Processed rice husk ash and method for producing same

Abstract

The present invention relates to processed rice husk ash. The present invention further relates to a method for producing processed rice husk ash. Rice husk ash as a raw material is processed into rice husk ash having an apparent density in a dry state of 400-800 g / L and a water content of 30% by mass or less.

Description

Processed rice husk ash and its manufacturing method

[0001] The present invention relates to processed rice husk ash and a method for producing the same. CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to Japanese Patent Application No. 2023-107013, filed June 29, 2023, the entire disclosure of which is incorporated herein by reference.

[0002] In recent years, there has been a growing demand for a sustainable and regenerative society to protect the global environment. As one concrete measure to achieve this, biomass technologies that do not use fossil fuels, such as biomass boilers and biomass power generation, are becoming more widespread in various regions. Facilities that use such technologies often use rice husk ash as fuel. However, rice husks contain many unburnable components (especially silica), and disposal of the combustion ash (rice husk ash) containing many unburned components has become a problem for these facilities.

[0003] As one method for solving this problem, attempts have been made in the past to dissolve rice husk ash, which contains a large amount of silica components, in an alkaline aqueous solution (an aqueous solution containing an alkali metal salt, such as an aqueous sodium hydroxide solution), produce an aqueous silicic acid solution containing an alkali metal component (hereinafter referred to as an alkaline silicate aqueous solution), and reuse this as a raw material for wet-process silica (for example, Patent Documents 1 and 2).

[0004] In addition to being a raw material for wet-process silica, aqueous alkali silicate solutions are also widely used in the fields of raw materials for synthetic zeolites, civil engineering ground hardening agents, detergents, and textiles and pulp.

[0005] Patent Document 1: JP-A-2007-510613 (WO2005 / 044727) Patent Document 2: JP-A-2007-522069 (WO2005 / 077828) The entire disclosures of Patent Documents 1 and 2 are incorporated herein by reference.

[0006] However, when an aqueous alkali silicate solution is produced for industrial use using rice husk ash discharged from a biomass boiler or the like based on conventional technology, the present inventors have found that new problems arise, such as the following.

[0007] (1) When rice husk ash is directly added to an alkaline aqueous solution, the rice husk ash absorbs the solvent during dissolution, increasing the viscosity and making it difficult to form a slurry. On the other hand, when rice husk ash is added to an alkaline aqueous solution together with water to form a slurry, only a low-concentration aqueous solution of alkali silicate can be produced.

[0008] (2) When rice husk ash is added to an alkaline aqueous solution, the rice husk ash floats on the top of the solution, resulting in poor dissolution efficiency, particularly poor initial sinking into the alkaline aqueous solution.

[0009] (3) Rice husk ash has a low apparent density and is bulky relative to its weight. Therefore, the amount of rice husk ash that can be transported at one time is small, meaning that transportation efficiency is poor. In particular, the longer the distance from the boiler facility to the melting facility, the higher the transportation cost. Furthermore, because rice husk ash is bulky, storage efficiency is poor and storage costs also increase.

[0010] The present invention has been made in view of the above problems, and aims to provide rice husk ash that is more suitable for producing an alkali silicate aqueous solution than conventional rice husk ash, and a method for producing the same.

[0011] The present inventors conducted extensive research to solve the above problems. As a result, they found that the above problems can be solved by increasing the apparent density of rice husk ash through compaction or pulverization. The present invention is described in <1>, and preferably in <2> and subsequent items. <1> Rice husk ash having an apparent density in a dry state of 400 to 800 g / L and a moisture content of 30 mass% or less. <2> Rice husk ash according to <1>, having a carbon content of 0.1 to 8.0 mass% and a silica content of 85 to 95 mass%. <3> Rice husk ash according to <1> or <2>, having a volume average particle diameter D50 measured by laser diffraction method of 5.0 to 200 μm. <4> A method for producing rice husk ash, comprising: compressing or pulverizing raw rice husk ash having an apparent density in a dry state of 350 g / L or less so that the apparent density in a dry state is 400 to 800 g / L; and optionally adjusting the moisture content of the raw rice husk ash to a range of 30 mass% or less. <5> The method according to <4>, wherein the compression or pulverization is carried out so that the volume average particle diameter D50 is 5.0 to 200 μm. <6> The compression or pulverization is carried out using a roller compactor to obtain a volume average particle diameter D50 of 1.2 x 10 ３ The manufacturing method according to <4> or <5>, wherein the manufacturing method is carried out at a linear pressure of N / cm or more.

[0012] The present invention provides rice husk ash that is more suitable for producing an aqueous alkali silicate solution than conventional rice husk ash, and a method for producing the same. The aqueous alkali silicate solution produced from the rice husk ash of the present invention can be used as a raw material for wet-process silica, as well as for applications similar to those of conventional aqueous sodium silicate solutions derived from silica sand.

[0013] Fig. 1 shows a photograph of the raw rice husk ash used in the examples and a photograph of the processed rice husk ash of one example of the present invention. Fig. 2 shows a photograph of the liquid surface after the unprocessed rice husk ash is poured into the sodium hydroxide aqueous solution and the water has sunk therein. Fig. 3 shows the change in the amount of SiO2 as a function of time after the rice husk ash is poured into the sodium hydroxide aqueous solution. ２ 10 is a graph showing the relationship between the concentration.

[0014] <Rice Husk Ash> The rice husk ash of the present invention has an apparent density in a dry state of 400 to 800 g / L and a moisture content of 30 mass % or less.

[0015] Usually, the apparent density of unprocessed rice husk ash after combustion is 350 g / L or less, and often 300 g / L or less, and the rice husk ash of the present invention has a higher apparent density than unprocessed rice husk ash. The rice husk ash of the present invention has a moderately high apparent density and a moderate weight, which improves the sinking property in the alkaline aqueous solution and reduces the absorption of the solvent. Compared to unprocessed rice husk ash, a larger amount of rice husk ash can be put into the container, and therefore, the SiO ２ The concentration can be increased. Furthermore, by keeping the moisture content at 30% by mass or less, the rice husk ash of the present invention maintains a weight within a range suitable for transportation and a moderate solid state. In addition, since the rice husk ash of the present invention has a moderately high apparent density, it is not bulky, improving transportation efficiency and storage efficiency. As a result, the rice husk ash of the present invention improves transportation efficiency, storage efficiency, and sinkability in an alkaline aqueous solution, resulting in a more concentrated slurry. Thus, the rice husk ash of the present invention is more suitable for producing an alkaline silicate aqueous solution than unprocessed rice husk ash after combustion.

[0016] The apparent density of the rice husk ash of the present invention is 400 to 800 g / L. An apparent density of 400 g / L or more appropriately reduces the bulk relative to weight, improving transportation efficiency, storage efficiency, and sinking property in an alkaline aqueous solution, resulting in a high-concentration slurry. From the viewpoint of further improving transportation efficiency, storage efficiency, and sinking property in an alkaline aqueous 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. An apparent density of 800 g / L or less creates an appropriate amount of space between rice husk ash particles, improving sinking property in an alkaline aqueous solution. From the viewpoint of further improving sinking property in an alkaline aqueous 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.

[0017] The apparent density is measured in a dry state (generally with a moisture content of less than 1.5% by mass). Unprocessed rice husk ash after combustion is usually in a dry state, and the apparent density can be measured as is. The apparent density of rice husk ash that is not in a dry state can be measured after sufficient drying treatment (e.g., 125°C, 6 hours).

[0018] The moisture content of the rice husk ash of the present invention is 30% by mass or less. By having a moisture content of 30% by mass or less, the rice husk ash of the present invention maintains a weight within a range suitable for transportation and a moderate solid state. The moisture content may be 25% by mass or less, 20% by mass or less, 15% by mass or less, or 10% by mass or less. When the rice husk ash of the present invention contains a certain amount of moisture, dust dispersion can be suppressed, and handling during packaging, transportation, and unpacking, as well as handling when the rice husk ash is added to an aqueous solution, are improved. In addition, when the rice husk ash of the present invention contains a certain amount of moisture, the rice husk ash's sinkability in an alkaline aqueous solution is improved due to a moderate increase in weight. The moisture content may be 0.1% by mass or more, 0.5% by mass or more, 0.7% by mass or more, or 1.0% by mass or more. The moisture content can be adjusted by any process, such as adding water to the rice husk ash or drying it. The moisture content of the rice husk ash of the present invention does not need to be adjusted at all, i.e., no water needs to be added at all.

[0019] The silica and carbon contents of the rice husk ash of the present invention are not particularly limited. However, since rice husk ash is used as a silica source, a high silica content is preferable, and a low carbon content as an impurity is preferable. The silica content of rice husk ash may be 85% by mass or more, and preferably 90% by mass or more. The upper limit of the silica content varies depending on the type of rice, growing region, and rice husk ash combustion method (boiler type, combustion conditions, etc.), but is typically approximately 95% by mass or less. The silica content of rice husk ash can be measured, for example, using an X-ray fluorescence analyzer or an ICP emission analyzer. On the other hand, the carbon content of the rice husk ash of the present invention is preferably 0.1 to 8.0% by mass. The carbon content of rice husk ash discharged from a typical combustion facility is approximately 0.1% by mass or more. The carbon content of rice husk ash may be 0.3% by mass or more, 0.5% by mass or more, or 1.0% by mass or more. By making the carbon content in the rice husk ash 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 the 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 the rice husk ash can be measured, for example, with a carbon analyzer.

[0020] For the rice husk ash of the present invention, the volume average particle diameter D50 measured by laser diffraction is preferably in the range of 5.0 to 200 μm. When the volume average particle diameter D50 is 5.0 to 200 μm, the apparent density is easily maintained in the range of 400 to 800 g / L. Furthermore, when the volume average particle diameter D50 is 5.0 μm or more, the generation of excessive dust is suppressed, improving handleability. When the volume average particle diameter D50 is 5.0 to 200 μm, the amount of void space between particles falls within an appropriate range, and when the rice husk ash is added to an alkaline aqueous solution, the rice husk ash absorbs the alkaline aqueous solution appropriately. As a result, combined with the appropriate weight, the time required for the rice husk ash to sink into the alkaline aqueous 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 small as possible, the volume average particle diameter D50 is preferably 10 to 150 μm, more preferably 15 to 100 μm, and particularly preferably 20 to 80 μm.

[0021] Particle size distribution measurement can be performed using a commercially available laser diffraction particle size distribution analyzer. Specifically, particle size distribution measurement is performed by dispersing a sample in water to a predetermined concentration, introducing the dispersion into the circulation system of the measuring device, and adopting the value when the D50 value stabilizes as the measured value. When rice husk ash is processed into a compacted compact 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.

[0022] The rice husk ash of the present invention may be in the form of either a powder or a molded body, and the shape of the molded body is not particularly limited. As described below, the rice husk ash of the present invention is processed by compaction or pulverization, and can be easily distinguished from raw rice husk ash after combustion based on the appearance, apparent density, particle shape, and particle size distribution of the rice husk ash observed visually or under a microscope. For example, in terms of appearance, the particles of the rice husk ash of the present invention have a relatively uniform particle size compared to raw rice husk ash, with almost no particles exceeding 1.0 mm. The apparent density of the rice husk ash of the present invention is 400 g / L or more, while that of raw rice husk ash is 350 g / L or less.

[0023] The rice husk ash of the present invention can contribute to improving transportation efficiency, whether in bulk or packaged transportation. Because the processed rice husk ash of the present invention has a high apparent density, it is possible to load a larger amount of rice husk ash per shipment, for example, compared to conventional rice husk ash. Additionally, when storing the rice husk ash in a packaging bag (such as a paper bag, polyethylene bag, or flexible container bag), the rice husk ash of the present invention can reduce the storage space per storage amount. Additionally, the rice husk ash of the present invention also has the advantage of suppressing dust generation when packaging or unpacking the packaging bag.

[0024] The rice husk ash of the present invention can be used as a silica source. For example, the rice husk ash of the present invention can be used alone or mixed with silica sand as a raw material for an alkaline silicate aqueous solution. The alkaline silicate aqueous solution can be produced by known methods, for example, by dissolving rice husk ash in an aqueous sodium hydroxide solution. The dissolution conditions are not particularly limited, and known dissolution methods can be used. Dissolution can be carried out at 70°C or higher under normal pressure, and at 100°C or higher in a pressurized container such as an autoclave. Generally, the silica component in rice husk ash dissolves in an alkaline aqueous solution at approximately 70-90% by mass. Therefore, the amount of rice husk ash added to the alkaline aqueous solution must be greater than the amount of silica required to achieve the target concentration. Because the rice husk ash of the present invention has a high apparent density, a larger amount of rice husk ash can be charged into a container than conventional rice husk ash. Furthermore, because the rice husk ash of the present invention has a moderately high apparent density, it absorbs little solvent, eliminating the need for additional water for slurry formation. Therefore, there is an advantage that a large amount of alkali silicate aqueous solution can be produced at one time, or a highly concentrated alkali silicate aqueous solution can be produced. For example, once a pressurized container such as an autoclave is sealed, it is difficult to add additional raw materials. Thus, when a large amount of silica source needs to be added at one time, the rice husk ash of the present invention, which can be charged in a larger amount than conventional rice husk ash, is more effective.

[0025] In addition, the rice husk ash of the present invention has a higher apparent density and a more appropriate weight than conventional rice husk ashes, which improves its sinking property in an alkaline aqueous solution. The use of the rice husk ash of the present invention can shorten industrial process time, reduce energy consumption, and improve production efficiency.

[0026] The aqueous alkali silicate solution of the present invention produced using rice husk ash as a raw material can be used as a raw material for wet-process silica, either alone or mixed with a conventional aqueous alkali silicate solution (e.g., one produced using silica sand as a raw material).Wet-process silica has a high BET specific surface area and micropores to mesopores, and is therefore widely used in a wide variety of applications, including reinforcing fillers for industrial rubber, tires, silicone rubber, etc., paper fillers, matting agents for paints, abrasives for toothpaste, etc., and antiblocking agents for films.

[0027] <Method for producing rice husk ash> The method for producing rice husk ash of the present invention includes compressing or pulverizing raw rice husk ash having an apparent density in a dry state of 350 g / L or less so that the apparent density in a dry state is 400 to 800 g / L, and optionally adjusting the moisture content of the raw rice husk ash to a range of 30 mass% or less.

[0028] The raw rice husk ash is not particularly limited as long as it is ash emitted by burning rice husks. ２ From the viewpoint of reducing the burden on the environment, such as emissions, it is preferable that the raw rice husk ash be rice husk combustion ash discharged from combustion equipment that uses biomass fuel, such as a biomass boiler for heat recovery or power generation or a biomass power generation system. The apparent density of the raw rice husk ash in a dry state is 350 g / L or less. In the production method of the present invention, the raw rice husk ash is processed by compaction or pulverization to adjust the apparent density to an appropriate range. The lower limit of the apparent density of the raw rice husk ash is not particularly limited, but from the viewpoint of processing efficiency, it is preferably 150 g / L or more, 200 g / L or more, 250 g / L or more, or 300 g / L or more. The equipment for burning the rice husks is not particularly limited, and may be, for example, a commonly used boiler or combustion furnace.

[0029] The silica and carbon contents of the raw rice husk ash are not particularly limited, but since these contents are reflected in the silica and carbon contents of the rice husk ash of the present invention, a high silica content is preferable, and a low carbon content is preferable. A low carbon content also contributes to improving the energy utilization efficiency of biofuel in combustion equipment. The silica content of the raw rice husk ash may be 85% by mass or more, and preferably 90% by mass or more. The upper limit of the silica content varies depending on the type of rice, growing region, and rice husk ash combustion method (boiler type, combustion conditions, etc.), but is typically approximately 95% by mass or less. On the other hand, the carbon content of the raw rice husk ash is preferably 0.1 to 8.0% by mass. The carbon content of rice husk ash discharged from a typical combustion equipment is approximately 0.1% by mass or more. Although it is possible to reduce the carbon content of rice husk ash to less than 0.1% by mass through high-temperature combustion, such by-production of silica crystalline components such as cristobalite is likely to occur in the rice husk ash, which may be undesirable when using rice husk ash as a silica source. For example, when using rice husk ash as a silica source to produce an alkali silicate aqueous solution, the silica crystalline components may not dissolve in the solvent and remain as foreign matter. The carbon content of the raw rice husk ash may be 0.3% by mass or more, 0.5% by mass or more, or 1.0% by mass or more. By reducing the carbon content of the raw rice husk ash to 8.0% by mass or less, the incomplete combustion components of the raw rice husk are reduced, improving the quality of the raw rice husk ash (e.g., the content of silica components). The carbon content of the raw rice husk ash is preferably 6.0% by mass or less, 5.0% by mass or less, or 4.0% by mass or less.

[0030] In the manufacturing method of the present invention, raw rice husk ash is subjected to a processing treatment of compaction or pulverization to increase its apparent density in a dry state to a desired range. Compaction refers to a process of compressing the raw rice husk ash, and pulverization refers to a process of crushing the raw rice husk ash. The processing treatment may be either compaction or pulverization, or both may be performed simultaneously or sequentially. Depending on the type and conditions of the processing treatment, it may be difficult to distinguish between compaction and pulverization, and it is not necessary to distinguish between them.

[0031] 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.

[0032] The compaction treatment is preferably carried out using a roller compactor, and the compaction treatment is carried out at a rate of 1.2 × 10 ３ It is more preferable to carry out the compaction at a linear pressure of 1.2 × 10 N / cm or more. The linear pressure is the value obtained by dividing the total pressure (N) applied to the rolls of the roller compactor by the width (cm) of the rolls in the direction of the rotation axis. ３ When the linear pressure during compaction is 2.9 × 10 ３ N / cm or more or 4.7 x 10 ３ The linear pressure during compaction is preferably 12.8 × 10 ３ N / cm or less, and ３ It is more preferable that the hardness is not more than N / cm.

[0033] In the manufacturing method of the present invention, the moisture content of the raw rice husk ash may be adjusted to a range of 30% by mass or less, as necessary. By adjusting the moisture content within this range, the final product, the rice husk ash of the present invention, maintains a weight within a range suitable for transportation and a moderate solid state. Typically, the moisture content of raw rice husk ash after combustion is approximately 1.0% by mass. The moisture content may be 5.0% by mass or more, 10% by mass or more, and 25% by mass or less, or 20% by mass or less. The moisture content can be adjusted by drying or adding moisture. The moisture adjustment can be performed before or after compaction or pulverization. Adding moisture before compaction or pulverization also contributes to suppressing dust generation during these processing steps. When adding moisture before compaction or pulverization, it is preferable to adjust the moisture content within a range that does not cause the raw rice husk ash to solidify in the equipment due to the influence of moisture. Adding moisture after compaction or pulverization also contributes to suppressing dust generation during transportation and packaging of the processed rice husk ash. When water is added after compaction or pulverization, it is preferable to mix the water and rice husk ash uniformly. Furthermore, if the water content of the rice husk ash of the present invention, which is the final product, is 30% by mass or less, the water content may be adjusted multiple times.

[0034] The present invention will be explained in more detail below with reference to examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below.

[0035] <Raw material rice husk ash> In the examples, the following raw material rice husk ashes A to C were used. The basic physical properties of the rice husk ash are shown in Table 1. Raw material rice husk ash A: Ash discharged from a fluidized bed type biomass boiler. Raw material rice husk ash B: Ash discharged from a fixed bed type biomass boiler. Raw material rice husk ash C: Ash produced by burning rice husks in a small muffle furnace (model: KDF S100G, manufactured by Denken Co., Ltd.). Specifically, 500 g of rice husks were placed in each of two stainless steel saggers (300 x 300 x 115 mm) with lids and air holes, and these were stacked in two layers and fired at 500°C for two hours and then at 700°C for two hours.

[0036]

[0037] <Analysis method for rice husk ash> Moisture content (heat loss) (mass %) The moisture content was determined from the mass loss of the sample after drying at 105°C for 2 hours in accordance with JIS K 5101-15-1 (pigment test method - heat loss).

[0038] Apparent density (g / L) Based on JIS K 5101-12-1:2004 (Pigment testing method - apparent density or apparent specific volume - static method), a dedicated measuring device (0.5 mm mesh sieve, funnel, 30 mL cylindrical receiver, receiver stand, and funnel stand) was used. The sample (rice husk ash) was dropped from the sieve into the funnel with a brush, and the heaped portion of the sample that had accumulated in the receiver was scraped off with a spatula, after which the mass of the sample was measured. The apparent density of rice husk ash, expressed in units of g / L, was calculated based on its mass using the following formula.

[0039]

[0040] Bulk Density (g / mL) Based on Section "7.8 Bulk Density" of JIS K 6220-1:2015 (Rubber Compounding Agents - Organic Chemicals - Test Methods - Part 1: General), dedicated measuring equipment (a cylinder made of general steel with an inner diameter of 22.00 ± 0.05 mm and an internal depth of 100 mm, and a hollow piston with an outer diameter of 21.80 ± 0.05 mm, a length of 115 mm, and a mass of 190 g) was used. 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 settled, and the height of the piston protruding from the top of the cylinder was measured. The bulk density (g / mL) was calculated using the change in the height of the piston protruding from the top of the cylinder and the area of ​​the base of the cylinder using the following formula:

[0041]

[0042] Volume average particle diameter (D50) The particle size distribution was measured using a laser diffraction particle size distribution analyzer (model: SYNC30, manufactured by Microtrac Bell), and the particle size value at which the volume integrated cumulative value in the particle size distribution becomes 50% was determined as D50.

[0043] Pore ​​volume by mercury porosimetry (cm ３ / g) (hereinafter referred to as "mercury pore volume") Using a mercury porosimeter (model: PASCAL440, manufactured by ThermoQuest), the pressure was increased from 100 kPa to 200 MPa to measure the mercury pore volume in the pore radius range of 3.6 to 6,400 nm.

[0044] Carbon content (mass%) in rice husk ash After drying the rice husk ash at 105°C for 2 hours as a pretreatment, the carbon content was quantified using the following method. The carbon content was measured using a carbon analyzer (model: CS744, manufactured by LECO Japan LLC) that uses oxygen flow combustion and non-dispersive infrared absorption. The sample was heated at a temperature of 1,350°C, oxygen inlet pressure of 0.24 MPa, and measurement time of 50 seconds. The carbon content was measured using an infrared detector (NDIR) in the analyzer. ２ The gas was measured by quantifying it.

[0045] Composition of inorganic components other than carbon in rice husk ash (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 other than carbon to confirm the types of inorganic impurities detected, and then a quantitative analysis of the detected inorganic impurities was performed. The measurement sample was made 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 %).

[0046] Comparative Example 1 (Unprocessed) The raw rice husk ash A was not subjected to compaction or pulverization processes, and the dissolution test (1) described below was carried out as it was to evaluate the suppression of dust when the rice husk ash was added to the alkaline aqueous solution, the sinking of the rice husk ash, and the viscosity after addition. The physical properties of the resulting alkaline silicate aqueous solution were evaluated as follows: SiO ２ concentration and SiO ２ / (Na ２ O+K ２O) The molar ratio was calculated.

[0047] Example 1 (Consolidation) Using a roller compactor (model: FR125x40, manufactured by Freund Turbo), the roller rotation speed was 9 rpm and the linear pressure was 5.8x10 ３ The raw rice husk ash A was compressed under conditions of a pressure of 100 N / cm and a screw rotation speed of 90 rpm to obtain processed rice husk ash.

[0048] After the processing, the physical properties of the rice husk ash were measured in the same manner as in Comparative Example 1, and the dissolution test (1) was carried out.

[0049] Example 2 (Dry Grinding) Raw rice husk ash A was pulverized using a pin-type high-speed pulverizer (model: Jiyuu Pulverizer M-2, manufactured by Nara Machinery Manufacturing Co., Ltd.) at a feed rate of 30 kg / h and a rotation speed of 7,600 rpm to obtain processed rice husk ash. After processing, the physical properties of the rice husk ash were measured and a dissolution test (1) was conducted in the same manner as in Comparative Example 1.

[0050] Example 3 (Wet Grinding) 500 g of raw rice husk ash A and 1,500 g of water were mixed to prepare a rice husk ash slurry with a concentration of 25% by mass. The total amount of this slurry was poured into 4,200 g of alumina balls (diameter 10 mm, media specific gravity 4.09 g / cm). ３ The rice husk ash slurry was placed in a ceramic pot (volume 3.6 L) containing about 1 L of rice husk ash and sealed. The ceramic pot was rotated at 700 rpm for 3 hours using a pot mill rotating table (model: ANZ-100S, manufactured by Nitto Kagaku Co., Ltd.) to wet-pulverize the rice husk ash slurry.

[0051] After wet grinding, the alumina balls and slurry were separated using a sieve with a mesh size of 1 mm. A 150 mm diameter magnetic Buchner funnel (Nutsche) was attached to a 3 L suction filter bottle, and a polyester plain woven filter cloth (mesh size approximately 50 μm x 400 μm) was placed in the funnel to prepare a suction filter. The slurry was further subjected to solid-liquid separation using the suction filter. Using a constant temperature dryer (model: NDO-420, manufactured by Tokyo Rikakikai Co., Ltd.), the solid fraction after solid-liquid separation was left to dry at 125 ° C for 6 hours to obtain processed rice husk ash. After processing, the physical properties of the rice husk ash were measured in the same manner as in Comparative Example 1, and a dissolution test (1) was performed.

[0052] Comparative Example 2 (Excessive Pulverization) Raw rice husk ash A was pulverized using a supersonic jet pulverizer (model: PJM-100NP, manufactured by Nippon Pneumatic Mfg. Co., Ltd.) under conditions of a feed rate of 2.7 kg / h, a pulverization pressure of 0.55 MPa, four nozzles, and a nozzle diameter of 3 mm, to obtain processed rice husk ash. After processing, the physical properties of the rice husk ash were measured in the same manner as in Comparative Example 1, and a dissolution test (1) was conducted.

[0053] Example 4 (Crushing and Compaction) Using a roller compactor (model: FR125x40, manufactured by Freund Turbo Corporation), the roller rotation speed was 9 rpm and the linear pressure was 5.8x10 ３ The rice husk ash was crushed in the same manner as in Comparative Example 2 under conditions of a pressure of 100 N / cm and a screw rotation speed of 90 rpm, and then compacted to obtain processed rice husk ash. After the processing, the physical properties of the rice husk ash were measured and a dissolution test (1) was conducted in the same manner as in Comparative Example 1.

[0054] Comparative Example 3 (Unprocessed) Raw rice husk ash B was not subjected to compaction or pulverization, and the physical properties of the rice husk ash were measured as is in the same manner as in Comparative Example 1, and dissolution test (1) was carried out.

[0055] Comparative Example 4 (Unprocessed) Raw rice husk ash C was not subjected to compaction or pulverization, and the physical properties of the rice husk ash were measured as in Comparative Example 1, and dissolution test (1) was carried out.

[0056] Comparative Example 5 (Wet Grinding) Raw rice husk ash B was subjected to processing (wet grinding and solid-liquid separation) in the same manner as in Example 3 to obtain processed rice husk ash. After processing, the physical properties of the rice husk ash were measured and a dissolution test (1) was conducted in the same manner as in Comparative Example 1.

[0057] Comparative Example 6 (Wet Grinding) Raw rice husk ash C was subjected to processing (wet grinding and solid-liquid separation) in the same manner as in Example 3 to obtain processed rice husk ash. After processing, the physical properties of the rice husk ash were measured and a dissolution test (1) was conducted in the same manner as in Comparative Example 1.

[0058] <Dissolution test (1)> Rice husk ash was dissolved in an alkaline aqueous solution according to the procedure of the following dissolution method (1) to obtain an aqueous alkali silicate solution. This aqueous alkali silicate solution was evaluated for dust suppression when adding rice husk ash to the alkaline aqueous solution, sinking of the rice husk ash, and viscosity after addition. The physical properties of the resulting aqueous alkali silicate solution were evaluated as follows: SiO ２ concentration and SiO ２ / (Na ２ O+K ２ O) The molar ratio was calculated.

[0059] Dissolution method (1) Dissolution method (1) is the dissolution of SiO in an alkaline silicate solution. ２ The goal is to prepare an alkali silicate aqueous solution with a concentration of 17-19% by mass and a molar ratio of 3.00-3.40. These values ​​are standard for commercially available alkali silicate aqueous solutions. 1,310 g of sodium hydroxide aqueous solution adjusted to 8.9% by mass was added to a 3 L stainless steel container equipped with a stirrer and heated to 90°C. Next, 410 g of rice husk ash was added to the container while continuing to stir, the container was covered, and mixing was continued for 6 hours while maintaining the temperature at 90°C. The aqueous solution was then cooled to room temperature. A 150 mm diameter magnetic Buchner funnel (Nuchner) was attached to a 3 L suction filter bottle, and filter paper (1 μm mesh, standard 5C, manufactured by Advantec) was placed in the funnel to prepare a suction filter. Undissolved residue in the aqueous solution was removed using the suction filter to obtain an alkali silicate aqueous solution.

[0060] Suppression of dust during addition In the dissolution method (1) above, the amount of dust generated when rice husk ash was added to the alkaline aqueous solution was visually evaluated. The less dust generated, the more the deterioration of the working environment can be suppressed. The evaluation was made on a 5-point scale, with Example 1, where almost no dust was generated, being rated as 5, and Comparative Example 3, where the most dust was generated, being rated as 1. If the evaluation was 4 or higher, the amount of dust generated was within the acceptable range, and the result was displayed as "GOOD", and if it was 3 or lower, it was displayed as "BAD".

[0061] Sinking speed: In the dissolution method (1) above, the time (seconds) from immediately after adding rice husk ash until all of the rice husk ash on the liquid surface (excluding rice husk ash scattered as dust) becomes wet and sinks was measured. The shorter the sinking time, the shorter the manufacturing process time, including the rice husk ash dissolution step, which contributes to improving productivity. In the results, sinking times of 60 seconds or less were displayed as "GOOD", and times of more than 60 seconds were displayed as "BAD".

[0062] Viscosity: In the dissolution method (1) described above, a portion (200 mL) of the aqueous solution was removed into a beaker immediately after the rice husk ash on the liquid surface (excluding the rice husk ash scattered as dust) had completely wetted and sunk. Using a commercially available B-type viscometer (model: TVB-10M, manufactured by Toki Sangyo Co., Ltd.), the viscosity of the sample was measured after rotating it for 1 minute at 60 rpm using a No. 20 rotor if the viscosity of the sample was 50 mPa·s or less. For other samples, the viscosity was measured after rotating it for 1 minute at 3 rpm using a No. 21 rotor. In the results, a viscosity of 50 mPa·s or less was labeled "GOOD," and a viscosity of more than 50 mPa·s was labeled "BAD."

[0063] SiO in alkaline silicate solution ２ Concentration and Molar Ratio Using a syringe equipped with a membrane filter having a mesh size of 1 μm, the aqueous alkali silicate solution sample obtained by dissolution method (1) was collected (approximately 0.2 mL) and diluted 10,000 times with pure water (standard solution for calibration curve: 20 ppm by mass). The SiO concentration of the diluted sample was measured using a high-resolution ICP optical emission spectrometer (model: PS3520DDII, manufactured by Hitachi High-Tech Science Corporation). ２ , Na ２ O and K ２ Quantitative analysis of O was performed. ２ , Na ２ O and K ２ The O mass concentration was converted to a molar amount, and the molar ratio of silicon dioxide to the total of sodium oxide and potassium oxide was calculated. Specifically, the molar mass of each component was 60.08 (SiO ２ ), 61.98 (Na ２ O) and 94.19 (K ２ O), and the molar ratio was calculated by the following formula.

[0064]

[0065] The properties of the rice husk ashes of Examples 1 to 4 and Comparative Examples 1 and 2 and the results of the dissolution test (1) are shown in Table 2.

[0066]

[0067] <Explanation of Results: Examples 1 to 4, Comparative Examples 1 to 6> Characteristics of Rice Husk Ash Fig. 1 is a photograph of raw rice husk ash A, raw rice husk ash C, and the rice husk ash obtained in Example 1. As can be seen from Fig. 1, it is easy to determine whether the rice husk ash has been processed or not based on information such as the presence or absence of particles with a plant-derived structure, the size of the largest particle, and the uniformity of particle size.

[0068] While the apparent density of the rice husk ash in Comparative Example 1 was 300 g / L, the apparent densities of the rice husk ashes in Examples 1 to 4, which were compacted or crushed, were all significantly higher. Therefore, by increasing the amount of rice husk ash filled into the packaging bag, significant improvements in transportation and storage efficiency can be expected. It was found that excessive crushing of rice husk ash did not significantly increase the apparent density (Comparative Example 2). In other words, even if the raw rice husk ash is the same, the characteristics of the resulting rice husk ash differ depending on the processing conditions, so it was found that rice husk ash processing must be carried out under appropriate conditions. In particular, it was found that crushing rice husk ash and then compacting it significantly increased the apparent density (Example 4). In addition, it was found that the evaluation differed depending on the apparent density even when the particle diameter was similar (comparison of Example 4 and Comparative Example 2). This is thought to be because the apparent density is a parameter that also reflects the state of the voids between particles. From the viewpoint of providing rice husk ash suitable for producing an aqueous alkali silicate solution, the state of voids is an important characteristic, and therefore, it can be said that the solution to the problem of the present invention cannot be determined by particle size alone.

[0069] Even in the rice husk ashes obtained using different rice husk burning methods (Comparative Examples 3 and 4), the apparent density was low, and so similar results to those in Comparative Example 1 were obtained. In addition, even when the same processing was performed, the properties of the resulting rice husk ash differed depending on the raw rice husk ash, and it was found that processing of rice husk ash must be carried out under appropriate conditions depending on the raw rice husk ash (comparison of Example 3 and Comparative Examples 5 and 6).

[0070] Regarding the evaluation of the dissolution test, in both the examples and the comparative examples, the target SiO ２ Aqueous alkali silicate solutions having concentrations (17-19% by weight) and molar ratios (3.00-3.40) could be prepared.

[0071] Figure 2 is a photograph showing the state of the liquid surface of the aqueous solution after the raw rice husk ash A had completely sunk into the aqueous sodium hydroxide solution and was then thoroughly stirred for several tens of seconds in the dissolution test (1) of Comparative Example 1. The rice husk ash of Comparative Example 1 sunk for a long time and was in a highly viscous state. In Comparative Example 1, the highly viscous state continued for a while, so the solubility was insufficient.

[0072] In contrast, the rice husk ashes of Examples 1 to 4 all generated little dust, had short sinking times, low viscosity, and excellent solubility. In the case of Comparative Example 2, in which excessive grinding was performed, dust was generated and the sinking time was long, but it was found that by further compacting the rice husk ash, it was possible to obtain appropriate rice husk ash that could suppress dust generation and had a short sinking time.

[0073] As described above, it was found that rice husk ash suitable for producing an alkali silicate aqueous solution can be provided by processing rice husk ash appropriately so that the apparent density becomes 400 to 800 g / L.

[0074] Example 5: Using the rice husk ash (apparent density: 590 g / L) obtained in Example 1, an attempt was made to produce a highly concentrated aqueous alkali silicate solution according to the following dissolution method (2). If a highly concentrated aqueous alkali silicate solution can be produced, it may be possible to improve the production efficiency of aqueous alkali silicate solutions of standard concentrations and further broaden the applications of rice husk ash as a silica source.

[0075] Dissolution method (2) Dissolution method (2) is the dissolution of SiO in an alkaline silicate solution. ２ The goal was to prepare an alkali silicate aqueous solution with a concentration of 28-29% by mass and a molar ratio of 3.00-3.40. 1,080 g of sodium hydroxide aqueous solution adjusted to 17.0% by mass was added to a 3 L stainless steel container equipped with a stirrer and heated to 90°C. Next, 649 g of rice husk ash was added to the container while continuing to stir, the container was covered, and mixing was continued for 6 hours while maintaining the temperature at 90°C. The aqueous solution was then cooled to room temperature. A 150 mm diameter magnetic Buchner funnel (Nuchner) was attached to a 3 L suction filter bottle, and filter paper (1 μm mesh, standard 5C, manufactured by Advantec) was placed in the funnel to prepare a suction filter. The undissolved residue in the aqueous solution was removed using the suction filter to obtain an alkali silicate aqueous solution.

[0076] Comparative Example 7: As a comparison with Example 5, an attempt was made to produce a highly concentrated aqueous alkali silicate solution using the rice husk ash of Comparative Example 1 (apparent density: 300 g / L) according to dissolution method (2) in the same manner as in Example 5. However, the rice husk ash did not all sink into the aqueous sodium hydroxide solution, and the load on the agitator increased, so the dissolution operation was interrupted. As a result, the rice husk ash of Comparative Example 1 could not be dissolved according to dissolution method (2).

[0077] <Explanation of Results and Discussion I> When the rice husk ash obtained in Example 1 was dissolved in an aqueous sodium hydroxide solution using dissolution method (1) and dissolution method (2), the amount of SiO ２ The relationship between the concentration and the SiO2 content is shown in Table 3 and Figure 3(a). When the rice husk ash of Example 1 (apparent density: 590 g / L) was used, the SiO2 content was 1.2 g / L. ２ It was possible to produce a highly concentrated aqueous alkali silicate solution with a concentration of 28 to 29% by mass. Since the thermal energy and time required to dissolve rice husk ash are almost the same for dissolution methods (1) and (2), this result shows that by using the rice husk ash of the present invention, more rice husk ash can be dissolved in an alkaline aqueous solution with the same thermal energy.

[0078]

[0079] Example 6 Using the rice husk ash (apparent density: 590 g / L) obtained in Example 1, an attempt was made to produce a highly concentrated aqueous alkali silicate solution by pressurized dissolution according to the dissolution method (3) below. If pressurized dissolution can be applied, the time required to produce a highly concentrated aqueous alkali silicate solution can be further shortened compared to Example 5. Note that the unprocessed rice husk ash (raw rice husk ashes A to C) had such a low apparent density that it was not even possible to charge the required amount of rice husk ash into a pressure vessel.

[0080] Dissolution Method (3) Dissolution Method (3) is a method for obtaining an alkali silicate aqueous solution of the same quality as Dissolution Method (2) in a shorter time than Dissolution Method (2). 450 g of a 17.0% by mass sodium hydroxide aqueous solution and 270 g of rice husk ash were placed in a 1 L pressure vessel (autoclave) equipped with a stirrer while stirring. The vessel was then capped and dissolution was carried out under a pressurized condition of 150 °C and 0.47 MPa. Because the vessel was sealed during dissolution under pressurized conditions, sampling was not possible. Therefore, multiple experiments were carried out with pressurized times of 0.5, 1, 2, and 3 hours. After dissolution was completed, the pressure was released, and the aqueous solution was cooled to room temperature. After cooling, a 150 mm diameter magnetic Buchner funnel (Nuchner) was attached to a 3 L suction filter bottle, and filter paper (mesh size 1 μm, standard 5C, manufactured by Advantec Co., Ltd.) was placed in the funnel to prepare a suction filter. Undissolved residue in the aqueous solution was removed using the suction filter, yielding an aqueous alkali silicate solution.

[0081] The rice husk ash obtained in Example 1 was dissolved in an aqueous sodium hydroxide solution using dissolution method (2) and dissolution method (3). The time from the time of addition and the amount of SiO ２ The relationship between the concentration and the SiO2 content is shown in Table 4 and Figure 3(b). When the rice husk ash (apparent density: 590 g / L) of Example 1 was dissolved in an alkaline aqueous solution under pressurized conditions, a highly concentrated aqueous alkali silicate solution could be produced in a shorter time than in the dissolution method (2). Specifically, ２ The time required to produce a high-concentration aqueous alkali silicate solution with a concentration of 28 to 29% by mass was 6 hours using dissolution method (2), whereas it was 2 hours using dissolution method (3).

[0082]

[0083] <Reference Example: Example of Production of Wet-Process Silica> It was confirmed whether the aqueous alkali silicate solution produced using rice husk ash of the present invention exhibits performance equivalent to that of a conventional aqueous sodium silicate solution derived from silica sand. Specifically, wet-process silica (hydrated silica) for rubber tires was produced, and silica properties and rubber performance were evaluated. The production of wet-process silica, evaluation of silica properties, and evaluation of rubber performance were carried out in the same manner as in Example 1 of WO 2020 / 031523. When there are differences in the evaluation equipment between the international publication and this specification, this means that the evaluation was carried out using the evaluation equipment described in this specification and in accordance with the conditions described in the international publication.

[0084] The aqueous alkali silicate solution of the Reference Example was prepared in the same manner as in Example 5, except that a 240 L vessel with stirring was used and the amount of material was increased by 60 times. A highly concentrated aqueous alkali silicate solution was prepared, and this was diluted with water to obtain SiO ２ Concentration 12.44% by mass, Na ２ O concentration 3.74% by mass, K ２ O concentration 0.41% by mass, SiO ２ / (Na ２ O+K ２ O) molar ratio was adjusted to 3.20.

[0085] The comparative sodium silicate solution was a commercial sodium silicate cullet (medium molar grade, Na ２ O: 24.51% by mass, SiO ２ SiO2: 75.44% by mass, purity 99.5% by mass or more, manufactured by Tokuyama Corporation) was dissolved in warm water and further diluted with water to obtain SiO2. ２ Concentration 12.44% by mass, Na ２ O concentration 4.01% by mass, SiO ２ / Na ２ The mol ratio of 1:1 was adjusted to 3.20.

[0086] These aqueous alkali silicate solutions were then used to produce wet-process silica. The physical properties of the wet-process silica are shown in Table 5. In addition, rubber compounds were produced using each of the wet-process silicas. The rubber performance evaluations are shown in Table 6.

[0087]

[0088]

[0089] The results in Table 5 show that the aqueous alkali silicate solution produced using rice husk ash of the present invention can also produce wet-process silica having properties equivalent to those of the aqueous sodium silicate solution derived from silica sand. Furthermore, the results in Table 6 show that the wet-process silica produced using rice husk ash of the present invention as a raw material imparts to rubber compounds rubber physical properties equivalent to those of wet-process silica produced using silica sand as a raw material.

[0090] As explained above, the rice husk ash of the present invention has a moderately high apparent density, and therefore the following effects can be expected: (1) Compared to unprocessed rice husk ash, a larger amount of rice husk ash can be put into a container, resulting in a highly concentrated slurry. (2) Compared to unprocessed rice husk ash, the rice husk ash has improved sinking properties in an alkaline aqueous solution and absorbs less solvent, thereby reducing the time and energy required to produce a slurry and improving the productivity of the alkaline silicate aqueous solution. (3) Compared to unprocessed rice husk ash, the rice husk ash is less bulky, allowing a larger amount of rice husk ash to be transported in one shipment and a larger amount of rice husk ash to be stored in a limited space, thereby improving transportation and storage efficiency. (4) Compared to unprocessed rice husk ash, the rice husk ash generates less dust and is easier to handle during packaging and unpacking, thereby improving the working environment for handling rice husk ash. (5) Although the rice husk ash of the present invention is a recycled waste material, the performance of an aqueous alkali silicate solution produced using the rice husk ash as a raw material is equivalent to that of an aqueous sodium silicate solution derived from silica sand. The rice husk ash of the present invention can be considered an environmentally friendly material that can be used as a substitute for silica sand.

[0091] The processed rice husk ash of the present invention can be used as a silica source in addition to producing aqueous alkali silicate solutions.

Claims

1. Rice husk ash having an apparent density in a dry state of 400 to 800 g / L and a moisture content of 30 mass% or less.

2. Rice husk ash as described in claim 1, having a carbon content of 0.1 to 8.0 mass% and a silica content of 85 to 95 mass%.

3. Rice husk ash according to claim 1 or 2, having a volume average particle diameter D50 measured by a laser diffraction method of 5.0 to 200 μm.

4. A method for producing rice husk ash, comprising: compressing or crushing raw rice husk ash having an apparent density in a dry state of 350 g / L or less so that the apparent density in a dry state is 400 to 800 g / L; and optionally adjusting the moisture content of the raw rice husk ash to a range of 30 mass% or less.

5. The method according to claim 4, wherein the compaction or pulverization is carried out so that the volume average particle size D50 is 5.0 to 200 μm.

6. The compaction or crushing is carried out using a roller compactor at a rate of 1.2 x 10 ３ The method according to claim 4 or 5, wherein the method is carried out at a linear pressure of 100 N / cm or more.

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