Extraction of precious metals from waste printed circuit boards using cyanide-free alkaline glycine solution in the presence of an oxidant

Waste printed circuit boards represent one of the most challenging parts of e-waste recycling. The extraction of precious metals (gold, silver, palladium and platinum) from waste printed circuit boards has commonly been done using highly toxic, corrosive, or expensive lixiviants, such as sodium cyanide, aqua regia and iodine/iodide. The present study provides a greener approach to extract precious metals from waste printed circuit boards using alkaline glycine solution in the presence of an oxidant. The common strong oxidant of potassium permanganate and the non-hazardous potassium ferricyanide were investigated and compared in alkaline media. Decrease of the particle size of sample, and the rise of oxidant concentration (0.04-0.16 M) enhanced gold extraction significantly. However, the increase of temperature (similar to 55 degrees C) and glycine concentration (0.5-1 M), and the staged addition of oxidant did not significantly improve gold extraction. Control of solution E-h could not reduce permanganate consumption whereas the ferricyanide consumption could be reduced by >50% in 72 h. Under the recommended conditions, 86.8% gold, 70.2% silver, 89.3% palladium and 87.9% copper could be extracted using glycine-permanganate leaching system. In comparison, 79.3% gold, 69.0% silver, 68.5% palladium and 83.1% copper could be extracted using glycine-ferricyanide leaching system. These extractions are comparable with what is achievable from cyanide (NaCN)-based leaching systems. Both of the leaching systems showed fairly good selectivity to copper and precious metals. The present study paves a way for the further development of non-NaCN glycine process for e-waste recycling.