JP2005281116A - Method for producing activated carbon using microwave - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing activated carbon having practically sufficient properties by a preliminary treatment of a wet woody waste material using microwaves and a one-step heat treatment. <P>SOLUTION: After the moisture content of the woody material such as wood, bamboo, and bark is controlled, the woody material is exposed to microwave radiation for a required time. Then the microwave radiation is stopped and the woody material is carbonized by an external heating in a nonoxidizing atmosphere. The moisture content of the woody material is controlled to 50-500 wt.% vs. that of the woody material in dry condition and the microwave radiation time is 3-10 minutes after the moisture begins boiling. Before the microwave radiation, the woody material is boiled in a large volume of water under normal or reduced pressure to make water penetrate into the inside of the woody material. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing activated carbon from a wood-based material, and in particular, activated carbon having good adsorption performance in a relatively simple process without providing an activation process from a wood-based waste material such as wood waste, bamboo waste, and bark. It relates to a method of manufacturing.

  In order to produce activated carbon, it is necessary to carbonize plant-based raw materials such as coconut shells and mineral-based raw materials such as lignite, and then increase pores in the activation step. The activation method is roughly classified into a gas activation method and a chemical activation method. In Japan, the gas activation method is mainly used. This is because the raw material after carbonization is heated to a predetermined temperature, and an oxidizing gas such as water vapor, oxygen, carbon dioxide gas and carbon are reacted to increase the amount of pores and the specific surface area that conduct to the outside. is there.

  Since activation by oxygen is locally overheated and it is difficult to control a uniform reaction, water vapor or carbon dioxide is mainly used as a reaction gas, but these do not react with carbon unless the temperature is higher than 800 to 900 ° C. . Further, since the reaction conditions are significantly different between the carbonization step and the activation step, they are usually processed in separate reaction vessels. Therefore, in the conventional activated carbon manufacturing method, there is a problem that energy cost and equipment cost increase, and it is difficult to reduce the manufacturing cost of activated carbon.

  On the other hand, in the production of activated carbon, particularly in the activation step, it is necessary to supply a large amount of heat to the reaction site, and it is usually heated from the outside by direct heating or indirect heating. However, with external heating, it is necessary to use a rotary furnace or to stir the carbonaceous material because of the demand for heat transfer enhancement, so there are several attempts to heat activated carbon from the inside by induction heating or microwave heating. Or has been proposed. For example, various methods using microwave heating have been proposed for heating and regeneration of used activated carbon (Patent Documents 1 and 2 below).

  Further, Patent Document 3 below describes that “in a state where an activated carbon raw material made of molded organic waste is placed in a heating furnace and activated gas is supplied, a microwave or a high frequency is applied to the activated carbon raw material, and this is internally heated. Thus, an activated carbon production method that performs carbonization and activation ”is disclosed. Further, in Patent Document 4 below, “carbon black is fed into a microwave chamber while preheating on a screw conveyor, heated to 200 ° C. or higher by microwaves, and then charged into a fluidized steam activation furnace. In other words, a method for producing activated carbon that activates water vapor at a high temperature of 700 to 900 ° C. is disclosed.

Japanese Patent Laid-Open No. 50-152994 JP 2001-89120 A JP 2000-34114 A JP 2002-308613 A

  From the viewpoint of effective use of wood resources, recycling of woody waste materials such as wood waste, bamboo waste and bark has become an issue. The present inventors have conducted various studies on the technology for recycling such waste raw materials into valuable materials, and as part of this, focused on the production of activated carbon from wood-based raw materials. Activated carbon has various uses, and there is a wide range of required adsorption performance. In the case of activated carbon using woody waste as a raw material, it seems to be a problem how to manufacture a product having a certain level of adsorption performance at a low cost.

Thus, in order to produce activated carbon at a low cost, first of all, it is considered to be an effective means to omit the activation step. However, for that purpose, the adsorption performance required as activated carbon must be ensured at the end of carbonization.
In addition, waste materials such as wood chips and bamboo scraps are often stored in a piled state, and in most cases contain a large amount of moisture. If such wet raw materials are once dehydrated and dried and then subjected to treatments such as carbonization and activation, the process becomes long and the workability is poor, so that it is difficult to process at low cost. Therefore, it is desirable that the raw material in a wet state is processed as it is, the processing container is not changed as much as possible, and the target product is obtained in one heating step.

The inventors of the present invention have focused on the fact that by irradiating a wet woody material with microwaves, the hydroxyl groups contained in moisture and the like can resonate, and the temperature can be instantaneously increased. As a result, it is considered that the amount of pores in the tissue can be greatly increased because moisture, crystallization water and reaction products soaked into the woody tissue are rapidly gasified. Therefore, there is a possibility that a practically necessary pore volume and specific surface area can be secured without providing an activation step.
Therefore, the present invention uses an internal heating by microwaves to pre-treat wet wood waste raw material, and to produce activated carbon having practically sufficient performance by one-stage heat treatment mainly for carbonization. It is an object to provide a means to obtain.

  The inventors of the present invention conducted various studies on the production of activated carbon from water-containing wood raw material, and carbonized the raw water-containing raw material without irradiating it with a microwave by irradiating it with microwaves for a predetermined time. It was found that the specific surface area of the product after carbonization can be greatly increased. Thereby, even if the activation process after carbonization was omitted, it was found that practically sufficient activated carbon could be obtained.

The method for producing activated carbon of the present invention based on this finding is as follows:
A method for producing activated carbon from a wood raw material mainly comprising at least one of wood, bamboo, and bark, the step of adjusting the water content of the wood raw material adjusted in particle size as necessary, and this A step of irradiating a wood raw material having a controlled water content with microwaves for a predetermined time, and then a step of carbonizing the wooden raw material by external heating in a non-oxidizing atmosphere after stopping the microwave irradiation. It is a feature.

The present invention is intended to increase the amount of internal pores by irradiating a water-containing wood raw material with microwaves, thereby oscillating a hydroxyl group and the like and rapidly heating it to promote the generation of water vapor from the inside of the wood tissue. The point that microwave irradiation is not merely a heating means but mainly used as a means for increasing the amount of internal pores is essentially different from a conventional method for producing or activating activated carbon using microwaves.
Actually, as shown in the examples described later, the specific surface area of the activated carbon after carbonization varies greatly depending on the presence or absence of microwave irradiation. The mechanism by which the specific surface area is increased by microwave irradiation is not clear enough, but the rapid gas generation inside the woody tissue makes the tissue loose and the pores remain after the gas is released. It is guessed that.

  In the method for producing activated carbon of the present invention, in order to efficiently generate such pores, it is important to adjust the water content of the woody raw material during microwave irradiation to an appropriate range. In the preceding water content adjustment step, the water content in the water-containing raw material is preferably set in the range of 50 to 500% in terms of the ratio to the weight of the dry woody raw material.

In the present invention, in the carbonization step, microwave irradiation is stopped, and carbonization is performed by external heating in a non-oxidizing atmosphere. This is because microwave irradiation has a high energy cost, and even if microwave irradiation is performed in the carbonization step, there is almost no effect of increasing the specific surface area. In the present invention, the non-oxidizing atmosphere is an atmosphere containing no oxidizing gas (including a vacuum atmosphere) or a combustion exhaust gas containing H ₂ O, CO _2, etc. An atmosphere in which the reaction between the oxidizing component and carbon does not occur substantially.

  In the present invention, in the microwave irradiation step, it is preferable to continuously perform irradiation for 3 to 10 minutes after the microwave irradiation is started and the contained water starts to boil. Note that the determination of the boiling start time is not necessarily performed visually, and some measuring means may be used. Alternatively, the time until the start of boiling may be grasped empirically (in relation to the water content), and microwave irradiation may be performed continuously for 3 to 10 minutes.

  In the present invention, in the water content adjustment step (or prior to this), the wood raw material is boiled in a large amount of water under normal pressure or reduced pressure to infiltrate the water into the raw material, Adjustments may be made. When woody materials such as wood chips are in a dry state, it is often not easy to sufficiently penetrate moisture into the tissue. In such a case, it is preferable to boil the woody material in advance in a large amount of water under normal pressure or reduced pressure. Thereby, moisture permeates well into the raw material, and the effect of increasing the specific surface area by microwave irradiation can be further increased. In addition, since the operation | work is easier when there is much quantity of water at the time of boiling, what is necessary is just to adjust a water content to the said range after boiling.

  Furthermore, in the present invention, microwave irradiation is performed in an atmospheric atmosphere in a reduced pressure state of 400 to 710 Torr (standard atmospheric pressure− (50 to 390) Torr) in absolute pressure or an inert gas atmosphere in the same reduced pressure state. May be. Thereby, the production | generation amount and production | generation speed | rate of an internal pore at the time of microwave irradiation become larger, and the specific surface area of the activated carbon after carbonization can be enlarged further.

  The present invention makes it possible to produce activated carbon having practically sufficient adsorption performance without performing activation treatment by carbonizing a wood raw material after pretreatment with a microwave. Moreover, in this invention, it can process in a water-containing state, without drying the wooden raw material of a wet state. As a result, the manufacturing process of the activated carbon is simplified and the energy cost is reduced, so that the activated carbon can be manufactured at a low cost. Therefore, valuable materials can be produced at low cost from woody wastes such as wood waste, bamboo waste, and bark, and the present invention is also significant from the viewpoint of recycling wood resources.

  The activated carbon production method of the present invention includes a step of adjusting the moisture content of the wood raw material (moisture content adjusting step), a step of irradiating the water-containing raw material with microwaves for a predetermined time (microwave irradiation step), and then a micro step. It comprises a step (carbonization step) in which the irradiation of the wave is stopped and the wood raw material is carbonized by external heating in a non-oxidizing atmosphere. Moreover, you may provide the particle size adjustment process which cut | disconnects and crushes an excessively large raw material, or size-regulates and granulates a powdery raw material before a moisture content adjustment process as needed. Hereinafter, preferred embodiments of each of these steps will be described.

  As the raw material for the activated carbon, one mainly composed of one or more of wood, bamboo, and bark is used. In general, cutting scraps, canna scraps, saw scraps, peeled bark, etc. generated from the lumbering process of wood or bamboo may be used as the main raw material. However, it is not necessary to limit to such wooden waste, and raw wood or raw bamboo before lumbering, or wood or bamboo after lumbering may be used as raw materials. The reason for defining “a raw material mainly composed of a wood material” is that the above wood material should be contained in a certain ratio or more, for example, about 70% or more. The remaining raw material may be any organic material, but in general, it is preferably a vegetable fiber raw material. Particularly, a part of plant fiber waste such as rice husk, rice scum, and waste paper may be mixed and used as the raw material.

  There is no special limitation on the size of the wood raw material, but if the size is too large, it will be difficult to fill the reaction container or the effect of microwave irradiation will be uneven. Is preferred. If it is cutting waste of wood or bamboo, canna waste, saw dust, etc., it can be used almost as it is. In addition, the raw material size may be adjusted in advance in consideration of the target size of the product activated carbon, but in general, the particle size of the product activated carbon may be adjusted after carbonization.

  In the water content adjustment step, the water content W in the water-containing raw material is a predetermined target value within a range of 50 to 500% in terms of a ratio W / M (hereinafter referred to as water content) to the weight M of the dry woody raw material. Adjust to. According to the knowledge of the present inventors, when the moisture content at the time of microwave irradiation is less than 50%, the effect of generating pores inside the wood tissue, that is, the effect of increasing the specific surface area of the product activated carbon is insufficient. . On the other hand, even if the moisture content exceeds 500%, the internal pore generation effect does not increase any more, and conversely, the microwave energy is consumed for the evaporation of excess moisture, which is not preferable.

  It should be noted that the adjustment of the moisture content does not have to be exactly the same as the target value, and may be within a range of about ± 20%. Measure or estimate the moisture content of the crushed raw material by some method (for example, dry weight measurement, specific gravity measurement of slurry raw material, etc.), add water as needed, add dry raw material, dehydrate That's fine. Dehydration can be easily performed with a sieve or filter paper.

  Further, in the present invention, it is extremely important for a sufficient amount of moisture to penetrate into the wood tissue during microwave irradiation in order to exert the effect of microwave irradiation. In general, it is often not easy to hydrate dry wood, especially sawdust. It has been observed that dried sawdust and the like have strong water repellency, and even when immersed in water for a long time, water does not penetrate so much into the wood tissue.

  Therefore, it is preferable to boil the raw wood material in advance so that water is sufficiently permeated into the wood tissue. The boiling time is usually about 2 to 10 minutes, although it depends on the type of raw material. Usually, it is easier to perform the boiling process in a large amount of water, so the water content of the wooden material may be adjusted after the boiling process. Needless to say, the boiling treatment may be performed by boiling water under microwave irradiation. Furthermore, this boiling treatment may be performed under normal pressure, but if this is performed under reduced pressure, gas will be easily released from the inside of the wood tissue, and water penetration into the inside of the tissue will be made more reliable. Can do. The degree of pressure reduction need not be strictly limited, but may be, for example, about 400 to 710 Torr in absolute pressure.

  In the microwave irradiation step, it is preferable that the irradiation is continuously performed for 3 to 10 minutes after the microwave irradiation is started and the contained water starts to boil. The effect of evaporating water inside the woody tissue and generating internal pores depends on the irradiation time after the water reaches a temperature at which it begins to boil. According to the knowledge of the present inventors, if the irradiation time after the start of boiling is less than 3 minutes, the effect of increasing the product specific surface area is not sufficient. On the other hand, even if this time is 10 minutes or more, the effect of increasing the specific surface area is not increased any more, and the microwave energy becomes excessive, which is not preferable.

  In the present invention, it is not necessary to particularly limit the method, structure, etc. of the apparatus for performing the microwave irradiation. For example, microwave irradiation may be performed for a predetermined time in a batch type processing container. During conveyance, microwave irradiation may be performed in a predetermined zone to continuously process. The microwave generator and the structure of the waveguide for guiding it to the processing chamber are not particularly limited. As the frequency of the microwave, a 2.45 GHz microwave that is commonly used in a microwave oven can be suitably used.

  The carbonization step is a step in which the wood raw material after microwave irradiation is heated in a non-oxidizing atmosphere to perform dry distillation and carbonization. At this time, the microwave irradiation is stopped, and the wood raw material is heated from the outside. The present invention is characterized in that activated carbon having the same adsorption performance as that obtained by the activation treatment can be produced by only one heating process mainly for the carbonization.

  The temperature pattern in the carbonization step is not particularly limited, but there are many cases where a temperature raising process for raising the temperature at a predetermined temperature raising rate and a temperature holding process for holding for a predetermined time within a predetermined temperature range are provided. The maximum heating temperature is about 500 to 800 ° C., and carbonization is sufficient. Activated carbon can be produced at a lower heating temperature than when an activation step is provided. The type of the reaction vessel used in this step is not particularly limited, but usually a batch type reactor in which the heating pattern is easily controlled is used. In order to make the temperature in the reactor uniform, a rotating container or a stirring blade may be used. Carbonization may be performed in the same container as that for microwave irradiation, or may be transferred to another container for carbonization.

The heating method in the carbonization process may be an external heating type or an internal heating type. Internal heating type heating is generally performed by circulating combustion exhaust gas. If combustion exhaust gas containing excess air is circulated in a relatively high temperature range of the carbonization step, the condition of a non-oxidizing atmosphere cannot be maintained, so it is necessary to distribute combustion exhaust gas not containing excess air. Note that H ₂ O and CO ₂ of the combustion exhaust gas hardly react with C in the temperature range (800 ° C. or less) of the carbonization process in the present invention, and therefore may be contained in the circulating gas.

In the carbonization step, it is considered that H ₂ O, CO ₂ in the atmosphere hardly reacts with carbon of the wood raw material. However, there is a sufficient possibility that oxygen in the raw material reacts with carbon and hydrogen to produce H ₂ O, CO ₂ . That is, cellulose, which is the main component of the woody material, is represented by the chemical formula (C ₆ H ₁₀ O ₅ ) _n and contains 5/6 O of C by atomic ratio. This cellulose is not completely decomposed to a considerably high temperature range, and undecomposed O escapes into the outside air as H ₂ O or CO in the latter stage of the carbonization process, and at that time, there are a large number of pores leading to the outside. The possibility that the same effect as activation is given is also considered.

In any case, the activated carbon produced by the above method has a specific surface area measured by the BET method of 500 m ² / g or more, as shown in the Examples below. Therefore, this activated carbon is suitable for uses such as deodorization, decolorization, and humidity control.

Three types of charcoal (sawdust), bamboo scrap, and bark chips are carbonized independently as raw materials for activated carbon, with and without microwave irradiation as a pretreatment for carbonization (Example) and after (Carbide) The specific surface area and microstructure of the products were investigated. Each of the above raw materials was prepared as a powder having a size of 1 mm or less, and 10 g each of the examples and comparative examples were used as test materials.
In the examples (with microwave irradiation), the raw material was boiled in water at normal pressure of 100 to 200 CC. The boiling time was 10 minutes in all cases. In the microwave irradiation, the moisture content was adjusted to about 250%, and a microwave of 2.45 GHz was irradiated for 10 minutes (about 7 to 8 minutes after the start of boiling of water) in a normal household microwave oven.

In the carbonization step, a crucible filled with a test material was placed in a quartz tube in an annular electric furnace, and heated while circulating nitrogen. The heating pattern was heated to 800 ° C. in about 80 minutes and held at this temperature for 60 minutes.
On the other hand, the comparative example directly carbonized the test material without providing the above boiling step and microwave irradiation step. The conditions for the carbonization step in the comparative example are the same as in the examples. With respect to the test materials carbonized in Examples and Comparative Examples, the specific surface area was measured and the microstructure was examined. The specific surface area was measured by the BET method, and the microstructure was compared with scanning electron micrographs.

Table 1 shows the measurement results of the specific surface area. As can be seen from the table, the specific surface area value is approximately 300 m ² / g or less in the comparative example, whereas all values are 500 m ² / g or more in the examples. The increase ratio of specific surface area is 8.6 times for wood waste, 6.3 times for bamboo waste, and 2.3 times for bark. Microwave irradiation can significantly increase the specific surface area after carbonization. It was confirmed.

  FIG. 1 shows a comparison of microstructures after carbonization with bamboo waste, and FIG. 2 shows a comparison of microstructures after carbonization with bark. 1 and 2, (a) is a photograph of a test material of a comparative example (carbonization without microwave irradiation) and (b) is an example (carbonization after microwave irradiation). Note that (a) and (b) in FIG. 1 have the same magnification, but in FIG. 2, (b) has a magnification twice that of (a). As can be seen in these micrographs, the large pores are clearly increased by the microwave irradiation. Moreover, although it is not clear enough in the photograph, it can be seen that the amount of small pores is greatly increased when observed in detail. Although not shown, it has been confirmed that even in the case of wood chips, large pores and small pores are increased in the same manner as described above by microwave irradiation.

FIG. 1A is a photograph showing an example of a microstructure of activated carbon made from bamboo scraps of this example, and FIG. 1A shows the case where there is no microwave irradiation, and FIG. FIG. 2A is a photograph showing an example of the microstructure of activated carbon made from the bark of this example, and FIG. 2B shows the case where microwave irradiation is not performed, and FIG. 2B shows the case where microwave irradiation is performed.

Claims (5)

  A method for producing activated carbon from a wood raw material mainly comprising at least one of wood, bamboo, and bark, the step of adjusting the water content of the wood raw material adjusted in particle size as necessary, and this A step of irradiating a wood raw material with microwaves for a predetermined time, and then a step of stopping the microwave irradiation and carbonizing the wooden raw material by external heating in a non-oxidizing atmosphere. Production method.   2. The method for producing activated carbon according to claim 1, wherein, in the moisture content adjusting step, the moisture content of the wood material is 50 to 500% in a weight ratio with respect to the material in a dry state.   3. The method for producing activated carbon according to claim 1, wherein in the microwave irradiation step, irradiation with microwaves is started and irradiation is continued for 3 to 10 minutes after moisture starts to boil. .   4. The water content is adjusted in the moisture content adjusting step, wherein the wood material is boiled under normal pressure or reduced pressure to allow moisture to penetrate into the material, and then the moisture content is adjusted. A method for producing the activated carbon according to claim 1. The method for producing activated carbon according to any one of claims 1 to 3, wherein the microwave irradiation step is performed in an air atmosphere or an inert gas atmosphere in a reduced pressure state of 400 to 710 Torr in absolute pressure.