When I was a graduate student at the University of Arizona I took a Surface Mine Design course. When the time came for final projects I studied a small copper/molybdenum deposit in South America. I used Maptek Vulcan Pit Optimiser to determine the ultimate pit limits, where to design push backs, and to determine the preliminary schedule and costs. I learned a lot about the deposit, as well as some best practices for using Vulcan Pit Optimiser.
The algorithm behind Pit Optimiser was developed in 1964 by two scientists working for IBM. Helmut Lerchs and Ingo Grossmann applied operations research to determine a graph equation for the ultimate 3D pit limits. The algorithm is based on maximising the undiscounted profits; based on known revenues and costs for each point in the deposit, and the geometrical constraints. Today it is an industry standard and is widely known as the Lerchs Grossmann (LG) algorithm.
After studying the algorithm in-depth, and now applying my industry knowledge of its capabilities, here are some noteworthy best practices:
1. The original 1964 algorithm does not explicitly discount block values to consider time value of money. Nowadays you can leverage off built-in discount adjustments and positional mining cost adjustment factors to quickly account for increased cost and decreased value according to depth.
2. Block models are the logical storage houses for the input financials and output pit variables. Today .bcf scripts can be used to populate the cost and revenue variables for millions of blocks in a repeatable manner. That coupled with the validation tools in advanced statistics and block viewing tools, and you can rest assured that the optimised pit is based on intended financials and has produced a valid result.
3. Using an optimiser block size similar to the minimum mining unit will work well to obtain realistic results in a reasonable amount of time. Pit Optimiser has a block resizer that allows you to re-block the model on the fly. Increasing the block size may allow you time to run and evaluate more scenarios, leading to better results.
4. Multiple pit analysis is a great tool to determine logical locations for push backs. If you see that an incremental financial adjustment yields a large change in pit tonnage, it means a pushback may be just what you need to target that higher grade material. The built-in analysis reports and charts are a great way to identify these sensitivities.
5. You are only at the beginning of the design process once LG pit strings are produced. Since Pit Optimiser is built into Vulcan, it is easy to analyse and incorporate all of the information the tool provides into a pit design. Additional tools like Haulage Profile and Chronos allow you to obtain an even better idea of your project’s cost, which can be fed back into Pit Optimiser for an improved model.
For more information about Vulcan Pit Optimiser, or Vulcan training and support, contact your local Maptek office.
Project Mining Engineer
July 31, 2013
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