Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Previous work has shown that microwave heating of mineral ores induces micro cracks within the ore
structure, which can be attributed to the difference in the adsorption of microwaves amongst the different
mineral phases. This reduces the energy required during subsequent grinding and enhances the liberation
of valuable minerals. In order to design microwave applicators for this purpose, knowledge of the
effective dielectric properties of the crushed ore is required. Of particular interest is the effective complex
permittivity of the bulk crushed ore. The measurement of the effective permittivity of a large volume of
crushed ore is most readily accomplished using the waveguide measurement technique. In this method a
representative sample of the material is placed in a defined and fixed volume in a standard size
rectangular section metallic waveguide. The magnitude and phase angle of the transmitted and reflected
low power microwaves through and from the sample are measured. The complex permittivity can be
extracted from these so-called scattering, or Sij parameters.
In this study the effective complex permittivities for two porphyry copper ores and a copper carbonatite
ore were determined as a function of particle size distribution (-26.5+2mm) using two sizes of waveguide
(WR284 and WR340). The sample holders incorporate dielectric windows for the location of the material
under test. The extraction of dielectric properties from Sij parameter measurements is problematic using
standard algorithms in such cases. Accordingly a new Database Extraction (DBE) Algorithm has been
developed. In this method, a database of scattering parameters is established through electromagnetic
modelling of the measurement system. A search algorithm is used to determine the effective complex
permittivity of the modelled load whose scattering parameters provide the best fit to the experimental
data. The goodness of the experimental fit of the simulated to the measured Sij parameters is determined
by a root mean squared deviation minimisation metric. Results show that the method can be used successfully to determine an effective complex permittivity for a
bulk volume of the crushed material. It is concluded that the dielectric property extraction over the full
operational frequency interval (2.3-3 GHz) is preferred as it has a larger degree of extraction confidence
and hence reliability.
Results show that with increasing particle size, the experimental fit between the simulated and measured
Sij parameters becomes increasingly poor, as wall effect become more prominent. The effect is most
prominent for the smaller WR284 waveguide size. It is shown that for a waveguide size of similar size to
the particle size, the Sij parameter fitting is poorer compared to when a larger waveguide size is used. The
extracted complex permittivity reproducibility between repeated dielectric property measurements is
improved for the WR340 waveguide size, as the extractions in the WR284 waveguide is dominated by the
combined particle size and wall-effects of the sample holder.
Ore mineralogy is identified as a key parameter that influences the dielectric properties of the crushed
ore. For ores with a dominant microwave absorbent mineral phase, the dielectric constant and loss factor
is found to be larger, compared with ores with a more dominant microwave transparent gangue mineral
phase. / AFRIKAANSE OPSOMMING: Navorsing toon dat die verhitting van mineraal erts, met mikrogolwe, mikroskaal frakture in die
mineraalstruktuur teweeg bring weens die verskil in die adsorpsie van mikrogolwe in die verskillende
mineraalfases. Gevolglik verminder die energievereiste vir die vergruising van die erts en verbeter die
vrystelling van waardevolle minerale wat vasgevang is in die mineraalmatriks. Vir die ontwerp van
mikrogolfapplikators vir dié doel, word die effektiewe diëlektriese eienskappe van die vergruisde erts
benodig. Van spesifieke belang is die effektiewe komplekse permittiwiteit van die erts. Die effektiewe
permittiwiteit van `n vergruisde materiaal monster word met behulp van die golfgeleier tegniek gemeet.
Vir dié tegniek word `n verteenwoordigende monster van die materiaal in `n rigiede volume in `n
standaard grootte reghoekige golfgeleier geplaas. Die grootte en fasehoek komponente van die
deurgelate en weerkaatste mikrogolwe deur en van die oppervlak van die materiaal word gemeet. Die
komplekse permittiwiteit van die vergruisde materiaal kan geëkstrakteer word vanaf hierdie sogenaamde
verspreide, of Sij parameters.
In hierdie studie word die effektiewe permitiwiteit van twee porforie koper ertse en `n koper karbonatiet
erts bepaal as funksie van partikel grootte (-26.5+2 mm) deur gebruik te maak van twee standaard
grootte golfgeleiers. Die monster houers inkorporeer diëlektriese vensters om die vergruisde materiaal
monster in posisie te hou. In so `n geval is die ekstraksie van die diëlektriese eienskappe vanuit die Sij
parameter metings problematies. Gevolglik is ‘n nuwe Databasis Ekstraksie Algoritme ontwikkel wat `n
databasis van verspreide parameters opstel deur die elektromagnetiese simulasie van die metingsisteem.
`n Soek-algoritme word gebruik om die effektiewe komplekse permitiwiteit van die gesimuleerde monster
te bepaal wat die beste ooreenstem met dié van die gemete eksperimentele Sij parameter data. Die mate
van ooreenstemming tussen die parameters, word bepaal aan die hand van die minimaliserings
prosedure. Resultate toon dat dié metode geskik is vir die bepaling van die effektiewe komplekse permitiwiteit van die
vergruisde monster. Dit word vasgestel dat die betroubaarheid van die geëkstraeerde Sij parameters, en
gevolglik die diëlektriese eienskappe van die erts, toeneem indien die algoritme oor `n groter frekwensie
band uitgevoer word.
Resultate toon verder dat met toenemende partikel grootte, die mate waartoe die absolute grootte en
fasehoek komponente van die gesimuleerde en gemete Sij parameters ooreenstem, versleg. Dit word
toegeskryf aan wand-effekte. Hierdie verskynsel is veral opmerklik vir die kleiner grootte golfgeleier. Dit
word getoon dat vir metings waar die golfgeleier dieselfde orde grootte geometriese afmetings het as die
vergruisde erts self, die passing tussen die gesimuleerde en gemete Sij parameters swakker is, wanneer dit
vergelyk word met metings waar dit nie die geval is nie. Die reproduseerbaarheid van die geëkstraeerde
diëlektriese eienskap waardes verbeter vir lesings wat uitgevoer word in `n groter grootte golfgeleier.
Laasgenoemde word toegeskryf aan die meer dominante wand-effekte wat kenmerklik is vir `n kleiner
golfgeleier.
Erts mineralogie word geïdentifiseer as `n sleutel parameter wat die diëlektriese eienskappe van die
vergruisde materiaal beïnvloed. Beide die diëlektriese konstante en verliesfaktor is groter vir ertse met `n
oorheersende mikrogolf absorberende mineraalfase.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/5230 |
| Date | 12 1900 |
| Creators | Groenewald, Nico Albert |
| Contributors | Bradshaw, S. M., University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 256 p. : ill. |
| Rights | University of Stellenbosch |
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