In many solvent extraction systems, addition of solutes to the aqueous phase increases the distribution coefficient of extractable components. Data in Table 4.2 and Fig. 4.6 show how addition of nitrates to an aqueous solution of uranyl nitrate increases the distribution coefficient of uranyl nitrate between the aqueous phase and diethyl ether [F2]. The increase in distribution coefficients with increased nitrate concentration is explained as follows: Analysis of

the ether phase shows that uranium is extracted in the form of un-ionized uranyl nitrate. Addition of nitrate ion tends to increase the concentration of un-ionized uranyl nitrate by shifting the equilibrium to the right, and thus

U022+ + 2N03- ^ U02(N03)2

converts more of the uranium to an extractable form. It may be noted that uranyl nitrate acts as a self-salting agent, probably also by displacement of this equilibrium. In addition, readily hydrated cations, such as Ca2+ and Al3+, tie up much of the water in the aqueous phase, and thus increase the effective concentration of uranyl nitrate.

When the organic complexing agent in the solvent is nearly all combined as extracted complexes, further increase in concentration of the complex-forming metal ions in the aqueous phase wih cause the distribution coefficient for metal extraction to decrease. This phenomenon has been observed for uranyl nitrate [G3, Ml, М2] and for zirconium and hafnium nitrates [H4] when extracted by TBP in kerosene.

Table 4.3 gives distribution coefficients for uranyl 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 [М2]:

U02 2*(aq) + 2N03~(«?) + 2TBP(o) ^ U02(N03)2-2TBP(o) and H>q) + N03‘(aq) + TBP(o) =* HN03 • TBP(o)

with the following equilibrium constants:

К = [UO:(NQ3)2-2TBP(0)]

U [U02 [N03 -(«?)]2 [TBP(o)]2 { J

Table 4.4 Distribution coefficients for Zr(N03)4 between water and TBP in kerosene

Aqueous phase: 3.0 M HNO3

3.5 M NaN03

Organic phase: 60 v/o TBP (2.19 M)

Moles Zr per liter

—————————— Distribution

Aqueous Organic coefficient

For the purposes of this chapter, activity coefficients of unity are assumed, so that the bracketed quantities in Eqs. (4.15) and (4.16) become identical with molar concentrations.

Assuming that at equilibrium all aqueous uranium is in the form of uranyl ion and all organic uranium is in the form of the U0j(N03)j ^TBP complex, the uranium distribution coefficient is

„ _ |UO,(NO,),-2TBP(o)] „ …

——— №’*(«,)]———— <4Л,)

and combining Eqs. (4.15) and (4.17),

Dv = Kv [NO* -(«7)J 2 [TBP(o)J 2 (4.18)

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.

Similar saturation effects are apparent from the data for the extraction of Zr(NOs)4 with TBP in kerosene [H4], as shown in Table 4.4.