Enhanced extraction of rare earth elements by novel tuned diglycolamides

Abstract

International audienceRare earth elements (REE) are essential for our modern economy, in relation to the development of new energy and communication technologies, however their recycling from electronic waste and end-of-life products (such as permanent NdFeB magnets, Ni-MH batteries, etc.) is still not sufficiently developped.1 Although substitution of these materials by less critical ones is growing faster and faster especially in Japan efficient and eco-designed recycling processes will be of great importance in a near middle term. Depending on their technoeconomics efficiency and environmental footprint, hydrometallurgical processes enabling the recovery of separated elements could be of particular interest.Typically these processes include a first pretreatment (crushing, milling, sieving,) followed by an acidic leaching step (with possibly selective precipitation substeps) and a solvent extraction step (SX) in order to separate and purify the REE.2 Recently, diglycolamides (DGAs) appeared as a very interesting group of extractants for the recovery of trivalent lanthanides from nitric acid solutions, particularly in the presence of metal ions commonly found in waste products.3 The TODGA extractant (N,N,N',N'-tetraoctyl diglycolamide) was successfully used for designing a full REE recycling SX process from used permanent magnets.4 Nevertheless its performances have not yet been validated against upscaling tests.Most works concerning the group of DGAs dealt with symmetrical extractants exhibiting different separation efficiencies for REE in nitric acid media. The chain length modification on one side of the DGA (asymmetrical DGAs) can lead to important variation in selectivity during the Eu/Am separation.5 Recently, new dissymmetrical DGAs with very short chains were reported for REE extraction, such as for instance MODGA (N,N'-dimethyl-N,N'dioctyl-diglycolamide),6 however their solubility in industrial diluents is rather limited.The present work describes the organic synthesis of several novel DGAs and their solvent extraction behaviour towards REE in several aqueous acid media which could increase the industrial interest of such SX process. These new ligands displayed a remarkable improvement of REE extraction efficiency compared to reference TODGA in acid media, while presenting a good solubility in industrial aliphatic diluents. Furthermore, the separation factors of REE towards major impurities such as Fe3+ are substantially improved. Figure Distribution ratio of a novel DGA compared to TODGA in an acid solutionNevertheless it will be of primary importance to check whether the REE can be quantitatively de-extracted from the organic phase without any impurity. These promising results will also contribute to the design of an optimized SX process for the separation of REE