kiln shell corrosion
TRANSCRIPT
Kiln Shell Corrosion
Cement
ICTM • R. Krischanitz • May 2008
29 Kiln Shell Corrosion
PC kiln, Ø 4.15 x 58 m, 2450 tpd, <20% alternative fuels:
Fracture of kiln shell at rm 38.5 - despite phosphating it during previous kiln stops
Kiln shell thickness:
Original thickness 20mm
Thickness within fracture area (groove) 8mm
Average thickness in brick joints 14mm
Thickness below bricks 17mm
Ref: DAVILA, J.C. et al., 1995, Influencia del Refractario en la Corrosion de la Coraza del Horno Rotatorio para Cemento, ALAFAR XXIV Congreso, Cartagena, pp 157-183
Fracture of kiln shell due to corrosion
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Preferred corrosion below joints, condensation of moisture during kiln stops
Corrosion of kiln shell (rm. 65, calcining zone)
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wet cardboards
Corroded kiln shell within the calcining zone
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Loss of shell thickness 6 mm/a
Fuels: Coal + pet coke, tyres, palletised sewage sludge
Kiln shell corrosion (rm. 29, second tyre)
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Thickness15 mm
Scales from the kiln shell
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© RHI Technical Centre Leoben, Austria
Dense layer of magnetite and hematite
mag
hem
Porous layer of hematite and KCl
hem
KCl
Microscopical appearances of scales
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Composition of scales6DPSOH A B C D E F G
Fe2O3 44 90 88 17 56 n.a. 81
SO3 11 7 5 1 1 -
K2O 24 - 2 43 16 6
Na2O 4 - - - 2 1
Cl- 6 1 4 36 11 6
F- 7 - - - - -
PbO - - 1 - - -
Fe3O4 [[[ [[[ [ [[ [[[ [[
Fe2O3 [[[ [ [[ [[[
FeS [[ [[ [ [ [[
FeS2 [[
Fe(OH)2 [
FeSO4.H2O [
K2SO4 [[
KCl [[ [ [[[ [[ [[ [[
Na2SO4 [ [
NaCl [
NaF [
Pb2(SO4)O [
&KHPLFDO�DQDO\VLV�(wt.%)
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Kiln Feed / Coating
Kiln gas
KCl, K2SO4KCl, K2SO4
KCl, K2SO4 KCl, K2SO4
Preheater
/LQLQJ
Evaporation of volatiles in preheating zone
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Lower melting points of chlorides in comparison to sulphates. Eutectic melts have even lower melting points. The lower the melting point the higher the infiltration depth.Phase diagram: acc. to O.S.Dombrovskaya 1933
Phase diagram KCl – K2SO4
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Spalling of basic bricks caused by KCl after 3 months operation
Infiltrated refractory bricks
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� Reduce chlorine input to the kiln (selection of appropriate raw materials / substitute fuels, install a by-pass)
� Keep evaporation rate of volatile components as low as possible (avoid reducing operation conditions, use of mineralizers, other process measures)
� Reduce permeability of lining (e.g. reduction of joints by use of ISO shapes in the basic zone, use of mortar lining)
� Extension of the basic lining area
� Install refractories with low (zero) permeability
� Apply gas-tight coatings on the kiln shell (paints, flame / plasma sprayedmetallic or ceramic layers etc.)
� Select corrosion resistant steel type for kiln shell
� Sacrifice layercf also: Jøns, E.S. & Østergård, 1999, Investigation into kiln shell corrosion of rotary cement kilns, ZKG International,
vol. 52, No. 2, pp. 68-79
Measures to prevent kiln shell corrosion
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� Heat resistant up to 400°C
� Chemical resistance against alkaline salts
� Chemical resistance against corrosive gases (H2S, HCl, SO2, CO2 and O2)
� Resistance against acids
� Abrasion resistant (movement of the lining)
� Coating must be applicable on sight
Requirements for coatings
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Materials tested by University of Leoben / RHID. Jellen: “Verringerung der Ofenmantelkorrosion von Drehrohröfen in der Zementindustrie durch Beschichtungen”
� 3DLQWLQJV: - polyamide resin with graphite pigments- silicon resin with FeO pigments and inhibitors- silicon-modified alkyd resin- zinc ethyl silicate- phosphoric acid
� 0HWDOOLF�FRDWLQJ, flame sprayed: - Al- NiCr 50/50 - Ni based Cr-B- Zn, Zn/Al 85/15
� &HUDPLF�FRDWLQJ, plasma sprayed: - Al2O3
Studied types of coating
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� Ignition test at 400°C:
kiln atmosphere: CO2 20%H2O 10% SO2 2%HCl 2% O2 13%N2 53%
additionally the specimens were coated with alkaline-salts
� Low temperature corrosion:at 40°C in an exsiccator which base has been covered with HCl (21%)
Weight differences were measured over a longer period of time (up to1000 h)
Performed tests
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� Paintings: embrittlement occurs at moderate temperatures (e.g. 200°C),loss of adhesion on substrate
� Phosphate treatment guarantees no protection against corrosion at hightemperatures. After the ignition tests no phosphate detectable anymore.
� Metallic coating: protective effect is good / very good (e.g. Al) but adhesion on substrate is difficult to maintain
� Ceramic coating: high porosity of the ceramic layer leads to corrosion of the substrate and finally spalling of coating
Spray procedure expensive and time consuming (14 days one layer 5rm, 3,8m �), rough surface – difficult to clean from mortar
Results of investigation
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Fused cast refractories
• Zero permeability
• Very expensive
• Very brittle / low thermal shock resistance
Silica-impregnated alumina bricks (RESISTAL IS)
• Reduced open porosity (15-20% less porosity)
• Greatly reduced permeability (80-90% less permeability)
• Formation of protective reaction layer of alkali silicates at hot face („reaction sealing“)
Refractories with low (zero) permeability
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What is a sacrifice layer?
Method developed in Mexican cement plants („lamina de sacrificio“)
A sheet of stainless steel between refractory lining and kiln shell
Steel type: 1.4301 (AISI 304), 1.4571 (AISI 316)
Dimension: (A) 900 mm wide endless coil, 0.38 mm thickness, or (B) 1.220 x 2.440 mm, 0.79 mm thickness, point welded
Sacrifice layer against kiln shell corrosion
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900 mm wide endless coil, 0.38 mm thickness
Sacrifice layer (A)
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Type of steel used for sacrifice layer
Sacrifice layer (A)
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Cutting pieces of 2-3 m length
Sacrifice layer (A)
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Four sheets ready for installation
Sacrifice layer (A)
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SL installed between ANKRAL bricks and the cleaned kiln shell
Sacrifice layer (A)
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Sacrifice layer (A)
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Plates of 0.79 mm thickness are point welded to the kiln shell
Sacrifice layer (B)
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Sacrifice layer (C)
vertical overlap: 2 – 5cmhorizontal overlap: 5 – 20cm
magnetswooden bar
refractory bricks
stainless steel sheets 1.4301 / AISI 3040,40 x 1000mm
lining rig
Stage 1 Stage 2
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Steel shell with sacrifice layer (rm 30-35): very light corrosion
Steel shell without sacrifice layer (rm 52): strong corrosion
Fuel: Combustóleo + alternative fuels (PET, tyres, industrial waste)
Effect of sacrifice layer in a PREPOL kilnØ 4.8 x 75m, 4.800 tpd
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� Davila, J.C. et al., 1995, Influencia del Refractario en la Corrosion de la Coraza del Horno Rotatorio para Cemento, ALAFAR XXIV Congreso, Cartagena, pp 157-183
� Jøns, E.S. & Østergård, 1999, Investigation into kiln shell corrosion of rotary cementkilns, ZKG International, vol. 52, No. 2, pp. 68-79
� Jellen, D., 1999, Verringerung der Ofenmantelkorrosion von Drehrohröfen in der Zementindustrie durch Beschichtungen. Thesis, Montanuniversität Leoben.
References
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