Separation of Cd and Ni from Ni-Cd batteries using thiosulfate as leaching, precipitating and separating agent via photochemical method

Abstract

In recent years, recycling spent batteries has become a critical issue. In this study, the thiosulfate system which is used as leaching, precipitating, and separating agent was adopted, while the process combined with thiosulfate leaching, precipitation recovery, and UV photolysis separation steps was carried out to recycle spent nickel-cadmium (Ni-Cd) batteries. In the leaching process, Cu²⁺ provided by copper sulfate (CuSO₄) synergistically catalyzes the efficient leaching of Ni and Cd through oxidation and the formation of soluble complexes with S₂O₃²⁻. The experimental results and statistical analysis revealed that the leaching rate of Cd is only 34.79 % without adding CuSO₄ as catalytic oxidant, while Ni cannot be leached. At 0.01 mol/L CuSO₄ concentration, the leaching efficiency of Cd at 70 °C and 8 h was 90.47 %. Meanwhile, Ni reached a leaching efficiency of 42.46 %, and the initial separation coefficient between Cd and Ni was 0.47. Rhodamine B (RhB) was degraded by leaching residue, and the degradation effect of 30 mg leaching residue on 15 mg/L RhB reached 95.87 %. Based on the photochemical differentiation of metal thiosulfate complexes, the photolysis process for Cd selectively separated from Cd and Ni containing leach solutions under UV photochemistry was explored. A UV light source with a wavelength of 254 nm was chosen, and the rate constant for the photolysis of the Cd thiosulfate complex at a Na₂S₂O₃ concentration of 0.3 mol/L was 0.00321 min⁻¹, while the Ni thiosulfate complex remained largely failed to photolysis. Finally, Cd in the solution was converted into CdS precipitate with a precipitation rate of 88.39 %. The separation coefficient between Cd and Ni was 0.12 × 10⁻³, achieving the selective separation of Cd and Ni.