I have a solution containing mostly gold and other metal bromides and smaller amounts of chlorides derived from a leach using HCL, sodium bromide, salt and an oxidizer. Soon after decantation of the clear/yellow supernatant from the ore residue, I did a test precip on small samples of the liquid using SMB and got decent amounts of chocolate brown precipitant indicitive of precipitated gold. Knowing that the free chlorine in this solution would likely re-dissolve the precipitant, I let the bulk of the liquid sit overnight to let the chlorine dissipate. This morning I did a second precip test and much to my dismay, got almost zero precipitant. I have read that gold bromide is much more stable than gold chloride so does it seem likely that SMB is simply not powerful enough in terms of reduction ability to precip the gold from the gold bromide? If that's part of the problem then why did I get the small amounts of precip yesterday from the same solution? Did the gold chloride shift to the more stable bromide and thus is unaffected by the relatively mild reducing activity of SMB? I'm baffled. Any theories?? :?
Is gold bromide reduction feasible using SMB?
- Thread starter jsargent
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Actually gold (III) bromide is more unstable than gold chloride.
It will reduce with metabisulfite and a lot of other reducing agents.
It will reduce with metabisulfite and a lot of other reducing agents.
Lou said:
Excellent. Thanks Lou. I'll dig around and see if I can find the relative stability data for gold halides. I could have sworn iodide was more stable than bromide and bromide was more stable than chloride. Dyslexia strikes again.
Jeff
Still too much HCL...........perhaps?
ps_jb
Member
jsargent said:
Actually, stability of compound is just thermodinamics value, in solution kinetics play a big role. Also in the case of Red/Ox reactions potential of reductant is more important than stability of reductive spicies.
Right now, I making small batch with NaI/I2 - just first time for me.
Up to now - looks quite strange, anyway - after leaching all gold - I will try to reduce it by MBS - and will write here the results.
As a by bypass - I have a hydrazine, so gold will be precipitated
Lou said:
I'm not imagining it after all. At this site: http://doccopper.tripod.com/gold/AltLixiv.html
I found this quote relating to gold bromide vs gold chloride stability: "Complexation of gold with bromide ions can generate stable Au(I) and Au(III) complexes. AuBr2- is stable between 0.90 V and 0.95 at pH < 9 with [Au] = 10-5 M and [Br] = 10-2 M. AuBr4- is stable above 0.95 V and is stable over a larger set of potential and pH than AuCl4-. In acidic solutions, bromine can oxidize gold in the presence of bromide ions." So... I took that to mean gold bromide is generally more stable than gold chloride. Splitting hairs probably but at least my memory is still intact.
Platdigger said:
Well the pH is about 2.5 so the HCL is not too hot I would think. I could inch the pH up a bit but I don't want hydroxides forming. I'm still wondering what the optimum pH of the solution should be before adding SMB.
It's just bizarre I should get a nice drop one day with SMB and no drop at all the next. Another odd thing I notice is that when I add the SMB the clear golden solution turns orange-red immediately then after a couple hours it clarifies to colorless.
That orange red thing I have seen. I figured it had to do with the bromides being reduced to bromine.
Or would it be "bromate"?
Randy
Or would it be "bromate"?
Randy
Yes you recall correctly.
However, bromide in solution verses bromide as a salt are two different things.
That link you provided has decent information, if very poorly presented.
I think what he means to say is that iodide is a better ligand than bromide, and bromide better than chloride. This is true. It is also true that they are also better leaches in that order, but also increase in cost in that order, as well as decreasing in initial reactivity.
Thermodynamically, pure gold (III) bromide is indeed less stable than gold (III) chloride, but stable to what? Stable to heat? Stable to light? Stable to air, abstraction of hydrogen, etc.? It is conditional upon the form of the compound and the circumstances it is involved in, mmk?
In solution it is a different matter and I shan't argue.
However, bromide in solution verses bromide as a salt are two different things.
That link you provided has decent information, if very poorly presented.
I think what he means to say is that iodide is a better ligand than bromide, and bromide better than chloride. This is true. It is also true that they are also better leaches in that order, but also increase in cost in that order, as well as decreasing in initial reactivity.
Thermodynamically, pure gold (III) bromide is indeed less stable than gold (III) chloride, but stable to what? Stable to heat? Stable to light? Stable to air, abstraction of hydrogen, etc.? It is conditional upon the form of the compound and the circumstances it is involved in, mmk?
In solution it is a different matter and I shan't argue.
