Distribution coefficients of neptunium in 30 v/o TBP depend on neptunium valence, tempera­ture, and concentrations of uranyl nitrate, nitric acid, and other nitrates. At the nitric acid concentrations below 4 M usually used in Purex processes, the distribution coefficient of hexavalent neptunium is higher than that of tetravalent neptunium at the same nitric acid and uranyl nitrate concentrations. Both are much higher than that of pentavalent neptunium. Both tetravalent and hexavalent neptunium are extracted as the complexes with two molecules of TBP, Nprv(N03)4*2TBP and NpVI02(N03)2 -2TBP.

Table 10.23 lists principal sources of information on distribution coefficients of neptunium between 30 v/o TBP and aqueous solutions of uranyl nitrate and nitric acid.

Distribution coefficients of tetravalent and hexavalent neptunium can be correlated conveniently in terms of the separation factor from hexavalent uranium, i. e., the ratio of the distribution coefficient of neptunium to that of uranium.

Tetravalent neptunium. Srinivasan et al. [SI8, SI9] measured distribution coefficients of tetravalent and hexavalent neptunium and hexavalent uranium as functions of nitric acid and uranyl nitrate concentrations. At 45 and 60°C, the ratio of the observed [S19] separation factor for tetravalent neptunium to that of hexavalent uranium can be correlated within an average deviation of 6 percent by Eq. (10.25),

j? NBgv) = 0.01129 exp (0.3208лгщо — + 0.03636r°c) (10.25)

^UfVI) 3

At 25°C the equation is less satisfactory, with an average deviation of 18 percent from observations by Srinivasan et al. [S18],

Hexavalent neptunium. Distribution coefficients of hexavalent neptunium at 25, 45, and 60°C measured by Srinivasan et al. [S18, S19] are simply related to measured distribution coefficients for hexavalent uranium by Eq. (10.26), with an average deviation of only 5 percent,

= 0.54 (10.26)

^U(VI)

at all uranium concentrations and at nitric acid molarities between 1 and 4 Af. Germain et al.’s observed [G6] Np(VI) distribution data at 22°C yield an average value of 0.47 for this ratio.

In the HA extracting and HS scrubbing sections of the Purex process, pentavalent neptunium is partially oxidized to the hexavalent state by nitrate ion,

2Npv02+ + N03* + 3H+ -*• 2NpVI022+ + HN02 + H20

when nitrous acid is present to act as catalyst. For the reaction to proceed at a useful rate, the

HN02 concentration of the aqueous phase must be over 0.00004 M. At equilibrium, neptunium in the aqueous phase is then divided between the hexavalent and pentavalent states. The ratio of hexavalent to pentavalent neptunium is given by Eq. (10.27), obtained from the equilibrium ratio ANp defined by Eq. (10.24), and plotted in Fig. 10.30.

[Np(VT)] = „ [H*]3/2 [N03 ~31/2

[Np(V)J Np [HN02 ]1/J

The total neptunium concentration in the aqueous phase X]sjp is

xNp = [Np(Vl)] + [Np(V)] = [Np(Vl)] (l + 0°.28)

Because pentavalent neptunium is essentially inextractable, the neptunium concentration in the organic phase yNp is related to the distribution coefficient of hexavalent neptunium ^Nptvi) by

^Np =^nP(vi)[Np(V1)] (10.29)

The apparent equilibrium distribution coefficient of neptunium, D&pp, defined as

(10.30)

is then given by

n ________ _____________ ^Nptvi)_____________

app 1 + [HN02] 1/2/A’np[H+]3/2 [N03_]1,1

Figure 10.31, calculated [G12] from Eq. (10.31), £>Np(vi), and the observed equilibrium ratios of Fig. 10.30, shows the dependence of Z? app on temperature and the concentrations of HN02 and HN03. Figure 10.31 is strictly valid only in the absence of nitrates other than nitric acid and traces of neptunium. When uranyl nitrate is present at appreciable molarity хи, DNp(VTj is given by Eq. (10.26), and the apparent equilibrium distribution coefficient for neptunium may be estimated from

Dv is given in Figs. 10.13 and 10.15 and ANp in Fig. 10.30. Complete ionization of HN03 and U02(N03)2 is assumed.