Philosopher's graphene: waste into gold

Researchers from the University of Manchester might have to adopt a new title: alchemists, as they may have discovered the 21st-century version of the philosopher's stone. But unlike the mystical substance of old, the modern golden transformation is thanks to a microscopically thin material that has been growing in popularity due to its miracle-like properties – graphene.

"Graphene turns rubbish into gold, literally," said Andre Geim, a professor from the University of Manchester, lead author on the published findings, and Nobel laureate responsible for the first isolation of the miracle material graphene in 2004.

Collaborating with Tsinghua University in China and the Chinese Academy of Sciences, the University of Manchester found that graphene can be a kind of philosopher's stone, allowing gold to be extracted from various waste material that contains trace amounts of this precious metal, down to a billionth of a percent.

This new, seemingly alchemistical application of graphene works quite straightforwardly: add graphene into a solution containing waste material, and after a few minutes, pure gold appears on graphene sheets, with no other chemicals or energy input required.

"This apparent magic is essentially a simple electrochemical process," said Yang Su from Tsinghua University, who led the research team. "Unique interactions between graphene and gold ions drive the process and also yield exceptional selectivity."

After this, the pure gold can be gathered by simply burning away the graphene.

"Only gold is extracted with no other ions or salts," added Su.

The findings, published in "Nature Communications," shows that one gram of graphene can be sufficient to extract nearly two grams of gold.

Urban gold mining

As graphene costs have significantly reduced in the years since its synthesis, purportedly as low as less than 10 cents per gram, the two-to-one graphene to gold recovery ratio can immediately become an extremely profitable venture, considering gold sells for nearly $60 per gram at current prices.

When you consider that electronic waste is the world's fastest-growing solid waste and that less than 20% of e-waste is currently recycled – primarily due to a lack of technologies with sufficient efficiency and economic viability to recover valuable elements from it – the potential for a pure gold extraction method alone suddenly makes urban mining a much more prospective endeavor.

"Not only are our findings promising for making this part of the economy more sustainable, but they also emphasise how different atomically-thin materials can be from their parents, well-known bulk materials," Geim added. "Graphite, for example, is worthless for extracting gold, while graphene almost makes the philosopher's stone."

Aside from its obvious value as a precious metal, gold is used in many industries, including consumer electronics. From mobile phones, laptops, and televisions to advanced medical devices, sensors, and research tools, gold is known for its exceptional conductivity, among other things, and a little goes a very long way.

Yet when the products are of no more use, they are easily discarded, leaving behind a small treasure trove that, when accumulated in the vast landfills of the United States, becomes a large hoard.

This graphene-based process, with its high extraction capacity and high selectivity, can reclaim close to 100% of the gold from electronic waste, offering an enticing solution for addressing the gold sustainability challenge and e-waste issues.

"With the continuing search for revolutionary applications of graphene, our discovery that the material can be used to recycle gold from electronic waste brings additional excitement to the research community and developing graphene industries," said Hui-ming Cheng, one of the main authors from the Chinese Academy of Sciences.

Transforming ore to gold?

While the University of Manchester researchers are excited about recycling gold from e-waste, carbon materials have been widely used in the mining industry to recover gold from solution for several decades now.

Leaching is a method widely used in extractive metallurgy, where ore is treated with chemicals to convert the valuable metals within into soluble salts while the impurity remains insoluble. These can then be washed out and processed to give pure metal, and the remnants left behind are known as tailings.

You can read an in-depth explanation on mining tailings at Mining mine waste for technology metals in the May 6, 2020, edition of Metal Tech News.

As the centuries of mining have reduced quantities of higher-grade metal, technologies to extract value from these low-grade ores have been developed to offset the costs of mining up less valuable rock.

While early technologies have used variations of cyanide, ammonia, alkali, or acids, the drawbacks of leaching include lower efficiency and often significant quantities of waste effluent and tailings produced, which are usually either highly acidic or alkali, as well as toxic.

Despite these factors, carbon-in-leach and carbon-in-pulp methods that capture gold dissolved into solution on carbon materials are widely used to recover this precious metal at mines around the world.

However, efforts to improve sustainable practices in leaching have been growing in recent years, with technologies using cleaner chemistries to extract metals to compensate for the potential costs that would harm the environment. Companies, academia, and governments alike have been seeking ways to improve yield without foregoing efficiency, all the while taking into account the consumption of energy and production of toxic byproducts.

Coupled with new gold leaching technologies, it is not a stretch to envision graphene used as a philosopher's stone that more efficiently and sustainably transforms ore into bars of gold.