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Oxidative thiosulfate leaching using Cu(II)-NH3 has been explored for both mining and recycling applications as a promising method for Au extraction. This study seeks to understand the dissolution behavior of Au from waste RAM chips using a Cu(II)-NH3-S2O3 solution. In the course of this work, bimodal leaching and Au loss were observed in a manner that we have not identified in the literature. Identification of the existence of a specific Au-Ni-Cu lamellar structure in the gold fingers from RAM chips by scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) revealed the possibility of interference between Au recovery and the existence of Cu and Ni. During leaching, the co-extraction of Ni was found to predict a negative impact on the Au recovery, as a result of chemical interactions from the Au-Ni-Cu interlayer. Decopperization as a pretreatment was found necessary to remove the pre-existing Cu and promote Au leaching. As part of the study parameters, such as Cu(II) concentration, aeration rates, thiosulfate and ammonia concentrations, particle sizes, and temperatures, were investigated. A satisfactory Au recovery of 98% was achieved using 50 mM Cu(II), 120 mL/min aeration rate, 0.5 M (NH3)2S2O3, and 0.75 M NH4OH (i.e., AT/AH ratio of 0.67) for 4 h residence time at room temperature (25 °C). However, there were several high recoveries prior to Au loss from the lixiviant. It was revealed that the main cause of lower Au recovery was due to a precipitation or cementation reaction that included a sulfur species formation. Because of the bimodal leaching, a composite response comprised of the time to Au loss and maximum recovery was developed, termed leaching proclivity, to facilitate statistical analysis. Furthermore, this study explores the interactions between Au-Ni-Cu and provides suggestions for improving Au thiosulfate leaching under the interference of co-existing metals from waste PCB materials.
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Supercritical CO 2 -induced alteration of polymer-metal matrix and selective extraction of valuable metals from waste printed circuit boards
The rapidly accumulating amounts of waste electrical and electronic equipment (WEEE) is one of the biggest environmental concerns in modern societies, and this problem will be further accelerated in the future. The use of supercritical CO2 (scCO2) mixed with acids has been proposed as a greener solvent system compared to conventional cyanide and aqua regia solvents, however, the mechanisms of scCO2 in metal extraction from WEEE are still poorly understood. Thus, this study focused on the physical, structural, and chemical interactions between scCO2/acid solvents and complex layered components in waste printed circuit boards (WPCBs), one of the common WEEEs. Our study showed that the use of scCO2-based pretreatment allows faster leaching of metals including copper (Cu) in the subsequent hydrometallurgical process using H2SO4 and H2O2, while allowing gold (Au) recovery as hydrometallurgically delaminated solids. This enhancement is due to the selective leaching of Ni and unique inner porous structures created by ScCO2/acid treatment via dissolving the Ca-silicate-bearing fiberglass within the WPCB. Thus, the scCO2-based pretreatment of WPCBs shows a multifaceted green chemistry potential relating to the reduction in solvent usage and targeted recovery of Au prior to shredding or grinding that would reduce any loss or dilution of Au in the subsequent waste stream.
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- Award ID(s):
- 1706905
- PAR ID:
- 10195688
- Date Published:
- Journal Name:
- Green Chemistry
- ISSN:
- 1463-9262
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0GC02521F
