pushbacks
Intermediate pit shells
show possible practical
pushbacks
Pits near RF of 1 are
used for sensitivity and
risk analysis
Pits near RF of 1 are
used for sensitivity and
risk analysis
Exercise 2 – Creating and selecting optimal shells in Whittle Pit Shells
Page 199 of 308
You can enter revenue factors as a range with a start, end, and step or single values or a combination of both. You can choose from two methods of revenue factor range generation: fixed intervals or geometric intervals. The fixed intervals, as the name suggests, provide a constant step size in revenue factor. The geometric intervals allow you to specify the required number of factors but will then produce more pits in lower revenue factors and less in the higher factors (handy if looking for starter pits as well as a final pit). This is illustrated in the following figure.
The generation of pit shells involves simple cashflow (it is not discounted) and does not take into account the time value of money, or when a particular block is mined. The optimal shell assumes that all blocks are mined today.
Ore Selection Method
Whittle has two methods of selecting parcels for processing and determining whether a parcel is ore or waste. It can use cut-offs or cash flow.
When ore selection is by cut-off, ore is selected by comparing the grades of the material with pre- calculated processing cut-offs. If material does not satisfy the cut-offs, it is treated as waste. In the case of multiple processes, the first eligible process is used. The grades are compared with the cut-offs of each in turn, in the order in which they are specified in the Block Model node. For an explanation of how multiple cut-offs are handled, see the Whittle help.
When ore selection is by cash flow, ore is selected by comparing the cash flow which would be produced by processing it and the cash flow which would be produced by mining it as waste. If the cash flow from processing it is higher, the material is treated as ore. If not, it is treated as waste. If more than one processing method is applicable, the one which produces the highest cash flow is used.
In the simple cases (one process, no block co-ordinate based expressions affecting cut-off), ore selection by cash flow produces the same result as that produced by the use of marginal cut-offs. However there are some cases where this is not true.
Cut-off calculations are based on processing cost, element processing cost, price, selling cost, and recovery and are calculated in a preprocessing stage before individual blocks are considered. If any of the items are based on expressions that:
are based on block position (IX, IY, IZ) or
are based on grade of another element
then you must use ore selection by cash flow, because the marginal cut-off, determined for the whole mine, cannot be calculated.
In the simple case of a single process and no expressions, these two mechanisms should produce the same result. In the case of multiple processes, where the cut-offs are not ranked, you must use ore selection by cash flow. If you want to use ranked cut-offs, you must use ore selection by cut-off.
Exercise 2 – Creating and selecting optimal shells in Whittle Pit Shells
Page 200 of 308
For a simple case, such as this example where the ore selection method does not affect the destination of a block, note that selection by cut-off includes the cut-off grade for each rock code in the report file. This cut-off is based on the data for the reference block of that particular rock code. If pcafs are used, which is true in our case, the cut-off grade shown in the report is NOT correct for any block with a pcaf that is not 1.
Value mode is where the block value is either stored in the block line of the mod file or calculated from the parcel values defined by the element indicated to store the parcel value in the user element tab. Both ways of defining the block value require the VALU module and are not covered in this course.
When ore selection is by Profit mode, the parcel revenue element is used to determine the process path which will return the best cashflow. In simple situations, the results are identical to ore selection by cashflow but profit mode has the ability to be used in Stockpile and Cut-Off Optimisation (SPCO) for multiple processes.
Adding a Pit Shells Node
1. From the Slope Set node, right-click, and choose Add Pit Shells.
Note that the new node icon has a red cross and requires further input before processing. 2. Click on the Mining tab, and enter the following information:
Note that you enter a 5% dilution as 1.05. If the mining recovery were 5%, you would enter it as a factor of 0.95. If you use both mining dilution and mining recovery, the order of application does not affect the result because mining dilution affects the grade but mining recovery does not.
The rock type CAFs are not used because you have provided the mcaf in the model file. You have the choice to give the costs as $/BCM or $/t. Currently it is set to $/t.
3. Click on the Processing tab, and, beside the Processing Paths section, click Add. 4. Enter the information for the HCAP rock code.
There is only one method of processing the ore, so you will call it MILL and use cut-off for the ore selection. The reference processing cost for all rock codes is 16.79. Recovery is 0.94.