Electrochemical Study of Dysprosium in Fused LiCl–KCl Eutectic for Production of High Purity Metal
Authors: Novoselova Alena V. , Smolensky Valery V. , Volkovich Vladimir A. 
Published: December 2020

High purity rare earth metals are essential for research and use as high-performance materials. Even a minute amount of impurities can have significant influence on physical and chemical properties of metals, so some special properties of rare earth materials are exhibited only when they are in the state of high purity. Molten salts are promising reaction media for extractive metallurgy. In particular, molten chlorides are good electrolytes for selective dissolution or deposition of pure reagents; using molten salts provides a promising route for treatment of raw materials. In addition, molten salts proved to be suitable media for metal electrowinning and electrorefining. Available experience in high-temperature electrochemistry allows producing metals in the solid form. One of the advantages of molten salts is their variety. So, it is possible to find a solvent, which chemical and electrochemical characteristics are suitable to carry out a given process.

The goal of this work was studying the mechanism of electrochemical reduction of dysprosium ions in molten LiCl–KCl eutectic for production of high purity dysprosium metal.

The cathodic reduction of Dy(III) ions was studied on inert molybdenum electrode in the temperature range of 723–843 K under inert atmosphere. Cyclic voltammograms contained one cathodic peak at–3.19±0.11 V and the corresponding anodic peak at –2.95±0.11 V vs. chlorine reference electrode, Fig. The cathodic peak potential was not constant and shifted to the negative values with increasing scan rate. The cathodic peak current was directly proportional to the square root of the polarization rate. It was found that increasing scan rate led to an expected increase of irreversibility of the cathode process. The number of electrons (n) of the electrode reaction for reduction of Dy(III) ions was determined by square-wave voltammetry and the calculated value was equal to 2.93±0.05. According to the theory of linear sweep voltammetry the redox system Dy(III)/Dy(0) is irreversible and controlled by the rate of the charge transfer.

So, cathodic reduction of dysprosium ions proceeded in one three-electron electrochemical step at the potential of –3.19 V versus the Cl–/Cl2 reference electrode:

DyCl6
3– + 3 e– = Dy + 6 Cl–.

Temperature dependence of Dy(III)/Dy couple apparent standard potential was determined by chronopotentiometry at the zero current. The experimental values are described by the linear equation:

E*Dy/Dy(III) = –(3.401±0.009) + (6.2±0.1)∙10–4∙T.

The apparent standard Gibbs energy change, enthalpy, and entropy of dysprosium trichloride formation from the elements in fused LiCl–KCl eutectic and activity coefficient of DyCl3 were calculated.

The influence of different parameters on the composition of the cathode product was investigated. It was found that for fine purification of dysprosium from its impurities, the electrolysis should be carried out in two stages, in which the first stage is the purification electrolysis, and the second one is the base electrolysis.

The reported study was funded by Russian Foundation for Basic Research according to the research project No. 20-03-00743.

Figure. Typical cyclic voltammograms for the reduction of dysprosium trichloride on molybdenum electrode (S = 0.14 cm2) in fused LiCl–KCl eutectic at 723 K. m(DyCl3) = 4.1∙10–2 mol/kg. Scan rates, V s−1: 1 – 0.075; 2 – 0.2; 3 – 0.5.

Figure 1