Method for producing high-purity silica particle dispersion, high-purity silica particle dispersion, and method for producing purified acidic silicic acid solution

Abstract

To provide a method for producing a high-purity silica fine particle dispersion capable of producing a high-purity silica fine particle dispersion.SOLUTION: A method for producing a high-purity silica fine particle dispersion, comprising the following steps 1 to 4: (Step 1) A step of subjecting an aqueous sodium silicate solution to a cation exchange treatment to obtain a silicate solution having a predetermined pH. (Step 2) A step of adding an inorganic acid and hydrogen peroxide to the silicic acid solution obtained in the step 1, mixing them, and lowering the pH thereof to obtain an acidic silicic acid solution. (Step 3) A step of subjecting the acidic silicic acid solution obtained in the step 2 to a cation exchange treatment to obtain a purified acidic silicic acid solution. (Step 4) a step of adding an aqueous alkali hydroxide solution to a part of the purified acidic silicic acid solution obtained in the step 3, adjusting the pH, and then heating and maintaining at a predetermined temperature, and further adding another part of the purified acidic silicic acid solution obtained in the step 3, and simultaneously adding a pH adjusting agent, thereby obtaining a high-purity silica fine particle dispersion.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a silica fine particle dispersion (silica sol), particularly a high-purity silica fine particle dispersion (high-purity silica sol). [Background technology]

[0002] Conventionally, silica sols substantially free of metal impurities have been proposed, and such high-purity silica sols can be preferably used as polishing agents for electronic materials such as semiconductor silicon wafers. If metal impurities are contained in the silica sol, the metal impurities will diffuse into the wafer during polishing, degrading the wafer quality. This will significantly reduce the performance of semiconductor devices formed using such wafers. As a method for producing high-purity silica sol, a method of synthesizing silica sol using purified silicon alkoxide as a raw material is widely known as a means of minimizing impurities. However, in this production method, the raw material silicon alkoxide (alkoxysilane) is expensive, so the use of silica sol obtained by such a production method has been limited to, for example, high-value-added applications or special applications (see Patent Document 1). Another known method for producing high-purity silica sol is one that involves synthesizing silica gel through a wet reaction between an alkali silicate and a mineral acid, followed by the addition of a quaternary ammonium hydroxide aqueous solution. However, this method has problems with production efficiency and economy, as the silica gel preparation process requires precipitation of the silica gel in an acidic region containing a chelating agent and hydrogen peroxide at an acid concentration of 1N or higher, followed by separation and washing (see Patent Document 2).

[0003] The method for producing high-purity silica sol in Patent Document 3 includes a step of acid-treating the silicic acid solution, specifically adjusting the pH to 0 to 2.5, to remove ionic impurities present in the siloxane skeleton that constitutes the silicic acid in the silicic acid solution. For example, in Example 1, 500 g of 35% hydrochloric acid is used for this acid treatment, which is a 5% silicic acid solution (estimated to be at least 4,000 g based on the amount of raw material input) obtained by cation-exchanging a diluted solution of No. 3 sodium silicate water. Preparation using an acidic silicic acid solution containing such a large amount of acid as a raw material impairs the stability of the silica sol, causing gelation and aggregation and precipitation, and therefore typically requires a washing or anion-exchange treatment. The method in Patent Document 3 also discloses the removal and purification of hydrochloric acid through treatments such as solvent substitution. Such a large amount of acid removal process is disadvantageous in terms of labor, production time, and economics when producing high-purity silica sol.

[0004] In the method for producing high-purity, large-particle silica sol in Patent Document 4, an acid is added to an aqueous colloidal solution of activated silicic acid to adjust the pH to 0-2.0, followed by aging, cation exchange, anion exchange, and then cation exchange again to prepare a high-purity activated silicic acid solution, which is then used as a raw material to produce a high-purity, large-particle silica sol. For example, in Example 1, this method uses 20.2 g of nitric acid (concentration 61.3% by mass) in 5950 g of an aqueous colloidal solution of activated silicic acid, followed by 48 hours of aging, cation exchange, anion exchange, and cation exchange. This method has the same problems as Patent Document 3. In particular, the method in Patent Document 4 requires 48 hours of aging of the activated silicic acid solution, which results in very poor production efficiency and economical issues.

[0005] Patent Document 5 describes a method for producing a high-purity aqueous silica sol, in which aqueous glass is subjected to cation exchange to obtain an acidic silicic acid solution, and a strong acid is then added to the solution, which is then aged at pH 0 to 2.0, 0 to 100°C for 0.5 to 120 hours, followed by anion exchange to obtain a high-purity silica sol. This method requires aging treatment at 0 to 100°C for 0.5 to 120 hours, and the long aging time results in poor productivity, so improvements have been required for practical use.

[0006] In the method for producing high-purity silica sol described in Patent Document 6, cation exchange is carried out by adding a salt of a strong acid to aqueous glass, and the salt of the strong acid is at least one of aminotris(methylenephosphonic acid), 1-hydroxyethane-1,1-diphosphonic acid, or a salt thereof. Silica sols containing such organic acids require time and cost to treat the wastewater generated during production, and silica sols containing organic acids have problems in that their applications are limited. In particular, for polishing applications, chemical components are added to silica sols to adjust the polishing performance, but it has been pointed out that organic acids may deteriorate the polishing performance.

[0007] The method for producing silica sol described in Patent Document 7 involves mixing a water-soluble alkali metal silicate or an aqueous alkali metal silicate solution with an acidifying agent to produce an acidic alkali metal silicate aqueous solution with a pH of less than 2, followed by specific anion exchange and cation exchange treatments to prepare a silicic acid solution, and using this silicic acid solution as a raw material to obtain a stable silica sol. In this method, when sodium silicate is converted into a silicic acid solution and then the anion exchange treatment is performed, the pH of the silicic acid solution rises, which causes polymerization of silicic acid to proceed, resulting in the instability of the silicic acid solution.

[0008] The method for producing high-purity silica sol described in Patent Document 8 involves adding and mixing oxalic acid and a strong inorganic acid to a silicic acid solution, and then performing anion exchange and cation exchange to prepare high-purity silica sol. This method requires oxalic acid, which is a deleterious substance and therefore requires careful handling. Furthermore, silica sol containing oxalic acid has limited uses, and while chemical components are added to adjust the polishing performance, particularly for polishing applications, it has been pointed out that oxalic acid may deteriorate the polishing performance.

[0009] Patent Document 9 describes a method for producing a high-purity aqueous silica sol, which involves dissolving a strong acid and an alkali metal silicate in water under specified conditions, cation-exchanging the aqueous silica sol, followed by anion-exchanging the aqueous silica sol, and then adding potassium hydroxide or sodium hydroxide to prepare an aqueous silica sol. The aqueous silica sol is then cation-exchanged and adjusted to a pH of 8 to 10.5 by adding ammonia, resulting in a stable, high-purity aqueous silica sol. In Example 1 of the same document, in addition to the above procedures, cation-exchanging is performed again after anion-exchanging. This method requires many ion-exchange treatments, resulting in poor production efficiency. Furthermore, anion-exchanging the acidic silicic acid solution removes anions necessary for stabilizing the acidic silicic acid solution, raising the pH of the silicic acid solution. This significantly reduces the stability of the silicic acid solution, making it prone to gelation.

[0010] The method for producing high-purity large-particle silica sol in Patent Document 4 has the problems seen in the method in Patent Document 3, and in addition, as mentioned above, requires a 48-hour aging treatment, which can be said to be a further disadvantage in terms of production. Patent Document 5 also has the problem that it requires a long time for aging (holding step).

[0011] The method for producing a high-purity silica sol described in Patent Document 6 generates wastewater containing organic acids during production, which is problematic in that it requires time and effort to treat the wastewater, and also has a problem in that it affects polishing performance.

[0012] The method for producing silica sol described in Patent Document 7 requires an acidifying agent as described above, which poses a safety problem. The method for producing high-purity silica sol described in Patent Document 8 requires oxalic acid, which is a deleterious substance, and has problems in terms of handling and the effect on polishing performance.

[0013] The method for producing a high-purity aqueous silica sol described in Patent Document 9 requires many operations and treatments, including the ion exchange operation, as described above, and improvements are needed for practical use. Because anions are removed from the silicic acid solution, the pH of the acidic silicic acid solution after anion exchange is about 4.0, and such acidic silicic acid solutions are very unstable and gelate in an extremely short time. Therefore, such methods for producing silica sols that require anion exchange have the problem of difficulty in stable production. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Japanese Patent Application Publication No. 01-278413 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-247625 [Patent Document 3] Japanese Patent Application Publication No. 158810 / 1983 [Patent Document 4] Japanese Patent Application Publication No. 63-285112 [Patent Document 5] Japanese Patent Application Publication No. 04-002606 [Patent Document 6] Special Publication No. 2003-514742 [Patent Document 7] Special Publication No. 2014-511330 [Patent Document 8] Japanese Patent Application Publication No. 04-231319 [Patent Document 9] Japanese Patent Application Publication No. 06-016414 Summary of the Invention [Problem to be solved by the invention]

[0015] An object of the present invention is to provide a method for producing a high-purity silica fine particle dispersion, which is capable of producing a high-purity silica fine particle dispersion. [Means for solving the problem]

[0016] According to one aspect of the present invention, there is provided a method for producing a high-purity silica fine particle dispersion, comprising the following steps 1 to 4: (Process 1) A process in which a sodium silicate aqueous solution (silica concentration in the range of 2.0 mass % to 7.0 mass %) is subjected to a cation exchange treatment to obtain a silicic acid solution in the range of pH 2.0 to 3.0. (Process 2) A process in which inorganic acid and hydrogen peroxide (however, the mass ratio of hydrogen peroxide to inorganic acid ([mass of hydrogen peroxide] / [mass of inorganic acid]) is limited to the range of 0.05 to 50) are added to the silicic acid solution obtained in step 1, mixed, and the pH is lowered to obtain an acidic silicic acid solution (however, the acidic silicic acid solution after the pH reduction is limited to the range of 1.5 to 2.5). (Step 3) A step of subjecting the acidic silicic acid solution obtained in step 2 to a cation exchange treatment to obtain a purified acidic silicic acid solution. (Step 4) A process for obtaining a high-purity silica microparticle dispersion by adding an aqueous alkali hydroxide solution to a portion of the purified acidic silicic acid solution obtained in step 3, adjusting the pH to a range of 10.0 to 13.0, heating the solution and maintaining the temperature in the range of 50°C to 98°C for 20 minutes or more, and then adding another portion of the purified acidic silicic acid solution obtained in step 3 at an addition rate in the range of 0.001 g / min g to 0.5 g / min g (calculated as silica solids), and simultaneously adding a pH adjuster.

[0017] According to one embodiment of the present invention, there is provided a high-purity silica microparticle dispersion liquid having, relative to the silica solid content, an Al concentration of 5 ppm to 50 ppm, a Ti concentration of 1 ppm to 40 ppm, an Fe concentration of 5 ppm to 25 ppm, a Ca concentration of 1 ppm to 8 ppm, a Na concentration of 1 ppm to 20 ppm, an Mg concentration of 1 ppm to 10 ppm, a Cr concentration of 0.05 ppm to 1 ppm, a Ni concentration of less than 1 ppm, a Cu concentration of less than 1 ppm, and a Zn concentration of less than 1 ppm (however, the total amount of these metals is limited to 120 ppm or less).

[0018] According to one aspect of the present invention, there is provided a method for producing a purified acidic silicic acid liquid obtained by the method for producing a high-purity silica microparticle dispersion liquid according to the one aspect of the present invention, the method comprising steps 1 to 3. [Effects of the Invention]

[0019] According to one aspect of the present invention, there is provided a method for producing a high-purity silica fine particle dispersion, which can produce a high-purity silica fine particle dispersion. DETAILED DESCRIPTION OF THE INVENTION

[0020] [Method of manufacturing high-purity silica fine particle dispersion] The method for producing a high-purity silica fine particle dispersion according to this embodiment (hereinafter also referred to as the "production method according to this embodiment") is a method including the following steps 1 to 4. According to the manufacturing method of this embodiment, it is possible to solve the following complicated problems. For example, if the number of ion exchange processes increases, it becomes necessary to install a tank to receive or store the silicic acid solution after ion exchange. Furthermore, there is a problem of a loss of silica components occurring in each ion exchange process, resulting in a decrease in yield. Furthermore, because maintenance of the tank installed to store the silicic acid solution and regeneration and cleaning of the ion exchange resin are also required, the amount of chemical solution and cleaning water required for regeneration increases, and the processing of these increases the process load. Generally, when silicic acid solution is anion-exchanged, the pH of the solution rises from weakly acidic to neutral, making the solution prone to gelation. For this reason, the silicic acid solution after anion exchange must be used quickly, for example, by being introduced into the next process within a very short time, which increases the burden of process management, time management, and quality control. Furthermore, when acid treatment is performed after an aging treatment in which a silicic acid solution is held for a predetermined time, the silicic acid solution cannot be used unless it is held for the predetermined time, so the silicic acid solution required for silica particle preparation must be prepared in advance and planned in accordance with the silica particle preparation process. Therefore, when multiple batches are continuously mixed in the silica particle preparation, multiple tanks for holding the acid treatment must be prepared in accordance with the holding time and the start time of the silica particle preparation process. In this case, management of multiple tanks and the acid treatment silicic acid solution is required, making production extremely complicated. In this specification, the silica fine particle dispersion liquid is also referred to as silica sol.

[0021] [Step 1: Preparation of silicic acid solution] In the manufacturing method according to this embodiment, a silicic acid solution is used as a raw material. A sodium silicate aqueous solution (silica concentration in the range of 2% by mass to 7% by mass) is subjected to a cation exchange treatment to adjust the pH to the range of 2.0 to 3.0. The sodium silicate aqueous solution used here can be so-called water glass (sodium silicate) or diluted water glass, in which sodium silicate is dissolved in water. The SiO2 concentration contained in the sodium silicate aqueous solution must be in the range of 2% by mass to 7% by mass. If the SiO2 concentration is in this range, gelation is unlikely to occur in the ion exchange resin layer during the ion exchange treatment described below, which is preferable. A sodium silicate aqueous solution having such a concentration range can be obtained, for example, by diluting commercially available water glass with water, ion-exchanged water, pure water, or ultrapure water. Such water glass is, for example, represented by the composition formula Na2O·nSiO2·mH2O, and has an SiO2 concentration in the range of 22% to 38% by mass, an Na2O concentration in the range of 5% to 19% by mass, an SiO2:Na2O (molar ratio) in the range of 0.5 to 4, and a pH of 9.0 or higher. In this specification, unless otherwise specified, the term "silicic acid solution" refers to an aqueous solution (sol-like solution) with a silica concentration of 2.0% by mass to 7.0% by mass, obtained by dealkalizing an aqueous alkali silicate solution as described above, and with a pH of 2.0 to 3.0. This "silicic acid solution" is used as the raw material in Step 1. In addition, the term "acidic silicic acid solution" in this specification refers to a solution with a pH of 1.5 to 2.5, obtained by using the "silicic acid solution" as the raw material and subjecting it to treatment in Step 2 (particularly to lowering its pH), as described below.

[0022] (cation exchange resin) The cation exchange resin used in the cation exchange treatment is not particularly limited, but for example, an H-type cation exchange resin is preferably used. The cation exchange resin may be in any form, such as beads or fibers. In this embodiment, a method in which the cation exchange resin is packed in a column and the solution is passed through the column is preferred.

[0023] (cation exchange temperature) Generally, in an ion exchange operation, the ion exchange temperature and the contact time (space velocity (SV)) between the liquid to be ion-exchanged and the ion exchange resin are important factors. Here, examples of the ion exchange temperature include the temperature of the solution before passing it through the ion exchange resin, the temperature inside the column packed with the ion exchange resin, the temperature of the solution containing the ion exchange resin, the temperature inside the ion exchange tower during ion exchange, and the temperature of the solution flowing out from the column outlet of the ion exchange tower. In this embodiment, the ion exchange temperature is defined as the temperature of the solution before passing it through the ion exchange resin, as it is a factor that has a large effect on the ion exchange reaction. Generally, the higher the temperature of the ion exchange reaction, the higher the ion exchange efficiency, and also the leaching effect described later. However, in the case of ion exchange using diluted water glass (sodium silicate aqueous solution) or acidic silicic acid solution, when the temperature is high, the acidic silicic acid solution in particular tends to polymerize and gel, and the stability of the acidic silicic acid solution decreases. Even if gelation does not occur, when particles are prepared using a polymerized silicic acid solution, self-nucleation by silicic acid may occur, and silica fine particles of the desired size may not be obtained.

[0024] The liquid temperature [T1] of the sodium silicate aqueous solution before passing it through a column packed with a cation exchange resin is preferably in the range of 3°C to 18°C. This temperature range provides excellent ion exchange efficiency and a stable acidic silicic acid solution. If the liquid temperature of the sodium silicate aqueous solution is below 3°C, cation exchange may not proceed efficiently, and further reduction in temperature tends not to significantly improve the stability of the silicic acid solution. If the liquid temperature of the sodium silicate aqueous solution exceeds 18°C, although the adsorption effect of impurities is enhanced, polymerization of the acidic silicic acid solution tends to progress, making it more prone to gelation, and the stability of the acidic silicic acid solution tends to decrease. Furthermore, when particles are prepared using such an acidic silicic acid solution, self-nucleation by silicic acid may occur, making it difficult to obtain particles of the desired size. The temperature of the solution before passing through the column is preferably in the range of 5°C to 15°C, even more preferably 6°C to 15°C, and most preferably 7°C to 15°C. The temperature of the aqueous sodium silicate solution can usually be passed through as is as long as it is within the above-mentioned temperature range. If the temperature exceeds the above-mentioned temperature range, the temperature is adjusted by cooling. It is also desirable to adjust the temperature of the cation exchange resin packed in the column to a range of 3°C to 18°C ​​in advance using means such as refrigeration in order to prevent a temperature increase during passing through the column. In this specification, in step 1, the liquid temperature before the aqueous sodium silicate solution is passed through a column packed with a cation exchange resin may be referred to as T1.

[0025] (space velocity) The space velocity [S1] when passing a sodium silicate aqueous solution through a column packed with cation exchange resin is 2h -1 Over 6 hours -1 When the space velocity is in this range, the contact time between the silicic acid solution and the cation exchange resin is at a sufficient level, which is preferable in terms of ion exchange efficiency. -1 If the space velocity is less than 6h, the ion removal effect will be improved, but the ion exchange will take time, which will result in poor productivity and economical problems. -1 If the ion exchange time is more than this, the ion exchange time is short and economical, but the ion removal efficiency decreases, and the solution may become unevenly distributed within the ion exchange column, causing the solution to leak out without being ion-exchanged, which tends to be problematic. More preferably, the space velocity is 2.5 h -1 Over 5.0 hours -1 The following ranges are recommended. Generally, the spatial velocity is used to express the speed at which a liquid passes through a filter material. In this specification, the spatial velocity when a solution is passed through a column packed with ion exchange resin is the liquid passing speed per volume of ion exchange resin, and is therefore referred to as the liquid passing speed (m 3 / h) ÷ resin volume (m 3 )=space velocity(h -1 ) relationship. In this specification, the space velocity when the aqueous sodium silicate solution is passed through a column packed with a cation exchange resin in step 1 may be represented as S1.

[0026] In step 1, the sodium silicate aqueous solution is subjected to cation exchange treatment until a silicic acid solution with a pH in the range of 2.0 to 3.0 is obtained. It is preferable for the pH value [P1] of the acidic silicic acid solution to be in the range of 2.0 to 3.0, as this improves the stability of the silicic acid solution. Generally, silicic acid solutions are unlikely to have a pH below 2.0 even after cation exchange. If the pH of the silicic acid solution exceeds 3.0, the stability of the silicic acid solution is impaired, polymerization of the silicic acid solution is likely to proceed, and in some cases, gelation may occur. It is more preferable that the pH of the resulting silicic acid solution be in the range of 2.2 to 2.8. In this application, the silicic acid solution (pH 2.0 to 3.0) obtained by the cation exchange treatment in step 1 is referred to as the "silicic acid solution" for convenience. On the other hand, the solution obtained in step 2 by adding an inorganic acid and hydrogen peroxide to the silicic acid solution and adjusting the pH to a predetermined value is referred to as the "acidic silicic acid solution" for convenience. In this embodiment, in step 1, it is desirable to carry out the cation exchange treatment so as to satisfy the reaction factor A described below. The reaction factor A is described below.

[0027] [Step 2: Adding inorganic acid and hydrogen peroxide to the silicic acid solution] In step 2, inorganic acid and hydrogen peroxide are added to the silicic acid solution obtained in step 1, which has a pH in the range of 2.0 to 3.0, in order to dissolve and remove impurity elements other than silicon. Specifically, inorganic acid and hydrogen peroxide are added to the silicic acid solution (pH range of 2.0 to 3.0) (however, the mass ratio of hydrogen peroxide to inorganic acid ([mass of hydrogen peroxide] / [mass of inorganic acid]) is limited to the range of 0.05 to 50), mixed, and the pH is lowered (however, the pH of the acidic silicic acid solution after the pH reduction is limited to the range of 1.5 to 2.5), to obtain an acidic silicic acid solution. Adding hydrogen peroxide to the silicic acid solution together with inorganic acid improves the stability of the resulting leaching acidic silicic acid solution, facilitating the leaching of metal impurities from the silicic acid framework. In particular, the formation of a peroxotitanium complex with Ti enhances the impurity reduction effect. The mass ratio of hydrogen peroxide to inorganic acid ([mass of hydrogen peroxide] / [mass of inorganic acid]) must be between 0.05 and 50. This range ensures sufficient metal impurity reduction, particularly excellent Ti removal. If the mass ratio of hydrogen peroxide to inorganic acid is less than 0.05, there is a problem of insufficient reduction of metal impurities, particularly Ti. On the other hand, if the mass ratio of hydrogen peroxide to inorganic acid exceeds 50, the metal impurity reduction effect converges, and the increased amount of hydrogen peroxide required reduces economic viability. Note that both the [mass of hydrogen peroxide] and [mass of inorganic acid] refer to masses converted to solids. That is, the former refers to the mass of H2O2, and the latter refers to the mass of H2SO4 when the inorganic acid is, for example, sulfuric acid. The mass ratio of hydrogen peroxide to inorganic acid is preferably in the range of 0.08 to 30, and more preferably in the range of 0.09 to 10. An inorganic acid and hydrogen peroxide are added to a silicic acid solution to lower its pH, yielding an acidic silicic acid solution. The pH of the resulting acidic silicic acid solution must be in the range of 1.5 to 2.5. If the pH of the acidic silicic acid solution is less than 1.5, the amount of acid will be excessive, which is undesirable as particles are more likely to aggregate during steps 3 and beyond. Furthermore, maintaining particle stability requires an anion removal process, which lengthens the process and increases the process load. If the pH of the acidic silicic acid solution exceeds 2.5, impurities in the acidic silicic acid solution are likely to be insufficiently removed. The pH of the resulting acidic silicic acid solution is preferably in the range of 1.5 to 2.4, more preferably in the range of 1.5 to 2.2, and even more preferably in the range of 1.5 to 2.0. In this specification, the pH of the acidic silicic acid solution after adding the inorganic acid and hydrogen peroxide in step 2 may be referred to as P2.

[0028] The amount of hydrogen peroxide used relative to the silicic acid solution in step 2 is determined by the mass ratio range with respect to the inorganic acid and the pH range of the resulting acidic silicic acid solution, as described above. The relationship between the silicic acid solution and hydrogen peroxide is particularly desirable as follows: The mass ratio of hydrogen peroxide (H2O2) used relative to the mass of silicic acid solution used is desirably between 10 ppm and 51,000 ppm. When the mass ratio of hydrogen peroxide used relative to the mass of silicic acid solution is within this range, the metal impurity reduction effect is fully exerted, and the Ti removal effect in particular tends to be excellent. When the mass ratio is less than 10 ppm, the reduction of metal impurities, especially Ti, tends to be insufficient. On the other hand, when the mass ratio exceeds 51,000 ppm, the metal impurity reduction effect converges, and the increased amount of hydrogen peroxide used tends to reduce economic efficiency. The amount (mass) of hydrogen peroxide used relative to the amount (mass) of silicic acid solution used is more preferably in the range of 50 ppm to 10,000 ppm. The ratio of the amount (mass) of hydrogen peroxide (H2O2) used relative to the amount (mass) of silicic acid solution used can be determined by calculating [amount (mass) of hydrogen peroxide (H2O2)] / [amount (mass) of silicic acid solution used]. In this specification, the ratio is expressed in units of ppm (×10 -6 ") is used to indicate the

[0029] (pH of silicate solution after acid leaching) Generally, the process of eluting impurities contained in a solid phase using an acid or alkali is called leaching, which refers to the operation of dissolving or eluting specific elements from the solid phase into an aqueous solution. The operation of adding an inorganic acid to an acidic silicic acid solution, as performed in Patent Document 5, is also leaching, and the metal impurities eluted by leaching are removed by ion exchange or the like. Leaching using an acid is also called "acid leaching." It is known that acid leaching is generally more effective in eluting specific elements from the solid phase the lower the pH of the acidic silicic acid solution after leaching, and in Patent Document 5, a strong acid is added to the silicic acid solution to adjust the pH to 0 to 1.54.

[0030] Impurity ions leached into the acidic silicic acid solution by leaching are removed by a subsequent cation exchange operation. In Patent Document 5, the cation exchange treatment is performed at a pH of 1.54 or less. However, since cation exchange resins that have adsorbed metal impurities are usually regenerated by the action of strong acids such as hydrochloric acid or sulfuric acid, the lower the pH of the acidic silicic acid solution, the more likely the impurity ions adsorbed on the ion exchange resin are to be eluted back into the acidic silicic acid solution. The inventors' actual experiments showed that when the pH of the acidic silicic acid solution is less than 1.5, alkali metals such as sodium are easily desorbed, and the resulting acidic silicic acid solution contains several tens of ppm or more of sodium per gram of dry silica.

[0031] As mentioned above, in order to enhance the leaching effect, it is necessary to lower the pH of the acidic silicic acid solution. However, if the pH is too low, the regeneration effect of the ion exchange resin will cause impurity ions to be desorbed, resulting in the problem of insufficient removal of impurities. Therefore, an optimal pH is required that takes into account the leaching effect and the regeneration effect of the resin. As a result of the studies conducted by the inventors, it was found that the most suitable condition for the pH of the acidic silicic acid solution is, as mentioned above, in the range of 1.5 to 2.5. Note that when ion exchange is performed on the acidic silicic acid solution, the pH of the acidic silicic acid solution may be adjusted to a range of 1.5 to 2.5 before ion exchange.

[0032] In the manufacturing method according to this embodiment, as described above, by using hydrogen peroxide together with an acid (inorganic acid) in the silicic acid solution, metal impurities are more likely to form complexes, and the leaching effect is promoted particularly by the action of forming a peroxotitanium complex with Ti.

[0033] The inorganic acid used in this embodiment may be, for example, sulfuric acid, hydrochloric acid, nitric acid, or phosphoric acid, which may be used as an aqueous solution of an appropriate concentration. The concentration of the inorganic acid is not particularly limited, but is usually in the range of 1.0 mass % to 10.0 mass %. The hydrogen peroxide used in this embodiment is usually used as an aqueous hydrogen peroxide solution. The concentration of the aqueous hydrogen peroxide solution is not particularly limited, but is usually in the range of 30% by mass to 40% by mass.

[0034] The acidic silicic acid solution obtained in step 2 preferably has a maximum absorbance value in the wavelength range of 300 nm to 750 nm in the wavelength range of 350 nm to 410 nm, a maximum absorbance value of 0.01 or greater, and an oxidation-reduction potential of 450 mV to 700 mV. Titanium components derived from the raw materials are usually present in silicic acid solutions. When a silica microparticle dispersion containing residual titanium components is used for semiconductor polishing, contamination of the polished substrate by the titanium components is likely to occur. In the present invention, by adding an inorganic acid and hydrogen peroxide to the silicic acid solution in step 2, the resulting acidic silicic acid solution turns yellow, and the absorbance of the acidic silicic acid solution increases to 0.01 or greater. This is believed to be due to the reaction of hydrogen peroxide with titanium components present as impurities to form peroxotitanic acid. The formation of such peroxotitanic acid is suitable for removing titanium components in subsequent steps and is also suitable for confirming that the titanium components have been removed. The oxidation-reduction potential is suitable for the formation of peroxotitanic acid if it is in the range of 450 mV or more and 700 mV or less, and the range of the oxidation-reduction potential is preferably 460 mV or more and 680 mV or less.

[0035] (Mixed with a static mixer) In step 2, inorganic acid and hydrogen peroxide are added to the silicic acid solution to lower the pH, thereby leaching out impurities. Therefore, it is extremely important that the silicic acid solution, inorganic acid, and hydrogen peroxide are mixed uniformly. As mentioned above, the lower the pH, the greater the leaching effect. Therefore, uneven mixing can result in localized areas where the pH is not lowered, resulting in insufficient leaching and insufficient reduction of impurities. Prolonged mixing can be achieved uniformly. However, excessively long processing times can reduce economic viability and can even cause gelation of the silicic acid solution. Even if gelation does not occur, polymerization of the silicic acid can progress, and using such a silicic acid solution to prepare particles can lead to problems such as self-nucleation by the silicic acid or failure to achieve the desired size. Therefore, thorough and uniform mixing within a short period of time is required. To achieve mixing in a short time, a method is known in which pipes for inorganic acid and hydrogen peroxide are connected to the pipe through which the silicic acid solution flows, but in order to thoroughly mix the three solutions, it is necessary to increase the liquid delivery speed and the Reynolds number in the pipe. However, since it is difficult to achieve uniform mixing with this method, it takes time to achieve sufficient mixing, which may result in poor leaching.

[0036] The inventors discovered that by using a static mixer, or so-called static mixer, with a specific range of element numbers and a predetermined range of Reynolds numbers, mixing can be achieved without leaching failure and in a relatively short time. A static mixer is a static mixer (line mixer) without a drive unit. Its structure, for example, consists of a required number of elements (structured like rectangular plates twisted 180 degrees in opposite directions) arranged alternately within a pipe. The required number of elements is determined based on the properties of the target fluid and the purpose of the process. Fluids entering the static mixer are sequentially stirred and mixed by the elements installed within the pipe. There are various types of static mixers, but an example is an element-type static mixer in which right and left elements are arranged alternately, allowing for division, conversion, or reversal of two types of fluids to be mixed uniformly. In this embodiment, an acidic silicic acid solution, an inorganic acid, and hydrogen peroxide are mixed uniformly. Here, if the number of elements in the static mixer is n and the Reynolds number is Re, and the product B (hereinafter referred to as the "mixing factor B") is set to a range of 1,000 to 1,000,000, a homogeneous mixture can be obtained in a short time without causing poor leaching. The range of this mixing factor B is more preferably 5,000 to 100,000, and even more preferably 8,000 to 50,000. The number of elements is set arbitrarily by the manufacturer of the static mixer depending on the purpose or application, and the static mixer is commercially available. The number of elements in a commercially available static mixer is not particularly limited, but known static mixers have the number set in the range of 5 to 70, for example. The blending factor B is as shown in the following formula (F1). B=Re×n (F1)

[0037] [Step 3: Preparation of purified acidic silicic acid solution] In step 3, the acidic silicic acid solution having a pH in the range of 1.5 to 2.5 obtained in step 2 is subjected to a cation exchange treatment to prepare a purified acidic silicic acid solution. The purified acidic silicic acid solution according to this embodiment can be produced up to step 3.

[0038] The liquid temperature [T3] of the acidic silicic acid liquid obtained in step 2 before passing it through a column packed with a cation exchange resin is preferably in the range of 3°C to 18°C. This temperature range provides excellent ion exchange efficiency and ensures stability of the resulting purified acidic silicic acid liquid. If the liquid temperature of the acidic silicic acid liquid is below 3°C, cation exchange may not proceed efficiently. If the liquid temperature of the acidic silicic acid liquid exceeds 18°C, the acidic silicic acid liquid tends to polymerize and gel, resulting in a decrease in the stability of the purified acidic silicic acid liquid. The liquid temperature of the acidic silicic acid liquid is more preferably in the range of 5°C to 15°C, even more preferably in the range of 6°C to 15°C, and most preferably in the range of 7°C to 15°C. The temperature of the acidic silicic acid liquid can usually be passed through the column as is as long as it is within the above temperature range. If the liquid temperature exceeds the above temperature range, the temperature is adjusted by cooling. It is also desirable to adjust the temperature of the cation exchange resin packed in the column to a range of 3°C to 18°C ​​in advance using means such as refrigeration in order to prevent the temperature from rising during the flow of the liquid. In this specification, the temperature of the liquid before passing it through the column packed with the cation exchange resin in step 3 may be referred to as T3.

[0039] The longer the contact time between the acidic silicic acid solution and the ion exchange resin, i.e., the slower the liquid passing through the ion exchange column filled with the ion exchange resin and the lower the space velocity, the higher the ion exchange efficiency. However, a low space velocity requires a long operation time, which reduces production efficiency. The flow rate [S3] of the acidic silicic acid solution through the cation exchange resin in step 3 is not particularly limited, but is preferably 2 h -1 Over 6 hours -1 The following range, more preferably 2.5h -1 Over 5.0 hours -1It is recommended to pass the liquid at a space velocity in the following range. In this specification, the space velocity when the acidic silicic acid solution is passed through a column packed with a cation exchange resin in step 3 may be represented by S3.

[0040] In step 3, the pH value [P3] of the purified acidic silicic acid solution after cation exchange is preferably in the range of 1.5 to 2.5. Although impurity ions are eluted from the solid phase to the liquid phase by the leaching treatment, the eluted impurities are on the order of ppm, so the pH of the acidic silicic acid solution hardly changes before and after cation exchange. Conversely, if the pH significantly deviates from this range, some kind of contamination is likely to have occurred during the process. The pH of the purified acidic silicic acid solution is preferably in the range of 1.5 to 2.4, more preferably in the range of 1.5 to 2.2, and even more preferably in the range of 1.5 to 2.0. In this specification, the pH value of the purified acidic silicic acid solution after cation exchange in step 3 may be referred to as P3. Also, in this specification, the cation exchange treatment in step 3 is also referred to as treatment 3.

[0041] In the purified acidic silicic acid liquid obtained in step 3, it is preferable that the Al concentration is 5 ppm or more and 50 ppm or less, the Ti concentration is 1 ppm or more and 40 ppm or less, the Fe concentration is 5 ppm or more and 25 ppm or less, the Ca concentration is 1 ppm or more and 8 ppm or less, the Na concentration is 1 ppm or more and 20 ppm or less, the Mg concentration is 1 ppm or more and 10 ppm or less, the Cr concentration is 0.05 ppm or more and 1 ppm or less, the Ni concentration is less than 1 ppm, the Cu concentration is less than 1 ppm, and the Zn concentration is less than 1 ppm, relative to the silica solid content (however, the total amount of these metals is limited to 120 ppm or less).

[0042] The purified acidic silicic acid solution obtained in step 3 preferably has a maximum absorbance value in the wavelength range of 300 nm or more and 750 nm or less in the wavelength range of 350 nm or more and 410 nm or less, a maximum absorbance value in the range of 0.0001 or more and less than 0.01, and is colorless and transparent. As mentioned above, when hydrogen peroxide is added to the silicic acid solution in step 2, peroxotitanic acid is formed, and the resulting acidic silicic acid solution turns yellow, with an absorbance of 0.01 or more. In this embodiment, the cation exchange treatment in step 3 is carried out so that the resulting purified acidic silicic acid liquid has a maximum absorbance value in the wavelength range of 300 nm to 750 nm in the wavelength range of 350 nm to 410 nm, and a maximum absorbance value in the range of 0.0001 to less than 0.01, making it colorless and transparent, thereby obtaining a high-purity silica microparticle dispersion liquid in which the concentration of titanium as an impurity is sufficiently reduced.

[0043] [Step 4: Preparation of silica fine particle dispersion] In step 4, an aqueous alkali hydroxide solution is added to a portion of the purified acidic silicic acid solution obtained in step 3, and the mixture is heated to a temperature in the range of 50°C to 98°C. The temperature is maintained at 50°C to 98°C for 20 minutes or longer. Further, a portion of the purified acidic silicic acid solution obtained in step 3 is added at a rate in the range of 0.001 g / min·g to 0.5 g / min·g (calculated as silica solids), and a pH adjuster is simultaneously added to obtain a high-purity silica microparticle dispersion.

[0044] Specifically, a part of the purified acidic silicic acid solution is used as a seed solution, and another part is used as a feed solution. After the seed solution is adjusted to an alkaline state, it is mixed with the feed solution. More specifically, an alkali is added to the seed solution to adjust the pH of the solution to within the alkaline range of pH 10.0 to 13.0, preferably pH 10.5 to 12.0. The type of alkali is not particularly limited, and examples thereof include potassium hydroxide, sodium hydroxide, aqueous ammonia, etc. The alkali is used in an amount sufficient to adjust the pH of the seed solution to a range of 10.0 to 13.0.

[0045] The temperature of the solution containing the seed solution and alkali adjusted to the pH range is set to a range of 50°C to 98°C, preferably a range of 60°C to 98°C, and more preferably a range of 65°C to 98°C, and this temperature is maintained for 20 minutes or more. A holding time of 20 minutes or more, more preferably 30 minutes or more, is recommended. There is no particular upper limit to the holding time, but 90 minutes is usually sufficient. This holding process causes dissolution and polymerization of the purified acidic silicic acid solution that serves as the seed, which becomes a precursor for uniform core particles and makes it easier to obtain uniform silica microparticles.

[0046] Following the retention treatment, a feed liquid is added to the solution containing the seed liquid and the alkali, and the amount of the feed liquid used is such that, in terms of silica [parts by mass], the silica solid content in the feed liquid is in the range of 1 part by mass to 100 parts by mass per 1 part by mass of the silica solid content in the solution containing the seed liquid and the alkali. The addition rate of the feed liquid is in the range of 0.001 g / min·g or more and 0.5 g / min·g or less (calculated as silica solids). When the addition rate of the feed liquid is in this range, efficient production is possible and self-nucleation of silicic acid does not occur. If the addition rate of the feed liquid is less than 0.001 g / min·g, the blending takes a very long time, resulting in poor economic efficiency. If the addition rate of the feed liquid exceeds 0.5 g / min·g, the addition rate of silicic acid is too fast, which makes self-nucleation by silicic acid more likely to occur, making it difficult to obtain the desired particle size. It is more preferable that the addition rate of the feed liquid be in the range of 0.002 g / min·g or more and 0.3 g / min·g or less. The unit of addition rate, "g / min g," refers to the amount of dry silica added per minute of the feed solution relative to the dry silica in the seed solution. The unit "g / min g" can also be expressed as "g / (min g)."

[0047] When the feed solution is added to the solution containing the seed solution and alkali, a pH adjuster is simultaneously added to prevent excessive aggregation due to a decrease in the pH of the preparation when the feed solution is added to the solution containing the seed solution and alkali. The amount of the pH adjuster used is an equimolar amount to the inorganic acid when the inorganic acid added in step 2 is a monovalent inorganic acid; when the inorganic acid is a divalent inorganic acid, an amount twice as molar as the inorganic acid; and when the inorganic acid is a trivalent inorganic acid, an amount three times as molar as the inorganic acid. The addition rate of the pH adjuster is not particularly limited, as it varies depending on the concentration of the aqueous alkaline solution. The type of pH adjuster is not particularly limited, but examples include potassium hydroxide, sodium hydroxide, and aqueous ammonia. The pH adjuster is added to maintain the pH of the preparation at an alkaline level, dissolve the purified acidic silicic acid solution that is added sequentially, and deposit it on the particles. During the addition of the feed solution, the pH adjuster is added to maintain the pH of the system consisting of the seed solution, alkali, and feed solution in the range of 10.0 to 13.0. The feed solution is added to the seed solution preferably at a temperature in the range of 1°C to 30°C, more preferably 1°C to 20°C.

[0048] Preferably, the feed liquid and the pH adjuster are gradually added to the seed liquid adjusted to the pH and temperature as described above, whereby silica microparticles grow in the seed liquid and a silica microparticle dispersion liquid (silica sol) is obtained. Here, after adding the feed liquid and the pH adjuster to the seed liquid, it is preferable to maintain the temperature in the range of 50°C to 98°C for about 15 minutes to 5 hours, as this facilitates the growth of silica microparticles.

[0049] In step 4, after adding the feed liquid in the first stage, the feed liquid may be further added in the second stage at a different rate. In the second stage, a feed liquid, which is a part of the acidic silicic acid liquid obtained in step 3, may be added at a rate in the range of 1.0 to 10.0 times the addition rate in the first stage to obtain a high-purity silica microparticle dispersion.

[0050] In the method for producing a high-purity silica fine particle dispersion according to this embodiment, the following aspect 1 is preferable. [Aspect 1: Reaction Factor A] In the method for producing a high-purity silica fine particle dispersion according to this embodiment, it is preferable to carry out the process so that the value of the reaction factor (A) represented by the following formula (F2) falls within the range of 56 or more and 225 or less. A=p×s×t (F2) p: the average value of the pH of the acidic silicic acid solution after the addition of the inorganic acid and hydrogen peroxide in step 2 and the pH of the acidic silicic acid solution after the cation exchange treatment in step 3, s: Space velocity (unit: h) when passing the liquid in each ion exchange treatment in the step 1 and the step 3 -1 ) average value, t: average temperature (unit: ° C) of the liquid before passing it through each ion exchange treatment in the steps 1 and 3

[0051] As mentioned above, p in formula (F2) is the average value of the pH [P2] of the acidic silicic acid solution after the addition of inorganic acid and hydrogen peroxide in step 2 and the pH [P3] of the purified acidic silicic acid solution after the cation exchange treatment in step 3. This relationship can be expressed as p = ([P2] + [P3]) / 2.

[0052] As described above, s in the formula (F2) is the space velocity (unit: h -1 ) In other words, s in formula (F2) means the average value of the liquid passage rate [S1] when the sodium silicate aqueous solution is passed through the cation exchange resin in step 1 and the liquid passage rate [S3] when the acidic silicic acid solution is passed through the cation exchange resin in step 3. This relationship can be expressed as s = ([S1] + [S3]) / 2.

[0053] As described above, t in formula (F2) represents the average temperature before passing the solution through each of the ion exchange treatments in steps 1 and 3. That is, t in formula (F2) represents the average temperature [T1] before passing the sodium silicate aqueous solution through the cation exchange resin in step 1, and the average temperature [T3] before passing the acidic silicic acid solution through the cation exchange resin in step 3. This relationship can be expressed as t = ([T1] + [T3]) / 2.

[0054] The present inventors have focused on the conditions for cation exchange of the raw water glass and the acidic silicic acid solution in the intermediate stage in order to efficiently reduce impurities in the method for producing high-purity silica sol, and have completed the present invention. That is, the inventors have found that if the product of p, s, and t is A, and A is in the range of 56 to 225, it will be economical, efficient in removing metal impurities, and maintain the stability of the acidic silicic acid solution and purified acidic silicic acid solution, as long as A is in the range of 56 to 225, it will be economical, efficient in removing metal impurities, and maintain the stability of the acidic silicic acid solution and purified acidic silicic acid solution. In this embodiment, due to the process settings, the pH value (P2) of the purified acidic silicic acid solution after addition of inorganic acid and hydrogen peroxide in step 2 is equal to the pH value of the purified acidic silicic acid solution before the cation exchange treatment in step 3.

[0055] In this embodiment, in formula (F2), the value of p is preferably in the range of 1.5 to 2.5, more preferably in the range of 1.5 to 2.4, and particularly preferably in the range of 1.5 to 2.3. The value of s is preferably 2h -1 Over 6 hours -1 The range is preferably 2.5 h -1 More than 5 hours -1The range is more preferably 3.2 h -1 More than 4.8h -1 The range is as follows, and 3.2 h is particularly preferable. -1 Over 4.6 hours -1 The value of t is preferably in the range of 3°C or more and 18°C ​​or less, more preferably in the range of 5°C or more and 15°C or less, even more preferably in the range of 6°C or more and 15°C or less, and particularly preferably in the range of 7°C or more and 15°C or less.

[0056] The value of the reaction factor A is preferably in the range of 60 or more and 175 or less, more preferably in the range of 65 or more and 160 or less, and even more preferably in the range of 70 or more and 144 or less.

[0057] In this embodiment, the average particle size of the silica particles in the obtained high-purity silica particle dispersion is not particularly limited. For example, a silica particle dispersion having an average particle size of 3 nm to 300 nm (dynamic light scattering method) can be obtained.

[0058] [High-purity silica particle dispersion] The high-purity silica microparticle dispersion liquid according to this embodiment can be obtained by the method for producing the high-purity silica microparticle dispersion liquid according to this embodiment described above. The high-purity silica microparticle dispersion liquid according to this embodiment has, relative to the silica solid content, an Al concentration of 5 ppm or more and 50 ppm or less, a Ti concentration of 1 ppm or more and 40 ppm or less, an Fe concentration of 5 ppm or more and 25 ppm or less, a Ca concentration of 1 ppm or more and 8 ppm or less, a Na concentration of 1 ppm or more and 20 ppm or less, an Mg concentration of 1 ppm or more and 10 ppm or less, a Cr concentration of 0.05 ppm or more and 1 ppm or less, a Ni concentration of less than 1 ppm, a Cu concentration of less than 1 ppm, and a Zn concentration of less than 1 ppm (however, the total amount of these metals is limited to 120 ppm or less). [Example]

[0059] Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. Furthermore, the methods for measuring various properties of high-purity silica sol in the examples and comparative examples were as follows, unless otherwise specified.

[0060] <Measurement of silica sol and purified acidic silicic acid solution> 1. Purity (1) Metal (Al, Ti, Fe, Ca, Mg, Cr, Ni, Cu, Zn, or Na) content 1) Approximately 10 g of sample (silica sol or purified acidic silicic acid solution) is placed on a platinum dish and weighed to the nearest 0.1 mg. 2) Add 5 mL of nitric acid and 20 mL of hydrofluoric acid, heat on a sand bath, and evaporate to dryness. 3) When the liquid volume decreases, add another 20 mL of hydrofluoric acid, heat on a sand bath, and evaporate to dryness. 4) After cooling to room temperature, add 2 mL of nitric acid and approximately 50 mL of water, and heat to dissolve on a sand bath. 5) After cooling to room temperature, place in a flask (100 mL) and dilute with water to 100 mL to make the sample solution. 6) The content of each metal present in the sample solution was measured using the following measuring device.

[0061] [Al, Ti, Fe, Ca, Mg, Cr, and Zn content] Measurements were performed using an inductively coupled plasma optical emission spectrometer (Seiko Instruments Inc., SPS5520, an instrument that introduces a dissolved sample into high-frequency inductively coupled argon plasma, excites each element in the sample to emit light, and performs quantitative and qualitative analysis based on the emission spectrum. The measurement wavelength range is 175 to 500 nm). [Ni and Cu content] An atomic absorption spectrophotometer (AA240Z, manufactured by Agilent Technologies, Inc.) was used. The sample was converted into atomic vapor using a flame, and the atomic vapor layer was irradiated with light of an appropriate wavelength. The intensity of the light absorbed by the atoms was measured, and this was used to quantify the element concentration in the sample. A graphite furnace was used. Measurement mode: atomic absorption, measurement wavelength range: 190 to 900 nm. [Na content] An atomic absorption spectrophotometer (Hitachi, Ltd., Z-2310, measurement wavelength range: 190 to 900 nm) was used.

[0062] (2) SiO2 concentration The SiO2 concentration in silica sol or purified acidic silicic acid solution was determined by adding 2 mL of 50% aqueous sulfuric acid solution to 10 g of sample, evaporating to dryness on a platinum dish, and baking the resulting solid at 1000°C for 1 hour, cooling, and weighing. Next, the weighed solid was dissolved in a small amount of 50% aqueous sulfuric acid solution, and 20 mL of hydrofluoric acid was added. The solution was then evaporated to dryness on a platinum dish, baked at 1000°C for 15 minutes, cooled, and weighed. The silica content was determined from the difference in weight.

[0063] (3) From the measurement results of (1) and (2) above, the concentration (impurity concentration) of each element (Al, Ti, Fe, Ca, Mg, Cr, Ni, Cu, Zn, or Na) relative to the silica solid content (dry silica) of the silica sol was measured.

[0064] 2. pH measurement The pH of the acidic silicic acid solution, purified acidic silicic acid solution, and silica sol (silica microparticle dispersion) was measured by placing approximately 50 g of the measurement sample in a polyethylene sample bottle, immersing it in a thermostatic bath at 25°C for at least 30 minutes, and then inserting the glass electrode of a Horiba, Ltd. pH meter F22, which had been calibrated with standard solutions of pH 4, 7, and 9.0.

[0065] 3. Measurement of average particle size The average particle size of the silica sol (silica fine particle dispersion) obtained in each of the examples and comparative examples was measured using nanoSAQLA, a particle size measuring device manufactured by Otsuka Electronics Co., Ltd., which uses a dynamic light scattering method.

[0066] 4. Calculating the Reynolds number In this specification, the Reynolds number (Re) is calculated by the following general formula: Reynolds number (Re) = Duρ / μ (D: pipe inner diameter [m], u: average fluid flow velocity [m / s], ρ: fluid density [Kg / m3 ], μ: viscosity of fluid [m 3 / s])

[0067] 5. Absorbance Measurement A spectrophotometer (Hitachi High-Technologies Corporation, U-2900 model, measurement wavelength range 300 to 750 nm) was used.

[0068] 6. Redox Potential Measurement Measurements were made at 25°C using an ORP meter (D-52S) manufactured by Horiba Ltd. and an ORP electrode (9300-10D) manufactured by Horiba Ltd. (unit: mV).

[0069] [Example 1] ·Process 1 2,800 g of No. 3 water glass (silica concentration 24.3% by mass) was diluted with pure water to prepare 13,608 g of a sodium silicate aqueous solution (SiO2 concentration 5.0% by mass). The resulting sodium silicate aqueous solution was cooled to 12°C and then passed through 6 L of a strongly acidic cation exchange resin (Duolite C255LFH, manufactured by Rohm and Haas) at a space velocity of 4.0 h -1 The liquid was passed through the column at 100°C, and 12,480 g of silicic acid liquid (SiO2 concentration: 4.5% by mass) (pH: 2.6) was obtained. Step 2 (preparation of leaching acidic silicate solution) A static mixer (T3-12, 12 elements, manufactured by Noritake Company Limited) was prepared, and 12,000 g of the silicic acid solution prepared according to the above "Preparation of silicic acid solution" was introduced into the static mixer at 300 g / min over 30 minutes, while simultaneously introducing 125.0 g of an aqueous sulfuric acid solution (sulfuric acid concentration 9.8% by mass) at 4.2 g / min and 183.6 g of an aqueous hydrogen peroxide solution (hydrogen peroxide concentration 35.0% by mass) at 6.1 g / min over 30 minutes into the static mixer, thereby mixing the liquids and carrying out a leaching treatment of the silicic acid solution. The weight ratio of hydrogen peroxide to acidic silicic acid solution used (ratio of hydrogen peroxide weight / acidic silicic acid solution weight) was 5,355 ppm. The pH of the resulting acidic silicic acid solution (leaching acidic silicic acid solution) was 1.7, the sulfuric acid concentration was 1,000 ppm, and the SiO2 concentration was 4.4 mass%. The Reynolds number when passing through the static mixer was 1,709. In this specification, an acidic silicic acid solution that has been subjected to leaching treatment by adding an inorganic acid and hydrogen peroxide is also referred to as a "leached acidic silicic acid solution." Step 3 (preparation of purified acidic silicic acid solution) Next, the obtained leached acidic silicic acid solution was cooled to 12°C and applied to 6 L of a strong acidic cation exchange resin at a space velocity of 4.0 h -1 The liquid was passed through the column at 400°C to obtain 9,270 g of a purified acidic silicic acid liquid (SiO2 concentration: 4.3% by mass) (pH: 1.7). Step 4 (preparation of silica microparticle dispersion) 341 g of the purified acidic silicic acid solution obtained in step 3 was placed in a separable flask having an internal volume of 10 L so that the dry silica content was 14.7 g. Next, 17.9 g of a 48.7 mass% aqueous potassium hydroxide solution was weighed out. Furthermore, 587 g of ultrapure water was weighed out so that the total amount of the purified acidic silicic acid solution, potassium hydroxide, and ultrapure water was 945.9 g, and the 48.7 mass% aqueous potassium hydroxide solution and ultrapure water were stirred until homogenous. This diluted aqueous potassium hydroxide solution was placed in a 10 L separable flask and thoroughly stirred. After stirring, the silica concentration of the solution in the 10 L separable flask was 1.55 mass %, and the pH was 11.1. Next, the solution in the 10 L separable flask was heated to 83°C while stirring, and after reaching 83°C, the temperature was maintained for 30 minutes. After the temperature was maintained, 53.11 g of the purified acidic silicic acid solution obtained in step 3 (1,235 g of purified acidic silicic acid solution) calculated as dry silica was added to the solution in the 10 L separable flask at a rate of 0.020 g / min g (calculated as silica solids) over 3 hours, and simultaneously 100 g of a previously prepared aqueous potassium hydroxide solution (KOH concentration 2.0 mass%) was added over 3 hours to obtain a precursor particle dispersion (first-stage addition). Next, 318.8 g of the purified acidic silicic acid solution obtained in step 3 (7,413 g of purified acidic silicic acid solution) was added to the precursor particle dispersion at a rate of 0.030 g / min g (silica solids equivalent) over 12 hours, and at the same time, 601 g of a potassium hydroxide aqueous solution (KOH concentration 2.0 mass%) that had been prepared in advance was also added to the precursor particle dispersion over 12 hours (second-stage addition). After the second addition, the mixture was maintained at 83°C for 1 hour, then cooled to room temperature, and concentrated to an SiO2 concentration of 12% by mass using an ultrafiltration membrane (SIP-1013, manufactured by Asahi Kasei Chemicals Corporation), and then concentrated to 40% by mass using a rotary evaporator to obtain a high-purity silica sol (high-purity silica microparticle dispersion). The metal impurity concentrations of the obtained purified acidic silicic acid solution and silica sol were measured, and the concentrations of each metal impurity relative to the silica solid content were as shown in Tables 2 and 3. The absorbance and oxidation-reduction potential of the obtained acidic silicic acid solution were measured, and the results are shown in Table 1. The production conditions are also shown in Table 1 (the same applies to the following Examples and Comparative Examples). The Reynolds number (Re), mixing factor B, and calculated values ​​of p, t, s, and reaction factor A are also shown in Table 1 (excluding cases where the relevant values ​​could not be measured or calculated; the same applies to the following Examples and Comparative Examples). The average particle size of the silica microparticles in the obtained silica sol was 38 nm. In the present specification, the silica solid content is also referred to as "dry silica."

[0070] [Examples 2 to 5 and Comparative Examples 1 to 3] A purified acidic silicic acid solution and a silica sol were obtained in the same manner as in Example 1, except that the preparation conditions were changed as shown in Table 1. The resulting purified acidic silicic acid solution and silica sol were evaluated in the same manner as in Example 1. The results obtained are shown in Tables 1 to 3. In addition, in Examples 1 to 5 and Comparative Examples 1 to 3, the color of the acidic silicic acid solution obtained in step 2, the wavelength and maximum value of the absorbance of the purified acidic silicic acid solution obtained by cation exchange in step 3, and the color of the purified acidic silicic acid solution obtained by cation exchange in step 3 are shown in Table 4.

[0071] [Table 1]

[0072] [Table 2]

[0073] [Table 3]

[0074] [Table 4]

Claims

1. A method for producing a high-purity silica fine particle dispersion, comprising the following steps 1 to 4: (Step 1) A process of subjecting an aqueous sodium silicate solution (silica concentration in the range of 2.0 mass % to 7.0 mass %) to a cation exchange treatment to obtain a silicic acid solution in the range of pH 2.0 to 3.

0. (Step 2) A process in which an inorganic acid and hydrogen peroxide (however, the mass ratio of hydrogen peroxide to inorganic acid ([mass of hydrogen peroxide] / [mass of inorganic acid]) is limited to the range of 0.05 to 50) are added to the silicic acid solution obtained in step 1, mixed, and the pH is lowered to obtain an acidic silicic acid solution (however, the acidic silicic acid solution after the pH reduction has a pH limited to the range of 1.5 to 2.5). (Step 3) A step of subjecting the acidic silicic acid solution obtained in step 2 to a cation exchange treatment to obtain a purified acidic silicic acid solution. (Step 4) A process for obtaining a high-purity silica microparticle dispersion by adding an aqueous alkali hydroxide solution to a portion of the purified acidic silicic acid solution obtained in step 3, adjusting the pH to a range of 10.0 to 13.0, heating the mixture, and maintaining the temperature in a range of 50°C to 98°C for 20 minutes or more, and then adding another portion of the purified acidic silicic acid solution obtained in step 3 at an addition rate in a range of 0.001 g / min g to 0.5 g / min g (converted to silica solids), and simultaneously adding a pH adjuster.

2. 2. The method for producing a high-purity silica microparticle dispersion according to claim 1, wherein in step 2, when the inorganic acid and hydrogen peroxide are added to the silicic acid solution and mixed, they are mixed using a static mixer, and at that time, the value of a mixing factor (B) represented by the following formula (F1) is in the range of 1,000 or more and 1,000,000 or less. B=Re×n...(F1) (B: mixing factor of the static mixer, n: number of elements in the static mixer, Re: Reynolds number)

3. 3. The method for producing a high-purity silica fine particle dispersion according to claim 1, wherein the reaction is carried out so that the value of a reaction factor (A) represented by the following formula (F2) falls within the range of 56 to 225. A=p×s×t...(F2) (p: the average value of the pH of the acidic silicic acid solution after the addition of the inorganic acid and hydrogen peroxide in step 2 and the pH of the purified acidic silicic acid solution after the cation exchange treatment in step 3, s: Space velocity (unit: h) when passing the liquid in each ion exchange treatment in the step 1 and the step 3 －１ ) the average value of t: average temperature (unit: °C) of the liquid before passing it through each of the ion exchange treatments in the steps 1 and 3

4. In the formula (F2), the value of p is in the range of 1.5 or more and 2.5 or less, and the value of s is in the range of 2h －１ Over 6 hours －１ 4. The method for producing a high-purity silica fine particle dispersion according to claim 3, wherein the temperature is in the range of 3°C or more and 18°C ​​or less.

5. 3. The method for producing a high-purity silica fine particle dispersion according to claim 1, wherein the amount (by mass) of the hydrogen peroxide used is in the range of 10 ppm to 51,000 ppm relative to the amount (by mass) of the silicic acid solution used.

6. The purified acidic silicic acid liquid obtained in step 3 has, relative to the silica solid content, an Al concentration of 5 ppm or more and 50 ppm or less, a Ti concentration of 1 ppm or more and 40 ppm or less, an Fe concentration of 5 ppm or more and 25 ppm or less, a Ca concentration of 1 ppm or more and 8 ppm or less, a Na concentration of 1 ppm or more and 20 ppm or less, a Mg concentration of 1 ppm or more and 10 ppm or less, a Cr concentration of 0.05 ppm or more and 1 ppm or less, a Ni concentration of less than 1 ppm, a Cu concentration of less than 1 ppm, and a Zn concentration of less than 1 ppm (however, the total amount of these metals is limited to 120 ppm or less).

7. 3. The method for producing a high-purity silica microparticle dispersion according to claim 1 or 2, wherein the acidic silicic acid solution obtained in step 2 has a maximum absorbance value in the wavelength range of 300 nm to 750 nm in the wavelength range of 350 nm to 410 nm, a maximum absorbance value of 0.01 or more, and an oxidation-reduction potential of 450 mV to 700 mV.

8. 8. The method for producing a high-purity silica microparticle dispersion liquid according to claim 7, wherein the purified acidic silicic acid liquid obtained in step 3 has a maximum absorbance value in the wavelength range of 300 nm to 750 nm in the wavelength range of 350 nm to 410 nm, the maximum absorbance value is in the range of 0.0001 to less than 0.01, and the purified acidic silicic acid liquid is colorless and transparent.

9. A high-purity silica microparticle dispersion liquid having, relative to the silica solid content, an Al concentration of 5 ppm to 50 ppm, a Ti concentration of 1 ppm to 40 ppm, an Fe concentration of 5 ppm to 25 ppm, a Ca concentration of 1 ppm to 8 ppm, a Na concentration of 1 ppm to 20 ppm, a Mg concentration of 1 ppm to 10 ppm, a Cr concentration of 0.05 ppm to 1 ppm, a Ni concentration of less than 1 ppm, a Cu concentration of less than 1 ppm, and a Zn concentration of less than 1 ppm (however, the total amount of these metals is limited to 120 ppm or less).

10. A method for producing a purified acidic silicic acid solution obtained by the method for producing a high-purity silica fine particle dispersion according to claim 1, comprising: The method includes steps 1 to 3. Method for producing purified acidic silicic acid solution.

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