3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Using the dem to relate drop ball tests to semi-autogenous grinding(2019) Samukute, Shwarzkopf OliverThe Drop Ball Test (DBT) is a common quality control procedure used in many grinding media-manufacturing units to evaluate the quality of manufactured balls by subjecting a sample to an impact fracture test. Whilst DBTs have provided reasonable data over many years, the quantitative comparison of the energy that the balls are subjected to during the DBT and in high impact loading environments such as Semi-Autogenous Grinding (SAG) mills remains a grey area. The Discrete Element Method (DEM) is a numerical technique that can provide a much more detailed description of the grinding media collision behaviour in both DBTs and SAG mills. The DEM allows simulation of the collision behaviour in various systems that involve interaction of many particles. The DEM model applied in this work uses a spring-sliderdashpot to calculate contact forces and the net resultant is used to compute acceleration, velocity and distance moved by the particles by applying Newton’s laws of motion. The objective of this work was to quantify the energy that grinding balls are exposed in both the DBT and SAG environments. Using the DEM, simulations where conducted in both environments to evaluate extent of ball impact loading. The impact energy spectra obtained from the DEM simulation of various ball sizes in the DBT was used to quantify the energy the balls are subjected to. The data showed that larger 125mm steel balls are exposed to relatively higher energy levels and have higher probability of fracture than smaller 115mm and 100mm balls. From the SAG mill simulations, ball trajectories were evaluated to determine the energy that the grinding media is exposed to. Increasing ore:ball ratios showed the extent of ore cushioning and reduction in energy that the balls are exposed to. Using the DBT data and DEM impact energy spectra obtained from both the DBT and SAG simulations, empirical models were developed that attempt to predict ball fracture in the DBT and try to relate ball fracture in the DBT to ball endurance in the SAG environment. A reasonable estimation of the energy that the balls are exposed in both the DBT and SAG mill was achieved. However, establishment of simulation parameters that specifically apply to the material of ball construction is recommended for future studies. From the results analysed, it was concluded that a more accurate determination of simulation parameters of the specific material of construction has the prospect of achieving improved ball fracture predictionsItem Development of a condition monitoring philosophy for a pulverised fuel vertical spindle mill(2016) Govender, AndréThe quantity and particle size distribution of pulverised coal supplied to combustion equipment downstream of coal pulverising plants are critical to achieving safe, reliable and efficient combustion. These two key performance indicators are largely dependent on the mechanical condition of the pulveriser. This study aimed to address the shortfalls associated with conventional time-based monitoring techniques by developing a comprehensive online pulveriser condition monitoring philosophy. A steady-state Mill Mass and Energy Balance (MMEB) model was developed from first principles for a commercial-scale coal pulveriser to predict the raw coal mass flow rate through the pulveriser. The MMEB model proved to be consistently accurate, predicting the coal mass flow rates to within 5 % of experimental data. The model proved to be dependent on several pulveriser process variables, some of which are not measured on a continuous basis. Therefore, the model can only function effectively on an industrial scale if it is supplemented with the necessary experiments to quantify unmeasured variables. Moreover, a Computational Fluid Dynamic (CFD) model based on the physical geometry of a coal pulveriser used in the power generation industry was developed to predict the static pressure drop across major internal components of the pulveriser as a function of the air flow through the pulveriser. Validation of the CFD model was assessed through the intensity of the correlation demonstrated between the experimentally determined and numerically calculated static pressure profiles. In this regard, an overall incongruity of less than 5 % was achieved. Candidate damage scenarios were simulated to assess the viability of employing the static pressure measurements as a means of detecting changes in mechanical pulveriser condition. Application of the validated pulveriser CFD model proved to be highly advantageous in identifying worn pulveriser components through statistical analysis of the static pressure drop measured across specific components, thereby demonstrating a significant benefit for industrial application.