Consider setting a price in your coins per weight for each metal type.
Sounds interesting and can be developed further if there is possibility to change scrap for coins and vice versa.
What about mining and this token's specifications?
This is fricking cool. A singularly-purposed idea to ascribe arbitrary value denomination, via cryptocurrency, to items of arbitrary value as junk: market value, salvage value, opportunity cost of storage space, component value...
One challenge will be for you to figure out shipping, which is NOT arbitrarily priced, and is expensive especially in relation to the junk item being shipped. If you go that route.