L-histidine as a green depressant for low-alkalinity separation of zinc‑sulfur minerals via differential copper coordination

The efficient flotation separation of sphalerite from pyrite remains a considerable challenge because of their similar floatability in Cu 2+ -activated systems, as well as the poor selectivity and environmental limitations of conventional depressants. In this study, L-histidine (L-HIS), an environmentally benign small organic molecule, was employed as a depressant for sphalerite/pyrite separation under low-alkalinity conditions (pH 8). Its selective depression mechanism toward Cu 2+ -activated pyrite was systematically investigated through flotation tests, multi-scale surface characterization, and theoretical calculations. The results showed that L-HIS selectively depressed pyrite while maintaining sphalerite floatability, producing a recovery difference of approximately 92.51%. This selectivity originated from the different states of Cu species on the two mineral surfaces. For pyrite, adsorbed Cu species were weakly bonded to surface S sites and coordinated strongly with the amino N sites of L-HIS, forming stable Cu N bonds. This weakened the Cu S interaction, promoted the partial removal of the Cu activation layer, and inhibited SEX adsorption. Meanwhile, L-HIS introduced hydrophilic functional groups onto pyrite, markedly reducing its hydrophobicity. In contrast, Cu species on sphalerite mainly existed as lattice-substituted Cu species with stable Cu S bonds. L-HIS showed only weak interaction with sphalerite, so SEX adsorption and surface hydrophobicity were preserved. This study provides a green depressant strategy for low-alkalinity flotation separation of Cu 2+ -activated sulfide minerals.