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Author: Subject: Gold precipitation
liquidlightning
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[*] posted on 21-8-2012 at 12:14
Gold precipitation


So, if I have about 3g of 14K gold (copper/gold) dissolved in HCl/H2O2, can I simply place a small rod of copper in to precipitate out the gold as per reactivity series?

[Edited on 21-8-2012 by liquidlightning]
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kristofvagyok
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[*] posted on 21-8-2012 at 13:40


I don't like that method. It works, but it won't be extra pure...

Pass through some SO2 (gas), it will drop out the gold from the solution, nothing else.




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Diablo
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[*] posted on 21-8-2012 at 23:08


How about citric acid, http://mrsec.wisc.edu/Edetc/nanolab/gold/index.html.



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AndersHoveland
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[*] posted on 22-8-2012 at 09:08


An alkaline solution of H2O2 can also be used to reduce HAuCl4

(2)AuCl4[-] + (3)H2O2 + (6)OH[-] --> (2)Au + (8)Cl[-] + (6)H2O + (3)O2

The gold usually separates out in a finely divided state, and appears brown by reflected light and greenish blue by transmitted light. If very dilte solutions are used, the gold sometimes separates out forming a yellowish film on the sides of the test tube.

HAuCl4 solutions can also be reduced with elemental phosphorous, which forms transparent red or blue solutions of colloidal gold. These brilliant colors can only be seen with transmitted light (hold the test tube up against a light source and look through the tube at the light). The solution appears to be only a murky brown by reflected light.
"A short manual of chemistry" p190, August Dupré, Henry Wilson Hake
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AJKOER
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[*] posted on 26-8-2012 at 06:58


Quote: Originally posted by kristofvagyok  
I don't like that method. It works, but it won't be extra pure...

Pass through some SO2 (gas), it will drop out the gold from the solution, nothing else.


Related to Kristofvagyok suggestion is the use of Sodium bisulphite, NaHSO3. Here is the reaction:

2 HAuCl4 + 2 NaHSO3 --> 2 Au + 4 HCl + Na2SO4 + SO2

Noted alternatives, per the same source, confirming Kristofvagyok SO2 are, to quote:

"There are alternative reagents to sodium bisulphite such as oxalic acid, sulphurous acid and sulphur dioxide."

Link: http://www.scribd.com/doc/17149083/Small-Scale-Gold-Refining...

[EDIT] Here is a video link: http://www.youtube.com/watch?v=-JH_0kwZRG4&feature=fvwre...

Another site (link: http://www.finishing.com/345/32.shtml ) notes that the yield using SO2 is only around 75% and to increase the recommendation is, to quote: "To drop a larger % you must slowly heat solution to simmering, just below 200 f (do not boil). Wait at least 1 hour before filtering, re-filter."
_________________________________

Another point is that the Gold was dissolved with HCl/H2O2, which is equivalent to using Chlorine water (effectively HOCl/HCl). Now, per US Patent 4,662,938 ( link: http://www.google.com/patents/US4662938 ): "The solid residue containing the gold is then leached with hypochlorous acid to dissolve the gold, and after separating the solids form the gold-containing hypochlorous acid leach solution, the gold is precipitated by contacting the solution with sulfur dioxide."

The patent gives the reactions as:

2 Au + Cl2 --> 2 AuCl (c)
2 AuCl (c) + Cl- --> AuCl2- (aq)
2 AuCl (c) + Cl- + Cl2 --> AuCl4- (aq)
AuCl4- (aq) + SO2 + 2 H2O --> AuCl2- (aq) + H2SO4 + 2 HCl
AuCl2- (aq) + SO2 + 2 H2O --> Au (c) + H2SO4 + 2 HCl

which implies to me the need for an excess of SO2 and sufficient reaction time to maximize gold yield.


[Edited on 26-8-2012 by AJKOER]
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