Optimisation of Fragmentation and Comminution at Boliden Mineral, Aitik Operation

The objective of this thesis is to optimise the fragmentation at Boliden Minerals open pit mine Aitik. Blasting is one of the main operations in mining, and its performance has a major impact on the overall economy of the mine. The fragmentation process affects not only the local productivity and unit costs of the mining department. It also affects the performance of the subsequent comminution steps, crushing and grinding. Local optimisation of parts of a production system, quite often results in a suboptimisation of the total system. Traditionally the mine and the mill have been divided into different cost centres, with the consequence that the mine has sub optimised the blasting for the mining costs, without consideration to the result further down the production. In this work the optimisation of the blast fragmentation has been aimed at the optimal for the whole process mine and mill, i.e. maximising the throughput. The primary grinding in the mill has been identified as the bottleneck of the production system mine and mill, therefore the blast optimisation has been aimed at maximising the throughput in these. The Aitik mine has equipment for tracking the ore from positions in the mine through the mill, which makes it possible to back track grinding results, e.g. throughput in primary grinding, to their origin in the mine. From the behaviour of the grinding mills, fill level, power and feed size, during production, an optimal change in fragmentation can be determined. Since the ore has similar fragmentation properties along the dip of the ore, it is assumed that the ore properties in underlying mining level is similar to what the current blast/domain has, which should give similar results with the same blast design. The optimal change in fragmentation is back tracked to the mine and a blast domain classification and blast design for the underlying mining level can be done. By using the optimal change in fragmentation for the primary mills, the blast design for the underlying mining level are determined by using a blast model and a crusher model. No laboratory work or field tests are used for the blast and crusher modelling. The input parameters are obtained from the size distribution, before and after crusher, achieved from the different domains. To investigate if finer fragmentation results in higher throughput in the primary grinding, five trial blasts were evaluated. The trial blasts were compared to the neighbouring blasts in the bench above. The trial blasts had 35 % higher powder factor than standard blasts. The location of the trial blast was not chosen with regard to the blast domains, but taken when and where possible. The average increase in throughput for the five trial blasts were eight percent, and for two of the trial blasts, which were located in the blasting domain where the expected increase in throughput was highest, the throughput increased by 14 and 22 percent. The gathered knowledge of the influence of the feed size on the grinding performance indicates that a finer fragmentation generally gives a higher throughput. Before the method with calculating an optimal change in fragmentation can be fully applied, further investigations needs to be done to ensure the relationships. Above all more trial blasts are needed. For the development of the blast and the crusher model the fragment size measurement has to be more reliable.