Electrochemical oxidation of covellite (CuS) in alkaline solution

The anodic oxidation of the surface of natural covellite in 0.1 M Na₂B₄O₇ (pH 9.2) has been investigated using cyclic voltammetry, chronoamperometry, chronopotentiometry, electrode impedance spectroscopy, and X-ray photoelectron spectroscopy. At potentials < 0.48 V vs S.C.E. (saturated calomel reference electrode), copper dissolution occurred without copper (hydr-)-oxide forming a passive film on the covellite surface; copper ion transport from the bulk covellite through a copper-depleted layer to the electrode/electrolyte interface was rate determining. In the potential range 0.48 to 0.92 V vs S.C.E., the copper (II) solubility was exceeded locally, so that a passivating copper (hydr-) oxide film covered the electrode surface. The oxidation process was then controlled by copper ion transport from the bulk covellite, through both a copper-depleted layer and a copper (hydr-)oxide film, to the electrode/electrolyte interface. The copper (hydr-)oxide film was thin and subsequently could be dissolved chemically in 1 min at open circuit potential. At potentials > 0.92 V vs S.C.E., the copper (hydr-)oxide film thickened, retarding the covellite anodic oxidation process; this film remained on the electrode surface for longer when the electrode was switched to open circuit potential. However, because a small amount of sulfur was oxidized to sulfate at the higher potentials, the passive layers were porous and showed a smaller polarization resistance.