Complete recycling of Printed Circuit Boards: From base metal bioleaching in a semi-continuous reactor with dual regulation to gold biosorption with brewer's yeast

Abstract

Bioleaching is a promising technology for recovering base metals and concentrating precious elements in secondary resources. In this study, the extraction of base metals from printed circuit boards (PCBs) was optimized using a dual control system including biomass encapsulation. Gold recovery was achieved in a second step by chemical dissolution followed by biosorption on brewer's yeast. A dual regulation system, controlling oxidoreduction potential (ORP) and copper concentration, was established. This system comprised two reactors interconnected in a loop: a leaching reactor and a biological reactor. Acidophilic bacteria in the biological reactor recycled ferrous into ferric iron in a low shear stress environment. The Fe3+ solution from the biological reactor recirculated to the leaching compartment (hosting the PCBs) based on ORP regulation (optimal setpoint: +410 mV vs Ag/AgCl). A second control aimed to maintain the copper concentration below the inhibitory level (15 g Cu2+ L−1) by addition of fresh medium and overflow of leachate. This configuration ensured the highest pulp density, operating consistently, in steady state, at the inhibitory concentration limit, the PCBs being added batch-wise when the stock within the leaching compartment was exhausted. Compared to planktonic cells, the encapsulated mode showed optimal base metals leaching yields: 100% for Cu, 24% for Al, 49% for Ni, and 100% for Zn, with a hydraulic retention time (HRT) of 92 h and an iron oxidation rate of 0.455 mg L−1 h−1. The reactor's productivity was 5.91 g Cu L−1d−1, corresponding to the leaching of 20.88 g PCBs L−1d−1. The gold enriched PCBs (from 290 to 450 mg Au kg−1) were then extracted using Aqua regia, with a 24:1 liquid/solid ratio at 80 °C for 2 h. Subsequent biosorption on brewer's yeast (10 g yeast L−1) selectively recovered gold, with 98% efficiency. Biosorption yields for “fresh” yeast resulting from a model brewing (90.8%) and commercial “active dry” yeast (88.7%) were comparable, indicating the potential use of spent yeast from brewing.