Oxidative Decomposition of Silver Telluride (Ag2Te) Using Hypochlorite in Different Acid Environments

Abstract

ABSTRACT Silver telluride represents one of the most important refractory silver mineral phases and requires the development of alternative processing routes. This work presents a detailed analysis of the effect of sodium and calcium hypochlorite species, HNO3, HCl, H2SO4 and pH on the kinetics and efficiency of silver telluride dissolution. The understanding of the reaction mechanism was complemented with the use of thermodynamic diagrams and characterization of the solid residues by the X-ray diffraction and SEM-EDS techniques. The results revealed that the presence of Ca(ClO)2 and H2SO4 promotes the secondary and massive precipitation of calcium sulfate. A comparison of the oxidative systems containing Ca(ClO)2 and NaClO showed that the solutions containing NaClO are more efficient and selective. The decrease in pH favors the tellurium dissolution due to an increase in the in-situ concentration of chlorine oxidizing species; the silver is transformed into silver chloride solid species on the surface of unreacted silver telluride. Thermodynamic and experimental studies revealed that silver chloride layer and the continuous consumption of hypochlorous acid promote the passivation of the system. The characterization of the solid residues evidenced that the type of acid contained in the systems modifies the morphology of the silver chloride layer. Finally, the surface characterization showed that passivation due to the formation of compact layer of silver chloride can be modified using a hypochlorite system containing nitric acid; this is due to a synergistic effect of nitrate and hypochlorite species on the kinetics of silver telluride dissolution. This effect is also discussed from a mechanistic viewpoint.