An examination of single particle rock breakage using an instrumented load cell, acoustic measurement and X-ray tomography.

Rodney Hocking (2009). An examination of single particle rock breakage using an instrumented load cell, acoustic measurement and X-ray tomography. PhD Thesis, Sustainable Minerals Institute, The University of Queensland.

       
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Author Rodney Hocking
Thesis Title An examination of single particle rock breakage using an instrumented load cell, acoustic measurement and X-ray tomography.
School, Centre or Institute Sustainable Minerals Institute
Institution The University of Queensland
Publication date 2009-03
Thesis type PhD Thesis
Supervisor Dr Peter Holtham
Total pages 326
Total colour pages 153
Total black and white pages 173
Subjects 04 Earth Sciences
Abstract/Summary Rock breakage is a highly energy intensive process for the mining industry. As energy becomes more expensive and there is an increasing drive toward sustainability the relationship between the energy efficiently of rock breakage processes become ever more important. As such, much effort is directed toward the development of predict models for breakage to maximise the efficiency of the process with respect to energy. To date, most models of rock breakage have focused on the correlation between energy and the efficiency of the breakage with little focus on how the rock breakage processes occur. In order to examine the cracking of rock samples in more detail a selection of small cylindrical samples 13.4 mm in diameter and 16mm long were tested. Samples of sandstone, basalt, marble and granite were tested from various quarries around Brisbane. A combination x-ray tomography and microscopy was used to map the damage that occurs between impacts and the results were correlated with acoustic measurements. The measurements enabled quantification of how energy is absorbed by rock samples v from low energy impacts and how it correlated with the high frequency acoustic emissions. For granite and basalt samples, it was found that upright loaded samples required more impacts to break at the same energy compared with sideways loaded samples. For brittle samples, the crack velocity was also shown to be higher for samples impacted in the sideways loading orientation. Analysis of the non contact acoustic measurement showed that there was no correlation between different frequency bandwidth and the energy applied or absorbed during the breakage event. Audio emissions above 5-15 kHz correlated well with crack velocity in the two brittle rocks tested. An analysis of the ultrasonic data using a short time period shows that the onset of cracking can be determined by using high frequency audio data. Analysis of brittle samples showed no apparent changes to the particle structure until the sample fails. In non-brittle sandstone samples, changes to the voids around the impact area are observed during repeated impacts. Texture related breakage is observed to occur, with tensile cracks shown to propagate around inclusions in both the granite and basalt rock samples. The contact area (between the impactor and the sample) is also important as there was cracking and fracturing around the impact area followed by tensile cracking outside the contact area. Simulation of the size of the contact area, using Hertz Midlin theory shows that the contact area stays approximately the same size for all sized particles and therefore the breakage around the contact area becomes more significant as the particle size gets smaller. A reanalysis of current comminution theories show that the t10 parameter is proportional to the surface area created using an impact for any given size fraction tested. A reanalysis of the work from Banini, 2000 shows that the size dependence that was observed is more likely an artefact of using the t10 parameter rather than surface area created. Using the data from Pauw and Mare, 1988 it can be concluded that if breakage occurs the surface area created is proportional to the energy applied vi on the single impact. If there is no breakage then there is no or little new surfaces created within the sample. A further examination of the work from Banini, 2000 shows that there is evidence of texture related breakage in 2 out of the 7 ore types tested, with self similar relationships not observed in all size fractions that were tested. This is consistent with some minerals cleaving along predetermined cleavage planes (such as Broken hill Galena ore), which are no longer evident as the samples get smaller. Naranayan (1985) and Awachie (1983) observed the size distribution of progeny particles had a self similar relationship. In 5 out of 7 rock types tested by Banini, it was found that the relationship between the t75 and the t10 parameter was uniform for all sized particles, however in 2 out of 7 this relationship changed. A cause of this is likely to be variability of rock texture of these samples. There have been a number of breakage models used in comminution to predict the relationship between the energy applied and the breakage achieved. The models are empirical in nature based on the results of controlled testing. They do not examine the breakage mechanisms. As energy efficiency becomes more important with the rising cost of electricity and a continued focus on sustainability, there is a need to understand breakage better so that the platform for improving breakage methods can be set. In order to examine the cracking to rock samples in more detail, small cylindrical rock samples of 13.4mm in diameter and approximately 16mm in length were impacted, repeatedly at low energies until the sample failed. The aims of this thesis is to use the short impact load cell to measure the energy and stress applied to the sample through repeated impacts and use acoustic measurement and x-ray tomography to quantify the damage that occurs to the samples between impacts. The energy that was applied to the samples was selected so that the samples did not generally fail on the first impact to the sample. The main aim of this these was quantifying how energy is absorbed by rock samples from low energy impacts. The information from the Short Impact Load Cell was used to measure how energy was absorbed by the rock samples. Audio and ultrasonic emissions were measured to vii show if there was any damage accruing to the rock samples from these low energy impacts. X-ray tomography and microscopy was used as a second means of quantifying damage to samples from impacts of low energy.
Keyword Comminution, single particle breakage, tomography, acoustics, ultrasonic
Additional Notes Colour print the following pages. 49,51,52,53,54,55,56,80,83,84,90,91,92,98,99,100,102,103,105 107,112,113,114,115,116,117,118,124,125,126,127,132,134,135,136,137,138,139,142,143,144,146,147,148,149,150,155,156,157,158,159,161,162,163,165,166,168,169,170,172,173,174,178,179,180,181,183,184,185,186,188,189,190,191,193,194,199,200,201,202,203,204,206,207,208,209,211,212,214,215,219,220 257-322

 
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Created: Thu, 23 Jun 2011, 10:04:01 EST by Mr Rodney Hocking on behalf of Library - Information Access Service