Access development is one of the most important and expensive things underground mines do. As deep mines access more orebodies that are distant from an existing shaft, driving single-heading accesses over several kilometres will become commonplace. Getting to the orebody as soon as possible is crucial because it increases the net present value of the deposit so improving the rate of advance in access drift development is critically important.
The activities in the conventional drill-blast cycle (removing broken rock, installing ground support, drilling face holes and charging explosives) in a deep Canadian mine add up to about 16 hours. Given the availability of equipment and crews in a multi-heading setting, completing one cycle can take 36 hours. In simple terms this is about 3 metres per day and even a 16-hour cycle with centralized blasting would result in a rate of 4.5 metres per day. Mine contractors can progress faster because they manage a more efficient shift change and blast at will.
Unfortunately, the effectiveness of the drill-blast process has decreased over time. Safety standards increased, accesses became bigger to accommodate larger equipment, and so has the need for more ground support because of the size increase, higher stresses and rockbursting at the face. Conventional drill-blast techniques responded poorly to these changes: installing face support every cycle to protect against face bursting reduced the advance rate to less than 3 metres per day.
Concurrent face activities
In our view at the Centre for Excellence in Mining Innovation (CEMI), one of the best ways to significantly increase the drill-blast advance rate in deep hard rock mines, is to introduce a system that allows concurrent activities to reduce the critical path cycle time. To be able to drill and charge the face while simultaneously installing ground support, the equipment and workforce in the heading requires temporary protection. We asked North Bay, Ont.-based Nordic Minesteel Technologies to build a set of canopies that would be strong enough to provide protection equal to or better than the permanent ground support to be installed. We also asked Pierre Labrecque, now with SRK Consulting, to use discrete event simulations to confirm that a critical path process less than 10.5 hours could be achieved. This analysis showed that this is possible, but the time taken to remove broken rock from the heading had to be less than one hour. Contracting the cycle time to less than 10.5 hours, makes it possible to complete two standard advances every day (9 metres per day) – a significant improvement on 3 metres a day.
Three steel-frame canopies make it possible to carry out drilling, charging and bolting activities in a fully manual operation, but as drilling and charging becomes autonomous, some canopies can be eliminated. The two front canopies protect operators against rock-related damage on the walls and roof of the drift and there is a face shield to protect against face bursting. The captive drill unit has the time necessary to execute iRing blast designs that will improve perimeter control and face fragmentation. The third canopy at the rear is for installing ground support, bolting through the frame to attach the removable mesh on the outside of the frame to the rock surface in the normal way. The external mesh on the third canopy is re-installed manually every cycle while the initiators are being connected up, eliminating the manipulation of mesh from the critical path. Nordic Minesteel is in the process of organizing a trial at a mine in Chile and the original prototype is being modified to prepare it for the trial.
A critical activity is preparing the face holes for explosives, cleaning the holes and installing the timed initiators. These tasks are easily completed with the dexterity of human fingers but making equipment to achieve the same tasks is difficult. Sudbury, Ont.-based TesMan Inc. has been successful at creating technology to accomplish these tasks remotely and since drill equipment has been automated for many years, it will soon be possible to execute all of the face activities without people in the heading.
Fast rock removal
The other important factor is removing broken rock from the heading in less than one hour. Using a batch activity such as a load-haul-dump (LHD) unit wastes a lot of time travelling to and from the dump point. A continuous loader can move rock at over 400 t/hour but loading into an LHD or truck forces the loader to stop operating while a full batch unit leaves and an empty one arrives. The Rail-Veyor and Torex Gold Resources’ Muckahi systems are recent attempts to solve this problem by relying on a rail system, either on the floor or on the roof.
An old technique, used when drifting, included installing rail transport, and used a simple loading machine to fill multiple self-discharging cars that were later pulled away by a locomotive. CEMI has had a new self-discharging system designed, called the Mascot, to solve the problem. The system relies on autonomous control of the units and so avoids the use of a rail system. There are two partners working with CEMI to build the first Mascot prototype, which is really just a re-build of a very old manual system to be operated by digital technology.
Each Mascot receives a fixed amount of rock from the loader and transports it autonomously to a dump point. The key is the autonomous movement of the units that allow them to be filled rapidly while allowing other units to move into place. Once broken rock is removed from the face, they allow the face equipment and canopies to move into the heading immediately. The concurrent face activities take around eight hours to complete and during this time the Mascots travel to the dump point and return to their parked position near the face. They can do this under battery power, being hauled by other vehicles or by connecting to a trolley assist system. Once the blast is ready for initiation, the canopies can be withdrawn to a safe distance and the face equipment can then travel past the waiting Mascots for maintenance and re-supply. After the blast, the continuous loader can re-enter the heading autonomously, with or without the canopies, followed by the Mascots in sequence, to begin the next cycle.
The Canopy and Mascot system relies on digital equipment control to offer the real possibility of a completely autonomous drill-blast development process. This can achieve two cycles per day, advancing a 5- to 6-metre square heading in poor ground conditions at about 10 metres per day, using a process that allows all equipment maintenance and re-supply to be completed off-cycle.
Continuous rock excavation
The poor performance of the drill-blast cycle has led several companies to invest in trials of continuous rock excavating machines for deep underground mines. This equipment has been successful in civil tunnelling projects in relatively shallow conditions with ‘soft’ rock where they can achieve up to 20 metres per day. Their performance depends on rock strength, but they can certainly achieve 10-12 metres per day in rock with up to 200 MPa uniaxial strength.
However, there are several reasons why we don’t believe that continuous rock excavators can provide a complete solution to speeding up advance times. There is an operational trade-off between drill-and-blast and continuous mining technology platforms. Cutting machines require a relatively long time to set up, and then compete very well in long tunnels once they are operating. In shorter tunnels, the long set-up works against them and few machines are able to create the complex geometries and sharp corners common in most hard rock mines. The drill-blast process is slower but is more flexible and has a shorter set-up time.
Continuous rock cutting technology is unlikely to ever completely eliminate explosives from the underground mine environment. In order to set up a cutting machine for a new excavation, the void in which the machine is configured has to be created by drill-blast equipment.
However, the fundamental difference between tunnelling and mining is the impact of post-completion stress changes. In transportation tunnels there are very few post-construction stress changes and virtually no need for repair or rehabilitation after completion. Mine access drifts at depth are routinely subjected to significant stress changes caused by adjacent, large-scale ore production activity. In the case of highly stressed, brittle rock masses, this results in damage from seismicity and rockbursting that has to be repaired quickly and efficiently. It is simply not feasible to bring back a large, continuous rock cutting machine to repair this kind of damage. Finally, massive cutting machines that almost completely fill the excavation are susceptible to being trapped by stress-induced damage and their weight makes them difficult to recover. Compared to the smaller, more manoeuvrable drill-blast equipment, there is a higher risk of long delays caused by rock instability issues.
Mining Innovation GPS
Given that drill-blast technology in deep mines is likely to remain essential for the foreseeable future, we have focused on making it as lean and efficient as possible. This leaning of the process is similar to the second stage of Industry 3.0, when manufacturing introduced just-in-time delivery along with automation. Although leaning production processes in mining – improving their time and energy efficiency – has been largely ignored to date, it is essential before Industry 4.0 can be introduced.
This redesign process is part of our Mining Innovation GPS (MI-GPS) approach, which considers how system constraints affect the effectiveness of technological options. This accelerates the arrival at the ideal ‘destination’ (solution) for each situation, rather than the slower and more expensive ‘try-it-and-see’ approach. CEMI has applied Mining Innovation GPS (MI-GPS) to every aspect of mining, including development, production, ventilation, backfill and tailings treatment systems. We think the resulting solutions are how mining processes have to be redesigned if the performance of the hard rock metal mining industry is to be improved significantly and so be able to meet all the demands of the future.
Doug Morrison is the president and CEO of the Centre for Excellence in Mining Innovation (CEMI). For more information on the MI-GPS, visit https://www.cemi.ca/innovation-gps/.