The generation of a mine sequence for a block caving mine is challenging given it represents the direction for opening draw points. In this new series we will present a new option for optimizing mine sequences using the concept of ‘best and worst case’ adopted from open pit mines. After introducing Footprint Finder and the Best and Worst Cases concepts in our previous post, we are now providing an example of this concept in real footprint.

By @DV 

APPLICATION OF THIS CONCEPT IN REAL FOOTPRINT

The Regal deposit (Bui, 2014) is a fictitious ore body but it is modeled as a massive porphyry copper deposit similar to many of the large block cave mines currently in operation. The overall view of the copper distribution in 3D and the plan view at the level of 1200 is shown in Figure 6.

An example of the application of mine sequence optimization using the concept of best and worst case was done in the Regal deposit using Footprint Finder shown in Table 1.

MINE SEQUENCE APPLIED FOR THE ENTIRE FOOTPRINT

For this example, only one level will be evaluated at 1200 where the optimum elevation was found based on a run of the entire block model. Figure 7 shows the result of the Footprint Finder evaluation for each block, where the height of draw is displayed at the left and the economic value at the right. A black line was digitized to limit the footprint to form a more realistic shape for a block cave footprint. Note the size of the footprint (1,500m long and 400m width in average) and the distribution of the economic value outlines three potential areas of high grade where the sequence could start.

An example of the application of mine sequence optimization using the concept of best and worst case was done in the Regal deposit using Footprint Finder shown in Table 1.

MINE SEQUENCE APPLIED FOR THE ENTIRE FOOTPRINT

For this example, only one level will be evaluated at 1200 where the optimum elevation was found based on a run of the entire block model. Figure 7 shows the result of the Footprint Finder evaluation for each block, where the height of draw is displayed at the left and the economic value at the right. A black line was digitized to limit the footprint to form a more realistic shape for a block cave footprint. Note the size of the footprint (1,500m long and 400m width in average) and the distribution of the economic value outlines three potential areas of high grade where the sequence could start.

The best and worst sequence was created for the footprint defined by the black line and the graphic results are shown in Figure 8, with the start of the sequence shown in warmer colors. Three points are highlighted as high economic values and offer potential initial points for the sequence.

Once the best and worst sequence is complete, the evaluation for other possible sequences is possible and the options are then evaluated:

• Start in the border of the footprint and advance with a flat shape or V-shape for cave front. Both options are common since there are geotechnical constraints to be considered (e.g. lead/lag, abutment stress, etc.) 
• Start in the center of the footprint and move in a diamond shape. This option is very attractive from the NPV perspective, since the sequence can start where the high grade is located, but it also generates many challenges from the operational perspective, since it concentrates a lot of the activity in the same area, meaning one production drift may support production, construction and development all at the same time.

Using Footprint Finder, it is possible to evaluate several scenarios in one step and compare and assess which of these options is closer to the best case and if it meets all of the design, geotechnical and operational considerations. Table 2 shows the list of the 55 runs done for the footprint described above: For each run a production schedule was created and the discounted cash flow was calculated to create a comparison between them all. A sample schedule is shown in Figure 9.

The summary results are shown in Figure 10 where it is possible to see the location of the best and worst sequence, two lines of runs for sequences created using a flat and arrow (45 deg.) cave front shape and one additional line starting at the centre of the footprint from 18 different initial points located in the highest economic value. The values of the operational sequences are in a range of 75% to 85% from the best option. In general, central options are optimal when compared with flat and V-shaped options.

Four sequence results are shown in Figure 11, with the start of the sequence shown in warmer colors. Due to the size of the footprint all of these options create very long faces (more than 500m) and are very difficult to maintain in practice. In addition, when comparing values for each option, the differences between them suggest initiation in the center of the footprint generates a more complex scenario for operation, construction and development. As a result of these studies, a new analysis was completed dividing the footprint into two zones (East and West) where each zone was evaluated independently to identify new options and a possible better overall sequence.

▶ In our next post we will present the results applied in two zones (East and West). Stay tuned!

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