Gold, with or without cyanide

Some things are unavoidably toxic, and some things were unavoidably toxic until a new, less toxic process was discovered. Less toxic is always a good thing. Sometimes it's less expensive in terms of direct costs such as how much the reagents cost, sometimes less expensive in terms of indirect costs, such as safety precautions and environmental protection.

Sodium hydroxide is one such; the old industrial method of making it involved mercury, which is highly toxic. The new industrial method doesn't. (There are still toxic chemicals involved, but they're not mercury.)

One thing that will hopefully one day be added to the past-tense version of unavoidably toxic is gold mining. Currently, if gold can't be panned from a streambed (placer mining) where it's present as pieces of fairly pure gold, it has to be dissolved out of the rocks, often using cyanide. A newly discovered process is being described as possibly displacing the cyanide.

The thing about gold, apart from being really expensive, is that it doesn't react with much. It doesn't rust, it's very stable, and all that also means it's really hard to dissolve so it can be extracted from the ores it's typically found in.

Cyanide is one of a few things that will dissolve gold. Aqua regia, a mixture of hydrochloric and nitric acid, is another. Neither of those are particularly safe!

Cyanide has a well-deserved frightening reputation, though I learned with great interest that basic sodium cyanide solution, such as is used in gold mining, doesn't produce much of the deadly gas HCN and in fact it's safe enough to stand next to a large open pond of the stuff—although I wouldn't do that without wearing a toxic gas detector badge, just in case.

Aqua regia is also quite dangerous, and produces some vile and toxic gases. Choose your poison, I guess.

In both cases, the dissolved gold then has to be treated to re-precipitate it in the form of a more pure gold. In the aqua regia method, the nitric acid has to be boiled off (toxic fumes!) before the gold can be precipitated. In the cyanide method, hot cyanide and hydrochloric acid are involved.

Ok, back to the news item that started this whole post. The researchers have a new process that makes gold without using cyanide and, they say, is more environmentally benign than the cyanide method.

Image from Zhichang Liu, Marco Frasconi, Juying Lei, Zachary J. Brown, Zhixue Zhu, Dennis Cao; Selective isolation of gold facilitated by second-sphere coordination with α-cyclodextrin; Nature Communications, May 14, 2013. Used with permission (CC-by-nc).

I note that instead of aqua regia (HCl and HNO3) they are using a mixture of HBr and HNO3 to dissolve the gold. As bromide and chloride are both halogens and are known to react in similar, but not identical, ways for that reason, this makes sense. Instead of a soluble gold chloride, they'll be making a soluble gold bromide.

HBr isn't exactly pleasant stuff, but the next few steps make a difference in process danger. Rather than boiling off the nitric acid, the solution is neutralized with KOH. Again, not harmless, but it doesn't boil off hot acid gases, so an improvement.

The part that's making all the news articles dance around saying it's a non-toxic process is the step after that, where a harmless starch called α-cyclodextrin is used to very selectively grab the gold bromide out of solution and leave behind all the other metal bromides. After that, simple filtration separates the gold from the other metals.

Image from Zhichang Liu, Marco Frasconi, Juying Lei, Zachary J. Brown, Zhixue Zhu, Dennis Cao; Selective isolation of gold facilitated by second-sphere coordination with α-cyclodextrin; Nature Communications, May 14, 2013. Used with permission (CC-by-nc).

There are still more steps needed, however; the solids here are not pure gold. The little gold bromide/cyclodextrin needles had to be reacted with a reducing agent to convert the soluble gold back to solid gold. They used sodium metabisulphite (Na2S2O5), which is not particularly nice stuff in high concentrations (as it would be used in an industrial situation). For one thing, it releases toxic SO2 gas on contact with acid. The process calls for neutralizing the aqua regia gold solution prior to adding it, but only to pH 4-6, which is still acidic, and it's still intended to react and reduce the gold to solid form. Other reducing agents with lower toxicity might be substituted here; the sodium metabisulphite doesn't end up in the final product in any way, it's there to donate some electrons to the gold to change it from soluble to solid form.

More filtration separates the fairly pure gold from the solution, and the α-cyclodextrin can be recycled by recrystallization and re-used. If a reducing agent that doesn't produce a gas when it's reacted is used, it would have to be separated from the α-cyclodextrin before that could be recrystallized.

So as usual, it looks like the media reports only got a tiny fraction of the reality, not helped by the press release's focus on only the α-cyclodextrin. It's a cool discovery, especially with how selective it is for gold, but it's not a non-toxic process. A well-run cyanide plant recycles its cyanide and thoroughly destroys what it can't recycle. There is always the risk of spills and leaks (as well as poor management), and that's why I'm in favour of replacing dangerous chemicals with less dangerous ones, even when there's a good recycling and management system available.

This new process would also have a few effluent streams to manage and detoxify. Those effluent streams (marked in red, along with the reagent inputs) were not described in detail in the paper, so the flowsheet above contains only a vague descripton of each. They'd need to be properly characterized and a good treatment plan worked out before this could even think of becoming a contender for replacing the cyanide method.

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