JPH0364450B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing boehmite having a small particle size and a uniform particle size distribution, and more specifically, for example, a method for producing boehmite suitable as a raw material for producing easily sinterable alumina. Regarding.

In recent years, a large amount of high alumina materials have been used as raw materials for porcelain for electronic parts, spark plug insulators for internal combustion engines, and catalyst carriers, but the raw material, alumina, has a high density (dense) after forming. In order to sinter (hereinafter referred to as "easy sinterability"), the particles
Fine particles of 1 μm or less and as uniform in particle size as possible are required, and Na ₂ O contained in alumina reduces electrical insulation and heat resistance, as well as its strength as a catalyst carrier, so it should be avoided as much as possible. It is required to reduce the content.

Alumina, which is currently produced industrially at low cost and in large quantities, is manufactured by firing aluminum hydroxide produced by the Bayer process, so it is usually 0.2 to 0.4% by weight due to the process. It contains Na ₂ O and cannot be used as is for the above purpose.

As a method for reducing the Na ₂ O content, a method has been proposed in which an aqueous slurry of aluminum hydroxide is treated at 117°C or higher in an autoclave to form boehmite, and the slurry is dehydrated and washed with water (USP 2774744, USP 3628914). In this method, when aluminum hydroxide transforms into boehmite, the soda present in the crystals or grain boundaries of aluminum hydroxide is released, so the Na ₂ O content can be easily reduced by simply washing with water. Is possible. However, since boehmite grows into large particles with good crystallinity, alumina produced by firing the boehmite produced also has a
The particle size becomes large, about ~50 μm, and the desired alumina with easy sinterability cannot be obtained. Even if this alumina is formed and sintered, it does not become denser, so it is difficult to obtain alumina with high strength and high wear resistance. It was not possible to obtain molding materials such as insulating materials.

In view of these circumstances, the present inventors conducted intensive studies on a method for producing boehmite, which has a small particle size and uniform particle size distribution and can be suitably used as a raw material for producing easily sinterable alumina. By adding a water-soluble carboxylic acid having two or more carboxyl groups when aluminum hydroxide is treated under heat and pressure to obtain boehmite, it has a small particle size and uniform particle size distribution, and has a low soda content. The present inventors have discovered that boehmite with a certain amount of boehmite can be obtained, and have completed the present invention.

That is, the gist of the present invention resides in a method for producing boehmite, which comprises treating aluminum hydroxide dispersed in water under heat and pressure in the presence of a water-soluble carboxylic acid having two or more carboxyl groups.

The present invention will be explained below.

The aluminum hydroxide used in the present invention is not particularly limited, and for example, aluminum hydroxide obtained from the Bayer process can be used. If the particle size is too large, high-temperature and long-time treatment will be required to turn it into boehmite, and if it is too small, it will be difficult to handle. Therefore, aluminum hydroxide with a particle size of 0.1 alumina or more, preferably 0.5 to 50 μm is preferred.

Examples of water-soluble carboxylic acids having two or more carboxyl groups include oxalic acid, succinic acid,
Examples include chain dicarboxylic acids such as tartaric acid and malonic acid, aromatic dicarboxylic acids such as phthalic acid, tricarboxylic acids such as citric acid and benzenetricarboxylic acid, and telluric acid such as benzenetetracarboxylic acid.

The amount added is 0.5 to 30 per aluminum hydroxide.
% by weight, preferably 2-20% by weight.

The aluminum hydroxide and the hydroxycarboxylic acid having two or more carboxyl groups are dispersed in water and subjected to hydrothermal treatment under heat and pressure to obtain boehmite.

The amount of water is 1 to 50 times the solid content, preferably 2 to 30 times the weight.

The conditions for the hydrothermal reaction vary depending on the particle size of the aluminum hydroxide used, but are usually 150-280℃,
5-65Kg/ ^cm2G for 0.1-20 hours, preferably 170-20 hours
The heating may be carried out at 250° C. and 8 to 40 kg/cm ² G for 0.5 to 10 hours.

The boehmite suspension obtained in this way is washed with water and dried in a conventional manner to obtain particles with a low soda content of 0.05% by weight or less and a particle size of 1 μm or less, especially 0.1 to 0.8 μm.
of boehmite was recovered.

To convert the boehmite into easily sinterable alumina, the above boehmite can be fired at a temperature of 500°C or higher, preferably 1100 to 1500°C, and after forming, it can be sintered at a temperature higher than the firing temperature used to produce alumina to create a high-density alumina. An alumina molded body is obtained and can be applied to porcelain for electronic parts, spark plug insulators for internal combustion engines, cutting tools, medical parts, etc.

The reason why the addition of a hydroxycarboxylic acid having two or more carboxyl groups is effective is not necessarily clear, but one reason is that by lowering the pH, the solubility of aluminum hydroxide is increased, and the nuclear It is conceivable that the particles are made finer by increasing the production rate and by adsorbing to a certain crystal face of aluminum hydroxide or boehmite and playing the role of the so-called mediocrystal effect.

The boehmite obtained by the method described above is a fine, uniformly-sized product with a low soda content, with an average particle size of 1 μm or less, and is useful as a raw material for producing easily sinterable alumina, and can be processed by ordinary drying and sintering. Since it does not agglomerate under certain conditions, it has the advantage of eliminating the need for a pulverization process when producing alumina, reducing energy consumption, and preventing contamination from impurities during the pulverization process. Value is great.

The present invention will be further specifically explained with reference to Examples below, but the present invention does not exceed the gist thereof.
The invention is not limited to the following examples.

Example 1 Aluminum hydroxide [Al(OH) ₃ , average particle size
3.3 μm, 100 parts of Na ₂ O (wt% content of 0.3 g) and 6 parts of citric acid monohydrate [C ₃ H ₄ (OH) (COOH) ₃ H ₂ O] and 1000 parts of water were added to make a suspension. I got it.

This suspension was heated in an autoclave at 220℃ for 24 hours.
The suspension was stirred and reacted for 6 hours under the condition of Kg/cm ² G to obtain a boehmite suspension, which was then washed with water, filtered, and dried at 100° C. for 1 hour. The boehmite thus obtained is
_Na2O content is 0.01% by weight or less, average particle size is 0.66μm
It was powder.

The above boehmite powder was fired at 1300℃ for 1 hour to convert it into α-Al ₂ O ₃ , and then this powder was converted into α-Al 2 O 3 at 1 ton/cm ²
When the molded product was pressure-molded to a bulk density of 1.57 g/cm ³ and then baked at 1550° C. for 2 hours, a molded product with a bulk density of 3.83 g/cm ³ was obtained. That is, the theoretical density of Al ₂ O ₃ is 3.99 g/cm ³
A dense molded body was obtained in which sintering progressed to 96.0%.

Comparative Example 1 The same treatment as in Example 1 was carried out except that citric acid monohydrate was not added, and the average particle size of the obtained boehmite powder was 1.9 μm.

This boehmite powder was prepared in the same manner as in Example 1 to obtain α
− into a powder of Al ₂ O ₃ and then convert this powder into
After pressure molding to a bulk density of 1.50g/ ^cm3 at 1ton/ ^cm2 ,
When sintered at 1550℃ for 2 hours, the bulk density was 2.99g/
A molded body of cm ³ was obtained. In this compact, sintering had progressed only to the theoretical density of Al ₂ O ₃ of 74.9.

Comparative Example 2 In place of citric acid monohydrate in Example 1, sodium citrate was added in an amount equivalent to 6 parts of citric acid monohydrate,
When treated under the same conditions, the average particle size was 1.6 μm.
boehmite powder was obtained. Example 1
Convert to α-Al ₂ O ₃ in the same manner as , then 1 ton/
After pressure molding to a bulk density of 1.56 g/cm ² in cm ² , it was fired at 155° C. for 2 hours. The bulk density of the obtained compact is
The density was 3.01 g/cm ³ (75.4% of the theoretical density).

Example 2 In Example 1, 6 parts of succinic acid [(CH ₂ ) ₂ (COOH) ₂ ] was added instead of citric acid monohydrate, and in the same manner, the amount of Na ₂ O was 0.01% by weight or less, Boehmite powder with an average particle size of 0.74 μm was obtained. This powder was converted to α-Al ₂ O ₃ in the same manner as in Example 1, then pressure-molded at 1 ton/cm ² to a bulk density of 1.32 g/cm ³ , and then calcined at 1550°C for 2 hours. density
A molded article having a density of 3.23 g/cm ³ (81.0% of the theoretical density) was obtained.

Comparative Example 3 In Example 1, 6 parts of palmitic acid [CH ₃ (CH ₂ ) ₁₄ COOH] was added instead of citric acid monohydrate, and boehmite powder with an average particle size of 2.3 μm was obtained in the same manner. Ta. This powder was prepared in the same manner as in Example 1 to obtain α-
Convert to Al ₂ O ₃ , then bulk density at 1 ton/cm ²
After pressure molding to 1.41 g/cm ³ , it was baked at 1550°C for 2 hours. The bulk density of the obtained molded product was 2.33g/cm ³
(58.4% of the theoretical density).

Example 3 In Example 1, 10 parts of citric acid monohydrate was added instead of 6 parts of citric acid monohydrate, and the other treatments were the same, resulting in boehmite powder with an average particle size of 0.52 μm. I got a body. This powder was converted into α-Al ₂ O ₃ in the same manner as in Example 1, and then 1 ton/
After pressure molding to a bulk density of 1.53 g/cm ² in cm ² , it was sintered in vacuum at 1700° C. for 2 hours. The bulk density of the obtained molded product was 3.91 g/cm ³ (98.0% of the theoretical density).

(Effects of the Invention) According to the present invention, boehmite, which is fine and has a uniform particle size and is free from impurity contamination, can be produced with low energy, and is useful as a raw material for producing easily sinterable alumina and the like.

Claims (1)

[Claims] 1. A method for producing boehmite, which comprises treating aluminum hydroxide dispersed in water under heat and pressure in the presence of a water-soluble carboxylic acid having two or more carboxyl groups.