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Under “Flotation Agents”, pages 7 to 17, it was stated that sphalerite (ZnS) must be activated by copper salts before it becomes floatable with xanthate or similar collectors. The copper-coated zinc mineral thus behaves in flotation as a copper mineral and its separation from natural copper minerals is extremely difficult. Therefore, as might be expected, the Cu-Pb-Zn ores are among the most difficult of all sulfide ores to treat.
Excellent Cu-Pb-Zn separations can be made on synthetic ores if the copper, lead, and zinc sulfides are ground separately and then mixed together. However, in naturally occurring copper-zinc ores, some copper salts have always coated or activated at least part of the zinc minerals.
If the zinc sulfides are finely disseminated in the gangue, the problem may be specially serious. Grinding of such ors does not produce clean zinc sulfide surfaces. Instead, it may oxidize the copper minerals, thus releasing more copper ions which activate additional zinc and iron sulfides.
Successful copper-zinc separations by flotation, therefore, always involve prevention of copper oxidation and dissolution, and frequently also the removal of a copper coating from the activated zinc sulfides. Sodium or calcium cyanide is most often used to remove and complex these interfering copper ions. However, as previous for copper sulfides and therefore must be used with considerable caution.
In general, the same reagent combinations are employed for Cu-Pb-Zn-Fe ores as for Pb-Zn ores. Sodium sulfite or bisulfite is perhaps more standard with the Cu-Zn ores than with Pb-Zn ores, and some cyanide is nearly always required. The use of these reagents is discussed under Pb-Zn ores, page 21.
The “standard” Cu-Pb-Zn-Fe ore flow sheet involves the flotation of a Cu-Pb concentrate, followed by Zn, and the pyrite flotation. In some mills it is not economic to separate the copper and lead but in others separation is made by treating the bulk concentrate. This may be done by either depressing the copper with cyanide, or the lead with SO2 or dichromate. The SO2 method is the most common and its application at the San Francisco Mines of Mexico has been reported a number of times in the literature.
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