Purification of rare earth bis(trifluoromethyl-sulfonyl)amide salts by hydrometallurgy and electrodeposition of neodymium metal using potassium bis(trifluoromethyl-sulfonyl)amide melts

Abstract This paper reports a novel bench-scale hydrometallurgical procedure and electrodeposition using potassium bis(trifluoromethyl-sulfonyl)amide (KTFSA) melts for the recovery of rare earth (RE) elements from Nd-Fe-B magnet waste. The investigations were performed at bench scale to assess the potential of a process based on leaching, deironization, and purification of RE amide salts. In the leaching process using 3.4 kg of oxidized Nd-Fe-B and 14.2 L of an aqueous solution of 1,1,1-trifluoro- N -[(trifluoromethyl)sulfonyl]methanesulfonamide (HTFSA), 83.0% Nd and 0.98% Fe were leached in 13 h, indicating that selective leaching of RE elements was performed at bench scale. Then, KOH or oxidized Nd-Fe-B was used as a precipitation agent in the deironization process and 100.0% Fe was successfully separated from RE components. Moreover, 4.07 kg of purified amide salts (M(TFSA) 3 , M = Pr, Nd, Dy, B, Al, and trace elements) were recovered from a spray dryer. The electrochemical behavior of Nd(III) in KTFSA melts containing M(TFSA) 3 (molar fraction of RE components: x RE  = 0.1) was investigated in this study. Electrochemical analysis revealed that the reduction peak of Nd(III) at around +1.0 V vs. K/K + was due to the following reaction: Nd(III) + 3e −  → Nd(0). The diffusion coefficient of Nd(III) was estimated to be 3.14 × 10 −10  m 2  s −1 at 483 K by semi-differential analysis, which is similar to that of Nd(III) in KTFSA melts containing pure Nd(TFSA) 3 salts ( x Nd  = 0.1). The electrodeposition of Nd was performed under potentiostatic conditions of +0.8 V vs. K/K + at 483 K. The electrodeposits had a fine surface morphology with small metal particles. The electrodeposits were confirmed to be Nd metal in the middle layer analyzed by scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. Finally, we demonstrated the effectiveness of the novel recovery process for practical use by estimating whole material flow.