Leaching manganese nodules with iron-reducing agents – A critical review

The accelerated growth in global demand for Manganese coincides with the continuing depletion of high-grade ores on the earth's surface. This element is essential for diverse markets such as steel production, dietary additives, fertilizers, cells, fine chemicals, and some chemical reagents. Interestingly, there are large mineral reserves in the ocean's depths, where marine nodules are an attractive option due to their high manganese content (between 16 and 24%). The dissolution of MnO₂ from manganese nodules needs a reducing agent. Iron is a feasible alternative with a low cost and can even be recycled from scrap or industrial waste like smelting slag, tailings, or scrap steel. This paper provides a comprehensive review of Manganese acid-reducing processes from marine nodules using different iron-reducing agents that include FeS₂, Fe²⁺, Fe⁰, Fe₂O₃, and Fe₃O₄. Fe⁰ has displayed the best performance in terms of dissolution kinetics and Mn extraction. This review further confers the chemistry and reactions involved in the reductive leaching and stresses the critical parameters that could be considered for optimization. In this sense, the concentration of reducing agent and temperature are highlighted as the most influential, making other parameters (e.g., particle size, stirring speed, acid concentration, and leaching time) almost irrelevant. Finally, it is concluded that the best way to extract Mn from marine nodules is to reuse steel scraps, working with high Fe⁰/MnO₂ ratios (2/1), low concentrations of sulfuric acid (0.1 mol/L), and short times (5 min), achieving extractions of 90%.