Browsing by Author "Bbosa, Lawrence Sidney"

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    Measurement of impact breakage properties of ore particles using a series of devices
    (2007) Bbosa, Lawrence Sidney; Mainza, Aubrey
    Single particle impact breakage experiments provide essential data to aid in the fundamental understanding of rock fracture in comminution. Such experiments, conducted using devices such as the drop weight tester, split Hopkinson bars and more recently the rotary breakage tester, have been successfully used to characterize ore breakage properties in relation to measured fracture energies. Current theoretical understanding of impact breakage is that there are three important energy regimes to the process. Below a certain energy value, Eo, breakage will never occur for an infinite number of impacts. Second is an intermediate energy zone for which breakage occurs after a number of consecutive impacts and after a critical energy value, Ecrit, is a regime where breakage typically occurs for a single impact. This work was therefore undertaken in order to identify the energy values described for impact breakage of a chosen homogenous ore and a conventional mining gold ore. Drop weight tests on gold ore were used to calculate A and b hardness parameters using both the standard JK breakage model and the modified Shi-Kojovic model. The A x b values with the modified model gave consistently higher values than the standard model, typically increasing by 2-5%. Split Hopkinson pressure bars were used to establish the ultimate compressive stress of blue stone through single impact breakage tests and the fraction of impact energy utilized to cause particle fracture. From these tests it was noted that less than 50% of available impact energy was utilized to cause fracture, with cylindrical specimens absorbing the highest fraction of 43%. The rotary breakage tester was used to conduct incremental breakage experiments with blue stone and gold ore. The probability to breakage at the impact energies tested was found to remain relatively consistent over consecutive impacts. This showed that a model could be fitted between the cumulative probability to breakage and the number of impacts at these energy levels. The values of Eo for blue stone and gold ore were calculated to be 0.0464 and 0.00366 respectively. Ecrit for 90% probability to first impact breakage for these two ores was 0.344 and 0.281 respectively. It was found that incremental breakage was much more inefficient than single impact breakage. From tests with both split Hopkinson pressure bars and the rotary breakage tester, breakage degrees for single impacts increased rapidly with increase in breakage energy whilst the breakage degrees obtained from incremental breakage tests for similar amounts of energy expended remained low.
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    Measuring the fracture energy of bed breakage using a short impact load cell
    (2017) Dube, Thobile Thenjiwe; Bbosa, Lawrence Sidney
    Particle fracture is the elementary process that governs comminution. In industrial machines particle breakage occurs mainly through three mechanisms: impact, abrasion and attrition. Of these mechanisms, impact breakage is known to be the most basic form of particle size reduction. Comminution devices are highly inefficient, as the energy used for particle breakage relative to that consumed by the equipment is low and reported to be between 1-2 %. As such, understanding the fundamentals of particle fracture is crucial for the development of energy efficient particle size reduction methods. Research done towards investigating particle fracture under impact loading has led to the development of several devices which include the twin pendulum device, drop weight tester, Split Hopkinson Pressure Bar, Rotary Breakage Tester and the Short Impact Load Cell. In this study the Short Impact Load Cell (SILC) was used to conduct bed breakage experiments on partially confined particles. Breakage tests using this device were conducted by vertically releasing a steel ball of known mass onto a bed of particles from a known height. The bed rested on a steel rod which was fitted with strain gauges to measure the particle response to impact loading. Tests were conducted on two ores, blue stone and UG2, to investigate the effect of three variables: steel ball mass, drop height and bed depth on the breakage behaviour of particles. The effect of each variable was investigated by evaluating the peak forces obtained, the particle fracture energy and the degree of particle breakage attained. For both ores it was found that the peak force increased linearly with increasing steel ball mass and drop height, and it was found that the drop height had a greater effect on the peak force than the steel ball mass. The maximum peak forces were obtained at one layer of particles and increasing the bed depth generally led to a reduction in the peak force. An exponential relationship was found between the peak force and bed depth, where the peak force decreased with increasing bed depth. It was found that the blue stone particles did not break at the range of input energies used in this work, therefore no fracture energy results were reported for blue stone. The fracture energy values for UG2 were low, where the maximum energy used for particle fracture was 2.7 % of the input energy. There was no direct correlation between the fracture energy and the steel ball mass, drop height and bed depth; however it was found that the bed depth had a larger effect on the fracture energy compared to both the steel ball mass and drop height. The greatest amount of energy used for fracture was generally obtained at the largest input energies using the 357 and 510 g balls. The optimum drop height which resulted in the highest fracture energy was generally found to be either 240 or 300 mm. A bed depth of five layers was found to be the optimum bed depth that allowed for the highest amount of energy to be utilized for breakage. No breakage results were obtained for blue stone due to the hardness and stiffness of the ore. For UG2, tests conducted at the same bed depth showed a trend in which the breakage initially increased greatly with increasing input energy; however at larger input energies the breakage obtained approached a constant value. Although the input energy was varied by changing both the steel ball mass and the drop height, the results showed that the degree of breakage was more dependent on the steel ball mass compared to the drop height. For all tests conducted, the maximum breakage was obtained at one layer of particles and increasing the bed depth led to a decrease in the breakage obtained. The results showed that the fracture energy and the degree of breakage were not directly related. It was found that there is an optimum amount of energy utilized for fracture that leads to the greatest breakage, where an in increase in the energy beyond the optimum point does not significantly affect the breakage obtained.
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    Ore breakage characterisation of UG2 deposits using the JK RBT
    (2017) Chikochi, Christopher; Bbosa, Lawrence Sidney; Mainza, Aubrey
    Ore breakage characterisation is a methodology that is used to determine the ore hardness, or resistance to breakage which can be compared across a database of different rock types. It thus develops a relationship between specific energy input and degree of breakage which can be applied to impact breakage in comminution devices. The present study is focussed on investigating the breakage properties of UG2 chromitite, pyroxenite, spotted anorthosite and mottled anorthosite grab samples from run-of-mine (RoM) ore stockpile (particle selection method) and cut drill core particles (cut core method). A mineralogical analysis of UG2 chromitite, pyroxenite, spotted anorthosite and mottled anorthosite was performed using Leica EZ4D optical microscope and QEMSCAN 650F to determine their mineral composition and texture. The presence of cracks in chromitite stockpile and cut drill core samples was also explored using a Nikon XTH 225 ST micro-focus X-ray system. RoM ore stockpile and cut drill core particles of each of these rock types were subjected to impact breakage in the JK Rotary Breakage Tester (RBT). The progeny particle size distributions and degrees of breakage of UG2 rock types obtained via the particle selection and cut core methods were compared. Standard breakage characterization models were fitted to the breakage data of different rock types and the relative hardness parameters compared. It was found that UG2 chromitite comprised mainly fine, isolated, round chromite grains in a plagioclase matrix. Pyroxenite samples were found to be made up of granular orthopyroxene, interstitial plagioclase and clinopyroxene. The mineralogical analysis also revealed that spotted anorthosite primarily contains plagioclase with orthopyroxene crystals forming isolated "spots" creating a poikilitic texture. Mottled anorthosite is made up of mainly plagioclase. Results from breakage tests showed that the progeny particle size distributions and the degrees of breakage for particles sourced from the RoM ore stockpile breaks into a finer product compared to cut drill core samples. This was attributed to the presence of cracks in the RoM ore particles as revealed by the tomographic scans. No visible cracks were found in the cut core particle. The ore hardness parameters were determined from fitting the breakage data to standard impact breakage characterisation models (t10 breakage and size dependent breakage model). Samples obtained via the particle selection method were consistently found to offer less resistance to impact breakage as shown by the higher Axb values compared to the cut drill core samples. Using the ore hardness classes presented by Napier-Munn et al (1999), UG2 chromitite, spotted anorthosite, mottled anorthosite and pyroxenite were thus classified as very soft, soft to very soft, soft to very soft and medium to soft respectively. The hardness indicator, 3600.M.fmat.x, for each size class determined using the parameters obtained from the size dependent breakage model decrease with an increase in the parent particle size. This shows that particles become more resistant to impact breakage as the initial particle size increases. However, for pyroxenite, spotted and mottled anorthosite, the indicator decreases between the particle sizes 14 to 28.6 mm but then increases for 41.1 mm.
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    Probability based models for the power draw and energy spectra of a tumbling mill
    (2013) Bbosa, Lawrence Sidney; Mainza, Aubrey
    Positron Emission Particle Tracking (PEPT) and the Discrete Element Method (DEM) are used to develop probability based models for the power draw and collision energy spectra of a tumbling mill. Experiments are conducted using dry spherical glass bead charge in a laboratory scale tumbling mill, which is mounted with a torque transducer and tachometer to measure mill power. Particle tracking information from PEPT is used to reconstruct the motion of glass beads and infer the overall charge behaviour, while DEM is employed to simulate particle motion and interaction, with collision mechanics calculated using the Hertz-Mindlin contact model. For both sets of data, the product of torque and average angular velocities in discrete cells are accumulated to obtain mill power. This method is found to be within statistical agreement with measured power for all tests. The information from both techniques is then used to develop a model for the power draw as a function of particle size, mill speed and volumetric filling. Predictions of the model match well with measured and calculated values. Based on frequency distributions of collision energies from DEM, a model for the energy spectra of each particle size per steady state mill revolution is developed. This model is found to predict collision frequencies within close agreement with DEM simulation data and follows trends consistent with existing work on tumbling mill modelling.