EP2951127A1

EP2951127A1 - Method for producing calcium carbonate gel and product obtained thereby

Info

Publication number: EP2951127A1
Application number: EP14702020.0A
Authority: EP
Inventors: Geert Jan Witkamp; Sergio Andrés PÉREZ ESCOBAR; Robert Sebastian GÄRTNER
Original assignee: Lhoist Recherche et Developpement SA
Current assignee: Lhoist Recherche et Developpement SA
Priority date: 2013-01-30
Filing date: 2014-01-30
Publication date: 2015-12-09
Also published as: CN104936900A; RU2015129524A; BE1021564B1; US20150344319A1; WO2014118281A1; JP2016508476A

Abstract

The invention relates to a method for preparing a calcium carbonate gel, comprising: a reaction between calcium hydroxide in a solid, dry state and alcohol, in such a way as to form an alcoholic suspension of calcium alcoholate; an injection of carbon dioxide into said suspension; and a gelification of the suspension in the form of a precipitated calcium carbonate alcogel, said alcogel being then able to be dried to form an aerogel or xerogel of calcium carbonate.

Description

"Process for preparing calcium carbonate gel

and the product thus obtained. "

The present invention relates to a process for the preparation of calcium carbonate gel and to the products obtained by means of such a process.

It is known to prepare calcium carbonate gels from a reaction between quicklime (CaO) and absolute methanol (without water), with the formation of a methanolate followed by an injection of C0 ₂ to give a calcium dimethyl carbonate which by reaction with water produces a calcium carbonate and methanol. Then, the gel obtained can be subjected to drying with CO2 so as to form an airgel of calcium carbonate in the form of a precipitate of vaterite particles of nanometric size aggregated to form an airgel type network (see for example J. Plank et al., Preparation and Characterization of a Calcium Carbonate Airgel, Hindawi Publishing Corporation, Research Letters in Materials Science, 2009, Article ID 138476, A. Buzagh, Kolloid Ueber Lôsungen der Erdalkalikarbonate, Kolloid-Zeitschrift, 38, 3, p.222-226, 1926, E. Berner, Uber die Einwirkung der Erdalkalioxide auf Alkohole, Berichte der deutschen chemischen Gesellschaft, 71, 9, p.2015-2021, 1938). However, it appeared that this production process was unreliable because it is not very reproducible. This lack of control over the properties and qualities of the gel obtained represents an unacceptable handicap when one wants to undertake a production on an industrial scale, in particular of aerogels. Other methods for preparing calcium-based gels or aerogels are known, for example those based on calcium alginate (see, for example, R. Horga et al., Lonotropic Alginates Aerogels and Precursors of Dispersed Oxide Phases, Applied Catalysis A, 325, 2, p.251-255, 2007).

EP 0 522 415 teaches a method of producing calcium carbonate by carbonating a calcium-based compound in monoethylene glycol followed by a dispersion-ripening step (see also M. Ryu et al. Synthesis of calcium carbonate in ethanol-ethylene glycol solvent, Journal of the Ceramic Society of Japan, 17 [1] 106-1 10, 2009).

Silica gels and aerogels have also been known for a long time. The reaction to form a network of silica nanoparticles, however, is slow and requires the use of polycondensation catalysts to accelerate production on an industrial scale. These catalysts, however, have the effect of altering the quality of the gel and its reproducibility. This results in a high cost of silica aerogels.

The preparation of the majority of silica aerogels uses tetramethyl or tetraethyl orthosilicate precursors which are highly toxic compounds.

For the gels of the prior art, an additional stage of maturation in an alcohol / water solution and, in the case of silica gels, a step of dipping in pure alcohol in order to extract the water are necessary. Indeed, any trace of water left in a silica gel will not be eliminated during drying by CO ₂ and will lead to an opaque and very dense airgel.

The gels of the prior art, in particular silica, are hydrophilic and even hygroscopic in nature. The absorption of water, in particular of the ambient air, leads to structural modifications, namely to the deterioration of the airgel, which usually requires prior chemical treatments of hydrophobation. The presumed reaction mechanism of the work of Piank et al. is the next :

a) Formation of a methanolate from quicklime

CaO + IMeOH → Ca (MeO) ₂ + H ₂ O

(b) Carbonation in dimethyl carbonate

Ca (MeO) ₁ + 2C0 ₂ → Ca (C0 ₂ MeO) ₂

c) Production in the presence of water of a carbonate for the formation of a gel

Ca (C0 ₂ MeO) ₂ + H ₂ O → CaC < ₃ + IMeOH + CO ₂

It is an object of the present invention to provide a process for the preparation of a gel which can be reliably controlled and thereby give rise to industrially reproducible gels. This method must advantageously be simple and thus allow industrial production including stable aerogels, preferably with a large BET specific surface, which are preferably mechanically resistant.

To solve these problems, there is provided according to the invention a calcium carbonate gel preparation process, comprising - a reaction between slaked lime in solid, dry form, and alcohol so as to form an alcoholic suspension calcium alkoxide,

an injection of carbon dioxide into said suspension, and

a geiification of the suspension in the form of a precipitated calcium carbonate alcogel.

A presumed reaction mechanism according to the invention is the following, in the case where the alcohol is methanol:

I) Formation of methanolate from slaked lime

Ca (OH) ₂ + MeOH → Ca (OH) (MeO) + H ₂ O

II) Carbonation of Methylicarbonate Hydroxide

Ca (OH) MeO) + C0 ₂ → Ca (C0 ₂ MeO) (OH)

III) Production of a carbonate for the formation of a gel

Ca (C0 ₂ MeO) (OH) → CaCO ₃ + MeOH The advantage of the use of slaked lime according to the present invention lies in the high reproducibility and excellent control of the quality of the reaction between this hydrated lime, solid, dry and alcohol. In this way, the solids content, the specific surface area and the bulk density of the gels obtained can be perfectly controlled, which was not the case with quicklime used in the prior state of the art.

According to the invention, the term "slaked lime" (Ca (OH) ₂ ) means a solid, dry composition which can contain only up to a few% by weight of free water. It can in no way be lime milk, because the water of such a suspension would contribute to a destructuring of the gel during production. Preferably, the slaked lime used is a powder having particles having a size of less than 1 mm, advantageously less than 500 μm, and preferably less than 90 μm; most of the particles are greater than 0.5 μm.

It should be noted that this solid slaked lime composition, which essentially comprises particles of calcium hydroxide, may further comprise the usual impurities of an industrial lime, namely phases derived from SiO ₂ , Al ₂ O ₃ , Fe ₂ O _3. , MnO, P2O5, K ₂ 0 and / or S0 ₃ , generally up to a few tens of grams per kilogram of slaked lime. Nevertheless, the sum of these impurities, expressed in the form of the aforementioned oxides, will not exceed 5%, preferably 3%, in particular 2% or even 1% by weight of the solid slaked lime composition.

Slaked lime according to the invention may also contain calcium oxide CaO which has not been hydrated during the extinction, or calcium carbonate CaC0 ₃ from either the initial limestone which is derived slaked lime ( uncooked part), or a partial carbonation reaction of the slaked lime in contact air. The calcium oxide content in the slaked lime according to the invention will not exceed 3% by weight. It will preferably be less than 2%, advantageously 1% by weight of the solid slaked lime composition. The calcium carbonate content will be less than 10%, preferably 6%, in particular 4% and even advantageously 3% by weight of the slaked lime composition.

The slaked lime according to the invention may also contain magnesium oxide 60 or phases derived from the Mg (OH) ₂ or MgCO _{3 type} . The sum of these impurities, expressed in the form of MgO, will not exceed 5%, preferably 3%, in particular 2% or even advantageously 1% by weight of the slaked lime composition.

The nature of the alcohol used according to the invention is not critical. Any alcohol known to those skilled in the art and forming an alcogel with lime is appropriate. However, it should be noted that it is desirable that the alcohol used contain as little water as possible, since, as mentioned above, this water could destroy the gel. On the other hand, it is desirable that the alcohol used has a relatively low boiling point, is not too viscous and has good solubility in supercritical CO ₂ .

In the context of the present invention, monoalcoals, alcohols whose formula has only one OH group, are therefore particularly preferred, those having a purity greater than or equal to 95% of technical grade, the residual 5% being typically formed of impurities and / or water or traces of glycoî, such as monoethylene glycol, which is meanwhile hardly soluble in supercritical CO ₂ .

Mention may also be made, by way of examples of usable alcohols, in particular of monoalcohols, of methanol, ethanol, propanol, butanol and isopropanol, although, for the reasons stated above, the use of Ethanol seems rather less favorable. The proportion of slaked lime with respect to the alcohol is preferably from 15 g / dm ³ to 200 g / dm ³ , in particular from 15 g / dm ³ to 00 g / dm ³ (gram of slaked lime by dm ³ d 'alcohol).

The reaction between slaked lime and alcohol is only moderately exothermic and rather slow. It is therefore possible to provide a slight heating of the reactor in which the reaction is to take place, for example at a temperature of between 20 and 70 ° C., preferably 30 and 70 ° C.

This reaction can last about 1 to 2 hours. Advantageously, the suspension obtained can then, before the injection step, be sieved through a sieve having a mesh size of between 20 and 250 μητι, in particular equal to or less than 45 μηη.

The injection of carbon dioxide into said suspension is preferably carried out at a temperature between 30 ° C and 70 ° C. Its purpose is to generate an alkylcarbonate in the alcoholic suspension of calcium alkoxide. It is advantageously stopped when this suspension has a pH of less than 9, in particular 8.7 and advantageously 8.3.

As the injection gas can be used, in addition to carbon dioxide, any gaseous mixture containing CO2 and at least one other gas, for example air.

After the injection, it is advantageous to screen the saturated suspension through a sieve having a mesh size of between 20 and 250 μm, in particular equal to or less than 45 μm.

After injection, the alcoholic suspension of alkylcarbonate (especially methylcarbonate) of calcium is allowed to gel to form a precipitated calcium carbonate alcogel.

According to a preferred embodiment of the invention, the method further comprises, before the injection and / or at the beginning thereof, seeding the alcoholic suspension of calcium alkoxide with calcium carbonate crystals. Particularly suitable calcium carbonate crystals are those selected from the group consisting of caicite crystals, aragonite, vaterite and mixtures thereof. The crystals of caite or aragonite are particularly preferred because they give rise to a very stable cation gel.

According to a particular embodiment of the invention, the method further comprises, before the injection and / or at the beginning thereof, an addition of at least one crystalline growth inhibitor of CaCO ₃ , in particular sugar, to said alcoholic suspension. Examples of crystalline growth inhibitors of CaCO 3 include sucrose, sucrose, a monosaccharide, in particular glucose, fructose or galactose, a disaccharide, in particular lactose, maltose or sorbitol, citric acid, polyacrylates, phosphates or metaphosphates or their corresponding acids or the soluble salts of strontium or magnesium. An addition of an amount of between 500 ppm and 5% by weight relative to the starting Ca (OH) _{2 is} preferably provided.

According to a particularly advantageous embodiment of the invention, the process further comprises drying the precipitated calcium carbonate alcogel so as to form an airgel of calcium carbonate. This drying can be carried out according to any method known to those skilled in the art. For example, it may be possible to subject the alcogel to known treatment with liquid or supercritical CO ₂ . Such a treatment is described rather vaguely, for example in the article by J. Plank et al. cited above.

Other methods of gel drying produce so-called xerogels. Unlike supercritical fluid drying, a contraction of the gel can not be avoided but is reduced. The best known methods are freeze drying (freeze drying) or cold drying. During lyophilization, the solvent in the gel is frozen and slowly sublimed by applying a vacuum or a very low partial pressure. In the case of methanol, temperatures below 175 K must be applied. This method has not only the disadvantage of requiring extremely low temperatures and long drying times, but also that a solidification of the solvent can break the structure of the gel. The cold drying method consists in evaporating the solvent from the gel at low temperature by applying a vacuum or a low partial pressure, in which conditions the recrystallization of the particles is avoided. For vaterite and calcite gels, the temperature should be less than 278 K. While xerogels achieve specific high surface areas and small particle sizes, they are commonly denser than aerogels dried under supercritical conditions. .

It appeared that a calcite gel obtained by seeding the alcoholic suspension of calcium alkoxide with crystals of calcite or aragonite gave rise after such drying to an airgel much more resistant to degradation and recrystallization. Depending on the seed crystals and their structure, the gel formed will have a preferred crystalline form which will comprise a proportion of vaterite preferably less than 97% by weight. The crystallinity of the calcite gel can be controlled by two parameters namely, the amount of calcite starting and its fineness (size of the calcite particles).

The higher the amount of calcite starting, the higher the rate of seeding, resulting in a high amount of calcite in the gel obtained. As for the fineness of calcite, the finer the calcite, the higher the rate of seeding.

Unlike a vaterite airgel, you! As described in the prior art, a calcite airgel has good stability in the presence of water, in particular air humidity. A calcium carbonate gel precipitated in the presence of a sugar addition is capable, after drying as provided above, of giving rise to an airgel with BET specific surface area and extremely high nitrogen pore volume, which exhibits surprising mechanical resistance.

By the terms "BET surface area" in the sense of the present invention is meant the specific surface area measured by nitrogen adsorption manometry and calculated according to the BET method.

By the term "particles" in the sense of the present invention is meant the smallest solid discontinuity of the inorganic filler observable by scanning electron microscopy (EB).

The present invention also relates to the gels obtained by a process according to the invention.

The alkogel obtained after gelling is advantageously constituted by 1 to 6% by volume of precipitated calcium carbonate nanoparticles having a particle size substantially between 5 and 600 nm, in particular between 5 and 300 nm, advantageously between 10 and 200 nm. nm, preferably between 10 and 50 nm, more preferably between 10 and 20 nm. This last interval is particularly characteristic of precipitates of vaterite. These nanoparticles are actually agglomerates of crystallite calcium carbonate, whose size is smaller than that of the nanoparticles.

The calcium carbonate airgel or the calcium carbonate xerogel obtained according to the invention advantageously has a BET specific surface area of 4 to 450 m ² / g, preferably 5 to 450 n ⁷ g.

Advantageously, the calcium carbonate airgel according to the invention has a BET specific surface area of between 40 and 450 m ² / g, preferably between 45 and 450 m ² / g, more preferably between 47 and 450 m 2 / g. and advantageously between 50 and 450 m ² / g, in particular from 100 to 450 m g. According to a preferred embodiment of the present invention, the calcium carbonate xerogel according to the invention has a specific surface area of between 4 and 50 m ² / g, preferably between 5 and 45 m ² / g, more preferably between 8 and 40 m ² / g.

According to a preferred embodiment of the present invention, the calcium carbonate xerogel according to the invention has a crystallite size of between 20 and 100 nm, in particular for the particles of caicity, in particular between 5 and 30 nm, in particular for the particles. of vaterite.

The calcium carbonate airgel according to the invention advantageously has a crystallite size of between 5 and 100 nm, especially for particles of caicity, in particular between 5 and 30 nm, in particular for the particles. vaterite, more particularly between 5 and 20 nm.

The calcium carbonate airgel or the calcium carbonate xerogel obtained according to the invention advantageously has an apparent density of between 0.01 and 0.15 g / cm ³ , preferably between 0.02 and 0.06 g. / cm ³ .

Advantageously, the airgel or xerogel according to the invention is characterized in that it consists of an airgel or xerogel of caicite, vaterite, aragonite or their mixtures.

In a preferred embodiment of the present invention, the airgel is characterized in that it has a BET specific surface area of between 4 and 40 m ² / g, or between 100 and 250 m ² / g, a crystallite size of between 5 and 30 nm in particular for vaterite and a particle size of between 60 and 600 nm or between 5 and 20 nm.

In the context of the present invention, particle sizes have been calculated by optical microscopy. The average thus obtained was retained to determine the ranges of values. In the context of the present invention, the airgel obtained according to the invention advantageously has a BET specific surface area of between 100 and 450 m ² / g, a crystallite size of between 5 and 20 nm, in particular for calcite and for vaterite, and a particle size of about 10 nm when obtained in the presence of a growth inhibitor.

In a particularly advantageous manner, the xerogel according to the invention is characterized in that it has a BET specific surface area of between 4 and 10 m ² / g, a crystallite size of between 20 and 100 nm, especially for calcite and between 15 and 30 nm, especially for vaterite and a particle size between 100 and 500 nm.

According to a particularly preferred embodiment, the xerogel according to the invention is characterized in that it has a BET specific surface of between 20 and 40 m ^z / g, a crystallite size between 20 and 100 nm, including calcite and between 15 and 30 nm, especially for vaterite and a particle size between 50 and 150 nm.

We will notice that during the production of ge! and calcium carbonate airgel according to the invention, the decomposition of calcium carbonate alkylate (especially methyl carbonate) hydroxide into calcium carbonate (step III) does not require water, contrary to the art previous (step c.) but depends only on the concentration of this compound. Consequently, the process according to the invention does not require steps of maturation and soaking of age! as in the case of the prior art.

In addition, the airgel thus obtained according to the present invention has properties of thermal and / or acoustic conductivity (22.2 mW / m / K for a packed density of 150 g / dm ³ ) which make this product, among others , an interesting candidate as an insulator. Other details and particularities of the invention will emerge from the description given below of nonlimiting exemplary embodiments.

FIG. 1a illustrates the influence of the addition of additives on the porous volume BJH of aerogels obtained according to the invention.

FIG. 1b shows the influence of adding additives on the average diameter of the airgel BJH pores obtained according to the present invention.

FIG. 2a illustrates the influence of the addition of additives on the porous volume BJH of aerogels obtained according to the invention.

FIG. 2b illustrates the influence of the addition of additives on the average diameter of the airgel BJH pores obtained according to the invention.

FIG. 3 represents the relationship between the BET specific surface area and the porous BJH volume of aerogels obtained according to the invention.

Figure 4 illustrates an SEM image of a calcite airgel obtained according to the invention.

Figure 5 illustrates a SEM image of a xeroge! of calcite obtained according to the invention.

FIGS. 6a and 6b show the influence of adding additives on the size of the crystals and the xerogel and aerogel particles obtained according to the invention.

For the purposes of the present invention, the term "pore volume BJH" is intended to mean the pore volume whose size is between 17 and 1000 Å (1.7 and 100 nm), as measured by nitrogen desorption manometry, obtained after degassing under vacuum at 190 ° C., and calculated according to the BJH method.

For the purposes of the present invention, the term "average pore diameter BJH" is intended to mean the average pore diameter, the size of which is between 17 and 1000 Å (1.7 and 100 nm), as measured by desorption manometry. nitrogen, obtained after degassing under vacuum at 90 ° C, and calculated according to the BJH method. The conditions of carbon dioxide injection influence the quality of the gel produced. In the context of the present invention, two different injection methods can be applied.

The first injection method uses a gas at atmospheric pressure, which contains approximately 15% by volume of C0 ₂ . In this method, a CO ₂ mixture in the presence of an inert gas can be used. Advantageously, this gaseous mixture has a CO ₂ content of between 2% and 100% by volume, preferably between 4% and 50% by volume, more preferably between 10% and 30% by volume. The gas injection is advantageously carried out under quasi-atmospheric pressure or under a low pressure, in this case at a pressure of less than 0.5 Pa, preferably less than 0.3 MPa.

This method ^achieves a uniform and almost complete conversion of the alkoxide. It is desirable to avoid too rapid gelation in the reactor before complete conversion has occurred. In this situation, the use of a diluted gas makes it possible to prolong the injection duration and to increase the volume of gas required. A diluted gas is preferred over a highly concentrated gas, because in the latter case, a much higher agitation is necessary to achieve adequate homogenization of the mixture in the reactor, which increases the risk ^of spontaneous and uncontrollable gelation.

In the second injection method, described in a non-limiting manner in Example 10, the homogenization is increased by the use of liquid C0 ₂ under pressure while accepting spontaneous gelling. In this method, the injection of carbon dioxide is advantageously carried out until a pressure of between 7 and 12 MPa is obtained, preferably between 8 and 11 MPa. The use of the same reactor to effect the drying makes it possible to combine the steps of carbonation of the alcoholate with gelling and drying in the same equipment. The use of liquid C0 ₂ is used to intensify the conversion reaction of ^the alkoxide, to increase the concentration of calcium carbonate in the gel and the density i'aérogel obtained. It was found that ^the increase in density ^of an airgel allows to significantly ^increase its mechanical strength.

In this second injection method, the conversion steps ^of calcium hydroxide into calcium alcoholate, the alcoholate conversion by injection of C0 _2, gelling and drying are conducted in a successive manner.

In the context of the present invention, the X-ray diffraction analysis (XRD) makes it possible to estimate the size of the crystallites using the Scherrer equation. This equation, taken again below, is valid for crystallites of size smaller than 100-200 nm:

Κλ WHERE

λ corresponds to the wavelength of the X-ray,

τ corresponds to the average size of the crystallite,

K is a dimensionless factor that depends on the shape of the crystallite. Its value is typically around 0.9.

β corresponds to the width at half height of a peak of the diffraction pattern.

Example 1

Preparation of an alcogel

A 3 dm ³ reactor having a height / diameter ratio of approximately 2 and equipped with a double-blade stirrer, an inlet for the gas at the bottom, and sensors for temperature, pH and conductivity. This reactor has a double wall and is made thermostatic by a heating / cooling bath.

2 dm ³ of analytical methanol are introduced into this reactor and a temperature of 30 ° C. is established therein, then 75 g of commercial slaked lime are added. The suspension obtained is mixed for 1-2 h at about 500 rpm which gives rise to the formation of a suspension of calcium methanolate in methanol. The temperature and pH remain stable until the end of the reaction, that is to say 30 ° C and respectively about 12.2 (this pH is however reached after 20 to 30 min reaction) . The slurry is then sieved through a sieve having a 45 μm rejection to remove coarse particles.

The sieved suspension is then placed in a reactor into which a gaseous mixture of carbon dioxide (15% by volume) and technical air (85% by volume) is injected at a rate of 4.75 dm ³ / min for 1 hour. -2 h. The injection is stopped at a pH of about 8.6, indicating that quantitative carbonation has occurred.

The slurry is then removed from the reactor and placed in a glass beaker where it is allowed to gel in the form of a precipitated calcium carbonate alcogel, which takes about 1 hour.

This alcogel is divided into two samples.

The first sample of about 1.5 dm ³ is left in the beaker to rest at ambient conditions of about 18 ° C with the top of the beaker covered with a plastic film. The gel remains stable, without degradation, for about 1 day. X-ray diffraction (XRD) analysis of the solid material reveals that the precipitated calcium carbonate mainly consists of vaterite with small traces of calcite.

Preparation of an airgel.

The second sample of about 50 cm ³ is placed in an autoclave at room temperature and atmospheric pressure. A thin layer of pure methanol (about 2 cm ³ ) is added above the gel to protect it during the first pressurization. The autoclave is then sealed and pressurized slowly by introducing carbon dioxide until a pressure of 0 MPa is reached at a rate of 0.1 to 0.2 MPa / min. The CO2 introduced has a temperature of 293 K. The autoclave is also maintained at this temperature. temperature during the aforementioned first step by thermostatization by means of a double jacket.

When the autoclave reached 10 MPa at 293 K and is filled with liquid C0 ₂ , stirring starts at 150 rpm and is maintained for 20 minutes. The opening of the inlet and outlet valves of the C0 ₂ of the autoclave then creates a continuous stream of liquid CO ₂ so as to replace gradually with pure CO ₂ the C0 ₂ mixed with methanol. This operation is continued for 30 minutes at a constant pressure of 10 MPa.

Then, the pressure in the autoclave is successively decreased and increased in order to accelerate the extraction of the methanol:

by a slight increase in the degree of opening of the CO ₂ outlet valve so as to achieve a very slow decrease of pressure up to 8 MPa in approximately 15 minutes, during which time a continuous flow of CO ₂ is maintained;

by reducing the degree of opening of the C0 ₂ outlet valve so as to increase the pressure up to 10 MPa, again over a period of about 15 minutes by maintaining a continuous flow of C0 ₂ .

These successive increases and decreases in pressure are carried out until there is no more methanoi detected at the outlet, that is to say that there are no more methanoi drops in the vase. of expansion of C0 ₂ . About two hours are sufficient for most of these samples (4 cycles of increase / reduction of pressure). When no more methanol is detected, all the valves are closed but agitation is maintained.

The autoclave is then placed in supercritical condition so as to allow the drainage of C0 ₂ out of the autoclave in the absence of a gas / liquid interface which could damage the airgel. To do this, the autoclave is heated up to 318 K in a period of about 20 minutes by means of the jacket. Since the temperature increases, the pressure should also increase. The pressure is however kept constant at a value of 10 MPa by opening the outlet valve. These supercritical conditions (CO2 at 318 K and 10 MPa) are maintained for 30 minutes.

At the end of this period, agitation is stopped and the autoclave is then slowly brought back to ambient pressure at a rate of 0.1 to 0.4 MPa / min.

Small pieces of airgel (powder or granules) are obtained on which, by analysis, a BET surface area of about 185 m ² / g (at plus or minus 10%) is observed and, by scanning electron microscopy (MEB), a particle size of about 10 to 20 nm. These particles therefore have a BET specific surface area raised to a point which could not be expected by the teaching of the state of the art. They are also much thinner with the consequence of an increase in the density of the particles, a greater interconnection of these particles and therefore a better mechanical stability of the airgel.

Example 2

An alkogel is prepared as in Example 1 except that the temperature in the reactor is set at 50 ° C instead of 30 ° C. The increase in temperature causes an increase in the rate of formation of the gel so that the age! It is already formed in the reactor at the end of the carbonation, shortly after the pH reaches a value of 8.6.

Example 3

The alkogel preparation conditions of Example 1 are repeated. An airgel powder having a bulk density, observed in accordance with the procedure described in EN459, of about 0.05 g / cm ³ and a BET surface area of 235 m ² / g (at plus or minus) is obtained. 10%). This suggests a size of spherical particles, ideal, about 10 to 20 nm, which is confirmed by SEM images.

The airgel sample is stored in 2 containers of 700 cm ³ each. One of the containers is left closed, the other is open regularly once a week. After 4 months, the airgel in the cios container decreased by 50% in volume. The airgel in the regularly open container degenerated to about 100 cm ³ of powder and resembles precipitated calcium carbonate (PCC) of micron size.

The powder from the closed container still has a BET specific surface area of about 180 m ² / g and a particle size of the order of 10 to 20 nm. The powder coming from the regularly open container, on the other hand, has a BET specific surface area of only 5 mg and a particle size of the order of one micron, which again shows a lack of stability, probably in the presence of the humidity of the air. . In fact, vaterite nanocrystals are unstable in contact with water and they recrystallize into larger crystals of aragonite and calcite.

Example 4

The procedure is repeated as in the preparation of alcogel of Example 1, except that before proceeding to the injection of the mixture of carbon dioxide and technical air, 0.3% is added to the suspension. by weight, in relation to the slaked lime used, calcium carbonate precipitated in the form of calcite and having a scalenohedron morphology (PCC paper grade) and an average particle diameter of 2.5 pm.

After gelation, 50 g of a sample are taken and subjected to the airgel preparation procedure described in Example 1. A XRD analysis reveals that the calcite content has increased to 99.8% by weight and that the airgel particles therefore consist entirely of calcite. The airgel has a BET surface area of only 7.6 m ² / g which corresponds to an ideal theoretical taifle of spherical particle of about 290 nm. A size of about 200 to 300 nm is confirmed by the observation of SEM images that show highly interconnected rhombohedral particles with high interparticle porosity and between agglomerates. This material was then stored for approximately 8 months in ambient air, with no signs of degradation, degeneration, or recrystallization.

Example 5

The procedure is repeated as in the preparation of alcogel of Example 1, except that before proceeding to the injection of the mixture of carbon dioxide and technical air, 0.3% is added to the suspension. by weight of sucrose, relative to the slaked lime implemented.

After gelation, 50 g of a sample are removed and subjected to the airgel preparation procedure described in Example 1. The analyzes reveal that the BET specific surface area of the airgel is 415 m ² / g (at plus or minus 10%). This suggests an ideal spherical particle size of about 5 nm. Furthermore, the analyzes reveal that the average BJH pore size increased by 10 nm in the vaterite aeragen of Example 1 to 32 nm in the airgel obtained in this example. By tactile pressure on the airgel obtained, it can be seen that it attests a significantly weaker brittleness than the other airgel described in the previous examples.

Example 6

An aicogel is prepared as indicated in Example 1, except that before proceeding with the injection of carbon dioxide, 0.3% by weight of caicite, which has a scalenohedron-type morphology, is added to the suspension. with a mean particle size of caicit of 1, 5 μιη, and 0.3% by weight of sucrose (table sugar) relative to the weight of the slaked lime. The gel obtained is dried in the autoclave as explained in Example 1 in the paragraph "preparation of an airgel."

At the end of the process, there is obtained a white translucent airgel having a BET surface area of 140 m 7 G and a BJH pore volume of 1, 47 cm ³ / g (for a pore size of 17 to 1000 A) and a size of crystallite, estimated using the Scherrer equation, of 30 nm for calcite.

The size of the calcite particles calculated from the BET surface area is 20 nm, which is confirmed by the observation of the SEM images. XRD analysis revealed that the material was calcite and no vaterite or aragonite particles were detected.

Example 7

A xerogel is prepared as in Example 1. The gel thus obtained is spread on a petri dish and dried in a drying oven for 8 hours at 50 ° C until the gel is stable by weight. A white powder is thus obtained and has a BET specific surface area of 23.2 m ² / g, a BJH pore volume of 0.091 cm ³ / g (for a pore size of 17 to 1000 Å), an estimated crystallite size using the Scherrer equation, 30 nm for calcite and 20 nm for vaterite and a particle size of 100 nm, calculated from the BET specific surface area of calcite, which is elsewhere confirmed to the observation of SEM images. XRD analysis revealed that the material consisted of 85% by weight of vaterite and 15% by weight of calcite.

Example 8

A xerogel is prepared as indicated in Example 1, except that before proceeding with the injection of carbon dioxide, 0.5% by weight of calcite, which has a scalenohedron-type morphology, is added to the suspension. with an average particle size of calcite of 1.5 μιτι, relative to the weight of the slaked lime. The gel thus obtained is spread on a petri dish and dried in a drying oven for 8 hours at 50 ° C until the gel is stable by weight. A white powder is obtained which has a BET specific surface area of 5.5 m 2 / g, a BJH pore volume of 0.014 cm ³ / g (for a pore size of 17 to 1000 Å), a crystallite size estimated at 1 using the Scherrer equation, 87.8 nm and a particle size of 440 nm, calculated from the BET specific surface area of calcite, which is confirmed by the observation of SEM images.

An XRD analysis reveals that the material is solely formed of calcite.

Example 9

A xerogel is prepared as in Example 1, with the difference that, prior to the injection of carbon dioxide, 0.3% by weight of calcite, which has a similar morphology, is added to the suspension. scalenohedron with a mean particle size of calcite of 1.5 μηι, and 0.3% by weight of sucrose relative to the weight of the slaked lime.

The gel thus obtained is spread on a petriery bunch and dried in a drying oven for 8 hours at 50 ° C until the gel is stable by weight. A gray-white granular xerogel (similar to the airgel obtained in Example 6) is obtained which has a BET specific surface area of 30.2 m ² / g, a BJH pore volume of 0.094 cm / g (for a size of pore from 17 to 1000 A), a crystallite size, estimated using the Scherrer equation, 29 nm for vaterite and 46 nm for calcite and a particle size of 80 nm, calculated from BET specific surface area of calcite, which is confirmed by the observation of SEM images. XRD analysis revealed that the material consisted of 55.7% by weight of vaterite and 44.3% by weight of calcite.

Example 10

In a reactor of 1 dm ³ , 0.5 dm ³ of analytical methanol, 60 g of slaked lime and 0.2 g of sucrose are introduced to prepare a airs of calcium carbonate. Then, the suspension thus obtained, in the closed reactor, is mixed at 500 rpm for 2 hours at a temperature of 55 ° C at atmospheric pressure. This makes it possible to form a suspension of calcium methanolate in methanol. Then, the temperature inside the reactor is brought to 20 ° C and a liquid mixture of carbon dioxide is injected to carry out a carbo-swimming. Excess CO2 is removed by an outlet valve. The carbonation reaction takes place for 2.5 hours at a pressure of 7.5 MPa. During the first half-hour, a pressure drop is observed due to the absorption of carbon dioxide (CO ₂ ) by the alcogel. Therefore, an injection of a gaseous mixture of carbon dioxide is performed several times so as to maintain the pressure of 7.5 MPa.

After one hour of reaction, the pressure increases spontaneously up to 8.5 MPa and is kept constant until the end of the reaction which ends an hour and a half later.

An injection of a gaseous mixture of carbon dioxide at a flow rate of 200 g / min is carried out for 4 hours at a pressure of 8.5 MPa to extract the solvent in the form of a mixture with the liquid C0 ₂ via the valve Release. The temperature is then brought to 45 ° C to place the autoclave in supercritical condition (C0 ₂ at 318 K and 8.5 MPa) so as to achieve the drainage of CO2 for 45 minutes. At the end of this period, depressurization is carried out for 15 minutes at a constant temperature of 45 ° C.

At the end of the process, an 85 g aerogel sample is recovered from the 560 g slaked lime slurry which means that a yield of 5.2% by weight has been obtained. The volume yield is between 120 and 140% since a volume between 0.6 and 0.7 dm ³ was produced from a suspension of 0.5 dm ³ . An XRD analysis reveals that the material consists of a mixture of vaterite and calcite. The airgel obtained has a BET surface area of 350 m ² / g and a BJH pore volume of 2.33 cm ³ / g for a pore size of 17 to 1000 A. The crystallite size of the airgel is 8 nm. for calcite and 9 nm for vaterite, the crystallite size being estimated by the Scherrer equation. The airgel has a particle size in the same order of magnitude as that of the crystallite size. The particle size is calculated from the BET specific surface area of calcite.

The apparent density of the airgel is 120 g / dm ³ and was obtained in compliance with the EN 459 standard. The thermal conductivity of the airgel corresponds to 22.2 mW / m / K for a packed density of 150 g / dm ³ . estimated using a Netzsch HF 436 Lambda high-speed conductometer. The value of 22.2 mW / m / K indicates that the airgel obtained can be used in the field of thermal insulation.

Exempt 11

Xerogeis and aerogels are prepared as indicated in the previous examples. The xerogeis and aerogels obtained are characterized with respect to the pore volume BJH for pore sizes ranging from 17 to 1000 A and with respect to the average pore diameter. These two parameters can be measured using devices commonly used to measure BET specific surfaces, such as Micromeritics Tristar.

The influence of the BET surface area, the size of the crystallites and the addition of additives to the two parameters mentioned above was analyzed.

It has been found that the surprisingly low density of aerogels and xerogeis results from the fact that each of said gels is composed of a network of particles which encircle the pores. It is preferable that the apparent particle size observed from SEM images (scanning electron microscopy) or calculated from BET specific surface area is relatively low. It is also advantageous if the ratio between the size of the apparent particles and the size of the crystallites is small.

FIGS. 1a and 1b show the influence of additives on the airgel BJH volume of aerogels and on the average diameter of the airgel BJH pores obtained according to the invention. It has been observed that the addition of sucrose in the aerogels obtained according to the invention makes it possible to significantly increase their pore volume BJH (FIG. 1 a) and their mean pore diameter BJH (FIG. 1b).

FIG. 1b also makes it possible to illustrate the structural difference that exists between aerogels having a BET specific surface area greater than 100 m ² / g and those having a specific surface area of less than 100 m ² / g. The latter have an average BJH pore diameter of about 70 to 150 A, as do the xerogels described below. We also observe that the porous volumes of these aerogels are similar to those of xerogefs.

FIG. 2a illustrates the influence of the addition of additives on the porous volume BJH and on the average diameter of the pores BJH of the xerogels obtained according to the invention. It can be seen in FIG. 2a that the addition of additives in xerogels obtained according to the invention makes it possible to increase both the pore volume BJH and the BET specific surface area of these gels. On the other hand, the increase of the BET surface area does not make it possible to improve the average pore diameter of these gels (FIG. 2b).

Figure 3 shows the relationship between the specific surface area

BET and the pore volume BJH of aerogels obtained according to the invention and having a BET specific surface area of less than 40 m ² / g.

FIGS. 4 and 5 respectively show SEM images of a caicite airgel and a caicate xerogel obtained according to the invention and having a BET specific surface area of 7.1 m ² / g. Figures 3 to 5 show that for comparable BET specific surfaces, xerogels and aerogels obtained according to the invention have very similar BJH porous volumes and pore sizes.

Figures 6a and 6b show the influence of adding additives on the size of the crystallites and xerogel and aerogel particles obtained according to the present invention. It can be seen in FIG. 6a that the size of the crystallites increases with the size of the apparent particles of the aerogels. This indicates that as BET surface area decreases, apparent particles and crystallite sizes increase.

We can also note that the increase in crystallite sizes is greater for calcite (Figure 6a) than for vaterite (Figure 6b). Therefore, if a sample contains a mixture of calcite and vaterite, the calcite crystallites will generally have a larger size.

For comparable BET specific surfaces, the xerogels and aerogels obtained according to the invention have very similar crystallite sizes.

Claims

claims

1. Process for the preparation of a calcium carbonate gel, comprising

a reaction between slaked lime in solid, dry form, and alcohol so as to form an alcoholic suspension of calcium alkoxide,

an injection of carbon dioxide into said suspension, and a gelation of the suspension in the form of a precipitated calcium carbonate alcogel.

2. Method according to claim 1, characterized in that the slaked lime used is a powder having particles less than 1 mm in size.

3. Method according to one of claims 1 and 2, characterized in that the proportion of slaked lime relative to the alcohol is between 15 g / dm ³ and 200 g / dm ³ .

4. Method according to one of claims 1 and 3, characterized in that the injection of carbon dioxide is stopped when said suspension has a pH below 9.

5. Process according to one of Claims 1 to 4, characterized in that the said injection of carbon dioxide into the said suspension takes place at a temperature of between 20 ° C and 70 ° C.

6. Process according to any one of claims 1 to 5, characterized in that it comprises, prior to said injection, sieving said suspension through a screen having a mesh size of between 20 and 250 μηι.

7. A method according to any one of claims 1 to 6, characterized in that, before the injection and / or at the beginning thereof, it further comprises a seeding of said suspension by calcium carbonate crystals.

8. Process according to claim 7, characterized in that the seed crystals are chosen from the group consisting of crystals of calcite, aragonite, vaterite and their mixtures.

9. Process according to any one of claims 1 to 8, characterized in that, before the injection and / or at the beginning thereof, it further comprises an addition of at least one crystalline growth inhibitor of CaCO3. to said alcoholic suspension.

10. Process according to any one of claims 1 to 9, characterized in that it comprises drying the precipitated calcium carbonate alcogel, so as to form an aeroge or xerogel of calcium carbonate.

1 1. Process according to Claim 10, characterized in that the drying of the alcogel takes place by subjecting it to liquid or supercritical CO ₂ .

12. Alcoge! calcium carbonate, as obtained by a process according to any one of claims 1 to 9.

13. Alkogel according to claim 12, characterized in that it consists of 1 to 6% by volume of precipitated calcium carbonate nanoparticles having a particle size substantially between 5 and 300 nm.

14. Alkogel according to claim 13, characterized in that said nanoparticles are agglomerates of crystallites of calcium carbonate.

15. Alkogel according to one of claims 13 and 14, characterized in that said nanoparticles are agglomerates of crystallites of calcite.

16. Aerocarbonate calcium carbonate, as obtained by a process according to one of claims 0 and 1 1.

17. Xerogel of calcium carbonate, as obtained by a process according to claim 10.

18. An aircraft according to claim 16 or xerogel according to claim 17, characterized in that it has a BET specific surface area of 6 to 450 m ² / g, preferably 5 to 200 m ² / g.

19. Airgel according to claim 16 or 18 or xerogel according to claim 17 or 18, characterized in that it has a bulk density of between 0.01 and 0.15 g / cm ³ , preferably between 0.02 and 0.06 g / cm ³ .

20. Aerogel or xerogel according to one of claims 16 to 19, characterized in that it consists of an airgel or xerogel of calcite, vaterite, aragonite or their mixtures.

21. Airgel according to claim 20, characterized in that it has a BET specific surface area of between 4 and 40 m ² / g, or between 100 and 250 m ² / g, a crystallite size of between 5 and 30 nm for vaterite and a particle size of between 60 and 600 nm or between 5 and 20 nm.

22. Airgel according to claim 20, characterized in that it has a BET surface area of between 100 and 450 m ² / g, a crystallite size of between 5 and 20 nm for calcite and for vaterite and a size of particles of about 10 nm when obtained in the presence of a growth inhibitor.

23. Xerogel according to claim 20, characterized in that it has a BET surface area of between 4 and 10 m ² / g, a crystallite size between 20 and 00 nm for calcite and between 15 and 30 nm for the vaterite and a particle size of between 100 and 500 nm.

2 ⁴ xerogel according to claim 20, characterized in that it has a BET specific surface of between 20 and 40 m ² / g, a crystallite size between 20 and 100 nm for calcite and between 15 and 30 nm for vaterite and a particle size of between 50 and 150 nm.