Страницы из Blasthole_Drilling_in_Open_Pit_Mining-11.pdf

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BLASTHOLE DRILLING IN OPEN PIT MINING 53 TALKING TECHNICALLY Free faces Forward displacement of blasted rock occurs if a blast shoots to a free face (Figure 1). Some movement of the rock mass is necessary to allow for crack pro- pagation. Increased movement assists crack propagation and can improve frag- mentation. This may not be the main objective in some operations (e.g. blast- ing in ore) so free faces may be limited (choked) to restrict ore dilution. Blasthole angle Vertical blastholes are usually used in surface metal mines because: Angled blastholes are more difficult to set up and drill; Some drills do not have an angled drilling capability; and • Drilling accuracy is greater with vertical blastholes. In free-face blasting, vertical front- row blastholes often leave variable and excessive burdens between the top and bottom of the charge (Figure 2). This variation is greater in high- or shallow- dipping faces and can cause hard, immovable toe. Front row blastholes collared near the crest to control the toe burden can cause explosion gases to blow out prematurely in the face. (See Figure 3 and 4) This blow out creates noise, airblast and flyrock and reduces blasthole pres- sure near the bench floor level, which may prevent adequate breakage and movement of the toe. This may neces- sitate the use of some angled blastholes in front rows. (Figure 5) Subdrilling and drilled length of blasthole Efficient excavation needs toe condi- tions that suit the digging equipment. Toe conditions are affected strongly by the amount of effective subdrilling. Subgrade or subdrilling is the length of the explosive charge, which lies beneath the designed bench floor level. Unavoidable fallback of drill cuttings and small rock fragments reduces the effective subdrilling to less than that originally drilled. It is good practice to drill a certain extra distance (which is longer for higher benches and weaker rocks) to allow for unavoidable fall- back. Priming The overriding concern in priming is to locate the primer in the explosives column and ensure operational safety and efficiency. The primer is generally placed at or near grade level. Some ope- rators place the primer at a known dis- tance above or below bench floor level to ensure that, should a misfire occur, the excavator operator does not dig di- rectly into a primer. This may be a valid reason for not placing the primer at bench floor level. Bottom priming has several advan- tages over top priming. They include: • Improved fragmentation, displace- ment and muckpile looseness; Reduced toe problems, better floors, and cleaner faces; Reduced noise, airblast, flyrock and surface overbreak; and • Fewer cut-offs and misfires. Charge distribution Distribution of the explosive char- ges in the rock mass is an important Effective free face Fig 1. Effective free face.
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