General

The reaction products of galvanic interaction may influence the flotation efficiency of composite minerals by direct depression or activation of minerals, or by affecting flotation froth characteristics. These reaction products, as will be shown, are not unique to galvanic processes, but their rate of formation may be enhanced by such interactions. The spatial separation of the anodic and cathodic reactions in galvanic interactions favours the kinetics in the sense that the reaction products formed at the

Figure 4 Mixed potentials of sulfide minerals alone and in contact with metallic iron as a function of xanthate concentration. Nonoxidizing conditions, argon purging - 400ccmin~1 natural pH, 25°C. (Reproduced with permission from Rao, Moon and Leja (1976) Flotation, A.M. Gaudin Memorial Vol. Copyright American Institute of Mining Metallurgical and Petroleum Engineers.)

Figure 4 Mixed potentials of sulfide minerals alone and in contact with metallic iron as a function of xanthate concentration. Nonoxidizing conditions, argon purging - 400ccmin~1 natural pH, 25°C. (Reproduced with permission from Rao, Moon and Leja (1976) Flotation, A.M. Gaudin Memorial Vol. Copyright American Institute of Mining Metallurgical and Petroleum Engineers.)

anodic and cathodic sites do not directly interact to deposit potentially reaction stifling product on the anodic site.

The reaction products of galvanic interaction between mineral species can be distinguished on the basis of their location. Firstly, the reaction product may take the form of a surface modification of the mineral, e.g. a metal-deficient sulfide layer, supported and to a greater or lesser degree stabilized, by the underlying, unaltered phase. Secondly, the product may be chemically distinct from, and physically attached to, the original mineral particle. Examples of this are elemental sulfur and ferrous hydroxide coatings. Finally, the reaction product may detach and remove itself from the original mineral, like sulfate anions, copper cations, or ferric hydroxide particulates.

The anodic reaction of sulfides is presently thought to lead to the formation of metal-deficient, sulfur-rich surface species, by releasing an active metal ion which may form a metal hydroxide (M(OH)2):

The formation of metal-deficient sulfide at the surface will tend to activate the surface and cause the metal hydroxy species to detach. However in the case of Fe2#, species remain largely attached, leading to a blanketing effect that tends to hinder particle-bubble attachment. At suitable pH values, the release of reactive cations may lead to the unwanted activation of sulfide minerals, notoriously by copper ions.

As a general precaution against this reaction path, dissolved oxygen levels can be lowered. However, a lowering of pulp oxidative potential tends to lead to a general depression of flotation. An alternative is the elimination of metal-deficient sulfide species, or elemental sulfur, by reaction with aqueous sulfur dioxide:

This is a possible mechanism for galena depression with sulfur dioxide in flotation, in addition to other mechanisms postulated, i.e. a lowering of copper ion activity in solution, xanthate decomposition and a lowering of the oxidative potential below that necessary for xanthate oxidation to dixanthogen.

A reaction path for the cathodic reaction of chal-copyrite, at neutral pH values and in oxygen-starved pulps, was also proposed by Li and Iwasaki:

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