Synergistic recovery of mercury and rare earth elements from waste phosphors by microwave alkali fusion - carbothermal reduction followed by acid leaching

Abstract

Waste phosphors (WP) containing mercury from discarded fluorescent lamps is not only a dangerous solid waste but also a secondary resource for recycling of rare earth elements (REEs) and mercury. This study aimed to investigate synergistic recovery of REEs and mercury from WP by microwave alkali fusion - carbothermal reduction followed by acid leaching. Phosphors and mercury compounds were decomposed by NaOH and carbon to rare earth oxides with low-valence and elemental mercury. When WP was roasted with alkali and carbon powder at 900 ^oC for 30 min, mercury removal efficiency reached 99.9%; the leaching efficiency of REEs reached 93.67% after HCl leaching without reducing agent. The addition of carbon enhanced alkali fusion according to thermodynamic and kinetic analysis. When carbon addition amount was 30% during microwave alkali fusion, the dissociation activation energy of Ce and Tb decreased by 4.33 and 15.8 kJ·mol^-1 respectively, the activation energy of mercury removal decreased by 15.05 kJ·mol^-1. Mixed rare earth oxides with purity of 91.2% were obtained from the acid leaching solution by oxalic acid precipitation followed by roasting. This work provided a new idea for synergistic recovery of REEs and mercury from real WP by microwave roasting with additives.