Gold stripping (de-plating) using sulfuric acid cell
This post being my first, let me start by thanking this forum for being an awesome source of information and inspiration! I was introduced to SM about
6 months ago, and am fascinated by what goes on here. Awesome forum, thank you!
I've also been itching to "get my feet wet" lol... I'm thinking metal surface treatments (plating, etching, oxide/nitride coatings, surface hardening,
enameling, etc.) of metals will be my start...
So, first project I'm planning on is stripping gold plating from some copper/brass parts and recovering the Au. Planning on using the process
described here:
https://patents.google.com/patent/US2185858A/en
This describes a bath of sulfuric acid with specific gravity of 1.65 (~73%). A "small amount" of CrO3 is said to improve the process. The Au plated
part to be stripped is the anode, and cathode is Ni (altho lead seems to be popular in YouTube videos). The patent recommends keeping temperature
below 100F.
The patent describes the operation of this as the sulfuric acid converting to persulfuric (H2SO5, Caro's acid) near the anode, that dissolving the Au,
then as the dissolved Au migrates away from the anode and reaches the regular sulfuric acid, precipitating out as mud.
The nice thing about this process is it's not supposed to attack the Cu and Zn, due to a protective oxide film formed when acid concentration is
above 60%. That is the reason I think this process is the best suited... Not attacking the base metal, making processing of the Au after stripping
relatively easy. Added bonus is not having to deal with Aqua Regia or CN-.
Was planning on doing this in a large beaker or pyrex casserole. 0-60V @ 15A lab power supply. As 100's of watts of heat will be going into the bath,
I'll either put it in a larger container with ice water, or maybe do a titanium tube coil with chilled water and/or glycol. The Ti cooling coil is
probably overkill for this but something I'm probably going to find useful for further experiments along this line.
What do you all think? I have no clue what the CrO3 is actually doing, or how much to add.... and CrO3 is something I'm nervous about playing with,
but also think will be useful down the road for other things (Cr plating and as a reagent)... It also doesn't seem to be necessary, so maybe I'll skip
that for starters?
Other thoughts.... After precipitating the Au mud out, the acid will be contaminated with some Cu, Zn, and Cr if I use that. Wonder if it's worth
distilling it rather than saving it as it is for further Au stripping. Opinions on dealing with the waste will be appreciated.
I'm still at least a week away from getting a fume hood, so not going to do this right away... I refuse to play around with hot acid without a fume
hood any more, last time I did, everything in the shop started to rust.
Thanks!
[Edited on 23-8-2020 by 1KEE]
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