Thermal decomposition behavior and kinetic mechanism of waste phosphors during sodium hydroxide roasting

Abstract

Waste phosphors are critical rare earth secondary resources, and their recycling can help alleviate the depletion of rare earth mineral resources. However, stable Al–Mg spinel structures exist in waste phosphors. The direct acid leaching method remains difficult to use for leaching cerium and terbium. In this paper, the waste phosphors were first pretreated via alkali roasting. NaOH was used as an alkali roasting agent, thus achieving the decomposition of the Al–Mg spinel structure. On the basis of thermodynamic calculations, thermal decomposition experiments were performed. The results showed that when the temperature was greater than 500°C, the waste phosphors could be decomposed into MgO, Eu2O3, Y2O3, CeO2, Tb4O7, NaAlO2, and BaO. The Ce3+ and Tb3+ in the waste phosphors were partially oxidized to Ce4+ and Tb4+, respectively, during the roasting process. The BET surface area and the average pore diameter for the roasted products also increased. Under the conditions of roasting temperature at 900°C, NaOH/sample mass ratio of 2.5, and roasting time of 150min, the leaching efficiencies of Y, Eu, Ce, and Tb could reach 99.4%, 99.1%, 98.5%, and 98.8%. The decomposition process of waste phosphors conformed to the unreacted shrinking core model. In addition, the conversion processes of Ce and Tb depended on the chemical reaction. The results may provide a new direction of development for the resource utilization of waste phosphors.