ИСТИНА

A problem of phosphogypsum utilization is of great importance these days. Although gypsum is a widely used material in the construction industry, phosphogypsum is usually not used mainly because of its weak radioactivity. Since long-term storage of phosphogypsum is no more favorable, the problem of removal of the radioactive contaminants (i.e. uranium and thorium compounds) from phosphogypsum is very vital. Several routes of purification of phosphogypsum are present, but still these processes are studied relatively poorly. To improve such technologies there is a need for information about thermodynamic properties and phase equilibria in the systems they deal with. To obtain required information thermodynamic modeling can be used. Many of the processes mentioned above have a prior interest in the UO22+, Ca2+, H+ // NO3-, H2O system because they require a nitric acid media. Current work is focused on this system either. The aim of the work is to develop a corresponding thermodynamic model. Logically, this work provides an extension to a previously reported thermodynamic description of the UO2(NO3) – HNO3 – H2O system. To represent thermodynamic properties of a liquid phase of the UO22+, Ca2+, H+ // NO3-, H2O system, Pitzer-Simonson-Clegg model was applied. Modeling was carried out stepwise with the number of components increasing gradually. Parameterization of the model was carried out by the method of least squares. The minimized objective function was the sum of squared residuals of calculated and experimental values (data on vapor-liquid and solid-liquid equilibria). Throughout the calculation gas phase considered to be ideal. The results of this work can be useful for the optimization of the complex processing of phosphogypsum. This work was financially supported by RFBR (№ 18-29-24167).