d h e m a t i t e , e q u a l 100, the strength d r o p s a l m o s t as f a r as to zero n e a r the wüstite p h a s e a n d t h e n i m p r o v e s w i t h a rising p o r t i o n of metallic iron. In principle, swelling a n d d e c r e a s e of c r u s h i n g strength of r e d u c e d pellets occur in o p p o s i t e direction. T h i s m e a n s a h i g h swelling is connected w i t h a low m e c h a n i c a l strength. Besides E d s t r ö m ' s t h e o r y o n swelling d u r i n g r e d u c t i o n , t h e r e are still o t h e r theories, such as: - I n c o r p o r a t i o n of c a r b o n at the p h a s e l i m i t b e t w e e n iron a n d w ü s t i t e 8 8 ) , - S e p a r a t i o n of f i b r o u s i r o n needles, so-called whiskers, or iron d r o p l e t s at p r e f e r r e d places of wüstite interfaces a n d t h u s d i s p l a c e m e n t of the surrounding particles89), - Intensification of iron s e p a r a t i o n s w h e n alkalines a r e p r e s e n t 9 0 ) . This additional volume increase results in an intensified or " c a t a s t r o p h i c " swelling. A n i m p o r t a n t c o n t r i b u t i o n to t h e e x p l a n a t i o n of the structural c h a n g e is m a d e by O t t o w 9 1 ) . A t first, he c o n f i r m s the v o l u m e increase d u r i n g r e d u c t i o n of h e m a t i t e to m a g n e t i t e as b e i n g a b o u t 11%. T h e m a g n e t i t e layer has b e c o m e thicker by this a m o u n t t h a n t h e p r e v i o u s h e m a t i t e layer. However, a c c o r d i n g to O t t o w ' s theory, a second, c r y s t a l l o g r a p h i c factor plays a n i m p o r t a n t role. O t t o w s t u d i e d t h e anisotropic b e h a v i o u r of h e m a t i t e crystals d u r i n g r e d u c t i o n . S u b s t a n c e s like h e m a t i t e crystallize in

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the h e x a g o n a l system 92) a n d , with regard to vectorial p r o p e r t i e s , o f t e n show a directionality. T h i s is called a n i s o t r o p i c b e h a v i o u r . T h e r e d u c t i o n velocity in h e m a t i t e grains, w h i c h , in parallel to the m a i n axis, p r o c e e d s m o r e slowly t h a n in vertical d i r e c t i o n to this axis, is also anisotropic. A p a r t f r o m the v o l u m e increase, m a g n e t i t e layers of d i f f e r e n t thickness are f o r m e d which, at the i n d i v i d u a l interfaces, b r i n g a b o u t tensions a n d a tearing of crystalline structure. In a pellet the h e m a t i t e phase is p r e s e n t as polycrystalline a g g l o m e r a t e , a r r a n g e d at r a n d o m . Therefore the crystalline destruction is highly intensified by the anisotropic effect o n m a n y crystallites. T h e t h r e e p h a s e s of structural d e s t r u c t i o n in conjunction with the a n i s o t r o p i c b e h a v i o u r are s h o w n b y O t t o w o n the sketches of Fig. 90. (A) shows a h e m a t i t e c u b e b e i n g partially r e d u c e d u n i f o r m l y in all directions a c c o r d i n g to E d s t r ö m . D u e to v o l u m e increase, the initial v o l u m e rose according to section s' to t h e new section s. A t the calculated v o l u m e increase of 11%, an angle of δ = 6 ° arises b y w h i c h the s t r u c t u r e is d a m a g e d . E d s t r ö m m e a s u r e d a n angle of 8 —10°. (B) shows the anisotropically formed.

d i f f e r e n t thickness of m a g n e t i t e

layers

(C) d i a g r a m m a t i c a l l y shows the d i f f e r e n t r e d u c t i o n velocity p r e v a i l i n g in one o r e g r a i n with two crystallites each of w h i c h has its c-axes in a d i f f e r e n t position.

Fig. 90. Influence of volume increase and anisotropic behaviour on the structure of a hematite cube during reduction

6.1 Change of Pellet Structure During Reduction

169

Thus, the g r e a t d r o p in structural s t r e n g t h of pellets low in g a n g u e d u r i n g r e d u c t i o n can be a t t r i b u t e d to the effect of the following m e c h a nisms: (a) V o l u m e increase d u r i n g conversion f r o m h e m a t i t e to the lower o x i d e stages, m a i n l y m a g n e t i t e , (b) A n i s o t r o p i s m of r e d u c t i o n velocity resulting in the f o r m a t i o n of m a g n e t i t e layers of d i f f e r e n t thickness. Both m e c h a n i s m s p r o c e e d if c o r r e s p o n d i n g r e a c t i o n c o n d i t i o n s allow this. In such a case, the structural d a m a g e c a n n o t be a v o i d e d . N e w — even if i n t e r m e d i a r y - phases arise w h i c h d e m a n d a greater v o l u m e causing a growth pressure. T o w h a t extent this p r e s s u r e m a y cause d a m a g e to or destruction of pellet structure is d e p e n d e n t o n t h e p r e v a i l i n g c o n d i t i o n s or those to be p r o v i d e d in o r d e r to c o u n t e r a c t a structural d a m a g e . S o m e of the factors p r o h i b i t i n g a g r o w t h pressure as r e f e r r e d to u n d e r i t e m 6.1.1 a n d having a specific effect are i n d i c a t e d below: (a) D u r i n g pellet i n d u r a t i o n n o hematite crystals are f o r m e d b u t o t h e r structures (e.g. m a g n e t i t e ) w h i c h , d u r i n g r e d u c t i o n , d o not h a v e a n a n i s o t r o p i c b e h a v i o u r a n d are not n o t a b l y w e a k e n e d . (b) T h e b o n d i n g forces of the g a n g u e p o r t i o n s are strong e n o u g h to w i t h s t a n d t h e growth pressure. (c) T h e g a n g u e is c h a n g e d b o t h c h e m i c a l l y a n d q u a n t i t a t i v e l y until s u f ficiently strong reacting forces b e c o m e active b y the reaction of g a n g u e c o m p o n e n t s with each o t h e r o r w i t h iron oxides. Since factors (b) a n d (c) in c o n j u n c t i o n w i t h g a n g u e c a n be m o s t s i m p l y varied, their i n f l u e n c e is first discussed.

6.1.2.2 Structural Change by Reaction of Gangue Components with Iron Oxides and with Each Other F r o m the o p e r a t i o n a l results o b t a i n e d in N o r t h A m e r i c a n blast f u r n a c e s it is well-known that only pellets with a g r e a t p o r t i o n of highly acid g a n g u e s h o w a low t e n d e n c y to swelling. T h e r e is also n o longer a n y d o u b t a b o u t t h e positive i n f l u e n c e of basic additives. Since T i g e r s c h i ö l d , it has been k n o w n that o t h e r a d d i t i v e s such as a l k a l i n e c o m p o u n d s h a v e a d e t r i m e n t a l e f f e c t on the swelling b e h a v i o u r . Since ever-increasing pellet q u a n t i t i e s with a low g a n g u e p o r t i o n are p r o d u c e d as well as processed in blast f u r n a c e s , t h e i m p o r t a n c e of t h e chemical c o m p o s i t i o n of g a n g u e , like t h a t of sinter, has b e c o m e a significant f a c t o r in pellet p r o d u c t i o n . In a large n u m b e r of pelletizing plants, basic a d d i t i v e s are n o w a d a y s used. In this connection, it h a d b e e n observed that at a specific ratio of basic to acid g a n g u e the swelling b e h a v i o u r changes r e m a r k a b l y . A t a basicity of 0.1—0.7 p a r t i c u l a r l y h i g h swelling indices were f o u n d 93 ).

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T h e experience a c q u i r e d in blast f u r n a c e s h a s m e a n w h i l e d e m o n s t r a t e d that a v o l u m e increase of u p to 20% still does n o t adversely a f f e c t t h e blast f u r n a c e o p e r a t i o n . H o w e v e r , if a greater v o l u m e increase occurs, it is considered as a b n o r m a l or even as a " c a t a s t r o p h i c " swelling w h i c h has, in any case, to b e a v o i d e d . T h e i m p o r t a n c e of additives a n d especially their c o m p o s i t i o n is wellk n o w n f r o m p u b l i c a t i o n s o f t e n dealing with q u i t e s p e c i f i c c o n d i t i o n s which c a n n o t b e generalized. T h e results of such p u b l i c a t i o n s , w h i c h w e r e o f t e n insufficiently described or b a s e d on d i f f e r e n t test conditions, s o m e times b r o u g h t a b o u t d i f f e r e n t a n d even controversial i n t e r p r e t a t i o n s . T h e r e f o r e , it was u n d e r s t a n d a b l e that in o t h e r places basic research with clearly d e f i n e d raw materials a n d additives was also c a r r i e d out. T h e p u r p o s e of s u c h tests was to investigate t h e i n f l u e n c e of q u a n t i t y a n d chemical c o m p o s i t i o n of additives in c o n n e c t i o n with the o r e c h a r a c t e r istics o n the pellet b e h a v i o u r d u r i n g r e d u c t i o n u n d e r controlled c o n d i tions. S o m e of such investigations with generally a p p l i c a b l e results are discussed below. M a n y of t h e m w e r e jointly a c h i e v e d by pellet c o n s u m e r s and pellet p r o d u c e r s .

6.1.2.3 Influence of Additives on Pellet Swelling U n d e r items 5 - 5 . 3 t h e i n f l u e n c e of a d d i t i v e s a n d , u n d e r i t e m 5.4, the influence of t h e r m a l t r e a t m e n t o n the m e c h a n i c a l p r o p e r t i e s of f i r e d pellets was discussed in detail. It h a d b e e n a n t i c i p a t e d t h a t these influences w o u l d also be similar d u r i n g reduction. In a n extensive study, H . K o r t m a n n 9 4 ) d e a l t with t h e reactions of additives with g a n g u e constituents a n d with i r o n oxides d u r i n g r e d u c t i o n . A h i g h - g r a d e Brazilian ore h a v i n g the f o l l o w i n g c h e m i c a l c o m p o s i t i o n was used in his tests: F e 2 O 3 = 99.15%; SiO 2 = 0.30%; A l 2 O 3 = 0.20% T h e total g a n g u e content of this ore was as l o w as 0.50% a n d its basicity was zero. A c c o r d i n g to the q u a n t i t y and c o m p o s i t i o n of g a n g u e , the optim u m firing t e m p e r a t u r e s r a n g e d f r o m 1300 to 1360 ° C . Silica, limestone, d o l o m i t e , m a g n e s i u m oxide, s o d i u m a n d p o t a s s i u m c a r b o n a t e a n d p o t a s s i u m f e l d s p a r served as additives. F r o m the r e d u c t i o n tests c a r r i e d out at t e m p e r a t u r e s of 900, 1000 a n d 1100 0 C , those p e r f o r m e d at 1000 ° C w e r e chosen for f u r t h e r consideration. T h e reaction t i m e was s u f f i c i e n t in o r d e r to achieve full r e d u c t i o n by using a r e d u c t i o n gas c o n t a i n i n g 40% C O a n d 60% N 2 . Because the results o b t a i n e d are of practical value, they are discussed below, in p a r t i c u l a r t h e i n f l u e n c e of silica and limestone, alone CaO a n d in mixtures. T h e ratio was selected as basis for the basicity. SiO 2

6.1 Change of Pellet Structure During Reduction

171

Fig. 91. Ternary phase diagram of C a O - S i O 2 - F e O n

T h e pellets were i n d u r a t e d at o p t i m u m c o n d i t i o n s f o r various levels of d i f f e r e n t additives. T h e swelling b e h a v i o u r of 33 pellet types thus f o u n d was tested d u r i n g r e d u c t i o n . T h e d i f f e r e n t swelling v a l u e s o b t a i n e d u n d e r specific c o n d i t i o n s w e r e plotted o n a triangle d i a g r a m , Fig. 91. T h e corners of the triangle are f o r m e d b y the reactants: C a O , SiO 2 a n d F e O n . Only t h e iron c o r n e r of the triangle is of interest. Swelling values of equal v o l u m e increase are c o n n e c t e d with e a c h other. T h u s , isochores of pellets of e q u a l v o l u m e increase b u t of v a r y i n g c h e m i c a l c o m p o s i t i o n and of d i f f e r e n t basicity w e r e derived. T h e isochore w i t h 20% v o l u m e increase is especially m a r k e d as this v a l u e is generally r e g a r d e d as the m a x i m u m p e r m i s s i b l e swelling d e g r e e in practice. C o n s e q u e n t l y this range of a c c e p t a b l e swelling with v o l u m e increases of less t h a n 20% is o u t s i d e t h e critical b o u n d a r y w h e r e a s inside this b o u n d a r y are located t h e pellet swelling values w h i c h are c o n s i d e r e d to b e u n a c c e p t a b l y high. T h e zones of h i g h e r swelling values b e c o m e c o n s i d e r a b l y n a r r o w e r t o w a r d s the iron corner. In this a r e a , very small v a r i a t i o n s in t h e p r o p o r t i o n s of gangue, their c h e m i c a l c o m p o s i t i o n o r basicity are s u f f i c i e n t to b r i n g a b o u t great f l u c t u a t i o n s of v o l u m e c h a n g e . T h e g r e a t e r the g a n g u e

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6 Behaviour of Indurated Pellets During Reduction

p r o p o r t i o n on t h e highly calcareous side, t h e b r o a d e r t h e r a n g e of acceptable swelling b e c o m e s a n d , in a c o r r e s p o n d i n g m a n n e r , the less sensitive the pellets react d u r i n g reduction. T h e extent a n d s h a p e of isochores t e n d to a range varying a r o u n d a n a v e r a g e v a l u e of a b o u t 0.33 basicity. By l e n g t h e n i n g the longitudinal side of the critical swelling zone, the basicity line is cut at a b o u t 0.1 a n d 0.6. T h u s , t h r e e ranges of d i f f e r e n t basicity are f o r m e d . At the s a m e time, the swelling range r u n n i n g in parallel is d i v i d e d into three c o r r e s p o n d i n g sections w h i c h m a r k the d i f ferent swelling indices. T h e s e three ranges represent the following zones: (a) acid g a n g u e with a basicity o f - 0 . 1 ; a c c e p t a b l e swelling values o f - 2 0 % ; (b) g a n g u e with a basicity of 0.1 - 0 . 6 ; critical swelling values o f + 2 0 % ; (c) basic g a n g u e with basicity increasing to + 0 . 6 a c c e p t a b l e swelling values of—20%. T h e i n f l u e n c e of basicity o n the swelling b e h a v i o u r decreases with rising g a n g u e p o r t i o n a n d loses its efficiency at a b o u t 10% of g a n g u e a m o u n t as m a r k e d by " L " in Fig. 91. T h e isochores established on the basis of l a b o r a t o r y tests were c o m p a r e d with the swelling values of seven pellet types p r o d u c e d in industrial plants w h i c h were plotted o n the

Fig. 92. Swelling rate of pellets during reduction with varying CaO and SiO2 content

6.1 Change of Pellet Structure During Reduction

173

d i a g r a m . T h e s e s h o w g o o d c o n f o r m i t y b e t w e e n t h e l a b o r a t o r y a n d industrial pellets. F r o m this it c a n be d e d u c e d that by v a r y i n g the basicity pellets can be p r o d u c e d f r o m d i f f e r e n t raw m a t e r i a l s , w h i c h fulfill the o p e r a t i o n a l d e m a n d s of blast f u r n a c e s a n d direct r e d u c t i o n plants with r e g a r d to the swelling b e h a v i o u r . T h e swelling values a s c e r t a i n e d in d e p e n d e n c e o n the c h a n g e d SiO 2 a n d C a O content, as p e r Fig. 91 are s e p a r a t e y s h o w n in Fig. 92. T h e swelling values first rise with increasing l i m e a d d i t i o n at low SiO 2 contents ( C u r v e s I—VI). T h e y t h e n exceed a m a x i m u m value a n d a f t e r this, d i m i n i s h with f u r t h e r l i m e a d d i t i o n . A s SiO 2 levels increase, t h e position of t h e swelling m a x i m u m m o v e s a n d the m a x i m u m level decreases. At 8% SiO 2 a n d a b o u t 1.2% C a O c o r r e s p o n d i n g to a basicity of approx. 0.15, the swelling m a x i m u m r e m a i n s b e l o w the critical swelling line. A t lower l i m e contents with the basicity a p p r o a c h i n g zero, f a v o u r a b l e swelling values can also b e a c h i e v e d w h e n t h e g a n g u e p r o p o r t i o n exceeds 5% as is s h o w n in Fig. 93. T h u s 5% g a n g u e at a basicity of zero is sufficient to k e e p the swelling value b e l o w the critical limit of 20% ( C u r v e IV). H o w e v e r , at basicities in the r a n g e of 0.1 to 0.6, increased g a n g u e contents of u p to a b o u t 10% are r e q u i r e d ( C u r v e s I—III). As basicities increase a b o v e 0.6, the g a n g u e p r o p o r t i o n s r e q u i r e d to r e m a i n below t h e critical swelling line g r a d u a l l y d i m i n i s h (Curves V - V I I I ) .

Fig. 93. Influence of basicity and gangue content on swelling

174

6 Behaviour of Indurated Pellets During Reduction

In general, t h e r e are three d i f f e r e n t ranges d e p e n d i n g o n the basicity: (a) Basicity range f r o m 0 to a b o u t 0.1, g a n g u e a m o u n t r e q u i r e d : a b o u t 5% by weight; (b) Basicity r a n g e f r o m 0.1 to a b o u t 0.6, g a n g u e a m o u n t r e q u i r e d : a b o u t 5 - 1 0 % by weight; (c) Basicity range of m o r e t h a n 0.6, g a n g u e a m o u n t r e q u i r e d : less t h a n 5% a p p r o x i m a t e l y . T h e a b o v e values are g u i d e lines in c o n j u n c t i o n with the i n f l u e n c e of basicity w h i c h are also c o n f i r m e d b y o t h e r a u t h o r s 9 5 ) , even if the i n t e r p r e t a t i o n s of the d a t a still greatly d i f f e r . V a r i a t i o n s d u e to o t h e r conditions are also possible. D u r i n g reduction, t h e r e is, in any case, a correlation b e t w e e n the g a n g u e and the v a r y i n g oxygen content of i r o n oxides at w h i c h mineralogical f o r m a t i o n s existing b e f o r e r e d u c t i o n m a y change or even new ones m a y arise. 6.1.2.3.1 Properties of Acid Pellets with a Basicity of less than 0.1. In t h e case of highly acid pellets with a basicity of less t h a n 0.1, the g a n g u e is p r e d o m i n a n t l y present as silica which partly reacts with h e m a t i t e u n d e r a seam-like cristobalite f o r m a t i o n 3 1 ) . T h e fired pellet strength is, to a c e r t a i n degree, d u e to h e m a t i t e bridges of polycrystalline structure. T h e s e pellets contain a large a m o u n t of o p e n pores. T h e r e d u c t i o n gas can quickly penetrate t h r o u g h these pores into the pellet core a n d s i m u l t a n e o u s l y attack t h e s t r u c t u r e in m a n y places. T h e structural c h a n g e begins very early at low t e m p e r a t u r e s over the w h o l e pellet v o l u m e . H o w e v e r , t h e structure c a n be held together p r i m a r i l y at high SiO 2 contents b y the s e a m - l i k e c o m p o u n d s b e t w e e n F e 2 O 3 a n d S i O 2 . As soon as the first wüstite arises, fayalite m a y be f o r m e d with silica of solid state, 2 F e O -SiO 2 . D u e to low reducibility, at high SiO 2 contents the pellet structure only changes slightly. In this way, the total r e d u c t i o n velocity is also d e c r e a s e d . M o r e o v e r , f a y a l i t e with a melting point: 1217 ° C m a y act as a b i n d e r w h e r e b y a f u r t h e r swelling a n d d e g r a d a t i o n is d i m i n i s h e d . C o n s e q u e n t l y , fayalite is to b e considered as a stabilizing c o m p o n e n t d u r i n g r e d u c t i o n of highly acid pellets. H o w e v e r , the g a n g u e p o r t i o n s h o u l d exceed 5% (Fig. 93) a n d i m p e d e the d e g r a d a t i o n at lower r e d u c t i o n t e m p e r a t u r e . 6.1.2.3.2 Properties of Pellets with a Basicity of 0.1 to 0.6. D u r i n g t h e firing of pellets with low C a O p r o p o r t i o n s at a basicity of 0.1 to a b o u t 0.6, a glassy slag p h a s e consisting of S i O 2 , C a O , F e 2 O 3 of varying m a y arise d u r i n g reduction. These, with fayalite, f o r m m i x e d crystals whose m i n i m u m m e l t i n g point is a b o u t 1117 0 C a n d c o r r e s p o n d s to a basicity of a p p r o x i m a t e l y 0.35. A t this t e m p e r a t u r e a n d consistency, the m e c h a n i c a l strength of the g a n g u e reaches a m i n i m u m so t h a t a

composition

6.1 Change of Pellet Structure During Reduction

175

stabilizing effect c a n n o t b e expected f r o m the slag p h a s e . In a d d i t i o n , t h e slag structure m a y vary if metallic i r o n s e p a r a t e s f r o m m i x e d crystals a n d the silicate s t r u c t u r e tends to fall a p a r t 94). In this basicity r a n g e of 0.35, the highest swelling values are observed. In Fig. 94 F r a s e r 9 6 ) s h o w e d t h e correlations b e t w e e n the m e l t i n g b e h a v i o u r of m i x e d crystals a n d t h e swelling of such pellets in c o n j u n c t i o n w i t h basicity. T h e lowest m e l t i n g t e m p e r a t u r e c o r r e s p o n d s with the h i g h e s t swelling degree. A rising a n d decreasing basicity raises the m e l t i n g p o i n t a n d the swelling b e h a v i o u r changes accordingly. If very h i g h g a n g u e p o r t i o n s are present, see Fig. 93, the structural d a m a g e is a v o i d e d , a n d the swelling d e g r e e can be k e p t b e l o w 20%. If, in olivines, o t h e r constituents, such as M g O , r e p l a c e p a r t of the C a O and the m e l t i n g t e m p e r a t u r e in t h e m i x e d crystals rises c o r r e s p o n d i n g l y , this results in a decrease of s w e l l i n g 9 7 ) . H o w e v e r , it m a y also h a p p e n that alkaline c o m p o u n d s enter t h e olivine s t r u c t u r e w h i c h causes a lowering of t h e m e l t i n g t e m p e r a t u r e a n d i n t e n s i f i e s t h e swelling 9 8 ). It can be stated that, w i t h i n a basicity r a n g e of 0.1 to a b o u t 0.6 of t h e g a n g u e constituents, l o w - m e l t i n g olivine crystals are f o r m e d w h i c h d o not i m p e d e pellet swelling d u r i n g r e d u c t i o n but, u n d e r c e r t a i n circumstances, c a n intensify such swelling. H o w e v e r , g a n g u e p o r t i o n s of a b o u t 10% a n d o v e r are c a p a b l e of r e d u c i n g t h e structural d e s t r u c t i o n b e c a u s e this g a n g u e is still solid or plastic a n d holds the pellet s t r u c t u r e t o g e t h e r like a supporting frame.

Fig. 94. Relationship between swelling degree, pellet basicity and phase diagram of Fe 2 SiO 4 to Ca 2 Si 4

176

6 Behaviour of Indurated Pellets During Reduction

6.1.2.3.3 Properties of Pellets with a Basicity of Higher than 0.7. If the pellet g a n g u e contains m o r e C a O t h a n c o r r e s p o n d s to a basicity of a b o u t 0.7, not only the glassy slag phase, consisting of C a O , SiO 2 a n d F e 2 O 3 , b u t also c a l c i u m ferrites, C a O F e 2 O 3 , are f o r m e d w h i c h m a y c a u s e a d i f f e r e n t s t r u c t u r e of partially closed pores. D u r i n g pellet firing, the availability of C a O considerably f a v o u r s the crystal g r o w t h of h e m a t i t e as is shown in Fig. 27 34 ). D u r i n g r e d u c t i o n of C a O . F e 2 O 3 , metallic i r o n arises on the entire s u r f a c e of calcium wüstite as a skin w i t h o u t f o r m a t i o n of needles. T h i s iron f o r m s concentric s e a m s a r o u n d the ferrite grains a n d i m p e d e s f u r t h e r swelling. It s h o u l d be noted t h a t pellets with a high C a O content a n d a basicity exceeding a b o u t 0.6 h a v e a high m e c h a n i c a l strength a f t e r pellet firing a n d also a f t e r r e d u c t i o n . T h e a d a p t a t i o n of the c h e m i c a l c o m p o s i t i o n of the acid g a n g u e constituents c o n t a i n e d in the ore or concentrate by m e a n s of C a O a d d i t i o n is thus a n efficient t e c h n i q u e for pellet p r o d u c t i o n . A f u r t h e r increase in the l i m e a d d i t i o n leads to n e u t r a l i z a t i o n of all t h e acid constituents a n d to the p r o d u c t i o n of self-fluxing or o v e r - f l u x e d pellets. I l m o n i a n d o t h e r Swedish a u t h o r s d e a l t with this p r o b l e m a n d additionally with the use of d o l o m i t e 99). A n o t h e r p u r p o s e of these studies was to increase the total slag a m o u n t with a high basicity since s o m e S w e d i s h concentrates h a v e a very low g a n g u e content. D u r i n g these studies, the necessity to m a i n t a i n a n o p t i m u m basicity was recognized. C o n s i d e r a t i o n was given to the p r o d u c t i o n of basic or over-basic pellets in a similar way to over-basic sinter. T h i s possibility was successfully investigated in tests run on a n industrial scale 100 ). In this c o n n e c t i o n the overall e c o n o m i c s of this p r o c e d u r e n e e d to b e s t u d i e d . T h e m e t h o d s f o r m e a s u r i n g the swelling b e h a v i o u r have, for a long time, b e e n t h e m o s t i m p o r t a n t ones for testing the pellet quality d u r i n g reduction. H o w e v e r , the i m p o r t a n c e of this m e t h o d decreases b e c a u s e it is t o o general a n d only i n f o r m s a b o u t t h e structural c h a n g e at a b o u t 1000 0 C as can be seen f r o m the test m e t h o d for m e a s u r i n g the swelling i n d e x (item 4.6.1.2.3). A t lower r e d u c t i o n t e m p e r a t u r e s of 4 0 0 - 6 0 0 0 C a n d u n d e r a gas a t m o s p h e r e with a low r e d u c t i o n potential — as exists in the u p p e r part of t h e blast f u r n a c e — insufficient i n f o r m a t i o n is given o n the structural change.

6.1.2.4 Influence of Gangue Bonds on Pellet Structure at Temperatures of 4 0 0 - 6 0 0 0 C Under Reducing Atmosphere In this t e m p e r a t u r e range, a l m o s t all g a n g u e phases are still in a solid state. T h e y act as a s u p p o r t i n g f r a m e against the destroying p r e s s u r e forces d u r i n g t h e conversion f r o m h e m a t i t e to m a g n e t i t e , w h i c h is connected with a m a j o r v o l u m e increase, see Fig. 88. A b o u t 75% of the total

Pellets from

Magnetite concentrate Magnetite concentrate Magnetite concentrate Magnetite concentrate Hematite

Chemical composition

Gangue

Fe t a

CaO

SiO 2

Basicity

63.1

0.3

8.4

68.9

0.2

1.3

63.2

3.9

4.1

62.9

4.0

65.8

2.1

Swelling degree in % (Required -20%)

Comments

Portion %

Low-temperature disintegration 500° C - 0 . 5 mm% -20%

-0.1

8.7

16.3

13.4

Migh gangue content

-0.2

1.5

17.8

40.0

Low gangue content

0.95

8.0

19.5

12.4

Basicity 0.7

4.0

1.0

8.0

17.7

11.3

Basicity 0.7

2.4

0.9

4.5

12.1

13.7

Basicity 0.7

6.1 Change of Pellet Structure During Reduction

Table 25. Properties of reduced pellets depending on varying basicity and gangue contents measured in industrial plants

177

178

6 Behaviour of Indurated Pellets During Reduction

v o l u m e increase a l r e a d y takes place d u r i n g t h e conversion to the m a g netite p h a s e a n d only a b o u t 25% d u r i n g t h e wüstite p h a s e . C o n s e q u e n t l y , great i m p o r t a n c e m u s t also b e a t t a c h e d to t h e structural d e s t r u c t i o n a t low temperatures. As was f o u n d by E d s t r ö m , O t t o w a n d others, a t r a n s f o r m a t i o n of crystalline structure, which a u t o m a t i c a l l y p r o c e e d s u n d e r given conditions, occurs d u r i n g r e d u c t i o n of h e m a t i t e to m a g n e t i t e a n d wüstite. T h i s t r a n s f o r m a t i o n m e a n s a w e a k e n i n g a n d d e s t r u c t i o n of crystalline structure w h i c h is i n d i c a t e d by a shattering of the entire pellets i n t o f r a g m e n t s a n d by a n increase of the fines p r o p o r t i o n b e l o w 0.5 m m . U n d e r blast f u r n a c e conditions, with low r e d u c t i o n t e m p e r a t u r e a n d gases with low r e d u c t i o n potential, this z o n e w i t h its i n e v i t a b l e c h a n g e of crystalline structure c a n n o t b e a v o i d e d . T a b l e 25 shows test results o b t a i n e d w i t h pellets p r o d u c e d in industrial plants. 6.1.2.5 Influence of Gangue Bonds at About 1000 0 C on Pellet Structure Under Reducing Atmosphere As set f o r t h u n d e r item 6.1.2.3.2, s o f t e n i n g p h e n o m e n a m a y p r i m a r i l y occur at a g a n g u e c o m p o s i t i o n with a basicity of 0 . 1 - 0 . 6 at w h i c h t h e pellets are d e f o r m e d u n d e r p r e s s u r e a n d thus the gas flow is i m p e d e d . A s at these t e m p e r a t u r e s , the v o l u m e increase m a y s i m u l t a n e o u s l y r e a c h a m a x i m u m , b o t h factors m a y jointly i n f l u e n c e decisively t h e gas p e r m e a bility of a pellet bed. T h e pellet b e h a v i o u r u n d e r these r e d u c t i o n conditions is tested by a d o p t i n g the pressure s o f t e n i n g m e t h o d d e s c r i b e d u n d e r item 4.7.1.2.4.

6.2 Behaviour of Indurated Pellets Consisting of Magnetite and Wüstite During Reduction T h e structural destruction observed d u r i n g r e d u c t i o n of pellets low in gangue can be p r e v e n t e d by varying the c h e m i c a l c o m p o s i t i o n a n d the quantity of g a n g u e constituents. H o w e v e r , t h e increasing a m o u n t of g a n g u e is d e t r i m e n t a l w h e n h i g h - g r a d e pellets are r e q u i r e d f r o m a m e t a l lurgical viewpoint. In particular, d u r i n g steel p r o d u c t i o n f r o m s p o n g e iron in the electric f u r n a c e , a small slag q u a n t i t y w o u l d be a d v a n t a g e o u s as f a r as energy c o n s u m p t i o n is concerned. T h e r e f o r e , considerations to p r e v e n t the structural d e s t r u c t i o n w i t h o u t gangue v a r i a t i o n are j u s t i f i e d . Both E d s t r ö m a n d O t t o w o b s e r v e d that, u n d e r certain circumstances, m a g n e t i t e crystals are not destroyed d u r i n g reduction. T h e r e d u c t i o n of m a g n e t i t e to metallic iron, see Fig. 88,

6.2 Behaviour of Indurated Pellets Consisting of Magnetite

179

curve 5, is even c o n n e c t e d with a v o l u m e decrease. If it were possible to p r o d u c e only m a g n e t i t e pellets instead of h e m a t i t e , t h e structural destruction m i g h t b e a v o i d e d . It is a l r e a d y k n o w n t h a t m a g n e t i t e pellets can be p r o d u c e d f r o m m a g n e t i t e ore u n d e r a n e u t r a l a t m o s p h e r e . H o w e v e r , for h e m a t i t e ores this is only possible b y first c o n v e r t i n g to m a g n e t i t e a n d then i n d u r a t i n g u n d e r a neutral a t m o s p h e r e . Pellets with a lower oxygen content t h a n that c o r r e s p o n d i n g to h e m a t i t e c a n also be p r o d u c e d by t h e m i x i n g of finely g r o u n d s p o n g e iron with ores o r c o n c e n t r a t e s as s h o w n in Fig. 87 and as d e s c r i b e d u n d e r items 5 . 3 . 1 . 8 - 5 . 3 . 1 . 8 . 2 as well as in Figs. 67 and 68. F o r c o m p a r i s o n , it s h o u l d be n o t e d t h a t t h e strength of high-grade h e m a t i t e pellets d r o p s s h a r p l y d u r i n g r e d u c t i o n w i t h gas d o w n to a m i n i m u m value in t h e wustite stage. O n the o t h e r h a n d , small q u a n t i t i e s of s p o n g e iron a d d i t i o n a l r e a d y result in a very h i g h strength. In a s i m i l a r way, the strength of m a g n e t i t e pellets can be i m p r o v e d by sponge iron a d d i t i o n . Also d u r i n g the r e d u c tion of such pellets with gas c o n t a i n i n g 80% C O a n d 20% N 2 at a t e m p e r a t u r e of 1000 0 C a n d d u r i n g a p e r i o d of 60 m i n u t e s , r e m a r k a b l e differences c o m p a r e d to h e m a t i t e pellets c a n b e o b s e r v e d , as is s h o w n in Fig. 95.

Fig. 95. Comparison of behaviour of various pellet types during reduction

180

6 Behaviour of Indurated Pellets During Reduction

D u r i n g r e d u c t i o n of h e m a t i t e pellets, curve I, a swelling of 43% is f o u n d whereas pellets p r o d u c e d f r o m sponge iron s h o w a clear s h r i n k a g e at different oxygen contents, curves II a n d III. T h e d i f f e r e n t strength development a b r a s i o n values of less t h a n 0.5% were c o r r e s p o n d i n g l y low. As can f u r t h e r be seen f r o m t h e d i a g r a m , the pellets p r o d u c e d with s p o n g e iron a d d i t i o n were practically r e d u c e d completely a f t e r 60 m i n u t e s while the r e d u c e d h e m a t i t e pellet still contains 12% oxygen.

6.3 Conclusions T h e following s u m m a r i z i n g c o m m e n t s can b e given o n this i m p o r t a n t section in w h i c h the b e h a v i o u r of pellets d u r i n g r e d u c t i o n is dealt with: (a) Only pellets with o p t i m u m firing strength, irrespective of their chemical c o m p o s i t i o n or origin of the ores f r o m w h i c h they are p r o d u c e d , fulfill the c o n d i t i o n s r e q u i r e d for m a i n t a i n i n g t h e necessary m e c h a n i c a l strength. (b) By a v a r i a t i o n of the c h e m i c a l c o m p o s i t i o n of g a n g u e constituents a d a p t e d to the o r e c o m p o s i t i o n , a potentially d a m a g i n g structural c h a n g e can be c o m p e n s a t e d for. (c) Pellets with a low g a n g u e content are also resistant to swelling a n d d e g r a d a t i o n w i t h o u t v a r i a t i o n of c h e m i c a l c o m p o s i t i o n of g a n g u e , if the h e m a t i t e crystalline structure is replaced b y a structure of iron o x i d e crystals with a lower oxygen p r o p o r t i o n , e.g. m a g n e t i t e .

r e p r e s e n t e d o n curves

7 Special Processes for Pellet Production

In this chapter, alternative pelletizing m e t h o d s are described which deviate f r o m the conventional ones. (a) Besides heat treatment of green pellets for induration, the h a r d e n i n g is achieved by additives with high bonding properties. (b) S o m e iron ores containing detrimental or valuable ingredients can be cleaned during heat treatment whereby these ingredients are either eliminated by volatilisation or m a d e soluble. T h e residual pellets are available for ironmaking.

7.1 Pellet Hardening by Using Binders T h e pellets are produced according to such processes designated as cold bond pelletizing 1 0 1 ). T h e y differ f r o m thermally indurated pellets by other bonding mechanisms which are based on hydraulic or hydrothermal reactions f r o m normal temperatures u p to a b o u t 250 0 C . T h e binders used should have such a chemical composition that no negative influence on the pellet properties is to be expected. S o m e processes, mostly known f r o m the literature are described below: T h e most significant stages of such processes are shown as simplified flowsheets in Fig. 96. 7.1.1 Grangcold Process102) T h e p u r p o s e of the development of this process was the intention to lower the pelletizing costs both for capital investment and energy consumption. It was developed by Grängesberg A. B. in Sweden. T h e quantity of cement used as binder varies between 8 and 12% by weight. According to the normal setting process for cement, a storage time of a b o u t 28 days is necessary to achieve an o p t i m u m strength within a justifiable h a r d e n i n g period. T h e strength as function of h a r d e n i n g time is shown in Fig. 97. At the beginning of the hydraulic reaction the green pellets have to be protected against abrasion and excessive pressure load. This is done in a

182

7 Special Processes for Pellet Production

Fig. 96. Other pellet hardening methods without firing

b e d of f i n e - g r a i n e d concentrates. W h e n a m i n i m u m strength is r e a c h e d , these concentrates are s e p a r a t e d f r o m the pellets b y s m o o t h screening a n d recirculated. T h e final h a r d e n i n g is t h e n a c c o m p l i s h e d in a s t o c k y a r d o r in an a d e q u a t e l y d i m e n s i o n e d storage plant. T h e c r u s h i n g strength of G r a n g c o l d pellets is lower t h a n that of t h e r m a l l y treated pellets. T h e h y d r a u l i c b o n d s disintegrate at a b o u t 600 0 C , i.e. f r o m this t e m p e r a t u r e onward, the pellets lose the greatest part of their m e c h a n i c a l strength. F o r this reason, the pellets should quickly pass t h r o u g h this t e m p e r a t u r e r a n g e d u r i n g reduction. T h e swelling b e h a v i o u r a n d o t h e r p r o p e r t i e s of pellets are s u f f i c i e n t d u r i n g reduction. A plant in J a p a n 103) is p r o d u c i n g a b o u t 45 000 tons of cold b o n d pellets per m o n t h .

7 . 1 . 2 C O B O and M T U P r o c e s s

104/101

)

T h e C O B O - " C O L D B O U N D " Process was d e v e l o p e d in the D i v i s i o n of Metal Processing of the Royal Institute of T e c h n o l o g y in S t o c k h o l m . First p u b l i c a t i o n s d a t e f r o m 1967. T h e raw m a t e r i a l s m u s t physically c o n f o r m to t h e conditions for green ball f o r m a t i o n . L i m e h y d r a t e a n d possibly o t h e r additives are used as binders. T h e pellets are f o r m e d in well-known facilities and, a f t e r p r e d r y i n g , are i n d u r a t e d for a b o u t 1 - 5

7.2 Chloridizing Volatilization of Non-Ferrous Metal Oxides

183

Fig. 97. Influence of time on hardening of Grang-cold pellets

hours at a p p r o x . 16 b a r pressure in a s t e a m a u t o c l a v e at t e m p e r a t u r e s of 1 5 0 - 2 5 0 ° C . T h e crushing strength of h a r d e n e d pellets ranges f r o m 7 5 0 - 1 3 0 0 N / p e l l e t a n d is less t h a n t h e c o r r e s p o n d i n g values of a p p r o x . 2000 N / p e l l e t r e q u i r e d f o r blast f u r n a c e o p e r a t i o n . T h e r e d u c t i o n p r o p e r ties are g o o d . T h e C O B O process is also s u i t a b l e for the pelletizing of other m e t a l oxides, such as c h r o m i t e ores, pyrite cinders a n d i n - p l a n t fines. T h e M T U 1 0 1 ) process is advertised especially in the U S A f o r the recirculation of in-plant fines in steel plants. T h i s process as well as t h e C O B O process c o r r e s p o n d to the s i m p l i f i e d f l o w s h e e t of Fig. 96.

7.2 Chloridizing Volatilization of Non-Ferrous Metal Oxides and Pellet Production Since pyrite - as the s u l p h u r basis f o r s u l p h u r i c acid p r o d u c t i o n — has mostly b e e n r e p l a c e d by elemental s u l p h u r f r o m o t h e r sources, t h e s u p p l y of pyrite cinders (calcines) as i r o n - b e a r i n g r a w m a t e r i a l f o r p i g - i r o n p r o d u c t i o n h a s drastically d i m i n i s h e d . N o w a d a y s , the m i n i n g of pyrite is no longer of p r i m a r y interest. H o w e v e r , p y r i t e is o f t e n o b t a i n e d as b y p r o d u c t d u r i n g flotation of zinc, lead or c o p p e r - b e a r i n g s u l p h i d e s . H o w e v e r , this pyrite s e p a r a t e d b y f l o t a t i o n is n o r m a l l y not f r e e f r o m nonferrous m e t a l sulphides. T o d a y , such pyrites a r e still of interest, especially in countries w i t h great p y r i t e deposits, e.g. S c a n d i n a v i a , S p a i n , P o r t u g a l , Greece, Soviet U n i o n . T h e s e countries are interested in the s e p a r a t i o n of

184

7 Special Processes for Pellet Production

n o n - f e r r o u s m e t a l oxides as chlorides f r o m p y r i t e cinders by c h l o r i d i z i n g at a t e m p e r a t u r e of a b o u t 1 2 0 0 - 1 2 5 0 ° C . F o r this p u r p o s e , the p y r i t e cinders are f o r m e d into green balls a f t e r a d e q u a t e grinding. As chlorinating agent C a C l 2 can b e a d d e d to the pellet mix. It is also possible to pass chlorine gas at a p p r o p r i a t e t e m p e r a t u r e t h r o u g h t h e hot pellet b e d . T h e t h e r m a l t r e a t m e n t a n d chlorination can b e a c h i e v e d either in a s h a f t f u r n a c e or in a rotary kiln. N o n - f e r r o u s m e t a l chlorides are volatilized a n d can be r e c o v e r e d f r o m the waste gas s t r e a m b y s c r u b b i n g . H o w e v e r , it is also possible to chlorinate the fine-grained calcines in a circulating fluid bed or in m u l t i p l e h e a r t h roasters in w h i c h s o d i u m c h l o r i d e is o f t e n u s e d as chlorinating agent. S u b s e q u e n t l y , the soluble chlorides are l e a c h e d a n d thus separated f r o m the calcines. T h e s e cleaned calcines, called p u r p l e ore, are m i x e d with o t h e r ores a n d a g g l o m e r a t e d in sinter o r pelletizing plants. Since, pyrites are mostly o b t a i n e d by f l o t a t i o n , they are a l r e a d y very fine-grained w h i c h is f a v o u r a b l e for pellet p r o d u c t i o n .

7.2.1 Chloridizing Volatilization and Pelletizing in the S h a f t F u r n a c e A s h a f t f u r n a c e plant of V u o k s e n n i s k a O Y in I m a t r a , F i n l a n d was o p e r a t e d for several years 1 0 5 ). T h e necessary q u a n t i t y of C a C l 2 , h a d a l r e a d y b e e n a d d e d to the pellet f e e d . T o p r e v e n t p r e m a t u r e dissociation of C a C l 2 in the presence of s t e a m d u r i n g pellet d r y i n g a n d heating, air d r i e d by using silica gel was r e g e n e r a t i v e l y h e a t e d to a b o u t 1200 0 C a n d passed into the pellet c h a r g e in the s h a f t f u r n a c e . T h e cleaned pellets were cooled a n d processed i n t o p i g - i r o n in electric r e d u c t i o n f u r n a c e s . T h e chlorides c o n t a i n e d in the waste gases w e r e electrostatically p r e c i p i t a t e d , w h i c h was c o n n e c t e d with m a n y c o r r o s i o n p r o b l e m s a n d h i g h o p e r a t i n g costs. T h i s process was only used on a pilot plant scale of a b o u t 150 tons/day. T h e s h a f t f u r n a c e process was f u r t h e r d e v e l o p e d a n d C a C l 2 was replaced by chlorine gas. A c o r r e s p o n d i n g pilot p l a n t with a b o u t 120 t p d capacity was o p e r a t e d for s o m e years at t h e D u i s b u r g e r K u p f e r h ü t t e , West G e r m a n y . T h e construction of an i n d u s t r i a l p l a n t a n d a w i d e r a p p l i c a t i o n of this process failed owing to t h e c h a n g e d m a r k e t s i t u a t i o n w h e n the c o n s u m p t i o n of pyrites a b r u p t l y d e c r e a s e d . This process repeatedly described in literature u n d e r the n a m e "LDK Process" ( L u r g i D u i s b u r g e r K u p f e r h ü t t e ) 1 0 6 ) is s h o w n in the flowsheet, Fig. 98. In o r d e r to achieve a u n i f o r m size distribution, the calcines f r o m v a r i o u s origins are s u b j e c t e d to dry grinding, m i x e d with b e n t o n i t e and f o r m e d into g r e e n balls. B e f o r e t h e green balls enter the s h a f t f u r n a c e , t h e y are largely d r i e d

corresponding

7.2 Chloridizing Volatilization of Non-Ferrous Metal Oxides

185

Fig. 98. Chlorination and pellet hardening of pyrite cinders in shaft furnace (LKD process)

o n a belt d r y e r with d r y i n g gases low in s t e a m . T h e s h a f t f u r n a c e , especially d e s i g n e d f o r this type of t h e r m a l t r e a t m e n t in the presence of chlorine gas, was d i v i d e d into t h r e e zones, as s h o w n in Fig. 99. T h e d r i e d pellets are i n d u r a t e d by hot c o m b u s t i o n gases in the u p p e r p a r t of the s h a f t . T h e waste gas f r o m this o p e r a t i o n is w i t h d r a w n at the s h a f t f u r n a c e feed end. T h e hot c h a r g e is t h e n t r e a t e d with a c h l o r i n e g a s - a i r mix in the c h l o r i n a t i o n zone, w h e r e t h e n o n - f e r r o u s m e t a l chlorides are f o r m e d . T h e gas arising in this zone a n d c o n t a i n i n g t h e volatile chlorides is t r e a t e d separately. T h e h e a t c o n t e n t of t h e i n d u r a t e d a n d c h l o r i n a t e d pellets is recovered b y f r e s h air in t h e cooling z o n e a n d separately e x h a u s t e d . T h e chemical analysis a n d m e c h a n i c a l q u a l i t y of t h e pellets p r o d u c e d b y c h l o r i n a t i o n m e t t h e pellet c o n s u m e r s ' r e q u i r e m e n t s . H o w e v e r , the a d d i t i o n a l c o n s u m p t i o n of c h e m i c a l s a n d h i g h e r o p e r a t i n g costs c o m p a r e d with n o r m a l l y p r o d u c e d pellets can only b e j u s t i f i e d if t h e n o n -

186

7 Special Processes for Pellet Production

Fig. 99. Gas flow in LKD shaft furnace

ferrous m e t a l s can be used economically. T h i s gas flow pattern, Fig. 99, can be a n a l o g o u s l y utilized for treating o t h e r ores.

7 . 2 . 2 C h l o r i d i z i n g V o l a t i l i z a t i o n and P e l l e t P r o d u c t i o n in a R o t a r y Kiln On the basis of the V u o k s e n n i s k a process, the K o w a - S e i k o process was d e v e l o p e d in J a p a n 107 ), according to w h i c h t h e volatilization and pellet i n d u r a t i o n are achieved in a rotary kiln. C a C l 2 is a d d e d to the pellet f e e d . T h e process is represented in Fig. 100. T h e f i n e l y - g r o u n d calcines are m i x e d with a C a C l 2 solution in a special device, the g r e e n balls are d r i e d on a belt d r y e r a n d h e a t e d u p to a b o u t 1250 ° C in a rotary kiln. T h e chlorides are w a s h e d out f r o m the waste gas a n d can b e recovered. T h e pellets are of a n excellent m e c h a n i c a l quality. Several plants with an overall c a p a c i t y of a b o u t 1 million tons of pellets per year are in o p e r a t i o n in J a p a n . In the U S A , a plant with the s a m e a n n u a l capacity is also in operation; f u r t h e r plants are u n d e r construction or in the design stage. The today's m a r k e t situation is not f a v o u r i n g this process. T a b l e 26 shows the chemical analyses of pyrite cinders a n d of the corr e s p o n d i n g o x i d e pellets p r o d u c e d with i n d i c a t i o n of physical-metal-

in %

Fe t o t

FeO

SiO 2

Cu

Pb

Zn

As

Chemical analysis of pyrite cinders

53 67

1-5

1-8

0.1-0.8

0.2-0.7

0.2-1.1

0.01-0.05

1-2

Range: chemical composition of chlorinated pellets

55-68

- 1

1-9

-0.03

-0.1

-0.2

-0.05

-0.04

UP

TO

in %

-

95

90

90

Degree of volatilisation of non-ferrous impurities Pellet size Compression strength N/pellet Abrasion - 0.5 mm in % Reduction behaviour

-

-

1 1 - 1 5 mm 2500 - 4000 1-3 like other good pellets

S

95

7.2 Chloridizing Volatilization of Non-Ferrous Metal Oxides

Table 26. Chemical composition of pyrite cinders and indurated pellets after chlorination and properties of pellets

187

188

7 Special Processes for Pellet Production

Fig. 100. Kowa Seiko chlorination and pelletizing process applied in a rotary kiln

lurgical properties. As can be seen f r o m this table, pellets f r o m chloridizing process can i n d e e d c o m p e t e w i t h m a r k e t a b l e pellets.

a

7.3 Recovery of Vanadium Pentoxide from Vanadium-Bearing Iron Ores V a n a d i u m is a n i m p o r t a n t alloying e l e m e n t in steel p r o d u c t i o n a n d as catalyst basis in the chemical industry. T h e largest reserves are a v a i l a b l e in v a n a d i u m - b e a r i n g magnetites or t i t a n o m a g n e t i t e s f r o m w h i c h the metal is p y r o m e t a l l u r g i c a l l y recovered as f e r r o - v a n a d i u m or as p e n t o x i d e , V 2 O 5 , by a c o m b i n a t i o n of p y r o - or h y d r o m e t a l l u r g i c a l steps. A slag, h i g h in v a n a d i u m , t r a d e d as i n t e r m e d i a t e p r o d u c t , is o f t e n used as r a w material. This slag is roasted u n d e r oxidizing a t m o s p h e r e , t o g e t h e r with alkaline salts a n d the calcines o b t a i n e d are leached. H o w e v e r , it is also possible to f o r m a m i x t u r e of v a n a d i u m - c o n t a i n i n g m a g n e t i t e concentrates a n d alkaline salts into green balls, w h i c h are i n d u r a t e d u n d e r a n oxidizing atmosphere. T h e water-soluble v a n a d a t e is separated by leaching

7.3 Recovery of Vanadium Pentoxide from Vanadium-Bearing Iron Ores

189

f r o m the pellets. By using these process v a r i a b l e s , n o t only v a n a d i u m salts processed into V 2 O 5 result, but also i n d u r a t e d pellets f o r iron p r o d u c t i o n are o b t a i n e d . U p to now, two variants are industrially a p p l i e d .

7.3.1 T h e r m a l Treatment of V a n a d i u m - B e a r i n g Pellets in a S h a f t F u r n a c e 1 0 8 ) Since 1953, such a process h a s b e e n a p p l i e d by R a u t a r u u k i Oy, Finland. T h e o r e used contains a b o u t : 38-40% magnetite 2 8 - 3 1 % ilmenite 1 - 2% p y r i t e with a b o u t 0.25% v a n a d i u m . F r o m the r a w ore, t h e following c o n c e n t r a t e s are p r o d u c e d by b e n e f i ciation: a b o u t 64% v a n a d i u m - c o n t a i n i n g m a g n e t i t e concentrate, a b o u t 35% i l m e n i t e c o n c e n t r a t e , as well as small a m o u n t s of p y r i t e c o n c e n t r a t e T h e m a g n e t i t e c o n c e n t r a t e f o r v a n a d i u m recovery has the following approximate composition F e tot. = 68.5%, T i O 2 = 3.1%, SiO 2 = 0.4%, C a O = 0.06%, M g O = 0.2%, A l 2 O 3 = 0.7%, V as V 2 O 5 = 1.14%. T h e concentrate is very finely g r o u n d to a b o u t 85% - 0 . 0 3 7 m m (400 m e s h ) a n d t h e specific s u r f a c e a r e a is i n d i c a t e d as 10 0 0 0 - 1 1 500 c m 2 / c m 3 . A c c o r d i n g to the p o r t i o n of V 2 O 5 , c o r r e s p o n d i n g q u a n t i t i e s of s o d i u m c a r b o n a t e are a d d e d to this concentrate. T h e following p a r a m e t e r s are specified: pellet d i a m e t e r 16-19 mm moisture content 8.5% H 2 O crushing strength of g r e e n pellets a b o u t 22 N / p e l l e t drop number 4x46 c m crushing strength of dry pellet 50—70 N / p e l l e t hot gas t e m p e r a t u r e 1100 - 1 1 5 0 ° C pellet t e m p e r a t u r e a p p r o x . 1250 0 C crushing strength of i n d u r a t e d pellet 1 0 0 0 - 1 3 0 0 N / p e l l e t . A f t e r cooling, the pellets are l e a c h e d in water, w i t h the v a n a d i u m recovery a p p r o x i m a t i n g 8 5 - 9 0 % , a n d are t h e n used f o r t h e blast f u r n a c e b u r d e n .

190

7 Special Processes for Pellet Production 7.3.2 Thermal Treatment of Vanadium-Bearing Pellets A c c o r d i n g to the G r a t e - K i l n P r o c e s s

T r a n s v a a l Alloys (Pty) Ltd. in t h e S o u t h A f r i c a n R e p u b l i c also p r o d u c e s V 2 O 5 f r o m a natural t i t a n o m a g n e t i t e w i t h o u t p r e v i o u s b e n e f i ciation. T h e ore has the following c o m p o s i t i o n 109 ): F e tot. 55.6 TiO2 12.7 SiO 2 2.2 V 0.9 T h e ore was g r o u n d to a b o u t 50%—0.09 m m a n d a specific s u r f a c e a r e a of a b o u t 2300 c m V g a n d m i x e d with N a 2 S O 4 as digesting agent. T h e following pellet crushing strength was a s c e r t a i n e d in the p r e h e a t i n g stage o n t h e travelling grate: green pellet strength a b o u t 30 N / p e l l e t dry pellet strength a b o u t 50 N / p e l l e t pellet strength at 900 0 C a b o u t 400 N / p e l l e t T h e p r e h a r d e n e d pellets enter the rotary kiln in w h i c h t h e y are h e a t e d u p to t e m p e r a t u r e s of a p p r o x . 1270 0 C for a b o u t 6 0 - 1 1 0 m i n u t e s . A f t e r cooling the pellets are leached in water. T h e l i q u o r still c o n t a m i n a t e d by o t h e r constituents is cleaned a n d t h e n processed t h r o u g h a m m o n i u m v a n a d a t e into m o l t e n V 2 O 5 scales. T h e pellets can, as f a r as t h e i r T i O 2 content allows, be f u r t h e r treated.

7.4 Dearseniflcation and Pelletizing of Iron Ores S o m e i r o n ores, a b o v e all pyrite cinders, c o n t a i n arsenic as a detrimental s u b s t a n c e w h i c h has to b e r e m o v e d b e f o r e i r o n p r o d u c t i o n . D u r i n g pyrite o x i d a t i o n , arsenic s u l p h i d e oxidizes to A s 2 O 5 , w h i c h , in this o x i d a t i o n stage, c a n n o t be volatilized a n d , u n d e r certain c i r c u m stances, has a t e n d e n c y to f o r m c a l c i u m arsenate. H o w e v e r , it is possible to convert t h e arsenic c o m p o u n d to As 2 O 3 w h i c h is volatile, a n d , in this state, can b e e l i m i n a t e d f r o m iron oxides. S u c h reactions are carried o u t either in a r o t a r y kiln, e.g. d u r i n g m a g n e t i z i n g roasting, o r in a s h a f t f u r n a c e as s h o w n in Fig. 99 in w h i c h a n e u t r a l to slightly r e d u c i n g a t m o s phere, r e q u i r e d f o r the reaction, can be p r o v i d e d . A calcine with the following composition: F e tot. = 53.70% Fe" = 6.60% S = 2.10% As = 0.22%

7.4 Dearsenification and Pelletizing of Iron Ores

191

was finely g r o u n d , t o g e t h e r with 2% coal c o r r e s p o n d i n g to 1% fixed c a r b o n a n d pelletized. A t a t e m p e r a t u r e of a b o u t 9 5 0 - 1 0 0 0 0 C a n d an oxygen content of less t h a n 3% in h e a t i n g gas, it was possible to lower the arsenic content d o w n to a b o u t 0 . 0 1 - 0 . 0 2 % a n d the s u l p h u r content to 0.02% 110). Such iron o r e pellets, largely l i b e r a t e d f r o m arsenic i m p u r i t i e s can be utilized for iron p r o d u c t i o n .

8 Balling Equipment

Raw material preparation, green ball f o r m a t i o n and pellet h a r d e n i n g are the decisive factors for the production of pellets with the characteristics required for their handling and reduction. F o r the p e r f o r m a n c e of these process parameters special e q u i p m e n t is used. In order to o b t a i n pellets of u n i f o r m and o p t i m u m quality, the design and function of the pertinent a p p a r a t u s must be sufficiently flexible for compensating for all of the very different properties related to the raw materials involved. Many of the devices used were already known. F o r many years some of them have been successfully utilized for similar purposes, which was advantageous for the introduction of pelletizing on an industrial scale. Nevertheless, it was necessary to a d a p t some of these devices to the conditions required for pelletizing. D u r i n g the design and layout of the whole pelletizing plant, these varieties of a p p a r a t u s have to be so d i m e n sioned and combined with each other that an o p t i m u m p e r f o r m a n c e and economy is ensured.

8.1 Homogeneity of Components for Pelletizing Mixtures A u n i f o r m chemical composition of the raw material constituents is one of the principal conditions for the quality of the pellets produced. As long as only one concentrate or one single ore is treated in a plant, there is no risk of an irregular composition. However, even in such a case, good mixing of bentonite is problematic to some extent. Therefore, at a very early stage attempts were m a d e to premix the small bentonite portions of approximately 0.7% with the great ore mass. These attempts were not very successful at that time. T h e question regarding the sufficient homogeneity of the mass to be pelletized becomes very acute and important if several solid constituents, either ores or additives have to be treated and to be uniformly distributed over each individual pellet.

8.2 Demands on the Mode of Operation of Balling Units

193

During the preparation of mixtures and green ball formation two functions have to be fulfilled: 1) Compensation of differences in the chemical composition of the various constituents and 2) Formation of green balls which is only done according to strict physical laws and is more or less independent of the chemical composition. Once several solid constituents are to be evenly incorporated into the pellets, suitable mixers should precede the balling devices for their release. It was already recognized at a very early stage that it is difficult to distribute uniformly the small bentonite amount of 0.7% over a great quantity of magnetite concentrate. Formerly, the Pekay mixer was mounted as mixing unit on the belt conveyor. This mixer was equipped with plough-like blades with which the bentonite, charged on to the concentrate surface, was to be blended during transportation. This system was not very effective. The difficulties increased when mixtures of different ores, concentrates and additives with different chemical compositions were processed. Instead of the mixers arranged above the belt conveyor, passage mixers were developed capable of homogenizing large quantities of up to 150tph 111). Before the design of the pelletizing plant for ore mixtures at Ijmuiden in Holland, mixing tests were carried out and different mixing systems were carefully investigated. On this occasion, it was found that the bentonite addition could be lowered from 0.7 to 0.3% if the mix were well homogenized. LKAB also performed extensive relevant tests and ascertained that good blending of the feed before pelletizing results in a reduction of bentonite consumption of up to 50%. In this way, it has been demonstrated that it is expedient to supply the balling units with a homogeneous mix.

8.2 Demands on the Mode of Operation of Balling Units To ensure that the ball formation alternatives and bonding mechanisms, described under items 2-2.1.2.3, as well as the influencing factors, specified under items 5 - 5 . 3 , become active, it is necessary to use adequate pelletizing equipment. In these units, the prepared loose pellet feed including bindere and other additives as well as water are to be formed into green balls. All the conditions for obtaining a good pellet quality are to be fulfilled for a good mechanical procedure of ball formation. In this connection, all movements and compression forces should become active in

194

8 Balling Equipment

o p t i m u m h a r m o n y with each other. T h e following d e m a n d s are m a d e on the balling e q u i p m e n t : — G o o d m i x i n g effect to e n s u r e — as far as possible — a u n i f o r m — continuous and smooth movements, — intensive relative m o v e m e n t s of c o n g l o m e r a t e s d u e to u n i f o r m m o i s tening of g r a i n surfaces, — intensive relative m o v e m e n t s of individual solid particles to each o t h e r to o b t a i n a m a x i m u m n u m b e r of contact p o i n t s a n d s u r f a c e s in o r d e r to f o r m as m a n y capillaries as possible, — m o v e m e n t s f o r p r o d u c i n g a slight pressure in o r d e r to r e d u c e t h e n u m b e r of cavities b e t w e e n solid particles a n d to f o r m p o r o u s c o n g l o m e r ates, — rolling m o v e m e n t in o r d e r to f o r m balls f r o m the a g g l o m e r a t e s w h i c h are partly still irregular.

8 . 2 . 1 Ball F o r m a t i o n and O p e r a t i o n o f t h e Principal Pelletizing U n i t s Pelletizing units should h a v e all constructional features so as to allow a m e c h a n i c a l t r a n s f o r m a t i o n of the process p a r a m e t e r s f o r ball f o r m a t i o n . T h e grain m i x t o be pelletized with a b o u t 8 0 % - 0 . 0 4 5 m m , of d i f f e r e n t grain s h a p e a n d s u r f a c e is m o v e d in such a w a y that the m o s t effective b o n d s r e q u i r e d f o r good ball f o r m a t i o n can arise. T h i s m o v e m e n t is achieved by u s i n g s u i t a b l e a p p a r a t u s and special tools, e.g. in so-called positive mixers. H o w e v e r , the most natural a n d the simplest m o v e m e n t is the freely rolling movement for w h i c h f a v o u r a b l e conditions for ball f o r m a tion are given b y nature. T h e rolling m o v e m e n t is also the basis for the most f r e q u e n t l y e m p l o y e d pelletizing units. It arises w h e n the angle of repose of an o r e pile is so steep that the gravity of particles o v e r c o m e s the frictional resistance of its e n v i r o n m e n t . W h e n a loose o r e mass is d u m p e d o n a plain s u p p o r t , a p y r a m i d e with a specific angle of repose is f o r m e d which results f r o m the frictional resistance of t h e pile. D r y grains f o r m a flat angle of r e p o s e while irregularly s h a p e d a n d wet grains with a m a j o r friction f o r m a s t e e p e r one. In t h e position of rest, the pile f o r m s the static angle of repose. W h e n the friction is o v e r c o m e at a steeper slope, the pile s u r f a c e u n d e r g o e s a rolling m o v e m e n t until t h e static angle of r e p o s e is again r e a c h e d a n d the pile is once m o r e in a position of rest. T h e rolling m o v e m e n t c a n b e m a i n t a i n e d if it is possible to k e e p the grains m o v i n g so that the angle of repose is b e l o w the d y n a m i c one. This can be achieved if the grains are p l a c e d on a n a d e q u a t e l y inclined s u p p o r t m o v i n g in o p p o s i t e d i r e c t i o n to gravity of the mass i.e. m o v i n g diagonally u p w a r d s .

distribution

of al

8.2 Demands on the Mode of Operation of Balling Units

195

In this way, t h e s u p p o r t c o n t i n u o u s l y m o v e s m a t e r i a l u p w a r d s d u e to the frictional resistance b e t w e e n s u p p o r t a n d ore. W h e n t h e angle of the inclined s u p p o r t b e c o m e s steeper, the grains are c o m p e l l e d to reverse a n d roll d o w n w a r d s over the ore surface. O n rolling d o w n , the wet grains tend to pick u p o t h e r particles a n d start to f o r m balls. If this d o w n w a r d m o v e m e n t can c o n t i n u e in t h e presence of new, fresh particles, the d i a m e t e r of agglomerates increases a n d green pellets of d e s i r e d size are finally f o r m e d . U n d e r these conditions, a c o m b i n a t i o n of balls of d i f f e r e n t d i a m e t e r d o w n to the original fines particle size develops. D u r i n g the d o w n w a r d rolling m o v e m e n t a certain sorting of the m i x occurs a c c o r d i n g to the d i f f e r e n t frictional resistance. T h e largest balls g a t h e r on t h e s u r f a c e of the ore charge a n d the fines w i t h the h i g h e s t f r i c t i o n a l resistance a c c u m u l a t e on the support. T h e balls a l r e a d y f o r m e d roll d o w n earlier w h e r e a s the very fine particles are c o m p e l l e d to reverse. If the s u p p o r t is p l a i n like the b o t t o m of a balling disc a c o r r e s p o n d i n g device is necessary to reverse the material flow. H o w e v e r , if the s u p p o r t is c u r v e d like t h e i n n e r wall of a d r u m , the m a t e r i a l a u t o m a t i c a l l y reverses at a certain wall slope. A c c o r d ing to their w e i g h t and gravity, t h e d o w n w a r d rolling balls give rise to slight hits to t h e s u r r o u n d i n g pellets. A s a result, the balls are consolidated or weak balls m a y b r e a k . T h e c o n d i t i o n of such a rolling ore m a s s with a w i d e size r a n g e is d i a g r a m m a t i c a l l y s h o w n in Fig. 101. If t h e s u p p o r t is moved in the o p p o s i t e d i r e c t i o n to gravity, it lifts a m i x t u r e of grains of d i f f e r e n t d i a m e t e r at a specific slope, d u e to its lifting p o w e r a n d frictional

Fig. 101. Factors influencing rolling movement of particles (gravity, frictional

resistance,

upward conveyance,

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resistance, until gravity o v e r c o m e s the frictional resistance. T h e rollingd o w n m o v e m e n t t h e n begins selectively a c c o r d i n g to the d i f f e r e n t frictional resistance w h i c h a u t o m a t i c a l l y brings a b o u t classification. O w i n g t o the classifying effect, zones of a g g l o m e r a t e s with d i f f e r e n t d i a m e t e r s a r e clearly p e r c e p t i b l e . A c c o r d i n g to the type a n d m o d e of o p e r a t i o n of t h e balling unit, the finished balls a u t o m a t i c a l l y s e p a r a t e f r o m the r e m a i n i n g mix as in t h e case of t h e balling disc. If there is n o s u f f i c i e n t sorting effect — as with the d r u m — t h e balling unit. T h e c r u s h e d oversize a n d undersize are recycled to t h e f r e s h feed. N o w a d a y s , mostly d r u m s a n d discs are used as balling e q u i p m e n t , the design a n d f u n c t i o n of w h i c h is the subject of the following considerations.

8.2.2 Balling Drum T h e balling d r u m was d e v e l o p e d f r o m t h e m i x i n g d r u m used in sinter plants a n d a d a p t e d to its new f u n c t i o n . D u r i n g t h e first years of d e v e l o p ment, the d r u m was the sole balling unit. All pelletizing plants o p e r a t e d on an industrial scale in the U S A a n d in S w e d e n were exclusively e q u i p p e d w i t h d r u m s u p to a b o u t 1956. O w i n g to the fact that t h e i r c o m p o n e n t p a r t s h a v e b e e n k n o w n a n d p r o v e d for m a n y years, as well as to their easy o p e r a t i o n a l control u n d e r relevant o p e r a t i n g conditions, they are n o w a d a y s also used f r e q u e n t l y in pelletizing plants.

8.2.2.1 The Principal Drum Component Parts and the Balling Operation 8.2.2.1.1 The M a i n Component Parts. T h e d r u m consists of a slightly inclined cylinder o p e n at b o t h ends, occasionally with a low r e t e n t i o n ring or cone at the feed end in o r d e r to prevent any b a c k w a r d flow of p e l l e t feed. F o r t h e i m p r o v e m e n t of the adhesive strength a n d roughness, the metallic s m o o t h inner wall is c o a t e d with a layer of moist fines a n d its thickness is controlled a n d limited by scrapers. T h e s e scrapers a r e stationary or rotating a n d a r r a n g e d in parallel to the d r u m axis. In m o d e r n plants, so-called rotating spiral scrapers are used w h i c h not only limit the b e d h e i g h t b u t also i m p a r t a h i g h e r frictional resistance to t h e inner face. In o r d e r to e n s u r e a successful o p e r a t i o n , t h e d i m e n s i o n s a n d f u n c t i o n s of the p r i n c i p a l d r u m c o m p o n e n t p a r t s m u s t b e variable, such as: - length a n d d i a m e t e r of d r u m - angle of inclination of d r u m axis to t h e h o r i z o n t a l - n u m b e r of revolutions.

discharged

8.2 Demands on the Mode of Operation of Balling Units

197

Fig. 102 shows d i a g r a m m a t i c a l l y t h e essential balling d r u m c o m p o n e n t parts, their d i m e n s i o n s a n d o p e r a t i n g characteristics as t h e y are used f o r industrial plants with capacities of 9 0 - 1 3 0 t p h g r e e n pellets for various o r e types.

Fig. 102. Main dimensions and influencing features of a drum for green pellet

8.2.2.1.2 Operation of the Balling Drum. T h e p r e p a r e d f e e d is c h a r g e d to the u p p e r end of the d r u m . If necessary, w a t e r is s p r a y e d o n d e t e r m i n e d areas for the a c h i e v e m e n t of o p t i m u m ball f o r m a t i o n . T h e m a t e r i a l rolls in flat spirals t o w a r d s the discharge end. A c c o r d i n g to t h e length, slope, rotating s p e e d a n d filling d e g r e e of d r u m , pellets with a certain size distribution are p r o d u c e d at a capacity d e p e n d e n t o n t h e o r e type used. D u e to the m o d e of o p e r a t i o n the d r u m h a s practically n o sorting effect. T h e d r u m d i s c h a r g e has to be screened a n d t h e pellets of d e s i r e d size are separated. A n y p r o d u c e d oversize is, a f t e r crushing, c o m b i n e d w i t h u n d e r size a n d fresh feed m a t e r i a l a n d j o i n t l y c h a r g e d to t h e d r u m . T h u s , t h e r e is a n inevitable circulating l o a d of recycled u n d e r s i z e w h o s e q u a n t i t y m a y vary between 1 0 0 - 4 0 0 % of the f r e s h feed d e p e n d i n g o n the o p e r a t i n g conditions. T h e small pellets c o n t a i n e d in t h e u n d e r s i z e serve, d u r i n g their repeated passage t h r o u g h t h e d r u m , as seeds f o r t h e f o r m a t i o n of p r o p e r l y sized pellets. A c c o r d i n g to the circulating l o a d involved, t h e raw m a t e r i a l passes r e p e a t e d l y t h r o u g h the d r u m b e f o r e f i n i s h e d pellets are discharged. T h e s e p a r a t i o n of the utilizable f r a c t i o n s is n o r m a l l y a c h i e v e d t h r o u g h a specially d e v e l o p e d , s m o o t h l y v i b r a t i n g screen in o r d e r to preserve the green ball q u a l i t y a n d to avoid w a t e r f r o m t h e capillaries b e i n g pressed to the surface. T h e screen capacity m u s t be s u f f i c i e n t to c o m p e n s a t e for m a j o r fluctuations. T h e n o r m a l l y used v i b r a t i n g screen is n o w a d a y s increasingly r e p l a c e d b y a roller screen especially d e v e l o p e d for g r e e n ball handling. Fig. 103 d i a g r a m m a t i c a l l y shows t h e g r e e n pellet a n d u n d e r s i z e circuit. T h e s e p a r a t i o n of pellets of p r o p e r size is a c h i e v e d a c c o r d i n g to alternative I - w i t h v i b r a t i n g screen - a n d a l t e r n a t i v e II - with roller

production

at a rate of 9 0 -

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Fig. 103. Flow sheet of pelletizing drum operation

screen. T h e o p e r a t i o n of the roller screen is s m o o t h e r . T h e f l u c t u a t i o n s of the circulating l o a d i n f l u e n c e the filling d e g r e e a n d p r o p e r t i e s of the d r u m a n d to s o m e extent, the size range of the pellets. T h i s is a d i s a d v a n t a g e of the d r u m o p e r a t i o n a n d is still the subject of special e n d e a v o u r s to r e d u c e or even to a v o i d it 112 ). H o w e v e r , it is such an i m p o r t a n t i n f l u e n c i n g f a c t o r that it has to be considered for the e v a l u a t i o n of all o t h e r factors. 8.2.2.2 Influencing Factors for Green Ball Formation in a Drum 8.2.2.2.1 Drum Rotating Speed. A specific r o t a t i n g s p e e d of d r u m is necessary for g r e e n ball f o r m a t i o n . A r o t a t i o n w h i c h is e i t h e r too slow or too fast h a s to b e a v o i d e d , as is d i a g r a m m a t i c a l l y s h o w n in Fig. 104. T h e b e h a v i o u r of t h e c h a r g e is o b s e r v e d at t h r e e d i f f e r e n t speeds: (a) T h e d r u m rotates too slowly. T h e c h a r g e does not roll b u t slides as a mass in swinging m o v e m e n t s u p w a r d s a n d d o w n w a r d s o n the wall. N o pellets are f o r m e d . (b) T h e d r u m rotates at such a speed t h a t d u e to t h e f r i c t i o n a l resistance p r e v a i l i n g o n the wall, the c h a r g e is lifted until t h e m a t e r i a l reaches a n d exceeds the d y n a m i c angle. O n t h e c h a r g e s u r f a c e w h e r e t h e friction reaches its m i n i m u m , the particles begin to roll d o w n w a r d s w h i l e the first a g g l o m e r a t e s emerge. At the s a m e t i m e the pellets are g r o w i n g d u e to t h e layering of fines o n the a l r e a d y existing nuclei f r o m t h e

8.2 Demands on the Mode of Operation of Balling Units

199

Fig. 104. Movement of pellet feed in a drum at various rotating speeds

rotating mass. A t this o p t i m u m s p e e d , the m a t e r i a l m a i n l y carries out a rolling m o v e m e n t while it is b e i n g t u r n e d over. T h i s m o v e m e n t is designated as c a s c a d e m o v e m e n t . (c) T h e d r u m rotates so fast t h a t the c h a r g e is m o v e d b e y o n d the d y n a m i c angle of r e p o s e a n d p r e s s e d against t h e wall. A t t h e steeper angle, the m a s s d u m p s onto the l o w e r layer, w i t h o u t rolling. U n d e r these conditions n o a g g l o m e r a t e s can b e f o r m e d . T h e basic criterion for all f u r t h e r c o n s i d e r a t i o n s is the o p t i m u m s p e e d of rotation. H o w e v e r , this speed is d e p e n d e n t on the frictional p r o p e r t i e s of t h e particles. It has to be ascertained b y tests for each p a r t i c u l a r pellet f e e d . F o r this reason, m a n y industrial d r u m s can be o p e r a t e d at varying s p e e d . 8.2.2.2.2 Length and Tilt Angle of Drum. B e f o r e t h e finished g r e e n ball is finally f o r m e d , t h e c h a r g e n e e d s a certain f o r m a t i o n p e r i o d d u r i n g which the necessary rolling distance is covered for w h i c h a certain d r u m length is required. E m p i r i c a l l y , a m u l t i p l i e r of 2 1/2-3 of d r u m d i a m e t e r is established f o r t h e d r u m length. A p a r t f r o m t h e length, the tilt angle of the d r u m also plays a n i m p o r t a n t role. It d e t e r m i n e s the passage time, Fig. 105, a n d the filling load at a given t h r o u g h p u t . T h e limits are determ i n e d by t w o d i f f e r e n t d r u m speeds. T h e lower r a n g e c o r r e s p o n d s to the passage t i m e of t h e w h o l e charge. T h e u p p e r p e r i o d only refers to the feed r a t e w h i c h c o r r e s p o n d s to the pellet p r o d u c t i o n rates. T h e a m o u n t of m a t e r i a l to be recycled is d e p e n d e n t on the d r u m slope, curve I in Fig. 106, d r u m length, c u r v e II, a n d screen o p e n i n g s f o r separating the g r e e n pellets of desired size, curve III. A g r e a t length decreases the q u a n t i t y of recycled m a t e r i a l a n d vice versa. T h e g r e a t e r the screen openings, t h e h i g h e r the a m o u n t of u n d e r s i z e or circulating load.

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Fig. 105. Influence of drum slope and rotating speed on retention time of green pellet feed and total load

Fig. 106. Influencing factors of drum operation on circuit load

8.2 Demands on the Mode of Operation of Balling Units

201

Fig. 107. Interior of a pelletizing drum equipped with rotating spiral scraper and loaded with a pellet charge

Fig. 107 shows the interior of a balling d r u m 1 1 3 ) , the rotating spiral w h i c h is a v a i l a b l e for the d o w n w a r d rolling m o v e m e n t s is i m p o r t a n t for the ball f o r m a t i o n .

scraper,

8.2.2.3 Balling Drum Capacity T h e capacity d e p e n d s very m u c h o n the p r o p e r t i e s of the m a t e r i a l to be pelletized. M a t h e m a t i c a l calculations c a n n o t yield exact data. O p e r a t i o n a l data f r o m l a b o r a t o r y tests allow the d e f i n i t i o n of reliable d i m e n s i o n s a n d Table 27. Dimensions, operating conditions a and production rate of pelletizing drums Drum diameter mm

Length mm

Green ball production tph

1800 2745 3050 3660

6100 9150 9755 9753

20 80 120 170

a Slope 6° Rotation speed 8 - 14/min depending on ore properties

-10°

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t i m e r e q u i r e d for the p r o d u c t i o n of green pellets of good quality. In o r d e r to ensure a flexible t h r o u g h p u t of the d r u m in a n industrial plant a n d to c o m p e n s a t e for fluctuations, t h e tilt angle a n d r o t a t i n g s p e e d of d r u m s c a n be varied. In principle, the capacity is d e p e n d e n t o n similar criteria as for the disc. A c c o r d i n g to the i n f o r m a t i o n o b t a i n e d f r o m a d r u m s u p p l i e r 113 ), the following correlations s h o w n in T a b l e 27, exist. These values m a y vary according to the o r e type, p r o p e r t i e s a n d quantity of a d d i t i v e s as well as to the desired pellet size. W h e n o p t i m u m o p e r a t i o n a l p a r a m e t e r s are m a i n t a i n e d , pellets of g o o d q u a l i t y are produced.

8.2.3 Balling Disc T h e disc was already used for balling of c e m e n t raw m e a l and in the fertilizer industry b e f o r e its a p p l i c a t i o n f o r i r o n ore pelletizing was investigated a b o u t 1949/50 114 ). W h e n balling f i n e ores o n a disc, this is done u n d e r the s a m e physical conditions as in a balling d r u m . T h e rolling m o v e m e n t is initiated by the inclination of t h e p l a i n disc b o t t o m instead of the concave d r u m wall. D u e to the design a n d o p e r a t i o n of the disc a classification by t h e ball d i a m e t e r already starts d u r i n g ball f o r m a t i o n . T o d a y , even t h o u g h at a m u c h later d a t e t h a n t h e d r u m , a c o n s i d e r a b l e p o r t i o n of world pellet p r o d u c t i o n is covered b y the disc. 8.2.3.1 The Principal Component Parts and Disc Operation 8.2.3.1.1 The Main Component Parts. In a s i m i l a r m a n n e r to the d r u m , the disc consists of the c o m p o n e n t parts necessary for ball formation. T h e y are represented in Fig. 108. T h e concentrically rotating, inclined disc is

Fig. 108. Main dimensions and influencing features of a disc for green pellet production at a rate of 90-140 t/h

8.2 Demands on the Mode of Operation of Balling Units

203

c o m p o s e d of a circular, p l a i n b o t t o m area. T o ensure s u f f i c i e n t f r i c t i o n a l resistance a n d a g o o d lifting effect for the pellet f e e d , a moist ore b e d of a b o u t 3 — 1 0 c m thickness is layered o n the b o t t o m a n d is controlled by scrapers. D i f f e r e n t designs were d e v e l o p e d such as stationary a n d oscillating scrapers or r o t a t i n g spirals. T h e c a p a c i t y of the disc is given by the side wall p o s i t i o n e d at a n angle of 9 0 0 to the b o t t o m . T h e h e i g h t of the rim is, to s o m e extent, d e p e n d e n t o n t h e disc d i a m e t e r . T h e speed c a n b e varied w i t h i n c e r t a i n limits d e p e n d i n g o n the pelletizing p r o p e r t i e s of r a w material. 8.2.3.1.2 Operation of the Balling Disc. A t t e m p t s s h o u l d b e m a d e to utilize a m a x i m u m part of the disc a r e a for g r e e n ball f o r m a t i o n . A t the beginning of the d e v e l o p m e n t work, t h e p o r t i o n of b o t t o m a r e a was as low as s o m e 50% a n d could be raised to a p p r o x . 90% in the c o u r s e of f u r t h e r d e v e l o p m e n t . Since d u r i n g ball f o r m a t i o n a classification occurs u p to the discharge of pellets of the d e s i r e d size, the q u a n t i t y of t h e necessary pellet feed c o r r e s p o n d s to t h e a m o u n t of pellets d i s c h a r g e d . T h i s results in a s i m p l e flowsheet for t h e m a t e r i a l flow s h o w n in Fig. 109. If, nevertheless, small a m o u n t s of u n d e r s i z e d e v e l o p w h i c h mostly occurs d u r i n g t r a n s p o r t a t i o n , these can be s e p a r a t e d by a roller screen a n d recycled. T o initiate the d o w n w a r d rolling m o v e m e n t , the disc is inclined at s u c h an angle that t h e m a t e r i a l layer c h a r g e d exceeds the d y n a m i c angle of repose. A c c o r d i n g to the lifting p o w e r of t h e r o u g h disc b o t t o m a n d the

Fig. 109. Flow sheet of pelletizing disc operation

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8 Balling Equipment

frictional resistance of t h e m a t e r i a l , t h e finest particles are lifted u p to t h e c u l m i n a t i o n p o i n t f r o m w h e r e they are f o r c e d to roll d o w n b y deflectors. T h e pellets a l r e a d y p r o d u c e d reverse earlier a n d roll d o w n w a r d s o v e r the surface of t h e c h a r g e while their d i a m e t e r increases, Fig. 110. By a n a d e q u a t e a r r a n g e m e n t of scrapers, the c h a r g e is so g u i d e d that d u r i n g its d o w n w a r d - r o l l i n g a classification is p r o m o t e d . T h e f e e d point of r a w m a t e r i a l s h o u l d b e there located w h e r e seeds are f o r m e d i.e. with a clock-

Fig. 110. Movement of pellet feed in the disc (formation of green pellets, size increase and discharge of finished pellets)

wise disc r o t a t i o n a p p r o x i m a t e l y 3—4 hrs. T h i s p o i n t can be v a r i e d f o r influencing the pellet d i a m e t e r a n d according to the o r e p r o p e r t i e s . If water is n e e d e d f o r ball f o r m a t i o n , this can be a d d e d at points w h e r e the pellets are still nascent. Fig. 110 shows the m a t e r i a l m o v e m e n t s of the d o w n w a r d rolling charge, a d d i t i o n a l l y g u i d e d by scrapers, as well as the raw material f e e d a n d discharge p o i n t of the f i n i s h e d pellets located at a b o u t 7 - 8 . 3 0 hrs. In a cross section the classification is p e r c e p t i b l e in m a n y layers according to the d i a m e t e r of t h e i n d i v i d u a l agglomerates. T h e sector of a balling disc with t h e recognizable s e p a r a t i n g effect is shown in Fig. 111. 8.2.3.2 Influencing Factors of Green Ball Formation on a Disc In case of d r u m s the concave s h a p e of the wall a n d t h e rotating s p e e d are the two factors initiating a n d controlling the d o w n w a r d rolling m o v e ment. F o r a disc, the inclined p l a i n b o t t o m , the side wall, the d e f l e c t o r and scrapers h a v e the f u n c t i o n of initiating a n d m a i n t a i n i n g the rolling

8.2 Demands on the Mode of Operation of Balling Units

205

Fig. 111. Size classifying effect in a disc m o v e m e n t . In this connection, it is i m p o r t a n t to p r o v i d e a m a x i m u m rolling-down d i s t a n c e f o r t h e m a t e r i a l to be pelletized by the inclination of the disc, its lifting p o w e r a n d t h e frictional resistance of the charge. 8.2.3.2.1 Disc Slope and Rim Height. T h e disc slope is d e t e r m i n e d by the d y n a m i c angle of repose of specific m a t e r i a l . T o o v e r c o m e this angle, t h e

Fig. 112. Relation between disc slope, rim height and angle of repose of a

fine-grained

ore pile

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8 Balling Equipment

disc slope m u s t always b e slightly steeper w h i c h is s h o w n by B h r a n y o n the sketch in Fig. 112. If a is the d y n a m i c angle of r e p o s e a n d β the tilt angle of t h e disc b o t t o m , the latter m u s t always be greater t h a n α. If it were smaller or equal, the m a t e r i a l w o u l d be b r o u g h t to a p o s i t i o n of rest. A t too steep a n angle, the charge w o u l d no longer b e lifted b y friction. As in all p r e v i o u s cases, the frictional factor, d e p e n d e n t on the ore type used, plays a decisive role. T h e tilt angle to the h o r i z o n t a l line of discs for high p r o d u c t i o n rates with a m i n i m u m d i a m e t e r of 6 m varies b e t w e e n 45 a n d 4 8 ° a c c o r d i n g to the frictional coefficient of o r e and the lifting coefficient. T h e rim h e i g h t is also d e t e r m i n e d by the tilt angle. T h e filling v o l u m e is d e p e n d e n t o n b o t h factors. 8.2.3.2.2 Disc Rotating Speed. T h i s is subject to t h e s a m e c o n d i t i o n s as for the d r u m . W i t h insufficient speed, the c h a r g e r e m a i n s in a relative p o s i t i o n of rest. N o rolling of m a t e r i a l occurs. A t a n excessive speed, the m a t e r i a l m o v e s u p w a r d s w i t h o u t rolling d o w n . D u e to the c e n t r i f u g a l force, it is slung against the disc r i m a n d n o longer c o m e s off so that n o relative rolling m o v e m e n t is possible. T h i s p h e n o m e n o n occurs at the critical speed. A c c o r d i n g to experience the o p t i m u m s p e e d must be b e l o w a b o u t 70% of this value. P r i m a r i l y the f i n e - g r a i n e d particles s h o u l d be practically lifted u p to the vertex a n d are t h e n forced to roll d o w n w a r d . In this way, the length a n d the greatest part of the s u r f a c e are utilized as rolling p a t h . T h e speed of discs in industrial plants ranges f r o m 6 to 7 rotations per m i n u t e at d i a m e t e r s of 6—7.5 m which m e a n s a d i a m e t e r a n d at a speed of 10—14 r p m , the c i r c u m f e r e n t i a l speed is also a b o u t 110—160 m / m i n .

8.2.3.2.3 Residence Time of Material in the Disc. T h e residence t i m e r e q u i r e d for ball f o r m a t i o n is a b o u t the s a m e in the d r u m for a given pellet size. T h e t i m e varies linearly with the pellet d i a m e t e r 25). B h r a n y f o u n d the following relation b e t w e e n t i m e a n d pellet size for the d i f f e r e n t ores: Pellet size in m m Residence T i m e in sec.

8 O r e A 120 O r e B 70

10 150 90

12 200 100

see also Fig. 105, item 8.2.2.2.2 for d r u m o p e r a t i o n . This results f r o m the disc filling d e g r e e w h i c h is d e t e r m i n e d by the d i a m e t e r , r i m height, tilt angle a n d desired pellet size. Besides t h e bulk density w h i c h m a y vary b e t w e e n 2 a n d 2.2 t / m 3 , the m o i s t u r e content of the c h a r g e plays a decisive part. If a pelletizable concentrate having the m o i s t u r e content r e q u i r e d for ball f o r m a t i o n is

circumferential

8.2 Demands on the Mode of Operation of Balling Units

207

involved, the residence t i m e of the c h a r g e can a l m o s t exclusively b e utilized for ball f o r m a t i o n . H o w e v e r , if w a t e r has to be a d d e d to the charge, the residence t i m e extends a n d results in a c o r r e s p o n d i n g larger capacity, as s h o w n in Fig. 48, item 5.2. 8.2.3.2.4 Disc Diameter. In view of the s i m p l e design and m o d e of o p e r a t i o n of the balling disc, it can easily b e used for small pelletizing capacities. As indicated u n d e r item 4.4.3, a l a b o r a t o r y t y p e balling disc has, for e x a m p l e , a d i a m e t e r of 0.8— 1 m a n d the c o m m e r c i a l disc m a y have any d i a m e t e r r e q u i r e d . In big pelletizing plants built recently, discs of d i a m e t e r s b e t w e e n 6.5 a n d 7.5 m are in o p e r a t i o n . T h e size of disc a r e a has no i n f l u e n c e on the pellet q u a l i t y a p a r t f r o m the fact t h a t the p o r o s i t y decreases slightly with increasing d i a m e t e r . H o w e v e r , t h e d i a m e t e r a n d disc area are decisive for the q u a n t i t y of pellets p r o d u c e d . All criteria f o r the f o r m a t i o n of balls of good q u a l i t y m u s t be given in a n o p t i m u m m a n n e r for each disc size, i.e. there is a c e r t a i n c o r r e l a t i o n d e p e n d e n t on the ore-type u s e d which s h o u l d be d e t e r m i n e d by tests b e t w e e n speed, tilt angle, rim height, filling d e g r e e a n d residence time. C o n s e q u e n t l y , there are direct relations b e t w e e n disc d i a m e t e r , disc surface and t h r o u g h p u t rate.

8.2.3.3 Balling Disc Capacity Subject to o p t i m u m q u a l i t y of g r e e n pellets, the t h r o u g h p u t rate is d e t e r m i n e d b y the filling d e g r e e of the disc, b u l k density a n d residence time. All factors for quality control can b e a s c e r t a i n e d p r e v i o u s l y in t h e l a b o r a t o r y w h e r e the capacity limit can b e easily d e t e r m i n e d . It is exceeded w h e n the pellet q u a l i t y decreases. T h e t h r o u g h p u t related to disc a r e a in t p h c a n then be converted to the d i m e n s i o n s of a n i n d u s t r i a l plant. In such a plant, values w h i c h are s o m e t i m e s b e t t e r t h a n those f o u n d in the l a b o r a t o r y are r e a c h e d . T a b l e 12, item 4 shows a c o m p a r i s o n of the values o b t a i n e d in industrial plants w i t h those in the l a b o r a t o r y . E x c e p t for m i n o r variations in f a v o u r of the i n d u s t r i a l p l a n t , a g o o d c o r r e l a t i o n is observed as is the case with g r e e n ball qualities. M a t h e m a t i c a l m o d e l s suggest that the capacity be d e t e r m i n e d t h r o u g h the balling disc d i a m e t e r . However, it a p p e a r s that the s i m p l e s t w a y to convert the capacity is t h r o u g h the balling disc area. T a b l e 28 indicates the d e p e n d e n c y of c a p a c i t y on the d i m e n s i o n s of different balling discs in o p e r a t i o n . T h e raw m a t e r i a l characteristics r e p r e s e n t a n i m p o r t a n t i n f l u e n c i n g factor for c a p a c i t y variations. A c c o r d i n g to t h e ore t y p e as well as c o m position and q u a n t i t y of a d d i t i v e s (see items 5 . 3 . 1 . 3 - 5 . 3 . 3 . 3 ) the c a p a c i t y

208

8 Balling Equipment

Table 28. Dimensions, operating conditions and production rate of pelletizing discs Disk diameter mm

Area

Rim height

m2

mm

5000 5500 6000 7000 7500

20 23.5 28 38 44

550/600 550/600 550/600 600/700 600/800

Tilt angle

Speed r.p.m.

Green pellet production tph/m 2

tph 45-48° 45-48° 45-48° 45-48° 45-48°

6.5/7.5 6.5/7.5 6/7 6/7 6/7

40506090 90-

60 70 90 120 140

2 -3 2.1-3 2.1-3.2 2.4-3.1 2.1-3.2

m a y fluctuate u p to 30%. T h i s calls for t h e p e r f o r m a n c e of c a r e f u l tests with each i n d i v i d u a l ore or m i x t u r e . M i n o r variations d u r i n g o p e r a t i o n can be c o m p e n s a t e d for b y a n a d j u s t a b l e disc slope, disc r i m h e i g h t a n d speed.

8 . 2 . 4 C o m p a r i s o n B e t w e e n B a l l i n g D r u m and Balling D i s c 115,42) Balling d r u m s a n d balling discs are the two units w i t h which practically all big pelletizing plants t h r o u g h o u t t h e world are e q u i p p e d . B o t h units o p e r a t e a c c o r d i n g to the s a m e process principle. T h e green balls f o r m e d are a l m o s t of the s a m e quality. W a t e r c o n s u m p t i o n , g r a n u l o m e t r i c p r o p e r t i e s a n d b i n d e r c o n s u m p t i o n d o not i n f l u e n c e either of the systems. T h e great d i f f e r e n c e is that in the case of t h e d r u m o p e r a t i o n a relatively high circulating load of undersize is m a i n t a i n e d b y screening. T h i s o f f e r s a certain m a t e r i a l b u f f e r to c o m p e n s a t e for m i n o r irregularities of f e e d rate. T h e d r u m is, to s o m e extent, less sensitive to f e e d rate variations t h a n the disc. L K A B carried out a c o m p l e t e c o m p a r i s o n b e t w e e n d r u m a n d disc at its travelling grate plant with a capacity of 2.5 million t p y pellets at M a l m b e r g e t 4 2 ) . T h e m o d e of o p e r a t i o n of a d r u m with a d i a m e t e r of 3.6 m a n d a length of 9.5 m was c o m p a r e d with t h a t of a balling disc of 7.5 m dia. T h e results of these tests can be s u m m a r i z e d as follows: — T h e flexibility of the d r u m to m a t e r i a l fluctuations is to s o m e extent better t h a n that of the disc. — T h e p r o d u c t q u a l i t y is equal for b o t h systems. — T h e pellets leaving the balling d r u m are screened a n d the u n d e r s i z e is recirculated. — T h e pellets leaving the balling disc are screened if it is desired to m a i n tain a very close size r a n g e for direct r e d u c t i o n . F o r blast f u r n a c e s screening is not necessary.

8.2 Demands on the Mode of Operation of Balling Units

209

Fig. 113. Development of green pellet production in drums and discs

Process technological a n d q u a l i t a t i v e d i f f e r e n c e s a p p a r e n t l y d o not exist for either system. T h e q u e s t i o n r e g a r d i n g d i f f e r e n t costs arises. T h e c o m parison m a d e b y L K A B yielded a n e c o n o m i c a d v a n t a g e in f a v o u r of the disc o p e r a t i o n . F o r the p r o d u c t i o n of blast f u r n a c e pellets w h e r e no screening is necessary, t h e costs are b y 4—8% lower as c o m p a r e d with d r u m o p e r a t i o n . In this connection, a pellet size range of 8 5 - 9 2 % b e t w e e n 9 - 1 5 m m a n d less t h a n 3% u n d e r 5 m m are e x p e c t e d w h e n a disc is used. Fig. 113 shows the d e v e l o p m e n t of pellet p r o d u c t i o n in d r u m s a n d discs. T h e use of discs increases c o m p a r e d w i t h t h a t of d r u m s .

8.2.5 Comparison of Vibrating and Roller S c r e e n s W i t h i n the scope of the c o m p a r a t i v e tests p e r f o r m e d with d r u m s a n d discs L K A B also investigated the g r e e n ball screening with v i b r a t i n g a n d roller screens, see flowsheet of Fig. 103. A v i b r a t i n g screen of 2.1 m w i d t h a n d 4.3 m length with d i f f e r e n t screen d e c k s was c o m p a r e d with a roller screen of 2.5 m w i d t h h a v i n g 49 rollers of 10.2 c m d i a m e t e r w i t h d i f f e r e n t gaps b e t w e e n t h e i n d i v i d u a l rolls. T h e p u r p o s e of this c o m p a r i s o n was to find out h o w distinct was the s e p a r a t i o n of u n d e r - a n d oversize f r o m the desired p r o d u c t size b e t w e e n 10 a n d 12.5 m m . In this connection, it was ascertained t h a t t h e efficiency of t h e roller screen u n d e r e q u a l conditions was higher b y a b o u t 25% t h a n that of the v i b r a t i n g screen. C o n s e q u e n t l y , roller screens w e r e installed in a n existing p l a n t . F u r t h e r m o r e , it was

210

8 Balling Equipment

a s s u m e d that pellets of a better s u r f a c e q u a l i t y could be o b t a i n e d b y using roller screens. In a d d i t i o n , the o p e r a t i o n of these roller screens p r o v e d to be s m o o t h e r with lower vibrations observed in the b u i l d i n g structure.

8.2.6 Other Balling Systems F o r b o t h systems, discs a n d d r u m s , a l t e r n a t i v e constructions w e r e d e v e l o p e d e.g. t h e stepwise balling disc called " f l y i n g s a u c e r " or a d r u m with inner r e t e n t i o n cones. In each case, the u n d i s t u r b e d , free, rolling m o v e m e n t r e q u i r e d for good ball f o r m a t i o n was p u r p o s e l y i n t e r r u p t e d which finally p r o v e d to be a disadvantage. N o w a d a y s , such systems are n o longer used. O n l y the f r e e d r u m space a n d the f r e e disc a r e a e n s u r e o p t i m u m rolling effect for the f o r m a t i o n of p e r f e c t green balls. 8.2.6.1 Balling Cone

116

)

In s o m e pelletizing plants of B e t h l e h e m Steel balling cones w e r e used on an industrial scale. Fig. 114 diagrammatically shows the most i m p o r t a n t c o m p o n e n t parts. T h e m o d e of o p e r a t i o n is h a l f w a y b e t w e e n that of d r u m a n d disc. T h e rolling principle of green ball f o r m a t i o n is identical. S e p a r a tion of undersize is unnecessary because a classification takes place inside the balling c o n e a n d only pellets of the greatest d i a m e t e r are d i s c h a r g e d . C o n e angle, d i a m e t e r and slope r e f e r r e d to the horizontal are variable. N o w a d a y s balling cones are n o longer used.

Fig. 114. Green pellet formation in a cone

8.2.6.2 M i x Granulator

117

)

This a p p a r a t u s operates intermittently. M i x i n g devices installed in a concentrically rotating circular vessel carry o u t partly eccentric circular m o v e m e n t s a n d thus force the particles to b e m i x e d and g r a n u l a t e d into contact with each o t h e r by a sliding effect u n d e r a slight pressure. In the

211

8.3 Handling and Feeding Devices

presence of water, capillary forces m a y t h u s d e v e l o p w h i c h b r i n g a b o u t the f o r m a t i o n of agglomerates. T h i s a p p a r a t u s successfully used in m a n y industries has not p r o v e d s u i t a b l e for the i r o n o r e pelletizing technology d u e to its low capacity c o m p a r e d with the h i g h t h r o u g h p u t r e q u i r e d in big pelletizing plants. 8.2.6.3 Vibrating Trough

114

)

A t t e m p t s h a v e always b e e n m a d e to a p p l y o t h e r m o v e m e n t principles apart f r o m rolling to ball formation. Most of these attempts are m o r e or less of theoretical value. Nevertheless, they are briefly d e s c r i b e d below. C u r v e d t r o u g h s are v i b r a t e d with t h e f r e q u e n c y a n d v i b r a t i o n a m p l i t u d e as well as the c u r v a t u r e b e i n g variable. T h e m a t e r i a l is f e d at o n e p o i n t a n d passes i n t o the zone of the v i b r a t i n g b o t t o m . O n this occasion, the direction of the trajectory p a r a b o l a o p p o s e s g r a v i t a t i o n so t h a t a d o w n ward m o v e m e n t is o b t a i n e d as resultant. A screening d e v i c e a r r a n g e d at the end of each pelletizing t r o u g h d i s c h a r g e s the u n d e r s i z e into the next trough a n d t h e f i n i s h e d pellets are d e l i v e r e d laterally. A p a r t f r o m the vibrating rolling m o v e m e n t s , slight i m p a c t forces t e n d to i m p r o v e the pellet strength. 8.2.6.4 Eccentrically Moving Unit

28

)

Such an a p p a r a t u s was c o n s i d e r e d for ball f o r m a t i o n . R a w m a t e r i a l a n d water were c h a r g e d into an eccentrically moving bowl a n d , d u e to horizontal rotation, ball f o r m a t i o n started. A s a result of the c e n t r i f u g a l force, the b u l k i e r pellets leave the bowl a n d d r o p i n t o the vertical downward tapering pipe w h i c h p e r f o r m s t h e s a m e eccentric, circular m o v e ments as the bowl. T h e pellets roll spirally d o w n w a r d s along t h e wall. In this way, it was possible to p r o d u c e g r e e n balls of high strength with a low water content. T h e a p p a r a t u s was t o o c o m p l i c a t e d a n d was never used on an industrial scale.

8.3 Handling and Feeding Devices T h e t r a n s p o r t of green balls f r o m the balling to the i n d u r a t i n g units makes, f r o m a m e c h a n i c a l viewpoint, the h i g h e s t d e m a n d s on t h e i r strength, d r o p resistance a n d t u m b l i n g b e h a v i o u r . T h e r e f o r e , the p r i n c i p a l f u n c t i o n of the h a n d l i n g e q u i p m e n t is to t r a n s p o r t the pellets to the s u b sequent process stage w i t h o u t causing a n y d a m a g e . It is w o r t h w h i l e to look for a d e q u a t e l y designed h a n d l i n g a n d f e e d i n g devices which e n a b l e careful h a n d l i n g of the s o f t g r e e n balls. It s h o u l d b e n o t e d that the

212

8 Balling Equipment

m e t h o d s for testing the green ball strength w e r e especially d e v e l o p e d for testing the pellet p r o p e r t i e s at the transfer points ( d r o p test) and d u r i n g their sliding over chutes or d u r i n g their d o w n w a r d m o v e m e n t in the s h a f t furnace (abrasion test). Normally, green balls are carried on conveyor belts to the i n d u r a t i n g reactors. T h e design of t h e reversing station of these conveyor belts d e t e r m i n e s the h e i g h t of pellet d r o p w h i c h m a y be u p to one meter. D e s p i t e the deficiency of these designs w h i c h was early recognized it t o o k a long t i m e until better devices were d e v e l o p e d .

8.3.1 Roller Conveyor T h e roller conveyor was especially d e v e l o p e d for green ball h a n d l i n g p r o b l e m s in pelletizing p l a n t N o . 1 of the I n t e r n a t i o n a l Nickel Co. at C o p p e r Cliff, C a n a d a a n d was p r o v e d on a n industrial scale (1958). T h e technical i n n o v a t i o n a n d a d e q u a t e design resulted in p a t e n t s being issued t h r o u g h o u t the w o r l d 1 1 8 ) . T h e f u n c t i o n i n g p r i n c i p l e is based o n t h e rotation of i n d i v i d u a l l y d r i v e n rolls which, as a c o m b i n e d system, t r a n s p o r t the g r e e n balls at a specific angle of inclination f r o m a h i g h e r point d o w n w a r d s . E a c h of the i n d i v i d u a l g r e e n balls rolls d o w n w a r d s with its rotating direction being o p p o s i t e that of t h e rollers, as is d i a g r a m matically s h o w n in Fig. 115. T h e roller d i a m e t e r a n d the d i s t a n c e b e t w e e n the rolls are in a certain r e l a t i o n s h i p to the ball d i a m e t e r . T h e pellet r o t a t i n g in the g a p b e t w e e n two rolls is forced out by the following ones a n d m o v e s over the roll a p e x to the next gap. A t t h e s a m e time, the balls are laterally p u s h e d off f r o m their straight-line m o v e -

Fig. 115. Functioning of roller conveyor and roller screen

8.3 Handling and Feeding Devices

213

Fig. 116. Industrial size roller conveyor in operation

m e n t a n d are thus d i s t r i b u t e d over the full w i d t h of the c o n v e y o r f e e d i n g the i n d u r a t i n g m a c h i n e . N o w a d a y s , the roller c o n v e y o r is used in travelling grate plants a n d is also increasingly a p p l i e d to o t h e r firing systems. Fig. 116 shows a p h o t o g r a p h of a roller c o n v e y o r on a n industrial scale.

8.3.2 Roller Screen T h e roller screen was d e r i v e d f r o m the roller c o n v e y o r w i t h the g a p widths b e t w e e n i n d i v i d u a l rolls h a v i n g b e e n a d a p t e d to t h e u n d e r s i z e particles to be s e p a r a t e d . T h u s , p a r t of the roller c o n v e y o r o p e r a t e s as a screen, b e f o r e the g r e e n balls are f e d to the i n d u r a t i n g m a c h i n e , see Figs. 115 a n d 116. In this way, fines a n d ball f r a g m e n t s can be s e p a r a t e d f r o m the f e e d a n d recirculated. T h e roller c o n v e y o r has t h e a d d i t i o n a l a d v a n t a g e t h a t the balls receive a very s m o o t h , u n i f o r m s u r f a c e by their continuous rolling m o v e m e n t .

8.3.3 Rolling Belt Conveyor

119

)

Another interesting alternative has already b e e n tested in Inco's C o p p e r Cliff Pelletizing plant, n a m e l y the rolling belt conveyor. A belt c o n v e y o r

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with a relatively steep angle of inclination r e f e r r e d to the horizontal moves u p w a r d s in o p p o s i t e direction to the d o w n w a r d m o v e m e n t of g r e e n balls. T h e accelerated speed of ball m o v e m e n t can be slowed d o w n b y swinging r u b b e r aprons. F i n e s a n d pellet f r a g m e n t s w h i c h d o not roll are discharged u p w a r d s over the h e a d pulley of belt conveyor a n d recycled. T h e rolling belt conveyor was also e x p e r i m e n t a l l y used as a balling unit. H o w e v e r , the p e r t i n e n t d e v e l o p m e n t work was n o t continued.

9 Heat Treatment Systems

T h e last step in pellet production, t h e r m a l treatment, consisting of drying, heating and cooling is carried out in several systems which already have been successfully employed for similar operations. Provided that the green pellet quality is sufficient, the various heating systems must be adapted to the new task. This means, modifications have to be made, if necessary. T h r e e important burning systems are presently used. T h e i r principal features are outlined in general before they are described in greater detail. (a) Shaft Furnace. D u e to its high t h e r m a l efficiency and its relatively simple operational requirements, it was used as the first unit both in the U S A and in Sweden for indurating pellets f r o m magnetite concentrates. As long as magnetite was the main raw material, the s h a f t f u r n a c e was of great importance. T h e green balls are charged on top of the f u r n a c e opening. They m o v e by gravity downwards through all thermal zones towards the discharge end. T h e pellets are in continuous m o v e m e n t and exposed to friction and increasing pressure. Control and influence of individual thermal steps f r o m outside are practically impossible. T h e process gases flow at varying temperatures through the pellet bed, absorbing heat in the cooling zone and emitting heat in the burning and preheating zone. (b) Travelling Grate - Rotary Kiln Combination. Following successful use with the burning of cement raw meal, the c o m b i n a t i o n of travelling graterotary kiln-cooler was also adapted to the firing of iron ore pellets. This process has become known throughout the world u n d e r the n a m e " G r a t e Kiln-Process" and is suitable for nearly all pelletizable ores. In the grate-kiln system three specific process units for different thermal conditions are connected with each other. — T h e travelling grate is used for drying, preheating and oxidation of magnetite. T h e green balls are in a relative position of rest. H o t waste gases f r o m the rotary kiln or additionally f r o m the cooler pass through the green ball layers in different

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directions a n d zones while t h e green balls are d r i e d a n d p r e h e a t e d . — In the rotary kiln, p r o v i d e d with refractory lining the p r e h e a t e d pellets are i n d u r a t e d m a i n l y by r a d i a t i o n h e a t to the r e q u i r e d t e m p e r a t u r e . D u r i n g the transfer f r o m the p r e h e a t i n g grate to the r o t a r y kiln, special care has to be taken in o r d e r to b r i n g the pellets in s o u n d c o n d i t i o n into the rotary kiln to avoid b r e a k a g e . R e a c t i o n of oxide d u s t with the hot lining a n d f o r m a t i o n of accretions or rings m u s t be a v o i d e d . A f t e r d r o p p i n g f r o m t h e travelling grate over a c h u t e into the rotary kiln the pellets m o v e in the f o r m of flat spirals t o w a r d s the discharge end. - S u b s e q u e n t l y , t h e hot pellets pass into the third unit, the cooler, w h e r e they are cooled with air. T h e h e a t e d cooling air flows chiefly into t h e rotary kiln a n d p a r t l y over the travelling grate. W i t h the possibility of h a v i n g t h r e e d i f f e r e n t process units, the h e a t s u p p l y can be p r o p o r t i o n e d a c c o r d i n g to the o r e type to be t r e a t e d . T h e use of the travelling grate with its v a r i o u s functions is i m p e r a t i v e for a successful o p e r a t i o n of the rotary kiln. F o r thermal, o p e r a t i o n a l and q u a l i t y reasons, the rotary kiln a l o n e w o u l d not be sufficient. (c) Travelling Grate. Since the turn of the century, the travelling grate h a d b e e n used f o r the sintering of f i n e - g r a i n e d iron ores a n d was a d a p t e d round a b o u t 1 9 4 8 - 5 0 to pellet firing a f t e r a d e q u a t e m o d i f i c a t i o n s h a d been m a d e . T o d a y travelling grates are successfully in o p e r a t i o n f o r practically all o r e types t h r o u g h o u t the world. T h e i n d i v i d u a l t h e r m a l zones of the travelling grate can be r a t h e r easily s e p a r a t e d into horizontal reaction a r e a s a n d a d a p t e d to t h e relevant r e q u i r e m e n t s by using process gas of controlled flow rates a n d t e m p e r a t u r e s . A f t e r the green balls h a v e been c h a r g e d to t h e grate, t h e y r e m a i n in a relative position of rest until they are d i s c h a r g e d . T h e last grate area serves to cool the hot pellets with f r e s h air. T h e recovered h e a t is recirculated to the system.

9.1 Shaft Furnace C o m p a r e d with o t h e r firing systems, the s h a f t f u r n a c e has s o m e advantages, such as: - s i m p l e construction, a small n u m b e r of m o v i n g parts, r e f r a c t o r y lining of total f u r n a c e with bricks of o p t i m u m c e r a m i c q u a l i t y d i f f e r i n g in various zones - intensive h e a t exchange b e t w e e n gases and solids d u e to c o u n t e r c u r r e n t flow. A d i s a d v a n t a g e of the s h a f t f u r n a c e is the little c h a n c e of i n f l u e n c i n g individual process stages a n d t h u s causing a lower flexibility. T o a v o i d

9.1 Shaft Furnace

217

corrective a c t i o n a n d to ensure t r o u b l e - f r e e o p e r a t i o n , s o m e i m p o r t a n t conditions h a v e to be fulfilled: (a) G r e e n balls of best q u a l i t y are r e q u i r e d . In p a r t i c u l a r , b e n t o n i t e is o n e of the p r i n c i p a l b i n d e r s for s h a f t f u r n a c e pellets. (b) U n i f o r m d i s t r i b u t i o n of pellet c h a r g e o v e r t h e cross sectional area. (c) C a r e f u l i n t r o d u c t i o n of process gases w i t h o u t o v e r h e a t i n g the f u r n a c e walls a n d m a r g i n a l p a r t s of the pellet charge. (d) U n i f o r m l y p r o p o r t i o n e d s u p p l y of b u r n i n g gases a n d cooling air as well as intensive m i x i n g in the t h e r m a l l y relevant firing a n d p r e h e a t ing zone. Each i m p a i r m e n t of the u n i f o r m p e r m e a b i l i t y of the feed causes a n irregular gas f l o w a n d thus an i r r e g u l a r h e a t s u p p l y w h i c h i n e v i t a b l y results in a d i f f e r i n g pellet quality.

9.1.1 Shaft Furnace Types In the c o u r s e of d e v e l o p m e n t , v a r i o u s p r o v e d constructions w e r e e m ployed. A q u i t e early i m p o r t a n t m o d i f i c a t i o n to t h e f u r n a c e d e s i g n was the c h a n g e f r o m the circular to the r e c t a n g u l a r s h a p e of the f u r n a c e cross section w h i c h is generally used n o w a d a y s . T h e g r e e n ball f e e d i n g d e v i c e was r e p e a t e d l y m o d i f i e d . T h e s h a p e , size a n d a r r a n g e m e n t of b u r n e r gas inlet o p e n i n g s into the f u r n a c e space was also c h a n g e d a n d especially the cooling m e t h o d so that s h a f t f u r n a c e s with coolers of d i f f e r e n t design are used. T w o of the b e s t - k n o w n types are d i a g r a m m a t i c a l l y s h o w n in Figs. 117/118.

Fig. 117. Long shaft furnace with internal cooling

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Fig. 118. Medium shaft furnace type with alternative external coolers In t h e case of the first f u r n a c e type, Fig. 117, the cooling is largely acc o m p l i s h e d in the s a m e space as the firing. T h e cooling zone is relatively long so that this f u r n a c e is designated as a long s h a f t - t y p e f u r n a c e . Such a cooling air flow rate is injected as is n e e d e d for o p t i m u m h e a t exchange. T h e c h a m b e r s for c o m b u s t i o n of the fuel s u p p l i e d are p r o v i d e d with fresh air. As far as the second f u r n a c e type, Fig. 118, is concerned, the m a j o r p a r t of cooling is carried out outside the s h a f t in s e p a r a t e cooling c h a m b e r s . T w o alternatives are possible for recovering the sensible pellet heat. According to alternative I, the hot cooling air is d e d u s t e d a n d , d e p e n d i n g o n the p e r t i n e n t r e q u i r e m e n t s , i n t r o d u c e d directly into the c o m b u s t i o n c h a m bers. As r e g a r d s alternative II, the heat e x c h a n g e is a c h i e v e d by using a recuperative system in which the cool, fresh air is indirectly h e a t e d a n d conveyed to t h e c o m b u s t i o n c h a m b e r s . At the present time, the long-type f u r n a c e , Fig. 117, is m a i n l y given preference for c o m p a r a t i v e l y high t h r o u g h p u t s . T h e following c o m m e n t s thus refer to this f u r n a c e type.

9.1.2 Process S t a g e s 9.1.2.1 Charging of Green Balls to the Furnace O n e of the m o s t i m p o r t a n t prerequisites to a r e g u l a r f u r n a c e o p e r a t i o n is a good gas p e r m e a b i l i t y of t h e charge. T o ensure this, the green balls

9.1 Shaft Furnace

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Fig. 119. Shaft furnace green pellet feeding device

h a v e to be loosely a n d evenly fed o n t o t h e c h a r g e surface. In the c o u r s e of d e v e l o p m e n t , a special f e e d i n g system was d e v e l o p e d with w h i c h the pellet surface is evenly s h a p e d by f o r m a t i o n of regular rills a n d valleys the d i f f e r e n c e b e t w e e n p e a k of rills a n d b a s e of valleys b e i n g a b o u t 15 cm. This system is s h o w n d i a g r a m m a t i c a l l y in Fig. 119. T h e pellets are fed in thin layers by m e a n s of reversible belt conveyors with p r o g r a m m e d m o v i n g r h y t h m with the rill s h a p e b e i n g p e r p e n d i c u l a r to the l o n g i t u d i n a l axis of the f u r n a c e port. T h e feed level is a b o u t 0.3 m b e l o w the f u r n a c e lip. F r o m h e r e , t h e green balls slide at a s p e e d of 3—4 c m p e r m i n u t e d o w n into the f u r n a c e . T h e process gases leave the f u r n a c e in the o p p o s i t e direction t h r o u g h the pellet b e d at a t e m p e r a t u r e of a b o u t 1 2 0 - 1 5 0 ° C . T o m a i n t a i n t h e d i f f e r e n t t e m p e r a t u r e ranges inside the f u r n a c e , it is i m p o r t a n t that t h e s u r f a c e of the pellet c h a r g e is k e p t always at the s a m e level. 9.1.2.2 Drying, Preheating and Firing A t a d e p t h of 1 2 0 - 1 5 0 m m b e l o w the b e d s u r f a c e a n d a f t e r 4 - 6 m i n u t e s residence time, the pellets are largely dry a n d p r e h e a t e d b e f o r e the o x i d a t i o n of m a g n e t i t e begins. O n c e the c h a r g e has m o v e d d o w n w a r d s by a b o u t 500 m m the o p t i m u m f i r i n g t e m p e r a t u r e of a p p r o x i m a t e l y 1350 0 C is r e a c h e d . T h e a m o u n t of h e a t a v a i l a b l e inside the f u r n a c e con-

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sists of the following c o m p o n e n t s if m a g n e t i t e c o n c e n t r a t e w i t h o u t a d ditional fuels is to be treated: (a) T h e sensible h e a t of hot pellet recovered f r o m the cooling zone is t r a n s f e r r e d with cooling air into the u p p e r zones w h e r e it is m i x e d with b u r n e r gases. (b) D u r i n g conversion of m a g n e t i t e to h e m a t i t e , the o x i d a t i o n heat, alm o s t 50% of total r e q u i r e m e n t s , is released a n d p r o d u c e s a n a d d i tional t e m p e r a t u r e rise. (c) T h e r e m a i n i n g h e a t originates f r o m h o t c o m b u s t i o n gases b e i n g p r o d u c e d by c o m b u s t i o n of oil or gas with air in the o u t s i d e c o m b u s tion c h a m b e r s a n d forced t h r o u g h c o r r e s p o n d i n g inlet o p e n i n g s at a t e m p e r a t u r e of a b o u t 1280 c C into the d e s c e n d i n g pellet c h a r g e 120 ). T o a c h i e v e a h i g h t h e r m a l effect, t h e cooling air flow is to be so controlled t h a t a n o p t i m u m firing t e m p e r a t u r e of a b o u t 1300—1350 0 C is o b tained in c o n j u n c t i o n with the o x i d a t i o n h e a t as m i x i n g t e m p e r a t u r e prevailing a b o v e t h e b u r n e r gas inlet level 121 ). A p p r o x . 3 5 - 4 0 % of the total process air v o l u m e is s u p p l i e d to the c o m b u s t i o n c h a m b e r . T h e rem a i n i n g process air v o l u m e is i n t r o d u c e d as cooling air. A u n i f o r m t e m p e r a t u r e d i s t r i b u t i o n over the entire f u r n a c e section is a n i m p o r t a n t p r e r e q u i s i t e to good pellet quality. T h e gas d i s t r i b u t i o n is d e p e n d e n t on the p e r t i n e n t feed porosity r e p r e s e n t i n g a specific resistance to flow a n d thus limiting the gas flow rate, see also items 13.2 and 13.4. According to o p e r a t i o n a l values f o u n d the r a t i o of process air to pellet charge is 1 : 1 by weight. A h i g h e r gas velocity at a n a d e q u a t e p r e s s u r e may result in e r u p t i o n s in the c h a r g e or in a f l u i d i z a t i o n of the pellet surface. D u e to the resistance of the c h a r g e to the gas flow, the p e n e t r a t i o n d e p t h of b u r n e r gases f r o m the f u r n a c e walls to t h e c h a r g e centre is limited a n d t h e r e f o r e also the calories a v a i l a b l e locally. F o r this

Fig. 120. Temperature isotherms in the upper stove of the shaft furnace

9.1 Shaft Furnace

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reason, magnetite concentrate is exclusively treated. If h e m a t i t e pellets h a d to be processed, the lack of h e a t w o u l d h a v e to be c o m p e n s a t e d f r o m outside and this w o u l d cause o v e r h e a t i n g at the f u r n a c e walls a n d in pellets near the wall. T h e lateral gas flow p r o b l e m s are a n i m p o r t a n t r e a s o n f o r l i m i t i n g the w i d t h of f u r n a c e section to a m a x i m u m of a b o u t 2.5 m. T o e n a b l e sufficient m i x i n g of d i f f e r e n t process gases a r r i v i n g s i m u l t a n e o u s l y at a p proximately the b u r n e r level, the necessary h e i g h t of the f u r n a c e s h a f t f r o m the b u r n e r level u p to the pellet c h a r g e s u r f a c e is a b o u t 2 . 5 - 3 m. As the d i f f e r e n t gas flows unite i s o t h e r m s are f o r m e d over t h e u p p e r stove section which are d i a g r a m m a t i c a l l y s h o w n in Fig. 120 122 ). 9.1.2.3 CooIing of P e l l e t s T h e m a j o r p a r t of the s h a f t f u r n a c e v o l u m e serves for cooling the h o t pellets. T h e cooling air is injected into the l o w e r f u r n a c e p a r t w h e r e t h e section is i n t e r r u p t e d by a series of swinging rolls ( c h u n k b r e a k e r s ) . T h e s e rolls, e q u i p p e d w i t h b r e a k i n g teeth s u p p o r t t h e w h o l e pellet c h a r g e a n d k e e p it loose. A n y a g g l o m e r a t e s w h i c h m a y h a v e b e e n f o r m e d by overheating, are b r o k e n in o r d e r not to c h o k e the discharge devices. T h e cooling air is b l o w n into the loosened b e d a p p r o x i m a t e l y at this level at such a pressure a n d flow rate that it flows evenly u p w a r d s t h r o u g h t h e entire charge a n d carefully cools the pellets. T h e cooling air flow rate is adjustable, thus controlling the pellet d i s c h a r g e t e m p e r a t u r e . 9.1.2.4 H e a t C o n s u m p t i o n D u r i n g the first years that s h a f t f u r n a c e s w e r e used, i n s u f f i c i e n t experience was a v a i l a b l e to profit by the possibilities o f f e r e d by this heat exchange system. In 1958, the h e a t c o n s u m p t i o n was a b o u t 1.1 million kJ a n d is t o d a y a p p r o x i m a t e l y 500,000 k J / t pellets in i n d u s t r i a l furnaces. Only pellets of m a g n e t i t e c o n c e n t r a t e are f i r e d successfully. N u m e r o u s a t t e m p t s h a v e b e e n m a d e to increase the h e m a t i t e p o r t i o n a n d to c o m p e n s a t e for the lack of o x i d a t i o n h e a t by t h e a d d i t i o n of finely g r o u n d coal 1 2 3 ). But n o c o n s i d e r a b l e success h a s yet b e e n a c h i e v e d w h e r e the pellet q u a l i t y is p r i m a r i l y concerned. So f a r a n a p p r o x i m a t e h e m a t i t e portion of u p to 15% in the m a g n e t i t e c o n c e n t r a t e is tolerable.

9.1.3 Furnace Dimensions, Capacity and Market Position In recent years, the biggest s h a f t f u r n a c e s with a design c a p a c i t y of a p proximately 500,000 t pellets p e r year w e r e b u i l t o n the basis of m a g n e t i t e concentrates in d i f f e r e n t countries t h r o u g h o u t t h e world. T h e c o r r e s p o n d -

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Fig. 121. Main dimensions, gas flow and temperature profile of shaft furnace

ing f u r n a c e s h a d roughly t h e s a m e d i m e n s i o n s a n d e q u i p m e n t as s h o w n in Fig. 121 124). T h e various zones, the residence t i m e of pellets in these zones a n d the p r i n c i p a l t e m p e r a t u r e s are also described. T h e f u r n a c e has a rectangular sectional area of a b o u t 2.5 x 6.5 m , i.e. a b o u t 16 m 2 . T h e f u r n a c e w i d t h is l i m i t e d d u e to the p e n e t r a t i o n d e p t h of h e a t i n g gases into the pellet charge. T h e length is m a i n l y d e p e n d e n t on the design of the lateral c o m b u s t i o n c h a m b e r s . T h e s h a f t s p a c e is vertically d i v i d e d into two zones, called u p p e r and lower stove. T h e u p p e r stove begins at the b u r n e r level a n d h a s a height of a b o u t 2 . 5 - 3 m. In this section, all h e a t i n g stages are c a r r i e d out. A t a descending s p e e d of c h a r g e of 3—4 c m per m i n u t e , the pellets r e m a i n for a b o u t o n e h o u r in this zone. U n d e r n e a t h this b u r n e r level, w h e r e n o external heat s u p p l y occurs, the cooling zone

9.2 The Grate-Kiln Combination

223

in the lower stove begins w h e r e a rest of the m a g n e t i t e m a y also oxidize. T h e lower stove ends at a b o u t the level of the air a d m i s s i o n inlets and c h u n k breakers. It has a height of 7—9 m. C o n s e q u e n t l y , the processefficient total h e i g h t of s h a f t f u r n a c e is a p p r o x i m a t e l y 1 1 - 1 2 m . T h e residence t i m e is a b o u t 4—6 h o u r s a n d d e p e n d s o n t h e relevant gas perm e a b i l i t y of the charge. T h e a n n u a l capacity of such a s h a f t f u r n a c e is a p p r o x . 500,000 t pellets. S o m e of these f u r n a c e s are o p e r a t i n g e.g. at t h e Erie M i n i n g Co. in M i n n e s o t a , U S A I furnace Griffith Mine in O n t a r i o , C a n a d a 3 furnaces LKAB in N o r t h e r n S w e d e n 1 furnace Sierra G r a n d e in A r g e n t i n a 4 furnaces In the c o u r s e of years, the initial g r e a t i m p o r t a n c e of the s h a f t f u r n a c e has decreased m o r e a n d more. A b o u t 25 m i l l i o n tons of the m a g n e t i t e concentrates t r e a t e d in pelletizing p l a n t s in 1978 a r e still i n d u r a t e d in s h a f t f u r n a c e s w h i c h c o r r e s p o n d s to a b o u t 9 - 1 0 % of world p r o d u c t i o n . Reasons for the d e c r e a s i n g i m p o r t a n c e of the s h a f t f u r n a c e use are: — limited t h r o u g h p u t — almost exclusive use of m a g n e t i t e — c o m p a r a t i v e l y high f u e l c o n s u m p t i o n — s o m e w h a t i n f e r i o r q u a l i t y c o m p a r e d to o t h e r systems. H o w e v e r , even if the q u a l i t y of s h a f t f u r n a c e pellets was slightly lower t h a n that o b t a i n e d in o t h e r firing systems, it is s u f f i c i e n t to m e e t the req u i r e m e n t s in blast f u r n a c e and direct r e d u c t i o n plants.

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)

T h e m a i n d e v e l o p m e n t work for c o n v e r s i o n of the successful c e m e n t b u r n i n g process to the t h e r m a l t r e a t m e n t of g r e e n balls p r o d u c e d f r o m iron ore was first p e r f o r m e d by Allis C h a l m e r s in M i l w a u k e e , Illinois, USA. Initial c o n s i d e r a t i o n s to d e v e l o p a new c e m e n t b u r n i n g process with a low heat c o n s u m p t i o n led at the t i m e to a g r e a t success. E n d o t h e r m i c reactions o n the travelling grate a n d t h e b u r n i n g of c e m e n t clinker at high t e m p e r a t u r e s in the rotary kiln were t h e best p r e r e q u i s i t e s to the a p p l i c a tion of this technology to the firing of iron ore pellets. D u e to t h e experience g a i n e d in the course of c o n s t r u c t i o n of m a n y c e m e n t plants, the m a c h i n e r y a n d e q u i p m e n t were of s u c h a s o u n d f u n c t i o n a l design t h a t n o substantial m o d i f i c a t i o n s were r e q u i r e d f o r green ball h e a t t r e a t m e n t . T h e three m a i n process units, n a m e l y travelling g r a t e - r o t a r y kiln - cooler, a n d their i n d i v i d u a l f u n c t i o n s are discussed on the basis of Fig. 122 126 ).

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Fig. 122. Functioning of grate-kiln process

9 . 2 . 1 T h e T r a v e l l i n g G r a t e a n d its F u n c t i o n s T h e travelling grate consists of 3 m a i n parts: the m o v a b l e grate, the stationary w i n d b o x e s below, a n d stationary h o o d above. T h e g r a t e consists of a n endless c h a i n of grate plates w h i c h are connected with the w i n d b o x e s in a gas-tight m a n n e r . T h e process gases are passed t h r o u g h a d e q u a t e l y d i m e n s i o n e d fans. T h e windboxes, as well as the d r y i n g a n d p r e h e a t i n g h o o d are d i v i d e d into individual process zones. O n the grate, the following t h e r m a l stages successively p r o c e e d : d o w n d r a u g h t or u p - d r a u g h t drying, p r e h e a t i n g , d e h y d r a t i o n a n d o x i d a t i o n of magnetite. T h e green balls are charged directly onto the grate plates w i t h o u t h e a r t h layer at a bed height of a b o u t 20 cm. N o w a d a y s , in m a n y plants this is achieved t h r o u g h a roller screen a n d roller conveyor. D u e to the relatively low b e d height a n d c a r e f u l feeding, a good gas p e r m e a b i l i t y is m a i n t a i n e d in the charge. T h e pellets travel into the first d r y i n g zone in which b o t h d o w n - a n d u p - d r a u g h t m a y be a p p l i e d . T h e d r y i n g gas flows t h r o u g h the b e d at a t e m p e r a t u r e of a p p r o x i m a t e l y 180—210 0 C a n d leaves it at a b o u t 1 0 0 ° C . In the second zone, the gas t e m p e r a t u r e is a l r e a d y so high that a dissociation of h y d r a t e s m a y start. T h e necessary energy is s u p p l i e d at a b o u t 400 0 C f r o m t h e p r e h e a t i n g zone w h i c h a l r e a d y serves for p r e h e a t i n g , dissociation of c a r b o n a t e s or m a g n e t i t e o x i d a t i o n . It is s u p p l i e d by the waste gases f r o m t h e rotary kiln. At the end of the grate, the pellet bed reaches a b o u t 1100 0 C . T h e first crystal c o m p o u n d s begin to f o r m p r i m a r i l y d u e to m a g n e t i t e o x i d a t i o n . In the

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case of other ores, e.g. h e m a t i t e or l i m o n i t e , the necessary h e a t energy can be m a d e a v a i l a b l e by a d d i t i o n a l h e a t i n g w h i c h , however, t h e n gives rise to a n extended residence time. T h e p r e f i r e d pellets leave the g r a t e over a chute and t u m b l e into t h e rotary kiln. This transfer p o i n t is a critical i m p e r f e c t i o n of this system. T h e r e is the risk that w e a k pellets are a b r a d e d or b r o k e n at this point. P r o v i s i o n is m a d e for collection of the u n d e s i r a b l e m a t e r i a l a n d for its s e p a r a t i o n f r o m the material flow b e f o r e it enters the kiln. T h i s serves to avoid reactions between the h e m a t i t e c o n t a i n i n g d u s t a n d the r e f r a c t o r y lining of the kiln wall. If such d u s t nevertheless enters the kiln, this results in accretions a n d ring f o r m a t i o n w h i c h cause great d i s t u r b a n c e s of the entire system. T h i s difficulty can be largely a v o i d e d by i m p r o v i n g the pellet strength t h r o u g h a higher p r e h e a t i n g t e m p e r a t u r e o n t h e travelling grate. In this way, the travelling grate a l r e a d y takes over part of the f u n c t i o n s of the rotary kiln. By a d j u s t i n g the fan capacity a n d process gas t e m p e r a t u r e , the travelling grate h a s a c o n s i d e r a b l e flexibility w h i c h m a y h a v e a c o m p e n sating effect p a r t i c u l a r l y in the case of ores with d i f f e r e n t properties. A l s o by changing the i n d i v i d u a l zone split a n d , a b o v e all, by e x t e n d i n g the preheating period, it is possible to p r o v i d e o p t i m u m c o n d i t i o n s for the rotary kiln operation. T h e kiln waste gases enter the p r e h e a t i n g h o o d at a b o u t 1100 0 C and leave it at a b o u t 100 0 C a f t e r they h a v e r e p e a t e d l y p a s s e d t h r o u g h the pellet charge, indicating a good h e a t exchange. T h e m o s t i m p o r t a n t f u n c t i o n of the travelling g r a t e is to s u p p l y the rotary kiln with p r e h e a t e d pellets of a s u f f i c i e n t resistance to b r e a k a g e a n d a b r a s i o n . T h e rotary kiln alone, w i t h o u t p r e c e d i n g travelling grate, w o u l d m o r e correspond to a n o d u l i z i n g kiln. T h e r e f o r e , t h e entire system only works successfully as a grate-kiln c o m bination.

9 . 2 . 2 T h e R o t a r y Kiln and its F u n c t i o n s In the rotary kiln, t h e pellets are b r o u g h t f r o m their relative p o s i t i o n of rest on the travelling grate i n t o a rolling m o v e m e n t . T h e y roll spirally in a very thin layer towards the d i s c h a r g e end. T h e pellets are u n i f o r m l y heated by r a d i a t i o n h e a t of the hot kiln wall a n d hot gases t h u s leaving the kiln as i n d u r a t e d pellets. T o e n s u r e that t h e pellets reach a t e m p e r a ture of a p p r o x i m a t e l y 1320—1340 ° C , the f u r n a c e wall t e m p e r a t u r e is higher, which particularly intensifies t h e risk of accretions. T h e kiln is h e a t e d by fuel a n d hot air f r o m t h e cooler in a central b u r n e r . F u e l can b e s u p p l i e d as oil, gas or, to a certain extent, p u l v e r i z e d coal i n s o f a r as q u a n t i t y and m e l t i n g p r o p e r t i e s of the coal ash p e r m i t this. T o o b t a i n a

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large heating s u r f a c e at o p t i m u m residence t i m e , the kiln d i a m e t e r is relatively great c o m p a r e d to the kiln length.

9.2.3 The Cooler T h e cooler is designed as an a n n u l a r cooler with lateral walls a n d grates which serve as t h e b o t t o m . T h e pellets leave t h e r o t a r y kiln at a b o u t 1300 0 C and are c h a r g e d as a layer of 7 5 - 8 0 cm d e p t h o n t o the cooler. T h e cooling air is forced t h r o u g h the bed into two zones. W h e n h e a t i n g pellets f r o m m a g n e t i t e concentrates, it m a y be that the residence t i m e a n d p r e h e a t i n g t e m p e r a t u r e o n the travelling g r a t e are insufficient f o r a c o m p l e t e m a g n e t i t e oxidation. Since the rotary kiln is not very s u i t a b l e for f u r t h e r o x i d a t i o n , the final o x i d a t i o n m u s t then be carried out in t h e cooler 71), w h i c h m a y s o m e t i m e s lead to a n initial slight t e m p e r a t u r e rise in the first cooling zone. T h e hottest p a r t of cooling air goes directly into the kiln. T h e cooling air of lower t e m p e r a t u r e can be b l o w n into the drying zone of travelling grate. T h e efficient r e c u p e r a t i o n of the sensible heat f r o m the i n d u r a t e d pellets is very i m p o r t a n t for a good h e a t recovery. M a n y efforts are successfully m a d e for d e c r e a s i n g the h e a t i n p u t 127 ). T h e cooling air a n d waste gas fans control the m a i n process gas s t r e a m . Fans which h a v e been installed b e t w e e n the p r e h e a t i n g a n d drying zone regulate partial gas streams. A grizzly is a r r a n g e d b e t w e e n rotary kiln outlet a n d cooler inlet in o r d e r to r e m o v e a n y m a j o r u n d e s i r a b l e l u m p s f r o m the system.

9.2.4 H e a t Consumption It should be b o r n e in m i n d that the grate-kiln process was at the t i m e chiefly d e v e l o p e d for saving h e a t energy for c e m e n t b u r n i n g by using the o p t i m u m h e a t t r a n s f e r m e t h o d s . In the rotary kiln, the h e a t t r a n s f e r is achieved by r a d i a t i o n a n d o n the grate by convection. T h i s a d v a n t a g e also applies to iron o r e pelletizing. T h e following h e a t c o n s u m p t i o n figures are available f r o m industrial plants over a long o p e r a t i n g p e r i o d : For pellets p r o d u c e d f r o m magnetite: a b o u t 3 0 0 - 2 5 0 , 0 0 0 k J / t pellet. F o r pellets p r o d u c e d f r o m a m i x t u r e consisting of 30% magnetite, 70% h e m a t i t e - l i m o n i t e , 8% d o l o m i t e : a p p r o x i m a t e l y 1,200,000 k J / t pellet. F o r pellets p r o d u c e d f r o m h e m a t i t e : a b o u t 8 5 0 , 0 0 0 - 1 , 0 0 0 , 0 0 0 kJ/pellet.

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In this connection, it s h o u l d be n o t e d t h a t the r e c u p e r a t i o n of sensible heat f r o m the h o t pellet b e d has r e a c h e d a h i g h stage of p e r f e c t i o n . H o w ever, in c o m p e t i n g firing systems the h e a t c o n s u m p t i o n does not greatly differ.

9 . 2 . 5 C u r r e n t S t a t e a n d M a r k e t P o s i t i o n o f the A l l i s C h a l m e r s Grate-Kiln P r o c e s s T h e t h r e e process-specific units a r e a d a p t e d to a s p e c i f i c o r e or o r e m i x in such a way t h a t residence t i m e a n d h e a t s u p p l y p e r m i t o p t i m u m results to be achieved with r e g a r d to p o w e r c o n s u m p t i o n , t h e r m a l efficiency a n d pellet quality. T h e possibility of c h a n g i n g the d i m e n s i o n s of the v a r i o u s a p p a r a t u s d u r i n g the design stage results in g r e a t flexibility w h e r e the use of d i f f e r e n t ores is concerned. T h u s , g r a t e kiln plants f o r o r e types of the greatest variety, e.g. m a g n e t i t e a n d h e m a t i t e c o n c e n t r a t e s as well as natural o r e fines a n d m i x t u r e s 126) are n o w a d a y s o p e r a t e d all over t h e world. It is also possible to p r o d u c e pellets of d i f f e r e n t basicity. T h e specific c a p a c i t y of the t h r e e t h e r m a l units is d e t e r m i n e d b y the p r o p e r t i e s of d i f f e r e n t o r e types, see Fig. 123. T h e s p e c i f i c c a p a c i t i e s r e f e r r e d to the travelling grate a r e a ( C u r v e I), the r o t a r y kiln t h r o u g h p u t with r e g a r d to kiln v o l u m e ( C u r v e II) a n d the cooling c a p a c i t y ( C u r v e III) were c o m pared.

Fig. 123. Influence of ore types on drying, induration and cooling rate in grate-kiln operation

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Fig. 124. Relation between kiln volume and grate area for various ore types

T h e r e are d i f f e r e n c e s in the travelling g r a t e section. T h e c a p a c i t y achieved with m a g n e t i t e pellets is h i g h e r by 25% t h a n with h e m a t i t e a n d h i g h e r by 50% t h a n with l i m o n i t e - c o n t a i n i n g h e m a t i t e pellets or a n ore mix. W h e n t h e p r e h e a t e d pellets h a v e r e a c h e d a certain strength a n d t e m p e r a t u r e a n d are oxidized u p o n leaving the grate, there are h a r d l y any differences for their residence t i m e in the rotary kiln. F o r cooling, only the pellet t e m p e r a t u r e a n d the specific calorific v a l u e are of i m p o r t a n c e unless the pellets s h o w great fluctuations in their b u l k density. F r o m the t i m e t h e first industrial p l a n t with a c a p a c i t y of a b o u t 350,000 tpy pellets was b u i l t at t h e H u m b o l d t M i n e in 1960 with a grate a r e a of a b o u t 62 m 2 a n d a kiln v o l u m e of a b o u t 350 m 3 , a p e r i o d of only two y e a n e l a p s e d b e f o r e plants with a capacity of 1 m i l l i o n tpy were erected. T h e d i m e n s i o n s of the v a r i o u s units were increased according to the rising d e m a n d s so t h a t n o w a d a y s plants with a capacity of 4 million tpy in o n e single line are a l r e a d y in o p e r a t i o n . T h e g r a t e a r e a is increased u p to 400 m 2 a n d t h e kiln v o l u m e s u p to a b o u t 2,200 m 3 . T h e r e l a t i o n s h i p b e t w e e n grate area a n d kiln v o l u m e is m o r e o r less linear but w i t h m i n o r differences caused by various ore types, as is a p p a r e n t f r o m Fig. 124. T h e grate a r e a for m a g n e t i t e pellets is s m a l l e r t h a n that f o r h e m a t i t e pellets at e q u a l kiln v o l u m e s while that for o r e m i x tures d e p e n d s o n the constituents. T h e a p p l i c a t i o n range of this process can be d e m o n s t r a t e d by the q u a n t i t y of pellets w o r l d w i d e p r o d u c e d f r o m d i f f e r e n t r a w m a t e r i a l s u p to 1978:

9.3 Travelling Grate Systems — m a g n e t i t e ores a p p r o x i m a t e l y 60 m i l l i o n tpy — h e m a t i t e ores a p p r o x i m a t e l y 19.5 million tpy - magnetite-hematite mix a p p r o x i m a t e l y 11.3 million tpy - h e m a t i t e - l i m o n i t e mix a p p r o x i m a t e l y 6.2 million tpy

229

= 3 8 % *) =31% *) =24% *) =22% *)

*) % of the corresponding world production from this ore type. U p to 1978 a b o u t 97 million tons of pellets w e r e p r o d u c e d in grate-kiln plants. T h i s c o r r e s p o n d s to a p p r o x i m a t e l y 33% of the total world p r o d u c tion, see Fig. 133. T h e real p r o d u c t i o n rates a r e o f t e n h i g h e r but c a n n o t be easily d e t e r m i n e d since m a n y i n d i v i d u a l p l a n t s are s o m e t i m e s o p e r a t e d at overload. T h i s latter s t a t e m e n t also a p p l i e s to o t h e r h a r d e n i n g systems. 9.2.6 Other Grate-Kiln Processes T h e d e s i g n a t i o n "grate-kiln p r o c e s s " f o r i r o n o r e pelletizing is connected with the n a m e Allis C h a l m e r s a l t h o u g h t h e original b u r n i n g system was first d e v e l o p e d in G e r m a n y in the thirties u n d e r the n a m e " L e p o l P r o c e s s " (LEllep-POLysius). A s licensor of t h e L e p o l process, Polysius also d e c i d e d in f a v o u r of o r e pelletizing even t h o u g h at a later d a t e t h a n Allis C h a l m e r s . T h e c o m b i n a t i o n of travelling g r a t e - r o t a r y kilncooler is generally the s a m e as for the grate-kiln process. U p to now, it was to a lesser extent, a p p l i e d to iron ores t h a n to o t h e r r a w materials, e.g. nickel, c h r o m e or v a n a d i u m ores, i n - p l a n t fines a n d p h o s p h a t e s 128 ).

9.3 Travelling Grate Systems T h e travelling g r a t e is o n e of the oldest a n d m o s t f r e q u e n t l y used units for the p r o d u c t i o n of a g g l o m e r a t e s f r o m f i n e - g r a i n e d ores. Its flexibility allows a w i d e a p p l i c a t i o n range in the v a r i o u s fields of a g g l o m e r a t i o n e.g.: - R a w materials: i r o n ores, n o n - f e r r o u s m e t a l s u l p h i d e s or oxides, fly ash, c e m e n t r a w m e a l , clay - Fuels: coke breeze, a n t h r a c i t e , s u l p h i d e s u l p h u r , oil, gas a n d c o m p o s i t e fuels - Process variants: d o w n - d r a u g h t , u p - d r a u g h t gas flow or a c o m b i n a t i o n of b o t h systems

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— Reactions: e n d o t h e r m i c reactions in w h i c h h e a t energy is c o n s u m e d or e x o t h e r m i c reactions in w h i c h h e a t energy is released. Its m a i n a p p l i c a t i o n r a n g e is the a g g l o m e r a t i o n of f i n e - g r a i n e d i r o n ores to be treated in the blast f u r n a c e . A b o u t 70% of the raw m a t e r i a l s used for the present pig iron p r o d u c t i o n originate f r o m iron o r e sinter a n d 30% f r o m pellets a n d l u m p ore. It is not s u r p r i s i n g that the travelling grate was also tested a n d successfully e m p l o y e d for pellet i n d u r a t i o n . T h e travelling grate c o n s t r u c t i o n was taken over f r o m the sinter strand a n d consists of three m a i n parts: — the central p a r t is m o v a b l e a n d consists of pallets, c o m p o s e d of a f r a m e and a s u p p o r t i n g structure into which g r a t e bars are inserted. T h e pallets are c o n n e c t e d b y m e a n s of sliding seal b a r s with the w i n d b o x e s in a gas-tight m a n n e r , — the b o t t o m p a r t is c o m p o s e d of the s t a t i o n a r y w i n d b o x e s c o n n e c t e d with the gas m a i n s a n d the m o v i n g grate, — the u p p e r p a r t c o m p r i s e s the h e a t energy a n d air s u p p l y systems in a stationary h o o d a b o v e the grate. T h e system serves to convey the necessary drying, h e a t i n g or cooling gases t h r o u g h the pellet bed. I m p o r t a n t process facilities are the fans with w h i c h the process gases are m o v e d t h r o u g h the charge. At one end t h e green balls are c h a r g e d and the i n d u r a t e d pellets leave the grate at the o p p o s i t e end. T h e e n t i r e t h e r m a l t r e a t m e n t is achieved d u r i n g one passage of pallets. T h e r e s i d e n c e time of pellets o n the grate is b e t w e e n 30 a n d 40 m i n u t e s d e p e n d i n g o n the ore t y p e treated, see Fig. 84.

9 . 3 . 1 A p p l i c a t i o n o f T r a v e l l i n g G r a t e s to T h e r m a l T r e a t m e n t of P e l l e t s D u e to the great challenge of travelling g r a t e a p p l i c a t i o n s d e v e l o p m e n t work was started s i m u l t a n e o u s l y a n d i n d e p e n d e n t l y at v a r i o u s places in the world. D i f f e r e n t ores and various h e a t i n g technologies were investigated. T h r e e p r o t o t y p e s were, in parallel, d e v e l o p e d a n d installed o n a n industrial scale, w h i c h are outlined below: (a) The travelling grate plant of Cleveland-Cliffs, Iron Co. T h i s p l a n t was started u p at I s h p e m i n g , M i c h i g a n in O c t o b e r 1956. F l o t a t i o n c o n c e n trates f r o m s p e c u l a r h e m a t i t e ores were t r e a t e d . T h e whole o p e r a t i o n , except for the ignition, was carried out in u p - d r a f t gas flow. Solid fuel, mostly a n t h r a c i t e or coke b r e e z e at a rate of a b o u t 4—5% was c h a r g e d in layers o n t o the s u r f a c e of green pellets. T h e final p r o d u c t o b t a i n e d consisted of pellets a n d a certain a m o u n t of pellet sinter.

9.3 Travelling Grate Systems

231

(b) The travelling grate plant of Reserve Mining Co. at Silver Bay, M i n nesota was started u p in O c t o b e r 1955. T h e ore basis was m a g n e t i t e concentrates. T h e f u e l s u p p l y consists of a n t h r a c i t e (12—15%) a n d oil (85—90%). A t the b e g i n n i n g the entire process was c a r r i e d o u t in d o w n d r a u g h t except for u p - d r a u g h t d r y i n g and, later on, u p - d r a u g h t cooling was a p p l i e d . (c) The travelling grate plant of International Nickel Co. of C a n a d a Ltd. at C o p p e r Cliff, O n t a r i o was started u p in F e b r u a r y 1956. In this plant, artificial m a g n e t i t e was treated. T h e only e x t r a n e o u s fuel s o u r c e w a s oil, a n d , later on, n a t u r a l gas. N o coal was a d d e d . In this p l a n t new e q u i p m e n t , especially d e v e l o p e d a n d designed for pellet f o r m a t i o n a n d i n d u r a t i o n a n d f u r t h e r new process variants w e r e a p p l i e d . 9.3.2 General Features S o m e f e a t u r e s of the travelling grate are f a v o u r a b l e for t h e t h e r m a l treatment of green balls. (a) T h e relatively low d e p t h of 20—40 cm of the green ball layer avoids excessive pressure a n d allows a u n i f o r m gas p e r m e a b i l i t y . (b) A n y irregularities in the gas f l o w and p e r m e a b i l i t y of the bed only influence a small p a r t of the layer. T h e y are quickly e l i m i n a t e d by the horizontal m o v e m e n t of the grate. (c) T h e a r r a n g e m e n t of w i n d b o x e s a n d the possibility of sealing t h e m against the m o v i n g pallets allows a n a d e q u a t e d e l i m i t a t i o n of ind i v i d u a l zones of d i f f e r e n t t e m p e r a t u r e s , gas flow rate a n d gas flow direction. (d) Gases of d i f f e r e n t t e m p e r a t u r e a n d a t m o s p h e r e can b e i n t r o d u c e d into the b u r n e r h o o d a b o v e t h e charge. (e) T h e possibilities of using d i f f e r e n t fuel types and c o n s e q u e n t l y b u r n e r designs p e r m i t a great flexibility in the c h o i c e of fuel types. (f) D u e to the v a r i a t i o n range of process p a r a m e t e r s , o p t i m u m firing conditions can be p r o v i d e d for a w i d e variety of o r e types at a good pellet quality. (g) Extensive r e c u p e r a t i o n of sensible h e a t of the fired pellets results in a relatively low h e a t c o n s u m p t i o n . (h) T h e construction of big units allows the p r o d u c t i o n of c o r r e s p o n d i n g l y large pellet q u a n t i t i e s in o n e single line. However, these f e a t u r e s d o not a p p l y e q u a l l y to the v a r i o u s travelling grate processes, which are separately d e s c r i b e d below.

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9 Heat Treatment Systems 9 . 3 . 3 U p - d r a u g h t I n d u r a t i o n P r o c e s s for S p e c u l a r H e m a t i t e

D u e to the fact that a flotation c o n c e n t r a t e is not s u i t a b l e for n o r m a l d o w n - d r a u g h t sintering, pelletizing in an e x p e r i m e n t a l s h a f t f u r n a c e was tested with a negative result. S u b s e q u e n t l y , the up-draught pelletizing process was jointly d e v e l o p e d by the C l e v e l a n d Cliffs Iron C o m p a n y , Ishpeming, M i c h i g a n a n d the M i n e s E x p e r i m e n t Station of the University of M i n n e s o t a in M i n n e a p o l i s 129). T h e i n d i v i d u a l green balls w h i c h are fed o n t o a travelling g r a t e of 1.8 m w i d t h and 50 m length are c o a t e d with the necessary solid fuel for the h e a t t r e a t m e n t .

Fig. 125. Up-draught induration on travelling grate

Fig. 125 shows the process flowsheet. T h e grate bars are first c o v e r e d with a layer of fired pellets onto w h i c h a t h i n layer of ignition coal is placed. T h e coal is ignited with an ignition f u r n a c e so that the pellet layer is h e a t e d in d o w n - d r a u g h t flow. T h e first green balls c o n t a i n i n g solid fuel are layered on the s u r f a c e of the red hot pellets at a d e p t h of a b o u t 20 cm. By reversing t h e air flow direction to u p - d r a u g h t this first pellet layer is dried, p r e h e a t e d a n d the coal t h e r e i n c o n t a i n e d is ignited. By f u r t h e r air injection, the f l a m e f r o n t is forced to travel u p w a r d s . W h e n it reaches the surface, a f u r t h e r layer of green balls is f e d a n d the air c o n t i n u e d to be forced t h r o u g h the bed. T h i s p r o c e d u r e is r e p e a t e d u p to a pellet b e d height of 80 cm. In the last grate section, the cooling takes place. F r o m this zone, p a r t of the sensitive h e a t of the pellets is r e c u p e r a t e d a n d b l o w n u n d e r the g r a t e as hot air of approx. 450 ° C . T h e d i s c h a r g e d p r o d u c t is screened and the oversize is c r u s h e d . A f t e r fine g r i n d i n g the undersize is recycled a n d c o m b i n e d with the c o n c e n t r a t e for balling. T h e final p r o d u c t is a v a r i a b l e m i x t u r e of pellets a n d pellet sinter. A n a d v a n t a g e of this process is that the pallet parts a n d grate b a r s never c o m e in contact with r e d - h o t m a t e r i a l or hot gases.

9.3 Travelling Grate Systems

233

O n the o t h e r h a n d , its d i s a d v a n t a g e is t h a t t h e v o l u m e a n d p r e s s u r e of the u p - d r a f t air flow is l i m i t e d in o r d e r to a v o i d any f l u i d i z a t i o n of the charge. At that t i m e the p r o d u c t q u a l i t y d i d n o t fully c o r r e s p o n d to t h e customer's expectations. T h e design c a p a c i t y was a p p r o x . 2000 t a g g l o m e r a t e / d a y . A f t e r a few years of o p e r a t i o n the p l a n t was s h u t d o w n .

9 . 3 . 4 T r a v e l l i n g G r a t e P r o c e s s A c c o r d i n g to A r t h u r G . M c K e e and C o m p a n y This i n d u r a t i o n process was for the first t i m e a p p l i e d o n an i n d u s t r i a l scale by the Reserve M i n i n g Co., Messrs. A r t h u r G . M c K e e a n d Allis Chalmers at Silver Bay, Minnesota, a f t e r it h a d been jointly developed a n d tested in the l a b o r a t o r y of the M i n e s E x p e r i m e n t Station of the U n i v e r s i t y of M i n n e s o t a 130 ). A pilot p l a n t with a c a p a c i t y of 1000 t / d a y was first built at B a b b i t , M i n n e s o t a . T h i s process was virtually based o n the d o w n - d r a u g h t principle of a n o r m a l sinter m a c h i n e . T h e first six m a c h i n e s h a d a suction a r e a of 1.8 m x 51 m = 91 m 2 . A belt c o n v e y o r a n d a v i b r a t i o n screen a r r a n g e d directly a b o v e the grate served as a g r e e n ball f e e d i n g device. Initially, a certain a m o u n t of coal was p o w d e r e d o n t o the m a g n e t i t e green pellet surface. In this case the following h e a t sources h a d to be so h a r m o n i s e d , that a n o p t i m u m t h e r m a l effect at a s p e c i f i c m o m e n t was ensured: — coal on the s u r f a c e of pellets — o x i d a t i o n of m a g n e t i t e inside the pellets — oil converted to hot gas — cooling of h e a t r e c u p e r a t e d f r o m hot pellets. This h a r m o n i s a t i o n could not be easily a c h i e v e d a n d the h e a t i n p u t r e q u i r e d for ensuring a g o o d pellet q u a l i t y was thus relatively high, viz. altogether a b o u t 1.5 million kJ per t pellets a n d was c o m p o s e d of: oil firing: coal c o n s u m p t i o n : o x i d a t i o n heat:

29% 59% 12% 100%

This process was also very s i m i l a r to the f i r i n g technology a p p l i e d d u r i n g d o w n - d r a u g h t sintering a n d only d i f f e r e d by a larger oil-fired h o o d instead of the n o r m a l ignition f u r n a c e . T h e side walls a n d grate bars of the pallets were not p r o t e c t e d by h e a r t h a n d sidewall layer against overheating. T h i s o v e r h e a t i n g of the m e t a l l i c parts initially c a u s e d the greatest d i f f i c u l t i e s even if specific alloys were used. T o avoid a n y excessive t h e r m a l stress, m a g n e t i t e ores h a v e h i t h e r t o been used for this

234

9 Heat Treatment Systems

travelling grate system only b e c a u s e the i n d u r a t i o n of m a g n e t i t e pellets can be carried o u t at lower t e m p e r a t u r e s t h a n h e m a t i t e pellets. Fig. 126 is a typical process flowsheet indicating the zones of d i f f e r e n t t h e r m a l treatment. In o r d e r to o p t i m a l l y control the i n d i v i d u a l process stages, t h e area a b o v e the 28 w i n d b o x e s is d i v i d e d into s e p a r a t e r e a c t i o n zones: (a) U p - d r a u g h t d r y i n g zone (b) D o w n - d r a u g h t d r y i n g a n d p r e h e a t i n g zone (c) Ignition zone (d) F i r i n g a n d h e a t exchange zone (e) U p - d r a u g h t cooling zone I (f) D o w n - d r a u g h t cooling zone II

= 7 windboxes =

25% of the suction

= 4 windboxes= = 2 windboxes =

14% of the suction 7% of the suction

= 9 windboxes =

32% of the suction

= 3 windboxes =

11% of the suction

= 3 windboxes =

11% of the suction

28 w i n d b o x e s = 100% of the suction N u m e r o u s a t t e m p t s were m a d e to i m p r o v e t h e o p e r a t i n g p a r a m e t e r s f o r which p u r p o s e pellet p r o d u c e r s and plant constructors h a d to c o o p e r a t e closely 131). - T h e green ball f e e d i n g can b e i m p r o v e d by roller conveyors a n d roller screens so that a h i g h e r porosity of the pellet b e d is to b e expected. - T h e d o w n - d r a u g h t cooling was converted to u p - d r a u g h t cooling. - T h e fuel s u p p l y was limited to oil or gas. C o a l was no longer used. In this way, it was possible to achieve a better pellet q u a l i t y a n d to lower the fuel c o n s u m p t i o n drastically, w h i c h is n o w a p p r o x i m a t e l y 700,000kJ tons per ton pellets.

Fig. 126. Principle of the Mc Kee travelling grate induration system

9.3 Travelling Grate Systems

235 2

— T h e suction a r e a was enlarged f r o m 1.8 m x 51 m = 91 m to 2.4 m x 7 3 m = 175 m 2 w h i c h allowed a h i g h e r p r o d u c t i o n rate. T h e zone division was also c h a n g e d : u p - d r a u g h t drying d o w n - d r a u g h t firing d o w n - d r a u g h t cooling u p - d r a u g h t cooling

5 8 8 7 28

windboxes windboxes windboxes windboxes windboxes

According to this system, A r t h u r G . M c K e e a n d the licensees b u i l t plants on the basis of m a g n e t i t e c o n c e n t r a t e with a total a n n u a l c a p a c i t y of approx. 12.5 million t pellets. Plants o p e r a t e d according to a s i m i l a r system with a total a n n u a l capacity of a b o u t 24 million tons of pellets w e r e erected in the Soviet U n i o n which c o r r e s p o n d s to a total a n n u a l c a p a c i t y of a p p r o x . 37 million tpy pellets a n d a b o u t 13% of the world p r o d u c t i o n , see Fig. 133.

9.3.5 Lurgi-Dravo Travelling Grate Process T h e d e v e l o p m e n t of this process v a r i a n t is b a s e d on the c o n s i d e r a t i o n s leading to the t h i r d d e v e l o p m e n t p h a s e , the pelletizing of a l m o s t all o r e types, see i t e m 1.2.3. T h i s all-round pelletizing process was p r e s e n t e d at the A I M E m e e t i n g held in N e w O r l e a n s in 1957 a n d is c h a r a c t e r i z e d as follows 1 8 / 1 3 2 ): " A pelletizing p l a n t in G e r m a n y , h o w e v e r , m u s t be able to process ores of d i f f e r e n t origin, h a v i n g q u i t e d i f f e r e n t c h a r a c teristics. T h i s r e q u i r e m e n t was of g r e a t i m p o r t a n c e for the d e v e l o p m e n t of the Lurgi process. F o r the p r o d u c t i o n of g r e e n pellets, a m a c h i n e w o u l d , therefore, h a v e to be c a p a b l e of d e a l i n g with all k i n d s of raw m a t e r i a l s , and h a r d e n i n g m u s t be d o n e in a f u r n a c e , t h e t e m p e r a t u r e of w h i c h is a d j u s t a b l e to t h e d i f f e r e n t t e m p e r a t u r e s of the ores". T h e first p u b l i c a t i o n a p p e a r e d in 1956 105 ) a f t e r several years of intensive d e v e l o p m e n t work w h i c h h a d been s t a r t e d in 1948/49 a n d yielded promising results. O w i n g to the political s i t u a t i o n at t h a t t i m e , c o m m u n i c a t i o n w i t h foreign experts was very d i f f i c u l t in contrast to the close contacts of A m e r i c a n engineers a n d metallurgists with their S w e d i s h counterparts. A s a result, all k n o w l e d g e on t h e state of sinter technology in G e r m a n y a n d all e l a b o r a t e d n e w process p a r a m e t e r s were r e p e a t e d l y c o m p a r e d with a n d weighed against each other. 9.3.5.1 I m p o r t a n t P r o c e s s F e a t u r e s T h e most i m p o r t a n t f u n c t i o n of this process was to pelletize all o r e types as efficiently as in sinter plants. F u n d a m e n t a l c o n s i d e r a t i o n s

236

9 Heat Treatment Systems

referred b o t h to process p a r a m e t e r s a n d process specific a p p a r a t u s e.g.: (a) T e c h n o l o g y of green ball f o r m a t i o n f r o m d i f f e r e n t ores o n the balling disc, (b) U s e of b i n d e r s , (c) G e n t l e h a n d l i n g of green balls, (d) P r o t e c t i o n of metallic parts against o v e r h e a t i n g , (e) T h e r m a l technology, type a n d q u a n t i t y of fuels, (f) Extensive r e c u p e r a t i o n of sensible h e a t f r o m the hot pellet charge. A c c o r d i n g to this p r o g r a m m e , the w o r k was first carried out in laboratories a n d , later on, in a continuously o p e r a t i n g pilot plant. T h e following results were o b t a i n e d : (a) A f t e r extensive orienting tests in 1948/49, the disc was f o u n d to b e a very s u i t a b l e balling unit and a l r e a d y used in 1953 on an industrial scale for the pelletizing of flotation pyrite c i n d e r s 1 0 5 ) . In this case, it was ascertained that a m a x i m u m f r e e b o t t o m a r e a is i m p o r t a n t for green ball f o r m a t i o n in contrast to the construction of the flying saucers. (b) T h e d e v e l o p m e n t of gentle h a n d l i n g facilities for g r e e n balls with low d r o p heights in o r d e r to be able to f o r e g o b e n t o n i t e addition. (c) T h e side wall layer a n d h e a r t h layer were i n t r o d u c e d for the protection of m e t a l l i c parts of pallet side walls a n d grate b a r s u p p o r t i n g structure against overheating. (d) F r o m the beginning, o n e single a d d i t i o n a l h e a t source was principally a i m e d at in o r d e r to control m o r e exactly the h e a t supply. (e) T o achieve a n extensive r e c u p e r a t i o n of sensible h e a t f r o m hot pellets, cooling air was b l o w n in p r o p o r t i o n e d q u a n t i t i e s t h r o u g h the hot pellet bed w h e r e u p o n pallets a n d h e a r t h layer w e r e first cooled. T h e initial cooling air, slightly p r e h e a t e d , passes t h r o u g h the hot pellet b e d w h e r e b y the pellet q u a l i t y is preserved. (f) D r y i n g was initially a c c o m p l i s h e d in d o w n - d r a u g h t only but, later on, was carried out in u p - d r a u g h t too. T h e b u r n e r s a n d f a n constructions are special f a b r i c a t i o n s of the relevant suppliers.

9.3.5.2 Application of the Process on an Industrial Scale Since, at the t i m e w h e n the process h a d r e a c h e d a certain d e v e l o p m e n t level (1953), t h e r e was n o real d e m a n d for a pelletizing plant in G e r m a n y , negotiations were c o n d u c t e d with potential f o r e i g n customers. At that time, t h e I n t e r n a t i o n a l Nickel Co. at C o p p e r Cliff, O n t a r i o , C a n a d a h a d the intention to recover the nickel c o m p o n e n t s f r o m nickel b e a r i n g pyrrhotite w h e r e b y highly reactive, f i n e - g r a i n e d artificial m a g n e t i t e was o b t a i n e d as b y - p r o d u c t . Inco accepted the p r o p o s a l s s u b m i t t e d by Lurgi. In 1954, the first plant was o r d e r e d which was started u p in 1956. T h e

9.3 Travelling Grate Systems

237

designed daily capacity of this p l a n t was a p p r o x i m a t e l y 1200 t width of 2.5 m a n d length of 48 m . T h e p l a n t was initially h e a t e d exclusively with oil a n d , later on, with n a t u r a l gas only. In this first plant, new constructions a n d process-technological d e v e l o p m e n t s were used. A f t e r solving s o m e initial difficulties, Inco s t a r t e d u p a second p l a n t of the s a m e system with an a n n u a l c a p a c i t y of 600,000 t pellets in 1963. O n c e it h a d been possible to pelletize o n e of the r a w m a t e r i a l s most difficult to treat, artificial m a g n e t i t e , n u m e r o u s f u r t h e r plants were erected on the basis of ores of d i f f e r e n t c h e m i c a l c o m p o s i t i o n a n d m o r p h o l o g y 1 3 3 ) . 9.3.5.2.1 Charging of Green Balls to Indurating Grate and the M o d e of Operation. T h e green balls pass via a n oscillating belt c o n v e y o r o v e r a roller screen a n d c o n v e y o r onto the i n d u r a t i n g m a c h i n e . F i n e particles and pellet f r a g m e n t s are s e p a r a t e d by screening a n d recycled. B e f o r e t h e green balls are f e d onto the grate bars, the latter are covered with a 10 c m thick h e a r t h layer consisting of f i r e d pellets f o r p r o t e c t i o n against overheating. T h e s a m e m a t e r i a l is also i n t r o d u c e d b e t w e e n the side walls a n d green ball c h a r g e t h r o u g h chutes a n d g u i d e plates. Fig. 127 shows the s c h e m e of the f e e d i n g station of the travelling grate. A cross section of the green pellet b e d within the q u a s i r e f r a c t o r y c a p s u l e , c o m p o s e d of m a t e r i a l already fired in the f o r m of side wall a n d h e a r t h layer is r e p r e s e n t e d in Fig. 128. T h i s also shows the t h e r m a l i n s u l a t i n g effect of the h e a r t h layer and contains i n f o r m a t i o n on the t e m p e r a t u r e d i s t r i b u t i o n over the entire pellet b e d height, h e a r t h layer, g r a t e bars a n d in the w i n d b o x o n t h e b o u n d a r y b e t w e e n the end of the firing a n d b e g i n n i n g of the cooling zones. ( T h e t e m p e r a t u r e s are m e a s u r e d a c c o r d i n g to t h e s c h e m e r e p r e sented in Fig. 39, i t e m 4.) In the entire pellet b e d ( m e a s u r i n g p o i n t s T 1 - T 3 ) , the t e m p e r a t u r e is, at this stage, 1 3 0 0 - 1 3 4 0 0 C a c c o r d i n g to the

Fig. 127. Green pellet, side and hearth layer feeding device of Lurgi-Dravo travelling grate system

pellets

253 9 Heat Treatment Systems

Fig. 128. Thermal insulating effect of hearth and side layer

ore type a n d a d d i t i v e s used. O n the h e a r t h layer surface, the t e m p e r a t u r e r e q u i r e d for firing prevails w h e r e a s it is lower by a b o u t 200 0 C o n the u n d e r s i d e of the h e a r t h layer ( m e a s u r i n g p o i n t T 4 ) a n d a b o v e the g r a t e bars. T h u s , it can be clearly seen that the h e a r t h layer has a n insulating effect a n d also that t e m p e r a t u r e s of 1340 0 C can still be a p p l i e d w h i c h , for instance, is essential for i n d u r a t i o n of h e m a t i t e pellets. A similar insulating effect a p p l i e s to the side wall layer. T h e grate b a r t e m p e r a t u r e s are a b o u t 5 2 0 - 6 5 0 ° C ( m e a s u r i n g p o i n t T 5 ), while the waste gas t e m p e r a t u r e in the w i n d b o x m a y rise to a t e m p e r a t u r e of 650—750 0 C ( m e a s u r i n g p o i n t T 6 ). W h e n the cooling air is i n t r o d u c e d , t h e m e t a l l i c parts a n d h e a r t h layer first cool d o w n b e f o r e the cooling air r e a c h e s the pellet bed. 9.3.5.2.2 Firing Pattern and Heat Consumption. T h e ore type, n a t u r e a n d quantity of additives, loss on ignition and m o i s t u r e content d e t e r m i n e the firing p a t t e r n r e q u i r e d , as is s h o w n in Fig. 84, i t e m 5. F r o m this p a t t e r n and the p e r i o d s n e e d e d for the various t h e r m a l stages, there results the division of the i n d i v i d u a l zones in the firing h o o d and w i n d b o x e s . Fig. 129 represents the h e a t i n g cycle a n d the r e f e r e n c e t e m p e r a t u r e s f o r t h e various zones. T h e p e r i o d s and percentages given for the entire t h e r m a l t r e a t m e n t in T a b l e 29 c o r r e s p o n d with the cycles for various ore types in Fig. 84. T h e p e r i o d s r e q u i r e d for the i n d i v i d u a l t h e r m a l stages vary a c c o r d i n g to the feed characteristics and thus the total i n d u r a t i o n time. F l u c t u a t i o n s between 33 m i n u t e s for m a g n e t i t e a n d 41 m i n u t e s for l i m o n i t e - h e m a t i t e -

9.3 Travelling Grate Systems

239

Table 29. Thermal treatment zones according to the pelletizing process Thermal treatment

Time in minutes a

%

Up-draught drying

4-

7

Down-draught drying

2-

5

6-12

Preheating Dehydration Calcination Oxidation

7-

9

17-22

Firing

7-11

17-28

After-firing

3-

12-17

5

8-12

Cooling

12-14

30-35

Total period

33.5-41.0

a

According to the ore characteristics

containing raw m a t e r i a l s m a y occur. T h i s is also a n i n d i c a t o r of t h e i n d u r a t i n g m a c h i n e capacity to be expected. A p a r t i c u l a r f e a t u r e of this system is s h o w n in Fig. 129, n a m e l y the direct r e c u p e r a t i o n of hot air f r o m the cooling zone w i t h o u t hot gas fans. T h e a r r a n g e m e n t of the process gas m a i n s and the m e t h o d of recycling the sensitive h e a t give rise to very low h e a t c o n s u m p t i o n . T h e designed a n d real h e a t c o n s u m p t i o n of m a n y big pelletizing plants p l a n n e d a n d o p e r a t i n g o n typical ores are s h o w n in T a b l e 30. 9 3 . 5 . 2 . 3 Capacity, Flexibility and Market Situation. T h e capacities given in tons of product per m 2 grate area and day are also indicated in Table 30. In conjunction with the d i a g r a m s of Fig. 84 a n d 129 it shows t h e

Fig. 129. Principle of Lurgi-Dravo travelling grate induration system

9 Heat Treatment Systems

240

great flexibility of the process. As a result m a n y plants are constructed w o r l d w i d e o n t h e basis of q u i t e d i f f e r e n t raw materials. Both acid a n d basic pellets of d i f f e r e n t basicity h a v e been p r o d u c e d . A s is a p p a r e n t f r o m the following T a b l e 31, r e f e r r i n g to 1978, the L u r g i - D r a v o travelling g r a t e process reaches a b o u t 44% a n d o t h e r grate processes attain a b o u t 13% of the world p r o d u c t i o n , see Fig. 133. T h e s e capacities only relate to design values. T h e effective p r o d u c t i o n rates are p r o b a b l y higher. T h e process e q u i p m e n t also shows a great flexibility. D u e to the d e m a n d for h i g h e r Table 30. Productivity and fuel consumption of pelletizing plants according to the Lurgi-Dravo method for various ore types Types of ores

Comments

Fuel consumption KJ/pellets

Pellet production t/m 2 • day design

operation

design

operation

100% Magnetite

26-30

32

460/550000

325000

Sweden

100% Hematite

22-24

23

ca. 1 • 106

850000

Brazil

50: 50 Magnetite Hematite

24-26

27.5

700/800000

700000

Netherland

Limonite Hematite

16-18

18.5

1.2-1.5 • 106 1.25 • 106

Australia

Table 31. World production of pellets in travelling grate systems in 1979 Ore types

Portion of world production 106 t/year

Magnetite Hematite Magnetite-Hematite Hematite-Limonite

34 42 33 21

%a 22 68 70 75

-

Lurgi-Dravo Other travelling grates

130 37

-

44 13

Total

167

-

57

World production

293

a b

% of the world production from this ore type on LURGI-DRAVO grates % of the total world production

9.4 Other Heat Treatment Systems

241

Fig. 130. Development of Lurgi-Dravo travelling grates

capacities of the individual units, it was not d i f f i c u l t to increase the pallet d i m e n s i o n s a n d travelling grate length. T h e c h a n g e of pallet d i m e n s i o n s a n d suction a r e a since 1960 as well as the pellet q u a n t i t i e s p r o d u c e d o n various m a c h i n e types a r e d e s c r i b e d in Fig. 130. A c c o r d i n g to the d e m a n d s m a d e over the years (abscissa), the p r o d u c tion rates are s h o w n in the d i f f e r e n t curves in c o n j u n c t i o n with p a l l e t widths and suction areas. C o m p l e t e c o n s t r u c t i o n a l d r a w i n g s are a v a i l a b l e for a 5 m w i d e m a c h i n e , c o r r e s p o n d i n g to a suction a r e a of 1000 m 2 or a p r o d u c t i o n c a p a c i t y of a b o u t 9 m i l l i o n tpy pellets p e r u n i t at a s p e c i f i c capacity of 27 t / m 2 / d a y . H o w e v e r , t h e q u e s t i o n arises w h e t h e r it is worthwhile or a d v i s a b l e to b u i l d a n d o p e r a t e such units.

9.4 Other Heat Treatment Systems T h e challenge to engineers a n d m e t a l l u r g i s t s in c o n n e c t i o n with the d e v e l o p m e n t of the pelletizing process was so great t h a t n u m e r o u s attempts were m a d e to d e v e l o p a n d design a pellet-specific process without r e f e r e n c e to systems a l r e a d y existing. H i g h expenses were incurred a n d m u c h work was d o n e for this p u r p o s e . H o w e v e r , in the c o u r s e

242

9 Heat Treatment Systems

of years, t h e t h r e e well-known processes were d e v e l o p e d to such a d e g r e e of p e r f e c t i o n t h a t pellets of high q u a l i t y can be p r o d u c e d . Nevertheless, the e n d e a v o u r s to d e v e l o p n e w ideas should not be neglected.

9.4.1 Circular Indurating Furnace Circular sinter m a c h i n e s have a l r e a d y b e e n used since the turn of the century p r i m a r i l y for sinter-roasting of s u l p h i d e ores in m a n y n o n - f e r r o u s metal smelters. T h e s i m p l e seal b e t w e e n m o v a b l e and stationary m a c h i n e parts was o n e of their a d v a n t a g e s c o n s i d e r i n g the conveyance of SO 2 containing roaster gases. S u b s e q u e n t l y , the circular m a c h i n e was also used successfully for sintering of iron ores a n d pyrite cinders. Because of its relatively s i m p l e o p e r a t i o n , it was also utilized for the p r o d u c t i o n of to the low c a p a c i t y of a b o u t 1000 t p d sinter p e r unit t h e interest to use this type of m a c h i n e decreased rapidly. It is to the c r e d i t of A r t h u r G . M c K e e & C o m p a n y to have tested a n d d e v e l o p e d t h e circular m a c h i n e type for pellet i n d u r a t i o n 134 ). Since 1970, a pilot plant h a s b e e n in o p e r a t i o n ; in 1974 an i n d u s t r i a l plant with a grate a r e a of a p p r o x . 160 m 2 , a n effective suction a r e a of 120 m 2 a n d a c a p a c i t y of 0.6 million tpy pellets was built a n d started u p in Mexico. T h e t h e r m a l t r e a t m e n t is similar to t h a t of the straight-line travelling grate. T h e gas a n d cooling air flow is d o w n - d r a u g h t w h i c h m a y give rise to a special risk of o v e r h e a t i n g f o r t h e metallic constructional parts. T h e g r a t e b a r s are protected by h e a r t h layer a n d the green pellet b e d height is a b o u t 20 cm so that a better gas p e r m e a b i l i t y is r e a c h e d a n d thus fans with a lower capacity are admissible. T h e side walls are insulated by r e f r a c t o r y lining. T h e fired pellets are r e m o v e d f r o m t h e grate by a s c r a p i n g device which is installed at a sufficient distance b e t w e e n g r a t e b a r a n d h e a r t h layer (depth: a p p r o x . 5 cm), to leave t h e h e a r t h layer in its position. M c K e e estimates a h e a t c o n s u m p t i o n of a b o u t 500,000 kJ p e r ton pellets p r o d u c e d f r o m m a g n e t i t e concentrate a n d a heat c o n s u m p t i o n of a p p r o x . 900,000 kJ for pellets p r o d u c e d f r o m h e m a t i t e ores. U p to now, only the p l a n t in M e x i c o is in o p e r a t i o n .

9.4.2 "Heat Fast" Process

135

)

As was o f t e n in the past e q u i p m e n t suppliers, p e l l e t - p r o d u c e r s a n d pellet-consumers jointly a t t e m p t e d to d e v e l o p a pellet-specific process in order to p r o d u c e in such a plant alternatively o x i d i z e d or r e d u c e d pellets.

light-weight

agg

9.5 Comparison of Important Pelletizing Systems

243

It was i n t e n d e d to e m p l o y specific e q u i p m e n t f o r the v a r i o u s t h e r m a l stages such as: (a) D r y i n g o n a belt d r y e r at a t e m p e r a t u r e of a p p r o x . 300 ° C . (b) T r a n s f e r of d r y pellets to a n a n n u l a r h e a r t h f u r n a c e for pellet p r e heating u p to a t e m p e r a t u r e of a b o u t 600 ° C in a layer with a d e p t h c o r r e s p o n d i n g to the d i a m e t e r of o n e single pellet. (c) I n d u r a t i o n a n d cooling in a s h a f t f u r n a c e , s i m i l a r to the f u n c t i o n s in a n o r m a l s h a f t f u r n a c e . T h e p r e l i m i n a r y tests led to the construction of an industrial plant w i t h a design capacity of 1.2 million tpy pellets. F o r several reasons, this p l a n t was shut d o w n a n d d i s m a n t l e d . T h e a l t e r n a t i v e process, p r e r e d u c t i o n , was only tested on a pilot p l a n t scale.

9.4.3 "Annular Furnace" of Huntington-Heberlein

136

)

This system was b a s e d on a circular h e a r t h f u r n a c e . T h e g r e e n balls are placed at a bed h e i g h t of a b o u t 80 c m on the grate. H o t gases are s u c k e d t h r o u g h the bed. T h e f u r n a c e has lateral, gas-tight walls. T h e pellet c h a r g e is h e a t e d with a c o n c u r r e n t gas flow. T h e waste gas is s u c k e d off u p w a r d s shortly b e f o r e the discharge point of the f u r n a c e . T h e hot pellets d r o p i n t o a cooling s h a f t f r o m w h i c h the hot cooling air passes into the system. A n industrial p l a n t h a s not yet been built.

9.5 Comparison of Important Pelletizing Systems Since the start of the first i n d u s t r i a l plants at the Erie a n d R e s e r v e Mining Co. in 1955/56, various firing processes w e r e c o m p e t i n g with each other. S o m e processes d e v e l o p e d d u r i n g this p e r i o d were not successful. A f t e r a short t i m e they d i s a p p e a r e d f r o m t h e m a r k e t . S o m e systems which, a b o v e all d u r i n g the years directly a f t e r 1955, p l a y e d a great p a r t are no longer i m p o r t a n t since the r a w m a t e r i a l p r o p e r t i e s c h a n g e d in the m e a n t i m e a n d they could not be a d a p t e d to these varied conditions. F u r t h e r systems w h i c h later on, f r o m a b o u t 1960 o n w a r d s , were introd u c e d o n the world m a r k e t m e a n w h i l e h o l d a l e a d i n g position, e.g. t h e grate-kiln a n d the L u r g i - D r a v o - t r a v e l l i n g grate process. S o m e reasons f o r this d e v e l o p m e n t are given below: (a) Flexibility to d i f f e r e n t ore types (b) H i g h capacity in o n e single unit (c) U n i f o r m l y high quality, irrespective of o r e type, q u a n t i t y a n d c o m position of a d d i t i v e s

244

9 Heat Treatment Systems

(d) L o w capital a n d o p e r a t i n g costs (e) H i g h p l a n t availability. A b o v e all, the d e m a n d s on the flexibility of processes were p a r t i c u l a r l y acute at the t i m e w h e n , besides m a g n e t i t e concentrates, ores c o n t a i n i n g h e m a t i t e or o r e m i x t u r e s of d i f f e r e n t c o m p o s i t i o n h a d to be pelletized.

9.5.1 C h a n g e of Ore B a s i s D u r i n g the early years u p to a b o u t 1960, m a g n e t i t e concentrates were exclusively used. F r o m this d a t e o n w a r d , h e m a t i t e ores or c o n c e n t r a t e s as well as m a g n e t i t e - h e m a t i t e m i x t u r e s were also treated. T h e s e t h r e e ore variants are still t h e p r i n c i p a l raw m a t e r i a l s for pellet p r o d u c t i o n . Fig. 131 shows the c h a n g e of o r e types f r o m the b e g i n n i n g of industrial p r o d u c t i o n up to 1978.

Fig. 131. Proportions of various ore types for pellet feed

9 . 5 . 2 P r o d u c t i o n F i g u r e s per U n i t o f V a r i o u s I n d u r a t i n g S y s t e m s W i t h t h e increasing c o m p e t i t i o n on the w o r l d w i d e ore m a r k e t , the pellet p r o d u c e r s w e r e c o m p e l l e d to k e e p the capital a n d o p e r a t i n g costs as low as possible w h i c h was partly achieved b y using bigger units. F r o m the capacity of the firing units according to the d i f f e r e n t firing systems, the choice of a s u i t a b l e system resulted a l m o s t a u t o m a t i c a l l y f o r high p r o d u c -

9.5 Comparison of Important Pelletizing Systems

245

Fig. 132. Production rates per unit of various heat treatment systems

tion rates, as s h o w n in Fig. 132. T h e p r o d u c t i o n figures per unit are plotted against years. T h e capacity values r e a c h u p to 5 million tons pellets per y e a r f o r the biggest, a n d d o w n to less t h a n 0.5 m i l l i o n tons f o r the smallest units. T h e m a j o r a m o u n t of s m a l l e r plants was built b e f o r e 1965. F r o m 1960 onwards, the capacities a l r e a d y a t t a i n e d u p to a b o u t 2 million tons of pellets a n d , f r o m 1965, the c a p a c i t i e s c o n t i n u o u s l y increased. F r o m a b o u t 1975, the first units with a capacity of 4 or 5 million tons pellets were erected. F o r capacities of u p to 0.5 million tons pellets, s h a f t f u r n a c e s a n d travelling grates were p r e d o m i n a n t l y used w h e r e a s for c a p a c i t i e s exceeding 0.5 m i l l i o n tons pellets all travelling grate processes a n d the grate-kiln process are a p p l i c a b l e . P r o d u c t i o n rates of a b o v e 2 million tons pellets per year in o n e single unit were only attained in L u r g i - D r a v o travelling grate or grate-kiln plants. F o r the present, only these two systems are s u i t a b l e for units with high capacities.

9 . 5 . 3 P r o p o r t i o n o f V a r i o u s F i r i n g S y s t e m s in the World Pellet Production T h e a d v a n t a g e s of greater flexibility r e g a r d i n g the a v a i l a b i l i t y of materials a n d the rising capacities p e r unit h a d s o o n e r o r later to lead to d i f f e r e n t m a r k e t positions of the v a r i o u s c o m p e t i n g processes. T h e respective d e v e l o p m e n t is s h o w n in Fig. 133 relating to the t i m e a n d t h e installed total capacity. T h e e v a l u a t i o n of the i n d i v i d u a l curves leads to s o m e interesting conclusions.

246

9 Heat Treatment Systems

Fig. 133. Shares of various heat treatment systems in world production

T h e s h a f t f u r n a c e process, o n e of the first and at that t i m e most i m p o r t a n t f i r i n g processes, has only c h a n g e d its position slightly in years. T h e travelling grate process of A r t h u r G . M c K e e was m e a n w h i l e m o d i f i e d , b u t its i m p o r t a n c e did not i m p r o v e . A l t h o u g h the grate-kiln a n d L u r g i - D r a v o travelling grate processes were d e v e l o p e d at a later d a t e , their p r o p o r t i o n in the world m a r k e t has increased rapidly. T h e use of travelling grates generally f o u n d the widest a p p l i c a t i o n range b e c a u s e even the grate-kiln process w i t h o u t p r e c e d i n g travelling grate could not be o p e r a t e d successfully.

9.5.4 Cost Comparison It is not the p u r p o s e of this b o o k to m a k e a cost c o m p a r i s o n in s u p p o r t of a decision regarding the choice of one or other indurating system. T h e n u m b e r s of plants h i t h e r t o o p e r a t e d according to the various processes m a y generally serve as a criterion. It m a y also be a s s u m e d t h a t t h e decision in f a v o u r of o n e or the o t h e r i n d u r a t i o n system has b e e n t a k e n a f t e r c o n s i d e r a t i o n of all cost factors with a good pellet q u a l i t y b e i n g imperative. T h e capital investment d e p e n d s on m a n y specific c o n d i t i o n s for each p a r t i c u l a r plant w h i c h m u s t not b e directly related to the c h o s e n firing system.

10 Plant Layout and Process Control

10.1 Plant Layout T h e choice of the most important a p p a r a t u s for the p e r f o r m a n c e of the various process stages, the a r r a n g e m e n t of individual plant items relative to each other as well as the total layout are d e p e n d e n t on various factors, the most important of which are discussed below: (a) Plant Site: Nowadays, pelletizing plants are built at the following locations: the mine, the port, the iron and steel plant. (b) R a w Material Basis: Ore types, chemical and physical properties, as well as their p r e p a r a t i o n stage d e t e r m i n e the selection of the various apparatus to be used. (c) Pelletizing and Induration Units of d i f f e r e n t design also influence the arrangement of individual plant items relative to each other. Fig. 134 is a general schema based on the travelling grate process for depicting the material flow in a pelletizing plant. This scheme is, in a varied form, also valid for other induration systems. F o r the purpose of this scheme, it is assumed that all raw material constituents have already been prepared for green ball formation. T h e first part covers the raw material bins and handling facilities, whereas the second part shows the mixing and prewetting e q u i p m e n t as well as the green ball f o r m a t i o n system. In the third part, the induration system is represented which consists either of a shaft furnace, grate-kiln combination or travelling grate. This scheme m a y also serve as a guide for process control.

10.2 Process Control

133

)

In order to have at any time a good surveillance of the pelletizing process, the plants are e q u i p p e d with the necessary measuring and control systems. T h e instruments are c o m b i n e d in a central control r o o m where all data is collected.

248

10 Plant Layout and Process Control

Fig. 134. Typical flow sheet of a pelletizing plant travelling grate system

(a) T h e s e controls refer to the process cycle w i t h a view to a c h i e v i n g a n o p t i m a l pellet q u a l i t y by - d i s t r i b u t i o n a n d p r o p o r t i o n i n g of m a t e r i a l masses - p r e p a r a t i o n of a h o m o g e n e o u s mix - f o r m i n g of green balls - i n d u r a t i o n a n d cooling of pellets. (b) T h e m e a s u r i n g e q u i p m e n t also has the f u n c t i o n of technical o p e r a tional control to p r o v i d e - low energy c o n s u m p t i o n - high a n d s t e a d y p r o d u c t i v i t y - h i g h availability of the plant. N o w a d a y s , such control facilities f o r m a n integral p a r t of each m o d e r n pelletizing plant a n d are a d a p t e d to the d e m a n d of various firing systems. By way of e x a m p l e , the principal control f u n c t i o n s in a travelling grate

10.2 Process Control

249

plant are e x p l a i n e d b e l o w with the i n d i v i d u a l p l a n t sections h a v i n g b e e n c o m b i n e d in the f o r m of three sections s i m i l a r l y to those s h o w n in Fig. 134.

1 0 . 2 . 1 D i s t r i b u t i o n and P r o p o r t i o n i n g o f M a t e r i a l F l o w s In the first section the m a t e r i a l s t r e a m s are d i s t r i b u t e d over t h e individual bins t h r o u g h travelling belt c o n v e y o r s a n d p l o u g h scrapers arranged a b o v e the bins. T h e s e bins are s u p p o r t e d by load cells allowing continuous control of their filling degree. A f t e r a specified filling w e i g h t has been a t t a i n e d , the belt conveyors are s w i t c h e d over to the next b i n according to a p r e d e t e r m i n e d p r o g r a m m e . 1 0 . 2 . 2 P r o p o r t i o n i n g o f S o l i d C o m p o n e n t s and W a t e r R a w m a t e r i a l is extracted f r o m t h e v a r i o u s raw m a t e r i a l bins a n d p r o p o r t i o n e d b y weigh feeders. T h e a d m i s s a b l e v a r i a t i o n is not g r e a t e r t h a n ± 0 . 5 p e r c e n t by weight. T h e necessary w a t e r a m o u n t is also m e a s u r e d a n d in s o m e plants controlled p r o p o r t i o n a t e l y to the solids q u a n t i t y with p a r t of the water b e i n g a l r e a d y a d d e d at the p r e m i x e r . In m a n y plants, the a m o u n t of water is still controlled by h a n d .

1 0 . 2 . 3 F o r m a t i o n and T r a n s p o r t o f G r e e n P e l l e t s In principle, the pelletizing unit is o p e r a t e d b y h a n d a f t e r t h e i n d i v i d u a l c o m p o n e n t s h a v e previously passed a mixer. T h e total q u a n t i t i e s of g r e e n balls f o r m e d originate, in most cases, f r o m several pelletizing units. T h e y are collected o n a belt c o n v e y o r a n d c a r r i e d to the i n d u r a t i n g m a c h i n e . T h e a m o u n t of g r e e n balls s u p p l i e d is r e c o r d e d by m e a n s of a belt weigher and corrected according to t h e q u a n t i t y of screened undersize. T h i s belt w e i g h e r is a n i m p o r t a n t control unit, is c o n n e c t e d to the pellet indurating m a c h i n e a n d controls its velocity.

10.2.4 Green Pellet Charging T h e grate velocity control is i m p o r t a n t in o r d e r to k e e p the pellet b e d height constant. In this connection, the a d d i t i o n a l a m o u n t s of h e a r t h layer and side wall layer are taken i n t o account. A c o r r e c t i o n to c o m p e n s a t e f o r the varied g r e e n ball b u l k density a n d t i m e lag c a u s e d by t h e d i s t a n c e between green ball belt w e i g h e r a n d pellet f e e d i n g p o i n t o n t o the grate is also ensured.

250

10 Plant Layout and Process Control 10.2.5 Thermal Treatment of Green Balls

T h e p r o c e d u r e of pellet i n d u r a t i o n is controlled b y process gases flowing at t h e rates p r e d e t e r m i n e d according to the h e a t i n g p a t t e r n a n d at a c o r r e s p o n d i n g t e m p e r a t u r e t h r o u g h the pellet bed. A n i n d i c a t i o n of a sufficient s u p p l y with the necessary h e a t energy results f r o m the t e m p e r a t u r e s p r e v a i l i n g in the wind boxes. In a similar way as for d o w n - d r a u g h t sintering, b u r n - t h r o u g h points can be used as g u i d e values. F o r specific process stages, they c o r r e s p o n d to d e f i n e d t e m p e r a t u r e s . W h e n the t e m p e r a t u r e s p r e d e t e r m i n e d in the heating p r o g r a m m e are exceeded, or lower t e m p e r a t u r e s prevail, the process gas f l o w rate is c h a n g e d . T h i s is a c h i e v e d by controlling the capacity of the m a i n f a n located b e h i n d the w i n d boxes of t h e firing zone. In the case of o t h e r process gas fans also, the necessary gas f l o w is f o r c e d t h r o u g h the b e d b y m e a n s of by-pass c o n n e c t i o n s as well as t h r o u g h p a r t i c u l a r inlets a n d outlets. In this way, the gas pressure in t h e b u r n e r h o o d is controlled t h r o u g h the cooling air fan, since the h o t cooling air f r o m the cooling h o o d is i n t r o d u c e d t h r o u g h a direct d u c t into the burner hood. T h e u p - d r a u g h t drying and waste gas fans control the gas flows in the f r o n t part of the i n d u r a t i n g m a c h i n e . T h u s , t h e process gas f a n capacities, varied according to the t e m p e r a t u r e s m e a s u r e d , virtually control the t h e r m a l p r o c e d u r e of pellet i n d u r a t i o n .

10.3 Development and Trends of Further Control Variants T h e very fast a n d successful d e v e l o p m e n t of digital systems p r i m a r i l y in control a n d c o m p u t e r technology led to an extensive use of these systems in pelletizing plants. T h e i r a p p l i c a t i o n r a n g e extends f r o m s i m p l e d a t a collection, f a u l t recording, coefficient c a l c u l a t i o n a n d f a u l t analysing t h r o u g h " o n - l i n e controls" to the conversion f r o m one o p e r a t i n g s c h e m e to another. T h e use of digital controllers f o r controlled systems with a n extreme t i m e b e h a v i o u r as well as for control circuits with several m a t h e m a t i c a l interactions, e.g. for controlling the i n d u r a t i n g m a c h i n e speed, p r o v e d p a r t i c u l a r l y successful. Also m a s t e r control f u n c t i o n s concerning m a t e r i a l h a n d l i n g or mix c o m p o s i t i o n s on the basis of c h e m i c a l analyses can be solved m o r e simply a n d reliably with a digital system t h a n by a d o p t i n g the conventional m e t h o d s . As new a u t o m a t i o n systems, including m o d e r n m e a s u r i n g m e t h o d s , undergo current, s y s t e m a t i c investigations a n d testing a n d as new m a t h e matical process m o d e l s are d e v e l o p e d , it can b e expected t h a t in the n e a r future, the m e a s u r i n g a n d control systems of pelletizing processes will b e greatly i n f l u e n c e d .

11 Pellets in the Blast Furnace Burden

Since it is possible to produce pellets of good quality f r o m a great variety of raw materials, as explained in detail in chapters 5 and 6, it would still be advisable to set out the ways in which the blast f u r n a c e operators accept this new agglomerate in practice. As long as pellets were only produced f r o m fine-grained magnetite concentrates difficult to sinter, they were considered as a s u p p l e m e n t to sinter. T h e i r use was first geographically limited to a few regions. However, when pellets f r o m compete with l u m p ore and sinter. In countries with high portions of imported ores, such as J a p a n or Western Europe, a highly developed sinter technology already exists according to which the blast f u r n a c e can be supplied with burdens of good quality. F u r t h e r m o r e , the sinter quality was continuously improved. A statement on the advantages or disadvantages of using pellets for the blast f u r n a c e b u r d e n can only be m a d e by a comparison with the quality of all blast f u r n a c e constituents and under controlled operating conditions. S o m e initially recognised advantages, such as the spherical shape or the high iron content of pellets, are nowadays no longer considered as superior. F u r t h e r m o r e , the other iron oxide constituents, such as sinter or l u m p ores and even coke, are available in a d e f i n e d size range after crushing and distinct classification. Today, the iron content of a well p r e p a r e d sinter b u r d e n is quite comparable with the analysis of m a n y varieties of pellets. However, before pellets influenced the positive d e v e l o p m e n t of the total blast furnace technology, m a n y attempts were necessary to render the pellets suitable for the blast f u r n a c e operation. At first, the mechanical pellet properties had to be developed so that they m e t the r e q u i r e m e n t s of handling and stresses in the blast f u r n a c e operation.

11.1 Influence of Mechanical Properties When the first m a j o r quantities of pellets were available for blast furnace tests, the expected success was not fully realised. On the contrary,

fine-grained

252

11 Pellets in the Blast Furnace Burden

a high fines p o r t i o n and a b o v e - a v e r a g e q u a n t i t i e s of flue dust in t h e blast f u r n a c e c o m p e l l e d the pelletizing plant o p e r a t o r s to i m p r o v e the m e c h a n i cal strength of the pellets. T h e partly d r a m a t i c d e v e l o p m e n t a n d the results o b t a i n e d were described by W. E. M a r s h a l l 4 7 ) , as is s h o w n in Fig. 85, item 6. T h i s w o r k was, so to say, p e r f o r m e d in a closed c o m b i n e , because the first pelletizing plants and blast f u r n a c e s were o w n e d by the s a m e c o m p a n i e s and their special d e m a n d s were of o v e r r i d i n g i m p o r tance. W i t h great interest, a large-scale test run on 150,000 t of pellets in a blast f u r n a c e of M a n n e s m a n n w e r k e in D u i s b u r g , West G e r m a n y in 1962/63 a n d the experience thus gained was r e c o r d e d 2 0 ) . F r o m the f a v o u r a b l e results of these tests, and tests in J a p a n , pellets were considered, also o u t s i d e the U n i t e d States, as b e i n g a n e q u i v a l e n t blast f u r n a c e constituent alongside sinter and l u m p ore. In the m e a n t i m e , the issue of g u i d i n g principles for the e v a l u a t i o n of the m e c h a n i c a l pellet quality h a d b e g u n . N o w a d a y s , such s t a n d a r d s are i n t e r n a t i o n a l l y ack n o w l e d g e d a n d described in c h a p t e r 4. A f t e r the m e t h o d s of i m p r o v i n g the m e c h a n i c a l strength of M e s a b i R a n g e pellets h a d b e c o m e k n o w n , m a j o r p r o b l e m s were n o longer encountered in the respective blast f u r n a c e operations. T h e M e s a b i pellets contain by n a t u r e , high p r o p o r t i o n s of acid g a n g u e with a basicity of a b o u t 0.2, at w h i c h no pellet d e g r a d a t i o n or pellet swelling is o b s e r v e d d u r i n g reduction. As a result of this d e v e l o p m e n t phase, it was clearly recognised that only pellets of a p e r f e c t m e c h a n i c a l strength high in acid g a n g u e can w i t h s t a n d any p r e m a t u r e d e g r a d a t i o n . T h e relevant conditions are d e s c r i b e d in c h a p t e r 5 a n d 6. F o r other pellet types with a high iron a n d low g a n g u e content, g r e a t difficulties arose, so that the d e p e n d e n c e of the metallurgical p r o p e r t i e s on the chemical c o m p o s i t i o n also h a d to be t h o r o u g h l y investigated.

11.2 Influence of Chemical Composition It was first recognised in S w e d e n that a h i g h m e c h a n i c a l strength of h i g h - g r a d e iron o r e pellets alone is insufficient to withstand the stresses d u r i n g r e d u c t i o n 7 / 8 1 ) . This was c o n f i r m e d b y a blast f u r n a c e test carried out with S w e d i s h pellets, low in g a n g u e , by the T h y s s e n h ü t t e s o ), in West G e r m a n y . In particular, the unsatisfactory results w h e n the first M a r c o n a pellets were treated in J a p a n 82) initiated a b r o a d discussion on the causes of d e g r a d a t i o n of these pellet types, a l t h o u g h their m e c h a n i c a l strength was perfect. M a n y efforts were m a d e to investigate and e x p l a i n this p h e n o m e n o n w i t h o u t a satisfactory e x p l a n a t i o n h a v i n g been f o u n d u p

11.4 Comparison of Pellets and Sinter

253

to now. O n the o t h e r h a n d , m e a s u r e s for d e c r e a s i n g or even p r e v e n t i n g pellet d e g r a d a t i o n or pellet swelling d u r i n g r e d u c t i o n are m e a n w h i l e known. Only a f t e r it was possible to forecast b o t h the m e c h a n i c a l a n d , to a h i g h degree, the metallurgical p r o p e r t i e s by controlled action, could pellets qualitatively c o m p e t e with the o t h e r blast f u r n a c e constituents n a m e l y sinter or l u m p ore.

11.3 Methods of Pellet Charging to the Blast Furnace Like all blast f u r n a c e constituents, t h e pellets are n o r m a l l y screened b e f o r e they are c h a r g e d into the blast f u r n a c e a n d the undersize is recirculated to the pellet cycle or sinter plant. In o r d e r to ensure full utilization of their u n i f o r m and high p e r m e a b i l i t y , they are c h a r g e d in s e p a r a t e layers, a l t e r n a t i n g with the o t h e r c o m p o n e n t s . Special a t t e n t i o n was paid to this d u r i n g the blast f u r n a c e test r u n by M a n n e s m a n n . N o w a d a y s , the c h a r g i n g of constituents in layers h a v i n g the s a m e gas p e r m e a b i l i t y is a generally a c k n o w l e d g e d c h a r g i n g technology. Pellets with good m e c h a n i c a l p r o p e r t i e s and a low t e n d e n c y to d e g r a d a t i o n d u r i n g reduction can be p r o d u c e d by using a n y pelletizing process w h a t s o e v e r . F u r t h e r possibilities of ensuring a m o r e e c o n o m i c pellet p r o d u c t i o n a n d of a d a p t i n g the blast f u r n a c e technology even m o r e efficiently to the use of larger pellet portions are being studied.

11.4 Comparison of Pellets and Sinter Experts still hold d i f f e r e n t o p i n i o n s c o n c e r n i n g the a d v a n t a g e s a n d disadvantages of the use of pellets in the blast f u r n a c e . Partly, b e c a u s e sufficient experience with the use of pellets is not a v a i l a b l e , a n d s o m e t i m e s , primarily d u r i n g b o o m periods, pellets of lower q u a l i t y enter the m a r k e t . It is difficult to m a k e a generally valid s t a t e m e n t e i t h e r way, since o f t e n too m a n y very d i f f e r e n t local o p e r a t i n g c o n d i t i o n s m a y i n f l u e n c e the result to a h i g h e r d e g r e e t h a n the q u a l i t y of o n e or the o t h e r pellet type. However, a c o m p a r i s o n nevertheless a p p e a r s interesting. By way of e x a m p l e , trials carried o u t in the blast f u r n a c e plant at Hoogovens, I j m u i d e n B. V., in the N e t h e r l a n d s a r e r e f e r r e d to 137 ). As is known, H o o g o v e n s h a v e to i m p o r t all the ores r e q u i r e d for pig o r e p r o d u c t i o n . T o achieve an o p t i m u m a g g l o m e r a t i o n situation, they o p e r a t e both a pelletizing a n d a sinter plant, each with an a n n u a l capacity of

11 Pellets in the Blast Furnace Burden

254

3 . 1 0 6 tons. F o r the test carried out over a p r o l o n g e d p e r i o d , acid a n d basic pellets h a v i n g a basicity of 0.15, 0.90 a n d 1.35, as well as o v e r b a s i c sinter h a v i n g a basicity of 2.0 were p r e p a r e d a n d c h a r g e d at d i f f e r e n t mixing p r o p o r t i o n s to the blast f u r n a c e . T h e p r o g r a m m e i n c l u d e d the tests s h o w n in T a b l e 32.

Table 32. Test programme for comparing influence of pellets with sinter in blast furnace burden 130 ) Hearth diameter of in m

Pellets in %

Basicity

Sinter in %

Basicity

Number of trial weeks

95 70 50

1.35 0.90 0.15

5 30 50

2.0 2.0 2.0

5.9 3 13.0 6 Normal practice

I n f o r m a t i o n available f r o m the blast f u r n a c e o p e r a t i o n s e.g. f r o m Japan, N o r t h A m e r i c a and W e s t e r n E u r o p e were i n c o r p o r a t e d into the evaluation of t h e trials carried out by H o o g o v e n s . T h e s e results a l t o g e t h e r cover a w i d e r a n g e of varying m i x t u r e s f r o m 95% fluxed a n d o v e r f l u x e d sinter u p to 95% acid and fluxed pellets in the blast f u r n a c e b u r d e n . A c o m p a r i s o n was m a d e for the fuel c o n s u m p t i o n a n d the f u r n a c e p r o d u c tivity. T h e results o b t a i n e d were recently c o n f i r m e d by i n f o r m a t i o n given at the JJSJ c o n f e r e n c e in Sidney, O c t o b e r 1979 137 ). (a) The fuel consumption is calculated as an a v e r a g e s t a n d a r d i z e d rate (coke + 1.16 x f u e l ) in kg coke per ton of hot metal. T h e following values were o b t a i n e d for the a b o v e m e n t i o n e d areas: (A) J a p a n e s e blast f u r n a c e s o p e r a t e d with 7 5 - 9 5 % self-fluxing sinter, b a l a n c e sized l u m p ore and pellets =530 kg f u e l / T H M (ton hot metal) (B) J a p a n e s e blast f u r n a c e s o p e r a t e d with 46% self-fluxing sinter, 45% self-fluxing pellets, b a l a n c e sized l u m p o r e = 5 1 4 kg f u e l / T H M (C) N o r t h A m e r i c a n f u r n a c e s o p e r a t e d with 75%—95% acid pellets a n d limestone = 5 4 2 kg f u e l / T H M (D) E u r o p e a n f u r n a c e s o p e r a t e d with 75 — 95% self-fluxing sinter = 5 3 2 kg f u e l / T H M (E) trials at I j m u i d e n including s t a n d a r d o p e r a t i o n 45% acid pellets a n d 45% o v e r f l u x e d sinter =528 kg f u e l / T H M

11.5 Pellet Proportion in the Blast Furnace Burden

255

T h e r e are practically n o d i f f e r e n c e s b e t w e e n (A) a n d ( D ) at roughly the s a m e blast f u r n a c e charge, the figures of (A), ( D ) a n d (E) are almost e q u a l with 45% acid pellets in the blast f u r n a c e b u r d e n , (C). T h e highest h e a t c o n s u m p t i o n is o b t a i n e d w h e n using acid pellets, a n d the lowest is achieved at e q u a l portions of self-fluxing pellets a n d o v e r b a s i c sinter, (B). Except for (C), the d i f f e r e n c e s are not very great. F r o m this, it can be seen that at a good p r e p a r a t i o n of sinter a n d pellets, the coke c o n s u m p t i o n is similar to that resulting f r o m the use of a high p o r t i o n of self-fluxing sinter. As a result, it is d e m o n s t r a t e d that pellets are just as s u i t a b l e for the blast f u r n a c e as sinter. (b) The resistance to gas flow in the blast f u r n a c e c h a r g e is the lowest at a pellet p o r t i o n of 95%. H o w e v e r , no r e m a r k a b l e d i f f e r e n c e s in the f u r n a c e c h a r g e against a well p r e p a r e d sinter f e e d were f o u n d . Also in this case there is no negative i n f l u e n c e of pellets o n the blast f u r n a c e b u r d e n , p r o v i d e d that the i n d i v i d u a l c o m p o n e n t s s h o w a g o o d b e h a v i o u r d u r i n g r e d u c t i o n 137 ). (c) Production costs. T o achieve these f a v o u r a b l e values r e g a r d i n g coke c o n s u m p t i o n , a d e q u a t e p r e p a r a t i o n of a g g l o m e r a t e s is r e q u i r e d , w h i c h m a y be reflected in c o r r e s p o n d i n g p r o d u c t i o n costs 138 ). T h e s e costs are a b o u t 1 0 - 1 2 % higher for basic pellets t h a n for acid pellets. T h e p r o d u c t i o n costs for basic sinter are roughly b e t w e e n these two values, so it is c h e a p e r to i n t r o d u c e the necessary basic constituents t h r o u g h a sinter product. T h u s , the s t a n d a r d mix in the I j m u i d e n plant is c o m p o s e d of a p p r o x i m a t e l y 50% acid pellets a n d 50% o v e r f l u x e d sinter.

11.5 Pellet Proportion in the Blast Furnace Burden F o r d i f f e r e n t reasons, the pellet utilisation in v a r i o u s countries has developed in very d i f f e r e n t ways a n d in d e p e n d e n c e on the existing sinter plants there, e.g. in J a p a n or W e s t e r n E u r o p e , as c a n be seen f r o m T a b l e 33 139 ). In this table a trend of the blast f u r n a c e b u r d e n u p to 1985 is shown for s o m e countries with m a j o r steel p r o d u c t i o n . Except for the U n i t e d States w h e r e pellets are p r e d o m i n a n t , in all o t h e r countries the higher p o r t i o n of a g g l o m e r a t e s consists of sinter, a p p r o x . 70%. In the long run a slight c h a n g e can be expected by raising the pellet p o r t i o n at t h e expense of l u m p ores. T h e i r a m o u n t will decrease f r o m a b o u t 25% to 18% while the pellet c o n s u m p t i o n will rise f r o m 17% to a b o u t 26%. In the U n i t e d States the s i t u a t i o n will be d i f f e r e n t . T h e pellet p o r t i o n will increase u p to 70% a n d the b a l a n c e will consist of sinter a n d l u m p ore.

256

11 Pellets in the Blast Furnace Burden Table 33. Composition of blast furnace burden in 1976 and future trends 139 ) 1976 in % Pellets

1. Japan 2. USSR 3. Fed. Rep. of Germany 4. Great Britain 5. USA 6. Western Countries 7. Eastern Block 8. World (total)

approx. approx. approx. approx. approx. approx. approx. approx.

11 15-18 7 56 22 5 17

1985 in % Sinter

Lump ore

Pellets Sinter

72 78 74 79 30 58 68 59

abt. abt. abt. abt. abt. abt. abt. abt.

13 20 23 25 73 30 21 26

17 6 18 21 13 20 27 24

70 75 65 65 16 53 59 56

Lump ore 12 5 12 10 11 17 20 18

A n o t h e r forecast u p to the year 2000 140) roughly shows the following tendency: the sinter p o r t i o n will m o v e u p to a r o u n d 70% a n d the pellet p o r t i o n to a b o u t 25%, which c o r r e s p o n d s to d o u b l e the present p o r t i o n , w h e r e a s the l u m p ore portion will f u r t h e r d i m i n i s h . T h i s is a n i n d i c a t i o n of the intention to p r e d e t e r m i n e the blast f u r n a c e b u r d e n p r o p e r t i e s as f a r as possible. T h e tendency to cover the increasing rate of pig iron a n d steel in the f u t u r e m a i n l y b y pellets is caused by v a r i o u s factors u n f a v o u r a b l e for the construction of new sinter plants, such as high capital investment, d e p e n d e n c y o n coke breeze as h e a t energy a n d a b o v e all g r e a t e r ecological p r o b l e m s in sinter plants. T h e severe prescriptions r e g a r d i n g air a n d water p o l l u t i o n control in u r b a n centres, w h e r e m o s t blast f u r n a c e s a n d sinter plants are located, almost i m p e d e the construction of new sinter plants. A c c o r d i n g to the i n f o r m a t i o n o b t a i n e d f r o m a c o m p e t e n t sinter a n d pelletizing plant constructor, the capital costs of such plants w i t h a c o m p l e t e e n v i r o n m e n t a l protection system, including SO 2 a b s o r p t i o n , are, at an annual capacity of 3 x 106 tons, a p p r o x i m a t e l y 1 0 - 1 2 % h i g h e r for sinter plants t h a n for pelletizing plants.

12 The Utilization of Pellets in Direct Reduction Plants

T h e high iron content of pellets, their s p h e r i c a l s h a p e , l i m i t e d size r a n g e and practically no fines were the typical p r o p e r t i e s of a g g l o m e r a t e s w h i c h gave a fresh i m p e t u s to the d e v e l o p m e n t of d i r e c t r e d u c t i o n , p r i m a r i l y o n the basis of gas, in the s h a f t f u r n a c e . T h e first positive e x p e r i e n c e was a l r e a d y m a d e in 1950 in W i b e r g plants in S w e d e n 7/8). W h e n H o j a l a t o y L a m i n a ( H y L ) converted their retort process d e v e l o p e d in Mexico in 1957 f r o m l u m p ore to pellets in 1970, the o p e r a t i o n a l results i m p r o v e d the capacity by a b o u t 30% a n d the f u e l c o n s u m p t i o n by a b o u t 15% 141). T h e M i d r e x process was d e v e l o p e d a n d c o m m e r c i a l l y i n t r o d u c e d exclusively on the basis of pellets. H o w e v e r , d u e to the high gas p e r m e a b i l i t y of the pellet charge, a t t e m p t s were m a d e to replace a p a r t of the expensive pellets by carefully selected, c h e a p e r l u m p ores, w h i c h h a v e a lower b e d porosity. T h e p e r c e n t a g e of l u m p ores in the c h a r g e is d e p e n d a n t o n the c h a n g e of gas p e r m e a b i l i t y of the entire m i x t u r e , which is i n f l u e n c i n g the s h a f t f u r n a c e p r o d u c t i v i t y , as s h o w n in Fig. 11. A n o t h e r i n f l u e n c i n g factor is the b e h a v i o u r of the n a t u r a l ores d u r i n g reduction, as f a r as swelling a n d d e g r a d a t i o n is c o n c e r n e d , w h e r e b y t h e gas p e r m e a b i l i t y can be varied. T h e basic c o n d i t i o n s for the pellet b e h a v i o u r d u r i n g direct r e d u c t i o n are the s a m e as for the blast f u r n a c e . H o w e v e r , a d d i t i o n a l d e m a n d s are Table 34. Pellet consumption in direct reduction plant with gaseous reductants In operation

Plants 1978 Process Midrex HyL Purofer Armco

a

Sum

6

Sum a

ores

Under construction

Pellet consumption in 10 t/year (approx.) 4.9 7.3 4.9 4.9 0.9 0.3 -

12.2 9.8 0.9 0.3

11.0

23.2

12.2

In the above processes it is also possible to use, to some extent, classified lum

258

12 The Utilization of Pellets in Direct Reduction Plants

existing as d e s c r i b e d u n d e r items 4.7 and 4.7.1.3. A b o v e all, the pellets shall not p r e m a t u r e l y soften, stick t o g e t h e r a n d t h u s f o r m l u m p s . F u r t h e r more, the s p o n g e iron pellets shall h a v e a s u f f i c i e n t cold crushing strength a n d resistance to a b r a s i o n in o r d e r to w i t h s t a n d t r a n s p o r t f r o m the r e d u c tion plant to the steel p l a n t w i t h o u t d a m a g e . T h e pellet q u a n t i t y in the most i m p o r t a n t gas r e d u c t i o n plants, w h i c h in 1978 were a l r e a d y in operation or u n d e r construction, is s h o w n in T a b l e 34. In r e d u c t i o n processes carried out with solid r e d u c t a n t s in rotary kilns, pellets a n d l u m p ores can b e utilized with less stringent restrictions p r o v i d e d that t h e l u m p ores used are generally s u i t a b l e for direct reduction. A l t h o u g h t h e pig iron curve p r o c e e d s flatly, Fig. 10, the pellet p r o d u c t i o n curve rises f u r t h e r . O n o n e h a n d , this is d u e to the increasing pellet p r o p o r t i o n in pig iron p r o d u c t i o n a n d on the o t h e r , to the raised steel p r o d u c t i o n by the a p p l i c a t i o n of the direct r e d u c t i o n processes. T h e latter allow a n e n l a r g e m e n t of the steel capacity in small i n c r e m e n t s a n d a closer p l a n t location to the c o n s u m e r s ' m a r k e t w h e r e b y the capital investment is lowered accordingly.

13 Some Theoretical Considerations

List of Frequently

used

Symbols

260

13 Some Theoretical Considerations

261 13 Some Theoretical Considerations

262

13 Some Theoretical Considerations

F o r a b e t t e r c o m p r e h e n s i o n of the process p a r a m e t e r s , a n d for s u p p l e m e n t i n g results of e x p e r i m e n t s a n d experience, s o m e theoretical considerations are set forth below. T o begin w i t h , the basic features for pellet f o r m a t i o n are discussed, followed by c o n s i d e r a t i o n s dealing with the t h e r m a l t r e a t m e n t of green pellets, with p a r t i c u l a r attention being given to drying. S o m e of t h e data, e q u a t i o n s a n d d i a g r a m s are of m o r e general character a n d are merely valid for p a r t i c u l a r applications. T h e y m a y rather serve as a g u i d e for better u n d e r s t a n d i n g of the i n f l u e n c e of s o m e factors a n d s h o u l d be read in close connection with the respective c h a p t e r s concerned.

13.1 Green Ball Formation U n d e r i t e m 2.1 b o n d i n g m e c h a n i s m s are described w h i c h b e c o m e active d u r i n g the f o r m a t i o n of green balls f r o m fine-grained iron ores, particularly in c o n j u n c t i o n with water, as s h o w n in Figs. 1 2 , 1 3 , 1 4 . O n t h e basis of test results a n d measurements carried o u t in l a b o r a t o r i e s a n d industrial plants, the significance of influencing factors is explained u n d e r items 5.1 a n d 5.2. F o r s u p p o r t i n g this k n o w l e d g e , s o m e m a t h e m a t i c a l considerations h a v e been m a d e a n d already r e p e a t e d l y discussed in the literature 2 3 - 2 6 ). S o m e of these investigations are based o n d e f i n e d spheres, such as glass or metal balls, which d i f f e r r e m a r k a b l y f r o m the s h a p e of finely g r o u n d ores. Part of the m a t h e m a t i c a l derivations are t h u s applicable to green ball f o r m a t i o n by way of c o m p a r i s o n only. T h r e e factors are decisive for the f o r m a t i o n of green balls a n d their m e c h a n i c a l strength: — the b o n d i n g forces b e t w e e n grain surface a n d water, — the g r a n u l o m e t r i c properties, such as size d i s t r i b u t i o n of particles, specific s u r f a c e , p r o p e r t i e s of particle s u r f a c e and wettability, — m e c h a n i c a l c o m p r e s s i o n a n d i m p a c t forces w h i c h b e c o m e active d u r i n g rolling of solids particles with w a t e r in d r u m s or discs.

1 3 . 1 . 1 B o n d s B e t w e e n W a t e r and F i n e - G r a i n e d P a r t i c l e s In general, pre-wetted ore particles are c h a r g e d to a pelletizing unit. T h e green balls are f o r m e d , if necessary, by f u r t h e r w a t e r a d d i t i o n the voids b e t w e e n the i n d i v i d u a l ore-grains g r a d u a l l y filling.

13.1 Green Ball Formation

263

D u r i n g this o p e r a t i o n , d i f f e r e n t b o n d i n g m e c h a n i s m s b e c o m e successively active. T h e liquid filling d e g r e e serves as a c r i t e r i o n for the w a t e r a m o u n t c o n t a i n e d in existing voids b e t w e e n the i n d i v i d u a l grains. This d e g r e e indicates the p o r t i o n of voids filled with w a t e r r e f e r r e d to the total void v o l u m e .

13.1.1.1 Bonding by Liquid Bridges In the case of a low w a t e r a d d i t i o n , l i q u i d b r i d g e s f o r m at f a v o u r a b l e c o o r d i n a t i o n points of i n d i v i d u a l ore particles at a l i q u i d filling d e g r e e Ψ of less t h a n 0.2, as d i a g r a m m a t i c a l l y illustrated in Fig. 12 B. T h i s first phase of b o n d s is designated as the bridge bonding phase 142 ). H o w e v e r , this kind of b o n d i n g is not s u f f i c i e n t for g r e e n pellet f o r m a t i o n . W i t h f u r t h e r water a d d i t i o n i n d i v i d u a l capillaries fill with w a t e r , Fig. 12 C a n d f o r m c o n g l o m e r a t e s held t o g e t h e r by capillary forces. T h e strength developing in the various stages of w a t e r filling is d e s c r i b e d in Fig. 135 144) which also indicates the i n f l u e n c e of o v e r s a t u r a t i o n with water.

Fig. 135. I n f l u e n c e of w a t e r filling d e g r e e of c a p i l l a r i e s o n t e n s i l e s t r e n g t h d u r i n g g r e e n ball f o r m a t i o n , r a w m a t e r i a l : l i m e s t o n e , ε = 0.41

264

13 Some Theoretical Considerations 13.1.1.2 Bonding Forces in Transition Range

As long as the voids are only partly filled with water, capillary tensile forces and b r i d g e b o n d s are jointly active. T h i s ball f o r m a t i o n stage is designated as the transition stage. T h e liquid filling d e g r e e Ψ has increased to a b o u t 0.4 142 ). H o w e v e r , the j o i n t b o n d i n g forces are not yet s u f f i c i e n t to ensure the necessary strength of pellets. T h e g r a d u a l void filling continues, as s h o w n in Fig. 12 D. 13.1.1.3 Bonding Forces in the Capillary Range T h e o p t i m u m strength is to be expected w h e n the voids b e t w e e n the individual o r e particles have n e a r l y filled w i t h w a t e r a n d the l i q u i d s u r f a c e of the w a t e r - f i l l e d capillaries on the pellet s u r f a c e shows concave menisci. T h e capillary tension thus arising is, a c c o r d i n g to Tigerschiöld, the m a i n cause of the green pellet strength. T h e liquid filling d e g r e e has n o w a t t a i n e d a v a l u e Ψ ranging f r o m 0.8 to 0.9 of the total voids. T h e b r i d g e b o n d practically n o longer exists. O n l y u n d e r certain conditions, it m a y r e m a i n active on the pellet s u r f a c e a c c o r d i n g to the size of particles involved. A s p e r calculations, the o p t i m u m b o n d i n g p o w e r by b r i d g i n g only reaches a v a l u e of a b o u t 35% of the fully efficient capillary b o n d i n g forces 23 ). These h a v e m e a n w h i l e m o v e d to the pellet s u r f a c e as is illustrated in Figs. 12 E and 13. T h e tension b u i l t u p in the capillaries c o r r e s p o n d s to the l i q u i d c o l u m n level in the v a r i o u s voids. It is d e p e n d e n t on the void r a d i u s a n d on the properties of the ore grain surface. T h e capillary u n d e r p r e s s u r e PK results f r o m the following factors:

T h e r a d i u s of c u r v a t u r e R d e p e n d s o n the capillary r a d i u s r a n d on the contact angle b e t w e e n meniscus a n d capillary wall. At a c o m p l e t e wetting of capillaries, r a d i u s of c u r v a t u r e R b e c o m e s e q u a l to capillary r a d i u s r. In this case, the tension can be expressed as follows:

This c a l c u l a t i o n a p p l i e s to a g g l o m e r a t e s for which a liquid degree of 0 . 8 - 0 . 9 has been a t t a i n e d in the voids. T h e capillary u n d e r p r e s s u r e PK can be r e p r e s e n t e d as

filling

13.1 Green Ball Formation

265

In this e q u a t i o n , m is d e s i g n a t e d as t h e h y d r a u l i c p o r e radius. It results f r o m the ratio of the p o r e v o l u m e to the s u r f a c e of the pores filled with water. In a c o n g l o m e r a t e of finely g r o u n d ores with a statistical size distribution, the average h y d r a u l i c r a d i u s is r e s p o n s i b l e for the v a l u e of capillary tension. O n this basis, T i g e r s c h i ö l d a n d I l m o n i established the following relationship:

If this term is i n t r o d u c e d into the f o r m u l a

the following e q u a t i o n for the capillary tension is d e v e l o p e d :

(1) According to this e q u a t i o n , the green pellet strength is p r i m a r i l y proportional to the pores and capillaries of the fine grains. T h e e q u a t i o n only shows the trend. E x p e r i m e n t a l values d i f f e r f r o m the calculated figures. T h e l i q u i d filling d e g r e e of 0.8—0.9 is o p t i m a l . If m o r e w a t e r is a d d e d , value Ψ= 1 is r e a c h e d a n d exceeded. 13.1.1.4 Oversaturation with Water If this is the case, the concave voids on the s u r f a c e are overfilled with water. T h e s u r f a c e is c o a t e d with a w a t e r skin a n d m o r e c o r r e s p o n d s to a solids — c o n t a i n i n g water droplet. T h e capillary b o n d i n g forces n o longer exist. Only t h e s u r f a c e tension of the w a t e r is active, Fig. 12 F. T h e s u r f a c e which previously a p p e a r e d dry a n d r o u g h b e c o m e s n o w b r i g h t a n d the pellet slightly m o r e plastic but m o r e resistant to d r o p p i n g . If the w a t e r a d d i t i o n is f u r t h e r c o n t i n u e d , the pellet m a y even be c o n v e r t e d to sludge. T h e d e p e n d e n c e of the c o m p r e s s i o n strength on the l i q u i d filling d e g r e e a n d the decrease of the resistance at o v e r s a t u r a t i o n are s h o w n in Fig. 135. T h e strength is here expressed as tensile s t r e n g t h in N e w t o n units.

13.1.2 Influence of Granulometric Properties on Green Pellet Strength T h e second i m p o r t a n t f a c t o r f o r g r e e n ball f o r m a t i o n is the n a t u r e of particles f r o m which the pellets are m a d e . R u m p f 143) calculated the tensile strength f r o m balls consisting of particles of equal diameter a n d f o u n d the s i m p l i f i e d r e l a t i o n s h i p

266

13 SomeTheoreticalConsiderations

In this f o r m u l a , H is the adhesive force at contact points a n d d is the d i a m e t e r of the spherical particle. D u r i n g g r i n d i n g of ores, a m i x t u r e of particles with a wide size range and a steady size d i s t r i b u t i o n is o b t a i n e d , in which the p o r t i o n of fines - 0.045 m m d i a m e t e r s h o u l d be a b o u t 85%. F o r the d e t e r m i n a t i o n of the strength of g r e e n pellets derived f r o m particles of d i f f e r e n t diameters, a representative particle size dh has to be established. If t h e range of particle size is not t o o widely s p r e a d , the particle size dh can be d e t e r m i n e d f r o m the calculated or m e a s u r e d specific surface. If this is not the case, the value dh has, according to a m o d e l consideration by R u m p f , to be graphically d e f i n e d by the fact that the p a r t i a l surface

f o r m e d by all m a j o r grains can just be c o a t e d by grains of

smaller size. T h e i r s u m of digits is T h e conditional e q u a t i o n for dh is then:

1 3 . 1 . 3 I n f l u e n c e of R o l l i n g F o r c e s D u r i n g M o v e m e n t o f Green Pellets M o v e m e n t forces are considered as the t h i r d factor influencing g r e e n ball f o r m a t i o n . D u r i n g rolling, fine-grained ore particles are pressed into the surface of a l r e a d y existing large agglomerates. T h i s is achieved by the pressure w h i c h , d u e to an acceleration i m p a c t , is exerted by the pellet rolling over a particle. T h e intensity of this i m p a c t P is expressed by the following e q u a t i o n 1 4 4 ) :

T h e ore particle is pressed at a specific force into the s u r f a c e of a rolling pellet. T h e c o r r e s p o n d i n g pressure K at the contact point b e t w e e n particles a n d pellets is as follows:

In this e q u a t i o n , c is d e p e n d e n t on the s h a p e a n d p o s i t i o n of particle on the pellet surface. If the particle has the cross sectionF = c . d 1 ²formulaa n d a cubic shape, c is equal to one, d1 a n d b1 equal to d a n d b. T h e f o r m u l a can be changed; K is then calculated as follows:

13.1 Green Ball Formation

267

This f o r m u l a implies that pressure K rises p r o p o r t i o n a l l y to the r a t i oR/dformula, as well as to t h e s q u a r e of m o t i o n velocity V of the pellets. D u r i n g rolling, the pellets are c o m p a c t e d by the accelerative force a n d they receive their spherical s h a p e which is schematically s h o w n in Figs. 14 A a n d C. T h e decisive factors for the f o r m a t i o n of green balls of g o o d m e c h a n i c a l quality are: - o p t i m u m water a d d i t i o n , e q u a t i o n (1), to p r o d u c e capillary forces - sufficient particle size range, e q u a t i o n (2) - i m p a c t and pressure d u r i n g rolling, e q u a t i o n (3). T w o types of a p p a r a t u s , the balling disc a n d the balling d r u m , are chiefly used for ball f o r m a t i o n . As a l r e a d y m e n t i o n e d , the t h e o r e t i c a l considerations are in practice not very i m p o r t a n t . All the necessary o p e r a t i n g d a t a h a v e to be f o u n d out by tests.

1 3 . 1 . 4 D e s i g n and O p e r a t i o n o f B a l l i n g D i s c and B a l l i n g D r u m As already described u n d e r item 8.2, d i m e n s i o n i n g , a d j u s t m e n t a n d o p e r a t i o n of pelletizing units are largely b a s e d o n e m p i r i c a l values. M a t h ematical ideas are e m p l o y e d to a c e r t a i n extent only. N e v e r t h e l e s s , s o m e m a t h e m a t i c a l relationships can be used for the c o n s t r u c t i o n a n d o p e r a t i o n of the two most i m p o r t a n t balling units n a m e l y discs a n d d r u m s . 13.1.4.1 Design and Dimensions of Balling D i s c s T h e m a i n dimensions of a disc r e q u i r e d for a s c e r t a i n i n g a given p r o d u c tion rate are the disc d i a m e t e r a n d h e i g h t of the wall, the rim. M a n y investigators h a v e f o u n d the r e l a t i o n s h i p b e t w e e n d i a m e t e r D a n d wall height H as: H/D fomula

— = c o n s t . 4 4 b) According to Pietsch a n d B o m b l e d means H =0.2

145

), this c o n s t a n t is a b o u t 0.2; this

D

146

According to K l a t t ) the constant of 0.2 is only valid u p to a disc d i a m e t e r of 4 meters, t h e n the constant is decreasing. W h e n e v a l u a t i n g the figures f r o m T a b l e 27 2 8 ) for large discs of 5 - 7 . 5 m d i a m e t e r , the values are no longer constant, they vary b e t w e e n 0.12 a n d 0.08. 13.1.4.2 Rotating Speed and Disc Slope T o p r o d u c e green pellets of good quality, the f i n e - g r a i n e d m a t e r i a l m u s t be b r o u g h t into rolling m o v e m e n t . F o r this p u r p o s e a n o p t i m u m r o t a t i n g

268

13 Some Theoretical Considerations

speed of the disc or d r u m is necessary to h a r m o n i s e the static, d y n a m i c a n d centrifugal forces. T h e so-called critical speed, at w h i c h the gravity is just c o m p e n s a t e d by c e n t r i f u g a l forces acting on the a g g l o m e r a t e s is of great i m p o r t a n c e . W h e n the critical speed is exceeded, t h e granules are slung against the disc wall d u e to h i g h e r c e n t r i f u g a l forces a n d n o longer b e c o m e d e t a c h e d f r o m the wall. T h e r e f o r e , t h e critical s p e e d n c r i t is decisive for g o o d balling and described in the f o r m u l a d e v e l o p e d by several investigators 4 4 / 1 4 5 / 1 4 6 ).

F u r t h e r m o r e , for a given m a t e r i a l , the d y n a m i c angle of r e p o s e (see Fig. 112) a n d the frictional resistance h a v e to b e c o n s i d e r e d . T o ensure a good o p e r a t i o n , the o p t i m u m speed is a b o u t 6 0 - 7 5 % of t h e critical rotating speed. According to K l a t t 1 4 5 ) the f o r m u l a

can be used as a t h u m b rule to e s t i m a t e the o p t i m u m speed. H o w e v e r , for large discs in o p e r a t i o n the o p t i m u m speed is, as i n d i c a t e d in T a b l e 28, lower and r e a c h e s 6 - 7 r . p . m . at disc d i a m e t e r s of 6 - 7 . 5 m. T h e o p t i m u m speed does not only allow good balling b u t also ensures a m a x i m u m utilization of disc area. 13.1.4.3 Balling D r u m and its Design D a t a In the literature, only a few m a t h e m a t i c a l i n d i c a t i o n s on the d i m e n sions, o p e r a t i n g m e t h o d as well as capacity a n d quality to be expected, are known. T h e y are essentially based on d a t a f r o m the construction of ball mills. T h e m a j o r p a r t of the i n f o r m a t i o n relies on e m p i r i c a l values b e i n g described u n d e r item 8.2.2. (a) T h e slope of the d r u m axis does not serve for controlling the ball f o r m a t i o n a n d green pellet quality. It is m a i n l y i n f l u e n c i n g the mass t h r o u g h p u t , respectively the retention time. (b) T h e d r u m length a n d diameter in c o m b i n a t i o n with the r e t e n t i o n time or r o t a t i n g s p e e d respectively are responsible for a g o o d balling a n d green pellet quality. (c) T h e rotating speed is limited by the critical speed, Fig. 104 a n d calculated a c c o r d i n g to the f o r m u l a

13.2 Thermal Treatment of Green Pellets A f t e r conversion of " C rit =

42.3 y j jformulafor a d r u m

with

269 3.6 m

diameter,

the rotating speed is 70% of the critical speed i.e. a b o u t 15 r . p . m . w h i c h is s o m e w h a t h i g h e r t h a n the values s h o w n in T a b l e 27 a n d Fig. 105. (d) T h e frictionar resistance varying a c c o r d i n g to the ore p r o p e r t i e s is a f u r t h e r leading factor controlling the r o t a t i n g speed.

13.2 Thermal Treatment of Green Pellets T h e f o r m a t i o n of green pellets of good q u a l i t y is followed by the m o s t i m p o r t a n t steps: Drying, h e a t i n g and cooling. T h e r e are v a r i o u s ways of t r a n s f e r r i n g the necessary energy f r o m the heat s o u r c e to t h e pellet charge. A c c o r d i n g to d i f f e r e n t firing m e t h o d s a d o p t e d t w o m a i n h e a t t r a n s f e r p r o c e d u r e s are applied: In the shaft furnace and on travelling grates the h e a t is s u p p l i e d at m o r e t h a n 90% by convection w h e n hot gases a r e passing t h r o u g h the pellet charge. In the rotary kiln, the m a j o r p o r t i o n of the heat energy is transferred by radiation. P r e h e a t i n g a n d firing always start with d r y pellets. T h e r e f o r e , drying must be finished b e f o r e h a n d . T h e necessary h e a t s u p p l y is a c c o m p l i s h e d only by convection w h e r e b y an a d d i t i o n a l factor, the w a t e r e v a p o r a t i o n , has to be considered. Since this is a d i f f e r e n t p r o c e d u r e c o m p a r e d to n o r m a l heat transfer the c o r r e s p o n d i n g steps f o r drying are discussed separately u n d e r item 13.3. Besides c o n v e c t i o n a n d r a d i a t i o n , t h e r e are other heat t r a n s f e r possibilities. T h e y all are discussed in c o n n e c t i o n with the i n d u r a t i o n systems n o w a d a y s m a i n l y used.

1 3 . 2 . 1 H e a t T r a n s f e r in a P e l l e t C h a r g e o n t h e T r a v e l l i n g G r a t e o r in t h e S h a f t F u r n a c e Fig. 136 represents a s i m p l i f i e d section t h r o u g h a pellet b e d o n a pallet of a travelling grate. T h e indicated figures r e f e r to the v a r i o u s possibilities of introducing h e a t into the bed. T h e s e t r a n s f e r m e c h a n i s m s can be described as follows: 1) H o t gas flowing t h r o u g h the b e d gives off h e a t by convection; its t e m p e r a t u r e d r o p s f r o m the inlet t e m p e r a t u r e prevailing at the t o p to the outlet t e m p e r a t u r e prevailing u n d e r n e a t h t h e g r a t e bars. 2) If the gas in the h o o d contains n o t a b l e p o r t i o n s of CO2, H 2 O a n d SO 2 and if the distance b e t w e e n the brick lining of the h o o d a n d the pellet

270

13 Some Theoretical Considerations

Fig. 136. Heat transfer to a pellet layer in position of rest

bed s u r f a c e is sufficiently great, a h e a t s u p p l y by gas radiation a d d i tionally takes place. H o w e v e r , r a d i a t i o n only influences the t o p pellet layer t e m p e r a t u r e . 3) If the bricklining of the h o o d has a h i g h e r t e m p e r a t u r e t h a n the pellet b e d s u r f a c e u n d e r n e a t h , h e a t is t r a n s f e r r e d to the pellet b e d by radiation. T h i s a g a i n only applies to the t o p pellet layer. A part of the wall r a d i a t i o n (3) is a b s o r b e d by gas r a d i a t i o n (2) and thus c a n n o t pass onto the pellet bed. 4) Inside the b e d , h e a t is transferred by c o n d u c t i o n inside the i n d i v i d u a l pellet a n d f r o m pellet to pellet t h r o u g h points of direct contact. T h i s kind of h e a t t r a n s f e r w o u l d also t a k e place if no gas flow (1) existed. T h e h e a t c o n d u c t i o n factor (4) also contains a certain p o r t i o n of r a d i a t i o n as f o r (2) a n d (3), b u t this r a d i a t i o n only b e c o m e s active at pellet t e m p e r a t u r e s exceeding 800 ° C . 5) A n "internal h e a t s u p p l y " b y e x o t h e r m i c r e a c t i o n occurs for m a g n e t i t e ores w h e n t h e m a g n e t i t e oxidizes to h e m a t i t e . This h e a t s u p p l y m a y roughly cover half of the total h e a t c o n s u m p t i o n a n d is to be introduced a d d i t i o n a l l y w h e n h e m a t i t e c o n t a i n i n g ores are treated.

1 3 . 2 . 2 H e a t T r a n s f e r of G r e e n P e l l e t s b y C o n v e c t i o n a n d G a s F l o w 13.2.2.1 G a s Flow F o r the h e a t t r a n s f e r by convection, the gas flow t h r o u g h a pellet b e d is of p a r t i c u l a r i m p o r t a n c e . T h e gas flow is d e p e n d i n g on t h e gas p e r m e ability or b e d resistance and highly i n f l u e n c e d by the physical consistency of the pellets. S o m e considerations are a d v i s a b l e concerning the gas flow

13.2 Thermal Treatment of Green Pellets

271

based on physical rules 147 ). T h e simplest w a y of t h e r m a l t r e a t m e n t is t h e single p h a s e gas flow t h r o u g h an u n m o v e d pellet bed. O n the basis of presently a v a i l a b l e d a t a 148 ), the nearly s p h e r i c a l s h a p e of pellets a n d t h e relatively n a r r o w size range of 9—15 m m pellet d i a m e t e r , a p r e d e t e r m i n a t i o n of the resistance to gas f l o w in a b e d is possible. If the resistance values are k n o w n , the basis for calculating the f a n c a p a c i t y is given, a l t h o u g h the final d i m e n s i o n s of the f a n are a d d i t i o n a l l y d e p e n d e n t o n o t h e r factors.

13.2.2.1.1 R e s i s t a n c e of a Charge of U n i f o r m Pellet Size to G a s Flow. T h e resistance to gas flow ΔPCH follows linearly the h e i g h t of a pellet charge and is d e p e n d e n t o n the following e q u a t i o n :

T h e void v o l u m e e can be calculated f r o m t h e specific density of pellets Qpformula,chargeQCHformulaa n d gas Qg'formulain c o n f o r m i t y with the following e q u a t i o n :

T h e p o r t i o n of void v o l u m e c a n vary b e t w e e n 0.40—0.49 for a pellet bed. It is of great i m p o r t a n c e for the resistance to gas flow. T h e flow resistance coefficient Ψ of the b e d , w h i c h is in close r e l a t i o n to the R e y n o l d s ' n u m b e r , is c o m p o s e d of a l a m i n a r a n d t u r b u l e n t p a r t 1 4 9 )

T h e relation with the R e y n o l d s ' n u m b e r is as follows:

Before the flow c o n d i t i o n in a b e d can b e e v a l u a t e d , it is necessary to k n o w the R e y n o l d s ' n u m b e r . F o r beds, it is expressed as follows:

13.2.2.1.2 R e s i s t a n c e of Pellet C h a r g e of D i f f e r e n t Pellet Size to G a s Flow. If the pellet d i a m e t e r greatly varies, a d d i t i o n a l factors h a v e to be considered. First, the b u l k densities gjCHformulaa n d the p a r t s of v o l u m e are to be d e t e r m i n e d f r o m t h e screen analysis 1 4 8 ).

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13 Some Theoretical Considerations

F r o m this, an average pellet d i a m e t e r dm is calculated a c c o r d i n g to the equation

(7) Vi p r o p o r t i o n a l v o l u m e V total v o l u m e S u b s e q u e n t l y , the average void v o l u m e factors of charges c o m p o s e d of grains of d i f f e r e n t d i a m e t e r s εm and of the smallest fractionεksubscritoare to be d e t e r m i n e d . F r o m this results a n empirical correction factor formula w h i c h is to be i n t r o d u c e d into the resistance e q u a t i o n . T h e resistance to flow of charges c o m p o s e d of pellets of d i f f e r e n t d i a m e t e r s results f r o m the following e q u a t i o n :

T h e d a t a for designing the process fans can be calculated by m e a n s of e q u a t i o n s (7) a n d (8) for charges of u n i f o r m a n d d i f f e r e n t pellet size. T h e s e c o n s i d e r a t i o n s only refer to the pellet bed. O t h e r factors m a y influence a d d i t i o n a l l y the resistance to flow of the whole firing system including d u c t work a n d dedusting facilities. In o r d e r to be able to cope with e x t r e m e c o n d i t i o n s d u e to p o o r pellet quality, a c o r r e s p o n d i n g capacity reserve is r e q u i r e d for the process fans. 13.2.2.1.3 R e s i s t a n c e of Pellet Charge and Travelling G r a t e to G a s Flow. A p a r t f r o m the pellet b e d , a n o t h e r resistance arises on the travelling grate d u e to the restriction of the space b e t w e e n the grate bars ΔPGR. T h e following r e l a t i o n s h i p exists:

T h e resistance to flow of the pellet bed a n d grate can b e calculated according to the e q u a t i o n :

Fig. 137 shows d i a g r a m a t i c a l l y the resistance to flow of a c h a r g e of u n i f o r m pellets with a d i a m e t e r of 10—12 m m having a height of 40 cm and of small pellets a n d particles of 3 —8 m m d i a m e t e r including t h e grate resistance as f o u n d in pot pelletizing tests. T h e grate resistance is only p e r c e p t i b l e in a b e d of a p p r o x i m a t e l y 5 cm height. F r o m this b e d height o n w a r d , b o t h curves are linear with t h e bed height. In the case of pellets of d i f f e r e n t d i a m e t e r s , the resistance to flow is substantially higher. T h a t is w h y the pellet fractions of 3—8 m m d i a m e t e r

13.2 Thermal Treatment of Green Pellets

273

Fig. 137. Influence of bed height on pressure drop of gas flow at a different gas flow velocity and pellet size

serve as an insulating layer b e t w e e n pallet side walls a n d the p o r o u s pellet feed in L u r g i - D r a v o travelling grates. A good gas p e r m e a b i l i t y and an efficient gas flow t h r o u g h a pellet bed are achieved if the pellet d i a m e t e r is w i t h i n a close range. 13.2.2.2 H e a t T r a n s f e r by C o n v e c t i o n T h e m a i n h e a t t r a n s f e r t h r o u g h a pellet c h a r g e is realized by convection. T h e following d a t a on heat t r a n s f e r by c o n v e c t i o n are available. T h e convective h e a t t r a n s f e r to a pellet b e d t h r o u g h w h i c h gas is flowing is represented in a m a n n e r generally valid by the n o n - d i m e n s i o n a l N u s s e l t number:

w h e r e αCH is the convective heat t r a n s m i s s i o n c o e f f i c i e n t r e f e r r e d to the pellet bed s u r f a c e ( W • m - 2 • K - 1 ) . T h e d e p e n d e n c e of the convective h e a t t r a n s m i s s i o n c o e f f i c i e n t o n flow conditions, c h a r a c t e r i z e d by the R e y n o l d s ' n u m b e r of the pellet b e d of

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13 Some Theoretical Considerations

u n i f o r m size can be r e p r e s e n t e d according to J e s c h a r

150

) as follows:

In the case of charges c o m p o s e d of pellets of d i f f e r e n t d i a m e t e r s , t h e R e y n o l d s ' n u m b e r a n d a v e r a g e pellet d i a m e t e r have to be calculated accordingly, as expressed in the following e q u a t i o n :

which refers to ideal a n d s m o o t h balls. In general, pellets s h o w n o t a b l e deviations f r o m the ideal spherical s h a p e , w h i c h for pellet charges a m o u n t to a b o u t 40—60% of t h e theoretical value. T h e e q u a t i o n s 11, 12 and 13 are typical for c o n d i t i o n s in the s h a f t f u r n a c e and on the travelling grate. Fig. 138 s h o w s the c o e f f i c i e n t of h e a t transmission α as a f u n c t i o n of the t e m p e r a t u r e s for b o t h gas velocities. T h e gas velocity is r e f e r r e d to 273 K, 1 b a r a n d to the voids inside the charge. As is a p p a r e n t f r o m t h e t e m p e r a t u r e d e p e n d e n c e , h e a t i n g a n d cooling processes in h i g h pellet b e d s can be calculated m o r e exactly if the total b e d h e i g h t is d i v i d e d into a certain n u m b e r of smaller layers. O n the o t h e r h a n d , a n a v e r a g e coefficient of h e a t t r a n s m i s s i o n m a y be a s s u m e d . T h e convective h e a t transmission in the rotary kiln m a i n l y refers to the area of the c h a r g e over which the gases flow. T h e r e f o r e , the α v a l u e c a n be neglected. T h e convective h e a t t r a n s f e r in the rotary kiln c h a r g e is

Fig. 138. Convective heat transfer by gas flow through a pellet bed

13.2 Thermal Treatment of Green Pellets

275

smaller by one o r d e r of m a g n i t u d e t h a n the h e a t t r a n s f e r o n the travelling grate and in the s h a f t f u r n a c e . 13.2.2.2.1 H e a t T r a n s f e r to the Pellet C h a r g e on the Travelling G r a t e . As concerns the pallet sectional a r e a s h o w n in Fig. 136 the h e a t t r a n s f e r rules are c o m p a r a t i v e l y simple, w h e r e a s they b e c o m e m o r e c o m p l i c a t e d if the pellets on the travelling grate are taken into c o n s i d e r a t i o n . In travelling direction the waste gas outlet t e m p e r a t u r e follows, with s o m e delays, t h e hot gas inlet t e m p e r a t u r e at the s a m e location. D u e to this "cross-flow effect", the calculation of t e m p e r a t u r e g r a d i e n t in the pellet b e d b e c o m e s complicated. In this case, h e a t balances of small d i f f e r e n t i a l layers of the total bed a n d the travelling t i m e are e s t a b l i s h e d as in a s i m i l a r way s h o w n for the drying progress in Fig. 144. T o c o m p e n s a t e for this delay, an afterfiring section is i n c o r p o r a t e d in the firing cycle, see Fig. 84. T h i s f i g u r e shows the t e m p e r a t u r e g r a d i e n t in c o n j u n c t i o n with the i n d i v i d u a l process stages and for d i f f e r e n t ore types o n the basis of t h e results of c o m p u t e r calculations. T h e s e calculations tally with m e a s u r e m e n t s m a d e in l a b o ratories a n d industrial plants. 13.2.2.2.2 H e a t T r a n s f e r to the Pellet C h a r g e in the S h a f t F u r n a c e . O n e advantage is the g o o d h e a t e c o n o m y . C o m p a r e d to m a t e r i a l s usually treated in s h a f t furnaces, pellets represent a u n i f o r m a n d p o r o u s bed. T h e pellets are m o v i n g d o w n w a r d s by gravity. F r i c t i o n b e t w e e n pellets a n d brickwork a n d b e t w e e n i n d i v i d u a l pellets m a y h a v e s o m e d e t r i m e n t a l effect on the pellet quality so that t h e residence t i m e , t e m p e r a t u r e p r o f i l e and gas d i s t r i b u t i o n m a y be i m p a i r e d . T h e u n i f o r m gas flow t h r o u g h the b e d sectional area, which is i m p o r t a n t for t h e h e a t transfer, is b e t t e r ensured in the travelling grate, w h e r e the pellets are t r a n s p o r t e d u n d e r controlled conditions. Fig. 122 shows d i a g r a m a t i c a l l y a pelletizing s h a f t f u r n a c e and the t e m p e r a t u r e profile. T h e h e a t t r a n s f e r m e c h a n i s m s (1), (4) and (5) described in Fig. 136 are also valid for the s h a f t f u r n a c e charge. Since, inside the f u r n a c e n o big cavities exist, the h e a t t r a n s f e r f r o m gas to pellets is a l m o s t completely a c c o m p l i s h e d b y convection (1). T h e s a m e applies to the h e a t emission f r o m t h e hot pellets by cooling air in the lower stove.

1 3 . 2 . 3 H e a t T r a n s f e r t o t h e P e l l e t C h a r g e in t h e R o t a r y K i l n T h e partial steps of h e a t transfer f r o m the h e a t sources to the pellet b e d in the rotary kiln are illustrated in Fig. 139 and m a r k e d by n u m b e r s corresponding to t h e various h e a t t r a n s f e r m e c h a n i s m s . T h e y s h o w s o m e substantial d i f f e r e n c e s c o m p a r e d to t h e travelling grate technology. W h e n

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13 Some Theoretical Considerations

Fig. 139. Heat transfer to a pellet layer moving in a rotary kiln

the pellets roll d o w n on the b e d a n d inner kiln shell, a p p r o x i m a t e l y 90% of the necessary h e a t energy is s u p p l i e d to t h e m by gas r a d i a t i o n (2) a n d b r i c k w o r k r a d i a t i o n (3). F o r a short t i m e the pellets reach t e m p e r a t u r e s , which are only slightly below the gas t e m p e r a t u r e of the f r e e f u r n a c e volume. H o w e v e r , this t e m p e r a t u r e rise only refers to a low b e d height of 3 - 4 pellet d i a m e t e r s . D u e to the kiln rotation, the surfaces of the highly h e a t e d pellets are pulled into the c h a r g e at the lower end of the pellet bed. T h e y give off s o m e of their s u r p l u s heat to cooler a d j a c e n t pellets. D u e to the lifting effect of the bricklining cooler pellets pass over to the c h a r g e s u r f a c e , w h e r e they are f u r t h e r h e a t e d by radiation. In the case of the rotary kiln, a n a d d i t i o n a l m e c h a n i s m (6) is a d d e d c o n t r i b u t i n g h e a t energy by the continuously varying contact b e t w e e n c h a r g e a n d brickwork. O w i n g to the heating of b r i c k w o r k a b o v e the pellet bed, a small a m o u n t of heat, the storage heat, is a b s o r b e d and partially given off to the cooler pellet c h a r g e if the b r i c k w o r k m o v e s u n d e r the latter. H o w e v e r , this effect is not i m p o r t a n t d u e to heat loss by r a d i a t i o n to the e n v i r o n m e n t . T h e m a g n e t i t e o x i d a t i o n (5) does not m a k e any notable c o n t r i b u t i o n as a h e a t source since practically n o c u r r e n t of air is flowing t h r o u g h the pellet c h a r g e a n d the o x i d a t i o n thus only p r o c e e d s slowly. T h e total heat transfer in the rotary kiln is c o m p l i c a t e d a n d still the subject of investigations. T h e c o m p o n e n t s (1), (2), and (3) are calculable to s o m e degree. As the h e a t t r a n s f e r by convection is insignificant, the considerations c o n c e r n i n g heat c o n d u c t i o n by r a d i a t i o n are m o r e important. 1 3 . 2 . 4 H e a t T r a n s f e r by R a d i a t i o n T h e h e a t transfer by r a d i a t i o n can be initiated by r a d i a n t gas molecules a n d a r a d i a n t solid, such as the inner s u r f a c e of the brick lining in the rotary kiln a b o v e the charge.

13.2 Thermal Treatment of Green Pellets

277

13.2.4.1 Gas Radiation T h e calculation of this h e a t transfer m e t h o d was p r i m a r i l y p r o m o t e d b y Hottel and his colleagues 1 5 1 ). A n extensive s u r v e y of the state of knowledge in this field is given by R. J e s c h a r l48 ). P o l y a t o m i c gases, such as C O 2 , H 2 O , C H 4 , SO 2 r a d i a t e a n d a b s o r b in certain regions of wavelength according to h o w the f r e q u e n c y of v i b r a t i o n s of a t o m s in a molecular c o m p o u n d tallies with the f r e q u e n c y of the e l e c t r o m a g n e t i c radiation. T h e calculation is c o m p l i c a t e d if t h e regions of wavelength for i n d i v i d u a l gas constituents intersect a n d if r e d - h o t solid particles are contained in the gas flow, which is called "solids r a d i a t i o n " 152 ). T h e e x a m p l e given below refers to r a d i a n t gases. R a d i a t i o n h e a t can be t r a n s f e r r e d to the pellet s u r f a c e (P) qG->P or to the f u r n a c e wall (w) q G - > w . T h e app r o x i m a t i o n m e t h o d d e v e l o p e d by G o d 153) is a d o p t e d for n u m e r i c a l evaluation. W i t h reference to Fig. 139 a n d a n a l o g o u s l y to Fig. 136 - firing space a b o v e the pellet bed - the f o r m u l a for the r a d i a t i o n h e a t transferred from gas to pellet bed s u r f a c e is the following:

CS is the r a d i a t i o n value of the black b o d y . T h e e m i s s i o n factor EG is to be individually calculated for each gas constituent, such as C O 2 a n d H 2 O, a n d to be a d d e d ; including correction f a c t o r AE

Each a d d e n d is a f u n c t i o n of gas layer thickness S t e m p e r a t u r e T a n d partial pressure PCO2 and PH2O. In the b u r n e r h o o d located a b o v e the travelling grate, distance H b e t w e e n the i n n e r h o o d s u r f a c e a n d the pellet s u r f a c e m a y be a s s u m e d for 5. F o r the rotary kiln, a n " e q u i v a l e n t " gas layer thickness SR is used, w h i c h , a c c o r d i n g to S c h a c k 1 5 4 ) is S'R = 0.9 X D OF the f u r n a c e d i a m e t e r . U n d e r the r a d i a t i o n c o n d i t i o n s d u r i n g pellet i n d u r a t i o n , the emission value can, a c c o r d i n g to Hottel 151 ), be a s s u m e d as 0.95. A c c o r d i n g to the e q u a t i o n s 14 a n d 15, h e a t is also given off by gas r a d i a t i o n to the wall a b o v e the pellet b e d q2 = qG->w with t h e s u r f a c e t e m p e r a t u r e TP having to be r e p l a c e d by the wall t e m p e r a t u r e TW in e q u a t i o n (14). T h i s c o m p o n e n t can b e neglected within the scope of the a b o v e considerations. 13.2.4.2 Kiln Lining Radiation T h e heat reflected by the b r i c k w o r k as solids r a d i a t i o n qW->P to the pellet bed s u r f a c e is also of i m p o r t a n c e a n d calculated a c c o r d i n g to the

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13 Some Theoretical Considerations

Between the vault a n d the pellet bed, and t h e r e f o r e d o e s not pass o n t o the r a d i a t i o n , which, according to e q u a t i o n h a d the wall t e m p e r a t u r e TW C o n s e q u e n t l y , the b r i c k w o r k r a d i a t i o n h e a t flows:

p a r t of qW->P is a b s o r b e d by gas pellet surface. This is the h e a t 14, the gas w o u l d emit itself if it (3) is the d i f f e r e n c e b e t w e e n t w o

T h e total h e a t a m o u n t transferred to the pellet bed by gas r a d i a t i o n (2) a n d b r i c k w o r k r a d i a t i o n (3) is then:

A c c o r d i n g to calculations the m a j o r p o r t i o n of h e a t is t r a n s f e r r e d via solids r a d i a t i o n a n d not via gas radiation. C o m p a r e d to the travelling grate and s h a f t f u r n a c e the h e a t c o n d u c t i o n by r a d i a t i o n is a b o u t t h r e e times as h i g h in the rotary kiln w h e r e a s the convective h e a t t r a n s f e r is p r e d o m i n a n t in the s h a f t f u r n a c e and on the travelling grate, T a b l e 35.

13.2.5 H e a t C o n d u c t i o n As shown in Figs. 136 and 138 there is yet a n o t h e r possibility of h e a t transfer by direct heat conduction. It can be effective in two ways: By c o n d u c t i n g t h e h e a t f r o m the pellet s u r f a c e to the core and b y direct contact f r o m pellet to pellet at d i f f e r e n t t e m p e r a t u r e s by points of contact. 13.2.5.1 H e a t C o n d u c t i o n Inside the Pellet T h e heat c o n d u c t i o n inside a n i n d i v i d u a l pellet is i m p o r t a n t b e c a u s e it influences the necessary m i n i m u m firing t i m e T m i n . T h i s is the t i m e n e e d e d to t r a n s f e r the heat, r e q u i r e d for p r o p e r i n d u r a t i o n f r o m the pellet s u r f a c e to the pellet core. In the case of the various h e a t t r a n s f e r m e t h o d s described above it is supposed that the pellets are already dry and n o other t h e r m a l influences b e c o m e active. T h e pellet is h e a t e d in a s u r r o u n d i n g gas m e d i u m of a specific t e m p e r a t u r e . T h i s t e m p e r a t u r e is not constant, but varies with d i f f e r e n t velocities a c c o r d i n g to process a p p l i c a t i o n in s h a f t furnaces, rotary kilns or travelling grate plants. F o r pellet h e a t i n g at a t e m p o r a l l y varying a m b i e n t t e m p e r a t u r e , e q u a t i o n s , whose e v a l u a t i o n is very c o m p l i c a t e d , were d e v e l o p e d 155 ). Starting f r o m a constant a m b i e n t t e m p e r a t u r e , it is possible to ascertain by an a p p r o x i m a t i o n 148) a theo-

13.3 Green Ball Drying

279

Fig. 140. M i n i m u m firing t i m e r e q u i r e d f o r h e a t t r a n s f e r i n t o single p e l l e t s of d i f f e r e n t size

retical m i n i m u m firing t i m e for d i f f e r e n t pellet d i a m e t e r s limited to a m e a n calorific value, as is d i a g r a m a t i c a l l y d e m o n s t r a t e d in Fig. 140. 13.2.5.2 H e a t C o n d u c t i o n by P o i n t s of C o n t a c t H e a t c o n d u c t i o n by points of contact is only possible if a d j a c e n t pellets are of d i f f e r e n t t e m p e r a t u r e s . Since on the travelling g r a t e the i n d i v i d u a l pellets have a relatively restful r e l a t i o n s h i p to e a c h o t h e r , the i n d i v i d u a l pellets situated at the s a m e level are s u p p l i e d with the s a m e a m o u n t of heat so that b e t w e e n a d j a c e n t pellets the necessary t e m p e r a t u r e d r o p d o e s not really exist, but only for pellets in two layers o n e a b o v e the other. In the shaft f u r n a c e and rotary kiln, the pellets are m o v i n g a n d the points of contact c h a n g e continuously. In the rotary kiln, a t e m p e r a t u r e b a l a n c e can be achieved b y the m i x i n g of hot pellets with colder ones d u r i n g the rolling m o v e m e n t . H o w e v e r , it is m o r e i m p o r t a n t that the rolling m o v e ment causes cooler pellets on the b e d s u r f a c e to be constantly f u r t h e r heated by gas a n d b r i c k w o r k r a d i a t i o n .

13.3 Green Ball Drying T o satisfy the relevant t h e r m a l r e q u i r e m e n t s it is necessary to c o o r d i nate the h e a t s u p p l y and the t e m p e r a t u r e of the h e a t i n g gas. T h i s is of particular i m p o r t a n c e d u r i n g g r e e n ball drying, b e c a u s e a n a d d i t i o n a l factor is to be considered, n a m e l y the w a t e r e v a p o r a t i o n .

280

13 Some Theoretical Considerations

U n d e r items 2.2.1 and 5.4.1, i m p o r t a n t factors for successful d r y i n g were e x p e r i m e n t a l l y investigated a n d described. In the light of the preceding considerations, the k n o w l e d g e of h e a t transfer specifically for drying is of great interest. In this connection, drying only m e a n s the expulsion of t h e w a t e r r e q u i r e d for green ball f o r m a t i o n , w h i c h is contained in t h e grain surface a n d in the capillaries b e t w e e n t h e individual grains. In the three w e l l - k n o w n firing processes, the drying is exclusively carried out w i t h hot gases flowing t h r o u g h the pellet charge. T h e r e f o r e , the following c o n s i d e r a t i o n s are limited to heat supply by convection of a ball charge at a p r e p o n d e r a n t l y t u r b u l e n t f l o w of heating gases. T h y drying process starts s i m u l t a n e o u s l y on a n i n d i v i d u a l pellet a n d in the pellet charge. F o r better u n d e r s t a n d i n g it is c o n s i d e r e d separately.

1 3 . 3 . 1 D r y i n g o f an Individual P e l l e t T h e drying process of a moist p o r o u s b o d y c o r r e s p o n d i n g to a n i n d i v i d u a l pellet is d i a g r a m a t i c a l l y shown in Fig. 141. H o t gas of t e m p e r a t u r e TD flows t o w a r d s the moist pellet, w h i c h has an initial m a s s of MW and the h e a t of the gas causes part of the w a t e r to e v a p o r a t e f r o m the pellet surface. T h e drying process results f r o m t h e c h a n g e in weight of the drying pellet. D r y i n g is t e r m i n a t e d w h e n the w e i g h t MD of the pellet no longer shows any change. T h e pellet m o i s t u r e X can then be a s c e r t a i n e d according to the following e q u a t i o n :

which c o r r e s p o n d s to the d i f f e r e n c e b e t w e e n wet and dry mass d i v i d e d by dry mass. In practical o p e r a t i o n , the m o i s t u r e content is o f t e n related to the wet mass, as is described u n d e r item 2.2.1.1. T h e drying p r o c e e d s f r o m

Fig. 141. Influence of hot gas flow on drying of a green pellet

13.3 Green Ball Drying

281

Fig. 142. D r y i n g p r o g r e s s of a m o i s t p e l l e t

the surface in concentric "skins", s h o w n as d o t t e d rings in Fig. 141, t h r o u g h various stages into the pellet core, as is also s h o w n in Fig. 15. T h e t i m e - r e l a t e d drying p r o c e d u r e X(t) is p r o p o r t i o n a l to w e i g h t decrease M(t) as can be seen f r o m Fig. 142 A. T h e greatest w e i g h t reduction takes place in p h a s e I, see Fig. 16. A s a result, p h a s e I also has the highest drying velocity mpontoDI(Fig. 142 B). It is r e p r e s e n t e d by the following e q u a t i o n :

If the s t e a m pressure on the pellet s u r f a c e PD is a p p r o x i m a t e l y e q u a t e d with the s a t u r a t e d s t e a m pressure PS at a s u r f a c e t e m p e r a t u r e TP e v a p o r a t i o n values of u p to 40 k g / m 2 h can be calculated f o r the individual pellet. A c o m p a r a b l e a m o u n t w o u l d be 1 0 0 - 1 5 0 k g / m 2 h for a sinter mix. In phase II, t h e water has a l r e a d y b e e n e v a p o r a t e d f r o m the surface. T h e drying f r o n t shifts t o w a r d s the pellet core. T h e s t e a m arising m u s t cover a longer d i f f u s i o n distance S t h r o u g h the capillaries w h i c h are

297 13 Some Theoretical Considerations

Fig. 143. Relation between drying gas and pellet surface temperature

already dry. C o n s e q u e n t l y , the drying velocity slows d o w n accordingly, m D I I . It d r o p s to zero w h e n the w a t e r in the capillaries also evaporates a n d a constant w e i g h t MDR is r e a c h e d , Fig. 142 B . If c o m b i n e d water is still present, p h a s e III, w e i g h t variations h a v e to be detected by using sensitive thermo-scales. T h e drying p r o c e d u r e d u r i n g the individual p h a s e s also results in a change in the pellet surface t e m p e r a t u r e , which c o r r e s p o n d s to the d r y i n g gas t e m p e r a t u r e , Fig. 142 C. F o r e v a p o r a t i o n of water f r o m hydrates, h i g h e r t e m p e r a t u r e s are necessary, p h a s e III. D u r i n g drying, the specific s u r f a c e t e m p e r a t u r e of pellets is o b t a i n e d for each relevant drying gas t e m p e r a t u r e . T h e s e t e m p e r a t u r e s are s h o w n in Fig. 143 for two d i f f e r e n t c l i m a t i c zones according to calculations by O. Krischer 156 ). 1 3 . 3 . 2 D r y i n g of a P e l l e t C h a r g e W h e n the d r y i n g takes place by a hot gas flow t h r o u g h a pellet charge, the f u n d a m e n t a l rules 157 ) d o not c h a n g e c o m p a r e d to the d r y i n g of a single pellet. H o w e v e r , the m a t h e m a t i c a l correlations b e c o m e m o r e c o m plicated, since the c o n d i t i o n of hot drying gases changes according to t i m e

13.3 Green Ball Drying

283

a n d place w h e n they enter the pellet charge. T h e drying p r o c e d u r e in three s u p e r i m p o s e d pellet layers is s h o w n in Fig. 144 A - E for consecutive drying steps u n d e r e q u a l conditions of d r y i n g gases. T h e drying p r o c e e d s in three phases I - I I b u t has a l r e a d y b e e n t e r m i n a t e d a f t e r p h a s e II w h e n it is a s s u m e d that only a d h e r i n g w a t e r w o u l d exist. In the initial stage A, b e f o r e drying starts, the entire layer is u n i f o r m l y moist, t e m p e r a t u r e and m o i s t u r e content are u n c h a n g e d , TA , XA . In step B, the drying has c o m m e n c e d . T h e drying gas with the t e m p e r a t u r e TDR a n d a m o i s t u r e content φDR flows u p w a r d s t h r o u g h the pellet bed. T h e m o i s t u r e content f r o m the b o t t o m layer has partly been expelled a n d the m o i s t u r e content is smaller t h a n XA. T h e d r y i n g air is o v e r s a t u r a t e d with s t e a m a n d the excess s t e a m condenses on t h e second, the cooler pellet layer, w h e r e b y its m o i s t u r e content b e c o m e s h i g h e r t h a n it initially was. T h e surface t e m p e r a t u r e is increased f r o m TA to TP by c o n d e n s a t i o n heat. T h e third a n d the following layers are u n c h a n g e d u n d e r given conditions. T h e e x h a u s t gas is nearly s a t u r a t e d . T h e drying process continues in step C. N o w the b o t t o m layer is already dry, the central layer is partially d r i e d a n d the t o p layer is overwetted. T h e exhaust gas is still nearly s a t u r a t e d . T h e pellet t e m p e r a ture is a b o u t the s a m e as in step B. At a f u r t h e r drying gas supply, the b e d b e c o m e s c o m p l e t e l y dry, D to E. T h e surface t e m p e r a t u r e of the pellets a n d t h e d r y i n g gas t e m p e r a t u r e as well as the h u m i d i t y of the exhaust a n d d r y i n g gas h a v e b a l a n c e d out. In an u n m o v e d b e d with m a n y pellet layers t h r o u g h w h i c h hot d r y i n g gases

Fig. 144. Water content variation in a green pellet layer during drying

284

13 Some Theoretical Considerations

are flowing, e v a p o r a t i o n velocity, drying t i m e a n d t e m p e r a t u r e g r a d i e n t can be calculated on the basis of similar conditions. T h e s p a c e - t i m e d e v e l o p m e n t of the d r y i n g process in a g r e e n ball layer can be s i m u l a t e d in a step-wise calculating p r o c e d u r e . O n the basis of a c o m p u t e r s i m u l a t i o n 1 5 8 ) , such a result is d i a g r a m a t i c a l l y illustrated in Fig. 145. T h e pellet charge of 30 cm thickness is d i v i d e d into 30 partial layers y, each 1 cm thick. T h e h e a t b a l a n c e for each p a r t i a l layer was established: " C o o l i n g of hot drying air = h e a t i n g of pellets + w a t e r e v a p o r a t i o n " . T h e drying s i t u a t i o n was ascertained a f t e r 1—4—7 m i n u t e s f r o m the m o m e n t of drying gas inflow in one direction as " u p d r a u g h t d r y i n g " . T h e following curves give i n f o r m a t i o n concerning the v a r i o u s d r y i n g times and the respective b e d heights: (a) the s i t u a t i o n a f t e r one minute of drying: (a) T h e t e m p e r a t u r e in the pellet b e d j u s t a b o v e the h e a r t h layer rapidly d r o p s f r o m a b o u t 175 0 C to a b o u t 40 0 C at a b e d h e i g h t of 10 cm f r o m b o t t o m a n d r e m a i n s u n c h a n g e d for the rest of the charge.

Fig. 145. Influence of time on the drying process of a green pellet bed

13.3 Green Ball Drying

285

(β) T h e a v e r a g e m o i s t u r e content in the lower p a r t of the b e d is less than the m o i s t u r e content of the g r e e n pellets but f r o m a level of a b o v e 10cm of bed height the m o i s t u r e c o n t e n t is h i g h e r t h a n that of green pellets. (dúvida) T h e h u m i d i t y of the d r y i n g gas is increasing a n d the gas is saturated at a b o u t 20 cm of bed height. (b) T h e situation a f t e r four minutes of drying: (a) A g r e a t e r part of the pellet b e d a l r e a d y has a h i g h e r t e m p e r a t u r e . But at the t o p of the bed the t e m p e r a t u r e of 40 0 C is still p r e v a i l i n g as before. (β) T h e pellet bed is already dry for the first 12 cm f r o m c h a r g e bottom. F r o m this level the m o i s t u r e content increases and exceeds the n o r m a l value just below the t o p level of the pellet bed. (ɣ) O n the b e d s u r f a c e the drying gas c o n t a i n s a b o u t 95% h u m i d i t y . (c) T h e situation a f t e r seven minutes d r y i n g is as follows: (a) T h e t e m p e r a t u r e in the b e d rises a n d r e a c h e s a b o u t 75 0 C on the bed surface. (β) T h e pellet bed is almost d r y o v e r a h e i g h t of a b o u t 25 cm. (ɣ) T h e h u m i d i t y of the escaping gas is not h i g h e r t h a n that of the incoming drying gas. T h e c o m p u t e d values explained a b o v e in Fig. 145 s h o w a g o o d conformity with the c o r r e s p o n d i n g m e a s u r e d values s h o w n in Figs. 1 7 , 1 8 and 144. Since the rules of the p r o p e r drying p r o c e d u r e s are k n o w n , it is possible to m a k e available dry pellets of good q u a l i t y for f u r t h e r t h e r m a l treatment.

Final Remarks

F o r f i n e - g r a i n e d i r o n o r e particles, pelletizing is an efficient alternative to d o w n - d r a u g h t sintering. D e s p i t e the generally a c k n o w l e d g e d f a v o u r e d pellet properties, the initially great expectations concerning the i m p r o v e ment of the blast f u r n a c e o p e r a t i o n were w o r l d w i d e not equally realized a l t h o u g h the metallurgical b e h a v i o u r of pellets is, in general, better t h a n that of m a n y l u m p ores and practically e q u i v a l e n t to that of sinter. Fig. 14647sobrescrito)explains the d i f f e r e n t b e h a v i o u r of l u m p ores, sinter a n d pellets of various basicity d u r i n g reduction. T h e u n i f o r m spherical s h a p e of the pellets c a u s e d s o m e difficulties by segregation f r o m the o t h e r c o m p o n e n t s of the blast f u r n a c e b u r d e n w h e n m a j o r quantities were used. T h i s p r o b l e m was o v e r c o m e b y the a p p l i c a -

Fig. 146. Reducibility of different ores, sinter and pellet types - correlation between two different testing methods

Final Remarks

287

tion of a m o r e s u i t a b l e charging t e c h n i q u e in w h i c h m a t e r i a l of s i m i l a r size and s h a p e is c h a r g e d in c o r r e s p o n d i n g layers. A n o t h e r a l t e r n a t i v e was to crush pellets of large size into pieces a n d to c h a r g e these pieces into the blast f u r n a c e 159). By c o m p a r i n g the b e h a v i o u r of this material with sinter a n d u n c r u s h e d pellets of s m a l l e r size, s o m e b e n e f i t s were f o u n d . T h e r e f o r e a s h a p e o t h e r t h a n the spherical s h a p e could be i m a g i n a b l e . T h e use of pellets is, no d o u b t , of decisive i m p o r t a n c e for the i n t r o d u c tion and progress of the direct r e d u c t i o n processes. In this c o n n e c t i o n a r e m a r k a b l e c o n t r i b u t i o n is m a d e to the successful d e v e l o p m e n t of the steel plants w i t h lower capacity (Mini-steel plants) in w h i c h steel is directly p r o d u c e d , bypassing the blast f u r n a c e . T h i s d e v e l o p m e n t is only at the beginning a n d will be of increasing i m p o r t a n c e for m a n y countries in the future. Since pelletizing is closely c o n n e c t e d with these new steel technologies, it will p a r t i c i p a t e in the f u t u r e g r o w t h accordingly. T h e a d v a n t a g e of the lower heat c o n s u m p t i o n for pellet firing as c o m p a r e d with sintering decreases in view of the fast increasing fuel oil prices. A t t e m p t s to partly replace the fuel oil by coal s h o u l d be continued 6 7 / 6 8 ). T h e ever increasing costs for a g g l o m e r a t i o n lead to c o n s i d e r a t i o n s for direct processing of f i n e - g r a i n e d iron ore particles into steel w i t h o u t agglomeration. Efforts in this d i r e c t i o n s h o u l d be i n t e n s i f i e d even if a notable success a p p e a r s h a r d l y realisable for the present.

References

Chapter 1. Definition and Development of Pelletizing Process 1. Bennet, R. L.; Lopez, R. D.: Agglomeration of iron ore concentrates. Chem. Eng. Progr. Sympos. Ser. (1963) 40—51 2. Cappel, F.; Wendeborn, H.: Sintern von Eisenerzen. Düsseldorf: StahleisenBücher, Bd. 19, Stahleisen 1973 3. Andersson, A. G.: Swedish patent No. 35124, Class 29 : a 1912 4. Brackelsberg, C. A.: Die Herstellung von Agglomeratkugeln. Sonderdruck aus Tech. Mitt. H. 15(1916) 1 - 8 5. Hüttemann, P.: Diskussionsbeitrag Stahl und Eisen 72 (1952) 1577-1579 6. Haley, K. M.; Apuli, W. E.: Pelletizing on a horizontal grate machine. Agglomeration (ed. W. A. Knepper) New York: Interscience 1962 pp. 931—964 7. Tigerschiöld, M.: Aspects on pelletizing of iron ore concentrates JISI May 1954. Special meeting in Sweden, pp. 13—24 8. Stalhed, J.: Die Herstellung von Eisenschwamm nach dem Wiberg-Verfahren. Stahl und Eisen 72 (1952) 459-466 9. Firth, C. V.: Agglomeration of fine iron ores. Proc. AIME, Blast Furn. Coke Oven Raw Mater 4, 46 (1944) 4 4 - 6 9 10. Bennet, R. L.; Hägen, R. E.; Mielke, M. V.: Nodulizing iron ores and concentrates at extaca. Min. Eng. 6 (1954) 3 2 - 3 8 11. Ramsey, R. H.: Teamwork on taconites. Eng. Min. J. 156 (1955) 7 2 - 9 3 12. Wendeborn, H.: Neuere Enwicklungen der Bandsinteranlagen. Stahl und Eisen 71 (1951) 1212-1218 13. Wenzel, W.; Gudenau, H. W.; Moeljono, I.: Filtrieren und Sintern von Feinerzschlämmen. Stahl und Eisen 93 (1973) 4 9 - 5 5 14. Meyer, K.: Stand der Entwicklung der Pelletierung von Eisenerzen, Stahl und Eisen 82 (1962) 147-154 15. Brandes, G.; Rausch, H.: Mixing and Conditioning of Sinterplant feed. Proc. AIME, Blast Furn. Coke Oven Raw Mater. 18 (1959) 233-249 16. Violetta, D. C.: Updraft pelletizing of specular hematite concentrates. J. Metals 10 (1958) 118-121 Hamilton, F. L.; Mean, H. F.: Production and properties of experimental pellet-sinter. Proc. AIME, Blast Furn. Coke Oven Raw Mater. (1955) 1 - 8 17. Rausch, H.; Meyer, K.: Einfluß der Mischfeuerung auf die Betriebsergebnisse von Sinteranlagen. Stahl und Eisen 78, 9 (1958) 600-606 Rausch, H.; Cappel, F.: Comparison between conventional hot air and mixed fired sinter. Agglomeration (ed. W. A. Knepper). New York: Interscience 1962, p p . 4 5 5 - 4 8 0

18. Meyer, K.; Rausch, H.: The Lurgi pelletizing process. J. Metals 10 (1958) 129-133

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19. Dailey, W. H.: Blast furnace performance pellets - vs - sinter. Meeting of Association of Iron & Steel Engineers Pittsburgh Feb. 1962 Kaas, W.; Gerstenberg, B.: Ermittelung der Kaltdruckfestigkeit an Hochofcn Pellets nach unterschiedlichen Prüfverfahren. Stahl und Eisen 99 (1979) 513-517 20. Petersen, U.; Kahlhöfer, H.; Send, A.: Betriebsversuche zur Leistungsteigerung des Hochofens durch Einsatz von Pellets. Stahl und Eisen 83 (1963) 1397-1407 21. Meyer, K.: Entwicklung der Pelletierung von Eisenerzen und Herstellung von Mischerzpellets. Stahl und Eisen 83 (1963) 1337-1344 22. Koen, W.: Two years of pelletizing at hoogovens. Metals Society London 1972, pp. 1 2 - 1 6 Chapter 2. Fundamentals of Pelletizing 23. Rumpf, H.: The strength of granules and agglomerates. Agglomeration (ed. W. A. Knepper). New York: Interscience 1962, pp. 3 7 9 - 4 1 8 24. Tigerschiöld, M.; Ilmoni, P.: Fundamental factors influencing the strength of green and burned Pellets made from fine magnetite-ore concentrates. Proc. Blast Furn., Coke Oven Raw Mater., Session on Ore and Agglomeration 9 (1950) 18-45 25. Sastry, K. V. S.; Fuerstenau, D. W.: Mechanism of agglomerate growth in green pelletizing. Powder Technol. 7 (1973) 9 7 - 1 0 5 26. Linkson, P. B.; Glastonbury, J. R.; Duffy, G. J.: The mechanism of granular growth in wet pelletizing. Trans. Inst. Chem. Eng. 51 (1973) 2 5 1 - 2 5 9 Bhrany, U. N.; Johnson, R. T.; Myron, T. L.; Pelczarski, E. A.: Dynamics of pelletization. Agglomeration (ed. W. A. Knepper). New York: Interscience 1962, pp. 229-259 Newitt, D. M.; Conway-Jones, J. M.: A contribution to the theory and practice of granulation. Trans. Inst. Chem. Eng. 36 (1958) 4 2 2 - 4 4 2 Schubert, H.: Tensile strength and capillary pressure of moist agglomerates. Agglomeration 77, AIME New York 1977, Chapt. 9, pp. 144-155 27. Krischer, O.; Jaeschke, L.: Trocknunssverlauf in durchströmten Haufwerken Chem. Ing. Techn. 33 (1961) 5 9 2 - 5 9 8 " Krischer, O.; Kast, W.: Die wissenschaftlichen Grundlagen der Trocknungstechnik, Vol. 1, 3rd. ed. Berlin, Heidelberg, New York: Springer 1978, pp. 258-263. 28. Information from the Laboratories and Engineering Department of Lurgi Chemie und Hüttentechnik Frankfurt/Main, F R G 29. Pietsch, W.: Die Festigkeit von Granulaten mit Salzbrückenbindung und ihre Beeinflussung durch das Trocknungsverhalten. Doctorthesis. Aufbereitungstech. (1967) 297-307 30. Vitiyugin, V. M.; Leonteva, T. G.; Trofimov, V. A.: Thermal stability of iron ore pellets during dicing. Steel USSR (1971) 5 8 7 - 5 8 9 31. Westenberger, H.: Über Phasenbestand, Kornbindung und Gefüge gebrannter Eisenerzpellets. Diplom Ingenieur Thesis. Johann Gutenberg-Universität Mainz 1967 32. Urich, D. M.; Tsu, Ming Han: A progress report on the effect of grind, temperature and pellet size upon the quality of specular hematite pellets. Agglomeration (ed. W. A. Knepper). New York: Interscience, pp. 669-719 33. Yefimenko, G. G. et al.: Thermal dissociation of hematite during oxidizing firing of iron fluxed briquettes. Steel USSR (1977) 3 0 7 - 3 0 8

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142/44. Sommer, K.; Herrmann, W.: Auslegung von Granulierteller und Granuliertrommel. Chem. Ing. Tech. (1979) 266-277 143. Rumpf, H.: Grundlagen und Methoden des Granulierens. Chem. Ina. Tech. (1958) 144-158 Rumpf, H.; Ebert, K. F.: Darstellung von Komverteilung und Berechnung der spezifischen Oberfläche. Chem. Ing. Tech. (1964) 523-537 (Special print) 144. Tarjan, G.: Der Einfluß von Bewegungskräften auf die Granulation. Aufbereitungstech. 1 (1966) 2 8 - 3 2 145. Pietsch, W.: Die Beeinflussungsmöglichkeiten des Granulierbetriebes und ihre Auswirkungen auf die Granulateigenschaften. Aufbereitungstech. (1966) 177-191 Papadakis, M.; Bombled, J. P.: La granulation des matieres premieres de cimenterie. Rev. Mater. Constr. Trav. Publics. (1961) 2 8 9 - 2 9 9 146. Klatt, H.: Die betriebliche Einstellung von Granuliertellern. Zement-KalkGips 11 (1958) 144-154 147. Molerus, O.: Neue Druckverlustgleichung für die Schüttungsdurchströmung im laminaren und im Übergangsbereich. VDI-GVC Fachsitzung „Mehrphasenströmung", Düsseldorf 1976 Marincic, L.; Gerstenberg, B.: Untersuchungen über die Struktur und den Druckverlust von Kugel- und Möllerschüttungen unterschiedlicher Kornbandbreiten. Stahl und Eisen 98 (1978) 1353-1359 148. Jeschar, R.: Druckverlust und Mehrkornschüttungen aus Kugeln. Arch. Eisenhüttenwes. 35 (1964) 9 1 - 1 0 8 VDI-Wärmeatlas, 2nd ed. 1974, pp. L E 1 - L E 2 149. Brauer. H.: Die Eigenschaften der Zweiphasenströmung bei der Rektifikation in Füllkörpersäulen. Dechema Monographien Vol. 37, pp. 7—78 150. Jeschar, R.: Analogie zwischen Wärme und Stoffübergang in Schüttungen aus Kugeln. Arch. Eisenhüttenwes. (1964) 955-961 Heiligenstaedt, W.: Wärmetechnische Rechnungen für Industrieöfen. Düsseldorf: Stahleisen 1966 151. Hottel, H. C.; Sarofim, A. F.: Radiative transfer. New York: Mc. Graw-Hill 1967 152. Lichtenberger, H.: Flammenstrahlung und Wärmeabbau in Feuerräumen. Techn. Mitt. 3 (1972) 110-115 153. God, Ch.: Berechnung der Strahlungswärmeübertragung von Verbrennungsgasen. Radex Rundsck 4 (1974) 2 5 0 - 2 7 3 154. Schack, A.: Der industrielle Wärmeübergang, 7th ed. 155. Carslaw, H. P.; Jaeger, J. C.: Conduction of heat in solids, 2nd ed. Oxford, pp. 230-254 156. Krischer, 0 . ; Jaeschke, L.: Trocknungsverlauf in durchströmten Haufwerken. Chem. Ing. Tech. 33 (1961) 592-598 157. Kroell, K.: Trockner und Trocknungsverfahren, 2nd ed. Berlin, Heidelberg, New York: Springer 1959, pp. 2 3 8 - 2 5 3 158. Computer Centre of Lurgi; F R G Final Remarks 159. Onoda, M.; Takenada, Y.: Kawaguchi, F.; Fujita, J.: Physical and metallurgical properties of crushed pellets. Trans. ISIJ, 18 (1978) 6 1 1 - 6 1 7

Subject Index

Additives Alkali compounds 56, 127, 189, 190 Bentonite 53-55, 110-115 analysis, composition 54,55 influence of green pellet behaviour 111 of dry pellet behaviour 112 of indurated pellet behaviour 113 of drop numbers 112 types 113,114 Calcium carbonate 55,56,115, 121-125,150 chloride 56, 115, 126, 183-188 hydroxide 55,56,115-120,150-153, 170-176 oxide 55, 115,116 Dolomite 115, 171, 175,176 Ores 127,128 Organic compounds 136, 137 Return material 131,132 Silica-Silicates 53,171-176 Sponge iron 131-135 Agglomerates, other Briquettes 3,4 Nodules 8 Pellet sinter 13 Sinter 4 - 6 , 9 - 1 5 , 2 2 9 , 2 3 3 , 2 3 5 , 253-256,286,287 Binders, see additives Equipment - Operation Ore preparation Agitating vessel 65 Air classifier 78 Air-plane-tire 78

BalIing cone 210 disc 7 7 - 7 9 , 2 0 2 - 2 0 9 capacity 207-209 diameter 202,207 dimensions 202 operation 203 rotating speed 202,206 scraper 202 side wall 202,207 slope 202,205 drum 77, 196-202,208,209 capacity 201 circuit 198 dimensions 197-199 operation 197,198 rotating speed 198-200 scraper 197,200 slope 199-201 units, others 78,210,211 Crusher cone crusher 73 gyratory crusher 73 Dewatering 66 Drying drum 66,67 Electrostatic separator 60,73 Extruder 54 Filter 66 aids 66 cake 76,77 clothing 66 Flotation 71,78 Gravity separation 57 Humphrey's spirals 57 Reichert cone 57 Grinding 64, 65 balls 64 circuit 64,65,74 costs 64 drum 64,65,74,75 energy 63,75,76

Subject Index Iiberation grinding 59,64 mill 64,75 pebbles 64,74 rods 64, 74 surface grinding 64 Homogenizer 192, 193 Hydrocyclone 64,65 Magnetizing roasting 59, 60,72, 73 in fluid bed 59 in rotary kiln 59 in shaft furnace 59 Magnets, drum 58. 59 high. Iow intensity 59,72 wet, dry 59,73 Mixer batch type mixer 79, 193 fluffer mixer 77 passage mixer 193 pekay mixer 193 screw mixer 198,203 Screens roller screens 198, 209,212,213,237 vibrating screens 198,209 Transportation, handling belt conveyor 198 roller conveyor 212,213,237 rolling belt conveyor 213 Equipment - Operation Pellet induration Annular furnace 243 Circular grate 242 Grate-kiln 215,223-229 charging device 224 cooler 226,229 dimensions 228,229 gas flow 224 heat consumption 226 treatment 224,225 kiln, rotary 225,229 travelling grate 224,229 Heat-Fast system 242 Shaft furnace 215,216-223 charging, feeding 219 chunk breaker 217,222 cooler 221 dimensions 222 firing, heating 220 gas flow 222 long furnace type 217 medium furnace type 218 operation, heat consumption 221, 222

299

Travelling grates 215 general features 231 Lurgi-Dravo system 235 charging device 237 cooler 239 dimensions 241 gas flow 239 hearth-side-layer 238 heat treatment 239 operation, heat consumption 238-240 pallets 238 McKeesystem 233-235 charging device 233,234 cooler 234, 235 dimensions 235 gas flow 234 heat consumption 233,234 treatment 234,235 Up-draught travelling grate 232, 233 Fuels, used Anthracite 138 Coals 138,225,232,233 Coke breeze 13,14,139 Fuel oil 222,225,233,239 Magnetic oxidation 220,224, 233,239 Natural gas 222,225,239 Iron bearing minerals Iron bearing material, secondary sources blast furnace dust 52,136 slag 52, 136 BOF dust 52, 136 slag 52, 136 inplant fines 52, 135, 136 iron ores, bearing tramp elements 48, 52, 184-191 leached products 189,190 Mill scale 52, 136 Pyrite. Pyrrhotite cinders 52, 184-188 return material 131,132 Iron compounds Chalcopyrite 52 Goethite 50 Hematite, concentrate 49, 62,81, 117, 134, 137,139, 147, 151,152,154,166 Hematite-Limonite 62,81 Hydrohematite 50

300

Subject Index

Ilmenite 189 Iron glance 49 Lepidocrocite 50 Limonite 50 Magnetite, natural concentrates 6, 7,48,62,81, 122, 125, 135, 137, 139, 146, 148, 149, 155, 156, 181 artificial concentrates 52, 59, 60, 149, 150 Magnetite-Hematite 62,81 Martite 49 Pentlandite 52 Pyrite 50 Pyrrhosiderite 50 Pyrrhotite 52 Siderite 50 Iron compounds, Products Calciumferrites 153 Fayalite 175 Wustite 166, 167 Iron ore type and some characteristic names Bomi Hill (Magnetite), Liberia 129 Chowgule (Hematite, Limonite), Goa, India 51 CVRD (Hematite), Brasil 22,51, 101,129 Earthy hematite 49 Hamersley (Hematite), W. Australia 22,51 Itabirite 48 Jaspilite 48 Kiruna(Magnetite), Sweden 129 Krivoj rog (Magnetite), USSR 129 Manoriver(Hematite) 129 Marcona (Magnetite), Peru 129 Minette 48 Robe river (Limonite), W. Australia 51 Specular hematite 49 Sydvaranger (Magnetite), Norway 129 Taconites 48 Venezuela fines (Hematite), Venezuela 12,51 Laboratory Application range 6 8 - 7 0 Davis tube 72 Flow sheet for magnetic separation 73 for ore preparation 70 for ore washing 71

Furnace for orienting tests 84 Pot grates 8 5 - 8 8 Surface determination of fine grained ores 7 4 - 7 5 B.E.T. method 63 Blaine method 74 Fisher subsieve sizer 75 Svenson method 74 Testing of pellet quality crushing strength 80,90 drop numbers 81 resistance 81 ; microporosity 91 moisture determination 79 tumbler resistance 90 Testing of pellets during reduction 91-98 reduction in blast furnace 9 3 - 9 5 crushing strength 90 Low-Temperature Disintegration test, static 93 Low-Temperature Disintegration test, dynamic 94 Reduction under Load test (RuL) 94 Reduction in furnace for direct reduction 9 6 - 9 8 Direct Reduction Disintegration Stability test(DRDS) 96 Low-Temperature Disintegration test, dynamic 96 Sticking test(RMC) 96 Swelling test 96 Sometimes used other methods 95 Gakushin test 95 Linder test 94 Pellets in blast furnace burden 2, 5, 20,21 Charging 253 Chemical composition 160-162 Mechanical properties 251 Pellet proportion, future trend 255,256 Pellets and Sinter 253-255 Pellets, definition 3 development stages 5 - 1 5 properties 17,81,89,97 size 17 distribution 17 transportability 20 typical plant locations 20

Subject Index Pellets for direct reduction 5,18, 19, 257,258 Future trend 258 Pig iron 5 Process features Ball formation, green pellets 24-28, 99-140,263-267 additives, binders 81,109-137, 140 balling alternatives, compacting 25,28 bonding forces 26—28,263—265 bridge formation, capillary range 26, 263-265 ' transition range 26,263-265 capillary bonding forces 24,26, 263-265 electrostatic bonding forces 24 magnetic bonding forces 24 surface tension bonding forces 24, 27,263,265 Van der Waals bonding forces 24 bulk density 83 crushing strength 80, 111, 112,114, 118, 119, 122, 125, 126, 128, 133, 137, 139,263,265 drop numbers 81, 112, 114, 119, 128 resistance 81 grain size 62-64, 100-104,265-267 distribution 62-64, 100-104,266 handling, screening, transportation 198,212,213,219,224,232,234, 237 moisture 26-28,79, 105-110, 263-265 saturation 265 rolling forces 25,27,28, 194, 195, 200,201,204,266,267 specific surface area 62-64,74,75, 100-104, 117-121,126 Benefication chlorination, chloridizing 183-188 electrostatic separation 60,61,73 elimination, recovery of trampelements 183-191 flotation 58,61,71 gravity separation 57,61,71 magnetic separation 58,59,71,72 magnetizing roasting 59,60,72 washing 57,61,70,71

301

Drying of green pellets crushing strength, dry pellet 36, 81, 108, 111, 112, 114, 119, 125 126,128, 133, 137, 144, 146 shock temperature 37,142 resistance 142 drying gas, flow direction 34, 35 gas temperature 37,141,146,281, 282 gas velocity 143, 146 of green pellet layer 3 3 - 3 5 , 141-146, 282-285 of individual balls 3 0 - 3 3 , 2 8 0 - 2 8 2 stages of individual balls 31, 32, 280, 282 time 141, 143, 156, 157 Induration of pellets by binders 181-183 Cobo, MTU process 182,183 Grangcold process 181 Induration of pellets by heat treatment abrasion 81,91, 103, 113, 120,126, 128, 130, 137, 160, 187 additives 45, 113, 120, 123, 125, 126, 128, 130, 134, 135,137, 139, 187, 189, 190 bonding forces, calcium ferrites 44, 153 crystal change 39,40,41 crystal growth 41, 42, 45 intergranular phases 4 3 - 4 5 cooling 45,46, 156, 157, 185, 186, 217,218,221,222,224, 226,232, 234,235,239, 242 crushing strength, see additives, 81, 90, 102, 103, 113, 120, 123, 128, 130, 147, 150, 152 firing, after 156,157 atmosphere 149 temperature 4 0 - 4 3 , 113, 123, 132, 134, 135, 147, 150-152, 156, 157, 184, 186, 189,190,220,222,224, 234,238,239 time 156,157,222,239 grain size, distribution 62-64, 100-103 hea ting pattern 155-158 hematite, decomposition 154,155 oxidation, Magnetite 148, 149 preheating 146,155-158 specific surface area, see process features, ball formation

302

Subject Index

Reduction, hematite pellets 159-178 structural change by additives 170-174 anisotropism 168, 169 basicity 173-176 crystal change 165-169 ferrites 176 gangue reaction 161,169,170,173 reduction temperature 176-178

Reduction, magnetite, wüstite pellets 178-181 reaction, metallic iron with oxides 164, 165 swelling 166-175 testing, see laboratory