A combination technique comprising hydrometallurgy and electrochemical deposition developed by researchers at the University of Camerino in Italy has boosted the recovery rate of silver from spent solar cells to 98.7 percent. Compared to conventional approaches, this approach is also environmentally friendly.
Commercially deployed solar cells use significant amounts of important metals such as aluminum, steel, copper, zinc, and silver. While aluminum and steel are usually used for racks and support systems and are easy to recover, copper and silver are much more difficult to recover as part of solar cells’ electrical circuits.
Need for recovering metals
With a greater demand shift for greener energy sources, large-scale solar power facilities are being installed. Since solar panels typically last up to three decades, scientists are also worried about the large-scale demand for their disposal once they reach the end of their lives.
Not only is metal recovery crucial due to its limited availability, but it can also help limit the search for more metals from the Earth. Conventional approaches for metal mining involve open-pit mining, which requires vast and energy-intensive operations. Moreover, they harm the environment and ecosystems of the region.
Since metals can be separated, molten, and reused with much less energy than in mining operations, it is much more economical to recover them from products without utility.
Separating copper and silver
Metals can typically be leached from a product, but since the reduction potential values of copper and silver are close, the leaching process is difficult to execute. To overcome this challenge, researchers proposed using a base-activated persulfate with ammonia, where the persulfate acts as an oxidizing agent. The reaction produces copper oxide, which acts as a protective layer and prevents its own leaching.
Images A and B show silver particles in silicon solar cell waste before leaching, while images c and D show silver particles in silicon solar cell waste after leaching. Image credit: University of Camerino.
The researchers tested the process with multiple variables, such as the concentration of ammonia in the solution, the amount of solar cell waste used, the concentration of persulfate used, and the total reaction time. Conditions such as temperature and a stirring speed of 300 rpm remained unchanged in the experiments.
The researchers found that ammonia concentrations of 0.5 M and 0.2 mol per liter of persulfate, along with a reaction time of 60 minutes, worked best and separated silver with 85 percent efficiency. However, the team was keen on further improving the efficiency and turned to using the electrochemical process.
The team decided to use the electrodeposition-redox replacement (EDRR) approach since it has previously been reported to successfully separate a highly pure form of silver from a leachate that contains copper ions.
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Another advantage of the approach was that it could work directly with the leachate and did not require adding other chemicals. The EDRR technique is highly selective of silver and recovers precious metals with a high efficiency of 98.7 percent, making it highly favorable over conventional processes.
Ameya Paleja Ameya is a science writer based in Hyderabad, India. A Molecular Biologist at heart, he traded the micropipette to write about science during the pandemic and does not want to go back. He likes to write about genetics, microbes, technology, and public policy.
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