Blast Furnace Iron Making, IIT,KGP, Oct 26, 2010

PRODUCTION OF LIQUID HOT METAL

Amit Chatterjee Adviser to the MD Tata Steel

IIT, Kharagpur, 26th October, 2010

PRODUCTION OF LIQUID HOT METAL

Amit Chatterjee Adviser to the MD Tata Steel

IIT, Kharagpur, 26th October, 2010

PART - I IRONMAKING IN BLAST FURNACES Present Scenario and General Features

IRON ORE TO LIQUID STEEL -

30 - Fe2O3

Iron ore

Direct molten steelmaking

%

n e 20 g y x O

10 -

Direct reduction

Blast furnace and Smelting Reduction Liquid steel

Remelting DRI, HBI

Scrap

0 Scrap

Refining

%

n5 o br a C -

| 0

Hot metal

| 200

| 400

| 600

| 800

| 1000

Temperature OC

| 1200

| 1400

| 1600

BLAST FURNACE IRONMAKING Amongst all the ironmaking processes, the blast furnace still holds the dominant position. The blast furnace has remained up-to-date competitive with the new technologies.

and

Hot metal production rates of 8000-10,000 tpd, fuel rates of around 450-470 kg/thm (270-275 kg coke plus 175-225 kg coal), furnace availability ranging between 95-98% and campaign life of 15-20 years are benchmarks today.

SIZE AND NUMBER OF BLAST FURNACES IN THE WORLD IN 2007 140   s 120    F    B 100    f   o 80   r   e    b 60   m 40   u    N 20

130

80 59

63 38

35

34

21 8

9

7

4

0   0   0   0  0    5  0  0   0  0  0    5  0  0   0  0  0    5  0  0   0  0  0    5  0  0   0  0  0    5  0  0   0  0    5  0   1  0   6   1   2   2   3   3   4   4    5    5              1   0   0   1   0  1   0  1   0  1   0  1   0  1    5   0  1   0  1   0  1   0  1    5    5  0   1  0    5   0    5   0    5   0    5   0   1   2   2   3   3   4   4    5

BF inner volume, m 3

In 2007, there were around 490 BFs varying in size from 500 to

CHANGE IN SIZE OF BLAST FURNACES AT POSCO, S.KOREA

BLAST FURNACES IN INDIA BF size, m3 (inner vol.)

No. of  Furnaces

Combined inner vol., ‘000 m3

Production, Mpta

> 3000

2

6.4

4.0

2000-3000

8

17.2

8.5

1500-1999

8

14.1

6.5

1000-1499

15

16.2

7.5

500-999

8

3.9

3.0

< 500

~ 30

~ 5.5

~ 2.5

Total

68

61

32

INCREASING BLAST FURNACE SIZE IN INDIA Steelworks Big BFs

Big BFs Under project planning stage

Inner vol., m3

Capacity, Mtpa

JWS

4019

3.0

Tata Steel - ‘G’ (after upgrdation)

2308

1.8

Tata Steel - ‘H’

3800

2.5

Tata Steel - ‘I’

3800

2.5

Vizag # 3

~ 4000

3.0

Bhilai # 0

~ 3800

2.7

IISCO

~ 3200

2.2

Bokaro 2 – rebuild

~ 2600

1.8

~ 4000 x 2

6.4

1680 x 2

3.0

Tata Steel – KPO Medium and small BFs under construction

JSW, JSPL, Bhusan

MACRO-FEATURES OF A BLAST FURNACE The furnace is a refractory lined steel shell filled with material viz. coke, iron ore, sinter, pellets, flux, etc. from the stockline down to the bottom. The process goes on continuously for  several years till the furnace is shut down for repairs and modification. The inputs and outputs are represented per  metric ton (i.e. tonne) of hot metal .

Preheated air at 1000-1250 O C is blown through tuyeres into the furnace. It may be enriched with some pure oxygen, moisture. Most modern furnaces also inject pulverised coal. Exothermic combustion of coke and coal by oxygen of air gasifies carbon into CO and also provides heat. The highest temperature zone of  the furnace (1900-2000OC) is at the level of tuyeres – the raceway.

BLAST FURNACE PLANT

BLAST FURNACE REFRACTORY LINING For containing heat, lining is important. It is subjected to: • Carbon monoxide attack. • Action of alkali and other vapours high temperature. • Abrasion by moving solid charges. • Attack by molten slag and metal. • Effect of furnace design and operation. Alumino-silicates and carbon are refractory materials most commonly employed for BF lining. Ordinary fireclay bricks containing 40-45% Al 2O3 are used in the upper stack. 60% Al 2O3 (known as high duty fireclay) is employed for lower stack, belly and bosh. There have been attempts to use silicon carbide bricks in the bosh region as inner refractory lining. Carbon is the popular refractory in the hearth. It has very high

HOT BLAST STOVES

HOT BLAST STOVES 25-40% of the total BF gas generated is consumed in pre-heating the blast in hot blast stoves. Each furnace has at least three stoves. The stove is a tall cylindrical (height 20-36 m, diameter 6-8 m) steel shell lined with insulating bricks inside. The interior of a stove has a combustion chamber, and a heat re-generator  unit, which consist of refractory bricks arranged as a checker work. As gases flows through the checker work, heat is exchanged with checker  bricks. The stoves operate in cycles. During heating cycle, the blast furnace gas is burnt with air in the combustion chamber. The hot flue gas heats up the bricks. This requires 2-4 hours. Then the combustion is stopped and air at room temperature is blown through the stove in the reverse direction. The air, blown by turbo-blowers, gets heated following contact with hot checker  bricks. Then flows into the blast furnace through tuyeres. This is the cooling cycle of the stoves. Lasts 1-2 hr. Since cooling is faster than heating, a minimum of 3 stoves are required – one on cooling and two on heating.

DEVELOPMENTS IN BF IRONMAKING

TECHNOLOGICAL IMPROVEMENTS IN BFs IN GERMANY AND EFFECT ON COKE RATE

IMPROVEMENTS IN BLAST FURNACES Maximum size of blast furnaces stabilised at about 15 m hearth diameter; inner volume of 5000-6000 m 3. Maximum productivity achieved 2.8-2.9 t/m 3  /day using conventional raw materials. Maximum output is 12000 tpd; equivalent to 4 Mtpa. Coke consumption (without coal injection or other fuel) is at best about 450 kg/thm, i.e 3.15 Gcal or 12.5 GJ, with recoverable excess BF gas of energy value 3-4 GJ. Iron ore beneficiation becoming mandatory for reduction of slag volume from 300-350 kg/thm to 200 kg or even 100-150 kg/thm using high grade pellets (66-68%).

OPERATIONAL FEATURES OF SOME BFs IN THE WORLD Parameter

Posco (S. Corus Korea) BF 6 (Netherlands) BF 7

Kimitsu 3 (Japan)

Nippon Steel (Japan)

G Blast Furnace (Tata Steel)

Production, t/day

8600

6750

10,233

10,051

5150

Working volume, m3

3225

2328

3790

NA

2308

Productivity, t/m3/day*

2.66

2.9

2.7

2.47

2.2

Top pressure, kg/cm2

2.5

1.67

2.25

2.2

1.3

Oxygen enrichment, %

1.6/2.0

4.9

4.0

2.4

4.6

Burden, % Sinter (S), Ore (O), Pellets (P)

85(S) 15(O)

50(S) 50(P)

50(S) 50(P)

93(S) 7(P)

70(S) 30(O)

1.85

1.54

1.54

1.84

2.4

Coke ash, %

11

9.5

9.5

10.2

15.4

Coke rate, kg/thm

390

339

365

392

410

PCI rate, kg/thm

100

161

125

71(Oil)

120

Sl

320

236

236

286

300

l2O3 in sinter, %

k /h

AUXILIARY FUEL INJECTION INTO BLAST FURNACES Injection of hydrocarbons through the tuyeres generates H 2 and CO in the combustion zone. H 2 gives several additional benefits, such as: • Faster gaseous reduction of iron oxides. • Higher thermal conductivity of the gas and consequently, faster heat transfer to the solid burden. • Better bed permeability in the furnace, since hydrogen has a much lower density than CO and N2.

TOTAL REDUCING AGENTS IN 1995 Country

Coke, kg/thm

Coal, kg/thm

Oil, kg/thm

Others, kg/thm

Total, kg/thm

Japan

414

98.8

-

1.2

514

USA

413

36.5

10.0

40

499.5

France

351

125.6

3.5

-

480.1

Germany

359

51

63

-

473

Italy

353

129.5

15

-

497.5

Netherlands

357

141

-

-

498

UK

394

43

55

-

492

India

480

120

-

-

600

Today, in many countries, coke consumption even as low as 270290 kg/thm has been achieved at coal injection rates of 190-220 kg/thm, with a coke to coal replacement ratio in the range of 

COAL INJECTION INTO BLAST FURNACE • Pulverised coal injection (PCI) is a of considerable current interest. • In most cases, 1 kg coal at best replaces 1 kg coke, referred to as Replacement ratio. Sometimes, RR can be more than 1. • Typically, coal is ground to about 80% below 75 micron (0.075mm). • Coal injection is normally accompanied by suitable oxygen enrichment of the air blast. • Coal injection rates above 100 kg coal/thm are quite common now-a-days and some modern furnaces have reached a level as high as 250 kg/thm. • Choice of appropriate coal in terms of its ability to combust easily in the raceway, depends on the nature of the coal (particularly its volatile matter content), particle size distribution and mode of injection. All these factors influence the Replacement ratio.

INCREASE IN GLOBAL AVERAGE PCI RATE

Higher PCI calls for better coke. Avg. PCI rate in 2008-09 : Japan, Korea, Taiwan – 120, China – 190, EU – 215,

PRESSURE DROP AT DIFFERENT COKE RATES

LIMITATION OF COAL COMBUSTION IN THE RACEWAY

FUEL RATE IN ‘G’ BLAST FURNACE,TATA STEEL 700

  m    h    t    / 600   g    k  ,   e 500    t   a   r   n 400   o    i    t   c   e 300    j   n    i    l   a 200   o    C    /   e    k 100   o    C

Coke rate

77

533

Coal rate

73 101

565

498

129

113

448

458

2006-07

2007-08

0 2003-04

2004-05

2005-06

Gradual decrease in coke and increase in coal is seen.

CHARGING SYSTEM: BELL TYPE

BELL-LESS TOP WITH ROTATING CHUTE

LATEST IS GIMBLE TOP CHARGING SYSTEM

ADVANTAGES ACCRUED FROM IMPROVED BURDEN DISTRIBUTION • Increased productivity, decreased coke rate, improved furnace life. • Improved wind acceptance and reduced hanging as well as slips. • Improved efficiency of gas utilisation and indirect reduction. • Lower silicon content in hot metal and consistency in the hot metal quality. • Reduces tuyere losses and minimisation of scaffold formation. • Reduced dust emission owing to uniform distribution of  fines.

CENTRAL WORKING AND WALL WORKING BLAST FURNACES

MONITORING BURDEN DISTRIBUTION The monitoring system for assessing distribution includes: • Heat flux monitoring equipment to measure the heat flow in different zones (both above and under the burden). • Profile meters for the measurement of surface profiles. • Thermocouples in the throat, stack and bosh regions to measure temperature. • Stack pressure monitoring and pressure drop measurement along the furnace height. • Special instruments such as infrared probes to monitor the burden surface temperature, devices in the stack region to measure individual layer thicknesses and local descent rate, and tuyere probes to sample materials at the tuyeres level. • Mathematical models for charge distribution control, overall heat and mass balance and interpretation of probe data.

BLAST FURNACE PROBING AND CONTROL

PART - II IRONMAKING IN BLAST FURNACES Mechanism of Reduction, Blast Furnace Reactions, Zones in a BF

NOMENCLATURE OF REACTIONS IN A BF The reduction of iron oxides by CO and H 2 is traditionally known as Indirect Reduction in blast furnace ironmaking. This is meant to distinguish it from the reduction by solid carbon, which is called Direct Reduction. Gas-solid reactions are much faster than reactions between two solids. Therefore, maximum of indirect reduction is the goal. Utilisation of hydrogen as a reductant has definite advantage. Disadvantage is -----.

BLAST FURNACE REACTIONS As the solid charges descend downwards, major reactions may be classified into the following categories viz.: • Removal of moisture from the raw materials. • Reduction of iron oxides by CO. • Gasification of carbon by CO 2. • Dissociation of CaCO3 (where raw limestone added). • Reduction of FeO by carbon. • Reduction of some other oxides of ore by carbon. • Combustion of coke and coal in front of tuyeres. The outputs from the furnace are: • Molten iron (i.e. hot metal) • Molten slag • Gas at a temperature of around 200 OC, containing CO,

IMPORTANT BLAST FURNACE REACTIONS Gasification reaction: 2C + O2 = 2CO

Exothermic reaction

Boudouard reaction: 2CO = 2CO2 + C

Endothermic/ Exothermic (beyond 1000°C) reaction

Solution loss reaction: C + CO2 = 2CO

Endothermic reaction

Water gas shift reaction: CO + H2O = H2 + CO2

Mild Exothermic reaction

MECHANISM OF IRON OXIDE REDUCTION • Transfer of reactant gas to the solid surface (CO or H 2) across the gas boundary layer around the piece of solid. 3 Fe2O3(s) + CO (g) = 2 Fe3O4 (s) + CO2 (g) Fe3O4(s) + CO (g) = 3 FeO (s) + CO2 (g) 3 Fe2O3(s) + H2 (g) = 2 Fe3O4 (s) + H2O (g) Fe3O4(s) + H2 (g) = 3 FeO (s) + H2O (g) • Inward diffusion of reactant gas through the pores of the solid chemical reaction FeO (s) + CO = Fe (s) + CO 2 (g) FeO (s) + H2 = Fe (s) + H2O (g) • Outward diffusion of the product gas (CO 2 or H2O) through the pores. • Transfer of the product gas from the solid surface into the bulk

IRON OXIDE REDUCTION Reaction kinetics of iron ore reduction determines the rate at which iron oxides are converted to metallic iron. The rate of any chemical reaction increases as the temperature increases. As a result, reaction kinetics is not generally a matter of great concern in blast furnaces. This is contrary to the reaction rates in DR processes. Owing to lower temperatures, the reactions are slower. Hence, the production rate of DR processes is generally lower than that in blast furnaces.

IRON OXIDE REDUCTION Iron oxide reduction is complex because the oxide charged undergoes a series of changes, step-by-step before the conversion to the final product. The slowest step in the entire process chain determines the overall reaction rate and is referred to as the rate controlling step. Chemical reactions are either homogeneous (if a single phase is involved) or heterogeneous (if two or more phases are involved). The solid-state reduction of iron oxides is heterogeneous, involving solid and gas phases separated by an interface. For the chemical reactions to occur, the reactants must reach the interface and the products must move away. The movements of reactants and products are affected by

RATE CONTROLLING STEPS IN IRON OXIDE REDUCTION Rate of iron oxide reduction depends on the rates of heat and mass transfer  across the gas-flow boundary layer at the outer surface of the solid phase. When the reaction rate is controlled by this factor, it is known as "Boundary Layer Control". Rate of diffusion of reducing gas inwards and product gas outwards through the reduced iron layer can control the rate of reduction. This phenomenon is generally associated with large ore particles, and known as "Gaseous Diffusion Control" or "Iron Pore Control." Chemical reaction at the wustite-iron interface can be rate controlling. In such a case, the rate of reduction per unit area of the remaining iron-oxide surface is found to be constant with time. Mechanism is known as "Interfacial Reaction Control" or "Phase Boundary Reaction Control". When both gaseous diffusion control and interfacial reaction control combine to influence the rate of reduction, the mechanism is referred to as

SCHEMATIC OF IRON ORE REDUCTION

REDUCTION OF IRON OXIDES

Topo-chemical type reduction

Partially reduced porous iron oxide

Importance of porosity assessed by microscopic examination of reduced iron oxide. Topo-chemical reduction has three concentric layers – magnetite, wustite, metallic iron – each layer same shape as outer surface in case of  dense oxides. Porous oxides has similar structure in individual particles.

REDUCTION OF IRON OXIDE BY H2 VS. CO

Reduction by hydrogen

Reduction by CO

• Initially, reduction by hydrogen is faster than carbon monoxide. • Magnetite reduced by hydrogen contains grains of wustite completely enveloped in dense layers of metallic iron. In CO reduction, metallic iron layers consist of almost pure pearlite. • Carbon can diffuse very rapidly in austenite so that at the interface between austenite and wustite, carbon is available to complete the reduction. In case of hydrogen, reduction is incomplete. • Solid-state diffusion of ferrous iron through wustite much greater than gaseous diffusion of hydrogen or CO through ore particles. Therefore, solid-state diffusion in the stack region is not the rate controlling step in BF

STRUCTURAL CHANGES IN IRON OXIDE REDUCTION Hematite → Magnetite → Wustite → Metallic iron • • • • • • • • •

• • •

In hematite, oxygen atoms arranged in close-packed hexagon structure. In magnetite and wustite, the structure is FCC. In first stage of reduction, oxygen atoms undergo major readjustment. Results in 25% increase in volume, opens-up structure, facilitates redn. During magnetite to wustite transformation, oxygen lattice is unchanged. Iron atoms diffuse to fill vacant sites in lattice; volume change is small. Wustite has variable composition – composition changes from equilibrium with magnetite to equilibrium with metallic iron. Nucleation and growth of iron crystals results in shrinkage and large increase in porosity of the metallic phase. Transformation of hematite to magnetite – 25% increase in volume. Further, 7-13% increase during transformation to wustite. Followed by shrinkage to metal phase. Total increase in volume during complete reduction of hematite:25-27%. For magnetite ores, no volume increase; 4-5% shrinkage in final product. Explains why reducibility of magnetite is very poor.

REDUCTION OF METAL OXIDES BY CO

REDUCTION OF IRON OXIDE BY CO

REDUCTION OF IRON OXIDE BY HYDROGEN

BF NOMENCLATURE

F B f o t h gi e H

ZONES IN A BLAST FURNACE

DETAILS OF THE ZONES Stack: Wall slopes outwards in downwards direction Extends from the stock line to the mantle level. In this zone the burden is completely solid. The charge gets heated from 200°C at the stock line to 1100-1200°C at the bottom of the stack. Most of reduction occurs by gas-solid contact.

Belly: The cylindrical portion below the stack  Metallic burden begins to soften and fuse as it travels.

Bosh: Below the belly and sloping inwards in downwards direction Burden begins to melt except coke. Gangue and flux combine to form the slag. The furnace walls are either parallel and then taper down, or are entirely tapering down resulting in reduction the sectional area by about 20-25% . This is because of decrease in the apparent volume of the charge.

Tuyere or Combustion Zone: End of slope; parallel walls Except central column of coke, entire charge is molten. Oxygen of the blast burns coke to CO. Number of combustion zones, in front of each tuyere exists. Thus, there is a ‘runway’ or ‘race-way’ in front of each tuyeres which is first horizontal and then becomes vertical while expanding.

Hearth: Below the tuyere region and cylindrical  Some coke descends into hearth to form the ‘deadman’. Entire charge is molten and

TUYERE AREA IN A BLAST FURNACE

MECHANISM OF SILICON REDUCTION

CONCEPT OF RAFT From sensible heat of the flame, its temperature is calculated. This is known as Raceway Adiabatic Flame Temperature (RAFT). Heat content of flame = mass of gas in the flame average specific heat of gas (RAFT 298) Change in Operating Variable

Change in RAFT , 0C

Blast temperature raised by 100 0C

+82

Blast oxygen raised by 1%

+53

Blast moisture raised by 5g/Nm 3

-28

1% methane added to blast

-56

PRODUCTIVITY OF BLAST FURNACES BF productivity is defined as tonnes of hot metal produced per  day per cubic metre of inner/working volume. Productivity can be increased by: • Screening of solid charges before charging into the furnace to eliminate fines below a certain size • Agglomeration of fines by sintering, pelletising • Proper top charging device to make the distribution of burden size as uniform as possible in any horizontal section. • Use of better quality coke. • Use of higher hot blast temperature. • Use of oxygen enriched blast. • Use of higher top pressure. • Use of superior quality iron oxide burden. • Improved facilities for metal and slag evacuation.

PRODUCTIVITY AS FUNCTION OF SLAG RATE

ACCEPTABLE COKE FOR BLAST FURNACES Suitability assessed in terms of: Room temperature strength High temperature strength Chemistry Size Reactivity For blast furnaces in India, ‘acceptable’ values are: • Room temperature strength : M 10 7.0 (max.) • High temperature strength: CSR 64 (min.), CRI 25 (max.). • Chemistry: Ash 15-17% min., Alkali 0.35% max. • Size : Suit iron oxide feed. Size at tuyere level? Lower productivity of Indian furnaces essentially on

TYPICAL COST BREAK UP FOR HOT METAL (1.2%) Fluxes

(25.4%) Ore & Scrap

(47.4%) Coke

(5.1%) Others (6.2%) Labour  & Admn. (2.5%) Maintenance (2.5%) Relining (1.3%) (4.7%) Blowing Refractories cost

(3.7%) Coal & Tar injection

PART - III SMELTING REDUCTION ALTERNATIVE METHOD OF IRONMAKING

PROGRESSIVE REDUCTION IN BF COKE CONSUMPTION OVER THE YEARS Year Coke rate, Injectant, kg/thm kg/thm

Total reductant, kg/thm

Comments

1950

1000

0

1000

Lean local ores

1965

600

0

600

Rich seaborne ores

1970

525

50

575

Oil injection/high blast temperature/ oxygen enrichment

1980

500

50

550

High top pressure/burden distribution and permeability control

1990

400

125

525

Coal injection/improved sinter coke quality

2000

325

175

500

Increased coal/gas/oil injection

2010

250

250

500

Continued use of metallics in the burden

DEPENDENCE OF BLAST FURNACES ON COKE Parameter

Case A

Case B

Case C

Case D

Coal, kg/thm

0

106

149

173

Coke, kg/thm

482

376

334

305

Total fuel, kg/thm

482

482

483

478

59.4

59.2

61.1

58.8

1129

1141

1159

1177

Humidity, g/Nm3

30

16

10

6

Oxygen in blast, %

21

21

22.4

22.4

Gas utilisation, %

49.2

50.6

51.0

48.2

Hot metal temperature, oC

1487

1475

1478

1482

0 24

0 27

0 28

0 33

Production, area/ d

t/m2

hearth

Blast temperature, oC

Si i h t

t l %

FORECAST OF TECHNOLOGIES TO BE ADOPTED FOR HOT METAL PRODUCTION 100 Non-coking coal

New SR technologies Fine ore

75

Pellets Lump ore 50 Coking coal Conventional BF route 25

2000

Sinter  Pellets Lump ore

2025-2030  YEAR

EMERGING TRENDS IN INPUTS TO IRON/STEELMAKING Future  Recent  Lump ore, sinter, pellets

Fine ores

Conventional 

Lump ore

DRI using NG, non- coking coal

Coke from metallurgical coal and PCI

Coke from metallurgical coal Scrap

Scrap

Scrap

SR hot metal using noncoking coal

Non-coking coal, NG, synthetic gases

FUNDAMENTALS OF SMELTING REDUCTION Off-Gas (Critical in economics of  all SR processes) Pre-reduction Unit

Iron Ore/ Pellets Hot Reducing Gas

Pre-Reduction

Pre-reduced Ore

Post Combustion Smelting Reduction Vessel

Final Reduction Coal Melting Coal Gasification

Oxygen/Air  Hot Metal

Pre-reduction Degree, Extent of Post Combustion and Heat

RAW MATERIALS USED IN BLAST FURNACES AND IN SMELTING REDUCTION Process

Oxide Feed

Reductant

Product

Blast furnace including mini blast furnace

Lump ore, Coke, coal, Hot metal sinter, pellets oil, tar, essentially for BOF natural gas steelmaking

Smelting reduction

Ore fines, lump ore, waste iron oxides

Coal, oxygen, electricity

Hot metal (synthetic hot metal) for EAF / EOF steelmaking

FLOWSHEET OF SINGLE STAGE, TWO STAGE AND THREE STAGE SR PROCESSES Ore

Gas

Ore

Gas Reduction

Ore

Reduction

Gas

Coke Coal

Reduction and melting

O2

DRI

Gas

Metal

Gasification

DRI

Coal Melting

Slag

Gas

Coke

Gas

O2 Melting

Single stage Slag

Metal

O2

Two stage Slag

Metal

Three stage

Coal

SALIENT FEATURES OF SR (1) • SR involves both reduction and smelting, i.e. melting accompanied by chemical reaction(s). • In an ideal SR reactor, in the strictest sense, all the reduction reactions should take place in the liquid state in a single step. • Any SR process involves the extraction of metal values from the ore following liquid phase reduction by non-coking coal. • In actual practice, most SR processes utilise two steps – the removal of oxygen from the ore in the solid state to varying extents in step one, followed by removal of the remaining oxygen in liquid phase reduction reactions in step two. • Compared with DR processes, the principal advantage of high temperature operation in SR processes is: faster rates of  reaction and prevention of sticking problems associated with solid-state reactions (intrinsic in DR processes).

SALIENT FEATURES OF SR (2) • Smelting reduction processes are thus either two-step processes with separate pre-reduction and smelting reduction steps (such as Corex and HIsmelt), • Simpler one-step processes involving simultaneous reduction and smelting still not fully proven. • All SR processes consume fairly large volumes of coal that generates large amount of export gas, the effective utilisation of the generated by-product gas is extremely important. • Generally, the use of export gas makes or breaks the cost structure of SR processes .

SALIENT FEATURES OF SR (3) • Unless the net export gas from any Corex plant can be utilised (extent of generation 1650 Nm 3  /thm; calorific value 7500KJ/Nm3) the process itself becomes totally uneconomical. • If no credit is given to the off- gas, the cost of the hot metal made can be as much as 50% higher than blast furnace hot metal. • Adequate credit can be obtained by selling cogenerated electrical power from the Corex off gas. • Another use of the export gas is in shaft furnace, DRI production adjacent to the Corex furnace.

REDUCTION STEPS IN SR • Reduction by solid carbon • Reduction by carbon dissolved in Iron • Reduction of molten FeO by CO

REDUCTION OF SLAGS BY SOLID CARBON A liquid boundary layer is expected to exist on the slag side of  the slag / carbon interface, FeO must be transported to the nucleation site in the following manner: FeO (l) + C (s) = Fe (l) + CO (g) FeO (bulk) transported to FeO (slag /gas interface) A halo is formed, after which the following steps come into play: FeO + CO = Fe + CO 2 (at gas / slag interface) CO2 (gas /slag interface) transported to CO 2 (carbon /gas interface). CO2 + C = 2CO (at carbon / gas interface)CO (carbon / gas interface) transported to CO (gas / slag interface).

REDUCTION OF SLAGS BY CARBON DISSOLVED IN IRON Taking desulphurisation as an example, a three step mechanism has been proposed: FeS(iron) = FeS (slag) FeS (slag) + CaO (slag) = FeO (slag) + CaS(slag) FeO (slag) + C (iron) = Fe(l) + CO(g)

REDUCTION OF MOLTEN FeO BY CO • Reduction rate of slags is independent of the FeO concentration in the range 67.7 to 48.0%. • FeO less than 48%, the reduction is dependent on the fraction reacted and the partial pressure of CO. • Reduction rate of iron oxide by CO follows a first order  rate law for the reversible reduction of stoichiometric FeO. The rate equation is: (R’ / Ao) = exp (-32300 / RT – 1.37). (1.0 - 0.7aSiO2). (aFeO. pCO - aFe. pCOk) where, R’ = Rate constant, mol/cm2 .s Ao = Reaction surface area, cm2 T = Reaction temperature, K The activation energy was determined to be 135 kJ/ mol.

NET ENERGY CONSUMPTION AND GAS CREDIT OF SR PROCESSES vis-a-vis BF 35 30   m    h    t    /    J    G  ,   y   g   r   e   n    E

Ga s Credit Net Energy

25 20 15 10 5 0 Fastme lt

Hismelt

Corex

Redsmelt

Blast Furnace

MAJOR SMELTING PROCESSES Vertical Shafts • MBF – Capacity : 30,000-1,125,000 tpa (covers very wide range) • Corex – 5 operating plants, Capacity : 300,000-900,000 tpa First and leading SR process. Very high volumes of off-gas; some coke often used. Coal properties can be varied over a small range (Indian coal?) • Finex – Capacity : 1.2-1.5 mtpa. Process development complete. Very promising. • Tecnored – Capacity : 150,000 tpa. Process still under development 

Bath Smelting Processes • HIsmelt – Capacity : 600,000-1,200,000 tpa. Process almost ready. • Ausmelt – Capacity : up to 2.5 mtpa. Process not proven. • Romelt – Capacity : 200,000-1,000,000 tpa . Russian process with tremendous  promise, but no plant despite efforts, including in India and 

Rotary Hearth Furnace (RHF)

Japan • ITmk3 – Capacity : 0.5 mtpa. Slag separation by partial melting is unique.

RHF Combined with Melting / Smelting

• Inmetco – Capacity - 60,000 tpa. Suitable for zinc-bearing iron ores. • FastMelt – 2 operating plants mainly for smelting solid wastes from ISPs .

COMMERCIALISED SR PROCESSES • Corex – Many operating units; most popular SR process. Typical iron oxide Lump ore

Pellets

Sinter  

Fetotal

62 - 65

62 - 65

50 - 55

Grain size, mm

8 - 20

8 - 16

10 - 30

• HIsmelt – Nucor, Rio Tinto, Mitsubishi and Shougang (of PR of  China) agreed to construct a 0.8 Mtpa plant at Kwinana in Western Australia after pilot plant tests were completed at the same location. The plant had produced more than 37,000 tonnes of hot metal in total till March 2006. Has been stopped thereafter.

FLOW SHEET OF THE COREX PROCESS

SOME KEY FEATURES OF COREX • Is a shaft furnace-based process. It can accept high alkali containing ores without any build-up inside the reactor. • Specific melting capacity is higher than in blast furnacesproductivity of the order of 3-3.5 t/m 3 /d can be achieved. • The process is capable of operating at 50-115% of its nominal capacity. • It takes only half an hour to stop the plant and four hours to restart it, whenever required. • Hot metal quality is comparable with blast furnaces (C=4.04.5%, Si=0.30-0.80%, S=0.02-0.09%, P depends on inputs). • It has outstanding environmental superiority in comparison with the blast furnace process in terms of generation of dust, SOx, NOx, phenol, cyanides, etc.

FLOW SHEET OF THE FINEX PROCESS

SOME KEY FEATURES OF FINEX • Posco developed Finex process to utilise iron ore/non-coking coal fines not suitable for charging to their Corex plant. • Corex in Posco uneconomical owing to restrictions on coal and iron oxide size (top size and under size). Also, often required 10% coke to ensure hearth permeability. • Finex actually able to utilise iron ore in the form of fines. • After fluidised bed reduction, fine DRI is hot briquetted before melting in a melter gasifier (similar to Corex). • Non-coking coal is also briquetted before use in melter gasifier. • Hot metal composition same as BF. • Range of complete scale of production yet to be established.

Finex is certainly of interest to ISPs. Whether it can be used to supply limited tonnages of hot metal to EAFs?

FLOW SHEET OF HISMELT PROCESS

HISMELT FLOW OVERVIEW

SOME KEY FEATURES OF HISMELT • Incorporates many BF features – hot blast stoves, air blast, etc. • Can be single stage process; better, with separate pre-reduction. • Pre-reduction of iron oxide and oxygen enrichment of hot air  (1200°C) blast provide substantial productivity enhancement. • Hot metal contains 4.3 ± 0.15% carbon, phosphorus and silicon levels are extremely low viz. < 0.05% P and