JPH0535086B2 - - 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 containing extremely low content of radioactive impurities such as uranium in addition to alkali metals and chlorine from an aqueous alkali silicate solution. High-purity silica is used for fillers, dispersants, etc.
In addition to being used as a raw material for transparent quartz glass and special ceramics, it is also expected to be used as a raw material for fillers in 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 No. 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. [Means for Solving the Problems] The present inventors have improved the above-mentioned problems in conventional methods, and have developed an efficient method for producing high-purity silica with extremely low impurity content using an aqueous alkali silicate solution as a raw material. In order to produce it efficiently and economically, we conducted intensive research to form a fine fibrous gel from an aqueous alkali silicate solution in a coagulation bath, and treated the resulting fibrous gel with an acid-containing solution and then with water to remove impurities. The present invention was completed based on the discovery that high purity silica can be obtained by extraction and removal. 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: M ₂ O・nSiO ₂ (where M is an alkali metal element, n is the number of moles of SiO ₂ and is 0.5 to 5)) is made of noble metals, noble metal alloys, or Extruded directly into a water-soluble organic medium from a spinning nozzle with a pore diameter of 1 mm or less, either made of tetrafluoroethylene resin or whose nozzle surface is coated with precious metals or tetrafluoroethylene resin. The resulting fibrous gel is heated to an acid concentration of 30% in the first stage of processing.
The gist of the present invention is a method for producing high-purity silica, which is characterized in that it is treated with a solution containing acid in a volume percent or less, and then washed with water to extract and remove impurities. The present invention will be explained below. An embodiment of the present invention consists of the following two 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. 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, 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 can be easily extracted by acid and water, so the above combination, together with the effect of feature (1) above, can significantly improve the efficiency of extracting impurities from silica. . 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 two steps will be sequentially explained, taking as an example a case where a sodium silicate aqueous solution is used as the alkali silicate aqueous solution in the method of the present invention. [Step-1: (Fibre-forming process)] The raw material sodium silicate aqueous solution 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. If a sodium silicate aqueous solution with a high SiO ₂ concentration and too high viscosity is used as a raw material, it should be diluted with water as appropriate before use. In the case of an aqueous sodium silicate solution containing about 30% SiO ₂ , the viscosity is low under normal conditions and the stringiness is insufficient, so sodium silicate is polymerized and used in order to impart stringiness to the solution. As a method 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 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 pulled off with a roller or placed on a belt conveyor and sent 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 may be 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,
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. Various methods can be used, such as adding a liquid low-boiling substance to the fiber 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. 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 in this process can be carried out in one step, but in order to extract and remove particularly trace amounts of impurities, the treatment should be divided into at least two steps, and the treatment solution used in each step should be 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. . 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, but within that range,
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 the variation in the impurity extraction composition. 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 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. 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 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. [Effects of the Invention] According to the method of the present invention, high purity silica having an extremely low content of impurities including radioactive elements such as uranium can be obtained using an aqueous alkali silicate solution as a raw material. The resulting silica has a higher purity than that produced by conventional techniques, so it can be used as fillers and dispersants, as well as raw materials for transparent quartz glass and special ceramics, as well as resin compositions for encapsulating electronic components. It is also expected to be used as a raw material for fillers. 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, equivalent to No. 3, hereinafter the same) (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. 10 g of the obtained short fibrous gel was immersed in a treatment solution - 5% by volume aqueous solution of sulfuric acid: 500 c.c., and heated to 100°C with stirring.
The treatment solution was treated with sulfuric acid 10% by volume.
Aqueous solution: 500 c.c. was used, and the second stage treatment was carried out in the same manner. The short fibrous silica thus obtained is washed with boiling water, filtered, deoxidized and dehydrated, dried at 150°C, and then crushed with an agate crusher to make the particle size distribution uniform. Obtained. In addition, in this example, the acid used was a special grade reagent manufactured by Hani Chemical, and the water used had an electrical conductivity of 1.0 μS/cm (25°C).
The following ion-exchanged water 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 still 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 in the silica obtained in Examples 1 and 2 above. Analysis of Cl, U and Th was carried out by activation analysis.

[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 through a PTFE resin-coated nozzle with a hole diameter of 0.1 mm and 50 holes into various coagulants to obtain transparent fibrous gels. This fibrous gel was treated in the same manner as in Example 1, and the results shown in Table 2 were obtained.

[Table] Example 4 and Comparative Example 1 (1) 10 g each of the short fibrous gel obtained by the same procedure as in Example 1 was used as the first-stage treatment solution at acid concentrations of 0.5, 10, and 10, respectively. 20 and 30, and for comparison 40 and 70% by volume sulfuric acid aqueous solution each 500
cc and then subjected to impurity extraction treatment according to Example 1. (Examples: 4-1 to 4, Comparative Examples 1-1 to 2) (2) In addition, compared to Comparative Example 1, only the order of the acid concentration of the treatment liquid in the acid treatment was changed, and the other treatments were the same. Ivy. (Examples: 4-5 to 6) The impurity content in the obtained silica is shown in Table 3 together with the results of Example 1.

[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 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, respectively, as the first treatment solution,
After treatment at 100° C. for 1 hour with 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. The subsequent processing is as in Example 1.
Silica particles were obtained by a method similar to . Table 4 shows the impurity content in silica.

[Table] 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. These stock solutions were made into fibers according to the procedure of Example 1. The status of each is shown in Table-5. In addition, the obtained fibrous gel was treated in the same manner as in Example 1, and the impurity extraction results obtained are shown in Table 5, representing the Na content in the silica. 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, respectively.
Gold-plated SUS-316 with 50 holes each, mmφ
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 silica.

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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 (where M is an alkali metal element,
n is the number of moles of SiO ₂ , 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. A method for producing high-purity silica, which comprises washing with water to extract and remove impurities. 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.