Microbial adhesion and surface modifications of sulphide minerals relevant to flotation and flocculation

Sammanfattning: Biological processes have been attracting attention in mineral processing industry due to their lower operating costs, environmental acceptability and flexibility in adaptation. Bio-hydrometallurgical methods to treat various sulphide and oxide ores have been developed during the years and these processes have been adopted by certain mineral industries. Bio-beneficiation processes involving the separation of value minerals from ores and materials using conventional flotation and flocculation methods have been shown to be promising in recent years. There is a tremendous potential to use microorganisms as flocculants, flotation collectors and/or depressants. A great progress in flotation of sulphides has been realised by using mineral environment - native bacterial strains, such as acidophilic sulphur oxidizing bacteria of Thiobacillus genus. In this work the alterations of surface properties of pyrite and chalcopyrite after biological conditioning with At. ferrooxidans and L. ferrooxidans cells and  with the cells adapted to higher concentrations of Cu and Zn ions, were investigated. Both strains are acidophilic, iron oxidizing microorganisms, with natural occurrence in ore deposits and mine water and high affinity towards sulphide minerals. The changes in surface properties of wild and metal ions adapted bacteria and minerals after bacterial treatment were evaluated by zeta-potential and adsorption, diffuse reflectance FT-IR and XPS studies. The flocculation of particles from settling behaviour in aqueous suspension and Hallimond microflotation tests were performed to quantify the effect of cell treatment on separation processes. Yeast cells of Saccharomyces cerevisiae and Yarrowia lipolytica were tested as a coagulating agent for very fine sulphide mineral separation from silicates. Measurement of contact angles on cell lawn and mineral surfaces using different test liquids were done to determine the hydrophobic/hydrophilic character of bacterial and mineral surfaces, and to estimate the different components ofsurface free energy; Lifshitz-van der Waals and polar, and polar component divided into acid and base components contributing to the total surface free energy. In addition, the Hamaker constant that is essential to construct the DLVO potential energy diagrams of bacterial cell interaction to mineral, has been estimated from the free energy of bacterial adhesion to minerals, which is determined from contact angle data. Although both strains are iron oxidizing, their genetic and metabolic pathways differ and consequently their surface properties. While ferrous grown At. ferrooxidans tends to be slightly negatively charged in the entire pH range studied, L. ferrooxidans cells exhibited higher magnitude of zeta potential and a clear iso-electric point (IEP) at pH 3.3. After zinc and copper ion adaptation the cells surface became less negatively charged and the IEP shifted to pH 2.2 after copper adaptation and to pH 3 after zinc ions adaptation. This shift in IEP is not due to adsorption of cations on the cellssurface but because of altered composition of surface compounds as revealed by XPS analysis. The DRIFT spectra of minerals treated with cells showed the absorbance bands corresponding to cells surface chemical composition identifying cells adhesion to minerals. At. ferrooxidans and L. ferrooxidans  DRIFT spectra are comparable and the metal ions adaptation resulted only in minor changes in absorption peak shapes and intensities.  The contact angle data showed that both cells have similar hydrophobic/hydrophilic properties. Copper and zinc adaptation increases the total surface energy and the polar character of bacterial surface. The total surface energy of S. cerevisae is higher but less polar compared to other bacteria. However, all cells possess a dominant electron donating character. At. ferrooxidans cells altered the surface properties of pyrite and chalcopyrite in different ways, where the IEP shifted to acidic and basic regions in case of pyrite and chalcopyrite respectively. The adhesion of L. ferrooxidans cells on minerals lowered the IEP of minerals, approaching close to that of the cells IEP. The changes in surface charge properties of minerals are corroborated with the results of settling tests. L. ferrooxidans cells selectively depressed the flotation of chalcopyrite but the flotation ability of pyrite remained intact. Yeast cells were successful in selective coagulation of sphalerite and galena fine particles but not silicates. Flotation found to be suitable method for the separation of selectively formed sulphide biocoagulates. The microbial adhesion is essential and critical for the success of bioflotation and bioflocculation processes. The adsorption of bacterial strains onto mineral surfaces is observed to be a fast process and the adsorption densities differed on pyrite, chalcopyrite and sphalerite minerals. Iron (II) grown At. ferrooxidans cells adhere more on pyrite than chalcopyrite but the cells adapted to copper and zinc ions adhered similarly on these minerals. Adhesion to sphalerite is the lowest in all the cases of bacteria. The adsorption of L. ferrooxidans cells on chalcopyrite is higher and also depresses its flotation more compared to pyrite flotation. Differences in adhesive characteristics are explained by electrostatic forces arising due to surface charges and/or considering the character of surfaces involved in the thermodynamic approach. But the extended DLVO theory incorporating dispersive, electrostatic and acid base interaction energies is found to be the more suited for microbial adhesion predictions onto minerals and was in a good agreement with the experimental flotation and flocculation results.