Preparing activated carbon primarily involves transforming carbon-rich materials through a two-step process: carbonization and activation.
Basically, there are two main steps in the preparation and manufacture of activated carbon: the carbonization of the carbonaceous raw material in the absence of oxygen to break down the cross-linkage between carbon atoms, followed by the activation of the carbonized product, known as char, for further pore development.
The Two Main Steps in Activated Carbon Preparation
The journey from raw carbonaceous material to highly porous activated carbon involves carefully controlled thermal and chemical treatments.
Step 1: Carbonization
The initial phase is carbonization. This is a fundamental step where the raw material, such as wood, coconut shells, peat, or coal, is heated to high temperatures in an inert atmosphere, meaning without oxygen.
- Process: The material is subjected to temperatures typically ranging from 400°C to 900°C (750°F to 1650°F) in environments devoid of oxygen, often using gases like nitrogen.
- Purpose: This thermal treatment drives off volatile organic compounds, breaks down the complex cross-linkages within the carbon structure, and significantly increases the carbon content of the material. The result is a solid carbon residue called char.
- Key outcome: Formation of a basic carbon skeleton with rudimentary pore structures.
Step 2: Activation
Following carbonization, the resulting char undergoes the activation process. This step is crucial for developing the extensive network of pores that gives activated carbon its massive surface area and adsorption capabilities.
Activation can be achieved through physical or chemical methods:
- Physical Activation:
- The char is exposed to oxidizing gases like steam, carbon dioxide, or air at elevated temperatures (often 800°C to 1100°C or 1470°F to 2000°F).
- These gases selectively react with the carbon surface, etching away parts of the carbon structure and widening the pores that were initiated during carbonization.
- This method is essentially controlled burning.
- Chemical Activation:
- The raw material or char is impregnated with activating agents such as phosphoric acid (H₃PO₄), potassium hydroxide (KOH), zinc chloride (ZnCl₂), or sulfuric acid (H₂SO₄).
- The impregnated material is then heated, usually at lower temperatures than physical activation (often 400°C to 800°C or 750°F to 1470°F).
- The activating agent acts as a dehydrating agent and catalyst, promoting the breakdown of organic material and the formation of pores during heating. After activation, the chemical agent is washed away.
- This method can sometimes combine carbonization and activation into a single thermal step.
Both activation methods lead to a significant increase in the internal surface area and pore volume of the carbon, making it highly effective for adsorption applications.
In summary, preparing activated carbon is a two-step process transforming raw materials into a porous adsorbent through controlled heating and activation treatments.
