JPH0262488B2 - - Google Patents

Description

[Detailed description of the invention]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing easily sinterable alumina powder, which provides a dense sintered body at relatively low temperatures by normal pressure or reduced pressure sintering. Prior Art Conventionally, various manufacturing methods including the Bayer method are known as methods for obtaining alumina powder for sintering. Among these methods, methods for producing easily sinterable alumina powder include firing alumina hydrate obtained by hydrolyzing aluminum alkoxide, alumina hydrate obtained by decomposing sodium aluminate with halohydrin, or A known method is to obtain alumina by thermally decomposing or calcining ammonium alum, aluminum ammonium carbonate hydroxide, or the like. However, these conventional methods have problems in that the raw materials used are expensive organic substances and the manufacturing process is complicated, resulting in high costs and severely limiting the field of application. On the other hand, attempts have been made to produce alumina powder for sintered bodies using the Bayer process, but the current situation is that nothing satisfactory in terms of sintering properties has yet been obtained. On the other hand, as an inexpensive method for producing alumina hydrate, a method for producing alumina hydrate by neutralizing an acidic substance and an alkaline substance, one of which is an aluminum compound (hereinafter abbreviated as the neutralization method), is known. The alumina hydrate obtained by this neutralization method is widely used in various fillers, catalyst supports, etc. However, although the conventional neutralization method described above is excellent in reducing manufacturing costs,
When the alumina hydrate obtained by this method is used as a raw material for forming a sintered body, there is a problem in that only one with relatively poor sintering properties can be obtained. Summary of the Invention The present invention has been made in view of the problems associated with the above-mentioned prior art, and it provides an easily sinterable alumina powder that has a simple manufacturing process, reduces costs, and has excellent sintering properties. The purpose is to provide a manufacturing method for. The present inventors have conducted extensive research on the relationship between these alumina hydrates and the alumina powder obtained by firing them. As a result, alumina hydrate obtained by neutralizing an acid aqueous solution and an alkaline aqueous solution in which monovalent and divalent anions coexist is different from the conventional method using a monovalent acid alone or a divalent acid alone. It has been found that the alumina powder has different properties compared to that of the alumina powder, and exhibits excellent sintering properties when fired to form an alumina powder. The method for producing easily sinterable alumina powder according to the present invention has been made based on the above-mentioned findings, and more specifically, it is possible to produce an acidic aqueous solution and an alkaline aqueous solution containing aluminum ions in one or both of the aqueous solutions. A method for producing alumina powder by firing an alumina hydrate obtained by reacting with It is characterized by the use of objects. According to the method of the present invention, an alumina hydrate with finer average particles can be obtained compared to conventional methods, and this alumina hydrate is calcined at a temperature of 1400°C or less under normal pressure or reduced pressure by calcination and pulverization. When sintered, it becomes an alumina powder with excellent properties that yields a dense sintered body with a theoretical density of over 97%. Alumina powder made from alumina hydrate produced by the conventional neutralization method had poor sintering properties and could only be applied to limited applications such as catalyst supports, but the present invention has improved sintering properties. Since it is possible to obtain easily sinterable alumina powder with dramatically improved properties, the range of its uses is also expanded. According to the present invention, since an inexpensive inorganic aluminum compound can be used as a raw material and the manufacturing process is simple, it is possible to significantly reduce costs compared to easily sinterable alumina powder obtained by conventional methods. becomes. Therefore, high-speed sintered bodies such as thread guides and cutting tools, in which alumina sintered bodies have traditionally been used,
It is expected to be used not only in acid-resistant and wear-resistant sintered bodies such as ceramic seals, but also in new applications such as ceramic substrates and general structural materials. In addition, since the alumina obtained by the present invention is a well-pulverized powder with a primary particle size of 0.2 μm or less, it can be used as a composite oxide raw material, filler, etc. for purposes other than sintered bodies, making use of its characteristics. It can also be expected to be used for DETAILED DESCRIPTION OF THE INVENTION Generally, the sintering properties of alumina are greatly influenced by the properties of its precursor. In the present invention, the sintering characteristics of easily sinterable alumina are largely due to the properties of the amorphous alumina hydrate, which is its precursor, and the main part of the present invention lies in the synthesis of the alumina hydrate. . The manufacturing method of the present invention will be explained in more detail below. Synthesis of alumina hydrate In the production process of the present invention, alumina hydrate as a precursor is produced by reacting an acidic aqueous solution with an alkaline aqueous solution containing aluminum ions in one or both of the aqueous solutions. This can be obtained by using a compound containing both monovalent anions and divalent anions. The molar ratio X ^- /X ^2- of monovalent anion X ^- and divalent anion X ^2- is 8.0 or less, preferably 0.5 or more 7.0
The following is desirable. Among the above aqueous solutions, the acidic aqueous solution can be prepared as follows. (a) Mixing a monovalent mineral acid and a divalent mineral acid. In this case, as the monovalent mineral acid, hydrochloric acid, nitric acid, etc. can be used, and as the divalent mineral acid, sulfuric acid, etc. can be preferably used. (b) Mixing an aluminum salt that releases a monovalent anion with a divalent mineral acid. As such an aluminum salt, polyaluminum chloride, aluminum chloride, aluminum nitrate, basic aluminum nitrate, etc. can be used. (c) Mixing an aluminum salt that releases divalent anions with monovalent and/or divalent mineral acids. As the aluminum salt in this case, aluminum sulfate, basic aluminum sulfate, etc. are preferably used. On the other hand, alkaline solutions include sodium aluminate, sodium hydroxide, potassium hydroxide,
Aqueous ammonia, ammonium hydrogen carbonate, sodium carbonate, etc. can be used alone or in combination and diluted to an appropriate concentration. In addition, the concentration of alkaline aqueous solution that does not contain aluminum ions is
Approximately 1.0 mol/liter is appropriate. In addition, in the above raw material aqueous solution, in order to obtain alumina hydrate with good properties, the relationship between the anions and Al ³⁺ ions in the acidic aqueous solution is such that the monovalent anion X ^- and the Al ³⁺ ion The molar ratio X ^- /Al ³⁺ is 0.2 or more, preferably 0.5 or more, and a divalent anion
The molar ratio of X ^2- and Al ³⁺ ions, X ^2- /Al ³⁺ , is 0.7 or more;
Preferably it is 1.5 or more. The neutralization reaction is preferably carried out under conditions such that the pH of the reaction system is maintained at 7.0 or lower, more preferably between 4 and 6. Specifically, it is a neutralization reaction from an acid range where the reaction completion pH is 7 or less, or a neutralization reaction by simultaneous addition, and the reaction temperature is room temperature to 50°C. Crystalline aluminum hydroxide is likely to be formed when the reaction completion pH is 7 or higher, or when the neutralization reaction starts from an alkaline region, and this form of aluminum hydroxide contains alkali-derived Na ⁺ and other substances in the crystal lattice. Since the cations are taken in, they cannot be removed by simple washing, and when alumina powder is used, various problems occur. If the reaction completion pH is 7 or less, the product will be amorphous, and residual Na can be easily removed by washing with water, and when it is made into alumina powder, the Na ⁺ concentration can be reduced to 50 ppm or less, for example. High purity alumina of 99.9% or higher can be easily obtained. Since increasing the reaction temperature leads to increasing the size of the primary particles, it is preferably as low as possible. In the product synthesized under the above conditions, for example, when hydrochloric acid is used as the monovalent acid, the residual amount of Cl ^- is 0.5% or less even though Cl ^- is mixed in the raw material, The amount of chlorine gas generated during firing to transform into alumina is not a problem. Note that when sulfuric acid is used as the divalent acid, the amount of residual SO ₄ ^2- in the product tends to increase as the reaction completion pH decreases, and depending on the conditions, sulfur dioxide gas generated during calcination may become a problem. However, this residual SO ₄ ^2- can be removed by washing with an alkaline aqueous solution. This amorphous alumina hydrate becomes a raw material for easily sinterable alumina produced under the following conditions. Production of easily sinterable alumina powder The alumina hydrate obtained by the above synthesis method is calcined to form α-alumina in a conventional manner, and then crushed as necessary to form easily sinterable alumina. can do. It is desirable that the calcination be performed at as low a temperature as possible so that the obtained α-alumina particles do not undergo grain growth. The firing temperature varies depending on the firing conditions, but is generally below 1200°C. The alumina powder obtained is a fine powder with an average temporary particle size of 0.2 μm or less, and this is considered to be one of the factors that improves the sintering properties. Although the reason why fine, easily sinterable alumina powder can be obtained by the present invention is not necessarily clear, it is clear that the particle size of the alumina hydrate produced under the above-mentioned conditions is fine and uniform. Therefore, it is presumed that this is because when this is fired, it becomes fine and uniform alumina as it is. For pulverization, a known pulverizer such as a ball mill, a vibration mill, an attritor, etc. can be used. As a desirable form, the alumina powder obtained by performing wet pulverization for 24 hours with an attritor using 2 mmφ alumina beads as a pulverizing medium has an average secondary particle size of 0.25 μm or less, and is molded at 1 ton/cm ² . In this case, a sintered body with a theoretical density of 97% or more is obtained at 1400℃. EXAMPLES The present invention will be explained below with reference to Examples, but the present invention is not limited to the description of these Examples. Example 1 Aluminum sulfate (Al ₂ O ₃ = 8%, SO ₄ ^2- = 23
%) 500g, HCl (35%) 114.2g, H ₂ SO ₄ (96%)
By mixing 100.8g and 1285g of pure water, Al ₂ O ₃ :
Prepare 2000 g of an acidic aluminum aqueous solution with a composition in which the molar ratio of SO ₄ ^2- :Cl ^- is 1:5.3:2.9, and add sodium aluminate (Al ₂ O ₃ = 20%, Na ₂ O/
Al ₂ O ₃ = 1.5) Prepare an alkaline aqueous aluminum solution by adding 200 g and 1800 g of water, and adjust the pH of the reaction phase.
Perform the simultaneous addition reaction at room temperature while maintaining the temperature at 5.0. The reaction was completed in about 1 hour, filtered, washed with pure water, and desorbed.
Add Na and dry. The product thus obtained was then calcined at 1200° C. for 2 hours to obtain α-alumina. This alumina powder is mixed with pure water and 2φmm
A suitable amount of alumina beads were added thereto to ensure sufficient pulverization, and the mixture was placed in an alkaline cylindrical container (attritor) and pulverized for 24 hours using a stirring blade. The powder thus obtained had an average primary particle size of 0.15 μm and an average secondary particle size of 0.20 μm. This alumina powder was molded using a hydrostatic press at 2000 Kg/cm ² to obtain a test piece with a molded density of 2.22 Kg/cm ³ .
The sintered densities when fired in the atmosphere at normal pressure in the temperature range of 1300°C to 1600°C were as shown in Table 1.

[Table] Example 2 Polyaluminum chloride (Al ₂ O ₃ = 10%, Cl = 10
%) 400g, H ₂ SO ₄ (96%) 240g and pure water 1365
The molar ratio of Al ₂ O ₃ :SO ₄ ^2- :Cl ^- is 1:
Acidic aluminum aqueous solution 2000 with a composition of 3:1.4
g, and then add water to 200 g of sodium aluminate (Al ₂ O ₃ = 20%, Na ₂ O / Al ₂ O ₃ = 1.5).
Prepare an aqueous alkaline aluminum solution to which 1800 g of aluminum has been added, and perform simultaneous addition reaction at 40°C while maintaining the pH of the reaction phase at 6.0. The reaction was completed in about 1 hour, filtered and washed with dilute NH ₄ HCO ₃ solution and pure water to remove sodium.
Remove SO ₄ and dry. As a result of analyzing the obtained dried product, Al ₂ O ₃ = 42.3%, Cl ^- = 0.2%, SO ₄ ^2- =
It was 2.9% amorphous aluminum hydroxide.
This material was calcined at 1200°C for 2 hours to obtain α-alumina. Pure water and 2φmm alumina beads were added to the alumina powder in an appropriate amount to enable sufficient pulverization, and the mixture was placed in an alumina cylindrical container and pulverized for 24 hours using a stirring blade. The powder thus obtained had an average primary particle size of 0.15 μm and an average secondary particle size of 0.20 μm. This alumina powder was molded using a 2000 kg/cm ² isostatic press to obtain a test piece with a molded density of 2.22 g/cm ³ . The sintered density was 3.90 g/cm ³ when fired in the atmosphere at normal pressure and temperature of 1400°C. Example 3 500 g of aluminum sulfate (Al ₂ O ₃ = 8%) and
Mix 100g of H ₂ NO ₃ (61%) and 1400g of pure water.
2000 _g of an acidic aluminum aqueous solution having a composition with _a molar ratio of Al 2 O 3 :SO ₄ ^2- :NO ₃ ^- of 11:3.0:18 was prepared, and this was reacted and calcined in the same manner as in Example 1.
It was ground into alumina powder. This alumina powder had an average primary particle size of 0.16 microns and an average secondary particle size of 2.1 microns. This alumina powder
Molded with a 2000Kg/ ^cm2 isostatic press to a molding density of 2.1
A test piece of g/cm ³ was obtained. The sintered density was 3.89 g/cm ³ when fired in the atmosphere at normal pressure and temperature of 1400°C. Comparative Example 1 Aluminum sulfate (Al ₂ O ₃ = 8%, SO ₄ ^2- = 23
%) dilute 500g with 2000g of pure water to obtain Al ₂ O ₃ :
An acidic aluminum aqueous solution having a composition in which the molar ratio of SO ₄ ^2- :Cl ^- was 1:3.1:0 was prepared, and this was used in Example 1.
Alumina powder was obtained by reaction, calcination, and pulverization in the same manner as above. This alumina powder had an average primary particle size of 0.20 microns and an average secondary particle size of 0.26 microns. This alumina powder was molded using a hydrostatic press at 2000 kg/cm ² to obtain a test piece with a molded density of 2.01 g/cm ³ . The sintered density was 3.80 g/cm ³ when fired in the atmosphere at normal pressure and temperature of 1400°C. Comparative example 2 Polyaluminum chloride (Al ₂ O ₃ = 20%, Cl = 10
%), 21.2 g of H ₂ SO ₄ (96%), and 1780 g of pure water were mixed to give a molar ratio of Al ₂ O ₃ :SO ₄ ^2- :Cl ^- of 1:
Acidic aluminum aqueous solution with a composition of 0.53:5.0
When 2000 g of the powder was prepared and reacted and fired in the same manner as in Example 1, the alumina powder obtained was so agglomerated that it could not be pulverized.

Claims (1)

[Claims] 1. Alumina powder is produced by firing an alumina hydrate obtained by reacting an acidic aqueous solution and an alkaline aqueous solution containing aluminum ions in one or both of the aqueous solutions. A method for producing easily sinterable alumina powder, characterized in that the acidic aqueous solution contains both monovalent anions and divalent anions. 2. The method according to claim 1, wherein the molar ratio X ^- /X ^2- of the monovalent anion X ^- and the divalent anion X ^2- is 8.0 or less. 3. The method according to claim 1, wherein the molar ratio X ^- /Al ³⁺ of monovalent anions X ^- and Al ³⁺ ions in the raw material is 0.2 or more. 4. According to any one of claims 1 to 3, wherein the molar ratio X ^2- /Al ³⁺ of divalent anions X ^2- and Al ³⁺ ions in the raw material is 0.7 or more. The method described. 5. The method according to any one of claims 1 to 4, wherein the PH of the reaction system is maintained at 7.0 or less.