The mechanism and kinetics of the carbothermic reduction of mamatwan manganese ore fines

Abstract

The kinetics of reduction of the manganese ore from Mamatwan mine in South Africa has been studied by thermogravimetric analysis (TGA), X-ray diffraction analysis (XRD), optical microscopy, Scanning Electron Microscopy (SEM) and energy dispersive analysis of X-rays (EDAX) between 1100° and 1350°C with graphite as the reductant under argon atmosphere. It has been observed that the rate and degree of reduction increased with increasing temperature and decreasing particle size. In the early stages of reduction, up to about 4 minutes of reaction time corresponding to about 30% reduction, reduction of higher oxides of manganese and iron (Mn 2O 3 and Fe 2O 3) to MnO and FeO, respectively, was observed. This stage was possibly controlled by diffusional process across the boundary layer between the solid phases. An apparent activation energy of 61.03kJ was calculated for this diffusional process. Metallization started as random nucleation of iron, which subsequently was carburized around MnO grains inside the particle. Also, after about 30% reduction, the formation of a silicate phase was observed. Up to 70% reduction at 1350°C, reduction rate was possibly controlled by chemical reaction between the oxide and gaseous phases for which an activation energy of 153.32kJ was calculated. The later part of the process proceeded by the reduction of MnO, covered by either the carbide or silicate phase, by carbon dissolved in the carbide phase (Mn, Fe) 5C 2. Diffusion of Mn 2+ ions in the oxide phase is the most likely rate determining step for this stage for which an apparent activation energy of 310.4kJ was found.