Gas dispersion in a laboratory Denver flotation cell was characterized by measuring the superficial gas velocity (Jg), bubble size (d b) and estimating the bubble surface area flux, Sb as a function of impeller speed, frother concentration (Dowfroth 250), and solids content (-30mum silica). The cell was operated by self-aspiration or forced air. The air flow rate was measured using a McMillan meter (self-aspiration) and a mass flow meter (forced air). The bubble size was measured by a photographic technique developed at McGill. From repeat experiments, the average relative standard deviation on Jg was ca. 2% and on db was ca. 13%. No effect of location on bubble size was found. / In self-aspiration mode Jg increased with impeller speed due to increased suction and decreased with increasing frother concentration and solids content. The effect of frother was attributed to increased recirculation of air as the bubble size was reduced. The effect of solids appears related to sedimentation. Bubble size increased with increasing impeller speed, which was attributed to the increase in Jg. Solids did not affect the bubble size. Bubble size decreased with frother most notably at low concentration then remained almost constant at high frother concentration. The critical coalescence concentration appeared to be close to 5 ppm for the present conditions. The bubble surface area flux Sb increased approximately linearly with impeller speed up to ca. 40s-1. / Using forced air the bubble size behaved as expected: it decreased with increasing impeller speed (due to higher shear) and decreasing Jg . The Sb ranged up to ca. 80s-1, higher than values quoted for a laboratory machine. To correspond more closely to industrial cells, operation with forced air is recommended. The use of higher impeller speeds than are commonly used in laboratory testwork will give more closely fit superficial gas velocity found in industrial plant practice.

Rockburst control measures have been in practice with continued efforts for improvements since the beginning of the 20th century. The thesis concentrates on the evaluation of destress blasting, which is an important pro-active rockburst control measure. The concept of destress blasting is based on the fracturing of highly stressed rock mass by detonating explosive charge within it. The concept has been carried out ever since the first reported use in a Canadian coal mine circa the early 1930s. Since then, many mines across the continents have applied this technique using a trial-and-error approach with mixed successes. To date, the application lacks scientific base. / The aim of this thesis is to identify the governing mechanisms associated with destress blasting applications. A holistic approach is undertaken, which involves a critical analysis of the reported field evidences, development of a numerical procedure and detailed investigations at the micro-mechanical level to investigate the fracturing of rock under confinement by different types and magnitudes of explosive energy. / A numerical procedure is developed in the thesis that carries promising potential to improve the understanding on rock fracturing by explosive energy as well as provides a platform to develop means for enhancing explosive energy utilization. The procedure is validated with reported field observations. / Analyses of destress blasting is made through dynamic modelling by simulating discrete fractures using the developed procedure. A normalized parameter ℓ ci is introduced to investigate fracturing extent after destress blasts. The investigations revealed that destress blasting produces limited fractures aligned along the principal stresses. The fracturing extent reduces with the increase in the confinement. The study indicates that the current practice of destress blasting seemingly provides more psychological benefits than factual benefits from the desired destressing. / The study also introduces a non-dimensional parameter, betaij , which characterizes destressing effects. The parameter not only adequately explains destressing phenomenon, but also offers clarifications to seemingly inexplicable reported field observations of destress blasting. Local fractures around the boreholes aligned along the principal stresses are found to be the cause of reported local stress concentration and rock stiffening post to destress blasting against the desired stress relaxation and softening.

The extensive damages resulting from static liquefaction re-emphasized the need for reliable criteria in predicting the possible development of this phenomenon. This research presents and evaluates elastic wave velocities in determining the boundary between contractive or dilative tailing materials in the laboratory. The effects of factors such as strain rate, confining pressure and fine contents on liquefaction potential of tailings are also discussed. / To achieve the above objectives, an experimental investigation of the behavior of undrained tailings at confining pressures ranging from 25 to 280 kPa and under two strain rates of 0.5 and 1 mm/min was carried out using a modified Wykeham Farrance triaxial apparatus. As part of this research, a bender element system consisting of piezo-ceramic transducers placed in the loading head and base pedestal of the triaxial cell, a control box and software to generate and receive signals were developed. The system was employed to measure shear and compressive wave velocities (Vs and Vp) during both consolidation and ultimate steady state. These elastic wave velocity measurements were used to develop relationships between void ratio (e), mean effective normal stress (P') and elastic wave velocities. After consolidation, the specimens were loaded in shear under constant strain rates to determine their ultimate steady state at large strains. / The analysis of the results obtained from this research shows limited strain softening response (limited liquefaction) in fine and medium-grained tailings whereas coarse-grained tailings illustrate a strain hardening response or dilative behavior. Shear wave velocities measured during the consolidation stage show better correlation with the ones obtained from the relationships, particularly in fine and medium-grained tailings. This is due to the fact that pore water may affect compressive wave velocity measurements and cause errors in the P wave measurements. / An increase in strain rate in fine-grained tailing does not increase contractiveness, but rather does exactly the opposite. In other words, higher strain rates are associated with higher stability for fine-grained tailings. This is in contrast with the effect of high strain rates on coarse and medium-grained tailings. The membrane effect as well as vibration induced by high strain rates and low volume compressibility in coarse-grained tailings may explain the lower liquefaction resistance in medium and coarse-grained tailings under high strain rates. It should be noted that the instability region becomes larger as confining pressures increase showing more liquefaction potential under higher confining pressures.

Interaction with MgO-minerals is one mechanism suspected to reduce selectivity in flotation of pentlandite from ultramafic ore. Understanding the surface chemistry of the minerals involved will lead to improved flotation conditions that maximize flotation selectivity. The first part of the thesis compares isoelectric point (i.e.p.) and point of zero charge (p.z.c.) determined using Mular-Roberts [M-R] titration technique of MgO-minerals chlorite, serpentine and talc. The M-R technique was unsuccessful with talc, attributed to Mg2+ acting as potential determining ion. For serpentine and chlorite, respectively, p.z.c. was pH 4.3 and 4.6 and i.e.p. pH 3.2 and <3. Dispersion index (DI) for chlorite suggested that aggregation/dispersion is controlled by both; serpentine remained dispersed, possibly due to hydration effects.The second part determines surface properties of pentlandite and serpentine isolated from an ultramafic ore. Zeta potential measurements were made on minerals alone and as mixtures with either indifferent electrolyte or supernatant derived from an ore suspension as background. Individual mineral results anticipated interaction due to electrostatic attraction. This was confirmed in the mixed mineral case, with Mg(OH)2 precipitate interaction as an additional factor. Scanning electron microscopy validated the findings.Aggregation/dispersion was determined by turbidimetry using a light scattering technique and optical microscopy. The effect of selected factors on aggregation/dispersion of pentlandite and serpentine was investigated by a design of experiment (DOE). Concentration of carboxymethyl cellulose, CMC, and the interaction between CMC and pH were the important factors. Contact angle measurements explored effects of several factors on pentlandite hydrophobicity and, using a DOE, small-scale flotation was used to investigate effects on pentlandite floatability. The pH was the most important factor, acidic pH increasing both hydrophobicity and floatability. Copper activation enhanced both properties, as well; magnesium affected hydrophobicity at alkaline pH, but it did not show significant effect on floatability; serpentine was detrimental to the process; and CMC was capable of partially restoring the hydrophobicity and floatability of pentlandite depressed with serpentine. / L'interaction de la pentlandite avec des minéraux d'oxide de magnésium (MgO-) est soupçonnée d'être à l'origine de la sélectivité réduite de la pentlandite dans les procédés de flottation de minerais ultrabasiques. Une meilleure compréhension de la chimie de surface des minéraux impliqués devrait mener à l'amélioration des conditions de flottation qui en maximisent la sélectivité. La première partie de la thèse compare le point isoélectrique (p.i.e) et le point de charge nulle (p.c.n), déterminés à l'aide de la technique de titrage Mular-Roberts [M-R], de minéraux d'oxide de magnésium tels que la chlorite, la serpentine et le talc. Dans le cas du talc, la technique M-R a échoué, probablement dû aux ions Mg2+ qui jouent le rôle d'ions déterminateurs de potentiel. Dans le cas de la serpentine et de la chlorite, les p.c.n. ont été déterminés à pH 4,3 et 4,6 respectivement et les p.i.e à pH 3,2 et <3 respectivement. Des tests de décantations ont suggéré que l'agrégation/dispersion observée pour la chlorite était contrôlée à la fois par le p.c.n et le p.i.e ; la serpentine au contraire est restée dispersée, possiblement dû à des effets d'hydratation. La deuxième partie détermine les propriétés de surface de la pentlandite et de la serpentine isolées à partir d'un minerai ultrabasique. Des mesures du potentiel zêta ont été effectuées sur les minéraux seuls et mélangés en présence d'un électrolyte, indifférent dans un cas et surnageant dans l'autre cas, préparé à partir d'une suspension de minerai utilisée comme milieu d'étude. Les résultats relatifs aux minéraux individuels et en particulier les forces d'attraction électrostatique observables ont permis d'anticiper leur interaction. Cette hypothèse a été confirmée dans le cas des minéraux mélangés, étant après avoir pris en compte l'interaction de précipités de Mg(OH)2 comme facteur additionnel. Des observations au microscope électronique à balayage ont permis de valider les résultats. L'agrégation/dispersion a été déterminée par décantation en utilisant des techniques de lumière diffuse et de microscopie optique. L'effet de certains facteurs sur l'agrégation/dispersion de la pentlandite et de la serpentine a été étudié sur la base d'un plan d'expériences (PE). Entre autres, la concentration en carboxymethyl cellulose (CMC) et l'interaction entre le CMC et le pH ont été considérés comme des facteurs importants. Des mesures d'angle de contact ont permis d'explorer l'hydrophobicité de la pentlandite et, à l'aide d'un PE, une étude de flottation à petite échelle a été réalisée pour investiguer la flottabilité de la pentlandite. Les résultats ont montrés que le pH était le facteur le plus important, un pH acide ayant pour effet d'augmenter à la fois l'hydrophobicité et la flottabilité. L'ajout de cuivre a également permis de renforcer ces deux propriétés par un effet d'activation; la présence de magnésium au contraire a affecté l'hydrophobicité de la pentlandite à pH alcalin mais n'a pas eu d'effet significatif sur ses propriétés de flottabilité; la présence de serpentine s'est trouvée être préjudiciable au procédé mais l'utilisation de CMC a pu être utilisée afin de restaurer partiellement l'hydrophobicité et la flottabilité de la pentlandite diminuées par la présence de la serpentine.

Aspects of stability of access tunnels with particular reference to the factors controlling face access tunnel closure upon the efficiency of coal production and safety of longwall mining operations are investigated. These are used to justify the present studies of tunnel stability and barrier pillar design in mines of the Cape Breton Coalfield. / An analysis of in situ deformation data of 26 face access tunnels by tunnel convergence measurement and a new photographic technique for approximately 40 km of tunnels are presented. Comparative studies of these techniques are made and deformation characteristics with respect to depth of cover, panel width, extracted seam height, barrier pillar width, type of steel arch supports and the effect of adjacent workings are investigated. / Due to the narrow range of field data, the effect of such parameters as extracted seam height, panel width, tunnel shape and tunnel size could not be examined, nor could the influence which the material properties may have on tunnel closure be identified. Hence, comprehensive parametric studies by two dimensional finite element simulation of the face access tunnels by use of MSAP2D are undertaken. Stress and displacement distributions and the extent of any failed zone around the tunnel with respect to the aforementioned parameters together with barrier pillar width, and the stiffness of the surrounding materials are considered. / The existing theories for tunnel convergence prediction are described. From this investigation it is found that some of these theories are inapplicable to soft rocks at depth. Those which are applicable, due to their limitations, are not suited for prediction of the vertical convergence profile from tunnel faceline in longwall coal mining. Hence, a new theoretical exponential formula for tunnel convergence prediction based on a modified Mohr-Coulomb failure criterion is developed and checked against in situ convergence measurements. / The current barrier pillar design criteria are also investigated. It is found that these criteria yield too conservative a design and are not suited for depths greater than 600 m below ground surface. Hence, two theoretical methods of designing barrier pillar width for longwall coal mining are developed and compared with field data and current barrier pillar design criteria.

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