The electrochemical behavior of a semiconducting natural pyrite in the presence of bacteria
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I.
INTRODUCTION
M A N Y of the gold and silver deposits now being developed contain precious metals finely disseminated in sulfide minerals such as pyrite. These ores often present considerable resistance to conventional processing technology and do not allow economical recovery of precious metals. The sulfide minerals, consisting principally of pyrite, must be oxidized to liberate the encapsulated precious metal and allow it to contact the leaching agent. This oxidation can be carried out by roasting or other pretreatment (either chemical or biological). The application of bioleaching to refractory precious metal ores is a relatively new concept in comparison to oxidation by roasting and chemical treatment and is receiving increasing consideration as a technically and economically acceptable alternative to conventional technology for the processing of these refractory ores. [1-51 The fact that fine gold inclusions within the pyrite must disrupt the lattice is of considerable importance because these regions are also sites of accelerated bacterial corrosion. [4] The obligate chemoautotrophic bacterium Thiobacillus ferrooxidans (T. ferrooxidans) has, since its isolation in 1947, t6a,sl proved to be of considerable importance in the recovery of metal from low-grade sulfide ores. The results of several hundred studies have been repeatedly summarized and discussed in review articles, [9-12] and it is now well established that the bacterium T. ferrooxidans is able to oxidize Fe 2ยง S ~ and metal sulfides, as well as other reduced inorganic sulfur compounds. The oxidation of pyrite by T. ferrooxidans is described by the reaction 4FeS2 + 1502 + 2 H 2 0 ~ 2Fe2(504)3 + 2H2504
[1]
This biological effect can be divided into two categories, direct and indirect metabolic reactions, leading to two mechanisms of contact. The direct contact mech-
I. PALENCIA, Assistant Professor, is with the Hydrometallurgical Group, School of Chemistry, Department of Chemical Engineering, University of Seville, Seville, Spain. R.Y. WAN, Senior Metallurgist, is with Newmont Exploration Limited Metallurgical Services, Salt Lake City, UT 84108. J.D. MILLER, Professor, is with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Manuscript submitted September 17, 1990. METALLURGICALTRANSACTIONS B
anism requires physical contact between bacteria and pyrite particles. In the indirect contact mechanism, the bacteria oxidize ferrous ion to the ferric state, thereby regenerating the ferric ion required for chemical oxidation of pyrite.
A. Direct Contact Mechanism The pyrite is directly attacked by the bacteria according to Reaction [1]. Panin et al. ]131 suggested that the direct contact mechanism is based on electrochemical interactions intensified by bacteria.
B. Indirect Contact Mechanism The pyrite is chemically oxidized by ferric ion: FeS2 + 7Fe2(SO4)3 + 8H20 ~ 15FeSO4 + 8H2SO4
[2] FeS2 +
Fe2(SO4) 3 ~
3 F e S O 4 + 2S
[3]
Reaction [2] is the sum of Reaction [3] with Reaction [4]: 2S + 6 F e 2 ( 5 0 4
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