Anodic Dissolution Of Zr-Dy Alloy In LiCl-KCl Molten Salt And Electrochemical Extraction Of Dy

Electrolytic refining is one of the most promising dry reprocessing of spent nuclear fuel.At present,zirconium alloys are used as cladding materials in commercial pressurized water reactors,boiling water reactors and heavy water reactors.Waste cladding materials contain fission elements.The separation of cladding materials and fission elements attracts more attention of researchers.In this paper,Zr and Dy were represented as the cladding materials rare earth elements to study the electrolytic refining process of Zr-Dy alloys in the LiCl-KCl molten salt,mainly focused on the electrochemical behavior,anodic dissolution and separation of Zr and Dy and extraction of Dy in Zr-Dy alloy.1.The electrochemical behavior of Zr(?)and Dy(?)on an inert W electrode was studied by a series of electrochemical methods in LiCl-KCl molten salt at 823 K.The results displayed that the reduction of Zr(?)proceeds in a two-step with two-electron transfer,while Dy(?)in a one-step three-electron transfer process.The electrochemical coreduction of Zr(?)and Dy(?)on W electrode was explored by cyclic voltammetry,chronopotentiometry and open-circuit chronopotentiometry.The redox signal peak of Zr-Dy solid solution was determined,as well as the redox potential of Zr,Zr-Dy solid solution and Dy.2.At 823 K,the anodic dissolution behavior of Zr and its passivation were first studied using open-circuit chronopotentiometry,linear scanning voltammetry,and square wave voltammetry.The content of Zr(?)ions in the molten salt increases with the anodic dissolution duration.Meanwhile,it was found that the passivation phenomenon occurs easily at large sweep speed,and the passivation potential shifted positively with the increase of the anodic dissolution time.Then,the anodic dissolution behavior and passivation of Zr-Dy alloy in the LiCl-KCl molten salt was investigated employing cyclic voltammetry and linear scanning voltammetry.The dissolution order of Zr-Dy alloy was found to be Dy>Zr-Dy>Zr,The passivation phenomenon occurs easily at large sweep speed with the increase of the anodic dissolution time.Anodic dissolution of Zr-Dy alloy were conducted in the LiCl-KCl molten salt at three different current densities,and the anodic dissolution process was monitored online using cyclic voltammetry and square wave voltammetry.It was found that with the proceeding of anodic dissolution,the oxidation process of Zr was found to be from two-step two-electron transfer to one-step four-electron transfer.The XRD and SEM-EDS were used to characterize the Zr-Dy alloy after anodic dissolution at different current densities.Anodic dissolution mainly occurred on the surface of the alloy.As the current density increased,the interior alloy gradually dissolved.3.At a temperature of 823 K,electrochemical extraction of Dy was performed by constant current electrolysis and constant potential electrolysis,using Zr-Dy alloy as the anode and Al as the cathode.Electrolytic refining results obtained at the constant current electrolysis showed that:increasing the anode current density and prolonging the electrolysis time can promote the anodic dissolution of Zr-Dy alloy.After 8 hours of the anodic dissolution at a current density of 100 mA/cm~2,the anodic dissolution ratio was 39.63%,and the anodic dissolution rate was 7.53 g/cm~2·h.No refined product was obtained on the cathode due to the product falls off in the molten salt.Electrolytic refining results obtained at the constant potential electrolysis showed that:both choosing a positive potential and prolonging the electrolysis time can promote the anodic dissolution of Zr-Dy alloy.When the applied potential at-1.20 V(vs.Ag/AgCl),and the electrolysis time for 8 hours,the anodic dissolution ratio could reach 9.46%,the anodic dissolution rate 1.58 g/cm~2·h,the cathode current efficiency 11.89%,and Dy was attained on the cathode.The results of SEM-EDS were found that Dy was distributed on the surface of the Al matrix,but Zr was not found,which indicated it is feasible to separat Zr and Dy by electrolytic refining.