Heap bioleaching of a complex sulfide ore: Part II. Effect of temperature on base metal extraction and bacterial compositions

The effect of low to moderate temperatures (7 to 50 °C) on the bioleaching of a low-grade, multi-metal black schist ore from Finland in which pentlandite was the main valuable mineral, was studied using columns at set temperatures. The iron and sulfur-oxidizing microbial culture used were enriched from the ore deposit water samples. At 7 °C and 21 °C, the leach liquor redox potential stabilized to 500–600 mV, whereas at 35 °C and at 50 °C it varied between 300 and 500 mV. Microbial iron oxidation started after a lag phase of 20 days at 7 °C and after 60 days of operation, total iron and Fetot/Fe2+-ratio were higher in the 7 °C column leach liquor than at other temperatures. At 50 °C, all dissolved iron remained in ferrous form and did not indicate microbial activity. Highest bioleaching recoveries of Ni (26%), Zn (18%) and Co (6%) were obtained after 140 days at 21 °C. At 50 °C, bioleaching decreased due to the lack of ferric ion. The microbial composition, as measured by Polymerase Chain Reaction (PCR)–Denaturing Gradient Gel Electrophoresis (DGGE)-sequencing approach, was affected by temperature. Acidithiobacillus ferrooxidans was the most common species in the leach liquor at 7 °C; while at 35 °C Leptospirillum ferrooxidans dominated and at 50 °C, Sulfobacillus thermotolerans was the most common organism. Total liquid-phase cell counts were higher at 7 °C than at other temperatures. Thus the boreal ore deposit enrichment culture was composed of microorganisms capable of being active over a wide temperature range.