Method for producing alkali silicate aqueous solution

Abstract

A method for producing an alkali silicate aqueous solution according to the present invention comprises: preparing a slurry of an alkali silicate aqueous solution using rice husk ash as a raw material; subjecting the slurry to solid-liquid separation into a residue and an alkali silicate aqueous solution; washing the residue with water; and repeating a cycle in which a post-washing solution obtained by the water washing is added to a raw material for preparing a slurry in a next cycle.

Description

Method for producing alkaline silicate aqueous solution

[0001] The present invention relates to a method for producing an aqueous alkali silicate solution. CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to Japanese Patent Application No. 2024-106877, filed July 2, 2024, the entire disclosure of which is incorporated herein by reference.

[0002] A sodium silicate aqueous solution is used as a raw material for wet-process silica. A sodium silicate aqueous solution is generally produced by mixing natural sand (e.g., silica sand, silica stone, silica clay, etc.), sodium hydroxide, and water (including the case where a sodium hydroxide aqueous solution is used) at a high temperature of approximately 70°C or higher, according to the following reaction formula: Chemical reaction formula: 2NaOH + nSiO ２ → Na ２ O.nSiO ２ +H ２ O n: Molar ratio (sodium silicate aqueous solution)

[0003] In recent years, there has been a demand for a sustainable society to protect the global environment. In the production of sodium silicate aqueous solutions, sustainable renewable silica materials have been considered as an alternative to natural sand. One such material is rice husk ash, which is produced when rice husks, which contain a high silica content, are burned in biomass boilers and biomass power generation plants (Patent Documents 1 and 2).

[0004] When rice husk ash is used as a raw material, the resulting aqueous solution usually contains potassium derived from rice husk ash in addition to sodium as an alkali metal component. Therefore, in the following explanation, a silicate compound in which the alkali metal component is specifically limited to sodium will be referred to as "sodium silicate" (Na ２ O.nSiO ２ ), and silicate compounds containing alkali metal components other than sodium are generally referred to as "alkali silicate." In the present invention and this specification, alkali silicate is meant to include sodium silicate. The "molar ratio" used as an index of the quality of sodium silicate and alkali silicate is "SiO ２ / Na ２In the case of alkali silicate, it means "SiO ２ / (Na ２ O+K ２ 0) molar ratio” (i.e., the ratio of the molar amount of silica to the total molar amount of sodium oxide and potassium oxide).

[0005] Patent Document 1: Japanese Patent Publication No. 2007-510613 (corresponding to US 2007 / 0276051 A1) Patent Document 2: Japanese Patent Publication No. 2007-522069 (corresponding to US 2008 / 0286383 A1) The entire disclosures of Patent Documents 1 and 2 are incorporated herein by reference.

[0006] When rice husk ash is used as the silica source, the aqueous alkali silicate solution can be obtained by mixing rice husk ash, sodium hydroxide, and water to prepare a slurry containing an aqueous alkali silicate solution with a predetermined silica concentration and molar ratio, and then subjecting the slurry to solid-liquid separation to separate the aqueous alkali silicate solution from the slurry. Conventionally, the residue after solid-liquid separation has been discarded as is.

[0007] However, the inventors' investigations revealed that the residue contains an aqueous alkali silicate solution in an amount approximately 2 to 5 times the mass of the solids. In typical aqueous alkali silicate solutions used in the production of wet-process silica, the molar ratio is approximately 3.0 to 4.0, and the silica concentration is approximately 13 to 28 mass%. These values ​​are relatively high, and disposal of the residue means that a considerable amount of silica is discarded as an aqueous alkali silicate solution along with the solids.

[0008] Therefore, in order to reduce waste of resources, it is desirable to increase the ratio of the amount of utilized silica to the total amount of silica in the rice husk ash prepared as a raw material (silica utilization rate). The method for calculating the silica utilization rate will also be explained in the section "Silica Utilization Rate" under <Analysis Method> in the Examples (paragraph 0075).

[0009] An object of the present invention is to provide a method for producing an aqueous alkali silicate solution using rice husk ash as a raw material, which can increase the utilization rate of silica.

[0010] The above-mentioned problem was solved by recovering silica from the residue after solid-liquid separation generated in the process of producing an aqueous alkali silicate solution, and using the recovered silica as a raw material for producing an aqueous alkali silicate solution. Specifically, the above-mentioned problem was solved by the invention [1] below, preferably by the inventions [2] and after. [1] A method for producing an aqueous alkali silicate solution comprising repeating the following steps (A) to (C) one or more times, ２ / (Na ２ O+K ２ O) A method for producing an aqueous alkali silicate solution, which obtains a target aqueous alkali silicate solution having a molar ratio in the range of 3.0 to 4.0 and a silica concentration in the range of 13 to 28 mass %: Step (A) mixing the following raw materials (1) to (3) or the following raw materials (1) to (4) to prepare a slurry containing an aqueous alkali silicate solution containing silica extracted from rice husk ash: (1) rice husk ash, (2) the aqueous alkali silicate solution supplied from step (C), (3) sodium hydroxide, (4) water, Step (B) performing solid-liquid separation of the slurry prepared in step (A) into the aqueous alkali silicate solution and a residue to obtain the target aqueous alkali silicate solution, Step (C) washing the residue obtained in step (B) with water and supplying at least a portion of the solution after washing to step (A) as raw material (2). [2] In the step (A), the raw materials are blended in a ratio that satisfies the following (a) and (b): (a) the amount of silica and potassium oxide based on the composition ratio of raw material (1), the amount of silica, the amount of sodium oxide, and the amount of potassium oxide supplied from raw material (2), and the amount of SiO calculated based on the sodium oxide equivalent of raw material (3), ２ / (Na ２ O+K ２ O) molar ratio of the SiO of the target aqueous alkali silicate solution ２ / (Na ２ O+K ２(b) the silica concentration calculated based on the amount of silica based on the composition ratio of raw material (1), the amount of silica supplied from raw material (2), and the amount of water in the raw materials is equal to or greater than the silica concentration of the target aqueous alkali silicate solution. [3] The manufacturing method according to [1] or [2], wherein in step (C), the amount of water used in washing with water is such that the silica concentration of the solution after washing is 0.5 to 15% by mass. [4] The manufacturing method according to any one of [1] to [3], wherein raw material (1) is rice husk ash having a silica content of 80% by mass or more. [5] The manufacturing method according to any one of [1] to [4], wherein no silica-based mineral is added in steps (A) to (C). [6] The manufacturing method according to any one of [1] to [5], wherein the solid-liquid separation in steps (B) and (C) comprises pressure filtration at a pressure of 0.15 to 0.8 MPa. [7] The manufacturing method according to [6], wherein the pressure filtration is performed using a filter press. [8] The manufacturing method according to [6] or [7], wherein the solid-liquid separation in step (B) and step (C) includes squeezing the residue after the pressure filtration under a pressure of 0.15 to 0.8 MPa. [9] The manufacturing method according to any one of [1] to [8], wherein the preparation of the slurry in step (A) is carried out under conditions of a temperature of 105 to 300°C and a pressure of 0.20 to 8.60 MPa.

[10] The manufacturing method according to any one of [1] to [9], wherein in the preparation of the slurry in step (A), the temperature of the mixture and the dissolution time are adjusted so that the dissolution rate of silica in the rice husk ash is 80 mass% or more. Here, the dissolution rate of silica in rice husk ash is calculated based on the following formula 2. Formula 2:

[11] The method according to any one of [1] to

[10] , wherein the rice husk ash of raw material (1) is processed rice husk ash having an apparent density of 400 to 800 g / L in a dry state and a moisture content of 30 mass% or less.

[12] In step (A), the raw materials are blended in a ratio that satisfies the following (a) and (b): (a) the amount of silica and potassium oxide based on the composition ratio of raw material (1), the amount of silica, sodium oxide, and potassium oxide supplied from raw material (2), and the amount of SiO calculated based on the sodium oxide equivalent of raw material (3). ２ / (Na ２ O+K２ O) molar ratio of the SiO of the target aqueous alkali silicate solution ２ / (Na ２ O+K ２ (b) the silica concentration calculated based on the amount of silica based on the composition ratio of raw material (1), the amount of silica supplied from raw material (2), and the amount of water in the raw materials is equal to or greater than the silica concentration of the target aqueous alkali silicate solution; in step (C), the amount of water used in washing with water is an amount such that the silica concentration of the solution after washing is 0.5 to 15 mass%; raw material (1) is rice husk ash having a silica content of 80 mass% or more; no silica-based mineral is added in steps (A) to (C); the solid-liquid separation in steps (B) and (C) comprises carrying out pressure filtration under a pressure condition of 0.15 to 0.8 MPa, the pressure filtration being carried out using a filter press; and the solid-liquid separation in steps (B) and (C) comprises squeezing the residue after the pressure filtration under a pressure condition of 0.15 to 0.8 MPa. The preparation of the slurry in step (A) is carried out under conditions of a temperature of 105 to 300°C and a pressure of 0.20 to 8.60 MPa, and in the preparation of the slurry in step (A), the temperature of the mixture and the dissolution time are adjusted so that the dissolution rate of silica in the rice husk ash is 80 mass% or more, and the dissolution rate of silica in the rice husk ash is calculated based on the following formula 2: The manufacturing method according to [1], wherein the rice husk ash of the raw material (1) is processed rice husk ash having an apparent density of 400 to 800 g / L in a dry state and a moisture content of 30 mass% or less.

[0011] According to the method for producing an aqueous alkali silicate solution of the present invention, the utilization rate of silica can be increased in the production of an aqueous alkali silicate solution using rice husk ash as a raw material.

[0012] Figure 1 is a graph showing the relationship between dissolution time and silica concentration, molar ratio, and silica dissolution rate in Reference Example 1. Figure 2 is a graph showing the relationship between dissolution time and silica concentration, molar ratio, and silica dissolution rate in Comparative Example 1. Figure 3 is a graph showing the relationship between dissolution time and silica concentration, molar ratio, and silica dissolution rate in Comparative Example 2. Figure 4 is a flow diagram showing an outline of each step in the present invention. Figure 5 is a graph showing the relationship between dissolution time and silica concentration, molar ratio, and silica dissolution rate in Example 3, compared with Comparative Example 2.

[0013] <Method for Producing an Aqueous Solution of Alkali Silicate> The method for producing an aqueous solution of alkali silicate of the present invention comprises repeating the following steps (A) to (C) one or more times, ２ / (Na ２ O+K ２ O) A method for obtaining a target aqueous alkali silicate solution having a molar ratio in the range of 3.0 to 4.0 and a silica concentration in the range of 13 to 28 mass %: Step (A) mixing the following raw materials (1) to (3) or the following raw materials (1) to (4) to prepare a slurry containing an aqueous alkali silicate solution containing silica extracted from rice husk ash: (1) rice husk ash, (2) the aqueous alkali silicate solution supplied from step (C), (3) sodium hydroxide, (4) water, Step (B) performing solid-liquid separation of the slurry prepared in step (A) into the aqueous alkali silicate solution and a residue to obtain the target aqueous alkali silicate solution, Step (C) washing the residue obtained in step (B) with water and supplying at least a portion of the solution after washing to step (A) as raw material (2).

[0014] In the production method of the present invention, a series of steps from step (A) to step (C) is defined as one cycle, and this cycle is repeated one or more times. The number of repetitions can be appropriately set taking into consideration productivity, work efficiency, equipment maintenance, etc. The number of repetitions is, for example, preferably two or more, three or more, or four or more, and more preferably ten or more.

[0015] The target aqueous alkali silicate solution obtained by the production method of the present invention has an SiO in the range of 3.0 to 4.0. ２ / (Na２ O+K ２ O) molar ratio and a silica concentration in the range of 13 to 28 mass%. By having the molar ratio and silica concentration within the above ranges, the aqueous alkali silicate solution obtained by the production method of the present invention can be used in the same applications as existing aqueous alkali silicate solutions. An aqueous alkali silicate solution having a molar ratio and silica concentration within the above ranges is particularly suitable as a raw material for producing wet-process silica (such as precipitated silica and gel-process silica). For example, in the case of precipitated silica, an aqueous alkali silicate solution with a molar ratio of 3.0 to 3.5 and a silica concentration of 10 to 18 mass% is preferably used, while in the case of gel-process silica, an aqueous alkali silicate solution with a silica concentration of 20 to 30 mass% is preferably used. The aqueous alkali silicate solution for the above-mentioned purpose can be used for these applications as is, or can be used for these applications after slight quality adjustment, for example, by adding water or commercially available silica. The wet-process silica produced in this manner can be used for many known applications, such as reinforcing fillers for rubber or tires, matting agents for paints, and toothpaste or industrial abrasives, just like conventional wet-process silica.

[0016] The molar ratio is preferably in the range of 3.1 to 3.7, more preferably in the range of 3.2 to 3.6. The silica concentration is preferably in the range of 15 to 25% by mass, more preferably in the range of 18 to 22% by mass.

[0017] Since the aqueous alkali silicate solution of the present invention is produced from rice husk ash, it contains a trace amount of potassium. In the aqueous alkali silicate solution produced using rice husk ash, for example, the molar ratio K of the potassium oxide concentration to the sodium oxide concentration, expressed in mol%, is ２ O / Na ２ Therefore, the alkali silicate aqueous solution produced using rice husk ash has a molar ratio K ２ O / Na ２ Based on the molar ratio K, it can be distinguished from aqueous alkali silicate solutions prepared using silica-based minerals (minerals containing silica components, such as silica sand, silica stone, and silicate clay). ２ O / Na ２ The molar ratio K is preferably 0.30 or less.２ O / Na ２ When O is 0.30 or less, the neutralization reaction is less likely to vary when neutralizing and synthesizing wet-process silica, and the quality of the wet-process silica is more likely to be stable.

[0018] Step (A) Step (A) is a step of mixing raw materials (1) to (3) or raw materials (1) to (4) to prepare a slurry containing an aqueous alkali silicate solution containing silica extracted from rice husk ash.

[0019] The raw material (1) is rice husk ash, a silica source. Rice husk ash contains a large amount of silica and has been attracting attention as a renewable material in the current era of demands for a sustainable society. The rice husk ash preferably has an apparent density of 400 to 800 g / L in a dry state and a moisture content of 30% by mass or less.

[0020] The apparent density of rice husk ash can be increased by subjecting it to compaction or pulverization. Typically, the apparent density of unprocessed rice husk ash after combustion is 350 g / L or less, often 300 g / L or less. In other words, the rice husk ash used in the production method of the present invention preferably has a higher apparent density than unprocessed rice husk ash. Rice husk ash with a moderately high apparent density and a moderate weight improves its ability to sink into the solution and reduces solvent absorption. As a result, a larger amount of rice husk ash can be added to the container compared to unprocessed rice husk ash, which has the advantage of making it easier to increase the silica concentration. Rice husk ash with a high apparent density is particularly useful when preparing a slurry in a small-capacity container. Furthermore, because the moisture content is 30% by mass or less, rice husk ash easily maintains a weight within a range suitable for transportation and a moderate solid state. In addition, rice husk ash has a moderately high apparent density, which makes it less bulky and improves transportation and storage efficiency.

[0021] From the viewpoint of further improving the transport efficiency, storage efficiency, and sinking property in a solution, and making it easier to obtain a high-concentration slurry, the lower limit of the apparent density range is preferably 450 g / L or more, more preferably 500 g / L or more, and even more preferably 550 g / L or more. From the viewpoint of further improving the sinking property in a solution, the upper limit of the apparent density range is preferably 750 g / L or less, more preferably 730 g / L or less, and even more preferably 720 g / L or less.

[0022] The apparent density is measured in a dry state (generally with a moisture content of less than 1.5% by mass). Unprocessed rice husk ash after combustion is usually in a dry state, and the apparent density can be measured as is. The apparent density of rice husk ash that is not in a dry state can be measured after sufficient drying treatment (e.g., 125°C, 6 hours).

[0023] The moisture content of the rice husk ash may be 25% by mass or less, 20% by mass or less, 15% by mass or less, or 10% by mass or less. When the rice husk ash contains a certain amount of moisture, dust scattering can be suppressed, and handling during packaging, transportation, and unpacking, as well as handling when the rice husk ash is added to an aqueous solution, are improved. In addition, when the rice husk ash contains a certain amount of moisture, the rice husk ash's ability to sink into a solution is improved due to a moderate increase in weight. The moisture content may be 0.1% by mass or more, 0.5% by mass or more, 0.7% by mass or more, or 1.0% by mass or more. The moisture content can be adjusted by any process, such as adding water to the rice husk ash or drying it. The moisture content of the rice husk ash does not need to be adjusted at all, i.e., no water needs to be added at all.

[0024] The silica and carbon contents in rice husk ash are not particularly limited. However, since rice husk ash is used as a silica source, a high silica content is preferable, and a low content of carbon impurities is preferable. The silica content in rice husk ash varies depending on the type of rice, the growing region, and the combustion method of rice husk ash (boiler type, combustion conditions, etc.). For example, when the combustion temperature of rice husks is about 500°C, the silica content tends to decrease and become less than 70% by mass. Furthermore, when the combustion temperature of rice husks is higher (e.g., 600°C or higher), rice husk ash with a higher silica content is likely to be obtained. The silica content in rice husk ash is, for example, 70% by mass or more, preferably 80% by mass or more or 85% by mass or more, and more preferably 90% by mass or more. The upper limit of the silica content is typically approximately 98% by mass or less. The silica content in rice husk ash can be measured, for example, using an X-ray fluorescence analyzer or an ICP emission analyzer. The carbon content in rice husk ash is preferably 0.1 to 8.0% by mass. The carbon content in rice husk ash discharged from general combustion equipment is generally 0.1% by mass or more. The carbon content in rice husk ash may be 0.3% by mass or more, 0.5% by mass or more, or 1.0% by mass or more. When the carbon content in rice husk ash is 8.0% by mass or less, the incomplete combustion components of the rice husk are reduced and the quality of the rice husk ash (e.g., the content of silica components) is improved. The carbon content in rice husk ash is preferably 6.0% by mass or less, 5.0% by mass or less, or 4.0% by mass or less. The carbon content in rice husk ash is more likely to decrease as the combustion temperature of the rice husk increases. The carbon content in rice husk ash can be measured, for example, using a carbon analyzer.

[0025] For rice husk ash, the volume average particle diameter D50 measured by laser diffraction is preferably in the range of 5.0 to 200 μm. A volume average particle diameter D50 of 5.0 to 200 μm facilitates maintaining the apparent density in the range of 400 to 800 g / L. Furthermore, a volume average particle diameter D50 of 5.0 μm or greater suppresses the generation of excessive dust, further improving handling. Furthermore, a volume average particle diameter D50 of 5.0 to 200 μm ensures that the amount of void space between particles is within an appropriate range, allowing the rice husk ash to adequately absorb the solution when added to the solution. As a result, combined with the appropriate weight, the time required for the rice husk ash to sink into the solution is shortened, further improving sinking properties. Since it is preferable that the proportion of fine particles of 1.0 μm or less in the particle size distribution is as low as possible, the volume average particle diameter D50 is preferably 10 to 150 μm, more preferably 15 to 100 μm, and particularly preferably 20 to 80 μm.

[0026] Particle size distribution measurement can be performed using a commercially available laser diffraction particle size distribution analyzer. Specifically, particle size distribution measurement is performed by dispersing a sample in water to a predetermined concentration, introducing the dispersion into the circulation system of the measuring device, and adopting the value when the D50 value stabilizes as the measured value. When rice husk ash is processed into a compact by compaction as described below, it is preferable to adjust the compaction conditions so that the volume average particle diameter D50 measured by the above method is in the range of 5.0 to 200 μm.

[0027] The rice husk ash may be in the form of either powder or a molded body, and the shape of the molded body is not particularly limited.

[0028] The rice husk ash having a high apparent density is obtained by subjecting raw rice husk ash to a compaction or pulverization process. Compaction refers to the process of compressing rice husk ash, and pulverization refers to the process of crushing rice husk ash. The processing may be either compaction or pulverization, or both processes may be performed simultaneously or sequentially. Depending on the type and conditions of the processing, it may be difficult to distinguish between compaction and pulverization, so it is not necessary to distinguish between them.

[0029] The types of compaction and pulverization are not particularly limited. The pulverization process may be dry or wet. Dry processing tends to generate dust, but does not require a pretreatment process, allowing for efficient processing. Wet processing requires pretreatment, such as a slurrying process, but can suppress dust generation. Dry pulverization can be carried out using commercially available dry pulverizers, such as roller mills, high-speed rotary mills (pin mills), jet mills, ball mills, and bead mills. Wet pulverization can be carried out using commercially available wet pulverizers, such as in-line mixers, jet mills, ball mills, and bead mills. Pulverization can destroy plant-derived structures in rice husk ash and reduce interparticle voids, thereby increasing the apparent density of the rice husk ash. Compaction can be carried out using commercially available granulators, such as roller compactors and tablet presses using compression rollers or briquetting rollers. Compaction can compress the rice husk ash and reduce the voids between particles, thereby increasing the apparent density of the rice husk ash while suppressing dust generation. When both compaction and pulverization are performed, the plant-derived structures in the rice husk ash can be destroyed and the rice husk ash can be compacted, making it easier to obtain a higher apparent density while suppressing dust generation.

[0030] The raw material (2) is the aqueous alkali silicate solution supplied from step (C). The raw material (2) contains silica recovered from residues generated during the production of the aqueous alkali silicate solution using rice husk ash as a raw material. By using the raw material (2) in the production of the aqueous alkali silicate solution, it is possible to reduce resource waste and increase the silica utilization rate.

[0031] As described below, raw material (2) is a solution obtained by washing the residue after solid-liquid separation in obtaining the target aqueous alkali silicate solution, and can therefore be considered a solution obtained by diluting the target aqueous alkali silicate solution with water. Since the molar amounts of silica and alkali metal components do not change even when diluted, the molar ratio of raw material (2) is the same as that of the target aqueous alkali silicate solution. However, due to dilution, the silica concentration of raw material (2) is lower than that of the target aqueous alkali silicate solution.

[0032] The amount of raw material (2) used can be adjusted appropriately depending on the molar ratio and silica concentration of the target aqueous alkali silicate solution and the molar ratio and silica concentration of raw material (2). The amount of raw material (2) used is, for example, 100 to 500 parts by mass, preferably 150 to 450 parts by mass, and more preferably 200 to 400 parts by mass, per 100 parts by mass of rice husk ash. When the amount of raw material (2) used is within the above range, the need to change the production conditions of the aqueous alkali silicate solution (particularly the blending balance of the raw materials in step (A)) for each cycle is reduced, and continuous production of the aqueous alkali silicate solution is promoted.

[0033] Raw material (3) is sodium hydroxide, and raw material (4) is water. Raw materials (3) and (4) may be added in the form of an aqueous sodium hydroxide solution. Raw materials (3) and (4) function as a solvent for dissolving rice husk ash. Since water is contained in raw material (2), if the water contained in raw material (2) is sufficient, raw material (4) does not need to be added separately.

[0034] The amount of raw material (3) used can be adjusted appropriately depending on the molar ratio and silica concentration of the target aqueous alkali silicate solution. The amount of raw material (3) used is, for example, 5 to 50 parts by mass, preferably 10 to 40 parts by mass, and more preferably 15 to 35 parts by mass, per 100 parts by mass of rice husk ash. The amount of raw material (4) used can be adjusted appropriately depending on the molar ratio and silica concentration of the target aqueous alkali silicate solution. The amount of raw material (4) used is, for example, 5 to 100 parts by mass, preferably 10 to 70 parts by mass, and more preferably 15 to 50 parts by mass, per 100 parts by mass of rice husk ash. When the amounts of raw materials (3) and (4) used are within the above ranges, the need to change the production conditions for the aqueous alkali silicate solution (particularly the blending balance of the raw materials in step (A)) for each cycle is reduced, and continuous production of the aqueous alkali silicate solution is promoted.

[0035] The temperature of the water in the raw material (4) is preferably warm water from the viewpoint of promoting dissolution of silica in the rice husk ash. The temperature of the warm water is, for example, 40 to 95°C, preferably 50 to 85°C, and may be 60 to 80°C. From the viewpoint of reducing energy consumption during production, it is preferable to use warm water that utilizes heat recovery.

[0036] In the production method of the present invention, raw materials other than raw materials (1) to (4) can be added. Examples of such raw materials include silica-based minerals that serve as additional silica sources. From the perspective of realizing a sustainable society, it is preferable to add a small amount of silica-based minerals, and it is even more preferable to add no silica-based minerals. That is, in the production method of the present invention, it is preferable not to add silica-based minerals in step (A), and further, in steps (A) to (C). In the production method of the present invention, only raw materials (1) and (2) are used as silica sources throughout the cycle of steps (A) to (C), making it possible to continuously produce an aqueous alkali silicate solution.

[0037] It is preferable to blend the raw materials by predicting the dissolution rate of silica in rice husk ash in advance and balancing the blending of raw materials (1) to (3) or raw materials (1) to (4) so ​​that the target molar ratio (3.0 to 4.0) and silica concentration (13 to 28 mass%) are obtained when the silica dissolution rate reaches the predicted value. By doing so, at the end of step (A), the aqueous alkali silicate solution in the mixture has a SiO in the range of 3.0 to 4.0. ２ / (Na ２ O+K ２ The silica solubility in rice husk ash is the ratio of the mass of silica dissolved in the aqueous solution to the total mass of silica in the rice husk ash, and a detailed calculation method will be described in the Examples section.

[0038] In step (A), the raw materials are preferably blended in a ratio that satisfies the following conditions (a) and (b). Condition (a) specifies that, assuming that the dissolution rate of silica in rice husk ash is 100% (all of the silica in the rice husk ash is dissolved), the molar ratio based on silica, sodium oxide, and potassium oxide supplied from raw materials (1) to (3) is equal to or greater than the molar ratio of the target aqueous alkali silicate solution. Condition (b) specifies that, assuming that the dissolution rate of silica in rice husk ash is 100%, the silica concentration based on silica supplied from raw materials (1) and (2) is equal to or greater than the silica concentration of the target aqueous alkali silicate solution. (a) The SiO2 content calculated based on the amounts of silica and potassium oxide based on the composition ratio of raw material (1), the amounts of silica, sodium oxide, and potassium oxide supplied from raw material (2), and the sodium oxide equivalent amount of raw material (3) is ２ / (Na ２ O+K ２ O) molar ratio of the SiO of the target aqueous alkali silicate solution ２ / (Na ２ O+K ２ (b) the silica concentration calculated based on the amount of silica based on the composition ratio of raw material (1), the amount of silica supplied from raw material (2), and the amount of water in the raw materials is equal to or greater than the silica concentration of the target aqueous alkali silicate solution.

[0039] In other words, conditions (a) and (b) stipulate that the amount of silica required to produce the target aqueous alkali silicate solution must be ensured solely from the amounts of silica supplied from raw materials (1) and (2). When conditions (a) and (b) are satisfied, the aqueous alkali silicate solution in the slurry will have the target molar ratio and silica concentration when the required amount of silica has been dissolved from the rice husk ash. As a result, the aqueous alkali silicate solution obtained as the filtrate in step (B) will directly become the target aqueous alkali silicate solution.

[0040] The solubility of silica in rice husk ash is preferably 80% by mass or more. From the viewpoint of reducing resource waste, the solubility of silica is preferably 83% or more, more preferably 85% or more. The upper limit of the solubility of silica is not particularly limited, and may be, for example, 90% or less, 95% or less, or 98% or less. A more accurate molar ratio can be calculated by taking into account the solubility of potassium oxide in rice husk ash in addition to the solubility of silica. Potassium oxide in rice husk ash quickly dissolves in water when exposed to water following the dissolution of silica, so the solubility of silica can be used. However, the amount of potassium oxide in rice husk ash is minute compared to the amount of silica (see, for example, Table 1 below), and the impact of differences in solubility is small.

[0041] The temperature and pressure when preparing the slurry by mixing the raw materials (1) to (4) can be adjusted as appropriate. From the viewpoint of promoting the dissolution of the rice husk ash, it is preferable to prepare the slurry under high-temperature and pressurized conditions, and in this case, it is particularly preferable to perform the preparation under conditions of a temperature of 105 to 300°C and a pressure of 0.20 to 8.60 MPa. The temperature when preparing the slurry is preferably 110 to 250°C, more preferably 115 to 200°C. The pressure when preparing the slurry is preferably 0.25 to 5.00 MPa, more preferably 0.30 to 2.00 MPa. The preparation of the slurry under high-temperature and pressurized conditions can be performed using a pressure vessel such as an autoclave.

[0042] The dissolution time of the raw materials when preparing a slurry by mixing raw materials (1) to (4) can be adjusted as appropriate. The slurry preparation in step (A) may be terminated when the molar ratio of the aqueous alkali silicate solution and the silica concentration in the slurry reach the target values. From the viewpoint of the dissolution rate of silica in rice husk ash and productivity, the dissolution time is 0.5 to 8 hours, preferably 1 to 7 hours, and more preferably 1 to 5 hours. The dissolution time tends to be shorter when dissolving under high-temperature and pressurized conditions than when dissolving under normal pressure. In particular, when preparing the slurry, it is preferable to adjust the temperature and dissolution time of the mixture so that the dissolution rate of silica in rice husk ash is 80% by mass or more.

[0043] In the production method of the present invention, the preceding step (C) to step (A) of the first cycle may be a step of dissolving rice husk ash by a conventional method, i.e., without using raw material (2), washing the residue obtained by solid-liquid separation, and obtaining a solution after this washing. That is, raw material (2) in step (A) of the first cycle may be a solution obtained by washing the residue obtained by a general method for dissolving rice husk ash.

[0044] Step (B) Step (B) is a step of subjecting the slurry prepared in step (A) to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain the target aqueous alkali silicate solution.

[0045] The method of solid-liquid separation is not particularly limited. Solid-liquid separation can be performed using a general solid-liquid separation device. Solid-liquid separation may be performed only once, or may be performed multiple times from the viewpoint of removing more foreign matter.

[0046] The solid-liquid separation is preferably carried out using a filtration device, which may be appropriately selected from known devices such as a centrifugal separator belonging to gravity filtration, a filter press and a sealed multistage filter belonging to pressure filtration, and a Nutsche, drum filter, or belt filter belonging to vacuum filtration.

[0047] From the overall viewpoint of filtering capacity, separation performance, etc., solid-liquid separation is preferably carried out by a filter press.

[0048] Solid-liquid separation using a filter press preferably includes pressure filtration at a pressure of 0.15 to 0.8 MPa. The pressure in the pressure filtration is preferably 0.2 to 0.7 MPa, more preferably 0.3 to 0.7 MPa.

[0049] Furthermore, after solid-liquid separation using a filter press, the method preferably includes squeezing the residue under a pressure of 0.15 to 0.8 MPa. By squeezing the residue, the aqueous alkali silicate solution remaining in the residue can be squeezed out, further increasing the amount of silica that can be recovered from the residue. The pressure in squeezing is preferably 0.2 to 0.7 MPa, and more preferably 0.3 to 0.7 MPa. From the viewpoint of facilitating water washing in the subsequent step (C), the squeezing in step (B) is preferably carried out at a pressure lower than that carried out after water washing. This makes it easier for the wash water to penetrate the residue after squeezing.

[0050] In step (B), the slurry is subjected to solid-liquid separation to obtain the target aqueous alkali silicate solution and a residue.

[0051] After the target aqueous alkali silicate solution is obtained in step (B), the quality of the aqueous alkali silicate solution may be adjusted as necessary after step (B). This quality adjustment includes, for example, adding a silica material such as silica sand or commercially available amorphous silica to adjust the silica concentration, and adding other aqueous alkali silicate solutions, sodium hydroxide, and known additives to adjust the molar ratio, pH, and other properties.

[0052] Step (C) Step (C) is a step in which the residue obtained in step (B) is washed with water, and at least a portion of the post-washing solution is supplied to step (A) as raw material (2). The present inventors have found that an aqueous alkali silicate solution remains in the residue in an amount approximately 2 to 5 times the mass of the solid content. In step (C), at least a portion of the aqueous alkali silicate solution remaining in the residue is extracted with water to recover silica. The "post-washing solution" refers to the solution discharged after water washing (including squeezing), and in the present invention, the post-washing solution contains the aqueous alkali silicate solution extracted from the residue. In the production method of the present invention, the utilization rate of silica can be increased by supplying at least a portion of the post-washing solution to step (A) as raw material (2).

[0053] In the production method of the present invention, the residue produced in step (B) or step (C) of the final cycle can be discarded.

[0054] The post-washing solution is a solution obtained by diluting the target aqueous alkali silicate solution with water. Because it is merely diluted, the molar ratio of silica to alkali metal components in the post-washing solution is the same as that of the target aqueous alkali silicate solution. In contrast, due to dilution, the silica concentration in the post-washing solution is lower than that of the target aqueous alkali silicate solution. The amount of water used during water washing is preferably an amount that results in a silica concentration of 0.5 to 15% by mass. A silica concentration of 0.5% by mass or more promotes use of the entire post-washing solution and ensures a sufficient silica content, making it easier to balance the formulation in step (A). Furthermore, a silica concentration of 15% by mass or less ensures a sufficient amount of water is used for water washing, facilitating extraction of the aqueous alkali silicate solution from the residue and increasing the amount of silica recovered. The amount of water used during water washing is more preferably an amount that results in a silica concentration of 1.0 to 10% by mass, and even more preferably an amount that results in a silica concentration of 1.5 to 6% by mass.

[0055] In addition, from the viewpoint that the silica concentration in the solution after water washing is in the range of 0.5 to 15 mass%, the amount of water used for water washing is preferably 100 to 700 parts by mass, and more preferably 200 to 600 parts by mass, per 100 parts by mass of the residue.

[0056] The temperature of the water used for washing is not particularly limited, and may be room temperature water or warm water. Warm water is preferred from the viewpoint of further increasing the amount of aqueous alkali silicate solution extracted from the residue. The temperature of the warm water is, for example, 30 to 80°C, preferably 40 to 75°C, or may be 50 to 70°C. From the viewpoint of reducing energy consumption during production, it is preferable to use warm water that utilizes heat recovery.

[0057] Furthermore, after washing with water, it is preferable to include squeezing the residue under a pressure of 0.15 to 0.8 MPa, as in step (B). By squeezing the residue, the aqueous alkali silicate solution remaining in the residue can be squeezed out, further increasing the amount of silica that can be recovered from the residue. The pressure in squeezing is preferably 0.2 to 0.7 MPa, more preferably 0.3 to 0.7 MPa.

[0058] In step (C), at least a portion of the post-washing solution obtained by washing with water is supplied to step (A) as raw material (2). From the viewpoint of reducing waste of resources, it is preferable that the amount of post-washing solution supplied to step (A) is large. For example, the amount supplied to step (A) is 50 mass % or more of the post-washing solution, more preferably 75 mass % or more, and it is even more preferable that the entire post-washing solution is supplied to step (A) as raw material (2).

[0059] <Effects of the Invention> The method for producing an alkali silicate aqueous solution of the present invention can increase the silica utilization rate in the production of an alkali silicate aqueous solution using rice husk ash as a raw material. Specifically, the production method of the present invention can increase the silica utilization rate to 70% or more, preferably 75% or more, and more preferably 80% or more.

[0060] When producing an aqueous alkali silicate solution with a molar ratio of 3.0 or more and a silica concentration of 18% by mass or more, conventional slurry preparation methods typically involve charging an excess amount of rice husk ash and stopping the slurry preparation when the molar ratio and silica concentration reach the target values. This is because, as the dissolution rate of silica in rice husk ash approaches 100% (e.g., Reference Example 1), the dissolution rate of silica drops dramatically, and even if the slurry is prepared assuming a 100% dissolution rate, it takes a long time for the dissolution rate to reach 100%, resulting in reduced productivity.

[0061] The above-described slurry preparation method assumes that a certain amount of rice husk ash remains undissolved as solids, and a large amount of residue is generated after solid-liquid separation. The inventors have found that the residue contains an aqueous alkali silicate solution with a mass approximately 2 to 5 times the mass of the solids. In particular, when the silica dissolution rate is set low during slurry preparation, the amount of aqueous alkali silicate solution remaining in the residue tends to increase in conjunction with an increase in the solids content in the residue. Such residue has traditionally been discarded, and much of the silica is not utilized in the production of an aqueous alkali silicate solution (low silica utilization rate).

[0062] The present inventors have therefore conducted extensive research into a method for increasing the utilization rate of silica throughout the entire production process of an aqueous alkali silicate solution, and have arrived at the present invention. In the present invention, at least a portion of the post-wash solution obtained after washing the residue with water is supplied to step (A) as raw material (2). The post-wash solution contains an aqueous alkali silicate solution extracted from the residue. In other words, in the present invention, silica contained in the aqueous alkali silicate solution, which has conventionally been discarded together with the residue, is recovered and used as a raw material for producing the aqueous alkali silicate solution, thereby achieving a higher utilization rate of silica than in conventional methods.

[0063] In particular, by taking into consideration the dissolution rate of rice husk ash in preparing the slurry and balancing the blending of raw materials (1) to (3) or raw materials (1) to (4), it is possible to continuously produce an aqueous alkali silicate solution using only raw materials (1) and (2) as the silica source throughout the cycle of steps (A) to (C).

[0064] The present invention will be explained in more detail below with reference to examples. The materials, amounts used, ratios, processing details, and processing procedures shown in the examples can be changed as appropriate within the scope of solving the problems of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below. In the examples, "parts" and "%" are based on mass unless otherwise specified.

[0065] <Analysis Method> Moisture content of rice husk ash (heat loss) (mass %) The moisture content was determined from the weight loss value after drying at 105°C for 2 hours, based on JIS K5101-15-1 (pigment test method - heat loss).

[0066] Apparent density (g / L) of rice husk ash was determined based on JIS K5101-12-1:2004 (Pigment testing method - apparent density or apparent specific volume - static method) using a dedicated measuring device (a sieve with 0.5 mm openings, a funnel, a 30 mL cylinder receiver, a receiver stand, and a funnel stand). The sample (rice husk ash) was dropped from the sieve into the funnel with a brush, and the heaped portion of the sample that had accumulated in the receiver was scraped off with a spatula, after which the mass of the sample was measured. The apparent density of rice husk ash, expressed in units of g / L, was calculated based on its mass using the following formula. Formula 3:

[0067] Bulk density (g / mL) of rice husk ash was measured based on Section "7.8 Bulk Density" of JIS K6220-1:2015 (Rubber Compounding Agents - Organic Chemicals - Test Methods - Part 1: General) using dedicated measuring equipment (a cylinder made of general steel with an inner diameter of 22.00 ± 0.05 mm and an internal depth of 100 mm, and a hollow piston with an outer diameter of 21.80 ± 0.05 mm, a length of 115 mm, and a mass of 190 g). The piston was allowed to fall naturally into the cylinder before the sample (rice husk ash) was placed, and the height of the piston protruding from the top of the cylinder was measured. Next, approximately 1 g of the weighed sample was placed in the cylinder, and the piston was slowly lowered over 5 seconds. The side wall of the cylinder was lightly tapped with a piece of wood to ensure the piston was fully seated, and the height of the piston protruding from the top of the cylinder was measured. The bulk density (g / mL) was calculated using the change in the height of the piston protruding from the top of the cylinder and the area of ​​the base of the cylinder using the following formula: Formula 4:

[0068] Volume average particle size (D50) of rice husk ash The particle size distribution was measured using a laser diffraction particle size distribution analyzer (model: SYNC30, manufactured by Microtrac Bell), and the 50% value (D50) of the volume integrated cumulative value in the particle size distribution was calculated. The sample was directly placed in the analyzer and measured without undergoing ultrasonic dispersion treatment or the like.

[0069] Carbon content (mass%) in rice husk ash or solid content: As a pretreatment, rice husk ash was dried at 105°C for 2 hours. After that, the carbon content was measured using a carbon analyzer (model: CS744, manufactured by LECO Japan LLC) that uses combustion in an oxygen stream and non-dispersive infrared absorption. The sample was heated at a temperature of 1,350°C, oxygen inlet pressure of 0.24 MPa, and measurement time of 50 seconds. The carbon content was measured using an infrared detector (NDIR) in the analyzer. ２ The gas was measured by quantifying it.

[0070] Composition (mass %) of inorganic components other than carbon in rice husk ash or solid content Using a wavelength dispersive X-ray fluorescence analyzer (model: ZSX Primus II, manufactured by Rigaku Corporation), a qualitative analysis was first performed on elements excluding carbon to confirm the types of inorganic impurities detected, and then a quantitative analysis of the detected inorganic impurities was performed. The measurement sample was prepared by placing rice husk ash in a ring-shaped mold and molding it under pressure. The quantified inorganic component composition (excluding carbon) was converted to oxides using the analysis software provided with the device to determine the concentration (mass %). Note that when it was difficult to mold the measurement sample, a commercially available binder for wavelength dispersive X-ray fluorescence analysis was added and molded before measurement.

[0071] Aqueous solution composition: An aqueous solution sample of alkali silicate (approximately 0.2 mL) was collected using a syringe equipped with a membrane filter with 1 μm openings and diluted 10,000 times with pure water (standard solution for calibration curve: 20 ppm by mass). The SiO ２ , Na ２ O and K ２ The mass concentrations of the components, such as O, were quantified.

[0072] Silica concentration and molar ratio in alkali silicate aqueous solution SiO obtained by measuring the component composition of the aqueous solution ２ , Na ２ O and K ２ The mass concentration of O was converted to a molar amount to obtain SiO ２ / (Na ２ O+K ２ Specifically, the molar mass of each component was 60.08 (SiO ２ ), 61.98 (Na ２ O) and 94.19 (K ２ O), and the molar ratio was calculated by the following formula: Formula 5:

[0073] Mass of solids and residual solution in residue after filtration The residue after filtration was washed with an equivalent amount of distilled water to remove the remaining aqueous alkali silicate solution, and the washed residue was dried in a dryer for a sufficient time (125°C x 6 hours) until the water had evaporated, after which the mass of solids contained in the residue after filtration was measured. The mass of the residual solution in the residue was calculated by subtracting the mass of the solids from the total mass of the residue after filtration.

[0074] Silica dissolution rate The silica dissolution rate was calculated using the following formula based on the total silica mass in the rice husk ash and the silica mass dissolved in the alkali silicate aqueous solution at each stage during or after dissolution. The silica mass dissolved in the alkali silicate aqueous solution was calculated from the silica concentration (mass%) of the aqueous solution at each stage and the amount of water used. Formula 2:

[0075] Silica Utilization Rate The silica utilization rate was calculated using the following formula based on the total silica mass in the rice husk ash and the silica mass used in the production of the alkali silicate aqueous solution. The silica mass used in the production of the alkali silicate aqueous solution is the sum of the mass of silica contained in the target alkali silicate aqueous solution and the mass of silica recovered from the residue and used as a recycled raw material for the production of the alkali silicate aqueous solution; in other words, it is the mass of silica that was input as rice husk ash and not discarded. Formula 1:

[0076] <Preparation of Rice Husk Ash Raw Material> Rice husk ash (apparent density: 300 g / L) discharged from a biomass boiler was prepared. It was compacted using a roller compactor (model: FR125x40, manufactured by Freund Turbo) at a roll rotation speed of 9 rpm and a linear pressure of 5.8 x 10 ３ This rice husk ash was compressed and granulated under conditions of a pressure of 100 N / cm and a screw rotation speed of 90 rpm to obtain a processed rice husk ash (hereinafter also referred to as "rice husk ash A") with an apparent density of 590 g / L. Rice husk ash A was used as the rice husk ash raw material in the production of the following alkaline silicate aqueous solution. The properties of rice husk ash A are shown in Table 1.

[0077]

[0078] Reference Example 1 First, the solubility of silica in aqueous sodium hydroxide solution in rice husk ash A was investigated. Assuming that 100% of the silica present in rice husk ash A was dissolved in aqueous sodium hydroxide solution (silica solubility 100%), raw materials were prepared with a composition such that the silica concentration and molar ratio were the target values ​​of 18.0 mass% and 3.3, respectively.

[0079] Specifically, 120 L of warm water (40-50 ° C water, the same applies below) and 26.42 kg of 48.0 mass% sodium hydroxide aqueous solution were added to a 240 L stainless steel dissolution tank equipped with a stirrer, and 36.00 kg of rice husk ash A was added over 20 minutes while continuing to stir. The mass ratio of rice husk ash A to the total mass of the raw materials was approximately 19.7%. Then, the temperature of the slurry was raised to 90 ° C while stirring, and stirring was continued for 8 hours while maintaining the liquid temperature at 90 ° C to obtain an alkali silicate slurry. During the dissolution of the rice husk ash, approximately 20 mL of sample was taken as appropriate, and after filtering with 5C filter paper (manufactured by ADVANTEC), the silica concentration and molar ratio of the resulting alkali silicate aqueous solution were measured.

[0080] The results of Reference Example 1 are shown in Table 2 and Figure 1. Although the liquid temperature was maintained at 90°C and stirring was carried out for 8 hours with the aim of achieving a silica dissolution rate of 100%, the silica dissolution rate remained at about 94.0%, and the silica concentration did not reach the target value of 18.0 mass%. Because the amount of sodium hydroxide aqueous solution added was adjusted so that the molar ratio would reach the target value when the silica dissolution rate was 100%, the molar ratio also did not reach the target value of 3.3.

[0081]

[0082] When the alkali silicate slurry obtained in Reference Example 1 after 8 hours is filtered to obtain an alkali silicate aqueous solution, a high silica utilization rate can be expected. However, there is a drawback in that the physical properties of the alkali silicate aqueous solution do not reach the target values, despite the long time required to dissolve the raw materials. In order to achieve the target values ​​for the physical properties of the alkali silicate aqueous solution, further dissolution time is required. The formulation in the dissolution method of Reference Example 1 needs to be improved from the viewpoint of improving productivity.

[0083] Comparative Example 1 Comparative Example 1 is an example of a dissolution method in which the raw material blending is improved so that the silica concentration and molar ratio of the alkali silicate aqueous solution reach the target values ​​(18.0 mass% and 3.3, respectively) in a shorter dissolution time than in Reference Example 1, specifically, in a dissolution time of 2 hours (assumed dissolution rate of approximately 78%). Based on the results of Reference Example 1, and taking into consideration that the silica dissolution rate in rice husk ash A reaches 78.0% after 2 hours, the ratio of the mass of rice husk ash A to the total mass of the raw materials in Comparative Example 1 was set to approximately 1.22 times that of Reference Example 1. The ratio of the amount of sodium hydroxide aqueous solution to the estimated amount of dissolved silica in Comparative Example 1 is the same as that in Reference Example 1.

[0084] The procedure for preparing the alkali silicate aqueous solution in Comparative Example 1 is as follows. Step (A): Mixing of raw materials and preparation of slurry. 93.50 kg of hot water and 20.43 kg of 48.0 mass% sodium hydroxide aqueous solution were placed in a 240 L stainless steel dissolution tank equipped with a stirrer, and 36.00 kg of rice husk ash A was added over 20 minutes while continuing to stir. The mass ratio of rice husk ash A to the total mass of raw materials was approximately 24.0%. The temperature of the slurry was then raised to 90 ° C while stirring, and stirring was continued for 2 hours while maintaining the liquid temperature at 90 ° C to obtain an alkali silicate slurry. During the dissolution of the rice husk ash, approximately 20 mL of sample was taken as appropriate, and after filtering with 5C filter paper (manufactured by ADVANTEC), the silica concentration and molar ratio of the resulting alkali silicate aqueous solution were measured. Step (B): Solid-Liquid Separation of Slurry The slurry obtained in step (A) was filtered using a filter press under a charge pressure of 0.4 MPa to obtain an aqueous alkali silicate solution and a residue.

[0085] The dissolution method using the raw material blend of Comparative Example 1 is generally the same as the method conventionally used when rice husk ash is used as the silica raw material. The mass ratio of rice husk ash A to the total mass of the raw materials can be appropriately adjusted depending on the dissolution time setting. Assuming a silica dissolution rate of less than 100% as described above, undissolved silica will remain in the rice husk ash residue.

[0086] Comparative Example 2 (Study on Improvement of Silica Utilization Rate) Comparative Example 2 is an example of a dissolution method improved based on the results of Reference Example 1 so as to improve the silica dissolution rate and utilization rate compared to Comparative Example 1. Specifically, the raw material blend was adjusted so that the silica concentration and molar ratio of the aqueous alkali silicate solution would reach the target values ​​(18.0 mass % and 3.3, respectively) within a dissolution time of 4 hours (assumed dissolution rate of approximately 85%).

[0087] The procedure for preparing the alkali silicate aqueous solution in Comparative Example 2 is as follows. Step (A): Mixing of raw materials and preparation of slurry. 101.90 kg of hot water and 22.33 kg of 48.0 mass% sodium hydroxide aqueous solution were placed in a 240 L stainless steel dissolution tank equipped with a stirrer, and 36.00 kg of rice husk ash A was added over 20 minutes while continuing to stir. The mass ratio of rice husk ash A to the total mass of raw materials was approximately 22.5%. The temperature of the slurry was then raised to 90 ° C while stirring, and stirring was continued for 4 hours while maintaining the liquid temperature at 90 ° C to obtain an alkali silicate slurry. During the dissolution of the rice husk ash, approximately 20 mL of sample was taken as appropriate, and filtered with 5C filter paper (manufactured by ADVANTEC Co., Ltd.). The silica concentration and molar ratio of the resulting alkali silicate aqueous solution were measured. Step (B): Solid-Liquid Separation of Slurry The slurry obtained in step (A) was filtered in the same manner as in Comparative Example 1 to obtain an aqueous alkali silicate solution and a residue.

[0088] <Results of Comparative Examples 1 and 2> The relationship between the dissolution time and the silica concentration, molar ratio, and silica dissolution rate in Comparative Examples 1 and 2 is shown in Table 3 below and Figures 2 and 3. Furthermore, Table 4 shows the input amount of each raw material and the component breakdown of each raw material in Comparative Examples 1 and 2. Table 5 shows the mass of the aqueous alkali silicate solution and the residue after solid-liquid separation, the component breakdown thereof, and the silica utilization rate in Comparative Examples 1 and 2.

[0089] In Table 4, the masses of the rice husk ash, sodium hydroxide aqueous solution, and water are actual measurements, the component breakdown in the rice husk ash is a calculated value based on the analytical values ​​of the carbon amount and inorganic component composition in the rice husk, and the component breakdown in the sodium hydroxide aqueous solution is a calculated value obtained from the aqueous solution concentration. In Table 5, the masses of the alkali silicate aqueous solution (filtrate) and residue are actual measurements, and the component breakdown in the alkali silicate aqueous solution (filtrate) is a calculated value based on the analytical values ​​of the component composition of the aqueous solution. In addition, the masses of the solids and residual solution are analytical values ​​obtained by measuring the mass of the solids in the residue and the residual solution, the component breakdown in the solids is a calculated value based on the analytical values ​​of the carbon amount and inorganic component composition in the solids, and the component breakdown in the residual solution is a calculated value based on the analytical values ​​of the component composition of the aqueous solution. The same applies to the other tables.

[0090]

[0091] Step (A): Mixing raw materials and preparing slurry

[0092] Step (B): Solid-liquid separation of slurry

[0093] <Explanation of the results of Comparative Examples 1 and 2> From the results in Table 3, it was found that the silica dissolution rate at each dissolution time was almost constant even when the ratio of the rice husk ash input amount to the total raw material was changed. In Comparative Example 1 (assumed dissolution rate of approximately 78%), as expected, the silica concentration and molar ratio were able to reach the target values ​​in just 2 hours, a shorter time than in Reference Example 1. In Comparative Example 2 (assumed dissolution rate of approximately 85%), as expected, the silica concentration and molar ratio were able to reach the target values ​​in just 4 hours, a shorter time than in Reference Example 1.

[0094] As shown in Table 5, the amount of the aqueous alkali silicate solution obtained in Comparative Example 1 was approximately 103 kg, and the amount of the aqueous alkali silicate solution obtained in Comparative Example 2 was approximately 124 kg. It can be seen that by setting the silica dissolution rate high, a larger amount of the target aqueous alkali silicate solution can be obtained from the same amount of rice husk ash (Table 4).

[0095] In addition, as shown in Table 5, the residue after filtration is a cake-like mixture containing solids that did not dissolve in the aqueous sodium hydroxide solution and the residual solution of the aqueous alkali silicate solution. The mass of the residue is approximately five times the solids, which indicates that a large amount of the aqueous alkali silicate solution that should have been extracted remains in the residue.

[0096] Specifically, the amount of residue in Comparative Example 1 was approximately 47 kg, of which approximately 37 kg was the aqueous alkali silicate solution that should have been extracted. Estimated from the concentration of the aqueous alkali silicate solution, the amount of silica that was dissolved in the solution but discarded and not utilized was approximately 7 kg (b5 in Table 5). Combined with the undissolved silica in the solid content, the amount of discarded silica was approximately 14 kg (b7 in Table 5). As a result, the silica utilization rate in Comparative Example 1 was a low 57.3% (b9 in Table 5).

[0097] The amount of residue in Comparative Example 2 was approximately 35 kg, of which approximately 28 kg was the aqueous alkali silicate solution that should have been extracted. Based on the concentration of the aqueous alkali silicate solution, the amount of silica that was dissolved in the aqueous solution but discarded and not utilized was estimated to be approximately 5 kg (b5 in Table 5). Because the silica dissolution rate in Comparative Example 2 was set higher than in Comparative Example 1, the amount of residue and the amount of discarded silica were reduced compared to Comparative Example 1. Even so, when combined with the undissolved silica in the solid content, the amount of discarded silica was approximately 10 kg (b7 in Table 5), meaning that a large amount of silica was still discarded. As a result, the silica utilization rate in Comparative Example 2 was 69.3% (b9 in Table 5), which was higher than in Comparative Example 1, but there is room for further improvement.

[0098] The results of Comparative Examples 1 and 2 reveal the following points (i) and (ii): (i) It is possible to improve the silica utilization rate to some extent by using a raw material blend with a high silica dissolution rate. (ii) However, in the conventional method, because a portion of the aqueous alkali silicate solution remains in the residue after filtration, even if the silica dissolution rate is set as high as about 85%, the silica utilization rate remains below 70%.

[0099] The present inventors have noticed that a portion of the aqueous alkali silicate solution remains in the residue after filtration, leaving behind silica that is dissolved in the solution but is not utilized. As a result of extensive research, the present inventors have discovered a new method that allows for the production of the desired aqueous alkali silicate solution while recovering such silica, thereby successfully improving the silica utilization rate. Specific examples are provided below.

[0100] Example 1 (Study on further improvement of silica utilization rate, first cycle) Example 1 is an example in which silica is recovered from the residue obtained in Comparative Example 2 and the recovered silica is used to produce the target aqueous alkali silicate solution. As in Example 1, the step (C) preceding step (A) in the first cycle of the present invention may be a step of washing with water the residue obtained by a general method for dissolving rice husk ash to obtain a solution after washing with water. Figure 4 is a flow diagram that schematically shows each step in the present invention.

[0101] Pre-process (C): Washing of residue 35.36 kg of the residue obtained in Comparative Example 2 was washed with 115 kg of warm water, and the post-wash solution was recovered in a recovery tank. Solid-liquid separation was performed using the same procedure as in process (B) of Comparative Example 1. The mass of the resulting post-wash solution was 115.00 kg. Approximately 20 mL of the post-wash solution was removed and used as a sample for component analysis. In addition, for the residue after water washing, the masses of the solids and residual solution were measured, and the components of the solids and residual solution were analyzed. The recovered post-wash solution was supplied to the next process (A).

[0102] Step (A): Mixing of raw materials and preparation of slurry In a 240 L stainless steel dissolution tank equipped with a stirrer, 115.00 kg of the post-wash solution (reference numeral 2 in FIG. 4) obtained in the previous step (C), 22.33 kg of a 48.0 mass% sodium hydroxide aqueous solution, and 9.62 kg of warm water for concentration adjustment were added, and 36.00 kg of rice husk ash A (reference numeral 1 in FIG. 4) was added over 20 minutes while continuing to stir. The solubility of silica in rice husk ash was set to about 85%, as in Comparative Example 2. The mass ratio of rice husk ash A to the total mass of raw materials was about 19.7%. Then, the temperature of the slurry was raised to 90 ° C while stirring, and stirring was continued for 4 hours while maintaining the liquid temperature at 90 ° C to obtain an alkali silicate slurry. During the dissolution of the rice husk ash, approximately 20 mL of the sample was taken at appropriate intervals, filtered through 5C filter paper (manufactured by ADVANTEC), and the silica concentration and molar ratio of the resulting aqueous alkali silicate solution were measured.

[0103] Step (B): Solid-Liquid Separation of Slurry The slurry obtained in step (A) was filtered in the same manner as in Comparative Example 1 to obtain a residue (reference numeral 3 in FIG. 4 ) and the target aqueous alkali silicate solution (reference numeral 4 in FIG. 4 ).

[0104] Step (C): Washing of Residue 35.36 kg of the residue obtained in step (B) was washed with 115.00 kg of warm water, and the post-wash solution was recovered in a recovery tank. The mass of the resulting post-wash solution was 115.00 kg. Approximately 20 mL of the post-wash solution was removed and used as a sample for component analysis. Furthermore, for the residue after washing, the masses of the solids and residual solution were measured, and component analysis of each of the solids and residual solution was performed. This step (C) is a pre-step of step (A) in Example 2, and the post-wash solution obtained here is used as a raw material in step (A) in Example 2.

[0105] Example 2 (Second Cycle) Example 2 is an example in which silica is recovered from the residue obtained in Example 1 and the recovered silica is used to produce the target aqueous alkali silicate solution.

[0106] Previous Step (C): Washing of Residue with Water The previous step to step (A) in Example 2 is step (C) in Example 1. 115.00 kg of the post-wash solution obtained in step (C) in Example 1 is used as a raw material in the next step (A).

[0107] Step (A): Mixing of raw materials and preparation of slurry A slurry was prepared in the same manner as in Example 1. That is, 115.00 kg of the post-wash solution obtained in the previous step (C), 22.33 kg of a 48.0 mass% sodium hydroxide aqueous solution, and 9.62 kg of warm water for adjusting the concentration were placed in a 240 L stainless steel dissolution tank equipped with a stirrer, and 36.00 kg of rice husk ash A was added over 20 minutes while continuing to stir. The mass ratio of rice husk ash A to the total mass of the raw materials was approximately 19.7%. The subsequent steps were the same as in Example 1.

[0108] Step (B): Solid-Liquid Separation of Slurry The slurry obtained in step (A) was filtered in the same manner as in Comparative Example 1 to obtain an aqueous alkali silicate solution and a residue.

[0109] Step (C): Washing of Residue 35.36 kg of the residue obtained in step (B) was washed with water in the same manner as in step (C) of Example 1, and measurements and analysis were carried out in the same manner. The mass of the resulting post-wash solution was 115.00 kg. This step (C) is a step preceding step (A) of the third cycle, and the post-wash solution obtained here is used as a raw material in step (A) of the third cycle. Since the third cycle and subsequent steps can be carried out in the same manner as in the second cycle, detailed explanations will be omitted.

[0110] <Results of Examples 1 and 2> Table 6 shows the water washing conditions in the preceding step (C) of Examples 1 and 2, as well as the masses of the post-wash solution and residue after filtration, and a breakdown of their components. Table 7 shows the input amounts of each raw material in step (A), and a breakdown of the components of each raw material. Table 8 shows the masses of the aqueous alkali silicate solution and residue after solid-liquid separation in step (B), as well as a breakdown of their components. Table 9 shows the water washing conditions in step (C) of Examples 1 and 2, as well as the masses of the post-wash solution and residue after filtration, as well as a breakdown of their components. Table 9 also shows the silica utilization rates in Examples 1 and 2.

[0111] Pre-process (C): Washing of residue with water

[0112] Step (A): Mixing raw materials and preparing slurry

[0113] Step (B): Solid-liquid separation of slurry

[0114] Step (C): Washing the residue with water

[0115] <Explanation of the Results of Example 1> As shown in Table 6, the water washing treatment (including filter press) in the pre-step (C) yielded 115.00 kg of washed solution and 35.36 kg of residue.

[0116] The silica concentration in the post-wash solution obtained in the previous step (C) was 3.6% by mass (c5 in Table 6), which was lower than the target value of 18.0% by mass due to dilution with the wash water. Meanwhile, the molar ratio was maintained at the target value of 3.3 before and after the water wash. The post-wash solution contained 4.09 kg of silica (c7 in Table 6). This silica was recovered from the residue obtained in Comparative Example 2.

[0117] The weight of the residue after washing was 35.36 kg (c8 in Table 6), which was unchanged from the weight before washing (c1 in Table 6). The residue still contained residual solution after washing. The amount of silica contained in the solid content of the residue did not change before or after washing (b4 in Table 5 and c9 in Table 6). However, the amount of silica contained in the residual solution after washing was 1.01 kg (c10 in Table 6), which was reduced to about one-fifth of the amount of silica contained in the residual solution before washing (5.09 kg (b5 in Table 5). This difference corresponds to the recovered silica.

[0118] The formulation shown in Table 7 was adjusted so that the silica concentration and molar ratio of the aqueous alkali silicate solution would reach the target values ​​(18.0 mass% and 3.3, respectively) after 4 hours, assuming a silica dissolution rate of approximately 85%.

[0119] As shown in Table 8, 147.59 kg (e1 in Table 8) of the target aqueous alkali silicate solution and 35.36 kg of a residue (e3 in Table 8) were obtained by solid-liquid separation of the slurry.

[0120] The production amount of the target aqueous alkali silicate solution increased by approximately 18% compared to the amount of 124.88 kg of the target aqueous alkali silicate solution obtained in Comparative Example 2 (b1 in Table 5). Focusing on the amount of silica, the amount of silica in the target aqueous alkali silicate solution obtained in Example 1 was 26.57 kg (e2 in Table 8), while the amount of silica in the target aqueous alkali silicate solution obtained in Comparative Example 2 was 22.48 kg (b2 in Table 5), a difference of 4.09 kg. It can be seen that the production amount of the target aqueous alkali silicate solution increased by using as a raw material the post-wash solution containing 4.09 kg of silica (d4 in Table 7) recovered from the residue in the preceding step (C).

[0121] The amount of residue in step (B) was 35.36 kg (e3 in Table 8), the same as in Comparative Example 2, corresponding to the amount of rice husk ash added and the dissolution rate, and the masses of the solids and residual solution and their component breakdown were also the same as in Comparative Example 2. These results show that, regardless of whether the present invention is implemented or not, the residue after obtaining the target aqueous alkali silicate solution contains a certain amount of silica, and from the perspective of reducing waste of resources, it can be said that it is preferable to repeat steps (A) to (C) specified in the present invention.

[0122] As shown in Table 9, the water washing treatment (including filter press) in step (C) yielded 115.00 kg of washed solution and 35.36 kg of residue.

[0123] The post-wash solution obtained in step (C) contained 4.09 kg of silica (f5 in Table 9), which is the same as the amount of silica in the post-wash solution obtained in the previous step (C), 4.09 kg (c7 in Table 6). The post-wash solution obtained in step (C) of Example 1 can be used as the raw material for step (A) of Example 2. In this case, the post-wash solution obtained in step (C) (f4 in Table 9) has the same component composition as the post-wash solution obtained in the previous step (C) (c4 in Table 6), so the production conditions in Example 2 do not need to be changed from those in Example 1. In this way, steps (A) to (C) specified in the present invention can be repeatedly performed under the same production conditions, allowing for continuous production of an aqueous alkali silicate solution.

[0124] The residue obtained in step (C) (f6 in Table 9) is discarded. The amount of silica contained in the solid matter to be discarded, 4.87 kg (f7 in Table 9), is the same as that in Comparative Example 2 (b4 in Table 5). In contrast, the amount of silica contained in the residual solution to be discarded is 1.01 kg (f8 in Table 9), which is reduced to about one-fifth of the amount of silica in Comparative Example 2, 5.09 kg (b5 in Table 5).

[0125] As shown in Table 9, the mass of silica used in the production of the alkali silicate aqueous solution in Example 1 was 26.57 kg (f11 in Table 9). Therefore, the silica utilization rate (the ratio of the mass of silica used to the total mass of silica in the rice husk ash) was 81.9%. Compared with the silica utilization rates of 57.3% and 69.3% in Comparative Examples 1 and 2, respectively, it can be seen that the silica utilization rate is significantly increased by the present invention.

[0126] <Explanation of the Results of Example 2> In Example 2, the target aqueous alkali silicate solution was produced from a raw material containing the post-wash solution obtained in step (C) of Example 1. This post-wash solution also contained silica recovered from the residue in the previous step. As a result, in Example 2, the target aqueous alkali silicate solution was also produced, and the silica utilization rate was increased as in Example 1.

[0127] Example 3 (Dissolution under Pressurized Conditions) An experiment to improve the dissolution rate of rice husk ash was conducted using a pressure vessel such as an autoclave. 365 g of warm water, 205.00 g of 24.0% by mass sodium hydroxide aqueous solution, and 165.00 g of rice husk ash were added to a 1 L autoclave equipped with a stirrer while stirring, and the vessel was then capped. The rice husk ash was then dissolved under high-temperature, pressurized conditions of 150 °C and 0.47 MPa. The dissolution experiment was conducted for 0.5 hours, 1 hour, 1.5 hours, and 2 hours. After dissolution was completed, the pressure was released, and the resulting slurry was allowed to cool to room temperature. A 150 mm diameter magnetic Buchner funnel (Nuchner) was attached to a 3 L suction filter bottle, and the cooled slurry was subjected to suction filtration using filter paper (1 μm mesh, standard 5C, manufactured by ADVANTEC) to remove undissolved residue, yielding an alkali silicate aqueous solution. The physical properties of the resulting aqueous alkali silicate solution were investigated.

[0128] <Explanation of the Results of Example 3> The results of Example 3 compared with Comparative Example 2 are shown in Table 10 and Figure 5. As shown in Table 10 and Figure 5, the method of Example 3, in which rice husk ash is dissolved under high-temperature and pressurized conditions, allowed the production of a highly concentrated aqueous alkali silicate solution in a shorter time than Comparative Example 2. Specifically, when producing an aqueous alkali silicate solution with a silica concentration of 18.0 mass % and a molar ratio of 3.3, it took 4 hours in Comparative Example 2, but it was possible to produce it in 1.5 hours in Example 3.

[0129] The method of dissolving rice husk ash under high-temperature and pressurized conditions can be used to dissolve rice husk ash in step (A) of Examples 1 and 2. The results of Example 3 show that it is possible to shorten the time required to dissolve rice husk ash in Examples 1 and 2, and therefore the time required to produce an aqueous alkali silicate solution.

[0130]

[0131] Example 4 (Pressing) 35.36 kg of the residue obtained in step (C) of Example 2 was pressed at a pressure of 0.7 MPa. As a result, the remaining solution could be further squeezed out, and the final mass of the residue was reduced by 25% to approximately 26.52 kg.

[0132] The pressing of the residue can be adopted in step (B) as well as step (C). The results of Example 4 show that the extraction of the alkali silicate aqueous solution from the residue in Examples 1 and 2 can be promoted, and thus the silica utilization rate can be further improved.

[0133] <Effects of the Invention> As explained above, when an aqueous alkali silicate solution is produced from rice husk ash by the method of the present invention, it is possible to improve the utilization rate of silica present in the rice husk ash while reducing the amount of residue that becomes industrial waste.

Claims

1. The method includes repeating the following steps (A) to (C) one or more times to obtain a SiO ２ / (Na ２ O+K ２ O) A method for producing an aqueous alkali silicate solution, which obtains a target aqueous alkali silicate solution having a molar ratio in the range of 3.0 to 4.0 and a silica concentration in the range of 13 to 28 mass %: Step (A) mixing the following raw materials (1) to (3) or the following raw materials (1) to (4) to prepare a slurry containing an aqueous alkali silicate solution containing silica extracted from rice husk ash: (1) rice husk ash, (2) the aqueous alkali silicate solution supplied from step (C), (3) sodium hydroxide, (4) water, Step (B) performing solid-liquid separation of the slurry prepared in step (A) into the aqueous alkali silicate solution and a residue to obtain the target aqueous alkali silicate solution, Step (C) washing the residue obtained in step (B) with water and supplying at least a portion of the solution after washing to step (A) as raw material (2).

2. The manufacturing method according to claim 1, wherein in step (A), the raw materials are blended in a ratio that satisfies the following (a) and (b): (a) the amount of silica and potassium oxide based on the composition ratio of raw material (1), the amount of silica, sodium oxide, and potassium oxide supplied from raw material (2), and the amount of SiO calculated based on the sodium oxide equivalent of raw material (3); ２ / (Na ２ O+K ２ O) molar ratio of the SiO of the target aqueous alkali silicate solution ２ / (Na ２ O+K ２ (b) the silica concentration calculated based on the amount of silica based on the composition ratio of raw material (1), the amount of silica supplied from raw material (2), and the amount of water in the raw materials is equal to or greater than the silica concentration of the target aqueous alkali silicate solution.

3. The method according to claim 1 or 2, wherein in step (C), the amount of water used for washing is such that the silica concentration in the solution after washing is 0.5 to 15% by mass.

4. The manufacturing method according to claim 1 or 2, wherein the raw material (1) is rice husk ash having a silica content of 80% by mass or more.

5. The manufacturing method according to claim 1 or 2, wherein no silica-based mineral is added in steps (A) to (C).

6. The production method according to claim 1 or 2, wherein the solid-liquid separation in step (B) and step (C) comprises carrying out pressure filtration under a pressure of 0.15 to 0.8 MPa.

7. The method of claim 6, wherein the pressure filtration is carried out by a filter press.

8. The production method according to claim 6, wherein the solid-liquid separation in steps (B) and (C) comprises squeezing the residue after the pressure filtration at a pressure of 0.15 to 0.8 MPa.

9. The production method according to claim 1 or 2, wherein the preparation of the slurry in step (A) is carried out under conditions of a temperature of 105 to 300°C and a pressure of 0.20 to 8.60 MPa.

10. The manufacturing method according to claim 1 or 2, wherein in the preparation of the slurry in step (A), the temperature and dissolution time of the mixture are adjusted so that the dissolution rate of silica in the rice husk ash is 80 mass % or more, where the dissolution rate of silica in the rice husk ash is calculated based on the following formula 2:

11. The manufacturing method according to claim 1 or 2, wherein the rice husk ash as raw material (1) is processed rice husk ash having an apparent density of 400 to 800 g / L in a dry state and a moisture content of 30 mass% or less.

12. In step (A), the raw materials are blended in a ratio that satisfies the following (a) and (b): (a) The amount of silica and potassium oxide based on the composition ratio of raw material (1), the amount of silica, sodium oxide, and potassium oxide supplied from raw material (2), and the amount of SiO calculated based on the sodium oxide equivalent of raw material (3). ２ / (Na ２ O+K ２ O) molar ratio of the SiO of the target aqueous alkali silicate solution ２ / (Na ２ O+K ２ (b) the silica concentration calculated based on the amount of silica based on the composition ratio of raw material (1), the amount of silica supplied from raw material (2), and the amount of water in the raw materials is equal to or greater than the silica concentration of the target aqueous alkali silicate solution; in step (C), the amount of water used in washing with water is an amount such that the silica concentration of the solution after washing is 0.5 to 15 mass%; raw material (1) is rice husk ash having a silica content of 80 mass% or more; no silica-based mineral is added in steps (A) to (C); the solid-liquid separation in steps (B) and (C) comprises carrying out pressure filtration under a pressure condition of 0.15 to 0.8 MPa, the pressure filtration being carried out using a filter press; and the solid-liquid separation in steps (B) and (C) comprises squeezing the residue after the pressure filtration under a pressure condition of 0.15 to 0.8 MPa. The preparation of the slurry in step (A) is carried out under conditions of a temperature of 105 to 300°C and a pressure of 0.20 to 8.60 MPa, and in the preparation of the slurry in step (A), the temperature of the mixture and the dissolution time are adjusted so that the dissolution rate of silica in the rice husk ash is 80 mass% or more, and the dissolution rate of silica in the rice husk ash is calculated based on the following formula 2: The manufacturing method according to claim 1, wherein the rice husk ash of the raw material (1) is processed rice husk ash having an apparent density of 400 to 800 g / L in a dry state and a moisture content of 30 mass% or less.

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