EP1252635A2

EP1252635A2 - Method for conditioning soda effluents in the form of nepheline

Info

Publication number: EP1252635A2
Application number: EP01907692A
Authority: EP
Inventors: Olivier Fiquet; Ronan Le Chenadec; Didier Gibert
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2000-01-26
Filing date: 2001-01-25
Publication date: 2002-10-30
Anticipated expiration: 2021-01-25
Also published as: WO2001056040A3; US20020198431A1; RU2257627C2; JP2003520975A; FR2804103A1; US6676915B2; RU2002122769A; WO2001056040A2; EP1252635B1; FR2804103B1

Abstract

The invention concerns a method for conditioning waste consisting of a possibly radioactive aqueous solution of NaOH 3 to 10 M. Said method consists in: a) adding to the aqueous solution a metakaolin powder in such amount as to obtain a suspension capable of aggregating and forming a zeolite-type crystalline phase A; b) introducing the suspension in a mould; c) allowing the suspension to aggregate in the mould to obtain a solid moulded 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 ranging between 1000 and 1500 °C.

Description

PROCESS FOR PACKAGING SODIUM EFFLUENTS

IN NEPHELINE FORM

DESCRIPTION

Field of the invention

The subject of the present invention is a process for conditioning waste constituted by aqueous sodium hydroxide solutions.

More specifically, it relates to the processing of radioactive soda solutions, obtained as waste from fast neutron nuclear reactors. 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 can contain radioactive elements, such as 22Na and other radioelements originating from nuclear power activity in general such as uranium, plutonium, cesium, cobalt, etc.

In order to reduce the potential chemical risk represented by the storage of sodium in metallic form, this waste is generally converted into 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 that they may contain, for storage.

State of the art

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

Al ₂ Si ₂ 0.2H ₂ 0 + 2NaOH + 0.52 NaN0 ₃ + 0.68H ₂ O → Na ₂ O. Al ₂ O ₃ .2 SiO ₂ .O, 68H ₂ O.0.52 NaN0 ₃ + 3H ₂ 0 (1)

Na ₂ O.Al ₂ O ₃ .2SiO ₂ .0, 68H ₂ 0.0, 52NaN0 ₃ Δ

2NaAlSiO ₄ .0.26Na ₂ O + 0.68H ₂ O + 0.26N ₂ O ₅ (2) The silico-aluminous clays, which can be used for this conversion, belong to the group comprising kaolin, bentonite, dickite, halloysite and pyrophillite. From the intermediate product (cancrinite) in diagram (1), during the calcination, either a loose powder or objects molded by compression of the intermediate product in the desired form are formed, followed by sintering to at least 600 ° C. In this process, it is therefore necessary to manipulate powders in order to obtain solid shaped products and to use mechanical or hydraulic presses to compress these powders.

Methods of synthesizing zeolite 4A are also known, from calcined kaolins, by reaction of these with sodium hydroxide, as described in Ind. Eng. Chem. Res. , 27,

1988, pages 1 291-1 296, [2], and in US-A-4 271 130,

[3]. In reference [2], a gel is formed by dissolving the calcined kaolin in the sodium hydroxide solution, then a solid product is prepared, by heating the gel, by crystallization. For this purpose, kaolin quantities such as the Na ₂ O / Si0 ₂ molar ratio are chosen, that is from 1.8 to 3.8, preferably 2.8. The amount of water is such that the molar ratio H ₂ 0 / Na ₂ 0 either in the range from 30 to 50, which corresponds to soda solutions 2.23 to 3.7 M. This gives zeolite powders usable in the formulation of detergents, the size of the particles being in the range of 1 to 10 μm for 99% by weight of the particles. In reference [3], a zeolite A is obtained by reaction of metakaolin with an alkaline aqueous medium, using a 7 to 30% solution of sodium hydroxide and an amount of NaOH, representing 1.3 to 3 times the stoichiometric quantity required for the formation of zeolite A. This corresponds to an Na ₂ 0 / Si0 ₂ ratio of 0.05 to 10 and an H ₂ 0 / Na ₂ 0 molar ratio of 15 to 70, i.e. a sodium hydroxide solution 1, 58 to 7.4 M. As before, this zeolite in powder form is intended for use in detergents and that is why we are looking for a brighter and less yellow zeolite A, a result which is obtained by using, as a product of departure, metakaolin.

Statement of the invention

The present invention specifically relates to a method of packaging a waste consisting of an aqueous sodium hydroxide solution, which makes it possible to obtain solid products, of the nepheline type, without having to treat a powder and compact it.

According to the invention, the process for conditioning a waste, consisting of an aqueous solution comprising 3 to 10 mol / 1 of NaOH soda, comprises the following steps: a) adding to the aqueous solution a metakaolin powder in an amount such that 'A suspension capable of solidifying and forming a crystalline phase of the zeolite A type is obtained; b) introducing the suspension into a mold; c) allow the suspension to solidify in the mold to obtain a solid molded 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 starting from a concentrated aqueous solution of sodium hydroxide and adding thereto metakaolin powder in an appropriate amount makes it possible to obtain a suspension capable of solidifying to form a crystalline phase of the zeolite type A.

According to the invention, this zeolite A can therefore be obtained directly in the form of a solid molded product, then transformed into the nepheline type phase by a heat treatment. This is very advantageous when the aqueous sodium hydroxide solutions contain radioactive products, since the handling of powder is eliminated, which could lead to a dispersion of the radioactivity, the investment in material is reduced since it is no longer necessary to use mechanical or hydraulic presses, and we can also reduce the size of the installation and produce molded products corresponding to the dimensions of the waste containers. In addition, the use of a nepheline-type phase makes it possible to confine the radioactivity to a stable phase, which prevents the leaching of the radioactive products trapped in this structure. In the process of the invention, the fact of starting from an aqueous soda solution, comprising practically no sodium nitrate and nitrite, makes it possible to obtain, by reaction with metakaolin, a crystalline phase of zeolite A type. .

In document [1], this phase could not appear, due to the presence of sodium nitrate and nitrite which led to the formation of a cancrinite type phase. In the invention, the reaction of metakaolin with sodium hydroxide corresponds to the following overall reaction scheme:

20-70 ° C 96NaOH + 48 [Al ₂ Si ₂ 0 ₇ ] + 168H ₂ 0>

Na ₉₆ Al ₉₆ Si ₉₆ 0 ₃₈ -216H ₂ O (3)

The possibility of obtaining the solidification of the sodium hydroxide solution by this reaction results from the choice of metakaolin, from the sodium hydroxide concentration of the starting aqueous solution and from the quantity of metakaolin added.

Indeed, the zeolite phase only appears in the case of the use of metakaolin. Depending on the reaction time, the quantity of metakaolin and the reaction temperature, a proportion of hydroxysodalite phase can form. However, this second phase, even in significant proportion, does not prevent solidification of the suspension and conversion to nepheline. To directly obtain solidification of the solution without bleeding, it is important to have a concentration of the sodium hydroxide solution greater than 3 mol / 1, preferably greater than 5 mol / 1. When the sodium hydroxide concentration is too low, the reaction takes place, leaving a supernatant liquid and a dense solid. When the sodium hydroxide concentration exceeds 10 mol / l, mixing problems are observed as well as a very short setting time, the suspension is then very viscous and solidification may prove to be too rapid to implement the process of the invention. It is therefore important to choose a 3 to 10 M sodium hydroxide solution to obtain this solidification. Furthermore, the quantity of metakaolin added should be chosen so that it corresponds substantially to the stoichiometry of the reaction (3) described above, i.e. a molar ratio of metakaolin to soda, which is close to the stoichiometry of the reaction, that is to say a molar ratio of 0.4 to 0.6, preferably about 0.5.

To obtain solidification of the suspension, the water content of the suspension obtained by adding metakaolin to the sodium hydroxide solution should also be adjusted to an appropriate value. The water content of this suspension depends on the sodium hydroxide concentration of the starting aqueous solution, since no addition of water is made after adding metakaolin to this solution. Starting from aqueous solutions containing 3 to 10 mol / l of sodium hydroxide, suspensions containing 30 to 70% are obtained. weight of water, in the case where the quantity of metakaolin added corresponds substantially to the stoichiometry of the reaction.

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

Other important parameters for obtaining solidification of the solution are: the method of preparation of the metakaolin used;

- the particle size of metakaolin;

- the processing temperature; and

- chemical species, other than NaOH, present in the starting aqueous solution. As regards metakaolin, this is obtained by calcining kaolin at temperatures of 500 to 1200 ° C. The lower the calcination temperature, the more reactive the product. However, the drop in reactivity can be compensated for by grinding. In the invention, the metakaolins obtained at temperatures of 800 to 1000 ° C. are preferred, having an average particle size of 1 to 50 μm, preferably from 1 to 10 μm.

The treatment temperature used for reaction (3) can be from 15 to 100 ° C, under atmospheric pressure. One could also operate at higher temperatures under pressure. Indeed, moderate heating of the suspension makes it possible to activate the solidification of the suspension.

The chemical species present in the starting aqueous solution may interfere with the reaction (3) for the formation of zeolite A. It is the case in particular of the N0 ₃ ^" and N0 ₂ ions present in the starting aqueous solution of the reference [1] which prevent the formation of the zeolite A phase and the solidification leading to the cancrinite phase. Also according to the invention , the total content of N0 ₃ ^~ and N0 ₂ ^" ions in the starting aqueous solution is preferably 0 to 0.5 mol / l.

When the process of the invention is used for conditioning aqueous radioactive soda solutions, the presence of these radioactive elements is not a problem since they are found in extremely small amounts in the solution.

The possibility of having a concentrated aqueous sodium hydroxide solution solidify by adding metakaolin is not apparent from the references [1] to [3] which teach neither the choice of metakaolin and the starting solution, nor the parameters essential for get that lump.

After solidification of the solution in the form of hydrated zeolite A, the molded product obtained by this solidification is subjected to drying, then to a heat treatment to transform it into nepheline.

Drying can be carried out after demolding the product at temperatures of 110 to 500 ° C. The heat treatment is then carried out on the dried product at temperatures of 1000 to 1500 ° C, to obtain the transformation into nepheline (between 500 and 850 ° C), then the densification of the products by removing the open porosity. Other characteristics and advantages of the invention will appear better on reading the description which follows, of exemplary embodiments given, of course, by way of nonlimiting illustration, with reference to the appended drawing.

Brief description of the drawing

The figure is a diagram representing the different stages 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 starting soda solution (waste) the desired amount of metakaolin powder, and knead the whole 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 calcination of a powdered kaolin at a temperature of 800 ° C., for 1 hour.

After obtaining a homogeneous mixture, the shaping step is carried out by casting or spinning. For exemple, . the dough can be poured into a sealed mold to obtain a product of the desired shape. The mixture crystallizes and solidifies in the minutes or hours that follow. The setting time depends on the temperature used. To activate the plug very strongly, you can operate at a temperature of 40 to 70 ° C by moderate heating. It would also be possible to promote the growth of the zeolite grains by inoculating the suspension with crystals of zeolite A to obtain a homogeneous germination or with crystals of Nepheline leading to heterogeneous germination.

After solidification, hardened products are obtained which have acquired sufficient mechanical strength to be handled by automated means. They can therefore be removed from the mold before subjecting them to drying and the final heat treatment. -

They can also be stored in the mold, if the latter consists of a completely incinerable and ashless material, for example made of polymer material or cellulose. The molded products are then dried, which is carried out slowly to evacuate 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 on the geometry of the molded products. We can operate at a temperature of 110 to 550 ° C, not exceeding the temperature of 100 ° C at first, to avoid cracking of the molded products, by a too violent departure of water vapor. This drying step corresponds to the following reaction equation:

Na ₉₆ Al ₉₆ Si ₉₆ θ ₃₈₄ .216H ₂ 0 → Na ₉₆ Al ₉₆ Si ₉₆ 0 ₃₈₄ + 216H ₂ 0 (4)

The dried products are then subjected to the final heat treatment, at least 1000 ° C. The purpose of this treatment is: 1) convert the zeolite phase to a nepheline type phase, conversion which typically takes place 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 densified nepheline phase is thus obtained in which the salts or radioactive elements which were liable to be present in the aqueous sodium hydroxide solution are trapped. The products obtained can then be sent to a storage site, possibly after baking.

The following examples of packaging of sodium hydroxide solutions are given by way of illustration and not limitation.

EXAMPLE 1 Packaging of an ION Soda Solution

At 20 ° C, 555 g of metakaolin are added

(PROLABO Kaolin product washed) with 500 ml of ION sodium hydroxide solution, with mechanical stirring. The homogenization is continued under stirring for 15 minutes, after the introduction of the metakaolin, the suspension is then poured into molds of Teflon ^®. The molds filled with the suspension are then introduced into an oven to accelerate setting, operating at a temperature of 40 ° C. After 24 h, the products are removed from the mold, then dried by gradually subjecting them to a maximum temperature of 110 ° C. at slow speed, for 24 h, and then subjecting them to sintering at 1250 ° C, maintaining this temperature for 2 h, with a heating rate of 2 ° C / min.

Monolithic products are thus obtained having satisfactory qualities for waste storage.

Example 2: Packaging of a 5N sodium hydroxide solution

At 20 ° C, 277.5 g of metakaolin are added; identical to that of Example 1, at 500 ml of the 5N sodium hydroxide solution, with mechanical stirring, then homogenization is carried out for 15 minutes, after the end of the introduction of metakaolin. The suspension is then poured into Teflon molds, then the molds are heated to 70 ° C. to accelerate the solidification of the suspension. After 24 h, the molded products are removed from the mold and dried, gradually subjecting them to a maximum temperature of 110 ° C. at low speed, for 24 h. They are then subjected to sintering at 1250 ° C. with a 2 h stage and a heating rate identical to that of Example 1.

Satisfactory products are thus obtained for waste storage.

References cited

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

[2] Ind. Eng. Chem. Res. , 27, 1988, pages 1,291-1,296 [3] US-4,271,130.

Claims

1. A method of packaging a waste consisting of an aqueous solution comprising 3 to 10 mol / 1 of NaOH soda, characterized in that it comprises the following steps: a) adding, to the aqueous solution, a metakaolin powder in quantity such that a suspension capable of solidifying and forming a crystalline phase of the zeolite A type is obtained; b) introducing the suspension into a mold; c) allow the suspension to solidify in the mold to obtain a solid molded 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.

2. The method of claim 1, wherein the amount of metakaolin added is such that the molar ratio of metakaolin to sodium hydroxide is from 0.4 to 0.6.

3. The method of claim 2, wherein the metakaolin / soda molar ratio is about 0.5.

4. Method according to claim 1, in which the metakaolin powder has an average particle size of 1 to 50 μm.

5. Method according to any one of claims 1 to 4, wherein the metakaolin is obtained by calcining kaolin at a temperature of 500 to 1,200 ° C.

6. The method of claim 1, wherein the amount of metakaolin added is such that the water content of the suspension is 30 to 70% by weight.

7. The method of claim 1, wherein the total content of N0 ₃ ^~ and N0 ^~ ions in the starting aqueous solution is 0 to 0.5 mol / 1.

8. The method of claim 1, wherein step c) is carried out at a temperature of 15 to 100 ° C.

9. The method of claim 1, wherein the drying of the molded product is carried out, after demolding of this molded product, at a temperature of 110 to 500 ° C.

10. Method according to any one of claims 1 to 9, wherein the starting aqueous solution is a radioactive solution.