Sulaiman
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Why use urea to neutralize Au in AR ?
Just to have done it I am having a go at recovering gold from CPUs and connectors
Small scale one time attempt.
The gold containing scrap is soaking in 36% HCl to remove non-precious metals
Next will be a rinse then Aqua Regia to dissolve the gold and other precious metals.
After that filter, neutralise with urea, filter, drop out the gold with sodium metabisulphite.
Could someone point me to references to why urea is prefix prefered for the neutralisation of the AR ?
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diddi
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use hot nitric to pull out the silver first.
Beginning construction of periodic table display
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Sulaiman
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Will do, no idea if there is any silver but I'll do it then test the solution for Ag+ , thanks.
Can anyone enlighten me on the use of urea,
I gather it is mainly to neutralise the oxidising nitric acid
But if I chose a base it would be NaOH, KOH, NaHCO3 etc.
What is special about urea ?
(is it because urea can neutralise the nitric but not the chloroauric acid ?)
This leads to a related question
Why sodium metabisulfite to drop out the gold ?
(is it just as a source of SO2 ?)
Sorry for such simple questions but I found a lot of anecdotal information but not a lot of relevant chemistry information, maybe most of the
processes have been developed empirically , or I'm a rubbish researcher.
[Edited on 19-11-2015 by Sulaiman]
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Nicodem
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I think you misunderstood whatever that source you did not cite said. It looks pretty obvious that the solution is never neutralized, but only the
excess aqua regia is reduced with urea.
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diddi
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the MBS is cheap. and unregulated so it is the choice of bucket chemists who go on youtube with painters dust masks and motorcycle helmets for PPE
Some of them can even pronounce the names of the chems they are using !
[Edited on 19-11-2015 by diddi]
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Sulaiman
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Quote: Originally posted by Nicodem  | | I think you misunderstood whatever that source you did not cite said. It looks pretty obvious that the solution is never neutralized, but only the
excess aqua regia is reduced with urea. |
This is where I get confused, I shall go and do a little more research.
I can follow one of the many 'recipies' but because I treat it as hobby chemistry, I would like a mental picture of what is going on chemically.
(I expect to spend more on chemicals than the value of the gold, based on my 'ingredients' vs. YT videos, if I get a 3mm sphere trophy I will be
happy
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Sulaiman
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thanks diddi
diddi, thank you for the heads up on using nitric acid before the aqua regia.
The HCl 'wash' was reactive and gave a transparent blue solution which I have saved as Solution A.
I rinsed the stock with distilled water twice and added the rinse water to Solution A for later analysis.
(a quick SnCl2 test was negative)
Three more distilled water washes then it was at the point that I would have used aqua regia.
Based on your post I started with 100 ml 1M HNO3 to avoid any passivation problems,
then whilst boiling I added c100 ml of 69% HNO3 in c10 ml batches ...
... this stage needed careful addition and temperature (and breath) control,
a very vigorous reaction releasing lots of visible NO2.
I am really glad that you suggested this step,
judging by the colour there must be a lot of copper in my stock,
which will now not be in the aqua regia, so maybe a purer product ?
Here is a photo' of the nitric acid digestion phase after over one hour.
apologies for a double post.
Edit: I realise that it is not just copper in there
but I am unfamiliar with the colours of various nitrate salt solutions
this will become Solution B for later analysis.
P.S. it is getting exciting now, lots of tiny gold flakes floating around
and adding more 69% made no more NO2,
so the flask is now stored outside, ready for the next phase.
[Edited on 22-11-2015 by Sulaiman]
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