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 firebricks, the melting point of the surface layer of silicon bricks will drop sharply, and stalactite droplets will appear. However, stalactite corrosion generally does not occur during normal operation. The diffusion of alkaline components to the middle of the brick body after contacting the brick surface also exists. However, its diffusion depth is much shallower than that on clay refractory materials. At the beginning of this alteration, R2O dissolves the bricks 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 refractory bricks from further corrosion. 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 invasion of Na2O, 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 silicon fire brick near the highest temperature area is cristobalite crystal. The temperature of the transformation of tridymite to tridymite is theoretically 1470°C, but the transformation temperature can be reduced to 1260°C when R2O coexists. Quartz begins to transform into tridymite at 870°C, and the temperature at this location can be inferred from this transformation. Whether it is recrystallization or polycrystalline transformation, it will weaken the firmness of the bond between particles in the brick body, and 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 is 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 tridymite is higher than that of the original brick, and the content of cristobalite is lower; the innermost is a light yellow untransformed tribute layer.
The silica brick has poor corrosion resistance to R2O liquid phase. The R2O 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, the silica firebrick masonry has small brick joints, and 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 fire 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 refractory bricks.
Usually the most severely eroded part of the silica fire 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 firebrick is eroded, although the gap on the top is small, there is often a large space slightly below it.
Therefore, on the one hand, silica brick masonry requires reducing brick joints, including using 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 its service life.
