Olddude have you considered pyrolysis of your 15 lbs of mylar then treating the ash to reclaim your silver.

Pyrolysis is essentially heating a material with no combustion during world war two many farmers ran their trucks and tractors from a wood gasifier - wood gas

If you use pyrolysis to reduce your mylars you will be left with ash containing silver. Much easier to work with a pound of ash versus 15 lbs of mylar, Saves time and chemicals.