Thermal Transformation of Discarded CRT Printed Circuit Boards for Recovery of Sn Values

A printed circuit board (PCB) is an essential and central component of electronic waste and comprises majorly valuable metallic values (Sn, Pb, Cu). This study utilizes a short, fast-heating, low-temperature pyrolysis process to recover value-added tin-based alloy from spent cathode ray tube (CRT) PCB. The optimized process condition (550 °C, 10 minutes) resulted in ~ 62 wt pct solid, ~ 8.5 wt pct oil, and ~ 29.5 wt pct gas products. Sn recovery of ~ 75 pct in the Pb-Sn metallic droplet form with the purity of Sn + Pb ~ 85.2 pct at 550 °C within 10 minutes is obtained. Sn-based alloy does not form at 400 °C due to incomplete decomposition of the polymeric fraction and limited surface tension, as revealed by characterization. At higher temperatures (550 °C), the molten alloy spreads over the PCB Cu tracks due to enhanced wetting force, leading to diffusion of Sn in Cu and separation of Pb. The diffusion of Sn in Cu leads to the formation of intermetallics (Cu₃Sn, Cu₆Sn₅) at higher residence times and temperatures, inhibiting the recovery of the alloy. Further, the quenching causes spinodal decomposition, leading to a zebra stripe pattern in the alloy, and XPS confirmed the absence of Br in the alloy. The organic functional group disappears from the residual solid char with temperature, as revealed by FT-IR. The gaseous product comprises CH₄, CO, CH₃OH, C₂H₆, C₂H₄, and CO₂ with gross heating value of ~ 310 kJ/m³. The pyrolyzed oil comprised ~ 60 pct phenol and phenolic compounds and can be re-utilized for synthesizing phenolic resins and epoxy resin. ~ 0.19 kg of Sn-based alloy (~ 85.2 pct purity) can be recovered from 1 kg of PCB as a feedstock for the tin-making industry and soldering applications.