Metal Fires Require Knowledge Of Proper Extinguishing Agents
People outside the fire service usually think of all metals as being noncombustible, but fire fighters know that this is simply not true.
While larger pieces of a metal can be difficult to ignite, ignition can be accomplished when metals are in such forms as powders, dusts, chips and lathe turnings. Molten metals can be especially hazardous.
The hazards of combustible metals and the need for special extinguishing agents are unique and, consequently, fires involving these fuels are categorized as class I) fires. These fires can be difficult to extinguish and mishandling them can cause explosions and spread fire over large areas.
Water cannot he used on many burning metals because the resultant chemical reaction can cause an explosion which would scatter flaming metal particles to great distances. It is best to use specially formulated dry powders, which often have a graphite base. Although dry sand is often recommended for extinguishing class D fires, even this can be poor. There are certain conditions under which the sand can react with a metal to generate even more heat. Other effective extinguishing agents include sodium chloride (common table salt), sodium bicarbonate, magnesium carbonate, magnesium oxide, and mixtures of these. Halogenated extinguishing agents must never be used on a burning metal as the resulting chemical reaction can be explosive.
Consideration of hazards
Each group of metals has its own chemical and physical properties. Thus, the hazards of each group must be considered separately. In the following discussion of combustible and reactive metals, we will give thought to their fire, explosion and toxicity hazards and to fire fighting methods and extinguishing agents.
Chemists have categorized all known chemical elements according to their physical and chemical properties. Elements with similar properties are grouped together in what scientists know as the periodic table of the elements. Let’s borrow the chemists’ classification system and discuss the potentially hazardous metals according to their group.
The six metals in Group IA are called alkali metals and they are among the most chemically reactive elements known. This extreme reactivity causes these metals to be especially hazardous. Cesium, rubidium and francium have very little commercial value and thus are not likely to be encountered by the average fire fighter. However, sodium, potassium and lithium have many industrial uses.
Sodium, a silvery bright metal, is the most common of the alkali metals and it is produced by the ton. It is used as the starting material for the synthesis of many highly reactive sodium compounds. Sodium burns in air with a distinctive yellow flame and produces sodium oxide as a combustion product. Sodium will react with water to release hydrogen. This reaction may also generate sufficient heat to ignite the highly explosive hydrogen. Thus, it is absolutely essential that sodium be protected from all sources of water and moisture. Be sure to remember such moisture sources as damp concrete, moist sand and, of course, avoid direct application of water.
The primary industrial use of potassium is in the production of the sodium-potassium alloy mentioned above. Potassium is also a silvery metal and has all of the hazards of sodium, plus several others. Potassium is much more reactive than either sodium or lithium. It can ignite spontaneously upon exposure to moist air at room temperature, burning with a purple flame. The reaction of potassium with water is quite violent and is accompanied by explosions which splatter the burning metal. The sodium-potassium alloys, NaK, like the free metals, also react violently with water.
Lithium, the third alkali metal, is also a soft, silvery, water-reactive element. It is used extensively in many industrial processes, including production of fungicides, bleaches, porcelain and pharmaceuticals. The lithium-water reaction occurs much more slowly than do those of sodium and potassium. Autoignition of lithium is rare. An external ignition source is usually required.
Water-reactive metals
To repeat, all alkali metals are water-reactive so water must not be applied to these burning metals. Also, lithium, sodium and potassium react with halogenated hydrocarbon extinguishing agents. When halogenated extinguishing agents are applied to burning alkali metals, carbon is produced and, since carbon is combustible, the fire continues to burn. Carbon dioxide likewise is ineffective in extinguishing alkali metal fires because it also reacts with the metal to release carbon. Even dry sand must not be used on burning alkali metals.
Sodium and potassium fires call for the use of soda ash, sodium chloride, nitrogen or graphite for extinguishment. Even graphite cannot be used without some caution, however, as it reacts with the metal at high temperatures to produce the metal carbide. Then, if the carbide is wetted, acetylene forms and ignites immediately.
Lithium differs from sodium and potassium in its reactivity, a factor that necessitates the use of other extinguishing agents. If sodium compounds, such as soda ash or sodium chloride, are used on a lithium fire, free sodium is produced. This is more dangerous than even the lithium. Lithium will also react with nitrogen so lithium fires are not extinguished even when covered by a nitrogen blanket. Lithium fires are best extinguished with either lithium chloride or graphite.
Alkali metal fires pose a severe threat to fire fighters’ health. The fumes are extremely caustic and irritating to the eyes, respiratory tract, mucous membranes and skin. Metallic oxide inhalation can cause serious lung injury. Thus, full protective clothing and selfcontained breathing apparatus are essential.
Dusts are explosive
Five GROUP IIA metals concern the fire fighter—calcium, strontium, magnesium, barium and beryllium. Resembling aluminum powder in appearance, each of these metals can be easily ignited by an external heat source when in powder form. The ignition hazard increases as the particle size decreases. Dusts suspended in air are explosive mixtures which can be ignited by a spark and, in the case of calcium, can spontaneously ignite.
Magnesium is a very common metal that today’s fire fighter is quite likely to encounter. It is a very lightweight metal and therefore is used extensively in aircraft and automobile frames, racing cars and engines, and other products where weight consideration is important. Magnesium burns with an intense white flame and intense heat. The flame is so bright that eye damage is possible from the ultraviolet radiation.
Application of water to burning magnesium produces hydrogen and if the water is applied at a slow rate, the hydrogen accumulation will likely ignite from the hot metal. However, water can be successfully and safely applied in large amounts so that rapid cooling of the metal is accomplished. Graphitebase dry powders are recommended for chips and shavings.
Water must not be applied to any other Group IIA metal fire, nor should carbon dioxide and halogenated agents. For example, magnesium oxide, the product of magnesium combustion, is so reactive that it combines with a halogenated agent. Neither can magnesium fires be extinguished by smothering or removing the oxygen supply.
Talc or dry powders used
Group IIA metal fires are best extinguished with powdered talc or with dry powders specially designed for the involved metal. These powders should be shoveled onto the fire until the metal fuel is completely blanketed. If the metal is burning on a combustible floor, it is best to spread a layer of extinguishing powder on a clear area, shovel the burning metal onto the powder, and then complete extinguishment by shoveling more powder onto the fire.
Beryllium fires warrant a special note of caution because this metal is an extremely toxic respiratory poison and eye irritant. Introduction of beryllium under the skin through lacerations, for example, can cause very slow-healing sores. As with all class D fires, beryllium fires require full protective clothing and self-contained breathing apparatus. After a beryllium fire has been extinguished, be certain to wash all beryllium dusts from your clothing and body before removing the facepiece of your breathing apparatus. Then bathe and change clothes as quickly as possible.
Titanium, hafnium and zirconium— Group IVA metals—are combustible, but here again their combustibility depends upon particle size; i.e., the smaller the metal particles (the larger the surface to volume ratio), the greater the fire and explosion hazard becomes. Purity of the metal is also an important factor in relation to its ignition potential. The greater the purity of the metal, the more serious is its fire and explosion hazard.
Zirconium is used extensively in the nuclear and steel industries. Finelydivided zirconium dust is one of the most pyrophoric metals known. It has been known to ignite spontaneously and explosively merely from the heat generated by component dust particles rubbing together. Zirconium, like magnesium, burns with an intense white flame. Water and carbon dioxide must not be used on a zirconium fire because hydrogen and carbon, respectively, will be liberated. Likewise, halogenated extinguishing agents cannot be used.
Let fire burn out
The safest procedure, perhaps, is to allow the fire to burn itself out. Specially formulated metal extinguishing powders can be used but caution must be exercised to avoid forming potentially-explosive dust clouds.
The hazards and means of handling hafnium fires are much like those of zirconium.
Metallic titanium is also hazardous but, although quite abundant in nature, its high production costs currently limit its use in industry. It is an ideal metal for ship construction, though, because it is resistant to corrosion by seawater. Titanium is also pyrophoric, but unlike zirconium, titanium also burns readily in bulk form. The specific extinguishing agents for titanium fires are the specially-designed dry powders.
Zinc (Group IIB) is the last of the pyrophoric metals. Zinc is commonly known by most people but, in reality, it is less common than either titanium or zirconium. Zinc is used in galvanizing iron, as a structural material, in dry cell batteries and as a component of brass. Zinc poses no particular hazard in these forms, but it is dangerous in pulverized forms. Zinc dust, when moistened, heats spontaneously and ignites, producing zinc oxide.
Zinc, like most other hazardous metals, reacts with water to produce hydrogen, carbon dioxide to form free carbon, and also with halogenated extinguishing agents. Fumes from burning zinc are highly toxic, causing a condition known as metal-fume fever when inhaled. Zinc fires are best fought with suitable dry powders that are absolutely dry.
Aluminum dust explosive
Pure aluminum dust can explode when ignited. The burning metal reacts with water, carbon dioxide and halogenated extinguishing agents. Aluminum siding and roofing materials can melt so rapidly under common structural fire conditions that they give the illusion of burning. The melting point of aluminum is 1220°F.
Many of the metals discussed are more familiar to chemists than to fire fighters. However, fire fighters may encounter them in storage, transportation and processing. Knowledge of hazardous metal storage and processing can best be obtained through inspections and pre-fire planning. There can be little excuse for a fire department being unaware of potentially dangerous metals in its district.
Transportation is an entirely different matter, however. There is no way a fire fighter can know when a hazardous material is being transported through his area. The only alternative is to remain constantly alert to the possibility that such materials may be involved when you approach a transportation incident.
