JPH0535087B2 - - Google Patents

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, 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 is
In addition to applications such as fillers and dispersants, and raw materials for transparent quartz glass and special ceramics,
In particular, it is expected to be suitably used as a filler for resin compositions for encapsulating electronic components. Synthetic resin compositions containing inorganic fillers such as silica are used as encapsulating materials for electronic components.Inorganic fillers have various physical properties such as expansion coefficient, thermal conductivity, moisture permeability, mechanical properties, 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 device malfunction has arisen, and this is due to the presence of tens to hundreds of silica-based fillers in the encapsulation materials used, especially silica-based fillers.
It is believed that this is caused by alpha rays emitted from trace amounts of radioactive elements such as uranium and thorium, which are contained in ppb units, and it is desired to further reduce the content of such impurities in silica. The present invention aims to meet such demands. [Prior Art] 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, a method of purifying an aqueous alkali silicate solution by treating it with an ion exchange resin (JP-A-60-42217, JP-A No. 60-42217, Showa 60
−42218, etc.) have been proposed. [Problems to be solved by the invention] Highly pure silica can be produced by these methods, but in the case of method (1), the average particle diameter of the obtained silica particles is on the order of mμ, and the specific surface area is small. is large, making it difficult to use as a filler in resin compositions for encapsulating electronic components. In addition, in method (2), since the purification operation is performed after diluting the SiO ₂ concentration of the aqueous alkali silicate solution to about 10% by weight or less, it is difficult to precipitate silica from the silica sol in terms of equipment efficiency. Since the operating conditions for separating and recovering from the mother liquor are complicated, there is a problem 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 the Problems] The present inventors have improved such problems in the conventional method, and have made a method using an alkali silicate aqueous solution as a raw material, which has an extremely low content of impurities, and
We have conducted extensive research to efficiently and economically produce high-purity silica with low hygroscopicity and a small specific surface area.
The aqueous alkali silicate solution is made into a fine fibrous gel in a coagulation bath, and the resulting fibrous gel is mixed into a liquid containing an acid.
Then, the impurities are extracted and removed by treatment with water, and the resulting silica is heat-treated at a temperature of 1000°C or higher, so that the content of impurities is extremely low, and
The present invention was completed by discovering that it is possible to obtain high-purity silica with low hygroscopicity and a small specific surface area. The present invention is an aqueous alkali silicate solution having a viscosity in the range of 2 to 500 poise (alkali silicate is
General formula: _M2O・_nSiO2 <where M is an alkali metal element, n is the number of moles of _SiO2 , and represents 0.5 to 5>
(represented by , directly extruded into a water-soluble organic medium from a spinning nozzle with a pore diameter of 1 mm or less to form a fine fibrous gel;
After treating the obtained fibrous gel with a solution containing an acid with an acid concentration of 30% by volume or less in the first stage of treatment,
The gist of the present invention is a method for producing high-purity silica, which is characterized 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. The embodiment of the present invention consists of the following three steps. ●Process-1: (Fiberization process) A highly viscous liquid with stringiness (hereinafter referred to as stock solution) is prepared from an aqueous alkali silicate solution, and this stock solution is coagulated in a coagulation bath using a fiberization device to form fine particles. It is made into a 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: (1) 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 with a hole diameter of 1 mm 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 poise. By combining features (1) and (2) above, it is surprisingly possible to obtain a fibrous gel with a hollow structure even when using a normal circular hole nozzle, and the coagulated part is a homogeneous high-density gel. Since it is obtained in a structure that maintains a swollen state and allows impurities to be easily extracted by acid and water, the above combination, together with the effect of feature (1) above, significantly improves the efficiency of extracting impurities from silica. I can do it. (3) When the obtained silica is heat-treated at a temperature of 1000°C or higher, the silanol groups present on the particle surface disappear, and the hygroscopicity of the silica can be significantly reduced. 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 material aqueous alkali silicate solution includes sodium salt, potassium salt,
Aqueous solutions such as lithium salts can be used. Hereinafter, the above three steps will be sequentially explained using an example in which a sodium silicate aqueous solution is used as the alkali silicate aqueous solution in the method of the present invention. [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 invention is 2 to
A range of 500 poise is preferred, and a range of 10 to 200 poise is particularly preferred. When a sodium silicate aqueous solution with a high SiO ₂ concentration and too high viscosity is used as a raw material, it is used after diluting it with water as appropriate. In the case of an aqueous sodium silicate solution containing approximately 30% SiO ₂ , the viscosity is low under normal conditions and the stringiness is insufficient, so sodium silicate is polymerized and used to impart stringiness to the solution. As a method for polymerizing sodium silicate,
Partial neutralization with acidic substances, dehydration and concentration methods, methods of adding polyvalent metal salts, 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 30 to 60°C, passed through an appropriate filtration device, and sent to a fiberizing 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 an aqueous sodium silicate solution solidifies while adhering to the nozzle surface, a strong bond is formed between the sodium silicate and the nozzle surface, and it is extremely difficult to separate this bond. 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. 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 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 it was made of precious metals such as gold and platinum,
Nozzles made of precious metal alloys such as gold-platinum alloys or tetrafluoroethylene (hereinafter referred to as TFE) resin, or whose nozzle surface is made of precious metals or
It has been found that the nozzle release properties of gelled alkali silicate are significantly improved when a product coated with TFE resin is used. The noble metals referred to in the present invention include gold, platinum, silver, and palladium, and can be coated on the nozzle surface by ordinary plating treatment. In addition to polytetrafluoroethylene (PTFE), the TFE resin used in the present invention refers to copolymers of TFE and hexafluoropropylene, copolymers of TFE and perfluoroalkyl vinyl ether, and copolymers of ethylene and TFE. copolymers, such as copolymers of ethylene and vinyl fluoride, copolymers of ethylene and vinylidene fluoride, and copolymers of ethylene and chlorotrifluoroethylene. The nozzle surface is coated with the TFE resin according to a conventional method, and if necessary, the 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. From the viewpoint of preventing adhesion to surfaces, the wet method is more advantageous than the dry method. 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 is
Either pick it up with a roller or put it on a belt conveyor and send it to the next step-2. The pore diameter of the nozzle used in this process is 0.05 to 1.0.
The range is preferably 0.1 to 0.3 mm. As the nozzle, a normal circular hole nozzle is used, but a modified cross-section hole nozzle or a hollow fiber spinning nozzle can 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,
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. The methods of 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. Various methods can be used, such as adding a liquid low-boiling substance to the fibers and heating the stock solution to form fibers, or 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, esters such as methyl acetate and ethyl acetate, ketones such as acetone and methyl ethyl ketone, and dimethylacetamide ( Examples include amides such as dimethylformamide (hereinafter referred to as DMAC), dimethylformamide (hereinafter referred to as DMF), and dimethyl sulfoxide. The coagulation rate of alkali silicate varies widely depending on the type of coagulant used, so it is difficult to determine the coagulation bath temperature unambiguously, but it is usually within 10 to
A temperature of around 60℃ is best. The fibrous gel can be picked up at a rate of 1 to 100 m/min with a roller type, and 0.1 to 100 m/min with a conveyor type.
It is normally operated at a speed of about 50 m. [Step-2: (Impurity extraction step)] The fibrous gel obtained in step-1 is treated with a liquid containing an acid in this step. The acids 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 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 . Generally, impurities are extracted using highly concentrated acids, but in the method of the present invention, the process
In order to maintain as much as possible the gel structure formed in step 1 from which impurities can easily escape, it is preferable that the acid concentration of the treatment liquid be 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 (100% of the treatment liquid
Acid contained per part by volume: means 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 efficiency of extracting impurities. Acid concentration 30% by volume in the first stage of treatment operation
If a treatment solution exceeding 20% is used, the structure of the silica produced by this treatment becomes too dense, making it difficult to extract the impurities remaining inside. Furthermore, if the acid concentration of the treatment solution 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 preferably in the range of 0.5 to 30% by volume, preferably in the range of 1 to 25% by volume, and more preferably in the range of 3 to 20% by volume. be. In the case of multi-stage processing, the acid concentration of the processing solution in the first stage must be 30% by volume or less, but there is no such restriction on the acid concentration of the processing solution in the second and subsequent stages, and it can be set arbitrarily. can be determined. The processing temperature in this process is not particularly limited, but it is 50℃
It is best to carry out the extraction operation at a temperature above. 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 temperature during pressurized extraction is preferably as high as possible, but in consideration of equipment corrosion due to acid and energy costs, the temperature is 100-150℃, preferably 110-150℃.
A range of 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 mm, including ^cm2 .
Approximately 10 mm is suitable. 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 impurity extraction operation, 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 30 minutes to 5 minutes in case of batch method.
In the case of a continuous type, the time is about 30 seconds to 30 minutes, preferably about 1 to 10 minutes. The silica fibers obtained by acid treatment are then washed with water at a desired temperature, and if necessary, combined with a filtration operation for deacidification and dehydration treatment. 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 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 the silica after acid treatment can be reduced to about 10 ppm or less for alkali metals, 3 ppm or less for chlorine, and about 3 ppb or less for uranium. [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. Adhering water easily evaporates when heated to 100°C or higher, whereas bound water is difficult to completely remove even at temperatures of 400°C or higher. In particular, silica obtained by a wet method has a large number of silanol groups (≡Si-OH) 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 moisture evaporates based on the dry weight of the silica, but the silanol groups remain as they are, and these absorb moisture from the atmosphere. Rehydration occurred.
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
After examining various treatment conditions, we found that by heat-treating the silica obtained in step-2 at a temperature of 1000°C or higher, the silanol groups surprisingly disappeared.
Moreover, it has been found that dense silica with a small specific surface area can be obtained. The present invention was completed based on this knowledge. The tendency of rehydration decreases as the heat treatment temperature increases, and when heat treatment is performed at a temperature of 1000° C. or higher, the tendency of rehydration is hardly observed. When high-purity silica is used as a filler for transparent quartz glass or resin compositions for encapsulating electronic components,
In particular, since the presence of moisture is a problem, 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℃ or higher, it cleaves through the countless microscopic cracks that exist in the fibers and changes to fine-grained silica, while the micropores collapse and form 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 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 heating temperatures of 1000°C or higher, practically 1100 to 1400°C.
It is recommended that the range be within the range of . 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 for practical purposes, air is preferable. The equipment for heat treatment is to heat silica to 1000°C.
It is sufficient if the temperature can be maintained at the above temperature, and in addition to tube furnaces, box furnaces, tunnel furnaces, etc., fluidized firing furnaces can be used, and heating methods include electric heat, combustion gas, etc. good. [Effects of the Invention] According to the method of the present invention, particles of high purity with extremely low impurity content including radioactive elements such as uranium, low hygroscopicity, and small specific surface area are produced using an aqueous alkali silicate solution as a raw material. Silica particles with a diameter of 1 to 100 μm can be obtained. 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] Hereinafter, the method of the present invention will be specifically explained using Examples and Comparative Examples. Example 1 Sodium silicate #3 (JIS K1408, No. 3 equivalent, the same applies hereinafter) (SiO ₂ : 28%, Na ₂ O: 9%,
U: 36 ppb) 3000 g was heated to 50° C. under reduced pressure and dehydrated and concentrated to obtain a stock solution for fiberization containing 32% SiO ₂ .
The viscosity of this stock solution was approximately 100 poise at 30°C, and the stringiness was also good. After filtering this stock solution, it was extruded using an extruder through a PTFE resin-coated nozzle with a hole diameter of 0.1 mmφ and 200 holes at a speed of 3 m/min into a coagulation bath (coagulant: DMAC) 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, and the fiber length was approximately 1
cm short fibers. 10g of the obtained short fibrous gel was immersed in a treatment solution - 5% by volume aqueous solution of sulfuric acid: 500CC, and heated to 100% with stirring.
C. for 1 hour, and then the treatment liquid was changed to a 10% by volume aqueous solution of sulfuric acid: 500 c.c., and a second stage treatment was carried out in the same manner. 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. The short fibrous silica due to the heat treatment was finely cleaved and turned into fine particulate silica, but in order to make the particle size distribution uniform, it was pulverized with an agate pulverizer to obtain silica particles. In addition, in this example and below, the acid used was a special grade reagent made by Hani Chemical, and the water used was made with an electrical conductivity of 1.0μS/cm.
(25°C) or below was used. Example 2 Sodium silicate #3 (same lot as Example 1)
While maintaining 5000 g at 30°C, finely powdered acidic sodium sulfate was slowly added little by little 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 30 poise was obtained. This stock solution was aerated and filled with air bubbles. This bubble-containing stock solution was extruded as it was from an extruder through a gold-platinum alloy nozzle with a hole diameter of 0.1 mmφ 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. Table 1 shows the impurity content and physical properties of the silica obtained in Examples 1 and 2 above. Analysis of CI, U, and Th was performed by activation analysis.

【table】

[Table] Example 3 Sodium silicate #3 (same lot as Example 1)
5000g was dehydrated and concentrated under reduced pressure using a vacuum pump while stirring in a kneader kept at 50℃.
SiO ₂ :31.8% was used to obtain a transparent stock solution. The viscosity of the stock solution was 50 poise at 30°C. This stock solution was extruded from an extruder into various coagulants through a PTFE resin-coated nozzle with a hole diameter of 0.1 mmφ and 50 holes to obtain a transparent fibrous gel. This fibrous gel was treated in the same manner as in Example 1, and the results shown in Table 2 were obtained.

[Table] Also, the physical properties of silica particles after heat treatment are; No. 3
-1 to 3 had a weight average particle diameter of 15 μm, a bulk density of 0.55 g/cm ² , and a water absorption rate of 0.0%. Example 4 and Comparative Example 1 (1) 10 g each of the short fibrous gel obtained by the same operation as in Example 1 was used as the first-stage treatment solution at acid concentrations of 0.5, 10, 20, and 30, respectively. , or for comparison 40 and 70% by volume aqueous sulfuric acid each
500 c.c., and then subjected to impurity extraction treatment according to Example 1. (Examples: 4-1 to 4,
Comparative Examples: 1-1 to 1-2) (2) Further, the same treatment as Comparative Example 1 was performed except that only the order of the acid concentrations of the treatment liquid in the acid treatment was changed. (Examples: 4-5 to 4-6) 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 silica particles after heat treatment are; No.
All of 4-1 to 6 had a weight average particle diameter of 15 μm, a bulk density of 0.55 g/cm ³ , and a water absorption rate of 0.0%.

[Table] Example 5 and Comparative Example 2 Using the stock solution prepared in Example 3, a short fibrous gel was obtained in the same manner as in Example 1. 10 g of each of these short fibrous gels were placed in 500 c.c. of nitric acid aqueous solutions with acid concentrations of 5, 10, and 20, respectively, and 40 and 60% by volume for comparison as the first-stage treatment solution.
After treatment at 100° C. for 1 hour while stirring, the treatment liquid was then changed to 500 c.c. each of a 10% by volume nitric acid aqueous solution, and a second stage treatment was carried out in the same manner. Silica particles were obtained by a method similar to that in Example 1 for subsequent treatments. The results obtained are shown in Table 4.

[Table] Also, the physical properties of silica particles after heat treatment are; No.5
-1 to 3 each had a weight average particle diameter of 15 μm, a bulk density of 0.55 g/cm ³ , and a water absorption rate of 0.0%. Example 6 and Comparative Example 3 Sodium silicate #3 (same lot as Example 1)
6000 g was dehydrated and concentrated under reduced pressure using a vacuum pump while stirring in a kneader maintained at 70°C 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 extracted from an extruder with pore sizes of 0.2, 0.5, 1.0, and 3.0 for comparison.
Gold-plated SUS6− with 50 holes each, mmφ
The gel was extruded into a coagulation bath through a 316 nozzle 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】

[Table] Example 8 and Comparative Example 5 The silica obtained in Example 1 after completion of impurity extraction and water washing was dried at 105° C. for 4 hours. Weigh out 10.00g each of the obtained dry silica, and add 400, 600, 800, 900, 1000 and
Heat treatment was performed at each temperature of 1200°C for 1 hour. After heat treatment, each sample was allowed to cool down to room temperature in a desiccator, the weight (W ₀ ) was measured, and then the sample was heated at 20°C.
The samples were left in a constant temperature and humidity 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 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. .

【table】

Claims (1)

[Claims] 1. An aqueous alkali silicate solution having a viscosity in the range of 2 to 500 poise (the alkali silicate has the general formula:
M ₂ O・nSiO ₂ <However, M is an alkali metal element,
n is the number of moles of SiO ₂ and is expressed as 0.5 to 5)
The material is made of precious metals, precious metal alloys, or tetrafluoroethylene resin, or the nozzle surface is coated with precious metals or tetrafluoroethylene resin, and the pore diameter is 1 mm or less. A fine fibrous gel is extruded directly from a spinning nozzle into a water-soluble organic medium, and the resulting fibrous gel is treated with a liquid containing an acid with an acid concentration of 30% by volume or less in the first stage of processing. After that, the impurities were extracted and removed by washing with water, and the obtained silica was heated at 1000℃.
A method for producing high-purity silica, characterized by heat treatment at a temperature above. 2. The method for producing high-purity silica according to claim 1, wherein the spinning nozzle has a hole diameter in the range of 0.05 to 1.0 mm. 3. The method for producing high-purity silica according to claim 1, wherein the diameter of the spinning nozzle is in the range of 0.1 to 0.3 mm. 4. The method for producing high-purity silica according to claim 1, wherein the fibrous gel contains air bubbles. 5 The acid concentration of the acid-containing solution used in the first step when treating fibrous gel with an acid-containing solution is 0.5 to 30.
The method for producing high-purity silica according to claim 1, wherein the silica content is in the range of % by volume. 6. The method for producing high-purity silica according to claim 1, wherein the acid treatment of the fibrous gel is carried out in at least two stages.