It is by far the most widely used explosive in coal mining, quarrying, metal mining, and civil construction: it accounts for an estimated 80% of the 6,000,000,000 pounds (2,700,000 metric tons) of explosive used annually in North America. It also sees service in improvised explosive devices, where it is also known as a fertilizer bomb.
ANFO under most conditions is considered a high explosive; it decomposes through detonation rather than deflagration and with a high velocity. It is a tertiary explosive consisting of distinct fuel and oxidizer phases and requires confinement for efficient detonation and brisance. Its sensitivity is relatively low; it generally requires a booster (e.g., one or two sticks of dynamite, as historically used, or, in more recent times, Tovex) to ensure reliable detonation. The explosive efficiency associated with ANFO is approximately 80% of TNT, also stated as (0.8) TNT equivalency. The most efficient mixed AN explosives using fuels other than fuel oil can exceed (1.6) TNT equivalency.
The basic chemistry of ANFO detonation is the reaction of ammonium nitrate (NH4NO3) with a long chain hydrocarbon (CnH2n+2) to form nitrogen, carbon dioxide and water. In an ideal stoichiometrically balanced reaction, ANFO is composed of approximately 94.3% AN and 5.7% FO by weight. The normal ratio recommended is 2 quarts of fuel oil per 50 pounds of ammonium nitrate. In practice, a slight excess of fuel oil is added, as underdosing results in reduced performance while overdosing merely results in more post-blast fumes. Thus, as recommended by the Wisconsin Conservation Department, while avoiding producing excessive yellow smoke, a ratio of "2.5 to 3 quarts of fuel oil per 50 pounds" of ammonium nitrate "produces excellent results". When detonation conditions are optimal, the aforementioned gases are the only products. In practical use, such conditions are impossible to attain, and blasts produce moderate amounts of toxic gases such as carbon monoxide and oxides of nitrogen (NOx).
Ammonium nitrate is widely used as a fertilizer in the agricultural industry. In many countries its purchase and use is restricted to buyers who have obtained the proper license. This restriction is primarily because it is an attractive and simple component used in the production of fertilizer bombs by terrorists.
In the mining industry, the term ANFO specifically describes a mixture of solid ammonium nitrate prills and No. 2 fuel oil (heating oil.) In this form, it has a bulk density of approximately 840 kg/m3. The density of individual prills is about 1300 kg/m3, while the density of pure crystalline ammonium nitrate is 1700 kg/m3. It is notable that AN prills used for explosive applications are physically different from fertilizer prills; the former contain approximately 20% air. These voids are necessary to sensitize ANFO: they create so-called "hot spots".
AN is highly hygroscopic, readily absorbing water from air. It is dangerous when stored in humid environments, as any absorbed water interferes with its explosive function. AN is also water soluble. When used in wet mining conditions, considerable effort must be taken to dewater boreholes.
Other explosives based on the ANFO chemistry exist; the most commonly used are emulsions. They differ from ANFO in the physical form the reactants take. The most notable properties of emulsions are water resistance and higher bulk density.
The popularity of ANFO is largely attributable to its low cost and high stability. In most jurisdictions, ammonium nitrate need not be classified as an explosive for transport purposes; it is merely an oxidizer. Many mines prepare ANFO on-site using the same No. 2 diesel fuel that powers their vehicles, although heating oil (No. 2 fuel oil), which is nearly identical, may cost less than No. 2 diesel fuel. Many fuels can theoretically be used; however, the low volatility and cost of No. 2 fuel oil makes it ideal.
Unmixed ammonium nitrate can decompose explosively and has been responsible for industrial disasters such as the Texas City disaster in Texas City, Texas in 1947 and the Ryongchon disaster of Ryongchon, North Korea in 2004. However, it is considered a somewhat inefficient explosive as it exhibits only (0.44) TNT equivalency.
ANFO has occasionally been used in terrorist bombings. First used in 1970 by student protesters at the University of Wisconsin-Madison, who learned how to make and use ANFO from a Wisconsin Conservation Department booklet entitled Pothole Blasting for Wildlife, the ANFO car bomb was soon adopted by the IRA, such as in the 1993 Bishopsgate bombing. It has also seen use by groups such as the FARC, ETA, and various Palestinian terrorists. A more sophisticated variant of ANFO (with nitromethane ammonium nitrate as the fuel called ANNM) was used in the 1995 Oklahoma City bombing. Improvised bombs made with agricultural-grade AN are less sensitive and less efficient than the explosive-grade variety.
ANNM or amonium nitrate and nitromethane is the most powerful type of AN explosive exceeding (1.6) TNT equivalency when mixed correctly. It usually contains a 60:40 mix of AN and NM (60% ammonium nitrate, 40% nitromethane by mass) but this results in a wet slurry. Sometimes more AN is added to reduce liquidity and make it easier to store and handle. ANNM is sometimes referred to as "poor man's RDX" as it has a similar RE factor to RDX when mixed correctly and it is far cheaper and easier to obtain. When the ANNM detonates the primary products are H2O, CO2 and N2 but NOx and other toxic gases are sometimes formed because of impurities. The balanced equation is as follows:
3NH4NO3 + 2CH3NO2 -> 4N2 + 2CO2 + 9H2O