ABSTRACT

ABSTRACT

Specimens of copper-bearing quartz monzonite were subjected to a plane shock wave simulating high compressional stresses in the proximity of a borehole wall. Fragmentation was studied as a function of stress levels between 1.6 GPa (23 x 104 psi) and 7.3 GPa(105.8x104 psi) and pulse durations ranging from 1.1 µs to 6.9 µs. Both explosive pressure and pulse duration were shown to have a strong effect on particle size distribution. While the effect of pressure is well recognized, the effect of pulse duration indicates that ultimate fragmentation increases with the time of application of a plane wave pulse. At high pulse durations, degree of fragmentation is more sensitive to changes in pressure than at low pulse durations.

INTRODUCTION

Recent interests in improving the efficiency in the comminution of ore minerals have led to studies involving the characterization of fracturing from explosive loading. Energy consumption in the mechanical crushing and grinding of ores for beneficiation is less than 1% efficient (Committee on Comminution and Energy Consumption, 1981). In 1978, electrical energy consumed to crush and grind copper ores was 18 times the energy consumed for explosive fragmentation. The increase in surface area per kilowatt-hour equivalent of explosives consumed is not well understood. Conventional rock breakage using explosives in cylindrical boreholes is performed for handling ore during mine operations; little attention is given during blasting to the amount of particle size reduction for processing. Fine crushing, however, occurs within a limited region surrounding the borehole. There is considerable interest to extend this zone of finely crushed rock during blasting. The mechanisms of rock breakage using multiple cylindrical charges are extremely complex. A number of experiments are being conducted in an attempt to understand the roles of stress waves, delayed gas pressures and their interaction with reflecting free surfaces on fragmentation. In the first stage of this investigation, the effects of the shock wave which produces the "rubblized" region in the proximity of the borehole was studied. In order to allow independent variation of the peak pressure and pulse duration, an impact technique utilizing a flyer plate was used. Ten experimental explosive events were conducted to determine the effects of compressive explosive pressure and pulse duration on fragmentation for copper porphyry in the absence of reflected waves. Nominal design pressures ranged from 1.2 to 8.0 GPa (14.5 to 116 x 104 psi) while pulse durations ranged from 1 to 6 µs. The axial loading is thought to simulate the high compression forces near the borehole wall during blasting. Rock specimens used for shock experiments are altered quartz monzonite porphyries consisting of quartz, orthoclase, and abundant sericite. Grain sizes range from 1 to 5 mm (0.04 to 0.2 in.) with 3% to 4% chalcocite as the chief ore mineral. The rock contains numerous open and healed fractures.

EXPERIMENTAL DESIGN AND SHOCK WAVE THEORY

Figure 1 shows the inclined plate plane-wave generator used to induce a uniaxial shock wave in a cylindrical specimen of copper porphyry (20.3 cm (8 in.) in diameter and 10.2 cm (4 in.) in height). The "mousetrap" assembly, described by Benedick (1972), uses a triangular line- wave generator and Detasheet (1) of varying thicknesses as the main explosive charge in direct contact with a 30.5 cm (12 in.) square plate of 2024 aluminum alloy.

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