THE revolutionary transition from non-electric to electronic delay detonators (EDDs) in blasting has brought many benefits to mining productivity, but there are still many advantages to be gained by blast pioneers prepared to think outside the box.
|Complex geology and block shapes can be addressed by the flexibility and accuracy of blast timing using EDDs. The plan at the top shows a pattern drilled on a block containing a rich ore zone in the centre. Timing using three different vectors in BlastMapIII allows resultant timing contours and blast movement vectors as in the plan above, effectively heaping ore into a well-defined pile.|
This is the message from BME technical director Tony Rorke, who highlights the focus in recent years on the flexibility in timing that EDDs allow and more specifically on the value of longer timing delays.
“When we were first exploring the capability of EDDs, we were impressed by the accuracy and reliability this technology allowed us,” he says. “This was a huge step forward, ensuring there were no out-of-sequence blasts.”
When using non-electric initiation systems certain traditional timing conventions were employed based on the fact that accuracy decreased significantly as the length of pyrotechnic delay elements increased and that the downlines may be severed (cut-offs) during a blast before the firing signal reaches the detonators leading to dangerous misfires.
“To get detonators to fire in some acceptable sequence, therefore, delay periods were in the past kept as short as possible. Longer down-hole delay has the dominant impact of firing time accuracy, so the risk of out-of-sequence initiation is a function of the in-hole detonator. Keeping this short as possible led to delay periods of between 350 and 550 milliseconds becoming standard for in-hole delays in surface blasting.”
The result of these delays was that surface delays had to be notably shorter, to ensure an adequate ‘burning front’ – so standard delays most commonly used today for surface connections are 17, 25, 42 and 67 milliseconds. These periods are generally suitable for hard rock, but the shorter 17 and 25 millisecond periods increase the risk of out-of-sequence firing or blast choking, due to the inaccuracy of longer in-hole periods. However, if the surface delay periods are increased to try and prevent non-sequential firing, there is a greater risk of cable cut-offs caused by short burning fronts.
“None of these problems exist with EDDs,” Tony Rorke says. “Their accuracy allows for short delays with little risk of non-sequential firing. Each detonator also counts down independently, irrespective of whether the cable attached to it is cut and blast planners can, therefore, program long delays without worrying about misfires caused by downline cut-offs.”
This greatly improves the flexibility in timing designs, allowing complex and effective timing designs. This is possible with powerful software like BME’s BlastMapIII and the result is a range of blast outcomes that can effectively change the mining method and significantly reduce costs.
He says rock takes around 30 milliseconds to react to the energy of an explosion, depending on geology. This has led blasters to lengthen timing delays in search of better results – and they have been rewarded. Blasting results have improved as timing delays increase, delivering improvements in a number of areas including ore dilution control, through-seam blasting, underground blasting, trim blasting and drill pattern variation.
The problem, however, is that adoption of EDDs is not always followed by any blasting practice innovation. “My experience is that when operations convert from non-electric initiation systems to electronic detonators there is a reluctance to give up the familiar 17x42 or 42x67 millisecond combinations, and miners impose these delay periods on EDDs,” he says. “This restricts the benefits of electronics to only the accuracy component and ignores the more powerful flexibility component.”
To achieve less ore dilution, for example, the advanced programming capability of EDDs needs to be harnessed. Using BME’s BlastMapIII design software with AXXIS electronic detonators, it is easy to transfer a complex timing design from a computer to the detonators.
“The advantages with this timing is that it provides strong visual cues for loading teams. It also ensures that displacement across ore and waste boundaries is limited, preventing dilution.”
Through-seam blasting is another valuable innovation successfully employed in the US and Australia. The principle is to separate the charge in the reef and in the overlying waste with an aggregate ‘waste deck’. Charges in the overlying waste are fired first, followed by those in the reef zone a few seconds later. This can only be achieved using EDDs, as the firing of the top layer cuts off downlines to detonators in the layer below.
“A time lag of a few seconds allows the waste to settle back on top of the reef,” he says, ”so that when the reef charges fire, the overlying waste effectively blankets and confines the reef portion of the blast, preventing heave and dilution.”
Tony Rorke urges mines to actively explore the power of EDDs to achieve timing designs and results that are not possible with non-electric initiation systems.