Selective leaching of valuable metals (Au, Ag etc.) from waste printed circuit boards (PCB)

This study was carried out to recover gold, silver and other valuable metals from the printed circuit boards (PCB) of waste computers. PCB samples were crushed to under 1mm by a shredder and initially separated into 30% conducting and 70% non-conducting materials by an electrostatic separator. The conducting materials, which contained the valuable metals, were then used as the feed material for magnetic separation where it was found that 42% was magnetic and 58% non- magnetic. The non-magnetic materials contained 0.227mg/g Au and 0.697mg/g Ag. Further leaching of the non-magnetic component using 2.0M sulfuric acid and 0.2M hydrogen peroxide at 85$^{\circ}C$ extracted more than 95% copper, iron, zinc, nickel and aluminium. Au and Ag were not extracted in this solution, however, more than 95% of Au and 100% of Ag were selectively leached with a mixed solvent (0.2M ammonium thiosulfate, 0.02M copper sulfate, 0.4M ammonium hydroxide). Finally, the residues were reacted with a NaCl solution to leach out Pb while sulfuric acid was used to leach out Sn. Recoveries reached 95% and 98% in solution, respectively.