Hydrometallurgical process for the recovery of yttrium from spent fluorescent lamp: Leaching and crystallization experiments

Present day rising necessity for rare earth elements (REE) is of great interest for its recovery through processing various waste materials concerning both economic and environmental benefits. The present study investigates the recovery potential of yttrium from fluorescent lamp waste using a hydrometallurgical process. Leaching of metals from the waste was studied by applying acids viz. hydrochloric acid (HCl), nitric acid (HNO₃) and sulfuric acid (H₂SO₄). Influence of various factors (solid:liquid ratio, reaction temperature, reaction time, and acid concentration) were conducted by full factorial design for the recovery of yttrium. Experimental variables such as decomposition, leaching and the oxide preparation were studied and the mechanisms responsible during the progress in each step was systematically investigated. The optimal experimental conditions attained with 40% solid/liquid ratio, at 45 °C in 0.5 h at 150 rpm, with 3N H₂SO₄ concentration. Whereas, HCl and HNO₃ leachants showed poor performance. Leaching process conducted in this study were best suited to ‘ash diffusion control dense constant size-spherical particles model’, which means that the diffusion process through the ash is the rate regulatory step in the leaching process. With increase in the concentration of oxalic acid as precipitating agent, a reduction in the nuclear induction period resulted indicating a higher sedimentation rate and shorter equilibration time. In addition, crystal nucleation rate and crystal growth rate, showed that the reaction velocity of the crystal nucleation ‘p’ is 2.88 and the crystal growth degree and value increases with the increase in the concentration of oxalic acid (n: 0.3437–0.4872). The precipitation rate was improved with the rise in temperature (from 25 °C to 45 °C) while the sedimentation rate was found negligible above 45 °C. This detailed study methodology can be considered as a feasible process thus creating a possibility to treat fluorescent waste lamp powders for various industrial applications.