Bioleaching of Ok Tedi copper concentrate in oxygen- and carbon dioxide-enriched air

Abstract

Bacterial leaching of sulphide minerals is fairly well established in heap, dump and in-situ leaching and the primary bacterial treatment of refractory gold concentrate using CSTRs has gained industrial prominence in recent years. Three cultures, a mesophile T.ferrooxidans (DSM 583), a moderate thermophile Sulfobacillus accidophilus (YTF1) and an extreme thermophile (Sulfolobus BC65) were used to bioleach the Ok Tedi copper concentrate under the optimal O 2 and CO 2 enriched air in laboratory CSTRs (2L) at 30% O 2 and 10% CO 2 by volume. The positive effect of using oxygen and carbon dioxide enriched air to bioleach the copper mineral was clearly demonstrated where increases in Cu leaching rate by factors of 2.8, 2.1 and 1.9 for DSM 583, YTF1 and Sulfolobus respectively were observed compared with shake flask tests. The lag phase observed in shake flasks or in CSTR with air, was eliminated when the mineral was leached under optimal gas enrichment and ≈80% of the Cu was extracted in the first 120 to 150 hours of leaching for all three cultures. The average observed dissolved oxygen concentrations in the pulp were 11.8, 8.0 and 7.2 mg/l for DSM 583, YTF1 and Sulfolobus respectively, indicating that O 2 was not limiting in these tests. The Fe released during the bioleaching of copper was non stiochiometric for a predominantly chalcopyrite concentrate and the low redox potential values (max 638mV) suggested galvanic interaction during the leaching of copper where chalcopyrite was leached preferentially in the presence of pyrite. Bioleaching of copper under optimum gas enrichment at different pulp densities showed that near total copper extraction occurred at pulp densities of 3, 10 and 20% (w/v). At high pulp densities, more so at 40% than 30% (w/v), the leaching rate decreased rapidly for experiments with YTF1 and Sulfolobus. Monitoring of dissolved O 2 shows that the O 2 demand increased with increasing pulp density and the average dissolved O 2 concentrations observed for DSM 583, YTF1 and Sulfolobus at 40% solids concentration were 3.0, 2.6 and 2.4 mg/l respectively, suggesting that oxygen and carbon dioxide are not limiting. The decrease in leaching rate observed at high pulp densities (30 & 40%) under optimum gas enrichment was accompanied by low metal dissolution and low redox potential values which was attributed to shear stress due to attrition of cells at high solids concentration. The toxicity of metal ions especially Ag, Fe and Cu, and the formation of precipitates and jarosites was confirmed from XRD analysis of leach residues. These factors appeared to impede the leaching process rather than limiting O 2 and CO 2 conditions.