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<p>The waste mylar sheets from keyboards consist of an organic layer of polyethylene terephthalate (PET) as a base layer onto which a thin layer of conductive paths of silver (Ag) is present. The study focuses on the holistic recycling of the mylar sheets to recover the Ag values and polymeric fraction, which are otherwise discarded or landfilled. Different routes are evaluated, such as direct leaching, pyrolysis, and alkaline hydrolysis (as pre-treatment). The direct leaching of mylar sheets in 2&#xa0;M HNO<sub>3</sub> (80&#xa0;°C, S/L ratio of 1/20, 2&#xa0;h) resulted in ~ 98.8% dissolution of the Ag values; however, a large amount of PET pieces (~ 99 wt.%) are left unutilized. Further, the acid-leached PET is partially degraded and subsequently either discarded or downcycled, leading to the loss of resources. The pyrolysis and alkaline hydrolysis methods target the PET degradation, which leads to Ag liberation. The pyrolysis enriches Ag values in the residue (Ag- 6.3%) and reduces the acid volume by 80% for the post-pyrolysis-leaching step. In this study, alkaline hydrolysis is a suitable pre-treatment method for depolymerization and Ag recovery. Hydrolysis converts PET into terephthalic acid and ethylene glycol, while the Ag tracks are retained in the solid form, thus achieving complete resource utilization. The process avoids the gaseous emissions (NO<sub>x</sub> and CO<sub>2</sub>) produced during conventional acid leaching and pyrolysis. At an optimal hydrolysis condition (NaOH:feed (g/g)-0.5, 300&#xa0;°C, L/S of 25, 30&#xa0;min), 98.7% depolymerization yields Ag flakes of purity ~ 87%. The hydrolyzed residue can be directly melted (1050&#xa0;°C, 1&#xa0;h) to produce Ag metal of purity 99.1% with an overall recovery of 96.8%. Therefore, the additional leaching/precipitation process for Ag recovery is eliminated. The processing of 100&#xa0;g of keyboard mylar sheets at optimal conditions yields 1.24&#xa0;g of Ag metal and 72.2&#xa0;g of terephthalic acid (TPA), thus mitigating the burden on primary extraction.</p> Graphical Abstract <p></p>

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Recovery of Silver Values from Discarded Keyboard Mylar Sheets

  • 期刊论文

  • 发表日期：

    2025年11月8日

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  • Shaila Mir,
  • Nikhil Dhawan

摘要

The waste mylar sheets from keyboards consist of an organic layer of polyethylene terephthalate (PET) as a base layer onto which a thin layer of conductive paths of silver (Ag) is present. The study focuses on the holistic recycling of the mylar sheets to recover the Ag values and polymeric fraction, which are otherwise discarded or landfilled. Different routes are evaluated, such as direct leaching, pyrolysis, and alkaline hydrolysis (as pre-treatment). The direct leaching of mylar sheets in 2 M HNO3 (80 °C, S/L ratio of 1/20, 2 h) resulted in ~ 98.8% dissolution of the Ag values; however, a large amount of PET pieces (~ 99 wt.%) are left unutilized. Further, the acid-leached PET is partially degraded and subsequently either discarded or downcycled, leading to the loss of resources. The pyrolysis and alkaline hydrolysis methods target the PET degradation, which leads to Ag liberation. The pyrolysis enriches Ag values in the residue (Ag- 6.3%) and reduces the acid volume by 80% for the post-pyrolysis-leaching step. In this study, alkaline hydrolysis is a suitable pre-treatment method for depolymerization and Ag recovery. Hydrolysis converts PET into terephthalic acid and ethylene glycol, while the Ag tracks are retained in the solid form, thus achieving complete resource utilization. The process avoids the gaseous emissions (NOx and CO2) produced during conventional acid leaching and pyrolysis. At an optimal hydrolysis condition (NaOH:feed (g/g)-0.5, 300 °C, L/S of 25, 30 min), 98.7% depolymerization yields Ag flakes of purity ~ 87%. The hydrolyzed residue can be directly melted (1050 °C, 1 h) to produce Ag metal of purity 99.1% with an overall recovery of 96.8%. Therefore, the additional leaching/precipitation process for Ag recovery is eliminated. The processing of 100 g of keyboard mylar sheets at optimal conditions yields 1.24 g of Ag metal and 72.2 g of terephthalic acid (TPA), thus mitigating the burden on primary extraction.

Graphical Abstract