ELECTROCHEMICAL OXIDATION OF COVELLITE (CUS) IN ALKALINE-SOLUTION

The anodic oxidation of the surface of natural covellite in 0.1 M Na2B 4O7 (pH 9.2) has been investigated using cyclic voltammetry, chronoamp erometry, chronopotentiometry, electrode impedance spectroscopy, and X -ray photoelectron spectroscopy. At potentials < 0.48 V vs S.C.E. (sat urated calomel reference electrode), copper dissolution occurred witho ut copper (hydr-)oxide forming a passive film on the covellite surface ; copper ion transport from the bulk covellite through a copper-deplet ed 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) solubi lity was exceeded locally, so that a passivating copper (hydr-)oxide f ilm covered the electrode surface. The oxidation process was then cont rolled by copper ion transport from the bulk covellite, through both a copper-depleted layer and a copper (hydr-)oxide film, to the electrod e/electrolyte interface. The copper (hydr-)oxide film was thin and sub sequently could be dissolved chemically in 1 min at open circuit poten tial. 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 s witched to open circuit potential. However, because a small amount of sulfur was oxidized to sulfate at the higher potentials, the passive l ayers were porous and showed a smaller polarization resistance. (C) 19 94 Academic Press, Inc.