In 1924, Bowen et al. published a crystalline phase diagram of a compound (3Al2O3·2SiO2) containing 70.5% Al2O3, a chemical component from the island of Scotland, named mullite. Under normal conditions, mullite (3Al2O3·2SiO2) is the most stable thermodynamic crystalline phase in the Al2O3-SiO2 binary system. It is precisely because of its good high temperature thermal stability, high temperature low creep rate, low thermal expansion coefficient and good dielectric properties, as well as its good chemical corrosion resistance, mullite has been widely used as a refractory material for ceramics and steelmaking industry kiln furniture.
The main raw materials for the production of mullite bricks are andalusite, kyanite or sillimanite. Andalusite is a natural mullite formation, and its expansion rate is lower than that of kyanite or sillimanite when it is fired at high temperature. The additives for the production of mullite bricks are usually activated alumina and refractory clay. Activated alumina can reduce the reaction temperature of mullite. Refractory clay helps to improve the plasticity of the clay during molding and activate the formation of mullite. To compensate for the thermal stress that occurs during firing.
Usually, we call refractories with mullite mineral content exceeding 80% as mullite bricks.And high-quality mullite bricks contain more than 97% mullite phase. As a neutral refractory material, mullite brick has certain corrosion resistance to batch materials and alkali dust. At the same time, mullite brick has excellent properties such as high strength, low thermal conductivity, low high temperature creep coefficient and high thermal shock stability. , Has a certain promotion value in the glass melting furnace
2 Performance analysis of refractory materials for glass kiln
2.1 Silica brick
Silica brick has good high temperature performance such as high softening temperature under load and low high temperature creep. At the same time, it has good high temperature corrosion resistance. Because of its mature production technology and low price, it is widely used in glass furnaces. As we all know, the mineral composition of silica bricks is composed of cristobalite, tridymite, residual quartz and a small amount of silicate liquid. Large volume changes result in poor thermal shock stability of silica bricks. When the temperature reaches 1470℃, the tridymite in the silica brick will be transformed into cristobalite. Cristobalite is the most stable crystal phase in the silica brick, which will enhance the physical and chemical properties of the silica brick. When some parts of the kiln are lower than At 1470℃, the presence of tridymite will reduce the performance of the silica brick, especially the alkali dust or alkali vapor will corrode the wollastonite (CaO·SiO2) in the silica brick at this time, and start to produce in the silica brick The glass phase, and the glass phase will gradually dissolve the large particles of silicon oxide in the silica bricks, causing the silica bricks to dissolve and peel off, thereby affecting the use of the silica bricks.
2.2 Alkaline brick
Alkaline bricks have strong corrosion resistance to alkali dust, but in the high temperature area of the grid body, the magnesia bricks are susceptible to the erosion of SiO2, V2O5 and CaO, resulting in the volume expansion of the grid bricks and spalling. In the middle and lower part of the grid, that is the condensation zone of the grid, magnesia bricks are prone to sulfate corrosion. Directly combining magnesia chrome bricks is the best choice for the lower grid body, but directly combining Cr2O3 in magnesia chrome bricks will pollute water resources. Some countries in Europe and the United States have banned the use of magnesia chrome bricks, and I believe that my country will restrict it soon.
The use of alkaline bricks for chromium-containing products has high thermal conductivity, large expansion turbulence and high creep rate, which will cause deformation during kiln baking, which is not conducive to the sealing of the kiln body. Compared with mullite bricks, the heat dissipation of the wall increases, which increases the insulation cost of the kiln.
2.3 Fused zirconium corundum brick
Fused zirconium corundum bricks have good anti-erosion properties of batch materials, and can be used in almost all high temperature areas of glass furnaces when the temperature is relatively stable. The biggest disadvantage of fused zirconia corundum brick is its poor thermal shock stability, which is not suitable for use in parts with large temperature fluctuations. At the same time, fused zirconia corundum bricks have high thermal conductivity, resulting in large heat dissipation of the kiln body, which is not conducive to energy saving and improving the operating environment. Compared with other refractory materials used in glass furnaces, the construction of fused zirconia corundum bricks is more difficult and expensive.
2.4 Mullite bricks
There are two types of mullite bricks. One is sintered mullite bricks, which are made of natural mullite as the main raw material and pressed and sintered with additives. Due to the unstable chemical composition and mineral composition of natural mullite raw materials, the sintered mullite bricks have poor stability of physical and chemical performance and high-temperature creep rate, but their high-temperature strength and thermal shock stability are high. They can be used in areas with low melting furnace temperatures such as regenerators’ middle-lower part. Table 1 shows the physical and chemical indexes of typical sintered mullite bricks.
The other type is re-sintered mullite bricks, which are made by sintering synthetic mullite as the main raw material with additives. Synthetic mullite comes from sintering and electric fusion. Due to the controllability of raw material properties,the physical and chemical properties of mullite brick products are stable, and the high temperature physical properties are good. Strong corrosion resistance, especially for re-sintered fused mullite bricks. Because mullite is more fully petrochemical, the products are more resistant to alkali dust and alkali vapor chemical erosion. They can be used in high temperature areas, such as small furnaces and upper parts of regenerator and other parts. Table 2 shows the physical and chemical properties of typical re-sintered fused mullite bricks.
Analysis and research believe that the alkali vapor in the flue gas has the strongest corrosion to refractory materials, and the mullite phase in the mullite brick will react with the alkali vapor (NaOH) in the flue gas at a temperature above 1270℃. The surface of the mullite brick forms a stable corundum phase and part of the glass phase, thereby effectively preventing the further erosion of the mullite brick by the alkali vapor, and improving the corrosion resistance of the mullite brick against the alkali vapor.
Table 1 Physical and chemical indexes of sintered mullite bricks
Table 2 Index of physical and chemical properties of re-sintered fused mullite brick
3 Application of mullite brick in glass melting furnace
3.1 Front wall
The front wall of the glass melting furnace is close to the batch area. Due to the large temperature fluctuations in this area and the large amount of raw material dust, the conditions are relatively harsh. The sintered mullite bricks have good resistance to rapid cold and heat, and also have good resistance to fit material dust erosion ability. Therefore, the suspended wall and the feeding wing wall of this part can be designed to use re-sintered fused mullite bricks
3.2 Card neck
The card neck is generally designed with a water bag and agitator. When the equipment is repaired, a large temperature difference will occur in the space of the card neck. Sintered mullite bricks are a better design choice for this place.
3.3 The upper wall of the small furnace and the regenerator
The use of fused zirconia corundum bricks for the small furnace is the best design choice. Because mullite bricks, especially those made of fused mullite as raw materials, also have good resistance to the corrosion of batch materials and alkali vapors. Re-sintering fused mullite bricks may be used in small furnaces, at least in the latter pairs of small furnaces, to improve the operating environment and reduce the cost of the small furnace. Similarly, re-sintered fused mullite bricks can also be used on the upper wall of the regenerator. Compared with alkaline bricks, mullite bricks have a lower thermal conductivity and expansion coefficient, which can reduce the expansion and deformation tendency of the kiln body. The cost of heat dissipation and heat preservation of the kiln body.
3.4 Ascents and tongue hangings of coal-fired glass furnace
As we all know, the life bottleneck of the gas kiln lies in the small furnace. The traditional design uses silica bricks with fused zirconia corundum brick tongues. Because the temperature here is lower than 1450℃, the use of silica bricks in this part is not the best Choose, the part is severely corroded, and the good corrosion resistance of mullite bricks can be used to extend the service life of this part. In addition, fused zirconia corundum bricks are used for the tongue anchors. Due to its poor thermal shock stability, the use of re-sintered fused mullite bricks may be a better choice.
3.5 Grid body of gas regenerator
The lattice body of the currently designed gas regenerator is made of clay bricks, and the general life cycle is 3 to 4 years, which cannot meet the demand of longer and longer kilns. The replacement of lattice bricks is not only labor-intensive, but also affects other parts of the kiln. Cause damage. The use of a certain amount of sintered mullite bricks on the top of the gas grid can increase the service life of the kiln.
3.6 Stove and bottom lattice body
The bottom of the regenerator lattice body will deposit flue gas condensate. The longer the kiln age, the more frequent the operation of the bottom regenerator. The grate and bottom lattice bricks need to experience more thermal shocks, and the sintered mullite bricks have good thermal shocks. Performance and corrosion resistance can increase the service life of this part.
3.7 Operating holes Mullite bricks have good thermal shock stability and high-temperature strength, and can be used in various operating holes of the furnace, such as observation holes, spray gun holes, and air supply holes.
3.8 Grid under special conditions
The latest NOx emission standards have prompted more and more glass manufacturers to improve their production processes and use a reducing atmosphere to reduce the NOx content. This will increase the concentration of alkali vapor (NaOH) in the flue gas and increase the corrosion of the grid. The re-sintered fused mullite brick has good resistance to alkali vapor invasion, and can be used as a lattice body configuration under this condition.
3.9 Configuration and use of special glass furnace
3.9.1 Water glass melting furnace
The main components of the water glass batch are soda ash and silica sand. The weight percentage of soda ash as a flux varies according to the water glass modulus, accounting for about 25-45%. Compared with other glass products, the water glass batch material has a lower melting temperature and a higher alkali vapor concentration in the flue gas. At present, silica bricks and high alumina bricks are mostly used in the flame space of water glass melting furnaces in China. Silica bricks and high-alumina bricks have certain advantages in price, but generally sodium silicate furnaces have a shorter service life and higher energy consumption. Comprehensive analysis shows that the use of sintered mullite bricks in the sodium silicate furnace is economical.
3.9.2 Oxygen glass melting furnace
Oxygen glass melting furnace uses oxygen instead of traditional air-assisted combustion, which reduces a large amount of N2 and reduces the amount of flue gas to save energy and reduce consumption. At the same time, the alkali in the flue gas The vapor concentration and water vapor concentration have also been greatly increased. The traditionally designed silica bricks have been replaced by fused zirconia corundum bricks and fused αβ bricks. Because fused bricks are expensive and difficult to construct, there are also sintered material substitutes such as calcium-free silica bricks produced by RHI and magnesia-aluminum spinel bricks produced by DSF. The re-sintered fused mullite bricks produced by SEFPRO have been successfully used in glass fiber kilns for many years. Perhaps, sintered mullite bricks can also be promoted and used in all-oxygen furnaces for other glass products.
There are many types of mullite bricks in the domestic market, and there is a big gap between product performance and price, so you should choose them reasonably when you use them. At the same time, in order to ensure product quality and performance requirements, the production monitoring and product testing of mullite bricks should be increased to narrow the gap with the international advanced level and reduce product prices.