Recovery of gold and silver from spent mobile phones by means of acidothiourea leaching followed by adsorption using biosorbent prepared from persimmon tannin

A new approach of recovering precious metals from printed circuit boards (PCBs) of mobile waste by acidothiourea leaching followed by selective adsorption on low-cost and environmentally benign biomass sorbent prepared from easily available agricultural waste is suggested. The influence of various parameters like thiourea (TU) concentration, acid concentration, pulp density, temperature, and contact time were studied for gold and silver leaching using a batchwise method. It was found that the PCB sample with smaller particle size distribution (53 to 75 μm) yielded higher amounts of gold and silver being leached into the solution. The optimum conditions for the leaching of gold were found to be 0.5 M TU in 0.05 M H2SO4 at 45 °C, while that of silver were 0.5 M TU in 0.01 M H2SO4 at 60 °C. Under the optimum leaching conditions, an average of 3.2 mg/g of gold and 6.8 mg/g of silver were extracted from incinerated sample of PCBs of mobile waste. Kinetic studies revealed that the complete leaching of silver was achieved in less than 2 h, while it took approximately 6 h at ambient temperature in the case of gold. Presence of 0.01 M ferric ions facilitated the gold dissolution rate and complete leaching was achieved within 2 h. Conventional copper cementation for recovery of gold and silver from pregnant leached liquor was not so effective. Adsorptive recovery of dissolved gold and silver using activated carbon as well as crosslinked persimmon tannin gel was also examined. Low-cost persimmon tannin extract crosslinked with sulfuric acid was found to be a promising material for the complete recovery of gold and silver from the leached liquor. This adsorbent not only adsorbed the dissolved precious metals selectively but also reduced the adsorbed cationic species of gold to the elemental gold.