Sulphamic acid possible degradation?

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Ultrax

Ultrax

Well-known member
Joined
Feb 10, 2022
Messages
366
Location
Ukraine
Hey there,
I'm trying to remove excess HNO3 from the aqua regia by adding the sulphamic acid solution, and I don't see ANY bubbles.
Thinking that all is OK and HNO3 was converted to sulfuric acid, I was proceeding to the precipitation step, and... gold doesn't precipitate on 100%.

Then trying to check the reason for failure, I evaporated a different portion of the same AR solution to dry powder
and clearly saw brown NOx fumes during heating.

I have only two possible hypotheses because I bought sulphamic acid last year.
Does it possible that my sulphamic acid has degraded and hasn't worked?

Or maybe the brown fumes of NOxes also form from decomposing nitrate impurities when heated to 500C?

Please advice,

Thanks!
 
Ultrax said:
Hey there,
I'm trying to remove excess HNO3 from the aqua regia by adding the sulphamic acid solution, and I don't see ANY bubbles.
Thinking that all is OK and HNO3 was converted to sulfuric acid, I was proceeding to the precipitation step, and... gold doesn't precipitate on 100%.

Then trying to check the reason for failure, I evaporated a different portion of the same AR solution to dry powder
and clearly saw brown NOx fumes during heating.

I have only two possible hypotheses because I bought sulphamic acid last year.
Does it possible that my sulphamic acid has degraded and hasn't worked?

Or maybe the brown fumes of NOxes also form from decomposing nitrate impurities when heated to 500C?

Please advice,

Thanks!
Click to expand...
The sulfamic must be hot. Around 70-80 Centigrade.
 
How much copper was in the AR? Sometimes an excess can cause
problems when neutralizing nitric.
Edited for spelling
 
Last edited:
No copper at all. Iron salts - up to 20% from total weight of PM in AR solution. Sometimes with small quantity (<1% of PM) of dissolved AgCl in complexes. More than 80% of precious metals in solution are (Pt) and (Au).
 
Shark said:
How much copper was in the AR? Sometimes an excess can cause
problems when neutralizing nitric.
Edited for spelling
Click to expand...
Very interesting. Could you please describe the chemical mechanism of this?
The excess copper should react with the excess HNO3 and create copper nitrate Cu(NO3)2, right?
How can copper coexist in solution with free nitric acid?
 
Ultrax said:
Very interesting. Could you please describe the chemical mechanism of this?
The excess copper should react with the excess HNO3 and create copper nitrate Cu(NO3)2, right?
How can copper coexist in solution with free nitric acid?
Click to expand...
No, I really can’t as I don’t understand the chemical reactions that well. It is something I have ran across several times when processing certain escrap such as ceramic IC’s or especially with ceramic CPU’s. A few times as well when running certain gold filled directly in AR. I have often thought it could be the reaction between the copper and the sulfamic as it converts to sulfuric but I have very little understanding of the reactions. It does not happen every time but shows up at random in my case. Enough times to be annoying. I have almost stopped trying to figure it out and change my method when it occurs to using heat and evaporation to the solution. I was actually hoping someone smarter in Chemistry would have a better idea.
 
Copper nitrate and or most any metal nitrate salt when mixed with an acid such as sulfuric acid (here the sulfuric which is generated by the reaction of sulfamic acid itself) can generate more free nitric acid in solution which forms soluble copper sulfate, this reaction could also occur with dissolved metal nitrate salts in solution such as the soluble lead nitrates when a lead is involved in the solution, the addition of sulfuric acid will form lead sulfate and nitric acid.

Heat as usual would drive the reaction forward.
 
Thanks butcher. It did seem to occur most when working with a fairly dirty solution. The more metals involved the more often it seems to occur.
 
butcher said:
Copper nitrate and or most any metal nitrate salt when mixed with an acid such as sulfuric acid (here the sulfuric which is generated by the reaction of sulfamic acid itself) can generate more free nitric acid in solution which forms soluble copper sulfate, this reaction could also occur with dissolved metal nitrate salts in solution such as the soluble lead nitrates when a lead is involved in the solution, the addition of sulfuric acid will form lead sulfate and nitric acid.

Heat as usual would drive the reaction forward.
Click to expand...
Thermodynamically, which of the reactions with sulfamic acid in AR solution has priority? (a) direct conversion of free nitric acid to sulfuric acid, or (b) exchange of nitrates of base metal impurities (including silver and lead) with sulfuric acid and release of the additional free nitric acid. Or are they competing?
So I guess, in theory, if there is lead nitrate or silver nitrate in the AR solution, then when sulfamic acid is added (with excess), a sulfate of lead or silver should drop down? And the excess of sulfamic acid should be enough to precipitate all base metals nitrates as sulfates from aqua regia solution? And only after that will the direct conversion of the remaining nitric acid into sulfuric acid begin, which, in turn, already has nothing to react with?

That is, we can assume that there shouldn't be irrelevant base metal nitrates in the AP solution, but only a HAuCl4, and as low as possible excess of free nitric acid (!). Only in this case the reaction of free nitric acid and sulfamic acid proceed 100% smooth as described in theory?

During the oxidation of iron hydroxides in aqua regia solution, which salts are formed first? Chlorides or nitrates? Judging by the logic of the reaction - chlorides, since if nitrates were formed first, then in the presence of free nitric acid and its reaction with sulfamic acid, gold would precipitate from the AR solution due to ferrous sulfate? This is true?
 
Last edited:
Golddigger76 said:
What materials are you refining and what color is your aqua regia now ?
Click to expand...
Sulfide concentrate after flotation and 700С roasting.
We have to use specifically AR because concentrate contains high percent of Pt.
Color of the solution is saturated orange.
 
I have not seen this happen when processing relatively clean solutions such when doing a second refine. I do very little karat gold but do not recall it ever happening when I have. On occasion I have stopped the nitric with large excesses of sulfamic and higher heat. Even then it took abnormally excess of time. I do recall twice when I just put it off until the next day and returned to a solution that seemed to have “fixed” it’s self. I assumed that the extra time after the heating had some effect on it.

I know none of this is technical but might help someone figure it out that can pass on a better understanding of it later.
 
Ultrax said:
Sulfide concentrate after flotation and 700С roasting.
We have to use specifically AR because concentrate contains high percent of Pt.
Color of the solution is saturated orange.
Click to expand...
Are you using premixed AR?
Have you tested the solution for Gold presence?
Which precipitant are you using?
 
Decades ago I was told by a very experienced metallurgist to always try to minimise inputs into any process, especially in wet chemistry.
The reasoning was that any input would have the possibility of affecting reactions and possibly causing side reactions as well as adding to the process cost.
Pretty well all metallurgists from this long ago era followed a similar plan during Aqua Regia digests.
The cost of chemicals was higher than the cost of heat so any excess nitric acid was removed from the liquor by simmering until all of the brown fumes had stopped evolving. This had the extra benefit in that there were no side reactions from added chemicals.
This was effective in an industrial setting but not suitable in residential locations without installing a fume hood and acid neutralising set up.
The above leads to people in residential areas using chemical reagents to remove excess nitric acid fumes from liquors.
Unfortunately the products of these reagent reactions still have to be dealt with, there are no free lunches.
Having got rid of the free excess nitric acid there was still the problem of the complexed nitric acid in the form of base metal nitrates.
The greatest problem was caused by divalent metal salts, especially copper nitrate but other metals such as divalent iron caused problems. Having just undergone an Aqua Regia digest, these metals were all in their highest valence form.
If a reducing agent such as a sulfite was added to an Aqua Regia digest liquor the agent would not only reduce gold to zero valence state but would reduce the base metals from their highest valence form to their lowest valence form.
This would release nitrate complexes into the solution which would lead to the-dissolution of some of the precipitated gold.
You could get around this problem by either adding excess precipitant into the solution until all of the bass metals had been reduced and so the source of the nitrate complexes was exhausted or by adding a chemical to the solution to react with the base metal complexes such that the nitrate complexes were fully degraded and could not be liberated as nitrate complexes by the addition of a reductant to the solution.
It was found that urea would attach more strongly to the nitrate complexes than the base metals thus neutralising the nitrate complex release mechanism caused by adding a reducing agent to the solution. The urea nitrates then played no part in any reactions during the reduction stage.
At that time urea was much cheaper than sulfites.
The above is why, if your starting material contains base metals, you have the options for processing laid out above. Usually the main considerations are financial in the cost of reagents but disposal costs also should be considered.
Each starting material and each regulatory statute will be different so there are no over-arching process pathways.
Deano
 

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