1. Influence of phase-contacting time

At 21 ℃, with phase ratio varying from 1:1 to 6:1, the stripping rates of Pu(IV) as function of phase-contacting time were obtained at the concentrations of HSC, H⁺, NO₃^- and Pu(IV) in the organic phase being 0.05 mol/L, 0.4 mol/L, 0.4 mol/L and 0.8 g/L, respectively. The results are given in Fig. 2. The stripping rates of Pu(IV) at all phase ratios increased with phase-contacting time. At a same contact time, the stripping rate decreased with increasing phase ratio. At a phase ratio of 1:1 and phase-contacting time of 5 s, the stripping rate was 92.8%; while for phase ratio of 4:1, the stripping rate was 58.7% and 84.5% at phase-contacting time of 5 s and 20 s, respectively. It is worth noting that the reduction stripping of Pu(IV) could go basically balance at 4:1 phase ratio and 30 s phase-contacting time, and the stripping rate was 91.9%. Therefore, at a large phase ratio, appropriate increase in contact time can improve the stripping rate of Pu(IV) significantly.

Fig. 2.Full-screen View

(Color online) The stripping rate of Pu(IV) at different phase ratios as function of phase-contacting time. c(Pu(IV))₀=0.8 g/L, c(HSC)=0.05 mol/L, c(H⁺)=0.4 mol/L, $c({\text{NO}}_3^{-})=0.4\text{ mol/L}$, T=21 ℃.

2. Influence of H+ concentration

At 21 ℃, phase-contacting time of 30 s and phase ratios of 1:1 and 4:1, the influence of H⁺ concentration on the stripping rate was investigated at the concentrations of HSC, NO₃^- and Pu(IV) in organic phase being 0.05 mol/L, 0.4 mol/L and 0.8 g/L, respectively. The results are shown in solid signs in Fig. 3. The stripping rate decreased with increasing H⁺ concentration. This is because that the reaction rate between HSC and Pu(IV) is inversely proportional, in a power of 0.43, to H⁺ concentration [4]. An increase of H⁺ concentration will decrease the amount of Pu(III) reduced by Pu(IV). Additionally, there is rapid extraction/back-extraction equilibrium of Pu(IV) between aqueous and organic (30% TBP/kerosene) phase, and Pu(III) is hardly extracted by 30% TBP/kerosene, hence the decrease of Pu(IV) stripping into aqueous phase. When the H⁺ concentration increased from 0.3 to 1.0 mol/L, the Pu(IV) stripping rate decreased from 89.2% to 70.7%. So, lower H⁺ concentration is preferred for back-extraction of Pu(IV) by HSC. Increasing the stripping time to 90 s and keeping the parameters of other conditions, the results are illustrated in blanket signs in Fig. 3. We suggest that extending the stripping time properly may increase the stripping rate if higher H⁺ concentration is needed according to the process.

Fig. 3.Full-screen View

(Color online) Effect of the H⁺ concentration of on the stripping of Pu(IV) at different phase ratios and stripping time. c(Pu(IV))₀=0.8 g/L, c(HSC)=0.05 mol/L, $c({\text{NO}}_3^{-})=0.4\text{ mol/L}$, T=21 ℃.

3. Influence of NO3- concentration

At 21 ℃, phase-contacting time of 30 s and phase ratios of 1:1 and 4:1, the influence of NO₃^- concentration on the stripping rate was investigated when the concentration of HSC, H⁺ and Pu(IV) in organic phase were 0.05 mol/L, 0.4 mol/L and 0.8 g/L, respectively. The result was given in Fig. 4. The stripping rate of Pu(IV) decreased with the increasing NO₃^- concentration. This is because that the reaction rate between HSC and Pu(IV) is inversely proportional, in a power of 0.58, to NO₃^- concentration, so an increase in NO₃^- concentration decreases, the amount of Pu(III) is reduced by Pu(IV), hence the decrease of Pu(IV) back-extraction to aqueous phase. Another reason is the salting out effect and synergistic extraction effect in the extraction process, so distribution ratio of Pu(IV) between TBP/kerosene and nitric acid aqueous solutions increase with the NO₃^- concentration, so the amount of Pu(IV) back-extraction to aqueous phase and the stripping rate of Pu(IV) decrease. From Fig. 4, at phase ratio =4:1, when the NO₃^- concentration increased from 0.4 to 1.0 mol/L, the stripping rate of Pu(IV) decreased from 85.3% to 77.6%. Thus, lower NO₃^- concentration is helpful for the back-extraction of Pu(IV) by HSC.

Fig. 4.Full-screen View

(Color online) Effect of NO₃^- concentration on the stripping of Pu(IV) at different phase ratios. c(Pu(IV))₀=0.8 g/L, c(HSC)=0.05 mol/L, c(H⁺) =0.4 mol/L, T=21 ℃, stripping time =30 s.

4. Influence of HSC concentration

At 21 ℃, phase-contacting time of 30 s and phase ratios of 1:1 and 4:1, the influence of HSC concentration on the stripping rate was investigated at concentrations of NO₃^-, H⁺ and Pu(IV) in organic phase being 0.4 mol/L, 0.4 mol/L and 0.8 g/L, respectively. As shown in Fig. 5, the stripping rate of Pu(IV) increases with HSC concentration. Below 0.05 mol/L, an increase in HSC concentration increases the stripping rate of Pu(IV) significantly, while over 0.05 mol/L, the HSC concentration affects just a little the stripping rate of Pu(IV). Therefore, in using HSC as a reductant for U/Pu separation, 0.01 mol/L is the best concentration.

Fig. 5.Full-screen View

(Color online) Effect of HSC concentration on the stripping of Pu(IV) at different phase ratios. c(Pu(IV))₀=0.8 g/L, c(H⁺) = 0.4 mol/L, $c({\text{NO}}_3^{-})=0.4\text{ mol/L}$, T=21 ℃, stripping time = 30 s.

5. Influence of U(VI) concentration

At 21 ℃, phase-contacting time of 30 s and phase ratios of 1:1 and 4:1, the influence of U(VI) concentration on the stripping rate was investigated at concentrations of NO₃^-, HSC, H⁺ and Pu(IV) in organic phase being 0.4 mol/L, 0.05 mol/L, 0.4 mol/L and 0.8 g/L, respectively. As shown in Fig. 6, the stripping rate of Pu(IV) increases with the U(VI) concentration. For phase ratio =4:1, the stripping rate increased by 7.5% at 5.0 g/L of U(VI) concentration.

Fig. 6.Full-screen View

(Color online) Effect of U(VI) concentration on the stripping of Pu(IV) at different phase ratios. c(Pu(IV))₀=0.8 g/L, c(HSC)= 0.05 mol/L, c(H⁺) = 0.4 mol/L, c(NO₃^-)=0.4 mol/L, T=21 ℃, stripping time = 30 s.

6. Influence of temperature on the reaction

At phase-contacting time of 30 s and phase ratios of 1:1 and 4:1, the influence of temperature on the stripping rate was investigated at concentrations of NO₃^-, HSC, H⁺ and Pu(IV) in organic phase being 0.4 mol/L, 0.05 mol/L, 0.4 mol/L and 0.8 g/L, respectively. As shown in Fig. 7, the stripping rate of Pu(IV) increased with temperature. This is because the Pu(IV) distribution of decreases with increasing temperature, hence the increase of Pu concentration in aqueous phase; also, a temperature increase accelerates the reaction rate between HSC and Pu(IV): a 10 ℃ increase accelerates the reaction rate by about 10 times. So, an appropriately high temperature increases the stripping rate of Pu(IV) in the process of U/Pu separation.

Fig. 7.Full-screen View

(Color online) Effect of temperature on the stripping of Pu(IV) at different phase ratios. c(Pu(IV))₀=0.8 g/L, c(HSC)= 0.05 mol/L, c(H⁺) = 0.4 mol/L, ${\text{c(NO}}_3^{-})=0.4\text{ mol/L}$, stripping tome = 30 s.

1. DMHAN-MMH as the reductant of Pu(IV)

The experiment, conducted according to the above conditions, achieved a steady state after 2.5 h. The concentration profiles of uranium, nitric acid and plutonium are shown in Fig. 8. At the stripping step, both the organic and aqueous phase had higher concentration of uranium, while the organic phase had a lower concentration of nitric acid (about 0.02 mol/L). At the supplement extraction step, the higher concentration of nitric acid in 1BS could help uranium extraction, thus a satisfactory result was achieved with the U concentration being only 5×10^-3 g/L in the 1BP outlet. And the Pu concentration in 1BU was 25 μgL^-1. The calculation results show that the U and PU yields are 99.99% and 99.96%, respectively; the material balances of U and HNO₃ are 99.9% and 98.5%, respectively; and the separation factor of Pu from U (SF_Pu/U) is 2.5×10⁴, and separation factor of U from Pu (SF_U/Pu) is 6.0×10⁴.

Fig. 8.Full-screen View

(Color online) The concentration profile of U, Pu and HNO₃ in the experiment of DMHAN-MMH.

In Fig. 8, the uranium concentration declined to 30 mg L^-1 when the stages for supplemental extraction is 4, offering a higher uranium yield of 99.99%; a plutonium concentration peak appeared at Stage 7, indicating a cumulative process of plutonium in aqueous phase. We attribute this to the oxidation product of DMHAN, namely formaldehyde.

2. HSC as the reductant of Pu(IV)

The experiment, conducted at the same conditions, achieved a steady state after 2.0 h. The concentration profiles of uranium, nitric acid and plutonium are shown in Fig. 9. At the stripping step, both the organic and aqueous phase had a higher concentration of uranium, while the organic phase had a lower HNO₃ concentration of about 0.02 mol/L. At the supplement extraction step, the higher HNO₃ concentration in 1BS could help uranium extraction, thus a satisfactory result was achieved with uranium concentration being only 6.11×10^-3 g L in the 1BP outlet. In the experiment contained no plutonium, the uranium yield was 99.998%, and the U and HNO₃ material balances were 99.92% and 97.5%, respectively.

Fig. 9.Full-screen View

(Color online) The concentration profile of U, Pu and HNO₃ in the experiment of HSC.

At supplement extraction step, uranium concentration in aqueous phase declined to 50 mg L^-1 at stage 2, offering higher yield of more than 99.99%. At higher stage numbers, it decreased little; while the distribution ratio of plutonium was almost the same as those in stages 1–6, without plutonium peak, unlike the result using DMHAN-MMH as reductant. So it is considered that there are no uncertainties in U/Pu separation process. Plutonium concentration in aqueous phase and organic phase is about 3.2 g/L and 0.2 g/L, respectively, the concentration difference is small between each stage. The distribution of nitric acid in the supplement extraction step is similar to that of plutonium.

At the stripping step, uranium concentration did not change obviously and a lower concentration in organic phase outlet agrees with the theory calculation; while in aqueous phase, it increased significantly in stages 7–11. In stages 11–16, the concentration was almost the same. In organic phase, plutonium concentration decreased significantly in stages 7–11, but kept nearly the same in stages 12–16; while in aqueous phase, plutonium concentration showed a trend of orders of magnitude lower in stages 7–11, but kept almost the same in stages 11–16. The HNO₃ concentration in organic phase decreased significantly in stages 7–13, but kept almost the same in stages 14–16; while the HNO₃ concentration in aqueous phase decreased obviously in stages 7–12, but kept almost the same in stages 13–16.

Calculation results show that the separation factor of plutonium from uranium (SF_Pu/U) is 2.8×10⁴, and separation factor of uranium from plutonium (SF_U/Pu) is 5.9×10⁴.

Compared the results of experiments using DMHAN-MMH or HSC as reductant for separating Pu from U, we can see that the separation factor of SF_Pu/U and SF_U/Pu are nearly the same. From the point of the view, HSC can replace DMHAN-MMH in separation Pu from U in APOR process.