Carbides

Carbides, compounds of carbon with metals or metalloids, formed by heating carbon with the corresponding oxide in an electric furnace. Examples include calcium carbide CaC₂, aluminum carbide Al₄C₃, and carborundum CSi. Most carbides decompose in water to form hydrocarbons (CaC₂ produces acetylene in this reaction). The greatest technical application of carbides is in the fixation of atmospheric nitrogen: when heated in a nitrogen atmosphere, CaC₂ converts to calcium cyanamide CaCN₂ (CaC₂ + N₂ = CaCN₂ + C), which serves as a material for the production of nitric acid and cyanide salts and is used as a fertilizer. Calcium carbide (CaC₂) is a gray or grayish-black non-combustible substance with a crystalline structure; it is obtained by fusing lime and coal in an electric furnace at temperatures above 3,000°. Calcium carbide is most unstable in water, where it forms acetylene and slaked lime with the release of a large amount of heat. The hot calcium carbide produced in factories is packed in hermetically sealed (to prevent moisture from entering) iron drums. Acetylene is obtained from calcium carbide in special apparatus by the action of water. Workers handling calcium carbide in an industrial setting may develop skin lesions, which manifest in two forms: dermatitis and localized ulcers (necrosis). Dermatitis develops mainly when working with dry, solid calcium carbide; if water gets on the hands in this case, acetylene and slaked lime are formed with the release of heat, which causes the formation of ulcers. The latter, in appearance and pathogenesis, resemble ulcers caused by caustic lime or fertilizers containing calcium cyanamide. Prevention. Prevention of dust formation and protection of hands, which should be smeared with vaseline before putting on gloves and then dusted with talc. For more on prevention, see below calcium-nitrogen.