The importance of draw control in block cave mining operations

Kim Nesbitt, Associate Geotechnical Engineer, Cartledge Mining and Geotechnics

Draw control is a critical aspect of block cave mining operations. In this article, Kim Nesbitt, Associate Geotechnical Engineer at Cartledge Mining and Geotechnics, explores how draw control plays a pivotal role in ensuring the efficiency, safety and viability of the large-scale underground mining process and the importance of accurate data in planning, monitoring and execution.

Draw control within block cave mining operations is considered crucial to avoid conditions of isolated draw, which may result in unnecessary waste ingress, compaction of ore and/or loss of ore reserves.

The objectives of draw control in cave mining are to:

• Avoid premature ingress of waste that dilutes the ore, diminishing quality, and causing recovery problems in the ore treatment process.

• From a geotechnical perspective, it is important that active drawpoints are sited in such a way that once undercutting has been achieved static columns of caved ore are not allowed to develop between drawpoints and particularly over major apices as, with time, these static columns can compact and crush production tunnels.

Effective draw control is dependent on:

• Accurate information as to tonnage drawn from individual drawpoints daily.

• An ability to collect and analyse the information for purposes of control and to establish trends over time.

Accurate and robust information-gathering systems are essential for optimal draw control. An ability to simulate particle flow in the cave is essential for a better understanding of ore movement. This is achieved using the Particle Flow Code (PFC). PFC is a general purpose, distinct-element modeling (DEM) framework that is available as a 2D and 3D program. PFC models simulate the independent movement (translation and rotation) and interaction of many rigid particles that may interact at contacts based on an internal force and moment. An improved understanding of ore flow might allow wider spacing of drawpoints in caves with coarse fragmentation. This could allow lower extraction ratios and more stable extraction level layout configurations.

Managing drawpoint hang-ups is essential

Drawpoint hang-ups frequently occur and need to be managed timeously. Different types of hang-ups are defined for operational reasons. Different secondary drilling and blasting methods are used to dislodge interlocking arches in the drawbells or break the hang-ups into a manageable size that can be transported to ore pass tips by the production LHDs.

• A large low hang-up is usually caused by one or two large fragments at the mouth of the drawpoint, too large to be moved by an LHD, that prevents production from the drawpoint.

• A low cluster is an interlocking arch of fragments close to the drawpoint mouth that prevents further loading unless the arch can be broken, usually by drilling and blasting. One or two key blocks stabilise the arch.

• A high cluster is an interlocking arch of fragments at a height of more than 5m from the mouth of the drawpoint that prevents ore flow in the drawpoint until the arch can be broken by drilling and blasting. One or two key blocks stabilise the interlocking arch.

• A large high hang-up is a single large fragment that lies across the top of the drawpoint, often extending from one side to the other, that prevents ore flow until the fragment is broken by drilling and blasting.

Within proper draw control processes, information on the location and type of hang-up is made available to production personnel to allow them to plan secondary drilling and blasting operations and drawpoint production daily. The Geotechnical Engineer ensures that problematic hang-ups, usually high cluster and large high hang- ups, are brought down quickly, as hang-ups that persist for a long time prevent interaction between adjacent drawpoints.

The fragmentation distribution of ore reporting to drawpoints generally varies with respect to tonnage reporting through the drawpoint. Up to 30 percent of the first 20,000 tons of ore that report to drawpoints can be larger than 2m3 in size.

Secondary drilling and blasting is therefore an expensive and time-consuming part of the mining cycle that constrains the production potential of any block cave. Standard drilling and blasting procedures control operations to ensure safe working conditions for personnel and equipment, and production efficiency.

Drilling and blasting are efficient in eliminating hang-ups. Drilling is carried out during the production shift, but secondary blasting is usually carried out at the end of each shift when personnel and equipment have been withdrawn from the production area. Procedures are in place that allow secondary blasting during the production shift if a lack of drawpoints from which to load ore constrains production.

Conclusion

Draw control is fundamental to block cave mining operations’ success, through ensuring optimal iron ore recovery and enhancing safety and viability.

To achieve all of this, accurate and robust information-gathering systems are essential. Information on ore flow and the availability of LHDs on site must be done with minimal human intervention for optimum safety.

An ability to simulate particle flow in the cave is essential for a better understanding of vertical and horizontal mixing in the ore column. The Particle Flow Code currently being used to simulate ore flow is a step in this direction. An improved understanding of ore flow might allow wider spacing of drawpoints in caves with coarse fragmentation, which could allow lower extraction ratios and more stable extraction level layout configurations.

An improved understanding of ore flow will positively impact draw control strategies, cave mining layouts and mining equipment. This could allow cave mining at greater depths and in ore bodies with increasingly coarse fragmentation.

References
Bartlett P.J. Planning a mechanised cave with coarse fragmentation in kimberlite. PhD thesis, Department of Mining Engineering, University of Pretoria, South Africa, 1998.

Bartlett P.J, and Nesbitt, K. Draw Control at Premier Mine. Publication is submitted for publication in MASS.MIN 2000

Lorig L. Unpublished. Presentation to Cave Study Group, 1998.