Optimizing the thiosulfate leaching of gold from printed circuit boards of discarded mobile phone

• The thiosulfate leaching of gold was controlled by the chemical leaching mechanism.
• The quadratic mathematical model for the rate of gold leaching has been observed.
• The rate of gold leaching was optimized using response surface methodology.
• The optimal conditions were applied for kinetic model of gold leaching process.

An environmentally benign process involving thiosulfate leaching was developed in order to recover gold from the printed circuit boards (PCBs) of discarded mobile phone. The effect of concentration of the reagents such as thiosulfate, copper(II) and ammonia on the leaching of gold was investigated in the temperature range 20–50 °C. Parameters were optimized through modeling of the leaching process using response surface methodology (RSM) based on central composite design (CCD). The optimum conditions for leaching of gold from PCBs were identified to be 72.71 mM thiosulfate, 10.0 mM copper(II) and 0.266 M ammonia. The initial rate of gold leaching was found to be 2.395 × 10− 5 mol∙m− 2∙ s− 1 under the optimum conditions. As regards the kinetics of gold leaching, the pseudo-second order kinetic model with chemical control was found to be applicable in the low concentration range (40–60 mM thiosulfate, 5–7 mM copper(II) and 0.22–0.247 M ammonia), compared to that of pseudo-first order kinetic model at mid concentration range of the reactants viz., 60–70 mM thiosulfate, 7–9 mM copper(II) and 0.247–0.263 M ammonia. The apparent activation energy of the reaction in the temperature range 20–50 °C was found to be 78.6 kJ∙mol− 1. The samples were characterized before and after leaching using scanning electron microscopy (SEM) which corroborated the chemical controlled leaching mechanism.