Thermite

A thermite reaction is a type of aluminothermic reaction in which aluminium metal is oxidized by the oxide of another metal, most commonly iron oxide. The name thermite is also used to refer to a mixture of two such chemicals. The products are aluminium oxide, free elemental iron, and a large amount of heat. The reactants are commonly powdered and mixed with a binder to keep the material solid and prevent separation. The reaction is used for thermite welding, often used to join rails.

Types

Black or blue iron oxide (Fe₃O₄), produced by oxidizing iron in an oxygen-rich environment under high heat, is the most commonly used thermite oxidizing agent because it is inexpensive and easily produced. Red iron(III) oxide (Fe₂O₃, commonly known as rust) can also be used to make thermite and yields a significantly more energetic reaction. Other oxides are occasionally used, such as in manganese thermite and chromium thermite, but only for highly specialized purposes. Both examples use aluminium as the reactive metal.

In principle, any reactive metal could be used instead of aluminum. This is rarely done, however, because the properties of aluminium are ideal for this reaction. It is by far the cheapest of the highly reactive metals; it also forms a passivation layer making it safer to handle than many other reactive metals. The melting and boiling points of aluminum also make it ideal for thermite reactions. Its relatively low melting point (660°C, 1221°F) means that it is easy to melt the metal, so that the reaction can occur mainly in the liquid phase^[1] and thus proceeds fairly quickly. At the same time, its high boiling point (2519°C, 4566°F) enables the reaction to reach very high temperatures, since several processes tend to limit the maximum temperature to just below the boiling point.^[2] Such a high boiling point is common among transition metals (e.g. iron and copper boil at 2887 °C and 2582 °C respectively), but is especially unusual among the highly reactive metals (cf. magnesium and sodium which boil at 1090 °C and 883 °C respectively).

Although the reactants are stable at room temperature, they burn with an extremely intense exothermic reaction when they are heated to ignition temperature. The products emerge as liquids due to the high temperatures reached (up to 2500 °C (4500 °F) with iron(III) oxide)—although the actual temperature reached depends on how quickly heat can escape to the surrounding environment. Thermite contains its own supply of oxygen and does not require any external source of air. Consequently, it cannot be smothered and may ignite in any environment, given sufficient initial heat. It will burn well while wet and cannot be extinguished with water. Small amounts of water will boil before reaching the reaction. If thermite is ignited underwater, the molten iron produced will extract oxygen from water and generate hydrogen gas in a single-replacement reaction. This gas may, in turn, burn by combining with oxygen in the air.

This raises the question of the possible effects of water ballast reacting with the quasi-thermite skin of the Hindenburg to actually intensify the fire which destroyed the airship. The Discovery series Mythbusters tested this theory and concluded that the skin has an effect, but to say that hydrogen had no involvement in the burn is incorrect.

Ignition

Conventional thermite reactions require very high temperatures for initiation. These cannot be reached with conventional black-powder fuses, nitrocellulose rods, detonators, or other common igniting substances. Even when the thermite is hot enough to glow bright red, it will not ignite as it must be at or near white-hot to initiate the reaction. It is possible to start the reaction using a propane torch if done correctly, but this should never be attempted for safety reasons. The torch can preheat the entire pile of thermite which will make it explode instead of burning slowly when it finally reaches ignition temperature.

Often, strips of magnesium metal are used as fuses. Magnesium burns at approximately the temperature at which thermite reacts, around 2500 kelvin (4000 °F). This method is notoriously unreliable: magnesium itself is hard to ignite, and in windy or wet conditions the strip may be extinguished. Also, magnesium strips do not contain their own oxygen source so ignition cannot occur through a small hole. A significant danger of magnesium ignition is the fact that the metal is an excellent conductor of heat; heating one end of the ribbon may cause the other end to transfer enough heat to the thermite to cause premature ignition. Despite these issues, magnesium ignition remains popular amongst amateur thermite users.

The reaction between potassium permanganate and glycerine is used as an alternative to the magnesium method. When these two substances mix, a spontaneous reaction will begin, slowly increasing the temperature of the mixture until flames are produced. The heat released by the oxidation of glycerine is sufficient to initiate a thermite reaction. However, this method can also be unreliable and the delay between mixing and ignition can vary greatly due to factors such as particle size and ambient temperature.

Another method of igniting is to use a common sparkler to ignite the mix. These reach the necessary temperatures and provide a sufficient amount of time before the burning point reaches the sample.

A stoichiometric mixture of finely powdered Fe(III) oxide and aluminum may be ignited using ordinary red-tipped book matches by partially embedding one match head in the mixture, and igniting that match head with another match, preferably held with tongs in gloves to prevent flash burns.

Safety

The near-impossibility of smothering and high temperatures generated make thermite reactions potentially hazardous. Appropriate precautions must be taken before igniting thermite. Thermite should not be used near flammable materials; small streams of molten iron released in the reaction can travel considerable distances and may melt through metal containers, ignite their contents, etc. Flammable metals with relatively low boiling points such as zinc, whose boiling point of 907°C (1665°F) is about 1370°C (2500°F) below the temperature at which thermite burns, should be kept away from thermite, as contact with such metals could potentially boil superheated metal violently into the air, where it could then burst into flame as it is exposed to oxygen. Thermite must be used with care in welding pipes or other items with air cavities, as thermal expansion of trapped gases may cause bursting. Generally, the ignition of thermite should be timed so individuals handling it have ample time to get away. As with any pyrotechnic composition, thermite that is not being used in a particular task should be kept far away from the site of ignition. When handled in a responsible manner by properly trained individuals, thermite can be reasonably safe.

The thermite reaction can take place accidentally in industrial locations where abrasive grinding and cutting wheels are used with ferrous metals. Using aluminium in this situation produces an admixture of oxides which is capable of a violent explosive reaction. ^[3]

Mixing thermite with water can be counterintuitively dangerous, causing a phreatomagmatic fragmentation explosion.

Military uses

Thermite grenades are used as incendiary devices to quickly destroy items or equipment when there is imminent danger of them being captured by enemy forces. Because of the difficulty in igniting standard iron-thermite, plus the fact that it burns with practically no flame and has a small radius of action, standard thermite is rarely used on its own as an incendiary composition. It is more usually employed with other ingredients added to enhance its incendiary effects. Thermate-TH3 is a mixture of thermite and pyrotechnic additives which have been found to be superior to standard thermite for incendiary purposes. Its composition by weight is generally thermite 68.7%, barium nitrate 29.0%, sulphur 2.0% and binder 0.3%. Addition of barium nitrate to thermite increases its thermal effect, creates flame in burning and significantly reduces the ignition temperature. Although the primary purpose of Thermate-TH3 is as an incendiary, it will also weld metal surfaces together.

A classic military use for thermite is disabling artillery pieces. Thermite can be used to permanently disable artillery pieces without the use of explosive charges and therefore operate with a reasonable amount of stealth. The 2nd Ranger Battalion used thermite grenades against the German artillery at Pointe du Hoc during the Allied invasion of Normandy. There are several ways to do this. One method is to weld the breech of the weapon closed by inserting an armed thermite grenade into it and then quickly closing the breech. This makes the weapon impossible to load. An alternative method is to insert an armed thermite grenade down the muzzle of the artillery piece, fouling the barrel. This makes the piece very dangerous to fire. Yet another method is to use thermite to destroy the traversing and elevation mechanism on the cannon, making it impossible to properly aim the gun.

Civilian uses

Thermite reaction proceeding for a railway welding. Shortly after this, the liquid iron flows into the mold around the rail gap.

The violent effects of thermite demonstrated in the Utah desert

Thermite reactions have many uses. Thermite was originally used for repair welding in-place thick steel sections such as locomotive axle-frames where the repair can take place without removing the part from its installed location. It can also be used for quickly cutting or welding steel such as rail tracks, without requiring complex or heavy equipment.

A thermite reaction, when used to purify the ores of some metals, is called the Thermite process. An adaptation of the reaction, used to obtain pure uranium, was developed as part of the Manhattan Project at Ames Laboratory under the direction of Frank Spedding. It is sometimes called the Ames process.

When thermite is made using iron (III) oxide, for maximum efficiency it should contain, by mass, 25.3% aluminum and 74.7% iron oxide. (This mixture is sold under the brand name Thermit as a heat source for welding.) The complete formula for the reaction using iron (III) oxide is as follows:

Fe₂O₃(s) + 2Al(s) → Al₂O₃(l) + 2Fe(l); ΔH = -851.5 kJ/mol

When thermite is made using iron (II,III) oxide, for maximum efficiency it should contain, by mass, 23.7% aluminium and 76.3% iron oxide. The formula for the reaction using iron (II,III) oxide:

3Fe₃O₄(s) + 8Al(s) → 4Al₂O₃(l) + 9Fe(l); ΔH = -3347.6 kJ/mol

The reaction using Fe₃O₄ produces a substantially larger amount of energy pr. mol reaction than the reaction using Fe₂O₃, which produces more energy pr. gram of thermite mixture.

Thermite should not be confused with a thermal lance.

Thermite in Popular Culture

On a fifth season episode of Mythbusters, hosts Jamie Hyneman and Adam Savage test a Hindenburg disaster theory using thermite, prompting Jamie to say, "I love the smell of thermite in the afternoon."

Humorous science show Brainiac employs thermite often, especially to eat through cars.

History

Thermite was invented in 1893 and patented in 1895 by German chemist Dr. Hans Goldschmidt. Consequently, the reaction is sometimes called the "Goldschmidt reaction" or "Goldschmidt process". Dr. Goldschmidt was originally interested in producing very pure metals by avoiding the use of carbon in smelting, but he soon realized the value in welding. The first commercial application was the welding of tram tracks in Essen, in 1899. Degussa, a corporate descendant of Goldschmidt's firm, is still today one of the world's largest producers of welding thermite.

References

^ or rather, where the solid oxide particles meet the liquid metal
^ i.e. loss of fuel and heat due to vaporization
^ Fireball from Aluminum Grinding Dust

L. L. Wang, Z. A. Munir and Y. M. Maximov, (1993). "Thermite reactions: their utilization in the synthesis and processing of materials". Journal of Materials Science. 28 (14): 3693–3708. doi:10.1007/BF00353167.{{cite journal}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
M. Beckert (2002). "Hans Goldschmidt and the aluminothermics". Schweissen und Schneiden. 54 (9): 522–526.
DEGUSSA page on thermite

External links

[liquid_phase-1] r rather, where the solid oxide particles meet the liquid metal

[boiling_point-2] .e. loss of fuel and heat due to vaporization

[3] Fireball from Aluminum Grinding Dust

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