Aug '13

Value in pieces

Smarter tools and bold ideas promise better blasting results

By Correy Baldwin

Smart blasting is proving to be an essential tool for operations bent on improving their margins. A mill runs best when it is fed ore that has been properly fragmented, with particles of consistent size, within a range best suited to that plant. Improve fragmentation during blasting and mineral recovery rates will go up, while energy costs of crushing and grinding will be kept in check.

“It’s far cheaper to use explosive energy to break rock than electrical energy in crushers and mills,” explains Gavin Yuill, the North American technical services manager for Orica. Blasting specialists are honing their technology – from the drilling of blast holes, and the use of explosives toelectronic detonators and post-blast image analysis.

Controlling fragmentation can begin at the drilling stage through measurement-while-drilling (MWD) technology. MWD analyzes geological conditions as blast holes are drilled, creating a more accurate, detailed picture than traditional geological assessments.

“MWD technologies have the potential to change how blasting is done, as we have immediate real data around the geology in the blast,” says Yuill. This detailed information allows companies to design more efficient blast sequences and choose the most effective explosives, as tying blasting energy to geological variations results in a more optimum blast.“It’s theoretically possible to custom load every hole, instead of using the same product across the whole bench,” says Yuill.

Another evolving technique involves setting up an array of seismographs around a blast site and firing a single “signature” hole. “You take the trace that comes on the seismographs and measure the transmission of the energy coming through the rock,” explains Austin Powder general manager Keith Taylor. “There are now programs such as our QED program where you can enter that data and it will give you the optimum firing time for that specific rock, across for every hole. We have found out in many cases that we should shorten the row-to-row or hole-to-hole firing, from a more traditional 25 milliseconds down to 10 milliseconds or even less.”

Greater range and precision

Explosive products have become much more sophisticated as well. “The biggest change in the last few years in explosives has been in emulsions,” explains Austin Powder general manager Keith Taylor. Emulsion explosives can be sensitized to detonate to produce an individualized blast strength, providing a versatility and control not available with conventional explosives. With the modern loading equipment, the emulsion can be programmed to different densities in the various parts of the hole.

Advances in emulsion explosives not only allow for greater detonation sensitivity, but greater explosive strength and range as well. Traditional explosives have a limited relative bulk strength (RBS) of between 100 and 170. New emulsion explosives greatly expand this range. Orica’s Flexigel explosive, for example, has a low RBS of 30, better suited for softer ground or sensitive areas, while their Vistis explosive can reach RBS 275, ideal for hard rock.

But more sophisticated explosives require more sophisticated detonation. “Electronic detonators have to be the biggest advance in drill and blast planning,” says Yuill. Electronic systems are more reliable, have a large delay range, remote blasting capabilities and, most importantly, incredible accuracy. Electronic detonation can be programmed down to the thousandth of a second, something simply not possiblewith standard non-electric detonators. “This allows an engineer to design the interaction between shock waves,” explains Yuill, “leading to a much more efficient usage of the explosive energy.”

According to Taylor, the industry has yet to use electronic detonation to its full potential: “On the whole we have yet to take advantage of what the increased timing accuracy allows us to do. In most cases, we’ve just timed the electronic detonators to the old firing times. That’s ok, but that’s not using the detonator to its optimum.”

Bigger blasts

Ultra high-intensity blasting (UHIB) is another advance that could have major promise. “The UHIB philosophy is to pack a lot more energy into the rock volume compared to traditional blasting,” explains Marcin Ziemski, who was part of the research team at the University of Queensland that modeled the method. “It maximizes rock fracture through explosive energy, thus reducing the amount of work required of traditional crushing and grinding. However, he notes: “To be practically usable, it must alleviate high energy blast problems associated with overt flyrock, blast movement and vibration.” This involves layering a conventionally charged level of blasts on top of a deeper layer that features powder factors four to five times higher. The blast begins with the detonation of the top layer which fragments the overlying rock. Following a delay, the lower, high-intensity charges are detonated and the top layer, like a heavy blanket, buffers the explosion and limits flyrock. The researchers’ models of the approach found that, though it inevitably pushes up the cost of drilling and blasting, it could improve fragmentation and increase mill throughput between 25 per cent for hard ores to 40 per cent for softer ones.

In February 2012, UHIB had its first field trial at Codelco’s Andina mine in Chile, where it was applied as part of a larger conventional blast. The test included tighter-spaced drill holes and a powder factor of 2.3 kilograms per bank cubic metre of rock on the lower layer – three times the amount of a conventional blast. According to the Orica team that conducted the test, excavation of the UHIB section was reported to be easier, produced more fines less than 25 millimetres, and generated lower vibration levels than the conventional blast. The first full scale UHIB blasts were carried out by Orica in May 2013 at Goldcorp’s Peñasquito mine in Mexico, where, according to Yuill, the same benefits were seen.

Improved feedback

Blast design software is essential to create sequences and anticipate end results, though developing an ideal model is an ongoing process which relies on improving the information used by the software.

“The models are good,” says WipWare president Tom Palangio,“but every operation has its own set of conditions, and there are so many variables that they can’t all be easily accommodated by a mathematical formula. There’s the hardness of the rock, the jointing in the rock, the explosive you’re using, the way the explosive couples to the rock – it’s the biggest jigsaw puzzle in the world.”

A number of size distribution analysis systems have the ability to quantify and analyze blast results, and thus help in modifying prediction modelling. For example, WipWare’s Reflex and WipFrag technology measures and quantifies the size distribution and shape measurements of the fragments on haulage trucks, scoop trams and rail cars.

“You can tell exactly what’s on the vehicle, and exactly where it came from. By taking that data from each vehicle and merging it all together, you can quantify the blast and get an exact idea of what the total blast was like.” The image analysis can accurately measure very fine particles, down to the micron range.

Comparing the prediction models to the measured results of a blast, explains Palangio, allows you to fine-tune your blast design. “You can start to benchmark. As you change the variables – maybe you change the pattern or the explosives or the timing – you can track and see the cause and effect. This really opens up some doors, because not only do you start to improve things, you start to understand the variables better.”

Still, presenting new technology can be a struggle. “The uptake is there, though the first question we still often get is not ‘what does it save us?’ but ‘what does it cost?’” says Taylor. There may be cost increases with the new technology, he says, but the end unit cost is cut dramatically, making for a quick return on investment.

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