For most people, the association with ammonium nitrate is with farming, particularly as it is used as a high-nitrogen containing fertilizer. However, the specific chemical composition of this nitrate salt of ammonium is also one of the most common options as an explosive, particularly when used in mining blasts.
While it can be used on its own as an explosive, it is commercially available in the form of ANFO or ammonium nitrate/fuel oil, which is estimated to be the explosive used in over 80% of all blasting completed in the United States and Canada.
The use on ANFO is heaviest in quarrying, mining for metals, as well as in coal mining. It is considered to be a low-cost option to other forms of explosives, and it also provides significant benefits over other blasting options. Able to detonate in small areas in a very controlled way, this is also the explosive of choice for avalanche control programs where precise detonations are used to remove potentially dangerous snow accumulations before an avalanche occurs.
The Blast Mechanism
ANFO and also ammonium nitrate is considered to be blasting cap-insensitive, so it is not a high explosive. An example of a high explosive would include dynamite, which is detonated with the use of a blasting cap.
To use ANFO in blasting operations a primer is required. The primer may also be called a booster, and it can use dynamite as a basic and simple type of primer. A more specific primer option is found in PETN or pentolite, which creates the desired explosion in a very precise and controlled fashion.
Water Concerns
AN, or the ammonium nitrate, is highly susceptible to absorbing water. This includes water from humidity in the air. This creates a potentially dangerous situation when trying to control a blast. Additionally, it is also very soluble in water, which is why it is an effective fertilizer as it does break down and readily absorb into the soil.
However, it is important to realize that the AN used in blasting is not the same in density as the fertilizer. The prills or the solid round pellets of AN used for blasting, have a density of about 1300 kg/m3 while the fertilizer has a density of about 1700 kg/m3. This is accomplished by the addition of air into the process of forming the explosive grade AN prills.
By adding the fuel oil to create the ANFO combination, the issues with moisture are greatly reduced. Used in emulsion form, it becomes a very stable, low cost, easy to transport blasting option that is ideal for a great many different blasting requirements.