I used the aforementioned cu powder for the cementation of pgms from stone removal aqua regia. I think I used a 5 lb batch on about 200 gallons of liquor. Ended up with copper bar that was about 10% pgm content, a good amount of silver and balance copper.
I carefully applied powder to surface of liquid so that it formed a uniform thin layer expecting it to sink as individual particles. I then mixed and tested with stannous. When I achieved a negative stannous, I added the same amount I had added to get to that point and let it sit. It worked wonderfully.
I do now prefer to add cu powder in a glass container, let it settle, then add to waste without filtering. When a good sized chunk of powder made it thru, it was not uncommon to see it actually melt in to the polypropylene barrel some....so I assume I lost some due to adhesion to the barrel wall.
One note, if you have killed off excess nitric with sulfamic, you may see silver sulfide in the melt, so use scrap iron in the crucible to reduce it.
I cannot remember your location, but if you are processing a lot of stone removal lots, you are well ahead to find a consultant to help you get set up with an appropriate technique. It will pay for itself.
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