Assessing alternative pre-treatment methods to promote metal recovery in the leaching of printed circuit boards

Printed circuit boards are a metals rich fraction of e-waste streams and their recycling to recover the metals promotes growth of a secondary resources economy. Reported successful cases of PCB recycling are carried out in large volumes using high-level technology and high capital investment smelter house operations. Hydrometallurgical processes have the potential to provide an option for the scalable processing of smaller volumes of PCBs and can be set up at relatively lower capital costs. However, to date, the hydrometallurgical treatment options available rely on precursor communition processes that also require high capital and energy costs. This study evaluates the options of using NaOH pre-treatment, milling, combustion and other alternative PCB pre-treatment methods to promote the leaching of metals which are otherwise inaccessible. A batch of custom-made PCBs of known and consistent metal content has been used to allow effective comparison. The effectiveness of each pre-treatment method is evaluated through carrying out an ammoniacal copper leach and recoveries are compared. X-ray CT scans are used to characterise the boards and map out accessibility of metals for leaching as well as to demonstrate the effectiveness of each pre-treatment method. Results from the X-ray CT scans showed 69% of the copper to be located in the inner layers of the PCB rendering chemical pre-treatment which only exposes metal on the outer surfaces of the board ineffective. It was concluded that an integration of pre-treatment methods effectively liberates metals without the use of heavily mechanised tools and at relatively minimum environmental impacts.