Introduction of Ammonium Nitrate
Before learning what dry blasting agents is, we have to know what ammonium nitrate (AN) is. Ammonium nitrate production is started from reaction of anhydrous ammonia gas and concentrated nitric acid, which about 83% concentration is then produced in the AN solution. Ather that, excess water is evaporated off to leave an AN concentration of 95 to 99.9%.
A small percentage of water is left in the solution when the prills are formed. When the prills are dried, the water is removed and it leaves voids in the prills. This is where the diesel fuel goes when added to the prills.
Dry Blasting Agent or ANFO
Dry blasting agents are the most common of all explosives used today. Approximately 80% of the explosives used in this country are dry blasting agents. The term dry blasting agent describes any material in which no water is used in this formulation. Early dry blasting agents employed fuels of solid carbon or coal combined with ammonium nitrate in various forms. Through experimentation, it was found that solid fuels tend to segregate in transportation and provide less than optimum blasting results. It was found that diesel oil mixed with porous ammonium nitrate prills gave the best overall blasting results. The term ANFO (ammonium nitrate and fuel oil) has become synonymous with dry blasting agents. An oxygen balanced mixture of ANFO is the cheapest source of explosive energy available today (Figure 1). Adding finely divided aluminum to dry blasting agents increases the energy output ans also increases cost. Dry blasting agents can be broken down into two categories, cartridged and bulk.
Before learning what dry blasting agents is, we have to know what ammonium nitrate (AN) is. Ammonium nitrate production is started from reaction of anhydrous ammonia gas and concentrated nitric acid, which about 83% concentration is then produced in the AN solution. Ather that, excess water is evaporated off to leave an AN concentration of 95 to 99.9%.
A small percentage of water is left in the solution when the prills are formed. When the prills are dried, the water is removed and it leaves voids in the prills. This is where the diesel fuel goes when added to the prills.
Dry Blasting Agent or ANFO
Dry blasting agents are the most common of all explosives used today. Approximately 80% of the explosives used in this country are dry blasting agents. The term dry blasting agent describes any material in which no water is used in this formulation. Early dry blasting agents employed fuels of solid carbon or coal combined with ammonium nitrate in various forms. Through experimentation, it was found that solid fuels tend to segregate in transportation and provide less than optimum blasting results. It was found that diesel oil mixed with porous ammonium nitrate prills gave the best overall blasting results. The term ANFO (ammonium nitrate and fuel oil) has become synonymous with dry blasting agents. An oxygen balanced mixture of ANFO is the cheapest source of explosive energy available today (Figure 1). Adding finely divided aluminum to dry blasting agents increases the energy output ans also increases cost. Dry blasting agents can be broken down into two categories, cartridged and bulk.
Figure 1: Blasting Agent Formulations
Cartridged Blasting Agents
For wet hole use, where blasthole are not pumped, an aluminized or densified ANFO cartridge can be used (Figure 2). Densified ANFO is made by either crushing approximately 20% of the prills and adding them into the normal prill mixture or by adding iron compounds to increase the density of the cartridge. In both cases, the object is to produce an explosive with a density greater than one 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 them in wet holes that were recently pumped and contain only small amounts of water.
Cartridge loading of explosives is more tedious and require more personnel since the cartridges have to be physically taken into the blast site and stacked by each hole. The cartridges are then dropped into the borehole during the loading process.
For wet hole use, where blasthole are not pumped, an aluminized or densified ANFO cartridge can be used (Figure 2). Densified ANFO is made by either crushing approximately 20% of the prills and adding them into the normal prill mixture or by adding iron compounds to increase the density of the cartridge. In both cases, the object is to produce an explosive with a density greater than one 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 them in wet holes that were recently pumped and contain only small amounts of water.
Cartridge loading of explosives is more tedious and require more personnel since the cartridges have to be physically taken into the blast site and stacked by each hole. The cartridges are then dropped into the borehole during the loading process.
Figure 2: Cartridged ANFO
Bulk ANFO
Bulk ANFo is prilled ammonium nitrate and fuel oil. It is often either blown or augered into the blasthole from a bulk truck. The mixed ANFO can be placed in some trucks which the dry ammonium nitrate (AN) and diesel oil (FO) can be mixed in the field as the material is being placed in the borehole. The blasting industry has a great dependence on dry blasting agents because of the large volume used. Dry blasting agent will not function properly if placed in wet holes for extended periods of time. For this reason, the blaster should know the limitation of his product.
Bulk ANFo is prilled ammonium nitrate and fuel oil. It is often either blown or augered into the blasthole from a bulk truck. The mixed ANFO can be placed in some trucks which the dry ammonium nitrate (AN) and diesel oil (FO) can be mixed in the field as the material is being placed in the borehole. The blasting industry has a great dependence on dry blasting agents because of the large volume used. Dry blasting agent will not function properly if placed in wet holes for extended periods of time. For this reason, the blaster should know the limitation of his product.
Water Resistance of Ammonium Nitrate
Ammonium nitrate, which is 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, detonation can occur with the production of rust colored fumes of nitrous oxide. The energy produced from such detonation is significantly less than the product would be capable of producing under normal conditions. For this reason, blasthole geyser, flyrock is thrown, and other problems arise from using ammonium nitrate fuel oil mixtures in wet blastholes. If ammonium nitrate is placed in wet blastholes, it will absorb water. When the water content reaches approximately 9%, it is questionable whether the ammonium nitrate will detonate regardless of the size primer used. Figure 3 indicated the effect of water content on the performance of ammonium nitrate. It indicates that as water content increases, minimum booster values also increases and detonation velocity decreases significantly.
Figure 3: Effects of Water in ANFO
Energy Output of ANFO
When ammonium nitrate fuel oil mixtures are made in the field, variations in oil content can easily occur. Bagged mixtures received from some distributors have similar problems. The amount of fuel oil placed on the ammonium nitrate is extremely critical from the standpoint of efficient detonation (Figure 4). To get optimum energy release, one would want about a 94.5% ammonium nitrate with a 5.5% diesel oil mixture (or waste oil filtered).
Figure 4: Effects of Fuel Oil Content on ANFO
Figure 5: VOD vs. Borehole Diameter Charged by ANFO
The critical diameter of poured ANFO is about 2.0 - 2.5 inches. Under this it will not detonate. If it is loaded pneumatically, the critical diameter can be increased to 1.0 inch.
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