JPH03232719A - Production of readily grindable alumina - Google Patents

Abstract

PURPOSE:To obtain easily grindable alumina by pyrolyzing Bayer's method aluminium hydroxide at a specific temperature-raising rate, calcining the product and further calcining the prepared intermediate calcination alumina containing alpha-alumina as a main component in the presence of a mineralizing agent. CONSTITUTION:Aluminum hydroxide prepared by Bayer's method is pyrolyzed at a temperature-raising rate of <=20 deg.C/min to produce gamma-alumina free from water of crystallization. The gamma-alumina is subjected to an intermediate calcination treatment to prepare intermediately calcined alumina, which is further calcined in the presence of a mineralizer (e.g. aluminum fluoride) preferably at a temperature of 600-1500 deg.C to provide the objective alumina. The grinding of the prepared alumina for a short time permits to provide spherical alumina powder having a narrow fine particle diameter distribution of approximately 1-3mum and suitable as a raw material for ceramics.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention is an easy-to-pulverize alumina powder that can be easily pulverized to obtain a uniform alumina powder using Bayer process aluminum hydroxide as a raw material. It concerns the manufacturing method.

<Prior Art> Alumina powder has been widely used as a cooling agent or abrasive for manufacturing various ceramic products. In recent years, the use of ceramic materials in electronic devices such as integrated circuit boards has expanded, and the quality required for alumina has become significantly stricter.

Conventionally, as alumina for ceramic raw materials, alumina produced by firing aluminum hydroxide by the so-called Bayer process and pulverized as finely as possible has been mainly used. However, the alumina obtained by directly firing aluminum hydroxide using the conventional Bayer process has a particle size of several pm to several tens of μm.
Primary particles are strongly aggregated secondary particles with a count of several μm to over 100 μm, and not only does it take an extremely long time to crush these agglomerated particles, but the particle size distribution of the alumina obtained by crushing is It has been difficult to obtain alumina powder of fine quality suitable for use as a raw material for ceramics over a wide range of areas.

<Invention tries to solve! I! ! > The present invention provides a method for producing calcined alumina that can easily be made into fine alumina powder with a uniform particle size by pulverization using Bayer process aluminum hydroxide as a coolant. Means for Solving> The present inventors have conducted various studies in order to obtain calcined alumina that can be easily made into fine alumina powder with a uniform particle size by pulverization using Bayer process aluminum hydroxide as a raw material. As a result, Bayer process aluminum hydroxide, used as an IQ material, was thermally decomposed into so-called γ-alumina by heating it at a very slow temperature increase rate of 20°C per minute or less, and then this was further calcined at a high temperature. Through a series of operations such as producing intermediate calcined alumina consisting of 90% or more of α-alumina, and then calcining this intermediate calcined alumina in the presence of a mineralizing agent, it is easily reduced to a fine particle size of about 1-3 μm by pulverization. We have discovered a new fact that it is possible to obtain alumina powder that is spherical and has a uniform particle size.

The present invention has been made based on the above findings.

In other words, 1. The present invention heats Bayer process aluminum hydroxide at a temperature increase rate of 20° C. or less per minute to thermally decompose it, and then calcinates it to produce intermediate calcined alumina consisting of 90% or more of α-alumina, Thereafter, this intermediate calcined alumina is calcined in the presence of a mineralizing agent to produce alumina, which is easy to crush.

The present invention will be explained in detail below.

The raw material-alumina used in the present invention is aluminum hydroxide subjected to 1iJif+ by the Bayer method. Conventional Bayer Process When aluminum hydroxide is used as a raw material for ceramic materials, it is usually fired in a continuous firing furnace such as a rotary kiln or in a fluidized bed firing equipment to produce α-alumina. used as a form. This firing is usually 1000
The alumina obtained in this way is formed by the strong agglomeration of primary particles of tlum-I number/j Ill, and has a diameter of several μm - several μm in diameter.
Because they are coarse particles, they had to be crushed before they could be used as ceramic raw materials. However, the conventional calcined alumina described above has extremely poor pulverization properties, and not only is it difficult to achieve sufficient pulverization even after long-term pulverization, but the particle size of the obtained alumina powder is mostly human, with a particle size of several μmm. Moreover, it was difficult to obtain a well-defined granular powder.

Bayer process aluminum hydroxide is a so-called gibbsite-based alumina that has three molecules of water of crystallization, but when it is heated, dehydration through thermal decomposition is completed at a temperature of about 500 to 600"C, resulting in so-called γ-alumina. Next history 10
By maintaining the temperature at 00°C or higher, it changes into α-alumina as an edge-sintered product.

In the method of the present invention, when the Bayer process aluminum hydroxide as described above is calcined to produce α-alumina, at least as described above, the aluminum hydroxide is thermally decomposed and releases crystal water to produce γ-alumina. The temperature increase rate is 20℃ or less 1g per minute! This calcination, which is preferably carried out at a very slow speed of 16° C. or lower, may be carried out in any rotary kiln, tunnel kiln, shuttle kiln, or the like.

The alumina that has been γ-oxidized after thermal decomposition is then subjected to intermediate calcination so that it becomes 90% or more α-alumina, but there are no particular restrictions on the rate of temperature increase in this calcination. If the degree of gelatinization of the obtained intermediately fired alumina is lower than 90%, coarse plate-like particles that are difficult to crush during the next step of firing with a mineralizer will grow. , so it should be avoided. More preferably, the firing should be performed so that the α-alumina content becomes 95% or more.

The intermediate calcined alumina thus obtained is calcined at 000-1500°C in the presence of a mineralizer. Mineralizing agents include hydrofluoric acid, aluminum fluoride, sodium fluoride,
Fluorine compounds such as cryolite can be suitably used. Also,
Chloride gas or chlorides such as hydrochloric acid may be used; this calcination converts the intermediate calcined alumina to a crystalline state that is highly susceptible to crushing. For firing, any rotary kiln, tunnel kiln, shuttle kiln, etc. can be used.

For example, when using hydrofluoric acid as a hardening agent, the ratio of mineralizing agent to alumina is 60"C if the concentration of hydrogen fluoride near the pregelatinized alumina is about 10% by volume.
When the hydrogen fluoride concentration is about 2.5% by volume, a fired product with a desired particle size can be obtained by heating to a certain degree.
Since the crystal growth rate is somewhat slow when fired at about 00℃, it is desirable to bake at about 800"C.
5%, it is desirable to calcinate at a temperature of 1000°C or higher. When calcining with aluminum fluoride added as a mineralizing agent to alumina, decomposition of fluorine compounds in the calcining furnace, rli emission, etc. Although it is necessary to increase the amount of fluorine added slightly in consideration of scattering outside the system, it usually takes 1 minute to add 0.05% or more of the fluorine compound to α-alumina.

The intermediate calcined alumina calcined as shown in F is highly pulverized due to the presence of the mineralizing agent and has a good shape, so that it can be easily reduced to a diameter of 1-3 μm using a crushing device such as a ball mill.
It is possible to obtain granular alumina powder with a size of about 100 m.

As mentioned above, in the method of the present invention, when baking Bayer process 7km aluminum to convert it into α-alumina, at least until aluminum hydroxide thermally decomposes and releases crystal water to become so-called γ-alumina. heating rate of 20 per minute
After heating at a very slow well speed of less than ゛C, the temperature is lowered to 1000 yen and calcined to the extent that at least 90% of the alumina is pregelatinized to produce intermediate calcined alumina, which is then mineralized. A method for producing alumina that is easy to crush by heating and calcining in the presence of a chemical agent, and the alumina calcined by the method of the present invention has the property of being extremely easy to crush.
Particle size l-3 that can be easily passed through ceramic raw materials by crushing
Granular alumina powder of about μm size can be obtained.

Effect> The reason why calcined alumina that is easy to crush can be obtained by the method of the present invention is not fully elucidated, but the method of the present invention allows for thermal decomposition at an extremely slow temperature increase rate. The Bayer process aluminum hydroxide becomes transitional alumina with somewhat large micropores, and when this is alpha-ized, α-alumina with a coarse structure and cocoon-shaped chains is obtained. When this is heated in the presence of mineralizing agents,
It is thought that mass transfer through the gas phase breaks the connections between the particles, forming an aggregate of small spherical particles with relatively uniform particle size and high uni-f property, which improves the crushability. .

In addition, when using the method of the present invention, the sodium content contained as an impurity in the aluminum hydroxide of J-Kyoryo tends to be volatilized and removed during the firing process of intermediate fired alumina with the mineralizing agent. It also has the advantage of being able to obtain product alumina with low sodium content.

<Example> Next, examples of the present invention are listed.

Example 1 Aluminum hydroxide obtained by the Bayer method was heated to 1300"C at a heating rate of 3.3°C per minute to pyrolyze it. The temperature was then raised to 1300°C and pregelatinized to give an intermediate Calcined alumina was used.The degree of gelatinization was 100%.

8 parts of aluminum fluoride per 100 parts of this intermediate calcined alumina! Add 0.08 part at 1250℃ x 1
Time smoldered.

Example 2゜Execution V! The same heat treatment as in Example 41 was performed except that the intermediate firing temperature of aluminum hydroxide used in Example 41 was 1180°C. The degree of gelatinization of the obtained intermediate fired alumina was 90%. This was fired at 1100°C for 1 hour in a 2.5vol% hydrogen fluoride atmosphere using a tubular furnace. Note that the hydrogen fluoride gas was generated by passing air through a 38 vt % hydrogen fluoride water W4 solution heated to 45°C.

Example 3 Intermediate calcined alumina with a degree of gelatinization of 100% obtained in the same manner as in Example 1 was heated in a tubular furnace in the same manner as in Example 2 with 10 vol of hydrogen fluoride.
% air flow for 600''CXIUf.Hydrogen fluoride gas was generated in the same manner as in Example 2 except that the heating temperature of the hydrogen fluoride aqueous solution was 75°C.

Example 4 Intermediately fired alumina with a degree of gelatinization of 100% obtained in the same manner as in Example 1 was fired in a tubular furnace in the same manner as in Example 2, except that it was fired at 800° C. for 0.5 hours.

Example 5 Intermediate calcined alumina with a degree of gelatinization of 100% obtained in the same manner as in Example 1 was calcined in a tubular furnace in the same manner as in Example 2. Hydrogen fluoride gas was generated in the same manner as in Example 2, except that the holding temperature of the hydrogen fluoride aqueous solution was 17°C.

Comparative Example 1 The same method as in Example 1 was carried out except that the thermal decomposition was rapidly carried out in an air flow furnace by contacting with high temperature gas at 1000°C.

Comparative Example 2゜Aluminum hydroxide was heated to 600℃ at a heating rate of 3.3℃ per minute.
The fired product was heated to 800°C for thermal decomposition.
After heating to a certain degree (degree of a-oxidation: 0%), it was fired in the same manner as in Example 1 at 1250"CX for 1 hour in the presence of aluminum fluoride.

Comparative Example 3 After thermal decomposition in the same manner as Comparative Example 1, and then calcined for a short time at a temperature of about 900"C (degree of gelatinization 0%), hydrogen fluoride was dissolved using a tube furnace in the same manner as in Example 2. It was fired at 1100° C. for 1 hour in an air stream.

Comparative Example 4 Bayer process Aluminum hydroxide was thermally decomposed by heating to 600''C at a rate of 36℃ per minute, and then 1
The intermediate calcined alumina obtained (heated to 300° C. and gelatinized to a degree of gelatinization of 100%) was calcined in the same manner as in Example 2 in a hydrogen fluoride atmosphere.

Comparative Example 5 Except that the degree of gelatinization in calcination after pyrolysis was 50%.
The same method as in Comparative Example 4 was used.

Comparative Example 6 The same method as Comparative Example 4 was carried out except that the degree of gelatinization in the calcination after pyrolysis was 75%.

Comparative Example 7 The same method as in Example 4 was carried out except that the firing time was 500"C for 0.5 hours in a tube furnace.

Comparative Example 8 The same method as in Example 4 was carried out except that the hydrogen fluoride concentration in the air flow during firing in an 8° tube furnace was adjusted to 0.3 vol%. Hydrogen fluoride gas was generated in the same manner as in Example 2, except that the holding temperature of the hydrogen fluoride aqueous solution was 14°C.

Table 1 shows the crushability of the graded alumina obtained in Example 1-5 and Comparative Example 1-8.

The degree of gelatinization of the intermediate fired alumina shown in the table was determined by the XM regeneration method (Geigerflex RA D-11 manufactured by Rigaku Denki).
lA CuKa line) (+04), (+13), (
+10) was determined from the diffraction line.

The pulverizability was measured by wet pulverization using the following pulverizer, and its particle size distribution was measured by the Sedigraph method, and expressed as the time required for pulverization until the average particle size reached 2 μm.

Tabletop planetary ball mill (P- manufactured by Fritsch Japan)
7) Grinding container 12m1 sintered alumina bowl
5 mm mφ Alumina ball M 12g Sample rim 2.5g Water bar Sodium birophosphate 0.0005 sol/
l Aqueous solution 2.5 g Container 470 rpm, bottom & 1900 rpm - rotation speed l <Effects of the invention> As stated in 1-1, the calcined alumina obtained by the method of the present invention is extremely easy to crush, and can be crushed in diameter by short-time crushing. It is possible to obtain ultrafine, uniform, spherical alumina powder with a particle size of about 1-371.

[Brief explanation of drawings]

The alumina obtained in Example 1 and FIGS. 2 to 5 are those of Comparative Example 1. Comparative Example 2 Alumina obtained in Comparative Example 5 and Comparative Example 7 is shown.

Claims (2)

[Claims] (1) Bayer method Aluminum hydroxide 20% per minute
After thermal decomposition by heating at a temperature increase rate of ℃ or less, this is calcined to produce an intermediate calcined alumina consisting of 90% or more of α-alumina, and then this intermediate calcined alumina is calcined in the presence of a mineralizing agent. A method for producing alumina that is easy to crush. (2) The method for producing alumina that is easy to crush according to claim 1, wherein a fluorine compound is used as the mineralizing agent. (3) The method for producing easily pulverized alumina according to claim 1 or 2, characterized in that the intermediate calcined alumina is calcined in the presence of a mineralizer at a temperature of 600 to 1500°C.