Thread: 'Green' gold extraction method replaces cyanide with starch

20 May 13 by Ian Steadman



Extracting gold from ore is a poisonous business. The most common method is cyanide leaching, where cyanide salts in solution are used to suck the gold out from its ore. You get gold, but you also get highly toxic byproducts.

However, chemistry postgraduate student Zhichang Liu has discovered -- by accident -- a new method of extracting gold, using (of all things) corn starch instead of cyanide. It could eliminate the need for an extremely damaging industrial practice around the world.

Liu was part of a team led by Sir Fraser Stoddart at Northwestern University, Illnois that was looking for something quite specific -- three-dimensional cubes made of gold and starch that could be used to store tiny bubbles of gas and other molecules. In the kind of chemistry experiment you can even try at home, Liu mixed two solutions together at room temperature in a beaker -- a gold-bromide solution, and a solution of sugary starches known as cyclodextrins, which are made up of rings of glucose molecules.

Much to his (and everyone else's) surprise, what was formed wasn't cages, but "needles" of gold. To test exactly what was causing the needles to form, Liu tested six more solutions -- cyclodextrins composed of rings of six (alpha), seven (beta) and eight (gamma) glucose molecules, each dissolved in either an aqueous solution of potassium tetrabromoaurate or potassium tetrachloroaurate.

The key ingredient turned out to be alpha-cyclodextrin, giving the best quality needles -- more than 4,000 of them, each 1.3 nanometres wide. Under an electron microscope it emerged that they were made of alternating rings stacked atop each other. One ring of gold ions in a ring of four bromine atoms, the other ring a potassium ion surrounded by six water ions. The needles grew parallel to each other in a bundle.

"There is a lot of chemistry packed into these nanowires," Stoddart said. "The elegance of the composition of single nanowires was revealed by atomic force microscopy, which throws light on the stacking of the individual donut-shaped alpha-cyclodextrin rings." The needles are easily harvested from the mixture.

It's an exciting discovery because of the implications for gold extraction on an industrial scale. The waste product from cyanidation is a disgusting and toxic gloop that can seep into rivers or poison groundwater, killing wildlife and rendering landscape sterile. Huge pits of ore can be subjected to cyanide leaching in mines, with the runoff a major environmental hazard that requires a significant cost to render safe. Despite this, its scaleability makes it by far the most common method for extracting gold from ore.

By contrast, Liu's method generates nothing more than a mildly alkali metal salt that is easily disposed of. It is also reportedly more efficient than cyanide leaching, an important consideration if the method is to win over industry.

The paper detailing the research has been published in Nature Communications.

'Green' gold extraction method replaces cyanide with starch (Wired UK)

Is the starch readily available and cheaper than cyanide? Then it will work in local non industrial no corporate gold extraction which is where the worst of the pollution occurs.

For industrial applications the next step is to find a sensor that can measure the concentration of the starch residue.