MICROSTRUCTURAL CHANGES IN CHROMITE CONCENTRATE DURING CALCINATION INAIR AND ARGON ATMOSPHERES

The main chemical and physical changes that took place during firing o f a Greek chromite concentrate in air are as follows. From 820 to 1170 degrees C, oxidation of FeO to Fe2O3 occurred. Owing to an excess of R(2)O(3) oxides over RO oxides in the chrome spinel unit cell, Fe2O3 c ombined with excess R(2)O(3) and, on cooling, solid solutions of Cr2O3 , Fe2O3, and Al2O3 separated from the spinel lattice. From 1240 to 137 0 degrees C, dissociation of Fe2O3 to FeO.xFe(2)O(3) occurred, leading to the resolution of the sesquioxide solid solutions which reentered the chrome spinel lattice. The serpentine matrix was partially transfo rmed to forsterite. Interaction of the chromite phase with the gangue resulted in the crystallisation of bonds joining neighbouring chromite particles, due to liquid and/or solid phase sintering. The changes th at occurred during calcining of the chromite in argon atmosphere were restricted to a partial transformation of the gangue to forsterite and to a localised sintering.