636 Under mixed-potential conditions, the dissolution rate of chalcopyrite is determined by the intersection of anodic and cathodic polarization curves (Figure 2). Experimental measurements show that the exchange current density for ferric reduction on chalcopyrite surfaces is low compared with noble metals, contributing to slow overall reaction kinetics (Ghahremaninezhad et al., 2012; Yue & Asselin, 2014). Figure 2 illustrates the conceptual mixed-potential framework governing chalcopyrite dissolution. The anodic polarization behaviour of chalcopyrite (solid blue curve) exhibits an active–passive transition. This process is often associated with the formation of surface reaction products and it can be measured experimentally at slow potentiodynamic scan rates or via stepped potentiostatic experiments (ViramontesGamboa et al., 2007; Ghahremaninezhad et al., 2013). The cathodic polarization curve (dashed red line) represents the reduction of ferric ions to ferrous ions in acidic solution. The intersection of the two curves defines the mixed potential (EM) and corresponding dissolution current density (rate of the electrochemical reaction), id, for the system. Under typical sulfate heap leaching conditions, solution potentials often fall within or near the passive region of the CuFeS2 polarization curve, resulting in slow dissolution kinetics despite relatively high bulk solution oxidation–reduction potentials (Ren et al., 2022; Viramontes-Gamboa et al., 2007). 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).
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