Preparation and Regeneration of Activated Carbon

The crystals of diamond or graphite or as fullerene spheres are generally available forms of Pure carbon. The atoms in graphite are arranged as sheets of hexagons and the atoms in diamond are arranged as tetrahedra. Activated carbon grains consist of random arrays of microcrystalline graphite. Such grains can be made from many different organic substances, but various grades of coal are most commonly used, because the product is hard, dense, and easy to handle.

The raw material is first carbonized at about 500°C in a nonoxidizing atmosphere. This produces a char that contains some residual organic matter and many small graphite crystals. The char is then subjected to a slow oxidation in air at about 500°C or steam or carbon dioxide at 800 to 950°C. The preferred activation atmospheres are steam and carbon dioxide, because the oxidation reactions are endothermic and more easily controlled. The slow oxidation removes residual organic matter and small graphite crystals and produces a network of microscopic pores. The higher temperature promotes the formation of larger graphite crystals, which reduces the randomness of their arrangement.

It is almost always economical to recover and regenerate spent carbon, unless the quantities are very small. This is true even of powdered carbon, if it can be easily separated from the process stream. The economic break point for on-site regeneration depends on carbon usage and varies somewhere between 500 and 2000 lb/day (Snoeyink, 1990). Larger amounts should be shipped to commercial regeneration plants.

Spent carbon can be regenerated as it was made by heating in rotary kiln, fluidized bed, multiple hearth, or infrared furnaces (von Dreusche, 1981; McGinnis, 1981). In multiple hearth furnaces, the required heat is supplied by the hot combustion gases of a fuel. The fuel is oxidized at only small excess air to minimize the amount of oxygen fed to the furnace. In infrared furnaces, heating is supplied by electrical heating elements. This method of heating provides somewhat better control of oxygen concentration in the furnace, but some oxygen enters with the carbon through the feedlock.

The usual regeneration stages in any furnace are as follows (McGinnis, 1981; Snoeyink, 1990):

• Drying stage — usually operated at about 200 to 700°C, depending on furnace type

• Pyrolysis stage — usually operated at about 500 to 800°C

• Coke oxidation stage — usually operated at about 700°C for water treatment carbon and at about
900°C for wastewater treatment carbons (Culp and Clark, 1983)

The pyrolysis stage vaporizes and cokes the adsorbed organics. The vaporized material diffuses into the hot gases in the furnace and either reacts with them or is discharged as part of the furnace stack gas. The coked material is oxidized by water vapor or by carbon dioxide, and the oxidation products exit in the furnace stack gases:

For water treatment carbons, the pyrolysis and activation stages are operated at temperatures below those required to graphitize the adsorbate char. Consequently, the newly formed char is more reactive than the original, largely graphitized activated carbon, and it is selectively removed by the oxidation process. Wastewater treatment carbons are oxidized at temperatures that graphitize as well as oxidize the adsorbate char. Significant amounts of the original carbon are also oxidized at this temperature.

Regeneration losses are generally on the order of 5 to 10% by weight. This is due mostly to spillage and other handling and transport losses, not to furnace oxidation. There is usually a reduction in grain size, which affects the hydraulics of fixed bed adsorbers and the adsorption rates. However, the adsorptive capacity of the original carbon is little changed (McGinnis, 1981).