JPH08217567A - Production of porous silicon nitride refractory - Google Patents

Abstract

PURPOSE: To easily produce porous silicon nitride refractories used as a filter and a heat insulator for an aluminum melting furnace at a low cost in a high yield. CONSTITUTION: An alkaline aq. soln. is added to a mixture of Si3 N4 powder with Si powder, they are molded while allowing the Si to self-foam and the resultant molded body is nitrided and fired at 1,200-1,450 deg.C firing temp. in an N2 atmosphere having <=500ppm concn. of O2 in the firing atmosphere to produce the objective silicon nitride refractories.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a silicon nitride porous refractory used for a heat insulating material for an aluminum melting furnace.

[0002]

2. Description of the Related Art Porous silicon nitride refractories are known for their excellent fire resistance, heat resistance and thermal shock resistance, and since they have a large wetting angle with aluminum, they are heat insulating materials and filters for aluminum melting furnaces. Widely used in. But,
The conventional method for producing porous silicon nitride has a high firing temperature,
This is a problem in that the product yield is reduced, such as deformation during firing and a degreasing step required for molding. Therefore, it has a drawback that the product cost becomes high.

[0003]

Generally, in order to produce a silicon nitride porous refractory, a method is used in which a foaming agent is added to a silicon nitride powder in the form of a slurry, followed by molding and firing. This method has a high sintering temperature and has a problem of deformation due to shrinkage during firing. Further, in the method of adding a large amount of a thermally decomposable resin or the like to the silicon nitride powder and thermally decomposing during sintering to generate pores, the method of sintering the silicon nitride powder has a high porosity. However, in order to obtain a high porosity, a degreasing step is required after adding a large amount of a thermally decomposable resin and molding. For this reason, the bonds between the particles become sparse during firing, and deformation due to firing shrinkage and the shape retention strength of the fired body are lacking, so that a decrease in yield due to cracking or the like cannot be avoided. As yet another manufacturing method, it has been proposed to impregnate urethane foam with slurry silicon nitride powder, but again, the sintering temperature is high, and there is a problem that deformation due to firing shrinkage occurs during firing. was there.

[0004]

The present invention has been studied with the object of eliminating and improving the above-mentioned conventional drawbacks. That is, in a method for manufacturing a porous refractory made of silicon nitride, an alkaline aqueous solution is added to a mixture consisting of Si ₃ N ₄ and Si, Si is self-foamed and molded, and the O ₂ concentration is 500 ppm or less. Nitriding firing is performed at 1200 ° C to 1450 ° C in an N ₂ atmosphere.

[0005]

The method for producing a porous silicon nitride refractory according to the present invention utilizes the phenomenon that metallic Si foams in an alkaline solution, and adds an alkaline aqueous solution to a mixture of Si ₃ N ₄ and Si to foam the mixture. While refining, the nitriding firing is performed in an N ₂ atmosphere. Here, the surface of Si used reacts with an alkaline aqueous solution,
SiO ₂ is generated while generating heat of formation. Since bubbles are generated during this reaction, the slurry in the mold undergoes foam expansion, and then the slurry is dried and solidified by the heat of reaction.
The generated SiO ₂ acts as a binder for the Si ₃ N ₄ and Si compacts, so that the compact can maintain the compact strength.
The pore diameter and porosity of the molded body can be controlled by adjusting the particle diameter of Si to be used and the pH and viscosity of the slurry. In addition, by performing nitriding firing in an N ₂ , N ₂ + H ₂ , NH ₃ atmosphere, S ₃ N _{4 in} the Si foam molded article
SiO ₂ generated on the surfaces of the i and Si particles causes a nitriding reaction to generate Si ₃ N ₄ . As the Si powder used here, it is preferable to use a powder having a particle size of 100 μm or less in order to completely nitride the molded body. The reason for setting the O ₂ concentration in the firing atmosphere to 500 ppm or less is that the O ₂ concentration is 500 ppm.
Above ppm, S and S generated on the surface of Si particles
This is because iO ₂ does not cause a nitriding reaction. Further, the reason for setting the firing temperature to 1200 ° C. to 1450 ° C. is the temperature for causing the nitriding reaction of Si and SiO ₂ generated on the surface of the Si particles, but Si sufficiently nitridates with N ₂ in the atmosphere, This is the temperature range required to generate Si ₃ N ₄ while generating heat. At this time, when the temperature of the fired body suddenly exceeds 1420 ° C. due to the heat of reaction, unreacted Si melts and agglomerates before the production of low-temperature α-Si ₃ N ₄ is completed, so that the inside of the fired body Therefore, the strength of the fired product varies and the wettability with metal aluminum is adversely affected. Therefore, it is more effective to control the N ₂ partial pressure during firing so that the temperature of the fired body does not exceed 1420 ° C., or finely control the firing temperature near the nitriding temperature. In order to accelerate this reaction, it is effective and preferable to use a carbon-containing binder in the molded body. The firing atmosphere may be nitriding firing in an N ₂ + H ₂ , N ₂ + NH ₃ atmosphere. Furthermore, the production method of the present invention is not limited to the method for producing a silicon nitride porous refractory,
By replacing the bone agent with SiC, it can be applied to a method for producing a silicon carbide silicon nitride composite porous refractory or the like.

[0006]

EXAMPLES The present invention will be described in detail based on examples. The results of each are shown in Table 1.

[0007]

Example 1 70% by weight of β-Si ₃ N ₄ powder, 30% by weight of Si powder, and 3.0% by weight of CMC as an organic binder.
Was mixed, and 25% by weight of 0.05N-sodium hydroxide aqueous solution as an alkaline aqueous solution was added thereto, and the mixture was stirred in a slurry form, poured into a 150 × 150 × 150 mm paper mold, and foam-molded to dry. Then, a molded body was obtained. The obtained molded body, together with the mold, had an O ₂ concentration of 500 p.
Nitriding firing was performed at 1380 ° C. in an N ₂ + H ₂ atmosphere of pm or less. The obtained fired body showed no deformation due to firing shrinkage, and a good silicon nitride porous refractory was obtained.

[0008]

Example 2 50% by weight of β-Si ₃ N ₄ powder, 50% by weight of Si powder and 5.0% by weight of CMC as an organic binder.
Which was mixed with 100% by weight of sodium silicate and 20% by weight of water as an alkaline aqueous solution and stirred into a slurry, and the mixture was poured into a paper mold of 150 × 150 × 150 mm and foam-molded. Was dried to obtain a molded body. The obtained molded body together with the mold had an O ₂ concentration of 500.
Nitriding firing was performed at 1380 ° C. in a N ₂ + H ₂ atmosphere of ppm or less. The obtained fired body showed no deformation due to firing shrinkage, and a good silicon nitride porous refractory was obtained.

[0009]

[Comparative Example 1] 100% by weight of β-Si ₃ N _{4 and} 3.0% by weight of CMC as an organic binder were mixed, and sawdust having an average particle size of 0.5 mm was added as a pore-forming material to the mixture to prepare water 5 %, 150 x 150 x 15
What was molded into 0 mm was dried to obtain a molded body. The obtained molded body was degreased in Air at 500 ° C. for 12 hours, and then nitrided and baked at 1900 ° C. in an N ₂ atmosphere. The obtained fired body had a large firing shrinkage and was significantly deformed.

[0010]

Comparative Example 2 100% by weight of β-Si ₃ N ₄ , dispersant 0.
5% by weight and 3.0% by weight of CMC as an organic binder were mixed, and 30% by weight of water was added to the mixture to make a slurry, which was impregnated into a foamable urethane foam to form 150 × 150.
The molded product of × 150 mm was dried to obtain a molded product.
The obtained molded body was degreased in Air at 500 ° C. for 12 hours, and then nitrided and baked at 1900 ° C. in an N ₂ atmosphere. The obtained fired body had a large firing shrinkage and was significantly deformed.

[0011]

Comparative Example 3 70% by weight of β-Si ₃ N ₄ powder, 30% by weight of Si powder and 3.0% by weight of CMC as an organic binder.
Mixing, adding sawdust with an average particle size of 0.5 mm as a pore-forming material, and adding 5% of water as prepared water,
What was molded in 150 × 150 × 150 mm was dried to obtain a molded body. The obtained molded body was mixed with 5 in Air.
After degreasing at 00 ° C. for 12 hours, 138 in N ₂ + H ₂ atmosphere
Nitrided and baked at 0 ° C. No deformation was observed in the obtained fired body due to firing shrinkage.

[0012]

Comparative Example 4 70% by weight of β-Si ₃ N ₄ powder, 30% by weight of Si powder, 0.5% by weight of dispersant, and 3.0% by weight of CMC as an organic binder were mixed, and 30% by weight of water was added thereto. Then, the slurry was impregnated into a foamable urethane foam and molded into 150 × 150 × 150 mm, and then dried to obtain a molded body. The obtained molded body was mixed with 5 in Air.
After degreasing at 00 ° C. for 12 hours, 138 in N ₂ + H ₂ atmosphere
Nitrided and baked at 0 ° C. No deformation was observed in the obtained fired body due to firing shrinkage.

[0013]

[Table 1]

[0014]

As described above, by utilizing the features of the method of the present invention, a porous refractory made of silicon nitride used for a filter for an aluminum melting furnace and a heat insulating material can be easily manufactured at a good yield and at a low cost. It will be possible.

Claims (1)

[Claims] 1. A method for producing a porous refractory made of silicon nitride, wherein an alkaline aqueous solution is added to a mixture of Si ₃ N ₄ powder and Si powder, and a compact formed while self-foaming Si is formed into O ₂ A method for producing a porous refractory material, which comprises nitriding firing in a N ₂ atmosphere having a concentration of 500 ppm or less and a firing temperature of 1200 ° C to 1450 ° C.