Kinetic model for the carbothermic reduction of manganese dioxide

Abstract

Mixtures of MnO 2 and graphite were reacted under argon at different constant temperatures, and the loss in mass was recorded continuously. The reduction path was assumed to be MnO 2 → Mn 3O 4 → MnO → Ma 5C 2 or Mn. Chemical conditions determine the ratio of Mn 5C 2/Mn formed in the reduction product. A multi-step shrinking-core model was proposed to describe the kinetics of reduction. Parameters in this model were estimated by performing a least-squares regression on the experimental data. Acceptable correspondence was obtained between predicted and experimental kinetic curves. The activation energy for the reduction of MnO 2 to Mn 3C 4 was only 71.3 kJ mol −1, which indicated a weak dependence on temperature. Further reduction steps revealed activation energies of about 207 kJ mol −1, and were of the same order of magnitude as the activation energy for the Boudouard reaction. The distribution of species at any point in time, which depended on the MnO 2/carbon ratio and the temperature, could be calculated by means of this proposed model.