Bacteria-sulfide mineral interactions with reference to flotation and flocculation

Abstract: 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 sulfide and oxide ores have been developed during the years and these processes have been adopted by certain 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 sulfides has been realised by using mineral environment - native bacterial strains, such as acidophilic sulfur oxidizing bacteria of Thiobacillus genus. In this work, the alterations of surface properties of pyrite and chalcopyrite after biological conditioning with A. ferrooxidans and L. ferrooxidans cells were studied. Both strains are acidophilic, iron oxidizing microorganisms, with natural occurence in ore deposits and mine water and high affinity towards sulfide minerals. The changes in surface properties of minerals after bacterial treatment were evaluated by zeta- potential and adsorption studies and recording diffuse reflectance FT-IR spectra. The flocculation of particles from aqueous suspension and their settling behaviour and Hallimond microflotation tests were performed to quantify the effect of cell treatment on separation processes. Measurement of contact angles on cell and mineral surfaces with different test liquids were done to determine the hydrophobic/hydrophilic character of bacterial and mineral surfaces and to estimate the different components of surface free energy; Lifshitz-van der Waals and acid-base components contributing to the total surface free energy. In addition, the Hamaker constants that are essential to construct the DLVO potential energy diagrams of bacterial cells interaction to minerals, have been estimated from the free energy of bacterial adhesion to minerals, determined from contact angle data. Although both strains are iron oxidizing, their genetics and metabolic pathways differ and consequently their surface properties. While ferrous iron grown A. 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. A. 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. Although the adsorption of bacterial strains onto mineral surfaces observed to be a fast process, the adsorption densities differred onpyrite and chalcopyrite. The adsorption of L. ferrooxidans cells on chalcopyrite is more and also depress its flotation more compared to pyrite flotation. The DRIFT spectra of minerals treated with cells showed the absorbance bands corresponding to cells surface chemical composition. The contact angle data showed that both cells have similar hydrophobic/hydrophilic properties. The surface thermodynamic and DLVO theory predictions revealed bacterial adhesion on minerals in agreement with the experimental results.