An efficient, multi-stage process for recovering gold from electronic waste involving autoclave pre-treatment and industrial-metal pre-extraction

Abstract

In recent years, the recovery of gold from electronic waste, especially waste printed circuit boards (PCBs), has grown in importance. PCBs consist of organic epoxy resins overlaid with valuable metals. The first step in recovering metals from waste PCBs is the removal of these resins. In this paper, autoclave-assisted alkaline pretreatment employing NaOH was used to remove resins from the PCBs. The process is shown to remove epoxy efficiently when conducted at 350 °C for 18 h using 0.8 mol·L−1 NaOH, 18 bar, and a liquid-to-solid (L/S) ratio of 18 mL·g−1. After autoclaving, the solid-phase sample (autoclaved powder) was treated with a mixture of FeCl3 and HCl solutions – a process known as “FeH extraction” – to remove industrial metals. Under the optimal conditions (0.8 mol·L−1 FeCl3, 0.6 mol·L−1 HCl, L/S 24 mL·g−1, 50 °C, 100 min, 300 rpm), this process proved highly efficient in removing industrial metals. Following this treatment, a KClO3-HCl-CaCl2 extraction system was employed to extract gold from the FeH-extracted sample. With this system, a 99.5% gold extraction efficiency was achieved under the optimal conditions (0.55 mol·L−1 KClO3, 1.2 mol·L−1 HCl, 1.5 mol·L−1 CaCl2, 60 °C, 80 min, L/S 12 mL·g−1, 350 rpm). Gold ions (AuCl4−) in the extracted solution were subsequently recovered by using hydroquinone as a reducing agent to form gold nanoparticles (AuNPs) with the assistance of NaOH. Finally, the AuNPs were stabilized using benzoquinone (an oxidized product of hydroquinone).