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  • 1 Tsinghua University Department of Chemical Engineering Beijing 100084 P. R. China
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In order to predict extraction equilibria for uranyl nitrate and nitric acid between aqueous and tributyl phosphate (TBP)-hydrocarbon diluent solutions, activity coefficient equations for the three components in the system HNO3-UO2(NO3)2-H2O were derived and the general equation for excess Gibbs energy, proposed by Clegg and Pitzer, simplified. The activity coefficient equations comprise a Debye-Hückel term and a Margules expansion carried out to the four suffix level, where the higher order electrostatic contribution was neglected. The binary parameter was determined from the thermodynamic properties of the two relevant aqueous solutions. The three mixing parameters were obtained by correlating data for the partial pressure of nitric acid over HNO3-UO2(NO3)2-H2O solutions at 298.15 K. By using the mixing parameters, the activity coefficients of the ternary system can be calculated with good accuracy, and the solubility of uranyl nitrate in aqueous nitric acid with concentration up to 14 mol/kg can be satisfactorily predicted.