Isothermal Reduction of Nchwaning Manganese Ore in CO/CO2/H2 Atmospheres

Abstract

In the industrial production of manganese ferroalloys, incomplete pre-reduction of higher manganese- and iron oxides in the furnace causes an excess consumption of carbon and energy. Pre-treatment of the manganese raw materials in an external unit may alleviate this issue. To optimize the pre-treatment process, a deeper understanding the solid-state reduction kinetics is needed. In this work, Nchwaning manganese ore have been reduced isothermally at different temperatures at two different oxygen partial pressures in CO/CO2 and CO/CO2/H2 gas mixtures, that is four different gas compositions. Isokinetic modeling was used to identify the activation energy range for the different cases and rate expressions were derived using the reaction order model. The derived model expressions adequately reproduced the experimental curves. The effects of temperature and gas composition were quantified based on the model expressions. By reducing the CO content in the CO/CO2 gas mixtures from 70 % to 30 %, the reduction rate was reduced by a factor 3. For the equivalent change in the CO/CO2/H2 gas mixtures, the rate was reduced by a factor 2. The addition of hydrogen increased the reaction rate by a factor of 1.9-2.8 for gases with equivalent thermodynamic potential within the experimental range. If partial combustion of the furnace gas is used to fuel a pretreatment unit, the addition of hydrogen will be beneficial to avoid significantly lowering the reduction rate of the ore.