Bioleaching of copper from low-grade ore using isolated bacteria and defined mixed cultures

Pure mesophilic bioleaching bacteria were isolated to compare their potential for oxidizing ferrous and sulfur in synthetic media and copper extraction from low grade ore with mixed bacterial community. A total of 160 samples were collected from various sites of different mines. Enrichment and isolation of ferrous-/sulfur- oxidizing bacteria were done in specific media. A total of 68 isolates were screened, 63 of which oxidized Fe2+; the rest oxidized sulfur at different rates. Three important types of bacteria were identified to be Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirillum ferrooxidans on the basis of morphological and physiological tests. The oxidation characteristics of both sulfur and ferrous were studied in pure isolates and mixed cultures. Oxidation behavior for all pure Fe2+-oxidizing isolates was properly modeled with Monod equation and a specific Fe2+-oxidation rate (?m) of 0.076 to 0.737 / h was reached. The rate of sulfur oxidation for pure sulfur-oxidizing isolates was 9 mg of sulfur / l/ h. Results of ferrous- or sulfur- oxidation for mixed cultures were in agreement with their bacterial community and pure isolates. The role of bacteria in releasing of copper was evaluated for pure isolates and mixed cultures. The results obtained showed that the simultaneous use of three types of isolates leads to more copper release and lower acid consumption compared to other communities. The positive effect of the initial concentration of Fe2+ showed that major portion of copper extraction is via indirect reactions.