ANFO and Dry Blasting Agents
Dry blasting agents are the most common of all explosives used today. Approximately 80% of the explosives used in the United States are dry blasting agents. The term dry blasting agent describes any material in which no water is used in the formulation. Figure 21 shows the commonly used AN-based blasting agent formulations.
Bulk ANFO is prilled ammonium nitrate and fuel oil. The prills are spherical particles of AN manufactured in a prilling tower with a similar process to that used in making bird shot for shotgun shells. AN prills are also used in the fertilizer industry although there are differences between the fertilizer grade and the blasting grade prills. The blasting prill is considered a porous prill, which better distributes the fuel oil and results in better blasting performance. Table 2 lists the difference in properties of fertilizer and blasting prill. Figure 22 shows ANFO prills alongside a typical set of car keys (for scale).
AN, when bulk loaded into a blasthole, has no water resistance. If the product is placed in water and shot within a very short period of time, marginal detonation can occur with the production of rust colored fumes of nitrous oxide. The ammonium nitrate will dissolve in water and the ammonium nitrate will slump and often break initiator leads. The liberation of nitrous oxide is commonly seen on blasts involving bulk AN when operators have not taken the care to load the product in a proper manner, which ensures that it will stay dry.
When such a marginal detonation occurs, the product produces significantly less energy than it would be capable of producing under normal conditions. For this reason, blastholes geyser, flyrock is thrown, and other problems arise from using AN fuel oil mixtures in wet blastholes. If AN is placed in wet blastholes, it will absorb water. When the water content reaches approximately 9%, the AN may not detonate regardless of the size primer used. Figure 23 indicates the effect of water content on the performance of AN. As water content increases, minimum booster values also increase and detonation velocity decreases significantly.
For wet hole use, where blastholes are not pumped, an aluminized or densified ANFO cartridge can be used. Densified ANFO is made by: (1) crushing approximately 20% of the prills and adding them back into the normal prill mixture or, (2) adding iron compounds to increase the density of the cartridge. In both cases, the object is to produce an explosive with a density greater 1 so that it will sink in water.
Another type of ANFO cartridge is made from the normal bulk ANFO with a density of 0.8. This cartridge will not sink in water. However, it is advantageous to use this type of cartridged ANFO when placing in wet holes that were recently pumped and that contain only small amounts of water.
Cartridge loading of explosives is more tedious and requires more personnel since the cartridges have to be physically taken to the blast site and stacked by each hole. The cartridges are then dropped into the borehole during the loading process. Heavy ANFO requires fewer personnel since the explosive is pumped directly into the blasthole from the bulk truck.
Heavy ANFO or ammonium nitrate blends are mixtures of ammonium nitrate prills, fuel oil, and slurries. The advantage to using heavy ANFO blends is that they can be mixed at the blasthole and quickly loaded into the hole (Figure 24). The ratio of the amount of slurry mixed with the ANFO can be changed to offer either a higher energy load or a load that is water resistant. The cost of heavy ANFO rises with increasing amounts of slurry. The advantage over cartridged products is that the entire blasthole is filled with energy and there is no wasted volume, such as would result from cartridge loading. A disadvantage with using the blends is that, since the explosive occupies the entire volume of the blasthole, any water in the hole is forced upward. This means that one may have to use the blend in the entire hole. Conversely with cartridge products, because of the annular space around the cartridge, one can build up to get out of water and then use the lower priced bulk ANFO.
Two-Component Explosives. Two-component explosives are often called “binary explosives” since they are made of two separate ingredients. Neither ingredient is explosive until mixed. Binary explosives are normally not classified as explosives. They can be shipped and stored as non-explosive materials. Commercially available, two-component explosives are a mixture of pulverized AN and nitromethane that have been dyed either red. These components are brought to the job site and only the amount needed are mixed. On mixing the material, it becomes cap sensitive and is ready to use. These binary explosives can be used in applications where dynamite or cap sensitive slurries would otherwise be used. Binary explosives can also be used as primers for blasting agents and bulk slurries. In most states, binary explosives are not considered explosive until mixed. They, therefore, offer the small operator a greater degree of flexibility on the job. Their unit price is considerably higher than that of dynamite. However, the money saved in transportation and magazine costs outweighs the difference in unit price. If large quantities of explosives are needed on a particular job, the higher cost per weight and the inconvenience of onsite mixing negates any savings that would be realized from less stringent storage and transportation requirements.