Jor
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Gold from dental fillings
I have a few dental fillings, from my mom, she is a dentist. I want to recycle the gold. I have done this before, but did it with not enough care, so
yield wasn't optimal.
I did it again. Let me share pictures. Any advice is very welcome.
The gold:
Aqua regia was made from 32% p.a hydrochloric acid and 65% nitric acid. I have p.a 37% HCl, but still in sealed bottle, and my technical 37%
colorless, but still technical.
I mixed 1:3,2-3,3 amounts of HNO3/HCl (because HCl is not 37%, I use more than 3).
It was heated on hotplate to about 80C. Gold was added. It vigorously dissolved:
Not everything dissolved. There were still large pieces of grey material left, wich did not dissolve on further addition of aqua regia, even when
boiling. I assumed this to be ceramic (my gold was not cleaned with HF). Also a yellow/brown powder settled, probably AgCl, looking this way because
of the dark brown solution.
I added a spatula of urea to destroy most nitrous acid, and I think also all nitrogen oxides, because this equillibrium shifts:
2 HNO2 <--> H2O + NO2 + NO
2 HNO2 + (NH2)2CO --> 2 N2 + 3 H2O + CO2
The solution became paler brown.
Next it was boiled to about 30-40% it's volume.
5ml conc. HCl was added, and it was boiled down to the smae volume. Another 2ml conc. HCl and evaporated again.
Then it was left to cool down, and diluted with water, forming a yellow suspension.
This is AgCl, probably colored yellow, because of the strong color of the solution. But to be sure, did anything else precitipate out? Maybe a
palladium/platinum salt?
This is where I am now.
I will filter tomorrow, and analyse the insoluble grey pieces. Look if they dissolve in dilute NaF solution in 2M HCl , maybe 0,5M HF. Is that still
dangerous, a few mL test tube scale, otherwise I don't do it.
After filtering I dispose of AgCl. Boil solution and add excess Na2SO3 (sodium sulfite) solution. This should precitipate gold.
Are there any mistakes? And how much palladium/platinum is present in dental alloys? Did I do something wrong by adding urea? I'm pretty sure I;m not,
but I have never seen the method as well, being used in refining.
Is it really true that very small amounts of platinum ions (as chloro-complexes) can cause very severe allergic reactions?
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sparkgap
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For your last question, it would seem to be true: http://www.cdc.gov/Niosh/pdfs/0520.pdf
sparky (~_~)
"What's UTFSE? I keep hearing about it, but I can't be arsed to search for the answer..."
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Contrabasso
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http://jdr.sagepub.com/cgi/reprint/48/6/1284
Gives Au as 70 - 95%
Pd and Pt each as up to 5% according to the hardness of the dental alloy.
Yes the chloroplatinate complexes are a serious risk.
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garage chemist
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Have you seen my method?
http://www.versuchschemie.de/ptopic,124915.html#124915
I did not add urea. After there was no more reaction of the mixed acids with the gold, I decanted the solution, added a second portion of acids,
heated again for a while, then decanted, boiled the residue with 10% HCl (to extract some palladium retained by the AgCl) and washed the residue with
plenty of distilled water.
The filtrate was evaporated 2-3 times with conc HCl until absolutely no more NOx evolved before and during heating.
I recommend not to add urea, it can create ammonium ions by hydrolysis in the acidic media which will precipitate platinum, which would then end up in
your Ag!
The NOx-free solution was then diluted with water, left to stand over night, and decanted from a few mg of AgCl that settled out.
Now the gold was precipitated with SO2 gas at 80-90°C.
Exact procedure, purification of the precipitated gold and isolation of Pd and Pt are found on VC.
The insoluble grey pieces are most likely AgCl. When the alloy contains enough Ag, the AgCl retains the shape of the original alloy part. Those lumps
are easily crushed with a glass rod, contrary to ceramics,.
[Edited on 15-3-2009 by garage chemist]
[Edited on 15-3-2009 by garage chemist]
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Jor
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Ah, I have seen your method quite a long time ago, but I totally forgot about it. Very nice pictures, etc.
I am not too worried that their is platinum in my AgCl precitipate. Im only doing a small amount of gold, a few grams at most, so the amount of
platinum is really small. Platinum, and especially palladium, are only worthwhile extracting on a larger scale.
I couldn't crush the insoluble pieces, but too be on the safe side, i have kept them, for later analysis.
I started heating the filtered solution to 70-80C. I added a solution of an excess (not weighed out) sodium sulfite in 1M HCl (the gold solution was
also very acidic). There was an observation of gas formation. After dinner, some brownish powder precitipated (gold), but the solution was still
yellow. Next I heated to 85C and added more sodium sulfite, now as solid. As soon as the solid touched the solution, there was vigorous SO2 evolution
so I vigorously stirred. I added quite some sodium sulfite, but hardly any gold precitipated, and the solution was still yellow. Weird.
I'm gonna wait until tomorrow and see if the solution has lost it's yellow color.
Maybe some nitric was still present, but it could have never been that much, I have added really a very large excess sodium sulfite.
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garage chemist
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The yellow color is from Pd and Pt.
A solution of divalent Pd is deep brown. I am surprised that your solution isn't almost completely black- all my dental alloy solutions still were
black after the gold was precipitated.
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Jor
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Aha, so that's why no more gold precitipated out!
This night I have left the hotplate on so the sulfur dioxide in solution could react with what I assumed was AuCl4(-).
I completely forgot the fact that the solution could also evaporate --> I came back this morning, and all solution was evaporated!!!
I was very mad. I decided to redissolve again in aqua regia. This was very fast, and I got a brown solution, with some insoluble precitipate. I am
going to do another reaction with sulfite, and now, I will filter when nomore gold forms.
It seems that there is only a small amount of gold in the fillings, as I have only seen a relatively small amount of gold precitipate from solution
yesterday, that's why i though most was in solution. Nowhere near 75% gold by mass in the fillings. But most of the material I had seems to be
ceramic.
Probably this is only going to yeild me maybe 1 gram of gold.
Well, it's still wort about 20 dollars...
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