Phase transition of Fe-containing phase in naturally cooled and water-quenched copper slags during high-temperature treatment

Iron (Fe) accounts for the highest-content of valuable metal recoverable from copper slag, which is most effectively recycled using pyrometallurgical high-temperature treatment. It is believed that the roasting temperature significantly affects the physical structure of copper slag. Therefore, this paper used thermogravimetry-differential scanning calorimetry (TG-DTG-DSC), X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), vibrating sample magnetometry (VSM), and scanning electron microscopy (SEM) to examine the Fe phase transformation in water-quenched copper slag (WQCS) and naturally cooled copper slag (NCCS) after high-temperature treatment (700–1100 °C). The main Fe-containing phases in the WQCS were represented by fayalite (Fe2SiO4) and magnetite (Fe3O4). As the temperature increased, the Fe2SiO4 underwent oxidative decomposition and re-formation, with a phase transition law of Fe2SiO4→Fe2O3+Fe3O4→FeSiO3+Fe2SiO4, while that of the Fe3O4 was Fe3O4→Fe2O3→Fe3O4. The phase transition processes were more complex in the NCCS, where the main Fe-containing phases were Fe2SiO4, Fe3O4 and chalcopyrite (CuFeS2). The oxidative decomposition temperatures of the Fe2SiO4 ranged from 700 °C to 850 °C. As the temperature increased, the phase transition law of Fe3O4 was Fe3O4→Fe2O3→Fe3O4→Fe2O3. Lastly, at 700 °C, the CuFeS2 began to oxidize and decompose, with a phase transition law of CuFeS2→FeS + Fe2O3→Fe3O4.