EXTRACTION AND COEXTRACTION OF Tc(VII), Zr(IV), Np(IV.VI), Pa(V) AND Nb(V) WITH TRIBUTYL PHOSPHATE FROM NITRIC ACID SOLUTIONS

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Abstract
Distribution equilibria of Np(IV,VI), Tc(VII), Zr(IV), Pa(V) and Nb(V) were studied in a system involving a 30 vol.% solution of tributyl phosphate (TBP) In dodecane as a solvent. The distribution of Np(IV) and Np(VI) was measured at 25 and 50°C. Combined with published distribution data, the results made it possible to give a general picture of Np extraction under the conditions of the Purex process. The distribution ratios of Np(IV) and Np(VI) are presented as functions of the concentrations of nitric acid and U(VI) in the aqueous phase and as a function of temperature. The extraction of Tc(VII) and Zr(IV) and, to a lesser extent, the extraction of Pa(V) and Nb(V), was studied at 25 and 55°C. Tc(VII) is strongly coextracted with Zr(IV) and is moderately coextracted with U(VI), due to the formation of the mixed complexes Zr(TcO4)(NO3)2(TBP)2 and UO2(TcO4)(NO2)(TBP)2 respectively. Equilibrium concentration quotients of the coextraction reactions were determined and a semi-empirical mathematical description of the coextraction equilibria was suggested. Nb(V) is coextracted with Zr(IV) at > 5 M nitric acid, but not at lower acid concentrations. The extraction of Pa is anomalously enhanced in the presence of ammonium hexanitratocerate(IV).

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