Composition containing synthetic mineral particles and a process for preparing the composition

Abstract

A composition comprising synthetic mineral particles, such as silicate or phyllosilicate mineral particles, is presented. The composition can be prepared by a process in which a hydrogel precursor of the synthetic mineral particles is produced by a coprecipitation reaction between at least one compound comprising silicon, such as sodium metasilicate, and at least one compound comprising at least one metal element, such as a dicarboxylate salt of the formula M(R.sub.1COO).sub.2, wherein R.sub.1 is H or an alkyl group having 1 to 4 carbon atoms. The coprecipitation reaction also takes place in the presence of at least one carboxylate salt of formula R.sub.2COOM wherein M is Na or K, and R.sub.2 is H or an alkyl group having 1 to 4 carbon atoms.

Claims

1. A composition comprising synthetic mineral particles, wherein the composition has, in X-ray diffraction, the following characteristic diffraction lines: a plane (001) situated at a distance between 9.40 and 9.90 ; a plane (002) situated at a distance between 4.60 and 4.80 ; a plane (003) situated at a distance between 3.10 and 3.20 ; and a plane (060) situated at a distance between 1.51 and 1.53 ; the intensity of the diffraction line characteristic of the plane (002) being greater than the intensity of the signal corresponding to a plane (020) situated at a distance between 4.40 and 4.60 , and the ratio between the intensity of the diffraction line characteristic of the plane (001) and the intensity of the diffraction line characteristic of the plane (003) being from 0.60 to 1.50, and wherein the composition does not have, in X-ray diffraction, a diffraction line characteristic of a plane situated at a distance between 12.00 and 18.00 .

2. The composition according to claim 1, wherein said synthetic mineral particles are phyllosilicate mineral particles having at least one non-swelling phase formed of a stack of elementary lamellae, said elementary lamellae being constituted by an association of two tetrahedral layers situated on either side of an octahedral layer, and having the chemical formula
(Si.sub.xGe.sub.1-x).sub.4M.sub.3O.sub.10(OH).sub.2 wherein x is a real number of the interval [0; 1], and M is 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], such that $\underset{i=1}{\overset{8}{.Math.}}y\left(i\right)=1.$

3. The composition according to claim 1, wherein the composition has, in near-infrared, a vibration band situated between 5000 cm.sup.1 and 5500 cm.sup.1, corresponding to the presence of water bonded at lamina edges.

4. The composition according to claim 2, wherein said composition has, in near-infrared, a vibration band situated between 5000 cm.sup.1 and 5500 cm.sup.1, corresponding to the presence of water bonded at lamina edges.

5. The composition according to claim 2, wherein said composition has, in near-infrared, a vibration band situated between 5200 cm.sup.1 and 5280 cm.sup.1, corresponding to the presence of water bonded at lamina edges.

6. The composition according to claim 2, wherein said composition has, in near-infrared, a vibration band situated at 7185 cm.sup.1.

7. The composition according to claim 1, wherein said synthetic mineral particles are silicate mineral particles.

8. The composition according to claim 1, wherein said synthetic mineral particles are phyllosilicate mineral particles.

9. The composition according to claim 2, wherein said synthetic mineral particles have the chemical formula Si.sub.4Mg.sub.3O.sub.10 (OH).sub.2.

10. The composition according to claim 2, wherein the elementary particles of said synthetic mineral particles have a particle size in a range between 20 nm and 100 nm.

11. The composition according to claim 1, wherein the composition is prepared by a process in which a hydrogel precursor of the synthetic mineral particles is prepared by a coprecipitation reaction between at least one compound comprising sodium metasilicate, and at least one compound comprising at least one metal element, wherein the coprecipitation reaction takes place in the presence of at least one carboxylate salt of the formula R.sub.2COOM, wherein M is Na or K, and R.sub.2 is H or an alkyl group having 1 to 4 carbon atoms, and wherein said carboxylate salt of the formula R.sub.2COOM is added before the coprecipitation reaction takes place.

12. The composition according to claim 11, wherein said at least one compound comprising at least one metal element is a dicarboxylate salt of the formula M(R.sub.1COO).sub.2, wherein R.sub.1 is H or an alkyl group having 1 to 4 carbon atoms, and M is 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], such that $\underset{i=1}{\overset{8}{.Math.}}y\left(i\right)=1.$

13. The composition according to claim 12, wherein R.sub.1 and R.sub.2 are each independently selected from the group consisting of CH.sub.3, CH.sub.3CH.sub.2, and CH.sub.3CH.sub.2CH.sub.2.

14. The composition according to claim 12, wherein the groups of R.sub.1 and R.sub.2 are identical.

15. The composition according to claim 11, wherein after said coprecipitation reaction, said hydrogel precursor is then subjected to a hydrothermal treatment to obtain said synthetic mineral particles.

16. The composition according to claim 15, wherein said hydrothermal treatment is carried out at a temperature of from 150 C. to 400 C.

17. The composition according to claim 15, wherein said hydrothermal treatment is carried out at a pressure of from 5 bar to 200 bar.

18. The composition according to claim 11, wherein said hydrogel precursor of said synthetic mineral particles is a hydrogel of the formula
(Si.sub.xGe.sub.1-x).sub.4M.sub.3O.sub.11, nH.sub.2O, wherein x is a real number of the interval [0; 1], and n is a number of molecule(s) of water associated with said hydrogel precursor.

19. The composition according to claim 11, wherein said at least one carboxylate salt of the formula R.sub.2COOM is present in an amount so as to give, based on silicon, a molar ratio R.sub.2COOM/Si of from 0.1 to 9.

20. The composition according to claim 2, wherein said composition is prepared by a process comprising: (i) preparing a hydrogel precursor of synthetic mineral particles by a coprecipitation reaction comprising: (a) providing a first solution comprising at least one sodium metasilicate and at least one carboxylate salt of the formula R.sub.2COOM, wherein M is Na or K, and R.sub.2 is H or an alkyl group having 1 to 4 carbon atoms; and (b) rapidly adding to the first solution a second solution comprising at least one dicarboxylate salt of the formula M(R.sub.1COO).sub.2, wherein R.sub.1 is H or an alkyl group having 1 to 4 carbon atoms, and 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], such that $\underset{i=1}{\overset{8}{.Math.}}y\left(i\right)=1;$ and (ii) after said coprecipitation reaction, subjecting said hydrogel precursor to a hydrothermal treatment to obtain said synthetic phyllosilicate mineral particles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1 to 3 show RX diffractograms with the relative intensity of the signal (number of counts per second) as a function of the inter-reticular distance in Angstroms. Curves 1 to 5 of FIGS. 1 and 2 and curves 3 and 10 of FIG. 3 show the results of analyses carried out by X-ray diffraction of a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention. FIG. 2 shows an enlargement of part of the diffractograms shown in FIG. 1.

(2) FIGS. 4 and 5 show near-infrared spectra with the intensity of the signal as a function of the wavelength expressed in cm.sup.1. Curves 11 to 15 of FIG. 5 and curves 13 and 20 of FIG. 4 show the near-infrared spectra of a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention. Curve 16 of FIG. 5 show the near-infrared spectra of a natural talc composition.

AGENERAL PROTOCOL FOR THE PREPARATION OF A COMPOSITION COMPRISING SYNTHETIC MINERAL PARTICLES ACCORDING TO THE INVENTION

(3) 1Preparation of a Silico/Germano-Metallic Hydrogel

(4) The silico/germano-metallic hydrogel can be prepared by a coprecipitation reaction involving, as reagent, at least one compound comprising silicon, at least one dicarboxylate salt of the formula M(R.sub.1COO).sub.2 in the presence of at least one carboxylate salt of the formula R.sub.2COOM wherein M denotes a metal chosen from the group formed of Na and K, and R.sub.2 is chosen from H and alkyl groups having fewer than 5 carbon atoms.

(5) By means of this coprecipitation reaction it is possible to obtain a hydrated silico/germano-metallic hydrogel having the stoichiometry of talc (4 Si for 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], such that

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

(7) The silico/germano-metallic hydrogel is prepared by a coprecipitation reaction carried out starting from: 1. an aqueous solution of penta-hydrated sodium metasilicate or an aqueous solution of sodium metagermanate, or a mixture of these two solutions in the molar proportions x:(1-x), 2. a solution of dicarboxylate salt(s) prepared with one or more dicarboxylate salt(s) of the formula M(R.sub.1COO).sub.2 diluted in a carboxylic acid, such as acetic acid, and 3. a solution of carboxylate salt(s) prepared with one or more carboxylate salt(s) of the formula R.sub.2COOM diluted in distilled water.

(8) The preparation of the silico/germano-metallic hydrogel is carried out following the protocol below: 1. the solutions of sodium metasilicate and carboxylate salt(s) of the formula R.sub.2COOM are mixed, 2. the solution of dicarboxylate salt(s) of the formula M(R.sub.1COO).sub.2 is added rapidly thereto; the coprecipitation hydrogel forms instantaneously.

(9) It is further possible to subject the preparation medium of said hydrogel to ultrasound.

(10) At the end of this precipitation there is obtained a silico/germano-metallic hydrogel (Si.sub.xGe.sub.1-x).sub.4M.sub.3O.sub.11, nH.sub.2O in an aqueous solution of carboxylate salt(s), said hydrogel being strongly hydrated and having a more or less gelatinous consistency.

(11) The silico/germano-metallic hydrogel (Si.sub.xGe.sub.1-x).sub.4M.sub.3O.sub.11, nH.sub.2O obtained in the presence of the carboxylate salt(s) of the formulae R.sub.2COOM and R.sub.1COOM is thus ready to be subjected directly to a hydrothermal treatment.

(12) The hydrogel can likewise be recovered after centrifugation (for example 3000 to 15,000 revolutions per minute, for 5 to 60 minutes) and removal of the supernatant (solution of carboxylate salt(s)), optionally washing with demineralized water (for example two successive washings and centrifugations) and then drying, for example in an oven (60 C., 2 days), by lyophilization, by drying by atomization or by drying under microwave irradiation. The particles containing silicon, germanium and metal of the formula (Si.sub.xGe.sub.1-x).sub.4M.sub.3O.sub.11, nH.sub.2O can accordingly be stored in the form of a powder (in the presence or absence of the carboxylate salts(s), depending on whether washing with water has been carried out or not) with a view to possible subsequent hydrothermal treatment.

(13) 2Hydrothermal Treatment of Said Silico/Germano-Metallic Hydrogel

(14) The silico/germano-metallic hydrogel (Si.sub.xGe.sub.1-x).sub.4M.sub.3O.sub.11, nH.sub.2O as obtained hereinbefore, which may or may not have been dried, is subjected to a hydrothermal treatment at a temperature of especially from 150 C. to 370 C.

(15) To that end: 1. the hydrogel as obtained after precipitation (where appropriate in suspension with the carboxylate salt(s) of the formulae R.sub.2COOM and R.sub.1COOM) or previously dried, is placed in a reactor/autoclave, 2. if necessary, an aqueous solution comprising at least one carboxylate salt of the formula R.sub.2COOM (in hydrated or anhydrous form) is added to said hydrogel, with stirring, 3. the liquid/solid ratio is optionally adjusted to a value of from 2 to 20, especially from 5 to 15 (the quantity of liquid being expressed in cm.sup.3 and the quantity of solid in grams, and denoting the quantity of dry hydrogel only, that is to say without taking into account the carboxylate salts(s)), 4. the reactor/autoclave is placed inside a furnace or an oven, at a predetermined reaction temperature (established from 150 C. to 350 C.) throughout the treatment.

(16) During the hydrothermal treatment, the silico/germano-metallic hydrogel gradually acquires a gelatinous consistency. The composition comprising mineral particles that is obtained at the end of the hydrothermal treatment has a crystallinity which can be observed in X-ray diffraction, that crystallinity increasing with the duration of the hydrothermal treatment and manifesting itself on the corresponding diffractograms by the rapid appearance of characteristic lines, which become finer and intensify rapidly during the treatment.

(17) Furthermore, it has been observed by near-infrared analysis that the intensity of the vibration band corresponding to the vibration of the Mg.sub.3OH bond also increases with the duration of the hydrothermal treatment.

(18) At the end of this hydrothermal treatment there is obtained a colloidal talcose composition comprising phyllosilicate mineral particles in suspension in an aqueous solution of carboxylate salt(s). At the end of this hydrothermal treatment, the talcose composition contained in the reactor is recovered by centrifugation (3000 to 15,000 revolutions per minute for 5 to 60 minutes) and then removal of the supernatant. The supernatant solution contains said salt(s) of the formula RCOOM and can be stored with a view to recovering the carboxylate salt(s) and recycling it/them.

(19) The composition comprising mineral particles that is recovered is then, preferably, washed with water, in particular with distilled or osmozed water, at least two cycles of washing/centrifugation being carried out.

(20) The composition comprising mineral particles that is recovered after the last centrifugation can then be dried: in an oven at a temperature of from 60 C. to 130 C. for from 1 to 24 hours, or by lyophilization, for example in a lyophilizer of the CHRIST ALPHA 1-2 LD Plus type, for from 48 hours to 72 hours, or by atomization.

(21) There is ultimately obtained a divided solid composition, the color of which is dependent on the nature of the dicarboxylate salt(s) of the formula M(R.sub.1COO).sub.2 used for the preparation of the silico/germano-metallic hydrogel (and also, where applicable, on the respective proportions of those dicarboxylate salt(s)).

(22) The inventors have accordingly found not only that a relatively short duration of the hydrothermal treatment is sufficient to allow the initial hydrogel to be converted into a crystallized and thermally stable material, but also that the synthetic mineral particles obtained have improved crystallinity.

(23) The phyllosilicate mineral particles contained in a talcose composition obtained by a process according to the invention have remarkable properties in terms of purity, crystallinity and thermal stability, and those properties are obtained with a duration of the hydrothermal treatment that is reduced significantly as compared with the duration of the hydrothermal treatment previously necessary in a known process for preparing a talcose composition.

(24) Furthermore, following the hydrothermal treatment, a composition, in particular a talcose composition, obtained by a process according to the invention can optionally be subjected to an anhydrous heat treatment, which is carried out at a pressure below 5 bar (0.5 MPa), at a temperature greater than 350 C. and lower than the degradation temperature of the synthetic mineral particles, especially at a temperature of from 450 C. to 600 C., for example for a duration of from 30 minutes to 24 hours, in particular from 1 to 15 hours.

BSTRUCTURAL ANALYSIS AND CHARACTERIZATION

(25) The analysis results of a talcose composition obtained following the protocol described hereinbefore are reported below. These results confirm that it is possible by means of the invention to effectively form 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 the choice of temperature and duration of implementation, the invention permits the extremely simple synthesis of synthetic mineral particles containing silicon and/or germanium and metal that are stable and pure and have a size and crystalline characteristics that are defined and foreseeable.

(26) The analyses were carried out especially by X-ray diffraction, by infrared and by observations under an electron microscope. The collected data are presented in the accompanying figure and in the examples, and are commented on hereinbelow.

(27) 1X-Ray Diffraction Analyses

(28) In X-ray diffraction (RX), a natural talc such as a talc from the ARNOLD mine (New York state, USA) is known to have the following characteristic diffraction lines (according to the publication of 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 the plane (001), a line situated at a distance of 9.34 ; for the plane (002), a line situated at a distance of 4.68 ; for the plane (020), a line situated at a distance of 4.56 ; for the plane (003), a line situated at a distance of 3.115 ; for the plane (060), a line situated at a distance of 1.52 .

(29) FIGS. 1 to 3 show RX diffractograms, on each of which there is shown the relative intensity of the signal (number of counts per second) as a function of the inter-reticular distance in Angstroms.

(30) FIGS. 1 and 2 show the results of analyses carried out by X-ray diffraction on: a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 3 hours (curve 1), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 6 hours (curve 2), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 18 hours (curve 3), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 24 hours (curve 4), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 10 days (curve 5).

(31) FIG. 2 shows an enlargement of part of the diffractograms shown in FIG. 1, that is to say only the part of the diffractograms corresponding to the inter-reticular distances between 2 and 4 , thus permitting better visualization of the lines corresponding to the plane (003).

(32) FIG. 3 shows the results of X-ray diffraction analyses on a synthetic talc composition of the formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment at 300 C. for 18 hours (curve 3), and on that same synthetic talc composition after anhydrous heat treatment for 5 hours at 550 C. (curve 10).

(33) The RX diffractograms shown in FIGS. 1 to 3 were recorded on a CPS 120 device marketed by INEL (Artenay, France). This is a diffractometer with a curved detector, allowing detection in real time over an angle domain of 120. The acceleration voltage used is 40 kV and the intensity 25 mA. The Bragg equation giving the structural equidistance is: d.sub.hkl=0.89449/sin (with the use of a cobalt anticathode).

(34) This X-ray diffraction analysis confirms that there is great structural similarity between the phyllosilicate mineral particles of the talcose compositions prepared according to the invention and the particles of natural talc.

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

(36) Furthermore, analysis of the RX diffractograms of the prepared talcose compositions also allows the coherent domain to be determined for each talcose composition, that is to say the number of elementary laminae that are stacked without a major defect in the c* direction (of the reciprocal space of the crystal lattice of the synthetic mineral particles). The coherent domain can be determined for the lines (001) and depends especially on the full width at half maximum of the corresponding line and on the corresponding diffraction angle. In the case of the synthetic talc, it is determined in particular starting from the line (003).

(37) A natural talc from the LYAONING province (China) has, for example, a coherent domain of 70 laminae.

(38) The talcose compositions prepared by a process according to the invention have coherent domains which are similar to the coherent domains of natural talcs and greater than the coherent domains of synthetic talcs of the prior art for comparable temperatures and durations of hydrothermal treatment.

(39) Analysis of the RX diffractograms of the prepared talcose compositions also allows the ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) to be determined and compared with that of a natural talc. For a natural talc from the LYAONING province (China), the ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 0.61.

(40) 2Near-Infrared Analyses

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

(42) FIGS. 4 and 5 show near-infrared spectra, on each of which there is shown the intensity of the signal as a function of the wavelength expressed in cm.sup.1. FIG. 5 shows the near-infrared spectra of: a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 3 hours (curve 11), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 6 hours (curve 12), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 18 hours (curve 13), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 24 hours (curve 14), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 10 days (curve 15), and a natural talc composition from the LYAONING province (China) (curve 16).

(43) FIG. 4 shows a near-infrared spectrum of a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel (i.e. containing silicon and metal) in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 18 hours (curve 13) as well as a near-infrared spectrum of that same synthetic talc composition after anhydrous heat treatment for 5 hours at 550 C. (curve 20).

(44) These spectra were acquired using a NICOLET 6700-FTIR spectrometer over a domain of 9000 cm.sup.1 to 4000 cm.sup.1.

(45) 3Microscopic Observations and Assessment of the Particle Size of the Particles

(46) In view of the considerable fineness of the powders of which the talcose compositions according to the invention can be constituted, the size and particle size distribution of the phyllosilicate mineral particles composing them were assessed by observation under a field-emission scanning electron microscope and under a transmission electron microscope.

(47) It is found that the particle size of the elementary particles varies between 20 nm and 100 nm.

(48) It has further been observed that the synthetic talc particles prepared by a process according to the invention exhibit a pearly effect which may be of interest in many industrial fields.

(49) The examples which follow illustrate the preparation process according to the invention and the structural characteristics of the compositions comprising synthetic mineral particles, and in particular of the talcose compositions comprising phyllosilicate mineral particles, so obtained.

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

(50) A first aqueous solution of sodium metasilicate comprising 42.43 g of pentahydrated sodium metasilicate Na.sub.2SiO.sub.3,5H.sub.2O and 140 ml of demineralized water is prepared, and the solution is stirred at ambient temperature (21 C.) for 10 minutes. 171 g of trihydrated sodium acetate CH.sub.3COONa.3H.sub.2O are then added to the solution, stirring of the solution being maintained for 10 minutes and the solution being maintained at a temperature of from 30 C. to 50 C. with the aid of a water bath. The concentration of sodium acetate in this first solution is 4 mol/1.

(51) A second solution of magnesium acetate comprising 32.17 g of tetrahydrated magnesium acetate Mg(CH.sub.3COO).sub.2.4H.sub.2O and 100 ml of 1M concentrated acetic acid is then prepared.

(52) Finally, when the first solution of sodium metasilicate and sodium acetate has returned to ambient temperature, the second solution is added rapidly to the first solution, with stirring and in a single batch.

(53) There is obtained a suspension of hydrogel precursor of synthetic talc particles, which is in the form of a hydrogel of milky consistency. At the end of the precipitation of the hydrogel, the concentration of sodium acetate in the hydrogel suspension is 4 mol/1, as during the subsequent hydrothermal treatment, the hydrogel obtained then being subjected directly to a hydrothermal treatment.

(54) The hydrothermal treatment of the hydrogel is carried out at a temperature of 300 C. for 3 hours at a pressure of 80 bar (8 MPa) (saturation vapor pressure of the water in the reactor).

(55) To that end, the hydrogel suspension so obtained is placed directly in a closed titanium reactor. The titanium reactor is then placed in a furnace at a temperature of 300 C. for 3 hours.

(56) After cooling to ambient temperature, the reactor is opened and the suspension obtained is centrifuged. After centrifugation there are recovered on the one hand a talcose composition and on the other hand a supernatant solution comprising especially sodium acetate, which can then be recovered and optionally recycled.

(57) The talcose composition that has been recovered is then subjected to two successive cycles of washing with demineralized water and centrifugation.

(58) The talcose composition recovered after centrifugation is finally dried by lyophilization for 72 hours.

(59) The X-ray diffractogram of the synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained is shown in FIG. 1 (curve 1). The X-ray diffractogram of this talcose composition has diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) situated at a distance of 9.77 (I=100); a plane (002) situated at a distance of 4.78 (I=33); a plane (020) situated at a distance of 4.55 (I=26); a plane (003) situated at a distance of 3.17 (I=61); a plane (060) situated at a distance of 1.52 (I=11).

(60) Such a talcose composition has a coherent domain of 12 laminae (using line (003)). The ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 1.49.

(61) The near-infrared spectrum of the synthetic talc composition obtained is shown in FIG. 5 (curve 11). It has a vibration band at 7185 cm.sup.1 representing the vibration of the Mg.sub.3OH bond of the talc. The near-infrared spectrum also has a vibration band at 5251 cm.sup.1 which is characteristic of a synthetic talc according to the invention and corresponds to the presence of water bonded to the talc at lamina edges.

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

(62) A hydrogel precursor of synthetic mineral particles is prepared according to the protocol described in Example 1.

(63) The hydrogel obtained is subjected directly to a hydrothermal treatment at a temperature of 300 C. for 6 hours at a pressure of 80 bar (8 MPa) (saturation vapor pressure of the water in the reactor). The concentration of sodium acetate during the hydrothermal treatment is 4 mol/1.

(64) The talcose composition that is recovered is then subjected to two successive cycles of washing with demineralized water and centrifugation and is finally dried by lyophilization for 72 hours.

(65) The X-ray diffractogram of the synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained is shown in FIG. 1 (curve 2).

(66) The X-ray diffractogram of this talcose composition has diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) situated at a distance of 9.45 (I=100); a plane (002) situated at a distance of 4.72 (I=34); a plane (020) situated at a distance of 4.57 (I=20); a plane (003) situated at a distance of 3.14 (I=98); a plane (060) situated at a distance of 1.53 (I=18).

(67) Such a talcose composition has a coherent domain of 19 laminae (using line (003)). The ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 1.06.

(68) The near-infrared spectrum of the synthetic talc composition obtained is shown in FIG. 5 (curve 12). It has a vibration band at 7185 cm.sup.1 representative of the vibration of the Mg.sub.3OH bond of the talc. The near-infrared spectrum also has a vibration band at 5251 cm.sup.1 which is characteristic of a synthetic talc according to the invention and corresponds to the presence of water bonded to the talc at lamina edges.

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

(69) A hydrogel precursor of synthetic mineral particles is prepared according to the protocol described in Example 1.

(70) The hydrogel obtained is subjected directly to a hydrothermal treatment at a temperature of 300 C. for 18 hours at a pressure of 80 bar (8 MPa) (saturation vapor pressure of the water in the reactor). The concentration of sodium acetate during the hydrothermal treatment is 4 mol/1.

(71) The talcose composition that is recovered is then subjected to two successive cycles of washing with demineralized water and centrifugation and is finally dried by lyophilization for 72 hours.

(72) The X-ray diffractogram of the synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained is shown in FIG. 1 (curve 3).

(73) The X-ray diffractogram of this talcose composition has diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) situated at a distance of 9.64 (I=100); a plane (002) situated at a distance of 4.74 (I=30); a plane (020) situated at a distance of 4.59 (slight shoulder); a plane (003) situated at a distance of 3.15 (I=100); a plane (060) situated at a distance of 1.52 (I=5).

(74) The intensity I of the corresponding lines that is given is normalized relative to the most intense line of the diffractogram, the intensity of the most intense line being taken as 100. It is observed that such a composition prepared by a process according to the invention has, in X-ray diffraction, lines corresponding to the planes (001) and (003) of very high intensities relative to the other lines, indicating crystallinity very similar to that of a natural talc. Furthermore, the line corresponding to the plane (002) has a higher intensity than the line corresponding to the plane (020), the diffraction line characteristic of a plane (020) being in part coincident with the diffraction line characteristic of a plane (002) and being present only in the form of a slight shoulder.

(75) Such a talcose composition has a coherent domain of 25 laminae (using line (003)). The ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 0.96. The near-infrared spectrum of the synthetic talc composition obtained is shown in FIGS. 4 and 5 (curve 13). It has a vibration band at 7185 cm.sup.1 representative of the vibration of the Mg.sub.3OH bond of the talc. The near-infrared spectrum also has a vibration band at 5251 cm.sup.1 which is characteristic of a synthetic talc according to the invention and corresponds to the presence of water bonded to the talc at lamina edges.

Example 4Preparation of a Composition Comprising Synthetic Mineral Particles According to the Invention

(76) The talc obtained in Example 3 is subjected to an anhydrous heat treatment or annealing for 5 hours at 550 C.

(77) The synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained after annealing was characterized by X-ray diffraction and near-infrared analysis.

(78) The X-ray diffractogram of the synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained is shown in FIG. 3 (curve 10).

(79) The X-ray diffractogram of this talcose composition has diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) situated at a distance of 9.51 (I=98); a plane (002) situated at a distance of 4.70 (I=29); a plane (003) situated at a distance of 3.14 (I=100); a plane (060) situated at a distance of 1.52 (I=5).

(80) The intensity I of the corresponding lines that is given is normalized relative to the most intense line of the diffractogram, the intensity of the most intense line being taken as 100.

(81) Relative to the X-ray diffractogram of the synthetic talc before annealing, an increase in the intensity of the line (002) as compared with the intensity of the line (020) is observed, the line (020) then being coincident with the diffraction line characteristic of plane (002). There is also observed an increase in the intensities of lines (001), (002) and (003), that is to say, more generally, of the intensities of the lines (004 Such annealing therefore allows the crystallinity of the synthetic talc prepared to be increased further, the structural characteristics of that talc then being even more similar to those of a natural talc.

(82) Such a talcose composition has, after annealing, a coherent domain of 28 laminae (using line (003)). The ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 0.98.

(83) The near-infrared spectrum of the synthetic talc composition obtained is shown in FIG. 4 (curve 20). The near-infrared spectrum of the synthetic talc composition obtained after annealing has a vibration band at 7185 cm.sup.1 representative of the vibration of the Mg.sub.3OH bond of the talc. The intensity of this vibration band has increased as compared with the near-infrared spectrum of the synthetic talc before annealing, which indicates an increase in the crystallinity of the synthetic talc obtained. There is also observed a decrease in the intensity of the vibration band corresponding to the presence of water bonded to the talc at lamina edges (situated at 5273 cm.sup.1) as compared with the curve 13 of the synthetic talc before heat treatment, revealing a reduction in the number of molecules of water bonded to the talc at lamina edges during the annealing.

Example 5Preparation of a Composition Comprising Synthetic Mineral Particles According to the Invention

(84) A hydrogel precursor of synthetic mineral particles is prepared according to the protocol described in Example 1.

(85) The hydrogel obtained is subjected directly to a hydrothermal treatment at a temperature of 300 C. for 24 hours at a pressure of 80 bar (8 MPa) (saturation vapor pressure of the water in the reactor). The concentration of sodium acetate during the hydrothermal treatment is 4 mol/1.

(86) The talcose composition that is recovered is then subjected to two successive cycles of washing with demineralized water and centrifugation and is finally dried by lyophilization for 72 hours.

(87) The X-ray diffractogram of the synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained is shown in FIG. 1 (curve 4).

(88) The X-ray diffractogram of this talcose composition has diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) situated at a distance of 9.54 (I=100); a plane (002) situated at a distance of 4.73 (I=31); a plane (003) situated at a distance of 3.15 (I=96); a plane (060) situated at a distance of 1.52 (I=7).

(89) It is observed that the line corresponding to the plane (002) has a higher intensity than the line corresponding to the plane (020), the diffraction line characteristic of a plane (020) being partially coincident with the diffraction line characteristic of a plane (002) and being present only in the form of a very slight shoulder.

(90) Such a talcose composition has a coherent domain of 24 laminae (using line (003)). The ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 0.96.

(91) The near-infrared spectrum of the synthetic talc composition obtained is shown in FIGS. 4 and 5 (curve 14). It has a vibration band at 7185 cm.sup.1 representative of the vibration of the Mg.sub.3OH bond of the talc. The near-infrared spectrum also has a vibration band at 5251 cm.sup.1 which is characteristic of a synthetic talc according to the invention and corresponds to the presence of water bonded to the talc at lamina edges.

Example 6Preparation of a Composition Comprising Synthetic Mineral Particles According to the Invention

(92) A hydrogel precursor of synthetic mineral particles is prepared according to the protocol described in Example 1.

(93) The hydrogel obtained is subjected directly to a hydrothermal treatment at a temperature of 300 C. for 10 days at a pressure of 80 bar (8 MPa) (saturation vapor pressure of the water in the reactor). The concentration of sodium acetate during the hydrothermal treatment is 4 mol/1.

(94) The talcose composition that is recovered is then subjected to two successive cycles of washing with demineralized water and centrifugation and is finally dried by lyophilization for 72 hours.

(95) The X-ray diffractogram of the synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained is shown in FIG. 1 (curve 5).

(96) The X-ray diffractogram of this talcose composition has diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) situated at a distance of 9.51 (I=92); a plane (002) situated at a distance of 4.73 (I=26); a plane (003) situated at a distance of 3.14 (I=100); a plane (060) situated at a distance of 1.52 (I=5).

(97) It is observed that the line corresponding to the plane (002) has a higher intensity than the line corresponding to the plane (020), the diffraction line characteristic of a plane (020) being coincident with the diffraction line characteristic of a plane (002) and difficult to distinguish therefrom.

(98) Such a talcose composition has a coherent domain of 34 laminae (using line (003)). The ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 0.81.

(99) The near-infrared spectrum of the synthetic talc composition obtained is shown in FIG. 5 (curve 15). It has a vibration band at 7185 cm.sup.1 representative of the vibration of the Mg.sub.3OH bond of the talc. The near-infrared spectrum also has a vibration band at 5251 cm.sup.1 which is characteristic of a synthetic talc according to the invention and corresponds to the presence of water bonded to the talc at lamina edges.

(100) It is thus noted that, the more the duration of the hydrothermal treatment increases (as with an anhydrous heat treatment), the more the synthetic talc composition obtained is similar to a natural talc, in particular with regard to the values of coherent domains and the ratio between the intensity of the RX diffraction line characteristic of plane (001) and the intensity of the RX diffraction line characteristic of plane (003) (for a same hydrogel precursor and a constant hydrothermal treatment temperature).

(101) It can thus be noted that the near-infrared spectra of the various synthetic talcs according to the invention are very similar to a near-infrared spectrum of a natural talc (curve 16, FIG. 5), with the exception of the vibration band between 5000 cm.sup.1 and 5500 cm.sup.1 which, for a natural talc, does not have such a high intensity as for synthetic talcs according to the invention.

(102) The invention can be the subject of many other applications and of different variants with respect to the embodiments and examples described above. In particular, said talcose composition can comprise phyllosilicate mineral particles in which different metals are situated at octahedral sites, such that in (Si.sub.xGe.sub.1-x).sub.4M.sub.3O.sub.11, nH.sub.2O, M has the formula (Co.sub.0.5Ni.sub.0.5).