CONTINUOUS CUTTING AND RAPID DEVELOPMENT
Rapid coarse ore transport
As the impact of climate change becomes more severe , the need to transition to a low-carbon economy within 20 years is becoming obvious .
Improving the productivity of the mining industry is the only way to achieve this transition by producing , delivering and storing more electricity , according to Doug Morrison , President and CEO of CEMI .
“ Since bringing new mines into production can take over 20 years , so increased supply can only be achieved by making existing mines produce more metal , at a lower cost ; higher-priced metals cannot displace low-cost carbon ,” he said .
Since the mechanisation of metal mine production in the mid-1980s , LHDs and trucks have been the stalwarts of coarse ore transport systems . After the primary crusher , ore is moved efficiently by conveyors , but coarse ore from drawpoints is too dense and abrasive for conventional belt conveyors .
Conventional batch vehicle transport of coarse ore has two critical limitations – energy consumption and time efficiency , according to Morrison . “ Typically , LHDs weigh three times their payload and return empty so that less than 20 % of the fuel is used to move the ore – most of the horsepower moves the LHD ,” he said .
Ventilation is the second largest cost in underground mines and is regulated by horsepower – reducing equipment horsepower reduces ventilation .
Second , a typical haulage cycle ( dig ore pile , travel to dump point , discharge , return ) is dominated by the travel time during which no other equipment touches the ore , Morrison said . The result is that the mine ’ s final product , created at great cost and ready for delivery to the customer , is utilised less than 10 % of the time .
“ No business can be successful leaving its final product untouched 90 % of the day ,” he said .
Continuous loader systems can engage the ore pile at least 80 % of the shift , they use about 20 % of the energy of LHDs and transport ore at least five times faster than mobile vehicles . Most continuous loaders today cannot meet these requirements – they would have to be made more wear-resistant and more powerful than now , according to Morrison .
“ The critical success factor is what the loader discharges to ,” he said . “ If the receiving vehicle is a truck , there is always a delay when the full truck is replaced by an empty one – and the loader is no longer continuous . A steel conveyor would be efficient , but the sources of broken ore in draw-points change too frequently for a conventional steel conveying system to be effective .”
CEMI has designed the Mascot System to , it says , continuously transport coarse ore – either from continuous sources in draw-points , or for fixed-tonnage sources such as drift development headings that break a similar tonnage every cycle .
All units are electric rubber-wheeled vehicles , designed to operate autonomously and align themselves as to operate in concert . While moving into position they can operate under battery power but , once aligned , they remain stationary while they transport the product and can be interconnected to use direct power , CEMI says . The units align themselves behind a continuous loader which also controls the maximum acceptable size of material by breaking large pieces of ore with a hydraulic chisel . Behind the continuous loader , each Mascot unit receives the product and discharges it to the unit behind it until all material is delivered to its final destination .
In a drift heading , the Mascot units ( Mascars ) have storage capacity for
Figure 1 : The sequence of Mascot Car movements to accelerate rock removal
about 20 t , so that a loader can fill 15 units with 300 t . The key is to keep the rock moving continuously using Mascars that can pass one another in the heading ( Figure 1 ). A row of 3-5 Mascars can be filled by the loader while a second row moves into position alongside them . Once the last car in the first row is filled , the loader switches the discharge tray to the second row and the full cars leave to be replaced by empty units .
Full Mascars can move back along the drift and park against one side of the drift to allow other vehicles to access the face , according to CEMI . Once all the broken rock has been moved out of the heading , the loader can withdraw to a park position to allow the face drill and bolting equipment to move into the face . The Mascars have around eight hours to deliver their load to the rock-bin and return to the park position in the drift heading , ready to repeat the process .
In the case of a continuous supply of ore from a draw-point , the wheeled Mascot units do not require ore storage capacity ; they act as a steel conveyor in a walled trough receiving and discharging ore . The units are called Dragoons ( Figure 2 ) and , once lined up along one wall of the drift , they can deliver ore continuously from the draw-point to the ore pass . A modified version of this unit can traverse drift intersections and allow utility and supply vehicles continuous access , CEMI says .
The larger number of units behind the continuous loader will cost about 10 times more than the LHDs they replace , but they move 40 times more ore with about 20 % of the energy used by LHDs . Computer simulations of a 100,000 t / d block caving operation estimate the operating and capital cost of the Mascot system to be 50 % and 60 %, respectively , of the current operating cost of conventional systems . In deep , hot , low-tonnage , highgrade mines , the application is to help achieve autonomous production .
Figure 2 : Series of Mascot Dragoons moving ore continuously
NOVEMBER / DECEMBER 2020 | International Mining