More on the cause of the Coahuila truck explosion

25 kilo sacks of AN-FOYou learn something every day and in this case it is ANFO, or ammonium nitrate-fuel oil mixture. As I mentioned in the update on the truck explosion in Coahuila on September 10, I could not imagine anyone shipping that chemical combination, but apparently it is very commonplace, especially in the U.S.

Here are the facts:


ANFO (or AN/FO) stands for ammonium nitrate/fuel oil (most often diesel fuel, sometimes kerosene or even molasses). 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 (2,700,000 metric tons) 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 requiring confinement for efficient detonation. Its sensitivity is relatively low: it generally requires a booster to ensure reliable detonation. The explosive efficiency associated with ANFO is approximately 80% of TNT, also stated as (0.8) TNT equivalency. The most efficient mixed AN explosives using fuels other than fuel oil can exceed (1.6) TNT equivalency.

The basic chemistry of ANFO detonation is the reaction of ammonium nitrate (NH4NO3) with a long chain hydrocarbon (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. 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, hydrogen sulfide, and oxides of nitrogen (NOx).

Discovery

Unmixed ammonium nitrate can decompose explosively and has been responsible for industrial disasters such as the Texas City disaster in Texas City, Texas in 1947 and the Ryongchon disaster of Ryongchon, North Korea in 2004. However, it is considered a somewhat inefficient explosive as it exhibits only (0.44) TNT equivalency.

Industrial use

Ammonium nitrate is widely used as a fertilizer in the agricultural industry. In many countries its purchase and use has been restricted only if the buyer has obtained the proper license. This restriction is due primarily because it is an attractive and simple component used in the production of fertilizer bombs by terrorists.

In the mining industry, the term ANFO specifically describes a mixture of solid ammonium nitrate prills and fuel 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. It is notable that AN prills used for explosive applications are physically different from fertilizer prills; the former contain approximately 20% air. These voids are necessary to sensitize ANFO: they create so-called “hot spots” in which the interaction of the detonation front with a spherical void concentrates energy. Blasting-grade AN prills are typically between 0.9 and 3.0 mm in diameter.

AN is highly hygroscopic; that is, it readily absorbs water from air. Care must be taken with its storage in humid environments, as any absorbed water interferes with its explosive function. AN is also water soluble. If ANFO is to be used in wet mining conditions, considerable effort must be taken to dewater boreholes.

Other explosives based on the AN/FO 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. Most mines prepare ANFO on-site using the same No. 2 diesel fuel that powers their vehicles. Many fuels can theoretically be used; however, the low volatility and cost of No. 2 diesel makes it ideal.

Terrorist use

AN/FO has occasionally been used in terrorist bombings. First improvised in 1970 by student protesters at the University of Wisconsin-Madison, the ANFO car bomb was soon adopted by the IRA. It has also seen use by groups such as the FARC, ETA, and various Palestinian terrorists. A more sophisticated variant of ANFO (with nitromethane as the fuel called ANNM) was used in the Oklahoma City bombing. It is noteworthy that improvised bombs made with agricultural-grade AN are less sensitive and less efficient than the explosive-grade variety.

Interesting stuff. That explains the intensity of the explosion. And apparently, this cargo was packed, placarded with all legal permits. Although the investigation is far from over, I imagine the end result will be the fault of the pick up truck which was on a “beer run”!