Isothermal oxidation kinetics of industrial South African chromite concentrates in air

Abstract

The oxidation of chromite pellets prior to smelting in a submerged arc furnace is key to ensuring that maximum benefit can be derived in the subsequent smelting step. Oxidized pellet feed material reduces operating costs by increasing energy and reductant efficiency, as well as increasing chrome recoveries.

There is significant compositional variance in chromites used in South African chromite smelting operations due to the different seams from which the materials are mined. The impact of these variances on the extent and rate of oxidation of chromite ores has not been studied before, and it was the impetus for this study.

A detailed kinetic study was carried out on three different types of chromite ores, originating from different commercially mined seams in South Africa, specifically the UG2, LG6 and MG seams. Thermogravimetric analysis was performed on these samples in air at temperatures ranging from 873 to 1473 K, which was used as input to the kinetic analysis. The proposed mechanism of chromite oxidation was found to occur in two stages: initially by random nucleation and growth of a sesquioxide phase, followed by a three-dimensional diffusion-controlled mechanism. The activation energy for oxidation was found to be similar for UG2 and LG6 ore, but higher for MG at all tested temperatures.

The effect of chromite composition, temperature, and time on the extent of oxidation (α) was further quantified and captured in a single regression equation, confirming that time and temperature increase the extent and rate of oxidation. The Ti content was found to be directly correlated with higher values of α for a given time and temperature. The regression equation partially resembles the functional form of the diffusivity equation for cations in chromite found in the literature. It was also found that the distribution between Cr and Al impedes oxidation, with higher Cr content resulting in lower values of α for a given time and temperature.