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mmonium nitrate fertilizer, chemically classified as an oxidizer, is commonly used in agriculture and for commercial and residential turf grass. Approximately 7 million tons of ammonium nitrate are produced annually in the U.S. and, under normal conditions, it is a relatively safe material in storage and use.Ammonium nitrate has been in the news during the past two decades when it was used as an oxidizer for homemade chemical explosives by domestic and foreign terrorists. Ammonium nitrate also has been involved in accidents and responsible for the deaths of emergency responders and civilians. This year in West, TX, a fire and explosion at a retail fertilizer facility killed 15 people, mostly emergency responders fighting the fire. Ammonium nitrate is also used as a chemical oxidizer in the manufacturing of the commercial explosive ANFO, a chemical mixture of ammonium nitrate and No. 2 fuel oil.
Handle with care
Ammonium nitrate, NH4NO3, is a strong inorganic oxidizer that can be an explosive by itself under certain conditions. It is primarily used as an agricultural fertilizer for its nitrogen content. Response personnel should deal with ammonium nitrate incidents with a great deal of caution. Ammonium nitrate is a colorless or white-to-gray crystal that is soluble in water. It decomposes at 210°C (392°F), releasing nitrous oxide gas and ammonia.
Although ammonium nitrate itself does not burn, as an oxidizer it supports and enhances combustion. When it comes in contact with other combustible materials, the fire hazard is increased. A fire involving ammonium nitrate in an enclosed space can lead to an explosion. Because it is an oxidizer, a fire involving ammonium nitrate can occur in the absence of atmospheric oxygen. Ammonium nitrate may explode when exposed to strong shock or high temperatures under confinement.
Contaminants may increase the explosion hazard of ammonium nitrate. Organic materials, such as chlorides and some metals, such as chromium, copper, cobalt and nickel, can make explosions involving ammonium nitrate more energetic.
Under National Fire Protection Association (NFPA) 704, Standard System for the Identification of the Hazards of Materials for Emergency Response, hazards for ammonium nitrate are health 1, fire 0, reactivity 3 and special information OX for oxidizer. The four-digit United Nations identification number is 1942 with an organic coating and 2067 as the fertilizer grade. Other mixtures of ammonium nitrate have four-digit numbers; they can be found in the Department of Transportation (DOT) Hazardous Materials Tables and Emergency Response Guide.
Factors in explosions
Several factors must be in place for an explosion to occur involving a chemical explosive such as ammonium nitrate. An explosion involving a chemical explosive is in actuality a fast-moving fire. In simple terms, an explosive that functions via a chemical reaction creates a rapidly burning fire that is made possible by the presence of a chemical oxidizer. Atmospheric oxygen does not provide enough oxygen for a chemical explosion to take place.
As a review of fire science, in order to have a fire, you need a fuel, oxygen and heat, which when combined under the right physical conditions create an ongoing chemical chain reaction. This process continues until the fuel is consumed, the heat is reduced, the oxygen is removed or something interrupts the chemical chain reaction.
Components that allow a fire to burn rapidly enough to produce an explosion are the presence of a fuel; instead of oxygen, you need a chemical oxidizer; plus heat or some type of initiator that creates the heat. Unlike fire, you need the chemical mixture to be confined for an explosion to occur. Confinement can be accomplished with the use of a piece of pipe, plastic tube, cardboard tube or any type of substantial container that will accomplish confinement. The material itself, if in an appropriate-size volume, can provide the confinement necessary for the explosion to occur.