Operation of a commercial-scale biooxidation heap for pretreatment of refractory gold ores demonstrates the propensity of heaps to heat concurrent with pyrite oxidation. Heap temperatures can reach 75 degreesC. The high temperature is lethal for mesophilic iron-oxidizing bacteria. Column testing was conducted to compare effects on biooxidation pretreatment of a sulfidic refractory gold ore and the microbes' response to temperature when bacteria and archaea were grown at different temperatures. Five columns were operated at a different temperature regime: 20-23 degreesC; 35 degreesC; 50 degreesC; 60 degreesC; and one column varied from 20-23 to 60 degreesC. For the variable temperature column, the temperature was increased stepwise from 20-23 degreesC, to 35 degreesC, to 50 degreesC, to 60 degreesC in 2-week increments; after 2 weeks at 60 degreesC, the temperature was decreased every 2 weeks in the same stepwise manner. The following inocula were used: (1) columns at 20-23 and 35 T, mixed culture of Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans; (2) column at 50 degreesC, moderate-thermophilic iron-oxidizing Sulfobacillus-type bacteria; (3) column at 60 degreesC, hyper-thermophilic archaea, Acidianus and Metallospheara species; and (4) an equal mix of all of the above for the variable temperature column. The hyper-thermophiles did not increase in numbers until the temperature was increased to 50 degreesC and above. Lowering the temperature resulted in a decrease in population density of this group of microbes. The moderately thermophilic bacteria increased in number until the temperature was raised to 60 degreesC; at that point, numbers decreased and remained stable as further decrease in temperature occurred. Decimation of the mesophilic iron-oxidizing bacteria occurred when the temperature was increased to 50 and 60 degreesC. A low-level population concentration was detectable at increased temperatures attributable to their presence in the ambient temperature solution reservoir. However, this population increased as the temperature was lowered. There was little difference in the amount of sulfide oxidation at either ambient room temperature or 35 degreesC. Increasing the biooxidation temperature from 35 to 50 degreesC increased apparent sulfide oxidation from about 38% to 48%. At 60 degreesC, sulfide oxidation was highest at about 51%. The variable temperature column also had higher sulfide oxidation, 41%, attributable to periods of high temperature biooxidation. The data suggest that there is little difference among the groups of microbes used in this study in terms of sulfide oxidation and improved gold recovery, with some apparent slight advantage in using the hyper-thermophilic microbes. (C) 2003 Elsevier B.V. All rights reserved.
