A wide range of organic products is suitable as feedstocks for the manufacture of activated carbon. Wood, sawdust, peat, coconut shells and even olive stones are the preferred uncarbonized feedstocks. Of the (already) carbonized feedstocks coal, low temperature lignite coke, charcoal and coke from acid sludges (e.g. from the manufacture of lubricants) are utilized. The properties of activated carbon are very much influenced by the type of feedstock utilized. There are two fundamentally different processes for converting these feedstocks into activated carbon: chemical activation and gas activation.

Chemical activation is generally carried out with uncarbonized feedstocks, gas activation generally with carbonized feedstocks. The aim of both processes is to convert the particular feedstock into a material with a high specific surface area (BET values between 400 and 2500 m²/g) and the optimum pore size distribution for the
required application. There are three types of pores:
(1)micropores with diameters of 0.4 to 2nm,
(2)mesopores with diamters of 2 to 50nm,
(3)macropores with diameters greater than 50nm.

Chemical Activation

The preferred feedstock for “chemical activation” (see Fig. 5.7-1 ) is wood, in the form of sawdust. These processes are based on the dehydration of the feedstock by reaction with dehydration agents and are particularly effective with cellulose-containing materials. Chemicals such as phosphoric acid or zinc chloride are utilized. Sawdust is mixed with phosphoric acid or zinc chloride, optionally formed, and then ed to 400 to 600°C when phosphoric acid is utilized or to 600 to 700°C when zinc chloride is utilized.

Processing of materials activated using this process is relatively difficult and complex, since on economic and ecological grounds most of the chemicals have to be recovered. They have to be repeatedly washed to obtain an activated carbon which is largely free of activation agent.

Powdered activated carbon is usually produced using these processes, but it is also possible to produce formed carbon by forming (granulating) mixtures of sawdust or peat and activation agent.

Gas Activation

"Gas activation" (see Figs 5.7-1 and 5.7-2) means the formation of pores in a carbon matrix by the removal of carbon by oxidation with steam or carbon dioxide or their
mixtures. Whereas oxidation with oxygen proceeds too rapidly, reaction with steam or carbon dioxide proceed at a moderate rate. Since these reactions are significantly
endothermic, oxygen is added to the H₂O/CO₂-mixtures, to ensure by exothermic combustion of hydrogen and CO that the reaction temperature does not drop below 800°C.

Activation with steam and carbon dioxide proceeds at 800 to 1000°C.

In the gas activation of uncarbonized feedstocks, a low temperature carbonization process, leading to carbonization, has to be carried out before activation. This low temperature carbonization process has a considerable influence on the pore distribution in the final product.

Coal, which contains a high fraction of volatile components, has to be preoxidized and then carbonized at low temperatures, whereas anthracite can in principle be activated directly. In practice, however, pulverization, briquetting with the aid of a binder, briquette communition and low temperature carbonization are generally carried out. This makes subsequent activation much easier. Lignite low temperature coke can be directly activated.

If formed carbon is manufactured from low temperature lignite coke, a low temperature carbonization of the binder (pitch, organic resin), which consolidates the ground coke, has to be carried out. In this way the binder is also carbonized (see Section 5.7-3).

The ash content of materials such as coal or lignite can, for example, be reduced by acid treatment (with hydrochloric acid, nitric acid) either before or after activation.

The methods used by different companies for the manufacture of activated carbon are generally regarded as company know-how, so that nothing is known of the actual units utilized for the manufacture of particular activated carbon types. In practice, units typically used for high temperature reactions, such as rotary kilns, multiple hearth furnaces, fluidized bed reactors, shaft furnaces etc., are used. Activation gas and feedstock can be fed in in cocurrent or countercurrent. Heating can proceed directly or indirectly. In direct heating, the gas formed upon activation (hydrogen, carbon monoxide) is immediately burnt in the reactor. There is also a process in which the gas formed is utilized for the indirect heating of a fluidized bed reactor.

The residence time in the reactors depends upon the particular process conditions used and has a considerable influence upon yield, pore size distribution and BET-surface area of the activated carbon produced.