Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals Using Ionic Liquids

Abstract

This paper reports a novel bench-scale hydrometallurgical procedure and electrodeposition using triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl)amide ([P 2225 ][TFSA]) ionic liquids (ILs) for the recovery of rare earth (RE) metals from spent Nd-Fe-B magnets. The hydrometallurgical treatments were carried out at bench scale to produce RE amide salts of high purity. In the leaching process employing 1.7 kg of oxidized Nd-Fe-B fine powder and 14.2 L of an acid medium of 1,1,1-trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide (H[TFSA]), selective leaching of RE ions (85.7±5.8% Nd) was performed at bench scale. Then, Fe (<99.9%) was successfully separated from RE ions in the deironization process. The total amount of the recovered amide salts through the evaporation treatment using a spray dryer was 3.57 kg. From the CV/EQCM measurements for Nd(III) at 373 K, a clear cathodic peak with the mass increased, and the ηρ decreased was observed at −2.79 V. Considering our previous investigations, the reduction of Nd(III)/Nd(0) was indicated as [Nd (III) (TFSA) 5 ] 2− + 3e − → Nd(0) + 5[TFSA] − . In addition, the M app value in the range of −2.49 V ~ −2.94 V was 46.8 g mol −1 , which was close to the theoretical value for the electrodeposition reaction of Nd(III)/Nd(0), 48.1 g mol −1 . Moreover, the electrodeposition of Nd(0) was carried out under the condition of −3.20 V versus Fc/Fc + at 373 K. The electrodeposits were identified with the metallic Nd in the middle layer investigated by X-ray diffraction and X-ray photoelectron spectroscopy. Finally, we demonstrated that the novel recovery process consisted of hydrometallurgy and electrodeposition using ILs was effective by calculating material flow.

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“…If, however, more purification or elemental separation must be performed, additional process steps are required such as: (i) chlorination using safer agents such as NH 4 Cl [220,221]; (ii) advanced leaching [222][223][224][225][226][227][228][229][230][231][232][233][234][235][236]; (iii) or even bioleaching using bacteria which avoids the use of strong acids [237]; (iv) hydrometallurgy (solid-liquid or liquid-liquid extraction) [15,210,[238][239][240][241][242][243][244]; as well as (v) electrodeposition at the reduction step, including in ionic liquids [245][246][247][248][249][250][251][252][253] and molten salts [254][255][256][257][258][259], with their wide electrochemical stability [260].…”