While the copper blast furnace is essentially a vertical shaft into which air is blown by nozzles or "tuyeres" at a suitable point near the bottom, the "converter" is a vessel, either cylindrical or pear-shaped, capable of rotation on trunnions. This is provided with air-nozzles so arranged that the molten charge can—by ro tating the vessel—be brought to cover the air-nozzles. When this is done the air is forced through the molten charge and rapid oxi dation takes place. The heat of combustion of the sulphur serves to maintain and to raise the temperature of the charge, so that no supply of fuel is required.
At one time it appeared probable that smelting by the blast furnace and the converter would become the means of production of the great bulk of the world's copper supply ; practice tended towards an ever-increasing size of unit. But flotation methods of concentrating copper ores have created a difficulty in the use of the blast furnace. The great rush of air in this type of furnace tends to blow finely-divided ore into the flues, necessitating ex pensive after-treatment and recovery of the flue dust. This diffi culty has to some extent been overcome by the development of the roasting and sintering machines such as those of Dwight Lloyd which convert the finely-divided product of flotation into a fairly hard and porous sintered product at the same time as roasting it. None the less, there has been a great development in the use of the reverberatory furnace for copper smelting. These furnaces consist essentially of shallow basins which hold the charge and the layer of covering slag while the heating flame plays over the charge and against the furnace crown. In principle, these reverberatory furnaces are similar to the refining furnaces and to the open-hearth steel furnaces used in ferrous metallurgy, al though the temperatures to be attained and, therefore, the meth ods of firing, are different. Another important factor in the development of the reverberatory furnace for copper smelting has been the introduction of pulverised coal firing. For the blast furnace it is necessary to employ coke, since the fuel must be sufficiently hard and strong to bear the superimposed weight of the charge without crushing unduly. Only in this way can the charge be kept sufficiently "open" to allow of the free passage of the gases. The production of coke involves expensive plant, even if the recovery of by-products more or less covers the cost of the coking operation itself. Only certain types of coal, more
over, lend themselves to the production of metallurgical coke. For pulverised coal firing, on the other hand, finely divided coal of very moderate quality, much of which had formerly to be re garded as practically waste, can be used. For furnaces where very high temperatures are required and pre-heating of the air is essential, the use of coal-dust firing offers some serious diffi culties. The ash, which represents anything from about eight to twenty per cent of the powdered coal, is necessarily blown into the furnace with the coal and most of it passes into the flues. In the high-temperature regenerative furnaces the hot products of combustion pass through regenerator chambers which serve the purpose of heating the incoming air, and the ash from the coal-dust tends to clog and destroy the brick-work of these chambers. This difficulty applies with great force to steel melting, but does not arise in the metallurgy of copper, since the tempera tures are not high enough to require regenerative furnaces.
The further refining of copper may be carried out either by furnace methods or electrolytically. In the former process— which is much the older—the crude "blister" copper is melted in reverberatory furnaces and subjected first to strongly oxidising in fluences—as by blowing air through the molten metal. When the impurities have been oxidised as completely as possible and have been removed by scraping away the resulting surface slag, the metal is subjected to a reducing action by the process of "poling." "Poles" of wood—sometimes the trunks of trees of moderate size—are pushed down into the molten mass, while charcoal is sprinkled on the surface. As the wood burns, the products of combustion reduce the cuprous oxide in the molten metal to cop per. The process is stopped when the metal has reached a condi tion known as "tough pitch" which is usually judged by the appearance of a small quantity which is ladled out and allowed to solidify. The examination of the fracture of a small bar is also a good guide. The "tough pitch" condition cannot, in the case of refinery copper, be defined by any specific oxygen content, since the proportion of oxygen required depends upon the amount of other impurities present, notably arsenic. Furnace refined copper is rarely if ever free from the latter impurity.