Recycling rare earth elements from waste cathode ray tube phosphors: Experimental study and mechanism analysis

Abstract

In this work, we developed a clean and efficient process for recycling rare earth elements from waste CRT phosphors. A mixture of sulfuric acid and hydrogen peroxide was used for the oxidative leaching of the rare earth elements, and the leaching efficiencies of both Y and Eu reached 99% under the optimal leaching conditions: 3 M H2SO4, 4 vol.% H2O2, 55 °C, and 1 h. The S2− was oxidized to S0, which efficiently avoided the sulfur pollution produced by existing technologies. The green solvent ionic liquid [OMIm][PF6] and the extractant Cyanex272 were employed for the separation of the rare earths. The optimal extraction parameters were determined to be 0.2 mol/L H2SO4, 0.4 vol.% of Cyanex272 in the organic phase, a 10:1 A/O ratio, and an extraction time of 20 min at room temperature, under which, the individual extraction efficiencies of Y, Eu, Zn, and Al were 99%, 87, 8%, and 0%, respectively, and the separation factor of rare earth to Zn reached 593. Additionally, the extraction system could be recycled and reused by stripping. The mechanisms of the leaching and extraction process were also analyzed. Based on a kinetics study, the leaching process was found to follow the model of diffusion-control via the product layer, and the apparent activation energies for Y and Eu were calculated to be 75.86 kJ/mol and 77.06 kJ/mol, respectively. Meanwhile, a simulated leaching model based on diffusion through the solid S0 layer was established. The synergistic extraction mechanism was also discussed, and a cation exchange reaction was speculated to occur between the rare earth elements and Cyanex272. The findings in this study are expected to provide an effective and practical method for the recovery and reutilization of waste CRT phosphors.