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1 Alkali Corrosion of Refractories in Cement Kilns.

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1 1 Alkali Corrosion of Refractories in Cement Kilns

2 2 Alkali Corrosion Topics 1.Introduction to alkali corrosion of refractories 2.Characterization of corroded industrial refractory materials 3.Behavior of alkali salts and alkali salt mixtures 4.Mechanisms of alkali corrosion 5.Investigation methods 6.Conclusions

3 3 Alkali Corrosion Corrosion attack in cement rotary kilns Deuna Zement GmbH, Informationsmaterial 2005 high temperature thermal insulation material combustion of fuels raw material preparationclinker burningclinker storage cement mill clinker burning heat exchanger electrostatic filter grate cooler rotary kiln refractory lining high temperature thermal insulation material metallic components Introduction to Alkali Corrosion of Refractories

4 4 Alkali Corrosion Reason of alkali accumulation in the cement rotary kilns cement dust returns into the burning process implementation of raw meal preheating first with the Lepol grate improved preheating of cement raw meal in Humboldt air-suspension preheater and intensified due to alkali circulation use of secondary fuels, i.e. use of combustible waste instead of powdered coal ore oil Sources of corrosive substances alkali:included in natural raw materials, coal, secondary fuels chlorine:included in secondary fuels sulfur:included in natural raw materials, coal, oil, secundary fuels Introduction to Alkali Corrosion of Refractories E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

5 5 Alkali Corrosion P. Scur, Mitverbrennung von Sekundärbrennstoffen wie heizwertreiche Abfälle und Tiermehl in der Zementindustrie am Beispiel Zementwerk Rüdersdorf. VDI-Berichte Nr. 1708, 2002, S The use of secondary fuels Introduction to Alkali Corrosion of Refractories

6 6 Alkali Corrosion Combustion of secundary fuels The chlorine is particularly inserting in burning process:  chlorine containing compounds, not pure gas The chlorine is mainly included in:  polyvinylchlorid (PVC)  used tires  common salts of domestic waste The chlorine appearance tends to result:  changing of the reaction process  intensification of the refractory corrosion Reasons for this behavior:  formation of low viscous and aggressive fused salts at relatively low temperatures  high amount of the corrosive compound is gaseous  gases an melts can simply pass trought pores and cracks of working refractory material to the metallic bars  attack by chemical reaction and dissolution the fire-proof material behind  condensate on the metallic components leads to excessive corrosion phenomena Introduction to Alkali Corrosion of Refractories E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

7 7 Alkali Corrosion Secondary fuels solid (plastic, rubber, battery, animal residues; tyres, domestic waste...) liquid (used oil, tar, chemical wastes...) gaseous (landfill, pyrolysis gas) Alkalibursting and chemical spalling of the refractories Gas corrosion (condensation) of the metal components Organic Compounds alkalis sulfates chlorides. and other corrosive compounds fireclay insulating brick after 3 years in use in a cement rotary kiln (feed end) Effect of the combustion of secundary fuels in cement rotary kilns Introduction to Alkali Corrosion of Refractories E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

8 8 Alkali Corrosion Post mortem investigations Roof of kiln hood of the DOPOL-kiln: E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials Characterization of corroded industrial refractory materials Hot side (refractory bricks or concrets) Cool side (metal jacket) Calcium silicate Insulating brick basic abrasion lining

9 9 Alkali Corrosion Post mortem investigation Alkali corroded calcium silicate thermal insulating material in the chamber at 600 – 700 °C:  X-ray analysis Hot side area:  based on KCl and CaSO 4  residualNaCl, futher chlorides, Cr- and Fe-sulfates Characterization of corroded industrial refractory materials Calcium silicate thermal insulating material (thickness 25 mm) after 18 month in use in the chamber between the preheater and the rotary cement kiln. Hot side (refractory concrete) Cool side (metal jacket) E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

10 10 Alkali Corrosion Post mortem investigation Alkali corroded fireclay brick in the hot zone at 800 °C:  X-ray analysis Area around the crack:  based mainly on leucit (K 2 O  Al 2 O 3  4SiO 2 )  residualsilica (SiO 2 ), mullite (3Al 2 O 3  2SiO 2 ) corundum (Al 2 O 3 ) Characterization of corroded industrial refractory materials Fireclay brick from the chamber between the preheater and the rotary tube of the cement kiln after use (18 month), left heat site with a temperature between 800 to 900 °C. Hot side Cool side E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

11 11 Alkali Corrosion Post mortem investigation Alkali corroded fireclay insulating brick in the hot zone at > 1000 °C:  X-ray analysis Hot side area:  based mainly onleucit (K 2 O  Al 2 O 3  4SiO 2 ), mullite (3Al 2 O 3  2SiO 2 )  residualsilica (SiO 2 ), kalsilit (K 2 O  Al 2 O 3  2SiO 2 ) larnit (2CaO  SiO 2 ) Characterization of corroded industrial refractory materials Fireclay insulating brick after 3 years in use in a cement rotary kiln (feed end), front heat site with a temperature > 1000 °C. Hot side Cool side Infiltration zone E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

12 12 Alkali Corrosion Post mortem investigation Alkali corroded magnesia brick in the sinter zone at > 1100 °C:  X-ray analysis Hot side area:  based mainly onleucit (K 2 O  Al 2 O 3  4SiO 2 ), mullite (3Al 2 O 3  2SiO 2 )  residualsilica (SiO 2 ), kalsilit (K 2 O  Al 2 O 3  2SiO 2 ) larnit (2CaO  SiO 2 ) Characterization of corroded industrial refractory materials Magnesia brick after 2 years in use in a cement rotary kiln (sinter zone), above on the heat site with a temperature > 1000 °C. Hot side Cool side E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

13 Post mortem investigation Alkali corroded refractory concrete from the wall of a bottom cyclone of cement:  SEM-Analysis (pore size 100 to 200 µm) In pores and reacted layers:  “A” and “B” present deposit KCl  bubbly microstructure of KCl-layer is an evidence for its primary liquid state  “B” present cracks in the KCl-layer as a indication for differences of the thermal linear expansion coeffizients 13 Industrial refractory brick from the wall of a bottom cyclone of cement kiln after 1 year usage. A B Alkali Corrosion Characterization of corroded industrial refractory materials E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

14 14 Alkali Corrosion Characterization of corroded industrial refractory materials Validation of the industrial refractory materials by alkali attack Refractories based on aluminum silicate:  formation of feldspar  volume increase  alkali bursting Refractories based on calcium silicate  not stable in the exhaust  disintegration to CaCO 3, CaSO 4, SiO 2 without volume change Refractory bricks and concretes (based on alumina or magnesia)  deposit of substances in pores  spalling (spall in layers)  The formation of feldspar, the alkali bursting, the cracks and the fractional dropout are caused due to alkali corrosion attack. E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

15 15 Alkali Corrosion Alkali compounds in corroded refractory bricks and concretes The most of analyzed samples contained: Feldspar, KCl, Alkali sulfate, NaCl, Other chlorides Other sulfates  In summery, K and K-compounds are more “common” than Na and Na-compounds. E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials Characterization of corroded industrial refractory materials

16 16 Behavior of alkali salts and alkali salt mixtures Alkali Corrosion High temperature behavior of alkali salts and alkali salt mixtures Salts after heating at 1100°C in crucibles: Solid saltafter 1100°C Na 2 SO 4, K 2 SO 4  molten Na 2 CO 3, K 2 CO 3  molten NaCl, KCl  evaporated CaSO 4  sintered The solid salts as most reactive and corrosive mixtures after heating at 1100°C in crucibles: Salt mixturesafter 1100°C SM 1K 2 SO 4 / K 2 CO 3  melting SM 2 K 2 SO 4 / K 2 CO 3 / KCl  gas SM 3 K 2 SO 4 / K 2 CO 3 / KCl / CaSO 4  solid E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

17 17 Behavior of alkali salts and alkali salt mixtures Alkali Corrosion High temperature behavior of alkali salts and alkali salt mixtures Thermal linear expansion coefficient (  lin ) of solid salts and salt mixtures:  highest value: K 2 SO 4  lowest value: CaSO 4  is reflected in the value of the salt mixtures Solid salt  lin measured /K (20/600 °C)  lin literature /K (0 °C) KCl5266,2 K 2 SO ,6 K 2 CO ,3 CaSO 4 16 SM 1 (K 2 SO 4 /K 2 CO 3 )58 SM 2 (K 2 SO 4 /K 2 CO 3 /KCl)50 SM 3 (K 2 SO 4 /K 2 CO 3 /KCl/CaSO 4 )34 Thermal linear expansion coefficient (  lin ) of solid salts and salt mixtures E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

18 18 Behavior of alkali salts and alkali salt mixtures Alkali Corrosion High temperature behavior of alkali salts and alkali salt mixtures Density of solid and molten salts (literature):  density difference between liquid and solid salts  volume increase during heating up Hygroscopicity:  K 2 CO 3 are hygroscopic  KCl, K 2 SO 4, CaSO 4 are not hygroscopic  The volume expansion during heating up combined with the hygroscopicity (K 2 CO 3 ) leads to the destruction of the refractory in humid atmospheres. Solid salt Density of solid g/cm³ Density of melt g/cm³ Volume increase % Hygroscopicity KCl1,991,5231no K 2 SO 4 2,661,8941no K 2 CO 3 2,431,9624hygroscopic* CaSO 4 2,96no *weight increase app. 15 % after 4 days on normal area (24 °C, 60 % rel. humidity) E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

19 19 Behavior of alkali salts and alkali salt mixtures Alkali Corrosion Behavior of satured water based solutions of alkali salts and alkali salt mixtures pH-values of satured water based salt solutions:  K 2 CO 3 -solution is high alkaline  KCl-, K 2 SO 4 -, CaSO 4 -solutions are neutral to alkaline  solutions of salt mixtures are mainly high alkaline  The acid effect is not identifiable of the corrosion products of sheet-matall jacket of rotary kiln too. Salt solution pH-value directly after 8 days KCl7,997,69 K 2 SO 4 7,278,34 K 2 CO 3 13,8313,74 CaSO 4 9,697,92 SM 1 (K 2 SO 4 /K 2 CO 3 )12,1012,23 SM 2 (K 2 SO 4 /K 2 CO 3 /KCl)12,0912,14 SM 3 (K 2 SO 4 /K 2 CO 3 /KCl/CaSO 4 )12,0812,14 pH-values of satured water based salt solutions as a function of time at 21 °C. E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

20 20 Behavior of alkali salts and alkali salt mixtures Alkali Corrosion Behavior of satured water based solutions of alkali salts and alkali salt mixtures Electrical conductivity of satured water based salt solutions:  K 2 SO 4 is more soluble than CaSO 4  the value of electrical conductivity of CaSO 4 is increased by a factor 16  The corrosion due several micro processes is supported by Cl - and SO4 2-.  One of the corrosion mechanisms is based on electrochemical corrosion. Salt solution Electrical conductivity directly after 8 days KCl K 2 SO K 2 CO CaSO SM 1 (K 2 SO 4 /K 2 CO 3 )161 SM 2 (K 2 SO 4 /K 2 CO 3 /KCl)184 SM 3 (K 2 SO 4 /K 2 CO 3 /KCl/CaSO 4 )178 Electrical conductivity in µS/cm of satured water based salt solutions as a function of time at 21 °C. E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

21 21 Alkali Corrosion 4 main alkali corrosion mechanisms Melt formation Change of density and volume of the solid phase Expansion as a result of salt stored in pores Corrosion due to water condensation Mechanisms of alkali corrosion E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

22 22 Mechanisms of alkali corrosion Alkali Corrosion 1.Melt formation Alkali salt + refractory material:  formation of melts at 750 – 1450 °C (from literature) Alkali salt mixtures + refractory material:  partially melt formation at 600 – 950 °C  completely melt formation at 700 – 1000 °C (from phase diagrams)  In addition: presence of K 2 O and Na 2 O as reactive and corrosive substances at high temperature and water vapour Refractory oxid / melting point [°C] Alkali compound Temperature of 1. melting [°C] MgO / 2840 K 2 SO 4 K 2 CO 3 Na 2 O K 2 O No miscibility CaO / 2580 KCl + NaCl CaSO 4 Na 2 O K 2 O No miscibility Cr 2 O 3 / 2200 KCl + K 2 O K 2 O Al 2 O 3 / 2050 Na 2 O K 2 O TiO 2 / 1830 K 2 SO 4 + K 2 O Na 2 O K 2 O SiO 2 / 1713 Na 2 O K 2 O MgO + Al 2 O 3 / 1925No dates Al 2 O 3 + SiO 2 / 1595 Na 2 O K 2 O MgO + SiO 2 / 1543 Na 2 O K 2 O CaO + SiO 2 / 1436 Na 2 O K 2 O CaO + Al 2 O 3 / 1395No dates Temperature from the 1. melting for refractory oxids or oxids mixturs with compounds of alkalis from the phase diagrams. E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

23 23 Mechanisms of alkali corrosion Alkali Corrosion 1.Melt formation Magnesia Phase diagram of the system K 2 SO 4 – MgO:  melt formation of eutectic at 1067 °C Phase diagram of the system K 2 CO 3 – MgO:  melt formation of eutectic at 895 °C similar behavior is due of the system KCl - MgO  MgO based refractory materials are not alkali resistant because melt formation at 895 °C. E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

24 24 Mechanisms of alkali corrosion Alkali Corrosion 1.Melt formation SiO 2 -based refractories Phase diagram of the system Na 2 O – SiO 2 :  melt formation at 782 °C resp. 789 °C  complete melt of by 26 % Na 2 O  no strength of solid structure (25 % melt) by 4 % Na 2 O at 1300 °C Phase diagram of the system K 2 O – SiO 2 :  melt formation at 769 °C  complete melt of eutectic by 27 % K 2 O  no strength of solid structure (25 % melt) by 4 % K 2 O at 1300 °C  25 % eutectic melt by 6,5 % Na 2 O or K 2 O at 800 °C  Strong effect of flux of the alkalis leads to damage of SiO 2 -based refractories at 700 and 800 °C by a melt formation E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

25 25 Alkali Corrosion Alkali Corrosion of Therml Insulating Material Based of Calcium Silicates 1.Melt formation Calcium silicate Phase diagram of the system Na 2 O – CaO – SiO 2 :  lower volume expansion of reaction products  melt formation of eutectic at 720 °C Phase diagram of the system K 2 O – CaO – SiO 2 :  melt formation of eutectic at < 720 °C  Refractory materials based on wollastonite no alkali resistant, because melt formation at 700 °C.

26 26 Mechanisms of alkali corrosion Alkali Corrosion 1.Melt formation Applied Temperatures in presence of alkali < 1300 °C, because of melt formation below 1100 °C:  refractory oxides MgO, CaO, Cr 2 O 3, TiO 2 and SiO 2  binary combinations Al 2 O 3 /SiO 2, CaO/SiO 2, MgO/SiO 2 Applied Temperatures in presence of alkali > 1300 °C:  refractory oxid Al 2 O 3  binary combinations Al 2 O 3 /MgO, Al 2 O 3 /CaO could be „suitable“ (no dates of melt formation) E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

27 2.Change of density and specific volume of the solid phase Alkali compounds unknown:  MgO, CaO Densities of refractory oxids:  > 3 g/cm³ (except SiO 2, CaO  SiO 2 ) Densities of new formed alkali compounds:  < 3 g/cm³ (most frequently)  The volume increase of solid phase of the refractory oxides containing alkali compounds leads to an attrition of microstructure and the damage of refractory lining. 27 Mechanisms of alkali corrosion Alkali Corrosion Refractory oxide Density g/cm³ New formed alkali compounds Density g/cm³ Volume change % Al 2 O 3 3,99(N,K) 1…6 A 1…11 2,63…3,42+17…+52 Cr 2 O 3 5,25NC4,36+20 SiO 2 2,65(N, K) 1…3 S 1…4 2,26…2,96-10…+17 3Al 2 O 3  2SiO 2 3,17 (N,K) 1…3 AS 1 …6 N 3 CA 3 S 6  (SO 4 ) 2,40…2,62+21…+32 CaO  6Al 2 O 3 3,69(N,K)C 0…14 A 4…11 3,03…3,31+11…+22 MgO  Al 2 O 3 3,55… 3,70 NM 0,8…4 A 5…15 3,28…3,33 +7…+13 2MgO  SiO 2 3,22(N,K) 1…2 M 1…5 S 3…12 2,56… 3,28 -2…+23 CaO  SiO 2 2,92(N,K) 1…2 C 1…23 S ,72…3,36-13…+7 Refractory oxids, possible alkali compounds (cement chemistry notation) from the phase diagrams, whose densities and change of volume („+“ expansion, „-“ shrinkage). E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

28 K 2 O  MgO  SiO 2 2.Change of density and specific volume of the solid phase Phase diagram of system MgO – SiO 2 – K 2 O with forsterite:  formation of solids at 1100 – 1300 °C 2MgO  SiO 2, MgO, K 2 O  MgO  SiO 2, K 2 O Change of densities e.g. specific volume by chemical reaction of forsterite with K 2 O:  expansion and shrinkage  Refractory materials based on forsterite no alkali resistant, because volume increase leads to destruction of the structure 28 Mechanisms of alkali corrosion Alkali Corrosion E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials Solid Density g/cm³ Specific volume cm³/g 2MgO  SiO 2 3,220,311 MgO3,590,279 K 2 O  MgO  SiO 2 2,760,362 K2OK2O2,330,429

29 Mullite Fireclay 1556 °C 29 Alkali Corrosion 2.Change of density and specific volume of the solid phase Phase diagram of system K 2 O – Al 2 O 3 – SiO 2 with mullite and fireclay:  formation of solids with lower densities at < 1556 °C mullite react to corundum fireclay react to alkali feldspar  first eutectic melts appear at 1556 °C similar behavior is due of the system Na 2 O – Al 2 O 3 – SiO 2  Lower density of products by reactions of K 2 O and Na 2 O with mullite and fireclay leads to:  high volume expansion  “alkali bursting”  damage of refractories Mechanisms of alkali corrosion E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

30 Volume expansion of mullite and fireclay by reaction with K 2 O or Na 2 O 30 Alkali Corrosion 2.Change of density and specific volume of the solid phase Calculated volume expansion of mullite and fireclay depend on the content of K 2 O or Na 2 O (from phase components and densities) Mullit: 22 % volume increase with 8 % linear expansion by formation of corundum Fireclay: volume expansion decrease at a K 2 O/Na 2 O-content of > 20 % Volume expansion in % Content of K 2 O or Na 2 O in % by weight Mullite + K 2 O Mullite + Na 2 O Fireclay + K 2 O Fireclay + Na 2 O Mechanisms of alkali corrosion E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

31 31 Alkali Corrosion 2.Change of density and specific volume of the solid phase Phase diagram of system K 2 O – CaO – Al 2 O 3 with hibonite:  formation of solids at 1100 °C with high volume expansion Phase diagram of system Na 2 O – CaO – Al 2 O 3 with hibonite:  more expansion of volume than with K 2 O  Refractory materials based on hibonite are not alkali resistant, because the volume expansion at 1100 °C leads to a damage of the structure (contrary to literature opinion) Mechanisms of alkali corrosion E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

32 32 Alkali Corrosion 2.Change of density and specific volume of the solid phase Phase diagram of system Na 2 O – Al 2 O 3 with alumina:  formation of solids at < 1300 °C  melt formation of eutectic at 1580 °C Phase diagram of system K 2 O – Al 2 O 3 with alumina:  formation of solids at < 1300 °C  melt formation of eutectic at 1910 °C  Refractory materials based on alumina are not alkali resistant, because the volume expansion up to 1000 °C leads to a damage of the structure  up to 1400 °C destruction of the aluminates (NaAlO 2, KAlO 2 ) and evaporation of alkalis  Exception:  -alumina with “alkali resistant considerations” Mechanisms of alkali corrosion E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

33 2.Change of density and specific volume of the solid phase The increased volume of the solid phases to 52% is leading to bursting of solid structures. Less known and in contrast to the general opinion are the following topics:  Alumina Al 2 O 3 reacts to alkali aluminates with a volume increase to 52 % and leads to a destruction of the products.  Cr 2 O 3 leads to expansion by reaction with alkalis.  The density modifications of SiO 2 and calcium silicates taking place by melting. The volume increase of solid parts by melting is not a problem, but the melt formation and the deformation of the products.  Fireclay reacts to feldspars and shows a volume increase between 21 to 32 %. This corrosion process is known as “alkali bursting”.  Hibonite, known as alkali-resistant, reacts to β-alumina, and presents a volume increase of about 22 %.  Spinel reacts to (Na 2 O ⋅ MgO ⋅ Al 2 O 3 )-compounds, like β-alumina, and leads to volume increase of approximately 13 %.  Forsterite reacts to alkali compounds and shows a volume increase to 23 %. Forsterite is also,( contrary to literature opinion), not alkali corrosion resistant. 33 Mechanisms of alkali corrosion Alkali Corrosion E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

34 34 Alkali Corrosion 3.Expansion phenomena Salt storage in pores of refractories:  evaporation of salt at high temperatures  condensation of salt in cooler range of refractory materials  pores are filled entirely with liquid or solid salts Destruction mechanisms:  thermal linear expansion of salts 5- to 10-fold more than refractory materials  thermal shock sensibility of refractory material is increased  volume increase between solid and liquid salt (change of densities)  hygroscopicity of salts and volume increase (destruction in humid atmosphere) Mechanisms of alkali corrosion Industrial refractory brick from the wall of a bottom cyclone of cement kiln after 1 year usage. A B E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials

35 35 Alkali Corrosion 4.Corrosion due to water condensation Satured water based salt solutions:  pH-values are neutral to alkaline (no acid!!) Metal corrosion  pH-value < 10  electrochemical corrosion  Investigations for the future Mechanisms of alkali corrosion E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials Alkali corrosion of a steel bar in a gradient furnace after treatment at 1000°C.

36 36 Alkali Corrosion Sumary of the alkali corrosion mechanisms  physical-chemical high temperature melting processes associated with solution, sintering and shrinkage  chemical material conversion under solid conditions and so modification of density of solid refractory phases causing bursting effects  mechanical stresses/bursting between solid salt in the pores and the refractory material  chemical material conversion followed by expansion and shrinkage due to water condensation and removal of water condensation products E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials Mechanisms of alkali corrosion

37 Coating of solid raw material with solid salt particles 37 Alkali Corrosion Investigations of the alkali resistance – “disc-test” Disc-test: pressed disc based on 70 % refractory powder and 30 % salt mixture (K 2 SO 4, KCl, K 2 CO 3 ) Change of sample diameter, weight and visual features of refractory/salt heat treated discs under periodic heating and cooling conditions Fireclay: diameter increase from 50 to 53 mm  linear expansion of 6 % due to alkali bursting Disc-test of fireclay salt briquette before and after heat treatment at 1100 °C for 5 hours unfired 1100 °C / 5 hours Investigation methods solid raw material particle layer of solid salt particles mixture of solid raw material + alkali salts U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels

38 Expansion and shrinkage of the different mixtures after treatment at 1100 °C and 5 h 38 Alkali Corrosion Investigations of the alkali resistance – “disc-test” Change of diameter after 1100 °C at 5 hours  high value of expansion Zirconia mullite Z72 Spinel MA 76 Spinel AR 78 Hibonite SLA-12 Hibonite Bonite Forsterite Olivin Aluminium titanate  high value of shrinkage Zirconia 3Y-TZP  suitable materials Zirconia 3,5Mg-PSZ Na-aluminate  -alumina Betacalutherm (dried, fired) U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels Salt mixtures SM 1K 2 SO 4 / K 2 CO 3 SM 2K 2 SO 4 / K 2 CO 3 / KCl SM 3K 2 SO 4 / K 2 CO 3 / KCl / CaSO 4 Investigation methods

39 Expansion and shrinkage of the different mixtures after treatment at 1300 °C and 5 h 39 Alkali Corrosion Investigations of the alkali resistance – “disc-test” Change of diameter after 1300 °C at 5 hours  high value of expansion Zirconia mullite Z72 Spinel AR 78 Hibonite SLA-12 Hibonite Bonite Forsterite Olivin Aluminium titanate  high value of shrinkage Zirconia 3Y-TZP Zirconia 3,5Mg-PSZ Na-aluminate Spinel MA 76  suitable materials  -alumina Betacalutherm (dried, fired) Salt mixtures SM 1K 2 SO 4 / K 2 CO 3 SM 2K 2 SO 4 / K 2 CO 3 / KCl SM 3K 2 SO 4 / K 2 CO 3 / KCl / CaSO 4 Investigation methods U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels

40 40 Alkali Corrosion Investigations of the alkali resistance – “disc-test” Change of diameter after 1300 °C at 50 hours  high value of expansion Spinel AR 78 Forsterite Olivin  suitable materials  -alumina Betacalutherm (dried, fired) Spinel MA 76 Salt mixtures SM 1K 2 SO 4 / K 2 CO 3 SM 2K 2 SO 4 / K 2 CO 3 / KCl SM 3K 2 SO 4 / K 2 CO 3 / KCl / CaSO 4 Expansion and shrinkage of the different mixtures after treatment at 1300 °C and 50 h Investigation methods U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels

41 41 Alkali Corrosion Investigations of the alkali resistance – “disc-test” Change of diameter after 1100 and 1300 °C, 5 and 50 hours hold time  high value of expansion Zirconia mullite Z72 Spinel MA 76 Spinel AR 78 Hibonite SLA-12 Hibonite Bonite Forsterite Olivin Aluminium titanate  high value of shrinkage Zirconia 3Y-TZP Zirconia 3,5Mg-PSZ Na-aluminate  suitable materials  -alumina Betacalutherm (dried, fired)  Betacalutherm and  -alumina are long-time and alkali resistant after that as the only fire-proof materials up to 1300 °C Samples for change of disc diameter after heating at 1300 °C and 5 h Investigation methods U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels

42 42 Alkali Corrosion Investigations of the alkali resistance – “disc-test” Influence of humidity of alkali-infiltrated used raw materials:  increase of sample weight 30 – 70 %  The sample weight had increased because the humidity had condensed in the pores of the sample structure. Increase of sample weight after heat treatment and storage time at 20 °C and 100 % rel. humidity. Salt mixtures SM 1K 2 SO 4 / K 2 CO 3 Investigation methods U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels

43 43 Alkali Corrosion Investigations of the alkali resistance – “disc-test” Influence of humidity of alkali-infiltrated used raw materials:  volume increase < 1 %  volume decrease < 1 %  The water absorption of alkali infiltrated samples took place with out or minor changes in volume at high humidity across month.  The alkali infiltrated Betacalutherm and  -alumina take in humidity and dehumidify without change in volume again and no destruction of the structure. Change of sample volume after heat treatment and 2 and 3 months storage time at 20 °C and 100 % rel. humidity. Salt mixtures SM 1K 2 SO 4 / K 2 CO 3 Investigation methods U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels

44 44 Alkali Corrosion Investigations of the alkali resistance – crucible test according DIN Crucibel test: DIN °C for 5 hours salt mixture K 2 SO4, K 2 CO 3 Refractory concrete on the base of Fireclay:  completely infiltration of the salt mixture  alkali bursting lead to critical cracks  damage of the crucible at low temperature and short exposure time Crucible test of castable gunning material according to DIN 51069, after heat treatment at 1000 °C for 5 hours bottom crucible upper crucible sealing alkali salt mixtur alkali gas E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials Investigation methods

45 45 Alkali Corrosion Investigations of the alkali resistance – crucible test according DIN Crucibel test: DIN °C / 800 °C for 5 hours salt mixture K 2 SO 4, K 2 CO 3 and salt mixture K 2 SO 4, K 2 CO 3, KCl, CaSO 4 Calcium silicate thermal insulating material:  infiltration with partly fluid salt melt at 700 °C  damage the crucible at 800 °C  partly dissolving of the calcium silicate in the salt melt Melt formation at low temperature (720 °C) Calcium silicate thermal insulating material with salt mixture K 2 SO 4 and K 2 CO 3 at 700 °C for 5 h Calcium silicate thermal insulating material with salt mixture K 2 SO 4, K 2 CO 3, KCl and CaSO 4 at 800 °C for 5 h Investigation methods

46 46 Alkali Corrosion Investigations of the alkali resistance – test in a gradient furnace Gradient furnace:  gradient of temperature °C  alkali atmosphere Thermal insulation material:  Betacalutherm Refractory material:  refractory concrete Steel bar:  austenitic steel with scaling resistance to 1000 °C Salt mixtures:  K 2 SO 4 / K 2 CO 3 / KCl Investigation methods Wall built-up for corrosion test in gradient furnace

47 47 Alkali Corrosion Investigations of the alkali resistance – test in a gradient furnace Thermal insulation material:  Betacalutherm with out corrosion effects Refractory material:  refractory concrete with cracks, volume increase (2-3%), formation of feldspar in the hot zone Steel bar:  scaling with volume increase (33-56 %) in the hot zone  Verification of the post mortem investigations of the industrial refractory materials Investigation methods Wall built-up after corrosion test in gradient furnace: left – scaling of the steel bar in the alkali corroded refractory material; right – Betacalutherm without corrosion effects

48 48 Alkali Corrosion Conclusions of alkali corrosion of the refractory materials Worst corrosion – bursting effect:  salt mixture of K 2 SO 4 / K 2 CO 3 No alkali resistant:  all refractory oxides  all refractory mixtures “alkali resistant considerations”:  low alumina content materials (  -alumina doped material)  -alumina:  alkali aluminate (5 to 11 mol Al 2 O 3, 1 mol Na 2 O or K 2 O)  melting point 1580 – 2053 °C   -alumina does not melt ore react with higher content of alkalis at temperatures below 1580 °C Conclusions E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials Refractory oxide Alkali oxid Damage by SiO 2 Na 2 OMelt up to 782 °C K2OK2OMelt up to 769 °C Calcium silicate Na 2 OMelt up to 720 °C K2OK2OMelt up to 700 °C Al 2 O 3 Na 2 O10% volume expansion by 3% Na 2 O K2OK2O10% volume expansion by 4% K 2 O Mullite Na 2 O10% volume expansion by 14% Na 2 O K2OK2O10% volume expansion by 17% K 2 O Fireclay Na 2 O10% volume expansion by 16% Na 2 O K2OK2O10% volume expansion by 15% K 2 O ForsteriteK2OK2O10% volume expansion by 34% K 2 O SpinelNa 2 O10% volume expansion by 7% Na 2 O Hibonite Na 2 O10% volume expansion by 5% Na 2 O K2OK2O10% volume expansion by 6% K 2 O Sumary of phase diagrams

49 49 Refractories for gasification process

50 Introduction to Refractories for Gasification Processes 50 Refractories for gasification process Wear mechanisms of refractories in slagging gasifiers J.P. Bennett, Refractory liner materials used in slagging gasifiers

51 Introduction to Refractories for Gasification Processes 51 Refractories for gasification process C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification Potential Refractory Problems in Coal Gasification 1alkali attack 2carbon monoxide disintegration 3silica volatilization 4steam-related reactions 5thermoelastic stresses 6erosion due to solid particulates 7corrosion and erosion due to molten coal slag and/ or iron 8iron oxide bursting dry ash gasifiers slagging gasifiers

52 Introduction to Refractories for Gasification Processes 52 Refractories for gasification process C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification Potential Refractory Problems in Coal Gasification Corrosion and Erosion by Molten Coal Slag and/or Iron High-purity alumina, chrome-magnesia, alumina-zirconia-silica, zirconia, SiC  Grand Forks Energy Technology Center (GFETC)  less than 10 h at 1550 °C lifetime  Ruhrchemie Texaco gasifier  hundreds of hours at 1600 °C lifetime  lifetime depends on conditions (unique for single gasifier) and coal/ slag (e.g. CaO/SiO 2 1) major mechanisms of the corrosion process: dissolution, penetration and disruption, and erosion higher velocity slag  rate of corrosion ↑  dissolution and/or erosion ↑

53 Introduction to Refractories for Gasification Processes 53 Refractories for gasification process C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification Potential Refractory Problems in Coal Gasification Corrosion and Erosion by Molten Coal Slag and/or Iron dense high-chromia content refractories  superior corrosion resistance to CaO/SiO 2 = high-iron oxide acidic coal slag at 1575  chrome-spinel (MgCr 2 O 4 )  low solubility of Cr 2 O 3 and MgCr 2 O 4 in SiO 2 -Al 2 O 3 -CaO liquids refractories containing > 30 % Cr 2 O 3  reaction with all types of coal slags to form complex spinels (slowly dissolution)  problems: poor thermal-shock resistance and susceptible to iron oxide bursting high alumina refractory intermediate in performance in acidic slags and poor in basic slags SiC + Fe x O y → ferrosilicon alloy (low melting) magnesia-chromite refractories better in basic slags than in acidic slags (dissolution of MgO in all cases)

54 Introduction to Refractories for Gasification Processes 54 Refractories for gasification process C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification Potential Refractory Problems in Coal Gasification Thermal Shock Resistance of Brick Linings only few data available (Fig.) dense high-chromia (~ 80 wt%) have significantly lower thermal shock resistance than sintered low-chromia bricks (e.g. 90 wt% Al 2 O wt% Cr 2 O 3 ) improvement of the thermal shock resistance by microstructural alteration heating and cooling rates have to be carefully controlled to avoid spalling

55 Introduction to Refractories for Gasification Processes 55 Refractories for gasification process C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification Potential Refractory Problems in Coal Gasification Iron Oxide Bursting absorbed iron oxides leads to failures in spinel containing refractories ferrite spinels have larger unit-cell sizes than chromites or aluminates (Fig.)  reactions with Fe x O y leads to internal stresses  spalling Fe +2 /Fe +3 ratio depends on partial O 2 -pressure (unit cell size alters) low porosity limits the penetration of iron oxides from the slag  spalling occurs only in a thin surface layer (problem: cracks due to thermal shock

56 Introduction to Refractories for Gasification Processes 56 Refractories for gasification process C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification Potential Refractory Problems in Coal Gasification Alkali Attack formation of low-melting low-viscosity liquids or dry alkali-alumino-silicate compounds problems occurs in the non-slagging regions of gasifiers most coal slags contain significant amounts of alkali (1-10%) Na (g) + atmosphere → NaOH NaOH + refractory (mullite) → NaAlSiO 4 + NaAl 11 O 17 (~ 30% volume expansion) minimizing the alkali attack by:  use of low-alkali coals  lower process temperatures (decrease efficiency)  higher density of refractories (limitation of the penetration)  use of high-silica refractories (60 wt%)  react with alkali to produce glass  sealing off of the surface

57 Introduction to Refractories for Gasification Processes 57 Refractories for gasification process C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification Potential Refractory Problems in Coal Gasification Carbon Monoxide Induced Disintegration 2 CO = C + CO 2 ( °C, red. atm.)  deposition of carbon  refractory failure caused by internal stresses accelerated by metallic iron, free iron oxides, iron carbides no reported failures but laboratory experiments (Fig.) rate of attack increases rapidly as the pressure increases small amounts of iron (0.25 wt%) affect the rate  alumina castables loose strength in pure CO alkali compounds increase the attack rate H 2 S retard attack

58 Introduction to Refractories for Gasification Processes 58 Refractories for gasification process C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification Potential Refractory Problems in Coal Gasification Reduction of Silica by H 2 (reducing and steam-containing atmosphere) loss of silica due to formation of volatile compounds e.g. 50% loss of silicate refractory in a secondary ammonia reformer after several years no changes of silica content at a depth of ~10 mm from the hot face  indicates extremely slow diffusion rate of SiO below 1200 °C SiO 2 (s) +H 2 → SiO (g) + H 2 O

59 Introduction to Refractories for Gasification Processes 59 Refractories for gasification process C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification Potential Refractory Problems in Coal Gasification Steam-Related Reactions coal gasification atmosphere containing high partial pressures of steam:  SiC disintegration  strength loss in phosphate- bonded refractories  no degradation of cement- bonded castables Results applicable to low-temperature sections of most gasifiers. ( °C)

60 Introduction to Refractories for Gasification Processes 60 Refractories for gasification process Refractory problems in coal gasification C – Physical wear – “spalling” J.P. Bennett, Refractory liner materials used in slagging gasifiers

61 Introduction to Refractories for Gasification Processes 61 Refractories for gasification process C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification Potential Refractory Problems in Coal Gasification Thermomechanical Degradation of Monolithic Linings cracking during initial dryout and heat-up of monolithic refractory lining mechanical reliability of the lining can be improved by: (1)minimizing the amount of linear shrinkage of the refractory (2)continuous, slow heat-up rate (3)elimination of long hold periods during the heating and cooldown (4)maintaining the vessel shell temperature as close to ambient as possible (5)using incompressible bond barriers (6)using anchor spacings greater than 1.5 times the lining thickness

62 Introduction to Refractories for Gasification Processes 62 Refractories for gasification process C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification Potential Refractory Problems in Coal Gasification Erosion of Refractory Materials Testing methods: direct-impingement: (dolomite and sand particles vs. refractory)  chrome castable more erosion resistant than high-alumina and lightweight castable fluidized-bed: (ambient temperature and 810 °C with dead-burned dolomite)  high- and intermediate-alumina castables more erosion resistant than chrome castable impingement-tube: (simulates hot-gas transfer lines with dolomite)  high- and intermediate-alumina castables performed well  erosion occurs primarily in the softer matrix

63 Introduction to Refractories for Gasification Processes 63 Refractories for gasification process Corrosion Mechanisms dissolution formation of an intermediate compound solid solution Kwong, et al., Wear Mechanisms of Chromia Refractories in Slagging Gasifiers

64 Introduction to Refractories for Gasification Processes 64 Refractories for gasification process Kwong, et al., Wear Mechanisms of Chromia Refractories in Slagging Gasifiers oxygen partial pressure in a gasifier range from to oxygen potential affects: (1)valence state of transition oxides such as iron and vanadium oxides (2)oxide basicity (3)basicity of slags formed from iron and vanadium oxides (4)melting point of the slags  oxygen potential influences slag – refractory reactions and the compounds formed

65 Introduction to Refractories for Gasification Processes 65 Refractories for gasification process Thermodynamic calculations - HSC Chemistry ® Kwong, et al., Wear Mechanisms of Chromia Refractories in Slagging Gasifiers V 3 O 5 should be stable phase in gasifiers environments FeO with some Fe 3 O 4 may be stable phase formed at oxygen partial pressure of to 10 -9

66 Introduction to Refractories for Gasification Processes 66 Refractories for gasification process Material development from the 1970’s until today R. Dürrfeld, Refractories in Coal Gasification Plants

67 Introduction to Refractories for Gasification Processes 67 Refractories for gasification process Evaluated materials in the 1970’s and 1980’s alumina-silicate high alumina chromia-alumina-magnesia spinels alumina and magnesia alumina and chrome SiC chrome materials with phosphate  only materials with high chrome oxide content (min. 75 wt.-%) (reaction between chromia and FeO) J.P. Bennett, Low chrome/ chrome free refractories for slagging gasifiers

68 Introduction to Refractories for Gasification Processes 68 Refractories for gasification process today’s researches – low /no chrome oxide alumina with ZrO 2, MgO and additives alumina-zirconia with MgO, SiC and additives HfO 2, HfSiO 4 ZrSiO 4 NiAl 2 O 4  researches still in progress J.P. Bennett, Low chrome/ chrome free refractories for slagging gasifiers M. Müller et al., Corrosion behaviour of chromium-free ceramics for liquid slag removal in pressurized pulverized coal combustion

69 69 Thank you for your attention!


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