The chemical compound ammonium nitrate, the nitrate salt of ammonia, has the chemical
formula NH4NO3. It is a white crystalline solid which is highly soluble in water. It
is predominantly used in agriculture as a high-nitrogen fertilizer. The compound is
used as explosives in mining, but also improvised explosive devices. It is the main component
of ANFO, a popular explosive, which accounts for 80% explosives used in North America.
It is used in instant cold packs, as hydrating the salt is an endothermic process.
Ammonium nitrate is found as a natural mineral in the driest regions of the Atacama Desert
in Chile, often as a crust on the ground and/or in conjunction with other nitrate, chlorate,
iodate, and halide minerals. Ammonium nitrate was mined there in the past, but virtually
100% of the chemical now used is synthetic.
Fertilizer Ammonium nitrate is an important fertilizer
with the NPK rating 34-0-0. It is less concentrated than urea, giving ammonium nitrate a slight
transportation disadvantage. Ammonium nitrate's advantage over urea is that it is more stable
and does not lose nitrogen to the atmosphere. During warm weather, urea is typically to
only be applied before imminent rain to minimize nitrogen loss.
Safety, handling, and storage Health and safety data are shown on the material
safety data sheets available from suppliers and found on the internet.
Heating or any ignition source may cause violent combustion or explosion. Ammonium nitrate
reacts with combustible and reducing materials as it is a strong oxidant. Although it is
mainly used for fertilizer, it can be used for explosives. It was sometimes used to blast
away earth to make farm ponds. Ammonium nitrate is also used to modify the detonation rate
of other explosives, such as ammonia-based dynamites, for example nitroglycerin and amatol.
Numerous safety guidelines are available for storing and handling ammonium nitrate. It
should not be stored near combustible substances or certain fertilizers, especially urea.
Ammonium nitrate has a critical relative humidity of 59.4%, above which 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. 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. Licenses are granted only to applicants with appropriate security measures in place
to prevent any misuse. 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.
Production The industrial production of ammonium nitrate
entails the acid-base reaction of ammonia with nitric acid:
HNO3 + NH3 → NH4NO3 Ammonia is used in its anhydrous form gas
and the nitric acid is concentrated. This reaction is violent owing to its high exothermic
nature. After the solution is formed, typically at about 83% concentration, the excess water
is evaporated to an ammonium nitrate content of 95% to 99.9% concentration, 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. The ammonia required for this process is obtained
by the Haber process from nitrogen and hydrogen. Ammonia produced by the Haber process is oxidized
to nitric acid. Another production method is used in the so-called Odda process.
Ammonium nitrate can also be made via metathesis reactions:
(NH4)2SO4 + 2 NaNO3 → 2 NH4NO3 + Na2SO4 (NH4)2SO4 + Ca(NO3)2 → 2 NH4NO3 + CaSO4
Sodium sulfate is removed by lowering the temperature of the mixture. Since sodium sulfate
is much less water-soluble than ammonium nitrate, it precipitates, and may be filtered off.
For the reaction with calcium nitrate, the calcium sulfate generated is quite insoluble,
even at room temperature. Reactions
Ammonium nitrate reacts with metal hydroxides, releasing ammonia and forming alkali metal
nitrate: NH4NO3 + MOH → NH3 + H2O + MNO3
Ammonium nitrate gives ammonium chloride and nitric acid upon reaction with hydrochloric
acid: NH4NO3 + HCl → NH4Cl + HNO3
Ammonium nitrate leaves no residue when heated: NH4NO3 → N2O + 2H2O
Ammonium nitrate is also formed in the atmosphere from emissions of NO, SO2, and NH3, and is
a secondary component of PM10. Crystalline phases
Transformations of the crystal states due to changing conditions affect the physical
properties of ammonium nitrate. These crystalline states have been identified:
The type V crystal is a quasicubic form which is related to caesium chloride, the nitrogens
of the nitrates and the ammoniums are at the sites in a cubic array where Cs and Cl would
be in the CsCl lattice. Health hazards
Health and safety data are shown on the material safety data sheets which are available from
suppliers and can be found on the internet. Ammonium nitrate is not very hazardous to
health and is usually used in fertilizer products. The chances of direct personal exposure to
the chemical are very low, because the fertilization of the soil by use of ammonium nitrate is
done at early stages of plant growth and usually does not remain detectable on the harvested
plants or when the plants reach the consumer. Ammonium nitrate has an LD50 of 2217 mg/kg,
which for comparison is about two-thirds that of table salt.
Acute health effects Short-term exposure to ammonium nitrate can
cause symptoms ranging from minor irritation to nausea, vomiting, gastric irritation, headaches,
dizziness, and hypertension. Long-term health effects
The toxicity of nitrates when ingested is due to in vivo conversion to nitrites. The
material safety data sheet considers chronic ingestion of more than 5 mgday unacceptable.
The primary overdose effects of chronic exposure are orthostatic hypotension and methemoglobinemia.
Other common effects include: faintness, fatigue, weakness, depression, mental impairment, dizziness,
shortness of breath, and reflex tachycardia; headache, nausea, vomiting, and nephritis
may also occur. Disasters
Ammonium nitrate decomposes into the gases nitrous oxide and water vapor when heated;
however, it 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. The two major classes of incidents resulting
in explosions are: 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, and Tessenderlo.
The explosion results from a fire that spreads into the ammonium nitrate itself, or from
a mixture of ammonium nitrate with a combustible material during the fire. The fire must be
confined at least to a degree for successful transition from a fire to an explosion. Pure,
compact AN is stable and very difficult to ignite, and numerous cases exist when even
impure AN did not explode in a fire. Ammonium nitrate-based explosives were used
in the Oklahoma City and 2011 Delhi bombings, the 2013 Hyderabad blasts, and the bombing
in Oslo 2011. Ammonium nitrate decomposes in temperatures
normally well above 200°C. However, the presence of impurities often reduce the temperature
point when heat is being generated. Once the AN has started to decompose, then a runaway
reaction will normally occur as the heat of decomposition is very large. AN evolves so
much heat that this runaway reaction is normally impossible to stop. This is a well-known hazard
with some types of N-P-K Fertilizers, and it is responsible for the loss of several
cargo ships. Under normal handling conditions, ammonium
nitrate is not harmful. However, inhalation of high concentrations of its dust can cause
respiratory tract irritation. Symptoms may include: coughing, sore throat, shortness
of breath, or even suffocation. When swallowed in high concentrations, ammonium nitrate may
cause headache, dizziness, abdominal pain, vomiting, bloody diarrhea, weakness, a tingling
sensation, heart and circulation irregularities, convulsions, collapse, and suffocation. It
forms a mild acid when mixed with water. This acid can cause irritation to the eyes, nose,
and skin. In November 2009, a ban on ammonium sulfate,
ammonium nitrate, and calcium ammonium nitrate fertilizers was imposed in the former Malakand
Division—comprising the Upper Dir, Lower Dir, Swat, Chitral, and Malakand districts
of the North West Frontier Province of Pakistan—by the NWFP government, following reports that
those chemicals were used by militants to make explosives. In January 2010, these substances
were also banned in Afghanistan for the same reason. After several cases, AN has now been
legalised due to the Pakistani forces of NWFP. Ammonium nitrate was suspected as the explosive
responsible for the fertilizer plant explosion in West, Texas on April 17, 2013. Investigators
said they believe it exploded following a fire that began in the plant's office.
Mixture with fuel oil
ANFO is a mixture of 94% ammonium nitrate and 6% fuel oil 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 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.
References
External links International Chemical Safety Card 0216
"Storing and Handling Ammonium Nitrate", United Kingdom Health and Safety Executive publication
INDG230 Chemical Advisory: Safe Storage, Handling,
and Management of Ammonium Nitrate United States Environmental Protection Agency
Calculators: surface tensions, and densities, molarities and molalities of aqueous ammonium
nitrate