RESEARCHERS in the UK and the US have discovered that a simple, non-toxic amide can extract gold selectively from electronic waste.
Though found in only very small quantities, the concentration of gold in electronic waste, such as computers and mobile phones, is 80 times higher than that found in natural gold ore. In fact, electronic waste is thought to contain as much as 7% of the world’s gold, and up to 300 t of gold is used to produce new electronics every year. Much of the waste is simply landfilled. The recycling processes which do exist generally use highly toxic cyanide compounds. Jason Love, a chemistry professor at the University of Edinburgh, UK, led a team to find a non-toxic, more efficient alternative.
Love and the team used a simple primary amide, dubbed ‘L’, which they prepared themselves with commercially-available reagents.
The team dissolved the metal from waste circuit boards using a hydrochloric leaching solution. A 4M solution was found to be the most effective. They then added the amide to toluene solvent and mixed it with the leaching solution. Using spectroscopy, the researchers found that the positively charged L amide ligand selectively binds to the negatively charged AuCl4 ions in the leaching solution, forming a highly stable complex. They say that such fundamental chemical understanding of the interactions will be extremely useful in developing metal recovery processes.
To extract the gold, bound to the amide in the organic phase, the researchers need only to add water to it. Around 88% of the gold in the organic phase transfers into the aqueous phase.
In tests, they found that even in highly mixed leaching solutions, with equivalent metal concentrations to that which would come from mobile phone waste, the L amide was still very selective and could extract 82% of the gold present, removing just 6.4% of iron present and 2.7% of the tin, with none of the copper, despite there being far more copper than any other metal. This is a far better performance than one commercially available extraction solution, DBC, which extracted 86% of the gold, but also 72% of the tin, 74% of the iron, 6.8% of the zinc and 0.8% of the copper.
“We are very excited about this discovery, especially as we have shown that our fundamental chemical studies on the recovery of valuable metals from electronic waste could have potential economic and societal benefits,” said Love.
Angewandte Chemie DOI: 10.1002/anie.201606113
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