aqua regia--extract gold from?

any ideas on how to extract gold from aqua regia?
i assume just diluting it would work as would neutralizing it also.
iwould assume that the gold would just pdrecipitate out

thanks
peter


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On Tue, 04 Nov 2003 19:15:59 -0800, in rec.crafts.jewelry ilaboo
wrote:

any ideas on how to extract gold from aqua regia?
i assume just diluting it would work as would neutralizing it also.
iwould assume that the gold would just pdrecipitate out

thanks
peter

Simple dilution doesn't do it, as the gold is now in the form of gold chloride,
which is water soluable. What you have to do is a process called cementation,
which, as you suggests, precipitates it out. It does it by replacing the gold
ion in the chloride, with a baser and therefor more reactive metal. Like Iron,
for example.

If you simply add steel wool to the acid, it will slowly react, at first just
using up remaing acid. Then after free acid is gone, the iron replaces the gold
in the chloride, and the gold then deposits back out. eventually, once the
process stops, the solution is one of iron chloride, and the gold is now plated
loosely all over whatever remains of the steel wool, or as a sludge on the
bottom It can be filtered, and after being very careful to remove all remaining
traces of acids or chloride compounds, you can then remove the remaing steel
wool by dissolving it in a straight acid like straight HCL, or better,
sulphuric. The results will be decent, but probably not highly pure, as it's
difficult to remove absolutely all the iron. Also, if you originally dissolved
gold alloy that contained any platinum group metals or copper, these will also
have precipitated out. The copper can be redissolved at the same time as the
iron, if you choose the acid appropriately, but the platinum group metals will
be difficult to seperate. Normally, one precipitates them first, with oxalic
acid, before throwing down the gold.

A cleaner and more selective precipitating agent is to use a solution of ferric
sulphate. Since that dissolves, it doesn't leave metallic iron sitting around
to contaminate the works, and it's a more effective precipitant as well, since
the sulphate ion also acts on this, though I don't recall exactly how. In
large refineries, the gold is usually precipitated out of the solution by
bubbling hydrogen sulphide gas through the solution. This is not for the small
operation however, since though tanks of H2S are cheap enough, the gas not only
smells of rotton eggs, but is also highly toxic. You need a fume hood and
appropriate scrubbers to use that method. Like the cementation method, the
ferric sulphate will also throw down any platinum group metals present, so if
they are there, you'd want to precipitate them first with oxalic acid. But the
ferric sulphate does not affect the copper remaining in the solution. That
stays there. So you get only the gold and platinum group metals in the
precipitate.

Hope that helps.

Peter Rowe
..

in article , ilaboo at
wrote on 5/11/03 3:15 am:

any ideas on how to extract gold from aqua regia?
i assume just diluting it would work as would neutralizing it also.
iwould assume that the gold would just pdrecipitate out

thanks
peter


Add Sodium Metabisulphite and the gold will precipitate out as a yellow
cloud.

NOTE sodium metabisulphITE NOT sodium Meta bisulphATE.

Sodium MetabisulphATE does not work but as its only one letter different
even chemical supply houses can make a mistake and send the wrong stuff.

I once was given a very good bottle of wine by a gold refiner for spotting
this mistake.

Sodium MetaBisulhite is used in home winemaking for sterilising bottles so
you can buy small quantities at home brew shops.

Just keep adding Sodium Metabisulphite and stirring it in with a glass rod
gently until no more gold precipitates out.

Then filter the aqua regia to recover the gold and melt it down.

Thats all there is to it.

Hillary

In article , Peter W. Rowe
pwrowe@ixDOTnetcomDOTcom wrote:
A cleaner and more selective precipitating agent is to use a solution of
ferric sulphate.

Peter,I think you mean ferrous sulphate (green vitriol), a reducing agent,
like sodium bisulphite/metabisulphite.

Ferric salts are oxidising agents, which is why ferric chloride is used to
etch copper.

I am always interested in your posts and have learnt a great deal from then
in the last couple of years

Thank you - G.H.Ireland

--
igor
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Acorn RISC OS4
_____________________________________________

On Thu, 06 Nov 2003 02:04:01 -0800, in rec.crafts.jewelry Mr G H Ireland
wrote:


Peter,I think you mean ferrous sulphate (green vitriol), a reducing agent,
like sodium bisulphite/metabisulphite.

You're quite right. Ferrous it is. I should know better than to try and
remember chemical names late in the evening... (grin)

Peter