JPH06104569B2 - Alumina powder manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a method for producing alumina powder, and more specifically, it has low firing cost, fine primary particles, small particle size after pulverization, and narrow particle size distribution. The present invention relates to a method for producing alumina powder.

<Prior Art> Alumina is chemically stable, has a high melting point, and has excellent physical properties such as mechanical strength, hardness, and electrical insulation.
It is widely used as a ceramic material, abrasive, and filler.

Further, in the field of various mechanical parts and electric parts, an alumina sintered body having a uniform sintered structure and excellent translucency obtained by adding a grain growth inhibitor such as MgO to alumina powder is also used. However, the characteristics of these sintered bodies are that the average particle diameter of the alumina powder, which is the raw material, the variation in particle diameter and particle shape,
Further, since it is significantly affected by the purity, there is a demand for high-purity alumina powder having fine particles and a narrow particle size distribution.

As a method for producing such alumina powder, there are conventional thermal decomposition methods of ammonium alum and hydrolysis methods of organic aluminum, but since these methods are expensive, the aluminum hydroxide obtained by the Bayer method is naturally filtered and dried. Alumina powder obtained by calcination and pulverization is applied. However, since the transition of aluminum hydroxide to α-alumina requires calcination at a high temperature in this method, crystal growth of primary particles during calcination is performed. The resulting alumina powder has a large average particle size, and when it is pulverized by a pulverizing operation, it requires not only long time pulverization but also the alumina powder after pulverization has a particle size. Has the drawback of large variation.

<Problems to be Solved by the Invention> In view of these circumstances, the present inventors have found that the Bayer method allows α-
In the method for producing alumina powder, the firing cost is low, the average particle size is small, as a result of earnestly studying to obtain α-alumina powder with little variation in particle size and particle shape,
When precipitating aluminum hydroxide from an alkaline aluminate solution, using α-alumina powder as seeds, it was found that the above physical properties are remarkably improved when the precipitated gibbsite is converted to boehmite and then baked, and the method of the present invention is used. Has been completed.

<Means for Solving the Problems> That is, the present invention is a method for producing alumina powder by precipitating gibbsite from an alkali aluminate solution by the Bayer method, filtering, drying, and firing, in the alkali aluminate solution as seeds. Another object of the present invention is to provide a method for producing an alumina powder, which comprises adding α-alumina powder to precipitate gibbsite, then converting the gibbsite into boehmite, and then firing.

Hereinafter, the method of the present invention will be described in more detail.

In carrying out the method of the present invention, the alkaline aluminate solution used is generally a calcined alkaline aluminate solution obtained by immersing an alumina-containing ore such as bauxite in an alkaline solution, extracting alumina, and then separating and removing the extraction residue (red mud). Is used.

The α-alumina powder used as seeds in the alkaline aluminate solution means one having a peak of α-alumina in X-ray diffraction, and preferably the α-alumination rate after firing is about 50% or more,
Preferably, it is about 80% or more of alumina, and it is used as it is or after being crushed to have a mean particle size of about 1 μm or less, preferably about 0.5 μm or less.

When alumina, which does not show a peak of α-alumina in X-ray diffraction, or alumina hydrate, etc. is used as seeds, no decrease in the firing temperature at the α-transition and conversion of the obtained gibbsite into boehmite are performed. However, it is not possible to obtain α-alumina powder which is fine and has a narrow particle size distribution and excellent characteristics even if it is fired and then pulverized if necessary.

The amount of α-alumina powder added to the alkaline aluminate solution is about 100 parts by weight of gibbsite to be precipitated.
It is used in the range of 0.1 to about 50 parts by weight, preferably about 3 to about 15 parts by weight.

If the addition amount is less than about 0.1 parts by weight, the conversion to α-alumina requires high-temperature and long-time calcination, so the calcination cost becomes high, and the resulting α-alumina also causes grain growth, Therefore, it is impossible to obtain fine α-alumina powder.

On the other hand, if the amount exceeds 50 parts by weight, there is no effect corresponding to the amount added and it is not economical, and the α-alumina powder as seeds aggregates and sinters to obtain the α-alumina powder having the desired physical properties. I can't.

The conditions for precipitating gibbsite from the alkaline aluminate solution by adding α-alumina powder are not different from the case where aluminum hydroxide is added as seeds in the Bayer method, and are usually 50 to 70 ° C. temperature conditions, 20 to 70 ° C.
It may be held for 150 hours to allow precipitation.

The particle size of gibbsite to be precipitated can be adjusted variously, but the average particle size of about 1 μm or less is preferable because the final α-alumina can be crushed easily and the particle shape is close to spherical.

The precipitated α-alumina-containing gibbsite is then converted to boehmite.

The conversion of gibbsite to boehmite may be carried out by ordinary hydrothermal treatment, in which the gibbsite is kept in an alkaline aqueous medium or in water, and the autoclave is heated at a temperature of 200 to 300 ° C for several minutes to 10 hours, preferably at 210 to 250 ° C. The heat treatment may be performed at a temperature for several minutes to 3 hours.

The boehmite-type alumina hydrate thus obtained may be filtered by a conventional method, washed with water, and calcined using a rotary kiln, an electric furnace, a shuttle kiln, a tunnel kiln, or the like, and the calcining condition is usually 1000 to 1400 ° C. 10 minutes at temperature
It may be fired for 6 hours.

Further, when the α-alumina powder after firing is further pulverized, a pulverizer commonly used in the art, for example, a jet mill, a micron mill, a ball mill, a vibration mill, a media mill or the like may be used. The time cannot be uniquely determined because it varies depending on the crushing model used for crushing, the crushing conditions, the desired average particle size, etc., but it is usually several minutes to several hours.

The α-alumina powder obtained after pulverization by the method of the present invention is about 1
It has an average particle size of less than μm, or about 0.7 μm in a steady state, and the ratio σ ^{2 of} the cumulative weight of 16% to the diameter of 84% is 2.5 or less, always 2.2 or less. It has a small variation and is suitable as a weight loss for a sintered body having a high density or excellent translucency.

Compared to the conventional seed aluminum hydroxide addition method by carrying out the method of the present invention, the firing cost is low, the primary particles of the α-alumina powder obtained are fine, and the average of the α-alumina powder obtained after further pulverization It is not clear why the particle size is small and the particle size distribution is narrow, but α-alumina in the alumina hydrate at the time of firing acts as an α-transition accelerator, and the α-transition temperature is lowered. As a result, an α-alumina powder having a small amount of primary grain growth and agglomerated grains can be obtained, and by converting gibbsite into boehmite, the conventional conversion sequence of gibbsite into α-alumina is x → k →
Since two series of α and boehmite → γ → δ → θ → α can be changed to one series of boehmite → γ → δ → θ → α,
The above-mentioned effect is further improved in combination with the seed effect of α-alumina, and it is believed that a particle which can be easily atomized by subsequent pulverization and has a small variation in particle diameter and particle shape can be obtained.

The α-alumina powder added to the alkaline aluminate solution in the method of the present invention may be used in combination with seed aluminum hydroxide or alumina gel other than α-alumina.

As for the addition method, it is of course possible to add it after it has been dispersed in the solution in advance, as well as in powder form.

<Example> Hereinafter, the method of the present invention will be described in more detail with reference to examples, but the examples do not limit the method of the present invention.

Example-1 Sodium aluminate solution (Na ₂ O / Al ₂ O ₃ molar ratio of 1.5, Na ₂ O110
(g / l) 100 parts by weight of gibbsite precipitates with an average particle size of 0.
5 parts by weight of 2 μm α-alumina powder and 10 parts by weight of alumina gel were added as seeds, and stirred at a temperature of 50 ° C. for 48 hours to deposit 0.5 μm of aluminum hydroxide having a gibbsite structure.

Then, the gibbsite obtained after filtration and washing with water is hydrothermally treated in an autoclave at a temperature of 200 ° C for 2 hours to dry the boehmite, which is then calcined in a shuttle kiln in the air at a temperature of 1200 ° C for 4 hours, and then in a vibration mill. Crushed for 1 hour.

The powder thus obtained had an average particle size of 0.2 μm and a particle size variation σ ² of 1.5.

The α-conversion rate of the powder after firing was 93%.

20 mm of the obtained α-alumina powder was applied using a rubber press.
When molded into a size of φ × 5 mm and sintered at a temperature of 1400 ° C., the sintered density was 3.9 g / cm ³ , and the sintered body sintered at 1600 ° C. had a uniform structure.

Comparative Example-1 Firing and pulverization were performed under the same conditions as in Example-1, except that gibbsite was not converted to boehmite in Example-1.

The alumina thus obtained has an average particle size of 0.5 μm.
The variation in grain size was σ ² = 2.0, and the α conversion after sintering was 90%.

The density of the sintered body at 1400 ° C. was 3.7 g / cm ³ , and the structure of the sintered body at the temperature of 1600 ° C. was uniform, but slightly worse than that of Example-1.

Comparative Example-2 In Example-1, 15 parts by weight of alumina gel was used as seeds instead of the α-alumina content, and calcination and granulation were performed under the same conditions as in Example-1 in which gibbsite was not converted to boehmite.

The alumina thus obtained contained a large amount of agglomerated particles and had a large average particle size of 0.6 μm, and the variation in particle size was σ ² = 4.8.

Further, the density of the sintered body at 1400 ° C. was 3.0 g / cm ³ , and the α-conversion rate of the powder after firing was 60%.

Example-2 High-purity sodium aluminate solution (Na ₂ O / Al ₂ O ₃ molar ratio 1.
5, Na ₂ O 110 g / l), 10 parts by weight of high-purity α-alumina powder having an average particle size of 0.2 μm and 5 parts by weight of alumina gel were added as seeds to 100 parts by weight of gibbsite to be precipitated, and the temperature was 50 ° C. After stirring for 48 hours, 0.5 μm gibbsite was deposited.

Then, the gibbsite obtained after filtration and washing with water was hydrothermally treated in an autoclave at a temperature of 220 ° C. for 2 hours to form a boehmite, and then the obtained boehmite was dried, and then dried in air using a shuttle kiln at a temperature of 1200 ° C. for 4 hours. It was calcined for an hour and then ground in a vibrating mill for 1 hour.

The powder characteristics of the α-alumina thus obtained and the physical properties of the sintered body were 0.2 μm in average particle diameter and σ ² was 1.
The ratio was α, and the sinter density at a temperature of 1400 ° C was 3.9 g / cm ³ .

Then, 0.05% by weight of MgO was added to the α-alumina powder,
It was mixed and molded, and sintered at 1800 ° C in a hydrogen stream.

The light transmittance of the sintered body thus obtained was 60% (in-line).

For comparison, the light transmittance of a sintered body obtained by molding and sintering the same method using α-alumina powder obtained by the same method as in Example-2 except that it was not converted into boehmite. Was 46%.

Example 3 In the same manner as in Example 1, except that 0.5 parts by weight of 0.2 μm α-alumina powder and 15 parts by weight of alumina gel were used as seeds and gibbsite was precipitated and boehmite formed in the same manner as in Example 1. After that, it was dried, calcined, and then ground.

The powder thus obtained had an average particle size of 0.4 μm and a uniform particle size variation σ ² of 2.0.

The α-conversion rate of the powder after sintering was 90%.

The density of the sintered body at 1400 ° C was 3.6 g / cm ³ , and the structure of the sintered body at a temperature of 1600 ° C was uniform.

Comparative Example-3 The calcination and pulverization treatment was carried out under the same conditions as in Example-3 except that the gibbsite was not converted to boehmite in Example-3.

The alumina thus obtained has an average particle size of 0.5 μm and a particle size variation of σ ² = 2.5.
%Met.

The density of the sintered body at 1400 ° C. was 3.5 g / cm ³ , and the structure of the sintered body at the temperature of 1600 ° C. was uniform, but slightly worse than that of Example-3.

<Effects of the Invention> According to the method of the present invention described in detail above, when aluminum hydroxide is precipitated from an alkaline aluminate solution,
Using α-alumina powder as seeds, a simple method of transferring the obtained gibbsite to boehmite and then baking, reduces the baking cost at the time of converting to α, and the primary particles of the obtained powder are fine and further It is possible to obtain an α-alumina powder having a small average particle size after crushing and a narrow particle size distribution, and its industrial value is extremely large.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Saburo Nabeshima, 5-15 Kitahama, Higashi-ku, Osaka City, Osaka Prefecture Sumitomo Chemical Co., Ltd. Incorporated (56) Reference JP 62-128918 (JP, A) JP 58-36923 (JP, A) JP 61-254685 (JP, A)

Claims (2)

[Claims] 1. A method for producing an alumina powder by precipitating gibbsite from an alkali aluminate solution according to the Bayer method, filtering, drying and firing the mixture, and adding α-alumina powder as seeds to the alkali aluminate solution to give gibbsite. A method for producing an alumina powder, which comprises: precipitating, converting the gibbsite into boehmite, and then firing. 2. The method for producing an alumina powder according to claim 1, wherein the addition amount of the α-alumina powder as seeds is 0.1 to 50 parts by weight with respect to 100 parts by weight of gibbsite to be precipitated.