kt5000
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Recovery of gold using HCl + H2O2
I tried to UTFSE, but most of the posts seemed to address other issues later in the process. I'm a beginner, but having a blast tinkering, so please
bear with me..
I have a handful of gold-plated computer scrap. I don't have easy access to aqua regia, so I'm trying an alternate method using HCl + H2O2 (link
below). I have 32% HCl and 3% H2O2, mixed in a 2:1 ratio.
My understanding is that this mix should dissolve everything except the gold. HCl doesn't dissolve copper, but the H2O2 seems to allow it. (does the
H2O2 act as a catalyst?)
I ran a couple batches and got a bunch of green/brown liquid, a bunch of gold flakes, and some solid black stuff. I'm thinking the liquid is a mix of
copper chloride, aluminum chloride, maybe some tin chloride(?). Now that I made it, is there a good (safe) way to dispose of it?
On the solids, I'm a bit puzzled about the black stuff. I tried reacting it with 32% HCl and also in 33% H2SO4, but nothing happened. I'm not sure
how to go about separating it. Aqua regia might do it by dissolving the gold and leaving the black solid to be filtered out, but I don't have aqua
regia handy. I read something about a chlorine bleach mixture being able to dissolve gold.
Reading back through the link below, I may have found something:
"When an AP batch is done, the reason why gold flakes remain is because the oxidizer is all used up, and gold chloride displaces base metals and gets
deposited as a black powdery film"
So that black stuff may be gold chloride, which would explain why HCl won't affect it.
http://www.scribd.com/doc/29035575/Peroxide-Acid-Method
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woelen
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Gold chloride is not black, but orange and its solutions are yellow.
HCl + H2O2 can dissolve gold as well. If you want to recover your gold, then you need H2O2 of higher concentration. You mix the H2O2 (e.g. 20%) one
part with two parts of your 32% HCl and put the metal scrap in the mix. All metal stuff will dissolve, the gold also.
After the reaction, you boil for a while to destroy remaining H2O2 and then you add bisulfite to precipitate the gold.
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kt5000
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I owe you a beer, woelen, if I ever visit the NL. Thanks for the clarification. I just got some MnO2, so I'll test that concentrated H2O2 I'm making
and see if I reached 20% 
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chemrox
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That was really nice. Thanks woelen.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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kt5000
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What's a readily available sulfite that I could use to precipitate the gold?
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barley81
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Some stump removers are sodium metabisulfite, in particular, Bonide Stump-Out. I've used it before, and it behaves like sodium metabisulfite, though I
am not sure about the purity. If you're lucky, Home Depot will have it in stock. Good luck with gold refining!
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Arthur Dent
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Another source of Sodium Metabisulfite (or alternately, Potassium Metabisulfite, both work equally well) is to go to your local Wine making/Beer
making supply shop. There you'll find high quality (food grade) Na or K Metabisulfite as well as many other nifty chemicals.
When you add a saturated Sodium Metabisulfite solution to your dissolved metal chloride solution, it will turn a dirty grayish brown almost opaque
color. That's your gold separating from the solution and turning into fine colloidal particles. Let the liquid settle for several hours. If all goes
well, there should be a brown "mud" at the bottom of your vessel: that's your gold.
Robert
--- Art is making something out of nothing and selling it. - Frank Zappa ---
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Fleaker
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If the solution is concentrated and free of excess oxidizer, you will get a nice light fluffy brown precipitate. Concentrated solution of Au(III) are
very orange-yellow. Bear in mind that gold ion is rather toxic and that you should use a container that doesn't allow for misting.
Typically, very fine colloidal gold from precipitation means one of two things:
solutions with excess base metals and tramp elements (Se/Te are also precipitated)
too dilute a solution
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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Pickardjr
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Would HCL/H2O2 dissolve platinum. I been thinking about trying to recover Pt from old spark plugs.
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Arthur Dent
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I don't believe it can, only Aqua Regia (25% Nitric Acid/75% Hydrochloric Acid) can dissolve Platinum, and then it does that quite slowly.
Robert
--- Art is making something out of nothing and selling it. - Frank Zappa ---
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shaheerniazi
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If you want to see some incredible video's of gold and platinum dissolving in Aqua Regia, look at these amazing videos:
http://www.youtube.com/watch?v=APxL87X92t4 for platinum
http://www.youtube.com/watch?v=XoqU1GfIOkI for gold
http://www.youtube.com/watch?v=AHsPkoO4wKw for gold recovery
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Random
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You might also be interested in paladium frim MLCCs if you are into this.
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kt5000
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The Bonide Stump-Out worked great. The solution turned a brownish-red and settled after about 2 hours, leaving the brownish mud and relatively clear
liquid. Adding the sodium metabisulfite instantly bubbled out what I assume to be pure chlorine gas. I wasn't expecting that and got a couple
breaths of the stuff. Ouch! I'll be sure to do that step outdoors next time.
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kt5000
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I found an explanation for the gasses..
Sodium metabisulfite added to water produces sodium bisulfite:
Na2S2O5 + H2O --> 2NaHSO3
Sodium bisulfite + gold(i) chloride yields elemental gold, HCl gas, and sodium bisulfate:
3NaHSO3 + 2AuCl + 3H2O --> 3NaHSO4 + 6HCl + 2Au
Damn HCl gas
[Edited on 2-12-2013 by kt5000]
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Random
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Not HCl but probably SO2
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kt5000
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I thought SO2 seemed likely too, but Wikipedia says SO2 has a rotten-egg sulfur smell, and there was nothing like that. It smelled just like a whiff
of 32% HCl vapor.
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DraconicAcid
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Quote: Originally posted by kt5000  |
I thought SO2 seemed likely too, but Wikipedia says SO2 has a rotten-egg sulfur smell, and there was nothing like that. It smelled just like a whiff
of 32% HCl vapor. |
No- H2S has a rotten-egg smell. SO2 is a different kind of nasty.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Random
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Yeah SO2 could be described as sharp smell more than rotten egg one. Matches could be more easily used for decription of the smell.
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Fleaker
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Adding any sulfite salt to an acidic solution can (and often does) liberate free SO2 gas.
SO2 is what is reducing the gold. Gold (III) oxidizes that to sulfate.
If you were to bubble SO2 through a chloroauric acid solution in HCl, you will end up with a sulfuric acid solution.
HCl/Cl2 does indeed dissolve platinum; it is preferred to use that over aqua regia unless the platinum is a bulk metal because aqua regia has much
faster dissolution rate. So if your Pt is powdered, HCl and H2O2 (or chlorate) will work fine. The solution should be simmered to remove dissolved
chlorine.
If aqua regia is used, it is required to evaporate the solution to a syrup, adding HCl (and perhaps a small bit of NaCl to prevent decomposition) to
destroy residual nitric acid. Failure to do so will leave considerably more Pt in the raffinate/filtrate than what would be recovered if the nitrates
were not removed.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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CrossxD
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OK I can dissolve gold in HCl with H2O2 and I can get gold back using copper can't I?
2HCl + H2O2 + 2Au -> 2H2O + 2AuCl ?
AuCl + Cu->CuCl + Au?
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blogfast25
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| Quote: Originally posted by CrossxD | OK I can dissolve gold in HCl with H2O2 and I can get gold back using copper can't I?
2HCl + H2O2 + 2Au -> 2H2O + 2AuCl ?
AuCl + Cu->CuCl + Au? |
Gold is trivalent. Copper is bivalent (in this context).
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aga
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Bivalaphobe. I knew it.
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