Here'sa readers digest version of my process for recovering the gold from a lot of P-Pros.
1) As I collect them, I usually try to wash off any thermal grease before putting them in the collection. Any of the grease on the chips will break down in any acid bath and will coat the chips and consequently the precious metals you are trying to reclaim. Incineration will also remove the grease as well as any other oils and would probably do it better but since I rarely pick up more than a few here or there, soap and water seems to work ok.
2) Grind or crush chips to a powder
3) Cover the powder or pieces half way with water and top off with nitric to remove base metals. I usually take a sample after the first nitric bath, rinse it and treat that again with a small dilute nitric acid to make sure the base metals have been dissolved. This is an often overlooked step but the fact of the matter is, those chips also contain silver so if you don't use the nitric, your aqua regia will convert that silver to a chloride which will coat the gold making recovery much more difficult .
4) After the nitric step you can either rinse the chips well or incinerate again to remove the nitric. Again, I only rinse a few times with water. Keep all of the rinse water together becuase it will all have silver in it.
5) once the nitric step is completed I will add HCl to recover the silver as a chloride and move the solution to a drum which contains no PM's and treat that later. You can also recover the silver by cementing with copper but I never had much luck with that and always got a cleaner finished silver by recovering as a chloride. The cementation method does not require as much work and is listed in several places here on the forum.
6) Turning attention back to the chips and after rinsing and you are sure you have removed the base, then you can use aqua regia or HCl/clorox to reclaim the gold. After the first AR bath you might want to again, take a small sample and rinse it clean and treat again with a dilute AR to make sure all of the gold has been recovered and repeat the AR process as necessary.
7) After AR rinse the chips clear of any remaining acid, which might contain dissolved gold and move all of the solution into a pyrex for evaporation.
8) After Evaporation you can precipitate with SMB.
After precipitation I start to seriously clean the recovered gold before a final refining. This includes an incineration, a dilute HCl boil an aqua ammonia boil followed by another dilute HCl boil followed by water rinses before dissolving again and repeating the cleaning process. This consistently yields gold of very good quality. Like I said this is a readers digest description but if you need any elaboration on any part, let me know and I would be happy to go into as much detail as you need.
