Effects of blasting on damage and grindability of impacted rock

CIM Bulletin, Vol. 1, No. 1091, 2006
P.D. Katsabanis, S. Gregersen, G. Kunzel, M. Pollanen, C. Pelley, and S. Kelebek
The file is a zipped PDF document.The importance of blasting to mill operations has been discussed in a series of publications, especially in the last decade. Literature suggests that there are two types of benefits from finer blast fragmentation: productivity increases and decreased energy requirements at the mill. Productivity increases have been linked to fragmentation improvements while energy requirements in the mill can be expected if the work index changes with blasting.In the present investigation, an effort was made to quantify the damage due to micro-fractures produced by detonating explosives, establish relationships between damage and easily measurable parameters of the rock, and finally examine relationships between damage and work index, as well as damage and timing of impact. The work index was measured using both standard tests and relative tests in a Bond ball mill and a Bond rod mill. The material used was a competent granodiorite representing a fairly coarse-grained rock.Initially, the level of damage was intentionally kept low so that material properties for the damaged samples could be measured. Later, powder factors were increased to examine the effect of realistic powder factors on work index. In the first series of tests, the uniaxial compressive strength, tensile strength, Poisson’s ratio, P-wave, S-wave velocity, point load test index, crushing, and grinding work indices were evaluated for samples without any blasting load and for samples that had received a variety of blast loads, ranging from a single impact with low-strength detonating cord, to a combination of impacts at different times, and more significant impacts with higher powder factors. It was shown that the P- and S-wave velocities and rock-strength parameters decrease even at low impacts, suggesting that the rock is damaged by microfractures, although macro fracturing was not observed. At impact levels lower than the ones necessary to produce fragmentation, a decrease of point load index was observed, showing potential benefits at the crusher level.Damage values appear to be affected by timing, suggesting that the superposition of the stress waves produced by the detonating explosives influenced the damage process. Since small, unstemmed charges were used, the gases produced by the explosive are not expected to have played a role on the damage process. Damage was also affected, as expected, by explosive energy concentration. Fragmentation was achieved at higher impact levels, introduced by elevated explosive consumption. Fragments are believed to carry microfractures as well and their work index was studied in a series of tests in a ball and rod mill. Some influence of blasting on work index was recorded in these tests. The tests revealed a reduction of work index, which appears to be affected by the powder factor and the distance of the fragments from the borehole. The reduction of the work index achieved for this rock was between 5% and 15%.Experimental data on unblasted rock and six types of blast configurations, as described, follows:

Samples blasted with a single length of 5.3 g/m detonation cord (Type 1).
Samples blasted with two lengths of 5.3 g/m detonation cord placed at diametrically opposite positions with no delay between the cords (Type 2).
Samples blasted with two lengths of 5.3 g/m detonation cord with 50 ms delay between the cords (Type 3).
Samples blasted with two lengths of 2.1 g/m detonation cord with no delay between the cords (Type 4).
Samples blasted with two lengths of 2.1 g/m detonation cord with 50 ms delay between the cords (Type 5).
Samples blasted with two lengths of 5.3 g/m detonation cord with 16 ms delay between the cords (Type 6).
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