Abstract: Copper is recovered from an aqueous pulp of an ore containing both sulfide minerals and oxidized minerals by continuously monitoring the EMF of the pulp and adding a water-soluble sulfide to the pulp in an amount from 0.05 to 7 pounds of contained sulfur per ton of ore whenever and for so long as the pump EMF is above about -30 millivolts with reference to a standard silver-silver chloride electrode, and discontinuing such addition whenever such EMF in less than about -30 millivolts. Thereafter the pulp is subjected to a froth flotation operation in the presence of a collector for copper sulfide minerals to produce a concentrate containing most of the sulfide minerals and a substantial part of the oxidized minerals of the ore.