The extraction of metals from waste printed circuit boards (WPCBs) in H2SO4-CuSO4-NaCl leaching system was studied. The effect of initial concentration of cupric (0.5-7.5 g/L Cu2+), chloride (4.7-46.6 g/L Cl-) and temperature (20-80 degrees C) on the leaching of copper was investigated using response surface methodology, i.e., three-level Box-Behnken design. Extraction of other metals including Fe, Ni, Ag, Pd and Au was also determined. The importance of the main effects of the leaching parameters on the extraction of copper was found to be in the decreasing order of temperature, the initial concentration of Cu2+ and Cl-. The findings have shown that the initial Cl-/Cu2+ molar ratio should be maintained sufficiently high to maximise extraction of copper. However, an excessively high Cl-/Cu2+ ratio can exert a detrimental effect on the process due to a decrease in the activity of Cu2+ as oxidant. It was found that the highest levels of all the parameters should be selected to achieve high leaching recoveries (>= 91%) for Cu, Fe, Ni and Ag. Under these conditions, the dissolution of palladium was limited to 58%. The effect of solids ratio (1-15% w/v) and air/oxygen (2-4 L/min) on the rate and extent of leaching were also tested. Increasing the solids ratio (1-15 w/v) was observed to adversely affect the leaching of metals with no copper extraction at 10 w/v in the absence of air/oxygen. Air/oxygen was confirmed to be a suitable oxidant to regenerate Cu2+ and hence, maintain high Cu2+/Cu+ ratios, i.e., redox potentials during the leaching process. The presence of air/oxygen led to a significant improvement in the leaching of metals, e.g., %14 Cu (no air/oxygen) cf. complete copper extraction at >= 2 L/min air/oxygen over 120 min. The current findings suggested that, particularly at high solids ratios (>= 5% w/v), the regeneration of Cu2+ by the introduction of air/oxygen is essential for high extraction of metals, Pd in particular. (C) 2013 Elsevier B.V. All rights reserved.