Thermodynamics Study On The Preparation Of U (Ⅳ) By U (Ⅵ) Catalyzed By Hydrazine In Nitric Acid

U (IV) is a widely used good Pu (IV) restore the stripping agent in Purex processcycle of spent-fuel reprocessing aimed to achieve decontamination of uranium andplutonium in total separatio. In a reprocessing plant, the U(IV) preparation methodsmainly include: sedimentation method, method of hydrogen, electrolysis,heterogeneous catalysis, and so on. In recent years, both at home and abroad onheterogeneous Catalysis with nitric acid as medium, hydrazine to restore stability, andPlatinum black catalyst, we undertook a preliminary study on the reaction ofhydrazine reduction catalyzed U(VI) to prepare U(IV). The results showed thatPlatinum-catalyzed hydrazine to reduce U(VI) is feasible under certain conditions.In order to get optimum conditions of Platinum-catalyzed hydrazine restoredU(VI) preparation of U(IV), accumulate and realize the basic data needed to processscale, it is necessary to get reaction heat, thermodynamics, kinetics, reactionmechanism and other related fundamental research in the course ofplatinum-catalyzed hydrazine to reduce U(VI) to prepare U(IV) in nitric acid medium.Through theoretical and experimental research, we got the following conclusion:(1)After researching the effects to reaction translation rate of nitrateconcentration, hydrazine concentration, catalyst dosage and the reaction temperaturein the reaction system of hydrazine catalytic reduction U(VI) to prepare U(IV), we gotbest technology conditions when initial U(VI) concentration is0.4mol/L: nitrateconcentration for0.56mol/L~0.78mol/L, hydrazine concentration for1.02mol/L,solid liquid rate for0.008kg/L, temperature for50℃.(2)In nitrate system, Platinum black for catalyst, hydrazine for reduction agent,under reaction liquid of technology conditions is initial uranium concentration0.9mol/L, initial hydrazine concentration1.0mol/L, initial nitrate concentration0.8mol/L, and Platinum black dosage0.2g/25mL, we derive the units volume theoryreaction hot calculation formula: Q=a△fHm,1Θ+△fHm,2Θ[C(N2H4)-C(UO22+)a/2-]/3,according to chemical formula of the reaction in the system. And the theoretical reaction heat during0.5h-4h was accumulated according to the above formula. Measured temperature difference at different moments of the reaction under the same conditions of40℃,50℃,60℃using the eslf-designed devise. The actual reaction heat can be achieved by the follow formula:Qt=[Cm1AT1+Cm2AT2]. The resuls of comparison of theoretical and actual reaction reaction heat show that the experimental and calculated values match in a hour.(3)For common uranium concentration ranging from0.21mol/L to0.79mol/L, we get kinetic rate equation:-d(UO22+)/dt=kC1.11(UO22-)C1.42(N2H5+)C-0.72(H+).When the solid-liquid ratio is0.008kg/L, temperature of50℃, the reaction rate constant is k=1.3×10-3(mol/L)-0.081/min. According to the Arrhenius equation, respectively, evaluated the diffusion-controlled reaction activation energy for10.42KJ/mol and dynamics control activation energy for24.7KJ/mol.(4) For the low-enriched uranium (0.021～0.079mol/L), under conditions of the temperature ranging between45℃～75℃, we obtain kinetic rate equation:-dC(UO22+)/dt=kC0.94(UO22+)C0.3(N2H5+)C-0.21(H+). when the solid-liquid ratio is0.008kg/L,Temperature of50℃, the reaction rate constant is k=2.89×10-2(mol/L)-0.03/min. According to the Arrhenius equation, respectively, evaluated the diffusion-controlled reaction activation energy for4.33KJ/mol and dynamics control activation energy for18.438KJ/mol.