EP1252635B1 - Method for conditioning soda effluents in the form of nepheline - Google Patents

Description

Field of the invention

The present invention relates to a waste packaging method consisting of aqueous solutions of soda.

It concerns, more precisely, the treatment of radioactive soda solutions, obtained as waste from nuclear reactors to fast neutrons.

Indeed, the development of fast neutron nuclear reactors, using metallic sodium as coolant, leads to the production of radioactive sodium waste. This waste can come from the operation of industrial and experimental reactors, but also from research laboratories. They may contain radioactive elements, such as ²² Na and other radioelements derived from electronuclear activity in general such as uranium, plutonium, cesium, cobalt, etc.

To reduce the chemical risk potential that sodium storage under metal form, this waste is generally converted in concentrated sodium hydroxide solution by a process of destruction with water.

These concentrated soda solutions must be transformed into solid waste, retaining the radioactive products they may contain, with a view to of their storage.

State of the art

US-A-4,028,265 [1] illustrates a liquid radioactive waste conversion process caustics containing sodium nitrate to a product solid insoluble. According to this method, an clay powder based on aluminum silicate, with an aqueous solution or suspension of the waste radioactive liquid, which has a concentration sodium hydroxide of 3 to 7 M and which contains sodium nitrate, to form a product of the type cancrinite, which is then transformed by calcination at least 600 ° C, in a mineral form, such as nepheline.

This conversion corresponds to the following reaction schemes: Al ₂ Si ₂ O.2H ₂ O + 2NaOH + 0.52 NaNO ₃ + 0.68H ₂ O
→ Na ₂ O.Al ₂ O ₃ .2SiO ₂ .O, 68H ₂ O.0.52 NaNO ₃ + 3H ₂ O Na ₂ O.Al ₂ O ₃ .2SiO ₂ .0.68H ₂ O.0.52NaNO ₃ · Δ
2NaAlSiO ₄ .0.26Na ₂ O + 0.68H ₂ O + 0.26N ₂ O ₅

Silico-aluminous clays, likely to be used for this conversion, belong to the group comprising kaolin, bentonite, dickite, halloysite and pyrophillite. From the intermediate product (cancrinite) (1), it is formed, during the calcination, a loose powder, either compression moldings intermediate product in the desired form, followed sintering at at least 600 ° C.

In this process, it is therefore necessary to handle powders to obtain products solid shaped and use presses mechanical or hydraulic to compress these powders.

Also known are processes for synthesizing zeolite 4A from calcined kaolins by reacting them with sodium hydroxide as described in Ind. Eng. Chem. Res., 27, 1988, pages 1,291-1296, [2], and in EP-A-0 012 248, [3]. In reference [2], a gel is formed by dissolving the calcined kaolin in the sodium hydroxide solution, and then, by heating the gel, a solid product is prepared by crystallization. For this purpose, kaolin quantities such as the Na ₂ O / SiO ₂ molar ratio, that is 1.8 to 3.8, preferably 2.8, are chosen. The amount of water is such that the molar ratio H ₂ O / Na ₂ O is in the range of 30 to 50, which corresponds to solutions of soda 2.23 to 3.7 M. zeolite usable in the formulation of detergents, the particle size being in the range of 1 to 10 microns for 99% by weight of the particles.

In reference [3], zeolite A is obtained by reaction of metakaolin with an aqueous alkaline medium, using a solution containing 7 to 30% of sodium hydroxide and a quantity of NaOH, representing 1.3 to 3 times the stoichiometric quantity required for the formation of zeolite A. This corresponds to a Na ₂ O / SiO ₂ ratio of 0.05 to 10 and an H ₂ O / Na ₂ O molar ratio of 15 to 70, ie a sodium hydroxide solution 1, As before, this zeolite in the form of a powder is intended to be used in detergents, which is why a glossier and less yellow zeolite A is sought, which result is obtained by using, as a product of departure, metakaolin.

Presentation of the invention

The present invention is specifically for object a method of packaging a waste consisting of an aqueous solution of soda, which allows to obtain solid products, nepheline type, without have to treat a powder and compact it.

a) ajouter à la solution aqueuse une poudre de métakaolin en quantité telle qu'on obtienne une suspension apte à prendre en masse et former une phase cristalline de type zéolithe A ;

b) introduire la suspension dans un moule ;

c) laisser la suspension prendre en masse dans le moule pour obtenir un produit solide moulé à base de zéolithe A ;

d) sécher le produit moulé ; et

e) convertir la phase zéolithe A en une phase de type néphéline par traitement thermique à une température de 1000 à 1500°C.

According to the invention, the method of packaging a waste, consisting of an aqueous solution comprising 3 to 10 mol / l of NaOH sodium hydroxide, comprises the following steps:

a) adding to the aqueous solution a metakaolin powder in an amount such that a slurry capable of forming and forming a crystalline phase of zeolite A type is obtained;

b) introducing the suspension into a mold;

c) allowing the slurry to set in the mold to obtain a molded solid product based on zeolite A;

d) drying the molded product; and

e) converting the zeolite A phase to a nepheline-type phase by heat treatment at a temperature of 1000 to 1500 ° C.

In the process of the invention, the fact of from a concentrated aqueous solution of soda and to add metakaolin powder in quantity appropriate means of obtaining a suspension to take in mass to form a crystalline phase zeolite type A.

According to the invention, this zeolite A can therefore be obtained directly as a product solid molded, then transformed into phase-type nepheline by a heat treatment. This is very advantageous when aqueous solutions of soda contain radioactive products because the handling of powder, which could lead to a dispersion of the radioactivity, one decreases investment in equipment since it is no longer necessary to use mechanical presses or hydraulic systems, and the size of installing and producing molded products corresponding to the dimensions of the waste containers. In addition, the use of a nepheline type phase allows to confine the radioactivity in a phase stable, which prevents leaching of products radioactive substances trapped in this structure.

In the process of the invention, the fact of from an aqueous soda solution, not including practically no nitrate and sodium nitrite, allows to obtain, by reaction with metakaolin, a crystalline phase of zeolite A.

In document [1], this phase could appear, due to the presence of nitrate and sodium nitrite that led to the formation of a cancrinite phase.

In the invention, the reaction of metakaolin with sodium hydroxide corresponds to the following overall reaction scheme:

The possibility of getting the mass of the soda solution by this reaction results from the choice of metakaolin, the concentration of sodium hydroxide starting aqueous solution and the amount of metakaolin added.

Indeed, the zeolite phase only appears in the case of the use of metakaolin. Next time the amount of metakaolin and the reaction temperature, a proportion of phase hydroxysodalite can be formed. However, this second phase, even in significant proportion, does not preclude not the caking of the suspension and the conversion in nepheline.

To directly take the mass of the solution without bleeding, it is important to have a concentration of the sodium hydroxide solution greater than 3 mol / l, preferably greater than 5 mol / l. When the soda concentration is too low, the reaction has place by leaving a supernatant liquid and a solid dense. When the sodium concentration exceeds 10 mol / l, mixing problems are observed as well as that a very short setting time, the suspension is then very viscous and caking can prove to be too fast to implement the process of the invention. It is therefore important to choose a 3 to 10 M sodium hydroxide solution to obtain this mass.

Moreover, it is advisable to choose quantity of metakaolin added, in such a way that corresponds substantially to the stoichiometry of the reaction (3) described above, a molar ratio from metakaolin to soda, which is close to stoichiometry of the reaction, ie a ratio molar ratio of 0.4 to 0.6, preferably about 0.5.

To get hold of the suspension, it is also appropriate to appropriate value the water content of the suspension obtained by adding metakaolin to the solution of soda. The water content of this suspension depends on the sodium hydroxide concentration of the aqueous solution of departure, since no additional water is added after addition of metakaolin to this solution. Starting from aqueous solutions containing 3 to 10 mol / l of sodium hydroxide, reaches suspensions containing from 30 to 70% in weight of water, in case the amount of metakaolin added substantially corresponds to the stoichiometry of the reaction.

Also, we can advantageously adjust the amount of metakaolin added so that the content of the water of the suspension is 30 to 70% by weight.

• le mode de préparation du métakaolin utilisé ;
• la granulométrie du métakaolin ;
• la température de traitement ; et
• les espèces chimiques, autres que NaOH, présentes dans la solution aqueuse de départ.

Other important parameters to obtain the solubility of the solution are:

• the method of preparation of the metakaolin used;
• the particle size of metakaolin;
• the treatment temperature; and
• the chemical species, other than NaOH, present in the starting aqueous solution.

With regard to metakaolin, this one is obtained by calcining kaolin at temperatures from 500 to 1200 ° C. The higher the calcination temperature is low and the product is reactive. However, compensate for the decrease in reactivity by grinding. In the invention, metakaolins obtained from temperatures of 800 to 1000 ° C, having a granulometry average of 1 to 50 microns, preferably 1 to 10 microns.

The treatment temperature used for the reaction (3) can be 15 to 100 ° C under pressure atmospheric. We could also operate on higher temperatures under pressure. Indeed, a moderate heating of the suspension allows to activate the caking of the suspension.

The chemical species, present in the starting aqueous solution, can interfere with the formation reaction (3) of the zeolite A. This is the case in particular of the NO ₃ ^- and NO ₂ ^- ions present in the starting aqueous solution. of reference [1] which prevent the formation of the zeolite phase A and the setting in mass leading to the cancrinite phase.

Also according to the invention, the total content of NO ₃ ^- and NO ₂ ^- ions of the aqueous starting solution is preferably from 0 to 0.5 mol / l.

When the process of the invention is used to condition aqueous solutions of radioactive soda, the presence of these elements radioactive is not a problem because they are in extremely small amounts in the solution.

The possibility to make mass an aqueous solution of concentrated sodium hydroxide by addition metakaolin does not appear in references [1] to [3] who do not teach the choice of metakaolin and starting solution, nor the essential parameters to get this en masse.

After caking the solution under form of zeolite A hydrated, the molded product is subjected obtained by this en masse to a drying, then to a heat treatment to turn it into nepheline.

Drying can be done after demolding the product at temperatures of 110 to 500 ° C.

The heat treatment is then performed on the dried product at temperatures of 1000 to 1500 ° C, to obtain the transformation into nepheline (between 500 and 850 ° C) and densification products by removing open porosity.

Other features and benefits of the invention will appear better on reading the description which follows, of examples of realization given, of course, by way of illustration and not limitation, in reference to the appended drawing.

Brief description of the drawing

The figure is a diagram representing the different steps of the process of the invention.

Detailed description of the embodiments

In Figure 1, we see that the first step of the process is to add to the solution of starting soda (waste) the desired amount of powder metakaolin, and to knead together to obtain a homogeneous paste.

Preferably, the average particle size of the metakaolin powder is less than 10 μm and this metakaolin was obtained by calcining a kaolin in powder at a temperature of 800 ° C for 1 hour.

After obtaining a homogeneous mixture, performs the shaping step by casting or spinning. For example, we can pour the dough in a mold tight to obtain a desired shaped product.

The crystallization and the setting in mass of the mixture occur within minutes or hours that follow. The setting time depends on the temperature used. To strongly activate the plug, you can operate at a temperature of 40 to 70 ° C by heating moderate.

We could also promote growth zeolite grains by seeding the suspension with zeolite A crystals to get a homogenous germination or with nepheline crystals leading to heterogeneous germination.

After caking, we obtain hardened products that have acquired mechanical strength sufficient to be handled by means automated. We can unmold them before submit to drying and final heat treatment.

We can also keep them in the mold, if it consists of a material totally incinerable and ashless, for example material polymer or cellulose.

The molded products are then slowly dried to remove the residual water, avoiding cracking of the products. The duration of the drying step obviously depends on the amount of water to be evaporated, as well as the geometry of the molded products. It can operate at a temperature of 110 to 550 ° C, not exceeding the temperature of 100 ° C in the early stages, to prevent cracking of the molded products, starting too violent water vapor. This drying step corresponds to the following reaction equation: Na ₉₆ Al ₉₆ Si ₉₆ O ₃₈₄ .216H ₂ O → Na ₉₆ Al ₉₆ Si ₉₆ O ₃₈₄ + 216H ₂ O

1°) de convertir la phase zéolithe en une phase de type néphéline, conversion qui a lieu typiquement entre 500 et 850°C ; et

2°) de densifier les produits moulés et supprimer la porosité ouverte, ce qui peut se produire à des températures allant de 850 à 1500°C.

The dried products are then subjected to the final heat treatment at at least 1000 ° C. This treatment aims to:

1 °) to convert the zeolite phase into a nepheline type phase, which conversion typically occurs between 500 and 850 ° C; and

2 °) densify the molded products and remove the open porosity, which can occur at temperatures ranging from 850 to 1500 ° C.

A nepheline phase is thus obtained densified in which are trapped the salts or radioactive elements that were likely to be present in the aqueous solution of starting soda. The products obtained can then be directed to a storage site, possibly after stuffing.

The following examples of conditioning solutions of soda are given for illustration and non-limiting.

Example 1: Conditioning a solution 10N soda

At 20 ° C., 555 g of metakaolin are added (washed PROLABO Kaolin product) 500 ml of the solution 10N sodium hydroxide, with mechanical stirring. We continue homogenization with stirring for 15 minutes, after the end of the introduction of metakaolin, then sinks the suspension in Teflon® molds. We then introduces the molds filled with the suspension in an oven to accelerate the setting, operating a temperature of 40 ° C. After 24 hours, we unmold products, then dried by submitting them gradually at a maximum temperature of 110 ° C to slow speed, for 24 hours, and then submit them to sintering at 1250 ° C, maintaining this temperature for 2 hours, with a heating rate of 2 ° C / min.

We thus obtain monolithic products having satisfactory qualities for storage of waste.

Example 2: Packaging a solution soda 5N

At 20 ° C., 277.5 g of metakaolin are added, identical to that of Example 1, 500 ml of the 5N sodium hydroxide solution, with mechanical stirring, then homogenize for 15 minutes, after the end of the introduction of metakaolin. We then sink the suspension in Teflon® molds, then heated the molds at 70 ° C to accelerate the caking of suspension. After 24 hours, the products are demolded molded and dried by submitting them gradually at a maximum temperature of 110 ° C to slow speed, for 24 hours. They are then submitted to a sintering at 1250 ° C with a 2 hour stage and a speed heating identical to that of Example 1.

This produces satisfactory products for waste storage.

References cited

[1] US-A-4,028,265.

[2] Ind. Eng. Chem. Res., 27, 1988, pp. 1,291-1296.

[3] US-4,271,130.

Claims (10)

1. Method for conditioning a waste constituted by an aqueous solution comprising 3 to 10 mol/l of sodium hydroxide NaOH, characterised in that it comprises the following stages:

   a) adding a quantity of metakaolin powder to the aqueous solution such that a suspension is obtained capable of solidifying and forming a crystalline phase of the zeolite A type;

   b) introducing, the suspension into a mould;

   c) leaving the suspension to solidify in the mould in order to obtain a moulded solid product based on zeolite A;

   d) drying the moulded product; and

   e) converting the zeolite A phase into a nepheline type phase by heat treatment at a temperature of 1000°C to 1500°C.
2. Method according to claim 1, characterised in that the quantity of metakaolin added is such that the molar ratio of metakaolin to sodium hydroxide is from 0.4 to 0.6.
3. Method according to claim 2, characterised in that the metakaolin/sodium hydroxide molar ratio is around 0.5.
4. Method according to claim 1, in which the metakaolin powder has an average granulometry of 1 to 50µm.
5. Method according to any one of claims 1 to 4, characterised in that the metakaolin is obtained by calcination of kaolin at a temperature of 500 to 1200°C.
6. Method according to claim 1, characterised in that the quantity of metakaolin added is such that the water content of the suspension is from 30 to 70% by weight.
7. Method according to claim 1, characterised in that the total content of NO₃ ^- and NO₂ ^- ions present in the aqueous solution of origin is from 0 to 0.5 mol/l.
8. Method according to claim 1, characterised in that stage c) is carried out at a temperature of 15 to 100°C.
9. Method according to claim 1, characterised in that the drying of the moulded product is carried out, after de-moulding said product, at a temperature of 110 to 500°C.
10. Method according to any one of claims 1 to 9, characterised in that the aqueous solution of origin is a radioactive solution.

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