JPH04285017A - Method for separating ruthenium and, as required, cesium and cobalt existing in aqueous solution such as flowing out from fuel reprocessing facility where radiation has already been irradiated - Google Patents

Abstract

PURPOSE: To provide a method for separating ruthenium and optionally cesium and cobalt present in an aqueous solution such as an effluent from an irradiated fuel preprocessing equipment.

CONSTITUTION: In order to replace iron of a ferrocyanide by ruthenium and thus form a ruthenocyanide, the aqueous solution is brought into contact with an alkali metal ferrocyanide, e.g. potassium or sodium, and next the ruthenocyanide is 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. It is possible to extract substantially all ruthenium existing in the aqueous solution.

COPYRIGHT: (C)1992,JPO

Description

[Detailed description of the invention]

0001

FIELD OF THE INVENTION The present invention relates to a method for separating ruthenium present in an aqueous solution. In particular, the invention has application to the removal of ruthenium present in aqueous solutions such as effluents from irradiated nuclear fuel reprocessing facilities. The invention can also be used to simplify the detection and analysis of the ruthenium content of natural waters, such as seawater.

0002

[Prior Art] Ruthenium is used in nuclear reactors (electro
It accounts for 6% of the total weight of artificial radioisotopes produced in nuclear reactors) and appears in the reprocessing cycle of irradiated nuclear fuel.

Ruthenium has four isotopes,
Their respective masses are 103, 105, 106 and 107, and their radiative half-lives are 39.8 days, 4.5 hours, 1 year and 4 minutes. Due to the short half-lives of the 105Ru and 107Ru isotopes and the time required to reprocess irradiated fuel, only 103Ru and 106Ru may require consideration.

Assuming all possible oxidation states between I and VIII from the electronic structure of ruthenium, III and IV are the most stable. In all its forms, ruthenium can form numerous complexes,
Some of them are extremely stable and very difficult to hydrolyze, causing problems in removing ruthenium from aqueous effluents from irradiated nuclear fuel reprocessing plants.

In such reprocessing plants, irradiated fuel is dissolved in concentrated nitric acid. Furthermore, ruthenium is oxidized at its maximum valence to form the tetraoxide form RuO
It becomes 4. This relatively unstable and highly volatile compound readily reacts with reducing ligands contained in the medium. Namely:

[0006]

[Chemical formula 1]

Ruthenium reacts predominantly with nitrosyl cations (NO+), which exist mostly in this form in acidic media. These bonds are stabilized by a highly pronounced inverted π-ligand-metal coordination favored by the charge of NO3+, as seen in Mössbauer spectroscopy. This results in the stable substance Ru(II)NO3+, which exists mostly in this form compared to Ru(III)NO3+, which is formed from neutral NO groups.

That is, most of the complexes present in the solution are generally mononuclear and hexa-coordinated Ru(II) nitrosyl complexes, and the other ligand is substantially a nitrosyl complex (NO3-). , nitro group (NO2, NO2-), hydroxy group (OH-) and aqua group (H2O).

Among these complexes, the general formula: RuNO
The nitro complex of nitrosyl-ruthenium according to (NO2)x(NO3)y(H2O)2(OH)t, where x, y and t are such that x+y+t=3, is extremely stable. It is difficult to separate because it is difficult to hydrolyze.

In the conventional irradiated nuclear fuel reprocessing process, the fission products are isolated by forming a complex with tributyl phosphate and extracted with a specific low-density, non-polar organic solvent. .

In the case of nitro complexes of nitrosyl-ruthenium, in particular the tetranitro derivative RuNO(NO2)4OH2-, it is not possible to replace the NO2 and OH groups with tributyl phosphate ligands, and therefore complexes of this type Remains in the aqueous phase.

[0012] Like cobalt sulfide precipitation in acidic or basic media, the above complexes are difficult to eliminate by conventional purification processes. 106Ru-Rh pair is L
a In the composition of the effluent from the Hague plant,
It occurs at a rate of 80% of the activity of gamma radionuclides.

Similarly, since such nitro complexes hydrolyze only very slowly, it is also difficult to extract them from nitrosyl-ruthenium by oxyhydroxide-based adsorbents. Another problem that arises with ruthenium is
Because it is difficult to separate, it is difficult to accurately measure the activity of ruthenium in analytical water samples.

The method currently used to analyze ruthenium in natural waters is coprecipitation with manganese dioxide. Cs+ ions, which have no affinity for dioxide, are hydrated ions Co(H2O) on the surface of ferrocyanide particles.
) 62+, the cesium level in water can also be determined by the incorporation of the cobalt-potassium ferrocyanide double salt in powder form.

However, even with this method, the nitro complex cannot be extracted from ruthenium, and the standard 11
The working efficiency in a 5 liter sample does not exceed 50-60% of the ruthenium actually present. Thus, it is only possible to analyze the hydrolyzed form of nitrosyl-ruthenium that can be adsorbed onto manganese dioxide, since ruthenium is not extracted with the commercially available powdered form of cobalt-potassium ferrocyanide double salt.

The present invention is particularly directed to a method for the separation of ruthenium present in aqueous solutions, which also includes nitrosyl-ruthenium nitro complexes, which are of great importance in the treatment of aqueous effluents and in the determination of ruthenium present in water samples. The present invention relates to a method capable of extracting substantially all of the ruthenium contained therein.

According to the invention, the method for separating ruthenium present in an aqueous solution comprises the steps of: replacing iron in ferrocyanide with ruthenium to form ruthenocyanide;
It consists of contacting an aqueous solution with an alkali metal ferrocyanide and then separating the ruthenocyanide.

That is, the present invention provides Fe(I) according to the following reaction.
Based on the exchange reaction between I) and Ru(II): Fe
(II)CN64-+Ru(II)NO3+→Ru(I
I) CN64-+Fe(II)NO3+.

The above reaction can take place due to the similarity of the Fe(II) CN64- ion and the Ru(II) NO3+ ion in terms of the degree of oxidation of the metals involved and the type of ligand.

This exchange reaction is preferably carried out using an excess of alkali metal ferrocyanide in order to convert all the ruthenium of the ruthenium complex to ruthenocyanide.

The alkali metal ferrocyanide can be, for example, potassium ferrocyanide or sodium ferrocyanide, preferably sodium ferrocyanide.

Generally, the amount of alkali metal ferrocyanide used is 4.10-4 mol/l or more, but in order to entrain all the ruthenium, the amount is 4.10-3 mol/l.
The above is preferable.

According to the invention, the ruthenocyanide thus formed is precipitated by a suitable reagent or
Separation can be achieved by fixation on an anion exchange resin or by evaporation of the solution.

According to a first embodiment of the method of the invention:
The method comprises: a) adding an alkali metal ferrocyanide to a ruthenium-containing aqueous solution to exchange the iron in the ferrocyanide with ruthenium; and b) precipitation of the alkali metal and copper double ferrocyanide with entrainment of the ruthenium. c) separating the precipitate thus obtained.

That is, although the method of the present invention allows the separation of ruthenium on the double ferrocyanide precipitate, this is not possible with the direct addition of commercially available double ferrocyanide in powder form.

In the first embodiment of the process of the invention, various copper salts can be used, especially copper nitrate.

Double ferrocyanides of alkali metals and copper, such as double ferrocyanides 5Fe(CN)6Cu2,
To form Fe(CN)6K4, it is important to use enough copper to precipitate any ferrocyanide present. Further, the molar ratio of copper salt/alkali metal ferrocyanide is preferably 2 or more.

When potassium ferrocyanide is used, the copper salt/potassium ferrocyanide molar ratio is preferably 3 or more. In the case of sodium ferrocyanide, the molar ratio is preferably 2 or more.

In a first embodiment of the method of the invention, the pH of the aqueous solution influences the efficiency of adsorption of ruthenium onto the double ferrocyanide precipitate.

In order to obtain the highest ruthenium adsorption efficiency, it is also advantageous to adjust the pH of the aqueous solution to a suitable value of 4 to 6 before adding the copper salt.

The pH of the aqueous solution can be adjusted using a suitable reagent, such as nitric acid, either before contacting the aqueous solution with the alkali metal ferrocyanide or after replacing the iron with ruthenium.

The first embodiment of the method according to the invention can also be used for the purification of aqueous effluents containing not only ruthenium but also cesium and/or cobalt. That is, in the presence of excess copper, the alkali metal and copper double ferrocyanide precipitate entrains all the cesium. Therefore, the use of special adsorbents for cesium may be unnecessary.

In the case of cobalt, potassium ferrocyanide replaces 60Co in the radioactive effluent by shifting the chemical equilibrium towards the more stable ferrocyanide form.
Particularly reactive with complexes. This mechanism is enhanced by the precipitation of double ferrocyanide.

According to a second embodiment of the method of the invention:
Separation of ruthenocyanide is carried out by immobilizing it on an anion exchange resin. In this case, in order to fix the alkali metal ferrocyanide to the anion exchange resin and then fix the ruthenium thereto,
A ruthenium-containing aqueous solution can be contacted with the resin. The anion exchange resin used is Dowex 1×
It can be of eight types.

[0035] During the contact, F of the ferrocyanide is
An exchange occurs between e(II) and Ru(II) in the aqueous solution, and ruthenium is fixed on the anion exchange resin.

In a variant of said second embodiment of the method of the invention, an alkali metal ferrocyanide is first added to the ruthenium-containing aqueous solution, and then said aqueous solution is treated with an anion exchange resin in order to fix the ruthenocyanide ions on the resin. bring into contact. Again, an anionic resin of the Dowex 1×8 type can be used.

In this case, the hexacyanoferrate and ruthenocyanide ions present in the solution after addition of the alkali metal ferrocyanide are highly negatively charged and have a very high affinity for this type of resin. .

In a third embodiment of the process of the invention, ruthenium is recovered by adding an alkali metal ferrocyanide to an aqueous solution and then concentrating the solution by evaporation.

That is, when an aqueous solution is brought into contact with an alkali metal ferrocyanide to exchange the iron of the ferrocyanide with ruthenium and the aqueous solution is evaporated, the ruthenium can be separated in the form of a residue.

In a third embodiment of the invention, ruthenium of valence II can be stabilized in the form of a stable complex by the addition of ferrocyanide, thus eliminating the highly volatile and extremely toxic Ru. (IV) Problems caused by the presence of the compound in the oxidizing medium are avoided.

[0041]

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the invention will become apparent from the following description of an illustrative, non-restrictive example, taken in conjunction with the accompanying drawings, in which: FIG.

Example 1 In this example, the effect of the pH of the aqueous solution on the level of ruthenium adsorption was investigated.

The starting product has a volume of 50 cm3 with a ruthenium activity of 3.10-4 Bq/l, determined by gamma spectrometer.
It was a seawater sample. The sample was placed on a magnetic stirrer and diluted with HN.
pH 1-9 by adding O3 or dilute NaOH
．． Adjusted to 5. While stirring, 1.56・10-
200 μl of a 1 mol/l potassium ferrocyanide solution was added to carry out an iron/ruthenium exchange corresponding to a ferrocyanide concentration of 6.24·10 −4 mol/l. Then the copper/ferrocyanide molar ratio is 2, corresponding to 3.5.
．． 73.10-1 mol/l Copper nitrate solution Cu (NO3
) 400 μl of 23H2O was added.

After standing for 12 hours, 25 ml of the supernatant was collected, and its residual ruthenium activity was measured by gamma spectrometer, and the extraction efficiency (%) was determined, which corresponds to the ratio of ruthenium activity in the precipitate to ruthenium activity in the starting solution. was calculated.

The results obtained are shown in FIG. This figure shows the ruthenium extraction efficiency (%) as a function of the pH of the aqueous solution. It can be seen that the best results are obtained in the pH range of 4 to 6.

Example 2 In this example involving environmental seawater, the ruthenium activity is approximately 170 mBq/l. Following the same working procedure as in Example 1, the effect of a copper/ferrocyanide molar ratio in the range of 0.5 to 5 on the amount of ruthenium extracted (expressed as %) was investigated. Each 115 liter seawater sample was brought to pH 4.5 in the presence of 0.1 molar potassium ferrocyanide. The results obtained are shown in FIG. 2. In Figure 2, the ruthenium extraction efficiency for the recovered precipitate with the highest activity is
It is arbitrarily fixed at 100%. It can be seen that the best results were obtained with a molar ratio of 3.

Example 3 This example further demonstrates the effect of copper/copper on ruthenium extraction levels.
The influence of the molar ratio of ferrocyanide was investigated. However, in labeled seawater with known ruthenium activity (3・10-4Bq/l), the ferrocyanide concentration is 6.24.
- The experiment was conducted at a concentration of 10-4 mol/l and a pH of 5.1.

The results obtained are shown in FIG. Figure 3 also shows copper/
It represents the change in ruthenium extraction efficiency as a function of ferrocyanide molar ratio. Here too, the best results were obtained with a molar ratio of 3 or more.

Example 4 In this example, the influence of the amount of potassium ferrocyanide used on the ruthenium extraction efficiency was investigated. 0.5・10
The same working procedure as in Example 1 was followed using amounts of ferrocyanide from -4 to 30.10-4 mol/l, a pH of 4.8 and a copper/ferrocyanide molar ratio of 3.5.

The variation in ruthenium extraction efficiency (%) as a function of the amount of potassium ferrocyanide added is shown in FIG. Excellent results are obtained with ferrocyanide concentrations above 4.10-4 mol/l, and it can be seen that as the ferrocyanide concentration increases, the extraction efficiency also increases, albeit slowly. At a potassium ferrocyanide concentration of 4·10 −3 mol/l, a limit corresponding to 100% fixation of ruthenium is reached.

Example 5 In this example, the method of the invention was used to remove ruthenium in 115 liters of water having a ruthenium activity of approximately 170 mBq/l. Water, 5・10-
Two moles of potassium ferrocyanide were added and stirring continued by bubbling for 5 minutes. The pH was then adjusted to 4.8 by adding concentrated nitric acid, then 2.10-1 mol of copper nitrate Cu(NO3)2.3H2O was added and stirring continued for 12 hours. After standing for 12 hours and removing the supernatant, the precipitate was filtered. The precipitate was dried at 60° C. and the ruthenium content was determined by gamma spectrometer. Virtually 100% of the ruthenium present was extracted.

Thus, all the ruthenium present in the effluent can be removed by the method of the invention. This is an important advance for the environment.

In the field of environmental science, the method of the invention can also be used to detect ruthenium used as a marker for sea or ocean waters. That is, the radioactivity half-life of 106Ru is short (1 year), making it an extremely advantageous marker for evaluating the transit period in water bodies. Rigorous measurements of ruthenium activity in seawater, whatever its physicochemical form, will give an accurate understanding of the processes involved in the dispersion of effluents in seawater.

Example 6 In this example the working procedure differs from Example 3 only by replacing potassium ferrocyanide with the sodium salt Fe(CN)6Na4. The effluent from the La Hague plant produced 106Ru labels (3 and 104B).
q/l) based on seawater, the sodium ferrocyanide concentration was 6.24 10-4 mol/l, the pH was 5.1, and the influence of the copper/ferrocyanide molar ratio on the ruthenium extraction level. investigated.

The results obtained are shown in FIG. In the case of potassium ferrocyanide, the copper/ferrocyanide molar ratio is 3.
It can be seen that when sodium ferrocyanide was used, the maximum ruthenium extraction was reached as soon as the copper/ferrocyanide molar ratio reached 2.

This difference in behavior between sodium ferrocyanide and potassium ferrocyanide is due to the fact that the single salt copper ferrocyanide, Fe(CN)6Cu2, is not formed from potassium ferrocyanide. That is, potassium salt has the general formula [Fe(CN)6Cu2]n, Fe(CN)6K4(
n in the formula is 1 with the copper ferrocyanide/potassium ferrocyanide molar ratio used to form the precipitate.
-5).

[Brief explanation of the drawing]

FIG. 1 is a graph showing the change in ruthenium extraction efficiency (%) as a function of pH in aqueous solutions labeled with radioactive effluent from the La Hague plant.

FIG. 2 is a graph showing the change in ruthenium extraction efficiency as a function of copper/ferrocyanide (potassium ferrocyanide) molar concentration ratio in seawater.

FIG. 3 is a graph showing the variation of ruthenium extraction efficiency as a function of copper/ferrocyanide (potassium ferrocyanide) molar concentration ratio in seawater labeled in the laboratory by radioactive effluent from the La Hague fuel reprocessing plant. .

FIG. 4 is a graph showing the change in ruthenium extraction efficiency as a function of potassium ferrocyanide loading in radioactive effluent labeled seawater.

FIG. 5: Graph showing the variation of ruthenium extraction efficiency as a function of copper/ferrocyanide (sodium ferrocyanide) molar concentration ratio in seawater labeled in the laboratory by radioactive effluent from the La Hague fuel reprocessing plant. It is.

Claims (15)

[Claims] 1. A method for separating ruthenium present in an aqueous solution, comprising: contacting the aqueous solution with an alkali metal ferrocyanide to exchange the iron of the ferrocyanide with ruthenium to form a ruthenocyanide; A process characterized in that it consists in separating the ruthenocyanide thus formed. 2. a) adding an alkali metal ferrocyanide to a ruthenium-containing aqueous solution to exchange the iron in the ferrocyanide with ruthenium; and b) precipitation of the alkali metal and copper double ferrocyanide with ruthenium entrained. adding a copper salt to the solution to form a product; c
2. Process according to claim 1, characterized in that it comprises the steps of: ) separating the precipitate thus obtained. 3. The method of claim 2, wherein the copper salt is copper nitrate. 4. The method according to claim 1, wherein the alkali metal ferrocyanide is fixed to an anion exchange resin, and the ruthenium-containing aqueous solution is brought into contact with the resin to fix ruthenium thereon. 5. The method of claim 1, wherein the alkali metal ferrocyanide is added to an aqueous solution containing ruthenium, and the aqueous solution is then contacted with an anion exchange resin to fix ruthenocyanide ions to the resin. . 6. The method of claim 1, wherein ruthenium is recovered by adding the alkali metal ferrocyanide to an aqueous ruthenium-containing solution and then evaporating and concentrating the solution. 7. The method according to claim 1, wherein the alkali metal ferrocyanide is potassium ferrocyanide. 8. The method according to claim 1, wherein the alkali metal ferrocyanide is sodium ferrocyanide. 9. A method according to claim 2, characterized in that before adding the copper salt to the aqueous solution, the pH of the solution is adjusted to a value of 4 to 6. 10. The method according to claim 2, wherein the molar ratio of the copper salt/alkali metal ferrocyanide is 2 or more. 11. The method according to claim 7, wherein the copper salt/potassium ferrocyanide molar ratio is 3 or more. 12. The method according to claim 8, wherein the copper salt/sodium ferrocyanide molar ratio is 2 or more. 13. The method according to claim 1, wherein the amount of the alkali metal ferrocyanide used is 4·10 −4 mol or more per liter of aqueous solution. 14. The method according to claim 13, wherein the amount of the alkali metal ferrocyanide used is 4·10 −3 mol or more per liter of aqueous solution. 15. Process according to claim 2, characterized in that the starting aqueous solution further contains cesium and/or cobalt, said cesium and cobalt being simultaneously entrained in the double ferrocyanide precipitate.