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A new tri-n-butylphosphate−nitric acid (TBP−HNO 3 ) adduct was prepared by combining TBP and fuming (90%) HNO 3 . The adduct was characterized, and its phase-equilibrium behavior in supercritical carbon dioxide is reported. Supercritical carbon dioxide (sc-CO 2 ) was modified with this new adduct [TBP(HNO 3 ) 5.2 (H 2 O) 1.7 ], and the extraction efficacies of selected rare earth oxides (Y, Ce, Eu, Tb, and Dy) at 338 K and 34.5 MPa were compared with those obtained using an adduct formed from concentrated (70%) HNO 3 and TBP [TBP(HNO 3 ) 1.7 (H 2 O) 0.6 ]. All rare earth oxides tested with both adduct species could be extracted with the exception of cerium oxide. The water and acid concentrations in the different adducts were found to play a significant role in rare earth oxide extraction efficiency.

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Rare earth elements (REEs) are vital to modern, high-tech devices. Recycling REEs from post-consumer electronics can potentially diminish supply chain risks. Toward that end, liquid-liquid solvent extraction of various REEs was investigated with tetrabutyl diglycolamide (TBDGA) in 1-octanol from hydrochloric acid media. Metal partitioning to the organic phase was shown to increase as [Cl-] increased. In contrast, increasing [H + ] did not improve extraction. The use of the polar diluent 1-octanol provided high extraction efficiency, especially for the partition of heavy lanthanides from solutions of high chloride concentration. Although the polar diluent also extracted molar amounts of water and acid, it was concluded that a neutral metal/TBDGA complex as mainly the di-solvate was extracted, and that complexation was observed to be exothermic. These results indicate that REE extraction from aqueous chloride solutions can be efficient without the use of high acid concentrations.

The electrochemical production of rare earth metals (REMs) in ionic liquids (ILs) has received much attention as a promising, sustainable replacement to molten salt electrolysis. Water additives have been suggested as a promoting strategy for the ionic liquid process; however, the fundamental understanding of the interfacial processes required to assess the overall viability for REM production is lacking. In this regard, a full investigation of the impact of water on dysprosium (Dy) electrodeposition in pyrrolidinium triflate (BMPyOTf) ionic liquid was carried out. Water introduction was revealed to involve an interplay of implications on the electrodeposition process, including coordination, speciation, reduction pathways, interfacial dynamics, nucleation, and metal stability and purity. Under highly dry conditions, the reduction occurs at a very negative potential (−3.3 V) in a consecutive pathway, resulting in negligible metal electrodeposition (low rate and efficiency) at the electrode surface. Small water concentrations (<500 ppm) lead to partitioning of the Dy complex between water and IL-coordinated speciation, giving rise to an additional wave at a more positive potential (−2.4 V). Probing the heterogeneous Dy speciation by spectroscopic analyses enabled uncovering of the reduction mechanism and evaluation of the mass transport properties. In addition to lowering the reduction thermodynamics, water introduction also improved the nucleation, deposition rate, and faradic efficiency. Despite these benefits, stripping voltammetric analysis predicts substantial chemical reactivity of the deposited Dy metal with water additives and/or electrolyte components, under long timescales. Surface characterization of the obtained product confirmed the instability of Dy metal as an oxidized/fluorinated material and its limited purity (∼60%). Moreover, high water introduction triggered a fast hydrogen evolution reaction (HER), downgrading the robustness of system efficiency. The overall impact of water additives seems to engender both promoting and mitigating effects on electrochemical REM production in IL, requiring a specific technoeconomic assessment and/or more innovative strategies to be sought.