Place coils of the thin ribbons of gold into a beaker with Aqua
Regina (nitric acid and sulfuric acid combined) and heat until the
Almost. It’s called Aqua Regia (no N.) And the formula is nitric
acid with Hydrochloric acid, not Sulphuric. 3 parts HCl to one part
Nitric. Nitric and sulphuric will not dissolve gold.
Also, what’s with that “Hyper load” with silver to drop out the
gold. Have you actually done this, Nanz? When gold dissolves in aqua
regia, it forms a gold chloride (the reason gold does not dissolve in
nitric plus sulphuric is that gold forms neither a gold nitrate, nore
a gold sulphate). To drop the gold out of the solution, you have to
get that chloride ion to jump to something else. The trouble with
suggesting silver, is that silver isn’t very soluable in hydrochloric
acid (or aqua regia). The reason is that silver chloride is not water
soluable Put silver in the acid, and it forms an impervious layer of
silver chloride on the surface, and the reaction stops. Normally,
when acid refining gold alloys, the acid dissolves the gold and the
copper and the platinum metals, but the silver componant of the
alloys remains behind in the sludge as silver chloride. It’s not
dissolved in the aqua regia.
The process of “dropping” the gold out of the solution by adding a
more reactive metal is, I think, called “cementation”. You can do it
with copper directly, or even easier and cheaper, with steel wool.
The trouble with cementation is that if not every bit of the new more
reactive metal dissolves, then you end up with a mix of pure gold AND
the remains of your other metal. If you use little enough so all the
(steel or whatever) DOES dissolve, then likely, you’ve still got
some gold remaining in the solution. So you have to use more steel
wool than needed, so there’s still some left mixed with the gold. So
then you neutralize all acid, and wash the mix. Now add sulphuric
acid to dissolve the iron without bothering the gold. It’s this part,
where sulphuric is added, that may have led to your confusion with
the aqua regia mix. You can also use nitric to get rid of the iron,
but if there is any trace of chloride iol left (traces of
hydrochloric acid), then adding nitric will not only get rid of the
iron, but it will also redissolve traces of your gold as well,
depending on how much HCl or chloride ion is present. So sulphuric is
safer for this part. But overall, dropping the metal out of solution
with steel wool, while it sounds easy, is a bit of a mess to do.
There’s an easier and cleaner way.
Better than using another metal to precipitate the gold out of the
solution, is to use a liquid agent. Ferrous sulphate, a pale green
crystaline chemical that’s rather inexpensive is a good and common
choice. Dissolve it in water, and add this to the gold bearing acid
until no more gold comes out of solution. Let it settle, decant, and
filter to get your gold (be sure to check the remaining liquid with a
bit more ferrous sulphate just to be sure you’ve got it all)
If you’re original alloy had any of the platinum group metals, then
you’ve got a problem, because the platinum metals will have come out
of solution at the same time as the gold. To prevent this, you have
to remove the platinum first. Oxalic acid is the agent added to
precipitate the platinum metals, without bothering the gold. Then,
after filtering out the platinum sludge, you can recover the gold.
Wash, dry, mix with a bit of boric acid, and melt. Done right, you
can get very close to 24K. Want to get to laboratory standards of
pure gold, then repeat the process, redissolving your precipitated
gold in new acid.
By the way. One final note. dissolving a bunch of gold in aqua regia
is a process you need to do with care. The fumes are toxic as hell,
and can mess up any steel tools in your workshop in addition to your
lungs. Also, the metal dissolving generates heat. So use a large
enough beaker or flask to hold the acid so there’s room for foaming
and the like. If you’re melting rolled out ribbon, as Nanz suggests,
it’s controllable, though you may wish to not add all the metal all
at once, but rather, start with some, add more as the first bunch
dissolves. But if you’re directly adding bench filings or sweeps,
which have a much higher surface area, the reaction can generate so
much heat as to boil the acid. Enough to get it to boil over rather
spectacularly, if you’re unlucky enough. And that, trust me on this,
makes one unholy mess… Fortunately, when I learned this lesson the
hard way, back in the 80s, gold was a lot less expensive, so my
losses weren’t quite as catastrophic. But it was still an expensive
lesson. Oh, and did I mention that the heat of the suddenly boiling
beaker can sometimes crack it? It shouldn’t have, given that it was
pyrex, but dang. It did just that… The plastic bin I had the
beakers in caught most of it, but there’s a house in Detroit where
the soil underneath the back deck probably still has more than a
slight trace of gold chloride in it… I caught most of the overflow,
but not all.
Also, remember where I described how silver does not dissolve well
in aqua regia? If your gold alloy has a high percentage of silver,
you will have the same problem getting the gold to dissolve, since it
can also form that silver chloride layer preventing the acid from
getting to the metal underneath. If your gold does have a lot of
silver, then when you melt it down to make the ingot, add a bunch of
copper to lower the karat as well as the percentage of silver.
All in all, you CAN refine your own gold. Get good at it, and you
can sometimes do it for less money than the pros charge. But in my
experience, you normally are not saving yourself enough money to make
it worth it, and sometimes, inexperience will lead to significant
losses. If you’re fascinated with metal and what you can do, then by
all means, try a small batch just so you’ve experienced it and
learned something abou it. It’s educational, after all. But for best
financial results (including the costs of acids, etc, which
themselves aren’t cheap), I personally recommend that you use a
professional refiner. They earn their fees.
Also, please do not consider this particular posting to be complete.
I no doubt have left out important little details. Do more research
before trying this, if you want good and safe results.