Coal Age

MAR 2019

Coal Age Magazine - For more than 100 years, Coal Age has been the magazine that readers can trust for guidance and insight on this important industry.

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30 www.coalage.com March 2019 blasting continued www.richwood.com www.richwood.com On-Track ® Belt Tracker Reliable Richwood Belt Tracking • Protect the belt and conveyor structure from damage caused by off -center tracking • Simple installation • No edge guide rollers • Designed for optimum work life with heavy duty components phone (304) 525-5436 email: info@richwood.com ©Richwood 2017 Innovative conveyor solutions for bulk material handling. It can then be seen that, just based on the rock, a powder factor design approach does not work and will disadvantage mines both in the cost of the drill and blast program and in poor performance. Dozens of other reasons exist that powder fac- tor will be a poor design tool, and a small sample of these will continue to be seen. Stiffness Ratio The physical dimensions of the charge also play a major role in the function of the powder factor. Short benches have extremely low bore- hole utilization and as such have very little weight of explosives. Bore- holes that are long and are drilled for a high bench have a large amount of borehole utilization and thus contain greater weight of explosive, com- paratively. This bench height, or the dimensionless alternative, which is termed stiffness ratio, also affects the powder factor. Assuming that a steady powder-factor design is used, a low bench would have close spacing, a medium bench would have slight- ly larger spacing, and a high bench would have no variations to spacing as the increased weight of explosive is nearly identical to the increased vol- ume from the bench height. However, in today's day and age, it is under- stood how the spacing is to be modi- fied based on the bench height. The relationship for the Inde- pendent Variable Konya Design change in spacing based on the bench height can then be viewed as a relationship between powder fac- tor and bench height. This is seen in Figure 1, which shows the chang- es in powder factors compared to changes in stiffness ratio (Bench Height divided by the Burden). As can be seen, a bench would then have variable powder factor with benches that have a stiffness ratio (bench height/burden) of 2 to 2.50 having the largest powder factor and high benches having the lowest powder factor. Typical powder-factor-design approaches rely on the powder factor, which is typically required to break bench- es of a stiffness ratio of 2 to 3. This means a mine blasting with a high bench and using powder factor design would not be taking advan- tage of the expanded spacing. By having a small spacing when nec- essary, the mine is increasing their drill and blast cost and suffering from poor fragmentation. Hole-to-Hole Timing Changes in the hole-to-hole tim- ing will change the borehole inter- action, which changes the spac- ing between boreholes. With an extremely short or instantaneous delay between boreholes, the spac- ing needs to be greatly expanded

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