Bioflotation

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Flotation is a well-known process. It is said that miners discovered it while washing their clothes. Certain chemicals of the washing powder adsorbed with metal sulfides and made them float on the washing solution. Today, flotation is widely used for producing concentrates of raw materials, e.g. for separating metal sulfides of interest from undesired ones or from gangue material. An aqueous suspension of finely grained ore is conditioned with flotation chemicals, making the metal sulfides either more hydrophobic (collector) or more hydrophilic (depressant). In the first case, small air bubbles are produced which aggregate with the hydrophobic particles and make them float foam-like on the suspension. In the second case, hydrophilic sulfides are depressed, i.e. kept in the liquid phase. Although this is a very useful method it is environmentally harmful as currently used depressants such as sulfur dioxide, cyanide, and chromate are very toxic. The solution to this problem could be the use of bacteria for flotation processes. Of particular interest are the so-called leaching bacteria, acidophilic metal sulfide oxidizing species, which are already used in hydrometallurgy. These bacteria selectively attach to sulfide surfaces (their substrate), forming a biofilm, i.e. a layer of bacterial cells plus excreted extracellular polymeric substances (EPS) [1, 2]. The slimy matrix of EPS consists of polysaccharides and lipids [3-5], sometimes also proteins and nucleic acids are found. Bacterial attachment alters sulfides to become more hydrophilic. Consequently, leaching bacteria such as Acidithiobacillus ferrooxidans have successfully been employed as a depressant in pilot-scale bioflotation experiments. However, our knowledge on the selectivity of certain bacterial species to certain sulfides is limited. In the future, free EPS and related compounds should be tested. The objective would be to replace the currently used toxic depressing chemicals with EPS-derived reagents consisting of the functional units of natural EPS from leaching bacteria.

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References

This article is based on the information from Bioflotation by Thore Rohwerder 2006.pdf

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