⑴ Decomposition stage (400-1200℃), in which diaspore dehydrates to form corundum pseudo-image, kaolinite dehydrates to form metakaolinite, and metakaolinite is converted into mullite and quartz. In this stage, there is a large volume shrinkage.
⑵ Secondary mullitization stage (1200-1400℃ or 1500℃), above 1200℃, the corundum formed by the dehydration of diaspore and the free SiO2 decomposed from kaolinite continue to react to form mullite, which is called secondary mullitization. This process is called secondary mullitization. This stage is accompanied by a volume expansion of about 10%.
⑶ Recrystallization stage (1400-1500℃). Above 1400-1500℃, the secondary mullite in bauxite has been completed and entered the recrystallization stage. The crystals of mullite and corundum grow and develop, and the pores shrink and disappear. The impurities form a liquid phase to fill the pores in the material block. Due to the dissolution and crystallization of the solid phase in the liquid phase and the diffusion between solid phase molecules, the material block gradually becomes dense.
