Guys,
I could swear (from your pictures) that this is what we jewelers call "tumbling shot""
http://www.ottofrei.com/store/product.php?productid=5769&cat=1064&page=1#DetImage
If so, the most economical way to process it would be to use a vibratory polisher with a coarse cutting compound:
http://www.shorinternational.com/TumblingMedia.htm
The heavy cut plastic should work fairly quickly. They last a really long time . The PMs will be removed as fine powder, along with the sanding grit. Filter the wash water and dissolve the metals in the filtrate. The rice can be rinsed in a strainer and tossed, or be run again with itself (alone) to burnish it for reuse in the plating process.
You can make a vibrating polisher pretty simply. It's a barrel, bucket or tub, set on 4 or more springs. An electric motor with a mis-balanced fan (put a bolt in one blade) provides the vibration. Go to a jewelers supply shop, look at one and study it. You will be amazed at how simple the construction is.
Glad I found this post. I have been a refiner and jeweler since 1977, and didn't know this use even existed. I love new sources of PMs!
-G