If you have an old propane tank that is no longer safe to use then check with a local propane company as they may take them for free or small fee to dispose of safely.
Some cities and counties may also have a hazard waste drop off location as well where you can drop off propane tanks and other hazardous waste for free or also for a small fee.
Or you can take your old propane tank to a retailer such as Homedepot and exchange it for a new filled propane tank when you buy propane from them.
Some scrap yards may take propane tanks but not all do so you'll have to check around.