New insights into selective depression of copper-activated sphalerite by zinc ions in chalcopyrite flotation

Effective separation of sphalerite and chalcopyrite by flotation is often compromised by the unintentional activation of sphalerite with copper ions, which creates a hydrophobic copper sulfide–like surface and renders its flotation performance comparable to chalcopyrite. Zinc sulfate is widely used to depress sphalerite; however, the atomic-scale origin of its selective action remains poorly understood. In particular, the mechanism by which zinc sulfate strongly depresses copper-activated sphalerite while exerting minimal influence on natural chalcopyrite, despite their similar surface chemistries, is unclear. This study aimed to elucidate the selective depression mechanism using a combined experimental and theoretical approach. Microflotation tests and contact-angle measurements were conducted to evaluate the depressive effects of zinc sulfate, while surface chemical states were examined using X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) calculations were employed to model the adsorption interactions of key zinc hydrolysis species on different mineral surfaces. The results showed that zinc hydrolysis species adsorbed weakly on chalcopyrite but interacted strongly with sphalerite through Zn-S bonding. Notably, on copper-activated sphalerite, these species formed particularly strong Cu-O coordination bonds with surface copper sites. This interaction was supported by the emergence of Cu(II)-O species in the XPS spectra and was further supported by high adsorption energies and distinct electron-density redistributions observed in the DFT calculations. These findings indicate that selective depression is primarily attributed to the formation of stable Cu–O bonds between zinc hydrolysis products and copper sites specific to the activated sphalerite surface. This atomic-scale insight provides a theoretical basis for optimizing reagent schemes in the flotation of copper–zinc sulfide ores.