11.66. The solvent-extraction method for separating the constituents of an aqueous solution can be used when one or more of these constituents are appreciably soluble, whereas the others are much less soluble, in an organic solvent which is essentially immiscible with water. When the or­ganic liquid is brought into intimate contact with the aqueous solution, the substances present will distribute themselves between the organic and aqueous phases. The constituent (or constituents) with the greatest solubility in the organic medium will tend to pass into that phase whereas the others will tend to remain in the aqueous solution. Thus, a partial separation of the constituents of the solution will have been achieved.

11.67. In choosing the organic liquid for a particular solvent-extraction procedure, an important property is its selectivity, that is its ability to extract a particular component (or components) of a solution in preference to all others that are present. The selectivity is expressed by the separation factor, i. e., the ratio of the distribution coefficients of the wanted and unwanted species when equilibrium is attained between the two phases. The distribution coefficient or distribution ratio D is defined as

P _ Cone, of component in organic phase Cone, of component in aqueous phase

at equilibrium, and the separation factor a is given by

_ D (product)

D (impurity) ’

A good solvent for extraction is one for which the distribution coefficient for one component is large and the separation factor is either large or small. In other words, it is desirable that D(product) shall be large, whereas D(impurity) should be small, or vice versa.

11.68. The extraction of an inorganic compound, such as a nitrate, from an aqueous solution by means of an organic solvent is influenced by a number of circumstances. Of particular importance are the presence of (1) salting agents, (2) complex-forming anions, and (3) oxidizing or reducing agents.

11.69. A salting agent is either a salt or an acid, having the same anion as the inorganic compound to be extracted, the presence of which in the aqueous solution increases the distribution ratio. In the extraction of uranyl nitrate, for example, either nitric acid or one if its salts, such as sodium, potassium, calcium, or aluminum nitrate, can serve as a salting agent. These substances are soluble in the aqueous phase but not in the organic solvent.

11.70. The extraction of a specified element from aqueous solution by an organic medium is dependent upon the particular form in which the element is present in the solution. For example, uranyl nitrate hexahydrate can be extracted by certain organic solvents, but the corresponding sulfate is not extractable by these solvents. The addition of a sulfate or other salt of a complex-forming anion to an aqueous solution of uranyl nitrate will thus decrease the extractability of the uranium, since a proportion of the element will be in some form other than the nitrate. The complex-forming anions decrease the distribution coefficient between the organic and aqueous phases, and so their effect is opposite to that of the common-ion salting agents.

11.71. The solvent-extraction processes for separating uranium and plu­tonium from the fission products and then from each other depend on the somewhat unusual chemical behavior of the heavy elements. Starting with actinium (atomic number 89), there is a series of 15 elements, called the actinide series, in the sixth period of the periodic system which resemble the rare-earth (or lanthanide) elements in the fifth period. The lanthanide elements all have similar chemical properties, based on a positive valence of 3, resulting from the presence of three relatively loosely bound outer electrons in each atom. In the analogous actinide series, there are also marked resemblances among the elements, especially in the formation of a tripositive (III) valence state. However, because some of the actinide elements have inner electrons which are not very tightly bound, it is possible to realize tetrapositive (IV), pentapositive (V), and hexapositive (VI) states.

11.72. Although in a given valence state the various actinide elements have similar chemical properties, these properties often are very different in the different oxidation states. For example, the nitrates of the (IV) and (VI) states are appreciably soluble in certain organic liquids, but the nitrates of the (III) states are virtually insoluble in these liquids. The relative stability of the different oxidation states varies with the atomic number of the element. Hence, by the use of appropriate reagents, it is possible to shift the oxidation and reduction states so that two (or more) elements in a given solution will be in different states with differing solubilities in an organic liquid. Separation of the elements can then be accomplished by solvent extraction.

11.73. It is evident from the foregoing discussion that the nitrates of the (IV) and (VI) states of the actinide elements will have large distribution coefficients and hence will be extractable by an organic liquid, but the lower oxidation (III) state will have a smaller distribution coefficient and be less extractable. One consequence of this difference in the distribution coefficients is that after an element has been extracted into an organic medium it can be back-extracted into an aqueous solution if the (IV) or (VI) state is reduced to the (III) state. Suppose two actinide elements have been extracted into an organic solvent in the (IV) or (VI) state. If one of the elements is reduced to the (III) state, it can be separated from the other element by back-extraction into an aqueous solution. The uranium and plutonium in spent reactor fuel are separated from one another in this way.

The purex process

11.74. The “Purex” process, using л-tributyl phosphate (TBP) as the extractant, is typical of solvent-extraction procedures employed in the treat­ment of spent fuel. In the form of nitrates, uranium (VI) and plutonium (IV) can be readily extracted from aqueous solution by TBP, whereas the fission products are taken up to a much smaller extent. TBP is relatively stable in the presence of fairly high concentrations of nitric acid, hence, the latter is used as the salting agent.

11.75. In the first cycle of the Purex process, of which an outline flow sheet is shown in Fig. 11.5, the feed consists of an aqueous solution con­taining uranium (VI), plutonium (IV), and fission product (FP) nitrates plus an excess of nitric acid. Sodium nitrite is added to make sure that the plutonium is entirely in the (IV) state, since this form is best extracted by TBP. The feed solution enters at the middle of the first (extraction) column, while the less dense organic extractant (TBP in a kerosene-type solvent) entering from the bottom flows upward. The uranium (VI) and plutonium (IV) nitrates are thereby extracted from the aqueous solution and pass into the organic medium. In the upper part of the column the organic phase is scrubbed with concentrated nitric acid. Most of the fission products that may have entered the organic solvent are now back-extracted into the aqueous phase, but the nitric acid, which acts as a salting agent, largely prevents back-extraction of the uranium and plutonium. The aqueous ef­fluent (raffinate) from the extraction column contains essentially all the fission products with little or no uranium or plutonium.

11.76. The organic phase containing the uranium and plutonium next passes into the second (partitioning) column where it flows upward and meets the downflowing aqueous strip solution containing a reducing agent to reduce plutonium (IV) to plutonium (III). In a modified Purex process, the reduction is performed electrolytically. The plutonium (III) nitrate is not soluble in the organic medium and so it is back-extracted into the aqueous phase. As this flows downward it is scrubbed with fresh TBP moving upward from the bottom of the column. Any uranium (VI) that has passed into the aqueous solution is thereby returned to the organic phase. The aqueous medium, containing plutonium (III) nitrate, leaves at the bottom of the partition column.

11.77. The organic solution of uranium (VI) nitrate, from which the plutonium and fission products have been almost completely separated, is now transferred to the bottom of the third (stripping) column where it flows upward and is stripped by dilute nitric acid flowing downward. In the absence of a salting agent, the uranium is back-extracted into the aqueous phase and then flows out of the bottom of the column. The spent solvent, leaving at the top, is sent to a recovery plant for purification and subsequent reuse in extraction.

11.78. For further purification, both the aqueous uranium (VI) and plutonium (III) nitrate solutions are submitted to a second and third cycle. The uranium purification in each cycle is essentially identical with the last two stages of the first cycle shown in Fig. 11.5. The aqueous uranium solution is first extracted into the TBP phase and scrubbed with a reducing solution; the organic phase is then stripped by dilute nitric acid in a second column. For each plutonium cycle, the plutonium (III) solution is converted into the (IV) state by means of sodium nitrite and nitric acid, extracted into the TBP medium, and scrubbed with nitric acid in the same column. The organic solution then passes to a stripping column where the plutonium is back-extracted into the aqueous phase by dilute nitric acid. Ion exchange has been employed for the final purification of plutonium following the first cycle of recovery by solvement extraction. The procedure is particu­larly valuable for the separation of zirconium and ruthenium, and their daughter products niobium and rhodium, respectively. The common prac­tice, however, is to use two additional stages of solvent extraction to remove residual fission products, as stated above.