Chalcopyrite and gangue mineral surface reactions controlling copper heap leaching: Grain-scale insights using low-voltage microprobe analysis and 3D imaging

• Geochemistry, grain-scale EPMA and µCT show mineral-fluid interface reactions. • Temperature is a critical control for secondary mineralization and pH/Eh stability. • Si-rich gangue dissolution creates pervasive residues coating on copper minerals. • Mineral replacement of Cu-sulfides can optimize Cu recovery via textural control. Copper is a critical material for energy systems and modern technology, necessitating economical extraction methods like heap leaching. This study investigates surface reaction mechanisms limiting copper recovery, focusing on gangue mineral interactions and copper sulfide replacement reactions in high-salinity water. Integrated batch and flow-through experiments (25–50 °C, pulp densities 5–20 g/100 mL, up to 30 days) were combined with low-voltage electron microprobe analysis (EPMA), reflective microscopy, micro-computed tomography (µCT) imaging, and aqueous speciation modelling. Copper dissolution displayed a characteristic ∼10-hour induction period dominated by preferential iron release and reductive-like surface transformation of chalcopyrite into other copper-enriched intermediates. After this period, copper extraction rates accelerated at 50 °C; however, a downward trend was observed across all solid loadings. This decline suggests either the precipitation of secondary mineral phases or the progressive depletion of reactive mineral surfaces. Finer particle sizes, elevated temperature, and longer fluid residence time substantially increased dissolved copper concentration. Secondary phases identified include: (1) secondary copper phases (covellite, chalcocite, tenorite, langite) within dissolution pits and fractures formed via pseudomorphic replacement of chalcopyrite. These phases are more reactive and their formation results in volume expansion during replacement, leading to micro-fractures and self-enhancing porosity; (2) siliceous residues from gangue dissolution that can coat mineral surfaces and inhibit leaching; (3) iron phases like goethite that limit jarosite formation; (4) evolving sulfur phases: mobile and minimally passivating within 7 days, transitioning to immobile amorphous layers by 8–30 days, progressively reducing hydraulic conductivity; and (5) iron hydroxy sulfates (e.g., jarosite) formed from gangue, brine and Fe ions interactions. These consume Fe 3+ required for oxidative chalcopyrite dissolution. Overall, the results offer new insights into the thermodynamic and kinetic controls of dissolution and precipitate growth. By predicting mineralogical alterations based on texture and spatial factors, this work informs strategies to manage secondary phases and optimize copper heap leaching efficiency.