MICROBIAL LEACHING OF CHALCOPYRITE CONCENTRATE AT DIFFERENT TEMPERATURES

1. MICROBIAL LEACHING OF CHALCOPYRITE CONCENTRATE AT DIFFERENT TEMPERATURES Objective of the research The main objective of the research was to study the possibilities for increasing the efficiency of copper extraction from chalcopyrite by means of comparative experiments for bioleaching using microorganisms growing at different temperatures. Materials and methods The cultures were adapted to the concentrate by conceptive transfers of late-log-phase subcultures into concentrate suspensions with increasing pulp densities in a nutrient medium with following composition (in g/l): (NH4)2SO4 1.0, KH2PO4 0.5, KCL 0.5, MgSO4.7H2O 0.5, Cu(NO3)2 0.01, yeast extract 0.5, with initial pH of 1.8-1.9. The nutrient media used for the cultivation of the mesophilic culture was without yeast extract. The adaptation procedures were carried out at the relevant temperatures i.e. 33oC, 55oC and 75oC. The mesophilic culture consisted of the species Acidithiobacillus ferrooxidans, At. thiooxidans, Leptospirilum ferrooxidans and Acidiphilium cryptum. The moderately thermophilic culture consisted of Sulfobacillus thermosulphidooxidance and At. caldus. Leaching conditions: agitated 2L buffled bioreactors containing 900 ml of the nutrient medium, 100 g concentrate (with particle size minus 0.074 mm) and 100 ml of an active microbial inoculum, consisting of a late-log phase culture preliminary adopted to the concentrate and containing about 109 cells/ml. The leaching was carried out at stirring rate of 800 rpm for 14 days (for batch leaching) at the relevant temperature. Apart from mechanical stirring , air containing 0.2% CO2 was injected into the bioreactors at a rate of 0.3 l/l mineral suspension. Ilieva R1., Iliev M1., Peev Z1., Groudeva V1., Groudev S2. 1-Department of Microbiology, Faculty of Biology, University of Sofia, Sofia 1421, Bulgaria 2-University of Mining and Geology, Sofia 1700, Bulgaria Table 1 Chemical composition of the concentrate Table 2 Microbial continues leaching of copper from the concentrate at different temperatures Results The leaching by the mesophilic and moderately thermophilic bacteria was relatively not very efficient and maximum copper extractions of 32.5% and 53.8% respectively, were achieved at 10% pulp density within a residence time of 14 days. The leaching by the extremely thermophilic Achaea was much more efficient and the highest copper extraction of 82.4% for 14 days was achieved by a mixed culture consisting of archaea of the genera Sulfolobus, Acidianus and Metallosphaera. It was found that the efficient leaching of chalcopyrite was connected with the ability of the relevant microorganisms to possess high sulphur-oxidizing activity at relatively low pH (about 1.3-1.5). No passivation films of elemental sulphur and jarosites were detected on the chalcopyrite surface in such microbial cultures. Conclusions The data from this study demonstrates that a real efficient microbial leaching of chalcopyrite can be achieved only by means of extremely thermophilic chemolitotrophic archaea possessing high iron- and sulphur-oxidizing activities. The direct contact of these archaea with the surface of this mineral as well as their ability to grow in a leach system, containing high concentrations of heavy metals makes the preliminary adaptation of the relevant microbial culture to the mineral substrate subjected to leaching an important stage in the development of the complex technological approach for processing chalcopyrite-bearing concentrates. Acknowledgements: The study is supported by Project 13/2013/SU