Electrochemical Formation of Dy Alloys in a Molten CaCl2-LiCl System
Authors: Konishi Hirokazu , Hua Hang , Ono Hideki , Oishi Tetsuo , Yasuda Kouji , Nohira Toshiyuki 
Published: February 2020

The molten salt electrochemical process has been studied by Ito and co-workers including the present authors and many other researchers. The authors reported a unique phenomenon during the electrochemical formation of Dy-Ni alloys in molten LiCl-KCl-DyCl3: DyNi2 film with thickness of 60 µm was formed in 2 h at 700 K. The growth rate of DyNi2 was extremely high, compared with the conventional solid phase diffusion at temperatures lower than half of the melting temperature of DyNi2, i.e., 1531 K. In addition, the growth rate was found to be strongly affected by electrochemical parameters such as potential and current density. This phenomenon was termed “electrochemical implantation”, because it was different from the ordinary concepts of electrodeposition followed by solid phase diffusion. The authors also reported that anodic polarization of the formed DyNi2 resulted in the rapid dissolution of Dy to form other Dy-Ni phases. The obtained phase and morphologies were dependent on the applied potential. This phenomenon was named “electrochemical displantation”.

The use of rare earth (RE) - iron group alloys has increased significantly in a number of industrial fields over the past few decades. In particular, the demand for Dy-added Nd-Fe-B magnets is rapidly increasing because these magnets are indispensable for high-performance motors in electric vehicles and hybrid electric vehicles. 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 RE metals because of the uneven distribution of RE resources. Against this background, it is necessary to develop an inexpensive and environmentally friendly recovery/separation process for RE metals, especially the recovery of Dy and Nd from magnet scraps.

We proposed a new separation and recovery process for RE metals from scraps using molten salt and an alloy diaphragm. The new process is based on our previously discovered phenomena, i.e., “electrochemical implantation” and “electrochemical displantation”. RE containing scrap is used as the anode. A RE-transition metal (TM) alloy is used as the diaphragm, which functions as a bipolar electrode. During electrolysis, all the RE metals in the anode are dissolved in the molten salt as RE ions. One or several specific RE ions are selectively reduced to form RE-TM alloys on the alloy diaphragm according to their formation potentials and/or alloying rates. Subsequently, the RE atoms chemically diffuse through the alloy diaphragm and are dissolved into the molten salt as RE ions in the cathode room. The permeated RE ions are finally deposited on the Mo or Fe cathode as RE metals. The RE ions remaining in the anode room can be collected by electrolysis using another cathode in the anode room. Almost all impurities remain in the anode room as residue or anode slime.

This new process was first applied to chloride melts, and the separation of Dy from Nd and Pr were investigated using Cu, Ni and Zn cathodic electrodes in molten LiCl-KCl-DyCl3-NdCl3 and LiCl-KCl-DyCl3-NdCl3-PrCl3 systems. The highest mass ratio of Dy/Nd+Pr in Dy-Nd-Pr-Ni alloy sample was found to be 50 at 0.65 V (vs. Li+/Li) for 1 h by ICP-AES. The present study focused on the electrochemical formation of Dy alloys in a molten CaCl2-LiCl system at 873 K. Furthermore, the separation of Dy and Nd was investigated with Fe electrodes.