Surface chemistry of Pb-activated sphalerite

Abstract

Activation of sphalerite flotation by lead ions (Pb) is an important aspect of selective separation of sphalerite and galena, but its molecular mechanism remains poorly understood. In this paper, the surface chemistry of sphalerite in the Pb activation process at pHs 4 and 9 was investigated using inductively coupled plasma-atomic mission spectrometry (ICP-AES), time of flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). ICP results showed that the ion exchange ratio of Pb:Zn is not 1:1 during the activation at both acidic and alkaline pHs, indicating multiple adsorption mechanisms. ToF-SIMS results showed that three kinds of Pb secondary ion fragments, i.e., Pb+, PbOH+ and PbS+ were determined on the Pb-activated sphalerite surface, which provided direct evidence for the possible mixed activation mechanisms. In addition, ToF-SIMS depth profile results exposed that the Pb species not only distribute on the outermost sphalerite surface but also diffuse into the crystal structure of the mineral. XPS studies revealed the formation of PbS and PbSO3 at pH 4 and PbS, PbO with Pb(OH)2 at pH 9. It was also revealed that Pb uptake was higher at pH 9 than pH 4. Furthermore, XPS depth profile results showed the formation of PbSO3, Pb(OH)2 and PbO have not exceeded 2 nm depth, representing chemical bonding on the mineral surface, while PbS diffusion took place down to around 10 nm depth through ion exchange mechanism. All these details have established that the activation of sphalerite by Pb involves a mixed activation mechanism of ion exchange and Pb-oxide as activating sites. The PbS species formed by the ion exchange not only distribute on the outermost layer of sphalerite surface but also penetrate the surface and diffuse into the bulkphase of sphalerite.