Efficient separation of precious metals from computer waste printed circuit boards by hydrocyclone and dilution-gravity methods

To fulfill the different aspects of green chemistry and to achieve full use of the secondary resources (waste printed circuit boards (WPCB)), the necessity of developing green methods for recovery of precious metals (Au, Pd, and Ag) is highly demanded. In this study, a novel environment-friendly physical separation approach; the combination of crushing, grinding, sieving as pretreatment steps alongside hydrocyclone and the dilution-gravity method (DGM) as the main final steps; is proposed. Inductively coupled plasma-mass spectrometry (ICP-MS), atomic absorption spectroscopy (AAS), and energy-dispersive X-ray spectroscopy (EDS) characterization methods were utilized to understand the effects of different separation steps applied in this research. The size and shape of grinded materials and the ones produced after hydrocyclone and DGM were evaluated using scanning electron microscopy. The results showed that the sieving step separated the highest gold fraction in the finer classification (<75 μm) while placed the copper (70 wt.%) into the coarser contents. The overflow to underflow outlet diameter ratio and inlet pressure was evaluated to determine the separation efficiency of a hydrocyclone effect of parameters. In the best-case scenario at 3 bar hydrocyclone operation pressure where the overflow to underflow outlet diameter (Do/Du) was 6.5, the highest metal fraction (87 wt.%) was achieved in the sink of the DGM. In this case, the total separation efficiency of gold, palladium, silver, and copper was 75%, 78%, 64%, 72%, respectively. Thus, this study demonstrates the feasibility of utilizing the sieving as mentioned above classification pretreatment steps followed by hydrocyclone and DGM methods as promising approaches for recovering precious metals from WPCBs that contain annually almost 50 million tons of e-waste.