X-ray tomography study on the leaching dynamics of, and pore evolution in, large chalcopyrite ore particles during chloride leaching

The dissolution of chalcopyrite (CuFeS₂) in a heap leaching environment is known to progress slowly; this is mainly attributed to the formation of a product layer that inhibits the dissolution of copper from the mineral. Transporting the dissolved ions from the inner regions of the larger particles to the bulk solution through cracks and/or pores might also slow the process. Therefore, a systematic long-term study has been undertaken to determine the presence and propagation of pores, fissures or crack networks in 12–17 mm particles of a sulfide ore containing mainly quartz (SiO₂), chalcopyrite and pyrite (FeS₂) during its dissolution under chloride-rich heap leaching conditions. Each particle was placed in a cylindrical receptacle and flooded with 0.1 mol/L H₂SO₄ solutions at 0, 20 and 150 g/L of chloride (as NaCl) using 0, 0.5 and 1 g/L initial cupric ion as the oxidant and leached for a period of up to 180 days at room temperature (18–22 °C). The generation of cracks and fissures and dissolution of mineral phases were observed using X-ray Computed Tomography (X-CT) at regular intervals during leaching. Additional experiments were run using 150 g/L Cl⁻ to evaluate acidity from pH -0.4 (1.0 mol/L H₂SO₄) up to pH 3, as well as the effect of temperature at room temperature and 50 °C.

The results indicated that copper dissolution increased with increasing chloride concentration at room temperature and that the initial presence of cupric ions somewhat enhances the extraction. It was found that between pH 1 and 0.2 (0.1 mol/L H₂SO₄) at 150 g/L Cl⁻ the dissolution is enhanced but inhibited at higher acid concentration. Low acidity (pH 3) promotes the precipitation of Fe that can block the pores and inhibit the dissolution of chalcopyrite. The X-CT scans confirmed that the gradual evolution of fissures and network of cracks over time enhanced the solution contact with the value mineral inside the particles, which then gradually disintegrated. In line with the leaching results, this effect seemed to be intensified at the higher chloride concentrations and when increasing temperature from 20 to 50 °C. The homogenous dissolution of chalcopyrite grains throughout the particle hints at the presence of galvanic coupling with pyrite grains as the key mode of chalcopyrite dissolution.