JPH0757685B2 - Method for producing high-purity silica - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing high-purity silica. More specifically, the present invention relates to a method for producing high-purity silica having a very low content of radioactive impurities such as uranium in addition to alkali metal and chlorine, a low hygroscopicity, and a small specific surface area from an aqueous alkali silicate solution.

The high-purity silica obtained by the method of the present invention is a filler.
It is expected to be suitably used as a filler for resin compositions for encapsulating electronic parts, in addition to any application of the dispersant, as a raw material for transparent quartz glass, special ceramics and the like.

A synthetic resin composition containing an inorganic filler such as silica is used as a sealing material for electronic parts. The inorganic filler has various physical properties such as expansion coefficient, thermal conductivity, moisture permeability, and mechanical properties. From the viewpoint of cost, it is considered to be advantageous to blend as much as the moldability allows, and silica-based fillers are said to be the most preferable. However, with the high integration of electronic component devices, the problem of device malfunction Is caused by the α rays emitted from radioactive elements such as trace amounts of uranium and thorium contained in tens to hundreds of ppb units in the sealing material used, especially in the silica-based filler. However, it is desired to reduce the content of such impurities in silica.

The present invention aims to meet such a demand.

[Conventional technology]

As a method for producing high-purity silica, 1) a method of reacting silicon tetrachloride purified by distillation, adsorption, liquid phase extraction or the like under an oxyhydrogen flame is known. And 2) as a method for producing high-purity silica from an alkali silicate aqueous solution as a raw material; 2-1) a method for purifying an alkali silicate aqueous solution by treating it with an ion exchange resin: (JP-A-60-42217) , JP-A-60-42218). 2-2) Method for purifying by treating alkaline silicate aqueous solution with acid: (JP-A-59-54632, JP-A-60-191016, JP-A-60-60163)
-204612) has been proposed.

[Problems to be solved by the invention]

High-purity silica can be produced by these methods, but in the case of the method 1), the obtained silica particles are fine particles having an average particle size of the order of mμ and a large specific surface area. It is difficult to use it as a filling agent.

In the method 2-1), since the purification treatment operation is carried out by diluting the SiO ₂ concentration of the alkali silicate aqueous solution at about 10% by weight or less, in terms of equipment efficiency, silica is precipitated from silica sol. Since the operating conditions for separating and recovering from the mother liquor are complicated, there is a problem in productivity.

The method 2-2) is simple and various attempts have been made.
Even if the alkaline silicate aqueous solution is treated with a high-concentration acid, phase separation locally occurs, and it is difficult to extract impurities. Also,
Even if the alkaline silicate aqueous solution is extruded into a dilute acid solution in an attempt to avoid local phase separation, the extruded alkali silicates adhere to each other to form a large mass, which significantly reduces the extraction rate of impurities. There are difficulties in terms of practicality.

Further, since the method 2) is a wet method, the obtained silica has a drawback that it has hygroscopicity because many silanol groups are present on the surface thereof.

The inventors of the present invention have improved such problems in the conventional method, using an alkali silicate aqueous solution as a raw material, and having an extremely low content of impurities, and a high-purity silica having a low hygroscopicity and a small specific surface area. The present invention has been completed through intensive research to produce it more efficiently and economically than ever before.

[Means for solving problems]

That is, the present invention is a high-purity by extracting an aqueous solution of alkali silicate into a fine fibrous gel in a coagulant, treating the resulting fibrous gel with a solution containing an acid, and then water to extract and remove impurities. In producing silica, an acid solution is used as a coagulant, and the resulting silica is subjected to heat treatment at a temperature of 1000 ° C. or higher to reduce hygroscopicity and to obtain a dense high-purity silica having a small specific surface area. "Aqueous alkali silicate solution having a viscosity in the range of 2 to 200 poise (alkaline silicate is represented by the general formula: M ₂ O.nSiO ₂ <where M is an alkali metal element and n is SiO 2). 0.5 in moles of ₂
To 5) is directly extruded from a spinning nozzle having a pore diameter of 1 mm or less into an acid solution having a concentration of 0.1 to 4 N to obtain a fine fibrous gel, and the obtained fibrous gel is obtained. Is immersed in a solution containing an acid having an acid concentration in the range of 0.5 to 4 N in the first stage of the treatment, and then washed with water to remove impurities, and the resulting silica is heat treated at a temperature of 1000 ° C or higher. A method for producing high-purity silica characterized by
Is the gist.

Hereinafter, the present invention will be described in detail. Embodiments of the invention include
It consists of the following three steps.

-Step-1: (fiberizing step). A highly viscous liquid having spinnability (hereinafter referred to as "stock solution") is prepared from an alkali silicate aqueous solution, and this stock solution is coagulated in an acid solution using a fiberizing device to form a fine fibrous gel.

-Process-2: (impurity extraction process). The obtained fibrous gel is treated with a liquid containing an acid (hereinafter referred to as a treatment liquid) and then with water to extract and remove impurities.

-Process-3: (heat treatment process). The obtained fibrous silica is heat-treated at a temperature of 1000 ° C. or higher.

The features of the present invention will be described below.

(1) An alkaline silicate aqueous solution is coagulated in an acid solution by using a fiberizing device equipped with a spinning nozzle having a pore diameter of 1 mm or less to form a fine fibrous gel.

Since the obtained fibrous gel has a fine diameter and a high surface area, the extraction efficiency of impurities is enhanced.

(2) When making an aqueous solution of alkali silicate into a fine fibrous gel;
Use a solution with a viscosity in the range of 2 to 200 poise, and make the acid concentration of the acid solution as a coagulant 4 N or less. According to the method of the present invention in which the above and (1) conditions are combined, surprisingly, a fibrous gel having a hollow structure can be obtained even by using an ordinary circular hole nozzle, and the coagulated body portion thereof is Since a homogeneous and highly swollen state is maintained and impurities are easily extracted by acid and water, the extraction efficiency of impurities in silica can be remarkably improved in combination with the effect of the above feature (1).

(3) The fibrous silica from which impurities have been extracted is heat-treated at a temperature of 1000 ° C or higher.

When the obtained silica is heat-treated at a temperature of 1000 ° C. or higher,
The silanol groups existing on the surface of the particles can be eliminated to significantly reduce the hygroscopicity of silica, and the fine pores of the silica particles are crushed to reduce the specific surface area and obtain dense silica particles with a high bulk density. You can

By the method of the present invention which combines the above characteristics, the extraction efficiency of impurities in silica can be remarkably improved, and low hygroscopicity and compactness can be imparted to the obtained silica.

The high-purity silica obtained by the method of the present invention is also suitable for transparent quartz glass and applications such as a filler for a resin composition for electronic component encapsulation, in which the presence of water is a problem.

In the method of the present invention, an aqueous solution of sodium silicate, potassium salt, lithium salt or the like of silicic acid can be used as the raw material alkali silicate aqueous solution.

Hereinafter, in the method of the present invention, an example of using an aqueous sodium silicate solution as the alkaline silicate aqueous solution,
The above three steps will be sequentially described.

[Step-1: (Fibring step)] A raw material solution is prepared by preparing an aqueous sodium silicate solution as a raw material in a viscosity range suitable for forming a fiber.

The viscosity range of the stock solution suitable for the method of the present invention is 2 to 200 poises, and particularly preferably 10 to 100 poises.

When an aqueous sodium silicate solution having a high SiO ₂ concentration and an excessively high viscosity is used as a raw material, it is appropriately diluted with water before use.

In the case of an aqueous sodium silicate solution containing about 30% SiO ₂ ,
In a normal state, the viscosity is low and the spinnability is not sufficient, so sodium silicate is polymerized and used in order to impart spinnability to this.

As a method of polymerizing sodium silicate, a partial neutralization method with an acidic substance, a dehydration concentration method, a method of adding a polyvalent metal salt, and the like have been proposed. The dehydration concentration method is the simplest method, and sodium silicate is polymerized to increase the viscosity by dehydration of several%.

A temperature suitable for fiberizing the prepared stock solution, for example, 30 to 60 ° C
Then, it is sent to the fiberizing device using a metering pump through an appropriate filtering device.

The fiberizing device is not particularly limited, and in general, an extruder equipped with a spinning nozzle (hereinafter referred to as a nozzle) can be used.

The biggest problem in using a nozzle is the occurrence of adhesion trouble of the stock solution extruded from the nozzle to the nozzle outlet surface.

As is well known, an aqueous solution of sodium silicate is a viscous liquid with a high affinity for metals, has the property of rapidly solidifying with a slight decrease in water content, and is also used as an adhesive. As can be seen, when a stock solution of an aqueous sodium silicate solution is solidified in a state of being attached to the nozzle surface, a strong bond is formed between the sodium silicate and the nozzle surface, and it is extremely difficult to peel it off. When the solidified body adheres to the nozzle surface, the undiluted solution extruded from the adjacent holes continues to adhere and solidify, and finally the fiberizing operation cannot be continued.

Such a phenomenon is likely to occur when the nozzle used has a small hole diameter and a large number of holes. A solution to this is to minimize the adhesion between the nozzle surface and the stock solution.

The present inventors have conducted various studies on the material of the nozzle to be used, and when making the alkali silicate aqueous solution into a fine fibrous gel, gold, platinum and other precious metals, and gold-platinum alloys and other precious metal alloys are used. Made of or made of tetrafluoroethylene (hereinafter referred to as TFE) resin nozzle, or the nozzle surface of which is coated with precious metal or TFE resin, the nozzle release property of gelled alkali silicate is significantly improved. However, it has been found that a remarkable effect of preventing the occurrence of the trouble of the alkali silicate adhering to the nozzle surface is obtained.

The noble metal as referred to in the present invention is gold, platinum, silver, palladium or the like, and the nozzle surface can be coated by an ordinary plating process.

The TFE resin referred to in the present invention means polytetrafluoroethylene (PTF
E), copolymers of TFE and hexafluoropropylene, copolymers of TFE and perfluoroalkyl vinyl ether, copolymers of ethylene and TFE, copolymers of ethylene and vinyl fluoride, ethylene Includes copolymers such as copolymers with vinylidene fluoride and ethylene and chlorotrifluoroethylene.

The TFE resin is coated on the nozzle surface according to a conventional method, and if necessary, the outer surface of the nozzle may be coated with a primer and then coated.

In addition to the wet method, various methods such as extruding an aqueous solution of alkali silicate into the air from the nozzle and then treating it with an acid solution to coagulate it can be used for fiberizing. From the viewpoint of preventing adhesion to the wet method, the wet method is more advantageous than the dry method.

In the method of the present invention, the stock solution is extruded from a nozzle immersed in a coagulation bath. The extruded stock solution is coagulated into a gel in the coagulation bath into a gel. This fibrous gel is taken up by a roller or sent to the next step using an appropriate transportation device such as a belt conveyor.

The hole diameter of the nozzle used in this step is preferably 0.05 to 1.0 mm, and more preferably 0.1 to 0.3 mm.

A normal circular hole nozzle is used as the nozzle, but a modified cross-section hole nozzle or a hollow fiber spinning nozzle can also be used.

According to the method of the present invention, a hollow fiber gel can be obtained without using a hollow fiber spinning nozzle, and a good impurity extraction effect can be obtained in step-2.

Mixing fine bubbles into the fibrous gel is also effective in increasing the efficiency of impurity extraction.

As a method of mixing fine air bubbles in the fibrous gel, a method of using a raw material prepared by stirring so that air is entrained in the liquid, a chemical foaming agent which decomposes by heating into the raw liquid to generate a gas or Add a low boiling substance that is liquid at room temperature,
Various methods such as a method of heating the raw material into fibers while heating it, or a method of utilizing the cavitation phenomenon of a pump which feeds the raw liquid into the fiber-forming apparatus can be adopted.

An acid solution is used as the coagulant used in the coagulation bath of the present invention. The acid is an inorganic acid such as sulfuric acid, nitric acid or hydrochloric acid, and sulfuric acid or nitric acid is preferably used. Further, as the acid solution, an aqueous solution of these acids is preferable in practice.

The acid concentration of the acid solution as a coagulant is 0.1 N or higher, 4
It is better to set the range above the regulation.

When an acid solution having an acid concentration of more than 4 N is used as a coagulant, the structure of the produced silica becomes too dense, and it becomes difficult to extract impurities contained inside in the next step-2. Also, the acid concentration of the acid solution as a coagulant is
If it is less than 0.1 N, the coagulation rate of the alkali silicate is too small and the fibrous gels are likely to adhere to each other, which is not practical.

Therefore, the acid concentration of the acid solution as a coagulant is
The range of 0.1 to 4 is preferable, and the range of 0.5 to 4 is preferable.
The range is 3 normal, and more preferably 1 to 2 normal.

When the conditions specified in this specification are combined as the viscosity of the stock solution, the pore size of the nozzle, and the acid concentration of the acid solution used as the coagulant, the alkali silicate has a moderate coagulation rate and surprisingly has a hollow structure. In addition, a highly swelled transparent fibrous gel can be obtained. The resulting gel has the following steps-
Also in No. 2, the swelling state is maintained, and dealkalization proceeds in this state.

The surface of the resulting fibrous gel has numerous scaly cracks, and the existence of these cracks makes it easy for the acid to penetrate into the gel, and thus has a fine and hollow structure. By the synergistic effect with the high surface area that is characteristic of fibers,
The extraction efficiency of impurities is significantly improved.

Further, the presence of cracks can significantly reduce the load of the pulverization step when adjusting the particle size of the obtained silica.

The highly swollen state of the gel as used herein means a state where the liquid content of the gel is high, and the degree of swelling can be indicated by the liquid content of the gel.

There is a preferable range for the liquid content of the gel in which impurities are easily extracted. The liquid content is calculated by the following formula; {liquid content, [%] = (w ₁ −w ₂ ) × 100 / w ₂ , w ₁ : About 10 g of the sample is taken and centrifuged at room temperature (1000
G × 10 minutes treatment), sample weight excluding the adhering liquid,
(G). w ₂ : After the above treatment, the sample was dried at 150 ° C. for 4 hours and allowed to cool to room temperature in a desiccator, and the weight was calculated as (g)}. For example, when using sodium silicate # 3 as a raw material, The preferred liquid content range of fibrous gel is 80-150%
It is a degree.

If the liquid content is less than 80%, it may be difficult to extract impurities contained in the gel in the subsequent step-2, probably because the structure of the formed gel becomes too dense.

On the other hand, when the liquid content exceeds 150%, the silica concentration in the gel is too low even if the obtained gel is transparent, or the volume shrinkage of the gel in Step-2 becomes large,
A devitrified silica which contains impurities inside is obtained. Even if the impurity extraction operation is repeated on silica in such a state, it is difficult to remove the impurities as intended by the present invention.

Further, even in the case of a fibrous gel having a liquid content within this range, the silica concentration in the gel is non-uniform and partially whitened, and the gel in a devitrified state contains impurities such as those of the present invention. Difficult to remove.

It is difficult to uniquely determine the coagulation bath temperature because the coagulation rate of the alkali silicate varies greatly depending on the type of acid used as the coagulant, but it is usually a temperature of about 10 to 60 ° C.

If the same type of acid solution used in the next impurity extraction step is used as a coagulant, the steps such as coagulant recovery and waste liquid treatment that are required when using a coagulant such as an organic solvent Is advantageous because it can be omitted.

Fibrous gel is collected by roller type at 1 to 10 per minute
It is usually operated at a speed of about 0 m and a conveyor type of about 0.1 to 50 m / min.

[Step-2: (Impurity Extraction Step)] In this step, the fibrous gel obtained in Step-1 is treated with a liquid containing an acid. The acid is an inorganic acid such as sulfuric acid, hydrochloric acid or nitric acid and an organic acid such as formic acid, and in practice, it is preferable to use sulfuric acid or nitric acid.

Further, as the treatment liquid, an aqueous solution of these acids is preferable in practical use.

As the acid treatment operation in this step, a method of treating in one step can be adopted, but particularly in order to extract and remove a trace amount of impurities, the treatment operation is divided into at least two steps, and the treatment to be used in each step A multi-step process of renewing the liquid can also be performed.

It is a general method to perform extraction of impurities using a high-concentration acid, but in the method of the present invention, in order to maintain the structure of the gel from which impurities are easily removed as much as possible, the treatment is performed. The acid concentration of the liquid is preferably low.

The acid concentration of the treatment liquid is 4 N or less, preferably 0.5 to 3 N, and more preferably 1 to 2 N.

The treatment temperature is not particularly limited, but the extraction operation is preferably performed at a temperature of 50 ° C or higher.

If the treatment is performed under pressure at a temperature higher than the boiling point of the treatment liquid at normal pressure, the time required for impurity extraction can be shortened. The temperature during pressure extraction is preferably as high as possible, but considering the corrosion of the equipment due to acid and the energy cost, it is 100-150.
C., preferably 110 to 140.degree. C. is practical.

It is desirable to carry out the treatment in this step with stirring. The fibrous gel easily has a length of 2-5 mm when stirred in the treatment liquid.
It becomes a short fiber shape.

When the gel is made into short fibers, the dispersibility of the gel by stirring in the treatment liquid becomes extremely good. The short fibrous gel is dispersed in the treatment liquid in a slurry form, which facilitates the operation of extracting impurities and improves the uniformity of the effect of extracting impurities, resulting in significantly less variation in impurity extraction results. Further, since the short fibrous gel also has bulkiness, which is a characteristic of a fibrous material, liquid separation is extremely easy even by washing and filtering operations after the impurity extraction treatment.

The acid treatment time is about 30 minutes to 5 hours in the case of the batch method, and about 30 seconds to 30 minutes in the case of the continuous method, preferably about 1 to 10 minutes.

The silica fiber obtained by the acid treatment is then washed with water at an arbitrary temperature and, if necessary, combined with a filtration operation for deoxidation dehydration treatment.

The acid used in the present invention is preferably a high-purity product called refined or electronic grade, and the water used for diluting the raw material or the acid used or washing the silica is preferably pure water containing few impurities.

By the treatment of this step, the content of the impurities containing the radioactive element in silica is extremely low. The content of impurities in the silica after acid treatment can be set to about 10 ppm or less for alkali metal, 3 ppm or less for chlorine, and about 3 ppb or less for uranium.

[Step-3: (Heat Treatment Step)] Although the fibrous gel is converted to silica by the acid treatment in Step-2, the silica still retains water. This water is divided into attached water and bound water. Adhering water is 100 ° C
Boiled water easily evaporates when heated above, whereas bound water
It is difficult to completely remove even at a temperature of 400 ° C or higher.

In particular, silica obtained by the wet method has a large number of silanol groups (≡Si—OH) on the particle surface, and these silanol groups bond with moisture in the atmosphere. When the silica obtained by the wet method was heat-treated at 800 ° C. for 1 hour, about 7% of water was evaporated based on the dry weight of silica, but the silanol groups remained as they were, and this absorbed water in the atmosphere. Rehydrated. When this product is left in the atmosphere at room temperature, it absorbs moisture, and when it is left for a long time, it returns to its original state. In order to solve this problem, the present inventors have studied various treatment conditions. As a result, the silica obtained in the step-2 was
It was found that silanol groups disappear unexpectedly and that dense silica with a small specific surface area can be obtained by heat treatment at a temperature of 1000 ° C or higher. The present invention has been completed based on such findings.

The tendency of rehydration decreases with increasing heat treatment temperature,
When the heat treatment is performed at a temperature of 1000 ° C or higher, the tendency of rehydration is hardly recognized.

When the use of high-purity silica is transparent quartz glass or a filler for a resin composition for encapsulating electronic components, the presence of water is a particular problem, so heat treatment at 1000 ° C or higher is an essential step. .

When the silica obtained in step-2 is subjected to heat treatment at a temperature of 1000 ° C. or higher, it is cleaved from the numerous minute cracks existing in the fiber, changes into fine particle silica, and the fine pores are crushed into a dense structure. Becomes

Silica particles having a specific surface area of about 10 m ² / g or less can be obtained.

Therefore, the silica after heat treatment can be used as it is as silica particles, but if necessary, it can be further pulverized to adjust the particle size.

The heat treatment condition for obtaining silica particles with low hygroscopicity, high bulk density and small specific surface area is 1000
℃ or more, practically in the range of 1100 ~ 1400 ℃ is good. The processing time may be appropriately determined in relation to the set temperature.

The atmosphere for the heat treatment may be oxygen or carbon dioxide gas, or if necessary, an inert gas such as nitrogen or argon. Practically, it should be air.

As a device for heat treatment, it is only necessary to maintain silica at a temperature of 1000 ° C. or higher, and a tubular furnace, a box furnace, a tunnel furnace, a fluidized firing furnace, or the like can be used. , Electric heat, combustion gas, etc.

〔The invention's effect〕

According to the method of the present invention, using an aqueous solution of alkali silicate as a raw material, the content of impurities including radioactive elements such as uranium is extremely low, and the hygroscopicity is low, the specific surface area is small, and the particle size is 1 to 1. 100 μm silica particles can be obtained.

The silica particles obtained by the method of the present invention have a high purity, a low hygroscopicity, and a dense structure as compared with the case of the prior art, so that they are particularly high in addition to raw materials such as transparent quartz glass and special ceramics. It can be used as a filler of a resin composition for sealing an integrated circuit.

Further, the method of the present invention has an advantage that the manufacturing cost can be reduced as compared with the conventional method.

〔Example〕

Hereinafter, the method of the present invention will be specifically described with reference to Examples and Comparative Examples.

. EXAMPLE -1 silicate soda # 3 No. (JIS K1408,3 No. equivalent, hereinafter the _{same) (SiO 2: 28%,} NaO: 9%, U: 36ppb) of 6000 g, under reduced pressure
It was heated to 50 ℃ and dehydrated and concentrated to obtain a fiberizing stock solution of SiO ₂ : 32%. The viscosity of this stock solution was about 100 poise at 30 ° C, and the spinnability was also good.

After filtering this undiluted solution, it was extruded through a gold-platinum alloy nozzle having a hole diameter of 0.1 mm and a number of 200 holes using an extruder at a speed of 6 m / min into a coagulation bath maintained at 30 ° C. and 1N aqueous solution of sulfuric acid 20. .

The extruded stock solution was neutralized with Na ₂ O and coagulated to form a transparent fibrous gel. This fibrous gel had a hollow structure and had a shape with a twist in which a thick portion and a thin portion were alternately formed, and numerous scale-like cracks were formed on the surface portion.

The fibrous gel was taken out of the coagulation bath by a belt conveyor at a conveyor speed of 1 m / min, and the dipping time of the fibrous gel in the coagulation bath was about 1 minute.

40 g of the obtained fibrous gel is treated with a treatment solution-1% sulfuric acid aqueous solution:
It was immersed in 500 CC and treated at 100 ° C. for 3 hours while stirring. The fibrous gel was finely cleaved into short fibers having a length of 2 to 5 mm.

Next, the obtained short fibrous silica was put in water: 500 CC, stirred for 10 minutes, and then dehydrated using a Nutsche. Sulfate concentration in silica after washing with water 5 times
It was 1 ppm or less.

The obtained silica was dried at 150 ° C. overnight and then heat-treated at 1200 ° C. for 1 hour.

The short fibrous silica was finely cleaved by the heat treatment and changed into fine particle silica, which was crushed with an agate crusher in order to obtain a uniform particle size distribution, and final silica particles were obtained.

Table 1 shows the content of impurities in the obtained silica and the physical properties of the silica particles.

The analysis of Cl, U and Th was performed by activation analysis.

In the following Examples, the acid used was a special grade reagent manufactured by Hanai Chemical Co., Ltd., and the water used was ion-exchanged water having an electric conductivity of 1.0 μS / cm (25 ° C.) or less.

Example-2 and Comparative Example-1 Sodium silicate # 3 (the same lot as in Example-1) 5000 g 5
The mixture was dehydrated and concentrated under reduced pressure using a vacuum pump while stirring in a kneader kept at 0 ° C to obtain SiO ₂ : 31.8% to obtain a transparent stock solution. The stock solution had a viscosity of 50 poise at 30 ° C.

This stock solution was extruded from an extruder into each of 20 sulfuric acid aqueous solutions of various concentrations through a SUS-316 nozzle coated with "Teflon" having a hole diameter of 0.2 mmφ and 50 holes to obtain a fibrous gel. The fibrous gels were all transparent.

Each of these fibrous gels was treated in the same manner as in Example-1, and the results shown in Table-2 were obtained.

When using a nozzle made of SUS-316 not coated with "Teflon", only a gel in a dumpling state in which the undiluted solution easily adheres to the nozzle was obtained, and it was difficult to obtain a fibrous gel. .

Example-3 and Comparative Example-2. Each of the stock solutions prepared in Example-2 was extruded to have a pore size of 0.2, 0.5, 1.0, and for comparison, a number of holes of 3.0 mmφ.
The fibrous gel was obtained by extruding into 50 sulfuric acid aqueous solution of 2N concentration through 50 "Teflon" -coated SUS-316 nozzles or gold-plated SUS-316 nozzles.

The obtained fibrous gel was treated in the same manner as in Example-1, and impurities were represented by the Na content in the obtained silica before heat treatment, and the extraction results are shown in Table 3 as a representative.

Among the obtained fibrous gels, the ones shown in Example-3 were all transparent, but the gels obtained in Comparative Example 2-1 were devitrified and exhibited white color and contained silica. The rates were partially different and non-uniform.

Example-4 and Comparative Example-3. 6000 g of sodium silicate # 3 (the same lot as Example-1) was dehydrated and concentrated under reduced pressure using a vacuum pump while stirring in a kneader held at 7 ° C, Stock solutions of various viscosities were obtained.

These stock solutions were made into fibers according to the procedure of Example-1.
Table 4 shows each state.

Further, the obtained fibrous gel was treated in the same manner as in Example-1, and the extraction results of impurities were represented by the Na content in the obtained silica before heat treatment as shown in Table-4.

Example-5. According to the procedure of Example-1, the stock solution prepared in Example-1 was extruded into a nitric acid aqueous solution 20 having a concentration of 1 N to obtain a fibrous gel.

40 g of the obtained fibrous gel was placed in 500 CC of 1N aqueous nitric acid solution and treated at 100 ° C. for 3 hours while stirring to obtain short fibrous silica having a length of 2 to 5 mm. The subsequent treatment was carried out according to the method of Example 1 to obtain silica particles.

The content of impurities in the silica before the heat treatment was; Na: 1.0 ppm, U: 2.0 ppb.

The physical properties of the silica particles after heat treatment were: weight average particle size: 15 μm, bulk density: 0.55 g / cm ³ , water absorption rate: 0.02%.

Example-6 and Comparative Example-4. The silica obtained in Example-1 after completion of extraction of impurities and washing with water was dried at 105 ° C for 4 hours.

10.00g each of the obtained dried silica was weighed, and 1 at each temperature of 400,600,800,900,1000 and 1200 ° C was measured.
Heat treatment was performed for an hour.

Each sample after heat treatment was allowed to cool to room temperature in a desiccator, weighed (W ₀ ), and then allowed to stand in a constant temperature / humidity chamber adjusted to 20 ° C, 80% RH to determine the change in weight over time. It was measured.

The measurement results are shown in Table 5, and no weight change was observed in the silica heat-treated at 1000 ° C. or higher. (Example: 6-1, 2). On the other hand, when the treatment temperature was 400 to 900 ℃, moisture in the air was adsorbed and the weight of silica particles increased with time. (Comparative example: 4-1 to 4). The dried silica obtained in each of Examples other than Example 1 also gave the same results as in the present Example after being subjected to a heat treatment at 1000 ° C. or higher, and moisture absorption due to standing in the air was also observed. There wasn't.

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Claims (2)

[Claims] 1. An alkali silicate aqueous solution having a viscosity in the range of 2 to 200 poise (the alkali silicate is represented by the general formula: M ₂ O.
nSiO ₂ (where M is an alkali metal element and n is 0.5 to 5 in terms of the number of moles of SiO ₂ )) is directly fed from a spinning nozzle having a pore diameter of 1 mm or less within a concentration range of 0.1 to 4 N. Is extruded into an acid solution to give a fine fibrous gel, and the obtained fibrous gel has an acid concentration of 0.5 to 4 at the first stage of treatment.
A method for producing high-purity silica, which comprises immersing in a liquid containing an acid within a specified range, then washing with water to remove impurities, and heat-treating the obtained silica at a temperature of 1000 ° C. or higher. 2. A patent in which the spinning nozzle is made of a noble metal, a noble metal alloy, or a tetrafluoroethylene-based resin, or a nozzle surface of which is coated with a noble metal or a tetrafluoroethylene-based resin. The method for producing high-purity silica according to claim 1.