RU2630112C1 - Method of producing a small-crystalline aluminate of magnesium - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method comprises the autoclaving of the initial mixture of water in the autoclave, comprising a molar ratio of 1.0-1.2 oxyhydroxide or aluminium hydroxide with magnesium oxide and an activating additive, which uses an aqueous solution of glycerol (C₃H₅(OH)₃) and aluminium chloride⋅ AlCl₃⋅6H₂O. The treatment is carried out first under subcritical conditions, at a temperature of 200 to 360°C and steam pressure from 1.5 to 19 MPa for 6-20 hours, and then under supercritical conditions, water vapor at a temperature of 370 to 420°C and steam pressure from 21.5 to 30.4 MPa for 1-12 hours, after which the water vapor is removed, the autoclave is cooled to room temperature, the magnesium aluminate obtained is discharged, washed with distilled water until a pH of 7 washing waters is obtained and dried at temperature up to 70°C to constant weight, while glycerin is taken in an amount of 1-12 wt %, and aluminium chloride in an amount of 0.01-0.8 wt % relative to the weight of the final product, magnesium aluminate, the weight ratio of aluminium chloride to glycerol being 0.01-0.1. A mixture of magnesium oxide powders and hydroxide or aluminium oxyhydroxide can be preliminarily sifted or processed in a planetary or ball mill to form a homogeneous mass.

EFFECT: invention makes it possible to obtain a single-phase fine-crystalline magnesium aluminate with an average crystal size of 0,2-0,5 mcm and a content of the main substance of not less than 99,99 percent.

2 cl, 1 dwg, 1 tbl, 6 ex

Description

Technical field

The present invention relates to the field of synthesis of inorganic materials, namely fine crystalline magnesium aluminate MgAl ₂ O _{4 of} high purity with an average crystal size of 0.2 to 0.5 microns, used as raw material for the manufacture of single crystals and light-transmitting aluminum-ceramic ceramics.

The main requirement for small-crystalline magnesium aluminate synthesized by the protected method of production is a low content of coloring mineral impurities (less than 0.01% in total), which determines high light transmission and a narrow distribution of synthesized spinel particles with an average size not exceeding 0.5 microns.

State of the art

Chinese researchers (Feng W, Wang Y and Hu W, International Patent application (WO 2012019521, 02.16.2012) have proposed a method for producing magnesia spinel using active magnesium oxide and powdered aluminum or recycled aluminum waste as precursors. At the first stage, magnesite is calcined (MgCO ₃₎ or brucite (Mg (OH) ₂₎ and the resulting material was ground to obtain a powder of the active magnesium oxide. Further, the active magnesium oxide is added powdered aluminum industrial waste or recycled aluminum production, to the mixture was mpaktiruyut and calcined at high temperatures. This method is positioned as one of the cheapest ways to synthesis spinel, although the impurity content of the product quality is low and it can not be used as a raw material for optical ceramics.

In the article (L.G. Karakchiev, E.G. Avvakumov, O.B. Vinokurova, A.A. Gusev, Spinel formation during heat treatment of mechanically activated mixtures of brucite and hydrargillite. Journal of Inorganic Chemistry, 2005, v. 50, No. 10 , pp. 1612-1616) it is shown that the mechanical activation of a mixture of aluminum hydroxide and magnesium hydroxide in the planetary activator EI-2x150 followed by heat treatment of the activation products in air at 750 ° C leads to the formation of highly dispersed spinel (MgAl ₂ O ₄ ). However, the method is energy-intensive and the resulting product is not clean enough to obtain optical ceramics.

There is a method of producing highly dispersed magnesium aluminate (RU 2457181 from 04/27/2011, Isupov V.P., Khusnutdinov V.R., Avvakumov E.G., Vinokurova O. B. Method for producing highly dispersed magnesium aluminate). The method consists in the fact that aluminum hydroxide and magnesium hydroxide in a weight ratio of 2.69: 1 are mixed and subjected to mechanical activation in highly stressed planetary centrifugal mills for 5-15 minutes with a ratio of the weight of the sample to the mass of the ball load of 1: 10-1: 30 and acceleration of 20-40 g. The products of mechanical activation are treated with water at temperatures of 20-95 ° C and a time of 0.25-2 hours, filtered, dried and subjected to heat treatment at a temperature of at least 800 ° C for 2-4 hours. The proposed method allows to obtain powdered magnesium aluminate with a specific surface area of from 80 to 260 m ² / g However, it does not allow to obtain a pure product in connection with the mechanical activation of raw materials in planetary-centrifugal mills. In addition, it consumes a lot of electricity during high-temperature heating of the intermediate.

A known method for the synthesis of magnesium aluminate (Yamamoto Toshio, Suda Akihiko, Sugiura Masahiro, EP No. 0990621 A2. MKI C01F 7/16, 04/05/2000, Spinel powder and spinel slurry), based on the processing of a mixture of aqueous solutions of aluminum and magnesium salts with an aqueous solution of ammonia separating the precipitate, drying it in air at 150 ° C for 10 hours, calcining the dried precipitate at 400 ° C for 5 hours, and finally calcining at 850 ° C for 5 hours. The obtained magnesium aluminate, after its additional dispersion in water, has an average particle size of 10-12 μm and a specific surface area in the range of 80-126 m ² / g. The disadvantage of this method is the large particle size, multi-stage process, the need for disposal of aqueous solutions of ammonium salts formed during water treatment.

Woohong Kim, Jasbinder S. Sanghera, Guillermo R. Villalobos, Shyam S. Bayya, Ishwar D. Aggarwal (US 20090220790, September 3, 2009) proposed the use of a coprecipitation process of a mixture of magnesium hydroxide and aluminum hydroxide to obtain MgAl ₂ O ₄ nanopowder. The resulting double hydroxide was precipitated, separated, and then washed in aprotic solvents, such as ethyl acetate, tetrahydrofuran, methyl ethyl ketone, acetonitrile, etc. After washing, the precipitate consisting of double hydroxides was dried and calcined at a temperature of 1300 ° C. The result was a spinel powder with a particle size of from 20 to 100 nm.

Known (US 7182929 B1, 02.27.2007, Nanostructured multi-component and doped oxide powders and method of making same, A. Singhal, G. Skandan, M. Jain) a method for the production of nanostructured multicomponent pure or doped oxides, including the stage of dissolution of cation salts for example, magnesium ions in the case of synthesis of MgAl ₂ O ₄ , in an organic solvent with the addition of a second component of the dispersed nanopowder into this solution, for example, Al ₂ O ₃ in the case of synthesis of MgAl ₂ O ₄ . The solution with both components is heated to facilitate the diffusion of magnesium ions into aluminum oxide nanoparticles. Then the solid phase is separated and heated to high temperatures to form the structure of crystals of magnesium aluminate.

As a result of the operations, multicomponent or doped nanoparticles of oxides (in particular MgAl ₂ O ₄ ) with a size of 5 to 500 nm, preferably 20-100 nm, are obtained. The average size of the secondary particles (aggregates) is in the range from 25 to 2000 nm, preferably less than 500 nm. The size of the primary crystals was about 20 nm. MgAl ₂ O ₄ nanopowders were obtained using three different solvents: hexanol (boiling point 155 ° C), decanol (since 250 ° C) and water (since 100 ° C). It was shown that the particle characteristics of the MgAl ₂ O ₄ nanopowder depend on two factors: the boiling point and type of solvent, and on the particle characteristics of the alumina powder used. The method is labor and energy intensive.

Known (M. BAR J, JF BOCQUET, K. CHHOR, C. POMMIER, Submicronic MgAI204 powder synthesis in supercritical ethanol, JOURNAL OF MATERIALS SCIENCE 27 (1992), 2187-2192) a method for producing submicron magnesium aluminate MgAl ₂ O ₄ by decomposition of double alkoxide Mg [Al (O-secBu) ₄ ] ₂ or Mg [Al (OC ₂ H ₅ ) ₄ ] ₂ in supercritical ethanol at T≥360 ° C and pressure Pc≥6.13 MPa. The reaction is carried out in a steel reactor. The decomposition of these alkoxides in ethanol starts from 200 ° C, and small crystals of magnesium oxide (MgO) and amorphous aluminum oxyhydroxide (AlOOH) are formed. As the temperature rises to 360 ° C and higher, magnesium ions diffuse into the interplanar space of the layered structure of aluminum oxyhydroxide with the formation of submicron crystals of magnesium aluminate MgAl ₂ O ₄ . The resulting magnesium aluminate powder is filtered off and heated at 1100 ° C to complete the construction of the spinel structure. It is assumed that the main role in the conversion of metal alkoxides to magnesium aluminate is played by the water contained in alcohol, which provokes hydrolysis and polycondensation processes. Hexane can also be used as a reaction medium. The method is multi-stage, unreasonably complex and highly costly.

The patent (Minoru Matsui, Tomonori Takahashi US 4,584,151, Method of producing a polycrystalline transparent spinel sintered body), published 04/22/1986, describes a method for producing MgAl ₂ O ₄ spinel powder for ceramic production. The method involves the synthesis of MgAl ₂ O ₄ powder from alcohol solutions of a mixture of magnesium alkoxide and aluminum alkoxide in a molar ratio of (0.502: 0.498) to (0.524: 0.476). Hydrolysis of the mixture, planting and heat treatment is carried out in an oxidizing atmosphere at temperatures of 900-1100 ° C. Then the powder is sintered in hydrogen at a temperature above 1700 ° C with a heating rate of 50 ° C per hour at temperatures of 800-1400 ° C. The resulting cake is crushed and lithium fluoride is added to the powder in an amount of 0.05-0.15 mass%. The disadvantages of this method are: high energy consumption and pollution of the obtained material when grinding sintered samples. The synthesized MgAl ₂ O ₄ powder can be used for the manufacture of aluminum-magnesia ceramics, but the resulting ceramics has a low light transmission.

Previously, the use of water vapor (aqueous fluid) with activating additives for the synthesis of crystalline materials was disclosed in (V.V. Lazarev, GP Panasyuk, IL Voroshilov, GP Boudova and MN Danchevskaya, SN Torbin, YD Ivakin, New Ecologically Pure Technologies of Fine -Crystalline Materials, Ind. Eng. Chem. Res., 1996, 35, 10, pp. 3721-3725). The authors investigated the structural transformations of amorphous or weakly crystalline samples of silica and alumina when treated with water vapor under supercritical conditions. Thermocouple treatment of the initial product, amorphous silica or aluminum hydroxide Al (OH) ₃ , was carried out in laboratory (V = 20 ml), pilot (V = 55 l), and technical (V = 4 m ³ ) autoclaves at temperatures from 375 to 450 ° C and water vapor pressure selected in the range of 20-28 MPa using activator microadditives: tetramethylammonium hydroxide ((CH ₃ ) ₄ NOH), hydrazine hydrate (N ₂ H ₅ OH), aliphatic alcohols (C _m H _2m OH), or tetraethylorthosilicate ((C ₂ H ₅ O) ₄ Si). It was found that the process of restructuring the structure of silica (or alumina) proceeded under conditions of quasiequilibrium of hydroxylation-dehydroxylation with the participation of activator molecules in the formation of a solid matrix of end products - quartz or corundum.

In patent RU 2340558, “Method for the production of finely crystalline doped lanthanum aluminate”, authors: Danchevskaya M.N., Ivakin Yu.D., Torbin S.N., priority of the invention 12/29/2006, the invention relates to the synthesis of doped finely crystalline lanthanum aluminate LaAlO ₃ used as refractories, catalysts and phosphors. In this method, a mixture of aluminum hydroxide, lanthanum oxide and dopants is subjected to thermocouple autoclaving in two stages. The first stage is carried out in the temperature range of 180-250 ° C, for 20-22 hours, and the second at 390-450 ° C for 48-72 hours. As alloying additives, oxides or salts of elements selected from the series: Mg, Cr, Eu, Ce are used in an amount of from 0.001 to 2 mass% by weight of lanthanum aluminate. The claimed method allows to obtain lanthanum aluminate with a low water content, doped with magnesium, chromium, europium or cerium. The method does not apply to the synthesis of especially pure lanthanum aluminate, since without alloying additives the process does not proceed completely. The disadvantage of this method is the length of the procedure for treating with a steam of water a mixture of reagents at both temperature steps.

The closest to the claimed technical essence and the achieved effect is the method described in the proceedings of the conference "14th European Meeting on Supercritical Fluids", Marseille (France), 18 May - 21 May 2014 [Danchevskaya MN, Ivakin Yu.D., Muravieva GP, Torbin SN "Synthetic magnesium spinel - raw material for optical ceramics))], where a method for producing fine crystalline magnesium aluminate (MgAl ₂ O ₄ ) from a mixture of magnesium oxide and aluminum hydroxide Al (OH) ₃ or AUON (boehmite) with an alloying agent is disclosed by stepwise synthesis of spinel sequentially in subcritical and then in supercritical conditions. AlCl ₃ or organic agents NH ₄ C ₂ H ₃ O ₂ , 1,2,3-propanetriol and 1,2-ethanediol were used as alloying agents. This method allows to obtain spinel (MgAl ₂ O ₄ ) with an average crystal size of from 0.2 to 2.0 microns and a shape from lamellar to spherical or octahedral. The disadvantage of this method is the duration of the synthesis at the second isothermal stage at a temperature higher than critical in the series (370-400) ° C in an atmosphere of supercritical aqueous fluid. For example, when using AlCl ₃ additive in an amount of 0.48 mass% with respect to the final product, 100% conversion of a mixture of aluminum hydroxide and magnesium oxide proceeds over a period of more than 15 hours at a temperature of 400 ° C (P _H2O = 26.4 MPa), after preliminary exposure to the first isothermal stage at 230 ° C (P _H2O = 3 MPa) for 20 hours.

Disclosure of invention

The objective of the proposed technical solution is to develop a simple, energy-saving and environmentally friendly method for producing magnesium aluminate with a low content of impurities, not more than 0.01 mass% by the sum of coloring elements, with an average crystal size of 0.2 to 0.5 microns.

The problem is solved by the proposed method, which consists in autoclaving with steam the water of the initial mixture, including 1.0-1.2 oxyhydroxide or aluminum hydroxide with magnesium oxide taken in a molar ratio, and an activating additive, which is used as an aqueous solution of glycerol (C ₃ H ₅ (OH) ₃ ) and aluminum chloride AlCl ₃ ⋅ 6H ₂ O, the treatment being carried out first under subcritical conditions, at a temperature of 200 to 360 ° C and a vapor pressure of 1.5 to 19 MPa for 6-20 hours and then under the conditions of supercritical state of water vapor at a temperature ature from 370 to 420 ° C and a steam pressure of 21.5 to 30.4 MPa for 1-12 hours, after which the water vapor is vented, the autoclave is cooled to room temperature, the obtained magnesium aluminate is discharged, washed with distilled water until the pH reaches = 7 washings and dried at temperatures up to 70 ° C to constant weight in room conditions.

Preferably, for the preparation of the activating additive, glycerol is taken in an amount of 1-12 mass% and aluminum chloride in an amount of 0.01-0.8 mass% relative to the weight of the final product - magnesium aluminate, while the weight ratio of aluminum chloride to glycerin is 0.01- 0.1.

Preferably, the mixture of powders of magnesium oxide and aluminum hydroxide or oxyhydroxide is pre-sieved or processed in a planetary or ball mill until a homogeneous mass is formed.

To create an atmosphere of water vapor, the required amount of water is placed on the bottom of the autoclave, under the container with the initial mixture. The amount of water required to create the desired pressure in the autoclave is calculated taking into account the water contained in the feed. To simplify the calculation of the amount of water required to create the required pressure in the autoclave, the initial mixture is impregnated with a solution of a mixture of glycerol and aluminum chloride and dried at a temperature of up to 70 ° C to constant weight, after which a sample is taken in which the amount of water in the feed is determined by high-temperature heating .

The technical result of the proposed method is to obtain highly dispersed magnesium aluminate with an average crystal size of 0.2-0.5 microns, with a content of coloring impurities of not more than 0.01%, at a lower temperature and for a shorter time period, in relation to the analogue, which achieved by using an additive of a mixture of glycerol and AlCl ₃ as an activator in the synthesis of MgAl ₂ O ₄ in an atmosphere of water vapor. The synthesized fine crystalline magnesium aluminate can be used as a raw material for the manufacture of single crystals and light-transmitting aluminomagnesium ceramics.

An essential distinguishing feature of the claimed technical solution is the use in the synthesis of magnesium aluminate from a mixture of aluminum oxyhydroxide (or hydroxide) and magnesium oxide when treating water vapor additives of a mixture of glycerol (C ₃ H ₅ (OH) ₃ ) and AlCl ₃ ⋅ 6H ₂ O to obtain 100% fine crystalline product (MgAl ₂ O ₄ ). The method allows you to adjust the size and shape of the crystals. An increase in activator concentration leads to a decrease in crystal size. With increasing water vapor pressure during spinel synthesis, faceting of spherical spinel particles occurs. The choice of temperature ranges of the first and second successive stages of synthesis is due to the features of the mechanism of formation of magnesium aluminate under the described conditions.

The first stage (200-360) ° C, at the corresponding equilibrium pressures of water vapor in a two-phase system, is determined by the temperatures of formation of an intermediate compound from the initial mixture - layered double magnesium and aluminum hydroxide with the chemical formula [Mg _x Al _1-x (OH) ₂ ] (OH) _1-x . The formation of such compounds at a certain stage is associated with the participation of water vapor molecules in the formation of the structure of magnesium aluminate. At the second stage (370-420) ° C, at a vapor pressure in the range from 21.5 MPa to 30.4 MPa, decomposition of double magnesium hydroxide and aluminum with the formation of MgAl ₂ O ₄ occurs in a medium of supercritical aqueous fluid. At T <370 ° C, decomposition of the intermediate is difficult. Above 420 ° C, there is no reason to raise the temperature, because at lower temperatures the process is quite complete.

With an increase in glycerol concentration above 12 wt%, contamination of the synthesized spinel with glycerol decomposition products is noted. A decrease in the concentration of glycerol in the reaction medium below 1% reduces the speed of the spinel formation process, and the process does not proceed completely. The use of the second component of the activator AlCl ₃ ⋅ 6H ₂ O with a concentration in the range of (0.01-0.8) mass% leads to an acceleration of the process. An increase in the concentration of AlCl ₃ ⋅ 6H ₂ O above 0.8 mass% relative to the final product of the synthesis of MgAl ₂ O ₄ can lead to corrosion of the autoclave.

Brief Description of the Drawings

The essence of the claimed method is illustrated by drawings.

In FIG. Figure 1 shows an electron microscopic image of MgAl ₂ O ₄ powder synthesized with an addition of a mixture of glycerol 3.5 mass% and AlCl ₃ ⋅ 6H ₂ O 0.2 mass% during heat treatment in water vapor first at a temperature of 280 ° C (P _H2O = 6 5 MPa) for 20 hours, then at 370 ° C (P _H2O = 21.5 MPa) for 10 hours.

The implementation of the invention

All reagents used are commercially available, all procedures, unless otherwise specified, were carried out at room temperature or ambient temperature, that is, in the range from 18 to 25 ° C.

Chemical symbols have their usual meanings: μm (micrometer (s)), μl (microliter (microliters)), μg (micrograms (micrograms)), M (mol (moles) per liter), l (liter (liters)), ml (milliliter (milliliters)), g (gram (grams)), mg (milligram (milligrams)), mole (mole), mmol (millimol (millimole)).

The method for producing fine crystalline magnesium aluminate is a two-stage synthesis. At the first stage, a previously prepared mixture of powders of oxyhydroxide or aluminum hydroxide with magnesium oxide is taken, taken in a molar ratio of Al ₂ O ₃ / MgO = 1-1.2 with an activating additive consisting of 1-12% glycerol and AlCl ₃ ⋅ 6H ₂ O in an amount of 0.01-0.8% by weight of the final product of the synthesis of MgAl ₂ O ₄ . The ratio of additives AlCl ₃ ⋅ 6H ₂ O / C ₃ H ₅ (OH) ₃ is taken in the range of 0.01-0.1. The resulting mixture is mixed to a homogeneous mass, followed by steam treatment of the water in an autoclave, in subcritical conditions, below the critical water temperature (374 ° C), namely, at a temperature selected in the range from 200 to 360 ° C and with an appropriate steam pressure of 1 , 5 to 19 MPa for 6-20 hours, and then at the second stage of synthesis, the treatment is carried out under the supercritical state of water vapor, followed by exposure at the temperature reached, in the range 370-420 ° C, namely 400 ° C, in isothermal conditions within 1-12 hours. Then the steam is vented, the autoclave is cooled and depressurized. The synthesized sample is removed from the autoclave, washed with distilled water to a pH of 7 drains, and dried in air at 70 ° C to constant weight for about 6 hours.

For the formation of a homogeneous mass of powders of the starting components, sieving through sieves with a hole size of 300 ± 10 μm or processing in a planetary or ball mill in a low-energy mixing mode (with the ratio of the mass of balls to the mass of the powder not more than 2 and lasting no more than 10 minutes) is used.

To increase the yield of the product and the possibility of increasing the loading of the initial mixture into the autoclave, it is possible to pre-dry the raw material at a temperature of up to 70 ° C to a constant weight after making an activator solution.

The given examples of a specific embodiment of the invention are provided to provide specialists in this field of technology for a complete description, conduct and application of the method according to the invention, and imply that the above examples do not limit the scope of the invention intended by the authors.

The essence of the proposed method for producing crystalline magnesium aluminate is illustrated by the following examples of the synthesis of spinel from a mixture of reagents with a molar ratio of Al ₂ O ₃ / MgO = 1.04.

Example 1

In a 18.5 ml autoclave container, 1 ml of a 18.7% aqueous solution of glycerol (12% by weight of synthesized MgAl ₂ O ₄ ) and 1 ml of a 0.16% aqueous solution of AlCl ₃ ⋅ 6H ₂ O (0.1% of the weight of the synthesized MgAl ₂ O ₄ ). 1.4712 g of AlOOH (boehmite) and 0.5288 g of magnesium oxide MgO were preliminarily mixed by sieving three times through a nylon sieve with a hole size of 300 μm and the mixture was poured into a container with an aqueous solution of activators. The mixture was stirred until a homogeneous mass was formed. A wet feed container was placed in an autoclave. At the bottom of the autoclave, under the container, 0.73 ml of distilled water was poured to create a water vapor pressure of 26.8 MPa at 400 ° C. The autoclave was sealed and heated to 220 ° C (P _H2O = 2.4 MPa). At this temperature, the autoclave was held for 18 hours. Then the autoclave was heated to 400 ° C (P _H2O = 26.8 MPa) and kept in isothermal conditions for 10 hours. After synthesis, the autoclave was cooled, depressurized, and the synthesis product was recovered. The synthesized powder was washed with distilled water to a pH of 7 washings and dried at 70 ° C to constant weight. The product was a fine crystalline magnesium aluminate with an average crystal size of 0.21 microns. The impurity content in the obtained sample No. 1 is presented in table. one.

Example 2

1.4712 g of boehmite (AlOOH) and 0.5288 g of magnesium oxide were mixed three times by sieving through a nylon sieve with a hole size of 300 μm and pouring 1 ml of a 1.56% glycerol solution (1% relative to the weight of the synthesized MgAl ₂ O ₄ ) and 1 ml 0.16% aqueous solution of AlCl ₃ ⋅ 6H ₂ O (0.1% by weight of synthesized MgAl ₂ O ₄ ). The mixture was stirred until a homogeneous mass was formed. The resulting mass was placed in an autoclave container. At the bottom of the autoclave with a volume of 18.5 cm ³ was poured 0.34 ml of distilled water. A raw material container was placed in an autoclave and sealed. The autoclave was heated to 270 ° C (P _H2O = 5.6 MPa) and held for 10 hours. Then the autoclave was heated to 410 ° C (P _H2O = 28.7 MPa) and kept at this temperature for 8 hours. After cooling the autoclave, the synthesis product was recovered. The synthesized powder was washed with distilled water to a pH of 7 washings and dried at 70 ° C. The synthesized powder was a fine crystalline magnesium aluminate with an average crystal size of 0.5 μm.

Example 3

1.4712 g of boehmite (AlOOH) and 0.5288 g of magnesium oxide were mixed by three sieving through a nylon sieve with a hole size of 300 μm and filled with 1 ml of a 6.25% glycerol solution (4% relative to the weight of the synthesized MgAl ₂ O ₄ ) and 1 ml 0.016% solution of AlCl ₃ ⋅ 6H ₂ O (0.01% by weight of MgAl ₂ O ₄ ) in distilled water. The mixture was stirred until a homogeneous mass was formed. The resulting wet mass was placed in an autoclave container. At the bottom of the autoclave with a volume of 18.5 cm ³ 1.26 ml of distilled water was poured. A container with raw materials was lowered into the autoclave and sealed. The autoclave was heated to 270 ° C (P _H2O = 5.6 MPa) and held for 6 hours. Then the autoclave was heated to 410 ° C (P _H2O = 28.7 MPa) and kept at this temperature for 2 hours. After cooling the autoclave, the synthesis product was recovered. The synthesized powder was washed with distilled water to pH 7 and dried at 70 ° C. The synthesized powder was a fine crystalline magnesium aluminate with an average crystal size of 0.4 μm.

Example 4

1.0966 g of finely divided aluminum hydroxide Al (OH) ₃ and 0.3348 g of magnesium oxide were mixed by three sieving through a nylon sieve with a hole size of 300 μm. 1 ml of 3.26% aqueous glycerol solution (3.3% relative to synthesized MgAl ₂ O ₄ ) and 1 ml of 0.49% aluminum chloride solution (0.5% relative to MgAl ₂ O ₄ ) were poured into the autoclave container, and poured into the container a mixture of aluminum hydroxide with magnesium oxide and stirred until a homogeneous mass is formed. At the bottom of the autoclave, under a container, 1.2 ml of distilled water was poured. A container with wet feed was placed in the autoclave and sealed. The autoclave was heated to 230 ° C (P _H2O = 2.9 MPa) and held for 12 hours. Then the autoclave was heated to 420 ° C (P _H2O = 30.4 MPa) with exposure at this temperature for 1 hour. After cooling the autoclave, the synthesis product was removed from it, washed and dried by the method described above. The synthesized powder was a fine crystalline magnesium aluminate with a crystal size of 0.48 μm.

Example 5

A mixture of 9.315 g of boehmite and 3.348 g of magnesium oxide was processed in a planetary mill in a low-energy mixing mode (with a ratio of ball mass to powder mass of not more than 2 and a duration of 10 minutes). The resulting mixture was mixed with 9 ml of an aqueous solution containing 0.3 g of glycerol (3% relative to synthesized MgAl ₂ O ₄ ) and 0.003 g AlCl ₃ ⋅ 6H ₂ O (0.03% relative to synthesized MgAl ₂ O ₄ ), and dried at 70 ° C. 2 g of the dried reaction mixture was placed in an autoclave container. 2.93 ml of distilled water was poured into the bottom of the autoclave. A raw material container was placed in the autoclave, and the autoclave was sealed. The synthesis of magnesium aluminate was carried out in water vapor in a two-stage mode: first at 250 ° C and a water vapor pressure of 4.1 MPa for 10 hours, then at a temperature of 400 ° C and a pressure of 26.8 MPa for 6 hours. After cooling the autoclave, the synthesis product was removed from it, washed and dried by the method described above. The average size of the resulting crystals is 0.45 microns. The impurity content in the obtained sample No. 5 are presented in table. one.

Example 6

1.4712 g of boehmite (AlOOH) and 0.5288 g of magnesium oxide were mixed three times by sieving through a nylon sieve with a hole size of 300 μm and pouring 1 ml of a 5.5% glycerol solution (3.5% relative to the weight of synthesized MgAl ₂ O ₄ ) and 1 ml of a 0.31% solution of AlCl ₃ ⋅ 6H ₂ O (0.2% by weight of MgAl ₂ O ₄ ) in distilled water. The mixture was dried at 70 ° C and placed in an autoclave container. 1.17 ml of distilled water was poured into the bottom of the autoclave with a volume of 18.5 cm ³ . A raw material container was placed in the autoclave and the autoclave was sealed. The autoclave was heated to 280 ° C (P _H2O = 6.5 MPa) and held for 20 hours. Then the autoclave was heated to 370 ° C (P _H2O = 21.5 MPa) and kept at this temperature for 10 hours. After cooling the autoclave, the synthesis product was recovered. The synthesized powder was washed with distilled water to pH 7. The product was dried at 70 ° C. The synthesized powder was a fine crystalline magnesium aluminate with an average crystal size of 0.3 μm.

The inventive method allows to obtain single-phase fine crystalline magnesium aluminate with a crystal size of 0.2-0.5 microns with a basic substance content of at least 99.99%.

Claims (2)

1. A method for producing fine crystalline magnesium aluminate, which includes autoclaving with steam the water of the initial mixture, including 1.0-1.2 oxyhydroxide or aluminum hydroxide with magnesium oxide taken in a molar ratio, and an activating additive, which is used as an aqueous solution of glycerol (C ₃ H ₅ (OH) ₃ ) and aluminum chloride AlCl ₃ ⋅ 6H ₂ O, the treatment being carried out first under subcritical conditions, at a temperature of 200 to 360 ° C and a vapor pressure of 1.5 to 19 MPa for 6-20 hours, and then under the conditions of supercritical state of water vapor at t temperature from 370 to 420 ° C and a steam pressure of 21.5 to 30.4 MPa for 1-12 hours, after which the water vapor is vented, the autoclave is cooled to room temperature, the obtained magnesium aluminate is discharged, washed with distilled water until the pH is reached 7 washing water and dried at a temperature of up to 70 ° C to constant weight, while taking glycerin in an amount of 1-12 wt. %, and aluminum chloride in an amount of 0.01-0.8 wt. % relative to the weight of the final product - magnesium aluminate, while the weight ratio of aluminum chloride to glycerol is 0.01-0.1. 2. The method according to claim 1, characterized in that the mixture of powders of magnesium oxide and aluminum hydroxide or oxyhydroxide is pre-sieved or processed in a planetary or ball mill until a homogeneous mass is formed.

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