Enrico Boschi (ETH Zurich), Raffaele Mezzenga (ETH Zurich), Mohammad Peydayesh (ETH Zurich)
Gold, the symbol of wealth and power, is now also found in our electronic devices – but mining it harms the environment. But now a surprising solution has arisen from cheese production. Researchers at ETH Zurich are using whey to extract gold from electronic waste. This is an environmentally friendly approach that turns two waste products into precious metal.
Picture: Raffaele Mezzenga (ETH Zürich)
Gold has always held special appeal throughout human history. Power and wealth were – and still are – displayed with gold. Today, gold is found not only in gold bars, coins and jewellery, but increasingly in electronic devices too.
But gold’s popularity has its downsides. Gold mining has significant environmental impacts. Indeed, to extract one tonne of gold, 100,000 tonnes of ore must be moved. Gold is extracted from rock mechanically using explosives or chemically using cyanide solutions, which poses health risks and causes environmental damage around the mines.
There is constant demand for new gold to satisfy the demand of our consumer society. Currently, 7 percent of available gold is bound in electronic waste, of which only 20 percent is recycled globally. The gold concentration in electronic waste exceeds that of gold mines by a hundredfold. So recycling from electronic waste is undoubtedly the order of the day. But how can it be recycled? While melting it down is a simple, efficient and environmentally friendly solution for recycling gold from jewellery or coins, the prerequisites for recycling gold from electronic waste are more challenging. Today, it is usually extracted from waste in numerous steps using energy-intensive processes and toxic chemicals – with negative impacts on the environment and high costs.
Metallic impurities in solution bind to the proteins in whey through weak molecular interactions and can thus be separated. This applies to all metals with a large molecular weight, including gold. Raffaele Mezzenga, Professor in the Department of Health Sciences and Technology at ETH Zurich, saw potential in this: “We wanted to exploit this affinity for gold and whey proteins to bind gold from electronic waste.”
But what form does the whey need to be in for this to happen? As part of his Bachelor’s thesis, Enrico Boschi established a process that turns whey into sponges. The proteins contained in whey are structurally destroyed at a low pH and high temperature, precipitated and then freeze-dried. This creates an aerogel, a highly porous solid. And then the miracle can begin: binding gold to the protein sponge.
Gold can only be adsorbed onto the sponges if the metal-containing components of the electronic waste are first isolated from the device, mechanically destroyed and ionised with standard acids. The solution created in this way contains 1 part per million gold, 1,000 parts per million copper, 1,000 parts per million iron and other metals in low concentrations. When an aerogel is incubated in the metal ion solution for an extended period of time – at least one day – the sponge contains 90% gold and 10% copper. Gold’s affinity for whey proteins is so high that it occupies almost all binding sites. This surprised even Raffaele Mezzenga: “Under certain conditions – in the form of an aerogel and with long incubation times – whey proteins selectively bind gold with a very high affinity and displace other metals. The aerogel can bind 20 percent of its own weight in gold.”
Once the gold is bound to the aerogel, there is no need for alchemy to obtain solid gold. The sponge is simply incinerated at temperatures above 1,000 degrees Celsius and the resulting gold nuggets can be collected. Raffaele Mezzenga described this fascinating process: “Elemental gold rises like a phoenix from the ashes.” Two waste materials – electronic waste and whey – are used to produce gold with 91 percent purity, which corresponds to 22 carats. Even when all the process costs are taken into account, this results in value creation by a factor of 50.
Mr Mezzenga’s group tested the process with motherboards from computers. A gold nugget weighing 450 milligrams was created from 20 of these circuit boards. Meanwhile, the researchers have shown that gold can also be successfully extracted from mobile phones and microchip manufacturing waste. The next step is to scale up the technology so that it can also be used outside of the ideal conditions of a laboratory in the coming months.
But the vision is even bigger. Raffaele Mezzenga hopes that aerogels from other waste streams in the food industry can be used to selectively bind and separate other metals or even hormones. Or, as he put it: “Let’s see what nature has in store for us. We might be in for a surprise!”
