Extraction studies of uranium into a third-phase of thorium nitrate employing tributyl phosphate and N,N-dihexyl octanamide as extractants in different diluents

Extraction behavior of 1 x 10(-2)-0.1 M U(VI) from aqueous phases containing 0.86 M Th(IV) at 4 M HNO(3) in 1.1 M tributyl phosphate (TBP) and 1.1 M N,N-dihexyl octanamide (DHOA) solutions in different diluents viz. n-dodecane, 10% 1-octanol + n-dodecane, and decahydronaphthalene (decalin) was studied. Third-phase formation was observed in both the extractants using n-dodecane as diluent. There was a gradual decrease in Th(IV) concentration in the third-phase (heavy organic phase, HOP) with increased aqueous U(VI) concentration [0.71 M (no U(VI))-0.61 M (0.1 M U(VI)) for TBP; 0.27 M (no U(VI))-0.22 M (0.1 M U(VI)) for DHOA]. The HOP volume in case of DHOA was similar to 2.2 times of that of TBP. Uranium concentration in HOP increased with its initial concentration in the aqueous phase [from 1.8 x 10(-2) M (0.01 M U(VI))-0.162 M (0.1 M U(VI)) for TBP; from 1.4 x 10(-2) M (0.01 M U(VI))-0.14 M (0.1 M U(VI)) for DHOA] suggesting that Th(IV) was being replaced by U(VI). An empirical correlation was developed for predicting the concentrations of uranium and thorium in HOP for both the extractants. No third-phase appeared during the extraction of uranium and thorium from the aqueous phases employing 10% 1-octanol + n-dodecane, or decalin as diluents, and therefore, were better choices as diluent for alleviating the third-phase formation during the reprocessing of spent thorium based fuels, and for the recovery of thorium from high-level waste solutions.