Overview
Using the same optimisation parameters as the previous exercise, we will add a positive and negative discount factor to the sale price of gold. Remember that positive discount values reduce the sale price of the gold whereas negative discount values increase the sale price.
As a result, different optimal pits will be generated for each different sale price and therefore a nested series of pit shells will be produced.
The data files required for this exercise are:
block_model.mdl topo1.dtm topo1.str gold_$unit.pop
Note that the optimisation parameters file gold_$unit.pop was created in the previous exercise, if you do not have this file, use the file gold_$unit_ex1.pop instead.
Requirements
This exercise assumes you have done the previous exercise and you are familiar with all the fields on the Pit Optimisation Parameters form and the data that was used.
If at any time you are unsure about any field on the form, refer back to the previous exercise.
Ore Discounting Factor
If you still have the previous results displayed on your screen, reset graphics.
Make sure you have the block model file gold.mdl open and that you have the block model menu displayed.
From the Block model menu, select Pit Optimisation , then enter gold_$unit on the form displayed and press Apply. There is no need to enter in the file extension for this field. If you do not have this file, use gold_$unit_ex1 instead.
This parameter file contains all the optimisation parameters that were entered into the Pit Optimisation Parameters form in the last exercise and these values will be displayed on the next form that pops up. This enables you to rerun the previous optimisation or make modifications to the parameters and then run a new schedule. Any changes made to the values on the Pit Optimisation Parameters form will be written back to this parameter file.
In this exercise, the only parameters we will be modifying are the discounting percentages that will be applied to the sale price. For the previous exercise, no discounting percentages were applied. This time we will compare the optimum pits produced for a 10% discount (a decrease in sale price to $20.70 per gram) and a -10% discount (an increase in sale price to $26.30 per gram).
Click on the Optimisation tab pane on the Pit Optimisation Parameters form and fill in the discount percentages as shown below:
Apply the form and the pit shells should be displayed in graphics and appear similar to the following image:
-10 % Discount
0 % Discount
As was discussed in exercise one, smaller optimum pit shells will be produced as the sale price of the ore material decreases. This is what can be seen from the above results. The thistle coloured pit corresponds to a discount of 10 % (sale price is reduced by 10 %). This pit is smaller than the optimum pit generated for the sale price with no discount (yellow coloured pit), which is smaller than the optimum pit generated for a discount value of -10 % (sale price increases by 10% - the blue coloured pit).
A series of nested pits have been calculated.
Report
Once again, a window will pop up to display a summary of each optimum pit and should appear as follows:
Discount Volume Value Output
-10.00 4,326,000.00 41,486,199.00 pit-10.dtm 0.00 3,860,000.00 31,872,712.00 pit0.dtm 10.00 2,386,000.00 23,806,499.00 pit10.dtm
From the above report it can be seen, the higher the sale price for the ore, the higher the value of the pit. Therefore the pit with the lowest price (the most discount) should be mined first because the pit is smaller and so has less risk associated with it. The pit is breakeven at a lower sale price.
A graph can be generated to show the changes in sale price with the net value of the pits:
$0
If a series of nested pits are created, the smallest pit containing the block will have its discount value written back to the block model in the Pit Attribute. To show this, do the following:
Reset graphics to clear the optimal pit shells from the screen. We do not need to save these as they have already been saved.
From the Block model menu, select Display, then Display block model, to display the block model. Apply the following defaults:
From the Block model menu, select Constraints, then Remove all graphical constraints, to remove any constraints applied on the block model.
Now constrain the model to only see the blocks that have a discount attribute value of > -9999.00 (the default value).
From the Block model menu, select Constraints, then New graphical constraints, enter in the following constraint:
By colouring the blocks by the discount factors, we will be able to see what blocks belong to each pit.
From the Block model menu, select Display then Colour Model by Attribute, and fill out the form as shown below:
As can be seen by the following image, each block will be populated with the discount value for the smallest pit that contains it:
Message Window
The message window gives a summary of the net value for the entire block model and the nested pits, from the highest sale price to the lowest sale price, are generated. The report should appear as below:
Total Resource Nett Value = -589,942,742.10 Number of Model Blocks = 139,785
Number of Positive Blocks = 913 Results are in the file pit-10.dtm
Total Resource Nett Value = 31,438,803.89 Number of Model Blocks = 74,503 Number of Positive Blocks = 453 Results are in the file pit0.dtm
Total Resource Nett Value = 22,915,224.05 Number of Model Blocks = 74,270 Number of Positive Blocks = 370 More Discounting
This time we will generate six nested optimal pits to compare the results of increasing discounting factors.
Reset graphics to remove the previous results.
From the Block model menu, select Pit Optimisation, then select the pit optimisation
parameters file that was used to generate the nested pits above. This file should be called either gold_$unit.pop or gold_$unit_ex1.pop.
Click on the Optimisation tab pane and enter in the following discounts leaving all the other parameters the same.
The following pit shells will be displayed in the graphics viewport:
10 % Discount
0 %
Discount 60 %
Discount
50 % Discount
By examining these results carefully, you can see that the more discount applied, the smaller the optimal pit and hence, the smaller the net value of the pit.
This is also shown by the report that is generated:
Discount Volume Value Output
0.00 3,860,000.00 31,872,712.00 pit0.dtm 10.00 2,386,000.00 23,806,499.00 pit10.dtm 20.00 722,000.00 18,669,302.00 pit20.dtm 30.00 664,000.00 14,603,836.00 pit30.dtm 40.00 642,000.00 10,771,492.00 pit40.dtm 50.00 614,000.00 7,036,592.00 pit50.dtm 60.00 454,000.00 3,650,717.00 pit60.dtm
The above results can be graphed as follows:
0
9.20 11.50 13.80 16.10 18.40 20.70 23.00 Sale Price ($/g)
Pit Value ($)
Producing a series of nested pit shells as above can also be used as a guide for the pit development (push backs) throughout the life of the mine.