JP4442214B2 - Method for producing fine α-alumina - Google Patents

Description

The present invention relates to a method for producing fine α-alumina.

Fine α-alumina is a fine particle of alumina [Al ₂ O ₃ ] whose main crystal phase is α-phase, and is widely used as a raw material for producing sintered bodies such as light-transmitting tubes. Yes. Such fine α-alumina is required to have a high α conversion, a high BET specific surface area, and a low necking in that a sintered body having excellent strength can be obtained.

Non-Patent Document 1 [A. Krell, Nano Structured Materials, V0l.11, 1141 (1999)] as a method for producing fine α-alumina with a high α conversion rate and a high BET specific surface area, dissolved aluminum nitrate in water. Then, an aqueous solution of aluminum nitrate is added, and ammonia is directly added thereto at 75 ° C. to hydrolyze to obtain an aluminum hydrolyzate. The hydrolyzate is dispersed in water together with seed crystal particles, and then water is removed to remove aluminum. A method is disclosed in which a powder mixture of hydrolyzate and seed crystal particles is formed and the powder mixture is fired.

However, in the conventional production method in which ammonia is directly added to an aluminum nitrate aqueous solution at 75 ° C. to hydrolyze, the obtained fine α-alumina has a problem that the particles are often necked.

A. Krell, Nano Structured Materials, Vol. 11, 1141 (1999)

Accordingly, the present inventors have intensively studied to develop a method capable of producing fine α-alumina having a low α-necking ratio and a high BET specific surface area, and as a result, an aluminum compound as an aluminum hydrolyzate was converted to ammonium in an aqueous medium. If the aqueous medium is removed from an aqueous mixture obtained by hydrolyzing with a salt and dispersed in an aqueous medium having a pH of 5 or less together with seed crystal particles, and the obtained powder mixture is fired, there is less necking, The inventors have found that fine α-alumina having a high α conversion rate and a high BET specific surface area can be obtained, and have reached the present invention.

That is, the present invention hydrolyzes an aluminum compound with an ammonium salt in an aqueous medium to obtain an aluminum hydrolyzate,
The obtained aluminum hydrolyzate is dispersed together with seed crystal particles in an aqueous medium having a hydrogen ion concentration of pH 5 or less to obtain an aqueous mixture,
The aqueous medium is removed from the obtained aqueous mixture to obtain a powder mixture containing aluminum hydrolyzate and seed crystal particles,
The present invention provides a method for producing fine α-alumina, which comprises firing the obtained powder mixture.

According to the production method of the present invention, it is possible to produce a fine α-alumina having a high α conversion rate and a BET specific surface area and little necking.

In the production method of the present invention, an aluminum compound is first hydrolyzed with an ammonium salt in an aqueous medium to obtain an aluminum hydrolyzate.

As an aluminum compound, what is water-soluble and does not contain metal components other than aluminum, for example, an aluminum salt is mentioned. Specific examples of the aluminum salt include aluminum nitrates such as aluminum nitrate and ammonium ammonium nitrate, aluminum inorganic salts such as ammonium alum and ammonium aluminum carbonate, aluminum such as aluminum oxalate, aluminum acetate, aluminum stearate, aluminum lactate and aluminum laurate Organic salts and the like can be mentioned, and aluminum inorganic salts are preferable, and aluminum nitrates are more preferable.

An aqueous medium is a medium containing water (H ₂ O) as a main component, and may contain, in addition to water, an organic solvent that volatilizes or decomposes at least at a firing temperature. Good. Examples of such organic solvents include polar organic solvents including alcohols such as methanol, ethanol, propanol and isopropanol, and nonpolar organic solvents such as carbon tetrachloride, benzene and hexane. The amount of the aqueous medium used is usually about 50 parts by mass or more and 100 parts by mass or less in terms of pure water per 100 parts by mass of the aluminum compound.

The ammonium salt is a salt of ammonia and an acid, and usually a water-soluble one is used. As the acid, a weak acid is preferable, and an ammonium weak acid salt which is a salt of ammonia and a weak acid is preferably used. As the weak acid, for example, an inorganic weak acid such as carbonic acid is preferably used, and as the ammonium inorganic weak acid salt, specifically, for example, ammonium hydrogen carbonate [(NH ₄ ) HCO ₃ ], ammonium carbonate [(NH ₄ ) ₂ CO ₃ ] And the like.

In order to hydrolyze an aluminum compound with an ammonium salt in an aqueous medium, for example, the aluminum compound may be mixed with an ammonium salt in an aqueous medium. The aluminum compound is preferably hydrolyzed in a state of being completely dissolved in the aqueous medium. For this reason, the aluminum compound is usually used as an aluminum compound aqueous solution previously dissolved in the aqueous medium. In the case of using an aluminum compound aqueous solution, the aluminum compound concentration is usually 0.1 mol / L or more, preferably 0.5 mol / L or more and a saturation concentration or less in terms of aluminum. The aluminum compound is preferably completely dissolved.

The ammonium salt is usually used as an aqueous ammonium salt solution in which an ammonium salt is dissolved in an aqueous medium. In the case of using an aqueous ammonium salt solution, the ammonium salt concentration is usually 0.1 mol / L or more, preferably 0.5 mol / L or more in terms of ammonium, and may be a saturation concentration or less.

For mixing, the aqueous aluminum compound solution may be added to the aqueous ammonium salt solution, or the aqueous ammonium salt solution may be added to the aqueous aluminum compound solution. The amount of ammonium salt used is usually such that the hydrogen ion concentration in the system is pH 3 or higher. By becoming pH 3 or more, an aluminum compound hydrolyzes and an aluminum hydrolyzate produces | generates. The time required for hydrolysis is usually about 1 hour to 24 hours.

The hydrolysis is usually performed at a temperature of 60 ° C. or lower, preferably 50 ° C. or lower, more preferably 35 ° C. or lower. Usually, the aqueous medium, the aluminum compound aqueous solution, or the ammonium salt aqueous solution is at or above the freezing temperature, preferably 0 ° C. or higher. Done in When hydrolyzing at a temperature exceeding 60 ° C., fine α-alumina with many necking particles tends to be obtained.

A hydrolysis mixture containing an aqueous medium and an aluminum hydrolyzate is obtained by hydrolyzing the aluminum compound with an ammonium salt in the aqueous medium. Since the aluminum hydrolyzate is usually insoluble in an aqueous medium, in such a hydrolysis mixture, the aluminum hydrolyzate is dispersed in the aqueous medium in the form of a sol or gel, or is precipitated as a precipitate. Yes.

In the production method of the present invention, the obtained aluminum hydrolysis product is dispersed together with seed crystal particles in an aqueous medium to obtain an aqueous mixture. The aqueous medium in this aqueous mixture has a hydrogen ion concentration of pH 5 or less. As the aqueous medium, water is used, and for example, an aluminum compound aqueous solution may contain the same solvent as described above.

As the seed crystal particles, a metal compound is usually used, and specific examples include metal oxide particles such as alumina, iron oxide, and chromium oxide. As such seed crystal particles, those having a particle size of usually about 0.01 μm or more and 0.5 μm or less are used, and preferably 0.05 μm or more. The BET specific surface area is preferably about 12 m ² / g or more and about 150 m ² / g or less, more preferably 15 m ² / g or more. As the seed crystal particles, those having a crystal structure of corundum type are preferably used, and those having no crystal water are preferably used. Examples of seed crystal particles having a corundum crystal structure and no crystal water include α-alumina particles, α-iron oxide particles, and α-chromium oxide particles. Alumina particles are preferably used because they are the same metal component as the fine α-alumina obtained.

The content of the seed crystal particles in the aqueous mixture is such that, when metal oxide particles are used, the aluminum hydrolyzate and seed crystal particles in terms of metal oxide are obtained in that a fine α-alumina with a high α conversion rate can be easily obtained. 1 part by mass or more, further 2 parts by mass or more, and particularly 4 parts by mass or more are preferable per 100 parts by mass of the total amount. Further, the amount of seed crystal particles used may exceed 50 parts by mass, but since the pregelatinization rate does not increase in accordance with the amount used, it is usually 50 parts by mass or less, preferably 40 parts by mass or less, and more preferably 30 parts. It is about the mass part or less.

The content of the aqueous medium in the aqueous mixture is usually about 50 parts by mass or more and 700 parts by mass or less, preferably about 100 parts by mass or more and 500 parts by mass or less per 100 parts by mass of the total amount of the aluminum hydrolyzate and seed crystal particles.

The aqueous mixture may be obtained by adding an aluminum hydrolyzate and seed crystal particles to an aqueous medium, but the hydrolyzed mixture after hydrolyzing the aluminum compound aqueous solution contains water and an aluminum hydrolyzate. Usually, an aqueous mixture is obtained by adding seed crystal particles to the hydrolysis mixture. In addition, seed crystal particles previously added to an aluminum compound aqueous solution, mixed with an ammonium salt, and hydrolyzed in the presence of the seed crystal particles are added to the aluminum hydrolyzate and the seed crystal particles previously added to water. An aqueous mixture in which is dispersed can also be obtained.

For example, the seed crystal particles may be added to the hydrolysis mixture or the aqueous aluminum compound solution in a powder state, or may be added in a state of being dispersed in a solvent such as water. In order to disperse the seed crystal particles in the solvent, for example, after mixing the seed crystal particles with the solvent, the seed crystal particles may be classified by a wet method using a ball mill, a vibration mill, a medium stirring mill, or the like. By classifying, seed crystal particles are dispersed in the solvent. The solvent may contain an organic dispersant, a pH adjuster and the like. Examples of the solvent include water and the same solvent as described above as a solvent that can be contained in the aqueous mixture.

In the aqueous mixture, the hydrogen ion concentration of the aqueous medium is pH 5 or less. If the hydrogen ion concentration of the aqueous phase of the hydrolyzed mixture after the previous hydrolysis is pH 5 or less, the seed crystal particles can be added thereto to make an aqueous mixture as it is. Moreover, when the hydrogen ion concentration of a hydrolysis mixture exceeds pH 5, you may adjust pH by adding a volatile acid to a hydrolysis mixture. The volatile acid may be an inorganic acid such as hydrochloric acid or nitric acid or an organic acid such as acetic acid.

In the production method of the present invention, the aqueous medium is removed from the obtained aqueous mixture. In order to remove the aqueous medium from the aqueous mixture, for example, the aqueous medium may be volatilized and evaporated to dryness. The aqueous medium can be volatilized by a usual method such as a freeze drying method or a vacuum drying method. The temperature at which the aqueous medium is volatilized is usually 100 ° C. or lower.

Further, the aqueous medium may be removed by a method of filtering the aqueous medium by a normal filtration operation and then drying the filtration residue. The filtration temperature is usually 100 ° C. or lower. The filtration residue after filtration may be dried by air drying or may be dried by heating. The drying temperature is usually 100 ° C. or lower. Drying may be performed in the atmosphere, may be performed in an inert gas such as nitrogen gas, or may be performed under reduced pressure.

Thus, by removing the aqueous medium from the aqueous mixture, a powder mixture containing the aluminum hydrolyzate and seed crystal particles can be obtained.

The powder mixture thus obtained is heated and fired. Heating is performed at a temperature increase rate of 150 ° C./h to 500 ° C./h, for example. The firing temperature is usually 600 ° C. or higher, preferably 700 ° C. or higher in that fine α-alumina having a high α conversion rate can be easily obtained, and 1000 ° C. or lower, preferably 950 ° C. or lower in terms of less necking between particles. It is.

Firing may be performed in the air or in an inert gas such as nitrogen gas or argon gas. Further, it may be fired while keeping the water vapor partial pressure in the atmosphere low.

Firing can be performed using a normal firing furnace such as a tubular electric furnace, box-type electric furnace, tunnel furnace, far-infrared furnace, microwave heating furnace, shaft furnace, reflection furnace, rotary furnace, roller hearth furnace, or the like. . Firing may be performed batchwise or continuously. Moreover, you may carry out by a stationary type and may carry out by a fluid type.

The firing time may be sufficient as long as the aluminum hydrolyzate is turned into α to obtain fine α-alumina having a high α conversion rate, depending on the type, amount, type of firing furnace, firing temperature, and firing atmosphere of the aluminum compound used. Although it differs, it is about 10 minutes or more and 24 hours or less, for example.

The fine α-alumina thus obtained has a particle diameter of about 0.01 μm or more and 0.1 μm or less, a high α conversion and a large BET specific surface area, for example, an α conversion of 90% or more, preferably 95% or more. The BET specific surface area is 13 m ² / g or more and 150 m ² / g or less, preferably 15 m ² / g or more and 100 m ² / g or less.

The obtained fine α-alumina may be pulverized. In order to pulverize the fine α-alumina, a medium pulverizer such as a vibration mill, a ball mill, or a jet mill can be used. The obtained fine α-alumina may be classified.

The fine α-alumina thus obtained is useful as a raw material for producing an α-alumina sintered body, for example. Examples of the α-alumina sintered body include those requiring high strength such as cutting tools, bioceramics, and bulletproof plates. Examples include semiconductor manufacturing equipment parts such as wafer handlers and electronic parts such as oxygen sensors. Light-transmitting tubes such as sodium lamps and metal halide lamps are also included. Examples thereof include a ceramics filter used for removing solids contained in a gas such as exhaust gas, filtering aluminum melt, filtering food such as beer, and the like, and a ceramic filter for selectively permeating hydrogen in a fuel cell. Further, the obtained fine α-alumina can be used as a raw material, and can also be used as a raw material for producing a conductive sintered body, a thermally conductive sintered body, and the like.

The obtained fine α-alumina is used as an additive to improve the head cleaning property and body wearability by adding it to the coating layer of the coating type magnetic media in the same manner as normal fine α-alumina as a powder. Can do. It can also be used as a toner. It can also be used as a filler added to the resin. It can also be used as an abrasive, for example, it can be used as a slurry dispersed in a solvent such as water, and can be used for polishing semiconductor CMP, hard disk substrates, etc. It can be used for precision polishing of a head or the like. It can also be used as a catalyst carrier for supporting a catalyst component on the surface.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited by these Examples.

Note that the alpha conversion rate of the fine α-alumina obtained in each example was determined from the diffraction intensity spectrum of the fine α-alumina obtained using a powder X-ray diffractometer, the peak intensity (I _{α (012 )} ) And the peak intensity (I _{θ (440 )} ) of the θ phase (440 plane),
α conversion rate = I _{α (012)} / (I _{α (012)} + I _{θ (440)} ) × 100 (%) (1)
Calculated by The BET specific surface area was determined by a nitrogen adsorption method. The average primary particle diameter was determined as the number average value of the measured values by measuring the maximum diameter in the fixed direction of each primary particle for 20 or more arbitrary particles in a transmission electron micrograph of fine α-alumina.

Example 1
[Production of seed crystal slurry]
Aluminum nitrate nonahydrate [Al (NO) ₃ · 9H ₂ O] (manufactured by Wako Pure Chemical Industries, special grade, powder) 375.13 g was dissolved in 777.90 g of pure water to obtain an aqueous solution. 0.5 g) was mixed with 990 mL (990 g) of pure water to obtain 1000 g of an aluminum nitrate aqueous solution having a hydrogen ion concentration of pH 2. In this aluminum nitrate aqueous solution, 125 g of α alumina particles (seed crystal particles) having a BET specific surface area of 16.0 m ² / g and a particle size of about 0.1 μm were added and dispersed, and then 350 g of alumina beads (diameter 2 mm) were filled. Wet classification was performed using a ball mill for 24 hours to obtain a seed crystal slurry.

[Production of fine α-alumina]
Aluminum nitrate nonahydrate [Al (NO ₃ ) ₃ · 9H ₂ O] (Wako Pure Chemical Industries, special grade, powder) 375.13 g (1 mol) was dissolved in 777.9 g of pure water to prepare an aluminum nitrate aqueous solution. (PH is 1.8) 1 L (1000 cm ³ ) was obtained. To this aluminum nitrate aqueous solution 250 cm ³ , 7.08 g of the seed crystal slurry obtained above [including 1.42 g of α-alumina particles and 0.012 g of aluminum nitrate [Al (NO ₃ ) ₃ ]] was added, and alumina beads [diameter of about 2 mm] after sufficiently stirring for 24 hours in a ball mill filled with 960 g, the alumina beads were removed to obtain an aluminum nitrate aqueous solution containing seed crystal particles.

An aqueous solution obtained by dissolving 12.5 parts by weight of ammonium hydrogen carbonate powder (manufactured by Wako Pure Chemical Industries, special grade) in 87.5 parts by weight of pure water, pH is 7.8] 150 g of stirring at room temperature (about 25 ° C.) While adding 127 g of the aluminum nitrate aqueous solution containing the seed crystal particles obtained above at a feed rate of about 6 g / min using a micro rotary pump, a hydrolyzed precipitate (aluminum hydrolyzate) and A slurry-like aqueous mixture containing seed crystal particles (α alumina particles) was obtained. This aqueous mixture had a hydrogen ion concentration of pH 4.1 in the aqueous phase.

The slurry was allowed to stand at room temperature (about 25 ° C.) for about 24 hours, and then water was volatilized using a vacuum dryer to obtain a powdery mixture. This powdery mixture contains 10 parts by mass of α-alumina particles (seed crystal particles) per 100 parts by mass in terms of oxide of the metal component.

This powder mixture is crushed in a mortar, placed in an alumina crucible, heated in a box-type electric furnace to 920 ° C. at a heating rate of 300 ° C./h in the atmosphere, and calcined at the same temperature for 3 hours. Fine α-alumina was obtained. This fine α-alumina had a gelatinization rate of 97%, a BET specific surface area of 22.1 m ² / g, and an average primary particle diameter of 51 nm. When the fine α-alumina was observed with a transmission electron microscope (TEM), there was little necking between the particles. A TEM photograph of the obtained fine α-alumina is shown in FIG.

Comparative Example 1
A slurry-like aqueous mixture was obtained in the same manner as in Example 1 except that the amount of aluminum nitrate aqueous solution containing seed crystal particles was 40 g. This aqueous mixture had a hydrogen ion concentration of pH 5.2 in the aqueous phase.

A powdery mixture was obtained in the same manner as in Example 1 except that the aqueous mixture obtained above was used instead of the aqueous mixture obtained in Example 1, and fine α-alumina was obtained. This fine α-alumina had an α conversion of 96%, a BET specific surface area of 18.5 m ² / g, and an average primary particle diameter of 100 nm. When this fine α-alumina was observed by TEM, many necking between particles was observed. A TEM photograph of the obtained fine α-alumina is shown in FIG.

2 is a transmission electron microscope (TEM) photograph of fine α-alumina obtained in Example 1. FIG. 2 is a transmission electron microscope (TEM) photograph of fine α-alumina obtained in Comparative Example 1.

Claims (2)

Aluminum nitrate nonahydrate is hydrolyzed with ammonium hydrogen carbonate in an aqueous medium in the presence of seed crystal particles to obtain an aluminum hydrolyzate,
The obtained aluminum hydrolyzate is dispersed together with seed crystal particles in an aqueous medium having a hydrogen ion concentration of pH 5 or less to obtain an aqueous mixture,
The aqueous medium is removed from the obtained aqueous mixture to obtain a powder mixture containing aluminum hydrolyzate and seed crystal particles,
A method for producing fine α-alumina, which comprises firing the obtained powder mixture. The manufacturing method of Claim 1 which hydrolyzes an aluminum nitrate nonahydrate at 60 degrees C or less.

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