Silicic acid solution and method for producing the same

Abstract

To provide a highly concentrated and stable silicic acid solution.SOLUTION: The silicic acid solution according to the present invention contains silica and an acid, has a pH of 3.0 or less, contains 8% or more of silica in terms of SiO2, and has a content of gel-like materials containing silica having a size of 3 μm or more of 1000 particles / ml or less. The silicic acid solution having these characteristics exhibits excellent long-term stability.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a silicic acid solution and a method for producing the same. [Background technology]

[0002] One of the uses of silica-based particles is in cosmetics, and silicic acid solution, which is obtained by removing cations from water glass, can be used as a suitable raw material. However, silicic acid solution generally has low stability and tends to thicken and gel over time, so it is often handled at a relatively low concentration to prevent this.

[0003] A method using a 0.5 to 10%, preferably 3 to 4%, silicic acid solution is known as a method for producing porous silica-based particles with surface smoothness (see, for example, Patent Document 1). One method for obtaining such a silicic acid solution is to dilute water glass with water and then treat it with a cation exchange resin to dealkalize it. However, this method has difficulty obtaining a stable silicic acid solution with a concentration exceeding 5%. Therefore, when the silicic acid solution is spray-dried as a silica raw material, most of the components become water, making it difficult to improve production efficiency. Furthermore, silicic acid solutions with a concentration exceeding 5% obtained by this method do not necessarily have high filterability, which can pose a production problem.

[0004] It has been well known that removing the alkali component from an alkali silicate solution, such as water glass, impairs its stability, resulting in thickening and gelation. This is because silicate ions become very unstable near neutral pH, causing polymerization reactions such as the formation of siloxane bonds between silicates. Further removal of the alkali component or acidification by adding an acid restores stability. It is known to first mix the alkali silicate with a large amount of acid to a pH of less than 2, and then contact the solution with a hydroxyl-type basic anion exchange resin and a hydrogen-type acidic cation exchange resin in any order to obtain a silicic acid solution (see, for example, Patent Document 2). However, this method of contacting the anion exchange resin makes it difficult to control the amount of remaining acid, and also creates a locally high pH environment near neutral or equivalent around the anion exchange resin, resulting in the formation of a gel, making it difficult to obtain a highly concentrated and stable silicic acid solution.

[0005] Also, a method of adding a stabilizer such as an acid to obtain a stabilized aqueous solution of active silicic acid has been disclosed (see, for example, Patent Document 3). In this method, a stabilizer is added after obtaining an active silicic acid solution to stabilize it, but when a high-concentration active silicic acid solution is obtained by a known method, a large amount of gel-like matter has already formed at that point, and this method is not necessarily suitable as a means for obtaining a high-concentration, stable, and highly filterable silicic acid solution.

[0006] Furthermore, a method of electrodialyzing water glass to dealkalize it is known (see, for example, Patent Document 4). In this method, the preferred conditions are to dilute water glass with water to a silica concentration of 4 to 7%, but this is not sufficient as a method for obtaining a silicic acid solution with a higher concentration. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-137806 [Patent Document 2] Special Publication No. 2014-511330 [Patent Document 3] International Publication WO2022 / 210195 [Patent Document 4] Japanese Patent Application Laid-Open No. 2004-99375 Summary of the Invention [Problem to be solved by the invention]

[0008] It is generally known that silicic acid is a substance that is prone to undergo polymerization reactions such as siloxane bond formation due to reactions between silicic acids at near-neutral pH, resulting in loss of fluidity and prone to thickening and gelation. As mentioned above, it has been difficult to provide a highly concentrated and stable silicic acid solution using conventional methods. [Means for solving the problem]

[0009] The silicic acid solution of the present invention contains silica and acid, has a pH of 3.0 or less, contains 8% or more silica in terms of SiO2, and contains 1000 or less gel-like particles of 3 μm or more containing silica per ml. Such a silicic acid solution has a concentration of 8% or more, has little change in viscosity, and is highly stable.

[0010] The method for producing a silicic acid solution according to the present invention comprises: a first step of mixing an acidic solution with an alkali silicate and simultaneously or alternately removing the alkali components while maintaining a pH of 5 or less, gradually increasing the silica concentration to obtain a silicic acid solution with a silica concentration of 8% or more; and a second step of recovering a silicic acid solution with a silica concentration of 8% or more and a pH of 3.0 or less from the silicic acid solution obtained in the first step. That is, if the silicic acid solution obtained in the first step has a pH of 3.0 or less, it is recovered as is. If the silicic acid solution obtained in the first step has a pH higher than 3.0, the alkali is further removed to adjust the pH to 3.0 or less before recovery. DETAILED DESCRIPTION OF THE INVENTION

[0011] The silicic acid solution according to the present invention contains silica and acid and has a pH of 3.0 or less, and contains 8% or more silica in terms of SiO2, and the content of gel-like particles containing silica and 3 μm or larger is 1000 pieces / ml or less.

[0012] The inventors discovered that silica-containing gel-like materials (hereinafter simply referred to as gel-like materials) of 3 μm or larger in size are the starting point for gelation in silicic acid solutions, accelerating the thickening and gelation of the silicic acid solutions. When performing dealkalization on high-concentration alkali silicate solutions, they noticed that even when a highly transparent silicic acid solution appears to be obtained, a large amount of gel-like materials (presumably lumps of water-containing Si-containing polymers formed by the reaction of silicic acid with other silicic acids) is produced. This is presumably because localized imbalances in the alkali or silicic acid concentration in the system during the dealkalization process make localized thickening and gelation particularly likely in high-concentration systems. Gel-like materials are typically difficult to observe because their component composition is nearly identical to that of the silicic acid solution. However, diluting the silicic acid solution creates a difference in composition with the solvent components, making these gel-like materials optically observable. A highly concentrated, stable silicic acid solution with little gel-like material and high filterability was obtained. The refractive index of amorphous silica is about 1.45, which is higher than that of water or silicic acid solution containing silicic acid components. Therefore, when silica particles with a particle size of 3 μm or more are dispersed in water or silicic acid solution, strong scattering occurs and transparency can be lost. In this way, there is a clear distinction between gel-like substances and silica particles.

[0013] Therefore, by reducing the gel-like particles to 1000 particles / ml or less, a highly concentrated and stable silicic acid solution can be obtained.

[0014] If there are 1,000 or more gel-like particles of 3 μm or more per ml, the gel-like particles will cause a decrease in filterability during production. Furthermore, since they may be the starting point for gelation of the silicic acid solution, they will cause a decrease in the stability of the silicic acid solution, making it impossible to obtain a highly concentrated and stable silicic acid solution. The number of particles is more preferably 800 or less per ml, and even more preferably 500 or less per ml.

[0015] It is usually difficult to achieve a pH range of 0.0 to 3.0 using only silicic acid without adding any acid. On the other hand, a large amount of acid must be added to lower the pH to 3.0 or below, which can cause problems as an impurity when used as a silica raw material. Furthermore, if the pH exceeds 3.0, sufficient stability at high concentrations is not obtained, and thickening and gelation over time are likely to occur.

[0016] Therefore, the amount of acid is preferably in the range of 100 to 10,000 ppm. If the acid content is less than 100 ppm, it is difficult to maintain the pH at 3.0 or less, and thickening and gelation tend to occur over time. On the other hand, if the acid content exceeds 10,000 ppm, impurities may become an issue when used as a silica raw material.

[0017] In addition, the acid concentration / alkali concentration ratio is preferably equal to or greater than 1. If the acid concentration / alkali concentration ratio is less than 1, the amounts of acid and alkali required to adjust the pH to 3 or less will increase, and the electrolyte concentration will increase, making it difficult to maintain the stability of the silicic acid solution.

[0018] Examples of the acid include sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, boric acid, etc. From the viewpoint of corrosion to equipment and the constituent components of cosmetic materials, sulfuric acid is preferred.

[0019] Examples of organic acids include carboxylic acids and hydroxycarboxylic acids (compounds having both a carboxyl group and an alcoholic hydroxyl group in one molecule). Preferred carboxylic acids include monocarboxylic acids (such as formic acid, acetic acid, and acrylic acid) and polycarboxylic acids (such as oxalic acid, malonic acid, succinic acid, and maleic acid). Preferred hydroxycarboxylic acids include lactic acid, tartaric acid, malic acid, glyceric acid, and citric acid.

[0020] From the viewpoint of productivity, the viscosity after 24 hours at 5°C is preferably in the range of 0.5 to 2.0 times the viscosity immediately after production. If the viscosity after 24 hours exceeds 2.0 times the viscosity immediately after production, this is undesirable because it will affect the particle size distribution and particle shape when preparing silica particles by spray drying. Furthermore, the viscosity after 24 hours is usually not less than 0.5 times the viscosity immediately after production.

[0021] <Method of manufacturing silicic acid solution> In order to obtain a silicic acid solution containing little gel-like matter, it is most preferable not to form gel-like matter in the dealkalization step. Therefore, a production method including the following steps is preferred.

[0022] (first step) While maintaining a pH of 5 or less, the acidic solution is mixed with alkali silicate and the alkali components are removed simultaneously or alternately, gradually increasing the silica concentration to prepare a silicate solution with a silica concentration of 8% or more.

[0023] (Second process) From the silicic acid solution obtained in the first step, a silicic acid solution with a silica concentration of 8% or more and a pH of 3.0 or less is recovered. That is, if the silicic acid solution obtained in the first step has a pH of 3.0 or less, it is recovered as is. If the silicic acid solution obtained in the first step has a pH of more than 3.0, it is further subjected to alkali removal to adjust the pH to 3.0 or less before being recovered.

[0024] According to this manufacturing method, the alkaline components can be removed while maintaining a low alkaline concentration in the system under acidic conditions in which silicic acid can exist stably, so that a highly concentrated and stable silicic acid solution can be obtained with little gel-like matter.

[0025] Each step will be described in detail below. <First step> (acidic solution) The acidic solution may be a solution of an inorganic acid or a solution of an organic acid. Furthermore, the solvent for the acidic solution is preferably water, but it may contain a solvent other than water. Examples of inorganic acids include hydrochloric acid, nitric acid, phosphoric acid, boric acid, and sulfuric acid. Among these, sulfuric acid is preferred from the viewpoints of equipment corrosivity and its use as a component in cosmetic materials. Examples of organic acids include carboxylic acids and hydroxycarboxylic acids (compounds having both a carboxy group and an alcoholic hydroxyl group in one molecule). Examples of preferred organic acids include monocarboxylic acids (such as formic acid, acetic acid, and acrylic acid) and polycarboxylic acids (such as oxalic acid, malonic acid, succinic acid, and maleic acid). Examples of hydroxycarboxylic acids include lactic acid, tartaric acid, malic acid, glyceric acid, and citric acid.

[0026] When the acidic solution is diluted, the pH of the system may fluctuate significantly locally when mixed with the alkali silicate solution, which may lead to the formation of a gel. Therefore, the pH of the acidic solution is preferably 5 or less, more preferably 4 or less, and even more preferably 3 or less.

[0027] (Alkaline silicate solution) The alkali silicate solution is a solution in which an alkali silicate, such as sodium silicate, potassium silicate, or lithium silicate, is dissolved in water. Here, alkali refers to alkali metals (Li, Na, K, Rb, and Cs). Water glass or diluted water glass, in which sodium silicate is dissolved in water, can be used as the alkali silicate solution. The silica concentration of the alkali silicate solution should be 8% or higher to obtain a high-concentration silicate solution. A sodium silicate solution in this concentration range can be obtained, for example, by diluting commercially available water glass with pure water. Examples of water glass include those represented by the formula Na2O·nSiO2·mH2O, with a silica concentration ranging from 22% to 38%, a Na2O concentration ranging from 5% to 19%, a SiO2 / Na2O (molar ratio) ranging from 0.5 to 4, and a pH of 9 or higher. However, if the alkaline component in the alkali silicate solution is high, mixing it with an acidic solution can cause significant localized pH fluctuations in the system, making it more likely to form a gel. Therefore, it is preferable to use a low alkaline component within a range that allows silicic acid to remain stable in a dissolved state. From this perspective, No. 2 water glass, No. 3 water glass, and No. 4 water glass are preferably used. Plants such as rice husks can also be used as a silicic acid source. Water glass made from rice husks consumes less energy in the manufacturing process and is therefore more environmentally friendly.

[0028] (Mixing and removal of alkaline components) The acidic solution and the alkali silicate solution can be mixed by any conventionally known method. The acidification of the alkali silicate solution by mixing the acidic solution and the alkali silicate solution is preferably carried out in a very short time to prevent the formation of gel-like substances. Specific examples include a method in which the alkali silicate solution is gradually added to the stirred acidic solution, or a method in which the two solutions are instantly mixed using a line mixer or the like. Among these, a method in which the alkali silicate solution is gradually added to the acidic solution while simultaneously removing the alkali components is particularly preferred, as it can increase the silica concentration in the solution without increasing the alkali concentration in the mixed solution of the acidic solution and the alkali silicate, i.e., while maintaining a pH of 5 or less, and while suppressing the formation of gel-like substances, which is particularly likely to occur near neutral. In this case, the pH is preferably 5 or less, more preferably 4 or less, and even more preferably 3 or less, from the viewpoint of preventing the formation of gel-like substances.

[0029] Any conventionally known method can be used to remove the alkaline component, such as a method using a cation exchange resin, an ion exchange membrane (electrodialysis), or an ultramembrane. The method using a cation exchange resin is preferably used because the acid required for stabilization remains in proportion to the amount charged.

[0030] (Temperature in alkaline component removal process) Generally, in the alkali component removal process, the higher the liquid temperature, the higher the removal efficiency. However, since this also promotes the polymerization reaction of silicic acid, a high temperature is not necessarily desirable, especially when preparing a highly concentrated silicic acid solution. The liquid temperature in the alkali component removal process is preferably 3°C to 18°C, more preferably 3°C to 10°C. If the liquid temperature is below 3°C, alkali removal may not proceed efficiently, and further reduction in temperature tends not to significantly improve the stability of the silicic acid solution. Furthermore, if the liquid temperature of the sodium silicate aqueous solution exceeds 18°C, a gel-like substance tends to form, and the stability of the resulting silicic acid solution tends to decrease. In addition, when using an ion exchange resin for removal, in this embodiment, the temperature of the solution before passing it through the ion exchange resin is the temperature during dealkalization. Furthermore, to prevent temperature rise during passage, it is preferable to previously adjust the temperature of the cation exchange resin packed in the column to 3°C to 18°C.

[0031] (cation exchange resin) When alkaline components are removed by cation exchange, it is preferable to use an H-type cation exchange resin. The cation exchange resin may be in any form, such as beads or fibers. In this embodiment, it is preferable to fill a column with the cation exchange resin and pass the liquid through it.

[0032] (space velocity) Generally, in ion exchange operations, the contact time (space velocity (SV)) between the liquid to be ion exchanged and the ion exchange resin is an important factor. The space velocity when the alkali silicate mixture is passed through a column packed with cation exchange resin is 2 h -1 More than 60h -1 When the space velocity is in this range, the contact time between the silicic acid solution and the cation exchange resin is at a sufficient level, which is preferable in terms of ion exchange efficiency. -1If the space velocity is less than 60 h, the pH of the added liquid tends to increase in the step of adding the alkali silicate described below, and a gel-like substance tends to be formed. -1 If the temperature exceeds this value, uneven flow of the solution occurs in the ion exchange column, causing uneven alkali concentration in the ion exchange column, which also tends to make gel-like matter more likely to form.

[0033] Here, the spatial velocity when passing a solution through a column packed with ion exchange resin is the liquid passing rate per volume of ion exchange resin, and is therefore referred to as the liquid passing rate (m 3 / h) ÷ resin volume (m 3 )=space velocity(h -1 ) relationship.

[0034] <Second process> The silicic acid solution having a silica concentration of 8% or more and a pH of 5 or less obtained in the first step is recovered as it is, or after further removing the alkaline component, as a silicic acid solution having a pH of 0.0 to 3.0.

[0035] When alkaline silicate is gradually added to a circulation tank containing an acidic solution while simultaneously circulating the solution through a column packed with cation exchange resin to remove alkaline components, the solution can be recovered from the column outlet upon completion of the alkaline silicate addition, resulting in a silicic acid solution with a pH of 0.0 to 3.0. To achieve this, the column is preferably packed with cation exchange resin at a level between 1 and 2 times its exchange capacity, more preferably between 1.2 and 2 times. If the cation exchange resin capacity is less than 1, alkaline components will remain in the silicic acid solution, necessitating additional measures such as additional alkaline removal. Furthermore, using cation exchange resin in excess of 2 times its ion exchange capacity is not generally sufficient to remove additional alkaline components, and requires a larger column to accommodate the cation exchange resin, which is undesirable.

[0036] (spray drying) Spray drying can be performed using a conventional method using a commercially available spray dryer (such as a disk rotation type or nozzle type). The high-concentration silicic acid solution of the present invention can be used as is or after filtration as the liquid to be sprayed. It may also be mixed with a dispersion of fine particles composed of silica, titania, alumina, iron oxide, or the like. Spray drying conditions include spraying the dispersion thus obtained into a hot air stream at a rate of 1 to 3 liters per minute. The hot air temperature is preferably in the range of 70 to 400°C at the inlet, and 100 to 300°C at the outlet, with a temperature of 40 to 60°C at the outlet. An inlet temperature below 70°C results in insufficient drying of the solids contained in the dispersion, while a temperature above 400°C results in distorted particle shape during spray drying, which is undesirable. An outlet temperature below 40°C is also undesirable because the solids are not sufficiently dried and adhere to the inside of the device.

[0037] It is also preferable to preliminarily adjust the dispersion to a solid content of 8 to 50% by weight, and then subject the dispersion to spray drying in a spray dryer. A solid content of less than 8% by weight is not economical, while a solid content of more than 50% by weight increases the viscosity of the slurry, distorting the shape of the spray-dried product, which is undesirable. When the dispersion is subjected to spray drying using a spray dryer, the silicon compound in the silicic acid solution is dehydrated and polycondensed to form a silica component.

[0038] The silica-based particles thus obtained have a particle diameter of approximately 0.1 to 50 μm, and have a generally spherical or nearly spherical shape.

[0039] The above-mentioned production method can produce a highly concentrated and stable silicic acid solution. Furthermore, when such a silicic acid solution is used to produce silica powder by spray drying, not only can the process time be shortened, but also the amount of water relative to the silica component can be reduced, thereby reducing the amount of energy used to produce the silica powder, thereby enabling economical production activities with reduced environmental impact.

[0040] Examples of the present invention will be specifically described below. [Example 1] 6.91 g of commercially available 25% sulfuric acid was mixed with 2817 g of pure water to produce 2824 g of 0.0612% sulfuric acid. 1976 g of No. 3 water glass (silica concentration 24.3%, Na2O concentration 8.0%) was also prepared. When the entire amount of the acidic solution and No. 3 water glass was mixed, the silica concentration was 10% and the sulfuric acid concentration was 360 ppm. The acidic solution was placed in a circulation tank and circulated through an ion exchange column packed with 4010 ml of strong acid cation exchange resin (DIAION™ SK1BH, Mitsubishi Chemical Corporation) at a space velocity of 40 / h. No. 3 water glass was then gradually added to the circulation tank over a 5-hour period. During this time, the pH in the circulation tank temporarily increased with the addition of No. 3 water glass, reaching a maximum value of 2.4. After the addition of the entire 1976g of prepared No. 3 water glass was completed and the silica concentration reached 10% and the sulfuric acid concentration reached 360ppm, the recovery of the liquid that had passed through the ion exchange column was started. Next, when the liquid in the circulation tank was gone, the liquid was switched to pure water and the recovery of the liquid continued. When the electrical conductivity of the recovered liquid reached 80% of that at the start of recovery, the recovery was stopped and a silicic acid liquid was obtained.

[0041] The silica powder of this example was prepared by spray-drying the silicic acid solution into a dry air stream at 105°C, with gas supplied to one of two-fluid nozzles at a flow rate of 5 kg / hr and the other nozzle at a pressure of 2 kg / hr. The solution was sprayed and dried in a dry air stream at 105°C to prepare silica powder. The weight of the silica powder recovered after 1 hour of spraying was measured. The silica powder was then heat-treated at 1000°C for 1 hour to confirm its residual rate, and the yield of the silica powder was evaluated using the formula: "silica powder yield (g / h) = weight of silica powder recovered in 1 hour (g / h) × residual rate after high-temperature treatment (%)."

[0042] The properties of the silicic acid solution obtained as described above were measured and evaluated as follows. The results, along with the preparation conditions, are shown in Table 1. The same procedures were carried out for the other examples and comparative examples.

[0043] [1] Silica concentration 3 to 5 g of sample was placed in a platinum crucible, dried in a water bath, fired with a burner, and fired at 1000°C for 1 hour, then cooled and weighed. The silica concentration was calculated by subtracting the mass of the alkali components (Na, K, Cs, Li, Rb) converted to oxide from the difference between the amount placed in the platinum crucible and the weighing after firing at 1000°C for 1 hour.

[0044] [2]Na, K, Cs, Li concentration To 30 g of sample, 5 mL of nitric acid (Kanto Chemical, special grade, 63%) and 20 mL of hydrofluoric acid (Kanto Chemical, special grade, 47%) were added and heated on a sand bath. When the liquid volume decreased, an additional 20 mL of hydrofluoric acid was added and heated on a sand bath to evaporate to dryness. After cooling to room temperature, 2 mL of nitric acid and approximately 50 mL of water were added and heated on a sand bath to dissolve. After cooling to room temperature, the mixture was placed in a 100 mL flask and diluted to 100 mL with water to obtain the sample solution. The concentrations of each component were measured using an atomic absorption spectrophotometer (Hitachi High-Tech Science Z3300, measurement wavelength range 190-900 nm).

[0045] [3]Rb concentration To 10 g of sample, 5 mL of nitric acid (Kanto Chemical, special grade, 63%) and 20 mL of hydrofluoric acid (Kanto Chemical, special grade, 47%) were added and heated on a sand bath. When the liquid volume decreased, an additional 20 mL of hydrofluoric acid was added and heated on a sand bath to evaporate and dry. After cooling to room temperature, 2 mL of nitric acid and approximately 50 mL of water were added and dissolved by heating on a sand bath. After cooling to room temperature, the sample was placed in a 100 mL flask and diluted to 100 mL with water to obtain the sample solution. The concentration was measured using an ICP device (Agilent, 5800).

[0046] [4] SO4, Cl, NO3 concentration 1 g of the sample was diluted with water to 100 mL to prepare a sample solution, which was then measured using an ion chromatograph (ICS-1100, manufactured by Dionex).

[0047] [5] Acetic acid concentration A 30 g sample was collected, and 50 ml of sulfuric acid (1+1) (Kanto Chemical) was added and steam distilled until the effluent volume reached approximately 450 ml. Next, a phenolphthalein solution was added to the effluent as an indicator, and neutralization titration was performed with 0.1 mol / L aqueous sodium hydroxide solution to determine the acetic acid concentration.

[0048] [6]pH measurement A 40 g sample was placed in a 50 ml PET screw tube, and the liquid temperature was adjusted to 25°C in a water bath. After that, measurements were taken using a tabletop pH / electrical conductivity meter (HORIBA, F-74) and a pH electrode (HORIBA, 9615S).

[0049] [7] Conductivity measurement A 40 g sample was placed in a 50 ml PET screw tube, and the liquid temperature was adjusted to 25°C in a water bath. After that, measurements were taken using a benchtop pH / electrical conductivity meter (HORIBA, F-74) and a general-purpose electrical conductivity cell (HORIBA, 3552).

[0050] [8] Gel-like material (3 μm or more) The sample was measured using a particle counter (AccuSizer 780APS, Entegris, Inc.) under the following conditions: loop size 1.0 ml, dilution ratio 1:30, dilution ratio 2:30, flow rate 60 ml / min, measurement time 120 seconds, and the silicate solution was diluted 900 times in the device. The number of gel-like particles 3 μm or larger was calculated using the analysis software attached to the device and used as the cumulative number (# / ml) of particles with a particle diameter of 3.05 μm.

[0051] [9] Filterability The sample was placed in a 50 ml syringe and filtered under pressure at 0.10 MPa using a Cytiva syringe filter (WHATMAN, 25 mm diameter, 0.45 μm PP). Filtration was stopped when the filtration rate became less than 5 drops per 10 seconds, and the weight of the filtrate was measured. When the entire sample in the syringe was used up, the syringe filter was removed, and the sample was repeatedly replenished while preventing negative pressure from building up inside the syringe and syringe filter, and the filterability evaluation was continued.

[0052]

[10] Initial viscosity Using a low viscosity adapter (L sleeve and L rotor) for a TVB-type viscometer on a B-type viscometer (Toki Sangyo Co., Ltd., TV-10), the liquid temperature was adjusted to 25.0°C in a water bath and measurements were taken while maintaining the temperature at 25.0°C. The initial viscosity was measured within 3 hours after preparation of the silicic acid solution, and the solution was kept refrigerated at 5°C until measurement.

[0053]

[11] Viscosity after 24 hours The sample was left in a water bath at 5°C for 24 hours, then removed, and the viscosity at 25.0°C was measured in the same manner as above.

[0054]

[12] Viscosity change rate (24h) The viscosity change rate (24 h) was calculated using the formula "viscosity change rate (24 h) = viscosity after 24 h / initial viscosity."

[0055]

[13] [Visible light absorption spectrum] The sample was placed in a quartz cell with an optical path length of 1 cm, and the transmittance in the visible light region (400 to 800 nm) was measured using an ultraviolet-visible spectrophotometer (V-750, manufactured by JASCO Corporation). Table 1 shows the minimum transmittance (%) in the above measurement range.

[0056] [Example 2] 2824 g of 0.143% sulfuric acid was prepared as an acidic solution, and when the total amount was mixed with 1976 g of No. 3 water glass, the silica concentration was 10% and the sulfuric acid concentration was 840 ppm. A silicic acid solution was obtained in the same manner as in Example 1, except that the No. 3 water glass addition time was 2 hours. During this time, the maximum pH value in the circulation tank was 2.2.

[0057] [Example 3] As an acidic solution, 36.0 g of acetic acid (Kanto Chemical Co., special grade) was mixed with 2787.9 g of pure water to prepare 2824 g of 1.27% acetic acid solution. When the total amount was mixed with 1976 g of No. 3 water glass, the silica concentration was 10% and the acetic acid concentration was 7,500 ppm. A silicic acid solution was obtained in the same manner as in Example 1, except that the No. 3 water glass addition time was 2 hours. During this time, the maximum pH value in the circulation tank was 3.8.

[0058] [Example 4] 2429 g of 0.198% sulfuric acid was prepared as an acidic solution, and when the total amount was mixed with 2371 g of No. 3 water glass, the silica concentration was adjusted to 12% and the sulfuric acid concentration to 1000 ppm. The sulfuric acid was circulated in an ion exchange column packed with 4810 ml of a strong acid cation exchange resin (DIAION™ SK1BH, manufactured by Mitsubishi Chemical Corporation) at SV40. A silicic acid solution was obtained in the same manner as in Example 1, except that the No. 3 water glass addition time was 2.5 hours. The maximum pH value in the circulation tank during this period was 2.2.

[0059] [Example 5] The silicic acid solution obtained in the same manner as in Example 2 was further passed through an ion exchange column packed with 250 ml of a strongly acidic cation exchange resin (DIAION™ SK1BH, manufactured by Mitsubishi Chemical Corporation) at SV40 to obtain a silicic acid solution.

[0060] [Comparative Example 1] 1976g of No. 3 aqueous glass (silica concentration 24.3%, Na2O concentration 8.0%) was mixed with pure water and 0.5% hydrochloric acid to prepare 4800g of diluted aqueous glass with a silica concentration of 5% and a hydrochloric acid concentration of 100ppm. This was passed through an ion exchange column packed with 4010ml of strong acid cation exchange resin (DIAION™ SK1BH, manufactured by Mitsubishi Chemical Corporation) at SV40 / h to obtain a silicic acid solution with a pH of 2.6. In this comparative example, the yield of silica powder was evaluated using this silicic acid solution in the same manner as in Example 1.

[0061] Comparative Example 2 Pure water was added to 1976 g of No. 3 aqueous glass (silica concentration 24.3%, Na2O concentration 8.0%) to prepare 4800 g of diluted aqueous glass with a silica concentration of 10%. An attempt was made to pass this through an ion exchange column packed with 4010 ml of strongly acidic cation exchange resin (DIAION™ SK1BH, manufactured by Mitsubishi Chemical Corporation) at an SV of 40 / h, but gelation occurred during the passage of the liquid.

[0062] Comparative Example 3 2824 g of sulfuric acid with a concentration of 0.0612% prepared in the same manner as in Example 1 was gradually added to 1976 g of No. 3 water glass (silica concentration 24.3%, Na2O concentration 8.0%) to obtain a mixed silicic acid solution with a silica concentration of 10%, a sulfuric acid concentration of 360 ppm, and a pH of 11.7. This solution was passed through an ion exchange column packed with 4010 ml of a strong acid cation exchange resin (DIAION™ SK1BH, manufactured by Mitsubishi Chemical Corporation) at a flow rate of 40 / h to obtain a silicic acid solution with a pH of 2.1.

[0063] Comparative Example 4 As in Example 1, 2824 g of 0.0612% sulfuric acid was placed in a circulation tank, and then circulated through an ion exchange column packed with 4010 ml of a strongly acidic cation exchange resin (DIAION™ SK1BH, manufactured by Mitsubishi Chemical Corporation) at a space velocity of 40 / h. Next, No. 3 water glass was gradually added to the circulation tank over 0.5 hours. During this time, the pH in the circulation tank continuously increased with the addition of No. 3 water glass, and when the pH reached 5.7, a tendency for viscosity increase was observed, and when the pH reached 6.4, gelation occurred.

[0064]

Table 1

Claims

1. A silicic acid solution containing silica and an acid and having a pH of 3.0 or less, The silica in the silicic acid solution is SiO ２ It contains more than 8% of the total. A silicic acid solution containing 1000 or less gel-like particles of 3 μm or larger containing silica per ml.

2. 2. The silicic acid solution according to claim 1, wherein the acid is contained in the silicic acid solution at a concentration of 100 to 10,000 ppm.

3. 2. The silicic acid solution according to claim 1, wherein the acid is sulfuric acid.

4. The silicic acid liquid according to claim 1, characterized in that when the viscosity of the silicic acid liquid is measured at 5°C, the viscosity after 24 hours is in the range of 0.5 to 2.0 times the initial viscosity.

5. A first step of mixing an acidic solution with an alkali silicate and simultaneously or alternately removing the alkali components while maintaining a pH of 5 or less, gradually increasing the silica concentration to obtain a silicate solution with a silica concentration of 8% or more; A method for producing a silicic acid liquid, comprising: a second step of recovering a silicic acid liquid having a silica concentration of 8% or more and a pH of 3.0 or less from the silicic acid liquid obtained in the first step.

6. 6. The method for producing a silicic acid solution according to claim 5, wherein the alkali component is removed at a temperature of 10° C. or less.

7. 6. The method for producing a silicic acid solution according to claim 5, wherein the alkaline components are removed using a cation exchange resin.

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