Separation of the metallic and non-metallic fractions of waste printed circuit boards - A review focused on the organic swelling

This paper reviews the procedures to separate the metallic (MF) and non-metallic (NMF) fractions of the waste printed circuit boards (WPCBs) for recycling their most valuable fractions. Firstly, a comprehensive overview of the whole process of recycling WPCBs, starting from the composition and structure of the WPCBs and exploring the currently available and implemented technologies (physical and/or chemical) is presented. In a second stage, the recent progresses made on the development of more environmentally friendly methods (supercritical fluid extraction and organic swelling) to separate particularly metals (the most valuable fraction of WPCBs) is reviewed, focusing particular attention on the organic swelling. This last strategy demonstrated to be an excellent pre-treatment in the recycling of WPCBs as it dissolves the brominated epoxy resin (BER) from the WPCBs and promotes the total delamination of WPCBs, which increases the recovery of the MF and NMF (and, thus, the economic revenue) minimizing the environmental impact of the WPCBs recycling processes comparatively to the traditional technologies. Additionally, two theoretical models (Hildebrand and Hansen), which are useful to predict the solubility between the solvent and the BER, and its mechanistical interpretation are presented. Finally, some challenges and opportunities of the WPCBs recycling processes are highlighted.