PROCESS FOR PREPARING A GEOPOLYMER-ZEOLITE COMPOSITE MATERIAL, MATERIAL THUS PREPARED AND ITS USES

Abstract

The present invention relates to a method for preparing a geopolymer-zeolite composite material comprising the following steps: (a) preparing a geopolymer foam from a geopolymeric slurry in which the Si / Al molar ratio is greater than or equal to 1.2 and less than 1.4 and the H2O / M2O molar ratio with M representing the compensation cation is greater than or equal to 8 and less than or equal to 10; (b) optionally allowing the geopolymer foam prepared in step (a) to harden; then (c) subjecting the geopolymer foam prepared in step a) optionally hardened during step (b) to a heat treatment at a temperature less than or equal to 80°C for a time greater than or equal to 3 hours, whereby a geopolymer-zeolite foam composite material of LTA structure and / or FAU structure is obtained. The present invention also relates to the composite material thus obtained and its uses.

Description

Title of the invention: PROCESS FOR PREPARING A GEOPOLYMER-ZEOLITE COMPOSITE MATERIAL, MATERIAL THUS PREPARED AND ITS USES Technical field

[0001] The present invention relates to the field of composite materials and in particular composite materials based on geopolymers and zeolites.

[0002] Thus, the present invention provides a method for preparing a composite material with a macroporous geopolymer matrix, the surface of which is covered with LTA type zeolites and / or FAU type zeolites, a material thus prepared and in particular a material in which the zeolites are LTA type zeolites optionally associated with FAU type zeolites and its uses in particular for the decontamination of liquid effluents of medium or low activity, the trapping of gases, the decontamination of toxic elements or even for extracting, concentrating and enabling the recovery of elements of interest. STATE OF THE PRIOR ART

[0003] Studies on geopolymer-zeolite composite materials are increasingly numerous, proof of the growing interest in such materials.

[0004] Indeed, these hybrid materials combine the advantageous properties of their two constituents. On the one hand, geopolymers exhibit high mechanical strength and good chemical stability, providing a solid and durable support for zeolites. Intrinsically, geopolymers are also good candidates as ion exchangers. On the other hand, zeolites display a high surface area, porosity and adsorption capacity. They are currently widely used as an absorbent for ions such as copper, zinc, cesium and strontium. Furthermore, depending on the family or structural type of zeolites, a selection of the ion to be exchanged can be chosen.

[0005] Thanks to these combined properties, such hybrid materials find numerous applications as absorbents, particularly useful for the decontamination of liquid effluents, the trapping of gases, the decontamination of toxic elements and / or the extraction, concentration and recovery of elements of interest.

[0006] As explained in the review by Rozek et al, 2019 [1], there are three methods for synthesizing geopolymer-zeolite composites: the first with the introduction of zeolites as a raw material during the synthesis of the geopolymer, the second by in-situ formation of zeolites in a geopolymer matrix and the last by hydrothermal synthesis. Hydrothermal synthesis is a method of synthesizing zeolites in which the raw materials composed of various elements such as the building blocks of the skeleton, i.e. silicon, aluminium, phosphorus, etc. are used for synthesis at a temperature typically above 90°C, or even above 100°C, in the presence of mineralisers such as OH or F anions in an aqueous medium.

[0007] From the information contained in the review [1] regarding the formation of zeolites in-situ in a geopolymer matrix, it appears that depending on the type of precursors, the activation solution, the Si / Al and Na / Al ratios or the firing conditions, it is possible to obtain different zeolite structures.

[0008] In particular, Wan et al, 2017 [2] cited in the journal [1] presents the synthesis of metakaolin-based geopolymers at different Si / Al ratios chosen from 1:1; 1.5:1; 3:1; 4:1 and 5:1. The Na2O / Al2O3 and Na2O / H2O ratios were set at 1:1 and 1:12 respectively. In the different syntheses, the bulk-type geopolymer was cast in a mold and then placed in a 60°C oven for 6 hours. The main result of this article is the formation of LTA zeolite in-situ for a Si / Al ratio of 1:1.

[0009] Finally, Candamano et al, 2022 [3] proposes the preparation of a geopolymer-FAU zeolite composite material. Two methods are presented in this article, the first by heat treatment at 90°C for 24h and the second by heat treatment at 50°C for 1h followed by a hydrothermal treatment at 90°C for 24h. For the second method, the authors used a foaming agent (H2O2) and ultimately obtained a composite material comprising a geopolymer foam and FAU zeolite. Furthermore, a geopolymer foam has inferior mechanical properties to a solid geopolymer, making the structure brittle and difficult to use to allow the continuous passage of a liquid effluent to be decontaminated.

[0010] Given the interest in geopolymer-zeolite composite materials, the inventors set themselves the goal of proposing an effective, simple and easily industrializable process for preparing such a composite material which does not have the disadvantages of the materials of the prior art and is particularly suitable for the selective trapping of radioactive elements such as cesium and strontium. Statement of the invention

[0011] The present invention makes it possible to achieve the goal set by the inventors. since the latter propose a process making it possible to obtain, in a single heat treatment step and without hydrothermal treatment, a composite material comprising a geopolymer foam functionalized by zeolites formed in situ.

[0012] In particular, the method according to the invention makes it possible to obtain geopolymer-zeolite composite materials of LTA structure and / or zeolites of FAU structure. particularly suitable for the selective trapping of radioactive elements such as cesium and strontium.

[0013] Unexpectedly, the inventors have shown that certain combinations of the ratios conventionally used in the field of geopolymers, namely the Si / Al molar ratio and the H2O / M2O molar ratio with M representing the compensation cation, make it possible to obtain a geopolymer foam with good mechanical properties preventing the composite material from being brittle. In other words, the process which is the subject of the invention makes it possible to obtain a composite material with high open porosity while maintaining good workability and sufficient mechanical strength.

[0014] Finally, since the process which is the subject of the invention is easy to implement without requiring excessive temperature or pressure conditions, it is easily industrializable and it is possible to prepare the composite material directly in a filtration column.

[0015] More particularly, the present invention relates to a process for preparing a geopolymer-zeolite composite material comprising the following steps: (a) preparing a geopolymer foam from a geopolymeric slurry in which the Si / Al molar ratio is greater than or equal to 1.2 and less than 1.4 and the H2O / M2O molar ratio with M representing the compensation cation is greater than or equal to 8 and less than or equal to 10; (b) optionally allowing the geopolymer foam prepared in step (a) to harden; then (c) subjecting the geopolymer foam prepared in step (a) optionally hardened during step (b) to a heat treatment at a temperature less than or equal to 80°C for a duration greater than or equal to 3 hours, whereby a geopolymer-zeolite foam composite material of LTA structure and / or FAU structure is obtained.

[0016] In a particular embodiment, the method for preparing a geopolymer-zeolite composite material comprises the following steps: (aO preparing a geopolymer foam from a geopolymeric grout in which the Si / Al molar ratio is greater than or equal to 1.25 and less than or equal to 1.35 and the H2O / M2O molar ratio with M representing the compensation cation is greater than or equal to 8 and less than or equal to 10; (bi) optionally allowing the geopolymer foam prepared in step (ai) to harden; then (ci) subjecting the geopolymer foam prepared in step (ai) optionally hardened during step (bi) to a heat treatment at a temperature less than or equal to 80°C for a duration greater than or equal to 3 hours, whereby a material geopolymer-zeolite foam composite of FAU structure and possibly LT A structure is obtained.

[0017] In other words, in this particular embodiment, the Si / Al molar ratio in the geopolymeric grout is greater than or equal to 1.25 and less than or equal to 1.35, whereby a geopolymer-zeolite foam composite material of FAU structure and optionally of LTA structure is obtained.

[0018] By “geopolymer-zeolite composite material” is meant, in the context of the present invention, a material comprising a geopolymer matrix or skeleton whose surface is covered with zeolites.

[0019] By "geopolymer matrix" or "geopolymer" is meant, in the context of the present invention, a solid, inorganic and porous material in the dry state, obtained following the hardening of a mixture containing finely ground materials (i.e. the aluminosilicate source) and a saline solution (i.e. the activation solution), said mixture being capable of setting and hardening over time. This mixture can be designated under the terms "geopolymeric grout", "geopolymeric mixture", "geopolymer mixture", "geopolymeric composition" or even "geopolymer composition". The hardening of the geopolymer is the result of the dissolution / polycondensation of the finely ground materials of the geopolymeric mixture in the saline solution such as a saline solution of high pH (i.e. the activation solution).

[0020] More particularly, a geopolymer or geopolymer matrix is ​​an amorphous alumino-silicate inorganic polymer. Said polymer is obtained from a reactive material containing essentially silica and aluminum (i.e. the alumino-silicate source), activated by a strongly alkaline solution, the solid / solution mass ratio in the formulation being low. The structure of a geopolymer is composed of a Si-O-Al network formed of silicate (SiO4) and aluminate (A1O4) tetrahedra linked at their vertices by sharing oxygen atoms. The substitution of a silicon atom (valence +IV) by an aluminum atom (valence +III) results in a charge deficit compensated by one or more charge compensating cation(s), also called compensation cation(s), which makes it possible to compensate for the negative charge of the A1O4 complex.The compensating cation(s) is (are) advantageously chosen from the group consisting of alkali metals such as lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs), alkaline earth metals such as magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba) and mixtures thereof. In particular, the compensating cation(s) is (are) advantageously chosen from the group consisting of sodium (Na), potassium (K) and mixtures thereof.

[0021] In the geopolymer-zeolite composite material prepared by the process according to the invention, the geopolymer is in the form of geopolymer foam.

[0022] By "geopolymer foam" is meant a geopolymer as previously defined, macroporous, mesoporous and possibly microporous, typically having a density of less than 0.9 g / cm3.

[0023] By "mesoporous and macroporous geopolymer" is meant a geopolymer having both macropores and mesopores. By "macropores" is meant pores or voids having an average diameter greater than 50 nm and in particular greater than 70 nm. By "mesopores" is meant pores or voids having an average diameter of between 2 and 50 nm and in particular between 2 and 20 nm.

[0024] Depending on the formulation used, a geopolymer foam may also have micropores. By "micropores" is meant pores or voids having an average diameter of less than 2 nm. When micropores are present, the microporosity is typically less than 15% and in particular less than 10% and, in particular, between 5 and 10% by volume relative to the total porosity of the geopolymer.

[0025] In a geopolymer foam, the total porosity corresponding to the macroporosity, the mesoporosity and the possible microporosity is greater than 70%, in particular greater than 75%, and, in particular, greater than 80% by volume relative to the total volume of the geopolymer.

[0026] The geopolymer foam used in the context of the present invention has open porosity, penetrating open porosity, connected (or interconnected) porosity and closed porosity. Advantageously, the geopolymer foam used in the context of the present invention has percolating pores which connect a first main surface of the geopolymer foam to a second main surface of the geopolymer foam. For the purposes of the present invention, by "main surface" is meant an external part of the geopolymer foam, which limits it with respect to its environment. The main surface(s) typically have(s) cavities, in particular macroscopic, which are not obstructed. Thus, a geopolymer foam can also be defined as a mesoporous and macroporous geopolymer whose macropores are connected.

[0027] In the geopolymer-zeolite composite material prepared by the process according to the invention, zeolites are present on the surface and in particular on the surface of the pores and voids of the geopolymer foam.

[0028] Zeolites are crystalline microporous minerals belonging to the family of hydrated aluminosilicates. Made up of alumina and silica tetrahedra, linked by oxygen atoms, zeolites and in particular sodalite are composed of small cages which form a larger cage by connection.

[0029] More particularly, the geopolymer-zeolite composite prepared by the process according to the invention may comprise (i) exclusively zeolites of FAU structure, (ii) exclusively zeolites with LTA structure or (ii) both zeolites with FAU structure and zeolites with LTA structure.

[0030] FAU structure zeolites also known as "faujasite structure zeolites" include natural faujasite as well as X zeolites and Y zeolites. They have a three-dimensional pore structure, composed of sodalite cages, which are connected in six-membered double rings. Such zeolites can be used in the adsorption of heavy metals, the separation of small molecules from gas or liquid, and as a catalyst in fluid catalysis cracking [1],

[0031] A zeolite of LTA structure also known as "Linde Type A structure zeolite" and "zeolite A" has an α-cage composed of 8 cu-boctahedra connected by 12 cuboids. Due to its microstructure, it has excellent ion exchange capacities, satisfactory hydrophilic properties and is environmentally neutral [1].

[0032] A person skilled in the art knows different techniques for preparing a geopolymer foam and which can be used in the context of step (a) or (ai) of the process of the invention. Thus, geopolymer foams can be synthesized indirectly, namely by using the "template" method or even directly by incorporating gas into a geopolymeric slurry. As for the so-called "template" method, it involves manufacturing an emulsion of the oil type in an alkaline activation solution and geopolymerizing the mineral phase around the drops.

[0033] In the case of direct synthesis, there are several variants. First, the macroscopic porosity of the geopolymer can be created by the water vapor generated during the curing of the geopolymer at 150°C. Alternatively, surfactants can be introduced into the geopolymeric slurry, still liquid, which during the mechanical stirring step entrains air, thus creating macroscopic porosity. Another way to manufacture a geopolymer foam is to use chemicals such as metallic aluminum, silica fume or even hydrogen peroxide. Indeed, the addition of metal to the geopolymer mixture generates hydrogen by reaction with alkalis while the addition of peroxide generates oxygen. In this variant, the gas-generating adjuvants can be combined with air-entraining adjuvants as described in international application WO 2016 / 173950 A1 [4].Finally, direct or reverse 3D printing can also be used to prepare geopolymer foam.

[0034] In a particular embodiment, the geopolymer foam is prepared, during step (a) or (ai) of the process according to the invention, using a gas-generating adjuvant and an air-entraining adjuvant.

[0035] In this particular embodiment, the preparation of the geopolymer foam during step (a) or (aj) of the process according to the invention comprises the following steps: (i) preparing an activation solution comprising at least one air-entraining adjuvant, (ii) adding, to the activation solution prepared in step (i), an alumino-silicate source and mixing the whole, whereby a geopolymer paste is obtained, (iii) adding, to the geopolymer paste prepared in step (ii), at least one gas-generating adjuvant and then mixing the whole whereby a geopolymer grout is obtained then a geopolymer foam once the volume expansion is complete.

[0036] Step (i) of the method according to the present invention consists of adding the air-entraining adjuvant to a previously prepared activation solution. The prior preparation of the activation solution is a conventional step in the field of geopolymers.

[0037] The expressions "activation solution", "high pH saline solution" and "strongly alkaline solution" are, in the present invention, similar and can be used interchangeably.

[0038] By "strongly alkaline" or "high pH" is meant a solution whose pH is greater than 9, in particular greater than 10, in particular greater than 11 and, more particularly, greater than 12. In other words, the activation solution has an OH concentration greater than 0.01 M, in particular greater than 0.1 M, in particular greater than 1 M and, more particularly, between 5 and 20 M.

[0039] The activation solution used in the context of the invention is a strongly alkaline aqueous solution which may optionally contain silicate components chosen in particular from the group consisting of silica, colloidal silica and vitreous silica. When the activation solution contains one or more silicate component(s), the latter is / are present in an amount of between 100 mM and 10 M, in particular between 500 mM and 8 M and, in particular, between 1 and 6 M in the activation solution.

[0040] The activation solution used in the context of the invention is an aqueous solution, which means that the solvent it contains is water. By "water", we mean, in the context of the invention, tap water, deionized water (or demineralized water), distilled water, ultrapure water (18.2 MQ) or one of their mixtures.

[0041] In addition, the activation solution used in the context of the invention comprises the compensation cation M or the mixture of compensation cations in the form of an ionic solution or a salt. Thus, the activation solution is in particular chosen from an aqueous solution of sodium silicate (Na2SiO3), potassium silicate (K2SiO2), sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)2), cesium hydroxide (CsOH) and their derivatives etc. In particular, the activation solution used in the invention is a strongly alkaline aqueous solution of sodium silicate (Na2SiO3). More particularly, the activation solution used in the invention is an aqueous solution prepared from BETOL®39T marketed by the company Woellner and made alkaline by adding NaOH.

[0042] The quantity of compensation cation(s) M is such that the molar ratio H2O / M2O with M representing the compensation cation is greater than or equal to 8 and less than or equal to 10 in the activation solution and therefore in the geopolymeric grout obtained in step (c) or (ci) of the process of the invention.

[0043] Similarly, the quantity of compensating cation(s) M is such that the molar ratio A12O3 / M2O with M representing the compensating cation is between 0.8 and 1.2 and in particular of the order of 1 (i.e. 1 ± 0.1).

[0044] By "air-entraining adjuvant" is meant an adjuvant capable of forming and / or stabilizing, in the geopolymer, microbubbles of air or gas uniformly distributed throughout the mass. In the context of the present invention, the microbubbles formed are essentially microbubbles of the gas produced by the gas-generating adjuvant, due to the use of such an adjuvant in addition to a gas-entraining adjuvant. In addition to these microbubbles, there are also the microbubbles of air trapped during the mixing of the geopolymer paste also comprising the gas-generating adjuvant.

[0045] Those skilled in the art know various air-entraining adjuvants which can be used in particular in the field of concrete. Advantageously, the air-entraining adjuvant used in the context of the present invention belongs to the field of surfactants which lower the surface tension of water and facilitate the formation of gas bubbles by reducing the energy required to create gas-water contact surfaces. Such surfactants also allow the formation of an insoluble and hydrophobic film around the gas voids.

[0046] Advantageously, the air-entraining adjuvant is chosen from anionic surfactants, cationic surfactants, non-ionic (or neutral) surfactants and a mixture thereof. Those skilled in the art will find additional information, particularly in terms of definitions and specific examples of these air-entraining adjuvants, in [4]. In a particular embodiment, the air-entraining adjuvant used in the invention is the adjuvant marketed under the brand name Sika®AER-5.

[0047] The air-entraining adjuvant is used in a content of between 1% and 5% by mass, in particular between 1.5% and 3% by mass and, in particular, of the order of 2% by volume (i.e. 2% ± 0.25%) relative to the mass of the geopolymeric grout.

[0048] The air-entraining adjuvant is added to the activation solution all at once or in several times and even drop by drop for an air-entraining adjuvant in liquid form or by sprinkling for an air-entraining adjuvant in solid form. Once the air-entraining adjuvant has been added to the activation solution, the resulting solution is mixed using a mixer, a stirrer, a magnetic bar, an ultrasonic bath or a homogenizer. The mixing / kneading during step (a) of the method according to the invention is carried out at a relatively high speed. By "relatively high speed", is meant, in the context of the present invention, a speed greater than 250 rpm, in particular greater than or equal to 350 rpm. Such stirring makes it possible to obtain a uniform solution, in particular a homogeneous solution or a solution of the micro-emulsion type.

[0049] Step (i) of the process according to the invention is carried out at a temperature between 10°C and 40°C, advantageously between 15°C and 30°C and, more particularly, at room temperature (i.e. 23°C ± 5°C) for a duration between 1 s and 40 s, in particular between 10 s and 30 s and, more particularly, between 15 s and 25 s.

[0050] Step (ii) of the process according to the invention therefore comprises the preparation of a geopolymer paste obtained following the addition of the aluminosilicate source to the activation solution containing at least one air-entraining adjuvant.

[0051] The alumino-silicate source is a reactive material containing essentially silica and aluminum that can be used to prepare the geopolymer foam used in the context of the invention. It is a solid source containing amorphous alumino-silicates. These amorphous alumino-silicates are in particular chosen from natural alumino-silicate minerals such as illite, stilbite, kaolinite, pyrophyllite, andalusite, bentonite, kyanite, milanite, grovenite, amesite, cordierite, feldspar, allophane, etc.; calcined natural aluminosilicate minerals such as metakaolin; synthetic glasses based on pure aluminosilicates; aluminous cement; pumice; calcined by-products or industrial mining residues such as fly ash and blast furnace slag respectively obtained from the combustion of coal and during the transformation of iron ore into cast iron in a blast furnace; and mixtures thereof. In a particular embodiment, the aluminosilicate source used is metakaolin.

[0052] Taking into account any silicate components that the activation solution may contain, the quantity of aluminosilicate source(s) is such that the Si / Al molar ratio in the geopolymeric grout obtained in step c) or Ci) of the process of the invention and therefore in the geopolymer foam is greater than or equal to 1.2 and less than 1.4 and, in certain forms of implementation, is greater than or equal to 1.25 and less than or equal to 1.35.

[0053] The aluminosilicate source can be poured in one or more times onto the activation solution containing at least one air-entraining adjuvant. In one form of special implementation, the aluminosilicate source can be sprinkled on the activation solution containing at least air-entraining adjuvant.

[0054] Advantageously, step (ii) of the process according to the invention is carried out in a mixer into which the activation solution containing at least one air-entraining adjuvant has been previously introduced. Any mixer known to those skilled in the art can be used in the context of the present invention. By way of non-limiting examples, mention may be made of a NAUTA® mixer, a HOBART® mixer, a HENSCHEL® mixer and a HEIDOLPH® mixer.

[0055] Step (ii) of the method according to the invention comprises mixing or kneading the activation solution, the air-entraining adjuvant with the aluminosilicate source. This mixing or kneading is initially carried out at a relatively slow speed. By "relatively slow speed" is meant, in the context of the present invention, a rotation speed of the mixer rotor less than or equal to 700 rpm / min, in particular greater than or equal to 100 rpm / min and, in particular, between 400 and 600 rpm / min. By way of non-limiting example, in the case of a standardized mixer, the stirring speed is 500 rpm / min. Once the mixing has been carried out and to obtain optimal homogenization, the stirring speed can be increased to a sustained speed, i.e. a speed greater than 1000 rpm / min, in particular greater than 1500 rpm / min and, in particular, of the order of 2000 rpm / min (i.e. 2000 rpm / min ± 200 rpm / min).

[0056] Step (ii) of the process according to the invention is carried out at a temperature between 10°C and 40°C, advantageously between 15°C and 30°C and, more particularly, at room temperature (i.e. 23°C ± 5°C). Step (ii) of the process according to the invention is carried out for a sufficient time to obtain a homogeneous geopolymer paste.

[0057] By "gas-generating adjuvant" is meant an adjuvant which, in situ i.e. in the geopolymeric mixture, is capable of reacting with at least one element present in the latter and / or of decomposing so as to produce, by endothermic or exothermic chemical reaction, a gas. Such a gas is in particular chosen from oxygen, hydrogen, nitrogen, carbon monoxide, carbon dioxide, ammonia, methane or one of their mixtures.

[0058] Those skilled in the art know various gas-generating adjuvants which can be used in particular in the field of concrete. Advantageously, the gas-generating adjuvant used in the context of the present invention is chosen from the group consisting of azo compounds, such as azodicarbonamides and their derivatives; hydrazine derivatives, such as p-toluenesulfonylhydrazide, 4,4-oxibis(benzenesulfonylhydrazide) and toluenesulfonylacetonehydrazone; semi-carbazides, such as p-toluenesulfonylsemicarbazide; tetrazoles, such as 5-phenyltetrazole, nitroso compounds such as N,N-dinitrosopentamethylenetetramine; organic peroxides; inorganic peroxides; carbonate compounds or their derivatives such as carbonates or bicarbonates of alkali or alkaline earth metals, in particular calcium carbonate and sodium bicarbonate, optionally used in a mixture with at least one activating agent, such as citric acid and a mixture thereof.

[0059] Those skilled in the art will find additional information, particularly in terms of definitions and specific examples of these gas-generating adjuvants, in [4]. In a particular embodiment, the gas-generating adjuvant used in the invention is hydrogen peroxide (H2O2).

[0060] The gas-generating adjuvant is used in a content of between 0.5% and 5% by volume, in particular between 0.2% and 3% by volume and, in particular, of the order of 1% by volume (i.e. 1% ± 0.1%) relative to the volume of the geopolymeric grout.

[0061] Following the addition of the gas-generating adjuvant to the geopolymer paste, the whole is mixed or kneaded at a relatively slow speed as previously defined. In fact, a gentle mixture is favored because the bubbles will form very quickly and we do not want them to break or escape.

[0062] Step (iii) of the process according to the invention is carried out at a temperature between 10°C and 40°C, advantageously between 15°C and 30°C and, more particularly, at room temperature (i.e. 23°C ± 5°C). Step (iii) of the process according to the invention is carried out for a sufficient time to obtain a homogeneous geopolymeric grout.

[0063] In a first embodiment, the geopolymer foam is used to prepare the zeolites in situ at the end of step (iii). In this first embodiment, there is therefore no step (b) or (bj) in the process according to the invention. The geopolymer foam is in the fresh state.

[0064] In a second embodiment, the geopolymer foam is used to prepare the zeolites in situ at the end of step (iii) and after having allowed the geopolymer foam to harden, step (b) or (bj) is therefore not optional. The conditions allowing hardening during step (b) or (bi) include the conditions typically implemented during a curing step of a geopolymer. This hardening can be carried out in the open air, under water, in various airtight molds, by humidifying the atmosphere surrounding the geopolymer foam or by applying an impermeable coating to this foam. This hardening can be carried out at a temperature between 10°C and 50°C, in particular between 20°C and 40°C and, in particular, at room temperature. It can last between 12 h and 2 days and in particular of the order of 24 h (i.e. 24 h ± 2 h).

[0065] Furthermore, following step (a) or (ai) of the method according to the invention and prior to the possible step (b) or (bi), the geopolymer foam can be placed in molds so as to give it a predetermined shape, in particular following this optional hardening. Typically, the composite material prepared by the method according to the invention is in the form of a monolith. In a particular embodiment, it can be placed in a filtration column.

[0066] Step (c) or (ci) of the process according to the invention consists in generating in situ the zeolites of the composite material. The zeolites are therefore obtained by conversion / reorganization of the chemical structure on the surface of the geopolymer foam, whereby zeolites line the surface of the geopolymer foam and in particular the surface of the pores that it comprises.

[0067] Step (c) or (ci) of the method according to the invention is a gentle heat treatment that the geopolymer foam undergoes, possibly following hardening. During this gentle heat treatment, the temperature used is less than or equal to 80°C, in particular between 50°C and 80°C and, in particular, of the order of 70°C (i.e. 70°C ± 5°C). Step (c) of the method according to the invention has a duration greater than or equal to 3 h, in particular between 3 h and 12 h and, in particular, of the order of 6 h (i.e. 6 h ± 1 h).

[0068] The heat treatment of step (c) or (ci) of the process according to the invention is clearly distinguished from a hydrothermal treatment as implemented in the processes of the state of the art. On the one hand, the temperature implemented during the heat treatment of step (c) or (ci) of the process according to the invention is lower than the temperature of a hydrothermal treatment typically greater than 90°C, or even greater than 100°C. On the other hand, during the heat treatment of step (c) or (ci) of the process according to the invention, the geopolymer foam is not placed in an aqueous solution and a fortiori in an aqueous solution comprising mineralizers such as OH or F anions.

[0069] The present invention also relates to a geopolymer-zeolite composite material prepared by a method as previously defined. More particularly, the composite material according to the present invention is in the form of a geopolymer foam whose surface is covered with zeolites of FAU structure or with both zeolites of FAU structure and zeolites of LTA structure. The composite material according to the invention is therefore not in the form of a geopolymer foam whose surface is covered solely with zeolites of LTA structure.

[0070] The present invention finally relates to the use of such a composite material as an absorbent for decontaminating liquid effluents of medium or low activity, for trapping gases such as carbon dioxide (CO2) and nitrogen oxides NOX with x representing 1 or 2, for the decontamination of toxic elements and / or for extracting, concentrating and allowing the recovery of elements of interest. In one embodiment In a particular embodiment, the composite material is used to extract cesium and / or strontium from a solution containing it.

[0071] In other words, the present invention relates to a method for separating cesium and / or strontium from a solution containing cesium and / or strontium, consisting of bringing a composite material as previously into contact with a solution containing cesium and / or strontium, whereby a solution depleted in cesium and / or strontium and a geopolymer foam having zeolites of FAU structure and optionally zeolites of LTA structure trapping cesium and / or strontium are obtained.

[0072] Alternatively, the present invention relates to a method for separating cesium and / or strontium from a solution containing cesium and / or strontium, consisting of preparing a composite material according to the method as previously defined and then bringing the composite material thus prepared into contact with a solution containing cesium and / or strontium, whereby a solution depleted in cesium and / or strontium and a geopolymer foam having zeolites of FAU structure and / or zeolites of LTA structure trapping cesium and / or strontium are obtained.

[0073] In other words, the extraction method according to the invention can be considered as a method for treating a solution containing cesium and / or strontium. By "treatment of a solution containing cesium and / or strontium" is meant decreasing or reducing the quantity of cesium and / or strontium present in the solution before implementing the method according to the invention, i.e. before contacting with the composite material according to the invention or the composite material prepared according to the method of the invention. This decrease or reduction may involve the partial or total elimination of these ions in the solution.

[0074] Indeed, the composite material according to the invention or the composite material prepared according to the method of the invention, due to its excellent properties such as excellent exchange capacity, excellent selectivity and high reaction speed, is particularly suitable for such uses.

[0075] In addition, the excellent mechanical strength and stability properties of the composite material according to the invention or of the composite material prepared according to the process of the invention, resulting from its specific structure, allow its conditioning in a column and the continuous implementation of the separation / fixing process, for example in a fluidized bed, which can thus be easily integrated into an existing installation, for example in a chain or treatment line comprising several stages.

[0076] At the end of the extraction process according to the invention, the cesium and / or strontium found in the solution to be treated are immobilized in the composite material according to the invention or the composite material prepared according to the process of the invention by sorption, that is to say by ionic exchange or adsorption within the zeolites, themselves physically linked to the surface of the foam.

[0077] The solution to be treated containing cesium and / or strontium is typically a liquid or effluent from the nuclear industry, nuclear installations and activities using radionuclides. In particular, mention may be made, for example, of cooling water from power plants, and all various effluents coming into contact with radioisotopes such as all wash water or resin regeneration solutions.

[0078] As the composite material according to the invention or the composite material prepared according to the method of the invention can be in the form of monoliths, the latter can be used as a packaging material trapping the cesium and / or strontium contained in this packaging and capable of flowing out of the latter.

[0079] Other characteristics and advantages of the present invention will become apparent to those skilled in the art upon reading the examples below given for illustrative and non-limiting purposes, with reference to the appended figures. Brief description of the drawings

[0080] [Fig.l] shows scanning electron microscopy (SEM) images, at different magnifications, of a zeolite-lined geopolymer foam obtained via protocol 1 defined below and with Si / Al = 1.2 and H = H2O / Na2O = 8.

[0081] [Fig.2] shows the X-ray diffraction (XRD) measurements associated with the SEM images of [Fig.l].

[0082] [Fig.3] shows the influence of the Si / Al ratio on the structure of the zeolites lining a geopolymer foam obtained via protocol 2 defined below and with H = H2 O / Na2O = 8. The LTA type zeolite is represented in the form of squares, the SiO2 in the form of diamonds and the FAU type zeolite in the form of triangles.

[0083] [Fig.4] shows the influence of the Si / Al ratio on the structure of the zeolites lining a geopolymer foam obtained via protocol 2 defined below and with H = H2 O / Na2O = 10. The LTA type zeolite is represented in the form of squares, the SiO2 in the form of diamonds and the FAU type zeolite in the form of triangles.

[0084] [Fig.5] shows the influence of the water content on the foam monoliths with, for the photograph on the left, a monolith obtained via protocol 2 defined below and with Si / Al = 1.25, R = 1% and H = 8 and for the photograph on the right, a monolith obtained via protocol 2 defined below and with Si / Al = 1.25, R = 1% and H = 10.

[0085] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0086] Example 1: Methods according to the invention implementing different protocols and different formulations

[0087] To synthesize geopolymer foams by direct foaming, the formulation is based on molar ratios commonly used in the literature. The ratio A12O3 / M2O with M representing the compensation cation is equal to 1 in order to compensate for the charge deficit of the aluminum.

[0088] To achieve direct foaming, hydrogen peroxide (a blowing agent gas generator) was chosen. To obtain homogeneous bubbles, the use of a blowing agent gas generator combined with a surfactant gas entrainer makes it possible to obtain a more homogeneous pore size distribution.

[0089] Therefore, for the present invention, the use of Sika®AER5 was chosen. To synthesize the activation solutions, BETOL®39T (Woellner), sodium hydroxide pellets and milli-Q water were used.

[0090] The water content directly impacts the geopolymerization mechanism, particularly during the dissolution-hydrolysis and polycondensation steps. Water is consumed during the dissolution-hydrolysis of metakaolin and is regenerated during polycondensation, allowing the formation of the porous network. Since the hydroxide dissolution reaction is exothermic, the solution is stirred by magnetic stirrer until the solution is at room temperature. After magnetic stirring of the activation solutions, the metakaolin (ARGICAL-M 1000 Imérys), the surfactant (Sika®AER5) and the pore-forming agent (H2O2) are added to the activation solution.

[0091] To designate the quantity of H2O2 added, the ratio R with R = Volume H2O2 / Volume of dough is often used in the literature, this ratio has been fixed in the invention at R = 1%. Quantitative data are given at the end of this paragraph.

[0092] According to the literature, to crystallize zeolite, it is necessary to have a final Si / Al molar ratio < 1.5. With the raw materials used, it is possible to reduce the Si / Al ratio to a minimum of 1.2 by reducing the amount of betol. To promote the crystallization of zeolites, it is necessary to carry out a heat treatment. It was decided to carry out a heat treatment in an oven at 70°C for 6 h. Before carrying out this heat treatment, two protocols are proposed:

[0093] - Protocol 1: Place the geopolymer in the oven directly after the end of the swelling,

[0094] - Protocol 2: Wait 24 hours after swelling then place the geopolymer in the oven.

[0095] In the case of protocol 1, by having a H2O / Na2O ratio = H = 8 and a Si / Al ratio = 1.2, a geopolymer foam with correct mechanical strength was obtained with uniform bubble sizes. This foam was then observed under a scanning electron microscope in order to characterize the surface condition.

[0096] It can be observed that the surface of the geopolymer is covered with grains of the order of a micrometer (between 1 and 3 pm). This is LT A type zeolite identifiable fa- clearly by its cube-shaped morphology ([Fig.l]).

[0097] To confirm the presence of LTA type zeolite within the geopolymer, X-ray diffraction measurements were carried out. The DRX measurement presented [Fig.2] clearly reveals the presence of LTA type zeolite. Thus, protocol 1 makes it possible to obtain macroporous geopolymers lined with LTA type zeolite.

[0098] To improve the mechanical strength of geopolymers, protocol 2 allows time for the geopolymerization reaction to take place before carrying out a heat treatment and thus consolidating the structure. Several formulations were tested with a view to controlling pore size and mechanical strength.

[0099] To do this, by adjusting the Si / Al ratios between 1.2 and 1.4 and / or H2O / Na2O between 8 and 10, it is possible to increase the pore size and improve the mechanical strength. After heat treatment of the geopolymers from protocol 2, the latter were subjected to DRX.

[0100] Protocol 2 allows us to observe the amorphous bump, centered around 28-29° in 20, characteristic of geopolymers, which allows us to confirm that the geopolymerization reactions have had time to take place.

[0101] Furthermore, the H2O / Na2O ratio does not seem to have any impact on the zeolites formed. Indeed, for Si / Al = 1.25, the DRX curves for H2O / Na2O = 8 and H2O / Na2O = 10 are the same.

[0102] As for protocol no. 1, zeolite is observable for Si / Al ratios between 1.25 and 1.35. However, for Si / Al = 1.4 there are no more zeolites.

[0103] At the end of this protocol, it was noticed that depending on the Si / Al ratio, the variety of the zeolite varies. The more the Si / Al ratio increases, the more FAU type zeolite is present until it replaces the LTA type zeolite for Si / Al = 1.35. In summary,

[0104] - for Si / Al = 1.2 (Protocol No. 1): only LTA type zeolite is present.

[0105] - for Si / Al = 1.25: Mainly LTA type zeolite and presence of FAU type zeolite,

[0106] - for Si / Al = 1.3: Reduction of LTA type zeolite (intensity) and majority of FAU type zeolite,

[0107] - for Si / Al = 1.35: Absence of LTA type zeolite and presence of zeolite of FAU type and

[0108] - for Si / Al >1.4: Absence of zeolite.

[0109] It is possible, by a fine analysis of the X-ray diffraction spectra, to quantify the different phases. Table 1 below reports the proportions of the phases for the formulation Si / Al = 1.25, R = 1% and H = 10. It can be seen that approximately 20% of zeolite relative to the total mass of the composite material was synthesized by this procedure.

[0110] [Tables 1] refinement result reduced to % of Si Normalization without Si LTA 48.27% 14.89 15.63% Faujasite 12.28% 3.79 3.98% quartz 23.24% 7.17 7.53% Si 16.21% 5.00 100 30.85 geopolymer 69.15 72.61%

[0111] Example 2: Preparation of a geopolymer foam according to protocol 2 with Si / Al=1.25, R=1% and H=10

[0112] The raw materials used are: - BETOL®39T from Woellner, - AER5 from Sika - soda tablets (NaOH), - distilled water, - metakaolin Ml000 from Imerys and - hydrogen peroxide (H2O2).

[0113] Preparation of the activation solution for a geopolymer of volume 50 ml

[0114] 11.64 g of NaOH pellets are mixed with 3.24 g of BETOL®39T and 24.90 g of distilled water. The mixture is stirred using a magnetic bar until the soda pellets are completely dissolved and the temperature returns to room temperature. The AER5, i.e. the air entrainer, is then added with a mass corresponding to 2% of the total mass of the geopolymer paste.

[0115] Preparation of 50 ml of geopolymer paste

[0116] 39.8 g of metakaolin are mixed with the activation solution under stirring at 500 rpm until the metakaolin is completely incorporated. The resulting paste is then mixed at 2000 rpm for 2 min.

[0117] Foaming and consolidation at room temperature

[0118] 0.5 ml of hydrogen peroxide is added to the 50 ml of geopolymer paste to initiate foaming (R = Volume H2O2 / Volume of dough = 1%). The foam thus generated is filmed to prevent drying on the surface and is left to consolidate at room temperature for 24 hours.

[0119] Precipitation of zeolite in-situ

[0120] Once the geopolymer foam has consolidated, it is placed in an oven at 70°C for 6 hours. References

[0121] [1] Rozek et al, 2019, “Geopolymer-zeolite composites: A review”, J. Clean. Prod., vol. 230, pages 557-579.

[0122] [2] Wan et al, 2017, « Geopolymerization reaction, microstructure and simulation of metakaolin-based geopolymers at extended Si / Al ratios », Cem. Concr. Compos., vol.79, pages 45-52.

[0123] [3] Candamano et al, 2022, « Préparation of foamed and unfoamed geopolymer / NaX zeolite / activated carbon composites for CO2 adsorption », J. Clean. Prod., vol. 330, page 129843.

[0124] [4] Demande internationale WO 2016 / 173950 Al publiée le 3 novembre 2016.

Claims

Claims

1. A method for preparing a geopolymer-zeolite composite material comprising the following steps: (a) preparing a geopolymer foam from a geopolymeric slurry in which the Si / Al molar ratio is greater than or equal to 1.2 and less than 1.4 and the H2O / M2O molar ratio with M representing the compensation cation is greater than or equal to 8 and less than or equal to 10; (b) optionally allowing the geopolymer foam prepared in step (a) to harden; then (c) subjecting the geopolymer foam prepared in step (a) optionally hardened in step (b) to a heat treatment at a temperature less than or equal to 80°C for a time greater than or equal to 3 hours, whereby a geopolymer-zeolite foam composite material of LTA structure and / or FAU structure is obtained.

2. Preparation method according to claim 1, characterized in that the Si / Al molar ratio in said geopolymeric slurry is greater than or equal to 1.25 and less than or equal to 1.35, whereby a geopolymer-zeolite foam composite material of FAU structure and optionally of LTA structure is obtained.

3. Preparation method according to claim 1 or 2, characterized in that said geopolymer foam is prepared, during said step a), using a gas-generating adjuvant and an air-entraining adjuvant.

4. Preparation method according to any one of claims 1 to 3, characterized in that the preparation of the geopolymer foam during said step a) comprises the following steps: (i) preparing an activation solution comprising at least one air-entraining adjuvant, (ii) adding, to the activation solution prepared in step (i), an alumino-silicate source and mixing the whole, whereby a geopolymer paste is obtained, (iii) adding, to the geopolymer paste prepared in step (ii), at least one gas-generating adjuvant and then mixing the whole whereby a geopolymer grout is obtained and then a geopolymer foam once the volume expansion is complete.

5. Preparation process according to claim 4, characterized in that said aluminosilicate source is metakaolin.

6. Preparation process according to claim 4 or 5, characterized in that said gas-generating adjuvant is hydrogen peroxide (H2O2

7. Geopolymer-zeolite composite material prepared by a process as defined in any one of claims 2 to 6, the composite material being in the form of a geopolymer foam whose surface is coated with zeolites of FAU structure or with both zeolites of FAU structure and zeolites of LTA structure.

8. Use of a geopolymer-zeolite composite material according to claim 7, for decontaminating liquid effluents of medium or low activity, for trapping gases such as carbon dioxide (CO2) and nitrogen oxides NOX with x representing 1 or 2, for the decontamination of toxic elements and / or for extracting, concentrating and enabling the recovery of elements of interest.

9. Use of a geopolymer-zeolite composite material according to claim 7, for extracting cesium and / or strontium from a solution containing it.

10. A method for separating cesium and / or strontium from a solution containing cesium and / or strontium, comprising contacting a composite material according to claim 7 with a solution containing cesium and / or strontium, whereby a solution depleted in cesium and / or strontium and a geopolymer foam having zeolites of FAU structure and optionally zeolites of LTA structure trapping cesium and / or strontium are obtained.

11. A method for separating cesium and / or strontium from a solution containing cesium and / or strontium, comprising - preparing a composite material according to the method according to any one of claims 1 to 6 and then - bringing said composite material thus prepared into contact with a solution containing cesium and / or strontium, whereby a solution depleted in cesium and / or strontium and a geopolymer foam having zeolites of FAU structure and / or zeolites of LTA structure trapping cesium and / or strontium are obtained.

12. Method according to claim 10 or 11, characterized in that said solution containing cesium and / or strontium is a liquid or effluent from the nuclear industry, nuclear installations and activities using radionuclides.

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