THE OXIDATION OF NEPTUNIUM(IV) BY NITRIC-ACID IN 100-PERCENT-TBP AND DILUTED TBP N-DODECANE SOLUTIONS

For the accurate computer simulation of the Purer process the kinetics of important chemical reactions occurring in both the aqueous and sol vent phases in the process must be included in the model. One importan t species in the Purer process is Np(IV), and so the kinetics of the o xidation of Np(IV) by HNO3, in TBP phases have been determined experim entally to aid the efficient modelling of Np(IV) in Purer process flow sheets. It was found that in 100% TBP solutions the oxidation is autoc atalytic, with a mechanism proceeding predominantly via Np(V) repropor tionation. The rate equation and rate constants have been determined f or both the non-catalysed and autocatalysed components. However, there are significant departures from these kinetics as the TBP solution is diluted by n-dodecane. These differences include the retardation of t he reaction and a change from an autocatalytic reaction to first-order kinetics, particularly at lower temperatures. Although this data is s uitable for inclusion into computer models, the slow reaction in 30% T BP solutions means that the solvent-phase oxidation of Np(TV) will be a minor effect. (C) 1998 Elsevier Science S.A.