| Lead oxide mineral is an important lead resource, and cerussite is a typical lead oxide mineral. With increasing lead consumption, lead sulfide ores cannot meet such requirements; lead oxide resources may be an important lead source and must be efficiently utilized to address the imbalance between metal lead supply and demand. The sulfidization flotation is the most commonly and commercially used method for concentration and pretreatment of lead oxide minerals, and sulfidization is the most critical stage in this process. Thus far, various sulfidization techniques have been used to pretreat lead oxide minerals. However, only the surface sulfidization method has been applied to industrial production because of technical and economic considerations. Nevertheless, the surface sulfidization method continues to exhibit certain defects. Therefore, surface sulfidization reinforcement is crucial to concentrate lead oxide minerals by flotation.The electronic structures and properties of cerussite crystal and surface were initially calculated using density functional theory (DFT). The dissolved species of cerussite as well as distribution coefficients of various lead hydroxyl complexes in aqueous solutions were discussed in detail through thermodynamics and solution chemical calculation, which served as a theoretical foundation for subsequent studies. Then, the flotation performance and sulfidization mechanism of cerussite in the direct sulfidization flotation system were analyzed through micro-flotation tests, distribution regularities of various S species in the Na2S solutions, surface adsorption experiments, zeta potential measurements, X-ray diffraction (XRD) measurement, scanning electron microscopy and energy dispersive spectrometer (SEM-EDS), as well as X-ray photoelectron spectroscopy (XPS) analyses. In addition, the effect of S species adsorption on the electronic structures and properties of cerussite surface was investigated through DFT. This investigation further verified the sulfidization mechanism of cerussite based on the electronic level, which also provided a basis for comparison for further discussion. The flotation and sulfidization behaviors of cerussite in the presence of chloride ions were also investigated and elucidated through micro-flotation tests, surface adsorption experiments, zeta potential measurements, XRD measurement, SEM-EDS, XPS analysis, and DFT. Compared with the results of direct sulfidization flotation, the contribution of chloride ions to the sulfidization flotation of cerussite was discussed and verified. Finally, the essence of surface sulfidization reinforcement was elucidated and revealed based on experimental and electronic levels through thermodynamics and solution chemical calculation, dissolution tests, zeta potential measurements, XPS analysis, and DFT. Surface sulfidization reinforcement is attributed to the increased number of surface active sites on the cerussite surface after it was pretreated with chloride ions.The results of electronic structures and properties of cerussite crystal and surface show that perfect cerussite has poor electrical conductivity because of its broad band gap. The valence electron configurations of Pb, C, and O atoms for cerussite after geometric optimization are Pb 5d106s1.876p0.85, C 2s0.902p2.43, and O 2s1.832p4.82, respectively. Both covalent and ionic bonds exist in the cerussite crystal, and the chemical bonds of C-O and Pb-O are covalent and ionic, respectively. The distance of the Pb-O bond is longer than that of the C-O bond, so the Pb-O bond can be broken first during crushing and grinding. The electronic structures and properties of cerussite (110) surface greatly differ from those of cerussite crystal, and the whole bands of the former move toward the direction of low energy. In addition, the Pb atom contributes the most to the density of states near the Fermi level, and it becomes the active site for subsequent surface reactions.The flotation performance of cerussite is significantly influenced by the existing species in the pulp solution. The calculation results of solution chemistry indicate that the distributions of dissolved species in the aqueous solution are closely related to the pH values of the solution. At pH values below 7.5, lead in the aqueous solution mainly exists as dissociated Pb2+; at 7.5<pH<9.7, Pb(OH)+ is the dominant species in the aqueous phase; when the solution pH is between 9.7 and 11.2, the lead in the aqueous solution mainly exists as Pb(OH)2; at 11.2<pH<11.7, Pb(OH)3 is the dominant species in the aqueous phase; at pH>11.7, the dominant lead species in the aqueous solution is Pb(OH)42-The results of micro-flotation tests showed that cerussite floatability initially increased and then decreased as the concentration of Na2S increased. The surface analysis and solution measurements of sulfidized cerussite revealed that the increase in Na2S concentration is beneficial to the formation of more lead sulfide species on the mineral surface. In this case, the residual concentrations of S in pulp solutions also increased, demonstrating that the inhibition of excessive sulfide ions on cerussite floatability is due to the effect of abundant residual sulfide ions on sulfidized cerussite. The lead sulfide species that formed on the mineral surface was composed of lead sulfide (PbS), lead disulfide (PbS2), and lead polysulfide (PbSn), and the proportional increase of disulfide (S22-) and polysulfide (Sn2-) in total S could improve the activity of lead sulfide species. The DFT calculation of the adsorption of S species on the mineral surface showed that HS- can be stably adsorbed onto the cerussite surface, after which some transferred electrons are evident between the Pb and S atoms. An obvious overlapping phenomenon occurs between Pb 6p and S 3p orbits near the Fermi level (i.e., the sulfidization behavior of cerussite occurs through the bonding interaction between the S atom from the HS- and Pb atoms on the mineral surface), thereby forming lead sulfide species.The addition of chloride ions prior to sulfidization significantly improved cerussite floatability at the same concentration of Na2S and sodium isoamylxanthate (NaAX) compared with the cases when chloride ions were absent. The percentage of increase in flotation recovery was 10%～16%. More sulfide ions in the pulp solution were consumed to produce lead sulfide films on the mineral surface in the presence of chloride ions, and the proportions of disulfide and polysulfide in total S increased in this case. The DFT calculation indicated that the number of transferred electrons of bonded atoms and the hybridization degree between the Pb and S atoms greatly increased after the mineral surface was pretreated with chloride ions, and new Pb 6p and S 3p state density peaks appear near the Fermi level.The dominant species of lead chloride complexes in the pulp solution is PbCl+ in the sulfidization flotation of cerussite with chloride ions. The solubility of cerussite in the NaCl solution was lower than that in deionized water, and its zeta potential in the NaCl solution was higher than that in deionized water. Moreover, the atomic concentrations of C and O decreased and the atomic concentration of Pb increased on the mineral surface after it was pretreated with chloride ions compared with the results without chloride ions. These results consistently indicated that the Pb content on the cerussite surface increased through migration of Pb species in the system after the mineral surface was pretreated with chloride ions (i.e., the number of active sites on the cerussite surface increased). The calculation results of DFT also revealed that both the number and activity of lead atoms on the cerussite surface increased and improved after PbCl+ was stably adsorbed onto the mineral surface. Thus, the content of lead sulfide species on the sulfidized cerussite surface increased in the presence of chloride ions.In this case, the dissolution of inner cerussite was preferably screened. Fewer dissolved lead ions were evident in this system, and thus a small amount of lead ions remained dissolved in the Na2S solution alone. The solubility of lead ions decreased significantly in the Na2S solution along with chloride ions.
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