643 sulfur is oxidized to sulfate, accumulation of Rfilm from sulfur deposits is suppressed, and the primary kinetic lever in Figure 3 is Rj(E) and Rct via the high driving force of the transpassive potential. 4.4 Elevated-temperature bio-heap leaching: the Nuton approach Nuton, a Rio Tinto venture, is a commercially deployed elevated-temperature bio-heap leaching system for primary copper sulfide ores. The technology integrates naturally occurring microorganisms cultivated in proprietary bioreactors, heap engineering, aeration management, and digital tools (Rio Tinto, 2025; Nuton, 2023). In late 2025, Nuton achieved first copper production at the Johnson Camp Mine in Arizona, a partnership with Gunnison Copper Corp., targeting approximately 30,000 tonnes of refined copper over a four-year demonstration period: the first industrial-scale validation of this technology (Rio Tinto, 2025). At the core, Nuton appears to be an elevated-temperature bioleaching process. As described publicly by Nuton's venture lead, the elevated temperatures are generated by bacterially driven sulfide oxidation, without the need for external heating, and under the right thermochemical conditions this delivers peak copper recovery from primary sulfides (Burley, quoted in International Mining, 2022). The microbes accelerate sulfide mineral oxidation, generate heat, and enable copper to dissolve into leach solution that is processed via conventional SX–EW. The NI 43-101 technical report for the Johnson Camp Mine confirms primary sulfide copper mineralization dominated by chalcopyrite, with column testing indicating copper leaching extractions up to ~84% under Nuton-related test conditions, and a total copper recovery estimate of 50.1% across all ore types including run-of-mine material (Gunnison Copper Corp., NI 43-101 Technical Report, 2025). Patent literature assigned to Rio Tinto entities discloses a range of potential process elements including silver catalysis (Brown et al., 2020; Hackl et al., 2020), pyrite addition during agglomeration (Fennel et al., 2022; Burley et al., 2022), and nitrogen-containing organic 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 elevatedtemperature bio-heap leaching, can be rationalized within a unified resistance framework in which the overall
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