Activated carbon

Overview

Activated carbon is a carbon material with great specific surface area and decolorization ability. In the 19th century, people apply sugar, wine and water for decolorization, de-flavor and purification. There is a history of 100 years for bone charcoal to be used for water filtration. During the First World War, it has begun to produce activated carbon as gas masks. To the 20th century, 90 years, the activated carbon has also gotten wide application in the sewage treatment, organic solvent concentration recovery, air purification and other fields such as environmental protection and gold extraction.
The carbon material remaining after the carbonization of the organic matter at 600 to 700 °C has the ability to be active or activated. However, these carbon materials can firmly adsorb some gaseous hydrocarbons and other substances, reducing their activation capacity need to increase the carbonization temperature to above the critical temperature for the formation of activated carbon. So that these adsorbed gaseous substances are decomposed and separated. Then apply water vapor and carbon dioxide for activation to improve the activation capacity to industrial applications.

Fig.1 Micropores of activated carbon fibers
The absorption of the activated carbon is due to the internal rich porous, namely a large surface area, therefore, the point for the production of activated carbon is the application of organic matter in the carbonization and activation process for generating as many pores as possible.
Activated carbon, according to its shape can be divided into powdered activated carbon, granular activated carbon and fibrous activated carbon. Activated carbon raw materials are:
(1) plants: wood, sawdust, shells (including coconut shell, hawthorn nuclear, rice husk, etc.), and cellulose, lignin and pulp residues.
(2) Animal categories: a variety of animal bones.
(3) Minerals: lignite, peat, anthracite and petroleum residues.
(4) Chemical: polyacrylonitrile, viscose, phenolic and other fibers. The commonality of these materials is that solid-phase carbonization takes place during the heat treatment.

Usage History

According to ancient Egyptian records, the application history of carbon can date back to 1550 BC. The body of the female corpse was unearthed in the Western Han Tomb of Mawangdui (190-190 BC) in Changsha, China. The charcoal was packed around the upper part of the wooden coffin with the thickness being 30 to 40 centimeters and the weight being more than 5,000 kilograms. This is exact the application of the adhesive property of the carbon. The industrial production of activated carbon started from 1900 to 1901 when the Dutch scientist Ostelijieke (Raphaelvon Ostrejko) obtained the patents of chemical and gas activation method. During World War I, activated carbon had been used in gas masks accompanied with the well-known adsorption theories such as capillary condensation theory, adsorption isotherm and pore size distribution theory, monomolecular layer and multi-molecular layer adsorption theory, etc. The understanding of activated carbon has promoted the development of activated carbon production as well as the expansion of its applications. By the 1930s, activated carbon production has become an industry. After the 40's, the proposed activated carbon pore classification and the theory of microporous filling based on adsorption potential theory have both obtained practical applications. In 1932, China began to study the production process of activated carbon and has gradually established the activated carbon industry to 1950s.

Activation principle

Activation methods include gas activation and chemical activation of two categories. Gas Activation method is also known as physical method. The carbonaceous material is activated to produce activated carbon at high temperature using some oxidizing gas such as water vapor, carbon dioxide (or flue gas) or oxygen (or air) as the activator. Wood, during the carbonization process, often has some tar residues remain in the charcoal. During the activation process, the activator can make residual tar and other carbon compounds subject to oxidative decomposition, removal of surface impurities, so that the original blocked pore is open; the activator can also erode the surface of the carbon to form new pores while the walls between the original pores may also be burned out so that the pore is expanded. This will result in the formation of highly developed pore structure, greatly increasing the specific surface area as well as the adsorption property of carbon. The temperature required for activation varies depends on the kind of activator. The activation temperature with water vapor is about 800-950 ℃. When activated by flue gas, the temperature is about 900-950 ℃. When air is used as the activator, due to the reaction between carbon and oxygen is very fierce at high temperature, generally activated at about 600 ℃.
Chemical activation method mostly applies zinc chloride as the activator. Mainly take advantage of the swelling, dehydration, erosion and dissolution effect of zinc chloride in the high temperature on the cellulose in plant raw materials, leading to the formation of pores, to achieve the purpose of activation. Zinc chloride method can generally lead to activated carbon with robust transitional pore, being suitable for the removal of macromolecular such as sugar and pigment impurities. Upon taking advantage of Zinc chloride for production of activated carbon, the ratio of zinc/ sawdust (ratio of anhydrous zinc chloride over absolute dry wood sawdust) has a great impact on the pore size of the obtained activated carbon. High ratio leads to relative developed transition hole of activated carbon; low ratio can lead to activated carbons with developed microporous. Almost all of the carbonaceous material can be used for making activated carbon through different processes.

Manufacturing method

The basic principle for the manufacturing of activated carbon includes physical and chemical methods. The physical method is through applying the raw material for heating decomposition in sealed, oxygen-free environment, leading the evaporation of hydrogen and other substances while leaving the carbon with numerous microporous, namely the carbonization process.
The carbonization temperature can be up to 1200 ℃. Then send water vapor or carbon dioxide in the micro-oxygen environment to further increase the porous, namely, the activation process with the activation temperature being generally 700~900 ℃, leading to activated carbon after cooling. Chemical method is through pre-mixing the raw material with chemicals such as ZnCl2, HCl, FeCl3, H3PO3 and H2SO4 for thorough stirring, followed by carbonization at a lower temperature, such as 500~600 ℃. In order to improve the specific surface area, after the carbonization, it is subject to activation in the water vapor or carbon dioxide atmosphere.

Gas activation method

The main equipment for the manufacture of activated carbon is the activation furnace. In China, the gas activation method commonly used saddle furnace, but also uses multi-tube furnace, fluidized furnace and converter. The United States also uses multi-layer furnace. Saddle furnace (Figure 1) consists of the furnace body, two regenerators and chimney in both left and right. The activated furnace is a square furnace made from saddle-shaped and other special-shaped bricks with many activation channels (Figure 2) and is divided into two halves around the furnace. The raw material carbon is charged into the raw material trough at the top of the furnace and slowly descended along the activation channel by gravity, and goes through in sequence along the preheating section, replenishing carbonization section, activated section and the cooling section and finally discharged from the lower part. Apply the length of the interval time of material discharge to control the rate of decline of carbon material. The steam activator is fed into the lower part of the left regenerator, preheated to 1000~1100 ℃, then fed into the left half furnace, activating the raw material carbon through direct contact it in the activation section. As the activation reaction is endothermic, so that the furnace temperature continues to decline.
The activated gas mixture is sent into the right half of the furnace. Send through the secondary air at different positions on the furnace to cause the combustion of the combustible gas with the generate heat causing the rise of the furnace temperature. The mixed gas can activate the right half of the activated charcoal. The released high-temperature flue gas enters into the right regenerative chamber, make the inside lattice brick be heated and be further discharged into the chimney. The flue, air, and steam valves are switched once every 30 minutes when water vapor flows in exactly the opposite direction when fed into the right regenerator so that the left and right halves can be alternately warmed and cooled to keep the temperature around the range for activation. The normal activation temperature is 850 to 950 ° C. We need take care of the positive pressure operation of the activation furnace. Activated material discharged from the activation furnace can be processed by removing sand, crushing, screening, pickling, washing, dehydration and drying to obtain the finished product.

Figure 2 Schematic diagram of saddle furnace structure
1. Preheat section; 2. Supplementary carbonization section; 3. Upper nearby flue; 4. activation section; 5. Upper flue; 6. Central flue 7. Combustion chamber; 8. Regenerator; 9. Lattice brick layer; 10. Upper far flue; 11.Underearth far flue; 12. Cooling section; 13. foundation; 14. Feed opening; 15. Material feeding tank;

Structure and properties

Activated carbon is an amorphous carbon, and although they do not have a crystalline structure such as diamond and graphite, it has been found from diffraction of X-rays that their structures contain essentially crystallites. Basic microcrystals are composed of several layers of carbon atoms arranged in a hexagonal network of irregular overlapping each other. In the activated carbon, in addition to the basic crystallites, there are a single network plane which does not constitute a parallel layer and irregular carbon, such as aliphatic chain structure carbon, carbon adhering to the edge of the aromatic structure, and so on. The element consisting activated carbon, in addition to the carbon element, also includes chemical combination of hydrogen, oxygen, etc., and many other elements in the form of ash, the surface of activated carbon also has acidic, alkaline surface oxides and functional groups.
The most important property of activated carbon is the adsorption properties. Activated charcoal is a porous substance containing a large number of pores including pores having a radius of 20 angstroms or less, transition pores having a radius of 20 to 1000 angstroms, and macropores having a radius of 1,000 to 20,000 angstroms. Because of the different pore sizes and different surface oxides and functional groups, the adsorption properties of activated carbon are different, and have the ability of selective adsorption. For example, sugar with activated carbon has more transitional pores with relative good absorption property on macromolecules sugar and pigment impurities while activated carbon having more porous is applicable to the gas adsorption and removal of small molecular impurities. The main quality index for powder-shaped activated carbon, the decolorizing power on 0.15% methylene blue is 8~14ml/0.1g activated carbon; the decoloring power of A method or B method caramel color is 90~100%; the total iron content is less than 0.05~ 0.1%; the chloride content is not higher than 0.2 to 0.25 percent; the pH value is 3 to 9; the burning residue is not more than 3 to 8 percent and the drying loss is not more than 10 percent. For detailed classification and classification criteria, see the activated carbon standard LY 216-79 of the Ministry of Forestry of the People's Republic of China.
Activated carbon must have sufficient mechanical strength to withstand the wear and tear when used. Appropriate reduction of activation temperature and shortening the activation time is conducive to the improvement of mechanical strength, but is accompanied with the adsorption capacity. In order to improve the adsorption capacity, the activation temperature and the activation time must be increased, which in turn leads to the loss of strength and the recovery rate of the catalyst. Therefore, we need to take comprehensive consideration based on the application demand in order to obtain the best efficacy and the lowest production costs. The post-operated activated carbon is easy to regenerate for re-use, which can avoid secondary environmental pollution.

Main indicators

The main indicators of activated carbon are: particle size, strength, water, water capacity, bulk density, particle density, true density, specific surface area, ash content and other indicators of physical properties and adsorption performance indicators including benzene adsorption rate and carbon tetrachloride activity.
Low temperature (400 ℃) activated carbon is called L-carbon; high temperature (900 ℃) activated carbon called H-carbon. The H-char must be cooled in an inert atmosphere, otherwise will change to L-char. The adsorption property of activated carbon is related to the nature and activation temperature of the activated gas and the chemical properties of the gas, the activation temperature and the degree of activation, the composition of the inorganic species and the content of the activated carbon, mainly depends on the chemical properties of the gas and the activation temperature. The carbon content, specific surface area, ash content and pH value of the aqueous suspension increase with the activation temperature. The higher the activation temperature, the more complete volatilization of residual volatiles, the more developed the microporous structure, the greater the specific surface area and adsorption activity. With the removal of volatile carbon, the ash content of carbon increases. The ash content in activated carbon mainly consists of K2O, Na2O, CaO, MgO, Fe2O3, Al2O3, P2O5, SO3, Cl-and other components. Ash composition and its content have great impact on the adsorption activity of carbon. It can be generally used of hydrochloric acid or hydrofluoric acid soaking and then washing to remove or reduce the ash of activated carbon. The ash content of activated carbon is related to the raw material of active carbon. The contents of hydrogen and oxygen in activated carbon decrease with the increase of activation temperature.The adsorption capacity of activated carbon on acid or alkali is related to the pH value of its aqueous suspension. The activated carbon enabling the decrease of pH value on distilled water has a strong adsorption capacity on alkali; the activated carbon enabling the increase of pH value on distilled water has a strong adsorption capacity on acid. The mechanical strength of activated carbon is related to the raw material and carbonization temperature of activated carbon. When the carbonization temperature exceeds 700 ℃, the mechanical strength of activated carbon will increase significantly. In addition to wide application in wastewater purification and analytical chemistry, activated carbon is mainly used for the extraction of gold and silver in chemical processing technology.

Uses

1. Beaching agent; deodorant; de-flavor agent; cleaning agent in food production; it is widely used in sugar, glucose, caramel, oil, fruit juice and wine drinks for decolorization purification, removal of colloidal substances and water treatment.
2. Used for PSA separation of air and preparation of nitrogen;
3. Mainly used in the Chinese or western original drug in pharmaceutical industry;
4. It is suitable for the decoloring and deodorization of brewing industry, production of edible oil and food additive with special decoloring capability on caramel and molasses.
5. It can be used for desulfurization, water purification, air purification, recovery of solvents, absorption and as a catalyst carrier;
6. It can be used in gas, coke oven gas, natural gas, carbon dioxide and shift gas to remove hydrogen sulfide.
7. Mainly used for the deodorization, dechlorination and liquid decolorization in food, beverage, pharmaceutical and high-purity drinking water; as well as widely used in the chemical industry for solvent recovery and gas separation.
8. Widely used in the pre-treatment of industrial water and domestic water and the deep purification treatment on chemical, printing and dyeing, electronics, coking, environmental protection and other industrial wastewater.
9. Widely used in the recycling of organic solvents including toluene, xylene, ether, ethanol, acetone, gasoline, trichloropropane and carbon tetrachloride.
10. For the absorption of gas, liquid hazardous substances, the removal of various organic vapors and filtering out harmful gases and air odor.

Chemical properties

It appears as black porous odorless material with grain shape varying from the cylindrical, coarse particles to fine powder particles. The particle diameter is generally 1~6mm, the length of about 0.7 to 4 times the diameter, or exhibiting irregular particles with a particle size of 6 to 120 mesh. It is odorless, tasteless and is insoluble in water and organic solvents. The packing density is about 0.3-0.6g/ml, the micropore volume is about 0.6-0.8ml/g, and the specific surface area is about 500-1500m2/g. It has a strong absorption force on the organic polymer material so having a high removal capacity on the trace elements, pigments, odor substances in the liquid phase. The most suitable pH value is 4.0~4.8, the optimum temperature is 60~70 ℃.

Usage limit

Approximately 3 g of the powder sample was placed in a glass stoppered conical flask containing 10 mL of 5% dilute hydrochloric acid, boiled for 30 s, and then cooled to room temperature. Add 100 ml Iodine test solution (TS-l24); add stopper and have strong shaking for 30s. After filtration, discard part of the initial filtrate and get 50 ml filtrate. Take another 10 ml iodine test solution and add water to 50ml as a reference solution. The sample solution should not be deeper than the reference solution, indicating that the sample has an adsorption effect.
Take the samples in the air for burning; there should be carbon monoxide and carbon dioxide volatilization, and residual ash.
Heating to red; slow combustion without flame;

Toxicity

ADI is not subject to restrictive regulations (FAO/WHO, 2001).
ORAS (FDA, §240.361, §240.236, §240.401, 2000);
FDA, §240.105l (2000): 0.9% wines, 0.25% sherry; 0.4% grape juice. Identification test

Hazards & Safety Information

Category : Spontaneous Combustion product
Stimulation data:  Activated carbon dust and particles can stimulate the eyes and mucous membranes
EXPLOSIVE HAZARD CHARACTERISTICS:  The dust can be explosive when exposed to heat, open flames, and oxide explosions
Flammability and Hazardous properties:  it is explosive in case of heat, open flames and oxides burning.
Storage and transportation characteristics:  Treasury: low temperature, ventilated and dry; store it separately from fire, high temperature, and oxidant.
Extinguishing agent:  water, carbon dioxide, foam, dry powder

Chemical Properties

Black, light powder free from grittiness

Uses

Activated carbons are internally porous microcrystalline, non-graphitic forms of carbon. The activated carbons possess large surface area (～1000 m²/g) and pore volumes which make them useful for applications in energy storage devices, catalysis, and for removal of impurities from gases and liquids.Our activated carbon has been modified with Bis(2-ethylhexyl)phosphate and is well suited for metal ion removal from water.

Uses

Activated charcoal is used as an oral absorbent in treating oral intoxication from many substances.

Uses

Adsorbent.

Definition

An amorphous form of carbon made by heating wood or other organic material in the absence of air. Activated charcoal is charcoal that has been heated to drive off absorbed gas. It is used for absorbing gases and for removing impurities from liquids.

Safety Profile

It can cause a dust irritation, particularly to the eyes and mucous membranes. Combustible when exposed to heat. Dust is flammable and explosive when exposed to heat or flame or oxides.