JPH11147709A - Production of granular silica - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing granular silica having smooth surface shape, capable of carrying out gelation under arbitrary gelation conditions and free from aggregation without using any additive such as surfactant and without carrying out mechanical pulverization in a method for producing granular silica by sol-gel method from a tetraalkoxysilane. SOLUTION: In this method for producing granular silica comprising hydrolyzing a tetraalkoxysilane to afford an aqueous solution of sol, gelatinizing the solution, drying the resultant gel and baking the dried gel, the aqueous solution of sol obtained by hydrolyzing the tetraalkoxysilane is mixed with an organic solvent which is not compatible with water under stirring to afford W/O type suspension and then, the aqueous solution of sol is gelatinized under stirring. Thereby, granular silica having about 40-500 μm size can efficiently be produced without passing through a pulverizing step.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing granular silica, and more particularly to the production of high-purity synthetic quartz glass products suitable for semiconductor-related fields such as optical fibers, crucibles for pulling silicon single crystals, and sealants for LSIs. The present invention relates to a method for producing granular silica for use in a high yield.

[0002]

2. Description of the Related Art Granular silica as a raw material for quartz glass products used in the semiconductor industry, the optical communication field, and the like has been conventionally used as a raw material obtained by pulverizing natural quartz. However, it contained various metal impurities and was not satisfactory in terms of purity and uniformity. On the other hand, in particular, as a means for improving purity, an oxyhydrogen flame method using a lump of fume obtained by attaching and growing fumes generated by decomposition of silicon tetrachloride in an oxyhydrogen flame to a substrate, tetraalkoxysilane A sol-gel method using a gel obtained by using an organic silicon compound such as the above as a raw material has been studied. Since the oxyhydrogen flame method is a gas phase reaction, the contamination with impurities can be extremely reduced, but has a disadvantage that it requires a lot of energy and has a high production cost. On the other hand, the sol-gel method is a liquid phase reaction at a low temperature, and has an advantage that a high-purity product can be obtained at low cost.

However, also in the sol-gel method, a step of pulverizing the obtained gel, or a crushed gel or granular silica obtained by calcining the gel, removes impurities derived from scraping the inner walls of equipment, piping and the like. The contamination cannot be completely prevented. Therefore, the above-mentioned sol-
There has been a demand for a method for producing granular silica having a granular and smooth surface shape so as to omit the pulverizing step in the gel method and not to scrape the inner walls of the apparatus, piping, etc. For this purpose, the gel is granulated at the stage of gelation of the sol aqueous solution obtained by hydrolyzing the tetraalkoxysilane, and dried to obtain a granular dry gel efficiently without aggregation,
Further, it is necessary to bake this to obtain granular silica.

[0004] Granular silica used for the above-mentioned purpose preferably has a particle size of about several tens to several hundreds of µm from the viewpoint of handling properties and the like. That is, an object of the present invention is to obtain high-purity granular silica of about several tens to several hundreds μm in high yield.

As a method for producing granular silica from a tetraalkoxysilane by a sol-gel method, there is a method in which a water-insoluble organic solvent and an aqueous sol solution are converted into a W / O emulsion in the presence of a surfactant to carry out a gelling reaction. (JP-A-01-103904) and a method of pulverizing an aqueous sol solution while drying it in a vibrating fluidized drier (JP-A-01-234318) are known.

[0006] Among them, in order to obtain a stable W / O emulsion, it is necessary to add a surfactant. Such a surfactant may remain as a carbon component in the process of drying and calcining the obtained gel to obtain granular silica, and is not suitable as a method for producing high-purity granular silica. In order to make it a W / O emulsion,
The particle size of the obtained granular silica is usually from 0.1 to several μm, and the particle size is too small for the intended use of the present invention.
In one method, mechanical pulverization is performed at the gelation stage, and it is still impossible to completely prevent impurities from being mixed.

[0007]

Therefore, as a method for producing granular silica from a tetraalkoxysilane by a sol-gel method, there is no need to use additives such as a surfactant and to carry out mechanical pulverization. Gelation is possible,
A method for producing granular silica having a smooth surface shape without agglomeration in high yield has been desired.

[0008]

Means for Solving the Problems The present inventors have made intensive studies to solve the above problems, and as a result, mixed a specific organic solvent with a sol aqueous solution obtained by hydrolyzing tetraalkoxysilane, It has been found that the above problem can be solved by performing gelation while stirring, and the present invention has been completed.

That is, the present invention provides a method for producing granular silica in which tetraalkoxysilane is hydrolyzed to form an aqueous sol solution, which is gelled, dried to form a dry gel, and then calcined. A method for producing granular silica, comprising mixing and stirring an aqueous sol solution obtained above and an organic solvent incompatible with water to form a W / O suspension, and then gelling the aqueous sol solution with stirring. It is.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The tetraalkoxysilane used in the present invention includes tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, tetrapentoxysilane, tetrahexyloxysilane, etc., and is preferably an alkoxy having 1 to 4 carbon atoms. It is preferable to use tetramethoxysilane and tetraethoxysilane because the reaction rate is high.

In the present invention, as a method for obtaining an aqueous sol solution by hydrolyzing tetraalkoxysilane, a mixture containing tetraalkoxysilane and water, and if necessary, a catalyst and an alcohol are added at 5 to 50 ° C., preferably 20 to 50 ° C. ~
What is necessary is just to keep stirring at a constant temperature of 40 ° C. and to stir. Water is charged about 1 to 20 equivalents with respect to the tetraalkoxysilane, and the hydrolysis reaction is allowed to proceed under standing or stirring to form an aqueous sol solution. Since water is consumed as the hydrolysis reaction proceeds, water may be added during the reaction. For the purpose of producing high-purity granular silica, it is preferable to use ion-exchanged water or ultrapure water as the water. Further, a catalyst may be appropriately added for the purpose of accelerating the reaction. As such a catalyst, a catalyst containing no metal is preferable, and hydrochloric acid, carbonic acid, acetic acid, or aqueous ammonia is preferable. The alcohol may be appropriately added for compatibilization of the tetraalkoxysilane with water, but is not necessarily added because the alcohol is generated as the hydrolysis reaction of the tetraalkoxysilane proceeds.
Such alcohols include methanol, ethanol,
And propanol and butanol.

In the present invention, when the aqueous sol solution is gelled, it is particularly important to mix and stir an organic solvent that is not compatible with water into the aqueous sol solution. Specifically, those having a solubility in water at 20 ° C. of 5 w / w% or less can be particularly preferably used. When the solubility in water is 5 w / w% or less, when mixed with an aqueous sol solution to form a gel, a W / O-type suspension can be easily obtained, and a granular gel can be obtained with high yield. . Further, since the obtained gel contains a small amount of solvent components, the possibility of remaining as a carbon component after the subsequent drying / firing step is small, which is preferable.

For the purpose of controlling the particle size of the granular silica obtained by the method of the present invention or improving the yield, it is particularly preferable to use an organic solvent described below.

That is, the organic solvent used in the present invention preferably has a surface tension of 33 dyn / cm or less. Since the surface tension of the sol aqueous solution is about 35 to 45 dyn / cm, if mixed with an organic solvent having a surface tension lower than that of the sol aqueous solution, the sol aqueous solution easily becomes droplets, and the W / O type suspension can be easily formed. Obtainable. Therefore, the surface tension is 33
By using an organic solvent of dyn / cm or less, a granular gel having a sharp particle size distribution can be obtained with a high yield. In addition, the larger the difference between the surface tension and the aqueous sol solution, the smaller the particle size of the obtained gel can be.

The organic solvent used in the present invention is described in Polymer Handbook 3rd. Ed. , John Wylie and Sons. ink. , 1989, VII. Solubility parameter values (Poymer Handboo
k 3rd.Ed., John Wiley & Sons. Inc., 1989, VII.SOLUBILI
TY PARAMETER VALUES) is preferably a solvent having a hydrogen bond strength of “medium” described in the paragraph of “TY PARAMETER VALUES”, specifically, one or more selected from organic solvents such as ketones, esters, and ethers. Two or more organic solvents. Note that the hydrogen bond strength refers to the molecular structure of a compound,
In other words, it indicates the strength of intermolecular interaction due to the hydrogen bond caused by the type of the functional group. In addition to the solubility parameter defined by the half power of the cohesive energy of the molecule, the compatibility between heterogeneous compounds is shown. It is used as a scale. Since such an organic solvent has an appropriate affinity for the sol aqueous solution, it is considered that when mixed with the sol aqueous solution, a stable W / O suspension is generated. By mixing and stirring such an organic solvent, the sol aqueous solution and W of the organic solvent are mixed.
/ O type suspension can be easily obtained, and the sol aqueous solution is gelled in the form of droplets, so that the sol aqueous solution can be obtained in high yield,
A granular gel having a desired particle size, without aggregation, and having a smooth surface shape can be obtained.

The organic solvent used in the present invention particularly preferably satisfies the above-mentioned conditions. Specific examples thereof include isopropyl ether, n-butyl ether, diethyl carbonate, diethyl ketone, methyl n
-Butyl ketone, di-n-propyl ketone, methyl isobutyl ketone, diisobutroketone, n-propyl acetate, n-butyl acetate, n-amyl acetate,
Isobutyl acetate, isopropyl acetate, isoamyl acetate, ethyl propionate, n-butyl propionate, isoamyl propionate, methyl butyrate, ethyl butyrate, isoamyl butyrate,
Isophorone and the like. In addition, in order to completely remove the solvent by drying and firing after gelation, volatile,
Diethyl carbonate or diethyl ketone, which has good thermal decomposability, is particularly preferable.

In the present invention, the mixing amount of the organic solvent is not particularly limited, but it is preferable to use about 50 to 200 parts by volume with respect to 100 parts by volume of the sol aqueous solution. When the mixing amount of the organic solvent is within the above range, the fluctuation in power required for stirring before and after gelation is small, and the particle size control of the granular gel particles is easy.

At least the surface of the container and the stirring device used in the present invention is made of a hydrophobic material such as polyethylene, polypropylene, and Teflon for the purpose of maintaining the sol aqueous solution in the form of droplets and gelling. Is preferred. The stirring conditions after mixing the organic solvent vary depending on the reaction vessel and the stirring mechanism for gelation, but stirring to the extent that a stable W / O suspension is formed, for example, from the range of 200 to 600 rpm. Adopt it. If one example is given, inner diameter 90mmφ, height 150mm
Cylindrical container and 75 × 20 mm rectangular blades
When the total amount of the sol aqueous solution and the organic solvent is about 300 g by using the attached stirring blade, the stirring may be performed at a stirring speed of about 250 to 350 rpm.

In the present invention, gelling conditions can be arbitrarily selected. Generally, gelation may be performed at 20 to 80 ° C. for about 10 minutes to 6 hours. In addition, in order to obtain high-purity granular silica, it is preferable to obtain a granular gel having a pore size large enough to completely volatilize residual alkoxy groups in the firing step. For this purpose, it is particularly preferable to perform gelation at 40 to 60 ° C. for about 30 to 60 minutes using an alkaline catalyst such as aqueous ammonia.

In the present invention, the particle size of the granular gel can be controlled by the amount of water used for preparing the aqueous sol solution, the type of organic solvent used for gelation, stirring conditions, and the like. A granular gel having a particle size of about 80 to 700 μm can be obtained.

The granular gel obtained by the present invention can be dried and calcined to obtain granular silica without going through a pulverizing step. Such drying is performed for the purpose of removing the residual solvent and moisture, and may be used without particular limitation as long as the apparatus has a pressure reducing mechanism and a heating mechanism. Specific examples of the dryer include a batch-type vacuum dryer, a conical dryer, and a cylindrical dryer.

The granular dry gel obtained according to the present invention is
Subsequently, baking is performed to completely remove residual silanol groups and moisture, and pores are closed to obtain granular silica. The firing furnace may be any of a stationary type, a fluidized bed type, or a moving bed type,
The firing may be performed at a temperature of 1000 to 1300 ° C. while flowing a gas such as air, nitrogen, argon, or helium as needed. Since the gel shrinks by firing, the particle size of the granular silica obtained by the method of the present invention is 40 to 50.
It is about 0 μm.

[0023]

According to the present invention, an aqueous solvent insoluble in water is mixed with an aqueous sol solution obtained by hydrolyzing tetraalkoxysilane and stirred to form a W / O suspension, and the aqueous sol solution is gelled. A method for producing granular silica, characterized by the fact that
Granulation is performed at the gelation stage, and the particle size is 80 to 7 without aggregation.
Since a granular gel having a smooth surface shape of about 00 μm can be obtained, it is possible to obtain a granular silica having a particle size of about 40 to 500 μm by directly firing without requiring a pulverizing step. Also, using additives such as surfactants,
It is not necessary to mechanically pulverize, it is possible to almost completely remove impurities from being mixed in the production process, and it is possible to produce granular silica in high yield.

[0024]

EXAMPLES The present invention will be described below in more detail with reference to Examples, but it should be understood that the present invention is by no means restricted to such specific Examples.

Examples 1 to 7 and Comparative Examples 1 to 3 A cylindrical polypropylene container having an inner diameter of 90 mmφ and a height of 150 mm was charged with 60 g of tetramethoxysilane and 0.01 times the amount of 0.01% ammonia water equivalent to 10 times the amount of tetramethoxysilane. It was charged, stirred at a speed of 200 rpm with a 75 × 20 mm Teflon stirring blade, and reacted at 20 ° C. for 60 minutes to obtain an aqueous sol solution. Subsequently, Table 1. The organic solvent shown in the above was charged in the same volume as the aqueous sol solution, and the
Gelation was performed at 60 ° C. for 60 minutes. The obtained granular silica gel was filtered and dried at 100 ° C. for 8 hours in a vacuum dryer.
It was dried at 160 ° C. for 2 hours. The obtained dried granular gel was sieved with a mesh having an appropriate opening diameter, the dried granular gel was separated, weighed, and the yield was calculated. The dried granular gel was observed with a video microscope, and the shape was observed, the presence or absence of aggregation, and the particle size were measured. Table 1 shows a series of measurement results. Are also shown. The dry granular gels obtained in Examples 1 to 5 were granular bodies having a smooth surface shape of spherical to elliptical shape without aggregation of 90 to 400 μm.
In addition, the dried granular gels obtained in Examples 6 and 7 were partially agglomerated, and the yield of the dried granular gel was 80 to 72%. Further, a part of the dried granular gel was obtained in an agglomerated state. These were subsequently fired to obtain granular silica. The dried gels of Comparative Examples 1 to 3 that could not be granulated were not fired. The shape of the dried granular gel of Example 4 is shown in FIG. It was shown to.

In the firing, the dried granular gels obtained in Examples 1 to 7 were placed in a quartz crucible and heated from room temperature to 800 ° C. at 2 ° C.
The temperature was raised from 800 ° C. to 1250 ° C. at a rate of 4 ° C./min, and the temperature was maintained at 1250 ° C. for 2 hours. The particle size of the fired granular silica and the presence or absence of air bubbles are determined by observation with a video microscope. The number of black spots is determined by spreading 2.5 g of the fired granular silica on white paper and counting the number of black spots visually. Was. As a result, the obtained granular silica had a particle size of 40 to 500 μm, and no extraneous particles or bubbles detected as black spots were mixed.
The granular silica of Example 4 was analyzed for residual hydroxy groups by FT-IR, and as a result, the content of hydroxy groups was 80 ppm or less.

[0027]

[Table 1]

[Brief description of the drawings]

FIG. 1 is a photograph showing the particle shape of the dried gel obtained in Example 4 taken with a video microscope.

Claims (1)

[Claims] 1. A method for producing granular silica in which tetraalkoxysilane is hydrolyzed to form an aqueous sol solution, which is gelled, dried to form a dry gel, and then calcined, the sol obtained by hydrolyzing tetraalkoxysilane. An aqueous solution and an organic solvent that is incompatible with water are mixed and stirred to form a W / O
A method for producing granular silica, comprising forming a mold suspension and then gelling the aqueous sol solution with stirring.