Pyrophoricity



A pyrophoric substance is a substance that ignites spontaneously: that is, its autoignition temperature is below room temperature. Examples are iron sulfide and many reactive metals including uranium, when powdered or sliced thinly. 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 (with few exceptions) nitrogen. Most pyrophoric fires should be extinguished with a class D fire extinguisher.

Uses
The creation of sparks from metals is based on the pyrophoricity of small metal particles. This can be useful, including: the sparking mechanisms in lighters and various toys, using ferrocerium; starting fires without matches, using a firesteel; the flintlock mechanism in firearms; and spark testing metals.

Liquids
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.

Solids
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 (such as Lithium Aluminum Hydride and Sodium Hydride) can be handled in the air for brief periods of time, but the containers must be flushed with inert gas before storage.

Disposal of pyrophoric materials
Small amounts of pyrophoric materials and empty containers must be disposed of carefully, by quenching the residue. Less reactive substances can be disposed of by diluting heavily with an unreactive solvent like hexane, placing the container in a cooling bath, and adding water dropwise. More reactive substances can quenched by slowly adding the dilute solution to dry ice, then adding a mildly reactive substance, which does not freeze in dry ice, to the mixture (wet diethyl ether, acetone, isopropyl alcohol, and methanol are often used)

Pyrophoric solids

 * Finely divided metals (magnesium, calcium, zirconium, uranium)
 * Alkali metals (sodium, potassium)
 * Metal hydrides or nonmetal hydrides (germane, diborane, sodium hydride, lithium aluminum hydride, uranium trihydride)
 * Grignard reagents (compounds of the form RMgX)
 * Partially or fully alkylated derivatives of metal and nonmetal hydrides (diethylaluminum hydride, trimethylaluminium, butyllithium, triethylboron)
 * Uranium-235 is pyrophoric, as shown by the vaporization of DU penetrator rounds into a tens of microns-fine burning dust upon impact with their targets.
 * Alkylated metal alkoxides or nonmetal halides (diethylethoxyaluminum, dichloro(methyl)silane)
 * Metal carbonyls (dicobalt octacarbonyl, nickel carbonyl)
 * Used hydrogenation catalysts such as Raney nickel (especially hazardous because of the adsorbed hydrogen)
 * Copper fuel cell catalysts, e.g., Cu/ZnO/Al2O3 [1]
 * Phosphorus (white, or yellow)
 * Plutonium
 * Methanetellurol (CH3TeH)

Pyrophoric gases

 * Arsine
 * Diborane
 * Phosphine
 * Silane

Pyrophoric liquids

 * Hydrazine
 * Metalorganics of main group metals (e.g. aluminum, gallium, indium, zinc and cadmium etc.)

External links and references

 * List of pyrophoric materials
 * Larger list of pyrophoric materials
 * Extensive description of pyrophoricity
 * US Dept. of Energy Handbook entry

Pyrophor