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

Reduction of manganese ores from the Wessel mine of South Africa has b een investigated in the temperature range 1100 degrees C to 1350 degre es C with pure graphite as the reductant under argon atmosphere. The r ate and degree of reduction were found to increase with increasing tem perature and decreasing particle sizes of both the ore and the graphit e. The reduction was found to occur in two stages: (1) The first stage includes the rapid reduction of higher oxides of manganese and iron t o MnO and FeO. The rate control appears to be mixed, both inward diffu sion of CO and outward diffusion of CO2 across the porous product laye r, and the reaction of carbon monoxide on the pore walls of the oxide phase play important roles. The values of effective CO-CO2 diffusiviti es 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 mix ed carbide of iron and manganese. The chemical reaction between the ma nganous oxide and carbon dissolved in the metal phase or metal carbide seems to be the rate-controlling process. The rate constant of chemic al reaction between MnO and carbide on the surface of the impervious c ore 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 ra nge from 102.1 to 141.7 kJ/kg/mol.