Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/836

Rh 786 ZINC In the Belgian process the reduction and distillation are carried out in cylindrical retorts of fire-clay, about a metre long and 15 metre wide inside. Some forty-six or more retorts, arranged in eight parallel horizontal rows, are heated in one furnace. The fur naces are square and open in front, to allow the outlet ends of the retorts to project ; they are grouped together by fours ; and their several chimneys are within the same enclosure. Each retort is provided with two adapters, namely, a conical pipe of fire-clay, about 4 metre long, which fits into the retort end, and a conical tube of black sheet iron, which fits over the end of the fire-clay pipe, and which at its outlet end is only 2 centimetres wide. To start a new furnace, the front side is closed provisionally by a brick wall, a fire lighted inside, and the temperature raised very gradually to a white heat. After four days heating the provisional front wall is removed piecemeal, and the retorts, after having been heated to redness, are inserted in corresponding sets. The charge of the re torts consists of a mixture of 1100 lb of roasted calamine and 550 lb of dry powdered coal per furnace. A newly-started furnace, how ever, is used for a time with smaller charges. Supposing the last of these preliminary distillations to have been completed, the resi dues left in the retorts are removed and the retorts, as they lie in the hot furnace, are charged by means of semi-cylindrical shovels, and their adapters put on. The charging operation being com pleted, the temperature is raised, and as a consequence an evolution of carbonic oxide soon begins, and becomes visible by the gas burst ing out into the characteristic blue flame. After a time the flame becomes dazzling white, showing that zinc vapour is beginning to escape. The iron adapters are now slipped on without delay, and left on for two hours, when, as a matter of experience, a consider able amount of zinc has gone out of the retort, the greater part into the fire-clay adapter, the rest into the iron cone. The former con tains a mixture of semi-solid and molten metal, which is raked out into iron ladles and cast into plates of 66 to 77 lb weight, to be sold as &quot;spelter.&quot; The contents of the iron recipient consist of a powdery mixture of oxide and metal, which is added to the next charge, except what is put aside to be sold as &quot; zinc dust.&quot; This dust may amount to 10 per cent, of the total production. As soon as the adapters have been cleared of their contents, they are replaced, and again left to themselves for two hours, to be once more emptied and replaced, &c. The complete exhaustion of the charge of a furnace takes about 11 hours. In the Silcsian process the distillation is conducted in specially constructed muffles, which are arranged in two parallel rows within a low- vaulted furnace, pretty much like the pots in a glass furnace. As a rule every furnace accommodates ten muffles. Through an orifice in the outlet pipe (which during the distillation is closed by a loose plug) a hot iron rod can be introduced from time to time to clear the canal of any solid zinc that may threaten to obstruct it. As soon as the outlet pipe has become sufficiently hot the zinc flows through it and collects in conveniently placed receptacles. About 6 or 8 hours after starting the distillation is in full swing, and in 24 hours it is completed. A fresh charge is then put in at once, the muffles being cleared only after three successive distillations. The distillate consists of a conglomerate of drops (&quot;drop zinc&quot;). It is fused up in iron basins lined with clay, and cast out into the customary form of cakes. In some modern works the muffles are heated by means of Siemens s regenerative gas furnace, by which a more uniform heat can be secured and maintained at a less cost. Of the several metallic impurities in zinc ores iron is at once the most common and the least objectionable, because it is absolutely non-volatile at the temperature of a zinc retort; whenever commercial zinc contains iron, this comes from its having been re-fused in iron vessels after its dis tillation. Lead, though hardly volatile by itself at a red heat, if present in the ore, is, so to say, carried over by the zinc vapour and passes at least partly into the distillate. Cadmium and arsenic being more volatile than zinc itself, if present, accumulate in the first fractions of the distillate, but may pervade it in traces to the end. Zinc made from oxidized ores is usually free from arsenic; that derived from blende is almost sure to contain it. This in practice is equivalent to saying that, while in former times it was easy, it is now very difficult, to obtain in commerce zinc free from arsenic. Traces of arsenic do not, however, in terfere with any of the technical applications of the metal. As for cadmium, it is not (metallurgically speaking) an impurity at all, but, like silver in lead, a rather desirable admixture which it may be worth while to extract. No reliable method is known by which commercial zinc could be purified so as to render it fit for all the purposes of the analyst ; the only way to obtain really pure zinc is to prepare it from pure oxide by distillation with charcoal in a non-metallic retort. Properties of Pure Zinc. Zinc, a bluish- white metal, fuses at 415 C. and under ordinary atmospheric pressure boils at 1040 C. (Deville and Troost). The molten metal on cooling deposits crystals, and at last freezes into a com pact crystalline solid, which may be brittle or ductile according to circumstances. According to Bolley, if zinc be cast into a mould at a red heat, the ingot produced is laminar and brittle ; if cast at just the fusing-point, it is granular and sufficiently ductile to be rolled into sheet at the ordinary temperature. According to some authorities, pure zinc always yields ductile ingots. A clue to the explanation of these anomalous facts is afforded by certain observations of Gustav Rose and others, from which it appears that zinc is dimorphous and may or may not crystallize in the regular system. Supposing a mass of molten zinc to freeze into, say, cubes, the ingot will be ductile ; an ingot of, say, rhombohedra, on the other hand, is almost bound to be brittle, because the crystals are orientated in a lawless fashion, and, as they cannot be expected to contract at the same rate in all directions, we must be prepared for a brittle ingot. Commercial &quot; spelter &quot; always breaks under the hammer ; but at 100 to 150 C. it is susceptible of being rolled out into even a very thin sheet. Such a sheet, if once produced, remains flexible when cold. At about 200 C., again, the metal becomes so brittle that it can be pounded in a mortar. The specific gravity of zinc cannot be expected to be perfectly constant ; according to Karsten, that of pure ingot is 6 9 15, and rises to 7 191 after rolling. The coefficient of linear expansion is 0-002,905 for 100 from upwards (Fizeau). The specific heat is 093,93 (Schiiller andWartha). Compact zinc is bluish white ; it does not tarnish much in the air. It is pretty soft, and clogs the file. If zinc be heated up to near its boiling-point, it catches fire and burns with a brilliant light into its powdery white oxide, which forms a reek in the air (lana philosophical). Boiling water attacks it appreciably, but no more, with evolution of hydrogen and formation of hydroxide, Zn(OH) 2. A rod of perfectly pure zinc, when immersed in dilute sulphuric acid, is so very slowly attacked that there is no visible evolution of gas; but, if a piece of platinum or other less basilous metal is brought into contact with the zinc, it dissolves readily, with evolution of hydrogen and formation of sul phate. The ordinary impure metal dissolves at once, the more readily the less pure it is. Cold dilute nitric acid dissolves zinc as nitrate, with evolution of nitrous oxide, N 2 O, and formation of nitrate of ammonia. At higher temperatures, or with stronger acid, nitric oxide, NO, is produced besides or instead of nitrous. Oxide of Zinc, ZnO. There is only this one oxide. It is pre pared chiefly in two ways, (1) by burning the metal, a method now being carried out industrially, the zinc vapour being some times produced extempore from a mixture of roasted ore and carbon, and (2) by heating the basic carbonate (see below). It is an infus ible solid, which is intensely yellow at a red heat, but on cooling be comes white. This at least is true of the oxide produced from the metal by combustion ; that produced from the carbonate, if once made yellow at a red heat, retains a yellow shade permanently. Oxide of zinc is insoluble in water, and does not combine directly with it ; it dissolves readily in all aqueous acids, with formation of &quot; zinc salts. &quot; It also dissolves in aqueous caustic alkalies, including ammonia, forming &quot;zincates&quot; (e.g., ZnO.KHO). Oxide of zinc is used in the arts as a white pigment ; it has not by any means the covering power of white lead, but offers the advantage of being non- poisonous and of not becoming discoloured in sulphuretted hydrogen. It is used also in medicine, chiefly externally. The hydrate, Zn(OH) 2, is prepared by precipitating a solution of any zinc salt with caustic potash. The alkali must be free from carbonate and an excess of it must be avoided, otherwise the hydrate re-dissolves. It is a white powder, and is insoluble in water. To acids and to alkalies it behaves like the oxide, but dissolves more readily.