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

• Published in: Separation and purification technology Vol. 170; pp. 417 - 426
• Main Authors: Ota, Hisashi, Matsumiya, Masahiko, Yamada, Takahiro, Fujita, Tetsuo, Kawakami, Satoshi
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.10.2016
• Subjects: Bis(trifluoromethyl-sulfonyl)amide melts; Electrodeposition; Ferrous alloys; Hydrometallurgy; Leaching; Magnetic materials; Melts; Neodymium; Neodymium metals; Purification; Rare earth metals; Bis(trifluoromethyl-sulfonyl)amide melts; Electrodeposition; Neodymium metals; Hydrometallurgy
• DOI: 10.1016/j.seppur.2016.06.044
• ISSN: 1383-5866

•The hydrometallurgical process based on leaching, deironization and purification of rare earth amide salts was performed at bench scale.•The electrodeposition of Nd metal in KTFSA melts was performed under potentiostatic condition.•We demonstrated the effectiveness of the novel recovery process by estimating whole material flow. 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 3.4kg of oxidized Nd-Fe-B and 14.2L of an aqueous solution of 1,1,1-trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide (HTFSA), 83.0% Nd and 0.98% Fe were leached in 13h, 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, 4.07kg 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: xRE=0.1) was investigated in this study. Electrochemical analysis revealed that the reduction peak of Nd(III) at around +1.0V vs. K/K+ was due to the following reaction: Nd(III)+3e−→Nd(0). The diffusion coefficient of Nd(III) was estimated to be 3.14×10−10m2s−1 at 483K by semi-differential analysis, which is similar to that of Nd(III) in KTFSA melts containing pure Nd(TFSA)3 salts (xNd=0.1). The electrodeposition of Nd was performed under potentiostatic conditions of +0.8V vs. K/K+ at 483K. 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.