Bacterial leaching of zinc and copper from mining wastes

Bacterial leaching of zinc from chat (chert) pile rock and copper from tailings pond sediment was studied in shake flask experiments. Thiobacillus ferrooxidans at 26°C leached 38% of zinc from pulverized rock in 15 days and extracted copper completely from native sediment in 24 h. Experiments omitting single medium constituents for the bacterial leaching of both zinc and copper indicated that the primary mechanism of bacterial leaching is very likely by the indirect route, in which the bacteria catalyze the ferric-ferrous redox cycle. The small particle size of the native tailings pond sediment (particles of ∼ 5 μm in size contributed almost all of the surface area) is a probable reason for the relatively short amount of time required for the copper leaching. Because of the rapidity of the extraction of copper with T. ferrooxidans, larger scale tests were carried out in a bioreactor with mechanical agitation and aeration. Complete extraction of copper was obtained in 12 h. A separate fluidization experiment showed that the tailings pond sediment could be kept suspended with a low fluid bulk velocity (0.3 cm/s). Two possible routes to scale up the bacterial leaching of copper from tailings pond sediment are discussed.