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

Abstract

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.

Keywords

H₂ reduction / kinetics / pre-exponential factor / activation energy / thermogravimetry / manganese ore / CO₂ emission / Engineering / Resources Engineering and Extractive Metallurgy

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