Role of oxygen and ammonium ions in silver leaching with thiosulfate–ammonia–cupric ions

The dissolution of silver with thiosulfate and copper ions was studied in a batch-stirred reactor at 25 °C in the absence and presence of various ammonium ions and oxygen concentrations in the aqueous solution. Leaching silver with 4 mg·L−1 oxygen in the aqueous solution requires high concentration of ammonium and thiosulfate ions. High silver dissolution is achieved at pH of high cupric tetraamine concentration, around pH 9.8, but a high degradation of thiosulfate ions is resulted from their oxidation by the presence of oxygen. When the oxygen concentration is below 1 mg·L−1, dissolved silver thiosulfate species are fairly stable and the degradation of thiosulfate ions in the aqueous solution is low. Under these conditions, high silver dissolution is attained using low thiosulfate concentration. At the oxygen concentration of 4 mg·L−1, dissolved silver thiosulfate species precipitate out of the aqueous solution significantly affecting silver dissolution. Precipitation of dissolved silver thiosulfate species occurs because of oxidation of the thiosulfate bonding to the silver.