When disposing of any vessel that contains flammable or dangerous materials, always follow your local/national safety regulations, this includes the disposal of propane tanks. While it may actually be possible to recycle the container, some areas may not have suitable recycling facilities, and therefore the container may have to be taken to a salvage yard or local authority dump to ensure proper disposal. It is imperative that you do not throw propane tanks into a skip or bin as other rubbish can puncture or crush the container which may result in an explosion.
The propane tank should be emptied through normal use i.e. cooking with a gas bbq. The next step is to call potential disposal sites and consult the phone book/web for propane tank disposal companies and scrap metal yards. The disposal companies may come to you and collect the tank; however they usually charge a disposal fee. The scrap metal yards will require the container to be delivered, and many yards will refuse containers of dangerous materials.
You can also try taking your old propane tank to a diy store, there are some stores that will take your tank when you buy a new one.
If you are transporting the tank, place it tank on the floor of the vehicle. You must do your best to avoid sharp turns or poorly paved roads that could potentially cause the tank to tip or roll.
To dispose of a small propane tank, the kind often used for camping, find a local recycling depot that can accept the item or notify your local refuse collection service. If the item can be collected by your regular bin-men, place the container on the ground to avoid the risk of your tank puncturing.
You should never just release propane gas into the air just by opening the valve as the slightest spark could start a fire.