JP6141710B2 - Method for producing high purity synthetic silica powder - Google Patents

Description

  The present invention relates to a method for producing high-purity silica powder that is a raw material for high-purity quartz, and in particular, high-purity quartz used for electric melting, oxyhydrogen melting, vacuum melting, and the like, which are currently used for producing quartz glass. The present invention relates to a method for producing a high-purity silica powder as a raw material.

  Natural quartz powder, which is produced by digging natural veins and refining them by floating selection, is used in the production methods of natural quartz glass, such as electric melting, oxyhydrogen melting or vacuum melting. Synthetic quartz glass has been used as a raw material with a silica-containing liquid such as silicon tetrachloride.

  By the way, natural quartz powder is worried about depletion of high purity grade. In addition, there are problems of environmental destruction due to mining, silicosis of workers, and environmental pollution due to a large amount of hydrofluoric acid waste liquid. On the other hand, silicon tetrachloride has a problem that its growth rate is slow when it is made into quartz glass in a flame, and its cost is high.

  In order to solve the above problem, Nippon Kasei Co., Ltd. sells synthetic silica powder produced by hydrolysis of methoxysilane, but the price is very high and it is used only for some applications. Further, the problem of quality due to carbon contained in methoxysilane has not been solved.

It has also been proposed to produce synthetic silica or colloidal silica by passing water glass through an ion exchange resin (Patent Documents 1 to 6).
However, in these methods, gelation occurs during the initial cation exchange, the production efficiency is lowered, the yield is reduced, and the acid and peroxide are removed to remove titanium and boron contained in the raw material. A process of ion exchange again after adding hydrogen was necessary, and further an acid treatment with hydrochloric acid containing hydrogen peroxide was required thereafter.

Japanese Patent No. 4504491 JP 2001-233628 A JP 2002-173314 A JP 2002-173315 A JP 2002-173316 A JP 2002-173317 A

  An object of the present invention is to provide a method for producing a high-purity synthetic silica powder, which can produce a high-purity synthetic silica powder at a low cost.

The present invention relates to a method for producing a high purity synthetic silica powder including the following steps.
(1) A step of reacting fumed silica with an alkali metal hydroxide aqueous solution to produce an alkali metal silicate aqueous solution.
(2) A step of dealkalizing the obtained alkali metal silicate aqueous solution to obtain a silica aqueous solution having a pH in the range of 9-11.
(3) A step of subjecting the obtained aqueous silica solution to cation exchange treatment to adjust the pH of the aqueous solution to 2 to 3.
(4) A step of concentrating and gelling the obtained aqueous silica solution.
(5) A step of drying the obtained gelled product.
(6) A step of pulverizing the dried gelled product to obtain a pulverized product.
(7) A step of treating the pulverized product with an acid aqueous solution.
(8) A step of firing the pulverized material treated with the acid aqueous solution in a dry gas.

  According to the present invention, a high-purity synthetic silica powder is manufactured at low cost in order to prepare a water glass from a fumed silica composed of a high-purity silica component and further obtain a silica powder through an aqueous silica solution. can do.

Hereinafter, the present invention will be described step by step.
<First step>
This first step is a step of reacting fumed silica with an alkali metal hydroxide aqueous solution to produce an alkali metal silicate aqueous solution. This step is a step of obtaining water glass using so-called high-purity silica made of fumed silica.

  Here, fumed silica is amorphous silica produced by a flame hydrolysis method and is also called fumed silica. Such fumed silica can be produced without going through a liquid phase, for example, by hydrolyzing silicon tetrachloride at high temperature in an oxyhydrogen flame. As for fumed silica, the average primary particle size of 100 arbitrary particles measured with a transmission electron microscope is about 5 to 100 nm.

  In addition, the silicon tetrachloride which is a starting material used for the fumed silica of the present invention may be a by-product generated from the polysilicon manufacturing process. Purity does not matter. This is evaporated and hydrolyzed with a flame. In the normal fumed silica production process, hydrogen gas and oxygen gas are used to create a high-temperature flame to produce very fine fumed silica. In the present invention, weak natural gas or shale gas may be used. That is, it is not necessary to stick to the particle size of fumed silica.

Next, the fumed silica is dissolved in an alkali metal hydroxide aqueous solution, that is, a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution, or a lithium hydroxide aqueous solution to obtain an alkali metal silicate aqueous solution, that is, water glass. . The corresponding alkali metal silicate aqueous solution is a sodium silicate aqueous solution, a potassium silicate aqueous solution, or a lithium silicate aqueous solution. As an alkali metal hydroxide, sodium hydroxide is better because it is cheaper in terms of cost. When making an alkali metal silicate aqueous solution (water glass) by dissolution with an ordinary aqueous alkali metal salt solution, it is carried out in an autoclave, but the fumed silica used in the present invention has a reaction temperature of 60 to 100 ° C. at normal pressure. Dissolve. When the reaction temperature is less than 60 ° C., the reaction rate is slow, which is not practical. On the other hand, since the reaction is performed at normal pressure, the reaction rate does not exceed 100 ° C., but the reaction rate is sufficiently fast. At this time, the weight ratio of SiO ₂ : M (where M represents an alkali metal such as Na) is about 20 to 40%, preferably 25 to 35%. If the alkali is less than 20% by weight, the reaction for forming water glass is not sufficient. On the other hand, if it exceeds 40% by weight, the load of ion exchange in the subsequent process is large, which is disadvantageous in cost.

  Since the reaction is completed in 1 to 3 hours, for example, about 2 hours, it is diluted with pure water to 5 to 15% by weight after completion of the reaction. Preferably it is 7 to 10% by weight. When the amount is less than 5% by weight, it takes time to gel later, while when it exceeds 15% by weight, there is a phenomenon that gelation occurs quickly, which is not preferable. Preferably, the solution is then filtered through, for example, 1 μm filter paper. At this time, it goes without saying that filtration under reduced pressure is performed in a shorter time.

<Second step>
The second step is a step of dealkalizing the alkali metal silicate aqueous solution (water glass) obtained in the first step to obtain a colloidal silica aqueous solution (colloidal silica) by adjusting the pH to the range of 9-11. It is.
In this invention, in order to obtain a silica, if water glass is synthesize | combined from fumed silica and this water glass is passed through a cation exchange resin and dealkalized at a stretch, it will gelatinize. In the present invention, prior to this, the alkali metal silicate aqueous solution (water glass) obtained in the first step is dealkalized under conditions such that the resulting colloidal silica aqueous solution has a pH of 9-11. Is. By passing through the second step, the colloidal silica aqueous solution is not gelled during the real dealkalization treatment with the cation exchange resin in the third step.

The dealkalization treatment used here is not particularly limited, and for example, a cation exchange resin method, an electrophoresis method, an electrodialysis method, or the like can be used, preferably a batch type H-type cation exchange resin. It is. Alkali is removed by the dealkalization treatment here to obtain a silica aqueous solution, but dealkal treatment is carried out under such conditions that the pH of the system is 9-11.
That is, for example, an H-type cation exchange resin is added to the alkali metal silicate aqueous solution filtered as described above, for example, until the pH is in the range of 11 to 9. When the pH exceeds 11, a large amount of alkali remains, and when the pH is less than 9, gelation occurs. When this solution is passed through an ion exchange column in which a column is filled with an H-type cation exchange resin, gelation occurs in the column, and washing is necessary when the ion exchange resin is regenerated.
Thus, by this second step, the alkali metal silicate aqueous solution such as sodium silicate becomes a silica aqueous solution substantially by the dealkalization treatment.

<Third step>
In the third step, the obtained aqueous silica solution is subjected to cation exchange treatment and further subjected to dealkalization treatment so that the pH of the aqueous solution is adjusted to 2 to 3.
The dealkalization treatment uses a cation exchange resin, preferably an H-type cation exchange resin. The H-type cation exchange resin used here is not particularly limited, and commercially available H-type cation exchange resins such as strongly acidic beads, fibers, and cloths can be used.
The method of passing the aqueous silica solution to these H-type cation exchange resins is not limited in any way. For example, a method of filling the column with the H-type cation exchange resin and passing the solution, or the aqueous solution and the H-type cation exchange resin are not limited. Well-known methods, such as processing an ion exchange resin by a batch system, can be used. The used H-type cation exchange resin can be regenerated using an ordinary method, that is, an acid such as hydrochloric acid, sulfuric acid, or nitric acid. Preferably, the silica aqueous solution obtained in the third step is passed through a column packed with an H-type cation exchange resin.

Here, the pH of the silica aqueous solution is adjusted so that the pH becomes 2 to 3 depending on the amount of the cation exchange resin. The pH is unlikely to be less than 2, but there is no effect if the amount of cation exchange resin is too large. On the other hand, when pH exceeds 3, impurities, such as a metal, cannot be removed completely. In the present invention, as in Japanese Patent No. 4504491, after the treatment with the H-type cation exchange resin, it is not necessary to add an acid and hydrogen peroxide and perform ion exchange again. This is because in the present invention, fumed silica itself used as a raw material is a purified raw material, and thus the fumed silica does not contain impurities such as titanium and boron.
The colloidal silica aqueous solution treated with the cation exchange resin thus obtained is a stable sol at a pH of 2 to 3.

<4th process>
In the fourth step, the colloidal silica aqueous solution thus obtained is concentrated and gelled. In this case, examples of the concentration method include a method using heating, a deep cooling method, an ultrafiltration membrane method, and the like, and the ultrafiltration membrane method is preferable. The pore diameter in the ultrafiltration membrane (Ultrafiltration Membrane) is about 0.01 to 0.001 μm, which is larger than the reverse osmosis membrane (RO membrane, NF membrane) and smaller than the microfiltration membrane (MF membrane). Although it is not impossible for ultrafiltration membranes to pass through the entire liquid in the same way as ordinary filters, the pores are small and the membranes are blocked in a short time due to blocked fine particles and impurities. For this reason, normally, the stock solution continues to flow in a certain direction along the surface of the membrane, and the water (concentrated water) in which fine particles and impurities are concentrated is discharged continuously or used while returning to the liquid feed side. The adhesion of impurities to the film surface is reduced. The flow along the membrane surface is called a cross flow, and the method of using a filtration membrane using the flow is called a cross flow method.

  The structure of the ultrafiltration membrane is a hollow fiber membrane (formed into a hollow thread with a diameter of about 0.5 to 30 mm in diameter, and usually filtered from the outside to the inside of the thread (the reverse type is also available)) , Spiral Membrane [A single filtration membrane is superposed on a mesh (support) to maintain strength, folded in two and bonded into a bag shape, and then the bag mouth is opened with a water collecting tube that has a cut in a straight line. It is sandwiched and rolled into a Date roll with this as the core. Pressurized from the cross-sectional direction of Date winding, obtain concentrated water from the opposite cross-section and permeate from the water collecting pipe], tubular membrane [hollow cylindrical shape, thicker than hollow fiber membrane. The diameter is up to about 10 cm. Although the flow rate of concentrated water can be increased and it is easy to prevent the impurities from adhering to the film surface, this increases the energy cost and the installation area. ], Flat membrane [flat membrane. In order to use it in the cross flow method, it is necessary to make it flow in the water tank or container at a high flow rate, and the energy cost is high, but there is an advantage that the membrane can be easily cleaned. In the present invention, any structure can be adopted.

  As described above, the colloidal silica aqueous solution treated with the cation exchange resin is a stable sol at a pH of 2 to 3. This is concentrated by removing water by passing through an ultrafiltration membrane, for example. The silica concentration after passing is 10 to 20% by weight. At this concentration, the silica sol gels. This method is very energy efficient compared to the method of concentrating by heating or the method of freezing and thawing.

<5th process>
The fifth step is a step of drying the obtained gelled product.
In the fifth step, the silica particles are dried prior to the next pulverization. Although a drying method is not specifically limited, For example, it can be made to dry at the temperature of 40-200 degreeC.
That is, the gel solution obtained in the fourth step naturally ripens and changes to silica monolysis having sufficient strength. Prior to grinding, this is heated and dried. The drying means is not particularly limited, and the gelled product may be heated and dried at a temperature of, for example, 40 to 200 ° C., preferably 80 to 120 ° C. by a heating means such as a hot air drying furnace, high frequency heating, or an infrared lamp. The heating time is usually about 1 to 2 hours. If the heating temperature is less than 40 ° C., the water evaporation rate is slow, while if it exceeds 200 ° C., bubbles are generated due to rapid water evaporation.
Thus, the gelled product obtained in the fourth step becomes amorphous silica after heat drying, and the water content is usually about 10 to 50% by weight, preferably about 15 to 30% by weight.

<6th process>
In the sixth step, the dried gelled product is pulverized to obtain a pulverized product.
In the sixth step, prior to the treatment with the acid solution in the next seventh step, the silica particles are pulverized to form fine particles to improve the cleaning effect. The pulverization method is not particularly limited, and a method usually used for pulverization of silica particles can be used.
In this step, the dried gelled product is pulverized by a method without known contamination and sieved to the desired particle size. For example, when manufacturing a quartz crucible, it is about 0.3 to 0.1 mm. When used for oxyhydrogen or vacuum melting, it is about 0.05 to 0.2 mm. For an EMC filler, It is better to be as fine as possible. For general applications, the particle size is preferably 0.1 to 0.4 mm. Since the particles shrink in the subsequent firing step, it is preferable to set the particle in this range. The particle size after firing is 0.07 to 0.30 mm. Here, the particle size is a particle size defined as a mesh opening when sieving.
Examples of the pulverizing means include a roll mill, a ball mill, a disk mill, and a jet mill. The pulverizing medium is preferably made of quartz glass because of contamination.
The particle size at the time of pulverization should be changed according to the application.

<Seventh step>
In the seventh step, the pulverized product (synthetic silica powder) is treated with an acid aqueous solution.
The seventh step of the present invention is to remove the impurities adhering to the silica by washing the silica obtained in the sixth step. The washing can be carried out by a method usually performed in water washing or the like. However, since iron may be mixed during the pulverization of the silica particles, washing with an acid aqueous solution is preferable. In this case, it is preferable to rinse with water (preferably ultrapure water) after washing with an acid aqueous solution.

Although it does not specifically limit as aqueous solution of an acid, For example, hydrochloric acid, a sulfuric acid, nitric acid etc. should just be used, and these may be used individually or in combination.
That is, in this step, the synthetic silica powder is treated with an acid aqueous solution such as a hydrochloric acid aqueous solution having a concentration of 2 to 7% by weight. At this time, of course, it is not necessary to add hydrogen peroxide. The concentration of acid such as hydrochloric acid is preferably 4 to 6% by weight. When the concentration is higher than 7% by weight, the impurities are hardly dissolved. On the other hand, when the concentration is lower than 2% by weight, the impurities may not be sufficiently dissolved. This acid treatment is preferably performed at 60 ° C. to 90 ° C. and for a time of preferably 10 minutes to 7 hours. Synthetic silica particles still have fine open cells from which impurities are dissolved. Impurities are metal components such as aluminum, iron, sodium, potassium, calcium, titanium and magnesium, and these impurities can be removed on the order of ppb by this acid treatment.
The treatment in this step is preferably extraction rather than dissolution. After acid treatment such as hydrochloric acid, acid content such as hydrochloric acid may be extracted in the same manner with pure water.
Here, dehydration is carried out by measuring the absence of acid components such as hydrochloric acid with a pH meter or a conductivity meter. The dehydration may be performed by a centrifugal dehydrator, but the vacuum dehydrator has a higher dehydration efficiency. The moisture content of the silica powder after dehydration is about 10 to 20% by weight.

<Eighth process>
In the eighth step, the pulverized product (synthetic silica powder) after the seventh step is fired in a dry gas.
In the eighth step, the silica obtained in the seventh step is baked to obtain high-purity quartz powder having an extremely low OH group content.
Here, the baking temperature and time may be the same temperature and time as those used in the conventional baking for obtaining high-purity quartz. High-purity quartz preferably has as little OH group content as possible, and if it is fired at a higher temperature for a longer time, quartz with a lower OH group content can be obtained. Conditions may be set as appropriate.

Examples of the dry gas include a gas produced by a deep cooling method such as dry nitrogen gas, dry helium gas, and dry air, and a gas that is cooled to remove moisture.
The firing temperature is 1,100 to 1,300 ° C., preferably 1,200 to 1,250 ° C., and the firing time is usually 10 to 30 hours, preferably 15 to 20 hours. When the firing temperature is less than 1,100 ° C., moisture is slowly released and is not efficient. On the other hand, firing at a temperature exceeding 1,300 ° C. causes the particles to sinter and necessitates pulverization after firing. Further, when the firing time is less than 10 hours, the moisture is not sufficiently detached, and even when it exceeds 30 hours, the OH group content does not decrease proportionally.
As a specific example of firing, the synthetic silica powder dehydrated in the seventh step is placed in a firing furnace and fired by flowing dry nitrogen gas. This firing temperature is 1,200 to 1,250 ° C. for 20 hours or more to sufficiently remove moisture.

  Thus, high purity synthetic silica powder can be obtained at low cost. The obtained high-purity synthetic silica powder has a small amount of other metal elements as impurities in the ppb order in the measurement with the ICP emission spectroscopic analyzer, and the OH group content is 50 ppm or less in the measurement with the FT-IR analyzer, The silica component is extremely pure.

  Hereinafter, although an Example is given and demonstrated regarding the manufacturing method of the highly purified synthetic silica powder of this invention, this Example only showed a part of this invention.

Example 1
Silicon tetrachloride, a by-product in the polysilicon production, was evaporated and passed through a burner flame. The flame at this time is natural gas and oxygen. Silicon tetrachloride used nitrogen gas as a carrier. The flow rate of nitrogen gas was 0.4 m ³ / H, and the total amount of nitrogen gas and silicon tetrachloride was 0.7 m ³ / H. The natural gas flow rate was 6 m ³ / H, and the oxygen gas flow rate was 3 m ³ / H. As a result, fine fumed silica was produced and collected by a cyclone made of quartz glass. The yield was 700 g / H.

  Next, in a reactor in which PP (polypropylene) coating is applied to the inner surface of SUS304 with a heating device, 500 g of the fumed silica obtained above, 10 liters of pure water, and 200 g of sodium hydroxide are placed and heated to 80 ° C., Stir. After 2 hours, heating and stirring were stopped, the mixture was cooled to room temperature, and filtered under reduced pressure using filter paper made of polytetrafluoroethylene (DuPont Teflon) having an opening of 1 μm.

  While stirring, H + type cation exchange resin (IR120B manufactured by Organo) was slowly added to the solution. The addition of H + type cation exchange resin was stopped when the pH dropped below 11. Thereafter, the mixture was slowly stirred for 30 minutes and placed in a centrifugal dehydrator to recover the H + type cation exchange resin, and the filtrate was transferred to a PP tank. The pH was 10.8.

  This solution was passed through a column packed with 150 liters of H + type cation exchange resin (IR120B manufactured by Organo). Finally, the solution was recovered under reduced pressure. The weight of this solution was 10.2 kg. The pH was 2.3.

  This solution was passed through an ultrafiltration membrane (Torefil manufactured by Toray Industries, Inc.) to remove water and concentrated. At this time, the silica concentration in the solution was 10.3% by weight. The pH of this solution was unchanged from 2.4. This was heated to 100 ° C. for 4 hours to be gelled and dried by heating (moisture content of 50% by weight).

  The hardened gel was pulverized with a quartz glass roll mill and sieved with a PP mesh to obtain silica powder having a silica gel particle size of 0.1 to 0.4 mm. The yield was 75%. In addition, the particle | grains less than 0.1 mm can be used as the raw material for EMC fillers (transparent sealing resin).

Next, a 5% by weight hydrochloric acid solution was added to silica gel and heated to 80 ° C. without stirring. After 2 hours, this was filtered through a PP mesh. The reason for using hydrochloric acid here is that it can be used repeatedly for the regeneration of the H + type cation exchange resin. Of course, sulfuric acid may be used. After 2 hours, filtration was performed, pure water was added, and the mixture was heated again at 80 ° C. for 2 hours. This was repeated twice.
This silica powder was heated at 120 ° C. for 2 hours in order to remove moisture and dried, and further heated at 350 ° C. for 2 hours in order to completely remove moisture and evaporate hydrochloric acid.

Next, this was put in an electric furnace and heated up to 700 ° C. over 1 hour, held for 2 hours, then heated up to 1,250 ° C. in dry nitrogen over 2 hours and held there for 10 hours. The silica powder was cooled to room temperature and sieved through PP and 50 meshes. The obtained synthetic silica powder had an average particle size (measured with a laser particle size distribution meter) of 0.19 mm.
Table 1 shows the results of measuring the purity of the silica powder with an ICP emission spectroscopic analyzer (ICPS-7510 manufactured by Shimadzu Corporation).
The OH group content was 38 ppm as a result of measurement with an FT-IR analyzer (4100 manufactured by JASCO Corporation).

Comparative Example 1
No. 3 water glass manufactured by Fuji Chemical Co., Ltd. was diluted with pure water to a SiO ₂ concentration of 6.5% by weight to prepare 1 liter. This was passed through a column containing 650 cc of H + type cation exchange resin (IR120B manufactured by Organo) to obtain a colloidal silica sol having a pH of 2.75. At this time, gelation occurred in the column, and considerable work was required to regenerate the ion exchange resin. Next, 2.3 g of 60% nitric acid and 2.5 g of 30% hydrogen peroxide were added and stirred. This was again passed through a column of 100 cc H + type cation exchange resin (same as above). The pH was 1.75. Aqueous ammonia solution was added thereto for gelation at pH 4.5.

This was put in an oven and kept at 50 ° C. for 2 hours, then kept in a freezer at −30 ° C. for 5 hours and filtered to obtain silica powder. The yield at this time was 30 g. To this, 200 g of 5% hydrochloric acid and 30% hydrogen peroxide water were added to 1%, and the mixture was heated at 80 ° C. for 3 hours. This was repeated twice. Then, it was performed twice at 70 ° C. for 3 hours with pure water. This was kept at 1,200 ° C. for 20 hours and sieved with a PP mesh. The average particle diameter of the obtained silica powder was 0.23 mm. Table 2 shows the results of measuring the purity of the silica powder in the same manner as in Example 1.
Further, the OH group content was measured in the same manner as in Example 1, and as a result, it was 60 ppm.

  As apparent from the above, in the production method of the present invention, for example, fumed silica obtained by flame hydrolysis of silicon tetrachloride is reacted with an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution at 60 to 100 ° C. Producing potassium silicate and mixing it with a batch cation exchange resin to bring the pH in the range 9-11, then passing through a column packed with the cation exchange resin, the pH in the range 2-3. The step of concentrating and gelling this through an ultrafiltration membrane, the step of drying at 80 to 120 ° C., the step of pulverizing this to a particle size of 0.1 to 0.4 mm, 2 A step of immersing in an aqueous solution of ˜7 wt% hydrochloric acid to extract impurities at 60 to 90 ° C., and baking this in dry nitrogen gas at 1,200 to 1,250 ° C.

  Thereby, the gelation problem of the efficiency fall in manufacture is solved in the ion exchange method from water glass. In addition, since fumed silica with high purity is originally used as a raw material, removal and purification of titanium or the like using hydrogen peroxide water becomes unnecessary. This shows that the purity of the produced silica powder is stable and high.

Since the production method of the present invention is carried out only with an alkaline aqueous solution and an acid such as hydrochloric acid, the salt produced by the neutralization reaction can be regenerated and used by utilizing a diaphragm. For example, when fumed silica and sodium hydroxide are reacted and passed through an ion exchange column, sodium adheres to the ion exchange resin. When this is regenerated, sodium chloride is produced when the aqueous hydrochloric acid solution used in the acid treatment is used. When a voltage is applied to this aqueous solution by the diaphragm method, sodium hydroxide and hydrochloric acid are separated. That is, the process of the present invention is a sealed process, and the environmental load is very low.
The silica powder obtained by the production method of the present invention is highly pure and inexpensive.

  The high-purity synthetic silica powder obtained by the present invention is useful for applications such as high-purity quartz products such as quartz crucibles and diffusion parts, and EMC fillers.

Claims (10)

The manufacturing method of the high purity synthetic silica powder for quartz glass including the following process.
(1) A first step in which fumed silica is reacted with an alkali metal hydroxide aqueous solution to produce an alkali metal silicate aqueous solution.
(2) A second step in which the obtained alkali metal silicate aqueous solution is dealkalized to obtain a silica aqueous solution with a pH in the range of 9 to 11. (3) The resulting silica aqueous solution is subjected to cation exchange treatment, 3rd process which makes pH of this aqueous solution 2-3.
(4) A fourth step of concentrating and gelling the obtained aqueous silica solution.
(5) A fifth step of drying the obtained gelled product.
(6) A sixth step of obtaining a pulverized product by pulverizing the dried gelled product.
(7) A seventh step of treating the pulverized product with an acid aqueous solution.
(8) The 8th process of baking the pulverized material processed with acid aqueous solution at 1,100-1,300 degreeC in dry gas. The method for producing high-purity synthetic silica powder for quartz glass according to claim 1, wherein the fumed silica is obtained by flame hydrolysis of silicon tetrachloride. The alkali metal hydroxide salt is sodium hydroxide, potassium hydroxide, or lithium hydroxide, the alkali metal silicate salt is sodium silicate, potassium silicate, or lithium silicate, and fumed silica and hydroxide. The manufacturing method of the high purity synthetic silica powder for quartz glass of Claim 1 or 2 whose reaction temperature with alkali metal salt aqueous solution is 60-100 degreeC. The method for producing high-purity synthetic silica powder for quartz glass according to any one of claims 1 to 3, wherein the dealkalization treatment is performed using a batch cation resin to adjust the pH to 9 to 11. The method for producing high-purity synthetic silica powder for quartz glass according to any one of claims 1 to 4, wherein the cation exchange treatment in the third step is performed by passing a silica aqueous solution through a column packed with an H-type cation exchange resin. The method for producing high-purity synthetic silica powder for quartz glass according to any one of claims 1 to 5, wherein the means for concentrating the silica aqueous solution is an ultrafiltration membrane. The method for producing a pure synthetic silica powder for high quartz glass according to any one of claims 1 to 6, wherein the means for drying the gelated product is an overheating method such as an oven, and the drying temperature is 40 to 200 ° C. The high-purity synthetic silica powder for quartz glass according to any one of claims 1 to 7, wherein the pulverizing means for the gelled product is a roll mill, a ball mill, or a jet mill, and the particle size of the pulverized product is 0.1 to 0.4 mm. Manufacturing method. The manufacturing method of the high purity synthetic silica powder for quartz glass in any one of Claims 1-8 whose acid aqueous solution is hydrochloric acid aqueous solution, sulfuric acid aqueous solution, or nitric acid aqueous solution. The production of high-purity synthetic silica powder for quartz glass according to any one of claims 1 to 9, wherein the dry gas is dry nitrogen gas, dry helium gas, and dry air, and the firing temperature is 1,100 to 1,250 ° C. Method.