In our previous article we talked about material flow and different algorithms for simulating the mixing in block caving. In this post we delve into mine sequencing and production scheduling, tools and strategies to achieve the production goals while meeting the constraints.
Drawpoint sequencing determines the starting point, sets the direction, and influences the economics for the entire block cave mine for the rest of its life. Production schedule answers the question of how much material to extract from each drawpoint in each period while meeting the strategic and operational constraints.
Block caving extracts ore from the base, rather than from the top, and the mining infrastructure is developed below the deposit before extraction begins. Tunnels provide access to drawpoints where ore is collected.
Which drawpoint to develop first, then second, and so on? This mine sequencing step fixes the point of initiation for the entire block caving project, or a part of it. It sets the direction that the cave will advance. The mine sequence influences the economics of the mine over its lifetime.
How drawpoint openings are sequenced and how much is extracted from each is based on economic value (typically from the highest to lowest economic value), the geotechnical conditions, and operational constraints.
Mine Sequence
Orebody shape, access to infrastructure, grade distribution, stress directions and magnitudes all influence the point at which to initiate the mining sequence. Determining the mining sequence, including decisions about which blocks to extract and the timing of their extraction represents the central goal of long-term mine production scheduling. The production schedule defines the management investment strategy. An optimal plan will reduce costs, increase equipment use, and optimize recovery of marginal ores while ensuring steady production rates and consistent product quality.
A systematic sequence design depends on many factors:
- Induced stresses for different scenarios
- Whether a sequence influences fragmentation
- Geological environment in terms of zoning of IRMR (in situ rock mass rating) and MRMR (mining rock mass rating)
- Ongoing mining and future dilution
- Mud rush potential
- Rockburst potential
- Potential of structural failure or massive wedge failures
- Influence of adjacent operations
Drawpoint sequencing generated using PCBC
Production Scheduling
When developing a practical production schedule, extraction per year from individual drawpoints (draw rate) and life of drawpoints (draw life) must also be considered. Availability of mining equipment, geotechnical conditions of the orebody and the host rock, geometry of drawpoints, and height of draw columns can also factor.
A practical production schedule with the possibility of generating higher revenues earlier in the project can significantly improve cash flow by increasing the net present value (NPV), changing a deep low-grade ore resource to a valuable ore reserve.
Early-stage caving projects usually carry a high degree of uncertainty in the input parameters. With uncertainty, multiple schedules must be generated to cover the likely range of key parameters. Such a process is time consuming with potential human errors. GEOVIA simulation optimization tools can be implemented to lead and facilitate this analysis.
Mine sequence optimization simulation and visualization technologies help assess multi-level scenarios to predict block caving propagation. Detailed production schedules can be generated based on targeted production rate as well as drawpoint opening strategy and sequence.
Geotechnical constraints within caving simulations can be incorporated directly in the mine planning process that allows correlations between key parameters within the model to have a more realistic picture for the economics of the project.
The Block Cave Scheduler tool in GEOVIA PCBC provides a robust suite of sequencing controls, including multiple ways to manage and
- input a sequence,
- ability to set rate of development of new drawpoints,
- establish Production Rate Curves (PRC) or maturing curves for each drawpoint,
- set variable production targets and shut-off grades per period.
- implement the material flow and mixing impact within the production schedule
- use different scheduling methods for different purposes
- run multiple scenarios at once
The software tool provides numerous scheduling methods to control the way a cave develops and advances and accounts for past, minimum, maximum, and total tonnage constraints per drawpoint. The Block Cave Scheduler features multiple display options, including draw column depletion, height of draw (HOD) surface propagation, sequence analysis and interactive charting in Microsoft Excel during a run.
Snapshot of the production schedule in a certain period generated by PCBC
In our next post, we look at monitoring, evaluating, calibration and the importance of continual updating throughout the life of the mine.
Firouz Khodayari is an industry consultant, author, and professional engineer with a PhD in mining engineering and more than a decade of international mine planning and optimization experience. He specializes in mathematical modelling and production schedule optimization for open pit and underground mining
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