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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Laser cladding of Ti-6Al-4V with carbide and boride reinforcements using wire and powder feedstock

Farayibi, Peter Kayode January 2014 (has links)
The growth in the use and wear of Ti-based alloy components in mining and offshore explorations has led to a search for techniques to re-engineer such components for reuse. The most desirable method of restoring/protecting the component surfaces is by hard-facing to enhance longevity in service. Laser cladding is one of the viable techniques to achieve a thick coating on such components which involves the addition of reinforcing particulates to improve surface properties such as hardness, wear and erosion resistance amongst others. A fundamental study and understanding of the resultant microstructure-property of the laser clad, hard-facing composite becomes necessary. In this study, laser cladding of Ti-6Al-4V wire with Spherotene particulate reinforcement and laser cladding of modified pre-blend of Ti-6Al-4V and TiB_2 powder were undertaken. The resulting physical and microstructural characteristics, hardness, and performance characteristics of laser clad composites were investigated. Samples from the as-deposited laser clad composites were characterised using optical microscopy, scanning electron microscopy (with chemical microanalysis) and X-ray diffraction. Performance characteristics were examined via erosion testing of the laser clad Ti-6Al-4V/Spherotene using plain and abrasive water jetting, and tensile testing of the laser clad Ti-6Al-4V/TiB_2 composite. The results showed that a crack and pore free clad containing as high as 76 wt.% Spherotene in the Ti matrix was achieved at an energy density of 150 J.mm^-2, 275 mm/min traverse speed, 700 mm/min wire feed rate and 30 g/min powder feed rate. The microstructure of the laser clad Ti-6Al-4V/Spherotene is characterised by nano-sized precipitates of reaction products (W and TiC) uniformly distributed in a beta-Ti solid solution matrix. Matrix hardness is enhanced by the presence of the reaction products in the Ti ranging between 410-620 kgf.mm^-2. Moreover, the modication made to the 90 wt.% Ti-6Al-4V/10 wt.% TiB2 feedstock by attaching the TiB2 to Ti-6Al-4V allowed uniform distribution of reinforcing element in the deposited composite. The composite microstructure on solidication is characterised by TiB eutectic needle-like features uniformly distributed in a Ti-rich primary phase. The hardness of the composite ranged between 440-480 gf.mm^-2. Tensile tests showed that the mean elastic modulus of Ti-6Al-4V/TiB_2 composite is 145 GPa, which is a 27% improvement when compared to that of Ti-6Al-4V. Erosion test indicated that the Ti-6Al-4V/Spherotene composite offered as high as 13 and 8 times resistance of that of Ti-6Al-4V when subjected to PWJ and AWJ impacts respectively.
22

Optimising roadheader performance based on laboratory and field work

Gollick, Michael John January 1999 (has links)
This thesis covers in detail a study of the excavation of rock salt by roadheader, the factors affecting performance and finally a specification with operational results of a new production machine to suit the South African Coal Mining Industry. Dosco Overseas Engineering Ltd. the author's employer, is introduced. Reference is made to how, over the years, performance prediction has radically changed from a mere approximation to a position where an accurate value with a performance guarantee is a necessity. Reference is made to the Universities of Newcastle-upon-Tyne, Nottingham and Leeds who have been the main suppliers of rock testing facilities. The University of Newcastle-upon-Tyne has had further responsibilities for a specific test programme, funded by Dosco, to establish a Performance Prediction Methodology. A general introduction to the trial site at Domtar Salt, located in Canada, is given, along with the current mining methods and the particular aspects requiring consideration if machine mining were to be adopted. A detailed study over a twelve month period covering three main topics; fines production, performance rates, and cutter pick suitability is described. Results are discussed at length and valuable conclusions are drawn. Extrapolation of the results to predict the performance of a larger machine suitable to Domtar's high production requirements is shown. The ability to relate this study to other applications and, in particular, the aspect of pick penetration and its effect on machine design is discussed. A prediction curve suitable for South African coal is shown, along with the necessary calculations to enable a high production rate and the corresponding effect on machine design. Specification features, such as boom force, cutter head design and cutter motor power, are considered at length. The implications for the machine manufacturer for even larger, more powerful machines is shown. An early correlation· of findings is established by comparison to field results from a smaller single boom, Dosco roadheader. The study concludes that current or new machine design can be favourably influenced to reflect varying market requirements and that accurate prediction of machine performance is possible. A later, overall study of the TB2500 shows achieved production rates and comments in particular, on machine mining rate and available mining time. Recommendations having particular reference to the equipment suppliers involvement in the sales procedure are given.
23

Treatment of produced water by simultaneous removal of heavy metals and dissolved polycyclic aromatic hydrocarbons in a photoelectrochemical cell

Igunnu, Ebenezer Temitope January 2014 (has links)
Early produced water treatment technologies were developed before carbon dioxide emissions and hazardous waste discharge were recognised as operational priority. These technologies are deficient in the removal of dissolved hydrocarbons and dissolved heavy metal ions which have been identified as major contributors to the high environmental impact factor of produced water. The simultaneous removal of heavy metals and polycyclic aromatic hydrocarbon (PAH) from produced water via photoelectrochemical process was identified in this work as a produced water treatment alternative with the potential to virtually eliminate the cost for chemical reagents and high energy input. Several grades of simulated produced water were synthesised and used to understand different parameters necessary for developing a successful photoelectrochemical treatment. The process demonstrated in this work followed a simple two–electrode photoelectrochemical cell where heavy metals were recovered on a platinum electrode with simultaneous degradation of PAH (phenanthrene) on a photoanode, with the aid of sunlight (simulated) and an applied cell voltage of 1.0 V. Multiwall CNT-TiO2 synthesised via a modified sol-gel method served as the photoanode after it was immobilised at a loading of 2.5 mg/cm2 on a titanium plate. The results obtained from the photoelectrochemical treatment showed a recovery of 1.6 g/cm2 of lead, 0.2 g/cm2 of copper and 0.1 g/cm2 of nickel from produced water on a 0.1 cm diameter platinum electrode after 24 hours of irradiation with simulated sunlight at 1.0 V cell voltage and a simultaneous degradation of up to 16 % phenanthrene on the photoanode, which gives a potential of scaling up the process to a commercial throughput.
24

Al-Co-Ce glass forming alloys and their corrosion behaviour

Li, Chunling January 2014 (has links)
There is growing interest in Al-TM (transition metal)-RE (rare earth) amorphous alloys because of the combination of their good mechanical properties and corrosion resistance. However, the high critical cooling rate required to form the amorphous structure leads to difficulties in generating bulk amorphous material. Therefore, there has been greater interest in producing amorphous Al-TM-RE alloys as surface layers. In this study, wedge mould casting, laser surface melting (LSM) and large area electron beam (LAEB) surface melting were used to fabricate Al Co-Ce alloys in both crystalline and amorphous form. An eutectic Al 33Cu (wt.%) alloy was also used to quantify the solidification conditions based on the well-known relationship. The microstructures formed by different processes were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, image analysis and X ray and glancing angle X-ray diffraction. In particular, LAEB surface melting was found to be able to provide a sufficiently high solidification velocity for the generation of an amorphous layer on the remelted surface of bulk crystalline Al Co-Ce alloys. Experimental results show that the LAEB treatment can remelt, homogenise the multiphase crystalline starting material and generate a predominantly amorphous layer, although it also caused cracking of the treated layer. However, the cracking was largely reduced in the laser refined starting microstructure. Laser microstructural refinement also improved the homogenisation and amorphisation generated by the subsequent LAEB treatment. The temperature field of multi-pulse LAEB irradiated Al-Co-Ce and Al-Cu alloys was numerically simulated through a finite difference method. The simulation results were generally consistent with the experimental results. The corrosion behaviour of Al-Co-Ce alloys with different microstructures was studied through potentiodynamic polarisation tests. Al-Co-Ce amorphous layer exhibited an enhanced corrosion resistance compared to the crystalline counterpart, although cracking in the amorphous layer greatly influenced the effectiveness of the amorphous layer protecting the substrate.
25

The design and control of mine refrigeration systems

Howes, Michael January 1992 (has links)
The research is directed towards modelling the chiller set, the heat rejection and the load subsystems of a complete mine refrigeration system and simulating the performance in order that the design can be optimised and the most cost effective control system determined. The refrigeration load profile for a mechanised mine is complex and primarily a function of surface climatic variations, the strongly cyclic sources of heat resulting from the operation of diesel powered mining equipment and the associated differences in thermal environmental acceptance criteria. Modelling of the central element of the system, the compressor, is based on empirical relationships which use the actual cooling duty and input power rather than general compressor curves using theoretical flow and head coefficients. This has a more general application and is not restricted to a single compressor type. The steady state modelling of five refrigeration systems has included two types of compressor, four types of evaporator, three types of condenser, two types of cooling tower and five types of mine cooling appliances. The research has extended modelling of refrigeration systems by incorporating fully the heat rejection and load subsystems and has demonstrated that relatively complex mine refrigeration systems can be modelled and the simulation results related to actual measurements with an acceptable accuracy. This has been further improved by testing the system elements and adjusting the theoretical performance analysis where necessary. These adjustments concern either the more difficult to assess factors such as evaporating and condensing heat transfer coefficients or factors influenced by unusual operating conditions. The research has shown that, despite the complexity of the load profile and the refrigeration system, modelling and simulation can be used effectively to optimise both the design and the control system.
26

A study of optimisation methods applied to methane recovery and mine ventilation systems

Moll, A. T. J. January 1993 (has links)
The current trends towards increased coal production from seams at greater depth has led to increased demands being placed on the methane drainage and mine ventilation systems. This study has been directed towards the development of computer models to simulate and analyse the gas flows in methane drainage ranges and the airflow and pressure distribution in mine ventilation networks. The first part of this thesis describes the conventional techniques of methane drainage currently employed and the mathematical approach used to model gas flow in the methane drainage range. It also discusses how, when combined with measured underground data, predicted results can be used to assess the performance of the range. Subsequent analysis techniques will suggest measures that can be undertaken to optimise this performance and assess the effect of selective network reconfiguration. The second part of the thesis includes a discussion of the increasing application of booster fans in U.K. underground coal mines and a critical analysis of the mathematical methods used to model airflow and pressure distribution in mine ventilation networks. One particular method was adopted in a simulation model used to analyse networks with booster fans and specified face airflows. Both simulation models developed have been used to analyse representative underground methane drainage ranges and mine ventilation networks. The thesis concludes that the accuracy of the methane drainage models can only be guaranteed when developed in conjunction with suitable measured data. The mine ventilation model requires user guidance through the solution and analysis procedure. Results are presented in a simple graphical manner and this approach, rather than the 'black box' non interactive approach, is to be favoured in the analysis of mine ventilation networks.
27

The selective heating of pyrite in coal using microwave energy

Wardle, Peter January 2011 (has links)
The objective of this research project was to investigate the effect of microwave treatment on the conversion of pyrite in coal in order to enhance its magnetic properties and to aid subsequent desulphurisation by magnetic separation. Key areas of this investigation include:- a review of the fundamental theoretical aspects of heating in an applied microwave field, a detailed literature survey of the potential applications of microwave heating to aid coal desulphurisation, a study of the transformation mechanisms of pyrite by microwave and conventional heating methods, a comprehensive review of the influence of heterogeneity on the dielectric properties of coals and the effect of high power microwave heating on the decomposition of pyrite in a high sulphur coal. A detailed survey of the literature has shown that although a significant amount of work has been reported on the microwave heating of coal, little information is available which examines the mechanism through which different coals heat in an applied microwave field. A study is therefore presented in this thesis which quantifies, for the first time, the structural ordering of carbon in a range of coals of various ranks and the relationship with their dielectric properties. The results show a clear relationship between the structural ordering of carbon in coal and dielectric loss factor. Essentially, as the structural ordering increases, the level of free charge (per unit volume) increases together with electron mobility which results in an increase in dielectric loss factor. The transformation of pyrite to form the more magnetic pyrrhotite and other related decomposition products by conventional radiant heating has been the focus of much research. However, little attention has been given to the effect of microwave heating on the mechanisms of pyrite transformation in inert and oxygen-containing atmospheres. The study presented in this thesis investigates the effect of temperature, particle size and heat treatment time to establish how these affect the transformation process and the subsequent nature of the final product. This study showed that exposure of pyrite to both conventional and microwave heating methods enhanced its magnetic properties as a result phase alterations at the particle surface. Conventionally heated pyrite samples showed the highest magnetic susceptibility increases, speculated to be due difficulties surrounding the temperature control and measurement of the microwave heated samples. From a review of the literature in this field of research and by SEM observations made for each sample test in this work a general pyrite transformation model as a result of microwave exposure is proposed. The effect of microwave radiation on the conversion of in-situ coal derived pyrite was also investigated. The study presented here extends previous work by others by assessing quantitatively the influence of higher applied microwave power levels on the conversion of pyrite in coal. Samples of run-of-mine high pyritic sulphur content (~12%wt) Kentucky coal were microwave pre-treated at varying exposure times using 8kW and 15kW of microwave power operating at a frequency of 2.45GHz. This work included initial material characterisation which consisted of TGA, optical microscopy and dielectric property measurement, followed by SEM/EDAX characterisation of both microwave treated and untreated sample specimens. This is the first time that microwave treated coal, embedded in a carnauba wax mounting medium, has been observed by this method with focus on the thermal alterations of the pyrite phase. It was demonstrated that with sufficient power and residence time, the conversion of pyrite to the more magnetic pyrrhotite is achievable using microwave irradiation. The amount of pyrrhotite formed as a result of microwave exposure varies with microwave residence time and forward power level. Comparisons of change in the iron sulphide species in the coal was made between microwave treated and untreated material. Using a multimode microwave cavity it was found that significant amounts of pyrrhotite were formed in 20s when using 15kW of microwave power. Treatment at lower power levels in this type of cavity was found to be less effective. The overall effect of treatment on the coal proximate character was minimal.
28

Accessing the extent of kerogen/bitumen interactions on source rock maturation and hydrocarbon generation

Azri, Mohammad Hazwan January 2013 (has links)
In order to ascertain whether kerogen and the heavy oil or bitumen generated during the initial stages of source rock maturation then go on to produce similar gas yields compared to the natural situation when they are present in source rocks, a study has been conducted on an immature type II Kimmeridge Clay Formation (KCF) source rock (Dorset, United Kingdom) with different total organic carbon contents (14, 25 and 30 % TOC), and all having an initial vitrinite reflectance (VR) of approx. 0.4 % Ro. Anhydrous and hydrous pyrolysis experiments were conducted on the whole immature source rock, the isolated partially matured bitumen-extracted source rock and the isolated bitumen (bitumen generated and extracted from the initial source rock) and mixtures of the isolated partially matured source rock and bitumen using two types of Hastalloy vessels (22 and 25 ml) at 320-420 °C for 7-48 h over a pressure range of 115-900 bar. Hydrocarbon gas yields and the increase in VR were greater from the whole rock experiments compared to the experiments on the isolated partially matured source rock and isolated bitumen (no mineral matter present), as well as the experiments on mixtures of the isolated partially matured isolated source rock and bitumen combined. The effect of particle size were also found to be of significance as higher gas yields were obtained from recombined mixture experiments using powdered samples (14 % TOC source rocks) compared to using rock chips (30 % TOC source rocks). Hydrocarbon gas yields, pyrobitumen yields, and VR values were found to decrease as pressure increases towards 500 and 900 bar compared to 180 bar at 350 °C where the effects of pressure retardation were found to be much more significant from the partially matured source rock maturation and bitumen in isolation than from the whole source rock. At 420 °C, gas generation, bitumen plus oil and coke yields were not affected by pressure, but VR decreases going from 310 bar to 500 and 900 bar. The results obtained demonstrate that the interplay of inherent mineral matter, reactant phase, source rock/kerogen, bitumen and pressure are key factors in determining the extent of hydrocarbon generation and source rock maturation in geological basins.
29

Microwave enhanced processing of ores

Kobusheshe, Joseph January 2010 (has links)
Recent research developments have suggested that microwave assisted comminution could provide a step change in ore processing. This is based on the fact that microwave-absorbent phases within a multi-mineral ore can be selectively heated by microwave energy hence inducing internal stresses that create fracture. A detailed review of existing literature revealed that little or no information is available which relates and examines the influence of hydrated minerals on microwave assisted fracture despite the fact that most important ores are associated with phyllosilicates, the vast majority of which are hydrated. A study was carried out on two Kimberlite diamond ores containing various types of hydrated minerals but devoid of any semiconducting minerals which are known to be good microwave heaters. The results confirmed that dehydration of minerals containing interlayer adsorbed water induces significant micro and macro fractures after microwave treatment. The significance of microwave induced fracture on beneficiation was investigated by conducting liberation and flotation tests on two porphyry copper ores. It was demonstrated that microwave pre-treatment improves beneficiation at sizes suitable for flotation and that higher improvements in degree of liberation are attained in coarser particle sizes between 212 and 425 µm. Flotation tests demonstrated a potential for real economic benefits in terms of value proposition. An increase of 8-10% in copper sulphides recovery from coarse sized particles (-400+200 µm) and an overall increase in grade/recovery of between 1-2% was obtained. The results also showed that microwave pre-treatment enhances selective mineral recovery as the grade-recovery of iron sulphides decreased in all but one microwave treated samples. The major drawback to further developments towards industrial scale application was found to be the lack of an effective continuous processing microwave applicator. Any future applicator designs must be able to ensure localised hot spots and confinement of all the microwave energy.
30

Predicting self-oxidation of coals and coal/biomass blends using thermal and optical methods

Avila, Claudio January 2012 (has links)
Self-oxidation and spontaneous combustion of coals is a problem of global concern. There are social, economic and environmental costs associated with this phenomenon and major incidents can, in extreme cases, lead to human casualties. More often however, damage is made to commercial facilities, the calorific value of the fuel is reduced and substantial release of noxious gases, particulate matter and CO2 may contribute to local and international pollution levels. This problem is not only restricted to coals, it also affects other carbonaceous materials such as biomass and potentially coal-biomass blends. A considerable amount of literature has been published, and whilst the causes are relatively well understood, the existing methods for predicting spontaneous combustion are not reliable enough for scientists or the coal industry. This research focuses on understanding the oxidation characteristics of coal, biomass and coal-biomass blends at low temperatures, with the aim of defining a set of experimental test procedures to identify coal and biomass propensity to spontaneously combust. Based on a comprehensive literature survey, two main research areas were identified as feasible sources of information to detect prone coals: microscopy and thermal analysis. Considering these, an extensive experimental program was carried out using 42 coals (including at least three well known samples prone to spontaneous combustion), 10 different biomass types, and a number of coal-biomass blends, including diverse particle and sample sizes, at different stages of the oxidation process. Initially, pulverized coal samples (size <106µm) were subjected to thermogravimetric and calorimetric tests (small sample size), and differential thermal analysis (large sample size) using a large scale reactor. From these tests, the link between the weight loss/gain of the samples and the reactivity at low temperature was confirmed, developing successfully two thermogravimetric tests to identify high risk samples. Afterwards, textural features of thermally altered coal samples (light reflectance and particle morphology) were studied by means of combined petrographic and image analysis techniques. Results showed that particle reflectance and textural changes depend on oxidation temperature, which are linked to the spontaneous combustion potential. Based on these results, two tests were proposed by comparing light reflectance before and after a slow oxidation, quantifying the formation of morphotypes associated with highly reactive samples. A similar approach was used to study biomass and coal-biomass blends, focusing on the quantification of intrinsic reactivity and particle morphology by TGA and optical microscopy. For raw and char biomass particles, results showed a significant correlation between the optical and reactivity properties. Additionally, several new morphotypes were identified from biomass char samples. These characteristics were associated with the thermal behaviour of large samples, although these results did not suggest any distinctive indicator to identify samples prone to self-heat, concluding that the low temperature oxidation of biomass is a completely different phenomenon to that experienced by coals. In the case of coal-biomass blends, the most relevant finding was a synergetic effect observed for the ignition temperature, which was always lower than the ignition temperature of the individual components. This finding has not been described in literature before, and further work is necessary in order to investigate this interaction in greater depth. Finally, a set of standardised procedures to assess the reactive properties of these materials has also been proposed.

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