Gold cleverly extracted from electronic waste

Small gold nuggets – obtained from old computer circuit boards. © ETH Zurich / Alan Kovacevic

Until now, the treasures of waste have been difficult to recover. But now researchers have developed an astonishingly effective and sustainable process for recovering gold from used electronics: using a protein sponge made from a by-product of cheese production, dissolved gold particles can be bound from the material and then transformed into gold nuggets. The developers say that the sustainable process is extremely profitable due to the considerable yield with little effort.

The rare precious metal is particularly known for its importance as a decorative element and as an investment - but gold is also very popular as a raw material in industry: its special properties mean it is used for numerous purposes. Above all, its special electrical conductivity makes gold an important element in microelectronics. Significant amounts of gold are used to produce smartphones, computers, etc. Once these devices have served their purpose, they and their valuable contents end up in electronic waste.

In view of the increasing demand, methods are currently being worked on to recover the gold from this waste. However, previous concepts are energy-intensive and often require problematic chemicals. But now researchers led by Mohammad Peydayesh from the Swiss Federal Institute of Technology in Zurich (ETH) are presenting a system that scores points in terms of profitability and sustainability. As the team explains, their process is based on binding gold through a so-called airgel. It is a type of sponge whose structures are formed from protein nanofibers.

A gold sponge made from whey protein

As a raw material for the production of their airgel, the scientists used a by-product of cheese production: whey. They hoped that the proteins contained therein would have a specific binding ability to dissolved metals. To convert the material into a usable form, the team subjected the whey proteins to heat and acid treatment. This initially created a gel that was formed by cross-linked structures made of protein nanofibers. The researchers then dried this substance, creating a sponge tissue from the whey proteins that is suitable for use in substance solutions.

For their purpose, the researchers then dissolved electronic circuit boards from 20 old computers in special acids. As a result, all metals contained were then present in the liquid in the form of ions. The whey protein aerogels were then inserted into this substance. As the researchers report, the soaked sponge material actually developed a golden shimmer.

Airgel loaded with gold ions. © Peydayesh M et al. Advanced Materials 2024

The analyzes showed that the whey airgel has a surprisingly high and selective gold adsorption capacity. Although other metal ions – especially copper – also accumulate, the precious metal is bound particularly intensively, the team reports.

Lucrative yield

In the next step, the scientists heated the sponge material to “harvest” the precious metal. As they explain, the metal ions crystallized into flakes that could be extracted. So they were finally able to melt the gold yield into a lump. As it turned out, the team used their process to obtain a nugget weighing around 450 milligrams from the 20 computer circuit boards, which consists of 91 percent gold and nine percent copper, which corresponds to almost 22 carats of gold.

Schematic representation of gold recovery. © Peydayesh M et al. Advanced Materials 2024, adapted

As the developers emphasize, the yield appears to be extremely interesting from an economic point of view given the low effort involved: In their study, they calculate that the bottom line is that the costs for producing the airgel, the energy requirements and the entire process technology are around 50 times lower than that Value of gold yield.

That's why the team is now working on scaling the technology to further develop it to market maturity. According to the researchers, it could also be worthwhile to mine for gold in other sources using the method. Some industrial waste also contains the precious metal in potentially lucrative quantities. “I particularly like the fact that we use a by-product of the food industry for our process,” concludes senior author Raffaele Mezzenga from ETH. In a sense, with this promising technology, two waste materials are always sustainably refined into gold.

Source: Swiss Federal Institute of Technology Zurich (ETH Zurich), specialist article: Advanced Materials, doi: 10.1002/adma.202310642

Recent Articles

Related Stories

Stay on op - Ge the daily news in your inbox