EP0296984A1 - Process for removing chloride ions from contaminated solid wastes, such as incineration ashes contaminated by actinides - Google Patents

Abstract

The invention relates to the removal of chloride ions present in contaminated solid waste such as actinide-contaminated incineration ash. The chloride ions are first dissolved in an aqueous solution, for example a nitric acid solution at 4 to 6 mol/l, then the chloride ions thus dissolved in the aqueous solution are oxidized, for example electrochemically, optionally in the presence of an oxidoreductive couple such as Co³⁺/Co²⁺, to remove them from the solution in the form of chlorine in a gaseous current. This dissolution and this oxidation can be carried out in the electrolyser (1), the gaseous current being introduced by (10) to be eliminated in (13). The actinides can then be dissolved directly in this electrolyser by electrolysis in the presence of Ag²⁺ ions.

Description

The subject of the present invention is a process for removing chloride ions present in contaminated solid waste such as incineration ash contaminated with actinides.

It is known that one of the problems frequently encountered in nuclear installations is to recover actinides such as plutonium and / or neptunium, present in solid waste originating either from the manufacture of nuclear fuel elements, or from the processing of fuels irradiated nuclear power. This waste may consist of ash from the incineration at 800-900 ° C of combustible waste highly contaminated with plutonium, which leads to the production of calcined ash in which the plutonium is in the form of oxide or other refractory compound, therefore in a form which is difficult to recover in solution. Among the wastes likely to be treated in this way, there are certain laboratory wastes, in particular wastes based on plastics such as polyvinyl chloride, which leads to the formation of ashes having high contents of chloride ions, up to 30% by weight.

Among the treatments that can be used to recover plutonium from such waste, methods are known based on the solubilization of plutonium in the form of Pu (VI) ions in nitric medium by oxidation using silver (II) which can be regenerated by electrolysis, as described in European patent application EP-A-158555 filed March 22, 1985. This process is very advantageous but when the solid waste contains chloride ions, it is necessary to remove all chloride ions first before proceeding with the solutilization phase of plutonium under the action of Ag (II). Indeed, the presence of chloride ions in the waste would lead to the elimination of the Ag (I) ions from the solution in the form of AgCl, thus making it impossible to regenerate Ag (II) by electrolysis and therefore the dissolution of the plutonium.

A known method for removing chloride ions, before proceeding with the solubilization of the plutonium, is to wash the waste with water as described in the aforementioned French patent. However, this technique of washing with water, although effective, has many disadvantages for implementation on an industrial scale.

Indeed, before carrying out the plutonium solubilization phase in an electrolyser, it is necessary to wash the waste with water, then to separate the waste from the aqueous washing solution, which requires the use of two successive specific devices. The solid waste which has been subjected to washing must then be transferred to the oxidizing dissolution apparatus, that is to say to the electrolyser. In addition, with this method of eliminating chloride ions, a new liquid effluent contaminated with alpha-emitting actinides, such as plutonium and americium, having a high concentration of Cl⁻ ions is obtained which is difficult to treat.

The present invention specifically relates to a process for removing chloride ions from contaminated solid waste, which avoids the drawbacks mentioned above.

The process according to the invention, for removing chloride ions present in contaminated solid waste, consists in dissolving the chloride ions in an aqueous solution and in oxidizing the chloride ions thus dissolved in the aqueous solution in order to remove them from the solution in the form chlorine in a gas stream.

Chloride ions can be dissolved by contacting the waste with an aqueous solution appropriate. Acid solutions can be used, in particular an aqueous nitric acid solution advantageously having a nitric acid concentration of 4 to 6 mol / l.

The oxidation of the chloride ions present in solution can then be carried out electrochemically. This oxidation can be carried out directly by passing an electric current through the solution, or indirectly via an oxidizing agent regenerated by electrolysis.

Thus, the chloride ions can be dissolved and then oxidized in the same apparatus, in particular in the electrolytic cell which will be used for the subsequent dissolution of actinide elements such as plutonium and neptunium. With the method of the invention, it is thus possible to avoid the use of different successive devices as well as the transfer of waste from the washing device to the electrolytic cell for dissolving plutonium and / or neptunium.

Furthermore, the solution used to dissolve the chloride ions can be an aqueous solution of nitric acid, the nitric acid concentration of which is suitable for the subsequent step of dissolving plutonium and / or neptunium.

To carry out the direct electrochemical oxidation of the chloride ions dissolved in the aqueous nitric acid solution, it suffices to apply a sufficiently high potential between the electrodes of the electrolysis cell that the oxidation reaction of the chloride ions in chlorine gas, whose apparent normal potential is 1.36 volt / ENH, can occur on the anode. This can be achieved by passing a sufficiently intense constant current through the electrolyser.

To indirectly carry out the electrochemical oxidation of chloride ions, an oxidoreductive couple is used whose apparent normal potential is greater than 1.36 volts / ENH by regenerating directly by electrolysis in the cell the oxidized species of the redox couple.

Among the redox couples likely to be used, there may be mentioned the couple Co³⁺ / Co²⁺ whose apparent normal potential is 1.83 volts / ENH.

It is of course possible to use other redox couples as long as these are compatible with the presence of Cl- ions in solution.

During this oxidation, the chlorine which is released is eliminated by entraining it by a gas stream, for example air or an inert gas such as nitrogen, and the chlorine can then be separated from this stream. gaseous by conventional techniques, for example by passing the gas stream over a soda lime trap.

Thus, the only new waste produced by the process of the invention can be a soda lime trap with an extremely low level of radioactive contamination, which can be stored inexpensively in surface sites. Thus, the treatment of this new waste poses no problem, which was not the case for the chlorinated radioactive effluents obtained with the process of the prior art.

The method of the invention can be implemented in an electrolysis cell of the conventional type, in particular in an electrolyser such as that described in European patent EP-A-160589 or European patent EP-A-0158555.

For this implementation, an aqueous nitric acid solution having a nitric acid concentration of 4 to 6 moles per liter is introduced into the electrolyser, then the contaminated waste, and the whole is subjected to stirring for one sufficient time to obtain the solubilization of at least 95% of the chloride ions. The surface of the solution is preferably also swept by means of a gas stream such as air, for the entire duration of the solubilization treatment which is generally less than 1 hour. It can operate at room temperature or at a higher temperature, ranging for example from 20 to 70 ° C.

After this operation, the electrolyser is produced oxidation of chloride ions to chlorine gas. This is done by passing a direct current through the electrolyser with sufficient intensity for the potential of the anode to be greater than 1.36 volts / ENH. During the electrolysis, the gas sweep is maintained to remove the chlorine released by the reaction from the gas stream. The duration of electrolysis is chosen as a function of the chloride ion concentration in the solution to ensure that the elimination of chloride ions is practically complete. Duration of 1 to 3 hours is generally sufficient.

Generally, the temperature is higher than the ambient temperature because the solution is heated by passage of the electric current. This is done at temperatures ranging from 40 to 60 ° C.

When the oxidation of chloride ions is carried out indirectly by means of an oxidoreductive couple, the same procedure is carried out after adding at least one of the species of the desired couple to the solution.

When the elimination of Cl⁻ ions in the form of Cl₂ is complete, the actinides such as plutonium, americium and uranium can be dissolved in the same electrolyser. For this purpose, a silver compound (II) such as silver nitrate is added to the solution, and a sufficient potential difference is maintained between the electrodes to continuously regenerate the Ag²⁺ ions which have been used to oxidize and dissolve the actinides present in the waste. At the end of the operation, the solid waste is separated from the solution which contains most of the radioactive elements such as plutonium, americium and uranium.

The process of the invention is thus entirely advantageous since it is possible to carry out the successive stages of elimination of the chloride ions and of dissolution of the contaminating radioactive actinide elements in the same electrolysis cell, which eliminates the liquid separation operations. solid and transfer needed before. Furthermore, it does not lead to the production of new aqueous effluents such as chlorinated effluents contaminated with plutonium, americium and / or uranium.

The invention will be better understood on reading the following examples given, of course, by way of illustration and without limitation, with reference to the appended drawing which schematically represents an electrolysis cell which can be used to implement the process of the invention.

In this figure, it can be seen that the electrolysis cell comprises a first tube 1 and a second tube 3 of identical diameter connected by conduits 5 and 7 arranged so as to allow the circulation (along the path of the arrows) of the waste mixture- solution of the second tube 3 to the first tube 1 by the pipe 5 in weir and from the first tube 1 to the second tube 3 by the pipe 7. The two tubes 1 and 3 have a safe geometry and they can for example have the following dimensions: a diameter of 16 cm and a total height of 1 m, which corresponds to a useful capacity of 20 l. Tubes 1 and 3 can be made of Plexiglas, tantalum or glass.

The first tube 1 includes the means required to ensure electrolysis, agitation of the solid-solution mixture and the trapping of the gases produced during the electrolysis. Thus, the first tube 1 comprises an anode 9 which is preferably of tubular shape in order to present a large surface area, and a cathode 11 disposed in a cathode compartment 12 delimited in part by a porous wall of electrical insulating material and connected to a circuit vacuum 13 provided with a soda lime trap and a gas washing column. A gas stream, for example air, is introduced at 10 into the first tube to effect a sweep of the surface of the liquid. Thus, the gaseous products formed during electrolysis, in particular chlorine, are captured and entrained after dilution in the sweeping gas through a soda lime trap retaining the chlorine and a washing column having as function retention of nitrogen products, traces of hydrogen possibly formed being evacuated by ventilation of the containment in which is arranged the device.

The cathode compartment which may for example have a diameter of 5 cm and a volume of approximately 0.5 l, is provided with more than one level detector 14 and one line 16 for introducing an electrolyte into the cathode compartment 12. The pipe 16 comprises a valve 18 controlled by the level 14 detection means in this compartment in order to control the opening of the valve 18 when the level is located below the desired value 22 and on the contrary to close valve 18 when the liquid level has reached the desired value. Furthermore, it is noted that the cathode compartment is disposed opposite the pipe 5, which makes it possible to facilitate the evacuation of the calories released in this compartment, by the flow of solid-liquid mixture entering the pipe 5. A turbine 15 which can be driven by an electric motor 20 ensures mixing and circulation of the solid-liquid mixture between the tubes 1 and 3.

The second tube 3 is provided with means for cooling the solid-solution mixture, which consist of a coil 17 immersed in the solution and traversed by a coolant. It also includes a feed hopper 19 for introducing the waste to be treated, means 21 for detecting the end of the dissolution of the actinides, means 23 for withdrawing the solution obtained and a pipe 24 for introducing the reagents into the device, or silver oxide, nitric acid and optionally the intermediate used for the oxidation of chloride ions.

To improve safety, the device further comprises a screen 25 made of neutron absorbing material, for example plaster or borated polyethylene, disposed between the two tubes.

The means 21 for detecting the end of the dissolution of the actinides can be constituted by an optical probe capable of locating the coloring of the AgII, which indicates that the silver ions are no longer used to oxidize the plutonium. It is also possible to use a probe for measuring the density of the liquid because the end of the reaction can be identified by obtaining a stable density. It is also possible to use a probe for measuring the gas evolution on the anode.

The means 23 for withdrawing the solution obtained comprise a drain pipe which opens at the lower end of the second tube.

The turbine 15 present in the first tube 1 can be replaced by a pump or any suitable means making it possible to ensure vigorous stirring in the first tube and to circulate the solid-liquid mixture from the first tube to the second tube in the direction indicated. by the arrows.

The porous partition wall of the cathode compartment 12 can be made of sintered glass or sintered ceramic, for example sintered alumina.

To treat waste containing chloride ions in the electrolysis cell described above, the procedure is as follows.

First of all, an aqueous solution of nitric acid having the desired concentration of nitric acid is introduced into the cell via line 24. The waste contaminated by the hopper 19 is then introduced and the turbine 15 is started to mix the waste-aqueous nitric acid mixture mixture and its circulation between the tubes 1 and 3. At the same time, a scanning of air from the first tube 1 by introducing the air through line 10 and extracting it through the vacuum circuit 13. When the dissolution of the chloride ions is complete, a potential difference is established between the anode 9 and the cathode 11 to pass a continuous electric current of sufficient intensity through the aqueous solution, while operating the turbine 15 to ensure mixing and circulation of the mixture and maintaining the air sweep at 10. During this electrolysis, the chloride ions are oxidized to chlorine gas which is eliminated by the gas stream in the vacuum circuit 13.

When all the chlorine has been removed, you can proceed to the dissolution of contaminating actinide elements. In this case, the desired reagent, for example silver oxide or nitrate, is introduced via line 24, and a suitable potential difference is maintained between the cathode and the anode to regenerate the silver II. This electrolysis is carried out for a sufficient time to obtain the dissolution of the contaminating elements by always keeping the turbine running to ensure mixing and circulation of the mixture and by maintaining the air sweep at 10. The end of the reaction is detected by the detector 21, and the solution is cooled by the coil 17 when necessary to maintain its temperature at the desired value.

Examples of treatment of contaminated waste containing chloride ions are described below by the method of the invention.

EXAMPLE 1 .

In this example, using an electrolyser laboratory glass having a platinum anode 36 cm² surface and a cathode constituted by a platinum wire placed in a compartment separated by a glass frit of porosity ^No. 4 which contains 8 ml of 8M nitric acid. The electrolyser is closed by a cover and connected to a vacuum line fitted with a silver nitrate bubbler.

100 ml of a 4 mol / l nitric acid solution are first introduced into the electrolyser, then 10 g of ash having a chloride ion content of 2.1% by weight is added and the mixture is stirred. mixture of ash and solution for 2 h 30 min using a magnetic bar. A potential difference is then applied between the anode and the cathode to cause a constant current of 1A to pass through the electrolyser for 1 hour, while simultaneously carrying out an air sweep of the electrolyser. During this electrolysis phase, the temperature in the electrolyser is brought to about 50 ° C as a result of the passage of electric current.

During this experiment, the concentration is determined in chloride ions of the solution before the electrolysis phase and at the end of electrolysis. The results obtained are as follows:
concentration of chloride ions after the dissolution step: 0.06 mol / l, and
- concentration of chloride ions after one hour of electrolysis 3,7.10⁻³ mol / l.

Thus, at the end of electrolysis, the concentration of chloride ions represents only 6% of the initial concentration of chloride ions in the solution.

We also note the presence of an abundant white precipitate of AgCl in the bubbler flask lined with the silver nitrate solution.

Thus, the process of the invention makes it possible to obtain a satisfactory elimination of the chloride ions in the form of Cl₂.

EXAMPLE 2 .

In this example, the same procedure is used as in Example 1 to treat 10 g of ash having a chloride ion content of 2.1% by weight using the same electrolyser, but by interposing a soda lime trap on the vacuum line before the bubbler filled with the silver nitrate solution.

• a) agitation du mélange pendant une heure,
• b) agitation du mélange à la température ambiante pendant une heure avec un balayage d'air, et
• c) agitation pendant une heure à une température de 60°C, avec un balayage d'air.

First of all, the chloride ions are dissolved by carrying out the following steps:

• a) stirring the mixture for one hour,
• b) stirring the mixture at room temperature for one hour with an air sweep, and
• c) stirring for one hour at a temperature of 60 ° C, with an air sweep.

Electrolysis is then carried out at a constant intensity of 1 A, with air sweeping, at a temperature of 60 ° C., for one hour as in Example 1.

The chloride ion concentration measurements of the solution carried out after these various steps gave the following results:
- 0.06 mol / l after step a),
- same results after steps b) and c), and
- 3.7.10⁻³ mol / l. at the end of the electrolysis.

It is thus noted that the quantity of Cl⁻ ions dissolved after one hour is identical to that obtained after 2 h 30 min of stirring. Furthermore, even with air sweeping, the concentration of Cl⁻ ions remains constant, which shows that the entrainment of Cl⁻ ions in the form of HCl by the air is a negligible process.

The residual concentration of chloride ions after the electrolysis phase represents, as in Example 1, 6% of the initial concentration.

It is also observed that there is no white precipitate in the silver nitrate bubbler, which shows that the trapping of chlorine by the soda lime trap is effective.

EXAMPLE 3 .

In this example, the electrolysis cell illustrated in the figure is used to treat active ash containing 23% by weight of chloride ions and approximately 3% by weight of plutonium. 6l of nitric acid at 4 mol / l are introduced into the electrolysis cell, then 554g of ash are added and the whole is stirred for 1 hour under air sweeping.

Electrolysis is then carried out by passing a current with an intensity of 80A for 3 hours at a temperature of 40 to 60 ° C under air sweep.

The concentration of chloride ions in the solution is determined at the end of the electrolysis. This is 10⁻³ mol / l. It is thus noted that only 0.17% of the chloride ions initially present in the ashes remain in solution.

An analysis of the chloride ion content of the solution during electrolysis made it possible to calculate the faradic yield. This is 50% during most of the electrolysis phase.

EXAMPLE 4 .

In this example, 916 g of ash identical to that of Example 3 are treated and the dissolution steps are carried out. Cl⁻ ions and electrolysis, under air sweep, under the same conditions. The residual concentration of chloride ions in the solution after three hours of electrolysis is equal to 4.5.10⁻³ mol / l.

Thus, the residual content of chloride ions represents 0.45% of the chloride ions of the starting ash. A calculation of the faradaic yield showed that it was 54% during most of the electrolysis phase.

Thus, the process of the invention is very effective for the removal of chloride ions.

EXAMPLE 5 .

In this example, 5 g of ash containing 7.8% by weight of Cl⁻ ions are treated in a laboratory electrolyser similar to that of Example 1 except that the platinum anode has an area of 3.33 cm² and that the sweep is with nitrogen.

These ashes are dispersed in 100 ml of nitric acid at 4 mol / l and the whole is stirred for one hour. Electrolysis is then carried out with an electrolysis current of 0.5 A for 118 min. at a temperature of 80 ° C. During dissolution and electrolysis, a nitrogen sweep is carried out at a flow rate of 1.3 l / h.

After electrolysis, the residual concentration of chloride ions in the solution is 1.9.10⁻³ mol / l, which corresponds to an elimination yield of 98.3%.

EXAMPLE 6 .

In this example, 5 g of ash identical to that of Example 5 are treated using the same procedure, but adding 0.1 mol / l of Co²⁺ ions to the solution during electrolysis (in the form of cobaltous nitrate) to effect the oxidation of Cl⁻ ions in solution.

After 82 min. electrolysis carried out under the same conditions as those of Example 5, the residual concentration of chloride ions is 1.9.10⁻³ mol / l.

It is thus noted that the presence of the couple CoII / CoIII results in a gain of 30% on the duration of electrolysis necessary to obtain the same rate of elimination of Cl⁻ ions in the form of Cl₂.

Claims (9)

1. A method of eliminating chloride ions present in contaminated solid waste, characterized in that it consists in dissolving the chloride ions in an aqueous solution and in oxidizing the chloride ions thus dissolved in the aqueous solution to remove them from the solution as chlorine in a gas stream. 2. Method according to claim 1, characterized in that the chloride ions are dissolved by bringing the waste into contact with an aqueous solution of nitric acid. 3. Method according to claim 2, characterized in that the aqueous nitric acid solution comprises from 4 to 6 mol / l of nitric acid. 4. Method according to any one of claims 1 to 3, characterized in that one carries out the oxidation of chloride ions in aqueous solution electrochemically. 5. Method according to any one of claims 1 to 3, characterized in that one carries out the oxidation of the chloride ions in aqueous solution by means of an oxidoreductive couple whose oxidized species is regenerated by electrolysis. 6. Method according to claim 5, characterized in that the redox couple is the couple Co³⁺ / Co²⁺. 7. Method according to any one of claims 1 to 6, characterized in that the chlorine removed from the solution is recovered by passing the gaseous stream containing the chlorine over a soda lime trap. 8. Method for recovering actinides present in solid waste also containing chloride ions, characterized in that it comprises the following successive steps: 1 °) bringing into contact with stirring in an electrolysis cell comprising an anode and a cathode, the solid waste with an aqueous solution of nitric acid having a nitric acid concentration of 4 to 6 moles / l for one sufficient time to dissolve at least 95% of the chloride ions; 2) applying between the anode and the cathode of the cell a sufficient potential difference to oxidize the chloride ions into chlorine, and entrain the chlorine gas thus released dan a gas stream; and 3) add a silver compound to the aqueous solution and maintain a sufficient potential difference between the anode and the cathode of the cell to continuously regenerate the Ag²⁺ ions which have been used to oxidize and dissolve the actinides present in the waste. 9. Method according to claim 8, characterized in that in the second step, a cobalt II compound is additionally added to the aqueous solution.

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