# Who needs cyanide?



## Harold_V (May 15, 2013)

Use cornstarch instead! 

What is not clear is if it will dissolve gold, or that it simply allows for selective precipitation. At any rate, you should find this interesting.

http://www.popsci.com/science/article/2013-05/cornstarch-replaces-cyanide-clean-new-gold-extraction-method

Harold


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## Geo (May 15, 2013)

Thank you Harold, thats a great bit of information. now if one of our more learned members can translate this into crazy hillbilly backyard refiner talk, maybe it can help me out. i tried to follow along as best i could but it is beyond me. im sure one of you guys can try this and share the experience with the rest of us.

http://www.nature.com/ncomms/journal/v4/n5/full/ncomms2891.html


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## butcher (May 15, 2013)

From what I can understand from the article on cornstarch, is that they already had gold dissolved in a solution of bromine (halide), and are able to reduce the gold from solution in a needle like precipitate, and have found Alpha-cyclodextrin a six glucose molecule (sugar) would reduce the gold, they seem to suggest it would reduce the gold selectively with this sugar if the gold is also in a solution with platinum.

At least this is what I can gather using this carbon/sugar to precipitate gold from a halide solution.
This seems to me like a long chain carbon based sugar or carbon with a lot of hydrogen and oxygen in an hydroxide carbon based chain Alpha-Cyclodextrin (C36 H60 O30), I bet that stuff would make a good fuel, what I can gather also is it is sold as some type of dietary fiber for weight loss (fat absorber), I guess it is a good gold absorber also. :lol: 

If I am understanding this right cornstarch has nothing to do with replacing cyanide, they had the gold dissolved in bromine, and as far as a certain type of sugar, or carbon based substance reducing gold from solution this is no surprise to me.

I may have this wrong but would like to learn more.
http://en.wikipedia.org/wiki/Alpha-Cyclodextrin


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## skippy (May 15, 2013)

It still needs the gold to be dissolved first by the bromine, and if you can get bromine to selectively dissolve the gold, who cares what you precipitate it with? I suppose if the bromine could dissolve gold simultaneously while the dextrine was being added... you would maybe get a fine precipitate that could be separated. You would have a reaction of bromine and sugar competing with that of the bromine dissolving the gold though, which might nix that idea. 

Call me cynical, but it seems like its green window dressing hastily thrown on a less than revolutionary discovery. I could certainly be missing something, but if I have it's in no small part because the authors explanation of the environmental benefits were vague.


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## Harold_V (May 15, 2013)

butcher said:


> From what I can understand from the article on cornstarch, is that they already had gold dissolved in a solution of bromine (halide), and are able to reduce the gold from solution in a needle like precipitate,


That's what I concluded, too, yet they talk as if this will eliminate cyanide, which *isn't* used in refining---just extraction. The article was rather misleading in that regard. I expect the author may not fully understand the procedure involved, and has (inadvertently) misrepresented what has been learned.

Harold


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## lazersteve (May 15, 2013)

I agree, the only thing they replaced cyanide with was HBr and nitric acid. The sugar cited is not dissolving the gold.







Steve


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## butcher (May 15, 2013)

From that chart it looks like he is also using SMB to precipitate the gold from the gold bromide solution, and then using the hydro carbon based sugar to collect what he may have missed crystallizing it and returning it into the process reduction process.


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## voeckel (May 19, 2013)

I attached their paper for those who want to dig deep in full details. They lost me very quickly there. :lol: 
I think, Steve's chart seems to be from the same source.

Peter


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## g_axelsson (May 20, 2013)

I agree, I have read the paper now and the only thing I can see this could be used for is precipitating a gold-sugar complex from a dirty gold bromide solution. Kind of how butyl diglyme could be used for extracting gold chloride from a solution but without the easy precipitation and separation.

The authors might be good chemists but too bad they have no idea of how refining is done in real life.

Interesting read though, makes you think... maybe it could be used to extract gold from dirty solutions... Take a real mess. Add α-cyclodextrin and NaBr and the effect should be a precipitate of brown gold bromide sugar crystals. You might have to centrifuge the sample to make it settle. The question is how effective it is at removing gold from a gold bromide solution.

But that's science. Someone discovers an interesting thing, some one else finds out how to use it in real life. The latter one is the one that get rich... :mrgreen: 

Göran


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## butcher (May 21, 2013)

I think I will just stick to cementing with copper, or collecting traces in the stock pot.


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## FPZA (Jul 14, 2013)

thank you for information. we use more than 2000 mtons annually.


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## freechemist (Jul 16, 2013)

In a try to clarify some points, I dare to add some comments.

*@Harold:*The title of the publication cited: *"cornstarch-replaces-cyanide-clean-new-gold-xtraction-method"* is misleading. Cyclodextrin (not simply cornstarch) does not extract gold out of ores. It forms a complex with already dissolved gold(III), in the form of the tetrabromoaurate-anion, AuBr4-, after dissolution of the metal in a HBr/HNO3-mix, followed by neutralization of most acid with KOH. Together with potassium (K+)-cations, surrounded by 6 water-molecules, the cyclodextrin-complexed AuBr4--anions form an only sparingly soluble, crystalline compound.

*@Butcher:* You're right. Cornstarch has nothing to do with replacing cyanide. The gold, in the precipitation-reaction with cyclodextrin is not reduced, it remaines in the +(III)- oxidation state, as AuBr4--anion.
SMB has in fact been used, to reduce the coprecipitated gold-cyclodextrin-complex, like written in the original paper:


> The co-precipitated α·Br complex—namely, recovered gold—was separated from impurities by filtration and then reduced with a reductant, such as Na2S2O5, to afford the recovered gold metal. The residual gold in the filtrate can be recycled while α-CD can be reused by recrystallisation from the filtrate. This laboratory scale process is highly selective for gold, as well as being economic, fast and feasible.



*@Practicability of the process:* The investigated process does not apply, to dissolve gold from ores, and thus is not apt to replace cyanide-leaching. The authors of the original paper themselves choose to dissolve small samples of gold-alloy-scrap (250 mg) in a mixture of conc. HBr and conc.HNO3 (3:1;v/v), similar to treatment with AR, and they didn't investigate really existing ore-samples. Such HBr/HNO3-mixtures may become very dangerous, being much more unstable, than AR, decomposing spontaneously in an unpredictable way, boiling over explosively
under evolution of huge amounts of bromine-vapours (Br2) and nitrogen-oxides (NOx).
In my opinion in gold-recovery/refining this KAuBr4/cyclodextrin-coprecipitation is not only a cost-intensive, but also an unnecessary additional step, compared to direct reduction of gold, dissolved in aqueous HBr.


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## Platdigger (Jul 16, 2013)

I think its only virtue (if it works at all) is, or may be to get the gold dissolved in a huge amount of very dilute liquid, gathered up if you will into a much smaller amount of stuff before being reduced.
If it actually works on ore, I think this may be the thought. More like a coagulant.


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