The reduction of chromite in the presence of silica flux

The mechanism and kinetics of the carbothermic reduction of a natural chromite was studied at 1300–1500 °C in the presence of silica. Thermogravimetry, X-ray diffraction (XRD) analysis, energy dispersive X-ray analysis (EDAX) and metallography were the experimental techniques used. Silica affected the reduction at and above 1400 °C. A two stage reduction mechanism was established. The first stage, up to about 40% reduction, is primarily limited to iron metallization and zoning is observed in partially reduced chromites. In this stage silica does not interfere with the reduction. The second stage is mainly confined to chromium metallization and formation of a silicate slag alters the reduction mechanism. Ion-exchange reactions between the reducible cations (Cr³⁺ and Fe²⁺) in the spinel and the dissolved cations (A1³⁺ and Mg²⁺) in the slag allow further reduction. Due to the very high driving force for the diffusion, the overall process is shifted toward a more chemical reaction controlled mechanism. A generalized rate equation was then applied to the individual metallization curves of iron and chromium from which respective rate constants and diffusion coefficients were derived. The rate constants were in the range 6.74 × 10⁻⁴–9.01 × 10⁻⁴ s⁻¹ for iron and 7.20 × 10⁻⁴–8.50 × 10⁻⁴ s⁻¹ for chromium reduction at 1500 °C in the presence of silica. At 1500 °C, the corresponding diffusion coefficients were in the range 3.14 × 10⁻⁸–4.78 × 10⁻⁸ m²/s for Fe²⁺ diffusion in the spinel and in the range 1.70 × 10⁻⁸–2.03 × 10⁻⁸ m²/s for the respective diffusion of Cr³⁺. Finally using Arrhenius plots activation energies were derived.