Personally I have never been a fan of using chemicals to de-NOx my AR & in fact abandoned that method very early in my refining work
As we know free nitric in AR is a problem we run into here on the forum all to often when "new" members join the forum & their first question is --- I recovered gold foils from fingers (or pins) with AP (or nitric) & then used AR 3/1 HCl/nitric to dissolve the foils but my gold does not drop when I add my SMB
To which the answer is - "you used to much nitric to dissolve your gold so you still have "fee" nitric in your AR which prevents your gold from precipitating so you need to de-NOx the AR to get rid of that free nitric --- which is of course the correct answer to the original question
There are then 2 follow up answers to the member with this problem ------------
1) You can de-NOx your AR by the evaporation method as described in C. M. Hokes book - evaporate down to a syrup add HCl until no more brown fumes evolve (may have to evaporate + HCl several times)
Then Deano has chimed in & told us you don't need to evaporate to syrup & add HCl - simply put on heat until no more brown fumes evolve
2) Or you can do it with sulfamic acid
Then there is the question of urea - which is frowned upon - since the "introduction" of sulfamic
I say - "since the
introduction of sulfamic" - because when I first joined the forum urea was actually suggested as a/the chemical method to de-NOx AR --- it was not until something like 3 or 4 years after joining the forum that someone (I "think" it "may" have been lazersteve) introduced sulfamic as a chemical means to de-NOx AR - with the side benefit of not needing to use sulfuric to drop lead out of AR before precipitating the gold
In other words - when I first joined this forum - urea was an excepted method to chemically de-NOx AR but sulfuric was also needed to drop lead (if lead was present) --- the introduction of sulfamic killed both those birds with one stone
That all said - I have of course used ALL of the above methods - but (at least in my experience) I have found that NONE of them are absolute sure fire methods
Will they de-NOx your AR enough to drop your gold - YES - BUT - there is a possibility/likely hood that you may or not re-dissolve "some" of your gold - especially if you do not filter your drop "right after" the drop but instead let it sit for sometime for settling
In other words - with any & all of the above methods - when you "first" drop your gold & you get a good negative stannous test for gold - but - if you allow "time" for settling - if you then stannous test again - it is possible - even likely that you will get positive gold test result (that positive may be "slight" &/or even great)
That is because with evaporation (&/or just heat) though you will get rid of the vast majority of NOx/free nitric you are likely to still have "traces" of NOx/free nitric & given time (for settling) those traces can/will re-dissolve "some" gold
With chem de-NOx methods - the "nitrates" (with urea) as explained by Deano can/will cause the re-dissolving of at least "some" gold --- & even with sulfamic this can happen as explained by both butcher & Deano in this thread ----------
https://goldrefiningforum.com/threads/sulphamic-acid-possible-degradation.31403/
As I said at the beginning of this post --- "Personally I have never been a fan of using chemicals to de-NOx my AR & in fact abandoned that method very early in my refining work"
The same is true with the evaporation &/or heating method
I abandoned ALL of those methods so long ago I don't even remember the last time I used any of them
IMO - the very best way to de-NOx AR is to use up the free nitric by loading it with metal
If it is a relatively clean solution (little or no base metals) I use Harold_V's method of putting a gold button in the bottom of the beaker & letting it sit on low heat over night so that the free nitric is used up by "loading" the AR with gold ions - I use this method when the solution is relatively clean to start with which should give you back 999 gold in a single drop with chemical precipitation (after normal washes of course)
If I am working with a very dirty (base metal) solution I simply cement it with copper - copper cementation does the same thing as the above gold button trick - just that you are de-NOxing the AR by "loading" it with copper ions instead of gold ions
In other words - copper - besides de-NOxing the AR - also drops your gold (all in the same step) as a "recovery" method for "dirty" solutions --- which (recovery) is likely all you are going to get from dirty solutions with chem precipitation anyway
When done right - copper cementation should give you back your gold very close to (plus/minus) 995
My point being - with copper cementation - it is a one step - de-NOx/gold drop recovery process for dirty solutions
For what is worth - I don't remember the last time I used any method - other then those last two methods
So - now - open to all the "ya buts" or "what abouts"
Kurt