JPH0741318A - Production of low soda alumina - Google Patents

Abstract

PURPOSE:To provide a producing method for a low soda alumina <=1500 deg.C in buring temp., excellent in sintering property at a low temp., small in shrinkage at the time of sintering and stable against variation of sintering temp. as the obtained product properties and capable of producing various ceramic products at a high dimensional accuracy. CONSTITUTION:The low soda alumina is produced by adding 0.02-0.3wt.% fluoride based mineralizer expressed in terms of fluorine and 0.5-10wt.% alpha-alumina <=1mum in average particle diameter into a raw material source composed of aluminum hydroxide obtained by Bayer method and/or a transition alumina and burning at <=1500 deg.C.

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 low-soda alumina, and more specifically, it has a low content of soda, is easily crushed after firing, and has a relatively low content. The present invention relates to a method for producing low-soda alumina that can be sintered at a temperature and can stably obtain low alumina in a wide temperature range where the shrinkage rate during sintering is high. It relates to a method enabling the production of low soda alumina.

[0002]

2. Description of the Related Art Alumina is used in various mechanical parts and electric parts because it has excellent chemical stability and physical properties based on excellent mechanical strength and thermal stability. Most of such industrially used alumina is produced in large quantities by firing aluminum hydroxide by the Bayer method.

However, the aluminum hydroxide produced by the Bayer method usually inevitably contains about 0.2 to 0.7% by weight of soda as Na ₂ O in terms of alumina, which is burned. If it is brought into alumina as it is, even if it is in such a degree, it is not preferable because it is used as a ceramic raw material for an electrical insulating material such as a spark plug or an IC substrate, which causes poor insulation and fire resistance. Therefore, conventionally, various methods for removing the soda component in the alumina have been studied, and with the removal of the soda component, a certain result has been obtained in reducing the soda content of the alumina powder.

However, in recent years, in the field of electronic material ceramics such as IC substrates and packages in which such low soda alumina is used, the technical progress has been remarkable, and not only low soda but also the particle size of alumina particles and With respect to the distribution, low temperature sinterability, low shrinkage factor during sintering, etc., development of alumina having a high level of characteristics has been demanded.

[0005]

Therefore, the inventors of the present invention calcined the raw alumina source composed of aluminum hydroxide and / or transition alumina obtained by the Bayer method to produce low soda alumina, and calcined it. Temperature is 1,5
The temperature is not higher than 00 ° C., and the obtained product has good sinterability at low temperature, and the shrinkage factor at the time of sintering is small so that it is stable against variations in sintering temperature. , And proceeded with a study on a method for producing low soda alumina capable of producing various ceramic products with high dimensional accuracy. That is, it is possible to reduce the manufacturing cost by setting the firing temperature to a low temperature of 1,500 ° C. or lower, and
The aim was to develop a method for producing low-soda alumina that allows the particles of α-alumina to be formed to grow to an appropriate degree and is easily disintegrated after firing.

However, generally, the firing temperature is 1,55
When the temperature is set to a low temperature of 0 ° C. or lower, the α crystal often does not grow sufficiently, and when crushed, it becomes a fine particle powder of 1 μm or less, and thus the fine powder α-alumina thus obtained is A ceramic product, such as an IC substrate, which has a large shrinkage factor at the time of sintering when it is molded into a desired shape using this and is sintered to manufacture various ceramic products, such as an IC substrate. It is said to be unsuitable for manufacturing.

On the other hand, when a raw material alumina source is fired, if a mineralizer is added, α crystal growth is promoted. By devising the amount of this mineralizer added, even at a firing temperature of 1,500 ° C. or lower. It is possible to sufficiently develop the growth of α-alumina crystals. However, when such a mineralizer is added during firing, it is greatly affected by firing conditions such as a slight temperature distribution in the firing furnace, and coarse crystals are generated in the high temperature zone, or primary particles are fused to each other. As a result, it becomes difficult to develop uniform particle growth, such as irregularly shaped particles, and the particle size distribution tends to have a so-called bimodal particle size distribution.

Further, the particle size of α-alumina primary particles suitable for producing a high alumina sintered body is usually in the range of 1.5 to 5 μm. However, even if calcined at a temperature of 1,550 ° C. or lower without adding a mineralizer, the growth of particles is slow and it is difficult to obtain α-alumina having a particle size of 1.5 μm or more. By adjusting the firing temperature and the addition or non-addition of the mineralizing agent and the addition amount thereof, small particles are manufactured as agglomerated particles composed of primary particles of 1 μm to large particles of about 10 μm. .

A method of adding α-alumina as seeds for crystallization when calcination of aluminum hydroxide such as gypsite to produce α-alumina to obtain α-alumina having good low-temperature sinterability Is also proposed. However, α-alumina obtained by the conventional method of this kind has fine particles and is excellent in low-temperature sinterability, but has a large shrinkage factor during sintering, which causes fluctuations in sintering temperature and non-uniformity. Therefore, it is not suitable for manufacturing ceramic products that have a problem in dimensional stability due to the above and require high dimensional accuracy.

Similarly, in JP-A-62-23061, water obtained by adding α-alumina powder as seeds to a sodium aluminate solution in which aluminum hydroxide is dissolved in the Bayer method and precipitating it A method of firing aluminum oxide is disclosed, and JP-A-62-230615 discloses a method of hydrothermally treating the aluminum hydroxide obtained by the above method to convert it into boehmite and firing it. It is disclosed. However, in any of these methods, the obtained α-alumina is fine particles and has the same problem as in the case described in JP-A-62-128918.

Further, in Japanese Patent Publication No. 63-35573, aluminum hydroxide or alumina particles are used as a raw material.
A method of adding a fluoride-based mineralizer and particles containing silicon oxide to this is disclosed. However, when a fluorine compound is used, it can be converted to α-alumina at a relatively low temperature and α crystalline particles can be grown, so the independence of the particles becomes relatively good, and as a result, the pulverizability is good. However, when the added amount exceeds a certain level, rapid particle growth occurs, and large hexagonal plate-shaped crystals of 5 μm or more are formed even in low-soda alumina having a soda content of 0.1 wt% or less. . And this large hexagonal plate crystal is
If a large amount is present in α-alumina, the sintered density and mechanical strength of the product will decrease, which is not preferable.

As described above, the use of conventionally proposed additives such as fluorine compounds, chlorine compounds, and boron compounds is remarkable for removing the soda component inevitably present in aluminum hydroxide or alumina. Although effective, these fluorine compounds, chlorine compounds, boron compounds and the like have an action as a mineralizer by themselves, but there are the problems as described above, and the present invention can be obtained only by using these additives. The target firing temperature is 1,500 ° C. or less, the obtained product characteristics are good sinterability at low temperature, and the shrinkage rate during this sintering is small and stable. It is difficult to produce low soda alumina suitable for producing various ceramic products with high dimensional accuracy.

In view of this point of view, the present inventors have conducted extensive studies to achieve the above-mentioned target, and as a result, have found that the raw material alumina source contains a fluoride mineralizer and a predetermined α-alumina powder. The present invention has been completed by finding that good results can be obtained by simultaneously adding and baking at a predetermined ratio in a well-balanced manner. Therefore, an object of the present invention is that the firing temperature is 1,500 ° C. or less, the obtained product characteristics are good sinterability at low temperature, and the shrinkage ratio at the time of sintering is It is an object of the present invention to provide a method for producing low soda alumina, which is small and stable, and is capable of producing various ceramic products with high dimensional accuracy.

[0014]

[Means for Solving the Problems] That is, the present invention relates to a raw material alumina source comprising aluminum hydroxide and / or transition alumina obtained by the Bayer method, and a fluoride-based mineralization for the alumina equivalent raw material alumina source. Fluorine is used as the agent.
In the method for producing low soda alumina, 02-0.3 wt% and α-alumina powder having an average particle size of 1 μm or less are added at a ratio of 0.5-10 wt%, respectively, and fired at a temperature of 1,500 ° C. or less. is there.

The raw material alumina source used in the method of the present invention is aluminum hydroxide obtained by the Bayer method and / or
Alternatively, it is a transition alumina. Here, as the aluminum hydroxide, it is economically advantageous to use gibbsite type aluminum hydroxide produced by the Bayer method that is industrially mass-produced and can be obtained at low cost, and a normal commercial product as it is, or It is possible to provide a product that has been subjected to a soda-free treatment such as acid cleaning in advance. Further, as the raw material transition alumina, one not containing α-alumina obtained by firing it at 500 to 1,000 ° C. is used (this may also be referred to as “intermediate alumina”). ). Of these aluminum hydroxide and transition alumina, it goes without saying that either one of them can be used alone,
Both of them can also be mixed and used in an appropriate ratio. Those having an average particle diameter of 5 to 150 μm are general-purpose, have an alumina conversion purity of 99.6% by weight or more, and have a total soda content of 0.05 to 50 in terms of Na ₂ O conversion.
It is preferably 0.4% by weight.

In the method of the present invention, in such a raw material alumina source, 0.02 to 0.3% by weight of a fluoride-based mineralizer as fluorine is used with respect to the raw material alumina source in terms of alumina, and an average particle diameter is 1 μm or less. 0.5 to 10 of α-alumina powder
Add each at a weight percentage and fire. Examples of the fluoride-based mineralizing agent used here include fluorides such as aluminum fluoride, sodium fluoride, cryolite and magnesium fluoride, and the average particle size thereof is preferably 100 μm or less, More preferably 60-100 μm
belongs to. If the average particle size is larger than 100 μm, the effect of addition cannot be sufficiently exhibited unless it is used after crushing. Further, the addition amount of these fluoride mineralizers is 0.02 to 0.3% by weight as fluorine, preferably 0.05 to 0.2, with respect to the alumina conversion amount of the raw material alumina source.
Weight% is good. If too much fluoride-based mineralizer is added,
Hexagonal plate-shaped crystals are easily formed, and the sintering characteristics of the product alumina are impaired. On the other hand, if the amount added is too small, the effect of promoting the growth of the raw material alumina source into α-alumina particles during firing is insufficient.

Further, the α-alumina powder used together with the above-mentioned fluoride mineralizer needs to have an average particle size of 1 μm or less, preferably 0.3 to 1.0 μm. The smaller the average particle size, the greater the number of particles in the same amount (weight) of addition, and the better the effect of addition. If the average particle size exceeds 1 μm,
If you do not increase the amount added, the desired effect cannot be obtained,
There is a problem that it is difficult to mix them uniformly. On the other hand, if it is made too fine, the cost will increase, which is not preferable. The amount of the α-alumina powder added is preferably in the range of 0.5 to 10% by weight, and when the amount is 0.5% by weight or less, the effect of the addition is insufficient, and addition of more than 10% by weight makes it desirable to grow particles. Is difficult to develop, which is not preferable. The α-alumina powder used here is obtained, for example, by firing aluminum hydroxide obtained by the Bayer method under the conditions of 1,150 to 1,200 ° C. for about 5 hours using a static furnace. You can

The method of adding α-alumina powder to the raw material alumina source is not particularly limited as long as it can uniformly mix the whole, and for example, a method of dry blending the whole in a powder state. , A method of uniformly dispersing the α-alumina powder in a raw material alumina source, a method of mixing the raw material alumina source and the α-alumina powder in a slurry form in water or an organic solvent, and filtering and deliquoring the same. Can be mentioned. When the above α-alumina powder is slurried and dispersed in the raw material alumina source, the slurry medium to be used may be water or an organic solvent (ethyl alcohol, toluene, acetone, etc.), and may be a suitable one if necessary. A dispersant (such as citric acid) may be used. When this slurry is added, it may be atomized with a nozzle or the like, or may be added while cutting out the raw material alumina source with a screw feeder or the like.

As for the apparatus used to uniformly disperse the α-alumina powder in the raw material alumina source, any apparatus can be used as long as the powder can be mixed so as not to be in this state, for example, a mortar mixer or a mix. Muller (Shinto Kogyo), Rocking Mixer (Aichi Denki),
A mixer such as a Henschel mixer (Mitsui Miike Industry) can be used. The effect of adding the above-mentioned fluoride mineralizer does not change even if it is added before mixing the raw material alumina source and the α-alumina powder, or after mixing,
For more uniform mixing, it is preferable to add the α-alumina powder to the above slurry.

By the way, when the raw material alumina source used in the method of the present invention contains 0.1% by weight or more of soda as Na ₂ O in terms of alumina, the raw material alumina source has a chlorine equivalent amount of 2%. Chloride-based soda removal agent is added in a proportion of not more than wt%, preferably 0.1 to 1.0 wt%, and the mixture is fired. The chloride-based soda removing agent used here may be one capable of reacting with the soda component when heated and capturing and removing the soda component, for example,
Examples thereof include chlorides such as hydrochloric acid, aluminum chloride and magnesium chloride. These may be used alone or in a mixture of two or more. Also,
The addition amount of this chloride-based soda removal agent is 2% by weight.
More may be added, but it is not preferable because not only the effect of adding a large amount cannot be obtained, but also the concentration of hydrogen chloride in the waste gas increases during firing, which may corrode the equipment used.

In addition to the function as a soda removing agent, these chloride-based soda removing agents include, for example, tunnel furnaces, shuttle furnaces, electric furnaces, and other stationary furnaces (hereinafter, these are referred to as "stationary furnaces"). ) Also functions as a blowout preventing agent during the firing of the raw material alumina source. Further, the chloride-based soda removing agent may be added and mixed either before or after the raw material alumina source and the α-alumina powder are mixed, or after the fluoride-based mineralizer is added. The effect is the same. This chloride-based soda removing agent may be added to the above slurry,
In this case, the pH of the slurry may become acidic, and therefore a dispersant suitable for it must be selected. Further, it is also effective to make a silica-based substance coexist as the other soda-removing agent, but depending on the use of these soda-removing agent, as a blowout preventing agent at the time of firing the raw material alumina source in the stationary furnace, No effect.

In the method of the present invention, the raw material alumina source, the fluoride-based mineralizer and the α-alumina powder are uniformly mixed, or a mixture thereof is further mixed with a chloride-based soda removing agent. The resulting mixture is usually 1,50
Baking is performed at 0 ° C or lower, preferably 1,100 to 1,500 ° C. If the firing temperature is higher than 1,500 ° C., on the contrary, the generated particles become too large and the pulverizability is deteriorated, which is not preferable. The firing temperature is 1,100 to 1,50.
Within the range of 0 ° C., the vapor pressure of the fluoride-based mineralizer is appropriately maintained, the growth of α-alumina particles is promoted, and the problem that the particles become too fine does not occur. For this calcination, for example, a rotary kiln, a fluidized bed calcination furnace, an airflow calcination furnace, a stationary furnace, or the like can be used, and a method of filling a calcination container in the furnace and calcination can also be applied. As long as these furnaces have a structure in which they can be continuously fired without being cooled, they can be appropriately combined and used. The holding time in the maximum temperature range in this firing furnace differs depending on the relationship between the form of the firing furnace and the heat transfer rate, but is usually preferably several minutes to 15 hours.

The method for crushing the alumina produced in this way is not particularly limited,
It can be carried out by using a commonly used pulverizer, such as a ball mill, a vibration mill, a jet mill, etc., whereby the desired low soda alumina can be obtained.
In this case, for example, when performing the crushing process with the same ball mill, in the case of Examples 1 to 3 described later, the crushing process time is about 60 minutes and the average particle size is almost constant, but in the case of Comparative Example 2 In addition, until the average particle diameter becomes almost constant, the average particle diameter is not constant until 240 minutes or more.

The low soda alumina obtained by the method of the present invention does not contain coarse particles of 6 μm or more or irregular particles,
The average particle size is within the range of 0.5 to 3.0 μm, and most of the particles are centered around the target particle size ±
It belongs to the range of 1.5 μm and is distributed almost evenly within the range. As a result, the low-soda alumina obtained by the method of the present invention can obtain a compact with a high density, has good sinterability at low temperatures, and has a low shrinkage rate during this sintering. It is stable and stable, and is optimal as a low soda alumina for manufacturing various ceramic products with high dimensional accuracy. Further, the obtained alumina has a soda concentration of 0.02 to 0.06% by weight, which is a sufficiently low level.

[0025]

The reason why the low soda alumina having such characteristics can be obtained by the method of the present invention is not clear, but it is considered as follows. That is, aluminum hydroxide and / or transition alumina produced by the Bayer method and α-alumina powder having an average particle size of 1 μm or less are allowed to coexist,
When a fluoride-based mineralizer acts at a temperature of 900 ° C or higher, transition alumina (including aluminum hydroxide changed during temperature increase) coexists in the process of converting to α-alumina. Mass transfer occurs selectively on the surface of the added α-alumina powder, and particles of the added α-alumina grow.
This effect is remarkable as the amount of the fluoride mineralizer added is large, and as the firing temperature is high. In addition, coexisting α
-The larger the number of alumina powders, the smaller the particle size of the grown α-alumina tends to be. Here, FIG. 1 shows such a relationship in firing in a shuttle furnace, in which a fluoride (here, aluminum fluoride) and an average particle diameter are 1 μm.
A desired average particle size (D _p 50) and a desired particle size distribution are obtained by appropriately selecting the addition amount (% by weight) of each α-alumina powder of m or less, and the firing temperature and time. It is shown that the α-alumina can be obtained.

Further, when the low soda alumina having the above-mentioned characteristics obtained by the method of the present invention is used as a ceramic raw material, the sintering temperature is low, so that a high sintered density is obtained and the shrinkage due to the sintering is caused. The sintered body can be stably obtained with a low shrinkage ratio without being affected by fluctuations in sintering temperature or unevenness in the sintering furnace. The reason for this is not clear, but it can be considered as follows. That is, at the time of sintering, the disappearance of pores due to the progress of sintering and the generation of grain boundary strain due to the growth of sintered particles occur simultaneously in parallel with each other in the sintered body, thereby suppressing shrinkage during sintering. It is considered that the stability of shrinkage is maintained in a wide sintering temperature range without the occurrence of partial abnormal grain growth.

As shown in FIG. 4, the difference in shrinkage due to the sintering of the formed sheet is 155 of the sintering temperature of the electric furnace.
It was as low as 0.2% or less between 0 ° C and 1650 ° C, and when α-alumina powder was not added and only fluoride was added, a remarkable difference of 0.4 to 0.6% was recognized. To be Further, since the firing temperature is 1,500 ° C. or lower, almost no fusion of α-alumina produced is observed, and the α-alumina particles are easily crushed to form primary particles.

[0028]

EXAMPLES The present invention will be specifically described below based on Examples and Comparative Examples. The present invention is not limited to these examples and comparative examples. The samples used in the following Examples and Comparative Examples were all prepared by the method of (1) Sample Preparation below, and the particle size distribution of the obtained α-alumina was measured by a sedigraph. The results are shown in Figure 2. Next, the sintering characteristics of the low soda alumina obtained in these Examples and Comparative Examples were measured in accordance with the following (2) Sintering test and (3) Characteristic measuring method. The results of this (3) characteristic measurement are shown in FIGS. 3 and 4.

(1) Sample Preparation Raw Material Alumina Source (Aluminum Hydroxide by Bayer Method) (A) (B) Total Soda (Na ₂ O Conversion, Weight%): 0.20 0.10 Other Metal Impurities (Oxide Conversion) , Wt%): 0.05 0.05 Adhesion moisture (wt%): 0.1 0.1 Average particle size (sieve particle size, μm): 70 70 α-alumina powder Total soda (Na ₂ O conversion, wt%) ): 0.05 Other metal impurities (oxide conversion, weight%): 0.05 Average particle size (Sedigraph method, μm): 0.5 True specific gravity: 3.92

A slurry of α-alumina powder having an average particle size of 1 μm or less was prepared by the following procedure. 100 parts by weight of α-alumina powder having an average particle diameter of 0.5 μm and pure water 5
0.4 part by weight of citric acid was added to 0 part by weight of the mixture, and a predetermined amount of aluminum fluoride as a fluoride mineralizer was added thereto. When the soda content contained in the raw material aluminum hydroxide and / or transition alumina is 0.1% by weight or more in terms of Na ₂ O, a predetermined amount of aluminum chloride is further added as a chloride-based soda removing agent. After that, nylon coated steel balls were used and mixed for 20 hours in a nylon rotary mill to obtain a slurry.

Next, the slurry of the α-alumina powder thus obtained is put into a mix muller together with the above-mentioned raw material alumina source and mixed for 15 minutes, and then the obtained mixture is transferred to a firing furnace at a predetermined temperature. Was calcined to obtain calcined α-alumina. Each 1 kg of this calcined α-alumina was placed in a vibrating ball mill having a pot with an internal volume of 5.4 liters, and 20 mmφ alumina balls 7,500 k
g was added and crushed for 4 hours to obtain a low-soda alumina as a sample for characteristic measurement. The above-mentioned fluoride mineralizer and chloride de-soda agent may be added in this mix muller, and regarding firing, the mixture should be filled in a chamotte-like container and fired. Is also an effective method.

(2) Sintering Test Next, using the low-soda alumina thus obtained, this was molded and fired as follows to prepare a sintered body. 480 parts by weight of α-alumina obtained by crushing,
12 parts by weight of silica, 2 parts by weight of magnesia, and 6 parts by weight of calcia are put into a pot mill, and a dispersant or organic solvent which is usually used is added thereto, and the mixture is rotatively mixed for about 16 hours. Then, a plasticizer, a binder or An organic solvent was added and mixed for another 5 hours. The obtained slurry was defoamed, and the gate height was set to 1 using a DP-150 type sheet forming machine manufactured by Tsugawa Seiki Seisakusho.
mm, the film speed was 178 mm / min. 50x4 after the formed sheet is naturally dried day and night
It was punched into a size of 0 mm and further dried at 90 ° C. for about 2 hours to obtain a raw sheet. An appropriate number of the obtained green sheets were stacked and placed on a setter, heated to a predetermined temperature in a Keramax heating element electric furnace, and held for about 2 hours to obtain a sintered sheet.

(3) Characteristic Measurement The pressed bulk density (molding density) of the low soda alumina obtained in the above (1) sample preparation was measured by the following method, and the sintered sheet was measured by the following method. The sintering properties were measured. Pressurized bulk density: 20% soda alumina powder obtained by crushing treatment at a molding pressure of 1,000 kg / cm ²
It was molded into a plate shape having a size of mm × 40 mm × 8 mm, and the molding density (pressure bulk density) was measured by the dimension method. Sintering density: The green sheet was fired at a predetermined temperature, and the density of the obtained sintered sheet was measured by the Archimedes method. Shrinkage rate: The linear shrinkage rate was calculated by the following equation from the length (L ₀ ) of the raw sheet and the length (L _S ) of the sintered sheet obtained by firing at a predetermined temperature. Linear shrinkage (%) = {(L ₀ −L _S ) / L ₀ } × 100

Example 1 7.4 kg of a slurry containing 5% by weight of α-alumina powder having an average particle size of 0.5 μm in 150 kg of aluminum hydroxide obtained by the Bayer method containing 0.2% by weight of soda as Na ₂ O. And aluminum fluoride (average particle size 60μ
m, purity 99% by weight) 0.1 kg and aluminum chloride aqueous solution 5.2 kg containing 19% by weight in terms of chlorine were added and mixed according to the prescription of the above (1) sample preparation, and then the resulting mixture was mixed. Fill a chamotte container, calcination in a shuttle furnace using natural gas as a heat source under conditions of 1,350 ° C. for 10 hours, crush the resulting calcined α-alumina, and soda with an average particle size of 2.1 μm. (Na ₂ O) concentration 0.0
3% by weight of α-alumina was obtained. The particle size distribution of the obtained α-alumina was measured. The results are shown in Figure 2.

Regarding this α-alumina, the above (2)
A green sheet was prepared according to the sintering test and (3) property measurement, and this was sintered to obtain a sintered sheet, and the pressed bulk density, the sintered density and the shrinkage ratio were measured. The results are shown in FIGS. 4 and 5, and the pressed bulk density of the molded body was 2.29 g /
cm ³ , the sintered density of the sintered sheet is 3.73 g / cm
³ (1,550 ° C) and the linear shrinkage ratio by sintering is 14.9.
% (1,550 ° C.).

Example 2 In this Example 2, except that firing was performed using a tunnel kiln furnace using heavy oil as a heat source as a firing furnace, and that firing treatment was performed in the same furnace at 1,350 ° C. for 10 hours, All were carried out according to the same procedure as in Example 1 above. Obtained α
- particle size distribution of the alumina is as shown in FIG. 2, soda (Na ₂ O) concentration was 0.03 wt%. The sintering characteristics were as shown in FIGS. 4 and 5. The pressed bulk density is 2.32 g / cm ³ , and the density of the sintered body is 3.74.
The linear contraction rate by g / cm ³ (1,550 ° C.) and sintering was 14.6% (1,550 ° C.).

Example 3 7.4 kg of a slurry containing 5% by weight of α-alumina powder having an average particle size of 0.5 μm in 150 kg of aluminum hydroxide by the Bayer method containing 0.1% by weight of soda as Na ₂ O. And 0.1 kg of aluminum fluoride were added and mixed according to the procedure of (1) Sample preparation described above, and then a rotary furnace equipped with a one-stage air flow heating device using kerosene as a heat source was used, and the alumina temperature at the focal point was 1 0.5 at 500 ° C
The calcined α-alumina thus obtained was calcinated for a period of time to obtain α-alumina. The obtained α-alumina (low soda alumina) had a soda (Na ₂ O) concentration of 0.02% by weight.
The measurement result of the particle size distribution is shown in FIG.

With respect to this α-alumina, a green sheet was prepared according to the above (2) sintering test and (3) characteristic measurement,
This was sintered into a sintered sheet, and the pressed bulk density, sintered density and shrinkage ratio were measured. The results are as shown in FIGS. 4 and 5, the pressure bulk density is 2.27 g / cm ^3, the density of the sintered body 3.75g / cm ³ (1,550 ℃) and by sintering linear shrinkage The rate was 14.6% (1,550 ° C).

Comparative Example 1 150 kg of aluminum hydroxide obtained by the Bayer method containing 0.2% by weight of soda as Na ₂ O, 0.07 kg of aluminum fluoride and 19% by weight as chlorine equivalent.
After adding and mixing 5.2 kg of the aluminum chloride aqueous solution contained therein in accordance with the procedure of (1) Sample preparation, the resulting mixture was filled in a chamotte container, and was put in a tunnel furnace using heavy oil as a heat source. A calcining treatment was performed at 400 ° C. for 10 hours, and the calcined α-alumina obtained was crushed to obtain α-alumina. The obtained α-alumina is soda (Na ₂ O)
The concentration is 0.06% by weight, and the measurement result of the particle size distribution is shown in FIG.

With respect to this α-alumina, a green sheet was prepared according to the above (2) sintering test and (3) characteristic measurement,
This was sintered into a sintered sheet, and the pressed bulk density, sintered density and shrinkage ratio were measured. The results show its sintering characteristics as shown in Fig. 4.
And as shown in FIG. 5, the pressurized bulk density is 2.23 g.
/ Cm ³ , the density of the sintered body is 3.69 g / cm ³ (1,5
The linear shrinkage ratio at 50 ° C and sintering is 15.3% (1,5
50 ° C.).

Comparative Example 2 Kerosene was used as a heat source as it was without adding a fluoride mineralizer or a chloride de-soda agent to aluminum hydroxide prepared by the Bayer method containing 0.1% by weight of sodium as Na ₂ O. Was fired for 0.5 hours at the alumina temperature of the focus of 1,650 ° C. in a rotary furnace equipped with an air flow heating device in one stage, and the obtained fired α-alumina was crushed to obtain α-alumina. . The obtained α-alumina is soda (Na ₂
The O) concentration is 0.06% by weight, and the measurement result of the particle size distribution is shown in FIG.

With respect to this α-alumina, a green sheet was prepared according to the above (2) sintering test and (3) characteristic measurement,
This was sintered into a sintered sheet, and the pressed bulk density, sintered density and shrinkage ratio were measured. The results show its sintering characteristics as shown in Fig. 4.
And as shown in FIG. 5, the pressed bulk density is 2.24 g.
/ Cm ³ , the density of the sintered body is 3.69 g / cm ³ (1,5
50%) and linear shrinkage due to firing is 15.8% (1,5
50 ° C.).

[0043]

According to the method of the present invention, the firing temperature is 1,5
The temperature is not higher than 00 ° C., and the product characteristics are good sinterability at low temperature, and the shrinkage factor at the time of sintering is small and stable against the fluctuation of sintering temperature. Low soda alumina, which can produce various ceramic products with dimensional accuracy, can be easily produced, and energy saving is achieved, which is of high industrial value.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the addition amount of α-alumina powder of 1 μm or less and the addition amount of aluminum fluoride for obtaining low soda alumina having a target average particle diameter.

FIG. 2 is a graph showing a particle size distribution of α-alumina obtained in each example.

FIG. 3 is a graph showing a particle size distribution of α-alumina obtained in each comparative example.

FIG. 4 shows α-obtained in each example and comparative example.
It is a graph showing the relationship between the sintering temperature (° C) of alumina and the sintering density (g / cm ² ).

FIG. 5 shows α-obtained in each example and comparative example.
It is a graph showing the relationship between the sintering temperature (° C) of alumina and the linear shrinkage ratio (%).

Claims (4)

[Claims] 1. A raw material alumina source comprising aluminum hydroxide and / or transition alumina obtained by the Bayer method,
0.02 to 0.3% by weight of fluorine-based mineralizer as fluorine with respect to the raw material alumina source in terms of alumina and average particle size 1 μ
A method for producing low soda alumina, which comprises adding α-alumina powder of m or less at a rate of 0.5 to 10% by weight and firing at a temperature of 1,500 ° C. or less. 2. The fluoride mineralizer is aluminum fluoride,
The method for producing low soda alumina according to claim 1, which is a mixture of one or more selected from sodium fluoride, cryolite and magnesium fluoride. 3. When the soda content in the raw material alumina source is 0.1% by weight or more as Na ₂ O in terms of alumina,
2. The method for producing low soda alumina according to claim 1, wherein a chloride-based soda removing agent is further added to the raw material alumina source at a ratio of 2% by weight or less in terms of chlorine. 4. The chloride-based soda removing agent is one or two selected from hydrochloric acid, aluminum chloride and magnesium chloride.
The method for producing low soda alumina according to claim 3, which is a mixture of at least one kind.