Three modes of gas-firing are to be noticed, each of which is
adapted to special local conditions. (a) The gas is made from the
fuel in a detached fireplace and conducted while hot into the
combustion chamber of the furnace, and the air for complete combustion
is heated by the products of combustion on their way to the chimney.
(b) Both the producer gas and the air are heated before they enter
the combustion chamber, as in the Siemens system of regenerative
firing. (c) Natural gas is piped to the furnace, where it meets
air heated by the chimney gases. The primary advantages of
gas-firing are that less fuel is required, that there is better control of
the heat in the furnace, and that larger and more accessible furnaces
can be built. In Silesia the introduction of gas-firing has led to
the use of furnaces containing eighty muffles. In the United
States, Belgian furnaces of type (a) are built to contain 864 retorts;
of type (b), to contain 300 to 400 retorts; and of type (c), preferably
about 600 retorts. The use of gas-fired furnaces greatly
simplifies manual labour. On a direct-fired furnace at least one
man, the brigadier, must be an expert in all the operations involved;
but with a gas furnace a division of labour is possible. One man
who understands the use of gaseous fuel can regulate the heat of
a thousand or more retorts. The men who charge and empty the
retorts, those who draw and cast the metal, and those who keep
the furnace in repair, need not know anything about the making
or using of gas, and the men who make the gas need not know
anything about a zinc furnace. Again, in direct-fired furnaces
there are commonly seven or eight rows of retorts, one above
another, so that to serve the upper rows the workman must stand
upon a table, where he is exposed to the full heat of the furnace
and requires a helper to wait upon him. With gas-firing the retorts
can be arranged in four horizontal rows, all within reach of a man
on the furnace-room floor. Furthermore, with the large furnaces
which gas-firing makes possible mechanical appliances may be
substituted for manual labour in many operations, such as removing
and replacing broken retorts, mixing and conveying the charge
drawing and casting the metal, charging and emptying the retorts,
and removing the residues and products.
Refining.—The specific effects of different impurities on the physical properties of zinc have only been imperfectly studied fortunately, however, the small amounts of any of them that are likely to be found in commercial zinc are not for most purposes very deleterious. It is generally recognized that the purest ores produce the purest metal. Grades of commercial zinc are usually based on selected ores, and brands, when they mean anything usually mean that the metal is made from certain ores. Chemical control of the metal purchased is not nearly as common as it should be, and the refining of zinc is at best an imperfect operation. To obtain the metal chemically pure a specially prepared pure oxide or salt of zinc is distilled. A redistilled zinc, from an ordinarily pure commercial zinc, is often called chemically pure but redistillation is seldom practised except for the recovery of zinc from galvanizer's dross and from the skimmings and bottoms of the melting furnaces of zinc rolling mills. The only other method of refining is by oxidizing and settling. A bath, even of very impure zinc, is allowed to stand at about the temperature of the melting-point of the metal for forty-eight or more hours, whereupon the more easily oxidizable impurities can be largely removed in the dross at the top, the heavier metals such as lead and iron settling towards the bottom. This method is rarely practised except by the rollers of zinc. A certain amount of refined zinc can be dipped from the furnace; a further amount, nearly free from iron, can be liquated out of the ingots cast from the bottom of the bath in a subsequent slow remelting, and it is sometimes possible to eliminate a zinciferous lead which collects in the sump of the furnace. Owing to the fact that at temperatures between its melting and boiling point zinc has a strong affinity for iron, it is often contaminated by the scraper while being drawn from the condenser, as is shown by the fact that the scraper wears away rapidly. As each retort in a furnace is in all essentials a separate crucible, and as the metal from only a few of them goes into a single ingot, there can be no uniformity either in the ingots made from the same furnace during a day's run or in those made from several furnaces treating the same ore. Some brassfounders break from a single ingot the quantity of zinc required to produce the amount of brass they wish to compound in one crucible, but when perfect uniformity is desired the importance of remelting the zinc on a large scale cannot be too strongly emphasized.
Electrolytic Separation of Zinc.—The deposition of pure zinc is beset with many difficulties. Zinc being more electro-positive even than nickel, all the heavy metals must be removed before its deposition is attempted. Moreover, unless the conditions are closely watched, it is liable to be thrown down in a spongy form. M. Kiliani found that the sponge was produced chiefly when a weak solution, or a low current-density, was used, and that hydrogen was usually evolved simultaneously; sound deposits resulted from the use of a current-density of 200 amperes, or more, per sq. ft. and strong solutions. The cause of the spongy deposit is variously explained, some (Siemens and Halske) ascribing it to the existence of a compound of zinc and hydrogen, and others, among whom are G. Nahnsen, F. Mylius and A. Fromm, F. Foerster and W. Borchers, trace it to the presence of oxide, produced, for example either by the use of a solution containing a trace of basic salt of zinc (to prevent which the bath should be kept just—almost imperceptibly—acid), or by the presence of a more electro-negative metal, which, being co-deposited, sets up local action at the expense of the zinc. Many processes have been patented, the ore being acted upon by acid, and the resulting solution treated, by either chemical or electrolytic means, for the successive removal of the other heavy metals. The pure solution of zinc is then electrolysed. E. A. Ashcroft patented a process of dealing with complex ores of the well-known Broken Hill type, containing sulphides of silver, lead and zinc but the system was abandoned after a long trial on a practical scale. A full account of the process (Trans. Inst. Min. and Met., 1898, vol. vi. p. 282) has been published by the inventor, describing the practical trial at the Cockle Creek Works. The ore was crushed roasted, and leached with sulphuric acid (with or without ferric sulphate); the solution was purified and then electrolysed for zinc with lead anodes and with a current-density of 5 amperes per sq. ft. at 2.75 volts when diaphragms were used, or 2.5 volts when they were dispensed with, or with 10 amperes per sq. ft. at 3 or 2.5 volts respectively, the electrolyte containing 1.2 ℔ of zinc in the form of sulphate and 12 to 34 oz. of sulphuric acid, per gallon. The current efficiency was about 83 per cent. Canvas diaphragms were used to prevent the acid formed by electrolysis at the anode from mixing with the cathode liquor and so hindering deposition. C. Hoepfner has patented several processes, in one of which (No. 13,336 of 1894) a rapidly rotating cathode is used in a chloride solution, a porous partiton separating the tank into anode and cathode compartments, and the chlorine generated by electrolysis at the anode being recovered. Hoepfner’s processes have been employed both in England and in Germany. Nahnsen’s process, with an electrolyte containing alkali-metal sulphate and zinc sulphate, has been used in Germany, and a process invented by Dieffenbach has also been tried in that country, Siemens and Halske have proposed the addition of oxidizing agents such as free halogens, to prevent the formation of zinc hydride, to which they attribute the formation of zinc-sponge. Borchers and others deposit zinc from the fused chloride. In Borchers process the chloride is heated partly by external firing, partly by the heat generated owing to the use of a current-density of 90 to 100 amperes per sq. ft.
Properties
Zinc is a bluish white metal showing a high lustre when freshly fractured. It fuses at 415° C. and under ordinary atmospheric pressure boils at 1040° C. Its vapour density shows that it is monatomic. The molten metal on cooling deposits crystals belonging to the hexagonal system, and freezes into a compact crystalline solid, which may be brittle or ductile according to circumstances. 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. Commercial “spelter” 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 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 6.915, and rises to 7.191 after rolling. The coefficient of linear expansion is 0.002,905 for 100° from 0° upwards (Fizeau). The specific heat is 0.09555 (Regnault). Compact zinc is bluish white; it does not tarnish much in the air. It is fairly soft, and clogs the file. If zinc be heated 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 philosophica, “philosopher’s wool”). Boiling water attacks it appreciably, but slightly, with evolution of hydrogen and formation of the 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, copper or other more electro-positive metal be brought into contact with the zinc, it dissolves readily, with evolution of hydrogen and formation of the sulphate. 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. At higher temperatures, or with stronger acid, nitric oxide, NO, is produced besides or instead of nitrous. Zinc is also soluble in soda and potash solutions, but not in ammonia.
Applications.—Zinc is largely used for “galvanizing” iron, sheets of clean iron being immersed in a bath of the molten metal and then removed, so that a coat of zinc remains on the iron, which is thereby protected from atmospheric corrosion. It is also a constituent of many valuable alloys; brass, Muntz-metal, pinchbeck, tombac, are examples. In technological chemistry it finds application as a reducing agent, e.g. in the production of aniline from nitrobenzene, but the use of iron is generally preferable in view of the cheapness of this metal.
