Mechanisms of the hydration effect on direct flotation of hemimorphite: An experimental and DFT study

Hemimorphite, as a most common Zn–O mineral, is an important source of zinc resources. Oleic acid direct flotation method is effective to recover hemimorphite. However, strong hydration of hemimorphite leads to flotation difficulties, the mechanism of hemimorphite hydration remains unclear. In this study, two hydrated surface models were established through the adsorption of H+ ions and –OH groups on hemimorphite surfaces to investigate the influence of hydration on direct flotation of hemimorphite using micro-flotation tests, zeta potential analysis, FTIR measurements and DFT calculations. Experimental results show that pH severely affects direct flotation of hemimorphite. At pH < 9, the surface adsorbs H+ ions; at pH > 9, –OH groups are adsorbed on the surface, leading to a rapid decrease in zeta potential. DFT results show that H adsorption shifts Zn 4s orbitals towards the Fermi energy level and leads to hemimorphite surface layer being in an electron-deficient state, promoting the stable sp3 hybridization of oleic acid (OA) with Zn atoms to form outer-orbital complexes. However, –OH adsorption increases the steric hindrance of Zn atoms and causes an electron-enrichment state of hemimorphite surface layer, hindering the OA-Zn interaction. The study provides new insights into the mechanism of hemimorphite hydration and helps to achieve interface regulation for direct flotation of hemimorphite.