Feasibility of gold leaching from waste printed circuit boards with an airlift bioreactor applying the condensed distillers solubles as a microbial nutrient and its recovery by reduction

To improve the bio-cyanide production by Chromobacterium violaceum, an airlift bioreactor was designed, and its performance was evaluated. Condensed distillers solubles, a byproduct of the ethanol production process, served as a nutrient source for C. violaceum. During bacterial cultivation, the produced cyanide was volatilized and captured in a 1M NaOH solution connected to an airlift bioreactor. Through the optimization of the bio-cyanide production process in the bioreactor, a cyanide concentration of 596.4 ± 14.8 mg/L was obtained in the NaOH collector after 8 days, which was utilized to leach Au from waste printed circuit boards (WPCBs). Na₂S₂O₄ was employed in the Au reduction process after Au leaching. Under optimal conditions with a reductant concentration of 1 g/L, a reaction temperature of 60 °C, and pH 11, 99.3 % of Au was recovered within 6 h. Additionally, based on the optimal conditions obtained in this study, a comprehensive economic feasibility assessment for the entire process of Au leaching using bio-cyanide and Au reduction with Na₂S₂O₄ was performed. Calculations indicated that a gross profit of USD 31,248.9 could be achieved per tonne of WPCBs with an Au content of 0.1 %. These findings suggest that bioleaching represents a cost-effective and environmentally friendly alternative to recover Au from WPCBs.