Study On Decomposition And Solution Combustion Characteristics And Reaction Kinetics Of Uranyl Nitrate

The thermal decomposition of uranyl nitrate（UNH）,as the spent fuel acid treatment product,into uranium oxides（UO₃,U₃O₈,UO₂）is an effective high-value utilization method.Preparation of uranium oxides with high specific surface area is of great significance for the subsequent sintering of UO₂.Meanwhile,the study of thermal decomposition of UNH products and the decomposition dynamics can also supports the development of the flame decomposition process at present.In this work,the effects of thermal decomposition parameters of UNH via thermal decomposition and solution combustion method,and reaction atmosphere on thermal decomposition product is investigated,and the decomposition mechanism of UNH is also explored.Firstly,in view of the influencing factors of UNH in thermal decomposition,the thermal decomposition of UNH was studied via TG-DSC technology.The results show that powdered samples was less affected by the mass transfer process during the thermal decomposition of UNH.When the heating rate increases,the UNH decomposition temperature range increases,and the decomposition time decreases.The decomposition under both inert gas N₂ atmosphere and air atmosphere,the deoxygenated process was inhibited,which was mainly caused by the product of water vapor.The formation of NO and O₂ has little effect on the thermal decomposition.The products were characterized by XRD,SEM,BET and XPS.It was found that the products at the temperature range from 300 to 800℃ under both N₂ and air atmosphere are UO₃ and U₃O₈,and the products under H₂ flow was changed to UO₃ and UO₂.The U species in U₃O₈ was consistent of U（IV）,U（V）and U（VI）valence,the higher proportion of U（V）facilitated reduction of the as-obtained product at low temperature.Among them,the specific surface area of the product obtained under the air atmosphere is relatively smaller.Next,the products obtained by thermal decomposition of UNH have small specific surface area and low activity.The UNH decomposition products were explored by adding combustion agents,and the effects of UNH on the decomposition and the properties of the products were systematically studied with different combustion agents and different proportions of combustion agents.The results show that the combustion process,intensity and duration are obviously different under different conditions.The crystal types of decomposition products obtained under different combustion agents are different.The uranium oxide obtained with citric acid as combustion agent has UO₂ phase.Different preparation methods can be reduced to UO₂ in the mixture of hydrogen and argon at 600℃.When glycine is used as the combustion agent and the F/O ratio is 1,the uranium oxide obtained is U₃O₈.At this time,the specific surface area of the product after reduction reaches 21.16 m²/g.Finally,in order to better optimize the thermal decomposition process of UNH under two atmospheres of N₂ and air,the Kissinger-Akahira-Sunose（KAS）method,Flynn-Wall-Ozawa（FWO）method,Crane method and Caoats-Redfem method were used for.The apparent kinetics of the mixing reaction at each stage of decomposition were calculated.Before the calculation,the decomposition process of each stage was first studied by TG-DSC.The results show that the thermal decomposition process of UNH both in N₂ and air atmosphere can be divided into five stages,the first stage to the third stage is dehydration process,the fourth stage is dehydration and denitration process,the fifth stage is deoxidation process.The five stages correspond to different decomposition activation energy,reaction order,pre-exponential factor and decomposition kinetics equation.The activation energies of the first three steps of dehydration under empty atmosphere were 75.49,81.97 and 121.60 kJ·mol^-1,respectively,and the fourth step was 114.72 kJ·mol^-1 under N₂ atmosphere.