Urea to eliminate excess Nitric Acid

Hai I am new to this forum and currently I am doing some refining gold from electronic scrap mostly are PCBs from mobile phones. This forum is very helpful for me as when something goes wrong I will look for the solution here.

So I managed to get the gold dust which I am not sure if it is the right dust as the color are brownish after I put the SMB into Aqua Regia solution. However there are things that I am not sure if I am doing it right and confused with the usage of Urea to eliminate the excess nitrate. Some people say in this forum to use Urea but some are not as Urea is very dangerous when mixing with Nitric Acid. I got scared and find another way to get rid of this nitrate such as evaporation and addition of water. But it does not work as when I put a lot of SMB into the solution the brownish fume came out and it does not precipitate the gold. So I tried to put Urea and SMB again and it worked. yes it did precipitate the gold.

So my question is if the using Urea is correct way? is it safe? since some says it could be explosive

Appreciate if some can answer my question.

Thanks

Firstly, welcome to the forum. If I understand correctly what you've explained:

I got scared and find another way to get rid of this nitrate such as evaporation and addition of water

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In the nitric evaporation process, you do not add water, but add HCl. Some free acid is required for the SMB to do its work.

solution the brownish fume came out and it does not precipitate the gold

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That brown cloud IS precipitation. It's only that excess nitric may then cause it to re-dissolve.

If you add your nitric patiently, in small increments, you will not have this problem.
Keep the urea for growing plants.

Welcome to the forum.
We advise using sulphamic to rid nitric if an excess has been used but the best advice is to cover your material with plenty of Hcl and slowly add small amounts of nitric until you get no reaction, this should eliminate the need to neutralise the nitric.

Wilo said:

This forum is very helpful for me as when something goes wrong I will look for the solution here.

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You are supposed to study this forum, to know what you are doing before "something goes wrong".

Things go better when something doesn't go wrong. It is always better to learn from others mistakes. Doing your homework helps to prevent hazardous or costly mistakes.

Jim

Wilo said:

I got scared and find another way to get rid of this nitrate such as evaporation and addition of water. But it does not work as when I put a lot of SMB into the solution the brownish fume came out and it does not precipitate the gold. So I tried to put Urea and SMB again and it worked. yes it did precipitate the gold.

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Hi and welcome to the forum.

Urea is used by some people but the consensus on the forum is that it isn't needed if you manage your nitric uses or add a gold button to use up extra nitric at the end. Urea doesn't guarantee to remove all the nitrates and in some cases could form explosive compounds. It can also complicate things if you have PGM:s in solution. For some other discussions, check the links on this page.
http://goldrefiningwiki.com/mediawiki/index.php/Urea

I have never used urea.

When using evaporation you should add HCl and not water. This will remove even nitrates. If you got a brown gas cloud when adding SMB then you still had some nitric acid left with your gold chloride. Properly denoxed solutions doesn't produce any red or brown fumes.
http://goldrefiningwiki.com/mediawiki/index.php/Denoxing

Göran

Göran,
The GoldRefiningWiki ( http://goldrefiningwiki.com/mediawiki/index.php/Main_Page ) has really come a long way, looks like you have put an awful lot of work into it Very Nice.

butcher said:

Göran,
The GoldRefiningWiki ( http://goldrefiningwiki.com/mediawiki/index.php/Main_Page ) has really come a long way, looks like you have put an awful lot of work into it Very Nice.

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Thanks, I add to it now and then when I have time.



This is how I envisioned the wiki from the start. A better place to collect references and in dept information, easy to find information and expand when new discussions on GRF appear. With links to good threads on the forum as references.
There is a lot more to do though, the wiki is merely started in my mind.

Göran

You are supposed to study this forum, to know what you are doing before "something goes wrong"

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Yes I did study this forum before I put to work but sometimes there are so many versions of it that I did not know who to follow

That brown cloud IS precipitation. It's only that excess nitric may then cause it to re-dissolve.

If you add your nitric patiently, in small increments, you will not have this problem.
Keep the urea for growing plants.

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What do you mean by add nitric?

When using evaporation you should add HCl and not water. This will remove even nitrates. If you got a brown gas cloud when adding SMB then you still had some nitric acid left with your gold chloride. Properly denoxed solutions doesn't produce any red or brown fumes.
http://goldrefiningwiki.com/mediawiki/i ... p/Denoxing

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I tried to add HCL also but there was no brown cloud coming out from the solution. it did not have any reaction just like adding water.

So I have managed to get the dust but now I am not sure whether it is the right dust since when I melted everything there was no gold found. I put the images below





The last image is after I melted it

Can anyone help to explain?

Thanks a lot

What do you mean by add nitric?

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This is what others are also trying to tell you. Instead of mixing up a 3:1 (or whatever) ratio of HCl and HNO3 and calling it Aqua Regia, which will have far too much nitric in it, add your nitric in small, incremental quantities.

I tried to add HCL also but there was no brown cloud coming out from the solution. it did not have any reaction just like adding water

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Most likely you did not evaporate far enough. It doesn't work unless you get down to a reasonably thick syrup. Check out Hoke's guide to evaporation, it's all you need.

Yes I did study this forum before I put to work but sometimes there are so many versions of it that I did not know who to follow

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Then stop. You really should not proceed until you understand what you are doing, why you are doing it, and the result you expect to achieve. You can't just play with this, you could get hurt.

Wilo said:

The last image is after I melted it
Can anyone help to explain?

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I don't mean this disrespectfully on a personal level, but perhaps being a little blunt is warranted.
Clean, light tan coloured, well refined gold powder IN = shiny gold OUT.
Dark mix of all sorts of random crap IN = ???

The good news is that if there's gold in there, it'll still be there when you've learned to refine clean powder. This site is full of example images of clean powder. When you can make that, you're ready to start melting it.

Walk before you run.

That does not look like gold, more like brown rust. I wonder if SMB + tin in solution would give a brown precipitate? SO₂ left after precipitating gold could give a brown false positive when testing with stannous so it seems logical to assume that a lot of tin chloride in solution would drop as a brown sludge of tin sulfide if mixed with SMB in acidic conditions.

This also means that if there is tin in solution there shouldn't be any gold chloride left. Maybe it exists as a colloid at this stage, maybe it's still with the solids filtered off.
- Did you save the solids when filtering the gold chloride?
- Did you save the "barren" solution after filtering off the "gold"?
- Was there a lot of gray sludge that was slow to filter?
- What exactly did you do before precipitating "gold"?
- How much scrap did you start with and how much gold did you expect?
In bullet form, please.

Before precipitating the "gold", did you test the solution with stannous?

Göran

g_axelsson said:

That does not look like gold, more like brown rust. I wonder if SMB + tin in solution would give a brown precipitate? SO₂ left after precipitating gold could give a brown false positive when testing with stannous so it seems logical to assume that a lot of tin chloride in solution would drop as a brown sludge of tin sulfide if mixed with SMB in acidic conditions.
Göran

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That is a really clever answer Göran, and I would bet money on you being correct.

The more I read about smb/->SO2 precipitation, the more I come to understand there are several instances where it is not truly selective.
Mainly that "non selective" nature comes from its converting chloride species to sulfur compounds. Silver, lead, palladium, (and now) tin...and I'm sure there is probably more.

Topher_osAUrus said:

g_axelsson said:

That does not look like gold, more like brown rust. I wonder if SMB + tin in solution would give a brown precipitate? SO₂ left after precipitating gold could give a brown false positive when testing with stannous so it seems logical to assume that a lot of tin chloride in solution would drop as a brown sludge of tin sulfide if mixed with SMB in acidic conditions.
Göran

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That is a really clever answer Göran, and I would bet money on you being correct.

The more I read about smb/->SO2 precipitation, the more I come to understand there are several instances where it is not truly selective.
Mainly that "non selective" nature comes from its converting chloride species to sulfur compounds. Silver, lead, palladium, (and now) tin...and I'm sure there is probably more.

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Yep! Kinda my observation and theory. Don't know the science behind it, but over gassing by adding to much smb will drop more than just gold.

Palladium said:

Yep! Kinda my observation and theory. Don't know the science behind it, but over gassing by adding to much smb will drop more than just gold.

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Yes and no. It really doesn't bring much extra down at all. You can literally shovel the stuff in (a metaphor if I may) and still end up with gold that will clean up beautifully with a proper wash procedure. The information that too much SMB creates low purity gold with a huge amount of base metals is a myth that has been propagated for too long. It doesn't.

Edit: the pictures show the results of a process carried out in a manner that needs improvement, not the results of too much precipitant.

anachronism said:

SMB creates low purity gold with a huge amount of base metals is a myth that has been propagated for too long. It doesn't.
.

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I would say it depends on what base metals are in the mix with the gold, and how much.
Well, not so much "I would say", as numerous published authors "say".
And, I would bet that TK Rose, F.E.Beamish, R Loewen, and H.F Collins didn't just go out on a limb guessing, but did actual true-to-science (and scientific approach) testing.

I agree though, that repeated wash and rinse cycles after precipitation can, and does clean up the precipitated gold greatly.

I have to take my son to preschool stuff here in a minute, but I will get you cited quotes shortly.

I understand his statement. But it's all in procedure and process. For the sake of the uneducated mind we teach to the method of the science. While your statement is somewhat acceptable to the more advanced refiner the newbies are taught to relate to the art of achieving perfection. Causing a problem that doesn't need to be and then justifying it with it will clean up while it may be true is not art or science. It's just getting it done.

The figures constantly quoted for the precipitation of gold using SMB are only applicable in a laboratory setting, or for precipitating from a pure solution of Gold Chloride. As soon as you add "reality" to the situation i.e. Base metals, residual excess nitric, and the resulting differing base metal ionic states among others, then those figures simply go out of the window.

You'd be hard pushed to precipitate 10g of gold from a dirty AR solution containing 10g of gold using 10g of SMB. It simply doesn't happen.

For example in many cases your base metals are actually effectively redissolving your gold by reacting with the chloride ions. That flies in the face of "accepted lore" however if you read Hoke's page on the stock pot she actually alludes to this. Like so much of what Hoke wrote it was interpreted in an over simplified way because it was presented in such a simplified language. The science itself was masked, and missed.

As such "getting it done" relies upon all these things and that should be what is taught rather than over simplified stoichiometry. Learning to refine is about learning about the real world and the interactions between everything that lead to the end result. Pretty shiny pure gold. 8)

Being an excellent chemist doesn't make for someone automatically being a good refiner.

Most likely you did not evaporate far enough. It doesn't work unless you get down to a reasonably thick syrup. Check out Hoke's guide to evaporation, it's all you need.

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I read Hoke's book yeah maybe I did not evaporate enough since I thought it will not turn into thick syrup

Then stop. You really should not proceed until you understand what you are doing, why you are doing it, and the result you expect to achieve. You can't just play with this, you could get hurt.

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I don't play with it. I read Hoke's book, I watch video, I read this forum, comments etc. knowing the theory won't get me the gold. I have to give it a try to get the experience and to perfect it. therefore I need your guide to make it perfect

- Did you save the solids when filtering the gold chloride?
- Did you save the "barren" solution after filtering off the "gold"?
- Was there a lot of gray sludge that was slow to filter?
- What exactly did you do before precipitating "gold"?
- How much scrap did you start with and how much gold did you expect?
In bullet form, please.

Before precipitating the "gold", did you test the solution with stannous?

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- yes I still keep the solution
- yes exactly there a lot of gray sludge when first filtering
- before precipitating, I put Urea into Aqua Regia that has gold flake dissolved in the solution. then I put SMB afterwards
- around 2kg of scrap PCB from smartphone. Before I poured into nitric acid I did some cleaning the PCB to make it clean from any IC

Unfortunately I did not test the solution with stannous as I do not have one. I will buy at this point since it is very important

Hi Wilo

If you started with 2Kg of scrap PCB from smart phones then you will struggle to see the gold precipitated from this. If it I had been old Nokia stuff from the 90s then the yields were quite good but the new stuff is not "good stuff."