Kinetics of Novel Extraction Systems Used in the Partitioning of Nuclear Waste

Both 2,6-di(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (n-Pr-BTP), and a synergistic mixture of di(chlorophenyl)dithiophosphinic acid and tri-n-octyl phoshine oxide, are able to selectively extract trivalent actinides over lanthanides from 0.5-1 M nitric acid. We focused on clearly identifying the extraction regime (chemical or diffusional) of these extraction systems. The dependency of extraction rates from stirring speed was measured in our special stirred cell. The n-Pr-BTP system showed extraction rates independent from stirring speed. This reveals that the mass transfer is controlled by a slow chemical complexing reaction. On the other hand the synergistic mixture showed a linear relationship between fluxes and stirring rate indicating that mass transfer is controlled by diffusion. Furthermore extraction rates are about ten times higher than for the n-Pr-BTP system.