Galvanic Interaction between Sulfide Minerals in a Pulp

As indicated earlier, galvanic interactions arise between two or more dissimilar minerals, and/or metals that are in electrical contact with each other and with an electrolyte. Electrochemical reactions at the min eral surfaces result in coupled current and ion flows. The cathodic reaction is generally the reduction of oxygen to hydroxide, while the oxidation reaction involves the oxidation of the sulfide mineral. The current flow depends on the surface area and conductivity of the mineral as well as the chemical composition of the electrolyte.

Minerals can only be separated by flotation if they are physically separate, i.e. liberated from each other. Short periods of galvanic contact between sulfide particles are unlikely to result in the development of the longer-term hydrophobicity that would be required for flotation. Polarization studies by Gardner and Woods on lead sulfides have indicated that the formation of hydrophobic substances, in this case lead xanthate, is reversible and thus unlikely to endure long enough for bubble contact to be established. In the context of the selective flotation of sulfides it would be the middlings, where the different sulfides would still be in physical contact, that would be the most influenced by galvanic interactions.

In the case of middlings it is possible that the floatability may even be better than that of pure minerals, due to the greater spatial separation and electric potential differentiation of the anodic and cathodic sites on such composite particles compared to single mineral particles. The possibility for spatial separation will increase with increasing conductivity of the solution and will be more important in solutions of high salinity.

As an indication of the galvanic interactions that may develop between different sulfides, a list of rest potential values has been reproduced in Table 1. The rest potential values mentioned were determined at near neutral pH values and will generally decrease with increasing pH. Because of this effect, many sulfides may be depressed by an increase in pulp pH, as their potentials move further away from the dixan-thogen/xanthate equilibrium potential.

This disregards any chemical changes that may occur on the mineral surfaces due to a rise in alkalinity. High pH conditions typically develop at the cath-odic sites, which favour the precipitation of metal hydroxides and would encroach on the anodic reaction site if the spatial separation of the sites is not large.

Consider the flotation of a middlings particle containing chalcopyrite and pyrite. In the absence of a xanthate collector, pyrite acts as a cathode of the local pyrite-chalcopyrite cell. Oxidation of the chal-copyrite surface is the predominant reaction balanced by the corresponding reduction reaction on the pyrite surface.

Buckley and Woods demonstrated that the col-lectorless floatability of chalcopyrite and pyrite middlings particles increases with the amount of quartz added. This was attributed to the adsorption of hydrophilic iron hydroxides from the sulfide mineral surfaces on the quartz surface.

The uptake of xanthate ion strongly depends on the rest potential of the sulfide mineral. For sulfide minerals with rest potentials above + 0.13V, xanthate ions are oxidized at the mineral surface to dixantho-gen, which imparts hydrophobicity to the mineral surface. For bornite and chalcocite, whose rest potential was below that of the xanthate/dixanthogen reversible couple, metal xanthate was identified. For sphalerite and stibnite, the reaction products could not be positively identified. Rao, Moon and Leja also indicated that contact between various sulfides and iron will result in the depression of the potential to such an extent that the oxidation of xanthate to dixanthogen will no longer be possible. This is indicated in Figure 4.

During electrochemical interaction between sulfide species, ionic charge transfer takes place through the flotation liquor, while electronic charge transfer takes place through the solid interface; solid phase conductivity, as well as water conductivity is thus important. As an example, it is the experience on the Phalaborwa igneous complex that plant water conductivities range generally between 180 mS (fresh industrial water) and extremes of ca. 500 ms, with a middle range of 200-300 mS.

For separate mineral particles, the solid phase charge transfer would rely on particle collision, in which the gangue particles have a shielding influence. This reduces the galvanic interactions to a point where electrochemical interactions between fully liberated minerals are unimportant in flotation plant practice, unless plant waters are highly conductive, and both pulp densities and sulfide mineral concentrations are high enough.

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