Advances in Recovering Noble Metals from Waste Printed Circuit Boards (WPCBs)

An extensive investigation on the noble metal (NM) content in different classes of waste printed circuit boards (WPCBs: random access memories, RAMs; network interface controllers, NICs; motherboards; TV, DVD/CD player, hard-drive, and mobile phone PCBs) has been performed to define the most appropriate case study and provide a robust database useful for workers in the waste valorization field. Following accurate selection, mechanical comminution, representative sampling, quantitative digestion, and analytical characterization (ICP-AES), RAMs and mobile phone PCBs confirmed to be the "richest" source, while TV PCBs are the "poorest" one in term of NM content. Accordingly, the RAM case study has been employed for the application of a new NMs recovery method, previously set up on finely comminuted waste electric and electronic equipment underwent materials enrichment by mechanical separation. Despite the very large amount of vitreous-plastic and metallic materials present in the mixture, satisfactory NM recovery yields (Cu 70%, Ag 92%, Au 64%) with limited byproduct formation have been obtained using safe and recyclable reagents in mild conditions: citric acid for base metal leaching, ammonia in oxidizing environment for Cu and Ag separation and recovery, triiodide aqueous solution for gold recovery, at room pressure, and 25-100 °C. The reported results provide useful quantitative parameters for assessing the profitability of an industrial scale-up of the new sustainable NMs recovery method.