Thermolysis Processing of Waste Printed Circuit Boards: Char-Metals Mixture Characterization for Recovery of Base and Precious Metals

Waste printed Circuit boards (WPCBs) encompasses 3% of electronic waste, one of the fastestgrowing categories of global waste. These waste materials contain base and precious metals with concentrations higher than those of primary resources. Therefore, recovery of the metals (especially precious metals) is the driving economic factor for recycling WPCBs. However, this effort requires the liberation of metals from non-metal fractions and each other. While comminution can effectively liberate metals from non-metals, it requires significant energy due to the high hardness and tenacity of WPCBs. Although the non-metal fraction has a low economic value, it could provide high energy recovery instead of being landfilled if the release of toxic flame retardants can be controlled. In this study, pilot scale testing of a patented thermolysis process was performed to demonstrate the capability of safely converting the plastic components of WPCBs into clean fuel gas substantially free of halogenated and volatile organic compounds, thereby liberating the metals from non-metal fractions. The char-metal mixture obtained from the process was then comprehensively characterized for the liberation and recovery of base and precious metals. The dioxin content analysis proved that dioxins were not generated, and the existing dioxins in the feedstock were cracked under chemical-reducing conditions of the process. The characterization of size and density fractions of the char-metal mixture revealed that most metals including Cu, Al, and Au are moderately and well liberated in particle size fractions smaller than 0.5 mm, and 0.149 mm, correspondingly. Based on these findings, a process flowsheet was proposed to liberate and recover base and precious metals from WPCBs. The process involves shredding and thermolysis processing of the WPCBs to convert the plastic components to fuel gas. The remaining char-metal mixture is then subjected to sizing to fractionate the sample to +0.595 mm, -0.595 mm+0.149 mm, and -0.149 mm size fractions (i.e., coarse, medium, and fine). Only the coarse fraction will require comminution for the metal liberation. The medium and fine fractions could be directly treated through physical separation to recover and separate base and precious metals. Through the proposed process, the feed to the comminution process, the most energy-consuming stage in recycling, is reduced by 65%.