ANFO stands for Ammonium Nitrate / Fuel Oil. 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 billion pounds of explosive used annually in North America.

ANFO under most conditions is considered a high explosive: it decomposes through detonation rather than deflagration and with a high velocity of detonation. It is a secondary explosive consisting of distinct fuel and oxidizer phases and requiring confinement for efficient detonation. Its sensitivity is relatively low: it generally requires a booster to ensure reliable detonation.

The basic chemistry of ANFO detonation is the reaction of ammonium nitrate (NH₄NO₃) with a long chain hydrocarbon (C_nH_2n+2) to form gasses of nitrogen, water, and carbon dioxide. In an ideal stoichiometrically balanced reaction, ANFO is comprised of approximately 96.3% AN and 5.7% FO by weight. 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. When detonation conditions are optimal, the aforementioned gasses are the only products. In practical use, such conditions are impossible to attain, and blasts produce moderate amounts of toxic gasses such as carbon monoxide, hydrogen sulfide, and oxides of nitrogen.

Ammonium nitrate is widely used as a fertilizer in the agricultural industry.

In the mining industry, the term ANFO specifically describes a mixture of solid ammonium nitrate prills and fuel oil. In this form, it has a bulk density of approximately 840kg/m³. The density of individual prills is about 1300kg/m³, while the density of pure crystalline ammonium nitrate is 1700kg/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" in which the interaction of the detonation front with a spherical void concentrates energy. Blasting-grade AN prills are typically between 0.90 and 3.00mm in diameter.

AN is highly hygroscopic; that is, it readily absorbs water from air. Care must be taken with its storage in humid environments, as any absorbed water interferes with its explosive function. AN is also water soluble. If ANFO is to be used in wet mining conditions, considerable effort must be taken to dewater boreholes.

Other explosives based on the AN/FO 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. Most mines prepare ANFO on-site using the same No. 2 diesel fuel that powers their vehicles. Many fuels can theoretically be used; however, the low volatility and cost of No. 2 diesel makes it ideal.

AN/FO has occasionally been used in terrorist bombings. It has seen use by groups such as the Provisional IRA, ETA, and various Palestinian extremists. A more sophisticated variant of the standard AN/FO reaction was used in the Oklahoma City bombing. It is noteworthy that improvised bombs made with agricultural-grade AN are less sensitive and less efficient than the explosive-grade variety.

See also

• Prill
• Sprengel explosive