Figure 1: Primer and Booster in Borehole
Selection Criteria for Primer
The two most critical criteria in primer selection are primer composition and primer size. The primer composition determines the detonation pressure which is directly responsible for the initiation of the main charge. Research conducted by Norm Junk at the Atlas Powder Company had demonstrated that primer composition significantly affected the performance of ANFO charges. Figure 3 is a graph illustrating the effect of detonation pressure for a 3-inch diameter ANFO charge and response of the ANFO at various distance from the primer. It will be noted that thermal primers of low detonation pressure actually caused a burning reaction to start rather than a detonation.
Primer size is also important to obtain a proper reaction. Very small diameter primers are not as effective as large diameter units. Figure 4 demonstrates the effect of primer diameter on ANFO response in 3-inch diameter charges at various distances from the primers. This research conducted by Norm Junk from Atlas Powder Company, indicated that small diameter primers become ineffective regardless of the composition of the material used.
Figure 2: Effect of Primer on Velocity of Detonation (VOD) - Olofsson
Figure 3: Explosive Composition and Primer Performance (after Junk)
Figure 4: Primer Diameter and Primer Performance (after Junk)
The steady state velocity depends on the density, the confinement and particle size of ANFO as well as the blasthole diameter. The VOD increases as the blasthole diameter increases and reaches its highest value at a blasthole diameter of 300 mm as shown in table below.
Table 1: The Steady State Velocity of ANFO for Different Blasthole Diameters
Primer Selection Guideline
The following are some general guidelines for priming:
- The detonation pressure of a primer must be above the level necessary to cause the main charge to detonate at or above its normal velocity. The specific gravity and confined detonation velocity can be used as indicators of detonation pressure if detonation pressure values are not available. A primer that has a specific gravity of approximately 1.2 with a confined detonation velocity greater than 15,000 ft/s would normally be adequate when priming non-cap sensitive explosives, materials such as ANFOs, blasting agents and most water gels. This combination of density and velocity produces a detonation pressure of about 60 kilobars. For explosives such as emulsions, which would detonate at higher velocites, more energetic primers would produce better results. A specif gravity of primer of 1.3 with a confined detonation velocity greater than 17,000 ft/s would be adequate to more quickly achieve the explosive's normal velocity. This combination of density and velocity produces a detonation pressure of about 80 kilobars.
- The diameter of the primer should be larger than the critical diameter of the explosive used for the main column charge.
- The primer must be sensitive to the initiator. A wide variety of the products are used as primers. These primers have different sensitivities. Some may be initiated by low energy detonating cord, while others may be insensitive to these initiators. It is important that the operator understand the sensitivity of the primer to ensure that detonation in the main column charge will properly occur.
- The explosive in the primer must reach its rated velocity of detonation within the length of the cartridge. If this is achieved, then additional cartridges of primer explosive serve no useful purpose.
- For most blasting applications, no more than two primers per blasthole are needed. The second primer, although technically not needed, is commonly used as a backup system should the first primer fail or fail to shoot the entire charge.
Booster are used to intensify the explosive reaction at a particular location within the explosive column. Booster are sometimes used between each cartridge of detonating explosive to ensure a detonation transfer across the ties of the cartridge. This is normally a poor excuse for the use of boosters, since booster cost can be considerable. The selection of an explosive in a cartridge which would not require a booster between each cartridge may be a more economical solution.
In general, boosters are used to put more energy into a hard layer within the rock column. They are sometimes also used to intensify the reaction around the primer which will put more energy at the primer location. This is commonly used when primers are near the bottom of the hole, since the bottom of the hole is the hardest place to break. Using a booster at hole bottom normally allows the increase in the burden dimension and better breakage at the toe of the shot. Boosters can be made of similar explosive materials as primer. Their sole function is to place more energy at point locations within the explosive column.
Figure 5: Primers/Boosters