1. Alkali Corrosion of Refractories in Cement Kilns
Alkali attack is a chronic problem in the most popular high temperature applications such as blast furnaces, gasifiers, glass furnaces and cement kilns.
Especially in the last years the problem of alkali corrosion is dramatically increased due to the waste burning and the combustion of the so called secondary fuels in kilns at high temperature processes. 1

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

3. Introduction to Alkali Corrosion of Refractories
raw material preparation
clinker burning
clinker storage
cement mill
Corrosion attack
in cement rotary kilns
clinker burning
high temperature thermal insulation material
electrostatic filter
heat exchanger
combustion of fuels
rotary kiln
grate cooler
An einem besonders krassen Beispiel einer starken korrosiven Beanspruchung feuerfester Wärmedämmstoffe durch den Einsatz s. g. Sekundärbrennstoffe in der Zementindustrie soll der Prozess der Alkalikorrosion erläutert werden.
In ähnlicher Weise ist eine solche Alkalikorrosion bei Müllverbrennungsanlagen, in der Glas-, - und Keramikindustrie, der Metallurgie, in Kesselfeuerungen u. a. Hochtemperaturanlagen, die alkalihaltige Roh- und Brennstoffe verwenden zu beobachten, wenn teilweise auch in abgeschwächter Form
Beim Zementherstellen werden im Zementdrehrohrofen die Rohklinker mit den Brennstoffen verbrannt. Die Abkühlung der heißen Verbrennungsgase erfolgt dabei in Wärmetauschern (bei ca. 800°C). Bei Verbrennung von Sekundärbrennstoffen können wegen der unterschiedlichen Heizwerte Temperaturschwankungen mit Spitzen bis 1400°C entstehen. Dadurch wird die Abgastemperatur erhöht und die Gase kühlen sich im Wärmetauscher langsamer ab. Die Diffusion der Gase in die porösen Feuerfest- und Wärmedämmstoffe führt zu einer Kristallisation von Chloriden und Sulfaten in den Poren. Diese zerstören das Material durch chemische Reaktionen, Auflösung in Schmelzen oder Kristallisationsdruck.
Durch Kondensation der Dämpfe infolge Unterschreitung des Taupunktes am Blechmantel des Ofens o. an den Metallankern für die Wärmedämmstoffe und Korrosion der Metallteile durch Schwefel- oder Salzsäure kann die gesamte Konstruktion eines solchen Ofens statisch instabil werden (Standsicherheit).
In den Brennprodukten (Zementklinker) wird der Clorid-, Sulfat- und Alkaligehalt erhöht.
Insgesamt führt das dazu, das die Lebenserwartung solcher Anlagen sich dramatisch verringert: von 5-10 Jahren auf 1-2 Jahre sinkt.
Ganz extrem wird nach Wartungsarbeiten an den HT-Analgen, bei denen das Aggregat auf Umgebungstemp. abgekühlt werden muss, dieser Korrosionsprozess beobachtet.
Soforthilfe in Industrie:
-Um Taupunkt zu verschieben wird auf Wärmedämmung ganz verzichtet  Überhitzung des Metallmantels  Verbrennungsgefahr, Anlage wird statisch instabil
- Fugen im ff. Mauerwerk leiten aggressive Gase vom ff. Material weg, aber WDS wird noch stärker angegriffen  Dichtungsschichten nicht 100%-dicht auftragbar
refractory lining
high temperature thermal insulation material
metallic components
Deuna Zement GmbH, Informationsmaterial 2005
Introduction to Alkali Corrosion of Refractories 3

4. Introduction to Alkali Corrosion of Refractories
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
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Introduction to Alkali Corrosion of Refractories 4

5. Introduction to Alkali Corrosion of Refractories
The use of secondary fuels
Der Anteil der Sekundärbrennstoffe bei der Verbrennung steigt ständig, vor allem in der Zementindustrie (blaue Balken).
P. Scur, Mitverbrennung von Sekundärbrennstoffen wie heizwertreiche Abfälle und Tiermehl in der Zementindustrie am
Beispiel Zementwerk Rüdersdorf. VDI-Berichte Nr. 1708, 2002, S
Introduction to Alkali Corrosion of Refractories 5

6. Introduction to Alkali Corrosion of Refractories
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
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Introduction to Alkali Corrosion of Refractories 6

7. Alkali Corrosion
Effect of the combustion of secundary fuels in cement rotary kilns
Secondary fuels
solid
(plastic, rubber, battery, animal residues; tyres, domestic waste...)
liquid
(used oil, tar, chemical wastes...)
gaseous
(landfill, pyrolysis gas)
Organic Compounds
alkalis
sulfates
chlorides .
and other corrosive compounds
fireclay insulating brick after 3 years in use in a cement rotary kiln (feed end)
Alkalibursting and chemical spalling of the refractories Gas corrosion (condensation) of the metal components
The secondary solid, liquid and gaseous fuels contain higher amounts of alkalis, sulfates, chlorides and other corrosive compounds, also organics.
According to the literature destruction of the refractory can occur by the formation of low-melting low-viscosity liquids, or, more usually by the formation of dry expansive alkali-alumino-silicate compounds that result to chemical spalling.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Introduction to Alkali Corrosion of Refractories 7

8. Characterization of corroded industrial refractory materials
Alkali Corrosion
Post mortem investigations
Roof of kiln hood of the DOPOL-kiln:
Hot side (refractory bricks or concrets)
Cool side (metal jacket)
Calcium silicate
basic abrasion lining
Insulating brick
Ansicht der Zustellung eines Ofenkopfes im Zementdrehrohrofen nach 8-jährigem Einsatz (Fa. Deuna Zement GmbH):
Zustellung von kalt nach heiß: RT 102 P (Calciumsilikat)  FL 75/24 (Feuerleichtstein)  ALMAG 85 (basisches Verschleißmauerwerk Magnesia-Spinell)
 Hier ist Calziumsilikatschicht in ihrer porösen Struktur erhalten, hat aber eine sehr geringe Festigkeit; keine Alkaliverbindungen gefunden, obwohl davor liegender Feuerleichtstein durch Alkalien korrodiert wurde, damit dürften keine Alkaliverbindungen zum Ofenblechmantel vorgedrungen sein
 Feuerleichtstein auf heißen Seite durch Alkalien korrodiert.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Characterization of corroded industrial refractory materials 8

9. Characterization of corroded industrial refractory materials
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 CaSO4
 residual NaCl,
futher chlorides,
Cr- and Fe-sulfates
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
Characterization of corroded industrial refractory materials 9

10. Characterization of corroded industrial refractory materials
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 (K2OAl2O34SiO2)
 residual silica (SiO2),
mullite (3Al2O32SiO2)
corundum (Al2O3)
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
Characterization of corroded industrial refractory materials 10

11. Characterization of corroded industrial refractory materials
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 on leucit (K2OAl2O34SiO2),
mullite (3Al2O32SiO2)
 residual silica (SiO2),
kalsilit (K2OAl2O32SiO2)
larnit (2CaOSiO2)
Cool side
Infiltration zone
Hot side
Fireclay insulating brick after 3 years in use in a cement rotary kiln (feed end), front heat site with a temperature > 1000 °C.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Characterization of corroded industrial refractory materials 11

12. Characterization of corroded industrial refractory materials
Alkali Corrosion
Post mortem investigation
Alkali corroded magnesia brick in
the sinter zone at > 1100 °C:
    X-ray analysis
Hot side area:
 based mainly on leucit (K2OAl2O34SiO2),
mullite (3Al2O32SiO2)
 residual silica (SiO2),
kalsilit (K2OAl2O32SiO2)
larnit (2CaOSiO2)
Hot side
Cool side
Magnesia brick after 2 years in use in a cement rotary kiln (sinter zone), above on the heat site with a temperature > 1000 °C.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Characterization of corroded industrial refractory materials 12

13. Characterization of corroded industrial refractory materials
Alkali Corrosion
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
Industrial refractory brick from the wall of a bottom cyclone of cement kiln after 1 year usage. A B
In the figure position “A” and “B” present the deposit of KCl in the refractory concrete pores (with a pore size of approx. 100 to 200 µm. )
This sample was taken from the wall of a bottom cyclone of cement kiln after one year usage.
The bubbly microstructure of KCl is an evidence for its primary liquid state.
The cracks in the KCl-layer in position “B” is an indication for differences of the thermal linear expansion coefficients as well as the elastical modulus between salt layer and refractory concrete.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Characterization of corroded industrial refractory materials 13

14. Characterization of corroded industrial refractory materials
Alkali Corrosion
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 CaCO3, CaSO4, SiO2 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
Characterization of corroded industrial refractory materials 14

15. Characterization of corroded industrial refractory materials
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.
A wide range of corrosive substances and new formed compounds after corrosion are listened in the literature. The most of the analyzed samples based on industrial corroded refractory bricks and concretes contained alkali compounds, such as feldspar, KCl, alkali sulfate, NaCl as well as other chlorides. In summary, the compounds of potassium are more “common” than the compounds of sodium.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Characterization of corroded industrial refractory materials 15

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 salt after 1100°C
Na2SO4, K2SO4  molten
Na2CO3, K2CO3  molten
NaCl, KCl  evaporated
CaSO4  sintered
The solid salts as most reactive and corrosive mixtures after heating at 1100°C
in crucibles:
Salt mixtures after 1100°C
SM 1 K2SO4 / K2CO3  melting
SM 2 K2SO4 / K2CO3 / KCl  gas
SM 3 K2SO4 / K2CO3 / KCl / CaSO4  solid
The high temperature behavior is different of salts and salt mixtures.
After heating at high temperature in crucibles, K2SO4 and K2CO3 are molten, KCl is evaporated and CaSO4 is sintered.
In contrast, the salt mixtures present a different picture at 1100 °C:
K2SO4 / K2CO3 predominant as melting
K2SO4 / K2CO3 /KCl as gas
K2SO4 / K2CO3 /KCl / CaSO4 as solid.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Behavior of alkali salts and alkali salt mixtures 16

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: K2SO4
 lowest value: CaSO4
 is reflected in the value of the
salt mixtures
Solid salt
lin
measured
10-61/K
(20/600 °C)
literature
(0 °C)
KCl 52
66,2
K2SO4 90
44,6
K2CO3 58
43,3
CaSO4 16
SM 1 (K2SO4/K2CO3)
SM 2 (K2SO4/K2CO3/KCl) 50
SM 3 (K2SO4/K2CO3/KCl/CaSO4) 34
The measured thermal linear expansion coefficients (lin) of solid salts and salt mixtures reaches this values ( between 16 and 90*10-6 1/K ).
The highest value achieves K2SO4 and the lowest CaSO4; this behavior is reflected also in the values of the salt mixtures.
The thermal shock sensibility of refractory materials is increased.
As a result, change of temperature leads to the destruction of the microstructure.
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
Behavior of alkali salts and alkali salt mixtures 17

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:
 K2CO3 are hygroscopic
 KCl, K2SO4, CaSO4 are
not hygroscopic
The volume expansion during heating up combined with the hygroscopicity (K2CO3)
leads to the destruction of the refractory in humid atmospheres.
Solid salt
Density of
solid
g/cm³
melt
Volume
increase %
Hygroscopicity
KCl
1,99
1,52 31 no
K2SO4
2,66
1,89 41
K2CO3
2,43
1,96 24
hygroscopic*
CaSO4
2,96
Moreover the density difference between liquid and solid salts produce a dramatically volume increase about 24 to 41 vol.-%.
The volume increase by melting is not a main problem, because the liquid melted salts flow out of the open pores.
Last but not least several salts, such as K2CO3 are hygroscopic combined with volume increase, and finally the destruction of the refractory structure in humid atmospheres is observed.
However, the frequently located salts in industrial samples,( CaSO4, KCl and K2SO4) are not hygroscopic.
*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
Behavior of alkali salts and alkali salt mixtures 18

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:
 K2CO3-solution is high alkaline
 KCl-, K2SO4-, CaSO4-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
KCl
7,99
7,69
K2SO4
7,27
8,34
K2CO3
13,83
13,74
CaSO4
9,69
7,92
SM 1 (K2SO4/K2CO3)
12,10
12,23
SM 2 (K2SO4/K2CO3/KCl)
12,09
12,14
SM 3 (K2SO4/K2CO3/KCl/CaSO4)
12,08
The literature opinion is: Below dew point we have a condensation of water solution of salts, acid or alkaline solutions, causing corrosion in refractories and metals.
Our experiments show the pH- and the electrical conductivity values of salt solutions and condensation products contribute to a different complex picture of corrosion.
The measured pH-values of water based salt solutions are neutral to extremely alkaline (pH-values between 7,22 to 13,83).
The acidic effect is not identifiable of the corrosion products of sheet-metal jacket of rotary kiln too.
The corrosion due several micro processes is supported by Cl- and SO42-, which dissociated in water based salts solutions.
One of these corrosion mechanisms is based on electrochemical corrosion.
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
Behavior of alkali salts and alkali salt mixtures 19

20. Electrical conductivity
Alkali Corrosion
Behavior of satured water based solutions of alkali salts and alkali salt mixtures
Electrical conductivity of satured
water based salt solutions:
 K2SO4 is more soluble than CaSO4
 the value of electrical conductivity
of CaSO4 is increased by a factor 16
The corrosion due several micro
processes is supported by Cl- and SO42-.
One of the corrosion mechanisms is
based on electrochemical corrosion.
Salt solution
Electrical conductivity
directly
after
8 days
KCl
378
381
K2SO4 91 90
K2CO3
173
172
CaSO4
1560
1655
SM 1 (K2SO4/K2CO3)
161
SM 2 (K2SO4/K2CO3/KCl)
184
SM 3 (K2SO4/K2CO3/KCl/CaSO4)
178
The corrosion due several micro processes is supported by Cl- and SO42-, which dissociated in water based salts solutions.
One of these corrosion mechanisms is based on electrochemical corrosion.
All used salt solutions conduct electricity.
The electrical conductivities of saturated salt solutions are different and range between 90 and 1655 μS/cm, and remain constant for at least 8 days.
The highest determined value of electrical conductivity belongs to CaSO4 (1655 μS/cm) and on the lowest to K2SO4 (90 μS/cm), in spite the fact that K2SO4 is more soluble than CaSO4.
Iron steel and low carbon steel are corrod in most of satured water based salt solutions (pH-value < 10). The acidic effect is not identifiable of the corrosion products of sheet-metal jacket of rotary kiln too. The corrosion due several micro processes is supported by Cl- and SO42-, which dissociated in water based salts solutions. One of these corrosion mechanisms is based on electrochemical corrosion
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
Behavior of alkali salts and alkali salt mixtures 20

21. 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
4 main alkali corrosion mechanisms
Corrosion due to water condensation
This work explores due to laboratory experiments supported partially by post mortem industrial trials the chemical interactions between alkali species and established refractory materials and illustrates four main alkali corrosion mechanisms:
1. Alkali corrosion due to melt formation
2. Reactions of refractory oxides with alkalis lead to change of density and specific volume of the solid phase
3. Expansion phenomena as a result of salt stored in pores
4. Corrosion due to water condensation.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion 21

22. Mechanisms of alkali corrosion
Refractory oxid /
melting point [°C]
Alkali compound
Temperature of 1. melting [°C]
MgO / 2840
K2SO4
K2CO3
Na2O
K2O
1067
895
No miscibility
CaO / 2580
KCl + NaCl
CaSO4
645
1365
Cr2O3 / 2200
KCl + K2O
366
669
Al2O3 / 2050
1410
1450
TiO2 / 1830
K2SO4 + K2O
804
986
950
SiO2 / 1713
789
742
MgO + Al2O3 / 1925
No dates
Al2O3 + SiO2 / 1595
732
695
MgO + SiO2 / 1543
713
685
CaO + SiO2 / 1436
725
720
CaO + Al2O3 / 1395
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 K2O and Na2O as
reactive and corrosive substances at high
temperature and water vapour
The alkali salts form with the refractory materials in a wide temperature range (between 750 up to 1450 °C) melts.
The alkali slat mixtures melts partially between 600 and 950 °C, completely between 700 and 1000 °C.
The interaction of K2O and Na2O as reactive and corrosive substances at high temperature and in present of water vapour is being investigated.
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
Mechanisms of alkali corrosion 22

23. Mechanisms of alkali corrosion
Melt formation
Magnesia
Phase diagram of the system K2SO4 – MgO:
 melt formation of eutectic at 1067 °C
Phase diagram of the system K2CO3 – 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
Mechanisms of alkali corrosion 23

24. Mechanisms of alkali corrosion
Melt formation
SiO2-based refractories
Phase diagram of the system Na2O – SiO2:
 melt formation at 782 °C resp. 789 °C
 complete melt of by 26 % Na2O
 no strength of solid structure (25 % melt)
by 4 % Na2O at 1300 °C
Phase diagram of the system K2O – SiO2:
 melt formation at 769 °C
 complete melt of eutectic by 27 % K2O
by 4 % K2O at 1300 °C
25 % eutectic melt by 6,5 % Na2O or K2O at 800 °C
Strong effect of flux of the alkalis leads to damage of SiO2-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
Mechanisms of alkali corrosion 24

25. Alkali Corrosion Melt formation Calcium silicate
Phase diagram of the system Na2O – CaO – SiO2:
 lower volume expansion of reaction products
 melt formation of eutectic at 720 °C
Phase diagram of the system K2O – CaO – SiO2:
 melt formation of eutectic at < 720 °C
Refractory materials based on wollastonite
no alkali resistant, because melt formation
at 700 °C.
Alkali Corrosion of Therml Insulating Material Based of Calcium Silicates 25

26. Mechanisms of alkali corrosion
Melt formation
Applied Temperatures in presence of alkali < 1300 °C,
because of melt formation below 1100 °C:
 refractory oxides MgO, CaO, Cr2O3, TiO2 and SiO2
 binary combinations Al2O3/SiO2, CaO/SiO2, MgO/SiO2
Applied Temperatures in presence of alkali > 1300 °C:
 refractory oxid Al2O3
 binary combinations Al2O3/MgO, Al2O3/CaO could be „suitable“
(no dates of melt formation)
In summery of this investigations, this refractory oxides (MgO, CaO, Cr2O3, TiO2 and SiO2) and their binary combinations (Al2O3/SiO2, CaO/SiO2 and MgO/SiO2) can not be applied at temperatures higher than 1300 °C because of formation of melts at temperatures below 1100 °C.
Only the refractory oxide Al2O3 and their binary combinations (Al2O3/MgO and Al2O3/CaO) could be “suitable”.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion 26

27. Mechanisms of alkali corrosion
Change of density and specific volume of the solid phase
Alkali compounds unknown:
 MgO, CaO
Densities of refractory oxids:
 > 3 g/cm³
(except SiO2, CaOSiO2)
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.
Refractory
oxide
Density
g/cm³
New formed
alkali compounds
Volume
change %
Al2O3
3,99
(N,K)1…6A1…11
2,63…3,42
+17…+52
Cr2O3
5,25 NC
4,36
+20
SiO2
2,65
(N, K)1…3S1…4
2,26…2,96
-10…+17
3Al2O32SiO2
3,17
(N,K)1…3AS1…6
N3CA3S6(SO4)
2,40…2,62
+21…+32
CaO6Al2O3
3,69
(N,K)C0…14A4…11
3,03…3,31
+11…+22
MgOAl2O3
3,55…
3,70
NM0,8…4A5…15
3,28…3,33
+7…+13
2MgOSiO2
3,22
(N,K)1…2M1…5S3…12
2,56… 3,28
-2…+23
CaOSiO2
2,92
(N,K)1…2C1…23S1...12
2,72…3,36
-13…+7
Reactions of refractory oxides with alkalis lead to change of density and specific volume of the solid phase.
Most of the refractory oxides, except MgO and CaO, and all refractory oxide mixtures react with alkalis. Alkali bursting of fire clay products is a well known corrosion process.
In this table, the refractory oxides, except SiO2 and CaOSiO2 have a density over 3 (g/cm³), the formatted most frequently alkali compounds below 3 g/cm³.
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
Mechanisms of alkali corrosion 27

28. Mechanisms of alkali corrosion
Change of density and specific volume of the solid phase
Phase diagram of system
MgO – SiO2 – K2O with forsterite:
 formation of solids at 1100 – 1300 °C
2MgOSiO2, MgO, K2OMgOSiO2, K2O
Change of densities e.g. specific volume
by chemical reaction of forsterite with K2O:
 expansion and shrinkage
Refractory materials based on forsterite no alkali resistant, because volume increase leads to destruction of the structure
K2OMgOSiO2
Solid
Density
g/cm³
Specific volume
cm³/g
2MgOSiO2
3,22
0,311
MgO
3,59
0,279
K2OMgOSiO2
2,76
0,362
K2O
2,33
0,429
The reactions of forsterite with Na2O are similar.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion 28

29. Mechanisms of alkali corrosion
Change of density and specific volume of the solid phase
Phase diagram of system
K2O – Al2O3 – SiO2 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
Na2O – Al2O3 – SiO2
Lower density of products by reactions of
K2O and Na2O with mullite and fireclay
leads to:  high volume expansion
 “alkali bursting”
 damage of refractories
Mullite
Fireclay
1556 °C
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion 29

30. Mechanisms of alkali corrosion
Change of density and specific volume of the solid phase
Calculated volume expansion
of mullite and fireclay depend
on the content of K2O or Na2O
(from phase components and
densities)
Mullit:
22 % volume increase with
8 % linear expansion
by formation of corundum
Fireclay:
volume expansion decrease at a
K2O/Na2O-content of > 20 %
Volume expansion in %
Content of K2O or Na2O in % by weight
Mullite + K2O
Mullite + Na2O
Fireclay + K2O
Fireclay + Na2O
Volume expansion of mullite and fireclay by reaction with K2O or Na2O
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion 30

31. Mechanisms of alkali corrosion
Change of density and specific volume of the solid phase
Phase diagram of system
K2O – CaO – Al2O3 with hibonite:
 formation of solids at 1100 °C
with high volume expansion
Na2O – CaO – Al2O3 with hibonite:
 more expansion of volume than with K2O
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)
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion 31

32. Mechanisms of alkali corrosion
Change of density and specific volume of the solid phase
Phase diagram of system
Na2O – Al2O3 with alumina:
 formation of solids at < 1300 °C
 melt formation of eutectic at 1580 °C
K2O – Al2O3 with alumina:
 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 (NaAlO2, KAlO2) and evaporation of alkalis
Exception: -alumina with “alkali resistant considerations”
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion 32

33. Mechanisms of alkali corrosion
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 Al2O3 reacts to alkali aluminates with a volume increase to 52 % and leads to a destruction of the products.
Cr2O3 leads to expansion by reaction with alkalis.
The density modifications of SiO2 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 (Na2O⋅MgO⋅Al2O3)-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.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion 33

34. Mechanisms of alkali corrosion
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)
Industrial refractory brick from the wall of a bottom cyclone of cement kiln after 1 year usage. A B
Expansion (phenomena) as a result of salt stored in pores.
Salt evaporate at high temperatures and condence in cooler range of porous refractory materials.
So the pores are filled entirely with liquid or solid salts. (In pores and reacted layers, the existence of alkali compounds has been proved. )
The following destruction mechanisms can be obeserved:
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion 34

35. Mechanisms of alkali corrosion
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
Alkali corrosion of a steel bar in a gradient furnace after treatment at 1000°C.
Iron steel and low carbon steel are corrod in most of satured water based salt solutions (pH-value < 10). The acidic effect is not identifiable of the corrosion products of sheet-metal jacket of rotary kiln too. The corrosion due several micro processes is supported by Cl- and SO42-, which dissociated in water based salts solutions. One of these corrosion mechanisms is based on electrochemical corrosion
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion 35

36. Mechanisms of 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
The mechanisms of alkali corrosion in high temperature thermal insulation materials are multifunctional and complicated.
In terms of this work four alkali corrosion mechanisms have been proposed and demonstrated due to laboratory experiments supported partially by post mortem industrial trials:
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 and
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 36

37. Investigation methods
Alkali Corrosion
Investigations of the alkali resistance – “disc-test”
Disc-test:
pressed disc based on 70 % refractory powder
and 30 % salt mixture (K2SO4, KCl, K2CO3)
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
solid raw
material particle
layer of solid salt
particles
mixture of solid raw material + alkali salts
Coating of solid raw material with solid salt particles
Disc-test of fireclay salt briquette before and after heat treatment at 1100 °C for 5 hours
unfired °C / 5 hours
This test based on mixtures of refractory materials and alkali salts with max. particle size 0,5 mm are mixed at the rate of 70 to 30.
The mixtur is pressed to discs with a diameter of 50 mm and thickness of 10 mm.
The samples are heated at 1100°C and 1300°C for 5 h and 50 h respectivly.
The change of sample diameter, weight and length are determined.
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Investigation methods 37

38. Investigation methods
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)
Salt mixtures
SM 1 K2SO4 / K2CO3
SM 2 K2SO4 / K2CO3 / KCl
SM 3 K2SO4 / K2CO3 / KCl / CaSO4
Betacalutherm:
Is a filter pressed, hydrothermal cured and high porous product.
New thermal insulation material combines the resistance capacity of alkali aluminates against alkaline with low thermal conductivity because of the pore structure.
The pore structure develops as fact of the wet processing of the raw material and the high stabilising water content which develops in the autoclave process.
Expansion and shrinkage of the different mixtures after treatment at 1100 °C and 5 h
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Investigation methods 38

39. Investigation methods
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 1 K2SO4 / K2CO3
SM 2 K2SO4 / K2CO3 / KCl
SM 3 K2SO4 / K2CO3 / KCl / CaSO4
Betacalutherm:
Is a filter pressed, hydrothermal cured and high porous product.
New thermal insulation material combines the resistance capacity of alkali aluminates against alkaline with low thermal conductivity because of the pore structure.
The pore structure develops as fact of the wet processing of the raw material and the high stabilising water content which develops in the autoclave process.
Expansion and shrinkage of the different mixtures after treatment at 1300 °C and 5 h
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Investigation methods 39

40. Investigation methods
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 1 K2SO4 / K2CO3
SM 2 K2SO4 / K2CO3 / KCl
SM 3 K2SO4 / K2CO3 / KCl / CaSO4
Expansion and shrinkage of the different mixtures after treatment at 1300 °C and 50 h
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Investigation methods 40

41. Investigation methods
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
Betacalutherm:
Is a filter pressed, hydrothermal cured and high porous product.
New thermal insulation material combines the resistance capacity of alkali aluminates against alkaline with low thermal conductivity because of the pore structure.
The pore structure develops as fact of the wet processing of the raw material and the high stabilising water content which develops in the autoclave process.
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Investigation methods 41

42. Investigation methods
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.
Salt mixtures
SM 1 K2SO4 / K2CO3
Water absorption and the expansion behaviour at normal atmosphere and 100 % humidity are of interest for thermal insulation materials and have been measured during a period of several months.
These experiments imitate the industrial practice of shutting-down kilns with alkali-infiltrated insulation materials.
Therefore after heat treatment and cooling, the alkali-infiltrated disc samples have been exposed several days at 20 °C and 100 % humidity.
The influences of humidity on the sample volume and sample weight after long-time-stabilitytest (1300 °C, 50 h hold time) were investigated. Diameter, height and weight of disc samples after 2, 3, 7, 14, 24, 56, 74 and 89 days respectively were determined for BETA CALUTHERM and β-alumina with salt mixture SM 1.
The maximal storage time of the samples was equal to the shut-down time of cement kilns for maintenance and repair in winter.
The sample weight had increased after 2 and 3 month, respectively, between 30 and 70 %, 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.
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Investigation methods 42

43. Investigation methods
Alkali Corrosion
Investigations of the alkali resistance – “disc-test”
Influence of humidity of alkali-infiltrated
used raw materials:
 volume increase
< 1 %
 volume decrease
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.
Salt mixtures
SM 1 K2SO4 / K2CO3
The volume of the samples has been calculated and its changes after 2 to 74 and 89 days respectively are presented in Fig. 7.
The volume increase and the volume decrease are <1 % and fall below a required value of 2 %.
This means, that the water absorption of alkali-infiltrated disc samples of BETA CALUTHERM and β-alumina took place without or minor changes in volume at high humidity across months.
The alkali infiltrated materials BETA CALUTHERM and β-alumina take in humidity and dehumidify without change in volume again.
One can assume that there will arise no destruction of the structure.
A proof can be made by the analysis of the mechanical properties.
Primarily the fired BETA CALUTHERM variants as well as the pure β-alumina after alkaline attack at high temperatures proved to be constant also at normal temperatures and high atmospheric humidity.
Change of sample volume after heat treatment and 2 and 3 months storage time at 20 °C and 100 % rel. humidity.
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Investigation methods 43

44. Investigation methods
Alkali Corrosion
Investigations of the alkali resistance – crucible test according DIN 51069
Crucibel test:
DIN 51069
1000 °C for 5 hours
salt mixture K2SO4, K2CO3
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
bottom crucible
upper crucible
sealing
alkali salt mixtur
alkali gas
Crucible test of castable gunning material according to DIN 51069, after heat treatment at 1000 °C for 5 hours
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Investigation methods 44

45. Investigation methods
Alkali Corrosion
Investigations of the alkali resistance – crucible test according DIN 51069
Crucibel test:
DIN 51069
700 °C / 800 °C for 5 hours
salt mixture K2SO4, K2CO3 and
salt mixture K2SO4, K2CO3, KCl, CaSO4
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 K2SO4 and K2CO3 at 700 °C for 5 h
Calcium silicate thermal insulating material with salt mixture K2SO4, K2CO3, KCl and CaSO4 at 800 °C for 5 h
Investigation methods 45

46. Investigation methods
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:
 K2SO4 / K2CO3 / KCl
Wall built-up for corrosion test in gradient furnace
Investigation methods 46

47. Investigation methods
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
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
Investigation methods 47

48. Alkali Corrosion
Conclusions of alkali corrosion of the refractory materials
Worst corrosion – bursting effect:
 salt mixture of K2SO4 / K2CO3
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 Al2O3, 1 mol Na2O or K2O)
 melting point 1580 – 2053 °C
-alumina does not melt ore react
with higher content of alkalis at
temperatures below 1580 °C
Refractory
oxide
Alkali oxid
Damage by
SiO2
Na2O
Melt up to 782 °C
K2O
Melt up to 769 °C
Calcium silicate
Melt up to 720 °C
Melt up to 700 °C
Al2O3
10% volume expansion by 3% Na2O
10% volume expansion by 4% K2O
Mullite
10% volume expansion by 14% Na2O
10% volume expansion by 17% K2O
Fireclay
10% volume expansion by 16% Na2O
10% volume expansion by 15% K2O
Forsterite
10% volume expansion by 34% K2O
Spinel
10% volume expansion by 7% Na2O
Hibonite
10% volume expansion by 5% Na2O
10% volume expansion by 6% K2O
The worst corrosion - bursting effect is caused by the mixture of K2SO4 and K2CO3, (salt mixture SM 1). In summary no refractory oxides and refractory oxide mixtures as tested are alkali-resistant. Low alumina content materials as well as β-alumina doped materials can be taken under an “alkali resistant consideration”. β-alumina is an alkali aluminate with a high content of Al2O3 (5 to 11 mol Al2O3 and 1 mol Na2O or K2O). The melting point varies (between 1580 to 2053 °C) depending on composition. This fact underlines the interesting alkali behavior, that β-alumina does not melt with higher contents of alkalis at temperatures below 1580 °C.
Sumary of phase diagrams
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Conclusions 48

49. Refractories for gasification process49

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

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

52. Refractories for gasification process
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/SiO2 < 1 or CaO/SiO2 > 1)
major mechanisms of the corrosion process: dissolution, penetration and disruption, and erosion
higher velocity slag  rate of corrosion ↑  dissolution and/or erosion ↑
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification 52

53. Refractories for gasification process
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/SiO2 =
high-iron oxide acidic coal slag at 1575  chrome-spinel (MgCr2O4)
low solubility of Cr2O3 and MgCr2O4 in SiO2-Al2O3-CaO liquids
refractories containing > 30 % Cr2O3
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 + FexOy → ferrosilicon alloy (low melting)
magnesia-chromite refractories better in basic slags than in acidic slags (dissolution of MgO in all cases)
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification 53

54. Refractories for gasification process
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% Al2O3-10 wt% Cr2O3)
improvement of the thermal shock resistance by microstructural alteration
heating and cooling rates have to be carefully controlled to avoid spalling
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification 54

55. Refractories for gasification process
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 FexOy leads to internal stresses  spalling
Fe+2/Fe+3 ratio depends on partial O2-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
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification 55

56. Refractories for gasification process
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) → NaAlSiO4 + NaAl11O (~ 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
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification 56

57. Refractories for gasification process
Potential Refractory Problems in Coal Gasification
Carbon Monoxide Induced Disintegration
2 CO = C + CO2 ( °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
H2S retard attack
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification 57

58. SiO2 (s) +H2 → SiO(g) + H2O Refractories for gasification process
Potential Refractory Problems in Coal Gasification
Reduction of Silica by H2
SiO2 (s) +H2 → SiO(g) + H2O
(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
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification 58

59. Refractories for gasification process
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)
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification 59

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

61. Refractories for gasification process
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:
minimizing the amount of linear shrinkage of the refractory
continuous, slow heat-up rate
elimination of long hold periods during the heating and cooldown
maintaining the vessel shell temperature as close to ambient as possible
using incompressible bond barriers
using anchor spacings greater than 1.5 times the lining thickness
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification 61

62. Refractories for gasification process
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
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification 62

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

64. Refractories for gasification process
oxygen partial pressure in a gasifier range from 10-7 to 10-9
oxygen potential affects:
valence state of transition oxides such as iron and vanadium oxides
oxide basicity
basicity of slags formed from iron and vanadium oxides
melting point of the slags
 oxygen potential influences slag – refractory reactions and the compounds formed
Kwong, et al., Wear Mechanisms of Chromia Refractories in Slagging Gasifiers 64

65. Refractories for gasification process
Thermodynamic calculations - HSC Chemistry®
V3O5 should be stable phase in gasifiers environments
FeO with some Fe3O4 may be stable phase formed at oxygen partial pressure of 10-7 to 10-9
Kwong, et al., Wear Mechanisms of Chromia Refractories in Slagging Gasifiers 65

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

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 67

68. Refractories for gasification process
today’s researches – low /no chrome oxide
alumina with ZrO2, MgO and additives
alumina-zirconia with MgO, SiC and additives
HfO2, HfSiO4
ZrSiO4
NiAl2O4
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 68

69. Thank you for your attention!69