Solvation and electrochemical analyses of neodymium complexes in TFSA-based ionic liquids dissolving the nitrates synthesized from spent Nd–Fe–B magnets

Abstract

In this study, rare earth (RE) nitrates, M(NO3)3·xH2O, were synthesized from Nd–Fe–B magnets in order to apply nitric acid as a low cost acid medium. The synthesized RE nitrates were identified by ICP-AES, XRD and TG-DTA. These results revealed that the RE nitrates were mainly composed of Nd(NO3)3·6H2O.In addition, the solubility of the RE nitrates in n-hexyl-trimethyl-ammonium bis(trifluoromethyl-sulfonyl)amide, [N1116][TFSA], ionic liquids (ILs) was investigated by UV–Vis–NIR and Raman spectra. According to the hypersensitive peak (4I9/2→2G7/2) of Nd3+, the solubility of the RE nitrates in [N1116][TFSA] was estimated to be 0.05moldm−3. The solvation of the neodymium complex in [N1116][TFSA] dissolved RE nitrates was also researched by UV–Vis–NIR spectra. The molar extinction coefficients and the peak wavenumbers of the neodymium complex in ILs differed from related complexes such as [Nd(TFSA)5]2−, [Nd(H2O)9]3+ and [Nd(NO3)5]2−. This result suggested that the solvated structure was [Nd(NO3)a(TFSA)5−a]2− and the bond strength between the central Nd3+ ion and the surrounding ligands was expressed in the following order: [NO3−]<[TFSA−]<H2O. However, the significant peak at 750cm−1 did not appeared in Raman spectrum of [Nd(NO3)a(TFSA)5−a]2− in [N1116][TFSA]. This result revealed that the [TFSA−] anion bound to the central Nd3+ ion in the complex: [Nd(NO3)a(TFSA)5−a]2− was unstable in [N1116][TFSA]. Therefore, the spectrochemical series would be applicable for limited conditions of [NO3−] in TFSA-based ILs.Furthermore, the electrochemical behavior of the [Nd(NO3)a(TFSA)5−a]2− complex in [N1116][TFSA] was investigated by cyclic voltammetry. A cathodic current peak for [Nd(NO3)a(TFSA)5−a]2−/Nd(0) was observed at −3.0V and this reduction potential was shifted to the more positive side compared to [Nd(TFSA)5]2−/Nd(0), so that the interaction between the central Nd3+ ion and the ligand of [Nd(NO3)a(TFSA)5−a]2− would be weaker than that of [Nd(TFSA)5]2−. The diffusion coefficient of [Nd(NO3)a(TFSA)5−a]2− in [N1116][TFSA] was calculated to be 3.5×10−13m2s−1, which is almost the same as that for [Nd(TFSA)5]2−. The series of results enabled us to demonstrate that direct dissolution from RE nitrates and the electrodeposition in a TFSA-based ILs system could be applied for the recovery process of Nd metal.