Development of energy-saving recycling process for rare earth metals from voice coil motor by wet separation and electrodeposition using metallic-TFSA melts

• The precipitation pH of Fe and RE depended on the concentration of the metal ions.
• The order of the precipitation formation was found to be Fe3 + < Fe2 + < Dy3 + < Nd3 + < Pr3 +.
• Fe3 + was selectively precipitated by the precipitation titration method.
• Nd metal and a part of oxides in electrodeposits were identified by XPS.
• The recycling process of RE from VCM wastes was proposed as a saving energy method.

In this study, we are focused on the precipitation titration method using alkali-metal (AM) hydroxide for the selective separation of Fe. For the titrand of 1.0 M HTFSA including Fe3 +, two equivalence points were observed; the first equivalent point around 1 < pH < 2 was derived from an acid–base neutralization reaction and the second equivalent point at 2.5 < pH corresponded to the precipitation reaction of Fe(OH)3. Then, we also investigate the concentration dependence of the precipitation reaction of AM hydroxide against Fe2 +, Fe3 + and RE3 + (RE = Pr, Nd, Dy). This result revealed that the Fe(OH)3 precipitates were easy to form in acidic pH and this precipitation behavior was consistent with HSAB rule. After the evaluation of the precipitation titration method, we applied this method as one of the wet separation process in order to recover RE metals from the practical wastes of Nd–Fe–B magnet. For this Nd–Fe–B magnet, we carried out a series of novel recycling process constituted from pre-treatment (demagnetization, chemical etching, roasting), wet separation (acid-leaching, precipitation titration, synthesis of metallic-bis(trifluoromethyl-sulfonyl) amide [TFSA] salts) and electrodeposition. After the precipitation titration, the metallic-TFSA salts were synthesized at high yield > 90% by evaporation treatment. A clear endothermic peak of this metallic-TFSA salts including RE and K appeared and the melting point was close to that of Nd(TFSA)3 and KTFSA. It was also revealed that a best method was to apply the metallic-TFSA salts containing much RE and K components as an electrolyte bath on the electrodeposition process. The electrodeposition of Nd was performed under potentiostatic condition of − 3.5 V vs. Pt quasi-reference electrode at 513 K. The surface morphology of the electrodeposits was fine fibrous structure with small metal particles. The electrodeposits were confirmed to be Nd metal on the middle layer analyzed by EDX and XPS. As for the Nd electrodeposits, the carbon content was extremely low and the oxygen content was smoothly decreased with the depth of the electrodeposits. Finally, we demonstrated the effectiveness of a multi-stage recycling process for a practical use by estimating for whole material flow.