Hoke didn't make it sound all that complicated. If you have the PDF, just search "oxalic", and you can hop right to the topic. Or just read below. Check it out, and tell me what you think.
PRECIPITATING GOLD WITH OXALIC ACID
Oxalic acid is often used to precipitate gold from aqua regia solutions,
but since it is relatively expensive, it is usually saved for occasions
where the purpose is to obtain specially pure gold; that is, when the gold
you start with is already nearly pure enough for your purpose. Thus,
suppose you had your gold content down to 6-k or less, had dissolved
out the base metals with nitric acid, and had obtained a gold residue that
was almost pure enough to be used again. This in turn you had dissolved
in aqua regia, and you are now ready to re-precipitate it. This
aqua regia solution therefore contains only small amounts of other metals.
This is a case where oxalic acid might well be chosen as the most suitable
reagent.
You will observe that copperas and sulphur dioxide were employed on
solutions that contained (in addition to dissolved gold) much dissolved
copper, zinc, and possibly nickel and other metals.
Oxalic acid has this advantage over copperas: it does not introduce any
iron or other material which, if melted in with the fine gold, could hurt
its quality. The disadvantages are its cost, and the fact that when using
it you must heat your solutions, both the aqua regia solution and the dissolved
oxalic acid that is added to it, preferably to the boiling point. Hot
solutions, especially when bulky, are awkward to handle, and may be
dangerous. For these reasons, as we said, oxalic acid is usually reserved
for jobs where the quantity of solution is relatively small, or when you
are working with a solution that contains little or no dissolved base
metals.
The method of using it, therefore, will be described in Chapter XV
on the special purification of gold. You should become acquainted with
this method, even though you do not plan to use it regularly.
***************
For each Troy ounce of gold you will need at least a Troy ounce
of oxalic acid crystals. If you failed to expel all the excess nitric
acid, you will need more.
Dissolve the oxalic acid in hot water. Add a little ammonia, say
a teaspoonful or less to an ounce of oxalic acid; add slowly to avoid
spattering.
Do not use too much ammonia the mixture should not smell of
ammonia after being stirred. If it does, add enough additional
oxalic acid to kill the ammonia smell. If you have litmus paper,
use it here, and let the mixture show a slight acid reaction. Have
the mixture in a big beaker or jar that will withstand heat.
Next, heat your aqua regia solution to boiling. Also have the
oxalic solution hot boiling if possible. Pour the gold solution
slowly into the oxalic acid solution. Gold comes down at once with
hot oxalic acid, but takes hours or even days if it is cold.
Some people pour the oxalic into the gold; we think it is better
to pour the gold into the oxalic.
There will be bubbles and fumes, so work slowly or the liquids
may boil over when they come together. If you failed to expel all
the excess nitric acid, brown fumes will appear here; if this happens
you may have to add much more oxalic acid in order to kill all the
nitric acid.
Let the mixture stand for some time, keeping it hot if possible.
The gold comes down as bright clean crystals, often very beautiful,
which usually settle promptly.
** test with stannous to make sure you have all the gold..
