Electric rock breaking for south african ore bodies
Date
2007-02-28T12:48:09Z
Authors
Ilgner, Hartmut Johannes
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Abstract
Although pulsed power has been used in many parts of the world over the last few decades to
initiate high-voltage discharges through rock, no systematic test work on South African ore
bodies and related rock types has been done so far.
As part of CSIR Miningtek’s integrated approach of combining underground comminution with
a novel Tore© hydrotransport system, which has been shown to operate well with coarse
particles up to 10 mm, various rock types were fragmented in single discharge mode under
laboratory conditions.
The work was conducted at the University of the Witwatersrand’s high-voltage laboratory with
a custom-designed test rig. The rig configuration was based on a critical review and analysis
of the literature and on assessments of existing test facilities elsewhere. Core samples with
diameters ranging from 16 to 48 mm were cut from test specimens with thicknesses ranging
from 8 to 48 mm. Rock types included Ventersdorp Contact Reef, Carbon Leader, Elsburg
Formation, UG2 and Merensky, as well as pure quartz, shales, lava and dykes.
A six-stage Marx generator provided a voltage rise time of 2 000 kV/μs to create a discharge
through the rock, in preference to a discharge through the surrounding water, which acts as
an insulator at ramp-up times faster than 0,5 μs. High-speed photography, and an analysis of
the voltage and current signals for various rock types and for water alone, were used to
quantify the potential benefits of rock breaking by electric discharge.
It was found that some Kimberlite specimens and mineralised gold-bearing reefs were much
easier to fragment than hanging wall or footwall material. Merensky reef appeared to be more
susceptible than the less brittle UG2 material. A correlation was derived between the dynamic
resistivity of various rock types, measured at 16 MHz excitation frequency, and the electrical
breakdown strength at which discharge took place.
The fragments created had a more cubical shape than would be created by conventional
impact crushing. However, the high voltage requirements of about 30 to 35 kV per millimetre
of rock thickness would necessitate not only efficient mechanical and electrical contact
between the electrodes and the rock, but also considerable safety features for underground
installations.
The clearly identified, preferential fracturing of reef rock types, compared with the hanging or
footwall materials, suggests that the greater benefit of electric rock breaking may lie in primary rock breaking as a mining method, rather than in secondary comminution of broken rock to
enable hydraulic transportation by pipeline to surface.
Description
Student Number : 9803381J -
MSc Dissertation -
Faculty of Engineering and the Built Environment
Keywords
dielectric breakdown, voltage ramp up rates, high voltage crushing, shock waves, mining, comminution