Application of Unified Model with TBP Dilution to Description of Extraction of Nitrates of Tetravalent Zirconium and Haf
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ication of Unified Model with TBP Dilution to Description of Extraction of Nitrates of Tetravalent Zirconium and Hafnium Actinides from Nitrate Solutions E. A. Puzikova,*, B. Ya. Zil’bermana, N. D. Goletskiia, I. V. Blazhevaa, and A. S. Kudinova a
Khlopin Radium Institute, St. Petersburg, 194021 Russia *e-mail: [email protected]
Received February 16, 2020; revised March 21, 2020; accepted March 22, 2020
Abstract—Published and authors’ own data have been used to create a unified mathematical model describing the distribution of uranyl nitrate, nitric acid, and tetravalent elements in TBP with consideration for its dilution in their distribution in neutral and acid systems in a wide range of component concentrations. As the base values of the concentration equilibrium constants were taken their values calculated for pure TBP. In this case, raising the diluent concentration leads to a linear dependence of the logarithms of the extraction constants on the TBP concentration with a parameter a the value of which reflects the influence exerted by the nature of a diluent. It is assumed that, simultaneously, there occur several complexation reactions in the aqueous and organic phases and a nonstoichiometric physicochemical interaction of the components. The values of the equilibrium constants of the extraction reactions and the corresponding parameters a were determined for tetravalent thorium, uranium, plutonium, neptunium, zirconium, and hafnium, which made it possible to describe their extraction with TBP solutions in a hydrocarbon diluent at concentration of the extractive agent of 5 to 100 vol %. The error in calculations of the distribution coefficients in terms of the model does not exceed 12%. Keywords: extraction, actinides(IV), zirconium, hafnium, nitric acid, tributyl phosphate, mathematical simulation DOI: 10.1134/S1066362220040062
In [1–3], a unified multireaction approach to describing the interphase equilibria involving tributyl phosphate (TBP) of any concentration in extraction of tri- and hexavalent elements from nitrate solutions was suggested. The unified model consists in that the distribution coefficients of components are related to their concentration, with an assumption made for describing the equilibrium that a component can be involved in several reactions occurring in parallel. As the base concentration of the extractive agent is taken pure TBP, and its dilution is taken into account by introduction of a linear dependence of the logarithm of the solvation constant on the TBP concentration, which enables a comparison of the effects of various diluents. The goal of the present study is to extend the unified model to description of extraction in TBP of various concentrations of tetravalent thorium, uranium, neptunium, plutonium, zirconium, and hafnium, including that in the presence of uranyl nitrate. The tetravalent actinides and zirconium are the conventional
participants of the Purex process and concentrated TBP solutions are also used to separate zirconium and hafnium in the industrial manufacture [4,
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