Nucleation kinetics of reactive crystallization of ammonium di-uranate

Abstract

ABSTRACT The reactive crystallisation of ammonium di-uranate (NH4U2O7) from uranyl nitrate (UO2 (NO3)2) and ammonia (NH3) is a key process in the nuclear fuel cycle towards uranium-bearing fuel preparation. However, the process kinetics and mechanism remain poorly understood and the modelling of the reactive crystallisation of NH4U2O7(ADU) is not available. Hence, this work aims to determine the kinetics and mechanisms of the nucleation and growth of ADU through induction time measurements by comparing sonochemical and conventional precipitation methods. Induction was measured as functions of initial supersaturation starting from S = 1.07 to S = 1.875 and in a temperature range of 30 °C to 80 °C. The solubility data of ADU were also found experimentally. The trends in the experimental data suggest that two nucleation mechanisms co-exist: heterogeneous and homogeneous nucleation at low and high supersaturations respectively. Due to acoustic cavitation of the ultrasound prompting micromixing, the induction time is markedly reduced in the sonochemical route owing to the reduction of the energy barrier for nucleation. The induction time data were also analysed to calculate the interfacial energy (γ) and other parameters including the radius of the critical nucleus (r*), critical free energy of the nucleus (ΔG*) and nucleation rate (J). Determination of nucleation kinetics gives us insights into the fundamentals of the ADU crystallisation process for control and optimisation. Highlights: Nucleation kinetics of the reactive crystallisation of ADU was studied through classical nucleation theory Induction time data with supersaturation are interpreted by classical nucleation theory and different crystallisation kinetic parameters were determined Homogeneous and heterogeneous nucleation was determined from the correlation A comparison of sonochemical and conventional precipitation shows that in the sonochemical route, the nucleation happens at lower supersaturation with lower induction time because of much lower activation energy requirement All the model data gives us an insight into the crystallisation kinetics to optimise and control the crystallisation process of ADU