Heap bioleaching of a complex sulfide ore: Part I: Effect of pH on metal extraction and microbial composition in pH controlled columns

The effect of pH on the bioleaching of a low-grade, black schist ore from Finland containing pyrrhotite, pyrite, pentlandite, chalcopyrite and other mineral sulfides was studied using columns containing 9.0 kg of agglomerated ore that was irrigated with nutrient supplemented surface water from the deposit at ambient temperature. Iron and sulfur-oxidizing enriched culture was used to inoculate the columns. Iron oxidation and metal leaching increased with decreasing pH. At pH 1.5, 59% Ni was bioleached after 140 days together with 59% Zn, 13% Cu and 16% Co. In comparison, at pH 3.0 only 15% Ni, 10% Zn, 5% Cu and 0.5% Co were leached; while at pH 2, 53% Ni was bioleached after 230 days. Based on an optimization between the leaching of metals, the acid consumption, the concentration of soluble iron and the dissolution of other gangue minerals, leaching at pH of 2.0 were recommended for this heap bioleaching application.The microbial composition as determined by a combination of Polymerase Chain Reaction (PCR)–Denaturing Gradient Gel Electrophoresis (DGGE)-sequencing approach was not significantly affected by pH. Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans were the dominant species in all the leach liquors. In addition, L. ferriphilum was detected for the first time in extracted leach residue liquor after 300 days of bioleaching. Sulfobacillus thermotolerans, A. caldus, A. thiooxidans and some unknown species were found to lesser extent. Archaeal species were also present in all leach liquors.