MODELING OF THE EXTRACTION OF NEODYMIUM AND PRASEODYMIUM NITRATES FROM AQUEOUS-SOLUTIONS CONTAINING A SALTING-OUT AGENT OR NITRIC-ACID BY TRI-N-BUTYLPHOSPHATE

A thermodynamic model that predicts the extraction of trivalent lantha nide (Pr, Nd) nitrates and water from aqueous solutions by tri-n-butyl phosphate (30, 50 and 100 vol%) in dodecane is presented. The Mikulin- Sergievskii-Dannus' model was used to predict the extraction isotherms of lanthanides (Ln) and water for Ln(NO3)(3)-H2O-TBP-dodecane system and Ln(NO3)(3)-NH4NO3-H2O-TBP-dodecane system. The parameters of the m odel were used in conjunction with those previously published for nitr ic acid extraction by TBP, to predict the extraction isotherms of lant hanides and nitric acid for Ln(NO3)(3)-HNO3-H2O-TBP 100 vol% system, T he stoichiometric mean activity coefficients of components in the aque ous phase were calculated from experimental data using Mikulin's equat ion, and those of the components in the organic phase were calculated by Sergievskii-Dannus' equation. A satisfactory description of the dis tribution of lanthanide nitrate and water is obtained over a wide rang e of lanthanide concentration in the aqueous phase by taking into acco unt the formation of the following complexes: TBP, TBP . H2O. (TBP)(3) . Nd(NO3)(3), (TBP)(4) . Pr(NO3)(3), (TBP)(3) . Pr(NO3)(3). It was sh own that in Ln(NO3)(3)-HNO3-W2O-TBP 100 vol% system, it is necessary t o consider an acid-complex, (TEP)(3) . HLn(NO3)(4), in order to obtain the best fit of the isotherms to the extraction data.