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.
The benefits derived from the adoption of flotation methods include:
- Increased recoveries of copper;
- simplification of the milling process with resultant decrease in the number of stages of treatment and the number of machines required;
- production of higher-grade concentrates by the elimination of insoluble and pyrite which had formerly been included in the concentrates produced by gravity methods;
- production of concentrates other than copper concentrates from plants essentially copper plants—for example, the production of pyrite concentrates by the Britannia Mining Co., the production of zinc concentrates at the Flin Flon and Sherritt Gordon plants, and the production of nickel-copper concentrates at the concentrator of the International Nickel Co., all by [differential] flotation methods;
- decreased first cost of concentrating plants per ton of daily capacity and decreased operating and repair costs per ton of ore treated;
- decreased costs of milling and mining operations and improved metallurgical results, permitting lower-grade copper ores to be mined and treated and thereby increasing the available ore reserves of many properties;
- general adoption of flotation process, influencing the development of the reverberatory furnace, which generally has replaced the blast furnace in smelting operations, with resultant benefits in the cost of copper smelting.
Types of Ores & Methods of Treatment
From the milling standpoint, copper ores may be classified roughly as follows:
- Ores in which the copper is present in the native or metallic form, represented principally by the ores of the Lake Superior district. These ores respond to gravity concentration of the coarser particles of copper, and the finer particles may be recovered by flotation or, as at Calumet & Hecla Conglomerate mill, the sand portion of the gravity- plant tails may be leached and the slime portion may be treated by flotation.
- Ores containing coarse sulfides of copper and oxidized ores containing cuprite, which respond to both gravity and flotation methods; these may be concentrated by gravity methods to recover the coarser particles and the tailing from the gravity machines by flotation; or fine-grinding and flotation only may be used. In most modern plants the gravity step is omitted.
- Ores in which the copper occurs in the form of fine sulfide mineral particles disseminated throughout the gangue and which are treated by fine grinding and flotation. These ores are represented notably by the so-called porphyry-copper ores.
- Oxidized ores in which malachite, azurite, and chrysocolla are the predominant copper minerals that do not respond either to gravity or simple flotation methods. Sulfide-filming of the carbonates followed by flotation of the filmed minerals, or direct leaching followed by precipitation of the copper from the solution may be employed for such ores.
- Ores containing sulfide copper minerals together with enough sulfides of zinc, lead, iron, or other metals to warrant use of differential flotation to separate them and to make a separate concentrate of each.
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-
flotation method. Table 58 summarizes the differences between results obtained from the earlier and later methods.
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.
Figure 154 is a flow sheet of the flotation section of the Britannia concentrator. Here “the flotation treatment comprises first the production of bulk concentrates which contain the chalcopyrite and
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.”