A carbonaceous substance can be used as the raw material for activated carbon.
Kuraray Chemical selects materials considering the difficulty in obtaining the material, amount of material required, price, reactivity with gas or chemicals, and appropriateness of quality for the products.
Materials for activated carbon in use worldwide are as follows:
| Powdered activated carbon |
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|---|---|
| Granulated activated carbon |
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| Fibrous activated carbon |
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Coconut shell
In addition to the more common raw materials discussed earlier, others can include waste tires, phenol formaldehyde resin, rice husks, pulp mill residues, corn cobs, coffee beans and bones.
Most of the developed nations have facilities to activate coconut shell, wood and coal. Third world countries have recently entered the industry and concentrate on readily available local raw materials such as wood and coconut shell. Coconut shell contains about 75% volatile matter that is removed, largely at source by partial carbonization, to minimize shipping costs. When producing coconut shell activated carbon from coconuts, only the shell (see fig.) is used and 50000 coconuts are needed to produce 1 ton of activated carbon.
The cellulosic structure of the shell determines the end product characteristics, which (at 30-40% yield on the carbonized basis) is a material of very high internal surface area consisting of pores and capillaries of fine molecular dimensions.
The ash content is normally low and composed mainly of alkalis and silica. Coal is also a readily available and reasonably cheap raw material. The activate obtained depends on the type of coal used and its initial processing prior to carbonization and activation.
It is normal procedure to grind the coal and reconstitute it into a form suitable for processing, by use of a binder such as pitch, before activation. (This is typical for extruded or pelletized carbon). An alternative method is to grind the coal and utilize its volatile content to fuse the powder together in the form of a briquette.
This method allows for blending of selected materials to control the swelling power of the coals and prevents coking. If the coal is allowed to “coke” it leads to the production of an activate with an unacceptably high proportion of large pores.
Blending of coals also allows a greater degree of control over the structure and properties of the final product. Wood may be activated by one of two methods, i.e. steam or chemical activation, depending on the desired product. A common chemical activator is phosphoric acid, which produces a char with a large surface area suitable for decolorization applications.
The carbon is usually supplied as a finely divided powder which since produced from waste materials such as sawdust, is relatively cheap and can be used on a “throw-away” basis. Since activated carbon is manufactured from naturally occurring raw materials, its properties will obviously be variable. In order to minimize variability it is necessary to be very selective in raw material source and quality and practice a high level of manufacturing quality control.
