Work conducted in Australia and the United States has shown that most sulphide minerals can, under certain conditions, be floated in the absence of collectors (Chander, 1988a; Woods, 1988; Ralston, 1991). All these studies imply that, if not oxygen itself, then at least an oxidising potential is required for collectorless flotation. It has been established that sulphide minerals oxidise through a continuum of metal-deficient sulphides of decreasing metal content through to elemental sulphur (Equations 12.5 and 12.6) by reactions of the type:

(12.25)

and

(12.26)

These sulphur-rich, metal-deficient zones can render the mineral hydrophobic, provided that the local conditions are such that the metal oxides/hydroxides formed by the reaction are solubilised. Excessive oxidation can produce thiosalts (Equation 12.7), and, ultimately, sulphate (Equation 12.8), together with metal ions which may re-adsorb, as hydrolysis products, on to the mineral, producing hydrophilic surfaces.

Buckley et al. (1985) studied the surface oxidation of galena, bornite, chalcopyrite, and pyrrhotite, and found that for each mineral the initial oxidation reaction is the removal of a metal component from the surface region to leave a sulphide with similar structure to the original mineral but with lower metal content. Metal-deficient sulphide layers containing high sulphur-metal ratios are probably stabilised by the underlying mineral because they have the same sulphur lattices. The authors showed that flotation of the minerals could be accomplished without the aid of collectors when a metal-deficient sulphide, rather than elemental sulphur, is formed. Naturally hydrophobic sulphide minerals, such as molybdenite,...