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The closure of a mine can be the result of various factors, including technical challenges, writes Atlantech Senior Mining Engineer David Shirley.
Reasons for mine closures may include adverse political settings, declination of demand for the commodity, unfavourable geological factors, and naturally, full exhaustion of the resource.
The ideal scenario
Resource exhaustion is the least disruptive reason to close an open-cut mine as long and medium-term planners always have that future state in mind and can guide management decision-making proactively. This enables all aspects of the business to prepare and adjust to closure requirements.
Years before the last saleable tonne reaches the surface, environment and community teams increase their focus on final landforms, finalise compliance criteria, and develop robust rehabilitation schedules.
In addition, corporate, legal, and commercial teams ensure the mine’s transition to closure has clear and compliant pathways, which uphold the company’s values while meeting government requirements and customer expectations.
Unexpected challenges
Often though, there are short-term technical and operational aspects that can impact a closure. In this blog post, we will explore the kind of technical challenges open-cut mine production teams face when the pit is at its deepest point. We also examine how to effectively deal with the prerequisite ever-shrinking mining footprint while maintaining safety without compromising on productivity.
Mining intensity
Mining Intensity quantifies the density of excavation machinery in a designated area. Low intensity may indicate an unfavourable pit production angle, while high intensity leads to congestion between machines and an imbalance in haul road and loading area space. For open-cut mines closing due to resource depletion, managing short and medium-term scheduling decisions, along with day-to-day optimisation, presents a significant challenge.
Safety
Vehicle interaction while having a tier 1 digger per strip is not unfavourable, provided pit intersections and the haulage network are well setup. This can prove difficult when one or two strips are completed and there are not any new strips coming online. This scenario is compounded at larger multi-pit sites when an entire pit is completed.
When there is reduced elbow room on the bench, elevated standards are required. Windrow height, shape and material needs to be fit-for-purpose. A high level of precision is needed on bund positioning on drill and blast pattern boundaries; defining work areas; vehicle parking areas; and geotechnical hazard offsets. The production or pit engineer’s role execution becomes more critical than ever. Close collaboration between engineers and supervisors leads to optimised spatial division, allowing multiple mining activities to co-exist in an ostensibly cramped space.
Ore sequencing
Within standard operations, reduced product demand or transport issues may result in high stock levels at ore processing facilities. Having many strips in multiple pits can help alleviate the problem as the schedule adapts and pivots to focus temporarily on waste production.
However, in the final twelve months or thereabouts, the reduced number of pits, strips and dig areas can severely hamper this scheduling flexibility. Schedulers must be mindful of not only the upcoming quantities of ore production, but also the possibility that pit or orebody parameters and geometry may make it difficult to avoid being ore bound. Ore rehandles from a temporary holding stockpile is undesirable but potentially unavoidable.
Engineers and schedulers need to identify these possibilities early so that the problem-solving process can commence. Influential people within the business may be able to help alleviate the stockpile capacity stress, while other key planning roles select and prepare the holding stockpile site as a contingency.
Productivity
Having a reduced in-pit working footprint often results in difficulty finding areas for equipment maintenance. This results in longer than usual walk times for all slow-moving equipment. Imminent lightning storms will force planning teams to make difficult decisions regarding holding off loading blast patterns with explosives in favour of mining production. This is due to the possibility of lightning exclusion zones encapsulating the entire remaining working areas, forcing detrimental weather delays. This is especially strenuous when blasted mineable inventory figures start to get uncomfortably low. Drill and blast pattern geometry needs to be optimised to increase blast crew productivity and reduce blast turnover time.
Strategic opportunities to optimise the closure date
As the mine nears its end of life, the significance of time in the net present value equation increases. For example, with only seven months remaining in the schedule until the last truckload of ore is hauled out of the pit, an opportunity to cut three weeks from the end date can have significant operating cost-saving implications for the business. These opportunities to optimise the schedule are not usually present during normal full-scale operation. For example:
- Finding ways to run tier 1 and tier 2 excavators beyond their scheduled retirement date.
- Optimising in-pit dump progression to make use of the abundance of recently uncovered pit floor to reduce hauling distance and alleviate manning and equipment deficiencies.
- In certain circumstances, excavators that are not usually tasked with ore production in favour of waste movement may be ideal for accelerating the advancement of an area. Assuming the potential for increased ore losses are within acceptable limits, this acceleration can unlock downstream mining processes such as in-pit dumping or the drill and blast phase.
Final landform and dump designs
In the production’s final stages, technical and environmental teams are solidifying the mine’s final landform. The relationship between the final landform design and the final dumped topography design is iterative. Any remaining flexibility in the final dump surface should be utilised while strictly complying with the mine’s development consent conditions. Mine designers and schedulers should diligently review final landform designs to, where possible, reduce unnecessary long hauls or inefficient long bulk push areas.
It is important to avoid simply accepting a final landform design. Their size and automatically generated nature lend themselves to overlooked opportunities. Furthermore, final short-term dump designs may, where safety permits, be done in such a way that places waste as close to its final resting place as possible. For example, if a final landform in a localised area calls for a 2-3% grade, rather than dumping flat and dozer shaping later, skilled production teams can save downstream rework by dumping the waste at that grade, using the final landform as their target surface design.
Final wall stability and advanced wall control blasting techniques
Open pits are destined to leave open voids and exposed walls, and depending on their long-term use, these will likely partially fill with tailings or water. The walls above and closer to the surface must be considerately constructed for longevity. Drill and blast design best practices must be adhered to on any final wall blast with geotechnical stability front of mind. However, as dump systems advance along final walls, they significantly shorten the geotechnical risk exposure time, especially lower in the pit.
Strata hazards and wall failures resulting from difficult geology and blasting can be managed and monitored as a temporary measure until such time that they are covered by the advancing dump face. On the other hand, final void exposed walls do not have this same luxury. Examples of final wall drill and blast best practices include but are not limited to:
- pre-splitting, which may require a long-term pit design change, giving the required catch bench width to allow the operation of a drill rig with an articulated mast,
- reduced hole diameter,
- reduced maximum instantaneous charge,
- angle of initiation,
- booster location in hole,
- air or stemming decks, etcetera.
Non-production work with Tier 1 assets
When production ends in an open pit, it gives way to the mine closure project, which increases its intensity. Opportunities can arise where a closure project task calls for the construction of a topographic feature that lends itself to the capabilities of larger equipment still in use.
As discussed in earlier topics, the flexible utilisation of tier 1 assets for explicitly operational purposes only is reduced. Before a large excavator makes its final walk to the decommissioning pad, it would prove advantageous to have prepared a list of tasks for it to complete prior to retirement. These include tasks that would otherwise occupy much smaller equipment far longer. For example:
- The construction of a staging dam on a final landform with a 50kBCM capacity could be completed in a matter of shifts, rather than in weeks using mining equipment.
- The bulk movement of a dump for rehabilitation using truck and shovel methods rather than dozers over lengthy distances.
- Disused major haul roads built on pit crests that are scheduled for remediation may require the side casting or removal of significant quantities of waste. This is easily achieved in a shorter period with larger mining equipment.
How we can help
At Atlantech, we provide technical excellence and innovative mining and environmental consulting services. Our team of experienced mining engineers can assist at identifying and planning for technical challenges that may arise across every stage of the mining life cycle. Please reach out to us for a confidential discussion about how we can help.
Sources and further reading:
- University of Queensland (Mine closure overview)
- AggNet (Limiting blast-induced damage on final pit walls)
- Australian Government (Mine closure leading practice guideline)
1 Comment
Nice summary, I hop along with mine planners there are plenty of other corporate professionals reading this.