The Compositional and Structural Changes in Chromite during Reduction with Carbon

Abstract

Russian chromite particles reduced to various stages of reduction with graphite were subjected to the analyses by an X-ray diffractometer and an electronprobe microanalyser.
At 1 100°C, the chromite was reduced only to a slight extent. At the temperatures between 1 150° and 1 250°C, metallic beads were formed on the surface of a chromite grain. Their Cr content was relatively low. The grain was divided into the core of almost unreduced chromite and the outer layer depleted in Fe and rich in Al and Mg.
At 1 300°C or above, metallic beads with a higher content of Cr were formed around the grain. In the later stage of reduction, the grain was divided into three zones, i.e., the outer layer mainly consisted of MgAl₂O₄, the inner layer depleted in Fe only, and the core of almost unreduced chromite. Metallic beads were formed in the outer layer and the inner layer separately and a darker doughnut-like zone was observed between them in a coarser grain (above 100μm). A considerably large content of MgO was observed in this zone. Metallic phase formed in the inner layer was vein-like and rich in Fe.
At 1 250°C or above, the transfer of Si from gangue into chromite grains was observed, which was considered to be the main reason of the acceleration of the reduction rate of chromite ore with SiO₂.