Opt to Act - Simple Action #35: Find a moment this week to be awed by nature.
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This can be confusing, I just want to add a couple of things.
The reason you need to puncture them is because when the canister no longer works it isn't really empty. The canister has two gases in it. One is a propellent, the other is the actual fuel the stove burns. It runs out of propellent just before it runs out of fuel. So when the canister stops working, there is still a little bit of propane inside it. With that bit of propane, they can't be recycled, which is why you have to make a hole, to vent the propane, so they can be recycled. We are talking about a minuscule amount of propane.
About ten years ago REI saw this problem - in some stores we had machines that would puncture and crush cans for recycling - but the machine was expensive. So we challenged our vendors to come up with a device to safely, easily puncture a can. Jetboil met our challenge and introduced the Jetboil Crunchit.
The crunchit screws onto the threads on top of the canister, and then has a punch like a can opener. This alleviates the risk of creating a spark, with a nail, or an ice axe - agreed, the most manly of methods! - and igniting the remaining fuel. If you do this when the canister is "empty" there is so little fuel the risk is very low. Do not try and puncture any fuel canister when it can still power a stove.
So when a canister is "empty" it still really has a little fuel in it.
I'm glad you are all recycling your canisters!