Extraction of precious metals from waste printed circuit boards using alkaline ferricyanide solutions: An exploratory study

Precious metals leaching using conventional processes presents challenges due to the use of toxic, expensive, and complex leaching systems, such as cyanidation, thio-systems, and halide systems comprising both lixiviant and oxidant. The present study employed a simple leaching system of alkaline ferricyanide solution with dual functions of leaching and oxidization, to extract precious metals from waste printed circuit boards (PCBs). The ferricyanide concentration, pH and temperature were found affecting Au extraction considerably. The selective extraction of Au, Ag and Pd over base metals reached 68.3, 93.0 and 74.0 % (10 g/L ferricyanide, pH 11 and 80℃), respectively. The recovery of precious metals from leachate using carbon adsorption and the regeneration of ferricyanide from barren solutions employing ferri/ferrocyanide redox cycle were shown feasible. Around 91.7 % ferricyanide could be regenerated by oxidising ferrocyanide using potassium permanganate. Finally, conceptual flowsheets were proposed to extract and recover precious metals and to regenerate and reuse the only reagent introduced ferricyanide. The study proved the feasibility of the simple leaching method, paving the way for future development of the new process.