Rice husk ash, method for producing same, calcium silicate molded body, and method for producing same

Abstract

Provided are: a rice husk ash suitable as a raw material for producing a xonotlite-based calcium silicate molded body that has preferable characteristics (for example, preferable mechanical strength) and that exhibits excellent incombustibility; and a method for producing the rice husk ash.　A rice husk ash according to the present invention exhibits, in a diffraction spectrum obtained by powder X-ray diffraction, a cristobalite (101) peak intensity of 10000 counts or less and an ignition loss of 3 mass% or less. A method for producing a rice husk ash according to the present invention comprises a step for subjecting rice husk to a combustion treatment. The time for the combustion treatment is 10 hours or longer. This rice husk ash is suitable as a raw material for producing a xonotlite-based calcium silicate molded body that has a preferable mechanical strength and that exhibits excellent incombustibility.

Description

Rice husk ash, method for producing the same, calcium silicate molded body, and method for producing the same 【0001】 The present invention relates to rice husk ash, a method for producing the same, a calcium silicate molded body, and a method for producing the same. 【0002】 Generally, inexpensive natural resources (such as silica) are used as silica raw materials for producing calcium silicate products. There is also a method of manufacturing calcium silicate products by substituting this silica raw material with rice husk ash silica. Such rice husk ash silica can be obtained, for example, by burning rice husks in a biomass boiler. As a method for producing rice husk ash from rice husks, Patent Document 1 discloses a method of burning carbonized rice husks and recovering the combustion ash generated by this combustion as silica, as well as a silica production apparatus. Further, Patent Document 2 discloses an apparatus and a production method for producing vegetable silica by burning biomass fuel or the like at a high temperature of 1000 degrees or more. 【0003】 JP-A-2023-002926 JP-A-2017-9240 【0004】 However, as a result of investigations by the present inventors, it was found that the quality of conventionally obtained rice husk ash (rice husk ash silica) tends to vary. Further, even when producing wollastonite-based calcium silicate using such rice husk ash, it was difficult to synthesize a wollastonite-based calcium silicate molded body having good properties (for example, good mechanical strength). 【0005】 The present invention has been made in view of the above, and an object thereof is to provide rice husk ash suitable as a raw material for producing a wollastonite-based calcium silicate molded body having good properties (for example, good mechanical strength and incombustibility), and a method for producing the same. Another object of the present invention is also to provide a calcium silicate molded body obtained using the above rice husk ash, and a method for producing the same. 【0006】The inventors of this invention conducted extensive research to achieve the above objectives and found that the objectives can be achieved by adjusting the cristobalite (101) peak intensity of rice husk ash to 10,000 counts or less and the ignition loss to 3% by mass or less, thereby completing the present invention. 【0007】 In other words, the present invention encompasses, for example, the subject matter described in the following sections: Section 1: Rice husk ash having a cristobalite (101) peak intensity of 10,000 counts or less in a diffraction spectrum obtained by powder X-ray diffraction, and a loss on ignition of 3% by mass or less. Section 2: A method for producing rice husk ash according to Section 1, comprising a step of burning rice husks, wherein the burning time is 10 hours or more. Section 3: A calcium silicate molded article obtained from the rice husk ash according to Section 1. Section 4: A method for producing a calcium silicate molded article, comprising a step of hydrothermally treating a raw material containing the rice husk ash according to Section 1 to obtain a slurry, and a step of molding the slurry. Section 5: A method for producing a calcium silicate molded article according to Section 4, comprising a step of obtaining the rice husk ash by the method described in Section 2, and using the energy generated by burning the rice husks for the production of the calcium silicate molded article. 【0008】 The rice husk ash of the present invention is suitable as a raw material for producing xonotlite-based calcium silicate molded articles, and in particular, it is suitable as a raw material for producing xonotlite-based calcium silicate molded articles that have good mechanical strength and excellent non-flammability. 【0009】 This is a schematic diagram illustrating an example of an apparatus for producing rice husk ash according to the present invention. 【0010】 Embodiments of the present invention will be described in detail below. In this specification, the expressions "containing" and "including" include the concepts of "containing," "including," "substantially consisting of," and "consisting only of." 【0011】 1. Rice husk ash The rice husk ash of the present invention has a cristobalite (101) peak intensity of 10,000 counts or less in the diffraction spectrum obtained by powder X-ray diffraction, and the rice husk ash of the present invention has a loss on ignition of 3% by mass or less. 【0012】 By using the rice husk ash of the present invention, a xonotlite-based calcium silicate molded article can be produced, and the resulting xonotlite-based calcium silicate molded article can have good mechanical strength and also has excellent non-flammability. 【0013】 When the cristobalite (101) peak intensity exceeds 10,000 counts, the mechanical strength of the resulting xonotlite-based calcium silicate molded product decreases. 【0014】 The cristobalite (101) peak intensity is preferably 8000 counts or less, more preferably 6000 counts or less, even more preferably 5000 counts or less, and particularly preferably 4000 counts or less. Furthermore, the cristobalite (101) peak intensity is preferably 100 counts or more, more preferably 1000 counts or more, even more preferably 1500 counts or more, and particularly preferably 2000 counts or more. 【0015】 The method for adjusting the cristobalite (101) peak intensity to 10,000 counts or less is not particularly limited. For example, by adjusting the combustion conditions of the rice husks, which are the raw material when producing rice husk ash, particularly the combustion time (residence time) described later, to an appropriate range, it becomes easy to adjust the cristobalite (101) peak intensity to 10,000 counts or less. 【0016】The cristobalite (101) peak intensity can be determined by measuring the rice husk ash of the present invention by powder X-ray diffraction. Specifically, it can be estimated based on the peak intensity (2θ = 21.9° ± 0.5) originating from the crystals of the cristobalite (101) plane in the diffraction spectrum obtained by powder X-ray diffraction of the rice husk ash of the present invention. The measurement device used is Rigaku's powder X-ray diffractometer "MiniFlex 600," and the rice husk ash is measured under the following conditions (X-ray source: Cu-Kα rays (wavelength 0.154 nm), tube voltage 35 kV, tube current 10 mA, measurement range 20° to 30°, scan speed 2.0000° / min, step width 0.0200°, slit 0.3 mm), and correction is performed with a standard sample. The standard sample was measured using NISI's SRM-640d, with the following conditions: X-ray source: Cu-Kα rays (wavelength 0.154 nm), tube voltage 35 kV, tube current 10 mA, measurement range 20° to 40°, scan speed 2.0000° / min, step width 0.0200°, and slit 0.3 mm. The peak intensity (2θ = 28.6° ± 0.5) derived from the silicon (111) plane crystals was estimated, and the measured value of the rice husk ash was corrected using the measured value of the standard sample with the following formula: Rice husk ash correction value (counts) = Rice husk ash measured value × Correction coefficient Correction coefficient = Standard material measured value immediately after tube replacement ÷ Standard material measured value for the current month Note that the standard sample was measured for a peak intensity of 28.4° based on Si(111), and the rice husk ash was measured for a peak intensity of 22.0° based on cristobalite(101). 【0017】 As mentioned above, the ignition loss of the rice husk ash of the present invention is 3% by mass or less. If the ignition loss exceeds 3% by mass, the mechanical strength of the resulting xonotlite-based calcium silicate molded article decreases, and its non-flammability also decreases. 【0018】 The ignition loss is preferably 2.9% by mass or less, more preferably 2.8% by mass or less, even more preferably 2.7% by mass or less, and particularly preferably 2.6% by mass or less. Furthermore, the lower limit of the ignition loss is not particularly limited and may be 0% by mass, 0.1% by mass or more, or even 0.5% by mass or more. 【0019】The method for adjusting the ignition loss to 3% by mass or less is not particularly limited. For example, by adjusting the combustion conditions of the rice husks, which are the raw material when producing rice husk ash, particularly the combustion time (residence time) described later, to an appropriate range, it becomes easy to adjust the ignition loss to 3% by mass or less. 【0020】 The ignition loss of the rice husk ash of the present invention can be measured by the following method. Approximately 5 g of rice husk ash dried at 150°C is weighed and placed in an electric furnace (Nabertherm "Muffle Furnace LT9 / 11 (Germany)"), and the temperature is raised to 1000°C over 150 minutes, followed by a heat treatment at 1000°C for 30 minutes. After the heat-treated rice husk ash is allowed to cool naturally to room temperature (25°C), it is removed from the electric furnace, and the ignition loss is calculated using the following formula: Ignition loss (%) = ((Weight before heating - Weight after heating) / Weight before heating) × 100. 【0021】 The rice husk ash of the present invention is not particularly limited in terms of other physical properties, as long as the cristobalite (101) peak intensity and ignition loss each satisfy the above ranges, and can, for example, be the same as known rice husk ash. Furthermore, the composition of the rice husk ash of the present invention is not particularly limited as long as the cristobalite (101) peak intensity and ignition loss each satisfy the above ranges. 【0022】 2. Method for Producing Rice Husk Ash The method for producing the rice husk ash of the present invention is not particularly limited. For example, the rice husk ash of the present invention can be obtained by burning the rice husks used as raw material. Preferably, the method for producing the rice husk ash of the present invention comprises a step of burning the rice husks, and the burning time is 10 hours or more. In this case, the rice husk ash of the present invention can be easily produced, and in particular, it becomes easy to adjust the cristobalite (101) peak intensity to 10,000 counts or less and the ignition loss to 3% by mass or less. 【0023】 When burning rice husks, the type of rice husk used is not particularly limited; for example, known types of rice husks can be widely applied in this invention. 【0024】The combustion treatment conditions are preferably divided into two stages, namely a first combustion treatment and a second combustion treatment. In this case, it becomes easier to adjust the cristobalite (101) peak intensity and ignition loss of the resulting rice husk ash to a desired range. The combustion treatment is not limited to two stages; it may be one stage or three or more stages. 【0025】 In the first combustion treatment, the combustion temperature can be set to 300 to 1100°C, as this makes it easier to adjust the cristobalite (101) peak intensity and the ignition loss to a desired range. The combustion time is preferably 30 to 60 seconds, excluding the heating time (i.e., from the time the desired temperature is reached), as this makes it easier to adjust the cristobalite (101) peak intensity and the ignition loss to a desired range. In the first combustion treatment, it is preferable to burn the rice husks in the presence of air. 【0026】 In the second combustion treatment, the combustion temperature can be set to 500 to 800°C, as this makes it easier to adjust the cristobalite (101) peak intensity and the ignition loss to a desired range. The combustion time is preferably 10 to 20 hours, excluding the heating time (i.e., from the time the desired temperature is reached), as this makes it easier to adjust the cristobalite (101) peak intensity and the ignition loss to a desired range. In the second combustion treatment, it is preferable to burn the rice husks in the presence of air. The temperature of the first combustion treatment may be higher than the temperature of the second combustion treatment, or the temperature of the first combustion treatment may be lower than the temperature of the second combustion treatment. In either case, the rice husk ash of the present invention can be obtained. 【0027】 In the combustion of rice husks, it is preferable that the combustion treatment time (excluding the heating time) be 10 hours or more. In this case, it becomes easier to adjust the cristobalite (101) peak intensity to 10,000 counts or less and the ignition loss to 3 mass% or less. The combustion treatment time referred to here means the total combustion time. Therefore, if the combustion treatment is divided into two stages, a first combustion treatment and a second combustion treatment, as described above, it means the total combustion time of the first and second combustion treatments. 【0028】Based on the above, one preferred embodiment of the present invention for producing rice husk ash is that it includes a step of burning rice husks, and the burning time is 10 hours or more. 【0029】 The process of burning the rice husks can be carried out in a boiler, and preferably in a biomass boiler designed to perform the burning process in two or more stages. 【0030】 Figure 1 is a schematic diagram illustrating the structure of a biomass boiler that can be used in the process of burning rice husks. 【0031】 The biomass boiler B shown in Figure 1 has a first combustion furnace 1 for performing a first combustion treatment of rice husks M1 and a second combustion furnace 2 for performing a second combustion treatment of rice husks M1, both located inside the boiler and adjacent to each other separated by a partition wall 3. The first and second combustion furnaces are designed to be heated in order to burn the rice husks M1. The biomass boiler B also has a raw material supply port 4 connected to the first combustion furnace 1 for supplying rice husks M1 to the first combustion furnace 1. 【0032】 The first combustion furnace 1 and the second combustion furnace 2 are designed to be supplied with air A from the outside from the bottom of the first combustion furnace 1 and the second combustion furnace 2, and an exhaust passage E is provided for exhausting the air introduced from the outside or the gas produced by combustion to the outside. The partition wall 3 is provided with a ventilation passage 5 for supplying high-temperature air generated in the first combustion furnace 1 to the second combustion furnace 2. 【0033】 The ventilation passage 5 is a pipe for guiding air A into the rice husks accumulated in the second combustion furnace 2, and is provided so as to penetrate the partition wall 3. In this embodiment of the biomass boiler B, because the ventilation passage 5 is provided, air A is supplied to the second combustion furnace 2 through the ventilation passage 5, so that sufficient oxygen reaches the inside of the rice husks accumulated in the second combustion furnace 2, and the combustion of the rice husk ash M2 is promoted. At the same time, since air A is supplied from the bottom, oxygen is supplied to the inside of the rice husks accumulated in the second combustion furnace 2, so that the biomass boiler B of this embodiment can continue burning for more than 10 hours in total. 【0034】 Conventional biomass boilers can promote the combustion of the surface of rice husks accumulated in the furnace, but it is difficult to burn the inside of the rice husk deposit for more than 10 hours. For this reason, it has been difficult to produce the rice husk ash of the present invention (rice husk ash with a peak intensity of 10,000 counts or less and a loss on ignition of 3% by mass or less). To overcome this difficulty, it is conceivable to increase the combustion temperature, but when processed at high temperatures, it is not possible to stably adjust the cristobalite (101) peak intensity to 10,000 counts or less. It is also conceivable to burn the entire rice husk uniformly and for a long time by making the combustion furnace longer, but in this case, the equipment becomes large and complex. 【0035】 In contrast, the biomass boiler B shown in Figure 1, by having the above-described configuration, can burn the rice husks M1 accumulated in the furnace uniformly, and can burn for more than 10 hours, thus making it easy to obtain the rice husk ash of the present invention. 【0036】 The rice husks that have undergone the first combustion treatment in the first combustion furnace 1 are supplied to the second combustion furnace 2 across the partition wall 3, where they undergo a second combustion treatment. This produces the desired rice husk ash M2. The produced rice husk ash M2 can be recovered through the feeder 6 and the recovery port 7. 【0037】 The rice husks supplied to the first combustion furnace of the biomass boiler B shown in Figure 1 can be continuously burned by air A supplied from the bottom once they are ignited at the start of operation. The rice husks M1, which have become lighter after combustion, are then lifted up by the airflow caused by the supplied air A and the heat of combustion, and the lifted rice husks M1 are cooled by the water drum 8 and multiple water pipes 10. The temperature and time of this cooling are not particularly limited, and the effects of the present invention are not impaired regardless of the cooling temperature. After cooling, the rice husks M1 fall into the second combustion furnace 2 due to their own heat of combustion and the decrease in airflow (dashed arrow in Figure 1). The second combustion furnace 2 is designed to burn continuously using the heat it has stored, the high-temperature air supplied from the ventilator 5, and the air A supplied from the bottom side. 【0038】 The combustion heat can be utilized (reutilized), for example, as various energy sources, and the method thereof is not limited. For example, similar to a known water tube boiler, the combustion heat can heat the water in the water drum 8 and the plurality of water tubes 10 to generate steam in the steam-water drum 9. Alternatively, the combustion heat can be utilized as a heat source for producing hot water. As will be described later, it can also be used as a heat source for obtaining steam in the production of a calcium silicate molded body. 【0039】 3. Calcium silicate molded body A calcium silicate molded body can be produced using the rice husk ash of the present invention. Since the cristobalite (101) peak intensity of the rice husk ash of the present invention is adjusted to 10,000 counts or less and the loss on ignition is 3% by mass or less, the calcium silicate molded body obtained using the rice husk ash can have good mechanical strength and is also excellent in nonflammability. 【0040】 The density of the calcium silicate molded body obtained using the rice husk ash of the present invention is not particularly limited. For example, it is in the range of 120 to 240 kg / m ３ . The shape and size of the molded body are also not particularly limited and can be appropriately designed according to the application. 【0041】 The calcium silicate molded body, for example, has a flexural strength of 20 N / cm ２ or more, preferably 30 N / cm ２ or more, more preferably 40 N / cm ２ or more. 【0042】 The linear shrinkage rate (heating linear shrinkage rate) when the calcium silicate molded body is heated at 1,000 °C for 3 hours is, for example, 2% or less, preferably 1% or less. 【0043】 The method for producing a calcium silicate molded body using the rice husk ash of the present invention is not particularly limited. For example, a known method can be widely applied in the present invention. For example, it is preferable to obtain a calcium silicate molded body by a production method including a step A of hydrothermally treating a raw material containing the rice husk ash of the present invention to obtain a slurry, and a step B of molding the slurry. 【0044】 In Step A, a raw material containing rice husk ash of the present invention is hydrothermally treated to obtain a slurry. The raw material containing rice husk ash can contain a calcareous component and water. 【0045】 Examples of the calcareous component include quicklime (calcium oxide), slaked lime (calcium hydroxide), carbide slag, calcium chloride, and the like. 【0046】 The raw material may contain other siliceous components together with the rice husk ash of the present invention. Examples thereof include silica, silica sand, silica gel, white carbon, diatomaceous earth, ferrosilicon dust, shirasu, and the like. The siliceous component contained in the raw material may be only the rice husk ash of the present invention. 【0047】 The raw material can further contain other components as long as the effects of the present invention are not impaired. Examples of the other components include gypsum, and other aluminosilicates such as kaolin, organic acids, and the like. 【0048】 In the raw material, the content ratio of the rice husk ash of the present invention is preferably 40% by mass or more, more preferably 45% by mass or more, and preferably 70% by mass or less, more preferably 60% by mass or less, based on the total amount of the solid content contained in the raw material. 【0049】 In the raw material, the content ratio of the calcareous component is preferably 40% by mass or more, more preferably 45% by mass or more, and preferably 60% by mass or less, more preferably 55% by mass or less, based on the total amount of the solid content contained in the raw material. 【0050】 In the raw material, the total amount of the rice husk ash of the present invention and the calcareous component is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, based on the total amount of the solid content contained in the raw material. 【0051】 The amount of water contained in the raw material is not particularly limited. For example, the amount of water can be adjusted so that the solid content concentration is 3 to 10% by mass. 【0052】The conditions for hydrothermal treatment of the raw materials are not particularly limited; for example, the temperature can be 150 to 260°C, preferably 160 to 250°C. The hydrothermal treatment time can be set appropriately according to the temperature, etc., preferably 2 to 10 hours, and more preferably 3 to 7 hours. The hydrothermal treatment of the raw materials can be carried out, for example, in a known pressure vessel. 【0053】 By hydrothermally treating the raw materials, an aqueous slurry containing xonotlite-based calcium silicate is obtained. The resulting xonotlite-based calcium silicate can take the form of a hydrate, for example. The xonotlite (xonotlite-based calcium silicate) in the slurry can form secondary particles. The particle size of the secondary particles is not limited, and is, for example, about 5 to 150 μm. 【0054】 In step B, the slurry obtained in step A is molded. The molding method is not particularly limited; for example, dehydration molding can be employed. The dehydration molding method is not particularly limited; for example, a wide range of known dehydration molding methods can be employed. 【0055】 When forming the slurry, various additives can be added to the slurry as needed. Examples of additives include fibrous materials, cement, fillers, pigments, dyes, polymers (resins), flocculants, and water repellents. 【0056】 Examples of fibrous materials include known organic and inorganic fibers such as pulp, glass fibers, cotton, ceramic fibers, vinylon fibers, aramid fibers, nylon fibers, polyester fibers, polyethylene fibers, polypropylene fibers, steel fibers, and carbon fibers. Examples of cement include Portland cement, white cement, alumina cement, blast furnace cement, fly ash cement, and blended cement. Examples of fillers include clay, bentonite, talc, and calcium carbonate. 【0057】The fibrous material content can be, for example, 20% by mass or less, preferably 15% by mass or less, and more preferably 10% by mass or less, relative to the total mass of solids in the slurry. The cement content can be, for example, 5 to 30% by mass, relative to the total mass of solids in the slurry. 【0058】 Examples of dehydration molding methods include press dehydration molding, papermaking, roll dehydration molding, and centrifugal molding. The dehydration molding conditions are not particularly limited, and known conditions can be widely adopted in this invention. 【0059】 After molding the slurry, drying treatment can be performed as needed. Known drying methods can be employed, including air drying, heat drying, hot air drying, vacuum drying, freeze drying, vacuum freeze drying, humidity-controlled drying, atmosphere-controlled displacement drying, and supercritical drying. The drying temperature and degree of dryness (moisture content) should be appropriately set according to the composition, purpose, and application of the molded product. 【0060】 The molding process in step B yields the xonotlite-based calcium silicate molded article of the present invention. The content of xonotlite-based calcium silicate in the calcium silicate molded article is, for example, 50% by mass or more, preferably 80% by mass or more, and more preferably 90% by mass or more. 【0061】 Here, the method for producing the calcium silicate molded body described above can be combined with the method for producing rice husk ash of the present invention described above. Specifically, the method for producing rice husk ash of the present invention involves combustion treatment of rice husks, and the energy contained in such a manufacturing method can be used in step A and / or step B. In particular, the combustion energy in the method for producing rice husk ash of the present invention can be used in the method for producing the calcium silicate molded body. 【0062】 For example, the combustion energy used in the rice husk ash production method can be used to generate steam, which can then be used in the hydrothermal treatment performed in step A. Alternatively, the combustion energy used in the rice husk ash production method can be used to generate steam, which can then be used to dry the calcium silicate molded body obtained in the molding process of step B. 【0063】 In other words, the present invention can provide a new manufacturing system for producing a calcium silicate molded body, and more specifically, it can provide a manufacturing method that comprises a step of obtaining rice husk ash by the method for producing rice husk ash of the present invention, and uses the energy generated by burning the rice husks to produce a calcium silicate molded body. 【0064】 In the above manufacturing system (manufacturing method), in addition to using silica obtained from rice husk ash as a raw material for manufacturing molded products, the combustion energy of rice husks can be used in the production process of calcium silicate products, thereby enabling energy conservation and reduction of carbon dioxide emissions. 【0065】 In specifying the inventions contained herein, the components (properties, structures, functions, etc.) described in each embodiment of this disclosure may be combined in any way. That is, this disclosure encompasses all subject matter consisting of any combination of the combinatable components described herein. 【0066】 The present invention will be described more specifically below with reference to examples, but the present invention is not limited to the embodiments of these examples. 【0067】 (Example 1A) Rice husks were prepared as raw material, and a two-stage combustion treatment was carried out using a biomass boiler (see Figure 1) equipped with a first combustion furnace for performing a first combustion treatment and a second combustion furnace for performing a second combustion treatment of the rice husks. Specifically, the rice husks were supplied to the first combustion furnace and subjected to a first combustion treatment at a temperature of 800°C for 34 seconds. After that, the rice husks that had undergone the first combustion treatment were supplied to the second combustion furnace and subjected to a second combustion treatment at a temperature of 700°C for 12.1 hours. Therefore, the total time for the combustion treatment of the rice husks (sum of the time for the first combustion treatment and the time for the second combustion treatment) was approximately 12.1 hours. The rice husk ash obtained in this way was crushed and used as rice husk ash 1a, which was used as a raw material for a calcium silicate molded body. 【0068】(Examples 2B to 2L) As shown in Table 1, rice husk ash was obtained in the same manner as in Example 1A, except that the time for the first combustion treatment of rice husks, the time for the second combustion treatment, and the total time for the combustion treatment were changed. 【0069】 (Comparative Examples 1A to 1D) As shown in Table 1, rice husk ash was obtained in the same manner as in Example 1A, except that the time for the first combustion treatment of rice husks, the time for the second combustion treatment, and the total time for the combustion treatment were changed. 【0070】 (Production Results of Rice Husk Ash) Table 1 shows the production conditions (combustion conditions) for rice husk ash carried out in each example and comparative example, as well as the measurement results of the cristobalite (101) peak intensity (Cri peak intensity), ignition loss (Ig. Loss), and BET specific surface area of ​​the obtained rice husk ash. From Table 1, the rice husk ash obtained in each example had a peak intensity of 10,000 counts or less and an ignition loss of 3% by mass or less. 【0071】 【0072】 (Example 2) Following the formulation shown in Table 2, a slurry was prepared by mixing 100 parts by mass of raw materials (the remainder being additives such as clay) containing 49.4 parts by mass of rice husk ash and 48.2 parts by mass of quicklime obtained in Example 1A with 1600 parts by mass of water. The slurry was then heated in an autoclave at a pressure of 14 kgf / cm². ２ The mixture was subjected to hydrothermal treatment (hydrothermal synthesis reaction) for 5 hours while being stirred under conditions of 200°C. This resulted in a molding slurry (CaO / SiO2). ２ A molar ratio of 1.050 was obtained. To 74.5 parts by mass of this molding slurry (calculated in terms of solid content, i.e., the amount of the raw materials), 2.6 parts by mass of pulp, 18.5 parts by mass of cement, and 4.4 parts by mass of glass fiber were added to obtain a molding slurry, which was then poured into a mold and heated at 150 kgf / cm². ２ Dehydration molding was performed under pressure. By drying the resulting molded body, a xonotlite-based calcium silicate molded body was obtained. 【0073】 (Example 3) A xonotlite-based calcium silicate molded body was obtained in the same manner as in Example 2A, except that the rice husk ash obtained in Example 1A was replaced with the rice husk ash obtained in Example 1L. 【0074】 (Comparative Example 2) A xonotlite-based calcium silicate molded article was obtained in the same manner as in Example 2A, except that the rice husk ash obtained in Example 1A was replaced with the rice husk ash obtained in Comparative Example 1B. 【0075】 (Comparative Example 3) A xonotlite-based calcium silicate molded body was obtained in the same manner as in Example 2A, except that the rice husk ash obtained in Example 1A was replaced with the rice husk ash obtained in Comparative Example 1C. 【0076】 (Comparative Example 4) A xonotlite-based calcium silicate molded article was obtained in the same manner as in Example 2A, except that the rice husk ash obtained in Example 1A was replaced with the rice husk ash obtained in Comparative Example 1D. 【0077】 (Manufacturing results of calcium silicate molded articles) Table 2 shows the results of the bending strength, heat shrinkage rate, and non-flammability tests for the calcium silicate molded articles obtained in each example and comparative example. 【0078】 【0079】 Table 2 demonstrates that calcium silicate molded articles obtained from rice husk ash with a cristobalite (101) peak intensity of 10,000 counts or less and a loss on ignition of 3% by mass or less possess good mechanical strength, excellent heat resistance due to their low thermal shrinkage rate, and also excellent non-flammability. 【0080】 (Evaluation method) The BET specific surface area (m²) of the rice husk ash obtained in each example and comparative example. ２ The peak intensity and ignition loss (Ig. Loss) of cristobalite (101) per g, as well as the flexural strength, thermal shrinkage rate, and non-flammability of the calcium silicate molded article, were evaluated using the following procedure. 【0081】 [BET Specific Surface Area] The BET specific surface area of ​​rice husk ash was measured using the nitrogen adsorption method with samples pretreated by vacuum heating at 150°C for 8 hours under reduced pressure of 40 mm Torr. The measurement device used was the Quantachrome "QUADRASORB SI". 【0082】[Cristobalite (101) Peak Intensity (Cri(101))] The cristobalite peak intensity of rice husk ash was estimated based on the peak intensity (2θ = 21.9° ± 0.5) originating from the cristobalite (101) plane crystals in the diffraction spectrum obtained by powder X-ray diffraction. The measurement device used was Rigaku's powder X-ray diffractometer "MiniFlex 600," and rice husk ash was measured under the following conditions (X-ray source: Cu-Kα rays (wavelength 0.154 nm), tube voltage 35 kV, tube current 10 mA, measurement range 20° to 30°, scan speed 2.0000° / min, step width 0.0200°, slit 0.3 mm), and correction was performed using a standard sample. Standard samples were measured using NISI's SRM-640d under the following conditions: X-ray source: Cu-Kα rays (wavelength 0.154 nm), tube voltage 35 kV, tube current 10 mA, measurement range 20° to 40°, scan speed 2.0000° / min, step width 0.0200°, slit 0.3 mm. The peak intensity (2θ = 28.6° ± 0.5) derived from the silicon (111) plane crystal was estimated, and the measured value of rice husk ash was corrected using the measured value of the standard sample with the following formula: Rice husk ash correction value (counts) = Rice husk ash measured value × Correction coefficient Correction coefficient = Standard material measured value immediately after tube replacement ÷ Standard material measured value for the current month. Note that the standard sample was measured for a peak intensity of 28.4° based on Si(111), and the rice husk ash was measured for a peak intensity of 22.0° based on cristobalite(101). 【0083】 [Loss on Ignition (Ig. Loss)] Approximately 5 g of rice husk ash dried at 150°C was weighed and placed in an electric furnace (Nabertherm "Muffle Furnace LT9 / 11 (Germany)"), and the temperature was raised to 1000°C over 150 minutes, followed by a heat treatment at 1000°C for 30 minutes. After the heat-treated rice husk ash was allowed to cool naturally to room temperature (25°C), it was removed from the electric furnace, and the loss on ignition was calculated using the following formula: Loss on ignition (%) = ((Weight before heating - Weight after heating) / Weight before heating) × 100. 【0084】 [Bending Strength] A 150 mm x 37 mm x 25 mm test piece was cut from the calcium silicate molded body, and a three-point bending test was performed on this test piece to calculate the bending strength of the calcium silicate molded body. 【0085】[Heat-induced linear shrinkage rate] A test piece measuring 150 mm x 37 mm x 25 mm was cut from a calcium silicate molded body. This was placed in an electric furnace and heated to 1000°C over 150 minutes, followed by a heat treatment at 1000°C for 3 hours. After the heat-treated molded body was cooled to room temperature (25°C), it was removed from the electric furnace, and the linear shrinkage rate in the longitudinal direction was calculated using the following formula: Linear shrinkage rate (%) = ((Dimensions before heating - Dimensions after heating) / Dimensions before heating) × 100. This was then defined as the heat-induced linear shrinkage rate of the calcium silicate molded body. 【0086】 [Non-flammability test] A cylindrical test specimen measuring Φ45 mm x 50 mm was cut from a calcium silicate molded body, cured at 60°C for 24 hours, and then allowed to cool in a desiccator for 1 hour. After confirming that the furnace temperature of 750°C was stably maintained for 10 minutes, the test specimen was placed in the furnace and heated for 30 minutes or more until equilibrium was reached. The surface temperature and the temperature difference between the core temperature and the furnace temperature of the test specimen were calculated using the following formulas: Core temperature (°C) = Maximum core temperature of the test specimen - Initial furnace temperature; Surface temperature (°C) = Maximum surface temperature of the test specimen - Initial furnace temperature. If both the core temperature and surface temperature were 30°C or less, it was judged to have excellent non-flammability and marked with "○". If either the core temperature or surface temperature exceeded 30°C, it was judged to have poor non-flammability and marked with "×". 【0087】 B: Biomass boiler 1: First combustion furnace 2: Second combustion furnace 3: Partition wall 3 4: Raw material supply port 5: Ventilation passage 6: Feeder 7: Recovery port A: Air E: Exhaust passage M1: Rice husks M2: Rice husk ash

Claims

1. Rice husk ash having a cristobalite (101) peak intensity of 10,000 counts or less in the diffraction spectrum obtained by powder X-ray diffraction, and a loss on ignition of 3% by mass or less.

2. A method for producing rice husk ash according to claim 1, comprising a step of burning rice husks, wherein the burning time is 10 hours or more.

3. A calcium silicate molded body obtained from rice husk ash as described in claim 1.

4. A method for producing a calcium silicate molded article, comprising the steps of: hydrothermally treating a raw material containing rice husk ash as described in claim 1 to obtain a slurry; and molding the slurry.

5. A method for producing a calcium silicate molded article according to claim 4, comprising the step of obtaining the rice husk ash by the method described in claim 2, wherein the energy generated by burning the rice husks is used for the production of a calcium silicate molded article.

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