Mullite comes in two crystalline forms

The production of mullite materials can generally be made of kaolinite, sillimanite group minerals, aluminum hydroxide or aluminum oxide and silicon dioxide directly synthesized. Clay materials react with alumina or sillimanite minerals with industrial alumina under heating conditions to form primary and secondary mullite. Once mullite is formed in the range of 1000~1200 ° C, further increasing the temperature only increases the crystallization. The formation of secondary mullite usually ends at 1650℃. In order to produce dense mullite products, the two-step sintering process is commonly used.
Mullite comes in two crystalline forms: acicular and prismatic. The refractoriness of acicular mullite is higher than that of prismatic mullite when the chemical composition of acicular mullite is the same. Kaolinite is rapidly heated to more than 1400 ° C to form acicular mullite. Otherwise, slow heating to a lower temperature forms prismatic mullite. There are also reports of tubular and spherical mullite, the former is presumed to be due to the silica and alumina tetrahedral size mismatch resulting in tension caused by tubular shape, the latter is called nitrogenous mullite. The characteristics of mullite thermal expansion anisotropy make it have good thermal stability. When the advanced mullite material is used as the feeder parts, it can be directly replaced to the running feeder without preheating.