Bioleaching of waste material from the Salobo mine, Brazil: Recovery of refractory copper from Cu hosted in silicate minerals

In this study we demonstrate the recovery of refractory copper from Cu-sulphide inclusions in silicate minerals of waste rocks by Acidithiobacillus ferrooxidans-mediated silicate dissolution. Acidophilic, iron-oxidising bacteria and waste rock samples were collected at the Salobo 3 Alpha Iron Oxide Copper Gold (IOCG) mine in Para State, Brazil. Bench-top column leaching trials of seven different waste rocks with complex gangue mineralogy were run with and without endemic A. ferrooxidans in acidic media. Pre- and post-leach materials characterisation and continuous monitoring of leaching column effluents were used to investigate the amenity of low-sulphide waste material to leaching by A. ferrooxidans and the mechanisms controlling solution chemistry. The data presented here suggests that, in-lieu of other Fe(II) sources, iron-oxidising bacteria utilise solubilised ferrous iron from Fe-bearing silicate minerals, such as chlorite, biotite and amphiboles as alternative energy sources. The microbially mediated weathering of Fe(II)-bearing silicates provides access to 'refractory' sulphides trapped as Cu-sulphide inclusions in these minerals. The dissolution of these silicate minerals controls the pH of the leach solution. A. ferrooxidans thereby indirectly causes the leach solution pH of biotic systems to buffer at pH 3.2 to 5.0.