Autocatalyzed Silver Dissolution from Different Phases Employing the Monoethanolamine–Water Alternative System: An Approach Towards the Sustainable Recovery of Silver

In this paper, the effect of pH, stirring, and the role played by hydrogen peroxide on silver dissolution kinetics were analyzed from a fundamental viewpoint using the monoethanolamine–water (MEA–H2O) alternative system. The oxidative dissolution of silver was studied through different approaches including thermodynamic simulations, electrochemical techniques, such as open circuit potential, linear voltammetry, and chronoamperometry, followed by conventional leaching tests using different silver phases. The thermodynamic and electrochemical tests revealed that H2O2 in the MEA–H2O system promotes the formation of the AgMEA+ complex, which accelerates silver dissolution, producing the stable Ag $$\left( {{\text{MEA}}} \right)_{2}^{ + }$$ complex. The voltammetric results showed that an increase in MEA from 0.05 to 0.80 M, promoted the silver dissolution kinetics. The analysis of electrochemical tests at different MEA concentrations showed that the rate determining step corresponds to the formation of the AgMEA+ complex with a reaction order of 1. When the MEA concentration was increased up to 1.50 M, the oxidative dissolution of silver attained a limit current density of 0.016 A/cm2. Furthermore, the chronoamperometric test carried out with silver at different anode potentials, revealed the possibility to maximize the oxidation current density while the MEA concentration is decreased from 1.50 to 0.80 M. This presents a plausible opportunity to reduce the MEA concentration without compromising the silver recovery. When the pH of the MEA–H2O system was reduced to 10.2 with sulfuric acid (H2SO4), silver oxidation was significantly promoted, e.g., the current density increased from 0.014 to 0.10 A/cm2 using 1.50 M MEA. Conventional silver leaching tests using the MEA–H2O system confirmed the synergistic effect of H2O2 and pH on silver dissolution kinetics, obtaining a silver recovery of 60% during the first 30 min. Voltammetric tests carried out with carbon paste electrodes containing silver sulfide (CPE–Ag2S) revealed the possibility of producing metallic silver which can be dissolved with the MEA–H2O system. Finally, the recovery of silver from silver sulfide was also evaluated using a cathodic process in ethylamine solutions with aluminum foil, followed by the conventional oxidative dissolution with the MEA–H2O system.