Kinetics study on the H2 reduction of Nchwaning manganese ore at elevated temperatures

Replacing solid carbon with hydrogen gas in ferromanganese production presents a forward-thinking, sustainable solution to reducing the ferro-alloy industry’s carbon emissions. The HAlMan process, a groundbreaking and eco-friendly method, has been meticulously researched and scaled up from laboratory experiments to pilot tests, aiming to drastically cut CO2 emissions associated with ferromanganese production. This innovative process could potentially reduce CO2 emissions by about 1.5 tonnes for every tonne of ferromanganese produced. In this study, a lab-scale vertical thermogravimetric furnace was used to carry out the pre-reduction of Nchwaning manganese ore, where direct reduction occurred with H2 gas under controlled isothermal conditions at 700, 800, and 900°C. The results indicated that higher pre-reduction temperatures (800 and 900°C) effectively converted Fe2O3 to metallic iron and Mn2O3 to MnO. By continuously monitoring the mass changes during the reduction, both the rate and extent of reduction were assessed. A second-order reaction model was applied to validate the experimental outcomes of H2 reduction at various temperatures, showing apparent activation energies of 29.79 kJ/mol for dried ore and 61.71 kJ/mol for pre-calcined ore. The reduction kinetics displayed a strong dependence on temperature, with higher temperatures leading to quicker and more complete reductions. The kinetics analysis suggested that the chemical reaction at the gas–solid interface between hydrogen and the manganese ore is likely the rate-limiting step in this process.