metalresearcher - 21-6-2011 at 03:46
Once I got rather pure gold from discarded 14 carat jewelry by:
- Dissolve in aqual regia
- Prepare an FeSO4 solution
- Add the solution to the brownish yellow dissolved golden solution
- A brownish precipitate will appear (Au3+ + 3Fe2+ => 3Fe3+ + Au )
The brown slurry can be filtered out and rinsed, That is rather pure gold.
Or are there better methods ?
cyanureeves - 21-6-2011 at 04:31
i use sodium metabisulphate instead of FeSO4 but your method is probably better because you can make it yourself. i evaporate the aqua regia then add
extra hcl acid and re-evaporate to drive out the nitric acid.i have never added urea to the aqua regia like some people do. my precipitate has always
been brown but some can get a purer yellow.
Mixell - 21-6-2011 at 04:58
How about reducing with aluminium?
Shouldn't solid gold flakes/ particles appear just like when copper(II) is reduced with aluminium?
theflickkk - 21-6-2011 at 05:12
I doubt using aluminium would be a good idea since it would probably also precipitate gold together with other metals used in the alloy.
Mixell - 21-6-2011 at 05:26
Ah,yes, I misread it to be 24 instead of 14 carat, what are the other metals in the alloy?
Jor - 21-6-2011 at 06:14
When I purify gold, I dissolve the alloy in aqua regia (4:1 37% HCl:65% HNO3), and once all is dissolved I evaporate the liquid intill it's syrupy,
add some 6M HCl, evporate again and repeat this untill no more brown vapour develops above the liquid when it is not boiling. Next I will disute the
concentrated solution with 2-3M HCl, heat to 70-80C and add a solution of sodium metabisulfite. Let it react for a while (say 30 minutes), as I have
found that after filtering the liquid after a few minutes some gold still forms in the filtrate. Next you can proceed to isolate the Pt and Pd as the
ammonium salts of the MCl6(2-) ions (M=Pd or Pt).
Fleaker - 21-6-2011 at 20:37
If you are taking gold from jewelery and want it to be of 4N purity for use in your experiments, then you will need to use a selective precipitating
agent.
I use sulfur dioxide for precipitating gold that has no palladium present. Palladium is present in dental alloys and most European white golds.
Sulfites and SO2 are notoriously bad for alloys containing significant palladium. That is when copperas (FeSO4), oxalic acid, or hydrazine sulfate (in
acidic milieu) are indicated.
10 and 14 K gold do not dissolve well in aqua regia at all because a crust of AgCl forms on the surface. To avoid this, one can alloy up your karat
scrap with gold to 20+ K and digest in aqua regia or alloy down with silver and part the base(r) metals from the gold with simple 30% nitric acid
solution.
Usually, jewelery is melted with three times its mass in pure silver to alloy it down. The goal is to get it around 4-6 K gold content (about 15-25%
Au by mass); this is called inquartation. It is not done commercially anymore, but for the purposes of the home chemist, it is the best thing to do as
PGMs ride along with silver, and the copper is mostly separated. This alloy is poured into water to generate what is called "corn flakes" (based on
their appearance), which are then digested in hot nitric acid to solvate the silver, copper, and other base metals and leave behind gold sponge.
The solution is filtered off and copper is used to precipitate the silver as 99% pure silver metal from the filtrate. The gold is then boiled in
nitric acid until the solution imparts no more color, the acid decanted, and then hydrochloric acid is added. At this point, one may dispense with the
nitric acid and use hydrogen peroxide or sodium chlorate (this simplifies the reduction step). The gold will all go into solution. Take this yellow to
deep orange solution and dilute by adding ice cubes. Filter it to remove any formed AgCl. This cold solution is then treated with sodium sulfite to
drop the gold. The light brown gold powder is then boiled in HCl, and rinsed well in the filter with conc. ammonia. It should be 99.95% or more in
purity.
5N pure gold can be obtained by taking the gold produced as above described and redigesting it in highly pure aqua regia, carefully evaporating, and
then diluting and precipitating with oxalic acid at 90*C to which enough ammonia has been added to make the solution pH around 6-6.8.