Silica bricks have poor corrosion resistance to alkaline oxides and are often used in the upper structure of tank kilns. Usually, the corrosive agent in tank kilns is mainly r2O (alkali metal oxides). After a large amount of R2O erodes silica bricks, the melting point of the surface layer of silica bricks will drop sharply, and stalactite-like droplets will appear. However, stalactite-like erosion generally does not occur during normal operation. The alkaline component also diffuses to the middle of the brick body after contacting the brick surface. However, its diffusion depth is much shallower than that on clay refractory materials. At the beginning of this alteration, R2O dissolves the silica brick from the surface and penetrates into the brick body through the pores, only forming a very thin low-melting-point metamorphic transition layer on the surface, which reduces the silica brick from further erosion. At this time, the alkaline component of the outer layer of the brick body is higher, and the concentration of the alkaline component suddenly drops from the inner layer. This is because the surface of the brick is dissolved, generating a new glass phase containing more SiO2. The viscosity of this glass phase is relatively high, which not only blocks the pores, but also hinders the diffusion and migration of alkali metal ions to the inner layer of the brick, preventing the brick from further erosion. Only when the flame is sprayed to the top of the arch, causing local overheating, and the glass phase on the surface of the brick is taken away, the brick is further eroded.
After being eroded, the surface of the large arch silica brick is white and smooth, and the metamorphic layer is very obvious. In addition to SiO2 crystals, there are no other crystals in the metamorphic layer. With the diffusion and intrusion of Na20, it has a good mineralization effect on the growth of tridymite. Therefore, in the alteration zone of siliceous refractory materials, the recrystallization of tridymite occupies a very important position. Moreover, tridymite has been in contact with the glass phase for a long time, and can also grow into a tubular column in the new glass phase produced during the replacement reaction. The inner surface of the silica brick near the highest temperature area is cristobalite crystals. The temperature at which quartz is converted to cristobalite is theoretically 1470°C, but the conversion temperature can be reduced to 1260°C when R20 coexists. Quartz begins to convert to tridymite at 870°C, and the temperature at that location can be inferred from this conversion. Whether it is recrystallization or polycrystalline conversion, the firmness of the bond between particles in the brick body will be weakened, and it may even be destroyed due to uneven expansion and contraction, resulting in loose peeling.
After the silica bricks in the high temperature area of the pool furnace melting pool are corroded, they are clearly divided into several layers: a very thin layer of high-viscosity glass on the surface; behind it are white and dense cristobalite crystals; behind it is a light green cristobalite crystal layer, which is light green due to the high content of FeO; behind it is a gray filter layer, in which the content of quartz is higher than that of the original brick, and the content of cristobalite is lower; the innermost is a light yellow undeformed tribute layer.
The silica brick has poor corrosion resistance to the R20 liquid phase. The R20 liquid phase first erodes the weak link of the binder in the brick, causing the loss of the binder and the loosening of the aggregate. If the furnace is improperly built or baked, and the silica brick masonry has small brick joints, the R2O gas phase in the furnace gas will enter the brick joints. Due to the low temperature inside the brick joints, the R2O gas will condense into liquid at around 1400°C. This high-concentration R2O (alkali metal oxide) liquid will quickly erode the silica bricks and form holes. At this time, if there is ventilation and cooling, it will accelerate the condensation of R2O gas, thereby accelerating erosion and causing serious damage to the silica bricks.
Usually, the most severely eroded part of the silica brick is 1/3 to 1/2 of its upper part, where the gas has condensed and the temperature is relatively high, so the erosion is the most serious. After the silica brick is eroded, although the flaming gap on the top is small, there is often a large cavity in the slightly lower part, as shown in the figure.
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Therefore, on the one hand, silica brick masonry requires reducing brick joints, including the use of large arch bricks; on the other hand, when the kiln temperature does not exceed 1600°C, the use of arch top insulation can prevent R2O from condensing in the brick joints, thereby reducing erosion. Therefore, large arch brick insulation can not only save fuel, but also protect the arch top and extend the service life.
The stones generated by the large arch of silica bricks are rarely seen under normal circumstances. Since the main component of silica bricks is SiO2, SiO2 is easily melted and diffused in the melting pool and homogenized in the glass liquid. This transparent lump containing more SiO2 contains crystals of quartz or tridymite, which can be seen as slightly yellowish green by naked eye. This is because silica bricks contain more Fe2O3. However, during high-temperature melting, due to the melting and downward flow of silica bricks on the kiln top, the electric fused casting bricks at the bottom are eroded by silicon flow and enter the glass liquid to produce refractory stones.
Silica bricks are very durable under normal operation (kiln temperature below 1600°C). A1203 in silica bricks is a harmful substance, and a slight increase in its content will significantly reduce its refractoriness. In recent years, kiln temperatures have continued to rise, requiring the use of high-quality silica bricks, which have a SiO2 content of up to 97%, an A1203 content of less than 0.3%, and other impurities below 0.5%. The load softening temperature is 30 to 40°C higher than that of ordinary silica bricks, so the tank kiln temperature can be increased by 20 to 30°C.
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