JPS62153118A - Production of easily sintering alumina powder - Google Patents

Abstract

PURPOSE:To produce easily sintering alumina powder having small particle diameter at a low cost, by adding alpha-alumina powder to an aluminum salt or hydrated alumina which gives alumina by thermal decomposition, calcining the mixture and pulverizing the product. CONSTITUTION:A starting raw material for alumina is added with alpha-alumina powder and the mixture is calcined and pulverized to obtain the objective alumina powder. The starting raw material used in the above process is selected from an aluminum salt giving alumina by thermal decomposition (e.g. ammonium alum), a hydrated alumina obtained by the hydrolysis of an organoaluminum compound such as alkylaluminum, a hydrated alumina produced by the spark discharge of aluminum in water and a hydrated alumina produced by reacting an aqueous solution of sodium aluminate with ethylene chlorohydrin. The amount of the alpha-alumina is preferably 0.1-50pts.wt. per 100pts.wt. of the starting raw material and the particle diameter of the powder is preferably <=1.0mum.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a method for producing alumina powder, and more specifically to a method for producing alumina powder, which is easy to sinter, has low firing costs, has fine particles, and has a small particle size after pulverization. The present invention relates to a method for producing alumina powder.

(Conventional technology) 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. The characteristics of alumina sintered bodies vary greatly depending on the crystal shape, purity, particle size, etc. of the raw material alumina powder, but in recent years, from the perspective of energy saving, alumina sintered bodies with high sintered density can be obtained from alumina powder at lower temperatures. is required.

As a method for producing such easily sinterable alumina powder,
Conventional methods include a method in which alumina hydrate obtained by hydrolyzing organic aluminum is calcined at a temperature of about 1,100°C or higher, and then pulverized, or a method in which ammonium alum, aluminum sulfate, ammonium aluminum carbonate, etc. is thermally decomposed and transferred to α-alumina. Methods such as pulverization are known. However, although the resulting alumina hydrate has extremely fine particles and a sharp particle size distribution, it requires calcination at high temperatures for transformation.
It has the disadvantage that crystal growth of the primary particles occurs during firing, resulting in an increase in the particle size of the alumina powder obtained.

(Problems to be Solved by the Invention) In view of the above circumstances, the present inventors have developed a method with low firing cost.
In addition, as a result of intensive studies to obtain α-alumina powder with fine alumina particles after pulverization and less variation in particle size and shape, we found that fine α-alumina powder was mixed in aluminum salt or after hydrolysis of alumina hydrate. The inventors have discovered that the above object can be satisfied by incorporating it into a product, and have completed the method of the present invention.

(Means for Solving the Problems) That is, the present invention provides an aluminum salt that becomes alumina through thermal decomposition, an alumina hydrate obtained by hydrolyzing an organic aluminum compound, and an alumina obtained by spark discharge of aluminum in water. hydrate, and alumina hydrate obtained by reacting an aqueous solution of sodium aluminate with ethylene chlorohydrin.
To provide a method for producing easily sinterable alumina powder, which comprises adding α-alumina powder to the aluminum salt or alumina hydrate in advance to make it present, followed by firing and pulverizing the alumina powder. It is in.

The method of the present invention will be explained in more detail below.

The alumina starting materials used in carrying out the method of the present invention include aluminum salts that become alumina through thermal decomposition;
Examples include ammonium alum, aluminum sulfate, ammonium aluminum carbonate, and the like. Alumina hydrates include those obtained by hydrolyzing organoaluminum compounds such as alkyl aluminum and alkoxy aluminum, alumina hydrates obtained by spark discharge of aluminum pellets in water, or ethylene chlorohydride of an aqueous solution of sodium aluminate and an organic acid. Examples include those in which phosphorus is reacted to precipitate fine alumina hydrate, which is then aged at a temperature of 50° C. or higher to produce pseudo-boehmite.

The α-alumina powder added to these aluminum salts or alumina hydrates is one that has an α-alumina peak in X-ray diffraction, and preferably has a α-alumina ratio of 5 after firing.
Any alumina with a content of 0% or more, preferably 80% or more, can be used as it is or by pulverizing it to obtain an alumina with an average particle size of 1 μm.
It is preferably used as a powder of 0.5 μm or less. When using alumina with no α-alumina peak in X-ray diffraction, or alumina hydrate, etc., as seeds, there is no decrease in the firing temperature during α-transformation, and even when crushed, fine particles are obtained. It is not possible to obtain α-alumina powder that is large and has a narrow particle size distribution.

The amount of α-aluminum powder added to the aluminum salt or alumina hydrate is 0.001 to 50.0% per 100 parts by weight of alumina in the aluminum salt or alumina hydrate.
M571 parts, preferably in the range of 3 to 15 parts by weight, is used, and the method of addition is to include α-alumina powder in the crystals for aluminum salts, and to add α-alumina powder during hydrolysis of organoaluminum compounds. It is preferable to include α-alumina powder in the alumina hydrate crystal.

If the amount added is less than 0.1 part by weight, the conversion to α-alumina requires a long time at high temperature, which increases the firing cost, and the resulting α-alumina also causes grain growth, which can lead to It is not possible to obtain fine-grained α-alumina powder. On the other hand, if the amount exceeds 50 parts by weight, the effect is not commensurate with the amount added, and it is not only uneconomical, but also the α-alumina powder added as seeds will aggregate and sinter, resulting in the α-alumina powder having the desired physical properties.
- Unable to obtain alumina powder.

The aluminum salt or alumina hydrate containing α-alumina powder obtained in this way may be fired and then crushed using an electric furnace, rotary kiln, shuttle kiln, tunnel kiln, etc., and the firing conditions are usually 1000~ 14
What is necessary is just to bake at the temperature of 00 degreeC for 10 minutes - 6 hours. Further, as a crusher, a jet mill, a micron mill, a ball mill, a vibration mill, a media mill, etc. may be used, and the crushing time varies depending on the type of crushing machine used for crushing, the crushing conditions, and the desired average particle size, so it is determined on a case-by-case basis. However, it is usually sufficient to grind for several minutes to several hours.

Although it is not clear why α-alumina with lower firing cost and smaller pulverized particle size can be obtained by carrying out the method of the present invention than with conventional methods, aluminum salt containing α-alumina powder Alternatively, alumina hydrate acts as a gelatinization transition accelerator during calcination, lowers the gelatinization transition temperature, and as a result, α-alumina powder with less primary grain growth and agglomerated grains can be obtained, making it easier to grind later. It is assumed that the particles become atomized.

(Examples) Hereinafter, the method of the present invention will be explained in more detail with reference to Examples, but the Examples are not intended to limit the method of the present invention.

Example 1 73.5 g of aluminum sulfate as a reagent was mixed with 75 g of warm water (
100'C) and ammonium sulfate 14.6
g in 25g of hot water at 100℃, then dissolve these 2
By mixing the seed solution, followed by stirring and cooling, 88 g of ammonium alum crystals were obtained. After dissolving this crystal again in 100 g of hot water at 100°C, the particle size was 0.2 with respect to 100 parts by weight of ammonium alum precipitated.
1 part by weight of α-alumina powder of μm was added, followed by stirring and cooling to obtain about 80 g of ammonium alum crystals containing α-alumina. The obtained crystals were calcined in air at 1200° C. for 4 hours using a shuttle kiln, and then ground for 1 hour using a vibration mill.

The obtained powder had a uniform particle size with an average particle size of 0.3 μm, and the gelatinization rate of the powder after firing was 93%.

The thus obtained α-alumina powder was molded using a rubber press at a pressure of It/c+J to a size of 20 mmφ x 5 fl, and sintered at 1400°C for 2 hours.
The sintered density was 3.9 g/cut.

Comparative Example 1 Crystallization, firing, and pulverization were carried out under the same conditions as in Example 1 except that α-alumina powder was not added. The alumina thus obtained contains many aggregated particles and has an average particle size of 0.
At 5 μm, the gelatinization rate of the powder was 70%.

Furthermore, as a result of sintering the compact obtained by the same method as in Example 1 at 1400°C for 2 hours, the density of the obtained sintered compact was 3.3.
g/cut.

Example 2 90 g of water was added to 100 g of aluminum sulfate as a reagent, heated, dissolved, and filtered, and 1.0 g of α-alumina powder with a particle size of 0.2 μm was added to 100 parts by weight of the precipitated aluminum sulfate.
After adding 5 parts by weight, it was cooled to precipitate aluminum sulfate crystals containing α-alumina.

The crystals thus obtained were calcined in air at a temperature of 12 Q O''c for 4 hours using a shuttle kiln and then ground for 1 hour in a vibratory mill.

The obtained powder had an average particle size of O,,',pm, and the particle size was uniform, and the gelatinization rate of the powder after firing was 909%.

The α-alumina powder subjected to fS in this way was pressed using a rubber press at a pressure of lt/c+J to 20-1φ×
When molded into a size of 51 cm and sintered at 1400°C for 2 hours, the sintered density was 3.8 g/ctl.

Comparative Example 2 Crystallization, firing, and pulverization were carried out under the same conditions as in Example 2 except that α-alumina powder was not added. The alumina thus obtained contains a large number of aggregated particles (mixed) with an average particle size of 0.
At 6 μm, the gelatinization rate of the powder was 60%.

Furthermore, as a result of sintering the compact obtained by the same method as in Example 2 at 1400°C for 2 hours, the density of the obtained sintered compact was 3.0.
g/cnf.

Example 3 0.2 μ in 2.0 molar ammonium bicarbonate solution
2,0 with addition of α-alumina having an average particle size of m
Drop the molar ammonium alum solution; 2.
An ammonium aluminum carbonate containing 5% α-alumina was obtained. This is heated in air using a shuttle kiln for 12 hours.
It was calcined at a temperature of 00°C for 4 hours and then ground in a vibrating mill for 1 hour.

The obtained powder had an average particle size of 0.4 μm, and the particle size was uniform, and the α-alumina powder after firing had a gelatinization rate of 90%. Using a rubber press, it was molded into a size of 20i φ x 5 fins at a pressure of IT/cffl, and sintered at 1400°C for 2 hours.
The sintered density was 3.9 g/Cl11.

Comparative Example 3 α-alumina was obtained under the same conditions as in Example 3 except that α-alumina powder was not used. The alumina thus obtained had an average particle size of 0.6 μm, and the gelatinization rate of the powder was 70%.

Furthermore, as a result of sintering the compact obtained by the same method as in Example 3 at 1400'C for 2 hours, the density of the obtained sintered compact was 3.1.
g/cd.

Example 4 Into an isopropanol solution containing 20% by weight of aluminum isopropoxide, 5 parts by weight of α-alumina powder of 0.2 μm was added to 100 parts by weight of boehmite, which is a hydrolysis product. After mixing, water was added in a molar ratio of 10 times that of aluminum isopropoxide to cause hydrolysis, thereby obtaining boehmite containing α-alumina.

After drying, this boehmite was calcined in air at a temperature of 1200° C. for 4 hours using a shuttle kiln, and then ground for 1 hour using a vibration mill.

The obtained powder had an average particle size of 0.3 μm and was uniform in particle size, and the gelatinization rate of the powder after firing was 90%.

The thus obtained α-alumina powder was molded using a rubber press at a pressure of 1 t/c- to a size of 20 cranes φ x 5 dragons, and sintered at 1400°C for 2 hours, resulting in a sintered density of 3.9g/cn? It became.

Comparative Example 4 α-alumina was obtained under the same conditions as in Example 4 except that α-alumina powder was not added. The average particle size of the obtained powder was 0.
At 5 μm, the gelatinization rate was 60%.

In addition, a molded body obtained in the same manner as in Example 4 was 1400"c 2
As a result of time sintering, the sintered body density of the obtained sintered body was 3.
It was 2 g/cra.

Example 5 0.2 μm α-alumina powder was precipitated into 100 parts by weight of alumina hydrate in a sodium aluminate solution (N a 20/A 1203 molar ratio 1.2, Af 2.03 15 g/4). 2 parts by weight of ethylene chlorohydrin (relative to 100 parts by weight of the sodium aluminate solution) were added, and the reaction temperature was increased to 7 parts by weight.
Alumina hydrate was obtained by holding at 0°C for 2 hours. After drying this alumina hydrate, it was immersed in air using a shuttle kiln.
It was calcined at a temperature of 200° C. for 4 hours and then ground in a vibrating mill for 1 hour.

The obtained powder had an average particle size of 0.4 μm and was uniform in particle size, and the gelatinization rate of the powder after firing was 90%.

The thus obtained α-alumina powder was molded into a size of 20 mmφ x 5 squares using a rubber press at a pressure of lt/cal and sintered at 1400°C for 2 hours, resulting in a sintered density of 3. It became .9g/cffl.

Comparative Example 5 α-alumina was obtained under the same conditions as in Example 5 except that α-alumina powder was not added. As a result of sintering the obtained powder for 2 hours, the density of the obtained sintered body was 3.0 g/
-It was.

(Effects of the Invention) According to the method of the present invention detailed above, α-alumina powder is preliminarily added to the aluminum salt or alumina hydrate specified in the present invention, and then preformed and pulverized. It is possible to provide α-alumina powder that is low in firing cost, has fine particles, has a small particle size after pulverization, and is easy to sinter with little variation in particle size, and can be used to produce a sintered body of the powder. When used as a raw material, it is possible to significantly lower the sintering temperature, and its industrial value is enormous.

Claims (1)

[Claims] 1) Aluminum salt that becomes alumina through thermal decomposition, alumina hydrate obtained by hydrolyzing an organic aluminum compound, alumina hydrate obtained by spark discharge of aluminum in water, and aluminic acid. At least one aluminum salt or alumina hydrate selected from the group consisting of alumina hydrate obtained by reacting an aqueous soda solution and ethylene chlorohydrin is calcined and pulverized to obtain alumina powder. Alternatively, a method for producing easily sinterable alumina powder, which comprises adding α-alumina powder to alumina hydrate in advance to make it present, followed by firing and pulverizing. 2) The amount of α-alumina added to the aluminum salt or alumina hydrate is 0.1 parts by weight per 100 parts by weight of the alumina hydrate in the aluminum salt or alumina hydrate.
The method for producing easily sinterable alumina powder according to claim 1, wherein the amount is 50 parts by weight. 3) The method for producing easily sinterable alumina powder according to claim 1, wherein the α-alumina powder added to the aluminum salt or alumina hydrate has a particle size of 1.0 μm or less. 4) The method for producing easily sinterable alumina powder according to claim 1, wherein the aluminum salt that becomes alumina by thermal decomposition is aluminum sulfate, ammonium alum, or ammonium aluminum carbonate. 5) The method for producing easily sinterable alumina powder according to claim 1, wherein the organoaluminum compound is alkyl aluminum or alkoxy aluminum.