| In recent years,molten salts electrolysis has been widely applied in many fields,most notably spent fuel pyroprocessing technology,which employs electro refining and co-deposition methods to separate actinides from fission products in LiCl-KCl eutectic salts.Rare earths,accounting for about a quarter of fission products in spent fuel,are known as neutron poisons which impede nuclear fission chain reaction in the reactor,therefore,the most detrimental elements during nuclear fuel cycle.However,chemical and electrochemical properties of actinides and rare earths are highly similar,which results in extremely difficult separation between them.To establish appropriate separation methods based on molten salts electrolysis,it requires elaborate electrochemical studies to obtain fine distinction of the electrodeposition processes between actinides and rare earths in molten salts.On the other hand,because of chemical similarity,people could simulate and predict the electrodeposition process of actinides in molten salts based on the study of rare earths,which is conductive to developing innovative extraction process and establishing standard research models for radioactive elements.Therefore,this thesis focuses on the study of rare earths in molten salts,including the electrochemical behavior and electrodeposition process.And extraction of rare earths from molten salts is attempted by co-deposition method.All these studies will provide elaborate data and research methods for further separation and extraction of actinides.This thesis mainly includes two parts of fundamental research.1.The electrochemical behavior of five different rare earths(La,Nd,Gd,Er and Y)in LiCl-KCl eutectic salts was studied,respectively,in the first part of this thesis.Thermodynamics,kinetics,and electrocrystallization process were determined by various electrochemical methods and the morphology of electrodeposits was observed by SEM.The electroreduction process of RE(?)ions in LiCl-KCl eutectic salts was studied by cyclic voltammetry,chronopotentiometry and square wave voltammetry.The reduction of La(?),Gd(?),Er(?),and Y(?)ions on an inert Mo electrode is an one-step reaction with three electrons exchanged.But the reduction of Nd(?)ion involves two steps with the intermediate ion of Nd(?).The equilibrium potentials at different temperatures were determined by the half-cell systems which were established by in-situ electrodeposition of rare earth metals on Mo electrodes.Moreover,according to Nernst equation the corresponding apparent potentials were calculated to build the relationship between temperatures and potentials.The differences of Gibbs energy of RECl3 between crystal state and molten salts state were used to calculate the activity coefficients of RE(?)ions in LiCl-KCl eutectic salts.The diffusion coefficients of RE(III)ions in LiCl-KCl eutectic salts at different temperatures were determined by chronopotentiometry or convolution technique and then the corresponding diffusion activation energies were deduced according to Arrhenius equation.The exchange current densities of RE(III)/RE(0)redox at different temperatures were deduced from the linear polarization curves according to Butler-Volmer equation.The relationship between the peak potentials/currents and scan rates of cyclic voltammograms were employed to qualitatively estimate the reversibility of the reduction process of RE(?)ions.The difference between anodic and cathodic peak potentials or the function of convolutive currents and potentials were used to calculate the standard rate constants of the reduction process.And then the acquired standard rate constants were employed to quantificationally estimate the reversibility.Chronoamperometry were employed to determine the nucleation and growth mechanism of electrocrystallization process of RE(?)ions on Mo electrodes.The concentration effect on electrocrystallization mechanism was evaluated.According to Scharifker-Hills models,at a low concentration of RECl3,the electrocrystallization process always accords with progressive nucleation;however,at a high concentration,the electrocrystallization process transforms to instantaneous nucleation.SEM was employed to visually observe the electrodeposits at different deposition conditions.Towards 2.0 wt.%LaCl3 and NdCl3,the deposits tend to be dendritic;however,for 2.0 wt.%GdCl3,ErCl3 and YCl3,the deposits are always continuous and compact metallic films.2.The co-deposition processes of reactive metal ions(Al(?),Mg(II),and Li(I))and RE ions(Eu(II)and Pr(?))were studied by different electrochemical methods.Futher,extraction of rare earth elements from molten salts was realized by preparation of RE alloys via co-deposition method of potentiostatic and galvanostatic electrolysis.The feasibility of extraction of rare earths from oxide feeds was evaluated in LiCl-KCl-AlCl3 molten salts.Taking advantage of the chlorination ability from AlCl3,the in-situ generation of Eu(?)or Pr(?)ions could be achieved in LiCl-KCl-AlCl3 molten salts by adding Eu2O3 or Pr6O11.Cyclic voltammetry,square wave voltammetry and open-circuit chronopotentiometry were employed to study the co-deposition behavior of Al(?)with Eu(?),Pr(?)or Li(?)ions to determine the formation potentials of intermetallics and to verify the feasibility of formation of Al-Li-RE alloys.Al-Li-Eu and Al-Li-Pr alloys were prepared by galvanostatic electrolysis and analyzed with XRD,SEM,EDS and ICP.The element and phase compositions of Al-Li-RE alloys are related to the AICl3 concentrations.The co-deposition behavior of Mg(?),Li(?)and Pr(?)ions was studied by cyclic voltammetry in LiCl-KCl-MgCl2 molten salts.And the formation potentials of Mg-Li and Mg-Pr intermetallics or alloys were evaluated by open-circuit chronopotentiometry.Mg-Pr and Mg-Li-Pr alloys were prepared by potentiostatic and galvanostatic electrolysis,respectively,and analyzed with XRD,SEM,EDS and ICP.The element and phase compositions of the Mg-Pr and Mg-Li-Pr alloys are related to the concentrations of MgC12 and PrCl3 in molten salts. |
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