Propane tanks are naturally recyclable since they're made from steel. However, recyclers can be reluctant to take these tanks since they're pressurized and can explode if they still contain fuel. Coleman introduced the Green Key program to make its canisters more easily recyclable, but discontinued the program in 2010. Follow the recycling process for either the general canisters or the Green Key canisters, which carry a bright green key and a label reading "Green Key." Not all recyclers take propane tanks; you may need to contact several before you find a match.
General Coleman Propane Recycling
Empty the propane canister first. Hook it up to your Coleman stove or lantern and light the device, then run the device until there is no more fuel and the flame sputters out. Your propane tank must be empty before recycling.
Search Earth911 for propane tank recyclers near you. Enter in "Propane Tanks" and your ZIP code to find local recycling options. Also check for scrap metal yards (by entering in "Scrap Metal") near you and contact them via telephone to determine whether they take propane.
Call local recyclers to determine their open hours. Let them know you have a Coleman propane tank to recycle and ask how you need to prepare the canister for recycling. As Earth911 notes, some recycling centers make you do something to "prove" the tank is empty (such as punching a hole in the tank).
Prepare the tank for recycling following the directions given. For example, saw open the propane canister if directed by your local recycler.
Take the tank to the local recycler or scrap metal recycler once its suitable prepared.
Recycling Green Key Canisters
Empty your Green Key Coleman canister in the same manner as other Coleman canisters: by hooking it up to your lantern or camp stove and running it until there is no more fuel and the flame dies out.
Insert the male end of the bright green key into the black cap at the end of your Green Key canister and push the key firmly into the cylinder. This indicates your tank is empty and is the only preparation you need to complete for recycling.
Find local propane recyclers near you by searching Earth911 by entering in "Propane Tank" or "Scrap Metal" and your zip code.
Call matching recyclers to find out whether you can drop off Coleman Green Key canisters and when they are open. Per Coleman Recycling, not all recyclers accept these canisters so don't assume.
Bring the empty canister to a recycler during open hours.