The Separation of Uranium and Plutonium by Electrolytic Reduction in

23 The Separation of Uranium and Plutonium by Electrolytic Reduction in the Purex Process H E JIAN-YU, ZHANG QING-XUAN, and LO LONG-JUN

Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: April 16, 1980 | doi: 10.1021/bk-1980-0117.ch023

Institute of Atomic Energy, Academic Sinica, Beijing, People's Republic of China

In the p a r t i t i o n i n g of U and Pu in the first cycle of the purex solvent extraction process f o r reprocessing spent nuclear f u e l from production reactors, ferrous sulfamate is still being used as the reducing agent f o r Pu(IV). I t s use seriously im pairs the economy in the treatment of medium active waste be cause of the introduction of scale-forming and corrosive ions to the e f f l u e n t . This drawback is more serious in the repro cessing of spent f u e l from power reactor, where the Pu-content would be so high that a tenfold increase in the amount of unde s i r a b l e salts introduced to the process stream would r e s u l t . Therefore, many alternatives f o r the ferrous sulfamate r e duction method have been studied(1,2,3); besides, the use of U(IV) as the reductant has actually found some p r a c t i c a l a p p l i cation. Since U(ΙV) is being prepared by means of e l e c t r o l y t i c reduction of U(VI), it is natural to go a step further, namely, to introduce e l e c t r o l y t i c reduction to the process stream it s e l f . In such an in-situ e l e c t r o l y t i c process, not only P(IV) would be expected to be d i r e c t l y reduciable to Pu(III), but any U(IV) formed would also be expected to serve as the reductant f o r Pu(IV). In f a c t , such a method has been investigated and developed both in the Federal Republic of Germany(4,5) and in the united States(6,7). This process has also been studied in the People's Republic o f China f o r some time and here are presented some of our experimental r e s u l t s . DBTBBMBATIOH OF CURBBHT-POTBHTIAL CUBTBS In order to help selecting a suitable current or potential f o r c o n t r o l l i n g the course of e l e c t r o l y s i s , current-potential curves f o r solutions containing one or more constituents of the substances present in the 1B battery were determined, with no s t i r r i n g in the c e l l . The solution of Pu(IT) was prepared from i t s stock containing 3M/L HHO3. Results on Ρt-cathode were shown in F i g . 1 and 2 , those on Τi-cathode shown in F i g . 3 · from

0-8412-0527-2/80/47-117-317$05.00/0 © 1980 American Chemical Society In Actinide Separations; Navratil, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


318

ACTINIDE SEPARATIONS

Figure 1. Current-potential curves for Ft cathode: S = 0.28 cm ; V = 20 mL; (Ο) 1.6M HNOs, 10 mg/mL Pu (IV); (A) L6M HN0 10 mg/mL Pu (IV), 70 mg/mL U (VI). 2

Pt

-1*00 -ZOO

0

+200

Î400

iéOO

3i

\ \

4

2

5

3 6 9 /2/5/6 2/ 14 H 30 t(/nim

In Actinide Separations; Navratil, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

23.

H E ET AL.

Electrolytic Reduction of U and

321

Pu

riment8 with HNO3 solutions of 0.5 H/L to 2 M/L which contains an i n i t i a l HHO2 concentration of around 10"^-10"" M/L. The r e s u l t s , as shown b y Fig.4, indicated that, whereas HHO3 is d i f f i c u l t to reduce at 0.5 H/L, i t s reduction becomes more and more easy as i t s concentration goes from 1 H/L to 2 H/L. Be sides, f o r the 2 H/L HHO3 in the presence of hydrazine, the rate of formation of HHO2, while being depressed at the be ginning of the e l e c t r o l y s i s , r i s e s sharply a f t e r a while. This l a s t experiment also proves that on Tl-cathode HHO3 could be reduced to H H 0 even when there is no H H 0 present in the i n i t i a l HHO3 solution. Therefore, f o r the separation of Pu and U by e l e c t r o l y t i c reduction in HNO3 solutions(as being practised in the Purex Process), enough hydrazine should be added as the supporting reducing agent. 5


2

2

ELECTROLYTIC RBDUCTIOH OF U(VI). Since U(VI) in the form of UO2 is present in large amount in the 1B battery in the first cycle of the purex process, i t s successful e l e c t r o l y t i c reduction to U(IV) would create a most favorable condition f o r the reduction of Pu(IV). Both Heal(l2) and Finlayson( have shown that H+ ion is Involved in the e l e c t r o l y t i c reduction of U(VI) to U(IV) as summarized by the following reaction: VO * 2

+

4H

+

+

2e~

«

4

U+

+

2H 0. 2

The object of our work is to measure the rate of reduction of UO2 & function of t y p i c a l operating parameters such as the applied cathode potential, HNO3 concentration and the amount of hydrazine added. As shown in Fig.5 and Fig.6, the reduction rate of U(VI) increases with increasing applied cathode p o t e n t i a l and HHO3 concentration. Fig.7 indicated that the addition of hydrazine e f f e c t i v e l y promotes the rate of reduction of U(VI) but t h i s effect becomes very small beyond 0.1 H/L hydrazine. We have decided to add 0.2 H/L hydrazine in our experi ments f o r the reduction of Pu(IV) to make sure that the d e t r i mental effect of HH0 could be counteracted. a s

2

ELECTROLYTIC REDUCTION OF Pu(IV). Cohen(14) had studied systematically the e l e c t r o l y t i c oxidation and reduction of Pu. His results showed that the overpotentials of Pu(I )-Pu(III) and Pu(VI)-Pu(V) couples are low, but it is high f o r Pu(V)Pu(IV) on Pt electrode. Our experiments showed that the overpotential of Pu(IV)-Pudll) on Ti-electrode is also high. How ever, in the p a r t i t i o n i n g step of the first cycle in the purex process, the large excess of U present would be expected to be b e n e f i c i a l f o r the reduction of Pu(IV). In our work, the effect of a c i d i t y and ϋ-content on the v


ACTINIDE


322

Figure 5. Electrolytic reduction of U (VI) as a function of cathode potential: S = 20 cm ; V = 20 mL; with stirring; composition: IM HNO , 0.2M N H , 70 mg/mL U (VI); (Ο) Ε = -300 mV/ SCE; (A) Ε 500 mV/SCE; (V) Ε = -600 mV/SCE; (X)E = -700 mV/ SCE; (·)Ε = -1000 mV/SCE.

SEPARATIONS

2

Ti

s

2

+

5

15

t (rnln)

Figure 6. Electrolytic reduction of U (VI) as a function of nitric acid con centration: S i = 20 cm ; V = 10 mL; with stirring; Ε 700 mV/SCE; (•) 0.5M HN0 ; (A) 1.0M HNO ; (Χ) I.5M HN0 . 2

T

3

s

3

Figure 7. Electrolytic reduction of U (VI) as a function of N H concentra tion; S = 20 cm ; V = 10 mL; Ε = —700 mV/SCE; with stirring; (Ο) 1M HNOs, 70 mg/mL U (VI), OM N H +; (A)1M HN0 70 mg/mL U (VI), 0.05M N H +; (X) 1M HNOs, 70 mg/mL U (VI), 0.1M N H ; (Π) 1M HNOs, 70 mg/mL U (VI), 0.5M N H \ 2

+

5

2

ri

2

5

3>

2

5

2

+

5

2

5


20


23.

H E ET AL.

Electrolytic Reduction of U and

Pu

323

Pu(IV) reduction rate were studied. Pig.8 shows that at large excess of U and low concentration of Pu, no e f f e c t of acid concentration on Pu(IT) reduction rate could be observed. After a period of e l e c t r o l y s i s of less than 30 seconds, nearly a l l of the Pu(lV) could be converted to Pu(III). This fact corresponds to the change of the potential of the e l e c t r o l y t e solution with time, which drops very rapidly a f t e r the start of the e l e c t r o l y s i s . The effect of O-eoneentration on the reduction rate of Pu(lV) is shown in P i g . 9 , from which it is c l e a r l y shown that the reduction rate of Pu(IV) depends very much on the amount of U r e l a t i v e to that of Pu in the e l e c t r o l y t e solution. The upper two curves showed that i f the weight r a t i o of U/Pu is near or more than one, the reduction rate of Pu(IV) could be greatly accelerated. This fact indicates c l e a r l y that here U(IV) plays an important role in the reduction of Pu(IV). On the other hand, i f the U-content in the solution is small compared to that of Pu, the rate of reduction of Pu(IV) is determined c h i e f l y by the e l e c t r o l y t i c reduction of Pu(IV) i t s e l f which is rather slow. This fact should be borne in mind in designing e l e c t r o l y t i c reduction equipments in the purex process. ELECTROLYTIC REDUCTION OF U(VI) IN A MONOCELL WITHOUT A DIAPHRAGM Schmieder(5) said that in the presence of hydrazine in a common cathode-ânode space, there happens only the formation of U(IV) on the cathode without any reoxidatlon of it on the anode u n t i l the course of e l e c t r o l y s i s reaches the stage of oxygen evolution on the anode. He said that t h i s is due to the fact that oxidation of U(IV) has a high oyerpotentlal on the anode. Then it should be possible to construct an e l e c t r o l y t i c reduct i o n apparatus without a diaphragm. We have conducted several experiments in such a set-up and the r e s u l t s were shown in F i g . 1 0 . From t h i s figure it could be seen that a f t e r 30 minutes of e l e c t r o l y s i s , appreciable amount of U(IV) is formed even when the area of anode is larger than that of cathode and the hydrazine remaining in the c e l l is 70% of the i n i t i a l amount added. In Fig.11 the current-potential curves of N 2 H $ , I T and HNO3 on Pt-anode are represented. I t could be seen that for the curve of U(IV) oxidation, there appear two waves, the first r e presenting U(IV) oxidation and the second having not yet been i d e n t i f i e d . Fig.12 shows the oxidation of U(IV) in HNO3 acid solution in the presence of large excess of hydrazine. The res u l t s of these experiments indicate that U^+and hydrazine could be simultaneously oxidized on the Pt-electrode before oxygen evolution. +

ELECTROLYTIC REDUCTION IN A MILLI TYPE MIXER-SETTLER FOR SEPARATION OF ϋ ANDTïï


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ACTINIDE

324

SEPARATIONS

1 8


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Figure 8. Electrolytic Pu (IV) reduc tion as a function of nitric acid concen tration at low Pu (IV) concentration: S i = 20 cm ; V = 10 mL; Ε = -700 mV/ SCE; (Ο) 0.5M HN0 , 0.56 mg/mL Pu (IV), 70 mg/mL U (VI); (A) 1M HN0 , 0.56 mg/mL Pu (IV), 70 mg/mL U (VI). T

2

3

3

30

60 tCsec)

90

Figure 9. Electrolytic Pu (IV) reduc tion as a function of U (VI) concentra tion: S = 20 cm ; V = 10 mL; Ε = —700 mV/SCE; with stirring; (Ο) I M HNOs, 0.2M N H \ 70 mg/mL U (VI), 0.56 mg/mL Pu (IV); (A) I M HN0 , 0.2M N H , 0.06 mg/mL U (VI), 0.1 mg/mL Pu (IV); (S7) I M HNO , 0.2M N H \ 0.06 mg/mL U (VI), 1 mg/mL Pu (IV); (X) I M HNOs, 0.2U N H \ 0.06 mg/mL U (VI), 8 mg/mL Pu (IV). 2

Ti

2

5

3

2

+

5

s

2

5

2

5


120


HE E T AL.

325

Electrolytic Reduction of U and Pu

t ( ni!n) Figure 10. Electrolytic reduction of U (VI) as a function of ratio of anode and cathode area in common space: S = 22 cm (cathode area); V = 40 mL; Ε = -350 mV/SCE; with stirring; composition: 1.5M HNO , 0.2M N H \ 70 mg/mL U (VI); (O) R = 2, i = 440 mA; (Q) R = J, i = 420 mA; (χ) R = .2, i = 400 2

Pt

s

2

5

Figure 11. Current-potential curves for Pt anode: S = 0.28 cm ; V = 30 mL; (X) 2M HNOs; (Ο) 1.5U HNO , 0.2M N H +; (A) I M HNO 24 mg/mL U (IV); 95% U (IV) 2

Pt

s

2

rnV/SCE

5

Si



326


I.

tCm/n)

Figure 12. Cycle electrolytic oxidation of N H -U (IV) solution: S = 280 cm ; V = 55 mL; Eanode h&00 mV/SCE; throughput, 10 mL/min. 2

IBXQ

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BX

+

5

2

IÔSQ

IBF©

3

Pt

es

'7

/BP© Figure 13. Flowsheet for partitioning of U and Pu: (S) electrolytic part, (O) flow ratio.

Figure 14. Schematic of electrolytic mixer settler: 1, mixing chamber; 2, settling chamber; 3, anode cell; 4, Pt anode; 5, Ti cathode plate; 6, insuhtor; 7, separator.



23.

H E E T AL.

327


The construction of the M i l l i - t y p e couter-current mixers e t t l e r extraction equipment is similar to that employed in the Federal Republic of Germany. A flow diagram and a sketch of the c e l l construction used in our experiments are shown in Fig.13 and Fig.14. In the apparatus there are 17 stages, 9 of which for extraction(the BX part) and the remaining ones f o r organic washing(the BS p a r t ) . The housing of the mixer-settler i t s e l f serves as the cathode which is made of Ti-metal. Pt-wire wrapped on a hollow insulated plate serves as the anode. The volume is about 8.4om3 f o r each mixing chamber and 25.2cm' f o r each s e t t l i n g chamber. Residence time of more than one minute is maintained in each mixing chamber and a i r pulse is used f o r mixing the organic and aqueous phases. The throughput of organic feed solution is 5ml per minute and the flow r a t i o of feed solution 1BF to aqueous extraction stream 1BX and to organic wash stream 1BS is 5/1/1. Reduction is carried out mainly in the s e t t l i n g chamber and constant current is used f o r cont r o l l i n g the e l e c t r o l y t i c process. Operation of the mixers e t t l e r has been found satisfactory, U and Pu products achieve acceptable l e v e l of quality. Data and r e s u l t s of the experiments are given in table I and the concentration p r o f i l e in the mixer-settler shown in Fig.15. From these the following points could be noted: 1. The acid concentration p r o f i l e in the mixer-settler plays an important role f o r removing U from Pu. With the proper current strength satisfactory results could be achieved when the concentration of HNO3 in the Pu product ranges from 1.4 M/L to 2 M/L. Increasing the acid concentration by a c i d i f y i n g the organic wash stream and increasing HNO3 in 1BF in the BS part could increase the S F substantially, as shown by comparing the results of experiments Ho. 2 and 3 · The effect of acid concentration in the BX part on SFp is not so important, as could be seen by comparing the results of experiments Ho. 2 to 4, where a change in the acid concentration of the BX part gives almost the same SFp . 2. I t seems that current density plays a d e f i n i t e role f o r obtaining a good separation of U and Pu. As shown in table I, low values of SFp are obtained in experiments No. 1 and 6 , f o r which the current strength is 10 to 30£ lower than that used in experiments 2 through 5, f o r which high values of SFp^dO ) are obtained. 3· The Influence of r a d i o l y t i c decomposition product d i butyl phosphate, which could form a strong complex with Pu(IY), has been examined in experiments 5 and 6. In each case 9*10"" more per l i t e r of HDBP has been added to the organic feed solution. No s i g n i f i c a n t difference could be seen, when a current strength of 100mA is used. Experiment No.6 gives a somewhat lower value of SFp but here a lower current strength, namely, 90mA has been used. To summarize, under the conditions of our experiments, saU

u

u

u

4

4

u



16

16

15

2

3

4

91.4

87.4

88.5

84.6

0.312 0.18 0.47 0.22 0.51 0.05

0.277 0.18 0.61 0.21 0.46 0.04

0.307 0.17 0.49 0.18 0.52 0.04

0.343 0.24 0.49 0.18 0.52 0.04

0.303 0.20 0.43 0.20 0.33 0.06

4

80.5

82.1

71.0

72.0

1.71

M/L

HHO3

0.024 1.42

0.007 1.78

0.009 1.70

0.008 2.00

1.07

64.0

1BU

90.0

2

1BS U Pu mg/ml jug/ml

1BX

U Pu HNO3 HHO3 H H$ HNO3 HHO3 mg/ml mg/ml M/L M/L M/L M/L M/L

1BP D I

0.19

0.10

0.24

0.03

0.1

1.24

1.42

1.27

1.60

1.38

1.58

36

6.4

2.2 11

4.0 37

1.0

%

70

mA

Cur rent

100

90

9.6 100

8.6 100

10.8

9.3 100

0.2 15.0

1.9 28

11

>5

SF U Pu Pu mg/ml mg/ml xio3 X10'

1BP

** 14 91.4 0.312 0.18 0.47 0.22 0.51 0.04 68.8 0.037 1.42 0.35 6 1BP throughput: 5ml/min.; flow r a t i o : 1BF/1BX/1BS«5/1/1. * Current efficiency based on Pu. ** 9X10~ M/L HDBP added.

5

*« 15

12

1

Ho.

Η ΦΙΗ

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Table I . Results of 0 and Pu Separation in the Mixer-settler


E T AL.


329


HE

Figure 15. Concentration profileinthe mixer settler: O, organic phase; A, aque ous phase.



330

t i s f a c t o r y results have been obtained for the separation of V and Pu by means of e l e c t r o l y t i c réduction»


SUMMARY Experiments on the e l e c t r o l y t i c reduction of U and Pu in the aqueous phase in presence of hydrazine were carried out to investigate the effect of various factors influencing the rate of reduction. The potentials of the aqueous solution, which can serve to indicate the course of the reduction process, were measured and operating parameters such as acid concentration, hydrazine concentration, applied potential on the cathode, etc., were investigated. Experimental results indicated that, on Ti-cathode n i t r i c acid could be reduced to nitrous even when there is no HN02 in the i n i t i a l HNO3 solution; and, with a U/Pu r a t i o ranging from 10" to 10 , Pu(IV) can be reduced readily when the U/Pu r a t i o is near or more than 1 at low concentration of Pu. In t h i s case, obviously U(IV) formed in the process plays an important role in the reduction of Pu(IV). To investigate the f e a s i b i l i t y for applying this method in the p a r t i t i o n i n g of U and Pu in the first cycle of the purex process, e l e c t r o l y t i c reduction experiments have been carried out in a M i l l i - t y p e mixer-settler counter-current extraction apparatus, with the first 9 stages serving as the e l e c t r o l y s i s c e l l s and the remaining ones for the back washing of uranium. With a flow rate of 1BF/1BX/1BS of 5/1/1» a separation factor SP varying from 10^ to 10^ was obtained, depending on the current density as well as the acid concentration in the BS part of the battery 1B. The organic wash stream was a c i d i f i e d to increase the acid concentration in the BS part so as to obtain a high SP . Under the conditions employed in our experiments sat i s f a c t o r y results for the separation on U and Pu have been obtained, 2

2

U

U

ACKNOWLEDGEMENT The authors are indebted to prof. Wang De-Xi for consultation and advice during the work, L i Zhao-Yi, Jiang Dong-Liang, Tian Bao-Sheng, Zhu ZhuZhong, Zhang J i a - J i u n , Zhan Yi-Zhu and Wu De-Zhu also took part in t h i s work, REFERENCES 1. 2. 3. 4. 5·

Mckibben, J. M.; Bercaw, J. E . DP-1248, 1970. R i c h a r d s o n , G. L. HEDL-TME-75-31, 1975. G o l d s t e i n , M. AEC-BNL-22443, 1976. Koch,; O c h s e n f e l d , W.; Schwind, E . KFK-990, 1969. Schmieder, H.; Baumgartner, F.; Goldacker, Η.; Hausberger,H. ORNL-tr-2999; Report KFK-2082, 1975.


23.

HE

ET

AL.


331


6. Krumpelt, M.; H e i b e r g e r , J.; S t e i n d l e r , M. J. ANL-7799,1971. 7· Krumpelt, M.; H e i b e r g e r , J.; S t e i n d l e r , M. J. ANL-7871,1972. 8. E l l i n g h a m , H. J. T. J. Chem. Soc., 1932, 1565. 9. Mirolyubov, E. N. J. A p p l . Chem.(Russian), 1962, 35, 132. 10. Schmidt, G.; Bunsenges, B. J. Phys. Chem., 1961,65, 531. Schmidt, G.; Krichel, G., i b i d . , 1 9 6 4 , 68, 531. 11. V e t t e r , K. J. " E l e c t r o c h e m i c a l Kinetics"; Academic P r e s s , New York, 1967, p.493. 12. H e a l , H. G. T r a n s . Faraday Soc., 1949, 45, 1. 13. F i n l a n y s o n , M. B.; Mowat, J. A. E l e c t r o c h e m . Tech., 1965, 3, 148. 14. Cohen, D. J. I n o r g . N u c l . Chem., 1961, 18, 207.

RECEIVED July 10, 1979.


The Separation of Uranium and Plutonium by Electrolytic Reduction in

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