Alkali roasting of Indian chromite ores: thermodynamic and kinetic considerations

Abstract

A lime free alkali roasting of chromite ores of Indian origin was investigated in the present study. Isothermal roasting experiments were carried out at different temperatures in order to study the effects of roasting process parameters such as charge composition, ore mineralogy, roasting time, temperature and also the kinetics of roasting. Thermal analyses of the roasting experiments were also carried out. The efficiency, determined by the mass balance analysis, of roasting process is found to be 65%. The reaction kinetics was best explained by Gistling and Brounshtein kinetic model. The reaction is diffusion controlled. The diffusion of cations: Cr³⁺ in the spinel and Na⁺ ions in the liquid phase apparently control the overall rate in the lower temperature regime from 973 to 1173 K, for which the activation energy is in the range of 205–220 kJ mol⁻¹. Above 1173 K, the apparent activation energy is much less than the low temperature reaction barrier and varies in the 45–48 kJ mol⁻¹ range, which shows that the rate controlling step is gaseous diffusion (O₂ and CO₂). The mechanism of roasting reaction is discussed with the help of experimental results.