Greener Gold: μεταλλουργική μέθοδος εξαγωγής χρυσού χωρίς κυάνιο ανακαλύφθηκε τυχαία!

Gold mining is using chemicals such as cyanide and mercury to separate the gold from the ore and other metals associated with it. These pollute the environment via mine tailings, and concentrate up the food chain. This dangerous process is currently used by most mining companies.

A chemistry “postdoc” made a lucky discovery that may well reduce this pollution when he found that a particular environmentally benign and cheap cornstarch derivative could replace the cyanide. The process was discovered in the test tube by accident, while trying to grow crystals of a particular cubic shape. He was initially disappointed when golden needles grew rapidly when he mixed the cornstarch with a gold solution. These turned out to be bundles of starch nanowires that had separated the gold out of solution and concentrated it. Follow up research revealed that the process was financially competitive and more efficient than current processes.

The process can be used to extract and separate the gold from other metals in a wide diversity of raw sources, from low grade ore to scrap electronic waste, another source of pollution in the modern world, in which cyanide is now the main reagent. The salts it leaves behind are relatively benign. The research was published recently in Nature Communications.

Given that the world’s appetite for gold is not going to decrease, anything that cleans up its extraction process or enables easier recycling, can only be a good thing. The fact that the ingredients and reactions are simple makes them cheap, and the team have already scaled up the process far enough to extract gold from electronic waste. The fact that it was discovered by pure luck illustrates well Asimov’s contention that good fundamental science starts with the words ‘that’s funny’, and that careful thought and observation is needed to spot these moments, followed by plenty of perspiration in order to understand and make use of the discovery.

Abstract

Gold recovery using environmentally benign chemistry is imperative from an environmental perspective. Here we report the spontaneous assembly of a one-dimensional supramolecular complex with an extended {[K(OH2)6][AuBr4] (α-cyclodextrin)2}n chain superstructure formed during the rapid co-precipitation of α-cyclodextrin and KAuBr4 in water. This phase change is selective for this gold salt, even in the presence of other square-planar palladium and platinum complexes. From single-crystal X-ray analyses of six inclusion complexes between α-, β- and γ-cyclodextrins with KAuBr4 and KAuCl4, we hypothesize that a perfect match in molecular recognition between α-cyclodextrin and [AuBr4]− leads to a near-axial orientation of the ion with respect to the α-cyclodextrin channel, which facilitates a highly specific second-sphere coordination involving [AuBr4]− and [K(OH2)6]+ and drives the co-precipitation of the 1:2 adduct. This discovery heralds a green host–guest procedure for gold recovery from gold-bearing raw materials making use of α-cyclodextrin—an inexpensive and environmentally benign carbohydrate.

Full Paper:
Selective isolation of gold facilitated by second-sphere coordination with α-cyclodextrin

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