AMC Consultants was recently involved in a project that required, amongst other activities, a review of options to measure blast fragmentation in an operating open pit mining environment.
Blast fragmentation measurement is usually depicted as a particle size distribution (PSD) curve relationship of percentage of rock passing verses rock size as shown below.
Figure 1 Typical rock PSD curve
Quantifying blast fragmentation is useful for measuring blasting performance to determine if the blast has effectively broken the rock mass to enable it to be loaded and that the ore oversize does not exceed the crusher open size. Improved fragmentation and consistent fragmentation across a blast profile will generally support higher extraction rates and therefore lower loading costs. Drill and blast costs may be reduced if it is determined that the fragmentation result is finer than required.
Blasting is an effective way to break large rocks into smaller rocks much like the role of a primary crusher but it can be cost prohibitive at producing very fine rock particles. Often on an operating mine the installed crushing and grinding capacity is fixed either by its design or by the available power. However, blasting capacity may present upside to the overall capacity of rock breakage and comminution leading to an increase in a mines ore throughput. In some projects reducing the ore top size specification allows an adjustment to the primary crusher setting resulting in an overall crushed product with more fines. This in turn provided a superior feed to the grinding circuit enabling higher tph to be achieved.
Successful blasting to optimize the overall project economics is part of the study of mine to mill optimization. Mine-to-Mill® optimization as coined by JKTech considers the fragmentation and the impact of the particle size distribution on the drill, blast and processing costs and process plant performance. AMC has been involved with a number of projects which also broaden the scope to consider other important factors such as; ore loss and dilution, the mining schedule, bench height optimization and mining operating costs.
To evaluate the effect of design changes in the blasting practice it is necessary to validate the consequences when the design changes are applied in practice. Modern techniques to measure blast fragmentation have been developed by several vendors using either optical imaging or laser scanning and proprietary algorithms to determine the particle size distribution. Figure 2 shows the fragmentation measurement process.
The techniques may be used on an ad hoc basis at the face where a portable camera is used or in production settings where the camera is fitted to the shovel arm or excavator boom to provide continuous monitoring. Positional records allow matching of fragmentation results to different zones in the blast.
Figure 2 Automatic digitization of rock particles
Vendors offering solutions to portable measurements include Metso, Motionmetrics, Innovative Machine Vision and Split Engineering. Various levels of scaling and allowance for the repose angle of the rill or face are required to achieve reliable PSD analysis. There is development in using laser scanning to capture fragmentation data which is considered to provide more accurate measurements and is less susceptible to reading errors caused by from dust and excess fines. Motionmetrics, Orica FRAGTrackTM and Split Engineering all offer shovel /excavator mounted options.
Figure 3 Hand held fragmentation measurement
In addition, measurement may be taken at the primary crusher to capture the sizing as material is tipped in to the hopper in real time. Vendors offering fixed at crusher installation solutions include MetsoPro and Split Engineering. Refer to figure 4.
Figure 4 Screen of Metso Visio Truck output
The location of the measuring point enables different information to be gathered with the portable method more suited to detailed blast analysis matching blast performance to blast design. The more automatic methods provide consolidated fragmentation data per blast but may lose the geo-spacing information of the rocks source.
Further rock fragmentation measurement located on the crusher discharge conveyor offers advantages and presents different but important data. Firstly, installation on a fixed conveyor is simpler and more reliable than applications in the pit or at the crusher, secondly the crushed ore PSD is a more critical measurement particularly to AG and SAG mill circuits but also to secondary crushing circuits.
In many operations the bottle neck is not the primary crusher but further downstream with the throughput being directly related to the PSD measured after the primary crusher. For a complete mine to mill optimization measurements of fragmentation may be required at multiple locations to understand and optimize the overall process.
This article is not intended to promote any products or vendors and AMC notes that each of these vendors offers specific solutions generally available as part of a greater mine to mill process optimization offering. The vendors are appropriately thanked for their contributions.
Principal Mining Engineer