Fragmentation and waste rock compaction in small-scale confined blasting

Abstract: Sub-level caving (SLC) is an important mass mining method, used at LKAB e.g. The caved rock or debris at the SLC interface reduces the fragmentation and the swelling of the blasted ring and it dissipates the explosive energy. These phenomena may immobilize the blasted ring, causing ore losses. There are two major factors that influence the mobilization of the blasted ring, fragmentation and swelling of the blasted material. The caving process is influenced by also the stiffness of the waste rock, which is dependent in some way by the compaction that the blast ring contributes to. To investigate these phenomena in large-scale is very difficult and nearly impossible due to the mining method itself. Therefore model scale tests have been made to understand the mechanisms of rock breakage and therefore fragmentation under relatively confined conditions. To minimize geometrical and geological effects, tests were conducted on Ø140 mm cylinders of magnetic mortar, which fractures like magnetite but is a less variable material. The cylinders were placed inside an Ø300 mm steel or plastic cylinder and confined by packed aggregate. PETN cord with different strengths in a centred hole gave a specific charge between 02 and 2.6 kg/m3. This thesis describes and discusses how fragmentation and compaction, the reverse of swelling, depend on the specific charge for different types of debris confinement. Four different types of debris have been tested and for one of them a confining pressure of about 0.42 or 0.86 MPa was induced by a slotting and bolting of the steel cylinder. The results show both that the passive confinement makes the fragmentation considerably coarser than from free cylinders and that the properties of the debris have a strong influence on both fragmentation and swelling. The effect of the confining pressure was relatively small however and interpreted mainly by changes in debris porosity. The magnetic mortar and the non-magnetic aggregate allow for post-blast magnetic separation. The setup provides extremely repeatable results. The fragmentation of the blasted mortar and the aggregate plus the compaction (reverse swelling) of the confined mortar cylinders have been measured. Free mortar cylinders were used as a reference To date, more than 160 cylinders have been shot and evaluated in terms of fragmentation or compaction. It has been clearly shown that the test set-up is robust and gives repeatable results. The small scale tests are a first step to increase the understanding of confined blasting as in sub-level caving. By using the acoustic impedance between the blasted material and the confining debris, a relationship for both fragmentation and compaction have been found depending on material, specific charge and physical properties of the debris. Regression analysis has been used for both tasks, where the two statistical hypotheses clearly have good agreement with actual data i.e. the prediction models can forecast both the fragmentation and compaction for this set-up with reasonable accuracy The results can be comparable with confined blasting in large scale, this both that it have representative design parameters that fulfil in many ways the scaling laws and the second is that it can be linked to other comparisons between large-scale and small-scale, where the similarities have been shown. The results from this thesis have shown to be a good input for numerical modelling, where confined blasting is one of the new tasks. This will be a part of the coming work to optimally design SLC-rings for maximum output of ore.