Urea question

[anachronism]

Basically Urea isn't as useless as it has been characterised. It's got a valid role, just not the one it was originally touted to be. Funnily, Hoke's stockpot chapter (if you read it from this perspective) does allude to redissolving of PMs. I remember having a discussion with Goran and possibly Kurt about this a few years back because there are layers in her book that go beyond the basics.

 

[ólom]

The usual way I treat gold chloride solution from aqua regia digestion is to simmer the solution on a hotplate until the fumes change colour from brown to white.
Give the beaker a further few minutes on the hotplate to be sure that residual nitric acid has been removed.
Add urea to instrumental reading 1.5 pH.
Drop your gold with metabisulfate or any precipitant of your choice.
If you prefer to remove free nitric acid by chemistry then replace the simmering step with a sulfamic acid addition.
You then add urea to remove any nitrates present such as copper nitrate.
If you add enough urea to get to instrumental pH 1.5 you will get a cleaner drop but this step is not necessary if you are happy to have dirtier gold from just the nitrates removal step at a lower pH.
After this step you precipitate the gold.
Deano

In this thread, Ultrax describes that the sulfamic acid did not react, what should be done in this case?https://goldrefiningforum.com/threads/sulphamic-acid-possible-degradation.31403/

 

[Yggdrasil]

In this thread, Ultrax describes that the sulfamic acid did not react, what should be done in this case?https://goldrefiningforum.com/threads/sulphamic-acid-possible-degradation.31403/

Read the whole thread, not just that post!

 

[ólom]

Read the whole thread, not just that post!

I read it but I didn't understand it. Now I read it more carefully, now I understand. Thank you!

 

[kurtak]

Basically Urea isn't as useless as it has been characterised. It's got a valid role, just not the one it was originally touted to be. Funnily, Hoke's stockpot chapter (if you read it from this perspective) does allude to redissolving of PMs. I remember having a discussion with Goran and possibly Kurt about this a few years back because there are layers in her book that go beyond the basics.

Per the bold print - yes - we did have that discussion - in fact a couple times

Kurt

 

[ólom]

If the sulfamic acid does not react, then I evaporate it and use urea, can I then add sulfuric acid because of the potential solder?

 

[Yggdrasil]

If the sulfamic acid does not react, then I evaporate it and use urea, can I then add sulfuric acid because of the potential solder?

Sulfuric is just a couple of drops, you will immediately see if there are Lead in solution.

 

[ólom]

Sulfuric is just a couple of drops, you will immediately see if there are Lead in solution.

Ok. Thank you!

 

[Dreamer]

This is my understanding as well. But used this way, it explains some of the issues with dropping gold and the reformation of NOx when running some ewaste material with a heavy copper base as a contaminants. These often need to be dropped and filtered immediately to stop the gold from resolving, given time, such as sitting over night, may completely redissolve small batch’s completely. I have ran into this in the past, but just recently started to understand why, now maybe how to stop it.

This was my understanding also, HNO3 + H3NSO3 → H2SO4 + N2O + H2O What am I missing ? What common divalent would interrupt the reaction or push it in the other direction?

 

[anachronism]

Sulfuric is just a couple of drops, you will immediately see if there are Lead in solution.

But not necessary if you have denoxxed with Sulphamic acid.

 

[Dreamer]

Wouldn't adding urea proceed as

Respectfully, Please correct my understanding. From what I am reading on this thread, "Urea destroys Nitric and Sulfamic does not" ? This is opposite from everything I've been taught and clearly I am missing something.

Would one of you kind fellow Members or Moderators help me out here?

 

[Shark]

My understanding, at this point is, urea destroys the NOx, sulfamic destroys the actual nitric acid. They are chemically different substances and need dealt with separately, although either one alone can potentially restart the AR process.

Edit for clarity


See next post down I am wrong here.

 

[anachronism]

Wouldn't adding urea proceed as View attachment 62638

Respectfully, Please correct my understanding. From what I am reading on this thread, "Urea destroys Nitric and Sulfamic does not" ? This is opposite from everything I've been taught and clearly I am missing something.

Would one of you kind fellow Members or Moderators help me out here?

It's basically this Dreamer.

Urea is no good as a denoxxing agent. It doesn't destroy free Nitric. But it does break down certain high valency Nitrates that may cause precipitated gold to redissolve when you are cleaning it with HCl.
Sulphamic acid DOES denox by destroying free Nitric but it doesn't do anything to the Nitrates.

Yes it's a step beyond the "established" knowledge however it's a step beyond that improves what home refiners are doing. Call it levelling up the skills if you will.

 

[Acidrain]

Is using those better than evaporating and adding HCI to get rid of excess nitric acid or just not using too much in the first place?

 

[Acidrain]

I'm still reading Hokes book and learning from other various sources and trying to understand this

 

[anachronism]

Is using those better than evaporating and adding HCI to get rid of excess nitric acid or just not using too much in the first place?

Using Sulphamic to denox is the way ahead. Yes far better than adding HCl and simmering down. Obviously less excess Nitric is always a good way to go.

 

[Dreamer]

It's basically this Dreamer.

Urea is no good as a denoxxing agent. It doesn't destroy free Nitric. But it does break down certain high valency Nitrates that may cause precipitated gold to redissolve when you are cleaning it with HCl.
Sulphamic acid DOES denox by destroying free Nitric but it doesn't do anything to the Nitrates.

Yes it's a step beyond the "established" knowledge however it's a step beyond that improves what home refiners are doing. Call it levelling up the skills if you will.

So the NO bound to other elements is captured by the urea converted to Urea Nitrate liberating the less electronegative elements to bind to H2O?

After we filter that off, do we capture the precipitants and heat on high? - Just kidding (DO NOT DO THIS) . For discussion purposes, what is the safest way to stabilize the Urea Nitrate for disposal?

 

[Dreamer]

Is using those better than evaporating and adding HCI to get rid of excess nitric acid or just not using too much in the first place?

The use of Urea is discouraged and personally speaking is a last resort. As Anachronism said Sulfamic is the way ahead. I've always poured off my waste onto copper, cement out anything still caught-up in solution and process it later. Less steps, less chemicals and very minimal difference in recovery at the end of the day.

I think this whole discussion was to bring to light when it might be considered and an alternative should sulfamic be difficult to source. But if you have the time and patience, evaporation and rehydration followed by slight neutralization (1-2 pH) will give a nice drop.

 

[Acidrain]

The use of Urea is discouraged and personally speaking is a last resort. As Anachronism said Sulfamic is the way ahead. I've always poured off my waste onto copper, cement out anything still caught-up in solution and process it later. Less steps, less chemicals and very minimal difference in recovery at the end of the day.

I think this whole discussion was to bring to light when it might be considered and an alternative should sulfamic be difficult to source. But if you have the time and patience, evaporation and rehydration followed by slight neutralization (1-2 pH) will give a nice drop.

I see, thanks Dreamer! That's something I understand

 

[kurtak]

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