ANFO (or AN/FO, for ammonium nitrate/fuel oil) is a widely used bulk industrial explosive. Its name is commonly pronounced as "an-fo".
ANFO has found wide use in coal mining, quarrying, metal mining, and civil construction in applications where its low cost and ease of use may outweigh the benefits of other explosives, such as water resistance, oxygen balance, higher detonation velocity, or performance in small-diameter columns. ANFO is also widely used in avalanche hazard mitigation.
The chemistry of ANFO detonation is the reaction of ammonium nitrate with a long-chain alkane (CnH2n+2) to form nitrogen, carbon dioxide, and water. In an ideal stoichiometrically balanced reaction, ANFO is composed of about 94.3% 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 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).
The fuel component of ANFO is typically diesel, but kerosene, coal dust, racing fuel, or even molasses have been used instead. Finely powdered aluminium in the mixture will sensitise it to detonate more readily.
ANFO is classified as a blasting agent, meaning that it decomposes through detonation rather than deflagration at a velocity higher than the speed of sound in the material but cannot be detonated with a No. 8 blasting cap without a sensitizer. ANFO has a moderate velocity compared to other industrial explosives, measuring 3,200 m/s in 130 mm (5 in) diameter, unconfined, at ambient temperature.
ANFO is a tertiary explosive, meaning that it cannot be set off by the small quantity of primary explosive in a typical blasting cap. A larger quantity of secondary explosive, known as a primer or a booster, must be used. One or two sticks of dynamite were historically used; current practice is to use Tovex or cast boosters of pentolite (TNT/PETN or similar compositions).
In the mining industry, the term ANFO specifically describes a mixture of solid ammonium nitrate prills and diesel fuel. 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.
While the density of pure crystalline ammonium nitrate is 1700 kg/m3, individual prills of explosive-grade AN measure approximately 1300 kg/m3. Their lower density is due to the presence of a small spherical air pocket within each prill: this is the primary difference between AN sold for blasting and that sold for agricultural use. 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; in commercial usages however, this has fallen out of favor due to cost.
ANFO has a bulk density of about 840 kg/m3. In surface mining applications, it is typically augured into boreholes by dedicated trucks that mix the AN and FO components immediately before the product is dispensed. In underground mining applications, ANFO is typically blow-loaded.
AN is highly hygroscopic, readily absorbing water from air. In humid environments, absorbed water interferes with its explosive function. AN is fully water-soluble; as such, it cannot be loaded into boreholes that contain standing water. When used in wet mining conditions, considerable effort must be taken to remove standing water and install a liner in the borehole; it is generally more productive to instead use a water-resistant explosive such as emulsion.
In most jurisdictions, ammonium nitrate need not be classified as an explosive for transport purposes; it is merely an oxidizer. Mines typically prepare ANFO on-site using the same diesel fuel that powers their vehicles. While many fuels can theoretically be used, the low volatility and cost of diesel make it ideal.
Ammonium nitrate is widely used as a fertilizer in the agricultural industry. It is also found in instant cold packs. In many countries, its purchase and use are restricted to buyers who have obtained the proper license.
Unmixed ammonium nitrate can decompose explosively and has been responsible for several industrial disasters, including the 1921 Oppau explosion in Germany, the 1947 Texas City disaster in Texas City, Texas, the 2004 Ryongchon disaster in North Korea, and the 2013 West Fertilizer Company 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 maliciously in 1970 when protests by students became violent 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.
ANFO used to be widely used by the FLNC (National Liberation Front of Corsica) in Corsica, along with f15 explosive, in order to fight against French colonists and symbols. Five containers of 500 kilos each were used to blow up the Tax Office building in Bastia on 28 February 1987.
The ANFO car bomb was adopted by the Provisional IRA in 1972 and, by 1973, the Troubles were consuming 47,000 lb of ammonium nitrate for the majority of bombs. The Ulster Volunteer Force (UVF) also made use of ANFO bombs, often mixing in gelignite as a booster. 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 and ETA. In 1992, Shining Path perpetrated the Tarata bombing in Lima, Peru, using two ANFO truck bombs.
The Shijiazhuang bombings (Chinese: 靳如超爆炸案 or 石家庄"3·16"特大爆炸案) rocked the city of Shijiazhuang, China, on 16 March 2001. A total of 108 people were killed, and 38 others injured when, within a short time, several ANFO bombs exploded near four apartment buildings.
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 kg 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 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 aluminium powder), increasing demolition power by 10–30% over plain ANFO, was used in the Oslo bombing.
On 13 April 2016, two suspected IRA members were stopped in Dublin with 67 kg of ANFO.
On 6 March 2018, 8 members of the extreme right neo-Nazi group «Combat 18 Hellas» arrested in Athens, Greece accused for multiple attacks on immigrants and activists with 50 kg of ANFO in their possession.
Ammonium nitrate and nitromethane (ANNM) 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 1.0 (ANNM+AL = R.E.F. of 1.0–1.1). 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 products 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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- Securing Ammonium Nitrate: Using Lessons Learned in Afghanistan to Protect the Homeland from IEDs: Hearing before the Subcommittee on Cybersecurity, Infrastructure Protection, and Security Technologies of the Committee on Homeland Security, House of Representatives, One Hundred Twelfth Congress, Second Session, 12 July 2012