different regions may experience different surface chemistries and therefore operate as parallel current pathways contributing to the overall dissolution current. The right side of Figure 3 illustrates this heterogeneity, where different surface domains with distinct film compositions and resistances operate in parallel. Each domain contributes a local current density (i1, i2, i3) corresponding to surface fraction θk, with the total surface current density given by isurface = θ₁i₁ + θ₂i₂ + θ₃i₃ and Σθk = 1. These domains may represent regions covered by different reaction products such as CuS, CuS + S⁰, or CuS with iron oxyhydroxides. Figure 3 therefore highlights that CuFeS2 passivation likely arises from a combination of transport limitations, surface film formation, and semiconductor effects acting simultaneously and heterogeneously across the mineral surface, rather than a single blocking mechanism. Of course, any such limitation is dependent on the acidic solution actually making its way to the mineral grain surface, which is a consistent challenge in percolation leaching. 3.1 Sulfur surface layers Oxidation of sulfide minerals commonly produces elemental sulfur or polysulfide species. These products may accumulate on the mineral surface and form partially blocking films that hinder transport of oxidizing species to the mineral surface (Hackl et al., 1995). However, the passivating role of elemental sulfur is itself contested: porous sulfur layers have been reported under some conditions that do not measurably impede leaching, suggesting that sulfur morphology and porosity, rather than mere presence, determine whether a transport barrier is established (Klauber, 2008). In the resistance framework shown in Figure 3, sulfur and related surface products contribute to a film resistance (Rfilm) whose magnitude depends on the physical character of the deposit rather than its composition alone. 3.2 Metal-deficient sulfides Preferential dissolution of iron from CuFeS2 may produce copper-rich sulfides resembling covellite (CuS). These phases possess markedly different electronic properties from CuFeS2 itself and may alter electron-transfer kinetics at the mineral/solution interface (Ghahremaninezhad et al., 2013). As leaching progresses and the Cu-rich surface layer develops p-type semiconductor character, the resulting p–n junction at the interface with the underlying n-type CuFeS2 introduces an additional electronic barrier to charge transfer (Ren et al., 2022). Within the framework of Figure 3, the copper-rich covellite-like overlayer contributes to Rfilm, while simultaneously generating the potential-dependent junction resistance Rj(E) at its interface with the underlying ntype CuFeS2. Rj(E) is not confined to CuS-covered domains. It is present across all surface domains as a property of the underlying CuFeS2 semiconductor interface, as elaborated in Section 3.4. 3.3 Ferric precipitates Under strongly oxidizing conditions, ferric hydroxysulfates such as jarosite or iron oxyhydroxides may precipitate on mineral surfaces. These phases can partially block reactive sites and further increase resistance to charge transfer and mass transport (Stott et al., 2000). Jarosite formation is favoured at elevated ORP and pH values above approximately 1.5 to 2 in the presence of K+ and/or Na+, conditions that are readily encountered in active bioleach heaps. Such precipitates contribute to the overall surface film resistance term Rfilm in Figure 3 and may be
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