JP7607002B2 - Alkaline silicate aqueous solution and method for producing same - Google Patents

Description

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

Sodium silicate, also known as "water glass," is widely used as a raw material for wet-process silica and synthetic zeolite in fields related to civil engineering ground hardening agents, detergents, fibers, pulp, etc. Commonly known methods for producing sodium silicate include the wet method (chemical formula A), in which natural sand (silica sand, silica stone, silicic clay, etc.) that is rich in silica ( _SiO2 ) is dissolved in an aqueous sodium hydroxide solution, and the dry method (chemical formula B), in which sand and soda ash are mixed and placed in a melting furnace, heated and melted, and when a transparent body is obtained, it is removed and cooled to solidify.
Chemical formula A
2NaOH+nSiO ₂ → Na ₂ O・nSiO ₂ +H ₂ O n: molar ratio
(Sodium silicate aqueous solution)
Chemical formula B
Na ₂ CO ₃ +nSiO ₂ → Na ₂ O・nSiO ₂ +CO ₂ n: molar ratio
(Sodium silicate cullet)

Wet-process silica, which is made from conventional sand-derived sodium silicate, has a high BET specific surface area and micropores to mesopores, and is therefore widely used in a wide range of applications, including reinforcing fillers for industrial rubber, tires, and silicone rubber, paper fillers, matting agents for paints, abrasives for toothpaste, and antiblocking agents for films.

In recent years, there has been a growing demand to create a sustainable and regenerative society in order to protect the global environment. As one concrete measure to achieve this, biomass technologies that do not use fossil fuels, such as biomass boilers and biomass power generation, are becoming more widespread in various regions. Facilities that use such technologies often use plants that are high in silica (rice husks, rice straw, wheat straw, bagasse, bamboo, and other grass family plants) as fuel. However, disposal of combustion ash (plant ash), which is high in silica, is an issue for these facilities.

As a method to solve this problem, attempts have been made to dissolve plant ash (particularly rice husk ash) that contains a lot of silica in an aqueous solution of sodium hydroxide to produce an aqueous solution of silicic acid that contains an alkaline component, and to reuse this as a raw material for wet-process silica (for example, Patent Document 1 and Patent Document 2).

In addition, aqueous silicic acid solutions using plant ash as a raw material usually contain potassium as an alkaline component in addition to sodium. Therefore, in the following description, except when the alkaline component is specifically limited to sodium, silicic acid containing an alkaline component will be referred to as "alkali silicate" instead of the commonly used term "sodium silicate" ( _{Na2O.nSiO2 )} . In the present invention and this specification, alkali silicate is intended to include sodium silicate.

Special Publication No. 2007-510613 Special Publication No. 2007-522069

Based on known techniques such as those disclosed in Patent Documents 1 and 2, it is not impossible to produce an aqueous alkali silicate solution from rice husk ash or other plant ashes and produce wet-process silica.

However, the aqueous alkali silicate solution derived from plant ash contains more impurities and foreign matter that cause coloring than that produced from sand, and wet-process silica produced from these colored aqueous alkali silicate solutions also contains many impurities and foreign matter, and is colored (low whiteness) and low-quality wet-process silica. Therefore, conventional wet-process silica produced from aqueous alkali silicate solutions derived from plant ash could only be used for limited purposes and usage, such as being used for cheap and general-purpose purposes, or being used by mixing a small amount of aqueous alkali silicate solution derived from plant ash with an aqueous alkali silicate solution derived from sand.

Since the quality of natural sand is consistent depending on the place of origin, with conventional sand-derived alkali silicate it is possible to address the quality of impurities by, for example, changing the place of origin of the sand depending on the intended use. However, when using plant ash, the plant ash itself contains a lot of impurities and foreign matter and is of unstable quality, making it difficult to find a way to solve the problems of impurities, foreign matter, and discoloration.

For these reasons, wet-process silica produced from an aqueous solution of alkaline silicate derived from plant ash has not become as widespread as expected.

In addition, conventional techniques, such as contacting the material with oxygen for a long period of time (Patent Document 1) or burning the material at high temperatures (Patent Document 2), generate large amounts of carbon dioxide and consume a lot of energy, and cannot be considered appropriate in light of recent considerations regarding environmental issues.

In addition, users of wet-process silica are calling for the supply of high-performance, high-quality wet-process silica that uses environmentally friendly raw materials (alkali silicate).

The present invention has been made in view of the above problems, and the object of the present invention is to provide an aqueous solution of alkaline silicate derived from plant ash that enables the production of wet-process silica with a high degree of whiteness equivalent to that achieved by using a conventional aqueous solution of alkaline silicate derived from sand.

The second object of the present invention is to provide a method for producing an aqueous alkali silicate solution that enables the production of such an aqueous alkali silicate solution of sufficient quality.

The inventors discovered that by producing wet-process silica using an aqueous alkali silicate solution derived from plant ash, which has high transmittance, as a raw material, it is possible to produce wet-process silica with a high degree of whiteness equivalent to that produced by using an aqueous alkali silicate solution derived from conventional sand, and thus completed the present invention.

The present invention is configured as follows.
[1]
An aqueous solution of alkali silicate in which the ratio of _{the P2O5} concentration to the _SiO2 concentration, _P2O5 / _{SiO2, expressed} in mass%, is 1.0 x ^10-3 or more, the _SiO2 concentration is 8.9 mass% to 16.7 mass%, and the transmittance (%T) at an optical path length of 1 cm and a wavelength of 500 nm is 65% or more.
[2]
The aqueous alkali silicate solution according to [1], wherein the ratio of the _SiO2 concentration to the ( _Na2O + _K2O ) concentration, _SiO2 /( _Na2O + _K2O ), expressed in mol%, is 2.8 to 3.5.
[3]
The aqueous alkali silicate solution according to [1] or [2], wherein the ratio of the K ₂ O concentration to the Na ₂ O concentration, K ₂ O/Na ₂ O, expressed in mol %, is 0.03 to 0.30.
[4]
[1] to [3], wherein when the aqueous alkali silicate solution is filtered through a filter having an opening of 1 μm, the amount of solid matter captured on the filter is less than 100 ppm by mass relative to the mass of the aqueous alkali silicate solution before filtration.
[5]
The aqueous alkali silicate solution according to any one of [1] to [4], which is used as a raw material for wet-process silica.
[6]
A method for producing an aqueous alkali silicate solution includes subjecting a raw material aqueous alkali silicate solution obtained by mixing plant ash containing a silica component, sodium hydroxide and water to one or both of a filtration process using a filter having an opening size of less than 50 μm and an adsorption process using an adsorbent to obtain an aqueous alkali silicate solution.
[7]
The method for producing an alkali silicate aqueous solution according to [6], wherein the plant ash is rice husk ash.

By using the aqueous alkali silicate solution derived from plant ash of the present invention as a raw material, it is possible to produce high-quality wet-process silica with a high degree of whiteness equivalent to that produced by using a conventional aqueous sodium silicate solution derived from sand.

The manufacturing method of the present invention also provides an aqueous alkali silicate solution derived from plant ash that enables the production of wet-process silica with a high degree of whiteness equivalent to that achieved when a conventional aqueous alkali silicate solution derived from sand is used.

In addition, the present invention allows the reuse of plant ash that would normally be discarded, making it possible to provide an environmentally friendly aqueous alkali silicate solution.

FIG. 1 is an image showing the filtration residue in the crude filtration of an aqueous alkali silicate solution. FIG. 2 shows the transmittance spectra of the aqueous alkali silicate solutions (Example 4, Comparative Example 1, and Comparative Example 4) at wavelengths of 200 nm to 800 nm. FIG. 3 shows the transmittance spectra of the aqueous alkali silicate solutions (Examples 1, 5, and Reference Example 1) at wavelengths of 200 nm to 800 nm. FIG. 4 shows the transmittance spectrum of the aqueous alkali silicate solution (Comparative Example 3) at wavelengths of 200 nm to 800 nm.

<Alkaline silicate aqueous solution>
The aqueous alkali silicate solution of the present invention is an aqueous alkali _silicate solution in which the ratio of the _P2O5 concentration to the _SiO2 concentration _{, P2O5} / _SiO2, expressed in mass%, is 1.0 x ^10-3 or more, the _SiO2 concentration is 8.9 mass% to 16.7 mass%, and the transmittance (%T) at a wavelength of 500 nm and an optical path length of 1 cm is 65% or more.

The aqueous alkali silicate solution of the present invention is produced using plant ash, which is the combustion ash of a plant containing a silica component. The plant ash contains a phosphorus component, and phosphoric acid (P ₂ O ₅ ) in terms of oxide is a component inherent to the plant ash. In the aqueous alkali silicate solution of the present invention using plant ash as the main silica source, the ratio of the P ₂ O ₅ concentration to the SiO ₂ concentration expressed in mass % is higher than that of an aqueous alkali silicate solution derived from sand, and the ratio P ₂ O ₅ /SiO ₂ is 1.0×10 ⁻³ or more. In general, the ratio P ₂ O ₅ /SiO ₂ in an aqueous alkali silicate solution using only sand as a silica source is not 1.0×10 ⁻³ or more. Therefore, the presence of P ₂ O ₅ based on the ratio P ₂ O ₅ /SiO ₂ makes it possible to determine that the aqueous alkali silicate solution is derived from plant ash (i.e., plant ash is used as at least a part of the silica source). In the alkali _silicate aqueous solution of the present invention, the ratio _P2O5 / _SiO2 may be ^1.5x10-3 or more, ^1.8x10-3 or more, or ^2.0x10-3 or more. The upper limit of _the ratio _P2O5 / _SiO2 is not particularly limited and depends on the type of plant ash, but is, for example, 20x10-3 or less. From the viewpoint of reducing impurities, the ratio _{P2O5 / SiO2} is preferably ^15x10-3 or less, more preferably ^13x10-3 or ^less , and even more preferably ^10x10-3 or less.

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

Usually, plant ash contains a lot of impurities and foreign matter. When an alkali silicate aqueous solution is produced by a conventional method using plant ash containing a lot of impurities and foreign matter, the impurities and foreign matter are mixed into the alkali silicate aqueous solution, resulting in a decrease in the transmittance (%T). In the alkali silicate aqueous solution of the present invention, the transmittance is set to 65% or more by reducing the amount of impurities and foreign matter. With the transmittance of 65% or more, it becomes easier to produce wet-process silica with a high degree of whiteness. In the alkali silicate aqueous solution of the present invention, the transmittance is preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more. In particular, if the transmittance is 80% or more, it becomes easier to produce wet-process silica with a degree of whiteness of 90 or more.

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

The _SiO2 concentration in the aqueous alkali silicate solution of the present invention is 8.9% by mass to 16.7% by mass. In general, the _SiO2 concentration of an aqueous alkali silicate solution used as a raw material for wet-process silica is within the above range. Therefore, by having the _SiO2 concentration within the above range, the aqueous alkali silicate solution of the present invention can be easily used in the same applications as existing aqueous alkali silicate solutions.

The aqueous alkali silicate solution of the present invention contains sodium and potassium components derived from plant ash. In the aqueous alkali silicate solution of the present invention, the ratio of the _SiO2 concentration to the ( _Na2O + _K2O ) concentration, _SiO2 /( _Na2O + _K2O ), expressed in mole percent, is preferably 2.8-3.5. In aqueous alkali silicate solutions generally used as raw materials for wet-process silica, the molar ratio _SiO2 /( _Na2O + _K2O ) (usually the amount of _K2O is very small) is in the range of 2.8-3.5. Therefore, when the molar ratio _SiO2 /( _Na2O + _K2O ) is in the above range, the aqueous alkali silicate solution of the present invention can be easily used in the same applications as existing aqueous alkali silicate solutions.

In the aqueous alkali silicate solution of the present invention, the ratio of the K ₂ O concentration to the Na ₂ O concentration, K ₂ O/Na ₂ O, expressed in mole percent, is preferably 0.03 to 0.30. When the molar ratio K ₂ O/Na ₂ O is 0.30 or less, the neutralization reaction is less likely to vary when neutralizing and synthesizing wet-process silica, and problems with the quality stability of the wet-process silica are less likely to occur. In addition, when the molar ratio K ₂ O/Na ₂ O is 0.03 or more, the molar ratio K ₂ O/Na ₂ O can be easily adjusted by mixing with another type of alkali silicate, such as an alkali silicate derived from sand. The molar ratio K ₂ O/Na ₂ O is preferably in the range of 0.04 to 0.20, more preferably in the range of 0.05 to 0.15.

The molar ratio _K2O / _Na2O in the alkali silicate can be one of the criteria for judging whether the alkali silicate is derived from plant ash or not. In alkali silicate derived from sand, the molar ratio _K2O / _Na2O is usually less than 0.01. Therefore, when the molar ratio _K2O / _Na2O is 0.01 or more, it is often said that the aqueous alkali silicate solution contains plant ash, except in cases where it is clear that the _K2O concentration has been increased in a post-treatment.

The total of the Na ₂ O concentration and the K ₂ O concentration (Na ₂ O + K ₂ O) can be, for example, 3.5 mass % to 5.0 mass %. In an aqueous alkali silicate solution generally used as a raw material for wet-process silica, the total concentration of Na ₂ O + K ₂ O (usually the amount of K ₂ O is very small) is in the range of 3.5 mass % to 5.0 mass %. Therefore, when the total concentration of Na ₂ O + K ₂ O is in the above range, the aqueous alkali silicate solution of the present invention can be easily used in the same applications as existing aqueous alkali silicate solutions.

In the aqueous alkali silicate solution of the present invention, when the aqueous alkali silicate solution is filtered through a filter with a mesh size (particle retention capacity) of 1 μm, the amount of solids captured on the filter is preferably less than 100 ppm by mass relative to the mass of the aqueous alkali silicate solution before filtration. The amount of solids is preferably less than 90 ppm by mass, and more preferably less than 80 ppm by mass, relative to the mass of the aqueous alkali silicate solution before filtration. In particular, when the adsorption treatment and filtration described below are performed, the amount of solids can be reduced to less than 50 ppm by mass, preferably less than 40 ppm by mass, and more preferably less than 30 ppm by mass.

The material of the filter used is not particularly limited as long as the mesh size is 1 μm. For example, the filter is filtered using a test filter paper (e.g., standard 5C, manufactured by ADVANTEC), the filter paper is thoroughly washed with pure water, and dried at 105° C. for 2 hours. Based on the solid mass (mass of undissolved residue) remaining on the filter paper and the mass of the aqueous alkali silicate solution used for filtration, the mass % concentration of the undissolved residue can be calculated by the following formula.

According to experiments conducted by the present inventors, it was found that the coloration problem in conventional wet-process silica derived from plant ash is caused by impurities and foreign matter in the aqueous alkali silicate solution derived from plant ash. In other words, many impurities and foreign matter are present in conventional aqueous alkali silicate solutions derived from plant ash, and these impurities and foreign matter cloud and color the aqueous alkali silicate solution. When wet-process silica is produced using such an aqueous alkali silicate solution, impurities and foreign matter are mixed into the wet-process silica, resulting in a problem of reduced whiteness of the wet-process silica.

As described later, plant ash contains carbon components remaining from unburned combustion, as well as insoluble foreign matter such as graphite-based foreign matter and vitrified silica components, and an aqueous solution of alkali silicate derived from plant ash is prone to producing undissolved residues. If an aqueous solution of alkali silicate containing a large amount of undissolved residues is used as the raw material to produce wet-process silica, many impurities and foreign matter will be mixed into the wet-process silica. For example, in matte applications, such foreign matter will be scattered on the surface after painting and become protrusions, which will reduce the matte performance and impair the aesthetic appearance. In addition, in applications such as industrial rubber, tires, and toothpaste, the inclusion of impurities and foreign matter in wet-process silica is undesirable because it will reduce the quality of the product. Therefore, in order to obtain wet-process silica with a high whiteness, the mass % concentration of undissolved residues in the aqueous solution of alkali silicate is adjusted to the above range to suppress the inclusion of impurities and foreign matter in wet-process silica.

The aqueous alkali silicate solution of the present invention can be obtained by the method for producing an aqueous alkali silicate solution of the present invention (the method for producing an aqueous alkali silicate solution described below) which involves carrying out one or both of a specified filtration process and an adsorption process. As shown in the examples, the aqueous alkali silicate solution can also be obtained by the method for producing an aqueous alkali silicate solution in which plant ash with a high whiteness is obtained by a specified combustion method described in this specification, and the plant ash is mixed with sodium hydroxide and water.

For comparison, in the methods of Patent Documents 1 and 2, for example, the obtained aqueous alkali silicate solution is not subjected to purification treatment such as filtration. Therefore, due to the presence of impurities and foreign matter in the plant ash, the conventional aqueous alkali silicate solution as described above is not transparent, and wet-process silica with high whiteness cannot be produced. This is demonstrated by the results of Comparative Example 3 described in the Examples.

<Method of producing an aqueous alkali silicate solution>
The method for producing an aqueous alkali silicate solution of the present invention includes subjecting a raw aqueous alkali silicate solution obtained by mixing plant ash containing a silica component, sodium hydroxide and water to one or both of a filtration process using a filter having an opening size of less than 50 μm and an adsorption process using an adsorbent to obtain an aqueous alkali silicate solution. By this production method, the aqueous alkali silicate solution of the present invention described above can be obtained.

<Plant ash containing silica components>
The plant ash used in producing the aqueous alkali silicate solution of 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 it is a plant and part containing a silica component. Examples of plants containing silica components include plants of the Gramineae family, such as rice (rice straw and rice husk), wheat, sugarcane, and bamboo. Rice husks after threshing are most preferred because they are less contaminated by sludge, are collected at collection sites, and are easily available.

The quality of the plant ash used in the present invention is not particularly limited, but from the viewpoint of industrially efficiently producing an aqueous alkali silicate solution with few impurities and foreign matter, it is preferable to use plant ash with a low carbon content and high whiteness. The amount of carbon in the plant ash is preferably 2 mass% or less, and preferably 1 mass% or less. The lower limit of the amount of carbon in the plant ash is not particularly limited, and in practice it is 0.05 mass% or more, and may be 0.1 mass% or more. The whiteness of the plant ash is preferably 30 or more, more preferably 40 or more, and even more preferably 50 or more. The upper limit of the whiteness of the plant ash is not particularly limited, and in practice it is 90 or less, and may be 70 or less.

To obtain plant ash with high whiteness and few impurities and foreign matter as described above, it is preferable to use a combustion furnace that can burn the plant material at 550°C to 900°C while flowing it. The combustion temperature is more preferably 600°C to 800°C, and even more preferably 650°C to 750°C.

Here, the relationship between the above-mentioned combustion method in which plant material containing silica components is burned at 550°C to 900°C while flowing, and impurities and foreign matter in plant ash will be explained. In order to produce plant ash with high whiteness, the inventors have investigated in detail the types of impurities and foreign matter in plant ash and the causes of their generation. As a result, it was found that when burning at a temperature lower than 550°C or when the burning is uneven due to insufficient material flow, the plant material is likely to be unburned and water-soluble hydrocarbon components are likely to remain in the plant ash. Water-soluble hydrocarbon components are difficult to dissolve in an alkali silicate solution and remove by filtration, and cause the solution to become discolored. It was also found that when sufficient oxygen is not supplied during burning, the hydrocarbon components in the plant material are carbonized and insoluble graphite-based foreign matter is generated in the plant ash. In addition, it was found that when plant material is burned all at once at a combustion temperature of 700°C or higher, vitrified silica components are likely to be generated in the plant ash as insoluble foreign matter. In this case, it was found that the silica components on the surface of the material were vitrified, which made it easy for the inside of the material to become unburned and oxygen-deficient, and that water-soluble hydrocarbon components and insoluble graphite-based foreign matter were also easily generated. If an alkali silicate solution is produced using plant ash obtained by burning at high temperatures all at once, in addition to the vitrified silica components, water-soluble hydrocarbon components and insoluble graphite-based foreign matter are also mixed into the alkali silicate solution, resulting in a significant decrease in the quality of the alkali silicate solution. In addition, even if the ash appears white at first glance, it was confirmed that a certain amount of impurities and foreign matter such as water-soluble hydrocarbon components, insoluble graphite-based foreign matter, and vitrified silica components are present in the plant ash. Considering the above-mentioned generation process of impurities and foreign matter in plant ash, in order to obtain plant ash with a high degree of whiteness, it is effective to burn the plant material uniformly under sufficient oxygen while suppressing the vitrification of silica components using the above-mentioned combustion method.

In the above-mentioned combustion method, it is preferable to use a combustion furnace for burning the plant material. The type 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 matter, it is preferable to use a combustion furnace equipped with a fluidizing device that fluidizes the plant material during combustion and capable of burning the plant material at 550°C to 900°C while fluidizing it. For example, the combustion furnace may be a direct combustion furnace in which the plant material is burned with oxygen, or a gasification combustion furnace in which the plant material is heated in an oxygen-free or low-oxygen state to generate organic gas and then burn the material. In addition, when the plant ash contains a large amount of graphite-based foreign matter, the carbon components may be removed by, for example, secondary combustion in the same combustion furnace while reusing the heat of the primary combustion.
According to such a method, the quality of the plant ash tends to be stable, so that the plant ash becomes more suitable as a silica source for the aqueous alkali silicate solution of the present invention.

<Sodium hydroxide solution>
The sodium hydroxide and water used in the production method of the present invention can be known. In the embodiment in which the aqueous sodium hydroxide solution and the plant ash are mixed, the concentration of the aqueous sodium hydroxide solution is not particularly limited, and known solutions can be used. In general, from the viewpoint of productivity of the aqueous alkali silicate solution, it is preferable to mix the aqueous sodium hydroxide solution and the plant ash at as high a concentration as possible, and then dilute with water to a desired _SiO2 concentration. On the other hand, since the apparent density of the plant ash is very small, if the concentration of the aqueous sodium hydroxide solution is too high, it may be difficult to put the plant ash into the aqueous sodium hydroxide solution. From the viewpoint of making it easy to mix the plant ash and the aqueous sodium hydroxide solution, the concentration of the aqueous sodium hydroxide solution is practically 20% by mass or less, and may be 15% by mass or less or 10% by mass or less.

<Detailed manufacturing conditions of the alkaline silicate aqueous solution>
The aqueous alkali silicate solution of the present invention is obtained by mixing plant ash containing silica components, sodium hydroxide and water. The order of mixing plant ash, sodium hydroxide and water is not particularly limited. Plant ash and sodium hydroxide may be added to water simultaneously or separately, a mixture of plant ash and sodium hydroxide may be added to water, plant ash may be added to an aqueous sodium hydroxide solution obtained by dissolving sodium hydroxide in water, or sodium hydroxide may be added to an aqueous solution obtained by dissolving plant ash in water.

The amounts and mixing ratios of water, sodium hydroxide, and plant ash used can be appropriately adjusted depending on the type of plant ash, the target _SiO2 concentration of the alkali silicate aqueous solution, the intended use, and other conditions. Since vitrified silica components are difficult to dissolve, it is not necessary to dissolve all of the silica components in the plant ash in the aqueous solution. If a method is used that dissolves approximately 90% by mass to 95% by mass of the silica components in the plant ash, the aqueous alkali silicate solution can be produced with good productivity in terms of energy consumption and time.

For example, if it is assumed that 90% of the silica component contained in the plant ash dissolves in the aqueous solution, then the amount of plant ash added and dissolved should be approximately three times the mass of sodium hydroxide in the aqueous solution, and it will be easy to adjust the ratio of the _SiO2 concentration to the ( _Na2O + _K2O ) concentration, _SiO2 /( _Na2O + _K2O ), expressed as molar percentage, to 2.8 to 3.5.

In the manufacturing method of the present invention, other conditions for mixing and dissolving the plant ash, sodium hydroxide, and water are not particularly limited.

Mixing and dissolving is carried out, for example, in a dissolving tank (container) with stirring, at normal pressure (atmospheric pressure) or under pressure, at a temperature between 60°C and 100°C for 1 hour to 5 hours as appropriate.

The manufacturing method of the present invention includes subjecting the raw alkali silicate aqueous solution obtained as described above to one or both of a filtration process using a filter with an opening of less than 50 μm and an adsorption process using an adsorbent to obtain an alkali silicate aqueous solution. As described above, conventional plant ash contains impurities such as graphite-based foreign matter, foreign matter in which silica components have been vitrified, and water-soluble hydrocarbon compounds. If these impurities or foreign matter are mixed into the alkali silicate aqueous solution and the wet-process silica, they will cause coloring. Therefore, for example, by simply dissolving conventional plant ash in a sodium hydroxide aqueous solution, it is not possible to obtain wet-process silica of the quality desired by the present invention. In the present invention, impurities and foreign matter in the alkali silicate aqueous solution are removed by subjecting the raw alkali silicate aqueous solution to one or both of the filtration process and the adsorption process.

The mesh size of the filter used in the filtration process is less than 50 μm in order to remove insoluble foreign matter. From the viewpoint of more thoroughly removing foreign matter, the mesh size of the filter is preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. If the mesh size of the filter is 10 μm or less, the filtering property (productivity) is significantly deteriorated, so it is sufficient to select an appropriate mesh size while considering productivity and quality. There are no particular restrictions on the material of the filter, and various filters such as filter paper, filter cloth, and membrane filters can be used. From the viewpoint of more thoroughly removing foreign matter, the filtration process using a filter with a mesh size of less than 50 μm may be performed multiple times. When the filtration process is performed multiple times, it is generally performed in order of increasing mesh size.

In addition, in the manufacturing method of the present invention, filtration using a filter with a mesh size of 50 μm or more may be performed before filtration using a filter with a mesh size of less than 50 μm. Filtration using a filter with a mesh size of 50 μm or more can be performed as necessary when there is a large amount of undissolved coarse particles, etc.

Adsorption processing can remove water-soluble impurities that are difficult to remove by filtration processing.

Whether or not to perform the adsorption treatment is determined appropriately depending on the amount and type of impurities in the aqueous alkali silicate solution. For example, it is preferable to perform both filtration and adsorption treatment for an aqueous alkali silicate solution whose transmittance (% T) at a wavelength of 500 nm is less than 10%.

On the other hand, if the transmittance (%T) of 65% or more can be achieved after either one of the treatments (for example, if there are few impurities or foreign matter in the plant ash), then only one of the filtration and adsorption treatments may be performed. Specifically, if it is sufficient to remove insoluble foreign matter, then only filtration may be performed, and if it is sufficient to remove water-soluble impurities, then only adsorption treatment may be performed. In addition, for an aqueous alkali silicate solution with the transmittance (%T) of 65% or more, one or both of the filtration and adsorption treatments may be performed to further improve the transmittance, or an adsorption treatment may be performed after filtration, and filtration may be performed again to remove remaining adsorbent.

The adsorbent used in the adsorption treatment is not particularly limited, and any known adsorbent can be used. Examples of adsorbents include diatomaceous earth, clay minerals, activated carbon, zeolite, silica gel, and activated alumina. The adsorbent is preferably activated carbon. The time of the adsorption treatment can be adjusted appropriately depending on the amount of impurities and foreign matter in the aqueous alkali silicate solution, and is, for example, 12 to 40 hours, and preferably 15 to 30 hours. The amount of the adsorbent used in the adsorption treatment can be adjusted appropriately depending on the amount of impurities in the aqueous alkali silicate solution, and is, for example, 5 g to 20 g, or may be 8 g to 15 g per 1,000 g of the aqueous alkali silicate solution.

Since the silica and alkali components in plant ash are not constant, the _K2O and _SiO2 concentrations in the alkali silicate solution are likely to vary. Therefore, if necessary, the final concentration adjustment and each molar ratio adjustment are performed after performing one or both of the above filtration and adsorption treatments. The _SiO2 mass% can be adjusted by diluting with water or adding additional silica material (e.g., commercially available silica powder). The ratio of the _SiO2 concentration to the ( _Na2O + _K2O ) concentration in mol% ( _SiO2 /( _Na2O + _K2O )) can be adjusted by adding sodium hydroxide.

<Other conditions for the alkaline silicate aqueous solution>
The aqueous alkali silicate solution of the present invention may use a silica material derived from sand (such as silica sand, silica stone, or silicic clay) as a part of the silica source. When the quality of the wet-process silica is important, the proportion of the silica material derived from sand can be increased, and when environmental consideration (reuse of plant ash) is important, the proportion of plant ash can be increased. From the viewpoint of environmental consideration, 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 even more preferably 90% by mass or more. In the preparation of the raw alkali silicate aqueous solution, it is also possible to use 100% by mass of plant ash without using a silica material derived from sand as the silica source.

The sand-derived silica material may be added as part of the silica source when preparing the above-mentioned aqueous alkali silicate solution. The sand-derived silica material may also be used in such a form that an aqueous alkali silicate solution derived from plant ash and an aqueous sodium silicate solution derived from sand are prepared separately and then mixed before use.

The aqueous alkali silicate solution of the present invention may contain additives as necessary. The additives are not particularly limited, and known additives may be used. For example, an aluminum oxide ( _{Al2O3 )} source such as sodium aluminate may be added. The wet process silica produced from the aqueous alkali silicate solution containing such additives has the effect of improving reinforcing performance as a reinforcing agent for silicone rubber, industrial rubber, and tires.

As described above, according to the manufacturing method of the present invention, it is possible to sufficiently remove impurities and foreign matter such as graphite-based foreign matter in plant ash, foreign matter resulting from vitrification of silica components, and water-soluble hydrocarbon compounds from the aqueous alkali silicate solution. As a result, it is possible to provide an aqueous alkali silicate solution derived from plant ash that can produce wet-process silica with a high degree of whiteness equivalent to that obtained when an aqueous alkali silicate solution derived from sand is used.

In addition, the present invention allows the reuse of plant ash that would normally be discarded, making it possible to supply an environmentally friendly aqueous solution of alkaline silicate. Furthermore, the aqueous solution of alkaline silicate obtained by the manufacturing method of the present invention contains a small amount of impurities and foreign matter, making it possible to manufacture high-quality wet-process silica.

<Uses of alkaline silicate aqueous solution>
The aqueous alkali silicate solution of the present invention can be used as a raw material for producing wet-process silica. Although the aqueous alkali silicate solution of the present invention is derived from plant ash, the transmittance (%T) is 65% or more, and the content of impurities and foreign matter that cause coloring is low. By using the aqueous alkali silicate solution of the present invention as a raw material, it is possible to produce wet-process silica with a high degree of whiteness equivalent to that of the conventional aqueous alkali silicate solution derived from sand, even though it is produced using plant ash. In addition, since the aqueous alkali silicate solution of the present invention has a low content of impurities and foreign matter, it is possible to produce high-quality wet-process silica with few impurities and foreign matter.

The type of wet-process silica produced from the aqueous alkali silicate solution of the present invention is not particularly limited, and may be, for example, precipitated silica (white carbon) or gel silica.The reason for this is that the process of precipitating silica from an aqueous alkali silicate solution for wet-process silica basically all show the same tendency.An example of the synthesis process of wet-process silica is shown below (chemical formula C).
Chemical formula C (Example of wet silica synthesis process)
M ₂ O・nSiO ₂ +H ₂ SO ₄ → nSiO ₂ +M ₂ SO ₄ +H ₂ O
Here, M represents an alkali including sodium and potassium, and n represents a molar ratio.

The wet-process silica produced using the aqueous alkali silicate solution of the present invention as a raw material has high whiteness and little foreign matter, and therefore has sufficient reinforcing properties to be used as a reinforcing filler for industrial rubber and tires. Furthermore, it can be used in applications where transparency and whiteness are important, such as reinforcing fillers for silicone rubber, paper fillers (anti-strike-through agents), matting agents for paints, abrasives for toothpaste, and anti-blocking agents for films, in the same way as known aqueous alkali silicate solutions derived from sand.

In addition, since the aqueous alkali silicate solution of the present invention contains few impurities and foreign matter, the aqueous alkali silicate solution of the present invention can be used for known applications other than wet-process silica, such as raw materials for synthetic zeolites, ground hardening agents and cleaning agents for civil engineering, and additives for fibers and pulp. By using the aqueous alkali silicate solution of the present invention, problems such as the above-mentioned contamination of materials and products with foreign matter are less likely to occur compared to conventional aqueous alkali silicate solutions derived from plant ash.

Specific examples are given below for explanation. However, there is no intention to limit the scope of the invention to these examples.

<Combustion conditions for plant ash>
Rice husks and rice straw were used as plant materials for producing plant ash. The rice husks and rice straw were thoroughly dried at 125°C for 2 hours. 500g of rice husks (apparent density: about 110g/L) or 250g of rice straw cut to about 10cm were placed in a 2mm thick stainless steel sagger (300mm x 300mm x 115mm (height)) equipped with an air hole and a lid, and these were stacked in two layers and burned in an air atmosphere in a tabletop muffle furnace (model: KDF S100G, manufactured by Denken Co., Ltd.). Since only about 200g of plant ash could be obtained at most at one time, the combustion process was carried out multiple times under the same conditions until the required amount of plant ash was obtained, and then mixed at the end. In addition, when burning at 900°C, a mullite sagger (outer dimensions 210 mm x 210 mm x 70 mm (height), inner dimensions 194 mm x 194 mm x 56 mm (height)) equipped with an air hole and a lid was used instead of the stainless steel sagger.

Six types of plant ashes, ash a to ash f, were obtained using the above method. The combustion conditions for each sample are shown in Table 1 below. In Table 1, combustion conditions showing multiple combustion treatments indicate that preliminary combustion (one or more times) was performed at 500°C, the removed ash was stirred, and then the next combustion treatment was performed. For example, in Table 1, the combustion conditions for ash b, "500°C x 2 hr, 600°C x 2 hr," indicate that preliminary combustion was performed once for two hours at 500°C, followed by combustion treatment for two hours at 600°C. Plant ash f is ash obtained by burning rice husks at 700°C for two hours in an air excess state without material flow, according to the method described in Patent Document 1.

<Analysis of plant ash>
● Whiteness of plant ash This was measured using a whiteness meter (model: NW-12, manufactured by Nippon Denshoku Industries Co., Ltd.).

Carbon content in plant ash (mass%)
The carbon content was measured by a carbon analyzer (model: CS744, manufactured by LECO Japan LLC) using the oxygen flow combustion-non-dispersive infrared absorption method, by subjecting the sample to heat treatment under conditions of a temperature of 1,350°C, an oxygen inflow pressure of 0.24 MPa, and a measurement time of 50 seconds, and quantifying CO gas and _CO2 gas with an infrared detector (NDIR) in the device. The plant ash was pre-dried at 105°C for 2 hours.

Composition of inorganic components other than carbon in plant ash (mass%)
Using a scanning X-ray fluorescence analyzer (model: ZSX Primus II, manufactured by Rigaku Corporation), first, a qualitative analysis was performed on elements other than carbon to confirm the type of inorganic impurities detected, and then a quantitative analysis of the detected inorganic impurities was performed. The measurement sample was prepared by putting plant ash into a ring-shaped mold and pressurizing it to form it. The quantitatively determined inorganic component composition (excluding carbon) was converted into oxides using the analysis software attached to the device to determine the concentration (mass%).

Ashing rate (mass%)
After thorough drying at 125° C. for 2 hours, the mass percentage of the combustion ash after combustion relative to the mass of the plants such as rice husks was measured.

Apparent density (bulk specific gravity) of plant ash (g/L)
The apparent density of the plant ash is determined using a dedicated measuring device (a sieve with 0.5 mm openings, a funnel, a 30 mL cylindrical receiver, a receiver stand, and a funnel stand) based on JIS K5101-12-1:2004 (Pigment testing method - apparent density or apparent specific volume - static method). The sample (plant ash) was dropped from the sieve into the funnel with a brush, and the heaped portion of the sample in the receiver was scraped off with a spatula, after which the mass of the sample was measured. The apparent density (bulk specific gravity) of the plant ash in g/L was calculated based on its mass using the following formula:

Bulk density (apparent specific gravity) of plant ash (g/mL)
The bulk density of the plant ash was measured based on the section "7.8 Bulk Density" of JIS K6220-1:2015 (Rubber Compounding Agents - Organic Chemicals - Test Methods - Part 1: General) using a dedicated measuring device (a cylinder with an inner diameter of 22.00 ± 0.05 mm and an inner depth of 100 mm made of general steel, and a piston with an outer diameter of 21.80 ± 0.05 mm, a length of 115 mm, a mass of 190 g, and a cavity inside). The piston was allowed to fall naturally into the cylinder before the sample (plant ash) was placed, and the height of the piston protruding from the top of the cylinder was measured. Next, about 1 g of the weighed sample was placed in the cylinder, the piston was slowly dropped over 5 seconds, the side wall of the cylinder was lightly tapped with a piece of wood to make the piston well blended, and the height of the piston protruding from the top of the cylinder was measured. The bulk density (apparent specific gravity) in g/mL was calculated using the change in the height of the piston protruding from the top of the cylinder and the bottom area of the cylinder according to the following formula.

The carbon content and inorganic composition other than carbon in the plant ash of each sample are shown in Table 1.

Combustion conditions and components of ash a to ash f:

<Method for preparing an aqueous alkali silicate solution>
● Primary processing (mixing and filtering materials)
1,740 g of sodium hydroxide solution adjusted to 5.8% by mass was added to a 3 L stainless steel container equipped with a stirrer and heated to 85 ° C. Then, 300 g of plant ash was added to the sample solution while continuing stirring, and dissolved over 2 hours while maintaining 85 ° C., and the sample solution was cooled to room temperature. Here, it was assumed that 90% of the SiO ₂ contained in the plant ash was dissolved in the sodium hydroxide aqueous solution, and the target SiO ₂ concentration was set to 12.0 mass%. A 150 mmΦ magnetic Buchner funnel (Nutsche) was attached to a 3 L suction filtration bottle, and a polyester plain woven filter cloth (mesh size about 50 μm × 400 μm) was used to perform coarse filtration while sucking the sample solution to remove undissolved residue. As a result, an alkali silicate aqueous solution after primary treatment was obtained.

Undissolved residue was found in all plant ashes, and the amount was 10-20% by mass on a wet basis. This amount corresponds to approximately 1-4% by mass in terms of solid content after drying. Figure 1 shows photographs of the undissolved residues of plant ash a-d and plant ash f. Although quantitative evaluation of the amount of undissolved residue is difficult because it contains moisture, no correlation was found between the total amount of undissolved residue and the final combustion temperature. In contrast, the color of the undissolved residue differed depending on the final combustion temperature. Specifically, the residue of plant ash d burned at 900°C was a pale yellow to white residue mainly composed of undissolved glass, while the others were gray to black in color. Thus, the lower the final combustion temperature, the darker the undissolved residue was. If the required amount of alkali silicate solution was not obtained after the above filtration, the treatment was repeated several times under the same conditions and finally mixed to obtain a sample.

● Measurement of alkali concentration and _SiO2 concentration (simple measurement for concentration adjustment)
For the aqueous alkali silicate solution after the primary treatment, the alkali concentration (mass % concentration of Na ₂ O + K ₂ O) and the SiO ₂ concentration (mass % concentration) were measured by the titration method shown below.

Alkaline concentration:
About 2 g of sample, accurately weighed to the nearest 1 mg, and about 100 mL of distilled water were placed in a 300 mL conical beaker, and 2 to 3 drops of methyl orange (0.1% by mass aqueous solution) were added as an indicator to the sample solution, which was then titrated with 1N-HCl using a 50 mL burette. The point at which one drop turned orange was set as the endpoint, and the alkali concentration was calculated from the amount of 1N-HCl consumed at that point using the following formula:

_SiO2 concentration:
5.0 g of sodium fluoride reagent was added to the alkali concentration titration end point solution and thoroughly mixed. Then, about 0.5 mL of methyl red xylene cyanol FF indicator was added to the sample solution and titrated with 1N-HCl. When the solution turned red, 2.0 mL of 1N-HCl was added and the titration amount a (mL) was recorded. Here, the methyl red xylene cyanol FF indicator was prepared by dissolving 0.8 g of methyl red and 0.2 g of xylene cyanol FF in 1,000 mL of ethanol.

Next, back titration was performed with 1N-NaOH using a 10 mL burette. When the color of the sample solution turned brown-orange, it was confirmed that the pH was in the range of 5.8 to 6.0, which was the end point. The titration volume b (mL) was recorded, and the _SiO2 concentration was calculated using the following formula.

- Secondary treatment of alkaline silicate solution (if necessary)
Based on the measurement results by the titration method, the components were finely adjusted by adding either wet-process silica or water or both so that the SiO ₂ concentration in terms of oxide approached the target SiO ₂ concentration of 12.0 mass% (additional dissolution or additional heat dissolution). In this example and comparative example, secondary treatment was performed on the aqueous alkali silicate solutions obtained using plant ash d, plant ash e, and plant ash f, respectively. For the aqueous alkali silicate solution derived from plant ash d, only wet-process silica (silica purity about 93 mass%, Nipsil LP, manufactured by Tosoh Silica Co., Ltd.) was added, and the SiO ₂ concentration was adjusted by heat dissolution. For the aqueous alkali silicate solution derived from plant ash e, water and the above wet-process silica were added, and the SiO ₂ concentration was adjusted by heat dissolution. For the aqueous alkali silicate solution derived from plant ash f, only the above wet-process silica was added, and the SiO ₂ concentration was adjusted by heat dissolution. Plant ash f was ash obtained by burning rice husk ash for 2 hours at 700 ° C. in an air excess state without flowing the material, according to the method described in Patent Document 1. In the case of plant ash f, the surface portion of the rice husk is vitrified first, and most of the vitrified components are removed as residues in the crude filtration during primary treatment. Therefore, compared with plant ash d and plant ash e, a larger amount of wet-process silica was required to adjust the _SiO2 concentration.

The sample solution that had undergone primary treatment and, if necessary, secondary treatment was used as the stock solution of the aqueous alkali silicate solution to be used in the examples and comparative examples. In the following explanation, the stock solution obtained using plant ash a will be referred to as "alkali silicate a," and similarly, the stock solutions obtained using plant ash b to plant ash f will be referred to as "alkali silicate b" to "alkali silicate f," respectively.

In addition, as Reference Example 1, an aqueous solution of alkali silicate was produced using commercially available silica sand as a raw material by the following method.
Aqueous sodium silicate solution derived from silica sand:
A 5L pressure vessel (autoclave) was charged with 3,000g of water, and 1,500g of sodium silicate cullet (medium molar grade, _Na2O : 24.51%, _SiO2 : 75.44%, purity 99.9% or more, manufactured by Tokuyama Corporation) produced using commercially available silica sand as a raw material was dissolved for 60 minutes under conditions of a pressure of 0.7MPa and a temperature of 160°C. 2,000g of sodium silicate aqueous solution was taken from the vessel, diluted twice with water, and cooled to room temperature. Then, a 150mmΦ magnetic Buchner funnel (Nutsche) was attached to a 5L suction filtration bottle, and crude filtration was performed while suctioning using a polyester plain woven filter cloth (mesh opening approximately 50μm×400μm). Undissolved residual components were removed by crude filtration to obtain an aqueous solution of sodium silicate (alkali silicate) derived from silica sand.

<Examples and Comparative Examples of Alkaline Silicate Aqueous Solution>
Plant ash a to plant ash f were used to prepare alkali silicate a to alkali silicate f, respectively, to obtain aqueous solutions of alkali silicate in Examples 1 to 3 and Comparative Examples 1 to 3. Each aqueous solution, including the aqueous solution of alkali silicate in Reference Example 1, was subjected to the following measurements.

Composition analysis and impurity measurement The alkali silicate aqueous solution sample extracted with a syringe equipped with a membrane filter with a mesh size of 1 μm was diluted with pure water. A high-resolution ICP emission spectrometer (model: PS3520DDII, manufactured by Hitachi High-Tech Science Corporation) was used to quantitatively analyze the sample (unit: mass%) based on the calibration standard solution for each element. The dilution ratio was 10,000 times (calibration standard solution: 20 ppm by mass) for the quantitative analysis of SiO ₂ , Na ₂ O, and K ₂ O, and 1,000 times (calibration standard solution: 10 ppm by mass) for the quantitative analysis of P ₂ O ₅ , CaO, MgO, Al ₂ O ₃ , Fe ₂ O ₃ , and MnO.

Molar ratio The mass concentrations of _SiO2 , _Na2O and _K2O measured above were converted into molar amounts to obtain the _SiO2 /( _Na2O + _K2O ) molar ratio and the _K2O / _Na2O molar ratio. 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 according to the following formula.

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

Measurement of undissolved residue (mass ppm) of alkali silicate aqueous solution The undissolved residue (amount of solid matter captured on a filter when filtered through a filter with a mesh size of 1 μm) of an alkali silicate aqueous solution, which is used as a raw material for wet-process silica, was measured. A 110 mmΦ magnetic Buchner funnel (Nuchsche) was attached to a 2 L suction filtration bottle, and suction filtration was performed on 1,000 g of alkali silicate aqueous solution using a filter paper with a mesh size of 1 μm (standard 5C, manufactured by ADVANTEC). After filtration, the solution was washed with 1,000 g of pure water and dried at 105° C. for 2 hours in a box dryer. The mass of the solid matter captured on the filter paper was then calculated, and the mass ppm concentration of the undissolved residue was calculated using the following formula.

●Specific gravity of alkaline silicate solution (reference value)
An aqueous solution of alkaline silicate adjusted to 20±0.5° C. was placed in a 250 mL measuring cylinder, and a hydrometer (float scale) was placed in the cylinder and the scale was read when the cylinder came to rest.

<Production of wet-process silica and evaluation of its physical properties>
For wet-process silica, the process of precipitating silica from an aqueous solution of alkali silicate basically shows the same tendency, so alkali silicate a to alkali silicate f were used to produce precipitated silica from among the wet-process silicas by the following method, and their physical properties were confirmed (Examples 1 to 3, Comparative Examples 1 to 3). In addition, precipitated silica was produced using an aqueous solution of sodium silicate derived from silica sand, and its 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 widely and generally used as raw materials for wet-process silica with high whiteness. The physical properties are shown in Table 2 below.

●Production of wet-process silica In a 2L stainless steel container equipped with a stirrer, 495mL of hot water and 36mL of the alkali silicate aqueous solution to be tested were charged so that the pH was 11.8±0.2, and the temperature was raised to 75°C. While continuing stirring to maintain the temperature and pH, 366mL of the alkali silicate aqueous solution and about 108mL of 20% by mass diluted sulfuric acid were dropped at a constant flow rate over 60 minutes. Thereafter, the dropping of the alkali silicate aqueous solution was stopped, and when the pH reached 3, the dropping of the diluted sulfuric acid was also stopped to terminate the neutralization reaction. A 150mmΦ magnetic Buchner funnel (Nutsche) was attached to a 2L suction filtration bottle, and the sample solution was subjected to suction filtration using a filter paper (standard 5A, manufactured by ADVANTEC) with an opening of 8μm, further washed with 600mL of pure water, and left to dry at 125°C for 8 hours, thereby obtaining wet-process silica (precipitated silica).

●Whiteness of wet-process silica The obtained wet-process silica powder was lightly re-ground in a mortar for about 3 minutes, and then the whiteness of the wet-process silica was measured using the same method as for measuring the whiteness of plant ash. If the whiteness of the wet-process silica is 85 or more, the aqueous alkali silicate solution, which is the raw material, was rated GOOD in order to satisfy the quality as high-performance wet-process silica, and if the whiteness of the wet-process silica is less than 85, the wet-process silica was judged to be a colored product (discolored product), and the aqueous alkali silicate solution, which is the raw material, was rated NG.

● BET specific surface area of wet-process silica (reference value)
The BET specific surface area of the wet process silica powder was measured by the single point method using a fully automatic specific surface area measuring device (Macsorb HM model-1210, manufactured by Mountec Co., Ltd.).

Carbon content in wet-process silica (reference value)
The carbon content in the wet process silica was measured in the same manner as in the case of measuring the carbon content in the plant ash.

The analytical results of Examples 1 to 3, Comparative Examples 1 to 3, and Reference Example 1 are shown in Table 2 below.

<Examples and Comparative Examples in the Case of Mixing an Alkaline Silica Solution that has been Filtered and Adsorbed and an Alkaline Silica Solution Derived from Sand>
As described below, the alkali silicate a and the alkali silicate b were subjected to either or both of filtration and adsorption treatment to evaluate the effect on the quality of the wet-process silica (Examples 4 and 5 of the aqueous alkali silicate solution and Comparative Example 4). Furthermore, the quality of a mixture of an aqueous alkali silicate solution derived from plant ash and an aqueous alkali silicate solution derived from sand was examined (Examples 6 to 8 of the aqueous alkali silicate solution).

<Comparative Example 4>
A 110 mmφ magnetic Buchner funnel (Nuchsche) was attached to a 3 L suction filtration bottle, and filtration alone was performed while suctioning alkali silicate a (about 2,000 g) using filter paper (1 μm mesh, 5C standard, manufactured by ADVANTEC). Hereinafter, filtration using the filter paper is referred to as "5C filtration". That is, in Comparative Example 4, 5C filtration was performed on the alkali silicate a of Comparative Example 1 to remove solid foreign matter present in the aqueous alkali silicate solution. Furthermore, wet-process silica was produced using the alkali silicate a after 5C filtration as a raw material, and its quality was analyzed.

Example 4
20 g of commercially available powdered activated carbon was added to the alkali silicate a (about 2,000 g) and allowed to stand for 24 hours to perform adsorption treatment of the components in the sample. After that, a 150 mmΦ magnetic Buchner funnel (Nutsche) was attached to a 3 L suction filtration bottle, and 5C filtration was performed on the sample. That is, in Example 4, the alkali silicate a of Comparative Example 1 was subjected to adsorption treatment to adsorb and remove impurities in the aqueous solution, and 5C filtration was performed to remove solid foreign matter present in the aqueous solution of the alkali silicate. Furthermore, wet-process silica was produced using the alkali silicate a after 5C filtration as a raw material, and its quality was analyzed.

Example 5
A 110 mmφ magnetic Buchner funnel (Nuchsche) was attached to a 3 L suction filtration bottle, and 5C filtration was performed on alkali silicate b (about 2,000 g). Furthermore, wet process silica was produced using alkali silicate b after 5C filtration as a raw material, and its quality was analyzed.

<Examples 6 to 8>
The aqueous alkali silicate solution obtained in Example 5 after 5C filtration and the aqueous alkali silicate solution obtained in Reference Example 1 were mixed at a ratio of 4:6 (Example 6: the aqueous solution derived from plant ash was 40% by mass, the same applies below), 6:4 (Example 7) and 8:2 (Example 8) to produce aqueous alkali silicate solutions. Furthermore, wet-process silica was produced using these aqueous alkali silicate solutions as raw materials, and the quality of the resulting product was analyzed.

The analytical results of Comparative Example 4 and Examples 4 to 8 are shown in Table 3 below.

Explanation of Results
From the results of plant ash b to plant ash d shown in Table 1, it can be seen that by performing preliminary combustion and thoroughly burning the plant material, the amount of carbon-containing impurities in the plant ash due to unburned combustion is reduced. In contrast, from the results of plant ash a and plant ash e, it can be seen that a large amount of carbon-containing impurities remains due to insufficient combustion temperature. Moreover, from the results of plant ash f, it can be seen that a large amount of carbon-containing impurities remains even when burning at high temperature without preliminary combustion. This is thought to be because the surface of the plant material is vitrified by high-temperature combustion, and the inside of the material becomes unburned due to reasons such as insufficient supply of oxygen.

From the results of Examples 1 to 3 shown in Table 2, when preliminary combustion is performed and the sample is sufficiently burned to produce an alkali silicate solution using plant ash, the amount of insoluble foreign matter and water-soluble carbon compounds in the alkali silicate solution is small, since the filtration residue is small and the transmittance is high, even if the alkali silicate solution is not subjected to filtration and adsorption treatment. Furthermore, when wet-process silica is produced using such plant ash as a raw material, impurities in the wet-process silica are also reduced, and wet-process silica with a sufficient degree of whiteness is obtained. In contrast, from the results of Comparative Examples 1, 2, and 3, it is found that a large amount of carbon-containing impurities remains when an alkali silicate solution is produced using plant ash with an insufficient combustion temperature or plant ash that has simply been burned at a high temperature. Furthermore, it is found that when wet-process silica is produced using such plant ash as a raw material, impurities in the wet-process silica are also increased, and wet-process silica with a sufficient degree of whiteness is not obtained.

The results of Comparative Example 4, Example 4, and Example 5 shown in Table 3 show that the quality of the aqueous alkali silicate solution is dramatically improved by subjecting the aqueous alkali silicate solution to either or both of filtration and adsorption treatment.

In particular, a comparison of Comparative Example 1, Comparative Example 4, and Example 4 shows that by subjecting an aqueous alkali silicate solution of insufficient quality to an adsorption treatment and filtration, the quality can be improved to a level that makes it possible to produce wet-process silica with a whiteness of more than 90.

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

Figure 3 shows the transmittance spectra (wavelengths 200 nm to 800 nm) of the aqueous alkali silicate solutions in Example 1, Example 5, and Reference Example 1. Example 1 is a case where alkali silicate b is produced using plant ash b burned at a final temperature of 600°C, and then wet-process silica is produced using this as a raw material. Example 5 is a case where alkali silicate b is subjected to 5C filtration, and then wet-process silica is produced using this as a raw material. Reference Example 1 is a case where wet-process silica is produced using an aqueous alkali silicate solution derived from silica sand.

Although alkali silicate b contained 0.41% by mass of filtration residue, which was a solid foreign matter, the colored components dissolved in the alkali silicate aqueous solution were barely 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 silicate b as a raw material was 85, which was a sufficient whiteness for a highly functional wet-process silica. In Example 5, in which 5C filtration was performed on alkali silicate b, the transmittance at a wavelength of 500 nm improved to 94.4%, and the whiteness of the precipitated silica also improved to 91. The quality was comparable to the whiteness of wet-process silica produced using an aqueous solution of alkali silicate derived from sand as a raw material.

The quality of the aqueous alkali silicate solution derived from plant ash may not be as good as that of the aqueous alkali silicate solution derived from sand as in Reference Example 1. In such cases, the quality of the aqueous alkali silicate solution derived from plant ash can be further improved by mixing it with the aqueous alkali silicate solution derived from sand as necessary (Examples 6 to 8).

Figure 4 shows the transmittance spectrum (wavelengths 200 nm to 800 nm) of the aqueous alkali silicate solution in Comparative Example 3. The aqueous alkali silicate solution derived from plant ash f produced according to the method described in Patent Document 1 had a transmittance of 55.4% at a wavelength of 500 nm, and the whiteness of the wet-process silica produced using this as a raw material was 73, meaning that wet-process silica with sufficient whiteness was not obtained.

The wet-process silica synthesized using the aqueous alkali silicate solution of the present invention as a raw material is useful as an environmentally friendly aqueous alkali silicate solution, since it can have high reinforcing properties for industrial rubber and tires, and can also be used as a reinforcing filler for silicone rubber where transparency and whiteness are important, as a filler for paper (anti-bleed-through agent), as a matting agent for paint, as an abrasive for toothpaste, and as an anti-blocking agent for films (similar to conventional aqueous alkali silicate solutions derived from sand).

Claims (11)

An aqueous solution of alkali silicate in which the ratio of _{the P2O5} concentration to the _SiO2 concentration, _P2O5 / _{SiO2, expressed} in mass%, is 1.0 x ^10-3 or more, the _SiO2 concentration is 8.9 mass% to 16.7 mass%, and the transmittance (%T) at an optical path length of 1 cm and a wavelength of 500 nm is 65% or more. 2. The aqueous alkali silicate solution according to claim 1, wherein the ratio of the SiO ₂ concentration to the (Na ₂ O+K ₂ O) concentration, SiO ₂ /(Na ₂ O+K ₂ O), expressed in mol %, is 2.8 to 3.5. 3. The aqueous alkali silicate solution according to claim 1, wherein the ratio of the concentration of K ₂ O to the concentration of Na ₂ O, K ₂ O/Na ₂ O, expressed in mol %, is 0.03 to 0.30. The aqueous alkali silicate solution according to claim 1 or 2, in which, when the aqueous alkali silicate solution is filtered through a filter having a mesh size of 1 μm, the amount of solid matter captured on the filter is less than 100 ppm by mass relative to the mass of the aqueous alkali silicate solution before filtration. The aqueous alkali silicate solution according to claim 1 or 2, which is used as a raw material for wet-process silica. A method for producing an aqueous alkali silicate solution, comprising subjecting a raw alkali silicate solution obtained by mixing plant ash containing silica components, sodium hydroxide, and water to both a filtration process using a filter with a mesh size of less than 50 μm and an adsorption process using an adsorbent to obtain an aqueous alkali silicate solution. The method for producing an aqueous alkali silicate solution according to claim 6, wherein the adsorption treatment is carried out after the filtration treatment. A method for producing an aqueous alkali silicate solution, comprising subjecting a raw alkali silicate solution obtained by mixing plant ash containing silica components, sodium hydroxide, and water to a filtration process using a filter having a mesh size of 50 μm or more, and then subjecting the raw alkali silicate solution to both a filtration process using a filter having a mesh size of less than 50 μm and an adsorption process using an adsorbent to obtain an aqueous alkali silicate solution. A method for producing an aqueous alkali silicate solution, comprising: mixing plant ash containing silica and having a whiteness of 30 or more, sodium hydroxide, and water to obtain a raw alkali silicate solution; filtering the resulting solution through a filter having a mesh size of less than 50 μm to obtain an aqueous alkali silicate solution. A method for producing an aqueous alkali silicate solution, comprising mixing plant ash containing silica and having a whiteness of 30 or more, sodium hydroxide, and water to obtain a raw aqueous alkali silicate solution, and subjecting the raw aqueous alkali silicate solution to an adsorption treatment using an adsorbent to obtain an aqueous alkali silicate solution. The method for producing an alkali silicate aqueous solution according to any one of claims 6 to 10 , wherein the plant ash is rice husk ash.