I got the method 2 years ago from a sterling refiner processing 30,000 ounce batches, we scaled down to 1000 ounces a day because we melt 350 ounces of karat gold scrap in with the 1000 ounces of silver (essentially inquarting) and digest the alloy in 50% nitric. The gold doesn't react and is filtered out and cleaned up in another nitric dissolve and melted with a borax, soda ash, manganese dioxide flux which scavenges any remaining silver and produces a 999.5 fine bar consistantly. The silver is processes as described and comes out 9999+ every time. The gold assay comes from a JM assay and the silver I assay with a Volhard titration in house because we sell the silver ourselves. We take care not to include any bars of gold with PGM's in the process because it will effect the gold purity, as it too will not dissolve completely. There is some on line literature about the method but it is sketchy and I go by the procedure I've outlined above. As with any process, familiarity will allow you to know when the drop is done just like you can look at a gold precip and know by looking that you've gassed enough. (of course I always have the guys check with stannous just in case) Recovery from the acid is complete and accountability is excellent. Just watch the pH, if the solution is stable at 1.5 when you start adding the reducing solution, and the reducing solution has been reduced to 4, you're OK. We make 55 gallon batches of reducing agent so it's on tap when we need it, it has never gone bad. One more thing, the digestion gets hot, which I'm sure you know, so heavy walled 316 stainless tanks hold up the best. Polypro is fine for the mixing and storing the sodium formate solution. Once you see the sponge this drops, you'll be hooked.
