The Relationship between Refractory Temperature and Alumina Content of High Alumina Bricks

The important working properties of high alumina bricks are load temperature and high-temperature creep. The load softening temperature increases with the increase of product AL₂O₃ content. The content of AL₂O₃-high alumina bricks is below 70%. The load softening temperature depends on the mullite crystal and liquid phase ratio. The content of AL₂O₃ mullite-corundum products is between 70% and 90%. As AL₂O₃ increases, the load softening temperature does not increase significantly. This is because the Fe₂O₃ and TiO₂ components in the raw material increase slightly with the increase of AL₂O₃, and the quantity and properties of the high-temperature liquid phase change accordingly.

The mullite crystal phase partially softens at high temperatures. Although the amount of corundum has increased, the skeleton cannot be formed, resulting in no significant increase in the load-softening temperature. Only when the AL₂O₃ content in the product exceeds 90% or even exceeds 95%, the main crystal phase in the product is corundum, and the direct bonding rate between crystal grains is significantly improved. The liquid phase only exists in the gaps between crystal grains, and its load-softening temperature is significantly increased.

The creep rate is used to express the high-temperature creep rate of high alumina bricks. For example, the creep rates of first- and second-level high alumina bricks are similar. At 1200°C, the creep rate is 0.25~0.29×10-5R.h. At the same temperature, the temperature of third-grade high alumina bricks is 3.5×10-5r·h, 10 times higher than that of first- and second-grade high alumina bricks. Physical analysis shows that the glass phase amount in the first and second-grade high alumina bricks is 7%~9%, and the third-grade high alumina brick is 20%. The creep rate is not only related to the glass phase amount but also to the composition of the glass phase and the high-temperature viscosity. At 1200°C, the liquid viscosity of the third-level high alumina brick is only half that of the first-level high alumina brick and 26% of that of the second-level high alumina brick.

Therefore, glass plays a dominant role in the creep behavior of the third-grade high-alumina bricks, while in addition to the glass effect in the first- and second-grade bricks, crystal creep plays an important role. The higher the direct bonding rate between crystals, the more obvious the creep effect between crystals. The key to improving high-temperature creep is to improve the purity of raw materials during the production process, change the chemical and mineral composition of the matrix, eliminate the number of small glass phases, and adjust the glass phase composition. It can also improve high-temperature volume stability and slag resistance.

Third-grade high alumina bricks have similar properties to clay bricks, and their main crystal phases are mullite and glass phases. Because its high-temperature performance is better than that of clay bricks, third-level high-aluminum products can be used in situations where clay bricks can be used. The main crystal phase of second-level high-alumina bricks is mullite, and the high-temperature performance of these products is significantly better than clay bricks. The main crystal phases of first-grade high alumina bricks are mullite and corundum. Since corundum has higher chemical stability and fire resistance than mullite, the higher the corundum content in the product, the higher the high-temperature resistance and corrosion resistance of the product. However, the thermal expansion coefficient of corundum is much greater than that of mullite, so the higher the corundum content, the lower its thermal shock resistance.