RU2117631C1 - Method of preparing aluminium oxide powder to manufacture transparent ceramic products - Google Patents

Abstract

FIELD: building industry, more particularly preparation of aluminium oxide powder. SUBSTANCE: aluminium oxide powder is prepared by liquid phase hydrolysis carried by mixing aluminium alkoxide with aqueous solution of one or more organic polymers, at least one of which is insoluble in alcohol resulting from hydrolysis. Said aqueous solution also contains sealing additive in dissolved form. The resulting mixture is stirred. Water to aluminium alkoxide molar ratio is from 2.5 to 5.0. Total amount of said organic polymers is 3- 30 wt % of equivalent content of aluminium oxide. Alcohol and water remnants are removed from the resulting hydrolysate till hydrolysate is converted to fragile state (xerogel). When removing alcohol and water remnants from hydrolysate, hydrolysate temperature should not exceed 200 C. The resulting aluminium hydroxide xerogel is ground to particle size of not greater than 50 mcm. Ground xerogel is heat treated in the air or in other oxidative medium at 1100-1350. EFFECT: increased specific efficiency of equipment and lower power consumption. 6 cl, 1 tbl

Description

 The invention relates to a technology of alumina powder used to produce transparent corundum ceramics, used in particular in the form of tubes of high-pressure sodium discharge lamps.

A known method for producing non-aggregated powder of α-Al ₂ O ₃ for the manufacture of ceramics (USSR Patent N 1709901, class C 01 F 7/44, 01/30/92). However, a fine powder (average particle size 0.16 - 1.35 μm) of aluminum hydroxide or γ-Al ₂ O _{3 is} used as a precursor substance in this patent. It is very difficult to obtain it on an industrial scale while maintaining the purity necessary to obtain a transparent ceramic . So, in the examples (USSR patent N 1709901, class C 01 F 7/44, 01/30/92) it is indicated that aluminum hydroxide was obtained as an aerosol with an aluminum alkoxide productivity of 8.9 g / h and a gas (nitrogen) flow rate of 750 l / h (about 900 g / h). Such dilution of the reagent to a mass concentration of about 1% reduces the specific productivity of the reactor and the purity of the product. In addition, (USSR patent N 1709901, class C 01 F 7/44, 01/30/92) indicates the need for instant heating of the precursor substance from a temperature not exceeding 850 ^o C to a temperature of 1150 - 1450 ^o C, i.e. not less than 300 ^o C, which is very difficult to implement on an industrial scale, without resorting to the use of complex and expensive equipment.

In (US Patent No. 5455019, class C 01 F 7/02, class 423/629, 3.10.95), a method is proposed for solving the problem of producing highly dispersed aluminum hydroxide (particle size 2-10 microns), not containing large particles by liquid phase hydrolysis aluminum alkoxide in an apparatus with very intensive mixing in the presence of organic substances introduced in the form of a solution with aluminum alkoxide and chemically modifying the latter. However (US patent 5455019, class C 01 F 7/02, class 423/629, 3.10.95) it is not indicated what structure the particles of Al ₂ O ₃ powder formed as a result of heat treatment of the hydroxide obtained by the method described in patent, and whether it can be applied to obtain transparent corundum ceramics. In addition, (US patent N 5455019, class C 01 F 7/02, class 423/629, 3.10.95) indicates the need for very high shear rates, which requires the use of a very powerful mixer. This leads to an increase in capital and energy costs.

Closest to the proposed one is the method described in (RF patent N 2083531, 20.10.97) and consisting in liquid-phase hydrolysis of an alcohol mixture of aluminum alkoxide with an aqueous solution of mineral salts and organic substances with stirring, drying the gel and aluminum hydroxide, grinding xerogel, heat treatment of ground hydroxide aluminum, forming blanks from powder α-Al ₂ O ₃ and firing blanks in a vacuum oven at a maximum temperature of 1800 ^o C.

The objective of the present invention is to increase the specific productivity of the equipment, reducing energy consumption, simplifying the equipment used while maintaining the quality of the obtained Al ₂ O ₃ powder.

The problem is achieved in that the liquid-phase hydrolysis is carried out by mixing aluminum alkoxide, which can be individually or in the form of an alcohol solution, while mixing with an aqueous solution of one or more organic polymers, at least one of which is insoluble in the alcohol formed during hydrolysis . The specified aqueous solution also contains a sealing additive in dissolved form. The mixture is then stirred, the molar ratio of water: aluminum alkoxide is from 2.5 to 5. The total amount of the mentioned organic polymers is 3 to 30 wt.% Of the equivalent content of aluminum oxide. Alcohol and residual water are removed from the hydrolyzate obtained until the hydrolyzate is in a brittle state (xerogel). The method of removing alcohol and water residues does not matter, however, the temperature to which the hydrolyzate must be subjected must not exceed 200 ^o C. The resulting xerogel is subjected to grinding to a particle size of not more than 50 microns. The ground xerogel is sent for heat treatment in air or in another oxidizing medium at a maximum temperature of 1100 - 1350 ^o C. The resulting powder is non-aggregated, it is mainly α-Al ₂ O ₃ and has a narrow particle size distribution in the range of 0.8 - 1, 6 microns. Samples formed from this powder after firing in vacuum or hydrogen at 1750 - 1850 ^o C are characterized by high light transmission, which is the main quality parameter of products made of transparent ceramics. Moreover, the linear shrinkage of the workpieces is in the technologically acceptable range of 16 - 18%.

 The difference of the proposed method from the prototype (RF patent N 2083531, class 20.10.97) lies in the fact that for hydrolysis can be used not only an alcohol solution of aluminum alkoxide, but also an individual aluminum alkoxide. This reduces energy costs for the evaporation of the volatile components of the mixture formed as a result of hydrolysis. In addition, in the hydrolysis of aluminum alkoxide, at least one organic polymer insoluble in the alcohol present in the hydrolysis apparatus must be used. This allows you to get the xerogel structure of aluminum hydroxide, ensuring the stability of the properties of ceramics.

Examples. The hydrolysis of sec-butoxide was carried out as follows. An aqueous solution of polyacrylamide and polymethylacrylic acid was added to second-butoxide at room temperature with stirring, the contents of which are 2 wt.% And 1 wt.%, Respectively, with respect to Al ₂ O ₃ , as well as sealing additives: magnesium, yttrium and lanthanum nitrates from the calculation of the content of oxides in Al ₂ O ₃ (wt.%): MgO-0,1; V ₂ O ₃ - 0.05; La ₂ O ₃ - 0.05. The paste formed during hydrolysis was kept under stirring for 10 minutes. Then, alcohol and water residues were removed by distillation at a temperature of 150 ^o C. The resulting aluminum hydroxide xerogel was subjected to dry grinding in a planetary mill with corundum balls in Teflon drums. The maximum particle size after grinding was determined petrographically. Next, the crushed powder, placed in a corundum crucible, was subjected to heat treatment in a furnace in air. The heating rate was 400 ^° C./h. After reaching the maximum temperature (1250 ^o C) followed by exposure at it for 2 hours, after which the powder was cooled with the oven. Samples in the form of disks 1 mm thick were molded from the obtained powder by the method of semi-dry pressing at a pressure of 100 MPa. After that, the samples were fired in a vacuum resistance furnace according to the regime: linear heating to 1250 ^° C for 100 minutes, linear heating to 1400 ^° C for 80 minutes, linear heating to 1800 ^° C for 8 minutes, holding at 1800 ^° C for 3 hours ., cooling at a rate not exceeding 80 ^o C / min. The transparency of the burned polished discs with a thickness of 0.5 mm was evaluated by the direct light transmission obtained on a Specord M 400 spectrophotometer at a wavelength of 660 nm. The results of experiments in which some technological parameters were varied are presented in the table.

 The aqueous solution has a pH of 2 to 4.

Claims (6)

 1. A method of producing aluminum oxide powder for the manufacture of transparent ceramics, including liquid-phase hydrolysis of aluminum alkoxide with an aqueous solution of a sealing additive with a molar ratio of water: aluminum alkoxide 25 - 5.0, removal of alcohol and water residues from the hydrolyzate, grinding of aluminum hydroxide xerogel and its heat treatment, characterized in that the hydrolysis is carried out by mixing aluminum alkoxide individually or in the form of an alcohol solution with stirring with an aqueous solution of one or more organically x polymers, at least one of which is insoluble in the alcohol formed during hydrolysis.  2. The method according to claim 1, characterized in that the total amount of organic polymers is 3 to 30 wt.% Of the amount of aluminum oxide formed from aluminum alkoxide. 3. The method according to claim 1, characterized in that when removing alcohol and residual water from the hydrolyzate, the temperature of the latter does not exceed 200 ^o C.  4. The method according to claim 1, characterized in that before heat treatment of aluminum hydroxide produce grinding of the latter to a particle size of not more than 50 microns.  5. The method according to claim 1, characterized in that the heat treatment of aluminum hydroxide is carried out in air or in any other oxidizing atmosphere. 6. The method according to claim 1, characterized in that the heat treatment of aluminum hydroxide is carried out at a temperature in the range of 1100 - 1350 ^o C.

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