Sugar solution to toxic gold recovery

The specific self-assembly between α-cyclodextrin and KAuBr4 leads to the precipitation of nanowires © Dennis CaoUS researchers have discovered a way to selectively isolate and recover gold from raw materials, including alloys, using a simple sugar derived from corn starch. The work could offer a greener and cheaper alternative to conventional processes, which use cyanide and often result in environmental contamination.

Gold is typically recovered from mined ore and waste materials, including electronic waste, using highly poisonous cyanide to convert gold into a water-soluble coordination complex through a process known as leaching. But while effective, this process poses a risk to workers and the environment.

Fraser Stoddart and colleagues at Northwestern University in Evanston, Illinois, have done away with toxic reagents and instead use α-cyclodextrin – a polysaccharide found in corn starch. When mixed with a gold bromide salt in water, a precipitate quickly forms in solution. A reductant, such as sodium metabisulfite, can then be used in a final step to get the gold in its pure form.

The discovery happened by chance when Zhichang Liu in Stoddart's lab mixed a solution of α-cyclodextrin with a dissolved gold salt. He was trying to make an extended, three-dimensional cubic structure suitable for storing gases and small molecules. However, needle structures rapidly formed instead.

X-ray crystallography revealed that the needles are composed of around 4000 nanowires bundled together thanks to supramolecular interactions. After further experiments with α-, β- and γ-cyclodextrins and two different gold salts, the team found α-cyclodextrin and potassium tetrabromoaurate (KAuBr₄) to be the best combination for the rapid formation of the needles.

'We are fairly confident that this is a big leap forward in terms of being very different from any of the much more toxic methods that rely on cyanide to isolate gold,' says Stoddart. 'We don't know of anything that is close to approaching this in terms of being environmentally benign and we think it could easily be increased in its scale.'

'Any way to reduce the reliance on cyanide in gold extraction is certainly going to be very attractive for both environmental and societal reasons,' says John Provis, a chemical engineer at the University of Sheffield, UK. 'The most appealing points are the ability to recover the gold with a fairly simple separation step, and its high selectivity for gold over platinum and palladium – these are both common challenges in halide-based gold processes.'

However, John Monhemius at Imperial College, London, UK, thinks any practical applications will be limited to the treatment of recycled scrap containing gold or in the refining of gold doré, an alloy of gold and silver, produced at gold mines. 'It is most unlikely that this new chemistry will have any impact on the long-established use of cyanide at gold mines worldwide for the recovery of gold from primary ores,' he says. 'It is a new precipitation method, not a new dissolution method and therefore is not applicable to leaching gold ores.'

References

Bundles of nano wires ~

Much goldmining involves thinking about 'leaching gold out of ore'. However, there are vast amounts of gold available where the gold is not bound up with silicates ~ for instance, ancient iron/nickel meteorites eventually rust away and leave behind a 2% gold lode. It will eventually (millions of years) dissolve in water and be spread across a broad area. Given a large enough meteorite thousands of acres could well be contaminated.

Eventually the asteroid belt will be mined and folks will have all the gold they can use but here is a chance to get access to the same thing.

The first step is to bind up the gold salts with a corn sugar. Then, flush that into a smaller area, e.g. a pond, where you can then use a food preservative to precipitate out the gold nano wires.

Alternatively, find a cornfield in an area where gold salts, albeit in small quantity, have been found. Build a small barrier to form a catchment basin at the lowest point on the field. Corn sugars will have been shedding into the soil for a couple of centuries in most cases ~ maybe thousands of years in other cases.

Every now and then toss some food preservative in the catchment basin ~ stir it up nicely, and come back next week to start up your portable tertiary gold retrieval device.

This beats the dickens out of the termite trick, but even there you could plough up a termite nest, stir up the mud and old termite remains with an appropriate corn sugar, then precipitate out the gold nano particles with food preservative.

Remember there's at least as much gold lying about in the top soil as has ever been found in all of history. TIME TO FARM!

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