Gold recovery from secondary waste of PCBs by electro-Cl2 leaching in brine solution and solvo-chemical separation with tri-butyl phosphate

Abstract

In the present study, sustainable recycling of gold from the secondary stream of waste printed circuit boards using brine leaching with electro-Cl2 and solvo-chemical strategies was investigated. More than 99% of gold was leached in 2.0 mol.L−1 NaCl solution at pH, 1.0; electro-Cl2 rate, 0.62 mmol min−1; temperature, 50 °C; and time, 75 min. The kinetic data best fitted to the logarithmic model, while activation energy (16.3 kJ mol−1) indicated an intermediate-controlled leaching process. Results corroborated with the solution chemistry and reaction thermodynamics of the Au–NaCl-(electro)Cl2 system could reveal the stepwise reaction mechanism. In-situ generated Cl2(g) first dissolved in a brine solution that forming Cl2(aq), which subsequently converted to NaClO and diffused on gold particles to liberate the gold-chloro complex. Gold from the brine leach liquor was quantitatively separated using 0.5 mol.L−1 TBP at pH(eq) = 0.5 and O:A = 1:1. The formation of [HAuCl4·2TBP.H2O]¯ complex through the ion-pair solvation into the organic phase was supported by advanced spectral analysis. Finally, a high-purity gold with >99% efficiency was stripped by contacting the Au-loaded organic phase with 0.2 mol.L−1 thiosulfate solution; however, using the same concentration of thiourea yielded only a 94% efficacy. This study successfully eliminates the use of highly toxic lixiviants (like aqua-regia and cyanide) in gold metallurgy along with the regeneration and reusable benefits of the organic solvent (TBP).