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Impurities In Mullite Bricks

Generally, mullite bricks contain impurities such TiO2, Fe2O3, CaO, MgO, K2O and Na2O. Mullite bricks made of natural materials contain more impurities than those made of synthetic materials.
Mullite Bricks
Those impurities work as fluxes in mullite bricks and can reduce the formation temperature and viscosity of the melt and increase the amount of liquid phase and the dissolution rate and the dissolution amount of the solid phase. 
TiO2 can form solid solution in mullite bricks and accelerate the development of crystals, or enter liquid phase and form glass body at high temperature. At high temperature, Fe2O3 has some solid solubility in Mullite and corundum to form a limited solid solution. It has higher solid solubility in corundum than in mullite. the formation of solid solution can accelerate the growth of mullite and corundum crystals. 
In short, with the increasing of Al2O3 in mullite bricks, its high temperature performance is improved, but with the increasing of flux, its high temperature performance decrease. Accordingly, strict control of impurities content, especially K2O, Na2O and Fe2O3, is an important measure to improve the high temperature performance of mullite bricks. Slags or gases containing alkali components have serious corroison to mullite bricks.