REFRATECHNIK
Report No. 55 Wear phenomena of refractory linings in the burning zone of cement rotary kilns M. Künnecke, R. Gocht
Refratechnik Cement GmbH Rudolf-Winkel-Strasse 1 37079 Göttingen Germany Phone +49 551 6941 0 Fax +49 551 6941 104
[email protected] www.refra.com
Published by: Refratechnik Cement GmbH, Göttingen All rights reserved. Reproduction in any format – including extracts – is only permitted with the approval of the publisher.
Report No. 55 Wear Wear phenomena phenomena of refractory refract ory linings linings in the burning zone of cement rotary kilns M. Künnecke, R. Gocht
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Photo 1: Perfect burning conditions
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Introduction Depending on the cement clinker production process type, the average lining life of burning zone refractories is 10-12 10-12 months being equivalent to approx. 500 g of refr actories per ton of produced clinker. If your lifetime is more than 1 year or the specific consumption is less than 50 0 g, you can be happy. Photo 1 shows a perfect , clear flame, good protective coating and satisfactor y process conditions. If your kiln looks similar, you may relax. If the lining performance however is less than 6 months and brick consumption is more than 800 g, the following case histories of damaged bricks and linings will hopefully contr ibute to an improvement. Refractory bricks installed in the burning zone of cement rotary kilns are considered as working lining and are thus sub jected jecte d to continuous contin uous wear through t hrough thermal, thermal , chemical chemi cal and mechanic al st resses. ress es. Numerous stresses of that kind do overlap in the kiln due to the interaction of kiln feed, flame, and kiln shell deformation as per Fig. 1. Refractories are sensitive ceramics and should be handled with care because: There is no cement without refractories!
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stresses on refractory linings
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Fig. 1: Stresses on the refractory lining in cement rotary kilns
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First of all we have to look a t the various responsibilities and influences regarding the lining performance (Fig. 2).
user’s responsibility thermal stress
installation of refractories
chemical stress
service conditions
dry storage
mechanical stress
lifetime of refractories
selection
installation drawing
quality of production
quality of raw materials
producer’s responsibility
Fig. 2: Responsibilities or influences regarding the lifetime of refracto ries
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Generally, extensive investigations are necessary to evaluate complicated complicated cases of premature wear. Nevertheless, some typical kinds of lining damage can already be identified on site immediately after having stopped the kiln by comparing by comparing the present lining situation in your kiln with the following case histories. histories. The attached phot os of lining damages were collected during inspections of kiln linings which showed premature wear due to different reasons. If you recognize such types of damage during shutdown of your kiln (Photo 2), you might be able to avoid future problems by t aking appropriate improvement measures. The following photos and figures are examples for typical damages due to mechanical, thermal, and chemical overstres ses: 1. 1.1 1.1 1.2 1.3 1.4 1.5 1.6
Mechanical wear Convex spalling Concentric stress cracks Spiralling Ovality Formation of grooves Pinch at retaining rings
Photo 3 Photo 4 Photo 5 Photo 6 Photo 7 Photo 8
2. 2.1 2.2 2.3 2.4
Thermal wear Concave melting pits Lava-like coating Excessive thermal load Thermal shock
Photo Photo Photo Photo
9 / 10 / 11 / 12 /
Fig. 9 Fig. 10 Fig. 11 Fig. 12
3. 3.1 3.2 3.3 3.4
Chemical wear Infiltration of alkali salts Corrosion of chrome ore Redox bursting Hydration cracks
Photo 13 / Photo 14 / Photo 15 / Photo 16 /
Fig. 13 Fig. 14 Fig. 15 Fig. 16
/ Fig. 3 / Fig. 4 / Fig. 5 / Fig. 6 / Fig. 7 / Fig. 8
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Photo 2: Lining inspection during a kiln stop
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1.
Mechanical Mechanic al wear
1.1 1.1
Convex spalling spallin g
1.2
Concentric stress cracks
1.3
Spiralling
1.4
Ovality
1.5
Formation of grooves
1.6
Pinch at retaining rings
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Photo 3: Convex spalling
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phenomena:
symptoms:
convex spallings in longitudinal direction of the kiln, so-called cobblestones
axial pressure generated by dilatation, brick resistance is exceeded thus causing spalling
convex spalling
reasons:
improvements:
• insufficient clearance in the expansion joints • installation without cardboard cardboard spacers • stoppage of operat ion during heating up, after cardboard spacers burnt off
• insertion of cardboard spacers according to installation installation drawings • use of bricks with preattached preattached cardboard spacers like all basic qualities from Refratechnik
Fig. 3: Mechanical wear 1
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Photo 4: Concentric stress cracks
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phenomena:
symptoms:
concentric stress cracks in 3-5 cm depth around the circumference
cracks in the reaction zone of steel plate-lined bricks and in bricks in the keying section
concentric stress cracks
reasons:
improvements:
• steel plates oxidize and react with the bricks forming a monolithic horizon of magnesioferrite with significant volume increase
• lining without steel plates (mortar or clench lining) • correct keying of the brick rings
Fig. 4: Mechanical wear 2
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Photo 5: Spiralling
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phenomena:
symptoms:
spiralling, tilting and edging, cold face wear from friction against the kiln shell
lining displacements displacements and st ress marks on the brick's cold face due to relative movements between kiln shell and lining
spiralling
reasons:
improvements:
• loose installation • increased kiln shell ovality • expansions and contractions due to frequent kiln stoppages • changing coating formation • deformation of the kiln shell
• uniform kiln operation • repair of bulged kiln shell • measuring the ovality by means of the REFRA-Shell II equipment • proper and tight installation with installation with staggered lining staggered lining and REFRA-Rig II • kiln alignment measurement
Fig. 5: Mechanical wear 3
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Photo 6: Ovality
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phenomena:
symptoms:
deep spalling of single bricks, inbet ween completely perfect brick sections
high kiln shell ovality in the tyre area (squeeze and release) during each rotation stressing the brick's hot face
ovality
reasons:
improvements:
• worn tyre shoes increase the clearance causing excessive ovality • limit for the ovality max. 1/10 of kiln diameter, e.g. 0.5% for a 5 m dia. kiln • maximum migration bet ween kiln shell and tyre 25 mm
• permanent monitoring of migration between kiln shell and tyre • measuring the ovality by means of the REFRA-Shell II equipment • kiln alignment measurement • insertion of new tyre pads • best flexible lining with mechanically enhanced refractory brick grades, like PERILEX® CF, TOPMAG® A1 or ALMAG® A1 • mortar installation
Fig. 6: Mechanical wear 4
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Photo 7: Formation of grooves
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phenomena:
symptoms:
parallel grooves (2-4 bricks wide) with deep spalling appearing appear ing along the kiln axis
orientated premature wear channels within uniform, unaffected lining
formation of grooves
reasons:
improvements:
• keying of the rings with excessive tightness • damaged bricks in the keying sec tion caused by jack hammer • more than one metal shim per joint or shims on both sides of one brick
• correct keying of brick rings • uniform insertion of liners in the whole joint • one steel plate per joint only • use of max. 2 mm thick keying shims
Fig. 7: Mechanical wear 5
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Photo 8: Pinch at retaining rings
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phenomena:
symptoms:
collapse of brick rings against ret aining ring or outlet segments due to pinch spalling
horizontal cuts and cracks of bricks at the upper edge of re taining rings and transpassing nose ring segments
pinch at retaining rings
reasons:
improvements:
• thrust and oscillations generate shear cracks and grinding • flexing kiln outlet or ovality accelerate the damage • loose brick lining
• solid kiln shell • use of appropriate retaining rings with max. 50 mm height • use of highly abrasion resistant brick grades like KRONAL® 63 AR
Fig. 8: Mechanical wear 6
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2.
Thermal wear
2.1 2.1
Concave melting pits
2.2
Lava-like coating
2.3
Excessive Exces sive thermal load
2.4
Thermal shock
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Photo 9: Concave melting pits
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phenomena:
symptoms:
concave wear, so-called duck nesting (looking like eutectic melting of alumina bricks)
overheating of bricks weakens the structure of bricks at their hot face
concave melting pits
reasons:
improvements:
• coating-free operation of standard grade bricks with moderate refractoriness • flame impingement on the lining • misalignment of the burner
• adjustment of burner • limits for silica modulus: SM < 2.6 • cross section load < 5.0 x 10 6 kcal/m2h • use of refractories with high thermochemical resistance, like ALMAG® AF with magnesiafused spinel or MAGNUM® E, the magnesia-zirconium oxide brick grade with balanced elastification
Fig. 9: Thermal wear 1
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Photo 10: Lava-like coating
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phenomena:
symptoms:
lava-like coating solidly adhering to the bricks and cracks behind the densified surface
overheating of clinker with liquid phase infiltration and densification of the hot face of bricks, loss of mechanical flexibility
falling coating takes off densified brick heads
lava-like coating
reasons:
improvements:
• overheating of clinker with formation of increased liquid clinker phase infiltrating the hot face, e.g. peaks of ferritic phase during production of SRC
• avoidance of burning conditions with increased liquid phase LPh < 26% (L & P) • uniform operation conditions • best possible homogeneity of raw materials • use of refractories with high thermochemical resistance, like ALMAG® AF with magnesiafused spinel or MAGNUM® E, the magnesia-zirconium oxide brick grade with balanced elastification
Fig. 10: Thermal wear 2
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Photo 11: Excessive thermal load
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phenomena:
symptoms:
change from uniform brick matrix with round grain to periclase needles with brittle structure
structural fatigue of high-quality bricks at coating-free operation due to high temperature
excessive thermal load
reasons:
improvements:
• overheating above 1700 °C without liquid phase • thermotactic recrystallisation recrystal lisation
• uniform and continuous operation conditions • use of refractories with high thermochemical resistance, like ALMAG® AF with magnesiafused spinel or MAGNUM® E, the magnesia-zirconium oxide brick grade with balanced elastification
Fig. 11: 11: Thermal wear 3
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Photo 12: Thermal shock
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phenomena:
symptoms:
split-like spallings in 2-3 cm thin layers
sudden changes in temperature generate thermal tensions that cause horizontal cracks
thermal shock
reasons:
improvements:
• quick heating up • sudden cooling down • coating losses
• rational heating-up at 50 °C/h • slowly cooling down • uniform operating conditions • stable coating • use of optimally elastified brick elastified brick grades with high thermal shock resistance of ≥ 80 cycles, such as PERILEX® CF, TOPMAG® A1, and ALMAG® A1
Fig. 12: Thermal wear 4
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3.
Chemical wear
3.1 3.1
Infiltration Infiltrat ion of alkali salts
3.2
Corrosion of chrome ore
3.3
Redox bursting
3.4
Hydration cracks
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Photo 13: Infiltration of alkali salts and corrosion of kiln shell
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phenomena:
symptoms:
a cut through the brick shows infiltration horizons, and the inner kiln shell is attacked by corrosion
gaseous alkali salt compounds migrate into the bricks, f illing the brick pores, condensating and solidifying them
infiltration of alkali salts
reasons:
improvements:
• deposits of mainly K 2SO 4 in the lower transition zone and burning zone, along with KCl in the upper transition zone
• alkali salt content to be reduced • alkali/sulphate ratio to be balanced in a range of 0.8-1.2 • use of brick grades with improved infiltration resistance: PERILEX® resistance: PERILEX® CF, REFRAMAG® AF, TOPMAG® AF or ALMAG® AF • cleaning of the kiln shell and us e of REFRACOAT® for protection of protection of the kiln shell • installation of sacrificial steel plates
Fig. 13: Chemical wear 1
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Photo 14: Corrosion of chrome ore
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phenomena:
symptoms:
yellow-green discoloration and brittle surface with cracks
with excessive alkalis the chrome ore is attacked and corroded, and toxic hexavalent alkali chromates are formed
corrosion of chrome ore
reasons:
improvements:
• with free K 2O, Na2O formation of K 2CrO 4, Na 2CrO 4 • loosening of the brick structure by dissolving of the chrome ore
• alkalis to be reduced and alkali/sulphate ratio to be balanced (refer to 3.1) • use of chrome ore-free bricks: PERILEX® CF or ore-free bricks: PERILEX® REFRAMAG®, TOPMAG® or ALMAG® grades
Fig. 14: Chemical wear 2
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Photo 15: Redox bursting
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phenomena:
symptoms:
bleaching of dark brown basic bricks to cappuccino colour and bursting of the structure
repeated change from oxidizing to r educing atmosphere (so-called redox) causes volume change between trivalent (red) and (red) and bivalent (green) iron
carbon deposits in the joints and on the kiln shell and in the pores of the bricks
redox bursting
reasons:
improvements:
temporarily reducing atmosphere due to: • insufficient combustion • use of fuels rich in ashes • course grain coal or petcoke • use of alternative fuels
• min. of 1.5% of O2 for complete combustion • improvement of coal/petcoke fineness (max. residue 50% on 90 micron mesh of volatile content • use of a chrome ore-free brick grades ALMAG® AF, iron content: ALMAG® with lowest iron content: ALMAG® 85, and ALMAG® A1
Fig. 15: Chemical wear 3
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Photo 16: Hydration cracks
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phenomena:
symptoms:
spider web cracks propagating from the surface into the bricks, even disintegration
CaO and MgO are sensitive to humidity. Magnesite bricks are by far less sensitive than dolomite bricks, but should be protected against seawater, rain and humid air, especially in subtropical climates.
hydration cracks
reasons:
improvements:
• MgO reacts with water to brucite Mg(OH) 2 under significant volume increase
• dry storage under roof • recommended storage time 6 months in subtropical climate • ventilated storage • max. ignition loss of humid bricks 0.3% seaworthy packing • use of REFRAPACK seaworthy packing with double pre-shrunk plastic foil
Fig. 16: Chemical wear 4
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Recommendations • Optimize uniformity of raw meal feed. • Try to obtain and maintain stable coating conditions in the burning zone. • Control kiln shell ovality in the tyre areas and kiln alignment periodically. • Balance thermal load. • Avoid frequent kiln stoppages. • Heat-up and cool down the kiln slowly. • Apply a suitable installation method with your permanent crew or a reliable subcontractor. • Do carefully select appropriate brick grades with Refratechnik.
Basic bricks for burning zones MAGNUM® E
magnesiazirconium oxide
lower transition zone subject to extremely high thermal load with cement clinker attack and redox conditions prevailing; also recommended for use in lime, dolomite and dolomite and magnesite kilns, chrome ore-free
ALMAG® AF
magn magnes esia ia-f -fuse usedd spin spinel el
uppe upperr and and lowe lowerr trans transit itio ionn zones zones subj subject ect to to extrem extremel elyy high high thermochemical thermochemical load in kilns using alternative fuels, chrome ore-free
TOPMAG® AF
magn magnes esia ia-f -fuse usedd spi spine nell
uppe upperr and and lowe lowerr tran transi siti tion on zones zones sub subje ject ct to to high high thermochemical thermochemical load in kilns using alternative fuels, chrome ore-free
ALMAG® A1
magn magnes esia ia-f -fuse usedd spin spinel el
uppe upperr tran transi siti tion on zon zone, e, tyre tyre sect sectio ionn subj subject ect to to high high the therm rmoomechanical load with redox conditions and extreme alkali and sulphate attack, chrome ore-free
TOPMAG® A1
magnes magnesiaia-fuse fusedd spinel spinel
upper upper transit transition ion zone, zone, tyre section section subjec subjectt to to hig highh mech mechani anical cal load with redox conditions prevailing and extreme alkali attack, chrome ore-free
ALMAG® 85
m a g n e s i a - s pi n e l
upper and lower t ransition z ones sub ject to severe se vere serv ice conditions with alkali att ack and redox conditions prevailing, chrome ore-free
REFRAMAG® AF
mag magnesi nesiaa-sp spiinel nel
upper pper and and lower ower tran transi siti tion on zone zoness as wel well as burn urning zone zone subject to thermochemical load in k ilns using alternative fuels, chrome ore-free
FERROMAG® F1
magnesia-spinel (fMA spinel)
burning zone and upper transition zone subject t o severe service conditions, good coatability, chrome ore-free
PERILEX® CF
magnesia-spinel (fMA spinel)
burning zone as well as upper transition zone, good coatability, chrome ore-free
PERILEX® 83
magn magnes esia ia-c -chr hrom omit itee
burn burnin ingg zon zonee as as wel welll as as upp upper er and and low lower er tran transi siti tion on zone zoness subject to severe service ser vice conditions, low chrome ore content
PERILEX® 80
magn magnes esia ia-c -chr hrom omit itee
burn burnin ingg zon zonee as as wel welll as as upp upper er and and low lower er tran transi siti tion on zone zoness sub ject t o normal nor mal servic se rvic e conditions condi tions,, low chrome ore conten c ontentt
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Photo 17:
Kiln without Refratechnik services
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Refratechnik Cement GmbH Alte Münzesheimer Strasse 13 76703 Kraichtal/Gochsheim Germany Phone +49 725 8 9112 9112 0 Fax +49 7258 9112 9112 27
[email protected] Refratechnik Ibérica S.A. Clot del Torrent, s/n. 08729 Gornal/Barcelona Spain Phone +34 977 16705 0 Fax +34 977 16705 4
[email protected] Refratechnik Italia S.r.L. Corso Casale 360 13039 Trino Italy Phone +39 0161 801213/801074 Fax +39 0161 804207
[email protected] Refratechnik México S. A. de C.V. Shakespeare No. 30 - 11° 11° Piso Col. Nueva Anzures C.P. 11590, Del. Miguel Hidalgo México, D.F. Mexico Phone +52 55 525074 66 Fax +52 55 525074 67
[email protected] Refratechnik North America, Inc. 530 Maryville Centre Drive Suite 320 St. Louis, Missouri 63141 USA Phone +1 314 336 0634 Fax +1 314 336 0639
[email protected] Refratechnik Asia Limited Suites 3107-09, 31st Floor Tower 1 The Gateway Harbour City, Kowloon Hong Kong Phone +852 3920 5000 Fax +852 3920 5050
[email protected] www.rtasia.hk
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