Selective dissolution and re-precipitation by pH cycling enables recovery of manganese from surface nodules†

Meeting global sustainable development and climate goals requires a rapid transition to renewable energy technologies. However, these emerging technologies rely on critical elements whose sourcing presents geopolitical and environmental challenges. In this study, we explore ferromanganese nodules from the Oacoma site in South Dakota as a viable feedstock for sourcing manganese, a critical element used in the production of battery cathodes, consumer electronics, and steel. The nodules are readily accessible from the surface site and primarily consist of rhombohedral metal carbonates, including manganese at 3.5–5.4 at% (9.2–14.1 wt%) relative to all the elements present in the nodules. Based on titration experiments and an equilibrium speciation model, we developed a strategy for extracting the manganese by selectively dissolving carbonate phases in acidic conditions, followed by selectively re-precipitating manganese oxide in alkaline conditions. Specifically, exposing the samples to pH 1.5–2 dissolved almost all the calcium and manganese ions, while retaining a significant portion of the iron and magnesium in the residual nodule powders. Subsequently, increasing the pH of the leachate to 5.7 resulted in the selective re-precipitation of predominantly iron hydroxide. Further increasing the pH of the leachate solution to 10.9 finally produced a relatively pure manganese oxide product. Our pH cycling approach recovered 65.7–74.2% of the manganese in the nodules at 70.3–85.4 at% (81.5–91.0 wt%) purity relative to the other metals, without the need for specialty chemicals, membranes, ligands, or resins, and without generating highly acidic wastes. We further performed a preliminary assessment of the scalability and industrial relevance of this process to explore these nodules as a feedstock for sustainable sourcing of manganese.