Electrochemical Formation of RE-Zn (RE=Dy, Nd) Alloys in a Molten LiCl-KCl System

Introduction

We proposed a new separation and recovery process for RE metals from Nd magnet scraps using molten salt electrolysis and an alloy diaphragm. The new separation and recovery process for RE metals was first applied to chloride melts, and the present study focused on the electrochemical formation of RE-Zn (RE=Dy, Nd) alloys in a molten LiCl-KCl system at 653 K. The separation of Dy was also investigated using a Zn electrode in a molten LiCl-KCl-DyCl₃-NdCl₃system.

Experimental

All experiments were performed in LiCl-KCl eutectic melts under dry argon atmosphere at 653 K. DyCl₃ or NdCl₃ was added directly to these melts. The working electrodes were Mo and Zn wires for the investigation of electrochemical behavior. Rectangular shaped plates of Zn were used as the working electrodes. The reference electrode was a Ag⁺/Ag electrode. All the potentials given hereafter were referred to Li⁺/ Li electrode potential on a Mo wire. The counter electrode was a glassy carbon rod. The alloy samples were prepared by potentiostatic electrolysis. After the electrolysis, the samples were analyzed by XRD, SEM and ICP-AES.

Results and discussion

Before the investigation of RE-Zn (RE=Dy, Nd) alloys formation, the electrochemical behavior of Li (I) was studied using a Zn or Mo wire as a working electrode in a molten LiCl-KCl system at 653 K. The scanning rate was set at 0.05 Vs^-1. During the scan in the negative direction using a Mo electrode, a large cathodic current was observed at 0 V (vs. Li⁺/ Li). Since Mo does not form alloy with Li, the cathodic current corresponds to Li metal deposition. On the other hand, a cathodic current was observed from 0.60 V using a Zn electrode. Moreover, three large cathodic current peaks were observed. These two cathodic current peaks except for one cathodic current peak of 0 V correspond to the formation of Li-Zn alloys. When the potential scan direction was reversed at -0.05 V, two anodic peaks were observed. These anodic peaks suggest the Li dissolution from the different Li-Zn alloys.

Taking into account the possibilities of the formation of various Dy-Zn alloys, the electrochemical behavior of Dy (III) was studied using a Zn or Mo wire as the working electrode in a molten LiCl-KCl-DyCl₃ (0.50 mol%) system at 653 K. The scanning rate was set at 0.05 Vs^-1. During the scan in the negative direction using a Mo electrode, a small cathodic current was observed at 0.50 V. Since Mo does not form alloy with Dy, the cathodic current corresponds to Dy metal deposition. Large and small cathodic current were observed from 0.60 and 1.10 V using a Zn electrode, respectively. Since these potential are more positive than that for Dy metal deposition (0.50 V), the cathodic current correspond to the formation of Dy-Zn alloys. When the potential scan direction was reversed at 0.50 V, three anodic peaks were observed. These anodic peaks suggest the Dy dissolution from the different Dy-Zn alloys.

In order to investigate the formation potential of Dy-Zn alloys in detail, open-circuit potentiometry was carried out with a Zn electrode after depositing Dy metal by potentiostatic electrolysis at 0.50 V for 30 s in a molten LiCl-KCl-DyCl₃(0.50 mol%) system at 723 K. There were three potential plateaus at (a) 0.57 V, (b) 0.95 V and (c) 1.20 V. The observed potential plateaus were considered to correspond to different coexisting Dy-Zn or Li-Zn phases, respectively. Based on the result of open-circuit potentiometry, an alloy sample was prepared by potentiostatic electrolysis at 0.70 V for 1 h. From the XRD pattern of the sample, the alloy phase was identified as DyZn₃.