| Zirconium has very low thermal neutron capture cross-section and excellent mechanical properties,making it an irrepleacable material in nuclear industry.At present,the industrial nuclear grade zirconium production process mainly includes two parts:zirconium and hafnium separation through the extraction process between the organic solvent and Zr(Hf)-containing aqueous solution,and zirconium metal production with magnesiothermic reduction process.The production process involves complex combination of hydro-and pyro-metallurgical operations,leading to high production costs and serious environmental issues.In this study,zirconium and hafnium were effectively separated via a molten salt extraction process in a low melting-point alloy system based on a novel process for nuclear grade zirconium production,and the investigation on the electrochemical behavion of zirconium in molten salt was performed for the molten salt electro-refining process of zirconium.This study aims to provide important theoretical basis for the green and efficient nuclear grade zirconium production process.In the present study,the effects of reaction temperature,equilibrium time,and molten salt extractant composition on the molten salt extraction Zr-Hf separation process were invesitigated.Transient electrochemical techniques such as cyclic voltammetry,square wave voltammetry,chronoamperometry,chronopotentiometry,and open circuit chronopotentiometry were conducted to investigate the zirconium electrochemical behavior and anodic dissolution process and mechanisms in both fluoride and chloride molten salt systems.The anodic dissolution process and mechanisms of Cu and Cu-Zr alloy in the chloride molten salt system were also investigated.Main experimental results and conclusions are summarized as following:(1)Thermodynamic analysis shows that a lower temperature is favourable for the Zr-Hf separation process either using chloride or fluoride salt as the extractant.Fluoride molten salt system has better selectivity to Hf in the molten alloy at the temperature below 1000?.(2)The NaF-CaF2-CuF2 molten salt extractant delivers better Zr-Hf separation efficiency in the Cu-Zr-Hf molten alloy system.The best result was obtained at 1000? with the Cu(II)/Hf mole ratio of 4,as to be approximately 48%for Hf removal efficiency and 2.0 for Zr-Hf separation factor.The addition of Zr(IV)into the molten salt system was not able to promote the Zr-Hf separation process,mainly due to the fact that the added ZrCl4/ZrF4 contains also Hf at the same level of the alloy phase,which is harmful for the Hf removal reaction during the equilibrium process.(3)The reduction of Zr(IV)in the LiF-KF-ZrF4 molten salt system was found to follow a three-step mechanism of Zr(IV)/Zr(II),Zr(II)/Zr(I),and Zr(I)/Zr.The diffusion coefficient of Zr(IV)in the LiF-KF-ZrF4 molten salt system at 600? was evaluated to be about 8.31×10-6 cm2/s according to the electrochemical results.The anodic dissolution process and mechanisms of Zr in the melt were also investigated.The K3ZrF7 complex was formed during the Zr anodic dissolution process which covers the surface of the electrode and decreases the efficiency of the electrolytic process.A kinetic model for the Zr anodic dissolution process was proposed,and the diffusion process between the electrode and molten salt was found to be the rate-determining step of Zr anodic dissolution process according to the kinetic equation.(4)The reduction of Zr(IV)in the LiF-NaF-K2ZrF6 molten salt system was found to follow a two-step mechanism of Zr(IV)/Zr(II)and Zr(II)/Zr.The diffusion coefficient of Zr(IV)in the LiF-NaF-K2ZrF6 molten salt system at 750? was evaluated to be about 1.78 ×10-5 cm2/s according to the electrochemical results.The anodic dissolution process and mechanisms of Zr in the melt were also investigated.The Na3ZrF7 complex was formed during the Zr anodic dissolution process which covers the surface of the electrode and decreases the efficiency of the electrolytic process.A kinetic model for the Zr anodic dissolution process was proposed,and the the rate-determining step of Zr anodic dissolution process was found to be the diffusion process between the electrode and molten salt according to the kinetic equation.(5)The reduction of Zr(IV)in the LiCl-KCl-K2ZrF6 molten salt system was found to follow a two-step mechanism of Zr(IV)/Zr(II)and Zr(II)/Zr.The diffusion coefficient of Zr(IV)in the LiCl-KCl-K2ZrF6 molten salt system at 550? was evaluated to be about 4.62×10-5 cm2/s according to the electrochemical results.No insoluble substance was found on the electrode during the anodic dissolution process of Zr in the LiCl-KCl-K2ZrF6 molten salt system.A kinetic model for the Zr anodic dissolution process was proposed,and the exchange current density evaluation for Zr in the melt delivered an average value of 5.25×10-4 A/cm2 according to the relationship between dissolution rate and reaction time under varied polarization conditions.The determined dissolution rate and reaction time relation in the experiments accord closely with the theoretical calculation results.(6)The anodic dissolution process and mechanisms of Cu and Cu-Zr alloy at 550? in the melt were investigated.The exchange current density evaluation for Cu in the melt delivered an average value of 2.06×10-3 A/cm2 according to the relationship between dissolution rate and reaction time under varied polarization conditions.The determined dissolution rate and reaction time relation in the experiments accord closely with the theoretical calculation results.In the anodic dissolution process of Cu-Zr alloy,Zr was preferentially oxidized and dissolved into the molten salt.The radius of the Cu-Zr core decreased gradually,in the meanwhile,the thickness of the unreacted Cu diffusion layer kept increasing,and the overall size of the anode kept the same as that of the Cu-Zr alloy before electrolysis.With the progress of the anodic dissolution process,the diffusion of Zr ions within the anode became more difficult,which further decreased the dissolution rate of Zr on the anode.By precisely controlling the electrode potential,selective oxidation of Zr in the Cu-Zr alloy can be achieved,and pure Zr metal can be finally obtained on the cathode. |
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