AMMONIUM NITRATE EXPLOSION HAZARDS

Ammonium nitrate (AN) primarily is used as a fertilizer but it is also very important compound in the production of industrial explosives. The application of ammonium nitrate in the production of industrial explosives was related with the early era of Nobel dynamite and widely increased with the appearance of blasting agents such as ANFO and Slurry, in the middle of the last Century. Throughout the world millions of tons of ammonium nitrate are produced annually and handled without incident. Although ammonium nitrate generally is used safely, accidental explosions involving AN have high impact resulting in loss of lives and destruction of property. The paper presents the basic properties of ammonium nitrate as well as hazards in handling of ammonium nitrate in order to prevent accidents. Several accidents with explosions of ammonium nitrate resulted in catastrophic consequences are listed in the paper as examples of non-compliance with prescribed procedures.


INTRODUCTION
Ammonium nitrate (AN) has extensive use in the area of nitrogen fertilizers and explosives (Elvers, 1989;Suslick,1992).AN is the main oxidizing salt in many types of industrial explosives.While the production of dynamite and powder AN/TNT and AN/NG explosives was strictly related to factory production, the production of blasting agents like ANFO, Slurry, emulsion and heavy ANFO approached to the mines as the places of consumption.The production of Slurry and ANFO at mines (on the spot -NALIM system) in Serbia was started in mid 70's of the last Century.The first production of ANFO by the system NALIM was on June 2 nd , 1975 while the first filling with Slurry explosives -MAJDANIT was started on October 29 th , 1976 in the mines of RTB Bor, Serbia (Kričak et al. 2010).
The next stage in the development of explosives was the invention of emulsion explosives and finally heavy ANFO as a mixture of emulsion and ANFO explosives.Bulk explosives such as ANFO, Slurry and heavy ANFO had the influence on rapid reduction of the production of dynamite and explosive sensitive powder explosives.
Ammonium nitrate has different forms depending upon its use.Ammonium nitrate in crystalline form, porous prills/granules or as saturated aqueous solution is mainly used for the production of industrial explosives.Ammonium Nitrate Based Fertilizers are uniform mixtures in prill or granular form containing ammonium nitrate as the main ingredient.
The application of AN in the production of different types of explosives is presented in Table 1, (Kričak et al. 2010).
Table 1 -Application of AN in the production of different types of explosives (Kričak et al. 2010) The

PHYSICAL AND CHEMICAL PROPERTIES OF AMMONIUM NITRATE
Ammonium nitrate is obtained by neutralization of nitric acid with ammonia.It is highly soluble in water and very hygroscopic.Therefore, it should be protected from moisture during storage and transportation.
Ammonium nitrate has a positive oxygen balance +20%.It enables the content of oxygen in excess as compared with the quantity of oxygen required for the oxidization of all the oxidizable compounds present in the explosive or deriving from its decomposition.For this reason, ammonium nitrate is the main oxidizing salt in many types of explosives.
Many properties of AN especially those relevant to its use as a component of explosive mixtures are well documented (Mellor, 1922; US Department of Commerce, 1972;Elvers,1989, Suslick, 1992).Pure ammonium nitrate is very stable and should meet specified quality requirements to be used in the production of industrial explosives.Some of these properties are summarized in Table 2.

THE MAIN CHEMICAL HAZARDS ASSOCIATED WITH AMMONIUM NITRATE
The main chemical hazards associated with ammonium nitrate are (European Fertilizer Manufacturers' Association, 2000): -Fire; -Decomposition; -Explosion

Fire
Ammonium nitrate itself does not burn.Being an oxidizing agent, it can facilitate the initiation of a fire and intensify fires in combustible materials.Hot AN solution can initiate a fire in fags, wooden articles etc., on coming into contact with them.Similarly, fertilizer products or dust contaminated with oil or other combustible materials can also start fires when left on hot surfaces (European Fertilizer Manufacturers' Association, 2000).
Fires involving AN cannot be extinguished by the prevention of air ingress because of the provision of oxygen from the AN.

Firefighting Measures
Fire may be caused by non-compliance with the instructions for use or not following the operating instructions (negligence, carelessness, lack of knowledge) (HIP-Azotara, 2015).
In case ammonium nitrate is caught by fire, plenty of water must be used and call the fire brigade.Water is the only known satisfactory extinguishing agent for an ammonium nitrate fire (Canadian Legal Information Institute, 2015).As an ammonium nitrate fire progresses, large quantities of very toxic gases are evolved.
Firefighters must be protected by wearing suitable protective clothing and selfcontained breathing apparatus.They must also be trained for carrying and properly using the equipment (HIP-Azotara, 2015).
If the initial fire is not extinguished but progresses, the next step is the evacuation of personnel in the safety distances.There is no possibility either method to predict at what point will explosion occur, but in almost all the explosions of ammonium nitrate affected by the fire there was always enough time for the evacuation (Kričak et al. 2010).

Decomposition
When pure ammonium nitrate is heated, a slow dissociation to nitric acid and ammonia becomes noticeable at a relatively low temperature, the reaction absorbing heat.At about 93ºC, the solid ammonium nitrate begins to sublime and the rate of sublimation increases until the melting point of 169ºC is reached.The molten material vaporizes at an rate as the temperature is raised (Compressed Gas Association, 1963).
At a temperature of about 210ºC, a reaction begins resulting the formation of nitrous oxide and steam according to the following reaction equation (Kričak et al. 2010): (1) This reaction releases heat and becomes more rapid as the temperature rises.When the ammonium nitrate is heated rapidly and the temperature allowed to rise to the range of 260ºC to 290ºC, the decomposition increasingly follows the various side reactions in which higher oxides of nitrogen as well as elemental nitrogen and oxygen and steam are formed.These reactions are highly exothermic and rapid, and if the ammonium nitrate and the products of the reaction are confined, so that the temperature or pressure rise cannot be controlled, the reaction may become explosive according to the reaction equation (Kričak et al. 2010): (2) These decomposition reactions refer to pure solid AN.The presence of certain impurities may lower the decomposition temperature.A number of materials have a strong catalytic effect on the thermal decomposition of AN.These include acids, chlorides, organic materials, chromates, dichromates, salts of manganese, copper and nickel and certain metals such as zinc, copper and lead (European Fertilizer Manufacturers Association, 2000).
The release of toxic fumes is one of the main hazards associated with the decomposition of AN.Decomposition may produce nitrogen oxides (NO, NO 2 , ...), ammonia and amines (HIP-Azotara, 2015).Pure solid ammonium nitrate does not heat spontaneously.

Explosion
According to EFMA (European Fertilizer Manufacturers Association, 2000), AN is especially difficult to detonate and neither flame, spark nor friction is known to cause detonation.Shocks derived from detonating gas mixtures (hydrogen/oxygen or acetylene/oxygen) have been found to be incapable of producing detonation in AN.AN fertilizer dust, being non-combustible in nature, does not give rise to a dust explosion such as those commonly associated with grain and organic dusts.Shock initiation in solid prilled AN needs a fairly substantial stimulus.Heating under confinement and shock initiation of hot or contaminated AN by projectile impact appear to be more credible mechanisms in the context of industrial operations (European Fertilizer Manufacturers Association, 2000).
Strongly acidic conditions and the presence of contaminants should be avoided to counter the explosion hazard in AN solutions.Explosions can occur when ammonium nitrate is heated under confinement in pumps (European Fertilizer Manufacturers Association, 2000).

CLASSIFICATION OF AMMONIUM NITRATE
In Safety Data Sheet of the Serbian Company for the production of fertilizers and nitrogen compounds "HIP -AZOTARA", Pančevo from May 15 th , 2015 (HIP-Azotara, 2015) ammonium nitrate 34.8% N is classified according to the Rulebook on classification, packaging, labeling and advertising of the chemical and certain article in accordance with the Globally harmonized system of classification and labeling of the UN ("The Official Gazette of the RS", no.64/10, 26/11, 5/12 and 105/13) as presented in Table 4.
As can be seen in Table 4, ammonium nitrate is classified as oxidizer which may intensify the fire.AN is not self-ignitable, however it can burn even in the absence of air.When heated it melts and may cause decomposition and release of toxic gases containing nitrogen oxides and ammonia.In the presence of reducing agents ammonium nitrate may transform into ammonium nitrite, which is not stable and may cause the risk of explosion (HIP-Azotara, 2015).

AMMONIUM NITRATE STORAGE
In many Countries there are specific legal requirements which must be followed regarding ammonium nitrate storage.These requirements generally cover the storage areas with respect to their structural and operational requirements and must be consulted for the relevant Country (European Fertilizer Manufacturers Association, 2000).
Ammonium nitrate is stable if kept in original containers in storage areas with natural ventilation and if protected against fire, ignition sources, wet floors and exposure to the atmosphere (HIP-Azotara, 2015).Ammonium nitrate is an oxidizing material that, at elevated temperatures, will support the combustion of materials such as wood, paper, fuel oil and sulphur; but self-sustaining burning reactions are not usually obtained unless more than one percent of combustible material is present (Canadian Legal Information Institute, 2015).
The storage area must be dry and well ventilated.The stored product must not be directly exposed to sunlight in order to avoid physical damage due to thermal decomposition.Smoking in the storage area is strictly forbidden.
Electrical installation in storage area must be resistant to ammonia vapors.Explosives, solid or liquid substances susceptible to decomposition by explosion, for example.organic peroxides or flammable liquids such as gasoline, benzene, toluene, ether, etc should not be stored in any warehouse or a part of a warehouse next to the one containing the AN (Kričak et al. 2010).
Organic substances, acids and other corrosive liquids and reactive substances such as chlorates, hypochlorites, chromates, nitrites, permanganates, zinc and its salts must not contaminate the AN in any case.
It should take care particularly to avoid contact AN with products that produce heat in a presence of moisture; quicklime, calcium cyanamide, slag as well as the products with poorly defined or undefined composition whose behavior in relation to the AN may be unsafe for example.pesticides, disinfectants, agents for the destruction of living organisms in the wood, etc. (Kričak et al. 2010).
Preferably, AN should be stored in closed warehouse.
The bags should be carefully put together in a pile, so that all sides can easily approach them.Piles should not be too close to walls or partition walls -at a distance of not less than 75 cm.Packaging requirements of AN are much stricter than those for fertilizers because of the hygroscopic properties of AN and the need for avoiding any danger that may occur due to spills of materials.Bags used for packaging of AN must be waterproof and sealed well to prevent entry of moisture.It must be strong enough to be able to withstand the damage during normal handling operations.At temperatures up to 55°C the bags must not show any signs of damage.
Water should be available near the warehouse in large quantities for immediate use.

AMMONIUM NITRATE ACCIDENTS
Although ammonium nitrate is not classified as explosive, there are many explosions incurred as a result of the fire of ammonium nitrate.The conditions necessary for an explosion were being examined in detail and it was quite certain that confined space and high temperature are key elements for the explosion of AN in fire conditions (Kričak et al. 2010).
The content of organic substances increases the sensitivity of AN, allowing the explosion even at a lower pressure and a lesser degree of coating.Warm AN is especially sensitive to contamination by organic or other combustible materials, especially when in the closed-limited space.Table 5 presents some of the Significant Accidents Involving Ammonium Nitrate -Post 1950 (Heather, 2002).
Fire and subsequent explosions incidents from 2003 to 2004 are listed in Table 6.
Table 6 -Fire and subsequent explosions incidents 2003-2004(Nygaard, 2006) ) Date/place Description October 2, 2003 St. Romain en Jarez, France (TF 1, 2003) Explosion in farmer's barn.The hay in a farmer's barn caught fire.The barn also contained some 3 000 kg to 5 000 kg fertilizer grade AN and about 3 000 plastic (polyethylene) boxes in piles.Approximately one hour and fifteen minutes after the fire was notified, an explosion took place causing 26 injuries (including 18 fire-men) and structural damage to 82 houses of varying severity.Damage was observed as far as 650 meter from the warehouse.February 18, 2004 Neyshabur, Khorasan province Iran (BBC, 2004) Railcar explosion.51 run-away train wagons derailed, of which 7 wagons were loaded with 420 t fertilizer AN in bags, 7 with 390 t urea in bags, 17 with crushed-in-bulk sulphur, 10 with gasoline and fuel oil, and 10 with cotton lint.The AN exploded after catching fire from sulphur and petroleum products, causing 300 fatalities.9 railway employees were charged for negligence.March 9, 2004Castellon, Spain (ThinkSpain, 2004) Truck explosion.A truck with 25 t of fertilizer AN in bulk (open truck, metal "bath-tube") collided with a car and rolled over on its back, with spill of fuel and a fire started.About 20-30 minutes later an explosion occurred.Two people were killed (the car driver and a truck driver which was hit by a stone in the explosion) and 5 injuries.Pieces were thrown 200 meters away.May 24, 2004Mihailesti, Romania (Highbeam Research, 2004) Truck explosion.A truck with 23 t of fertilizer AN in bags skid off the road and turned over.The truck cabin started burning.An hour later the cargo exploded, just when a fire crew was about to start the water hoses.20 people were killed and several houses in nearby village were damaged.Neither the truck nor the driver was qualified for dangerous goods transport.April 22, 2004North Korea Ryongchon (GlobalSecurity, 2004) Railcar explosion.An oil road tanker was reported to have collided with two railcars loaded with AN and knocking down an electric pole.It is thought that electric sparks caused a fire and explosion of the AN, with 161 fatalities and 1300 injuries.Due to the size of the crater officials have questioned whether military explosives were involved instead of AN. 6.1.1. Basf Plant Oppau, Germany, September 21, 1921 Ex explosives factory now making fertiliser -a 50/50 mixture of ammonium sulphate and ammonium nitrate called "mischsaltz" (Safety In Engineering, 2013).This highly hygroscopic mixture had the disadvantage of clogging together under the pressure of its own weight during storage.It was common practice to loosen the    In this case, AN was affected by a fire in an enclosed space without the possibility of leakage of dissolved AN, which accelerated the growth of temperature and pressure conditions and turned the fire into the explosion (Kričak et al. 2010).AN was packed in paper bags and coated with a 1% wax used as an anticaking agent, which increased the explosive sensitivity of AN taken by fire and also the energy potential of explosion (Kričak et al. 2010).

AZF Toulouse Disaster, September 21, 2001
A fertiliser factory containing ammonium nitrate storage facilities exploded.The factory employed 470 persons, was located 3 km from the centre of Toulouse on an island of the Garonne River surrounded by an (urban) environment.The explosion produced a crater measuring about forty meters in diameter and 7 m in depth related to the natural ground (Safety In Engineering, 2013).About 200 t to 300 t is said to be involved in the explosion (Kričak et al. 2010).The exact cause remains unknown.
Figure 4 presents crater resulting from the detonation and the warehouse destroyed by the blast.Explosion equivalent to 3.4 on Richter scale.22 people were killed on the factory site and 8 persons outside.In total 2 500 persons were injured.Various structures were distorted within a radius of 400 m to 700 m.Buildings were damaged within a radius of 700 m to 1 500 m and many windows broken even at a larger distance (Safety In Engineering, 2013).
In total about 30 000 buildings were touched of which 10 000 heavily damaged including schools, Universities and a hospital.Transportation facilities and electrical power lines and telephone communication were also disrupted.The financial consequences were estimated to be 2.5 billion euros.
The last three accidents represent a real disasters because of explosions of large amounts of ammonium nitrate.However, examples of accidents listed in Tables 5  and 6 show that the negligence in handling of ammonium nitrate during the transportation and storage may also cause fire or explosions.

CONCLUSIONS
Ammonium nitrate is a stable compound and generally is difficult to explode when it is in solid or molten form or in solution.However, ammonium nitrate may explode when it is exposed to strong shock or to high temperature under confinement.The presence of certain contaminants may increase the explosion hazard of ammonium nitrate.While certain inorganic contaminants, including chlorides and some metals, such as chromium, copper, cobalt, and nickel may sensitize ammonium nitrate, organic impurities increase the energy of ammonium nitrate explosions.As ammonium nitrate solution becomes more acidic, its stability decreases, and it may be more likely to explode.
In a large quantity of ammonium nitrate, localized areas of high temperature may be sufficiently confined by the total quantity to initiate an explosion.The explosion of a small quantity of ammonium nitrate in a confined space (e.g., a pipe) may initiate the explosion of larger quantities (e.g., in an associated vessel).
Low density areas, such as bubbles, in molten ammonium nitrate or solutions, also may increase the possibility of an explosion and enhance the propagation of an explosion.
Ammonium nitrate by itself does not burn, but in contact with other combustible materials, it increases the fire hazard.It can support and intensify a fire even in the absence of air.Fires involving ammonium nitrate can release toxic nitrogen oxides and ammonia.A fire involving ammonium nitrate in an enclosed space could lead to an explosion. FigureOpau Figure 3 -

Figure 4 -
Figure 4 -a) Crater resulting from the detonation, b) Warehouse destroyed by the blast (French Ministry of Sustainable Development, 2013)

Table 2 -
(Kričak et al. 2010)of AN for the production of industrial explosives(Kričak et al. 2010)

Table 3
(HIP-Azotara, 2015)physical and chemical properties of ammonium nitrate with 34.8% N produced by the Company for the production of fertilizers and nitrogen compounds "HIP-AZOTARA" Pančevo(HIP-Azotara, 2015)

Table 3 -
Properties of AN 34.8%N produced by the Company " H I P -A Z O T A R A " P a n čevo (HIP-Azotara, 2015)