Electrolytic refining has increased greatly. The process con sists in using the crude copper as anode or positive plate in an electrolytic cell. The copper of the anode is dissolved, under the action of an electric current, in the copper sulphate solution which constitutes the electrolyte of the cell, pure copper being deposited on the negative plate or cathode. The impurities pres ent in the crude copper accumulate as a slime at the bottom of the electrolytic cell and this slime is collected and treated for the recovery of other metals, particularly platinum, gold and silver. The recovery of these by-products plays an important part in the economy of the electrolytic refining process.
The product of electrolytic refining, known as "cathode cop per" is a metal substantially free from all impurities except a little sulphur, the latter due to the mechanical enclosure of parti cles of the sulphate solution used in the electrolysis. For some purposes the cathode copper is used in that condition—as, for example, in the preparation of alloys. As produced in the refining cell, however, it is not suitable for mechanical treatment, such as rolling or drawing and where it is to be used as raw material for such purposes, remelting is required. This is done in large rever beratory or open-hearth furnaces, conditions during melting being kept oxidising in order to eliminate any traces of sulphur. This oxidising melting is followed by "poling" in order to lower the oxide content and bring the metal to the "tough pitch" condition. The relative merits of electrolytic and furnace-refined copper were, for a long time, the subject of acute discussion and there are still a few metallurgists who prefer the "best selected" cop per—i.e., the refinery product—to the high-purity material. The choice between electrolytic and refinery copper is now, however, almost solely a question of price.
Much attention has been given in recent years to an accurate study of the effects of impurities in copper, some very thorough work on the subject having been carried on for a number of years for the British Non-Ferrous Metals Research Association at the National Physical Laboratory, England, although much im portant work on the subject had been done earlier, as for exam ple by Hampe in Germany and by Skowronski in America. All impurities are found to render copper harder and less ductile and to lower the electrical conductivity. Bismuth is particularly po tent, while arsenic, on the other hand, appears to make the metal tougher rather than more brittle unless the proportion is very high. In considering these effects of impurities, however, it must
be borne in mind that a certain small amount of oxygen in the form of cuprous oxide is always present in copper that has been melted. While the effects of small proportions of oxide both upon the physical and electrical properties of copper are small com pared with those of phosphorus or bismuth, these effects must be taken into account when the simultaneous presence of another im purity is considered. Thus it has been shown that arsenic tends to counteract the effects of oxygen and it would not be surprising to find that arsenic has a similar action in regard to the deleterious effects of antimony and even of bismuth. If this is the case it may be found possible to admit proportions of antimony and bismuth which, in the absence of a suitable amount of arsenic, are known to be prejudicial.
The metallurgical processes which have been considered in outline above, serve as illustrations of the processes and methods used in the reduction of non-ferrous metals. Every individual metal, of course, requires the use of special processes and meth ods, but it will not be possible here to deal with further examples of such processes, except in regard to two metals typical of en tirely different conditions. One of these, zinc, owes its interest to the fact that it is volatile at the temperatures required to bring about its reduction by carbon, while the other—aluminium, is a metal whose chemical activity especially with regard to oxygen, is so high that reduction by carbon and heat alone is not possible.
Until a comparatively recent time the whole of the world's zinc supply was produced by distillation in the retort. In this process, oxide of zinc is mixed with finely divided carbon, gen erally in the form of coal, and heated in long fire-clay tubes or "retorts." The product of the reaction in these retorts is a va pour consisting of metallic zinc and oxygen—probably volatilised zinc oxide. This is caught in receivers, placed at the mouths of the retorts outside the furnace, where metallic zinc is condensed. A certain proportion of the product, however, condenses as a fine bluish-white powder, known as "blue powder" which is a mixture of metallic zinc and zinc oxide. It is not commercially feasible to recover the zinc from this powder and its production consti tutes a loss in the process, although for chemical purposes a reasonably satisfactory market for a certain amount of this pow der has, in recent years, been found.