What factors should you take into account when calculating a mineable ore reserve computation for a block cave mine? Discover the 1st set of parameters in our previous posts to consider when starting a new project. Discover in this post the following parameters of your checklist: mining sequence and historyand non-linear material flow.

By Tony Diering, Ph.D. VP Caving Business Unit Dassault Systèmes, GEOVIA

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MINING SEQUENCE AND HISTORY

Mining sequence can have a direct or indirect effect on the computed ore reserve for a block cave. There are various indirect influences to be considered. For larger projects, a variable shut-off grade may be employed. In this case, the tonnes will generally be decreased from newer draw points using the higher shut-off grade. This requires that the Best HOD process be imbedded within the production schedule, so that if the sequence changes, different draw points will be mined at different points in time and have different reserves. This process can be quite complex especially when there are other constraints active simultaneously such as maximum and minimum HOD. It should also be mentioned that the choice of mining sequence can have a very large effect on the NPV of a block cave even if the same ore reserve is mined, depending on when the higher grade portions of the deposit are mined.

Another common constraint is that sequencing should be set up with a single initiation point for a block cave mine. Once separate panels are used, the sequencing becomes more flexible.

Once a block cave has been in operation for several years and actual mining history is available, then the process of updating reserves is further complicated by this mining history. Areas will be encountered where the official reserve (in the model) has already been mined out, but the sampled grades at the draw points are sufficient to justify mining from those areas. Other areas will be encountered where the grades are too low and the draw points are required to be closed early. A tool exists within PCBC called LSQ (Least Squares) which can use the local draw point samples to update the estimate of material in the draw column for short term corrections in the forecast.

In general, the sequencing of draw points requires a careful trade-off between the geotechnical constraints from overall stresses and undercut face shape combined with the need to mine higher grade draw points earlier in the sequence. The example in Figure 4 (Richter 2008) from the Finsch mine shows the undercutting sequence. It was a good compromise between targeting higher grades and geotechnical constraints.

NON-LINEAR MATERIAL FLOW

Another consequence of the mining sequence manifests itself when working with more advanced flow models, which are imbedded in the PCBC production scheduler. The mineable reserve is a direct function of the mining sequence. As soon as a mixing algorithm is used when material moves between draw points or from outside the draw point domain into the cave zone, then the mined grades will depend on mining sequence or rate. Draw points, which are mined first or faster, will tend to “rob” material from other adjacent draw points or sources. The dilution model and recovered grades will then vary and the total mineable reserve will also depend on the sequence. This can be confusing to those not familiar with flow models in a caving environment. This is perhaps the key reason why cave management and good draw control are crucial for successful block cave operation. As soon as there is poor draw control, there will also be a loss of ability to generate reliable forecasts. The other consequence of poor draw control is an increase in dilution or lower recovery of ore or both.

In our next post​​​​​​​ we will detail residual material in multi-lift  situations and share other considerations. Stay tuned!

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