Jump into this series and get an overview of the factors affecting the calculation of a mineable ore reserve computation for a block cave mine. In our previous posts, we introduced the list of parameters to consider when starting a new project. Discover in this post the following parameters of your checklist: fragmentation, economics, max/min hod, haircut and time.

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

FRAGMENTATION

Though fragmentation is fundamentally important in any block cave evaluation, the direct assessment of fragmentation is a geotechnical responsibility beyond the scope of this paper. However, it must be emphasized that the cavability of the rock mass and the subsequent fragmentation during the extraction process will directly influence all the flow characteristics mentioned above. Fragmentation will strongly influence draw point spacing, dilution entry (flow dilution) and overall ore recovery. There are many ways the fragmentation characteristics will influence the other inputs into PCBC such as layout, fines migration and vertical mixing inputs and, in production schedules, the production rate from each draw point.

ECONOMICS

Given that an ore reserve is what can be economically mined (in today’s terms), a discussion of the economics of the block cave is justified. The common workflow within PCBC to assess the mining reserve (based on metal prices, recoveries and operating costs) is as follows:

• Set an undiscounted dollar value to each tonne of rock in the input model.
• Calculate the draw column above each draw point.
• Evaluate the economic limit for each draw point. (independently of others)
• Adjust this to lower and upper limits if too low or too high.
• Adjust the values vertically to take some account of the time required to mine a tall column of rock
• Make adjustments to irregular heights or to trim isolated high column heights. (Haircut)
• Refine the above to take account of non-vertical or non-linear flow mechanisms. This is done by dynamic Best HOD calculations within the production scheduler.
• Refine the above to take account of variable shut-off grade with time.

Within PCBC, the initial approach would be to build the draw point layout and draw columns, do pre-vertical mixing and run the Best HOD tool, which gives a report of the tonnes and value contained within the draw column above each draw point. This is convenient for initial assessments and for set up of the detailed draw point layout. Later, within the production scheduler, the more complex adjustments due to mixing and time dependencies would be included.

The following topics are discussed in more detail here:

• Maximum HOD
• Minimum HOD
• Haircut
• Time or vertical discounting

MAXIMUM HOD

It is economically compelling to try to mine high columns from a single layout. There is a high capital cost to set up a layout, but the cost per tonne can be reduced if a higher column can be mined. The key factors to be considered here are:

• Geotechnical risk associated with the way in which the cave will propagate. If the cave does not go where you want it to go, you will not recover all the material
• Geotechnical risk associated with the draw point stability. If the draw point fails before you are finished mining, you will not recover all the material.
• Flow risk. The higher you mine, the more difficult it will be to predict how the draw columns will interact and how material will flow into the draw points.
• Economic risk. It takes longer to mine a taller column, so the higher it goes, the higher the risk of economic impact (e.g. metal prices or time discounting etc.)
• Aspect ratio risk. If the height to width ratio is high, then using a higher maximum HOD is high risk.
• Cave management risk. If the block cave is poorly managed during operation, then there will be increased risk of loss of draw point infrastructure. This is an extension of the draw point pillar stability mentioned above.
• Limited by past mining and residual material. In a multi-lift situation, the maximum HOD is usually limited to the base of the previous mining horizon even if there is ore grade material left from the previous mining. This is due to reluctance or inability to classify the residual material as ore.

In weaker rock, maximum HOD values would often be set around 250m to 300m. Under more favourable circumstances, maximum HOD values can go higher. Some mines plan to mine up to 1000m columns (or more). This can be done, but the above risks should be taken into account in terms of ore reserve statement. A block of highly drilled material for which the grade is well estimated, but is 700m above a layout and to the side of the deposit, is high risk material for inclusion in the ore reserve.

MINIMUM HOD

Often, due to the hydraulic radius requirements of cave initiation or to reduce stress loading of extraction tunnels, it is required to mine material from draw points, which are below shut-off grade. The question arises as to what minimum HOD should be used. Economics would push towards a low value, while geotechnical considerations would require a higher value. Typical values range from 50m to 300m. It is useful to look at the economic impact of different minimum values and also to take cognisance of where these columns are located within the overall footprint. Pillar strength would be another consideration. Ceasing to mine low grade draw points can result in convergence issues affecting the extraction of adjacent higher grade draw points.

HAIRCUT

An ideal block cave would have all draw points with a similar maximum HOD (e.g. 400m). However, there are circumstances where some very irregular top profiles are encountered. In these circumstances, a haircut is well justified:

• A small high grade zone high up in the ore column can yield a very irregular HOD profile. In this case, the zone should be inspected and consideration given as to whether it is realistic to expect it to be extracted. This ties in to the concept of aspect ratio. To expect a few draw points to extract material up to 500m adjacent to many draw points, which are extracting only 300m, is unrealistic. The Haircut tool in PCBC will help to assess whether to chop out the high grade bit or increase the draw in the low HOD areas to help with the extraction of the high grade section
• A follow on from the previous item arises in multi-lift situations. One will often encounter draw points on the edges of the deeper layout, which extract tall columns adjacent to the previous mining lift. This is especially likely since the lower lift will often be evaluated at higher metal prices than were present during the mining of the upper lift. However, whether or not this side material can be extracted should be closely evaluated. In stronger rock, it is likely that the cave sides will migrate into the upper footprint and reduce the chances of extraction of this side material.

TIME OR VERTICAL DISCOUNTING

Often an ore reserve would not be sensitive to vertical discounting of a draw column. However, there are considerations for taking a closer look at vertical discounting:

• Variable shut-off grade is expected for a large deposit.
• The deposit contains many variable grade “pods.” A small high grade zone lying above a large low grade zone is unattractive. Unless some vertical discounting is implemented, the value of high above low or low above high will be the same. But, the reality is that high grade material at the base of a draw column is much more valuable and hence a better candidate to include in an ore reserve.

In our next post​​​​​​​, we will present the following 8 parameters, among them excavation geometry. Stay tuned!

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