Extraction Of Lanthanides (Pr,Tb,Ho,Tm) By Molten Salt Electrolysis On Solid And Liquid Electrodes

Molten salt electro-refining of the spent fuel is considered to be one of the most promising dry reprocessing technologies.In order to select electrode materials for the separation of molten salt electrolysis,it is necessary to investigate the electro-deposition mechanism of actinides and lanthanides on different electrodes.In Li Cl-KCl molten salt,the solid metals（W,Mg and Cu）and liquid metals（Zn and Bi）as the working electrodes,the electrochemical reduction process of lanthanides on different electrodes were studied.From the view of the thermodynamics and kinetics,the comparison of electrochemical extraction rates and efficiencies of lanthanides were performed on different electrodes.（1）Electrochemical methods（square wave voltammetry,cyclic voltammetry,linear polarization,open-circuit chronopotentiometry）were employed to study the electrochemical behavior and reaction mechanism of Pr（III）on W and Zn film electrodes.The electrochemical reduction of Pr（III）on W electrode was detected to be a one-step three-electron transfer and quasi-reversible process.The dynamic properties,for instance,exchange current density and reaction resistance of Pr（III）on W electrode were determined by linear polarization method,and the activation energy was calculated to be 21.13 k J mol^-1.The electro-reduction mechanism of Pr（III）on the Zn film electrode was studied:Pr（III）+x Zn+3e^-→Pr Zn_x.The deposition potential of Pr on Zn film electrode shifts positively due to the formation of Pr Zn_x,and the depolarization values of Pr on Zn film electrode were estimated.The equilibrium potentials of Pr-Zn intermetallic compounds were determined,and the activities of Pr and the partial molar Gibbs free energies in Pr-Zn alloys and the standard molar Gibbs free energies（35）G_f（Pr Zn_x）,enthalpies（35）H_f（Pr Zn_x）and entropies（35）S_f（Pr Zn_x）for the formation of Pr Zn_x were evaluated.Moreover,the Pr-Zn alloy was extracted by constant current electrolysis in molten salt.The characterization results of SEM-EDS and XRD indicated that the alloy consisted of Pr Zn₁₁,Pr Zn_17/2,Pr Zn_58/13,Pr Zn_11/3 and Pr Zn alloy phases.（2）The electrochemical methods were used to study the electro-reduction mechanism of Tb（Ⅲ）on Cu and Zn electrodes,respectively.Tb（Ⅲ）was reduced to form TbCu_x on Cu electrode and Tb Zn_x on Zn electrode,which led to the deposition potential of Tb on Cu and Zn film electrodes shifts positively.The depolarization values of Tb on Cu and Zn film electrodes were estimated,and found that the maximum depolarization value of Tb（Ⅲ）on Zn electrode（0.71 V）is larger than that on Cu electrode（0.38 V）.It was suggested that Tb was more easily extracted on Zn electrode.The equilibrium potentials of Tb-Cu和Tb-Zn compounds were determined.Thus,the thermodynamic data of Tb in Tb-Cu alloys and Tb-Zn alloys,and（35）G_f（TbCu_x）and（35）G_f（Tb Zn_x）were evaluated.It is found that the standard molar Gibbs free energies（（35）G_f）of Tb Zn₁₂ and Tb Zn_17/2 were smaller than（35）G_f（TbCu_x）.It was speculated that the formation of Tb Zn₁₂ and Tb Zn_17/2were easier than TbCu_x.The exchange current densities（j₀）of Tb（Ⅲ）/TbCu₅ and Tb（Ⅲ）/Tb Zn₁₂ couples on electrodes were studied by linear polarization method.At the range of 723～823 K,j₀(Tb Zn₁₂/Tb（Ⅲ）)was found to be greater than j₀（TbCu₅/Tb（Ⅲ））.The charge-transfer resistance of Tb Zn₁₂/Tb（Ⅲ）is smaller than TbCu₅/Tb（Ⅲ）.And the activation energy of Tb Zn₁₂/Tb（Ⅲ）is also smaller than Cu₅Tb/Tb（Ⅲ）.It was speculated that the reaction rate of Tb（Ⅲ）on Zn electrode may be faster than that on Cu electrode at the same temputure..Metallic Tb was extracted on Cu and Zn electrodes under the same conditions by constant current electrolysis.The products were characterized by SEM-EDS and XRD.Meanwhile,the concentration of Tb（Ⅲ）in the molten salt was determined by ICP-AES,and the extraction rates and efficiencies were calculated at different duration.After the extraction for 2 h,the average extraction rates of Tb on Cu and Zn electrodes were 0.96×10^-5 mol cm^-3h^-1 and 1.98×10^-5 mol cm^-3 h^-1,respectively.This is consistent with the results of thermodynamics and kinetics.Furthermore,the interaction between the Zn electrode material or the extracted product of Tb-Zn alloy and the molten salt was investigated,and found that the dissolution of Zn in the molten salt could be inhibited under polarized conditions.The dissolution rate can be reduced by an order of magnitude.（3）Electro-reduction mechanisms of Ho（III）on W,Mg film and Zn film electrodes were detected by electrochemical methods.Similarly,the electrochemical reduction of Ho（III）on W electrode was detected to be a one-step three-electron transfer and quasi-reversible process.The equilibrium potential and apparent potential of Ho（III）/Ho（0）on W electrode were determined by open-circuit chronopotentiometry.Ho（III）was reduced to form Ho Mg_x on Mg film electrode and Ho Zn_x on Zn film electrode,which led to the deposition potential of Ho（III）on Mg and Zn film electrodes shifts positively.The depolarization values of Ho（III）on Mg and Zn film electrodes were estimated,and found that the maximum depolarization value of Ho（III）on Mg electrode（0.09 V）is smaller than that on Zn electrode（0.61 V）.It was suggested that Ho was more easily extracted on Zn electrode.The equilibrium potentials of Ho-Mg and Ho-Zn compounds were determined.Thus,the thermodynamic data of Ho in Ho-Mg and Ho-Zn alloys,and the standard molar Gibbs free energies（35）G_f（Ho Mg_x）and（35）G_f（Ho Zn_x）were evaluated.It is found that（35）G_f（Ho Zn_x）was smaller than（35）G_f（Ho Mg_x）.It was speculated that the formation of Ho Zn_x was easier than Ho Mg_x.Metallic Ho was extracted on Mg and Zn electrodes by constant potential electrolysis.The products were characterized by SEM-EDS and XRD.The results indicated that the Ho-Mg alloys consisted of Ho₅Mg₂₄、Ho Mg₂ and Ho Mg phases,while Ho-Zn alloy consisted of Ho₂Zn₁₇and Ho Zn₂phases.The extraction rates and efficiencies were calculated at different duration using ICP-AES.After extraction for 7.5 h,the average extraction rate and efficiency of Ho on Mg electrode were 0.58×10^-5mol cm^-3 h^-1 and 67.7%,and 1.03×10^-5 mol cm^-3 h^-1 and 79.9%on Zn electrode after extracted 5 h.（4）The electro-reduction mechanisms of Tm（III）on W,Cu and Bi film electrodes were detected by cyclic voltammetry and square wave voltammetry.The electrochemical reduction of Tm（III）on W electrode was detected to be two-step reaction:Tm（III）→Tm（II）→Tm（0）.The electro-reduction mechanisms of Tm（III）on Cu and Bi film electrodes were different from W electrode,and the one-step three-electron transfer is existed:Tm（III）+x M+3e^-→Tm M_x（M=Cu,Bi）.The electrode not only affects the electrode reaction,but also changes the electro-reduction mechanism.Tm（III）was reduced to form Tm Cu_x on Cu electrode and TmBi_x on Bi film electrode,which led to the deposition potential of Tm on Cu and Bi film electrodes shifts positively.The depolarization values of Tm on Cu and Bi film electrodes were estimated,and found that the maximum depolarization value of Tm（III）on Cu electrode（0.68 V）is smaller than that on Bi film electrode（0.79 V）.It was suggested that Tm was more easily extracted on Bi electrode.Metallic Tm was extracted on Cu and Bi electrodes by constant potential electrolysis.The products were characterized by SEM-EDS and XRD.The results indicated that Tm Cu₅,Tm Cu₂ and Tm Cu intermetallic compounds were prepared on Cu electrode by different applied potentials;while the Tm-Bi alloy consisted of TmBi phases.