AN IONIC-DIFFUSION MECHANISM OF CHROMITE REDUCTION

The occurrence of the various phases that form during the solid state carbothermic reduction of chromite is explained by a point defect model. In this model each chromite particle is considered to be comprised of concentric layers of spinel unit cells (Fe**2** plus and Mg**2** plus occupy tetrahedral sites, Cr**3** plus , Al**3** plus , and Fe**3** plus occupy octahedral sites). The phases that form are a result of the interchange of cations between unit cells within the particle effected by the presence of carbon monoxide at the surface of the particle. Four stages of reduction are identified: (1) the formation of a slightly iron-enriched core comprising distorted spinel unit cells surrounded by a region of a normal spinel unit cells, effected by the reduction of Fe**3** plus to Fe**2** plus ; (2) the formation of Cr-Al sesquioxide and Mg-Cr-Al spinel phases effected by the metallization of Fe**2** plus ions and subsequent production of Cr**2** plus ions; (3) reduction of Fe**2** plus interstitials in the spinel core; and (4) metallization of chromium ions via the Cr**2** plus intermediate. The non-appearance of the sesquioxide phase at high temperatures is explained by considering the effect of temperature on the magnitudes of the diffusion coefficients of Cr**2** plus , Mg**2** plus , and interstitial Fe**2** plus ions.