2. Hard rock cutting for production
Hard-rock cutting technologies hold huge potential, not just for development purposes but also in a production setting.
For many years, the coal industry along with soft-rock applications such as salt, trona and potash, have been reliant on machines such as longwalls and shearers, road headers and borer miners to safely and cost-effectively cut material from the face rather than drill and blast it free.
The use of these technologies in hard-rock mines has until now been constrained by the ability of cutting tools to deal with the rock strengths encountered and associated costs. However, a number of miners and manufacturers have been focusing their efforts in this field and trials are now beginning to bear some very promising fruit.
Caterpillar has been working on its Rock Header and Rock Straight systems – essentially a road header and a longwall-type cutting system – for some time. The company first presented them at the MassMin conference in 2012 and showcased its activated cutting technology (ACT), which was first trialled at KGHM’s Polkowice mine in Poland, at MINExpo later the same year.
ACT utilises picks (rather than discs) to undercut the rock mass, based on the fact that the tension and shear strength of rock is generally less in terms of magnitude than the compressive strength. Cutting forces and, in turn, tool wear, are significantly lower in undercutting compared with conventional frontal attack methods, making it more suitable for hard-rock cutting.
It was reported after MINExpo that the Cat RH35 concept had been designed and built; RH stands for Rock Header and the numbers refer to the machine’s 3m x 5m heading dimensions.
The Rock Straight system is designed for fully mechanised mining of low seams and narrow reefs, such as copper, gold or platinum applications. It comprises a hard-rock miner (HRM220) cutting head, a hard-rock chain conveyor (HRC30) and a hydraulic roof support (HRS1220).
The link to the system on the Caterpillar website has recently been removed. However, the HRM220 was listed as being 6.8m long, while the conveyor is thought to be 100m in length and the hydraulic roof support can extend to 2.15m.
Caterpillar confirms to MM that the Rock Header is closer to commercialisation than the Rock Straight system, but the company is reluctant to give further details at this time.
Additionally, in early 2014, competitor Joy Global successfully trialled CRCMining’s Oscillating Disc Cutter (ODC) for hard-rock mining. The technology, which was branded Dynacut by Joy Global following its licensing to the company in 2006, combines a number of rock-breakage concepts into a single technology enabling the continuous excavation of hard rock using relatively lightweight equipment in both surface and underground mining applications. The ODC uses lower forces than those associated with conventional disc cutting and has the added advantage of producing large chips and little dust.
The company cited safety and an expected 20% increase in advance rates as key development drivers along with less disturbance to the rock mass and a smooth final rock profile. Two trials ensued soon after with Joy Global; one at Anglo Platinum’s Bathopele mine in South Africa, and a second at a Joy Global test site in Wollongong, Australia, although the results are yet to be announced.
Sandvik Mining officially inaugurated a new hard-rock cutting test rig on March 3, at its competence centre for mechanical cutting in Zeltweg, Austria. The engineering design, planning, construction and assembly of the rig started in August 2013 and were completed at the end of 2014. Since the beginning of the year, various cutting tests have taken place that Sandvik says will “contribute to the development of different cutting technologies for applications in rapid mine development and production machine concepts”.
The company is also working on its MN220 reef miner. The prototype was originally launched in 2000, and from 2001 to 2006 the machine was operated at both Lonmin and Impala platinum mines, but “unfortunately it was not able to initiate the transformation from drill and blast to mechanical excavation”, Uwe Restner, product line and sales support manager for hard-rock continuous mining at Sandvik tells MM.
At the time, the cost of drill-and-blast operations versus mechanical excavation was not favourable given platinum prices and the idea was shelved. However, “due to the fact that labour costs have significantly increased over the last few years, the picture today looks different, and this is why mechanical excavation is being given a second chance now. Currently, we do everything to use this chance and to meet the excavation cost target,” Restner adds.
The MN220, which also uses the undercutting technique and incorporates simultaneous cutting and roof bolting, is currently under trial at Bathopele and Sandvik is working to make the machine more cost-effective.
The aim is to optimise the cutting process and cutting-tool consumption as well as the loading and conveying of the cut material. “For material loading and conveying we also break new ground by applying pneumatic ore-handling systems based on air flow suction,” said Restner. “Out of the experience of this trial we also plan an upgrade of the MN220 to a new machine version optimised for narrow-reef platinum mining, but also keeping an eye on narrow-reef gold mining. Most probably, this should happen within the next three years.”
When asked about the potential of hard-rock cutting technologies in mining, Restner states: “In my opinion mechanical excavation will offer a step change, both in mine development and ore production. But this change will require some time and patience and very good co-operation between mining companies and equipment manufacturers. Only this tight co-operation will be the key to success.
“Looking back on the coal-mining industry, the step change from conventional mining to mechanised and partly automated mining easily took 10-20 years. Now, mechanical excavation is the standard in coal mining. I believe that a technology change in hard-rock mining would also require a similar time period.”
Restner cites production costs and safety (removing miners from hazardous working environments) as key drivers for the introduction of hard-rock cutting. “Concerning production-cost reduction, there are also other aspects that have to be considered such as selective mining (for platinum seams less than 1.1m in height). This reduces waste generation and optimises materials handling and ore processing,” he explains.
“Mechanical excavation eliminates secondary crushing; mining without man access eliminates rock-support installation; smoother mechanical excavation maintains rock stability and this also saves rock-support installation costs.”
The greatest benefit of mechanical excavation will be seen at operations where the ore occurs in concentrated seams such as platinum and gold reefs, some copper deposits as seen at KGHM operations in Poland, and tungsten mines such as Panasqueira in Portugal.
“I do not really see mechanical excavation as a production method for mass mining and very disseminated orebodies,” Restner states. “Production rates are not such an issue as they very much depend on geological conditions. Production cost is the key factor. If mechanical excavation machines produce at lower rates and costs compared with conventional methods, then they are still interesting as you could use a higher number of machines and you still would produce at lower cost compared with drill and blast.”
Restner believes that lack of patience, resistivity to change and the conservativeness of the mining business have been holding back the adoption of hard-rock cutting technologies.
“Mechanical excavation equipment must also be cost-effective in terms of production performance and tool consumption, which is where a lot of mining equipment and tool manufacturers struggle. For example, our reef miner in South Africa must also achieve a certain production rate and tool-consumption requirement to achieve a certain excavation/production cost target. If this target is not met, then the reef miner would not be attractive enough to motivate the mining industry to change to mechanical excavation,” he says.
“Of course, there might also be some other cost aspects having a huge effect on production cost. But here, quite frequently, the first look is at excavation cost and later the entire production cost. The overall picture has to be looked at and finally considered.”
Mining Magazine’s top 10 technologies to watch:
2. Hard rock cutting for production
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