Pre-reduction of Nchwaning manganese ore in CO/CO2, H2/H2O, and H2 atmospheres

Hydrogen (H2), a relatively underexplored reductant in ferromanganese (FeMn) production, offers an attractive avenue for mitigating gaseous carbon (C) emissions. The reduction behaviour of South African Nchwaning manganese (Mn) ore using gaseous CO/CO2, H2/H2O, and H2 atmospheres was investigated experimentally in the temperature variation of 700, 800, and 900 °C. The effect of different gas compositions and temperatures was studied using a vertical thermogravimetric (TG) tube furnace. During pre-reduction, two parallel reactions occurred, namely the reduction of higher Mn- and iron (Fe) -oxides, and the decomposition of carbonates. After each test, decrepitation, chemical composition, phase transformation, and porosity were characterised. Using the rate of mass loss, a kinetic model was obtained to predict kinetic constants. The oxidation state of the higher Mn- and Fe-oxides was lowered during CO/CO2 and H2/H2O pre-reduction. Only during pure H2 pre-reduction was Fe2+ reduced to its metallic state, Fe0. The majority of carbonates decomposed in the presence of the CO/CO2 atmosphere at 900 °C, whereas in the presence of H2 in the reducing atmosphere carbonates decomposed at a higher rate and lower temperatures. Additionally, the extent and rate of mass loss were expedited by increasing the temperature, employing H2-containing atmospheres, and lowering the oxygen partial pressure (pO2) of the H2-containing atmospheres. No significant trends were observed in ore decrepitation and porosity across various atmospheres and temperatures, except for decrepitation in the water vapour-containing atmosphere. The utilisation of a pure H2 atmosphere has a significant ability for pre-reducing Mn ores with carbonate content by expediting carbonate decomposition and promoting Fe-oxide metallisation, thereby enhancing the efficiency of ore treatment in metallurgical applications.