Rock Products

JAN 2019

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40 • ROCK products • January 2019 www.rockproducts.com Drilling Scenario One: Borehole Diameter = 3.5 in. Burden = 7.5 ft. Spacing = 10.25 ft. Bench Height = 22.5 ft. Scenario One: Borehole Diameter = 3.5 in. Burden = 7.5 ft. Spacing = 10.25 ft. Bench Height = 30 ft. Table 3 shows the common fragmentation effects of stiffness ratio. As the bench is lower the fragmentation of the blast contains more fines and more oversize – the opposite of what is desired in any situation. This is because the goals of frag- mentation are most often two parts, the first part is what is the size of the rock. Typically, this is defined as a P50 or P80; however, the second and one of the most important goals is how evenly distributed can the fragmentation be. Imagine the scenario of an aggregates producer who desires that blasted material, on average, be around 6 in. They bring two powder companies in to conduct side-by-side blasts to see which can achieve this result. Powder Company A conducts their blast and produces an average size of 6.5 in. material, with 95 percent of material between 4 and 8 in. Powder Company B conducts its blast and produces an aver- age size of 6 in. material, but minimal material is between 4 and 8 in. Instead there is a large amount of material that is under 2 in. and a large amount of material that is over 12 in. Who will get the contract? Powder Company A, because distribution is in many cases more important than having the exact size. Larger stiffness ratios produce better distributed product than lower stiff- ness ratio, if boulders and fines are costing an operation achieving proper stiffness ratio is one method to eliminate these problems. Selection of Borehole Sizes As many mines have a certain bench size requirement, the main way a mine can engineer an improved stiffness ratio is through the selection of a proper borehole diameter. This is because the burden of a blast is directly proportional to the diameter of the borehole. Therefore, selecting a borehole diameter that produced a higher stiffness ratio is typically better. What does this mean though? Typically this goes against what many think is a proper method to reduce costs, instead of going to a larger diameter borehole the use of a smaller diameter borehole is preferred. While this increases drilling and initiator costs, the ability to spread the spacing out and obtain uniform fragmentation offset this cost. Additionally, through the use of scientifically based, not sales based, priming recommendations and the ability to spread the patterns out further on higher benches many mines actually see a large decrease in overall mine costs when conducting mine-to-mill optimization techniques when going to an appropriately sized, smaller borehole. What is the proper borehole size for an operation? Well, it varies based on the operation but typically an operation should design there borehole so that they achieve a stiffness ratio above a three, after the burden has been calculated. For an upper limit, it is typical that at slightly above four will give the best fragmentation while keeping costs minimal. While sites can blast at stiffness ratios well above four, they may be able to reduce costs while keeping fragmentation similar if they move closer to a stiffness ratio of four. Conclusion Mining is an extremely competitive environment, especially for aggregate producers that are constantly trying to achieve minimal costs through an operation. The typical bottleneck for many operations though is not in the blasting but in the crushing and processing; this leads aggregate producers to greatly gain by introducing mine-to-mill optimization pro- grams and considering the total cost of an operation, not just of individual units. This path will often introduce the goal of obtaining proper sizing and distribution of blasted materi- als to ensure that the crusher(s) are running at maximum capacity and minimal re-handling/secondary blasting costs are introduced. In order to achieve these goals, it is often more economical to select a borehole that will produce a stiffness ratio in the range of 3 to 4.5, giving the mine a good fragmentation dis- tribution and sizing while also keeping costs manageable. In many mining situations, the way to achieve this will be to go to a smaller diameter borehole that is typically the oppo- site of what producers first imagine when looking at lowering costs. This is because at first this can appear costlier, but with proper spacing arrangements oftentimes powder factors remain the same as with a larger borehole. Increases in cost then arise from additional drilling and initiators. However, this will lead to properly sized product where the reduction in crushing and secondary breakage costs, as well as reduction of waste fines, which will ultimately put the producer into a better economic condition with a lower overall cost. Dr. Calvin Konya is the president of Precision Blasting Services, and Anthony Konya is a project engineer for the company. They can be reached at 440-823-2263, or anthony@idc-pbs.com.

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