The soda-ash roasting of chromite minerals: Kinetics considerations

Soda-ash roasting of the chromite mineral is commonly used worldwide for the production of water-soluble sodium chromate. The formation of sodium chromate during the soda-ash roasting reaction depends on the oxygen partial pressure and availability of oxygen at the reaction front. The effects of temperature, oxygen partial pressure, charge composition, and their roles on the overall roasting reaction were studied in order to analyze the reaction mechanism. The influence of process parameters such as the addition of alkali and process residue as the filler material on the overall reaction rate is discussed. The rate-determining steps for the soda-ash roasting reaction are analyzed. The importance of the binary Na2CO3-Na2CrO4 liquid phase during the reaction in determining its speed is also examined. It is shown that the experimental results for the roasting reaction can be best described by the Ginstling and Brounshtein (GB) equation for diffusion-controlled kinetics. From the measured kinetics data, the apparent activation energy for the roasting reaction was calculated to be between 180 and 190 kJ.mol(-1) in the temperature range from 1023 to 1210 K and between 35 and 40 kJ.mol(-1) above 1210 K.