Structural changes and kinetics in the gaseous reduction of hematite

Abstract

The mechanism of the gaseous reduction of hematite grains to magnetite was studied. Grav-imetric measurements were carried out for the reduction of Carol Lake hematite pellets and grains in CO-CO₂ atmospheres over the temperature range 500 to 1100°C. The pore size distribution in the reduced magnetite was measured by mercury porosimetry. Partially reduced grains were examined by optical microscopy. At temperatures below 800°C, the reduction of a hematite grain to magnetite occurred at a well-defined shrinking-core inter-face. The average pore size in magnetite formed at 600°C was found to be 0.03 µm. An es-timate of the rate of CO diffusion through pores of this size indicated that the reaction rate at 600°C was controlled by a step near the hematite-magnetite interface. At temperatures above 800°C, the reaction mechanism became altered due to the preferential growth of magnetite along a single direction in each hematite grain. The reduction rate decreased with an increase in temperature, and no microporosity was present in magnetite formed at 1000°C and above. It was postulated that the reaction rate was controlled by the rate of formation of fresh nuclei and by their rate of subsequent growth.