Recycling of rare earths from lamp phosphor waste: Enhanced dissolution of LaPO4:Ce3+,Tb3+ by mechanical activation

Abstract

With the promoted use of compact fluorescent lamps and the increasing amount of stockpiled compact fluorescent lamp waste, lamp phosphors are becoming an interesting secondary source for critical metals (yttrium, europium and terbium). This paper explores on the mechanism of a new potential hydrometallurgical route to improve the recovery efficiency of rare-earth elements from the green phosphor LaPO4:Ce3+,Tb3+ using mechanical activation as a pretreatment step prior to leaching. By applying an intense frictional action, the leaching yields of rare-earth elements were enhanced from 0.9% to 81% at room temperature, as a consequence of the change in activation energy. The shrinking core model was employed for modelling the leaching kinetics of the rare-earth elements and to calculate the decreasing apparent activation energy of the unmilled and milled samples (from 68 kJ mol−1 to 1.4 kJ mol−1). This difference can be explained by the physicochemical changes, including structural decomposition, specific surface area increase and particle size reduction, which were related to the corresponding leaching pattern. The optimized mechanical activation procedure was successfully applied to lamp phosphor waste, containing a mix of different phosphors. After sequential removal of the halophosphate phosphor and the red YOX phosphor, 99.0%, 87.3% and 86.3% of La, Ce and Tb present in the LaPO4:Ce3+,Tb3+ phosphor could be dissolved. These observations provide more insight in the mechanical activation process and may contribute to a more sustainable alternative route to the rare-earth element recycling industry.