Extraction of pure Co, Ni, Mn, and Fe compounds from spent Li-ion batteries by reductive leaching and combined oxidative precipitation in chloride media

Abstract

The redox equilibria – also taking chloro–complexation into account – and the thermodynamics of the HCl – Co – Ni – Mn system have been examined. The leaching efficiencies of the LiCoO2 (LCO), LiNiMnCoO2 (NMC111), and LiFePO4 (LFP) black masses were studied in the 1 – 10 M HCl concentration and the 25 – 95 °C temperature ranges. A minimum of 2 M HCl concentration with an 80-minute contact time at 95 °C was sufficient to achieve > 98 % leaching yields from the LCO material. By increasing the HCl concentration to 6 M, the leaching time could be halved. The NMC cathode material could even be leached completely at 80 °C with 2, 4, 8, and 10 M HCl in 50, 40, 10, and 8 min, respectively. The same yield could be achieved even at only 25 °C using 6, 8, and 10 M HCl for ∼ 310, 125, and 50 min, respectively. A relation was found between the chloride ion activity and the leaching yields, while the evolution of Cl2 was also examined to help explain leaching kinetics. The purity of the graphite residue reached 99.9 % (referring to Co, Ni, Mn, Li) after two cross-current leaching steps. The oxidative precipitation – using hypochlorite solutions - partially generated by utilizing the Cl2 from leaching - was examined in the 1–6 pH range showing that selectivity significantly increases with decreasing pH, but the degree of reagent utilization also decreases. Kinetic factors were also pointed out to markedly affect selectivity. The combined hydrolytic–oxidative precipitation process could recover 96–98 % pure raw oxides, which were further refined to 98–99.8 % purity by scrubbing in acidic MnSO4 and CoSO4 solutions.