Cyanide consumption minimisation and concomitant toxic effluent minimisation during precious metals extraction from waste printed circuit boards

Abstract

Waste printed circuit boards (WPCBs) constitute a hazardous material with up to 40 different metals, including numerous many heavy metals and environmentally harmful metals. Most hydrometallurgical processing approaches use high concentrations of toxic reagents and generate significant amounts of harmful effluents. This research investigates the use of cyanide-starved glycine solution containing no free cyanide in the leachate to extract precious metals from WPCBs, with most of copper and base metals pre-removed by upstream glycine-only leaching. Under the optimised conditions, 90.1% Au, 89.4% Ag and 70.1% Pd were extracted together with 81.0% Cu and 15.0% Zn. The extraction of other base metals remained low at 8.3% for Al and <5% for Pb, Ni, Co, Fe and Sn, indicating a fairly good selectivity of the leaching system. By comparing with stoichiometric and intensive cyanidation, the cyanide-starved glycine system showed comparable or better performance on precious metals extraction, but cyanide use was reduced by >70% whereas the glycine can be reused. Analysis indicates that with an initial cyanide dose of 250 ppm, the leaching solution was starved of cyanide after 4 h with no free cyanide, which minimises safety and health risks significantly compared with traditional intensive cyanidation ([CN] > 3500 ppm). Glycine and cyanide dose, pH, and particle size dominated leaching kinetics, while staged addition of cyanide did not enhance the extractions. A significant (70–90%) reduction in required cyanide use and cyanide-bearing effluents can be achieved while performing polymetallic metal removal and allowing reagent recycling.