ORE DRESSING is an important process in the field of mining (q.v.). It may be defined as mechanical concentration whereby valuable minerals in an ore are separated from worthless impurities or gangue and is distinguished from metallurgy which employs chemical methods for recovering metals and metallic compounds from rich ores or from the concentrated products of the ore dresser. Without this relatively cheap process of enrich ment as compared to expensive metallurgical processes, the cost of common metals, such as lead, zinc and copper and many mineral products, would be far above present prices, and, as richer ore deposits continue to be exhausted, ore dressing will become of still greater importance. As an illustration of what ore dressing is doing for copper alone, the so-called porphyry-copper mills may be cited. In some of these mills ioo tons of ore assaying not over I % copper will yield three tons of concentrates assaying 3o% copper and 97 tons of waste tailings assaying about o.i % copper. Thus the copper lost in the tailings is not over one-tenth of the total copper in the original ore and the expensive metallurgical process has to treat only three tons of concentrates as compared to 'co tons of original ore if concentration were not used. A single mill may treat as high as 20,000 tons of ore in 24 hours. Terms also used more or less as practically synonymous with ore dressing are milling, washing and concentration.
Ores in the majority of cases do not consist of a single mineral in a high degree of purity, but are a more or less complex mixture of different minerals, some valuable and some worthless. Native gold, tin, stone and many gems may occur mixed with gravel in placer deposits. Native copper is found sprinkled through rock masses. Diamonds are enclosed in a matrix of hard rock. Native gold is also found encased in quartz veins. The minerals of base metals, such as lead, zinc and copper, are commonly sulphides, oxides or other chemical compounds of these metals and occur distributed through solid rock or gangue in veins and other types of ore deposits. A given deposit may contain minerals of only one metal, or two or more metals may be represented and in the latter case the problem of the ore dresser becomes harder as he has not only to eliminate the waste gangue but also to separate the min erals of one metal from those of another. Beside minerals of the
metals, there are many non-metallic minerals which likewise require elimination of gangue by concentration before these min erals become marketable. Among such minerals are mica, graphite, feldspar, asbestos, fluorspar, phosphate rock and the abrasive min erals garnet, emery and corundum. Even common sand and gravel may be washed to get rid of undesirable constituents and likewise clay to free it from grit and mica. The best examples of ores which do not require concentration are the iron ores of the Lake Superior region. Much of our bituminous coal is also sufficiently free from impurities to be marketed direct but even on iron and coal considerable washing is in use for lower grade deposits and this is bound to increase as the richer deposits are worked out. The subject will be considered in three divisions: (I) Properties of minerals which aid their separation; (2) Individual machines and unit operations; (3) Complete mill systems.
Specific Gravity has been the most important property in concentration. Valuable minerals are usually heavy and gangue minerals light, although there are exceptions. Thus the rate of settling in water or air or the tendency of a mass of grains to stratify when acted on by currents of water or air makes it possible to separate a lighter mineral from a heavier one. Size has to be considered also since a large grain will settle faster than a small grain of the same mineral and, consequently, large grains of light minerals are equal settling with small grains of heavy minerals. Separation based on specific gravity is commonly called gravity concentration. Colour, lustre and fracture are of aid when valuable minerals are being separated from waste by the process of sorting or picking by hand selection. These properties also enable the operator to make the proper line of division between the products coming off a concentrating machine. Frac ture and cleavage will affect the form of the grains produced by crushing. Flat grains settle less readily than rounded or cubical grains. Hardness, toughness, friability and brittleness affect the power used in crushing ores. Decrepitation which causes some minerals to fly to fine pieces when heated enables such minerals to be separated, by subsequent screening, from other minerals which do not decrepitate and thus remain coarse.