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Hydrothermal leaching of silver from photovoltaic panels

Athanasiadou Rafaela

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Summary

(In the context of sustainable development, there has been a rapid growth of photovoltaic (PV) technologies, highlighting the importance of developing efficient methods for the recovery, treatment, and recycling of waste generated at the end of their life. Particular interest lies in the recovery of valuable metals, such as silver (Ag), which are embedded within the structure of PV panels and can be reused in the manufacture of new electrical and electronic equipment. This is, precisely, the aim of the present thesis – the recovery of silver from end-of-life monocrystalline silicon (m-Si) photovoltaic panels. To achieve this objective, silver leaching from waste monocrystalline silicon (m-Si) photovoltaic panels was studied using hydrothermal treatment. Specifically, this was performed using citric acid as a leaching agent. The experimental parameters studied included citric acid concentration, hydrothermal treatment time and temperature. Prior to the leaching process, two pre-treatment procedures were applied. Initially, the back insulating plastic backsheet, typically white and made of Tedlar, was manually removed from the panel pieces. These samples were then subjected to high-temperature thermal treatment to thermally decompose the EVA polymer. In the second pre-treatment method, the panel pieces were submerged in toluene to remove EVA and the backsheet, and to separate the cells, glass, and ribbons. Following pre-treatment, the pieces underwent leaching with 4 M HCl to remove aluminum from the cells. Subsequently, hydrothermal leaching with citric acid was applied to recover silver. During the experimental procedure, various hydrothermal parameters were studied, including acid concentration (1–1,5–2 M), processing time (60–105–150 min), and temperature (150–180–210°C), while the liquid-to-solid ratio (L/S) was kept fixed at 30 ml:1g, based on preliminary tests. To optimize the hydrothermal leaching parameters, the Response Surface Methodology (RSM) was used. Based on the results of this study, it was found that excessively long treatment times (more than 150 min), as well as very short times (less than 60 min), did not improve the leaching efficiency. Furthermore, no significant interactions were observed between concentration and temperature, or between time and temperature, while only mild interactions were noted between concentration and time. As a final step, for comparative purposes, leaching was also conducted using nitric acid (HNO₃, 65%). The strong inorganic acid proved significantly more effective in recovering silver, with the efficiency of citric acid (C₆H₈O₇) being less than 5%, compared to 65% for HNO₃. In conclusion, the optimal conditions for hydrothermal leaching with citric acid were determined to be 210°C, 95 min and 2M citric acid.

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