The reduction mechanism of a natural chromite at 1416 °C

Abstract

The behavior of a natural chromite from the Bushveld Complex, Transvaal, South Africa, during reduction at 1416 °C by graphite was studied by means of thermogravimetric analysis, X-ray diffraction (XRD) analysis, energy-dispersive X-ray analysis (EDAX), and metallographic analysis. Experimental runs were allowed to proceed up to 120 minutes, resulting in 99 pct reduction. The specific objective of this study was to delineate the reduction mechanism of chromite by graphite. Zoning was observed in partially reduced chromites with degrees of reduction of up to about 70 pct. The inner cores were rich in iron, while the outer cores were depleted of iron. Energy-dispersive X-ray analysis revealed that Fe²⁺ and Cr³⁺ ions had diffused outward, whereas Cr²⁺, Al³⁺, and Mg²⁺ ions had diffused inward. The following mechanism of reduction, which is based on the assumption that the composition of the spinel phase remains stoichiometric with increasing degree of reduction, is proposed, (a) Initially, Fe³⁺ and Fe²⁺ ions at the surface of the chromite particle are reduced to the metallic state. This is followed immediately by the reduction of Cr³⁺ ions to the divalent state, (b) Cr²⁺ ions diffusing toward the center of the particle reduce the Fe³⁺ ions in the spinel under the surface of the particle to Fe²⁺ at the interface between the inner and outer cores. Fe²⁺ ions diffuse toward the surface, where they are reduced to metallic iron, (c) After the iron has been completely reduced, Cr³⁺ and any Cr²⁺ that is present are reduced to the metallic state, leaving an iron- and chromium-free spinel, MgAl₂O₄.