Enhanced photoelectrocatalytic breakdown of Cu-cyanide complexes and copper recovery using photoelectrogenerated free chlorine

Abstract

The breakdown of Cu–cyanide complexes (decomplexation) and recovery of the associated copper was explored using a photoelectrocatalytic treatment incorporating Cl− in the electrolyte (PEC-Cl) and with WO3 as a photoanode. Using the PEC-Cl process, the efficiency of decomplexation improved in the presence of a high concentration of Cl−. However, when the concentration of Cl− exceeded 20 mM, the deposited Cu could be oxidized to copper compounds by free chlorine (Cl2, HOCl, Cl and OCl), resulting in a decline in Cu recovery. The total nitrogen removal increased from 7.9% (PEC without Cl−) to 95.6% for the PEC-Cl process, which can be attributed to the generation of chlorine radicals (OCl and/or Cl). A mechanism for Cu–cyanide decomplexation and Cu recovery during the PEC-Cl process is proposed. Cu–cyanide complexes are destroyed by free chlorine, generating CNO− and Cu2+. The Cu2+ is then reduced onto the cathode and the CNO− is further oxidized into NH4+ and HCOO−. NH4+ is attacked by chlorine radicals and finally converted into N2 while HCOO− is gradually degraded to CO2 and H2O. Keywords: Cu–cyanide complex, Copper recovery, Free chlorine, Photoelectrocatalysis