Friday, March 5, 2010

Calcium Carbide

The first production of calcium carbide was an accident. In 1892 T.L. Willson was attempting to prepare metallic calcium from lime and tar in an electric furnace as Spray N.C. The product obtained obviously not calcium, was thrown into a nearby stream, and Willson was amazed to note that it liberated great quantities of combustion gas. The first factory for the production of calcium carbide was built at Nigeria Falls in 1896.

Uses And Economics
Calcium carbide is utilized for the manufacture of Cyanamid by combining it with nitrogen, and for the preparation of acetylene of by reacting with water. Cyanamid is made by heating calcium carbide in an atmosphere of nitrogen.

Calcium carbide was used formerly in large quantities for the manufacture of acetylene, but hydrocarbon based acetylene is some much more common. US production of calcium carbide has fallen from 1 x 106 t in 1940s to 242,000 t in 1979.

Manufacture:
Calcium carbide is prepared from quicklime and carbon at 2000 to 2200oC.

CaO (c) + 3 C (amorph) → Ca C2 (l) + CO (g)

The source of carbon is usually coke, anthracite, or petroleum coke. Coke is the most widely used. It should be compact and have a low ash content, a low ignition point, and high electrical resistivity, so that the bulk of the furnace charge will be highly resistant to the flow of energy. Thus the energy is concentrated, resulting in a more rapid and complete reaction. Phosphorus should be absent, since it forms a phosphide which is converted to poisonous phosphine (PH3) when the carbide is made into acetylene. The quicklime is produced by burning limestone containing at least 97% calcium carbonate. Impurities such as magnesia, silica, and lime hamper production and give a less pure carbide.

The carbide furnace is not a true are resistance furnace, but has been developed from the familiar are furnace. Ingot furnaces, similar to those producing fused aluminum oxide, have during molten carbide. The furnace consists of a steel with the side walls lined with extremely hot, alkaline conditions. Most of the large furnaces use three phase electric current and have three vertical electrodes suspended in the shell. Improvements include and utilized and Soderberg continuous self baking electrodes, which permit larger capacity furnaces than the old prebaked electrodes. The capacity range of the furnaces is generally between 18 to 65 MJ or higher and a three phase tapping furnace of 90 MJ produces about 180 t of commercial product (usually 85% carbide) per day. The approximate consumption of materials per ton of carbide is 860 kg lime, 590 kg coke, 16 kg electrode paste, and 108 MJ energy.

The lime and coke are charged continuously with intermittent or continuous tapping of the liquid product directly into cast iron chill pts of about 5 t capacity each. The carbide is cooled, crushed and sized then packed in 4 -5 to 100 kg steel drums or up to 5 t container for shipping.

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