Applied thermodynamics in nuclear industry modelling of precipitation processes
Résumé
The driving force of precipitation reactions is given by supersaturation, which characterizes the deviation from the equilibrium state. Precipitation mechanisms, such as nucleation, crystal growth or agglomeration, are governed by supersaturation, which is therefore a key parameter for process control. In nuclear industry, the solutions usually involve many salts with constituting ions of high valence, which leads to strong ionic strengths. The deviation from thermodynamic ideality has therefore to be taken into account to determine the thermodynamic properties of solutions. The activity coefficients are then required to predict the thermodynamic driving force for precipitation processes. This work focuses on the description of the thermodynamic studies of precipitations operating in nuclear industry neodymium oxalate and uranium peroxide precipitations. Oxalates are characterized by very low solubility so that oxalate precipitation is widely used in the rare earth and/or actinides separation, in both analytical chemistry and treatment of high-level liquid wastes, mainly in nitric acid medium. Regarding to uranium peroxide precipitation, it is involved in the processing of ores to recover uranium from purified leach solution as yellow cake.
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