Zinc is a bluish-white brittle metal, possessing a crystalline structure, and is reported to have been found native in Australia. It melts at 520 C., and has a specific gravity of about 7.15. At a temperature between 100 C and 150 C, it may be rolled out into sheets or drawn into wire, but at 300 C. it reverts to a brittle condition, and may be readily powdered under the hammer. It becomes superficially tarnished in moist air, and is soluble in dilute acids. Zinc is obtained by heating in retorts at a high temperature its roasted or calcined ores in admixture with coal or coke. The zinc oxide is reduced to metal, which, being volatile, distils, and is condensed. Smelting in electric furnaces has been experimented with, but considerable difficulties must yet be overcome before this method can be pronounced a success. Metallic zinc (spelter) is used chiefly for coating (galvanizing) iron, and in the manufacture of the various alloys with copper, known as brass. Zinc oxide and zinc sulphide are both used as pigments, and are sometimes specified in place of white lead; they are less poisonous and retain their color better than white lead, although their covering power is inferior. Metallic zinc is also used for electrical storage in dry batteries. Zinc dust or zinc shavings are used to precipitate gold and silver from its solution in the cyanide process. Zinc chloride is used as a wood preservative.
The chief sources of zinc are sphalerite and smithsonite. The former often exists in vast quantities associated with galena, which is an objectionable constituent from the point of view of the zinc smelter. The metallurgical treatment of this "refractory sulphide ore" has long been a problem, and although great improvements have been effected in mechanically separating the two values by wet dressing, oil flotation, etc., and various chemical and metallurgical processes have been devised, it does not yet appear to have been solved by the introduction of a process of universal application. Principally owing to the improved methods of mechanical separation, large deposits of zinc-lead sulphide ores are being opened up in Siberia, Burma, etc., in addition to those in Australia, the western US and elsewhere.
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