JP7833376B2 - Method for producing a high-purity alkaline silica solution, apparatus for producing a high-purity alkaline silica solution, and method for producing a high-purity silica sol. - Google Patents

Description

This invention relates to a method for producing a high-purity alkali silicate solution, an apparatus for producing a high-purity alkali silicate solution, a high-purity alkali silicate solution, and a high-purity silica sol and a method for producing the same.

For silica used as a raw material in semiconductor manufacturing, there is a constant demand for higher purity silica. A widely known method for producing high-purity silica involves synthesizing silica using purified silicon alkoxide as a raw material, minimizing impurities. However, because the alkoxide raw material is expensive, the use of silica produced by such methods has been limited to high-value-added applications or specialized uses. Another known method involves synthesizing silica through a wet reaction between alkali silicate and mineral acid. This method allows for relatively inexpensive silica production. However, the silica produced by this method contains metal impurities in the order of several ppm to tens of ppm, thus requiring higher purity. In particular, in methods for producing high-purity silica using alkali silicate as a raw material, the purity of the resulting silica depends on the impurity content of the alkali silicate raw material. Therefore, there has been a demand for the provision of alkali silicate with the highest possible purity.

Traditionally, alkali silicate solutions were manufactured by either pulverizing alkali silicate cullet under high temperature and pressure, or by dissolving highly soluble amorphous silica in an alkaline aqueous solution. However, the resulting alkali silicate solution contained undissolved residues and flat, plate-like microparticles generated during manufacturing. These impurities caused problems such as filtration failures in subsequent processes and defects in the final product. Therefore, these impurities were removed by filtration using filter aids and methods such as filter presses. However, because the flat, plate-like microparticles range in size from microns to submicrons, they could not be removed by the aforementioned filtration processes.

Patent Document 1 describes a method for producing a high-purity alkali silicate solution, which involves diluting an alkali metal silicate with pure water, contacting it with an H-type cation exchange resin to decation, adding an acid to make it strongly acidic, then contacting it again with an H-type cation exchange resin and an OH-type anion exchange resin to decation and deanionize, adding an alkali and heating to colloidize the silica, concentrating the solution, and then adding KOH to obtain an aqueous potassium silicate solution.

Furthermore, as a method for removing impurities in the process of manufacturing colloidal silica and silica gel using a conventional alkaline silicate solution, Patent Document 2 describes a method for producing high-purity colloidal silica. This method involves diluting the alkaline silicate solution with pure water, contacting it with an H-type strongly acidic cation exchange resin to dealkalize it and obtain an aqueous solution of activated silicic acid, then adding acid to make it strongly acidic, contacting it with an H-type strongly acidic cation exchange resin and an OH-type strongly basic anion exchange resin to obtain high-purity activated silicic acid, and finally growing the particles to produce high-purity colloidal silica.

Patent Document 3 describes a method for producing high-purity alkali silicate, characterized by using silica with a water content of 10% or more, a _SiO2 purity of 99.9% or more excluding water, and a specific surface area of 100 ^m² /g or more, and an aqueous solution of alkali metal hydroxide.

Special Publication No. 41-3369 Japanese Patent Application Publication No. 5-97422 Japanese Patent Publication No. 2003-183018

The manufacturing method described in Patent Document 1 aimed to achieve high purity through ion exchange and acid treatment. The manufacturing method described in Patent Document 2 was basically the same as that described in Patent Document 1, but both had limitations in terms of achieving high purity. Furthermore, the manufacturing method described in Patent Document 3 required preparing silica (or silica gel) from sodium silicate, and then repeatedly acid-washing the silica to obtain high-purity silica for use as a raw material. This method had problems in that it was a long process and costly.

The present invention aims to provide a method for producing a high-purity alkali silicate solution, which allows for the inexpensive industrial production of high-purity alkali silicate through a relatively easy-to-control process; an apparatus for producing a high-purity alkali silicate solution; a high-purity alkali silicate solution; and a high-purity silica sol and a method for producing the same. High-purity alkali silicate is a favorable raw material for producing high-purity silicate compounds.

According to one aspect of the present invention, a method for producing a high-purity alkali silicate solution is provided, which includes a preparation step of preparing a reaction solution by adding silicon fine particles to an aqueous alkali hydroxide solution, and satisfies all of the following conditions 1) to 3).
1) The average particle size of the silicon nanoparticles is in the range of 0.03 μm to 10 mm.
2) The average addition rate (B) of silicon nanoparticles to the alkali hydroxide aqueous solution is in the range of 0.05 g/min·L to 5.0 g/min·L relative to the volume of the reaction solution.
3) The value of the reaction parameter A, represented by formula (F1), is in the range of greater than 0 and less than or equal to 10,000.
A=B×C×D×T...(F1)
(In formula (F1), B is the average addition rate of silicon nanoparticles to the alkali hydroxide aqueous solution (g/min·L), C is the specific surface area of the silicon nanoparticles ( ^m² /g), D is the molar ratio of alkali hydroxide to silicon nanoparticles (alkali hydroxide/silicon nanoparticles), and T is the reaction temperature (°C).)

According to one aspect of the present invention, a manufacturing apparatus for use in the method for producing a high-purity alkali silicate solution according to the aforementioned aspect of the present invention is provided, comprising: a suspension tank for suspending silicon nanoparticles in water to obtain a silicon nanoparticle suspension; a stirring device attached to the suspension tank; a reaction tank for adding the silicon nanoparticle suspension to an aqueous alkali hydroxide solution and reacting it to obtain a high-purity alkali silicate solution; a temperature control device and a stirring device attached to the reaction tank; a transfer pipeline for supplying the silicon nanoparticle suspension from the suspension tank to the reaction tank; and a control device for controlling the addition rate of the silicon nanoparticle suspension.

According to one aspect of the present invention, a high-purity alkali silicate solution is provided, obtained by reacting a suspension of silicon microparticles with an aqueous alkali hydroxide solution, wherein the concentrations of Al and Ni relative to the silica solid content are each 200 ppm or less, the concentrations of Mg, Ca, Fe, and Zn are each 30 ppm or less, and the concentrations of Ti, Cr, Cu, Ag, and Pb are each 5 ppm or less.

According to one aspect of the present invention, a method for producing a high-purity silica sol is provided, using a high-purity alkali silica solution obtained by the method for producing a high-purity alkali silica solution according to the aforementioned aspect of the present invention as a raw material.

According to one aspect of the present invention, a high-purity silica sol is provided in which the Al concentration relative to the silica solid content is 20 ppm or less, the Ca, Ni, and Na concentrations are each 10 ppm or less, and the Mg, Ti, Cr, Fe, Cu, Zn, Ag, and Pb concentrations are 5 ppm or less.

According to the present invention, it is possible to provide a method for producing a high-purity alkali silicate solution that can be manufactured industrially at low cost using a relatively easy-to-control process, an apparatus for producing a high-purity alkali silicate solution, a high-purity alkali silicate solution, and a high-purity silica sol and a method for producing the same.

This is a schematic diagram showing a manufacturing apparatus that can be used in a method for producing a high-purity alkali silicate solution according to an embodiment of the present invention.

[Method for producing high-purity alkaline silicate solution]
First, the method for producing a high-purity alkali silicate solution according to this embodiment will be described.
The method for producing a high-purity alkali silicate solution according to this embodiment includes a preparation step of adding silicon fine particles to an aqueous alkali hydroxide solution to prepare a reaction solution, and satisfies all of the above conditions 1) to 3). Conditions 1) to 3) are described below.

The inventors of this invention have discovered a method for producing high-purity alkali silicate by reacting alkali hydroxide with metallic silicon. Based on this finding, they conducted thorough research and ultimately completed the present invention.
In the method for producing high-purity alkali silicate using metallic silicon and alkali hydroxide as raw materials, there were practical problems such as those described below (i) to (iv).

(i) When using relatively large particulate metallic silicon with a particle size exceeding 10 mm as a raw material, the reaction rate with alkali hydroxide is extremely slow due to the small specific surface area of metallic silicon, resulting in the problem of requiring a long time to dissolve the metallic silicon in alkali hydroxide. Furthermore, there were issues of resource consumption and cost associated with introducing an external heat source for the dissolution process, including heating.

(ii) Metallic silicon in the form of lumps or granules with a particle diameter exceeding 10 mm tends to settle in solutions such as aqueous alkali hydroxide solutions. Therefore, a method is usually employed in which the metallic silicon is first suspended by stirring in a reaction vessel filled with pure water. In this case, it is necessary to maintain a very high stirring speed in order to suppress the settling of the metallic silicon. As a result, the metallic silicon lumps may abrade the bottom or sides of the reaction vessel, causing the vessel to wear down, generating foreign matter and impurities, which can impair the purity of the resulting alkali silicate.

(iii) When metallic silicon nanoparticles with a particle size of 10 μm or less are used as a raw material, the metallic silicon nanoparticles may aggregate during the drying process in the manufacturing stage, forming aggregates of several millimeters to several centimeters in size. In this case, the large size of the aggregates may lead to increased sedimentation and contamination with metallic impurities due to abrasion of the reaction vessel. Furthermore, when metallic silicon nanoparticles with a particle size of 0.03 μm or less are used as a raw material, the specific surface area of the metallic silicon is sufficiently large, resulting in a remarkably fast reaction rate, which causes a rapid rise in the temperature of the reaction solution. The hydrogen gas generated in this reaction can cause the reaction solution to bump or overflow in the reaction vessel.

(iv) The method for producing liquid alkali silicate usually involves using sodium carbonate and silica sand as raw materials, melting the mixture to obtain sodium silicate cullet, and then pulverizing it under high temperature and pressure of 100°C or higher using equipment capable of creating high temperature and pressure inside with saturated steam, such as an autoclave, to prepare liquid alkali silicate. During this process, due to the high temperature and pressure conditions, elongated layered silicates (flat, microscopic particles) are generated. When silica sol is prepared using alkali silicate containing such elongated layered silicates, these particles may remain in the silica sol, potentially reducing its filterability. When such silica sol is used as a dispersion of abrasive particles for polishing various substrates, elongated layered silicates (flat, microscopic particles) remain on the substrate after polishing, and even after cleaning the substrate, resulting in defects such as scratches.
Here, regarding the generation of the aforementioned elongated layered silicates (flat, plate-like microparticles), it is known that when the _SiO₂ / _Na₂O molar ratio of sodium silicate cullet (solid form) is within a specific range, and furthermore, when the heating temperature for the cullet is within a specific range, there is a tendency for elongated layered silicates (flat, plate-like microparticles) to be generated. And since the pulverization conditions normally applied to solid sodium silicate cullet used as a raw material for silicate alkali preparation overlap with such conditions, we believe that the generation of elongated layered silicates (flat, plate-like microparticles) was unavoidable.

In this invention, the problems described in (i) to (iv) above are solved as follows.

Regarding the problem described in (i) above, the manufacturing method of the present invention utilizes the reaction heat generated when silicon nanoparticles are added to an aqueous alkali hydroxide solution to perform the dissolution reaction of silicon nanoparticles. Therefore, the use of external heat sources can be suppressed, resources can be saved, and the method is economically superior.

Regarding the problem described in (ii) above, in the manufacturing method of the present invention, the size of the silicon nanoparticles (metallic silicon) used as raw materials is in the range of an average particle diameter of 0.03 μm to 10 mm, and since they are relatively resistant to settling, it is not necessary to increase the stirring speed. Furthermore, because the particle size of the silicon nanoparticles is not large enough to cause wear on the reaction vessel, the generation of foreign matter and impurities caused by erosion of the inner wall of the reaction vessel can be suppressed, which is advantageous for producing a high-purity alkali silicate solution.

Regarding the problem described in (iii) above, the manufacturing method of the present invention selects an appropriate amount of alkali hydroxide (molar ratio) relative to the amount of silicon nanoparticles used, the addition rate, and the reaction temperature in relation to the specific surface area of the silicon nanoparticles used as raw material, thereby enabling an appropriate reaction rate. Therefore, even when using silicon nanoparticles with a large specific surface area and high reactivity as raw materials, bumping of the reaction solution and overflow of the reaction solution can be prevented.

Regarding the problem described in (iv) above, the manufacturing method of the present invention does not use sodium silicate cullet, which causes the formation of flat, plate-shaped microparticles, as a raw material. As a result, a high-purity alkali silicate solution free from flat, plate-shaped microparticles can be obtained.

In the method for producing a high-purity alkali silicate solution according to this embodiment, it is necessary to satisfy the following condition 1).
1) The average particle size of the silicon nanoparticles is in the range of 0.03 μm to 10 mm.

In conventional methods for producing alkali silicate solutions using sodium silicate cullet as a raw material, a high-purity alkali silicate solution cannot be obtained because the cullet contains a large amount of metal impurities derived from silica sand. On the other hand, in the production method of this embodiment, the use of high-purity silicon nanoparticles (metallic silicon) as a raw material, without using sodium silicate cullet, is one of the reasons why a high-purity alkali silicate solution can be produced. Furthermore, in the manufacturing process, high purity of metallic silicon is achieved by reducing metal impurities during the reduction process from silica to metallic silicon, or during the purification process.

If the average particle size of the silicon nanoparticles is less than 0.03 μm, the specific surface area is too large, causing the reaction with alkali hydroxide to proceed rapidly. This leads to a rapid increase in the temperature of the reaction solution, and the hydrogen gas generated in this reaction causes the reaction solution to bump or overflow in the reaction vessel. While the reaction rate can be suppressed by reducing the rate at which silicon nanoparticles or alkali hydroxide are added, the reaction time increases, reducing economic efficiency.
When the average particle size of silicon nanoparticles exceeds 10 mm, the silicon nanoparticles can easily erode the bottom or inner surface of the reaction vessel, generating foreign matter and impurities that reduce the purity of the resulting alkali silicate. Furthermore, because silicon nanoparticles have a small specific surface area and a remarkably slow reaction rate, complete dissolution requires a long time.
From the same viewpoint as above, the average particle size of the silicon nanoparticles is preferably 0.03 μm or more and 500 μm or less, more preferably 0.03 μm or more and 50 μm or less, even more preferably 0.03 μm or more and 10 μm or less, and particularly preferably 0.03 μm or more and 3 μm or less.

In this embodiment, the average particle diameter of silicon nanoparticles is calculated from the specific surface area measured by the BET specific surface area measurement method described in (i) below, using the following formula (F3) with a silicon density of 2.33, and the average particle diameter is calculated as the equivalent sphere particle diameter. However, if the average particle diameter of the silicon nanoparticles exceeds 1 μm as a result, that average particle diameter value is not adopted, and the value obtained by measurement and calculation using the laser diffraction/scattering method described in (ii) below is used as the average particle diameter of the silicon nanoparticles. In this case as well, if the average particle diameter of the silicon nanoparticles exceeds 1000 μm, that average particle diameter value is not adopted, and the average particle diameter obtained by measurement and calculation using calipers described in (iii) below is used as the average particle diameter of the silicon nanoparticles.
Average particle diameter = 6000/[specific surface area (SA) x density (ρ)]... (F3)
(i) Particle size conversion method from specific surface area measured by the BET specific surface area measurement method (surface area measuring device, manufactured by Mountec, model number: Mascsorb HM-1220) (ii) Laser diffraction/scattering method (laser diffraction/scattering particle size distribution analyzer, manufactured by Horiba, Ltd., model number: LA-950V2)
(iii) A method of measuring the longest diameter of 100 or more silicon microparticles using calipers and taking the arithmetic mean.

In the method for producing a high-purity alkali silicate solution according to this embodiment, it is necessary to satisfy the following condition 2).
2) The average addition rate (B) of silicon nanoparticles to the alkali hydroxide aqueous solution is in the range of 0.05 g/min·L to 5.0 g/min·L relative to the volume of the reaction solution.
Note that this addition rate represents the addition rate of the silicon nanoparticles themselves. As described later, when silicon nanoparticles are usually added to an aqueous alkali hydroxide solution, they are added to the aqueous alkali hydroxide solution in the form of a silicon nanoparticle suspension in which the silicon nanoparticles are dispersed. Therefore, the addition rate of the silicon nanoparticle suspension is carried out in accordance with the addition rate range in terms of the solid content of the silicon nanoparticles.
Furthermore, the unit of addition rate, "g/min·L," refers to the mass of silicon particle suspension (in terms of solid content) added per minute to a unit volume (1 L) of alkali hydroxide aqueous solution. Here, the unit "g/min·L" can also be expressed as "g/(min·L)."

When silicon nanoparticles are added to an aqueous alkali hydroxide solution and dissolved, heat of reaction is generated due to the dissolution. In this invention, this heat of reaction is used as the heat source for the dissolution reaction. However, during the dissolution reaction, the size of the silicon nanoparticles gradually decreases, so the surface area decreases, the reaction rate decreases, and the reaction temperature tends to decrease. On the other hand, since silicon nanoparticles are added to the reaction solution as they are added, the newly added silicon nanoparticles dissolve, contributing to the temperature rise. In addition, the temperature decreases due to heat dissipation of the reaction solution and increases due to friction also occur. Therefore, when using the heat of dissolution of silicon as the heat source for the reaction, it is necessary to appropriately control these factors. Specifically, it is necessary to control reaction parameter A within a predetermined range, and in particular, to control the average addition rate (B), which has the greatest influence on the heat of reaction, within a predetermined range.
If the average addition rate (B) is less than 0.05 g/min·L, the temperature rise due to the heat of reaction during dissolution will not be greater than the temperature decrease due to heat dissipation, making it impossible to maintain the target temperature. Furthermore, the slow addition rate will result in a longer reaction time, reducing economic efficiency.
If the average addition rate (B) exceeds 5.0 g/min·L, the temperature rise due to the heat of reaction during dissolution excessively exceeds the temperature decrease due to natural cooling, making it impossible to maintain the target temperature. In addition, the large amount of hydrogen gas generated by the increased reaction rate can cause the reaction solution to bump or overflow in the reaction vessel.
From the same viewpoint as above, the average addition rate (B) is preferably 0.1 g/min·L or more and 3.0 g/min·L or less, more preferably 0.15 g/min·L or more and 1.0 g/min·L or less, and even more preferably 0.2 g/min·L or more and 0.8 g/min·L or less.

In the method for producing a high-purity alkali silicate solution according to this embodiment, it is necessary to satisfy the following condition 3).
3) The value of the reaction parameter A, represented by formula (F1), is in the range of greater than 0 and less than or equal to 10,000.
A=B×C×D×T...(F1)
In formula (F1), B is the average addition rate (g/min·L) of silicon nanoparticles to the alkali hydroxide aqueous solution, C is the specific surface area ( ^m² /g) of the silicon nanoparticles, D is the molar ratio of alkali hydroxide to silicon nanoparticles (alkali hydroxide/silicon nanoparticles), and T is the reaction temperature (°C).

The value of reaction parameter A can be greater than 0. Furthermore, the smaller the value of reaction parameter A, the lower the reactivity.
When the value of reaction parameter A exceeds 10,000, the reaction rate increases, and the large amount of hydrogen gas generated causes the reaction solution to bump or the reaction vessel to overflow.
From the same viewpoint as above, the value of reaction parameter A is preferably 50 or more and 5,000 or less, and more preferably 100 or more and 1,000 or less.

The value of B, which represents the average addition rate (B) (g/min·L) of silicon nanoparticles to an alkali hydroxide aqueous solution, is as described above.

The value of C, which is the specific surface area ( ^m² /g) of silicon nanoparticles, shall be determined by the BET method if it is measurable by that method. If it is not measurable by the BET method (lower limit of measurement 2.5 ^m² /g or less), the specific surface area converted value calculated by equivalent sphere conversion of the average particle diameter measured by the aforementioned laser diffraction/scattering method shall be applied. If it is not measurable by the laser diffraction/scattering method (lower limit of measurement for specific surface area converted value 0.0025 ^m² /g or less), the specific surface area converted value calculated by equivalent sphere conversion of the average particle diameter obtained by particle diameter measurement using the aforementioned caliper shall be applied. Furthermore, the value of C, which is the specific surface area ( ^m² /g) of silicon nanoparticles, is preferably 0.00025 ^m² /g or more and 86 ^m² /g or less, more preferably 0.025 ^m² /g or more and 70 ^m² /g or less, and particularly preferably 2.5 ^m² /g or more and 60 ^m² /g or less.
If the specific surface area is above the aforementioned lower limit, the dissolution time of silicon nanoparticles can be shortened, improving productivity. Furthermore, since the silicon nanoparticle size is not too large, there is no need to excessively increase the stirring speed to suppress sedimentation, and wear on the reaction vessel can be prevented.
If the specific surface area is below the aforementioned upper limit, the reactivity will not increase significantly, making it easier to control the amount added and the amount of hydrogen generated, and preventing overflow and bumping of the reaction solution.

The molar ratio of alkali hydroxide to silicon nanoparticles (alkali hydroxide/silicon nanoparticles), D, is preferably 0.2 or more and 0.7 or less.
If the molar ratio (alkali hydroxide/silicon nanoparticles) is above the lower limit, the alkali for the silicon nanoparticles will be sufficient, and the dissolution time can be shortened.
If the molar ratio (alkali hydroxide/silicon nanoparticles) is below the aforementioned upper limit, there will be no excessive alkali relative to the silicon nanoparticles, thus reducing the amount of ion exchange resin required when desalting the resulting alkali silicate to prepare the silica solution. Furthermore, the amount of acid used to regenerate the ion exchange resin that has adsorbed alkali can be reduced.

The reaction temperature (°C), T, is preferably 40°C or higher and 98°C or lower, more preferably 45°C or higher and 90°C or lower, and particularly preferably 50°C or higher and 85°C or lower.
If the reaction temperature is above the aforementioned lower limit, the reaction rate can be increased and the dissolution time can be shortened.
If the reaction temperature is below the aforementioned upper limit, the reaction rate will not become excessively fast, making it easier to control the amount of hydrogen generated and thus preventing overflow from the reaction vessel. Furthermore, an autoclave is not required at such reaction temperatures. The upper and lower limits of the reaction temperature are within a range that can be controlled by the reaction heat of the silicon nanoparticles. In particular, when performing reactions that exceed the upper limit temperature, it is difficult to reach the required temperature using only the reaction heat of the silicon nanoparticles, and other heat sources are necessary.

In the method for producing a high-purity alkali silicate solution according to this embodiment, in the preparation step, the addition of silicon nanoparticles is carried out in two stages with different silicon nanoparticle addition rates (in terms of solid content), and when the addition rate of silicon nanoparticles in the first stage is S1 and the addition rate of silicon nanoparticles in the second stage is S2, it is preferable that the value of the addition rate ratio R, expressed by the following formula (F2), is in the range of greater than 1 and less than or equal to 10.
R=S1/S2...(F2)

The addition rate S1 of silicon nanoparticles in the first stage of addition is the rate at which silicon nanoparticles are added to an alkali hydroxide aqueous solution at room temperature until the target temperature is reached. The addition rate S2 of silicon nanoparticles in the second stage of addition is the rate at which the temperature is maintained after the target temperature is reached. The ratio R of these two rates is a parameter that indicates the ratio of the addition rate S1 to S2, and it is preferable to control this within the range described above. When the reaction solution is heated and maintained at a predetermined temperature to allow the reaction to proceed, the energy required to raise the temperature to the predetermined temperature is greater than the energy required to maintain the temperature. Therefore, the addition rate ratio R will inevitably be greater than 1. A value of addition rate ratio R of 1 or less means that the addition rate of the second stage of addition is equal to or greater than that of the first stage. In this case, depending on the setting conditions of S1, if the predetermined temperature has been reached at the end of the addition of S1, the addition of S2 will cause a further temperature increase. In this case, the reaction temperature will exceed the desired temperature, making it easy for bumping or overflow to occur. Furthermore, depending on the settings for S1, the predetermined temperature may not be reached when S1 is added. In this case, dissolution cannot be performed at the predetermined temperature and time, resulting in inefficient production.
If the addition rate ratio R exceeds 10, the opposite may occur: the addition rate in the second stage is too slow, and the temperature is maintained below the target temperature reached in the first stage. Alternatively, the addition rate in the first stage is too fast, causing the temperature to exceed 100°C, which could lead to a runaway dissolution reaction and cause an overflow, potentially creating safety problems.
From the same viewpoint as above, the value of the addition rate ratio R is preferably 1.5 or more and 7.5 or less, and more preferably 2 or more and 5.0 or less.

In the method for producing a high-purity alkali silicate solution according to this embodiment, it is preferable in the preparation step to add the silicon fine particles to the alkali hydroxide aqueous solution in the form of a suspension of silicon fine particles in which the silicon fine particles are dispersed.
When adding powdered or fine-particle silicon nanoparticles to a reaction solution, improper handling can lead to dust explosions due to friction, and inhaling the dust can cause health problems. Even with sufficient equipment countermeasures, the investment in equipment can be excessive. Furthermore, adding silicon nanoparticles in suspension form makes it easier to control the rate at which the silicon nanoparticles are added.
The solid content concentration (silicon microparticle concentration) of the silicon microparticle suspension is not particularly limited. The solid content concentration of the silicon microparticle suspension is preferably in the range of 0.1% by mass or more and 50% by mass or less, and more preferably in the range of 1% by mass or more and 40% by mass or less.

Furthermore, it is preferable that the silicon microparticle suspension requires a stirring power of 0.1W to 3,000W when suspending the silicon microparticles in pure water.
The required power for stirring is the total energy used to stir the fluid in the stirring tank; a higher value indicates stronger stirring capability.
When the required stirring power is 0.1W or less, insufficient stirring capacity can cause silicon particles to settle or the reaction system to become non-uniform, leading to problems such as bumping and overflow. When the required stirring power exceeds 3,000W, excessive stirring can cause the inner wall of the reaction vessel to be eroded by the silicon particles, or the silicon dissolution reaction can proceed too quickly, making it impossible to control the reaction temperature. These problems can be prevented by keeping the required stirring power within the range of 0.1 to 3,000W.
From the same viewpoint as above, the required power for stirring is more preferably 0.1W to 1,000W, even more preferably 0.1W to 100W, and particularly preferably 0.1W to 10W.

The high-purity alkali silicate solution obtained by the manufacturing method of this embodiment can be further purified by known means. Specifically, the high-purity alkali silicate solution obtained by the above manufacturing method can be purified by applying known purification methods, such as ultrafiltration, to reduce undissolved residue (including inorganic metal oxides). If a clear high-purity alkali silicate solution cannot be obtained after filter filtration, a filter with a smaller pore size or ultrafiltration may be used.

In the method for producing a high-purity alkali silicate solution according to this embodiment, the following embodiment 1 is preferred.
[Aspect 1]
A method for producing a high-purity alkali silicate solution, comprising a preparation step of adding silicon nanoparticles to an aqueous alkali hydroxide solution to prepare a reaction solution, wherein all of the following conditions 1) to 3) are met,
1) The average particle size of the silicon nanoparticles is in the range of 0.03 μm to 10 mm.
2) The average addition rate (B) of silicon nanoparticles to the alkali hydroxide aqueous solution is in the range of 0.05 g/min·L to 5.0 g/min·L relative to the volume of the reaction solution.
3) The value of the reaction parameter A, represented by formula (F1), is in the range of greater than 0 and less than or equal to 10,000.
A=B×C×D×T...(F1)
(In formula (F1), B is the average addition rate of silicon nanoparticles to the alkali hydroxide aqueous solution (g/min·L), C is the specific surface area of the silicon nanoparticles ( ^m² /g), D is the molar ratio of alkali hydroxide to silicon nanoparticles (alkali hydroxide/silicon nanoparticles), and T is the reaction temperature (°C).)
Furthermore, the value of B, which is the average addition rate of the silicon fine particles to the alkali hydroxide aqueous solution, is in the range of 0.05 g/min·L to 5.0 g/min·L relative to the volume of the reaction solution.
The value of C, which is the specific surface area ( ^m² /g) of the silicon nanoparticles, is between 0.00025 ^m² /g and 86 ^m² /g.
The value of D, which is the molar ratio of alkali hydroxide to silicon nanoparticles (alkali hydroxide/silicon nanoparticles), is 0.25 or more and 0.5 or less.
A method for producing a high-purity alkali silicate solution, wherein the value of T, which is the reaction temperature (°C) of silicon nanoparticles and an aqueous alkali hydroxide solution, is between 40°C and 98°C.

Furthermore, in the method for producing a high-purity alkali silicate solution according to this embodiment, the following embodiment 2 is preferred.
[Aspect 2]
A method for producing a high-purity alkali silicate solution, comprising the preparation step of the above embodiment 1, in which a first-stage addition and a second-stage addition are performed with different addition rates (in terms of solid content) of silicon nanoparticles, and when the addition rate of silicon nanoparticles in the first stage is S1 and the addition rate of silicon nanoparticles in the second stage is S2, the addition rate ratio R, represented by the following formula (F2), is in the range of 1 to 10 when added to an aqueous alkali hydroxide solution.
R=S1/S2...(F2)

Furthermore, in the method for producing a high-purity alkali silicate solution according to this embodiment, the following embodiment 3 is preferred.
[Appearance 3]
A method for producing a high-purity alkali silicate solution, wherein in the preparation step of the 2nd embodiment, the silicon nanoparticles are added to the alkali hydroxide aqueous solution in the form of a suspension of silicon nanoparticles in which the silicon nanoparticles are dispersed.

Furthermore, in the method for producing a high-purity alkali silicate solution according to this embodiment, the following embodiment 4 is preferred.
[Aspect 4]
A method for producing a high-purity alkali silicate solution, wherein, in the preparation step of the above embodiment 3, the required stirring power when suspending the silicon microparticle suspension in pure water is in the range of 0.1 W to 3,000 W.

Furthermore, in the method for producing a high-purity alkali silicate solution according to this embodiment, the following embodiment 5 is preferred.
[Aspect 5]
A method for producing a high-purity alkali silicate solution, wherein the high-purity alkali silicate solution obtained in the preparation step of the above embodiment 4 contains 0.1 or fewer flat, plate-shaped fine particles, which, as measured by image analysis, have a major axis of 500 nm or more and 1500 nm or less, a minor axis of 100 nm or more and an aspect ratio (major axis/minor axis) of 3 or more.

[High-purity alkaline silicate solution]
Next, the high-purity alkali silicate solution according to this embodiment will be described.
The high-purity alkali silicate solution according to this embodiment is obtained by the method for producing the high-purity alkali silicate solution according to this embodiment described above. The solvent used in the high-purity alkali silicate solution according to this embodiment may be water. Furthermore, when the solvent used in the high-purity alkali silicate solution is entirely water, it is also referred to as a "high-purity alkali silicate aqueous solution."
The high-purity alkali silicate solution according to this embodiment has Al and Ni concentrations of 200 ppm or less relative to the silica solid content, Mg, Ca, Fe, and Zn concentrations of 30 ppm or less, and Ti, Cr, Cu, Ag, and Pb concentrations of 5 ppm or less. According to the method for producing the high-purity alkali silicate solution according to this embodiment, such high-purity alkali silicate can be produced inexpensively through an industrially simple process.
Furthermore, the metal concentration (metal content) relative to the silica solids refers to the ratio of the mass of metal to the mass of silica (solids after drying) in a high-purity alkali silicate solution.

In this high-purity alkali silicate solution, it is preferable that the content of flat, plate-shaped fine particles, which have a major axis of 500 nm to 1500 nm, a minor axis of 100 nm to 500 nm, and an aspect ratio (major axis/minor axis) of 3 or more, as measured by image analysis, is 0.1 particles/mL or less.
If the content of such flat, fine particles is 0.1 particles/mL or less, then when silica sol is prepared using this high-purity alkali silicate and applied to polish various substrates, it is possible to suppress the occurrence of scratches on the substrate and to suppress the residue of flat, fine particles on the substrate.

[Equipment for manufacturing high-purity alkaline silicate solution]
Next, the apparatus for producing the high-purity alkali silicate solution according to this embodiment will be described.
The apparatus for producing high-purity alkali silicate solution according to this embodiment is a production apparatus that can be used in the method for producing high-purity alkali silicate solution according to this embodiment described above.
As shown in Figure 1, the apparatus 100 for producing a high-purity alkali silicate solution according to this embodiment includes a suspension tank 1 for suspending silicon nanoparticles in water to obtain a silicon nanoparticle suspension, a stirring device 2 attached to the suspension tank 1, a reaction tank 3 for adding the silicon nanoparticle suspension to an aqueous alkali hydroxide solution and reacting them to obtain a high-purity alkali silicate solution, a temperature control device 4 and a stirring device 5 attached to the reaction tank 3, a transfer pipeline 6 for supplying the silicon nanoparticle suspension from the suspension tank 1 to the reaction tank 3, and a control device 7 for controlling the addition rate of the silicon nanoparticle suspension. The apparatus may further include a storage tank 8 for storing the obtained high-purity alkali silicate solution, a transfer pipeline 9 for supplying the high-purity alkali silicate solution from the reaction tank 3 to the storage tank 8, and a control device 10 for controlling the transfer rate of the high-purity alkali silicate solution.

Suspension tank 1 is a tank used to suspend silicon nanoparticles in water to obtain a silicon nanoparticle suspension. Directly adding silicon nanoparticles to reaction tank 3 in powder or aggregate form is difficult to control the amount added and raises safety concerns. Therefore, this equipment is designed to suspend and disperse a predetermined amount of silicon nanoparticles in water in order to add them to the reaction tank at a constant rate.
The material of the suspension tank 1 may be resin or metal. However, if the material of the suspension tank 1 is metal, it is preferable that the inner surface is coated with resin or glass lining.
The stirring device 2, which is attached to the suspension tank 1, is installed inside the suspension tank 1 and is intended to uniformly disperse the silicon fine particles when suspending them in water. The size of the stirring device 2 depends on the suspension tank 1. Furthermore, it is generally desirable that the material of the stirring device 2 be the same as that of the suspension tank 1.

Reaction tank 3 is a tank used to obtain a high-purity alkali silicate solution by adding a suspension of silicon nanoparticles to an aqueous alkali hydroxide solution and reacting them. In other words, it is equipment intended to prepare high-purity alkali silicate by dissolving silicon nanoparticles in alkali hydroxide.
The material of the reaction vessel 3 may be resin or metal. However, if the material of the reaction vessel 3 is metal, it is preferable that the inner surface is coated with resin.

The temperature control device 4 attached to the reaction vessel 3 is a piece of equipment intended to control the liquid temperature inside the reaction vessel 3 to a constant temperature, and to cool the reaction vessel to lower the liquid temperature in the event of an abnormal temperature rise.
The manufacturing method of this embodiment utilizes the heat of dissolution of silicon nanoparticles as a heat source, but an external heat source may be used as a supplement. Examples of external heat sources include electric heating, steam heating, or hot water heating.
The stirring device 5, which is attached to the reaction vessel 3, is installed inside the reaction vessel 3 and is intended to uniformly mix and react the added silicon fine particles when they are dissolved in alkali hydroxide. The size of the stirring device 5 depends on the reaction vessel 3. In addition, it is generally desirable that the material of the stirring device 5 be the same as that of the reaction vessel 3.

The transfer pipeline 6 is a pipeline that supplies the silicon microparticle suspension from the suspension tank 1 to the reaction tank 3. The typical size of the transfer pipeline 6 depends on the volume of the reaction liquid and the transfer time calculated from the cycle time of the manufacturing process. Furthermore, the material of the transfer pipeline 6 is typically preferably the same as that of the suspension tank 1 or the reaction tank 3.

The control device 7 is a device that controls the rate at which the silicon nanoparticle suspension is added.
Storage tank 8 is a facility intended for storing a high-purity alkali silicate solution obtained by dissolving silicon nanoparticles in alkali hydroxide in reaction tank 3.
The material of the storage tank 8 may be resin or metal. However, if the material of the storage tank 8 is metal, it is preferable that the inner surface is coated with resin. When the material of the storage tank 8 is resin or when the metal surface is coated with resin, no metal fragments are generated even if the inner wall is cut, so the purity of the resulting alkali silicate solution is not reduced, which is desirable. In addition, the removed components tend to float in the alkali silicate solution, making them easy to remove.

The transfer pipeline 9 is a pipeline that supplies high-purity alkali silicate solution from the reaction vessel 3 to the storage tank 8. The typical size of the transfer pipeline 9 depends on the amount of reaction solution and the transfer time calculated from the cycle time of the manufacturing process. Furthermore, the material of the transfer pipeline 9 is typically preferably the same as that of the reaction vessel 3 or the storage tank 8.
The control device 10 is a device that controls the rate at which high-purity alkali silicate solution is added.

[High-purity silica sol and method for producing the same]
Next, a high-purity silica sol according to this embodiment and a method for producing the same will be described.
The method for producing high-purity silica sol according to this embodiment is a method that uses the high-purity alkali silicate solution obtained by the method for producing high-purity alkali silicate solution according to this embodiment described above as a raw material.
Furthermore, the high-purity silica sol according to this embodiment has an Al concentration of 20 ppm or less relative to the silica solid content, Ca, Ni, Cu, and Na concentrations of 10 ppm or less each, and Mg, Ti, Cr, Fe, Zn, Ag, and Pb concentrations of 5 ppm or less.
The metal concentration (metal content) relative to the silica solids is as described above.

[Effects of this embodiment]
According to this embodiment, the following effects can be achieved.
(1) According to this embodiment, high-purity alkali silicate can be prepared by using silicon as a raw material.
(2) With the particle size of the raw material silicon in this embodiment, the generation of foreign matter and contamination with impurities caused by scraping the inner surface of the reaction vessel during the process are suppressed.
(3) According to this embodiment, a rapid rise in temperature during the preparation process is suppressed, or overflow is prevented.
(4) According to this embodiment, the reaction heat generated when silicon nanoparticles are added to an aqueous alkali hydroxide solution is used to promote the dissolution reaction of silicon nanoparticles. Therefore, the use of an external heat source can be suppressed, resources can be saved, and it is economically superior.

[Other embodiments]
It should be noted that the present invention is not limited to the embodiments described above, and any addition of steps or modifications or improvements to the manufacturing method of the present invention, to the extent that they include the manufacturing method of the present invention and achieve the objectives of the present invention, are also included in the present invention.

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 the various properties of high-purity alkali silicate or high-purity silica sol in the examples and comparative examples were carried out as described below.

<Measurement of high-purity alkali silicate>
[1] Content of plate-shaped fine particles A 100 g solution of high-purity alkali silicate was prepared by diluting a high-purity alkali silicate solution with ultrapure water to a 5% _SiO₂ concentration. High-purity alkali silicate was filtered under reduced pressure using a 0.1 μm membrane filter (ADVANTEC Toyo Co., Ltd., T010A047A). After passing 100 g of ultrapure water through the filter to wash it, 100 g of 5% diluted high-purity alkali silicate was passed through to separate the plate-shaped fine particles contained in the high-purity alkali silicate solution. Subsequently, 50 g of ultrapure water was passed through the filter once, 100 g of ultrapure water twice, and 100 g of ultrapure water three times to wash the plate-shaped fine particles remaining on the filter. The plate-shaped fine particles separated in this way were photographed using an electron microscope S-5500 (Hitachi High-Technologies Corporation, 100,000x or 20,000x magnification) to obtain an SEM image. This SEM image was analyzed using image analysis methods to measure the content of flat, plate-shaped microparticles with a major axis of 500 nm to 1500 nm, a minor axis of 100 nm to 500 nm, and an aspect ratio (major axis/minor axis) of 3 or more.

[2] Impurity concentration (1) Metal (K, Mg, Al, Ti, Ca or Fe) content 1) Take about 10 g of the high-purity alkali silicate solution of the sample into a platinum dish and weigh it to the nearest 0.1 mg.
2) Add 8 mL of sulfuric acid and 20 mL of hydrofluoric acid, and heat on a sand bath until evaporated to dryness. 3) When the liquid volume has decreased, add another 20 mL of hydrofluoric acid and heat on a sand bath until evaporated to dryness.
4) After cooling to room temperature, add 5 mL of hydrochloric acid and approximately 50 mL of water, and heat and dissolve on a sand bath.
5) After cooling to room temperature, transfer to a flask (250 mL) and dilute with water to 250 mL to prepare the sample solution.
6) The content of each metal present in the sample solution was measured using the following measuring device.

[K content]
An atomic absorption spectrophotometer (Hitachi, Ltd., Z-2310, with a measurement wavelength range of 190–900 nm) was used.

[Content of Mg, Al, Ti, Ca, and Fe]
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.

(2) Metal (Cr, Zn, Ag, Pb, Ni or Cu) content 1) Take about 10 g of high-purity alkali silicate solution of the sample into a platinum dish and weigh it to the nearest 0.1 mg.
2) Add 5 mL of nitric acid and 20 mL of hydrofluoric acid, and heat on a sand bath until evaporated to dryness. 3) When the liquid volume has decreased, add another 20 mL of hydrofluoric acid and heat on a sand bath until evaporated to dryness.
4) 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.
5) After cooling to room temperature, transfer to a flask (100 mL) and dilute with water to 100 mL to prepare the sample solution.
6) The content of each metal present in the sample solution was measured using the following measuring device.

[Content of Cr, Zn, Ag, and Pb]
Inductively coupled plasma emission spectrometer (SPS5520, manufactured by Seiko Instruments Inc.)
This device 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 using the emission spectrum. The measurement wavelength range is 175 to 500 nm.

[Cu and Ni 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, thereby quantifying the elemental concentration in the sample. A graphite furnace is used. Measurement mode: atomic absorption, measurement wavelength range: 190-900 nm.

(3) _SiO2 concentration The _SiO2 concentration in the high-purity alkali silicate solution is determined by neutralizing the alkali silicate solution with hydrochloric acid and sodium hydroxide aqueous solution, then adding potassium fluoride solution, and titrating the resulting alkalinity with hydrochloric acid. Specifically, the _SiO2 concentration is measured based on the following reaction.
H ₂ SiO ₃ +6KF+H ₂ O→K ₂ SiF ₆ +4KOH
(4) Based on the measurement results from (1) to (3) above, the impurity concentrations of each of the 12 elements (K, Mg, Al, Ca, Ti, Cr, Fe, Ni, Cu, Zn, Ag, or Pb) were measured relative to the silica content (silica solids) of the high-purity alkali silicate.

[3] Molar Ratio The high-purity alkali silicate solution is preferably obtained by processing a raw material solution in which the molar concentration ratio of _SiO₂ to alkali oxide ( _SiO₂ /alkali oxide) is 3.4 or less. That is, if the high-purity alkali silicate solution is a high-purity aqueous sodium silicate solution, it is preferable that the molar concentration ratio of _SiO₂ to _Na₂O ( _SiO₂ molar concentration/ _Na₂O molar concentration) is 3.4 or less.
Here, the _SiO₂ concentration in the high-purity alkali silicate solution is measured by the method described above. Furthermore, the alkali concentration in the alkali silicate solution is determined, for example, in the case of a high-purity sodium silicate aqueous solution, by neutralizing it with hydrochloric acid, adding an excess of hydrochloric acid, and then back-titrating with a sodium hydroxide solution to quantify _Na₂O . The same procedure is followed for other alkalis.

[4] Molecular weight The high-purity alkali silicate is preferably of an average molecular weight of 30,000 or less, and more preferably of 10,000 or less.
Here, the average molecular weight is the value obtained by filtering a high-purity alkali silicate solution using a Millipore 0.22 μm filter MILLEX-GV, filling it into a plastic cell, measuring it in auto mode using a nanoSAQLA manufactured by Otsuka Electronics, and converting the resulting average particle size to the average molecular weight according to the calculation formula below.
Average molecular weight = {22 x π x (average particle diameter) ³ }/6
The above calculations apply the formula described on page 68 of "The Science of New Industrial Materials, Series B, Vol. 8: Silica and Alumina," published by Kinbara Publishing.

<Measurement of high-purity silica sol>
[1] Purity (1) Metal (Mg, Al, Ca, Ti, Cr, Fe, Zn, Ag, Pb, Ni, Cu, or Na) content 1) Take about 10 g of high-purity silica sol of the sample into a platinum dish and weigh it to the nearest 0.1 mg.
2) Add 5 mL of nitric acid and 20 mL of hydrofluoric acid, and heat on a sand bath until evaporated to dryness.
3) When the liquid volume decreases, add another 20 mL of hydrofluoric acid and heat on a sand bath until evaporated to dryness.
4) 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.
5) After cooling to room temperature, transfer to a flask (100 mL) and dilute with water to 100 mL to prepare the sample solution.
6) The content of each metal present in the sample solution was measured using the following measuring device.

[Content of Mg, Al, Ca, Ti, Cr, Fe, Zn, Ag, and Pb]
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.
[Cu and Ni 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, thereby quantifying 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.

(2) _SiO2 concentration The _SiO2 concentration in high-purity 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 firing the resulting solid at 1000°C for 1 hour, after which it was cooled and weighed. 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, fired at 1000°C for 15 minutes, and then cooled and weighed. The silica content was determined from the weight difference between these measurements.

(3) Based on the measurement results from (1) to (2) above, the impurity concentrations of each of the 12 elements (Mg, Al, Ca, Ti, Cr, Fe, Ni, Cu, Zn, Ag, Pb, or Na) relative to the silica solid content (silica dry) of the high-purity silica sol were measured.

[Example 1]
Sodium hydroxide (48% by mass concentration) was added to ultrapure water to obtain 3272 g of a 19% by mass sodium hydroxide aqueous solution (hereinafter referred to as Solution A).
Next, metallic silicon powder (average particle size = 0.08 μm) was added to ultrapure water and stirred for 4 hours with a stirring power of 2 W to disperse it, thereby obtaining 2728 g of silicon fine particle suspension with a silicon solid content of 25% by mass (hereinafter also referred to as Solution B).
Next, while stirring solution A, and while blowing nitrogen gas into the reaction vessel at a rate of 10 L/min, solution B was added at room temperature for 0.5 hours at an addition rate of 1.2 g/min·L for the silicon microparticle suspension (solid content equivalent) in the first stage addition. During this time, the reaction heat from the dissolution of metallic silicon by sodium hydroxide caused the liquid temperature to rise. When it reached 60°C, solution B was added for 5.5 hours at an addition rate of 0.4 g/min·L for the silicon microparticle suspension (solid content equivalent) in the second stage addition. After the addition of solution B was completed, the liquid temperature was maintained at 60°C for 8 hours using an external heat source. The maintenance of 60°C was terminated after confirming that the hydrogen gas concentration in the reaction vessel was below 100 ppm.
Next, the solution was cooled to room temperature to obtain 5949 g of a high-purity alkali silicate solution with a solid content of 23.20% by mass of _SiO₂ .
The impurity concentrations of 12 elements relative to the silica solid content of the obtained high-purity alkali silicate were measured using the method described above, and the impurity concentrations of each metal are shown in Table 2.
Furthermore, the average particle size of the obtained high-purity alkali silicate was measured using the method described above, and the molecular weight was calculated by converting it using the above formula. The average molecular weight was found to be 227.
The measurement of impurity concentration and calculation of average molecular weight in the following examples and comparative examples were all performed in the same manner as in Example 1.

(Preparation of high-purity purified silica solution)
Ultrapure water was added to the high-purity alkali silicate solution obtained as described above to obtain 12,020 g of a high-purity alkali silicate solution with an _SiO₂ concentration of 5%. This solution was passed through 6 L of a strongly acidic cation exchange resin (Duolite C255LFH, manufactured by Rohm & Haas) at a space velocity of 2.75 ^h⁻¹ to obtain 11,280 g of acidic silicate solution with a pH of 2.7. The _SiO₂ concentration of the obtained acidic silicate solution was 4.7% by mass.
Next, the entire volume of the acidic silicic acid solution was passed through 6 L of chelate ion exchange resin (CR-11, manufactured by Mitsubishi Chemical Corporation) at a space velocity of 2.75 ^h⁻¹ to obtain 9,660 g of purified silicic acid solution with a pH of 2.7. The _SiO₂ concentration of the obtained purified silicic acid solution was 4.5% by mass.
Furthermore, the entire volume of the purified silicic acid solution was passed through a separate 6 L of strongly acidic cation exchange resin (Duolite C255LFH, manufactured by Rohm & Haas) at a space velocity of 2.75 ^h⁻¹ to obtain 8,720 g of high-purity purified silicic acid solution with a pH of 2.7. The _SiO₂ concentration of the obtained high-purity purified silicic acid solution was 4.2% by mass.

(Preparation of high-purity silica sol)
A portion of the high-purity purified silica solution obtained as described above (318 g) was taken out as the seed solution, and another portion of the high-purity purified silica solution (8,070 g) was taken out as the feed solution.
Next, 15 g of a 48.8% by mass potassium hydroxide aqueous solution was added to 524 g of ultrapure water, and then seed solution was added and the mixture was heated. After reaching 83°C, the temperature was maintained for 30 minutes, and while maintaining this temperature, the feed solution was added at a constant rate over 3 hours, then the addition rate was increased to 1.5 times and the mixture was added for another 12 hours. After the entire amount of feed solution was added to the seed solution, the mixture was maintained at 83°C for 1 hour and then cooled to room temperature.
The obtained solution was concentrated to 12% by mass using an ultrafiltration membrane (SIP-1013, manufactured by Asahi Kasei Chemicals), and then concentrated to 40% by mass using a rotary evaporator.
The metal impurity concentration of the obtained high-purity silica sol was then measured using the method described above, and the concentrations of each metal impurity relative to the silica solid content are shown in Table 3.

[Example 2]
After preparing solutions A and B in the same manner as in Example 1, solution B was added to a stirrer reactor at room temperature for 0.5 hours at an addition rate of 1.8 g/min·L for the silicon microparticle suspension (solid content equivalent) in the first stage, while blowing nitrogen gas at 10 L/min of solution A into the reactor. During this time, the liquid temperature rose due to the reaction heat when the metallic silicon was dissolved by sodium hydroxide, and when it reached 80°C, solution B was added for 3 hours at an addition rate of 0.5 g/min·L for the silicon microparticle suspension (solid content equivalent) in the second stage. After the addition of solution B was completed, the liquid temperature was maintained at 80°C for 6 stages using an external heat source, and the maintenance of 80°C was terminated after confirming that the hydrogen gas concentration in the reaction vessel was 100 ppm or less. Thereafter, the same procedure as in Example 1 was carried out to obtain a high-purity alkali silicate solution. Subsequently, this was used as a raw material to obtain a high-purity silica sol.

[Example 3]
After preparing solutions A and B in the same manner as in Example 1, solution B was added to the reaction vessel with a stirring bar at room temperature while blowing nitrogen gas at 10 L/min. The addition rate of the silicon microparticle suspension (in terms of solid content) in the first stage was maintained at 0.4 g/min·L for 7 hours. During this time, the reaction heat from the dissolution of metallic silicon by sodium hydroxide caused the liquid temperature to rise, and after reaching 60°C, the temperature was maintained without changing the addition rate. After the addition of solution B was completed, an external heat source was used to maintain the liquid temperature at 60°C for 8 hours. The maintenance at 60°C was terminated after confirming that the hydrogen gas concentration in the reaction vessel was 100 ppm or less. Thereafter, the same procedure as in Example 1 was performed to obtain a high-purity alkali silicate solution. Subsequently, this was used as a raw material to obtain a high-purity silica sol.

[Comparative Example 1]
3581 g of pure water was placed in a 10 L autoclave container and stirred while 29.5 g of sodium hydroxide aqueous solution (48% by mass) was added as a molar ratio adjuster. Next, sodium silicate cullet, crushed to a size of a few centimeters or less, was added to adjust the molar ratio ( _SiO₂ / _Na₂O ) to 3.19. Subsequently, the temperature was raised to 151°C and maintained for 3 hours to dissolve. After cooling to below 40°C, the resulting water glass was filtered through quantitative filter paper (No. 5A, particle retention capacity 7 μm) to obtain 5240 g of alkali silicate solution with an _SiO₂ solid content of 24.4% by mass. Then, silica sol was prepared using the obtained alkali silicate solution.
(Preparation of acidic silica solution)
Ultrapure water was added to the alkali silicate solution obtained as described above to obtain 12,020 g of a high-purity alkali silicate solution with an _SiO₂ concentration of 5%. This solution was passed through 6 L of a strongly acidic cation exchange resin (Duolite C255LFH, manufactured by Rohm & Haas) at a space velocity of 2.75 ^h⁻¹ to obtain 11,120 g of acidic silicate solution with a pH of 2.6. The _SiO₂ concentration of the obtained acidic silicate solution was 4.6% by mass.
(Preparation of silica sol)
A portion of the acidic silica solution obtained as described above (39 g) was taken out as the seed solution, and another portion of the acidic silica solution (8,078 g) was taken out as the feed solution.
Next, 51 g of alkali silicate was added to 833 g of ultrapure water, and then seed solution was added and the mixture was heated. After the temperature reached 83°C, it was maintained at this temperature for 30 minutes, and while maintaining this temperature, the feed solution was added at a constant rate over 11 hours. After the entire volume of feed solution was added to the seed solution, the temperature was maintained at 83°C for 1 hour and then cooled to room temperature.
The obtained solution was concentrated to 12% by mass using an ultrafiltration membrane (SIP-1013, manufactured by Asahi Kasei Chemicals), and then concentrated to 40% by mass using a rotary evaporator.
The impurity concentrations of the obtained silica sol were then measured using the method described above, and the Mg concentration relative to the silica solid content was 19.8 ppm, Al concentration was 129.9 ppm, Ca concentration was 15.1 ppm, Ti concentration was 96.8 ppm, Cr concentration was 0.6 ppm, Fe concentration was 38.8 ppm, Ni concentration was 0.3 ppm, Cu concentration was 0.2 ppm, Na concentration was 7,349 ppm, and Zn, Ag, and Pb concentrations were 0.0 ppm.

[Manufacturing conditions and results of various measurements]
The manufacturing conditions in the examples are shown in Table 1. The results of various measurements in the examples and comparative examples are shown in Tables 2 and 3.

1…Suspension tank, 2…Agitator, 3…Reaction tank, 4…Temperature control device, 5…Agitator, 6…Transfer pipeline, 7…Control device, 8…Storage tank, 9…Transfer pipeline, 10…Control device, 100…High-purity alkali silicate solution manufacturing apparatus.

Claims (4)

A method for producing a high-purity alkali silicate solution, comprising a preparation step of adding silicon nanoparticles to an aqueous alkali hydroxide solution to prepare a reaction solution, wherein all of the following conditions 1) to 6) are met,
In the above preparation step, a first-stage addition and a second-stage addition are performed with different addition rates (in terms of solid content) of the silicon nanoparticles.
When the addition rate of silicon nanoparticles in the first stage of addition is S1, and the addition rate of silicon nanoparticles in the second stage of addition is S2, the value of the addition rate ratio R, expressed by the following formula (F2), is in the range of 1 to 10.
R=S1/S2...(F2)
In the preparation step, the silicon nanoparticles are added to the alkali hydroxide aqueous solution in the form of a silicon nanoparticle suspension in which the silicon nanoparticles are dispersed.
The aforementioned silicon microparticle suspension is characterized by a stirring power requirement of 0.1W to 3,000W when suspending the silicon microparticles in pure water.
A method for producing a high-purity alkaline silicate solution.
1) The average particle size of the silicon nanoparticles is in the range of 0.03 μm to 10 mm.
2) The average addition rate (B) of silicon nanoparticles to the alkali hydroxide aqueous solution is in the range of 0.05 g/min·L to 5.0 g/min·L relative to the volume of the reaction solution.
3) The value of the reaction parameter A, represented by formula (F1), is in the range of greater than 0 and less than or equal to 10,000.
A=B×C×D×T...(F1)
(In formula (F1), B is the average addition rate of silicon nanoparticles to the alkali hydroxide aqueous solution (g/min·L), C is the specific surface area of the silicon nanoparticles ( ^m² /g), D is the molar ratio of alkali hydroxide to silicon nanoparticles (alkali hydroxide/silicon nanoparticles), and T is the reaction temperature (°C).)
4) The value of C, which is the specific surface area (m²/g) of the silicon nanoparticles, is in the range of 0.00025 ^m² / g or more and 86 m²/g ^or less ^.
5) The value of D, which is the molar ratio of alkali hydroxide to silicon nanoparticles (alkali hydroxide/silicon nanoparticles), is in the range of 0.2 or more and 0.7 or less.
6) The value of T, which is the reaction temperature (°C), is in the range of 40°C to 98°C. In the resulting high-purity alkali silicate solution, the content of flat, plate-shaped fine particles, measured by image analysis, having a major axis of 500 nm to 1500 nm, a minor axis of 100 nm to 500 nm, and an aspect ratio (major axis/minor axis) of 3 or more, is 0.1 particles/mL or less.
A method for producing a high-purity alkali silicate solution according to claim 1 . A manufacturing apparatus used in the method for producing a high-purity alkali silicate solution according to claim 1 or claim 2,
A suspension tank for suspending silicon nanoparticles in water to obtain a silicon nanoparticle suspension,
A stirring device attached to the suspension tank,
A reaction vessel comprising: adding the aforementioned silicon fine particle suspension to an aqueous alkali hydroxide solution and reacting to obtain a high-purity alkali silicate solution;
A temperature control device and a stirring device attached to the reaction vessel,
A transfer pipeline for supplying the silicon fine particle suspension from the suspension tank to the reaction tank,
The system includes a control device for controlling the rate at which the silicon microparticle suspension is added.
A manufacturing apparatus for high-purity alkali silicate solution. A method for producing high-purity silica sol, using a high-purity alkali silicate solution obtained by the method for producing a high-purity alkali silicate solution according to claim 1 or claim 2 as a raw material.