Chlorination And Anodic Dissolution Of Zirconium In Molten Chlorides

With the rapid growth of the demand for clean energy,nuclear power has a strategic significance to optimize the energy structure,reduce environmental pollution and promote sustainable development of the energy and economy.Zirconium is an important rare metal for nuclear applications due to its low neutron-capture cross-section,high thermal conductivity and good mechanical properties.At present,most of the nuclear grade zirconium used in China mainly relies on import,and the production and refining of high purity zirconium have drawn great attention.In the traditional electrorefining technology of zirconium,a solid anode and batch production were conducted,with high temperature,high consumption and poor continuity.Moreover,ZrCl₄ is directly introduced to prepare zirconium-containing electrolyte,which not only causes the inconvenience of storage and transportation,but also leads the sublimation of ZrCl₄ in the introduction process due to a low sublimation temperature（331 ℃）.Therefore,it is necessary to find a new electrorefining process of Zr,which could improve the batch production and simplify the introduction of ZrCl₄ electrolyte,aiming to achieve continuous electrorefining of Zr.In view of the existing defects in refining process of zirconium,a continuously production process is proposed to refine zirconium.In this study,in-situ preparation ZrCl₄ electrolyte for refining process was investigated via the chlorination of zirconium by CuCl and SnCl₂.Dynamic electrochemical behaviors in the chlorination process were monitored by electrochemical tests.In addition,electrochemical behaviors of Cu（Ⅰ）,Sn（Ⅱ）and Zr（Ⅳ）in LiCl-KCl melt were studied,and the anodic dissolution behaviours of Cu,Sn,Zr and their alloy were also investigated,which laid a theoretical foundation for subsequent electrorefining of Zr.The main results are summarized as following.The electrochemical behaviors of Cu（Ⅰ）and Zr（Ⅳ）in eutectic LiCl-KCl melt were investigated on Mo electrode at 500 ℃ by a series of electrochemical techniques,such as cyclic voltammetry,square wave voltammetry and chronopotentiometry.The reduction potential of Cu（Ⅰ）is obtained at-0.45 V（vs.Ag/AgCl）,and the reduction process is a quasi-reversible process controlled by diffusion mass transfer.The diffusion coefficients of Cu（Ⅰ）were calculated by the results of cyclic voltammetry and chronopotentiometry,which were found to be 1.79×10^-5 and 1.26×10^-5 cm2· s-1,respectively.Three reduction potentials of zirconium ions are obtained at-1.15,-1.22 and-1.65 V,which correspond to the reduction of Zr（Ⅳ）to Zr（Ⅱ）,Zr（Ⅰ）and Zr,respectively.The in-situ preparation of LiCl-KCl-ZrCl₄ melt was investigated by the displacement reaction between Zr and CuCl in LiCl-KCl melt at 500 ℃,and the reaction progress was also investigated by a series of electrochemical techniques.The electrochemical signals show that the concentration of Zr（Ⅳ）ions increases gradually and then reaches to the maximum value as the reaction time increases,while the concentration of Cu（Ⅰ）ions decreases rapidly and drops below the detection limit of the electrochemical tests.Meanwhile,the concentrations of Cu and Zr ions in the melt were determined by ICP analysis and the results are in good agreement with the electrochemical tests.Finally,LiCl-KCl melts with ZrCl₄ are obtained,and the final electrochemical behavior of Zr（Ⅳ）produced in the melt is similar to that directly introduced.The electrochemical behavior of Sn（Ⅱ）in LiCl-KCl melt was studied,and the reduction potential of Sn（Ⅱ）is obtained at-0.40 V,The reduction process is a quasi-reversible process controlled by the diffusion mass transfer.The diffusion coefficients of Sn（Ⅱ）were calculated by the results of cyclic voltammetry and chronopotentiometry,which are found to be 1.54×10^-5 and 1.69×10^-5 cm²·s^-1,respectively.LiCl-KCl-ZrCl₄ melt was prepared by in-situ displacement reaction between Zr and SnCl₂ in LiCl-KCl melt at 500 ℃,and the progress of the reaction between Zr and SnCl₂ was also investigated by dynamically electrochemical measurements.The results reveal that the concentration of Zr（IV）increases gradually and reaches to a maximum value as the reaction time increases,while the concentration of Sn（Ⅱ）decreases gradually and drops below the detection limit.In addition,the chemical analyses of Zr（IV）and Sn（II）in the melt at various times were also carried out and the results are in good agreement with those of the electrochemical measurements.Finally,LiCl-KCl-ZrCl₄ melts with low concentration of Sn（II）were obtained.The electrochemical behaviors of Cu（I）and Sn（II）in LiCl-KCl-CuCl-SnCl₂ were investigated.The results show that the reduction potential of Cu（I）is slightly negative than that of Sn（Ⅱ）,and the two potentials are nearly coincident when they both exist in the molten salt.Zirconium was chloridized by both CuCl and SnCl₂ when the molar ratios of Cu（Ⅰ）/Sn（Ⅱ）were 1:1 and 2:1,respectively,and the reaction process was monitored by electrochemical tests.When the molar ratio of Cu（Ⅰ）/Sn（Ⅱ）is 1:1,both the concentrations of Cu（Ⅰ）and Sn（Ⅱ）drop rapidly during the reaction process,while the falling rate of Cu（Ⅰ）is faster than that of Sn（Ⅱ）.When the molar ratio of Cu（Ⅰ）/Sn（Ⅱ）is 2:1,although both concentrations of Cu（Ⅰ）and Sn（Ⅱ）are falling,the concentration of Cu（I）is higher than Sn（Ⅱ）during the reaction process,and the two are nearly identical（<0.01 mass%）when the reaction finished.When excessive zirconium reacted with different mass ratios of Cu（Ⅰ）/Sn（Ⅱ）,chlorination reaction time increases gradually with the decrease of the mass ratios.The anodic dissolution potentials of Cu,Sn,Zr,Cu-Sn-Zr and Cu-Zr alloys were determined by cyclic voltammetry and anodic polarization test,and the initial dissolution potentials of Cu,Sn and Zr are-0.51,-0.58 and-1.0,respectively.There is a large potential difference between Cu,Sn and Zr,and Zr can be preferentially dissolved from alloy in theory.Compared with the anode dissolution curves of Cu-Zr and Cu-Sn-Zr alloy,the former has a higher current density,which corresponds to the higher content of Zr in the alloy.After the potentiostatic anodic dissolution of Cu-Zr and Cu-Sn-Zr alloy,SEM and metallographic microscopy results show that dissolution of Zr in the alloy is obvious,and there is a clear boundary between the dissolution zone and alloy matrix.Additionally,a clear dissolution transition zone and a Cu-rich zone appear in Cu-Zr dissolved alloy.The dissolution loss of Zr increases with the increase of dissolution potential and dissolution time,while the dissolution rate increases with the increase of dissolution potential,and decreases with the prolong of dissolution time.