Solvent extraction chemistry and kinetics of zirconium

The solvent extraction behavior of zirconium in the HNO₃- tributyl phosphate (TBP) system can be explained based on the existence of four principal aqueous species, Zr⁴⁺, ZrOH³⁺, Zr₃(OH)⁸⁺ ₄, and oxo-polymers. The Zr⁴⁺ and Zr₃(OH)³⁺ species are extractable and are in equilibrium with inextractable Zr₃(OH)⁸⁺ ₄. The oxo-polymers are formed by heat, are inextractable, and are not in equilibrium with the other species. The aqueous equilibria and their equilibrium quotients have been previously determined. In the present study, these equilibria were used along with both tracer and macro zirconium concentrations (oxo-polymers excluded by extraction and back scrubbing) to determine the distribution equilibrium constants for both the Zr⁴⁺ and ZrOH³⁺ ions. The four equilibrium constants give excellent fits to both tracer and macro-zirconium distribution data. The concentrations of the extractable zirconium species which are calculated from the equilibria have been used to begin examining the extraction kinetics of zirconium in the HNO3-TBP system. In relatively concentrated nitric acid, approximately 3 M and greater, Zr⁴⁺ ion predominates, and the rate of extraction of zirconium increases as approximately the second power of the TBP concentration. In low acid (1 M and less) ZrOH³⁺ ion predominates, and the rate of extraction of zirconium increases as approximately the third power of the nitrate concentration. This is in significant contrast with the behavior of uranium, which shows only a small dependence of the extraction rate on TBP concentration, and no dependence on nitrate concentration. This suggests that operation of a kinetic separations system at low TBP and nitrate concentrations will significantly improve separations over those achieved at equilibrium.