Recovery of metals from pyrolysed PCBs by hydrometallurgical techniques

- Vacuum pyrolysis of WPCBs followed by physical beneficiation separates low and high metal concentrates.
- Nitric acid leaching of high metal concentrates of poly-cracker ash of PCBs for recovery of metals
- 98% recovery of acid and metals (Cu, Fe, Pb and Ni) from the leach liquor of poly-cracker ash of PCBs

Present paper is a part of developed novel process to recover various metals (Cu, Fe, Ni and Pb) environmentally from waste printed circuit boards (WPCBs). The process consists of pyrolysis-physical beneficiation-leaching-solvent extraction-electrowinning-cementation. The PCBs were pyrolysed at a temperature of 300 °C and the obtained poly-cracker ash of PCBs was subjected to comminution for the separation of metallic from non-metallic fractions using scutter crusher and grinding mills. Further, the ground poly-cracker ash of PCBs was scrubbed/washed with water without using any additives to enrich the metallic concentrate up to 98%. The scrubbed sample was further screened to separate low and high metal concentrates. Further, the high metal concentrate (+ 10 mesh i.e. > 2 mm size) was leached in 4 M HNO3 at 90 °C for 60 min maintaining pulp density 30 g/L and stirring speed 250 rpm. Result showed the complete dissolution of the metals viz. Cu, Fe, Pb and 57.5% Ni etc. present in the concentrate. Systematic scientific studies were also carried out to recover acid and metal from leach liquor of the poly-cracker ash of PCBs. To recover nitric acid from the leach liquor, the experiments were carried out using 60% TBP diluted in kerosene, which resulted in the extraction of 98% HNO3 in three stages at O/A ratio 3/1 in 12 min. Subsequently, air sparging was performed to separate and recover Fe as precipitate. After the recovery of acid and iron, the leach liquor was further processed to recover Cu using 25% LIX 84IC diluted in kerosene. The copper was found to be extracted 99.99% in two stages at pH 2.5, O/A ratio 1/1 in 20 min, leaving Ni and Pb in the raffinate. Cu solution was stripped from loaded organic using 10% H2SO4. Cu metal sheet was obtained from the strip solution by electrowinning process in 4 h at temperature 30 °C using a cathode current density of 200 A/m2. Further, the raffinate was processed for the recovery of Ni and Pb. Based on the experimental results a process flow-sheet is proposed for the extraction of all major metals (Cu, Fe, Ni and Pb) from the waste PCBs. The process has the potential to be commercialized after scale-up studies.