Separation of Dy from Nd-Fe-B Magnet Scraps Using Molten Salt Electrolysis

Abstract

IntroductionThe demand for Dy-added Nd-Fe-B magnets is rapidly increasing because these magnets are indispensable for high-performance motors in electric vehicles (EVs) and hybrid electric vehicles (HEVs). These magnets need to possess sufficient thermal stability for use in such motors in high-temperature environments. The addition of Dy is necessary to improve the thermal stability of Nd-Fe-B magnets. However, there is the concern about a shortage of rare earth metals because of the uneven distribution of rare earth resources. Against this background, it is necessary to develop an inexpensive and environmentally friendly recovery/separation process for rare earth metals, especially the recovery of Dy from Nd-Fe-B magnet scraps.We proposed a new separation and recovery process for Dy from Nd magnet scraps using molten salt electrolysis and an alloy diaphragm [1-3]. The new separation and recovery process was first applied to chloride melts [4-6], and the present study focused on the separation of Dy from Nd-Fe-B magnet scraps in a molten LiCl-KCl system. The anodic dissolution of RE (Dy, Nd, etc) using Nd-Fe-B magnet electrodes and electrowinning of Dy using Ni electrodes were carried out in a molten LiCl-KCl system at 723 K.ExperimentalAll chemicals were anhydrous reagent grade. The LiCl-KCl eutectic (LiCl:KCl = 58.5:41.5 mol%) was placed in a high purity alumina crucible, and kept under a vacuum for more than 24 hrs at 473 K to remove water. All experiments were performed in LiCl-KCl eutectic melts under a dry Ar atmosphere at 723 K. A chromel-alumel thermocouple was used for temperature measurements. The anodic working electrodes were Nd-Fe-B magnets (20 mm × 10 mm × 1.5 mm) wound Ni wires (5 mm × f 1 mm, 99 %). The composition of Nd-Fe-B magnet was 20 mass%Nd, 5mass%Dy, 65mass%Fe, 1mass%B, etc. The cathodic working electrodes were Mo and Ni plates (20 mm × 10 mm × 0.2 mm, 99 %). The reference electrode was an Ag wire immersed in LiCl-KCl containing 1 mol% of AgCl. The potential of this reference electrode was calibrated with reference to that of a Li+/Li electrode, which was prepared by electrodepositing Li metal on a Mo wire. The counter electrode was a glassy carbon rod (50 mm × f 5 mm, Tokai Carbon Co., Ltd.). The samples were prepared by potentiostatic electrolysis. After electrolysis, the samples were rinsed with distilled water. Cross-sections of these samples were also observed by SEM. The compositions of the samples were analyzed by EDX.Results and discussionAnodic potentiostatic electrolysis at 1.70 V and 2.20 V for 12 h were conducted using Nd-Fe-B magnet electrodes. From a cross-section of a sample obtained at 1.70 V, it was found that RE in the outer layer was selectively dissolved but RE in the inner layer remained. On the other hand, the Nd-Fe-B magnet was almost dissolved, and the original form was disappeared.After anodic potentiostatic electrolysis at 2.20 V, cathodic potentiostatic electrolysis was conducted at 1.00 V for 5 h using a Mo plate in order to removed Fe dissolved into the bath from the Nd-Fe-B magnet. The electrodeposited Fe was observed on a Mo plate. Furthermore, based on the results of previous works [5], cathodic potentiostatic electrolysis was conducted at 0.65 V for 4h, 12 h using Ni plates in order to recover Dy selectively. The SEM analysis showed that the alloys formed. From the EDX analysis of formed alloy, the molar ratio of Dy/Nd in the alloy samples are found about 10 for 4 h and 16 for 12 h. These results suggested that the separation of Dy and Nd could be achieved.