Method for producing rice husk activated carbon

Abstract

Provided is a method for producing a rice hull activated carbon at low cost and with a small number of steps. The method for producing a rice hull activated carbon includes: an alkali etching step of performing alkali etching on a carbonized rice hull obtained by carbonizing a rice hull; and an acid adding step of adding an acid to a carbonized rice hull suspension liquid obtained by suspending the carbonized rice hull subjected to the alkali etching in the alkali etching step in a solvent.

Description

[0001] This application is a divisional application of the application filed on July 24, 2018, with application number 201810818961.1 and invention title "Method for manufacturing rice husk activated carbon". Technical Field

[0002] This invention relates to a method for manufacturing rice husk activated carbon. Background Technology

[0003] For a long time, most of the unused parts of plants such as vegetables and grains have been discarded. Therefore, in order to protect and improve the Earth's environment, there is a strong demand for the effective utilization of these unused plant parts. As an example of the effective utilization of unused plant parts, research is underway on using carbon materials produced by carbonizing rice husks as electrochemical devices, drug carriers, adsorbents, etc. (for example, see Patent Document 1).

[0004] Among them, the carbonization treatment of rice husks is being studied in the following way: after carbonizing the rice husks, an alkaline treatment is performed to remove silicon in the microporous structure in order to form a microporous structure (for example, see Patent Document 2).

[0005] However, in the above method, after alkali treatment, it is necessary to perform a running water rinse with tap water or similar water, which requires a considerable amount of water. If large-scale production of rice husk activated carbon is considered, water will account for a large portion of the manufacturing cost. Therefore, a low-cost manufacturing method for rice husk activated carbon with fewer processes and reduced water usage is desired.

[0006] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2008-273816 Patent Document 2: Japanese Patent Application Publication No. 2013-112572 Summary of the Invention

[0007] Technical issues The objective of this invention is to solve the aforementioned problems and achieve the following goal: to provide a low-cost method for manufacturing rice husk activated carbon with fewer processing steps.

[0008] Technical solution The means to solve the aforementioned problem are as follows. That is, <1> A method for manufacturing rice husk activated carbon includes: an alkaline etching step, in which the activated carbon obtained by carbonizing rice husks is alkali-etched; and an acid addition step, in which an acid is added to an activated carbon suspension obtained by suspending the activated carbon that has been alkali-etched in the alkaline etching step in a solvent.

[0009] <2> As described <1> The described method for manufacturing rice husk activated carbon involves adding acid to a carbonized suspension with a pH of 10.0 to 11.0 during the acid addition step.

[0010] <3> As described <1> or <2> In the described method for manufacturing rice husk activated carbon, the pH of the fumigated carbon suspension containing the acid added in the acid addition step is 5.6 to 6.2.

[0011] <4> As described <1> ~ <3> The method for manufacturing rice husk activated carbon as described in any one of the methods further includes, after the acid addition step, an impregnation step in which the activated carbon is impregnated in a solvent of a activated carbon suspension containing the acid for 8 to 20 hours.

[0012] <5> As described <1> ~ <4> The method for manufacturing rice husk activated carbon as described in any one of the methods is an organic acid.

[0013] <6> As described <5> The method for manufacturing rice husk activated carbon described herein, wherein the organic acid is malic acid, acetic acid, or citric acid.

[0014] <7> As described <1> ~ <6> The method for manufacturing rice husk activated carbon as described in any one of the methods further includes, after the acid addition step, a pH adjustment step, to adjust the pH of the activated carbon suspension to 8.0 to 8.6.

[0015] <8> As described <1> ~ <7> The method for manufacturing rice husk activated carbon as described in any one of the methods further includes, after the acid addition step, an extraction step of filtering the charcoal to extract the rice husk activated carbon.

[0016] Technical effect According to the present invention, the aforementioned problems can be solved and the aforementioned objectives can be achieved, providing a low-cost method for manufacturing rice husk activated carbon with fewer steps.

[0017] That is, the present invention can significantly reduce the cost of cleaning water, which accounts for a large portion of the manufacturing cost of rice husk activated carbon, and enable the production of rice husk activated carbon at low cost. Attached Figure Description

[0018] Figure 1 This is a flowchart illustrating the various steps of the method for manufacturing rice husk activated carbon according to the present invention.

[0019] Figure 2 (a) and (b) are graphs showing the relationship between the number of soaking times and pH in Example 1.

[0020] Figure 3 This is a graph showing the relationship between the alkaline etching temperature (°C) in the alkaline etching process and the amount (wt%) of silica components (ash) remaining in the manufactured rice husk charcoal.

[0021] Figure 4 This is a graph showing the relationship between the alkaline etching time (h) in the alkaline etching process and the amount (wt%) of silica content (ash) remaining in the manufactured rice husk charcoal. Detailed Implementation

[0022] (Manufacturing method of rice husk activated carbon) The method for manufacturing rice husk activated carbon of the present invention includes at least an alkaline etching step and an acid addition step, and may include other steps as needed.

[0023] <Alkali Etching Process> The alkaline etching process is a process of alkaline etching the carbonized charcoal obtained by carbonizing rice husks.

[0024] <<Crust>> The husks mentioned are industrial waste discharged during the harvesting of rice, barley, wheat, naked wheat, barnyard millet, millet, etc.

[0025] There are no particular limitations on the type of rice husk used, and it can be selected appropriately depending on the purpose. For example, rice husks themselves, dried rice husks, crushed rice husks, fermented rice husks, baked rice husks, and extracted rice husks can be used. One type of rice husk can be used alone, or two or more can be used together.

[0026] Among these grain husks, those with low silica content are preferred because they have a higher amount of fixed carbon.

[0027] It should be noted that rice husks can be obtained from agricultural cooperatives, wineries, and food companies, for example.

[0028] <<Smoked charcoal>> The charcoal is obtained by carbonizing the rice husks.

[0029] The charcoal is porous, possessing mesopores with a pore size of 2 nm to 50 nm and micropores with a pore size less than 2 nm. Silicon components (e.g., silicon dioxide) are attached to the micropores of the charcoal. Therefore, the number of micropores on the charcoal surface increases by removing the silicon components.

[0030] -Carbonization- The carbonization refers to the heat treatment of the rice husks to transform them into carbonaceous material (e.g., see JIS M0104-1984). It should be noted that the rice husks may also undergo the following pretreatment prior to carbonization.

[0031] --Pretreatment for carbonization-- There are no particular limitations on the pretreatment for the carbonization process, and it can be appropriately selected according to the purpose. Examples include pre-washing the rice husk material, crushing the rice husk to the desired particle size, grading the crushed rice husk, alcohol impregnation of the rice husk, and pre-carbonization treatment of the rice husk. These pretreatments can be used alone or in combination with two or more.

[0032] ---Alcohol Impregnation--- The alcohol impregnation refers to impregnating the rice husks in an alcohol (e.g., methanol, ethanol, or isopropanol) before carbonizing or pre-carbonizing them.

[0033] The alcohol impregnation process reduces the mineral content and / or moisture content of the rice husks, and also prevents off-flavors that may develop during carbonization.

[0034] ---Preparatory carbonization treatment--- The pre-carbonization treatment refers to heating the rice husks at a temperature lower than the carbonization temperature (e.g., below 400°C) before carbonizing them, in a state where oxygen is isolated.

[0035] The aforementioned pre-carbonization treatment allows for the extraction and reduction of tar components generated during carbonization.

[0036] -Carbonized Environment- There are no particular limitations on the carbonization environment used for the carbonization process, and it can be appropriately selected according to the purpose. For example, examples include oxygen-isolated environments (e.g., vacuum environments, inert gas environments such as nitrogen and / or argon, environments that dry-steam the plant-derived material). These environments can be used alone or in combination.

[0037] In these environments, an inert gas environment is preferred because the amount of fixed carbon content will not decrease.

[0038] -Carbonization temperature- There are no particular limitations on the carbonization temperature, which can be appropriately selected according to the purpose, but it is preferably 400°C to 800°C, more preferably 500°C to 700°C, and particularly preferably 600°C to 700°C.

[0039] If the carbonization temperature is less than 400°C, a large amount of tar components remain, and there may be no carbonization at all. If the carbonization temperature exceeds 800°C, the properties may sometimes deteriorate. On the other hand, if the carbonization temperature is within the more preferred or particularly preferred range, it is advantageous that a fine structure can be developed through the volatile components and / or carbon molecules in the porous carbon material.

[0040] - Rate of heating to carbonization temperature - There is no particular limitation on the heating rate to the carbonization temperature, and it can be appropriately selected according to the purpose, but it is preferably 1°C / min or more, more preferably 3°C / min or more, and particularly preferably 5°C / min or more.

[0041] If the heating rate to the carbonization temperature is less than 1°C / minute, the nitrogen consumption will increase due to the longer processing time, which may sometimes result in higher costs.

[0042] On the other hand, if the rate of increase to the carbonization temperature is within the more preferred or particularly preferred range, it is advantageous that the tar will separate while leaving the desired fixed carbon components.

[0043] -Carbonization time- There is no particular limitation on the upper limit of the carbonization time, which can be appropriately selected according to the purpose, but it is preferred to be 10 hours, more preferably 7 hours, and particularly preferably 5 hours.

[0044] There is no particular limitation on the lower limit of the carbonization time, which can be appropriately selected according to the purpose, but a time that can reliably carbonize the rice husks is preferred.

[0045] <<Alkali Etching>> The silicon component is removed from the carbonized carbon by the alkaline etching.

[0046] As for alkaline etching, there are no particular limitations as long as an alkali is used for etching, and it can be appropriately selected according to the purpose. For example, treatments such as immersing the carbonized charcoal in an alkaline aqueous solution, heating the carbonized charcoal in an alkaline aqueous solution, causing the carbonized charcoal to undergo a gas-phase reaction with the alkali, and stirring the carbonized charcoal in an alkaline aqueous solution can be used. These treatments can be used alone or in combination with two or more.

[0047] Of these processes, immersing the carbonized charcoal in an alkaline solution is preferred because it allows for the simple and low-cost removal of silicon components.

[0048] -Immersion in alkaline solution- There are no particular limitations on the method of impregnation in the alkaline solution, and it can be appropriately selected according to the purpose. For example, methods such as impregnating the charcoal in an aqueous solution with a pH of 11.0 or higher for 8 to 20 hours, or impregnating the charcoal in an aqueous solution with a pH of 11.0 or higher and heating it in an oven or the like can be used. These methods can be used alone or in combination.

[0049] Among these methods, the method of immersing charcoal in an aqueous solution with a pH of 11.0 or higher and heating it in an oven or the like is preferred because it can remove silicon components easily and at low cost.

[0050] -Alkali- There are no particular limitations on the base used, and it can be appropriately selected according to the purpose. Examples include sodium hydroxide, potassium hydroxide, and lithium hydroxide. One of these substances can be used alone, or two or more can be used together.

[0051] Among these substances, sodium hydroxide is preferred because of its high efficiency in removing silica and the fact that the material can be obtained at low cost.

[0052] It should be noted that silicon dioxide is believed to react with sodium hydroxide to produce sodium silicate (Na2SiO3), which can be removed from the charcoal.

[0053] <Acid Addition Process> The acid addition process is the process of adding acid to the charcoal suspension, which is obtained by suspending the charcoal that has been alkali-etched in the alkali etching process in a solvent.

[0054] <<pH of the suspension before adding acid>> The pH of the suspension before the addition of acid is not particularly limited and can be appropriately selected according to the purpose, but it is preferably 10.0 to 11.0, more preferably 10.2 to 10.8, and particularly preferably 10.4 to 10.6.

[0055] If the pH of the suspension before adding the acid is less than 10.0, the aqueous solution used for cleaning the charcoal may not be sufficiently reduced. If the pH of the suspension before adding the acid is greater than 11.0, the removal of silicon may become insufficient. On the other hand, if the pH of the suspension is within the more preferred or particularly preferred range, it is advantageous to achieve both the reduction of the aqueous solution used for cleaning and the removal of silicon.

[0056] <<Acid Addition>> The pH of the suspension after adding the acid is not particularly limited and can be appropriately selected according to the purpose, but it is preferably 5.6 to 6.2, more preferably 5.7 to 6.1, and particularly preferably 5.8 to 6.0.

[0057] If the pH is less than 5.6, a large amount of water may be needed to restore it to neutral; if the pH is greater than 6.2, gelation may occur; and sometimes the pH may revert to alkalinity without acid immersion. On the other hand, if the pH is within the more preferred or particularly preferred range, it is advantageous to be able to perform rinsing with a stable volume of water.

[0058] <<Acid>> There are no particular limitations on the type of acid used, and it can be appropriately selected according to the purpose. Examples include organic acids and inorganic acids. One of these acids can be used alone, or two or more can be used together.

[0059] Among these acids, organic acids are preferred due to their high safety profile.

[0060] -Organic acids- There are no particular limitations on the organic acids mentioned, and they can be appropriately selected according to the purpose. Examples include malic acid, acetic acid, and citric acid. These acids can be used alone or in combination of two or more.

[0061] Of these acids, citric acid is preferred due to its high safety and ease of storage.

[0062] <Other processes> Other steps included in the method for manufacturing rice husk activated carbon of the present invention are not particularly limited and can be appropriately selected according to the purpose. Examples include dilution steps, impregnation steps, pH adjustment steps, and extraction steps. These steps can be used individually or in combination.

[0063] <<Dilution Process>> The dilution process is a process that, after the alkaline etching process, dilutes the carbon fumigation suspension with water or the like until the carbon fumigation suspension reaches the desired pH.

[0064] - Desired pH of charcoal suspension - The desired pH of the fumigation suspension is not particularly limited and can be appropriately selected according to the purpose, but it is preferably 10.0 to 11.0, more preferably 10.2 to 10.8, and particularly preferably 10.4 to 10.6.

[0065] If the pH of the suspension is less than 10.0, it may sometimes be insufficient to reduce the amount of water used for cleaning the charcoal. If the pH of the suspension exceeds 11.0, the removal of silicon may sometimes become insufficient. On the other hand, if the pH of the charcoal suspension is within the more preferred or particularly preferred range, it is advantageous to achieve both the reduction of the aqueous solution used for cleaning and the removal of silicon.

[0066] <<Immersion Process>> The impregnation process involves immersing the smoked charcoal in a solvent within a smoked charcoal suspension containing added acid. This impregnation process can be performed after the acid addition process.

[0067] The impregnation time in the impregnation process is not particularly limited and can be appropriately selected according to the purpose. However, it is preferably 8 hours to 20 hours, more preferably 14 hours to 16 hours, and particularly preferably 15 hours.

[0068] If the impregnation time is less than 8 hours, the ash content may sometimes increase. If the impregnation time exceeds 20 hours, the process preparation time may sometimes be lengthened. On the other hand, if the impregnation time is within the more preferred range or the particularly preferred range, it is advantageous that stable characteristics (pore volume, ash content) can be obtained.

[0069] <<pH Adjustment Process>> The pH adjustment process is a process of adjusting the pH of the smoked charcoal suspension to 8.0 to 8.6, and can be carried out after the acid addition process. In this pH adjustment process, if the pH of the smoked charcoal suspension becomes 8.0 to 8.6, it can be transferred to the extraction process described below.

[0070] As the method of the pH adjustment process, there is no particular limitation and it can be appropriately selected according to the purpose. For example, a method of continuously supplying neutral water to the smoked charcoal suspension for dilution, a method of replacing the solvent in the smoked charcoal suspension with neutral water, etc. can be cited. These methods can be used alone or two or more of them can be used in combination.

[0071] Among these methods, the method of continuously supplying neutral water for dilution is preferred because the working hours do not increase.

[0072] <<Extraction Process>> The extraction process is a process of filtering the smoked charcoal to extract rice husk activated carbon, and can be carried out after the acid addition process. The rice husk activated carbon obtained in the extraction process is dried.

[0073] As the method of the extraction process, there is no particular limitation and it can be appropriately selected according to the purpose. For example, a method of extracting in a paste form into a mesh bag using neutral water, a method of extracting only the activated carbon after only discharging neutral water, etc. can be cited. These methods can be used alone or two or more of them can be used in combination.

[0074] Among these methods, the method of extracting in a paste form into a mesh bag using neutral water is preferred because of the operability of the operation.

[0075]

Examples

[0076] (Example 1) In Example 1, rice husks as raw materials were carbonized to transform into smoked charcoal, and then through an alkali etching process, a dilution process, an acid addition process, an impregnation treatment, a pH adjustment process, and an extraction process, rice husk activated carbon was prepared.

[0077] <Carbonization of rice husks> Weighed 400 to 600 grams of crushed rice husks (produced in Kagoshima Prefecture, rice husks of Isehikari), put them into an alumina crucible, and carried out preliminary carbonization by heating at 300 °C for 4 hours in a nitrogen stream (20 L / min). Then, it was heated to 600 °C at a heating rate of 5 °C / min in a nitrogen stream (20 L / min). Then, carbonization was carried out at 600 °C for 6 hours to transform into smoked charcoal, and then cooled to room temperature. Nitrogen was continuously flowed during the carbonization and cooling processes.

[0078] <Alkali etching process> Alkali etching was carried out by impregnating 2.7 kg of the above-mentioned obtained smoked charcoal in 1.68 kg of an 8.25 mass% sodium hydroxide aqueous solution at 90 °C for 15 hours.

[0079] <Dilution process> The solvent (sodium hydroxide aqueous solution) in the smoked charcoal suspension used in the above alkali etching process was separated from the smoked charcoal and discarded, and 15 L of tap water was supplied to the recovered smoked charcoal and shaken for 0.5 hours. Again, the solvent (sodium hydroxide aqueous solution) in the smoked charcoal suspension was separated and discarded to recover the smoked charcoal, and 15 L of tap water was supplied to this smoked charcoal. In this way, the supply of 15 L of tap water was repeated 5 times in total. The pH of the suspension obtained by the 5th supply of tap water became 10.0.

[0080] <Acid addition process> It was confirmed that the pH of the smoked charcoal suspension obtained by the above dilution process was 10.0, and 8 g of 1M citric acid (8 g / kg) was added to each 1 kg of the smoked charcoal. The pH of the smoked charcoal suspension after adding the acid became 5.9.

[0081] <Impregnation process> The above-mentioned acid-added suspension was impregnated at room temperature for 12 hours.

[0082] <pH adjustment process> The solvent in the above-mentioned acid-added smoked charcoal suspension was separated from the smoked charcoal and discarded, and 15 L of tap water was supplied to the recovered smoked charcoal and shaken for 0.5 hours. Again, the solvent was separated and discarded to recover the smoked charcoal, and 15 L of tap water was supplied to this smoked charcoal. In this way, the supply of 15 L of tap water was repeated 12 times in total. The pH of the smoked charcoal suspension obtained by the 12th supply of tap water became 8.2, and the subsequent extraction process was carried out.

[0083] <Extraction Process> The above-mentioned fumigation suspension with a pH of 8.2 was passed through a filter used to filter the fumigation suspension, and the resulting rice husk activated carbon was recovered. The obtained rice husk activated carbon was dried using a hot air circulating dryer to become the sample of Example 1.

[0084] <Evaluation of the Residual Silicic Acid Components> Weigh 1 gram of the rice husk activated carbon obtained by the above method and burn it completely in air at 600°C in an alumina crucible. The residual silica content in the rice husk activated carbon is determined by measuring the weight of the ash residue after combustion. A higher weight of ash residue indicates a greater amount of residual silica in the rice husk activated carbon. The evaluation results are shown in Table 1 below.

[0085] <Evaluation of Tap Water Usage> The amount of tap water used for cleaning the charcoal during the pH adjustment process (per 1 kg of charcoal) is shown in Table 1 below.

[0086] <pH Evaluation of Fumigated Charcoal Suspension> exist Figure 2 The graph shows the number of times the solvent in the fumigation suspension was replaced after the alkaline etching process, plotted on the horizontal axis, and the pH of each suspension after replacement, plotted on the vertical axis. pH measurements were performed according to JIS standard JIS-Z-8802. The pH of each fumigation suspension was measured 2 hours after the solvent for the fumigation suspension was supplied to the fumigation charcoal.

[0087] Table 1

[0088] It can be seen that the manufacturing method of Example 1 is a manufacturing method that can significantly reduce water consumption and effectively reduce costs.

[0089] like Figure 2 As shown, it can be confirmed that by changing the solvent in the activated carbon suspension, the pH after the alkaline etching process is close to neutral. It can be seen that, in Example 1 regarding the method for manufacturing activated carbon using the rice husk of the present invention, the number of solvent changes required in the pH adjustment process until a pH of 8.0 to 8.6 is reduced.

[0090] Furthermore, from Figure 3 and Figure 4 It can be seen that by adjusting the alkaline etching temperature (°C) and alkaline etching time (h) in the alkaline etching process, the amount of residual silica components (ash) in the manufactured rice husk charcoal can be reduced.

[0091] Industrial utilization potential The method for manufacturing rice husk activated carbon of the present invention is applicable to the cost-effective manufacturing of carbon materials derived from rice husks with a specific microporous structure.

[0092] Specifically, according to previous methods, approximately 2.6m³ of material is needed to produce 1kg of rice husk activated carbon. 3 / kg of water, but according to the method for manufacturing rice husk activated carbon according to the present invention, only about 0.14m³ of water is used. 3 It can be manufactured using water at a volume of 1 / kg, or using about 1 / 20th of the volume of water as in previous methods.

Claims

1. A method for manufacturing rice husk activated carbon, characterized in that, include: The alkaline etching process involves alkali etching the charcoal obtained by carbonizing rice husks to remove silicon components from the charcoal. The dilution process involves suspending the alkali-etched charcoal in a solvent to obtain a charcoal suspension, separating the solvent from the charcoal and discarding it. Tap water is then supplied to the recovered charcoal, and the suspension is diluted to a pH range of 10.2 to 10.8 by shaking. In the acid addition step, after the dilution step, acid is added to the fumigation charcoal suspension, and the pH of the fumigation charcoal suspension with added acid is 5.6~6.

2. The impregnation process involves, after the acid addition process, continuously impregnating the charcoal in the charcoal suspension containing the acid for 8 to 20 hours. The pH adjustment step involves adjusting the pH of the fumigation suspension to 8.0–8.6 after the impregnation step; and In the extraction process, the fumigation suspension is filtered to extract rice husk activated carbon.

2. The method for manufacturing rice husk activated carbon according to claim 1, characterized in that, The acid is an organic acid.

3. The method for manufacturing rice husk activated carbon according to claim 2, characterized in that, The organic acid is malic acid, acetic acid, or citric acid.

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