Fluorination Of Uranium Oxide And Separation Of Uranium And Lanthanum By Molten Salt Electrolysis

As a kind of green energy,nuclear energy plays an important role in reducing carbon emissions in China.One of the challenges the nuclear industry faces today is the reprocessing of spent fuels.In order to make full use of the limited uranium resources,the dry reprocessing of spent fuel using molten salt electrolysis faces enormous challenges.Thus,in this work,UO₂and U₃O₈ were used as raw materials,and the fluorination of uranium oxide was explored.The electrochemical behaviors of UF₄ and La F₃ as well as the influence of F^-on the electrode process kinetics of La³⁺and U⁴⁺were studied in molten Li Cl-KCl salt.Meanwhile,the feasibility of electrochemical separation and extraction of U and La from molten salt were explored using Al as cathode.The details were as follows:（1）UO₂ was firstly prepared by thermal reduction of Al-Li alloy or Li metal using sodium uranate as the raw material.Then the fluorination reaction mechanism of UO₂ with NH₄HF₂was studied at 423 K,and the intermediate of NH₄UF₅ containing UF₄ was prepared,and NH₄UF₅ was completely decomposed to form UF₄ in the Li Cl-KCl eutectic system at 613 K.The reaction mechanism of UO₂ and NH₄HF₂ was confirmed by the determination of the redox signal peaks of U⁴⁺and U³⁺employing electrochemical methods:UO₂（s）+NH₄HF₂（s）→NH₄UF₅（s）→UF₄（s）+NH₃（g）+HF（g）.At the same time,the fluorination reaction of U₃O₈ and NH₄F was also studied at 973 K.The reaction feasibility of U₃O₈ and NH₄F was analyzed by thermodynamic calculation.The products were characterized by XRD and Raman spectroscopy,and the fluorination mechanism of U₃O₈ and NH₄F was determined:U₃O₈（s）+NH₄F（s）→UO₂F₂（s）→UF₄（s）and U₃O₈（s）+NH₄F（s）→UO₂（s）→UF₄（s）.（2）The electrochemical behaviors of La F₃ and UF₄ on W electrode were studied by electrochemical methods.La³⁺was reduced to metal La through one-step three-electron transfer,U⁴⁺proceeded in two-step one electron process,namely U⁴⁺→U³⁺and U³⁺→U,and the signal peak of UO₂→U was also detected.Therein,the reduction of La³⁺to La is a soluble/insoluble quasi-reversible reaction controlled by diffusion,the reduction of U⁴⁺to U³⁺a soluble/soluble reversible reaction,and the reduction of U³⁺to U soluble/insoluble quasi-reversible reaction controlled by diffusion.The effects of F^-concentration on electrode process of U and La were investigated.As the concentration of F^-gradually increased,the diffusion coefficient of La³⁺decreased and the activation energy of diffusion increased and the exchange current density of U³⁺/U pair decreases due to the complexes formation of[La Cl₅F]^3-,[La Cl₄F₂]^3-and[La Cl₄F₃]^4-as well as[UF_xCl_6-x]^2-.The electrochemical signal of[UF_xCl_6-x]^2-complex was also detected.The precipitate deposit was analyzed by XRD and XPS,and found to be Li F and K₂UCl₅.（3）The electrochemical behaviors of U⁴⁺and La³⁺were studied on Al film electrode,and the under-potential deposition of U⁴⁺and La³⁺was found on Al film electrode.According to the open-circuit chronopotential curves of U⁴⁺and La³⁺recorded on Al film electrode,combined with Al-U and Al-La alloy phase diagrams,Al-U alloys of Al₄U,Al₃U,Al₂U and Al-La alloys of Al₁₁La₃,Al₃La,Al₂La,Al La,Al La₃ were formed.The thermodynamics of Al_mU and Al_mLa_nwere calculated at 773 K.The electrochemical behaviors of U⁴⁺and La³⁺were studied,and the under-potential deposition of U⁴⁺and La³⁺were found on Al electrode.Furthermore,the extracting U and La from molten salt was conducted using reactive Al electrode by constant potential electrolysis,respectively.XRD result showed that Al₃U and Al₄U and Al₁₁La₃ were obtained,respectively.Constant potential electrolysis was also carried out in molten salt containing U⁴⁺and La³⁺.The obtained alloy was characterized by XRD to be Al₃U,and the extraction rate of U was 93.25%,which realized the separation of U and La.