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Flitch or multi-pass mining is as contentious a subject as is politics. On paper, it looks like a viable method to reduce dilution and increase grade. Why then, do some operations avoid it? Reduced production efficiency, implementation aggravation, and poor grade recovery results have caused some mines to adhere to traditional bulk mining methods, even though their ore bodies appear to be ideally suited for flitch mining methods. Is there a way to improve grade recovery, thereby soothing the operational pain flitch mining can bring? This paper examines this question by presenting analyses of several blasts from a variety of narrow-vein gold mines.

Background
For precious metal open-cut mines, operational constraints and priorities often dictate the mining method. This includes how many flitches, or passes, will be mined in each bench. The type of equipment being used, the shape of the blasts, the throw, whether the blast is choked or free-faced, the heave, and the rock type can all influence this decision. From an ore control perspective, the sampling method and model confidence should be critical factors for determining if this is viable.

The decision to mine in single or multiple flitches is usually made before blasting. But, it is well-established that blast movement alters the shape of the ore body (La Rosa, 2011) (Thornton, DM, 2009), and in some cases, completely changes the dip (Hunt, William, 2015). What starts as a 90 degree dipping structure may not end up vertically-dipping at all (Figure 1).

Figure 1. Dip and Shape of Ore Changing Due to Blasting in Cross-Section

If a decision is made to mine in flitches, the vertical movement and heave must be considered in order to optimize ore control in the post-blast state. Consider the simple case given in Figure 2, where waste overlays ore in horizontally-deposited structures.

Before blasting, ore resides in the bottom flitch, with waste in the top. However, due to unequal vertical heave due to blasting, the interface changes from a flat plane into a folded surface.

Figure 2. Vertical Movement of Pre-Blast Flitch Boundary (Cross-Section)

The method developed by past research was to apply a two-dimensional solution to the complex three-dimensional problem by measuring movement at the flitch interface, thereby raising the flitch boundary to accommodate this movement ( (Hunt, William, 2015)). There are several problems with this approach:

  1. Raising mining interfaces between each flitch has to be standardized if applied, to prevent confusion in operations and allow it to be practically implemented.
  2. Each blast moves differently (Reference Hunt, Thornton Modelling Vs. Monitoring), so raising a flitch boundary across a bench is very inaccurate.
  3. Even in the same blast, the vertical component of the blast induced movement can be extremely variable (reference).
  4. The vertical movement indicated by movement monitors placed at the flitch boundaries often gives inaccurate vertical movement data.

Raising flitch boundaries has been the dominant solution imposed on the heave problem since blast movement began to be recognized and monitored. This was the best solution to the problem until recently, as the industry didn’t have a way to calculate the post-blast interface of the flitch boundary, or the aggregated value of each block above and below the planned flitch boundaries

Method
Three-dimensional ore control software, called “OrePro 3D™”, allows for examination of a blast as single or multiple flitch scenarios. A post-blast block model is generated from inputs, including:

  • in-situ grade control model,
  • movement vectors,
  • post-blast survey of the muck pile, and the blast design.

A post-blast model is constructed from these inputs, which is then sliced at the intended flitch boundaries. A purpose built optimizer then generates the most profitable ore blocks based on cutoff grades for each flitch, along with grades and tonnes calculated with swelled volume (post-blast density). This also works for a single pass, where no boundary is entered.

To determine if there is a financial benefit for mining multiple flitches as opposed to a single pass, the results can then be compared. The net gain in recoverable ore should be balanced with operational costs to determine whether or not flitching is economically attractive.
An additional consideration is the possibility that the minimum ore block size for a high working face (single flitch) would potentially be much larger than if smaller faces were mined.

Essentially, more-selective mining is possible with shorter flitches. For example, it is not feasible to mine a 5 m x 5 m block with a single flitch when the height of the muck pile is 20 m, even with small equipment. But, this same ore block may be mined if the muck pile was being mined in four flitches of five-meters each.

The mining face angle will also likely be reduced significantly if shorter flitches are mined. From the author’s research at several operations, the mining face angle for working faces between 10 m and 20 m varies between 60 and 70 degrees (Figure 3). When shorter flitches are mined, the working face can sometimes be mined at nearly 90 degrees. This factor, as well as the minimum mining dig width and length and dig direction, are taken into account when the ore block boundaries are calculated.
Figure 3.

Figure 3. Mining Face Angle

This means that dig direction becomes critical to prevent ore loss and dilution (Figure 4 and Figure 5), and that the optimal ore locations staked on the muck surface must account for this post-blast shape. The challenge in optimizing the ore/waste interface is to determine the optimal direction to mine post-blast, and defining what effect different directions have on the recovery.

Figure 4. Favorable Dig Direction to Dip
Figure 5. Unfavorable Dig Direction to Dip

In the example given in Figure 6, the problem with the unfavorable dig direction is illustrated. Should the ore control block include a lot of dilution to get the ore, or should it be kept narrow to minimize dilution at the expense of ore loss?

Figure 6. Where is the Optimal Ore Block Layout?

By mining in flitches, the effect of unfavorable dig direction is somewhat mitigated, as long as the post-blast shape of the ore is targeted, not the pre-blast shape with a 2-D translation.

Although there are benefits to mining in flitches, there are also drawbacks. If the mine is using blast hole assays for the creation of the grade control model, sampling at each flitch can create bias, in addition to the potential of samples becoming contaminated with material from other flitches. Operations personnel can sometimes contaminate the upper surface of the next flitch by blading material while preparing the surface for trucks.

This paper does not attempt to answer these questions, nor to make a definitive statement as to whether one should or should not flitch mine. That is a very site-specific question to answer, and in some cases, blast-specific. The method and tools presented here allow mines to determine if there is significant benefit in recoverable ore to warrant exploration into mining flitches.

Blasts Evaluated
This section examines three blasts where single and multiple flitches are evaluated. These blasts occurred in narrow-vein gold mines with variable bench heights. These deposits are structural, with different dip angles in each, making it imperative to correctly target the ore/waste interfaces. In each blast, a gold price of USD1300 and recovery of 93% were assumed. The cutoff grade and processing costs at each operation are different.

Blast 1
Blast 1 was fired towards the north with a free face. This blast occurred on a ten-meter bench, with very little heave. The minimum polygon size assumed for this blast was 5×5 m when two flitches were processed, and 10×10 m when a single flitch was processed. The assumed mining face angle was 75 degrees during two-flitch mining, and 60 degrees in a single flitch. The blast was mined toward the West (Figure 7). The area shown in Figure 8 contained a boulder, which choked the blast in the immediate vicinity. The grade control model at this mine is created using blast hole sampling, with two samples per blast hole, resulting in 5 m high blocks.

Figure 7. Mining Direction for Blast 1
Figure 8. Un-blasted Boulder Location

The differences between single and two-flitch boundaries are shown in Figure 9. The single flitch optimal ore blocks are larger than the individual flitch blocks as expected.

Figure 9. Blast 1 Single and Two-Flitch Optimal Ore Blocks

The results of each scenario are shown in Table 1. When adjusted for processing cost, only $11,000 additional revenue was recovered when mining two flitches instead of one. It is important to understand that the optimal ore block calculation is what makes this possible. If a mine were to simply project ore block boundaries on the surface and at the flitch boundary, there would likely be a significant ore loss and dilution, with a definite impact on recovered value.

Blast 2
Blast 2 was fired to the east with a choked face. This blast occurred on a nine-meter bench, with several meters of heave. The minimum polygon size assumed for this blast was 8×4 m in single and two-flitch scenarios. This was determined in conjunction with site personnel. The assumed mining face angle was 75 degrees during two-flitch mining, and 60 degrees in a single flitch. The blast was mined toward the West (Figure 10), but it was possible to mine the blast toward the South, so this direction was evaluated as well. The grade control model at this mine was created using RC data, with 2 m composites.

Figure 10. Mining Direction for Blast 2

The differences between single and two-flitch boundaries are shown in Figure 11 and Figure 12. The single flitch optimal ore blocks are larger than the individual flitch blocks as expected. It is also evident that the dig direction results in very different ore block stakeouts.

Figure 11. Blast 2 Single and Two-Flitch Optimal Ore Blocks (South Direction)
Figure 12. Blast 2 Single and Two-Flitch Optimal Ore blocks (West Direction)

The results of each scenario are shown in Table 2. When adjusted for processing cost, an additional $37,000 can be obtained by mining from the South in two flitches instead of one. This is mainly due to the reduction in dilution that is included in the optimized ore blocks when comparing single and two-flitch scenarios.

Blast 3
Blast 3 was fired towards the north with an open face, with a centerline dividing the blast into two movement domains along a Northwest line. This blast occurred on a ten-meter bench, with little heave. The minimum polygon size assumed for this blast was 10×10 m in single, and 2×2 m in a four-flitch scenario. The assumed mining face angle was 90 degrees during four-flitch mining, and 60 degrees in a single flitch. The blast was mined toward the Southwest (Figure 13). The grade control model at this mine is created using RC data, with 2.5 m composites.

Figure 13. Mining Direction for Blast 3

This blast contains very isolated mineralization that varies drastically with depth (Figure 14). This is why the generated polygons on each flitch are different.

Figure 14. Mineralization with Waste Removed from Viewer

The differences between single and two-flitch boundaries are shown in Figure 16. The single flitch optimal ore blocks are larger than the individual flitch blocks as expected. It is also evident that dig direction results in very different ore block stakeouts.

Figure 15. Blast 3 Single-Flitch Optimal Ore Blocks
Figure 16. Blast 3 Four-Flitch Optimal Ore Blocks

The results of each scenario are shown in Table 3. When adjusted for processing cost, an additional $35,000 can be obtained by mining in four flitches instead of one. This is mainly due to the reduction in dilution that is included in the optimized ore blocks when comparing the single and four-flitch scenarios.

Summary
In the three blasts evaluated, the value of flitch mining varies between $11,000 and $35,000 per blast, mostly due to a reduction in mining dilution. If the optimized ore blocks were not used, and mining direction was not considered, the difference would likely be much, much higher. Recall the example described in Figure 4. When the ore is structural, with sharp gradational boundaries, there is a heavy penalty to pay if ore blocks are not optimized for digging direction. An example of that principal from Blast 2 is shown in Figure 17.

Figure 17. Mitigation of Dig Direction through Flitch Mining

Mining in flitches mitigates the dig direction issue somewhat, with steeper working faces and ore blocks that target the ore in each flitch. However, the swell must be considered when creating the optimized blocks due to the change in material location caused by blasting.

Overall, the benefit to mining in flitches compared to a single flitch is blast and site-specific. However, if flitches are not desirable or practical, controlling dig direction and using optimized ore blocks that consider dig direction can provide maximum achievable value.

References

  1. Hunt, William. (2015). Increasing Recovered Grade in a Narrow Vein Surface Deposit through Blast Movement Monitoring. Africa/Australia. Adelaide: AUSIMM.
  2. La Rosa, D. a. (2011). Blast Movement Modelling and Measurement. APCOM. Wollongong.
  3. Thornton, DM. (2009). The Application of Electronic Monitors to Understand Blast Movement Dynamics and Improve Blast Designs. 9th International Symposium on Rock Fragmentation by Blasting – Fragblast 9.