Purification of Mullite by Reduction and Volatilization of Impurities

Mullite materials usually contain a residual glassy phase rich in SiO 2 , which concentrates impurities as Na 2 O, K 2 O, Fe 2 O 3 , and other minority compounds. A suitable way to minimize this glassy phase is the reduction and volatilization of its components by calcination at high temperatures (1300–1450°C) in atmospheres with a very low partial pressure of O 2 . Over 95% of the Na 2 O, K 2 O, and Fe 2 O 3 in mullite can be removed in this way, leaving concentrations lower than 0.02% by weight. To avoid the degradation of mullite that occurs when the partial pressure of O 2 is too low, the material to be purified is covered with TiO 2 plates.