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Hydrothermal Synthesis of Light-Weight Insulating Material Using Fly-ASH
WasfeUateriaL in Construction J.J.J.R. Goumons, H.A . von der Sloot snd Th.G. Aolben (Edrrors)
659
8 1991 Elsevier Science Publishers B. V . All righis reserved.
HYDROTHERMAL SYNTHESIS OF LIGHT-WEIGHT INSULATING MATWIAL USING FLY-ASH
B. BORST and P. KRIJGSMAN Ceramic Design International Holding BV, Hattem (The Netherlands)
P.O.Box
68,
8050
AB
SUHUARY
Fly-ash and lime are used as raw materials for the hydrothermal production of a light-weight insulating material. The process is carried outo in an aqueous suspension heated to a temperature of 190-240 C. Compared to a hydrothermally produced calcium-silicate material made from silica-fume and lime, the limiting temperature is much lower. Fly-ash can also be used as a free additive for the material made from silica-fume. This reduces the thermal shrinkage of the material. INTRODUCTION Hydrothermal processes are very promising for making added value products out of fly-ash (1,2). Alumina and Silica are the interesting constituents of the fly-ash that can be used for the hydrothermal synthesis of ceramic materials. The reactivity is high due to its amorphous nature. Because hydrothermal processes are carried out at moderate temperatures, the negative influence of certain impurities is reduced. An industrial process is known ( 3 ) and in progress for the hydrothermal production of Xonotlite (Ca,Si.O,,(OH).) made from lime and silica-fume. Other silica raw materials can be used instead as well (4). Silica-fume is a waste material from the production of ferro-silicon or silicon metal. The Xonotlite is used for the production of light-weight insulating material with a high limiting temperature (1100 OC). Adding some extra silicafume after the hydrothermal reaction reduces the thermal shrinkage of the material ( 5 ) . The aim of this project is to investigate the possibilities of using fly-ash as a raw material for the production of lightweight insulating material or as a free additive to the Xonotlite-product. 1.
660
EXPERIMENTAL Lime together with fly-ash or silica-fume are mixed with water in an stirred autoclave, The lime-silica ratio on a molar basis was 1.04:l in all cases. The reaction mixture is then heated to a temperature of 190-240 ‘ C (depending on the reactivity of the raw materials) and the pressure corresponds to the saturated steam pressure. After completion of the reaction, the reaction product is transferred from the autoclave to a receiving vessel at a sub-stantially constant flow rate. Just prior to this transfer, the pressure in the receiving vessel is brought up to the pressure in the autoclave by passing gas from the autoclave to the receiving vessel. A small pressure difference is created between the two vessels by letting down some pressure from the receiving vessel and the reaction product flows through the heatexchanger to this vessel (see Fig. 1). This process is developed to be able to operate a batchwise hydrothermal process on an industrial scale. 2.
Fig. 1. Lay-out of the process showing reactor, heat-exchanger and receiving vessel. Free additives like fly-ash or silica-fume are mixed with the thus obtained slurry. A fibrous material like wood pulp is added as well to give strenght to the finished product. Slabs are pressed using a filterpress. Drying at 120 - C removed the residual water. 2.
RESULTS
2.1 Crystal Structure When silica-fume is used as raw material, the reaction product consists entirely out of the crystal Xonotlite. It is a mixture of Tobermorite and Hydrogrossular when fly-ash is used. The ratio between these two crystal structures depends on the ratio
66 I
of A1 and Si in the fly-ash. Upon calcination at 1000 'C the material is transformed to Wollastonite when silica-fume is used and to a mixture of Wollastonite and Gehlenite when fly-ash is used. 2.2 Thermal Properties Slabs are pressed from reaction mixtures made from fly-ash/ lime and from silica-fume/lime. The properties of the two different materials can be compared. The results are summarized in tabel 1.
Raw Material
Free Additive
Density
[ k9/m31 360 10 wt% FA + 3 wt% WP 210 Silica-fume 1 0 wt% SF + 3 wt% WP '1 FA = Fly-ash, SF = Silica-fume, WP
Temp. limit
Thermal shrinkage
L.O.I.
1.7 % at 650 OC 1.0 at 1000 c
13.5
%
9.0
%
LOCI
Fly-ash
650
1100 =
Wood Pulp
One can see that the material made from fly-ash has a much lower temperature limit. This is because it is a much more hydrated material with a higher level of impurities compared to the Xonotlite material. The density of the fly-ash material is higher due to the lower porosity of the particles. This also effects the thermal conductivity as can be seen in figure 2.
,--. 0,18 Y E
\
0,17
0,l 6
P 0,15 v i0.14
c .->
c U
2
TI
c
0 0
0
E
a) c
+
0,13 0,12
1
Silica-fume
1
f
/
Figure 2. Thermal conductivity of the two materials of tabel 1.
0,ll
0,lO 0,09 0,08 0,07 0,06 0,05
7-
0
200
I -7.7 -
400
Mean Temperature, (oC)
600
We can conclude that fly-ash can be used as a raw material for the hydrothermal production of light-weight insulating mate-
662
rial although it can only be used for lower temperature applications. The temperature limit depends on the ratio between Si and A1 in the fly-ash and on the level of impurities. The effect on some thermal properties i s measured when flyash is used as a free additive to a Xonotlite product. The results are summarized in tabel 2 together with the results obtained with some other free additives. Tabel 2. Thermal properties of slabs pressed from Xonotlite slurry together with different free additives. Calcination at 1000 10 Wt% FA
2.7 2.9
"c)
3.0 1.6
3 wt% WP is added in all cases
11
Addition of fly-ash effectively reduces the thermal shrinkage although not as good as the addition of silica-fume. Addition of fly-ash has no effect on the drying shrinkage whereas silica-fume is giving a reduction. 3.
CONCLUSIONS
A light-weight insulating material with good insulating properties can be made using fly-ash as one of the raw materials in a hydrothermal process. The working temperature of the resulting material is limited to 650 'C. Fly-ash can also be used as a free additive for a Xonotlite product. This reduces the thermal shrinkage of the insulating material made from this mixture. REFERENCES 1 E.P. Stambaugh, Materials & Design, 10 (1989) 175-185 M. Hirato and Y. Ninomiya, SPEY 16 (Reports of special 2 3
4
5
project research on energy under grant in aid of scientific research, Japan) (1987) 117-122 P. Krijgsman, US-Patent 4,753,787 and 4,366,121 T. Mitsuda, J. Saito and E. Hattori, in: S. Somiya (Ed.), Proceedings of the 1st International Symposium on Hydrothermal Reactions, Japan, 22-26 March 1982, pp. 823-838 P. Krijgsman, US-Patent 4,545,970
659
8 1991 Elsevier Science Publishers B. V . All righis reserved.
HYDROTHERMAL SYNTHESIS OF LIGHT-WEIGHT INSULATING MATWIAL USING FLY-ASH
B. BORST and P. KRIJGSMAN Ceramic Design International Holding BV, Hattem (The Netherlands)
P.O.Box
68,
8050
AB
SUHUARY
Fly-ash and lime are used as raw materials for the hydrothermal production of a light-weight insulating material. The process is carried outo in an aqueous suspension heated to a temperature of 190-240 C. Compared to a hydrothermally produced calcium-silicate material made from silica-fume and lime, the limiting temperature is much lower. Fly-ash can also be used as a free additive for the material made from silica-fume. This reduces the thermal shrinkage of the material. INTRODUCTION Hydrothermal processes are very promising for making added value products out of fly-ash (1,2). Alumina and Silica are the interesting constituents of the fly-ash that can be used for the hydrothermal synthesis of ceramic materials. The reactivity is high due to its amorphous nature. Because hydrothermal processes are carried out at moderate temperatures, the negative influence of certain impurities is reduced. An industrial process is known ( 3 ) and in progress for the hydrothermal production of Xonotlite (Ca,Si.O,,(OH).) made from lime and silica-fume. Other silica raw materials can be used instead as well (4). Silica-fume is a waste material from the production of ferro-silicon or silicon metal. The Xonotlite is used for the production of light-weight insulating material with a high limiting temperature (1100 OC). Adding some extra silicafume after the hydrothermal reaction reduces the thermal shrinkage of the material ( 5 ) . The aim of this project is to investigate the possibilities of using fly-ash as a raw material for the production of lightweight insulating material or as a free additive to the Xonotlite-product. 1.
660
EXPERIMENTAL Lime together with fly-ash or silica-fume are mixed with water in an stirred autoclave, The lime-silica ratio on a molar basis was 1.04:l in all cases. The reaction mixture is then heated to a temperature of 190-240 ‘ C (depending on the reactivity of the raw materials) and the pressure corresponds to the saturated steam pressure. After completion of the reaction, the reaction product is transferred from the autoclave to a receiving vessel at a sub-stantially constant flow rate. Just prior to this transfer, the pressure in the receiving vessel is brought up to the pressure in the autoclave by passing gas from the autoclave to the receiving vessel. A small pressure difference is created between the two vessels by letting down some pressure from the receiving vessel and the reaction product flows through the heatexchanger to this vessel (see Fig. 1). This process is developed to be able to operate a batchwise hydrothermal process on an industrial scale. 2.
Fig. 1. Lay-out of the process showing reactor, heat-exchanger and receiving vessel. Free additives like fly-ash or silica-fume are mixed with the thus obtained slurry. A fibrous material like wood pulp is added as well to give strenght to the finished product. Slabs are pressed using a filterpress. Drying at 120 - C removed the residual water. 2.
RESULTS
2.1 Crystal Structure When silica-fume is used as raw material, the reaction product consists entirely out of the crystal Xonotlite. It is a mixture of Tobermorite and Hydrogrossular when fly-ash is used. The ratio between these two crystal structures depends on the ratio
66 I
of A1 and Si in the fly-ash. Upon calcination at 1000 'C the material is transformed to Wollastonite when silica-fume is used and to a mixture of Wollastonite and Gehlenite when fly-ash is used. 2.2 Thermal Properties Slabs are pressed from reaction mixtures made from fly-ash/ lime and from silica-fume/lime. The properties of the two different materials can be compared. The results are summarized in tabel 1.
Raw Material
Free Additive
Density
[ k9/m31 360 10 wt% FA + 3 wt% WP 210 Silica-fume 1 0 wt% SF + 3 wt% WP '1 FA = Fly-ash, SF = Silica-fume, WP
Temp. limit
Thermal shrinkage
L.O.I.
1.7 % at 650 OC 1.0 at 1000 c
13.5
%
9.0
%
LOCI
Fly-ash
650
1100 =
Wood Pulp
One can see that the material made from fly-ash has a much lower temperature limit. This is because it is a much more hydrated material with a higher level of impurities compared to the Xonotlite material. The density of the fly-ash material is higher due to the lower porosity of the particles. This also effects the thermal conductivity as can be seen in figure 2.
,--. 0,18 Y E
\
0,17
0,l 6
P 0,15 v i0.14
c .->
c U
2
TI
c
0 0
0
E
a) c
+
0,13 0,12
1
Silica-fume
1
f
/
Figure 2. Thermal conductivity of the two materials of tabel 1.
0,ll
0,lO 0,09 0,08 0,07 0,06 0,05
7-
0
200
I -7.7 -
400
Mean Temperature, (oC)
600
We can conclude that fly-ash can be used as a raw material for the hydrothermal production of light-weight insulating mate-
662
rial although it can only be used for lower temperature applications. The temperature limit depends on the ratio between Si and A1 in the fly-ash and on the level of impurities. The effect on some thermal properties i s measured when flyash is used as a free additive to a Xonotlite product. The results are summarized in tabel 2 together with the results obtained with some other free additives. Tabel 2. Thermal properties of slabs pressed from Xonotlite slurry together with different free additives. Calcination at 1000 10 Wt% FA
2.7 2.9
"c)
3.0 1.6
3 wt% WP is added in all cases
11
Addition of fly-ash effectively reduces the thermal shrinkage although not as good as the addition of silica-fume. Addition of fly-ash has no effect on the drying shrinkage whereas silica-fume is giving a reduction. 3.
CONCLUSIONS
A light-weight insulating material with good insulating properties can be made using fly-ash as one of the raw materials in a hydrothermal process. The working temperature of the resulting material is limited to 650 'C. Fly-ash can also be used as a free additive for a Xonotlite product. This reduces the thermal shrinkage of the insulating material made from this mixture. REFERENCES 1 E.P. Stambaugh, Materials & Design, 10 (1989) 175-185 M. Hirato and Y. Ninomiya, SPEY 16 (Reports of special 2 3
4
5
project research on energy under grant in aid of scientific research, Japan) (1987) 117-122 P. Krijgsman, US-Patent 4,753,787 and 4,366,121 T. Mitsuda, J. Saito and E. Hattori, in: S. Somiya (Ed.), Proceedings of the 1st International Symposium on Hydrothermal Reactions, Japan, 22-26 March 1982, pp. 823-838 P. Krijgsman, US-Patent 4,545,970