Optimization of Sustainable Processes for the Extraction of Precious Metals from End-of-life Printed Circuit Boards

Abstract

The technology advancement and innovation has been very significant and fast, in the last decades. This aspect is particularly evident in the electric and electronic equipment (EEE) field, where new devices are produced every year. The manufacture development is translated in two main issues: the highest metal required and the greatest waste flows to manage. To solve this double criticality, the old linear management method, in which every product became a waste, must evolve in a circular approach based on the principle “resource-product-regenerated resource” (urban mining). The end-of-life printed circuit boards (PCB) represent one of the most significant wastes from electric and electronic equipment (WEEE). In addition to their availability, the interest for these scraps is mainly justified by the high precious metal concentration (e.g. gold, silver, and palladium), which makes them a promising secondary resource. Several approaches are developed for precious metal recoveries from PCB, mainly by pyrometallurgy or hydrometallurgy. Biohydrometallurgical and green hydrometallurgical processes, using more sustainable reagents, are gaining increasing prominence, for their possibility to decrease the environmental costs, compared to the most traditional hydrometallurgy with chemical cyanide lixiviant. The present work aims to supply two innovative solutions: biotechnology by Pseudomonas aeruginosa for the biological cyanide production, and a green-hydrometallurgical process, using the thiosulphate as lixiviant agent for precious metal (i.e. Au and Ag) recovery. The central composite design of a response surface methodology (CCD-RSM) is used to optimize the cyanide production by P. aeruginosa evaluating the influence of both initial pH and glycine concentration. The same statistical approach (CCD-RSM) is used to find the best operative conditions for the precious metal leaching from PCB by thiosulphate, assessing the interaction among sodium thiosulphate, total ammonia/ammonium, particle size and time. The experimental results are further enhanced by the carbon footprint assessment which has quantified the possible environmental advantages of the developed solutions of PCB recycling, able to integrate the circular economy principles.