JPS6335573B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing low soda alumina. More specifically, a method for producing low-soda alumina in which a silica-based substance is added to and mixed with aluminum hydroxide or alumina containing a soda content, heat-treated in the presence of a fluorine-based substance, and then the silica-based substance and alumina are separated. It is related to.

In recent years, high alumina materials have been used in large quantities for spark plug insulators for internal combustion engines, insulating materials such as porcelain for electronic components, and heat-resistant and wear-resistant porcelain. Because it reduces electrical insulation and fire resistance, it is required to reduce its content as much as possible.

However, since alumina, which is currently produced industrially at low cost and in large quantities, is produced by the Bayer process, the resulting alumina usually contains 0.2 to 0.7% by weight of sodium as Na ₂ O due to the process. Therefore, this alumina cannot be used directly as a raw material for the above purpose.

Therefore, various methods have been studied to remove the soda content from the alumina obtained by the Bayer process. Cleaning method Adding boric acid and baking to form water-soluble sodium borate, followed by washing to remove the soda component Aluminum fluoride, boric acid, chlorine, etc. are added and mixed with aluminum hydroxide or alumina. Method of post-calcining to volatilize soda compounds Mix silica-based material with a larger particle size than aluminum hydroxide or alumina and sinter it to make the soda adhere to the silica-based material, then sieve the silica-based material and alumina. Separation method Adding a silica-based substance and hydrochloric acid or aluminum chloride to aluminum hydroxide or alumina,
A method has been proposed in which, after mixing and firing, the silica-based material is sieved from the alumina.

However, the above methods and methods are complicated and do not remove the soda content sufficiently, and the method requires extremely high temperatures to achieve the soda removal effect because the soda content contained in alumina is difficult to volatilize. It is difficult to say that this is an appropriate method as it requires treatment and causes significant corrosion of the firing furnace material. Another method is to lower the vapor pressure of the soda in the furnace by reacting the volatilized soda with a silica-based material, making it easier to volatilize the soda than alumina, but this method also requires high-temperature treatment. As a result, the firing furnace material is severely corroded, and the silica-based material deteriorates at high temperatures, causing surface peeling and reducing the purity of the alumina. A further method is to add hydrochloric acid or aluminum chloride and react with soda to form NaCl, which is easily volatilized, and to adsorb this onto a silica-based material, simply by adding only the silica-based material and heating. Although the soda removal rate is excellent compared to the method of adsorbing and removing the soda content, the firing temperature is not much different from the above method and is still not fully satisfactory.

Under these circumstances, the inventors of the present invention made extensive studies to improve the above-mentioned drawbacks, and found that when fluorine-based substances and silica-based substances are present during sintering of aluminum hydroxide or alumina, the sintering temperature can be lowered than before. They found that the soda removal rate could be lowered and the soda removal rate improved, and the method of the present invention was completed.

That is, the present invention is characterized in that a silica-based substance is added to and mixed with aluminum hydroxide or alumina containing a soda content, heat-treated in the presence of a fluorine-based substance, and then the silica-based substance and alumina are separated. The present invention provides a method for producing low soda alumina.

The soda-containing aluminum hydroxide or alumina to be treated in the method of the present invention is not particularly limited, and includes aluminum hydroxide or partially calcined alumina usually obtained from the Bayer process;
Examples include calcined alumina.

On the other hand, as a silica-based substance, the particle size is 0.3 mm to 2 mm.
degree of silica, quartz, silica sand, siyamoto, mullite, sillimanite, magnesium silicate,
Alumina silicate etc. can be used.

If the particle size of the silica-based material is smaller than 0.3 mm, it will be difficult to separate it from the alumina after firing, which is undesirable.If it is larger than 2 mm, the desodalizing effect will be poor. This is not preferable because it requires a large amount of addition, which lowers both economic efficiency and workability.

The amount added is 1 to 20% by weight (calculated as SiO ₂ ), preferably 3 to 10% by weight, based on alumina (dry weight basis).

On the other hand, examples of fluorine-based substances include hydrogen fluoride gas, fluorine gas, aluminum fluoride, calcium fluoride, magnesium fluoride, ammonium fluoride, and the like.

The amount of the fluorine-based substance added to aluminum hydroxide or alumina is 0.02% by weight or more (in terms of _F2 ), more preferably 0.1 to 0.3% by weight, based on the dry weight of alumina. If the amount of the fluorine-based substance added is less than 0.02% by weight, the effect of the addition of the fluorine-based substance will not be sufficient, and it is difficult to expect a soda removal effect at low temperature firing. In addition, there is no particular upper limit on the amount of fluorine-based substances added, but even if a large amount is added, there will be no firing temperature lowering effect or soda removal effect commensurate with the added amount, so it is recommended to add fluorine-based substances for reasons of economy and equipment protection. The limit is determined by itself.

When carrying out the method of the present invention, the firing conditions are not unique depending on the firing furnace used, but firing is usually carried out at a temperature of 1000°C or higher, more preferably 1100 to 1400°C for several minutes or more, generally 10 minutes to 3 hours.

If the firing temperature is lower than 1000°C, it is difficult to remove soda from the alumina, while if it exceeds 1400°C, fluorine volatilization will be severe, which is undesirable, for example, causing large wear and tear on the bricks of the kiln wall.

The type and model of the firing furnace used is not particularly limited as long as the firing temperature can be obtained, and examples include a rotary kiln, a roller hearth kiln, and an electric furnace. Among other things, the use of a rotary kiln built into the Bayer process for calcination of aluminum hydroxide makes it possible to omit the operation of uniformly mixing silica-containing substances and fluorine-containing substances in a separate process, and requires additional calcination energy. This is the most recommended method from the economic point of view, as it is not necessary.

The alumina calcined in this manner is then sieved to separate silica-based substances to obtain low soda alumina.

According to the method of the present invention described in detail above, compared to the method of adding only silica-based substances, the soda content in alumina or aluminum hydroxide is more likely to volatilize due to the action of fluorine present in the firing system.
It is now possible to remove soda by firing at lower temperatures.
In other words, by implementing the method of the present invention at the same firing temperature, an excellent soda removal effect can be achieved,
Furthermore, compared to a method in which aluminum hydroxide or alumina is impregnated with hydrochloric acid, etc. and mixed with a silica-based material and fired, it has the advantage of superior soda removal efficiency at the same firing temperature, although the reason is not clear. Its industrial value is enormous.

The method of the present invention will be explained in more detail with reference to Examples below, but the Examples show one embodiment of the method of the present invention, and the method of the present invention is not limited thereby.

In the examples, all percentages are by weight unless otherwise specified.

Example 1 Aluminum hydroxide by Bayer method at 500℃
To calcined alumina (Na ₂ O, 0.30%), 0.2% aluminum fluoride as F ₂ and 10% silica sand of 0.5 to 1.19 m/m were added and mixed, placed in an alumina crucible, and heated in an electric furnace. It was baked at 1150°C for 2 hours.
After cooling, silica sand was removed using a 149μ sieve to obtain alumina. Na ₂ O in this alumina is 0.03%,
SiO ₂ was 0.02%. SiO ₂ in raw material alumina is
Since it was 0.01%, there was almost no silica contamination.

For comparison, alumina calcined without adding aluminum fluoride and treated under the same conditions as above.
Na ₂ O was 0.22%.

Further, the Na ₂ O content in alumina treated under the same conditions as in Example 1 except that silica sand was not added was 0.26%.

Example 2 Aluminum fluoride was added to the same calcined alumina used in Example 1 as _F2 at 0.1%, 0.5 to 1.19 m/
5% of silica sand was added and mixed, and the mixture was placed in an alumina crucible and fired at 1250°C for 2 hours in an electric furnace. After cooling, silica sand was removed using a 149μ sieve to obtain alumina. This alumina contained 0.03% Na ₂ O and 0.01% SiO ₂ .

Example 3 The alumina was calcined and treated in the same manner as in Example 1 except that uncalcined Bayer process aluminum hydroxide was used as the raw material, and the Na ₂ O content in the alumina was 0.02%.

Example 4 Alumina in alumina calcined and treated in the same manner as in Example 1 except that alumina (Na ₂ O, 0.32%) with a pregelatinization rate of 35% obtained from the Bayer process was used as a raw material.
Na ₂ O was 0.04%.

Example 5 0.2% aluminum fluoride as F was added to the same calcined alumina as used in Example 1, 0.5 to 1.19 m/
Add 2% of quartz sand, mix and heat in an electric furnace for 1200 m.
It was baked at ℃ for 2 hours. After cooling, silica sand was removed using a 149μ sieve to obtain alumina. in this alumina
Na ₂ O was 0.04%.

For comparison, Na 2 O in alumina was treated under the same conditions as in Example 5 except that 0.4% hydrochloric acid was added as HCl instead of aluminum fluoride. Na ₂ O in alumina was 0.08%.
It was hot.

Example 6 The same treatment as in Example 1 was carried out except that aluminum fluoride in Example 1 was added instead of 0.2% calcium fluoride in terms of _F2 . Na ₂ O and SiO ₂ in the alumina thus obtained are each
It was 0.03%.

Example 7 The same treatment as in Example 1 was carried out except that aluminum fluoride in Example 1 was replaced with 0.2% ammonium fluoride in terms of F ₂ . The alumina thus obtained contains 0.03% Na ₂ O and SiO ₂
It was 0.02%.

Example 8 Fluorine gas was introduced into the same calcined alumina used in Example 1, and after adsorbing 0.2% _F2 , 0.5-
1.19 mm of silica sand was added at 10%, mixed, placed in an alumina crucible, and fired at 1150°C for 2 hours in an electric furnace. After cooling, silica sand was removed using a 149 μm sieve to obtain alumina. The alumina thus obtained contained 0.03% Na ₂ O and 0.02% SiO ₂ .

Claims (1)

[Claims] 1. A low-soda alumina characterized by adding and mixing a silica-based substance to aluminum hydroxide or alumina containing a soda content, heat-treating the mixture in the presence of a fluorine-based substance, and then separating the silica-based substance and alumina. manufacturing method.