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Title: Application of high voltage breakage to a range of rock types of varying physical properties
Author: van der Wielen, Klaas Peter
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2013
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Abstract:
High voltage breakage is a relatively novel comminution technology that uses highly energetic electrical discharges to induce electrical breakdown in rocks. Advantages of the technology in terms of weakening of rocks to ease comminution, as well as improved liberation compared to mechanical fragmentation methods have been demonstrated. However, a detailed understanding of the fragmentation mechanism and its selectivity, as well as how to optimise the process in terms of efficiency and treatment outcomes was still lacking prior to this thesis. The focus of this study was on how process variables and rock properties interact with high voltage breakage to enable more tailored treatment depending on the desired processing result. Twenty different rock types were extensively characterised in terms of geomechanical, mineralogical and electrical properties and treated at different voltages, number of pulses and discharges, electrode gaps and pulse rates. The resulting particle size distribution was investigated in detail, as well as liberation and weakening of selected rock types. In addition, process mineralogical aspects of the treatment were investigated using QEMSCAN® and a scanning electron microscope. Data in this thesis suggest total spark energy input is the main variable determining fragmentation and liberation outcomes of high voltage treatment. Some materials were found to exhibit a threshold voltage below which less fragmentation than expected occurred, but the main controlling factor for spark energy input is the number of discharges applied to a sample. The process efficiency was found to be strongly dependent on the discharge ratio, but also exhibited a strong rock-specific aspect. In general, low energy inputs and process water conductivity combined with a high voltage gradient and pulse rate were found to be most conducive to efficient high voltage processing. Based on fragmentation and weakening results, as well as liberation and process efficiency it is suggested that treatments in the 1 – 5 kWh t 1 range are most suitable for weakening and liberation applications of the technology. Voltages above 140 kV should be sufficient for most purposes, but this depends on the minimum voltage gradient required to reliably develop discharges in a rock type. Furthermore, feed sizes above 14 mm were found to be more suited to high voltage breakage, which is likely the result of the number of discharges available relative to the number of particles being treated. The voltage of a discharge dictates how many discharges are required to achieve a given energy input, and therefore the exact voltage chosen for a high voltage treatment is a function of feed size as well as efficiency and fragmentation considerations. The evolution of P80 of a high voltage treatment product with energy can be estimated with reasonable accuracy from a relationship incorporating porosity and acoustic impedance. Additionally, the decrease of the mass percentage of feed size material after a given energy input was found to be strongly correlated to a function including tensile strength and relative bulk permittivity. Other rock properties that were found to correlate significantly to high voltage breakage include mica and quartz content. Based on correlations between high voltage breakage indicators, tensile strength and acoustic impedance, as well as imaging of the alteration left by several plasma streamers it is concluded that shock waves are the dominant fragmentation mechanism, and that fragmentation occurs predominantly in a tensile stress regime. There is evidence that the selective fragmentation observed during high voltage breakage is a result of both fracturing along grain boundaries (inter-granular fragmentation) and preferential fracturing of certain mineral phases (intra-granular fragmentation). Intra-granular breakage behaviour is clearly evident from some of the data presented in this thesis. Quartz seems to respond strongly to high voltage treatment-induced stresses, which may be favourable from a process mineralogical perspective. Direct imaging of fractures has also yielded evidence for inter-granular selective fracturing, and strong enrichment of sulphides after treatment at low energy inputs also indicates selective, inter-granular breakage. In addition to the selective fragmentation there is also a selective component to the electrical efficiency of the process. Consequently, the selective nature of high voltage breakage is a feature that recurs in several aspects of the technology.
Supervisor: Pascoe, R. D. Sponsor: European Social Fund ; Institute of Minerals ; Materials and Mining ; Minerals Engineering Society
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.590894  DOI: Not available
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