Anodic Dissolution Of Al-Y (Sm) Alloy And Electrochemical Extraction Of Y And Sm From Molten Salt

Molten salt electrorefining with its unique advantages has been regarded as one of the most promising methods for spent fuel reprocessing.It consists of anodic dissolution and cathodic electrodeposition.Therein,anodic passivation can inhabit the anodic dissolution progress,and the selection of cathode electrode material can determine the extraction efficiency of electrochemical extraction.In order to improve the electrorefining rate and extraction efficiency,it’s necessary to carry out basic research on anodic dissolution and cathodic electrodeposition.Therefore,in this paper,the anodic dissolution of solid/liquid Al-Y(Sm)alloy was studied by electrochemical method,and the passivation phenomenon and mechanism during anodic dissolution were explored in LiCl-KCl molten salt;And the electrochemical extraction of Y and Sm were studied on Mg film cathode and liquid Bi,Sn cathode.The main research contents are as follows:(1)The kinetics of anodic dissolution using solid Al-Y alloy as anode were studied by potentiodynamic polarization method.With the increase of temperature,the exchange current density increased from 0.79 mA cm-2 at 773 K to 1.65 mA cm-2 at 923 K.The passivation phenomenon of solid Al-Y alloy during anodic dissolution was explored,and the anode surface after anodic dissolution was analyzedby XRD and SEM-EDS.The passivation mechanism of solid Al-Y alloy was proposed.The elecctrochemical impedance method showed that the resistance of the passivated film decreased from 108.68 Ωm-2 to 1.30 Ω cm-2 with the introduction of F-ions.The mechanism of introducing F-ion into molten salt to inhibit passivation was also proposed.Moreover,the electrochemical reaction of liquid Al-Y alloy at 1123 K was studied by chronopotential method.The diffusion coefficient of Y in liquid Al-Y alloy was calculated by Sand’s equation to be D=6.61 10-6 cm2 s-1.The kinetics of anodic dissolution on liquid Al-Y anode was studied by potentiodynamic polarization method.The exchange current density increased gradually from 13.87 mA cm-2 at 1123 K to 20.09 mA cm-2 at 1213 K with the increasing temperature.The selective dissolution of Y on liquid Al-Y alloy anode could be achieved by potentiostatic anodic solution at-1.30 V.(2)Anodic dissolution of solid/liquid Al-Sm alloy was studied in LiCl-KCl eutectic salt by a series of electrochemical techniques.The passivation phenomenon on solid Al-Sm alloy was found during anodic dissolution,and the passivation mechanism was proposed.The kinetics of solid AlSm alloy during anodic dissolution was studied by potentiodynamic polarization method.The exchange current density increased from 0.40 mA cm-2 at 773 K to 0.81 mA cm-2 at 923 K.The effect of F-on anodic dissolution passivation was studied during anodic dissolution.The exchange current density increased from 0.40 mA cm-2 to 30.32 mA cm-2 with the increase of AlF3 concentration.The kinetics of liquid Al-Sm alloy during anodic dissolution was also studied by potentiodynamic polarization method.The exchange current density increased from 31.55 mA cm’2 at 1123 K to 37.67 mA cm-2 at 1213 K with increasing temperature.(3)The electrochemical behaviors of Sm(Ⅲ)were studied on Mg film cathode and liquid Sn and Bi cathodes in LiCl-KCl molten salt by steady-state and transient electrochemical methods.The results of cyclic voltammetry and square wave voltammetry showed that the formation of various Sm-M(Sn,Bi,Mg)alloys resulted in the underpotential deposition of Sm(Ⅲ)on the Sn,Bi,Mg electrodes.The exchange current density of Sm(Ⅱ)/SmMx pair was measured by Linear polarization(LP)and Tafel techniques,and the reaction activation energies of Sm(H)/SmMx pair were calculated to be 14.08 kJ mol-1(Sm(Ⅱ)/SmSn3)and 15.78 kJ mol-1(Sm(Ⅱ)/SmBi),respectively.Metallic Sm was extracted on liquid Sn and Bi electrodes using potentiostatic and galvanostatic electrolysis,the extracted products were to be SmSn3 and Sm5Sn3 on Sn electrode,and SmBi on Bi electrode.(4)The electrochemical behaviors and extraction of Y(Ⅲ)were studied on Mg film electrode and liquid Bi electrode.The underpotential deposition of Y on Bi/Mg membrane electrodes are due to the reduction of Y(Ⅲ)on Bi/Mg membrane to form Y-Bi/Mg intermetallic compounds.The standard molar formation of Gibbs free energy for BiY and Bi3Ys intermetallic compounds were calculated to be-205.51 kJ mol-1 and-882.83 kJ mol-1 according to the open circuit chronopotential curve.Rare earth Y was extracted by constant current on solid Mg cathode and liquid Bi cathode,and the concentration change of Y(Ⅲ)in molten salt was detected by square wave voltammetry and ICP-AES.The extraction rate and efficiency of Y were 1.82 10-5 mol cm-3 h-1 and 78.1%on solid Mg electrode,and 2.16 10-5 mol cm-3 h-1 and 91.5%on liquid Bi electrode,respectively.