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.
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. It also sees service in improvised explosive devices, where it is also known as a fertilizer bomb.[1]
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.[2] 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).
Industrial use
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 versions of ANFO which use prills are generalled called explosives grade, low density, or industrial grade ammonium nitrate. These voids are necessary to sensitize ANFO: they create so-called "hot spots".[3] Finely powdered aluminum can be added to ANFO to increase both sensitivity and energy; however, this has fallen out of favor due to cost. Other additions include perlite, chemical gassing agents, or glass air bubbles to create these voids. [4]
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.
Disasters
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 (0.42) TNT equivalency.
Terrorist use
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,[2][5] 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 terrorists when they first tried to destroy the World Trade Center towers in the 1990s. 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
ANNM or ammonium 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. ANNM is also more sensitive to shock than standard ANFO and so is easier to detonate which along with its power makes it a popular explosive among recreational users. 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
References
- ^ Jo Thomas (1997-09-29). "Jury to Be Picked in 2d Oklahoma Bomb Trial". The New York Times.
- ^ a b Mathiak, Harold A. (1965). Pothole Blasting for Wildlife. Wisconsin Conservation Department, Madison, Wisconsin 53701. p. 11.
- ^ It was found by the IRA, in response to using low brisance AN fertilizers, that "hot spots" can be created by blending powdered sugar into the ANFO mixture, effectively sensitizing the mixture to mining-standard prilled ammonium nitrate effectiveness in which the interaction of the detonation front with a spherical void concentrates energy. Blasting-grade AN prills are typically between 0.9 and 3.0 mm in diameter.
- ^ Mine Health & Safety Management, Michael Karmis
- ^ Mike Davis, Buda's Wagon: A Brief History of the Car Bomb (Verso: New York, 2007): 53.