Electrochemical reduction method for leaching terbium and cerium from waste phosphors†

Rare earth elements are commonly used in various fields due to their unique physical and chemical properties. Waste phosphors represent an important secondary source of rare earth elements, and their recovery is a crucial means of improving the utilization of these resources. When using FeCl₂ as a reducing agent to reduce Tb⁴⁺ and Ce⁴⁺ from washed phosphors, a large amount of reducing agent is required, and the Fe²⁺ cycle becomes complicated. In this study, the electrochemical reduction process was employed to treat washed phosphors. Under optimum conditions of FeCl₂ concentration of 0.10 mol L⁻¹, current density of 100 A m⁻², HCl concentration of 3 mol L⁻¹, temperature of 80 °C, L/S ratio of 30 mL g⁻¹, and time of 60 min, the leaching efficiencies of Tb and Ce reached 99.2% and 98.5%, respectively. The apparent activation energies of Tb and Ce were calculated as 17.71 and 18.46 kJ mol⁻¹, respectively, illustrating that the leaching processes of Tb and Ce depend on the diffusion. By applying an external electric field, Fe²⁺ can be recycled in the system, thereby reducing the amount of reducing agent needed and achieving a higher leaching efficiency for Tb and Ce. These results are beneficial for the resource utilization of waste phosphors and have significant implications for the sustainable development of rare earth resources.