Probably shouldn't put "empty" propane canisters in the trash and even if it's safe when totally empty, that's a lot of metal to just throw away, and I'm sure the recycle bin isn't ready for them either. What should be done with them? Does (or could) REI or any other outdoor retailer offer a recycling path?
My hiking buddy has a JetBoil stove that uses canisters of several sizes, and after we get back from a trip with empties he takes them out to a firing range (he's a former USAF PJ) for some target practice, then puts the punctured canisters into the recycling bin. A hammer-driven screwdriver or punch would have the same effect.
It really doesn't answer your questions, but this item is a refillable propane canister and it's legal to refill it.
Got some at REI a while back but they don't carry them anymore I don't think.
You can still buy them elsewhere.
There is a new 1lb cylinder exchange program in California that uses the Flameking cylinder with retail sales and exchanges to reduce waste. You can get it from local stores around the SF Bay - it's called Little Kamper propane. Getting convenient access to a safe alternative to disposable tanks has got to be the future of 1lb tanks.
While I dont believe it's technically legal, 1LB bottles can be refilled from a larger (ex 20lb) bulk tank. Lot's of vids on youtube and the attachment available on Amazon. It works well and saves $$. Keeps those bottles out of landfills.
Classic hillbilly tech. The Department of Transportation, at least, doesn't approve. Indeed, if that propane got loose inside a closed vehicle, it could be gnarly.
Inverting a 20lb tank to transfer liquid propane into a DOT 39 (non-refillable) container is a classic example of how YouTube and frugality can create a toxic cocktail. The manufacturer of the disposable tanks clearly disclaims the safety of their devices on the tank label and none of the refillable tank manufacturers suggest or instruct the use of their tanks as liquid propane dispensers.
Pressurized liquids that vaporize into flammable gas at 270x their volume are dangerous enough to warrant a thoughtful moment before choosing a safer solution.
Find another way to save money and switch to DOT 4BA (refillable) tanks.
It's a hack that has worked well for me and others I know. Obviously some care and common sense has to be taken in when/where one does the transfer i.e.. not near any open flame.....like the care one does when using propane in general. The "legit" refillable kit requires the inversion of the large propane tank as well. You can buy the kit including just a single 1lb tank for $49. Or, you can buy a three pack of 1lb's for around 7 or 8 bucks and the adapter for around $8. I'm not seeing a difference between the two different 1lb tanks but, if you do, I'd love to know what it is.