A tomographic exploration of the internal structure of coal and its role in single particle breakage
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The unintentional production of coal fines, or coal degradation, during coal beneficiation can lead to significant financial losses for coal producers. Coal fines in a closely sized feed can also lead to channelling, hotspots, and other hydrodynamic inefficiencies in reactors. Coal degradation occurs at any step in the beneficiation process where the coal is subjected to mechanical stresses or where the coal is subjected to a rapid increase in temperature. A number of particle and material properties influence the degradation characteristics of coal including the initial particle size, the composition of the particle, and weathering; the degradation characteristics can also be influenced by the properties of the unit processes used to beneficiate or utilise the coal, such as the breakage energy and temperature. The mechanisms that are used to describe the degradation of coal particles differ for mechanical and thermal degradation. Mechanical degradation is described by fracture and attrition, while thermal degradation is described by fragmentation and exfoliation; however, none of these mechanisms are thoroughly understood. In order to minimise the degradation that occurs across the coal mining, beneficiation, and utilisation process value chains, a better understanding of the factors that influence the degradation and the mechanisms whereby degradation occurs is necessary. To this end, this thesis describes research that elucidates the influence of various factors on the breakage and degradation characteristics of coal, and describes attempts to model these influences using a modification of the t10 degradation model. The factors influencing the degradation characteristics that were considered were particle shape, the particle’s orientation relative to the impact surface (impact orientation), and the particle microstructure. The influence of the microstructures was investigated using micro-focus X-ray computed tomography (μCT). The applicability of μCT as an analytical probe to identify and track the changes that occur within a particle during degradation is first confirmed for compressive breakage, and is then applied to single particles during compressive, impact, and thermal loading. The microstructures that were identified and tracked were pre-existing cracks within a particle, the orientation of the particle bedding plane relative to the applied force, the boundaries between lithotypes of varying density, and the boundaries along mineral inclusions. The influence of particle shape and impact orientation was studied and it was found that when slab-like particles impacted onto a larger surface area more degradation products formed compared to slab-like particles that impacted onto a distinct protrusion. This was due to the protrusion disintegrating thereby dissipating the breakage energy and protecting the rest of the particle from degradation. It was also found that the t10 degradation model could not predict the degradation behaviour of the specific South African coals tested due to the t10 degradation model being unsuitable for modelling the degradation of brittle, bimodal natural resources like coal. While studying the influence of particle shape and impact orientation, characteristic breakage patterns were observed, and the microstructure of coal suggested as a possible cause for these distinctive breakage patterns. The next phase of the investigation applied μCT to identify and track the changes that occur in a particle during degradation. Tomograms were generated before, during, and after degradation. The tomograms were compared, changes identified, and conclusions drawn regarding the influence of the microstructures on coal degradation namely: • All of the microstructures had the potential to contribute to a particle’s degradation. The microstructures either remained unchanged, acted as an initiation site for a new crack, aided the propagation of a crack, or halted the propagation of a crack. The possible influence of a specific feature on degradation could not be predicted from the tomograms generated before degradation. • Of all of the observable microstructures considered during this study, the pre-existing cracks in a particle had the most pronounced influence on the final crack distribution within the particle. • For both the mechanical and thermal degradation of a particle, the lower density microlithotypes present showed more new crack formation compared to the higher density microlithotypes. The lower density microlithotypes are the vitrinite rich microlithotypes which is known to be more brittle than both inertinite and carbominerite rich microlithotypes. • The orientation of a particle’s bedding plane relative to the applied mechanical force influenced the propagation of cracks in the particle by either aiding the propagation along the microlithotype boundaries when the force was applied along the bedding plane, or halting the propagation of cracks at the microlithotype boundaries when the force was applied across the bedding plane. It was initially hypothesised that, if the distribution of microstructures in a particle is known, then the particle size distribution of the progeny can be predicted. Due to the fact that no indication of whether a specific feature will contribute to the degradation of a particle, or what the contribution will be, the progeny size distribution of a single particle could not be predicted; however, the progeny size distribution of a population of degraded particles could be fitted using a Rosin-Rammler size distribution.
- Engineering