Towards understanding breakage and flow in sublevel caving (SLC) : Development of new measurement techniques and results from full-scale tests

Abstract: Blast function, fragmentation and gravity flow are core elements for sublevel caving (SLC). For the ore recovery to be as high and the waste rock dilution as low as possible all three elements have to work as planned. In the past some boundary conditions were changed: The scale and layout of SLC changed tremendously and production advanced to greater depths. As a consequence, blast performance and material flow are believed to have changed. The difficulties in simulating SLC relate to the physical scale as well as the broad range of time-scales involved, which results in a large number of unknowns and uncertainties. The present thesis contributes with the development of new measuring techniques, full-scale tests and analysis of results in all the above three core elements. Based upon that a better understanding of breakage and flow is obtained and this supports future process improvements.A remotely operated 3D photogrammetry system was designed and used to study the blast results from SLC rings in opening and hang-up situations. Various blasting situations were observed and this allowed a deeper study of the i) geometry of blasted cavity, ii) over- and underbreak and its effect on subsequently blasted rings, iii) interaction effects between adjacent holes/rings, iv) mapping of geological structures and its influence on the blast result and v) drilling accuracy based on identified boreholes. An attempt to identify malfunctions and categorize breakage problems was made. Some of the identified problems for initiation were unexpected and require a closer examination to increase blasting efficiency and to mitigate nitrate leakages. Potential improvements for blasting initial openings and scaling-up of rings are presented.Confined drift wall blasting tests in which the blasted burden was instrumented with various sensors to study movement and breakage were carried out. The main focus was the development of measuring equipment that could be scaled-up to full-scale SLC blasting and installed behind the rings. Results from an unconfined and confined blast situation are presented. A new measurement system to measure burden movement was developed, which combined the initial movement recorded by an accelerometer designed to minimize zero-shift with the later slower movement recorded by a photoelectric sensor (“fibre-optic zebra gauge”). In addition, time domain reflectometry (TDR) proved to be a reliable method for the verification of burden breakage (over- and underbreak).Fragmentation characteristics for blasted ore loaded from a specific ring were studied by sieving. For comparison lab-scale data from crushing, grinding and blasting, and also historical full-scale data from the mine, were analyzed. This data confirms that the material follows the NBC criteria (NBC = “natural breakage characteristics”) in the fines region (0 – 10 mm). However, a relative flattening of the sieving curves due to “selective” breakage in the mid-range (25 - 75 mm) and increased amount of fines was observed. The main conclusion is that magnetite behaves like normal hard rock from a fragmentation point of view despite this flattening deviation from Swebrec distribution behaviour as this is likely to be caused by the internal flow mechanisms in the SLC process.Gravity flow and the effects from confined blasting in hang-up situations were studied with the 3D imaging system and allowed the extension of conceptual models for breakage and flow. Evidence is given that gravity flow was disturbed and occurred in shallow, crescent-shaped flow zones. The front part of the burden was fractured, but immobilized. For the subsequent blast this meant that swell and compaction was limited to that zone with only marginal dilation and disaggregation of the burden at the sides. The operating figures for the blasted rings with observed flow zones indicated a disturbance-free extraction. Inflow of caving debris might occur either from above or in front when the loader penetrates into the flow zone of the previous ring. As a result of flow disruption in this narrow channel a temporary hang-up might evolve and the flow scheme be altered so that material is then loaded from previous blasts. With continued extraction, the cavity enlarges and finally becomes unstable, collapses and both previously “frozen ore” and waste rock enters the draw point.At the end a pivotal question remains: Are the recent observations of disturbed flow an inherent part of blasting SLC rings? This raises also the follow up question: Can we expect an undisturbed gravity flow if all borehole charges and rings detonate and break as planned? With continued investigations an answer seems to be obtainable in the near future. It is also recommended that the methods developed in this thesis be used in instrumented confined blasting tests in full-scale rings and be combined with gravity flow measurements.