Vanry
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problem precipitating gold
hello everybody.
I tried to get gold from electronic scraps, (not for economical value before the question pop) so i get on to dissolve it in aqua regia, and
precipitate it with sodium metabisulfate (SO2).
I first did it month ago on RAM. Everything went well, except i clearly overshot the amount of aqua regia needed, leading to a lot of losses during
filtration, as i was using coffe filter on a very fine powder.
But then I tried to use the same method to gold pins, I ended up with a red precipitate instead of a fine black powder/yellow-brownish stuff, that
completely vanished when i tried to melt it. I have absolutely no idea, of what it is.
What have I did precisely:
- I was hopping to get 1-2g of gold from this. First i put metal pins, plastic, cpu boards, cables into a beaker, and dissolved most of the metals
using HCl and H2O2. I got a blue solution (mainly copper was dissolve, seems logical)?
- I remove all the big platic and metal parts. The solution was filtered, to get the remaining gold flakes and plastic dust. Took that solid stuff,
rince it, before letting it sit for a while in conc nitric acid (heavy release of NO2 and blue solution again).
At this point, i could not see any solid metal in this exept the gold flakes. I assumed that almost every non-noble metal were dissolved. Again:
- filtered, rinced, put everything in a new beaker (gold + coffee filters + plastic dust)
- added aqua regia, made of: ~140mL of homemade azeotropic nitric acid and ~500mL of commercial 20% HCl. Let it sit for 24h at ~80°C (yup, I
overshooted the amount of aqua regia again, can't help being stupid...) I was left with a yellow solution, and no more gold flakes. I was pretty happy
at this point.
- filtered the crap left in the solution using a cotton ball inside a funnel.
- then i prepared a solution of sodium metabisulfate. I prepared around 2L of a saturated solution of SMB in distilled water, before adding it to my
yellow solution of dissolved gold.
- it bubbled a LOT lost its yellow color and i got a nice whiff of SO2, everything was looking nice. But even after 15 minutes, I didn't
notice any precipitate forming, even as it should be almost instantaneous.
- Being a bit disturb I heated it to around 90°C, let it stir overnight.
When i came back, the solution was pink/red, due to a fine reddish precipitate.
- filtered it on buchner on ash-less filters, got A LOT of red powder like 3-4 time the volume of gold I was expecting.
- I tried to melt it, i used borax as flux, heated it in a foundry, and as a result, nothing but a borax glass pellet, and very little traces of a red
stuff.
- at this point i'm completely lost. I tried to add more SMB to my solution, again, a lot of red dust is forming. I don't know how many time I could
do this before nothing more preipitate. My beaker is stirring it as I write this.
I read somewhere that a red dust could be a palladium pollution, but that bothers me, as i rinced everything with nitric acid before dissolving the
gold in aqua regia. I'm not really supposed to have contaminant at this point :/
So here comes the questions:
- what can this red dust be ?
- how could i test it to know what it is ?
- how can I test my solution for gold ?
- if gold is left in there, what should i try to recover it ?
Thanks for reading, hope you'll have an idea about this mess, and have a nice day !
Edit: added a missing step of filtration.
[Edited on 26-4-2019 by Vanry]
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RogueRose
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When you say you got a blue solution was that the color the whole time everything was dissolving? I've found that my solutions start out yellow and
very quickly turn green and can get a very deep green color. Only once did my solution turn blue and that was when I ran out of HCl in the solution
(I think) b/c as soon as I added a little more it turned back to dark green.
I'm still trying to figure out what the green/blue is indicating. Also, did you get any precipitate on the bottom of the beaker when it turned blue -
possibly whitish grey?
Finally, did you find that some of the gold foils or plating dissolved in the HCl/H2O2 mix? I think some of mine did and am wondering if it might
have to do with the other metal chlorides in the solution.
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Vanry
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The blue solution was blue all along the dissolution process. I only got a greenish solution when i was using HNO3 to make my last dissolution, but it
quickly turned blue when the boiling stopped.
Yup I got a grey precipitate, but it look really more like some plastic that has been destroyed by the acidic treatment than a metalic residue, but i
might be wrong. Why are you asking ? Some guess about what it was ?
And to answer your last part, I was very carefull about the amount of H2O2 I added, it was done with 200mL in a bit less than 1l of HCl over a week,
using air bubbling to bring more oxygen in the solution. I didn't felt like I was loosing gold flakes, but I can't be sure as I got no way of knowing.
I'm going to try to test this mess this WE if I got the time. In fact i got an idea, because my ventilation system is suspended above my big beaker
and I wonder if some SO2 could have attacked the metal (mainly aluminium i think), and felt back in my beaker while i was adding SMB. But Aluminium
sulfate is white not red...
sometimes I wish I have some advanced determination apparel... I'm pretty sure TGA could save the day here.
anyway, thank for you'r time, and if someone has an idea, I'm still reading 
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Vanry
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Hi got some news !
I runned some X-ray fluorescence on the red-ish precipitate and the supernatant.
So all my gold is in the water phase, with some copper zinc and iron (but mostly copper). The precipitate is mainly made of iron and copper, probably
into a sulfur-based complex.
Ok so I'm going to filtrate this mess first. But I'll still got copper in the aqueous layer. My teacher, that happened to work in materials and metals
fields told me to apply 0.5V to make the gold precipitate without touching any other metal. First would it work ? Second, any other idea ? because
even if a lab urrent supply is quite usefull, it is quite expansive too...
well, thanks for any advice, and have a nice day ^^
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Gru
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Quote: Originally posted by Vanry  | | My teacher, that happened to work in materials and metals fields told me to apply 0.5V to make the gold precipitate without touching any other metal.
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Has anybody done this?
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Tsjerk
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I think it could work, but I guess it works best on a concentrated solution.
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Calculon
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I think the problem are: too much aqua regia, and too much nitric acid in aqua regia.
It's enough 3-4ml of nitric acid added at 30ml HCl for dissolve few grams of gold, and if the solution is too diluited the precipitation is more
difficult.
View on youtube the videos of Sreetips are very accurate.
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