Sylvie Willis
Ammonium nitrate is a widely used bulk industrial high explosive that has been manufactured as a fertilizer for over a century. When mixed with fuel oil, it is known as ANFO (ammonium nitrate/fuel oil). The mixture is composed of 94% porous prilled ammonium nitrate and 6% number 2 fuel oil (diesel). The low volatility and cost of diesel make it an ideal choice for this mixture. ANFO is highly insensitive as an explosive and is commonly used in mining, quarrying, and civil construction. It has been associated with several industrial disasters and has also been used in terrorist attacks.
| Characteristics | Values |
|---|---|
| Composition | 94% porous prilled ammonium nitrate (NH4NO3) and 6% number 2 fuel oil (FO) (road) diesel |
| Chemistry | Reaction of ammonium nitrate with a long-chain alkane (CnH2n+2) to form nitrogen, carbon dioxide, and steam |
| Colour | Commercially available as pale white, grey, yellow, or pink oil-covered prills; non-commercially produced forms may be darker |
| Odor | Petroleum |
| Solubility | Soluble in water |
| Sensitivity | Highly insensitive as an explosive, requiring a quantity of secondary explosive, known as a primer or a booster, to be detonated |
| Toxicity | Produces moderate amounts of toxic gases such as carbon monoxide and nitrogen oxides (NOx) |
| Uses | Blasting rock, mining (coal mining, quarrying, metal ore mining), civil construction, explosives (car bombs, truck bombs) |
| Safety | May explode and throw fragments 1600 meters (1 mile) or more if fire reaches cargo; large quantities of toxic gases are generated in fires |
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What You'll Learn
ANFO: a mixture of ammonium nitrate and diesel fuel
ANFO (or AN/FO, for ammonium nitrate/fuel oil) is a mixture of 94% porous prilled ammonium nitrate (NH4NO3) and 6% number 2 fuel oil (FO) (road diesel). The ammonium nitrate acts as the oxidizing agent and absorbent for the fuel. The chemistry of ANFO detonation is the reaction of ammonium nitrate with a long-chain alkane (CnH2n+2) to form nitrogen, carbon dioxide, and steam.
ANFO is a widely used bulk industrial high explosive that has found applications in coal mining, quarrying, metal ore mining, and civil construction. It is favoured for its low cost and ease of use, although other explosives may offer benefits such as water resistance and higher detonation velocity. Mines typically prepare ANFO on-site using the same diesel fuel that powers their vehicles. While many fuels can theoretically be used, diesel is ideal due to its low volatility and cost.
The use of ANFO originated in the 1950s with the development of prills, which are small spherical or ovoid beads, 2-3 mm in diameter. These beads are manufactured by passing concentrated AN solution droplets down a drying tower against a rising stream of hot air. The increased porosity of the lower-density prills allows the liquid fuel (fuel oil or diesel) to absorb readily into the prills, creating an intimate fuel/oxidizer mixture.
ANFO is legally classified as a blasting agent (tertiary explosive) and not a high explosive due to its insensitivity to detonation under most conditions. However, unmixed ammonium nitrate can decompose explosively and has been responsible for several industrial disasters. When used in blasts, ANFO produces moderate amounts of toxic gases such as carbon monoxide and nitrogen oxides (NOx).
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ANFO's use in explosives and mining
ANFO, or AN/FO, stands for ammonium nitrate/fuel oil. It is a widely used bulk industrial explosive, consisting of 94% porous prilled ammonium nitrate (AN) and 6% number 2 fuel oil (FO), usually diesel. The use of ANFO began in the 1950s, and it is commonly used in coal mining, quarrying, metal ore mining, and civil construction.
ANFO is highly insensitive as an explosive, requiring a larger quantity of secondary explosive, known as a primer or booster, to detonate. This means it is classified as a tertiary explosive or "blasting agent", and not a high explosive. The low sensitivity of ANFO means that it is often used in applications where its low cost and ease of use are more important than the benefits of other explosives, such as water resistance, oxygen balance, and higher detonation velocity.
In the mining industry, ANFO is typically prepared on-site, using the same diesel fuel that powers the vehicles. The low volatility and cost of diesel make it an ideal fuel source for ANFO. ANFO is usually blow-loaded in underground mining applications. It is important to note that ANFO is highly hygroscopic, absorbing water from the air, which can interfere with its explosive function. As such, it is generally not used in boreholes that contain standing water.
The use of ANFO has been adopted by various groups for explosives and car bombs, including the Provisional IRA, the Ulster Volunteer Force (UVF), the Revolutionary Armed Forces of Colombia, and ETA.
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Toxic gases produced by ammonium nitrate fires
Ammonium nitrate (NH4NO3) is a widely used bulk industrial high explosive that has been manufactured as a fertilizer for over a century. It is also used in coal mining, quarrying, metal ore mining, and civil construction. When subjected to excessive heat, ammonium nitrate decomposes, releasing various gases, including toxic ones.
Ammonium nitrate fires generate large quantities of toxic gases, and first responders should wear appropriate personal protective equipment (PPE). Evacuating or sheltering downwind communities should also be considered.
The gases released by burning ammonium nitrate may be reddish-brown in colour and are highly toxic, but their effects usually have delayed results. One of the gases, nitrogen dioxide (NO2), attacks the lower respiratory function by dissolving in the moisture in the lungs and forming nitric acid (HNO3). Over time, nitric acid can destroy the alveoli, reducing lung capacity. Depending on exposure, irreversible lung damage can occur within 12 to 48 hours, causing shortness of breath, chest pains, cold sweats, and radiating chest pain—all symptoms that can be mistaken for a heart attack. Death follows swiftly.
The specific toxic gases produced by ammonium nitrate fires depend on the composition of the substance it is mixed with. For example, when decomposed, ammonium nitrate-based fertilisers release fumes containing water vapour, oxides of nitrogen, hydrogen chloride, ammonia, and chlorine. The fumes may also contain ammonium chloride and ammonium nitrate, which, along with the water vapour, can significantly reduce visibility.
In addition to the toxic gases released by burning ammonium nitrate, there is also a risk of explosion. Under certain conditions, such as confined spaces, elevated pressure, or contamination, ammonium nitrate can detonate. This has led to several tragic incidents, including the Texas City, TX, explosion in 1947, which killed 468 people.
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The chemistry of ANFO detonation
ANFO, or ammonium nitrate–fuel oil, is a widely used explosive for civilian purposes, especially in the mining industry. Its main advantages are its low price and simple method of production. Its main disadvantages are its low water resistance and problems related to non-ideal detonation, which can be a potential hazard.
ANFO is technically a high explosive as it decomposes through detonation rather than deflagration at a velocity higher than the speed of sound in the material. However, its low sensitivity means that it is not generally regulated as such. 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. It is described as a non-ideal high explosive as its explosive velocity is far from the thermodynamic ideal due to its porosity and the phase separation of its two components.
ANFO is highly insensitive, making it a tertiary explosive (or a "blasting agent"). Without a sensitizer, it cannot be detonated by a typical blasting cap. Finely powdered aluminium in the mixture will sensitise it to detonate more readily. Detonation waves in ANFO confined by aluminium tubes have been observed experimentally, with velocities ranging from 1.5 to 4.0 mm/s depending on explosive properties, charge size, and degree of confinement. The sound-speed of aluminium exceeds the detonation velocity, preventing a shock from forming in the tube wall and allowing the aluminium to transport energy from behind the detonation front to the undisturbed explosive upstream.
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The preparation of ANFO mixtures
ANFO (or AN/FO) is an explosive consisting of ammonium nitrate and fuel oil. It is a widely used bulk industrial explosive due to its low price and ease of production. ANFO is commonly used in coal mining, quarrying, metal ore mining, and civil construction.
To prepare ANFO, the ammonium nitrate is loaded into a hopper and fed into a mixing chamber. During this step, the fuel oil is injected into the ammonium nitrate feeding pipe, resulting in the formation of ANFO.
In some cases, ANFO can be further modified to create heavy ANFO or weight ANFO. This involves blending the ANFO with a bulk emulsion, which may be gassed or ungassed. The ratio of ANFO to bulk emulsion can be optimized depending on the specific requirements of the blast site. Heavy ANFO is typically prepared in a bulk truck or mixing truck, while weight ANFO is generally prepared in a truck mixer.
It is important to note that ANFO and its derivatives are explosives that require proper safety precautions and handling.
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Frequently asked questions
ANFO stands for ammonium nitrate/fuel oil. It is a widely used bulk industrial high explosive that consists of porous prilled ammonium nitrate and number 2 fuel oil (diesel).
The chemistry of ANFO detonation is the reaction of ammonium nitrate with a long-chain alkane to form nitrogen, carbon dioxide, and steam.
ANFO has been used in coal mining, quarrying, metal ore mining, and civil construction. It is also used as a car bomb in terrorist attacks.
