Oxidation Mechanism of Silver Selenide

The mechanism of selenium vaporisation from silver selenide, the major carrier of selenium in copper depleted anode slimes, was studied using isothermal oxidation of synthetic, massive Ag₂Se in flowing, dry oxygen and oxygen + 20 % sulfur dioxide mixtures at 450–550 °C. The reaction rates expressed as mass change rate indicate that in pure oxygen at 550 °C the reactions essentially cease within 1 h. Silver selenite scales were identified on the interface by SEM–EDS observations. The melting point of silver selenite is 540 °C, and above it selenite is formed in the molten state, where metallic silver as condensed deselenization product is embedded. Metallic silver as a roasting product of Ag₂Se will be partly sulfated in SO₂-bearing atmospheres. A small fraction of selenium is transformed into silver selenite, as the direct oxidation product of Ag₂Se. The reactions proceed much faster into the selenite matrix along the grain boundaries than the surface reactions. The reaction product layer is composed of very fine Ag₂SeO₃ and Ag₂SO₄ particles on the primary Ag₂Se, and significant porosity is generated next to the reaction zone.