Mine design from the “inside out”
June 25, 2018
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Whether designing an underground mine using pencil and paper or sophisticated software packages, the best approach is to start on the inside and then move towards the outside.
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Looking at the ore body—the inside of the proposed mine—allows the mine plan to focus on what really matters: economically recovering the minerals.
Preparing a mine design involves three key items: ore body, mine plan, and infrastructure. The designer starts on the inside and designs outwards: The ore body (provided by nature and interpreted by geologists) is the basis for the mine plan (the mining method, recovery sequence, and productive capacity). The mine plan is supported by the mine infrastructure to make it function (ore/waste handling, ventilation, electrical/mechanical systems, etc.). These key items must be addressed in sequence for the mine to be successful, efficient, and profitable.
Ore Body
Typically, mine designers are provided with a three-dimensional block model developed from a mineral resource with associated tonnage and grade distribution. The block model provides a footprint in terms of lateral and vertical extent of the resource. From the data, a distribution of tonnes per vertical metre can be determined and is often an early indicator of the productive capacity of the resource using various guides or principles. (Refer to The Hard Rock Miner’s Handbook Rules of Thumb authored by Jack de la Vergne. PEng, and published by Stantec Consulting Ltd.)
Because deposits can often be mined using several different methods, the goal of the mine design is to determine the method(s) that optimize the net present value of the resource. Initial design efforts take into consideration the geological and geomechanical aspects including geometry, proximity to surface, footprint, thickness, faulting, dykes, compressive strength of the resource and host rock, etc.
Mine Plan
For an underground mine, there are three primary categories of mining methods to be considered; caving, bulk, and selective, with multiple subsets for each category. One approach for mine design involves establishing a long list of all potential mining methods. For each method a list of advantages and disadvantages is prepared in an effort to discount or eliminate certain methods while advancing other methods to a short list. Based on the geological / geomechanical / environmental information provided, some methods will be discounted early in the design process.
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Mineralization in the ground before the mine design begins.
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From the short list of potential mining methods, a series of weighted ranking tables can be prepared to narrow down the list in order to select a base case mining method for subsequent study purposes. To arrive at a base case, a trade-off study may be required to review, in more detail, the specifics of each short listed method.
Based on preliminary economics, a cut-off grade is determined and used to establish a resource from the block model. Based on the base case mining method and the block model resource, specialized stope shape optimizer software tools can be used to determine the in-situ recoverable tonnes and grade, including planned dilution. The results are a three-dimensional wireframe with independent stope shapes. From here, the software provides the capability to evaluate different mining methods, stope dimensions, cut-off grades, and then compare the results.
The initial results from the optimizer are considered “high level” and not the final mineable shapes, however the results allow for an early comparison of mining methods—including recovery and dilution—and a confirmation of the selected mining method that sets the basis for the mine design moving forward. At this stage appropriate peer reviews can be completed in order to further confirm the mining method.
The mine design can now begin by using the high-level stopes shapes and adjusting the stope outlines to more finite shapes while analyzing the grade and tonnage distribution within the model. The value of the resource is in the ground, and the “time value of money” provides a focus to recover the higher value areas of the deposit early in the mine life where possible. This information is then used to develop the recovery sequence i.e. using bottom-up, or top-down mining, or mining on single or multiple horizons concurrently.
The next step in the design is to develop the mine levels that will allow access to the deposit, as well as the ability to move ore, waste, personnel, and materials on the mining horizons. At this stage the mine planner considers footwall or hanging wall access (or both), or locating the level development directly in the ore to minimize excavations in waste rock.
Mine Infrastructure
The design then moves from the excavation design to the overall infrastructure, with ore/waste handling and ventilation being two major systems that require early consideration. Material handling systems are required to remove waste rock which allows access to the ore. In addition, the production capacity determined for the resource will determine the rate at which the ore must be moved. Ventilation is required to meet the needs of the operation and considers contaminants from equipment exhaust, heating, cooling, recirculating / reusing where appropriate, or directing used air immediately to an exhaust system.
Other mine systems include electrical distribution, communications, dewatering, backfill (if applicable), and maintenance. As the major systems surrounding the deposit begin to evolve within the mine design, secondary systems which support mining on the active horizons can now be included, for example process water supply and drainage, compressed air, backfill distribution, explosives storage and distribution, refuge, and secondary egress.
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Observing the deposit allows the mining plan to focus on what really matters: mineral recovery.
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The stopes, level development, and infrastructure systems must then be connected to surface where the mine development will be initiated. Based on the depth and lateral extent of the deposit, access could involve a ramp, shaft, ramp/shaft combination, multiple ramps, or multiple shafts. All openings to surface should support the systems for ore/waste handling, personnel/materials movement, electrical distribution, and ventilation intake and exhaust.
Closing
With a vision of how the mine will be developed from the first blast in waste to the first stope in ore, a schedule can be prepared from the initial mine development through to steady state production. Bringing the value of the minerals in the ground to surface drives the results of the net present value calculations. Using appropriate scheduling software, in conjunction with the grade/tonnage data provided by the stope shapes, various iterations of mine development and stope by stope production can be analyzed to optimize the NPV, and several iterations may be required before the ultimate mine design is achieved.
For scheduling, keep in mind that initial development advance rates are determined by the number of headings/faces available and the capacity of the equipment used to complete the development. Forced-ventilation via duct work is necessary in blind headings, and the mine design must consider the heading size to provide the appropriate level of ventilation. As the mine development continues, the designer must consider questions like:
- At what stage of development will it be possible to establish another opening to surface to provide a flow-through system and increase the volume of ventilation air to the underground?
- Is waste development limited by truck haulage or hoisting via a temporary arrangement? When will it be possible to establish high capacity waste handling either via multiple trucks, conveyor system (lateral, inclined, or vertical), hoisting system (single or multiple skips with loading pocket arrangement) or some combination of each?
Unlocking the value of minerals in the ground is an exciting prospect! To do so, the mine design process is aided greatly by starting on the inside and moving outwards, all while focusing on the end product.
This content was originally published in Tecnología Minera.
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