Ore Treatment

The product of the zinc-distillation furnace suffers from a serious disadvantage. The ores of zinc and the concentrates which are treated in the furnace always contain considerable proportions of lead. This lead is rather less volatile than zinc, but sufficiently so to bring about a contamination of the zinc produced which may amount to one or two percent. Formerly no great importance was attached to this impurity, but the effects of impurities in metals and alloys have come to be more fully recognised and there is now an increasing demand for metals of the highest possible purity.

Special interest attaches to the rapid development, during re cent years, of electrolytic zinc production both in America and Australia—at Port Risdon in Tasmania. The zinc produced is of very high purity as compared with the product of the distilla tion furnace. Although the cost is still somewhat higher than that of furnace-produced zinc, the pure electrolytic metal is rapidly winning wide markets, thanks to the absence of lead and the com plete regularity of composition. Cadmium is a by-product of Australian zinc production and this metal is now becoming in dustrially important. It is finding uses in electroplating, for solders of higher melting point than the ordinary lead-tin solders (alloys of zinc and cadmium) and as an alloying element which, in small quantities, serves to harden and strengthen copper con ducting wires without decreasing the electrical conductivity so much as other additions. Electrolytic zinc production is also making rapid advances in America and has attained an output which is about one fifth of the world's total zinc production.

Aluminium.

Brief reference has been made above to the production of aluminium by fusion electrolysis. (See also ELEC TROMETALLURGY.) It is, in this respect, typical of a group of metals whose oxides cannot be reduced by heating them with carbon or by the action of ordinary reducing gases. Among these magnesium, calcium, sodium and potassium, and latterly, beryl lium may be mentioned. The processes differ widely in detail, but are all based on similar principles. The essential features are the use of a sufficiently fusible electrolyte in which an oxide or a salt of the metal can be dissolved and from which the pure metal is separated by electrolysis. The whole technique of fusion elec trolysis differs widely from the aqueous process as practised in the refining of copper. The necessity of keeping the electrolyte in the molten condition requires the application of heat which may be furnished separately or by the electrolysing current itself. Heavy currents and relatively high current densities are conse quently required. Usually, these electrolytic processes are only feasible where very cheap power is available, usually from water power Alternatively, very cheap fuel may serve the same pur pose, as at the great works at Bitterfeld, Germany, where cheap brown coal fuel is now employed mainly for the production of magnesium. This is produced by the electrolysis of a chloride

electrolyte from which chlorine gas is also evolved, and this gas is itself an important product.

The production of aluminium is carried out on a much larger scale than any of the other fusion-electrolysis processes and has been brought to a high degree of efficiency. The electrolyte, like most of those used in processes of this type, is of a halide char acter. It consists of double fluorides of aluminium and sodium (cryolite) which have the power to dissolve aluminium oxide. This oxide is decomposed during electrolysis and is replaced by fresh oxide fed to the furnaces. In the fusion electrolysis of other metals it is usually necessary to convert the metal from the ore into a chloride or fluoride before it can be incorporated in the electrolyte, which is usually also a double fluoride.

Another important feature which applies to most of the metals produced by fusion electrolysis is that they are of low density— so-called "light metals." This has an important effect on the methods which have to be employed in their production, since there is a strong tendency—unless the relative densities are care fully regulated—for the metal to rise to the surface of any molten bath and there to become rapidly oxidised or burnt away. Even in the case of aluminium, great care has to be taken to maintain the electrolyte at a density sufficiently low to allow the fluid metal to accumulate at the bottom of the bath. In the case of the very light metals such as magnesium and beryllium this would be almost impossible and instead of carrying out the electrolysis at such a temperature that the resulting metal is fluid—as is done in the case of aluminium—the temperature employed leaves the deposited metal just solid. Deposition occurs at the end of a rod cathode which, as it is built up by deposited metal, is gradu ally raised out of the bath. The result is the production of a solid stick or rod of the metal attached to each of these rising cathodes. Even with such readily oxidisable metals as calcium and beryllium this method has proved successful. It entails, how ever, the task of finding an electrolyte sufficiently fusible to be liquid at a temperature at which the metal itself is solid. In the case of beryllium, with a melting point above 1,250° C this is not difficult, but the problem is more serious with metals of rela tively low melting points like magnesium or calcium.