EP0189799B1 - Process for separating cesium ions from aqueous solutions - Google Patents

Description

The invention relates to a process for the selective separation of cesium ions from acidic aqueous radioactive solutions, in which a precipitant is added to the aqueous solution and the resulting precipitate containing the Cs ⁺ ions is separated from the solution.

In its capacity as a hard gamma emitter, Cs-137 is a particularly undesirable fission product in medium-radioactive, aqueous waste (MAW). A selective separation of the Cs-137 would make the processing of medium-active waste considerably easier. After the Cs-137 has been separated from the MAW, the shielding for the concentrates and / or the solidified repository can be omitted entirely or at least partially. Furthermore, such a method can also be used well for the extraction or separation of Cs isotopes from highly active waste solutions, such as those which occur in the reprocessing of nuclear fuels in the first extraction cycle. Here, the extraction of pure isotopes or isotope mixtures of cesium would be of practical importance for radiochemical applications and as a radiation or heat source.

Attempts have been made to precipitate the Cs ⁺ ions with sodium tetraphenylborate (trade name Kalignost), but it has been found that such a precipitation can neither be carried out selectively nor in an acidic environment.

The separation of cesium was mainly carried out by co-precipitation reactions using other known methods. However, the co-precipitation did not provide satisfactory decontamination factors (DF values) for Cs. For this reason, other methods were sought which should enable selective separation of the cesium radionuclides.

The extraction processes previously developed for Cs ⁺ ions are not suitable for the separation of Cs ^* from a typical MAW with its high content of NaN0 ₃ and free nitric acid.

J. Rais and P. Selucky proposed an extraction system for the separation of Cs ⁺ from aqueous solutions, which 2,3,11,12-dibenzo-1,4,7,10,13,16-hexa-oxa-cyclooctadeca-2 , 11-diene (Dibenzo-18-crown-6; short name DB-18-C-6) used in conjunction with sodium tetraphenylborate (CS-PS 149.404). However, the process is limited to alkaline Cs ⁺ solutions ( _PH 11 to 13): sodium tetraphenylborate is also hydrolyzed in this solution. The process also only works well in the absence of larger amounts of Na ⁺ and K ⁺ .

The invention has for its object to provide a method according to the type mentioned, with which cesium selectively over other alkali metal cations, such as. B. Li ⁺ , Na ⁺ and K ^* , can be separated with high effectiveness from acidic aqueous radioactive waste solutions.

• a) auf eine Cs^+-Konzentration im Bereich von 10-¹ bis 10^-3 mol/1 eingestellt wird,
• b) der Lösung aus Schritt a) das Fällungsmittel zugegeben und die entstehende Fällung abgetrennt wird und
• c) im Falle einer erwünschten Dekontamination von vorhandenem Cs-137 der Schritt a) mit inaktivem Cäsium (als Schlepper) und Schritt b) einmal oder mehrmals wiederholt werden.

The object is achieved in that the precipitant used is a sodium or lithium tetraphenyl borate bearing fluorine substituents on the phenyl rings, in which the phenyl rings are each disubstituted in the 2,4-position or substituted fourfold in the 2,3,5,6-position or are substituted five times in the 2,3,4,5,6-position. An effective implementation of the method is characterized in that the addition of the precipitant and / or the precipitation reaction as such takes place or is carried out at a temperature between 239 K and 303 K. The precipitant is preferably added to the solution in a slight excess with respect to the cesium content, for example from 1.2 times to 5 times the stoichiometrically required amount. A particularly good separation is obtained with the process according to the invention if the solution containing cesium ions

• a) is set to a Cs ⁺ concentration in the range from 10- ¹ to 10 ^-3 mol / 1,
• b) the precipitant is added to the solution from step a) and the resulting precipitate is separated off and
• c) in the case of a desired decontamination of existing Cs-137, step a) with inactive cesium (as a tug) and step b) are repeated one or more times.

The precipitation reaction can be carried out in the presence of an acid concentration in the range from 0 to 6 mol / l.

The acid stability of the precipitant molecule and the resulting poorly soluble precipitate is increased by the introduction of at least two fluorine substituents in the phenyl rings of the molecule, which largely prevent positive charges on the phenyl rings from stabilizing and thus initiate the breakdown of the molecule. The electron-withdrawing substituents protect the phenyl rings from electrophilic attacks.

• Natriumtetrakis (2.4-difluorphenyl) borat :
  (Siehe Schema Seite 3f.)
  
  

The synthesis of precipitants which can be used for the process according to the invention can, for example, proceed according to the following scheme:

• Sodium tetrakis (2,4-difluorophenyl) borate:
  (See diagram on page 3f.)
  
  

2,4-difluorobromobenzene is mixed with n-butyllithium (n-BuLi) at -78 ° C in di-ethyl ether and a BC1 ₃ solution in hexane is added dropwise to the phenyllithium derivative formed. After warming up to room temperature, hydrolysis is carried out, the ether is stripped off over water, the aqueous phase is mixed with a little activated carbon, filtered off and mixed with aqueous trimethylamine solution. The resulting trimethylammonium salt is recrystallized from methanol / water and dried. It is converted with sodium hydride into the corresponding alkali salt, which can be recrystallized from chloroform / acetone if necessary.

The compound lithium tetrakis (2,3,5,6-tetrafluorophenyl) borate was prepared in the same way (with lithium hydride) instead of sodium hydride.

Lithium tetrakis (pentafluorophenyl) borate production is by A.G. Massey, A.J. Park: J. Organometal. Chem., 2 (1964), pp. 245 to 250.

The products were characterized using IR, NMR and elemental analysis. To make the salts pure, the «detour via the trimethylammonium salts is necessary. For precipitation reactions, however, the aqueous solutions of the precipitation reagents are sufficient, the concentrations of which can be easily determined by quantitative precipitation with trimethylamine.

Solubilities of the corresponding Cs salts in pure water (298 K):

The solubilities were determined using radiometry.

All reagents form poorly soluble precipitates with Cs ⁺ , but not with potassium. Coprecipitation of potassium only occurs with lithium tetrakis (2,3,5,6-tetrafluorophenyl) borate with a K to Cs ratio ≥ 100, with sodium tetrakis (2,4-difluorophenyl) borate and lithium tetrakis (pentafluorophenyl) borate only with K ⁺ - to Cs ^{+ -} ratio> 100.

Slightly soluble Cs ⁺ precipitates also form the sodium tetrakis (4-fluorophenyl) borate known from US Pat. No. 3,468,959 and lithium tetrakis (2,4,6-trifluorophenyl) borate, the first compound in the neutral and alkaline region with good selectivity, the second compound Even in acidic to 3 molar acid, however, coprecipitation with K ⁺ takes place from the ratio K ⁺ : Cs ⁺ as 1: 1.

Example 1 (Cs ^{+ -} Precipitates from MAW Simulate)

The composition of the simulate used is listed in Table 1.

Inactive Cs ^{+ was} added to the MAW simulate (Cs ⁺ concentrations 1.0 · 10 ^-3 or 1.0 · 10-2 mol / I); the solutions were doped with Cs-137, regardless of the inactive Cs ⁺ concentration with the same activity (1 µ Ci / mi). The precipitant was added in duplicate, regardless of whether it was added as a solution or as a solid. After about 24 hours, samples were taken, filtered off, the activity of the filtrate was measured and the Cs ^μ concentration was then calculated by calibration. The results are shown in Tables 2 to 4.

• (1) Natriumtetrakis(2,4-difluorphenyl) borat
• (2) Lithiumtetrakis(2,3,5,6-tetrafluorphenyl) borat
• (3) Lithiumtetrakis(pentafluorphenyl) borat
  
  
  

The following were used as precipitants:

• (1) Sodium tetrakis (2,4-difluorophenyl) borate
• (2) Lithium tetrakis (2,3,5,6-tetrafluorophenyl) borate
• (3) Lithium tetrakis (pentafluorophenyl) borate
  
  
  

Example 2 (Cs ⁺ Precipitation from 5m Nitric Acid)

Carried out as described in Example 1, but only with compound (1).

The composition of the solution used was chosen so that it can simulate a HAW concentrate solution in this context (HAW = highly radioactive waste).

5 molar HN0 ₃ was treated with inactive Cs ⁺ (Cs ^{+ -} concentration 1.0 · 10 ^-2 mol / I). The solutions were doped with Cs-137 (1 µm Ci / ml). The precipitant was added in double excess. After 24 hours, samples were taken, filtered, the activity of the filtrate measured and the Cs ⁺ concentration calculated by calibration. The results are shown in Tables 5 and 6.

Sodium tetrakis (2,4-difluorophenyl) borate (compound 1) is acid-stable up to 6m-HN0 ₃ and at tempera tures up to 293 K. Under conditions such as those prevailing in radioactive waste solutions, the Cs salt exhibits the lowest solubility of the investigated compounds. Depending on the temperature (239 to 293 K), the solubilities range between 1.0 10-5 and 8.0 · 10-5 mol / l.

(In the case of Kalignost, such a solubility determination cannot be carried out because the decomposition of the compound proceeds too quickly under the test conditions). The precipitation of Cs ⁺ with compound (1) is not affected by K ⁺ . There is no coprecipitation with the potassium compound.

The precipitation endpoints are determined by the solubility of the corresponding Cs ⁺ salts.

Example 3 (Cs ⁺ Precipitation from HAW Simulate)

The composition of the HAW simulate can be seen in Table 7. The simulate solution was 5 molar in HN0 ₃ and contained most of the elements in the nitrate salt form.

The solution was doped with Cs-137 (1 µCi / MI). The precipitation was carried out as described in Example 2, but only with compound 3. The result is shown in Table 8:

Example 4 (Efficacy)

• 1. Einstellen der MAW-Lösung auf eine inaktive Cs⁺-Konzentration von 1.0 · 10^-3 mol/l. Fällung mit (1) in doppeltem Überschuß und Abtrennung des Niederschlags (durch Filtration oder Zentrifugation) liefert einen Dekontaminationsfaktor (DF) von 17. Die resultierende Cs⁺-Konzentration von Ca. 6.0 · 10-⁵ mol/l wird durch inaktives Cs⁺ wieder auf 1.0 · 10^-3 mol/I eingestellt, erneut gefällt und das ganze beliebig oft wiederholt. Beim viermaligen Cyclieren wird so ohne großen Materialaufwand ein DF für das aktive Cs von ca. 80 000 erreicht (Fällungstemperatur jeweils 293 K).
• 2. Einstellen der MAW-Lösung auf eine inaktive Cs⁺-Konzentration von 1.0 · 10-² mol/I. Verfahren wie oben. Die erste Fällung liefert einen DF von 170, beim nächsten Cyclus einen DF von 29000 usw. (Fällungstemperatur jeweils 293 K).
• 3. Wie bei 1., Fällungstemperaturen jetzt aber 277 K. Die erste Fällung liefert einen DF von 26, bei der vierten Fällung ist der DF größer als 400 000 .
• 4. Wie bei 2., Fällungstemperaturen jetzt aber 277 K. Die erste Fällung liefert einen DF von 280, die zweite Fällung bereits einen DF größer 78 000.
• 5. Wie bei 1., Fällungstemperaturen aber 260 K. Die 1. Fällung liefert einen DF von 62, bei der 3. Fällung ist der DF >230 000.
• 6. Wie bei 2. Fällungstemperaturen aber 260 K. Die 1. Fällung lieferte einen DF von 770, die 2. Fällung bereits einen DF > 590 000.
• 7. Einstellen der 5m HN0₃ auf eine inaktive Cs⁺-Konzentration von 10-² mol/l. Verfahren sonst wie bei 1). Die 1. Fällung liefert einen DF von 384, die 2. Fällung bereits einen DF von 148 000 (Fällungstemperatur jeweils 293 K).
• 8. Wie bei 7., Fällungstemperaturen aber 260 K. Die. 1. Fällung liefert einen DF von 667, die 2. Fällung bereits einen DF von 444 000.

Precipitation for Cs-137 can now be achieved by precipitation, for example with compound (1), in various ways:

• 1. Adjust the MAW solution to an inactive Cs ⁺ concentration of 1.0 · 10 ^-3 mol / l. Precipitation with (1) in double excess and separation of the precipitate (by filtration or centrifugation) gives a decontamination factor (DF) of 17. The resulting Cs ⁺ concentration of approx. 6.0 · 10- ⁵ mol / l is set to 1.0 · 10 ^-3 mol / I again by inactive Cs ⁺ , precipitated again and the whole process repeated as often as desired. When cycled four times, a DF for the active Cs of approx. 80,000 is achieved without great expenditure of material (precipitation temperature in each case 293 K).
• 2. Setting the MAW solution on an inactive Cs ⁺ concentration of 1.0 x 10- ² mol / I. Procedure as above. The first precipitation gives a DF of 170, for the next cycle a DF of 29000 etc. (precipitation temperature 293 K each).
• 3. As for 1., but precipitation temperatures now 277 K. The first precipitation gives a DF of 26, with the fourth precipitation the DF is greater than 400,000.
• 4. As for 2., but precipitation temperatures now 277 K. The first precipitation gives a DF of 280, the second precipitation already a DF greater than 78,000.
• 5. As for the 1st, but precipitation temperatures but 260 K. The 1st precipitation gives a DF of 62, with the 3rd precipitation the DF is> 230,000.
• 6. As with the 2nd precipitation temperature but 260 K. The 1st precipitation gave a DF of 770, the 2nd precipitation already a DF> 590,000.
• 7. Setting 5m HN0 ₃ on an inactive Cs ⁺ concentration of 10- ² mol / l. Otherwise proceed as for 1). The 1st precipitation gives a DF of 384, the 2nd precipitation already a DF of 148,000 (precipitation temperature 293 K each).
• 8. As for 7th, but precipitation temperatures 260 K. Die. 1st precipitation gives a DF of 667, the 2nd precipitation already a DF of 444,000.

Example 5 (Separation of the Cs Precipitation from Compound 3 by Liquid Extraction from Aqueous Solution)

Inactive Cs ^{+ was} added to water (Cs ⁺ concentration 1.0 · 10 ^-3 mol / I). As in the previous examples, the solutions were doped with Cs-137. The precipitant was added in a simple excess. After 24 h, samples were taken (to compare the effectiveness of the precipitation separation methods, on the one hand filtration, on the other hand extraction), filtered and the residual concentration of Cs ⁺ in the filtrate solutions of the samples was determined. It was 6.5 10 ^-5 mol / l.

The solutions containing the precipitates were then extracted with various organic solvents and the residual Cs ⁺ concentration in the aqueous phase was measured. The results are shown in Table 9.

Example 6 (Separation of the Cs ⁺ Precipitation from Compound 3 by Liquid Extraction from HAW Simulate)

The experiments and the comparison of the separation methods were carried out as described in Example 5, but the precipitant was added here in a double excess. The residual Cs ⁺ concentration after filtration of the samples was 7.2 · 10 ^-4 mol / l.

The results of the extractions are shown in Table 10.

Other organic solvents can also be used as extractants, but their effectiveness has not been investigated.

Claims (6)

1. Process for selectively separating caesium ions from acidic aqueous radioactive solutions, wherein a precipitating agent is added to the aqueous solution, and the resultant precipitate containing the Cs⁺ ions is separated from the solution, characterised in that a sodium or lithium tetraphenyl borate. which carries fluorine substituents on the phenyl rings, is used as the precipitating agent, wherein the phenyl rings are each disubstituted in the 2,4 positions or substituted four times in the 2,3,5,6 positions or substituted five times in the 2,3,4,5,6 positions.

2. Process according to claim 1, characterised in that the addition of the precipitating agent and/or the precipitation reaction as such is effected or respectively carried out at a temperature between 239 K and 303 K.

3. Process according to any of the preceding claims, characterised in that an excess amount of precipitating agent is added, from 1.2 times to 5 times the stoichiometrically required quantity, in relation to the Cs⁺ concentration in the solution.

4. Process according to claim 1, characterised in that

a) the solution containing caesium ions is adjusted to a Cs^* concentration in the range of 10-¹ to 10^-3 mol/I ;

b) the precipitating agent is added to the solution originating from step a), and the resultant precipitate is separated ; and

c) if a decontamination of existing Cs-137 is desired, step a) with inactive caesium and step b) are repeated once or several times more.

5. Process according to claim 1, characterised in that the precipitation reaction is carried out in the presence of an acid concentration in the range of 0 to 6 mol/I.

6. Process according to claim 1, characterised in that the separation of the precipitate from the solution is effected by means of extraction with an organic solvent selected from the group of chloroform ; diethyl ether/petrol ether (40-60) 2 : 1 [Vol./Vol.] ; 4-methyl-2-pentanone (5 % by vol. in toluene).