Figure 2 – Conceptual mixed-potential diagram illustrating CuFeS2 dissolution during ferric sulfate leaching. The active-to-passive transition potential and the onset of transpassive dissolution are strongly dependent on temperature, mineral source, solution composition, and measurement method. Under typical industrial heap leach conditions, solution potentials often fall within or near the passive region, though the precise boundaries of this region vary with local mineralogy and operating temperature. 3. MECHANISMS OF CHALCOPYRITE PASSIVATION The passivation of CuFeS2 during acidic leaching has been studied extensively for over fifty years but remains an area of active research and controversy. Numerous mechanisms have been proposed, including the formation of sulfur-rich surface layers, copper-rich sulfide phases, ferric precipitates, and semiconductor effects within the mineral lattice. Despite this sustained effort, no single mechanism has been conclusively established as the primary cause of passivation, and the relative importance of each contribution remains disputed (Hackl et al., 1995; Klauber, 2008; Watling, 2013). Rather than being mutually exclusive, these mechanisms are better interpreted as multiple resistances to electron transfer and mass transport, acting in series within any given surface domain, and in parallel across domains with different surface chemistries (Figure 3). The controlling resistance is neither spatially uniform nor temporally fixed. It evolves across the surface as reaction products accumulate and as the local chemistry develops during leaching. As a result, the measured dissolution rate reflects an ensemble average over surface domains with different film compositions, electronic properties, and kinetic behaviour, rather than a single ratelimiting step acting uniformly across the entire mineral surface. Within a given surface patch, the overall reaction rate can be viewed as the result of several resistances in series, including charge-transfer resistance at the mineral surface (Rct), electronic barriers associated with semiconductor junctions (Rj(E)), resistive surface films (Rfilm), and diffusion limitations within the boundary layer (Rdiff). Across the mineral surface, however,
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