Thiosulfate leaching of gold catalyzed by hexaamminecobalt(III): Electrochemical behavior and mechanisms

Investigation on Gold-Ligand Interaction for Complexes from Gold Leaching: A DFT Study

Gold leaching is an important process to extract gold from ore. Conventional alkaline cyanide process and alternative nontoxic lixiviants including thiosulfate, thiourea, thiocyanate, and halogen have been widely investigated. However, density functional theory (DFT) study on the gold complexes Au(CN)₂^-, Au(S₂O₃)₂^3-, Au[SC(NH₂)₂]₂⁺, Au(SCN)₂^-, and AuCl₂^- required for discovering and designing new highly efficient and environmentally friendly gold leaching reagents is lacking, which is expected to support constructive information for the discovery and designation of new high-efficiency and environmentally friendly gold leaching reagents. In this study, the structure information, electron-transferring properties, orbital interaction, and chemical bond composition for complexes Au(CN)₂^-, Au(S₂O₃)₂^3-, Au[SC(NH₂)₂]₂⁺, Au(SCN)₂^-, and AuCl₂^- depending on charge decomposition analysis (CDA), natural bond orbital (NBO), natural resonance theory (NRT), electron localization function (ELF), and energy decomposition analysis (EDA) were performed based on DFT calculation. The results indicate that there is not only σ-donation from ligand to Au⁺, but also electron backdonation from Au⁺ to ligands, which strengthens the coordinate bond between them. Compared with Cl^-, ligands CN^-, S₂O₃^2-, SC(NH₂)₂, and SCN^- have very large covalent contribution to the coordinate bond with Au⁺, which explains the special stability of Au-CN and Au-S bonds. The degree of covalency and bond energy in Au-ligand bonding decreases from Au(CN)₂^-, Au(S₂O₃)₂^3-, Au[SC(NH₂)₂]₂⁺, Au(SCN)₂^-, to AuCl₂^-, which interprets the stability of the five complexes: Au(CN)₂^- > Au(S₂O₃)₂^3- > Au[SC(NH₂)₂]₂⁺ > Au(SCN)₂^- > AuCl₂^-.

Use of the O2-Thiosemicarbazide System, for the Leaching of: Gold and Copper from WEEE & Silver Contained in Mining Wastes

Environmental pollution today is a latent risk for humanity, here the need to recycle waste of all kinds. This work is related to the kinetic study of the leaching of gold and copper contained in waste electrical and electronic equipment (WEEE) and silver contained in mining wastes (MW), using the O2-thiosemicarbazide system. The results obtained show that this non-toxic leaching system is adequate for the leaching of said metals. Reaction orders were found ranging from 0 (Cu), 0.93 (Ag), and 2.01 (Au) for the effect of the reagent concentration and maximum recoveries of 77.7% (Cu), 95.8% (Au), and 60% (Ag) were obtained. Likewise, the activation energies found show that the leaching of WEEE is controlled by diffusion (Cu Ea = 9.06 and Au Ea = 18.25 kJ/Kmol), while the leaching of MW (Ea = 45.55 kJ/Kmol) is controlled by the chemical reaction. For the case of stirring rate, it was found a low effect and only particles from WEEE and MW must be suspended in solution to proceed with the leaching. The pH has effect only at values above 8, and finally, for the case of MW, the O2 partial pressure has a market effect, going the Ag leaching from 33% at 0.2 atm up to 60% at a 1 atm.