Sampling waste printed circuit boards: Achieving the right combination between particle size and sample mass to measure metal content

Abstract

The current worldwide expansion of waste PCB (WPCB) deposits represents both a pressing environmental issue and an economic opportunity, fostering the development of numerous recycling processes across the world. An important input for designing such processes is the metallic content of WPCBs, which is assayed by grinding and leaching samples taken from the stack of WPCBs to be recycled. The content values come with substantial uncertainties, arising mainly from the uneven distribution of the metals within the structure of WPCBs.This study aims to quantify the effects on these uncertainties of the particle size, the mass of the sample digested and the number of digestion replicates. It focused on the abundance of six metals in WPCBs: Cu, Fe, Zn, Pb and Ni, and also Co, which is a critical element for the EU. A batch of 485 kg of WPCBs was put through several shredding and splitting steps to produce three fractions: one shredded to 2 mm, and two ground to 750 μm and 200 μm. From each sample, 16 samples of 0.5 g, 2 g or 5 g were digested in hot aqua regia. Bootstrapping of the results allowed the error around the mean content to be estimated, for each metal and for all the experimental conditions. Considering the largest sample masses and three replicated digestions, the uncertainties for Zn (resp. Ni) were reduced from 35% to 10% (resp. from 70% to 10%) when the particle size was reduced from 2 mm to 200 μm.