ORE TREATMENT Where Metallurgy Begins.—Beginning with the quarry or mine, it is difficult to determine precisely where the province of mining ends and that of metallurgy begins. Metals occur in nature only rarely in the free state, but usually in the form of "ores"—i.e., of minerals in which the metal exists in chemical combination with other elements, chiefly oxygen, sulphur, arsenic and silicon. A great many important ores are sulphides. When a metalliferous deposit is first discovered, the ore—for example in many of the American copper deposits—is of the nature of a carbonate, having been altered by the chemical action of the atmosphere from the original sulphide condition. These "oxidised" surface ores are easily worked and afford an initial advantage in opening up a new mineral region. As the ore is pursued to greater depths, however, its character usually changes and the carbonates and oxides are replaced by sulphides which are much more difficult to work. There are, of course, exceptions to this rule, as in the case of aluminium and of gold. The latter usually occurs in the free metallic state and at an early stage in the working of an auriferous region is generally obtained by the simple process of washing or "panning" the alluvial mud of the rivers. Platinum and osmiridium are obtained in similar ways. Such alluvial deposits, however, indicate the existence of gold-bearing rock at higher levels. In the rock the gold is usually associated with quartz and may occur either in a comparatively massive form as fine veins or streaks, or in a very finely almost colloidal form, as in the quartz lodes of the Witwatersrand.
The extraction of alluvial gold is an example of a very simple type of ore treatment which is, however, typical of the methods adopted. The metal-bearing mineral, whether it be oxide, car bonate, sulphide or free metal, always occurs intimately associated or mixed with non-metalliferous material, often constituting the country rock in which the mineral "lode" or "vein" is em bedded. In mining operations efforts are made to separate the
mineral as much as possible from t adjacent valueless rock or "gangue," but usually the separation is very incomplete until special processes are applied to the product. These separating or "ore dressing" processes depend on differences of properties between the mineral and the gangue. In the case of gold "wash ing" the property utilised is the higher density of the gold as compared with the mineral matter of the surrounding mud. Similar gravity separation in flowing water is frequently used, but can as a rule be applied only after the product of the mine has been ground to a fine powder. After grinding, the ore is frequently "graded" by various methods, such as screening or sieving, in order that the product subjected to the next operation may be of a standard degree of fineness.
Apart from the separation of the heavier mineral from the lighter gangue particles by means of water, other methods are also employed. In some cases magnetic separation is possible, when the mineral is either markedly attracted by powerful electromagnets or—less strongly—repelled by them. Such methods can, however, be used only for minerals, like iron, nickel or cobalt, which possess well-marked magnetic properties.