Our previous post introduced the first step: determining whether block caving is appropriate and viable. Here we survey the footprint of a block cave operation. The goal is to develop optimum level (levels) for extraction, best height of draw (BHOD), and the best tools for making critical decisions for extraction.
Mine footprint and BHOD determine the long-term economic viability of the entire block caving operation based on how much minable reserves will flow down through multiple columns of ore over the life of the mine.
Footprint and layout
The footprint lies beneath the orebody and is designed at a certain level, or levels for multi-lift operations. Selecting a level (or levels) represents a critical step and should be optimized based on the resource model, constraints, and the project's short- and long-term goals. For example, a smaller lift to start and bigger lift in lower levels for later years can reduce initial capital costs in early phases of a project. The GEOVIA Cave Long-Term Planner (Footprint Finder in GEOVIA GEMS™ PCBC) is a well-known tool to facilitate this process, it enables mining engineers to define different parameters and run multiple scenarios for optimizing the value of project. This kind of analysis is done with a resolution of blocks, using the estimated block model with added economic parameters.
In the next step, the mine layout is designed in the selected level (levels) to determine the location and geometry of tunnels and how many, how far apart, and where draw points should be located. In this planning stage, the capital costs are estimated based on the design requirements such as excavations, ventilation, reinforcement, and so on.
Footprint finder and layout design in PCBC
BHOD
The height of each draw column is critical in determining the viability of the operations over the life of the mine. The draw column height represents the economic height of diluted ore that can be drawn from that extraction level under the defined capital costs and profit conditions, according to Sandra’s Cave Mining Handbook. “There is no room for the philosophy that ‘we’ll maybe it will work.’”
In determining BHOD, the draw column height represents the total minable mineral reserves in that column. The ore will become diluted with other waste material toward the top of the column near the surface. Generally, the higher the draw column, the lower the dilution will be, assuming that the ore/dilution interface is maintained as a distinct zone and dilution only enters the column at the height of the interaction zone. The BHOD production level is calculated to that level before waste material begins mixing with the minable ore. This height is used as an input in the production schedule in the next steps. BHOD calculation can be done in PCBC by defining economic parameters and geotechnical constraints. The available visualization tools can significantly save engineers’ time for making critical decisions in this step.
In some cases, coarse fragmentation creates a “contact zone” rather than a distinct line, which affects the calculation of dilution, along with other factors such as excavation stability and the geometry of the draw zone. The non-ore material, however, has a life of its own we call “flow” that also must be factored in an optimized production schedule.
Calculated BHOD using PCBC
▶ We talk more about material flow and different algorithms for simulating the mixing in block caving in our next article.
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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