JPH0216243B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing aluminum hydroxide powder, and further relates to a method for producing easily sinterable alumina powder, which is one method of using the same. 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, these α-alumina powders have poor sinterability, and in order to improve the sinterability, a method has been used in which the raw alumina powder is further finely pulverized using a pulverizer. That is, in order to make the alumina powder easily sinterable, it is necessary to make the average particle size of the alumina powder 1 μm or less, which requires extremely large crushing energy. Even worse,
Alumina is a highly hard material, so when it is milled for a long time using a ball mill or vibration mill, the mill media is worn out and the resulting alumina powder is contaminated, and as a result, the purity of the alumina powder decreases. It was declining. As a result of conducting studies to overcome the drawbacks of the conventional methods described above, the present invention has developed a method for producing easily sinterable alumina powder without pulverizing the alumina powder by micronizing aluminum hydroxide, which is a raw material for producing alumina powder. It was discovered and completed that a powder can be obtained. That is, in the present invention, an organoaluminum compound is alcohololyzed in a nonpolar solvent, and then water is added in an amount of 12 moles or less per 1 mole of the organoaluminum compound, and the mixture is stirred in the presence of a mixed medium. Producing aluminum hydroxide powder by hydrolyzing and further drying the hydrolyzate,
If necessary, the alumina powder is further calcined to produce alumina powder. The organoaluminum compounds used in the present invention include trimethylaluminum, triethylaluminum, tri-n-propylaluminum, triiso-propylaluminum, tri-n-
Examples include trialkylaluminum such as butylaluminum and triiso-butylaluminum, and alkylaluminum hydride such as diethylaluminum hydride, di-n-propylaluminum hydride, and diisobutylaluminum hydride. These organoaluminum compounds are used by diluting and dissolving them to a concentration of usually 0.1 to 3 mol/preferably 0.5 to 2 mol/using a nonpolar solvent for the purpose of making the reaction mild. Preferred nonpolar solutions used in this case 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. , and mixtures thereof or petroleum fractions, such as petroleum ether, petroleum benzine, etc. Alcohols used for alcoholysis include methyl alcohol, ethyl alcohol, n-
propyl alcohol, iso-propyl alcohol,
n-butyl alcohol, iso-butyl alcohol,
These include sec-butyl alcohol, tert-butyl alcohol, pentyl alcohol, etc., and the alcohol used is usually 3 to 9 mol, preferably 3 to 6 mol, per 1 mol of the organoaluminum compound. Next, in the present invention, it is necessary to hydrolyze the reaction product using water in an amount of 12 times or less per mole of the organoaluminum compound,
It is preferably hydrolyzed with water in an amount of 2 to 8 moles. When hydrolyzed with 12 moles or more of water, the primary particles of the resulting aluminum hydroxide powder tend to aggregate, and the aluminum hydroxide powder obtained by drying exhibits a partially consolidated state. When this is calcined to transform it into α-alumina, sintering occurs in the solidified part, and α-alumina is made of fine particles with uniform particle size, which is an essential condition for exhibiting easy sinterability. It becomes difficult to form into powder, and the purpose 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 times the amount by mole, the reaction product will often exist in a partially decomposed 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 1100° C. or higher, so that the material in a terminally decomposed state is also changed into α-alumina in this process.
Therefore, the aluminum hydroxide may contain a substance in a terminally decomposed state as long as it is a small amount. The permissible content of the substance in the terminally decomposed state is generally not more than 10% by weight, and preferably not more than 5% by weight, as carbon content per dry aluminum hydroxide. The hydrolysis reaction is carried out by bringing water into contact with the reaction product as described above. There is another invention by the present inventors regarding a method of distilling the reaction product to isolate an aluminum alkoxide corresponding to the added alcohol, and then dissolving it in an organic solvent and hydrolyzing it. One of the advantages of the present invention is that aluminum alkoxides can be hydrolyzed without isolation. The reaction product is easily hydrolyzed in large amounts of water, but becomes difficult to decompose in the presence of small amounts of water, as in the method 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, in addition to using the non-polar solvent mentioned above, it is necessary to optimize the mixing and stirring means. This means is a mixing/stirring means capable of sufficiently contacting water and the reactant, and is a stirring or rolling mixing/stirring means containing a mixed medium, such as beads, balls, rods, or the like. A type of mixer containing a mixing medium in the form of a shape having the function of 1, or a mixing and agitating means such as that found in a pulverizer or a dispersion machine such as a ball mill or a sand mill is employed. The purpose of the present invention cannot be achieved using a conventional stirred tank type reactor, such as one that uses a stirring blade, and in a type of stirring that has high-speed moving parts, impurities due to abrasion of the stirrer or container may be added to the water. This is not preferable because it mixes into the decomposition and reduces the purity of the aluminum hydroxide powder. Furthermore, when using the method of the present invention, it is necessary to distill off the solvent and reactants and dry it by reducing pressure and/or heating the resulting hydrolyzate, leaving behind the aluminum hydroxide produced. This method is economically advantageous because it requires far less energy than conventional methods that distill off a large amount of water.
Furthermore, since it is not necessary to evaporate, it is possible to avoid caking of the wet cake that occurs during evaporation. 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 solvent and reactant used, or after distillation of the solvent and reactant, by a conventional method. In the present invention, the aluminum hydroxide obtained by hydrolysis has an amorphous or pseudo-boehmite structure, and is estimated to correspond to mono- or trihydrate of alumina (Al ₂ O ₃ ) based on its loss on ignition. When the amount of water for hydrolysis is increased, 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 a known technique. 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 higher the α-alumina content. In the present invention, this α
-Transition to alumina does not necessarily have to be 100% complete, and intermediate transition aluminas such as γ-, δ-, η-, θ- may be mixed. From the viewpoint of ease of sintering, the α-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 an alumina single crystal). When (MgO) or the like is added as a sintering aid, a highly transparent 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 same 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. EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited thereto. Example 1 114 g (1 mol) of triethylaluminum and n
-Pour 550 g (5.5 moles) of heptane into a nitrogen-substituted reactor (equipped with a stirring device, temperature control device, alcohol injection/dropping device, condenser, etc.), maintain the temperature at 60°C, and stir at 100 rpm. 200 g (3.3 mol) of isopropyl alcohol was added dropwise at a rate of 4 g/min. Approximately 1.6 ethane gas was generated per minute by the alcoholysis of triethylaluminum, and the generated gas was released to the outside of the system through the condenser. After the dropwise addition was completed, the mixture was cooled to room temperature to obtain a reaction product 1 in the form of a transparent homogeneous solution (a solution containing 1 mole of aluminum isopropoxide). Next, the above reaction product 1 and 108 g of low conductivity water (6
mol) was placed in an alumina pot mill (inner volume: 2, containing 2 kg of alumina balls), and tumbling stirring was performed for 2 hours to obtain a slurry of aluminum hydroxide. Next, this slurry was transferred to an eggplant-shaped flask, and the solvent (including heptane, isopropyl alcohol produced by hydrolysis, and excess water) was distilled off under reduced pressure using a rotary evaporator.
It was dried at 110°C for 20 hours. The obtained dry powder is
Loss on ignition at 1200℃ for 1 hour is 21% by weight, residual carbon content (carbon content detected by organic elemental analysis) is 0.7% by weight, specific surface area is 360m ² /g by BET method, identified as pseudo-boehmite by powder X-ray diffraction analysis. It was made of ultrafine aluminum hydroxide powder. Next, this aluminum hydroxide powder was heated at 1200℃ for 1
After calcining for a period of time, alumina powder with a specific surface area of 9.8 m ² /g was obtained. Powder X-ray diffraction analysis revealed that it was an alumina powder consisting only of α-alumina. Next, using a known technique, 0.1% by weight of magnesia was added as a sintering aid, and the mixture was mixed to a thickness of 1.5 mm and a diameter of 20 mm.
It was press-formed into a disc shape of mm with a pressure of 1000Kg/cm ² , pre-baked in air at 1200℃ for 1 hour, and then baked in vacuum at 1850℃ for 2 hours, resulting in a density of 3.98g/cm ³
A translucent alumina sintered body with extremely excellent transparency was obtained. Examples 2-8 Similar to Example 1, various organoaluminum compounds were alcoholyzed and then hydrolyzed using various alcohols in various non-polar solvents under the conditions shown in Table 1. The solvent was distilled off and dried to obtain aluminum hydroxide powder. As shown in Table 1, the characteristics of the obtained aluminum hydroxide differed somewhat depending on the example, but all were fine powders with good dispersibility. Next, the obtained aluminum hydroxide powder was
Calcination was performed at the temperature and time shown in the table to obtain alumina powder. The obtained alumina powder was then treated and fired in the same manner as in Example 1, and as a result, sintered bodies with good translucency were obtained in all cases. The properties of the obtained alumina and sintered body are shown in Table 1. Comparative example 1 114 g (1 mol) of triethyl aluminum and n
- 550 g (5.5 mol) of heptane was charged into a reactor purged with nitrogen, and while the temperature was kept at 60°C and stirred at 100 rpm, 200 g (3.3 mol) of isopropyl alcohol was added dropwise at a rate of 4 g/min. After finishing dropping,
Cool to room temperature and obtain reaction product 1 in the form of a transparent homogeneous solution.
I got it. Next, the above reaction product 1 was charged into two flasks, the temperature was maintained at 22°C, and while stirring at 250 rpm,
After injecting 108g (6 moles) of low conductivity water for 3 minutes,
Stirring was carried out for 2 hours to obtain a slurry of aluminum hydroxide. Next, this slurry was transferred to an eggplant-shaped flask, and the solvent was distilled off under reduced pressure using a rotary evaporator, followed by drying at 110°C for 20 hours.
The obtained dry powder has a loss on ignition of 25% by weight, a residual carbon content of 2.0% by weight, and a specific surface area of 320m ² /g, and is an ultrafine aluminum hydroxide identified as pseudo-boehmite. , containing small lumps of slightly hard aggregates. Next, the aluminum hydroxide powder thus obtained was calcined and sintered in exactly the same manner as in Example 1 to obtain α-alumina powder and a sintered body. The obtained sintered body has a density of 3.86 g/cm ³ , and compared with the theoretical density (density of single crystal α-alumina) of 3.98 g/cm ³ , it contains about 3% by volume of voids, and is white. It showed no transparency at all. Comparative Example 2 Triethylaluminum was alcoholyzed under the conditions shown in Table 2, then hydrolyzed and dried to obtain aluminum hydroxide powder. Next, the obtained aluminum hydroxide powder was calcined and sintered in the same manner as in Example 1 to obtain α-alumina powder and a sintered body. The properties of these aluminum hydroxide, alumina, and sintered bodies are shown in Table 2. As is clear from Table 2, when the amount of water added during hydrolysis exceeds 12 moles per mole of organoaluminum compound, the dry aluminum hydroxide powder becomes lumpy or fine powder containing small lumps. .
In addition, α-alumina powder obtained by calcining such aluminum hydroxide powder is partially sintered, and a relatively large force is required to crush it.
Furthermore, even when crushed, it was difficult to obtain a powder with a uniform particle size distribution. Therefore, when these α-alumina powders were sintered, a sintered body with good translucency and transparency could not be obtained.

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Claims (1)

[Claims] 1. Alcoholyze an organoaluminum compound in a non-polar solvent, then add 12 moles or less of water per mole of the organoaluminum compound, and mix and stir in the presence of a mixed medium. 1. A method for producing aluminum hydroxide powder, the method comprising: hydrolyzing the aluminum hydroxide powder, and then drying the hydrolyzate. 2 Alcoholyze an organoaluminum compound in a non-polar solvent, then add 12 moles or less of water per mole of the organoaluminum compound, hydrolyze it while stirring in the presence of a mixed medium, and further A method for producing alumina powder, which comprises drying and calcining the hydrolyzate.