You cannot throw away empty propane tanks in your garbage as the garbage people will not accept them and will not take them.
To dispose of empty propane tanks you can drop the empty propane tanks off at a propane suppliers place or a hazardous waste facility or if you can safely do so, you can remove the valve from the propane tank and then fill the propane tank with some water and cut it open and then a scrap metal yard will accept it.
You can take the empty propane tanks or gas canisters to a hazardous waste facility or a landfill that accepts hazardous waste.
Things you can do with empty propane tanks and empty propane cans are take them to a hazardous waste center or event or remove the needle valve from the propane canister with a valve stem removal tool and take them to a recycling center.
If you take the needle valve out with a valve stem removal tool you can then cut the empty propane tank open safety with a saw or grinder.
When I have the small empty propane cans I take the valve out and then I use a grinder with a cut off wheel and cut open the propane canister by cutting it in half.
After the propane canister is cut open in half the scrap metal yard here will take them mixed in with other metal.
Don't just throw the empty propane tanks in the garbage as they could potentially explode in the garbage truck and could injure or possibly kill someone.
Larger empty propane tanks that are empty can be refilled if in good condition.
But if not you can exchange the larger propane bottle for a better propane bottle with propane in it.
You can also if you have the right tools remove the valve from the larger propane tank and then fill it with water and cut it open and then sell it for scrap.
Small propane canisters can also be refilled at home using an adapter for the larger propane tanks that allow you to refill the smaller propane canisters.