Enlarge /. The polymer, known as COP-180, selectively traps gold after being leached out of electronic waste.
One thing that hinders the recycling of electronic waste is the actual recycling process. We need cheaper, safer, cleaner, or more effective ways to separate and recover the valuable elements from electronics before we can make the whole company more attractive and profitable. Some current methods use large amounts of energy to melt down components, but chemistry could offer some tempting alternatives.
A new study, led by Yeongran Hong from the Korea Advanced Institute of Science and Technology, looks at a chemical with an impressive affinity for gold. Expose some circuit boards to acid treatment to release their materials, and this material collects all of the dissolved gold. And after it releases the gold, it can be used again.
The researchers' gold washer is based on an organic compound called porphyrin. Linked together in a polymer, it has many, many small pores that want to energetically take up a metal atom. Chemists are looking for this type of structure to help with recycling.
The researchers have subjected their polymer to various tests to find out which metals it works best on and how much it can trap. It is most effective with a small number of precious metals, especially gold. Compared to the number of pores in the polymer, they found that it captured about ten times as many gold atoms. With other elements like platinum, each pore contains only one atom (responsible atomic social distance, let's say). But gold atoms seemed to be having a party at every pore.
Enlarge /. These are the elements that play well with the polymer. But because its affinity for gold is highest, it tends to fill with it first.
This behavior was verified by measurements and explained by some modeling. The researchers found that the polymer would interact with the gold atom – assisted by ultraviolet light – and give it some electrons, making it possible for more gold atoms to bond into a lump. Sure enough, repeating the test with different amounts of ultraviolet light had an impact, even though the picture was still quite high without it.
Finally, the polymer was subjected to a fairly authentic test. The researchers took seven printed circuit boards from a scrap yard and placed them in an acid bath to remove the metals. Then they mixed in their polymer, adjusted the solution and stirred it for a few days. (Although other tests showed that 99 percent of the gold can be flushed in about 30 minutes.) Filtering separated the polymer and its gold transport. By adding acid again, the polymer releases the gold, which turned out to be a solid nugget that made up 94 percent of the gold leached from the circuit boards.
Given the results, it seems easy to find the economic reason for this technique. The researchers say that producing the polymer costs around $ 5 per gram, and that this gram can trap $ 64 gold. And since the polymer can be reused, it would become significantly cheaper over time, which adds little to the overall cost of a recycling process.
"Although (printed circuit boards) contain more precious metals than mine ores," the team writes, "80 percent of this waste is still landfilled, mainly because of the lack of selective, high-yield, non-cyanide recovery processes." As it turns out that similar processes can harvest other elements more easily, all this waste will increasingly look like an economic opportunity – and help close the loop by turning old equipment into new one instead of garbage.
PNAS, 2020. DOI: 10.1073 / pnas.2000606117 (About DOIs).