Process for the continuous preparation of phyllomineral synthetic particles

Abstract

A process for preparing phyllomineral synthetic particles formed from constituent chemical elements in stoichiometric proportions including at least one chemical element selected from the group formed from silicon and germanium, and at least one chemical element selected from the group formed from divalent metals and trivalent metals, by a continuous solvothermal treatment at a pressure above 1 MPa and at a temperature between 100 C. and 600 C., by making the reaction medium circulate continuously in a solvothermal treatment zone of a continuous reactor (15) with a residence time of the reaction medium in the solvothermal treatment zone that is suitable for continuously obtaining, at the outlet of the solvothermal treatment zone, a suspension including the phyllomineral synthetic particles.

Claims

1. A process for preparing phyllomineral synthetic particles formed from chemical elements, named constituent chemical elements, in predetermined proportions, named stoichiometric proportions, said constituent chemical elements comprising at least one chemical element selected from the group consisting of silicon and germanium, and at least one chemical element selected from the group consisting of divalent metals and trivalent metals, via a solvothermal treatment of a reaction medium comprising a liquid medium and containing said stoichiometric proportions of said constituent chemical elements of said phyllomineral synthetic particles, said phyllomineral synthetic particles belonging to the group of non-swelling phyllosilicates, in which: said solvothermal treatment is performed continuously at a pressure greater than 1 MPa and at a temperature of between 100 C. and 600 C., the reaction medium is continuously circulated in a solvothermal treatment zone of a continuous reactor with a residence time of the reaction medium in said solvothermal treatment zone that is suitable for continuously obtaining, at the outlet of said solvothermal treatment zone, a suspension comprising said phyllomineral synthetic particles.

2. The process according to claim 1, wherein a constant-volume continuous reactor is used.

3. The process according to claim 1, wherein the solvothermal treatment zone of the reactor comprises at least one pipe, named the reaction pipe, in which the reaction medium continuously circulates between at least one inlet suitable for allowing the continuous introduction of at least one starting composition and at least one outlet via which the suspension comprising said phyllomineral synthetic particles is continuously recovered.

4. The process according to claim 3, wherein the temperature is controlled by controlling the temperature of the reaction pipe.

5. The process according to claim 1, wherein said solvothermal treatment is performed at a pressure of between 2 MPa and 50 MPa.

6. The process according to claim 1, wherein the duration of the continuous solvothermal treatment is adjusted by controlling the residence time of the reaction medium in the solvothermal treatment zone, in which said reaction medium is subjected to the temperature and pressure of the solvothermal treatment.

7. The process according to claim 1, wherein the reaction medium is continuously circulated in the reactor so that said reaction medium has a residence time in the solvothermal treatment zone of less than 10 minutes.

8. The process according to claim 1, wherein said reaction medium is introduced into the solvothermal treatment zone with a flow rate chosen to obtain the appropriate residence time.

9. The process according to claim 1, wherein said solvothermal treatment is performed under temperature and pressure conditions such that said reaction medium is under supercritical conditions.

10. The process according to claim 1, wherein the reaction medium is prepared continuously from at least one first starting composition comprising at least one compound, named mineral compound, chosen from silicates and/or germanates, solid solutions thereof and mixtures thereof, and of at least one second starting composition comprising at least one metal salt of at least one metal M selected from the group consisting of divalent metals and trivalent metals, said first and second compositions being placed in contact continuously upstream of at least one inlet of said solvothermal treatment zone.

11. The process according to claim 10, wherein said first starting composition is introduced continuously into at least one first pipe portion and said second starting composition is introduced continuously into at least one second pipe portion, each of the first pipe portion and of the second pipe portion being connected together upstream of the solvothermal treatment zone to allow the continuous placing in contact of these two compositions.

12. The process according to claim 11, wherein the first pipe portion and the second pipe portion join together upstream of at least one inlet of the solvothermal treatment zone, in a third pipe portion connecting together each of the first and second pipe portions and the inlet of the solvothermal treatment zone.

13. The process according to claim 1, wherein, for the preparation of phyllosilicate synthetic particles belonging to the group consisting of lamellar silicates, lamellar germanates, lamellar germanosilicates and mixtures thereof, wherein a precursor silico/germano-metallic hydrogel is used as precursor gel, and said solvothermal treatment is performed in the form of a continuous hydrothermal treatment of this precursor silico/germano-metallic hydrogel.

14. The process according to claim 13, wherein use is made, as precursor gel, of a precursor silico/germano-metallic hydrogel comprising: 4 silicon and/or germanium atoms according to the following chemical formula: 4 (Si.sub.xGe.sub.1-x), x being a real number of the interval [0; 1], 3 atoms of at least one metal M, M denoting at least one divalent metal having the formula Mg.sub.y(1)Co.sub.y(2)Zn.sub.y(3)Cu.sub.y(4)Mn.sub.y(5)Fe.sub.y(6)Ni.sub.y(7)Cr.sub.y(8) in which each y(i) represents a real number of the interval [0; 1], and such that $\underset{i=1}{\overset{8}{.Math.}}y\left(i\right)=1,$ (10) oxygen atoms ((10) O), being a real number of the interval [0; 10], (2+) hydroxyl groups ((2+) (OH)), being a real number of the interval [0; 10], and said hydrothermal treatment is performed so as to obtain continuously a suspension comprising phyllosilicate particles having the chemical formula (II) below:
(Si.sub.xGe.sub.1-x).sub.4M.sub.3O.sub.10(OH).sub.2(II) in which: Si denotes silicon, Ge denotes germanium, M denotes at least one divalent metal having the formula Mg.sub.y(1)Co.sub.y(2)Zn.sub.y(3)Cu.sub.y(4)Mn.sub.y(5)Fe.sub.y(6)Ni.sub.y(7)Cr.sub.y(8); each y(i) representing a real number of the interval [0; 1], and such that $\underset{i=1}{\overset{8}{.Math.}}y\left(i\right)=1,$ x is a real number of the interval [0; 1].

15. A process for preparing phyllomineral synthetic particles formed from chemical elements, named constituent chemical elements, in predetermined proportions, named stoichiometric proportions, said constituent chemical elements comprising at least one chemical element selected from the group consisting of silicon and germanium, and at least one chemical element selected from the group consisting of divalent metals and trivalent metals, via a solvothermal treatment of a reaction medium comprising a liquid medium and containing said stoichiometric proportions of said constituent chemical elements of said phyllomineral synthetic particles, said phyllomineral synthetic particles belonging to the group of non-swelling phyllosilicates, in which: said solvothermal treatment is performed continuously at a pressure greater than 1 MPa and at a temperature of between 100 C. and 600 C., the reaction medium is continuously circulated in a solvothermal treatment zone of a continuous reactor with a residence time of the reaction medium in said solvothermal treatment zone that is suitable for continuously obtaining, at the outlet of said solvothermal treatment zone, a suspension comprising said phyllomineral synthetic particles, wherein the concentration relative to the volume of the liquid medium of said constituent chemical elements of said phyllomineral synthetic particles introduced into the inlet of the solvothermal treatment zone of the reactor is between 10.sup.3 mol/L and 1 mol/L.

Description

(1) Other aims, characteristics and advantages of the invention will emerge on reading the following description of one of the preferential embodiments thereof, which is given as a non-limiting example, and which refers to the attached figures, in which:

(2) FIG. 1 is a schematic view of a device for preparing phyllomineral synthetic particles used in a process according to the invention,

(3) FIGS. 2, 5, 7, 8 and 9 represent x-ray (XR) diffractograms of phyllomineral particles obtained via the examples given below with a process according to the invention,

(4) FIGS. 3, 4 and 6 represent Fourier-transform infrared absorption spectra of phyllomineral synthetic particles obtained via the examples given below with a process according to the invention,

(5) FIGS. 10 and 11 are field-effect scanning electron microscopy photographs of phyllomineral synthetic particles obtained in an example given below with a process according to the invention.

(6) AGeneral Protocol for Preparing Phyllomineral Synthetic Particles According to the Invention

(7) 1/Device for Preparing Phyllomineral Synthetic Particles

(8) In a process according to the invention, a reactor 15 for continuously preparing phyllomineral synthetic particles (as illustrated in FIG. 1) is used, comprising: a first pipe portion 11 into which is introduced a first aqueous solution 20 comprising at least one mineral compound chosen from silicates, germanates, solid solutions thereof and mixtures thereof, a second pipe portion 12 into which is introduced a second aqueous solution 21 comprising at least one metal salt of at least one metal M, a third pipe portion 13 arranged after the first pipe portion 11 and the second pipe portion 12 and continuing up to an inlet 9 of a reaction chamber 16, the first pipe portion 11 and the second pipe portion 12 joining together at a point 17 from which the third pipe portion 13 begins, a reaction pipe 14 extending from the inlet 9 into the reaction chamber 16, and after the third pipe portion 13.

(9) A peristaltic pump 18 allows the first pipe portion 11 to be continuously fed under pressure with the first aqueous solution 20 contained in a stirred tank 30. A second peristaltic pump 19 allows the second pipe portion 12 to be continuously fed under pressure with the second aqueous solution 21 contained in a stirred tank 31.

(10) For the purposes of controlling the temperature inside the reaction pipe 14, the reaction chamber 16 is an oven comprising a heating sleeve comprising resistors made of ceramic material. The reaction pipe 14 is in the general form of a coil wound into multiple spires inside the heating sleeve, until it exits therefrom via an outlet 8 constituting the outlet of the reaction chamber 16.

(11) A co-precipitation reaction of a precursor gel of phyllomineral particles takes place in the third pipe portion 13, upstream of the inlet 9, i.e. before the solvothermal treatment. The temperature of the precursor gel composition in the third pipe portion 13 is close to room temperature. The length of the third pipe portion 13 may be surprisingly short, of the order of a few centimetres, and is, for example, between 10 cm and 20 cm. In the examples, this length is about 15 cm. The residence time in the third pipe portion 13 (i.e. between point 17 and the inlet 9 of the reaction chamber 16) is also very short and may be less than 5 minutes, especially less than 1 minute or even less than 30 seconds. The total time for preparing phyllomineral synthetic particles via a process according to the invention is thus less than 15 minutes, and in particular less than 10 minutes or even less than 5 minutes or about 1 minute.

(12) In addition, it is possible to introduce other solutions such as particle grafting or functionalisation solutions or to add a solvent in different points of the device, for example at inlets 4, 5 located before the solvothermal treatment zone, at an inlet 6 located in the solvothermal treatment zone or alternatively at an inlet 7 located after the solvothermal treatment zone and before the outlet for the suspension obtained.

(13) A pressure regulator 2 is placed downstream of the reaction chamber 16 in connection with a fifth pipe portion 10 extending from the outlet 8 of the reaction pipe 14 and of the reaction chamber 16 up to a container 25 in which is recovered a suspension comprising the phyllomineral synthetic particles obtained.

(14) Closure of a valve 32 positioned on the fifth pipe portion 10 makes it possible to circulate the suspension obtained at the outlet 8 of the reaction pipe 14 in a branch circuit 33 for passing this suspension through a porous sinter 34 suitable for retaining the particles and allowing their recovery. The porous sinter 34 is immersed in an ice tank 35 for cooling the suspension exiting the reactor. In this case, valves 36 and 37 located on the branch circuit 33 are opened. The porous sinter 34 is chosen so as to retain the phyllomineral particles synthesised by separating them from the liquid medium which transports them. The sinter is made, for example, of 316L stainless steel, with a porosity size of 50 m. When the porous sinter 34 is clogged with phyllomineral particles, it suffices to open valve 32 and to close valves 36 and 37 in order directly to recover the suspension in container 25, this suspension being cooled by passing through the ice tank 35, and then washed and centrifuged several times to recover the phyllomineral particles, which may then be dried, for example in an oven. In another variant (not shown), it is of course also possible to provide several sinters in parallel, which makes it possible to direct the suspension obtained at the outlet of the reaction pipe 14 to another sinter as soon as the preceding one is clogged with phyllomineral particles.

(15) 2/Preparation of a Silico/Germano-Metallic Precursor Gel

(16) The silico/germano-metallic gel may be prepared via a co-precipitation reaction involving, as reagent, at least one mineral compound comprising silicon and/or germanium, at least one dicarboxylate salt of formula M(R.sub.1COO).sub.2 (M denoting at least one divalent or trivalent metal and R.sub.1 being chosen from H and alkyl groups comprising less than 5 carbon atoms) in the presence of at least one carboxylate salt of formula R.sub.2COOM in which M denotes a metal chosen from the group formed by Na and K, and R.sub.2 is chosen from H and alkyl groups comprising less than 5 carbon atoms.

(17) This co-precipitation reaction makes it possible to obtain a hydrated silico/germano-metallic hydrogel having the stoichiometry of talc (4 Si/Ge per 3 M, M having the formula Mg.sub.y(1)Co.sub.y(2)Zn.sub.y(3)Cu.sub.y(4)Mn.sub.y(5)Fe.sub.y(6)Ni.sub.y(7)Cr.sub.y(8); each y(i) representing a real number of the interval [0; 1], and such that

(18) $\underset{i=1}{\overset{8}{.Math.}}y\left(i\right)=1,$

(19) The silico/germano-metallic gel is prepared via a co-precipitation reaction performed using:

(20) 1. an aqueous solution of sodium metasilicate pentahydrate or an aqueous solution of sodium metagermanate, or a mixture of these two solutions in molar proportions x/(1-x),

(21) 2. a solution of dicarboxylate salt(s), prepared with one or more dicarboxylate salts of formula M(R.sub.1COO).sub.2 diluted in a carboxylic acid, such as acetic acid, and

(22) 3. a solution of carboxylate salt(s), prepared with one or more carboxylate salts of formula R.sub.2COOM diluted in distilled water.

(23) The preparation of this silico/germano-metallic hydrogel is performed according to the following protocol:

(24) 1. the solutions of sodium metasilicate and/or metagermanate and of carboxylate salt(s) of formula R.sub.2COOM are mixed,

(25) 2. the solution of dicarboxylate salt(s) of formula M(R.sub.1COO).sub.2 is rapidly added thereto; the co-precipitation hydrogel forms instantaneously.

(26) In addition, it is possible to subject the medium for preparing said hydrogel to ultrasound.

(27) On conclusion of this precipitation, a silico/germano-metallic hydrogel is obtained, comprising: 4 (Si.sub.xGe.sub.1-x), 3 atoms of at least one metal M, M denoting at least one divalent metal having the formula Mg.sub.y(1)Co.sub.y(2)Zn.sub.y(3)Cu.sub.y(4)Mn.sub.y(5)Fe.sub.y(6)Ni.sub.y(7)Cr.sub.y(8) in which each y(i) represents a real number of the interval [0; 1], and such that

(28) $\underset{i=1}{\overset{8}{.Math.}}y\left(i\right)=1,$ (10) oxygen atoms ((10) O), being a real number of the interval [0; 10], (2+) hydroxyl groups ((2+) (OH)), being a real number of the interval [0; 10],

(29) in an aqueous solution of carboxylate salt(s), said hydrogel being highly hydrated (water molecules being bound to the hydrogel particles without this being water of constitution) and having a more or less gelatinous consistency.

(30) The hydrogel may also be recovered after centrifugation (for example between 3000 and 15 000 rpm, for 5 to 60 minutes) and removal of the supernatant (solution of carboxylate salt(s)), optionally washing with demineralised water (for example two successive washes and centrifugations) followed by drying, for example in an oven (60 C., 2 days), by lyophilisation, drying by atomisation or alternatively drying under microwave irradiation. The silico/germano-metallic particles of formula (I) below:
4 (Si.sub.xGe.sub.1-x) 3 M ((10) O) ((2+) (OH))
may thus be stored in the form of a powder (in the presence or absence of the carboxylate salt(s) depending on whether or not washing with water has been performed) for the purpose of an optional subsequent hydrothermal treatment.

(31) The precursor gel may be prepared continuously as envisaged in the phyllomineral particle preparation device described above, or, on the contrary, beforehand, i.e. outside the phyllomineral particle preparation device described above, and may then be introduced continuously, depending on the need, directly into the third pipe portion 13 or directly into the inlet 9 of the reaction pipe 14.

(32) In each case, it is important to control the dilution of the precursor gel introduced into each pipe portion and in the reaction pipe 14 so as to allow continuous circulation of the reaction medium in the reaction pipe 14, and in all of the pipes for conveying said precursor gel composition up to the inlet 9 of the reaction chamber 16. The concentration of precursor hydrogel in said precursor gel composition introduced into the inlet of the reaction chamber 16 is advantageously between 10.sup.3 mol/L and several mol/L, for example about 0.01 mol/L. It should be noted that this concentration is much lower than the concentrations used in the processes for preparing phyllomineral synthetic particles such as phyllosilicates of the prior art.

(33) 3/Hydrothermal Treatment of Said Silico/Germano-metallic Hydrogel

(34) The abovementioned optionally dried precursor hydrogel of formula (I), as obtained previously, is subjected to a hydrothermal treatment in the reaction pipe 14 when it enters the reaction chamber 16.

(35) The hydrothermal treatment is a solvothermal treatment which may be performed in particular under supercritical or subcritical conditions, and in particular homogeneous subcritical conditions. Thus, the temperature and pressure at which this solvothermal treatment is performed may be chosen so that the precursor gel composition introduced into the reactor inlet, and in particular the solvent(s) it comprises, is (are) under supercritical conditions or under homogeneous subcritical conditions, i.e. above the liquid-gas equilibrium curve of the solvent, and so that the solvent is in liquid form and not in the form of a liquid-gas mixture, or of gas alone.

(36) On conclusion of this hydrothermal treatment, a suspension is obtained comprising phyllosilicate mineral particles in an aqueous solution of carboxylate salt(s). At the end of this hydrothermal treatment, the suspension obtained is recovered by filtration, for example using a ceramic sinter, or alternatively by centrifugation (between 3000 and 15 000 rpm, for 5 to 60 minutes), followed by removal of the supernatant. The supernatant solution contains one or more salts of formula R.sub.1COOM and/or R.sub.2COOM and may be kept for the purpose of recovering this (these) carboxylate salt(s) and recycling it (them).

(37) The composition recovered comprising mineral particles may optionally be washed with water, in particular with distilled or osmosed water, by performing, for example, one or two washing/centrifugation cycles.

(38) The composition comprising mineral particles recovered after the final centrifugation may then be dried: in an oven at a temperature of between 60 C. and 130 C., for 1 to 24 hours, or alternatively by lyophilisation, for example in a lyophiliser of CHRIST ALPHA 1-2 LD Plus type, for 48 hours to 72 hours, by microwave irradiation, by atomisation, or alternatively via any powder-drying technique.

(39) A divided solid composition whose colour depends on the nature of the dicarboxylate salt(s) of formula M(R.sub.1COO).sub.2 used for the preparation of the silico/germano-metallic gel (and also, where appropriate, on the respective proportions of this (these) dicarboxylate salt(s)) is finally obtained.

(40) The inventors have thus been able to note not only that an extremely short time (less than 1 minute) of hydrothermal treatment under supercritical conditions is sufficient to allow conversion of the initial gel into a heat-stable crystalline material, but also that the synthetic mineral particles obtained have improved crystallinity.

(41) The phyllosilicate mineral particles contained in a talc composition obtained via a process according to the invention have noteworthy properties in terms of purity, crystallinity and thermal stability, and this being so for a hydrothermal treatment time that is extremely short (relative to the hydrothermal treatment time previously necessary in a known process for preparing a talc composition), and without the need for a subsequent anhydrous heat treatment (annealing).

(42) B/Analysis and Structural Characterisation

(43) The results of analysis of a talc composition obtained by following the protocol described previously are reported below. These results confirm that the invention effectively makes it possible to arrive at the formation of synthetic phyllosilicate mineral particles having structural characteristics (especially lamellarity and crystallinity) that are very similar to those of natural talcs. They also show that, especially by means of the choice of the temperature and time used, the invention makes it possible to synthesise, in an extremely simple manner, stable and pure synthetic silico/germano-metallic mineral particles, with defined and predictable crystalline characteristics and size.

(44) The analyses were especially performed by x-ray diffraction, infrared and by electron microscopy observations. The data collected are presented in the attached figures and in the examples, and are commented below.

(45) 1/X-Ray Diffraction Analyses

(46) On x-ray (XR) diffraction, a natural talc such as a talc originating from the ARNOLD mine (New York State, USA), is known to have the following characteristic diffraction lines (from the publication by Ross M., Smith W. L. and Ashton W. H., 1968, Triclinic talc and associated amphiboles from Gouverneur mining district, New York; American Mineralogist, volume 53, pages 751-769): for plane (001), a line located at a distance of 9.34 {acute over ()}; for plane (002), a line located at a distance of 4.68 {acute over ()}; for plane (020), a line located at a distance of 4.56 {acute over ()}; for plane (003), a line located at a distance of 3.115 {acute over ()}; for plane (060), a line located at a distance of 1.52 {acute over ()}.

(47) FIGS. 2, 5, 7, 8 and 9 show XR diffractograms of the particles obtained in the examples below, each of which representing the relative intensity of the signal (number of counts per second) as a function of the diffraction angle 2.

(48) The XR diffractograms represented were recorded on a CPS 120 machine sold by the company INEL (Artenay, France). It is a curved-detector diffractometer allowing real-time detection over an angular range of 120. The acceleration voltage used is 40 kV and the current is 25 mA. The Bragg relationship giving the structural equidistance is: d.sub.hkl=0.89449/sin (with the use of a cobalt anticathode).

(49) This x-ray diffraction analysis confirms that there is great structural similarity between the phyllosilicate mineral particles of the talc compositions prepared in accordance with the invention and the natural talc particles.

(50) In particular, the diffraction lines which correspond, respectively, to the planes (003) and (060) have positions that coincide perfectly with those of the reference diffraction lines for natural talc.

(51) 2/Near-infrared Analyses

(52) On infrared, it is known that natural talc has, in near-infrared, a vibration band at 7185 cm.sup.1 representative of vibration of the Mg.sub.3OH bond.

(53) The spectra presented in FIGS. 3, 4 and 6 were acquired using a NICOLET 6700-FTIR spectrometer over a range from 9000 cm.sup.1 to 4000 cm.sup.1.

(54) 3/Microscope Observations and Assessment of the Particle Size of the Particles

(55) Given the great fineness of the powders that may constitute the talc compositions in accordance with the invention, the size and particle size distribution of the phyllosilicate mineral particles of which they are composed were assessed by field-effect scanning electron microscopy and transmission electron microscopy observation.

(56) The examples that follow illustrate the preparation process according to the invention and the structural characteristics of compositions comprising the synthetic mineral particles, and in particular of the talc compositions comprising phyllosilicate mineral particles, thus obtained.

EXAMPLE 1

(57) A magnesium acetate solution is first prepared by adding 1.60817 g of magnesium acetate tetrahydrate (Mg(CH.sub.3COO).sub.2.4H.sub.2O) to 5 mL of acetic acid CH.sub.3COOH at 1 mol/L and 245 mL of distilled water.

(58) Separately, a sodium metasilicate solution is prepared by adding 2.12136 g of sodium metasilicate pentahydrate (Na.sub.2OSiO.sub.2.5H.sub.2O) to 250 mL of distilled water.

(59) The peristaltic pumps 18, 19 convey the two solutions separately via steel pipes with an outside diameter of inch (3.175 mm) and an inside diameter of 1.57 mm, and at a flow rate of 2 mL/min each, i.e. a total flow rate of 4 mL/min at point 17 where the mixing of the two solutions takes place continuously, a few centimetres before the inlet 9 of the reaction pipe 14. The temperature in the chamber 16 is 400 C., and the pressure in the reaction pipe 14 is maintained (by means of the pressure regulator 2) above 22.1 MPa (between 25 MPa and 27 MPa), so that the reaction medium which circulates in the reaction pipe 14 in the chamber 16 is under conditions above the critical point of water (374 C., 221 bar).

(60) The precursor gel, derived from the mixing and co-precipitation of the two solutions taking place in the third pipe portion 13 upstream of the inlet 9 of the reaction pipe 14, thus undergoes a hydrothermal treatment in the reaction chamber 16, which makes it possible to transform this precursor gel into a suspension of synthetic talc. The residence time in the reaction pipe 14 between the inlet 9 and the outlet 8 is 23 seconds.

(61) After cooling, the suspension obtained from the outlet 8 of the reactor 15 is a colloidal suspension of synthetic talc particles in saline aqueous medium (sodium acetate). It has the appearance of a milky white composition which settles over several tens of minutes. This suspension is subjected to a centrifugation cycle (10 min at 8000 rpm). After centrifugation, a talc composition, on the one hand, and a supernatant solution especially comprising sodium acetate, on the other hand, the latter then being able to be recovered and optionally recycled, are recovered.

(62) The recovered talc composition is then subjected to two successive cycles of washing with demineralised water and centrifugation (10 min at 8000 rpm).

(63) The recovered talc composition after centrifugation is finally dried in an oven at 60 C. for 12 hours.

(64) The XR diffractogram of the talc particles obtained according to the invention is represented by curve 40 in FIG. 2. The XR diffractogram of this talc composition shows diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) located at a distance of 10.05 {acute over ()}; a plane (002) located at a distance of 4.96 {acute over ()}; a plane (020) located at a distance of 4.59 {acute over ()}; a plane (003) located at a distance of 3.19 {acute over ()}; a plane (060) located at a distance of 1.53 {acute over ()}.

(65) Curve 40 is similar to that obtained via the process of WO 2013/004979 at 300 C. but with a hydrothermal treatment of 3 hours. Curve 44 in FIG. 2 is a comparative diffractogram of the talc particles obtained via the process of WO 2013/004979 at 300 C. with a hydrothermal treatment of 6 hours, which are considered as a reference.

(66) It is furthermore noted that by repeating this example several times, virtually identical diffractograms are obtained, demonstrating the excellent reproducibility of the process according to the invention.

(67) FIG. 3 represents the infrared absorption spectra of the particles obtained according to the invention in this example 1 (curve 37), comparing them with the infrared absorption spectrum of the talc particles obtained via the process of WO 2013/004979 at 300 C. with a hydrothermal treatment of 1 hour (curve 38) and with the infrared absorption spectrum of the talc particles obtained via the process of WO 2013/004979 at 300 C. with a hydrothermal treatment of 2 hours (curve 39).

(68) FIG. 4 represents the infrared absorption spectra of the particles obtained according to the invention in this example 1 according to the invention in 23 s (curve 45), and of talc particles obtained via the process of WO 2013/004979 at 300 C. with a hydrothermal treatment of 3 hours (curve 46); of talc particles obtained via the process of WO 2013/004979 at 300 C. with a hydrothermal treatment of 2 hours (curve 47); of talc particles obtained via the process of WO 2013/004979 at 300 C. with a hydrothermal treatment of 1 hour (curve 48).

(69) FIGS. 10 and 11 are photographs taken with a field-effect scanning electron microscope (FE SEM) illustrating phyllosilicate particles obtained in this example. Submicron talc particles are obtained (which are visible in the photographs of FIGS. 10 and 11 in a form in which the particles are agglomerated together) having a largest dimension of the order of 200 {acute over ()} to 3000 {acute over ()}, a thickness of less than 100 {acute over ()} corresponding to a few stacked sheets.

EXAMPLE 2

(70) A magnesium acetate solution is first prepared by adding 3.216 g of magnesium acetate tetrahydrate (Mg(CH.sub.3COO).sub.2.4H.sub.2O) to 10 mL of acetic acid CH.sub.3COOH at 1 mol/L and 490 mL of distilled water.

(71) Separately, a sodium metasilicate solution is prepared by adding 4.24284 g of sodium metasilicate pentahydrate (Na.sub.2OSiO.sub.2.5H.sub.2O) to 500 mL of distilled water.

(72) In this example, the two solutions 20, 21 are fed by the pumps 18, 19 at a flow rate of 4 mL/min each, i.e. a total flow rate of 8 mL/min of reaction medium in the reaction pipe 14. The residence time in the reactor (in the reaction pipe 14 between the inlet 9 and the outlet 8) is 11 seconds. The other reaction conditions are identical to those of example 1.

(73) The XR diffractogram of the talc particles obtained is represented by curve 41 in FIG. 2. The XR diffractogram of this talc composition shows diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) located at a distance of 10.47 {acute over ()}; a plane (020) located at a distance of 4.57 {acute over ()}; a plane (003) located at a distance of 3.19 {acute over ()}; a plane (060) located at a distance of 1.53 {acute over ()}.

(74) Curve 41 is similar to that obtained via the process of WO 2013/004979 at 300 C. but with a hydrothermal treatment of 1 hour.

EXAMPLE 3

(75) A magnesium acetate solution is first prepared by adding 3.2165 g of magnesium acetate tetrahydrate (Mg(CH.sub.3COO).sub.2.4H.sub.2O) to 10 mL of acetic acid CH.sub.3COOH at 1 mol/L and 490 mL of distilled water.

(76) Separately, a sodium metasilicate solution is prepared by adding 4.24325 g of sodium metasilicate pentahydrate (Na.sub.2OSiO.sub.2.5H.sub.2O) to 500 mL of distilled water.

(77) The reaction conditions are identical to those of example 1.

(78) The ceramic sinter 34 is used at the outlet so as to separate out the talc particles by filtering the suspension. The particles are recovered manually from the sinter (without washing or centrifugation) and then dried in an oven. Separately, the saline solution may be recovered at the sinter outlet and then dried to recover the salt.

(79) When the sinter 34 is filled with talc particles, the remainder of the synthesised product may be recovered in container 25, without passing through the sinter. This portion of the product is centrifuged, and then washed/centrifuged twice. The talc composition then recovered is subsequently dried in an oven.

(80) The XR diffractograms of the talc particles obtained in this example 3 are represented by curves 42 and 43 in FIG. 2. Curve 42 is obtained with the particles recovered by the sinter. Curve 43 is obtained by the particles recovered by washing and centrifugation without the sinter.

(81) The XR diffractogram of the talc composition represented by curve 42 shows diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) located at a distance of 10.08 {acute over ()}; a plane (002) located at a distance of 4.90 {acute over ()}; a plane (020) located at a distance of 4.53 {acute over ()}; a plane (003) located at a distance of 3.20 {acute over ()}; a plane (060) located at a distance of 1.52 {acute over ()}.

(82) The XR diffractogram of the talc composition represented by curve 43 shows diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) located at a distance of 10.54 {acute over ()}; a plane (002) located at a distance of 4.91 {acute over ()}; a plane (020) located at a distance of 4.56 {acute over ()}; a plane (003) located at a distance of 3.19 {acute over ()}; a plane (060) located at a distance of 1.52 {acute over ()}.

(83) Curves 42 and 43 are similar to that obtained via the process of WO 2013/004979 at 300 C. but with a hydrothermal treatment of 2 hours.

(84) FIG. 5 compares the XR diffractograms of the particles obtained in example 1 according to the invention in 23 s (curve 53), and of talc particles obtained via the process of WO 2013/004979 at 300 C. with a hydrothermal treatment of 3 hours (curve 54); of particles obtained on the sinter in example 3 according to the invention in 23 s (curve 55) and of talc particles obtained via the process of WO 2013/004979 at 300 C. with a hydrothermal treatment of 2 hours (curve 56); of particles obtained in example 2 according to the invention in 11 s (curve 57) and of talc particles obtained via the process of WO 2013/004979 at 300 C. with a hydrothermal treatment of 1 hour (curve 58).

(85) FIG. 6 compares infrared absorption spectra of particles obtained according to the invention in example 1 (curve 62), in example 2 (curve 63) and in example 3 (curve 64 for those obtained from the sinter; curve 65 for those obtained without the sinter).

EXAMPLE 4

(86) A magnesium acetate solution is first prepared by adding 3.2165 g of magnesium acetate tetrahydrate (Mg(CH.sub.3COO).sub.2.4H.sub.2O) to 10 mL of acetic acid CH.sub.3COOH at 1 mol/L and 490 mL of distilled water.

(87) Separately, a sodium metasilicate solution is prepared by adding 4.24325 g of sodium metasilicate pentahydrate (Na.sub.2OSiO.sub.2.5H.sub.2O) to 500 mL of distilled water.

(88) Three successive tests are performed under reaction conditions identical to those of example 1, by varying the temperature of the hydrothermal treatment and the flow rates according to the following table:

(89) TABLE-US-00001 Test 1 2 3 Temperature ( C.) 350 375 400 Flow rate of each 7.5 6 2 salt (mL/min) Total flow rate 15 12 4 (mL/min) Curve 72 73 74

(90) The XR diffractograms of the phyllosilicate particles obtained are represented by curves 72, 73 and 74, respectively, in FIG. 7. Curve 72 corresponds to the temperature of 350 C., curve 73 corresponds to the temperature of 375 C. and curve 74 corresponds to the temperature of 400 C. (similar to that of example 1). Curves 72 and 73 are virtually identical. As may be seen, a process according to the invention also makes it possible to obtain phyllosilicate particles having the structure of a talc under homogeneous subcritical conditions. In addition, under supercritical conditions, the structural characteristics of the particles are even better and, as may be seen by means of curve 74, the crystallinity of the particles obtained under these conditions is exceptional and similar to that of a natural talc.

(91) The XR diffractogram of the talc composition represented by curve 72 shows diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) located at a distance of 12.09 {acute over ()}; a plane (020) located at a distance of 4.57 {acute over ()}; a plane (003) located at a distance of 3.25 {acute over ()}; a plane (060) located at a distance of 1.53 {acute over ()}.

(92) The XR diffractogram of the talc composition represented by curve 73 shows diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) located at a distance of 11.96 {acute over ()}; a plane (020) located at a distance of 4.55 {acute over ()}; a plane (003) located at a distance of 3.25 {acute over ()}; a plane (060) located at a distance of 1.53 {acute over ()}.

(93) The XR diffractogram of the talc composition represented by curve 74 shows diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) located at a distance of 10.21 {acute over ()}; a plane (002) located at a distance of 4.98 {acute over ()}; a plane (020) located at a distance of 4.61 {acute over ()}; a plane (003) located at a distance of 3.22 {acute over ()}; a plane (060) located at a distance of 1.53 {acute over ()}.

(94) FIG. 8 compares the XR diffractograms of the particles obtained in test 2 (at 375 C.) according to the invention (curve 80), and of talc particles obtained according to the protocol described by WO 2013/004979 at 230 C. with a hydrothermal treatment of 6 hours (curve 81), and of talc particles obtained via the process of WO 2013/004979 at 300 C. with a hydrothermal treatment of 1 hour (curve 82).

(95) FIG. 9 compares the XR diffractograms of the particles obtained in test 3 (at 400 C.) according to the invention (curve 90), and of talc particles obtained via the process of WO 2013/004979 at 300 C. with a hydrothermal treatment of 3 hours (curve 91).

(96) It is found that the mean size of the elementary particles obtained in the above examples is generally less than 3000 {acute over ()}. The particle size may, of course, vary as a function especially of the residence time and of the temperature in the hydrothermal treatment zone, an increase in the residence time allowing, for example, an increase in the particle size essentially in the (a, b) plane of the crystal lattice of the particles (i.e. width and length of the particles).

(97) The above examples also show that it is easy to precisely adjust the structural characteristics of the phyllosilicate particles obtained by modifying the residence time, i.e. the duration of the solvothermal treatment, and/or the temperature of the solvothermal treatment.

(98) The invention may form the subject of numerous embodiment variants. In particular, it is possible to envisage several main pipes arranged in parallel in the same reactor; it is possible to prepare the precursor gel (or particles corresponding to this precursor gel) beforehand in order to be able to use it as need be to perform the solvothermal treatment; the device for continuously applying the temperature and pressure of the solvothermal treatment to the reaction medium initially constituted by the precursor gel may form the subject of different embodiment variants, etc.