Activated carbon, also called activated charcoal or activated coal is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions. It has a high degree of microporosity, just one gram of activated carbon has a surface area in excess of 500 m2.
Suitable raw materials
Activated carbon can be produced from carbonaceous source materials such as agricultural by-products like nutshells, peat, wood, coir, almond shells, apricot stones, cherry stones and grape seeds and lignite, coal, petroleum pitch etc.
Uses
The carbon adsorbents can be applied for the removal of tri halomethanes and metal ions from water, purification of waste solutions, water treatment, separation and concentration of trace elements and radioactive isotopes, production and analysis of high purity substances, fuel gas treatment and hazardous waste remediation etc. It is also used for the removal of undesired colour, smell and other impurities in many types of industries including basic drugs, fine chemicals, glucose, sugar, electroplating plasticizers and so on.
Production processes
Activated carbon can be produced by one of the following processes:
Physical reactivation: The precursor is developed into activated carbons using gases. This is generally done by using one or a combination of the following processes:
Carbonization: Material with carbon content is pyrolyzed at temperatures in the range 600–900 °C, in absence of oxygen (usually in inert atmosphere with gases like argon or nitrogen). Activation/Oxidation: Raw material or carbonized material is exposed to oxidizing atmospheres (carbon monoxide, oxygen, or steam) at temperatures above 250 °C, usually in the temperature range of 600–1200 °C.
Chemical activation: Prior to carbonization, the raw material is impregnated with certain chemicals. The chemical is typically an acid, strong base, or a salt (phosphoric acid, potassium hydroxide, sodium hydroxide, zinc chloride, respectively). Then, the raw material is carbonized at lower temperatures (450–900 °C). It is believed that the carbonization / activation step proceeds simultaneously with the chemical activation. Chemical activation is preferred over physical activation owing to the lower temperatures and shorter time needed for activating material.
Simple one-step method for agricultural by-products
Agricultural by-products have also proved to be promising raw materials for the production of activated carbons because of their abundant availability at a low price at the source of production. Examples are almond shells, nut shells, apricot stones, cherry stones and grape seeds. A simple one-step method for the production of activated carbons is a feasible alternative to the traditional two-stage process for the production of activated carbons by consecutive carbonization of the raw material and high temperature activation (900–1000°C) of the solid product from carbonization.
Agricultural by-products can be subjected to steam pyrolysis-activation at treatment at temperatures of 600 –700°C. The presence of water vapour during pyrolysis leads to a considerable increase in the liquid and gas product yields in addition to reducing the sulphur content in liquid and solid products. The properties of carbon adsorbents obtained by steam pyrolysis of depend on the treatment conditions such as the temperature and duration of treatment and the choice of raw materials.
Preparation of activated carbon from biomass
The activated carbons can be prepared by one-step pyrolysis of almond shells, nut shells, apricot stones, cherry stones and grape seeds, in the presence of steam.
The raw material is heated in an atmosphere of pure water vapour at a heating rate of 15°C per min to a final carbonization temperature of 800°C for duration of one hour. After the treatment it is left to cool. In order to obtain the carbons with steady content of surface oxides the steam flow is interrupted at a temperature of 300°C. The carbon particles preserve the shapes of the raw materials particles of regular shape.
Preparation of carbon adsorbents
Tar from steam pyrolysis and furfural are treated with 5% H2SO4 and the mixture is heated to 160°C with continuous stirring. The solid products obtained are heated to 600◦C in a covered container at a heating rate of 10°C per min under nitrogen. Oxidation of carbonizates with air at 400°C is used to obtain activated carbon with acidic surface properties. Steam activation with water vapour at 800◦C for one hour is used to obtain activated carbon with alkaline surface.
