JPH06115924A - Method for producing anhydrous silica fine powder and water-repellent silica fine powder - Google Patents

Abstract

(57) [Summary] [Objective] To provide anhydrous silica fine powder and water-repellent silica fine powder having an ultrafine particle size at low cost by a wet method. [Structure] Silica hydrogel is deflocculated, then azeotropically dehydrated with an azeotropically dehydratable organic solvent, and then dried or calcined to produce anhydrous silica fine powder. Azeotropic dehydration is carried out in the presence of an acid catalyst and dried to obtain a water-repellent silica fine powder, which is further calcined to obtain anhydrous silica fine powder.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for inexpensively producing anhydrous silica fine powder by a wet method. For more details,
The present invention relates to a method for extracting particles from silica hydrogel in which ultrafine particles of 10 nm or less are aggregated in a network form in the ideal form as anhydrous silica fine powder and water-repellent silica fine powder.

[0002]

2. Description of the Related Art As a method for producing fine powder of anhydrous silica, a silicon halide produced from metal silicon as a raw material, which was developed by Degussa in Germany in 1940, is hydrolyzed at high temperature. The dry method (SiCl ₄ + 2H ₂ O → SiO ₂ + 4HCl) is known, and the dry method has been technically exported to major countries in the world, and is currently industrially produced by Cabot of the United States and Japan's Aerosil of Japan. Has been done. However, in this method, it is necessary to use an expensive raw material and perform a complicated treatment step, and there is a drawback that the application is limited because of an expensive product. Also, it has no internal surface area (Kunio Goto,
Practical Handbook for Additives for Plastics and Rubber, p. 554
<Yoshiaki Hirata>, Chemical Industry Co., Ltd., 1970) for 380
A high specific surface area of m ² / g or more cannot be obtained.

In addition, there is also an airgel method for obtaining anhydrous silica fine powder by substituting water in a silica hydrogel produced by a wet method with an organic solvent and vaporizing the organic solvent in an autoclave at a critical temperature and a critical pressure. Are known. However, this method is expensive and requires high temperature and high pressure, so that the cost is high as in the dry method and there is a danger. Further, the silica obtained by this method has a drawback that the surface activity of the silica is lost because it is produced under severe conditions.

Further, the water-repellent silica has a good affinity when blended with a polymer material and gives a high physical property value. As a manufacturing method thereof, there is a method of treating hydrophilic silica surface with a silylating agent such as silicone oil or a silane coupling agent to make it water repellent, but since these treating agents are very expensive, silicone rubber, Its applications such as heat resistant grease are limited. These methods require an additional surface treatment step for anhydrous silica.

In order to solve the above-mentioned conventional problems, therefore, an object of the present invention is to provide anhydrous silica fine powder and water-repellent silica fine powder having an ultrafine particle size at a low cost by a wet method.

According to the method of the present invention, excellent water-repellent silica can be obtained in the process of producing anhydrous silica, and the water-repellent silica can be produced at a lower cost. That is, unlike the conventional method, it also includes a method for producing water-repellent silica which is surface-treated during the process for producing anhydrous silica.

[0007]

In order to solve the above problems, according to the present invention, the silica hydrogel is peptized and then azeotropically dehydrated with an organic solvent capable of azeotropic dehydration to remove water in the silica hydrogel. It is proposed to produce an anhydrous silica fine powder that is substantially free of water by dehydrating by substituting with an organic solvent and then drying or firing.

The above azeotropic dehydration is carried out in the presence of an acid catalyst and dried to obtain a water-repellent silica fine powder, which is further calcined to obtain an anhydrous silica fine powder.

Also, an alkali silicate solution is used, for example, Na
₂ O ・ xSiO ₂ + H ₂ SO ₄ → xSiO ₂・ nH ₂ O + Na
_It is preferable to obtain a silica hydrogel by decomposing it by a neutralization reaction of ₂ SO ₄ .

That is, the present invention is to extract the particles in the ideal form as anhydrous silica or water-repellent silica fine powder from silica hydrogel in which ultrafine particles of 10 nm or less are aggregated in a network.

[0011]

By such a manufacturing process, anhydrous silica fine powder and water-repellent silica fine powder, which are as bulky as the dry method and have a smaller primary particle size and a larger specific surface area than the dry method, can be manufactured at low cost.

In the first step, as in the conventional method for producing hydrated silica (white carbon), silica hydrogel produced by using inexpensive sodium silicate and acids such as sulfuric acid and hydrochloric acid as raw materials is peptized to obtain silica sol. And Then, in the second step, the moisture in the silica sol and the organic solvent are replaced by azeotropic dehydration for dehydration. Further, in the third step, the obtained cake-like silica is dried or calcined to obtain anhydrous silica fine powder. On the other hand, azeotropic dehydration in the second step is carried out in the presence of an acid catalyst, and drying in the third step is carried out to obtain water-repellent silica. The organic solvent used at that time can be dehydrated by a distillation or adsorption method, circulated and reused, and can be manufactured at an extremely low cost as compared with the conventional method for manufacturing anhydrous silica.

Further, the anhydrous silica fine powder and the water-repellent silica fine powder obtained by the present invention have very weak bonding between fine particles and do not form a hard agglomerate even when dried and fired, and the powder is placed in a container. With a weak force such as putting and shaking, the powder easily becomes bulky to the submicron order, and the energy required for pulverization is extremely small. Therefore, when pulverizing this powder, it is possible to easily pulverize it with a machine such as a vibrating sieve or a mixer.
Further, it is naturally possible to use a well-known crusher or fine crusher. In such a case, it is expected that the operating time will be very short.

The above structure of the present invention and the saliency of its operation will be explained more clearly from the following description.

In the silica hydrogel, generally very fine primary particles of 10 nm or less are aggregated to form a two-dimensional or three-dimensional network-like hydrogel structure. That is, there are voids between the contact particles in the agglomerate of the primary particles, and also between the secondary particles, the voids contain a large amount of water, and have a supporting structure over the entire system, which maintains its shape. However, once dried, a very strong contraction force due to dehydration works, and silanol groups on the particle surface also condense to form a siloxane bond, resulting in a hard xerogel and it is not possible to take out the original fine primary particles. It will be possible. This is silica gel used as a so-called desiccant. Therefore, the present inventors physically deflocculate the silica hydrogel, azeotropically dehydrating it with an organic solvent capable of azeotropic dehydration to replace water in the silica hydrogel with an organic solvent for dehydration, It has been found that by drying or firing, the very fine primary particles of the silica hydrogel can be taken out in an ideal form as anhydrous silica fine powder and water-repellent silica fine powder which do not substantially contain water.

The silica hydrogel used in the present invention is obtained, for example, by decomposing an alkali silicate solution by a neutralization reaction.

Examples of the alkali silicate solution include silicates such as sodium, potassium and lithium, and although not particularly limited, sodium silicate is preferable from the viewpoint of cost.

The silica concentration during the neutralization reaction of the alkali silicate solution is generally 2 to 15%, preferably 4 to 8%. If the silica concentration is less than 2%, the amount produced will decrease,
Manufacturing facilities need to be enlarged. If it is 15% or more, the viscosity of the produced gel becomes high and uniform stirring is difficult. In addition, a hard gel is formed and peptization becomes difficult.

PH during the neutralization reaction of the alkali silicate solution
Is preferably 2.0 to 6.0. The pH during the neutralization reaction is 2.
When it is 0 or less, the time required for gelation becomes long and the cleaning property in the next step becomes poor. If the pH is above 6.0,
It instantly turns into a hard gel and becomes difficult to peptize. The temperature during the neutralization reaction may be room temperature, but when silica hydrogel is produced at a relatively low pH, by appropriately heating it, the time required for gelation can be shortened.

The silica hydrogel thus obtained is washed to remove by-product salts such as sodium sulfate and peptized to break the network-like hydrogel structure to obtain silica sol.

As the method for deflocculating the silica hydrogel, for example, a method of physically deflocculating using a known wet pulverizer is applied. As a wet mill, a known pot mill,
Examples thereof include a ball mill such as a tube mill, a conical mill, a vibrating ball mill and a planetary mill, a tower mill, an attritor, a dyno mill, a sand grinder, an aniler mill and other medium stirring mills, and other emulsifying and dispersing machines. In sol formation, water or an organic solvent used in the next step is added in advance. As the peptizing method, in addition to the mechanical method adopted by the present inventors, there are scientific methods that have been used for a long time, such as washing with a coagulant with water, an ion exchange method, and a peptizing agent. There is a method of obtaining silica sol by adding

When the solvent of the silica sol obtained by peptizing the silica hydrogel is only water, an organic solvent is added, and when the solvent is a mixed liquid of water and an organic solvent, it is added as it is, or an organic solvent is further added. Then, azeotropic dehydration is performed.

As the organic solvent capable of azeotropic dehydration, acetone, benzene, glycerin, ethyl acetate, ethanol,
Methanol, propanol, butanol, pental,
Examples include, but are not limited to, hexanol, heptanol, octanol, nonanol, and the like.

By azeotropic dehydration with an organic solvent capable of azeotropic dehydration, water in the silica sol is replaced with the organic solvent to be dehydrated, and the silanol group on the silica surface is transesterified with the alkyl group of the organic solvent to repel water. Aqueous surface (Si-O
R) is formed. Therefore, by drying or calcining the organosol after azeotropic dehydration, anhydrous silica fine powder containing substantially no water can be obtained. Furthermore, when this esterification reaction is carried out in the presence of an acid, the esterification reaction proceeds more completely and the silanol groups on the silica surface are completely replaced with water-repellent alkyl groups. A water-repellent silica fine powder is obtained. As the acid, mineral acids such as sulfuric acid and hydrochloric acid are preferable, and boronic acid fluoride, aluminum chloride, zinc chloride and the like can also be used. This acid acts as a catalyst for the esterification reaction. The acid is added so that the pH of the reaction solution becomes 0.5 to 4. In the absence of this acid catalyst, the esterification reaction and the ester hydrolysis reaction proceed at the same time, so the esterification reaction proceeds only slowly and requires several days to complete the reaction. Not preferable.

In the azeotropic dehydration, it is effective to replenish the organic solvent as much as the reaction liquid has decreased during the process to increase the absolute amount of the organic solvent in order to complete the esterification reaction quickly. Is. Further, it is possible to collect water and an organic solvent that evaporate by heating with a condenser, and to separate and collect the recovered liquid.

After completion of the reaction, the mother liquor is separated by filtration, centrifugation, etc., and the added acid is washed and then dried or calcined. The dried product obtained by the method of the present invention has a characteristic that it is a powder that easily loosens and becomes a submicron powder with a weak force such as shaking in a container, as shown in Examples 1, 2 and 3 described later. ing. On the other hand, when not azeotropically dehydrated with the azeotropically dehydratable organic solvent, the dried product becomes aggregated particles as shown in Comparative Examples 1 and 2.

The temperature when firing to obtain anhydrous silica is preferably 800 ° C. or lower. Preferably 600 ° C
It is a degree. When the temperature is 800 ° C. or higher, the pores included therein are sealed, the specific surface area is reduced, and the burning energy is wasted. However, when a low specific surface area is required, the specific surface area can be controlled by firing at an arbitrary temperature of 800 ° C. or higher.

The firing temperature for obtaining anhydrous silica having no alkyl group on the surface is preferably 300 to 800 ° C. At 800 ℃ or higher, the same thing as above occurs,
When the temperature is 300 ° C. or lower, the alkyl group covering the surface of the silica particles does not disappear.

The anhydrous silica fine powder and the water-repellent silica fine powder thus obtained are commercially available products 1 by the dry method.
Similar to (commercial name: Aerosil 200) and water-repellent treated commercial product (commercial name: Aerosil R805), it is very bulky (50 g / l or less) and has a smaller primary particle size (10 nm or less) than the dry method. , High specific surface area (4
It is possible to obtain those having a high structural property of 00 m ² / g or more.

When the water-repellent silica of the present invention was measured with a Fourier transform infrared spectrophotometer (FT-IR), a peak of an alkyl group was observed, which means that the silica surface was substituted with a water-repellent alkyl group. Shows. Moreover, when this powder is dropped on water, it floats stably for a long time. This is 300-8
The anhydrous silica fine powder obtained by firing at 00 ° C. has a peak similar to that of anhydrous silica obtained by the dry method, etc., because the peak of the alkyl group disappears.

Further, the anhydrous silica fine powder and the water-repellent silica fine powder obtained in the present invention have very small bonding force between particles and easily become fine powder. This is because the silanol group on the particle surface is transesterified with the alkyl group of the organic solvent by azeotropic dehydration with an organic solvent capable of azeotropic dehydration and the surface is covered with a water-repellent alkyl group, so that the particles of the particles during drying are It is considered that the shrinkage force of the gel is weakened during drying because the silanol group does not form a siloxane bond and the solvent is replaced with alcohol having a small surface tension.

With respect to the water-repellent silica, for example, in the commercially available product 2, the surface is treated with octylsilane to be water-repellent, the specific surface area is 150 m ² / g, and the primary particle diameter is 20 nm.
However, in the present invention, as shown in Examples 1 and 2,
A water-repellent silica fine powder having a high specificity and a specific surface area of 400 m ² / g or more and a primary particle diameter of 10 nm or less can be obtained.
The water-repellent silica thus obtained is suitable for use as an expensive commercial product of anhydrous silica, for silicone oil and grease, and as a reinforcing filler for silicone rubber. Not only can it be replaced, but its effect can be further enhanced because it has higher structural properties than commercial products.

[0033]

EXAMPLES The present invention will be described more specifically below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

Example 1 190 g of 25% sulfuric acid was diluted with 180 g of pure water, and an aqueous solution of sodium silicate (SiO ₂ : 8%) was added to it with stirring to give 11 parts.
20 g was added to produce silica hydrogel at pH 4.5. After 40 minutes from the formation of the silica hydrogel, the mixture was diluted with water to a SiO ₂ concentration of 3% and aged at 20 ° C. for 4 hours. This was filtered, washed with water and dehydrated to obtain a washed silica hydrogel.

To 200 g of washed silica hydrogel, 500 ml of 1-butanol was added and peptized by a ball mill to obtain silica sol.

The total amount of this silica sol was poured into a three-necked flask having a capacity of 1 liter, and azeotropic dehydration was performed while heating with a mantle heater and stirring.

The sol in the three-necked flask evaporates water and 1-butanol by heating, so it was cooled by a condenser and collected. In addition, a new 1
-Butanol was added at any time.

When the temperature in the flask reached the boiling point of 1-butanol (117.3 ° C.), the heating was stopped, and after cooling, the mother liquor was separated by filtration.

The filter cake is dried at 120 ° C. for 2 minutes.
After drying for 4 hours, a dried product of anhydrous silica was obtained. Further, the dried product was calcined in an electric furnace at 600 ° C. for 3 hours to obtain a calcined product of anhydrous silica.

The specific surface area of the dried and calcined product of the obtained anhydrous silica was measured by the BET method.
00m ² / g, the calcined product was obtained a value of 450 m ² / g.
In addition, a transmission electron microscope (made by JEOL: JEM-201
As a result of observation in 0), primary particles of about 7 nm were both confirmed.

Both the dried and calcined products of the obtained anhydrous silica were bulky, and easily became fine powder with a weak force such as shaking in a container. When these fine powders were measured with a centrifugal sedimentation type particle sizer (manufactured by Shimadzu Corporation: SA-CP3L), the average particle sizes were 0.76 and 0.81, respectively.
μm and both were submicron fine powders.

The dried product of the anhydrous silica was measured by FT-IR (FT-IR-7300 manufactured by JASCO Corporation).
A butyl group peak was observed, and this powder was suspended when dropped in a beaker containing water.

In the calcined product of the above anhydrous silica, the peak of the butyl group by FT-IR disappeared and the surface state was the same as that of the dry process anhydrous silica.

The results are summarized in Table 1 together with the commercial products 1 and 2.

Example 2 A silica sol was obtained in the same manner as in Example 1, pH was adjusted to 1 by adding concentrated sulfuric acid, and azeotropic dehydration was performed in the same manner as in Example 1.

The temperature of the reaction solution is the boiling point of n-butanol (1
(17.3 ° C.), the heating was stopped, and after cooling, filtration and washing were carried out to separate the mother liquor.

The filter cake is dried at 120 ° C. for 2 minutes.
After drying for 4 hours, a dried product of silica was obtained. Further, a part of this dried product was baked in an electric furnace at 600 ° C. for 3 hours to obtain a baked product of silica.

The specific surface area of the dried product and the calcined product of the obtained anhydrous silica was measured by the BET method.
A value of 20 m ² / g and a value of 780 m ² / g for the fired product were obtained.
Moreover, as a result of observing with a transmission electron microscope, both are 7 nm.
The degree of primary particles was confirmed.

The dried and calcined products of anhydrous silica thus obtained were both bulky, and easily became fine powder with a weak force such as shaking in a container. Furthermore, when this fine powder was measured by a centrifugal sedimentation type particle sizer, the average particle diameters were 0.72 and 0.63 μm, respectively, and both were submicron fine powders.

When the dried product of the above anhydrous silica was measured by FT-IR, a butyl group peak was observed. Even if the powder was dropped into a beaker containing water and stirred, it floated stably for a long time without suspending, and a fine silica powder having strong water repellency was obtained.

In the calcined product of the above anhydrous silica, the peak of the butyl group by FT-IR disappeared and the surface state was the same as that of the dry process anhydrous silica.

The results are summarized in Table 1 together with the commercial products 1 and 2.

Example 3 190 g of 25% sulfuric acid was diluted with 180 g of pure water, and an aqueous sodium silicate solution (SiO ₂ : 8%) was added to it with stirring to 14 g.
50 g was added to produce a silica hydrogel at pH 5.6. After 40 minutes from the formation of the silica hydrogel, the mixture was diluted with water to a SiO ₂ concentration of 3% and aged at 20 ° C. for 4 hours. This was filtered, washed with water, and dehydrated to obtain washed silica hydrogel.

200 g of washed silica hydrogel was peptized by a ball mill to obtain silica sol. Mix well with 500 ml of 1-pentanol and add hydrochloric acid to adjust the pH to 2
After adjusting to 1, the mixture was poured into a 1-liter three-necked flask, and azeotropically dehydrated while heating with a mantle heater and stirring.

The sol in the three-necked flask evaporates water and 1-pentanol by heating, so it was cooled by a condenser and collected. In addition, new 1-pentanol was added at any time during the azeotropic dehydration.

When the temperature in the flask reached the boiling point of 1-pentanol (138.2 ° C.), the heating was stopped, and after cooling, the mother liquor was separated by filtration.

The filter cake is dried at 140 ° C. for 2 minutes.
After drying for 4 hours, a dried product of anhydrous silica was obtained. Further, the dried product was calcined in an electric furnace at 600 ° C. for 3 hours to obtain a calcined product of anhydrous silica.

The specific surface area of the dried and calcined product of the obtained anhydrous silica was measured by the BET method.
A value of 30 m ² / g and a value of 630 m ² / g for the fired product were obtained.
Moreover, as a result of observing with a transmission electron microscope, both are 5 nm.
The degree of primary particles was confirmed.

The dried and calcined products of anhydrous silica obtained were both bulky, and easily became fine powder with a weak force such as shaking in a container. Further, when the fine powder was measured with a centrifugal sedimentation type particle sizer, the average particle diameters were 0.45 and 0.42 μm, respectively, and both were submicron fine powders.

When the dried product of the above anhydrous silica was measured by FT-IR, a pentyl group peak was observed. When this powder was dropped in a beaker containing water, it floated stably for a long time without suspending, and a silica fine powder having strong water repellency was obtained.

In the calcined product of the above anhydrous silica, the peak of the pentyl group by FT-IR disappeared and the surface state was the same as that of the dry process anhydrous silica.

The results are summarized in Table 1 together with the commercial products 1 and 2.

Comparative Example 1 A silica sol was obtained in the same manner as in Example 1. This silica sol was dried at 120 ° C. for 24 hours in a dryer without azeotropic dehydration. Further, this dried product was baked in an electric furnace at 600 ° C. for 3 hours.

The dried and calcined products of silica obtained are:
Both of them formed a hard agglomerate, and the agglomerates were hardly broken by a force such as putting them in a container and shaking them.

The results are summarized in Table 1 together with the commercial products 1 and 2.

Comparative Example 2 A washed silica hydrogel was obtained in the same manner as in Example 1.
This silica hydrogel was dried at 110 ° C. for 24 hours in a dryer without deflocculation and azeotropic dehydration. Furthermore, 6
It was dried at 00 ° C. for 3 hours.

The dried and calcined products of silica thus obtained were roughly crushed in a mortar and the specific surface area was measured by the BET method. The dried product was 600 m ² / g and the calcined product was 630 m ² / g.
The value of was obtained. In addition, as a result of observing them with a transmission electron microscope, primary particles of about 7 nm were both observed.

Both the dried and calcined products of the above-mentioned silica have an apparent volume before drying and calcining reduced to about 1/10, forming a very hard lump, so-called xerogel, which is put in a container. It was almost not crushed by the force of shaking and kept its original shape.

The results are summarized in Table 1 together with the commercial products 1 and 2.

Comparative Example 3 190 g of 25% sulfuric acid was diluted with 180 g of pure water, and an aqueous sodium silicate solution (SiO ₂ : 8%) was added to 16 while stirring.
40 g was added and a silica hydrogel was formed at pH 6.5. This silica hydrogel was formed instantly and was a hard gel, and uniform stirring after formation was not possible. After 40 minutes from the formation of the silica hydrogel, the mixture was diluted with water to a SiO ₂ concentration of 3% and aged at 20 ° C. for 4 hours. This was filtered, washed with water, and dehydrated to obtain washed silica hydrogel.

200 g of the washed silica hydrogel was deflocculated by a ball mill, but many gel lumps that could not be deflocculated remained. This was thoroughly mixed with 500 ml of 1-butanol, poured into a 1-liter three-necked flask, and azeotropically dehydrated while heating with a mantle heater and stirring.

The sol in the three-necked flask evaporates water and 1-butanol by heating, so it was cooled by a condenser and collected. In addition, a new 1
-Butanol was added at any time.

When the temperature in the flask reached the boiling point of 1-butanol (117.3 ° C.), the heating was stopped, and after cooling, the mother liquor was separated by filtration.

The filter cake is dried at 120 ° C. for 2 minutes.
After drying for 4 hours, a dried product of anhydrous silica was obtained. Further, the dried product was calcined in an electric furnace at 600 ° C. for 3 hours to obtain a calcined product of anhydrous silica.

The specific surface area of the dried and calcined product of the obtained anhydrous silica was measured by the BET method.
A value of 80 m ² / g and a calcined product of 300 m ² / g were obtained.
Moreover, as a result of observing with a transmission electron microscope, both are 5 nm.
Uneven primary particles before and after were confirmed.

The dried and calcined products of the above anhydrous silica are
Although partially differentiated, the lump of gel after deflocculation formed a hard agglomerate as it was, and the agglomerate was hardly broken by a weak force such as putting it in a container and shaking it.

The results are summarized in Table 1 together with the commercial products 1 and 2.

[0078]

[Table 1]

[0079]

As is apparent from the above description, the present invention has the following effects. The anhydrous silica fine powder and the water-repellent silica fine powder according to the present invention are different from the conventional dry method in that the primary particles are ideally taken out by the wet method from the silica hydrogel in which ultrafine particles of 10 nm or less are gathered in a network form. . Further, regarding the water-repellent silica, a surface treatment step is not required for making the water-repellent, and excellent water-repellent silica can be obtained during the manufacturing process of anhydrous silica. Furthermore, the anhydrous silica fine powder and the water-repellent silica fine powder obtained by the present invention have very weak bonding between fine particles, are very bulky at the time of drying or firing, and are almost pulverized. It can be easily pulverized to submicron order by shaking.
Therefore, the grinding energy required for micronization may be very small.

The anhydrous silica fine powder and the water-repellent silica fine powder obtained by the method of the present invention have high structural properties such as a specific surface area of 400 m ² / g or more and a primary particle diameter of 10 nm or less. Therefore, it is suitable not only for silicone oils and greases, which are expensive commercial products of anhydrous silica, but also for reinforcing fillers of silicone rubbers, and it is not only possible to replace them with the products of the present invention, but also commercially available products. Since it has a higher structural property than the product, it is possible to further enhance the effect.

Front Page Continuation (72) Inventor Masahiro Maeno 1-8 Sakasedai, Asaka, Takarazuka-shi, Hyogo A305

Claims (7)

[Claims] 1. A method for producing fine anhydrous silica powder, which comprises deflocculating silica hydrogel to obtain silica sol, then azeotropically dehydrating with an organic solvent capable of azeotropic dehydration, and then drying or firing. 2. The azeotropic dehydration is carried out in the presence of an acid catalyst,
The method for producing fine anhydrous silica powder according to claim 1, wherein the method is drying and further calcining. 3. The silica hydrogel is obtained by decomposing within a pH range of 2.0 to 6.0 by adding an alkali silicate solution into an acid by a neutralization reaction of an alkali silicate solution. It is the obtained silica hydrogel, The manufacturing method of the anhydrous silica fine powder of Claim 1 or Claim 2 characterized by the above-mentioned. 4. The silica concentration during the neutralization reaction of the silica hydrogel is 2 to 15%.
4. The method for producing anhydrous silica fine powder according to any one of items 1 to 3. 5. A water-repellent silica fine powder characterized in that silica hydrogel is deflocculated to give a silica sol, which is then subjected to azeotropic dehydration with an organic solvent capable of azeotropic dehydration in the presence of an acid catalyst and then dried. Production method. 6. The silica hydrogel is obtained by adding an alkali silicate solution into an acid and decomposing it within a pH range of 2.0 to 6.0 by a neutralization reaction of the alkali silicate solution. 6. The method for producing a water-repellent silica fine powder according to claim 5, wherein the silica hydrogel is a prepared silica hydrogel. 7. The silica concentration during the neutralization reaction of the silica hydrogel is 2 to 15%.
Alternatively, the method for producing the water-repellent silica fine powder according to claim 6.