JPH0417896B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing aluminum hydroxide powder and 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, and wear resistance, 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 an average particle size of much more than 1 μm and have poor sinterability, so a method is used to improve the sinterability by further pulverizing the raw alumina powder using a mill etc. I've been exposed to it. That is,
In order to obtain an easily sinterable alumina powder, it is necessary that the average particle size of the alumina powder be at least 1 μm or less, but such fine grinding requires extremely large amounts of energy. In addition, since alumina is a very hard substance, when it is ground for a long time using a ball mill or vibration mill, the mill media is worn out and the abrasive powder gets mixed into the alumina.
The resulting alumina powder was contaminated, and as a natural result, the purity of the alumina powder was reduced. The inventors of the present invention focused on the drawbacks of the conventional method described above and proceeded with their studies. As a result, the inventors developed alumina powder by using specific hydrolysis conditions to micronize aluminum hydroxide, which is the raw material for producing alumina powder. It was discovered that easily sinterable alumina powder could be obtained without pulverizing the alumina powder, and the proposal was made earlier (patent application 1983-
17587), but as a result of further research, it was discovered that by adding alumina fine particles during hydrolysis, an alumina powder with easier sinterability and therefore higher translucency could be obtained, and the present invention was developed. This is the completed version. That is, the present invention provides: (1) aluminum alkoxide dissolved in a hydrophobic organic solvent;
When performing hydrolysis by mixing and stirring in the presence of a small amount of water and a mixed medium of 12 times the mole or less, fine alumina particles are added to the mixed system to perform the hydrolysis, and then the hydrolyzate is and (2) drying an aluminum alkoxide dissolved in a hydrophobic organic solvent.
When performing hydrolysis by mixing and stirring in the presence of a small amount of water and a mixed medium of 12 times the mole or less, fine alumina particles are added to the mixed system to perform the hydrolysis, and then the hydrolyzate is The present invention provides a method for producing alumina powder, which is characterized by drying and calcining 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 the organic solvents described below. 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 the object of the present invention cannot be achieved by reducing aggregates that are easily dispersed, in other words, partially consolidated. In other words, to express this phenomenon more sensually, aluminum hydroxide becomes a gel when it is hydrolyzed in the presence of a large amount of water or in an aqueous solvent, and when it is dried, it solidifies as a bulk. It does not become a fine powder. Examples of such preferable hydrophobic 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. A mixture thereof or a petroleum fraction, such as a non-polar organic solvent such as petroleum ether or petroleum benzene. In the present invention, aluminum alkoxide needs to be hydrolyzed in the presence of 12 moles or less of water, preferably in the presence of 2 to 8 moles of water. If hydrolysis is carried out with more than 12 moles of water, the primary particles of aluminum hydroxide obtained as described above tend to aggregate unevenly, and the aluminum hydroxide obtained by drying becomes a powder with good fluidity. Instead, it becomes partially solidified. Therefore, when this is calcined to change into α-alumina, sintering occurs in the solidified portion, and α-alumina with uniform particle size is an essential condition for exhibiting easy sinterability. It becomes difficult to make alumina powder,
The purpose of the invention cannot be achieved. Note that if the amount of water added is too small, for example, if it is less than 2 moles, the aluminum alkoxide will often exist in an undecomposed state, and the purity of the obtained aluminum hydroxide will decrease. However, when changing the aluminum hydroxide to α-alumina,
Since it undergoes a calcination process at 1100°C or higher, undecomposed substances also change into α-alumina during 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. In the present invention, during hydrolytic mixing, fine alumina particles are present in the mixing system to carry out the hydrolysis. Alumina (Al ₂ O ₃ ) used here
The fine particles are preferably alumina powder having a specific surface area of 2 m ² /g or more, particularly those having a primary particle size of 1 μm or less and having a substantially spherical shape. Note that high-purity alumina powder such as that produced by the method of the present invention is suitable as alumina to be present in the mixed system, and can be used as it is. The alumina powder to be added is Al ₂ O ₃ per mole of aluminum alkoxide to be hydrolyzed.
The amount may be at least 5×10 ⁻⁴ times by mole, but preferably from 1×10 ⁻³ times to 0.1 times by mole.
Incidentally, adding more than 0.1 mole amount does not cause any disadvantage to the performance of the α-alumina powder finally obtained by calcining, but it becomes impractical in terms of production cost. Moreover, if this amount is too small, less than 5×10 ⁻⁴ times by mole, a sintered body with high light transmittance, which is the effect of the present invention, cannot be obtained. In the present invention, hydrolysis is carried out by mixing and stirring in the presence of a mixed medium. Originally, aluminum alkoxides are easily hydrolyzed in large amounts of water, but decomposition becomes extremely difficult in the presence of small amounts of water in hydrophobic organic solvents 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 hydrolysis. To achieve this objective, it is necessary to add a mixing medium to the organic solvent and to optimize the mixing and stirring means. The mixing medium refers to beads, balls, rods, or objects having similar functions, and includes stirring or rolling mixing and stirring means such as mixers, ball mills, sand mills, etc. A crusher or a disperser is preferable as the mixing and stirring means of the present invention. Of course, alumina porcelain is used as the material for containers and mixed media in these mixing and stirring means, but surprisingly, it is soft and lightweight, so it is thought that it does not have sufficient mixing and stirring functions. Synthetic resin materials such as nylon, polypropylene, polyethylene,
Preferable results can also be obtained using a ball mill or rod mill made of vinyl chloride, Teflon, or the like. Another feature of the present invention is that ultra-high purity alumina can be obtained even when a synthetic resin-based mixing and stirring means is used. It should be noted that the purpose of the present invention cannot be achieved when using an ordinary stirred tank type reactor without a mixed medium, such as a type that is simply stirred with a stirring blade; This is not preferable because impurities due to wear of the container mix into the hydrolyzate and reduce the purity of the aluminum hydroxide powder. Furthermore, when using the method of the present invention, it is necessary to reduce the pressure and/or heat the resulting hydrolyzate to distill off the solvent and dry it, but the energy required for distilling off the organic solvent requires a large amount of water compared to the conventional method. It is much smaller and economically advantageous than when distilling off. Also, it doesn't necessarily have to be, so
The caking of the wet cake that occurs during filtration can be avoided. 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 by a conventional method simultaneously with distilling off the organic solvent used or after distilling off the organic solvent. 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 if the amount of water for hydrolysis is increased, some crystalline trihydrate such as nordstrandite or bayerite may coexist, but this does not essentially change the effect of the present invention. The aluminum hydroxide powder containing fine alumina particles obtained by the present invention can be calcined using a known technique to transform it into α-alumina powder. For example, when calcined at 1000 to 1300°C for 30 minutes to 10 hours, it transforms into alumina, but the higher the calcining temperature and the longer the calcining time, the higher the α-alumina content. 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. Hereinafter, the present invention will be explained in more detail with reference to Examples. Example 1 Heptane solution of aluminum isopropoxide (concentration 1 mol/g) 1 and specific surface area 8 m ² /g α
-Alumina powder 0.51g (0.005mol) and low conductivity water
108 g (6 mol) of the aluminum hydroxide was placed in a resin pot mill with a capacity of 3 along with 3 kg of iron cored resin 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 was distilled off under reduced pressure using a rotary evaporator, followed by drying at 110° C. for 20 hours.
According to powder X-ray diffraction analysis, the obtained dry powder is a mixture of aluminum hydroxide having a pseudo-boehmite structure and added α-alumina, has a specific surface area of 290 m ² /g, a loss on ignition of 22% by weight, and According to organic elemental analysis, it contained 0.5% by weight of carbon. Next, this aluminum hydroxide powder was heated at 1250℃ for 1
After calcining for a period of time, α-alumina powder with a specific surface area of 7.5 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.
The theoretical density was determined by adding 0.1% by weight and press-forming it into a disk shape with a thickness of 1.5 mm and a diameter of 20 mm, pre-calcining it in air at 1200℃ for 1 hour, and then sintering it in a vacuum at 1900℃ for 2 hours. A sintered body with the same density of 3.98 g/cm ³ and extremely excellent transparency was obtained. The obtained α-alumina powder was analyzed for impurities, and the results shown in Table 2 were obtained. It can be seen that ultra-high purity alumina with a purity of 99.999% or more can be obtained by the method of the present invention. Examples 2 to 7 According to each preparation composition shown in Nos. 2 to 7 of Table 1,
The same operation as in Example 1 was performed to obtain α-alumina powder. The obtained α-alumina powders were evaluated for sintering in the same manner as in Example 1, and it was found that each alumina powder had a density of 99.5 to 100.0% of the theoretical density of 3.98 g/cm ^3. have,
A sintered body with extremely excellent transparency was obtained. Comparative example A heptane solution of aluminum isopropoxide (concentration 1 mol/) and 108 g (6 mol) of low conductivity were charged into a resin pot mill with a capacity of 3 along with 3 kg of iron cored resin balls, and tumbling stirring was performed for 2 hours. A slurry of aluminum hydroxide was obtained. 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 and a carbon
The aluminum hydroxide powder contained 2.0% by weight, had a loss on ignition of 23% by weight, and had a specific surface area of 320m ² /g. This aluminum hydroxide powder was calcined at 1250°C for 1 hour to form α-alumina, and sintered in the same manner as in Example 1, resulting in a density of 3.94 g/cm ³ (99% of the theoretical density).
It became a dense sintered body with some translucency,
It was not possible to obtain a material having a high degree of translucency as in the present invention. Comparative Examples 2 to 3 The charging composition of Nos. 9 and 10 in Table 1, i.e., the amount of charging water in Examples 1 and 2 was changed from 6 mol to 15 mol, and the same operation as in Examples 1 and 2 was performed. α-alumina powder was obtained. Although each α-alumina powder was sintered in the same manner as in Example 1, a sintered body with good translucency could not be obtained. (Light transmittance evaluation method of α-alumina) The light transmittance of the examples and comparative examples shown in Table 1 was evaluated as follows. In other words, for a translucent sintered body of α-alumina produced under the same conditions, the linear light transmittance at a wavelength of 600 mm was calculated using the Shimadzu manufactured
Measurements were carried out using an integrating sphere adapter installed in a UV-240 self-recording spectrophotometer. The thickness of each sintered body is approximately 1.2
mm, but it differs somewhat, so the transmittance at a thickness of 1 mm was calculated using the following formula for the measured transmittance, and was taken as the linear transmittance of the sintered body. T=(1-R) ² exp(-αt) R=(n-1/n+1) ^2Here , T is transmittance, t is sample thickness, α is absorption coefficient, R is reflectance, and n is refraction. rate. From literature
Using n=1.768 of single crystal alumina (Saphire) for the NaD line, T=0.852exp(-αt) is obtained. The in-line transmittance of each sintered body sample is listed in the rightmost column of Table 1. The sintered bodies made using the α-alumina of Examples Nos. 1 to 7 according to the present invention all showed good in-line transmittance of 70% or more, whereas the sintered bodies made using the α-alumina of the comparative example In some cases, it can be seen that the in-line transmittance is as low as 40% or less.

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Claims (1)

[Scope of Claims] 1. Hydrolysis of aluminum alkoxide dissolved in a hydrophobic organic solvent by mixing and stirring in the presence of a small amount of water and a mixed medium of 12 moles or less per mole of the aluminum alkoxide. ,
A method for producing aluminum hydroxide powder, which comprises adding fine alumina particles to the mixed system to carry out the hydrolysis, and then drying the hydrolyzate. 2. When hydrolyzing an aluminum alkoxide dissolved in a hydrophobic organic solvent by mixing and stirring in the presence of a small amount of water and a mixed medium of 12 moles or less per mole of the aluminum alkoxide,
A method for producing alumina powder, which comprises adding fine alumina particles to the mixed system to carry out the hydrolysis, and then drying and calcining the hydrolyzate.