complexing agents (Kittelty et al., 2020), though the relationship of these patents to the deployed Nuton technology has not been publicly confirmed. In the framework of Figure 3, the Nuton approach addresses multiple resistances simultaneously. Elevated temperature directly reduces Rct by increasing exchange current densities and accelerating interfacial reaction rates, potentially shifting the controlling regime away from the slow surface steps that dominate at ambient conditions. Improved oxidant regeneration, through microbially enhanced Fe²⁺ oxidation and sustained Fe³⁺ supply, strengthens the cathodic branch in Figure 2 without requiring high external oxidant addition, and engineered aeration and moisture management directly target Rdiff, which is frequently rate-limiting in large heaps. The semiconductor/p–n junction interpretation of chalcopyrite passivation (Ren et al., 2022) implies that simply increasing bulk Eh may not overcome Rj(E) unless the surface state is modified or breakdown thresholds are crossed. The Nuton strategy appears to address this indirectly: by sustaining elevated temperature and active microbial oxidation, the system maintains conditions in which the passive regime is less stable and the mineral surface is continuously renewed by ongoing dissolution, rather than relying on a specific chemical or electrochemical depassivation mechanism. The exact ORP control strategies are not specified in public descriptions, but, as of writing, the approach is best understood as a multifaceted engineering solution that avoids prolonged residence in the pseudo-passive regime through a combination of temperature, biology, and heap design rather than through potential escalation or chemical surface modification alone. 5. OVERALL SYNTHESIS CONCLUSIONS The four approaches to overcoming chalcopyrite passivation reviewed in this paper, thiocarbonyl catalysis, chloride-assisted Eh-window control, nitrate-driven transpassive oxidation, and elevated-temperature bio-heap leaching, can be rationalized within a unified resistance framework in which the overall dissolution rate reflects the combined influence of charge-transfer resistance (Rct), semiconductor junction resistance (Rj(E)), surface film resistance (Rfilm), and diffusion resistance (Rdiff). Each technology targets a different subset of these resistances, and their relative maturity and commercial viability differ substantially. Table 1 summarizes the key distinguishing features of each approach within this framework. The most commercially advanced approaches for CuFeS2 leaching, the Jetti thiocarbonyl system and Nuton elevated-temperature bioleaching, achieve their results through fundamentally different mechanistic strategies. The Jetti approach chemically dismantles the p–n junction at the chalcopyrite surface, restoring the mineral to its native resistor state and allowing dissolution to proceed at lower mixed potential. The Nuton approach circumvents the passive regime through sustained elevated temperature and active microbial oxidation, maintaining conditions in which passivation is continuously destabilized rather than chemically removed. The BHP chloride systems occupy an intermediate position, engineering the solution potential to remain within an active dissolution window and exploiting the beneficial morphology of sulfur formed under chloride conditions. The nitrate-based approaches are mechanistically distinct in operating at what are likely transpassive potentials, where the passive regime is bypassed entirely by oxidative force,
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