Alkali metal silicate aqueous solution and method for producing the same

Abstract

The aqueous alkali metal silicate solution of the present invention is the following aqueous alkali metal silicate solution, wherein, expressed in mass%, the ratio of the P2O5 concentration to the SiO2 concentration, P2O5 / SiO2, is 1.0×10 ‑3 or more, the SiO2 concentration is 8.9% by mass to 16.7% by mass, and the transmittance (%T) at a light path length of 1 cm and a wavelength of 500 nm is 65% or more. The manufacturing method of the present invention includes using the above aqueous alkali metal silicate solution as a raw material for wet-process silica.

Description

Technical Field

[0001] The present invention relates to an aqueous solution of alkali metal silicate (derived from plants) produced using plant ash and a method for producing the same.

[0002] Cross-reference to Related Applications

[0003] This application claims the priority of Japanese Patent Application No. 2022-062996 filed on April 5, 2022, the contents of which are incorporated herein by reference. Background Art

[0004] Sodium silicate, also known as "water glass", is widely used as a raw material for wet-process silica and synthetic zeolite, and in fields such as civil engineering and construction for ground solidifying agents, cleaning agents, fibers, and pulp. As a general method for producing sodium silicate, there are widely known: a wet method (chemical formula A) in which natural sand (silica sand, silica stone, silicic acid clay, etc.) rich in silicon dioxide (SiO2) is dissolved in an aqueous sodium hydroxide solution; or a dry method (chemical formula B) in which sand and soda ash are mixed and placed in a melting furnace, heated and melted, and taken out and cooled and solidified when it becomes a transparent body.

[0005] · Chemical formula A

[0006] 2NaOH + nSiO2 → Na2O·nSiO2 + H2O n: molar ratio

[0007] (aqueous sodium silicate solution)

[0008] · Chemical formula B

[0009] Na2CO3 + nSiO2 → Na2O·nSiO2 + CO2 n: molar ratio

[0010] (sodium silicate cullet)

[0011] Conventionally, wet-process silica produced from sand-derived sodium silicate has a high BET specific surface area and has micropores to mesopores, so it is widely used as a filler for reinforcement in industrial rubber, tires, and silicone rubber, a filler for paper, a matting agent for coatings, an abrasive for dentifrices, and an anti-blocking agent for films.

[0012] In recent years, there has been a demand for a sustainable and renewable society for protecting the global environment. As one of the specific countermeasures for this, the popularization of biomass technologies that do not use fossil fuels, such as biomass boilers and biomass power generation, is being promoted everywhere. In equipment using such technologies, plants rich in silicon dioxide (grasses such as rice husks, rice straws, wheat straws, bagasse, and bamboo) are mostly used as fuels. However, in these equipment, the disposal of combustion ash (plant ash) rich in silicon dioxide has become a problem.

[0013] Therefore, as one of the methods for solving this problem, the following attempts have been made in the past: Plant ash containing a large amount of silica components (especially rice husk ash) is dissolved in an aqueous sodium hydroxide solution to produce an aqueous silicic acid solution containing an alkali component, and wet-process silica is produced from this aqueous alkali metal silicate solution (for example, Patent Document 1 and Patent Document 2).

[0014] It should be noted that an aqueous silicic acid solution using plant ash as a raw material usually contains potassium as an alkali component in addition to sodium. Therefore, in the following description, unless the alkali component is particularly limited to sodium, instead of the commonly used term "sodium silicate" (Na2O·nSiO2), silicic acid containing an alkali component is referred to as "alkali metal silicate". In the present invention and this specification, the meaning of alkali metal silicate includes sodium silicate.

[0015] Patent Document 1: Japanese Patent Application Laid-Open No. 2007-510613

[0016] Patent Document 2: Japanese Patent Application Laid-Open No. 2007-522069

[0017] All the descriptions of Patent Documents 1 and 2 are incorporated herein by reference. Summary of the Invention

[0018] Problems to be Solved by the Invention

[0019] Based on known technologies such as Patent Document 1 and Patent Document 2, it is not impossible to produce an aqueous alkali metal silicate solution from rice husk ash and other plant ash and produce wet-process silica.

[0020] However, the aqueous alkali metal silicate solution derived from plant ash contains more coloring impurities and foreign substances than the substance made from sand. The wet-process silica produced from these colored aqueous alkali metal silicate solutions can only be low-quality wet-process silica with many impurities, foreign substances, and coloring (low whiteness). Therefore, the conventional wet-process silica produced from the aqueous alkali metal silicate solution derived from plant ash can only be used for limited uses and applications: for inexpensive and general uses; or for mixing a small amount of the aqueous alkali metal silicate solution derived from plant ash into the aqueous alkali metal silicate solution derived from sand and using it; and so on.

[0021] The quality of natural sand is constant in each production area. Therefore, for the conventional aqueous alkali metal silicate derived from sand, it is possible to cope with the quality of impurities, etc. by changing the origin of the sand according to the purpose of use. However, in the case of using plant ash, since plant ash itself contains a large amount of impurities and foreign substances and its quality is unstable, it is difficult to find a countermeasure for solving the problems of impurities, foreign substances, and coloring.

[0022] For these reasons, wet-process silica produced from an aqueous solution of alkali metal silicate derived from plant ash has not been popularized as expected.

[0023] In addition, the methods of the prior art, such as contacting with oxygen for a long time (Patent Document 1) and burning at a high temperature (Patent Document 2), are methods with a large amount of carbon dioxide generation and high energy consumption, and cannot be said to be appropriate methods from the aspect of considering recent environmental problems.

[0024] Furthermore, users who use wet-process silica require the supply of high-functional and high-quality wet-process silica while using raw materials (alkali metal silicates) that take environmental considerations into account.

[0025] The present invention has been completed in view of the above problems, and an object of the present invention is to provide an aqueous solution of alkali metal silicate derived from plant ash, which can produce high-quality wet-process silica having the same degree of whiteness as when using an aqueous solution of alkali metal silicate derived from conventional sand.

[0026] Another object of the present invention is to provide a method for producing an aqueous solution of alkali metal silicate, which can produce such an aqueous solution of alkali metal silicate with sufficient quality.

[0027] Means for Solving the Problems

[0028] The present inventors have found that by using an aqueous solution of alkali metal silicate derived from plant ash with high transmittance as a raw material to produce wet-process silica, high-quality wet-process silica having the same degree of whiteness as when using an aqueous solution of alkali metal silicate derived from conventional sand can be produced, thereby completing the present invention.

[0029] The constitution of the present invention is as follows. [1]

[0031] An aqueous solution of alkali metal silicate, wherein, in terms of mass%, the ratio P2O5 / SiO2 of the P2O5 concentration to the SiO2 concentration is 1.0×10 -3 or more, the SiO2 concentration is 8.9 mass% to 16.7 mass%, and the transmittance (%T) at a light path length of 1 cm and a wavelength of 500 nm is 65% or more. [2]

[0033] The aqueous solution of alkali metal silicate according to [1], wherein, in terms of mol%, the ratio SiO2 / (Na2O + K2O) of the SiO2 concentration to the (Na2O + K2O) concentration is 2.8 to 3.5. [3]

[0035] The aqueous solution of alkali metal silicate as described in [1] or [2], wherein, in terms of mol%, the ratio K2O / Na2O of the K2O concentration to the Na2O concentration is 0.03 to 0.30. [4]

[0037] The aqueous solution of alkali metal silicate according to any one of [1] to [3], wherein when the aqueous solution of alkali metal silicate is filtered using a filter with a mesh size of 1 μm, the amount of solid components captured on the filter is less than 100 mass ppm relative to the mass of the aqueous solution of alkali metal silicate before filtration. [5]

[0039] The aqueous solution of alkali metal silicate according to any one of [1] to [4] is used as a raw material for wet-process silica. [6]

[0041] A method for producing an aqueous solution of alkali metal silicate, comprising: subjecting the raw material aqueous solution of alkali metal silicate obtained by mixing plant ash containing a silica component, sodium hydroxide, and water to one or both of a filtration treatment using a filter with a mesh size less than 50 μm and an adsorption treatment using an adsorbent material to obtain an aqueous solution of alkali metal silicate. [7]

[0043] The method for producing an aqueous solution of alkali metal silicate according to [6], wherein the plant ash is rice husk ash.

[0044] Advantages of the Invention

[0045] By using the aqueous solution of alkali metal silicate derived from plant ash of the present invention as a raw material, it is possible to produce high-quality wet-process silica having the same degree of whiteness as when using the aqueous sodium silicate solution derived from conventional sand.

[0046] In addition, by the production method of the present invention, it is possible to provide an aqueous solution of alkali metal silicate derived from plant ash, which can produce high-quality wet-process silica having the same degree of whiteness as when using the aqueous solution of alkali metal silicate derived from conventional sand.

[0047] Furthermore, in the present invention, it is possible to recycle the plant ash that is usually discarded, and thus it is possible to produce an aqueous solution of alkali metal silicate in consideration of the environment. Description of the Drawings

[0048] Figure 1 It is an image showing the filter residue in the rough filtration of the aqueous solution of alkali metal silicate.

[0049] Figure 2 It is the transmittance spectrum of the aqueous solution of alkali metal silicate (Example 4, Comparative Example 1, Comparative Example 4) in the wavelength range of 200 nm to 800 nm.

[0050] Figure 3 It is the transmittance spectrum of an aqueous solution of alkali metal silicate (Example 1, Example 5, Reference Example 1) within the wavelength range of 200 nm to 800 nm.

[0051] Figure 4 It is the transmittance spectrum of an aqueous solution of alkali metal silicate (Comparative Example 3) within the wavelength range of 200 nm to 800 nm. Detailed implementation manners

[0052] <Aqueous solution of alkali metal silicate>

[0053] The aqueous solution of alkali metal silicate of the present invention is the following aqueous solution of alkali metal silicate. Among them, expressed in mass%, the ratio P2O5 / SiO2 of the P2O5 concentration to the SiO2 concentration is 1.0×10 -3 Above, the SiO2 concentration is 8.9 mass% to 16.7 mass%, and the transmittance (%T) at a light path length of 1 cm and a wavelength of 500 nm is 65% or more.

[0054] The aqueous solution of alkali metal silicate of the present invention is manufactured using the combustion ash of plants containing a silica component, that is, plant ash. The plant ash contains a phosphorus component, and phosphoric acid (P2O5) when converted into an oxide is an inherently contained component of the plant ash. In the aqueous solution of alkali metal silicate of the present invention using plant ash as the main silica source, expressed in mass%, the ratio of the P2O5 concentration to the SiO2 concentration is higher than that of the aqueous solution of alkali metal silicate sourced from sand, and the ratio P2O5 / SiO2 is 1.0×10 -3 Above. Generally speaking, the ratio P2O5 / SiO2 in the aqueous solution of alkali metal silicate using only sand as the silica source will not be 1.0×10 -3 Above. Therefore, by the presence of P2O5 based on the ratio P2O5 / SiO2, it is possible to discriminate that the aqueous solution of alkali metal silicate is sourced from plant ash (that is, at least a part of the silica source uses plant ash). In the aqueous solution of alkali metal silicate of the present invention, the ratio P2O5 / SiO2 can be 1.5×10 -3 Above, 1.8×10 -3 Above, or 2.0×10 -3 Above. There is no particular limitation on the upper limit of the ratio P2O5 / SiO2, which depends on the type of plant ash. For example, it is 20×10 -3 Below. From the aspect of reducing impurities, the ratio P2O5 / SiO2 is preferably 15×10 -3 Below, more preferably 13×10 -3 Below, and further preferably 10×10 -3 Below.

[0055] In the aqueous solution of alkali metal silicate of the present invention, the transmittance (%T) at a light path length of 1 cm and a wavelength of 500 nm is 65% or more.

[0056] Generally, plant ash contains a large amount of impurities and foreign substances. When manufacturing an aqueous solution of alkali metal silicate using plant ash with many impurities and foreign substances by a conventional method, the impurities and foreign substances are mixed into the aqueous solution of alkali metal silicate, and the above-mentioned transmittance (%T) decreases. In the aqueous solution of alkali metal silicate of the present invention, by reducing the amount of impurities and foreign substances, the above-mentioned transmittance is 65% or more. By making the above-mentioned transmittance 65% or more, it is easy to manufacture wet-process silica with high whiteness. In the aqueous solution of alkali metal silicate of the present invention, the above-mentioned transmittance is preferably 70% or more, more preferably 80% or more, and further preferably 90% or more. In particular, if the transmittance is 80% or more, it is easy to manufacture wet-process silica with a whiteness of 90 or more.

[0057] The transmittance (%T) refers to the measured value at a wavelength of 500 nm measured by a spectrophotometer using a 10 mm square colorimetric cell (standard type, light path length 1 cm) made of borosilicate glass with pure water as the comparison reference. The wavelength of 500 nm is a wavelength near the boundary between blue and green in the visible light wavelength range (380 nm to 780 nm), and it is a wavelength that is easily correlated with the whiteness of silica.

[0058] The SiO2 concentration in the aqueous solution of alkali metal silicate of the present invention is 8.9% by mass to 16.7% by mass. Generally, the SiO2 concentration of the aqueous solution of alkali metal silicate used as a raw material for wet-process silica is within the above range. Therefore, by making the SiO2 concentration within the above range, it is easy to use the aqueous solution of alkali metal silicate of the present invention in the same applications as existing aqueous solutions of alkali metal silicate.

[0059] The aqueous solution of alkali metal silicate of the present invention contains a sodium component and a potassium component derived from plant ash. In the aqueous solution of alkali metal silicate of the present invention, the ratio of the SiO2 concentration to the (Na2O + K2O) concentration, SiO2 / (Na2O + K2O), is preferably 2.8 to 3.5 in terms of mol%. Generally, in the aqueous solution of alkali metal silicate used as a raw material for wet-process silica, the molar ratio SiO2 / (Na2O + K2O) (usually, the amount of K2O is very small) is in the range of 2.8 to 3.5. Therefore, by making the molar ratio SiO2 / (Na2O + K2O) within the above range, it is easy to use the aqueous solution of alkali metal silicate of the present invention in the same applications as existing aqueous solutions of alkali metal silicate.

[0060] In the aqueous solution of alkali metal silicate of the present invention, the ratio of K2O concentration to Na2O concentration, K2O / Na2O, is preferably 0.03 to 0.30 in terms of mol%. By making the molar ratio K2O / Na2O 0.30 or less, it is not easy to cause deviations in the neutralization reaction and the like during the synthesis of wet-process silica by neutralization, and it is not easy to cause problems with the quality stability of wet-process silica. In addition, if the molar ratio K2O / Na2O is 0.03 or more, it is easy to adjust the molar ratio K2O / Na2O by mixing with other types of alkali metal silicates such as alkali metal silicate from sand source. The molar ratio K2O / Na2O is preferably in the range of 0.04 to 0.20, more preferably in the range of 0.05 to 0.15.

[0061] The molar ratio K2O / Na2O in alkali metal silicate can be one of the criteria for judging whether the alkali metal silicate is from plant ash. Generally, in alkali metal silicate from sand source, the molar ratio K2O / Na2O is less than 0.01. Therefore, when the molar ratio K2O / Na2O is 0.01 or more, in most cases, it can be said that the aqueous solution of alkali metal silicate contains plant ash, except when adjustments have been explicitly made to increase the K2O concentration during post-treatment.

[0062] The total of Na2O concentration and K2O concentration (Na2O + K2O) can be, for example, 3.5 mass% to 5.0 mass%. Generally, in the aqueous solution of alkali metal silicate used as a raw material for wet-process silica, the total concentration of Na2O + K2O (usually, the amount of K2O is very small) is in the range of 3.5 mass% to 5.0 mass%. Therefore, by making the total concentration of Na2O + K2O in the above range, it is easy to use the aqueous solution of alkali metal silicate of the present invention in the same applications as existing aqueous solutions of alkali metal silicate.

[0063] In the aqueous solution of alkali metal silicate of the present invention, when filtering the aqueous solution of alkali metal silicate with a filter having a mesh (particle retention capacity) of 1 μm, the amount of solid components captured on the filter is preferably less than 100 mass ppm relative to the mass of the aqueous solution of alkali metal silicate before filtration. The amount of the above solid components is preferably less than 90 mass ppm, more preferably less than 80 mass ppm relative to the mass of the aqueous solution of alkali metal silicate before filtration. In particular, in the case where the adsorption treatment and filtration described later are carried out, the amount of the above solid components can also be made less than 50 mass ppm, preferably less than 40 mass ppm, more preferably less than 30 mass ppm.

[0064] The material of the filter used is not particularly limited as long as the mesh size is 1 μm. For example, filter with a test filter paper (e.g., No. 5C, manufactured by ADVANTEC), thoroughly wash the filter paper with pure water, and dry it at 105 °C for 2 hours. Based on the mass of the solid components remaining on the filter paper (the mass of the undissolved residue) and the mass of the alkali metal silicate aqueous solution subjected to filtration, the mass% concentration of the undissolved residue can be calculated by the following formula.

[0065] [Equation 1]

[0066]

[0067] According to the experiments conducted by the present inventors, it is known that the cause of the coloring problem in the wet-process silica derived from plant ash in the past lies in the impurities and foreign matters in the alkali metal silicate aqueous solution derived from plant ash. That is, a large amount of impurities and foreign matters exist in the conventional alkali metal silicate aqueous solution derived from plant ash, and these impurities and foreign matters make the alkali metal silicate aqueous solution turbid and colored. Moreover, as a result of using such an alkali metal silicate aqueous solution to produce wet-process silica, there will be a problem that impurities and foreign matters are mixed into the wet-process silica, and the whiteness of the wet-process silica is reduced.

[0068] As described later, plant ash contains insoluble foreign matters such as carbon components remaining due to incomplete combustion, graphite-based foreign matters, and vitrified silica components. In the alkali metal silicate aqueous solution derived from plant ash, undissolved residues are likely to be generated. When an alkali metal silicate aqueous solution with a large amount of undissolved residues is directly used as a raw material to produce wet-process silica, a large amount of impurities and foreign matters will be mixed into the wet-process silica. For example, in the case of an extinction application, such foreign matters are scattered on the surface after coating and become protrusions, which causes a reduction in the extinction performance and impairs the aesthetic appearance. In addition, in applications such as industrial rubber, tires, and toothpaste, it is also not preferable to mix impurities and foreign matters into wet-process silica in order to reduce the product quality. Therefore, in order to obtain wet-process silica with high whiteness, by adjusting the mass% concentration of the undissolved residue in the alkali metal silicate aqueous solution to the above range, the mixing of impurities and foreign matters into the wet-process silica is suppressed.

[0069] The alkali metal silicate aqueous solution of the present invention can be obtained by a method for producing an alkali metal silicate aqueous solution of the present invention (the method for producing an alkali metal silicate aqueous solution described later) that implements one or both of a specified filtration treatment and an adsorption treatment. In addition, as shown in the examples, it is also possible to obtain plant ash with high whiteness by a specified combustion method described in this specification, and obtain it by a method for producing an alkali metal silicate aqueous solution by mixing the plant ash, sodium hydroxide, and water.

[0070] In comparison, for example, in the methods of Patent Documents 1 and 2, the obtained aqueous solution of alkali metal silicate is not subjected to refining treatments such as filtration. Therefore, due to the presence of impurities and foreign matters in the plant ash, the conventional aqueous solution of alkali metal silicate as described above is opaque, and it is impossible to produce wet-process silica with high whiteness. This is confirmed by the results of Comparative Example 3 described in the examples.

[0071] <Method for manufacturing aqueous solution of alkali metal silicate>

[0072] The method for manufacturing an aqueous solution of alkali metal silicate according to the present invention includes subjecting a raw material aqueous solution of alkali metal silicate obtained by mixing plant ash containing a silica component, sodium hydroxide, and water to one or both of a filtration treatment using a filter having a mesh size smaller than 50 μm and an adsorption treatment using an adsorbent material to obtain an aqueous solution of alkali metal silicate. By this manufacturing method, the aqueous solution of alkali metal silicate of the present invention as described above can be obtained.

[0073] <Plant ash containing silica component>

[0074] The plant ash used in manufacturing the aqueous solution of alkali metal silicate according to the present invention is the combustion ash of a plant material containing a silica component. The type and part of the plant material are not particularly limited as long as they are plants and parts containing a silica component. Examples of plants containing a silica component include plants of the Gramineae family such as rice (rice straw, rice husk), wheat, sugarcane, and bamboo. For reasons such as less pollution such as sludge adhesion, aggregation at the aggregation site, and easy availability, threshed rice husks (rice hulls) are most preferred.

[0075] The quality of the plant ash used in the present invention is not particularly limited. From the aspect of industrially and efficiently producing an aqueous solution of alkali metal silicate with few impurities and foreign matters, it is preferable to use plant ash with a low carbon content and high whiteness. The carbon content in the plant ash is preferably 2% by mass or less, more preferably 1% by mass or less. The lower limit of the carbon content in the plant ash is not particularly limited, and is actually 0.05% by mass or more, and may be 0.1% by mass or more. The whiteness of the plant ash is preferably 30 or more, more preferably 40 or more, and further preferably 50 or more. The upper limit of the whiteness of the plant ash is not particularly limited, and is actually 90 or less, and may also be 70 or less.

[0076] In order to obtain plant ash with high whiteness and few impurities and foreign matters as described above, it is preferable to use a combustion furnace capable of burning the plant material while flowing at 550°C to 900°C. The combustion temperature is more preferably 600°C to 800°C, and further preferably 650°C to 750°C.

[0077] Here, the relationship between the above-described combustion method in which a plant material containing a silica component is burned at 550°C to 900°C while flowing and impurities and foreign substances in the plant ash will be described. In order to produce plant ash with high whiteness, the present inventors have investigated in detail the types and causes of impurities and foreign substances in the plant ash. As a result, it has been found that when burning at a temperature lower than 550°C and when the combustion is uneven due to insufficient material flow or the like, the plant material is likely to remain unburned, and the water-soluble hydrocarbon component is likely to remain in the plant ash. The water-soluble hydrocarbon component dissolves in the aqueous solution of alkali metal silicate and is difficult to remove by filtration, causing the coloring of the aqueous solution. In addition, it has been found that when insufficient oxygen is supplied during combustion, the hydrocarbon component in the plant material is carbonized, generating insoluble graphite-based foreign substances in the plant ash. Furthermore, it has been found that when the plant material is burned at once at a combustion temperature of 700°C or higher, the vitrified silica component is likely to be generated as an insoluble foreign substance in the plant ash. In this case, it is understood that due to the vitrification of the silica component on the material surface, an unburned and oxygen-deficient state is likely to occur inside the material, and water-soluble hydrocarbon components and insoluble graphite-based foreign substances are also likely to be generated. If an aqueous solution of alkali metal silicate is produced using the plant ash obtained by burning at a high temperature at once, in addition to the vitrified silica component, water-soluble hydrocarbon components and insoluble graphite-based foreign substances are also mixed into the aqueous solution of alkali metal silicate, and the quality degradation of the aqueous solution of alkali metal silicate becomes significant. In addition, it has been confirmed that even in the case of ash that appears white at first glance, there are a certain amount of impurities and foreign substances such as water-soluble hydrocarbon components, insoluble graphite-based foreign substances, and vitrified silica components in the plant ash. Considering the above-described generation process of impurities and foreign substances in the plant ash, in order to obtain plant ash with high whiteness, it is effective to uniformly burn the plant material under sufficient oxygen while suppressing the vitrification of the silica component by the above-described combustion method.

[0078] In the above-described combustion method, it is preferable to use a combustion furnace for burning the plant material. The model of the combustion furnace is not particularly limited. As described above, in order to obtain plant ash with high whiteness and few impurities and foreign substances, it is preferable to use a combustion furnace equipped with a flow device that causes the plant material to flow during combustion and that can burn the plant material at 550°C to 900°C while flowing. For example, the combustion furnace may be a direct combustion furnace that burns the plant material using oxygen, or a gasification combustion furnace that heats the plant material in an anaerobic or low-oxygen state to generate organic gas and then burns the material. In addition, in the case where the plant ash contains a large amount of graphite-based foreign substances, for example, the carbon component can be removed while reusing the heat of the primary combustion by secondary combustion in the same combustion furnace.

[0079] According to such a method, the quality of the plant ash is likely to be stable, and thus the plant ash becomes a better plant ash as the silica source for the aqueous solution of alkali metal silicate of the present invention.

[0080] <aqueous sodium hydroxide solution>

[0081] In the production method of the present invention, the sodium hydroxide and water used can be publicly known sodium hydroxide and water. In the method of mixing an aqueous sodium hydroxide solution with plant ash, the concentration of the aqueous sodium hydroxide solution is not particularly limited, and publicly known concentrations can be used. Generally speaking, from the perspective of the production rate of the aqueous alkali metal silicate solution, it is preferable to mix an aqueous sodium hydroxide solution with the highest possible concentration with plant ash and then dilute it with water to the desired SiO2 concentration. On the other hand, since the apparent density of plant ash is very small, if the concentration of the aqueous sodium hydroxide solution is too high, it may sometimes be difficult to add plant ash to the aqueous sodium hydroxide solution. From the aspect of facilitating the mixing of plant ash and the aqueous sodium hydroxide solution, the actual concentration of the aqueous sodium hydroxide solution is 20% by mass or less, and can also be 15% by mass or less, or 10% by mass or less.

[0082] <Detailed production conditions of aqueous alkali metal silicate solution>

[0083] The aqueous alkali metal silicate solution of the present invention is obtained by mixing plant ash containing a silica component, sodium hydroxide, and water. The mixing order of plant ash, sodium hydroxide, and water is not particularly limited. Plant ash and sodium hydroxide can be added to water simultaneously or separately, a mixture of plant ash and sodium hydroxide can be added to water, plant ash can be added to an aqueous sodium hydroxide solution obtained by dissolving sodium hydroxide in water, or sodium hydroxide can be added to an aqueous solution obtained by dissolving plant ash in water.

[0084] The amounts and mixing ratios of the water, sodium hydroxide, and plant ash used can be appropriately adjusted according to conditions such as the type of plant ash, the target SiO2 concentration of the aqueous alkali metal silicate solution, and its use. Since the vitrified silica component is difficult to dissolve, it is not necessary to dissolve all the silica components in the plant ash in the aqueous solution. If a method is used to dissolve approximately 90% to 95% by mass of the silica component in the plant ash, the aqueous alkali metal silicate solution can be produced with good productivity in terms of energy consumption and time.

[0085] For example, assuming that 90% of the silica component contained in the plant ash is dissolved in the aqueous solution, if plant ash with a mass approximately three times that of the mass of the sodium hydroxide introduced into the aqueous solution is used for dissolution, in terms of mol%, it is easy to adjust the ratio of the SiO2 concentration to the (Na2O + K2O) concentration, SiO2 / (Na2O + K2O), to 2.8 to 3.5.

[0086] In the production method of the present invention, there are no particular limitations on other conditions for mixing and dissolving plant ash, sodium hydroxide, and water.

[0087] Mixing and dissolving are carried out, for example, in a dissolution tank (container) with stirring, appropriately selecting within the range of 60 °C to 100 °C and 1 hour to 5 hours under normal pressure (atmospheric pressure) or under pressure.

[0088] The manufacturing method of the present invention includes: subjecting the above-mentioned raw material aqueous alkali metal silicate solution to one or both of a filtration treatment using a filter with a mesh size smaller than 50 μm and an adsorption treatment using an adsorbent material to obtain an aqueous alkali metal silicate solution. As described above, in the conventional plant ash, there are impurities such as graphite-based foreign substances, foreign substances formed by vitrification of silica components, and water-soluble hydrocarbon compounds. If these impurities and foreign substances are mixed into the aqueous alkali metal silicate solution and wet-process silica, they will cause their coloring. Therefore, for example, simply dissolving the conventional plant ash in an aqueous sodium hydroxide solution cannot obtain the wet-process silica with the target quality of the present invention. In the present invention, by subjecting the raw material aqueous alkali metal silicate solution to one or both of the above-mentioned filtration treatment and adsorption treatment, the impurities and foreign substances in the aqueous alkali metal silicate solution are removed.

[0089] In order to remove insoluble foreign substances, the mesh size of the filter used in the filtration treatment is smaller than 50 μm. From the aspect of more fully removing foreign substances, the mesh size of the filter is preferably 40 μm or less, more preferably 30 μm or less, and further preferably 20 μm or less. When the mesh size of the filter is 10 μm or less, the filterability (productivity) deteriorates significantly. Therefore, the mesh size of the filter can be appropriately selected while considering productivity and quality. The material of the filter is not particularly limited, and various filters such as filter paper, filter cloth, and membrane filters can be used. From the aspect of more fully removing foreign substances, the filtration treatment using a filter with a mesh size smaller than 50 μm can be carried out multiple times. When carrying out this filtration treatment multiple times, it is usually carried out in the order of decreasing mesh size.

[0090] In addition, in the manufacturing method of the present invention, before carrying out the filtration treatment using a filter with a mesh size smaller than 50 μm, a filtration treatment using a filter with a mesh size of 50 μm or more can also be carried out. The filtration treatment using a filter with a mesh size of 50 μm or more can be appropriately carried out as needed, such as when there are many undissolved coarse particles.

[0091] The adsorption treatment can remove water-soluble impurities that are difficult to remove by the filtration treatment.

[0092] The implementation of the adsorption treatment is appropriately determined according to the amount and type of impurities in the aqueous alkali metal silicate solution. For example, it is preferred to carry out both the filtration and adsorption treatments on the aqueous alkali metal silicate solution with a transmittance (%T) of less than 10% at a wavelength of 500 nm.

[0093] In contrast, as long as the above transmittance (%T) of 65% or more can be achieved after any one of the treatments (for example, when there are few impurities and foreign substances in plant ash), the filtration and adsorption treatments can be either one of them. Specifically, filtration alone may be carried out as long as it is sufficient to remove insoluble foreign substances, and adsorption treatment alone may be carried out as long as it is sufficient to remove water-soluble impurities. In addition, for an aqueous solution of alkali metal silicate with the above transmittance (%T) of 65% or more, in order to further improve the transmittance, one or both of the filtration and adsorption treatments may be carried out, or adsorption treatment may be carried out after filtration, and filtration may be carried out again to remove the remaining adsorbent.

[0094] There is no particular limitation on the adsorbent material used in the adsorption treatment, and known adsorbent materials can be used. Examples of the adsorbent material include diatomaceous earth, clay minerals, activated carbon, zeolite, silica gel, and activated alumina. The adsorbent material is preferably activated carbon. The time of the adsorption treatment can be appropriately adjusted according to the amount of impurities and foreign substances in the aqueous solution of alkali metal silicate, for example, it is 12 hours to 40 hours, preferably 15 hours to 30 hours. The amount of the adsorbent material used in the adsorption treatment can be appropriately adjusted according to the amount of impurities in the aqueous solution of alkali metal silicate, for example, it is 5 g to 20 g relative to 1,000 g of the aqueous solution of alkali metal silicate, or it can be 8 g to 15 g.

[0095] The silica component and alkali component in plant ash are not fixed, so the concentrations of K2O and SiO2 in the aqueous solution of alkali metal silicate are likely to deviate. Therefore, if necessary, after one or both of the above filtration treatment and adsorption treatment, final concentration adjustment and adjustment of each molar ratio are carried out. The adjustment of the mass% of SiO2 can be carried out by dilution with water or additional addition of a silica material (for example, commercially available silica powder). The adjustment of the ratio of the SiO2 concentration to the (Na2O + K2O) concentration, expressed in mol%, SiO2 / (Na2O + K2O), can be carried out by adding sodium hydroxide.

[0096] <Other conditions of the aqueous solution of alkali metal silicate>

[0097] The aqueous alkali metal silicate solution of the present invention can use silica materials from sand sources (such as silica sand, silica stone, kaolin silicate, etc.) as part of the silica source. When emphasizing the quality of wet-process silica, the proportion of silica materials from sand sources can be increased, and when emphasizing environmental considerations (reuse of plant ash), the proportion of plant ash can be increased. From the perspective of environmental considerations, it is preferable to use as much plant ash as possible. The proportion of plant ash in the silica source is preferably 60% by mass or more, more preferably 80% by mass or more, and further preferably 90% by mass or more. In the preparation of the raw material aqueous alkali metal silicate solution, it is also possible not to use silica materials from sand sources as the silica source and make the amount of plant ash 100% by mass.

[0098] The silica material from the sand source can be added as part of the silica source when preparing the above-mentioned aqueous alkali metal silicate solution. In addition, the silica material from the sand source can also be used in a form where an aqueous alkali metal silicate solution from a plant ash source and an aqueous sodium silicate solution from the sand source are separately prepared and mixed before use.

[0099] The aqueous alkali metal silicate solution of the present invention can contain additives as needed. There is no particular limitation on the additives, and known additives can be used. For example, an alumina (Al2O3) source such as sodium aluminate can be added. The wet-process silica produced from the aqueous alkali metal silicate solution containing such additives has the effect of improving the reinforcement performance as a reinforcing agent for silicone rubber, industrial rubber, and tires.

[0100] As described above, according to the manufacturing method of the present invention, it is possible to sufficiently remove impurities and foreign substances such as graphite-based foreign substances in plant ash, foreign substances formed by vitrification of the silica component, and water-soluble hydrocarbon compounds from the aqueous alkali metal silicate solution. As a result, it is possible to provide an aqueous alkali metal silicate solution from plant ash that can produce high-quality wet-process silica with the same degree of whiteness as when using an aqueous alkali metal silicate solution from a sand source.

[0101] In addition, in the present invention, it is possible to reuse plant ash that is usually discarded, so it is possible to supply an aqueous alkali metal silicate solution that takes environmental considerations into account. Moreover, in the aqueous alkali metal silicate solution obtained by the manufacturing method of the present invention, the content of impurities and foreign substances is small, so it is possible to produce high-quality wet-process silica.

[0102] <Use applications of the aqueous alkali metal silicate solution>

[0103] The aqueous alkali metal silicate solution of the present invention can be used as a raw material for manufacturing wet-process silica. Although the aqueous alkali metal silicate solution of the present invention is derived from plant ash, the above-mentioned transmittance (%T) is 65% or more, and the contents of impurities and foreign matters that cause coloring are small. By using the aqueous alkali metal silicate solution of the present invention as a raw material, high-quality wet-process silica with the same degree of whiteness as that obtained when using an aqueous alkali metal silicate solution derived from conventional sand can be manufactured even when using plant ash. In addition, since the contents of impurities and foreign matters in the aqueous alkali metal silicate solution of the present invention are small, high-quality wet-process silica with few impurities and foreign matters can be manufactured.

[0104] There is no particular limitation on the type of wet-process silica manufactured using the aqueous alkali metal silicate solution of the present invention as a raw material. For example, it can be precipitated silica (white carbon black) or gel-process silica. The reason is that for wet-process silica, the process of precipitating silica from the aqueous alkali metal silicate solution basically shows exactly the same tendency. An example of the synthesis process of wet-process silica is shown below (Chemical Formula C).

[0105] ·Chemical Formula C (Example of synthesis process of wet-process silica)

[0106] M2O·nSiO2 + H2SO4 → nSiO2 + M2SO4 + H2O

[0107] Here, M represents an alkali including sodium and potassium, and n represents a molar ratio.

[0108] The wet-process silica manufactured using the aqueous alkali metal silicate solution of the present invention as a raw material has sufficient reinforcing properties as a reinforcing filler for industrial rubber or tires due to its high whiteness and few foreign matters. Furthermore, it can also be used in applications that emphasize transparency and whiteness, such as reinforcing fillers for silicone rubber, fillers for paper (anti-back-through agents), matting agents for coatings, abrasives for toothpaste, and anti-blocking agents for films, in the same manner as known aqueous alkali metal silicate solutions derived from sand.

[0109] In addition, since the contents of impurities and foreign matters in the aqueous alkali metal silicate solution of the present invention are small, the aqueous alkali metal silicate solution of the present invention can also be used in known applications other than wet-process silica, such as raw materials for synthesizing zeolites, foundation curing agents for civil engineering and construction, cleaning agents, and additives for fibers and pulp. By using the aqueous alkali metal silicate solution of the present invention, problems such as the mixing of foreign matters into the above-mentioned materials and products are less likely to occur compared with conventional aqueous alkali metal silicate solutions derived from plant ash.

[0110] Examples

[0111] Examples are specifically given below for illustration. However, it is not intended to be limited thereto.

[0112] <Combustion Conditions of Plant Ash>

[0113] The plant materials used for manufacturing plant ash are rice husks and rice straws. The rice husks and rice straws are sufficiently dried at 125 °C for 2 hours. 500 g of rice husks (apparent density: about 110 g / L) or 250 g of rice straws cut into about 10 cm are placed in a 2-mm-thick stainless-steel crucible (300 mm × 300 mm × 115 mm (height)) equipped with air holes and a lid, and they are overlapped in two layers and burned in an air atmosphere using a bench-type muffle furnace (model: KDF S100G, manufactured by DENKEN Co., Ltd). Since at most about 200 g of plant ash can be obtained at one time, combustion treatment is carried out multiple times under the same conditions until the required amount of plant ash is obtained, and finally they are mixed. In addition, when burning at 900 °C, a mullite crucible (outer dimensions 210 mm × 210 mm × 70 mm (height), inner dimensions 194 mm × 194 mm × 56 mm (height)) equipped with air holes and a lid is used instead of the stainless-steel crucible.

[0114] Six kinds of plant ash, namely ash a to ash f, are obtained by the above method. The combustion conditions of each specimen are shown in Table 1 below. In Table 1, the combustion conditions for multiple combustion treatments are shown: pre-combustion is carried out at 500 °C (more than once), the taken-out ash is stirred, and then the next combustion treatment is carried out. For example, in Table 1, the combustion conditions of ash b, "500 °C, 2 hours, 600 °C, 2 hours", mean that after pre-combustion at 500 °C for 2 hours once, combustion treatment is carried out at 600 °C for 2 hours. Plant ash f is the ash obtained by burning rice husks at 700 °C for 2 hours without allowing the material to flow in a state of excess air according to the method described in Patent Document 1.

[0115] <Analysis of Plant Ash>

[0116] ● Whiteness of plant ash

[0117] Measurement is carried out using a whiteness meter (model: NW-12, manufactured by Nippon Denshoku Industries Co., Ltd).

[0118] ● Carbon content (mass %) in plant ash

[0119] The carbon content is measured as follows: Using a carbon analysis device (model: CS744, manufactured by LECO JAPAN CORPORATION) that utilizes combustion in an oxygen stream - non-dispersive infrared absorption method, the specimen is heat-treated under the conditions of a temperature of 1,350 °C, an oxygen inflow pressure of 0.24 MPa, and a measurement time of 50 seconds, and CO gas and CO2 gas are quantified using an infrared detector (NDIR) inside the device, thereby measuring. As a pretreatment, the plant ash is dried at 105 °C for 2 hours.

[0120] ● Composition of inorganic components other than carbon in plant ash (mass%)

[0121] Using a scanning fluorescence X-ray analyzer (model: ZSX PrimusII, manufactured by Rigaku Corporation), first perform qualitative analysis on elements other than carbon to confirm the types of inorganic impurities detected. Then, perform quantitative analysis of the detected inorganic impurities. The test sample used is a sample prepared by pressing plant ash into a ring-shaped mold. For the quantitative composition of inorganic components (excluding carbon), perform oxide conversion using the analysis software attached to the device to obtain the concentration (mass%).

[0122] ● Ashing rate (mass%)

[0123] Measure the mass% of the combustion ash after combustion relative to the mass of plants such as rice husks that have been fully dried at 125°C for 2 hours.

[0124] [Number 2]

[0125]

[0126] ● Apparent density (bulk specific gravity) of plant ash (g / L)

[0127] The apparent density of plant ash is determined based on JIS K5101-12-1:2004 (Pigment test methods - Apparent density or apparent specific volume - Static method) using a dedicated measuring device (sieve with a mesh size of 0.5 mm, funnel, cylindrical receiver with a capacity of 30 mL, receiver stand, and funnel stand). Use a brush to drop the test sample (plant ash) from above the sieve into the funnel, shave off the peak part of the sample piled up in the receiver in a mountain shape with a spatula, and then measure the mass of the sample. Based on this mass, calculate the apparent density (bulk specific gravity) of plant ash in g / L using the following formula.

[0128] [Number 3]

[0129]

[0130] ● Bulk density (apparent specific gravity) of plant ash (g / mL)

[0131] The bulk density of plant ash is measured using a dedicated measuring instrument (a cylinder made of ordinary steel with an inner diameter of 22.00 ± 0.05 mm and an inner depth of 100 mm, and a piston with an outer diameter of 21.80 ± 0.05 mm, a length of 115 mm, a mass of 190 g, and internal voids) in accordance with the item "7.8 Bulk Density" of JIS K6220-1:2015 (Compound Agents for Rubber - Organic Chemicals - Test Methods - Part 1: All). The piston is allowed to fall naturally into the cylinder before loading the sample (plant ash), and the height of the piston protruding from the upper part of the cylinder is measured. Then, approximately 1 g of the weighed sample is placed into the cylinder, the piston is slowly lowered in 5 seconds, and the side wall of the cylinder is gently tapped with a wooden piece to make the piston fit well. The height of the piston protruding from the upper part of the cylinder is measured. Using the change in the height of the piston protruding from the upper part of the cylinder and the bottom area inside the cylinder, the bulk density (apparent specific gravity) in g / mL is calculated by the following formula.

[0132] [Equation 4]

[0133]

[0134] The carbon content and the composition of inorganic components other than carbon in the plant ash of each sample are as recorded in Table 1.

[0135] Combustion conditions and contained components of Ash a - Ash f:

[0136] [Table 1]

[0137]

[0138] <Method for Preparing Aqueous Solution of Alkali Metal Silicate>

[0139] ● Primary treatment (mixing and filtering of materials)

[0140] 1,740 g of a sodium hydroxide aqueous solution adjusted to 5.8 mass% is put into a 3 L stainless steel container equipped with a stirring device and heated to 85°C. Then, while continuing to stir, 300 g of plant ash is put into the sample solution, dissolved at 85°C for 2 hours, and the sample solution is cooled to room temperature. Here, assuming that 90% of the SiO2 contained in the plant ash dissolves in the sodium hydroxide aqueous solution, the target SiO2 concentration is set to 12.0 mass%. A 150 mmΦ magnetic Buchner funnel (suction filter) is installed on a suction flask with a capacity of 3 L, and using a polyester plain weave cloth (mesh size of approximately 50 μm × 400 μm), the sample solution is roughly filtered while aspirating to remove undissolved residues. Thus, an aqueous solution of alkali metal silicate after the primary treatment is obtained.

[0141] Undissolved residues are confirmed in all of the plant ash, and the amount is 10 - 20 mass% on a wet basis. When this amount is converted to the dried solid component, it is equivalent to approximately 1 - 4 mass%.Figure 1 This is a photograph of the undissolved residues of plant ash a - d and plant ash f. Since the amount of undissolved residues contains moisture, it is difficult to conduct a quantitative evaluation, and the correlation between the total amount of undissolved residues and the final combustion temperature cannot be confirmed. In contrast, the color of the undissolved residues varies according to the final combustion temperature. Specifically, the residue of plant ash d burned at 900 °C is a light yellow - white residue mainly composed of undissolved glass, and the others are gray - black residues. The lower the final combustion temperature, the darker the black of the undissolved residues. It should be noted that in the case where the required amount of alkali metal silicate aqueous solution is not obtained after the above filtration, the treatment is repeated multiple times under the same conditions and finally mixed to obtain a sample.

[0142] ● Determination of alkali concentration and SiO₂ concentration (simple determination for concentration adjustment)

[0143] For the alkali metal silicate aqueous solution after one - time treatment, the alkali concentration (mass % concentration of Na₂O + K₂O) and SiO₂ concentration (mass % concentration) are determined by the titration method shown below.

[0144] Alkali concentration:

[0145] Add approximately 2 g of the sample weighed to the nearest 1 mg and approximately 100 mL of distilled water to a conical beaker with a capacity of 300 mL. Add 2 - 3 drops of methyl orange (0.1 mass % aqueous solution) as an indicator to the sample solution, and titrate with 1N - HCl using a 50 mL burette. Take the point where one drop turns orange as the end - point, and calculate the alkali concentration from the consumption of 1N - HCl at this time by the following formula.

[0146] [Equation 5]

[0147]

[0148] SiO₂ concentration:

[0149] Add 5.0 g of sodium fluoride reagent to the alkali - concentration titration end - point solution and mix well by shaking. Then, add approximately 0.5 mL of methyl red xylene cyanol FF indicator to the sample solution and titrate with 1N - HCl. When the liquid turns red, further add 2.0 mL of 1N - HCl and record its titration volume a (mL). Here, the methyl red xylene cyanol FF indicator is prepared by dissolving 0.8 g of methyl red and 0.2 g of xylene cyanol FF in 1,000 mL of ethanol.

[0150] Next, use a 10 mL burette to conduct a back - titration with 1N - NaOH. When the color of the sample solution is tea - orange, confirm that the pH is in the range of 5.8 - 6.0 as the end - point, record its titration volume b (mL), and calculate the SiO₂ concentration by the following formula.

[0151] [Number 6]

[0152]

[0153] ● Secondary treatment (if necessary) of the aqueous solution of alkali metal silicate

[0154] Based on the measurement results of the above titration method, in such a manner that the SiO2 concentration in terms of oxide conversion is close to the target SiO2 concentration of 12.0 mass%, one or both of wet-process silica and water are added for fine adjustment of the composition (additional dissolution or additional heat dissolution). In this example and the comparative examples, the secondary treatment was carried out on the aqueous solutions of alkali metal silicates obtained using plant ash d, plant ash e, and plant ash f, respectively. For the aqueous solution of alkali metal silicate derived from plant ash d, only wet-process silica (silica purity of about 93 mass%, Nipsil LP, manufactured by Tosoh Silicone Co., Ltd.) was added and heated for dissolution to adjust the SiO2 concentration. For the aqueous solution of alkali metal silicate derived from plant ash e, water and the above wet-process silica were added and heated for dissolution to adjust the SiO2 concentration. For the aqueous solution of alkali metal silicate derived from plant ash f, only the above wet-process silica was added and heated for dissolution to adjust the SiO2 concentration. Plant ash f is the ash obtained by burning rice husks at 700°C for 2 hours without allowing the material to flow in a state of excess air according to the method described in Patent Document 1. In the case of plant ash f, the surface portion of the rice husks is first vitrified, and most of the vitrified components are removed as residues in the coarse filtration during the primary treatment. Therefore, compared with plant ash d and plant ash e, a larger amount of wet-process silica is required to adjust the SiO2 concentration.

[0155] The sample solutions that have undergone the primary treatment and the secondary treatment if necessary are used as the stock solutions of the aqueous solutions of alkali metal silicates used in this example and the comparative examples. In the following description, the stock solution obtained using plant ash a is referred to as "alkali metal silicate a", and similarly, the stock solutions obtained using plant ash b to plant ash f are referred to as "alkali metal silicate b" to "alkali metal silicate f", respectively.

[0156] In addition, as Reference Example 1, an aqueous solution of alkali metal silicate was prepared using commercially available silica sand as a raw material by the following method.

[0157] Aqueous sodium silicate solution from silica sand:

[0158] 3,000 g of water was charged into a 5 L pressure vessel (autoclave), and 500 g of sodium silicate cullet (medium molar grade, Na₂O: 24.51%, SiO₂: 75.44%, purity 99.9% or more, manufactured by Tokuyama Corporation) made from commercially available silica sand was dissolved under the conditions of a pressure of 0.7 MPa and a temperature of 160 °C for 60 minutes. 2,000 g of the sodium silicate aqueous solution was taken from the container, diluted with water to twice the volume, and cooled to room temperature. Then, a 150 mm Φ magnetic Buchner funnel (suction filter) was installed on a 5 L suction flask, and coarse filtration was carried out while sucking using a plain woven polyester cloth (mesh size of about 50 μm × 400 μm). The undissolved residue components were removed by coarse filtration to obtain an aqueous solution of sodium silicate (alkali metal silicate) derived from silica sand.

[0159] <Examples and Comparative Examples of Aqueous Solution of Alkali Metal Silicate>

[0160] Alkali metal silicates a to f were respectively prepared using plant ashes a to f to obtain aqueous solutions of alkali metal silicates of Examples 1 to 3 and Comparative Examples 1 to 3. The following measurements were carried out on each aqueous solution, including the aqueous solution of Reference Example 1.

[0161] ● Composition analysis and impurity determination

[0162] The sample of the alkali metal silicate aqueous solution drawn with a syringe equipped with a membrane filter with a mesh size of 1 μm was diluted with pure water. Using a high-resolution ICP emission spectroscopic analysis device (model: PS3520DDII, manufactured by Hitachi High-Technologies Corporation), quantitative analysis of the sample was carried out using a standard solution based on the calibration curve of each element (unit: mass %). The dilution factor was 10,000 times for the quantitative analysis of SiO₂, Na₂O, and K₂O (standard solution for calibration curve: 20 mass ppm), and 1,000 times for the quantitative analysis of P₂O₅, CaO, MgO, Al₂O₃, Fe₂O₃, and MnO (standard solution for calibration curve: 10 mass ppm).

[0163] ● Molar ratio

[0164] The mass concentrations of SiO₂, Na₂O, and K₂O measured above were converted into molar amounts, and the SiO₂ / (Na₂O + K₂O) molar ratio and K₂O / Na₂O molar ratio were calculated. Specifically, the molar masses of each component were set to 60.08 (SiO₂), 61.98 (Na₂O), and 94.19 (K₂O), and each molar ratio was calculated by the following formula.

[0165] [Equation 7]

[0166]

[0167] ● Measurement of the transmittance (%T) of the aqueous alkali metal silicate solution

[0168] The transmittance at wavelengths from 200 nm to 800 nm was measured using a commercially available ultraviolet-visible spectrophotometer (model: V-730, manufactured by JASCO Corporation). During the measurement, a spectrophotometric standard cuvette made of borosilicate glass with an optical path length of 10 mm (model: S10-G-10, manufactured by GL Sciences Inc.) was used, and the measured value in pure water was used as the comparison reference. The transmittance (%T) at a wavelength of 500 nm (near the boundary between blue and green), where coloring is easily visible in the visible light wavelength range (wavelength 380 nm to 780 nm), was taken as the transmittance (%T) of the aqueous alkali metal silicate solution.

[0169] ● Measurement of the undissolved residue (mass ppm) of the aqueous alkali metal silicate solution

[0170] The undissolved residue (the amount of solid components captured on the filter) of the aqueous alkali metal silicate solution, which is the raw material for wet-process silica, was measured. A magnetic Buchner funnel (suction filter) with a diameter of 110 mm was installed on a suction flask with a capacity of 2 L, and 1,000 g of the aqueous alkali metal silicate solution was suction-filtered using a filter paper with a mesh size of 1 μm (specification 5C, manufactured by ADVANTEC). After the filtration was completed, it was further washed with 1,000 g of pure water and dried in a box dryer at 105 °C for 2 hours. Then, the mass of the solid components captured on the filter paper was determined, and the mass ppm concentration of the undissolved residue was calculated using the following formula.

[0171] [Equation 8]

[0172]

[0173] ● Specific gravity (reference value) of the aqueous alkali metal silicate solution

[0174] The aqueous alkali metal silicate solution adjusted to 20 ± 0.5 °C was added to a 250 mL graduated cylinder, and a hydrometer (liquid hydrometer) was placed in it, and the scale at the stationary moment was read.

[0175] <Manufacture and Physical Property Evaluation of Wet Process Silica>

[0176] In the case of wet-process silica, the process of precipitating silica from an aqueous solution of alkali metal silicate shows substantially the same tendency. Therefore, using alkali metal silicates a to f respectively, precipitated silica in wet-process silica was manufactured by the following method, and its physical properties were confirmed (Examples 1 to 3, Comparative Examples 1 to 3). In addition, precipitated silica was manufactured using an aqueous solution of sodium silicate from silica sand, and the quality was confirmed as Reference Example 1. The sodium silicate cullet and its aqueous solution in Reference Example 1 contain almost no impurities and are generally widely used as raw materials for high-quality wet-process silica with high whiteness. The physical properties are shown in Table 2 below.

[0177] ● Manufacture of wet-process silica

[0178] In a 2L stainless steel container equipped with a stirring device, 495 mL of warm water and 36 mL of the test alkali metal silicate aqueous solution were added so that the pH became 11.8 ± 0.2, and the temperature was raised to 75°C. While continuously stirring to maintain the temperature and pH, 366 mL of the alkali metal silicate aqueous solution and about 108 mL of 20 mass% dilute sulfuric acid were added dropwise over 60 minutes at a constant flow rate. Then, the dropwise addition of the alkali metal silicate aqueous solution was stopped, and when the pH reached 3, the dropwise addition of the dilute sulfuric acid was also stopped, and the neutralization reaction was ended. A 150 mmΦ magnetic Buchner funnel (suction filter) was installed on a suction flask with a capacity of 2 L, and the sample solution was suction filtered using a filter paper with a mesh size of 8 μm (specification 5A, manufactured by ADVANTEC), and further washed with 600 mL of pure water, and left to dry at 125°C for 8 hours, thereby obtaining wet-process silica (precipitated silica).

[0179] ● Whiteness of wet-process silica

[0180] After gently re-crushing the obtained wet-process silica powder with a mortar for about 3 minutes, the whiteness of the wet-process silica was measured by the same method as when measuring the whiteness of plant ash. When the whiteness of the wet-process silica is 85 or more, it meets the quality of high-functional wet-process silica, so the alkali metal silicate aqueous solution as its raw material is evaluated as good. When the whiteness of the wet-process silica is less than 85, the wet-process silica is judged to be a colored product (discolored product), and the alkali metal silicate aqueous solution as its raw material is evaluated as bad.

[0181] ● BET specific surface area of wet-process silica (reference value)

[0182] Using a fully automatic specific surface area measuring device (model: Macsorb HM model-1210, manufactured by Mountech), the BET specific surface area of the wet-process silica powder was measured by the single-point method.

[0183] ● Carbon content in wet-process silica (reference value)

[0184] The carbon content in wet-process silica is determined by the same method as that for determining the carbon content in plant ash.

[0185] The analysis results of Examples 1 to 3, Comparative Examples 1 to 3, and Reference Example 1 are shown in Table 2 below.

[0186] [Table 2]

[0187]

[0188] <Examples and Comparative Examples when mixing the alkali metal silicate aqueous solution that has been filtered and adsorbed and the alkali metal silicate aqueous solution derived from sand>

[0189] As described below, one or both of filtration and adsorption treatment are performed on alkali metal silicate a and alkali metal silicate b, and the influence on the quality of wet-process silica is evaluated (Examples 4 and 5 and Comparative Example 4 of the alkali metal silicate aqueous solution). Furthermore, the quality confirmation is carried out when mixing the alkali metal silicate aqueous solution derived from plant ash and the alkali metal silicate aqueous solution derived from sand (Examples 6 to 8 of the alkali metal silicate aqueous solution).

[0190] <Comparative Example 4>

[0191] A magnetic Buchner funnel (suction filter) with a diameter of 110 mm is installed on a suction flask with a capacity of 3 L, and a filter paper (mesh 1 μm, specification 5C, manufactured by ADVANTEC) is used. While sucking alkali metal silicate a (about 2,000 g), only filtration is performed. Hereinafter, the filtration using the above filter paper is referred to as "5C filtration". That is, in Comparative Example 4, 5C filtration is performed on the alkali metal silicate a of Comparative Example 1 to remove solid foreign matters present in the alkali metal silicate aqueous solution. Furthermore, wet-process silica is manufactured using the alkali metal silicate a after 5C filtration, and its quality is analyzed.

[0192] <Example 4>

[0193] 20 g of commercially available powdered activated carbon is added to alkali metal silicate a (about 2,000 g), and it is left standing for 24 hours to perform the adsorption treatment of the components in the sample. Then, a magnetic Buchner funnel (suction filter) with a diameter of 150 mm is installed on a suction flask with a capacity of 3 L, and 5C filtration is performed on the sample. That is, in Example 4, adsorption treatment is performed on the alkali metal silicate a of Comparative Example 1 to adsorb and remove impurities in the aqueous solution, and 5C filtration is performed to remove solid foreign matters present in the alkali metal silicate aqueous solution. Furthermore, wet-process silica is manufactured using the alkali metal silicate a after 5C filtration, and its quality is analyzed.

[0194] <Example 5>

[0195] An 110 mm Φ magnetic Buchner funnel (suction filter) was installed on a 3 L suction flask, and alkali metal silicate b (about 2,000 g) was filtered at 5°C. Further, wet-process silica was manufactured using the alkali metal silicate b filtered at 5°C as a raw material, and its quality was analyzed.

[0196] <Examples 6 to 8>

[0197] The aqueous alkali metal silicate solution filtered at 5°C obtained in Example 5 and the aqueous alkali metal silicate solution of Reference Example 1 were mixed at 4:6 (Example 6: the aqueous solution derived from plant ash was 40% by mass. The same applies hereinafter), 6:4 (Example 7), and 8:2 (Example 8) respectively to manufacture an aqueous alkali metal silicate solution. Further, wet-process silica was manufactured using these aqueous alkali metal silicate solutions as raw materials, and their quality was analyzed.

[0198] The analysis results of Comparative Example 4 and Examples 4 to 8 are shown in Table 3 below.

[0199] [Table 3]

[0200]

[0201] <Result Explanation>

[0202] From the results of plant ash b to plant ash d shown in Table 1, it can be seen that by performing pre-combustion to fully burn the plant material, the carbon-containing impurities caused by non-combustion in the plant ash are reduced. In contrast, from the results of plant ash a and plant ash e, it can be seen that due to insufficient combustion temperature, a large amount of carbon-containing impurities remain. In addition, from the result of plant ash f, it can be seen that a large amount of carbon-containing impurities also remain when burning at a high temperature without pre-combustion. It is considered that this is because due to high-temperature combustion, the surface of the plant material is vitrified, and for reasons such as insufficient oxygen supply, it becomes a non-combustible state inside the material.

[0203] From the results of Examples 1 to 3 shown in Table 2, it can be seen that in the case of pre-burning to fully burn the sample and using plant ash to produce an aqueous solution of alkali metal silicate, even without subjecting the aqueous solution of alkali metal silicate to filtration and adsorption treatment, since there is less filtration residue and a high transmittance, the amount of insoluble foreign matter and water-soluble carbon compounds in the aqueous solution of alkali metal silicate is also small. Furthermore, it can be known that in the case of producing wet-process silica using such plant ash as a raw material, the impurities in the wet-process silica are also reduced, and wet-process silica with sufficient whiteness can be obtained. In contrast, from the results of Comparative Example 1, Comparative Example 2, and Comparative Example 3, it can be seen that when using plant ash with an insufficient combustion temperature and plant ash that has only been subjected to high-temperature combustion to produce an aqueous solution of alkali metal silicate, a large amount of carbon-containing impurities remain. Furthermore, it can be known that in the case of producing wet-process silica using such plant ash as a raw material, the impurities in the wet-process silica also increase, and wet-process silica with sufficient whiteness cannot be obtained.

[0204] From the results of Comparative Example 4, Example 4, and Example 5 shown in Table 3, it can be seen that by subjecting the aqueous solution of alkali metal silicate to one or both of filtration and adsorption treatment, the quality of the aqueous solution of alkali metal silicate is improved dramatically.

[0205] In particular, from the comparison of Comparative Example 1, Comparative Example 4, and Example 4, it can be seen that by subjecting the aqueous solution of alkali metal silicate with insufficient quality to adsorption treatment and filtration, its quality can be improved to the extent that wet-process silica with a whiteness exceeding 90 can be produced.

[0206] Figure 2 The transmittance spectra (wavelength 200 nm to 800 nm) of the aqueous solutions of alkali metal silicate in Comparative Example 1, Comparative Example 4, and Example 4 are shown. The higher the transmittance (%) T at a wavelength of 500 nm, the higher the transparency. The transmittance of alkali metal silicate a in Comparative Example 1 (plant ash a burned at 500 °C, only subjected to one treatment) at a wavelength of 500 nm was 4.4%, and the whiteness of the precipitated silica produced using alkali metal silicate a as a raw material was 58. In Comparative Example 4 where 5C filtration was performed on alkali metal silicate a, although the coloring components dissolved in the aqueous solution of alkali metal silicate remained, the transmittance at a wavelength of 500 nm was improved to 32.8%. The whiteness of the wet-process silica using the aqueous solution of alkali metal silicate in Comparative Example 4 was also improved to 75, but the quality of the wet-process silica was insufficient. In contrast, in Example 4 where the above adsorption treatment and 5C filtration were performed on alkali metal silicate a, the transmittance at a wavelength of 500 nm was increased to 94.5%, and the whiteness of the wet-process silica was also increased to 94, obtaining wet-process silica with sufficient quality.

[0207] Figure 3The transmittance spectra (wavelength: 200 nm to 800 nm) of the aqueous alkali metal silicate solutions in Example 1, Example 5, and Reference Example 1 are shown. In Example 1, alkali metal silicate b was produced using plant ash b burned at a final temperature of 600 °C, and then used as a raw material to produce wet-process silica. In Example 5, alkali metal silicate b was subjected to 5C filtration and then used as a raw material to produce wet-process silica. In Reference Example 1, wet-process silica was produced using an aqueous alkali metal silicate solution derived from silica sand.

[0208] In alkali metal silicate b, the filtration residue as solid foreign matter was 0.41% by mass, but the coloring components dissolved in the aqueous alkali metal silicate solution were in a state where they were hardly visible to the naked eye, and the transmittance at a wavelength of 500 nm was 65.7%. The whiteness of the wet-process silica produced using alkali metal silicate b as a raw material was 85, which was sufficient whiteness for a highly functional wet-process silica. In Example 5 where alkali metal silicate b was subjected to 5C filtration, the transmittance at a wavelength of 500 nm was improved to 94.4%, and the whiteness of the precipitated silica was also increased to 91. The quality was comparable to the whiteness of the wet-process silica produced using an aqueous alkali metal silicate solution derived from sand.

[0209] The quality of the aqueous alkali metal silicate solution derived from plant ash is sometimes inferior to that of the aqueous alkali metal silicate solution derived from sand as in Reference Example 1. In such a case, if necessary, by mixing and using it with the aqueous alkali metal silicate solution derived from sand, the quality of the aqueous alkali metal silicate solution derived from plant ash can be further improved (Examples 6 to 8).

[0210] Figure 4 The transmittance spectrum (wavelength: 200 nm to 800 nm) of the aqueous alkali metal silicate solution in Comparative Example 3 is shown. In the aqueous alkali metal silicate solution derived from plant ash f produced according to the method described in Patent Document 1, the transmittance at a wavelength of 500 nm was 55.4%, and the whiteness of the wet-process silica produced using it as a raw material was 73, and a wet-process silica with sufficient whiteness could not be obtained.

[0211] Industrial Applicability

[0212] The wet-process silica synthesized using the aqueous alkali metal silicate solution of the present invention as a raw material can have high reinforcing properties in industrial rubber or tire applications, and can be used as a reinforcing filler for silicone rubber that emphasizes transparency or whiteness, a filler for paper (anti-backing agent), a matting agent for coatings, an abrasive for toothpaste, etc., an anti-blocking agent for films, etc. (the same as the conventional aqueous alkali metal silicate solution derived from sand), and is therefore useful as an aqueous alkali metal silicate solution considering the environment.

Claims

1. An aqueous solution of alkali metal silicate, expressed in mass %, the ratio of P2O5 concentration to SiO2 concentration, P2O5 / SiO2, is 1.0×10 -3 or more, the SiO2 concentration is 8.9 mass % to 16.7 mass %, and the transmittance at a light path length of 1 cm and a wavelength of 500 nm, i.e., %T, is 65% or more, where When filtering an aqueous alkali metal silicate solution using a filter with a mesh size of 1 μm, the amount of solid components captured on the filter is less than 100 mass ppm relative to the mass of the aqueous alkali metal silicate solution before filtration.

2. The aqueous solution of alkali metal silicate according to claim 1, wherein, Expressed in mol%, the ratio of the SiO2 concentration to the (Na2O + K2O) concentration, SiO2 / (Na2O + K2O), is 2.8 to 3.

5.

3. The aqueous solution of alkali metal silicate according to claim 1 or 2, wherein, Expressed in mol%, the ratio of the K2O concentration to the Na2O concentration, K2O / Na2O, is 0.03 to 0.

30.

4. The aqueous alkali metal silicate solution according to claim 1 or 2, which is used as a raw material for wet-process silica.

5. A method for manufacturing an aqueous solution of an alkali metal silicate, which is a method for manufacturing the aqueous solution of an alkali metal silicate according to claim 1 or 2, wherein, Comprising: Subjecting a raw material aqueous alkali metal silicate solution obtained by mixing plant ash containing a silica component, sodium hydroxide, and water to both a filtration treatment using a filter with a mesh size of less than 50 μm and an adsorption treatment using an adsorbent material to obtain an aqueous alkali metal silicate solution.

6. The method for producing an aqueous solution of alkali metal silicate according to claim 5, wherein, The plant ash is rice husk ash.

7. A method for manufacturing an aqueous solution of an alkali metal silicate, which is a method for manufacturing the aqueous solution of an alkali metal silicate according to claim 1 or 2, wherein, Comprising: Subjecting a raw material aqueous alkali metal silicate solution obtained by mixing plant ash containing a silica component and having a whiteness of 30 or more, sodium hydroxide, and water to a filtration treatment using a filter with a mesh size of less than 50 μm to obtain an aqueous alkali metal silicate solution.

8. A method for manufacturing an aqueous solution of alkali metal silicate, which is a method for manufacturing the aqueous solution of alkali metal silicate according to claim 1 or 2, wherein, Comprising: Subjecting a raw material aqueous alkali metal silicate solution obtained by mixing plant ash containing a silica component and having a whiteness of 30 or more, sodium hydroxide, and water to an adsorption treatment using an adsorbent material to obtain an aqueous alkali metal silicate solution.

9. The method for producing an aqueous solution of alkali metal silicate according to claim 7, wherein, The plant ash is rice husk ash.

10. The method for manufacturing an aqueous solution of alkali metal silicate according to claim 8, wherein, The plant ash is rice husk ash.

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