Pauliina Nurmi M.Sc.(Tech.) completed her doctorate at the Department of Chemistry and Bioengineering at Tampere University of Technology (TUT), a study of microbiological methods for oxidising and removing iron in bioleaching. Bioleaching is a process used in the mining industry where microbes are utilised to dissolve metals from minerals.
The first part of Nurmi's study explored factors influencing iron oxidation in bioleaching by determining the oxidation kinetics at pH 1 of a culture containing Leptospirillum ferriphilum bacteria.
"The culture in question tolerated really high concentrations of the test metals. The results are excellent," Pauliina Nurmi says.
She also tested and developed mathematical models to predict iron oxidation rates in bioreactors. The combined effects of multiple metals on iron oxidation kinetics have not previously been modelled. The formulas developed here enable the predicting of iron oxidation reaction rates in bioleaching.
PROMISING FINDINGS ALREADY PUT TO USE
Bioleaching is more environmentally friendly than the traditional smelting in extracting metals from ores. The process generates no gaseous emissions and does not require as much energy. Moreover, it is a profitable method even for low-grade ores. The method has also been tested for recovering metals from mining waste and junk metal. However, the problem with bioleaching is that it generates acidic effluents with a high iron content.
In the second part of her study, Nurmi developed a method based on iron oxidation and precipitation to remove iron and sulphate from process waters and effluents in mining. Iron and sulphate removal was studied in a laboratory-scale bioprocess treating acidic process effluent from heap bioleaching. Neural network modelling was employed to predict the amount of ferric iron precipitated in a variety of conditions.
"Up to 96-99 per cent of the iron precipitated in the process. Under optimal process conditions, we also managed to precipitate on average 66 per cent of the sulphate," Nurmi says.
Talvivaara Mining Company Plc., the company exploiting a multi-metal ore deposit in Sotkamo using bioleaching, is further developing this method in a pilot project together with TUT and is also applying for a patent. Some bioreactors have already been developed for treating acidic mine drainage, but these are designed for substantially lower iron content. Biological methods for removing iron and sulphate from bioleaching process effluents have not previously been described in a scientific publication.