ES2924598B2 - Procedure for the preparation of ceramic, bioceramic and refractory materials of a basic nature with a high content of forsterite by means of reactive grinding - Google Patents

Description

DESCRIPTION

Procedure for the preparation of ceramic, bioceramic and refractory materials of a basic nature with a high content of forsterite by means of reactive grinding

TECHNIQUE SECTOR

The present invention refers to a process for the preparation of ceramic materials that contain magnesium and silicon combined in their chemical composition, with a high content of magnesium orthosilicate ( ² MgO.SiO ² ) crystalline phase called forsterite, preparation of bioceramics and refractories of character basic, as well as its applications and uses of interest. Technical Sector: Chemistry. Various non-metallic mineral products. Various manufacturing industries and recycling. Ceramic and refractory materials. Structural material for the construction of high temperature furnaces (> 1500 °C). Steel and metallurgy. Bioceramics (biocompatible ceramics).

BACKGROUND OF THE INVENTION

As "basic refractories" are designated a certain number of refractory materials and products where the sum of magnesium, calcium and chromium oxides is predominant [Singer, F. and Singer S., Industrial Ceramics, Vols. 1, 2 and 3, Ed Urmo, Barcelona, 1971; Norton, F.H., Refractories, translation 3rd American edition, Ed Blume, Barcelona, 1972; Baumgart, W., Process Mineralogy of Ceramic Materials, F. Enke Publications, Stuttgart, 1984]. This group includes chromite and forsterite refractories which, although they should be considered neutral, can partly replace purely basic refractories. Transition products are very numerous and interesting for their technical applications, such as those of magnesia-chromite [Alper, A.M., Refractory Materials, Vol. 5: High-Temperature oxides, Part I: Magnesia, Lime and Chrome Refractories, Academia Press , New York and London, 1970].

An extensive review of the research and state of the art on forsterite refractory materials has been previously carried out, particularly considering this crystalline phase as the main component of refractory materials classified as basic in nature [Alper, A.M., Refractory Materials, Vol. 5 : High-Temperature oxides, Part I: Magnesia, Lime and Chrome Refractories, Academy Press, New York and London, 1970].

Within the general classification of basic refractory materials, the materials Forsterite refractories (magnesium ortho-silicate MgSiO ⁴ ) with a molar chemical composition of 2MgO.SiO2 and a melting point of 1910 °C, have been prepared by heat treatment from natural silicates using raw materials with high magnesium content, such as olivine from Norway and dunites and serpentines from Russia and India. Its name is due to Adolarius Jacob Forster (1739-1806). The application of olivine rock in the field of refractory materials began as early as 1925 by Goldschmidt [Singer, F. and Singer S., Industrial Ceramics, Vols. 1, 2 and 3, Ed Urmo, Barcelona, 1971]. It should then be referred to as an "olivine refractory" since olivine is the original raw material, but the term "forsterite refractory" is nevertheless often applied as the end product of manufacture. The raw materials most commonly used to obtain forsterite are olivine from Norway and dunite and serpentine from Russia and India.

The natural rock olivine is olive green in color, to which its name from the Latin "olive" alludes, it is composed of mixed crystals of forsterite and fayalite (iron silicate FeO.SiO ² ), together with small amounts of serpentine and sometimes , talc, varying the relative proportions of the different mineral phases with the origin of the rock, hence compositions such as (Mg ⁰ . ⁹ Fe ⁰ . ¹ ) ² SiO ⁴ [Alper, AM, Refractory Materials, Vol. 5 : High-Temperature oxides, Part I: Magnesia, Lime and Chrome Refractories, Academy Press, New York and London, 1970]. The only mineral phase with refractory properties is forsterite, with fayalite being a constituent of metallurgical iron slag; Talc and serpentine produce significant amounts of liquid phase above 1500 °C, which adversely affects the refractoriness of the material, even more so if it is subjected to a load or thermomechanical stresses [Ford, WF, The Effect of Heat on Ceramics , Text-Book Series, Ed MacLaren and Sons Ltd., London, 1978].

Furthermore, forsterite refractories do not have appreciable reactivity with magnesite, they have very low thermal conductivity and can be used in combination with magnesite and chrome-magnesite refractories up to 1500 °C, with high alumina refractories up to 1500 °C and with dolomite up to 1400 °C, with good thermomechanical properties. Olivine with a high forsterite content is a very useful refractory material and as it does not present notable contractions or expansions by heat treatment (from -1 to 0%, compared to -60% for magnesite, -15% for refractory clay and 15% for quartzite), nor crystalline modifications or evolution of gases during the thermal process, can be cut from the natural rock in the desired shape and used directly for the bottoms or surfaces of furnaces, heating furnaces and other applications in which it is useful a type of refractory with hardly any porosity. Olivine can also be used crushed and molded according to interest to apply [Alper, AM, Refractory Materials, Vol. 5: High-Temperature oxides, Part I: Magnesia, Lime and Chrome Refractories, Academic Press, New York and London, 1970]. In general, the melting point of a good forsterite refractory material would be around 1700 -1900 °C, depending on the impurities present. Suitable binders are usually added to hold the refractory particles together, such as chromite and caustic magnesite. As a technical requirement, a true "forsterite refractory" should be one in which this silicate is present in large proportion as magnesium orthosilicate (chemical composition ² MgO.SiO ² ) and iron in the form of non-silicon type compounds, such as, for example, magnesium ferrite or magnesium ferrite, for which magnesium oxide is added. Other crystalline phases may exist, such as periclase (MgO), enstatite, and spinel.

A typical composition of this type of forsterite refractory materials would be that with 45-51% by weight of MgO and 40-43% of SiO ² as major components, together with 7-8% of Fe ² O ³ , 0.2-0 .8% CaO and 1.8-2% other oxides such as chromium, titanium, aluminum, nickel and cobalt. It should not present an appreciable weight loss on calcination, since a certain proportion would indicate the presence of silicates with structural water not removed, such as serpentine [Watson, I., Industrial Minerals 1981 Refractories Survey, Ed BM Coope and EM Dickson, Metal Bulletin Ltd., 1981; Harben, P., The Industrial Minerals HandyBook 2nd Edition, Guide to Markets, Specifications & Prices, Ed Warwick Printing Company Ltd., England, 1995]. Forsterite insulating refractory materials, with total porosity 44-70%, have been obtained from mixtures of an igneous rock composed almost entirely of olivine and called dunite, with magnesite, studying the influence of the starting raw materials on the properties of the refractories obtained [Stamenkovic, I., and Sigulinski, F., Proc. 3rd. Int. Meeting Modern Ceramics and Technologies, Rimini, Italy, 1976, 27-31].

In Spain, forsterite refractory materials have been prepared and studied since the 1970s from mixtures of Spanish serpentines, sometimes magnetic (containing Fe ³ O ⁴ magnetite), and various commercial magnesites, comparing the materials obtained with typical refractories in the literature [Aleixandre, V. and González, J. Ma, Bol. Soc. Esp. Ceram. See, 1972, 11, 3-10]. In these investigations, in addition to carrying out an exhaustive bibliographical study on the state of the art in terms of obtaining forsterite up to the sixties [Aleixandre, V. and González, J.Ma, Bol. Soc. Esp. Ceram. See 1963, 2, 425-444; idem., Bol. Soc. Esp. Ceram. V. 1967, 6, 19-42], the effect of various mineralizers on the synthesis of forsterite was analyzed and it was revealed that it is possible to use forsterite refractories in furnaces where substances sensitive to the action of forsterite are treated. silica, contrary to what occurs with silicoaluminous.

Forsterite refractories are also useful for lining the walls of the firing zones of cement rotary kilns [Baumgart, W., Silicates Ind., 1961, 24, 191-197]. Also, basic dolomite and magnesite refractories have a much higher resistance to slag attack than forsterite, but have a higher thermal conductivity: therefore, heat loss increases to such an extent that protection with an insulating material is needed. . Kiln linings that use forsterite bricks do not need this type of protection, which is a clear advantage over other materials. On the other hand, another property of this type of refractory is that it apparently becomes more elastic at high temperatures and, as a consequence, the crucibles made with this material do not crack, which has been known for decades [Konopicky, K., Les matériaux de construction réfractaires, Société d'éditions scientifiques, techniques et artistiques, Paris, 1961; Wecht, P., Sprechsaal, 1962, 95, 275-278]. Its use has been indicated in rotary cement kilns and kiln regenerators (Martin kiln) and in the glass industry, in this case it is used in glass kiln recuperators, as well as in copper and lead metallurgy [Aleixandre, V., Bol. Soc. Esp. Ceram. See, 1962, 1, 255-258; Aleixandre, V. and González, J.Ma, Bol. Soc. Esp. Ceram. See 1967, 6, 19-42].

Likewise, forsterite ceramics and forsterite porcelains have been obtained by reacting magnesium oxide with raw materials containing magnesium silicates, such as serpentine or talc [Kock, W.J. and Berry, J., Ceramic Industry, 1963, 80, 68-69; Harman, R., Lebeda, S. and Slosiar, J., Silikaty, 1968, 12, 19-28], starting a reaction in the solid state at 1000 °C that becomes quantitative at 1200 °C, following the following scheme ( 1-4):

1) Serpentine, with the formula Si ⁴ Mg ⁶ O ¹⁰ (OH ⁾⁸ decomposes into forsterite and silica with elimination of the structural water from the silicate.

2) Silica combines with magnesium oxide to give more forsterite.

3) Talc, with the formula Si ⁸ Mg ⁶ O ²⁰ (OH ^{) 4} decomposes into enstatite and silica with elimination of structural water.

4) enstatite and silica combine by reaction with magnesium oxide to give rise to forsterite, with a melting point of 1910 °C when it is a pure phase, as the final product of said reaction in a quantitative way.

Thus, for example, the synthesis of refractory materials has been studied using also serpentine residues with MgO addition [Scheng, TW, Ding, YC, Chiu, JP, Minerals Engineering, 2002, 15, 271-275]. On the other hand, ionic substitutions can occur in the forsterite crystals that are formed by chemical reactions in the solid state, in such a way that magnesium can be replaced by calcium and iron, with composition variations such as Ca(Mg,Fe)SiO ⁴ as demonstrated by chemical analysis of magnesium refractories [ Lampropolou, PG, Katagas, CG and Papamentellos, DC, J. Am. Ceram. Soc., 2005, 88, 1568-1574]. Likewise, it is important to mention that olivine and forsterite are reduced by carbon at temperatures above 1630 °C, as well as by ferrosilicon and calcium carbide, among other compounds, giving rise to the formation of magnesium and silicon monoxide vapors. , also volatile. Iron itself can also replace magnesium in forsterite crystals at elevated temperatures [Smith, PL, Liddle, J. and White, J., Trans. J.Br. Ceram. Soc., 1985, 84, 62-69], although cast iron does not seem to affect forsterite refractories up to 1800 °C [Gilghrist, J., Combustibles y Refractarios, Ed Alhambra, Madrid, 1967]. Forsterite can be produced from talc but in the presence of carbon, through a process of carbothermal reduction (carboreduction) and reactive nitriding, in a nitrogen atmosphere, which in turn gives rise to the formation of nitrides and oxynitrides [Sugahara, Y ., Fukaishi, A., Kuroda, K. and Kato, C., Clay Science, 1987, 29-35].

Spanish Patent ES 2080650 describes a process for preparing silicon, aluminum and magnesium nitrides and oxynitrides from a hydroxylated silicate with aluminum and magnesium in its structure (vermiculite) by means of carbothermal reduction and nitriding.

It is important to mention in the state of the art that the direct reaction in the solid state between the two high purity oxides, silicon dioxide or silica SiO ² and magnesium oxide or periclase MgO, to give rise to forsterite has been studied until 1500 °C [Ford, WF, The effect of heat on ceramics, Institute of Ceramics Textbook Series, McLaren and Sons Ltd., London, 1967; Alper, AM, Refractory Materials, Vol. 5: High-Temperature oxides, Part I: Magnesia, Lime and Chrome Refractories, Academic Press, New York and London, 1970]. It has been found that, although solid-solid chemical reactions are slow, two silicates are formed, with the formula Mg ² SiO ⁴ (forsterite, 2MgO.SiO2 and melting point 1910 °C) and enstatite MgSiO ³ (MgO.SiO ² and melting point 1550 °C), as predicted by the binary phase diagram, with residual amounts of the two oxides unreacted. Only the phase with the highest melting point, forsterite, is produced in a majority proportion, maintaining a temperature of 1500 °C for long periods of time (over 1 hour), also depending on various factors, such as the nature and chemical composition of the starting raw materials, even forming a thin layer of the enstatite phase between forsterite and silica, a phase that then it reacts with MgO to give rise by reaction and diffusion to more forsterite [Brindley, GW and Hayami, R., Philosophical Magazine, 1965, 12, 505-512; Mikhail, SA and King, PE, Thermochimica Acta, 1992, 207, 95-102].

The North American patent US 4977024 describes the preparation of a magnesium oxide granule coated continuously and uniformly with silica or forsterite for its application as a filler with resins in electronics, with excellent insulating properties and high resistance to deterioration.

Forsterite has also been obtained under laboratory conditions from magnesium nitrate hexahydrate as a precursor of MgO oxide and a silica raw material, such as "Cabosil 600", a commercial silica gel with a high specific surface area, first decomposing nitrate and with prolonged heat treatments at 1500 K for three days, cooling, mixing with water, subsequent grinding and final heat treatment at 1550 K [Ball, M.C. and Sutherland, I., J. Mater. Chem., 1995, 5, 1459-1462].

During the last decades, the preparation of ceramic materials with a high forsterite content has undergone a certain development considering solgel-type processing and the formation of nanocomposites and multiphase materials [Brinker, C.J., Clark, D.E. and Ulrich, D.R., eds., Better Ceramics through Chemistry, Vol. 3, Ed Elsevier, Amsterdam, 1987; Hench, L. L. and West, J.K., Chem. Rev., 1990, 90, 33-72]. In this way, forsterite ceramic materials obtained from reactive precursors have been prepared and densified, starting with magnesium nitrate, absolute ethanol and a silicon alkoxide at 60 °C, with stirring until a sol is formed; then it gels, is treated at 100 °C and calcined at 400 °C and then at 1200 °C and 1500 °C. Materials with forsterite are obtained in a majority proportion, with densities of 97 % of the theoretical, starting from diphasic gels and 91 % from monophasic gels, but having to subject the material to heat treatment at 1200 °C for 30 hours to eliminate enstatite [ Kazakos, A., Komarmeni, S. and Roy, R., Mater. Letters, 1990, 9, 405-409]. In short, long and diverse heat treatments of the precursors.

With this sol-gel procedure, starting from alkoxides, using acetic acid for more intimate mixing, and under appropriate conditions to form the gel obtained in the form of fibers, it has also been possible to prepare nanocrystalline forsterite fibers at the laboratory level that crystallize at temperatures as low as 550 °C, with grain growth at 800 °C, which gives rise to crystals of 20-30 nanometers but which require a final treatment at 1300 °C [Tsai, MT, Materials Research Bulletin, 2002, 37, 2213-2226; Tsai, MT, J. Am. Ceram. Soc., 2002, 85, 453-458].

The synthesis of forsterite without containing enstatite has also been achieved in the state of the art by obtaining organometallic polymeric precursors of the poly(methacrylate) type, via copolymerization of alkaline salts of methacrylic acid, with various combinations of siloxane-methacrylate and methoxy -silyl-methacrylate with magnesium methacrylate, giving rise to a conversion of the precursor oxides during pyrolysis in air that causes yields of 77% of theory [Hogan, M., Martin, E., Ober, C.K., Hubbard, C.R., Porter, W.D. and Cavin, O.B., J. Am. Ceram. Soc., 1992, 75, 1831-1838]. Likewise, in the synthesis of forsterite and enstatite by the sol-gel route, even magnesium metal itself and tetramethoxysilane or tetraethoxysilane have been used, in various molar ratios, in the presence of hydrochloric acid as a catalyst for the hydrolysis reaction, thus that forsterite crystallizes at 500 °C [Ban, T., Ohya, Y. and Takashi, Y., J. Am. Ceram. Soc., 1999, 82, 22-26]. Other procedures by the sol-gel route are of interest in obtaining nanostructured forsterite particles with a narrow particle size distribution [Sanosh, K.D., et al., J. Alloys Comp., 2010, 495, 113-115] or by a modified method using the sol-gel route combined with a grinding treatment followed by heat treatment at 600-1000 °C [Mirhadi, S.M., et al., Ceram. Int., 2016, 42, 7974-7979].

A renewed interest in the preparation of forsterite ceramic materials occurred in 1987 due to the technical discovery of the optical properties, as LASER-type materials, of crystals of forsterite containing approximately 0.4 atomic % Chromium, in the range near infrared of 1,167-1,345 |jm. The active species are the Cr(IV) ions that occupy tetrahedral positions in the forsterite crystal. This has meant a development of great technical interest [Burlich, JM, Beeman, ML, Riley, B. and Kohlstedt, DL, Chem. Mater., 1991, 3, 692-698; Park, DG, Burlich, JM, Geray, RF, Dieckmann, R., Barber, DB and Pollock, CR, Chem. Mater., 1993, 5, 518-524; Beecroft, LL and Ober, CK, Adv. Mater., 1995, 7, 1009-1012]. In this way, it has been possible to obtain Chromium-doped forsterite nanoparticles by pyrolysis of a precursor by evaporation, condensation and oxidation using high temperature in the gas phase [Tani, T., Saeki, S., Suzuki, T. and Ohishi, Y. , J. Am. Ceram. Soc., 2007, 90, 805-808].

US Pat. No. 6,632,757 of October 14, 2003 describes the preparation of a forsterite glass-ceramic material from a glassy precursor that can be doped with 1% Chromium to give rise to its optical activity.

Starting from organometallic precursors of the polymethacrylate type, already mentioned above, used in the synthesis of forsterite, this type of chromium-doped forsterite materials of interest for their applications in LASER technology have also been prepared [Hogan, M., Martin, E. , Ober, C.K., Hubbard, C.R., Porter, W.D. and Cavin, O.B., J. Am. Ceram. Soc., 1992, 75, 1831-1838; Park, D.G., Hogan, M., Martin, E., Ober, C.K., Burlitch, J.M., Cavin, O.B., Porter, W.D. and Hubbard, C.R., J. Am. Ceram. Soc., 1994, 77, 33-40].

On the other hand, forsterite ceramics are difficult to sinter [Shiono, T., Sato, R., Shiomi, H., Minagi, T., and Nishida, T., J. Soc. Mater. Sci. Japan, 1999, 48, 554-558] and hence the porosity they present. However, Japanese researchers have achieved the synthesis of high-density and transparent forsterite ceramic materials using nanometer-sized precursors and high specific surface area, studying the effect of sintering temperature, variation in particle size, and degree of dispersion of two hydroxylated magnesium oxide precursors in the material finally obtained, with heat treatments up to 1700 °C for 10 hours [Sano, S., Saito, N., Matsuda, S., ICAO, N., Haneda, H., Arita, Y. and Takemoto, M., J. Am. Ceram. Soc., 2006, 89, 568-574].

Likewise, forsterite-containing ceramic materials have already been considered in advance also as potential high-frequency insulators [Ghosh, PK and Das, AR, Trans. Indian Ceramic. Soc., 1979, 38, 758-762] and for their application as resistor substrates in the form of thin films [Somogyi, A. and Szebeni, S., 13th Szilikatip. Szilikattu Konference, Budapest, 1981, 290-295]. Forsterite ceramics show excellent dielectric properties in the microwave field, but the variation in resonance frequency with temperature limits their practical applications, so they have been modified by adding titanium oxide to magnesium oxides and silicon (99.9% purity), used as raw materials, which also alters their sintering and dielectric properties [Tsunooka, T., Andréu, M., Higashida, Y., Sugiura, H. and Ohsato, H., J. Eur. Ceram. Soc., 2003, 23, 2573-2578]; or by replacing magnesium with calcium and manganese to improve these properties [Sugiyama, T., Tsunooka, T., Kakimoto, K. and Ohsato, H., J. Eur. Ceram. Soc., 2006, 26, 2097-2100], even with addition of lithium borosilicate to forsterite materials obtained by synthesis of MgO and SiO ² precursors treated at 1350 °C for 4 hours [Sasikala, TS, et al., J. Alloys Comp., 2008, 461, 555-559].

In recent decades, the progress of modern metallurgy and new technologies have associated the use of materials with better performance and, in this sense, high quality refractory materials are important. In addition to this, the need for energy savings requires the use of insulating refractory materials, such as forsterite. Although the refractoriness of materials with forsterite in the majority proportion is high, theoretically forsterite melts at 1910 °C as already mentioned, its resistance to sudden changes in temperature or thermal shock is relatively low [Alper, AM, Refractory Materials, Vol. 5: High-Temperature oxides, Part I: Magnesia, Lime and Chrome Refractories, Academic Press, New York, 1970]. To improve the resistance to thermal shock of forsterite refractories, various modifications have been made in their preparation. One of them, from much earlier research, has been to use various additions of magnesia and rutile, studying the influence of said additions on the properties of the refractories obtained [Ringh, R., et al., Bull. Soc. Franc. Ceram., 1957, 35, 21-35]. Other modifications have achieved a decrease in the sintering temperature of the forsterite refractory and the properties are also improved with the addition of 2% rutile [Birgis, LG and Fahim, MA, Ceramurgia, 1977, 315-320]. It has even been possible to prepare chromium-forsterite refractories with properties similar to those of chromium-magnesite, but at a lower cost [Uzberg, A., Bron, VA and Vidrina, JA, Ogupory, 1970, 35, 23-31]. An important improvement in the mechanical properties and resistance to thermal shock of this type of forsterite refractory materials has been achieved by obtaining it from zirconium silicate (zircon) and magnesium oxide by solid-state reaction, which gives rise to to a material composed of forsterite and zirconium oxide, the latter being responsible for this improvement [Yangyun, S. and Brook, RJ, Ceram. Int., 1983, 38-46].

The North American patent US 5204298 describes a basic monolithic refractory with a chemical composition of 65-96% MgO and containing cubic zirconium oxide and forsterite as crystalline phases, with apparent porosity of 7% or less, density of 3 .2 g/cm3 and excellent resistance to penetration and corrosion by slag, in addition to excellent performance in service, with application in the steel industry in steel manufacturing furnaces. The North American patent US 5453408 describes the preparation of a refractory sand with a high forsterite content for use in the steel industry, with a MgO:SiO ² ratio by weight of at least 1.2, a density of 3 g/cm3 and a point of melting at 1610 °C, being 50% by weight of forsterite, at least 5 to 15% of an agent with carbon, as a binder, and 8 to 27% by weight of synthetic maghemite. Another type of synthetic forsterite refractory sand composition is described in US Pat. No. 5,576,255. On the other hand, mechanical activation by reactive grinding is a useful procedure in materials synthesis, producing mechanochemical reactions. In these reactions In mechanochemical processes, processes occur that would not take place under other conditions and in which both the plastic deformation of the particles and the increase in microstresses and defects in them are involved, even at the molecular level and chemical bonds, which act in one direction. favorable for achieving important transformations in unconventional solid-state processes and that can be more favored by subsequent heat treatment [Tkácová, K., Mechanical Activation of Minerals, Elsevier, Amsterdam, 1989; Yariv, S., and Lapides, I., J. Mater. Synth. Proc., 2000, 8, 223-233; Beyer, MK, and Clausen-Schaumann, H., Chemical Reviews, 2005, 105, 2921-2948; Wieczorek-Ciurowa, K., and Gamrat, K., Mechanochemical syntheses as an example of green processes, J. Thermal Anal. Calorim., 2007, 1, 213-217; Baláz, P., et al., Chem. Soc. Rev., 2013, 42, 7571-7637].

Specifically, in the synthesis of forsterite using this type of treatment, various inorganic precursors have been used, such as amorphous silica and magnesium carbonate [Fathi, M.H. and Kharaziha, M., Mater. Letters, 2008, 62, 4306-4309], as well as high purity talc and magnesium carbonate [Tavangarian, F. and Emadi, R., J. Alloys Comp., 2009, 485, 648-652; Tavangarian, F., et al., Powder Technol., 2010, 198, 412-416]. With 10-100 hours of mechanical activation by grinding and treatments at 1000 °C -1200 °C, materials with a high forsterite content can be obtained. If high purity talc and MgO are used as starting raw materials, with grinding treatments of 5 hours followed by heat treatment at 1000 °C for 1 hour, forsterite particle sizes of 40 nanometers can be obtained, or with a grinding for 20-60 hours and treatments at 1200 °C for 1 hour for better sintering of the resulting material [Tavangarian, F. and Emadi, R., Mater. Res. Bull., 2010, 45, 388-391].

As for forsterite-based bioceramic materials for bone regeneration, they have generally been obtained by sol-gel synthesis from different precursors, such as colloidal silica and magnesium nitrate hexahydrate, aging for 24 hours at 60 °C, drying at 120 °C for 48 hours and heat treatment between 1100-1300 °C for 3 hours [Ni, S., et al., Ceram. Int., 2007, 33, 83-88]. Other methods of synthesis of forsterite as a bioceramic material have been carried out precisely from talc and MgCO ³ with adjustment of a ² :1 or 5:1 MgO:SiO 2 molar ratio and formation of nanostructured precursors [Tavangarian, F and Emadi, R., Mater. Letters, 2011,65, 740 743; Tavangarian, F., and Emadi, R., Ceram. Int., 2011, 37, 2275-2280].

Forsterite materials with high porosity for biomedical applications have also been obtained using magnesium hydroxycarbonate and silica by grinding for 10 hours and heat treatment at 900 °C [Saidi, R., et al., J. Sol-Gel Sci. Technol., 2017, 81, 734 740]. In recent years, the fabrication of silk-forsterite composites using inorganic precursors and the silk fibroin protein for tissue engineering applications has also been studied [Teimouri, A., et al., Ceram. Int., 2014, 40, 5405-6411]. Nanocrystalline forsterite materials have recently been obtained by solid-state reaction from rice husk ash, an industrial by-product that contains SiO ² in a majority proportion, with previously calcined MgO in stoichiometric amounts for the synthesis of forsterite, using a grinding treatment. of 12 hours and a subsequent thermal treatment of the resulting product at 700-1000 °C, finding a majority proportion of forsterite at 1000 °C [Mathur, L., et al., Bol. Soc. Esp. Ceram. Glass., 2018, 57, 112-118].

As has been previously explained in detail, there are several procedures in the state of the art with manifest utility for obtaining and manufacturing ceramic, bioceramic and refractory materials that contain forsterite in a majority proportion and that show a high technological interest, but it is based on raw materials in the form of very compact rocks, such as olivine, dunite or serpentine, or high-purity metal-organic reagents. These last precursors also appear in a liquid state at room temperature and, in general, tend to be easily flammable, which is associated with another series of drawbacks, such as the use of non-aqueous solvents, often also flammable, with high cost. relative and special handling conditions in protected or inert atmospheres to avoid humidity since it can even cause its hydrolysis, in addition to causing residues and environmental problems due to its toxicity.

As a major drawback, the high cost that would be associated with scaling up this type of invention procedures, until now at the laboratory level, is pointed out, restricting them to obtaining small amounts of precursor raw materials, of high purity, for the manufacture of pieces of small size intended for special or very specific applications. In short, little attention has been paid to the preparation of materials with a high forsterite content starting from relatively affordable inorganic raw materials, with medium purity and at moderate or low cost, without using metal-organic reagents.

The Spanish patent P200900844 (Procedure for obtaining a ceramic material of forsterite), with international publication number WO 2010/109051 A1 and international application number PCT/ES2010/070175, describes a procedure for obtaining a ceramic material from medium purity talc that comprises mixing an aqueous suspension thereof with MgO , drying the suspension obtained at a temperature of up to 350 °C and treating the product at a temperature of up to 1600 °C. Furthermore, the invention relates to a ceramic material obtainable by the described process comprising forsterite in a proportion of between 70% and 95% by weight of the ceramic material and its applications as a refractory material.

A clear advantage, both of the previous process of the invention and of the present one and others of the state of the art, is the use of talc as raw material. Talc is a mineral silicate with relative abundance in Spain, in provinces such as León or Málaga, and other European countries. It can be obtained finely divided from the original rock, with high or medium purity and at a relatively low cost [Rodas, M., Galán, E. and La Iglesia, A., Bol. Geol. Min., 1979, 3, 18-24; Harben, P.W., The Industrial Minerals Handy Book, second edition, Warwick Printing Company Limited, England, 1995]. The use of talc with impurities is also a clear advantage in the present process of the invention, as long as they are from other silicates, such as the aforementioned serpentine, or those of the chlorite type, a group of layered silicates that can contain iron and magnesium. in its structure [Brindley, G.W. and Lemaitre, Chemistry of Clays and Clay minerals, Monograph No. 6, ed. A.C.D. Newman, The Mineralogical Society, London, 1987].

The object of the invention is a procedure for the preparation of ceramic, bioceramic and refractory materials of a basic nature with a high content of magnesium orthosilicate with the formula 2MgO.SiO2 or also Mg ² SiO ⁴ and which is called forsterite. It starts with another but hydroxylated magnesium silicate, preferably talc, and it is mixed with magnesium oxide powder in a stoichiometric proportion to achieve the synthesis of forsterite and that its content is the maximum possible. The mixture is subjected to a mechanical treatment of mechanochemical activation through intensive high-energy grinding, at room temperature, which favors both the homogenization and intimate mixing of the compounds used, as well as the mechanochemical-type reaction between the decomposition products of talc with the added magnesium oxide.

The reactive precursor as a powder obtained in the milling is heat treated at a temperature between 1000-1200 °C so that the mechanochemically activated reaction occurs, which gives rise to the formation of forsterite. Therefore, it is not necessary reach temperatures as high as 1400-1600 °C to obtain a material with a high forsterite content, as reflected in the state of the art using this same type of inorganic precursors, particularly in a previous Spanish Patent (P2345648). to the state of the art.

In a preferred embodiment of the ceramic material with the process of the invention, ceramic, bioceramic and refractory materials are obtained with the following characteristics: apparent density of 2.76 g/cm3 or less; open porosity between 12% and 15% by volume; average coefficient of thermal expansion between 20 and 900 °C between 11.52 x 10-6 x °C-1 and 11.82 x 10-6 x °C-1, resistance to sudden changes in temperature, in number of cycles from room temperature to 950 °C and cooling in water, a minimum of 3 cycles; chemical composition (% by weight): SiO ² , 41%; MgO, 53%; Fe ² O ³ , < 3%, TiO ² <0.5%; CaO < 1% and a molar ratio MgO:SiO ² of 2:1, presenting a proportion of forsterite (2MgO.SiO2) that can be >90% by weight.

DESCRIPTION OF THE INVENTION

The invention refers to a process for preparing ceramic materials that contain magnesium and silicon combined in their chemical composition, with a high content of magnesium orthosilicate ( ² MgO.SiO ² ) or forsterite, preparation of bioceramics and refractories of a basic nature, as well as well as the applications and uses of interest of these materials. A mechanochemical activation of the raw materials used is carried out to favor both their intimate mixing and the forsterite synthesis reactions of the reactive precursor obtained with a subsequent heat treatment.

It starts with relatively affordable inorganic raw materials, not necessarily of high purity or even as by-products and at moderate or low cost, without using metal-organic precursors and solvents. The invention has a technical advantage in that metal-organic precursors are not used as raw materials, generally flammable and of high purity, with a high price, which implies the use of non-aqueous solvents, also generally flammable, with high relative cost and special conditions of handling such as protected or inert atmospheres to avoid hydrolysis, with generation of waste and environmental problems.

An advantage of the procedure of the invention is the use of a natural magnesium silicate such as talc used as raw material, of relative abundance in Spain and other countries, which can be easily obtained in a finely divided state (hardness 1 on the Mohs Scale) at low cost. The use of talc as a raw material, even with impurities that are thus revalued as an industrial by-product, is a clear advantage as long as they are silicates, such as serpentine and/or chlorite, or even magnesium carbonate, since all these impurities they contain Silicon and Magnesium in different relative proportions of interest for obtaining forsterite.

The effect of the mechanical treatment by intensive high-energy grinding of the raw materials used acts advantageously in the process of the invention for the synthesis of forsterite, with a notable increase in chemical reactivity through a mechanochemical-type reaction pathway to thus favor the formation reaction of said phase in a majority proportion, also being combined with a subsequent heat treatment of the product originated in the milling.

With the procedure of the invention, ceramic, bioceramic and refractory materials with forsterite (magnesium orthosilicate) in a majority proportion are obtained with advantage and said procedure allows easy scaling.

The industrial applications of the materials with a high forsterite content that are obtained with the process of the invention are the following: as a rigid substrate, filter, thermal insulation, catalyst support and for the manufacture of advanced materials for use in high temperature applications. , being 1800°C the maximum temperature of use, for areas exposed to heat (cement kilns, metallurgy, steel industry, etc.), as well as in obtaining engineering materials for possible LASER applications and bioceramic materials.

Detailed description of the invention

To carry out the process of the present invention it is possible to use as starting compounds hydroxylated layered silicates, silicates characterized by a two-layer structure or expandable silicates characterized by a three-layer structure. As examples of layered silicates that can be used, it is worth mentioning kaolinite, muscovite, pyrophyllite, talc, vermiculite or chlorite, with talc being the one that presents the most advantages due to its high content of magnesium and silicon oxides in relation to the rest.

It starts with talc, a hydroxylated layered silicate contained in the majority proportion in the rock where it comes from as raw material, and can be crushed relatively easily given its low hardness on the Mohs scale. This raw material, finely divided as it can be disposed of industrially, is preferably mixed with calcined magnesium oxide and also finely divided in a stoichiometric proportion to obtain forsterite.

The talc rock is previously crushed until it reaches a particle size with an average diameter of at least 0.100 mm and is mixed with the stoichiometric proportion of magnesium oxide, finely divided, by homogenizing the resulting paste with water; preferably the aqueous suspension should also contain acetone (propanone) to avoid, as far as possible, the hydration of the smallest grains of magnesium oxide which are the most reactive when placed in an aqueous medium.

Therefore, a first aspect of the invention refers to a process for preparing ceramic, bioceramic and refractory materials of a basic nature with a high content of forsterite by means of reactive grinding, which comprises the following stages:

a) Mix an aqueous suspension of talc of composition:

-SiO ² between 46.3% and 63.1% by weight,

- MgO between 29.10% and 32.90% by weight

- AhO3 between 0.29% and 3% by weight

- Fe ² O ³ between 0.15% and 3.4% by weight

- TiO ² between 0.05% and 0.13% by weight

- CaO between 0.19% and 1.50% by weight

- Na ² O between 0.04% and 0.10% by weight

- K ² O between 0.00% and 0.41% by weight

- loss on ignition at 1000°C between 5.04% and 8.02%

with calcined magnesium oxide at a minimum temperature of 1700 °C and with a minimum content of 97% by weight of particles with a size of less than 0.063 nm in a proportion between 70 and 80% by weight of talc and between 20 and 30% of magnesium oxide; b) Dry the suspension obtained in step (a) at a temperature of up to 350 °C;

c) Grinding of the product of step (b) for homogenization, reduction of particle size and induction of mechanochemical activation in a planetary ball mill with a jar made of steatite and balls of the same material;

d) Uniaxial pressing of the product obtained in stage c); and

e) Heat treatment of the product of step (c) at a temperature of up to 1200 °C.

In a preferred embodiment of the process of the invention, the talc and magnesium oxide from stage (a) are previously ground to a particle size with a mean diameter of at least 0.100 mm.

In another preferred embodiment, the aqueous suspension of stage (a) contains acetone in a proportion of 50% by volume.

In a preferred embodiment, the drying of stage (b) is carried out at a temperature between 250 °C and 350 °C for a time of between 30 and 120 minutes.

In another preferred embodiment, the drying of stage (b) is carried out in two sub-stages:

b1) Air-drying at a temperature between 90 °C and 120 °C, for up to 15 hours.

b2) Drying after step (b1) at a temperature between 250 °C and 350 °C, for a time between 30 and 120 minutes.

The powdered product obtained is subsequently subjected to a stage of heat treatment, which would be stage (e). There are possible implementation alternatives:

In a preferred embodiment, the heat treatment of stage (e) is carried out in the following substages:

e1) Heat treatment at a temperature of up to 800°C at a heating rate of between 40°C and 80°C per hour, for a time of between 60 and 150 minutes;

e2) The product obtained in (e1) is treated at a temperature of up to 1000°C at a heating rate of between 40° and 80°C per hour, for a time of between 60 and 150 minutes; and

e3) The product obtained in (e2) is treated at a temperature between 1100°C and 1200°C at a heating rate of about 60°C per hour for a time between 60 and 150 minutes.

Alternatively, the heat treatment of step (e) is carried out at a temperature between 1100°C and 1200°C at a heating rate of about 60°C per hour for a time between 60 and 150 minutes.

In another alternative, the heat treatment of stage (e) is carried out at a temperature of 1200 °C for a time of between 30 and 150 minutes.

This stage (e) can be carried out in different types of ovens and can also be carried out continuously, for example, in tunnel-type ovens and an inert atmosphere is not necessary, simply in an air atmosphere. The synthesis of forsterite 2MgO.SiO2 (57.14% by weight of MgO and 42.85% of SiO ² ), as magnesium orthosilicate with the formula Mg ² SiO ⁴ , proceeds by mechanochemical activation reaction and subsequent thermal activation in solid state.

The precursor obtained from grinding is heat treated at a temperature of between 1,000 and 1,200 °C so that the mechanochemically activated reaction occurs, leading to the formation of forsterite. Therefore, it is not necessary to reach temperatures as high as 1500 °C-1600 °C as described in other procedures starting from similar raw materials, mentioned in the state of the art, to obtain a material with a high forsterite content.

The thermal decomposition reaction of talc at 800 °C-1000 °C with formation of magnesium silicate MgSiO3 and silica ( ^SiO2 ) is as follows:

Si8Mg6O20(OH)4 = ^{6MgSiO3 2H2O2SiO2}

The solid state reaction induced by heat treatment at 1000 °C-1300 °C between magnesium silicate MgSiO ³ and MgO in the presence of silica to form magnesium orthosilicate, forsterite, Mg ² SiO ⁴ is as follows:

3MgSiO3 ^SiO2 + 5MgO = 4 Mg2SiO4

In addition to the reaction between enstatite and MgO that is induced by heat treatment at 1000 °C-1400 °C:

MgSiO3 MgO = Mg2SiO4

The solid state reaction induced by heat treatment at 1200 °C -1600 °C between silica and MgO to directly form the magnesium silicate, forsterite, Mg ² SiO ⁴ is as follows: SIO ² + 2MgO — Mg2SiO4

However, the mechanochemical decomposition reaction for talc, in which dehydroxylation of the silicate is induced at room temperature using a high-energy grinding treatment, is as follows:

Si8Mg6O20(OH)4 = amorphous material with composition MgO.SiO ² + ² H ² O

This amorphous and highly reactive material, considered the precursor of forsterite, on the one hand would form forsterite directly when activated thermally, according to the scheme:

amorphous material of composition MgO.SiO ² = Mg2SiO4 SiO ²

On the other hand, this silica-rich amorphous material would react with the MgO that has been added (in a manner analogous to what has already been described above) and a greater relative proportion of fosterite is then produced, according to the scheme:

amorphous material of composition MgO.SiO ² + MgO = Mg ² SiO ⁴

All forsterite formation reactions induced by heat treatment are thermodynamically favorable, one of the most favorable being the direct synthesis between silica, as a reactive product of the decomposition of talc, with magnesium oxide. Consequently, it is not necessary to add silica to the mix. The silica produced by the thermal decomposition reaction of talc may be in an amorphous state or it may crystallize as cristobalite, but in any case it is a reagent for the formation of forsterite with MgO. All of this is circumvented by the formation of forsterite directly by mechanochemical activation, as the first step, followed by a heat treatment, as the second step. Therefore, the solid state reactions produced by a heat treatment without this previous step, already proposed in a previous invention procedure (Spanish Patent with publication number 2345648), are modified by the mechanical activation treatment by high energy grinding. which gives rise to a reactive precursor, which, subjected to subsequent heat treatment, produces forsterite at a temperature of 1000 1200 °C and without other residual phases.

The high energy mill that can be used is a closed container, made up of a material such as steatite (compact metamorphic rock that contains silicates of magnesium). In this closed container or grinding jar, provided with a lid that fits in the same material as the jar, the raw materials are introduced. These materials have a high resistance to wear due to mechanical action and internal pressure. The grinding elements can preferably also be made of the same types of materials mentioned and in various shapes, such as spherical (balls) or cylindrical.

The grinding procedure for mechanochemical activation of the starting raw materials can be optimized considering various work parameters. Among others more outstanding, the following can be mentioned:

1) The quantity of raw materials and the total useful volume of the container or jug suitable for grinding.

2) The nature of the grinding medium, as well as the geometry of the grinding elements that form said medium.

3) The ratio between the weight of the mixture of raw materials subjected to grinding and the total weight of the grinding medium used.

4) The speed of rotation, in revolutions per minute (rpm), when using a planetary mill and the treatment time to induce and progress the mechanochemical activation reactions.

The amount of raw material to be treated, introduced into the container or jug used for grinding, and the weight and number of grinding elements that make up the grinding medium must also be taken into account. In addition, another factor is the possible effect of contamination of the raw material itself due to wear and tear when grinding for long periods of time, both of the material with which the container is made and of the grinding medium. Likewise, experience indicates that the volume of the container should be at least 3 times the volume of the mixture of raw materials for the grinding treatment to be effective.

On the other hand, the presence in the raw materials of a relatively small content of impurities of other metallic oxides, such as those of iron, titanium or aluminum, in addition to alkaline or alkaline earth metals, must be as low as possible to obtain of an excellent ceramic or refractory material. In any case, these oxides will give rise to vitreous phases, liquid at high temperatures, which up to certain limits can be favorable to the forsterite synthesis process and even act as mineralizers for its maximum possible content in the product and influence a better development of the crystals.

For example, iron can replace magnesium in the structure as a solid solution, so that the silicate would have a composition like (Mg,Fe)SiO ⁴ .

The persistence of certain phases, such as enstatite or cristobalite that are produced in the thermal decomposition of talc, if the mechanochemical activation treatment by grinding has not been carried out correctly, may limit the applications of the product resulting from the process of the invention as a ceramic material or refractory, in particular its pyroscopic or melt resistance, which is theoretically 1910 °C.

If there are silicate-type impurities that accompany the talc, such as serpentine or chlorite, or if it is talc as an industrial by-product, these two silicates decompose with the action of mechanochemical and thermal treatment at temperatures lower than that produced in the talc, giving rise to forsterite in very small crystals, which would act as nuclei for the formation of more forsterite by synthesis between the added magnesium oxide and the decomposition products of the talc. Forsterite is the final product of the process of the invention and is obtained in a majority proportion from the mixture of talc and magnesium oxide, once said mixture has been subjected to mechanical activation by grinding, followed by heat treatment.

High-energy grinding treatment times, under the conditions described herein, range from 20 to 40 hours, at a rotation speed of 300 to 500 rpm, using a one-volume grinding jar or container. less than 500 mL and that is loaded with a sample weight of less than 100 grams, with heat treatments at 1000 1200 °C for 30 and 60 minutes.

In all cases, the basic ceramic or refractory material obtained, as well as the refractory sand or forsterite chamotte, will have a certain porosity that will condition its properties and, consequently, part of its applications. It can be treated at a higher temperature, between 1300 °C -1400 °C, to achieve a higher degree of sintering of the material and to achieve minimal porosity (<5% by volume).

Optionally, the method of the invention further comprises:

i) Crushing the material obtained in the heat treatment of stage (d);

ii) Shape the previously crushed material in (i), preferably by uniaxial pressing; and

iii) Heat treatment of the product formed in (ii) at a temperature between 1200 °C and 1400 °C, for a time between 60 and 240 minutes, with the optimum temperature being 1200 °C and the optimum time being 120 minutes.

Another object of the present invention is the ceramic material obtained by the previously described procedure, which:

- It comprises forsterite in a proportion between 90% and 95% by weight of the ceramic material.

- the molar ratio of magnesium oxide and silicon dioxide is 2:1.

- it has an apparent density between 2.65 and 2.76 g/cm3;

- it has a porosity between 12% and 15% by volume;

- It has a coefficient of thermal expansion at a temperature between 20 °C and 900 °C of between 11.52 x 10-6 and 11.82 x 10-6 °C-1;

- it is resistant to temperatures up to 1800 °C.

The product obtained by means of the process of the invention has applications as a basic structural and refractory ceramic material due to its high content of magnesium oxide, in addition to being useful as a rigid substrate and filter to be used at high temperatures, an insulating ceramic material and a support for catalysts.

Likewise, the chamotte, calcine or forsteritic sand obtained with the process of the invention, with a suitable degree of crushing and granulometry established by mechanical separation or sieving, is also a synthetic raw material for the preparation of advanced ceramic materials with high thermomechanical performance. , with interesting applications in engineering.

In particular, as structural components capable of being used in devices subjected to high temperatures (> 1500 °C), with a maximum of 1800 °C, also as refractory linings or furnace support parts, limited to not being subjected to excessive changes. sudden changes in temperature and its porosity, which will condition its thermomechanical resistance and attack by penetration and corrosion of molten slag.

An important application of the product of the process of the invention is as a basic refractory material for use in the steel industry in steel manufacturing furnaces.

Likewise, the product of the process of the invention, made up of forsterite in a majority proportion, has potential application in the preparation of LASER-type materials that contain Chromium, and in the preparation of bioceramic materials, since forsterite is a biocompatible material.

PREFERRED EMBODIMENT OF THE INVENTION

Throughout the description and claims, the word "comprises" and its variants are not intended to exclude other technical characteristics, additives, components or steps. For those skilled in the art, other objects, advantages and characteristics of the invention will emerge in part of the description and in part of the practice of the invention The following example serves to illustrate the invention and is not intended to be construed as limiting thereof.

Example of realization:

In this particular embodiment of the invention, an industrial talc with a typical chemical composition such as the following (% by weight) is used as raw material: SiO ² , 54.72%; MgO, 30.21%; Al ² O ³ , 2.89%; Fe ² O ³ , 3.23%, TiO ² , 0.10%; CaO, 0.84%; Na ² O, 0.06%; ^K2O , 0.00; loss on ignition at 1000 °C, 7.84%, total sum 99.89%. This chemical composition of industrial talc differs from the theoretical values of talc in terms of silicon and magnesium oxide content and loss on ignition (SiO ² , 63.36% by weight; MgO, 31.89%), since it contains as an impurity another silicate with iron, aluminum and magnesium in the structure, identifying by X-ray diffraction that it is chlorite. The loss on calcination at 1000 °C of pure talc is 4.75%, so the difference observed with that of this industrial talc must be associated with hydroxylated impurities, such as silicates from the chlorite group.

The mineralogical composition of the industrial talc that is determined in the embodiment of the invention is 70% by weight of talc as hydroxylated magnesium silicate and 30% by weight of another mineral, in this particular case chlorite containing iron and magnesium in its structure. Likewise, the thermogravimetric analysis confirms the presence of chlorite that decomposes by heat treatment between 400 and 800 °C, with talc dehydroxylation between 850 and 1100 °C.

Other types of talcs can be used as raw materials to obtain forsterite. industrial, abundant in European countries such as Spain, which present variable impurity content, and even materials from the purification of talc destined for the pharmaceutical or cosmetic or ceramic industry in which it is limited, for example, to the minimum content of iron oxides or to the content of accessory minerals, such as chlorite and serpentine or magnesite (magnesium carbonate). This also revalues the use of by-products from the extraction of these minerals, mainly talc and serpentine. The talc is subjected to prior grinding in such a way that the granulometric analysis by sedimentation in water, after dispersing the particles, of a sample of the milled product indicates that the size of the particles has an average diameter of at least 0.100mm.

Magnesium oxide (commercial MgO, minimum 90-99% by weight) is also used as raw material, preferably obtained by heat treatment of an oxide precursor such as magnesite (magnesium carbonate) or hydroxide (brucite), calcined as minimum at 1700 1800 °C and ground. It is not convenient to start directly from magnesite or brucite to avoid relatively high weight losses due to the elimination of carbon dioxide, in one case, or water in the other.

The average size of the magnesium oxide particles must be less than 100 pm, preferably at least 97% by weight must be less than 63 pm.

Pure magnesium oxide as a raw material is commercially available in various qualities. Magnesium oxide as a raw material for refractories can be first grade or "first grade" periclase, with a minimum content of 96-99% in MgO, considered the most suitable for carrying out the invention, and with magnesium oxide content. Iron oxide less than 0.2% by weight, a CaO:SiO ² ratio of 3:1 or 4:1, Boron content less than 0.01% by weight and density of 3.44 g/cm3. Second quality magnesium or "second quality" for refractories have a minimum content of 95 % MgO, iron oxide less than 1% and a CaO:SiO ² ratio of 2:1, low Boron content and density of 3.4 g/cm3. Another type of magnesium oxide or periclase powder that can be used is dead burned, with a 90-95% MgO content, a maximum SiO ² content of 6-4% by weight, 3-5% CaO, 1-2% M ² O ³ , 1-2% Fe ² O ³ with a maximum loss on ignition at 1000 °C of 0.5% by weight.

A magnesium oxide powder of a lower quality or with a higher content of impurities can be used, depending on the content of iron, aluminum, boron, calcium or silica, in addition to the cost, but in this case the resulting ceramic or refractory material will also be of inferior quality due to the increase in the glassy phase and phases other than forsterite, to the detriment of mechanical properties.

The proportion of talc and magnesium oxide that is necessary and sufficient to achieve the maximum formation of forsterite per reaction is based on its stoichiometric ratio, that is, 2 moles of MgO and 1 mole of silica, theoretically equivalent to 57, 14% by weight of MgO and 42.85% by weight of SiO ² . Accordingly, starting from an ideal talc (SiO ² , 63.36% by weight, MgO, 31.89%), 60-70% by weight of talc should be mixed, the optimum being 65.53% by weight, and 30-40% by weight of magnesium oxide, the optimum being 34.46% by weight of magnesium oxide.

In the example of the invention starting from industrial talc, whose chemical characteristics have already been described above, and magnesium oxide of 99% by weight, 70-80% by weight of this industrial talc are used, the optimum being 71.60%. by weight, and 20-30% by weight of magnesium oxide of 99% by weight, the optimum being 28.40% by weight.

To carry out the mixture, since the talc does not become wet when water is added and the particles agglomerate due to hydrophobicity, it is necessary to modify the mixing medium, in addition to avoiding, as far as possible, the calcined magnesium oxide from hydrating in the medium and give rise to hydroxide. For this, a mixture of deionized water and acetone (minimum 98% by weight) is available in a 1:1 volume ratio. The exactly weighed amount of talc that corresponds to the percentage by weight of the mixture is added and the powdered magnesium oxide begins to be added, stirring continuously with a Selecta vigorous agitator equipped with a propeller. The mixing time must be between 50 and 90 minutes, the optimum being 60 minutes.

Once the two raw materials have been mixed and homogenized, the liquid medium used is eliminated by evaporation in the air with a gentle thermal treatment of drying under controlled conditions at 110 °C. Once the mass has dried, the agglomerates are broken up and the powder obtained, now dry, is subjected to a heat treatment between 250 and 350 °C in air, the optimum being 300 °C, and a duration of the same between 30 and 120 minutes, with 90 minutes being optimal. With this treatment, the elimination of the water and acetone retained in the mass that had not been eliminated, in turn, in the first drying is achieved, and the hydration of the magnesium oxide particles is reduced as much as possible, because if it has occurred, the magnesium hydroxide will then decompose with the release of water.

The product resulting from drying, which contains the forsterite precursor, is then subjected to a fundamental stage in this invention patent, which is the grinding and mechanochemical activation treatment, using a high-energy mill such as a ball mill and preferably a planetary ball mill. This step is essential, first of all, to achieve a very high degree of homogenization of the raw materials when mixed with each other, also producing a reduction in the size of the particles and, most importantly, to induce the mechanochemical activation that takes place using a high energy mill. Preferably, the high energy mill used is a ball mill and more preferably a planetary mill. In this way, it is possible to induce mechanochemical reactions in the starting raw materials.

On the other hand, the rotation speed that must be used for this treatment is between 300 and 500 rpm, with 400 rpm being optimal. And it is complemented with another parameter to take into account, which is the treatment time inside the device, which must be between 10 and 50 hours. Experience indicates that the optimal time in the example of the invention is 40 hours, since it ensures a greater degree of amortization of the talc, examined by X-ray diffraction, without having to provide additional energy with more grinding.

In the example of the invention, a Retsch model S-1 planetary ball mill is used and the preferred device is a jug or container made of steatite, with a volume of 300 mL, using a load of 14 balls of the same material, 7 of them of 17 mm in diameter and the other 7 of 15 mm in diameter. The same amount of sample, 60.00 grams, is always used to carry out the mechanochemical activation treatment by grinding.

Another device made of a different material that makes up the grinding jar can be used, such as steel with balls of the same material. By using a soapstone jug, a compact metamorphic rock that contains magnesium silicates, the possible contamination of the resulting raw material for the following treatment is minimized. For an expert in the field, by using jars made of other much harder materials, such as steel or tungsten carbide, the mechanical activation time by grinding could be reduced, but the degree of contamination of the resulting raw material would be more higher than when using steatite, since this material only provides silica and magnesium, which are the same constituents of forsterite.

The powder resulting from this grinding treatment, which is the forsterite precursor, is processed by conventional methods used in the manufacture of shaped materials.

For example, tablets or pellets are prepared starting from the ground powder, 10-30 grams, by uniaxial pressing at 50 MPa, with 5% humidity, in two times, using a cylindrical metal mold in a 120 Tm press, model CMH -120 AF. Thus, by using 30 grams of powder, cylindrical tablets of 50 mm in diameter and variable height (ca. 10 mm) are obtained.

Next, the step of heat treatment of the shaped material is carried out. It is advisable to carry out the heat treatment in several stages to avoid defects in the specimens and to favor thermal transformations. In a first stage, the previously dried powder is treated at 110 °C and 300 °C, first up to 800 °C at a heating rate between 40 and 80 °C per hour, the optimum being 60 °C per hour, during a residence time at that temperature between 60 and 150 minutes, the optimum being 120 minutes to eliminate all traces of moisture present in the ground powder.

Subsequently, the temperature is raised to 1000 °C at a heating rate of 60 °C per hour and is maintained at this temperature for a minimum of 60 minutes to favor the growth of the forsterite crystalline nuclei. From this treatment, the temperature is raised to 1,100-1,200 °C, the optimum being 1,200 °C, at a heating rate between 40 and 80 °C per hour, the optimum being 60 °C per hour, for a period of time between 60 and 150 minutes, the optimum being 120 minutes.

It is verified by X-ray diffraction and analysis of crystalline phases that the formation of forsterite, as a crystalline phase, is detected at 1000 °C and increases in relative proportion and crystalline character. At a temperature of 1200 °C, with a heat treatment between 60 and 150 minutes, the optimum being 120 minutes, the proportion of crystalline phases in the resulting product can be estimated at 90-95% of forsterite as the majority phase, 5% of magnesium oxide or periclase that remains unreacted and less than 5% magnesium ferrite (MgO.Fe2Ü3).

An example of ceramic material or refractory material that can be obtained with the process of the invention after heat treatment of the forsterite precursor powder previously subjected to mechanochemical activation treatment by grinding, previously pressed at 50 MPa, at a temperature of 1200 °C for 120 minutes, it has the following physical-chemical characteristics:

- Water absorption capacity: 4.2% by weight.

- Bulk density: 2.76 g/cm3.

- Open porosity: 12.6% by volume.

- Average coefficient of thermal expansion between 20 and 900 °C of 11.52 x 10-6 x °C-1.

- Resistance to sudden changes in temperature, or resistance to thermal shock, measured as the number of cycles that the refractory material resists without cracking or losing its cohesion, carrying out the test from room temperature to 950 °C and cooling in water, a minimum of 3 cycles.

- Chemical composition (% by weight): SiO ² , 41.22%; MgO, 52.98%; AhO3, 2.35%; Fe2O3, 2.70%, ^TiO2 , 0.11%; CaO, 0.56%; Na ² O, 0.05%; K ² O, 0.00 and sum total of 99.97%.

- MgO/SiO ² molar ratio of the refractory material obtained = 1.89 -1.98 (theoretical 2.00 of 2MgO.SiO2).

- Mineralogical composition: 90-95% forsterite.

Likewise, by grinding the product obtained according to the procedure of the invention following this example, a chamotte, calcine or sand with a high forsterite content is obtained, which can also be used in various applications and as raw material for the preparation of refractories with certain properties. physical, such as controlled porosity, with applications as an insulating material, or after adding chromium salts, for LASER applications.

The ceramic material obtained by sintering at 1400 °C for 120 minutes has the same chemical composition as that described above, with an apparent density of 2.85 g/cm3 and open porosity of 4% by volume. If it is subjected to a grinding treatment to reduce the size of the particles of the shaped and sintered material, using a hammer mill or a jaw mill, various granulometries can be obtained by sieving, one being very convenient for the purposes of its applications which has 30% by weight of a fraction less than 0.100 mm and a fraction less than 0.040 mm of 50% by weight, with an average diameter of the particles of 0.063 mm.

On the other hand, the general characteristics of forsterite refractories, according to the UNE standard (UNE 61027 Standard) that establishes a single quality, based mainly on the physical characteristics of these materials, are the following:

- Fe ² O ³ content in percentage (%) by weight, less than 7%.

- AhO ³ content in percent by weight, less than 1.5%.

- CaO content in percentage, less than 0.5%.

- Maximum porosity, in percentage by volume, 20%.

Comparing these values with those corresponding to the basic ceramic and refractory materials obtained, according to the procedure of the invention based on this illustrative and non-limiting example of the invention, it can be deduced that their iron oxide content perfectly meets the specifications given. in the described standard.

The alumina content is somewhat higher, but not too high, and is avoided by using as starting raw material an industrial talc with a lower alumina content than that used in this example.

Finally, in terms of porosity, the ceramic and refractory materials obtained as a result of the procedure of the invention have values even lower than those specified in the standard.

Claims (9)

1. - Procedure for the preparation of ceramic, bioceramic and refractory materials of a basic nature with a high content of forsterite by means of reactive grinding, characterized in that it comprises the following stages: a) Mix an aqueous suspension of talc of composition: -SiO ² between 46.3% and 63.1% by weight, - MgO between 29.10% and 32.90% by weight - AhO3 between 0.29% and 3% by weight - Fe ² O ³ between 0.15% and 3.4% by weight - TiO ² between 0.05% and 0.13% by weight - CaO between 0.19% and 1.50% by weight - Na ² O between 0.04% and 0.10% by weight - K ² O between 0.00% and 0.41% by weight - loss on ignition at 1000°C between 5.04% and 8.02% with magnesium oxide calcined at a minimum temperature of 1700 °C and with a minimum content of 97% by weight of particles with a size of less than 0.063 mm in a proportion between 70 and 80% by weight of talc and between 20 and 30% of magnesium oxide; b) Dry the suspension obtained in stage (a) at a temperature of up to 350 °C; c) Grinding of the product of stage (b) for homogenization, reduction of particle size and induction of mechanochemical activation in a planetary ball mill with a jar made of steatite and balls of the same material; d) uniaxial pressing of the product obtained in stage (c); and e) Thermal treatment of the product of stage (d) at a temperature of up to 1200 °C. 2. - Process according to claim 1, wherein the talc and magnesium oxide from step (a) are previously ground to a particle size with an average diameter of at least 0.100 mm. 3. - Process according to any of claims 1 to 2, wherein the aqueous suspension of step (a) contains acetone in a proportion of 50% by volume. 4. - Process according to any of claims 1 to 3, wherein the drying of stage (b) is carried out at a temperature between 250 °C and 350 °C for a time between 30 and 120 minutes. 5. - Procedure according to any of claims 1 to 3, wherein the drying of stage (b) is carried out in two substages: b1) Air-drying at a temperature between 90 °C and 120 °C, for up to 15 hours. b2) Drying after step (b1) at a temperature between 250 °C and 350 °C, for a time between 30 and 120 minutes. 6. - Procedure according to any of claims 1 to 5, wherein the heat treatment of stage (e) is carried out in the following substages: and l) Heat treatment at a temperature of up to 800°C at a heating rate of between 40°C and 80°C per hour, for a time of between 60 and 150 minutes; e2) The product obtained in (e1) is treated at a temperature of up to 1000°C at a heating rate of between 40° and 80°C per hour, for a time of between 60 and 150 minutes; and e3) The product obtained in (e2) is treated at a temperature between 1100°C and 1200°C at a heating rate of about 60°C per hour for a time between 60 and 150 minutes. 7. - Process according to claims 1 to 5, wherein the heat treatment of stage (e) is carried out at a temperature between 1100 ° C and 1200 ° C at a heating rate of about 60 ° C per hour for a time between 60 and 150 minutes. 8. - Process according to any of claims 1 to 7, further comprising: i) Crushing the material obtained in the heat treatment of stage (e); ii) Shape the previously crushed material in (i); and iii) Heat treatment of the product formed in (ii) at a temperature between 1200 °C and 1400 °C, for a time between 60 and 240 minutes. 9. - Procedure according to claim 8, wherein the shaping of the product or material is carried out by uniaxial pressing.