Comparative Method of Calculating the Thermodynamic Parameters of Some Uranium Hexafluoride Solutions

It is shown that the thermodynamics characteristics of uranium hexafl uoride solutions with volatile hexaand pentafl uoride elements are a consequence of the strictly symmetric confi guration of their molecular structure in the gaseous and liquid states. Simple methods are proposed for evaluating the properties of binary mixtures of elemental penta- and hexafl uorides. These methods are based on a comparison of the element–fl uorine bond length in the constituent molecules of system and the known enthalpy of mixing of one solution with the potential energy in the formation of a pair of unlike molecules of a different solution. Researchers looking for a universal method of deep purifi cation of uranium hexafl uoride have turned their attention to a rectifi cation method, for which there are no limitations on the depth of purifi cation, and a list of impurities to be removed, including metal impurities which are diffi cult to remove by means of water-extraction conversions of uranium concentrates and give volatile fl uorides (molybdenum, tungsten, ruthenium, technetium, and others). Moreover, its specifi c productivity is high, the energy consumption is low, and the separation effi ciency is high. These advantages are due to the special properties of uranium hexafl uoride as a working substance for rectifi cation: high density of liquid (more than 3.5 times higher than the density of water) and gaseous uranium hexafl uoride, low viscosity comparable to that of water, good wettability of the metals which are corrosion resistant to fl uoride media, and low heat of evaporation of liquid uranium hexafl uoride (less than 1/10th that of water). Deep purifi cation of uranium hexafl uoride raw material by rectifi cation has been used in the Republic of South Africa for several decades now [1]. In our country, we have experience in prolonged operation of a large-tonnage rectifi cation process for purifying sublimate and high-enrichment uranium hexafl uoride [2–4]. In order to develop a rectifi cation process, it is necessary to have information on the phase equilibrium of mixtures based on uranium hexafl uoride and to predict the properties of systems with which it is diffi cult to experiment. Computer methods of calculating steam-water systems have now been developed and are being improved; examples are models of local compositions Wilson, NRTL, UNIQUAC, equations of state SRK, Peng Robinson, and the group model UNIFAC, TTA [5–7]. They are based on the equation of thermodynamic equilibrium between coexisting phases and are generalizations of the Van-der-Waals equation of phase equilibrium, expressed in terms of partial properties [8]. Some forms of this equation are identical to a previously used van-Laar equation expressing the dependence of the coeffi cient of activity of a component on the composition of the solution [9]. Such methods of calculating the liquid–vapor equilibrium in nonideal systems take account of the effect of variable temperature and are convenient for modeling mixtures with an appreciable difference of the boiling temperature of pure components, making it possible to obtain good agreement between the computed and experimental parameters for some solutions. The application of a comparative method for calculating the properties of pure substances and solutions and the characteristics of phase and chemical equilibria is illustrated in the monograph [10]. In the present work, a method is proposed for fi nding parameters that can be used to calculate the vapor–liquid equilibria of mixtures of elemental hexa- and pentafl uorides by means of a comparative method.