Ammonium Nitrate and Fuel Oil


In December of 1956 Melvin Cook's invented a new blasting agent using an unusual mixture of ammonium nitrate, aluminum powder, and water. The safety and efficiency of this new explosive were apparent, and the use of water was revolutionary. Tests that followed resulted in the development of a new field of explosives: slurry explosives. This invention converted the commercial explosives industry from "dangerous dynamite" to "safe slurry" and dry blasting agents. In 1972 Cook developed the BLU-82, the largest and most powerful chemical bomb, using aluminized slurry.
ANFO
[20] (or AN/FO, for ammonium nitrate / fuel oil) is a widely used explosive mixture. The oil used is most often No. 2 fuel oil, or diesel fuel, but sometimes kerosene, coal dust, or even molasses.
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) to ensure reliable detonation. The explosive efficiency associated with ANFO is approximately 80% of TNT, also stated as TNT equivalency. The most efficient mixed AN explosives using fuels other than fuel oil can exceed 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 stechiometrically balanced reaction, ANFO is composed of approximately 94.3% AN and 5.7% FO by weight.
Ammonium nitrate-fuel oil has largely replaced dynamites and gelatins in bench blasting. Denser slurry blasting agents are supplanting dynamite and gelatin and dry blasting agents. The most widely used dry blasting agent is a mixture of ammonium nitrate prills (porous grains) and fuel oil. The fuel oil is not precisely CH2, but this is sufficiently accurate to characterize the reaction.
Uniform mixing of oil and ammonium nitrate is essential to development of full explosive force. Some blasting agents are premixed and packaged by the manufacturer. Where not premixed, several methods of mixing in the field can be employed to achieve uniformity.
As in other combustion reactions, a deficiency of oxygen favors the formation of carbon monoxide and unburned organic compounds and produces little, if any, nitrogen oxides. An excess of oxygen causes more nitrogen oxides and less carbon monoxide and other unburned organics. For ammonium nitrate and fuel oil (ANFO) mixtures, a fuel oil content of more than 5.5 percent creates a deficiency of oxygen.

The simple removal of a tree stump might be done with a 2-step train made up of an electric blasting cap and a stick of dynamite. The detonation wave from the blasting cap would cause detonation of the dynamite. To make a large hole in the earth, an inexpensive explosive such as ANFO might be used. In this case, the detonation wave from the blasting cap is not powerful enough to cause detonation, so a booster must be used in a 3- or 4-step train. The yield from the blasting caps and safety fuses used in these trains are usually small compared to those from the main charge, because the yields are roughly proportional to the weight of explosive used, and the main charge makes up most of the total weight.

Advantages of insensitive dry blasting agents are their safety, ease of loading, and low price. Ammonium nitrate is water soluble so that in wet holes, some blasters pump the water from the hole, insert a plastic sleeve, and load the blasting agent into the sleeve. Special precautions should be taken to avoid a possible building up of static electrical charge, particularly when loading pneumatically. When properly oxygen- balanced, the fume qualities of dry blasting agents permit their use underground. Canned blasting agents, once widely used, have unlimited water resistance, but lack advantages of loading ease and direct coupling to the borehole.