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 w as found that silica had a negligible effect on the reduction kinetics at 1325 degrees C. For the reduction with a constant lime addition at and above 1360 degrees C, however, the reduction process could be spl it into 2 stages. Silica exerted little effect on the early stage up t o a reduction level of about 50%, while the reduction extent went thro ugh a trough with increasing silica addition in the 2nd stage. This wa s partially attributed to the competing effects of an enhanced chromit e reduction associated with either lime or silica on the one hand, and the bonding oi silica and lime on the other which suppressed the posi tive effects of both additives. Besides heat transfer control, the ear ly stage up to a reduction level of about 50%, was suggested to be mai nly controlled by nucleation and/or chemical reaction with an apparent activation energy of 135 kJ/mol independent of silica addition. Excep t for the runs with 25% silica at and above 1460 degrees C, the data f or 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 we re non-linear and the mechanism involved was attributed to the dissolu tion of chromite grains in the slag phase. The results of this study a lso suggested that (i) heat treatment significantly affected the reduc tion kinetics, (ii) outward transport of gaseous reduction products fr om inside the pellets was not rate-limiting, and (iii) lime rather tha n silica should be used in the prereduction stage, the addition of sil ica in the smelting stage would be of advantage. (C) Munksgaard, 1997.