Selective co-extraction of critical metals from deep ocean polymetallic nodule leachate and preparation of battery-grade Ni-Co-Mn solution

[Display omitted] •A novel selective co-extraction of Ni, Co and Mn from DPN leaching solution.•The synergistic extraction organic system DNNSA and 4PC.•Direct production of battery-grade Ni-Co-Mn mixed sulfate solution.•High-efficient and economic alternative for polymetallic nodules processing. The high-efficient and economic recovery of Ni, Co, and Mn from the leaching solution is critical for the industrial application of deep-ocean polymetallic nodules processing. To reduce the reagent consumption and wastewater generation presented in the individual metal element separation processes, a synergistic solvent extraction (SSX) process that selectively co-extracted Ni, Co, and Mn by the mixed extractant of pyridine carboxylate ester and naphthalene sulfonic acid was developed. Furthermore, the unit operation optimization and the extraction mechanism were systematically investigated. Experimental results demonstrated that almost all the Ni and Co were selectively extracted and left the impurities such as Ca and Mg in the raffinate by the synergistic extraction under optimal conditions. Moreover, a quantitative extraction of Mn could be achieved by controlling the excess coefficient of the organic extractant. By the aid of FTIR, HRMS, and the slope method, the molecular formula of loaded organic was deduced as Ni(C16H25NO2)2(C28H43O3S)2·2H2O, in which structure nickel forms a stable six-coordination complex with the extractant and H2O. Then, a battery-grade Ni/Co/Mn sulfate solution (Ca less than 0.02 mg/L, Mg less than 0.05 mg/L) was produced by stripping with 0.75 mol/L H2SO4 solutions. In conclusion, recovery of Ni, Co, and Mn from the leaching solution of DPN by the co-extraction with the SSX system is a promising alternative for industrial application due to its highly efficient and economic efficiency brought by the short process, less reagent consumption and wastewater discharge.