High Molecular Weight Pyridine Amines as Solvents for Uranium(VI) and Its Separation from Thorium

Three high-molecular-weight pyridine amines, 5-(4-pyridyl)nonane, 2-hexylpyridine, and diphenyl-2-pyridylmethane, have been studied as components of solvent extraction systems. Results are presented to show the dependence of the uranium extraction coefficient (D a o = [U org ]/[U aq ]) on equilibrium concentrations of hydrochloric, nitric, and sulfuric acid solutions with and without thiocyanate ions. The optimal conditions for the extraction have been carefully selected from an extensive and critical investigation of the various factors involved; e.g., the effects of diluents, concentration of the mineral acids, thiocyanate ions, salting and complexing agents, and the concentration of the solvents. The extraction mechanism and composition of the extracted complexes of uranium have been studied from partition and slope-analysis data. The results obtained give an orderly picture of the mechanism of extraction of uranium thiocyanate complexes (partly in relation to the hydration and solvation of the compounds extracted). Anomalous extraction behavior was observed at solvent concentrations greater than 0.05 M. The results have been interpreted on the basis of the formation of micelles of the salt molecules of the solvents. It has been shown that these pyridines will extract the metal efficiently and reversibly from dilute acid chloride, nitrate, and sulfate solutions containing thiocyanate. Common salts have no depressing effect on extraction. Distribution coefficients and separation factors of several metal ions, with respect to uranium(VI), are reported for the three mineral acid systems, and a method for the separation of thorium-234 from uranium is also described.