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The aim of the research programme is the optimization of a bacterial leaching process for the treatment of auriferous arsenical pyrites.

The Olympias pyrite concentrate produced in Chalkidiki, northern Greece, will be examined as the case study. The Olympias concentrate comprises approximately 25% arsenopyrite and 68% pyrite and assays 26 parts per million gold. The implementation of the research programme will include the following phases.

Phase 1 will involve: the exchange of information among the cooperating partners referring to biooxidation testwork results previously conducted on Olympias concentrate and other refractory arsenical pyrites; the definition of experimental procedures to be followed; and the characterization of T. ferrooxidans cultures available within the research groups.

Phase 2 will involve basic studies in small scale pachuca reactors to enable modelling of bacteria growth and its association to pyrite and arsenopyrite leaching rates. Variables to be investigated will include surface area (particle size and solids concentration variation), biomass (bacterial numbers and protein levels), and dissolved metal concentrations (ferric, iron, total iron and arsenic). Distribution of bacteria between solid and liquid phases will be examined. The effect of solution chemistry on bacterial activity, leach rates and precipitates formation will be investigated.

Phase 3 will involve laboratory batch tests in pachua reactors at high pulp densities maintaining increased concentrations of iron(3+) ions in solution. Analytical techniques for bacteria activity monitoring will be evaluated. The reactor system to be employed will facilitate control of dissolved metals at specific levels without reducing bacteria or solids concentration. Optimum leaching conditions for mixed pyrite and arsenopyrite samples and the Olympias concentrate will be defined.

Phase 4 will involve continuous laboratory tests in miniplant to examine and improve bioleaching kinetics for Olympias pyrite concentrate. Basic research findings will be used to define optimum operating conditions related to pH, electromotive force(EMF), solids concentration, solution chemistry and bacterial activity. Gold recovery achieved after biooxidation will be determined. Stability of solid residues produced after the neutralization of biooxidation liquid effluents will be studied with particular attention given to arsenic(3+) ions.

Phase 5 will involve optimal process flowsheet development. Olympias concentrate will be examined as the case study. Materials and energy balance will be concluded.

Phase 6 will involve a pilot scale run if the targets set at the initiation of this programme are satisfied (ie increased solids concentration and short residence time coupled with high gold extractions after biooxidation). The run will be carried out at the 3.2 m3 BRGM pilot plant facilities but is not included in the programme.

Phase 7 will involve economic evaluation of the optimized bacterial leaching method as a process alternative for the treatment of refractory pyrite ores and concentrates.


Aegean Metallurgical Industries SA
Eratoshenous St 1
11635 Athinai

Participants (2)

Bureau de Recherches Géologiques et Minières (BRGM)
Avenue De Concyr
45060 Orléans
University of Wales, Cardiff
United Kingdom
Newport Road
CF2 1XH Cardiff