Synthetic mineral compound, composition including such a compound and method for manufacturing such a compound

Abstract

The invention relates to a mineral compound, referred to as synthetic mica, with formula A.sub.t(Si.sub.x-Ge.sub.1x).sub.4M.sub.zO.sub.10(OH).sub.2, wherein: A designates at least one monovalent interfoliar cation of a metal element, A having the formula Li.sub.w(1)Na.sub.w(2)K.sub.w(3)Rb.sub.w(4)Cs.sub.sw(5), each instance of w(i) representing a real number in the interval [0; 1], such that the sum of the instances of w(i) is equal to 1; t is a real number in the interval [0.3; 1]; x is a real number in the interval [0; 1]; M designates 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, each instance of y(i) representing a real number in the interval [0; 1], such as the formula (A); and z is a real number in the interval [2.50; 2.85]. The invention also relates to a composition comprising such a compound and a method for preparing such a compound. $\underset{i=1}{\overset{5}{.Math.}}w\left(i\right)=1^{″}$$\underset{i=1}{\overset{8}{.Math.}}y\left(i\right)=1^{″}$

Claims

1. A non-fluorinated synthetic mica compound having the following formula (I):
A.sub.t(Si.sub.xGe.sub.1-x).sub.4M.sub.zO.sub.10(OH).sub.2  (I) wherein: A is at least one monovalent interfoliary cation of a metal element having the formula Li.sub.w(1)Na.sub.w(2)K.sub.w(3)Rb.sub.w(4)Cs.sub.w(5); wherein each w(i) represents a real number of the interval [0; 1], such that Σ.sub.i=1.sup.5w(i)=1, t is a real number of the interval [0.3; 1], x is a real number of the interval [0; 1], 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); wherein each y(i) represents a real number of the interval [0; 1], such that Σ.sub.i=1.sup.8y(i)=1 z is a real number in the range [2.50; 2.85], and t+2z is a real number of the interval [5.3; 6.0].

2. The non-fluorinated synthetic mica compound according to claim 1, wherein y(3) is different from 1.

3. The non-fluorinated synthetic mica compound according to claim 1, wherein in formula (I), A denotes potassium.

4. The non-fluorinated synthetic mica compound according to claim 1, wherein it has, in X-ray diffraction, at least one diffraction line characteristic of a plane (001) located at a distance of between 9.70 Å and 10.70 Å.

5. The non-fluorinated synthetic mica compound according to claim 1, wherein said compound is organized according to a solid structure formed of sheets separated from each other by at least one interfoliary space, each cation A being disposed in said interfoliary spaces.

6. A composition comprising at least one non-fluorinated synthetic mica compound according to claim 1.

7. The composition according to claim 6, wherein the composition is free of iron.

8. The composition according to claim 6, wherein the composition is free of aluminum.

9. The composition according to claim 6, wherein the composition comprises particles of said compound having an average size of between 10 nm and 400 nm, as observed by electron microscopy.

10. A method for preparing a compound of following formula (I):
A.sub.t(Si.sub.xGe.sub.1-x).sub.4M.sub.zO.sub.10(OH).sub.2  (I) wherein: A is at least one monovalent interfoliary cation of a metal element having the formula Li.sub.w(1)Na.sub.w(2)K.sub.w(3)Rb.sub.w(4)Cs.sub.w(5); wherein each w(i) represents a real number of the interval [0; 1], such that Σ.sub.i=1.sup.5w(i)=1, t is a real number of the interval [0.3; 1], x is a real number of the interval [0; 1], 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); wherein each y(i) represents a real number of the interval [0; 1], such that Σ.sub.i=1.sup.8y(i)=1 z is a real number in the range [2.50; 2.85], and t+2z is a real number of the interval [5.3; 6.0]; said method comprising: preparing a precursor gel of the compound of formula (I) by a co-precipitation reaction between: 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 and potassium metagermanate, at least one metal salt of the divalent metal M, the molar proportion (Si.sub.xGe.sub.1-x)/M during the preparation of the precursor gel being in the range [2/1.425; 1.6] adding at least one hydroxide of formula AOH to the precursor gel so that the molar proportion of A/M is at least equal to t/z, and carrying out a solvothermal treatment of the precursor gel at a temperature of between 300° C. and 600° C.

11. The method according to claim 10, wherein, prior to said solvothermal treatment and following the precipitation of the precursor gel, the precursor gel is washed with a rinsing fluid.

12. The method according to claim 10, wherein said solvothermal treatment is carried out continuously.

13. The method according to claim 10, wherein said solvothermal treatment is carried out in aqueous medium.

14. The method according to claim 10, wherein after said solvothermal treatment, an anhydrous thermal treatment is carried out at a temperature between 500° C. and 600° C.

15. The method according to claim 11, wherein the rinsing fluid is water.

Description

(1) Other objects, features and advantages of the invention will become apparent upon reading the following description of one of its preferred embodiments given by way of a non-limiting example, and which refers to the appended figure, wherein:

(2) FIG. 1 shows a schematic view of a device making it possible to implement a method according to the invention in which the solvothermal treatment is carried out continuously,

(3) FIG. 2 represents an X-ray diffractogram of a composition comprising a compound of formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C.,

(4) FIG. 3 represents thermograms obtained by thermogravimetric analysis (TGA) and by thermodifferential analysis (TDA) of a composition comprising a compound of formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C.,

(5) FIG. 4 represents an X-ray diffractogram of a composition comprising a compound of formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C., followed by an anhydrous heat treatment at 550° C. for 5 hours,

(6) FIG. 5 represents thermograms obtained by thermogravimetric analysis (TGA) and by thermodifferential analysis (TDA) of a composition comprising a compound of formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C., followed by an anhydrous heat treatment at 550° C. for 5 hours,

(7) FIG. 6 represents an X-ray diffractogram of a composition comprising a compound of formula K.sub.0.3Si.sub.4Mg.sub.1.35O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C.,

(8) FIG. 7 represents an X-ray diffractogram of a composition comprising a compound of formula K.sub.0.3Si.sub.4Mg.sub.1.35O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C., followed by a treatment anhydrous thermal at 550° C. for 5 hours,

(9) FIG. 8 represents an X-ray diffractogram of a composition comprising a compound of formula Li.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C.

(10) FIG. 9 represents an X-ray diffractogram of a composition comprising a compound of formula Li.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C., followed by an anhydrous heat treatment at 550° C. for 5 hours

(11) FIG. 10 represents an X-ray diffractogram of a composition comprising a compound of formula KSi.sub.4Mg.sub.2.5O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C.,

(12) FIG. 11 represents an X-ray diffractogram of a composition comprising a compound of formula KSi.sub.4Mg.sub.2.5O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C., followed by an anhydrous heat treatment at 550° C. for 4 hours.

A—GENERAL PROTOCOL FOR THE PREPARATION OF A COMPOUND AND A COMPOSITION ACCORDING TO THE INVENTION

(13) 1/—Preparation of a Precursor Gel of a Compound of Formula (I) The precursor gel of a compound of formula (I) may be prepared by a coprecipitation reaction involving, as a reagent, at least one source of silicon and/or at least one source of germanium chosen from the group formed of potassium metasilicate and potassium metagermanate, and at least one metal salt of a divalent 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); Mg denoting magnesium, Co denoting cobalt, Zn denoting zinc, Cu denoting copper, Mn denoting manganese, Fe denoting iron, Ni denoting nickel, and Cr denoting chromium; and each y(i) representing a real number of the interval [0; 1], such that Σ.sub.i=1.sup.8y(i)=1

(14) This coprecipitation reaction makes it possible to obtain a precursor gel exhibiting the stoichiometry of a synthetic mica corresponding to the formula (I) of a compound according to the invention.

(15) The precursor gel is prepared by a coprecipitation reaction implemented from: 1. an aqueous solution in which at least one metal salt of a divalent metal M is dissolved, for example, in an aqueous solution of a metal sulphate 2. a solution of sulfuric acid (H.sub.2SO.sub.4), and 3. an aqueous solution of potassium metasilicate or an aqueous solution of potassium metagermanate, or a mixture of these two solutions in the molar proportions x/(1-x).

(16) The molar proportion (Si.sub.xGe.sub.1-x)/M during the preparation of this precursor gel is in the range [2/1.425; 1.6], and, in particular, in the range [2/1.3; 1.6].

(17) The preparation of this precursor gel is carried out according to the following protocol: 1. the solution comprising at least one metal salt is mixed with the sulfuric acid solution, 2. the aqueous solution of potassium metasilicate and/or potassium metagermanate is then added thereto, and the precursor gel is formed instantly.

(18) The resulting suspension comprising the precursor gel may be stirred at room temperature (for example at 22.5° C.) for 5 to 30 minutes and then subjected to several cycles of washing and centrifugation, or may be directly subjected to these washing cycles and centrifugation.

(19) The precursor gel may also be recovered after centrifugation (for example between 3000 and 1500 rpm, for 5 to 60 minutes) and elimination of the supernatant (potassium sulfate solution) and washing with demineralised water (for example three washes and successive centrifugations).

(20) The precursor gel washed and separated from the solution comprising potassium sulphate is then subjected to a solvothermal treatment as effected at the end of the last centrifugation, or possibly after having been dried (for example in an oven or by freeze-drying).

(21) At least one hydroxide of formula AOH is then added to the precursor gel so that the molar ratio A/M is at least equal to t/z.

(22) A suspension of precursor gel and hydroxyl AOH is thus obtained.

(23) 2/—Solvothermal Treatment of the Precursor Gel

(24) The precursor gel as previously obtained (after the addition of the hydroxide of formula AOH) is subjected to a solvothermal treatment at a temperature of in particular between 300° C. and 600° C.

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

(26) To do this, the precursor gel is placed in a reactor/autoclave which is placed inside an oven at a predetermined reaction temperature (established between 300° C. and 600° C.), during the entire course of the solvothermal treatment.

(27) Beforehand, the liquid/solid ratio may be adjusted to a value of between 2 and 80, in particular between 5 and 50 (the quantity of liquid being expressed in cm.sup.3, and the quantity of solid, in grams, and denoting the amount of dry gel only).

(28) In particular, it is preferable to place the reactor or the autoclave under the conditions of temperature and pressure of the solvothermal treatment for less than 6 hours, especially less than 3 hours, and more particularly less than one hour, after having added the hydroxide of formula AOH to the precursor gel.

(29) During the hydrothermal treatment, the precursor gel gradually acquires a gelatinous consistency. The composition obtained at the end of the solvothermal treatment has an observable crystallinity in X-ray diffraction, this crystallinity increasing with the duration of the solvothermal treatment and results in the corresponding diffractograms with the rapid appearance of characteristic lines which are refined and intensify rapidly during treatment.

(30) At the end of this solvothermal treatment, a composition is obtained comprising mineral particles of synthetic mica according to formula (I) of a compound according to the invention in suspension in a solution, in particular an aqueous solution. At the end of this solvothermal treatment, the composition contained in the reactor is recovered by centrifugation (between 3000 and 15000 rpm, for 5 to 60 minutes) and then elimination of the supernatant.

(31) The composition comprising mineral particles recovered after the last centrifugation may then be dried: in the oven at a temperature between 60° C. and 130° C., for 1 to 24 hours, or, by lyophilization, for example in a freeze-dryer of the CHRIST ALPHA® 1-2 LD Plus type, for 48 hours to 72 hours, or by atomization.

(32) In a second variant of a method according to the invention, the solvothermal treatment of the precursor gel is carried out continuously.

(33) In a method according to the invention in which the solvothermal treatment is carried out continuously, a reactor 15 for the preparation of mineral particles of a compound according to the invention is used continuously (as illustrated in FIG. 1) comprising: a first portion 11 of conduit in which a first aqueous solution 20 comprising the precursor gel is introduced, a second conduit portion 12 in which a second aqueous solution 21 comprising at least one hydroxide of formula AOH (KOH for example) is introduced, a third conduit portion 13 arranged after the first conduit portion 11 and the second conduit portion 12 and extending to an inlet 9 of a reaction chamber 16, the first conduit portion 11 and the second conduit portion 12 joining at a point 17 from which the third conduit portion 13 begins, a reaction conduit 14 extending from the inlet 9 into the reaction chamber 16, and after the third conduit portion 13.

(34) A peristaltic pump 18 continuously feeds the first conduit portion 11 with the first aqueous solution contained in a tank 30 with stirring. A second peristaltic pump 19 continuously feeds the second conduit portion 12 with the second aqueous solution 21 contained in a tank 31 with stirring.

(35) Alternatively, the hydroxide of the formula AOH may also be added to the precursor gel in the reservoir 30, water allowing adjustment of the dilution of the precursor gel that may be disposed in the reservoir 31.

(36) For the purposes of controlling the temperature within the reaction conduit 14, the reaction chamber 16 is an oven comprising a heating sleeve comprising resistors of ceramic material. The reaction conduit 14 is in the general shape of a coil wound in multiple turns inside the heating sleeve, until it leaves the latter through an outlet 8 constituting the outlet of the reaction chamber 16.

(37) The mixture inside the third conduit portion 13 is close to ambient temperature. The third conduit portion 13 is optional, and the point 17 and the input 9 may be combined. In the embodiment shown in FIG. 1, the third conduit portion 13 has, for example, a length of between 10 cm 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 minutes, and in particular less than 15 minutes, or even less than 5 minutes, or of the order of one minute.

(39) In addition, it is possible to introduce other solutions and, in particular, to adjust the amount of solvent at different levels of the device, for example using inlets 4, 5 located before the solvothermal treatment zone, the inlet 4 being located before the point 17, while the inlet 6 is located at the level of the solvothermal treatment zone, while the inlet 7 is located after the outlet of the solvothermal treatment zone and before the outlet of the suspension obtained.

(40) A pressure regulator 2 is disposed downstream of the reaction chamber 16 in connection with a fifth conduit portion 10 extending from the outlet 8 of the reaction conduit 14 and the reaction vessel 16 to a vessel 25 in which a suspension comprising the mineral particles obtained is recovered.

(41) Closing a valve 32 interposed on the fifth conduit portion 10 makes it possible to circulate the suspension obtained at the outlet 8 of the conduit 14 in a reaction circuit 33 arranged to pass this suspension through a porous sinter 34 that is adapted to retain the particles and allow their recovery. The porous sinter 34 is immersed in an ice container 35 to cool the suspension leaving the reactor. In this case, valves 36 and 37 that are arranged on the branch circuit 33 are open. The porous sinter 34 is chosen to retain the synthesized mineral particles by separating them from the liquid medium which carries them. The sintered material is, for example, made of 316 L stainless steel, with a porosity of 50 μm. When the porous sinter 34 is clogged with mineral particles, it is sufficient to open the valve 32 and to close the valves 36 and 37 in order to directly recover the suspension in the container 25, this suspension being cooled by passing it through the ice container 35, then washed and centrifuged several times in order to recover the mineral 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 allows the suspension obtained to be directed towards the outlet of the reaction conduit 14 and to another sinter as soon as the previous sinter is clogged by the mineral particles.

(42) Alternatively, in the case where a solution comprising the precursor gel and the hydroxide of formula AOH is initially prepared, the same and only portion of the conduit replaces the first conduit portion 11 and the second conduit portion 12. In another variant, it is also possible for the tank 30 to contain a solution comprising the precursor gel and for the tank 31 to contain the hydroxide of formula AOH.

(43) In each case, it is important to control the dilution of the precursor gel introduced into each portion of the conduit and into the reaction conduit 14 in order to allow continuous circulation of the reaction medium in the reaction conduit 14, and in all the conduits feeding the precursor gel composition to the inlet 9 of the reaction chamber 16. The concentration of precursor gel in the precursor gel composition introduced at the inlet of the reaction chamber 16 is advantageously between 10.sup.−3 mol/L and several mol/L, for example of the order of 0.01 mol/L. Note that this concentration is much lower than the concentrations used in the methods for preparing synthetic mineral particles such as phyllosilicates of the prior art.

(44) The solvothermal treatment carried out in the reaction conduit 14 is a solvothermal treatment which may, in particular, be carried out under supercritical or subcritical conditions, and, in particular, under homogeneous subcritical conditions. Thus, it is possible to choose the temperature and the pressure at which this solvothermal treatment is carried out so that the precursor gel composition introduced at the reactor inlet, and in particular the solvent(s) it comprises is under supercritical conditions or under homogeneous subcritical conditions, i.e. above the liquid-gas equilibrium curve of the solvent, so that the solvent is present in the liquid state and not in the form of a liquid-gas mixture or gas alone.

(45) At the end of this solvothermal treatment, a suspension is obtained comprising mineral particles in solution, in particular in aqueous solution. At the end of this solvothermal treatment, the suspension obtained is recovered by filtration, for example using a ceramic sinter, or else by centrifugation (between 3000 and 15000 rpm, for 5 to 60 minutes) then elimination of the supernatant.

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

(47) The composition comprising mineral particles recovered after the last centrifugation may then be dried: in the oven at a temperature between 60° C. and 130° C., for 1 to 24 hours, or, by lyophilization, for example in a CHRIST ALPHA® 1-2 LD Plus lyophilizer, for 48 hours to 72 hours, by irradiation of microwaves, by atomization, or by any other powder drying technique.

(48) The inventors have thus been able to note that not only an extremely short time (less than one minute) of solvothermal treatment under supercritical conditions is sufficient to allow conversion of the initial gel into a crystallized and thermally stable material, but also that the synthetic mineral particles obtained have a crystallinity comparable to that of natural micas.

(49) The mineral particles contained in a composition obtained by a method according to the invention have remarkable properties in terms of purity, crystallinity and thermal stability, and for an extremely short duration of solvothermal treatment.

B—ANALYSIS AND STRUCTURAL CHARACTERIZATION

(50) 1—X-Ray Diffraction Analysis

(51) FIGS. 2, 4 and 6 to 11 show X-ray diffractograms, each of which shows the relative intensity of the signal (number of strokes per second) as a function of the inter-reticular angstrom distance.

(52) A compound according to the invention has, in X-ray diffraction, at least one diffraction line that is characteristic of a plane (001) located at a distance of between 9.80 Å and 10.20 Å. Such a diffraction line is characteristic of micas.

(53) FIGS. 2, 4 and 6 to 11 respectively show the results of analyses carried out in X-ray diffraction on: a synthetic mica composition comprising a compound of formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C. (FIG. 2), a synthetic mica composition comprising a compound of formula K.sub.0.3Si.sub.4Ni.sub.1.35Mg.sub.1.35O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C. followed by an anhydrous heat treatment at 550° C. for 5 hours (FIG. 4), a synthetic mica composition comprising a compound of formula K.sub.0.3Si.sub.4Ni.sub.1.35Mg.sub.1.35O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C. (FIG. 6), a synthetic mica composition comprising a compound of formula K.sub.0.3Si.sub.4Ni.sub.1.35Mg.sub.1.35O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C. followed by an anhydrous heat treatment at 550° C. for 5 hours (FIG. 7), a synthetic mica composition comprising a compound of formula Li.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C. (FIG. 8), a synthetic mica composition comprising a compound of formula Li.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C. followed by an anhydrous heat treatment at 550° C. for 5 hours (FIG. 9), a synthetic mica composition comprising a compound of formula KSi.sub.4Mg.sub.2.5O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C. (FIG. 10), a synthetic mica composition comprising a compound of formula KSi.sub.4Mg.sub.2.5O.sub.10(OH).sub.2 obtained after a 24-hour hydrothermal treatment at 300° C. followed by an anhydrous heat treatment at 550° C. for 5 hours (FIG. 11).

(54) The diffractogram RX shown in FIG. 2 was recorded on a CPS 120 device marketed by 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 intensity is 25 mA. The Bragg relation giving the structural equidistance is: d.sub.hk1=0.89449/sin θ (with the use of a cobalt anticathode).

(55) The X-ray diffractograms represented in FIGS. 4 and 6 to 11 were recorded on a Panalytical MPDPro® diffractometer sold by Panalytical® (The Netherlands). This is a theta/theta multiconfiguration diffractometer (transmission, reflection, variable temperature) equipped with a fast linear detector. The Bragg relation giving the structural equidistance is: d.sub.hk1=0.7703/sin θ (with the use of a copper anticathode).

(56) 2—Thermal Analyses

(57) FIGS. 3 and 5 show curves obtained by thermogravimetric analysis (TGA) (curves 48 and 57) and by thermodifferential analysis (TDA) (curves 49 and 58) of a composition comprising a compound (Al.sub.2Si.sub.2O.sub.5(OH).sub.4 according to the invention obtained after a hydrothermal treatment of 24 hours at 300° C. (FIG. 3) and the same composition was further subjected to an anhydrous heat treatment at 550° C. for 5 hours.

(58) In FIGS. 3 and 5, the curves 49 and 58 (thermodifferential analysis, in solid lines in FIGS. 3 and 5) represent the heat released or absorbed by the analyzed composition sample (in mW on the y-axis located on the left side of the thermogram) as a function of the temperature (from 0° C. to 1,000° C.). In FIG. 3, the curves 48 and 57 (thermogravimetric analysis, in dashed lines in FIGS. 3 and 5) represent the mass variation of the composition sample analyzed (in % on the ordinate axis situated on the right side of the thermogram) as a function of the temperature (from 0° C. to 1,000° C.).

(59) The thermograms obtained are characteristic of micas, with dehydroxylation starting at 550° C., and a phase transformation at around 800° C. The dehydroxylation temperature is slightly lower than for a natural mica, which is explained by the smaller size of the particles obtained.

(60) The ATD and ATG analyses were carried out with a Diamond TG/TDA® thermobalance sold by PERKIN ELMER® (USA) in a temperature range extending from 30° C. to 1,000° C. under air, and with a heating rate of 10° C./min.

(61) 3—Microscopic Observations and Assessment of Particle Size

(62) Given the great fineness of the powders that can constitute the compositions according to the invention, the size and particle size distribution of the mineral particles that compose them were assessed by observation using scanning electron microscopy and field effect and electron microscopy in transmission.

(63) It can be seen that the average size of the elementary particles varies between 10 nm and 400 nm. In particular, the particles have a thickness of between 1 nm and 60 nm and a largest dimension of between 10 nm and 500 nm.

(64) In addition, it has been observed that the synthetic mica particles prepared by a method according to the invention have a pearlescent effect which may be of interest in many industrial fields.

(65) The following examples illustrate the preparation method according to the invention and the structural characteristics of the compounds thus obtained.

EXAMPLE 1

Preparation of a Composition Comprising Mineral Particles According to the Invention

(66) 300 ml of an aqueous solution of magnesium sulphate (33.27 g or 0.135 mol) and sulfuric acid (120 g of a 0.5M solution) are prepared.

(67) A solution of potassium metasilicate is then prepared by diluting 59.35 g (i.e. 0.2 mol) of an aqueous solution of potassium metasilicate (K.sub.2SiO.sub.3) containing 52% solids in 150 ml of demineralized water. This solution of potassium metasilicate is added to the previous solution and a white precipitate is formed instantly.

(68) The resulting suspension is stirred for 5 minutes. Three washing cycles are then carried out with distilled water and centrifugation at 8,000 rpm for 10 minutes each time the centrifugation is repeated. These successive washes with elimination of the supernatant solution after each centrifugation make it possible to remove the potassium sulphate formed during the precipitation reaction of the precursor gel. Finally, the recovered white precipitate is suspended in demineralised water to a final volume of 500 ml and subjected to ultrasound with magnetic stirring for 10 minutes until a homogeneous suspension of precursor gel with a white color is obtained.

(69) 988 mg of hydrated potassium hydroxide of formula KOH are then added to the precursor gel (containing 85% of potassium hydroxide and 15% of water, i.e. 0.015 mol of pure potassium hydroxide added) previously diluted in 30 ml of demineralized water, and the suspension obtained is stirred magnetically for 5 minutes at room temperature (22.5° C.).

(70) The precursor gel placed in a closed titanium reactor placed in an oven is then subjected to a hydrothermal treatment at a temperature of 300° C. for 24 hours under the saturated vapor pressure of the water in the reactor.

(71) After cooling to room temperature, the reactor is opened and the suspension obtained is centrifuged. After centrifugation, a composition comprising at least 80% by weight of particles of formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 is obtained.

(72) The composition of particles recovered after centrifugation is dried in an oven for 12 hours at 120° C. and then ground in a mortar. The resulting composition is in the form of a white powder.

(73) The X-ray diffractogram of the composition of particles of formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 thus obtained is shown in FIG. 2. The X-ray diffractogram of this composition has the following characteristic diffraction lines: a plane (001) located at a distance of 10.15 Å (line 40); a plane (002) located at a distance of 5.03 Å (line 41); a plane (020) at a distance of 4.53 Å (line 42); planes (003) and (022) located at a distance of 3.34 Å (line 43); a plane (131) located at a distance of 2.60 Å (line 44); a plane (005) located at a distance of 2.01 Å (line 45); a plane (060) located at a distance of 1.52 Å (line 46).

(74) Curves 48 and 49 obtained by TGA-TDA of the composition of particles of formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 thus obtained are shown in FIG. 3. Such a thermogram is characteristic of micas, with a dehydroxylation from 550° C., and a phase transformation at 800° C. These transformation temperatures are slightly lower than for a natural mica because the size of the particles obtained is smaller.

(75) The composition is then subjected to an anhydrous heat treatment at 550° C. in an oven for 5 hours. The composition obtained after the anhydrous heat treatment remains white.

(76) The X-ray diffractogram of the composition of particles of formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 obtained after an anhydrous heat treatment at 550° C. is shown in FIG. 4.

(77) The X-ray diffractogram of this composition, after the anhydrous heat treatment, has the following characteristic diffraction lines:

(78) a plane (001) located at a distance of 10.24 Å (line 50); a plane (002) located at a distance of 5.02 Å (line 51); a plane (020) at a distance of 4.56 Å (line 52); planes (003) and (022) located at a distance of 3.37 Å (line 53); a plane (131) located at a distance of 2.60 Å (line 54); a plane (005) located at a distance of 2.02 Å (line 55); a plane (060) located at a distance of 1.52 Å (line 56).

(79) As may be seen in FIG. 4 compared with FIG. 2, such an anhydrous heat treatment makes it possible to increase the proportion of synthetic mineral particles of formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 in the composition obtained. In particular, it may be observed that the shoulder initially present at the base of the peak corresponding to the plane (001) in FIG. 2 has disappeared in FIG. 4.

(80) The curves 57 and 58 obtained by TGA-TDA of the composition of particles of formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 thus obtained after such an anhydrous heat treatment are shown in FIG. 5. As in FIG. 3, such a thermogram is characteristic of micas, with dehydroxylation from 550° C., and a phase transformation at 800° C. These transformation temperatures are slightly lower than for a natural mica because the size of the particles obtained is smaller.

(81) In FIGS. 3 and 5, the curves 49 and 58 (thermodifferential analysis, in solid lines) represent the heat released or absorbed by the analyzed composition sample (in mW on the ordinate axis situated on the left side of the thermogram) as a function of the temperature. In FIG. 3, curves 48 and 57 (thermogravimetric analysis, in dotted lines) represent the mass variation of the analyzed composition sample (in % on the ordinate axis located on the right side of the thermogram) as a function of the temperature.

EXAMPLE 2

Preparation of a Composition Comprising Mineral Particles According to the Invention

(82) 300 ml of an aqueous solution of magnesium sulphate (16.64 g or 0.0675 mol), nickel sulphate (17.74 g or 0.0675 mol) and sulfuric acid (120 g of 0.5M solution) are prepared.

(83) A solution of potassium metasilicate is then prepared by diluting 59.35 g (0.2 mol) of a first aqueous solution of potassium metasilicate at 52% solids in 150 ml of demineralized water. This solution of potassium metasilicate is added to the previous solution and a green precipitate is formed instantly.

(84) The resulting suspension is stirred for 5 minutes. Three washing cycles are then carried out with distilled water and centrifugation at 8,000 rpm for 10 minutes each time the centrifugation is repeated. These successive washes with elimination of the supernatant solution after each centrifugation make it possible to eliminate the potassium sulphate formed during the precipitation reaction of the precursor gel. Finally, the recovered green precipitate is suspended in demineralized water to a final volume of 500 ml and subjected to ultrasound with magnetic stirring for 10 minutes until a homogeneous suspension of precursor gel of green color is obtained.

(85) 988 mg of hydrated potassium hydroxide (containing 85% of potassium hydroxide and 15% of water, i.e. 0.015 mol of pure potassium hydroxide added), previously diluted in 30 ml of demineralized water, are then added to the precursor gel, and the suspension obtained is stirred magnetically for 5 minutes at room temperature (22.5° C.).

(86) The precursor gel placed in a closed titanium reactor placed in an oven is then subjected to a hydrothermal treatment at a temperature of 300° C. for 24 hours under the saturated vapor pressure of the water in the reactor.

(87) After cooling to room temperature, the reactor is opened and the resulting suspension is centrifuged. After centrifugation, a composition comprising at least 80% by weight of particles of compound of formula K.sub.0.3Si.sub.4Ni.sub.1.35Mg.sub.1.35O.sub.10(OH).sub.2.

(88) The composition of particles recovered after centrifugation is dried in an oven for 12 hours at 120° C. and then ground in a mortar. The particle composition obtained after drying is in the form of a green powder.

(89) The X-ray diffractogram of the composition of particles of formula K.sub.0.3Si.sub.4Ni.sub.1.35Mg.sub.1.35O.sub.10(OH).sub.2 thus obtained is shown in FIG. 6. The X-ray diffractogram of this composition shows the following characteristic diffraction lines: a plane (001) located at a distance of 9.70 Å (line 60); a plane (020) located at a distance of 4.48 Å (line 61); planes (003) and (022) located at a distance of 3.27 Å (line 62); a plane (131) located at a distance of 2.58 Å (line 63); a plane (060) located at a distance of 1.51 Å (line 64).

(90) The composition is then subjected to an anhydrous heat treatment at 550° C. in an oven for 5 hours.

(91) The X-ray diffractogram of the composition of the formula K.sub.0.3Si.sub.4Ni.sub.1.35Mg.sub.1.35O.sub.10(OH).sub.2 obtained after an anhydrous heat treatment at 550° C. is shown in FIG. 7. The X-ray diffractogram of this composition has the following characteristic diffraction lines: a plane (001) located at a distance of 9.95 Å (line 65); a plane (020) located at a distance of 4.48 Å (line 66); planes (003) and (022) located at a distance of 3.27 Å (line 67); a plane (131) located at a distance of 2.58 Å (line 68); a plane (060) located at a distance of 1.51 Å (line 69).

(92) As may be seen in FIG. 7 compared with FIG. 6, such an anhydrous heat treatment makes it possible to increase the crystallinity of the particles of the composition obtained. In particular, it may be seen that the peak corresponding to the (001) plane has increased in intensity and has become finer.

EXAMPLE 3

Preparation of a Composition Comprising Mineral Particles According to the Invention

(93) 300 ml of an aqueous solution of magnesium sulphate (33.27 g or 0.135 mol) and sulfuric acid (120 g of a 0.5M solution) are prepared.

(94) A solution of potassium metasilicate is then prepared by diluting 59.35 g (i.e. 0.2 mol) of an aqueous solution of potassium metasilicate (K.sub.2SiO.sub.3) containing 52% solids in 150 ml of demineralized water. This solution of potassium metasilicate is added to the previous solution and a white precipitate is formed instantly.

(95) The resulting suspension is stirred for 5 minutes. Three washing cycles are then carried out with distilled water and centrifugation at 8,000 rpm for 10 minutes at each repeated centrifugation. These successive washes with elimination of the supernatant solution after each centrifugation make it possible to eliminate the potassium sulphate formed during the precipitation reaction of the precursor gel. Finally, the recovered precipitate is suspended in demineralized water to a final volume of 500 ml and subjected to ultrasound with magnetic stirring for 10 minutes until a homogeneous suspension of precursor gel with a white color is obtained.

(96) 629 mg of lithium hydroxide (LiOH) (i.e. 0.015 mol of LiOH), previously diluted in 30 ml of demineralized water, are then added to the precursor gel, and the suspension obtained is stirred magnetically for 5 minutes at room temperature (22.5° C.).

(97) The precursor gel placed in a closed titanium reactor placed in an oven is then subjected to a hydrothermal treatment at a temperature of 300° C. for 24 hours under the saturated vapor pressure of the water in the reactor.

(98) After cooling to room temperature, the reactor is opened and the suspension obtained is centrifuged. After centrifugation, a composition comprising at least 80% by weight of particles of compound of formula L.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 is recovered.

(99) The composition of particles recovered after centrifugation is dried in an oven for 12 hours at 120° C. and then ground in a mortar. The particle composition obtained after drying is in the form of a white powder.

(100) The X-ray diffractogram of the composition of particles of compound of formula L.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2, thus obtained is shown in FIG. 8. The X-ray diffractogram of this composition has the following characteristic diffraction lines: a plane (001) located at a distance of 10.28 Å (line 70); a plane (002) located at a distance of 4.94 Å (line 71); a plane (020) located at a distance of 4.48 Å (line 72); planes (003) and (022) located at a distance of 3.30 Å (line 73); a plane (131) located at a distance of 2.60 Å (line 74); a plane (060) located at a distance of 1.51 Å (line 75).

(101) The composition is then subjected to an anhydrous heat treatment at 550° C. in an oven for 5 hours.

(102) The X-ray diffractogram of the composition of particles of compound of formula Li.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 thus obtained after an anhydrous heat treatment at 550° C. is shown in FIG. 9. The X-ray diffractogram of this composition exhibits, after the anhydrous heat treatment, the following characteristic diffraction lines: a plane (001) located at a distance of 10.12 Å (line 76); a plane (002) located at a distance of 4.95 Å (line 77); a plane (020) located at a distance of 4.50 Å (line 78); planes (003) and (022) located at a distance of 3.30 Å (line 79); a plane (131) located at a distance of 2.59 Å (line 80); a plane (060) located at a distance of 1.52 Å (line 81).

(103) As may be seen in FIG. 9 compared with FIG. 8, such an anhydrous heat treatment makes it possible to increase the crystallinity of the particles of the composition obtained. In particular, it may be seen that the peak corresponding to the (001) plane has increased in intensity and has become finer.

EXAMPLE 4

Preparation of a Composition Comprising Mineral Particles According to the Invention

(104) 300 ml of an aqueous solution of magnesium sulphate (30.81 g or 0.125 mol) and sulfuric acid (120 g of a 0.5M solution) are prepared.

(105) A solution of potassium metasilicate is then prepared by diluting 59.35 g (i.e. 0.2 mol) of an aqueous solution of potassium metasilicate (K.sub.2SiO.sub.3) containing 52% solids in 150 ml of demineralized water. This solution of potassium metasilicate is added to the previous solution and a white precipitate is formed instantly.

(106) The resulting suspension is stirred for 5 minutes. Three washing cycles are then carried out with distilled water and centrifugation at 8,000 rpm for 10 minutes at each repeated centrifugation. These successive washes with elimination of the supernatant solution after each centrifugation make it possible to eliminate the potassium sulphate formed during the precipitation reaction of the precursor gel. Finally, the recovered white precipitate is suspended in demineralised water to a final volume of 500 ml and subjected to ultrasound with magnetic stirring for 10 minutes until a homogeneous suspension of precursor gel with a white color is obtained.

(107) 3.42 g of hydrated potash of formula KOH (containing 85% of potassium hydroxide and 15% of water, i.e. 0.052 mol of pure potassium hydroxide added), previously diluted in 30 ml of demineralized water, are and the suspension obtained is stirred magnetically for 5 minutes at room temperature (22.5° C.).

(108) The precursor gel placed in a closed titanium reactor placed in an oven is then subjected to a hydrothermal treatment at a temperature of 300° C. for 24 hours under the saturated vapor pressure of the water in the reactor.

(109) After cooling to room temperature, the reactor is opened and the resulting suspension is centrifuged. After centrifugation, a composition comprising at least 80% by weight of particles of compound of formula KSi.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 is recovered.

(110) The composition of particles recovered after centrifugation is dried in an oven for 12 hours at 120° C. and then ground in a mortar. The resulting composition is in the form of a white powder.

(111) The X-ray diffractogram of the composition of particles of compound of formula KSi.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 thus obtained is represented in FIG. 10. The X-ray diffractogram of this composition has the following characteristic diffraction lines: a plane (001) located at a distance of 10.14 Å (line 85); a plane (002) located at a distance of 5.04 Å (line 86); a plane (020) located at a distance of 4.51 Å (line 87); a plane (003) located at a distance of 3.33 Å (line 88); a plane (131) located at a distance of 2.30 Å (line 89); a plane (060) located at a distance of 1.52 Å (line 90).

(112) The composition is then subjected to an anhydrous heat treatment at 550° C. in an oven for 4 hours. The composition obtained after the anhydrous heat treatment remains white.

(113) The X-ray diffractogram of the composition of particles of the formula KSi.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 obtained after an anhydrous heat treatment at 550° C. is shown in FIG. 11. The X-ray diffractogram of this composition shows, after the anhydrous heat treatment, the following characteristic diffraction lines: a plane (001) located at a distance of 10.12 Å (line 91); a plane (002) located at a distance of 5.09 Å (line 92); a plane (020) located at a distance of 4.52 Å (line 93); a plane (003) located at a distance of 3.40 Å (line 94); a plane (131) located at a distance of 2.60 Å (line 95); a plane (060) located at a distance of 1.52 Å (line 96).

(114) As may be seen in FIG. 11 compared with FIG. 10, such an anhydrous heat treatment makes it possible to increase the crystallinity of the particles of the composition obtained. In particular, it may be seen that the peak corresponding to the (001) plane has increased in intensity and has become finer.

EXAMPLE 5

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

(115) 300 ml of an aqueous solution of magnesium sulphate (33.27 g or 0.135 mol) and sulfuric acid (120 g of a 0.5M solution) are prepared.

(116) A solution of potassium metasilicate is then prepared by diluting 59.35 g (i.e. 0.2 mol) of an aqueous solution of potassium metasilicate (K.sub.2SiO.sub.3) containing 52% solids in 150 ml of demineralized water. This solution of potassium metasilicate is added to the previous solution and a white precipitate is formed instantly.

(117) The resulting suspension is stirred for 5 minutes. Three washing cycles are then carried out with distilled water and centrifugation at 8,000 rpm for 10 minutes at each repeated centrifugation. These successive washes with elimination of the supernatant solution after each centrifugation make it possible to eliminate the potassium sulphate formed during the precipitation reaction of the precursor gel. Finally, the recovered white precipitate is suspended in demineralised water to a final volume of 500 ml and subjected to ultrasound with magnetic stirring for 10 minutes until a homogeneous suspension of precursor gel with a white color is obtained.

(118) 988 mg of hydrated potassium hydroxide (containing 85% of potassium hydroxide and 15% of water, i.e. 0.015 mol of added pure potassium hydroxide), previously diluted in 30 ml of demineralized water, are then added to the precursor gel, and the suspension obtained is stirred magnetically for 5 minutes at room temperature (22.5° C.).

(119) The precursor gel diluted in 300 ml of pure water is then placed in the tank 30 (see FIG. 1) Pure water which also makes it possible to adjust, if necessary, the dilution of the precursor gel in the portion 13 of the conduit 20 is placed in the tank 31.

(120) The peristaltic pumps 18, 19 make it possible to convey the two solutions separately via steel conduits having an outer diameter of ⅛ of an inch (3.175 mm) and an internal diameter of 1.57 mm, and at a flow rate of 2 mL/min each, i.e. a total flow of 4 mL/min at point 17 where the mixture of the solution containing the precursor gel and pure water occurs continuously, a few centimeters before the inlet 9 of the reaction conduit 14. The temperature in the chamber 16 is 400° C., and the pressure in the reaction conduit 14 is maintained (by virtue of the pressure regulator 2) greater than 22.1 MPa (of the order of 25 MPa), so that the reaction medium which circulates inside the reaction conduit 14 in the enclosure 16 is under conditions above the critical point of water (374° C., 221 bar).

(121) The precursor gel thus undergoes a hydrothermal treatment in the reaction chamber 16, which makes it possible to convert this precursor gel into a suspension of synthetic mineral particles of the formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2. The residence time in the reaction conduit 14 between the inlet 9 and the outlet 8 is 40 seconds.

(122) After cooling, the suspension exiting the outlet 8 of the reactor 15 is a colloidal suspension of synthetic mineral particles of the formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2. It has the appearance of a white milky composition which decays in several tens of minutes This suspension is subjected to a centrifugation cycle (10 min at 8,000 rpm). After centrifugation, on the one hand, a composition comprising synthetic mineral particles of formula KSi.sub.4Mg.sub.2.7O.sub.10(OH).sub.2, and, on the other hand, a supernatant aqueous solution, is obtained.

(123) The composition of particles recovered after centrifugation is dried in an oven (120° C. for 12 hours) and then ground with a mortar. The resulting composition is in the form of a white powder.

(124) The X-ray diffractogram of the composition of particles of formula K.sub.0.3Si.sub.4Mg.sub.2.7O.sub.10(OH).sub.2 has the following characteristic diffraction lines: a plane (001) located at a distance of 10.65 Å; a plane (002) located at a distance of 5.06 Å; a plane (020) located at a distance of 4.55 Å; planes (003) and (022) located at a distance of 3.35 Å; a plane (131) located at a distance of 2.62 Å; a plane (060) located at a distance of 1.53 Å.

(125) The invention may be subject to many variants. In particular, it is possible to prepare other compounds than those exemplified and corresponding to formula (I) by varying the nature of the metal salts and the hydroxide employed or also by using a source of germanium in addition to a silicon source during the preparation of the precursor gel.