Treating bioleach effluent
Pilot trials are ongoing to remove iron and sulphate, using a bioprocess, from the acidic effluent that results from heap bioleaching metals from minerals. The work is being undertaken at the Talvivaara Mining Company’s nickel mine in Sotkamo, Finland.
The technique, developed at the Tampere University of Technology in Finland, is said to be more cost effective and environmentally friendly than current methods for iron removal using hydroxide precipitation with lime or limestone.
Pauliina Nurmi, one of the researchers, claims, ‘This is the first scientific study published to systematically address iron and sulphate removal from solutions associated with bioleaching’. She explains that, in the new bioprocess, ‘the ferrous iron is oxidised to ferric iron in a low-pH fluidised-bed reactor (FBR) with an activated carbon matrix used as a carrier material for the iron-oxidising micro-organisms. [An alkali] is then added to increase the pH and precipitate the ferric iron’. A gravity settler enables final iron and sulphate removal, with the solution recycled back to the FBR.
Nurmi says up to 96-99% iron and, on average, 66% sulphate has been precipitated at laboratory scale.
‘The operating costs of this technology are about half the costs of the traditional chemical treatment,’ claims Marja Riekkola-Vanhanen at Talvivaara. She says it consumes far less chemicals and produces a smaller and easier to handle waste product for disposal. ‘The process does not produce gypsum, like the chemical processes, but jarosite, which contains both iron and sulphate and is stable in low pH values. The product is easy to separate from the solution and does not need as much storage area.’
Bioleaching results in an acidic waste solution that is high in iron content due to the use of iron and sulphur oxidising bacteria to catalyse the reactions. Iron’s natural abundance in sulphidic ores also means it is extracted during hydrometallurgical processing.
‘Recirculation of iron-containing leach solutions back to the process is common but leads to high concentrations of dissolved iron. The iron has to be removed because it may interfere with subsequent metal recovery and the effluents cause adverse effects on vegetation and fauna if dispersed in the environment,’ says Nurmi. ‘The other iron removal methods utilising biological iron oxidation have not been developed for bioleaching solutions, but generally for acid mine drainage, which usually contains significantly less iron.’
Having completed some initial trials at Talvivaara, plans are afoot for a larger-scale pilot test run to optimise the process parameters. The company is exploring the reactor’s pH and temperature, the dissolved oxygen and redox potential, metal concentrations, and ferrous iron oxidation, hydraulic retention time and influent flow rate. The results are being compared to the company’s chemical iron removal plant, which currently runs at over 1,000m3/h.
Miguel Diaz at AMEC Earth and Environmental, consultants to the mining industry, based in Ashford, UK, sees the potential for this work but is cautious about some of the claims made. He notes, ‘If you have a high density sludge plant, the separation of gypsum and ferric hydroxide is not a problem, so this jarosite will have to settle really well to make a significant difference’, and the statement that jarosite is stable would need to be ‘backed up by real data’.
Making a model
The technique, developed at the Tampere University of Technology in Finland, is said to be more cost effective and environmentally friendly than current methods for iron removal using hydroxide precipitation with lime or limestone.
Pauliina Nurmi, one of the researchers, claims, ‘This is the first scientific study published to systematically address iron and sulphate removal from solutions associated with bioleaching’. She explains that, in the new bioprocess, ‘the ferrous iron is oxidised to ferric iron in a low-pH fluidised-bed reactor (FBR) with an activated carbon matrix used as a carrier material for the iron-oxidising micro-organisms. [An alkali] is then added to increase the pH and precipitate the ferric iron’. A gravity settler enables final iron and sulphate removal, with the solution recycled back to the FBR.
Nurmi says up to 96-99% iron and, on average, 66% sulphate has been precipitated at laboratory scale.
‘The operating costs of this technology are about half the costs of the traditional chemical treatment,’ claims Marja Riekkola-Vanhanen at Talvivaara. She says it consumes far less chemicals and produces a smaller and easier to handle waste product for disposal. ‘The process does not produce gypsum, like the chemical processes, but jarosite, which contains both iron and sulphate and is stable in low pH values. The product is easy to separate from the solution and does not need as much storage area.’
Bioleaching results in an acidic waste solution that is high in iron content due to the use of iron and sulphur oxidising bacteria to catalyse the reactions. Iron’s natural abundance in sulphidic ores also means it is extracted during hydrometallurgical processing.
‘Recirculation of iron-containing leach solutions back to the process is common but leads to high concentrations of dissolved iron. The iron has to be removed because it may interfere with subsequent metal recovery and the effluents cause adverse effects on vegetation and fauna if dispersed in the environment,’ says Nurmi. ‘The other iron removal methods utilising biological iron oxidation have not been developed for bioleaching solutions, but generally for acid mine drainage, which usually contains significantly less iron.’
Having completed some initial trials at Talvivaara, plans are afoot for a larger-scale pilot test run to optimise the process parameters. The company is exploring the reactor’s pH and temperature, the dissolved oxygen and redox potential, metal concentrations, and ferrous iron oxidation, hydraulic retention time and influent flow rate. The results are being compared to the company’s chemical iron removal plant, which currently runs at over 1,000m3/h.
Miguel Diaz at AMEC Earth and Environmental, consultants to the mining industry, based in Ashford, UK, sees the potential for this work but is cautious about some of the claims made. He notes, ‘If you have a high density sludge plant, the separation of gypsum and ferric hydroxide is not a problem, so this jarosite will have to settle really well to make a significant difference’, and the statement that jarosite is stable would need to be ‘backed up by real data’.
Making a model
The work at Tampere has also explored the factors that influence the iron oxidation reaction necessary for bioleaching. Nurmi has, in turn, developed mathematical models to predict iron oxidation rates based on the combined effects of multiple metals (inhibitors). ‘The simultaneous effects of multiple inhibitory
metals have not previously been systematically studied or modelled,’ says Nurmi. Her research has found Fe2+ oxidation to
Fe3+ occurs by a Leptospirillum ferriphilum culture even at high concentrations of Fe3+, nickel and zinc.
‘The model can be used for developing kinetic models for various bioleaching
processes, for determining the retention times and dimensions of Fe2+ oxidation bioreactors,’ she says. ‘High-rate biological iron oxidation at elevated metal concentrations is a prerequisite for the emerging technique of two-stage tank bioleaching.’
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