Open pit mines constitute more than half of global minerals production. Yet most of the large, high-grade, and close to the surface deposits have been depleted or are currently in production. Besides, volatility in commodity prices and stringent environmental regulations limit the up-scaling expansions in large open pits. Consequently, the mines are determined to increase their operational efficiency in order to thrive. This has recently led to major metallurgical improvements in the processing of ores; whereas the improvements in mining of the said ores are relatively overdue in terms of efficiency and technological advancement. This thesis concentrates on the mining activities and their efficiency in open pits with a focus on drilling, blasting, loading, and crushing. As all of these tasks revolve around the fragmentation of run-of-mine ore, their relationships and efficiencies are explored within the context of fragmentation.
Fragmentation is a result of complex interactions between rockmass, blasting geometry, explosive, and timing sequence of blast holes. The influence of rockmass and timing sequence on fragmentation and efficiency are explored, as well as the target fragmentation for efficient loading and crushing. Moreover, the techniques for measuring fragmentation are evaluated as to whether they can benefit mines in terms of efficiency. As the circumstances in open pits are essentially site-specific, these issues are addressed as a case study of the Aitik mine in Sweden.
The research comprised four elements. First, the influence of rockmass fractures on blast results and downstream efficiency was evaluated via full-scale field trials. The fractures in and around the case study mine were mapped using a photogrammetric technique and six production blasts were adapted to the major fracture sets to evaluate the effect of initiation direction on downstream efficiency. Second, the influence of the timing sequence of blast holes was explored within the theories of stress waves interaction and their consequent effect on fragmentation. Theoretical and numerical solutions were accompanied by six field trials in full-scale to evaluate the influence of short delay times on fragmentation and efficiency. Third, an empirical study was conducted to correlate fragmentation to the efficiency of loading and crushing; this was done to define a target fragmentation for the studied case. Finally, the techniques to assess fragmentation were discussed both quantitatively and qualitatively.
The findings indicated that rockmass fractures have a significant influence on the quality of blasts and efficiency of downstream tasks. In the case study mine, adjustments to orientation of drill pattern and initiation direction of blasts suggested that careful experimentation in this regard can yield a favourable initiation direction with respect to existing discontinuities. Finer fragmentation and higher loading efficiencies can be achieved by adapting the blast designs to the existing fractures, which can lead to significant savings in the long run. On the contrary, the influence of stress waves interaction on blast results turned out to be marginal. Neither the theoretical and numerical solutions nor the field trials showed any significant improvements in blast results from short delays. In fact, it was found rather implausible to expect any noticeable improvements by using short delays.
The empirical method to evaluate target fragmentation proved useful as well. It was shown that by incorporating different data from various sources in a mine, one can follow the ore from muckpile to loaders and then to crushers. Having a qualitative understanding of the fragmentation, and by developing tools to measure efficiency, one can estimate what fragmentation is most favourable for an efficient operation. Finally, two image-based methods to assess fragmentation were discussed in terms of repeatability and statistical significance. It was found that the scatter in both methods is rather large, introducing a certain ambiguity in representativeness of their results. Admittedly, it was found that in matters of long-term efficiency, the number, size and representativeness of assessed samples are of more importance compared to the accuracy of individual measurements.
Luleå, 2016. , 60 p.