If the fire extinguisher is empty, simply remove the head and recycle the body (which is typically made of steel).
If the fire extinguisher is full, it needs to be treated as hazardous waste and disposed of at a Household Hazardous Waste (HHW) event or by going to Earth 911 and finding a drop off location. Please note, there may be a small fee for recycling.
Never let fire extinguishers go to waste. If you have old fire extinguishers that are not empty yet, check with your local fire department as you may be able to have them recharged or refilled at no cost.
If you dispose of a fire extinguisher, remember to replace it with a new one immediately.