JPH10139424A - Low temperature transition alpha-alumina and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain active α-alumina capable of being sintered at a low temperature, large in a BET specific surface area and excellent in sintering characteristics, etc., by adding and homogeneously dispersing α-alumina powder produced by neutralizing a part of diaspore powder or particles in a process for depositing gypsite by Bayer method. SOLUTION: This α-alumina is obtained by adding α-alumina powder produced by neutralizing a part of diaspore powder or particles in a process for depositing aluminum hydroxide (gypsite), sufficiently mixing and dispersing the α-alumina with aluminum hydroxide deposited from a sodium aluminate aqueous solution or mixing the α-alumina powder as seed nuclei for depositing aluminum hydroxide, dehydrating and drying the precipitated mixture or deposited products and subsequently calcining the product at <=1100 deg.C. The size of the added α-alumina powder may be the same as that of gypsite for the seeds. Namely, the diameters of 50wt.% of the powder are 0.01-50μm. The α-alumina powder is added in an amount of 0.01-100wt.% based on the gypsite. The method gives the α-alumina powder having a BET specific surface area of >=10m<2> /g.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an alpha alumina (α-A1 ₂ O ₃₎ which has been transitioned at a temperature lower than a normal transition temperature.
Hereinafter, it is referred to as α-alumina in this specification. ) And a method for producing the α-alumina. More specifically, an alumina-containing ore such as bauxite and a caustic soda solution are charged into an extraction device, the alumina content in the alumina-containing ore is extracted as sodium aluminate, and after separation and removal of insoluble components, the sodium aluminate is hydrolyzed. In the so-called Bayer method, aluminum hydroxide is precipitated in a precipitation step of aluminum hydroxide, and diaspore powder or α-alumina powder in which some of the particles are hydrated are added from outside the system, and water is used as seeds. As a crystal nucleus of aluminum oxide or as a mixture with aluminum hydroxide, α-alumina having partially hydrated diaspores or particles is uniformly dispersed in aluminum hydroxide, and the mixture is fired at a temperature of 1100 ° C. or lower. Method for producing low-temperature transition α-alumina and α-aluminum produced by the method It relates. The α-alumina produced by the method of the present invention has a BET specific surface area larger than that produced by the conventional method, and is excellent in sinterability, reactivity and the like.

[0002]

2. Description of the Related Art At present, the most widely used industrial method for producing α-alumina is to extract an alumina content from a raw material bauxite with an alkaline solution such as caustic soda, and to obtain water as an aluminum hydrate. This is a method in which aluminum oxide is formed, and this is fired. The method of extracting aluminum hydroxide from caustic soda solution from bauxite is referred to as the Bayer method, and the aluminum hydroxide produced by this method is usually gypsite (A1 ₂ O ₃ .3H ₂ O), which is a trihydrate. ).

[0003] As is well known, other than diaspore, gypsite, aluminum hydroxide such as bayerite and boehmite and amorphous alumina hydrate such as alumina gel are dehydrated by calcination, and η-alumina. , Χ
After passing through various intermediate aluminas such as alumina, γ-alumina, κ-alumina, θ-alumina, etc., it finally becomes the most stable α-alumina. It is well known that this transition has a transition sequence specific to the starting material, firing conditions and atmosphere. However, in any of the transition series, the final transition temperature to α-alumina is substantially constant, at least 1100 ° C. or higher, and the temperature at which transition to α-alumina is almost completely 1200 ° C. or higher.

[0004] Gypsite obtained by the above-described viar method changes to α-alumina at 11500 ° C to 1250 ° C via χ ^- alumina and κ-alumina by firing in a normal atmosphere. Therefore, in an alumina maker that produces α-alumina, it is usually fired at a high temperature of 1200 ° C. or higher to obtain a product α-alumina.

On the other hand, the only exception is the case of diaspore. The diaspore transitions directly to α-alumina at fairly low temperatures around 600 ° C. However, diaspores cannot be made by the usual via method, and pressure equipment such as an autoclave is required for synthesizing diaspores. A method for making a large amount of diaspores at low cost is not known at present. Also, the method of beneficiating ores mainly composed of diaspore such as natural alumite shale and firing at a low temperature of about 600 ° C. to obtain α-alumina has a limit in removing impurities, and α-alumina originated from a single-buyer method is used. When the same purity is required, it is difficult to sufficiently cope.

[0006]

The α-alumina used for refractories, ceramics, etc. is 1200 ° C. in a rotary kiln or a reverberatory furnace of an alumina maker.
It is obtained by calcining aluminum hydroxide (gypsite) at the above temperature, and its equipment and heat energy are enormous. Manufacturers are making thorough heat management and working to improve firing costs, but no significant improvement can be expected as long as the transition temperature to α-alumina is constant.

Further, the primary particle size and sinterability of α-alumina powder originating from the Bayer method will be described. The primary particles of α-alumina originating from the Bayer method, that is, the size of a single α particle, are approximately the same as those described above in the case where aluminum hydroxide having a normal particle size is used, since the powder is fired at a high temperature of about 1200 ° C. It grows to a diameter of about 1 to 5 μm. This one
The particle size of 5 μm is too large for applications where sintering characteristics are important, and sintering with the particle size as it is results in 1
A sintering temperature of 600 ° C. or higher is required. Further, when the grains grow to about 5 μm or more, α-alumina belongs to the hexagonal system, so hexagonal plate-like crystals develop, and when used as a powder for sintering, anisotropy appears, May cause physical problems.

Accordingly, α-alumina, which is used particularly in applications where sinterability is important, that is, easily sinterable α-alumina, uses aluminum hydroxide (gypsite) having fine primary crystal grains as a raw material, The primary particle size of the fired α-alumina is controlled to be 1 μm or less, and this is pulverized to produce easily sintered α-alumina. However, even when the sinterable α-alumina is produced, the transition temperature of 1200 ° C. does not change, and the cost increases due to adjustment of fine aluminum hydroxide and pulverization after sintering.
-Alumina requires about 2 to 5 times the cost of normal α-alumina. In addition, α-alumina obtained by firing at such a high temperature tends to cause grain growth during sintering, adversely affects sinterability such as inducing abnormal grain growth, and mechanical properties of the sintered body. It is said to hinder improvement.

[0009] From the above, the problems that triggered the present invention are:
If the transition to α-alumina occurs at a lower temperature than in the conventional case, and α-alumina can be produced at a lower temperature, the alumina firing cost can be reduced. In addition, if the transition to α-alumina can be caused at a lower temperature than before, the single crystal particles of α-alumina as primary particles can be made finer, and physical properties such as sintering characteristics can be obtained. Can be expected to improve.

[0010]

In view of such circumstances, the present inventor has conducted intensive studies on a method for causing a transition to α-alumina at a lower temperature than the conventional method, and as a result, produced aluminum hydroxide. In the so-called Bayer method, it has been found that the object can be achieved by applying the method described below, and the present invention has been completed.

That is, the present invention provides a so-called Bayer method for treating an alumina-containing ore such as bauxite with a caustic soda solution, dissolving and separating only the alumina component in the ore, and then recovering and refining aluminum hydroxide by hydrolysis. Addition of diaspore powder or α-alumina powder with some of the particles hydrated during the aluminum precipitation process, mixed and dispersed with the aluminum hydroxide sufficiently precipitated from the sodium aluminate solution, or precipitation of aluminum hydroxide And the mixed sediment or sediment is
A method for producing a low-temperature transition α-alumina characterized by firing at a temperature of 00 ° C. or lower, and α-alumina having a large BET specific surface area, high activity and excellent sinterability produced by this method. Is to provide. The low-temperature transition referred to here is higher than a normal firing temperature (about 1200 ° C.) for firing α-alumina by firing aluminum hydroxide (gypsite) manufactured by a conventional Bayer method.
It clearly refers to the transition to α-alumina at low temperatures.

Hereinafter, the method of the present invention will be described in more detail.
According to the present invention, an alumina-containing ore and a caustic soda solution are charged into an extraction device, a soluble alumina content in the alumina-containing ore is extracted to form a sodium aluminate solution having a high alumina concentration, and then an insoluble residue is separated from the solution. After removal, aluminum hydroxide as a seed is added to the sodium aluminate solution, and the sodium aluminate is hydrolyzed and aluminum hydroxide is precipitated with the seed as a nucleus.
In the so-called Bayer method, in the aluminum hydroxide precipitation step, in addition to aluminum hydroxide added as seeds, diaspore powder and / or α-alumina powder in which some of the particles are hydrated are added, and the precipitated water is added. Characterized in that the mixture is homogeneously mixed with aluminum oxide, dehydrated and dried, and then the mixture is fired at a temperature of 1100 ° C. or lower.
This is a method for producing alumina. The present invention also provides the above-described Bayer method, wherein the aluminum hydroxide is deposited in the step of:
Α- hydrated part of diaspore powder or / and particles as seed for aluminum hydroxide precipitation
Alumina powder was added, and aluminum hydroxide was precipitated using the seed as a core.
A method for producing α-alumina, characterized by firing at a temperature of not more than ° C. Further, α-alumina powder having a BET specific surface area of 10 m ² / g or more produced by these methods.

In order to explain the step of depositing aluminum hydroxide in the viar method, its outline will be described. The Bayer method is a method of refining aluminum hydroxide mainly by dissolving ore such as bauxite in caustic soda.
1886, K. J. Established by Bayer.
After that, various improvements have been made. The steps are roughly divided into an ore dissolving step, an insoluble residue separating step, and an aluminum hydroxide precipitation step.

In the ore dissolving step, the crushed or pulverized ore is charged into a dissolving and extracting apparatus such as an autoclave together with a sodium hydroxide solution containing 100 to 200 g / 1 of Na ₂ O.
The soluble alumina content in the ore is extracted as sodium aluminate at a temperature of about 10 to 250 ° C. In general, ore is preferably gypsite-type bauxite, which is trihydrate as an alumina component, in view of easy dissolution in caustic soda, but bauxite partially containing boehmite, which is monohydrate, is also preferable. It is used. Diaspore, which is also a monohydrate, is more difficult to dissolve in caustic soda than boehmite, and therefore is not used as a raw material ore in the Bayer method except in some countries such as China.

In the next step of separating the insoluble residue, an insoluble solid is deposited in the caustic soda solution in a multi-stage thickener.
This is a step of separating and removing this from the sodium aluminate solution. This insoluble residue is usually called red mud, and is a mixture mainly composed of oxides such as iron, silicon, and titanium such as hematite, silica, and anatase in bauxite. This red mud usually contains desiliconized material (usually called sodalite) obtained by a desiliconization operation for removing silica dissolved in the caustic soda solution.

In the step of depositing aluminum hydroxide, sodium hydroxide which has been separated and clarified from red mud is hydrolyzed in a precipitation tank to deposit aluminum hydroxide. In the precipitation tank, the solution is cooled to 50 to 70 ° C., and mechanical stirring or air stirring is performed. At this time, it is common to add aluminum hydroxide seeds in order to promote the precipitation reaction. Normally, the aluminum hydroxide seed to be added uses a relatively fine portion of the precipitated aluminum hydroxide. The amount of the aluminum hydroxide to be added cannot be generally specified, but is generally about 5 to 20.
It is about 0% by weight. Of course, the added species is preferably effective in a small amount, and the aluminum hydroxide precipitated in the precipitation step is usually gypsite, which is a trihydrate. Is common. The precipitated aluminum hydroxide is dehydrated and dried to become dried aluminum hydroxide (dry gypsite), and a series of buyer processes is completed.

Α-alumina is produced by firing the aluminum hydroxide thus purified by the Bayer method in a rotary kiln or a reverberatory furnace. As described above, aluminum hydroxide obtained by the Bayer method is gypsite, and is fired in a normal atmosphere to form 1150 to 1250 via χ-alumina and κ-alumina.
Transforms to α-alumina at ° C. On the other hand, the α-alumina transition temperature of diaspore is about 600 ° C., and the transition to α-alumina is performed at an extremely low temperature as compared with the case of gypsite.

According to the present invention, when the aforementioned diaspore or / and partially hydrated α-alumina is added to aluminum hydroxide (gypsite) produced by the Bayer method or the like within a certain range, these additives are used. It exhibits a seed effect and is based on the fact that the gypsum transition temperature of the entire gypsite is lowered. The details are described in Japanese Patent Application No. 8-258645.

The method of using α-alumina powder (anhydride) itself as a seed core is described in the past literature and is generally known. For example, Rajendran
(J. Mater. Sci., Vol. 29, pp. 5)
664-72, 1994). However, rather than α-alumina powder itself, α
-Alumina powder and diaspore powder have a greater effect of lowering the temperature of α-alumina transition. With the diaspore powder, under appropriate conditions, the pregelatinized transition temperature can be lowered from about 1200 ° C. to about 300 ° C. The diaspore particles are α-
In the firing process of alumina, the transition to α-alumina having a large BET specific surface area and high activity during temperature rise is very convenient as a seed core for low-temperature transition. However, the theoretical elucidation as to why the presence of these seed nuclei lowers the whole α-transition temperature has not been sufficiently made.

The method for producing a partially hydrated α-alumina powder is not particularly limited, and can be achieved by hydrating a portion of the existing α-alumina, such as the surface of particles, by some method. For example, grinding powder of α-alumina by a wet method or wear powder such as α-alumina balls may be used. Also,
α-alumina may be treated with hot steam or the like.

In order to lower the transition temperature to α-alumina by the seed effect, it is necessary to reduce the particle size of the diaspore powder and / or α-alumina powder obtained by partially hydrating particles, Is an important point. For this, a wet or dry mixing method using a shaker or a crusher can be considered. However, other than the wet method, the mixing and dispersion are often insufficient, and a special mixing operation is required, which causes an increase in cost. The method of the present invention solves the above-mentioned problems, and in the precipitation process of the Bayer method, in the liquid phase and during the precipitation of aluminum hydroxide, α-alumina in which diaspore and a part of particles are hydrated is added. Because of the method, uniform dispersion is ideally performed, and simultaneously eliminates the problem of requiring additional mixing and dispersion operations.

In the precipitation step of the Bayer method, the method of adding the diaspore powder or α-alumina powder obtained by partially hydrating particles is the same as the conventional method of adding gypsite seed (or alumina gel or the like). Not different. It may be mixed with gypsite seed and added in a slurry state, or may be added independently of the seed. The order of addition with the gypsite seeds may be determined in consideration of various conditions. The point is that aluminum hydroxide (gypsite) particles and diaspore particles or / and α-alumina particles partially hydrated in the precipitation step are homogeneously present in the mixed precipitate. It is more effective in terms of homogeneous dispersion that the diaspore particles and / or α-alumina particles in which a part of the particles are hydrated become the precipitated species and become the composite particles in which gypsite is crystallized around.

With respect to the particle size of the diaspore powder and / or the α-alumina powder partially hydrated,
Generally, it is preferably fine, but it may be about gypsite for seeds. That is, the 50% by weight diameter of the α-alumina powder obtained by partially hydrating the diaspore powder and / or the particles is 0.01 to 50 μm. It is practically difficult to reduce the 50% by weight diameter to 0.01 μm or less, and if it is 50 μm or more, homogeneous mixing becomes insufficient and the effect of lowering the transition temperature cannot be sufficiently exhibited. In the case of α-alumina in which a part of the particles is hydrated, the difference in specific gravity from gypsite is larger than that in diaspore, and the difference in sedimentation velocity is large. The weight% diameter is about 0.01 to 10 μm.

The addition amount of the diaspore powder and / or the α-alumina powder partially hydrated with respect to the aluminum hydroxide (gypsite) precipitated is 0.01 to 100% by weight. is there. The effect of addition of diaspore powder and / or α-alumina powder partially hydrated on lowering the transition temperature is extremely effective even with a small amount, and is usually about 0.1 to 5% by weight. However, when the seed nucleus is added for the purpose, it can be increased to about 100% by weight.
If it is less than 0.01% by weight, the effect of low temperature transition cannot be exhibited,
If the amount is more than 100% by weight, the economy is poor, and in any case, the purpose of the present invention is not met. Also, if the addition amount is too large,
Since α-alumina particles and diaspore particles which have been deposited in the precipitate and could not be transferred may remain in the Bayer solution and circulate to cause inability or inconvenience, the addition amount is preferably suppressed to the minimum necessary. Of course, the seed nucleus which is insufficient for the total aluminum hydroxide (gypsite) to be precipitated may be added in the form of gypsite or alumina gel by a conventional method in an appropriate amount.

One of the reasons that α-alumina, in which a part of the particles is hydrated, has a greater transition effect on gelatinization than ordinary α-alumina is that part of the surface of the α-alumina particles is hydrated and It is presumed that the consistency with the gypsite particles existing in the surroundings is related to the natural structure. For diaspore,
During firing, the BET specific surface area is very large and the activity is high.
It is considered that this may enhance the effect because it becomes alumina.

[0026] In the precipitation tank of the Bayer method, mechanical stirring or air stirring is performed, but those having reached an appropriate particle size are settled and concentrated. At this time, for homogeneous dispersion of the additive for causing a low-temperature transition, it is more preferable that the difference in specific gravity from gypsite is as small as possible, and α-alumina in which a part of particles is hydrated, and further that diaspore has α -It is clear that it is more advantageous than when alumina itself is used as a seed. Incidentally, the specific gravities of α-alumina, diaspore and gypsite are 3.9 to 4.
1, 3.4 and 2.4 (Ryoichi Kiriyama, "Structural Inorganic Chemistry III", Kyoritsu Zensho). The hydrate of α-alumina has a value closer to the specific gravity of gypsite than α-alumina itself (anhydride).

The diaspore used in the present invention may be an artificially synthesized diaspore or a natural diaspore. Synthetic diaspores are effective because they can be obtained with high purity in terms of purity, but they have problems in economics such as price. The most industrially advantageous method is, for example, a method using diaspore ore in alumite shale existing in large quantities in China and the like. In general, alum shale contains diaspore and kaolin as main components, and contains silica, titania, iron oxide, and the like as minor components, and iron oxide, titania, and the like are less than ordinary so-called bauxite. Therefore, in order to obtain a high purity diaspore, processing usually applied to ore such as magnetic separation or flotation is effective. In particular, for the removal of silica, reverse flotation using quaternary amines as a cation collector seems promising (Japanese Patent Publication No. 7-47301).

Thus, a mixed precipitate of the aluminum hydroxide obtained by the method of the present invention and α-alumina having partially hydrated diaspore powder or / and particles is obtained by ordinary calcination in the atmosphere. Completely transitions to α-alumina at 9000 ° C. to 1100 ° C. Therefore, the firing conditions of the method of the present invention are as follows:
Firing at a temperature of 0 ° C. or less, preferably 1000 ° C. or less. Above 1100 ° C., there is no difference from the firing temperature of the conventional Bayer gypsite, which is not the object of the present invention. The method of the present invention enables a significant reduction in energy costs, particularly in the production of α-alumina from gibbsite in the Bayer process, and has industrially extremely useful value.

Further, the α-alumina produced by the method of the present invention has a BET specific surface area larger than that of α-alumina produced by the conventional Bayer method and calcined at about 1200 ° C. whereas ~10m is about ² / g, ^{10m 2} / g or more, usually 15~40m ² /
g. The α-unit particles calculated from the specific surface area are fine and show excellent properties in reactivity and sinterability.

[0030]

According to the method of the present invention described in detail above, in the precipitation step of aluminum hydroxide (gypsite) in the Bayer method, diaspore powder and / or α-alumina powder in which a part of the particles are hydrated. Is added and homogenously dispersed, the precipitated mixture can be fired at a temperature of 1100 ° C. or lower to produce low-temperature transition α-alumina. For this reason, the firing cost of α-alumina can be greatly reduced, which greatly contributes to the alumina manufacturer and its industrial value is extremely large.
Furthermore, α-alumina produced by the method of the present invention can provide α-alumina having a large BET specific surface area, high activity, and excellent sintering characteristics.

Example 1 Sodium aluminate solution (molar ratio: Na ₂ O / Al ₂ O ₃₎
−1.5, Na ₂ O = 110 g / 1), with respect to aluminum hydroxide (gypsite) precipitated, diaspore powder for specimen having a diameter of 0.7 μm with a 50% by weight diameter (produced in Liaoning Province, China, specific gravity: 3. 1) 5% by weight of alumina gel (specific gravity:
2.2) 10% by weight as seeds,
After stirring for 8 hours, a mixture mainly composed of gypsite was precipitated and precipitated. The precipitate was filtered, washed and dried, and then subjected to differential thermal analysis. As a result, the precipitate began to exhibit an exothermic peak of a transition from about 960 ° C., and was almost completely transformed at 1040 ° C. Separately, the above precipitate was placed in an electric furnace in an air atmosphere for 10 minutes.
As a result of identification by X-ray diffraction, the powder calcined at 50 ° C. for 2 hours was completely α-alumina, and had a BET specific surface area of 17 m ² / g. The powder fired in an electric furnace at 1050 ° C. for 2 hours is dry-pulverized in an alumina ball mill for 1 hour, and 1 g of the powder is reduced to 10 mm × 10 mm by 150 mm.
CIP molding at 1 MPa, and then the molded piece is 1450 ° C.
For 2 hours. As a result of measuring the density of this sintered body by the Archimedes method, the theoretical density of 98.
It was 0%.

Example 2 In the same sodium aluminate solution as in Example 1, α-alumina powder (specific gravity: 50% by weight) was prepared by hydrating a part of particles having a diameter of 0.3 μm with respect to the precipitated gypsite. 3.4)
5% by weight and 10% by weight of alumina gel were added. The partially hydrated α-alumina is wear powder of alumina balls obtained by wet-pulverizing only high-purity alumina balls (Al2O3: 99.8%) and water in a ball mill, and identifies the presence of gypsite by X-ray. And it was confirmed that a part was hydrated. Then, as in Example 1, 5
The mixture was stirred at 0 ° C. for 48 hours to precipitate a mixture mainly composed of gypsite. After treating this precipitate in the same manner as in Example 1,
As a result of differential thermal analysis, the precipitate began to change into a gelatinized state at about 980 ° C., and was almost completely converted at 1050 ° C. Separately, the powder obtained by calcining the precipitate at 1080 ° C. for 2 hours was completely identified as α-alumina by X-ray identification. As with Example 1, 1
A sintered body was prepared by sintering at 450 ° C. for 2 hours, and the density of the sintered body was measured to be 97.2% of the theoretical density.

Comparative Example 1 In Example 2, α-alumina powder having no hydrated part (specific gravity:
3.95, 50% by weight: 0.3 μm) was added in the same manner as in Example 2 except that 5% by weight was added to the precipitated gypsite.
An exothermic peak of α transition was observed from about 1050 ° C. to 1130 ° C. A powder obtained by firing the precipitate at 1150 ° C. for 2 hours was used to produce a sintered body in the same manner as in Example 2. The measured sintered body density was 93% of the theoretical density.

Comparative Example 2 The procedure of Example 1 was repeated except that no diaspore was added and 15% by weight of alumina gel was added to the precipitated gibbsite. As a result, in the differential thermal analysis, an exothermic peak of a transition from 1150 to 1230 ° C. ° C. was observed. The precipitate is fired at 1250 ° C.
The line was confirmed to be α-alumina, and the density of the sintered body was measured by the same operation. As a result, it was 86% of the theoretical density.

Claims (5)

[Claims] 1. An alumina-containing ore and a caustic soda solution are charged into an extraction device to extract a soluble alumina content in the alumina-containing ore to form a sodium aluminate solution having a high alumina concentration, and then an insoluble residue is separated from the solution. After removal, aluminum hydroxide as a seed is added to the sodium aluminate solution, and the sodium aluminate is hydrolyzed and aluminum hydroxide is precipitated with the seed as a nucleus.
In the so-called Bayer method, in the aluminum hydroxide precipitation step, in addition to aluminum hydroxide added as seeds, diaspore powder or / and α-alumina powder partially hydrated particles were added and precipitated. Characterized in that the mixture is homogeneously mixed with aluminum hydroxide, dehydrated and dried, and then the mixture is fired at a temperature of 1100 ° C. or less.
-A method for producing alumina. 2. The method according to claim 1, wherein
In the aluminum hydroxide precipitation step, diaspore powder or / and / or as seeds for aluminum hydroxide precipitation.
And adding α-alumina powder having partially hydrated particles, precipitating aluminum hydroxide using the seeds as nuclei, dehydrating and drying, and firing the precipitate at a temperature of 1100 ° C. or lower. To produce α-alumina. 3. A 50% by weight diameter of the diaspore powder and / or α-alumina powder partially hydrated to be added in the precipitation step is 0.01 to 50 μm. A method according to claim 1 or claim 2. 4. The total amount of the diaspore powder and / or α-alumina powder partially hydrated to be added in the precipitation step is 0.01 to 100% by weight based on aluminum hydroxide to be precipitated. %. 5. An α-alumina powder having a BET specific surface area of 10 m ² / g or more, produced by the method of claim 1 or 2.