I recently read that sodium thiosulfate can be used for gold and silver leaching in much the same way as cyanide, but with no worries about toxicity. This intrigues me: aqua regia is a pain to work with, and cyanide is too toxic for an amateur, but thiosulfate is already one of my favorite reagents - cheap, nontoxic, and has so many uses. I thought I'd ask here just in case anybody has any experience with thiosulfate leaching or knows any good resources.

As a little test yesterday, I mixed up a little silver nitrate with NaCl to generate AgCl precipitate, then added an excess of thiosulfate. Presto - it all dissolved in a matter of seconds.

I do amateur chemistry as a hobby, and I bought around 60 lb of paydirt for gold extraction several months ago which I haven't gotten around to experimenting with. Just so I understand the process, let's pretend that it's all composed of minerals that leach well in thiosulfate.

So here's what I want to do. After ball milling the paydirt into as fine a powder as possible, I'd like to try using a thiosulfate solution to leach out the gold, silver, and anything else that forms a complex with thiosulfate. But I don't know anything about the process. I'm going to dig through Google Scholar and try to find some good references about it before doing anything, but at the moment all I know is that it also requires some ammonia and maybe a small amount of a copper salt. I'm not sure of the concentrations of each, and the few papers I've skimmed so far seem to have differing opinions about it.

Some questions I have right now:

-What concentration of thiosulfate would work best? What about ammonia, and is Cu(II) necessary as well? And am I missing anything else?

-What pH does this work best at? Is ammonia the only base I need, or should I also add Ca(OH)2 or something similar? Can I substitute an ammonium salt for ammonia, provided I keep the pH near optimal levels with Ca(OH)2 or a different base?

-What metals dissolve in thiosulfate besides silver and gold?

-Once I've filtered off the insoluble stuff and just have the thiosulfate solution, how do I precipitate the metals out? I have several reducing agents including sodium metabisulfite, sodium borohydride, and zinc powder. I tried borohydride for my AgCl experiment, and it did precipitate the silver but also reduced the thiosulfate to H2S and elemental S, which is obviously less than ideal and suggests it's too strong a reducing agent. I'm going to rerun it with metabisulfite and then zinc and see what works best.

After I finally get a crude metal precipitate, I'll refine it again with nitric acid and then aqua regia in order to separate gold from silver, and both from base metals.

Does anyone have any knowledge or resources about thiosulfate leaching, or suggestions about my process? If so, please chime in! I'll post anything useful I come across in the meanwhile.