Electrochemical Formation of Nd-Ni Alloys in a Molten Cesium Chloride System

Abstract

We have been investigating a new recovery process of Dy and Nd from the magnet scraps using molten salt electrolysis and alloy diaphragms [1-14]. The key step in this process is the alloying and de-alloying of rare earth (RE) elements on both sides of the diaphragm in order to effectively separate them in the molten salt. Here, the alloying and de-alloying were termed electrochemical implantation and electrochemical displantation, respectively, in a molten LiCl–KCl [15]. The features of these alloying and dealloying were different from those expected from the ordinary concept of solid phase diffusion. The growth rate of DyNi2 phase was extremely high compared with the conventional solid phase diffusion at temperatures lower than half of the melting temperature of DyNi2, and the dissolution rate of Dy from the formed DyNi2 was also high. So far, we have reported that Dy can be selectively alloyed with Ni by potentiostatic electrolysis in a molten LiCl–KCl–DyCl3(0.50 mol%)–NdCl3(0.50 mol%) system at 723 K [5]. The selective permeation of Dy through the RE-Ni alloy diaphragm from an anolyte containing DyCl3 and NdCl3 to a catholyte has been also confirmed [14]. However, the current density was of the order of 10 mA cm-2 or less, which is insufficient for industrial electrolysis. So, in this study, we focused on the CsCl system as a candidate melt to achieve a higher current density due to its high operation temperature. Since the alloying and de-alloying of REs in this system is unknown, the electrochemical formation of Dy-Ni alloys was investigated in a molten CsCl–DyCl3 system at 973 K as a first step in studying this system. On the other hand, in this study electrochemical formation of Nd-Ni alloys was investigated in a molten CsCl-NdCl3 (0.50 mol%) system at 973 K. Open-circuit potentiometry tests were carried out with Mo and Ni flag electrodes after depositing Nd metal by potentiostatic electrolysis at -0.15 V (vs. Nd3+/Nd) for 150 s and at -0.10 V for 1 h, respectively. A potential plateau was observed at 0.00 V with a Mo electrode. Since Mo does not form any intermetallic compounds with Nd, the observed potential is interpreted as the Nd3+/Nd equilibrium potential. Also five potential plateaus were observed at 0.04 V, 0.11 V, 0.18 V, 0.38 V and 0.57 V with a Ni electrode, which possibly correspond to the potentials of two-phase coexisting states of different Nd–Ni alloys. The alloy samples were prepared by potentiostatic electrolysis at 0.08 V for 1 h with a Ni plate electrode. The XRD analysis of obtained sample indicated the formation of NdNi2. The formed NdNi2 layers at 0.08 V were transformed to other phases such as NdNi3 at 0.14 V, Nd2Ni7 at 0.22 V and NdNi5 at 0.44 V due to anodic dissolution of Nd, depending on the applied potentials.References Oishi, H. Konishi, T. Nohira, M. Tanaka and T. Usui, Kagaku Kogaku Ronbunshu, 36(4), 299 (2010). [in Japanese]H. Konishi, H.Ono, T. Nohira and T. Oishi, MOLTEN SALTS, 54(1), 21 (2011). [in Japanese]S. Kobayashi, K. Kobayashi, T. Nohira, R. Hagiwara, T. Oishi and H. Konishi, J. Electrochem. Soc., 158(12), E142 (2011).S. Kobayashi, T. Nohira, K. Kobayashi, K. Yasuda, R. Hagiwara, T. Oishi and H. Konishi, J. Electrochem. Soc., 159(12), E193 (2012).H. Konishi, H.Ono, T. Nohira and T. Oishi, ECS Trans., 50(11), 463 (2012).T. Nohira, S. Kobayashi, K. Kondo, K. Yasuda, R. Hagiwara, T. Oishi and H. Konishi, ECS Trans., 50(11), 473 (2012).H. Konishi, H.Ono, T. Nohira and T. Oishi, ECS Trans., 53(11), 37 (2013).H. Konishi, H. Ono, E. Takeuchi, T. Nohira and T. Oishi, ECS Trans., 61(28), 19 (2014).H. Konishi, H. Ono, E. Takeuchi, T. Nohira and T. Oishi, ECS Trans., 64(4), 593, (2014).H. Konishi, T. Oishi, T. Nohira, H. Ono and E. Takeuchi, ECS Trans., 75(15), 105 (2016).H. Konishi, H. Hua, H. Ono, Y. Koizumi, T. Oishi and T. Nohira, ECS Trans., 86(14), 321 (2018).Y. Watanabe, Y. Norikawa, K. Yasuda and T. Nohira, Mater. Trans. 60(3) 379 (2019).K. Yasuda, T. Enomoto, Y. Watanabe, T. Oishi and T. Nohira, Mater. Trans., 61(12), 2329 (2020).T. Oishi, M. Yaguchi, Y. Katasho, H. Konishi and T. Nohira, J. Electrochem. Soc., 167(16), 163505 (2020).H. Konishi, T. Nohira and Y. Ito, J. Electrochem. Soc., 148(7), C506 (2001).