ACTIVATED CARBON {GRANULAR AND POWDER}

Carbon is probably the most widely distributed element in nature. It occurs in two allotropic crystalline forms viz. graphite (hexagonal system) and diamond (isomeric system), the former is soft and black while diamond is hard and transparent. Charcoal, coke and carbon black, classified as emorphous carbon; are considered by some to represent a third allotropic form. They are said to be composed of very minute crystals of graphite by others. Carbon is an essential constituent of all vegetable and animal matter in which it occurs in combination with hydrogen, nitrogen, oxygen and other elements in immense variety of compounds. In combination with hydrogen it occurs as hydrocarbons in petroleum. It is also found in carbon dioxide in air (0.03% as sodium bicarbonate in sea water, and as calcium and magnesium carbonate in sedimentary rocks such as chalk and dolomite.

Many carbons of industrial value are prepared from coal and from organic vegetable and animal matter. The resulting amorphous products include charcoal, coke and petroleum coke. Several carbon products are prepared and used in the electrical and electro-chemical industry.

Carbon 13, a stable isotope of carbon (At wt 13) has recently come into prominence as a tracer element employed in the study of biological process. It is obtained from carbon compounds by concentrating the minute quantity of the heavier isotope, normally present in them, by thermal diffusion methods. Carbon 14 or radioactive carbon, a product of the uranium atomic pile, is used also as a tracer element in the study of plant and animal metabolism.

The term Activated carbon, active carbon, or active charcoal is usually applied to amorphous carbons possessing higher absorption capacities than wood or animal charcoal. Many processes were developed during world war for the production of effective absorbents for use in gas masks. Industrial activated carbons in the form of pellets, granules or fine powders, and with many industrial applications, are now available in the market under different trade names.

Commercial absorbent carbons may be grouped into decolorizing, gas absorbant, metal absorbant, and medicinal carbons according to their physical structure, properties, and applications. No one type of carbon can be used for all purposes. A large variety of raw materials are available for the manufacture of these products. Coal, petroleum coke, and wood charcoal are activated by gas activation. Industrial waste e.g. raw dust, bagasse, molasses, straw, coconut pericarp and shell, corn cobs, paddy and ground nut husk, corn bean shell, distillery slop, waste Mahua flowers, waste wood pulp laquor, and mud from sugar factories have been utilized for the production of active carbons by chemical activation.

DECOLORISING CARBONS

Decolourizing carbons are manufactured by gas activation, in which the raw materials are first carbonized and the resulting charcoal heated to a high temperature in an oxidizing atmosphere Chemical activation in which the raw materials are impregnated with a chemical extruded and carbonized and deposition of carbon on porous inorganic base activation is needed in this case.

In the gas activation process, the raw material is carbonized under controlled conditions in closed retorts, the resulting charcoal is crushed, screened and heated in a second retort at 1000oC for 10 to 12 hours in an atmosphere of air, carbon dioxide, chlorine, super heated steam or a mixture of steam and air. Raw materials which do not possess the necessary density and structure for direct conversion are briquetted prior to carbonization. Prebriquetting gives a higher yield and a better product. In the process for the direct conversion of coal to activated carbon the crushed-materials, screened to 11/8 in pieces, is carbonized at 450o – 500oC and steam activated at 950o in continuous vertical retorts, the yield is about 12 1/2% of the coal taken.

For activation by chemical treatment, the raw material is ground and formed into paste with chemicals, e.g. chloride of zinc, calcium and magnesium, alkalies, sulphuric acid, phosphoric acid, sodium, silicate, boric acid, potassium sulphide, lime, ferric chloride, or potassium thiocynate. The paste is extruded under pressure dried and carbonized in gas retorts at about 1000oC. The charcoal is cooled, washed with hydrochloric acid and water to remove inorganic residues, and finally dried at about 300oC. A fluidized technique has been developed in France for the production of activated carbon.

The third process gives a product with a porous structure and appreciable mechanical strength. The raw materials viz. saw dust, sea weed, peat molasses, etc. is mixed with a insoluble salts and the mixture is strongly heated. The carbon gets deposited on the porous inorganic base. A similar product is obtained when a high ash vegetable product, such as paddy husk, containing an appreciable percentage of silica, is carbonized.

GAS ABSORBENT CARBONS

Gas and vapour absorbent carbons are obtained by carbonizing coconut shells, apricot stones, vegetable ivory, and anthracit. In recent years, methods have been developed for using softer materials which are rendered hard and dense by briquetting. The carbons are gas activated. A preparation useful for gas masks has been obtained by chlorinate bituminous coal (6-20) mesh until a 100% increased in weight takes place, pelleting and chlorinated material with hydrolyzed starch as binder, baking, crushed and powdered to (8-20) mesh, and steam activated at 800oC. Gas absorbent carbons are available in granular form of specified mesh range, e.g. 4 x 6, 412, 20, i.e. retained on 6- and 20 mesh sieve and passing through 4.12 mesh sieve.

METAL ABSORBENT CHARS

Metal absorbent chars are prepared by alkali activation. Structurally, they are identical with decolorizing carbons and are converted into the lather by acid treatment. An active product is obtained by heating bone charcoal with alkali at 850oC. The product obtained is negative changed material and important of its metal absorbing power. It however, possesses the properties of a decolorizing char. Treatment of flocculated material with alkali does not restore the metal absorbent properly.

MEDICINAL CARBONS

Activated carbon finds application in the preparation of pills and digestive tablets. Its absorptive properties are utilized in the treatment of the stomach due to hyper acidity. It removes toxic amines, organic acids of decomposed foods, and probably also bacteria from the intestinal tract and many other purposes.