A near-zero waste process for the full-component utilization of deep-sea polymetallic nodules based on reductive leaching with SO2 followed by separation and recovery

Recovery of critical metals from marine minerals is a promising option to ensure the sustainable supply of critical metals, but existing recovery processes suffer from technical bottlenecks in terms of low resource utilization and large waste discharge. This paper presents a complete hydrometallurgical route for the full-component utilization of deep-sea polymetallic nodules (DPN). Leaching experiments were first carried out in the presence of SO2 because cheap SO2 can destroy the original mineral structure based on a reductive mechanism. The leaching efficiencies of Ni, Co, Mn, Cu, La, Ce, and Fe were reached 99.2%, 99.3%, 98.7%, 95.9%, 95.3%, 95.4%, and 91.4%, respectively, under the optimal conditions of 30 degrees C, L/S ratio of 6:1 mL/g, H2SO4 dosage of 37.5 wt%. The downstream process aimed at the sequential recycling of metal ions from the pregnant leach solution (PLS) consists of Fe recovery by the hematite process, selective solvent extraction of Cu and rare earth elements (REEs), co-extraction of Ni/Co/Mn, and Mn electrowinning. As a result, high-grade hematite (60 wt% of Fe), industrial -grade CuSO4, mixed rare earth oxides, battery-grade (Ni, Co, Mn)SO4 solution, and electrolytic manganese were obtained. During the whole process, the recovery efficiency of Ni, Co, Mn, Cu, La, Ce, and Fe reached 97.1%, 97.1%, 96.9%, 91.5%, 91.1%, 91.1%, and 91.3%, respectively. Since the hydrometallurgical process achieves the resource utilization of all the DPN constituents, it has the advantages of near-zero waste and low energy consumption.