JPH0255366B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing aluminum hydroxide powder, and further to a method for producing easily sinterable alumina powder, which is one use of the aluminum hydroxide powder.

Alumina sintered body is a material with excellent heat resistance, chemical resistance, electrical insulation, wear resistance, etc., and in order to obtain this sintered body, it is required to develop an excellent raw material alumina powder. Conventionally, alumina powder has generally been produced by calcining aluminum salts such as aluminum hydroxide and aluminum sulfate obtained by various methods and converting them into α-alumina. However, since these α-alumina powders have poor sinterability, a method has been used in which the raw alumina powder is further pulverized using a mill or the like to improve the sinterability. In other words, in order to obtain alumina powder that is easily sinterable, the average particle size of the alumina powder must be 1 μm or less, and since pulverization requires extremely large amounts of energy and alumina is a highly hard substance, a ball mill is used. When grinding is performed for a long time using a vibrating mill or the like, the alumina powder obtained is contaminated due to abrasion of the mill media, and as a natural result, the purity of the alumina powder is reduced.

As a result of conducting studies focusing on the drawbacks of the conventional method described above, the present invention was developed by making aluminum hydroxide, which is the raw material for producing alumina powder, fine particles, thereby making it easy to sinter without pulverizing the alumina powder. This was completed after discovering that alumina powder could be obtained.

That is, in the present invention, an aluminum alkoxide dissolved in an organic solvent is hydrolyzed by mixing and stirring in the presence of water in an amount of not more than 12 moles per mole of the aluminum alkoxide and in the presence of a mixed medium, and then the hydrated Aluminum hydroxide powder is produced by drying the decomposed product, and if necessary, further calcined to produce alumina powder.

Examples of the aluminum alkoxide used in the present invention include aluminum methoxide, aluminum ethoxide, aluminum propoxide, aluminum isopropoxide, aluminum butoxide, aluminum isobutoxide, aluminum.sec.butoxide, aluminum.
Such as tert-butoxide, preferably one having solubility in an organic solvent. Hydrophobic organic solvents are preferably used as the organic solvent for dissolving the aluminum alkoxide, and particularly preferred are organic solvents in which the solubility of water in the organic solvent is 1% by weight or less. The reason for this is that if the solubility of water in the organic solvent exceeds 1% by weight, the fine primary particles of aluminum hydroxide obtained by hydrolysis tend to aggregate, and the aluminum hydroxide powder obtained by drying is easily formed. This is because aggregates that are difficult to disperse, in other words, are partially consolidated, making it impossible to achieve the object of the present invention. Preferred organic solvents include paraffinic hydrocarbons such as pentane, hexane, heptane, and octane, cycloparaffinic hydrocarbons such as cyclopentane and cyclohexane, and aromatic hydrocarbons such as benzene, toluene, and xylene. mixtures or petroleum fractions, such as petroleum ether, petroleum benzine, etc.

In the present invention, aluminum alkoxide is
Hydrolysis must be carried out in the presence of 12 moles or less of water, preferably 2 to 8 moles of water. When hydrolyzed with 12 moles or more of water, the resulting aluminum hydroxide powder primary particles tend to aggregate,
The aluminum hydroxide powder obtained by drying exhibits a partially consolidated state. When this is calcined and converted into α-alumina, sintering occurs in the solidified part,
It becomes difficult to obtain α-alumina powder with uniform particle size, which is an essential condition for exhibiting easy sinterability, and the object of the present invention cannot be achieved. Furthermore, if the amount of water added is too small, for example, if it is less than 2 moles, aluminum alkoxide often exists in an undecomposed state, resulting in a decrease in the purity of the aluminum hydroxide obtained. However, in order to convert the aluminum hydroxide into α-alumina, a calcining process is performed at a temperature of 1100° C. or higher, and undecomposed substances are also changed into α-alumina in this process. Therefore, the aluminum hydroxide may contain a small amount of the aluminum alkoxide or partially decomposed substances (hereinafter referred to as undecomposed substances), and the permissible content of the undecomposed substances is usually 10% by weight. % or less, preferably 5% by weight or less.

The hydrolysis reaction is preferably carried out by contacting the aluminum alkoxide with water in a hydrophobic organic solvent. Aluminum alkoxides are easily hydrolyzed in large amounts of water, but decomposition becomes difficult in the presence of small amounts of water, as in the process of the present invention. Therefore, in order to achieve the object of the present invention, it is essential to allow the hydrolysis reaction to proceed efficiently and to improve the dispersibility of the hydrolyzate. In order to achieve this objective, it is necessary to optimize the mixing and stirring means in addition to the above-mentioned organic solvent. This mixing/stirring means is a mixing/stirring means capable of bringing water and aluminum alkoxide into sufficient contact with each other, and is a stirring type or rolling type mixing/stirring means containing a mixed medium, such as beads, balls, rods, or the like. A type of mixer containing a mixing medium having a shape similar to that of the above, or a mixing and agitating means such as that found in a pulverizer or dispersion machine such as a ball mill or a sand mill is employed. Therefore, the purpose of the present invention cannot be achieved when using a conventional stirred tank type reactor such as a type that uses a stirring blade, and when using a type of agitation that has high-speed moving parts, the abrasion of the agitator or container may cause problems. This is not preferred because impurities are mixed into the hydrolyzate and reduce the purity of the aluminum peroxide powder.

Furthermore, when using the method of the present invention, it is necessary to distill off the solvent and dry the resulting hydrolyzate by reducing pressure and/or heating, but the energy required for distilling off the organic solvent requires a large amount of water as in the conventional method. It is much smaller than distillation and is economically advantageous. Furthermore, since it is not necessary to strain the liquid, it is possible to avoid the caking of the wet cake that occurs during straining. As a result, the crushing step, which was essential in the prior art, can be omitted, which is very convenient. Drying of the obtained aluminum hydroxide can be carried out simultaneously with distillation of the organic solvent used or after distillation of the organic solvent by a conventional method.

In the present invention, the aluminum hydroxide obtained by hydrolysis has an amorphous or pseudo-boehmite structure, and its loss on ignition indicates that it corresponds to mono- or trihydrate of alumina (Al ₂ O ₃ ). It is estimated that by increasing the amount of water for hydrolysis, crystallinity 3 of nordstrandite or bayerite etc.
Although hydrates coexist, this does not essentially change the effects of the present invention.

The aluminum hydroxide powder obtained according to the present invention can be converted into α-alumina powder by calcining using known techniques. For example, 30 at 1000-1300℃
When calcined for 10 minutes to 10 hours, it transforms into alumina, and the higher the calcining temperature and the longer the calcining time, the more the α-alumina content increases.

In the present invention, the transition to α-alumina does not necessarily have to be 100% complete;
Intermediate transition type aluminas such as δ-, η-, θ-, etc. may be mixed. From the viewpoint of easy sinterability, α
- The alumina content is not very limited, and if molded and fired using a known method, a sintered body can be obtained that is extremely close to the theoretical density (density of alumina single crystal). When added as a transparent alumina sintered body with high transparency, a translucent alumina sintered body can be obtained. However, the shrinkage rate before and after firing is an industrially important property, and in order to prevent the shrinkage rate from becoming too large, it is necessary to preferably control the α-alumina content to 90% by weight or more.

According to the present invention, aluminum hydroxide powder with good dispersibility and α-alumina powder with uniform particle size obtained using the aluminum hydroxide powder can be produced without the need for a pulverization step unlike conventional methods.
The resulting aluminum hydroxide powder and α
-Alumina powder has sufficient performance as a raw material for high-purity, easily sinterable α-alumina, as well as various fillers. The present invention will be explained in more detail below with reference to Examples.

Example 1 A heptane solution of aluminum isopropoxide (concentration 1 mol/) 1 and 108 g of low conductivity water were charged into an alumina pot mill (capacity 2, 20 mm diameter alumina balls 2 kg), and hydration was carried out by rolling stirring for 2 hours. An aluminum slurry was obtained. Next, this slurry was transferred to an eggplant-shaped flask, and the solvent was distilled off under reduced pressure using a rotary evaporator.
Dry at ℃ for 20 hours. The obtained dry powder has a pseudo-boehmite structure according to powder X-ray diffraction analysis, contains 0.7% by weight of carbon according to organic elemental analysis, has a loss on ignition of 21% by weight, and has a specific surface area of 360 m ² /g. It was aluminum oxide. The aluminum hydroxide powder particles thus obtained are shown in FIG.

Next, this aluminum hydroxide powder was heated at 1200℃ for 1
After calcining for a period of time, α-alumina powder with a specific surface area of 9.8 m ² /g was obtained, and it was confirmed by powder X-ray diffraction that it had been transformed into α-alumina.

Next, magnesia is added using a known technique.
Added 0.1% by weight, press-molded into a disk shape with a thickness of 1.5 mm and a diameter of 20 mm, pre-fired in air at 1200°C for 1 hour, and then sintered in vacuum at 1850°C for 2 hours.
A sintered body having the same density as the theoretical density of 3.98 g/cm ³ and extremely excellent transparency was obtained.

Example 2 A dry powder obtained in the same manner as in Example 1 using a heptane solution (concentration 1 mol/) of aluminum n-butoxide had a pseudo-boehmite structure, contained 1.9% by weight of carbon, and had a loss on ignition of 24 % by weight, and was ultrafine aluminum hydroxide with a specific surface area of 390 m ² /g.

This aluminum hydroxide powder was calcined at 1155℃ for 5 hours to produce α-alumina with a specific surface area of 15m ² /g.
As a result of sintering the α-alumina powder containing 95% or more in the same manner as in Example 1, it had a density of 3.98 with excellent transparency.
A sintered body of g/cm ³ was obtained.

Example 3 A solution of aluminum isopropoxide in pentane (concentration 1 mol/mole) and 108 g of low conductivity water were charged into a pot mill equipped with a cooling device together with a nylon-based synthetic resin ball with an iron core, and the mixture was heated while cooling to 7°C. After 5 hours of rolling stirring, an aluminum hydroxide slurry was obtained. The solvent of this slurry was distilled off under reduced pressure, and the slurry was dried at 110°C for 20 hours. The obtained powder has a pseudo-boehmite structure, contains 1.1% by weight of carbon, has a loss on ignition of 22% by weight, and has a specific surface area of
It was 310 m ² /g of ultrafine aluminum hydroxide.

As a result of sintering the α-alumina powder obtained by calcining this aluminum hydroxide powder in the same manner as in Example 1, a sintered body with excellent transparency and a density of 3.98 g/cm ³ was obtained.

Example 4 In Example 1, a toluene solution of aluminum isopropoxide (concentration: 1 mol/mole) was used instead of the heptane solution. The obtained dry powder is
It has a pseudo-boehmite structure and contains 0.9% by weight of carbon,
It was ultrafine aluminum hydroxide with a loss on ignition of 20% by weight and a specific surface area of 290 m ² /g.

Then, it was calcined at 1200℃ for 1 hour to achieve a specific surface area of 7.9m ² /
g of α-alumina was obtained. Further, as a result of sintering in the same manner as in Example 1, a sintered body with a density of 3.98 g/cm ³ and excellent transparency was obtained.

Comparative Example 1 of a heptane solution of aluminum isopropoxide (concentration: 1 mol/mole) and 108 g of low conductivity water were placed in a separable flask, and stirred with a stirring blade (200 rpm) for 2 hours to obtain a slurry of aluminum hydroxide. The solvent of this slurry was distilled off under reduced pressure and the powder, which was dried at 110°C for 20 hours, had a pseudo-boehmite structure, contained 2.0% by weight of carbon, and lost weight on ignition.
It was ultrafine aluminum hydroxide with a specific surface area of 320 m ² /g. As shown in FIG. 2, the aluminum hydroxide powder particles had fine primary particles, but most of them aggregated into large secondary particles.

This aluminum hydroxide powder was calcined at 1200°C for 1 hour to produce α-alumina, and sintered in the same manner as in Example 1, resulting in a density of 3.86 g/cm ³ (97% of the theoretical density).
The sintered body was white in color, and a dense and highly transparent sintered body as in the present invention could not be obtained.

[Brief explanation of the drawing]

FIG. 1 is an example of a 50,000 times enlarged photograph taken with a transmission electron microscope of the particle structure of aluminum hydroxide powder obtained by the method of the present invention. Second
The figure shows the particle structure of aluminum hydroxide powder obtained by a conventional method using a transmission electron microscope.
This is an example of a photograph enlarged 10,000 times.

Claims (1)

[Claims] 1. Hydrolyzing an aluminum alkoxide dissolved in an organic solvent by mixing and stirring in the presence of water in an amount not more than 12 times the mole of aluminum alkoxide and in the presence of a mixed medium, and then A method for producing aluminum hydroxide powder, which comprises distilling off the solvent and drying the hydrolyzate under reduced pressure and/or heating. 2. The manufacturing method according to claim 1, wherein the mixing medium is a bead, a ball, a rod, or a shaped object having a similar function. 3. Hydrolyze aluminum alkoxide dissolved in an organic solvent by mixing and stirring in the presence of 12 moles or less of water per mole of aluminum alkoxide and in the presence of a mixed medium, and then subject the hydrolyzate to reduced pressure. and/or a method for producing alumina powder, which comprises heating to distill off the solvent and drying and calcining. 4. The manufacturing method according to claim 3, wherein the mixing medium is a bead, a ball, a rod, or a shaped object having a similar function.