Process Hazards - hdbk10810041

Spontaneous Heating and Pyrophoricity

DOE-HDBK-1081-94

PYROPHORIC METALS

Process Hazards

Liquid alkali metal is valuable as a high temperature heat transfer medium. For

example, it is used in hollow exhaust valve stems in some internal combustion engines

and in the transfer of heat from one type of nuclear reactor to a steam generator. In

the latter process or other large-scale use of molten alkali metal, any equipment leak

may result in a fire. Where molten alkali metal is used in process equipment, steel pans

should be located underneath to prevent contact with concrete floors. Contact of

molten alkali metal with concrete will cause spalling of the concrete and spattering of

the metal.

Processing of alkali metal is essentially remelting it to form sticks or bricks or to add

as a liquid to closed transfer systems. During this handling, contact with moist air,

water, halogens, halogenated hydrocarbons, and sulfuric acid must be avoided.

Extinguishing Fires in Sodium, Lithium, NaK, and Potassium

The common extinguishing agents, such as water, foam, and vaporizing liquids,

should never be used because of the violent reactions upon application to alkali

metals. Class D dry powders developed for metal fires, dry sand, dry sodium

chloride, and dry soda ash are effective. These finely divided materials blanket the

fire while the metal cools to below its ignition temperature. Alkali metal burning in

an apparatus can usually be extinguished by closing all openings. Blanketing with

nitrogen is also effective. In the case of lithium, argon or helium atmospheres should

be used.

Zirconium and Hafnium

Properties

Zirconium

The combustibility of zirconium increases as the average particle size

decreases, but other variables, such as moisture content, also affect its ease of

ignition. In massive form, zirconium can withstand extremely high

temperatures without igniting, whereas clouds of dust in which the average

particle size is 3 microns have ignited at room temperature. Dust clouds of

larger particle size can be readily ignited if an ignition source is present, and

such explosions can occur in atmospheres of carbon dioxide or nitrogen as well

as in air. Zirconium dust will ignite in carbon dioxide at approximately 621 C

(1,150 F) and nitrogen at approximately 788 C (1,450 F). Tests have also

indicated that layers of 3-micron-diameter dust are susceptible to spontaneous

ignition. The depth of the dust layer and its moisture content are important

variables for ignition. Spontaneous heating and ignition are also possibilities

with scrap chips, borings, and turnings if fine dust is present. Layers of 6-

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Pyrophoricity