Selection of Corundum, Mullite, and Chrome Corundum Castables at Different Temperatures

Corundum castables, corundum mullite castables, and chrome corundum refractory castables are all high-grade castable castables, but because of their different corrosion resistance at different temperatures, they should be selected according to different temperatures when used.

Regardless of whether it is corundum, mullite, or chrome corundum refractory castables, pure calcium aluminate cement is used as a binder. Although the same binder is used, the compressive strength, apparent porosity, and bulk density of the three castables at different temperatures are different. The corrosion resistance at different temperatures is also different.

Mullite, corundum, and the main phase of corundum refractory castables is corundum, while the main phases of chrome corundum and mullite do not change after being corroded. However, at 800°C, the strength of chrome corundum refractory castables will decrease after reacting with alkali, and the amount generated is small, but mullite and corundum refractory castables are not significantly corroded by alkaline. Alkali has little erosion on mullite castables, and the morphology and structure of mullite are complete.

At 800℃, the corundum refractory castable also reacts very little with alkali. There is a layer of flaky material on the surface of the corundum particles, β-AL2O3, which is generated in small amounts, and the degree of damage to the corundum castable is not very obvious. In other words, at 800℃, the strength change rate of mullite and corundum castables is positive, and the strength increases after erosion. A small amount of erosion enters the interior of the castable, blocking some pores, and the erosion of the castable is relatively light, and no new phases are generated. After erosion, the strength increases, the volume density increases, and the porosity decreases. The refractory castable has strong resistance to alkali erosion, and the alkali has little effect on its microstructure.

Chromium oxide is easy to form potassium chromate under alkaline conditions. At low temperatures, chrome corundum castables react with alkali to form potassium chromate. Although the strength change rate of chrome corundum castables is positive, the change rate is small, the strength is low, the porosity is large, the structure is loose, and the alkali easily penetrates the castable, causing serious damage to the chrome corundum castable.

At 1200℃, the mullite castable particles are loose in the structure after being corroded by alkali, and bright white substances and flaky products gather on the mullite particles. Mullite and corundum castables react seriously with alkali, and the strength change rate is negative. The larger the absolute value, the lower the strength after corrosion, the lower the volume density, and the higher the porosity, resulting in a loose material structure, which seriously damages the microstructure of mullite and corundum, and a large volume expansion occurs, and the corrosion becomes more and more serious.

At 1200℃, there are many needle-shaped substances and bright white substances wrapped in the chrome corundum castable. As the temperature rises, the strength decreases. After alkali erosion, it fills between the chrome corundum crystals in an island shape. The viscosity is relatively high. The high-viscosity liquid phase covers the surface of the reaction layer. The structure of the castable is relatively dense, which can effectively prevent the diffusion of alkali vapor and reduce the erosion rate.

In other words, mullite and corundum refractory castables have better effects in furnace linings with severe erosion at temperatures between 800℃ and 1000℃. If the temperature is between 1200℃ and 1350℃, it is better to use chrome corundum castables.