At the risk of digressing:
On your site in the section on making CNC controller boards you wondered about rejuvinating ferric chloride etchant. Bell labs did just that:
- Drop in a couple electrodes.
- Apply a small current (suitable for electroplating) between them. (A couple volts and a current-limiting resistor - say, a small low-voltage light bulb.)
Result:
- Electrolytically refined copper at the negative electrode.
- Ferrous->Ferric ion conversion at the positive electrode creating rejuvinated solution.
Repeat tens of times before the stuff gets too cruddy to use. BIG drop in the amount of hazardous waste you need to dump - and really pure copper (rather than horribly poisonous copper ions in solution) as a by-product.
I think they used graphite for the positive electrode. Try a couple graphite rods for starters - copper will plate out on one.
Keep the voltage low enough and things should just stop when the copper is all plated out and the solution is back near full strength, rather than disintegrating the positive electrode, generating chlorine gas, etc. (Try this outdoors first, though. B-) That way if it screws up you're only out some already-waste solution, rather than your lungs.)
It's tempting to try to do it in the etching bath for a continuous process but I bet the current would make the etching uneven. You could do it with a pump to a separate rejuvinating chamber - but that's WAY overkill for anything short of a large-scale commercial board house.
Haven't tried this myself so far. If you decide to be the guinea pig let us know how it works out.
