The effect of crusher type on printed circuit board assemblies' liberation and dust generation from waste mobile phones

Waste electrical and electric equipment entering our recycling system poses various challenges in different stages of the whole treatment chain. Losses of valuable metals in the crushing of electrical devices through dusts and fines have been identified as one such challenge. Crushing is nevertheless crucial in order to liberate metals from each other for efficient separation in the sequential mechanical unit processes. This study investigated the relation of crushing mechanism on Printed Circuit Assemblies' (PCAs) liberation and fines generation in the size reduction of waste mobile phones using two crusher types. The results revealed that a fast-rotating hammer mill produced better liberated PCAs with an overall PCA grade, presenting the purity of PCA fraction of 77% compared to a slow-rotating cutting mill with 58% PCA grade. However, the hammer mill produced over two times the amount of fines compared to the cutting mill. The fines fraction mainly comprised silicon and base metals but also noble metals and harmful elements which need to be taken into account when further treatment is considered. Even though the same elements could be found in the fines from both crushers, differences in the concentrations were observed. In the cutting mill, higher concentrations of ductile materials such as gold and copper were observed in the fines fraction with particle size below 4 mm compared to the hammer mill.