Computer simulation studies of thiol collectors adsorption on sulphide mineral, for flotation process

Abstract

Surface properties of pyrite (FeS2), chalcopyrite (CuFeS2), galena (PbS) and sphalerite (ZnS) most thermodynamically stable surfaces have been studied using first priniciples density functional theory. The most stable surfaces showed the highest surface exposure as it covered a higher percentage of the surface area on morphologies. The interaction of water with such sulphide surfaces was also investigated; the structures of sulphide minerals surfaces were changed in the presence H2O molecules. The surfaces of FeS2 and ZnS relax most while those of CuFeS2 and PbS surfaces change slightly in the presence of H2O molecules. The results on the effect of chain length of DTPs and DTCs on the enthalpies of adsorption on pyrite, galena, chalcopyrite and sphalerite have shown that an increase in chain length of the DTPs resulted in an increase in the enthalpies of adsorption trend for pyrite, galena and sphalerite. This is an important observation since the ligand is the same in all cases and therefore the effect is due to the role of the alkyl group Moreover, we noted a decrease of enthalpies of adsorption with an increase of DTCs chain lengths on pyrite, galena, chalcopyrite and sphalerite. The effect of the branching of the hydrocarbon chain length of the dithiocarbamates on the enthalpy of adsorption of pyrite was investigated. The results show that the configuration of the alkyl chain length of the same carbon number has an influence on the enthalpy of adsorption. Furthermore, the results indicate that there was minimal enthalpy of adsorption when DeDTP was dosed to galena, chalcopyrite and sphalerite minerals as compared to DeDTC and ethyl xanthate. On the other hand, the enthalpies of adsorption of DeDTP on pyrite were very high which represented a greater exothermic reaction than for any of the DeDTC and ethyl xanthate. The bond distance between thiol collector and the surface is consistent with the corresponding calculated adsorption energies. The Mulliken population of S-Fe/S-Pb/S-Zn bond for the adsorption of eX on CuFeS2 surface are high compared to PbS and ZnS surfaces, which indicated that there is a strong covalent bond between S and Fe atoms as compared to S-Pb and Pb-Zn bonds. Such observations are consistent with results of other thiol collectors. It was seen that the Mulliken atomic charges populations of CuFeS2, PbS and ZnS surface layers are different before and after DTPs adsorption. The charges of Fe atom reduce, which indicates that the Zn and Pb atoms become more positive and the Fe atom becomes more negative. In addition, there are changes in the charges of S atoms in ZnS and PbS surface layers before and after H2O absorption, suggesting that the presence of water would affect the adsorption of thiol collector. The densities of states (DOS) of the thiol collectors on surfaces of sulphide minerals have shown a strong hybridisation between the S 3p-orbital HOMO, metals (Fe, Pb, Zn) 3d-orbital for pyrite and chalcopyrite, 6p-orbital for galena and (3d and 4s)-orbitals for sphalerite. The collector S 3p-orbital reduces to zero states on the surfaces of Fe, Pb and Zn atoms. The Fe-S bond population for DeDTP is lower than that of DispDTP and DbDTP in pyrite, respectively. For chalcopyrite the DeDTP Fe-S population is higher than both DispDTP and DbDTP: similar trends were observed for Pb-S and Zn-S, however, the Pb-S bonding was less covalent as compared to the Fe S in chalcopyrite. The DTPs Fe-S bond population is generally higher in chalcopyrite than in pyrite. Mulliken charges analysis indicated that the DTPs S atoms lost charges and the metals gained with a decreasing DTPs chain length for pyrite: a similar trend was observed for chalcopyrite. The DTPs gained electrons from galena and sphalerite surface. For DTCs pyrite and chalcopyrite surface Fe atoms gain more electrons in the presence of DeDTP than other DTPs, while galena and sphalerite lost most electrons in the presence of DbDTC than other DTCs. As for xanthate, the Mulliken bond charges indicated that the S atoms and the metals lost charges, suggesting that some charges reside at the internuclear region between the metals and sulphurs (M– S). These show that electron charges are collector and mineral dependent; collector would be an electron acceptor or donor depending on the mineral makeup. A comparison of the computational results, isothermal titration calorimetry (ITC) and microfloatation experiments for the interaction of DeDTP, DeDTC and eX with pyrite and galena was made. The calculated adsorption energies between thiol collectors and mineral surfaces were always more exothermic than the experimentally determined ones. In computational calulations, water adsorption was found to reduce the reactivity of Fe and Pb atom for the interaction with thiol collectors and bring the adsorption energies closer to the magnitude of the experimental values. FeS2 (100) surface heat of adsorption depict similar trends to experimental recoveries from microfloation for DeDTP, DeDTC and eX, while FeS2 (111) heats of adsorptions for DeDTP and DeDTC are consistent with ITC experimental results. Lastly, calculated DeDTP and DeDTC adsorptions on the PbS (100) are also consistent with experimental recoveries.