KINETICS OF THE SOLID-STATE CARBOTHERMIC REDUCTION OF WESSEL MANGANESE ORES

Abstract

Reduction of manganese ores from the Wessel mine of South Africa has been investigated in the temperature range 1100 degrees C to 1350 degrees C with pure graphite as the reductant under argon atmosphere. The rate and degree of reduction were found to increase with increasing temperature and decreasing particle sizes of both the ore and the graphite. The reduction was found to occur in two stages: (1) The first stage includes the rapid reduction of higher oxides of manganese and iron to MnO and FeO. The rate control appears to be mixed, both inward diffusion of CO and outward diffusion of CO2 across the porous product layer, and the reaction of carbon monoxide on the pore walls of the oxide phase play important roles. The values of effective CO-CO2 diffusivities generated by the mathematical model are in the range from 2.15 x 10(-5) to 6.17 x 10(-5) cm(2).s(-1) for different ores at 1300 degrees C. Apparent activation energies range from 81.3 to 94.6 kJ/kg/mol. (2) The second stage is slower during which MnO and FeO are reduced to mixed carbide of iron and manganese. The chemical reaction between the manganous oxide and carbon dissolved in the metal phase or metal carbide seems to be the rate-controlling process. The rate constant of chemical reaction between MnO and carbide on the surface of the impervious core was found to lie in the range from 1.53 x 10(-8) to 1.32 x 10(-7) mol.s(-1).cm(-2) Apparent activation energies calculated are in the range from 102.1 to 141.7 kJ/kg/mol.