Depression of pyrrhotite superstructures in copper flotation: A synchrotron X-ray powder diffraction and DFT study

Pyrrhotite naturally occurs in various superstructures including magnetic (4C) and non-magnetic (5C, 6C) types, each with distinct physicochemical properties and flotation behaviors. Challenges in accurately identifying and quantifying these superstructures hinder the optimization of pyrrhotite depression in flotation processes. To address this critical issue, synchrotron X-ray powder diffraction (S-XRPD) with Rietveld refinement was employed to quantify the distribution of superstructures in the feed and flotation concentrates of a copper–gold ore. To elucidate the mechanisms influencing depression, density functional theory (DFT) calculations were conducted to explore the electronic structures and surface reactivity of the pyrrhotite superstructures toward the adsorption of water, oxygen and hydroxyl ions (OH⁻) as dominant species present in the flotation process. S-XRPD analysis revealed that flotation recovery rates of pyrrhotite followed the order of 4C<6C<5C. DFT calculations indicated that the Fe 3d and S 3p orbital band centers exhibited a similar trend relative to the Fermi level with 4C being the closest. The Fe 3d band center suggested that the 4C structure possessed a more reactive surface toward the oxygen reduction reaction, promoting the formation of hydrophilic Fe-OH sites. The S 3p band center order also implied that xanthate on the non-magnetic 5C and 6C surfaces could oxidize to dixanthogen, increasing hydrophobicity and floatability, while 4C formed less hydrophobic metal-xanthate complexes. Adsorption energy and charge transfer analyses of water, hydroxyl ions and molecular oxygen further supported the high reactivity and hydrophilic nature of 4C pyrrhotite. The strong bonding with hydroxyl ions indicated enhanced surface passivation by hydrophilic Fe–OOH complexes, aligning with the experimentally observed flotation order (4C<6C<5C). These findings provide a compelling correlation between experimental flotation results and electronic structure calculations, delivering crucial insights for optimizing flotation processes and improving pyrrhotite depression. This breakthrough opens up new opportunities to enhance the efficiency of flotation processes in the mining industry.