Interpretation of the Extraction Mechanism of the Purex and Thorex Processes from Kinetics Data

The rate of transfer of species between phases in solvent extraction is expected to be controlled by chemical reaction kinetics, diffusion, or a combination of these steps. Transfer kinetics data for uranium(VI), thorium(IV), and zirconium(IV) indicate that the chemical reaction is the rate-controlling step for most dispersed systems. The evidence for this conclusion is: (1) the transfer rate constants for the extraction of uranium were the same for Lewis cell, single drop, and Kenics mixer tests; (2) the forward transfer rate constants increase with increasing TBP concentration (the viscosity of the organic phase also increases) in all cases; (3) the reverse transfer rate varies dramatically with changes in nitrate concentration in the aqueous phase (same phase viscosities); and (4) the difference in the activation energies of the forward and reverse reactions for uranium(VI) transfer is equal to the heat of reaction of uranium with TBP. A proposed mechanism for the solvent extraction kinetics which is consistent with the experimental results includes the formation or decomposition of a charged interfacial complex as the rate-controlling step and equilibration of the interfacial complex with the bulk organic complex as the rapid step.