An efficient extractant (2-ethylhexyl)(2,4,4′-trimethylpentyl)phosphinic acid (USTB-1) for cobalt and nickel separation from sulfate solutions

Abstract

With the rapid growth of new energy vehicles, more and more spent LIBs have been generating. Ternary cathodes account for more than half of the LIB cathode market share and their recycling draws the most attention. Solvent extraction is the most efficient and common method to separate and recover Co and Ni from spent ternary cathode leachates. Efficient extractant is the key. In this paper, we first studied (2-ethylhexyl)(2,4,4′-trimethylpentyl)phosphinic acid (USTB-1) on separating Co from high Ni/Co ratio solutions. It extracts Co over Mg and Ca and there would be no CaSO4 precipitation problem during Co scrubbing and stripping. Its Co/Ni separation performance is superior to MEXTRAL 507P and Cyanex 272. Its ΔpH0.5 of Co and Ni was 1.9. At saponification ratio of 40%, the βCo/Ni of USTB-1 reached to 1.05 × 104, while that of MEXTRAL 507P and Cyanex 272 were only 187 and 2697, respectively. USTB-1 has larger Co saturation capacity than Cyanex 272. It loaded 99.3 mg/L of Co, while Cyanex 272 only loaded 75.1 mg/L at the same conditions. Loaded Co can be easily stripped by mineral acids HNO3, HCl and H2SO4, among which H2SO4 is the most effective. The extraction mechanism was investigated through equilibrium pH-Log D relationship in the aid of FT-IR and ESI-MS spectra of the extraction complex. USTB-1 extracts Co through cation exchange and coordinated with Co via its two oxygen atoms. In conclusion, USTB-1 was more efficient than MEXTRAL 507P and Cyanex 272 for separating Co from high Ni/Co ratio solutions. It is highly suited to separating and recovering Co and Ni from spent ternary cathode leachates. It can reduce reagent and power consumptions and lead to cleaner production.