Electrochemical Separation And Extraction Of Zr(Ⅳ) And Sr(Ⅱ)from LiCl-KCl Melt

Nuclear power has obvious advantages in the development of low-carbon economy.The amount of radioactive waste accumulation is more and more with the wide application of nuclear energy products.Zirconium based alloy cladding materials as the second safety barrier for the safe and stable operation of nuclear reactor occupies a large proportion in the nuclear waste.In order to recover Zr metal and reduce the volume of geological repository,Zr must be separated from other fission elements.At present,in the advanced nuclear fuel cycle,molten salt electrolysis technology is considered to be one of the most advantageous nuclear waste reprocessing technologies.Thus,in this work,Zr and Sr were selected as typical nuclear waste to study their separation.In Li Cl-KCl-K₂Zr F₆(1.03×10^-4 mol/cm³)-Sr Cl₂（0.097 mol/cm³）molten salt system,liquid Zn was used as cathode to separate metal Zr.Meanwhile,the molten salt waste containing Sr（Ⅱ）was purified to reduce the volume of high radioactive molten salt waste.（1）The electrochemical behavior of Zr（Ⅳ）on an inert W electrode was studied by a series of electrochemical methods.The results of cyclic voltammetry and reverse chronopotentiometry showed that the reduction process of Z r（I V）?^2e→Zr（I I）?^2e→Zr at sweep rate of 0.07 V/s-0.20 V/s,Zr（Ⅳ）/Zr（Ⅱ）and Zr（Ⅱ）/Zr were soluble/soluble and insoluble reduction processes and Zr（Ⅱ）was found proceed in a quasi-reversible process controlled by diffusion.The diffusion coefficient was detected to be 5.65×10^-5 cm²·s^-1.Tafel method was used to measure the kinetic properties at different K₂Zr F₆ concentration,and found that the exchange current density of Zr（Ⅱ）/Zr on W electrode increased and the charge transfer resistance decreased with the increase of K₂Zr F₆ concentration.The effect of F^-concentration on reduction mechanism of Zr（Ⅳ）was explored.It was found that the reduction potential of Zr（Ⅳ）/Zr（Ⅱ）and Zr（Ⅱ）/Zr shifted negatively with the increase of F^-concentration due to the formation of long chain complexes,resulted in the diffusion coefficient of Zr（Ⅱ）decreased from 1.55×10^-5cm²·s^-1 to 0.72×10^-5 cm²·s^-1.（2）The electrochemical behavior of Zr（Ⅳ）on Zn film and liquid Zn electrode was studied to explore the feasibility of separation of Zr on Zn electrode.The reduction of Zr（Ⅳ）on Zn film electrode proceeded in two-step process with two electron,and depolarization effect was observed,which resulting in the formation of Zr-Zn alloy;while the reduction of Zr（Ⅳ）on liquid Zn electrode was one-step four electron and diffusion controlled quasi-reversible process.The diffusion coefficient was measured to be 8.07×10^-5 cm²·s^-1.The exchange current density and charge transfer resistance of Zr（Ⅳ）/Zr（Zn）at different scanning speeds were determined by Tafel method.It is found that with the increase of scanning speed,the exchange current density increases and the charge transfer resistance decreases.Zr was separated from molten salt by potentiostatic electrolysis.A kind of Zr-Zn intermetallic compound Zn_21.8Zr₁ was obtained by XRD and SEM-EDS characterization.（3）The electrochemical behavior of Sr（Ⅱ）on Zn film electrode was investigated to explore the electrochemical separation of Sr（Ⅱ）on reactive Zn electrode.The reduction of Sr（Ⅱ）on Zn film electrode proceeded in one-step process with two electron,and depolarization effect was observed,which resulting in the formation of Sr-Zn alloy.According to the electrochemical behaviors of Zr（Ⅳ）and Sr（Ⅱ）on Zn electrode,combined with the decomposition voltage of Sr Cl₂,KCl and Li Cl,the applied potential was selected for the electrochemical separation of Zr（Ⅳ）and Sr（Ⅱ）at about-0.96 V～-1.44 V.Zr（Ⅳ）and Sr（Ⅱ）were separated by potentiostatic electrolysis at-1.30 V on liquid Zn electrode in Li Cl-KCl-K₂Zr F₆-Sr Cl₂（0.097 mol/cm³）.The results of XRD and SEM-EDS showed that only Zr was separated from molten salt.The molten salt containing Sr（Ⅱ）was electrochemically purified.Sr was separated from molten salt by galvanostatic electrolysis and potentiostatic electrolysis.XRD and SEM-EDS analysis showed that Sr Zn₁₃ compounds was obtained.