The potential of valuable metal recovery from low-grade mixed waste printed circuit boards: Perspectives from mechanical preprocessing

• Effective size reduction of waste PCBs is achieved by shredding and cryo-milling • A nearly full mass retention is achieved with modified ventilation in cryo-mill • Copper and other base metals concentrate in milled particles with size ≥ 1 mm • Concentrations of higher-value metals grow in milled particles < 0.5 mm • Concentration of lead increases with decreasing particle size and must be mitigated To recover valuable metals from electronic waste (e-waste), industry prefers feedstock streams by category to improve efficiency. This though ignores the massive mixed waste printed circuit board (WPCBs) that are fragmented, hard to separate, and continue presenting risks to the environment and human health. This work explores the potential of valuable metal recovery from the abundant low-grade mixed WPCBs with perspectives from mechanical preprocessing. A two-stage comminution process involving shredding and cryogenic milling, with a two-stage elemental composition analysis method, is introduced to investigate the concentrations of select valuable metals. The cryogenic mill is specially configured to minimize airborne fines, ensure safe laboratory environment, and achieve over 99% mass retention. The milled particles are screened successively from 4 to 0.045 mm for accurate analysis of metal concentrations in each size class. Copper is dominant regarding mass percentage and concentrated in larger particles, followed by aluminum. The concentration of barium grows consistently with particle size decreasing from 4 to 0.045 mm, while tin exhibits a peak concentration around 0.5 mm. Beryllium is nearly non-detectable. Optimal milling time for the shredded PCBs depends on the target metals. Our findings indicate a potential to recover target metals from the low-grade mixed WPCBs in separate particle sizes, i.e., larger particles for base metals like copper and smaller particles for higher-value metals like barium and tin. Lead concentration is found higher in smaller particles, highlights the necessity of the proposed fines containment method for milling process.