zinc

The Columbia Encyclopedia, 6th ed.

zinc

zinc, metallic chemical element; symbol Zn; at. no. 30; at. wt. 65.38; m.p. 419.58°C; b.p. 907°C; sp. gr. 7.133 at 25°C; valence +2. Zinc is a lustrous bluish-white metal. It is found in Group 12 of the periodic table. It is brittle and crystalline at ordinary temperatures, but when heated to between 110°C and 150°C it becomes ductile and malleable; it can then be rolled into sheets. It is a fairly reactive metal.

Although zinc is not abundant in nature, it is of great commercial importance. It is used principally for galvanizing iron, but is also important in the preparation of certain alloys, e.g., Babbitt metal, brass, German silver, and sometimes bronze. It is used for the negative plates in certain electric batteries and for roofing and gutters in building construction. Since the metal reacts with dilute mineral acid to liberate hydrogen, it is often used for this purpose in the laboratory.

Zinc compounds are numerous and are widely used. Perhaps most important is zinc oxide, or zinc white, a versatile compound with many uses. Other zinc compounds include zinc chloride, used as a wood preservative, in soldering fluxes, as a mordant in dyeing textiles, and in adhesives and cements; and zinc sulfide, used in making lithopone as well as television screens and X-ray apparatus. The chromate, zinc yellow, serves as a pigment; sodium zincate, as a water softener and as a flocculating agent in water purification. The crystalline sulfate is known commonly as white vitriol.

Zinc is essential to the growth of many kinds of organisms, both plant and animal. It is a constituent of insulin, which is used in the treatment of diabetes. Zinc supplements, taken at the first appearance of symptoms, may reduce the severity of the common cold. Research suggests that a zinc deficiency can lead to excessive inflammation when the immune system responds to infection, and that zinc acts to slow the immune response, limiting inflammation.

Chief sources of zinc are the sulfide ore, zinc blende, or sphalerite (called also blende or "black Jack" ); zincite, an oxide; calamine, a silicate; and smithsonite, the zinc carbonate. Zinc ores are widely and abundantly distributed throughout the world. The United States is the leading producer. The metallurgy of zinc depends upon the ore used. The sulfide ore is roasted to the oxide, then mixed with coal and heated to 1,200°C. The zinc vaporizes and is condensed outside the reaction chamber and cast into blocks called spelter. In another method the ore is processed by flotation, filtering, roasting, and leaching; the resulting solution is filtered and the zinc removed by electrolysis.

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