Ammonium nitrate

Ammonium nitrate
Chemical name	Ammonium nitrate
Other Names	Saltpeter
Chemical formula	NH4NO3
SMILES
Molecular mass	80.04 g/mol
Appearance	White crystal
CAS number	6484-52-2
Properties
Density	1.73 g/cm3
Solubility	118 g in 100 g water
Melting point	169 °C
Boiling point	210 °C
Crystal structure	rhombohedral
Hazards
Main hazards	Shock Sensitive
NFPA 704
R/S statement	R: 8, 20, 21, 22, 36, 37, 38 S: 17, 26, 36
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa)

The chemical compound ammonium nitrate, the nitrate of ammonia with chemical formula NH₄NO₃, is commonly used in agriculture as a high-nitrogen fertilizer.

Use in explosives

Ammonium nitrate has found many uses as a strong oxidizer, primarily as a component of explosives. In this case, it is mixed with a hydrocarbon, usually Diesel fuel (oil) or, less commonly, kerosene. Because of the ready availability in bulk of the raw materials, ammonium nitrate/fuel oil (ANFO) mixtures have occasionally been used for improvised bombs, for example by the Provisional IRA and in the Oklahoma City bombing by Timothy McVeigh and Terry Nichols. It is also used in military explosives such as the daisy cutter bomb, and as a component of amatol. It has also found use as a solid rocket propellant, though ammonium perchlorate is frequently considered preferable due to higher performance and faster burn rates.

Fertilizer-grade ammonium nitrate (FGAN) is manufactured in more compact form, with much lower porosity, in order to achieve more stability and less sensitivity to detonation, while technical grade ammonium nitrate (TGAN) granules are made to be porous for better absorption of fuel and higher reactivity.

Disasters

Ammonium nitrate decomposes into gases including oxygen when heated (non-explosive reaction); however, ammonium nitrate can be induced to decompose explosively by detonation. Large stockpiles of the material can be a major fire risk due to their supporting oxidation, and may also detonate, as happened in the Texas City disaster of 1947, which led to major changes in the regulations for storage and handling.

There are two major classes of incidents resulting in explosions:

• In the first case, the explosion happens by the mechanism of shock to detonation transition. The initiation happens by an explosive charge going off in the mass, by the detonation of a shell thrown into the mass, or by detonation of an explosive mixture in contact with the mass. The examples are Kriewald, Morgan, Oppau, Tessenderloo, and Traskwood.
• In the second case, the explosion results from a fire that spreads into the ammonium nitrate itself (Texas City, Brest, Oakdale), or to a mixture of an ammonium nitrate with a combustible material during the fire (Repauno, Cherokee). The fire must be confined at least to a degree for successful transition from a fire to an explosion (a phenomenon known as "transition from a decomposition or deflagration", or DDT). Pure, compact AN is stable and very difficult to initiate. However, there are numerous cases when even impure AN didn't explode in a fire.

Ammonium nitrate decomposes in temperatures above 200 °C. Pure AN is stable and will stop decomposing once the heat source is removed, but in presence of catalysts (combustible materials, acids, metal ions, chlorides...) the reaction can become self-sustaining (known as self-sustaining decomposition, SSD). This is well-known phenomenon with some types of NPK fertilizers, and is responsible for loss of several cargo ships.

Historically significant accidental explosions

• In 1918, a fire broke out in the amatol loading plant in Morgan, New Jersey. Over 30,000,000 pounds (13,500 tons) of explosives were present on site, of that up to 9,000,000 lb (4,000 tons) of ammonium nitrate were involved in the explosion. The rest of the ammonium nitrate, stored in other sites within the area, was exposed to the shock and fire, but did not detonate.
• On September 21, 1921 a heavy explosion of 9,000,000 pounds (4,500 tons) of a mixture of ammonium nitrate and ammonium sulfate detonated during attempts to break up the caked pile with blasting charges (an operation apparently performed several times before), with 535-561 casualties (according to other sources, 450 or over 1000), occurred in the city of Oppau (on the grounds of BASF near Ludwigshafen in Germany). Over 700 homes were destroyed, the plant disappeared entirely and was replaced with a crater 50 ft deep and 250 ft in diameter, and the shock was felt 150 miles away. This was the largest man-made disaster in German history. [1]
• In 1924, in Nixon, New Jersey, ammonium nitrate was being recovered from amatol in order to be used as a fertilizer, when a fire broke out.
• In 1925 April 4 and May 3, two carloads, each containing 220 barrels of ammonium nitrate, were dispatched from Muscle Shoals, Alabama and caught fire in transportation. The barrels were stored in a warehouse with varying humidity for 6 years, so it is believed that they were ignited by friction of their nitrate-impregnated manilla paper lining. Other shipments were reportedly more successful.
• In 1941, an explosion of 300,000 pounds (136 tons) killed 100 people in Tessenderloo, Belgium.
• In 1944, an explosion occurred at a bomb-making plant in Milan, Tennessee, killing four.
• The Texas City Disaster on April 16, 1947, when 17,000,000 pounds (7,700 tons) of ammonium nitrate caught fire and then exploded on board of the SS Grandcamp, heavily damaging the port and killing 581 people, injuring over 5,000 more, and igniting the SS High Flyer, which exploded later, adding to the death toll. This is the best-known ammonium-nitrate related disaster.
• In September 21, 2001, at 10:15 am in the AZF (Azote de France) fertilizer factory in Toulouse, France. The explosion occurred in a warehouse where the off-specification granular AN was stored flat, separated by partitions. About 200-300 tons is said to be involved in the explosion, resulting in 31 people dead and 2,442 injured, 34 of them seriously. The blast wave shattered windows in 1.5-3 kilometer distance and the resulting crater was 10 meters deep and 50 meters wide. The exact cause remains unknown. The material damage was estimated at 2.3 billion Euros. [2]
• The Ryongchon disaster in April 22, 2004, in North Korea is suspected to have a shipment of ammonium nitrate involved, together with other explosives.

(Incidents not involving explosion, resulting only in self-sustaining decomposition or a fire, are not listed here.)

Other uses

The most common use of ammonium nitrate is in fertilizers.

Ammonium nitrate is also used in instant cold packs. In this use, ammonium nitrate is mixed with water in an endothermic reaction, which absorbs 26.2 kilojoules of heat per mole of reactant.

Due to its low temperature, non-toxic decomposition products, it finds use in gas generator applications such as airbags.

Ammonium nitrate is also used in the treatment of titanium ores.

Ammonium nitrate fertilizer is also used in the synthesis of methamphetamine.

Ammonium nitrate is used in survival kits mixed with zinc dust and ammonuim chloride because it will ignite on contact with water.

History

Ammonium nitrate, NH₄NO₃, is prepared by neutralizing nitric acid with ammonia, or ammonium carbonate, or by double decomposition between potassium nitrate and ammonium sulfate. It can be obtained in three different crystalline forms, the transition points of which are 35 °C, 83 °C and 125 °C. It is easily soluble in water, a considerable lowering of temperature taking place during the operation; on this account it is sometimes used in the preparation of freezing mixtures. On gentle heating, it is decomposed into water and nitrous oxide. Berthelot showed in 1883 that if ammonium nitrate is rapidly heated the following reaction takes place with explosive violence: 2NH₄NO₃ → 4H₂O + 2N₂ + O₂. In combination with gasoline or other liquid hydrocarbons it is a widely used industrial explosive, being particularly useful in open pit mining and is known as ANFO. The detonation rate is about 3000 feet per second (900 m/s); relatively slow compared to high explosives, which detonate at over 25,000 ft/s (7,600 m/s). This explosive combines the advantages of low cost and stability, requiring a high velocity explosive primer to begin detonation. It is sometimes used in small packets to break up snow cornices in avalanche control. Ammonium nitrate confined in large quantities (such as might be found in a ship's cargo hold) can detonate explosively if combined with hydrocarbons and heated sufficiently by a fire. A fire in a ship carrying ammonium nitrate waterproofed with wax was the cause of a devastating explosion resulting in the Texas City, Texas disaster.. On July 28, 1987 two teenage boys mixed zinc dust, ammonium nitrate and ammonium chloride from one of their father's lab. Water hit their pile and started a violent fire. The fire destroyed 6 homes and killed one of the boys.

Production

Industrial production is quite easy. A simple reaction of ammonia with nitric acid easily gives a solution of ammonium nitrate. The Haber process combines nitrogen and hydrogen to produce ammonia, part of which can be oxidized to nitric acid and combined with the remaining ammonia to produce the nitrate. Another production method is used in the so called Odda process.