Sustainable production of ultra-low-carbon ferromanganese and calcium aluminate slag from pre-reduced manganese ore using aluminium

The production of ultra-low-carbon ferromanganese is both energy- and carbon-intensive, motivating the search for greener alternatives. This study investigates a sustainable process, using hydrogen pre-reduced Nchwaning manganese ore (NPO) and aluminium as a reductant, to produce ultra-low-carbon ferromanganese with leachable mono-calcium aluminate (CaO·Al2O3, krotite) slag. NPO (<1 mm) was blended with CaO to achieve CaO/Al2O3 molar ratios of 1.0, 1.2, and 1.4 and smelted with varying aluminium additions at 1500°C. The resulting alloys (0.005–0.013%C, 74.98–78% Mn, 18.8–24.4% Fe) were of high quality, while slags were dominated by calcium aluminates with minor gehlenite and tricalcium aluminate. During smelting, the slag system transformed from MnO–CaO to CaO–Al2O3, and the metal system evolved from Al–Fe to Mn–Fe–Si through rapid interfacial reactions. Mn and Si distributions between metal and slag were closely linked to slag composition. The optimum condition was a CaO/Al2O3 ratio of 1.2 and 20 wt% excess Al, yielding high metal recovery and desired slag. Excess lime increased Al2O3 dissolution from the crucible (8–13 g), slightly reducing Mn recovery. This process offers a cleaner route to ultra-low-carbon ferromanganese with usable slags.