Ammonium nitrate

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Ammonium nitrate

By François NR RENAUD, honorary professor

The explosion in the port of Beirut, on August 4, 2020, of 2,750 tons of ammonium nitrate, which destroyed a large part of the Lebanese capital, invites us to remember the usefulness of this product but also its drawbacks and in particular its explosive aspect.

History

The spectre of world famine

The British economist Thomas Malthus (1776-1834) warned that the world’s population is growing faster than the food we can provide. His colleague chemist William Crookes (1832-1919), then tried to intensify wheat production in England because the big wheat producers, the USA and Russia, were going to stop exporting to feed their own population. Nitrogenous fertilizers were then brought by South American guano or saltpeter from Chile, whose reserves were dwindling.

Many chemists then began to produce nitrogen fertilizers by taking nitrogen from the atmosphere, which accounts for 78% of the air. In 1909, the German company BASF bought Fritz Haber’s patent, which consisted in manufacturing ammonia by reacting nitrogen with hydrogen according to the reaction N2 + 3H2 to 2NH3 at high temperature, high pressure and in the presence of a catalyst. He was awarded the Nobel Prize in 1918.

Carl Bosch industrialized the process which then became the Haber-Bosch process in 1913, i.e. just before the war. He was awarded the Nobel Prize for Chemistry in 1931.

Remember that it was the same Fritz Haber who developed the first chlorine attack in Ypres on April 22, 1915. He then centralized the research and production of chemical weapons in Germany. He was therefore at the origin of chemical warfare agents and more generally of the CBRN.

The manufacture of explosives

Under the guise of improving soil fertility, in fact German chemists aimed above all to exploit the explosive properties of ammonium nitrate in order to participate in the “war effort”.

Indeed, at the beginning of the First World War, Germany was strangled by the maritime blockade and lacked nitrates for the manufacture of explosives. Chemists were recalled from the front and worked to manufacture nitric acid HNO3 and then ammonium nitrate, NH4NO3 from ammonia and the manufacture of explosives could resume in 1915.

Ammonium nitrate is a powerful explosive agent, which is easily manufactured: added to a small amount of fuel oil (6% of the total volume), a mixture of 94% porous ammonium nitrate pellets acts as an oxidizing agent and can be used to create a deadly explosive. As an example, this mixture represents about 80% of the 2.7 million tons of explosives used each year in the USA.

Ammonium nitrate then has two different faces: as a fertilizer, it is estimated that more than half of the increase in the world’s population relies on the existence of nitrogen fertilizers, a balance that can be compared to the devastating effect of the explosives used during armed conflicts, for example..

Ammonium nitrate

Properties

Ammonium nitrate is a salt of nitric acid and ammonia with the formula NH4NO3. It is one of the most important nitrogen compounds. Its preparation is relatively simple:
NH3 +HNO3 ——> NH4NO3

Molecular weight M = 80, crystallized, it has a density d = 1.72 and a melting point F = 169 °C. However, ammonium nitrate is unstable above 170 °C and at 300 °C it decomposes, sometimes explosively, releasing a lot of gas and heat:
NH4NO3 ——> N2 + 2H2O + 0.5 O2 (V = 981 cm3/g and ΔQ = -1 465 J/g)

Stability

Ammonium nitrate is stable in its pure state, insensitive to shocks and friction, but it can be transformed into an explosive in the presence of certain products such as fuel oil or gasoline and from a simple spark. It is an “oxidizing agent”, i.e. without necessarily being combustible, it can, generally by yielding oxygen, cause or promote the combustion of other materials.

The decomposition of ammonium nitrate can occur under the effect of strong heat sources: in case of fire for example, it melts at 170°C and is easily contaminated. The decomposition can possibly lead to deflagration or even detonation. Such a phenomenon is very difficult to achieve when heating pure product. On the other hand, the risk is considerably increased in the presence of products such as organic matter (fuel oil, coal, sugar, fats, oils…), elemental sulphur, reducing agents, powdered metals, chlorates…; even in small quantities, these lower the critical temperature and accelerate the reaction speeds”.

As an explosive, mixed with TNT (trinitrotoluene) or pentrite, it is used in construction, mines and quarries. In ANFO (Ammonium Nitrate Fuel Oil), composed of 94% ammonium nitrate and 6% oil, it is not very sensitive and is suitable for civil engineering works, as it has a low propagation velocity of about 3,000 m.s-1 compared to that of dynamite, 6,000 m.s-1.

Ammonium nitrate is also a powerful oxidizer. Molten nitrate oxidizes metals such as zinc and lead, which dissolve under its action.

Fertilizers

As a fertilizer it is mainly used as a nitrogen fertilizer for leafy legume crops. Commercial ammonium nitrates must contain less than 0.02% chlorine and less than 0.2% combustible compounds. Clay, marl, or dolomite is added to it to have nitrogen contents of around 33% in France.

About ten million tons of mineral fertilizers are used in France over one year, according to the Ministry of Ecological Transition, including 5 to 6 million tons of nitrogen fertilizers.

A distinction is made between: simple fertilizers whose only nutrient is the nitrogen contained in ammonium nitrate = ammonitrate: 28% < N < 33.5% (If N> 34.5% and mixed with a fuel it becomes an industrial explosive (building mining)), and compound fertilizers containing nitrogen, phosphorus and potassium. Compound fertilizers are not explosive.

Human Toxicity

Ammonium nitrate is toxic to humans. By inhalation of its dust, it irritates the respiratory tract; by prolonged exposure it causes weakness, headaches and by contact, skin irritation.

Major accidents

Here and there, explosions can occur here and there, always devastating, on farms that store ammonium nitrate-based fertilizers. But, in some cases, major accidents causing considerable damage and accompanied by high mortality have been recorded in history.

The Oppau explosion occurred in Germany on 21 September 1921. It destroyed most of Oppau (a commune attached to that of Ludwigshafen in 1938), killed 561 people and wounded about 2,000. The town was devastated. It was a mixture of 4,000 tons of ammonium nitrate and sulfate that caused it.

On April 16, 1947, 581 people lost their lives in Texas City after a fire caused a French ship containing more than 2,000 tons of ammonium nitrate to explode in its port. Mobilized to prevent the disaster, the city’s firefighters were exterminated: only one survived the explosion. There were also more than 3,000 injured.

The Ocean Liberty is a Norwegian cargo ship, of a Liberty ship model, whose explosion, during unloading on July 28, 1947, caused extensive damage in Brest. It killed 26 people and injured hundreds. It was the explosion, in highly confined conditions, of a mixture of ammonium nitrate (between 740 and 3,000 tons, depending on the source) and liquid fuels.

In 1988, the explosion at the Pepcon plant in the United States was caused by a fire in the ammonium perchlorate stockpile.

On February 18, 2004, in Iran, the derailment of a train carrying 420 tons of ammonium nitrates caused a fire and then an explosion killing 328 people.

December 13, 1994, Sioux City, United States. On the manufacturing site, explosion of 75 t of nitrate in the reactor and the neighboring reserve; the death toll was 4 and 18 injured.

In France, the explosion of 300 tons of ammonium nitrate at the AZF Toulouse plant on September 21, 2001, left 2,500 people injured and 31 victims. Even today, we do not know what the trigger might have been.

The explosion of the West Fertilizer Company occurred in April 2013 at a nitrogen fertilizer depot located in the town of West, Texas, following an arson attack. Again, it was an ammonium nitrate depot that exploded, killing 15 people and injuring 200. The plant stored 240 tons of ammonium nitrate and 50 tons of anhydrous ammonia on this site.

On August 4, 2020, 2,750 tons of ammonium nitrate exploded in Beirut, killing 200 people and injuring thousands. There were 2 successive explosions whose blast was felt as far as Cyprus 200 km away.

U.S. terrorist Timothy McVeigh (executed in 2001) blew up a truck loaded with two tons of fertilizer in front of a federal building in Oklahoma City killing 168 people on April 19, 1995.

Transport

Ammonium nitrates are classified as oxidizing agents by the legislation on the transport of dangerous goods. The particular risks, known since 1921, imply a particular regulation concerning the storage of this product.

In port areas ammonium nitrate is transported in 25 or 50 kg big bags. The heaps of bags must not exceed 250 tons and the batches are separated by a distance of 4 m.

Ammonitrates are available in the form of solid granules or prills. They can be packaged in 50 kg plastic bags (leakproof containers), in 500 or 600 kg flexible containers, or in bulk.

Road transport is regulated and farms that store ammonium nitrate do not keep more than 250 tons.

Stockage

In France, one hundred and eight sites classified as “Seveso” store ammonitrates, and sixteen warehouses are listed as “Seveso high threshold” (more than 2,500 metric tons of ammonitrates).

Fertilizers are classified in 2 categories

They are considered dangerous above 28% nitrogen.

  • High dosage ammonitrates which contain 28 to 34.5% nitrogen and therefore sometimes more than 80% ammonium nitrate. To be marketed in France, they must meet a certain number of criteria, including a prior test of detonability              
  • Medium dosage ammonitrates: they contain 20 to 28% nitrogen (N), i.e. less than 80% ammonium nitrate.

Ammonium nitrate-based compound fertilizers are known as NP, NK or NPK fertilizers (N for nitrogen, P for phosphorus, K for potassium). Their ammonium nitrate content varies considerably from 3 to more than 80%.

Storage of concentrated ammonium nitrate falls under Seveso low threshold status from 350 tons of ammonium nitrate and Seveso high threshold status from 2,500 tons (lower quantities nevertheless apply for certain particular forms of ammonium nitrate).

Conclusion

On the human side, ammonium nitrate, a two-sided product, has enabled millions of people to feed themselves. The explosive side has also helped mankind in its “useful” version (quarrying…) but it is the cause of millions of deaths when it is used during conflicts for example.

As for accidents, always dramatic, they will always exist as long as the regulations concerning the transport or storage of these products are not respected .

References

Usine nouvelle 5 août 2020

The conversation 10 août 2020

La tribune 7 août 2020

Wikipédia  Consulté le 18 septembre 2020

Le monde, daté du vendredi 14 aout 2020 « En France, le stockage du nitrate d’ammonium sous surveillance »

Le Progrès de l’Ain, daté du lundi 24 août 2020 « Des engrais à base de nitrate d’ammonium sous surveillance »

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