Activation

Activation is the process of processing a material that is intrinsically adsorbent, materials such as zeolite or carbon, to enhance its adsorbent properties. Heat and steam treatment activates carbon by helping to create large numbers of small pores of a selected size, and chemical treatments create the proper electrostatic environment to attract and hold onto specific molecules. The most effective activated carbons receive both steam and chemical treatment, and the millions of pores together give in an enormous internal surface area enabling large quantities of selected molecules to be held. Both the electrostatic and the van der Waals forces are weak enough that the adsorption process is reversible by heating the carbon. This makes the process practical and economical, because the carbon can be reused many times.

We tend to think of activated carbon as being primarily useful for removing the molecules that contribute to unpleasant flavors in beverages, or for removing noxious gases when used in respirators. However, highly colored organic compounds tend to have strong electrical charges and are readily adsorbed by activated carbon, making it useful for removing both unpleasant flavors and unwanted colors. One of the first uses of activated carbon was to decolorize the raw product of the sugar industry and make it look more palatable.

Most activated carbon is prepared from plant or animal material, treated to remove everything but the carbon in its structure. Heating these materials in the absence of air converts them to charcoal, which retains much of the porous structure of the biological cells. Unfortunately, it also retains all the salts and many other non-volatile compounds that were present in the original material. The activating process removes many of these compounds, creating a purer and more effective carbon for adsorption, but also leaves behind other salts and residues.

Activated charcoal is prepared in three main forms:

• As granules - irregular shaped particles with sizes ranging from 0.2 to 5mm. This type is used for treating both liquids and gases.

• As powder - pulverized carbon with a size predominantly less than 0.18mm (US Mesh 80). This type is mainly used for treating liquids and for flue gas treatment.

• As pellets, or "prills" - extruded cylindrical shapes with diameters from 0.8 to 5mm. This type is mainly used for gases (but can be used for liquids too) because they offer least resistance to gas flow, have high mechanical strength, and low dust content.

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