Engineering & Mining Journal

JAN 2019

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BLASTING 30 E&MJ • JANUARY 2019 www.e-mj.com limits, but are far from widespread rules for design in the industry and are nowhere near optimal. Today, powder factor can be used as an economical tool to quickly give information on the costs associated with a blast, but not as a design tool. Modern Spacing Methods Modern spacing of boreholes is not based on setting a powder factor or using a pre- selected design that only works in unique situations. Instead the spacing of blast- holes should be selected based on the de- sired outcomes of the blast. The spacing will then be designed in accordance with the bench height, burden, and the hole- to-hole timing. For the rest of this arti- cle, it will be assumed that the boreholes along a row are not fired instantaneously along a row. In this case, one can simplify the spacing design further to where we as- sume that the spacing will scale with the burden. In this case, the spacing ratio will be defined as the spacing between bore- holes divided by the burden of a borehole. This spacing ratio will then be changed based on the stiffness ratio of the blast, where the stiffness ratio is defined as the bench height divided by the burden. When the stiffness ratio of the blast is less than 4, the bench will be considered a low bench. The lower the stiffness ratio of a blast, the worse the blast will function. For stiffness ratios near 1, the bench will break with a cratering mechanism. This same effect will also cause the spacing ratio of the blast to be reduced and, when the bench height is equal to the burden, the spacing ratio will also equal 1. As the stiffness ratio then increases, the spacing ratio will also increase. Assuming that the increase in stiffness ratio is from an in- crease in bench height, the powder factor variations are shown in Figure 3. The graph presented in Figure 3 may appear strange at first as the powder fac- tor dramatically increases, then quickly falls, and then again slowly rises. This will be explained further and why this phe- nomena occurs, but it may be easier to understand by comparing the powder fac- tor not to the bench height, but instead to its stiffness ratio. This also allows for this phenomena to be considered for all situa- tions of blasting, independent of borehole size or bench height. What is clear to see in Figure 4 is that two distinct actions change the powder factor based on the stiffness ratio. The first is in the low bench category and the second is in the high bench category. These two scenarios will both be explained in further detail in the following sections. Before an analysis of the mechanism on why these changes occur, it is important to first understand the differences between modern design techniques as compared to the old school blast design techniques. It has been known for decades that blasting with higher benches produces better re- sults, leading to improved fragmentation and throw. However, older design tech- niques showed that, with a stiffness ratio below 3, a large decrease in powder factor was available over a taller bench. As pow- der factor is an economic tool to determine how much explosive will be required for a blast, this led operators to have to decide between good blast effects at a higher cost or poor blast effects at a lower cost. Many operators then met somewhere in the mid- dle and choose to blast near a stiffness ra- tio of 2.6; which is the national average for stiffness ratios in mining operations. However, as Figure 5 shows, these old school techniques have not only cost op- erators good results from blasting with a larger stiffness ratio, but they have been costing operators much more in direct costs. From Figure 5 it is clear that with modern design approaches an operation could blast with a stiffness ratio of 4.0 and maintain lower powder factors than blasting with a 2.5 with the old approach. Better results and a smaller powder bill. It is critical for this discussion to under- stand that the powder factor of a blast does not influence the fragmentation, throw, or other blast results to any major extent. Variables such as the confinement, shape of charge, timing and method of breakage are much more critical and as a result us- ing more explosive to achieve similar or worse results is a waste of money. In the end it is not how much explosive is used, but rather the manner in which the explo- sive is used that achieves superior results. Figure 4—Stiffness ratio to powder factor based on modern spacing design. Figure 5—Modern blast design compared to old school blast design on powder factor compared to stiffness ratio.

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