RU2737298C1 - Method of producing nanoporous ceramics based on mullite - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to production of porous mullite materials and articles made therefrom and can be used in production of filtration elements for various fields of national economy, in particular, for purification of drinking water. Of Al(NO₃)₃⋅9H₂O salt, diluted solution with concentration of 0.1 M is prepared, from which is deposited aluminum hydroxide with aqueous solution of NH₄OH (~1 M) at temperature 0-2°C. Gel-like residue Al(OH)₃ is subjected to vacuum filtration with subsequent heat treatment at 150°C to form boehmite (AlOOH). To obtain amorphous SiO₂, silicic acid is subjected to thermal decomposition in the temperature range of 100-250°C. Mixing is performed γ-Al₂O₃ and amorphous SiO₂ in ratio Al₂O₃ : SiO₂ = 3:2 in dry grinding mode, then into mixture of 3Al₂O₃⋅2SiO₂ introducing a pore-forming component, in form of ammonium carbonate in amount of 10 or 20 wt% to total weight of powder and stirred for 10 minutes for homogeneous distribution (NH₄)₂CO₃ in whole volume of mixture, after which produced powders are compacted at pressure of 70 or 100 MPa and burnt at 1300°C for 2 hours.

EFFECT: method enables synthesis of single-phase mullite with controlled open porosity and narrow monomodal pore distribution.

1 cl, 2 tbl, 6 dwg

Description

The invention relates to the field of creating porous mullite materials and products from them and can be used in the production of filtration elements for various areas of the national economy.

The technical problem solved by the invention is as follows

Ceramic membrane filters based on zirconium dioxide (ZrO ₂ ) and aluminum oxide (Al ₂ O ₃ ) are used for fine purification of liquid and gaseous media in various industries and utilities. This is due to the fact that this ceramic has a special set of properties: high chemical and corrosion resistance, thermal stability at operating temperatures ≥500 ° C, dimensional stability, long service life and the possibility of multiple regeneration. Despite the fragility and relatively high cost of ceramic filters, they quickly pay off due to their high performance and long service life (up to 10 years and more instead of 1 year for polymer membranes) [1].

In general, a ceramic membrane filter consists of a porous carrier substrate that provides mechanical strength, and a thin membrane layer that determines the filtration fineness [2]. However, it is difficult to obtain a defect-free ceramic membrane of small thickness; therefore, the most widespread methods of production of filtration membranes are nowadays, which include two stages: the manufacture of a porous carrier substrate and the formation of a membrane layer on it.

Compositions based on α-Al ₂ O ₃ and aluminum-magnesium spinel (MgAl ₂ O ₄ ) are most often used as a porous base [3-6], on which selective layers are applied by slip casting or sol-gel method, allowing to obtain a pore size of ~ 100 -200 nm and make their distribution monomodal [1, 7, 8]. But at the same time, it should be noted that in the manufacture of membrane layers, a big problem is the elimination of gas bubbles that appear during the preparation of the slip and its fixation during heat treatment on a ceramic substrate, which is very likely to contribute to the formation of voids in the membrane layer and cracking itself. layer. In this regard, it is advisable to develop a technology for the production of porous ceramics with open porosity> 40% and a monomodal pore size distribution, the average size of which does not exceed 300 nm. This will allow this ceramics to be used as a full-fledged filtration membrane, eliminating the stage of deposition of membrane layers on a porous substrate.

As a porous oxide material, it is rational to use a compound of aluminum oxide with silicon dioxide - mullite (3Al ₂ O ₃ ⋅2SiO ₂ ) [9]. When synthesizing mullite from natural aluminosilicates (kaolin, chamotte, quartz sand, silica) at temperatures ≥1500 ° C, even in relatively pure materials, the practical yield of mullite is no more than 50-60% of the theoretical [10].

There is a known method for producing mullite according to RF patent No. 2463275, including mixing a natural aluminosilicate compound and an aluminum-containing compound, mechanical activation of the resulting mixture followed by heat treatment, characterized in that an aluminosilicate composition corresponding to the formula Al ₂ O ₃ ⋅SiO _{2 is} used as a natural aluminosilicate raw material, and as an aluminum-containing compound - technical alumina with the following ratio of components, wt. %: natural aluminosilicate Al ₂ O ₃ ⋅SiO ₂ - 62-72 technical alumina - the rest, after which the initial mixture is processed in a flow-type centrifugal mill with a mechanical load of the grinding bodies corresponding to an acceleration of at least 20 g, followed by heat treatment.

The main difficulty in producing such a way mullite is the necessity of using high temperatures to increase the diffusion mobility of cations of aluminum and silicon through a layer of 3Al ₂ O ₃ ⋅2SiO ₂ formed on the border between particles of the starting oxides [11], and this prevents the formation of the desired pore structure in mullite ceramics. Therefore, it is important to conduct research in the direction of developing technological methods for obtaining 3Al ₂ O ₃ ⋅2SiO ₂ from synthetic components with given parameters of open porosity and pore size distribution. At the moment, there is practically no information in the scientific literature on the conditions for the formation of a porous structure in mullite with a pore size of <300 nm.

A porous refractory mullite material according to RF patent No. 2182569 is known from the prior art, containing mullite of composition x Al ₂ O ₃ ⋅ySiO ₂ , obtained by exothermic reaction between ground components of a mineral charge, including silicon dioxide and aluminum, mixed with liquid glass, characterized in that the material has a porosity of 55-65%, a thermal conductivity at 20 ° C 0.06-0.10 W / m⋅K and contains 50-78 wt. % mullite with a content of x 66-72 wt. % and 28-34 wt. %, while the exothermic reaction takes place in the volume of the solidified porous material obtained by mixing the mixture with a gasifier - crystalline silicon, pre-activated by fine grinding to a particle size of <100 μm and mixed with water glass with pH> 8, with the following mass ratios of components: dioxide silicon: aluminum = (1.5-9.0): 1, liquid glass: silicon = (2-6): 1 and charge: liquid glass = (1.0-1.5): 1. A method of obtaining this material, including the preparation of a slip composition by mixing a charge, including silicon dioxide and aluminum, with water glass, molding, dehydration and heating to the initiation temperature of exothermic synthesis 700-800 ° C, followed by the synthesis of mullite at 1400-1700 ° C, which is different the fact that a ground mineral charge is used, a gas former is introduced into the slip composition - crystalline silicon, pre-activated by fine grinding to a particle size of <100 μm and mixed with liquid glass with pH> 8, at the following mass ratios of components: silicon dioxide: aluminum = ( 1.5-9.0): 1, liquid glass: silicon = (2-6): 1 and charge: liquid glass = (1.0-1.5): 1, dehydration of the slip composition to obtain a hardened porous material is carried out by evaporation of water during the interaction of the blowing agent with liquid glass, and the synthesis of mullite is carried out in the volume of a cured porous material.

This technical solution, as the closest to the one declared in terms of the technical essence and the achieved result, was adopted as its prototype. This technology makes it possible to obtain ceramics 3Al ₂ O ₃ ⋅2SiO ₂ with open porosity of ~ 50% and a particle size of 10-100 microns in the temperature range 1400-1700 ° C.

The disadvantages of the known technical solution are the high temperature of obtaining mullite ceramics and the fact that in such a coarse ceramic material it is extremely difficult to achieve a uniform pore size distribution, not to mention the creation of a pore structure necessary for filtration membranes. To reduce the temperature of mullite formation and create fine-porous ceramics on its basis, it is desirable to use dispersed components [13], which will make it possible to form a nanoporous structure in the final product.

It is known to use mechanochemical activation (MA) in the synthesis of 3Al ₂ O ₃ ⋅2SiO ₂ , which makes it possible to increase not only the dispersion of powders, but also creates additional structural defects that allow intensifying the process of formation and sintering of mullite ceramics at temperatures ≤1300 ° C [14] ... In studies [15-18] devoted to the mechanochemical activation of mullite synthesis, a positive effect of MA on the formation of 3Al ₂ O ₃ ⋅2SiO _{2 is} also noted and the possibility of reducing the sintering temperature of mullite ceramics by 200-300 ° C in comparison with the method of solid-phase synthesis is noted. ...

The objective of the claimed invention is to synthesize single-phase mullite using mechanochemical activation of γ-Al ₂ O ₃ and amorphous SiO ₂ and obtain on its basis nanoporous ceramics with controlled open porosity (-40-45%) and pore size less than 200 nm.

The essence of the claimed technical solution is expressed in the following set of essential features sufficient to solve the technical problem indicated by the applicant and obtain the technical result provided by the invention.

According to the invention a process for preparing nanoporous ceramics based on mullite, including mullite-phase synthesis using a mechanochemical activation of γ-Al ₂ O ₃ and amorphous SiO _2, characterized in that the salts of Al (NO _{3) 2} O ₃ ⋅9H prepared diluted solution with a concentration of 0.1 M, from which aluminum hydroxide is precipitated with an aqueous solution of NH ₄ OH (~ 1 M) at a temperature of 0-2 ° C, a gel-like precipitate of Al (OH) _{3 is} subjected to vacuum filtration followed by heat treatment at 150 ° C to form boehmite (AlOOH) , while to obtain amorphous SiO _2, silicic acid is subjected to thermal decomposition in the temperature range 100-250 ° C, then γ-Al ₂ O ₃ and amorphous SiO ₂ are mixed in the ratio Al ₂ O ₃ : SiO ₂ = 3: 2 in the mode dry grinding in a planetary mill for 15 minutes at a ratio of the mass of grinding balls made of high-density alumina ceramic to the total weight of the powder 10: 1, then a pore-forming agent is introduced into the mixture 3Al ₂ O ₃ ⋅2SiO ₂ component, which is used as ammonium carbonate in an amount of 10 or 20 wt. % to the total mass of the powder and stirred for 10 min for homogeneous distribution of (NH ₄ ) ₂ CO ₃ throughout the mixture, after which the resulting powders are compacted at a pressure of 70 or 100 MPa, and then fired at 1300 ° C for 2 hours.

The claimed set of essential features ensures the achievement of the technical result, which consists in the fact that after pressing MA-mixtures containing (NH ₄ ) ₂ CO ₃ in an amount of 10 wt. % and 20 wt. % under a pressure of 70 MPa, ceramics with a bimodal pore size distribution are obtained, an increase in the amount of a pore former leads to an increase not only in the pore size, but also in the intervals of their distribution. The use of (NH ₄ ) ₂ CO ₃ in an amount of 20 wt. % increases the pore size by approximately 1.5 times. An increase in the pressing pressure of MA-mixtures to 100 MPa promotes the formation of ceramics with a fairly narrow monomodal pore size distribution (80-110 and 90-150 nm). The claimed method makes it possible to further intensify the sintering process and form a nanoporous structure in ceramics at a temperature of 1300 ° C (2 h), and the use of ammonium carbonate as a pore former makes it possible to create a sufficiently developed system of channel-shaped pores.

The essence of the proposed technical solution is illustrated by the drawing, in which FIG. 1 shows the diffraction patterns of powders obtained by thermal decomposition of aluminum hydroxide, a - boehmite (150 ° C), b - γ-Al ₂ O ₃ (500 ° C), Designations: ○ - AlO (OH), □ - γ-Al ₂ O ₃ , in FIG. 2 - thermograms of AlOOH (a) and SiO ₂ ⋅nH ₂ O (b), in Fig. 3 - diffraction patterns of powders obtained by thermal decomposition of silicic acid, a - amorphous SiO ₂ (200 ° C), b - α-cristobalite (900 ° C), in Fig. 4 - diffraction patterns of mullite obtained at 1100 ° С (a) and at 1300 ° С (b). Designations: ◊ - 3Al ₂ O ₃ ⋅2SiO ₂ , □ - α-cristobalite, in Fig. 5 - pore size distribution in ceramics 3Al ₂ O ₃ ⋅2SiO ₂ (P _press. = 70 MPa, 1300 ° C, 2 h), а - 10 wt. % (NH ₄ ) ₂ CO ₃ ; b - 20 wt. % (NH ₄ ) ₂ CO ₃ , in Fig. 6 - pore size distribution in ceramics 3Al ₂ O ₃ ⋅2SiO ₂ (P _pres. = 100 MPa, 1300 ° C, 2 h), а - 10 wt. % (NH ₄ ) ₂ CO ₃ ; b - 20 wt. % (NH ₄ ) ₂ CO ₃ . The claimed method is implemented as follows.

Taking into account the results of work [19], in which it is noted that in the synthesis of mullite from aluminosilicates, the dispersion of aluminum oxide is of significant importance, and also taking into account the research on the selection of optimal components for obtaining 3Al ₂ O ₃ ⋅2SiO ₂ [15-18], for the synthesis of single-phase mullite used nanodispersed γ-Al ₂ O ₃ and amorphous SiO ₂ , obtained from the reagents given in table. 1.

Obtaining γ-Al ₂ O ₃ . A dilute solution with a concentration of 0.1 M was prepared from the Al (NO ₃ ) ₃ ⋅9H ₂ O salt, from which aluminum hydroxide was precipitated with an aqueous solution of NH ₄ OH (~ 1 M). The deposition process was carried out at a temperature of 0-2 ° C in a water-ice bath to reduce the degree of agglomeration of A1 (OH) ₃ . The pH in the reaction vessel was maintained at 9.0-9.5 units to avoid dissolution of the already formed aluminum hydroxide. A gel-like precipitate of Al (OH) ₃ was subjected to vacuum filtration and then heat treated at 150 ° C (1 h) in a SNOL drying oven to form boehmite (AlOOH), Fig. 1a.

The process of AlOOH thermolysis was studied by the method of differential thermal analysis (derivatograph Q-1000 from MOM), Fig. 2a. Boehmite dehydration occurs in the temperature range 100-300 ° C, then the DTA curve shows an exothermic effect (420-485 ° C), corresponding to the crystallization of aluminum oxide. XRD powder (diffractometer D8-Advance Bruker company), AlOOH conducted after calcination at 500 ° C (1 hour), indicating the formation of γ-Al ₂ O _3, the average crystallite size which is 18-20 nm and the specific surface area of ~ 115 m ² / g (Quantachrom NOVA 1200e gas sorption analyzer). Exoeffects at 760 ° C and 980 ° C indicate phase transitions γ-Al ₂ O ₃ → δ-Al ₂ O ₃ and δ-Al ₂ O ₃ → α-Al ₂ O _3, respectively.

Obtaining SiO ₂ . To obtain amorphous SiO _2, silicic acid was subjected to thermal decomposition. The thermogram, Fig. 2b, in the temperature interval 100-250 ° C endotherm is recorded deep characterizing dehydration SiO ₂ ⋅nH ₂ O, in this temperature range there is the main loss in sample mass (~ 54%, TG curve) and silicic acid, proceeds in the amorphous SiO ₂ , fig. 3a. The specific surface area of silica after heat treatment at 250 ° C (1 h) is about 300 m ² / g. In the temperature range 250-850 ° C, according to XRD data, silicon dioxide does not undergo phase changes and retains an X-ray amorphous structure; above 850 ° C, a slight exothermic effect is observed, due to XRD data, Fig. 3b, crystallization of α-cristobalite from amorphous silicon oxide.

In studies [15-18], it is noted that the aluminosilicate structure begins to form during MA and subsequent heating of the MA-powder leads to the formation of a phase with a spinel structure, which turns into mullite at 1100 ° C [15]. Considering the above, the mixing of γ-Al ₂ O ₃ and amorphous SiO ₂ in the ratio Al ₂ O ₃ : SiO ₂ = 3: 2 (as in mullite) was carried out in the mode of dry grinding in a Fritch planetary mill for 15 min (the ratio the mass of grinding balls made of high-density alumina ceramic to the total mass of the powder was 10: 1).

For the formation of a porous structure in 3Al ₂ O ₃ ⋅ 2SiO ₂ , the method of burnout additives was used, based on the introduction and subsequent burning of a pore-forming component, this method is promising for the production of permeable ceramics with channel-shaped pores [20]. In the MA-mixture was added a blowing agent (ammonium carbonate) in an amount of 10 and 20 wt. % to the total weight of the powder and stirred for another 10 min for homogeneous distribution of (NH ₄ ) ₂ CO ₃ in the entire volume of the mixture, in order to obtain a uniform porous structure in the final product after its decomposition and burnout. Tablets (d = 30 mm, h = 2-3 mm) were pressed from MA-mixtures of powders by the method of uniaxial double-sided pressing on a hydraulic press (PGR-10) at pressures of 70 and 100 MPa. The pressed samples were fired in air at temperatures of 1100, 1200, and 1300 ° C (2 h), the heating rate was ~ 400 ° / h, since the chemical activity of the components used to a greater extent is retained during intense heating [9, 10]. The cooling of the samples was not controlled and was carried out together with the oven. According to the results of XRD analysis after firing at 1100 ° C, samples 3Al ₂ O ₃ ⋅2SiO ₂ with a small admixture of the α-crystallite phase were obtained, Fig. 4a, and open porosity of 53 and 57%, depending on the amount of (NH ₄ ) ₂ CO ₃ , the average grain size of mullite after firing is 50-60 nm. It should be noted that the sintered samples turned out to be fragile and fractured under the influence of small mechanical loads. With a further increase in temperature to 1200 ° C, the α-cristobalite phase dissolves in 3Al ₂ O ₃ ⋅2SiO ₂ , forming a solid solution of silicon dioxide in mullite [21]. After firing at 1300 ° C, single-phase samples 3Al ₂ O ₃ ⋅2SiO _{2 were} obtained, Fig. 4b. The values of shrinkage, porosity and average grain size in sintered mullite specimens are shown in Table 2.

Since porous permeable alumina ceramics are in demand as filter elements operating under conditions of exposure to both acids and alkalis, an assessment of the chemical resistance of the obtained mullite samples was carried out.

To determine the chemical resistance, 3Al ₂ O ₃ ⋅ 2SiO ₂ samples sintered at 1300 ° C were selected. Before the studies, the samples were dried to constant weight at a temperature of 120 ° C, and then placed in nitric acid (concentration 2 M) or sodium hydroxide (concentration 2 M) and incubated for 1 h at a temperature of 60 ° C. After that, the samples were washed with distilled water, dried to constant weight, and the chemical resistance was determined by weight loss. Samples of porous mullite have rather high values of acid resistance and alkali resistance, table. 2. The data obtained correlate well with the results presented in [22].

The porous structure of 3Al ₂ O ₃ ⋅ 2SiO ₂ samples sintered at 1300 ° C was investigated by mercury porosimetry (porometer manufactured by Carlo Erba Strumentazione, model 2000). It was found that after pressing MA-mixtures under a pressure of 70 MPa, ceramics with a bimodal pore size distribution are obtained. The use of (NH ₄ ) ₂ CO ₃ in an amount of 20 wt. % increases the pore size by approximately 1.5 times, compared with 10 wt. % ammonium bicarbonate, fig. 5. An increase in the pressing pressure of MA-mixtures to 100 MPa leads to the formation of ceramics with a rather narrow monomodal pore size distribution: 80–125 nm for 10 wt. % (NH ₄ ) ₂ CO ₃ and 102-153 nm for 20 wt. % (NH ₄ ) ₂ CO ₃ , Fig. 6. Table 2 shows the pore volume in all studied samples. It was found that the porous structure of mullite affects the ultimate strength of mullite ceramics in compression, the smaller the size and volume of pores, the greater the strength values, table. 2.

Considering the fact that the pore size in the 3Al ₂ O ₃ ⋅ 2SiO ₂ samples does not exceed 200 nm, and the open porosity is 40-47%, the obtained mullite ceramics can be used without selective layers in cases of filtration of liquids and gases.

The proposed method for producing single-phase nanoporous mullite ceramics includes the use of initial components (γ-Al ₂ O ₃ and amorphous SiO ₂ ) in a dispersed state and mechanochemical activation of the charge together with a pore former (ammonium carbonate), which allows further intensification of the sintering process and the formation of nanoporous structure at a temperature of 1300 ° C (2 h). The use of ammonium carbonate as a pore-forming agent made it possible to create a sufficiently developed system of channel-like pores. A strong, chemically resistant ceramics based on 3Al ₂ O ₃ ⋅2SiO ₂ with a grain size of less than 100 nm has been obtained.

The effect of the amount of ammonium carbonate introduced into the MA-mixture and the pressing pressure on the open porosity of the ceramic, pore volume, and pore size distribution was established. The possibility of obtaining ceramics 3Al ₂ O ₃ ⋅2SiO ₂ with an open porosity of -45% and a monomodal pore distribution in the range from 80 to 150 nm is shown.

The developed method for producing nanoporous ceramics based on mullite avoids the problems arising from the deposition of membrane layers on a macroporous substrate when creating traditional filtration elements, and makes it simpler in technological execution. Therefore, this method of obtaining nanoporous mullite ceramics is advisable to use in the production of filtration elements for the separation of liquids, as well as in drinking water treatment systems.

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Claims (1)

A method for producing nanoporous ceramics based on mullite, including the synthesis of single-phase mullite using mechanochemical activation of γ-Al ₂ O ₃ and amorphous SiO ₂ , characterized in that a dilute solution with a concentration of 0.1 M is prepared from the Al (NO ₃ ) ₃ ⋅9H ₂ O salt , from which the precipitation of aluminum hydroxide is carried out with an aqueous solution of NH ₄ OH (~ 1 M) at a temperature of 0-2 ° C, the gel-like precipitate of Al (OH) _{3 is} subjected to vacuum filtration, followed by heat treatment at 150 ° C until boehmite (AlOOH) is formed, at to obtain amorphous SiO _2, silicic acid is subjected to thermal decomposition in the temperature range 100-250 ° C, then γ-Al ₂ O ₃ and amorphous SiO ₂ are mixed in the ratio Al ₂ O ₃ : SiO ₂ = 3: 2 in dry grinding mode in a planetary mill for 15 min at a ratio of the mass of grinding balls made of high-density alumina ceramic to the total mass of the powder 10: 1, then a pore-forming component is introduced into the 3Al ₂ O ₃ ⋅2SiO ₂ mixture, as a cat Orgo, ammonium carbonate is used in an amount of 10 or 20 wt.% to the total mass of the powder and stirred for 10 min for homogeneous distribution of (NH ₄ ) ₂ CO ₃ in the entire volume of the mixture, after which the resulting powders are compacted at a pressure of 70 or 100 MPa, and then fired at 1300 ° C for 2 hours.

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