Maximization of the silver recovery with the “monoethanolamine-copper-ammonium sulfate” novel system. A step towards the development of a less toxic leaching technology

Abstract

In the present research, the monoethanolamine‑copper-ammonium novel system was studied systematically to develop an alternative sustainable process for the dissolution of silver. This paper presents the effect of varying the concentration of cupric sulfate, ammonium sulfate, monoethanolamine and temperature on the silver leaching kinetics. The study was complemented with SEM-EDS, X-ray diffraction characterization techniques and thermodynamic analysis in order to understand the phenomena taking place in this novel leaching system. The results revealed that an increase in the cupric sulfate and ammonium sulfate concentration favors the silver dissolution rate and recovery. However, a massive increase in the concentration of cupric sulfate and ammonium sulfate, e.g., 0.4M and 1.06M, respectively, promotes the significant precipitation of Cu(OH)2, which hinders the silver dissolution. On the other hand, when the system is operated with a Cu2+ and NH4+ concentration lower than 0.3M and 0.8M, respectively, the silver dissolution is favored. The thermodynamic and SEM-EDS analysis along with the leaching tests revealed that Cu(NH3)42+, Cu(MEA)42+ and Cu(OH)2 (partially precipitated on the silver particles) are the oxidizing agents for silver. Oxidized silver can be complexed by the free ammonia produced in the system and monoethanoamine (MEA). Temperature is an important variable to accelerate the silver dissolution kinetics, the results showed that 83% of silver can be leached at 360min employing 0.3M Cu2+, 0.8M NH4+, 0.05M MEA at 45°C. Finally, the addition of MEA in the system is fundamental to decrease the massive precipitation of Cu(OH)2 species, promoting the silver dissolution rate at 45°C.