641 Ag/AgCl than controls while simultaneously delivering higher copper extraction: the catalyst may shift the mixed-potential intersection to higher current density at lower potential, as predicted by the mixed-potential framework of Figure 2. Independently of these film-related effects, Olvera et al. (2018) demonstrated that TU and FDS directly reduce both the anodic and cathodic charge-transfer resistances of CuFeS2, suggesting that decreasing Rct represents a third, distinct mechanistic lever operating independently of passive film dissolution. The surfactant component of the extended patent family may address a complementary resistance: Rdiff at the heap and particle scale. Improved wettability reduces the hydrophobicity of elemental sulfur deposits on grain surfaces, promotes more uniform reagent distribution through the ore bed, and enhances oxygen and ferric ion transfer to reactive mineral sites. These effects are particularly important in heaps where unsaturated preferential flow and channeling frequently limit effective leaching kinetics more than surface chemistry alone. 4.2 Chloride-assisted heap leaching and Eh-window control The BHP chloride heap leach patent family comprises several related disclosures addressing different aspects of Eh-window management for chalcopyrite dissolution. The foundational patent (Muller, Basson and Nicol, 2011) makes the core mechanistic claim: CuFeS2 dissolution is maximized when the mineral surface potential is maintained below ~600 mV vs SHE, and preferably within the 550–600 mV vs SHE window, using acidic chloride or mixed chloride/sulfate media in the presence of dissolved oxygen, at pH ~1–2, dissolved oxygen >1 ppm, and chloride concentrations of ~5–100 g/L from NaCl, MgCl₂, saline or sea water. These potentials are well below the passive region of CuFeS₂. The patent provides kinetic evidence that higher potentials are detrimental: copper dissolution in batch flask experiments reached ~70.4% under low-potential conditions (no ferric) versus ~6.87% under high-potential conditions (with ferric) over ~912 h. In controlled-potential tests at 35 °C, copper dissolution after ~1000 h showed a pronounced optimum: ~73.1% (550 mV), ~79.2% (580 mV), ~76.2% (600 mV), but only ~22.5% at 620 mV, with the 550 mV test reaching ~88% after 1240 h (Muller, Basson and Nicol, 2011). These are among the clearest direct demonstrations in the patent literature that more oxidizing conditions can drive CuFeS₂ into an unfavorable kinetic regime. Work by Velásquez-Yévenes et al. (2010) and Nicol et al. (2010) corroborates these findings, reporting enhanced dissolution in the 550–620 mV vs SHE window in dilute chloride solutions with dissolved oxygen essential for maintaining linear kinetics. A subsequent BHP Chile patent (Patiño Martinez, Nicol and Rautenbach, 2017) extended the concept by disclosing a pre-treatment phase using a high-chloride acid solution at solution potentials exceeding 700 mV vs SHE in the absence of microorganisms, followed by an active leach cycle. This apparently contradicts the low-potential Eh-window strategy of the 2011 patent, but the pre-treatment is directed at secondary sulfides (particularly chalcocite) where high-potential cupric chloride oxidation is beneficial, rather than at CuFeS₂ directly. The Eh-window concept was further developed in the SaL process (Rautenbach, 2018), which couples chloride leaching with an alternating rest-and-irrigate heap operating cycle. During extended rest periods the mineral surface is exposed to chloride-rich leach solution under controlled aeration, promoting passive-layer dissolution and reagent diffusion into ore particles before the next irrigation cycle flushes dissolved copper to the PLS circuit. Column data disclosed in the patent show the SaL cycle (referred to therein as "sal Q1 and Q2") outperforming both conventional chloride irrigation and thermophile bioleaching at ambient temperature on 85% chalcopyrite ore. The 2018 patent notes that seawater or brine may serve as the chloride source, which would be particularly relevant for water-scarce Chilean operations, though this is presented as a chloride supply option rather than a distinct process variant. In the framework of Figure 3, the rest-and-irrigate cycle can be interpreted as a strategy to allow sufficient contact time for Rfilm reduction via chloride complexation of the passive layer while also improving oxygen availability during rest periods to maintain potential within the active dissolution window.
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