Uranium, distribution coefficient

Goishi and Libby have investigated the extraction of pertechnetate from alkali solutions with pyridine. Later work showed that a better extraction is obtained using a mixture of sodium hydroxide and sodium carbonate as the aqueous phase. Since the uranyl carbonate complex is not extracted into pyridine, this system may be used for the separation of technetium from uranium. Distribution coefficients of fission products in pyridine are given in Table 4. Substituted pyridine such as 2,4-dimethylpyridine or 4-(5-nonyl)pyridine ) are useful for separating technetium from solutions containing appreciable amounts of aluminum nitrate. 

A comparison of the effect of the structure of phosphate esters on uranium extraction from nitrate media shows that the esters from secondary alcohols give higher uranium distribution coefficients (D s) than those from primary alcohols, phenyl esters extract uranium less strongly than alkyl esters, and benzyl esters are intermediate in extractant strength for uranium (24). 

Table 4.3 gives distribution coefficients for uranyi nitrate between aqueous nitric acid and 40 percent TBP in kerosene observed by Goldschmidt et al. [G3]. At each nitric acid concentration, the uranium distribution coefficient decreases with increasing uranium concentration. This can be attributed to the following overall reaction equilibria [M2] ... 

Assuming that at equilibrium all aqueous uranium is in the form of uranyl ion and all organic uranium is in the form of the U02(N03)2"2TBP complex, the uranium distribution coefficient is... 

The TBP concentration appearing in this equation is that of uncombined TBP. For a given total amount of TBP in the organic phase, the uncombined TBP is lower the higher the concentration of uranium, and the uranium distribution coefficient should decrease as the uranium concentration increases. This is confirmed by the experimental data in Table 4.3. 

Uranium distribution coefficients calculated from Eq. (4.23) and from the above data are listed in Table 4.3. 

Uranium extraction by TBP may in some cases become poorer in the presence of other extractable components because of depletion of free TBP by components other than uranium. Such behavior is illustrated by the U02(N03)2-HN03-TBP system analyzed above, as shown by the data [Ml ] in Fig. 4.7 for the distribution coefficient of uranium as affected by nitric acid concentration. For acid concentration less than about 5 M, the uranium distribution coefficient is greater the higher the acid concentration, because of the salting effect of nitrate ion from the acid. At acid concentrations greater than about S M, increasing acid concentration inhibits uranium extraction, because enough nitric acid has been extracted so that less free TBP is available to form the extractable complex with uranium. Smilar effects have been observed in the extraction of other elements with TBP [Ml]. 

The uranium distribution coefficient decreases with increasing S04 concentration, as shown in Fig. 5.8, owing to reversal of the preceding equilibrium. This permits stripping uranium from the amine by aqueous sulfate solution, as practiced at the Exxon miU, Table 5.19. The Kerr-McGee mill strips with 1.5 M NaQ solution by the reaction... 

Figure 5.8 Variation of uranium distribution coefficient in 0.1 M tri-n-octylamine with aqueous sulfate concentration. pH = 1 0.01 mol uranium/ liter in solvent.

The next step in purification is separation of uranyl nitrate from the other metallic impurities in the dissolver solution by solvent extraction. Practically aU uranium refineries now use as solvent tributyl phosphate (TBP) dissolved in an inert hydrocarbon diluent. The first U.S. refinery used diethyl ether as solvent and later refineries have used methyl isobutyl ketone or organic amines, but practically all have now adopted TBP. It is nonvolatile, chemically stable, selective for uranium, and has a uranium distribution coefficient greater than unity when the aqueous phase contains nitric acid or inorganic nitrates. 

Amine Phase ratio, aqueous/organic Uranium distribution coefficient... 

The extracting section is usually run at or near room temperature, to reduce solvent degradation and because the uranium distribution coefficient is higher the lower the temperature. It has been found advantageous to operate the scrubbing section at higher temperature, around 60°C, primarily because decontamination of ruthenium is more complete at higher temperature. 

Uranium stripping. Uranium in solvent leaving the IB column is stripped into the aqueous phase by counterflowing 0.01 M HNO3 in the 1C column. This is run at 50°C to reduce the uranium distribution coefficient. 

A similar batch ether process was operated on a scale of 1 ton/day by the U.S. Mallinckrodt Works as early as 1942. It was, however, modified in 1946 to a counter-current acid ether process using 40 ft high columns of about four theoretical stages. These columns had centrifugal pumps and jet mixers at 4 ft intervals, and relatively calm zones between. Advantage was taken of the increased uranium distribution coefficient in the presence of excess nitric acid, to reduce the solvent-to-aqueous fiow ratio and achieve a... 

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. 

These variations permit the separation of other components, if desired. Additional data on uranium, plutonium, and nitric acid distribution coefficients as a function of TBP concentration, solvent saturation, and salting strength are available (24,25). Algorithms have also been developed for the prediction of fission product distributions in the PUREX process (23). 

The effect of irradiation on the extractability of sulfoxides towards plutonium, uranium and some fission products were studied by Subramanian and coworkers . They studied mainly the effect of irradiation on dihexyl sulfoxide (DHSO) and found that irradiation did not change the distribution coefficient for Ru, Eu and Ce but increases the distribution coefficient for Zr and Pu. When comparing DHSO and tributyl phosphate (TBP), the usual solvent for the recovery and purification of plutonium and uranium from spent nuclear fuels, the effect of irradiation to deteriorate the extraction capability is much larger in TBP. Lan and coworkers studied diphenyl sulfoxides as protectors for the gamma radiolysis of TBP. It was found that diphenyl sulfoxide can accept energy from two different kinds of excited TBP and thus inhibits the decomposition of the latter. 

One of the first bed materials was based on the extractant diamyl amylphosphonate (DAAP marketed under the name U-TEVA-Spec ) and was designed for purification of the tetravalent actinides (U (IV), Th (IV), Pu (IV)) and hexavalent uranium (U(VI)). This material is characterized by high (>10-100) distribution coefficients for U and Th in significant (>3 M) concentrations of both nitric and hydrochloric acids, and so is useful for both U and Th purification (Horwitz et al. 1992 Goldstein et al. 1997 Eikenberg et al. 2001a). 

An additional material based on the extractant octyl-phenyl-N,N-diisobutyl-carbamoylmethylphosphine oxide, or CMPO, (marketed under the name TRU-Spec) has also been widely utilized for separations of transuranic actinides (Horwitz et al. 1993a) but is also useful for uranium-series separations (e.g., Burnett and Yeh 1995 Luo et al. 1997 Bourdon et al. 1999 Layne and Sims 2000). This material has even greater distribution coefficients for the uranium-series elements U (>1000), Th (>10000), and Pa. As shown in Figure 1, use of this material allows for sequential separations of Ra, Th, U, and Pa from a single aliquot on a single column. Separations of protactinium using this material (Bourdon et al. 1999) provide an alternative to liquid-liquid extractions documented in Pickett et al. (1994). 

Payne XE, Edis R, Fenton BR, Waite XD (2001) Comparison of laboratory nraninm sorption data with in situ distribution coefficients at the Koongarra uranium deposit. Northern Australia. J Environ Radioact 57 35-55... 

Numerous independent cost studies [170] have shown that for a sorbent to be acceptable, it must be able to concentrate the initial uranium feed upon contact by a factor (see distribution coefficient, Eq. (4)) of 10 -10. Such enrichment of the uranium by the sorbent corresponds to a uranium capacity of 0.3-5.0 mg per gram of dry resin [171]. 

Payne T. E., Eenton B. R., and Waite T. D. (2001) Comparison of in-situ distribution coefficients with experimental Rd values for uranium (VI) in the Koongarra Weathered Zone. In Surface Complexation Modeling of Uranium (VI) Adsorption on Natural Mineral Assemblages (ed. J. A. Davis). Nuclear Regulatory Commission, Washington, DC, pp. 133-142. 

Uranium(VI) and plutonium(IV) are separated from each other and from fission products by TBP, although phosphonates have better distribution coefficients than phosphates for Th and Carbamoylmethylphosphoryl (CMP) extractants are... 

Equal volumes (20 ml) of the TOPO solution in the organic solvent and uranyl sulphate solution containing sulphuric acid were shaken for 10 min in 50 ml stoppered conical flasks in a thermostatic water-bath at the required temperature. Preliminary experiments showed that equilibration is complete in 10 min. The mixture was centrifuged and separated, and uranium was stripped from the organic phase with 0.5 M ammonium carbonate solution, and then the distribution coefficient (the ratio of the equilibrium concentration of uranium in the organic phase to that in the aqueous phase, [U]org/[U]aq) was obtained. 

Influence of oxidizing reagents on uranium (VI) and americium distribution coefficients. 

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