Leaching of gold and palladium with aqueous ozone in dilute chloride media

The recycling of gold and palladium from metallic scraps can be carried out by ozone-leaching at ambient temperature and low (∼0.1 M) H+ and Cl− concentrations. Rh and Pt remain un-reacted, whereas metals such as Cu, Ni, Ag, can be previously eliminated through O2/H+ and O2/O3/H+ leaching pretreatments. Gold and palladium are dissolved in O3/Cl−/H+ with formation of AuCl4− and PdCl42−. Leaching studies showed a passive region, basically located at < 0.01 and < 0.05 M Cl− for Au and Pd, respectively. In the non-passive region, rates were only slightly dependent on either H+ and Cl−. Secondary formation of chlorine or hypochlorous acid was negligible at ≤ 0.1 M Cl−. 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. Rates increased with temperature up to about 40 °C, but decreased at higher temperatures due to the fall in the O3 solubility. The ozone mass transfer coefficients showed an activation energy < 20 kJ/mol. Gold leaching rate gradually diminished for pH > 2, as consequence of the influence of the [H+] on transfer control. The electric power consumption associated with O3 generation was in the range 4–8 kWh/kg metal leached.