Leaching of metallic silver with aqueous ozone

The recycling of silver from metallic scraps can be performed through O3 leaching at an ambient temperature and low (∼0.1 M) H2SO4 concentration. The main by-product is O2, which can be recycled to the O3 generation or used as leaching agent in a pretreatment step. The stoichiometry and the effects of the stirring speed, ozone and acid concentration and temperature on the leaching of silver were investigated. Silver dissolved as Ag2+(aq) in the range 10−3-1 M H2SO4, but for pH ≥4, insoluble Ag2O2 was the main reaction product. Kinetics appeared to be controlled by mass transfer of O3(aq) to the solid-liquid interface, showing first order dependency with respect to [O3]aq and PO3. Specific rates were only slightly dependent on the temperature in the interval 10-50 °C, but decreased at 60 °C due to the fall in O3 solubility. The mass transfer coefficients showed an average activation energy of 17 kJ/mol. No significant effect of [H2SO4] on mass transfer coefficients was observed for 10−2-1 M. Leaching rate gradually diminished for pH >2, as a consequence of the influence of the [H+] in the transport control.