Calcined bauxite is one of the components of alumina-magnesia-carbon refractories for steelmaking applications when in these refractories two types atmosphere at the surface and inside of the refractories exist in service conditions. In this study, Iranian bauxite powders were pressed with and without graphite additions, fired at 1300 oC and 1600 oC in air or reducing atmospheres. The phase evolution of samples was investigated using X-ray diffraction. It was shown that the samples fired at 1600 oC in an air atmosphere, contain corundum, 3 : 2 mullite, tialite, rutile and in a reducing atmosphere corundum, alumina-rich mullite (Al1.7Si0.15O2.85), rutile and a reduced form of titanium oxide (Ti2O3) and titanium nitride (TiN) were present. For an explanation of alteration the 3 : 2 mullite to alumina-rich mullite and decomposition of tialite into the parent oxides, the microstructures of samples fired at 1600 oC were investigated. Also the effect of a reducing atmosphere at high temperatures on high purity TiO2 and a mixture of high purity TiO2 plus Al2O3 in the stoichiometric ratio of tialite were inspected. From these results, a new explanation for the mechanism of the alteration 3 : 2 mullite to alumina-rich type and decomposition of tialite were established.

Keywords: Bauxite, TiO2, Reducing atmosphere, Tialite, Mullite.