Gold foils not dissolving in Aqua Regia.

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gaurav_347

Well-known member
Joined
Nov 17, 2013
Messages
169
Hello All,
I hope all of you are doing well. I have been facing some issues treating foils in AR. The problems are as follows :-

So at the moment I am processing Rams around 50 kgs. I recovered 4.2 kgs of clean cut fingers. I treated these fingers in dilute nitric acid to release the foils. Then I filtered the solution to get the foils. I was trying to dissolve these foils in AR but it wouldn't dissolve. While preparing the aqua Regia before treating the foils there was a strong reaction when nitric acid was being added to hcl. Like a strong/violent fizzing reaction. Secondly the foils won't dissolve in this AR. I tried to give it excess heat but still it won't. I have always used lab grade acids in the past to recover and refine the gold. This time I am using commercial grade acids. Could someone give their insights as to what is happening. I also performed a stannous test after one hour of treating these foils in AR but the tests came negative. The foils are still at the bottom of the SS reaction vessel. I might be wrong but I believe one of these acids have lost their potency.
 
gaurav_347 said:
The foils are still at the bottom of the SS reaction vessel. I might be wrong but I believe one of these acids have lost their potency.
Stainless steel is fine for working with nitric acid, but AR will eat through it. Get your foils and acid out of it before the vessel fails and you have a real mess on your hands.

I would suspect that the acid is being consumed reacting with the steel. Any gold that does dissolve will cement back out as the steel dissolves.

You also shouldn't premix your AR. In a beaker, cover the foils with HCl, then add the nitric in small increments till the foils are dissolved.

Dave
 
Are you sure it was HCl you mixed with your nitric acid? Sulfuric acid would give the same fizzing reaction and not being able to dissolve the gold foils.

Stainless steel is NOT compatible with aqua regia, so if you made a mistake with the acids then you might have been saved from a much worse catastrophe when the bottom of your vessel would have been dissolved and spilling out aqua regia and dissolved gold.

Göran
 
gaurav_347 said:
SS reaction vessel.

:shock:

HCl by it's self does & will react with SS - although it is a "very slow" reaction it will sooner or later ruin/distroy the SS vessel

The addition of nitric acid (a STRONG oxidizer) to HCl increases the reaction of the acids on the SS --- Like a strong/violent fizzing reaction (depending somewhat on the amount of nitric added) --- which will ruin the SS vessel - likely to the point of the vessel failing

Nitric acid - by it's self - does not react with SS (at least not with non magnetic 300 series SS)

Therefore - using nitric (by it's self) to dissolve metals like copper or silver in a SS vessel is not a problem

On the other hand - HCl should NEVER be used with a SS vessel - it will sooner or later lead to the vessel failing

Depending on the thickness of the SS &/or the amount of nitric added to HCl (to make AR) could lead to the SS vessel failing the first time out - if not - the vessel will sooner or later certainly fail

Also - although AR will react with (dissolve) gold --- it will react "more so" with the SS --- therefore - though it may dissolve "some" gold - it will dissolve the SS more readily - & any gold that may dissolve will then cement out as a "fine" powder


Kurt
 
Thank you for your suggestions. I know hcl reacts with ss and same goes with AR. I tried treating the foils in a glass beaker but the same problem persists. I am pretty sure that it's not sulphuric acid that I am adding to the hcl. It's nitric acid as it was the same acid used to treat the fingers. . Plus the containers are clearly marked with the stamps. The nitric acid being used is pretty old. Almost 8 months. Could this be the issue?

I have never mixed Ar before hand. I always add nitric acid in small increments. I only mixed it to see if the fizzing still persisted and it did. My theory is that the acids have gone bad or something has changed molecularly because of the age of the acids. These are commercial grade acids and I will get lab grade acids and try again.

Thank you
Gaurav
 
gaurav_347 said:
(1) It's nitric acid as it was the same acid used to treat the fingers.

(2) The nitric acid being used is pretty old. Almost 8 months. Could this be the issue?

(3) These are commercial grade acids and I will get lab grade acids and try again.

(1) If the nitric worked to dissolve the copper under the gold plating thereby releasing the gold foils from the fingers there is "NO" reason why the nitric wont work to make AR & dissolve your gold --- there is something else going on here - such as has already been suggested that you by "mistake" grabbed the wrong acid in place of ether the HCl or the nitric (mistakes happen) --- or ????


(2) the nitric being 8 months old is not the problem ether - as I said in the underlined above --- I have nitric acid that is 3 plus years old which I am still using to refine silver with & it still works perfectly fine

(3) there is "NO" need to buy "lab grade" acids for "refining" commercial grade acids will work perfectly fine when refining --- depending on location &/or sources lab grade acids can cost 10 times (or more) then commercial acid so there is just no need to spend the extra money on lab grade acid - you only need to use lab grade acids in the assay lad for analyzing "solutions" such as AA or ICP assay of "solutions"

If you did not (by mistake) use the wrong acid in place of ether the HCl - or nitric --- then there is something else going on that is reacting with & consuming the acid before it has a chance to dissolve the gold - such as copper &/or nickel still mixed in with your foils which is reacting with the acid & consuming it before it is able to dissolve the gold --- or --- ???


pictures would be helpfull


Kurt
 
I asked about the hydrochloric acid, not the nitric acid.

I bought some nitric acid in 1987 and it was still fine in 2010 when I used up the last of it. I had to replace the bottle though, the rubber seal on the original container failed.

Göran
 
Thank you all for all the help . I have been refining scraps on large scale for the last 3 years . I too have never faced such an issue before . I went ahead and bought different acids . The acids which came at the factory were from a different supplier. I believe they made a mistake while labeling the HCL or it was a manufacturing defect . I have since got the refund from the supplier. With the new acids I was able to dissolve the gold successfully.

Regards,
Gaurav
 
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