A pyrophoric substance is a substance that ignites spontaneously in air at or below or within 5 minutes after coming into contact with air. Examples are iron sulfide and many reactive metals including plutonium and uranium, when powdered or thinly sliced. Pyrophoric materials are often water-reactive as well and will ignite when they contact water or humid air. They can be handled safely in atmospheres of argon or nitrogen. Class Dfire extinguishers are capable of dealing with pyrophoric fires.
Uses
The creation of sparks from metals is based on the pyrophoricity of small metal particles, and pyrophoric alloys are made for this purpose. This has certain uses: the sparking mechanisms in lighters and various toys, using ferrocerium; starting fires without matches, using a firesteel; the flintlock mechanism in firearms; and spark-testingferrous metals.
Handling
Small amounts of pyrophoric liquids are often supplied in a glass bottle with a PTFE -lined septum. Larger amounts are supplied in metal tanks similar to gas cylinders, designed so a needle can fit through the valve opening. A syringe, carefully dried and flushed of air with an inert gas, is used to extract the liquid from its container. Pyrophoric solids require the use of a sealed glove box flushed with inert gas. Glove boxes are expensive and require maintenance. Thus, many pyrophoric solids are sold as solutions, or dispersions in mineral oil or lighter hydrocarbon solvents. Mildly pyrophoric solids can be handled in the air for brief periods of time, but the containers must be flushed with inert gas before storage.
Uranium is pyrophoric, as shown in the disintegration of depleted uranium penetrator rounds into burning dust upon impact with their targets; in finely divided form it is readily ignitable, and uranium scrap from machining operations is subject to spontaneous ignition
Several compounds of plutonium are pyrophoric, and they cause some of the most serious fires occurring in United States Department of Energy facilities
