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Author: Subject: Extraction of HAuCl4 With Ether
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[*] posted on 17-1-2013 at 18:13
Extraction of HAuCl4 With Ether


I'm looking to extract some HAuCl4 from a solution of aqua regia with diethyl ether. Total volume is 50ml of aqua regia and 50ml of ether.

Not blowing myself to bits is important and I have a few questions regarding safety and handling.

- Impact: Allegedly ether can be dangerous if subjected to mechanical impact. This seems to be a problem since extracting the gold chloride requires shaking it with the ether, which seems to fall under that risk? So could it combust simply from being shaken too hard?

- Oxidizers and Halogens: Apparently ether reacts with oxidizers and halogens. Aqua regia has nitric acid and chlorine. Is the aqua regia solution supposed to be set out to decay for a while before the ether is mixed in?

I've seen this extraction process described in a few places, and none of them seem to address these apparent contradictions. They all just say to dissolve the gold in aqua regia and then once it's dissolved, shake it with ether to extract the chloride.
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[*] posted on 17-1-2013 at 18:21


Ether is not an explosive. it's HIGHLY flammable though. so watch out for open flames!

The nitric acid and chlorine will be gone if the gold solution is boiled with excess hydrochloric acid to react all the nitric acid away and drive the chlorine out. the leftofver HCl will not doing anything dangerous with the ether.
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[*] posted on 18-1-2013 at 06:35


Not only open flames, but any hot surfaces too! Its autoignition temperature is only 160 C, so in many cases it can ignite simply from the heat of a nearby hot plate.
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[*] posted on 18-1-2013 at 06:52


Impact sensitivity can occur in the presence of organic peroxides, which ether naturally forms over time. If you suspect their presence, test your ether. If it comes back positive, don't use it -- dispose of it as safely as possible. Search for ether precautions, much has been written on the subject -- stabilization, testing, treatment and disposal (the proper authorities, chemicals, etc.).

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[*] posted on 18-1-2013 at 07:02


The problem with ether (beyond its extreme flammability and volatility) is that if it is stored for prolonged periods of time exposed to air/oxygen (eg if there is a singificant volume of air in the bottle over the ether), then it forms organic peroxides. And these -are- impact/friction sensitive, and that is probably which led you to believe (erroneously, fortunately) that ether itself is a sensitive explosive.

If you use fresh ether and don't store it for long periods of time, don't worry too much about it. There have been previous threads on this, the search engine should find them.

Ether can react violently with halogens, but this means the free, elemental halogens: Fluorine, chlorine, bromine, Iodine (and astatine :) ).
When hydrochloric acid dissociates, you get a chloride ion. This is a very different animal and won't react with ether at all.




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[*] posted on 18-1-2013 at 07:29


Quote: Originally posted by MrHomeScientist  
Not only open flames, but any hot surfaces too! Its autoignition temperature is only 160 C, so in many cases it can ignite simply from the heat of a nearby hot plate.


This is bullshit, it is far higher than 160°C.




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[*] posted on 18-1-2013 at 08:20


I assumed we were talking about diethyl ether, and that's the number I found for it. I'd always heard of its extreme flammability, but have never used it myself. Looking now at Sigma's MSDS for it, it has a listed "ignition temperature" of 180 C with a note next to that of "auto-flammability" (but also says 'no data available' for autoignition temperature). Fisher Scientific lists autoignition at 160 C. Where did you find that it is higher than this?
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[*] posted on 18-1-2013 at 08:31


Quote: Originally posted by MrHomeScientist  
I assumed we were talking about diethyl ether, and that's the number I found for it. I'd always heard of its extreme flammability, but have never used it myself. Looking now at Sigma's MSDS for it, it has a listed "ignition temperature" of 180 C with a note next to that of "auto-flammability" (but also says 'no data available' for autoignition temperature). Fisher Scientific lists autoignition at 160 C. Where did you find that it is higher than this?


I did an experiment - I put almost orange-hot wire into DEE, it just boiled. So it must be higher than that.




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[*] posted on 18-1-2013 at 09:06


Thanks for the help all. Apparently the diethyl ether needs to be added very slowly to the aqua regia because the reaction with nitric is slightly exothermic.
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[*] posted on 18-1-2013 at 09:18


There must not be any nitric acid present when adding diethyl ether. Please do not make the typical beginner's mistake of not evaporating your gold solution three times with additional HCl to get rid of the nitric acid.
Solvent extraction is also not a recommended method of refining gold. You're better off using the usual precipitation with sulfur dioxide or sodium metabisulfite, followed by filtering and washing the gold precipitate, redissolving and precipitating a second time with ferrous sulfate solution.
In all cases, be it extraction or precipitation, the nitric acid must be removed before adding the extractant or precipitant. A large amount of your gold will be left in the aqueous solution if you do not do this.




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[*] posted on 18-1-2013 at 09:22


Quote: Originally posted by Adas  
Quote: Originally posted by MrHomeScientist  
I assumed we were talking about diethyl ether, and that's the number I found for it. I'd always heard of its extreme flammability, but have never used it myself. Looking now at Sigma's MSDS for it, it has a listed "ignition temperature" of 180 C with a note next to that of "auto-flammability" (but also says 'no data available' for autoignition temperature). Fisher Scientific lists autoignition at 160 C. Where did you find that it is higher than this?


I did an experiment - I put almost orange-hot wire into DEE, it just boiled. So it must be higher than that.


The question isn't how high a temperature sometimes doesn't light it, but how low a temperature sometimes does.
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[*] posted on 18-1-2013 at 11:50


Quote: Originally posted by Adas  
I did an experiment - I put almost orange-hot wire into DEE, it just boiled. So it must be higher than that.
Auto-ignition is specified for a gas mixed with ordinary atmosphere at some ratio. You're talking about not only a liquid, but a situation without any atmospheric oxygen.

All this talk about temperatures alone is not the whole story. There the LEL and HEL, the low and high explosive limits, and these are temperature dependent. the 160 °C figure cited may only be true for a relatively narrow LEL-HEL range.
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[*] posted on 19-1-2013 at 09:29


Quote: Originally posted by garage chemist  
You're better off using the usual precipitation with sulfur dioxide or sodium metabisulfite, followed by filtering and washing the gold precipitate, redissolving and precipitating a second time with ferrous sulfate solution.
In all cases, be it extraction or precipitation, the nitric acid must be removed before adding the extractant or precipitant. A large amount of your gold will be left in the aqueous solution if you do not do this.


Hmmm... that's not really my experience. If you're referring to the gold dissolving in nitric, it's not supposed to do that.

I've separated gold from base metals like copper on that basis: silver, copper and crap dissolve, gold is left behind.

And when recently I dissolved some 30 g of sterling silver in 70 % nitric, I obtained a residue I thought might be Rhodium. Further investigation showed it was just plain gold: it hadn't dissolved in the strong nitric.

I agree that solvent extraction of HAuCl4 with ether is a waste of time. And I didn't realise ferrous solution can be used to reduce the gold but it makes sense...


[Edited on 19-1-2013 by blogfast25]




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[*] posted on 19-1-2013 at 12:21


Nah, he means removing the unspent nitric acid in the aqua regia before reducing -- otherwise the reducing agent doesn't work (or at least, as well).

Tim




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[*] posted on 19-1-2013 at 12:32


I meant the removal of nitric acid from aqua regia in which gold alloys have been dissolved.
I usually process dental alloy scraps for their gold and PGM content. These generally contain over 50% and often over 70% gold, 0-10% Pt and Pd, a little silver and less than 10% base metals. Nitric acid doesn't attack these alloys at all.




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[*] posted on 19-1-2013 at 12:40


Yup. People use urea to remove excess nitric, right?



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[*] posted on 19-1-2013 at 17:57


This is aint so easy as it seems.

HCl4Au have a really high solubility in water, much-much higher than in ether or any else organic solvent, the ratio of the gold in the water and the ether phase will be really poor for the ether.

There is a recipe in the Los Alamos technical reports what I have tried out... They said that from an acidic cc NH4NO3 water solution the uranyl-nitrate could be extracted with ether almost completely. It was not a great surprise that almost non of the uranyl nitrate came out with the ether.

I think that the ether extraction of HCl4Au from water would end up with a similar result, but it's worth a try.




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[*] posted on 20-1-2013 at 10:48


So I actually went ahead and mixed the two without boiling down with HCl or whatever. The only safety precaution I took was setting the aqua regia out in the sunlight for some time. I assume since nitric acid is sensitive to light it would've decayed or something.

I started by adding ether dropwise from afar. There was no explosion or rain of acid. I took this as a good sign so then I dumped a whole 5ml or so of ether into the 25ml aqua regia and nothing happened. The two are not extremely soluble at all. I mixed the two with a graphite rod and sure enough something happened. The aqua regia with gold, which was once a brilliant yellow, had now turned a very pale yellow and was much more like water. The ether layer had taken on the brilliant yellow, which I assume it was extracting the HAuCl4. The ether forms a very noticeable layer atop the aqua regia as it rises after mixing, like oil.

So in conclusion. No need to boil down or whatever. Just let it decay in the sun. Easier.

[Edited on 20-1-2013 by crestind]
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[*] posted on 20-1-2013 at 11:00


Quote: Originally posted by kristofvagyok  
This is aint so easy as it seems.

HCl4Au have a really high solubility in water, much-much higher than in ether or any else organic solvent, the ratio of the gold in the water and the ether phase will be really poor for the ether.



One thing we may be overlooking is that the HAuCl4 is completely dissociated in water (it's a strong acid) but not in ether. That could explain the distribution coefficient he seems to be experiencing. But it's highly unusual...




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[*] posted on 20-1-2013 at 13:43


Leaving it out in the sun for a little while is no guarantee of decomposition. Indeed, to quantitatively eliminate the nitrate would require a huge amount of exposure; the solution would be bubbling, and get rather hot from the reaction, not to mention absorption, if it actually decomposed in that time.

Moreover, the presence of light sensitizes ether, and the presence of radicals, in the air, in the ether and in the acid solution (various nitro- molecules) compounds the problem. Peroxides probably aren't stable in acid like that, but plenty of interesting nitro- and nitroso- things could form (come to think of it, I wonder what all would happen, and how sensitive the products are).

In short: you read our warnings, disregarded them, and proceeded in an even less safe manner.

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[*] posted on 20-1-2013 at 14:09


Quote: Originally posted by blogfast25  
One thing we may be overlooking is that the HAuCl4 is completely dissociated in water (it's a strong acid) but not in ether. That could explain the distribution coefficient he seems to be experiencing. But it's highly unusual...
Just a thought but ether is a Lewis base and AuCl3 is a strong Lewis acid. Maybe this could be why the gold went to the ether from a water solution in such high percent.



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[*] posted on 20-1-2013 at 15:05


Quote: Originally posted by kristofvagyok  
Quote: Originally posted by blogfast25  
One thing we may be overlooking is that the HAuCl4 is completely dissociated in water (it's a strong acid) but not in ether. That could explain the distribution coefficient he seems to be experiencing. But it's highly unusual...
Just a thought but ether is a Lewis base and AuCl3 is a strong Lewis acid. Maybe this could be why the gold went to the ether from a water solution in such high percent.

Is that idea of softness and hardness coming into mind for you? Maybe that little difference might become a huge difference in such at a priori contacted then fully separeted phase solution.
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[*] posted on 21-1-2013 at 17:20


Actually, in many instances, solvent extraction is preferable to direct reduction from solution, especially in cases where the base metal concentration is very high. Facing such a situation, a suitable organic solvent can be chosen that effectively removes the gold but leaves the base metals. As alluded to, HSAB is a good guideline in choosing the extractant. Ether is very effective for gold, but its extreme flammability/volatility make it unsuitable for industrial use and the shelf-life is horrible. That and all of your employees would be sky-high from its narcotic effects. Instead, we use dibutyl diglyme which is a more effective polyether that is less miscible with water and thus more reusable. I only use freshly distilled ether for very pure AuCl3 when I am wanting to make high quality salts for sale to universities.

That can then be back extracted with acid or (in our case), just reduced directly from the solution. The extractant can then be reused.

The common reducing agents for gold are:
  • Sulfites (that generate SO2 in situ).
  • Ferrous sulfate (so called "copperas")
  • zinc, as used in the Merril-Crowe process (typically for cyanides)
  • Oxalic acid (usually at pH 3-4)
  • Hydrazine and its salts (which ignore the presence of nitric acid)
  • Hydroquinone (other alcohols will reduce gold)


Used correctly, all are quantitative. Each has their own indication for use. Ferrous sulfate, of all, gives the lowest quality of gold.

Sulfites are indicated for the reduction of gold when iron, nickel, and copper are present (although a great excess of sulfite can precipitate CuCl which must be removed with dilute, warm HCl). PGMs will largely ride down with the gold when sulfur dioxide is used.

Oxalic acid ignores the PGMs but will drag down copper and iron with the gold. It must be done very hot, and preferably by slowly raising the pH of the boiling gold solution with caustic. This is really a polish step, and is preferred for the last refining. Oxalic acid and sulfites should be used consecutively to obtain gold of high fineness as the methods compliment one another.

In any case, as garage_chemist mentioned, it's wise to expel nitrogen oxides and residual nitric acid from the gold solutions so that a gross excess of reductant isn't consumed.

The exception to this "de-NOx" rule is hydrazine sulfate. An excess of this in solution (even slight) will prevent any gold from going back into solution. It will produce 99,9% Au from solutions laden with nickel and iron. It will also bring down palladium when the ORP drops below 680-690 mV.





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[*] posted on 24-1-2013 at 16:13


Quote: Originally posted by 12AX7  
Leaving it out in the sun for a little while is no guarantee of decomposition. Indeed, to quantitatively eliminate the nitrate would require a huge amount of exposure; the solution would be bubbling, and get rather hot from the reaction, not to mention absorption, if it actually decomposed in that time.

Moreover, the presence of light sensitizes ether, and the presence of radicals, in the air, in the ether and in the acid solution (various nitro- molecules) compounds the problem. Peroxides probably aren't stable in acid like that, but plenty of interesting nitro- and nitroso- things could form (come to think of it, I wonder what all would happen, and how sensitive the products are).

In short: you read our warnings, disregarded them, and proceeded in an even less safe manner.

Tim

To be fair I have done this process two times now with no problems. I think it is safe to say that while in theory there could be a dangerous reaction, actual experimentation suggests otherwise, and this procedure is reasonably safe. I have seen it documented in a patent as well.

[Edited on 25-1-2013 by crestind]
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[*] posted on 25-1-2013 at 18:57


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