The reduction mechanism of chromite in the presence of a silica flux

Abstract

The reduction behavior of a natural chromite from the Bushveld Complex of South Africa was studied at 1300 °C to 1500 °C. Reduction was by graphite in the presence of silica. Thermo-gravimetric analysis, X-ray diffraction (XRD) analysis, energy-dispersive X-ray analysis (EDAX), and metallographic analysis 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 a reduction level of about 40 pct, is primarily confined to iron metallization, and zoning is observed in partially reduced chromites. In this stage, silica does not interfere with the reduction, which proceeds by an outward diffusion of Fe²⁺ ions and an inward diffusion of Mg²⁺ and Cr²⁺ ions. The second stage is primarily confined to chromium metallization, and formation of a silicate slag alters the reduction mechanism. The slag phase agglomerates and even embeds partially reduced chromite particles. An ion-exchange reaction between the re-ducible cations (Cr³⁺ and Fe²⁺) in the spinel and the dissolved cations (Al³⁺ and Mg²⁺) in the slag allows further reduction. Once the reducible cations are dissolved in the slag phase, they are reduced to the metallic state at sites where there is contact with the reductant.