Coleman propane tanks are made of metal and are a recyclable material but it can be hard to find places where you can recycle them.
Some landfills may have a designated area where you can dispose of old propane tanks including the mini Coleman propane tanks for recycle.
Some scrap yards may take them but others may not.
I've found that if I cut them open then the scrap yard here will take them with the other steel material in a load of other scrap metal but they must be cut open.
Just drain all the propane out and then use a SawZall to cut them open which is really easy to do.
Just be safe and make sure all the propane is out of the propane tank before cutting it open with the saw.