JP3087403B2 - Method for producing spherical alumina - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing finely divided spherical alumina having a size of 3 .mu.m or less, which is easily crushed, using a Bayer-processed aluminum hydroxide as a raw material.

[0002]

2. Description of the Related Art Alumina powder is widely used as a raw material for producing various ceramic products, an abrasive, a refractory or various fillers. In any of these applications, the required quality of the raw material alumina is becoming stricter year by year. For example, in manufacturing alumina sintered bodies such as IC substrates, fine-grained spherical alumina is required in order to obtain high-density and excellent uniformity, and abrasives have surface defects (such as scratches). To reduce the amount, similar fine and spherical alumina is required. However, heretofore, for these applications, finely pulverized alumina obtained by continuously firing the Bayer method aluminum hydroxide in one step has been applied. Such alumina is manufactured through a crushing operation and a crushing operation in order to change the state from several tens μm to one hundred and several tens μm to several μm. However, there are problems in that the pulverizing and pulverizing operations require a long time, so that the production cost increases, impurities are mixed, and the obtained alumina particles are often irregular in shape and size. Therefore, various methods have been proposed, such as a method of calcining aluminum hydroxide obtained in the form of fine particles at the precipitation stage, and a method of calcining aluminum hydroxide having a large particle size once and then calcining it. The manufacturing method has not been established yet. The present inventors have studied the calcination process of aluminum hydroxide, and after regulating the rate of thermal decomposition, divide the calcination process after pyrolysis into a step of obtaining intermediate calcined alumina and a crystal growth step of the intermediate calcined alumina. Thereby, the knowledge that the shape of alumina can be controlled to some extent and that the particle size of alumina after recalcination is affected by the total soda content in the intermediate calcined alumina have been obtained, leading to the present invention. Was.

[0003]

SUMMARY OF THE INVENTION The present inventors have determined that the conditions for producing the raw material aluminum hydroxide are appropriate and that the method of controlling the particle size can be controlled by improving the firing method. An object of the present invention is to propose a method for producing spherical alumina having a narrow particle size distribution. More specifically, the present invention performs the thermal decomposition of the raw material aluminum hydroxide to intermediate alumina slowly, and controls the total soda content before and after the intermediate calcination step of producing an intermediate calcination alumina having a degree of gelatinization of 90% or more. The intermediate calcined alumina is re-calcined in the presence of a fluorine mineralizer to divide it into a re-calcination stage in which the particle size is controlled, and then fine spherical alumina can be obtained by performing a conventional crushing treatment. It provides a possible manufacturing method. Then, in that case, the total soda content in the raw material aluminum hydroxide is adjusted to a predetermined range from the beginning, or the total soda content is adjusted to a predetermined value by the effect of adding the desolder agent in the firing process after thermal decomposition. It is proposed to control the total soda content in the intermediate calcined alumina by means for adjusting to a range or means for washing the intermediate calcined alumina with hot water.

[0004]

That is, the present invention provides an intermediate calcined alumina having a total soda content of 0.5% by weight.
A Bayer method aluminum hydroxide having a soda content of below 700 ° C. at a rate of 20 ° C. or less per minute.
After dehydration and pyrolysis, the mixture is heated and heated at an intermediate temperature of 1400 ° C. or lower to obtain an intermediate fired alumina having a degree of α of 90% or more. A method for producing spherical alumina, comprising recalcining at 600 to 1500 ° C. in the presence of a fluorine mineralizer in a proportion of from 0.5 to 1.0 part by weight to control the particle size, and thereafter dissolving by a conventional method. Crush. Further, particularly when it is desired to obtain fine and spherical alumina by controlling the total soda content in the intermediate calcined alumina to 0.2% by weight or less, it is necessary to reduce the influence of soda in the recalcining step. As a first method, in the present invention, in the intermediate calcination treatment, the intermediate calcination is performed by adding a soda removal agent so that the total soda content in alumina after the intermediate calcination becomes 0.2% by weight or less, After the intermediate calcined alumina is obtained, the above-described recalcination is performed, and a method for producing spherical alumina is employed. Also, as a second method,
After the intermediate calcination treatment as it is or after the intermediate calcination treatment with the addition of the soda removal agent, washing with warm water is performed to reduce the total soda content to 0.1.
A method for producing spherical alumina is characterized in that recalcining is performed after obtaining an intermediate calcined alumina controlled to 2% by weight or less. Hereinafter, the present invention will be described specifically. Here, the total soda content (hereinafter, simply referred to as “soda content”) is the sum of the soda component bonded to alumina and the soda component simply attached in a free state, and is calculated in terms of alumina. And the degree of α-formation indicates the proportion of α-alumina in the total alumina.

The raw material aluminum hydroxide used in the present invention is a gibbsite type aluminum hydroxide produced by the Bayer method which is industrially mass-produced and available at low cost and has three molecules of water of crystallization. Can provide a spherical alumina. The particle size is not particularly limited, but is usually several tens to one hundred and several tens μm
Can be applied for general purposes. Further, in order to obtain an intermediate calcined alumina having a controlled soda content, an aluminum hydroxide prepared from the beginning to have a soda content of 0.5% by weight or less (in terms of alumina) may be used as it is, or the soda content may be reduced. A mixture obtained by mixing different raw materials of aluminum hydroxide and adjusting the total to 0.5% by weight or less is used. In this case, if the soda content exceeds 0.5% by weight, it is difficult to obtain fine spherical alumina with good yield and stably even after the firing step in the present invention, which is not preferable. Such aluminum hydroxide is dehydrated and pyrolyzed at 700 ° C. or lower, preferably 500 to 600 ° C., to obtain an intermediate alumina. At this time, the present invention is carried out at a low heating rate. That is, the rate of temperature increase until the aluminum hydroxide is thermally decomposed into intermediate alumina, particularly γ-alumina, is set to 2 / min.
0 ° C. or lower, preferably 2 to 15 ° C. and the upper limit is 70 ° C.
The temperature is set to 0 ° C. or less. Further, in that case, if necessary, a two-stage or multi-stage heating curve is used such that a heating rate of 10 to 20 ° C. per minute up to about 200 ° C. and a heating rate of 2 to 10 ° C. per minute thereafter Is also good. As a result, the pore diameter of the generated intermediate alumina is increased, and the spherical particles can be easily obtained by recalcining in the presence of the fluorine mineralizer.

The thermally decomposed intermediate alumina is then heated to 1400 ° C. or lower, preferably 1100 to 1300 ° C.
Intermediate firing at 30 ° C. for 30 minutes to 5 hours gives an intermediate fired alumina having a degree of gelatinization of 90% or more. This firing is carried out in a furnace connected with a heating furnace of the same type as the pyrolysis treatment or in an independent furnace,
The heating rate may be any appropriate one. In the intermediate baking step, if the degree of pregelatinization is 90% or less, the growth of coarse plate-like particles tends to occur in refiring, which is not preferable. That is appropriate. Further, in the intermediate firing, in order to develop uniform crystal growth by refiring in the next step, it is necessary to stop only the formation of intermediate fired alumina having a degree of gelatinization of 90% or more, and the firing condition is α- Since the alumina needs to be in a stage where it stops in an ungrown state, when the heating temperature is high, the calcination is preferably performed for a short time, for example, at 1300 ° C. for 5 hours or less.

[0007] The intermediate calcined alumina thus obtained is then reprocessed at 600 to 1500 ° C in the presence of a fluorine mineralizer in a proportion of 0.05 to 1.0 parts by weight per 100 parts by weight of alumina in terms of fluorine. Baking and crystal growth. As the fluorine mineralizer, a fluorine compound such as hydrofluoric acid, aluminum fluoride, sodium fluoride and cryolite is preferably used. Although chlorine-based mineralizers such as chlorine gas and hydrochloric acid are equally applicable, they are inferior due to problems such as the persistence of the effect of solid addition. The amount of addition of the fluorine mineralizer is 100 parts by weight in terms of fluorine, taking into account the scattering of fluorine compounds in the firing furnace due to the decomposition and volatilization of the fluorine compounds, or the concentration with soda in alumina. 0.05 to 1.0 part by weight, more preferably 0.07 to
It is preferably added in a proportion of 0.5 parts by weight. In this case, if the amount is less than 0.05 parts by weight, there is no effect of addition,
Even if added in excess of parts by weight, no improvement in the effect is observed. When hydrofluoric acid is used in a gaseous state, the concentration is controlled by the atmosphere concentration in the furnace, but becomes about 0.5 to 10% by volume as hydrofluoric acid near the alumina particles. It is preferable to control the concentration so that the higher the firing temperature, the lower the concentration. Refiring treatment is 600-1
500 ° C., more preferably 1000 to 1400 ° C.
Heat treatment for minutes to 2 hours. In this case, when heated to 1500 ° C. or more, crystal growth becomes remarkable and it is difficult to control the particle size, and it is not appropriate to obtain spherical alumina having fine particles of 3 μm or less, while crystallization hardly proceeds at 600 ° C. or less. It is not preferable. The firing furnace in each of the above steps may be any type of heating furnace, such as a continuous type, a flow type, a batch type, or a burner furnace or an electric furnace, as long as the heating conditions can be satisfied. , Tunnel kiln, shuttle kiln, etc. are applied. After re-firing, the powder is crushed by a suitable crusher, but the crushed material obtained through the above-mentioned steps can be easily crushed. For example, it is processed by a ball mill or the like. The calcined alumina obtained by the above steps is fine alumina particles having a narrow particle size distribution which are easily crushed in a short time.

When the treatment is carried out at the above-mentioned concentration of the fluorine mineralizer, it varies depending on the calcination conditions. However, when the treatment is carried out under general-purpose conditions, if the soda content in the intermediate calcination alumina is controlled to 0.15% by weight or less, it is easy. It is possible to obtain spherical alumina having a particle size of 0.5 to 2.0 μm and a soda content of 0.1 to 2.0 μm.
1.5 to 2.5 μm when controlling to 5 to 0.3% by weight
When the spherical alumina of m is controlled to 0.3 to 0.5% by weight, the spherical alumina of 2.0 to 3.0 μm can be easily obtained. Therefore, in order to stably obtain spherical alumina of 2.0 μm or less using aluminum hydroxide having a high soda content, it is necessary to control the soda content in the intermediate calcined alumina to 0.2% by weight or less. Therefore, as a first method, according to the present invention, in the intermediate calcination treatment, a soda removal agent is added so that the soda content in alumina after the intermediate calcination becomes 0.2% by weight or less. Intermediate firing is performed in the same manner as described above, and re-firing is performed. The method of using a silica-based material such as mullite, chamotte, alumina silicate, or silica sand or a molded ball as a silica source is a simple method for removing the soda. The mullite grains are added. As another method, it is also preferable to add impregnated hydrochloric acid at a rate of 0.5 to 2% by weight in the thermal decomposition step so that the soda can be easily volatilized and then add mullite grains or the like before the intermediate firing. As a result, for example, the soda content is transferred to the mullite grains, so that the soda treatment in the alumina can be performed. Then, after the completion of the intermediate firing or after the refiring, the mullite grains are removed by the sieving means. As a second method for obtaining spherical alumina having a diameter of 2.0 μm or less stably, a method of performing soda treatment by washing with warm water after a normal intermediate calcination treatment, and then performing re-calcination through a drying step is adopted. is there. In this case, the hot water washing is performed by repulping the intermediate calcined alumina with hot water at 70 to 100 ° C. By these methods, it is usually the case that
The content of soda can be 1% by weight or less, which facilitates generation of fine and spherical alumina at the time of refiring.

[0009]

The reason why the method of the present invention produces calcined alumina which can be easily crushed has not been sufficiently elucidated yet, but is considered as follows. That is, the Bayer aluminum hydroxide thermally decomposed at a slow heating rate becomes intermediate alumina particles having large micropores while maintaining the original aluminum hydroxide. Then, when the material is turned into α by intermediate firing, an uncrystallized growth of intermediate fired alumina having a rough structure in the form of a cocoon is obtained. When this uncrystallized growth is recalcined in the presence of a mineralizer, the connection between primary particles is broken by mass transfer through the gas phase, and aggregate crystals of highly independent spherical alumina particles with relatively uniform particle size are obtained. It is presumed that it will be easily broken up because it grows. In addition, if the intermediate calcined alumina having a high soda content is used in the recalcining process, the soda volatilized there captures the fluorine ions from the mineralizer and stays in the high-temperature atmosphere, and the fluorine ions react with the alumina crystal surface by the fluorine ions. It promotes the difference in growth action and changes the shape and particle size of alumina. However, when the treatment is performed in the quantitative region in the present invention, the crystal growth is performed uniformly without the directionality of the crystal growth, so that spherical alumina crystals grow. It is inferred. According to the method of the present invention, since the soda component contained in the raw material aluminum hydroxide is removed in the intermediate de-soda process and re-sintering process, the effect of obtaining a spherical alumina having a low soda content is also exhibited. Is done.

[0010]

The present invention will be described in more detail with reference to the following Examples and Comparative Examples, but it should not be construed that the invention is limited thereto. Further, in the following Examples and Comparative Examples, processing and measurement were performed in common under the following conditions. That is, α of calcined alumina
The degree of conversion is determined by the X-ray diffraction method (Geigerflex R manufactured by Rigaku Denki).
(104) ・ (11) with AD-IIIA CuKa wire)
3) It was determined from the diffraction line of the (116) plane. In addition, the disintegration is performed by a table type planetary ball mill (P-7 manufactured by Flitch Japan).
Pulverizing container: 12 ml made of sintered alumina Ball: 12 g of alumina ball having a diameter of 5 mm Sample: 0.0005 mol / l of sodium pyrophosphate
2.5 g of the sample was placed in 2.5 g of the aqueous solution. Rotation speed: Wet crushing was performed for 40 minutes under the conditions of a container of 470 rpm and a bottom plate of 1900 rpm. The particle size distribution after crushing was measured by a sedigraph method, the average particle size was determined, and the particle shape was observed with a scanning electron microscope.

Example 1 Gibbsite type aluminum hydroxide obtained by the Bayer method (BLU72, trade name, manufactured by Nippon Light Metal Co., Ltd., total soda content: 0.1%).
13% by weight and an average particle diameter D _{P50 of} 100 μm) was heated to 600 ° C. at a rate of 5 ° C./min in a siliconite-type electric furnace, and was thermally decomposed by holding at that temperature for 60 minutes. Subsequently, the temperature was raised to 1250 ° C., maintained at that temperature for 1 hour, and intermediately calcined to obtain intermediate calcined alumina (having a degree of pregelatinization of 100%). Aluminum fluoride was added and mixed at a rate of 0.10 part by weight in terms of fluorine with respect to 100 parts by weight of the intermediate calcined alumina, and again heated at 1250 ° C. in a siliconite type electric furnace.
For 1 hour. The obtained calcined alumina after crushing is spherical alumina having an average particle diameter D _{P50 of} 1.6 μm.
The soda content was 0.03% by weight. Example 2 Gibbsite type aluminum hydroxide obtained by the Bayer method (trade name BN52, manufactured by Nippon Light Metal Co., Ltd .; total soda content: 0.2)
The treatment was carried out under the same conditions as in Example 1 except that the average particle diameter D _P50 was 5 μ% and the _refiring temperature was 1300 ° C. Those of disintegrated obtained calcined alumina in spherical alumina having an average particle diameter _D P50 2.2 .mu.m, soda content was 0.08 wt%.

Example 3 Gibbsite-type aluminum hydroxide obtained by the Bayer method (trade name B52, manufactured by Nippon Light Metal Co., Ltd .; total soda content: 0.35)
(Average particle diameter _DP50 40 μm in weight%) and the same conditions as in Example 1 except that the refiring temperature was 1300 ° C. The obtained calcined alumina after crushing was spherical alumina having an average particle diameter D _{P50 of} 2.6 μm and a soda content of 0.10% by weight. Example 4 Gibbsite type aluminum hydroxide obtained by the Bayer method (trade name B52, manufactured by Nippon Light Metal Co., Ltd .; total soda content: 0.35)
After adding 1% by weight of hydrochloric acid to the average particle diameter D _{P50 (} 40 μm in terms of% by weight) in terms of alumina, the temperature was raised to 600 ° C. at a rate of 7 ° C./minute in a siliconite type electric furnace. It was thermally decomposed by holding and heating for 1 minute. Then, after adding 8% by weight of the mullite grains to the alumina, the mixture was held at 1250 ° C. for 1 hour in a siliconite-type electric furnace and was subjected to intermediate firing to obtain intermediate fired alumina (having a degree of α of 100%). After sieving the mullite grains, 0.10 parts by weight of aluminum fluoride was added and mixed with 100 parts by weight of the intermediate calcined alumina in terms of fluorine, and the mixture was again placed in a siliconite-type electric furnace.
It was kept at 00 ° C. for 45 minutes and fired again. The obtained calcined alumina after crushing was spherical alumina having an average particle diameter D _{P50 of} 1.3 μm and a soda content of 0.07% by weight.

Example 5 100 parts by weight of the intermediate calcined alumina obtained in Example 3 was repulped and washed with 300 parts by weight of hot water at 90 ° C.
After filtration and separation, further substituting and washing with 200 parts by weight of warm water and air-drying, the same re-firing treatment as in Example 3 was performed. The obtained calcined alumina after crushing has an average particle diameter D _{P50 of} 1.5 μm.
m spherical alumina having a soda content of 0.08% by weight.

Comparative Example 1 Gibbsite type aluminum hydroxide obtained by the Bayer method (BLU72, trade name, manufactured by Nippon Light Metal Co., Ltd., total soda content: 0.1%).
13% by weight and an average particle diameter D _{P50 of} 100 μm) was heated to 600 ° C. at a rate of 5 ° C./min in a siliconite-type electric furnace, and was thermally decomposed by holding at that temperature for 60 minutes. Subsequently, intermediate calcination was carried out so that the degree of pregelatinization became 30%, to obtain an intermediate calcined alumina. To 100 parts by weight of this intermediate calcined alumina, 0.10 parts by weight of aluminum fluoride was added and mixed in terms of fluorine, and then mixed again in a siliconite type electric furnace.
It was kept at 50 ° C. for 1 hour and fired again. The obtained calcined alumina after crushing is alumina having an average particle diameter D _{P50 of} 2.5 μm, most of which are hexagonal plate-like particles, spherical alumina is extremely small, and its soda content is 0.05% by weight. Met. Comparative Example 2 Gibbsite type aluminum hydroxide obtained by the Bayer method (trade name BN52, manufactured by Nippon Light Metal Co., Ltd .; total soda content: 0.2
Thermal decomposition was carried out under the same conditions as in Comparative Example 1 using an average particle diameter ( _{DP50 of} 35 μm at 5% by weight). Subsequently, the degree of alpha conversion is 30
%, To obtain an intermediate calcined alumina, and then recalcined under the same conditions as in Comparative Example 1. The resulting calcined alumina after crushing was alumina having an average particle size of 3.2 μm, most of which were hexagonal plate-like particles, the amount of spherical alumina was extremely small, and the soda content was 0.08% by weight. Was. Comparative Example 3 Gibbsite-type aluminum hydroxide obtained by the Bayer method (trade name: B52, manufactured by Nippon Light Metal Co., Ltd .; total soda content: 0.35)
Using an average particle diameter ( _{DP50 of} 40 μm in weight%), thermal decomposition was performed under the same conditions as in Comparative Example 1. Subsequently, the degree of pregelatinization is 30%
After intermediate firing to obtain intermediate fired alumina,
It was refired under the same conditions as in Comparative Example 1. After crushing of the obtained calcined alumina, alumina having an average particle size of 3.8 μm,
Most of the particles were hexagonal plate-like particles, the amount of spherical alumina was extremely small, and the soda content was 0.12% by weight.

[0015]

According to the present invention, the particle size distribution is narrowed by a simple means of improving the firing process, and the particle size distribution is narrow.
For example, spherical alumina having an average particle size of 0.5 to 3 μm and having a low soda content can be obtained. Since it can be used for various purposes, it is possible to obtain industrially excellent economic effects. I can do it. Further, for example, when it is used as a sintered body, it is possible to obtain a high-density and uniform body having excellent sinterability.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-59-97528 (JP, A) JP-A-62-46922 (JP, A) JP-A-3-232719 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) C01F 7/44

Claims (3)

(57) [Claims] 1. A Bayer-processed aluminum hydroxide having a soda content such that the total soda content becomes 0.5% by weight or less when converted into an intermediate calcined alumina at a temperature rising rate of 20 ° C. or less per minute at 700 ° C. After dehydration and pyrolysis, the mixture is heated and heated at an intermediate temperature of 1400 ° C. or lower to obtain an intermediate fired alumina having a degree of α of 90% or more. 05-
600 to 600 parts by weight in the presence of 1.0 part by weight of a fluorine mineralizer
A method for producing spherical alumina, wherein the particle size is controlled by refiring at 1500 ° C. 2. In the intermediate calcination treatment, an intermediate calcination was performed by adding a soda removal agent so that the total soda content in the intermediate calcination alumina was 0.2% by weight or less, and the total soda content was controlled. The method for producing spherical alumina according to claim 1, wherein an intermediate calcined alumina is obtained. 3. The method according to claim 1, wherein after the intermediate calcination treatment, washing with warm water is performed, and after the intermediate calcination alumina whose total soda content is controlled to 0.2% by weight or less is obtained, recalcination is performed. 3. The method for producing spherical alumina according to 1.).