Fused zirconium mullite is used as the main raw material for the production of slide plates and three major parts used in steel mills. Containing zirconium is greater than or equal to 35%. In order to further improve the chemical corrosion resistance, thermal shock resistance and reduce the expansion coefficient of mullite, the introduction of ZrO2 into the Al2O3-SiO2 system can improve the structure of mullite. This is mullite containing ZrO2, commonly known as zirconium mullite, which is generally produced by electric melting.
The ideal zircon-mullite mineral composition is mullite and baddeleyite. However, when we analyzed the Al2O3-SiO2 binary system, it has been pointed out that since Al2O3 can form a solid solution with 3Al2O3 2SiO2, especially the mullite that is cooled and crystallized after melting, the best composition point should be the mullite solid solution. The critical limit composition of the melt, namely 2Al2O3·SiO2, is close to the high-temperature eutectic point composition in the Al2O3-SiO2 system at this time, which is beneficial to the formation of fine crystals and uniform distribution.
It has a wide range of chemical composition of zircon mullite, ZrO2 content can be from 7% to 40%. But its best ratio is 2Al2O3·SiO2·ZrO2, the contents are Al2O3 52.7%, SiO2 15.6%, ZrO2 31.7%. This composition is equivalent to the composition of the ternary lowest eutectic point in the Al2O3-SiO2-ZrO2 ternary system (Al2O3 53%, SiO2 17%, ZrO2 30%). When the melt formed by this composition is crystallized, mullite, corundum and baddeleyite can be precipitated at the same time, mullite and corundum form a eutectoid, and the primary crystals of baddeleyite are distributed in short columns and needles in the form of string columns. Around the mullite crystals and eutectoids, a staggered structure is formed, and the resistance to chemical attack is much better than when each component forms a separate crystal. Due to the presence of impurities in the raw materials, there is also a glass phase in fused zirconia mullite, but the content is generally less than 5%, and the corundum phase is also less.
The main raw materials of electrofusion method are industrial alumina and zircon. For low-zircon mullite with ZrO2 content of 7%~9%, bauxite or kyanite can be used as raw material and zircon can be added. The requirements for bauxite are Al2O3>60%, SiO2<25%, and total impurities<8%.
It is mainly used for new-type pouring steel slides, sizing nozzles and long nozzles of continuous casting ladles, key parts of glass furnaces, etc.