(652c) A Two-Step Size Reduction Method for Fine Milling of Electronic Waste to Improve Recycling

Electronic waste (e-waste) recycling is important for waste treatment and recovery of valuable materials, as it can reduce the extraction of virgin ores and extend the life of those resources. Hydrometallurgical recovery of valuable metals from printed circuit boards (PCBs) is widely used and consists of at least two main unit operations of leaching and recovery. During leaching, a smaller particle size results in a greater dissolution of valuable metals, due to higher surface area and higher proportion of metals exposed to the leaching fluid. Conventional size reduction process involves shredding and hammer milling, for which industries reported as much as over 60% of valuable elements is lost through those unit operations in output streams (milling dusts), resulting in an overall reduction of the recycling efficiency and implying the need for improvement. A novel two-step mechanical size reduction method for pretreatment of e-waste materials is being developed for generating high-efficiency leaching-ready e-waste feedstock. This method involves a shredder mill and a cryogenic mill at Idaho National Laboratory’s (INL’s) Biomass Feedstock National User Facility (BFNUF). In a conventional two-step milling process, the hammer mill was found prone to generating sparks and presenting fire hazard during operation. In addition, loss of mass in dust and fines is not well mitigated. To re-risk the operation and improve efficiency, a new cryogenic mill is used instead and re-configured for reducing mass loss in processing e-waste. The mill chamber is filled with liquid nitrogen during operation, which significantly increased the brittleness of the particles and enhanced fragmentation. Both the discharge and vent valves are specially configured to allow stable ventilation of nitrogen gas and retention of fine particles. The size of the resultant fine particles ranges from sub-100 microns up to a few millimeters. Mostly importantly, the mass loss rate is below 5%, a huge improvement over the current industrial level.