Characterization of industrially pre-treated waste printed circuit boards for the potential recovery of rare earth elements

Rare earth elements (REE) are classified as critical raw materials and the environmental impact of mining them is of growing concern. The recovery of REE from electronic waste (e-waste) could offer a more sustainable practice. Waste printed circuit boards (WPCBs) are an important resource in the e-waste stream due to their content of valuable materials. However, data regarding the concentration and distribution of REE in WPCBs is very limited. The aims of this research were: (a) to analyse the chemical composition of comminuted WPCBs prior to processing (industrially pre-treated) with emphasis on REE, and (b) to determine the distribution of REE and other metals in different size fractions of the pre-treated WPCBs. The samples were supplied by commercial e-waste recycling companies, which makes them representative of the e-waste processing industry in the UK. Correlation between elemental concentrations and particle size was analysed using Spearman's rank correlation. Most REE concentrations were inversely correlated to the particle size. Concentrations of Y, La and Gd were found up to a thousand times higher in the smaller particle size compared with coarser particles. However, most of base metals including Cu, Sn, Pb and Zn did not show this trend. The present study highlights the occurrence of REE in comminuted WPCBs, and fine fractions as potential sources of these critical elements, currently not recovered during recycling process. A cost-effective sieving step is proposed to enrich the REE content for further recovery, prevent the possible loss of REE and maximize the total material recovered from WPCBs.