JP2016160154A - Method for producing fine silica powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently producing a fine silica powder that is derived from organic waste and has an average particle diameter of 3 μm or less.SOLUTION: The method for producing a fine silica powder includes: a step of primarily calcinating a silica-containing organic waste to produce a carbide; a step of comminuting the composite carbide made of carbide and silica to make it into a fine carbide powder having an average particle diameter of 3 μm or less; and a step of secondarily calcinating the fine carbide powder and decompose and remove the carbon component to produce a fine silica powder having an average particle diameter of 3 μm or less.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method for efficiently producing fine silica powder used in various applications.

  Fine silica powder (silicon oxide) is used, for example, as an admixture for cement. A silica powder as a general cement admixture is a mineral-derived silica powder. Silica in the cement reacts with calcium hydroxide produced by the hydration reaction (pozzolanic reaction) and promotes densification and hardening of the cement.

  In the case of mineral-derived silica powder, since the reaction rate with calcium hydroxide is slow, it takes a relatively long time for the cement to harden.

  Compared to mineral-derived silica powder, silica powder extracted from plant-based (organic) waste such as rice husks is porous, so it has good reactivity with calcium hydroxide, and the time until the cement hardens. It can be shortened. In particular, mineral silica has a crystal structure, whereas silica derived from organic waste has an amorphous structure and thus has high reaction activity. Therefore, silica powder derived from organic waste having an amorphous porous structure is more suitable for the progress of the pozzolanic reaction. Therefore, in recent years, attempts have been made to use silica powder derived from organic waste as a cement admixture.

  The manufacturing method of the silica powder derived from organic waste is disclosed, for example, in International Publication No. WO2007 / 026680 (Patent Document 1) and International Publication No. WO2008 / 053711 (Patent Document 2).

  The method for producing an amorphous silicon oxide powder described in Patent Document 1 is a step of preparing, as an input material, a wood material, a crop or a plant containing at least one of hexose and pentose and silicon oxide. And hydrolyzing the input raw material with nitric acid, sulfuric acid, phosphoric acid or organic acid to reduce the hexose content to 10 wt% or less and / or the pentose content to 20 wt% or less, And a step of burning the residue obtained by the hydrolysis treatment at 400 ° C. or more and 1200 ° C. or less.

  The method for producing amorphous silica described in Patent Document 2 includes a step of preparing an organic waste containing silicon oxide as a starting material, and a step of immersing the organic waste in a carboxylic acid aqueous solution having a hydroxyl group. And a step of washing the organic waste with water and a step of heating the organic waste in the air.

  If it is the manufacturing method of the silica powder (silicon oxide powder) by the said method, a high purity silica powder with few impurity contents will be obtained, but if it is a use as a cement admixture, so much high purity is not requested | required.

International Publication Number WO2007 / 026680 International Publication Number WO2008 / 053711

  If silica powder derived from organic waste is used as a cement admixture, the time until the cement hardens can be shortened. In some cases, cement may be injected and hardened in a gap such as a bedrock. In that case, if the particle size of the silica powder is large, the fluidity is deteriorated and it is difficult to completely close the gap in the bedrock.

  In order to improve the fluidity of the silica powder, it is necessary to pulverize the silica powder into a fine silica powder. For example, if it is silica fine powder derived from organic waste with an average particle size of 3 μm or less, the specific surface area increases, and in addition to showing better reactivity, the fluidity is also improved. Crevices can be completely closed with cement, and the curing time can be shortened.

  The problem here is that the time for the pulverization treatment to make coarse silica powder of about several millimeters after firing into a fine powder having an average particle diameter of 3 μm or less is very long. When the inventor of this application mechanically pulverized silica powder derived from organic waste (carbon content 0.08%) with a rocking mill, the processing time until the average particle size reaches about 4 μm is about 4 hours. The processing time until the average particle size reached about 3 μm was about 8 hours, and the processing time until the average particle size reached about 1 μm was about 14 hours. Such a long pulverization treatment is an obstacle to practical use.

  An object of the present invention is to provide a method for efficiently producing fine silica powder derived from organic waste and having an average particle size of 3 μm or less in a relatively short time.

  The method for producing fine silica powder according to the present invention includes a step of first firing organic waste containing silica to produce brittle carbide, and a composite carbide composed of the carbide and silica is pulverized to have an average particle size of 3 μm or less. And a step of obtaining a fine silica powder having an average particle size of 3 μm or less by subjecting the fine carbide powder to secondary firing to decompose and remove the carbon component.

  A preferred temperature range for primary firing is 450 ° C to 700 ° C.

  Preferably, in the primary firing, the time for maintaining the organic waste in the temperature range of 450 ° C. to 700 ° C. is in the range of 30 minutes to 50 minutes.

  Preferably, the primary firing is performed in an atmosphere in which inflow of air is blocked.

  The carbon content in the carbide after the primary firing is preferably in the range of 20% to 50% on a mass basis.

  The carbide grinding time is preferably 60 minutes or less.

  A preferable temperature range of the secondary firing is 750 ° C to 900 ° C.

  In the secondary firing, the time for holding the fine carbide powder in the temperature range of 750 ° C. to 900 ° C. is in the range of 20 minutes to 50 minutes.

  The secondary firing is preferably performed in an atmosphere containing air.

  The fine silica powder after the secondary firing has an amorphous and porous structure. Preferably, the carbon content in the fine silica powder after the secondary firing is 0.1% or less. More preferably, the silica purity of the fine silica powder after the secondary firing is 95% or more.

  Organic waste is from the group consisting of rice husk, rice straw, rice bran, straw, wood, sawdust, bark, bagasse, corn, sugarcane, sweet potato, soybean, peanut, cassava, eucalyptus, fern, pineapple, bamboo, rubber, and waste paper Is one of the chosen.

  According to the production method of the present invention, it is possible to efficiently produce fine silica powder derived from organic waste and having an average particle size of 3 μm or less.

It is a figure which shows the relationship between the grinding | pulverization processing time when the silica powder obtained from rice husk combustion ash is grind | pulverized, and the average particle diameter of the powder after a grinding | pulverization process. It is a figure which shows the relationship between the grinding | pulverization processing time when a rice husk carbide | carbonized_material is grind | pulverized, and the average particle diameter of the powder after a grinding | pulverization process. It is a figure which shows the relationship between the carbon content of a rice husk carbide | carbonized_material, and the average particle diameter of the powder after a grinding process.

  Embodiments of the present invention will be described based on the results of various experiments conducted by the present inventors. The organic waste used as the starting material is rice husk, but organic waste containing silica can be used as the starting material as long as it is silica-containing organic waste. For example, rice straw, rice bran, straw, wood, sawdust, bark, bagasse, corn, sugar cane, sweet potato, soybean, peanut, cassava, eucalyptus, fern, pineapple, bamboo, rubber, waste paper, and the like may be used as starting materials.

[Crushing of rice husk combustion ash (comparative example)]
Rice husk was prepared as an input raw material. The rice husk was subjected to an acid washing treatment with a citric acid solution (50 ° C., 1% concentration), and then washed with water and taken out. The rice husk after the water washing treatment was baked at 800 ° C. for 30 minutes in an electric furnace in an atmosphere containing air to obtain silica powder.

  The silica purity of the silica powder was 99.52%, and the carbon content was 0.08% by mass.

  The silica powder was pulverized with a rocking mill (using an alumina ball having a diameter of 2 mm), and the relationship between the pulverization time and the average particle size of the pulverized powder was examined. The results are shown in Table 1 and FIG.

  As is apparent from Table 1 and FIG. 1, the pulverization time until the silica powder obtained from the rice husk ash after baking at a temperature of 800 ° C. is made into a fine powder is very long. Specifically, in order to obtain a fine silica powder having an average particle diameter of 3 μm, which increases the utility value as a cement admixture, a pulverization treatment of about 8 hours is required. Such a long pulverization process is a major obstacle to practical use.

[Crushing of rice husk carbide (Example of the present invention)]
Rice husk was prepared as an input raw material. The rice husk was blocked from flowing air in an electric furnace and held at 600 ° C. for 30 minutes to obtain rice husk carbide (composite carbide comprising carbide and silica).

  The carbon content of the rice husk carbide was 39.2% by mass.

  The above rice husk carbide was pulverized with a rocking mill (using alumina balls having a diameter of 2 mm), and the relationship between the pulverization time and the average particle size of the pulverized powder was examined. The results are shown in Table 2 and FIG.

  As is apparent from Table 2 and FIG. 2, the time for pulverizing the rice husk carbide to fine powder is much shorter than that for the rice husk combustion ash after baking at 800 ° C. Specifically, it is only necessary to perform a grinding process for about 30 minutes in order to obtain a fine powder having an average particle diameter of 3 μm which increases the utility value as a cement admixture. Such a short pulverization process is a great advantage for practical use.

[Relationship between carbon content of rice husk carbide and grinding treatment]
The effect of carbon content of rice husk carbide on the grinding process was investigated. Specifically, nine types of samples with different carbon contents of rice husk carbide were prepared, and the average particle size of the fine powder obtained after pulverizing for 1 hour under the same conditions was examined. . The results are shown in Table 3 and FIG.

  As is apparent from Table 3 and FIG. 3, in order to make the average particle size 3 μm or less in a pulverization treatment time of 1 hour, the carbon content needs to be 20% or more on a mass basis. The upper limit of the carbon content is preferably about 50%.

  The reason why finer powder can be obtained by pulverizing rice husk carbide (composite carbide consisting of carbide and silica) with a higher carbon content than pulverizing rice husk combustion ash with a lower carbon content is as follows. To consider.

  Rice husk contains 70 to 80% of carbohydrates (polysaccharides) as cellulose, hemicellulose, and lignin, which are the main components of cell walls and fibers. In the polysaccharide state, they are elastic and easily deformable. That is, since these carbohydrates have ductility, they have a structure that is difficult to grind, which is an obstacle to fine grinding of the silica component.

  When rice husk or rice straw is heated to a temperature of 400 to 500 ° C. or higher, the molecular structure of polysaccharides such as cellulose and hemicellulose is broken, and hydrogen and oxygen are removed to generate carbides. Such a carbide is brittle and is encapsulated in silica inside a rice husk or the like, and silica and carbide are alternately present in a layered structure. Therefore, brittle carbides are pulverized during the pulverization process, and at the same time silica is pulverized.

  For the above reasons, it is considered that the pulverizability improves as the carbon content increases. In other words, it is considered that as the carbon content is increased, miniaturization can be realized by a short pulverization process.

[Conditions for primary firing to obtain preferable carbon content]
In order to shorten the pulverization time required for pulverizing organic waste carbides such as rice husks into a fine powder with an average particle size of 3 μm or less (for example, within 1 hour), the carbon content of the carbides should be 20% or more. Is preferable. Therefore, the inventor of the present application examined the conditions for the primary firing treatment for setting the carbon content of the carbide to 20% or more. The conditions are the holding temperature and holding time of primary firing in a state where the inflow of air is blocked. The results are shown in Table 4.

  What can be understood from the results in Table 4 is that the carbon content is affected by temperature and holding time. Apparently, if the holding time is 30 minutes or more and less than 60 minutes at a temperature of 300 ° C. or more, the total carbon content is 20% or more.

  However, in the sample subjected to primary firing at a temperature of 300 ° C. for 30 minutes, about 5% of the whole was not sufficiently carbonized and remained in a rice husk shape. This is because the exothermic peak of hemicellulose is 332 ° C. and the exothermic peak of cellulose is 438 ° C.

  Considering the above points, the preferred primary firing temperature is 450 ° C. or higher.

  If the primary firing temperature for rice husks is set to a high temperature of 700 ° C. or higher, ashing of the carbide proceeds and the carbon content may decrease. However, even at a temperature of 700 ° C., if the holding time is 30 minutes, the carbon content of 22% can be maintained. On the other hand, when the holding time was 60 minutes at a temperature of 600 ° C., the carbon content was 7.5%.

  From the above results, it is recognized that the preferred primary firing temperature is in the range of 450 ° C. to 700 ° C., and the preferred holding time is in the range of 30 minutes to 50 minutes.

  Sample 1 (300 ° C. × 30 minutes), Sample 2 (450 ° C. × 30 minutes), Sample 4 (600 ° C. × 45 minutes) with different primary firing conditions was subjected to a pulverization treatment for 1 hour, and the average particle size was measured. The results are shown in Table 5.

  As is clear from Table 5, with respect to Sample 2 and Sample 4, the average particle diameters of the fine powder after pulverization were 1.07 μm and 0.90 μm, respectively, and sufficient fine powder formation could be realized. On the other hand, with respect to Sample 1, since the primary calcination temperature was as low as 300 ° C., rice husks that were not completely carbonized remained, the average particle size of the powder after pulverization was 415.88 μm.

  Sample 1, sample 2 and sample 4 after the pulverization treatment are subjected to secondary firing in an oven containing air at a temperature of 800 ° C. for 30 minutes, and the carbon content after the secondary firing is measured. did. The results are shown in Table 6.

  As apparent from Table 6, with respect to Sample 2 and Sample 4, the carbon content after the secondary firing is 0.042% or less, and in the secondary firing process, the carbon component after the primary firing is in the air. It was found that the carbon content can be greatly reduced by burning and decomposing with oxygen. With respect to Sample 1 including those remaining as chaff, the carbon content after secondary firing was 0.084%, which was higher than Sample 2 and Sample 4.

  From the above results, if the rice husk is made into carbide, pulverized in the form of the carbide to a fine powder having an average particle size of 3 μm or less, then subjected to secondary firing at a temperature of about 800 ° C. in an atmosphere containing air. It was recognized that the silica purity can be improved by greatly reducing the carbon content.

[Preferred secondary firing conditions]
After crushing rice husk carbide to an average particle size of 3 μm or less, secondary calcination is performed to reduce the carbon content and increase the silica purity. This inventor performed the secondary baking process on the following conditions.

  After pulverizing the carbide after the primary firing, the fine carbide powder after pulverization was put into an electric furnace, and the furnace temperature was raised to 500 ° C. while supplying air into the furnace. When the furnace temperature reached 500 ° C, the inflow of air was stopped, the temperature was raised to 800 ° C, and the temperature was maintained at 800 ° C for 30 minutes.

  As described in Table 6, the carbon content of Sample 2 after the secondary firing was 0.042%, and the carbon content of Sample 4 was 0.042%.

  Regarding the secondary firing temperature, when the holding temperature is 750 ° C., it is difficult to make the carbon content 0.1% or less, which is inconvenient for obtaining high-purity silica. When holding temperature exceeds 900 degreeC, there exists a possibility that a silica may crystallize and we are anxious about the bad influence on a human body.

  Therefore, a preferable secondary firing temperature is in the range of 750 ° C. to 900 ° C., and the holding time is in the range of 20 minutes to 50 minutes.

[Cleaning of rice husk before primary firing]
This inventor investigated the influence of the presence or absence of the washing process with respect to the rice husk before primary baking. The results are shown in Table 7.

  In Table 7, the details of the samples and conditions are as follows.

  “Untreated”: a sample obtained by subjecting rice husks to primary firing without washing.

  “20 ° C. normal temperature water”: A sample of rice husk washed with normal temperature water at 20 ° C.

  “50 ° C. hot water”: a sample obtained by washing rice husk with hot water of 50 ° C.

  “50 ° C. citric acid 0.5%”: A sample obtained by subjecting rice husks to an acid washing treatment with citric acid at a concentration of 0.5% and 50 ° C.

  “50 ° C. citric acid 1%”: a sample obtained by subjecting rice husks to acid cleaning treatment with citric acid at a concentration of 1.0% and 50 ° C.

“Primary firing” condition: 600 ° C. × 30 minutes “Crushing treatment” time: 60 minutes “Secondary firing” condition: 800 ° C. × 30 minutes As is clear from the results in Table 7, cleaning treatment before primary firing Regardless of the presence or absence, the average particle diameter of the fine powder after the pulverization treatment is 1 μm or less, and the carbon content after the secondary firing is 0.06% or less. Regarding the silica purity, the untreated one was the lowest and the acid washed one was the highest. Therefore, when obtaining a high-purity fine silica powder having an average particle size of 3 μm or less and 99% or more, it is preferable to perform an acid cleaning treatment before the primary firing. When fine silica powder is used as a cement admixture, it is not necessarily inconvenient even if the silica purity is about 95%. Therefore, it is not always necessary to perform the washing treatment before the primary firing.

  Table 8 shows the elemental analysis results after secondary firing of each sample described in Table 7.

  For each sample in Table 8, the rice husks are primarily fired to form carbides, and the carbides are pulverized to an average particle size of 3 μm or less, and then the finely divided carbide powders are secondarily fired to obtain fine silica powders. It is a thing. As a comparative example, Table 9 shows the silica purity and carbon content of silica powder obtained by a conventional method (primary firing followed by secondary firing) in which the carbide is not pulverized.

  With respect to the silica purity and carbon content of the finally obtained silica powder, there is no particular change depending on the presence or absence of carbide pulverization in the middle stage. The greatest merit of grinding carbide in the middle is that a fine powder having an average particle size of 3 μm or less can be obtained in a short time. If the average particle size is reduced to 3 μm or less by the carbide pulverization treatment in the middle stage, the average particle size of the final silica powder obtained after the secondary firing also becomes 3 μm or less. The fine silica powder after the secondary firing has an amorphous and porous structure. The preferable carbon content of the fine silica powder is 0.1% or less on a mass basis. Moreover, the purity of preferable fine silica powder is 95% or more.

  In this experiment, rice husk was chosen as the starting material as a representative example of organic waste, but similar results would be obtained using other organic waste containing silica as the starting material.

  The present invention can be advantageously used as a method for efficiently producing fine silica powder.

Claims (13)

A step of primarily firing organic waste containing silica to generate carbides;
Crushing a composite carbide composed of the carbide and silica to obtain a fine carbide powder having an average particle size of 3 μm or less;
A method of producing a fine silica powder, comprising: subjecting the fine carbide powder to secondary firing to decompose and remove a carbon component to obtain a fine silica powder having an average particle size of 3 μm or less.   The method for producing fine silica powder according to claim 1, wherein the temperature range of the primary firing is 450C to 700C.   3. The method for producing fine silica powder according to claim 2, wherein in the primary firing, the time for maintaining the organic waste in a temperature range of 450 ° C. to 700 ° C. is in a range of 30 minutes to 50 minutes.   The said primary baking is a manufacturing method of the fine silica powder in any one of Claims 1-3 performed in the atmosphere which interrupted | blocked inflow of air.   The method for producing fine silica powder according to any one of claims 1 to 4, wherein a carbon content in the carbide after the primary firing is in a range of 20% to 50% on a mass basis.   The method for producing fine silica powder according to any one of claims 1 to 5, wherein a pulverization time of the carbide is 60 minutes or less.   The manufacturing method of the fine silica powder in any one of Claims 1-6 whose temperature range of the said secondary baking is 750 to 900 degreeC.   The method for producing fine silica powder according to claim 6, wherein in the secondary firing, the time for holding the fine carbide powder in a temperature range of 750C to 900C is in a range of 20 minutes to 50 minutes.   The said secondary baking is a manufacturing method of the fine silica powder in any one of Claims 1-8 performed in the atmosphere containing air.   The method for producing a fine silica powder according to any one of claims 1 to 9, wherein the fine silica powder after the secondary firing has an amorphous porous structure.   The carbon content in the fine silica powder after the said secondary baking is a manufacturing method of the fine silica powder in any one of Claims 1-10 which is 0.1% or less on a mass reference | standard.   The manufacturing method of the fine silica powder in any one of Claims 1-11 whose silica purity of the fine silica powder after the said secondary baking is 95% or more.   The organic waste is a group consisting of rice husk, rice straw, rice bran, straw, wood, sawdust, bark, bagasse, corn, sugarcane, sweet potato, soybean, peanut, cassava, eucalyptus, fern, pineapple, bamboo, rubber, waste paper The manufacturing method of the fine silica powder in any one of Claims 1-12 which is either selected from.

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