Efficient separation and recovery of Co and Ni from ammonium sulfate roasting-water leaching solutions of oceanic cobalt-rich crusts by sulfide precipitation and P507–Cyanex 272 synergistic extraction

Oceanic cobalt-rich crusts contain many valuable metals including Co, Ni, Cu, and Mn, of which Co and Ni are scarce metal resources on terrestrial land. Co, Ni, Cu, and Mn in cobalt-rich crusts can be efficiently extracted by the ammonium sulfate roasting-water leaching (ARWL) process, but separating and recovering Co and Ni from the leaching solution containing high concentrations of Mn is a challenging problem. In this study, a combination of sulfide precipitation and P507–Cyanex 272 synergistic extraction is proposed for the separation and recovery of Co and Ni from the ARWL solutions of oceanic cobalt-rich crusts. Under the optimum sulfide precipitation conditions, Co and Ni were precipitated with 99.85% and 99.63% efficiency, respectively, while Mn was coprecipitated with only .79% efficiency. The Co–Ni mixed sulfides were dissolved by dilute sulfuric acid under oxidative conditions to obtain a solution containing 12.584 g/L Co, 11.012 g/L Ni, and a small amount of Mn. Subsequently, Co and Mn were extracted from this solution using P507–Cyanex 272 synergistic extraction, the single-stage extraction efficiencies of Co and Mn were 95.76% and 99.73%, respectively, and the coextraction efficiency of Ni was 3.02%, under the conditions of a feed solution pH of 3.0, an extractant saponification degree of 70%, a total extractant concentration of 20% (P507 to Cyanex 272 ratio of 3:7), and an O/A ratio of 1.1:1. The results of the thermodynamic study showed that the reaction of Co extraction by this mixed extraction system was exothermic. Ni in the organic phase was washed with an H₂SO₄ concentration of .12 mol/L, followed by stripping of Co and Mn with an H₂SO₄ concentration of .6 mol/L. Mn in the stripping solution was removed by oxidative precipitation with (NH₄)₂S₂O₈, after which the Co₃O₄ product was obtained by precipitation with (NH₄)₂C₂O₄ and calcination. Similarly, NiC₂O₄·2H₂O can be obtained from the raffinate by (NH₄)₂C₂O₄ precipitation. Finally, a flowsheet was developed for the separation and recovery of Co, Ni, Cu, and Mn from the ARWL solutions of oceanic cobalt-rich crusts. The study provides a promising scheme for the recovery of various valuable metals from deep-sea cobalt-rich crusts or manganese nodules.