JPH01148764A - Lightweight refractory and its manufacture - Google Patents

Abstract

PURPOSE:To obtain the title refractory having the crystal structure consisting essentially of mullite and corundum and having lightness and high strength as well as high heat resistance and durability by constituting it with alumina short fiber, alumina long fiber and refractory powder are mutually bonded by mullite. CONSTITUTION:The lightweight refractory is the porous molded body in which the alumina short fiber having <=2000mu length, the alumina long fiber having 1-40mm length and refractory powder are mutually bonded by mullite. Its crystal structure consists essentially of mullite and corundum, the amount of mullite is regulated to >=12mol.% for the total amount of mullite and corundum, and does not contain free silica substantially. The lightweight refractory can be obtd. by the following method: 10-30wt.% amorphous silica powder is added to the mixture of polycrystal alumina short fiber having 20-2000mu length and the polycrystal alumina long fiber having 1-40mm length or the mixture further added with the refractory powder of their equivalentant or below; the wholly mixture is molded and then sintered at 1400-1600 deg.C till the presence of cristobalite is made nonrecognized.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a lightweight refractory having a high degree of heat resistance. , ferrite elements, thermistors, varistors, etc.), ceramic sliding materials, general ceramics, etc., and firing aids such as saggers and stands used to support the object to be fired during the firing process. The present invention relates to a lightweight refractory that can withstand repeated heating and cooling and is suitable for heat shield plates, heating element supports, etc. in various furnaces.

[Conventional technology]

The above-mentioned firing aids and kiln constituent materials must have a high degree of heat resistance that can withstand repeated high-temperature heating and cooling, as well as mechanical strength appropriate to the intended use. In order to reduce the thermal energy required for heating and cooling, and to shorten the time required for heating and cooling, thereby reducing energy costs and improving productivity, it is desirable that the device be lightweight and have as small a heat capacity per volume as possible. In particular, in the case of baking aids, it is strongly desired that they be lightweight in order to facilitate transportation and other handling.

As one of the lightweight refractories to meet such demands, Japanese Patent Application Laid-Open No. 59-88378 describes a powder of refractory materials such as alumina, mullite, etc. 90 to 50 w[%] and alumina, mullite, etc. O-5 of amorphous silica is added to 100 parts by weight of aggregate consisting of 10 to 50 vL% of refractory fibers.
〜10 parts by weight was added, and then 1450〜
A refractory obtained by firing at 1600°C is described. However, since this lightweight refractory is made of high alumina and has a large coefficient of thermal expansion, there is room for improvement in terms of spalling resistance.

[Problem that the invention seeks to solve]

The object of the present invention is to provide a lightweight refractory without the drawbacks mentioned above;
That is, the object of the present invention is to provide a lightweight refractory that is lightweight and has high strength, yet exhibits high heat resistance and durability, and a method for producing the same.

[Means for solving problems]

The lightweight refractory provided by the present invention is a porous material in which short alumina fibers with a length of 2000μ or less, continuous alumina fibers with a length of 1 to 40 mm, and refractory powder are bonded together by mullite. A molded body whose crystal structure is mainly composed of mullite and corundum, and the amount of mullite is 1 of the total amount of mullite and corundum.
It is characterized by having a content of 2 mol % or more and substantially free of free silica.

The present invention also provides a particularly advantageous method for producing the lightweight refractories of the present invention, namely polycrystalline alumina short fibers cut to a length of 20 to 2000 μm and cut to a length of 1 to 40 m.
10 to 30% by weight of polycrystalline alumina continuous fibers cut into 50 mm or a mixture of these and an equal or less amount of refractory powder.
The present invention provides a manufacturing method characterized by adding and mixing an amorphous silica powder, molding the resulting mixture, and then firing at 1400 to 1600°C until the presence of cristobalite is no longer recognized.

First, to explain the above manufacturing method, in the firing process after forming the raw material mixture, the surface layer of the polycrystalline alumina fiber reacts with the amorphous silica powder or the crystalline silica/cristobalite produced from it, resulting in a moofite ( 3AlzOx・2Si02) is produced, and conomulite acts as a binder to form a strong molded body.

The unreacted fiber core remains as alumina (corundum) and maintains its fiber shape. Compared to the conventional method described above, this manufacturing method is characterized by cutting a portion (preferably most) of the polycrystalline alumina fibers into fine pieces, preventing the fibers from becoming tangled in the raw material mixing process and separating them from other powders. Raw materials (especially amorphous silica)
The key is to increase the blending ratio of fiber and amorphous silica so that the fibers and amorphous silica are mixed uniformly. A high fiber ratio helps improve product strength, and a high blending ratio of amorphous silica allows it to completely react with alumina (
In other words, making it mullite) lowers the alumina ratio in the product and helps improve the product's heat resistance.

Polycrystalline alumina short fibers of 2000 μm or less, which are part of the raw material, are prepared by cutting polycrystalline alumina fibers using a suitable wet or dry grinder.

Although alumina fibers having a length of 1 to 40 mm or more are used, the length of this extremely shortened fiber group must be 2000 μm or less. When the fibers are made short in this way, the fibers as a whole exhibit properties similar to those of powder, and mix well with other powder raw materials. Fibers with a fiber length exceeding 2000μ, especially when the fiber diameter is small, tend to become entangled during the raw material mixing process, making it difficult for other powder raw materials to enter therein. Therefore, if a large amount of amorphous silica is blended, the amorphous silica (or its cristobalite in the firing process) and polycrystalline alumina fibers will not react uniformly, and some of the silica will remain unreacted. It remains in the product as cristobalite and deteriorates the durability of the product (cristobalite itself has good heat resistance, but
The stable crystal forms are different in the high-temperature region and the low-temperature region bordering C, and the process of dislocation between these two crystal forms causes a volume change, so if cristobalite is present in a refractory, cracks may occur due to heating and cooling. cause the occurrence. ). However, if the fiber length is too short, it will be difficult to obtain a product with low specific gravity and excellent strength and durability.
It is desirable to set the lower limit to 0μ. A particularly preferred fiber length is about 50 to 500 microns, with an average of 200 microns.

Another polycrystalline alumina continuous fiber is 2m long.
m or more, preferably 1-41-4O optimally 2-20 m
Let it be m. Since this fiber is mainly used for product reinforcement, it should be used in a small amount (preferably 0.05% of the total amount of all polycrystalline alumina fibers and refractory powder).
It is necessary to pay attention to uniformly dispersing the weight pitch by 5 to 10% by weight. The preferred length is 2-20 Lam. Using fibers that are too long or using an excessive amount will result in a product with a non-uniform structure, which is likely to cause deterioration in physical properties.

Note that this fiber may have a length of 2,000 μm or less and only differs in length from the polycrystalline alumina short fiber used, but it is better to use a slightly thicker fiber with a fiber diameter of 5 to 20 μm.
It is preferable because it does not easily become tangled in the process of mixing the molding material, and also serves as a nucleus for agglomeration of the powder raw materials to improve moldability. Particularly suitable fibers are those obtained by cutting continuous fibers, such as chopped strands (but weakly converged so that they can be easily opened). Those prepared from continuous fibers have uniform fiber diameters and lengths both within and between single fibers, so it is possible to achieve a large reinforcing effect with a small amount by using those with optimal properties.

Examples of polycrystalline alumina fibers include Al2O, which has α-alumina as its main crystalline phase, approximately 95% and 5i025%, and A12037, which has mullite as its crystalline phase.
2%, about 510□5%, AlzOs 80%, which has both a-alumina and mullite as the crystal phase, 5I0
About 220%, and A I x Os S
i Includes Ox-B 203 series.

The amorphous silica powder accounts for 10 to 30% by weight of the total amount of polycrystalline alumina fibers and refractory powder. If the blending ratio of amorphous silica is less than 10% by weight, there is little risk that free silica will remain after firing, but the product will have a high alumina content. On the other hand, the blending ratio of amorphous silica is 3
If it exceeds 0% by weight, even if the polycrystalline alumina fibers are cut to achieve uniform mixing and sufficient firing is performed, some of the amorphous silica remains unreacted in the form of cristobalite. , which tends to cause poor heat resistance. The blending ratio of amorphous silica is the total A when also considering alumina or mullite introduced in the form of refractory powder.
I203/S io 2 weight ratio is 70/30 to 90/1
It is desirable to set it to 0.

As the amorphous silica, for example, silica sol can be used.

As the refractory powder, it is desirable to use high-purity crystalline powder such as alumina powder, mullite powder, and aluminumite powder in a range of 5 to 100 parts by weight per 100 parts by weight of polycrystalline alumina fibers, but in addition, , about 5% by weight
Up to this point, cordierite, chamotte, fireclay, kaolin, etc. may be used in combination.

These raw materials are mixed in the above-mentioned ratio, and an appropriate amount of water is added before and after mixing to make the whole into a wet state or a slurry state. Next, the raw material mixture is molded by a conventional method of dehydration molding.
It is desirable to conduct the test under conditions such that the ratio is 1.5 to 1.5. When the obtained molded body is dried and then fired at 1400 to 16000 C, the polycrystalline alumina fibers maintain their fibrous form and, as mentioned above, part of the surface layer becomes amorphous silica or amorphous silica. It reacts with cristobalite to form mullite, but the remaining part is stabilized as corundum. This reaction takes about 1 to 10 hours to completely consume the cristobalite, and when the firing process is carried out, a product with a crystal structure consisting mostly of corundum and mullite, of which mullite accounts for about 12 to 90 mol%, is obtained. It will be done. Insufficient calcinations that leave cristobalite exhibit the drawbacks of listoparite content as described above. Disappearance of cristobalite can be confirmed by ordinary powder X-ray diffraction method.

In the lightweight refractory of the present invention obtained as described above, corundum fibers having a fiber shape of 2000μ or less, although they are short, corundum fibers having a longer reinforcing effect, and refractory powder are present at their contact points. It is bonded by mullite and has a large amount of microscopic voids. The standard and preferred porosity is about 50-80%, so that the refractory has a bulk specific gravity of 0.5-1.5, and about 1/3 of the dense alumina-based refractory per unit volume. indicates the heat capacity of

The lightweight refractory of the present invention can be used as it is, or after being subjected to cutting or heat-resistant surface coating (for example, zirconia coating) as necessary, as a firing aid or a kiln constituent material as described above.

〔Effect of the invention〕

The lightweight refractory according to the invention has a large amount of mullite, which has better thermal properties than alumina, and is substantially free of free silica. Due to the high efficiency of reinforcement with polycrystalline alumina fibers, it exhibits far superior high-temperature durability than conventional high-alumina lightweight refractories even though it is obtained using polycrystalline alumina fibers as a raw material.

Moreover, even if the bulk specific gravity is about 1.0 or less, it is easy to obtain a material showing sufficient strength for practical use, and it is also easy to cut.

〔Example〕

The present invention will be described below with reference to Examples and Comparative Examples.

The raw materials used on each side are as follows.

Polycrystalline alumina fiber untreated product: fiber diameter 3μ, average fiber length approximately 50mmAl, 0
．． 95%, 5i025%.

Ultra-shortened product: The above-mentioned untreated product is opened and cut with a pulper to have a fiber length of about 50 to 500μ.

Reinforcing fiber = A1□O, -B 20, -3 io. Polycrystalline alumina fiber (A120368%, Si0□2
7%, 820.5%, single fiber fiber diameter 15μ, length 10m
Chopped strand shape of m; However, Comparative Example 8 is 1
mm, 30 mm for Comparative Example 9) Refractory powder: Sintered alumina Amorphous silica: Silica sol Among the raw materials above, polycrystalline alumina fibers and refractory powder are first dispersed in water (chopped strand reinforcement). After that, amorphous silica is added and stirred, followed by suction dehydration molding.

After drying the obtained molded body with hot air,
Bake at C for 3 hours.

Table 1 shows the results of experiments conducted by variously changing the mixing ratio of raw materials and processing conditions in the above manufacturing method.

The test method for the characteristic values shown in the table is as follows.

Bending strength: For a test piece with a thickness of 6 mm, width of 25 mm, and length of 75 mm, span of 50 mm, loading rate Q, 2H
/min, measured at room temperature or 1400°C.

Spalling resistance: A test piece with a thickness of 6 mm, a width of 200 mm, and a length of 200 mm was heated to 600°C! , placed in a furnace at 700°C or 800°C, heated for 1 hour, air cooled, and examined for cracks. If there is no crack, repeat the same operation again up to 30 cycles. The numerical values shown in the table are the number of times of heating that resulted in cracking.

Further, an X-ray diffraction chart showing the crystal structure of the product of Example 1 is shown in FIG.

[Brief explanation of the drawing]

Figure 1: X-ray diffraction chart showing the crystal structure of the refractory according to Example 1

Claims (7)

[Claims] (1) Alumina short fibers with a length of 2000μ or less,
A porous molded body made of alumina continuous fibers with a length of 1 to 40 mm and refractory powder bonded to each other by mullite, and the crystal structure is mainly composed of mullite and corundum, and the amount of mullite is equal to that of mullite. A lightweight refractory, characterized in that the amount is 12 mol % or more based on the total amount of corundum, and that free silica is substantially free. (2) The lightweight refractory according to claim 1, having a bulk specific gravity of 0.5 to 1.5. (3) A mixture of polycrystalline alumina short fibers with a length of 20 to 2000 μm, polycrystalline alumina continuous fibers with a length of 1 to 40 mm, or a mixture of these and an equal or less amount of refractory powder. 30% by weight of amorphous silica powder is added and mixed, the resulting mixture is molded, and then heated at 1400 to 160° C. until no cristobalite is present.
A method for producing lightweight refractories characterized by firing at 0°C. (4) The manufacturing method according to claim 3, in which powdered alumina, mullite, or aluminumite is used as the refractory powder. (5) The manufacturing method according to claim 3, wherein the polycrystalline alumina continuous fiber is used in an amount of 0.5 to 10% by weight based on the total amount of all polycrystalline alumina fibers and refractory powder. (6) Length 2-20 as polycrystalline alumina continuous fiber
3. The manufacturing method according to claim 3, wherein a material having a diameter of 5 to 20 μm is used. (7) The manufacturing method according to claim 3, in which cut continuous fibers are used as the polycrystalline alumina continuous fibers.