Cobalt extraction from spent lithium-ion battery cathode material using a sulfuric acid solution containing SO2

To increase the degree of cobalt (Co) extraction, the process of the cathode material leaching was performed in a sulfuric acid (H₂SO₄) solution containing sulfur dioxide (SO₂) as a reducing agent. To provide a high resolution of the obtained results, frequent monitoring of Co concentrations in leached solution was conducted using an ultraviolet–visible spectrophotometer with several specific modifications related to the connection of the reaction vessel with the instrumental cuvette. The maximum degree of Co leaching (99.4%) was achieved with H₂SO₄ concentration of 3 mol/L, solid phase concentration of 33 g/L, temperature of 85 °C, SO₂ volume flow of 2 L/min, and leaching time of 60 min. The results of the performed kinetic analyses indicated that the Avrami equation best describes the investigated leaching process, which later was supported by the results of X-ray diffraction and scanning electron microscopy–energy-dispersive X-ray spectroscopy analyses. Also, the activation energy of 28 ± 3 kJ/mol is in favor of the fact that the process of Co leaching was controlled by the factors, such as diffusion and chemical reaction. The results of this study indicated that SO₂ can be used as an effective reducing agent in the investigated process.