Extraction Y And Dy By Molten Salt Electrolysis On Solid (W,Cu) And Liquid (Zn,Sn,Pb) Electrodes

In recent years,molten salt electrolysis has been regarded as one of the most effective methods for spent fuel reprocessing due to its unique advantages.The existence of rare earth elements(REE)in melts can seriously hinder the recovery of actinides(An).In order to promote the recovery of An,it is necessary to extract REE from melts so as to purify melts.Thus,the electrode process and extraction of REE(Y and Dy)were studied in Li Cl-KCl melts using solid(W,Cu),liquid(Zn,Pb,Sn)and double cathodes(Cu-Zn)as working electrodes by a series of electrochemical methods such as cyclic voltammetry(CV),square wave voltammetry(SWV),chronopotentiometry(CP),chronoamperometry(CA),open circuit chronopotentiometry(OCP)and linear polarization(LP).The main research contents are as follows:(1)Kinetics and thermodynamics of Y(?)and Dy(?)on inert W electrode were studied.The results of CV?SWV?CP and CA showed that the reductions of Y(?)and Dy(?)on W electrode are one-step three-electron transfer processes controlled by diffusion,and are reversible.The diffusion coefficients of Y(?)and Dy(?)in Li Cl-KCl melts are in the range of 10^-5-10^-6 cm² s^-1.The exchange current density of Y(?)/Y(0)and Dy(?)/Dy(0)couples measured by LP are 7.20-12.50 m A cm^-2 and 5.58-11.59 m A cm^-2,and the reaction activation energy are 35.59 k J mol^-1and 36.20 k J mol^-1,respectively.The results of CA indicated that the nucleation of Y on W electrode belongs to instantaneous nucleation,and the nucleation number densities were found to be 2.148×10⁵-5.038×10⁵ cm^-2 at different potentials.(2)The electrode reaction and extraction of Dy(?)on liquid Zn and Sn electrodes were studied.CV,SWV and reverse chronopotentiometry(RCP)experiments displayed that the underpotential deposition of Dy(?)on liquid Zn and Sn electrodes can occur and the Dy-Zn and Dy-Sn solid solutions were formed.The half-wave potential of Dy(?)on liquid Zn electrode was determined,and found its value is constant with the change of scanning rate.The solubility and diffusion coefficient of metallic Dy in liquid Sn were estimated by coulomb titration(CT)and CP,and found to be 5.84 g/100 g Sn and 7.88×10^-6 cm² s^-1,respectively.The co-deposition results of Dy(?)with Zn(II),Sn(II)showed that Dy-Zn and Dy-Sn alloys were formed on predeposition Zn and Sn metals.When the concentration of Dy(?)in melts is high,it is easy to form Dy-richer Dy-Zn alloy phase.Dy(?)was successfully extracted from melts using liquid Zn and Sn as working electrodes.Dy Zn₂ and Dy₂Zn₁₇ were prepared by co-deposition in melts at different Dy(?)concentration.(3)The electrochemical behaviors and extraction of Y(?)on Pb and Sn electrodes were researched by electrochemical methods.The results indicated that the reductions of Y(?)on liquid Pb and Sn electrodes were quasi-reversible and controlled by diffusion.The exchange current density,charge transfer resistance and reaction activation energy of Y(?)/Y(0)on liquid Pb,Sn and W electrodes were measured and compared,indicating that metallic Y is easier to deposit on liquid electrodes.The solubility and diffusion coefficient of metallic Y in liquid Sn metal were determined by electrochemical methods and found to be 1.64 g/100 g Sn and 8.88×10^-6 cm² s^-1,respectively.The co-deposition behaviors of Y(?)with Pb(II)/Sn(II)were studied by CV,SWV and CP,and Y-Pb and Y-Sn intermetallic compounds could be formed on predeposition Pb and Sn metals.The extraction of Y from melts were conducted on liquid electrode,the analysis results of XRD and SEM-EDS showed that the products were YPb₃ and YSn₃.(4)The reaction mechanisms and extraction of Y(?)on solid(Cu)and liquid(Zn)electrodes were studied,and kinetic and thermodynamic data of electrode process were determined.Double cathodes electrolysis cell was designed to extract metal Y.The result of CV indicated that the redox reaction of Y(?)on liquid Zn electrode belongs to solution/solution and irreversible controlled by diffusion.The reduction of Y(?)on Cu electrode is controlled by diffusion,and Cu₂Y is easier to form.The extraction of Y on liquid Zn and solid Cu electrodes were conducted,and the products are YZn₁₂ and YCu₂,respectively,indicating that it is feasible to extract Y using Cu and Zn electrodes.The thermodynamic properties of Y(?)on liquid Zn and solid Cu electrodes were studied by CV and OCP,indicating that the depolarization value of Y(?)on liquid Zn electrode was larger than that on Cu electrode and the Gibbs free energy of formation of Y-Zn solution is smaller than that of Y-Cu intermetallic compounds.LP experiments proved that Y(?)/(Y-Zn)_solution couple has higher exchange current density and smaller charge transfer resistance and reaction activation energy than those of Cu₇Y/Y(?)couple.The extraction of Y using liquid Zn,solid Cu and solid-liquid(Cu-Zn)double cathodes indicated that the extraction rate was faster on liquid Zn electrode.The deposition amount of metallic Y on liquid Zn and solid Cu in double cathodes electrolysis were 80.4%and 19.6%,respectively.Combined with the results of XRD and SEM-EDS,the deposition of Y(?)on liquid Zn electrode is easier.Comparision the dynamics and thermodynamics properties of Y(?)/Y(0)couple on liquid Zn,Pb and Sn electrodes,the preferred order of extracting Y with liquid electrode is Zn,Sn and Pb.