Non-toxic, high selectivity process for the extraction of precious metals from waste printed circuit boards

doi:10.1007/s11783-023-1723-z

Front. Environ. Sci. Eng.

2023, Vol. 17

Issue (10) : 123 https://doi.org/10.1007/s11783-023-1723-z

RESEARCH ARTICLE

Non-toxic, high selectivity process for the extraction of precious metals from waste printed circuit boards

Giulia Merli, Alessandro Becci(

), Alessia Amato, Francesca Beolchini

Department of Life and Environmental Sciences, Polytechnic University of Marche, Ancona 60131, Italy

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Abstract

● Au, Ag and Pd were recovered from WPCBs with high efficiencies.

● Au leaching is strictly dependent on WPCB size and reagent concentration.

● High Ag extraction efficiencies are achieved regardless of the WPCB size.

● Pd leaching works better with small and medium WPCB sizes.

● The leaching results suggest the possibility of selective recovery of metals.

The work presented here focused on the extraction of gold (Au), silver (Ag) and palladium (Pd) from electronic waste using a solution of ammonium thiosulfate. Thiosulfate was used as a valid alternative to cyanide for precious metal extractions, due to its non-toxicity and high selectivity. The interactions between sodium thiosulfate, total ammonia/ammonium, precious metal concentrations and the particle size of the waste printed circuit boards (WPCBs) were studied by the response surface methodology (RSM) and the principal component analysis (PCA) to maximize precious metal mobilization. Au extraction reached a high efficiency with a granulometry of less than 0.25 mm, but the consumption of reagents was high. On the other hand, Ag extraction depended neither on thiosulfate/ammonia concentration nor granulometry of WPCBs and it showed efficiency of 90% also with the biggest particle size (0.50 < Ø < 1.00 mm). Pd extraction, similarly to Au, showed the best efficiency with the smallest and the medium WPCB sizes, but required less reagents compared to Au. The results showed that precious metal leaching is a complex process (mainly for Au, which requires more severe conditions in order to achieve high extraction efficiencies) correlated with reagent concentrations, precious metal concentrations and WPCB particle sizes. These results have great potentiality, suggesting the possibility of a more selective recovery of precious metals based on the different granulometry of the WPCBs. Furthermore, the high extraction efficiencies obtained for all the metals bode well in the perspective of large-scale applications.

Keywords Thiosulfate Printed circuit boards Precious metals Leaching Hydrometallurgy

Corresponding Author(s): Alessandro Becci

About author:

*These authors equally shared correspondence to this manuscript.

Issue Date: 28 April 2023

Cite this article:

Giulia Merli,Alessandro Becci,Alessia Amato, et al. Non-toxic, high selectivity process for the extraction of precious metals from waste printed circuit boards[J]. Front. Environ. Sci. Eng., 2023, 17(10): 123.

URL:

https://academic.hep.com.cn/fese/EN/10.1007/s11783-023-1723-z
https://academic.hep.com.cn/fese/EN/Y2023/V17/I10/123

Fig.1 WPCBs with different granulometries: lower than 0.10 < ? < 0.25 mm (a), 0.25 < ? < 0.50 mm (b) and 0.50 < ? < 1.00 mm (c) tested by SEM analysis.

Tab.1 Factors and levels for the CCD-RSM

Fig.2 Au dissolution in the final liquor with small size WPCBs (0.10 < ? < 0.25 mm, in green): ● 0.2 mol/L NH₃ 0.1 mol/L thiosulfate, ■ 1 mol/L NH₃ 0.1 mol/L thiosulfate, ? 0.2 mol/L NH₃ 1 mol/L thiosulfate, ▲ 1 mol/L NH₃ 1 mol/L thiosulfate; big size WPCBs (0.50 < ? < 1.00 mm, in pink): ● 0.2 mol/L NH₃ 0.1 mol/L thiosulfate, ■ 1 mol/L NH₃ 0.1 mol/L thiosulfate, ? 0.2 mol/L NH₃ 1 mol/L thiosulfate, ▲ 1 mol/L NH₃ 1 mol/L thiosulfate; and medium size (0.25 < ? < 0.50 mm, in black): * 0.6 mol/L ammonia/ammonium 0.55 mol/L thiosulfate.

Fig.3 Simulated data vs. real data for Au mobilization.

Tab.2 Summary of ANOVA results for RSM for Au mobilization

Fig.4 Contours of thiosulfate vs. ammonium (a) and time vs. granulometry (b) for Au mobilization.

Fig.5 Ag dissolution in the final liquor with small size WPCBs (0.10 < ? < 0.25 mm, in green): ● 0.2 mol/L NH₃ 0.1 mol/L thiosulfate, ■ 1 mol/L NH₃ 0.1 mol/L thiosulfate, ? 0.2 mol/L NH₃ 1 mol/L thiosulfate, ▲ 1 mol/L NH₃ 1 mol/L thiosulfate; big size WPCBs (0.50 < ? < 1.00 mm, in pink): ● 0.2 mol/L NH₃ 0.1 mol/L thiosulfate, ■ 1 mol/L NH₃ 0.1 mol/L thiosulfate, ? 0.2 mol/L NH₃ 1 mol/L thiosulfate, ▲ 1 mol/L NH₃ 1 mol/L thiosulfate; and medium size (0.25 < ? < 0.50 mm, in black): * 0.6 mol/L ammonia/ammonium 0.55 mol/L thiosulfate.

Fig.6 Simulated data vs. real data for Ag mobilization.

Tab.3 Summary of ANOVA results for RSM for Ag mobilization

Fig.7 Contours of thiosulfate vs. ammonium (a) and time vs granulometry (b) for Ag mobilization.

Fig.8 Pd dissolution in the final liquor with small size WPCBs (0.10 < ? < 0.25 mm, in green): ● 0.2 mol/L NH₃ 0.1 mol/L thiosulfate, ■ 1 mol/L NH₃ 0.1 mol/L thiosulfate, ? 0.2 mol/L NH₃ 1 mol/L thiosulfate, ▲ 1 mol/L NH₃ 1 mol/L thiosulfate; big size WPCBs (0.50 < ? < 1.00 mm, in pink): ● 0.2 mol/L NH₃ 0.1 mol/L thiosulfate, ■ 1 mol/L NH₃ 0.1 mol/L thiosulfate, ? 0.2 mol/L NH₃ 1 mol/L thiosulfate, ▲ 1 mol/L NH₃ 1 mol/L thiosulfate; and medium size (0.25 < ? < 0.50 mm, in black): * 0.6 mol/L ammonia/ammonium 0.55 mol/L thiosulfate.

Tab.4 Summary of ANOVA results for RSM for Pd mobilization

Fig.9 Simulated data vs. real data for Pd mobilization.

Fig.10 Contours of thiosulfate vs. ammonium (a) and time vs. granulometry (b) for Pd mobilization.

Tab.5 Metal concentrations in the different particle sizes (mg/kg)

Fig.11 Changing in the molar ratio between thiosulfate (a) or ammonia (b) and the precious metal concentrations in the different experimental plans.

Fig.12 Principal component analysis (PCA) results.

Fig.13 Au, Ag and Pd extraction efficiencies (%) in the different experiments.

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