Make sure the propane canister is empty by attaching it to a stove or lantern and operating it until there is no trace of fuel remaining.
Obtain Coleman's "Green Key Tool" at a retailer that sells camping equipment. In some instances, these plastic devices are available at no charge when Coleman canisters are initially purchased. Push the "Green Key Tool" into the valve at the top of the cylinder. Allow the canister to sit for at least two minutes before attempting to recycle.
Take the canister, with the "Green Key Tool" still inserted in the valve, to the recycling center and explain that tool is proof that the canister is completely empty and safe for recycling.