In metalliferous mining operations,subdrill is that portion of the blast hole that is drilled below the target grade elevation,and in most cases loaded with explosives.Its primary aim is to enable efficient excavation down to the target floor level.Subdrill is often described as being expensive and that excessive subdrill should be avoided,as it can result in damage to the subsequent floor,lead to difficult drilling conditions and higher vibration levels.As a result,many operations aim to apply the bare minimum amount of subdrill in order to excavate to the target floor.At a large open cut copper operation,challenging geological structure resulted in slabby and blocky fragmentation in the stemming region,resulting in poor crusher throughput due to bridging.To overcome the poor fragmentation in the top of the bench,longer than conventional subdrill was used to precondition the top third of the bench.The technique significantly improved the run of mine fragmentation and resulted in feed to the in-pit crushers that is more consistent,which produced less crusher downtime due to bridging.A new method of quantifying the energy in a blast has been created.Termed the linear powder factor,or energy factor,the calculation considers the energy contributed by the sub-drill of the upper bench and its impact on the energy in the lower bench.The technique enables blast designs to be optimised to deliver finer fragmentation and more consistent blast outcomes.To support the layer of preconditioned rock at the top of the bench,an innovative new device was developed.The device is inserted into the drill collar immediately after drilling and remains in position throughout the subsequent steps of hole depth measurement,priming,loading and removed prior to stemming.The device not only supports the broken rock at the top of the bench,but also minimises the amount of drill cuttings that fall back into the hole.Furthermore,the device helps protect the hole from damage by external factors such as weather events,drill tracks,vehicle tyres and movement around the collar during measurement and loading of the blast hole.
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