A final sonication with 70% hot sulfuric acid can be used to clean up the gold markedly in purity.
Lou mentioned this above but it seems nobody bit on it so maybe I should explain the process and the value of the technique in cleaning up gold that isn't quite up to standards.
About 5 or 6 years ago there was a discussion amongst the moderators about getting high purity gold in a single pass. Lots of suggestions were discussed but most involved chelation and that always throws a monkey wrench into waste treatment so, for me, it isn't worth the effort. Especially if there is something simpler.
I mentioned a method I had used which I called pulping. Basically it involves tumbling in a violently agitated tumbler, gold sponge and adding 25% concentrated sulfuric acid to 75% hot distilled water. I was working on a large project and looking to gas gold in a single pass from direct aqua regia digestions and get a purity of .9999. The way I was doing it was to gas the solution with SO
2 and stop before the drop was complete. The gold that drops first is cleaner and the gold assayed high first drop. The balance of the gold in solution was filtered off and the gassing was completed, resulting in the balance of the gold which required a second pass or re-refine. I put this gold aside and continued work on the first drop high purity process I was hired to implement. I have worked with gold all my adult life and because of that I have a sizeable mental block assigning value to piles of in process gold. So while I was blissfully playing with the single drop process, a pile of gold was accumulating which was worth well over a million dollars, probably closer to 2 million. The boss was apparently watching and noticed, after all it was his money. He came to me and asked me to please make that gold sale-able. Like yesterday if possible!
I took a sample of the sponge before treatment and then used the pulping process in a large oblique barrel with a blade mixer and did the sulfuric water thing and ran it for 15 minutes. Filtered, rinsed, and resampled the gold. The gold before the process was .999+ and the gold after the process was .99999. This was run in an ICP by difference. I had used the process before and knew it worked but never actually quantified how effective it was in the lab. Although the first few runs came out 5 nines, I would be satisfied claiming 4 nines which we consistently hit in production.
Since this was the gold dropped at the end of a SO
2 drop it was the dirtiest of the gold dropped. I realized we could drop it all with gas, forget the extra filtration step, and use the pulping to process the entire lot and bring up the purity. This process was effective at bringing .999 gold up to .9999 gold easily without major waste treatment ramifications.
Lou had never heard of this before and since he has a well equipped lab with all of the big boys toys he gave it a try. The thing is he didn't have the oblique barrel and he didn't have the gold quantity to require the big barrel anyway so he used ultrasonics to provide the agitation. He used the ultrasonic to process lots up to 2 kilo's or so in size of sponge if I remember correctly. And his testing also verified the usefulness of the process.
So now, with a more detailed description of sonification as Lou called it, a good rinse protocol, followed by the sonication process, this may be the answer to cleaning up your gold sponge before you melt it. You need to do this in a beaker because most ultrasonic cleaners will not withstand the hot sulfuric. And I think the time requirements go up with an ultrasonic as well, maybe Lou can refresh my memory on that detail.
