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?
In my area, they recommend taking them to the household hazardous waste facilities. They say:
"It is illegal to put pressurized gas cylinders and tanks in the trash. They can cause explosions even if they are empty."
I'm not sure what the hazardous waste place does with them - I rather doubt they capture the metal.
I'm hopeful that someone else has a route toward recycling.
Use them until they're completely empty, then punch a hole in them with a screwdriver or nail, then they can be recycled as tin. We do this at work with 25lb refrigerant cans. Just make sure it's completely empty. If you're talking isobutane canisters, there is a punch tool available for those. Our recycler says that if they have a hole punched in them, the boiling point of the gas is so low that it will boil off quickly and therefore are not considered hazardous.
Be advised that if you take a refilled single-use cylinder on a public right of way you are in violation of the DOT regulation (49 U.S.C. 5124) that applies to that device. The penalty, if you are caught, can be a fine up to $500,000 or 5 years imprisonment. Please consider switching to a refillable cylinder and recycling your single-use tanks.
Far better than using a device that is designed to be reused?
The valve and shell of that single-use tank are not built for the rigors of refilling or reuse. They belong in the household hazardous waste collection stream, not the waste stream - and if you want to help keep our parks clean they belong in your "I don't use that anymore" stream.
Isn’t the reason for the special recycling to dispose of the remaining contents properly? The remaining contents of propane and refrigerant cans have high Global Warming Potential. One can may be small but wouldn’t a habit of this be harmful?
The special recycling process is for safety and stewardship. Waste handlers and processors regularly use compactors that exert enough pressure to rupture the tanks, causing explosions and fires. The proper recycling process for these tanks includes the extraction and reuse of any residual liquid propane but that work is "dull, dirty, and dangerous".