Alkali Corrosion of Refractories in Cement Kilns
1
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
2
Alkali Corrosion
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
Deuna Zement GmbH, Informationsmaterial 2005
refractory lining
high temperature thermal insulation material
metallic components
Introduction to Alkali Corrosion of Refractories
3
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
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Introduction to Alkali Corrosion of Refractories
4
Alkali Corrosion
The use of secondary fuels
P. Scur, Mitverbrennung von Sekundärbrennstoffen wie heizwertreiche Abfälle und Tiermehl in der Zementindustrie am
Beispiel Zementwerk Rüdersdorf. VDI-Berichte Nr. 1708, 2002, S. 189 - 20
Introduction to Alkali Corrosion of Refractories
5
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
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Introduction to Alkali Corrosion of Refractories
6
Alkali Corrosion
Effect of the combustion of secundary fuels in cement rotary kilns
Secondary fuels
Organic Compounds
solid
alkalis
sulfates
chlorides
..
.
and other corrosive
compounds
(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
fireclay insulating brick after 3 years in use in a
cement rotary kiln (feed end)
Gas corrosion
(condensation) of the
metal components
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Introduction to Alkali Corrosion of Refractories
7
Alkali Corrosion
Post mortem investigations
•
Roof of kiln hood of the DOPOL-kiln:
Cool side (metal jacket)
Calcium silicate
basic abrasion
lining
Insulating brick
Hot side (refractory bricks or concrets)
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Characterization of corroded industrial refractory materials
8
Alkali Corrosion
Post mortem investigation
•
Alkali corroded calcium silicate thermal
insulating material in the chamber
at 600 – 700 °C:

•
Hot side (refractory concrete)
X-ray analysis
Hot side area:
 based on KCl and CaSO4
 residual NaCl,
futher chlorides,
Cr- and Fe-sulfates
Cool side (metal jacket)
Calcium silicate thermal insulating material
(thickness 25 mm) after 18 month in use in the
chamber between the preheater and the
rotary cement kiln.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Characterization of corroded industrial refractory materials
9
Alkali Corrosion
Post mortem investigation
•
Alkali corroded fireclay brick in the hot zone
at 800 °C:

•
Hot side
Cool side
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.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Characterization of corroded industrial refractory materials
10
Alkali Corrosion
Post mortem investigation
•
•
Alkali corroded fireclay insulating brick in
the hot zone at > 1000 °C:
 X-ray analysis
Cool side
Infiltration zone
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
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
Alkali Corrosion
Post mortem investigation
•
Alkali corroded magnesia brick in
the sinter zone at > 1100 °C:
 X-ray analysis
•
Hot side area:
Hot side
 based mainly on leucit (K2OAl2O34SiO2),
mullite (3Al2O32SiO2)
 residual silica (SiO2),
kalsilit (K2OAl2O32SiO2)
larnit (2CaOSiO2)
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
Alkali Corrosion
Post mortem investigation
•
Alkali corroded refractory concrete from the
wall of a bottom cyclone of cement:
A
 SEM-Analysis
(pore size 100 to 200 µm)
•
In pores and reacted layers:
B
 “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.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Characterization of corroded industrial refractory materials
13
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
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
15
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
SM 2
K2SO4 / K2CO3 / KCl 
SM 3
K2SO4 / K2CO3 / KCl / CaSO4 

melting
gas
solid
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Behavior of alkali salts and alkali salt mixtures
16
Alkali Corrosion
High temperature behavior of alkali salts and alkali salt mixtures
• Thermal linear expansion
coefficient (lin) of solid salts and
salt mixtures:
Solid salt
lin
lin
measured literature
10-61/K
10-61/K
(20/600 °C)
 highest value: K2SO4
(0 °C)
 lowest value: CaSO4
KCl
52
66,2
 is reflected in the value of the
salt mixtures
K2SO4
90
44,6
K2CO3
58
43,3
CaSO4
16
SM 1 (K2SO4/K2CO3)
58
SM 2 (K2SO4/K2CO3/KCl)
50
SM 3 (K2SO4/K2CO3/KCl/CaSO4)
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
Behavior of alkali salts and alkali salt mixtures
17
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:
Density of
solid
g/cm³
Density of
melt
g/cm³
Volume
increase
%
Hygroscopicity
KCl
1,99
1,52
31
no
K2SO4
2,66
1,89
41
no
K2CO3
2,43
1,96
24
hygroscopic*
CaSO4
2,96
Solid salt
 K2CO3 are hygroscopic
 KCl, K2SO4, CaSO4 are
not hygroscopic
no
*weight increase app. 15 % after 4 days on normal area
(24 °C, 60 % rel. humidity)
 The volume expansion during heating up combined with the hygroscopicity (K2CO3)
leads to the destruction of the refractory in humid atmospheres.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Behavior of alkali salts and alkali salt mixtures
18
Alkali Corrosion
Behavior of satured water based solutions of alkali salts and alkali salt mixtures
• pH-values of satured water based
salt solutions:
pH-value
Salt solution
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
12,14
 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.
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
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
after
directly
8 days
KCl
378
381
K2SO4
91
90
K2CO3
173
172
CaSO4
1560
1655
SM 1 (K2SO4/K2CO3)
161
161
SM 2 (K2SO4/K2CO3/KCl)
184
184
SM 3 (K2SO4/K2CO3/KCl/CaSO4)
178
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
Behavior of alkali salts and alkali salt mixtures
20
Alkali Corrosion
Melt formation
Change of
density and
volume of the
solid phase
4 main alkali
corrosion
mechanisms
Expansion as a
result of salt
stored in pores
Corrosion due to water
condensation
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion
21
Alkali Corrosion
Refractory oxid /
melting point [°C]
Alkali compound
Temperature of
1. melting [°C]
MgO / 2840
K2SO4
K2CO3
Na2O
K2O
1067
895
No miscibility
No miscibility
CaO / 2580
KCl + NaCl
CaSO4
Na2O
K2O
645
1365
No miscibility
No miscibility
Cr2O3 / 2200
KCl + K2O
K2O
Al2O3 / 2050
Na2O
K2O
TiO2 / 1830
K2SO4 + K2O
Na2O
K2O
804
986
950
Na2O
K2O
789
742
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
SiO2 / 1713
(from phase diagrams)
 In addition: presence of K2O and Na2O as
reactive and corrosive substances at high
temperature and water vapour
Temperature from the 1. melting for refractory oxids or oxids
mixturs with compounds of alkalis from the phase diagrams.
366
669
1410
1450
MgO + Al2O3 / 1925
No dates
Al2O3 + SiO2 / 1595
Na2O
K2O
732
695
MgO + SiO2 / 1543
Na2O
K2O
713
685
CaO + SiO2 / 1436
Na2O
K2O
725
720
CaO + Al2O3 / 1395
No dates
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion
22
Alkali Corrosion
1. Melt formation
Magnesia
•
Phase diagram of the system K2SO4 – MgO:
3000
2850o
 melt formation of eutectic at 1067 °C
2500
•
Liquid
Phase diagram of the system K2CO3 – MgO:
 melt formation of eutectic at 895 °C
2000
o
T, C
MgO + Liq.
•
similar behavior is due of the system
KCl - MgO
 MgO based refractory materials
are not alkali resistant because melt formation
at 895 °C.
1500
(2%)
1069o
1000
1067o
Hex-K2 SO4 + MgO
588o
500
Ortho-K2 SO4 + MgO
0
K2 SO4
20
40
Mol %
60
80
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion
23
100
MgO
Alkali Corrosion
1. 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
 no strength of solid structure (25 % melt)
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
Alkali Corrosion
1. 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
Alkali Corrosion
1. 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)
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion
26
Alkali Corrosion
2. Change of density and specific volume of the solid phase
•
Refractory
oxide
Density New formed
g/cm³
alkali compounds
Density
g/cm³
Volume
change
%
Al2O3
3,99
(N,K)1…6A1…11
2,63…3,42
+17…+52
 > 3 g/cm³
(except SiO2, CaOSiO2)
Cr2O3
5,25
NC
4,36
+20
SiO2
2,65
(N, K)1…3S1…4
2,26…2,96
-10…+17
Densities of new formed alkali
compounds:
3Al2O32SiO2 3,17
(N,K)1…3AS1…6
N3CA3S6(SO4)
2,40…2,62
+21…+32
 < 3 g/cm³ (most frequently)
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
Alkali compounds unknown:
 MgO, CaO
•
•
Densities of refractory oxids:
 The volume increase of solid phase
(N,K)1…2M1…5S3…12 2,56… 3,28 -2…+23
of the refractory oxides containing 2MgOSiO2 3,22
alkali compounds leads to an
CaOSiO2
2,92
(N,K)1…2C1…23S1...12 2,72…3,36 -13…+7
attrition of microstructure and
the damage of refractory lining. 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
Alkali Corrosion
2. 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:
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
K2OMgOSiO2
 expansion and shrinkage
 Refractory materials based on forsterite no alkali resistant, because volume
increase leads to destruction of the structure
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion
28
Alkali Corrosion
2. 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
Fireclay
 first eutectic melts appear at 1556 °C
1556 °C
•
similar behavior is due of the system
Na2O – Al2O3 – SiO2
Mullite
 Lower density of products by reactions of
K2O and Na2O with mullite and fireclay
leads to:  high volume expansion
 “alkali bursting”
 damage of refractories
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion
29
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 K2O or Na2O
(from phase components and
densities)
Mullit:
22 % volume increase with
8 % linear expansion
by formation of corundum
•
Mullite + K2O
Mullite + Na2O
Volume expansion in %
•
Fireclay + K2O
Fireclay + Na2O
Fireclay:
volume expansion decrease at a
K2O/Na2O-content of > 20 %
Content of K2O or Na2O in % by weight
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
Alkali Corrosion
2. 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
•
Phase diagram of system
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
Alkali Corrosion
2. 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
•
Phase diagram of system
K2O – Al2O3 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 (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
Alkali Corrosion
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 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
Alkali Corrosion
3. Expansion phenomena
•
Salt storage in pores of refractories:
 evaporation of salt at high temperatures
A
 condensation of salt in cooler range of
refractory materials
 pores are filled entirely with liquid or
solid salts
•
B
Destruction mechanisms:
 thermal linear expansion of salts
5- to 10-fold more than refractory materials
 thermal shock sensibility of refractory
material is increased
Industrial refractory brick from the wall of a bottom
cyclone of cement kiln after 1 year usage.
 volume increase between solid and
liquid salt (change of densities)
 hygroscopicity of salts and volume increase
(destruction in humid atmosphere)
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion
34
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
Alkali corrosion of a steel bar in a gradient furnace
after treatment at 1000°C.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Mechanisms of alkali corrosion
35
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
36
Alkali Corrosion
Investigations of the alkali resistance – “disc-test”
•
layer of solid salt
particles
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
solid raw
material particle
mixture of solid raw material +
alkali salts
Coating of solid raw material with solid salt
particles
Fireclay:
diameter increase from 50 to 53 mm
 linear expansion of 6 % due to
alkali bursting
unfired
1100 °C / 5 hours
Disc-test of fireclay salt briquette before and
after heat treatment at 1100 °C for 5 hours
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Investigation methods
37
Alkali Corrosion
Investigations of the alkali resistance – “disc-test”
•
Change of diameter after 1100 °C at 5 hours
 high value of expansion
Salt mixtures
SM 1
K2SO4 / K2CO3
SM 2
K2SO4 / K2CO3 / KCl
SM 3
K2SO4 / K2CO3 / KCl / CaSO4
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)
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
Alkali Corrosion
Investigations of the alkali resistance – “disc-test”
•
Change of diameter after 1300 °C at 5 hours
 high value of expansion
Salt mixtures
SM 1
K2SO4 / K2CO3
SM 2
K2SO4 / K2CO3 / KCl
SM 3
K2SO4 / K2CO3 / KCl / CaSO4
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)
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
Alkali Corrosion
Investigations of the alkali resistance – “disc-test”
•
Change of diameter after 1300 °C at 50 hours
 high value of expansion
Salt mixtures
SM 1
K2SO4 / K2CO3
SM 2
K2SO4 / K2CO3 / KCl
SM 3
K2SO4 / K2CO3 / KCl / CaSO4
Spinel AR 78
Forsterite Olivin
 suitable materials
-alumina
Betacalutherm (dried, fired)
Spinel MA 76
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
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)
Samples for change of disc diameter after heating at 1300 °C and 5 h
 Betacalutherm and -alumina are long-time and alkali resistant after that as the only
fire-proof materials up to 1300 °C
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Investigation methods
41
Alkali Corrosion
Investigations of the alkali resistance – “disc-test”
•
Salt mixtures
SM 1
K2SO4 / K2CO3
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.
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Investigation methods
42
Alkali Corrosion
Investigations of the alkali resistance – “disc-test”
•
Influence of humidity of alkali-infiltrated
used raw materials:
Salt mixtures
SM 1
K2SO4 / K2CO3
 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.
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Investigation methods
43
Alkali Corrosion
Investigations of the alkali resistance – crucible test according DIN 51069
•
upper
crucible
Crucibel test:
DIN 51069
alkali gas
sealing
1000 °C for 5 hours
salt mixture K2SO4, K2CO3
•
bottom
crucible
alkali salt
mixtur
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
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Investigation methods
44
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
Calcium silicate thermal insulating material with
salt mixture K2SO4 and K2CO3 at 700 °C for 5 h
 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, K2CO3, KCl and CaSO4 at 800 °C for 5 h
Investigation methods
45
Alkali Corrosion
Investigations of the alkali resistance – test in a gradient furnace
•
Gradient furnace:
 gradient of temperature
100 - 1300 °C
 alkali atmosphere
•
Thermal insulation material:
 Betacalutherm
•
Refractory material:
 refractory concrete
•
Steel bar:
Wall built-up for corrosion test in gradient furnace
 austenitic steel 1.48.28 with
scaling resistance to 1000 °C
•
Salt mixtures:
 K2SO4 / K2CO3 / KCl
Investigation methods
46
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:
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
 scaling with volume increase
(33-56 %) in the hot zone
 Verification of the post mortem investigations of the industrial refractory materials
Investigation methods
47
Alkali Corrosion
Conclusions of alkali corrosion of the refractory materials
•
Worst corrosion – bursting effect:
 salt mixture of K2SO4 / K2CO3
•
No alkali resistant:
Damage by
Na2O
Melt up to 782 °C
K2O
Melt up to 769 °C
Na2O
Melt up to 720 °C
K2O
Melt up to 700 °C
Na2O
10% volume expansion by 3% Na2O
K2O
10% volume expansion by 4% K2O
Na2O
10% volume expansion by 14% Na2O
K2O
10% volume expansion by 17% K2O
Na2O
10% volume expansion by 16% Na2O
K2O
10% volume expansion by 15% K2O
Forsterite
K2O
10% volume expansion by 34% K2O
Spinel
Na2O
10% volume expansion by 7% Na2O
Na2O
10% volume expansion by 5% Na2O
K2O
10% volume expansion by 6% K2O
Calcium
silicate
 all refractory mixtures
“alkali resistant considerations”:
 low alumina content materials
(-alumina doped material)
•
Alkali
oxid
SiO2
 all refractory oxides
•
Refractory
oxide
Al2O3
Mullite
-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
Fireclay
Hibonite
Sumary of phase diagrams
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Conclusions
48
Refractories for gasification process
49
Refractories for gasification process
Wear mechanisms of refractories in slagging gasifiers
J.P. Bennett, Refractory liner materials used in slagging gasifiers
Introduction to Refractories for Gasification Processes
50
Refractories for gasification process
Potential Refractory Problems in Coal Gasification
1
alkali attack
2
carbon monoxide disintegration
3
silica volatilization
4
steam-related reactions
5
thermoelastic stresses
6
erosion due to solid particulates
7
corrosion and erosion due to molten coal slag and/ or iron
8
iron oxide bursting
dry ash
gasifiers
slagging
gasifiers
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
Introduction to Refractories for Gasification Processes
51
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
Introduction to Refractories for Gasification Processes
52
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 =
0.2-1.7
• 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
Introduction to Refractories for Gasification Processes
53
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
Introduction to Refractories for Gasification Processes
54
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
Introduction to Refractories for Gasification Processes
55
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 + NaAl11O17
•
(~ 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
Introduction to Refractories for Gasification Processes
56
Refractories for gasification process
Potential Refractory Problems in Coal Gasification
Carbon Monoxide Induced Disintegration
2 CO = C + CO2
(400-700°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
Introduction to Refractories for Gasification Processes
57
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
Introduction to Refractories for Gasification Processes
58
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 phosphatebonded refractories
 no degradation of cement-
bonded castables
Results applicable to low-temperature sections
of most gasifiers. (1000-1100 °C)
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
Introduction to Refractories for Gasification Processes
59
Refractories for gasification process
Refractory problems in coal gasification
C – Physical wear – “spalling”
J.P. Bennett, Refractory liner materials used in
slagging gasifiers
Introduction to Refractories for Gasification Processes
60
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:
(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
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
Introduction to Refractories for Gasification Processes
61
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
Introduction to Refractories for Gasification Processes
62
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
Introduction to Refractories for Gasification Processes
63
Refractories for gasification process
•
oxygen partial pressure in a gasifier range from 10-7 to 10-9
•
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
Kwong, et al., Wear Mechanisms of Chromia Refractories in Slagging Gasifiers
Introduction to Refractories for Gasification Processes
64
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
Introduction to Refractories for Gasification Processes
65
Refractories for gasification process
Material development from the 1970’s until today
R. Dürrfeld, Refractories in Coal Gasification Plants
Introduction to Refractories for Gasification Processes
66
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
Introduction to Refractories for Gasification Processes
67
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
Introduction to Refractories for Gasification Processes
68
Thank you for your attention!
69