ZINC 785 defined, though small, rhombohedra. The ordinary ore is massive, and is contaminated, often largely, with clay, silica, oxide of iron, and the like. Sp. gr. 4 to 4 5. Sometimes colourless, but as a rule light grey, yellow, or buff-coloured. Lustre, vitreous. It is found in association with silicates of zinc, zinc-blende, and lead ores, chiefly in limestone and dolomitic strata, at the Kelmisberg or Vieille Montagne in Belgium, in Derbyshire and Northumber land, and in Silesia ; but these last deposits are well nigh exhausted. Irregular deposits occur near Santander and Cartagena in Spain, and in Sardinia. At Wiesloch in Baden a yellow variety is found, which contains as much as 3 per cent, of cadmium. Smaller per centages of cadmium are met with in many other zinc ores. (4) Electric Calamine, the German Kieselzinkerz (ZnOSi0 2 + H-/)), is also called hemimorphite on account of the marked hemimorphism in its (ortho-rhombic) crystals. Sp. gr. 3 35 to 3-5. Sometimes colourless, but more frequently grey, yellow, red, green, brown, blue, always, however, light-coloured. Lustre, vitreous. The crystals, when heated, exhibit electric polarity ; hence the name. As a rule they are small and united into cuneiform, spheroidal, or kidney-shaped masses ; there are also granular, dense, and earthy varieties. It occurs with willemite and calamiue at the Altenberg near Aix-la-Chapelle, with blende and lead ore at Raibel and Blei- berg in Carinthia, near Iserlohn in Westphalia, at Matlock in Derbyshire, near Tarnowitz in Silesia, at Olbucs, Rezbanya in Hun gary, and Nertchinsk in Siberia. American sources are at Phcenix- ville and Friedensville in Pennsylvania and in the Austin mine in Virginia. (5) Willemite, anhydrous ZnOSi0 2 , occurs in New Jersey and else where ; it is a comparatively rare ore. (6) Zinc-Blende, or shortly Blende (ZnS). The five ores mentioned above, as indeed all oxidized zinc ores, having become scarce, most of the zinc which now occurs in commerce is derived from zinc- blende. This ore crystallizes in combinations of the two tetrahedra and other forms of the regular system. Sp. gr. 3 9 to 4 2. Colour, green, yellow, red, but mostly brown or black. Colourless crystals are scarce. Lustre, fatty or diamond-like. The ordinary ore forms crypto-crystalline or fibrous or granular masses, which sometimes present the form of kidneys, consisting of concentric layers. The finest crystals are found in Franklin, New Jersey, which are colour less and consist of pure ZnS, and in the Penas de Europa, Asturias (Spain), in which liquid enclosures are often met with. The darker varieties, which always include more or less of foreign sulphides, are found in a great number of places. In Cornwall, Wales, Alston Moor in Cumberland, Teesdale in Yorkshire, Derbyshire, and the Isle of Man dark-coloured blende is found in the lead- mining districts with galena, quartz, and limestone. In Belgium and on the Rhine massive blende occurs with iron pyrites and galena ; this ore requires a great deal of sorting to become fit for smelting. In Sweden blende is frequently found : at Ammeberg on Lake Wetter a vast deposit occurs in the gneiss. To English miners blende is known as "black Jack," to the South Americans as "chumbe." The principal American deposits are in Missouri. Metallurgy. Oxide of zinc, like most heavy metallic oxides, is easily reduced to the metallic state by heating it to redness with charcoal ; but, as zinc has the exceptional property of being readily volatile at the temperature of its reduction, the operation must be carried out in some kind of retort, and the zinc be recovered as a distillate. To pure red zinc ore the operation of distilling with charcoal might be applied quite directly ; and the same might be done with pure calamine of any kind, because the carbonic acid of carbonate of zinc goes off below redness and the silica of silicate of zinc only retards, but does not prevent, the reducing action of the charcoal. Zinc-blende, however, being sulphide of zinc, is not directly reducible by char coal ; but it is easy to convert it into oxide by roasting : the sulphur goes off as sulphurous acid, whilst the zinc re mains in the (infusible) form of oxide, ZnO. In practice, however, we never have to deal with pure zinc minerals, but with complex mixtures, which must first of all be subjected to mechanical operations, to remove at least part of the gangue, and if possible also of the heavy metallic impurities (see METALLURGY, vol. xvi. p. 59 sq.). And, supposing this to be done, the ore, even if it is not blende, must be roasted, in order to remove all volatile components as com pletely as possible, because these, if allowed to remain, would carry away a large proportion of the zinc vapour during the distillation. If the zinc is present as blende, this operation offers considerable difficulties, because in the roasting process the sulphide of zinc passes in the first instance into sulphate, which demands a high temperature for its conversion into oxide. Another point to be con sidered in this connexion is that the masses of sulphurous acid evolved, being destructive of vegetable life, are an intolerable nuisance to the neighbourhood in which the operations take place. Hasenclever and Helbig have constructed a furnace by which some two-thirds of the sulphurous acid can be conducted into chambers and condensed into the useful form of oil of __ a vitriol. Figs. 1, 2, and 3 show how the furnace is constructed, k is a Siemens gas furnace, n being e the orifice for the introduction of the fuel ; the gases go out at m, where they mix with air ; the flame travels between the sole g and the bottom of the muffle c, and then goes along the top of the muffle to the flue q ; it passes below the slanting canal bb and keeps it at a temperature above the fusing-point of antimony (432 C.). The ore is introduced through the funnel a ; it slides down the /S{ slanting canal bb ; the FIG. 1. Longitudinal section along A B of fig. 2. partition walls d, d, which follow each other at distances of half a metre, compel it to spread out evenly into a thin layer. The partition walls, as shown by fig. 3, are arranged so that the gases FIG. 2. Horizontal section along C D of fig. 1. coming from the muffle must travel along an undulating line and lick up as much as possible of the sulphur of the ore. From the last compartment e the gaseous product goes first to a cooling chamber, and thence to the / - James500 (talk), : . , . James500 (talk) vitriol chamber. The hoi- low cylinder /, which is cooled internally by air while made to revolve, con veys the ore in instalments from the bottom end of the canal to the muffle. At in tervals of two hours it is spread evenly on the bot- s.
torn of the muffle, and at last it is drawn out through o and trans ferred to the sole g, to be finally roasted (" todtgerb stet ") there. The apparatus works very satisfactorily even with ores poor in sulphur. An ore containing 20 per cent, of sulphur contained at the end/ of the inclined canal 10 per cent., at the end o of the muffle 6 4 per cent, and at last, when "todtgerostet," only 1 2. About one-third of the sulphur is lost, i.e., goes out through the chimney, as S0 2 . The distillation process in former times, especially in England, used to be carried out " per descensum." The bottom of a crucible is perforated by a pipe which projects into the crucible to about two-thirds of its height. The powdery mixture of ore and charcoal is put into the crucible around the pipe, the crucible closed by a luted-on lid, and placed in a furnace constructed so as to permit of the lower end of the pipe projecting into the ash-pit. The zinc vapour produced descends through the pipe and condenses into liquid zinc, which is collected in a ladle held under the outlet end of the pipe. For manufacturing purposes a furnace similar to that used for the making of glass was employed to heat a circular row of crucibles standing on a shelf along the wall of the furnace. This system, however, has long been abandoned ; at present one or other of the following methods is used as a rule.
XXIV. 99