JPH0478599B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for removing free carbon contained in fibrous silicon carbide whiskers by fluidized combustion in a fluidized bed.

[Prior Art] Silicon carbide whiskers are manufactured industrially mainly by reacting silicon oxide with carbon. Since the silicon carbide whiskers produced by the above method inevitably contain more than the stoichiometric amount of free carbon, a silicon carbide whisker purification process to remove this excess free carbon is important. It is becoming.

In order to remove the free carbon contained in the silicon carbide whiskers, it is industrially most efficient to burn the free carbon with a combustion gas. Possible methods include combustion in a bed, combustion in a moving bed in a rotary furnace such as a rotary kiln, and combustion while fluidizing the sample with combustion gas. Among these methods, the contact efficiency between free carbon and combustion gas is low, so it takes a long time to completely burn free carbon.
A further drawback was that the silicon carbide whiskers were also subject to considerable oxidation. In a particular method, silicon carbide whiskers were noticeably attached to the furnace wall, making long-term operation difficult. On the other hand, the method is disclosed in JP-A-55-100214. However, this requires a large gas flow rate,
This method burns the raw material powder while passing through a fluidized medium particle bed, and the refined powder is ejected out of the combustion furnace and recovered.This method is effective for processing particulate matter, but it is effective in treating fibrous silicon carbide whiskers. In this case, it is difficult to stably charge the raw material into the fluidized bed using airflow, and particles are segregated between the fluidized medium particle layer and the thin layer of fine particles that fly out from the top. Since aggregated coarse particles accumulate in the fluidized bed, oxidation of silicon carbide whiskers increases, and it is not possible to efficiently separate and recover fluidized medium particles and purified silicon carbide whiskers in a fluidized state.

[Problems to be Solved by the Invention] The present invention enables fluid combustion of free carbon contained in fibrous silicon carbide whiskers, for which it has been difficult to efficiently burn and remove free carbon, in a short period of time. It is an object of the present invention to provide a method for suppressing oxidation of silicon carbide whiskers as much as possible and efficiently separating and recovering them from fluidized medium particles.

[Means for Solving the Problems] According to the present invention, silicon carbide whiskers containing free carbon are charged into a fluidized bed of a fluidized bed combustion furnace, and the combustion temperature is maintained at 750 to 900°C. Burning the carbon and agglomerating the silicon carbide whiskers into granules, reducing the flow rate of the fluidized combustion gas and moving the granules to the upper surface of the fluidized bed,
Provided is a method for fluidized combustion removal of free carbon contained in silicon carbide whiskers, which is characterized by discharging and recovering outside the furnace.

In other words, by setting specific fluidized combustion conditions, the free carbon of the fibrous silicon carbide whiskers is burned off, the fibrous silicon carbide whiskers are aggregated and granulated in the fluidized bed, and are efficiently separated and recovered from the fluidized bed. It is something to do.

Typically, silicon carbide whiskers containing free carbon have a diameter of 0.1 to 3 μm (average diameter of 1.0 μm or less)
and has a length of 10 to 300 μm (average length of 50 μm or more). However, since the whiskers are highly adhesive and form secondary agglomerations, it is difficult to directly fluidize these whiskers with combustion gas and burn off free carbon. Therefore, by charging these whiskers into a bed of fluidized media particles with good fluidity, the silicon carbide whiskers can be fluidized well together with the fluidized media particles, and since the two are sufficiently mixed, the temperature inside the bed can be made uniform. Free carbon can be easily burned and removed while retaining it.

According to the present invention, the combustion temperature is 750°C or higher, 900°C or higher.
It is necessary to keep the temperature below ℃. At temperatures below 750℃, there is a large amount of unburned carbon in refined silicon carbide;
In the above case, the production rate of silicon dioxide is about 4% or more, which is not preferable as a silicon carbide product.

According to the present invention, the fluidized medium particles are preferably inert inorganic particles, such as alumina particles and silica particles, which have stable properties in the combustion temperature range and do not react with silicon carbide whiskers and combustion gas. In addition, the average particle diameter of the fluidized medium particles forming the fluidized bed is in the range of 50 to 200 μm;
It is preferable that the particle shape is spherical in order to obtain a uniform fluidity or mixing state, and also to form silicon carbide whisker particles of an appropriate size. If the average particle size of the fluidizing medium particles is 50 μm or less, the refined silicon carbide whiskers will not aggregate and form sufficient granules, making it difficult to fluidize them from the fluidizing medium particles, and if the average particle size is 200 μm or less. In the above case, many silicon carbide whiskers containing free carbon are unevenly distributed in the upper part of the fluidized medium particle layer, so the upper part of the layer becomes locally high temperature, and the silicon carbide whiskers are also significantly oxidized.

Further, according to the present invention, the height of the static bed of the fluidized medium particle bed is preferably in the range of 2 to 4 times the column diameter of the combustion furnace. If the bed height is less than twice the column diameter, it is difficult to involve the silicon carbide whiskers in the fluidized medium particle bed, and a uniform mixing state cannot be formed. On the other hand, when the bed height is increased by 4 times or more, the fluidized medium particle layer flows in the furnace while fluctuating greatly up and down, so the silicon carbide whiskers remain unevenly distributed at the top of the bed, and the fluidized medium particle layer It becomes a state where it is hardly affected by the mixing effect of.

Further, according to the present invention, it is preferable that the amount of silicon carbide whiskers containing free carbon charged is 20% or less of the sum of the amount charged and the amount of fluidizing medium particles. Charge of silicon carbide containing free carbon is 20%
If the above is the case, miscibility with the fluidized medium particles is poor, and since many silicon carbide whiskers are unevenly distributed in the upper part of the layer, the combustion temperature becomes extremely high, and the silicon carbide whiskers are easily oxidized, which is not preferable.

In the present invention, as free carbon is burnt and removed, silicon carbide whiskers are aggregated and granulated, the particle size is larger than the fluidized medium particles, and the apparent specific gravity is smaller. Therefore, after combustion purification, by lowering the flow rate of the fluidized combustion gas, silicon carbide whiskers can be separated on the surface of the fluidized medium particle layer and efficiently recovered.

FIG. 1 shows an outline of a fluidized bed combustion apparatus to which the method of the present invention is applied. The combustion furnace 21 is installed in the center of the electric heating furnace 7, and the combustion gas (usually air) from the blower 1 is dehydrated by the moisture remover 2, and is guided to the bottom of the furnace via the flow meter 3 and then passed through the dispersion plate. The fluidized medium is ejected from the small holes 4 and fluidized to form a fluidized bed 5. Raw material silicon carbide is charged into the fluidized bed from a powder quantitative feeder 22 through a raw material charging port 23 that penetrates the furnace side wall and opens directly above the distribution plate 4 . Refined silicon carbide after the combustion reaction is separated at the upper part of the fluidized bed by reducing the combustion gas flow rate, collected on the upper surface of the fluidized bed, and taken out from the product outlet 24 opened at the upper part of the fluidized bed on the side wall of the furnace. On the other hand, the combustion exhaust gas is discharged to the outside of the system via the dust collector 8. After discharging the product, a certain amount of raw material is charged from the raw material charging port, and fluidized bed combustion is performed again. By repeating this process, free carbon can be burned and removed semi-continuously while the fluidized medium particles remain in the furnace. Of course, batch operation is also possible. The temperature inside the electric heating furnace 7 and the fluidized bed 5 is detected by a digital thermometer 9 via a thermocouple 6, recorded on a temperature recorder 10, and transmitted to a temperature program controller 11 to determine the heating temperature of the electric heating furnace. to maintain the furnace temperature at a predetermined temperature.

Using the apparatus shown in FIG. 1, conditions for fluidized combustion of silicon carbide containing free carbon were investigated. Air flow rate is 0.75 l/min (value at room temperature, cavity air flow rate at bed temperature is 13 to 17.5 cm/sec), raw material charging amount/
(raw material charging amount + fluidized medium amount) is 8.0%, and the height of the static bed of fluidized medium particles (spherical alumina with a particle size of 75 to 125 μm is used) is 8 to 6.5 cm (2.5 to 3.25 cm of the column diameter).
Figure 2 shows the carbon content remaining in an unburned state in silicon carbide whiskers when free carbon is burned by changing the combustion temperature in the range of 617℃ to 908℃ while FIG. 3 shows the production rate of silicon dioxide by oxidation of silicon carbide whiskers.
The free carbon content in the recovered silicon carbide whiskers decreases as the maximum temperature in the layer increases, and
It becomes a low value of about 1% at temperatures above ℃. Furthermore, the production rate of silicon dioxide due to oxidation of silicon carbide whiskers increases from about 2% as the temperature inside the layer increases.
Even at a temperature of 900°C, the production rate of silicon dioxide is about 4%. That is, if combustion is performed at a temperature in the range of 750 to 900°C, oxidation of silicon carbide whiskers can be suppressed to a low level and free carbon can be burned efficiently.

In the method of the present invention, the unburned residual carbon is 3 to 5% in the conventional rotary kiln method, and the silicon dioxide production rate is about 4 to 5% in the rotary kiln method and the fixed bed method. The silicon production rate can be lowered by more than 50%. Furthermore, the particle size of the stationary silicon carbide whisker aggregates obtained by the method of the present invention is approximately 0.7 mm at 50% median diameter,
Separation from the fluidized medium particles was also good, and no spherical alumina particles were found to be mixed into the recovered silicon carbide whiskers.

[Example] Next, the present invention will be explained in more detail with reference to Examples.

The unrefined silicon carbide whiskers used contain 38.3% free carbon by weight, have an average diameter of 0.5 μm, and an average length of approximately 100 μm. Charge,
A combustion removal test was conducted under conditions where the temperature inside the furnace was uniform.

The apparatus used had a structure similar to that shown in Figure 1, and a stainless steel cylindrical tube with an inner diameter of 10 cm was used as the combustion furnace.

Example Spherical alumina particles with a particle size of 74 to 125 μmφ are used as the fluidizing medium, and the height of the static bed is 2.5 of the column diameter.
It was doubled. Initially, the bed temperature of the fluidized medium was set to 690°C, and the cavity standard gas velocity of this air was set to 14.2.
The raw materials were charged into the fluidized bed at a total ratio of 8.0% by weight in cm/sec, and burned for 20 minutes. As a result, the temperature inside the layer reached a maximum of 775℃.

After fluidized combustion, when the gas flow rate was lowered, the silicon carbide whiskers, which had aggregated and become granular, floated to the top of the layer. This was collected from the product outlet and analyzed for composition, and the free carbon content was 0.4%, and the content of silicon dioxide produced by oxidation was 3.2%. Note that no fluid medium component was observed.

Comparative Example Spherical alumina particles with a particle size of 250 to 500 φμm were used as fluidized medium particles, and the height of the static bed was 50 cm (5 times the column diameter). In addition, the initial bed temperature of the fluidized medium was set to 800°C, the air cavity standard gas velocity at this temperature was set to 15.1 cm/sec, and the raw materials were added to the fluidized bed at a ratio of 8.0% by weight as a whole. Charge and burn for 20 minutes. However, the charged raw material did not mix well with the fluidized medium particles, and most of it was burnt while being unevenly distributed at the top of the bed, resulting in the temperature of the raw material section reaching a maximum of 950°C.

Although the free carbon content of the recovered silicon carbide whiskers was low at 0.3%, the amount of silicon dioxide produced by oxidation of silicon carbide was high at about 7%. Also,
Agglomerates and granules of silicon carbide whiskers were hardly formed in the fluidized bed, and it was observed that the fluidized medium was mixed into the recovered silicon carbide whiskers.

[Effects of the Invention] As is clear from the above, according to the fluidized combustion removal method of free carbon contained in silicon carbide whiskers of the present invention, due to the fluidized medium particle layer with good fluidity, fluidization has been difficult until now. Since silicon carbide whiskers containing free carbon can be fluidized appropriately, oxidation of silicon carbide whiskers can be suppressed to a low level under uniform combustion temperature conditions, and free carbon can be burnt off extremely stably and efficiently. Moreover, the produced silicon carbide whiskers can be easily separated from the fluidized medium particle layer and can be efficiently recovered, making it an extremely useful method industrially.

[Brief explanation of the drawing]

Figure 1 is a diagram showing an outline of the equipment used to carry out the present invention, Figure 2 is a diagram showing the relationship between combustion temperature and carbon content during fluidized combustion, and Figure 3 is a diagram showing the relationship between combustion temperature and carbon content. It is a drawing showing the relationship with silicon dioxide production rate. 1... Blower, 2... Moisture remover, 3... Flow meter, 4... Dispersion plate, 5... Fluidized bed, 6... Thermocouple, 7... Electric heating furnace, 8... Dust collector, 9... ...Digital thermometer, 10...Temperature recorder, 11...Temperature program controller, 21...Combustion furnace, 22...
Powder quantitative feeder, 23... Raw material charging port, 24...
Product outlet.

Claims (1)

[Claims] 1. Silicon carbide whiskers containing free carbon are charged into a fluidized bed of a fluidized bed combustion furnace, and the combustion temperature is set to 750 to 750.
By maintaining the temperature at 900°C, the free carbon is burnt, and the silicon carbide whiskers are aggregated and granulated, and the granules are moved to the upper surface of the fluidized bed by reducing the flow rate of the fluidized combustion gas, and are discharged outside the furnace.
A method for fluid combustion removal of free carbon contained in silicon carbide whiskers, the method comprising recovering free carbon from silicon carbide whiskers. 2 The fluidized medium of the fluidized bed combustion furnace has an average particle size of 50~
It consists of at least one kind selected from heat-resistant spherical alumina particles and spherical silica particles of 200 μm, has a cavity standard gas velocity of combustion reaction gas in the fluidized bed at combustion temperature of 8 to 30 cm/sec, and contains free carbon. 2. The method according to claim 1, wherein the charged amount of silicon carbide whiskers is 20% or less of the sum of the charged amount and the weight of the fluidized medium, and the height of the fluidized medium stationary bed is 2 to 4 times the diameter of the combustion furnace column. A method for fluid combustion removal of free carbon contained in silicon carbide whiskers.