Activated carbon is a form of carbon material processed to exhibit an extremely high degree of internal porosity. This unique, sponge-like structure gives it a vast surface area, often exceeding 1,200 square meters per gram, making it highly effective at adsorption. Adsorption is the mechanism by which contaminants, molecules, or ions adhere to the carbon’s internal surfaces, allowing it to purify liquids and gases. Manufacturing this material is a precise, multi-step thermal and chemical process designed to strip away non-carbon components and intentionally create this extensive network of microscopic channels.
Choosing the Right Carbon Source
The production process begins with selecting a suitable carbonaceous raw material, known as a precursor. Common precursors include coconut shells, various types of coal (bituminous and lignite), and wood products, such as sawdust and peat. The choice of precursor material dictates the final properties of the activated carbon, particularly its pore size distribution.
Manufacturers select these materials based on high fixed-carbon content and low levels of impurities, such as ash. For instance, activated carbon derived from coconut shells is known for its high hardness and small pores, making it ideal for gas purification and water treatment. Conversely, wood-based precursors yield a product with larger pores, which is better suited for decolorization applications in liquid phase systems.
The First Transformation: Preparing the Char
Before the material can be activated, it must undergo a foundational thermal process called carbonization, or pyrolysis. This step involves heating the raw precursor material in an inert atmosphere, meaning it contains very little or no oxygen. Temperatures for this transformation typically range between 600°C and 900°C.
Carbonization thermally decomposes the precursor, driving off volatile components like moisture, tars, and other non-carbon elements. This process leaves behind a carbon-rich solid residue, referred to as “char.” The char is significantly denser and has a higher concentration of carbon, making it a suitable foundation for the subsequent pore-creation stage.
Creating Porosity: Physical and Chemical Activation
The char created in the previous step still possesses a low surface area. The final stage, activation, is where the massive internal pore structure is developed. There are two main industrial methods for this: physical activation and chemical activation, each producing a different type of final product.
Physical Activation
Physical activation, sometimes called thermal or steam activation, is a two-step method where the char is subjected to high temperatures and an oxidizing gas. The char is heated to temperatures ranging from 800°C to 1100°C. At these temperatures, activating gases, such as steam or carbon dioxide, are introduced.
The gas reacts selectively with the carbon atoms, removing them through a process known as gasification. This controlled burning of carbon atoms erodes the solid structure, widening existing pores and creating new ones, dramatically increasing the internal surface area. The use of carbon dioxide or steam facilitates process control because their reactions with carbon are endothermic, which helps regulate the temperature.
Chemical Activation
Chemical activation often bypasses the initial carbonization step, applying the chemical agent directly to the raw precursor material. The precursor is impregnated with a strong chemical agent, most commonly phosphoric acid (H3PO4) or zinc chloride (ZnCl2). The impregnated material is then heated to a lower temperature range, usually between 450°C and 900°C.
The chemical agent acts as a dehydrating and cross-linking agent within the carbon structure, inhibiting the formation of tar and other unwanted byproducts. Upon heating, the chemical compounds swell the structure and prevent shrinkage, which directs the development of the pores. After heat treatment, the chemical agent is washed out, leaving behind a highly porous carbon structure that favors the formation of medium-sized pores (mesopores).