JPH08301615A - Method for producing synthetic quartz powder and method for producing molded quartz glass - Google Patents

Abstract

(57) [Summary] [Purpose] Effective production of synthetic quartz powder without bubbles during melt molding. [Constitution] In a synthetic quartz powder manufacturing method including a step of heat-treating silica gel powder while flowing, the silica gel powder is heat-treated while being continuously supplied from one end of a rotary kiln. Production method. [Effect] It is possible to effectively produce synthetic quartz powder that does not generate bubbles during melt molding.

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. It is manufactured by using a synthetic quartz powder obtained by firing silica gel obtained by liquefying and melting it 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 as 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 earnest 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, in the baking of silica gel, in order to eliminate contamination of impurities from the container, silica gel is charged in a container made of quartz and heated in an electric furnace or the like. Especially when industrially manufactured, a large-diameter quartz crucible is used. However,
Since silica gel has a lower bulk density than quartz powder, the container used for firing cannot be used efficiently, productivity is poor, and manufacturing costs are high. Therefore, increasing the bulk density of the powder charged in the crucible is an important issue for improving productivity.

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, the unreacted alkoxy group and a part of the by-produced alcohol remain even if the alcohol by-produced by drying is removed. Actually, when the carbon concentration in the dried silica gel powder is measured, it is 1 to 3% though it varies depending on the drying conditions. 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 the silica gel is sealed, and the rate of temperature rise in the temperature raising process is important. However, as mentioned above, when industrially producing synthetic quartz powder, a crucible with a large diameter is used,
During the heating process, the temperature inside the crucible became uneven, and as a result, synthetic quartz powder with residual carbon was partially generated in some cases, and micro bubbles were generated in the molded product using the synthetic quartz powder. The phenomenon of doing is seen.

In order to solve such problems and efficiently produce high-quality synthetic quartz powder, heating using a rotary cylindrical heating device, a so-called rotary kiln, was also tried. However, since the material of the core tube is limited to quartz or the like for the purpose of preventing contamination, there is a problem that the life of the core tube is shortened due to heat shock caused by repeating the temperature raising step in the heat treatment. In addition, since a large amount of gas is generated from silica gel due to heat treatment, and the gas generation amount changes with time, it is difficult to treat exhaust gas with a rotary kiln, and when the peak time is set, the device becomes large and the silica gel powder inside may be blown off. .

Further, in the step of gradually sealing the silica gel, the solvent, adhering water, residual organic groups, etc. are removed sufficiently and accurately to obtain the desired high-quality synthetic quartz powder.
It is difficult to do this with a rotary kiln charged with a large amount of powder, since it is necessary to strictly control the conditions of heat treatment, especially the temperature raising step, and moreover, the strength of heating tends to increase in the axial direction of the rotary kiln. Leading to product variations,
There was also a problem. Further, it was difficult to sufficiently prevent contamination at the time of supplying and taking out the powder and uniforming the quality of each batch.

In view of the above-mentioned problems, the present inventors have made further studies, and as a result, the powder obtained by heat-treating silica gel under appropriate conditions and operations has a bulk density of quartz powder after firing. It is possible to make them equivalent, and since the alkoxy group and the hydroxyl group can be sufficiently removed, it is convenient to subject such powder to calcination at a higher temperature. By the treatment method, it was found that the space required for the production of synthetic quartz powder, manpower, the amount of gas supplied during heating can be reduced, aeration during heating can be easily performed, and the productivity can be greatly improved. It came to completion. That is, the present invention is a method for producing synthetic quartz powder, which comprises a step of performing heat treatment while flowing silica gel powder, wherein the silica gel powder is heat treated while being continuously supplied from one end of the rotary kiln. Manufacturing method, etc.

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

The alkoxysilane used as a raw material is preferably a C1-C4 lower alkoxysilane such as tetramethoxysilane or tetraethoxysilane or an oligomer thereof. The amount of water used is usually selected from the range of 1 to 10 equivalents of the alkoxy groups in the alkoxysilane. At this time, organic solvents such as alcohols and ethers which 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.

For 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 may be subdivided and then dried, or may be dried and subdivided. In any case, the particles are subdivided so that the particle diameter after drying is 10 to 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, the content of water expressed by the standard moisture content is 1 to 30% by weight.

The dried silica gel powder thus obtained is heat-treated under the specific conditions described below. That is, heat treatment is performed while flowing the silica gel powder. Then, at this time, it is a requirement of the present invention to perform the heat treatment while continuously supplying the silica gel powder from one end of the rotary kiln.

That is, while the dry silica gel is continuously supplied from one end of the core tube of the rotary kiln, the heat treatment is carried out in the core tube, and the powder after the treatment is continuously discharged from the core tube. It is necessary to select a material for the core tube that does not cause contamination of the treated powder.
Particularly, quartz is preferable. In the case of quartz, since there is a limit in the size of the core tube in manufacturing, it is possible to arrange a plurality of rotary kilns in series and perform heat treatment depending on conditions.

The temperature range for heat treatment is 200 to 11
It is 00 ° C. In particular, the removal of residual carbon in silica gel mainly proceeds in the region of 300 to 600 ° C., so that the operation is performed carefully. 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.

When performing the operations continuously in this way,
It is necessary to divide the heating zone of the furnace core tube into a plurality of zones and control the heating intensity so that the temperature of the powder rises in a predetermined gradient in accordance with the traveling direction of the treated powder. The temperature of the supplied powder is raised as it moves in the rotary kiln in the traveling direction. Therefore, the temperature rising rate of the powder is naturally obtained from the temperature distribution in the rotary kiln and the moving speed of the powder in the traveling direction. For example, if the distance between the temperature measurement points is 1 m, the temperature difference is 200 ° C., and the moving speed of the powder in the traveling direction is 0.5 m / Hr, the heating rate is 400 ° C./H.
r. The rate of temperature rise 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, 450
It is also effective to provide a zone where the temperature hardly changes in the region of up to 600 ° C. and set the passage time of the zone to 0.5 to 5 hours. For example, the heater can be divided into a plurality of heaters in the axial direction of the rotary kiln to control the heating intensity in the axial direction. In this case, it is desirable to relatively increase the heating strength and narrow the zone range in the temperature range where the water evaporates from around room temperature and the temperature range where the water evaporates. Next, it is desirable to control so that the temperature of the powder slowly rises without extreme heating in a temperature range where the heat of the sensible heat of silica gel is supplied to raise the temperature of the powder.

On the other hand, it is desirable that the supply rate of the silica gel powder to the rotary kiln be constant. If the heat treatment conditions for the powder are not constant, the quality of the treated powder obtained will vary, so it is desirable to keep the supply rate constant. For this purpose, it is desirable to supply by a quantitative device such as a table feeder, a rotary valve, a choke valve or the like.

The atmosphere for the heat treatment is clean air or an 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 method of supplying the gas is not particularly limited, but in the heat treatment in the region where the water content contained in the silica gel evaporates, it is desirable to supply the gas from the same one end as the silica gel powder and discharge them from the other end. This is because the temperature of one end of supplying the powder is close to room temperature, and therefore, when the powder is supplied in a countercurrent, water vapor is condensed near the supply port of the powder and the powder becomes a book, which may deteriorate the fluidity. When gas is supplied by a nozzle or the like, the gas supply nozzle is provided above the powder supply nozzle,
It is desirable that the nozzle direction be horizontal or upward with respect to the axis of the core tube. At the same time, if the powder supply nozzle is made horizontal or downward with respect to the axis of the core tube, it is possible to prevent the powder short path in the core tube. It is desirable that the linear velocity of the supply gas in the core tube is set to 1 m / s or less, preferably 0.5 m / s or less to prevent the rolling powder from being rolled up and short-passing the gas layer.

The treated powder that has undergone the heat treatment is discharged from the core tube together with the supply gas, so that the treated powder and the supply gas are separated. At this time, a separator such as a cyclone is designed so that the linear velocity of the gas is 1 m / s or less, preferably 0.5 m / s or less, to reduce the yield due to a part of the treated powder being entrained in the exhaust gas. It is desirable to prevent it. In order to prevent contamination of the product, it is desirable to refine the supply gas in advance. In particular, it is desirable to pass a filter capable of cutting 0.02 μm or less.

The gas supply rate is usually 30 to 300 liters / oxygen equivalent per 1 kg of powder continuously supplied.
It is Hr. In order to realize the heat treatment of silica gel by such a continuous operation, in the present invention, it is suitable to perform the inclination angle of the rotary kiln at 3 ° or less, preferably 1 ° or less. Surprisingly, in the heat treatment of the silica gel powder, even if the inclination angle is substantially 0 °, that is, it is completely horizontal, it does not hinder the continuous heat treatment.
On the contrary, it turned out to be desirable. At this time, it is desirable to provide a donut type weir or the like and make the diameter of the supply port smaller than the diameter of the discharge port. The angle of inclination of the rotary kiln is 3
Above 0 ° C, silica gel that has undergone heat treatment may cause backflow. That is, the silica gel does not move in the rotary kiln as a piston flow in the traveling direction. In such a case, slippage of the silica gel, short-cut, and the burnt-out of the silica gel due to these are likely to occur.

The above-described aspect of the present invention allows for substantially steady state at each point in the rotary kiln. That is, the physical properties of the treated powder at each point do not change with time, and the physical properties become a function in the axial direction of the rotary kiln. Then, the above-mentioned object of the present invention can be achieved.

Further, in the present invention, heat treatment is carried out while flowing silica gel powder in a rotary kiln. Here, when the powder “flows”, for example, will be described with reference to FIG. 2 showing a cross section of a rotary kiln in a rotating state in which the powder is charged, in comparison with FIG. 1 showing a cross section of the rotary kiln in a stationary state, the powder ( There is substantially no slippage between 1) and the powder contact surface (3) of the wall of the core tube (2), the powder is lifted by the wall of the core tube, and the powder falls on the wall surface above the repose angle. It refers to the state where they are separated and flow down to the bottom of the core tube wall (in the direction of the black arrow in FIG. 2). By continuously supplying silica gel powder to the rotary kiln and performing heat treatment while flowing this,
The reduction of carbon is promoted and a homogeneous treated powder is obtained. By such heat treatment, the carbon concentration in silica gel is reduced to about 50 to 1000 ppm.

The treated powder from which the residual carbon has almost disappeared is continuously heated, and the final temperature of the 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 while continuously supplying silica gel powder from one end of the rotary kiln. Also in this case, the 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 is adopted. The gas supply is usually
3 to 5 in terms of oxygen per 1 kg of powder continuously supplied
It is 0 liter / Hr. The heat treatment in this temperature range is also performed using a rotary kiln while flowing the powder. 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 to
It rises to about 1.2 g / ml.

When silica gel powder is subjected to heat treatment according to the present invention, synthetic quartz powder is obtained.
000 ppm or more remains. Therefore, usually, firing is performed in a temperature range further increased. For the container used for firing, a material that does not generate contamination of synthetic quartz powder with impurities, for example, a quartz crucible is used. In this firing, since carbon in the powder to be fired has been substantially completely removed, it is not necessary to pay particular attention 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. Also, in advance,
The bulk density of the powder is sufficiently increased, the bulk density of the powder before firing and the powder after firing do not significantly change, and the container can be efficiently used, so that the productivity can be improved.

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 compact. The forming method varies depending on the use of the formed body. For example, when the use is a crucible, the arc melt method is used, and when the use is an IC jig, the ingot is once produced by the Bernoulli method using an oxyhydrogen flame. Examples thereof include a molding method and a fusion method in which a carbon mold is used to heat and melt under vacuum. In either case,
By using the synthetic quartz powder obtained by the present invention, it is possible to obtain a molded product with extremely few bubbles, so that the quality of the molded product and the product yield are greatly improved.

[0027]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples unless it exceeds the gist. 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 using the rotary kiln shown in FIG. 1 in FIG. 1 is a dry gel hopper,
7 is a table feeder, 8 is a core tube, 9 is a supply port, 1
0 is a supply port donut-shaped weir, 11 is an air supply pipe, 12 is an outlet, 13 is an outlet donut-shaped weir, 14 is a treated powder receiver,
Reference numeral 15 is a first heater, 16 is a second heater, 1
7 is a third heater, 18 is a fourth heater, 19
Is a fifth heater and 20 is dry silica gel powder.

The core tube (3) is made of quartz and has a length (heating zone) of 2 m, an inner diameter of 200 mm, a donut-shaped weir opening diameter of the supply port: 40 mm, a donut-shaped opening diameter of the discharge port: 40 m.
It has a dimension of m. The core tube has an inclination angle of 0.5 °.
Adjusted so that First, set each heater to 500
The temperature was raised to ℃, while rotating the core tube at 4 rpm, dry silica gel powder at 6.2 kg / Hr and air at 4500.
It was supplied from the supply port in liter / Hr and heat-treated.
The powder discharged at 4, 6 and 8 hours after the start of the feeding operation was analyzed and found to have the values shown in Table 1.

[0030]

[Table 1]

Next, using the same rotary kiln, the treated powder obtained by the above operation was 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. 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 2.

[0032]

[Table 2]

(Baking) 60 kg of synthetic quartz powder obtained by the heat treatment was placed in a quartz crucible having a diameter of 560 mm, and heated in an electric furnace to be fired. Furnace heating rate 240 ℃ /
With Hr, after raising the ultimate temperature to 1200 ° C, 60 at the same temperature
Held for hours. At this time, the dew point was -50 ° C above the crucible.
Of clean dry air of 1900 liters / 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 59 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 60 kg of the dry silica gel powder obtained in (Preparation of dry silica gel powder) of Example 1 was directly charged into a crucible and heated in an electric furnace without being subjected to heat treatment. Firing was performed by the same operation as (firing). The synthetic quartz powder thus obtained was 47 kg. When this synthetic quartz powder was analyzed at each sampling location, the values were as shown in Table 4. This synthetic quartz powder was molded into ingots by the Bernoulli method at each sampling location. Table 4 shows the foaming state in the ingot.

[0037]

[Table 4]

[0038]

Industrial Applicability According to the present invention, it is possible to efficiently produce synthetic quartz powder that does not foam 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] Cross section 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 (10)

[Claims] 1. A synthetic quartz powder comprising a step of heat-treating silica gel powder while flowing, wherein the silica gel powder is heat-treated while being continuously supplied from one end of a rotary kiln. Manufacturing method. 2. The final temperature of the heat treatment is 900 to 1100.
The method for producing a synthetic quartz powder according to claim 1, wherein the temperature is 0 ° C. 3. The method for producing synthetic quartz powder according to claim 1, wherein the heat treatment is performed at 1100 ° C. or lower and then firing is performed at a temperature higher than 1100 ° C. 4. The method for producing synthetic quartz powder according to claim 1, wherein the inclination angle of the rotary kiln is 3 ° or less. 5. The method for producing synthetic quartz powder according to claim 1, which is carried out using a multistage rotary kiln. 6. The method for producing synthetic quartz powder according to claim 1, wherein the material of the core tube of the rotary kiln is quartz. 7. The method for producing synthetic quartz powder according to claim 1, wherein the silica gel powder is obtained by hydrolysis of tetraalkoxysilane. 8. The method for producing synthetic quartz powder according to claim 1, wherein the heating strength in the axial direction of the rotary kiln is controlled by providing a plurality of heaters in the axial direction of the rotary kiln. 9. The method for producing synthetic quartz powder according to claim 1, wherein the silica gel powder is obtained by hydrolysis of tetraalkoxysilane. 10. A synthetic quartz powder obtained by performing heat treatment while continuously supplying silica gel powder from one end of a rotary kiln during heat treatment while flowing the silica gel powder is further melted. A method for producing a molded quartz glass body.