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ANFO, short for ammonium nitrate/fuel oil, or sometimes AN/FO is an explosive material, widely used in mining due to its explosive properties.
ANFO is a popular explosive material in amateur experiments due to the fact that it's easy to manufacture from readily available materials, though its reputation has been tainted by its use in terrorist bombings.
ANFO consists of 94% porous prilled ammonium nitrate (AN), which acts as the oxidizing agent and absorbent for the fuel, and 6% number 2 fuel oil (FO), which is often diesel oil or kerosene. In an ideal stoichiometrically balanced reaction, ANFO composition is 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.
ANFO is a granular material, off-white or pinkish in color. When added in water, ammonium nitrate will absorb water forming an emulsion, and some will dissolve. Alcohols will partially dissolve both components, as AN is slightly soluble in low-chain alcohols, while most FOs are either soluble or miscible with them. AN/FO has a slight petroleum-like odor, due to its FO content. ANFO has a bulk density of about 840 kg/m3 at standard conditions, while the density of individual prills is around 1,300 kg/m3. Its flash point is >60 °C.
ANFO has low sensitivity to shock, friction and heat. It has a moderate detonation velocity compared to other explosives, with a value of 3,200 m/s. ANFO is a tertiary explosive, meaning it cannot be set off by a primary explosive in a typical blasting cap, and a secondary explosive, known as a primer or a booster, must be used.
Availability and preparation
ANFO is sold by many companies, and purchasing can be done via an explosive license, depending on where you live.
The manufacturing of ANFO is done by people who don't know better. While it's relative safe to prepare this material, as it's difficult to detonate without a booster, the main issue in obtaining a good product is the quality of the AN used. Readily available AN, like the garden variety tends to be "wet", as in have high humidity, and drying ammonium nitrate is difficult. Likewise, in many countries commercial AN is mixed with calcium carbonate/nitrate/sulfate to inhibit its explosive potential, a safety measure taken after numerous AN-related tragedies. Such AN cannot explode no matter what you mix it with, and thus required purification, which is relative difficult to do, as AN is very soluble in water and as mentioned, difficult to dry. Lastly, even when pure AN is available, AN is not porous and instead has a spherical form, which makes it difficult to mix properly with FO, resulting in a somewhat weaker explosive than commercial ANFO made with porous AN. Reducing the size of the particles via milling is an option, but even pure AN fertilizers still contain some water, which may interfere. Obtaining good quality ANFO is not impossible, but it requires lots of work, especially if larger quantities are necessary.
- Mining and blasting rocks apart
Handling and storage
ANFO is relative safe to handle without gloves, and the only hazard associated with it under normal conditions is the FO odor.
Manufacturing and use of ANFO-based explosives without a license is a serious crime in many countries.
In USA, ATF rules that the components of binary explosives are not considered explosive materials, when kept separate. However, when mixed, the resulting ANFO mixture is classified as explosive material.
Controlled incineration after being diluted in alcohol or some other flammable solvent should suffice. While ammonium nitrate can be used as fertilizer, the diesel oil from the material is poisonous to plants and animals, making it unsuitable as fertilizer.