Fire extinguishers must be disposed of by a competent person. If they are under pressure they are extremely dangerous and should never be placed in general waste. The contents must be disposed of in different ways depending on what they are. The bodies, once empty and unpressurised, are mostly metal and will be sent for recycling once made safe by a fire extinguisher engineer. To make it simple we can deal with this for you while servicing.
Any extinguishers over 2kg, and a 2kg CO2, should have their handles mounted at 1m from the floor level. Any fire extinguishers smaller than this, should have their handles mounted at 1.5m from the ground.