Pre-oxidation treatment of Indian chromite ores: kinetics and phase transformation behavior relevant to ferrochrome manufacturing and pelletization

Oxidation treatment of chromite ores is a commercially applied process for producing mechanically strong pellets for smelting operation in submerged arc furnace. The oxidation behaviour of chromite ores which essentially determines pellet quality bears a strong relationship with ore chemistry, heat treatment, temperature and time. The present study focuses on investigating the relationship between the oxidation behaviours of low FeO Indian chromite ore with process conditions (oxidation temperature and holding time) using a combination of experimental and simulation techniques. Non-isothermal DSC and TG analysis were conducted in temperature range of 100–1100°C to deduce the phase transformations during the oxidation process. In addition, isothermal TG analyses of chromite ores were conducted at 700, 800 and 1000°C with holding time varying between 0 and 180 min. The effect of temperature and time on the phase transformation of chromite spinel to sesquioxide phase was evaluated using XRD, SEM-EDX, and EPMA. Further, thermodynamic calculations were performed using FactSage ^TM to study the distribution of different elements in chromite and sesquioxide phases during the oxidation process. In addition, kinetic analysis was conducted to investigate the rate controlling phenomena. The results show that phase transformation kinetics are accelerated with increasing temperature and time and 60–120 min oxidation treatment at 1000°C is sufficient to attain equilibrium conditions. The kinetic analysis results in conjunction with experimental oxidation data indicates that phase transformation is both chemical reaction and diffusion controlled.