JP6760583B2 - How to make activated carbon - Google Patents

Description

The present invention relates to a method for producing activated carbon from a plant raw material (silicon-accumulated biomass) containing silica, and an activated carbon production system.

Activated carbon is used as an adsorbent for harmful substances in gases and liquids, a catalyst carrier, and a capacitor capable of repeatedly charging and discharging by adsorbing and desorbing ionic molecules.

Activated carbon is produced by carbonizing and activating organic matter. Organic substances as raw materials include plant-based substances such as wood, sawdust, coconut shells, and pulp waste liquid, as well as coal-petroleum-based substances such as coal, petroleum heavy oil, coal and petroleum-based pitch, and synthetic polymers. In addition to carbon, these organic substances contain elements such as oxygen and hydrogen.

The carbonization step is performed to heat-treat the organic matter of the raw material to produce a carbon-concentrated carbide to form the basic pore structure of the activated carbon. The activation step is carried out to prepare a porous activated carbon having a more developed pore structure from the above-mentioned carbide. The activation method includes a chemical activation method and a gas activation method.

In the chemical activation method, carbides are impregnated with an activator such as zinc chloride, phosphoric acid, calcium chloride, and calcium sulfide in an appropriate mass ratio, heated in an inert gas, and subjected to an oxidation reaction involving dehydration and carbon extraction. This is a method of increasing pores.

The gas activation method is a method in which a carbide is contact-reacted with a gas such as water vapor, carbon dioxide, or oxygen. In the gas activation method, the pore structure of carbide develops through the following process. First, in the initial heating process, the unassembled portion (the portion where the pores are not developed) of the carbide is selectively decomposed, and the closed pores inside the carbide are opened, so that the internal surface area is increased. Brought to you. Next, the carbon around the fine pores reacts to form pores with a large pore diameter.

An excellent activated carbon adsorbent preferably has a large adsorption capacity per unit amount and a large adsorption rate, and for that purpose, it is desirable that it has a large specific surface area and uniformly has a large number of pores having a diameter suitable for the adsorbed species.

For example, it has been reported that activated carbon having a BET specific surface area of 3000 m ² / g or more is produced from petroleum coke by a chemical activation method using potassium hydroxide (Non-Patent Document 1 and Patent Document 1).

In addition, silicon-accumulated biomass such as rice husks, which is a raw material, is mixed with an alkali metal compound and fired to activate alkali, and silica is eluted and removed as alkali silicate to form a porous material with a specific surface area of 2000 to 4000 m ² / g. A method for producing a quality silica-carbon composite material is known (Patent Document 2). Further, a method for producing a porous carbon material by treating a plant-derived material containing silicon with an acid or an alkali is known (Patent Document 3). Note that the KOH activation is chemical activation method, comparing the H _{2 O} (steam) activation is a gas activation method, it is known that there are differences as follows. That is, in KOH activation, the specific surface area increases without consuming carbon, and the number of pores having a pore diameter of 2 nm or more increases.

On the other hand, the H 2 _O activated, increases the specific surface area only by consuming carbon, easily generates the following pore pore diameter 2nm, but pores over pore diameter 2nm does not generate almost (Non-Patent Document 2 ).

U.S. Pat. No. 4,082,694 Japanese Patent No. 2506600 Japanese Patent No. 4618308

H. Marsh, D.M. Crawford, Carbon, 20,419 (1982) Yuzo Sanada, Motoyuki Suzuki, Kaoru Fujimoto, New Edition Activated Carbon Basics and Applications, Kodansha Scientific, p. 68 (2000)

A good activated carbon has a high specific surface area, a high adsorption rate, and many pores having a diameter suitable for the adsorbed species. The pore distribution of activated carbon depends on conditions such as raw material, carbonization temperature, activation method and temperature. A high specific surface area is obtained by carbonizing silicon-accumulated biomass according to a conventional method, mixing the obtained carbide with an alkali metal compound (potassium hydroxide, etc.), activating the carbon component with an alkali, and removing silica. Activated carbon can be obtained. The pores of the obtained activated carbon are widely distributed in the regions of macropores (pore diameter 50 nm or more), mesopores (pore diameter 2 to 50 nm), and micropores (pore diameter 2 nm or less).

However, such activated carbon is not suitable for gas adsorption because the ratio of micropores having a pore diameter of 2 nm or less is small. Therefore, an object of the present invention is to provide a method capable of producing activated carbon suitable for gas adsorption and an activated carbon production system.

The method for producing activated carbon according to the present invention comprises a step of carbonizing silicon-accumulated biomass to obtain carbides, and mixing an alkali compound with the carbides to activate alkali and reacting silicon to generate alkali silicate. It is characterized by having a step of removing the alkali silicate to obtain a porous carbon material and a step of gas-activating the porous carbon material to obtain activated carbon. The present invention also provides an activated carbon production system.

In the present invention, silicon-accumulated biomass such as rice husks is carbonized, then the carbides are activated with an alkali metal compound, and activated carbon having a high specific surface area is obtained by removing silica as an alkali silicate, and then gas activation is further performed. As a result, activated carbon having a high specific surface area and developed micropores can be produced. Therefore, according to the method of the present invention, since the specific surface area is high and the ratio of micropores is high, activated carbon suitable for gas adsorption can be produced.

Hereinafter, embodiments of the present invention will be described.
As a raw material, silicon-accumulated biomass such as rice husk, rice straw, sugar cane, corn, and bamboo, which can be expected to have a high specific surface area activated carbon, is used. Here, rice husks will be described as an example. Rice husks have a composition containing 71-87% organic matter (cellulose, hemicellulose, lignin, etc.) and 13-29% inorganic matter in mass%. Inorganic substances include 87-97% silica and 3-13% other oxides (sodium oxide, potassium oxide, alumina, etc.). The ratio of these components does not depend on the type of rice and the place of origin (L. Sun, K. Kong, Silicon-Based Materials from Rice Hucks and Their Application, Ind. Eng. Chem. Res. 2001, 40, 5861-5877. ).

In the present invention, the content of silica in the silicon-accumulated biomass may be 0.1% or more in mass%. First, the organic matter contained in the silicon-accumulated biomass, which is a raw material, is heat-treated in a non-oxidizing gas to carbonize it to obtain a carbide. Impurities other than organic substances in the raw material may be removed by acid cleaning before the carbonization treatment, or the raw material may be carbonized as it is without acid cleaning. The carbonization treatment decomposes organic substances such as cellulose to produce gaseous decomposition products.

The carbonization treatment is carried out in an inert gas stream or in a combustion gas in which oxygen is consumed, at rest or at a space velocity of 0.1 min ^-1 or more, preferably 0.3 to ¹ min ^-1 . The temperature of the carbonization treatment is 500 to 1000 ° C, preferably 500 to 800 ° C. The temperature is raised to a predetermined temperature at a rate of 1 ° C./min or more, preferably 5 to 20 ° C./min. The holding time after reaching the predetermined temperature is 0 to 5 hours, preferably 0 to 2 hours. The content of carbon and silica in the carbides obtained as a result of such carbide treatment is 50 to 60% carbon and 50 to 40% silica for carbides containing impurities, and about 50% carbon and about 50% silica for carbides from which impurities have been removed. It will be 50%.

Next, the obtained carbide and the alkali compound are mixed and heat-treated to activate the alkali, and the silica is reacted with the alkali compound to generate an alkali silicate.

Examples of the alkaline compound include hydroxides, carbonates, hydrogencarbonates, chlorides, sulfates, nitric acids and the like such as potassium, sodium and lithium. The form of the alkaline compound may be an aqueous solution, a powder, or a granular compound having a diameter of 5 mm or less. The amount of the alkali compound added to the carbide is 2 to 10 equivalents, preferably 2 to 5 equivalents, of alkali activation, in terms of alkali metal contained in the alkali compound, with respect to 1 mol of silica in the carbide for removing silica. For this purpose, it is 0.1 to 10 equivalents, preferably 1 to 4 equivalents, in terms of alkali metal contained in the alkali compound with respect to 1 mol of carbon in the carbide. The amount of the alkaline compound added to the carbide is the sum of the amount added for removing silica and the amount added for alkali activation.

A mixture of carbide and alkaline compound is placed in a nickel crucible and heat treated. This heat treatment is carried out at rest or in an inert gas such as nitrogen or argon having a space velocity of 0.1 min ^-1 or more, preferably a space velocity of 0.3 to ¹ min ^-1 . The heat treatment temperature is equal to or higher than the melting point of the alkaline compound, preferably 500 ° C. or higher and 1000 ° C. or lower, more preferably 700 to 900 ° C. The temperature is raised to a predetermined temperature at a rate of 1 ° C./min or more, preferably 5 to 20 ° C./min. The holding time after reaching the predetermined temperature is 0 to 5 hours, preferably 0 to 2 hours.

The reaction between silica and an alkaline compound is represented by the following reaction formula when the alkaline compound is potassium hydroxide or sodium hydroxide, respectively.
SiO ₂ + 2KOH → K ₂ SiO ₃ + H ₂ O
SiO ₂ + 2 NaOH → Na ₂ SiO ₃ + H ₂ O

Alkali silicate is produced from silica by these reactions. Alkali silicate is soluble in water and can be separated from carbon. After heat treatment, allow to cool to room temperature. Add water to the sample and boil to dissolve the alkali silicate. Boil until the alkali silicate dissolves. The boiling time is 30 minutes to 2 hours, preferably 30 minutes to 1 hour. Then, it is filtered to obtain activated carbon as a residue. The obtained activated carbon is washed with water and dried. It is preferable to use warm water of about 30 ° C. for washing. Washing is performed until the pH of the filtrate reaches about 7.0. Drying is performed at 80 to 100 ° C. for 30 minutes to 2 hours under atmospheric pressure or under reduced pressure of 0.01013 MPa (0.1 atm) or less, preferably at 100 ° C. for 30 minutes to 1 hour under reduced pressure of about 0.01013 MPa. Do.

The obtained porous carbon material (alkali-activated activated carbon) has a specific surface area of 2000 to 4000 m ² / g, and has macropores (pore diameter of 50 nm or more), mesopores (pore diameter of 2 to 50 nm), and micropores. It shows a pore distribution widely distributed in a region of pores (pore diameter 2 nm or less).

Next, the obtained alkali-activated activated carbon is gas-activated. Gas activation is performed by heat treatment in an atmosphere containing a gas such as water vapor, carbon dioxide, or oxygen.
The reaction between carbon and these gases is as follows.
In the case of water vapor C + H ₂ O → CO + H ₂ C + 2H ₂ O → CO ₂ + 2H ₂
In the case of carbon dioxide C + CO ₂ → 2CO
In the case of oxygen C + O ₂ → CO ₂ 2C + O ₂ → 2CO

By abstracting carbon from the alkaline activated carbon by these reactions, the pores having a diameter of 2 nm or less are increased. Of the above reactions, the reaction using water vapor or carbon dioxide is an endothermic reaction, while the reaction using oxygen is an exothermic reaction. Since it is difficult to control the temperature of the exothermic reaction, it is preferable to activate the gas using steam or carbon dioxide.

The reaction is initiated by the contact of water vapor or carbon dioxide with carbon. At this time, the molecular diameter of H ₂ O or CO ₂ affects the diameter of the pores to be formed. Since the van der Waals diameters of H ₂ O and CO ₂ are 4.82 Å and 5.26 Å, respectively, it is preferable to use H ₂ O in order to form pores having a small diameter.

In consideration of these points, the following conditions are adopted when the gas activation is performed in an inert gas-steam mixed gas stream. Inert gas - flow rate of the water vapor mixed gas space velocity 0.01～100Min ^-1 or higher, preferably 1~50min ^-1. The amount of water vapor supplied is 0.01 to 1 mol per minute, preferably 0.05 to 0.5 mol per minute, relative to 1 mol of carbon. The temperature of gas activation is 500 to 1000 ° C. Below 500 ° C, gas activation becomes insufficient. Gas activation in the range of 500 to 1000 ° C. is advantageous for obtaining a high specific surface area. It is considered that this is because the ratio of micropores is high. However, even if the gas is activated at a temperature exceeding 1000 ° C., the effect of increasing the ratio of micropores is saturated. The preferred gas activation temperature is 700-900 ° C. The temperature is raised to a predetermined temperature at a rate of 1 ° C./min or more, preferably 5 to 20 ° C./min. The holding time after reaching the predetermined temperature is 0 to 5 hours, preferably 1 minute to 2 hours. Since the pore diameter tends to increase as the holding time increases, it is preferable to avoid lengthening the holding time.

The activated carbon obtained as described above (alkali-activated-gas-activated activated carbon) has a high specific surface area and a high proportion of micropores suitable for gas adsorption, and thus can be advantageously applied to gas adsorption. The obtained activated carbon powder may be granulated and molded according to the intended use. Further, the activated carbon has a carbide acquisition unit that carbonizes silicon-accumulated biomass to obtain carbides, and an alkali compound is mixed with the carbides to activate alkali, and silicon is reacted to generate alkali silicate, and then the silica is produced. It will be molded by an activated carbon production system including a porous carbon material acquisition unit for removing acid alkali to obtain a porous carbon material and an activated carbon acquisition unit for gas activating the porous carbon material to obtain activated carbon. .. The activated carbon production system may be configured by integrally including a carbide acquisition unit, a porous carbon material acquisition unit, and an activated carbon acquisition unit, or may be configured in a state in which each portion is separated. Further, the activated carbon production system may be provided with other functional parts as long as it does not interfere with the present invention.

Hereinafter, examples of the present invention will be described. Regarding Example 1A in Table 1, 50 g of raw rice husks were taken as a raw material in a beaker, 500 mL of distilled water was added thereto, and the mixture was washed for 30 minutes, then rinsed with 1000 mL of distilled water, and dried in an oven at 100 ° C. for 24 hours. I let you. The dried rice husks were vacuum dried in an oven at 100 ° C. for 1 hour. To 50 g of washed rice husks by dry mass, add an aqueous solution of hydrochloric acid having a concentration of 3% HCl (v / v), which is a mixture of 80 mL of hydrochloric acid (special grade reagent) and 920 mL of distilled water, and heat the solution. Distillation was carried out for 2 hours (reaching), and suction filtration was performed after the solution had cooled. Then, the rice husks were washed with about 8 L of distilled water until the filtrate became about pH 7, and then dried in an oven at 100 ° C. for 24 hours.

Rice husks (leached rice hills: LRH) having a dry mass of 15 g (volume 150 cc) after the washing and leaching treatment were placed in an electric furnace. After replacing the inside of the electric furnace with nitrogen three times, the temperature is raised to 700 ° C at a nitrogen flow rate of 100 mL / min and a heating rate of 5 ° C / min, held at 700 ° C for 1 hour, carbonized, and naturally dissipated. As a result, rice husk carbide was obtained. The composition of rice husk carbide, _{carbon: 48.58%, SiO 2: 51.42} %, K 2 O , etc. impurities: was 0.05%.
A dry mass of 4 g of rice husk carbide and 16 g of KOH were placed in a nickel crucible and mixed for 1 minute, then aged in an oven at 70 ° C. for 2 hours, and the mixture was placed in an electric furnace. After replacing the inside of the electric furnace with nitrogen three times, the temperature is raised to 700 ° C. at a nitrogen flow rate of 100 mL / min and a heating rate of 5 ° C./min, and the temperature is maintained at 700 ° C. for 1 hour to activate alkali and react silica. The mixture was allowed to generate alkali silicate and naturally dissipated heat. About 200 mL of distilled water was added to the treated sample, and the sample was heated for 30 minutes to boil to dissolve the alkali silicate, and suction filtration was performed to remove the alkali silicate to obtain a solid substance. The obtained solid was washed with about 8 L of warm water until the filtrate reached about pH 7, and then vacuum dried in an oven at 100 ° C. for 1 hour to obtain KOH-activated rice husk activated carbon.

0.3 g of KOH-activated rice husk activated carbon with a dry mass was taken in a combustion boat and placed in an electric furnace. After replacing the inside of the electric furnace with nitrogen three times, the temperature is raised to 700 ° C at a nitrogen flow rate of 100 mL / min, distilled water inflow rate of 0.04 mL / min, and temperature rise rate of 5 ° C / min, and held at 700 ° C for 1 hour. Then, H ₂ O activation was carried out, and natural heat was dissipated to obtain KOH activated −H ₂ O activated rice husk activated carbon.

Example 1B in Table 1, with respect to 1C, except that the conditions of _{H 2} O activated, was changed to 1 hour at 800 ° C. (Example 1B) or 900 ° C. for 1 hour (Example 1C) are identical to Example 1A in conditions, KOH activated -H ₂ O activated chaff charcoal (example 1B, 1C) was obtained.

For KOH activated chaff charcoal (Comparative Example 1) and KOH activated -H ₂ O activated chaff charcoal (Example 1A-1C), BET specific surface area, nitrogen relative pressure _P / _P 0 = 0.99 for (diameter up to 300 nm) The total pore volume was measured. The micropore volume was measured by the t-plot method, and the ratio of the micropores to the total pore volume was determined.
The results are shown in Table 1. Table 1 also shows the results of commercially available coconut shell activated carbon 1 (Reference Example 1) for reference.

As can be seen from Table 1, KOH activated -H 2 _O activated chaff charcoal, compared with KOH activated chaff charcoal, the total pore volume despite decreases, micropore volume is increasing. Compared with KOH-activated rice husk activated carbon, H ₂ O-activated KOH activated carbon at 700 ° C-H ₂ O-activated rice husk activated carbon has a reduced specific surface area, but H ₂ O-activated KOH activated at 800 ° C or 900 ° C- H 2 _O activated chaff charcoal has increased specific surface area. 800 ° C. or 900 KOH activated -H ₂ O activated chaff activated carbon _{H 2} O activated at ° C., the total pore volume and micropore volume than _{H 2} O activated the KOH activated -H ₂ O activated chaff charcoal at 700 ° C. , The ratio of micropores is also increasing. These results, while macropores and mesopores are broken by H 2 _O activation is thought to be due to micropores develops. The specific surface area of KOH activated -H 2 _O activated chaff charcoal, total pore volume and micropore volume is large compared to most coconut shell active pores are micropores.

According to the present invention, by a chaff carbide to KOH activated, can produce activated carbon having a wide pore from the macropores have a high specific surface area to micropores, by further KOH activated chaff charcoal to H _{2 O} activated , The volume of micropores increased, and activated carbon with a high ratio of micropores in all pores could be produced.

With reference to Table 2, KOH was was replaced NaOH in the same manner as in Example 1 to produce a NaOH activated -H ₂ O activated chaff charcoal (Example 2). The resulting NaOH activated -H ₂ O activated chaff charcoal, the BET specific surface area was measured.

Next, KOH activated-H ₂ O activated (700 ° C.) rice husk activated carbon (Example 1A), NaOH activated-H ₂ O activated (700 ° C) rice husk activated carbon (Example 2), and commercially available coconut shell activated carbon 1 (reference). Each of Example 1) was left in a sealed container containing benzene at a temperature of 20 to 25 ° C. for 24 hours, and the amount of benzene adsorbed was calculated from the amount of increase in mass.

It is known that benzene vapor is well adsorbed on micropores with a diameter of 2 nm or less. As shown in Table 2, none of the KOH activated -H 2 _O activated chaff charcoal and NaOH activated -H 2 _O activated chaff charcoal, palm compared to shell activated carbon has many benzene adsorption.

According to the present invention, by alkali activation rice husk carbide, can produce activated carbon having a high specific surface area, further the alkali activation chaff charcoal by H _{2 O} activated, micropore volume is increased, the total pore in Activated carbon with a high proportion of micropores could be produced, and the amount of benzene adsorbed could be increased.

In addition, a commercially available coal-based activated carbon (Reference Example 2A) was prepared and steam-activated to prepare a commercially available coal-based activated carbon-steam activation (Reference Example 2B). Commercially available coconut shell activated carbon 2 (different from the commercially available coconut shell activated carbon 1 shown in Reference Example 3A, Table 1 and Table 2) is prepared and steam activated to activate the commercially available coconut shell activated carbon-steam activation (Reference Example). 3B) was prepared. In each case, the conditions for steam activation were the same as the conditions of Example 1A described above (700 ° C., 1 hour). For Reference Examples 2A, 2B, 3A and 3B, the BET specific surface area and the total pore volume were measured.

In Table 3, the specific surface area and the total pore volume of both the commercially available coal-based activated carbon (Reference Example 2A) and the commercially available coconut shell activated carbon 2 (Reference Example 3A) are reduced by steam activation. Most of the pores of these commercially available activated carbons are micropores, and it is considered that the micropores are broken by steam activation, resulting in a decrease in specific surface area and pore volume. Carboniferous activated carbon and coconut shell activated carbon are optimized to maximize the specific surface area and pore volume in the commercially available state. However, when commercially available coal-based activated carbon or commercially available coconut shell activated carbon is further steam-activated, the specific surface area and pore volume decrease, and the micropores decrease.

Summarizing the above results, we can conclude as follows. Alkali activation of rice husk activated carbon develops micropores, mesopores and macropores, resulting in a large specific surface area. Further, when the alkali activation chaff charcoal H _{2 O} to activation, broken macropores and mesopores, micropores develops instead. When steam activation is performed at 700 ° C., micropores develop, but the degree of development is not sufficient. As a result, the macropores and mesopores are broken, resulting in a large decrease in specific surface area and pore volume. When steam activation is carried out at 800 ° C. or 900 ° C., the development of micropores is remarkable, so that both the specific surface area and the pore volume are increased as compared with the case where steam activation is carried out at 700 ° C. Also, there appear to be optimal conditions for the water vapor activation temperature, which results in an increase in specific surface area and pore volume. In the case of this example, steam activation at 800 ° C. was optimal.
Hereinafter, the inventions described in the claims at the time of filing the application of the present application will be added.
[1] A process of carbonizing silicon-accumulated biomass to obtain carbides,
A step of mixing an alkali compound with the carbide to activate the alkali and reacting silicon to generate an alkali silicate, and then removing the alkali silicate to obtain a porous carbon material.
A method for producing activated carbon, which comprises a step of gas-activating the porous carbon material to obtain activated carbon.
[2] The method for producing activated carbon according to [1], wherein the silicon-accumulated biomass contains 0.1% or more of silica in mass%.
[3] The method for producing activated carbon according to [1] or [2], wherein the silicon-accumulated biomass is acid-treated at 100 ° C. or lower to remove impurities.
[4] The method for producing activated carbon according to any one of [1] to [3], wherein the alkaline compound is potassium hydroxide or sodium hydroxide.
[5] The method for producing activated carbon according to any one of [1] to [4], wherein the alkali activation is carried out at 500 to 1000 ° C.
[6] The method for producing activated carbon according to any one of [1] to [5], wherein the gas activation is carried out in an atmosphere of an inert gas containing water vapor.
[7] The method for producing activated carbon according to any one of [1] to [6], wherein the gas activation is carried out at 500 to 1000 ° C.
[8] A carbide acquisition unit that carbonizes silicon-accumulated biomass to obtain carbides, and an alkali compound is mixed with the carbides to activate the alkali, and silicon is reacted to generate alkali silicate, and then the alkali silicate is used. An activated carbon production system including a porous carbon material acquisition unit that is removed to obtain a porous carbon material, and an activated carbon acquisition unit that gas-activates the porous carbon material to obtain activated carbon.

Claims (1)

A step of removing impurities by acid-treating silicon-accumulated biomass containing 0.1% or more of silica by mass at 100 ° C. or lower.
The step of carbonizing the silicon-accumulated biomass to obtain a carbide and
A step of mixing potassium hydroxide with the carbide to activate alkali at 500 to 1000 ° C. and reacting silicon to generate alkali silicate, and then removing the alkali silicate to obtain a porous carbon material.
A method for producing activated carbon, which comprises a step of activating the porous carbon material with a gas at 800 to 900 ° C. in an inert gas atmosphere containing water vapor to obtain activated carbon.