PROCESS FOR PREPARING A SILICOTITANATE MATERIAL WITH A SITINAKITE STRUCTURE USABLE FOR DECONTAMINATION OF AN AQUEOUS EFFLUENT

Abstract

The present invention relates to a process for preparing a sitinakite-structured silicotitanate comprising the following steps: a) a step of forming a gel by mixing silicon alkoxide, titanium dioxide TiO2, sodium hydroxide and water according to the following molar ratios: - the molar ratio of the total number of moles of water in the mixture to the number of moles of titanium dioxide, called the molar ratio (H2O / Ti), ranging from 100 to 180; - the molar ratio of the number of moles of sodium hydroxide to the number of moles of titanium dioxide, called the molar ratio (Na / Ti), ranging from 5 to 10; and - the molar ratio of the number of moles of silicon alkoxide to the number of moles of titanium dioxide, called the molar ratio (Si / Ti), ranging from 0.8 to 1.7; b) a step of hydrothermal treatment of the gel obtained from step a).

Description

Title of the invention: PROCESS FOR PREPARING A SILICOTITANATE MATERIAL WITH A SITINAKITE STRUCTURE USABLE FOR THE DECONTAMINATION OF AN AQUEOUS EFFLUENT Technical field

[0001] The invention relates to a process for preparing a silicotitanate material of sitinakite structure which can be used for the decontamination of an aqueous effluent, in particular a radioactive aqueous effluent, such as a radioactive aqueous effluent comprising cesium and / or strontium.

[0002] Generally speaking, the invention is likely to find applications in all situations where it is necessary to decontaminate an aqueous medium, in particular of certain radioactive elements, such as strontium and / or caesium and, in particular, in the nuclear industry to treat aqueous liquid effluents containing these elements, which are likely to be produced by nuclear installations, either in the context of their normal operation (for example, in the context of an activity of reprocessing irradiated nuclear fuels, of the dismantling / cleaning up of installations or of the maintenance of installations) or accidentally. STATE OF THE PRIOR ART

[0003] The nuclear industry and, more specifically, reprocessing plants for irradiated nuclear fuels, generate radioactive waste, in particular aqueous liquid effluents, known as low and intermediate activity (LA-MA), which contain a certain number of radionuclides resulting from nuclear fission and corrosion of fuel cladding. In addition to the effluents from the reprocessing of irradiated nuclear fuels, there are effluents from washing contaminated equipment as well as effluents linked to the dismantling / cleaning up of installations, the maintenance of installations or possible nuclear incidents. To the extent that they cannot be discharged directly into the environment, all these effluents need to be treated, in particular with a view to being able, if necessary, to be discharged into the environment, provided that their radioactivity levels are below the limits imposed by the country's regulations.

[0004] Cesium 137 and strontium 90 are among the radionuclides, the extraction of which is more specifically targeted because they are abundant in irradiated nuclear fuels (and therefore in aqueous liquid effluents from the reprocessing of these fuels). combustibles) and their half-life of around 30 years makes them particularly radiotoxic. Moreover, this half-life is too long for decay to take place within a reasonable time frame.

[0005] Also, radioactive liquid effluents are required to undergo decontamination processes, involving the implementation of physicochemical treatments in treatment stations, during which the objective is to extract the radioactivity from the liquid effluent and then trap it in the form of solid waste compatible with the conditioning processes.

[0006] Physicochemical treatment processes for decontaminating radioactive liquid effluents are few in number on an industrial scale, due to the complex compositions of the effluents, their high ionic strength and also a large variability in pH. At present, two distinct processes for managing radioactive liquid effluents are implemented:

[0007] - the evaporation of said radioactive liquid effluents; and

[0008] -chemical treatment by entrainment of said radioactive liquid effluents.

[0009] Concerning evaporation, this applies essentially to radioactive liquid effluents with a low concentration of radioisotopes (in particular, low and medium activity effluents) and consists of concentrating all of the ions present in said effluents in a solid waste, which constitutes the evaporation residue. This solid waste is then confined in a specific matrix compatible with the waste stream, for example, cement or glass. However, this process is very energy-intensive and is not applicable to all types of effluents. In particular, when the effluents are saline, it can be harmful to the equipment in which it is implemented, because the presence of certain ions can cause scaling and / or hot corrosion of said equipment.

[0010] Concerning the entrainment, this method consists of transferring the radioisotopes present in the aqueous effluent to be treated into a solid phase by entrainment from solid particles capable of capturing said radioisotopes. This thus results in solid particles rich in radioelements and an aqueous effluent depleted in radioisotopes, which can be rejected, while the solid particles thus enriched are recovered by filtration or decantation, before being confined in a suitable matrix.

[0011] This type of process makes it possible to treat effluents with very variable compositions. What is more, its implementation is simple and its cost is attractive.

[0012] However, the training processes currently implemented generate significant quantities of sludge which are difficult to reconcile with current industrial waste outlets, such as glass or cement matrices.

[0013] Given the disadvantages of current decontamination processes, many nuclear researchers and industrialists are currently looking for alternative routes. for the treatment of these aqueous effluents, such as the search for ion exchange materials with high selectivity for the ions to be extracted.

[0014] These ion exchange materials can be in the form of an organic or inorganic material. Organic materials, such as ion exchange resins, are generally structurally in the form of a polymer, for example, from the family of polyamides, polysulfones, polyethersulfones carrying ion exchange groups, while inorganic materials are generally oxides (such as silica, alumina, zirconia, titanates).

[0015] Even though organic materials are much more widespread and less expensive, inorganic materials are preferred for decontamination applications in aggressive environments (chemical and radioactive), because they have much superior mechanical, chemical, thermal resistance properties and resistance to radiolysis. In addition, inorganic materials are directly compatible with the existing waste outlet, and in some cases can even be considered as containment materials.

[0016] Among a possible candidate from the titanate family and particularly suitable for the decontamination of aqueous media, particularly those containing cesium, mention may be made of silicotitanates of sitinakite structure. Indeed, the sitikanite phase is known for its ion exchange capacities in aqueous media. Sitinakite has a quadratic crystal structure and an ideal chemical formula Na2Ti2O3SiO4.2H2O. In this structure, the titanium atoms are arranged in clusters of 4 groups along the 42 axis, two at the top and two at the bottom rotated 90°. Each titanium forms a coordination octahedron and each vertex of the octahedron is shared in such a way that the center of an entity formed by four Ti atoms and four oxygen atoms forms a distorted cubane-type structure. These cubane-type structures are linked to each other through silicate bridges along crystallographic a and b axes.Thus, a tunnel propagating along the c axis is formed by TiO6 octahedra sharing their vertices. The Na atoms are distributed between two sites, the first site being located in the tunnels in which the Na atoms and the water molecules arrange themselves in a disorderly manner. The second site, perpendicular to these tunnels in cavities corresponds to structural defects, in which the Na atoms organize themselves by binding to four oxygens of the silicate entities and two water molecules located in adjacent tunnels. It is these Na ions that are responsible for the ion exchange.

[0017] Silicotitanates of sitinakite structure have been the subject of various preparation methods in the prior art.

[0018] Thus, US 2017 / 021681 describes a process for preparing a silicotitanate comprising a first step of synthesizing a gel by mixing a source of acid silicic acid (e.g. sodium silicate), a sodium compound (e.g. sodium hydroxide), titanium tetrachloride and water and a hydrothermal treatment step of the gel thus formed, the disadvantage of this process being the use of titanium tetrachloride which, in the presence of air, reacts rapidly to form toxic vapors and TiO2, thus requiring the establishment of a controlled atmosphere.

[0019] US 10343136 describes a process for preparing a silicotitanate also comprising a first step of forming a gel by mixing an inorganic titanium compound (such as titanium sulfate, titanium oxysulfate, titanium meta-titanate or titanium tetrachloride), an inorganic silicon compound (such as sodium silicate, silica sol, fumed silica or precipitated silica), water and an alkali metal hydroxide and a second step of crystallizing the gel.

[0020] In view of what exists, the authors of the present invention proposed to develop a process for preparing a silicotitanate having a si-tinakite structure useful for the treatment of water contaminated by radioelements presenting a danger to living organisms, such as cesium and strontium, the process having to, by a motivated choice of ingredients and operating conditions, allow the said structure to be obtained, which process having to be simple to implement and involving precursors which are simple to handle and having a sufficiently high content of metallic element so as not to have to resort to the use of large volumes and therefore large volume reactors. Statement of the invention

[0021] Thus, the invention relates to a process for preparing a silicotitanate of sitinakite structure comprising the following steps:

[0022] a) a step of forming a gel by mixing a silicon alkoxide, titanium oxide TiO2, sodium hydroxide and water according to the following molar ratios:

[0023] -the molar ratio of the total number of moles of water in the mixture to the number of moles of titanium oxide, called the molar ratio (H2O / Ti), ranging from 100 to 180;

[0024] -the molar ratio of the number of moles of sodium hydroxide to the number of moles of titanium oxide, called the molar ratio (Na / Ti), ranging from 5 to 10; and

[0025] -the molar ratio of the number of moles of silicon alkoxide to the number of moles of titanium oxide, called molar ratio (Si / Ti), ranging from 0.8 to 1.7;

[0026] b) a step of hydrothermal treatment of the gel resulting from step a).

[0027] Thanks to this motivated choice of ingredients (in particular, a silicon alkoxide for the silicon precursor and a titanium oxide TiO2 for the titanium precursor) and specific operating conditions (in particular, in terms of molar ratios), the process makes it possible to obtain a silicotitanate of sitinakite structure which has a significant capacity for sorption of radioelements present in an aqueous medium and, in particular, a significant capacity for sorption of cesium and strontium.

[0028] As mentioned above, the method of the invention comprises, firstly, a step of forming a gel by mixing a silicon alkoxide, titanium oxide TiO2, sodium hydroxide and water according to the specific molar ratios mentioned above.

[0029] More precisely, the silicon alkoxide can correspond to the formula (RO)4Si with the R, identical or different, representing, independently of one another, an alkyl group, linear or branched, which can contain from 1 to 10 carbon atoms or a phenyl group, an example of a particularly advantageous silicon alkoxide being tetraethyl orthosilicate (known by the abbreviation TEOS).

[0030] For the gel formation step, it is specified that the titanium oxide TiO2 is introduced as is in this step and therefore does not result from the transformation of a titanium precursor during mixing. Advantageously, the titanium oxide is in the form of a TiO2 powder.

[0031] It is understood that the total number of moles of water in the mixture corresponds to the number of moles of water added to the mixture to which is added, where appropriate, the number of moles of water included in the other ingredients, such as in sodium hydroxide, when the latter is in the form of an aqueous solution of sodium hydroxide.

[0032] For the gel formation step, the sodium hydroxide may be introduced in the form of an aqueous sodium hydroxide solution, for example, a 6.32 M aqueous sodium hydroxide solution.

[0033] As mentioned above, the molar ratios used for step a) are as follows:

[0034] -a molar ratio (H2O / Ti) ranging from 100 to 180, for example, 173;

[0035] -a molar ratio (Na / Ti) ranging from 5 to 10, for example, ranging from 6.8 to 7.5; and

[0036] -a molar ratio (Si / Ti) ranging from 0.8 to 1.7, for example, ranging from 1 to 1.2.

[0037] According to a particular embodiment, the gel formation step may consist of mixing tetraethyl orthosilicate, TiO2, an aqueous sodium hydroxide solution and water according to the following molar ratios:

[0038] -a molar ratio (H2O / Ti) equal to 146;

[0039] -a molar ratio (Na / Ti) equal to 7.29; and

[0040] -a molar ratio (Si / Ti) equal to 1.

[0041] In accordance with the process of the invention, the gel formed during step a) is subjected to a hydrothermal treatment step.

[0042] It is specified that, by hydrothermal treatment, it is understood, classically, a reaction carried out in a solvent (here, water) above ambient temperature and at a pressure greater than 1 atmosphere in a closed system.

[0043] From a practical point of view, for the implementation of step b), the gel obtained in step a) can be placed in a closed metal tube which is then placed in a reactor (for example, a stainless steel reactor) subjected to heating, for example, a temperature ranging from 160 to 190°C for a period ranging, for example, from 72 hours to 240 hours, for example, a temperature of 170°C for 72 hours.

[0044] The optimum temperature and duration may be set by a person skilled in the art by preliminary tests where pair tests (temperature, duration) will be implemented followed by an analysis by X-ray diffraction of the product obtained. The pairs (temperature, duration) retained may be those for which the si-tinakite phase is obtained.

[0045] At the end of the hydrothermal treatment step, the process of the invention may comprise a step of isolating the product thus obtained (in this case, the silico-titanate in sitinakite form), for example, by Buchner filtration, followed by a possible step of washing the isolated product to remove the reagents which have not reacted. The product thus isolated and optionally subjected to the washing step may finally be subjected to a drying step, for example, in air, in an oven.

[0046] Other characteristics and advantages of the invention will appear more clearly on reading the additional description which follows, which relates to an example of preparation in accordance with the invention and to a comparative example.

[0047] Of course, these examples are given only as an illustration of the subject of the invention and do not in any way constitute a limitation of this subject. Brief description of the drawings

[0048] [Fig. 1] represents a diffractogram obtained for the product produced in the context of example 1.

[0049] [Fig.2] represents a diffractogram obtained for the product produced within the framework of the comparative example.

[0050] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS EXAMPLE 1

[0051] This example illustrates the preparation of a silicotitanate of sitinakite structure in accordance with the process of the invention.

[0052] To do this, 359.5 mg of TiO2 powder and 1.008 mL of tetraethyl orthosilicate (TEOS) and 5.18 mL of a 6.32 M aqueous sodium hydroxide solution are mixed. Immediately after bringing these ingredients into contact, 6.82 mL of milliQ water were added to the mixture previously obtained, the molar ratios being as follows:

[0053] -a molar ratio (H2O / Ti) equal to 146;

[0054] -a molar ratio (Na / Ti) equal to 7.29; and

[0055] -a molar ratio (Si / Ti) equal to 1.

[0056] The resulting gel was stirred vigorously for 60 minutes and then transferred to a Teflon® line such that the resulting gel occupied approximately 50% of the line volume. The line was then sealed in a stainless steel reactor and heated for 3 days at 170°C. After cooling the reactor to room temperature, the product was recovered by Buchner filtration, washed extensively with milliQ water and ethanol to remove excess unreacted reagents and facilitate drying. Drying was done in air in a study preheated to 70°C overnight. The resulting product is analyzed by X-ray diffraction using a Bruker D8 Advance diffractometer and Cu-Ka radiation under conditions of 40 kV tube voltage, 40 mA tube current and 0.02° / s scan rate.

[0057] The diffractogram obtained is shown in Figure 1 attached in the appendix (Intensity I in arbitrary units ua on the ordinate and the angle 2® in ° on the abscissa) and attests to the obtaining of the silicotitanate in the majority form of sitinakite.

[0058] Once analyzed, the product was subjected to a sorption test in aqueous medium comprising strontium.

[0059] To do this, 5 mg of the silicotitanate were brought into contact for 24 hours with 10 mL of a milliQ water solution, in which approximately 100 ppm of strontium had been previously dissolved. After this contact time, the supernatant was filtered using a 0.45 μm cellulose acetate membrane and analyzed by X-ray fluorescence using a SPECTRO XEPOS X-ray fluorescence spectrometer marketed by AMETEK, the X-ray beam being generated from a palladium wire and the energy ranges being selected by reflection on a molybdenum target.

[0060] The product thus tested demonstrates excellent results in terms of strontium sorption with a maximum sorption capacity value Qmax (in mg.g ') of 180. COMPARATIVE EXAMPLE

[0061] This example illustrates the preparation of a silicotitanate of sitinakite structure according to a process not in accordance with the invention.

[0062] To do this, 359.6 mg of TiO2, 324.6 g of SiO2 and 5.68 mL of a 6.32 M aqueous sodium hydroxide solution were mixed. Immediately after bringing these ingredients into contact, 6.32 mL of milliQ water were added to the mixture previously obtained, the molar ratios being as follows:

[0063] -a molar ratio (H2O / Ti) equal to 146;

[0064] -a molar ratio (Na / Ti) equal to 7.98; and

[0065] -a molar ratio (Si / Ti) equal to 1.2.

[0066] The resulting gel was stirred vigorously for 60 minutes and then transferred to a Teflon® line such that the resulting gel occupied approximately 50% of the line volume. The line was then sealed in a stainless steel reactor and heated for 3 days at 170°C. After cooling the reactor to room temperature, the product was recovered by Buchner filtration, washed extensively with milliQ water and ethanol to remove excess unreacted reagents and facilitate drying. Drying was done in air in a study preheated to 70°C overnight.

[0067] The product obtained is analyzed by X-ray diffraction in accordance with the protocol set out in Example 1 above.

[0068] The diffractogram obtained is shown in Figure 2 attached in the appendix (Intensity I in arbitrary units ua on the ordinate and the angle 20 in ° on the abscissa) and attests to the obtaining of silicotitanate in different forms (sitinakite, natisite and nonatitanate).

[0069] Once analyzed, the product was subjected to a sorption test in aqueous medium comprising strontium according to a protocol similar to that defined in example 1 above.

[0070] The product thus tested demonstrates considerably reduced results in terms of strontium sorption compared to the product obtained in Example 1 with a maximum sorption capacity value Qmax (in mg.g ') of 77.

Claims

Claims

1. A process for preparing a silicotitanate of sitinakite structure comprising the following steps: a) a step of forming a gel by mixing a silicon alkoxide, titanium oxide TiO2, sodium hydroxide and water according to the following molar ratios: -the molar ratio of the total number of moles of water in the mixture to the number of moles of titanium oxide, called the molar ratio (H2O / Ti), ranging from 100 to 180; -the molar ratio of the number of moles of sodium hydroxide to the number of moles of titanium oxide, called the molar ratio (Na / Ti), ranging from 5 to 10; and -the molar ratio of the number of moles of silicon alkoxide to the number of moles of titanium oxide, called the molar ratio (Si / Ti), ranging from 0.8 to 1.7; b) a step of hydrothermal treatment of the gel resulting from step a).

2. A process for preparing a silicotitanate according to claim 1, in which the silicon alkoxide has the formula (RO)4Si with the Rs, identical or different, representing, independently of one another, a linear or branched alkyl group, or a phenyl group.

3. A process for preparing a silicotitanate according to claim 1 or 2, wherein the silicon alkoxide is tetraethyl orthosilicate.

4. A preparation process according to any one of the preceding claims, wherein the sodium hydroxide is in the form of an aqueous sodium hydroxide solution.

5. A preparation process according to any preceding claim, wherein the molar ratio (Na / Ti) is from 6.8 to 7.

5.

6. A preparation method according to any one of the preceding claims, wherein the molar ratio (Si / Ti) is from 1 to 1.

2.

7. A preparation method according to any one of the preceding claims, wherein the gel-forming step consists of mixing tetraethyl orthosilicate, TiO2, an aqueous sodium hydroxide solution and water according to the following molar ratios: - a molar ratio (H2O / Ti) equal to 146; - a molar ratio (Na / Ti) equal to 7.29; and - a molar ratio (Si / Ti) equal to 1.

8. A preparation process according to any one of the preceding claims- preceding, comprising, after the hydrothermal treatment step, a step of isolation of the product thus obtained.

9. Preparation process according to claim 8, comprising, after the isolation step, a step of washing the isolated product.

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