Leaching of yttrium and europium from fluorescent lamp phosphor powder using nitric acid: kinetics and optimization

Rare earth elements (REEs) are recognized as one of the most critical elements and considered essential for the development of sustainable energy technologies, lasers, electronics, alloys, catalysts, and other applications. In order to meet the increasing demand to ensure those essential usages, the supply of REEs is however limited from the primary sources. Recovery of REEs from secondary resources, such as discarded electrical and electronic equipment, referred to as electronic waste (e-waste) is consequently crucial. Fluorescent lamps among such wastes are of special concern due to the high quantity of REEs present in them. This article discusses a potential hydrometallurgical method for recovering REEs (yttrium and europium) from fluorescent lamp waste via nitric acid leaching. The effects of leaching factors including acid concentration and temperature were examined. To assist the leaching yields of those metals, the effect of alkaline fusion was specifically examined. The kinetics of both processes, direct- and alkaline fusion-assisted leaching were investigated based on the shrinking core model. In the case of direct leaching, the metal dissolution was controlled by surface chemical reaction with apparent activation energies of 43.9 and 48.1 kJ/mol for yttrium and europium, respectively, in the temperature range 303–333 K. Pretreatment, viz., the alkaline fusion changed the subsequent leaching mechanism to diffusion control with apparent activation energies of 7.4 and 9.5 kJ/mol for the respective metal in the same temperature range. Optimization of the alkaline fusion-assisted acid leaching process was undertaken by applying response surface methodology (RSM) based on the central composite design (CCD). Under the alkaline fusion conditions such as soda/fluorescent powder mass ratio of 0.65 g/g, temperature of 1223 K and reaction time of 7200 s followed by acid leaching (nitric acid concentration – 3.5 M, temperature-323 K, pulp density – 30 g/L and leaching duration – 1200 s), the leaching efficiencies of 92.6% and 99.5% for yttrium and europium, respectively, could be achieved.