JPH08301614A - Production of synthetic silica powder and production of silica glass molded form - Google Patents

Abstract

PURPOSE: To increase the bulk density of silica gel powder to be charged into a crucible and prevent the development of fine bubbles in the resultant silica glass molded form by heat treating silica gel powder in fluidization in a rotary kiln with its maximum stack height at a specified level or higher. CONSTITUTION: First, dry silica gel 10-1000μm in particle diameter is obtained by drying at 50-200 deg.C and pulverizing wet silica gel powder obtained by a sol-gel process such as hydrolysis or gelation of e.g. a tetraalkoxysilane. Secondly, the dry silica gel powder is heat-treated under fluidization in a rotary kiln with the core pipe made of quartz. In this case, there occurs no slippage between the powder 1 and the contact surface 3 of the core pipe 2 wall, the powder 1 is raised along the wall as the core pipe 2 is rotated and then allowed to fall (as indicated by arrows) off the wall at an angle of repose or above. To stabilize the fluid state of the powder 1, the maximum stack height of the powder 1 defined below is kept at >=75mm and the powder 1 is heated at 900-1100 deg.C (The maximum stack height is the maximum length of perpendiculars drawn from the powder surface 5 in contact with the space 4 inside the core pipe 2 toward the contact surface 3).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an efficient method for producing synthetic quartz powder and quartz glass moldings.

[0002]

2. Description of the Related Art In recent years, in glass products used in the fields of optical communication, semiconductor industry, etc., very strict control has been carried out regarding trace impurities and minute bubbles in the products. Such high quality glass is mainly used for refining natural quartz, for fume generated by decomposition of silicon tetrachloride in an oxyhydrogen flame to adhere to and grow on a substrate, and for hydrolysis and gel of silicon alkoxide. The synthetic silica powder obtained by firing the silica gel obtained by calcination is used, and it is manufactured by a method such as melting this to obtain a molded body.

[0003]

However, the method (1) has a limit in reducing the content of trace impurities, and the method (2) has a problem that the manufacturing cost is extremely high. On the other hand, with the method of firing silica gel, synthetic quartz powder having a low content of trace impurities can be obtained at a lower cost than the method of 1, but it cannot be said that the required level is necessarily satisfied. In addition, this method has a problem that minute bubbles may be generated in the molded product which is the final product, and the minute bubbles may cause various troubles.

[0004]

DISCLOSURE OF THE INVENTION The present inventors have found the above-mentioned problems in the method for producing synthetic quartz powder by firing silica gel, that is, the formation of fine bubbles in a molded product obtained by melting the synthetic silica powder is extremely small. As a result of diligent studies to find out a method for producing quartz powder and an industrially advantageous method for producing the quartz powder, the following points were found. That is, the silica gel is fired by charging the silica gel in a quartz container and heating it in an electric furnace or the like in order to eliminate impurities from the container. Particularly, in industrial production, a quartz crucible having a large diameter is used. However, since silica gel has a lower bulk density than quartz powder, the container used for firing cannot be efficiently used,
Productivity is poor and manufacturing costs are high. Therefore,
In order to improve productivity, increasing the bulk density of the powder charged in the crucible is an important issue.

Further, in the production of a molded body using quartz powder, the generation of fine bubbles during the production of the molded body is affected by the temperature rising process of the firing step during the production of the quartz powder. In the silica gel powder obtained by hydrolysis of tetraalkoxysilane, unreacted alkoxy groups and a part of by-produced alcohol remain even if the by-produced alcohol is removed by drying. Actually, when the carbon concentration in the dried silica gel powder is measured, it varies depending on the drying condition.
%. When this silica gel powder is fired in an oxygen-containing gas, most of the carbon is burned and removed during the temperature rising process, but some of it may be trapped as unburned carbon in the synthetic quartz powder. When this synthetic quartz powder containing unburned carbon is used, it becomes CO or CO ₂ gas at the time of melt molding and causes bubbles. Therefore, it is necessary to remove substantially all unburned carbon before sealing the silica gel, and the rate of temperature increase in the temperature increasing process is important. However, as mentioned above, in the case of industrial production of synthetic quartz powder, a crucible with a large diameter is used, and the temperature in the crucible during the heating process becomes non-uniform, and as a result,
In some cases, synthetic quartz powder in which carbon remains is partially generated, and micro bubbles are generated in a molded body using the synthetic quartz powder.

In view of the above-mentioned problems, the present inventors have conducted further studies, and as a result, heat-treated silica gel under appropriate conditions and operations before firing to obtain the bulk density of the powder to be fired after firing. The inventors have found that the silica powder can be made equivalent to the quartz powder and the alkoxy group and the hydroxyl group can be sufficiently removed, and the present invention has been completed. That is, the present invention is a method for producing synthetic quartz powder, which comprises a step of heat-treating silica gel powder while flowing it using a rotary kiln, wherein the maximum layer height of the powder in the rotary kiln is 75 mm or more. The method for producing the powder, etc.

Hereinafter, the present invention will be described in detail. The synthetic quartz powder of the present invention is a synthetic quartz powder obtained by heating silica gel powder obtained by hydrolysis / gelling of alkoxysilane etc. to make it non-porous. The method for producing the silica gel powder is not particularly limited, and various known techniques can be adopted, but the so-called sol-gel method by hydrolysis / gelling of alkoxysilane or the like is preferable from the viewpoint of easily achieving high purity. Hydrolysis of alkoxysilane by the sol-gel method is performed by reacting alkoxysilane with water according to a known method.

As the alkoxysilane used as a raw material, an alkoxysilane capable of being hydrolyzed and gelled is sufficient, but it is a C1 to C4 lower alkoxysilane such as tetramethoxysilane or tetraethoxysilane which is directly bonded to silicon. Alkoxysilanes substantially free of Si-C bonds due to hydrocarbon groups, or oligomers thereof are preferred.

The amount of water used is usually selected from the range of 1 to 10 times the equivalent of the alkoxy groups in the alkoxysilane. At this time, organic solvents such as alcohols and ethers that are compatible with water may be mixed and used, if necessary. Typical examples of the alcohol used include low aliphatic alcohols such as methanol and ethanol.

In this hydrolysis reaction, hydrochloric acid as a catalyst,
An acid such as acetic acid or an alkali such as ammonia may be added. It should be understood that all substances introduced into the reaction system such as water and catalyst used here are of high purity.
Gelation of the hydrolysis product can be carried out under heating or at room temperature. When heating is performed, the rate of gelation can be improved, and thus the gelling time can be adjusted by adjusting the degree of heating.

The obtained gel is wet silica gel containing a large amount of water, and it may be subdivided and dried, or it may be dried and subdivided. In any case, the particle size of the dry silica gel obtained by drying is 10
˜1000 μm, preferably 100 to 600 μm.

It is efficient to carry out the drying while heating at atmospheric pressure or under reduced pressure. The heating temperature varies depending on the conditions, but is usually 50 to 200 ° C. The operation can be performed either batchwise or continuously.
The degree of drying is usually from 1 to 30% by weight based on the content of water expressed as wet standard moisture. The silica gel powder thus obtained is heat-treated under the specific conditions described below. That is, the silica gel powder is heated to a specific maximum bed height in the rotary kiln while rotating the rotary kiln to cause the silica gel powder to flow.

The material of the core tube of the rotary kiln must be selected so that the treated powder does not cause contamination of the material, and quartz is particularly preferable. In the case of quartz, since there is a limit in the size of the core tube in manufacturing, heat treatment can be performed using a plurality of rotary kilns depending on the conditions. Although the cause is not clear, when a quartz core tube is used and the maximum bed height of the powder is low, the powder in the core tube may slip and the powder may not flow. Therefore, it is a feature of the present invention that the heat treatment is performed while keeping the maximum bed height of the powder flowing in the rotary kiln at 75 mm or more.

The term "flowing" of the powder in the present invention means that, for example, FIG. 2 showing the cross section of the rotary kiln in which the powder is charged is rotated and compared with FIG. 1 showing the cross section of the rotary kiln in the stationary state. That is, there is substantially no slip between the powder (1) and the powder-contacting surface (3) of the wall of the core tube (2), and the powder (1) follows the rotation of the core tube (2).
Is lifted by the wall of the core tube (2), and the powder (1) leaves the wall above the angle of repose and flows down to the lower part of the tube wall (in the direction of the black arrow in Fig. 2). The silica gel in the rotary kiln. The stable achievement of such a fluidized state of the powder can be easily achieved by keeping the maximum bed height of the silica gel powder at 75 mm or more, as described below.

The term "maximum bed height" as used herein means that a perpendicular line is drawn from the powder surface (5) where the powder (1) comes into contact with the void (4) in the core tube, in the direction of the powder contact surface (3) of the core tube. It means the maximum distance when The pipe diameter of the rotary kiln is not particularly limited, but as a result of the inventors of the present invention studying rotary kilns having various pipe diameters, the inner diameter of the pipe is 100 m.
When it is m or more, the effect of the present invention is particularly remarkable, preferably 120 mm or more, and more preferably 150 mm or more. The maximum diameter is not particularly limited, but the inner diameter of the tube is generally 600 mm or less, more preferably 500 mm or less, due to its structure, uniformity of mixing and heating, and the like. In the case of 150 mm or less, it is necessary to provide a special insertion and ejection mechanism at each pipe end.

When the maximum bed height is lower than 75 mm, the present inventors deteriorate the fluidity of the powder in the core tube and cause the powder to flow intermittently. It was found that the powder in the core tube flowed continuously. When the flow is intermittent, local heating occurs on the wall surface of the core tube, and when the flow starts, steam or methanol is rapidly generated, which makes it difficult to perform stable operation.

For the operation, a dry silica gel powder is preliminarily charged in a furnace core tube of a rotary kiln so that the maximum bed height is 75 mm or more, and heat treatment is carried out, or a continuous drying is performed from one end of the furnace core tube of the rotary kiln. A continuous method in which silica gel powder is supplied and heat treatment is performed while adjusting the amount of powder in the core tube so that the maximum bed height is 75 mm or more, and the powder after treatment is continuously discharged from the core tube. However, the latter method is more preferable in terms of economy and operability.

Particularly in the case of the continuous method, if the flow is intermittent, the powder is intermittently discharged from the core tube, and stable operation cannot be performed. Therefore, continuous flow of the powder is essential. . When carrying out by a continuous method, you may heat-process using a some rotary kiln in series. At this time, each rotary kiln is adjusted so that the maximum layer height of the powder is 75 mm or more. The rotation speed of the rotary kiln is not particularly limited and may be selected within a practical range.
Specifically, 1 to 20 rpm, preferably 3 to 10 rpm
It is a degree.

The temperature range for heat treatment is 200 to 11
It is 00 ° C. Especially in the range of 300 to 600 ° C,
Care should be taken as the removal of residual carbon in the silica gel proceeds. When the temperature exceeds 600 ° C., the silica gel starts to be sealed. Therefore, it is necessary to eliminate almost all the remaining carbon before the powder temperature reaches this temperature range. Otherwise, unburned carbon remains in the obtained synthetic quartz powder and bubbles are generated during melt molding.

The temperature rising rate in the region where the removal of residual carbon in silica gel proceeds is 1000 ° C./Hr or less, preferably 500 ° C./Hr or less. Also, 400 to 600
It is also effective to maintain the temperature in the range of 0.5 to 5 hours. Further, this heat treatment is performed in a clean air or oxygen-containing gas atmosphere. Specifically, a method of supplying clean air or oxygen-containing gas from one end of the core tube and discharging it from the other end can be used. The amount of gas supplied is usually 10 to 100 liters / hr in terms of oxygen in the batch method in the case of batch method, and 30 to 30 in terms of oxygen in the continuous method per kg of powder continuously supplied.
It is 300 liters / Hr. In the present invention, the heat treatment is carried out while maintaining the maximum bed height of the silica gel powder flowing in the rotary kiln at 75 mm or more. By using a rotary kiln to keep the maximum bed height at 75 mm or more and performing heat treatment while continuously flowing silica gel powder, carbon reduction is promoted and a homogeneous treated powder is obtained. By this treatment, the carbon concentration in silica gel is reduced to about 50 to 1000 ppm.

The treated powder from which the remaining carbon has almost disappeared is continuously heated, and the temperature of the final powder is 900
~ 1100 ° C, preferably raised to 950-1050 ° C. The heating rate at this time is usually 100 to 1000 ° C.
/ Hr. The heat treatment in this temperature range is also performed in a clean air or oxygen-containing gas atmosphere. Also in this case, as in the heat treatment in the temperature range up to 600 ° C., a method of supplying clean air or oxygen-containing gas from one end of the core tube and discharging it from the other end is adopted. The amount of gas supplied is usually 1 to 50 liters / hr of oxygen per 1 kg of powder in the case of the batch method, and 3 to 10 per 1 kg of powder continuously supplied in the case of the continuous method. 50 liters / hr
Is. The heat treatment in this temperature range is also carried out in the rotary kiln while keeping the maximum layer height of the powder at 75 mm or more. By using a rotary kiln to perform heat treatment while flowing the powder, uniform heating is performed, and uniform treated powder is obtained. By this treatment, the sealing of the silica gel was almost completed, and the bulk density (hereinafter referred to as “bulk density”) of the powder, which was about 0.7 to 0.8 g / ml, was 1.0.
~ 1.2 g / ml. When such an operation is performed in batches, the whole is heated directly or indirectly from the outside while rotating the core tube, and the heating strength is increased over time for processing. On the other hand, in the case of continuous operation, a method is used in which the heating zone of the core tube is divided into a plurality of zones, and the heating temperature is controlled so that the temperature of the powder rises in a predetermined gradient according to the flow direction of the treated powder. be able to.

As a result of studies conducted by the present inventors, the inclination angle of the rotary kiln should be 3 ° or less, preferably 1 ° or less in the heat treatment. This is because the silica gel powder to be treated can effectively prevent back mixing in the axial direction of the rotary kiln when the tilt angle is within this range. When the silica gel powder is subjected to the above-mentioned heat treatment according to the present invention, synthetic quartz powder is obtained, but usually, 1000 ppm or more of silanol remains. Therefore, the silanol is usually removed by performing firing in a temperature range further raised.

The container used for firing is made of a material which does not generate contamination of synthetic quartz powder with impurities, for example, a quartz crucible. In this firing, since the carbon in the powder to be fired has been substantially completely removed,
No special attention needs to be paid to the heating rate. Therefore, since the variation in the temperature rising rate in the container does not affect the quality, a homogeneous product can be obtained, and a container having a large capacity can be used as compared with the conventional product. In addition, since the bulk density of the powder has been sufficiently increased in advance, there is no significant change in the bulk density of the powder before firing and the powder after firing, and the container can be used efficiently, improving productivity. Planned.

The firing temperature is usually 1100 to 1300 ° C.
Is. The temperature rising rate is not particularly limited, and is 100 to 2000
It is appropriately selected from the range of ° C / Hr. Although the firing time depends on the firing temperature, it is usually 10 to 100 hours, and the silanol concentration in the synthetic quartz is 100 ppm or less, preferably 60.
It is continued until it becomes below ppm. Further, it is preferable that the heating is carried out while flowing air containing substantially no water or an inert gas, because the rate of reduction of silanol groups is accelerated. As a matter of course, substantially no carbon is present in the synthetic quartz powder after firing.

The synthetic quartz powder thus obtained can be molded into a molded body. The forming method varies depending on the use of the formed body. For example, when the use is a crucible, the arc melting method is used, and when the use is an IC jig,
Examples thereof include a method of once forming an ingot by Bernoulli method using an oxyhydrogen flame, and a fusion method of heating and melting under vacuum using a carbon mold. In any case, by using the synthetic quartz powder obtained by the present invention, a molded product with extremely few bubbles is obtained, and therefore the quality of the molded product and the product yield are greatly improved.

[0026]

The present invention will be described in detail with reference to the following examples. Example 1 (Production of dry silica gel powder) High-purity tetramethoxysilane was reacted with water to give a lumpy water content of 30% by weight.
The above silica gel (hereinafter referred to as "wet silica gel") was obtained. The lump-shaped wet silica gel was crushed with a net crusher and then dried by heating under reduced pressure to obtain powdery dry silica gel. The powdery dry silica gel was classified by a vibration sieving machine to obtain particles of 500 μm or less and 100 μm or more. When the dry silica gel (hereinafter referred to as "dry silica gel powder") after the classification was analyzed, the water content was 19.5% by weight and the carbon concentration was 1.1% by weight. The bulk density of this powder was 0.92 g / ml.

(Heat Treatment) Using the dry silica gel powder thus obtained, heat treatment was carried out by a rotary kiln shown in FIG. In FIG. 3, 6 is a dry gel hopper, 7 is a table feeder, 8 is a core tube, 9 is a supply port, 10 is a supply port donut-shaped weir, 11 is an air supply pipe, 12 is an exhaust port, and 13 is an exhaust port donut. Shaped weir, 14 treated powder receiver, 15 first heater, 16 second heater, 17 third heater, 18 fourth heater, 19 fifth heater, 20 dry silica gel powder Is. The core tube is made of quartz, the heating zone length: 2 m, inner diameter: 200 mm, supply port donut-shaped weir opening diameter: 40 mm, discharge port donut-shaped weir opening diameter: 40 mm
And the maximum layer height of the dry silica gel powder supplied into the core tube was set to 80 mm. Also,
The core tube was adjusted so that the inclination angle was 0.5 °.

In the heat treatment, first, each heating heater is set to 50.
The temperature was raised to 0 ° C., the core tube was rotated at 4 rpm, the dry silica gel powder was 9.3 kg / Hr, and the air was 678.
It was supplied from the supply port at 0 liter / Hr. The powder in the core tube was continuously flowing all the time. After starting the feeding operation,
When the powder discharged at 4, 6, and 8 hours was analyzed, the values shown in Table 1 were obtained. The residual carbon concentration is E
Obtained by melting a sample powder together with a flux using a MIA610 CS analyzer and quantitatively quantifying generated CO ₂ by infrared absorption method (quantity of CO generated is converted to CO ₂ with a catalyst) Is.

[0029]

[Table 1]

Next, using the same rotary kiln, the powder obtained by the above operation was further heat-treated under the following conditions. First heating heater: 600 ° C, second heating heater: 700 ° C, third heating heater: 850 ° C, fourth heating heater: 1000 ° C, fifth heating heater: 1060 ° C
The temperature was raised to 1, and while the core tube was rotated at 4 rpm, powder was supplied at 6.5 kg / Hr and air was supplied at 1000 liter / Hr from the supply port.

During the above heat treatment, the powder in the core tube was continuously and continuously flowing. 4, 6, 8 after starting the feeding operation
Analysis of the synthetic quartz powder discharged at the hour
It was the value shown in. The number of black particles is the number of black spots visually confirmed by uniformly laying 10 g of the product on a petri dish to a thickness of about 1 mm.

[0032]

[Table 2]

(Baking) 60 kg of synthetic quartz powder obtained by the above heat treatment was placed in a quartz crucible having a diameter of 560 mm, and heated in an electric furnace to be fired. The furnace has a heating rate of 24
After the temperature was raised to 1200 ° C. at 0 ° C./Hr, the temperature was maintained for 60 hours. At this time, the dew point is − on the upper part of the crucible.
Clean dry air at 50 ° C. was passed at 1900 liter / Hr. After the end of the holding, heating was stopped and the temperature was cooled to room temperature. Clean air was also circulated during cooling. The synthetic quartz powder obtained after firing was 58 kg. When the obtained synthetic quartz powder was analyzed at each sampling location, the values shown in Table 3 were obtained.

[0034]

[Table 3]

(Molding) The synthetic quartz powder obtained by firing was
An ingot was formed by the Bernoulli method at each sampling location. No bubbling was observed in the ingot.

Comparative Example 1 Example 1 was repeated, except that the opening diameter of the outlet donut-shaped weir was set to 60 mm and the maximum layer height was set to 70 mm.
Using the same rotary kiln, the dry silica gel powder obtained in Example 1 (production of silica gel powder) was heat-treated. The powder discharged at 4, 6, and 8 hours after the start of feeding was analyzed, and the values were as shown in Table 4.

[0037]

[Table 4]

Next, similarly, using a rotary kiln having a discharge donut-shaped weir opening diameter of 60 mm and a maximum bed height of 70 mm, the powder obtained by the above operation was treated under the following conditions. Further heat treatment was performed. 1st heating heater: 600 ° C, 2nd heating heater: 700 ° C, 3rd
Heater: 850 ° C, 4th heater: 1000
℃, 5th heater: heated to 1060 ℃, while rotating the core tube at 4 rpm, the powder at 6.5kg / Hr,
Air was supplied from the supply port at 1000 liter / Hr.

In the powder in the core tube, the latter half of the powder was continuously flowing from beginning to end, but the first half was intermittently flowing, and generation of more than normal gas was observed when transitioning from stationary to flowing. Was done. When the synthetic quartz powder discharged 4, 6 and 8 hours after the start of the feeding operation was analyzed, the values were as shown in Table 5.

[0040]

[Table 5]

Example 2 The dry silica gel powder obtained in (Production of dry silica gel powder) of Example 1 was used, and the inner diameter of the tube was 400 m.
The same (heat treatment) as in Example 1 was performed using the same rotary kiln as that used in Example 1 except for m.
The powder in the core tube was continuously flowing all the time. The synthetic quartz powder obtained by the heat treatment was fired by the same operation as in (baking) of Example 1, and the fired synthetic quartz powder was molded into an ingot by the same operation as in Example 1 (molding). No foam was observed in any of the ingots.

Comparative Example 2 The same operation as in Comparative Example 1 was carried out except that the same rotary kiln as that used in Example 1 was used except that the inner diameter of the tube was 400 mm. The powder in the core tube flowed continuously from beginning to end in the latter half, but in the first half, it intermittently flowed, and generation of more than normal gas was observed when transitioning from stationary to flowing,
The same state as in Comparative Example 1 was shown.

[0043]

EFFECTS OF THE INVENTION According to the present invention, it is possible to efficiently produce synthetic quartz powder that does not generate bubbles during melt molding.

[Brief description of drawings]

[Fig. 1] Cross-sectional view of a rotary kiln in a stationary state charged with powder.

FIG. 2 is a sectional view of a rotary kiln in a rotating state in which the charged powder is in a fluid state.

FIG. 3 is a diagram showing an example of a rotary kiln used in the present invention.

[Explanation of symbols]

 1 powder 2 core tube 3 powder contact surface 4 void in core tube 5 powder surface 6 dry gel hopper 7 table feeder 8 core tube 9 supply port 10 supply port donut shaped weir 11 air supply pipe 12 discharge port 13 discharge port donut shape Weir 15 1st heater 16 2nd heater 17 3rd heater 18 4th heater 19 5th heater

Claims (8)

[Claims] 1. A method for producing a synthetic quartz powder, which comprises a step of heat-treating a silica gel powder while flowing it using a rotary kiln, wherein the maximum bed height of the silica gel powder in the rotary kiln is 75 mm or more. Method for producing quartz powder. 2. The method for producing synthetic quartz powder according to claim 1, wherein the heat treatment is carried out while continuously supplying silica gel powder from one end of the rotary kiln. 3. The method for producing synthetic quartz powder according to claim 1, which is carried out using a multistage rotary kiln. 4. The final temperature of the heat treatment is 900 to 1100.
The method for producing the synthetic quartz powder according to claim 1, which is at a temperature of ℃. 5. The method for producing synthetic quartz powder according to claim 1, wherein the material of the core tube of the rotary kiln is quartz. 6. The method for producing synthetic quartz powder according to claim 1, wherein the silica gel powder is obtained by hydrolysis of tetraalkoxysilane. 7. After heat treatment at 1100 ° C. or lower,
Furthermore, the manufacturing method of the synthetic quartz powder in any one of Claims 1-6 baked at the temperature exceeding 1100 degreeC. 8. When the silica gel powder is heat-treated while being fluidized using a rotary kiln, the synthetic silica powder obtained by heat-treatment with the maximum bed height of the silica gel powder in the rotary kiln being 75 mm or more is further melted. A method for producing a characterized quartz glass molded body.