JPH06316407A - Production of silica sol - Google Patents

Abstract

PURPOSE:To efficiently produce a spherical silica uniform in size within the range of 3-100nm dispersed particle diameter when a silica sol is produced by hydrolyzing an alkyl silicate with an alkali catalyst. CONSTITUTION:A reactive medium is kept at 0.002-0.1mol/l alkali concn. and >=30mol/l water concn. and alkyl silicate is added to the medium so that Si atoms are contained by 7-80mol per 1 mol alkali. This alkali silicate is allowed to react in the medium at 45 deg.C to the b.p. of the resulting reactive mixture.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a method for producing spherical silica sols containing no impurities. Particularly, the present invention is 3
It is suitable for producing silica sols having uniform sizes of -100 nanometers (nm). Since the silica sol obtained by the production method of the present invention does not contain impurities such as Al, Fe, other polyvalent metals and free anions, it is useful as a raw material for high-purity silica gel, zeolite, etc., a binder for catalysts, an abrasive, etc. Is.

[0002]

2. Description of the Related Art A method for obtaining silica sol from water glass as a raw material by a method such as neutralization with an acid or ion exchange has been known for a long time. Further, by pyrolyzing silicon tetrachloride, high-purity fine silica powder containing almost no impurities such as metals can be obtained. On the other hand, a method of obtaining highly pure silica particles by hydrolyzing an alkyl silicate in an alcohol solution containing ammonia as a catalyst is already known. For example, Journal of Colloid and Interface Science (J. Co.
lloid and Interface Sci.) Volume 26 (1968) 62-69
The authors Stabber et al. On page cite the addition of 0.28 mol / l tetraethyl silicate to an alcoholic solution containing several mol / l of ammonia and several mol / l to 15 mol / l of water to hydrolyze 50- 90
It has been reported that 0 nm silica particles can be obtained.

Japanese Patent Laid-Open No. 62-275005 discloses a water-alcohol dispersion liquid containing seed particles (alcohol 35 to 97%).
There is described a method of obtaining silica particles having a particle size of 0.1 to 10 µm by adding an alkoxide and hydrolyzing it while maintaining alkaline with ammonia or the like. JP 63-74911
In the publication, an alkaline catalyst such as ammonia is used in a molar ratio of 0.5 to 10 with respect to an alkoxysilane, and water is added in an amount of 5 to 20.
In an alcohol solution containing a concentration of mol / liter,
A method for obtaining silica particles of 100 nm or less by hydrolyzing an alkoxysilane at a temperature of 30 ° C. or higher is described. The publication discloses that the higher the reaction temperature, the smaller the particle size of the produced silica.

Japanese Patent Laid-Open No. 61-209910 discloses 0.5-10%.
Quaternary ammonium hydroxide and 0.01 to 0 as a dispersant.
A method of obtaining a silica sol having a particle size of about 10 nm by a method of hydrolyzing a silicate ester in the presence of 001% of a surfactant and then concentrating at 60 to 70 ° C. is described.

[0005]

In the method of obtaining silica sol by neutralizing or ion-exchanging water glass as a raw material, impurities such as metals and free anions cannot be completely removed. The silica fine powder obtained by the thermal decomposition method of silicon tetrachloride is agglomerated particles, and a monodispersed sol cannot be obtained even when dispersed in water.

Generally, in the method of hydrolyzing an alkyl silicate in alcohol, the produced silica particles have a size of 50 nm to several μm, and especially when the size is 100 nm or less, the monodispersibility is not good. In the method of Staver et al., The particle size of silica produced can be controlled by varying the concentrations of ammonia and water in the reaction solution, but the higher the ammonia concentration, the larger the particle size of silica produced. It is in.

According to the method disclosed in Japanese Patent Laid-Open No. 63-74911,
In order to control the particle size of the produced silica, very strict temperature control is required, and it is difficult to obtain silica having the target particle size with good reproducibility. In addition, the particle size distribution of the generated particles is wide and the particle shape is likely to be distorted. In the method of the above-mentioned JP-A-61-209910, it is necessary to add a surfactant as a dispersant to the reaction system, and the surfactant in the produced sol resulting from this addition is used in the application field of the sol. May give unfavorable results. With this method, it is difficult to obtain particles larger than 10 nm because hydrolysis and particle growth do not easily occur even if a surfactant is used, and the stability of the produced silica is insufficient and high concentration is required. I can't. Further, it is difficult to control the particle size and particle size distribution.

The present invention solves the problems in these conventional methods to provide Al, Fe and other polyvalent metals and free anions,
An object of the present invention is to provide a method for efficiently producing an aqueous sol of spherical silica which does not contain impurities such as a surfactant and has a particle size of 3 to 100 nm and a uniform size.

[0009]

In the method for producing an aqueous silica sol of the present invention, the reaction medium is used in an amount of 0.0
While maintaining an alkali concentration of 02 to 0.1 mol and a water concentration of 30 mol or more, the reaction medium is mixed with 1 mol of the alkali.
Add an amount of 7-80 moles of alkyl silicate as Si atom to hydrolyze the alkyl silicate at a temperature of 45 ° C to the boiling point of the reaction medium or lower, and to advance polymerization of silicic acid generated by the hydrolysis. Colloidal silica having a particle size of 3 to 100 nm is produced in this reaction medium.

The alkali in the reaction medium used in the present invention is a hydroxide of an alkali metal such as ammonia, Na, K and Li, a water-soluble amine, a quaternary ammonium hydroxide, and the like. The product is highly reactive and is preferably used. When it is desired to obtain a silica sol containing no alkali metal, the above-mentioned nitrogen-containing basic compound is used. The concentration of these alkalis in the reaction medium is maintained at a concentration of 0.002 to 0.1 molar per liter of reaction medium. The alkali concentration is calculated based on the number of moles of alkali metal atoms in the reaction medium in the case of alkali metal hydroxides, and also ammonia, water-soluble amines,
For quaternary ammonium hydroxide and the like, the calculation is based on the number of moles of nitrogen atoms contained in the compound in the reaction medium.

The alkyl silicate used in the present invention is an alkyl ester of a silicic acid monomer or a silicic acid oligomer having a degree of polymerization of 2 to 10. The alkyl group preferably has 1 to 3 carbon atoms. Also, a mixed ester having different alkyl groups in the molecule, or a mixture of these alkyl silicates can be used.
Examples of preferable alkyl silicates include tetramethyl silicate, tetraethyl silicate, and methyl triethyl silicate.

At the end of the reaction according to the method of the present invention, the alkyl silicate is 4 mol / liter or less as Si atoms derived from the alkyl silicate in the entire reaction solution,
It is preferably used in such an amount that a concentration of 0.1 to 2 mol / liter is obtained. This alkyl silicate is equivalent to 1 mol of alkali in the reaction medium,
It is added to the reaction medium in a ratio of 7 to 80 mol, preferably 10 to 60 mol, as atoms. When the amount of this alkyl silicate is large, the concentration of by-product alcohol in the reaction solution increases with the progress of the reaction, but it is preferable to maintain this by-product alcohol concentration at 8 mol / liter or less until the end of the reaction. The concentration of this by-product alcohol in the reaction solution can be easily maintained by distilling and removing this by-product alcohol from the reaction system.

The alkyl silicate is added to the reaction medium, and the alkyl silicate is hydrolyzed by contacting the reaction medium with the alkyl silicate at a temperature between 45 ° C. and the boiling point of the reaction medium or lower. The polymerization reaction of silicic acid in the reaction medium can proceed. As the silicic acid polymerization reaction proceeds, core particles of silica are produced in the reaction medium, and the particle growth thereof produces colloidal silica having a uniform particle size of 3 to 100 nm, preferably 5 to 100 nm.

This hydrolysis reaction can be carried out under any of reduced pressure, normal pressure and increased pressure, but at a temperature of 45 ° C. or higher, preferably 55 ° C. to the boiling point of the reaction mixture or lower with sufficient stirring. Good to do. This hydrolysis reaction is continued until the alkyl silicate disappears from the reaction mixture formed by adding the alkyl silicate to the reaction medium, and can usually be completed in about 1 to 24 hours.

For the production of the aqueous silica sol according to the present invention,
Various operations can be employed. For example, the reaction medium and the alkyl silicate in the above molar ratio relative to the alkali in the medium may be charged into a container and then sufficiently stirred while maintaining the pressure and temperature. Alternatively, while maintaining the temperature by heating the reaction medium in the container, the reaction medium under sufficient stirring, the alkyl silicate in the above molar ratio,
It can be carried out all at once or intermittently, or as a more preferable method, by adding continuously at a rate of 0.02 to 2 mol / l of the reaction medium, preferably continuously.

The reaction mixture from which the alkyl silicate has disappeared is further subjected to heat aging at 70 ° C. or higher, preferably 80 to 100 ° C., for 30 minutes or longer, preferably for 1 to 6 hours. This heat aging may be carried out while distilling off the by-product alcohol in the reaction mixture, or after this distilling.
The method for producing an aqueous silica sol according to the present invention may be carried out in a liquid in which seed particles having the ability of particle growth are previously contained. The seed particles may be composed of any substance as long as silica deposition can occur on the surface thereof and have a particle size smaller than the particle size intended for production, but fine particles of silica are preferred. .

The silica sol obtained by the method of the present invention can be concentrated by a usual method such as an evaporation method and an ultrafiltration method. This concentration gives a stable silica sol up to about 40%.

[0018]

[Function] In the usual production of silica particles by hydrolyzing an alkyl silicate with an alkali catalyst in an alcohol solvent, silica particles having a large diameter are produced.
It is surprising that, according to the present invention, hydrolyzing an alkyl silicate in an essentially aqueous medium produces a stable aqueous sol of spherical colloidal silica with a uniform particle size of 5 to 100 nm.

Probably, the formation of the colloidal silica particles having the small diameter is considered as follows. That is, in the production of ordinary silica particles by hydrolyzing an alkyl silicate in an alcohol solvent with an alkali catalyst, the silicic acid produced by this hydrolysis is unstable in the alcohol solvent and immediately aggregates, resulting in the alcohol solvent. Silica with a large particle size is generated.

On the other hand, 30 mol / liter or more of water
In a reaction medium containing 0.002 to 0.1 mol / liter of alkali, the silicic acid produced by the hydrolysis of the alkyl silicate, when the Si / alkali molar ratio is low,
Silicic acid monomer or its low polymerization degree oligomer
It is stabilized by a sufficient amount of alkali with respect to Si and remains in its stable solution state in the reaction medium without causing agglomeration.

With the progress of hydrolysis, as the concentration of the silicic acid monomer or its low polymerization degree oligomer in the reaction medium increases, that is, as the Si / alkali molar ratio increases, the reaction medium becomes silicic acid monomer or its low polymerization degree. Degree, it becomes impossible to keep the oligomer in the dissolved state, and finally fine silica core particles are precipitated in this reaction medium,
After that, silicic acid generated in the reaction medium is polymerized around the core particles to cause particle growth of silica. The method of the present invention is characterized in that particles of silicic acid generated at this temperature are immediately grown.

Even if the alkyl silicate is hydrolyzed in the reaction medium, at a low Si / alkali molar ratio of 7 or less, particle growth of silica hardly occurs. However, the higher the molar ratio, the higher the silica ratio. It was found that the particle growth proceeded rapidly and that when the molar ratio was higher than 80, the hydrolysis of the alkyl silicate was significantly delayed.

When a reaction medium having an alkali concentration higher than 0.1 mol / liter is used, the concentration of silica core particles precipitated in the reaction medium becomes high, and the core particles are apt to aggregate with each other, resulting in uniform size. A silica particle sol cannot be obtained. Conversely, the use of a reaction medium having an alkali concentration lower than 0.002 mol / l results in a significantly slower hydrolysis rate of the alkyl silicate.

Generally, when the reaction temperature, the type of alkali, the concentration of alkyl silicate and the like are the same, Si
The higher the molar ratio of / alkali, the larger the particle size of the silica produced. Further, when the amount of the alkyl silicate to be used in the reaction is as large as 4 mol / liter or more in the total reaction liquid at the end of the reaction, the silica particles produced tend to agglomerate and a silica sol with good dispersibility cannot be obtained. . When the concentration of alcohol produced by hydrolysis further increases,
The hydrolysis of the alkyl silicate may not progress sufficiently, or the silica particles produced may aggregate, so
The alcohol concentration during the reaction is preferably maintained at 8 mol / liter or less.

The silicate having an alkyl group having 4 or more carbon atoms is not easily hydrolyzed, and the alcohol produced by the hydrolysis has low solubility in water, which is not preferable for use in the present invention. The reaction temperature particularly affects the dehydration condensation of silicic acid. When the reaction temperature is lower than 45 ° C, particle growth of colloidal silica is difficult to occur, and a large amount of silica particles remaining in a fine size before growth adsorb a large amount of alkali in the liquid, so that it is free in the liquid. There is less alkali in the state and the hydrolysis of the alkyl silicate is slower. At a preferable temperature of 45 ° C. or higher, when the reaction is carried out under the same other conditions, the diameter of the produced colloidal silica particles tends to increase as the reaction temperature increases.

By the stirring preferably carried out during the reaction,
The hydrolysis of the alkyl silicate in the reaction medium and the deposition of the resulting silicic acid on the silica particles by polymerization proceed uniformly. Further, even if a part of the alkyl silicate is insoluble in the reaction medium and is only in contact with the reaction medium, the alkyl silicate is maintained in a suspended state in the reaction medium by sufficient stirring of the reaction mixture. Thus, hydrolysis by contact with the reaction medium or dissolution in this liquid proceeds smoothly.

In the method of adding the alkyl silicate to the reaction medium containing the seed particles, since the silica growth generated by the hydrolysis of the alkyl silicate is bonded to the surface of the seed particles from the beginning of the reaction, the growth silica is grown. The particle size is not easily affected by the Si / alkali molar ratio and reaction temperature. In this method, a reaction medium containing 0.01 to 2% by weight of seed particles is convenient for producing silica particles having a particularly uniform particle size.

The aging at 70 ° C. or higher performed after completion of the reaction eliminates ungrown particles remaining in the reaction mixture,
It is effective in producing a stable sol.

[0029]

【Example】

Example 1 A 3 liter stainless steel reaction vessel equipped with a stirrer and a condenser was charged with 1773 g of pure water and 13.3 g of 10% NaOH aqueous solution, and the temperature of the liquid in the reaction vessel was adjusted to 80 by an oil bath.
It was kept at ℃. Then 214 g of commercial tetraethyl silicate was continuously fed into this vessel under stirring over 2.5 hours. About 1 hour after the start of this supply, the liquid in the container exhibited colloidal turbidity.

When the above supply is completed, the liquid temperature in the container is set to 88
The temperature was raised to 0 ° C., and the mixture was refluxed at this temperature for 1 hour. Then, the liquid in the container was evaporated and the vapor was discharged to the outside of the container to concentrate the liquid until the liquid temperature reached 95 ° C. Then, the total amount of the liquid in the container was taken out of the device and concentrated to 300 g by a rotary evaporator.
A silica sol having a SiO ₂ concentration of%, a pH of 10.6 and a viscosity of 2.6 mPa · s at 25 ° C. was obtained. When the amount of ethanol remaining in this sol was measured by gas chromatography, it was 0.1% by weight or less.

The particle diameter of this sol measured by the BET method is 13.5 nm.
Met. Colter Submicron Particle Analyzer N4
The particle size by dynamic light scattering method (hereinafter referred to as "N4") was 18 nm. The colloidal silica of this sol is shown in Figure 1.
It is shown as a 200,000 times electron micrograph. The average particle size measured from this photograph was 16 nm, and the standard deviation was 2 nm. The colloidal silica of this sol is spherical particles having a narrow particle size distribution.

Separately, the above sol was treated with a cation exchange resin to obtain a SiO ₂ concentration of 19.8 wt%, a pH of 4.7 and a pH of 2.5.
An acidic silica sol having a viscosity of 25 ° C. of mPa · s was obtained. Example 2 The same reaction vessel used in Example 1 was charged with 1629 g of pure water and 13.7 g of a 10% NaOH aqueous solution, and the liquid temperature in the vessel was kept at 80 ° C. by an oil bath. Then, 357 g of commercially available tetraethyl silicate was fed into this vessel under stirring over 5 hours. After the end of this supply, the product was concentrated to 500 g in the same manner as in Example 1 to obtain a SiO ₂ concentration of 20% by weight, 10.
A silica sol having a pH of 5 and a viscosity of 2 mPa · s at 25 ° C. was obtained. The silica particles of this sol had a BET method particle size of 18 nm, a "N4" particle size of 28 nm and an electron micrograph particle size of 25 nm.

Comparative Example 1 The same reaction vessel used in Example 1 was charged with 1706 g of pure water and 80 g of 10% NaOH aqueous solution, and the liquid temperature in the vessel was kept at 80 ° C. by an oil bath. Then 214 g of commercial tetraethyl silicate was fed into this vessel under stirring over 2.5 hours. The reaction liquid in this container became turbid due to the formation of an agglomerated gel during the supply. After standing overnight, the gel was removed from the resulting solution and the supernatant was collected. The recovered liquid was a silica sol having a pH of 11.4, and the colloidal silica had a BET method particle size of 7.2 nm and an "N4" particle size of 34 nm.

Comparative Example 2 The same reaction vessel used in Example 1 was charged with 1773 g of pure water, 13.3 g of 10% NaOH aqueous solution and 214 g of commercially available tetraethyl silicate, and the liquid temperature in the vessel was set to 30 with stirring. It was kept at ℃. Although the water layer and the oil layer were initially mixed in this container, the oil layer disappeared and the liquid in the container became uniform about 10 hours after the charging. The homogeneous liquid was clear and had a pH of 9.5 but no growing particles. When this reaction solution was heated at 88 ° C for 1 hour, the solution remained transparent and no particle growth was observed.

Example 3 In the same reaction vessel used in Example 1, 50 g of the silica sol having a SiO ₂ concentration of 20.2% by weight obtained in Example 1 and pure water 1579.
g and 10% NaOH aqueous solution 13.7 g were charged, and the liquid temperature in the container was kept at 80 ° C. by an oil bath. Then, 357 g of commercially available tetraethyl silicate was fed into this vessel under stirring over 5 hours. After the end of this supply, by concentrating in the same manner as in Example 1 until the amount of liquid in the container reached 550 g, a SiO ₂ concentration of 20 wt%, a pH of 10.4 and 2 mPa · s of ₂ mPa · s was obtained.
A silica sol having a viscosity of 25 ° C. was obtained. The colloidal silica of this sol has a BET method particle size of 27 nm and “N of 43 nm”.
4 "particle size and 38 nm electron micrograph particle size with a standard deviation of 2 nm. A 200,000 times electron micrograph of the silica particles is shown in FIG. It can be seen that its particle size distribution is very narrow. This means that the silicic acid produced by the hydrolysis of the supplied alkyl silicate does not form a new nucleus in the reaction solution and is deposited on the colloidal silica particles that are present in the solution from the beginning, and It means that it was consumed for growth.

Example 4 In the same reaction vessel used in Example 1, 50 g of the silica sol having a SiO ₂ concentration of 20% by weight obtained in Example 3 and 1579 g of pure water were used.
And 13.7 g of 10% NaOH aqueous solution were charged, and the liquid temperature in the container was kept at 80 ° C. by an oil bath. Then, 357 g of commercially available tetraethyl silicate was fed into this vessel under stirring over 5 hours. After the end of this supply, by concentrating in the same manner as in Example 1 until the amount of liquid in the container reached 550 g, a SiO ₂ concentration of 20 wt%, a pH of 10.4 and a pH of 2 mPa · s of 25 were obtained.
A silica sol having a viscosity of ° C was obtained.

The colloidal silica of this sol has a B of 63 nm.
It had an ET method particle size, a 110 nm "N4" particle size and an 85 nm particle size according to an electron micrograph. Example 5 1769 g of pure water was placed in the same reaction vessel used in Example 1.
17 g of 20% by weight tetramethylammonium hydroxide aqueous solution
And were charged, and the liquid temperature in the container was kept at 80 ° C. by an oil bath. Then 214 g of commercial tetraethyl silicate was fed into this vessel under stirring over 1.6 hours. After the end of this supply, the amount of liquid in the container was 300 g as in Example 1.
20% by weight SiO ₂ concentration,
A silica sol having a viscosity of 9.4 and 2.5 mPa · s at 25 ° C. was obtained.

The colloidal silica of this sol has a B of 12 nm.
It had an ET method particle size, a 20 nm "N4" particle size and a 17 nm electron micrograph particle size. Example 6 1775 g of pure water was added to the same reaction vessel used in Example 1.
11 g of 10% by weight Ammore water was charged and the liquid temperature in the container was kept at 80 ° C. by an oil bath. Then 214 g of commercial tetraethyl silicate was fed into this vessel under stirring over 1.4 hours. After the end of this supply, by concentrating in the same manner as in Example 1 until the liquid amount in the container reached 300 g, a SiO ₂ concentration of 20% by weight, a pH of 8.5 and a pressure of 2.5 mPa · s was obtained.
A silica sol having a viscosity of 25 ° C. was obtained.

The colloidal silica of this sol has a B of 13 nm.
It had an ET method particle size, 18 nm "N4" particle size and 16 nm electron micrograph particle size. Example 7 The same reaction vessel used in Example 1 was charged with 1784 g of pure water and 2.1 g of monoethanolamine, and the liquid temperature in the vessel was kept at 80 ° C. by an oil bath. Then, in this vessel under stirring, 214 g of commercially available tetraethyl silicate was added to 1.5
Supplied over time. After the end of this supply, by concentrating in the same manner as in Example 1 until the amount of liquid in the container reached 300 g, a SiO ₂ concentration of 20 wt%, a pH of 9.9 and 2.2 mPa · s.
A silica sol having a viscosity of 25 ° C. was obtained.

The colloidal silica of this sol had a BET method particle size of 19.4 nm, a "N4" particle size of 14 nm, and an electron micrograph particle size of 20 nm. Example 8 1500 g of pure water and 13.7 g of 10% NaOH aqueous solution were charged in the same reaction vessel as used in Example 1 and the liquid temperature was kept at 80 ° C. in an oil bath. Then, in a container under stirring, 357 g of commercially available tetraethyl silicate and 50 g of a 1% NaOH aqueous solution,
At the same time, each was supplied to the reaction vessel over 5 hours. After the end of this supply, the product was concentrated to 500 g in the same manner as in Example 1, whereupon a SiO ₂ concentration of 20 wt%, a pH of 10.5, and a concentration of 2 mPa · s of 25
A silica sol having a viscosity of ° C was obtained. The silica particles of this sol had a BET method particle size of 15 nm, a “N4” particle size of 22 nm, and an electron microscope particle size of 18 nm.

Example 9 1500 g of pure water and 8 g of pure water were placed in a 3-liter stainless steel reaction vessel equipped with a stirrer and a condenser with a rectification tube.
A 10% NaOH aqueous solution was charged and the liquid temperature was kept at 87 ° C in an oil bath. Then, 520 g of commercially available tetraethyl silicate and 240 g of 0.5% NaOH aqueous solution were simultaneously supplied to the reaction container in the stirred container for 5 hours each. As the reaction progressed and ethanol was produced, the temperature of the reaction solution was almost
The reaction was carried out while distilling the hydrous alcohol so that the temperature was kept at 88 ° C. After the supply was completed, the same procedure as in Example 1 was performed.
Concentrated with 7 g, 30 wt% SiO ₂ concentration, 10.6 pH, 2 m
A silica sol having a viscosity of 25 ° C. of Pa · s was obtained. The silica particles of this sol have a BET method particle size of 20 nm and "N4" of 28 nm.
It had a particle size and an electron microscope particle size of 26 nm.

[0042]

EFFECTS OF THE INVENTION According to the present invention, it is possible to produce a uniform-sized spherical silica sol having a particle diameter of several nm to 100 nm. And this sol can be concentrated to a silica concentration of several tens wt% and is stable for a long period of time.
An acidic silica sol can be obtained by treating the silica sol obtained by the method of the present invention with a cation exchange resin. Further, by removing the by-product alcohol contained therein by distillation from the silica sol obtained by the method of the present invention, an aqueous silica sol containing no alcohol can be obtained. Furthermore, by replacing the liquid medium of the silica sol obtained by the method of the present invention with an organic solvent, an organic solvent-dispersed silica sol called an organosol can be obtained.

The silica particles obtained by the method of the present invention do not contain impure components such as Al, Fe and other polyvalent metals and anions, and are raw materials such as polishing agents for silicon wafers and high-purity silica gel for liquid chromatography carriers. It is also useful as a binder for catalysts, a raw material for special zeolites, microfillers added to coating materials for electronic materials, microfillers for polymer films, and the like.

[Brief description of drawings]

1 is a 200,000 times electron micrograph showing the particle structure of the colloidal silica obtained in Example 1. FIG.

FIG. 2 is a 200,000 times electron micrograph showing the particle structure of the colloidal silica obtained in Example 3.

Claims (1)

[Claims] 1. The reaction medium is 0.0 per liter.
While maintaining an alkali concentration of 02 to 0.1 mol and a water concentration of 30 mol or more, the reaction medium is mixed with 1 mol of the alkali.
Add an amount of 7-80 moles of alkyl silicate as Si atom to hydrolyze the alkyl silicate at a temperature of 45 ° C to the boiling point of the reaction medium or lower, and to advance polymerization of silicic acid generated by the hydrolysis. A process for producing an aqueous silica sol, characterized in that colloidal silica having a particle size of 3 to 100 nm is produced in this reaction medium.