ANFO

Ammonium nitrate prills used in ANFO at a potash mine by the K+S AG

25-kilogram (55 lb) sacks containing ANFO

ANFO (or AN/FO, for ammonium nitrate/fuel oil) is a widely used explosive mixture. It consists of ammonium nitrate (NH₄NO₃) and, commonly, heating oil or diesel fuel; any carbon source such as kerosene, coal dust, or even molasses and sugar can be used. Nitromethane is one of the most effective carbon sources 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. It also sees service in improvised explosive devices, where it is also known as a fertilizer bomb.

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 trinitrotoluene (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 (hydro)carbon (C_nH_2n+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 oxides of nitrogen (NO_x).

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.

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 which 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.

Disasters

Unmixed ammonium nitrate can decompose explosively and has been responsible for industrial disasters, such as the Texas City disaster in Texas City in 1947 and the Ryongchon disaster in North Korea in 2004.

Terrorist use

ANFO has 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, the ANFO car bomb was soon adopted by the Provisional IRA, such as in the 1993 Bishopsgate bombing. It has also seen use by groups such as the Revolutionary Armed Forces of Colombia (FARC), ETA, and Ramzi Yousef's terrorists when they first tried 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. 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.

In July 2011, an aluminium-powder enriched ANFO explosive (150 kg of aluminum powder and 6 tonnes of ammonium nitrate), increasing demolition power by 10-30 percent over plain ANFO, was used in the Oslo bombing.

ANNM

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 NO_x and other toxic gases are inevitably formed because of impurities. The balanced equation is as follows:

3NH₄NO₃ + 2CH₃NO₂ -> 4N₂ + 2CO₂ + 9H₂O

However, depending on the detonation impetus (for example a #6 versus a #10 detonator), the products of the detonation can be decidedly unstoichiometric.

External links

• Video of ANFO being used at an open pit mine
• Video showing detonation of a 5 kg ANFO charge