A closed and zero-waste loop strategy to recycle the main raw materials (gold, copper and fiber glass layers) constitutive of waste printed circuit boards

Abstract

The main aim of this work was to develop a simple, efficient, and environmentally friendly technology for recycling all main raw materials [gold, copper and fiber glass (FG) layers] from waste printed circuit boards (WPCBs). Firstly, the connecting terminals (CT) containing gold fingers (GF) were cut from the WPCB using a mechanical process. Then, the CT were totally delaminated by swelling in a closed reactor [optimal conditions achieved with dimethyl sulfoxide (DMSO) or N,N-dimethyl pyrrolidone (NMP) at a liquid/solid (L/S) ratio of 5, WPCB area of 60 mm2, at 180 °C for 30 min and a stirring of 1000 rotations per minute]. GF were separated from the dried swollen mixture containing also copper and FG layers, using physical (magnetic separation followed by sieving) processes. Subsequently, gold was purified from the GF by leaching the base metals under microwave-assisted mild oxidizing acidic conditions (4.7 mol/L nitric acid, 60 s and a L/S = 12), as a solid, with high yield (99.9%) and purity grade (74.0%). Copper and nickel, present in the leachate, were recovered by sequential two stepwise-wise alkaline precipitations at pH 6.1 and 9.3, as solids, with recovery yields of 99.9 and 84.2 %, respectively. After swelling the remaining WPCB under similar conditions, the resultant mixture was mixed with the mixture remaining from the physical separation of GF and copper and FG layers were physically separated by air sorting. The economic and energetic assessment of the process developed evidence high economic advantages with low impact of energy consumption.