The four major steps in the production of marketable copper are mining, concentrating, smelting, and refining. In a few instances, however, leaching takes the place of concentrating, smelting, and refining. At present, although considerable leaching and direct-smelting ores are produced, the bulk of the copper ore mined is concentrated.
The milling of copper ores as practiced in the larger concentrators has changed to such an extent that comparatively few of the machines in use at the beginning of the period remain in service today. Primary and secondary crushing by machines of the Blake and gyratory types and intermediate and fine crushing by rolls has survived, but in the grinding field the development of pebble-mill grinding, the substitution of balls for pebbles, and the parallel development of drag-type classifiers have all but eliminated Chilean and Huntington mills. In the concentrating field, machines which effected separations on the basis of difference in specific gravity between copper and gangue minerals have been almost completely replaced by flotation equipment. In the Lake Superior district jigs and tables have, of course, been retained, and in a few concentrators which treat sulphide copper ores tables have been retained owing to unusual conditions at the plants or the smelters that treat the mill concentrates.
The flotation process, which was responsible for the almost complete change in equipment, has also undergone marked changes since its introduction in large-capacity concentrators. Flotation, when first introduced between 1913 and 1916, was used primarily to reduce losses of copper in the fine tailings of gravity plants. From an accessory to gravity methods, flotation very rapidly became a major process and finally, from 1923 to 1927, all but eliminated the gravity method in the treatment of low-grade sulfide copper ores.
The rapid development of ball-mill grinding must also be attributed to the adoption of the flotation process, since it was the incentive for developing grinding methods which produced considerable copper minerals too finely divided for successful recovery by existing gravity methods.
As with gold and other ores, details of practice vary because of differences in the ores or on account of economic considerations. Five figures are presented to illustrate in a general way the concentrating methods employed for treating the different types of ores.
Figure 150 is the flow sheet of one unit of the gravity concentration section of the Calumet & Hecla Conglomerate mill. The sand tailing from the mill is treated by ammonia leaching and the slime tailing by flotation. Since metallic copper is malleable, it cannot be broken and pulverized as can the more friable minerals, and after first picking out the larger lumps or nuggets of copper by hand, crushing is done by steam stamps; pebble mills instead of ball mills are employed for grinding because of the abrasive qualities of the gangue.
Figure 151 is the flow sheet of one section of the Cananea Consolidated Copper Co. mill as it was in 1929. This is a simple straight-flotation process that replaced an earlier combined gravity and bulk-
Figure 152 is the flow sheet of the Miami Copper Co. concentrator as it was in 1932; A shows the crushing plant and B the grinding and flotation units. Figure 153 is the flow sheet of the Miami concentrate re-treatment and filter plants. The ores are composed of chalcocite and pyrite with subordinate amounts of oxidized copper minerals disseminated mainly in a quartz-sericite schist.
pyrite with minor amounts of gold and silver. The bulk concentrates are dewatered and, after additional grinding, are again subjected to flotation. The latter operation produces finished copper concentrate, finished pyrite concentrate, and middlings which are re-treated.