Preparation Of UF₄ And Its Electrochemical Behavior In LiCl-KCl Molten Salt

The reprocessing of spent fuel is the key to restricting the sustainable development of nuclear energy.At present,the molten salt electrolytic refining technology,having many advantages,is regarded as the most promising technology for reprocessing of spent fuel.The spent metal fuel still contains a large amount of unburned fuel,wherein,uranium content is about 96 wt.%.In order to improve the overall utilization rate of uranium,uranium in spent fuel must be separated and recycled.In this work,anhydrous UF₄ were prepared by dry and wet methods using high-value uranium compounds as raw materials.Subsequently,the electrochemical behavior of uranium ions in LiCl-KCl molten salt and the on-line monitoring of the electrolytic extraction of uranium were studied by a variety of electrochemical methods.The details are as follows:（1）UF₄ was prepared by dry reprocessing.The high-valent uranium compounds（UO₃,U₃O₈,UO₂F₂,UO₂Cl₂,Na₂U₂O₇）were reduced to UO₂ by Al or Al-Li alloy.It was found that Al-Li alloy was more suitable as a reducing agent than Al,and the above five compounds were completely converted into pure UO₂.Anhydrous UF₄ product was then obtained by fluorination of UO₂ with NH₄HF₂.The reaction mechanism of fluorination process was UO₂→（NH₄）₂UF₆→NH₄U₃F₁₃→UF₄.And the optimal fluorination conditions were obtained.The total yields of UF₄ prepared by UO₃,U₃O₈,Na₂U₂O₇,UO₂F₂ or UO₂Cl₂ were 82.7%,83.5%,82.3%,85.3%,86.5%,respectively.UF₄ was prepared by hydrometallurgy reprocessing.A certain amount of UO₃ was dissolved in hydrochloric acid and then SnCl₂ and hydrofluoric acid were added.The XRD characterization of the product was UF4·1.5H2O,and the reaction process was UO3→UO₂²⁺→U⁴⁺→UF₄·1.5H₂O.However,after UO₃ was dissolved in hydrochloric acid introduced withNH₄⁺,SnCl₂ and hydrofluoric acid were added.The XRD characterization of the product was the mixture of（NH₄）₂UF₆ and NH₄U₃F₁₃.The mixture was further heated at 723K for 1 h,and anhydrous UF₄ was gained.The total yield could reach 95.1%.（2）The reduction process of U（Ⅳ）in molten LiCl-KCl-UF₄ on W electrode was determined to be U（Ⅳ）→U（Ⅲ）→U（0）by a series of electrochemical methods.The electrode reaction of U（Ⅳ）reduction to U（Ⅲ）was a reversible reaction controlled by diffusion,while the electrode reaction of U（Ⅲ）reduction to U（0）was a quasi-reversible reaction controlled by diffusion.The diffusion coefficients of U（Ⅳ）in LiCl-KCl molten salt at different temperatures were measured,and the diffusion activation energy of U（Ⅳ）was calculated as21.40 k J mol^-1.The nucleation mechanism of uranium on W electrode was found to be continuous nucleation using chronoamperometry.In addition,the activation energy of U（Ⅳ）/U（Ⅲ）electrode reaction was determined as 40.65 k J mol^-1 by cyclic voltammetry.The kinetic properties of U（Ⅲ）/U（0）electrode reaction were determined by linear polarization and Tafel curves,and the activation energies were calculated to be 13.03 k J mol^-1 and 14.25 k J mol^-1,respectively.（3）The electrode reaction of uranium ion on Al film electrode was studied by various electrochemical methods.It was found that the underpotential deposition of uranium on Al electrode owing to the formation of Al-U alloy through two steps:U（Ⅳ）+e^-→U（Ⅲ）+Al→Al_xU.The Al-U alloy was prepared by electrochemical reaction on Al electrode.The results of XRD and SEM-EDS showed the Al₄U and Al₃U were formed.The concentration change of uranium ion was online monitored,during the electrolysis of uranium extraction on Al electrode.The extraction efficiency of U could reach 92.7%and the average extraction rate 1.23×10^-5 mol cm^-3 h^-1 after electrolytic extraction for 7 h.