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Scientists use cheese-making waste to recover gold from electronic waste

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Scientists use cheese-making waste to recover gold from electronic waste

From 20 old computer motherboards, the researchers retrieved a 22-​carat gold nugget weighing 450 milligrams.
March 7, 2024

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In a double whammy for sustainability, researchers have extracted high purity gold from electronic waste using a material that they derive from food waste. And they say the process could be profitable thanks to the low-cost raw materials.

The study, published in the journal Advanced Materials, explores the use of tiny protein fibers derived from whey, a dairy industry side-stream, to recover gold from electronic waste. Compared to previously reported methods, “our approach uses food by-products, so the starting raw materials are extremely inexpensive and sustainable,” says Raffaele Mezzenga, a professor of materials and health sciences and technology department at ETH Zurich.

The world produced about 61.3 million tons of e-waste in 2023, and it is the fastest growing solid waste globally. Electronics in smartphones and computers contain many valuable metals, including gold, which is extremely good at conducting electricity and easy to mold.

 

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Recovering gold from e-waste can be highly profitable. Current methods to do that, however, are inefficient and resource-intensive. “They use extremely harsh chemical and are not environmentally friendly,” says Mezzenga.

He and his colleagues have shown recently that protein nanofibers are good at removing heavy metals from water. They decided to test the material for gold recovery. They treated whey proteins at high temperatures in acidic conditions. The protein nanofibers clumped together to form a gel. The team dried the gel, creating a highly porous sponge made of the protein fibers.

How the gold is recovered: Gold ions adhere to a sponge of protein fibrils. (Source: Peydayesh M et al. Advanced Materials, 2024, adapted)

To test the material, the researchers salvaged electronic motherboards from 20 old computers. After removing the plastic and metal casing, they smashed the motherboards into small pieces and dissolved them in a mix of nitric and hydrochloric acid, which created metal ions.

When the researchers placed the protein fiber sponge in the metal ion solution, the gold ions stuck to the protein fibers much more efficiently than other metal ions. The gold ions grow into flakes on the fibers. Finally, the team heated the gold-covered aerogel at 1,000°C for two hours. The aerogel burned and created ash, and the gold melted into a gold nugget.

In the end, the researchers had a 450-milligram nugget that was 91 percent gold—the rest was copper—which corresponds to 22 carats.

The value of the recovered gold is 50 times higher than the raw materials and energy costs for the process, Mezzenga says. Plus, the methods requires simple machinery such as freeze drying units and fume hoods to correctly evacuate toxic fumes from dissolution of e-waste. “I think the possibility to scale up and make it commercially profitable are very high,” he says. “And indeed, we are working on it.”

Source: Mohammad Peydayesh et al. Gold Recovery from E-Waste by Food-Waste Amyloid Aerogels. Advanced Materials, 2024.

Photograph: ETH Zurich / Alan Kovacevic

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