On the Role of the Electronic Bulk Properties of Semiconducting Minerals in their Interaction with Electrolytes

The solid-state properties of semiconducting natural minerals have often been considered to influence their interactions with electrolytes, although the importance of these properties in this context has never been unequivocally established. For instance, differences in semiconducting properties have been proposed by Paul et al. [1] to be responsible for observed variations in the electrochemical behaviour of natural galena, whereas Springer [2] had not found an influence of electronic conduction properties in pyrite, chalcopyrite, and galena on their electrical polarization behaviour. Simkovich [3], and Madeley and Boorman [4] observed that the point-defect structure of grown, heavily doped minerals appreciably effect their flotation behaviour, whereas Gam boa et al. [5] found no such influence with synthetic galena. Such disagreement is perhaps not too surprising, since it can easily be overlooked that the structure of the double layer at a mineral-electrolyte interface, not only depends on the bulk properties of the solid, but is also extremely sensitive to contamination from the ambient. For synthetic semiconductors this was shown by Boddy and Brattain [6], who described the effect of traces of cupric ion on the electrical characteristics of the germaniumelectrolyte interface. Further, Larrabee [7] concluded from radioactive tracer studies of the adsorption of metal ions on the compound semiconductors GaAs and InSb that traces of those elements, the electrochemical potential of which, in solution, is more positive than the potential at the semiconductor surface, will be deposited irreversibly at that surface. We found that also with natural galena (n type, 5 • 10~5 flm), trace amounts of impurities which were not originally present in the material could cause considerable changes to its reactivity. Furthermore, when low levels of certain contaminants were introduced, similar or identical effects could be observed that had been attributed by other workers to variations in the electronic bulk properties of semiconductors. Specifically it was found that traces of copper have an appreciable influence on (a) dissolution rates, (b) electrical polarization, and (c) the electrochemical potential of galena.