Bioleaching of marmatite flotation concentrate by Acidithiobacillus ferrooxidans

Bioleaching of a marmatite flotation concentrate by Acidithiobacillus ferrooxidans was carried out at 35 degreesC and an initial pH of 2.0 on an orbital shaker at 160 min(-1). Experimental results indicated that the adapted strains markedly increased the dissolution rate of marmatite when compared with the sterile control and the original strains. The fact that Fe3+ ions played a key role in the dissolution of marmatite was demonstrated by the chemical leaching of the concentrate with different concentrations of Fe3+ ions. The main role of the microorganisms was to oxidize Fe2+ ions into Fe3+ ions during the bioleaching process. S.E.M. observations showed the morphological features of the mineral sample and leach residues. The cross-section image of residue particles revealed two different regions, a periphery consisting of reaction products and an undissolved core. It was shown by X-ray diffraction and EDX analysis that the leach residues were composed of elemental sulfur, precipitate of jarosite and undissolved mineral impurities. These facts suggested that the elemental sulfur formed was not completely oxidized and dissolved only partially in the marmatite bioleaching even in the presence of bacterial strains. The sulfur product coated the residue particles and negatively affected the dissolution of marmatite. The marmatite was dissolved preferentially during the bioleaching process by Acidithiobacillus ferrooxidans, as compared with other sulfide minerals such as pyrite and galena in the concentrate. (C) 2004 Elsevier B.V. All rights reserved.