Biological and Chemical Oxidation of Sulfidic Minerals at Alkaline pH

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Lee, Jung
Biooxidation , Chemical Oxidation , Pyrite , Gold Recovery
The oxidation of sulfidic minerals to liberate gold is currently performed with the utilization of microorganisms and chemical reagents. Biooxidation is a hydrometallurgical oxidation method which can be a pre-treatment method for gold recovery. Iron- and sulfur-oxidizing microorganisms with varying physiological characteristics liberate sulfide matrices by using reduced sulfur compounds for growth, which exposes minerals such as gold to lixiviants. Compared to conventional methods, biooxidation is an environmentally friendly and more economic pre-treatment method. The chemical oxidation of sulfide minerals has been performed in all pH ranges. However, operating in higher pH environments can decrease operating and material costs. The aim of this investigation is to test the oxidative capabilities of alkaline bacteria and chemical reagents in three pyrite concentrates. Two alkaliphilic, halo-tolerant, and chemolithoautotrophic bacteria Thiomicrospira aerophila (DSMZ 13739) and Thioalkalivibrio versutus (DSMZ 13738) were used for this study. For chemical oxidation, the carbonate effect which occurs due to the presence of sodium carbonate and bicarbonate was evaluated. Various parameters such as pH, temperature, pulp density, time, and changes in media were altered to determine the optimal oxidative parameters. Investigations with bacteria suggest that iron precipitates produced during oxidation passivated onto the particles, preventing them from attaching to the particle surface. This resulted in all experiments without the inoculation of bacteria oxidizing more pyrite than the experiments with bacteria. Therefore, chemical oxidation via the carbonate effect was investigated. Complete oxidation of the sample was observed at 11 days using 20 g/L Na2CO3 and 10 g/L NaHCO3 with parameters of 80 °C, air supplied at 0.1 L/min, 20% pulp density, and stirred at 500 rpm. Further, qualitative, and quantitative analysis were performed on the oxidized samples to investigate physical and chemical changes in the samples. Gold leaching tests with conventional cyanidation techniques were performed on oxidized samples to determine the relationship between oxidation and gold recovery.
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