| Summary: | This work represents the study of the biological oxidation of iron pyrite, FeS_2. The chief objective of the study was to investigate the possibility of biological oxidation of iron pyrite and to determine, if possible, the physical and chemical conditions under which the oxidative process occurs. The apparatus used to study the problem consisted of an air-lift percolator containing Ottawa sand as a dispersing medium for the finely divided pyrite. A nutrient solution which would support bacterial growth was the lixiviant. The microorganisms used in the study were autotrophic, iron-oxidizing bacteria obtained from the mine waters of Bingham Canyon, Utah. Studies were made by inoculating the solutions in the percolators with actively growing bacteria and comparing the amount of iron oxidized in the inoculated percolators with the amount of iron oxidized in a sterile, control sample. The effect of autoclaving, mercuric chloride, temperature, light, and carbon dioxide on the oxidative process was studied. The acidity produced in the oxidation of the pyrite was measured. The effect on the activity of the bacteria of ammonium ion and cupric ion was studied. The results of the above studies showed that iron-oxidizing autotrophic bacteria do oxidize iron pyrite. Sterile control samples contained only five percent of the amount of iron in solution that appeared in inoculated solutions. Autoclaving and mecuric chloride killed the micro-organisms and therefore stopped the oxidative process. A reduction in temperature to 0° C. decreased the bacterial activity by an average of eighty-seven per cent. The bacterial activity was increased when the reaction vessels were placed in total darkness. The absence of carbon dioxide or oxygen in the atmosphere of the bacteria slows down the oxidative rate. Results showed that the ammonium ion or the nitrate ion is necessary for the normal growth and activity of the microorganisms. The bacteria in the problem have a tolerance for high concentration of cupric ion. They grew and were active in 500 ppm Cu^++. The possibilities in converting discarded pyrite waste to useful ferric sulfate by biological oxidation should prove to be valuable throughout the Intermountain area.
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