JPH0357207B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing alumina fibers, and particularly to a method for producing alumina fibers having a high specific surface area and suitable as catalyst carriers.

Alumina fibers are mainly used as high-temperature insulation materials, but their use as catalyst carriers is also being considered. In addition to fire resistance, catalyst carriers are generally required to have a large specific surface area, but alumina fibers produced by conventional methods from aluminum oxychloride solutions have a relatively small specific surface area and often do not meet this requirement. . It is known that alumina fibers with a high specific surface area can be obtained by performing a modification treatment such as steam treatment on precursor fibers before firing, which are formed from an aluminum oxychloride solution by a conventional method, and then firing them. This method requires extra costs and is not advantageous as an industrial method.

The present inventors have discovered that by including urea in the spinning dope, alumina fibers with a high specific surface area can be produced without any special treatment during the process.

The present invention was completed based on such knowledge, and its gist is to fiberize a spinning dope containing aluminum oxychloride, a silicon compound, and urea,
Next, there is a method for producing alumina fiber, which is characterized by firing at a temperature of 500°C or higher.

To explain the present invention in more detail, the spinning dope used in the present invention includes aluminum oxychloride,
It is necessary to contain silicon compounds and urea,
Furthermore, it is preferable that an organic polymer is included in addition to these. The method for producing an aluminum oxychloride solution is known, and can be easily produced, for example, by dissolving metallic aluminum in hydrochloric acid or an aqueous aluminum chloride solution. The Al/Cl atomic ratio of aluminum oxychloride is usually 1.0 to 2.0, preferably 1.6 to 1.9. If this ratio is too small, it will be difficult to obtain a solution with an appropriate aluminum concentration as a spinning dope; on the other hand, if this ratio is too large, the solution will become unstable and there is a risk that an aluminum hydroxide gel will precipitate.

Silica sol is preferred as the silicon compound, but
Water-soluble silicon compounds such as tetraethyl silicate and water-soluble siloxane derivatives are also used. These silicon compounds also change into silica during the firing process of the precursor fiber obtained by spinning, and have the effect of suppressing the conversion of alumina into α-alumina and suppressing the crystal growth of alumina. The ratio of aluminum oxychloride and silicon compound in the spinning dope is Al ₂ O ₃ and
The ratio to SiO ₂ is preferably in the range of 99:1 to 80:20. If the amount of silicon compound is less than this range, the alumina constituting the fiber will become α-
It easily turns into alumina, and the alumina particles tend to become coarse and the fibers become brittle. On the other hand, if the amount of silicon compound is too large, the specific surface area of the alumina fibers produced decreases due to the production of mullite (3Al ₂ O ₃ .2SiO ₂ ) in addition to the decrease in the alumina content in the alumina fibers.

Urea usually has a molar ratio of chlorine contained in aluminum oxychloride (CO(NH ₂ ) ₂ /Cl ₂ ).
It is present in the spinning dope so that the ratio is 1.0 or more.
It is thought that urea reacts with chlorine to form ammonium chloride during the firing process of the precursor fiber obtained by spinning, thereby promoting volatilization of chlorine. Therefore, it is sufficient to use urea in an amount necessary to convert chlorine in the spinning solution into ammonium chloride, that is, in an amount equivalent to chlorine, and the effect will not be further increased even if it is used in excess. A suitable amount of urea present is in the range of 1.0 to 1.1 as a molar ratio of urea to chlorine contained in aluminum oxychloride [CO( _NH2 ) ₂ / _Cl2 ]. If the abundance ratio of urea becomes too large, the spinning dope may become cloudy.

Preferably, an organic polymer is present in the spinning dope. Although it is possible to carry out the present invention using a spinning dope that is simply an aqueous solution of aluminum oxychloride added with a silicon compound and urea and concentrated to a predetermined concentration, if an organic polymer is present in the dope, Spinnability improves.

As the organic polymer, various natural or synthetic polymer compounds capable of forming fibers can be used. For example, soluble derivatives of starch and cellulose such as acetated starch, hydroxyethyl starch, methylcellulose, and carboxymethylcellulose, and water-soluble synthetic polymer compounds such as polyvinyl alcohol, polyethylene glycol, and polyacrylamide are used. When selecting an organic polymer, care must be taken to ensure that the spinning stock solution does not become cloudy or precipitate. The organic polymer is added to the spinning dope so that the viscosity is optimal for the spinning method applied, but generally it may be added so that the spinning dope has a viscosity of 1 to 1000 poise.

The spinning stock solution can be prepared by adding a silicon compound, urea and an organic polymer to an aqueous aluminum oxychloride solution and concentrating the solution to a predetermined aluminum concentration. In addition, at temperatures above 80℃, urea decomposes and increases the pH of the solution, which may result in precipitation of aluminum hydroxide, so concentrate under reduced pressure and raise the temperature of the solution above 80℃. It is preferable to do what you want. Also, if desired, the silicon compound, urea and organic polymer may be added to the solution during or after concentration. In particular, organic polymers may cause foaming during concentration, and in such cases it is preferable to add the organic polymer after concentration.

Precursor fiberization of the spinning dope can be carried out by any known spinning method, such as an extrusion method, a stretching method, a blowing method, or a centrifugation method. For example, in the case of the blowing method, spinning is performed by extruding a spinning stock solution adjusted to 5 to 100 poise into a high-speed air stream through pores of 0.1 to 0.5 mmφ. The extruded spinning stock solution is stretched and dried in an air stream to become precursor fibers. This method requires that the precursor fibers be sufficiently dried before being collected from the air stream. If drying is insufficient, the collected precursor fibers may adhere to each other or become droplets due to elastic recovery, resulting in the formation of shatter. Therefore,
If necessary, heated air may be used to promote evaporation of the solvent without causing foaming.

Firing of the precursor fiber obtained by spinning is also carried out by a conventional method. Firing at 500℃ or higher, preferably 600-1000℃
It is carried out at ℃. At temperatures below 500°C, the alumina fibers obtained have low strength and are brittle, making them unsuitable for practical use.
Moreover, when heated to 1200°C or higher, α-alumina formation progresses, and the specific surface area of the resulting fiber becomes smaller.

According to the present invention, alumina fibers with a high specific surface area can be easily produced by simply allowing urea to be present in the spinning solution without any special treatment during the process.

The present invention will be explained in more detail with reference to Examples below, but the present invention is not limited to the following Examples unless it exceeds the gist thereof.

Example 1 Aluminum oxychloride aqueous solution (aluminum content 70 g/l, Al/Cl (atomic ratio) = 1.8) 1,
35 g of 20% silica sol solution (manufactured by Nissan Chemical Industries, Ltd., trade name: Snowtex 0), 278 g of 5% aqueous polyvinyl alcohol solution (manufactured by Nippon Gosei Kagaku Kogyo Co., Ltd., trade name: GH-17), and 450 g of 10% aqueous urea solution.
was added and mixed well. The mixture was heated to 50℃ under reduced pressure.
It was concentrated to obtain a spinning stock solution (viscosity: 22 poise, alumina content: 26.5 wt%). Diameter from this spinning dope
Precursor fibers were spun by a blowing method using a 0.5 mm nozzle, and then fired in air at 800° C. for 1 hour to obtain alumina fibers. This one has an average diameter of 3μ
The specific surface area measured by the BET method was 163 m ² /g.

Comparative Example 1 In the method of Example 1, a mixture having exactly the same composition except that it did not contain urea was prepared, and this was heated under reduced pressure.
It was concentrated at 50°C to obtain a spinning stock solution (viscosity 24 poise, alumina content 25 wt%). Alumina fibers were produced from this spinning dope in exactly the same manner as in Example 1. The resulting fibers are brittle and have a specific surface area of 10
m ² /g or less.

Claims (1)

[Scope of Claims] 1. A method for producing alumina fiber, which comprises forming a spinning dope containing aluminum oxychloride, a silicon compound, and urea into fibers, and then firing the fiber at a temperature of 500°C or higher. 2. The method according to claim 1, wherein the spinning dope contains an organic polymer. 3 The ratio of aluminum to silicon in the spinning stock solution is
In terms of ratio of Al ₂ O ₃ and SiO ₂ , it is 99:1 to 80:20
A method according to claim 1 or 2, characterized in that the method is within the scope of. 4 Molar ratio of urea to chlorine contained in aluminum oxychloride in the spinning dope [CO(NH ₂ ) ₂ /
Cl ₂ ] is in the range of 1.0 to 1.1, the method according to any one of claims 1 to 3.