JPS623012A - Production of high-purity silica - Google Patents

Abstract

PURPOSE:To make it possible to produce high-purity silica having low water vapor absorption and small specific surface area, by making an aqueous solution of an alkali silicate into fibrous gel in a coagulating solution, treating the gel with an acid solution, washing with water and further heat-treating it at high temperature. CONSTITUTION:An aqueous solution of an alkali silicate shown by the formula M2O.nSiO2 (M is alkali metal of K, Na, Li, etc., n is number of mols of SiO2 and 0.5-5), having 2-500 poise viscosity, is extruded from a nozzle having <=1mm hole diameter, made of a noble alloy such as Au-Pt, etc., or of tetrafluoroethylene resin to an organic solvent such as methanol, ethanol, etc., to give foam-containing fibrous gel. The gel is cut into 5-50mm length, treated firstly with <=30% solution of an acid such as sulfuric acid, hydrochloric acid, nitric acid, etc., then with an acid solution with any concentration, washed with water to extract and to remove impurities and successively heat-treated at >=1,000 deg.C. High-purity silica having low water vapor absorption, small specific surface area and extremely low content of impurities such as alkali metals, chlorine, uranium, etc., is produced.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for producing high-purity silica.

Specifically, the present invention relates to a method for producing high-purity silica containing extremely low content of radioactive impurities such as uranium in addition to alkali metals such as sodium and chlorine from a hydrous alkali silicate.

The high-purity silica obtained by the method of the present invention has a filler and
In addition to applications such as dispersants and raw materials for transparent quartz glass and special ceramics, it is expected to be suitably used as a filler in resin compositions for encapsulating electronic components.

Synthetic resin compositions containing inorganic fillers such as silica are used as encapsulating materials for electronic components.

In terms of physical properties such as expansion coefficient, thermal conductivity, moisture permeability, and mechanical properties, as well as cost, it is considered advantageous to incorporate inorganic fillers in as large a quantity as moldability allows, and silica fillers are most preferred. has been done.

However, with the increasing integration of electronic components, the problem of malfunction of the elements has arisen, and this problem is caused by trace amounts of tens to hundreds of ppb contained in the sealing material used, especially the silica filler. It is said to be caused by alpha rays emitted from radioactive elements such as uranium and thorium.

Therefore, it is desired to further reduce the content of such impurities in silica. The method of the present invention is aimed at meeting these needs.

[Conventional technology]

As methods for producing high-purity silica, there are known methods: 1) obtaining high-purity silica by reacting silicon tetrachloride purified by 7 distillation, adsorption, liquid phase extraction, etc. under an oxyhydrogen flame. 2) As a method for producing high-purity silica using an aqueous alkali silicate solution as a raw material, there is a method of purifying an aqueous alkali silicate solution by treating it with an ion exchange resin (JP-A-60-42217, JP-A-60 -42218) is being proposed.

(Problems to be Solved by the Invention) Highly pure particulate silica can be produced by these methods, but in the case of the method (■), the average particle size of the obtained silica particles is on the order of a micron. It has a large specific surface area and is difficult to use as a filler in the resin composition for encapsulating electronic components. In addition, in the method 2), in terms of equipment efficiency, it is necessary to dilute the SiO Since the operating conditions for separating and recovering from the water are complicated, there are problems in terms of productivity. Furthermore, since it is a wet method, the obtained silica has a large number of silanol groups and has the disadvantage of being hygroscopic.

In order to solve these problems, the present inventors have developed a method of producing high-purity silica with extremely low impurity content and no hygroscopicity using hydrous flukasilicate as a raw material more efficiently and economically than before. We conducted research on sharp sounds in order to produce a sharp sound, and arrived at the present invention shown below.

[Means for solving problems]

That is, the present invention relates to a method for producing high-purity silica using an inexpensive hydrous alkali silicate as a starting material and by a relatively simple operation.
JO・n5io, HmHz.

The hydrated alkali silicate represented by is made into a fine fibrous gel, and the resulting fibrous gel is then treated with acid, washed with water to remove impurities, and then heat treated at a temperature of 1000°C or higher. Characteristic method for producing high-purity silica”
The main points are as follows.

The present invention will be explained in detail below. Person a implementing the present invention,
It consists of the following three steps.

1) A highly viscous liquid with stringiness (hereinafter referred to as stock solution) is prepared from a hydrous monolukasilicate, and this stock solution is made into a fine fibrous gel using a fiberizing device.

2) 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.

3) After heat-treating the obtained fibrous silica at a temperature of 1000° C. or higher, it is pulverized if necessary to prepare silica particles of any particle size.

The first feature of the present invention is that the hydrous alkali silicate is formed into a fine fibrous gel.

By making the gel into a fibrous form, not only the alkaline part of the gel in the swollen state is extracted by acid and water in step 2), but also the uniform fine diameter and high surface area of the fibrous gel prevents impurities. contributes to significantly increasing the extraction efficiency of

The second feature of the present invention is that the fibrous gel is treated with acid in the impurity extraction step.

This acid treatment can be carried out in one stage or in two stages.If the acid concentration of the treatment liquid used in the first stage of the treatment is set to 30% by volume or less, the impurity extraction efficiency will be increased. Much improved.

The third feature of the present invention is that the fibrous silica from which impurities have been extracted is heat treated at a temperature of 1000°C or higher.
Silica produced by the formula method has many silanol groups remaining on its particle surface, so it easily adsorbs moisture in the atmosphere. This adsorbed water evaporates even after heat treatment at around 500°C, but the silanol groups remain, which adsorb moisture in the atmosphere and cause rehydration. However, the tendency for rehydration decreases as the heat treatment temperature increases, and almost disappears at temperatures above 1000°C. When silica is used as a filler for transparent quartz glass or resin compositions for encapsulating electronic components, the presence of moisture is a particular problem, so 1000%
Heat treatment above ℃ is an essential step.

By combining the above characteristics, the method of the present invention can significantly improve the extraction efficiency of impurities in silica, and can also impart low hygroscopicity to the obtained silica.

As the hydrous alkali silicate used as a raw material in the method of the present invention, hydrous silicic acid-sodium salt, potassium salt, lithium salt, etc. can be used.

Next, the above three steps will be explained in the following 8174 steps using the method of the present invention in which hydrated sodium silicate is used as the hydrated alkali silicate.

■) Fiberization process Adjust the hydrated sodium silicate to a viscosity range suitable for fiberization and use it as a stock solution. The viscosity range of the stock solution is 10-1000
The poise level is good, and a range of 50 to 500 voids is particularly suitable for fiberization.

When using hydrous sodium silicate as a raw material, which has a high SiO□ strength and an excessively high viscosity, it is used after diluting it with water as appropriate.

In the case of hydrated sodium silicate containing about 30% of 5iOt,
Under normal conditions, the viscosity is low (the stringiness is not sufficient), so in order to give it stringiness, sodium silicate is polymerized and used.

Various methods have been proposed for this polymerization, such as a partial neutralization method using an acidic substance, a dehydration concentration method, and a method of adding a polyvalent metal salt. Among these methods, the dehydration concentration method is the simplest method; dehydration of a few percent causes the sodium silicate to polymerize and increase its viscosity.

The prepared stock solution is heated to a temperature suitable for fiberization, e.g.
It is kept at 0° C. and sent to a fiberizing device using a metering pump after iF if5.

The fiberizing device is not particularly limited, and generally an extruder (hereinafter simply referred to as an extruder) equipped with a spinning nozzle (hereinafter referred to as a nozzle) can be used. In this case, either a wet method or a dry method can be employed, but since polymerized sodium silicate tends to adhere to the nozzle surface, the NW method is more advantageous than the dry method.

The biggest problem when using a nozzle is the adhesion of the stock solution extruded from the nozzle to the nozzle exit surface. As is well known, polymerized hydrous sodium silicate is a viscous liquid with high affinity for metals, and has the property of rapidly solidifying with a slight change in water content. As can be seen from the fact that hydrated sodium silicate is also used as an adhesive, when a stock solution of hydrated sodium silicate adheres to the nozzle surface and solidifies, a strong bond is formed between the sodium silicate and the nozzle surface. is formed, so it is extremely difficult to peel it off. Once the coagulated material adheres to the nozzle surface, the stock solution extruded from adjacent holes will adhere and coagulate one after another, until it becomes impossible to continue the fiberizing operation.

Such a phenomenon is more likely to occur when the nozzle has a small hole diameter and a large number of holes. As a countermeasure against this, it is preferable to reduce the adhesion between the nozzle surface and the stock solution as much as possible.

As a result of various studies, the inventors of the present invention found that the nozzle surface was made of gold.
It has been found that coating with a noble metal alloy such as a platinum alloy or a tetrafluoroethylene (hereinafter referred to as TFE) type resin can have a remarkable effect on improving the nozzle releasability of a hydrous alkali silicate.In the present invention, Say TF
What is E-based resin? Polytetrafluoroethylene (PTFE)
In addition, TFE-hexafluoropropylene copolymer (
PUP), TFE-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-TFE copolymer (ETFE), ethylene-vinyl fluoride copolymer (EVF), ethylene-vinylidene fluoride copolymer (EVdF), ethylene - including copolymers such as chlorotrifluoroethylene copolymer (ECTFE). The nozzle surface is coated with TFE resin according to the usual method, and if necessary, after applying a brimer to the nozzle outer surface,
Coating may also be performed.

In the wet method, the stock solution is forced through a nozzle immersed in a coagulation bath. The extruded stock solution coagulates into fibers and becomes a gel.

This fibrous gel is pulled off with rollers or placed on a wire mesh belt conveyor and sent to the next process.

The nozzle used preferably has a hole diameter of 0.05 to 0.3 mφ. The nozzle holes are usually circular, but irregular cross-section hole nozzles or hollow fiber spinning nozzles can also be used. In particular, hollow fibrous gel produced using a hollow fiber spinning nozzle is suitable for increasing the impurity extraction efficiency in the impurity extraction step described later.

In addition, mixing fine air bubbles into the fibrous gel is also effective in increasing the impurity extraction efficiency.

Methods for mixing fine air bubbles into the fibrous gel include using a stock solution prepared by stirring so that air is drawn into the liquid, using a chemical blowing agent that decomposes into the stock solution by heating to generate gas, or using a chemical blowing agent that generates gas by heating the stock solution. Add a liquid low boiling point substance at
Various methods can be used, such as a method of fiberizing the stock solution while heating it, or a method of utilizing the cavitation phenomenon of a pump that sends the stock solution to the fiberization device.

The fibrous gel can be removed at a rate of 1 to 1 per minute using a roller type.
0m, and is a conveyor type and is normally operated at a speed of about 0.1 to 5m per minute.

As the coagulant used in the coagulation bath, a liquid that exhibits a dehydrating effect on the hydrous alkali silicate can be used. As such a liquid, various organic media that are miscible with water can be used. For example, alcohols such as methanol, ethanol, and n-propanol, esters such as methyl acetate and ethyl acetate, acetone, Examples include ketones such as methyl ethyl ketone, amides such as dimethylacetamide (hereinafter referred to as DQ8), dimethylformamide (hereinafter referred to as DMF), and dimethyl sulfoxide. Since the coagulation rate varies widely depending on the type of coagulant, it is difficult to determine the temperature of the coagulation bath, but a temperature of about 10 to 60°C is usually sufficient.

2) Impurity extraction step The fibrous gel obtained in the above step is treated with a solution containing an acid (treatment solution) in this step. The acids mentioned here include inorganic acids such as sulfuric acid, hydrochloric acid, and nitric acid, and organic acids such as formic acid. Practically speaking, it is preferable to use sulfuric acid, nitric acid, and the like.

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

This acid treatment operation can be carried out in one step, but in order to extract and remove especially trace amounts of impurities, the treatment operation should be divided into at least two steps, and a different treatment solution should be used for each step. Preferably, a multi-step process is carried out.

The acid concentration of the treatment liquid at the initial stage of the treatment operation is preferably 30% by volume or less. (Treatment solution: Acid contained per 100 parts by volume means 30 parts by volume or less, and the same shall apply hereinafter.) In the region where the acid IL1 degree of the processing solution is 30% by volume or less, the fibrous gel remains in a swollen state. , dealkalization proceeds in this state. Moreover, the synergistic effect of the fine diameter and high surface area, which are the characteristics of fine fibers, significantly improves the extraction efficiency of impurities.

If a treatment liquid with an acid concentration of 30% by volume or more is used in the initial stage of treatment, the structure of the silica produced by this treatment will become too dense, making it difficult to extract the impurities remaining inside.

Further, if the acid concentration of the treatment liquid is 0.5% by volume or less, it is not practical from the viewpoint of treatment operation.

For this reason, the acid concentration of the treatment liquid used in the first stage of this treatment is preferably in the range of 0.5 to 30% by volume, preferably 1 to 25% by volume, and more preferably 3 to 20% by volume.
is within the range of

In the case of multi-stage treatment, the acid concentration of the treatment solution in the first step must be 30% by volume or less, but there is no such restriction on the acid concentration of the treatment solution in the second and subsequent stages, and it can be set arbitrarily. can be determined.

Although the treatment temperature is not particularly limited, it is preferable to carry out the extraction operation within the range of 50° C. to the boiling point of the treatment liquid.

The silica fibers obtained by acid treatment are then washed with pure water at a desired temperature, and are subjected to deoxidation and dehydration treatment in combination with a filtration operation if necessary.

Although this step can be carried out continuously in the form of long fibers, in the case of batch treatment, it is preferable to cut the long fiber gel obtained in the above step into short fibers. A common cutter for cutting glass fibers can be used to shorten the fibers. The cutting length is usually 5 to 50 lengths, and preferably around 10 lengths. By making the gel into short fibers, the dispersibility of the gel during stirring in the treatment liquid can be improved. In the treatment liquid, the short fibrous gel is dispersed in the form of a slurry, which facilitates the impurity extraction operation, significantly improves the uniformity of the impurity extraction effect, and significantly reduces variations in the impurity extraction results. In addition, since the short fibrous gel also has the high potency characteristic of fibrous materials, it can be dehydrated very easily during washing and filtration operations after the extraction process.

Through this treatment, the content of the impurities containing radioactive elements in the obtained silica becomes extremely low.

When two-stage acid treatment is performed, alkali metal
10 ppm or less, chlorine 39 pm or less, and uranium about 3 ppb or less.

3) Heat treatment process Although the fibrous gel changes into silica by the acid treatment in step 2), the silica still retains water.

This water is divided into attached water and bound water. Adhered water is 1
If heated to 00°C or higher, it will easily form a depression, but it is difficult to completely remove bound water even at a temperature of 400°C or higher.

In particular, in the case of silica obtained by a wet method such as the method, there are many silanol groups (ESi-Of()) on the silica surface, which bond with moisture in the atmosphere. For example,
When heat treated at 800°C for 1 hour, 7.4% of water evaporates, but when this material is left in the air at room temperature, it adsorbs moisture, and the longer it is left, the more it returns to its original state.

In order to solve this problem, the present inventors investigated various heat treatment conditions, and found that by heat-treating the silica obtained in step 2) at a temperature of 100°C or higher, the silanol group was unexpectedly reduced. was found to disappear. The present invention was completed based on this knowledge.

When the silica obtained in step 2) is subjected to heat treatment at 1000°C or higher, it cleaves through the countless microscopic cracks that exist in the fibers, changes to fine particulate silica, and collapses the micropores to create a dense structure. Become.

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

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

The heat treatment conditions for obtaining silica particles with large bulk density and small specific surface area are as follows:
℃ or higher, practically 1100-1400''C (7)
It is better to set it to i 11. The processing time may be set as appropriate.

The atmosphere during the heat treatment may be oxygen, carbon dioxide, or the like, and if necessary, an inert gas such as nitrogen or argon may be used.

For practical purposes, it is preferable to use air.

The heat treatment can be carried out using a tube furnace, a box furnace, a tube furnace, a box furnace, etc.
In addition to the tunnel furnace, a fluidized furnace or the like can be used, and the heating method may be electric heat, combustion gas, or the like.

〔Effect of the invention〕

According to the method of the present invention, it is possible to obtain silica particles that have an extremely low content of impurities including radioactive elements such as uranium, have a particle size of 1 to 100 μm, and are not hygroscopic. The silica particles obtained in this way have lower production costs than conventional techniques, and are highly pure (and have no hygroscopicity, so they can be used as raw materials for transparent quartz glass, special ceramics, etc.) It can be used as a filler for resin compositions for encapsulating integrated circuits.

The method and effects of the present invention will be explained below using Examples and Comparative Examples.

〔Example〕

Example-1 Sodium silicate #3 (equivalent to JIS K1408.3, the same applies hereinafter) (SiOz: 28χ, NazO: 9χ,
U: 36 ppb) 3000 g was heated to 50° C. under reduced pressure and dehydrated and concentrated to obtain a stock solution for fiberization of 5 iOz: 32%. The viscosity of this stock solution was about 100 voids at 30° C., and the stringiness was also good. This stock solution was mixed with peracid and made into PTFE with a pore diameter of 0.1 flφ and 200 pores using an extruder.
through a resin-coated nozzle at a speed of 3 m/min.
It was extruded into a coagulation bath (coagulant: DMAC) maintained at .degree.

The extruded stock solution was dehydrated by DM8, solidified, and turned into a transparent fibrous gel. This fibrous gel was cut using a cutter to obtain short fibers with a fiber length of about 1c11.

The obtained short fibrous gel Log was immersed in a treatment solution - 500cc of a 5% by volume aqueous solution of sulfuric acid, and treated at 100°C for 1 hour with stirring, then the treatment solution was changed to 500cc of a 10% by volume aqueous solution of sulfuric acid. The second stage of processing was performed.

The short fibrous silica thus obtained was washed with boiling water, filtered, deoxidized, and dehydrated, pre-dried at 150°C, and then heat-treated at 1200°C for 1 hour.

Due to the heat treatment, the short fibrous silica was finely cleaved and turned into fine particulate silica, but in order to make the particle size distribution uniform, it was pulverized using an agate pulverizer to obtain the final silica particles.

Example-2 Sodium silicate #3 (same as Example 1) 500
0 g was maintained at 30° C., and finely powdered acidic sodium sulfate was slowly added while stirring. The viscosity of the sodium silicate solution increased as the amount of acidic sodium sulfate added increased, and a stock solution with a viscosity of 500 voids was finally obtained.

This stock solution was aerated and filled with air bubbles. This stock solution containing air bubbles is directly passed through an extruder with a pore size of 0.1
Fiφ was extruded through a nozzle with 200 holes into a coagulation bath using DMAC as a coagulant to obtain a fibrous gel. Many fine air bubbles were present in this fibrous gel. The fibrous gel containing air bubbles was cut into short fibers, and the same treatment as in Example 1 was performed to obtain silica particles.

Example-3 Sodium silicate #3 (SiOz: 28.5χ, NazO
:9.5χ, U:150ppb) 5000g in Example-
The solution was dehydrated and concentrated to SiO□:32.3χ according to 1 to obtain a stock solution. The viscosity of this stock solution was about 500 voids at 30°C. After leaving the stock solution overnight to defoam, it was extracted from an extruder with an outer hole diameter of 0.1 mφ, an inner hole diameter of 0.05 snφ, and a number of holes of 10.
The gel was extruded through two hollow fiber spinning nozzles into IIMAC to obtain a transparent hollow fiber gel. The obtained gel was treated in the same manner as in Example-1 to obtain silica particles.

The impurity content in the silica before heat treatment and the physical properties of the silica particles after heat treatment obtained in Examples 1 to 3 above are shown below.
Shown in 1.

Note that CI, U, and Th were analyzed by activation analysis.

Table-1 (l) Important impurity content in silica before heat treatment-1
(2) Changes in physical properties of silica particles after heat treatment were measured and expressed as water absorption.

Sample weight before being left in the atmosphere [g] Example-4 Sodium silicate No. 113 (same loft as Example-3) 50
00g was dehydrated and concentrated under reduced pressure using a vacuum pump while stirring in a Ni-Goo maintained at 50°C, and 5iOz: 31
．． 8″1 to obtain a transparent stock solution.The viscosity of the stock solution was 30°C.
It was 50 boise. This stock solution was extruded from an extruder through a PTFE-coated nozzle with a hole diameter of 0.1 mm and 50 holes into various coagulants to obtain transparent fibrous gels.

This mature fibrous gel was treated in the same manner as in Example-1.
The results shown in 2 were obtained.

Table 2 (l) Impurity content in silica before heat treatment Et-
OH: Ethanol (2) Physical properties of silica particles after heat treatment; NQ4-1 to NQ4-3 all had a weight average particle size of 15 μm, a bulk density of 0.96 g/cJ, and a water absorption rate of 0%.

Example-5 Using an extruder, the stock solution prepared in Example-3 was heated to a pore size of 0.
．． It was dropped into the air through a PTFE-coated nozzle with a diameter of 1 crane and 50 holes. A heating cylinder was placed at the nozzle outlet, and the temperature inside the cylinder was maintained at 150°C.

The stock solution discharged from the nozzle solidified due to evaporation of water and became a fibrous gel. This fibrous gel was mixed with 5% sulfuric acid by volume.
It was dropped into a bath containing an aqueous solution and subjected to preliminary acid treatment.

Next, the fibers were washed with water and cut into short fibers with a fiber length of about 10 m.

When this short fiber was treated according to Example-1,
As impurity content: Na: 2.2 ppla, U: 1
ppb or less, as physical properties after heat treatment; weight average particle size:
15μm, bulk density: 0.96g/c+J, water absorption rate:
0% silica particles were obtained.

Example-6 and Comparative Example-1 l) 10 g each of the short fibrous gels obtained by the same operation as in Example-1 were used as the first-stage treatment liquid at concentrations of 10 and 20, respectively, and for comparison. The samples were placed in 500 cc of aqueous sulfuric acid containing 40 and 70% by volume, and impurity extraction treatment according to Example 1 was performed.

(Example: 6-1 to 2, Comparative Example = 1-1 to 2) 2) In addition, compared to Comparative Example 1, only the order of the M concentration of the treatment solution in the acid treatment was changed, and the other treatments were the same. went. (
Examples = 6-3 to 6-4) The impurity content in the obtained silica is shown in Table 3 with the results of Example 1 added.

Table-3 Effect of acid concentration [sulfuric acid] in treatment solution ・Processing time: IHr.

Example 7 and Comparative Example 2 Short fibrous gels were obtained using the stock solution prepared in Example 4 in the same manner as in Example 1. Each Log of this short fibrous gel was used as the first stage treatment solution at a concentration of 5. l
O and 20. Also for comparison, 40 and 60% capacity
Pour into 5 oocc of nitric acid solution and add 10 ml while stirring.
After treatment at 0° C. for 1 hour, the treatment liquid was then changed to 500 cc of a 10% by volume nitric acid aqueous solution, and a second stage treatment was performed in the same manner. Silica particles were obtained by a method similar to Example 1 for subsequent treatments.

Table 4 shows the impurity content in the silica before heat treatment and the physical properties of the silica particles after heat treatment.

Table-4 Effect of acid concentration [nitric acid] in treatment liquid Table-4 (l) Impurity content in silica before heat treatment *Acid concentration: volume %, (concentrated nitric acid, CC/treatment liquid: 100cc)
Table 4 (2) Physical properties of silica particles after heat treatment * Weight average particle diameter [μm] is shown.

Example 8 and Comparative Example 3 The silica obtained in Example 1 after the impurity extraction and water washing was dried at 105° C. for 4 hours. Weigh out 10.00 g of each of the obtained dry silica, and weigh 40 g of each.
0,600,800,900.1000 and 1200
Heat treatment was performed at each temperature of 1 hour. After each heat-treated sample was allowed to cool to room temperature in a desiccator, its weight (Wo) was measured, and then it was left in the air to measure the change in weight over time.

The measurement results are shown in Table 5. No weight change was observed in the silica heat-treated at 1000°C or higher, but 1. When the heat treatment temperature was 400 to 900°C, the weight of the silica particles increased over time by adsorbing moisture in the atmosphere.

In addition, no moisture absorption was observed in the dried silica obtained in Examples other than Example 1 after being subjected to heat treatment at 1000° C. or higher when left in the air.

Table-5 Procedural Amendment (Voluntary) September 18, 1988 Commissioner of the Patent Office Kuro 1) Mr. Akio 1, Indication of the case 1985 Patent Application No. 1391.46 2, Name of the invention High purity silica 3. Relationship with the case of the person making the amendment. Patent applicant: 5-1 Marunouchi-chome, Chiyoda-ku, Tokyo, 100. Description 1. Name of the invention. Process for producing high-purity silica 2. Scope of claims. A method for producing high-purity silica, characterized by:

3) The manufacturing method according to claim 1, wherein the fibrous gel contains air bubbles.

4) Acid treatment of the fibrous gel is performed at an acid concentration of 30% by volume at first.
The manufacturing method according to claim 1, which is carried out using the following treatment liquid.

5) The manufacturing method according to claim 1 or 4, wherein the acid treatment of the fibrous gel is carried out in at least two stages.

3. Detailed description of the invention [Industrial application field] The present invention relates to a method for producing high-purity silica.

Specifically, the present invention relates to a method for producing high-purity silica, which has an extremely low content of radioactive impurities such as uranium in addition to alkali metals and chlorine, and has 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 has a filler and
In addition to applications such as dispersants and raw materials for transparent quartz glass and special ceramics, it is expected to be suitably used as a filler in resin compositions for encapsulating electronic components.

Synthetic resin compositions containing inorganic fillers such as silica are used as encapsulating materials for electronic components, but inorganic fillers have various physical properties such as expansion coefficient, thermal conductivity, moisture permeability, mechanical properties, and From the viewpoint of cost, it is considered advantageous to blend as much as moldability allows, and silica-based fillers are said to be the most preferred. However, with the increasing integration of electronic component elements, the problem of malfunction of the elements has arisen.
This is said to be caused by alpha rays emitted from trace amounts of radioactive elements such as uranium and thorium contained in silica, and it is desired to further reduce the content of such impurities in silica.

The present invention aims to meet such demands.

[Conventional technology]

Known methods for producing high-purity silica include: 1) A method in which silicon tetrachloride purified by distillation, adsorption, liquid phase extraction, etc. is reacted under an oxyhydrogen flame. 2) As a method for producing high-purity silica using an aqueous alkali silicate solution as a raw material, there is a method of purifying an aqueous alkali silicate solution by treating it with an ion exchange resin (JP-A-60-42217, JP-A-6 (1983)). No. 1-42218, etc.) have been proposed.

[Problem that the invention seeks to solve]

Highly pure silica can be produced by these methods, but in the case of method 1), the obtained silica particles are fine particles with an average particle size of the mμ order and have a large specific surface area, and are not suitable for resin compositions for encapsulating electronic components. It is difficult to use it as a filler.

In addition, in the method 2), since the purification operation is performed after diluting the SiO concentration of the aqueous alkali silicate solution to about 10% by weight or less, it is difficult to improve the efficiency of the equipment, and also because silica is precipitated from the silica sol and the mother liquor is Since the operating conditions for separating and recovering from the water are complicated, there are problems in terms of productivity. Furthermore, since it is a wet method, the obtained silica has a problem in that it has hygroscopic properties due to the presence of a large number of silanol groups on its surface.

[Means for solving problems]

The present inventors have improved these problems in the conventional method, and have created high-purity silica with extremely low impurity content, low hygroscopicity, and small specific surface area using an aqueous alkali silicate solution as a raw material. We conducted extensive research to produce it efficiently (and economically) by turning an aqueous alkali silicate solution into a fine fibrous gel in a coagulation bath, and treating the resulting fibrous gel with an acid-containing solution and then with water. By extracting and removing impurities and heat-treating the obtained silica at a temperature of 1000°C or higher, it is possible to obtain high-purity silica with extremely low impurity content, low hygroscopicity, and small specific surface area. Heading The invention has been completed.

The present invention is based on the general formula: M zO·ns+Oz (where M is an alkali metal element and n is the number of moles of SiO□0.
After extruding an aqueous solution of an alkali silicate represented by 5 to 5 from a spinning nozzle into a water-soluble organic medium to form a fine fibrous gel, and treating the resulting fibrous gel with a liquid containing an acid. , followed by washing with water to extract and remove impurities, and heat-treating the obtained silica at a temperature of 1000°C or higher.''

The present invention will be explained below. Embodiments of the invention include:
It consists of the following three steps.

, Step-1: (fiberization step).

A highly viscous liquid with stringiness (from an aqueous alkali silicate solution)
(hereinafter referred to as a stock solution) is prepared, and this stock solution is coagulated in a coagulation bath using a fiberizing device to form a fine fibrous gel.

-Step-2: (Impurity extraction step).

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.

-Step-3: (heat treatment step).

The obtained fibrous silica is heat-treated at a temperature of 1000°C or higher.

Features of the present invention are: +11 An aqueous alkali silicate solution is coagulated into a fine fibrous gel in a coagulation bath using a fiberizing device equipped with a spinning nozzle (hereinafter referred to as nozzle).

For fiberization, it is preferable to use a nozzle having a hole diameter of l+n or less.

The resulting fibrous gel has a fine diameter and a high surface area, increasing the efficiency of extracting impurities.

(2) When forming an aqueous alkali silicate solution into a fine fibrous gel, the aqueous alkali silicate solution is coagulated in a water-soluble organic medium.

The viscosity of the aqueous alkali silicate solution is preferably in the range of 2 to 500 voids.

By combining the above characteristics (1) and (2), surprisingly, a fibrous gel with a hollow structure can be obtained even using a normal circular hole nozzle, and the coagulated part is in a homogeneous and highly swollen state. , and impurities are easily extracted by acid and water.
Combined with the effect of 1, the above combination can significantly improve the efficiency of extracting impurities from silica.

(3) When the obtained silica is heat-treated at a temperature of 1,000°C or higher, the silanol groups present on the particle surface disappear, so the hygroscopicity of the silica can be significantly reduced, and the silica particles' The micropores are crushed, and dense silica particles with a small specific surface area and a large bulk density can be obtained.

By combining the above steps, the method of the present invention can significantly improve the extraction efficiency of impurities in silica, and can also impart low hygroscopicity and compactness to the obtained silica. The high-purity silica obtained by the method of the present invention is also suitable for applications where the presence of moisture is a problem, such as transparent quartz glass and fillers for resin compositions for encapsulating electronic components.

In the method of the present invention, the raw alkali silicate aqueous solution includes sodium salts and potassium salts of silicic acid.

Aqueous solutions such as lithium salts can be used.

Hereinafter, the case where a sodium silicate aqueous solution is used as the alkali silicate aqueous solution in the method of the present invention will be described as an example.
The above three steps will be explained in order.

[Step-1: (Fibre-forming step)] A sodium silicate aqueous solution as a raw material is adjusted to a viscosity range suitable for fiber-forming, and used as a stock solution.

The viscosity range of the stock solution suitable for the method of the present invention is preferably in the range of 2 to 500 voids, particularly preferably in the range of 10 to 200 voids.

540□ When a sodium silicate aqueous solution with high concentration and too high viscosity is used as a raw material, it is used after appropriately diluting with water.

In the case of an aqueous sodium silicate solution containing about 30% SiO□, the viscosity is low under normal conditions and the stringiness is not sufficient, so sodium silicate is polymerized and used in order to impart stringiness to it.

As methods for polymerizing sodium silicate, a partial neutralization method using an acidic substance, a dehydration concentration method, a method of adding a polyvalent metal salt, etc. have been proposed. Among these, the dehydration and concentration method is the simplest method, in which sodium silicate polymerizes and increases in viscosity by dehydration of a few percent.

The prepared stock solution is heated to a temperature suitable for fiberization, e.g.
The mixture is kept at 0° C., passed through an appropriate filtration device, and sent to a fiberization device using a metering pump.

The fiberizing device is not particularly limited, and generally an extruder equipped with a spinning nozzle can be used.

The biggest problem when using a nozzle is the occurrence of adhesion problems of the stock solution extruded from the nozzle to the nozzle exit surface.

As is well known, sodium silicate aqueous solution is a viscous liquid that has a high affinity with metals, and 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 the stock solution consisting of 8 sodium silicate solutions solidifies while adhering to the nozzle surface, a strong bond is formed between the sodium silicate and the nozzle surface, which is extremely difficult to separate. It is. When the coagulated material adheres to the nozzle surface, the stock solution extruded from adjacent holes will adhere and coagulate one after another, until it becomes impossible to continue the fiberizing operation.

This phenomenon tends to occur when the nozzle used has a small pore diameter and a large number of holes.The solution to this problem is to minimize the adhesion between the nozzle surface and the stock solution.

The present inventors conducted various studies on the material of the nozzle to be used, and found that noble metal alloys such as gold-platinum alloys or TF
E-based resin nozzle or nozzle surface made of precious metal or T
It has been found that when a nozzle coated with an FE resin is used, the nozzle separation properties of gelled alkali silicate are significantly improved.

The noble metals referred to in the present invention include gold, platinum, gold, and palladium, and can be coated on the nozzle surface by ordinary plating treatment.

The TFE resin referred to in the present invention is polytetrafluoroethylene (
PTFE), copolymers of TFE and hexafluoropropylene, copolymers of TFE and perfluoroalkyl vinyl ether, copolymers of ethylene and TFE, copolymers of ethylene and vinyl fluoride, ethylene and vinylidene fluoride, and copolymers of ethylene and chlorotrifluoroethylene.

The nozzle surface is coated with TFE resin according to the usual method.
If necessary, coating may be performed after applying a primer to the outer surface of the nozzle.

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

In the present invention, the stock solution is extruded through a nozzle immersed in a coagulation bath. The extruded stock solution coagulates into fibers in a coagulation bath and becomes a gel. This fibrous gel can be pulled off with a roller, or
Alternatively, it is placed on a belt conveyor and sent to the next step-2.

The hole diameter of the nozzle used in this process is 0.05 to 1. OI
The range of m is good, preferably 0.1 to 0.31.

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

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

Incorporating fine air bubbles into the fibrous gel is also effective in increasing impurity extraction efficiency.

Methods for mixing fine air bubbles into the fibrous gel include using a stock solution prepared by stirring so that air is drawn into the liquid, using a chemical blowing agent that decomposes into the stock solution by heating to generate gas, or using a chemical blowing agent that generates gas by heating the stock solution. Add a liquid low boiling point substance at
Various methods can be adopted, such as a method of fiberizing the stock solution while heating it, or a method of utilizing the cavitation phenomenon of a pump that sends the stock solution to the fiberization device.

A water-soluble organic medium is used as the coagulant used in the coagulation bath of the present invention. Water-soluble organic media have a high affinity for water, but have little affinity for alkali silicates. The coagulation of alkali silicates is thought to occur due to the so-called dehydration effect. Examples of the water-soluble organic medium used in the method of the present invention include alcohols such as methanol, ethanol, and n-propanol, vinegar, etc. Esters such as methyl and ethyl acetate, acetone.

Ketones such as methyl ethyl ketone, dimethylacetamide (hereinafter referred to as DMAC), dimethylformamide
(hereinafter referred to as DMF), dimethyl sulfoxide, and the like.

Since the coagulation rate of the alkali silicate varies greatly depending on the type of coagulant used, it is difficult to determine the coagulation bath temperature unambiguously, but a temperature of about 10 to 60°C is usually good.

The fibrous gel can be removed at a rate of 1 to 1 per minute using a roller type.
00 m, conveyor type 0.1 to 50 m/min
Normally operated at moderate speeds.

[Step-2: (Impurity extraction step)] The fibrous gel obtained in step-1 is treated with a liquid containing an acid in this step. The acid is sulfuric acid 8-hydrochloric acid.

In practical use, monoacids such as nitric acid and organic acids such as formic acid
It is preferable to use sulfuric acid, trinitric acid, or the like.

Further, as the treatment liquid, an aqueous solution of these acids is practically preferable.

The acid treatment operation in this process can be carried out in one step, but in order to extract and remove especially trace amounts of impurities, the treatment operation is divided into at least two steps, and the treatment solution used in each step is It is also possible to perform a multi-step process of updating the .

The -a method is to use highly concentrated acid to extract impurities, but in the method of the present invention, in order to maintain as much as possible the gel structure formed in step-1, from which impurities can be easily extracted. It is preferable that the acid concentration of the treatment liquid is low.

In the method of the present invention, the acid concentration of the treatment liquid at the first stage of the treatment operation is 30% by volume or less (meaning acid contained per 100 parts by volume of treatment liquid = 30 parts by volume or less, the same shall apply hereinafter). is preferable.

In a region where the acid concentration of the treatment solution is 30% by volume or less, the fibrous gel remains in a swollen state, and dealkalization proceeds in this state. Moreover, the synergistic effect with the high surface area characteristic of fine hollow fibers significantly improves the extraction efficiency of impurities.

If a treatment solution with an acid concentration exceeding 30% by volume is used in the initial stage of the treatment operation, the structure of the silica produced by this treatment will become too dense, making it difficult to extract the impurities remaining inside.

Furthermore, if the acid concentration of the treatment liquid is less than 0.5% by volume, it is not practical in terms of acid treatment efficiency.

For this reason, the acid concentration of the treatment liquid used in the first stage of this treatment is 0.5 to 25% by volume, preferably 1 to 25% by volume, and more preferably 3 to 25% by volume.
The range is 20% by volume.

In the case of multi-stage treatment, the acid concentration of the treatment solution in the first step must be 30% by volume or less, but there is no such restriction on the acid concentration of the treatment solution in the second and subsequent stages, and it can be adjusted arbitrarily. can be determined.

Although the treatment temperature in this step is not particularly limited, it is preferable to perform the extraction operation at a temperature of 50° C. or higher.

When 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 extracting impurities can be shortened. The higher the temperature during pressure extraction, the better, but considering the corrosion of the equipment due to acid and the energy cost, the temperature should be 100 to 1
A temperature of 50°C, preferably in the range of 110 to 140°C is practical.

It is desirable that this step be performed while stirring. Although the operation of this step can be carried out continuously in the form of long fibers, in the case of batch treatment, it is preferable to cut the long fiber gel obtained in step-1 above into short fibers.

A common cutter for cutting glass fibers can be used to shorten the fibers.

The cutting length is usually 5 to 50 m, and preferably about 10 m.

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 processing liquid in the form of a slurry, which facilitates the operation of impurity extraction, improves the uniformity of the impurity extraction effect, and significantly reduces variations in the impurity extraction results. In addition, since the short fibrous gel has bulkiness which is a characteristic of fibrous materials, liquid separation is extremely easy during washing and filtration operations after impurity extraction treatment.

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

The silica fibers obtained by the acid treatment are then washed with water at a desired temperature, and if necessary, combined with a filtration operation to undergo deacidification and dehydration treatment.

In addition, it is preferable that the acid used in the present invention be a highly purified product called purified or electronic grade, and that the water used for diluting the raw material or the acid used or for washing the silica be pure water with few impurities. .

By the treatment in this step, the content of the impurities including radioactive elements in the silica becomes extremely low.

The impurity content in silica after acid treatment is alkali metal:
It can be reduced to about 10 ppm or less, chlorine to 3 ppm or less, and uranium to about 3 ρppb or less.

[Step-3: (Heat treatment step)] The fibrous gel is changed into silica by the acid treatment in Step-2, but the silica still retains water. This water is divided into attached water and bound water. Adhered water is 100
While it easily evaporates when heated to temperatures above 400°C, it is difficult to completely remove bound water even at temperatures above 400°C.

In particular, silica obtained by a wet method has a large number of silanol groups (asi-014) on the particle surface, which bond with moisture in the atmosphere. For example, when silica obtained by a wet method is heat-treated at 800°C for 1 hour, about 7% of the water evaporates based on the dry weight of the silica, but the silanol groups remain as they are, and these absorb moisture from the atmosphere. caused rehydration. When this material is left in the air at room temperature, it absorbs moisture, and the longer it is left, the more it returns to its original state.

In order to solve this problem, the present inventors investigated various treatment conditions and found that by heat-treating the silica obtained in step-2 at a temperature of 100°C or higher, the silica was unexpectedly treated with silanol. It was discovered that the groups disappeared and dense silica with a small specific surface area was obtained. The present invention was completed based on this knowledge.

The tendency of rehydration decreases with increasing heat treatment temperature;
When heat treatment is performed at a temperature of 1000° C. or higher, there is almost no tendency for rehydration.

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

When the silica obtained in Step-2 is heat-treated at a temperature of 1000°C or higher, it cleaves through the countless microscopic cracks existing in the fibers, changes to fine particulate silica, and collapses the micropores to form a dense structure. becomes.

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

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

The heat treatment conditions for obtaining silica particles with low hygroscopicity, high bulk density, and small specific surface area are as follows:
000°C or higher, preferably in the range of 1100 to 1400°C. The processing time may be appropriately determined in relation to the set temperature.

The atmosphere during the heat treatment may be oxygen, carbon dioxide, or the like, and if necessary, an inert gas such as nitrogen or argon may be used, but it is practically preferable to use air.

The equipment for heat treatment only needs to be able to maintain the silica at a temperature of 1000°C or higher, and in addition to a tube furnace, a single box furnace, a tunnel furnace, and a fluidized sintering furnace, there are various heating methods. This can be done using electric heat, combustion gas, etc.

〔Effect of the invention〕

According to the method of the present invention, using an aqueous alkali silicate solution as a raw material, it has high purity with an extremely low content of inert substances including radioactive elements such as uranium, has low hygroscopicity, has a small specific surface area, and has a particle size of 1. Silica particles of ~100 μm can be obtained.

The silica particles obtained by these methods have higher purity, lower hygroscopicity, and a denser structure than those obtained by conventional techniques, so they can be used as raw materials for transparent quartz glass, special ceramics, and especially in highly integrated materials. It can be used as a filler for circuit sealing resin compositions.

Furthermore, the method of the present invention also has the advantage that manufacturing costs can be reduced compared to conventional methods.

〔Example〕

The method of the present invention will be specifically explained below using Examples and Comparative Examples.

Example-1 Sodium silicate No. 43 (equivalent to JIS K1408.3, the same applies hereinafter) (StOz: 28Z, NazO: 9X, U
: 36 ppb) 3QOOg was heated to 50°C under reduced pressure and dehydrated and concentrated to obtain a stock solution for fiberization of 5iCh: 32%. The viscosity of this stock solution is approximately 100 voids at 30°C.
Threadability was also good. This stock solution was filtered with acid and extruded using an extruder through a PTFE resin-coated nozzle with a hole diameter of 0.1 nφ and 200 holes at a speed of 3 m/min into a coagulation bath (coagulant: DHAC) maintained at 30°C.

The extruded stock solution was dehydrated and solidified by DMAC, turning into a transparent fibrous gel. This fibrous gel was cut using a cutter to obtain short fibers with a fiber length of about 11.

The obtained short fibrous gel Log was immersed in a treatment solution - 500CC sulfuric acid 5% aqueous solution by volume, and treated at 100°C for 1 hour with stirring, and then the treatment solution was replaced with 10% sulfuric acid aqueous solution 500CC. , The second stage of processing was performed in the same manner.

The short fibrous silica thus obtained was washed with boiling water, filtered, deoxidized and dehydrated, and pre-dried at 150°C.
Heat treatment was performed at 1200°C for 1 hour.

Due to the heat treatment, the short fibrous silica was finely cleaved and turned into fine particulate silica, but in order to make the particle size distribution uniform, it was pulverized using an agate pulverizer. ) Silica particles were obtained. In the following examples, a special grade reagent manufactured by Hanui Chemical was used as the acid, and ion-exchanged water with an electrical conductivity of 1.0 μS/am (25° C.) or less was used as the water.

Example-2 Sodium silicate No. 43 (same loft as Example-1) 5000
While keeping the temperature at 30° C., finely powdered acidic sodium sulfate was slowly added little by little while stirring. The viscosity of the sodium silicate solution increases as the amount of acidic sodium sulfate increases,
A stock solution with a viscosity of 30 boids was obtained.

Air was mixed into this stock solution, and the droplets were filled with air bubbles. The stock solution containing air bubbles is directly passed through the extruder with a pore diameter of Q, 1
The mixture was extruded through a gold-platinum alloy nozzle having a diameter of 4 m and 200 holes into a coagulation bath using DMAC as a coagulant to obtain a fibrous gel. Many fine air bubbles were present in this fibrous gel. The fibrous gel containing air bubbles was cut into short fibers, and the same treatment as in Example 1 was performed to obtain silica particles.

The impurity content and physical properties of the silica obtained in Examples 1 and 2 above are shown in Table 1. CI, U, and Th were analyzed by activation analysis.

Table-1fl), impurity content in silica before heat treatment;
Table-1F21, Physical properties of silica particles after heat treatment; *()
The numerical value indicates the weight average particle diameter.

**The silica after heat treatment was left in a thermostatic chamber adjusted to 20° C. and 80% RH, and the change in weight was measured and expressed as water absorption rate.

Water absorption rate, C%) = (W, -W,)X　100/w.

W+: Weight of sample after 72 hours at 20°C and 80χRH, (g) Wo: Weight of sample left to cool to room temperature in a desiccator after heat treatment, (g).

Example-3゜Silicate So No. 143 (same as Example-1)) 50
SiO□:ai
, sz to obtain a transparent stock solution. The viscosity of the stock solution was 50 voids at 30°C. This stock solution is passed through an extruder with a pore size of 0.
．． Transparent fibrous gels were obtained by extrusion into various coagulants through a PTFE resin-coated nozzle with a diameter of 1 mm and 50 holes. This fibrous gel was treated in the same manner as in Example-1,
The results shown in Table 2 were obtained.

Table-2, impurity content in silica before heat treatment;
The physical properties of the silica particles after heat treatment are; 1IkL3-1~3
All weight average particle diameters: 15 μm.

Bulk density: 0.55 g/J, water absorption rate = 0.0%.

Example-4 and Comparative Example-1゜1) Each log of the short fibrous gel obtained by the same operation as in Example-=1 was used as the first-stage treatment liquid at an acid concentration of 0.5 and 10, respectively. .20 and 30, and for comparison, 40 and 70% by volume of sulfuric acid aqueous solution 500CC, and impurity extraction treatment according to Example 1 was carried out. (Example = 4-1 ~ 4. Comparative example: 1-1 ~
2) 2) In addition, the same treatment was performed as in Comparative Example-1 except that only the order of the acid concentrations of the treatment liquid in the acid treatment was changed. (Examples = 4-5 to 6) The impurity content of the obtained silica is shown in Table 3 together with the results of Example 1.

In addition, the physical properties of the silica particles after heat treatment are; Floors 4-1 to 6
All weight average particle diameters: 15 μm.

Bulk density: 0.55 g/csl, water absorption rate 20.0%.

Table 3. Impurity content in silica before heat treatment; *Acid concentration: Volume %, (Concentrated sulfuric acid, CC/Treatment liquid: 100CC)
Example 5 and Comparative Example 2 Short fibrous gels were obtained using the stock solution prepared in Example 3 in the same manner as in Example 1. Each log of this short fibrous gel was used as a first-stage treatment solution with an acid concentration of 5,
10 and 20. For comparison, each of 40 and 60% by volume of nitric acid was placed in 500cc of aqueous solution and treated at 100°C for 1 hour with stirring.Then, the treatment solution was changed to 500cc of each 10% by volume of nitric acid solution, and the second stage was prepared in the same manner. was processed. Silica particles were obtained by a method similar to Example 1 for subsequent treatments.

The results obtained are shown in Table-4.

Table-4, Impurity content in silica before heat treatment; *Acid concentration: Volume %, (Concentrated nitric acid, CC/Treatment liquid: 100CC)
In addition, the physical properties of silica particles after heat treatment are;
All weight average particle diameters: 15 μm.

Bulk density: 0.55 g/cj, water absorption rate 20.0%.

Example-6 and Comparative Example-3 Sodium silicate #3 (same loft as Example-1) 6000
Dehydrated under reduced pressure using a vacuum pump while stirring in a Ni-Goo maintained at 70°C. ! The mixture was compressed to obtain stock solutions of various viscosities. Table 5 shows the state of these stock solutions when they were made into fibers according to the procedure of Example 1.

The obtained fibrous gel was treated in the same manner as in Example 1, and the impurity extraction results are shown in Table 5, represented by the Na content in the silica before heat treatment.

Example-7 and Comparative Example-4 The stock solution prepared in Example-1 was extruded using an extruder with pore sizes of 0.2, 0.5, 1.0, and 3.0 for comparison. O
Gold-plated 5O5-316 with 50 holes each, Uφ
The gel was extruded through a manufactured nozzle into a coagulation bath in the same manner as in Example 1 to obtain a fibrous gel.

The obtained fibrous gel was treated in the same manner as in Example 1, and the impurity extraction results are shown in Table 6, represented by the Na content in the silica before heat treatment.

Table -5゜ Value at a temperature of 30°C during fiberization.

Table-6　Example-8 and Comparative Example-5　The silica obtained in Example-1 after the impurity extraction and water washing was completed was dried at 105°C for 4 hours.

10.0% of each of the obtained dry silica. Weigh out OOg and weigh 400, 600, 800, 900.100 for each.
Heat treatment was performed at each temperature of 0 and 1200° C. for 1 hour.

After the heat treatment, each sample was allowed to cool down to room temperature in a desiccator, the weight (Wo) was measured, and then the sample was heated to 20°C.

The sample was left in a constant temperature/constant room adjusted to 80% RH, and changes in weight over time were measured.

The measurement results are shown in Table 7. No weight change was observed in silica that was heat-treated at 1000°C or higher, but when the treatment temperature was 400-900°C, it adsorbed moisture in the air. The weight of silica particles increased over time.

Note that the same results as in this example were obtained for the dried silica obtained in Examples other than Example-1 after heat treatment at 1000°C or higher, and no moisture absorption was observed when left in the air. Ta.

Table -7゜ *In a constant temperature/constant room adjusted to 20°C and 180% RH.

Claims (1)

[Claims] 1. General formula: M_2O・nSiO_2・mH_2O [where M is an alkali metal and n is the number of moles of SiO_2 and is 0
．． 5 to 5, m is the number of moles of H_2O and is an integer. ] The hydrated alkali silicate represented by is made into a fine fibrous gel, then the obtained fibrous gel is treated with acid, washed with water to remove impurities, and then heat treated at a temperature of 1000°C or higher. A method for producing high-purity silica, characterized by: 2. The manufacturing method according to claim 1, wherein the fine fibrous gel contains air bubbles. 3. The manufacturing method according to claim 1 or 2, wherein the fine fibrous gel is a hollow fibrous gel. 4. The manufacturing method according to claim 1, wherein sulfuric acid or nitric acid is used as the acid. 5. The acid treatment of the fibrous gel was performed at an acid concentration of 30% by volume at first.
The manufacturing method according to claim 1, which is carried out using the following treatment liquid. 6. The manufacturing method according to claim 1, wherein the acid treatment of the fibrous gel is carried out in at least two stages.