Page:The American Cyclopædia (1879) Volume XVI.djvu/850

 820 ZINC the smokeless combustion of anthracite afford the necessary conditions, was successfully car- ried on by both the New Jersey and Lehigh companies, according to a plan invented by Mr. Wetherill. Spelter ingots and rolled sheet zinc are now manufactured in the United States by a considerable number of establish- ments, enumerated below. The chief deposits of zinc ores in Europe are in Silesia and Carin- thia, and near Li6ge in Belgium, particularly the mine of Vieille-Montagne or Altenberg, between Li6ge and Aix-la-Ohapelle. Spain, Sweden, Great Britain, and other countries also furnish either calamine or blende. It is said that Swedish ores are now taken to Vieille- Montagne, the deposit there being partially exhausted. The chief deposits in the United States are those of calamine and blende at Friedensville, Lehigh co., Pa., in the lower Silurian magnesian limestone, the red oxide deposits of New Jersey in the same rock, the calamiue deposits near Knoxville, Tenn., and in Arkansas, the calamine and blende depos- its of the Silurian rocks of S. W. Wisconsin, and the subcarboniferous rocks of S. W. Mis- souri. Metallurgy. There are four principal ores of zinc, viz. : 1, the carbonate (smithson- ite, formerly called calamine), containing when pure 52 per cent, of zinc, white or pale yellow or green, vitreous to resinous in lustre, of sp. gr. 4'2, hardness 4'5, crystals rhombohedral ; 2, the silicate, which is either anhydrous (wil- lemite), containing 58 per cent, zinc, white, gray, or pale yellow, of vitreous lustre, sp. gr. 4, hardness 5'5, crystals hexagonal ; or hy- drous (calamine or electric calamine), white or pale yellow, of vitreous lustre, sp. gr. 3 - 16 to 8'9, hardness 4'75, crystals hemihedral ; 3, the sulphide (sphalerite, zinc blende), 66 - 9 per cent, zinc, yellow, red, brown, or black, rarely green or white, of adamantine lustre, sp. gr. 4, hardness 4, brittle, crystals isometric, contain- ing as accidental constituents iron, cadmium, copper, manganese, antimony, and lead, in traces ; 4, the oxide (zincite, red zinc ore), 80 - 2 per cent, zinc, deep red to orange yellow, of subadamantine lustre, sp. gr. 5*5, hardness 4, brittle, crystals hexagonal, occurring exclu- sively in the neighborhood of Franklin, N. J., and usually mixed with willemite, franklinite, and magnetite. To these natural ores may be added artificial zinciferous by-products, such as furnace calamine, zinc crusts from desilver- izing processes, &c. The manufacture of zinc is essentially a reduction, by means of carbon, of the zincic oxide, formed by a preliminary calcination of the ore. Since the temperature at which the oxide is reduced is about 1300 0. (2372 F.), or near the fusing point of cast iron, while the zinc melts at 412 0. and boils at 1200 C., the metal when reduced is a vapor, which by cooling is condensed into a liquid. Hence the distillation is performed in retort furnaces, which must be of the best fire-proof material. The price of fuel, its nature (giving long or short flame), the price and quality of clay, and the nature and purity of the ore, control the choice of methods and apparatus from among the three chief forms about to be described. A special difficulty in the manu- facture arises from the property of the metal at nearly the temperature of its reduction to become oxidized again in the presence of car- bonic acid, the formation of which cannot be entirely avoided. By heating zincic oxide and carbon, zinc and carbonic oxide are formed, the latter of which, acting upon another portion of zincic oxide, reduces it to metal and itself becomes carbonic acid. At sufficiently high temperature, this carbonic acid is again re- duced to oxide by another portion of the car- bon ; but if the temperature falls to a certain degree below white heat, the carbonic acid is reduced by the metallic zinc vapors instead, oxidizing these in turn to zincic oxide. Hence the retorts should not be allowed to cool du- ring distillation. In order that the zinc vapors passing out of the retorts into the nozzles or condensers maybe as little as possible oxidized by air and carbonic acid there present, the lat- ter vessels must be kept so cool (say 400 C= 752 F.) that the zinc may at once become liquid; but in spite of this, a portion is oxi- dized, causing loss and extra expense in the repeated reduction of the oxide. Too much cooling of the nozzles, or too slow generation of the zinc vapors, causes the formation, in- stead of liquid zinc, of a zinc powder, which must be transformed into bars at further ex- pense and loss. Metals more volatile than zinc, particularly cadmium and lead, pass over chiefly at the commencement of the process, and are caught in vessels beyond the nozzles. Aqueous vapor oxidizes energetically the vapors of zinc. The operations preliminary to distil- lation are calcination, crushing, and the prepa- ration of the charge. Calcination is applied to the carbonate and silicate ores to expel car- bonic acid and water, and to facilitate by the disintegration of the ore the subsequent action of carbonic oxide gas. For this purpose kilns may be used, in which the ore (in fragments) is either played upon by flame, or is charged in alternate layers with the fuel. The latter method saves fuel and gives a more uniform result, but may involve a reduction and vola- tilization of zinc when the fuel is in excess, and also adds to the ore the impurities con- tained in the cinder and ashes of the fuel. For finely crushed ore reverberatories are employed. The practice, formerly common, of using the waste gases from the distilling furnaces to heat the calcining furnaces, or even, as in the old Silesian furnaces, of building chambers for calcination in the same furnace, in another part of which distillation was conducted, is now pretty generally discarded, as interfering with the proper control of the latter and more delicate operation. The roasting of zinc blende has for its special object the removal of sulphur and the formation of zincio oxide. Owing to its density, this ore must in general be reduced