I've been wondering if gold could be easily refined,
could you give more explicit instructions on how to do
this? Like what voltage, what kind of transformer etc. Is
this just a plating bath of cyanide? I know nothing about
plating at this point,
Sigh.
I thought I'd put that idea at least a little to bed, as it
were, already.
Plating/electrolysis, is not generally a practical way to
properly refine small quantities of gold, especially with
cyanide solutions, since both copper and silver will cross
over as well as the gold, at least a little, and plating
voltages required for the three metals are close enough
together that unless you have an exceptionally well filtered
power supply, the variances just due to ripple in the voltage
supply will be sufficient to mess up the purity of your end
deposit, even if the behavior of plating solutions
(depletion effects right at the anode in particular) were not
already capable of doing it. One exception to this
statement, though, would be the seperation of gold from
platinum group metals, which are not cyanide soluable, and
which will drop off the cathode as a sludge.
You CAN, of course, use electrolysis to simply recover
dissolved gold in a cyanide or other solution, and
electrolytic refining does indeed exist. For gold, though,
the usual process is only economical on a large scale.
called the Wohlwill process, it uses gold chloride, not
cyanide, as the electrolyte. Usually, the starting anodes
will be above 850 purity before starting, or the electrolyte
too quickly becomes completely saturated with the baser
metals. In this process, only copper and gold and platinum
group metals go into the solution, with the silver dropping
out as a silver chloride sludge. In the correct
applications, wohlwill processing can give you acceptably
pure gold to market as such, but that application is
generally part of an overall process sequence, where the
exact nature of the impurities is already known. Jewelers
bench scrap doesn't meet that condition, and the results
you'll get from a home attempt will be spotty at best.
By far, the best ways to refine small quantites of gold in
the small shop are chemical methods rather than electrolytic.
The general technique involves first melting the scrap to an
ingot, along with up to four times the scrap's weight in
either copper or silver, depending on the original karat of
the scrap. The resulting ingot should be no more than 25%
gold, and as such, is soluable in simple nitric acid (which
must NOT contain any hydrocloric acid). The ingot is rolled
out to as thin a ribbon as possible in the rolling mill,
coiled up into a loose coil, and placed in a beaker of
diluted nitric acid (usually about 50% dilution, which
actually acts faster than stronger concentrations. Weaker
concentrations can also be used if you wish, and may be
desireable as it slows the rate of reactions, which can
otherwise be fairly vigorous. You want to avoild boil
over...). The low karat ingot dissolves/goes to pieces
pretty completely. Copper, silver, nickle, etc, all dissolve
completely, as does even some small percentage of certain
platinum group metals. The gold does not dissolve, and will
be left in the bottom of the beaker as a mix of sludge and
small flakes, which can be dried and washed and remelted. If
the scrap contained no platinum group metals, and not a lot
of green golds or certain white golds, then the gold
recovered this way can be quite high purity. Usually,
though, to be sure, the resulting gold is then reprocessed by
dissolving in aqua regia. You can also simply start without
diluting the scrap with copper or silver, and go directly to
the aqua regia, which is four parts HCL to one part Nitric
(Concentrated acids, technical grade). In this acid, the
gold and copper and platinum and some other metals dissolve,
while silver remains as a sludge. filter the liquid once
dissolving is complete, and you then have all your gold in
the gold chloride solution, along with other chlorides. The
gold is then precipitated out by the addition of Ferrous
sulphate, or bubbline Hydrogen sulphide gas through it, or
by "cementation" by putting in base metals such as iron,
which dissolve causing the gold to plate out in the same way
that copper plates out in your pickle pot when iron gets in
there.
This process IS actually pracitical for small shops, but is,
as you can guess, rather more involved than is described
here, and invovled some serious safety precautions as well.
Books have been written on just this stuff, and if you really
want to do it, then I'd strongly recommend that you spend
the money on a copy of such a book. My favorite is C.M
Hoke's "Refining Precious Metal Wastes". The explanations
given there are exhaustive and complete. It takes a little
study to learn, but it does work. One advantage to buying
the book rather than trying to cut corners is those pesky
platinum group metals. We use so many of them these days,
that it pays to be aware of how to treat them and recover
them in mixed scrap. It's not actually all that much more
complex, once you're already getting the gold, but when we
send our scrap to the commercial refiners, asking to get BOTH
the gold and the platinum back from a single lot of mixed
scrap seems to cost a whole lot more, and reduce yeilds...
I should mention as well that tool suppliers like Gesswein
also sell commercial small shop refining equipment. There
are two flavors of this I'm aware of. One is I.Shor's
electrolytic cell, which is a specialized plating tank
designed so you can electroplate your gold only INTO the
solution (salt water is used, so as gold dissolves, it forms
gold chloride) rather than across to plate back out of the
anode. This results in a liquid you then precipitate your
gold back out of the same as with the acid refining
techniques, but does not require acids. Nice idea. Not
quite as simple to use as they make it seem though, and you
cannot process filings and polishing scrap unless you can
melt it to a single ingot first. The other "flavor" of
equipment I've seen is simply a nice lab setup of beakers,
air filters, tubes, valves, hot plates, and all that jazz,
designed to do the acid refining process in a closed and
safely controlled setup. More complex looking, but probably
a more versatile setup too. It's big advantage over a home
brew setup is it's air filtration stuff, since doing this in
open beakers over a hot plate generates some seriously nasty
fumes.
Bottom line: There are no really simple, practical,
reliable, safe and simple methods of refining your own gold
that don't require at least a decent working knowledge of the
range of chemistries involved and what you can or cannot do.
Simple dissolution of low karat gold in nitric is simple,
but the thick brown fumes generated are noxious as hell, for
example. do this only with proper precautions, or outside in
a good breeze with no neighbors close downwind. This is not
to say you cannot do it. Just be prepared to do your
homework first. I know several small shop "old time"
jewelers who don't trust their scrap to anyone else but
themselves...
In addition to the book I mentioned, there is a nice simple
explanation of the basic gold refining sequence in the back
of Murry Bovin's "Centrifugal or lost wax casting" book. And
there are several others on the subject that escape my mind
just at the moment.
For all the trouble involved, though, in insuring a
consistant purity of the metal you work with, I'd still
strongly recommend the "three by five" method of gold
refining. This is where you properly and securely package
your gold scrap, which you've carefully kept seperate from
platinum scrap and silver scrap, in a three by five inch
mailing box (or larger, as your needs may require) and send
it by registered and insured mail to the commercial refiner
of your choice, who even after his fees and percentages, is
still likely to return more to you than your own efforts
(minus the time and cost of doing it yourself, of course)
would have yeilded you.
Out of curiosity, years ago, I taught myself to refine my
own gold, silver, and platinum. Got pretty good at it. But
once learned, and then analized for costs, I realized that
unless I was doing a who lot more of it than i was doing, it
just was costing me more money than it saved.
hope this helps.
Peter Rowe