Purification of mullite by reduction and volatilization of impurities

Mullite materials usually contain a residual glassy phase rich in SiO2, whi ch concentrates impurities as Na2O,K2O, Fe2O3, and other minority compounds . A suitable way to minimize this glassy phase is the reduction and volatil ization of its components by calcination at high temperatures (1300-1450 de grees C) in atmospheres with a very Low partial pressure of O-2. Over 95% o f the Na2O, K2O, and Fe2O3 in mullite can be removed in this way, leaving c oncentrations lower than 0.02% by weight. To avoid the degradation of mulli te that occurs when the partial pressure of O-2 is too low, the material to be purified is covered with TiO2 plates.