Experimental investigation of recycling rare earth elements from waste fluorescent lamp phosphors

Abstract

Characterization techniques and experimental measurements were used to evaluate a process for recycling rare earth elements (REEs) from spent fluorescent lamp phosphors. QEMSCAN analysis revealed that 70% of the rare earth bearing minerals was less than 10 µm in size. Feeds of varying characteristic were received throughout the course of the experimental analysis. A representative sample of the as-received feed contained 5.8% total rare earth elements (TREE) and upon sieving to below 44 µm, the grade increased to 16.5% TREE. By sieving further to below 10 µm, the grade increased to 19.8% TREE. Hydrochloric acid was used as lixiviant in batch leach experiments on the phosphor powder. The maximum extraction obtained was 90% for europium and yttrium at the following conditions: 1.5 M HCl, 70 degrees C, 1 hr, 30 g/L and 200 rpm. However, the solubility of cerium, lanthanum and terbium remained low under these conditions. Multistage leaching and calcination followed by leaching processes also resulted in poor extraction of cerium, lanthanum and terbium. Based on experimental results a new process for extracting the chief REEs from end of life fluorescent lamps has been developed. The proposed process employs a multistage acid leach using HCl under both mild and strong leaching conditions in addition to thermal treatment of the powder. Using this process, about 90% of the europium and yttrium is extracted in the first stage leach and over 90% of lanthanum in the second stage leach. There is also over 80% of cerium and terbium extracted which marks a significant improvement. Precipitation using oxalic acid as precipitant and sodium hydroxide for pH adjustment was able to recover 100% of the REE from the leach liquor. However, the purity of the mixed rare earth oxides produced is very low because of co-precipitation of impurities from the leach liquor. The process needs to be optimized for potential industrial application.