Kinetics analysis and Cr(6+)-free reaction mechanism of the alkali reduction of South African chromite ores for Cr2O3 extraction

The generation of the Cr6+-ion containing chromite ore processing residue (COPR) from the oxidative alkali roasting is still a significant environmental challenge faced by the chromium chemicals industry. The oxidative roasting of Cr2O3 in the presence of alkali yields water-soluble sodium chromate (Na2CrO4), traces of which remains trapped within the interconnected porous structures of COPR in spite of multiple leaching and washing stages. In this article, we present an alternative route and have investigated the kinetics of a novel reductive alkali roasting reaction which eliminates the formation of Cr6+ state by maintaining Cr3+-ion as sodium chromite (NaCrO2), which is insoluble in water. The reaction kinetics of reductive alkali roasting of South African chromite ore were investigated in an inert atmosphere in the temperature range of 850 °C to 1050 °C. The effects of process parameters, namely, time, temperature, and alkali-to-carbon ratio, on the reducibility of South African chromite ore were investigated. It was observed that the highest reducibility of South African chromite was observed for a reactant mixture in a weight ratio of chromite: Na2CO3:C = 1:1:0.2 which was reduced at 1050 °C for 2.5 h. Using the isothermal kinetic data, the activation energy analysis for the alkali reduction of chromite was carried out which confirms a diffusion-controlled process, as described by the Ginstling and Brounstein model. The derived value apparent activation energy (Ea) was found to be 206.0 ± 0.8 kJ mol−1 per mole of Cr2O3. Based on the apparent diffusion model, the phase changes during the alkali reduction reaction are explained.