KINETICS AND MECHANISM OF REDUCTION OF CARBON-CHROMITE COMPOSITE PELLETS

Experiments on the solid state reduction of cylindrical pellets made f rom graphite-chromite mixtures were carried out at 1000-1500 degrees C under Ar-CO atmospheres. The reduction was favoured by high temperatu res, small pellet size, and a fine particle size of reductant. The eff ect of reductant addition depended on its particle size. For pellets m ade with 1-2 mu m graphite, the reduction extent increased with increa sing weight ratio of graphite to chromite W-g/W-c to a value of simila r to 0.26, and then a decrease followed. For pellets made with 45-53 m u m graphite, however, the reduction percentage increased continuously with increasing W-g/W-c to a value of 0.49. For pellets with 7.94 mm diameter, the data for the early stage of reduction fit both an expone ntial law for nucleation control and an equation for chemical control with an apparent activation energy of 114 kJ mol(-1), while gas/solid diffusion through the solid product layer was most likely to be rate c ontrolling in the late stages with an apparent activation energy of 22 1 kJ mol(-1). The results of the present study also showed that heat a nd mass transfer inside large pellets may also be rate controlling in the early stage of reduction at high temperatures.