N9700845

BARC/1996/E/017

ELECTROLYTIC DESTRUCTION OF OXALATE IONS IN PLUTONIUM OXALATE SUPERNATANT

by

K. M. Michael, S. G. Talnikar, U. Jambunathan, S. C. Kapoor A. Ramanujam and N. Venkataraman

Fuel Reprocessing Division

1996

VOL 28 NS 1 9

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017

GOVERNMENT OF INDIA ATOMIC ENERGY COMMISSION

B ARC/1996/E/017

ELECTROLYTIC DESTRUCTION OF OXALATE IONS IM PLUTONIUM OXALATE SUPERNATANT

byK.M. Michael, S.G. Talnikar, U. Jambunathan, S.C. Kapoor,

A. Ramanujam and N. Venkataraman Fuel Reprocessing Division

BHABHA ATOMIC RESEARCH CENTRE MUMBAI, INDIA

1996

BARC/1996/E/017BIBLIOGRAPHIC DESCRIPTION SHEET FOR TECHNICAL REPORT

(as per IS t 9400 — 1980)

Ol Security classification t Unc1assified02 Distribution * External03 Report status « New04 Series t BARC External05 Report type i Technical Report06 Report No. * BARC/1996/E/01707 Part No. or Volume No. i08 Contract No. %to Title and subtitle c Electrolytic destruction of oxalate

ions in plutonium oxalate supernatant

11 Collation % 17 p., 1 fig., 6 tab*.13 Project No. t

20 Personal author (*> i K.M. Michael $ S.Q. Talnikar;U. Jambunathan; S.C. Kapoor<A. Ramanujan; N. Venkataraman

21 Affiliation of author (s> i Fuel Reprocessing Division, Bhabha Atomic Research Centre, Mumbai

22 Corporate author(s) c Bhabha Atomic Research Centre,Mumbai-400 083

23 Originating unit t Fuel Reprocessing Division, BARC,Mumbai

24 Sponsor(e) Name * Department of Atomic EnergyType s Government

30 Date of submission * August 199631 Publication/Issue date September 1996

contd...(ii)1

(ii)40 Publisher/Distributor i Head, Library

Bhabha Atomicand Information Research Centre

Division, , Mumbai

42 Form of distribution i Hard Copy50 Language of text : English51 Language of summary I English52 No. of references * 4 refs.53 Given data on :

60 Abstract s A simple and efficient electrolytic method is describedfor the destruction of the oxalate ions present in plutonium oxalate supernatant. Usin^ platinum electrode and very little KMnQ4, in situ generation of Mn * ions is achieved which in turn destroys the oxalate. The use of lower current density helps in achieving maximum current efficiency. The end point is easily detectable by the pink colour of permanganate. By reversing the current, this slight excess of permanganate can be destroyed, thus avoiding the use of hydrogen peroxide. By this simple electrolytic method, the corrosive oxalate ion is completely destroyed and the salt content of the waste solution is considerably reduced.

70 Keywords/Descriptors c CURRENT DENSITY; ELECTROLYSIS; PLUTONIUM; OXALATES; OXALIC ACID; URANIUM; PUREX PROCESS; PLUTONIUM OXIDES; QUANTITATIVE CHEMICAL ANALYSIS

71 Class No. s INIS Subject Category t B162099 Supplementary elements i

Electrolytic Destruction of Oxalate Ions in PlutoniumOxalate Supernatant

K.M.Michael, S.G.Talnikar, U. Jambunathan, S.C.Kapoor, A.Ramanujam, N.VenkatarumanFuel Reprocessing Division

Bhabha Atomic Rsearch Centre Trombay, Mumbai - 400 085

1.INTRODUCTIONIn fuel reprocessing plants where plutonium is separated

from spent fuel, the final precipitation of plutonium is carriedout with oxalic acid. The plutonium oxalate is filtered andignited to oxide. The supernatant generated during this processcontains plutonium in the range of 30-100 mg/1 depending on theconditions of precipitation. The nitric acid molarity of thissolution is in the range of 3-4M containing oxalic acid upto 0.1M. Several hundreds of litres of this solution have to berecycled for the recovery of plutonium. As oxalate ions arehighly corrosive they are to be destroyed before recycling. Theconventional method of destruction of oxalate is by the additionof excess KMnC>4 and destroying the remaining KMnO^ by HgOz-Alternately, the oxalate solution can be concentrated to aterminal acidity of 10 M^\ Both these procedures have got

their own inherent deficiencies. In the first process, extravolume and increase in salt content of the waste to be handledcreates problem in the final disposal. In the second method,corrosion of the evaporator vessel poses operational problems.Photochemical destruction of oxalate using ultra violet lamps in

(2)small scale (60 ml) by adding uranium has been reported . This method cannot be employed where large volumes of solutions have

1

to be handled. To reduce the salt generation in the process of destruction of oxalate, an electrolytic technique has been studied in detail. It is observed that electrolytic destruction is feasible and presence of small amounts of Ce or KMn04 increases the rate of destruction and complete destruction is possible. In the absence of Ce or KMn04, the rate of destruction is very slow and complete destruction is not possible. As KMnO* is already being used for this purpose further study has been carried out using only KMnO*.

2.EXPERIMENTALInitially, experiments were carried out using inactive

solution containing nitric acid and oxalic acid only. The solution was subjected to electrolysis, by taking different volumes ranging from 0.5 L to 10 L, in the absence and in the presence of Ce or KMn04. As mentioned earlier, detailed study was carried out using KMn04 only. Platinum foil was used as working electrode (anode) while a thick platinum wire was used as cathode. The influences of current density, cone, of KMn04 and anode-cathode separation were assessed. Finally, few experiments were carried out using the actual supernatant solution containing Pu and U. During electrolysis, the solution was analysed with standard KMn04 at fixed intervals to know the amount of oxalic acid destroyed. The experimental setup is shown in Fig. 1.

2

3.RESULTS AND DISCUSSION3.1 The effect of Ce or KM11O4

It can be seen from Table-1 that under the experimental conditions used and in the absence of Ce or KMn04 it takes almost 6 hours to destroy about 88% of the oxalate ions with an overall efficiency of 13 %. While in the presence of both Ce andKMn04, 100% can be destroyed in 5 hours with an efficiency of18%. The anode size used in these experiments is 30 sq. cm. and also the cathode separation has not been done. Hence, more time is required for complete destruction of oxalate. When Ce is used, at the end point the solution turns lime yellow. The reaction involved is given below.

2Ce4+ + H2C2O4 > 2Ce3+ + 2CO2 + 2H+ (1)

2KMnO* + 6HNO3 + 5H2C2O4 > 2KNO3 .+ 2Mn(NO3)2 + 8H2O + IOCO2 (3)—........—........-.....— ^ © ( 4 )

2Mn3+ + H2C2O4 > 2Mn2+ + 2CO2 + 2H+ (5)

The Mn+2 produced during the destruction process again gets

oxidised and the reaction continues until all the oxalate isdestroyed. At the end point, a pink colour is visible whichfades, on keeping for some time. This is due to the

3+ 2+disproportionation of Mb . Although presence of excess Mnretards the disproportionation, acidity above 3 M is required forstabilising Mn3 + *3* If the electrolysis is continued for

some more time, the colour deepens and becomes deep pink due tothe permanganate ion generated from the subsequent oxidation of

3

Mn3*. This excess MnO* formed can be destroyed by reversing the

current (i.e. by carrying out the reduction) thus avoiding the use of H2O2 for the destruction of excess MnO* . Appearance of visible rosy red colour indicates the complete destruction. The reduction potential of Mn ions in acid solution is shown below*

Oxidation states+VII +VI

-0.56 V 2 Mn04 Mn04I1 1.70 V

+ IV +III -HI 02.26 V 0.95 V 3+ 1.51 V 2+-1.18 V... — .... ............. —•• iMin

II I............. ... •» •...... ^ —............. ~

3.2 The Effect of Separation of CathodeIn this experiment, the cathode was separated by a 40 micron

pore size polypic tube. It can be seen from Table 2 that withseparation of the electrodes, the time required for the completedestruction of oxalate has been reduced from 13 hrs to 9 hrs withconsiderable increase in current effeciency (from 15% to 22%).This experiment was carried out using current density of 60

2mA/cm , hence low current efficiency. Current efficiency can be increased by decreasing the current density as explained in Table4. When cathode is not separated, some of the Mn3+ gets reducedat the cathode thus decreasing the available Mn3+ for thedestruction of oxalate ions.

3.3 The effect of variation of MnC>4In Table 3, the effect of variation of KMn04 cone. on the

destruction of oxalate ions is shown. It can be seen that upto about 80 % destruction there is not much change in the rate of

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destruction for both 0.02 and 0.06 N KMn04. Only at the end, tailing takes place in case of 0.02 N KMn04 with considerable decrease in current efficiency. Hence 0.06. N KMnO* was chose;; for further experinents. In all the above experiments where KMUO4 orcerium have been used, corrections have been made for the initial.

+4consumption of oxalate by either Ce or KMnO*.

3.4 Effect of Variation of Current DensityIn Table 4, the effect of variation of current density on

current efficency is shown for complete destruction, of oxalate.It is evident from the Table, that destruction should be carriedout at lower current densities in order to achieve higher currentefficiency. This can be achieved by using an electrode with largesurface area. Different anode sizes varying from 30 sq. cm. to400 sq. cm have been used for these experiments. Current density

2of 12.5 mA/cm was found to be ideal for the efficient destruction of oxalate.

3.5 Oxalate Destruction in Uranium and Plutonium Oxalate Solutions In Table 5, the results obtained, where the experiment was

carried out in presence of 1 g/1 uranium solution, are shown. Table 6 gives the results of the experiment carried out with five litres of actual supernatant solution containing 70 mg/1 Pu. It can be seen that the destruction of oxalate ion in the plutonium oxalate supernatant can be done by the insitu generation of Mn+^ ions with a current effeciency better than

65%. The experiment when repeated at 10 ltrs scale gave similar result as in Table 6.

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The acidity of the oxalate supernatant after the destruction of oxalate was tested and found to be 2.9 M. Thus there is no significant decomposition of nitric acid during the electrolysis. In all the above experiments, mechanical stirrers have been used for uniform mixing of the electrolyte. When large volumes are to be handled it would be better, if air sparging is done.

4.CONCLUSIONThe use of insitu generated MnO* can considerably reduce

the salt generation in the waste solution to be handled during destruction of oxalate ions in the plutonium oxalate supernatant. Since, one equivalent weight of oxalate ion is destroyed by one Faraday of electric current at 100% current efficency, from the current used and known current efficency for the particular experimental set up, the time required for the complete destruction can be predicted and sampling can be done accordingly. The end point is marked by a distinguishable pink colour. The excess MnO* is destroyed by reversing the current. Thus this method gives a clean process for the destruction of oxalate ions in the plutonium oxalate supernatant, with minimum increase in the salt content.

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5.REFERENCES1) J.F.Tracer Jr, and K.M.Harmon, H.W. 31186, 1954.2) T.G. Srlnivasan, S.K.Nayak, R.Damodharan and P.R.Vasudeva Rao

Radiochemistry and Radiation chemistry symposium, B.A.R.C. 1988, Section CT-9

3) F.Albert cotton, Geoffrey Wilkinson, Advanced Inorganic chemistry, Thirteenth wiley Eastern print, October 1990.PP.849

4) J.D.Lee, A New concise Inorganic chemistry, Van Nostrand Rein hold (International) ELBS edition 1989. PP. 335.

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Table-1Effect of Ce and Mn04 on Destruction of Oxalate Ions.

without cathode separation Volume of experimental solution =500 ml.Nitric acid molarity =3MOxalic acid molarity = 0.1 Mcurrent = 3 AmpsAnode = 30 cin

Time Percentage of meg. of oxalate destroyed and current eff.in absence of in presence of in presence ofCe or KMn04 Ce (0.02N) KMn04 (0.02N)

d e d e d e

1 25 22 25 22 25 222 47 21 56 25 56 253 60 18 77 23 70 214 72 16 84 19 91 205 82 15 100 18 100 186 88 13 -- — — --

d - % destroyed e - % current efficiency

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Table - 2Effect of Separation of Electrodes.

Electrolyte volume HNO3 cone.KMnC>4 concentrationCurrentAnodeInitial cone, of oxalate ions. Material used for cathode separation

Time in hrs

5 ltrs 3 N0.02 N 12 amps 200 cnT0.18 N (900 meq)

40 micron polypic tube.meq. current

destroyed efeciency (with cathode separation)

(%) (%)

meq. currentdestroyed efficiency

(without cathode separation) (%) (%)

11.0 22.325.6 25.735.5 23.951.1 25.760.0 24.167.8 22.877.8 22.383.3 20.985.5 19.1

10 91.1 18.311 94.4 17.012 97.8 16.013 98.9 15.0

20.0 40.038.9 39.056.7 37.975.6 37.986.7 34.890.0 35.893.3 37.597.8 24.6

100.0 22.0

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Table-3Effect of Concentration of KMnO*

Electrolyte volume HNO3 cone.Oxalic acidCurrentAnodeOxalate taken Material used for cathode separation

5 ltrs 3 N 0.2 N 12 Amps 200 cm 700 meq.40 micron poiypic tube

Time in hrs

KMn04 0.02 N KMn04 0.06 Nmeq

destroyed%

currentefficiency

%

meqdestroyed

%

currentefficiency

%

17.1 31.2 20.0 40.251.4 41.3 52.8 41.378.4 43.1 77.1 40.287.1 38.9 100.0 41.690.0 35.1 — ---- -

95.7 29.9 — —

98.6 16.4 - —

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Table-4Effect of Current Density on Oxalate Destruction

HNO3 cone. = 3 N

S .No Volume Oxalic acid current one. of Anode Currencone. density KMnO* size

(enr)effeci

(ltrs) (N) (mA/cm2) (N) (%)1 5 0.2 60 0.02 30 222 5 0.2 30 0.02 30 353 5 0.2 30 0.06 200 464 5 0.2 12.5 0.06 400 795 10 0.2 12.5 0.06 400 876* 10 0.2 12.5 0.06 400 927** 10 0.3 12.5 0.06 400 88*In presence of 1 g/1 U**Pu supernatant solution containing 70 mg/1

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Table-5

Volume of electrolyte HNO3 cone.Oxalic acid cone, uranium cone.KMn04 concn.Anode size CurrentMaterial used for cathode separation

Destruction of Oxalate10 ltrs 3 N 0.2 N 1 g/1 0.06 N_400 cm 5 Amp.40 micron polypic tube

in Uranium Oxalate Solution

Time meq. oxalic acid currentin hrs destroyed effeciency (%)0 15201 7.9 64.32 17.2 68.93 26.3 70.34 36.8 85.85 46.0 75.16 57.9 78.97 68.2 78.18 77.6 77.89 85.5 77.2

10 96.0 78.010.2 100.0 73.9

12

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l^K

lO

Table-6in Plutonium Oxalate Solution

Electrolyte volume Nitric acid cocn. Oxalic acid cocn. Plutonium cone. Current Anode size Material used for cathode separation

Destruction of Oxalate5 Itrs 3 N 0.2 N 70.54 mg/1 5 amps.400 cmz40 micron polypic tube

Time Mrs.

.5

. of oxalic currentdestroyed effeciency

(%) (%)

29.3 72.357.6 69.785.8 69.7100.0 65.5

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51

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1. Pt Foil Anode2. Pt Wire Cathode3. 40 pm Polypic Tube4. Stirrer5. Electrolyte6. D.C. Power Supply

W±<3 . 1 ELECTROLYTIC SETUP FOR THEOXALATE ION DESTRUCTION EXPERIMENT

A

Published by : Dr. M. R. Balakrishnan, Head Library & Information Services Division Bhabha Atomic Research Centre, Mumbai - 400 085, INDIA.