Activated carbon is one of the most effective media for removing a wide range of contaminants from industrial and municipal waste waters, landfill leachate and contaminated ground. As the world’s most powerful adsorbent, it can cope with a wide range of contaminants.Different contaminants may be present in the same discharge and carbon may be used to treat the total flow, or it may be better utilized to remove specific contaminants as part of a multistage approach.
Activated carbon is made from any substance with a high carbon content, and activation refers to the development of the property of adsorption. Activated carbon is important in purification processes, in which molecules of various contaminants are concentrated on and adhere to the solid surface of the carbon. Activated carbon is generally nonpolar, and because of this it adsorbs other nonpolar, mainly organic, substances. Extensive porosity (pore volume) and large available internal surface area of the pores are responsible for adsorption.
Any inexpensive material with a high carbon content and a low inorganic content can be used as potential
raw material in producing activated carbons. The most often used raw materials for manufacturing activated carbons include wood, peat, bituminous coal, lignite, coconut shell, nutshells, lignin, and others.
Activated carbons can be manufactured by either a physical or a chemical activation process. The physical activation process generally is used to manufacture activated carbon in a two-step activation process—the carbonization of raw materials in the absence of O2 followed by the activation of carbonized products. Steam and carbon dioxide (CO2) are the activating reagents most commonly used in physical activation, significantly influencing the porosity of the activated carbons. The chemical activation process is used to manufacture activated carbons usually in a single step, using zinc chloride, phosphoric acid, and potassium hydroxide as the activating reagents.The carbonaceous materials are converted into activated carbons depending upon the nature of the raw materials, the nature of the activating reagent, and the conditions of the activation process.Furthermore, activated carbon can be impregnated with sulfur (S), chloride, or iodine to increase its adsorptive capacity, making the impregnated activated carbon much more effective than un-impregnated activated carbon for removing gaseous mercury (Hg) from flue gases at low temperatures.
Utility of Activated Carbon
Activated carbon may be used to treat a number of contaminants in liquid wastes including Non-biodegradable organic compounds (COD), Adsorbable Organic Halogens (AOX), Toxicity ,Colour compounds and dyestuffs ,Inhibitory compounds for biological treatment systems ,Aromatic compound including phenol and bis-phenol A (BPA),Chlorinated/halogenated organic compounds ,Pesticides and a host of toxic substances.
Air Pollution Control
In the context of air pollution control, Volatile Organic Compounds (VOCs) from air and other gases can be removed to below the detection limit.Noxious compounds such as hydrogen sulphide and mercaptans are readily trapped through adsorption and help to prevent odours. Incinerators dealing with Municipal Solid Waste (MSW), hazardous industrial waste, medical waste, sewerage sludge and crematoria results in the formation of a flue gas containing a range of pollutants. Dioxins and heavy metals such as mercury and cadmium are not normally removed to low enough concentrations by conventional treatment.
A range of industrial inorganic compounds and materials can be removed from gas streams, before venting to the atmosphere, using specialised impregnated or catalytic (Centaur) carbons.
In the Food and Beverage Industry
Activated carbons can be used in the Food and Beverage industry to decolourise,dechlorinate,deozonate,decaffeinate,debitter,deodorise a number of food products.
In the Pharmaceutical Industry
In the Pharmaceutical industry activated carbons are used to provide superior removal of colour compounds, odour compounds, proteins and other contaminants that could be present in the raw materials or that form during production.
As a Catalyst
With its large surface area, purity and relative hardness, activated carbon is an ideal carrier for catalytic metals or a catalyst by itself.Activated carbons have been successfully used as a catalyst in the manufacture of dry cell batteries, production of biodegradable herbicides like cyanuric chloride glyphosate, mercaptan removal in petroleum distillates and in the production and destruction of phosgene.
In the production of natural gas
Activated carbons have also been used for removal of mercury from natural gas for the production of L.N.G. , the removal of mercury from liquid hydrocarbons and the removal of mercaptans/thiols , hydrogen sulphide and amine solutions from natural gas and natural gas scrubbing.
In the brewing industry
Industrial uses of activated carbon also include its use for the purification of hydrocarbon contaminated Carbon Dioxide generated from the conversion of sugars to alcohol and its reuse for carbonation in the brewing Industry thereby eliminating the need from purchasing Carbon Dioxide from outside sources.
For the storage of gases
It can also be used to remove trace lubrication oils from waste streams and in fruit storage for gas storage(under pressurised condition the extensively developed carbon porosity provides for greatly enhanced volume storage of either a pure gas, such as carbon dioxide or nitrogen, or a gas mixture such as air) and delivery and also in applications that provide alternatives to greenhouse gas emissions.
In the Caustic soda Industry
Caustic Soda is used extensively in, and is also a by-product from, the chloralkali industry. Mercury cells are used for production of chlorine, hydrogen, and sodium (in a few plants potassium) hydroxides by electrolysis of a brine solution, and this can cause contamination. Steam activated and impregnated carbons are proven for this application.
For the purification of Electroplating Chemicals
Electroplating chemicals that become contaminated with organics and metal finishing residues can be purified by the use of steam activated carbon and the chemicals can be recycled for reuse.Chromium can be recovered from electroplating solutions.
In Gold recovery applications
Coconut shell based granular activated carbons are used in gold recovery applications employing the Carbon in Leach (CIL) or Carbon in Pulp (CIP) processes.
In Process Water treatment
In process water treatment, activated carbons are used to remove tastes and odors ,disinfection byproducts like chloramines, free and combined chlorine,trihalomethanes and halocarbons, for pH and alkalinity control,condensate recovery and treatment systems in steam generating operations,personal and collective protective masks against toxic gases includind Industrial and military respirators.
Other applications include within air filtration systems in archives and museums,ozone management in ponds and aquaria,treatment of swimming pool water for removal of organic matter, chloroform ,ozone,chloramine and other bonded chlorine and in cigarette filters,filters for treating cabin air in automobiles.
Recycling of spent activated carbon
Once granular carbon is saturated or the treatment objective is reached, it can be recycled, by thermal reactivation, for reuse. Reactivation involves treating the spent carbon in a high temperature reactivation furnace to over 800°C. During this treatment process, the undesirable organics on the carbon are thermally destroyed. Recycling by thermal reactivation is a highly skilled process, to ensure that spent carbon is returned to a reusable quality. Recycling activated carbon by thermal reactivation meets the environmental need to minimise waste, reducing CO2 emissions and limiting the use of the world’s resources.