Kinetics of metallic silver leaching in the O2–thiosulfate system

Abstract In recent years, many researchers have studied processes for extracting gold and silver from ores using alternatives that are less harmful and aggressive to the environment than cyanides. Thiosulfate solutions represent one such alternative. Metallic silver can be dissolved in O 2 –thiosulfate in different experimental conditions. Silver dissolution was obtained without the formation of passivation layers on the surface of the silver plate used. The kinetic study conducted indicated that the process is affected by a stirring speed of between 8.3 and 15 s − 1 (the reaction rate increased threefold), and the reaction orders were 1 (oxygen partial pressure 0.2–1 atm), 0.41 (thiosulfate concentration 25–200 mol m − 3 ), 0 (thiosulfate concentration 200–600 mol m − 3 ), 0.35 (hydronium concentration 1 · 10 − 8 –1 · 10 − 7  mol m − 3 ) and 0 (hydronium concentration 1 · 10 − 6 –1 · 10 − 2  mol m − 3 ). The apparent activation energy of the process was 4.5 kJ mol − 1 . Kinetics is controlled by a mass transfer of oxygen in the solid–liquid interface. In the presence of copper ions and oxygen, the reaction rate increased by 30% with respect to the process without copper. However, the thiosulfate partially decomposed and a layer of silver sulfide formed on the surface of the silver. The process was applied to a metallic silver powder, which showed a similar behavior to the silver plate.