Thorium compounds can be extracted from aqueous solution by many of the immiscible organic solvents that have been used for extraction of uranium (Table 5.14). As with uranium, tributyl phosphate (TBP) is now the universal choice for extracting thorium from aqueous nitric acid solutions and for purifying thorium compounds by solvent extraction. However, in sulfuric acid solutions of thorium compounds or in solutions containing phosphoric acid such as are obtained from acid leaching of monazite, thorium is too strongly complexed to be readily extracted by TBP. To extract thorium from such solutions, processes using other organo — phosphorus compounds or organic amines have been developed, just as they have for uranium. Audsley and co-workers [Al] conducted pilot-plant experiments on extraction of thorium from solutions simulating the composition of sulfuric acid leach liquors from Canadian uranium ores after removal of uranium. They found that di(2-ethylhexyl)phosphoric acid, the solvent used in the Dapex process (Chap. 5, Sec. 8.6), was a satisfactory extractant, provided that ferric iron in the leach liquor (which would extract with thorium) was first reduced to ferrous by reaction with iron filings.

Because of the complications introduced by ferric iron, Audsley et al. concluded that a solvent that would be selective for thorium in the presence of ferric iron and that would not be inhibited by phosphate would be preferable to the Dapex solvent. They concurred with the conclusion of Crouse and co-workers at Oak Ridge National Laboratory [C5], that long-chain primary amines selectively extract thorium in the presence of uranyl, ferric, and phosphate ions. Compounds of this type are now the preferred extractant for thorium in such systems. Application of this so-called Amex process to thorium extraction from monazite is described in Sec. 8.6.