Extraction of rare earth ions by tri-n-butylphosphate/phosphonium ionic liquids and the feasibility of recovery by direct electrodeposition

• Published in: Separation and purification technology 2014-06, Vol.130, p.91-101
• Main Authors: Matsumiya, Masahiko, Kikuchi, Yuya, Yamada, Takahiro, Kawakami, Satoshi
• Format: Article
• Language: English
• Subjects: Anions; Cations; Electrodeposition; Ferrous alloys; Ionic liquids; Rare earth metals; Rare earths; Recovering; Solvent extraction; Tri-n-butyl phosphate
• ISSN: 1383-5866; 1873-3794
• DOI: 10.1016/j.seppur.2014.04.021

•Ionic liquids are useful as extraction and electrodeposition media.•Rare earths were easily extracted by tri-n-butylphosphate with phosphonium ionic liquids.•The extraction mechanism was evaluated from the slope analysis.•The complexation was investigated by Raman spectroscopic studies.•The Nd electrodeposits were recovered by direct electrodeposition. As the preliminary experiments for development of the recycling process to recovery of rare earths (REs) composed of Nd–Fe–B magnets by direct electrodeposition from an organic phase after extraction procedures, solvent extraction of RE such as Pr, Nd and Dy by tri-n-butylphosphate (TBP) with ionic liquids (ILs) were performed. Triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl)amide ([P2225][TFSA]), which is beneficial as an electrolyte for electrodeposition of electrochemically negative metals such as REs were employed as the IL. It was revealed that the extraction performance of RE(III) was enhanced significantly in TBP with [P2225][TFSA] when extraction of RE ions was carried out from an aqueous phase including TFSA− anions. Considering that the extraction mechanism was supposed to the cation-exchange of the IL with solvating the RE3+ cations by TFSA− anions, the mechanism was investigated by variation of concentrations of TBP, TFSA− anions in the aqueous phase and [P2225][TFSA]. As the results, the stoichiometry of both TBP and TFSA− anions in this IL extraction system with RE(III) was determined to be 3:1. According to UV–Vis–NIR spectroscopic analysis, the complexation of RE(III) extracted by TBP with [P2225][TFSA] was altered due to the coordination of TBP to RE(III). Estimation of the number of the TFSA− anion solvated to the centered Nd3+ cation in TBP with [P2225][TFSA] by Raman spectroscopy resulted in the centered Nd3+ cation solvated by three TFSA− anions and this result was consistent with the extraction mechanism analysis. From the electrochemical analysis, the extracted [Nd(TBP)3]3+ complex in TBP/[P2225][TFSA] was reduced to the following reaction: [Nd(TBP)3]3++3e−→Nd(0)+3TBP at −2.4V. The direct electrodeposition of the extracted [Nd(TBP)3]3+ in TBP/[P2225][TFSA] was carried out using a three-electrode system at 373K. The chemical bonding energy of Nd electrodeposits was investigated by XPS and the middle layer of electrodeposits was the metallic state, while a part of the top surface was the oxidation state.