METALLURGY OF ZINC. Zinc may be extracted from ores and furnace products by (1) the dry process, or (2) the electro-metallurgical pr9cess. :Metallic zinc can only be obtained in the dry way and by electro-metallurgical processes. As zinc cannot be separated from its solutions by any of the metals that resist ordinary tempera tures, the wet method only admits of the produc tion of compounds of zinc from which the metal mnst be extracted in the dry way. The wet process can, therefore, be considered only as an accessory process to the dry method of zinc ex traction. The extraction of zinc from ores and furnace products in the dry way is performed by converting the zinc into an oxide and then re ducing the oxide to a metal by means of carbon. Vurnaee products which consist of mixtures of zinc oxide and metal are submitted to direct reduction. From alloys with metals less volatile than zinc the latter is obtained by simple dis tillation. The extraction of zinc by the com bined wet. and dry method is performed by dis solving out the zinc, converting the zinc in solu tion into an oxide, and then reducing the latter by means of carbon. The electrolytic extraction of zinc: is performed by obtaining zinc in the form of aqueous solutions, from which zinc is separated by means of the .leetric current. If the zinc exists in the form of an alloy this alloy may be used as the anode in the electric current. Of the three methods, the dry method is given the preference whenever the ores or furnace products are suf ficiently rich in zinc. In spite of its great de ficiencies, no success has attended attempts to replace the dry process by either of the others. The combined wet and dry method has been tried experimentally on poor ores, hut hue hitherto failed on account of a number of defects and of its expense. it has only been employed in cases in which the object was not the extraction of metallic zinc, but the separation of zinc from other metals or metallic compounds as a by product in the form of commercial compounds such as sulphate, chloride, basic carbonate, or oxide of zinc. Good authorities, however, think it not impossible that it may find employment in zinc extraction proper as all auxiliary to the dry method for the prepa ration of compounds rich in zinc from ores or products poor in that metal. The electro metallurgical method has, up to the present, been used only on an experimental scale and with varying results upon zinc propel., containing no other valuable meal except zinc. It can be used with advantage for alloys which form anodes soluble in the electric circuit. Finally, it remains to be noted that the zinc obtained by either the dry process or the combined wet and dry process, is in most cases contaminated by impurities and has to be subjected to a refining process previous to use.
From the preceding general discussion it will be evident that the dry process of zinc reduction is the only one which is much used in the pro duction of this useful metal, and is, therefore, the only one which needs particular mention. Briefly summarized, this process consists of two separate operations: (1) the preparation of the ores for the process of reduction by calcination or roasting; (2) the reduction process proper, or the extraction of the zinc from the calcined ores. The preliminary operation comprises first the crushing of the ore, and, second, its burning or calcining in open heaps or in furnaces to change it into oxidized form. The reduction of the zinc from the calcined ores is performed by heating the latter with carbon to a white heat in fire-clay vessels. Zinc is then separated in the form of vapor and is collected in clay vessels as receiv ers or adaptors. Two forms of reduction fur naces are now used which are known as the Bel gian and the Silesian. The Belgian furnace con sists of from GO to SO small fire-clay retorts, each about 3 feet 6 inches long, by S inches in diameter, and set in a series of rectangular com partments, filling up all arched chamber. There is a clay nozzle or condenser attached to the front of each retort, and on the end of this nozzle there is a sheet-iron receiver for the con densed zinc, with a pit to collect the residue f the retorts. These are charged with ground and roasted ore. mixed with small coal free from sulphur. As the upper retorts receive less heat than the louver. they are not so heavily charged.
and they are, moreover. supplied with less pure ores. .1t the end of every six hours the receivers are emptied of their melted zinc.
In the Silesian furnact. fire-clay retorts, about 4 feet long by 1 foot 6 inches in diameter, are arranged in two rows, back to hack, and placed horizontally on a flat furnace bed, with a fire place on a lower level running along between the backs of the retorts. A condensing appa ratus comes away with e curve from the upper part of the front of each retort, and descends some two feet below it. From this, the zinc, on C01111vnsing, drops on the ground, or into a tray placed to receive it.
Zinc is applied with great success to the coat ing of sheet iron fur roofing and other purposes, and also for coating various kinds of iron wire, especially telegraph wire. See GALVANIZED IRON.