Electrochemical Formation of RE-Sn (RE=Dy, Nd) Alloys Using Liquid Sn Electrodes in a Molten LiCl-KCl System

Abstract

We proposed new separation and recovery process for RE metals from Nd magnet scraps using molten salt electrolysis and an alloy diaphragm(1-3). This new process was first applied to chloride melts, and the separation of Dy and Nd were investigated using Ni and Cu cathodic electrodes in molten LiCl-KCl-DyCl3-NdCl3systems(4-6). The highest mass ratio of Dy/Nd in Dy-Nd-Ni alloy samples was found to be 72 by ICP-AES. But there is a problem in the process. In this process, an alloy diaphragm can’t be durable against a long electrolysis.In order to solve this problem, we focused on the liquid metals as the alloy diaphragms. Liquid metals have higher endurance and diffusion rates of elements than solid metals during the electrolysis. Therefore, liquid metals are desirable for our process. For the first step, in this work a Sn electrode was used as a liquid metal electrode. The alloy samples were prepared by potentiostatic electrolysis using liquid Sn electrodes in a molten LiCl-KCl added DyCl3(0.50 mol%) and NdCl3(0.50 mol%) at 723 K.All experiments were performed in LiCl-KCl eutectic melts under dry argon atmosphere at 723 K. DyCl3 or NdCl3 was added directly to these melts. The working electrodes were liquid Sn electrodes for the investigation of electrochemical behavior. 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 SEM and XRF. Before the investigation of RE-Sn(Re=Dy, Nd) alloys formation, the electrochemical behavior of Li was investigated using a liquid Sn electrode as a working electrode in a molten LiCl-KCl system at 723 K. Cyclic voltammetry was conducted with a liquid Sn electrode. The scanning rate was set at 0.05 Vs-1. During the scan in the negative direction, a cathodic current was observed from 1.10 V. Since Sn can form alloy with Li, the cathodic current corresponds to the formation of Li-Sn alloy. On the other hand, taking into account the possibilities of the formation of various Dy, Nd-Sn alloys, the electrochemical behaviors of Dy(III) and Nd(III) were investigated using liquid Sn electrodes as working electrodes. In the negative scan for the DyCl3 added system, a cathodic current was observed from 1.15 V. This cathodic current might correspond to the formation of Dy-Sn alloy. In the negative scan for the NdCl3 added system, a cathodic current was observed from 1.30 V. This cathodic current might correspond to the formation Nd-Sn alloys. In the voltammogram using a liquid Sn electrode after the addition of both DyCl3(0.50 mol%) and NdCl3(0.50mol%), a cathodic current of the formation RE-Sn alloy was observed from 1.30 V. Based on the above results, alloy samples were prepared by potentiostatic electrolysis at 0.50-0.80 V for 0.50 h in a molten LiCl-KCl-DyCl3-NdCl3 system. From the XRF analysis of all samples, the existence of Dy and Nd in liquid Sn electrodes were confirmed. This result indicated the formation Dy-Nd-Sn alloys. The highest mass ratio of Nd/Dy in the alloy sample was 2.08 at 0.60 V. References T. Oishi, H. Konishi, T. Nohira, M. Tanaka and T. Usui, Kagaku Kogaku Ronbunshu, 36, 299 (2010). S. Kobayashi, K. Kobayashi, T. Nohira, R. Hagiwara, T. Oishi and H. Konishi, J. Electrochem. Soc., 158, E142 (2011). S. Kobayashi, T. Nohira, K. Kobayashi, K. Yasuda, R. Hagiwara, T. Oishi and H. Konishi, J. Electrochem. Soc., 159, E193 (2012). H. Konishi, H.Ono, T. Nohira and T. Oishi, MOLTEN SALTS, 54, 21 (2011). H. Konishi, H. Ono, T. Nohira and T. Oishi, ECS Transactions, 50, 463 (2012). H. Konishi, H. Ono, T. Nohira and T. Oishi, ECS Transactions, 53, 37 (2013).