It is dangerous to put empty BBQ propane tanks in recycling or garbage, whether they are empty, partial or full. For propane and helium tanks 20lbs or less, bring them to a Drop Off Depot that accepts hazardous waste. For large propane cylinders ,you will need to contact the supplier for disposal options as they will not be accepted at the depots. Large propane cylinders are anything larger than the typical BBQ propane sized propane cylinder (over 24" in height or over 20lbs).
Do not remove valves from propane cylinders. Keep gas valve fully closed while transporting for disposal to prevent gas leaks. A filled, leaking propane tank is considered very dangerous as it is considered an uncontrolled release of a very flammable and potentially explosive gas. If you encounter this situation, 911 should be called.
Additional details on Drop Off Depots can be found on the City of Toronto website.