Oxidation of pentavalent neptunium by nitric acid. Oxidation of pentavalent neptunium to hexavalent by nitric acid requires catalysis by nitrous acid. The kinetics of this reaction have been studied by Siddall and Dukes [S16], Swanson [S24] and Mouline [М9]. Siddall and Dukes reported that the reaction was first order in neptunium concentration, independent of nitrous acid concentration if greater than 5 X 10"s M, and depended on temperature T (K) and nitric acid molarity xH as can be represented by Eq. (10.33):

kN = 7.03 X 10’7 е7062/гл$280 min’1 (10.33)

fcfj is the specific rate constant in the equation

/Jy

^ = -*N(x5-;t?) (10.34)

where xs is the aqueous molarity of Np(V) and x° is its equilibrium molarity.

Swanson’s results appear very different. He reported that the first-order reaction rate constant in Eq. (10.34) was independent of nitric acid concentration and proportional to nitrous acid concentration:

kN = fc,[HN02] (10.35)

Values of к і are 46 Af-^min-1 at 24°C and 250 M~’ — min-1 at 46°C. However, at concentrations of nitric and nitrous acid used in reprocessing, values of kN from Eqs. (10.33) and (10.35) are not far apart.

Mouline’s experiments partially explain the apparent discrepancy. When the nitrous acid molarity is less than that of neptunium, the rate is proportional to nitrous acid concentration. At nitrous acid/neptunium concentration ratios above unity, the rate is independent of nitrous acid concentration. Values of the first-order rate constant observed by Mouline at 35°C for the latter condition are compared below with ones calculated from Eq. (10.33) correlating Siddall and Duke’s data.

First-order constant fc^, min

Nitric acid molarity, xH Average observed, Mouline Eq. (10.33)

2 0.013 0.0127

3 0.059 0.048

4 0.14 0.124

These reaction rates are too low to explain the appreciable extraction of neptunium obtained in the short-residence-time HA contactors used at Hanford and elsewhere. Swanson [S24] reported that radiolysis reaction products of TBP and nitric acid present in Purex solutions increased the neptunium oxidation rate and provided a possible explanation. He found that the oxidation rate could be increased several orders of magnitude by adding a synthetic “rate-accelerating material” (RAM) produced by reacting the aciform of nitropropane, C2H5(CH)(NO)(OH), with nitric and nitrous acids, and recommended addition of such a catalyst to Purex feed if increased neptunium extraction were desired.

Oxidation of pentavalent neptunium by pentavalent vanadium. Oxidation of pentavalent neptunium by pentavalent vanadium proceeds at a practical rate without catalyst. Dukes [D4] found that the rate of reaction could be represented by

=-kv[H+]J[V02+]*5 (10.36)

at

with values for the specific rate constant ky given in the second column of Table 10.24. The third column gives values for the first-order rate constant ky [H+] 2 [V02+] for conditions to be recommended in the HA contactor, 2.5 M HN03 and 0.01 M V02+. The rate is much greater than the rate with nitrous acid catalysis and is high enough for a practical process. Srinivasan et al. [S20] extracted more than 90 percent of the neptunium in laboratory mixer-settler experiments with V02+ as oxidant.

Reduction of neptunium. To separate neptunium from plutonium in the Purex process, plutonium is reduced to inextractable Pu(III) while neptunium is reduced from extractable

Source: E. K. Dukes, “Oxidation of Neptunium (V) by Vanadium (V),” Report DP-434, 1959.

Np(VI) through inextractable Np(V) to extractable Np(IV). Reduction to Np(V) is rapid, but reduction to Np(IV) is slow, probably because of need to remove oxygen from Np02+. Of the three reductants considered, ferrous iron reacts most rapidly, but must be present in such great excess for complete reduction to Np(IV) that one of the stronger reductants, tetravalent uranium or hydroxylamine, is preferred.

Reduction with tetravalent uranium. Newton [N3] found the rate of reduction of hexavalent neptunium to pentavalent to be rapid and given at 25°C by

_ d[^g(V^ = 21 7[Np(VI)] [U(IV)] (10.37)

The rate of reduction of pentavalent neptunium to tetravalent is much slower. Shastri et al. [S10] made an extensive study of the reduction of Np(V) by U(IV). In one series of experiments at 25°C, [H*] =0.1, and an ionic strength of 0.6 M, the rate of increase of Np(IV) molarity x4 could be represented by

y-= (0.039*5 + 0.26*4 )[U(IV)] (10.38)

^*min

The rate varied inversely as [H+]2. At hydrogen ion molarity [H+] and when U(IV) is present in sufficient excess to remain effectively constant during reduction, the integrated rate equation with *4 = 0 at t = 0 is

In the process example to be used in Sec. 7.7, where [H+] = 0.06, [U(IV)] = 0.044 and *4/*! = 0.99; t = 35 min.

Reduction with hydroxylamine. No comparable rate data for reduction of Np(V) by hydroxyl­amine are available. Barney [B2] reported that the initial rate of reduction of Pu(IV) by 0.1 M hydroxylamine nitrate (HAN) was about one-fourth the initial rate of reduction of Pu(IV) by U(IV) at 25°C. If the same ratio applies to reduction of Np(V) by HAN or U(IV), a reaction time of around (4X35)= 140 min might be required. Use of hydroxylamine would have the advantage of not requiring reduction of uranium to U(IV) and its subsequent recycle.