Electrobioleaching of base metal sulfides
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I.
INTRODUCTION
THIOBACILLUSferrooxidans is the primary organism implicated in the leaching of several sulfides. Its commercial potential in the leaching of lean grade and refractory ores, desulfurization of coals, and in mineral beneficiation has been well established. [~ 7] In addition to the above applications, bioprocessing has also been found to be very promising in the leaching of copper, zinc, and pyrite concentrates, and substantial metal concentrations in the leach liquor could be built up using specially designed reactors. IS-u] However, in many cases, slowness of the biooxidation reactions and practical difficulties associated with the harvesting of sufficient biomass of these organisms have limited the scope and wider commercial utilization of this biotechnology. Bacterial activity as well as cell biomass are two important parameters influencing bioleaching rates. Besides microbiological factors, several physicochemical, electrochemical, as well as mineralogical factors also influence biooxidation reactions. For example, the redox potential of the mineral and the medium, type of crystal structure and conductivity of the minerals, medium acidity, concentrations of ferrous, ferric, and other soluble anions and cations, temperature, availability of oxygen and carbon dioxide, solubility of products of oxidation, particle size, and pulp density of the leaching system are all parameters to be controlled, t~-3,7] Electrochemical principles could be used with advantage in bioleaching. Galvanic interactions among sulfide minerals in a bioleaching system would enhance the selective biooxidation of active minerals. [~4 i7] Mineral dissolution in a leaching medium could also be controlled by application of direct current (DC) potentials. Some electrochemical aspects of pyrite and zinc sulfide oxidation in the presence of Thiobacillus ferrooxidans have been reported recently, t~sa91 Activity as well as cell biomass of Thiobacillus ferrooxidans could be enhanced through electrochemical means. Enhanced yields of iron oxidizing bacteria may be achieved by in situ electrolysis of the soluble iron in the growth medium. 12~24j The role of applied DC potentials on the activity and growth of Thiobacillusferrooxidans has also been recently studiedY 5]
K.A. NATARAJAN, Professor, is with the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted January 2, 1991. METALLURGICAL TRANSACTIONS B
In this article, the role of electrochemical factors such as galvanic and applied potential effects on the dissolution behavior of base metal sulfides such as pyrite, chalcopyrite, and sphalerite in the presence and absence of Thiobacillus ferrooxidans is discussed. Mechanisms involved in the electrobioleaching of the above sulfides are outlined. Dissolution behavior of various sulfides when present alone or in different binary combinations is analyzed with respect to three contributing factors, namely, galvanic, applied potential, and microbiological.
II.
EXPERIMENTAL
A strain of Thioba
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