ANFO

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ANFO (or AN/FO, for ammonium nitrate / fuel oil) is a widely used explosive mixture. The oil used is most often No. 2 fuel oil, or diesel fuel, but sometimes kerosene, coal dust, or even molasses. Nitromethane is one of the most effective fuels used in this sort of explosive.

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 (2.7×109 kg) of explosive used annually in North America.[1] It also sees service in improvised explosive devices, where it is also known as a fertilizer bomb.[2]

Contents

Chemistry

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 or cast boosters of TNT/PETN or similar compositions) to ensure reliable detonation.

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 U.S. quarts of fuel oil per 50 pounds of ammonium nitrate (80 ml/kg). In practice, a slight excess of fuel oil is added, i.e., 2.5 to 3 quarts of fuel oil per 50 pounds of ammonium nitrate, as underdosing results in reduced performance while overdosing merely results in more post-blast fumes.[3] 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).

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