Reduction of chromite by graphite with CaO-SiO2 fluxes

Experimental studies were conducted on the effect of a combination of lime and silica fluxes on the carbothermic reduction of chromite. It was found that silica had a negligible effect on the reduction kinetics at 1325°C. For the reduction with a constant lime addition at and above 1360°C, however, the reduction process could be split into 2 stages. Silica exerted little effect on the early stage up to a reduction level of about 50%, while the reduction extent went through a trough with increasing silica addition in the 2nd stage. This was partially attributed to the competing effects of an enhanced chromite reduction associated with either lime or silica on the one hand, and the bonding of silica and lime on the other which suppressed the positive effects of both additives. Besides heat transfer control, the early stage up to a reduction level of about 50%, was suggested to be mainly controlled by nucleation and/or chemical reaction with an apparent activation energy of 135 kJ/mol independent of silica addition. Except for the runs with 25% silica at and above 1460°C, the data for the late stage of reduction (>65%) fitted a kinetic model for solid-state diffusion control with an activation energy of about 416 kJ/mol. For the reduction with high silica additions, the Arrhenius plots were non-linear and the mechanism involved was attributed to the dissolution of chromite grains in the slag phase. The results of this study also suggested that (i) heat treatment significantly affected the reduction kinetics, (ii) outward transport of gaseous reduction products from inside the pellets was not rate-limiting, and (iii) lime rather than silica should be used in the prereduction stage, the addition of silica in the smelting stage would be of advantage.