JP3387555B2 - Method for producing α-alumina - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is an α-type resin having a shape close to an equiaxed crystal form and excellent chipping resistance, which is obtained by firing aluminum hydroxide in two steps in the presence of a specific mineralizer.
The present invention relates to a method for producing alumina.

[0002]

2. Description of the Related Art Alumina powder is used as electronic components such as plugs and IC alumina substrates, refractories, abrasives, and various ceramic raw materials. As such an alumina powder, α-alumina powder obtained by the Bayer method is usually used because it is inexpensive and easily available. In this method, aluminum hydroxide as seeds is added to a supersaturated sodium aluminate solution to precipitate aluminum hydroxide, which is then filtered and washed with water, and then a rotary kiln, a fluidized drying firing furnace or a tunnel kiln is used. Calcination is performed in a calcination facility at a temperature of about 1000 ° C to 1500 ° C to obtain α-alumina.

In general, for the purpose of obtaining α-alumina at a low temperature or in a short time, it is known to carry out calcination in the presence of a so-called mineralizer such as a small amount of a fluorine compound, a boron compound or a chlorine compound. However, since α-alumina produced by this method is conventionally formed from a plate-shaped product in which crystals are developed in the direction perpendicular to the C-axis, it is necessary to use an abrasive, a fluidity or a filling property, such as a resin filler or It was disadvantageous compared with other inorganic powders as a raw material for a ceramic molded body that requires molding accuracy.

Further, since the alumina powder forms secondary agglomerated particles in which individual primary particles are firmly bonded and agglomerated in the firing process from aluminum hydroxide as a raw material to α-alumina, it is used as a raw material for the above-mentioned use. When provided, a ball mill, a vibration mill, a bead mill or the like is usually used to pulverize the particles to a desired particle size. In the pulverization and mixing process, not only the secondary agglomerated particles but also the primary particles themselves are ground (chipping). Fine particles are generated in the. When such fine particles are used as a raw material for a ceramic molded body, they hinder the outgassing of the binder in the firing process, which lowers the sintering density and causes defects (voids).
Further, the fine particles become agglomerated particles, which causes a decrease in polishing performance and a decrease in slurry fluidity, which is not suitable for use as an abrasive or a resin filler.

Japanese Examined Patent Publication No. 4-65012 discloses that aluminum hydroxide is added to a temperature higher than that required for the conversion to α-alumina by adding a mineralizer in the form of a compound containing boron and / or fluorine. In a method for producing crystalline alumina (α-alumina) by baking; aluminum hydroxide is added at a ratio of 0.1 to _{2 based on} Al ₂ O _3.
Up to and including 0.05% by weight of Na ₂ O
Has a concentration; a mineralizer containing ammonia (NH ₄ ⁺ ) is added; at least 80% of the crystals of crystalline alumina have a size of 1 to 10 μm, preferably 3 to 8 μm, and a D / H of A method of producing crystalline alumina characterized in that the ratio is at most 2. The α-alumina obtained by the above method is C
Although spherical (dice-shaped) alumina, in which crystals are grown not only in the axis-vertical direction but also in the C-axis horizontal direction, that is, so-called equiaxed alumina is obtained, it is not satisfactory in terms of the effect of improving chipping.

[0006]

In view of such circumstances, in view of the above circumstances,
The inventors have found that the alumina crystals after firing have a size of about 1 μm to about 10 μm, and the ratio of the height H to the diameter D (D / H).
To have a spherical shape close to 1 to 3 which is relatively close to an equiaxed crystal shape, or a shape close to a dice shape, and which is excellent in chipping resistance even when pulverized and has excellent chipping resistance. As a result of intensive studies, it was found that when firing aluminum hydroxide under specific conditions, alumina satisfying the above characteristics can be obtained, and the present invention has been completed.

[0007]

That is, according to the present invention, aluminum hydroxide is calcined in the presence of a fluorine compound, the alpha conversion is 10% to less than 100%, and the F content is 50 ppm to 30%.
Alumina of 00 ppm was prepared, and then this alumina was baked in the presence of a boron compound at a firing temperature of 800 ° C to 1500 ° C, 1
Another object of the present invention is to provide a method for producing α-alumina having a D / H of 1 to 3 and an average primary particle diameter of 1 μm to 10 μm, which is characterized by firing for at least an hour.

The method of the present invention will be described in more detail below. The aluminum hydroxide used in the method of the present invention is obtained by the Bayer method. The particle shape of the raw material aluminum hydroxide is not particularly limited. In the present invention, the raw material aluminum hydroxide is calcined in the presence of a fluorine compound so that the aluminum hydroxide has an alpha conversion rate of 10% to less than 100%, preferably 20% to 90% (hereinafter, this step In some cases the firing is expressed as primary firing).

The fluorine compound may be any fluorine-containing substance known in the art as a mineralizer,
For example, HF, AlF ₃ , CaF ₂ , NaF or Na ₃
AlF ₆ or the like is used. These may be mixed and added to the raw material aluminum hydroxide, or may be quantitatively supplied into the firing furnace, and the α-conversion rate of alumina after primary firing is 10% to less than 100%, and Has an F content of 50 ppm to 3000 ppm, more preferably 10
It may be in the range of 0 ppm to 1000 ppm.

The firing method is not particularly limited, but industrially, it may be carried out in a rotary kiln or a tunnel kiln, and the firing temperature and time are not particularly limited, but generally the firing temperature is about 800 ° C to 1300 ° C. It is carried out in the range of 1 hour to 5 hours.

Alumina obtained by calcining aluminum hydroxide as a raw material in the presence of a fluorine compound until the α content is in the range of 10% to less than 100% and the F content in the range of 50 ppm to 3000 ppm is followed by the presence of the boron compound. The firing is performed at a firing temperature of 800 ° C. or higher until the alumina after firing becomes α-alumina (hereinafter, firing at this stage may be referred to as secondary firing).

The calcination is not particularly limited as long as it is the above-mentioned condition, that is, the condition that α-alumina can be obtained in the presence of a boron compound, but it may be industrially carried out in a rotary kiln, a tunnel kiln, etc., and the calcination temperature and time. Is not particularly limited, but generally, the firing temperature is about 800 ° C to 1500 ° C,
It is preferably carried out at 900 ° C. to 1200 ° C. for 1 hour to 5 hours. Industrially, it is recommended to add a boron compound continuously in the same firing furnace after firing in the presence of a fluorine compound in order to reduce the energy cost, but the invention is not limited thereto.

In the present invention, since the growth in the C-axis horizontal direction is promoted in parallel with the crystal growth in the C-axis vertical direction during firing in the presence of the boron compound, the addition of the boron compound is preliminary performed. The amount, the firing temperature, and the time may be selected to set the conditions for obtaining the desired particle shape. Generally, the higher the firing temperature,
The higher the boron concentration in the firing atmosphere, the faster the particle growth, but the average primary particle diameter is about 1 μm to about 10 μm, and the ratio of the height H to the diameter D (D / H) is 1 to 3 which is relatively spherical or dice-shaped. Α-alumina in the range of about 0.05% by weight to about 0.5% by weight, preferably about 0.1% by weight to 0.2% by weight, based on the raw material alumina, at a firing temperature of 800.
It can be obtained by firing at 1500C for 1 hour to 5 hours.

The boron compound used in the present invention is not particularly limited as long as it is used as a mineralizer in the field, but usually boric acid, calcium borate, magnesium borate, sodium borate. Etc. are used. When firing in the presence of a boron compound, it may be used in combination with a fluorine compound or a chlorine compound. The fluorine compound used here may be the same as that used in the primary firing step, such as HF, AlF ₃ , and CaF.
₂ , NaF or Na ₃ AlF ₆ is used. Further, as the chlorine compound, NaCl, NH ₄ Cl or the like is used. The amounts of boron and fluorine added as mineralizers vary depending on the particle size of α-alumina to be finally obtained, but normally, in the primary firing, the fluorine compound is about 0. 05% by weight to about 0.5
It is desirable that the fluorine compound is 0 wt% to about 0.5 wt% as F and the chlorine compound is Cl ₂ is 0 wt% to about 2 wt% with respect to the raw material alumina in the second firing.

Further, when a low soda or low boron product is desired as the α-alumina product, a conventionally known method, for example, using low soda aluminum hydroxide as a raw material, or after primary firing And / or it is possible to apply water washing or pickling to the alumina after the secondary calcination.

In carrying out the present invention, it is essential that the baking in the presence of fluorine is carried out so that the α-conversion rate of alumina after baking is in the range of 10% to less than 100% and the F content is in the range of 50 ppm to 3000 ppm. And When the α-conversion rate and the F content are not within the above ranges, the average primary particle diameter is about 1 μm to about 10 μm and the C of the crystal is C, even if the subsequent step of firing in the presence of a boron compound is performed within the above range. The ratio of the height H to the diameter D (D / H) in which the crystal growth in the horizontal axis direction is promoted is 1
.Alpha.-alumina excellent in chipping resistance (no generation of fine particles due to crushing of particles) at the time of crushing or the like can be obtained. In addition, although the reason is not clear, in the primary firing, a boron compound alone or a fluorine compound and a boron compound are used as a mineralizer in aluminum hydroxide, and this is fired under these conditions or in the secondary firing. When α-alumina is obtained by firing under the same conditions as in the method of the present invention, a thick α-alumina in which crystal growth in the horizontal direction of the C-axis is promoted is obtained, but the chipping resistance is improved. No effect is seen.

[0017]

According to the method of the present invention described in detail above, aluminum hydroxide is calcined to a specific state in the presence of a fluorine compound, and then calcined in a boron atmosphere to obtain α-alumina. By a simple operation, α-alumina having an average primary particle size of about 1 μm to about 10 μm and a D / H of 1 to 3 having a shape relatively close to an equiaxed crystal form and excellent in chipping resistance can be obtained. Then, spark plugs and ICs
Suitable for applications requiring debinding properties for alumina substrates, fine ceramic applications where defects due to agglomerated particles pose a problem, or special abrasive applications,
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 conversion after primary firing of aluminum hydroxide, the primary particle diameter of alumina, the BET specific surface area, the central particle diameter, the molding density, the crystal form D / H of alumina and the chipping resistance are as follows. It was measured by the method.

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

Primary particle diameter: Calculated from the BET specific surface area according to the following formula. Primary particle size = 6 / ρ · BET (ρ: true specific gravity of alumina 3.99 g / cm ³ )

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.

Molding density: The sample was crushed with a ball mill, pre-molded with a die uniaxial press at 200 kg / cm ² , and then molded with an isostatic press at 500 kg / cm ² to obtain a measurement sample. The density was measured by the mercury method.

Crystal form D / H: The diameter (D) and thickness (H) of particles present per unit area were measured from an electron micrograph, and the average value was calculated.

Chipping resistance: After evaluation of the chipping resistance after firing, 350 g of alumina and 2950 g of 15 mmφ alumina balls were placed in a 3.3 liter alumina pot and dry ball mill grinding was carried out at a rotation speed of 80 rpm for 24 hours. , BET specific surface area (BET1) of alumina before pulverization and BET specific surface area (BET of alumina after pulverization)
ET2) was measured, and it was used as an index of the degree of generation of fine particles by pulverization from BET2 / BET1.

Example 1 0.6% by weight of aluminum fluoride (AlF ₃ ) was added to aluminum hydroxide (secondary average particle size: about 80 μm, Na ₂ O content: 0.2% by weight) obtained by the Bayer method. ,
After mixing, the mixture was put in a high alumina sheath and baked in a small electric furnace at 1100 ° C. for 2 hours. The obtained alumina had an alpha conversion rate of 40%, BET of 50.9 m ² / g, and a fluorine content of 800 ppm. Next, 0.9% by weight of H ₃ BO ₃ was added to and mixed with the alumina after the primary calcination, and the mixture was put into a high-alumina sheath and calcined at 1300 ° C. for 2 hours in a small electric furnace. The average primary particle diameter (μm) and BET specific surface area (BET1, m ² / g) of the obtained alumina, D
After measuring / H, the chipping resistance of this alumina was measured. The results are shown in Table 1.

Example 2 0.6% by weight of AlF ₃ as a mineralizer and 0.9% by weight of H ₃ BO ₃ were added to and mixed with the alumina after the primary calcination obtained in Example 1. After that, it was put in a high alumina sheath and fired at 1300 ° C. for 2 hours in a small electric furnace. The average primary particle diameter (μm) of the obtained alumina and B
After measuring the ET specific surface area (BET 1, m ² / g) and D / H, the chipping resistance of this alumina was measured. The results are shown in Table 1.

Examples 3 and 4 Using 0.1% by weight of AlF ₃ as a mineralizer in the primary calcination, the calcination temperature was 1150 ° C. in Example 3 and 1 in Example 4.
Primary firing and secondary firing were carried out in the same manner as in Example 1 except that the temperature was set to 200 ° C. The alpha conversion of the alumina obtained after the primary calcination was 27% in Example 3, 81% in Example 4, the BET specific surface area was 28.6 m ² / g in Example 3, and 3.7 m ^{2 in} Example 4.
/ G, the fluorine content of Example 3 was 130 ppm, Example 4 was 120 ppm. The average primary particle diameter (μm) and BET specific surface area (B
ET1, m ² / g) and D / H were measured, and then the chipping resistance of this alumina was measured. The results are shown in Table 1.

Comparative Example 1 The same raw material aluminum hydroxide used in Example 1 was mixed with Al.
After 0.6 wt% of F ₃ was added and mixed, the mixture was put into a high alumina sheath and fired at 1300 ° C. for 2 hours in a small electric furnace. The average primary particle diameter (μm) of the obtained alumina and B
After measuring the ET specific surface area (BET 1, m ² / g) and D / H, the chipping resistance of this alumina was measured. The results are shown in Table 1.

Comparative Example 2 The same raw material aluminum hydroxide used in Example 1 was added with H ₃
After 0.9 wt% of BO ₃ was added and mixed, the mixture was placed in a high alumina sheath and fired at 1300 ° C. for 2 hours in a small electric furnace. After measuring the average primary particle diameter (μm), BET specific surface area (BET1, m ² / g) and D / H of the obtained alumina, the chipping resistance of this alumina was measured. The results are shown in Table 1.

Comparative Example 3 The same raw material aluminum hydroxide as used in Example 1 was mixed with Al.
After adding and mixing 0.6% by weight of F ₃ and 0.9% by weight of H ₃ BO ₃ , the mixture was put in a high-alumina sheath and placed in a small electric furnace.
Firing was performed at 300 ° C. for 2 hours. The average primary particle diameter (μm) and BET specific surface area (BET1, m of the obtained alumina)
² / g) and D / H were measured, and then the chipping resistance of this alumina was measured. The results are shown in Table 1.

Comparative Example 4 Alumina was obtained in the same manner as in Example 1 except that H ₃ BO ₃ was not added during the secondary firing in the method of Example 1.
After measuring the average primary particle diameter (μm), BET specific surface area (BET1, m ² / g) and D / H of the obtained alumina,
The chipping resistance of this alumina was measured. The results are shown in Table 1.

Comparative Example 5 Example 1 was repeated except that 0.6% by weight of AlF ₃ was added and mixed in place of H ₃ BO ₃ during the secondary firing in the method of Example 1.
Alumina was obtained in the same manner as in. The average primary particle diameter (μm) and BET specific surface area (BET1, m of the obtained alumina)
² / g) and D / H were measured, and then the chipping resistance of this alumina was measured. The results are shown in Table 1.

[0034]

[Table 1]

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-59-97528 (JP, A) JP-A-5-310419 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C01F 7/44

Claims (2)

(57) [Claims] 1. Aluminum hydroxide is calcined in the presence of a fluorine compound to obtain an alumina having an alpha conversion of 10% to less than 100% and an F content of 50 ppm to 3000 ppm, and then calcined in the presence of a boron compound. Temperature 800 ℃
〜1500 ° C, D characterized by baking for 1 hour or more
/ H is 1 to 3 and the average primary particle diameter is 1 μm to 10 μm α
-Alumina production method. 2. The method for producing α-alumina according to claim 1, wherein the α-alumina conversion rate of alumina calcined in the presence of a boron compound is 20% to 90%.