Zinc is not one of the most common or widely distributed metals, and never occurs native. Three of its ores are utilized, viz.:- Red zinc ore, an oxide coloured red by associated iron and manganese oxides, and containing 80 per cent. zinc; blende or black jack, a sulphide yielding 67 per cent. of the metal; and calamine or zinc-spar, a carbonate affording 52 per cent. Of these, blende is by far the most common, forming veins in the sandstones and limestones of lead-yielding districts, and generally accompanied by galena and pyrites; calamine is second in importance, and favours the same geological formations as blende; while red zinc ore is comparatively rare. Of even less individual importance, though frequently associated with calamine, is electric calamine or zinc silicate, giving 64 to 66 per cent. of the oxide. A few-analyses of typical examples will be interesting:-
Zinc carbonate ...........
Iron Carbonate ..........
Iron Hydrate ..........
Zinc oxide ..
Zinc carbonate ..
Tin oxide ..
The principle which governs the extraction of zinc from its ores is that the metal is readily volatilized and easily recon-densed. The first step necessary with ores other than oxides is a calcination to remove the sulphur or other base and convert the zinc to an oxide; the next step is to smelt the oxide with carbon, by which the carbon and oxygen are combined to form gaseous carbonic acid, while the thus deoxidized metal distils over in vapour and is precipitated again in the solid form.
In the case of ordinary calamine, this preliminary calcination may be dispensed with, as the carbonic acid present is as effectually removed in the subsequent smelting; but with blende and silicious calamine, it is indispensable, in the former instance to drive off the sulphur, and in the latter to get rid of the silica. The ore is previously carefully picked over to free it fls far as possible from galena, the lead oxide derived from which in the smelting process would have a corrosive effect upon the crucibles. Next it is crushed and dressed, as other ores, and then submitted to calcination.
The calcination of blende is usually performed in a reverberatory furnace, having either a single hearth, or two hearths one above the other, or a series of three divisions at varying heights in one long hearth. Whatever plan is adopted, the flame from a coal fire is made to play upon the surface of the ore till all the sulphur is expelled, a result which is facilitated by frequently raking the mineral over, so as to expose fresh surfaces. There are numerous plans for arranging the furnaces, with a view of economising fuel and labour, each adapted to its own peculiar circumstances. The calcination of calamine is more often conducted in a sort of lime-Kilo with a null blast, as shown In Fig. 180.
The mixture of powdered ore and fuel (coal or coke) is charged into the furnace at the top a, and as the discharging of the calcined mineral takes place at the bottom b, the mass gradually descends into increasingly hot parts of the furnace, till it encounters at c the full force of the blast issuing from the twyers d. The carbonic acid and any sulphur present escape in vapour from the top a. The blast is not always availed of. The loss in calcination is about 1/2 the weight of the ore charged.
The calcined ore contains the zinc in the form of oxide. It is next reduced to very fine powder, passed through a sieve, and mixed with half its weight of coke or anthracite coal-dust, ready for smelting. If it is associated with much iron oxide and lime, it is mixed with an argillaceous ore, in order that the clay in the latter may be acted upon, rather than the material of the crucibles. There are 3 smelling methods in vogue, known as the Belgian, English, and Silesian, though the English system has been pretty generally superseded by the.
The furnace employed at Vieulle Montagne is constructed in sets of 4, and of the form indicated by Fig. 181. The principal parts are a fireplace a, tiers of fireclay tubes b, flues c, for heating the tubes and conveying the products of combustion to the chimneys d, each provided with a separate damper at the outlet. The mixture of calcined ore and fuel is filled into the tubes b, which measure 3 to 4 ft. long and 8 to 8 in. in internal diameter. To their mouth - ends are fitted short sections of conical cast-iron pipe e, penetrating the brickwork, and terminating outwardly in tapering wrought-iron tubes , about 2 ft. long and only 1 in. in diameter at the orifice; the opposite ends of the tubes 6 are closed. The number of tubes arranged in a single furnace varies from 40 to 80. The breastwork of the furnace is strengthened with cast-iron plates and wrought-iron tie-bars. When starting anew, the furnace is gradually heated for 4 days till it attains a white heat; quantities of carbonic oxide are evolved from the mouths of the wrought-iron pipes, and burn with a blue flame. As the volume diminishes the colour changes to greenish-wllite, and then copious white fumes appear, denoting that the metal has begun to volatilize.
That portion of the distillate which condenses and collects in the cast-iron pipes e is the purest; the deposit ac-30 per cent. of the metal it contained; the furnaces endure about 2 months' uninterrupted work. and are then thrown out for repairs. A further proportion of the zinc remains in the residue beyond the power of any known process to extract it at a profitable price; hence ores containing less- than 40 per cent. of zinc do not pay to work by this process. The fused zinc obtained is poured into ingot-moulds of generally 70 to 85 lb. each.