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, the results of this study demonstrate well the
feasibility of utilizing the abovementioned sieving classification
pretreatment steps followed by hydrocyclone and DGM methods as promising
approach for recovery of precious metals from WPCBs which contain
annually almost 50 million tons of e-waste.