Rate and Mechanism of Reduction-Dissolution of Chromite in Liquid Slags

The dissolution of chromite from the Bushveld Complex of South Africa in liquid slags was studied in the temperature range 1550 degrees to 1665 degrees C under argon gas. The slag compositions were similar to those of ferrochromium production and stainless steel making. Empirical relations between the slag composition and the dissolution of chromite were established through the use of a statistical model. The dissolution process was investigated by using the rotating cylinder technique and measured by the chemical analysis of the samples taken from the melt and the SEM-EDAX analysis of the reacted chromite cylinder samples. The chromite grains were depleted in iron and chromium as the dissolution progressed, leaving behind an alumina and magnesia rich spinel. The experimental data was evaluated by using kinetic models and mass transfer coefficients of chromium, iron and oxygen ions through the phase boundary between the solid chromite cylinder and the liquid slag were determined. The dissolution of chromite in liquid slags increases with increasing stirring rate. SEM-EDAX studies on the reacted chromite cylinders showed that coring took place within the chromite grains subjected to dissolution for sufficient length of time. Chromium and iron concentrations in the chromite were decreasing from the centre towards the surface of the grains while aluminum and magnesium contents were increasing at the edges compared to the centre of the chromite grains. Furthermore, the slag rich in alumina and magnesia diffuses in bulk into the chromite with a net result of increase in the concentration of these elements. The rate of dissolution of chromite in liquid slags was found to be controlled by the mass transfer of oxygen ions (O2-) through the liquid phase boundary between the solid chromite and the liquid slag. The activation energy for the mass transfer of O2- ions was calculated as 30.61 kCal/mol (128.07 kJ/mol).