Method for preparing synthetic mineral particles

Abstract

The invention relates to a method for preparing synthetic mineral particles with formula (Al.sub.yM.sub.1-y).sub.2(Si.sub.xGe.sub.1-x).sub.2O.sub.5(OH).sub.4, wherein M designates at least one trivalent metal selected from the group made up of gallium and the rare earths, which comprises the following steps: preparing a gel which is a precursor of said synthetic mineral particles by a co-precipitation reaction of at least one salt of metal selected among aluminium and M with at least one silicon source selected from the group made up of potassium metasilicate, sodium metasilicate, potassium metagermanate and sodium metagermanate, the molar ratio of (Al.sub.yM.sub.1-y) to (Si.sub.xGe.sub.1-x) during the preparation of said precursor gel being equal to 1, at least one base being added during said co-precipitation reaction; and performing a solvothermal treatment of said precursor gel at a temperature of 250° C. to 600° C.

Claims

1. A method for preparing synthetic mineral particles of the following formula (I):
(Al.sub.yM.sub.1-y).sub.2(Si.sub.xGe.sub.1-x).sub.2O.sub.5(OH).sub.4  (I) wherein Al is aluminium, Si is silicon, M is at least one trivalent metal selected from the group consisting of gallium, iron, and the rare earth elements, y is a real number between 0 and 1, Ge is germanium, x is a real number between 0 and 1, O is oxygen, and H is hydrogen, said method comprising: providing a precursor gel of the synthetic mineral particles of formula (I) that is prepared by a co-precipitation reaction between: at least one salt of a metal selected from the group consisting of aluminium and M, at least one source of at least one chemical element selected from the group consisting of silicon and germanium, said source of the chemical element selected from the group consisting of silicon and germanium being selected from the group consisting of potassium metasilicate, sodium metasilicate, potassium metagermanate, and sodium metagermanate, the molar ratio of (Al.sub.yM.sub.1-y)/(Si.sub.xGe.sub.1-x) over the course of the preparation of the precursor gel being equal to 1, at least one base being added over the course of the co-precipitation reaction; and subjecting said precursor gel to continuous solvothermal treatment at a temperature between 250 and 600° C. for a period selected so as to allow synthetic mineral particles of formula (I) to be obtained, wherein said solvothermal treatment is carried out for a period of less than 6 hours in a continuous piston-flow reactor; wherein the concentration of the precursor gel is less than or equal to 0.1 mol/L; and wherein the synthetic mineral particles of formula (I) obtained by the method have an average size of between 20 nm and 600 nm.

2. The method according to claim 1, wherein said aluminium salt is selected from the group consisting of aluminium chloride and aluminium nitrate.

3. The method according to claim 1, wherein said solvothermal treatment is carried out using a constant-volume continuous reactor.

4. The method according to claim 1, wherein said solvothermal treatment is carried out in an aqueous medium.

5. The method according to claim 1, wherein said solvothermal treatment is carried out at a pressure greater than 1 MPa.

6. The method according to claim 1, wherein said solvothermal treatment is carried out at a pressure between 22 and 30 MPa.

7. The method according to claim 1, wherein the duration of said solvothermal treatment is greater than 10 seconds and less than 6 hours.

8. The method according to claim 1, wherein said precursor gel is washed at least once prior to said solvothermal treatment.

9. The method according to claim 1, wherein said base is selected from the group consisting of NaOH and KOH.

10. A method for preparing synthetic mineral particles of the following formula (I):
(Al.sub.yM.sub.1-y).sub.2(Si.sub.xGe.sub.1-x).sub.2O.sub.5(OH).sub.4  (I) wherein Al is aluminium, Si is silicon, M is at least one trivalent metal selected from the group consisting of gallium, iron, and the rare earth elements, y is a real number between 0 and 1, Ge is germanium, x is a real number between 0 and 1, O is oxygen, and H is hydrogen, said method comprising: providing a precursor gel of the synthetic mineral particles of formula (I) that is prepared by a co-precipitation reaction between: at least one salt of a metal selected from the group consisting of aluminium and M, at least one source of at least one chemical element selected from the group consisting of silicon and germanium, said source of the chemical element selected from the group consisting of silicon and germanium being selected from the group consisting of potassium metasilicate, sodium metasilicate, potassium metagermanate, and sodium metagermanate, the molar ratio of (Al.sub.yM.sub.1-y)/(Si.sub.xGe.sub.1-x) over the course of the preparation of the precursor gel being equal to 1, at least one base being added over the course of the co-precipitation reaction; and subjecting said precursor gel to continuous solvothermal treatment at a temperature between 250 and 600° C. for a period selected so as to allow synthetic mineral particles of formula (I) to be obtained, wherein said solvothermal treatment is carried out for a period of less than 6 hours in a continuous plug-flow reactor; wherein the concentration of the precursor gel is of less than or equal to 0.1 mol/L; and wherein the synthetic mineral particles of formula (I) obtained by the method have an average size of between 20 nm and 600 nm.

Description

(1) Other objectives, characteristics, and advantages of the invention will become apparent upon a reading of the following description of one of its preferred embodiments, which is provided by means of example and without limitation, and by reference to the attached drawings, in which:

(2) FIG. 1 is a schematic view of a device allowing the execution of a method according to the invention in which the solvothermal treatment is carried out continuously,

(3) FIG. 2 is an X-ray diffractogram of a composition comprising a synthetic kaolinite according to the invention (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) obtained following 24 h of hydrothermal treatment at 300° C.;

(4) FIG. 3 is an infrared spectrum of a composition comprising a synthetic kaolinite according to the invention (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) obtained following 24 h of hydrothermal treatment at 300° C.;

(5) FIG. 4 shows thermograms obtained by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of a composition comprising a synthetic kaolinite according to the invention (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) obtained following 24 h of hydrothermal treatment at 300° C.;

(6) FIG. 5 is an X-ray diffractogram of a composition comprising a synthetic kaolinite according to the invention (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) obtained following 96 h (4 days) of hydrothermal treatment at 300° C.;

(7) FIG. 6 is an X-ray diffractogram of a composition comprising a synthetic kaolinite according to the invention (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) obtained following continuous hydrothermal treatment at 400° C.;

A. GENERAL PROTOCOL FOR A PREPARATION METHOD ACCORDING TO THE INVENTION

(8) 1. Preparation of a Precursor Gel for a Synthetic Kaolinite of Formula (I) According to the Invention

(9) The precursor gel of the synthetic mineral particles of formula (I) may be prepared by a co-precipitation reaction having as a reagent at least one source of silicon and/or at least one source of germanium, selected from the group consisting of potassium metasilicate, potassium metagermanate, sodium metasilicate, and sodium metagermanate, and at least one aluminium salt and/or a metal salt of a metal M such as aluminium sulphate or aluminium nitrate.

(10) This co-precipitation reaction allows a precursor gel having the stoichiometry of a synthetic kaolinite corresponding to formula (I) according to the invention to be obtained.

(11) The precursor gel is prepared by a co-precipitation reaction starting from:

(12) 1. An aqueous solution of potassium or sodium metasilicate or an aqueous potassium or sodium metagermanate solution, or a mixture of such solutions in the molar ratio x/(1−x);

(13) 2. A basic solution, in particular of sodium hydroxide or potassium hydroxide, and

(14) 3. An aqueous solution in which at least one aluminium salt and/or one salt of a metal M, e.g., an aqueous aluminium nitrate solution (Al(NO.sub.3).sub.3) is dissolved.

(15) The molar ratio of (Al.sub.yM.sub.1-y)/(Si.sub.xGe.sub.1-x) over the course of the preparation of the precursor gel is substantially equal to 1.

(16) The basic solution may be prepared, e.g., by dissolving sodium hydroxide in water, or using any compound suitable to generate at least one of sodium hydroxide or potassium hydroxide by reacting with the solvent to which said compound is added or in which the solvothermal treatment will be carried out. Sodium or potassium hydroxide may be generated in part, e.g., by adding an alkaline alcoholate to the solvothermal treatment medium such as sodium ethylate or potassium ethylate (the hydrolysis of which allows the formation of sodium hydroxide or potassium hydroxide and ethanol).

(17) This precursor gel is prepared in accordance with the following protocol:

(18) 1. The aqueous metasilicate and/or metagermanate solution is mixed with the basic solution,

(19) 2. The solution in which the aluminium and/or M salt is dissolved is then added; the precursor gel forms instantaneously.

(20) The inventors' measurements show that the pH in the resultant solution comprising the precursor gel is between 4 and 5.5.

(21) The resultant solution comprising the precursor gel may (or may not) be agitated at room temperature (RT) (e.g., 22.5° C.) for 5 to 30 min.

(22) The resultant precursor gel is subjected to several cycles of washing and centrifugation.

(23) For example, the precursor gel may be recovered following centrifugation (e.g., between 3000 and 15,000 RPM for 5 to 60 min) and elimination of the supernatant (e.g., potassium or sodium nitrate) and washing in demineralised water (e.g., three successive washes and centrifugations).

(24) The washed precursor gel is then subjected to solvothermal treatment in the form obtained following the final centrifugation or possibly after having been dried (e.g., in a proofer or by lyophilisation). In particular, there is no need to dry the mineral particles contained in the precursor gel thus obtained. Drying may, however, be carried out if the solvothermal treatment is not carried out rapidly following the preparation of the precursor gel and it is preferred to preserve it in powder form.

(25) 2. Solvothermal Treatment of the Precursor Gel for a Synthetic Kaolinite of Formula (I) According to the Invention

(26) The precursor gel obtained above is subjected to solvothermal treatment at a temperature between 250 and 600° C., in particular a temperature between 280 and 500° C., and in particular a temperature between 280 and 450° C.

(27) In a first embodiment of a method according to the invention, the solvothermal treatment of the precursor gel is carried out in a closed reactor.

(28) To this end, the precursor gel obtained following precipitation is placed in a reactor/autoclave placed inside a furnace or proofer at a predetermined reaction temperature (between 250 and 600° C.) for the entire duration of the solvothermal treatment.

(29) If necessary, the liquid/solid ratio is adjusted in advance to a value between 2 and 80, in particular between 5 and 50 (the amount of liquid being expressed in cm3 and the amount of solid in grams, and indicating solely the amount of dry hydrogel).

(30) The composition resulting from the solvothermal treatment has a crystallinity observable by X-ray diffraction, which increases with the duration of the solvothermal treatment and reflected in the corresponding diffractograms by the rapid appearance of characteristic lines that narrow and intensify rapidly over the course of the treatment.

(31) Following this solvothermal treatment, a composition comprising synthetic kaolinite mineral particles according to formula (I) according to the invention is obtained in suspension in a solution, in particular an aqueous solution. At the end of this solvothermal treatment, the composition contained in the reactor may be recovered by centrifugation (between 3000 and 15,000 RPM, for 5 to 60 min), followed by elimination of the supernatant, or stored in the form of a suspension.

(32) The composition comprising mineral particles that is recovered following the final centrifugation may then be dried: in a proofer at a temperature between 60 and 130° C. for 1 to 24 h, or by lyophilisation, e.g., in a CHRIST ALPHA® 1-2 LD Plus freeze-dryer for 48 to 72 h, or by atomisation.

(33) In a second embodiment of a method according to the invention, the solvothermal treatment of the precursor gel is carried out continuously. Such a continuous solvothermal treatment has the advantage that the duration of the solvothermal treatment is further reduced; for example, a duration of less than 6 h, in particular less than 2 h, is sufficient.

(34) In a method according to the invention in which the solvothermal treatment is carried out continuously, a reactor 15 for continuous preparation of mineral particles of a compound according to the invention (as shown in FIG. 1) is used, comprising: A first duct portion 11 in which a first aqueous potassium or sodium metasilicate solution 20 or an aqueous potassium or sodium metagermanate solution, or a mixture of these solutions, as well as a basic solution, is added; A second duct portion 12 in which a second aqueous solution 21, into which at least one aluminium salt and/or one salt of at least one metal M is dissolved, is added; A third duct portion 13 arranged after the first duct portion 11 and the second duct portion 12 and extending up to an inlet 9 of a reaction enclosure 16, with the first duct portion 11 and the second duct portion 12 joining at a point 17 at which the third duct portion 13 begins, A reaction duct 14 extending from the inlet 9 in the reaction enclosure 16 and after the third duct portion 13.

(35) A peristaltic pump 18 allows for continuous pressurised supply of the first duct portion 11 with the first aqueous solution 20, which is contained in a reservoir 30 that is agitated. A second peristaltic pump 19 allows for continuous pressurised supply of the second duct portion 12 with the second aqueous solution 21, which is contained in a reservoir 31 that is agitated.

(36) In order to control the temperature within the reaction duct 14, the reaction enclosure 16 is a furnace comprising a heating mantle, which comprises ceramic resistors. The reaction duct 14 is generally serpentine-shaped having multiple loops inside the heating mantle until it exits the latter via an outlet 8, which constitutes the outlet of the reaction enclosure 16.

(37) The mixture within the third duct portion 13 is near room temperature. The third duct portion 13 is optional, and the point 17 and the inlet 9 can be combined. In the embodiment shown in FIG. 1, the third duct portion 13 has, e.g., a length between 10 and 20 cm.

(38) The total residence time in the device for preparing synthetic mineral particles by a method according to the invention is less than 30 min, in particular less than 15 min, or even less than 5 min or on the order of 1 min.

(39) Furthermore, it is possible to introduce other solutions, and, in particular, to adjust the amount of solvent to different levels of the device, e.g., using inlets 4, 5 located before the solvothermal treatment area, the inlet 4 being located before the point 17, the inlet 6 being located at the level of the solvothermal treatment area, the inlet 7 being located after the solvothermal treatment area and before the outlet for the resultant suspension.

(40) A pressure regulator 2 is arranged downstream of the reaction enclosure 16 and connected with a fifth duct portion 10 that extends from the outlet 8 of the reaction duct 14 and the reaction enclosure 16 up to a receptacle 25 in which a suspension comprising the mineral particles obtained is recovered.

(41) By closing a valve 32 interposed on the fifth duct portion 10, the resultant suspension obtained at the outlet 8 of the reaction duct 14 can be circulated in a branched circuit 33, which allows this suspension to pass through a porous fritted filter 34 suited to retain the particles and allow for their recovery. The porous fritted filter 34 is plunged into an ice bath 35, allowing the suspension leaving the reactor to be cooled. In this case, valves 36 and 37 arranged on the branched circuit 33 are open. The porous fritted filter 34 is selected so as to retain the synthesised mineral particles by separating them from the liquid medium in which they are transported. The filter is made, e.g., of 316 L stainless steel and has a pore size of 50 μm. When the porous fritted filter 34 is clogged with mineral particles, it is sufficient to open the valve 32 and close the valves 36 and 37 in order to directly recover the suspension in the recipient 25, this suspension being cooled by passing through the ice bath 35, then washed and centrifuged several times in order to recover the mineral particles that may then be dried, e.g., in a proofer. In another variant (not shown), it is certainly also possible to provide several fritted filters in parallel, which allows the resultant suspension to be directed at the outlet of the reaction duct 14 towards another fritted filter once the previous filter has been clogged by the mineral particles.

(42) In one variant, if a solution comprising the precursor gel is first prepared, a single duct portion replaces the first duct portion 11 and the second duct portion 12.

(43) In any case, it is important to control the dilution of the precursor gel that is introduced into each duct portion and in the reaction duct 14 so as to allow for continuous circulation of the reaction medium in the reaction duct 14 and in all of the ducts supplying said precursor gel composition to the inlet 9 of the reaction enclosure 16. The concentration of the precursor gel in the precursor gel composition introduced into the inlet of the reaction enclosure 16 is advantageously between 10.sup.−3 mol/L and several mol/L, e.g., on the order of 0.01 mol/L. It should be noted that this concentration is much lower than the concentrations used in prior-art methods for preparing synthetic mineral particles such as phyllosilicates.

(44) The solvothermal treatment carried out in the reaction duct 14 is a solvothermal treatment that may, in particular, be carried out under supercritical or subcritical conditions, in particular homogeneous subcritical conditions. Thus, the temperature and pressure at which the solvothermal treatment is carried out can be selected such that the precursor gel composition introduced into the inlet of the reactor, and in particular the solvent(s) it comprises, is/are in supercritical conditions or homogeneous subcritical conditions, i.e., above the liquid-gas equilibrium curve of the solvent, and such that the solvent is in the liquid state and not in the form of a liquid-gas mixture or in purely gaseous form.

(45) Following this solvothermal treatment, a suspension comprising mineral particles in solution, in particular aqueous solution, is obtained. Upon completion of this solvothermal treatment, the suspension obtained is recovered by filtration, e.g., using a fritted ceramic filter, or by centrifugation (between 3000 and 15,000 RPM for 5 to 60 min), followed by elimination of the supernatant.

(46) The recovered composition, comprising synthetic mineral particles of formula (I), may be washed with water, in particular with distilled or reverse-osmosis-purified water, e.g., by carrying out one or two cycles of washing/centrifugation.

(47) The composition comprising synthetic mineral particles of formula (I) that is recovered following the final centrifugation may then be dried: in a proofer at a temperature between 60 and 130° C. for 1 to 24 h, or by lyophilisation, e.g., in a CHRIST ALPHA® 1-2 LD Plus freeze-dryer for 48 to 72 h, by microwave irradiation, by atomization, or by any other powder drying technique.

(48) The inventers have thus found that not only is an extremely short solvothermal treatment time (less than 1 min) in supercritical conditions sufficient to allow conversion of the initial gel into a crystallised, thermally stable material, but also that the synthetic mineral particles obtained have a crystallinity comparable to that of natural kaolinites.

(49) The synthetic mineral particles of formula (I) contained in a composition obtained by a method according to the invention have remarkable properties in terms of purity, crystallinity, and thermal stability despite a solvothermal treatment time that is significantly reduced compared to the times that are normally necessary in known-art methods.

B. ANALYSIS AND STRUCTURAL CHARACTERISATION

(50) 1. X-Ray Diffraction Analyses

(51) FIGS. 2 and 5 show X-ray diffractograms, each of which shows the relative signal intensity (number of hits per second) as a function of the interplanar distance in angstrom.

(52) In X-ray diffraction, a composition according to the invention has at least one diffraction line characteristic of a plane (001) situated at a distance of 7.00 to 7.30 Å. Such a diffraction line is characteristic of kaolinites.

(53) FIGS. 2 and 5 respectively show the results of X-ray diffraction analyses carried out on: A composition comprising a synthetic kaolinite Al.sub.2Si.sub.2O.sub.5(OH).sub.4 obtained following 24 h of hydrothermal treatment at 300° C. (FIG. 2), A composition comprising a synthetic kaolinite Al.sub.2Si.sub.2O.sub.5(OH).sub.4 obtained following 96 h of hydrothermal treatment at 300° C. (FIG. 5), A composition comprising a synthetic kaolinite Al.sub.2Si.sub.2O.sub.5(OH).sub.4 obtained following 30 min of continuous hydrothermal treatment at 400° C. (FIG. 6).

(54) The X-ray diffractogram shown in FIG. 2 was recorded on a Panalytical MPDPro® diffractometer marketed by Panalytical® (Netherlands). This is a multi-configuration theta/theta diffractometer (transmission, reflection, variable temperature) equipped with a rapid linear detector. The Bragg relationship that gives the structural equidistance is: d.sub.hkl=0.7703/sin θ (using a copper anticathode).

(55) The X-ray diffractogram shown in FIG. 5 was recorded on a CPS 120 device marketed by INEL (Artenay, France). This is a diffractometer with a curve detector that allows for real-time detection over an angular range of 120°. The acceleration voltage used is 40 kV, and the intensity is 25 mA. The Bragg relationship that gives the structural equidistance is: d.sub.hkl=0.89449/sin θ (using a cobalt anticathode).

(56) 2. Infrared Analysis

(57) FIG. 3 shows an infrared spectrum showing signal intensity as a function of wavelength expressed in cm.sup.−1. FIG. 3 is the near infrared spectrum (curve 50) of a composition comprising a synthetic kaolinite according to the invention (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) obtained following 24 h of hydrothermal treatment at 300° C.

(58) The spectra obtained show four vibration bands between 3620 and 3700 cm.sup.−1, which are representative of elongation vibrations of the hydroxyl groups (—OH) of a kaolinite.

(59) These spectra were acquired with a NICOLET 6700-FTIR spectrometer over a range of 9000 to 4000 cm.sup.−1.

(60) 3. Thermal Analyses

(61) FIG. 4 shows curves obtained by thermogravimetric analysis (TGA) (curve 55) and differential thermal analysis (DTA) (curve 56) of a composition comprising a synthetic kaolinite according to the invention (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) obtained following 24 h of hydrothermal treatment at 300° C.

(62) In FIG. 4, curve 56 (differential thermal analysis) represents the heat released or absorbed by the sample of the composition analysed (in mW on the ordinate axis on the left side of the thermogram) as a function of temperature (from 0 to 1200° C.). In FIG. 4, curve 55 (thermogravimetric analysis, dotted in FIG. 4) represents the variation in the mass of the sample of the composition analysed (in % on the ordinate axis on the right side of the thermogram) as a function of temperature (from 0 to 1200° C.).

(63) The thermograms obtained are characteristic of kaolinites, with dehydroxylation starting from 400° C., and recrystallisation in spinel structure after 960° C. These transformation temperatures are slightly lower than those of a natural kaolinite, which is explained by the smaller size of the particles obtained.

(64) The DTA and TGA analyses were carried out on a Diamond TG/TDA® thermobalance marketed by PERKIN ELMER® (USA) over a temperature range of 30 to 1000° C., in air, and with a heating rate of 10° C./min.

(65) 4. Microscopic Observations and Assessment of Particle Granulometry

(66) Taking into account the great fineness of the powders that may constitute the compositions according to the invention, the size and granulometric distribution of the synthetic mineral particles of which they consist were assessed by scanning and field-effect electron microscopy.

(67) It was found that the average size of the elementary particles varies between 20 and 600 nm, in particular between 20 and 500 nm.

(68) The following examples illustrate the method of preparation according to the invention and the structure characteristics of the synthetic mineral particles thus obtained.

Example 1—Preparation of a Composition Comprising Synthetic Mineral Particles According to the Invention

(69) An aluminium nitrate solution is prepared with 37.51 g (0.1 mol) aluminium nitrate nonahydrate in 200 ml pure water.

(70) A potassium metasilicate solution is also prepared from 29.67 g of an aqueous potassium metasilicate solution (K.sub.2SiO.sub.3) having a dry extract of 52% (i.e., 0.1 mol potassium metasilicate), 100 ml of potash (KOH) at 1M, and 200 ml of pure water.

(71) The first aluminium nitrate solution is added with stirring to the potassium metasilicate solution, and a white precipitate forms instantaneously.

(72) The suspension obtained is stirred for 5 min. Then, three cycles of washing with distilled water and centrifugation at 8000 RPM for 10 min are carried out for each new centrifugation. These successive washes with elimination of the supernatant after each centrifugation allow for the elimination of the potassium nitrate formed over the course of the precipitation reaction of the precursor gel.

(73) Then, the precursor gel, placed in a closed titanium reactor placed in a furnace, is subjected to hydrothermal treatment at a temperature of 300° C. for 24 h under the saturation vapour pressure of the water in the reactor.

(74) After cooling to RT, the reactor is opened, and the suspension obtained is centrifuged. After centrifugation, a composition comprising particles of the compound having the formula (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) is recovered.

(75) The particle composition recovered following centrifugation is dried in a proofer (120° C., 12 h), then ground in a mortar. The composition obtained is in the form of a white powder.

(76) The X-ray diffractogram of the composition of particles of the compound of the formula (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) thus obtained is shown in FIG. 2. The X-ray diffractogram of this composition shows the following characteristic diffraction lines: one plane (001) situated at a distance of 7.15 Å (line 40); one plane (020) situated at a distance of 4.46 Å (line 41); one plane (110) situated at a distance of 4.37 Å (line 42); one plane (111) situated at a distance of 4.16 Å (line 43); one plane (021) situated at a distance of 3.80 Å (line 44); one plane (002) situated at a distance of 3.56 Å (line 45); one plane (130) and one plane (201), situated at a distance of 2.56 Å (line 46); one plane (131) and one plane (200), situated at a distance of 2.50 Å (line 47); one plane (202) and one plane (131), situated at a distance of 2.33 Å (line 48); one plane (060), one plane (331), and one plane (331), situated at a distance of 1.49 Å (line 49).

(77) The infrared spectrum of the synthetic kaolinite composition obtained is shown in FIG. 3 (curve 50). It shows four vibration bands at 3620 cm.sup.−1, 3651 cm.sup.−1, 3667 cm.sup.−1, and 3693 cm.sup.−1, which are representative of elongation vibrations of the hydroxyl groups (—OH) of the synthetic kaolinite.

(78) The curves obtained by TGA-DTA of the composition of particles of the compound of the formula (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) thus obtained are shown in FIG. 4. Such a thermogram is characteristic of kaolinites, with dehydroxylation starting from 400° C., and recrystallisation in spinel structure after 960° C. These transformation temperatures are slightly lower than those of a natural kaolinite, which is explained by the smaller size of the particles obtained.

(79) One advantage of the particles obtained is that this transformation to a spinel structure around 975° C. (peak maximum), i.e., at a lower temperature than natural kaolinites, should, for example, allow for the manufacture of ceramics at a lower temperature, thus reducing the energy cost of the production of such ceramics.

Example 2—Preparation of a Composition Comprising Synthetic Mineral Particles According to the Invention

(80) An aluminium nitrate solution is prepared with 37.51 g (0.1 mol) aluminium nitrate nonahydrate in 200 ml pure water.

(81) A solution of sodium metasilicate is also prepared with 21.21 g sodium metasilicate pentahydrate Na.sub.2SiO.sub.3, 5H.sub.2O (0.1 mol) in 100 ml soda (1M) and 200 ml pure water.

(82) The first aluminium nitrate solution is added with stirring to the sodium metasilicate solution, and a white precipitate forms instantaneously.

(83) The suspension obtained is stirred for 5 min. Then, three cycles of washing with distilled water and centrifugation at 8000 RPM for 10 min are carried out for each centrifugation. These successive washes with elimination of the supernatant after each centrifugation allow for the elimination of the sodium nitrate formed over the course of the precipitation reaction of the precursor gel.

(84) Then, the precursor gel, placed in a closed titanium reactor placed in a furnace, is subjected to hydrothermal treatment at a temperature of 300° C. for 96 h under the saturation vapour pressure of the water in the reactor.

(85) After cooling to RT, the reactor is opened, and the suspension obtained is centrifuged. After centrifugation, a composition comprising synthetic mineral particles having the formula (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) is recovered.

(86) The particle composition recovered following centrifugation is dried in a proofer (120° C., 12 h), then ground in a mortar. The composition obtained is in the form of a white powder.

(87) The X-ray diffractogram of the composition of particles of the compound of the formula (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) thus obtained is shown in FIG. 5. The X-ray diffractogram of this composition shows the following characteristic diffraction lines: one plane (001) situated at a distance of 7.17 Å (line 60); one plane (020) situated at a distance of 4.46 Å (line 61); one plane (110) situated at a distance of 4.38 Å (line 62); one plane (111) situated at a distance of 4.18 Å (line 63); one plane (021) situated at a distance of 3.78 Å (line 64); one plane (002) situated at a distance of 3.58 Å (line 65); one plane (130) and one plane (201), situated at a distance of 2.57 Å (line 66); one plane (131) and one plane (200), situated at a distance of 2.50 Å (line 67); one plane (202) and one plane (131), situated at a distance of 2.34 Å (line 68); one plane (060), one plane (331), and one plane (331), situated at a distance of 1.49 Å (line 69).

Example 3—Continuous Preparation of a Composition Comprising Synthetic Mineral Particles According to the Invention

(88) An aluminium nitrate solution is prepared with 37.51 g (0.1 mol) aluminium nitrate nonahydrate in 200 ml pure water.

(89) A solution of sodium metasilicate is also prepared with 21.21 g sodium metasilicate pentahydrate Na.sub.2SiO.sub.3, 5H.sub.2O (0.1 mol) in 100 ml soda (1M) and 200 ml pure water, to which 100 ml soda (1M) is added.

(90) The first aluminium nitrate solution is added with stirring to the sodium metasilicate solution, and a white precipitate forms instantaneously.

(91) The suspension obtained is stirred for 5 min. Then, three cycles of washing with distilled water and centrifugation at 8000 RPM for 10 min are carried out for each centrifugation. These successive washes with elimination of the supernatant after each centrifugation allow for the elimination of the sodium nitrate formed over the course of the precipitation reaction of the precursor gel.

(92) The diluted precursor gel is then placed in 300 ml pure water in the reservoir 30 (cf. FIG. 1). Pure water that also allows for adjustment of the dilution of the precursor gel in the duct portion 13 is disposed in the reservoir 31.

(93) The peristaltic pumps 18, 19 allow the two solutions to be separately passed through steel ducts having an external diameter of ⅛ inch (3.175 mm) and an internal diameter of 1.57 mm. The temperature in the enclosure 16 is 400° C., and the pressure in the reaction duct 14 is maintained (using the pressure regulator 2) above 22.1 MPa (between 25 and 27 MPa), such that the reaction medium circulating in the reaction duct 14 in the enclosure 16 is in conditions greater than the critical point of water (374° C., 221 bar).

(94) Thus, the precursor gel undergoes hydrothermal treatment in the reaction enclosure 16, allowing the precursor gel to be transformed into a suspension of synthetic mineral particles of formula (Al.sub.2Si.sub.2O.sub.5(OH).sub.4). The residence time in the reaction duct 14 between the inlet 9 and the outlet 8 is 30 min.

(95) After cooling, the suspension exiting the outlet 8 of the reactor 15 is a colloidal suspension of synthetic mineral particles of the formula (Al.sub.2Si.sub.2O.sub.5(OH).sub.4). It has the appearance of a milky white composition that decants in several tens of minutes. This suspension is subjected to a cycle of centrifugation (10 min at 8000 RPM). After centrifugation, on the one hand, a composition comprising synthetic mineral particles of the formula (Al.sub.2Si.sub.2O.sub.5(OH).sub.4), and, on the other, an aqueous supernatant solution, are obtained.

(96) The particle composition recovered following centrifugation is dried in a proofer (120° C., 12 h), then ground in a mortar. The composition obtained is in the form of a white powder.

(97) The X-ray diffractogram of the composition of particles of the compound of the formula (Al.sub.2Si.sub.2O.sub.5(OH).sub.4) thus obtained is shown in FIG. 6. The X-ray diffractogram of this composition shows the following characteristic diffraction lines: one plane (001) situated at a distance of 7.09 Å; one plane (020) situated at a distance of 4.44 Å; one plane (111) situated at a distance of 4.13 Å; one plane (002) situated at a distance of 3.56 Å; one plane (130) and one plane (201), situated at a distance of 2.55 Å; one plane (131) and one plane (200), situated at a distance of 2.49 Å; one plane (202) and one plane (131), situated at a distance of 2.33 Å; one plane (060), one plane (331), and one plane (331), situated at a distance of 1.49 Å.

(98) Numerous embodiments of the invention are possible. In particular, it is possible to prepare other compounds than those exemplified and corresponding to formula (I), with the silicon being replaced in whole or in part by germanium, or using iron or gallium instead of aluminium.