Thesis (PhD (Process Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Previous work has shown that microwave heating of mineral ores induces fractures
around grain boundaries due to the differences in absorption of microwaves and the
resulting differential thermal expansion among the various mineral phases in the ore
particles. As a consequence, this reduces the energy required in subsequent grinding
and enhances liberation of valuable minerals. In this study, first, the influences of
different variables on bulk strength reduction of microwave treated ores have been
investigated. Nine different binary ore models were constructed by randomly
disseminating 10 vol.% microwave absorbing minerals in transparent matrices.
Computational simulations of heating, thermal damage and unconfined compressive
strength (UCS) tests on the conceptual binary ores have been undertaken by using
finite-difference modelling techniques. The influence of thermo-mechanical properties
of minerals on strength reduction of microwave treated ores was examined. It was
shown that in general the thermal properties of the microwave absorbing mineral and
the mechanical properties of the transparent matrix have the most significant effect on
the strength reduction. Binary ores containing a microwave absorbing mineral that
has a high thermal expansion coefficient in a strong transparent matrix achieved
higher reductions in strength. The influence of absorbent phase grain size on strength
reduction of ores was also quantified. It was shown that for the same energy inputs
and mineral types, the reductions in strength were much higher in coarse-grained
ores. It has also been shown that for the same mineralogy and treatment condition,
ores with poorly disseminated heated phase achieved much higher strength reduction.
The effect of microwave treatment on the mechanical state of an ore sample was also
examined. It was demonstrated that unconfined compressive strength is less sensitive
to microwave-induced micro-fractures and found to be a poor descriptor of liberation
behaviour.
A new method of characterizing damage in microwave treated ore using a continuum
approach was developed. The method measures the damage around the grain
boundary regions during the heating process. Using the method, it was possible to
elucidate in detail the influences of power density, mineralogy, ore texture on
microwave treatment of ore. It was shown that the amount of grain boundary damage
incurred at a specific power density and energy input is dependent both on the ore mineralogy and its texture. The energy inputs that were required for significant (>
10%) grain boundary damage in the ores range from 0.09 to 7.06 kWh/t depending on
the power density applied, the ore mineralogy and its texture. It was also shown that
for a given mineralogy and ore texture there is a power density level below which no
further increase in grain boundary damage is possible by increasing exposure time.
The effect of pulse repetition frequency on grain boundary damage was also
elucidated using the method. It was found that high pulse repetition frequencies (³ 50
Hz) resulted in an amount of grain boundary damage that was indistinguishable from
that caused by continuous wave operation for a fixed energy input. It has also been
shown that for a fixed microwave energy input the best result would be obtained by
using the lowest possible pulse repetition frequency and highest peak pulse power.
The effect of microwave treatment of ores at different treatment conditions on the
extent of damage and crack pattern was also investigated in detail using bondedparticle
model (BPM). It has been shown that the amount of micro-cracks and also
the cracks pattern in an ore sample after microwave treatment significantly depend on
its mineralogy, microwave treatment condition (power density) and absorbent phase
grain size. It has also been shown that a minimum power density is required to
localize damage around the grain boundary in an ore sample. This minimum power
density was found to strongly depend on the ore mineralogy and its texture.
Initial simulation test work concerning the effect of microwave treatment on liberation
of minerals is also presented. It has been shown that microwave irradiation
considerably changed the fracture pattern of an ore in simulated single particle
crushing. The fracture pattern of the ore treated at high power density (Pd = 0.1 kW
/mm3abs for 1 ms) was along the grain boundary and the absorbent mineral was
intact. In the ore treated at lower power density for the same energy input (Pd = 1
W/mm3abs for 0.1 s) both intergranular and transgranular fractures were observed.
However, in all cases the fracture patterns were preferentially localized around the
grain boundary compared to that of the untreated ore. / AFRIKAANSE OPSOMMING: Vorige studies het getoon dat mikrogolfverhitting van mineraalertse tot breuke om die
ertskorrelgrense aanleiding gee, omdat die verskillende mineraalfases in die
ertsdeeltjies die mikrogolwe verskillend absorbeer, en dus ook verskillend uitsit.
Korrelgrensbreuke verminder die vereiste energie vir latere slypwerk, en verhoog die
vrystelling van waardevolle minerale. Hierdie studie het eerstens die uitwerking van
verskillende veranderlikes op die algehele sterktevermindering van
mikrogolfbehandelde ertse ondersoek. Hiervoor is nege verskillende binêre
ertsmodelle vervaardig deur mikrogolfabsorberende minerale met ʼn
volumepersentasie van 10% lukraak in deursigtige matrikse te versprei. Met behulp
van eindigeverskilmodelleringstegnieke is berekeningsimulasies van verhitting,
warmteskade en onbegrensde druksterkte (“unconfined compressive strength”) op die
konseptuele binêre ertse uitgevoer. Die invloed van termomeganiese
mineraaleienskappe op die sterktevermindering van mikrogolfbehandelde ertse is
eerste onder die loep geneem. Daar is bevind dat die warmte-eienskappe van die
mikrogolfabsorberende mineraal, en die meganiese eienskappe van die deursigtige
matriks, die beduidendste uitwerking op sterktevermindering het. Binêre ertse wat ʼn
mikrogolfabsorberende mineraal bevat met ʼn hoë warmte-uitsettingskoëffisiënt in ʼn
sterk deursigtige matriks, het groter sterkteverminderings getoon. Die invloed van
korrelgrootte in die absorbeerfase op die sterktevermindering van ertse is volgende
versyfer. Die studie het getoon dat, op grond van dieselfde energie-insette en
mineraalsoorte, grofkorrelrige ertse groter sterktevermindering ondergaan het.
Eweneens is bewys dat, met dieselfde mineralogie en behandelingsomstandighede,
ertse met ʼn swak verspreide verhittingsfase ook groter sterktevermindering ervaar.
Die uitwerking van mikrogolfbehandeling op die meganiese toestand van ʼn
ertsmonster is boonop ondersoek. Die studie het getoon dat onbegrensde druksterkte
minder gevoelig vir mikrogolfgeïnduseerde mikrobreuke is, en as ʼn swak aanwyser
van vrystellingsgedrag beskou word.
ʼn Nuwe metode om skade by mikrogolfbehandelde ertse te tipeer is gevolglik met
behulp van ʼn kontinuumbenadering ontwikkel. Dié metode meet die skade rondom die
korrelgrens gedurende die verhittingsproses. Deur middel van voormelde metode was
dit dus moontlik om die invloed van kragdigtheid, mineralogie en ertstekstuur op die
mikrogolfbehandeling van erts deeglik te ondersoek. Daar is bevind dat die mate van korrelgrensskade by ʼn bepaalde kragdigtheid en energie-inset, van sowel die
ertsmineralogie as ertstekstuur afhang. Na gelang van die toegepaste kragdigtheid,
die ertsmineralogie en ertstekstuur, het die vereiste energie-insette vir beduidende
(>10%) korrelgrensskade van 0,09 tot 7,06 kWh/t gewissel. Dit het voorts geblyk dat
enige bepaalde mineralogie en ertstekstuur oor ʼn minimum kragdigtheidsvlak beskik,
onder welke vlak geen verlenging in blootstellingstyd enige verdere korrelgrensskade
kan veroorsaak nie. Die uitwerking van pulsherhaalfrekwensie op korrelgrensskade is
ook met behulp van bogenoemde metode verklaar. Die studie het getoon dat, op grond
van ʼn vaste energie-inset, hoë pulsherhaalfrekwensies (≥50 Hz) en gelykgolfwerking
presies dieselfde hoeveelheid korrelgrensskade tot gevolg het.
Volgende is daar met behulp van ʼn gebondedeeltjiemodel (“bonded-particle model”)
noukeurig ondersoek ingestel na die uitwerking van verskillende
mikrogolfbehandelingsomstandighede op die hoeveelheid skade en die kraakpatroon
by ertse. Die studie het getoon dat die hoeveelheid mikrokrake sowel as die
kraakpatroon in ʼn mikrogolfbehandelde ertsmonster in ʼn groot mate van die betrokke
erts se mineralogie, mikrogolfbehandelingsomstandighede (kragdigtheid) en
korrelgrootte in die absorbeerfase afhang. Daar is ook bevind dat ʼn minimum
kragdigtheid nodig is om skade tot die gebied om die korrelgrens te beperk, welke
minimum kragdigtheid oënskynlik grotendeels deur die ertsmineralogie en -tekstuur
bepaal word.
Die studie bevat ook die resultate van aanvangsimulasietoetse oor die uitwerking van
mikrogolfbehandeling op mineraalvrystelling. Die toetse het getoon dat
mikrogolfbestraling ʼn beduidende verandering tot gevolg het in die ertsbreekpatroon
met gesimuleerde enkeldeeltjievergruising. Die breekpatroon van die erts wat by hoë
kragdigtheid (Pd = 0,1 kW/mm3abs vir 1 ms) behandel is, het ál langs die korrelgrens
gestrek, terwyl die absorberende mineraal nog ongeskonde was. In die erts wat by
laer kragdigtheid dog dieselfde energie-inset behandel is (Pd = 1 W/mm3abs vir 0,1
s), is sowel tussenkorrel- as oorkorrelbreuke opgemerk. In teenstelling met die
onbehandelde erts, was die breekpatrone by die behandelde erts egter in alle gevalle
steeds merendeels rondom die korrelgrens geleë.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/3988 |
| Date | 03 1900 |
| Creators | Ali, Abubeker Yimam |
| 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 | English |
| Type | Thesis |
| Format | 392 p. : ill. |
| Rights | University of Stellenbosch |
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