Bioleaching of sulfide minerals in continuous stirred tanks

Abstract

Bioleaching in stirred tanks for the selective recovery of precious and base metals from sulfide concentrates is a fascinating subject for at least two reasons. The first reason is the high potential of industrial-scale applications of this technology that has acted as a driving force for countless investigations required to make the economics of this process really attractive. Around forty years ago, this was just a concept imagined by microbiologists and metallurgists, but already at the end of the 1970s studies at pilot-scale in Russia, Canada and South Africa showed that the concept was technically sound. At the middle of the next decade, the experiment in industrial context was undertaken by Gencor at Fairview mine after the works led by Eric Livesey Goldblatt on an arsenical sulfide concentrate containing gold refractory to direct cyanidation. Since that time, a steady growth of projects using this technology has persisted as illustrated by the Figure 1. The second reason is the growth selectivity and the steadiness of the microbial ecosystems in the bioreactors compared to the diversity of the natural environment that have given the opportunity to study the interactions between micro-organisms and minerals in privileged conditions. Bioleaching in agitated tanks at industrial scale is only relevant to the treatment of sulfidic concentrates and consist in using the catalytical enhancement exerted by some micro-organisms to oxidise sulfides to release valuable metals they contain. The sulfide compounds to (bio)-oxidize in the existing industrial plants are essentially pyrite and arsenopyrite in various proportions. Many successful investigations and demonstration operations have been run on other metal-bearing sulfides, like sphalerite (ZnS), pentlandite ((FeNi)9S8), covellite (CuS), chalcocite (Cu2S), and chalcopyrite (CuFeS2). From the chemical engineering point of view, bioleaching in agitated tanks is a continuous-flow steady-state process. The circuit of bioreactors configured in series or in parallel or combination of the two is fed with the slurry of the sulfidic concentrate, nutrients and air to bring the oxygen required to the sulfide oxidation. Micro-organisms are injected once at the beginning of the start-up of an operation and a batch culture is maintained until a certain point as close as possible to the middle of the exponential phase of the bacterial growth when the feed in fresh substrate can begin. A continuous flow of substrate and nutrients through the tanks is then ensured to keep optimum growth of micro-organisms required for the fastest degradation of the sulfidic minerals. The situation of equilibrium between substrate feed rate and stable microbial growth is called chemostat. This chapter provides an overview of the operating conditions of this technology and therefore to emphasize the field of applications for the present and the future. © 2007 Springer.