A comprehensive review on the recycling of discarded printed circuit boards for resource recovery

Printed circuit boards (PCBs) are an essential and central component of electronic waste. The rapid depletion of natural resources, massive generation of end-of-life PCBs and inherently metal-loaded values inevitably call for recycling and recovery. This review critically discusses the systematic and sequential processes adopted for PCB metallic recoveries via physical, pyrometallurgical, hydrometallurgical, and combined technologies. Pre-treatments play a decisive and significant role in upgradation and efficient metal extraction. A novel combination of different pre-treatments and hybrid thermal-chemical routes are often reported for improved separation efficiency and performance. Selective recovery (using solvent extraction, precipitation, polymer inclusion membrane, adsorption, ion exchange) of high purity product from multi-elemental leach solution has recently gained interest and is reviewed. Current recycling techniques at a commercial scale are preferably based on pyrometallurgy (smelting-refining), where electronic waste is only a fraction of the total feed stream. Electronic components such as monolithic ceramic capacitors, tantalum capacitors, integrated circuits, and central processing units mounted on the PCBs are important due to precious metals' presence. The futuristic recycling perspective should treat base and precious metal-rich components separately with minimal environmental effect, end product usage, and maximum economic benefit. Sustainable processing routes for converting discarded PCBs into value-added products should also be attempted, as amplified in this review. An integrated, definite framework for full resource recovery from waste PCBs was proposed.