The reprocessing of hydrometallurgical sulphidic tailings by bioleaching: The extraction of metals and the use of biogenic liquors

Mineralurgical and metallurgical processes generate a large amount of harmful residues. These wastes usually contain a considerable fraction of sulphides and a low percentage of target metals. Due to their characteristics, the main environmental issues concern the acid mine drainage generation and the release of metals. The bioleaching of these tailings can be considered as a win-win process due to the controlled oxidation of sulphides avoids the main environmental issues, while target metals can be recovered. In this work, a residue composed of pyrite (50.4%) and quartz, containing various non-ferrous metals, was reprocessed by bioleaching and brine leaching. Copper and zinc sulphides exhibited a refractory behaviour in bioleaching due to these tailings were previously leached, being the dissolution rate controlled by mass transfer. Cobalt is directly related to the pyrite matrix, requiring the removal of pyrite to extract it. Pyrite dissolution was also controlled by mass transfer, increasing the dissolution rate when the bioreactor conditions were improved. After 10 days of bioleaching in a stirred tank reactor, 87.2% Co, 43.6% Zn, 40.4% Cu, and 96.0% FeS2 were dissolved. In addition, after bioleaching, 55.9% Pb and 37.9% Ag were dissolved through chloride leaching with moderate conditions (100 g/L NaCl, 10 g/L H2SO4, 60 degrees C, and 1 h). Considering the metals extraction and the saving in management costs of tailings, the reprocessing of these tailings was not economically feasible due to the low grade of target metals. Nonetheless, during bioleaching, a great amount of ferric ion and sulphuric acid was released that could be used in the hydrometallurgical circuit as leaching liquor. This saving increased the revenues from tailings reprocessing, obtaining an internal rate of return of 10% with a feed rate of 260 t/d.

Automated summary

Mineralurgical and metallurgical processes generate harmful residues containing a low percentage of target metals. Bioleaching is an environmentally friendly process that avoids acid mine drainage and allows for the recovery of target metals. However, due to the low grade of target metals, reprocessing these residues is not economically feasible. Nonetheless, the bioleaching process releases ferric ion and sulfuric acid that can be utilized in the hydrometallurgical circuit.