Bioleaching of cobalt from Cu/Co-rich sulfidic mine tailings from the polymetallic Rammelsberg mine, Germany

A microbial consortium of mesophilic and acidophilic bacteria and archaea was applied in shake flasks as well as in 2 L stirred tank reactors (STR) to bioleach cobalt, copper, and other valuable metals from sulfidic mine tailings (Rammelsberg polymetallic massive sulfide deposit, Harz Mountains, Germany). After succession from low to high pulp density, the microbial consortium was well adapted to 10% pulp density and showed high bioleaching efficiency. Microbial activity and abundance were measured by microcalorimetry, microscopy, and quantitative, real-time PCR. The adapted mesophilic microbial consortium consisted mainly of Acidithiobacillus (At.) ferrooxidans and At. thiooxidans which achieved 91% cobalt and 57% copper extraction from the bulk tailings (Co 0.02%; Cu 0.12%) after 13 days in STR. Bioleaching tests with a tailings flotation concentrate (Co 0.06%; Cu 0.57%) showed a recovery of 66% cobalt and 33% copper. In addition, mineralogical analysis showed that cobalt occurred on the surface of framboidal pyrite and was mainly leached by microbial attack. Attached cells were microscopically observed on the surface of solid particles of the bulk tailings and tailings flotation concentrate. The amount of sulfides (mainly pyrite) in the tailings was sufficient to sustain microbial growth and thus no additional substrate was required for tailings bioprocessing. Bioleaching is considered to be an important processing step in the concept for reprocessing of the Rammelsberg mine tailings, and for many sulfidic mine tailings worldwide.