Coal Age

JAN-FEB 2017

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Page 42 of 51

January-February 2017 41 operating ideas continued stronger charge would only be located in the bottom of the blast. This was a com- mon practice until bulk loading of emul- sion began. The bottom charge is considered to be that which lies below the desired bottom (subdrill) and up to one burden distance above grade or floor. For example, if a blast has a burden of 20 feet (ft), the bottom charge is from 6 ft underneath the desired floor to 20 ft above the desired floor. Now that length of the bottom charge can be quantified. There are two methods for im- proving this bottom charge. Bottom Charge Improvement: Method One The first method of increasing the bot- tom charge of a blast is to increase the explosive energy present in this section. This is typically done by increasing the weight of the explosive, and is also why primers are typically placed near the floor of a blast. The primer is a more en- ergetic explosive and gives the bottom of the blast extra power. Another method is the use of boosters (no cap inserted) in this region. These boosters are a denser, more energetic explosive than the col- umn charge. While these methods work, especial- ly at helping prevent toes with a proper design, they will not significantly expand the burdens. How can one then increase this bottom charge in a simple and prac- tical manner? The answer to this comes from vary- ing the explosive used in a borehole. For example, assume an ANFO is used in an 8 in. borehole and has a 17-ft burden in the initial design. ANFO is a very light ex- plosive with a specific gravity of around 0.84 to 0.86. In this case, with normal de- sign parameters, this burden would be designed to pull the bottom burden and "over-shoot" the middle burden. What if one then replaces just the area of the bot- tom charge with a straight emulsion that has a specific gravity of 1.28? This would allow the burden to then be increased to where either the middle charge (straight ANFO) or the bottom charge (emulsion) was insufficient to pull their own bur- den. This could increase the burden from 17 ft to about 20 ft, saving the mine sig- nificant money. This method is extremely reliable and easy to implement through new loading procedures at most opera- tions. Further- more, different higher energy blends could be used as the bottom charge to increase this burden as well. In this situation, if a 30% emul- sion and 70% ANFO blend was used when the remainder of the hole was loaded with ANFO, the burden could likely be ex- panded to near 20 ft. This then adds min- imal cost to the operation and drastically increases the volume of material moved. With packaged product, the bottom could use a larger diameter, more energetic charge than the top charge and achieve the same results. Bottom Charge Improvement: Method 2 The second method for improvement to the bottom charge is achieved by angled drilling and can be used with method one for further improvements. This is com- monly done in Europe, but not in the USA. This method does have drawbacks. First, many large hole rotary drills cannot be angled substantially. In addition, the drillhole deviation is significantly larger on an angle than on a straight hole. How- ever, if the deposit is uniform, the drill has proper capabilities and blastholes are short then this can increase the amount of material blasted. When drilling an angled hole, the burden is still assessed as the perpen- dicular distance from the hole to relief at the time the hole detonates. Rock at the bottom of the bench breaks easier if the angle of the hole to the grade or floor is larger than 90°. The larger the breakage angle of the blast the easier the blast can break the bot- tom burden. This allows for an increased burden that can be significant. For exam- ple, with the previous 8-in. borehole that is straight (90°), the burden was 17 ft. If this hole is angled to where the breakage angle is 108° (3:1 slope on borehole) then the burden could again increase to more than 20 ft. If both of these methods are com- bined, a burden of around 23-plus ft could be achieved. This expansion in burden would then be more than 6 ft, and the spacing could be increased as well to accommodate the new burden. This could result in an additional 127 cubic yard (68-ft bench) of additional material per borehole by using an angled hole and using an emulsion charge in- stead of a bottom charge. Increasing the bottom charge of a blast allows for the use of larger burdens while achieving similar blasting as the previous pattern. This method can be used to drastically decrease the cost per ton from drilling and blasting, leading to improved productivity and efficiency. This can be done by either increasing the explosive energy in the bottom of the borehole or by increasing the break- age angle with angled holes. While these methods may require changing standard operating procedures, they can save mines hundreds of thousands of dollars annually. Combine this with other meth- ods of cost reduction and watch the bot- tom line shrink. Anthony Konya is a project engineer and Cal Konya, Ph.D., is the principal for Precision Blasting Services, a consulting firm that specializes in helping mines improve fragmentation programs. Anthony Konya can be reached at 440-823- 2263 or Figure 4—Breakage angle for angled holes.

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