Damaged or empty propane tanks and cylinders are hazardous waste and should never be put in the garbage or recycling as they could become punctured during the collection process and cause a fire or injury. 

Empty, good condition tanks can be exchanged or refilled where you got it (e.g. 1lb or 5-20lb propane cylinders, SodaStream gas cylinders) or have the supplier come pick it up. Look for a label to contact and return an unwanted cylinder to. Old, rusty or unknown cylinders may be taken to a scrap metal company once they have been completely emptied with the valves safely removed. Companies that specialize in gas cylinders may offer this service.