Selective Formation of Rare Earth-Nickel Alloys via Electrochemical Reactions in NaCl-KCl Molten Salt
We have proposed a new separation and recovery process for RE metals from magnet scraps using molten salts and an alloy diaphragm as shown in the figure . According to our previous studies, a specific RE element can be alloyed and de-alloyed rapidly with iron-group (IG) metals in molten salts by using electrochemical methods. In the proposed process, Nd, Dy, and Pr are separated because of differences in both the formation potentials and formation rates of the RE–IG alloys used as the diaphragm. In this study, selective formation of RE–Ni was investigated in molten NaCl–KCl–RECl3at 973 K.
Based on our previous studies [2-4], the effective potential range for the separation of Dy from Pr and Nd is expected to be between 0.39 V and 0.48 V vs. Na+/Na. In this potential range, the thermodynamically stable phases are PrNi3, NdNi3, and DyNi2. Since the formation of DyNi2 is considerably faster than that of PrNi3 and NdNi3, it is expected that the formation of DyNi2 would proceed preferentially to those of NdNi3 and PrNi3.
Then, the electrochemical formation of RE–Ni alloys were carried out at 0.42 V for 60 min in NaCl–KCl–0.50mol%RECl3. The thickness of the formed alloy layer was 4 µm for Nd-Ni, 2 µm for Pr-Ni, and 50 µm for Dy-Ni. These results suggest that Dy-Ni alloy is selectively formed in the melt containing both of NdCl3, DyCl3, and PrCl3.
The sample was prepared by potentiostatic electrolysis at 0.42 V for 60 min in NaCl–KCl–0.50mol%NdCl3–0.50mol%DyCl3–0.50mol%PrCl3. The sample was analyzed by cross-sectional SEM and EDX. The formed alloy layer has a thickness around 60 µm with a composition of Nd:Dy:Pr = 1.3:29.8:0.7 at%. The high composition ratio, xDy / (xNd + xPr), corresponding to 14.9 clearly indicates that Dy-Ni alloy can be selectively produced at this potential, and that RE-Ni alloy can be utilized as an alloy diaphragm with high separation ability for the proposed process. The results at different potentials are discussed in the presentation.
 T. Oishi, H. Konishi, T. Nohira, M. Tanaka, and T. Usui, Kagaku Kogaku Ronbunshu, 36, 299 (2010) (in Japanese).
 K. Yasuda, S. Kobayashi, T. Nohira, and R. Hagiwara, Electrochim. Acta, 92, 349 (2013).
 K. Yasuda, S. Kobayashi, T. Nohira, and R. Hagiwara, Electrochim. Acta, 106, 293 (2013).