Electrochemical Extraction Of Ln(Dy,Ho,Yb) By Molten Salt Electrolysis And Liquid Metal Reduction Extraction Of Ln(Ce,Sm)

With the wide application of nuclear energy,spent fuel reprocessing has become one of the key issues which restrict the sustainable development of nuclear energy.Lanthanides?Ln?in fission products are the most harmful products in the nuclear fuel cycle because of their large neutron absorption cross section which hinders nuclear fission.Dry reprocessing is considered as a promising technology for spent fuel reprocessing.Actinides?An?and Ln are separated and U,Pu and minor actinides recovered using molten salt electrolysis and molten salt/liquid metal extraction to realize the recycling and reusing spent nuclear fuel.Therefore,the electrochemical extraction of Ln on solid?W,Cu?and liquid?Pb,Bi,Sn?electrodes were studied in LiCl-KCl eutectic salts to realize the separation of Ln and An.Meanwhile,the reduction extraction of Ln was also discussed in molten salt LiCl-KCl-LnCl₃/liquid Bi-Li alloy system.The main research contents are as follows:1.The electrochemical behavior of Ln???on inert W electrode was studied and the diffusion coefficient of Ln???were determined at different temperatures.The diffusion coefficients of Dy???,Ho???and Yb???ions at 773 K were found to be 0.73×10^-5cm² s^-1,1.66×10^-5cm² s^-1and 1.86×10^-5 cm² s^-1,respectively.The electrochemical parameters such as the equilibrium and apparent electrode potentials of Ho???/Ho couple were determined by open circuit chronopotentiometry?OCP?.2.The electrochemical behaviors,electrode process kinetics and thermodynamics and electrodeposition mechanism of Dy???and Ho???on Cu electrode were studied by transient and steady state electrochemical methods.The change of Ln???ion concentration during electrolytic extraction was detected by electrochemical method.?1?The deposition potential and equilibrium potential of each Cu-Dy and Cu-Ho intermetallic compound were determined,and found the deposition potential of Dy???and Ho???on the Cu electrode shifted positively,that is so called“the depolarization effect”.?2?The thermodynamic parameters,such as standard mole Gibbs free energy,enthalpy and entropy of formation for each Cu-Dy and Cu-Ho intermetallic compound,were calculated.?3?The exchange current densities?j₀?of electrode reactions for Dy???/Cu₅Dy and Dy???/Cu_9/2Dy couples were measured using linear polarization technique?LP?.According to the linear relationship between j₀ and temperature,the activation energies?E_a?of the corresponding electrode reactions were calculated to be 26.08 kJ mol^-1and 22.55 kJ mol^-1,respectively.?4?Dy and Ho were extracted by potentiostatic electrolysis.The result of XRD and SEM-EDS indicated that five Cu-Dy compounds and four Cu-Ho intermetallic compounds were prepared under different conditions.The maximum extraction rates of Dy and Ho by potentiostatic electrolysis extraction at-2.15 V for 16 h and 20 h were 99.2%and 99.4%,respectively.3.The electrodeposition mechanism,electrode process kinetics and thermodynamics of Ho???on Bi electrode were studied.The variation of Ln???ion concentration during electrolytic extraction was monitored by electrochemical method.?1?The deposition potential and equilibrium potential of each Bi-Ho intermetallic compounds were determined.The deposition potential of Ho???was found to shift positively and depolarization effect was observed.?2?The standard mole Gibbs free energies,enthalpies and entropies of the formation for Bi-Ho intermetallic compound were calculated by EMF.?3?The charge transfer coefficient???of Ho???/Ho?0?redox on liquid Bi electrode at 773 K was calculated to be0.45-0.49 by cyclic voltammetry?CV?.?4?Ho was electrochemically extracted from LiCl-KCl-HoCl₃ molten salt by galvanostatic?-0.1 A?/potentiostatic?-1.5 V?electrolysis for9 h,the extraction products were characterized to be BiHo phase.Take advantage of the calibration curve of the relationship between the concentration of Ho???and the peak current density of square wave voltammetry?SWV?,the change of Ho???concentration during the extraction process was on-line monitored by SWV.The relative error was about±6.1%by ICP-AES analysis.The feasibility of on-line monitoring of molten salt electrolysis by SWV was confirmed.The extraction efficiency was also calculated.The highest extraction rate was95.9%and the average extraction rate was 2.95×10^-6mol cm^-3h^-1at-1.6 V constant potential electrolysis for 27 h.4.The electrodeposition mechanism,electrode process kinetics and thermodynamics of Dy???,Ho???and Yb???ions on Pb electrode were studied.?1?The deposition potential and equilibrium potential of each Pb-Ln intermetallic compounds were measured.The deposition potential of Dy???,Ho???and Yb???on liquid Pb electrode were found to shift positively and depolarization effect were observed.The reaction mechanism of variable valence Yb???on liquid Pb cathode is a two-step reaction:Yb???+e^-?Yb?II?,Yb?II?+2e^-?Yb?0?.?2?The standard molar Gibbs free energies,enthalpies and entropies of Pb-Dy intermetallic compounds were calculated by EMF.?3?The j₀ and charge transfer coefficient?of Dy???/Dy?Pb?couple were determined by Tafel curve.According to the linear relationship between j₀ and temperature,the electrode reaction activation energy was calculated to be 23.12 kJ mol^-1.The charge transfer coefficient?of Ho???/Ho?0?redox was also calculated by CV.?4?The electroextraction of Ln?Dy?Ho?Yb?was performed by potentiostatic and galvanostatic electrolysis,and the products were characterized by XRD and SEM-EDS.The resutls indicated the formation of different Pb-Ln intermetallic compounds.Meanwhile,the highest extraction efficiency and average extraction rate of Dy were estimated and found to be 97.2%and 6.17×10^-6mol cm^-3h^-1,respectively,by potentiostatic electrolysis at-1.52 V for 20 h.5.The electrodeposition mechanism and electrode process kinetics of Yb???were studied on liquid Sn electrode.The deposition potential of each Sn-Yb intermetallic compounds was determined and found the deposition potential of Yb???was positively shifted and the depolarization effect was obvious.The reaction mechanism of variable valence Yb???on liquid Sn electrode was found to proceed in a two-step reaction:Yb???+e^-?Yb?II?,Yb?II?+2e^-?Yb?0?.LP was used to determine the j₀ of Yb???/Sn₃Yb couple in various temperature range and the activation energy E_a of electrode reaction was obtained.Sn₃Yb and SnYb alloys were obtained by galvanostatic electrolysis at-0.015 A for12 h,and only Sn₃Yb phase was obtained by potentiostatic electrolysis at-1.3 V,-1.4 V and-1.45 V for 20 h,respectively.6.The reduction extraction of Ln?Ce,Sm?in molten salt LiCl-KCl-LnCl₃/liquid Bi-Li system was studied.The concentration and valence of Ln???ions were monitored by electrochemical method.?1?The effects of Li content in Bi-Li alloys,temperature and stirring speed on extraction rates,efficiencies and distribution ratios for Ce and Sm were determined,and found that the extraction rates,efficiencies and distribution ratios for Ce and Sm increase with improving the temperature,the content of Li in Bi-Li alloys and stirring speed.The extraction efficiency of Ce was much higher than that of Sm.The extraction efficiency of Ce?Sm?in single Ln component molten salt is higher than that in mixed component molten salt Ce?Sm?.The reduction extraction mechanism of Sm was studied by XPS and linear sweep voltammetry?LSV?.It was found that Sm?II?was produced in the extraction process.The extraction products were characterized by XRD and SEM-EDS.The results proved that Ce???,Sm???and Sm?II?were successfully reduced from molten salt to Ce and Sm and extracted into liquid Bi-Li alloy.?2?On-line monitoring the reduction extraction of Ln?Ce,Sm?was carried out in LiCl-KCl/Bi-Li system employing SWV,according to the calibration curve of the relationship between the concentration of Ln?Ce,Sm?ion and the peak current density of SWV at 673-873 K.The concentration of Ln???measured by calibration curve is consistent with that of ICP-AES,and the relative error is about±2.1%.The result showed that SWV is an effective way of on-line monitoring of Ln extraction in LiCl-KCl-LnCl₃/Bi-Li system.