Interfacial reconstruction by a PBX/PAX–BHA collector pair enables malachite sulfidization flotation in high-Mg2+ recycled water

High Mg 2+ induces hydrophilic deposits that passivate malachite. • Mg 2+ deposition suppresses Cu–S film formation during sulfidization. • Mixed collectors improve malachite flotation in high-Mg 2+ recycled water. • XPS, SEM–EDS and ToF-SIMS reveal Mg coverage and Cu–S film evolution. The flotation recovery of malachite from complex polymetallic ores is often severely depressed by high concentrations of Mg 2+ in recycled water; however, the underlying mechanism and effective mitigation strategy remain unclear. In this study, microflotation experiments combined with surface characterization and spectroscopic analyses were conducted to elucidate the inhibitory role of Mg 2+ in malachite sulfidization flotation and to evaluate the performance of a mixed-collector system. The results showed that in the sodium hydrosulfide (NaHS)–potassium butyl xanthate/potassium amyl xanthate (PBX/PAX) system, increasing the Mg 2+ concentration from 0 to 100 mmol/L decreased malachite recovery by 37.13%. In contrast, the addition of benzohydroxamic acid (BHA) to the PBX/PAX system markedly improved flotation recovery under high-Mg 2+ conditions. Scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS), zeta-potential, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) revealed that Mg 2+ promoted the formation of a heterogeneous Mg(OH) 2 –Cu–S composite deposit layer on the malachite surface, which blocked active Cu sites and suppressed the formation of the Cu–S sulfidized film. Contact-angle and adsorption results further showed that the synergistic interaction between BHA and PBX/PAX accelerated and enhanced collector adsorption, leading to a more stable hydrophobic surface. Fourier transform infrared (FTIR) spectra indicated that xanthates chemisorbed on Cu–S sites through Cu–S–C bond formation, whereas BHA coordinated with surface Cu 2+ via Cu–O/N chelation. Their cooperative adsorption resulted in the formation of a Cu–S/Cu–O/N composite interfacial layer, which effectively alleviated Mg 2+ -induced surface passivation. These findings demonstrated that the mixed-collector system restored the hydrophobicity and flotation performance of malachite in high-Mg 2+ water through synergistic interfacial reconstruction.