Urban mining: recovery of metals from printed circuit boards

Waste PCBs (WPCBs) contain many types of valuable but environmentally harmful metals. Thus, the recovery of metals from WPCBs has a high environmental consequence and is essential for resource conservation. The study aims to develop eco-friendly methods for the recovery of metals from WPCBs. Through this work, a systematic procedure for the separation of components and extraction of metals is demonstrated. PCB was disintegrated by sequentially dissolving the solder in methane sulfonic acid (MSA), epoxy in sodium hydroxide, N-Methyl-2-Pyrrolidone (NMP), and metal interconnects dissolved separately in citric acid, hydrochloric acid, and sulphuric acid. Various processing conditions were evaluated to enhance the extraction. ICP-AES analysis was done for all leaching solutions, and it was observed that the solution of 3.5 M MSA and 0.5 M H₂O₂ completely dissolves the solder (Sn and Pb) in 1.5 h at 28ºC. Both citric acid and HCl have shown more than 75% recovery of copper and metals like Ag, Al, Pb, Ni, and Fe in the leaching solution in 4 h and 24 h, respectively, at 28ºC. Similarly, leaching using 15wt% H₂SO₄ aqueous solution with 10 ml of H₂O₂ has shown more than 95% extraction of Ag, Al. Fe and Ni in the leachate. Citric acid has shown very competitive results among all three leachates with low extraction time. It was concluded that citric acid could be used as an eco-friendly replacement for inorganic acids. Subsequently, recovery of copper using iron powder was performed, and 95% recovery was achieved using 4 g of iron powder, which was deduced using ICP-AES analysis. The work demonstrates the feasibility of sequential delamination and removal of layers in a PCB without prior pulverization. A sustainable process for extracting copper and potentially other metallic components in PCB, in durations much shorter than those reported, is demonstrated. Notably, citric acid resulted in over 83% recovery of copper in less than 4 h, comparable and possibly better than some other inorganic acids. Additionally, there is no need for pre-crushing, and the non-metallic and polymeric components are obtained intact, which may potentially be used for different applications.