Activated carbon is a generic term for a family of highly porous carbonaceous materials, none of which can be characterized by a structural formula or by chemical analysis. The volume of the pores in activated carbons is generally defined as being greater than 0.2mL/g, and the internal surface area is generally larger than 400m2/g as measured by the nitrogen BET method. The width of the pores varies from 3Å to several thousand angstroms(Å), and the pores are generally classified for convenience in terms of their diameters: Macropores 500 to 20000 Å; Transitional pores (mesopores) 100 to 500 Å; Micropores 8 to 100 Å. In cross-section, the pores appear to be cylindrical or rectangular in shape, but can also occur in a variety of irregular shapes. Pores may also have constrictions or bottlenecks. The category and classification of activated carbon are not unified yet, but there are some names are broadly and commonly adopted: Coal-based activated carbon, wood activated carbon, shell activated carbon, synthetic material based activated carbon. The following photos are several products of activated carbon.
Figure 1. Photos of various AC products
Activated carbon can be manufactured from any carbonaceous raw material, such diverse starting materials as various ranks of coal (peat, lignite, bituminous coal, and anthracite), coconut shells, peach and apricot stones, waste tyres, wood, sawdust, petroleum heavy oil, cellulose, rice husks, corn cobs, sugar, bones, etc.，the starting materials used in relatively large-scale manufacture are readily available and, hence, are reasonably priced. In the Western World, the companies entering the business in the past 30 years have tended to concentrate on using coal (mostly lignite, bituminous coal, and anthracite), whereas, in the developing world, new entrants have concentrated on using coconut-shell charcoal.
Important physical properties are the number and size distribution of the pores, bulk density, dry impact hardness, wet abrasion resistance, and particle-size distribution. From the chemical point of view, comparisons are based on the ability of the different products to adsorb various selected substances from the gaseous phase, e.g. benzene, carbon tetrachloride, and nitrogen, as well as from the aqueous phase, e.g. iodine, molasses, phenol, methylene blue, and tannin.
Since the manufacturing process involves the removal of volatile matter from the raw material, the economic relationship between the price, availability, and quality of the raw material on the one hand, and its volatile content on the other, is important. For example, one of the advantages of bituminous coal and anthracite is their relatively low volatile content and, hence, their high yield of produces.
Although the general outlines of the various manufacturing processes are well known, the exact details of the procedures employed by the manufacturers are jealously guarded secrets. In general, the methods most commonly employed for the manufacture of activated carbon are of two main types: the wet-chemical process (basically a single-stage process), and the physical or thermal-gas process (essentially a two-stage process). As would be expected, chemical methods use chemicals for activation, whereas thermal processes use gases (usually carbon dioxide and air), vapours (steam), or mixtures of gases and vapours. The followings are several classical flow charts for AC manufacture process.
(1)The process schematic diagram for wood powder AC
(2)The process schematic diagram for shell AC
(3)The process schematic diagram for Coal-based AC
Figure2. Schematic diagram of the manufacture process for commonly various raw AC products
