Optimising Pre-Stripping in Whittle

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GEOVIA Whittle™ is designed to maximize the mining project value using the DCF method. It optimizes the project value by accelerating the realization of income and delaying costs. At first glance, pre-stripping (mining without processing) can be seen to have a negative effect on the project Net Present Value (NPV). However, pre-stripping is actually an opportunity to mine during construction and will speed up the delivery of ore. Whittle can be used to optimize pre-stripping whenever there is a lag in ore processing.

There are three good ways to set up pre-stripping in Whittle, defined in a Scenario node for a limited period of time:

  1. Lowering the processing limit, but not to zero.
  2. Raising the cut-off grade above any deliverable mining grades.
  3. Folding the pre-stripping tonnage into the first year of processing by increasing the mining limit.

The third option is most ideal. Whittle will determine the optimal amount of pre-stripping to maximize the project value. To achieve this goal, the first period mining limit should be raised high as Whittle will stop mining when it reaches one tonnage of both pre-stripping and mining at the processing level. The mining tonnage can then be split using either spreadsheets or reporting.

In this scenario, any module in Whittle can be used—including stockpiles, as ore will not be lost. While stockpiles are not required, they will most likely increase pre-stripping and the NPV. Furthermore, this method starts discounting at the first period of processing.

In the first two options, stockpiles are required to save ore extracted during pre-striping and ensure it is not wasted (rejected). The NPV will also be comparatively lower for the same schedule.

Pre-striping does require the adjustment of capital and operating costs, accordingly. And, the cost of pre-striping should be capitalized—however, Whittle will not do this automatically.

Here is a simple example of pre-stripping optimization:

  • A project is planned with a mining rate of 20 million tonnes per year, including 10 million tonnes of ore to be processed for two years after construction.
  • At the end of the first year, Whittle will determine (without mining limits) that 10 million tonnes of ore is available if 35 million tonnes of rock (including ore) is mined.
  • The required pre-stripping is 15 million tonnes—either before processing starts or at 9 months of full capacity mining.
  • Any ore found during that pre-striping period will be processed.
  • A buffer stockpile is not required.

It should be noted that Milawa Balance requires the use of reasonable mining limits at all times.

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