What is activated carbon?
There are many different definitions of activated carbon. But to simply understand that activated carbon is a processed carbon product to create a pore structure to enhance the specific surface area of coal.
The main component of activated carbon is carbon in amorphous form combined with a small fraction of carbon in the form of graphite crystals. In addition C activated carbon also includes many other chemical elements such as silicon, oxygen, hydrogen, sulfur, nitrogen, … These elements, can be available in input materials or penetrate in too Activated carbon production process.
Application of activated carbon
Use activated carbon since ancient times
Activated carbon has been used by humans for a long time. For the first time activated carbon was used by Egyptians and Sumerians in 3750 BC, before the Pharaoh Tutankhamun period (reigned 1332-1323 BC), to recover copper, zinc, tin and reduced metals. impurities in the production of bronze.
Around 1500 BC the first time activated carbon was used in medicine to treat wounds. Activated carbon is used to deodorize and heal wounds in addition to infection.
Ancient Indian Hindus already know how to use charcoal (a form of activated carbon but have a low specific surface area) to filter drinking water. During the First World War, activated carbon was used to make gas masks. Thanks to that, many lives of soldiers were saved from dangerous toxic gases.
Use activated carbon today
Today activated carbon is widely used in many areas of life such as:
In wastewater treatment: activated carbon is used to treat polluted water from factories, factories, hospitals, schools, etc. These wastewater sources, often contain a large amount of organic waste, When decomposing will cause ambient pollution and there are very unpleasant odors. After being treated through activated carbon, these organic substances will be absorbed into coal, waste water will no longer have bad smell, rotten as well as ensure environmental hygiene standards.
Removal of dangerous heavy metals: wastewater from some industrial sectors contains a large number of heavy metals such as Hg, Cd, Pb, As, Sb, Cr, Cu, Zn, Mn, … If these metals are not completely removed, they will enter the water source. Some may cause serious illnesses, others may reduce the quality of life. Humans, as well as animals, drink this water, and over time will accumulate and produce many diseases. Activated carbon is capable of absorbing many heavy metals and allows recovery of these metals after absorption.
In chemical technology: activated carbon can be used as a catalyst for many chemical processes, or used as a raw material, a carrier, a biological medium for many other processes.
In health: Activated carbon is used in the treatment of food poisoning as well as being used in the production of many drugs and pharmaceuticals such as: in the composition of Carbogast to treat stomach and intestinal pain, Carbotrim treat bacterial infections and food poisoning, …
Making filter materials: Activated carbon is used to filter water for daily use. Currently, activated carbon is used in most popular water filtration systems on the market. In addition, activated carbon is also used for air purification, bactericidal and applied in air conditioning systems, used in cigarette filters, in respirators, …
In the mining industry: the use of activated carbon in the extraction of gold, silver and other precious metals is a new technology and is very effective.
In the cosmetic industry: Activated carbon can be used to make white skin masks, used in toothpaste, as well as activated carbon soap.
Parameters of activated carbon
Activated carbon is used in many different industries. In each industry there are different requirements for activated carbon. Therefore, before being applied in any field, we need to know the characteristics of activated carbon such as:
Size, pore volume and specific surface area:
The size of the pore is calculated by the distance between the two sides of the groove or the diameter of the porous tube. According to IUPAC standards, the pore size is divided into 3 types: micro pore size less than 2 nm, meso pore size from 2 to 50 nm and macro pore ∼ over 50 nm.
The specific surface area of activated carbon is the total surface area calculated for 1 g of coal, expressed as [m2 / g]. The specific surface of coal was determined by the BET method, so it was called BET as the specific surface of the coal (the first three letters of the names of three scientists Stephen Brunauer, Paul Hugh Emmett, and Edward Teller came in 1938 ). BET is a very important parameter for activated carbon, indicating the absorption capacity of activated carbon. 95% of the specific surface area of activated carbon is the area of micro pores, meso pores contributes no more than 5%, while the macro pores contribute insignificantly to BET.
This is also an important indicator of activated carbon, typical for BET of coal. The iodine index is calculated in the number of mg iodine absorbed by 1 g of coal [mg/g]. The bigger iot, the higher the BET of activated carbon. However, iodine atoms are large in size so it is difficult to penetrate into micro pores so scientists must have a suitable conversion factor to BET.
Characteristic for abrasion resistance of activated carbon, this is also an interesting parameter because during use, activated carbon also has to suffer from physical effects such as being placed under liquid or gas flow. , under the impact of pressure, impact, rubbing on each other, .. therefore coal needs to have sufficient stiffness to keep the structure intact during use and recovery. The hardness of activated carbon depends very much on the input materials for fabrication, as well as the structure and BET of activated carbon.
Particle size distribution
Particle size greatly affects the accessibility of the adsorbed material to the surface of the activated carbon. The smaller the size, the easier it is to access and the faster the absorption process. This is especially significant when absorbed in low pressure gas systems. Careful calculation of particle size distribution makes it possible to select the optimal pressure parameters to minimize energy consumption.
Manufacturing activated carbon
Raw materials for activated carbon production:
Because the main component of activated carbon is carbon, all sources of carbon-rich materials can be used to make activated carbon. Currently, activated carbon is made from the main sources of raw materials such as:
Coal: This is a source of fossil fuels formed from plants that have been preserved from oxidation and decomposition in swamp ecosystems. Because carbon is the main ingredient, coal is a good source of raw materials for the production of activated carbon.
Most of the world’s coal reserves have been formed for a long time, from the carbon period more than 300 million years ago. Initially, when the plant died, the stalks were not decomposed but accumulated into large masses deep in the swamp. Over time, these marshes are intruded by sea water, storms and floods, buried in sediment. Over time, the stalks are under decaying, hard and dry sediments that form brown coal or linhit. More and more coal layers are buried, the temperature and pressure also increase turning brown coal into bituminous coal (tar, asphalt). In some cases the pressure increases very high, then bituminous coal turns into anthracite (thin coal).
Petroleum products: Activated carbon can be made from products of petroleum like tar, alkanes, aromatic carbon. They are all carbon-rich materials that can be used as input materials in the production of activated carbon.
Coconuts, peanuts, bamboo and firewood: These are all plants that contain a large amount of carbon, which is a good source of raw materials for the production of activated carbon. In Vietnam today, activated carbon from coconut skulls is being used in many fields both domestically and exported to world markets, bringing a significant source of revenue to the state budget.
Rice husk: Considering rice husk as a raw material source in activated carbon production is a new direction now. Vietnam is an agricultural country with huge annual rice production. The amount of rice husk produced annually reaches nearly ten million tons. The results of analysis of the composition of rice husk show that, in rice husk containing from (13 ÷ 15)% SiO2, (18 ÷ 20)% is the remaining fixed carbon is moisture and other organic volatile substances. Therefore, the production of activated carbon from rice husk is absolutely promising. Production of activated carbon from rice husk is not new, but it is worth mentioning that making it with high productivity and efficiency and minimizing environmental pollution. Currently in Vietnam, the topic of producing activated carbon from rice husk is chaired by Assoc. Dr. Nguyen Van Tu, Hanoi University of Science and Technology, has been successfully researched and put into practical production. This not only solves the environmental pollution problem because rice husk waste is not used reasonably, but also opens a direction to produce activated carbon from renewable materials to replace fossil fuel sources. are running out.
Technology of manufacturing activated carbon
Basically, the technology of activated carbon production is carried out from two main steps: coalization and activation. The choice of implementation depends on the input materials as well as the quality of the output coal.
Carbonization of material input:
Carbonization aims to convert input materials into carbon, creating the first pore system for subsequent activation. For input materials with a plant nature such as wood, bamboo, coconut skull, peanut shells, rice hulls, … they are made up of large molecular systems. Use temperature to decompose and break down bonds to create smaller molecules. In this process, some decomposers are usually released as gases or liquids leaving the pores as mentioned above. Coal is obtained, so there is always a smaller proportion of input material. The remaining carbon atoms will move at very small distances (<1nm) in the material block, to more sustainable locations.
For other types of materials, the way to form the initial pores as well as the mechanism of coal formation is different and completely different. Therefore, different sources of raw materials will produce different activated carbon structures.
If the input materials are aromatic, tar, and carbon ring circuits, the re-calcification takes place in the liquid phase with a different mechanism than the solid phase above. The coal produced is non-porous graphite. In order to increase the porosity of coal, there is a need for impact on the graphene layers.
Carbonization can also take place in gaseous form when input materials are alkanes such as methane, propane or Bezen. Under the effect of the pyrolysis process, the destroyed input materials create carbon-containing debris. These fragments bind to compatible substrates in conjunction with the mechanism of carbon atom displacement to create a six-sided blade structure of graphite.
Activation is the process of improving the activity of coal after the carbonization process. The nature of activation is the process of taking away carbon atoms in the volume of coal particles under the effect of temperature and activating agents. There are two activation mechanisms: physical chemistry and chemical activation.
Physical activation: using steam or CO2 gas as an activating agent, at high temperatures around (800 ÷ 850) oC these activating agents perform reactions with carbon atoms in the coal as follows:
C + CO2 → 2CO
C + H2O → CO + H2
For each C atom to be reclaimed, it forms a pore in the structure of coal or breaks down the bonding walls to create capillary circuit systems that increase the surface area of the coal.
Physical activation can use for all types of coal derived from coal, peat, coconut skull, peanut shell, wood .. If coal is used as a raw material for coal production, when activating steam Overheating (at about 1300 degrees Celsius) is blown into an activated furnace at a temperature of about 1000 degrees Celsius. In addition to the mechanism of forming pores due to the removal of carbon atoms, there is also the presence of air pockets available in coal. At high temperatures, these air sacs are broken out to leave pores in the structure of coal. Initially these pores were small, but over time activation, the surrounding air pockets also escaped leaving new pores. Many pores combine to form larger pores. When the activation process takes place too long, it forms very large pores that reduce the activity of coal. Therefore activation time is one of the very important parameters that determine the quality of the activated carbon obtained.
Chemical activation: Common activating factors are chemical compounds such as ZnCl2, Na2CO3, K2CO3, H3PO4, … Chemical activation is often used to enhance the activity of plant-derived coal. Coalification and activation processes can also occur simultaneously. Input materials are mixed with activating agents and processed at temperatures from (500 ÷ 800) oC. In this condition, activators react with C and create pores in the structure of coal:
2C + K2CO3 – 2K + 3CO
Activated by physical methods, although activated carbon can be obtained with a large specific surface area, the recovery efficiency is not high due to the large amount of coal loss during activation.
Activation by chemical method has the advantage of higher coal recovery efficiency, lower productivity, more difficult mechanization and automation.
Working mechanism of activated carbon
The working mechanism of activated carbon consists of 2 stages: crude filtration and adsorption. In the early stages, impurities, large organic substances are retained on the surface of coal because they are larger than the diameter of the pores. The next stage is the adsorption stage, small organic molecules, heavy metal ions are adsorbed into the surface of the pore system of coal. Macro-sized pores (> 50 nm) are responsible for transporting adsorbents to meso pores and microphones. In micro pores the absorption mechanism is filled, the gravitational force here is very large due to the doubling effect of the two walls. In meso pores (2 <d <50nm), the adsorption mechanism is capillary condensation.
Video of the ability to purify the color of activated carbon
Activated carbon is increasingly widely used in many industries, the demand for activated carbon is increasing. Besides, the current sources of activated carbon production are gradually exhausted. Therefore, it is imperative to find new sources of raw materials to replace existing materials. Production of activated carbon from rice husk has been realized but more investment is needed in research to optimize the technology to minimize production costs as well as limit the environmental pollution.