Disasters

Ammonium nitrate is a chemical compound with the formula NH4NO3. It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, although it does not form hydrates. It is predominantly used in agriculture as a high-nitrogen fertilizer.

Its other major use is as a component of explosive mixtures used in mining, quarrying, and civil construction. It is the major constituent of ANFO, a popular industrial explosive which accounts for 80% of explosives used in North America; similar formulations have been used in improvised explosive devices.

Many countries are phasing out its use in consumer applications due to concerns over its potential for misuse. Accidental ammonium nitrate explosions have killed thousands of people since the early 20th century. Global production was estimated at 21.6 million tonnes in 2017. By 2021, global production of ammonium nitrate was down to 16.7 million tonnes

Ammonium nitrate is found as the natural mineral gwihabaite (formerly known as nitrammite) – the ammonium analogue of saltpetre (mineralogical name: niter) – in the driest regions of the Atacama Desert in Chile, often as a crust on the ground or in conjunction with other nitrate, iodate, and halide minerals. Ammonium nitrate was mined there until the Haber–Bosch process made it possible to synthesize nitrates from atmospheric nitrogen, thus rendering nitrate mining obsolete.

The industrial production of ammonium nitrate entails the acid-base reaction of ammonia with nitric acid:

HNO₃ + NH₃ → NH₄NO₃

The ammonia required for this process is obtained by the Haber process from nitrogen and hydrogen. Ammonia produced by the Haber process can be oxidized to nitric acid by the Ostwald process. Ammonia is used in its anhydrous form (a gas) and the nitric acid is concentrated. The reaction is violent owing to its highly exothermic nature. After the solution is formed, typically at about 83% concentration, the excess water is evaporated off to leave an ammonium nitrate (AN) content of 95% to 99.9% concentration (AN melt), depending on grade. The AN melt is then made into "prills" or small beads in a spray tower, or into granules by spraying and tumbling in a rotating drum. The prills or granules may be further dried, cooled, and then coated to prevent caking. These prills or granules are the typical AN products in commerce.

Another production method is a variant of the nitrophosphate process:

Ca(NO₃)₂ + 2 NH₃ + CO₂ + H₂O → 2 NH₄NO₃ + CaCO₃

The products, calcium carbonate and ammonium nitrate, may be separately purified or sold combined as calcium ammonium nitrate.

Ammonium nitrate can also be made via metathesis reactions:

(NH₄)₂SO₄ + Ba(NO₃)₂ → 2 NH₄NO₃ + BaSO₄

(NH₄)₂SO₄ + Ca(NO₃)₂ → 2 NH₄NO₃ + CaSO₄

NH₄Cl + AgNO₃ → NH₄NO₃ + AgCl

As ammonium nitrate is a salt, both the cation, NH+4, and the anion, NO−3, may take part in chemical reactions.

Solid ammonium nitrate decomposes on heating. At temperatures below around 300 °C, the decomposition mainly produces nitrous oxide and water:

NH₄NO₃ → N₂O + 2 H₂O

At higher temperatures, the following reaction predominates.

2 NH₄NO₃ → 2 N₂ + O₂ + 4 H₂O

Both decomposition reactions are exothermic and their products are gas. Under certain conditions, this can lead to a runaway reaction, with the decomposition process becoming explosive.
See § Disasters for details. Many ammonium nitrate disasters, with loss of lives, have occurred.

The red–orange colour in an explosion cloud is due to nitrogen dioxide, a secondary reaction product.

A number of crystalline phases of ammonium nitrate have been observed. The following occur under atmospheric pressure.

Phase	Temperature (°C)	Symmetry
(liquid)	(above 169.6)
I	169.6 to 125.2	cubic
II	125.2 to 84.2	tetragonal
III	84.2 to 32.3	α-rhombic
IV	32.3 to −16.8	β-rhombic
V	below −16.8	tetragonal[15]

The transition between β-rhombic to α-rhombic forms (at 32.3 °C) occurs at ambient temperature in many parts of the world. These forms have a 3.6% difference in density and hence transition between them causes a change in volume. One practical consequence of this is that ammonium nitrate cannot be used as a solid rocket motor propellant, as it develops cracks. Stabilized ammonium nitrate (PSAN) was developed as a solution to this and incorporates metal halides stabilisers, which prevent density fluctuations.

Ammonium nitrate is an important fertilizer with NPK rating 34-0-0 (34% nitrogen).  It is less concentrated than urea (46-0-0), giving ammonium nitrate a slight transportation disadvantage. Ammonium nitrate's advantage over urea is that it is more stable and does not rapidly lose nitrogen to the atmosphere.

Ammonium nitrate readily forms explosive mixtures with varying properties when combined with explosives such as TNT or with fuels like aluminum powder or fuel oil. Examples of explosives containing ammonium nitrate include:

• Amatex (ammonium nitrate, TNT and RDX)
• Amatol (ammonium nitrate and TNT)
• Ammonal (ammonium nitrate and aluminum powder)
• ANFO (ammonium nitrate and fuel oil)
• Astrolite (ammonium nitrate and hydrazine rocket fuel)
• Goma-2 (ammonium nitrate, nitroglycol, nitrocellulose, dibutyl phthalate and fuel)
• Minol (explosive) (ammonium nitrate, TNT and aluminum powder)
• Nitrolite (ammonium nitrate, TNT and nitroglycerin +)
• DBX (ammonium nitrate, RDX, TNT and aluminum powder)
• Tovex (ammonium nitrate and methylammonium nitrate)

ANFO is a mixture of 94% ammonium nitrate ("AN") and 6% fuel oil ("FO") widely used as a bulk industrial explosive. It is used in coal mining, quarrying, metal mining, and civil construction in undemanding applications where the advantages of ANFO's low cost, relative safety, and ease of use matter more than the benefits offered by conventional industrial explosives, such as water resistance, oxygen balance, high detonation velocity, and performance in small diameters.

Ammonium nitrate-based explosives were used in the Sterling Hall bombing in Madison, Wisconsin, 1970, the Oklahoma City bombing in 1995, the 2011 Delhi bombings, the 2011 bombing in Oslo, the Myyrmanni bombing and the 2013 Hyderabad blasts.

In November 2009, the government of the North West Frontier Province (NWFP) of Pakistan imposed a ban on ammonium sulfate, ammonium nitrate, and calcium ammonium nitrate fertilizers in the former Malakand Division – comprising the Upper Dir, Lower Dir, Swat, Chitral, and Malakand districts of the NWFP – following reports that those chemicals were used by militants to make explosives. Due to these bans, "Potassium chlorate – the material which allows safety matches to catch fire – has surpassed fertilizer as the explosive of choice for insurgents."

Ammonium nitrate is used in some instant cold packs, as its dissolution in water is highly endothermic. In 2021, King Abdullah University of Science and Technology in Saudi Arabia conducted experiments to study the potential for dissolving ammonium nitrate in water for off-grid cooling systems and as a refrigerant. They suggested that the water could be distilled and reused using solar energy to avoid water wastage in severe environments.

It was once used, in combination with independently explosive "fuels" such as guanidine nitrate, as a cheaper (but less stable) alternative to 5-aminotetrazole in the inflators of airbags manufactured by Takata Corporation, which were recalled as unsafe after killing 14 people. The current USA death total is 27.

Numerous safety guidelines are available for storing and handling ammonium nitrate. Health and safety data are shown on the safety data sheets available from suppliers and from various governments.

Pure ammonium nitrate does not burn, but as a strong oxidizer, it supports and accelerates the combustion of organic (and some inorganic) material.

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