| Summary: | Hydrometallurgy may be an alternative to the currently practiced smelting process for
copper extraction from chalcopyrite (CuFeS₂). However, the low temperature
hydrometallurgical processes for chalcopyrite continue to face challenges, mostly relating to
their slow dissolution rates or high sulfuric acid production. The slow dissolution rate of the
mineral is strongly linked to the formation of the passive film on its surface. However,
despite 40 years of research on this topic, there is still not a complete agreement between
researchers about the composition and stability of chalcopyrite’s passive film in sulfuric acid
solutions. In this work, the nature of chalcopyrite’s passive film and its stability were studied
by application of a variety of electrochemical techniques. Additionally, the electrochemical
results of the chalcopyrite study were compared to those obtained for a pyrrhotite electrode
(Fe₁₋xS), as pyrrhotite electrochemistry represents a simplified case of the chalcopyrite
system. X-ray photoelectron spectroscopy (XPS) was used to analyze the composition of the
product layers formed on the surface.
It is shown that the chalcopyrite electrode is passive for potentials up to 0.90 VSHE.
Above this potential, transpassive dissolution occurs. Results of XPS studies have suggested
that a metal-deficient sulfide film (Cu₁₋xFe₁₋yS₂₋z) is the most plausible copper and iron
containing sulfide phase which passivates the surface of chalcopyrite. In addition, an outer
layer of iron oxyhydroxide (FeOOH) forms on the passive film. FeOOH forms via oxidation
of the passive film’s ferrous sulfide phases. The thickness of the sulfide passive film was
calculated to be approximately 6.7 nm. It is demonstrated that the transpassive dissolution of
chalcopyrite is significantly linked to oxidation of sulfur (from sulfide in the passive film to
elemental sulfur and maybe sulfur species with higher oxidation states, e.g. thiosulfate). No
elemental sulfur or polysulfide species were detected on the surface for potentials below 0.90
VSHE.
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