Applications of heterotrophic microorganisms to the mineral processing

Biotechnological processing of minerals is generally considered an environmentally benign approach to concentrate valuable minerals, extract valuable metals from ores and secondary resources, or to remove contaminants from industrial minerals. The method may have lower energy consumption and operational costs in comparison with the conventional chemical processes. Despite the varied application of the chemolithoautotrophic microorganisms in the acidic dissolution of base metals mostly from sulfidic resources, or bio-recovery of metal bearing minerals, chemoorganoheterotrophic microbes can be applied to a wider range of sources including ores, wastes, and industrial residues to beneficiate valuable minerals or to dissolve various elements. Nevertheless, the requirement of neutral conditions and sugar-containing media enhances the risk of microbial contamination and limits the development of heterotrophic bioleaching on an industrial scale.BACKGROUND AND OBJECTIVESHeterotrophic microorganisms such as fungi and bacteria exploit organic carbon as an energy and carbon source. They degrade the organic substrates to produce metabolites which can interact with minerals. Microbe-mineral or metabolite-mineral interactions are applied to different mineral beneficiation and hydrometallurgical processes such as bio-flocculation, bio-flotation, bio-sorption, bioleaching, and bio-precipitation.Heterotrophs may leach elements from resources through the bio-electrochemical reduction of metal oxide compounds. Reduction of ferric to ferrous iron by some bacteria in anaerobic condition or by oxalic acid secreted to the culture medium is an example. Protonation is another way to solubilize metals from minerals. Acidic metabolites generate H+ that attacks the mineral surface and destabilize the atomic bonds of the mineral crystals. In contrast to acidification, microbes can alkalify their growth environment by degrading proteins to ammonium ions using the protease enzyme. The alkaline conditions may lead to the dissolution of quartz or plagioclase minerals. The other leaching mechanism is metal complexation by organic ligands or chelators excreted to the culture media. They form stable complexes which enhance metal dissolution.Furthermore, the microbial cells per se and their extracellular polymeric substances can be applied as flotation or flocculation reagents. Proteinic and polysaccharidic substances may be absorbed on the mineral particle surface and make them hydrophobic or hydrophile, respectively. Being adsorbed on the mineral surface, the gram-negative and positive bacterial cells may act as depressants or collectors depending on the cell wall composition.Despite the undeniable capabilities of the heterotrophic microorganisms in metal leaching,there are some constrictions to apply them on industrial scales. The main reason is that theconditions they need to survive, and flourish are also suitable for many other organisms.Therefore, sterilization is required to prevent microbial contamination. However, sterilizationof the ores and culture media is nearly impossible.MATERIALS AND METHODSThe present paper reviews some recent achievements of the author and his team in theapplication of heterotrophic microorganisms to the biological leaching of metals from ores andsecondary resources.RESULTS AND DISCUSSIONAspergillus niger is one of the widely used fungi which is applied by the researchers aroundthe world due to its high organic acid production capabilities that can be applied to leachvarious metals from primary and secondary resources.Hosseini et al. (۲۰۰۷) employed A. niger isolated from pistachio shell and NCIM۵۴۸ to removeiron contaminant from a kaolin sample. About ۴۳% of iron was removed after one month usingthe strain isolated from pistachio shell at ۲۰ g/l pulp density. Results are illustrated in Fig. ۱.The continuous decline of pH during the process was the result of oxalic and citric acidexcretion to the medium. The mentioned organic acids can dissolve iron oxides/hydroxide fromthe ore by complexation, reduction, or protonation mechanisms.Later in ۲۰۰۹, Aghaei et al. applied response surface mythology to model and optimize theoxalic and citric acid production by A. niger (isolated from pistachio shell) along with ironremoval.Fig. ۲ shows the application of the Bacillus licheniformis cells and metabolite to hematite andgoethite flocculation investigated by Sadeghizadeh et al. (۲۰۱۷). Obviously, increasing pH value enhances the settling of both minerals by different bioreagents. Also, the bacterial cells and polysaccharides have the highest flocculation effect on either hematite or goethite.