Method for producing alkali silicate aqueous solution

Abstract

To enhance the utilization rate of silica in the production of an alkali silicate aqueous solution using rice husk ash as a raw material.SOLUTION: The method for producing an alkali silicate aqueous solution of the present invention includes repeating a cycle of preparing a first slurry of an alkali silicate aqueous solution using rice husk ash as a raw material, subjecting the first slurry to solid-liquid separation into an alkali silicate aqueous solution and a residue, preparing a second slurry of an alkali silicate aqueous solution using the residue as a raw material, subjecting the second slurry to solid-liquid separation into an alkali silicate aqueous solution and a residue, and adding the alkali silicate aqueous solution derived from the second slurry to a raw material for preparing the first slurry of the next cycle or mixing the alkali silicate aqueous solution derived from the second slurry with the alkali silicate aqueous solution derived from the first slurry of the current cycle.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a method for producing an aqueous alkali silicate solution. [Background technology]

[0002] Aqueous sodium silicate solution is used as a raw material for wet-process silica. Sodium silicate solution is generally produced by mixing natural sand (such as silica sand, silica stone, and silica clay), sodium hydroxide, and water (including the case of using aqueous sodium hydroxide) at a high temperature of approximately 70°C or higher, according to the following reaction formula: Chemical reaction formula 2NaOH+nSiO2 → Na2O·nSiO2+H2O 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, and 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 (Patent Documents 1 and 2).

[0004] When rice husk ash is used as the raw material, the resulting aqueous solution typically contains potassium derived from the rice husk ash in addition to sodium as an alkali metal component. Therefore, in the following description, silicate compounds in which the alkali metal component is specifically limited to sodium will be referred to as "sodium silicate" (Na2O·nSiO2), while silicate compounds containing alkali metal components other than sodium will generally be referred to as "alkali silicate." In this invention and this specification, alkali silicate includes sodium silicate. The "molar ratio" used as an indicator of the quality of sodium silicate and alkali silicate refers to the "SiO2 / Na2O molar ratio" in the case of sodium silicate, and the "SiO2 / (Na2O + KO) molar ratio" (i.e., the ratio of the molar amount of silica to the total molar amount of sodium oxide and potassium oxide) in the case of alkali silicate. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2007-510613 [Patent Document 2] Special Publication No. 2007-522069 Summary of the Invention [Problem to be solved by the invention]

[0006] When rice husk ash is used as a 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 contains undissolved silica, but is simply discarded.

[0007] In particular, 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, the conventional slurry preparation method described above generally involves adding an excess amount of rice husk ash and terminating the slurry preparation when the molar ratio and silica concentration reach the target values, resulting in a high proportion of undissolved silica in the residue. This is because, as the dissolution rate of silica in rice husk ash approaches 100%, the dissolution rate of silica drops dramatically (e.g., Reference Example 1 described in this specification below). 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.

[0008] Furthermore, the inventors' investigations have revealed that the residue contains an aqueous alkali silicate solution with a mass approximately 2 to 5 times the mass of the solid content. In typical aqueous alkali silicate solutions used in wet-process silica production, 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 when the residue is discarded, a considerable amount of silica is discarded as an aqueous alkali silicate solution along with the solid content.

[0009] As described above, in conventional methods for producing an aqueous alkali silicate solution, the undissolved silica contained in the solid content of the residue after solid-liquid separation and the silica contained in the aqueous alkali silicate solution remaining in the residue are discarded.

[0010] 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).

[0011] 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. [Means for solving the problem]

[0012] The above-mentioned problem can be solved by recovering silica from the residue after solid-liquid separation generated in the production process of an alkali silicate aqueous solution, and using the recovered silica in the production of an alkali silicate aqueous solution. Specifically, the above-mentioned problem can be solved by the invention of [1] below, and preferably by the invention of [2] and subsequent inventions. [1] A method for producing an aqueous alkali silicate solution, comprising repeating the following steps (A1) to (D1) one or more times to obtain a target aqueous alkali silicate solution having an SiO / (NaO+KO) molar ratio in the range of 3.0 to 4.0 and a silica concentration in the range of 13 to 28 mass %, ·Process (A1) A step of mixing the following raw materials (1) to (3) or the following raw materials (1) to (4) to prepare a first slurry containing an alkali silicate aqueous solution containing silica extracted from rice husk ash; (1) Rice husk ash, (2) the second aqueous alkali silicate solution provided from step (D1); (3) sodium hydroxide, (4) Water; ·Process (B1) a step of subjecting the first slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain a first aqueous alkali silicate solution, which is the target aqueous alkali silicate solution, and a residue; ·Process (C1) a step of mixing the residue obtained in step (B1) and the following raw materials (5) and (6) to prepare a second slurry containing an aqueous alkali silicate solution containing silica extracted from the residue; (5) sodium hydroxide, (6) Water; ·Process (D1) a step of subjecting the second slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue, and supplying at least a portion of the solid-liquid separated second aqueous alkali silicate solution to step (A1) as raw material (2). [2] The step (D1) further comprises washing the residue obtained by solid-liquid separation of the second slurry with water, and supplying at least a part of the solution after washing to the step (A1) as the raw material (7); The manufacturing method according to [1], wherein in step (A1), the first slurry is prepared by mixing raw materials (1) to (3) and (7) or raw materials (1) to (4) and (7). [3] The manufacturing method according to [1] or [2], wherein in step (C1), the blending of raw materials and mixing conditions when preparing the second slurry are adjusted so that the SiO2 / (Na2O + KO) molar ratio of the alkali silicate aqueous solution in the second slurry is 2.6 or less. [4] A method for producing an aqueous alkali silicate solution, comprising the following steps (A2) to (E), for obtaining a target aqueous alkali silicate solution having an SiO / (NaO+KO) molar ratio in the range of 3.0 to 4.0 and a silica concentration in the range of 13 to 28 mass %: ·Process (A2) A step of mixing the following raw materials (1), (3), and (4) to prepare a first slurry containing an aqueous alkali silicate solution containing silica extracted from rice husk ash; (1) Rice husk ash, (3) sodium hydroxide, (4) Water; ·Process (B2) a step of subjecting the first slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain a first aqueous alkali silicate solution and a residue; ·Process (C2) a step of mixing the residue obtained in step (B2) and the following raw materials (5) and (6) to prepare a second slurry containing an aqueous alkali silicate solution containing silica extracted from the residue; (5) sodium hydroxide, (6) Water; ·Process (D2) a step of subjecting the second slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain a second aqueous alkali silicate solution and a residue; ·Process (E) A step of mixing the first and second aqueous alkali silicate solutions to obtain the desired aqueous alkali silicate solution. [5] The step (D2) further comprises washing the residue obtained by solid-liquid separation of the second slurry with water, and supplying at least a part of the solution after washing to the step (A2) as the raw material (7); The manufacturing method according to [4], wherein in step (A2), the first slurry is prepared by mixing raw materials (1), (3), and (7) or raw materials (1), (3), (4), and (7). [6] The manufacturing method according to any one of [1] to [5], wherein in step (A1) or (A2), the raw materials are mixed in a ratio that satisfies the following (a) and (b): (a) The SiO2 / (Na2O+K2O) molar ratio 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 materials (2) and (7), and the amount of raw material (3) converted to sodium oxide exceeds the SiO2 / (Na2O+K2O) molar ratio of the target aqueous alkali silicate solution; (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 materials (2) and (7), and the amount of water in the raw materials exceeds the silica concentration of the target aqueous alkali silicate solution. [7] The method according to any one of [1] to [6], wherein the raw material (1) is rice husk ash having a silica content of 80% by mass or more. [8] The production method according to any one of [1] to [7], wherein no silica-based mineral is added in steps (A1) to (D1) or steps (A2) to (D2). [9] The method according to any one of [1] to [8], wherein the solid-liquid separation in step (B1), step (D1), step (B2) or step (D2) comprises carrying out pressure filtration under a pressure condition of 0.15 to 0.8 MPa.

[10] The method according to [9], wherein the pressure filtration is carried out using a filter press.

[11] The production method according to [9] or

[10] , wherein the solid-liquid separation in step (B1), step (D1), step (B2) or step (D2) comprises squeezing the residue under a pressure of 0.15 to 0.8 MPa.

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

[11] , wherein the preparation of the slurry in step (A1), step (C1), step (A2) or step (C2) is carried out under conditions of a temperature of 105 to 300°C and a pressure of 0.20 to 8.6 MPa.

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

[12] , wherein in the preparation of the first slurry in step (A1) or step (A2), the temperature of the mixture and the dissolution time are adjusted so that the dissolution rate of silica in the rice husk ash is 70 mass % or more and less than 90 mass %: Here, the dissolution rate of silica in rice husk ash is calculated based on the following formula 2. Formula 2:

number

[14] The manufacturing method according to

[13] , wherein in the preparation of the second slurry in step (C1) or step (C2), the temperature and dissolution time of the mixture are adjusted so that the dissolution rate of silica in the rice husk ash is a cumulative 90 mass% or more.

[15] The method according to [2] or [5], wherein the amount of water used in washing is such that the silica concentration in the solution after washing is 0.5 to 15 mass %.

[16] The manufacturing method according to any one of [1] to

[15] , 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.

[17] [1] The manufacturing method according to [1], The step (D1) further comprises washing the residue obtained by solid-liquid separation of the second slurry with water, and supplying at least a part of the solution after washing to the step (A1) as the raw material (7); In step (A1), the first slurry is prepared by mixing raw materials (1) to (3) and (7) or raw materials (1) to (4) and (7), In step (C1), the blending and mixing conditions of the raw materials when preparing the second slurry are adjusted so that the SiO / (NaO+KO) molar ratio of the alkali silicate aqueous solution in the second slurry is 2.6 or less; In step (A1), the raw materials are mixed in a ratio that satisfies the following (a) and (b): (a) The SiO2 / (Na2O+K2O) molar ratio 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 materials (2) and (7), and the amount of raw material (3) converted to sodium oxide exceeds the SiO2 / (Na2O+K2O) molar ratio of the target aqueous alkali silicate solution; (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 materials (2) and (7), and the amount of water in the raw materials exceeds the silica concentration of the target aqueous alkali silicate solution. The raw material (1) is rice husk ash having a silica content of 80% by mass or more, No silica-based mineral is added in steps (A1) to (D1), The solid-liquid separation in step (B1) or step (D1) includes performing pressure filtration under a pressure condition of 0.15 to 0.8 MPa, Pressure filtration is carried out using a filter press, The solid-liquid separation in step (B1) or step (D1) includes squeezing the residue under a pressure of 0.15 to 0.8 MPa; The preparation of the slurry in step (A1) or step (C1) is carried out under conditions of a temperature of 105 to 300°C and a pressure of 0.20 to 8.6 MPa; In the preparation of the first slurry in step (A1), the temperature and dissolution time of the mixture are adjusted so that the dissolution rate of silica in the rice husk ash is 70% by mass or more and less than 90% by mass; Here, the dissolution rate of silica in rice husk ash is calculated based on the following formula 2: Formula 2:

number

[0013] 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. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a graph showing the relationship between the dissolution time, the silica concentration, the molar ratio, and the silica dissolution rate in Reference Example 1. [Figure 2] FIG. 2 is a graph showing the relationship between the dissolution time, the silica concentration, the molar ratio, and the silica dissolution rate in Comparative Example 1. [Figure 3] FIG. 3 is a graph showing the relationship between the dissolution time, the silica concentration, the molar ratio, and the silica dissolution rate in Comparative Example 2. [Figure 4] FIG. 4 is a flow chart schematically showing each step in the first production method of the present invention. [Figure 5] FIG. 5 is a graph showing the changes in silica concentration and molar ratio of the aqueous alkali silicate solution during steps (A1) to (D1) for Examples 1 and 2. [Figure 6] FIG. 6 is a flow diagram schematically showing each step in the second production method of the present invention. [Figure 7] FIG. 7 is a graph showing the changes in silica concentration and molar ratio of the aqueous alkali silicate solution during steps (A2) to (D2) for Examples 3 and 4. [Figure 8] FIG. 8 is a graph showing the relationship between the dissolution time, silica concentration, molar ratio, and silica dissolution rate in Example 5, in comparison with Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention is a method for producing a target aqueous alkali silicate solution having a predetermined molar ratio and silica concentration using rice husk ash as a raw material, which involves dissolving rice husk ash to prepare a slurry, subjecting the slurry to solid-liquid separation, dissolving the residue after solid-liquid separation in an aqueous sodium hydroxide solution, and recovering the silica component from the residue. The recovered silica component is used as part of the raw material in the production of the aqueous alkali silicate solution (first method for producing an aqueous alkali silicate solution), or as part of the product (second method for producing an aqueous alkali silicate solution).

[0016] <Method for producing first aqueous alkali silicate solution> The first method for producing an alkali silicate aqueous solution of the present invention (hereinafter simply referred to as "first production method") comprises repeating the following steps (A1) to (D1) one or more times to obtain a target alkali silicate aqueous solution having an SiO / (NaO+KO) molar ratio in the range of 3.0 to 4.0 and a silica concentration in the range of 13 to 28 mass %: ·Process (A1) A step of mixing the following raw materials (1) to (3) or the following raw materials (1) to (4) to prepare a first slurry containing an alkali silicate aqueous solution containing silica extracted from rice husk ash; (1) Rice husk ash, (2) the second aqueous alkali silicate solution provided from step (D1); (3) sodium hydroxide, (4) Water; ·Process (B1) a step of subjecting the first slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain a first aqueous alkali silicate solution, which is the target aqueous alkali silicate solution, and a residue; ·Process (C1) a step of mixing the residue obtained in step (B1) and the following raw materials (5) and (6) to prepare a second slurry containing an aqueous alkali silicate solution containing silica extracted from the residue; (5) sodium hydroxide, (6) Water; ·Process (D1) a step of subjecting the second slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue, and supplying at least a portion of the solid-liquid separated second aqueous alkali silicate solution to step (A1) as raw material (2).

[0017] In the first production method of the present invention, a series of steps from step (A1) to step (D1) is considered 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.

[0018] The target aqueous alkali silicate solution obtained by the first production method of the present invention has an SiO2 / (Na2O+K2O) molar ratio in the range of 3.0 to 4.0 and a silica concentration in the range of 13 to 28 mass%. Since the molar ratio and silica concentration are within the above ranges, the aqueous alkali silicate solution obtained by the production method of the present invention can be used for the same purposes 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 can be used for these purposes as is, or it can be used for these purposes after slight quality adjustment, for example, by adding water or commercially available silica. The wet-process silica so produced, like conventional wet-process silica, can be used in many known applications, such as reinforcing fillers in rubber or tires, matting agents for paints, and toothpaste or industrial abrasives.

[0019] 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 mass %, more preferably in the range of 18 to 22 mass %.

[0020] The alkali silicate aqueous solution of the present invention is produced from rice husk ash and therefore contains trace amounts of potassium. In alkali silicate aqueous solutions produced using rice husk ash, the molar ratio (K2O / Na2O) of the potassium oxide concentration to the sodium oxide concentration, expressed in mole percent, is typically 0.01 or greater. Therefore, alkali silicate aqueous solutions produced using rice husk ash can be distinguished from alkali silicate aqueous solutions produced using silica-based minerals (silica sand, silica stone, silica clay, and other silica-containing minerals) based on this K2O / Na2O molar ratio. The K2O / Na2O molar ratio is preferably 0.30 or less. A K2O / Na2O molar ratio of 0.30 or less reduces variations in the neutralization reaction during neutralization synthesis of wet-process silica, facilitating stable quality of the wet-process silica.

[0021] Process (A1) Step (A1) is a step of mixing raw materials (1) to (3) or raw materials (1) to (4) to prepare a first slurry containing an aqueous alkali silicate solution containing silica extracted from rice husk ash. (1) Rice husk ash (2) The second aqueous alkali silicate solution provided in step (D1) (3) Sodium hydroxide (4) Water

[0022] The raw material (1) is rice husk ash, which is a silica source. Rice husk ash contains a large amount of silica and has been attracting attention as a renewable material in the current situation where a sustainable society is being sought. 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.

[0023] The apparent density of rice husk ash can be increased by compacting or pulverizing the rice husk ash. Typically, the apparent density of raw 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 raw rice husk ash. Rice husk ash with a moderately high apparent density and 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 raw 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.

[0024] From the viewpoint of further improving transportation efficiency, storage efficiency, and sinking property in 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 sinking property in 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.

[0025] 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 (for example, 125°C for 6 hours).

[0026] 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. By containing a certain amount of moisture in the rice husk ash, dust scattering can be suppressed, improving handling during packaging, transportation, and unpacking, as well as handling when the rice husk ash is added to an aqueous solution. Additionally, by containing a certain amount of moisture in the rice husk ash, the rice husk ash's ability to sink into the 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 optional processes such as adding water to the rice husk ash and 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.

[0027] The silica and carbon contents in rice husk ash are not particularly limited. However, because rice husk ash is used as a silica source, a high silica content is preferable, and a low content of carbon, which acts as an impurity, is preferable. The silica content in rice husk ash varies depending on the type of rice, the growing region, and the combustion method of the rice husk ash (such as the type of boiler and combustion conditions). For example, when the combustion temperature of rice husks is about 500°C, the silica content tends to decrease, becoming 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 higher, preferably 80% by mass or higher or 85% by mass or higher, and more preferably 90% by mass or higher. The upper limit of the silica content is typically approximately 98% by mass or lower. 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.

[0028] The volume average particle diameter D50 of rice husk ash 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, improving handling. Furthermore, a volume average particle diameter D50 of 5.0 to 200 μm ensures that the amount of void space between particles falls 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 size D50 is preferably 10 to 150 μm, more preferably 15 to 100 μm, and particularly preferably 20 to 80 μm.

[0029] 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 using 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.

[0030] 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.

[0031] The rice husk ash with 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 there is no need to distinguish between them.

[0032] 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.

[0033] Raw material (2) is the second aqueous alkali silicate solution supplied in step (D1). As described in steps (C1) and (D1), raw material (2) is a solution (second aqueous alkali silicate solution) obtained by preparing a second slurry by mixing the residue obtained after solid-liquid separation in obtaining the first aqueous alkali silicate solution with aqueous sodium hydroxide again. When preparing the second slurry, undissolved silica in the solid content of the residue is dissolved in the slurry solution, and the aqueous alkali silicate solution remaining in the residue is extracted into the slurry solution. In this way, the silica component in the residue is recovered in the slurry solution, and the second slurry is subjected to solid-liquid separation to obtain a second aqueous alkali silicate solution containing the recovered silica component. Using the second aqueous alkali silicate solution as raw material (2) in the production of an aqueous alkali silicate solution can reduce resource waste and increase silica utilization.

[0034] 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, 50 to 300 parts by mass, preferably 60 to 275 parts by mass, and more preferably 70 to 250 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 for the aqueous alkali silicate solution (particularly the blending balance of the raw materials in step (A1)) for each cycle is reduced, and continuous production of the aqueous alkali silicate solution is promoted.

[0035] Raw material (3) is sodium hydroxide, and raw material (4) is water. Some or all of 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 also contained in raw material (2) and raw material (7) described below, if the water contained in raw materials (2) and (7) is sufficient for preparing the first slurry, raw material (4) does not need to be added separately.

[0036] 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 8 to 40 parts by mass, and more preferably 10 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, or may be 10 to 70 parts by mass or 15 to 50 parts by mass, per 100 parts by mass of rice husk ash. By using raw materials (3) and (4) in 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 (A1)) for each cycle is reduced, and continuous production of the aqueous alkali silicate solution is promoted.

[0037] The temperature of the water in raw material (4) is preferably warm water from the viewpoint of promoting dissolution of silica in 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.

[0038] In the first production method of the present invention, as described below, when step (D1) further includes washing the residue obtained by solid-liquid separation of the second slurry with water and supplying at least a portion of the washed solution to step (A1) as raw material (7), the first slurry can be prepared in step (A1) by mixing raw materials (1) to (3) and (7) or raw materials (1) to (4) and (7). Because raw material (7) contains silica recovered from the residue, adding raw material (7) to the raw materials for the first slurry can further increase the utilization rate of silica.

[0039] The amount of raw material (7) used can be appropriately adjusted 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 (7). The amount of raw material (7) used is, for example, 10 to 500 parts by mass, preferably 50 to 400 parts by mass, and more preferably 100 to 300 parts by mass, per 100 parts by mass of rice husk ash.

[0040] In the first production method of the present invention, raw materials other than raw materials (1) to (4) and (7) can be added. Examples of such raw materials include silica-based minerals, which 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 first production method of the present invention, it is preferable not to add silica-based minerals in step (A1), and further in steps (A1) to (D1). In the production method of the present invention, it is possible to continuously produce an aqueous alkali silicate solution by using only raw material (1) and the aqueous alkali silicate solution supplied from step (D1) as silica sources throughout the cycle of steps (A1) to (D1).

[0041] The blending of raw materials is preferably balanced so that the silica dissolution rate in rice husk ash is estimated in advance and the target molar ratio (3.0-4.0) and silica concentration (13-28% by mass) are achieved when the estimated silica dissolution rate is reached. This allows the aqueous alkali silicate solution in the first slurry to have a SiO2 / (Na2O+K2O) molar ratio in the range of 3.0-4.0 and a silica concentration in the range of 13-28% by mass at the end of step (A1). The silica dissolution rate in rice husk ash is the ratio of the mass of silica dissolved in the aqueous alkali silicate solution to the total mass of silica in the rice husk ash; a detailed calculation method is described in the Examples section.

[0042] In step (A1), 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 the 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 the raw materials exceeds the molar ratio of the target aqueous alkali silicate solution. Condition (b) specifies that, assuming that the dissolution rate of silica in the rice husk ash is 100%, the silica concentration based on silica supplied from the raw materials exceeds the silica concentration of the target aqueous alkali silicate solution. Note that, if raw material (7) is not present, the amounts of silica, sodium oxide, and potassium oxide supplied from raw material (7) are zero and need not be taken into consideration. (a) The SiO2 / (Na2O+K2O) molar ratio 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 materials (2) and (7), and the amount of raw material (3) converted to sodium oxide exceeds the SiO2 / (Na2O+K2O) molar ratio of the target aqueous alkali silicate solution; (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 materials (2) and (7), and the amount of water in the raw materials exceeds the silica concentration of the target aqueous alkali silicate solution.

[0043] In other words, in the first production method of the present invention, conditions (a) and (b) stipulate that the amount of silica provided from raw materials (1) and (2), and further from raw material (7), if present, exceeds the amount of silica required to produce the desired aqueous alkali silicate solution. When conditions (a) and (b) are satisfied, the aqueous alkali silicate solution in the slurry will have the desired molar ratio and silica concentration once the required amount of silica has been dissolved from the rice husk ash. This allows the first aqueous alkali silicate solution obtained as the filtrate in step (B1) to be directly converted into the desired aqueous alkali silicate solution.

[0044] In preparing the first slurry in step (A1), it is preferable to adjust the temperature and dissolution time of the mixture so that the dissolution rate of silica in the rice husk ash is 70% by mass or more but less than 90% by mass. The dissolution rate of silica in step (A1) is preferably 73% or more, more preferably 75% or more, from the viewpoint of reducing resource waste. The dissolution rate of silica in step (A1) is preferably 87% or less, more preferably 85% or less, from the viewpoint of shortening the dissolution time. A more accurate molar ratio can be calculated by taking into account the dissolution rate of potassium oxide in rice husk ash in addition to the dissolution rate of silica. Potassium oxide in rice husk ash rapidly dissolves in water when exposed to water in conjunction with the dissolution of silica, so the dissolution rate of silica can be used. However, the amount of potassium oxide in rice husk ash is minute compared to the amount of silica (e.g., Table 1 below), and the impact of differences in dissolution rate is small.

[0045] The temperature and pressure when the raw materials are mixed to prepare the first slurry can be adjusted as appropriate. From the viewpoint of promoting dissolution of the rice husk ash, the first slurry is preferably prepared under high-temperature and pressurized conditions, preferably at a temperature of 105 to 300°C and a pressure of 0.20 to 8.6 MPa. The temperature when preparing the first slurry is preferably 110 to 250°C, more preferably 115 to 200°C. The pressure when preparing the first slurry is preferably 0.25 to 5.0 MPa, more preferably 0.30 to 2.0 MPa. The preparation of the first slurry under high-temperature and pressurized conditions can be carried out using a pressure vessel such as an autoclave.

[0046] The dissolution time of the raw materials when preparing the first slurry by mixing the raw materials can be adjusted appropriately. The preparation of the first slurry in step (A1) 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 the 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.

[0047] In step (A1) of the first production method of the present invention, the second aqueous alkali silicate solution obtained through the preceding steps (C1) and (D1) is used as raw material (2). The preceding step (C1) of the first cycle prior to step (A1) may be a step of dissolving rice husk ash residue obtained by a conventional method in aqueous sodium hydroxide to prepare a slurry. Furthermore, the preceding step (D1) of the first cycle prior to step (A1) may be a step of solid-liquid separating the slurry obtained in the preceding step (C1) into an aqueous alkali silicate solution and a residue, and supplying at least a portion of this solid-liquid separated aqueous alkali silicate solution to step (A1). That is, the raw material (2) in step (A1) of the first cycle may be a solution obtained by dissolving a residue obtained by a conventional method for dissolving rice husk ash and performing solid-liquid separation.

[0048] Process (B1) Step (B1) is a step of subjecting the first slurry prepared in step (A1) to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain a first aqueous alkali silicate solution, which is the target aqueous alkali silicate solution, and a residue.

[0049] 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.

[0050] 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.

[0051] From the overall viewpoint of filtering capacity, separation performance, etc., solid-liquid separation is preferably carried out by a filter press.

[0052] Solid-liquid separation using a filter press preferably involves pressure filtration at a pressure of 0.15 to 0.8 MPa. The pressure in pressure filtration is preferably 0.2 to 0.7 MPa, more preferably 0.3 to 0.7 MPa.

[0053] Furthermore, the solid-liquid separation using a filter press 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 may be 0.3 to 0.7 MPa.

[0054] In step (B1), the first slurry is subjected to solid-liquid separation to obtain a first aqueous alkali silicate solution, which is the target aqueous alkali silicate solution, and a residue.

[0055] The quality of the target aqueous alkali silicate solution obtained in step (B1) may be adjusted as necessary after step (B1), for example, by adding silica sand to adjust the silica concentration, or by adding other aqueous alkali silicate solutions, sodium hydroxide, and known additives to adjust the molar ratio, pH, and other properties.

[0056] Process (C1) Step (C1) is a step of mixing the residue obtained in step (B1) and raw materials (5) and (6) to prepare a second slurry containing an aqueous alkali silicate solution containing silica extracted from the residue. (5) Sodium hydroxide (6)Water

[0057] The present inventors have found that the residue contains undissolved silica and also contains an aqueous alkali silicate solution in an amount approximately 2 to 5 times the mass of the solid content. The steps (C1) and (D1) are intended to dissolve the undissolved silica, extract the aqueous alkali silicate solution remaining in the residue, and recover the silica as a new aqueous alkali silicate solution.

[0058] A part or all of the raw materials (5) and (6) may be added in the form of an aqueous sodium hydroxide solution. The raw materials (5) and (6) function as a solvent for dissolving the residue.

[0059] The amount of raw material (5) 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 (5) used is, for example, 5 to 50 parts by mass, preferably 8 to 40 parts by mass, and more preferably 10 to 35 parts by mass, per 100 parts by mass of the residue. The amount of raw material (6) 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 (6) used is, for example, 50 to 500 parts by mass, or may be 60 to 400 parts by mass or 70 to 300 parts by mass, per 100 parts by mass of the residue. By using raw materials (5) and (6) in the above ranges, dissolution of undissolved silica in the residue is promoted.

[0060] The temperature of the water in raw material (6) is preferably warm water from the viewpoint of promoting dissolution of silica in the residue. 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.

[0061] The temperature and pressure when preparing the second slurry can be adjusted as appropriate. From the viewpoint of promoting dissolution of silica in the residue, the second slurry may be prepared under high-temperature and pressurized conditions. When preparing the second slurry under high-temperature and pressurized conditions, it is preferable to perform the preparation under conditions of a temperature of 105 to 300°C and a pressure of 0.20 to 8.6 MPa. The temperature when preparing the second slurry is preferably 110 to 250°C, more preferably 115 to 200°C. The pressure when preparing the second slurry is preferably 0.25 to 5.0 MPa, more preferably 0.30 to 2.0 MPa. The preparation of the second slurry under high-temperature and pressurized conditions can be performed using a pressure vessel such as an autoclave.

[0062] In the preparation of the second slurry in step (C1), it is preferable to adjust the temperature and dissolution time of the mixture so that the dissolution rate of silica in the rice husk ash is 90 mass % or more in total, which can further increase the utilization rate of silica.

[0063] In step (C1), it is preferable to adjust the blending and mixing conditions of the raw materials when preparing the second slurry so that the molar ratio of the aqueous alkali silicate solution in the second slurry is 2.6 or less. The residue obtained in step (B1) contains a large amount of silica components that are difficult to dissolve. When the molar ratio of the aqueous alkali silicate solution in the second slurry is 2.6 or less (i.e., the molar amount of silica is relatively low), the difficult to dissolve silica components are easily dissolved, and the dissolution of undissolved silica in the residue is further promoted. It is more preferable that the molar ratio of the aqueous alkali silicate solution in the second slurry is 2.5 or less, 2.4 or less, 2.3 or less, 2.2 or less, or 2.1 or less.

[0064] Process (D1) Step (D1) is a step of subjecting the second slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue, and supplying at least a portion of the solid-liquid separated second aqueous alkali silicate solution to step (A1) as raw material (2). In the first production method of the present invention, supplying at least a portion of the second aqueous alkali silicate solution to step (A1) as raw material (2) can increase the utilization rate of silica.

[0065] The method of solid-liquid separation is not particularly limited. Solid-liquid separation can be performed using the same general solid-liquid separation device as described in step (B1). From the overall viewpoint of filtering capacity, separation performance, etc., solid-liquid separation is preferably performed using a filter press. Solid-liquid separation may be performed only once, or may be performed multiple times from the viewpoint of removing more foreign matter.

[0066] Solid-liquid separation using a filter press preferably involves pressure filtration at a pressure of 0.15 to 0.8 MPa. The pressure in pressure filtration is preferably 0.2 to 0.7 MPa, more preferably 0.3 to 0.7 MPa.

[0067] Furthermore, solid-liquid separation using a filter press preferably includes squeezing the residue under a pressure condition 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 may be 0.3 to 0.7 MPa. The squeezing performed before water washing is preferably performed at a lower pressure than the squeezing performed after water washing. This can promote penetration of the wash water into the residue during water washing.

[0068] A part or all of the second aqueous alkali silicate solution is supplied to step (A1) as raw material (2). From the viewpoint of reducing waste of resources, it is preferable to supply a large amount of the second aqueous alkali silicate solution to step (A1). For example, the amount supplied to step (A1) is 50% by mass or more, more preferably 75% by mass or more, of the second aqueous alkali silicate solution obtained in step (D1), and it is even more preferable to supply the entire amount to step (A1).

[0069] Preferably, step (D1) further comprises washing the residue obtained by solid-liquid separation of the second slurry with water and supplying at least a portion of the post-washing solution to step (A1) as raw material (7). The "post-washing solution" refers to the solution discharged after water washing (including squeezing), and in the present invention, the post-washing solution contains an aqueous alkali silicate solution extracted from the residue. As described above, the inventors have found that the aqueous alkali silicate solution remains in the residue in an amount approximately 2 to 5 times the mass of the solid content. By washing with water, the aqueous alkali silicate solution remaining in the residue can be extracted, allowing silica to be recovered. The post-washing solution containing the recovered silica is then supplied to step (A1) and used as raw material (7) in step (A1), thereby further improving the utilization rate of silica.

[0070] The post-washing solution is a solution obtained by diluting an aqueous alkali silicate solution with water, the molar ratio and silica concentration of which are the same as those of the second aqueous alkali silicate solution. Because the solution is merely diluted, the molar ratio of silica to alkali metal components in the post-washing solution is the same as that of the second aqueous alkali silicate solution. However, due to dilution, the silica concentration of the post-washing solution is lower than that of the second aqueous alkali silicate solution. The amount of water used for washing with water 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 facilitates the use of the entire post-washing solution and ensures a sufficient silica content, making it easier to balance the formulation in step (A1). Furthermore, a silica concentration of 15% by mass or less ensures a sufficient amount of water for washing with water, facilitating the extraction of the aqueous alkali silicate solution from the residue and increasing the amount of silica recovered. The amount of water used for washing with water 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.

[0071] In addition, from the viewpoint of easily adjusting the silica concentration of the solution after washing to the range of 0.5 to 15 mass%, the amount of water used for 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.

[0072] The water used for washing is not particularly limited and may be room temperature water or hot water, with hot water being preferred from the viewpoint of increasing the amount of aqueous alkali silicate solution extracted from the residue. The temperature of the hot water is, for example, 30 to 80°C, preferably 40 to 80°C, or may be 50 to 70°C. From the viewpoint of reducing energy consumption during production, it is preferred to use hot water that utilizes heat recovery.

[0073] The water washing preferably includes squeezing the residue under a pressure of 0.15 to 0.8 MPa. The preferred conditions for squeezing are the same as those for squeezing in step (B1).

[0074] A part or all of the post-washing solution can be supplied to step (A1) as raw material (7). From the viewpoint of reducing waste of resources, it is preferable to supply a large amount of the post-washing solution to step (A1). For example, the amount of raw material (7) supplied to step (A1) is 50% by mass or more of the post-washing solution obtained in step (D1), more preferably 75% by mass or more, and even more preferably the entire amount is supplied to step (A1). The post-washing solution of raw material (7) may be mixed with the second aqueous alkali silicate solution of raw material (2) and then supplied to step (A1), or it may be supplied to step (A1) separately from raw material (2).

[0075] In the first production method of the present invention, the residue produced in step (D1) can be discarded. Furthermore, the residue produced in step (B1) of the last cycle can be discarded.

[0076] <Effects of the first manufacturing method> The first 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, more preferably 80% or more, and particularly preferably 90% or more.

[0077] 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 adding 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.

[0078] The above-described slurry preparation method assumes that a certain amount of rice husk ash remains undissolved as a solid, and a large amount of residue is generated after solid-liquid separation. The present inventors have discovered that this residue contains undissolved silica components, and that an aqueous alkali silicate solution remains in an amount approximately 2 to 5 times the mass of the solid content. In particular, when the dissolution rate of silica 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 solid 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 utilization rate of silica).

[0079] The present inventors have therefore conducted extensive research into methods for increasing the utilization rate of silica throughout the entire production process of an aqueous alkali silicate solution, resulting in the present invention. The first production method of the present invention includes the steps of dissolving the residue of the first slurry in an aqueous sodium hydroxide solution to prepare a second slurry, and then subjecting the second slurry to solid-liquid separation to obtain a second aqueous alkali silicate solution. At least a portion of the second aqueous alkali silicate solution is supplied to step (A1) as raw material (2) and serves as the raw material for producing the target (first) aqueous alkali silicate solution. The second aqueous alkali silicate solution contains silica recovered by dissolving undissolved silica and extracting the aqueous alkali silicate solution remaining in the residue. As a result, the amount of silica discarded is reduced compared to conventional methods, and the utilization rate of silica is increased in the production process of an aqueous alkali silicate solution.

[0080] In particular, by balancing the raw material blending in consideration of the dissolution rate of rice husk ash in the preparation of the first and second slurries, it is possible to continuously produce an aqueous alkali silicate solution using only raw material (1) and the aqueous alkali silicate solution supplied from step (D1) as the silica source throughout the cycle of steps (A1) to (D1).

[0081] <Method for producing second aqueous alkali silicate solution> The second method for producing an aqueous alkali silicate solution of the present invention (hereinafter simply referred to as the "second production method") comprises the following steps (A2) to (E), and is a method for obtaining a target aqueous alkali silicate solution having an SiO / (NaO+KO) molar ratio in the range of 3.0 to 4.0 and a silica concentration in the range of 13 to 28 mass %: ·Process (A2) A step of mixing the following raw materials (1), (3), and (4) to prepare a first slurry containing an aqueous alkali silicate solution containing silica extracted from rice husk ash; (1) Rice husk ash, (3) sodium hydroxide, (4) Water; ·Process (B2) a step of subjecting the first slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain a first aqueous alkali silicate solution and a residue; ·Process (C2) a step of mixing the residue obtained in step (B2) and the following raw materials (5) and (6) to prepare a second slurry containing an aqueous alkali silicate solution containing silica extracted from the residue; (5) sodium hydroxide, (6) Water; ·Process (D2) a step of subjecting the second slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain a second aqueous alkali silicate solution and a residue; ·Process (E) A step of mixing the first and second aqueous alkali silicate solutions to obtain the desired aqueous alkali silicate solution.

[0082] In the second production method of the present invention, a series of steps from step (A2) to step (E) constitute one cycle, which can be 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.

[0083] The target aqueous alkali silicate solution obtained by the second production method of the present invention has a SiO2 / (Na2O+K2O) molar ratio in the range of 3.0 to 4.0 and a silica concentration in the range of 13 to 28 mass %, similar to the first production method described above. Other characteristics (properties, uses, etc.) of the target aqueous alkali silicate solution are the same as those of the first production method.

[0084] Process (A2) Step (A2) is a step of mixing the following raw materials (1), (3), and (4) to prepare a first slurry containing an aqueous alkali silicate solution containing silica extracted from rice husk ash. (1) Rice husk ash (3) Sodium hydroxide (4) Water

[0085] The raw material (1) is rice husk ash, which is a silica source. As in the first manufacturing method, 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 mass% or less. The rice husk ash is the same as in the first manufacturing method.

[0086] Raw material (3) is sodium hydroxide, and raw material (4) is water. Some or all of 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 also contained in raw material (7) described below, if the water contained in raw material (7) is sufficient for preparing the first slurry, raw material (4) does not need to be added separately.

[0087] 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 8 to 40 parts by mass, and more preferably 10 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, 10 to 500 parts by mass, or may be 50 to 400 parts by mass or 90 to 250 parts by mass, per 100 parts by mass of rice husk ash. By using raw materials (3) and (4) in 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 (A2)) for each cycle is reduced, and continuous production of the aqueous alkali silicate solution is promoted.

[0088] The temperature of the water in 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.

[0089] In the second production method of the present invention, as described below, when step (D2) further includes washing the residue obtained by solid-liquid separation of the second slurry with water, subjecting the washed solution to solid-liquid separation from the residue, and supplying at least a portion of the solid-liquid separated washed solution to step (A2) as raw material (7), the first slurry can be prepared in step (A2) by mixing raw materials (1), (3), and (7) or raw materials (1), (3), (4), and (7). Because raw material (7) contains silica recovered from the residue, adding raw material (7) to the raw materials for the first slurry can further increase the utilization rate of silica.

[0090] The amount of raw material (7) used can be appropriately adjusted 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 (7). The amount of raw material (7) used is, for example, 10 to 500 parts by mass, preferably 50 to 400 parts by mass, and more preferably 100 to 300 parts by mass, per 100 parts by mass of rice husk ash.

[0091] In the second production method of the present invention, raw materials other than raw materials (1), (3), (4), and (7) can be added. Examples of such raw materials include silica-based minerals, which 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 second production method of the present invention, it is preferable not to add silica-based minerals in step (A2), and thus in steps (A2) to (E). In the production method of the present invention, it is possible to continuously produce an alkali silicate aqueous solution by using only raw material (1) and the alkali silicate aqueous solution supplied from step (D2) as silica sources throughout the cycle of steps (A2) to (E).

[0092] The blending of raw materials is preferably balanced so that the silica dissolution rate in rice husk ash is estimated in advance and the target molar ratio (3.0-4.0) and silica concentration (13-28% by mass) are achieved when the estimated silica dissolution rate is reached. This allows the aqueous alkali silicate solution in the first slurry to have a SiO2 / (Na2O+K2O) molar ratio in the range of 3.0-4.0 and a silica concentration in the range of 13-28% by mass at the end of step (A2). The silica dissolution rate in rice husk ash is the ratio of the mass of silica dissolved in the aqueous alkali silicate solution to the total mass of silica in the rice husk ash; a detailed calculation method is described in the Examples section.

[0093] In step (A2), 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 the rice husk ash is 100% (all of the silica in the rice husk ash is dissolved), the molar ratio of silica, sodium oxide, and potassium oxide supplied from the raw materials exceeds the molar ratio of the target aqueous alkali silicate solution. Condition (b) specifies that, assuming that the dissolution rate of silica in the rice husk ash is 100%, the silica concentration based on silica supplied from the raw materials exceeds the silica concentration of the target aqueous alkali silicate solution. Note that in the second production method of the present invention, raw material (2) does not exist and therefore need not be taken into consideration. Furthermore, if raw material (7) is not supplied to step (D2), the amounts of silica, sodium oxide, and potassium oxide supplied from raw material (7) are zero and need not be taken into consideration. (a) The SiO2 / (Na2O+K2O) molar ratio 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 materials (2) and (7), and the amount of raw material (3) converted to sodium oxide exceeds the SiO2 / (Na2O+K2O) molar ratio of the target aqueous alkali silicate solution; (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 materials (2) and (7), and the amount of water in the raw materials exceeds the silica concentration of the target aqueous alkali silicate solution.

[0094] In other words, in the second production method of the present invention, conditions (a) and (b) stipulate that the amount of silica supplied from raw material (1), and further from raw material (7), if present, exceeds the amount of silica required to produce the target aqueous alkali silicate solution. When conditions (a) and (b) are satisfied, the aqueous alkali silicate solution in the slurry can have a molar ratio and silica concentration equal to or greater than the target values ​​when the required amount of silica is dissolved from the rice husk ash.

[0095] In the preparation of the first slurry in step (A2), it is preferable to adjust the temperature of the mixture and the dissolution time so that the dissolution rate of silica in the rice husk ash is 70% by mass or more and less than 90% by mass. From the viewpoint of reducing waste of resources, the dissolution rate of silica in step (A2) is preferably 73% or more, more preferably 75% or more. From the viewpoint of shortening the dissolution time, the dissolution rate of silica in step (A2) is preferably 87% or less, more preferably 85% or less.

[0096] The temperature and pressure when the raw materials are mixed to prepare the first slurry can be adjusted as appropriate. From the viewpoint of promoting dissolution of the rice husk ash, the first slurry is preferably prepared under high-temperature and pressurized conditions, preferably at a temperature of 105 to 300°C and a pressure of 0.20 to 8.6 MPa. The temperature when preparing the first slurry is preferably 110 to 250°C, more preferably 115 to 200°C. The pressure when preparing the first slurry is preferably 0.25 to 5.0 MPa, more preferably 0.30 to 2.0 MPa. The first slurry can be prepared under high-temperature and pressurized conditions using a pressure vessel such as an autoclave.

[0097] The dissolution time of the raw materials when preparing the first slurry by mixing the raw materials can be adjusted appropriately. The preparation of the first slurry in step (A2) 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 the 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.

[0098] In step (A2) of the second production method of the present invention, the post-wash solution obtained through the preceding steps (C2) and (D2) may be used as raw material (7). In this case, the preceding step (C2) of step (A2) in the first cycle may be a step of dissolving rice husk ash residue obtained by a conventional method in a sodium hydroxide solution to prepare a slurry. Furthermore, the preceding step (D2) of step (A2) in the first cycle may be a step of solid-liquid separating the slurry obtained in the preceding step (C2) into an alkali silicate aqueous solution and a residue to obtain a second alkali silicate aqueous solution and a residue. In other words, the raw material (7) in step (A2) of the first cycle may be a solution obtained by dissolving residue obtained by a conventional rice husk ash dissolution method, solid-liquid separating the solution, and washing with water.

[0099] Process (B2) The step (B2) is a step of subjecting the first slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain a first aqueous alkali silicate solution and a residue.

[0100] 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.

[0101] 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.

[0102] From the overall viewpoint of filtering capacity, separation performance, etc., solid-liquid separation is preferably carried out by a filter press.

[0103] Solid-liquid separation using a filter press preferably involves pressure filtration at a pressure of 0.15 to 0.8 MPa. The pressure in pressure filtration is preferably 0.2 to 0.7 MPa, more preferably 0.3 to 0.7 MPa.

[0104] Furthermore, the solid-liquid separation using a filter press 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 may be 0.3 to 0.7 MPa.

[0105] Process (C2) Step (C2) is a step of mixing the residue obtained in step (B2) and raw materials (5) and (6) to prepare a second slurry containing an aqueous alkali silicate solution containing silica extracted from the residue. (5) Sodium hydroxide (6)Water

[0106] The present inventors have found that the residue contains undissolved silica and also contains an aqueous alkali silicate solution in an amount approximately 2 to 5 times the mass of the solid content. The steps (C2) and (D2) are intended to dissolve the undissolved silica, extract the aqueous alkali silicate solution remaining in the residue, and recover the silica as a new aqueous alkali silicate solution.

[0107] A part or all of the raw materials (5) and (6) may be added in the form of an aqueous sodium hydroxide solution. The raw materials (5) and (6) function as a solvent for dissolving the residue.

[0108] The amounts of raw materials (5) and (6) used are the same as in the first production method. The temperature of the water in raw material (6) is the same as in the first production method. The temperature and pressure when preparing the second slurry are the same as in the first production method.

[0109] In the preparation of the second slurry in step (C2), it is preferable to adjust the temperature and dissolution time of the mixture so that the dissolution rate of silica in the rice husk ash is 90 mass % or more in total, which can further increase the utilization rate of silica.

[0110] In step (C2), it is preferable to adjust the blending of raw materials and mixing conditions when preparing the second slurry so that the molar ratio of the aqueous alkali silicate solution in the second slurry is 2.6 or less. The residue obtained in step (B2) contains a large amount of silica components that are difficult to dissolve. By making the molar ratio of the aqueous alkali silicate solution in the second slurry 2.6 or less (i.e., the molar amount of silica is relatively low), the difficult to dissolve silica components are easily dissolved, and the dissolution of undissolved silica in the residue is further promoted. It is more preferable that the molar ratio of the aqueous alkali silicate solution in the second slurry is 2.5 or less, 2.4 or less, 2.3 or less, 2.2 or less, or 2.1 or less.

[0111] Process (D2) Step (D2) is a step of subjecting the second slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain a second aqueous alkali silicate solution and a residue.

[0112] The method of solid-liquid separation is not particularly limited. Solid-liquid separation can be performed using the same general solid-liquid separation device as described in step (B2). From the overall viewpoint of filtering capacity, separation performance, etc., solid-liquid separation is preferably performed using a filter press. Solid-liquid separation may be performed only once, or may be performed multiple times from the viewpoint of removing more foreign matter.

[0113] Solid-liquid separation using a filter press preferably involves pressure filtration at a pressure of 0.15 to 0.8 MPa. The pressure in pressure filtration is preferably 0.2 to 0.7 MPa, more preferably 0.3 to 0.7 MPa.

[0114] Furthermore, solid-liquid separation using a filter press preferably includes squeezing the residue under a pressure condition 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 may be 0.3 to 0.7 MPa. The squeezing performed before water washing is preferably performed at a lower pressure than the squeezing performed after water washing. This can promote penetration of the wash water into the residue during water washing.

[0115] Preferably, step (D2) further comprises washing the residue obtained by solid-liquid separation of the second slurry with water and supplying at least a portion of the washed solution to step (A2) as raw material (7). By washing with water, the aqueous alkali silicate solution remaining in the residue can be extracted to recover silica. The washed solution containing the recovered silica is then supplied to step (A2) and used as raw material (7) in step (A2), thereby further improving the utilization rate of silica.

[0116] The post-washing solution is a solution obtained by diluting an aqueous alkali silicate solution with water, the molar ratio and silica concentration of which are the same as those of the second aqueous alkali silicate solution. Because the solution is merely diluted, the molar ratio of silica to alkali metal components in the post-washing solution is the same as that of the second aqueous alkali silicate solution. In contrast, the dilution results in a lower silica concentration in the post-washing solution than that of the second aqueous alkali silicate solution. The amount of water used for washing with water 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 facilitates the use of the entire post-washing solution and ensures a sufficient silica content, making it easier to balance the formulation in step (A2). Furthermore, a silica concentration of 15% by mass or less ensures a sufficient amount of water for washing with water, facilitating the extraction of the aqueous alkali silicate solution from the residue and increasing the amount of silica recovered. The amount of water used for washing with water 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.

[0117] The amount of water used for washing with water per 100 parts by mass of the residue is the same as in the first production method. The temperature of the water used for washing with water is the same as in the first production method.

[0118] The water washing preferably includes squeezing the residue under a pressure of 0.15 to 0.8 MPa. The preferred squeezing conditions are the same as those in the step (B2).

[0119] A part or all of the post-washing solution can be supplied to step (A2) as raw material (7). From the viewpoint of reducing waste of resources, it is preferable to supply a large amount of the post-washing solution to step (A2). For example, the amount of raw material (7) supplied to step (A2) is 50% by mass or more of the post-washing solution obtained in step (D2), more preferably 75% by mass or more, and even more preferably the entire amount is supplied to step (A2).

[0120] In the second production method of the present invention, the residue produced in step (D1) can be discarded. Furthermore, the residue produced in step (B1) of the last cycle can be discarded.

[0121] Process (E) In step (E), the first and second aqueous alkali silicate solutions are mixed to obtain the desired aqueous alkali silicate solution. In the second production method of the present invention, the utilization rate of silica can be increased by adding the second aqueous alkali silicate solution containing silica recovered from the residue to the first aqueous alkali silicate solution.

[0122] After the target aqueous alkali silicate solution is obtained in step (E), the quality of the aqueous alkali silicate solution may be adjusted as necessary after step (E). This quality adjustment includes, for example, adding silica sand 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.

[0123] <Effects of the second manufacturing method> The second 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, more preferably 80% or more, and particularly preferably 90% or more.

[0124] The second production method of the present invention, like the first production method described above, also includes the steps of dissolving the residue of the first slurry in aqueous sodium hydroxide to prepare a second slurry, and then subjecting the second slurry to solid-liquid separation to obtain a second aqueous alkali silicate solution. The second aqueous alkali silicate solution is mixed with the first aqueous alkali silicate solution obtained in step (A2) to obtain the target aqueous alkali silicate solution. The second aqueous alkali silicate solution contains silica recovered by dissolving undissolved silica and extracting the aqueous alkali silicate solution remaining in the residue. As a result, the amount of silica discarded is reduced compared to conventional methods, and the utilization rate of silica in the production method of an aqueous alkali silicate solution is increased.

[0125] In particular, by taking into consideration the dissolution rate of rice husk ash and balancing the blending of raw materials in the preparation of the first and second slurries, it is possible to continuously produce an aqueous alkali silicate solution throughout the cycle of steps (A2) to (E) using only raw material (1) and the aqueous alkali silicate solution supplied from step (D2) as the silica source. [Example]

[0126] 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.

[0127] <Analysis method> Moisture content of rice husk ash (loss on heating) (mass%) The water content was determined from the weight loss value after drying at 105°C for 2 hours in accordance with JIS K5101-15-1 (pigment testing method - heat loss).

[0128] ●Apparent density of rice husk ash (g / L) Based on JIS K5101-12-1:2004 (Pigment Test Method - Apparent Density or Apparent Specific Volume - Static Method), the density was determined using a dedicated measuring device (0.5 mm mesh sieve, funnel, 30 mL cylindrical receiver, receiver stand, and 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 the rice husk ash, expressed in g / L, was calculated based on its mass using the following formula. Formula 3:

number

[0129] ●Bulk density of rice husk ash (g / mL) Based on Section 7.8, "Bulk Density," of JIS K6220-1:2015 (Rubber Compounding Agents - Organic Chemicals - Test Methods - Part 1: General), measurements were performed using a dedicated measuring device (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 in, 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. The height of the piston protruding from the top of the cylinder was then 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 equation: Formula 4:

number

[0130] ●Volume average particle size of rice husk ash (D50) 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. Note that the sample was directly placed in the analyzer and measured without ultrasonic dispersion treatment or the like.

[0131] Carbon content in rice husk ash or solids (mass%) As a pretreatment, the 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 method. The sample was heated at a temperature of 1,350°C, with an oxygen inlet pressure of 0.24 MPa and a measurement time of 50 seconds, and CO gas and CO gas were quantified using the infrared detector (NDIR) inside the analyzer.

[0132] Composition of inorganic components other than carbon in rice husk ash or solids (mass%) 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 type 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 instrument 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.

[0133] ●Component composition of aqueous solution An aqueous alkali silicate solution sample (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 mass ppm). The mass concentrations of the components contained, such as SiO2, Na2O, and K2O, were quantified using a high-resolution ICP optical emission spectrometer (model: PS3520DDII, Hitachi High-Tech Science Corporation).

[0134] Silica concentration and molar ratio in alkali silicate solution The mass concentrations of SiO2, Na2O, and K2O obtained by measuring the component composition of the aqueous solution were converted into molar amounts to determine the SiO2 / (Na2O + K2O) molar ratio. Specifically, the molar masses of each component were set to 60.08 (SiO2), 61.98 (Na2O), and 94.19 (K2O), and the molar ratio was calculated using the following formula: Formula 5:

number

[0135] Mass of solids and remaining solution in the 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 the solid content in the residue after filtration was measured. The mass of the remaining solution in the residue was calculated by subtracting the mass of the solid content from the total mass of the residue after filtration.

[0136] 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 and after dissolution. The "aqueous solution" in the formula refers to the alkali silicate aqueous solution. 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:

number

[0137] ●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 silica mass contained in the target alkali silicate aqueous solution and the silica mass recovered from the residue and recycled and used as a raw material for producing the alkali silicate aqueous solution; in other words, it is the mass of silica that was input as rice husk ash but not discarded. Formula 1:

number

[0138] <Preparing rice husk ash as 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: FR125 × 40, manufactured by Freund Turbo) at a roll rotation speed of 9 rpm and a linear pressure of 5.8 × 10 3 This rice husk ash was compressed and granulated under conditions of a density of 1000 N / cm and a screw rotation speed of 90 rpm to obtain 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.

[0139] [Table 1]

[0140] <Reference example 1> First, we investigated the solubility of silica in aqueous sodium hydroxide solution in rice husk ash A. Assuming that 100% of the silica present in rice husk ash A was dissolved in aqueous sodium hydroxide solution (silica solubility rate 100%), raw materials were prepared with a composition that would result in the target silica concentration and molar ratio of 18.0 mass% and 3.3, respectively.

[0141] Specifically, 120 L of warm water (40-50°C, hereinafter the same) and 26.42 kg of 48.0% by mass sodium hydroxide 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 the raw materials was approximately 19.7%. The slurry temperature was then 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. Approximately 20 mL of samples were taken at appropriate intervals during the rice husk ash dissolution process, and after filtering with No. 5C filter paper (manufactured by Advantec), the silica concentration and molar ratio of the resulting alkali silicate aqueous solution were measured.

[0142] 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 goal 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% by 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.

[0143] [Table 2]

[0144] 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.

[0145] <Comparative Example 1> Comparative Example 1 is an example of a dissolution method in which the raw material blending was improved so that the silica concentration and molar ratio of the alkali silicate aqueous solution reached the target values ​​of 18.0 mass% and 3.3, respectively, in a shorter dissolution time than in Reference Example 1, specifically, in 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 was the same as that in Reference Example 1.

[0146] The procedure for preparing the aqueous alkali silicate solution in Comparative Example 1 is as follows. Step (A): Mixing raw materials and preparing slurry 93.50 kg of hot water and 20.43 kg of 48.0% by 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 the 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. Approximately 20 mL of samples were taken at appropriate intervals during the rice husk ash dissolution process, 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.

[0147] 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 a 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.

[0148] Comparative Example 2 (Study on improving silica utilization rate) Comparative Example 2 is an example of a dissolution method improved based on the results of Reference Example 1 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 alkali silicate aqueous solution reached the target values ​​of 18.0 mass% and 3.3, respectively, within a dissolution time of 4 hours (assumed dissolution rate of approximately 85%).

[0149] The procedure for preparing the aqueous alkali silicate solution in Comparative Example 2 is as follows. Step (A): Mixing raw materials and preparing slurry 101.90 kg of hot water and 22.33 kg of 48.0% by 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 the 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. Approximately 20 mL of samples were taken at appropriate intervals during the rice husk ash dissolution process, 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 In the same manner as in Comparative Example 1, the slurry obtained in step (A) was filtered to obtain an aqueous alkali silicate solution and a residue.

[0150] Comparative Example 3 (Change in molar ratio) Comparative Example 3 is an example of a dissolution method in which the molar ratio of silica was changed without changing the dissolution 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 reached the target values ​​of 18.0 mass% and 3.7, respectively, over a dissolution time of 2 hours (assumed dissolution rate of approximately 78%).

[0151] The procedure for preparing the aqueous alkali silicate solution in Comparative Example 3 is as follows. Step (A): Mixing raw materials and preparing slurry A 240 L stainless steel dissolution tank equipped with a stirrer was charged with 95.77 kg of hot water and 18.13 kg of 48.0 mass% sodium hydroxide aqueous solution. While stirring, 36.00 kg of rice husk ash A was added over 20 minutes. The mass ratio of rice husk ash A to the total mass of the 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. Approximately 20 mL of the sample was taken at appropriate intervals during the dissolution of the rice husk ash. 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 In the same manner as in Comparative Example 1, the slurry obtained in step (A) was filtered to obtain an aqueous alkali silicate solution and a residue.

[0152] <Results of Comparative Examples 1 to 3> The relationships between the dissolution time and the silica concentration, molar ratio, and silica dissolution rate in Comparative Examples 1 to 3 are shown in Table 3 below. In particular, FIG. 2 is a graph showing the relationships between the dissolution time and the silica concentration, molar ratio, and silica dissolution rate in Comparative Example 1, and FIG. 3 is a graph showing the relationships between the dissolution time and the silica concentration, molar ratio, and silica dissolution rate in Comparative Example 2. Furthermore, Table 4 shows the amounts of each raw material charged and the component breakdown of each raw material in Comparative Examples 1 to 3. Table 5 shows the masses of the aqueous alkali silicate solutions and residues after solid-liquid separation, their component breakdown, and silica utilization rates in Comparative Examples 1 to 3.

[0153] In Table 4, the masses of rice husk ash, sodium hydroxide solution, and water are measured values, the component breakdown in the rice husk ash is a calculated value based on the analytical values ​​of the carbon content and inorganic component composition in the rice husk, and the component breakdown in the sodium hydroxide 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 measured values, and the component breakdown in the alkali silicate aqueous solution (filtrate) is a calculated value based on the analytical values ​​of the aqueous solution's component composition. 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 content 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 aqueous solution's component composition. The same applies to the other tables.

[0154] [Table 3]

[0155] Step (A): Mixing raw materials and preparing slurry [Table 4]

[0156] Step (B): Solid-liquid separation of slurry [Table 5]

[0157] <Explanation of the results of Comparative Examples 1 to 3> The results in Table 3 show that the silica dissolution rate at each dissolution time remains almost constant even when the ratio of rice husk ash to the total raw material and the target molar ratio are changed. In Comparative Example 1 (estimated dissolution rate of approximately 78%, target molar ratio of 3.3), the silica concentration and molar ratio reached the target values ​​in just 2 hours, as expected, compared to Reference Example 1. In Comparative Example 2 (estimated dissolution rate of approximately 85%, target molar ratio of 3.3), the silica concentration and molar ratio reached the target values ​​in just 4 hours, as expected, compared to Reference Example 1. In Comparative Example 3 (estimated dissolution rate of approximately 78%, target molar ratio of 3.7), a higher molar ratio of 3.7 was achieved in a dissolution time similar to that of Comparative Example 1, as expected.

[0158] As shown in Table 5, the amount of aqueous alkali silicate solution obtained in Comparative Examples 1 and 3 was approximately 103 kg, and the amount of 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 desired aqueous alkali silicate solution can be obtained from the same amount of rice husk ash (Table 4). The results of Comparative Examples 1 and 3 show that the molar ratio of the aqueous alkali silicate solution can be appropriately changed by balancing the blending of raw materials.

[0159] In addition, as shown in Table 5, the residue after filtration is a cake-like mixture containing solids that did not dissolve in the sodium hydroxide aqueous solution and the remaining solution, the alkali silicate aqueous solution. The mass of the residue is approximately five times the solids, which indicates that a large amount of the alkali silicate aqueous solution that should have been extracted remains in the residue.

[0160] Specifically, the amount of residue in Comparative Examples 1 and 3 was approximately 46.7 kg (b3 in Table 5), of which approximately 9.3 kg (b10 in Table 5) was undissolved solids, and the remaining approximately 37.4 kg (b11 in Table 5) was the alkali silicate aqueous solution that should have been extracted. Component analysis revealed that the amount of silica in the solids was approximately 7.1 kg (b4 in Table 5). Estimated from the concentration of the alkali silicate aqueous solution, the amount of silica that was dissolved in the aqueous solution but discarded and not utilized was approximately 6.7 kg (b5 in Table 5). Therefore, the amount of discarded silica is the sum of these, approximately 13.8 kg (b7 in Table 5). As a result, the silica utilization rate in Comparative Examples 1 and 3 was a low 57.3% (b9 in Table 5).

[0161] The amount of residue in Comparative Example 2 was approximately 35.4 kg (b3 in Table 5), of which approximately 7.1 kg (b10 in Table 5) was undissolved solids, and the remaining approximately 28.3 kg (b11 in Table 5) was the alkali silicate aqueous solution that should have been extracted. Component analysis revealed that the amount of silica in the solids was approximately 4.9 kg (b4 in Table 5). Estimated from the concentration of the alkali silicate aqueous solution, the amount of silica that was dissolved in the aqueous solution but discarded and not utilized was approximately 5.1 kg (b5 in Table 5). Because the silica dissolution rate in Comparative Example 2 was set higher than in Comparative Examples 1 and 3, the amount of residue and the amount of discarded silica were reduced compared to Comparative Example 1. Nevertheless, when combined with the undissolved silica in the solids, 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 still room for further improvement.

[0162] The results of Comparative Examples 1 to 3 revealed 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 conventional methods, undissolved silica components remain in the residue after filtration, and in addition, a portion of the aqueous alkali silicate solution remains in the residue. Therefore, even if the silica dissolution rate is set to about 85%, the silica utilization rate remains below 70%.

[0163] The present inventors have noticed that, as described above, in conventional methods for preparing a slurry, undissolved silica components are present in the residue after filtration, and that part of the aqueous alkali silicate solution remains in the residue, resulting in silica that is not utilized in the production of the aqueous alkali silicate solution. As a result of extensive research, the present inventors have discovered a new method that enables the production of the desired aqueous alkali silicate solution while recovering such silica, thereby successfully improving the silica utilization rate.

[0164] <Examples of the present invention> The present invention will be described in detail below using examples. Examples 1 and 2 are examples of the first method for producing an aqueous alkali silicate solution of the present invention, corresponding to the first and second cycles, respectively. Examples 3 and 4 are examples of the second method for producing an aqueous alkali silicate solution of the present invention, corresponding to the first and second cycles, respectively.

[0165] Example 1 (First manufacturing method, first cycle) In Example 1, the residue obtained in Comparative Example 1 is further dissolved in a sodium hydroxide solution to recover silica as a second aqueous alkali silicate solution. The second aqueous alkali silicate solution is then used as a raw material in the preparation of a first slurry in step (A1), and the first slurry is subjected to solid-liquid separation to obtain the first aqueous alkali silicate solution, which is the target aqueous alkali silicate solution. As in Example 1, the pre-step (D1) prior to step (A1) of the first cycle of the present invention may be a pre-step (C1) in which a slurry is prepared using a residue obtained by a general method for dissolving rice husk ash, followed by solid-liquid separation of the slurry.

[0166] FIG. 4 is a flow chart schematically showing each step in the first production method of the present invention.

[0167] Pre-process (C1): Preparation of second slurry 46.71 kg of the residue obtained in Comparative Example 1, 11.49 kg of a 48.0 mass% sodium hydroxide aqueous solution, and 55.00 kg of warm water were placed in a 180 L stainless steel dissolution tank equipped with a stirrer, and the temperature of the slurry was raised to 90°C while stirring. Thereafter, while maintaining the liquid temperature at 90°C, the slurry was stirred until the molar ratio of the solution reached 2.0 (approximately 2 hours) to prepare a second slurry. During the slurry preparation process, approximately 20 mL of the sample was suitably taken and filtered with 5C filter paper (manufactured by ADVANTEC), after which the silica concentration and molar ratio of the obtained aqueous alkali silicate solution were measured.

[0168] Pre-process (D1): Solid-liquid separation of the second slurry and washing with water ·Solid-liquid separation The second slurry obtained in the previous step (C1) was filtered using a filter press at a charge pressure of 0.4 MPa to obtain 100.05 kg of a second aqueous alkali silicate solution and 13.15 kg of a residue. Approximately 20 mL of the second aqueous alkali silicate solution was removed and used as a sample for component analysis. For the residue after solid-liquid separation, the masses of the solids and residual solution were measured, and the components of the solids and residual solution were analyzed. ·Washing After solid-liquid separation, 13.15 kg of the residue was washed with 62.00 kg of warm water, yielding 11.57 kg of residue. 63.58 kg of post-wash solution was recovered in a recovery tank. The post-wash solution had a molar ratio of 2.12 and a silica concentration of 1.7% by mass. The resulting post-wash solution was mixed with the second aqueous alkali silicate solution to yield 163.63 kg of diluted aqueous alkali silicate solution. Approximately 20 mL of the diluted aqueous alkali silicate solution was removed and used as a sample for component analysis. 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.

[0169] This previous step (D1) is a previous step to step (A1) of Example 1 (next cycle), and the diluted aqueous alkali silicate solution contains a second aqueous alkali silicate solution corresponding to raw material (2) and a post-wash solution corresponding to raw material (7), and is supplied to step (A1) of Example 1 (next cycle) and used as a mixture of raw material (2) and raw material (7).

[0170] Step (A1): Mixing raw materials and preparing first slurry A 240 L stainless steel dissolution tank equipped with a stirrer was charged with 163.63 kg of the diluted alkali silicate aqueous solution (including the second alkali silicate aqueous solution and the post-wash solution) (reference numeral 2 in FIG. 4) obtained in the previous step (D1), 14.35 kg of a 48.0 mass% sodium hydroxide aqueous solution (reference numeral 3 in FIG. 4), and 8.15 kg of warm water (reference numeral 4 in FIG. 4) for adjusting the concentration. While continuing to stir, 36.00 kg of rice husk ash A (reference numeral 1 in FIG. 4) was added over a 20-minute period. The silica dissolution rate in the rice husk ash was set to approximately 78%, the same as in Comparative Example 1. The mass ratio of rice husk ash A to the total mass of the raw materials was approximately 16.2%. 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 (first 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.

[0171] Step (B1): Solid-liquid separation of first slurry The first slurry obtained in step (A1) was filtered using a filter press at a charge pressure of 0.4 MPa to obtain 46.71 kg of a residue and 175.41 kg of a first aqueous alkali silicate solution, which was the target aqueous alkali silicate solution having a molar ratio of 3.3 and a silica concentration of 18.0 mass%.

[0172] Step (C1): Preparation of second slurry 46.71 kg of the residue obtained in step (B1), 11.49 kg of a 48.0% by mass aqueous solution of sodium hydroxide, and 55.00 kg of hot water were placed in a 180 L stainless steel dissolution tank equipped with a stirrer, and the temperature of the slurry was raised to 90°C while stirring. Subsequently, while maintaining the liquid temperature at 90°C, the slurry was stirred until the molar ratio of the solution reached 2.0 (approximately 2 hours) to prepare a second slurry. During the slurry preparation process, approximately 20 mL of the sample was taken at appropriate intervals, filtered with 5C filter paper (manufactured by ADVANTEC), and the silica concentration and molar ratio of the resulting aqueous alkali silicate solution were measured.

[0173] Step (D1): Solid-liquid separation of the second slurry and washing with water ·Solid-liquid separation The second slurry obtained in step (C1) was subjected to solid-liquid separation to obtain 100.05 kg of a second aqueous alkali silicate solution and 13.15 kg of a residue. The solid-liquid separation was carried out in the same manner as in step (B) of Comparative Example 1. Approximately 20 mL of the second aqueous alkali silicate solution was removed and used as a sample for component analysis. For the residue after solid-liquid separation, the masses of the solids and residual solution were measured, and the components of the solids and residual solution were analyzed. ·Washing After solid-liquid separation, 13.15 kg of the residue was washed with 62.00 kg of warm water, yielding 11.57 kg of residue. 63.58 kg of post-wash solution was recovered in a recovery tank. The post-wash solution had a molar ratio of 2.12 and a silica concentration of 1.7% by mass. The resulting post-wash solution was mixed with the second aqueous alkali silicate solution to yield 163.63 kg of diluted aqueous alkali silicate solution. Approximately 20 mL of the diluted aqueous alkali silicate solution was removed and used as a sample for component analysis. 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.

[0174] Step (D1) of Example 1 (first cycle) is a preceding step to step (A1) of Example 2 (second cycle). The diluted aqueous alkali silicate solution contains a second aqueous alkali silicate solution corresponding to raw material (2) and a post-wash solution corresponding to raw material (7), and is supplied to step (A1) of Example 2 (second cycle) and used as a mixture of raw material (2) and raw material (7).

[0175] Example 2 (First manufacturing method, second cycle) Example 2 is a second cycle following Example 1 (first cycle), in which silica is recovered from the residue obtained in step (D1) of Example 1 (first cycle), and the recovered silica is used to produce the target aqueous alkali silicate solution.

[0176] Pre-process (D1) The step preceding the step (A1) of Example 2 is the step (D1) of Example 1. 163.63 kg of the diluted aqueous alkali silicate solution (including the second aqueous alkali silicate solution and the post-washing solution) obtained in the step (D1) of Example 1 is used as a raw material in the next step (A1).

[0177] Step (A1): Mixing raw materials and preparing slurry A slurry was prepared using the same procedure as in Example 1. Specifically, 163.63 kg of the diluted aqueous alkali silicate solution (including the second aqueous alkali silicate solution and the post-wash solution) (reference numeral 2 in FIG. 4) obtained in the previous step (D1) (reference numeral 3 in FIG. 4), 14.35 kg of a 48.0 mass% aqueous sodium hydroxide solution (reference numeral 3 in FIG. 4), and 8.15 kg of warm water (reference numeral 4 in FIG. 4) 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 (reference numeral 1 in FIG. 4) 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 16.2%. A first slurry was then obtained using the same procedure as in Example 1.

[0178] Step (B1): Solid-liquid separation of first slurry The first slurry obtained in step (A1) was filtered in the same manner as in Example 1 to obtain 46.71 kg of a residue and 175.41 kg of a first aqueous alkali silicate solution, which was the target aqueous alkali silicate solution having a molar ratio of 3.3 and a silica concentration of 18.0 mass%.

[0179] Step (C1): Preparation of second slurry A second slurry was prepared using 46.71 kg of the residue obtained in step (B1) in the same procedure as in step (C1) of Example 1, and measurements and analyses were carried out in the same manner.

[0180] Step (D1): Solid-liquid separation of the second slurry and washing with water In the same procedure as in step (D1) of Example 1, the second slurry obtained in step (C1) was subjected to solid-liquid separation and washed with water to obtain 11.57 kg of a residue and 163.63 kg of a diluted aqueous alkali silicate solution.

[0181] Step (D1) in Example 2 (second cycle) can be a preceding step to step (A1) in the third cycle. The diluted aqueous alkali silicate solution contains a second aqueous alkali silicate solution corresponding to raw material (2) and a post-wash solution corresponding to raw material (7), and is supplied to step (A1) in the third cycle and can be used as a mixture of raw material (2) and raw material (7). The third and subsequent cycles can be carried out in the same manner as the second cycle, and therefore detailed explanations are omitted.

[0182] <Results of Examples 1 and 2> Table 6 shows the amounts of each raw material used to prepare the second slurry in the preceding step (C1) in Examples 1 and 2. Table 7 shows the masses of the second aqueous alkali silicate solution and residue after solid-liquid separation in the preceding step (D1), as well as a breakdown of their components. Table 8 shows the masses of each material before water washing in the preceding step (D1), as well as the masses of the diluted aqueous alkali silicate solution and residue obtained by mixing the post-wash solution with the second aqueous alkali silicate solution, and a breakdown of the components of these materials after water washing. Table 9 shows the amounts of each raw material used in step (A1), as well as a breakdown of the components of each raw material. Table 10 shows the masses of the first aqueous alkali silicate solution and residue after solid-liquid separation in step (B1), as well as a breakdown of the components. Table 11 shows the amounts of each raw material used to prepare the second slurry in step (C1). Table 12 shows the masses of the second aqueous alkali silicate solution and residue after solid-liquid separation in step (D1), as well as a breakdown of the components. Table 13 shows the mass of each material before washing in step (D1), the mass of the diluted aqueous alkali silicate solution and residue obtained by mixing the post-washing solution with the second aqueous alkali silicate solution, and the breakdown of the components of these post-washing materials.

[0183] Pre-process (C1): Preparation of second slurry [Table 6]

[0184] Pre-process (D1): Solid-liquid separation of second slurry [Table 7]

[0185] Pre-process (D1): Washing residue with water [Table 8]

[0186] Step (A1): Mixing raw materials and preparing first slurry [Table 9]

[0187] Step (B1): Solid-liquid separation of first slurry [Table 10]

[0188] Step (C1): Preparation of second slurry [Table 11]

[0189] Step (D1): Solid-liquid separation of second slurry [Table 12]

[0190] Step (D1): Washing the residue with water [Table 13]

[0191] <Explanation of the results of Example 1> As shown in Tables 6 and 7, the preparation of the second slurry and solid-liquid separation in the preceding steps (C1) and (D1) yielded 100.05 kg of a second aqueous alkali silicate solution (d1 in Table 7) and 13.15 kg of a residue (d3 in Table 7). The second aqueous alkali silicate solution contained 11.93 kg of silica (d2 in Table 7). This silica was recovered from the residue by preparing the second slurry. The residue consisted of 2.89 kg of solids (d4 in Table 7) and 10.26 kg of residual solution (d6 in Table 7). The amount of silica in the solids was 0.71 kg (d5 in Table 7), a decrease from the value of 7.14 kg (b4 in Table 5) before the preparation of the second slurry. This indicates that the preparation of the second slurry successfully dissolved the undissolved silica.

[0192] As shown in Table 8, washing the residue from the previous step (D1) with water (amount of water: 62.00 kg) yielded 163.63 kg of diluted aqueous alkali silicate solution (e5 in Table 8) and 11.57 kg of residue (e7 in Table 8). The diluted aqueous alkali silicate solution contained 13.00 kg of silica (e6 in Table 8). This silica represents the total amount of silica recovered from the residue by preparing the second slurry, solid-liquid separation, and washing with water. The diluted aqueous alkali silicate solution was supplied to step (A1) and used as a raw material for preparing the first slurry.

[0193] Table 9 shows the blending of raw materials adjusted so that the silica concentration and molar ratio of the aqueous alkali silicate solution in the first slurry reach the target values ​​of 18.0 mass% and 3.3, respectively, after 2 hours, assuming a silica dissolution rate of approximately 78%.

[0194] As shown in Table 10, 175.41 kg (g1 in Table 10) of a first aqueous alkali silicate solution and 46.71 kg of a residue (g3 in Table 10) were obtained by solid-liquid separation of the first slurry. The first aqueous alkali silicate solution was the target aqueous alkali silicate solution having a molar ratio of 3.3 and a silica concentration of 18.0 mass%.

[0195] The production volume of the target alkali silicate aqueous solution increased by approximately 70% compared to the 103.18 kg (b1 in Table 5) amount of target alkali silicate aqueous solution obtained in Comparative Example 1. Focusing on the amount of silica, the amount of silica in the target alkali silicate aqueous solution obtained in Example 1 was 31.58 kg (g2 in Table 10), while the amount of silica in the target alkali silicate aqueous solution obtained in Comparative Example 1 was 18.57 kg (b2 in Table 5), a difference of 13.01 kg, which corresponds to the amount of silica contained in the diluted alkali silicate aqueous solution (including the second alkali silicate aqueous solution and the post-wash solution) supplied from the previous step (D1). It can be seen that the production volume of the target alkali silicate aqueous solution increased by using the alkali silicate aqueous solution supplied from the previous step (D1) as a raw material.

[0196] The dissolution rate of silica at the end of step (A1) was 78.0 mass% (g10 in Table 10), as expected. Therefore, the amount of residue in step (B1) was 46.71 kg (g3 in Table 10), the same as in Comparative Example 1, corresponding to the amount of rice husk ash added and the dissolution rate. The masses of the solids and residual solution, as well as their component breakdown, were also the same as in Comparative Example 1. These results demonstrate that, regardless of whether the present invention is implemented, the residue after obtaining the target aqueous alkali silicate solution contains a certain amount of silica. From the perspective of reducing resource waste, it is preferable to repeat the steps specified in the first production method of the present invention.

[0197] Table 11 shows the blending of raw materials adjusted to dissolve undissolved silica in the residue and to reach a molar ratio of about 2.0 in the aqueous alkali silicate solution in about 2 hours in the preparation of the second slurry in step (C1). Since step (C1) is a step for dissolving the hardly soluble silica in the residue, setting the molar ratio low throughout step (C1) promotes the dissolution of silica.

[0198] As shown in Table 12, solid-liquid separation of the second slurry in step (D1) yielded 100.05 kg of a second aqueous alkali silicate solution (i1 in Table 12) and 13.15 kg of a residue (i3 in Table 12). The second aqueous alkali silicate solution contained 11.93 kg of silica (i2 in Table 12). This silica was recovered from the residue by preparing the second slurry. The residue consisted of 2.89 kg of solids (i4 in Table 12) and 10.26 kg of a residual solution (i6 in Table 12). The amount of silica in the solids was 0.71 kg (i5 in Table 12), a decrease from the value of 7.14 kg (g5 in Table 10) before preparing the second slurry. This indicates that the undissolved silica was dissolved by preparing the second slurry.

[0199] As shown in Table 13, washing the residue in step (D1) with water (amount of water: 62.00 kg) yielded 163.63 kg of a diluted aqueous alkali silicate solution (j5 in Table 13) and 11.57 kg of a residue (j7 in Table 13). The diluted aqueous alkali silicate solution contained 13.00 kg of silica (j6 in Table 13). This silica represents the total amount of silica recovered from the residue by preparing the second slurry, separating the solid-liquid mixture, and washing with water.

[0200] By preparing a second slurry that promotes dissolution of undissolved silica, the final silica dissolution rate increased to 97.8 mass% (j14 in Table 13), and the silica utilization rate also increased to 97.4 mass% (j17 in Table 13). This demonstrates that the method of Example 1 can significantly improve silica utilization rate compared to the starting silica utilization rate of 57.3% in Comparative Example 1. Furthermore, the difference between the silica dissolution rate and utilization rate was 20.7 mass% in Comparative Example 1 (dissolution rate 78 mass%), while it was only 0.4 mass% in Example 1. Thus, the present invention enabled almost all of the dissolved silica to be utilized in the production of an aqueous alkali silicate solution.

[0201] The diluted aqueous alkali silicate solution obtained in step (D1) was supplied to step (A1) of Example 2 (second cycle) and used as a raw material for preparing a slurry. The amount of the diluted aqueous alkali silicate solution obtained in step (D1) and the silica content therein (j5 and j6 in Table 13) were the same as those of the diluted aqueous alkali silicate solution obtained in the previous step (D1) (e5 and e6 in Table 8). Therefore, by using the diluted aqueous alkali silicate solution obtained in step (D1) as a raw material for step (A1) of the next cycle, the second and subsequent cycles can be repeated in the same manner.

[0202] Figure 5 is a graph showing the changes in silica concentration and molar ratio of the aqueous alkali silicate solution over the periods of steps (A1) to (D1) for Examples 1 and 2. Table 14 shows the changes in silica concentration, molar ratio, and silica dissolution rate over the periods of steps (A1) and (C1), respectively.

[0203] [Table 14]

[0204] As shown in (a) of Table 14, at the start of dissolution in step (A1) of preparing the first slurry, the slurry solution contained silica derived from the second aqueous alkali silicate solution and the post-wash solution, and therefore had a silica concentration of 6.99% by mass and a molar ratio of 1.15. The dissolution rate of silica in the newly added rice husk ash increased with dissolution time, and when the dissolution rate reached the expected 78% by mass, the silica concentration reached the target value of 18.0% by mass, and the molar ratio reached the target value of 3.3.

[0205] As shown in Table 14(b), at the start of dissolution in step (C1) for preparing the second slurry, the slurry solution had a silica concentration of 6.48 mass% and a molar ratio of 1.09. After 2 hours, the silica concentration increased to 11.92 mass%, and the molar ratio increased to 2.12. This slurry solution was separated into a second aqueous alkali silicate solution by solid-liquid separation, diluted by mixing with the washed solution, and then supplied to step (A1).

[0206] In this way, by repeating the steps (A1) to (D1) defined in the present invention, it is possible to continuously produce an aqueous alkali silicate solution with a high dissolution rate and utilization rate of silica.

[0207] The residue obtained in step (D1) of 11.57 kg (j7 in Table 13) may be discarded. This amount of waste is significantly reduced compared to 46.71 kg in Comparative Example 1 and 35.36 kg in Comparative Example 2 (b3 in Table 5), and the present invention can be said to have a smaller environmental impact.

[0208] In Example 1, the second aqueous alkali silicate solution was mixed with the post-wash solution and supplied to step (A1) as a diluted aqueous alkali silicate solution, but in the first production method of the present invention, mixing with the post-wash solution is not essential. That is, in the first production method of the present invention, the water washing in step (D1) performed in Example 1 may not be performed, and only the second aqueous alkali silicate solution may be supplied to step (A1). Even in this case, silica contained in the second aqueous alkali silicate solution can be recovered, thereby improving the utilization rate of silica compared to conventional methods.

[0209] <Explanation of the results of Example 2> In Example 2, the target aqueous alkali silicate solution was produced from raw materials containing the diluted aqueous alkali silicate solution obtained in step (D1) of Example 1. The diluted aqueous alkali silicate solution used as a mixture of raw materials (2) and (7) in Example 2 had the same quantity and quality as the diluted aqueous alkali silicate solution used as a mixture of raw materials (2) and (7) in Example 1. Therefore, as shown in Tables 6 to 13, by repeating the same procedure as in Example 1, similar results to those in Example 1 were obtained in Example 2. This demonstrates that continuous production of an aqueous alkali silicate solution is possible by repeating steps (A1) to (D1) defined in the first production method of the present invention.

[0210] Example 3 (Second manufacturing method, first cycle) In Example 3, the residue obtained in Comparative Example 3 is further dissolved in aqueous sodium hydroxide to recover silica as a second aqueous alkali silicate solution. The second aqueous alkali silicate solution is then mixed with the first aqueous alkali silicate solution obtained in step (B2) to obtain the target aqueous alkali silicate solution. As in Example 3, the pre-step (D2) to step (A2) of the first cycle of the present invention may be a pre-step (C2) of preparing a slurry using a residue obtained by a general method for dissolving rice husk ash, followed by solid-liquid separation of the slurry.

[0211] FIG. 6 is a flow chart showing the steps of the present invention.

[0212] Pre-process (C2): Preparation of second slurry 46.71 kg of the residue obtained in Comparative Example 3, 7.00 kg of a 48.0 mass% sodium hydroxide aqueous solution, and 16.69 kg of warm water were placed in a 180 L stainless steel dissolution tank equipped with a stirrer, and the temperature of the slurry was raised to 90°C while stirring. The slurry was then stirred for 2 hours while maintaining the liquid temperature at 90°C to prepare a second slurry. During the slurry preparation process, approximately 20 mL of the sample was taken at appropriate intervals and filtered with 5C filter paper (manufactured by ADVANTEC), after which the silica concentration and molar ratio of the obtained aqueous alkali silicate solution were measured.

[0213] Pre-process (D2): Solid-liquid separation of the second slurry and washing with water ·Solid-liquid separation The second slurry obtained in the previous step (C2) was filtered using a filter press at a charge pressure of 0.4 MPa to obtain 45.20 kg of a second aqueous alkali silicate solution and 25.20 kg of a residue. Approximately 20 mL of the second aqueous alkali silicate solution was removed and used as a sample for component analysis. For the residue after solid-liquid separation, the masses of the solids and residual solution were measured, and the components of the solids and residual solution were analyzed. ·Washing After solid-liquid separation, 25.20 kg of the residue was washed with 60.49 kg of warm water to obtain 16.80 kg of residue, and 68.89 kg of post-wash solution was recovered in a recovery tank. The post-wash solution had a molar ratio of 2.53 and a silica concentration of 4.2% by mass. Approximately 20 mL of the resulting post-wash solution was removed and used as a sample for component analysis. 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.

[0214] This pre-process (D2) is a pre-process of the process (A2) in Example 3 (next cycle). The post-wash solution is supplied to the process (A2) in Example 3 (next cycle) and used as the raw material (7).

[0215] Step (A2): Mixing raw materials and preparing first slurry A 240 L stainless steel dissolution tank equipped with a stirrer was charged with 68.89 kg of the post-wash solution (reference numeral 2 in FIG. 6) obtained in the previous step (D2), 17.12 kg of a 48.0 mass% sodium hydroxide aqueous solution (reference numeral 3 in FIG. 6), and 44.00 kg of warm water (reference numeral 4 in FIG. 6) for adjusting the concentration. While stirring, 36.00 kg of rice husk ash A (reference numeral 1 in FIG. 6) was added over a period of 20 minutes. The silica dissolution rate in the rice husk ash was set to approximately 78%, as in Comparative Example 3. The mass ratio of rice husk ash A to the total mass of the raw materials was approximately 21.7%. 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, yielding an alkali silicate slurry (first 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.

[0216] Step (B2): Solid-liquid separation of the first slurry The first slurry obtained in step (A2) was filtered using a filter press at a charge pressure of 0.4 MPa to obtain 46.71 kg of a residue and 119.30 kg of a first aqueous alkali silicate solution having a molar ratio of 3.7 and a silica concentration of 18.0 mass%.

[0217] Step (C2): Preparation of second slurry 46.71 kg of the residue obtained in step (B2), 7.00 kg of a 48.0% by mass aqueous sodium hydroxide solution, and 16.69 kg of hot water were placed in a 180 L stainless steel dissolution tank equipped with a stirrer, and the temperature of the slurry was raised to 90°C while stirring. The slurry was then stirred for 2 hours while maintaining the liquid temperature at 90°C to prepare a second slurry. During the slurry preparation process, approximately 20 mL of the sample was taken at appropriate intervals, filtered with 5C filter paper (manufactured by ADVANTEC), and the silica concentration and molar ratio of the obtained aqueous alkali silicate solution were measured.

[0218] Step (D2): Solid-liquid separation of the second slurry and washing with water ·Solid-liquid separation The second slurry obtained in step (C2) was filtered using a filter press at a charge pressure of 0.4 MPa to obtain 45.20 kg of a second aqueous alkali silicate solution and 25.20 kg of a residue. Approximately 20 mL of the second aqueous alkali silicate solution was removed and used as a sample for component analysis. For the residue after solid-liquid separation, the masses of the solids and residual solution were measured, and the components of the solids and residual solution were analyzed. ·Washing After solid-liquid separation, 25.20 kg of the residue was washed with 60.49 kg of warm water to obtain 16.80 kg of residue, and 68.89 kg of post-wash solution was recovered in a recovery tank. The post-wash solution had a molar ratio of 2.53 and a silica concentration of 4.2% by mass. Approximately 20 mL of the resulting post-wash solution was removed and used as a sample for component analysis. 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.

[0219] Step (D2) of Example 3 (first cycle) is a pre-step of step (A2) of Example 4 (second cycle). The post-wash solution is supplied to step (A2) of Example 4 (second cycle) and used as raw material (7).

[0220] Step (E): Mixing the first and second aqueous alkali silicate solutions The first aqueous alkali silicate solution obtained in step (B2) and the second aqueous alkali silicate solution obtained in step (D2) were mixed to obtain 164.50 kg of the target aqueous alkali silicate solution having a molar ratio of 3.3 and a silica concentration of 17.7 mass%.

[0221] Example 4 (Second manufacturing method, second cycle) Example 4 is the second cycle following Example 3 (first cycle), in which silica is recovered from the residue obtained in step (D2) of Example 3 (first cycle) and the recovered silica is used to produce the target aqueous alkali silicate solution.

[0222] Pre-process (D2) The step preceding step (A2) of Example 4 is step (D2) of Example 3. 68.89 kg of the post-wash solution obtained in step (D2) of Example 3 is used as raw material (7) in the next step (A2).

[0223] Step (A2): Mixing raw materials and preparing a slurry A slurry was prepared using the same procedure as in Example 3. Specifically, 68.89 kg of the post-wash solution (reference numeral 2 in FIG. 6) obtained in the previous step (D2), 17.12 kg of a 48.0 mass% sodium hydroxide aqueous solution (reference numeral 3 in FIG. 6), and 44.00 kg of warm water (reference numeral 4 in FIG. 6) 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 (reference numeral 1 in FIG. 6) was added over a period of 20 minutes while continuing to stir. The mass ratio of rice husk ash A to the total mass of the raw materials was approximately 21.7%. A first slurry was then obtained using the same procedure as in Example 1.

[0224] Step (B2): Solid-liquid separation of the first slurry The first slurry obtained in step (A2) was filtered in the same manner as in Example 3 to obtain 46.71 kg of a residue and 119.30 kg of a first aqueous alkali silicate solution, which had a molar ratio of 3.7 and a silica concentration of 18.0 mass%.

[0225] Step (C2): Preparation of second slurry A second slurry was prepared using 46.71 kg of the residue obtained in step (B2) in the same procedure as in step (C2) of Example 3, and measurements and analyses were carried out in the same manner.

[0226] Step (D2): Solid-liquid separation of the second slurry and washing with water In the same procedure as in step (D2) of Example 3, the second slurry obtained in step (C2) was subjected to solid-liquid separation and washed with water to obtain 16.80 kg of a residue and 68.89 kg of a washed solution.

[0227] Step (D2) in Example 4 (second cycle) can be a pre-step of step (A2) in the third cycle. The post-wash solution can be supplied to step (A2) in the third cycle and used as raw material (7). The third cycle and subsequent steps can be carried out in the same manner as the second cycle, so detailed explanations are omitted.

[0228] <Results of Examples 3 and 4> Table 15 shows the amount of each raw material used to prepare the second slurry in the preceding step (C2) in Examples 3 and 4. Table 16 shows the mass of the second alkali silicate aqueous solution and the residue after solid-liquid separation in the preceding step (D2), as well as a breakdown of their components. Table 17 shows the mass of each material before water washing in the preceding step (D2), as well as the mass of the solution and residue after washing, and a breakdown of the components of these materials after water washing. Table 18 shows the amount of each raw material used in step (A2), as well as a breakdown of the components of each raw material. Table 19 shows the mass of the first alkali silicate aqueous solution and the residue after solid-liquid separation in step (B2), as well as a breakdown of the components. Table 20 shows the amount of each raw material used to prepare the second slurry in step (C2). Table 21 shows the mass of the second alkali silicate aqueous solution and the residue after solid-liquid separation in step (D2), as well as a breakdown of the components. Table 22 shows the mass of each material before washing with water in step (D2), the mass of the solution and residue after washing, and the component breakdown of the materials after washing with water. Table 23 shows the silica concentration, molar ratio, and silica content for the first aqueous alkali silicate solution, the second aqueous alkali silicate solution, and the target aqueous alkali silicate solution obtained by mixing these two solutions.

[0229] Pre-process (C2): Preparation of second slurry [Table 15]

[0230] Pre-process (D2): Solid-liquid separation of second slurry [Table 16]

[0231] Pre-process (D2): Washing residue with water [Table 17]

[0232] Step (A2): Mixing raw materials and preparing first slurry [Table 18]

[0233] Step (B2): Solid-liquid separation of the first slurry [Table 19]

[0234] Step (C2): Preparation of second slurry [Table 20]

[0235] Step (D2): Solid-liquid separation of second slurry [Table 21]

[0236] Step (D2): Washing the residue with water [Table 22]

[0237] Step (E): Mixing of alkali silicate aqueous solution [Table 23]

[0238] <Explanation of the results of Example 3> As shown in Tables 15 and 16, the preparation of the second slurry and solid-liquid separation in the preceding steps (C2) and (D2) yielded 45.20 kg of a second aqueous alkali silicate solution (m1 in Table 16) and 25.20 kg of a residue (m3 in Table 16). The second aqueous alkali silicate solution contained 7.61 kg of silica (m2 in Table 16). This silica was recovered from the residue by preparing the second slurry. The residue consisted of 5.04 kg of solids (m4 in Table 16) and 20.16 kg of residual solution (m6 in Table 16). The amount of silica in the solids was 2.85 kg (m5 in Table 16), a decrease from the value of 7.14 kg (b4 in Table 5) before the preparation of the second slurry. This indicates that the preparation of the second slurry allowed the undissolved silica to be dissolved.

[0239] As shown in Table 17, washing the residue in the previous step (D2) with water (amount of water: 60.49 kg) yielded 68.89 kg of washed solution (n4 in Table 17) and 16.80 kg of residue (n6 in Table 17). The washed solution contained 2.90 kg of silica (n5 in Table 17). This silica was recovered from the residue by washing with water. The residue consisted of 5.04 kg of solids (n7 in Table 17) and 11.76 kg of residual solution (n9 in Table 17). Because the pressure conditions for squeezing during water washing were higher than those used during solid-liquid separation of the second slurry, the amount of residual solution in the residue (11.76 kg) was reduced from the amount before washing (20.16 kg, m6 in Table 16). Furthermore, the amount of silica in the residual solution was 0.50 kg (n10 in Table 17), a decrease compared to the amount before washing (3.40 kg (m7 in Table 16). It can be seen that washing with water enabled extraction of the highly concentrated aqueous alkali silicate solution remaining in the residue before washing. The solution after washing was supplied to step (A2) and used as the raw material for preparing the first slurry.

[0240] Table 18 shows the blending of raw materials adjusted so that the silica concentration and molar ratio of the aqueous alkali silicate solution in the first slurry reach the target values ​​of 18.0 mass% and 3.7, respectively, assuming a silica dissolution rate of approximately 78%.

[0241] As shown in Table 19, the solid-liquid separation of the first slurry yielded 119.30 kg (p1 in Table 19) of a first aqueous alkali silicate solution and 46.71 kg of a residue (p3 in Table 19). The first aqueous alkali silicate solution had a molar ratio of 3.7 and a silica concentration of 18.0 mass%.

[0242] The dissolution rate of silica at the end of step (A2) was 78.0 mass% (p10 in Table 19), as expected. Therefore, the amount of residue in step (B2) was 46.71 kg (p3 in Table 19), the same as in Comparative Example 3, corresponding to the amount of rice husk ash added and the dissolution rate. The masses of the solids and residual solution, as well as their component breakdown, were also the same as in Comparative Example 3. These results demonstrate that, regardless of whether the present invention is implemented, the residue obtained after obtaining the first aqueous alkali silicate solution contains a certain amount of silica. From the perspective of reducing resource waste, it is preferable to repeat the steps specified in the second production method of the present invention.

[0243] Table 20 shows the blending of raw materials adjusted to dissolve undissolved silica in the residue and reach a molar ratio of about 2.5 in the alkali silicate aqueous solution in about 2 hours in the preparation of the second slurry in step (C2). Since step (C2) is a step for dissolving the hardly soluble silica in the residue, setting the molar ratio low throughout step (C2) promotes the dissolution of silica.

[0244] As shown in Table 21, solid-liquid separation of the second slurry in step (D2) yielded 45.20 kg of a second aqueous alkali silicate solution (r1 in Table 21) and 25.20 kg of a residue (r3 in Table 21). The second aqueous alkali silicate solution contained 7.61 kg of silica (r2 in Table 21). This silica was recovered from the residue by preparing the second slurry. The residue consisted of 5.04 kg of solids (r4 in Table 21) and 20.16 kg of residual solution (r6 in Table 21). The amount of silica in the solids was 2.85 kg (r5 in Table 21), a decrease from the value of 7.14 kg (p5 in Table 19) before preparing the second slurry. This indicates that the undissolved silica was dissolved by preparing the second slurry.

[0245] As shown in Table 22, washing the residue with water in step (D2) (amount of water: 60.49 kg) yielded 68.89 kg of washed solution (s4 in Table 22) and 16.80 g of residue (s6 in Table 22). The washed solution contained 2.90 kg of silica (s5 in Table 22). This silica was recovered from the residue by washing with water. The residue consisted of 5.04 kg of solids (s7 in Table 22) and 11.76 kg of residual solution (s9 in Table 22). Because the pressure conditions for squeezing during water washing were higher than those during solid-liquid separation of the second slurry, the amount of residual solution in the residue (11.76 kg) was reduced from the amount before water washing (20.16 kg) (r6 in Table 21). Furthermore, the amount of silica in the residual solution was 0.50 kg (s10 in Table 22), which was a decrease compared to the amount before washing, 3.40 kg (r7 in Table 21). It can be seen that washing with water enabled extraction of the highly concentrated aqueous alkali silicate solution remaining in the residue before washing.

[0246] As shown in Table 23, in step (E), the first aqueous alkali silicate solution obtained in step (B2) and the second aqueous alkali silicate solution obtained in step (D2) were mixed to obtain 164.50 kg of the target aqueous alkali silicate solution having a molar ratio of 3.3 and a silica concentration of 17.7 mass%.

[0247] The amount of the target aqueous alkali silicate solution obtained in Comparative Example 3 was 103.18 kg (b1 in Table 5), and the production amount of the target aqueous alkali silicate solution increased by approximately 60%. Focusing on the amount of silica, the amount of silica in the target aqueous alkali silicate solution obtained in Example 3 was 29.09 kg (t6 in Table 23), while the amount of silica in the target aqueous alkali silicate solution obtained in Comparative Example 3 was 18.57 kg (b2 in Table 5). The difference was 10.52 kg, which corresponds to the sum of the amount of silica recovered as the second aqueous alkali silicate solution in the preceding step (D2), 7.61 kg (m2 in Table 16), and the amount of silica recovered as the post-wash solution, 2.90 kg (n5 in Table 17). It can be seen that recovering silica from the residue in the preceding step (D2) increased the production amount of the target aqueous alkali silicate solution.

[0248] By preparing a second slurry that promotes dissolution of undissolved silica, the final silica dissolution rate increased to 91.2 mass% (s13 in Table 22), and the silica utilization rate also increased to 89.7 mass% (s16 in Table 22). This demonstrates that the method of Example 3 can significantly improve silica utilization rate compared to the starting silica utilization rate of 57.3% in Comparative Example 3. Furthermore, the difference between the silica dissolution rate and utilization rate was 20.7 mass% in Comparative Example 3 (dissolution rate 78 mass%), while it was only 1.5 mass% in Example 3. Thus, the present invention enabled almost all of the dissolved silica to be utilized in the production of an aqueous alkali silicate solution.

[0249] The post-washing solution obtained in step (D2) was supplied to step (A2) of Example 4 (second cycle) and used as a raw material for preparing a slurry. The production amount of the post-washing solution obtained in step (D2) and the silica content therein (s4 and s5 in Table 22) were the same as those of the post-washing solution obtained in the previous step (D2) (n4 and n5 in Table 17). Therefore, by using the post-washing solution obtained in step (D2) as a raw material for step (A2) of the next cycle, the second and subsequent cycles can be repeated in the same procedure.

[0250] Figure 7 is a graph showing the changes in silica concentration and molar ratio of the aqueous alkali silicate solution during steps (A2) to (D2) for Examples 3 and 4. Table 24 shows the changes in silica concentration, molar ratio, and silica dissolution rate during steps (A2) and (C2), respectively.

[0251] [Table 24]

[0252] As shown in (a) of Table 24, at the start of dissolution in step (A2) of preparing the first slurry, the slurry solution contained silica derived from the post-wash solution and therefore had a silica concentration of approximately 2.4% by mass and a molar ratio of 0.55. The dissolution rate of silica in the newly added rice husk ash increased with dissolution time, and when the dissolution rate reached the expected 78% by mass, the silica concentration reached the target value of 18.0% by mass and the molar ratio reached the target value of 3.7.

[0253] As shown in Table 24(b), at the start of dissolution in step (C2) to prepare the second slurry, the slurry solution had a silica concentration of 10.3 mass% and a molar ratio of 1.55. After 2 hours, the silica concentration increased to 16.8 mass%, and the molar ratio increased to 2.53. This slurry solution was separated into a second aqueous alkali silicate solution by solid-liquid separation, and mixed with the first aqueous alkali silicate solution in step (E) to produce the desired aqueous alkali silicate solution.

[0254] In this way, by repeating the steps (A2) to (E) defined in the present invention, it is possible to continuously produce an aqueous alkali silicate solution with a high dissolution rate and utilization rate.

[0255] The residue obtained in step (D2), 16.80 kg (s6 in Table 22), may be discarded. This amount of waste is significantly reduced compared to 46.71 kg in Comparative Example 3 and 35.36 kg in Comparative Example 2 (b3 in Table 5), and the present invention can be said to have a smaller environmental impact.

[0256] In Example 3, the post-wash solution was supplied to step (A2) as raw material (7), but in the second production method of the present invention, supplying the post-wash solution to step (A2) is not essential. That is, in the second production method of the present invention, the water washing step (D2) performed in Example 3 may not be performed, and the recovered aqueous alkali silicate solution containing silica may not be supplied to step (A2). Even in this case, the silica contained in the second aqueous alkali silicate solution can be recovered, thereby improving the utilization rate of silica compared to conventional methods.

[0257] <Explanation of the results of Example 4> In Example 4, the target aqueous alkali silicate solution was produced using raw materials containing the post-wash solution obtained in step (D2) of Example 3. The post-wash solution used as raw material (7) in Example 4 had the same quantity and quality as the post-wash solution used as raw material (7) in Example 3. Therefore, as shown in Tables 15 to 23, by repeating the same procedures as in Example 3, similar results to those in Example 3 were obtained in Example 4. This demonstrates that continuous production of an aqueous alkali silicate solution is possible by repeating steps (A1) to (E) defined in the second production method of the present invention.

[0258] Example 5 (Dissolution under pressurized conditions) Experiments were conducted to improve the dissolution rate of rice husk ash using pressure vessels such as autoclaves. 365 g of hot water, 205.00 g of 24.0% by mass sodium hydroxide solution, and 165.00 g of rice husk ash were added to a 1 L autoclave equipped with a stirrer while stirring, and the container was then capped. The rice husk ash was then dissolved under high-temperature and pressurized conditions of 150 °C and 0.47 MPa. The dissolution experiments were carried out for treatment times of 0.5 hours, 1 hour, 1.5 hours, and 2 hours, respectively. After dissolution was complete, the pressure was released, and the resulting slurry was allowed to stand and 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 and obtain an aqueous alkali silicate solution. The physical properties of the resulting aqueous alkali silicate solution were investigated.

[0259] <Explanation of the results of Example 5> The results of Example 5 compared to Comparative Example 2 are shown in Table 25 and FIG. As shown in Table 25 and Figure 8, the method of Example 5, in which rice husk ash is dissolved under high-temperature and pressurized conditions, enabled the production of a highly concentrated aqueous alkali silicate solution in a shorter time than that of 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 5.

[0260] The method of dissolving rice husk ash under high-temperature and pressurized conditions can be used when dissolving rice husk ash or residue in steps (A1) and (D1) of Examples 1 and 2, and also when dissolving rice husk ash or residue in steps (A2) and (D2) of Examples 3 and 4. The results of Example 5 show that it is possible to shorten the dissolution time of rice husk ash or residue in Examples 1 to 4, and therefore the time required to produce an aqueous alkali silicate solution.

[0261] [Table 25]

[0262] <Example 6> (Compression) 11.57 kg of the residue (j7 in Table 13) obtained in step (D1) of Example 2 was squeezed at a pressure of 0.7 MPa, which allowed the remaining solution to be further squeezed out, and the final mass of the residue was reduced by about 25% to about 8.67 kg.

[0263] The pressing of the residue can be employed not only in step (D1) but also in step (B1), and further in steps (B2) and (D2) of Examples 3 and 4. The results of Example 6 show that the extraction of the alkali silicate aqueous solution from the residue in Examples 1 to 4 can be promoted, and further improvement of the silica utilization rate can be achieved.

[0264] <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 the 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 (A1) to (D1) one or more times, 2 / (Na 2 O+K 2 O) A method for producing an aqueous alkali silicate solution, which provides 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 %: ・Process (A1) A step of mixing the following raw materials (1) to (3) or the following raw materials (1) to (4) to prepare a first slurry containing an alkali silicate aqueous solution containing silica extracted from rice husk ash; (1) Rice husk ash, (2) the second aqueous alkali silicate solution provided in step (D1); (3) sodium hydroxide, (4) Water, ・Process (B1) a step of subjecting the first slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain a first aqueous alkali silicate solution, which is the target aqueous alkali silicate solution, and a residue; ・Process (C1) a step of mixing the residue obtained in the step (B1) and the following raw materials (5) and (6) to prepare a second slurry containing an aqueous alkali silicate solution containing silica extracted from the residue; (5) sodium hydroxide, (6) Water; ・Process (D1) a step of subjecting the second slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue, and supplying at least a portion of the solid-liquid separated second aqueous alkali silicate solution to step (A1) as raw material (2).

2. The step (D1) further includes washing the residue obtained by solid-liquid separation of the second slurry with water, and supplying at least a part of the solution after washing to the step (A1) as the raw material (7); 2. The production method according to claim 1, wherein in step (A1), the first slurry is prepared by mixing the raw materials (1) to (3) and (7) or the raw materials (1) to (4) and (7).

3. In step (C1), the SiO of the aqueous alkali silicate solution in the second slurry 2 / (Na 2 O+K 2 3. The method according to claim 1, wherein the blending and mixing conditions of the raw materials when preparing the second slurry are adjusted so that the molar ratio of hydroxybenzoates to hydroxybenzoates is 2.6 or less.

4. The method includes the following steps (A2) to (E): 2 / (Na 2 O+K 2 O) A method for producing an aqueous alkali silicate solution, which provides 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 %: ・Process (A2) A step of mixing the following raw materials (1), (3), and (4) to prepare a first slurry containing an aqueous alkali silicate solution containing silica extracted from rice husk ash; (1) Rice husk ash, (3) sodium hydroxide, (4) Water, ・Process (B2) a step of subjecting the first slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain a first aqueous alkali silicate solution and a residue; ・Process (C2) a step of mixing the residue obtained in the step (B2) and the following raw materials (5) and (6) to prepare a second slurry containing an aqueous alkali silicate solution containing silica extracted from the residue; (5) sodium hydroxide, (6) Water; ・Process (D2) a step of subjecting the second slurry to solid-liquid separation into an aqueous alkali silicate solution and a residue to obtain a second aqueous alkali silicate solution and a residue; ・Process (E) A step of mixing the first and second aqueous alkali silicate solutions to obtain the desired aqueous alkali silicate solution.

5. The step (D2) further includes washing the residue obtained by solid-liquid separation of the second slurry with water, and supplying at least a part of the solution after washing to the step (A2) as the raw material (7); 5. The method according to claim 4, wherein in step (A2), the first slurry is prepared by mixing the raw materials (1), (3), and (7) or the raw materials (1), (3), (4), and (7).

6. The method according to any one of claims 1, 2, 4 and 5, wherein in step (A1) or (A2), the raw materials are mixed in a ratio that satisfies the following (a) and (b): (a) SiO calculated based on 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 materials (2) and (7), and the amount of sodium oxide converted from 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 O) molar ratio is greater than (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 materials (2) and (7), and the amount of water in the raw materials exceeds the silica concentration of the target aqueous alkali silicate solution.

7. The method according to any one of claims 1, 2, 4 and 5, wherein the raw material (1) is rice husk ash having a silica content of 80% by mass or more.

8. The method according to any one of claims 1, 2, 4 and 5, wherein no silica-based mineral is added in steps (A1) to (D1) or steps (A2) to (D2).

9. The solid-liquid separation in step (B1), step (D1), step (B2), or step (D2) comprises performing pressure filtration under a pressure of 0.15 to 0.8 MPa. The production method according to any one of claims 1, 2, 4, and 5.

10. The method according to claim 9, wherein the pressure filtration is carried out by a filter press.

11. The production method according to claim 9, wherein the solid-liquid separation in step (B1), step (D1), step (B2), or step (D2) comprises squeezing the residue under a pressure of 0.15 to 0.8 MPa.

12. The production method according to any one of claims 1, 2, 4 and 5, wherein the preparation of the slurry in step (A1), step (C1), step (A2) or step (C2) is carried out under conditions of a temperature of 105 to 300°C and a pressure of 0.20 to 8.6 MPa.

13. The method according to any one of claims 1, 2, 4 and 5, wherein in the preparation of the first slurry in step (A1) or step (A2), the temperature of the mixture and the dissolution time are adjusted so that the dissolution rate of silica in the rice husk ash is 70 mass% or more and less than 90 mass%. Here, the dissolution rate of silica in rice husk ash is calculated based on the following formula 2. Formula 2: [Equation 1]

14. The manufacturing method according to claim 13, wherein in the preparation of the second slurry in step (C1) or step (C2), the temperature and dissolution time of the mixture are adjusted so that the dissolution rate of silica in the rice husk ash is 90 mass% or more in total.

15. The method according to claim 2 or 5, wherein the amount of water used in washing is such that the silica concentration in the solution after washing is 0.5 to 15% by mass.

16. The method according to any one of claims 1, 2, 4 and 5, 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% by mass or less.

17. The method of claim 1, The step (D1) further includes washing the residue obtained by solid-liquid separation of the second slurry with water, and supplying at least a part of the solution after washing to the step (A1) as the raw material (7); In step (A1), the first slurry is prepared by mixing raw materials (1) to (3) and (7) or raw materials (1) to (4) and (7); In step (C1), the SiO of the aqueous alkali silicate solution in the second slurry 2 / (Na 2 O+K 2 O) adjusting the blending and mixing conditions of the raw materials when preparing the second slurry so that the molar ratio is 2.6 or less; In step (A1), the raw materials are mixed in a ratio that satisfies the following (a) and (b): (a) SiO calculated based on 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 materials (2) and (7), and the amount of sodium oxide converted from 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 O) molar ratio is greater than (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 materials (2) and (7), and the amount of water in the raw materials exceeds the silica concentration of the target aqueous alkali silicate solution. The raw material (1) is rice husk ash having a silica content of 80% by mass or more, No silica-based mineral is added in steps (A1) to (D1), The solid-liquid separation in step (B1) or step (D1) comprises carrying out pressure filtration under a pressure of 0.15 to 0.8 MPa; The pressure filtration is carried out by a filter press; The solid-liquid separation in step (B1) or step (D1) includes squeezing the residue under a pressure of 0.15 to 0.8 MPa; The preparation of the slurry in step (A1) or step (C1) is carried out under conditions of a temperature of 105 to 300°C and a pressure of 0.20 to 8.6 MPa; In the preparation of the first slurry in step (A1), the temperature and dissolution time of the mixture are adjusted so that the dissolution rate of silica in the rice husk ash is 70% by mass or more and less than 90% by mass; Here, the dissolution rate of silica in rice husk ash is calculated based on the following formula 2: Formula 2: [Equation 2] In the preparation of the second slurry in step (C1), the temperature and dissolution time of the mixture are adjusted so that the dissolution rate of silica in the rice husk ash is 90 mass% or more in total; 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, The method for producing rice husk ash, 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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