Uranyl nitrate has an unusual property, shared only by nitrates of a few other actinides, of being very soluble in a number of organic solvents. When such an organic solvent is immiscible with water, it can be used in a solvent extraction process to extract uranium from aqueous solutions and separate it from associated impurities. Such applications of solvent extraction are very important in extracting and purifying uranium from leach solution of uranium ores or from nitric acid solution of irradiated nuclear fuel. Examples of extractants that have been used for such separation processes are listed in Table 5.14.

The ability of diethyl ether to extract uranyl nitrate from aqueous solution has been known for a hundred years and was the method chosen by the Manhattan Project to purify the uranium used in the first nuclear chain reactors. This solvent has numerous disadvantages. It is very volatile, very flammable, and toxic, and it requires addition of sodium, aluminum, or calcium nitrate to the aqueous phase to enhance extractions. When solvent extraction was first applied to recovery of uranium and plutonium from irradiated fuel, other oxygenated solvents less volatile than diethyl ether that were first used were methyl isobutyl ketone, dibutyl

carbinol, and triglycoldicliloride. They had the disadvantages either of reacting with nitric acid or of requiring addition of solid salting agents. For solvent extraction of irradiated fuel, these have all been superseded by tributyl phosphate (TBP), in the Purex process described in Chap. 10. TBP has the advantage of being able to extract uranium efficiently from nitric acid solution without addition of solid nitrates. TBP is also used to purify natural uranium (Sec. 9.2 of this chapter).

The five solvents just discussed extract uranium in the form of neutral complexes of uranyl nitrate. With TBP the complex-forming equilibrium is

U02(N03)2 -6HsO(aq) + 2TBP(o) — U02(N03)2 -2TBP(o) + 6H20(aq)

where (aq) denotes the aqueous phase and (o) the organic.

The last two solvents in Table 5.14 are sometimes called liquid ion-exchangers because they react with water-soluble uranium-bearing ions to form organic-soluble compounds. Di(2- ethylhexyl) phosphoric acid is an example of a liquid cation exchanger, which acts through the equilibrium

О О

Ч, II

гССвНпО^РОНСо) + U022*(aq) * [(C8H170)2P0]2U02(o) + 2H+(aq)

Because of the long octyl group, both the acid and the uranyl salt are soluble in a hydrocarbon diluent and insoluble in water.

Trioctylamine is an example of a liquid anion-exchanger, which acts through the equi­librium

2[(C8H17)3NH]2S04(o) +U02(S04)34′(«?)- [C8H17NH]4U02(S04)3(o) + 2S04[16] [17] [18]-(«?)

Because of the long octyl group, both the amine and the uranyl sulfate complex are soluble in a hydrocarbon diluent and insoluble in water.

These liquid ion-exchangers have two advantages over TBP for extraction of uranium from leach liquors. Distribution coefficients are higher, so that uranium may be extracted at higher concentration from dilute leach liquors. The involvement of hydrogen or sulfate ion in the distribution equilibrium makes it possible to drive the reaction to favor either the organic or aqueous phase by adjusting the H2S04 concentration of the aqueous phase. Clegg and Foley [Cl], Merritt [М3], and Brown et al. [B8] describe many other long-chain amines and organophosphorus compounds that have been used to extract uranium from leach liquors. These may be used either for the hexavalent uranyl or the tetravalent uranous ion.