JPH0217643B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing highly flexible alumina fibers. Conventionally, alumina fibers have been manufactured, for example, as follows. First, an aluminum salt, for example basic aluminum chloride (Al ₂ (OH) ₅ Cl.
A viscous spinning solution is prepared by heating and concentrating an aqueous solution of 2.4H ₂ O). Next, this spinning solution is put into a centrifugal spinner having a large number of holes on the outer circumferential surface and rotated at high speed.
The cotton is collected in a cotton collection room to obtain spun fibers. Subsequently, the spun fibers are dried to obtain dry fibers. Continuing,
The dried fibers are preheated at about 600° C. to remove most of the water and chlorine and convert them into oxides to obtain primary fibers. Next, this primary fiber is placed in the air,
Heat treatment is performed at approximately 1000℃ to obtain the final alumina fiber. However, the alumina fibers obtained by the conventional method described above have a drawback of poor flexibility at high temperatures of 1500° C. or higher. The present invention has been made in order to eliminate the above-mentioned drawbacks, and aims to provide a method for producing alumina fibers that have high flexibility even at high temperatures. Alumina fibers (usually with a diameter of 3-5 μm)
The flexibility of fibers at high temperatures is affected by the size of the crystallites that make up the fibers and the number of pores in the crystallites, and it is thought that the larger the crystallites and the more pores, the poorer the flexibility. . Therefore, the present inventors conducted various studies on conditions that could eliminate the causes that adversely affect the flexibility of alumina fibers as described above, and first investigated the effect of high-temperature heat treatment in a hydrogen atmosphere. That is,
If the primary fiber obtained by preliminary heat treatment (dehydration and dechlorination) at 600℃ is heat treated at 1000℃ or higher in a hydrogen atmosphere, the pores in the crystallites that make up the final alumina fiber will be eliminated. can do. However, it has been found that the flexibility of alumina fibers cannot be sufficiently improved only by the above-described high-temperature heat treatment in a hydrogen atmosphere. In other words, when we investigated how the weight of 100 parts of spun fiber changes depending on the subsequent steps, we found that the dry fiber was 77.4 parts by weight, the primary fiber after preliminary heat treatment was 42.4 parts by weight, and the weight of 100 parts by weight of spun fiber was 77.4 parts by weight after preliminary heat treatment. The final alumina fiber after heat treatment is
The amount was 35.9 parts by weight, and it was found that the volumetric shrinkage during preliminary heat treatment of dry fibers was large. Therefore,
Pores and cracks occur due to volumetric shrinkage of the fibers during preliminary heat treatment, and high-temperature heat treatment in a hydrogen atmosphere alone cannot sufficiently eliminate these pores and cracks, and also causes abnormal growth of crystallites. It was found that the flexibility of alumina fibers could not be sufficiently improved because of the inability to sufficiently prevent this. Therefore, as a result of further investigation, the present inventors found that
In addition to the condition of performing high-temperature heat treatment in a hydrogen atmosphere, the researchers discovered that it is sufficient to add an appropriate amount of magnesium salt to the starting material, aluminum salt. In other words, if an appropriate amount of magnesium salt is added at the beginning, magnesia, which is converted into an oxide and generated during preliminary heat treatment, can suppress the volumetric shrinkage of the fibers and also prevent abnormal growth of crystallites during high-temperature heat treatment in a hydrogen atmosphere. can do. In addition, since an appropriate amount of magnesia is effective in eliminating pores both during preliminary heat treatment and high-temperature heat treatment in a hydrogen atmosphere, it is possible to improve the flexibility of the alumina fiber. Furthermore, we investigated the amount of magnesium salt added and found that if the magnesium salt is added so that the amount of aluminum salt and magnesium salt is 6% by weight or less in terms of magnesia in the final alumina fiber that has been converted into alumina and magnesia, the alumina fiber can be improved. We have discovered that flexibility can be significantly improved. That is, the method for producing alumina fibers of the present invention involves heating and concentrating an aqueous solution containing an aluminum salt and a magnesium salt to form a viscous spinning solution, drying the spun fibers, and then preheating them to dehydrate and dechlorinate them. Furthermore, it is characterized by containing 6% by weight or less of magnesia by heat treatment at high temperature in a hydrogen atmosphere. In the present invention, the lower limit of the amount of magnesium salt added to the aluminum salt is preferably about 0.1% by weight in terms of magnesia in the final alumina fiber. Further, the high temperature heat treatment in a hydrogen atmosphere in the present invention is desirably performed at a temperature of 1000°C or higher, preferably 1200°C or higher. This is because at temperatures below 1000°C, the effect of eliminating pores in the crystallites constituting the fibers is small. Examples of the present invention will be described below. Examples 1 to 6 and Comparative Examples 1 to 4 First, in a 50% aqueous solution of basic aluminum chloride [Al ₂ (OH) ₅ Cl2.4H ₂ O), when converted into Al ₂ O ₃ and MgO, in terms of MgO, Magnesium chloride (MgCl ₂ .6H ₂ O) was added in the weight percent shown in the table below. Next, 120 parts by weight of lactic acid was added to this aqueous solution based on 100 parts by weight of the solid content of the basic aluminum chloride and magnesium chloride.
Added parts by weight. Subsequently, the mixture was heated and concentrated to prepare a spinning solution having a viscosity of 150 poise at room temperature. Next, this spinning solution was put into a centrifugal spinner with a large number of holes with a diameter of 0.4 mm, and rotated at a speed of 3600 rpm.
The fibers were collected in a collection room with a relative humidity of 50% to obtain spun fibers. The average diameter of the resulting spun fibers was 4 μm. Next, this spun fiber was processed at room temperature with a relative humidity of 50°C.
% in air to obtain dry fibers. Subsequently, the dried fibers were passed through a furnace at a maximum temperature of 600°C using a net conveyor at a heating rate of 35°C/min. This preheat treatment removes most of the water and chlorine and converts the starting material to oxide.
As a result, primary fibers were obtained which were white in color, easily powdered, and opaque under a microscope. Next, this primary fiber was placed in a sealed furnace, and the inside of the furnace was replaced with N ₂ gas, and then replaced with H ₂ gas, and then heated to 1000°C.
Alternatively, final alumina fibers were obtained by rapidly raising the temperature to 1200°C and maintaining it for 1 hour. Each of the obtained alumina fibers was made into a 12 mm thick blanket, and the number of times of rupture and bending of this blanket at 1500°C and 1600°C was determined and is also listed in the table below. In addition, in Comparative Examples 1 and 4 in the table below, the amount of magnesium salt added (magnesia content in the alumina fiber in the table) is outside the range of the present invention, and in Comparative Examples 2 and 3, the magnesia content is outside the range of the present invention. However, the high-temperature heat treatment was performed in air instead of in a hydrogen atmosphere.

【table】

[Table] As is clear from the above table, the alumina fibers of Comparative Examples 1 and 4 have MgO contents outside the range of the present invention, so both have a small number of bending times at break and are inferior in flexibility. Furthermore, the alumina fibers of Comparative Examples 2 and 3 had poor flexibility because they were subjected to high-temperature heat treatment in air rather than in hydrogen. On the other hand, Examples 1-
All of the alumina fibers of Example 6 have improved flexibility, and especially those of Examples 2 and 3 show high flexibility even at 1600°C. As described in detail above, according to the present invention, it is possible to provide a method for producing alumina fibers having high flexibility even at high temperatures.

Claims (1)

[Claims] 1. By heating and concentrating an aqueous solution of aluminum salts and magnesium salts to form a viscous spinning solution, and drying the fibers spun from this, preheat treatment is performed to dehydrate and dechlorinate, and further heat treatment at high temperature in a hydrogen atmosphere. A method for producing alumina fiber, characterized by containing 6% by weight or less of magnesia.