Abstract In this work, a reactive melting procedure was implemented for the inertization of the manganese-containing cathodes of spent alkaline batteries. The process consisted of adding the cathode powder mixed with borax on top of molten aluminum at 1000 °C. By means of an aluminothermic reaction, manganese oxides are reduced and incorporated into the alloy. The presence of borax in the reactive mixture was found to be fundamental to obtain the Al-Mn alloy. The characterization of the metal phase and the slag phase by scanning electron microscopy and X-ray diffraction showed that manganese is reduced to form manganese borides in the slag and Al-Mn intermetallics in the alloy. This process could make feasible to use aluminum casting shops as processors of spent alkaline batteries.