Thorium third-phase formation

Suresh et al. investigated the extraction of uranium and thorium by TsBP and TiBP (isomers of TBP with branched carbon chain) as an alternative choice for TBP (47). Higher homologues of TBP, for example, THP and TEHP, were reported to have higher extraction ability with reduced tendency toward third-phase formation (50, 51). The esters with bulkier substituents in place of the butyl group were proposed to be of practical value for the process applications in uranium and thorium separation (54). The LOC of thorium in equilibrium with aqueous nitric acid-thorium nitrate was reported to decrease in the order THP > TAP > TBP. Pathak et al. showed that TEHP can be a better choice for U/Th separation compared to TBP and TsBP (55). 

Since all sources of thorium are associated with a certain amount of uranium, the first solvent-extraction cycle is designed solely to eliminate this element. The proportion of uranium to thorium can vary, but is often of the order of 5 per cent. The uranium distribution coefficient into tributyl phosphate is much higher than that of thorium under comparable conditions, e.g. 20 and 0-5 respectively for 40 per cent TBP/xylene or 6 and 0 04 respectively for 5 per cent TBP/xylene. The more highly diluted solvent is used for the uranium separation cycle in view of the higher ratio of the two distribution coefficients, or separation factor . Xylene is chosen in preference to, for example, odourless kerosene owing to the danger of formation of a third phase rich in thorium with the latter diluent. 

The U.K. process has been tested by Fareedudin et alJ in India and a similar degree of purification obtained. It was found possible to use kerosene as the diluent in place of xylene and the formation of a third phase was avoided by maintaining the thorium concentration in the solvent below 70-80 g Th02/1. 

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