Lead and Zinc Ores
Minerals of lead and zinc commonly are rather closely associated in nature; zinc-free lead ores and lead-free zinc ores are exceptional rather than common, although there are such.
Lead and zinc ores usually contain some silver, but those of the Mississippi Valley region are silver-free. Indeed, lead-zinc ores often are commercially valuable chiefly because of their silver (and sometimes gold) content. The silver and gold usually are intimately associated with the base metal minerals, are recovered with them in the smelter, and are finally separated in the refinery.
Because of this close association of zinc and lead ores, it is necessary to consider them together in discussing their concentration.
An earlier summary of practices and costs of concentrating lead and zinc ores was published by the Bureau of Mines in 1935. Most of the data in the present bulletin were abstracted from the earlier publication.
The development of differential or selective flotation has had an important influence upon the economics of lead and zinc mining and milling. Higher recoveries and better separation of the lead and zinc minerals into different concentrate products than was possible by gravity concentration has made it profitable to mine and treat many ores that formerly could not be worked profitably. Zinc concentrates with a high pyrite content produced by gravity methods sometimes required flash roasting and magnetic separation of the iron from the roasted material to produce a zinc concentrate of the required grade. With flotation, however, it is possible to depress the iron sulfide and float the zinc sulfides. With straight gravity treatment high slime losses were often unavoidable, but high recoveries can be made by flotation.
Gravity concentration still retains a place in the flow sheet of some mills where it is employed mainly on the coarse product from primary or secondary crushing before fine grinding, either to recover quickly coarse ore particles or to make a quick reject of gangue minerals, thus reducing the amount of material that must pass to expensive fine grinding.
Screens, classifying cones, or kindred settling devices, jigs, and shaking tables comprise the essential gravity concentration equipment. Tables are used incidentally in many flotation plants to check the performance of the flotation machines or the fineness of grinding or for other purposes.
Methods of Treatment
Because of the wide variations in tenor or ore, nature of ore and associated gangue minerals, fineness and distribution of ore particles in ores from different localities there is no standard method of treatment, and flow sheets must be worked out for each particular ore.
However, for the purpose of the present discussion, methods of treatment may be classified roughly as follows:
(a) Concentration of sized material.
(b) Concentration by roughing and cleaning.
2. Table concentration:
(а) Concentration of different classifier sand products on separate tables.
(b) Concentration by roughing and retreatment of concentrate and middling products.
(a) Following gravity concentration; flotation of primary slimes, of screen undersize from sizing ahead of gravity concentration, or of reground middlings or tailings from gravity concentration.
(b) Straight bulk flotation producing a combined lead-zinc concentrate (when either lead or zinc occurs in very small proportion to the other).
(c) Differential or selective flotation by which the ground mill feed is first treated in a lead circuit by roughing and cleaning in one or more stages and producing finished lead concentrate, followed by reconditioning and flotation of the lead-circuit tailing to recover the zinc in one or more stages of roughing and cleaning, the zinc-circuit tailing being final tailing.
(d) Flotation of oxidized lead and zinc minerals by the use of special reagents for sulphidizing or otherwise conditioning the pulp and for stage-addition in the process, usually involving a high reagent cost. (See Chief Consolidated, table 60.)
Various combinations of these methods of concentration are employed to suit different ores.
Figure 155 is the flow sheet of a concentrator employing gravity methods followed by flotation (typical of practice in the Tri-State district). Figure 156 is the flow sheet of a simple selective flotation plant in which lead and zinc concentrates are each cleaned once and the middling products are returned to the heads of their respective circuits for retreatment.
Table 60 presents data on a number of typical lead and zinc ore concentrators and summarizes the methods employed, metallurgical results obtained, and the operating costs.