MICROSTRUCTURAL ASPECTS OF REDUCTION OF ZIMBABWE CHROMITE TO HIGH-CARBON FERROCHROMIUM

Examination by transmission-electron microscopy of natural and metallu rgically reduced chromites indicates that the microstructures of the c hromite spinels have a profound influence on the metallurgical charact eristics of the ores. Spinesl of refining ores have relatively defect- free structures with a low density of dislocations and inclusions, whe reas non-refining ores are characterized by many crystal defects that include dislocations, inclusions, silicate veins and microfractures. T hese defects influence the reactions that occur on the reduction of ch romite ore to high-carbon ferro-chromium. Nucleation of the alloy is f avoured at crystal imperfections, which have higher free energies than the surrounding defect-free lattice. The growth, morphology and orien tation of the precipitates are controlled by the crystallographic plan es and axes of the spinel. Dislocations in the spinel assist the growt h of the precipitates by providing easy paths for the diffusion of cat ions.