Electrochemical Behaviors Of La(Ⅲ) And Gd(Ⅲ) In Molten Salts And Anodic Dissolution Of Bi-La-Li Alloys

Molten salts electrolysis has been widely applied in many fields, especially in spent fuel dry reprocessing, in which electrorefining and co-deposition methods were employed to separate actinides from fission products in LiCl-KCl eutectic salts in order to realize the recycling of nuclear fuel. However,it is very dir=fficult to separate actinides from rare earths because of the similar properties. To establish appropriate separation methods and technique conditions for molten salts electrolysis, it is necessary to investigate electrochemical behaviors of actinides and rare earths in molten salts. Therefore, in this paper, the electrochemical behaviors of rare earths were researched in molten salts. And extraction of rare earths from molten salts was performed by electrochemical method of co-deposition, liquid cathode and anode dissolution. The main research works are as follows:1. The electroreduction process of La(Ⅲ) and Gd(Ⅲ) ions was studied in LiCl-KCl eutectic salts by cyclic voltammetry and square wave voltammetry. The results showed that the reduction reactions of La(Ⅲ) and Gd(Ⅲ) ions on W electrode were one-step process with three electrons exchanged. The equilibrium potentials of Ln(Ⅲ)/Ln(Ln=La, Gd) at different temperatures were determined by open circuit chronopotentiometry on W electrode. The apparent potentials of Ln(Ⅲ)/Ln and the Gibbs energy of formation for LnCl3(Ln=La, Gd) were calculated.2. The electrochemical behaviors of La(Ⅲ) and Gd(Ⅲ) in LiCl-KCl eutectic salts on Bi film electrode were studied by cyclic voltammetry and square wave voltammetry. The results reveal that underpotential deposition was due to the formation of different metal compounds Ln-Bi. The partial molar Gibbs free energy and activity in Ln-Bi alloys and standard Gibbs free energy for Ln-Bi intermetallic compounds at different temperature were calculated by open circuit chronopotentiomery.’ And according the relationship of standard Gibbs free energy with temperature, entropy and enthalpy of different intermetallic compounds for Bi-Ln were also obtained.To confirm the underpotential deposition of La on liquid Bi, galvanostatic and potentiostatic electrolysis were carried out on W electrode in the LiCl-KCl-LaCl3-BiCl3 melts. The La-Bi alloys prepared were always residue-shaped in moten salts whether at low or high temperature,we could not collect the La-Bi alloys. Therefore, galvanostatic and potentiostatic electrolysis were executed on liquid Bi pool electrode in the LiCl-KCl-LaCl3 melts and the XRD pattern shows that the sample is comprised of LaBi2, LaBi,Li3Bi and Bi phases.3. The anodic dissolution of Bi-La-Li alloy in LiCl-KCl eutectic salts was studied by cyclic voltammetry and the results showed that the atom with more negative reduction potential was dissolved firstly. The effects factors on dissolution process were studied by stripping voltammetry. We found that the longer of deposition time, the stronger the peak current.Stripping peak current intensity increases with the content of La in alloy. Finally, the diffusion coefficients of La and Gd in liquid metal Bi were 1.81×10-5cm2 s-1 and 1.59×10-5 cm2 s-1 at 773 K, respectively, by chronopotentiometry.