Can old miniatures be recycled?
Yes.
Can any of this usefully be donated to figure manufacturers?
Certainly.
Could they incentivise us to do so by offering a couple of free packs in exchange for a set weight of "scrap" metal?
Absolutely!
The real question, as with most science and technology questions related to industry is not "can" this be done, but rather do we want to incur the costs to receive the benefit?
Costs are not necessarily fiscal. Often times they can be transformed into a fiscal cost such as equating human effort, time, and skill into labor cost. Even so, skills are often rare and time is inelastic, so there is an associated effort cost in finding and maintaining them.
Black Hat Miniatures "hit or miss" above is a good top level description of part of the trade space. In this case "metal" is not "metal". So there is a cost associated with potential incompatibility, possibly up to 100% if Manufacturer X's mix just wont cast in Manufacturer Y's molds (you know … the ones you want).
There's a lot more to quality control than that. We're assuming these are all free of paint and other contaminants. Either you wash them or the manufacturer does, raising their costs, and thus the amount of metal you need to send to make it worth their while to "give" you one figure.
The metal mix is harder, unless you are sending one Manufaturer's figures back to them, it's likely one end has to test the mix. If it is your end, then you need some type of quality control guarantee (certification). I doubt many manufacturers even have this skill set. They buy specific materials and work with what they know, not buy random materials and figure out how to work with them.
Unless you live right near an amenable manufacturer and they will let you on the premises, there is a double-shipping factor. This would easily be rolled into fiscal cost since a manufacturer can likely charge for shipping and receive money without needing to have you pay in extra metal. You can likely pay to send the stuff to them. But you are doubling the figure cost. At least. But the manufacturer is also creating and maintaining a new receiving, handling, storage, inventory, marketing, and accounting process. Some part of that may not be trivial to them or may limit the size of transaction they can support, limiting economies of scale in shipping.
Don't forget zero-product return shipping if "Nope … we can't use these. Try someone else." Or perhaps you could go out and create and maintain a cross-compatibility database.
Considering loss, overhead, and shipping, this could easily run into a 20:1 figure recycling ratio. Or more.
Then there's the reality of these amounts. If you have a lot of never to be painted figures, maybe 20:1 is OK for you. But on the manufacturer's side, the size of the run has to be worth the overhead. For example, the manufacturer may need to fill a whole mold of 20 figures, so at 20:1, you need to send in multiples of 400.
With respect to spears and cavalry straps … how many of those do you need to accumulate to equal one figure?
There's more that goes into it. But this is a decent baseline idea.
That doesn't mean that the recycling idea is a bad one. It just means that figure recycling is most likely to happen if you "know a guy", set up your own casting process, or find a way to make some of the costs transparent … like talking a manufacturer at a show you already go to into taking some stuff from you and bringing something back next year.
That last option brings up a cost we haven't discussed yet. It's entirely possible you and the manufacturer both think 5:1 will work, but then six months later you find out it is 15:1. I am not saying this is you, dear reader, but in such a situation it is possible one side might overreact on the Internet … just a bit. This creates a cost in terms of reputation risk for the manufacturer. Reputation risk is often a barrier to entry.
thanks everyone who helped after her family's recent fire.
