Bioleaching of cobalt and zinc from pyrite ore in relation to calcitic gangue content

Bioleaching of a pyrite ore containing high concentrations of cobalt (0.1%) and zinc (0.065%) was affected by small amounts of calcitic gangue (from 0.01 to 1.01%). Results from an air-lift percolator and from Erlenmeyer flask experiments show that a small percentage of calcite raises the pH and arrests the growth of the acidophilic bacterium Thiobacillus ferrooxidans. In percolator experiments, when calcite is completely removed by the continuous addition of small quantities of acid, and the pH of the liquor becomes acid, the micro-organism begins to grow and to bio-oxidize the pyrite ore. The growth of T. ferrooxidans shows different lag phase spans (from 13 to 190 days) depending on carbonate dissolution. The metals Fe, Zn and Co are released into the leaching solution together at different rates after a lag-time which depends on calcite concentrations in pyrite gangue. Metal ratios in the mineral bulk are different from those in the liquor, Zn dissolving 5 times more readily than Co. Bioleaching rates for metal removal from pyrite are higher in percolator (for Fe, from 5 to 15 mg/l/h) than in flask experiments (from 0.5 to 2 mg/l/h), but the lag phases are shorter (from 2 to 65 days). The differences between the two systems are related to calcite dissolution and gypsum precipitation.