A new application of solvent extraction to separate copper from extreme acid mine drainage producing solutions for electrochemical and biological recovery processes

Over the last decade, AMD waters have gained more attention as a potential source of metals due to the emerging need to recover or recycle metals from secondary resources. Metals recovery supports sustainability and the development of a circular economy with benefits for resource conservation and the environment. In this study, five extractants (Acorga M5640, LIX 54, LIX 622, LIX 622 N, and LIX 864) diluted (15% (v/v)) in Shell GTL with 2.5% (v/v) octanol were compared and evaluated for Cu recovery from an extreme AMD sample (5.3 +/- 0.3 g/L Cu) collected at the inactive Sao Domingos Mine in the Iberian Pyrite Belt of Portugal. Of the five extractants, Acorga M5640 showed the best selective efficiency. Further tests showed that 30% (v/v) of this extractant was able to selectively extract approximate to 96.0% of the Cu from the AMD in one extraction step and all of the remaining Cu (to below detection) in three steps. Among the different stripping agents tested, 2 M sulfuric acid was the most efficient, with approximate to 99% of the Cu stripped, and the recyclability of the organic phase was confirmed in five successive cycles of extraction and stripping. Furthermore, contact time tests revealed that the extraction kinetics allows the transfer of approximate to 97% of the Cu in 15 min, and aqueous to organic phase ratios tests demonstrated a maximum loading capacity of approximate to 16 g/L Cu in the organic phase. Raising the concentration of Cu in the stripping solution (2 M sulfuric acid) to approximate to 46 g/L through successive striping steps showed the potential to recover elemental Cu using traditional electrowinning. Finally, a biological approach for Cu recovery from the stripping solution was evaluated by adding the supernatant of a sulfate-reducing bacteria culture to make different molar ratios of biogenic sulfide to copper; ratios over 1.75 resulted in precipitation of more than 95% of the Cu as covellite nanoparticles.