JPH07101723A - Production of alpha alumina powder - Google Patents

Abstract

PURPOSE:To obtain an alpha alumina powder excellent in disaggregating property less in the number of coarse particles and sharp in particle size distribution in a high yield by exposing aluminum hydroxide obtained by Bayer method to an atmosphere of temp. under a specific condition and firing the obtained transition alumina under a specific condition. CONSTITUTION:The aluminum hydroxide obtained by Bayer method is exposed to the atmosphere of 400-1200 deg.C for 0.1sec to 5min and instantaneously calcining to form the transition alumina. This alumina is fired until the ratio of converting into alpha becomes >=80%. And the instantaneous calcining is usually executed by using an air flow type kiln so as to be <=5% in the alumina content occupied in the crystal phase in the obtained alumina. By this way, the objective alphaalumina is obtained with a relatively simple operation.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing α-alumina powder. More specifically, the present invention relates to a method for producing α-alumina powder having excellent crushability, few coarse particles and a sharp particle size distribution.

[0002]

2. Description of the Related Art The average primary particle diameter (size of α crystal) is 2 to
The α-alumina powder having a particle size of 3 μm or less is used for fine ceramics, fine abrasives, and refractories. These α-alumina powders are generally produced by firing aluminum hydroxide produced by the Bayer method in a continuous firing furnace such as a rotary kiln or a tunnel kiln or a batch firing furnace. Generally, aluminum hydroxide produced by the Bayer process has a density of about 0. It is a secondary agglomerated particle of about 10 to about 100 μm in which primary particles of a few μm to about 30 μm are agglomerated, and a strong interparticle bond is generated at the time of α conversion. Therefore, in order to obtain α-alumina powder having a particle size of 2 to 3 μm or less after firing, the particle size is adjusted by crushing or crushing with a ball mill or the like, but such an operation consumes a large amount of energy and has a drawback of increasing manufacturing cost. . In addition, since the α-alumina powder obtained by this method contains coarse particles and has a wide particle size distribution, the molded product obtained using this is inferior in debinding property and degassing property in the molded product during sintering. Further, the coarse particles cause defects in the sintered body, and have a problem of lowering mechanical strength.

Therefore, efforts have conventionally been made to improve the crushability of alumina after sintering. For example, a method is known in which polyethylene glycol, aluminum alkoxide, or the like is added to alumina powder as a grinding aid. However, although this method shortens the pulverization time, it has a drawback that the pulverization aid adsorbed on the alumina surface adversely affects the dispersibility and the like in forming a slurry.

Further, in Japanese Patent Laid-Open No. 61-26554, "A ceramic precursor or a metastable phase material is used in a method for producing a ceramic sintered body which is formed and fired by using a ceramic precursor or a metastable phase material as a starting material. A substance which forms a more stable phase obtained by heating is added to the starting raw material in an amount of 0.1 to 100% of the number of particles of the starting raw material, followed by molding and firing. A method of manufacturing a ceramic sintered body is disclosed. Further, Japanese Patent Application Laid-Open No. 62-128918 discloses "a method for producing an alumina powder containing α-alumina crystals having a size smaller than 1 μm, in which α-alumina precursor is 1 μm.
Smaller α-alumina particles were added as seed crystals, and the seeded precursor was added at 900-1350 ° C.
A method of making an alumina powder is disclosed which comprises the steps of sintering at least a portion of the alumina for a time sufficient to convert it to alpha alumina and grinding the sintered product into a powder of particles smaller than 1 μm. ing. However, although these seeding methods are expected to substantially improve the disintegration property, they have a drawback that the precipitation rate and growth rate of aluminum hydroxide are reduced and the production cost is increased.

Japanese Unexamined Patent Publication (Kokai) No. 3-93617 discloses that "boehmite phase is generated when aluminum hydroxide undergoes a phase transition in the process of baking aluminum hydroxide obtained by hydrolyzing alkali aluminate to produce alumina. A method for producing easily sinterable alumina is disclosed, which comprises firing so that the amount of alumina passing through becomes 10% or less of the whole. However, the content rate of beammite is 1
In order to reduce the content to 0% or less, (1) it is necessary to use alumina hydroxide having small primary particles, or (2) it is necessary to use high-aluminum hydroxide, which requires firing under high vacuum. However, there is a drawback that it is not economical because it is necessary to fire under extremely special firing conditions.

Further, Japanese Unexamined Patent Publication (Kokai) No. 5-43224 discloses that "when the intermediate-sintered alumina is used, the total soda content is 0.
Bayer aluminum hydroxide having a soda content of 5% by weight or less was dehydrated and pyrolyzed at 700 ° C. or less at a heating rate of 20 ° C. or less per minute, and then heated to 140.
Intermediate calcining is performed at 0 ° C. or lower to give intermediate calcined alumina having a degree of α conversion of 90% or more, and thereafter, the intermediate calcined alumina is fluorine-mineralized at a ratio of 0.05 to 1.0 part by weight based on 100 parts by weight of alumina in terms of fluorine. 600 to 1500 ° C in the presence of the agent
The method for producing spherical alumina is characterized in that the particle size is controlled by re-baking. However, this method has a drawback that the temperature rising rate during the first-stage firing is slow and it is not efficient.

[0007]

In view of such circumstances, in view of the above circumstances,
The present inventors have conducted extensive studies for the purpose of finding a method for producing α-alumina, which has excellent disintegration properties, has few coarse particles, and has a sharp particle size distribution with good yield, and is inexpensive. When firing under the conditions, it was found that α-alumina powder satisfying the above characteristics was obtained, and the present invention was completed.

[0008]

That is, according to the present invention, aluminum hydroxide obtained by the Bayer method is used at 400 ° C to 120 ° C.
An α-alumina powder, which is obtained by contacting an atmosphere at a temperature of 0 ° C. for 0.1 second to 5 minutes to perform a temporary calcination to obtain a transition alumina, and then calcining the transition alumina until the α conversion rate becomes 80% or more. In order to provide the manufacturing method of.

The present invention will be described in more detail below. In carrying out the present invention, aluminum hydroxide (gibbsite) obtained by the Bayer method is used as the raw material aluminum hydroxide. The particle shape and particle size of the raw material aluminum hydroxide are not particularly limited, but usually about 0.1. Primary particles of several μm to about 30 μm are aggregated, about 10 to about 100 μm
A Bayer process aluminum hydroxide comprising secondary agglomerated particles of is used.

In the present invention, the raw material aluminum hydroxide is first subjected to dehydration pyrolysis by instantaneous calcination. The term "instantaneous calcination" as used herein means that aluminum hydroxide as a raw material is 400 ° C to 1200
Contact for 0.1 seconds to 5 minutes under a temperature atmosphere of ℃, usually about 500 ℃ to about 1200 ℃, linear velocity of about 5 m / s to about 50
The raw material aluminum hydroxide is entrained in a hot air flow of m / sec and the firing is performed for a contact time of about 0.1 seconds to about 10 seconds. Alumina after such firing has a rehydration capacity (100 ° C or lower) at 100 ° C or lower.
Any transitional alumina having a rehydration ability of about 5% or more of the alumina present in the reaction zone at the following temperature and being coexistent with water is said to have rehydration ability, In general, the crystal phase in the obtained alumina after firing is substantially χ (kai), ρ (low), or amorphous alumina and does not substantially contain α-alumina (about 5% or less). And the BET surface area is about 150 to about 3
It may be 50 m ² / g, usually in the range of about 200 to about 350 m ² / g of transition alumina.

The structure of the firing is not particularly limited as long as it is a firing furnace capable of firing under the above-mentioned conditions. Normally, an airflow type firing furnace is used in which aluminum hydroxide is introduced into hot air for firing. It Alternatively, a method may be used in which aluminum hydroxide is charged into the flame and fired. The hot air may be directly heated by fuel or indirectly heated.

The transition alumina powder after the instantaneous calcination is usually separated and recovered from the air stream by a known method such as a cyclone, a bag filter and an electrostatic precipitator. The separation temperature from the air flow is determined by the inlet temperature of the raw material powder, the heat radiation of the calcined portion, the material of the separated portion, etc., but in the practice of the method of the present invention, the alumina after the instantaneous calcination has rehydration ability. The transition temperature is not particularly limited as long as it is a transition alumina powder.

The transitional alumina powder after the instant calcination is then α
Baking is performed until the conversion rate becomes 80% or more, preferably 90% or more. The calcination may be any calcination that can convert the transition alumina powder into α, and examples of the calcination device include a continuous calcination furnace such as a rotary kiln and a tunnel kiln, a batch calcination furnace, and an airflow calcination furnace. The firing temperature and time are not particularly limited, but generally the firing temperature is about 900 to about 15.
It is carried out at 00 ° C., preferably about 1000 to about 1300 ° C. for a period of about 1 hour to about 5 hours. When the α-conversion rate of the alumina after firing is less than 80%, when used as an alumina raw material powder for sintering, shrinkage during sintering of the molded body facilitates the generation of pores and deteriorates the sintered body characteristics. . In the alumina powder after firing, the primary particles are lightly fixed between the particles in the firing process to form agglomerated particles, so it is preferable to crush the agglomerates with a known pulverizer such as a ball mill, a vibration mill, or a media stirring mill. .

The alumina powder of the present invention obtained by firing in this manner has an α-conversion rate of about 80% or more, usually about 90% or more, and an average secondary particle diameter of about 1 μm or less, and +1 μm or more. The α-alumina powder has a coarse particle content of 10% by weight or less, a rosin-lamellar gradient of 2.5 or more, excellent crushability, few coarse particles, and a sharp particle size distribution.

Further, when the particle shape of the α-alumina powder obtained or the α-alumina powder with grown crystals is desired, a mineralizing agent which exerts an effect matching the purpose at the time of firing after the instantaneous calcination is added. Baking is recommended. Examples of such a mineralizer include a fluorine compound, a boron compound and a chlorine compound, which are used alone or simultaneously. The amount of these mineralizers added is 0.01% by weight to about 0.5% as F in the case of a fluorine compound with respect to the raw material alumina.
% By weight, 0.01% by weight as B for boron compounds
~ About 0.5% by weight, 0.0 in the case of chlorine compounds as Cl
5% to 2% by weight is suitable. The form of addition of these mineralizers may be contained in the starting aluminum hydroxide in advance, or may be added during firing. When α-alumina powder having a low soda content is desired, a conventionally known method, for example, using low-soda aluminum hydroxide as a raw material, or after primary firing and / or secondary firing It is also possible to wash the alumina powder with water or pickle. The α-alumina powder obtained by calcination in the presence of the mineralizing agent also has an α-conversion rate of about 80% or more, usually about 90% or more, and a rosin-Ramura-gradient of crushing of 2.5 or more. Α-alumina powder with excellent properties, few coarse particles and sharp particle size distribution.

As described in detail above, in the present invention, it is essential that the raw material aluminum hydroxide obtained by the Bayer method is instantaneously calcined to give transitional alumina and then further calcined to obtain α-alumina powder. Easy to sinter with a sharp particle size distribution, even without firing through dehydration pyrolysis by instantaneous firing, even when fired under firing conditions with a relatively slow heating rate (about 100 ° C / min or less), such as in ordinary electric furnaces and rotary kilns. Alumina cannot be obtained. Also, when α-alumina powder is obtained by using a rotary kiln equipped with a suspension preheater, there are few coarse particles and the α-particle has a sharp particle size distribution.
Alumina powder cannot be obtained. In the present production method, it is not clear why the α-alumina powder is easily disintegratable, has few coarse particles, and has a sharp particle size distribution. Since a myriad of cracks are generated in the direction perpendicular to the C axis of alumina, it is presumed that the α-aluminized alumina that has passed through such transition alumina can be easily crushed by a simple crushing treatment. . Also, in the case of a rotary kiln with a suspension preheater, because of the structure of the equipment, the fine alumina circulates in the system, so that the alumina burned in the rotary kiln is already α
It is considered that α-alumina obtained after firing does not have α-alumina powder having a large number of coarse particles and a sharp particle size distribution, as in the method of the present invention, because the converted alumina is made of alumina having a different degree of firing.

[0017]

EFFECT OF THE INVENTION According to the method of the present invention described in detail above, it is easy to disintegrate and the number of coarse particles is small by the relatively simple operation of instantaneously firing aluminum hydroxide and then firing it under normal conditions. In addition, α-alumina powder having a sharp particle size distribution can be obtained, and it is most suitable for fine ceramics, special abrasives, amorphous refractories, etc., and its industrial value is enormous.

[0018]

EXAMPLES The method of the present invention will be described in more detail below.
The examples are one embodiment of the method of the present invention and are not intended to limit the method of the present invention. In the method of the present invention, the α-ized rate, BET specific surface area, central particle size, and particle size distribution of aluminum hydroxide after firing were measured by the following methods.

Α-factor; powder X-ray diffraction method (Rigaku Denki Co., Ltd. rotor flex RAD-BCuK α-ray (11
6) Obtained from the diffraction line)

BET specific surface area; nitrogen gas adsorption method (Betasorb automatic surface area meter model 420 manufactured by Nikkiso Co., Ltd.)
0).

Central particle diameter: X-ray transmission method (Semigraph 5100, particle size distribution analyzer manufactured by Micromeritics)
It was measured by.

+1 μm: Cumulative weight% of particles having a particle size of 1 μm or more in the particle size distribution measured by the X-ray transmission method (manufactured by Micromeritics Co., Ltd., particle size distribution analyzer SEDIGRAPH 5100).

Particle size distribution (n value): The particle size distribution measured by an X-ray transmission type particle size distribution analyzer is Rosin-Ramura-
Rammler) This is the slope when plotted on the diagram and is displayed in tan θ.

Example 1 Aluminum hydroxide obtained by the Bayer method [central particles
Diameter (average secondary particle diameter) 50 μm, Na₂O content 0.2
% By weight] was instantly fired in a gas stream furnace. Inlet temperature in air flow furnace
Is 800 ° C, outlet temperature is 500 ° C, aluminum hydroxide
Retention time of about 0.5 seconds (combustion gas linear velocity 15m / second)
Met. Primary calcined alumina obtained by flash calcining
Has a BET specific surface area of 300 m²/ G, crystalline phase is α
There was no mina and was essentially χ and ρ alumina. Next
The primary calcined alumina (transition
Lumina) into a high-alumina sheath, and in a small electric furnace,
Firing (secondary firing) was performed at 1250 ° C. for 2 hours. Obtained
The alpha conversion of alumina is 90%, and the BET specific surface area is 5.5.
m²/ G. Alumina 3 thus obtained
50 g and 2950 g of 15 mmφ alumina balls 3.
Enclosed in a 3 liter alumina pot, rotation speed 80r
Dry milled at pm for 24 hours. Aluminum obtained after crushing
As a result, the central particle size (D ₅₀) Is 0.30μ
m, D₉₀/ D₅₀Is 1.9, the particle size distribution (n value) is 2.7,
The ratio of coarse particles of +1 μm or more was 4.0% by weight.
It was

Example 2 Instantly fired alumina (BE obtained in the same manner as in Example 1)
T specific surface area is 300m ²/ G, crystalline phase is χ and ρ aluminum
The heating temperature was changed to 1300 ° C in a small electric furnace.
Others were the same as in Example 1 and fired for 2 hours. Obtained a
Lumina alpha conversion is 98%, BET specific surface area is 3.5m²
/ G. In addition, after crushing treatment in the same manner as in Example 1,
When the obtained alumina was measured, the median particle diameter (D₅₀)
Is 0.45 μm, D₉₀/ D₅₀Is 1.7 and the particle size distribution (n
Value) is 2.8, and the ratio of coarse particles of +1 μm or more is 3.0
% By weight.

Comparative Example 1 The same aluminum hydroxide as used in Example 1 was placed in a high alumina sheath and heated to 500 ° C. at a heating rate of 1.5 ° C./min in a small electric furnace and heated for 2 hours. After holding, it was cooled to room temperature. The obtained alumina has a BET specific surface area of 10
0 m ² / g, the crystal phase was about 50% χ alumina, about 40% γ alumina, and the balance was boehmite, which had no rehydration ability at a temperature of 100 ° C. or lower. Then, this primary calcined alumina was put into a high alumina-like sheath as in Example 1 and secondarily calcined at 1200 ° C. for 2 hours in a small electric furnace. The alpha conversion of the obtained alumina is 90%, BET
The specific surface area was 6.0 m ² / g. Further, when the alumina obtained after pulverization was measured in the same manner as in Example 1, the central particle size (D ₅₀ ) was 0.29 μm, the D ₉₀ / D ₅₀ was 3.4, and the particle size distribution (n value) was The ratio of coarse particles of 1.2 or +1 μm or more was 10.0% by weight.

Comparative Example 2 Alumina was obtained in the same manner as Comparative Example 1 except that the secondary firing temperature was 1300 ° C. and 2 Hr. Α of the obtained alumina
The conversion was 94% and the BET specific surface area was 4.5 m ² / g. Further, when the alumina obtained after pulverization was measured in the same manner as in Example 1, the median particle diameter (D ₅₀ ) was 0.36.
μm, D ₉₀ / D ₅₀ is 2.8, and the particle size distribution (n value) is 2.
The proportion of coarse particles of 0, +1 μm or more was 9.0% by weight.

Comparative Example 3 Alumina was obtained in the same manner as in Example 1 except that the secondary firing temperature was 1100 ° C. The alpha conversion of the obtained alumina is 7
The BET specific surface area was 0% and was 8.9 m ² / g. Further, when the alumina obtained after pulverization was measured in the same manner as in Example 1, the median particle diameter (D ₅₀ ) was 0.25 μm,
D ₉₀ / D ₅₀ is 2.2, particle size distribution (n value) is 2.3, +1
The proportion of coarse particles of μm or larger was 8.0% by weight.

Comparative Example 4 A rotary kiln with a suspension preheater,
Aluminum hydroxide having a central particle diameter of 50 μm and a Na ₂ O content of 0.2 wt% was fired. The suspension pre-heater section consists of three cyclone heat exchangers and a duct connecting them, and uses combustion gas from a rotary kiln with a gas temperature of 1100 ° C (maximum) and a duct section linear velocity of 10 m / sec to heat it. Then ambient temperature 1300
Secondary firing was performed using a rotary kiln at ℃ to produce α-alumina. The alumina collected from the suspension preheater at the inlet of the rotary kiln had a BET specific surface area of 130 m ² / g, the crystal phase contained about 30% α-alumina, and the balance was χ and ρ alumina. The alpha conversion rate of alumina obtained by firing in a rotary kiln is 90%, BE
The T specific surface area was 4.0 m ² / g. In addition, Example 1
When the alumina obtained after pulverizing in the same manner as above was measured, the central particle size (D ₅₀ ) was 0.60 μm, the D ₉₀ / D ₅₀ was 4.2, and the particle size distribution (n value) was 2.2, +1 μm. The ratio of the above coarse particles was 25.0% by weight.

Example 3 In Example 1 and Comparative Example 1, when measuring the physical properties of alumina after the secondary calcination, the grinding time in a ball mill was shortened to 10 hours, and the obtained alumina was measured. In Example 1, the central particle size (D ₅₀ ) is 0.53 μm, and D _{90 is}
/ D ₅₀ is 1.8, particle size distribution (n value) is 2.9, +1 μm
The ratio of the above coarse particles was 8% by weight, Comparative Example 1 had a median particle diameter (D ₅₀ ) of 0.41 μm, and D ₉₀ / D ₅₀ of 3.2.
The particle size distribution (n value) was 2.4, and the ratio of coarse particles of +1 μm or more was 15% by weight. The results show that the alumina powder obtained by the method of the present invention is easily disintegrated.

Claims (2)

[Claims] 1. Aluminum hydroxide obtained by the Bayer method is exposed to an atmosphere of a temperature of 400 ° C. to 1200 ° C. for 0.1 seconds to.
A method for producing α-alumina powder, which comprises contacting for 5 minutes to perform instantaneous calcination to obtain transition alumina, and then firing the transition alumina until the α-conversion rate becomes 80% or more. 2. The instantaneous calcination is performed by using a gas stream type kiln, and the instantaneous calcination is performed so that the α-alumina content in the crystal phase in the obtained alumina is 5% or less. 1. A method for producing α-alumina powder.