Application of High Voltage Breakage to a Range of Rock Types of Varying Physical Properties
van der Wielen, Klaas Peter
Date: 12 June 2013
Thesis or dissertation
Publisher
University of Exeter
Degree Title
PhD Earth Resources
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 ...
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 t1 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.
Doctoral Theses
Doctoral College
Item views 0
Full item downloads 0