Flotation of Sulfide Minerals

In mineral processing plant, Sulfide minerals are semiconductors, enabling electrochemical phenomena to occur in these systems. These minerals, therefore, develop a potential, termed the rest potential, when placed in an aqueous solution. Rest potentials of various sulfide minerals have been established under flotation conditions, as Table 1 shows for a solution containing 6.25 × 10–4 mol/L ethyl xanthate at pH 7.
Galena Flotation

The flotation response of galena with 1 × 10–5 mol/L ethyl xanthate in the presence of air is presented in Figure 1. The figure shows that complete flotation is effected in the range from pH 2 to pH 10.
Xanthate is present in two forms on a galena surface under conditions in which flotation occurs. One form is xanthate chemisorbed at monolayer coverage; the other is bulk-precipitated lead xanthate adsorbed at multilayer coverage. Dixanthogen does not form under these conditions and is not observed. Confirmation of the presence of bulk lead xanthate on the galena surface has been provided by infrared spectrometry.
The multilayers of lead ethyl xanthate are held together by van der Waals bonding of the hydrocarbon chains of the xanthate, and these layers can be dissolved with organic reagents such as acetone. However, the xanthate chemisorbed at monolayer coverage cannot be leached from the surface.
In the chemisorption of xanthate at monolayer coverage, one xanthate ion adsorbs on each surface lead ion to form an unleachable phase of lead xanthate. Electrochemical measurements suggest that monolayer adsorption involves charge transfer, with the discharged xanthate ion being fixed at the galena surface and hydroxyl ion being formed). Ion exchange of xanthate ion for hydroxyl ion may also occur under these conditions.
Adsorption of xanthate on galena, then, apparently occurs in two stages. The first stage comprises chemisorption of one xanthate ion on each surface lead ion. The second stage comprises the formation and adsorption of bulk precipitated lead xanthate formed by metathetic replacement of sulfur-oxy species and carbonate on the surface. 
Copper Sulfide Flotation
Chalcocite (Cu2S) and chalcopyrite (CuFeS2) are the two most commonly floated copper sulfide minerals. Bornite (Cu5FeS4), covellite (CuS), and enargite (Cu3AsS4) are normally present in smaller quantities. Both chalcocite and chalcopyrite are floated readily with common sulfhydryl collectors. Figures 2 and 3 show the responses of chalcocite and chalcopyrite to flotation with ethyl xanthate.
The active species of collector when xanthate is added to the chalcocite system is xanthate ion. Dixanthogen does not form on the chalcocite surface.
Xanthate adsorption on chalcocite is a two-stage process similar to that for galena. The presence of an unleachable xanthate species on the chalcocite surface following xanthate adsorption was demonstrated by Gaudin and Schuhmann. These authors also demonstrated that following the formation of an unleachable chemisorbed layer, multilayers of cuprous xanthate form and adsorb on the surface.
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