A critical assessment on resource recovery from electronic waste: Impact of mechanical pre-treatment

Commonly used pre-treatments during e-waste recycling, e.g., manual dismantling, removal of components, size reduction, crushing etc. can cause significant local area contamination and environmental pollution. This study seeks to critically assess the impact of mechanical pre-treatments on average resource recovery, generation of high-value material concentrates and on concentrations of hazardous elements. Two sets of initial processing were carried out on a variety of waste printed circuit boards (PCBs). Electronic components were not removed from these boards. In Set I, PCBs were powdered using sorting, crushing, hammering followed by separation into heavy and light fractions through froth floatation. In Set II, PCBs were cut into small sized pieces. These were heat treated in a horizontal furnace in inert atmosphere at 850 °C for 15 min, followed by in-depth characterization of pyrolysis residues. The average recovery of main metal copper was quite comparable for both sets (Set I: 27.6 wt %; Set II: 26.3 wt %). In case of precious metals, there were higher recoveries for Au and Ti in Set I, whereas Ag, Pd, Pt showed better recoveries in Set II. A mixed trend was observed for minor metals Mn, Zn Sn, Sb and Si. In Set I, most elements showed a mixed presence in the heavy and light fractions after pyrolysis. On the other hand, Set II showed a significantly higher elemental segregation and generation of high value concentrates. Potential toxic hazardous elements (Pb, Cr, Co, Cd, Be) were detected in much higher amounts in Set I as compared to Set II. Waste processing under Set II was found to be more efficient, cost-effective and eco-friendly as compared to Set I. This study suggests a fresh look at traditional labour-intensive e-waste recycling practices towards limiting social and environmental impacts while enhancing the efficiency and cost-effectiveness of resource generation from e-waste.