An electrochemical study of natural enargite under conditions relating to those used in flotation of sulphide minerals

The electrochemistry of natural enargite (Cu3AsS4) in an alkaline solution of berate buffer is studied. Photoelectrochemical experiments confirm that the samples studied are p-type semiconductors. The potential range in which the photocurrent is noticeable (below -0.4 +/- 0.2 V vs. SCE) is more negative than the potential range of flotation (near 0.0 V vs. SCE). The electrochemical behaviour of the electrode is studied by electrochemical impedance spectroscopy (EIS) around this potential and by voltammetry. The time dependence of the impedance over some tens of minutes leads us to believe that a surface layer forms over the potential range studied, and the law for the growth of this layer corresponds to two processes: the formation and dissolution of the layer. The voltammetric study, interpreted in the light of a treatment assuming two successive transfers with a surface intermediate, supports this interpretation. Between 1 Hz and 10 kHz, the impedance spectra could be analysed with an equivalent circuit including a constant phase element, valid whatever the applied potential in the range from -0.1 to 0.2 V vs. SCE in the presence or absence of a collector (ethyl xanthate), while an equivalent circuit with simple elements was found with galena under similar conditions. This result is tentatively discussed in relation to particular structural properties of ternary compounds before and after electrochemical oxidation. The effects of xanthate additions are studied by voltammetry and EIS, also. No proof of the underpotential deposition of xanthate is observed. It seems that surface oxidation and reactivity with the collector are closely linked, and a process of ion exchange occurring at potentials more positive than 0.0 V is proposed to explain our results.