Eager for Emmentaler: Early particle scale porosity creation on rock fragments due to reactive gangue mineral dissolution for low-grade chalcopyrite leaching

Hydrometallurgical treatment via heap or in-situ leaching are cost-effective alternatives to conventional pyrometallurgy for Cu-recovery from low-grade ores, but are hampered by slow kinetics and low recovery. This study examined the impact of pH pre-treatment (0 followed by 2, or vice-versa) and NaCl addition on ferric-activated (5 g/L Fe 3+ ) sulfuric acid leaching of low-grade chalcopyrite ores from the Cadia porphyry copper deposit. The overall reactivity during 20 days of leaching was calcite > clinochlore > muscovite > K-feldspar > quartz, depending on lixiviant pH and the cm-scale abundance, texture, grain size, reactivity, and spatial location of gangue minerals in the ore fragments. Addition of NaCl at seawater concentrations had neglectable effect on the pH-driven early dissolution of gangue minerals. Early particle-scale porosity was induced by calcite dissolution, but quickly led to gypsum precipitation and pore clogging. Dissolution of calcite veins penetrating deep into the samples allowed the growth of reaction rims along dissolving veins and pores. Calcite dissolution was followed by clinochlore dissolution, forming lamellar siliceous residues. These siliceous residues locked and shielded chalcopyrite grains from lixiviant early in the leaching process; ion sorption on these residues can also retain Cu or other ions used as additives or catalysts. This study highlights that many processes that limit the efficiency and rate of Cu recovery are controlled by the nature, composition, abundance, as well as the texture of gangue minerals, in addition to ore-mineral colocation. Detailed multi-scale characterisation, from cm- down to μm, is required to optimize Cu-recovery in complex low-grade ores. • Gangue minerals control Cu leaching via porosity & passivation. • Calcite dissolves fast, clinochlore leaves pore-clogging residues. • pH 0–2 dissolves gangue but risks silica gel & precipitates. Pre-treat at pH 2, then pH 0 for optimal Cu recovery. • NaCl aids Cu solubility but not gangue dissolution. • Mineralogy and textures dictate heap leaching efficiency & challenges.