GB2249864A

GB2249864A - Process for the separation of Ruthenium and optionally Cesium and Cobalt present in an aqueous solution

Info

Publication number: GB2249864A
Application number: GB9122825A
Authority: GB
Inventors: Remi Gandon
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1990-11-16
Filing date: 1991-10-28
Publication date: 1992-05-20
Anticipated expiration: 2011-10-28
Also published as: GB2249864B; JPH04285017A; GB9122825D0; FR2669460A1; FR2669460B1

Abstract

A process for the separation of ruthenium and optionally cesium and cobalt present in an aqueous solution, such as an effluent from an irradiated fuel reprocessing installation consists of contacting said solution with an alkali metal ferrocyanide, e.g. potassium or sodium, in order to exchange the iron of the ferrocyanide by ruthenium and thus form a ruthenocyanide, which is then separated from the solution, either by precipitation by means of a copper salt, or by fixing on an anion exchange resin, or by concentrating the solution by evaporation.

Description

PROCESS FOR THE SEPARATION OF RUTHENIUM AND OPTIONALLY CESIUM AND COBALT PRESENT IN AN AQUEOUS SOLUTION SUCH AS AN EFFLUENT FROM AN IRRADIATED FUEL REPROCESSING INSTALLATION DESCRIPTION The present invention relates to a process for the separation of the ruthenium present in an aqueous solution. It more particularly applies to the elimination of the ruthenium present in aqueous solutions such as effluents from irradiated nuclear fuel processing installations. It can also be used for facilitating the detection and analysis of the ruthenium content of natural water, e.g. sea water.

Ruthenium represents 6% of the total weight of artificial radioisotopes produced in electronuclear reactors and therefore appears in the reprocessing cycle of irradiated nuclear fuels.

Ruthenium has four isotopes, whose mass numbers are 103, 105, 106 and 107 and whose radioactive half-life periods are respectively 39.8 days, 4.5 hours, 1 year and 4 minutes. In view of the short half-life period of 105Ru and 107Ru isotopes and the time necessary for the reprocessing of irradiated fuels, consideration can only be given to 103RU and 106Ru.

The electronic structure of ruthenium enables it to assume all the possible oxidation states between I and Veil, the most stable being III and IV. In all its forms, ruthenium is able to form numerous complexes, some of which are very stable and very difficult to hydrolyze, which causes problems in connection with the elimination of ruthenium from aqueous effluents from irradiated nuclear fuel reprocessing plants.

In these reprocessing plants, the irradiated fuel is dissolved in concentrated nitric acid. In addition, the ruthenium is oxidized at its maximum valency in tetroxide form Ru04. This relatively unstable and very volatile compound easily reacts with the reducing ligands contained in the medium, i.e.

Ruthenium preponderantly reacts with the nitrosyl cations (NO+) present in majority form in the acid medium. These bonds are stabilized by a very marked ligand - metal fl retrocoordination, favoured by the charge of NO3+, as was observed in Mossbauer spectrometry. This leads to a stable entity Ru(II) NO3+, which is present in majority form compared with Ru#1#1)NO3+formed from the neutral NO group.

Thus, the complexes present in solution are mostly nitrosyl complexes of RU(II), which are generally mononuclear and hexacoordinated, the other ligands being essentially nitrato groups (NO3 ), nitro groups (NO2, NO2 ), hydroxy groups (OH ) and aquo groups (H2O).

Among these complexes, the nitro complexes of nitrosylruthenium complying with the general formula: RuNO (NO2)x (NO3)y (H20)2 (OH)t in which x, y and t are such that x + y + t = 3 are very difficult to separate, because they are very stable and very difficult to hydrolyze.

In the conventional irradiated nuclear fuel reprocessing processes, the fission products are isolated by forming complexes thereof with tributyl phosphate and they are extracted with the aid of a low density, non-polar specific organic solvent.

In the case of nitro complexes of nitrosyl-ruthenium and in particular the tetranitro derivative RuNO(NO#)4OH2#. , it is impossible to substitute the NO2 and OH groups by tributylphosphate ligands and consequently this type of complex remains in the aqueous phase.

It is then difficult to eliminate it by conventional decontamination treatments, such as cobalt sulphide precipitation in an acid or basic medium. The 106Ru-Rh pair occurs in the composition of the effluents from the La Hague plant at a rate of 80% of the activity of the y emitter radionuclides.

In the same way, it is difficult to extract these nitro complexes from nitrosyl-ruthenium by oxyhydroxide-based adsorbents, because they only hydrolyze very slowly. Another problem which occurs with ruthenium is the difficulty of accurately measuring the activity of the ruthenium in the analyzed water samples because it is difficult to separate the same.

Thus, the process used at present for analyzing ruthenium in natural water makes use of coprecipitation by manganese dioxide. The determination of the cesium level in the water can take place jointly by the incorporation of double cobalt - potassium ferrocyanide in powder form, because the Cs ions, which have no affinity for the dioxide, are easily exchanged with the hydrated ions Co(H20)62 on the surface of the ferrocyanide grains.

However, this process does not make it possible to extract the nitro complexes from the ruthenium and the efficiency of the operation for a standard 115 1 sample does not exceed 50 to 60% of the ruthenium effectively present. Thus, it is only possible to analyze the hydrolyzed forms of the nitrosylruthenium adsorbable on the manganese dioxide, because the ruthenium is not extracted by the commercially available, powder form double cobalt-potassium ferrocyanide.

The present invention specifically relates to a process for the separation of the ruthenium present in an aqueous solution, which makes it possible to extract virtually all the ruthenium, even in the form of nitrosyl-ruthenium nitro complexes, which is of great interest for the treatment of aqueous effluents and for the determination of the ruthenium present in water samples.

According to the invention, the process for the separation of the ruthenium present in an aqueous solution consists of contacting the latter with an alkali metal ferrocyanide for exchanging the iron of the ferrocyanide by ruthenium and so as to thus form a ruthenocyanide, followed by the separation of the latter.

Thus, the inventive process is based on an exchange reaction between Fe(ll) and Ru(ll) according to the following reaction: Fe(II)CN64- + Ru(II)NO3+ --- > Ru(II)CN64- + Fe(II)NO3+ This reaction can take place due to the similarity between the Fecal) CN64 ion and the Ru(ll) N03 ion both from the standpoint of the oxidation degree of the metal and the type of ligand involved.

Preferably, this exchange reaction is carried out with an alkali metal ferrocyanide excess in order to transform all the ruthenium of the ruthenium complexes into ruthenocyanide.

The alkali metal ferrocyanide can e.g. be potassium ferrocyanide or sodium ferrocyanide, preference being given to the latter.

Generally, the alkali metal ferrocyanide quantity used is at least 4*10-4 mole/l and is preferably at least 4910-3 mole/l in order to entrain all the ruthenium.

According to the invention, it is possible to separate the thus formed ruthenocyanide either by precipitation by means of an appropriate reagent, or by fixing on an anion exchange resin, or by evaporation of the solution.

According to a first embodiment of the inventive process, the latter comprises the following stages: a) adding to the aqueous ruthenium-containing solution an alkali metal ferrocyanide in order to exchange the iron of the ferrocyanide by ruthenium, b) adding to said solution a copper salt in order to form a double alkali metal and copper ferrocyanide precipitate entraining the ruthenium and c) separating the thus obtained precipitate.

Thus, with the process according to the invention, it is possible to separate the ruthenium on a double ferrocyanide precipitate, whereas this is impossible when direct addition takes place of commercially available, powder-form double ferrocyanide.

In this first embodiment of the inventive process, it is possible to use various copper salts and in particular copper nitrate.

In order to form a double alkali metal and copper ferrocyanide, e.g. the double ferrocyanide 5Fe(CN)6Cu2, Fe(CN)6k4, it is vital to use sufficient copper to precipitate all the ferrocyanides present. In addition, the copper salt/alkal; metalsferrocyanide molar ratio is preferably at least 2.

When using potassium ferrocyanide, the copper salt/potassium ferrocyanide molar ratio is preferably at least 3.

In the case of sodium ferrocyanide, the molar ratio is preferably at least 2.

In this first embodiment of the inventive process, the pH of the aqueous solution influences the adsorption efficiency of the ruthenium on the double ferrocyanide precipitate.

It is also advantageous to adjust the pH of the aqueous solution before adding the copper salt to an appropriate value between 4 and 6, in order to obtain a maximum ruthenium adsorption efficiency.

The pH of the solution can be adjusted either before contacting it with the alkali metal ferrocyanide, or after the exchange of the iron by the ruthenium, by using appropriate reagents, e.g. nitric acid.

This first embodiment of the inventive process can be used for the decontamination of aqueous effluents containing not only ruthenium, but also cesium and/or cobalt. Thus, in the presence of excess copper the double alkali metal and copper ferrocyanide precipitate entrains all the cesium. It is therefore possible to obviate the use of specific adsorbents for cesium.

In the case of cobalt, the potassium ferrocyanide more particularly reacts with complexed 6OCo in radioactive effluents by displacement of the chemical equilibrium towards the more stable ferrocyanide form, the mechanism being accentuated by the precipitation of the double ferrocyanide.

According to a second embodiment of the process according to the invention, separation of the ruthenocyanide takes place by fixing the latter to an anion exchange resin. In this case, it is possible to fix the alkali metal ferrocyanide to an anion exchange resin and then contact the rutheniumcontaining aqueous solution with said resin in order to fix the ruthenium thereto. The anion exchange resin used can be of type Dowex 1 X 8.

During said contacting, an exchange occurs between the Fe(II) of the ferrocyanide and the Ru(II) in solution, so that the ruthenium is fixed to the anion exchange resin.

According to a variant of said second embodiment of the process according to the invention, addition firstly takes place to the ruthenium-containing aqueous solution of the alkali metal ferrocyanide and then said aqueous solution is contacted with an anion exchange resin in order to fix the ruthenocyanide ions to the resin. Once again, it is possible to use anionic resins of the Dowex 1 X 8 type.

Thus, the hexacyano ferrate and ruthenocyanide ions present in the solution after adding alkali metal ferrocyanide are highly negatively charged and have a very great affinity for this type of resin.

According to a third embodiment of the process according to the invention, the alkali metal ferrocyanide is added to the aqueous solution and then the ruthenium is recovered by concentrating said solution by evaporation.

Thus, after contacting the solution with an alkali metal ferrocyanide in order to exchange the iron of the ferrocyanide by ruthenium, the aqueous solution is evaporated, which makes it possible to separate the ruthenium in residue form.

In this third embodiment of the process, the ferrocyanide addition makes it possible to stabilize the ruthenium at valency II in the form of stable complexes, thus obviating the problems caused by the presence in the oxidizing medium of highly volatile and extremely toxic Ru(IV) compounds.

Other features and advantages of the invention can be gathered from the following illustrative and non-limitative description of examples, with reference to the attached drawings, wherein show: Fig. 1 A graph showing the variations of the ruthenium extraction efficiency (as a %) as a function of the pH of the aqueous solution, labelled by a radioactive effluent from the La Hague plant.

Figs. 2 and 3 Graphs showing the variations of the ruthenium extraction efficiency as a function of the ratio of the copper/ferrocyanide molar concentrations; Fig. 2 relating to a sea water and Fig. 3 to a sea water labelled in the laboratory by a radio active solution from the La Hague fuel reprocessing plant.

Fig. 4 A graph showing the evolution of the ruthenium extraction efficiency as a function of the added potassium ferrocyanide quantity, in a radio active effluent-labelled sea water.

Figs. 1, 2, 3 and 4 correspond to tests using potassium ferrocyanide.

Fig. 5 is a graph showing the evolution of the ruthenium extraction efficiency as a function of the copper/ferrocyanide molar concentration ratio, in a sea water labelled in the laboratory by a radioactive solution of effluents from the La Hague fuel reprocessing plant and differing from Fig.

3 by the use of sodium ferrocyanide in place of potassium ferrocyanide.

EXAMPLE 1 This example studies the influence of the pH of the aqueous solution on the ruthenium adsorption level.

The starting product is constituted by aqueous sea water samples with a volume of 50 cm3 having a ruthenium activity measured by y spectrometry of 3~10-4Bq/l. The samples are subject to magnetic agitation and their pH is adjusted to between 1 and 9.5 by adding dilute HNCt or NaOH solutions. Whilst still accompanied by stirring, to them is then added 200 p1 of a 1.56p10-1 mole/I potassium ferrocyanide solution in order to bring about the iron/ruthenium exchange, which corresponds to a 6.24e10-4 mole/I ferrocyanide concentration.This is followed by the addition of 400 p1 of a 2.73*10-1 mole/I copper nitrate solution Cu(NO3)23H20, which corresponds to a copper/ferrocyanide molar ratio of 3.5.

It is allowed to settle for 12h, followed by the sampling of 25 ml of supernatant and the measurement of its residual ruthenium activity by gamma spectrometry, in order to calculate the extraction efficiency (in %), which corresponds to the ratio of the ruthenium activity in the precipitate to the ruthenium activity in the starting solution.

The results obtained are given in Fig. 1 showing the ruthenium extraction efficiency (in %) as a function of the pH of the aqueous solution. It can be seen that the best results are obtained in the pH range 4 to 6.

EXAMPLE 2 In this example relating to environmental sea water, the ruthenium activity is approximately 170 mBq/l. The same operating procedure as in example 1 is followed and the influence of the copper/ferrocyanide molar ratio in the range 0.5 to 5 is studied on the extracted ruthenium quantity (as a %). Each 115 litre sea water sample is brought to pH 4.5 in the presence of 0.1 mole of potassium ferrocyanide. The results are given in Fig. 2. In the latter the ruthenium extraction efficiency with respect to the collected precipitate with the maximum activity is fixed in arbitrary manner at 100%. It can be seen that the best results are obtained with a molar ratio of 3.

EXAMPLE 3 This example also studies the influence of the copper/ferrocyanide molar ratio on the ruthenium extraction level, but working takes place at a pH of 5.1 with a ferrocyanide concentration of 6.24~10-4 mole/l in labelled sea water, where the ruthenium activity is known (3~104 Bq/l).

The results obtained are given in Fig. 3, which also represents the evolution of the ruthenium extraction efficiency as a function of the copper/ferrocyanide molar ratio. Here again the best results are obtained as from a molar ratio of 3.

EXAMPLE 4 This example studies the influence of the potassium ferrocyanide quantity used on the ruthenium extraction efficiency. The same operating procedure as in example 1 is followed using ferrocyanide quantities between 0.5~10-4 and 30e10-4 mole/l, a pH of 4.8 and a copper/ferrocyanide molar ratio of 3.5.

The results are given in Fig. 4, which represents the variations of the ruthenium extraction efficiency < as a %), as a function of the added potassium ferrocyanide quantity. It can be seen that good results are obtained as from a ferrocyanide concentration of 4~10-4 mole/I and if the ferrocyanide concentration is increased, the efficiency also increases, but slowly. The limit corresponding to 100% fixed ruthenium is reached for a potassium ferrocyanide concentration of 4X10-3 mole/l.

EXAMPLE 5 In this example, use is made of the process according to the invention for eliminating the ruthenium in 115 1 of water with a ruthenium activity of approximately 170 mBq/l. To the water is added 5.10-2 mole of potassium ferrocyanide and agitation is maintained by bubbling for 5 minutes. This is followed by an adjustment of the pH to 4.8 by adding concentrated nitric acid and then 2g10-1 mole of copper nitrate Cu(NC3)2, 3H20 is added and stirring is maintained for 12 hours. It is allowed to settle for 12 hours and the precipitate is filtered after eliminating the supernatant. The precipitate is dried at 600C and conditioned for measuring its ruthenium content by gamma spectrometry. There is a substantially 100% extraction of the ruthenium present.

Thus, the process according to the invention makes it possible to eliminate all the ruthenium present in the effluents, which is an important advance for the environment.

In 4-ln the field of environmental sciences, the process according to the invention can also be used for detecting the ruthenium employed as a marker for bodies of water in seas and oceans. Thus, the short radioactive half-life of 106Ru (1 year) makes it a very advantageous marker for evaluating the transit time of bodies of water. Thus, the precise measurement of the ruthenium activity of sea water, no matter what its physicochemical form, provides an accurate understanding of the processes involved in the dispersion of effluents in sea water.

EXAMPLE 6 In this example, the operating procedure differs from that of example 3 solely through the replacement of potassium ferrocyanide by the sodium salt Fe(CN)#Na#. On the basis of a sea water 106Ru-labelled by an effluent from the La Hague plant (39104Bq/l), a study is made of the influence of the copper/ferrocyanide molar ratio on the ruthenium extraction level at a pH of 5.1 with a sodium ferrocyanide concentration of 6.24*10-4 mole/I.

The results obtained are given in Fig. 5. It can be seen that the use of sodium ferrocyanide makes it possible to reach the maximum ruthenium extraction as soon as the copper/ferrocyanide molar ratio is equal to 2, whereas in the case of potassium ferrocyanide, said maximum extraction requires a ratio of 3.

This difference of behaviour between sodium and potassium ferrocyanides is due to the fact that the single copper ferrocyanide Fe(CN)6Cu2 does not appear to form from potassium ferrocyanide. Thus, the potassium salt gives addition compounds complying with the general formula [ Fe(CN)6Cu2 ] n,Fe(CN)6K4, in which n varies with the copper/potassium ferrocyanide molar ratio used for forming the precipitate between 1 and 5.

Claims

1. Process for the separation of the ruthenium present in an aqueous solution, characterized in that it comprises contacting said solution with an alkali metal ferrocyanide in order to exchange the iron of the ferrocyanide by ruthenium and thus form a ruthenocyanide, followed by the separation of the thus formed ruthenocyanide.

2. Process according to claim 1, characterized in that it comprises the following stages: a) adding to the ruthenium-containing aqueous solution an alkali metal ferrocyanide in order to exchange the iron of the ferrocyanide by ruthenium, b) adding to said solution a copper salt in order to form a double alkali metal and copper ferrocyanide precipitate entraining the ruthenium and c) separating the thus obtained precipitate.

3. Process according to claim 2, characterized in that the copper salt is copper nitrate.

4. Process according to claim 1, characterized in that the alkali metal ferrocyanide is fixed to an anion exchange resin and in that the ruthenium-containing aqueous solution is contacted with said resin to fix the ruthenium thereto.

5. Process according to claim 1, characterized in that alkali metal ferrocyanide is added to the rutheniumcontaining aqueous solution and in that the aqueous solution is then contacted with an anion exchange resin to fix the ruthenocyanide ions to the resin.

6. Process according to claim 1, characterized in that alkali metal ferrocyanide is added to the rutheniumcontaining aqueous solution and then the ruthenium is recovered by concentrating said solution by evaporation.

7. Process according to any of the claims 1 to 6, characterized in that the alkali metal ferrocyanide is a potassium ferrocyanide.

8. Process according to any one of the claims 1 to 6, characterized in that the alkali metal ferrocyanide is a sodium ferrocyanide.

9. Process according to claim 2, characterized in that the pH of the aqueous solution is adjusted to a value from 4 to 6 prior to adding a copper salt to the solution.

10. Process according to claim 2, characterized in that the copper salt/alkali metal ferrocyanide molar ratio is at least 2.

11. Process according to claims 7 and 10, characterized in that the copper salt/potassium ferrocyanide molar ratio is at least 3.

12. Process according to claims 8 and 10, characterized in that the copper salt/sodium ferrocyanide molar ratio is at least 2.

13. Process according to any one of the claims 1 to 7, characterized in that the alkali metal ferrocyanide quantity used is at least 4.10-4 molefl of aqueous solution.

14. Process according to claim 13, characterized in that the alkali metal ferrocyanide quantity used is at least 4~10-3 mole/I of aqueous solution.

IS. Process according to claim 2, characterized in that the starting aqueous solution also contains cesium and/or cobalt and in that the cesium and cobalt are simultaneously entrained on the double ferrocyanide precipitate.

16. A process according to claim 1 and substantially as described with reference to the Examples herein and with reference to the accompanying drawings.