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Most lead – zinc ores contain iron (pyrite). To recover the iron, the lead and zinc are first recovered in the usual manner and the iron is then floated from the zinc tailings. The iron which has been depressed in the lead and zinc flotation must be reactivated prior to flotation. This can be accomplished by reducing the pulp pH, or by precipitating the calcium ions with soda ash, or both. The pyrite may then be floated with a xanthate using 0.10 to 0.25 pounds per ton of ore.
Some ores may contain both pyrite and pyrrhotite and separating them may be desirable because of precious metal content of the pyrite, or other reason. Such a separation was developed in the Dow laboratories by the use of soda ash and pH control. The zinc flotation tailings were conditioned with soda ash at a pH of 9.0 ato 9.5, and the pyrite was floated with xanthate. The pyrite flotation tailings were then conditioned with sulfuric acid to a pH of 6.5 to 7.5 and more xanthate was added for the pyrrhotite flotation.
If a lime-burning plant were located near the mill building, the CO2 flue gas could be used to replace soda ash for the reduction of pulp pH and Ca++ precipitation. This unique variation is understood to be employed in at least one Pb-Zn mill for the reactivation of pyrite from the zinc flotation tailings.
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