Activated carbon, often referred to as activated charcoal, is a highly porous form of carbon widely used for its ability to capture contaminants. It functions by attracting and holding molecules onto its vast internal surface, a process known as adsorption. This material’s unique structure allows it to purify liquids and gases in various applications, ranging from water filtration to air purification. Understanding its production reveals how this common substance gains its remarkable purifying capabilities.
Choosing Raw Materials
Activated carbon production begins with selecting carbon-rich source materials. Materials chosen possess high carbon and low ash content, with suitable pore structure potential. Common raw materials include wood (for larger pore sizes) and coconut shells (for micropores and high carbon content). Coals (lignite, bituminous, anthracite), peat, and agricultural byproducts like fruit pits and nut shells are also used. The specific raw material influences the final activated carbon’s properties, including its pore structure and adsorptive capacity.
Pre-Activation Carbonization
After selection, raw material undergoes pre-activation carbonization, or pyrolysis. This involves heating the material in a low-oxygen environment, typically 400°C to 800°C. Volatile components like moisture, tars, and gases are driven off. The process transforms organic material into a carbon-rich char, serving as the foundation for subsequent activation. This char, though carbonized, lacks the extensive pore network of activated carbon; its purpose is to prepare the material for activation.
Activating the Carbon
Activation is the core step, dramatically increasing the material’s internal pore structure and surface area. This can be achieved through two primary methods: physical activation or chemical activation. Each method develops the porous network, but they differ in process and resulting pore characteristics.
Physical Activation
Physical activation heats carbonized char to 800°C-1100°C with an oxidizing gas. Steam, carbon dioxide, or a mixture of both are commonly used. These gases react with the carbon structure, enlarging existing pores and creating new ones. Steam activation allows precise pore size control, while carbon dioxide activation yields higher microporosity. This method effectively etches the carbon surface, leading to a highly developed porous network.
Chemical Activation
Chemical activation involves impregnating raw material or char with a chemical activating agent. Common agents include phosphoric acid, zinc chloride, or potassium hydroxide. This impregnated material is then heated to lower temperatures, typically 400°C to 700°C. Chemical agents promote dehydration, inhibiting tar formation and other byproducts during carbonization. They also swell the material and erode its fibers, creating a highly porous structure after heating and washing.
Final Processing
After activation, activated carbon undergoes finishing steps for commercial use. Chemically activated carbon requires thorough washing, typically involving multiple stages and filtration, to remove residual agents and ensure purity. After washing, both types of activated carbon are dried to remove excess moisture. The material is then crushed, ground, and sized, preparing it into various forms (granular, powdered, or pelletized) for specific applications. These final processing steps ensure the activated carbon is ready for its diverse roles in purification and adsorption.