Tailored separation of light rare earth elements using combined oxidative precipitation and multi-stage solvent extraction techniques

Abstract

• Combined oxidative precipitation and solvent extraction for efficient light REE separation. • Achieved over 99.8 % selective Ce precipitation using oxidative precipitation with minimal co-precipitation. • Optimized the combination of Cyanex 572 − TBP system in selective extraction of LREEs using statistical methods. • Production of individual light REOs with over 90 % purity from their mixture. • Developed a scalable multi-stage solvent extraction process for selective REE separation. The similar chemical properties of rare earth elements (REEs) and their closely matched ionic radii pose a significant challenge in their separation, making it difficult to produce individual REE products. This research focuses on the selective separation of lanthanum (La), cerium (Ce), didymium (praseodymium and neodymium), and samarium (Sm) from a highly pure mixed REE oxide powder derived from waste NiMH batteries. Initially, Ce was selectively precipitated as Ce(OH) 4 using potassium permanganate, achieving a precipitation efficiency of 99.8 % while maintaining co-precipitation of other REEs below 1.5 %. The remaining REE solution was subjected to solvent extraction using Cyanex 572 as the extractant and TBP as the phase modifier. Optimization studies determined the optimal conditions for maximizing separation efficiency, achieving a selective extraction of Sm with 46% efficiency in a single stage at pH 1.5, while limiting didymium co-extraction to less than ∼3%. In the next stage, didymium was extracted at pH 2.5 with ∼45 % efficiency, achieving a separation factor (β Nd/La ) exceeding 100, ensuring complete retention of La in the aqueous phase. Multi-stage solvent extraction significantly enhanced separation, with Sm reaching 97 % cumulative extraction efficiency after eight batchwise extraction-stripping stages. Didymium was efficiently separated from La with a cumulative extraction efficiency of 95 % after five stages, yielding a final La raffinate with purity exceeding 99 %. The stripped solutions and La raffinate were further processed for REE oxalate precipitation followed by calcination, resulting in individual REO products with over 90 % purity. XRD and EDS analyses confirmed high-purity lanthanum, didymium, and samarium oxides with minor residual impurities. This study demonstrates a scalable and efficient approach for separating adjacent light REEs, providing a robust methodology for sustainable REE recovery from secondary sources. The findings highlight the effectiveness of the Cyanex 572-TBP system in selective separation and offer an optimized process with reduced reagent consumption and high product purity, making it a viable option for industrial-scale implementation.

Topics & Concepts