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

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 DyNi 2 phase was extremely high compared with the conventional solid phase diffusion at temperatures lower than half of the melting temperature of DyNi 2 , and the dissolution rate of Dy from the formed DyNi 2 was also high. So far, we have reported that Dy can be selectively alloyed with Ni by potentiostatic electrolysis in a molten LiCl–KCl–DyCl 3 (0.50 mol%)–NdCl 3 (0.50 mol%) system at 723 K [5]. The selective permeation of Dy through the RE-Ni alloy diaphragm from an anolyte containing DyCl 3 and NdCl 3 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–DyCl 3 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-NdCl 3 (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. Nd 3+ /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 Nd 3+ /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 a