• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 19
  • 16
  • 11
  • 9
  • 8
  • 2
  • 2
  • 2
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 823
  • 212
  • 30
  • 28
  • 27
  • 21
  • 19
  • 18
  • 16
  • 16
  • 16
  • 15
  • 12
  • 12
  • 11
  • 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.

Application of high voltage breakage to a range of rock types of varying physical properties

van der Wielen, Klaas Peter January 2013 (has links)
High voltage breakage is a relatively novel comminution technology that uses highly energetic electrical discharges to induce electrical breakdown in rocks. Advantages of the technology in terms of weakening of rocks to ease comminution, as well as improved liberation compared to mechanical fragmentation methods have been demonstrated. However, a detailed understanding of the fragmentation mechanism and its selectivity, as well as how to optimise the process in terms of efficiency and treatment outcomes was still lacking prior to this thesis. The focus of this study was on how process variables and rock properties interact with high voltage breakage to enable more tailored treatment depending on the desired processing result. Twenty different rock types were extensively characterised in terms of geomechanical, mineralogical and electrical properties and treated at different voltages, number of pulses and discharges, electrode gaps and pulse rates. The resulting particle size distribution was investigated in detail, as well as liberation and weakening of selected rock types. In addition, process mineralogical aspects of the treatment were investigated using QEMSCAN® and a scanning electron microscope. Data in this thesis suggest total spark energy input is the main variable determining fragmentation and liberation outcomes of high voltage treatment. Some materials were found to exhibit a threshold voltage below which less fragmentation than expected occurred, but the main controlling factor for spark energy input is the number of discharges applied to a sample. The process efficiency was found to be strongly dependent on the discharge ratio, but also exhibited a strong rock-specific aspect. In general, low energy inputs and process water conductivity combined with a high voltage gradient and pulse rate were found to be most conducive to efficient high voltage processing. Based on fragmentation and weakening results, as well as liberation and process efficiency it is suggested that treatments in the 1 – 5 kWh t 1 range are most suitable for weakening and liberation applications of the technology. Voltages above 140 kV should be sufficient for most purposes, but this depends on the minimum voltage gradient required to reliably develop discharges in a rock type. Furthermore, feed sizes above 14 mm were found to be more suited to high voltage breakage, which is likely the result of the number of discharges available relative to the number of particles being treated. The voltage of a discharge dictates how many discharges are required to achieve a given energy input, and therefore the exact voltage chosen for a high voltage treatment is a function of feed size as well as efficiency and fragmentation considerations. The evolution of P80 of a high voltage treatment product with energy can be estimated with reasonable accuracy from a relationship incorporating porosity and acoustic impedance. Additionally, the decrease of the mass percentage of feed size material after a given energy input was found to be strongly correlated to a function including tensile strength and relative bulk permittivity. Other rock properties that were found to correlate significantly to high voltage breakage include mica and quartz content. Based on correlations between high voltage breakage indicators, tensile strength and acoustic impedance, as well as imaging of the alteration left by several plasma streamers it is concluded that shock waves are the dominant fragmentation mechanism, and that fragmentation occurs predominantly in a tensile stress regime. There is evidence that the selective fragmentation observed during high voltage breakage is a result of both fracturing along grain boundaries (inter-granular fragmentation) and preferential fracturing of certain mineral phases (intra-granular fragmentation). Intra-granular breakage behaviour is clearly evident from some of the data presented in this thesis. Quartz seems to respond strongly to high voltage treatment-induced stresses, which may be favourable from a process mineralogical perspective. Direct imaging of fractures has also yielded evidence for inter-granular selective fracturing, and strong enrichment of sulphides after treatment at low energy inputs also indicates selective, inter-granular breakage. In addition to the selective fragmentation there is also a selective component to the electrical efficiency of the process. Consequently, the selective nature of high voltage breakage is a feature that recurs in several aspects of the technology.

Gas flow pattern in the freeboard above a bubbling fluidised bed

Yórquez-Ramírez, María Isabel January 1999 (has links)
At present, a thermal cracking process is being developed at pilot plant scale at BP's research facilities in Scotland. The process converts a mixture of waste plastic into a hydrocarbon intermediate, which can be used for petrochemical or refinery processes. A problem of the process is the formation and growth of small amount of fibrous carbon in the freeboard space of the fluidized bed, used as the main reactor. The fibres are believed to form on active metallic sites, detach and grow in the recirculating areas in the freeboard. This can be tolerated but there are economic advantages in their reduction. The aim is to characterise the hydrodynamics of the flow in the freeboard to understand and control the growth of the fibres. A cold model of the fluidised bed used in the pilot plant have been constructed, following scaling laws that allow similar hydrodynamics behaviour of the two beds. Particle Image Velocimetry, PIV, a full-field non-intrusive optical velocity measurement technique is used to analyse the gas pattern in the freeboard. Due to the complexity of the flow and the limitations of the technique PIV was implemented using image shifting. The gas flow in the freeboard of a fluidised bed is strongly dependent on bubble eruption at the bed surface. Here, the gas flow above an erupting bubble has been studied by the injection of single bubbles in an incipiently fluidised bed. Gas velocity vector maps of the vertical central plane above the bed surface, i.e. a plane parallel to the direction of the overall flow, have been determined using image shifting. A mechanism has been proposed to describe the process of single bubble eruption.

Electro-biosorpative recovery of economic metals from waste streams

Hughes, Paul January 2012 (has links)
With increased industrial uses of precious metals, their price has increased considerably over the past decade. It has been suggested that biosorption may be exploited in the remediation of waste-water by removing contaminating entities but more recently the emphasis on exploitation of this phenomenon has been re-directed to the more lucrative sequestration and recovery of precious and semi-precious species, e.g. gold, silver and platinum. Locally sourced biomass materials including distillery yeast (Saccharomyces cerevisiae) obtained from the Old Bushmills Distillery, peat and seaweed (Fucus serratus), were shown to be suitable biosorbent materials and were able to sequester several metals from solution. The distillery yeast was found to work well with equilibrium dialysis, which allowed convenient separation of biomass from the sorbate. A biosorption process using electric fields to assist contact between membrane-enclosed biomass and the relevant sorbate solution was described and termed "Electric FieldMediated Biosorption". In this system, the biomass was enclosed in a dialysismembrane with an electrode, a counter electrode was placed in the sorbate solution and an established potential across the electrodes facilitated electrokinetic movement of the sorbate to the biosorbant material. This system was adapted to increase the contact between the distillery yeast biomass and the sorbate solution by immobilising the biomass in a PV A matrix. This modified-EFMB system better presented the biomass to the sorbate solution forcing ions into contact with the biomass. The PV A allowed better physical handling and this durability increased the life-time of the PV A matrix system, reducing the cost of replacing damaged and fouled dialysis membranes. During the study, the generation of gold nanoparticles was observed and the EFMB system may provide an alternative to the Turkavitch et al. method. The range of metals that the system is capable of actively sequestering demonstrates the functionality, efficiency and application range of the system

Tantalum and niobium mineralogy and recovery from kaolinised Cornish granite

Neighbour, Matthew W. January 2010 (has links)
The china clay industry in the UK produces vast quantities of benign waste through the production of clay and aggregate products. These wastes contain a wide array of minor quantities of accessory minerals, none of which have been quantitatively studied or properly classified in detail. In view of the potential for recovering rare elements, this project investigates the presence of these accessory minerals with particular focus on the tantalum and niobium ore mineral series, columbite-tantalite. Although these minerals are already known to be present, no research on their distribution within the potential ore body has been undertaken and knowledge of the structure of these and other tantalum minerals present is very limited. The thesis sets out to assess the potential resources available through analysis and interpretation of samples collected from Goonvean Ltd. SW England. Initial samples were taken from five operating pit faces to confirm the presence of tantalum mineralogy across the three areas worked by Goonvean Ltd (Goonvean, Rostowrack and Trelavour Downs's pits) for clay extraction. Detailed information of the elemental and mineralogical differences between these areas was gathered, using a variety of analytical techniques. To determine which single site had the most potential for concentrating tantalum efficiently. Based on element abundance, mineralogical content and characteristics of ore grains to upgrade, (Le. grain size, density and liberation) material from Trelavour Downs pit was selected for investigation. Drill core samples were collected and elemental distribution analysed, including Ta, Nb, Sn, Wand other trace level elements. Using geostatistical techniques the presence of trends or patterns within the area, which can give an insight into the mineralization, was investigated. This information was used to generate an estimate of grade and tonnages available. Although literature reports the presence of tantalum and niobium within the area, only one paper investigates the mineralogy from concentrates, (Scott et al. 1998) which from a small number of grains of columbite-tantalite showed a high variation in chemistry; Manning and Hill (1990) also recorded traces of the niobium and tantalum containing mineral ilmenorutile. In this study, QEMSCAN is applied to measure thousands of grains instead of tens of grains, providing improved detail into the mineralogical variability. The drill core samples were combined into composite samples which respect the geology. This produced eighteen samples which were analysed in full detail utilising mineral processing coupled with analytical techniques. Through this analysis, the tantalum minerals were classified, and other mineral species previously undiscovered in the area were identified. Notably QEMSCAN analyses enabled identification of the minerals present, including the gangue mineralogy and their characteristics. Laboratoryscale tests were conducted to assess the viability and variability of gravity separation for production of a tantalum pre-concentrate as well as monitoring the behaviour of other elements, notably niobium, tin and tungsten. QEMSCAN and XRF analyses of gravity separation products from the composite samples allowed insight into the effect of varying tantalum mineralogy on separation efficiency and a model was devised based on these results to predict the grade and recovery of tantalum for the eighteen composites. Compared to other operations, Tailson's Wodgina mine produces a pre-concentrate grade at 8% Ta20S, Haddington resources Ltd. Produces a pre-concentrate at 7.5% and Noventa's Morrua mine in Mozambique at 10% Ta205 (Serjak, 2004, Antonio, 2008).Tantalum grade after a rougher and cleaner Mozley table circuit was measured at 8.9% Ta20S and the combined tantalum recovery was 66%; this is greater than Tanco's sand recovery circuit at 55% , which has a similar size fraction (200-20 ~m), although their overall recovery reaches 70% (Flemming et aI, 1982) and Haddington's preconcentrate tantalum recovery is 65% (Serjak, 2004). The topaz granite contains variable levels of topaz which is heavy gangue mineral with a density ranging between 3.5-3.6 glcm3 (Barthelmy, 2010). This mineral is dominant in any concentrate produced from heavy liquid and gravity tests. Although its content is significantly reduced through gravity separation on the Mozley table, it remains at high levels and dilutes the concentrates. Niobium and tin were also shown to respond well to gravity separation. However, tungsten responds very poorly due its very fine grain size. It is the opinion of the author that further investigation into the extraction and refinement of these metals may lead to an economic added value product(s) if demand and prices for these metals is favourable. Although concentration by DMS upgraded the grain counts of tantalum mineralogy analysed by QEMSCAN, the grade was still too low to discern the variability of tantalum minerals within a single sample. This resulted in the model incorrectly predicting the Mozley table concentrate grade and recovery. Finally, a relationship between the mineralogy, mineral density and the tantalum and niobium assays of composites is proposed as well as a methodology to predict the mineralogical properties which affect the performance of mineral grains on the Mozley table. An integration of this estimated data is proposed and its uses outlined.

An investigation on the potential of bauxsol™ based grout and sprayed concrete to reduce acid rock drainage

O'Donoghue, V. January 2014 (has links)
This thesis investigated the potential of Bauxsol™ based grout and sprayed concrete to reduce acid rock drainage (ARD). The problem of ARD and the current methods to prevent and treat ARD were reviewed. Then, the current cementitious technologies used for and relevant to mining applications were reviewed. An overlap in the treatment and prevention of ARD with cementitious technologies was observed. The cost of, maintenance required and unreliability of current prevention and treatment methods for ARD meant that industry was seeking more effective and reliable solutions for the ARD problem. It was determined that there was potential to use a new filler material (BauxsoITM) in cementitious systems to prevent and treat ARD. Firstly, grouts and sprayed concrete suitable for application in mines were developed. Once the grouts and sprayed concrete were developed they were tested for their interaction with ARD. The success of the ARD was based on how efficiently the grout and sprayed concrete could immobilise deleterious substarices found in the ARD. Findings concluded that there was potential for BauxsoFM based grouts to prevent and treat ARD, however, the limits of the prevention and treatment observed needs to be determined before they are applied in mines. The ultimate success of the Bauxsol™ grouts and sprayed concrete could be determined by replicating the laboratory testing from this study in the field.

The environmental conditions in mines, with special reference to pneumoconiosis and dust problems

Chakravortty, Jatish Changra January 1956 (has links)
No description available.

Molecular simulation of adsorption and diffusion in a microporous carbon membrane

Vieira Linhares, Alexandre Manual January 2003 (has links)
In this thesis, we have particular interest in the hydrogen recovery from a hydrogen/hydrocarbon refinery waste mixture. Hydrogen is one of the clean, affordable and environmentally friendly energy sources. However, the current industry is not focused on the production or use of hydrogen as an energy carrier or a fuel for energy generation. Membrane separations are an economic alternative to either pressure swing adsorption separations or cryogenic separations. Transport across thin membranes can produce chemical and physical separations at a relatively low price. Thus, the diffusion of fluid gas-mixtures inside a porous material is an important factor in membrane separations. This research involves the mathematical modelling of adsorption and diffusion in microporous carbon membranes and particularly the Selective Surface Flow (SSF) carbon membranes developed by Air Products and Chemicals, Inc. Molecular simulations are used to predict the performance of the SSF membranes for hydrogen/hydrocarbon mixture separation under realistic conditions of temperature pressure and bulk gas compositions. Non-Equilibrium Molecular Dynamics (NEMD) gives a fully rigorous account of the dynamics of adsorption and diffusion at an atomic level, by integrating the equations of motion of adsorbed molecules interacting with each other, and with the surface, according to specified intermolecular potentials. In the NEMD simulations performed, it is assumed that all the pores in the membranes are identical, unconnected and open to the surface. However, this single-pore assumption is unlikely to occur in a real material. A real membrane contains pores of different sizes, connected together in a pore network, allowing the possibility of connectivity effects that are not accommodated by a single-pore model. Thus, the fundamentals of critical path analysis (CPA) are used to characterise the pore network structure. The CPA shows that species are selectively transported essentially through distinct sub-networks within the pore network of the membrane. The simulation results are compared with experimental permeabilities obtained from the Air Products selective surface flow membrane for a mixture of hydrogen/methane, relevant to the e.g. recovery of hydrogen from catalytic reformer offgas.

A theoretical and practical investigation of large scale rock blasting

Westwater, Robert January 1953 (has links)
No description available.

Bayesian statistical inference applied to reservoir modelling and earthquake scaling

Li, Lun January 2006 (has links)
This thesis is the first to apply the novel concept of a parsimonious statistical reservoir model to the accurate prediction of the short-term response of a hydrocarbon reservoir to perturbations in effective stress at well sites from water injection and hydrocarbon production. The inversion for the Statistical Reservoir Model is done by combining multivariate linear regression, a Bayesian Information criterion (BIC) for model optimisation, and Bayesian statistical modelling, to establish which well pairs have statistically significant correlations in monthly flow rate data. The statistical method is tested on real flow rate data from the Gullfaks oil field in the North Sea, and independently verified using the output of a physical reservoir simulation model with known characteristics. The results clearly show that long-range correlations of flow rate at well sites are sensitive to both the present-day stress field and pre-existing fault structures. Significantly spatio-temporally correlated well pairs align with respect to the direction of maximum principal horizontal stress or at preferred angles of ~30°, implying a response in the directions of incipient tensile or shear failure. This confirms that geo-mechanical effects exert a strong control on the reservoir response to fluid injection and withdrawal. A principal component analysis of the regression matrix reveals structures that can be interpreted as hydraulically reactive features, mapping in position and orientation on to the pattern of main faults in the field. These results demonstrate that the Statistical Reservoir Model contains relevant information on the hydraulic structure and geo-mechanical state of the reservoir. It can therefore be applied to improving reservoir description as well as improved short term predictive power. The former will help with longer tem prediction of reservoir response by conventional physical models, and the latter as an independent test of their short-term predictive power.

The dynamic and static behaviour of resin bonded rock bolts in tunnelling

Xu, Haixue January 1993 (has links)
The literature on the design, construction, testing and performance of resin bonded rock bolts has been surveyed, and particularly focused on vibration prediction and the behaviour of rock bolts when subjected to dynamic loading. The dynamic load transfer mechanism, the dynamic and static service behaviour of two-speed resin bonded rock bolts in microdiorite, and the dynamic behaviour of rock anchorages in mudstone when subjected to tunnel blasting have been investigated through two extensive full-scale field and laboratory tests. The investigation on two-speed resin bonded rock bolts has been performed with twenty four rock bolts installed within 1.1 to 5.7 m from the blasting face. All rock bolts were instrumented using load cells and accelerometers fixed on the anchor heads to monitor the instantaneous dynamic load and residual static load, and axial dynamic vibration of the bolts. Eight of the bolts were also instrumented with five inserted load cells along their length to monitor the dynamic load transfer mechanism and static load distribution. The dynamic load transfer mechanism, the dynamic response of rock bolts with scaled distance, and safe distance for the installation of permanent resin bonded rock bolts have been established. The effects of prestress load (from 0 to 100 kN) and distance to blast source have been assessed. For the investigation on mudstone anchorages, nine anchorages were instrumented with accelerometers at the anchor heads and in the vicinity of the fixed anchor zone to monitor vibration levels when subjected to nearby tunnel blasting. Residual loads were checked by lift-off test A relationship of vibration between the anchor head and fixed anchor has been established, and a safe peak particle velocity of 48 mm / s has been established. A physical model, which simulates the dynamic and static behaviour of resin bonded bolts subjected to blast loading, has been developed. Three instrumented bolts were tested, and different prestress and confining pressure levels were applied to the instrumented bolts.

Page generated in 0.0317 seconds