Featured Article: Blast And Loose

The drill and blast technique is the most common method of mining in open-pit mines. It is often used when a deposit can be accessed from the surface. The process uses a drilling rig to create a series of drill holes before explosives are detonated in these holes to break the rock into small pieces that are removed during the blasting process. This process affects almost every other phase of the mine-to-mill operation.

• However, blasting movement is not a well-understood process. One reason is the difficulty of measurement, and this lack of data has led to myths and conflicting theories. Today, open pit mines around the world use a blasting movement monitoring solution to understand blasting movement, optimise dilution, and increase ore recovery.

When drilling and blasting engineers design the blasting pattern, they should consider the pit boundary, geology, geotechnical specifications and bench height, among other factors – in addition to input from the blasters, drillers, and mine supervisors. The design should include the geometric design, the amount of explosives required per borehole, the BCM blasted, initiation system and firing sequence, etc.

Good blasting starts with good preparation and cleaning of the blasting pattern. Surveyors should check the wall and boundaries of the blast pattern to start setting out the design. This can be done by mine supervisors, blasters, drillers, and engineers. The importance of this step stems from reducing the amount of fly rock, avoiding oversize and good drilling conditions to reach the planned depth.

When it comes to the initiation sequence, drilling and blasting engineers should plan the blast pattern firing sequence based on the available geological and geotechnical information, and take into account the expected ore movement to reduce ore dilution and maximise ore recovery.

Open pit ore losses and dilution are two of the most important factors that must be considered when designing and operating an open pit mine. Ore losses refer to the amount of ore that is lost due to mining activities, while dilution refers to the amount of waste material that is mixed with the ore. Both of these factors can have a significant impact on the economics of a mining operation.

• Ore losses: Ore losses occur when ore is mined but not recovered. This can happen due to a variety of reasons, including blasting, fragmentation, and misclassification of ore. Ore losses can be minimised by careful blasting and fragmentation techniques, as well as by using accurate ore classification systems.
• Dilution: Dilution occurs when waste material is mixed with ore during mining. This can happen due to a variety of reasons, including blasting, fragmentation, and misclassification of ore. Dilution can be minimised by careful blasting and fragmentation techniques, as well as by using accurate ore classification systems.
• Impact on economics: Ore losses and dilution can have a significant impact on the economics of a mining operation. Ore losses can reduce the amount of ore that is recovered, while dilution can reduce the grade of ore that is recovered. Both of these factors can reduce the profitability of a mining operation.
• Mitigation strategies: There are a number of strategies that can be used to mitigate the effects of ore losses and dilution. These include careful blasting and fragmentation techniques, as well as accurate ore classification systems. Additionally, ore recovery systems can be used to recover ore that has been lost due to mining activities. Finally, ore blending techniques can be used to reduce the impact of dilution on ore grade.

This paper will discuss the most common blasts firing sequences and the expected movement of each.

Blast movement

In open pit mines, the vast majority of blast movements are horizontal. The rock is fractured along the line of the blast and the fragments are pushed outwards. The amount of movement depends on the amount of explosives used, the type of rock being blasted, and the geometry of the blast. Blast movements can also be vertical, or even diagonal. These types of blasts are less common but can be used to break up large blocks of rock or to create slots for ventilation shafts.

The amount of material that is moved by a blast depends on several factors, including:

• The amount of explosives used.
• The type of explosives used.
• The size and shape of the charge.
• The depth of buried charges.
• The type of rock being blasted.
• The moisture content of the rock.
• The angle at which the charges are placed.

There are a few different types of blast movement that can be used in open pit mines. The first is called linear blast movement. This is where the explosives are placed in a line and then detonated. This type of blast movement is often used when there is a need to move a large amount of material at once.

The second type of blast movement is called radial blast movement. This is where the explosives are placed in a circle around the target area. When the explosives are detonated, the material will be pushed outwards from the centre of the circle. This type of blast movement is often used when there is a need to create a large crater.

The third type of blast movement is called directional blast movement. This is where the explosives are placed in a specific direction. When the explosives are detonated, the material will be pushed in that direction. This type of blast movement is often used when there is a need to move material away from sensitive areas or to create a trench.

Common firing sequences

V-Shape and Zigzag / row by row firing sequence

If the blasts have only one free face, the optimal firing sequence is usually V-shaped (Figure 1). The expected movement will be in the direction of the free face with a shallow V and the movement is uniform throughout. The throughout angle can be determined by the surface delay and the position of the initiation point. The expected surface profile will be flat. This firing sequence with a flat V angle allows for easy movement control, resulting in less ore loss and dilution regardless of monitoring.

Figure 1. V-Shape firing sequence with one free face.

Figure 1 also shows the number of blasted holes during the blasting time in milliseconds. The number of holes blasted at the same time (instantaneous maximum charge) increases gradually up to a certain point and then decreases again, which can lead to ground vibrations.

The expected outcome from a zigzag/row by row firing sequence will be the same as V-shape (Figure 2). The main differences are that zigzag firing sequence is much easier to apply in the field and the throughout will be less than the V-shape firing sequence. Furthermore, the number of holes blasted at the same time will be less than V-shape.

Figure 2. Zigzag firing sequence with one free face.

Echelon firing sequence

If a blast has two free faces, the suggested firing sequence configuration is the echelon, as shown in Figures 3. The figures show how the direction of movement can be varied by changing the timing of the control and echelon rows. The initiation point is at the corner of the two free faces.

Figure 3. Echelon firing sequence 25 ms control line.

The pattern minimises the possibility of tight toe problems, and it is easier to connect up as the paths can be easily monitored. The expected movement will be uniform in a given direction with a shallow angle (180°). The direction of movement can be controlled by a suitable surface delay.

An effective echelon firing sequence increases the percentage of the rocks that move consistently. The main reason for the consistent movement is that the intervals between timing contours are consistent, resulting in a uniform movement. With a right pattern size and surface delay the number of blasted holes at the same time will be uniform during blasting, resulting in less ground vibration.

Box cut and centre lift

Box cut and centre lift is recommended when blasting has no free face. For deeper blasts, coupling with the centre lift is preferred. For shallower blasts, especially ramps, the box cut firing sequence is recommended (bearing in mind that the initiation point should be at the deepest point of the ramp to allow adequate movement).

The central lift (Figure 4) consists essentially of four echelons. Each sub-echelon is smaller, so the movement does not take as long to settle down to ‘normal’. Also, the number of mid-echelons and power troughs doubles compared to the previous firing sequences. The number of holes blasted simultaneously depends on the length of the branch, with the possibility of stemming ejection due to confinement movement.

Figure 4. Centre Lift firing sequence 25 ms control line.

The box cut consists of a single control line in the centre of the shot with two branches radiating from the control line (Figure 5). The number of holes blasted can be consistent if the two branches are the same length.

Figure 5. Box Cut firing sequence 25 ms control line.

One of the main challenges in controlling ore is not in tracking movement, but in the fact that there is inherent dilution in these regions caused by differential displacement at different depths. That is, ore can be pushed under waste and vice versa.

Conclusion

The firing sequences/firing sequence patterns discussed in this article can be leveraged to not only improve fragmentation, but also to better control blast movement and mitigate associated risks, such as ore loss and dilution. If a blast has only one free face, the optimal firing sequence is usually V-shaped. The expected movement will be in the direction of the free face with a shallow V and uniform throughout. Zig zag/ row by row firing sequences are similar but more difficult to apply in the field. An effective echelon firing sequence increases the percentage of the rocks that move consistently. For locked shallower blasts, especially ramps, box cut firing sequence is recommended and the initiation point should be at the deepest point of the ramp to allow adequate movement. Box cut is used mainly in creating sumps or opening a locked bench.

Notes

The author sincerely appreciates early comments and suggestions made by Farhad Faramarzi.

References

1. THORNTON, D.M., ‘The Application of Electronic Monitors to Understand Blast Movement Dynamics and Improve Blast Designs’, Blast Movement Technologies, (2009).
2. BRENT, G.F. and SMITH, G.E., ‘The detection of blast damage by borehole pressure measurement’, South African Institute of Mining and Metallurgy, (1999).
3. RORKE, A. J., and MILEV, A. M., ‘Near field vibration monitoring and associated rock damage’, South African Institute of Mining and Metallurgy, (1999).

Read the article online at: https://www.globalminingreview.com/special-reports/10052023/featured-article-blast-and-loose/