JPS5834405B2 - Method for producing silicon carbide mainly composed of β-type crystals - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for continuously producing silicon carbide consisting of fine β-type crystals using silica and carbon as starting materials.

Currently, the industrial manufacturing method for silicon carbide is a discontinuous method using a classical Acheson-type electric resistance furnace, which has various drawbacks such as poor workability and low thermal efficiency. It has not been possible to produce silicon carbide in large quantities and at a high yield, which has a low content of silicon and impurities, and is composed of fine β-type crystals with uniform particle size.

Conventionally, many methods for continuously producing silicon carbide have been proposed. For example, in West German Patent No. 1186447, the aim is to obtain α-type silicon carbide, and a vertical furnace is used in an intermediate step to continuously produce β-type silicon carbide. Although the method has been disclosed,
This method is characterized by raw material processing in which silica sand is coated with carbonaceous material for continuity, and the behavior of SiO gas, which will be discussed in detail later, is not taken into consideration, resulting in low raw material yield and thermal efficiency, and furthermore, SiO Since the exhaust gas passage is blocked by the gas precipitation reaction, it is unsuitable for stable continuous operation.

As mentioned above, an economical and industrial-scale method for continuously producing silicon carbide comprising β-type crystals has not yet been known.

From this point of view, the inventors of the present invention previously filed a patent application filed in 1973-
Patent Application No. 126425 (Japanese Unexamined Patent Publication No. 51-57800) published a patent application No. 51-605 on a method for continuous production of β-type silicon carbide.
No. 01 (Japanese Unexamined Patent Publication No. 52-142697), mainly β
This paper proposes an invention related to a method for continuously producing silicon carbide made of molded crystals.

The present invention relates to an improvement of the method previously invented and patented by the present inventors, and the present invention improves the thermal efficiency of the preheating zone and the automatic lowering of the charged raw materials, thereby stabilizing the furnace condition. The purpose of the present invention is to provide a method for continuously producing silicon carbide mainly composed of β-type crystals while maintaining a low power consumption, and silica and carbon are mixed at a C/SiO2 molar ratio of 3.2 to 5.0.
The SiC formation reaction is carried out by indirectly electrically heating the raw materials mixed within the range of 1,650 to 2,100°C while continuously or intermittently lowering the weight of the raw materials in a reaction vessel having a pre-heating zone, a heating zone, and a cooling zone. The bulk specific gravity of the blended raw materials is 0.
The object of the present invention can be distantly achieved by maintaining the height of the raw material filling layer in the preheating zone within the range of 0.2 to 0.9 m. be able to.

Next, the present invention will be explained in detail.

The reaction formula of silicon carbide produced from silica and carbon is generally shown by the following formula (1).

5i02+3C-+SiC+2CO...(1)
However, the actual main generation mechanism is (2) below.
It is known that SiO gas is generated according to the formula, and the SiO gas and carbon react according to the formula (3) to generate silicon carbide.

Sio2+C-+SiO+CO・・・・・・・・・(2
)SiO+2C-')SiC formation reaction...
...(3) For the present invention, silica stone,
High-purity silica selected from silica sand or powder thereof, and carbon selected from various granular or powdered carbon materials are used.

When blending this raw material, it is not desirable to use a blended raw material in which the blending ratio of carbon to silica is close to or low in chemical equivalent, as in the conventional method using an Acheson furnace.

This is because, in accordance with the present invention, if a blended raw material similar to that of the conventional method is used, S produced according to the above formula (2)
More of the iO gas is not converted into SiC, and the amount of SiO gas in the reaction vessel increases.

The SiO gas is generated in the preheating zone according to the following equations (4), (5), and (
6) The reaction shown in the formula is caused.

2S iO−+S iO2+S i ・・・
...(4) SiO+CO-+SiO+C...
(5) 3SiO+CO → 2Si02+SiC...
-(6) As a result, the amount of precipitated semi-molten material mixed with 5in2, Si, SiC, C, etc. is increased.

Since the precipitate is sticky, the raw materials coagulate with each other,
The smooth movement and descent of the most important raw material for the continuous production of silicon carbide is significantly hindered, making stable continuous operation over a long period of time difficult.

Therefore, according to the present invention, the blended raw material consisting of silica and carbon is a mixture of silica powder and carbon particles, a mixture of silica powder and carbon powder and formed into aggregates, or the aggregates include wood chips, rice husks, coconut shells, etc. , charcoal grains, coke grains, and lime grains can be used. However, the blended raw materials are blended with an excess of carbon, and the blended molar ratio C/SiO2 is 3. It needs to be within the range of 2 to 5.0.

The reason for this is that if the blending ratio is smaller than 32, it becomes difficult to react the silica sufficiently and maintain the smooth fall of the raw material over a long period of time as described above, while if the blending ratio is larger than 5.0, the carbon does not contribute to the reaction. This is not favorable for economically producing β-type silicon carbide because the thermal efficiency is lowered due to heating to a high temperature and the cost required for the carbon raw material increases.

According to the present invention, the C/S i 02 molar ratio is 3.
The bulk specific gravity of the raw materials blended to 2-5.0 is 0.45-0.
The reason for limiting the bulk specific gravity to within the range of 90 is that a lighter bulk specific gravity is preferable in terms of breathability and other aspects, but in order to make a blended raw material with a lower bulk specific gravity than 0.45, large amounts of expensive charcoal and wood chips must be used.
Or, it is necessary to make the particle size of the aggregates extremely uniform,
On the other hand, if the bulk specific gravity is larger than 0.90, the permeability of the reaction product gas will be poor and heat exchange between the high temperature gas and the raw material will not be carried out sufficiently. The bulk specific gravity of the blended raw materials should be adjusted to between 0.45 and 0.90 because they are susceptible to the influence of precipitates from gases, which significantly impedes the smooth fall of the raw material's own weight, making it difficult to maintain stable operation over a long period of time. It is necessary to limit the value within the range, and the best results are obtained within the range of 0.55 to 0.80.

The raw materials blended as described above are continuously or intermittently charged into the preheating zone from the upper part of the reaction vessel, which has a preheating zone, a heating zone button, and a cooling zone, and the charged raw materials are charged into the preheating zone of the reaction vessel. The interior is brought to the heating zone while continuously or intermittently lowering its own weight, and indirect electrical heating is performed horizontally within the heating zone.
When carrying out the SiC formation reaction within the temperature range of 650 to 2100°C, the most important thing for the present invention is to keep the height of the raw material packed layer, which is also used in the preheating zone, within the range of 0.2 to 0.9 m. It is important to maintain this.

If the height of the raw material layer is lower than 0.2 m, high-temperature CO gas will not be sufficiently exchanged with the raw material and will be released outside the system, and will also be transmitted through the reaction vessel and dissipated in the pre-heating zone. Since the amount of heat increases, the thermal efficiency decreases, and at the same time, the amount of SiO gas released outside the system along with the CO gas increases, resulting in large heat loss and raw material loss, making it difficult to economically produce β-type silicon carbide. On the other hand, if the height of the raw material-filled layer is set higher than 0.9 m, most of the SiO gas rising from the heating zone becomes the precipitates and is not released outside the system, so the SiO gas partial pressure inside the reaction vessel gradually increases. However, as the amount of precipitation in the preheating zone increases, the smooth descent of the raw material under its own weight is significantly inhibited, making stable continuous operation for a long period of time difficult. It is necessary to maintain within the range of 2 to 0.9 m,
Best results are obtained with a button within the range of 0.3 to 0.7 m.

3. In the present invention, the height of the raw material layer buttoned in the preheating zone is determined by means for indirectly electrically heating the charge in the heating zone;
That is, it is the vertical length from the heating part of the heating element or the upper end of the induction coil to the surface 1 of the top layer of the raw material layer.

In the present invention, the surface of the raw material layer is held at a constant position for a long time within the height range of the raw material layer, or
Alternatively, the position of the surface of the raw material layer may be pulsated intermittently.

The reaction temperature for carrying out the SiC conversion reaction was set to 165
The reason for limiting the temperature to 0 to 2100'C is that if the temperature is higher than 2100'C, the removed β-type silicon carbide will grow crystals and transform into α-type silicon carbide, forming a sintered mass, making continuous operation difficult. If it is lower than C, the reaction rate is extremely slow and uneconomical, so the reaction temperature needs to be limited within the range of 1650 to 2100°C.

In accordance with the present invention, the height of the top layer of the raw material layer in the preheating zone is maintained within the range of 0.2 to 0.9 m, and the mass velocity of CO gas passing through the top layer of the raw material layer in the preheating zone is set to 500 kg. It is preferable to operate at less than /m”·hr.

If the mass speed is increased above 500 kg/m2・hr, the amount of SiO gas passing through a unit horizontal cross section of the top layer of the raw material layer will increase, and precipitates from the SiC gas will condense in the raw material layer at a high concentration. This is because it becomes difficult to maintain the smooth weight fall of the raw material over a long period of time.

In other words, it is undesirable to increase the production amount by using a vertically long reaction vessel, since this results in a worsening of the weight-lowering properties of the raw materials.

Furthermore, if the mass speed is too low, the amount of production per hour for the unit production equipment will be low and it will be uneconomical, so the mass speed is most preferably in the range of 150 to 400.9/m2.hr.

Next, an example of a manufacturing apparatus that can be used to carry out the present invention will be described with reference to the drawings.

As shown in FIG. 1, the apparatus used in the method of the present invention has a raw material charging port 1, a pre-heating zone 2, a heating zone 3, a cooling zone 4, and a sealable product discharge port 5, which are arranged vertically. The tubes 8 forming the heating zone are made of graphite and are equipped with means 10, 11, 12 for indirect electrical heating of the charge in the heating zone, and are equipped with at least A heat insulating layer 9 made of carbon or graphite fine powder is provided on the outside of the heating zone 3.

The reaction vessel 6 is installed in the center of the apparatus, and is provided with indirect heating means 10. The IL 12 includes a graphite heating element 10 vertically installed on the outside of a cylinder 8 forming a heating zone, and a graphite heating element 10 installed close to the outside of the heat generating part 11 of the heating element 100 and inside the heat insulating layer. It consists of a reflecting tube 12.

The graphite heating element 10 has a U-shape, and is placed close to the outside of the cylinder forming the heating zone so that the heat generating part 11 of the heating element corresponds to the vertical direction of the heating zone, and It is designed to heat the charge inside as uniformly as possible.

The graphite heating element 10 is located in a space surrounded by a cylinder 8 forming a heating zone and a graphite reflection cylinder 12, and into the space, for example, argon, helium, nitrogen, - Carbon oxide, hydrogen, and other non-oxidizing gases are sealed to prevent oxidative consumption of the graphite heating element due to air intrusion.

Next, examples of the present invention will be described.

Example 1 Silica powder (S102 299.7 hiro 80 mesh or less)
100 parts by weight, 76 parts by weight of petroleum coke powder (C298.7%, ash content 205 parts, 325 mesh or less), and high pitch powder (C=50.4%).

7 parts by weight (200 mesh or less) were mixed in a vertical screw mixer for 10 minutes.

The raw materials were molded using a dish granulator while spraying a 0.5% aqueous solution of carbomethylcellulose, sized with a sieve and a burr grizzly, and then placed in a band-type ventilation dryer for 90 minutes with hot air at 150C. Dry average particle size 10.5
A molding raw material having yfIm, bulk specific gravity of 0.60, and C/5i02 molar ratio of 4.0 was obtained.

3. The forming raw material is charged from the top of the manufacturing equipment having the specifications shown in Table 1, the height of the filled layer in the pre-heating zone of the raw material layer is 0.5 m, and the mass velocity of CO gas is 250 to 2601<g.
/m”・hr, while lowering the temperature of the outer wall of the graphite tube in a heating zone controlled to 2150°C, indirect electrical heating is performed to perform the SiC formation reaction at about 1850°C,
After cooling to 450° C. in the generated CO gas, the reaction product was continuously discharged from the discharge port.

The composition ratio of the obtained product excluding impurities is 5iC71,
Section 6, 5i022.3 Hiroshi Free Carbon (F, C0) 26.1
Of the SiC, those that are β-type crystals are 96
．． 5%, and its average particle size was 10.3μ.

*Example 2 and Comparative Example 1 The same operations as in Example 1 were performed using the same blended raw materials as in Example 1 and changing the bulk specific gravity by changing the particle size distribution.

The results are shown in Table 2. Example 1 according to the method of the present invention
, 2, we were able to operate extremely smoothly and continuously for a long period of time.

In the case of Comparative Example 1 using raw materials with large bulk specific gravity, S
Even if only a small amount of precipitates are deposited between iO gas and raw material particles, crosslinking due to the agglomerates will immediately occur between the raw material particles, the weight drop of the raw material will be significantly worsened, the blow-up phenomenon of gas and raw materials will occur significantly, and continuous operation will be interrupted. It was difficult to do so, and there were also safety issues in terms of work.

Example 3 and Comparative Example 2 Using the same manufacturing equipment as used in Example 1, Example 1 was produced by changing the height of the filling layer in the preheating zone of the raw material layer.
performed the same operation.

The results are shown in Table 3.

According to the method of the present invention as in Example 1.3-1.3-2,
The plant was able to operate extremely smoothly and continuously for a long period of time.

When operating with the raw material layer kept low as in Comparative Example 2-1,
High-temperature CO gas was discharged and shattered, and at the same time, SiO gas was discharged outside the system, and the electric power consumption was poor and it was not economical.

In addition, when the raw material layer is high as in Comparative Example 2-2, it is expected that the heat exchange efficiency between the reaction product gas and the raw material will be improved, but in reality, the SiO gas in the reaction product gas is transferred to the preheating zone more than necessary. As a result, the amount of precipitates increased, and troubles due to adhesion and adhesion between raw material particles occurred frequently, resulting in a stagnation in the weight fall of the raw materials, making continuous operation difficult.

As explained above, according to the method of the present invention, silicon carbide consisting of fine β-type crystals with a uniform particle size and a low content of α-type silicon carbide and impurities can be produced in a stable furnace condition for a long period of time, and at a high temperature. It has the effect of allowing continuous production with high yield, and is extremely useful industrially.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view of a vertical continuous manufacturing apparatus used in Examples and Comparative Examples of the present invention, and FIG. 2 is a cross-sectional view of the manufacturing apparatus shown in FIG. It is a front view. DESCRIPTION OF SYMBOLS 1... Raw material charging port, 2... Pre-preparation zone, 3... Heating zone, 4... Cooling zone, 5... Product discharge port, 6... Reaction vessel, 8... Processing Cylinder forming a tropical zone, 9... Heat insulating layer, 10... Graphite heating element, 11... Heat generating part of graphite heating element, 12... Graphite reflective tube, 13... Non-oxidizing Gas filling inlet, 14...Guiding electrode, 15...Fusbar 16...Temperature measurement hole, 18...Produced gas discharge port.

Claims (1)

[Claims] 1. Silica and carbon in a C/5i02 molar ratio of 3.2 to 5.
The raw materials blended within the range of 0 are indirectly electrically heated to a temperature of 1,650 to 2,100°C to carry out the SiC reaction while continuously or intermittently lowering their own weight in a reaction vessel having a pre-heating zone, a heating zone, and a cooling zone. In the method for producing silicon carbide mainly composed of β-type crystals, the bulk specific gravity of the blended raw materials is set within the range of 0.45 to 0.90, and the height of the raw material-filled layer entering the preheating zone is set to 0.45 to 0.90. A method for producing silicon carbide mainly consisting of β-type crystals, characterized in that the operation is carried out while maintaining the crystal diameter within a range of 2 to 0.9 m. 2. Set the height of the raw material filling layer in the preheating zone to 03 to 0.
．． A method for producing silicon carbide mainly composed of β-type crystals according to claim 1, characterized in that the crystal diameter is maintained within a range of 7 m. 3. Production of silicon carbide mainly composed of β-type crystals according to claim 1 or 2, characterized in that the mass velocity of the CO gas passing through the raw material-filled layer is 500 kg/m''·nr or less Method 4. The main method according to claims 1 to 3, characterized in that the mass velocity of the CO gas passing through the raw material-filled layer is within the range of 150 to 400 kg/m'·nr. A method for producing silicon carbide consisting of β-type crystals.