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. The central strategy across the BHP patent family appears to be Eh-window management: keeping EM below the onset of CuFeS₂'s passive region. The collapse in dissolution at 620 mV vs SHE relative to 550–600 mV aligns with a transition into a passive state where Rfilm grows rapidly and/or Rj(E) becomes prohibitive. Chloride additionally stabilizes Cu(I) chloride complexes, alters sulfur morphology toward porous, non-adherent deposits, and thereby reduces Rfilm. Both the original patent and the companion peer-reviewed studies emphasize that dissolved oxygen is essential even under low-potential conditions, pointing to Rdiff control via oxygen supply and underscoring the importance of careful heap aeration design. 4.3 Nitrate-assisted heap leaching Nitrate-based approaches to chalcopyrite heap leaching have attracted patent activity but have not yet achieved commercial deployment; they are summarized here for completeness and for their mechanistic interest within the resistance framework of Figure 3. 4.3.1 Nitrate–nitrite heap oxidation with NOx management (BHP) A recent BHP patent (Chibwana, 2024) discloses heap irrigation with acidic nitrate solution plus nitrite at ~50–500 ppm to oxidize sulfide minerals in-heap, combined with air/oxygen injection to convert evolved NO to NO₂, followed by hydrolysis to recover nitric/nitrous acid and reduce nitrate loss. Disclosed nitrate concentrations are high (~25–80 g/L, preferably ~30–50 g/L). NOx containment relies on an impermeable heap cover with suction withdrawal and external scrubbing, with combined losses claimed below ~5%. The fundamental commercialization challenge is well recognized: NO and NO₂ are toxic and their emission from an uncovered heap would create an unacceptable atmospheric hazard. H₂O₂ addition most likely re-oxidizes evolved NO and NO₂ back to NO₃⁻. In the presence of nitrate there would be no bacterial activity in the heap, potentially eliminating the need for aeration, which may partially mitigate gas discharge risk given the low sulfide content of typical heap ores. The mechanistic basis for nitrate efficacy in both the BHP and Ceibo systems is best understood as a brute-force transpassive approach: the nitrosyl ion (NO⁺), formed under acidic conditions, has sufficiently high oxidation potential to set the mixed potential well above the passive region and into the transpassive regime, where chalcopyrite dissolves without forming a persistent passive layer. In this regime elemental sulfur is further oxidized to sulfate, so the concern about Rfilm from sulfur accumulation does not apply — a key distinction from lower-potential chloride or thiocarbonyl systems. In the framework of Figure 3, the cathodic branch is shifted so far upward that the mixed potential intersection bypasses the passive region entirely, and both Rfilm and Rj(E) become less relevant because the system operates above their potential range of influence.
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