Abstract
Leaching of silver from minerals has been studied for decades. Recently, research has been focused on the substitution of cyanide (because of its high toxicity) from hydrometallurgical operations. In this sense, the development of alternative-green leaching solutions for the recovery of metallic silver from minerals and urban mines is necessary. In this research, a thermodynamic-kinetic study of the alternative system “monoethanolamine (MEA)-ammonium sulfate-cupric sulfate” was performed for the dissolution of metallic silver, varying the pH and the cupric ion concentration at room temperature. Furthermore, this research aims to evaluate the silver dissolution behavior at a pH lower than 10.2, elucidating the rate determining step of the silver dissolution process at these conditions of pH and cupric ion concentration. Thermodynamic results confirm that the MEA and NH3 can complex silver generating the species: Ag(MEA)2 +, AgMEA-, Ag(NH3)2 +, AgNH3 + respectively. It was evaluated the effect of varying the pH (8.8, 9.9 and 9) and the cupric sulfate concentration (0.1, 0.15 and, 0.2 M at pH 9) on the kinetics of silver dissolution. The results revealed, the possibility to leach a 39% of silver using a leaching solution containing: 0.15 M cupric sulfate, 0.05 M MEA and 0.4 M ammonium sulfate at pH 9. The kinetic analysis showed that the silver dissolution is controlled by the diffusion of fluid species through a porous layer composed of copper oxides, which was ratified by SEM and EDS analysis. Finally the present paper, open the possibility to optimize the molar ratio MEA:NH3 to maximize the silver dissolution kinetics. Keywords: Leaching, metallic silver, MEA, ammonium.
Highlights
Cyanidation is the most employed process to carry out the dissolution of precious metals such as: silver
2.1 Thermodynamic study In order to elucidate the predominant species in the MEA-ammonia-copper-silver system, species distribution diagrams were constructed at room temperature using the Medusa Software and considering the equilibrium data contained in the Hydra database of the same software [29]
It is possible to observe in this diagram, that MEA can form complexes with silver at pH ≥ 7; for example, if the leaching system is operated at pH 9, the formation of the silver monoMEA and diMEA (AgMEA+ and Ag(MEA)2+) complexes takes place
Summary
Cyanidation is the most employed process to carry out the dissolution of precious metals such as: silver. It is extremely known, that the scientific community is investigating the development of alternative processes that are less toxic to the environment. In this sense, during several decades, the research has been focused in the silver leaching with thiourea and thiosulfate, respectively. It has been proposed the thiosulfate systems as the most promising leaching systems for silver and gold. On the other hand, according to AYLMORE and MUIR [4], the instability of the ammoniacal thiosulfate solutions is related to the presence of cupric ions, which promote the irreversible thiosulfate degradation to tetrathionates or other thiosalts and affects the silver dissolution kinetics and the process profitability
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