Primer and Boosters
The difference between a primer and booster is in its use, rather than in its physical composition or makeup. A primer is defined as an explosive unit that contains an initiator. For example, if a blasting cap is placed into a cartridge of dynamite, that cartridge with initiator becomes the primer. A booster, on the other hand, is an explosive unit of different composition than the borehole charge and does not contain a firing device. The booster is initiated by the column charge adjacent to it. A booster is used to put additional energy into a hard or tough rock layer (Figure 33).
If a rock mass contains considerable numbers of mud seams or open joints, confinement on the main charge could be lost during the detonation process. In this case it is common to find operators placing additional primers in the blasthole to cause the explosive charge to fire more rapidly, thereby reducing possible problems due to loss of confinement. If the blaster is working in a rock that contains mud seams, it may require a second primer to get efficient detonation throughout the total length of the charge. Conversely, in most cases from a purely technical standpoint for competent rock only one primer is needed for a single column charge of explosive. In these cases where more than one primer is used, it would be assumed that the bottom primer would be firing first.
If two or more primers are being placed in a blasthole, normally the second primer would be placed on a later delay period since the first primer location may be critical for the shot to perform properly. The second delayed primer would act only as a backup unit should the first one fail to initiate at the proper time.
Primers can be found in many sizes and in many varying compositions. Primer diameters can vary in size ( Figure 34) and come in many different compositions. Various grades of dynamite are used as primers as well as water gels, emulsions, and densified AN compounds. Various types of cast explosives of high density, high velocity, and high costs are also used for priming. Because of the vast number of sizes and compositions of primers, it is confusing for the operator. Improper selections are often made that can cause less than optimum results. Figure 35 shows a typical primer.
Primer size is also important to obtain a proper reaction. Very small diameter primers are not as efficient as large diameter units. Figure 37 shows the effect of primer diameter on ANFO response in 30 in. diameter charges at various distances from the primers. This research conducted by Atlas Powder Company (Junk, 1968 and Morhard, 1987), decades ago, indicated that small diameter primers become inefficient regardless of the composition of the material used.
