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The flowsheet and data shown in Figure 6 illustrate the practice at one southwestern U.S. plant treating a porphyry copper ore containing chalcopyrite, bornite, pyrite, and minor amounts of secondary minerals including chalcoite and native copper. Several of the xanthates show equal metallurgy but sodium isopropyl xanthate is more often used because of lower cost.
The copper minerals in this ore are closely associated with the pyrite. Therefore, to depress the finely ground pyrite after regrinding, it is necessary to add cyanide to the regrind mills. This practice also is recommended for other in which pyrite depression is a problem – unless complications arise form the presence of major amounts of chalcoite.
The same general reagent combination, with minor variations, finds application on a number of higher grade ores. Higher lime alkalinity is required if free pyrite floats, and more xanthate is therefore necessary. In some instances, small amounts of cyanide, either NaCN or Ca (CN)2, also are used if lime foes not sufficiently depress the iron sulfides. However, an excess of cyanide will also depress copper minerals; hence it is ordinarily added only to the cleaner stages.
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