A Visual Insight into the Oxidation of Sulfide Minerals During Bioleaching and Chemical Leaching of a Complex Ore

Abstract

A scanning electron microscope (SEM) study was performed to provide a visual insight into the oxidation patterns of sulfide minerals during chemical and bacterial leaching of a complex ore for 3 days. The mineral grains were studied under SEM before and after bacterial and chemical leaching with or without the addition of ferrous iron to generate ferric iron in situ by bacteria or chemical oxidant (MnO₂). Both mesophilic and moderately thermophilic cultures of bacteria were used in bioleaching tests. A limited oxidation of sphalerite and pyrite, similar to those in acid leaching (control), was observed to occur when no ferrous iron was added. However, the initial addition of ferrous iron into bioleaching media was shown to significantly improve the oxidation of sphalerite and pyrite. Galena was readily oxidized in the presence or absence of bacteria. Sphalerite was oxidized more extensively/selectively than chalcopyrite and pyrite, consistent with their respective nobility/electrochemical activity. Provided that chemical/biological oxidation of sphalerite was intensive, a sulfur-rich layer appeared to form on mineral surface. But, no such layer on pyrite surfaces was discernable. Supplementary bioleaching data were also provided to support SEM observations and to further elucidate the bioleaching characteristics of these sulfide phases. It can be inferred from this study that the oxidation of sulfides proceeds most discernibly via “indirect mechanism” and the generation of ferric iron by bacteria in sufficient quantity is essential for the effective oxidation of sulfide minerals.

Keywords:

• acidophilic bacteria
• bioleaching
• complex sulfide
• leaching
• pyrite
• sphalerite

ACKNOWLEDGMENTS

The authors would like to express their appreciation to the late Dr. C.V. Phillips for his invaluable comments and support, to Mr. S. Pendray and Mr. J. Curnow (Camborne School of Mines) for technical assistance to prepare the resin mounts, to Mr. E.Y. Yazici (Division of Mineral Processing, Department of Mining Engineering, Karadeniz Technical University) for his help during preparation of the manuscript, and to Turkish Ministry of Education for financial support to Dr. H. Deveci during his studies at Camborne School of Mines, University of Exeter.