Separation of Dy from Nd-Fe-B Magnet Scraps Using Molten Salt Electrolysis
The 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.
All 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 discussion
Anodic 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 , 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.
1. T. Oishi, H. Konishi, T. Nohira, M. Tanaka and T. Usui, Kagaku Kogaku Ronbunshu , 36, 299 (2010).
2. S. Kobayashi, K. Kobayashi, T. Nohira, R. Hagiwara,T. Oishi and H. Konishi, J. Electrochem. Soc., 158, E142 (2011).
3. S. Kobayashi, T. Nohira, K. Kobayashi, K. Yasuda, R. Hagiwara, T. Oishi and H. Konishi, J. Electrochem. Soc., 159, E193 (2012).
4. T. Nohira, S. Kobayashi, K. Kondo, K. Yasuda, R. Hagiwara, T. Oishi and H. Konishi, ECS Transactions, 50(11), 473 (2012).
5. H. Konishi, H. Ono, T. Nohira and T. Oishi, ECS Transactions, 50(11), 463 (2012).
6. H. Konishi, H. Ono, E, Takeuchi, T. Nohira and T. Oishi, ECS Transactions, 53(11), 37 (2013).