Efficient and sustainable
separation of metals is gaining increasing
attention, because of the essential roles of many metals in sustainable
technologies for a climate-neutral society, such as rare earths in
permanent magnets and cobalt, nickel, and manganese in the cathode
materials of lithium-ion batteries. The separation and purification
of metals by conventional solvent extraction (SX) systems, which consist
of an organic phase and an aqueous phase, has limitations. By replacing
the aqueous phase with other polar solvents, either polar molecular
organic solvents or ionic solvents, nonaqueous solvent extraction
(NASX) largely expands the scope of SX, since differences in solvation
of metal ions lead to different distribution behaviors. This Review
emphasizes enhanced metal extraction and remarkable metal separations
observed in NASX systems and discusses the effects of polar solvents
on the extraction mechanisms according to the type of polar solvents
and the type of extractants. Furthermore, the considerable effects
of the addition of water and complexing agents on metal separations
in terms of metal ion solvation and speciation are highlighted. Efforts
to integrate NASX into metallurgical flowsheets and to develop closed-loop
solvometallurgical processes are also discussed. This Review aims
to construct a framework of NASX on which many more studies on this
topic, both fundamental and applied, can be built.
Neodymium and dysprosium can be efficiently separated by solvent extraction, using the neutral extractant Cyanex 923, if the conventional aqueous feed phase is largely replaced by the green polar organic solvent polyethylene glycol 200 (PEG 200). While pure aqueous and pure PEG 200 solutions in the presence of LiCl or HCl were not able to separate the two rare earth elements, high separation factors were observed when extraction was performed from PEG 200 chloride solutions with addition of small amounts of water. This addition of water bridges the gap between traditional hydrometallurgy and novel solvometallurgy and overcomes the challenges faced in both methods. The effect of different variables was investigated: water content, chloride concentration, type of chloride salt, Cyanex 923 concentration, scrubbing agent. A Job plot revealed the extraction stoichiometry is DyCl 3 •4L, where L is Cyanex 923. The McCabe-Thiele diagram for dysprosium extraction showed that complete extraction of this metal can be achieved by a 3-stage counter-current solvent extraction process, leaving neodymium behind in the raffinate. Finally, a conceptual flow sheet for the separation of neodymium and dysprosium including extraction, scrubbing, stripping, and regeneration steps was presented. The nonaqueous solvent extraction process presented in this paper can contribute to efficient recycling of rare earths from end-of-life neodymium-iron-boron (NdFeB) magnets.
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