Elucidating the Interfacial Interactions of Copper and Ammonia with the Sulfur Passive Layer during Thiosulfate Mediated Gold Leaching

Characterization of copper and ammonia interactions with the passive layer that impedes gold dissolution with thiosulfate based electrolytes at extended time frames was performed with electrochemistry and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Generalized two-dimensional correlation spectroscopy (2DCOS) was also employed to assist interpretation of the collected SHINERS spectra. Growth of the complex passive layer mixture on a polycrystalline gold electrode immersed in a 0.10 M Na2S2O3 electrolyte was found to initially proceed through formation of sulfides adsorbed to the gold electrode surface (SERS band at ca. 326 and 309 cm−1) followed by rapid transformation to cyclo-S8 (474, 218, and 152 cm−1) and polysulfides/polythionates (459 cm−1). After stable formation of the passive film, the addition of Cu2+ resulted in the partial removal of the passive layer. The removal of the components of the passive layer proceeded according to the following sequence; polysulfides were fastest, cyclo-S8 next, followed by . The end product was observed to be a combination of copper and gold sulfides. In an independent study, the addition of ammonia displayed a similar ability to remove the passive layer constituents, albeit at much slower rates. From these observations it can be concluded that copper and ammonia play a vital function in preventing passive layer formation, allowing increased mass transport of thiosulfate and the oxidant across the electrode-electrolyte interface, thereby allowing a higher gold extraction efficiency.