Leaching of metallic silver with aqueous ozone

The recycling of silver from metallic scraps can be performed through O-3 leaching at an ambient temperature and low (similar to0.1 M) H2SO4 concentration. The main by-product is O-2, which can be recycled to the O-3 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 greater than or equal to 4, insoluble Ag2O2 was the main reaction product. Kinetics appeared to be controlled by mass transfer of O-3(aq) to the solid-liquid interface, showing first order dependency with respect to [O-3]aq and P-O3. Specific rates were only slightly dependent on the temperature in the interval 10-50degreesC, but decreased at 60degreesC due to the fall in O-3 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. (C) 2004 Elsevier B.V. All rights reserved.