Rare-earth recycling using a functionalized ionic liquid for the selective dissolution and revalorization of Y2O3:Eu3+ from lamp phosphor waste

Abstract

The supply risk for certain rare-earth elements (REEs) has sparked the development of recycling schemes for end-of-life products like fluorescent lamps. In this paper a new recycling process for lamp phosphor waste is proposed based on the use of the functionalized ionic liquid betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf₂N]. This innovative method allows the selective dissolution of the valuable red phosphor Y₂O₃:Eu³⁺ (YOX) without leaching the other constituents of the waste powder (other phosphors, glass particles and alumina). A selective dissolution of YOX is useful because this phosphor contains 80 wt% of the REEs although it only represents 20 wt% of the lamp phosphor waste. The proposed recycling process is a major improvement compared to currently used hydrometallurgical processes where the non-valuable halophosphate (HALO) phosphor (Sr,Ca)₁₀(PO₄)₆(Cl,F)₂:Sb³⁺,Mn²⁺ is inevitably leached when attempting to dissolve YOX. Since the HALO phosphor can make up as much as 50 wt% of the lamp phosphor waste powder, this consumes significant amounts of acid and complicates the further processing steps (e.g. solvent extraction). The dissolved yttrium and europium can be recovered by a single stripping step using a stoichiometric amount of solid oxalic acid or by contacting the ionic liquid with a hydrochloric acid solution. Both approaches regenerate the ionic liquid, but precipitation stripping with oxalic acid has the additional advantage that there is no loss of ionic liquid to the water phase and that the yttrium/europium oxalate can be calcined as such to reform the red Y₂O₃:Eu³⁺ phosphor (purity >99.9 wt%), effectively closing the loop after only three process steps. The red phosphor prepared from the recycled yttrium and europium showed excellent luminescent properties. The resulting recycling process for lamp phosphor waste consumes only oxalic acid and features a selective leaching, a fast stripping and an immediate revalorization step. Combined with the mild conditions, the reusability of the ionic liquid and the fact that no additional waste water is generated, this process is a very green and efficient alternative to traditional mineral acid leaching.