Dispose of most propane cylinders by taking the empty or broken cylinder to the retailer from which you purchased the cylinder. Most reputable propane retailers dispose of their own cylinders free of charge, and some also provide a free or discounted replacement cylinder. Never try to dismantle or dispose of the propane cylinder yourself, as the pressurized gas is highly flammable and presents a serious danger of fire, explosion and injury.
The disposal process for very large stationary propane cylinders is quite different from the process for the smaller cylinders used in grills, camp stoves and other small propane-fueled equipment. The first step in disposing of a large propane cylinder is to drain all of the remaining gas. This is a task for propane professionals, as they have the necessary equipment to safely recover all of the gas without harming humans or the environment.
After the tank is completely empty, you have several options. You can repurpose the tank for storing other materials, provided that you outfit it with the proper valves and regulators and that the cylinder is still structurally sound. You can also salvage the cylinder and collect its scrap metal value.
Typically, disposing of a large, underground stationary propane cylinder is not economical. In this case, professionals remove all of the residual gas from the tank and then fill it with sand, water or another inert material. Once it is filled with a harmless material, the tank presents no threat to the surrounding environment.