Rare earth elements recovery from fluorescent lamps. A new thermal pretreatment to improve the efficiency of the hydrometallurgical process

The literature is rich of scientific works regarding the recovery of yttrium and europium from red phosphors of lamps and it is poor of papers about the extraction of terbium, cerium and lanthanum from green phosphors. The red phosphors are constituted by rare earth oxides and they are more easily dissolved in the leaching step respect to the green phosphors that are rare earth phosphates. This paper was mainly focused on the recovery of rare earths from green phosphors, with major attention on terbium extraction that is the most valuable metal. Hence, an innovative process with a thermal pretreatment was proposed. Alkaline fusion with barium hydroxide and the following leaching were studied under various experimental conditions to optimize the process parameters. Statistical design of experiments and analysis of variance were performed in order to determine the main effects and interactions between the investigated factors (barium hydroxide/powders ratio, sulfuric acid concentration, leaching temperature, pulp density and leaching time) for the dissolution of each rare earth elements. The results, for most of REE considered, especially for terbium showed that barium hydroxide/powder ratio and temperature were significant with a positive effect. Concentration of sulfuric acid and pulp density were significant with a negative effect. Time of leaching instead had a very slightly positive effect. The best extraction yields, obtained under the optimal conditions, were: 99% for yttrium and europium, 80% for terbium, 65% for lanthanum, 63% for gadolinium and 60% for cerium. Therefore, rare earth oxides were produced after oxalic acid precipitation and calcination. The grade was 82.22% of yttrium oxide, 8.38% of europium oxide, 2.43% of cerium oxide, 2.29% of gadolinium oxide, 1.77% of lanthanum oxide and 1.52% of terbium oxide.