Abstract
The production of both over- and undersized coal fragments is a constant problem to open pit operators in the Powder River Basin. Operators strive to achieve production blasts that economically minimize the weight percentage of over- and undersize coal. However, natural fractures influence blast fragmentation, and can impose limits on the final fragment size distribution. ARCO's Anaconda Minerals Company studied the limits placed by natural fracturing as part of a wider geotechnical study of the Wyodak Coal in the eastern Powder River Basin, Campbell County, Wyoming. Measurements of fracture orientation, size and spacing were made at ARCO's Black Thunder Mine. These data were used to form statistical distributions for natural fracture size, spacing and orientation. An algorithm was created to select fracture properties at random from these distributions and to form hypothetical blocks. The blocks were then numerically "sieved" to derive possible particle size distribution curves. These curves are compared to fragmentation results from samples taken from three test blasts. All test blasts yield very similar lognormal fragment size distributions. The median particle size varies from 1.25 to 2.0 in. (3.2 to 5.1 cm), the mean from 4.0 to 8.5 in. (10.2 to 21.6 cm). The standard deviation was about 1.6 in. (4.2 cm). Coal fines (fragments<0.25 in. or 0.64 cm) are about 15%. Calculated fragment distributions for the natural fractures predict the median size well but underestimate the variance. Algorithm modifications to account for processes known to occur in the comminution zone near the blast hole significantly improve the match in the fine coal size range. The match suggests that spacing of fractures, rather than fracture length, controls the formation of fine- and medium-sized fragments. On the other hand, the oversize coal is controlled by the fracture lengths, rather than the spacing. Data from subsequent test blasts support this idea. By increasing delay times from 8 msec to 17 msec, engineers were able to reduce fines to 7% - 10% but could achieve no further improvement, despite changes in shot pattern geometry, decking and hole size. The hypothetical fragment distributions show that, even without blast enhancement, about 5% of the natural coal blocks are fine-sized.
Introduction
Prior to blasting, a rock mass is partially broken into incomplete fragments by naturally occurring fractures. Subsequent blasting of the rock mass extends these pre-existing fractures, as well as creates new one. Thus, the size distribution of the fragments created during blasting is related to the original density, size and orientations of the natural fractures. For example, if prior to blasting, 10% of the incipient fragments formed by the natural fractures were fine-sized (defined here as fragments less than 0.25 in or 0.64 cm), then at least 10% of the fragments after blasting should be fine-sized. The production of both over- and undersized coal fragments is a constant problem for open pit coal producers operating in the Powder River Basin, Wyoming. Excessive fines are particularly troublesome because they create explosion and fire hazards in the plant, contribute to fugitive dust problems in the pit, and may result in monetary penalties imposed upon delivery.
