The gangue sponge: implications for in-situ/heap leaching of chalcopyrite

As the demand for copper increases, processing increasingly low-grade ore has become a critical challenge. Most of these ores are dominated by aluminosilicate gangue minerals, among which Fe-bearing phyllosilicates are the most reactive. The effect of Fe-bearing minerals on the acid leaching of chalcopyrite, the most common primary Cu ore mineral was investigated by comparing mixtures of chalcopyrite with (i) chamosite, (ii) berthierine-chamosite, and (iii) pyrite. The lixiviant consisted of solutions with 0.1 m Fe₂(SO₄)₃ + 0.3 m H₂SO₄, and the experiments were conducted in an orbital shaking water bath at 50 °C for 43 days. The solution pH increased and Eh decreased for all systems, resulting in conditions that became less optimal for Cu recovery. Scanning electron microscope imaging highlighted the porous nature and variable elemental composition of the (partially) dissolved phyllosilicate minerals/amorphous-silica-residue after leaching. Micro X-ray Absorption Near Edge Structure (μXANES) measurements revealed that sulfur exists in multiple oxidation states both within the silicate gangue and around chalcopyrite, emphasising how the formation of an amorphous-silica-residue increased the complexity of the system. This complexity is related to the fact that (partial) dissolution of phyllosilicate gangue minerals during acid leaching results in the release of ions that change the solution chemistry; and form an amorphous-silica-residue, that contains numerous pores. These pores can act as local micro-reactors with non-equilibrium conditions and promote a heterogenous chemical composition. Subsequent mineral surface-fluid interactions in (partially) dissolved phyllosilicate/amorphous-silica-residue pores can vary, depending on the local chemical composition, making the leaching behaviour of chalcopyrite highly dependent on local conditions and on the mineralogical composition of the ores.