Method for producing high-purity silica gel, method for producing high-purity water glass, and method for producing a dispersion of high-purity silica fine particles.

Abstract

To provide a method for producing high-purity silica gel that does not require complicated processing. [Solution] A method for producing high-purity silica gel comprising the following steps 1 and 2. (Process 1) A process to obtain an acid solution of silica gel by adding an aqueous alkali silicate solution to an acid solution obtained by mixing an inorganic acid and hydrogen peroxide at an addition rate of 0.001 g / min·g to 0.01 g / min·g (calculated in terms of silica solids) under conditions where the pH of the acid solution is 1.0 or less and the temperature is in the range of 68°C to 90°C, thereby generating silica gel, and then aging the mixture at a temperature in the range of 68°C to 90°C to obtain an acid solution of silica gel. (Process 2) A step to separate silica gel from the silica gel acid solution obtained in step 1, and wash the silica gel with water until the pH of the water wash water for the silica gel is 4.8 or higher and the conductivity is 10 μS / cm or less, thereby obtaining high-purity silica gel.

Description

[Technical Field]

[0001] This invention relates to methods for producing silica gel, water glass (alkali silicate aqueous solution), silica microparticle dispersion (silica sol), and especially high-purity silica gel, high-purity water glass, and high-purity silica microparticle dispersion (high-purity silica sol). [Background technology]

[0002] Conventionally, silica sols that are 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 the silica sol contains metal impurities, these impurities will diffuse into the wafer during polishing, degrading the wafer quality. As a result, the performance of semiconductor devices formed using such wafers will be significantly reduced. As a method for producing high-purity silica sol, a widely known method involves synthesizing silica sol using purified alkyl silicates (alkoxysilanes) as raw materials, as a means of minimizing impurities. However, in this method, the alkyl silicates used as raw materials are expensive, so the use of silica sol obtained by such a method has been limited to, for example, high-value-added applications or special applications (see Patent Document 1).

[0003] Another known method for producing high-purity silica sol involves synthesizing silica gel through a wet reaction with alkali silicate and mineral acid, and then removing metal impurities by acid treatment to produce a silica fine particle dispersion using high-purity silica gel as the silica source.

[0004] As an example of a method for producing high-purity silica gel that can serve as a silica source, Patent Document 2 (Method for producing high-purity silica) can be cited. This document substantially discloses a method for producing high-purity silica gel, and in the method for producing high-purity silica gel described in this document, water glass is added to an acid solution with an acid concentration of 1 N or more that contains a chelating agent and hydrogen peroxide, and silica gel is produced by placing it in an acidic region and at a high temperature. Furthermore, the obtained silica gel is further washed with an acid solution similar to the above acid solution to remove metal impurities contained in the silica gel. For example, in Example 1 of Patent Document 2, silica gel is produced by adding No. 3 water glass (silica concentration 28.5 mass%) to an aqueous nitric acid solution (concentration 19.3 mass%) containing oxalic acid and hydrogen peroxide. Furthermore, after separating the obtained silica gel from the acid solution, it is dispersed again in an acid solution similar to the above acid solution and acid treatment is performed while stirring. However, in this method, since an organic acid is used as a chelating agent, the chemical oxygen demand (COD) of the wastewater becomes high, so wastewater treatment is required, which has the problem of increased burden on the process and increased costs.

[0005] Patent Document 3 discloses a method for producing high-purity silica gel, in which alkali silicate is added to an acidic solution in the acidic range of 1 N or more acid concentration, silica gel is produced while further contacting it with a chelating agent and hydrogen peroxide, and metal impurities can be removed by further washing the silica gel with an acidic solution similar to the aforementioned acidic solution. For example, in Manufacturing Example 1 of the same document, silica gel is produced by adding No. 3 water glass (silica concentration 28.5% by mass) containing EDTA (ethylenediaminetetraacetic acid) to an aqueous nitric acid solution (concentration 19.3% by mass). After separating the obtained silica gel from the acid solution, it is dispersed in an acid solution containing hydrogen peroxide and subjected to acid treatment while stirring. However, in this manufacturing method, an organic acid is used as a chelating agent to capture impurities, resulting in a high COD of the wastewater, which poses the problem of the need for wastewater treatment and increased costs. Furthermore, in this manufacturing method, it is presumed that silica gel in the intermediate stage (before additional washing with the acid solution) tends to have a relatively large primary diameter and a relatively small pore volume. In the case of silica gel with a relatively large primary diameter, metal impurities incorporated into the primary particles have difficulty coming into contact with the acid during the subsequent additional washing with the acid solution. Similarly, if the pore volume of the silica gel is relatively small, the diffusion of acid into the silica gel is not easy during the additional washing with the acid solution. In either case, it is expected that the metal impurity removal effect will not be sufficient. Thus, the method for producing high-purity silica gel described in Patent Document 3 requires a two-step process to remove metal impurities, which involves the use of chelating agents in the silica gel preparation process and subsequent additional acid treatment of the silica gel. This presents problems in terms of manufacturing efficiency and economic viability.

[0006] In the method for producing silica described in claim 3 of Patent Document 4, it is disclosed that high-purity silica from which metal impurities have been removed can be obtained by adding alkali silicate in the form of drops to an aqueous sulfuric acid solution containing a chelating agent (concentration of 1% by mass or more) and hydrogen peroxide (concentration of 1% by mass or more) in a range in which the neutralization rate of sulfuric acid is 40% or less. For example, in the example, silica gel is produced and metal impurities are removed by adding No. 3 water glass (silica concentration 24% by mass) to a sulfuric acid aqueous solution (39% by mass) containing citric acid (1% by mass) and hydrogen peroxide (1% by mass) in a range of 19-38% neutralization rate of sulfuric acid. In this manufacturing method, since silica gel is produced by dropping a high concentration of water glass into a high concentration sulfuric acid aqueous solution, the primary diameter of the silica particles tends to be large. Therefore, in order to improve the efficiency of metal impurity removal, it was necessary to contact the silica with a chelating agent or hydrogen peroxide before the polymerization of silicic acid progressed to remove the metal impurities.

[0007] Patent Document 5 discloses a method for producing high-purity silica in which metal impurities can be removed by contacting an alkaline silicate aqueous solution or silica gel with a chelating agent having phenolic hydroxyl groups and sulfone groups as ligands. For example, in Example 1, a commercially available sodium silicate aqueous solution (silica concentration 27.0% by mass) is reacted with disodium ethylenediaminetetraacetate and disodium chromotropate, then sulfuric acid and quaternary ammonium hydroxide aqueous solution are added to gel the silica gel, and the resulting silica gel is washed with hydrochloric acid (acid concentration 5 N) or oxalic acid aqueous solution. However, this manufacturing method has the problem that the COD of wastewater is high because organic acids are used as chelating agents, and wastewater treatment and its costs are high. In addition, there are washing steps using acids and chelating agents in addition to the silica gel preparation step, which also presents problems in terms of manufacturing efficiency and economics.

[0008] Patent Document 6 describes a method for producing high-purity silica gel, in which alkali silicate is added to a mineral acid containing a water-soluble organic compound having a hydroxyl group or a ketone group to produce silica gel, and metal impurities can be removed by repeatedly aging in the mineral acid solution and washing with water. For example, in Example 1, silica gel is produced by adding a solution containing sulfuric acid (acid concentration 36 N) and polyethylene glycol to No. 3 water glass (silica concentration 29% by mass), and this gel is aged in an aqueous sulfuric acid solution (acid concentration 6 N) for 6 hours, and then aged in hydrochloric acid (acid concentration 6 N) for 4 hours, repeating this process three times. However, this manufacturing method has problems with production efficiency and economics because the silica gel is aged in acid multiple times. It also has the problem of introducing metal impurities from polymer flocculants. Furthermore, the addition of polymer flocculants results in water glass with a large molecular weight, and when particles are prepared using this water glass, self-nucleation by silicic acid occurs, resulting in the problem that silica fine particles of the desired size cannot be obtained.

[0009] Patent Document 7 describes a method for producing high-purity silica gel, in which silica gel is generated in an alkaline region with a pH of 7 or higher by adding mineral acid to alkali silicate, and metal impurities can be removed by repeatedly aging in a mineral acid solution with a pH of 1 or lower and washing with water. For example, in Example 1, silica gel is generated by adding sulfuric acid (acid concentration 6 N) to a diluted product of No. 3 aqueous glass, and this gel is aged for 6 hours in an aqueous sulfuric acid solution (acid concentration 6 N), and then aged for 4 hours in hydrochloric acid (acid concentration 6 N), and this process is repeated three times. However, this manufacturing method has problems in terms of production efficiency and economics, such as generating silica gel in an alkaline region and performing multiple acid agings of the silica gel.

[0010] Patent Document 8 describes a method for producing high-purity silica gel, in which silica gel is produced by directly extruding alkali silicate into a mineral acid of a specific concentration, this silica gel is dissolved in an aqueous solution of alkali hydroxide, the resulting alkali silicate is directly extruded into a mineral acid of a specific concentration to produce silica gel, and the silica gel is further immersed in a mineral acid of a specific concentration to remove metal impurities. For example, in Example 1, silica gel is produced by extruding No. 3 water glass (silica concentration 24% by mass) into a mixed solution of hydrogen peroxide (concentration 1.5% by mass) and sulfuric acid (concentration 15% by mass) at a speed of 0.48 m / s, this gel is dissolved in an aqueous solution of sodium hydroxide (concentration 48% by mass), the resulting aqueous solution of sodium silicate is gelled again under the above conditions, the resulting silica gel is immersed in sulfuric acid (concentration 15% by mass) for 1 hour, and finally washed with water. However, this manufacturing method has problems with manufacturing efficiency and economics, as it involves two metal impurity removal steps and a silica gel dissolution step, and separation and washing are required between these steps. Furthermore, in the manufacturing method described in Patent Document 8, alkali silicate is extruded into mineral acid using an extrusion tube. However, the alkali silicate extruded into the mineral acid maintains a cylindrical shape, and gelation occurs instantaneously on its outer surface upon contact with the mineral acid. As a result, the mineral acid has difficulty penetrating the central part of the alkali silicate in this shape. Therefore, the removal of metal impurities from the alkali silicate in this shape is inefficient, and it was necessary to repeat the metal impurity removal treatment to achieve sufficient removal.

[0011] Patent Document 9 discloses a method for producing high-purity silica gel, in which a wet reaction of alkali silicate and mineral acid is carried out in an acidic region with an acid concentration of 1 N or higher and at a high temperature to produce silica gel, thereby removing metal impurities. For example, in Example 1 of Patent Document 9, a diluted product of No. 3 aqueous glass is added to hydrochloric acid (concentration 36.5% by mass) to precipitate silica gel. However, although this method is simple, it does not involve acid aging of the silica gel, so it fails to completely remove metal impurities, resulting in a high concentration of impurities. In fact, when the present inventors conducted a trace experiment, they obtained silica gel with a high concentration of Ti (approximately 20 ppm). Furthermore, in this method, since the hydrochloric acid is heated to 90°C, a large amount of fuming hydrogen chloride is generated during silica gel preparation, posing a safety problem. In addition, because a large amount of hydrochloric acid volatilizes due to high-temperature heating, it is difficult to control the concentration of the acid solution. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Application Publication No. 01-278413 [Patent Document 2] Japanese Patent Application Publication No. 07-242411 [Patent Document 3] Japanese Patent Publication No. 2000-247625 [Patent Document 4] Japanese Patent Publication No. 2014-210678 [Patent Document 5] Japanese Patent Application Publication No. 02-074515 [Patent Document 6] Japanese Patent Application Publication No. 60-204614 [Patent Document 7] Japanese Patent Application Publication No. 60-204613 [Patent Document 8] Japanese Patent Publication No. 2005-179159 [Patent Document 9] Japanese Unexamined Patent Publication No. 60-191016 [Overview of the Initiative]

Problems to be Solved by the Invention

[0013] An object of the present invention is to provide a method for producing high-purity silica gel, a method for producing high-purity water glass, and a method for producing a high-purity silica fine particle dispersion liquid, which do not require complicated processes.

Means for Solving the Problems

[0014] According to one aspect of the present invention, there is provided a method for producing high-purity silica gel, comprising the following step 1 and the following step 2. (Step 1) An aqueous alkali silicate solution is added to an acid solution obtained by mixing an inorganic acid and hydrogen peroxide under the conditions that the pH of the acid solution is 1.0 or less and the temperature is in the range of 68°C or higher and 90°C or lower at an addition rate in the range of 0.001 g / min·g or more and 0.01 g / min·g or less (in terms of silica solid content) to generate silica gel, and then the silica gel is aged in the range of 68°C or higher and 90°C or lower to obtain an acid solution of silica gel. (Step 2) Silica gel is separated from the acid solution of silica gel obtained in Step 1, and the silica gel is washed with water until the pH of the washing water of the silica gel is 4.8 or higher and the conductivity is 10 μS / cm or lower to obtain high-purity silica gel.

[0015] According to one aspect of the present invention, there is provided a method for producing high-purity water glass, comprising the following step 3 following Step 1 and Step 2 in the method for producing high-purity silica gel according to one aspect of the present invention. (Step 3) The high-purity silica gel obtained in Step 2 is dissolved in an aqueous alkali hydroxide solution to obtain high-purity water glass in which the Al concentration is less than 4.0 ppm, the Ca concentration is less than 3.0 ppm, the Ti concentration is less than 2.5 ppm, and the concentrations of Mg, Cr, Fe, Ni, Cu, and Zn are each less than 1 ppm with respect to the silica solid content.

[0016] According to one aspect of the present invention, a method for producing a high-purity silica fine particle dispersion is provided, comprising the following steps 4 and 5, following steps 1, 2 and 3 in the method for producing high-purity water glass according to one aspect of the present invention. (Step 4) A step in which the high-purity water glass obtained in step 3 is subjected to cation exchange to obtain a purified silica solution. (Step 5) A step to obtain a high-purity silica fine particle dispersion in which, in addition to a portion of the purified silicic acid solution obtained in step 4, an aqueous alkali hydroxide solution is added to a portion of the purified silicic acid solution obtained in step 4, the pH is adjusted to a range of 10 to 13, the solution is heated and the temperature is maintained at a range of 50°C to 98°C for 20 minutes or more, and further, another portion of the purified silicic acid solution obtained in step 4 is added at an addition rate of 0.001 g / min·g to 0.5 g / min·g (in terms of silica solids), and the Na concentration is less than 1.5 ppm, the Al concentration is less than 2.5 ppm, the Ti concentration is less than 1.5 ppm, and the concentrations of Mg, Ca, Cr, Fe, Ni, Cu, and Zn are each less than 1 ppm. [Effects of the Invention]

[0017] According to one aspect of the present invention, it is possible to provide a method for producing high-purity silica gel that does not require complicated processing, a method for producing high-purity water glass, and a method for producing a high-purity silica fine particle dispersion. [Modes for carrying out the invention]

[0018] [Method for producing high-purity silica gel] When alkali silicate and an acid are neutralized under strongly acidic conditions, silica precipitates, and depending on the conditions, primary particles of a few nanometers to tens of nanometers in size are produced. These primary particles immediately aggregate to form silica gel with a multi-order aggregated structure. Under strongly acidic conditions with a pH of 1.0 or lower, the aggregation of primary particles occurs while the growth of silica primary particles is suppressed, and silica gel is produced. The mechanism by which metal impurities contained in alkali silicate are removed is presumed to be as follows: Alkali silicate usually forms structures such as monomers and dimers, but in this state, it is presumed that many metal ions exist in the solvent in the form of hydroxides. When alkali silicate is neutralized with an acid and adjusted to a strongly acidic state, polymerization of monomers proceeds, resulting in the form of silica polymers and primary particles. Metal hydroxides are also cationized. During this polymerization, primary particles are formed while incorporating metal impurities present in the solvent into the silica backbone and primary particles. Under these conditions, where the pH is sufficiently low, silicic acid tends to form silicate cations, which repel metal cations electrically, thereby inhibiting their penetration into the silicic acid. Furthermore, metal impurities incorporated into low-polymer silicic acid or primary particles are leached under strongly acidic conditions and eluted into the solvent. It is presumed that these metal impurities eluted into the solvent are removed by post-treatment such as washing or ion exchange. It should be noted that leaching tends to proceed more effectively when the pore volume and pore diameter of the silica's multi-order aggregate structure are large. Prior art, for example, Patent Document 2, describes a reaction in which a high concentration of alkali silicate is added to an acid solution under strongly acidic and high-temperature conditions for a short period of time. In this case, the localized increase in silica concentration causes polymerization of silicic acid to proceed rapidly, generating a large amount of primary silica particles. Since the salt concentration in the system is very high, the primary silica particles rapidly aggregate and gel. When a high concentration of alkali silicate is used as the alkali silicate, silicic acid with a relatively large molecular weight is generated, and this silicic acid further polymerizes, so the primary diameter of the primary silica particles tends to be relatively large, for example, about 7 nm. Furthermore, because a relatively dense multi-order aggregated structure is formed by the short-time addition of high concentration alkali silicate, the pore volume of the silica gel tends to decrease. Under such manufacturing conditions, leaching is performed before silicic acid polymerization and in the low-polymerization state, but leaching from silica gel with large primary particles and a small pore volume results in poor metal impurity removal because metal impurities do not easily come into contact with the acid. On the other hand, in this embodiment, a low concentration of alkali silicate is added over a long period of time to an acid solution obtained by mixing an inorganic acid and hydrogen peroxide, under strongly acidic and high-temperature conditions with a pH of 1.0 or less and a temperature of 68°C to 90°C. In this case, silica with a small molecular weight is produced, and furthermore, because the polymerization rate is slow under such conditions, small primary particles of about 4 nm are produced. Also, by adding alkali silicate at a low rate over a long period of time, the silica concentration in the system gradually increases. Here, the generated primary particles gradually aggregate, and silica gel with a relatively large pore volume is produced. Therefore, in this embodiment, the penetration of metal impurities into the silica backbone and silica primary particles is suppressed first, and leaching occurs in the low-polymer silica and then in the silica primary particles. Subsequently, compared to the prior art (the invention of claim 3 of Patent Document 4), the silica gel of the present invention has smaller primary particles and a larger pore volume, so the diffusion of acid into the silica gel is easy, and leaching from the primary particles is likely to occur. As a result, under the manufacturing conditions of the present invention, silica gel advantageous for removing metal impurities can be obtained, and high-purity silica gel can be achieved through a simple process. The present embodiment relates to a method for producing high-purity silica gel, which involves carrying out a wet reaction between alkali silicate and an acid in a strongly acidic and high-temperature region to generate an acidic solution of silica gel, simultaneously extracting metal impurities from the raw material alkali silicate and the generated silica gel, and then separating the silica gel from the acidic solution, washing it, and obtaining high-purity silica gel. More specifically, the method for producing high-purity silica gel comprises the following steps 1 and 2. (Process 1) A process to obtain an acid solution of silica gel by adding an aqueous alkali silicate solution to an acid solution obtained by mixing an inorganic acid and hydrogen peroxide at an addition rate of 0.001 g / min·g to 0.01 g / min·g (calculated in terms of silica solids) under conditions where the pH of the acid solution is 1.0 or less and the temperature is in the range of 68°C to 90°C, thereby generating silica gel, and then aging the mixture at a temperature in the range of 68°C to 90°C to obtain an acid solution of silica gel. (Process 2) A step to separate silica gel from the silica gel acid solution obtained in step 1, and wash the silica gel with water until the pH of the water wash water for the silica gel is 4.8 or higher and the conductivity is 10 μS / cm or less, thereby obtaining high-purity silica gel. According to this embodiment, the multi-stage metal impurity removal process, which consists of silica gel preparation and acid washing of silica gel, as seen in conventional methods for producing high-purity silica gel, is unnecessary. Furthermore, according to this embodiment, organic acids, which pose a problem in terms of wastewater treatment costs, are unnecessary. In other words, the method for producing high-purity silica gel according to this embodiment consists of only a single-stage metal impurity removal process using a mixed acid solution of inorganic acid and hydrogen peroxide, without the need for multi-stage metal impurity removal processes or the addition of organic acids.

[0019] [Process 1] (Inorganic acid) The type of inorganic acid used in the preparation of silica gel is not particularly limited. Examples of inorganic acids include hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid. The amount of these inorganic acids used will vary depending on the type of inorganic acid and the concentration of the aqueous solution of the inorganic acid, but it is not particularly limited as long as the pH range of the acid solution of the silica gel after the silica gel is prepared in step 1 is within the range of 1.0 or less.

[0020] (hydrogen peroxide) In this embodiment, silica is produced by a neutralization reaction between an aqueous alkali silicate solution containing an alkali silicate such as sodium silicate and an inorganic acid, and then silica gel is produced by polymerization of the silica. Here, it is known that the aqueous alkali silicate solution usually contains polyvalent metal impurities, particularly Ti and Al. Since such Ti and other metals are easily incorporated into the silicate skeleton during silica production, in this embodiment, hydrogen peroxide is added to the inorganic acid beforehand to prevent the incorporation of Ti and other metals into the silicate skeleton. Hydrogen peroxide forms a complex structure with polyvalent metal ions and has the function of stabilizing the metal ions in the liquid phase. In particular, hydrogen peroxide readily reacts with Ti to form a peroxotitanium complex, thus promoting the removal of Ti from the alkali silicate by the inorganic acid. The amount of hydrogen peroxide used is preferably 5 parts by mass or more and 35 parts by mass or less per 100 parts by mass of silica contained in the alkali silicate aqueous solution. Within this range, the silica gel obtained by the manufacturing method according to this embodiment exhibits a sufficient metal impurity removal effect, which is preferable. If the amount of hydrogen peroxide used is less than 5.0 parts by mass, the metal impurity removal effect of the silica gel obtained by the manufacturing method according to this embodiment will not be sufficient. If the amount of hydrogen peroxide used exceeds 30 parts by mass, the metal impurity removal effect commensurate with the amount used will not be obtained, and it will only result in a high concentration of hydrogen peroxide in the wastewater, increasing the effort required for wastewater treatment. More preferably, the range of hydrogen peroxide used is recommended to be 5 parts by mass or more and 20 parts by mass or less. In this embodiment, an aqueous solution of hydrogen peroxide is typically used as the hydrogen peroxide. In this specification, the aqueous solution of hydrogen peroxide is also referred to as "hydrogen peroxide water."

[0021] (Preparation of acid solution) The acid solution can be prepared, for example, by adding an aqueous inorganic acid solution to a reaction vessel and continuously or intermittently adding hydrogen peroxide while stirring. Preferably, it is recommended to heat the aqueous inorganic acid solution to room temperature or higher before continuously or intermittently adding hydrogen peroxide. Heating the aqueous inorganic acid solution is preferable because it promotes the reaction between the alkali silicate and the inorganic acid, making the silicate easier to gel and promoting the acid dissolution of metal impurities. The temperature of the aqueous inorganic acid solution is recommended to be in the range of 68°C to 90°C. The amounts of inorganic acid aqueous solution (inorganic acid equivalent mass) and hydrogen peroxide (H2O2 equivalent mass) used relative to the alkali silicate aqueous solution (silica equivalent mass) are as described above. The mixing ratio of inorganic acid aqueous solution (inorganic acid equivalent mass) and hydrogen peroxide (H2O2 equivalent mass) when preparing the acid solution is not particularly limited as long as it is within the range of the amounts used relative to the alkali silicate aqueous solution (silica equivalent mass). Typically, the mass ratio of inorganic acid aqueous solution (inorganic acid equivalent mass) to hydrogen peroxide (H2O2 equivalent mass) is desirable to be in the range of 100:1 to 100:50 (however, this is limited to the range that satisfies the above restrictions).

[0022] Generally, the process of dissolving impurities contained in a solid phase with an acid or alkali is called leaching, and it refers to the operation of dissolving or dissolving specific elements from the solid phase into an aqueous solution. Acid leaching is also specifically called "acid leaching." It is known that the lower the pH after acid leaching, the more effective it is in dissolving specific elements from the solid phase (silica component) in water glass.

[0023] [Preparation of an acidic solution of silica gel] In this embodiment, silica gel is produced by adding an alkaline silicate aqueous solution at an addition rate of 0.001 g / min·g to 0.01 g / min·g (calculated in terms of silica solids) to an acid solution obtained by mixing an inorganic acid and hydrogen peroxide as described above, under conditions where the pH of the acid solution is 1.0 or less and the temperature is in the range of 68°C to 90°C. Silica gel is then produced, and subsequently, the mixture is aged within the aforementioned temperature range to obtain an acid solution of silica gel. Here, the acid solution used when adding the alkaline silicate aqueous solution must have a pH of 1.0 or lower and a temperature between 68°C and 90°C. A pH of 1.0 or lower in the acid solution is preferable because it facilitates the acid dissolution of metal impurities. If the pH exceeds 1.0, the acid dissolution of metal impurities becomes insufficient, leading to a significant decrease in the metal impurity removal effect. For example, at a pH of 2.0, Na, Al, Ti, or Fe will remain at concentrations of tens to tens of ppm. Furthermore, a temperature of 68°C to 90°C in the acid solution is preferable because it promotes the reaction between the alkaline silicate and the inorganic acid, making it easier for the silica to gel and facilitating the extraction of metal impurities into the liquid phase (acid solution). If the temperature is below 68°C, the gelation of the silica may be insufficient, and the extraction of metal impurities into the liquid phase (acid solution) may be insufficient. If the temperature exceeds 90°C, evaporation of water and decomposition of the inorganic acid and hydrogen peroxide will occur, hindering the preparation of silica gel. The pH of the acid solution is more preferably 0.5 or lower. The temperature of the acid solution is more preferably in the range of 70°C to 88°C, and even more preferably in the range of 70°C to 85°C.

[0024] (Alkali silicate solution) In step 1, an aqueous alkali silicate solution is added to a mixed acid solution of an inorganic acid and hydrogen peroxide. The aqueous alkali silicate solution used here can be an aqueous sodium silicate solution or an aqueous potassium silicate solution, etc. In particular, so-called water glass (sodium silicate) or diluted water glass can be suitably used as the aqueous sodium silicate solution. The silica concentration (A) in the aqueous alkali silicate solution is preferably in the range of 10.5% by mass or more and 18.0% by mass or less. A silica concentration (A) within this range is preferable because it makes rapid gelation due to contact between the alkali silicate and the acid solution less likely to occur during the silica gel preparation process. More preferably, the silica concentration (A) in the aqueous alkali silicate solution is recommended to be in the range of 11.0% by mass or more and 17.5% by mass or less. Even more preferably, a range of 12.0% by mass or more and 17.0% by mass or less is recommended. Such alkali silicate aqueous solutions within this concentration range can be obtained, for example, by diluting commercially available water glass with water, deionized water, pure water, or ultrapure water in the case of sodium silicate. Examples of such water glass include those represented by the chemical formula Na2O·nSiO2·mH2O, with an SiO2 concentration in the range of 22% to 38% by mass, a 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 or higher.

[0025] The rate at which the alkali silicate aqueous solution is added to the acid solution must be in the range of 0.001 g / min·g to 0.01 g / min·g (calculated in terms of silica solids). When the rate of addition of the alkali silicate aqueous solution is within this range, it is preferable because it suppresses rapid gelation due to contact between the alkali silicate and the acid solution, and allows for easy extraction of metal impurities into the liquid phase (acid solution) before the polymerization of silicate proceeds. If the rate of addition of the alkali silicate aqueous solution is less than 0.001 g / min·g, tiny silica gel particles that are difficult to separate solid and liquid are formed. On the other hand, if the rate of addition of the alkali silicate aqueous solution exceeds 0.01 g / min·g, the silicate gels rapidly, resulting in insufficient removal of metal impurities, which is undesirable. The addition rate of the alkali silicate aqueous solution to the acid solution is preferably in the range of 0.001 g / min·g to 0.008 g / min·g. More preferably, it is in the range of 0.001 g / min·g to 0.005 g / min·g. Here, the unit of addition rate "g / min·g" refers to the amount of silica solids supplied per unit time by the alkali silicate aqueous solution added per minute relative to the inorganic acid in the acid solution. Here, the unit "g / min·g" can also be expressed as "g / (min·g)". The addition time (B) of the alkali silicate aqueous solution to the acid solution is preferably in the range of 2.0h to 7.0h. An addition time (B) in the range of 2.0h to 7.0h is preferable because it suppresses rapid gelation due to contact between the alkali silicate and the acid solution, and allows for easy extraction of metal impurities into the liquid phase (acid solution) before the polymerization of silica proceeds. If the addition time (B) is less than 2.0h, rapid gelation of silica proceeds, resulting in insufficient removal of metal impurities. Furthermore, if the addition time (B) exceeds 7.0h, fine silica gel that is difficult to separate from the solid-liquid is generated, which is undesirable. In addition, the long addition time reduces production efficiency and impairs economic viability. More preferably, an addition time (B) of 2.5h to 6.5h is recommended, and even more preferably, a range of 3.0h to 6.0h is recommended. In this specification, the unit "h" or "hrs" means hours. For example, 1.0h means 1.0 hour.

[0026] By adding an aqueous alkali silicate solution to an acid solution under the conditions described above, silica gel formation proceeds. After the addition of the aqueous alkali silicate solution to the acid solution is complete, the acid solution in which silica gel formation is underway is aged in a temperature range of 68°C to 90°C (the same temperature range as when the aqueous alkali silicate solution was added). This allows for the extraction of metal ions from within the silica skeleton mainly by acid leaching, thereby obtaining an acid solution containing silica gel. The aging temperature (E) of the silica gel must be in the range of 68°C to 90°C. Below 68°C, the gelation of silicic acid is insufficient, and the extraction of metal impurities into the liquid phase (acid solution) is insufficient. Above 90°C, evaporation of water and decomposition of inorganic acids and hydrogen peroxide occur, which is undesirable. More preferably, the aging temperature (E) of the silica gel is recommended to be in the range of 70°C to 88°C. Even more preferably, a range of 70°C to 85°C is recommended. Furthermore, the maturation time (D) of the silica gel is preferably in the range of 0.16h to 7.0h. A maturation time (D) within this range is desirable because it allows for sufficient extraction of metal impurities from the silica gel. If the maturation time (D) is less than 0.16h, the extraction of metal impurities from the silica gel will be insufficient, and in particular, Al or Ti will be incorporated into the silica skeleton, which is undesirable. If the maturation time (D) exceeds 7.0h, the effect of removing metal impurities from the silica gel will saturate, which is undesirable. More preferably, a maturation time (D) of 0.20h to 6.8h is recommended. Even more preferably, a range of 0.23h to 6.5h is recommended. Furthermore, the silica concentration (C) of the silica gel acid solution prepared in step 1 is preferably in the range of 6.0% by mass or more and 7.5% by mass or less. When the silica concentration (C) of the silica gel acid solution is in the range of 6.0% by mass or more and 7.5% by mass or less, silica gel that is relatively easy to separate solid and liquid is obtained while suppressing rapid gelation due to neutralization or polymerization of alkali silicate, and metal impurities can be easily extracted into the liquid phase (acid solution) before the polymerization of silicic acid proceeds, which is preferable. If the silica concentration (C) of the silica gel acid solution is less than 6.0% by mass, siloxane bonds develop densely, causing the silica gel to become finer, which is problematic as it produces silica gel that is difficult to separate solid and liquid. On the other hand, if the silica concentration (C) of the silica gel acid solution exceeds 7.5% by mass, it may lead to rapid gelation due to contact between alkali silicate and inorganic acid, which is undesirable. The silica concentration (C) of the silica gel acid solution is more preferably in the range of 6.1% by mass or more and 7.2% by mass or less. Furthermore, it is even more preferably in the range of 6.2% by mass or more and 7.0% by mass or less. In this specification, the acidic solution of silica gel is also referred to as "silica gel acidic solution." Furthermore, unless otherwise specified, when simply referred to as "acidic solution," it refers to the solution used in step 1. This refers to "an acidic solution obtained by mixing an inorganic acid and hydrogen peroxide."

[0027] [Process 2] In step 2, silica gel is separated from the silica gel acid solution obtained in step 1, and the silica gel is washed with water until the pH of the rinse water (filter water) is 4.8 or higher and the conductivity is 10 μS / cm or lower to obtain high-purity silica gel. To separate silica gel from silica gel acid solution, methods such as filtration separation or centrifugation can be applied. The silica gel separated from the silica gel acid solution is purified by filtration washing with hot water until the rinse water (filtrate) has a pH of 4.8 or higher and a conductivity of 10 μS / cm or less, thereby obtaining high-purity silica gel. Here, a pH of 4.8 or higher in the rinse water is desirable because it sufficiently removes metal ions, metal complexes, or inorganic anions from the silica gel. If the pH of the rinse water is less than 4.8, the removal of metal ions, metal complexes, or inorganic anions from the silica gel is insufficient, which is undesirable. Furthermore, it is desirable that the conductivity of the rinse water be 10 μS / cm or less. A conductivity of 10 μS / cm or less is desirable because it sufficiently removes metal ions, metal complexes, or inorganic anions from the silica gel. On the other hand, if the conductivity exceeds 10 μS / cm, the removal of metal ions, metal complexes, or inorganic anions from the silica gel is insufficient, which is undesirable. The pH of the rinse water is preferably 5.0 or higher. Furthermore, its conductivity is more preferably 9.0 μS / cm or lower.

[0028] The properties of the high-purity silica gel obtained through step 2 include primary particles formed by the polymerization of silicic acid, secondary particles formed by the aggregation of primary particles, or multi-order aggregates formed by further aggregation of secondary particles. The primary diameter (G) (particle diameter equivalent to specific surface area) of this high-purity silica gel is preferably in the range of 3.5 nm to 4.5 nm. This range is preferable because it facilitates the diffusion of inorganic acids into the primary particles and the diffusion (removal) of metal impurities from the primary particles. If the primary diameter (G) of the high-purity silica gel is less than 3.5 nm, this is the lower limit of particle size at which silica particles can exist as primary particles. Even if primary particles smaller than 3.5 nm can be prepared, this poses a problem in terms of solid-liquid separation of the silica gel during the washing process. Furthermore, if the primary diameter (G) of the high-purity silica gel exceeds 4.5 nm, the diffusion of inorganic acids into the primary particles becomes difficult, resulting in insufficient removal of metal impurities. The method for measuring the primary diameter (particle diameter equivalent to specific surface area) is described later in this specification.

[0029] The average particle size (H) (laser diffraction / scattering method) of high-purity silica gel is preferably in the range of 25.0 μm to 53.0 μm. A particle size (H) within this range is advantageous because it facilitates contact between the inorganic acid and the silica gel, and allows for relatively easy solid-liquid separation. If the secondary diameter (H) of the high-purity silica gel is less than 25.0 μm, while contact between the inorganic acid and the silica gel is easy, solid-liquid separation is problematic. On the other hand, if the average particle size exceeds 53.0 μm, while it is advantageous for solid-liquid separation, the diffusion of the inorganic acid into the silica gel is difficult, potentially leading to poor cleaning during the washing process, and is therefore undesirable. More preferably, the average particle size (H) of high-purity silica gel is recommended to be in the range of 30.0 μm to 53.0 μm. The method for measuring the average particle size (laser diffraction / scattering method) is described later in this specification.

[0030] The pore volume (I) of the high-purity silica gel (the pore volume of silica gel determined by the nitrogen adsorption method) is preferably in the range of 0.50 mL / g to 1.50 mL / g. If the pore volume (I) of the high-purity silica gel is within this range, inorganic acids diffuse easily into the silica aggregate, which is advantageous in terms of removing metal impurities from within the silica framework. If the pore volume (I) of the high-purity silica gel is less than 0.50 mL / g, inorganic acids do not diffuse easily into the silica aggregate, and similarly, if the pore volume (I) exceeds 1.50 mL / g, the metal impurity removal effect becomes saturated. More preferably, the pore volume (I) of the high-purity silica gel is recommended to be in the range of 0.60 mL / g to 1.00 mL / g. The method for measuring the pore volume (I) (the pore volume of silica gel determined by the nitrogen adsorption method) is described later in this specification.

[0031] In this embodiment, it is preferable that the high-purity silica gel has a Na concentration of less than 3.5 ppm, an Al concentration of less than 4.0 ppm, a Ca concentration of less than 3.0 ppm, a Ti concentration of less than 2.5 ppm, a K concentration of less than 1.5 ppm, and concentrations of Mg, Cr, Fe, Ni, Cu, and Zn of less than 1 ppm each, relative to its silica solid content.

[0032] [F1, the dominant factor in silica gel preparation] In the method for producing high-purity silica gel according to this embodiment, the following embodiment 1 is preferred. [Aspect 1: Dominant factor F1 in silica gel preparation] In this embodiment, it is preferable to prepare the silica gel in such a way that the value of the silica gel preparation governing factor (F1), represented by the following formula (1), is within the range of 1,370 to 595,350. F1 = A × B × C × D × E ... (1) A: Concentration of the added alkali silicate aqueous solution B: Addition time of alkaline silicate solution C: Silica concentration at the end of preparation D: Silica gel maturation time E: Silica gel maturation temperature

[0033] The inventors focused on the formulation conditions of silica gel in order to efficiently reduce metal impurities in a method for producing high-purity silica gel, and completed the present invention. In other words, when adding an aqueous sodium silicate solution to a mixed acid solution of inorganic acid and hydrogen peroxide in step 1, if F1 is the product of the concentration of the added alkaline silicate solution (A), the time of addition of the alkaline silicate solution (B), the silica concentration at the end of preparation (C), the silica gel maturation time (D), and the silica gel maturation temperature (E), it was found that if F1 is in the range of 1,370 to 595,350, the removal efficiency of metal impurities is good, and the impurity concentration can be reduced to the same level as conventional multi-stage processes with just one metal impurity removal step. Note that the "silica concentration at the end of preparation (C)" refers to the silica concentration (C) of the silica gel acid solution prepared in step 1.

[0034] The value of F1, the dominant factor in silica gel preparation, is more preferably in the range of 2,340 to 490,100, even more preferably in the range of 3,500 to 405,800, and most preferably recommended to be between 14,000 and 350,000.

[0035] In this embodiment, in formula (1), the value of A is preferably in the range of 10.5 (mass%) to 18.0 (mass%), more preferably in the range of 11.0 (mass%) to 17.5 (mass%), and even more preferably in the range of 12.0 (mass%) to 17.0 (mass%). The value of B is preferably in the range of 2.0 (hrs) to 7.0 (hrs), more preferably in the range of 2.5 (hrs) to 6.5 (hrs), and even more preferably in the range of 3.0 (hrs) to 6.0 (hrs). The value of C is preferably in the range of 6.0 (mass%) to 7.5 (mass%), more preferably in the range of 6.1 (mass%) to 7.2 (mass%), and even more preferably in the range of 6.2 (mass%) to 7.0 (mass%). The value of D is preferably in the range of 0.16 (hrs) to 7.0 (hrs), more preferably in the range of 0.20 (hrs) to 6.8 (hrs), and even more preferably in the range of 0.23 (hrs) to 6.5 (hrs). The value of E must be in the range of 68°C to 90°C, more preferably in the range of 70°C to 88°C, and even more preferably in the range of 70°C to 85°C.

[0036] [Physical properties of silica gel F2] In this embodiment, it is preferable that the silica gel obtained satisfies the range of 43 to 358 for the value of the silica gel physical property factor (F2) represented by the following formula (2). F2 = G × H × I ... (2) G: Primary diameter of silica gel (particle diameter converted to specific surface area) H: Average particle size of silica gel (laser diffraction / scattering method) I: Pore volume of silica gel determined by nitrogen adsorption method

[0037] The inventors have found that, within the range where the dominant factor for silica gel preparation (F1) is between 1,370 and 595,350, if F2 is the product of the primary diameter (G), average particle diameter (H), and pore volume (I) of the prepared silica gel, then if F2 is within the range of 43 to 358, then the impurity concentration can be reduced to a level equivalent to that of conventional multi-stage processes with only a single-stage metal impurity removal process.

[0038] The value of the physical property factor F2 of silica gel is preferably in the range of 43 to 358, and more preferably in the range of 60 to 220.

[0039] In this embodiment, in formula (2), the value of G is preferably in the range of 3.5 nm to 4.5 nm, and more preferably in the range of 3.5 nm to 4.3 nm. The value of H is preferably in the range of 25.0 (μm) to 53.0 (μm), and more preferably in the range of 30.0 (μm) to 50.0 (μm). The value of I is preferably in the range of 0.50 mL / g or more and 1.50 mL / g or less, and more preferably in the range of 0.60 mL / g or more and 1.00 mL / g or less.

[0040] [Method for manufacturing high-purity water glass] The method for producing high-purity water glass according to this embodiment is a method comprising the following step 3, following steps 1 and 2 described above. (Step 3) A step to obtain high-purity water glass in which the high-purity silica gel obtained in step 2 is dissolved in an aqueous alkali hydroxide solution, and the Al concentration is less than 4.0 ppm, the Ca concentration is less than 3.0 ppm, the Ti concentration is less than 2.5 ppm, and the concentrations of Mg, Cr, Fe, Ni, Cu, and Zn are each less than 1 ppm, relative to the silica solid content. When dissolving the high-purity silica gel obtained through step 2 in an aqueous alkali hydroxide solution, the dissolution is promoted by stirring the aqueous alkali hydroxide solution containing the high-purity silica gel. To further accelerate the dissolution of the high-purity silica gel, the aqueous alkali hydroxide solution containing the high-purity silica gel may be heated while stirring, if desired. Here, the temperature after heating is preferably in the range of 40°C to 98°C. More preferably, a temperature range of 50°C to 98°C is recommended. The high-purity silica gel can be dissolved in the aqueous alkali hydroxide solution to obtain high-purity aqueous water glass (also referred to herein as "gel-dissolved aqueous water glass" or "high-purity alkali silicate aqueous solution").

[0041] [Method for producing a high-purity silica microparticle dispersion] The method for producing a high-purity silica fine particle dispersion according to this embodiment is a method comprising the following steps 4 and 5, following steps 1 to 3 described above. (Step 4) A step in which the high-purity water glass obtained in step 3 is subjected to cation exchange to obtain a purified silica solution. (Step 5) A step to obtain a high-purity silica fine particle dispersion in which, in addition to a portion of the purified silicic acid solution obtained in step 4, an aqueous alkali hydroxide solution is added to a portion of the purified silicic acid solution obtained in step 4, the pH is adjusted to a range of 10 to 13, the solution is heated and the temperature is maintained at a range of 50°C to 98°C for 20 minutes or more, and further, another portion of the purified silicic acid solution obtained in step 4 is added at an addition rate of 0.001 g / min·g to 0.5 g / min·g (in terms of silica solids), and the Na concentration is less than 1.5 ppm, the Al concentration is less than 2.5 ppm, the Ti concentration is less than 1.5 ppm, and the concentrations of Mg, Ca, Cr, Fe, Ni, Cu, and Zn are each less than 1 ppm.

[0042] In step 4, the high-purity water glass obtained in step 3 is subjected to cation exchange to obtain a purified silica solution. (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 preferred. The cation exchange resin can be in the form of beads, fibers, etc., and its form is not limited. In this embodiment, it is preferred to fill the column with the cation exchange resin and pass the liquid through it. The cation exchange may be performed once or multiple times as needed.

[0043] (Cation exchange temperature) Generally, in ion exchange operations, the ion exchange temperature and the contact time [space velocity (SV)] between the liquid being exchanged and the ion exchange resin are important factors. Here, the ion exchange temperature can be the temperature of the solution before it is passed through the ion exchange resin, the temperature inside the column packed with ion exchange resin, the temperature of the solution containing the ion exchange resin, the temperature inside the ion exchange column during ion exchange, and the temperature of the solution flowing out from the column outlet of the ion exchange column. In this embodiment, the ion exchange temperature was set to the temperature of the solution before it is passed through the ion exchange resin as a factor that greatly affects the ion exchange reaction. Generally, the higher the temperature of an ion exchange reaction, the higher the ion exchange efficiency and the greater the leaching effect, which will be discussed later. However, in the case of ion exchange with an alkaline silicate aqueous solution, if the temperature is high, a silicate solution in which polymerization has progressed is obtained. If particles are prepared using this silicate solution, self-nuclei are formed by silica, and silica nanoparticles of the desired size may not be obtained.

[0044] The temperature of the alkali silicate aqueous solution before passing it through a column packed with cation exchange resin is preferably in the range of 3°C to 18°C. Within this temperature range, ion exchange efficiency is excellent, and the resulting acidic silicate solution is stable. If the temperature of the alkali silicate aqueous solution is below 3°C, cation exchange may not proceed efficiently, and further lowering the temperature does not tend to significantly improve the stability of the silicate solution. If the temperature of the alkali silicate aqueous solution exceeds 18°C, although the adsorption effect of metal impurities increases, polymerization of the acidic silicate solution tends to progress, making it more prone to gelation, and thus the stability of the acidic silicate solution tends to decrease. Furthermore, when particles are prepared using such an acidic silicate solution, self-nucleation by silica may occur, resulting in the inability to obtain particles of the desired size. The temperature of the alkali silicate aqueous solution before passing it through the column is preferably in the range of 5°C to 15°C, more preferably in the range of 6°C to 15°C, and most preferably in the range of 7°C to 15°C. Generally, if the temperature of the alkali silicate aqueous solution is within the above temperature range, it can be passed through the column as is. If the temperature exceeds the above temperature range, it should be cooled to adjust the temperature. Furthermore, it is desirable to pre-condition the cation exchange resin packed in the column to a temperature of 3°C to 18°C ​​using means such as refrigeration to prevent temperature rise during passage.

[0045] (space velocity) The space velocity when passing an alkaline silicate aqueous solution through a column packed with cation exchange resin is 2h -1 6 hours -1It is preferably within the following range. When the space velocity is within this range, the contact time between the alkaline silicate aqueous solution and the cation exchange resin becomes a sufficient level, which is preferable in terms of ion exchange efficiency. When the space velocity is less than 2 h -1 Although the ion removal effect increases, it takes time for ion exchange, so the productivity is poor and there is a tendency for economic problems. When the space velocity exceeds 6 h -1 Although the ion exchange time is short and the economy is good, there is a tendency for problems because the ion removal efficiency decreases, or a solution bypass occurs in the ion exchange column and there is a possibility that a solution that is not ion-exchanged flows out. This space velocity is more preferably within the range of 2.5 h -1 or more and 5.0 h -1 or less. Note that when generally representing the velocity at which a liquid passes through a filter medium, the space velocity is used. In the case of this specification, the space velocity when passing a solution through a column filled with a cation exchange resin is the flow rate per unit volume of the cation exchange resin. Therefore, the relationship is (flow rate (m 3 / h)÷resin volume (m 3 ) = space velocity (h -1 ).

[0046] In Step 5, an aqueous alkaline hydroxide solution is added to a part of the purified silicic acid solution obtained in Step 4, heated within the range of 50°C or higher and 98°C or lower, held at a temperature within the range of 50°C or higher and 98°C or lower for 20 minutes or longer, and further, a part of the purified silicic acid solution obtained in Step 4 is added within the range of an addition rate of 0.001 g / min·g or higher and 0.5 g / min·g or lower (in terms of silica solid content conversion) to obtain a highly pure silica fine particle dispersion liquid.

[0047] Specifically, a portion of the purified silicic acid solution obtained in step 4 can be used as the seed solution, and another portion as the feed solution. The seed solution is also called a "seed particle dispersion," and usually refers to a dispersion containing the silicic acid polymer (seed particles) contained in the silicic acid solution. The feed solution is the purified silicic acid solution supplied to grow the seed particles and consists of the same components as the seed solution. Since the purified silicic acid solution is unstable, undergoing polymerization rapidly at room temperature, it is desirable to use the purified silicic acid solution obtained in step 4 as either the seed solution or the feed solution immediately after preparation. However, as long as the stability of the purified silicic acid solution used in step 5 is maintained at the desired level, the "seed solution from a portion of the purified silicic acid solution" obtained in step 4 may be replaced with the purified silicic acid solution obtained in the same process as steps 1 to 4, and the "another portion of the purified silicic acid solution" obtained in step 4 may be replaced with the purified silicic acid solution obtained in the same process as steps 1 to 4 as the feed solution. It is desirable to add alkali to the seed solution to adjust the pH of the solution to a range of 10 to 13, preferably 10.5 to 12. Next, the temperature of this solution is raised to a range of 50°C to 98°C, more preferably 60°C to 98°C, and even more preferably 65°C to 98°C. After raising the temperature, the temperature is maintained for a predetermined time, and then the feed solution is added to mix the seed solution and the feed solution, allowing for particle growth. The holding time at this time is recommended to be in the range of 20 minutes or more, preferably 25 minutes or more.

[0048] The type of alkali used is not particularly limited, but for example, potassium hydroxide, sodium hydroxide, and organic alkalis can be used. The amount of alkali used should be enough to adjust the pH of the seed solution to a range of pH 10 to 13. The amount of feed solution used is 100:100 to 100:10,000 parts by mass (silica equivalent) relative to the seed solution. The rate at which the feed solution is added is more preferably in the range of 0.002 g / min·g to 0.3 g / min·g, and even more preferably in the range of 0.01 g / min·g to 0.09 g / min·g. Here, the unit of addition rate "g / min·g" refers to the amount of silica solids supplied per minute of feed solution relative to the silica solids in the seed solution. The unit "g / min·g" can also be expressed as "g / (min·g)".

[0049] Here, the feed solution is added to the seed solution at a temperature preferably in the range of 1°C to 30°C, more preferably in the range of 1°C to 20°C. Furthermore, a pH adjuster may be added simultaneously with the feed solution, if necessary.

[0050] Preferably, when the feed solution is gradually added to the seed solution adjusted to the pH and temperature as described above, silica microparticles grow in the seed solution, and a silica microparticle dispersion (silica sol) is obtained. After adding the feed solution to the seed solution, it is preferable to maintain the mixture at a temperature of approximately 50°C to 98°C for several tens of minutes to several hours, as this allows the added purified acidic silicic acid solution to sufficiently deposit on the silica microparticles, making it easier for the silica microparticles to grow.

[0051] In step 5, after adding the feed solution in the first stage, the rate at which the feed solution is added may be changed in the second stage, and the feed solution may be added again. In the second step, a feed solution, which is part of the purified acidic silicic acid solution obtained in step 4, may be added at a rate of 1.0 to 10.0 times the rate of the first step to obtain a high-purity silica fine particle dispersion.

[0052] Furthermore, in this embodiment, the average particle size of the silica microparticles in the resulting high-purity silica microparticle dispersion is not particularly limited. For example, a silica microparticle dispersion with an average particle size of 3 nm to 300 nm (dynamic light scattering method) can be obtained.

[0053] [High-purity silica fine particle dispersion] The high-purity silica microparticle dispersion according to this embodiment can be obtained by the method for producing the high-purity silica microparticle dispersion according to this embodiment described above. The impurity concentration of the high-purity silica fine particle dispersion according to this embodiment is such that the Na concentration is less than 1.5 ppm, the Al concentration is less than 2.5 ppm, the Ti concentration is less than 1.5 ppm, and the concentrations of Mg, Ca, Cr, Fe, Ni, Cu, and Zn are each less than 1 ppm relative to the silica solid content. [Examples]

[0054] Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited in any way by these examples. Furthermore, unless otherwise specified, the methods for measuring various properties of high-purity silica gel, etc., in the examples and comparative examples were carried out by the methods described below. Hereafter, unless otherwise specified, the concentration indication displayed following the indication "sulfuric acid aqueous solution," etc., means sulfuric acid concentration. If "sulfuric acid" is another substance, it means the concentration of that substance. On the other hand, if there is a limitation before the concentration indication, that limitation shall be followed.

[0055] <Measurement of silica gel, water glass, and silica sol> 1. Purity Metal content (Al, Ti, Fe, Ca, Mg, Cr, Ni, Cu, Zn, K, or Na) 1) Place approximately 10 g of the sample (silica gel, water glass, or silica sol) into a platinum dish and weigh it to the nearest 0.1 mg. 2) Next, add 5 mL of nitric acid and 20 mL of hydrofluoric acid, heat on a sand bath and evaporate to dryness. When the liquid volume has decreased, add another 20 mL of hydrofluoric acid and heat on a sand bath and evaporate to dryness. 3) After cooling to room temperature, add 2 mL of nitric acid and approximately 50 mL of water, and heat and dissolve on a sand bath. 4) After cooling to room temperature, the obtained solution is placed in a flask (100 mL) and diluted with water to 100 mL to prepare the sample solution. 5) The content of each metal present in the sample solution was measured using the following measuring device.

[0056] [Content of Al, Ti, Fe, Ca, Mg, Cr, Zn, and K] The measurements were taken using an inductively coupled plasma atomic emission spectrometer (SPS5520, manufactured by Seiko Instruments Inc.). This instrument introduces a solution-containing sample into a high-frequency inductively coupled argon plasma, causing 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 (Agilent Technologies, Inc., AA240Z) was used. The sample is vaporized using a flame, and the atomic vapor layer is irradiated with light of an appropriate wavelength. The intensity of the light absorbed by the atoms is measured to quantify the elemental concentration in the sample. A graphite furnace is used. Measurement mode: atomic absorption, measurement wavelength range: 190-900 nm. [Na content] An atomic absorption spectrophotometer (Hitachi, Ltd., Z-2310, with a measurement wavelength range of 190-900 nm) was used.

[0057] 2. Silica concentration The silica concentration in silica gel, water glass, and silica sol was determined by adding 2 mL of 50% sulfuric acid aqueous solution to 10 g of the sample, evaporating it to dryness on a platinum dish, and then calcining the resulting solid at 1000°C for 1 hour, followed by cooling and weighing. Next, the weighed solid was dissolved in a small amount of 50% sulfuric acid aqueous solution, and then 20 mL of hydrofluoric acid was added. The mixture was then evaporated to dryness on a platinum dish, calcined at 1000°C for 15 minutes, cooled, and weighed. The silica content was determined from the weight difference between these measurements.

[0058] Based on the measurement results from 1 and 2 above, the concentrations (imperfection concentrations) of each element (Al, Ti, Fe, Ca, Mg, Cr, Ni, Cu, Zn, K, or Na) relative to the silica solid content of silica gel, water glass, and silica sol were measured.

[0059] 3. pH measurement pH measurements of silica gel acid solution or rinse water were performed by taking approximately 50g of the sample into a polyethylene sample bottle, immersing it in a 25°C constant temperature bath for at least 30 minutes, and then inserting the glass electrode of a Horiba F22 pH meter, which had been calibrated with pH 4, 7, and 9 standard solutions.

[0060] 4. Measurement of the primary diameter of silica gel For each example and comparative example, 0.03 g of silica gel obtained was calcined at 300°C for 1 hour, placed in a measuring cell, and degassed under vacuum at 300°C for 3 hours. Nitrogen was adsorbed onto the sample using a Microtrac-Bel pore distribution analyzer BELSORP-mini (standard method) to obtain an adsorption isotherm. The specific surface area was determined from this adsorption isotherm by the BET analysis method. The specific surface area-converted particle diameter (primary diameter: G) obtained from the specific surface area of ​​the silica gel by equivalent sphere conversion was calculated using the following formula. G(nm)=6000 / (ρ・Specific surface area) Here, ρ is the particle density, which is 2.20 g / cm³. 3 Let's assume that.

[0061] 5. Measurement of the average particle size of silica gel For each example and comparative example, silica gel was added to ultrapure water to a silica solid content of 0.1% by mass, and stirred for 30 minutes to obtain an aqueous silica gel solution. The particle size distribution of this solution was measured using a laser diffraction / scattering particle size distribution analyzer manufactured by HORIBA Corporation, and the average particle size was determined.

[0062] 6. Measurement of pore volume of silica gel For each example and comparative example, 0.03 g of silica gel obtained was calcined at 300°C for 1 hour, placed in a measuring cell, and degassed under vacuum at 300°C for 3 hours. Using a Microtrac-Bel pore distribution analyzer BELSORP-mini (standard method), nitrogen was adsorbed onto the sample to obtain an adsorption isotherm. The pore volume at a relative pressure of 0.990 was determined from this adsorption isotherm by BET analysis.

[0063] 7. Measurement of the average particle size of silica microparticles in silica sol The average particle size of the silica sol (silica microparticle dispersion) obtained in each example and comparative example was measured using nanoSAQLA, a particle size analyzer manufactured by Otsuka Electronics Co., Ltd., which uses the dynamic light scattering method.

[0064] [Preparation of sodium silicate aqueous solution] 5,206.4 g of sodium silicate aqueous solution (silica concentration 21.6%) was diluted with ultrapure water to prepare sodium silicate aqueous solution (silica concentration 15.4% by mass).

[0065] [Preparation of potassium hydroxide aqueous solution (20% by mass)] A potassium hydroxide aqueous solution (20% by mass) was prepared by adding 1,629.6 g of ultrapure water and 1,137.6 g of potassium hydroxide aqueous solution (48.7% by mass concentration) to a 10 L Teflon®-coated separable flask and stirring until homogeneous.

[0066] [Example 1] [Preparation of high-purity silica gel] (Process 1) 692 g of ultrapure water and 8121.8 g of sulfuric acid (concentration 24.9% by mass) were added to a reaction vessel, and the temperature was raised to 80°C while stirring. After reaching 80°C, 378.3 g of hydrogen peroxide solution (concentration 31.1% by mass) (10.5 parts by mass per 100 parts by mass of silica solids) was added over 5 minutes to obtain an acid solution. To this solution, 7,302.4 g of an aqueous sodium silicate solution (containing 1,125 g of silica solids, with a silica concentration of 15.4% by mass) was added over 3 hours at an addition rate of 0.003 g / min·g (calculated based on silica solids). After the addition was complete, the solution was kept at 80°C for 2 hours, and then cooled to 40°C to obtain 16,494.5 g of silica gel acid solution (silica concentration of 6.8% by mass). The pH of this silica gel acid solution was 0.1. (Process 2) The silica gel acid solution obtained in step 1 was separated from the acid solution by filtration, and washed with ultrapure water at 50°C until the pH of the filtrate was 4.8 or higher and the conductivity of the filtrate was 10 μS / cm or lower, yielding 5,403.7 g of high-purity silica gel (silica concentration 18.4% by mass). The concentration of metal impurities in the obtained high-purity silica gel was measured, and the concentrations of each metal impurity relative to the silica solid content are shown in Table 2. Furthermore, the primary diameter (particle diameter converted to specific surface area), average particle diameter (laser diffraction / scattering method), and pore volume of the obtained silica gel are also shown in Table 2.

[0067] Table 2 shows the values ​​of the dominant factor (F1) for silica gel preparation in step 1 and the physical property factor (F2) for high-purity silica gel.

[0068] [Preparation of high-purity water glass] (Step 3) 2,771.6 g of potassium hydroxide aqueous solution (concentration 20% by mass) was mixed with 5,000 g of high-purity silica gel obtained in step 2 over 30 minutes. After the entire amount was added, the mixture was heated to 95°C while stirring to obtain silica gel aqueous solution. This silica gel aqueous solution was further stirred at 95°C for 4 hours to dissolve the silica gel, yielding 7,771.6 g of gel-dissolved water glass (high-purity water glass) (silica concentration 11.8% by mass) (potassium silicate aqueous solution, molar ratio SiO2 / K2O = 3.1). The concentration of metal impurities in the obtained high-purity water glass was measured, and the concentrations of each metal impurity relative to the silica solid content are as shown in Table 3. In this specification, water glass obtained by dissolving silica gel in an aqueous potassium hydroxide solution is referred to as "gel-dissolved water glass" or "high-purity water glass."

[0069] [Preparation of high-purity silica microparticle dispersion] (Step 4) 6,000 g of the gel-dissolved water glass obtained in the previous step was diluted with ultrapure water to prepare 14,208 g of potassium silicate aqueous solution (silica concentration 5.0% by mass). After cooling the obtained potassium silicate aqueous solution to 12°C, it was poured into 6 L of strongly acidic cation exchange resin (Duolite C255LFH, manufactured by Rohm & Haas) at a space velocity of 4.0 h. -1 The solution was passed through the material to obtain 12,940 g of acidic silicic acid solution (silica concentration 4.5% by mass) (pH 2.6). (Step 5) 356.56 g of the acidic silicic acid solution obtained in step 4 (containing 16.05 g of silica solids) was placed into a 10 L separable flask. Subsequently, a pre-prepared potassium hydroxide aqueous solution (concentration 1.34% by mass, obtained by stirring 18.67 g of potassium hydroxide aqueous solution (concentration 48.7% by mass) and 658.95 g of ultrapure water until homogeneous) was added to the flask and thoroughly stirred. After stirring, the silica concentration of the solution in the flask was 1.55% by mass, and the pH was 11.2. Next, the solution was heated to 83°C while stirring, and the temperature was maintained for 30 minutes after reaching 83°C. After maintaining the temperature at 83°C, 1,808.86 g of acidic silicic acid solution (containing 81.40 g of silica solids) was added over 3 hours at an addition rate of 0.020 g / min·g (calculated based on silica solids) (first stage addition). Subsequently, 7,235.42 g of acidic silicic acid solution (containing 325.59 g of silica solids) was added over 12 hours at an addition rate of 0.030 g / min (calculated based on silica solids) (second stage addition). After the addition was completed, the temperature was maintained at 83°C for 1 hour, and then cooled to room temperature to obtain a high-purity silica fine particle dispersion (high-purity silica sol). The obtained high-purity silica microparticle dispersion was concentrated to a silica concentration of 12% by mass using an ultramodule (SIP-1013, manufactured by Asahi Kasei Corporation), and then concentrated to a silica concentration of 40% by mass using a rotary evaporator. The concentration of metal impurities in the obtained silica microparticle dispersion was measured, and the concentrations of each metal impurity relative to the silica solid content are shown in Table 5. Furthermore, the average particle size of the silica microparticles in the obtained silica microparticle dispersion was 37 nm.

[0070] [Examples 2-10] Silica gel, water glass, and silica sol were prepared in the same manner as in Example 1, except that the conditions shown in Tables 1 to 4 were changed. The obtained silica gel, water glass, and silica sol were then evaluated in the same manner as in Example 1. The results are shown in Tables 1 to 5.

[0071] [Comparative Examples 1-4] Silica gel, water glass, and silica sol were prepared in the same manner as in Example 1, except that the conditions shown in Tables 1 to 4 were changed. The obtained silica gel, water glass, and silica sol were then evaluated in the same manner as in Example 1. The results are shown in Tables 1 to 5.

[0072] [Table 1]

[0073] [Table 2]

[0074] [Table 3]

[0075] [Table 4]

[0076] [Table 5]

Claims

1. A method for producing high-purity silica gel, comprising the following steps 1 and 2. (Step 1) A process to obtain an acid solution of silica gel by adding an aqueous alkali silicate solution to an acid solution obtained by mixing an inorganic acid and hydrogen peroxide at an addition rate of 0.001 g / min·g to 0.01 g / min·g (calculated in terms of silica solids) under conditions where the pH of the acid solution is 1.0 or less and the temperature is in the range of 68°C to 90°C, thereby producing silica gel, and then aging the mixture at a temperature in the range of 68°C to 90°C to obtain an acid solution of silica gel. (Step 2) A step to separate silica gel from the silica gel acid solution obtained in step 1, and wash the silica gel with water until the pH of the water wash water for the silica gel is 4.8 or higher and the conductivity is 10 μS / cm or less, thereby obtaining high-purity silica gel.

2. The method for producing high-purity silica gel according to claim 1, wherein the high-purity silica gel has a Na concentration of less than 3.5 ppm, an Al concentration of less than 4.0 ppm, a Ca concentration of less than 3.0 ppm, a Ti concentration of less than 2.5 ppm, a K concentration of less than 1.5 ppm, and the concentrations of Mg, Cr, Fe, Ni, Cu, and Zn are each less than 1 ppm.

3. A method for producing high-purity silica gel according to claim 1 or claim 2, wherein in step 1, the value of the dominant factor (F1) for silica gel preparation, represented by the following formula (1), is in the range of 1,370 or more and 595,350 or less. F1=A×B×C×D×E...(1) (A: Concentration of added alkali silicate aqueous solution, B: Time of addition of alkali silicate aqueous solution, C: Silica concentration at the end of preparation, D: Silica gel maturation time, E: Silica gel maturation temperature)

4. A method for producing high-purity silica gel according to claim 3, wherein in the formula (1) above, the value of A is in the range of 10.5% by mass or more and 18.0% by mass or less, the value of B is in the range of 2.0 h or more and 7.0 h or less, the value of C is in the range of 6.0% by mass or more and 7.5% by mass or less, the value of D is in the range of 0.16 h or more and 7.0 h or less, and the value of E is in the range of 68°C or more and 90°C or less.

5. The method for producing high-purity silica gel according to claim 3, wherein the value of the silica gel property factor (F2) represented by the following formula (2) in the high-purity silica gel is in the range of 43 to 358. F2=G×H×I...(2) (G: Primary diameter of silica gel (particle diameter converted to specific surface area), H: Average particle diameter of silica gel (laser diffraction / scattering method), I: Pore volume of silica gel determined by nitrogen adsorption method)

6. A method for producing high-purity silica gel according to claim 5, wherein in the formula (2) above, the value of G is in the range of 3.5 nm or more and 4.5 nm or less, the value of H is in the range of 25.0 μm or more and 53.0 μm or less, and the value of I is in the range of 0.50 mL / g or more and 1.50 mL / g or less.

7. A method for producing high-purity water glass, comprising the following step 3, following step 1 and step 2 in the method for producing high-purity silica gel according to claim 1 or claim 2. (Step 3) A step to obtain high-purity water glass in which the high-purity silica gel obtained in step 2 is dissolved in an aqueous alkali hydroxide solution, and the Al concentration is less than 4.0 ppm, the Ca concentration is less than 3.0 ppm, the Ti concentration is less than 2.5 ppm, and the concentrations of Mg, Cr, Fe, Ni, Cu, and Zn are each less than 1 ppm, relative to the silica solid content.

8. A method for producing a high-purity silica fine particle dispersion, comprising the following steps 4 and 5, following the steps 1, 2 and 3 of the method for producing high-purity water glass according to claim 7. (Step 4) A step in which the high-purity water glass obtained in step 3 is subjected to cation exchange to obtain a purified silica solution. (Step 5) A step to obtain a high-purity silica fine particle dispersion in which, in addition to a portion of the purified silicic acid solution obtained in step 4, an aqueous alkali hydroxide solution is added to a portion of the purified silicic acid solution obtained in step 4, the pH is adjusted to a range of 10 to 13, the solution is heated and the temperature is maintained at a range of 50°C to 98°C for 20 minutes or more, and further, another portion of the purified silicic acid solution obtained in step 4 is added at an addition rate of 0.001 g / min·g to 0.5 g / min·g (calculated in terms of silica solids), and the Na concentration is less than 1.5 ppm, the Al concentration is less than 2.5 ppm, the Ti concentration is less than 1.5 ppm, and the concentrations of Mg, Ca, Cr, Fe, Ni, Cu, and Zn are each less than 1 ppm.

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