Silver leaching with the nitrite–copper novel system: A kinetic study

• Nitrite-copper is a totally new and promising silver leaching system.
• NO2–Cu2 + system is capable of complexing silver metal without thiosulfate.
• Nitrite ions act as a complexing as well as oxidizing agent.
• Leaching is controlled by chemical reaction of lixiviant at the Ag surface.
• About 96% silver can be recovered at 55 °C and 6 h.

This work presents a novel leaching system involving NO2–Cu to oxidize and complex silver without the use of cyanide, ammonia or thiosulfate. It is possible to oxidize metallic silver through the reduction reaction of the nitrite ions, the cupric-nitrite complexes seem to be responsible for increasing the silver leaching kinetics; the nitrite ions act as a complexing agent for the silver ions in the solution. The effect of cupric ions concentration (0.05 M, 0.1 M and 0.2 M) and nitrite ions concentration (1.0 M, 1.35 M and 1.5 M) on the dissolution kinetics of silver was evaluated. Based on the results the best leaching conditions were worked out as: 1.5 M nitrite ions and 0.2 M copper ions at a natural pH of 5.0. This leaching solution dissolves 80% of silver within 6 h. The effect of temperature on the dissolution kinetics of silver was studied at the room temperature till 30, 45, 50 and 55 °C, showing a progressive increase in silver recovery from 7.69% to 10.38%, 28.62%, 44.7% and 53.93%, respectively in 30 min. The kinetic analysis revealed that silver leaching depends on the temperature and rate controlling step corresponds to the silver dissolution through the chemical reaction. The apparent activation energy of the reaction is found to be 50.92 kJ/mol, along with the enthalpy and entropy of activation values of 48.34 kJ/mol and − 139.05 J/mol K, respectively. The kinetic model corresponding to the silver leaching can be represented by the expression: 1 − (1 − x)1/3 = k0 exp. (− 50.92/RT) t. The analysis of solid residue obtained after leaching at pH 5 by XRD and SEM with elemental mapping showed the formation of a copper-hydroxysulfate species, brochantite, formed in the bulk solution and not on the silver particle surface. The leaching results of silver by the NO2–Cu system were also ratified by the species distribution diagrams.

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