EP0157683B1 - Process for the bituminization of radioactive wastes constituted by cation-exchange resins and/or anion-exchange resins - Google Patents

Description

The present invention relates to a conditioning process by coating in bitumen radioactive waste consisting of ion exchange resins.

The coating of radioactive waste in bitumen is generally carried out by mixing an aqueous suspension of radioactive waste with bitumen in the fluidized state, then evaporating the water from the suspension and pouring the mixture thus obtained into a container to solidify it.

French patent 1,315,162 filed on December 6, 1961 by the French Atomic Energy Commission describes a bituminization process of this type, according to which the suspension of waste is mixed with fluidized bitumen in the presence of an agent. surfactant which facilitates the separation of the water contained in the suspension, most of this water is then separated either by decantation or by means of a mechanical device, and the product obtained after separation of the water is then kneaded at a temperature such that it is sufficiently fluid, and finally poured so as to obtain by cooling solid blocks of suitable plasticity.

French patent 2,052,093 of the Atomic Energy Commission filed on July 15, 1969 describes an installation for coating a suspension of radioactive products with bitumen using a thin film evaporator and a gear pump to extract the products mixes obtained at the base of the evaporator.

In French patent FR-A-2 356 246, an improvement to the processes for coating radioactive waste in bitumen is described, which consists in subjecting the waste to a pretreatment with salts such as calcium and barium chlorides , in order to improve the resistance to leaching of the solidified products obtained.

In this case, the treated waste is evaporation concentrates and / or precipitation sludges which contain salts such as sodium carbonates and sulphates.

There are also known installations for continuously coating suspensions of radioactive waste using a twin or quadruple screw extruder, in which the waste is mixed with the bitumen and the suspension is dried.

Generally, before proceeding to coating the suspension of waste, it is subjected to a neutralization treatment with sodium hydroxide. In fact, it is not recommended to coat the acid suspensions with the bitumen, in particular with free nitric acid, to avoid any risk of degradation of the bitumen at the end of the operation when, after evaporation of the water, the bitumen found in the presence of concentrated acid. In addition, prior neutralization before asphalting limits the corrosion phenomena of the installations.

When the radioactive waste consists of organic ion exchange resins, in particular of the anionic type in OH- and / or CI- form, the implementation of these conditioning processes by coating in bitumen does not make it possible to obtain a capacity. sufficient treatment of the asphalt installation. In addition, the bituminous mixes obtained have the major drawback of swelling when they are then immersed in water. In this case, the increase in volume of the coated materials can reach 20%, and exceed 100% in certain extreme cases, which leads to disaggregation of the coated waste.

There are also known methods of conditioning ion exchange materials in thermosetting resins or in cement, as described in French patent FR-A 2361724 and in Japanese patent 48-28899. According to these methods, in order to obtain packaged products free of cracks, it is necessary to carry out a pretreatment of the ion exchange resins to replace the H ⁺ ions with other cations, in order to prevent the resins from being able to fix some of the reagents necessary to harden the thermosetting resin or set the cement.

German patent DE-A-3 102 473 also describes a process for pretreatment of a mixture of cation exchange resins and anion exchange resins which then makes it possible to condition the resins separately by incorporation in bitumen or cement. According to this patent, in order to separate the cation exchange resins from the anion exchange resins, the mixture of resins is brought into contact with an aqueous solution of a salt, such as a sulfate, a chloride, a nitrate or an acetate. of alkali metal, to replace the H ⁺ and / or Na ⁺ ions of cationic resins with sodium and the OH- ions of anionic resins with other anions. The resins can then be separated by means of a liquid which has a density between that of the grains of one kind of resin and that of the grains of the other kind of resin. Thus, this patent does not deal with the problem of swelling in bituminous mixes in water.

The present invention relates specifically to a method of conditioning by bituminization of radioactive waste constituted either by cation exchange resins or by a mixture of cation exchange resins and anion exchange resins, which comprises a pretreatment step allowing solve the problems mentioned above.

• a) à soumettre lesdites résines à un prétraitement au moyen d'un sel pour remplacer les ions H⁺ et/ou Na⁺ des résines échangeuses de cations par des ions choisis dans le groupe comprenant Ca⁺⁺, Sr⁺⁺ et Ba⁺⁺,
• b) à mettre en suspension dans l'eau la ou les résines ainsi prétraitées, et
• c) à soumettre ladite suspension à l'opération de bitumage.

According to the invention, the method of conditioning by bituminization of radioactive waste constituted by cation exchange resins is characterized in that it consists:

• a) subjecting said resins to a pretreatment using a salt to replace the H ⁺ ions and / or Na ⁺ cation exchange resins with ions chosen from the group comprising Ca ⁺⁺ , Sr ⁺⁺ and Ba ⁺⁺ ,
• b) suspending the pretreated resin (s) in water, and
• c) subjecting said suspension to the asphalting operation.

When the waste also comprises anion exchange resins, the pretreatment is carried out by means of a salt such that the H ⁺ and / or Na ⁺ ions of the cation exchange resins are replaced by ions chosen from the group comprising Ca ⁺⁺ , Sr ⁺⁺ , and, Ba ⁺⁺ and, the OH- and / or CI- ions of anion exchange resins by an anion chosen from the group comprising NO ₃ ^- , HCO ₂ ^- and CH ₃ CO ₂ ^- .

According to a characteristic of the invention, the pretreatment is carried out by bringing said resins into contact, preferably with stirring, with an aqueous solution of a calcium, strontium or barium salt, said salt being chosen from the group comprising: nitrates, acetates and formates.

This embodiment of the pretreatment is in particular suitable for the treatment of a mixture of cation exchange resins and anion exchange resins because it allows the replacement of OH- or CI- ions of anionic resins by anions simultaneously. nitrates, acetates or formates and also replace the H ⁺ ions of the cationic resins with calcium, strontium or barium.

In this case, an aqueous solution of barium nitrate or acetate is generally used to carry out the pretreatment. Indeed, the use of barium nitrate is preferable because the anion NO ₃ ^- is generally present in the effluents, in particular in the effluents of the reprocessing centers and the nuclear study centers.

Thanks to the pretreatment of the invention, the structure of the three-dimensional network of ion exchange resins is modified by introducing into the molecular chain ions such as Ba ⁺⁺ and / or NO ₃ ^- or CH _a COO-, which occupy a place more important than the H ⁺ , OH- and CI- ions initially present. This makes it possible to prevent the penetration of water into the denser macromolecular network of the pretreated resin and thus to decrease the degree of hydration of the resin.

Indeed, the exchange, ion, anionic and cationic resins have the particularity of swelling in water in a significant manner. Their increase in volume is generally of the order of 20% for anionic resins in OH- and / or CI- form and greater than 50% for cationic resins in Na ⁺ form. This swelling is due to the penetration of water into the three-dimensional skeleton of the resins, which is particularly hydrophilic in the case of anionic resins which contain quaternary ammonium or amine groups and in the case of cationic resins which contain sodium sulfonate groups grafted to both types of resins on a polystyrene radical. As a result, the processing capacity of bituminization plants is lower when dealing with resins of these types. In addition, the bituminous mixes obtained after the bituminization treatment have the disadvantage of swelling in water.

On the other hand, when a pretreatment of the ion exchange resins is carried out according to the invention, the hydration capacity of the macromolecular network of the resins is reduced, and therefore the treatment capacity of the bituminization plants can be increased and limit swelling of the asphalt mixes obtained in contact with water.

Thus, in the process of the invention, the step of pretreatment of ion exchange resins with a salt such as barium nitrate, does not have the same object as the step of pretreatment of prior processes (FR- A-2361724 and Japanese patent 48/28899), since it is not intended to prevent the ion-exchange resins from consuming some of the reagents necessary for the reaction to form the coating matrix.

Likewise, the pretreatment step of the invention does not have the same object as the pretreatment step of French patent FR-A-2,356,246. In this patent, waste consists of concentrates of and / or radioactive material sludge such as chemical precipitation sludge containing salts, and the problem to be solved is to transform salts such as sulphate and sodium carbonate into salts having a less tendency to capture molecules of water.

In contrast, in the invention the waste is ion exchange resins, which do not contain. salts, and the problem to be solved is to decrease the hydration capacity of the macromolecular network of ion exchange resins.

For the asphalting operation, the operation is carried out conventionally using for example the techniques described in French patents 1,315,162 and 2,052,903 or American patent 3,298,961. The devices used are also of a conventional type and can be made up of thin-film evaporators, double or quadruple screw extruders, etc.

The method of the invention is particularly applicable to the treatment of cationic and optionally anionic resins with a polystyrene skeleton in beads and / or ground.

Examples of such resins include resins in the form of beads such as Iss resins sold under the brand Duolite by Diaprosim or resins sold under the brand Amberlite by Rohm and Haas. By way of example of ground resins, mention may be made of the Microionex resins sold by Diaprosim.

When, according to the invention, anionic and cationic resins are treated as a mixture, any proportion of anionic resins in the range between 0 and 100% can be used.

For the implementation of the process of the invention, the pretreatment step is preferably carried out by introducing into a tank a certain amount of an aqueous solution of a salt of Ba ⁺⁺ , Ca ⁺⁺ or Sr2 + containing N0 ₃ ^- , HCO ₂ ^- or CH ₃ CO ₂ ^- ions. The resins to be pretreated are suspended in the solution and the whole is stirred for a sufficient time which is chosen, depending on the salt concentration of the solution, so as to obtain the desired saturation rate of the exchange resins. ions.

Preferably, the salt concentration and the treatment duration are chosen so as to obtain a saturation rate of anions: N0 ₃ -, HCO ₂ ^- or CH ₃ CO ₂ ^- anion exchange resins close to 100%.

• 1) augmentant la capacité de traitement de l'installation de bitumage de 50% puisqu'on réduit le volume des résines mises en suspension dans l'eau et qu'on augmente la capacité d'évaporation du dispositif de bitumage,
• 2) supprimant dans les distillats les produits de dégradation des résines anioniques, en particulier NH₃ et CH₃NH₂, grâce à une stabilisation chimique des groupes fonctionnels ammonium quaternaire ou amine tertiaire, ce qui permet de récupérer des condensats neutres et non plus basiques, et
• 3) limitant le processus de gonflement des enrobés bitumineux dans l'eau, leur augmentation de volume ne dépassant pas 5% comme on le verra ci-après.

Thanks to the pretreatment of ion exchange resins according to the process of the invention, the bituminization process can be improved by:

• 1) increasing the treatment capacity of the asphalt installation by 50% since the volume of resins suspended in water is reduced and the evaporation capacity of the asphalt device is increased,
• 2) removing in the distillates the degradation products of anionic resins, in particular NH ₃ and CH ₃ NH ₂ , thanks to a chemical stabilization of the functional groups quaternary ammonium or tertiary amine, which makes it possible to recover neutral and no more basic condensates , and
• 3) limiting the process of swelling of bituminous mixes in water, their increase in volume not exceeding 5% as will be seen below.

Other characteristics and advantages of the invention will appear better on reading the following examples given of course by way of illustration and not limitation with reference to the appended drawing in which Figures 1 and 2 are diagrams representing the percentage increase in volume bituminous mixtures of cation exchange resins, obtained either by the process of the prior art, or by the process of the invention, depending on the time of immersion in water.

Example 1

This example illustrates the treatment of cation exchange resins Amberlite IR 120 H.

The resins are transferred by means of a hydro-ejector using demineralized water, to a treatment tank provided with a mechanical agitator and with an ultrasonic detection device of the level of the solid-liquid interface.

After decanting the resin mixture for a period which can range from 24 to 48 hours depending on whether the resins are in the form of beads or in the ground state, the supernatant water is removed, either by pumping or by siphoning under vacuum. . Additional quantities of resins are again transferred and the decanting, pumping and siphoning operations are repeated until the volume V of decanted resins reaches 50% of the volume of the tank.

In an auxiliary tank, a 0.37 mol.l ^-1 solution of barium nitrate is prepared and a volume V of this solution is introduced into the treatment tank, that is to say a volume identical to that occupied by decanted resins. The whole is stirred for two hours. It is then left to settle for 1 h and the supernatant solution is removed by means of a pump or a steam ejector or by vacuum siphoning. In this case, the volume of the solution removed corresponds to 1.22 V, which shows that not only was the pretreatment solution but also a certain amount of the water absorbed by the resins, this amount represents 14 % of the initial volume of decanted wet resins.

This operation is repeated three times to obtain a saturation rate in Ba "of the resins close to 100% due to the concentration of Ba (N0 ₃ ) ₂ in the solution (0.37 mole.l ^-1 ) and the duration of contact.

The saturation pretreatment causes a release of 10 to 15% of the initial activity of the resins. Also, the supernatant radioactive solution is returned to the head of the liquid effluent treatment station, either upstream of an evaporator, or upstream of a chemical coprecipitation chain.

After decantation and elimination of the supernatant solution, the pretreated resins are washed with demineralized water using a volume of washing water equal to 0.65 V, ie identical to that of the pretreated and decanted resins. The pH of the suspension of resins is adjusted to 7.5 ± 0.2 using a baryte milk at 300 gl ^- 'of Ba (OH) ₃ . This washing operation is repeated four times, controlling the concentration of N0 ₃ ^- of the washing water after each operation, until an N03 concentration of the washing water is obtained of less than 2 g ^-1 .

In fact, it is necessary to wash the pretreated resins before packaging them in bitumen, because if barium nitrate in excess was added to the dry extract of the resins, this would have the disadvantage of increasing, not only the volume of the final residue, but to further promote the leaching of this water-soluble salt.

The transfer and washing waters are very weakly radioactive and they are sent to the effluent treatment station where their level of radioactivity is checked.

The pretreated and washed resins are then resuspended in demineralized water using 0.4 to 0.45 V of water for a proportion of 0.6 to 0.55 V of the resin mixture. The suspension of pretreated organic resins is then sent to the asphalt plant, which is of the quadruple screw drying extruder type.

The properties of the asphalt leaving the asphalt plant are then checked.

In Table 1, the results obtained with respect to hourly capacity are reported. evaporation of the extruder and the composition of the coating obtained during a bituminization operation carried out with ball resins.

The swelling properties in water of the coated materials obtained which have been poured and solidified in the form of cylindrical test pieces 48 mm in diameter and 90 mm in height are then evaluated.

To determine the swelling in water, the mixes are immersed in ordinary non-renewed water or non-renewed demineralized water, and the volume increase (in%) of the mixes is periodically measured as a function of the duration immersion (in days).

The volume ratio between the water and the mixes is 4.5 and the surface / volume ratio of the mixes is 10.5 cm ^-1.

The results obtained are given on curves 1 of FIGS. 1 and 2 which represent the percentage increase in the volume of the coated materials as a function of time (in days). Figure 1 illustrates the results obtained with plain water and Figure 2 illustrates the results obtained with demineralized water.

In view of these results, it can be seen that the swelling of the coated materials after 365 days of immersion in ordinary water is 4.2% by volume, ie 4.45% by weight, and that it is 5, 0% by volume, or 5.1% by weight, after 365 days of immersion in demineralized water.

Example 2

As in Example 1, cation exchange resins Amberlite IR 120H are treated, but using a 1.5 mol.l ^-1 aqueous solution of barium acetate instead of the 0.37 mol aqueous solution. l ^-1 barium nitrate.

In an auxiliary tank, a 1.5 mol.l ^-1 solution of barium acetate is prepared and a volume V of this solution is introduced into the treatment tank, that is to say a volume identical to that occupied by decanted resins. The whole is subjected to stirring for two hours, which makes it possible to obtain a saturation rate of Ba2 + in the resins greater than 90% due to the concentration of Ba (NO ₃ ) ₂ in the solution (1.5 mole.l ^-1 ) and the duration of contact.

Then allowed to settle for one hour and the supernatant solution is removed by means of a pump or a steam ejector or by vacuum siphoning.

In this case, the volume of the solution eliminated corresponds to 1.35 V, which shows that not only the pretreatment solution, but also a certain amount of the water absorbed by the resins has been eliminated, this amount represents 21% of the volume of decanted wet resins.

After decantation and elimination of the supernatant solution, the pretreated resins are washed with demineralized water using a volume of washing water equal to 0.83 V, ie identical to that of the pretreated and decanted resins.

The pH of the resin suspension is adjusted to 7.5 ± 0.2 using a baryte milk containing 300 g of Ba (OH) ₃ .

This washing operation is repeated three times, controlling the concentration of CH ₃ CO ₂ - of the washing water after each operation until a concentration of CH ₃ C02 in the washing water of less than 2 g is obtained. . The operation is continued as in Example 1 and the asphalting operation is carried out under the same conditions using blown bitumen MR 90/40.

The properties of the asphalt leaving the asphalt plant are then checked.

The results obtained are given on curves 2 of FIGS. 1 and 2 which represent the percentage increase in volume of the mixes as a function of the time (in days) during which they were immersed in ordinary non-renewed water (FIG. 1 ) or in non-renewed demineralized water (Figure 2).

In view of these figures, it can be seen that the swelling of the coated materials after 120 days of immersion in ordinary water is 2.45% by volume, ie 3.4% by weight, and that the swelling after 120 days d immersion in demineralized water is 4.9% by volume, ie 5.0% by weight.

Example 3

In this example, Amberlite IR 120H resin beads are treated, as in Example 1, but using a 1.5 mol.l ^-1 soda solution instead of using the barium nitrate solution, which has the effect of bringing the resins in Na ⁺ form. The resins pretreated are coated in MR 90/40 bitumen under the same conditions as those of Example 1, and the properties of the mixes obtained are then determined as in Example 1.

The results obtained are given in Table 1 and on curves 3 of Figures 1 and 2, which represent the percentage increase in volume of the mixes as a function of the time (in days) during which they were immersed in water. regular (Figure 1) or in renewed demineralized water (Figure 2).

In view of these results, it can be seen that the swelling of the coated materials after 15 days of immersion in demineralized water or in ordinary water is very significant. Indeed, the swelling is 38.7% by volume, or 25.1% by weight, in the case of demineralized water, and 50.1% by volume, or 45.6% by weight, in the case of ordinary water.

Thus, it can be seen that, in the process of the invention, the choice of salt used for the pretreatment has a very significant effect on the results obtained.

Claims (6)

1. Process for conditioning by bituminizing radioactive waste constituted by ion exchange resins and incorporating cation exchange resins, characterized in that it comprises:

a) subjecting said resins to a pretreatment using a salt for replacing the H⁺ and/or Na⁺ ions of the cation exchange resins by ions chosen from the group including Ca⁺⁺, Sr⁺⁺ and Ba⁺⁺,

b) suspending the thus pretreated resins in water, and

c) bituminizing said suspension.

2. Process according to Claim 1 for conditioning by bituminizing radioactive waste constituted by cation exchange resins and anion exchange resins, characterized in that the treatment is performed by means of a salt such that the H⁺ and/or Na⁺ ions of the cation exchange resins are replaced by ions chosen from the group including Ca⁺⁺, Sr⁺⁺ and Ba⁺⁺ and the OH- and/or CI- ions of the anion exchange resins by an anion chosen from the group including N0₃-, HC02 CH₃CO₂ ^- .

3. Process according to Claims 1 or 2, characterized in that pretreatment is carried out by contacting said resins with an aqueous solution of a calcium, strontium or barium salt, the latter being chosen from the group including nitrates, acetates and formates.

4. Process according to Claim 3, characterized in that the pretreatment is carried out by contacting said resins with an aqueous barium nitrate solution.

5. Process according to Claim 3, characterized in that the pretreatment is carried out by contacting the resins with an aqueous barium acetate solution.

6. Process according to Claim 2, characterized in that the pretreatment is carried out in such a way as to obtain an approximately 100% NO₃ ^-, HCO₂ ^- or CH₃CO₂ ^- saturation level of the anion exchange resins.

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