Electrochemical Extracting Rare Earth Ytterbium And Dysprosium In LiCl-KCl Molten Salts On Cu Electrode And Its On-line Monitoring

The reprocessing of spent nuclear fuel will promote the development of nuclear energy, and it is of great significance meaning for resource saving and protection of environment. In this paper, extracting rare earth from fission elements by electrolysis was investigated on an active electrode. The feasibility of extracting .ytterbium and dysprosium by electrodepositing intermetallic compounds on the Cu electrode and on-line monitoring the change concentration of RE(Yb and Dy) ions during the electrolysis process in the LiCl-KCl melts were investigated.The details were listed as follows:(1) In order to extract variable valence ytterbium, the electrochemical behavior of Yb(III) in LiCl-KCl-YbC13 melts was studied on inert W and reactive Cu electrodes by cyclic voltammetry,square wave voltammetry and open circuit chronopotentiometry. The results indicate that the reduction of Yb(?) on inert W electrode proceeds in two steps: (1) Yb(?) + e?Yb(?); (2)Yb(?) + e-?Yb(0) . However, the system of Yb(?)/Yb(0) was not observed within the electrochemical windows, which inhibits the extraction of Yb from the melts on an inert electrode. In contrast, the deposition of Yb(?) occurs at a more positive potential on Cu electrode than that on the inert W electrode, due to the formation of various Yb-Cu intermetallic compounds during the under potential deposition. The extraction of ytterbium on Cu electrode was performed by galvanostatic and potentiostatic electrolysis to prepare Yb-Cu alloys,respectively. The alloy samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). XRD results show that we obtained not only thermodynamic stable phases, YbCu, YbCu2 and YbCu5, but also thermodynamic metastable phases, YbCu6.5 and Yb0.1Cuo.99. In order to estimating the extraction process, the concentration of Yb(?) in the melts was measured by inductive coupled plasma atomic emission spectrometer (ICP-AES), and the working electrode was replaced and characterized by SEM and XRD during electroextraction every 3 h. It was found that thermodynamic metastable phase was easily formed even if the concentration of Yb(?) was very low in LiCl-KCl melts. The extraction efficiency was about 99.9% for Yb(?) after potentiostatic electrolysis at -2.3 V for 18 h in LiCl-KCl-YbC13 melts.(2) The feasibility of extracting dysprosium by electrodepositing Dy-Cu intermetallic compounds on the Cu electrode in the LiCl-KCl melts was investigated. The electrochemical behavior of Dy(?) and formation mechanism of Dy-Cu alloy were studied in LiCl-KCl-YbCl3 by cyclic voltammetry, square wave voltammetry, chronopotentiometry and open circuit chronopotentiometry. The reduction of Dy(?) on inert W electrode takes place in only one step with three electrons exchange.The diffusion coefficients of Dy(?) ions on the electrodes were calculated by Berzins-Delahay and Sand equation. The values are 1.2×10-5 cm2·s-1 and 0.73 x10-5cm2·s-1,respectively. On the Cu electrode,the underpotential deposition of Dy(?) was found due to the formation of six intermetallic compounds. The oxidation peak potentials of six intermetallic compounds were -1.84 V?-1.69 V?-1.50 V?-1.37 V?-1.29 V?-1.16 V,respectively. The extraction of Dy was performed by galvanostatic electrolysis and the Dy-Cu alloy sample was characterized by XRD and SEM-EDS. The results revealed that the Dy-Cu alloys consist of Dy2Cu and DyCu phases and the thickness of Dy-Cu alloy film is about 300?m.(3) On-line monitoring of the concentration of rare earth (Yb and Dy) ions by electrochemical method was conducted during electroextraction in molten salts. The working curve about the relationship between the concentration of rare earth (Yb and Dy) ions and peak current density were obtained by square wave voltammetry. The relationship between the reductive peak current density of rare earth (Yb and Dy) and electrolysis time were obtained.According to the working curves of rare earth ions at different temperature, the concentrations which correspond to the current densities in molten salts were monitored during the electrolysis process.