Selective leaching of base/precious metals from E-waste of cellphone printed circuit boards (EWPCB): Advantages and challenges in a case study

Abstract

Complex materials, such as those contained in e-waste, offer new challenges for recovering metals of interest. This investigation exemplifies the difficulties encountered when treating multimetallic or refractory materials, from which specific values are to be separated and recovered. The case study involves the selective leaching of gold from e-waste printed circuit boards (EWPCB), containing ≈200 g Au per ton. To achieve a selective process, chemical inhibitors were employed; their function is to avoid the co-dissolution of base- metal impurities, which could accumulate in the leaching solutions and contribute to leaching agents' degradation, impeding gold extraction.Solutions of thiourea (CS(NH2)2) and hydrogen peroxide (H2O2) were proposed to leach the gold; however, strong competition for these reagents, especially by copper and iron contained in the EWPCB, completely blocked gold dissolution. After a pretreatment that removed the bulk of selected base metals (elimination of 98% of Cu and > 80% of Fe, Zn and Ni), the presence of these and other metals still obstructed gold extraction. The incorporation of chemical inhibitors was analyzed.Unfortunately, although oxalic acid did diminish the presence of both copper and iron ions in solution, it fomented the competitive leaching of aluminum and tin. The CS(NH2)2-H2O2-C2H2O4 system at ambient temperature (25 °C) allowed a gold recovery of ≈87% by suppressing most of the base metal interference. Not only the dissolution of the metal of interest is important, but also the fate of the accompanying metals and constituents.