Sequential recovery of metals from waste printed circuit boards using a zero-discharge hydrometallurgical process

Abstract

Several hydrometallurgical routes have been proposed in the literature to treat printed circuit boards (PCBs) from waste electronic equipment. These employ different chemical reagents for the recovery and separation of metals as metals or their salts. The recovery of multiple chemical reagents (unreacted acid, neutralising agent, metal salts) in a process requires several additional downstream steps affecting economic feasibility. There is a need to develop a safe, eco-friendly, and economically feasible process to recover metals from PCBs. In this work, we achieve this using a hydrometallurgical process for treating PCB with nitric acid by extracting metals sequentially. The focus is on tin, lead, and copper, which are present in significant quantities in PCBs. The process is scalable and is based on exploiting the physio-chemical interactions between the different metals and the acid. Tin and lead present in the solder are selectively dissolved at low acid concentrations. Tin comes out in the form of colloidal particles of metastannic acid. The low solubility of lead nitrate in concentrated nitric acid is exploited by evaporating the solution containing unreacted nitric acid and lead nitrate. In the concentrated acid, lead nitrate crystals are precipitated out, and the concentrated nitric acid is recycled for dissolution. The different nitrogen oxide gases generated are absorbed in water and recycled. Copper present in copper tracks was recovered by reacting with a higher concentration of acid in a downstream step. The copper nitrate obtained was separated from nitric acid by extracting nitric acid using Tri-Butyl Phosphate. Tin, lead, and copper are sequentially extracted as tin oxide (s), lead nitrate (s), and copper nitrate (aq.) with an efficiency of 77%–97%, 51%–85%, and 100%, respectively.