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ANFO (or AN/FO, for ammonium nitrate/fuel oil) is a widely used bulk industrial explosive mixture.
ANFO has found wide use in coal mining, quarrying, metal mining, and civil construction in undemanding applications where the advantages of ANFO's low cost and ease of use matter more than the benefits offered by conventional industrial explosives, such as water resistance, oxygen balance, high detonation velocity, and performance in small diameters.
It accounts for an estimated 80% of the 6,000,000,000 pounds (2.7×109 kg) of explosives used annually in North America.
The use of ANFO originates in the 1950s.
ANFO under most conditions is cap-insensitive, and so it is classified as a blasting agent and not a high explosive; it decomposes through detonation rather than deflagration with a moderate velocity of about 3,200 meters per second in 5-inch diameter, unconfined, at ambient temperature. It is a tertiary explosive consisting of distinct fuel and oxidizer phases and requires confinement for efficient detonation and brisance. Because it is cap-insensitive, it generally requires a primer, also known as a booster (e.g., one or two sticks of dynamite, as historically used, or in more recent times, Tovex or cast boosters of pentolite (TNT)/PETN or similar compositions) to ensure continuation of the detonation wave-train.
The chemistry of ANFO detonation is the reaction of ammonium nitrate (NH4NO3) with a long chain alkane (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. 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 nitrogen oxides (NOx).
Variants of ANFO using diesel fuel, kerosene, coal dust, racing fuel, or even molasses in place of the red diesel (Nº 2 fuel oil) have been used, and finely-powdered aluminium in the mixture will sensitise it to detonate more readily.
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.
In the mining industry, the term ANFO specifically describes a mixture of solid ammonium nitrate prills and 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. AN prills used for explosive applications are physically different from fertilizer prills; the former contain approximately 20% air. These versions of ANFO that use prills are generally called explosives grade, low density, or industrial grade ammonium nitrate. These voids are necessary to sensitize ANFO: they create so-called "hot spots". Finely powdered aluminium 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.
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 diesel fuel that powers their vehicles, although heating oil, which is nearly identical, may cost less than diesel fuel due to lower fuel tax. Many fuels can theoretically be used; the low volatility and cost of fuel oil makes it ideal.
Unmixed ammonium nitrate can decompose explosively and has been responsible for several industrial disasters. Notable industrial disasters include the 1947 Texas City disaster in Texas City, Texas, the 2004 Ryongchon disaster in North Korea, and the 2013 West Fertilizer Co. Plant Explosion in West, Texas. Environmental hazards include eutrophication in confined waters and nitrate/gas oil contamination of ground or surface water.
ANFO was 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, resulting in the Sterling Hall bombing.
The ANFO car bomb was adopted by the Provisional IRA in 1972 and, by 1973, the Troubles were consuming 47,000lbs of ammonium nitrate being used for the majority of bombs. The IRA detonated an ANFO truck bomb on Bishopsgate in London in 1993 killing one and causing £350 million in damage. It has also seen use by groups such as the Revolutionary Armed Forces of Colombia (FARC), and ETA.
Ramzi Yousef who was closely associated with Al Qaeda used ANFO to try to destroy the World Trade Center in 1993. A more sophisticated variant of ANFO (ammonium nitrate with nitromethane as the fuel called ANNM) was used in the 1995 Oklahoma City bombing.
In 2001 The Shijiazhuang bombings (Chinese: 靳如超爆炸案 or 石家庄“3·16”特大爆炸案) were a series of bomb blasts that rocked the city of Shijiazhuang, China on March 16, 2001. A total of 108 people were killed, and 38 others injured when within a short period of time several ANFO bombs exploded near four apartment buildings, and were characterized by China scholar Andrew Scobell as perhaps the worst terrorist act in the history of the People's Republic of China.
Improvised bombs made with agricultural-grade AN are less sensitive and less efficient than the explosive-grade variety. In November 2009, a ban on ammonium sulfate, ammonium nitrate and calcium ammonium nitrate fertilizers was imposed in the former Malakand Division – comprising the Upper Dir, Lower Dir, Swat, Chitral and Malakand districts of the North West Frontier Province (NWFP) of Pakistan, by the NWFP government, following reports that those chemicals were used by militants to make explosives.
In April 2010, police in Greece confiscated 180 kilograms of ANFO and other related material stashed in a hideaway in the Athens suburb of Kareas. The material was believed to be linked to attacks previously carried out by the "Revolutionary Struggle" terrorist group.
In January 2010, President Hamid Karzai of Afghanistan also issued a decree banning the use, production, storage, purchase or sale of ammonium nitrate, after an investigation showed that militants in the Taliban insurgency had used the substance in bomb attacks.
On 22 July 2011, an aluminium-powder enriched ANNM explosive, with total size of 950 kg (150 kg of aluminum powder), increasing demolition power by 10-30 percent over plain ANFO, was used in the Oslo bombing.
A formula developed by Sandia National Laboratories which combines iron sulfate with ammonium nitrate results in an effective and cheap fertilizer which is useless as a component in an ANFO explosive; "the iron ion “grabs” the nitrate and the ammonium ion takes the sulfate ion. Iron sulfate becomes iron nitrate and ammonium nitrate becomes ammonium sulfate. This metathesis reaction occurs if someone tries to alter the fertilizer to make it detonable when mixed with a fuel." The formula was not patented, but, as of 2013, had not been marketed widely in regions such as Afghanistan where ammonium fertilizer is both needed and abused, where the formula could provide considerable mitigation against fertilizer misuse. Both iron and sulfate ions are effective fertilizers in themselves, especially in alkali soils.
ANNM, or ammonium nitrate and nitromethane, is one of the most powerful improvised types of AN-based explosives. The relative effectiveness factor of ANNM varies depending on the mix but does not exceed 1 (annmal = RE 1-1.1). ANNM usually contains a 60:40 (kinepak) mix of AN and NM (60% ammonium nitrate, 40% nitromethane by mass), though this results in a wet slurry. Sometimes more AN is added to reduce liquidity and make it easier to store and handle, as well as providing an oxygen-balanced mix. ANNM is also more sensitive to shock than standard ANFO and is therefore easier to detonate. When ANNM detonates, the primary byproducts produced are H2O, CO2 and N2, but NOx and other toxic gases are inevitably formed because of a negative oxygen balance. The balanced equation is as follows:
3NH4NO3 + 2CH3NO2 -> 4N2 + 2CO2 + 9H2O
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