Ammonium nitrate

Template:Short description Template:Use dmy dates Template:Chembox

Ammonium nitrate is a chemical compound with the formula Template:Chem2. It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, but does not form hydrates. It is predominantly used in agriculture as a high-nitrogen fertilizer.<ref name=Ullmann>Template:Cite encyclopedia</ref>

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, an 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.<ref name=Dallas>Ammonium nitrate sold by ton as U.S. regulation is stymied. Template:Webarchive – The Dallas Morning News</ref> Accidental ammonium nitrate explosions have killed thousands of people since the early 20th century.<ref name=Dallas /><ref name="auto">Template:Cite web</ref> Global production was estimated at 21.6 million tonnes in 2017.<ref>Template:Cite web</ref> By 2021, global production of ammonium nitrate was down to 16.7 million tonnes.<ref name="auto"/>

Occurrence

Ammonium nitrate is found as the natural mineral gwihabaite (formerly known as nitrammite)<ref>Template:Cite web</ref> – the ammonium analogue of saltpetre (mineralogical name: niter)<ref>Template:Cite web</ref><ref>Template:Cite web</ref> – 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, rendering nitrate mining obsolete.

Production, reactions and crystalline phases

The industrial production of ammonium nitrate entails the acid-base reaction of ammonia with nitric acid:<ref>Template:Cite patent</ref>

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

Reactions

As ammonium nitrate is a salt, both the cation, Template:Chem2, and the anion, Template:Chem2, 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.<ref>Template:Greenwood&Earnshaw2nd</ref>

2 NH₄NO₃ → 2 N₂ + O₂ + 4 H₂O

Both decomposition reactions are exothermic and their products are gases. Under certain conditions, this can lead to a runaway reaction, with the decomposition process becoming explosive.<ref name="CEN"/> See Template:Section link 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.<ref name="CEN">Template:Cite web</ref>

Crystalline phases

Several 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<ref>Template:Cite journal</ref>

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.<ref>Template:Cite journal</ref>

Applications

Fertilizer

Ammonium nitrate is an important fertilizer with NPK rating 34-0-0 (34% nitrogen).<ref>Template:Cite web</ref> 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.

Explosives

Template:See also Ammonium nitrate readily forms explosive mixtures with varying properties when combined with explosives such as TNT or with fuels like aluminium 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)

Mixture with fuel oil

Template:Main ANFO is a mixture of 94% ammonium nitrate ("AN") and 6% fuel oil ("FO") widely used as a bulk industrial explosive.<ref name=Cook>Template:Cite book</ref>Template:Rp 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.<ref name=Cook/>Template:Rp

Terrorism

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 KPK (previously termed as NWFP) of Pakistan imposed a ban on ammonium sulfate, ammonium nitrate, and calcium ammonium nitrate fertilizers in the former Malakand Division Template:Sndcomprising 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 Template:Sndthe material which allows safety matches to catch fireTemplate:Sndhas surpassed fertilizer as the explosive of choice for insurgents."<ref>Template:Cite web</ref>

Niche uses

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.<ref>Template:Cite news</ref>

It was once used, in combination with independently explosive "fuels" such as guanidine nitrate,<ref>Template:Cite patent</ref><ref>Airbag Compound Has Vexed Takata for Years – The New York Times</ref> 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.<ref>A Cheaper Airbag, and Takata's Road to a Deadly Crisis. – The New York Times</ref> The current USA death total is 27.<ref>Template:Cite web</ref>

Safety, handling, and storage

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.<ref name="epa_safe_storage">Chemical Advisory: Safe Storage, Handling, and Management of Ammonium Nitrate United States Environmental Protection Agency</ref><ref>Template:Cite web</ref><ref>Template:Cite web</ref>

Pure ammonium nitrate does not burn, but as a strong oxidizer, it supports and accelerates the combustion of organic (and some inorganic) material.<ref name="epa_safe_storage"/><ref>Template:Cite book</ref><ref>Template:Cite web</ref> It should not be stored near combustible substances.

While ammonium nitrate is stable at ambient temperature and pressure under many conditions, it may detonate from a strong initiation charge. It should not be stored near high explosives or blasting agents.

Molten ammonium nitrate is very sensitive to shock and detonation, particularly if it becomes contaminated with incompatible materials such as combustibles, flammable liquids, acids, chlorates, chlorides, sulfur, metals, charcoal and sawdust.<ref name="orica">Template:Cite web</ref><ref name="epa_safe_storage"/>

Contact with certain substances such as chlorates, mineral acids and metal sulfides, can lead to vigorous or even violent decomposition capable of igniting nearby combustible material or detonating.<ref>Template:Cite web</ref><ref>Template:Cite web</ref>

Ammonium nitrate begins decomposition after melting, releasing [[NOx|Template:NOx]], HNO₃, [[Ammonia|NHTemplate:Su]] and H₂O. It should not be heated in a confined space.<ref name="epa_safe_storage"/> The resulting heat and pressure from decomposition increases the sensitivity to detonation and increases the speed of decomposition. Detonation may occur at 80 atmospheres. Contamination can reduce this to 20 atmospheres.<ref name="orica" />

Ammonium nitrate has a critical relative humidity of 59.4% at 30 °C. At higher humidity it will absorb moisture from the atmosphere. Therefore, it is important to store ammonium nitrate in a tightly sealed container. Otherwise, it can coalesce into a large, solid mass. Ammonium nitrate can absorb enough moisture to liquefy. Blending ammonium nitrate with certain other fertilizers can lower the critical relative humidity.<ref>Template:Cite web</ref>

The potential for use of the material as an explosive has prompted regulatory measures. For example, in Australia, the Dangerous Goods Regulations came into effect in August 2005 to enforce licensing in dealing with such substances.<ref>Template:Cite web</ref> Licenses are granted only to applicants (industry) with appropriate security measures in place to prevent any misuse.<ref>Ammonium Nitrate-Regulating its use, Balancing Access & Protection from Template:Cite web</ref> Additional uses such as education and research purposes may also be considered, but individual use will not. Employees of those with licenses to deal with the substance are still required to be supervised by authorized personnel and are required to pass a security and national police check before a license may be granted.

Health hazards

Ammonium nitrate is not hazardous to health and is usually used in fertilizer products.<ref name="MSDS">Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite news</ref>

Ammonium nitrate has an LD₅₀ of 2217 mg/kg,<ref name=MSDS1>Template:Cite web</ref> which for comparison is about two-thirds that of table salt.

Disasters

Template:Main Ammonium nitrate decomposes, non-explosively, into the gases nitrous oxide and water vapor when heated. However, it can be induced to decompose explosively by detonation.<ref>Template:Cite journal</ref> Large stockpiles of the material can also be a major fire risk due to their supporting oxidation, a situation which can easily escalate to detonation. Explosions are not uncommon: relatively minor incidents occur most years, and several large and devastating explosions have also occurred. Examples include the Oppau explosion of 1921 (one of the largest artificial non-nuclear explosions), the Texas City disaster of 1947, the 2015 Tianjin explosions in China, and the 2020 Beirut explosion.<ref>Template:Cite news</ref>

Ammonium nitrate can explode through two mechanisms:

Shock induced detonation. An explosive charge within or in contact with a mass of ammonium nitrate causes the ammonium nitrate to detonate. Examples of such disasters are Kriewald, Morgan (present-day Sayreville, New Jersey), Oppau, and Tessenderlo.
Deflagration to detonation transition. The ammonium nitrate explosion results from a fire that spreads into the ammonium nitrate (Texas City, TX; Brest; West, TX; Tianjin; Beirut), or from ammonium nitrate mixing with a combustible material during the fire (Gibbstown, Cherokee, Nadadores). The fire must be confined at least to a degree for successful transition from a fire to an explosion.