WO1996011478A1

WO1996011478A1 - METHOD FOR PARTIALLY α-DECONTAMINATING AN AQUEOUS EFFLUENT

Info

Publication number: WO1996011478A1
Application number: PCT/FR1995/001288
Authority: WO
Inventors: Roger Guilard; Hervé CHOLLET; Philippe Guiberteau; Michel Guerin
Original assignee: Commissariat A L'energie Atomique
Priority date: 1994-10-05
Filing date: 1995-10-04
Publication date: 1996-04-18
Also published as: FR2725552A1; FR2725552B1

Abstract

Method for partially α-decontaminating an aqueous effluent containing at least one pollutant selected from copper, zinc, tantalum, gold and elements from the actinide or lanthanide series. The method comprises the steps of contacting the aqueous effluent with a silica gel and separating the contaminated effluent and the silica gel that has bound at least one of the pollutants, the effluent to be treated having a pH higher than or equal to 3. The method is particularly suitable for the treatment of effluents discharged during radiated fuel reprocessing and effluents discharged from nuclear power plants.

Description

PROCESS FOR PARTIAL α DECONTAMINATION OF AN AQUEOUS EFFLUENT

DESCRIPTION

The present invention relates to a decontamination process using silica gel to treat an α-contaminated aqueous effluent and recover pollutants present in said effluent. This process thus makes it possible to recover elements from the family of actinides or lanthanides, or even other heavy metals, such as copper, zinc, tantalum or gold.

Currently, the effluents from plants for reprocessing irradiated nuclear fuels and technological effluents from nuclear centers are treated in an industrial evaporation tower, that is to say a column with trays in which distillation is carried out under reduced pressure. Such a technique made it possible to deplete effluents with low α contamination, that is to say for example for the CEA center (Commissariat à l'Energie Atomique) in Valduc, a contamination of less than 1.5.10 ⁷ Bq / m ³ in element α and less than 50 g / 1 in saline charge. A concentrate is obtained containing almost all of the α activity and a salt load of the order of 300 g / 1. This concentrate is further processed and conditioned in concrete. This technique also made it possible to bring the evaporates which constitute 90% of the aqueous phase of the treated effluents, to an activity of 300 to 500 Bq / m ³ , therefore much lower than the rejection value currently authorized (1000 Bq / m ³ ). This allowed, until now their release in the environment, after control of toxic radiological and chemical elements.

However, from 1994, the new standards imposed for the discharge into the environment of effluents are much more stringent. Thus, for the center of the Valduc CE for example, any effluent discharge must present a contamination α at most of 1 Bq / m ³ . These effluents will therefore have to be completely purified in order to reach the "zero" release to the environment.

The current evaporation technique does not make it possible to reach the new authorized discharge standard, since the phenomenon of entrainment of α vapor contamination cannot be eliminated. Thus, this technique is not effective and leads to very weakly charged solutions whose radioactive activity is between 200 and 300 Bqα / m ³ (less than 1000 Bqα / m ³ ).

It is therefore necessary to develop a process for the treatment of evaporates from the industrial evaporation tower, in order to eliminate the last traces of α emitting elements. Furthermore, it would also be useful to develop a process for treating the effluents upstream of the evaporation tower in order to possibly be able to replace this evaporation tower, while obtaining effluents with activities that comply with the new standards.

The extraction of heavy metals, actinides and in particular uranium, plutonium and americium has given rise to a great deal of research. Derivatives "crown ethers" not soluble in water and fixed on silica have been used in extraction processes to extract lead in particular, see for example US-A-4,943,375. Likewise, these derivatives of ¹ crown ethers have also been used in an adsorption process on resin, see for example patent application EP-A-0 433 175. This process makes it possible to recover part of the plutonium from concentrated solutions fission products or effluents from reprocessing plants. However, such a process does not allow the values of the new discharge standards currently authorized for the Valduc center to be reached. Also known from the prior art ultrafiltration techniques for the treatment of effluents. However, these techniques make it possible to trap plutonium or americium but do not make it possible to retain uranium. Finally, the liquid / liquid extraction methods also known from the prior art are not effective in the pH and concentration ranges envisaged.

Finally, from US-A-4 283 370, a process is known for separating the uranium present in natural water or sea water, using a silica gel. However, this method relates to the treatment of water having a much lower salt content than that of the decontamination effluents which it is proposed to treat in the invention.

Consequently, the object of the invention is to remedy the aforementioned drawbacks of the prior art.

To this end, the invention relates to a process for partial decontamination α of an aqueous effluent from nuclear fuel reprocessing installations and nuclear centers containing at least one polluting element chosen from copper, zinc, tantalum, gold, actinides or lanthanides.

- According to the characteristics of the invention, this process consists in passing said aqueous effluent with a silica gel and separating said decontaminated aqueous effluent and the silica gel having fixed at least one of said polluting elements; the effluent to be treated having a pH greater than or equal to 3 and the silica gel used having a particle size between 35 and 70 mesh.

This process makes it possible to carry out an effective, albeit partial, decontamination of the aforementioned polluting elements thanks to the affinity of silica gel for these elements. This decontamination makes it possible to remove a large part of the polluting elements using an inexpensive technique which is easy to implement. Subsequently, another decontamination process can be implemented, in order to eliminate the last traces of polluting elements.

The method according to the invention makes it possible to treat solutions for dissolving spent nuclear fuels, that is to say solutions which precisely include actinides and many metals. The method makes it possible to treat aqueous effluents from fuel reprocessing installations and nuclear centers. It applies more particularly to the recovery of the aforementioned pollutants in effluents of low activity α, (that is to say less than 15 MBq / m ³ ), which corresponds to the nature of the effluents present upstream or downstream of the industrial evaporation tower.

In addition, the process makes it possible more particularly to extract uranium, plutonium, americium, the aforementioned aqueous effluents. This is of interest for the treatment of effluents from military nuclear centers.

Preferably, the contact time between the aqueous effluent and the silica gel is from 1 to 30 minutes in dynamic mode and the effluent to be treated must be at a pH greater than 4.5.

In addition, according to an alternative embodiment of the method of the invention, a second step is carried out consisting in recovering the polluting elements extracted from the silica gel by re-extraction in a solution of nitric acid, preferably between 1 and 5N. This makes it possible to obtain the pH below 3 necessary for the release of the polluting elements out of the silica gel. This gel can therefore be recycled several times.

Other characteristics and advantages of the invention will appear better on reading the following description and examples given by way of illustration and not limitation. This description is made with reference to the accompanying drawings, in which: FIG. 1 is a curve illustrating the evolution of the partition coefficient Kd as a function of the pH for a synthetic solution of uranyl nitrate, - FIGS. 2 and 3 are graphs respectively illustrating the partition coefficient Kd and the percentage of uranium eluted from silica gel for ten successive treatment cycles, and FIG. 4 represents the ^" curves giving an account of the variation in the concentration of different cations, at the outlet a column of silica gel, as a function of the volume of initial effluent that has flowed.

In all of the following tests, use is made of a silica gel having a particle size of approximately 35 to 70 mesh, without prior treatment and marketed under the brand name KIESELGEL 60, by the company MERCK. This particle size has made it possible to obtain good results, however, these values are not critical for the good performance of the process. Test 1: Determination of the partition coefficient Kd as a function of the pH for a synthetic solution of uranyl nitrate

Among the different ways of defining the partition coefficient Kd, the following formula has been chosen:

K d z (moles of solute fixed) / (gram of silica gel)

(moles of solute remaining) / (milliliter of solution)

[S ₀ 1- [S] V or for a solute S, Kd = ^L 1. ^{l J} -

[S] m ^or t ^s θl ⁼ activity of the solute S in the initial solution,

[S] = activity of the solute S in the liquid phase after contact with the silica gel, V = volume of solution introduced onto the silica gel (in milliliter), m = mass of silica gel introduced (in grams).

The partition coefficient Kd directly reflects the ability of the material to fix the metal in question.

Procedure

0.1 g of silica gel is introduced into different flasks. 15 ml of a synthetic solution of uranyl nitrate with a concentration close to 1.6 g / l are then added to each flask. Each solution is adjusted to different pH values with 1.0 N sodium hydroxide. Then, the flasks are stirred for 3 minutes at room temperature (23 ° C. In this embodiment, the flasks then remain without stirring for 18 hours, so as to ensure sufficient contact time between the silica gel and the synthetic solution of uranyl nitrate to reach the maximum rate of fixation of the metal on the material. Finally, the solutions are filtered so as to separate on the one hand the silica gel which has fixed the uranium and on the other hand the filtrates. These are analyzed by plasma emission spectrometry.

The results obtained are shown in Figure 1 attached and mentioned in the table below.

PH 3.25 4.00 4.30 4.82 5.12

Kd 8.86 24.21 42.46 52.95 67.54

It is noted that the pH of the treated solution must be greater than 3 to start obtaining an extraction of uranium by silica gel. For a pH greater than 4.5, the partition coefficient Kd is close to 50. Finally, under optimal pH conditions, that is to say for a pH greater than or equal to 5, a partition coefficient of l order of 70 is observed. This is particularly advantageous during the industrial implementation of the process since the effluents present upstream of the evaporation tower have a pH generally greater than or close to 7 and that the effluents from said tower have a pH greater than 4.5 . This process can therefore be easily used after the evaporation tower or as a replacement for it.

Test 2: Fixation of the uranium by silica gel and elution of the latter using a nitric acid solution

We weigh exactly 0.1 g of silica gel which is placed in a flask and 15 ml of a synthetic solution of uranyl nitrate with a content close to 1.6 g / l and adjusted to a pH 4.82. The solution is stirred for 3 minutes at room temperature (23 ° C), then the mixture is left without stirring for a contact time of 18 hours. Then, the solution is filtered so as to separate it from the gel. The filtrate is analyzed by plasma emission spectrometry so as to deduce therefrom the Kd value of the silica gel.

According to the second step of the process of the invention, said silica gel is washed with distilled water, 5 ml of a 2N nitric acid solution are added and the whole is stirred for 3 minutes at room temperature. The acid solution is filtered and then analyzed by plasma emission spectrometry and the results are reproduced in FIG. 3. Furthermore, the silica gel which has been used for the first time is isolated, washed and dried then placed again in a flask containing 15 ml of a synthetic uranyl nitrate solution identical to that previously used, so as to undergo a second treatment cycle identical to that which has just been described. 10 successive decontamination treatments are thus carried out. The results are illustrated in Figures 2 and 3.

FIG. 2 shows that the partition coefficient Kd is relatively constant and is between 50 and 60. FIG. 3 shows that the nitric acid 2N makes it possible to re-extract in a constant manner over time practically 100% of the uranium previously fixed on silica gel. As a result, the silica gel is regenerable and its extraction capacity remains almost unchanged even after 10 commissioning cycles, which makes it completely suitable for industrial use.

Tests were also carried out during a dynamic process consisting in placing the gel of silica in a column and percolate, the effluent to be treated.

- Experimental apparatus :

In all the following tests, a glass column with a diameter of 1.5 cm and a height of 30 cm is used, containing 30 g of silica gel placed between a frit and polytetrafluoroethylene

(Teflon, registered trademark). The column can be fed either from the top or from the bottom, via calibrated pipes. During the decontamination process, the supply of contaminated effluent α takes place through the base of the column, at a speed of approximately 2.8 ^{× 10} −2 _c / _s (approximately 180 ml / h), which corresponds to a residence time of the effluent to be treated approximately 18 minutes.

Test 3: fixing of uranium 238 present in a real effluent coming from an industrial evaporation tower In this example, 15 liters of an effluent are treated containing as radioactive element only uranium 238 at a concentration of 0 , 6 μg / 1.

This effluent is passed through the aforementioned silica gel column. The initial effluent and the effluent obtained after passage over the silica gel are analyzed by plasma mass spectrometry (ICPMS). La final concentration ^'in uranium 238 of the effluent after passage through the silica gel is of 0.05 mg / 1, which shows that virtually all the uranium present in the original effluent was absorbed on the silica gel after a single pass on the column. (Silica gel retained approximately 91.7% of the initial uranium present). This effluent has therefore been partially purified from the emitting element α. The techniques of the prior art do not allow such a purification of uranium.

Note that the detection limit of this technique during this test is 0.02 μg / 1.

Test 4: fixation of uranium 238 present in a synthetic solution of uranyl nitrate

In the following two cases, 2 1 of a solution containing uranium 238 is treated. The initial effluent and the treated effluent, obtained after passing over silica gel, were determined by plasma mass spectrometry (ICPMS). ).

Effluent at 180 μg / 1 1, 3 mg / 1 treat (U ²³⁸ )

Treated effluent 0.22 μg / 1 1 μg / 1 _(U 238 ₎

Percentage 99.8% 99.9% of uranium 238 fixed by silica gel

The detection limit of this technique during this test is 0.03 μg / 1.

Test 5 Saturation of a column filled with silica by fixing a solution of uranium, cerium and europiu

In this test, 1.4 l of an effluent containing:

- around 53 mg / 1 of uranium,

- approximately 45 mg / 1 of cerium, and - approximately 17 mg / 1 of europium.

Cerium and europium are used to model the behavior of plutonium and americium. Regular samples are taken at the outlet of the silica gel column and the solutions obtained and the initial effluent are analyzed by plasma emission spectrometry. Furthermore, the cations previously fixed are eluted with a volume of 2N nitric acid corresponding substantially to the volume of the column. The results are illustrated in FIG. 4 attached, representing the evolution of the concentration of the different cations at the outlet of the column, as a function of the volume of initial effluent passed. The results are also given in the table below.

Volume Ce Eu U

0 0 0 0

50 0 0 0

100 0 0 0

150 5.2 0 0

200 36.6 5.68 0

250 45.56 13.29 0.3

300 44.3 18 2

350 44.5 17.76 14

400 42.1 16.88 18.5

450 43.5 16.34 30.2

500 44 17.16 38.1

550 44.1 17.97 42

600 43.5 16.88 44

650 42.9 17.05 50

700 44 17.3 53

750 44.5 16.4 52

800 44.2 16.9 52.2

900 44.1 17.04 52.6

1000 43.9 17.4 52.9

1100 44 17 52.4 1200 44.5 16.7 51.8

1300 44.4 16.5 51.6

1400 43.8 17.1 51.9

It can be seen that the saturation limit of the silica gel column is around 100 ml of solution for europium and cerium and 200 ml for uranium. The cation eluting first is detected after the flow of a hundred milliliters. This volume corresponds to the fixation of approximately 15 mg of cations (uranium, cerium and europium) for 30 g of silica gel contained in the column. The elution of the different cations fixed by a nitric acid solution, made it possible to recover nearly 100% of the metals and thus to completely regenerate the column.

Test 6: fixing of plutonium 239 and americium 241 present in a real effluent coming from an industrial evaporation tower

In this test, 15 1 of an effluent containing 62 Bq / m ³ of plutonium 239 and 213 Bq / m ³ of americium 241 are treated. The effluents before and after passage over the silica gel column are analyzed by α counting. . The effluent obtained contains 4 Bq / m ³ of plutonium and 14 Bq / m ³ of americium. The silica gel therefore retains respectively 93.5% of the plutonium and 93.4% of americium.

It can be seen that almost all of the actinides present in the effluent before the treatment have been fixed on the silica gel and that the effluent after passing over the column is partially purified of α emitting elements. Test 7: analysis of various cations present in the effluents before and after the passage over a column of silica gel

A quantitative and qualitative analysis of cations present in solution before and after the passage over a column of silica gel was carried out in order to determine the various elements liable to be retained by the silica gel. The cations tested were: Na, Mg, Ca, Sr, Ba, Si, Al, Ga, Pb, Hg, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Cd, Ta and Au.

The results obtained are illustrated in Table 1 below.

Table 1

Elements Na Mg Ca Sr Ba

Content of cation in 400 2 71 0.06 0.7 stock solution (mg / 1)

Content of cation in 395 1.8 73 0.06 0.6 solution after passage over silica gel (mg / 1)

Elements Si Al Ga Pb Hg

Content of cation in 13 1.12 0.02 0.17 0.03 stock solution (mg / 1)

Content of the cation in the 47 1.09 0.02 0.15 0.03 solution after passage over the silica gel (mg / 1)

Elements Ti Cr Mn Fe Ni

Content of cation in 0.36 0.05 0.04 4.6 0.06 stock solution (mg / 1)

Content of the cation in the 0.34 0.03 0.04 4.07 0.06 solution after passage over the silica gel (mg / 1)

Elements Cu Zn Cd Ta Au

Content of cation in 1.16 4.1 0.08 1, B 0.03 stock solution (mg / 1)

Content of the cation in the 1.02 3.65 0.06 0.03 <LD solution after passage over the silica gel (mg / 1)

LD: detection limit close to 0.01 mg / 1 It is found that only certain elements such as copper, zinc, tantalum and gold and to a lesser extent iron, are trapped by silica gel. On the other hand, the alkaline and alkaline-earth elements, always present in the effluents (and in particular the sodium) are not trapped by the silica. This prevents the column from being saturated too quickly and losing its effectiveness.

Test 8 test on alkaline effluents heavily loaded with plutonium and americium

In this test, 1/2 liter of a highly charged solution containing 19,840 kBq / m ³ of plutonium 239 and 750 kBq / m ³ of americium 241 is treated. The initial effluent and the effluent obtained after passage over the silica gel are analyzed by α counting. The treated effluent has a concentration of 2477 kBq / m ³ of plutonium 239 and 87 kBq / m ³ of americium 241. The depletion factor is 8 for plutonium and 8.6 for americium.

In addition, a qualitative and quantitative analysis of the cations present in the initial effluent and in the effluent obtained after treatment on ^' silica gel was carried out. The analysis was performed by plasma torch. The results are illustrated in Table 2.

Table 2

Al 1 1

If 20 36

Mn 3 3

Fe 380 290

Cd 17 17

This shows the good selectivity of the silica gel which does not retain the abovementioned cations.

Claims

1. Process for partial α decontamination of an aqueous effluent from nuclear fuel reprocessing installations and nuclear centers, containing at least one polluting element chosen from copper, zinc, tantalum, gold, actinides or lanthanides, characterized in that it consists in passing said aqueous effluent over a silica gel and in separating said contaminated effluent and the silica gel having fixed at least one of said polluting elements; the effluent to be treated having a pH greater than or equal to 3 and the silica gel used having a particle size between 35 and 70 mesh.

2. decontamination method according to claim 1, characterized in that the effluent to be treated is at a pH greater than 4.5.

3. decontamination method according to claim 1, characterized in that the aqueous effluent passes through the silica gel placed in a column and in that the residence time of the effluent in the column is at least 1 minute.

4. Decontamination method according to claim ^" 3, characterized in that the residence time is between 1 and 30 minutes approximately.

5. Decontamination process according to claim 1, characterized in that the pollutants extracted from the silica gel are recovered by re-extraction in a nitric acid solution.

6. decontamination method according to claim 5, characterized in that a nitric acid solution of between 1 and 5N is used.

7. decontamination method according to any one of the preceding claims, characterized in that the treated aqueous effluent is an effluent whose activity α is less than 15 MBq / m ³ .

8. A decontamination method according to claim 1, characterized in that it makes it possible to fix plutonium, uranium or americium on the silica gel.