A novel technique for selectively coagulating and separating coal from dispersed mineral matter has been developed at Virginia Tech. The process, which is known as Selective Hydrophobic Coagulation (SHC), differs from oil agglomeration, shear or polymer flocculation, and electrolytic coagulation processes in that it does not require reagents or additives to induce the formation of coagula. In most cases, simple pH control is all that is required to (i) induce the coagulation of coal particles and (ii) effectively disperse particles of mineral matter. If the coal is oxidized, a small dosage of reagents may be used to enhance the coagulation process. During the SHC development, it was discovered that the hydrophobic coagula were very difficult to separate from dispersed mineral matter due to their very small size and their susceptibility to breakage. Using the SHC technique, an evaluation of new methods for coagula recovery was conducted. In this effort, several methods for improving the separation of the coal coagula from dispersed mineral matter were examined. These included lamella thickening, centrifugal sedimentation, vacuum filtration, drum screening, and froth flotation. Each separation method was optimized using statistically-designed test matrices to determined the best separation method based on overall process performance. The thickener was found to be the best method for separating hydrophobic coagula from dispersed mineral matter based on overall process performance (e.g., recovery and grade), unit capacity, and engineering feasibility. Further testing of the thickener separation unit was conducted in an attempt to improve the process performance and the unit throughput. / Master of Science

M.S.

M.S.

Sulfur dioxide emissions, which are produced through the combustion of coal, are thought to be a leading contributor to acid rain. A number of postcombustion techniques for the reduction of sulfur dioxide emissions are being tested; however, the reduction in the pyritic sulfur content of coal through physical cleaning methods may be the most economically viable alternative to the S02 problem. In this investigation, the microbubble column flotation process (MCF), developed at VPI&SU, was tested as a means of reducing the pyritic sulfur content of several high-sulfur coals targeted by the u.S. Department of Energy. A wide variety of pyrite rejection schemes were tested including the use of pyrite depressants, dispersants and elevated pH conditions. The overall efficiency of the MCF process was characterized using a technique known as "release analysis". This technique was used to provide the optimum grade versus recovery relationship for a given coal and a given set of reagent conditions. It was also used as a means for evaluating the various schemes for rejecting coal pyrite. The results of this work indicate that the MCF process is capable of producing a separation very close to that generated by release analysis. The release analysis technique was also found to be an effective means of characterizing pyrite liberation and pyrite rejection for a given coal. In general, it was found that liberation was the most important factor in the rejection of pyrite, although elevated pH conditions seemed to provide improvements for some coals. / Master of Science

A new fine-coal cleaning circuit, with potential near-term applications, has been evaluated for treating fine coal (i.e., 28 mesh x 0). This circuit combines a surface-based separator known as Microcel™ column flotation with an enhanced gravity separator known as the Multi-Gravity Separator (MGS). The synergistic effect of combining both processes in a single circuit resulted in improved ash and pyritic sulfur rejection with minimal losses in energy recovery. In addition, technical and economic analyses of this processing scheme suggest it compares favorably with existing post-combustion desulfurization techniques. A detailed study of the MGS included the development of a model based on fundamental principles of fluid mechanics and mineral processing. The theoretical analyses identified drum speed as the most important MGS operating parameter. To validate these findings, a detailed parametric test program was conducted using coal samples from the Pittsburgh No. 8 and Illinois No. 6 seams. A statistical analysis of the test data also showed that drum speed was the most important variable in controlling the performance of the MGS. The other controlling parameters, i.e., feed percent solids, feed rate and wash-water addition rate, were found to be of lesser importance. The experimental test results were found to be in good agreement with the theoretical predictions obtained using the model. / Ph. D.

The probability of particle capture by an isolated bubble rising through a suspension of particles has been determined using fundamental principles of fluid mechanics. This analysis has allowed the rate constant for flotation to be evaluated as functions of bubble sine, particle size, flotation column diameter, air flow rate and critical film rupture thickness. The last parameter is a measure of the hydrophobicity of the particles to be floated. Using the theoretically determined rate constant, a population balance model has been developed for the flotation of fine coal in a column. The model is capable of predicting the dynamic response of the flotation column to changes in a wide range of operational conditions. Model simulations have been found to be in reasonable agreement with experiments conducted using a bench-scale column. / Ph. D. / incomplete_metadata

The critical rupture thicknesses (H_C) of thin liquid films between air bubbles and solid surfaces have been measured using an optical interferometry technique. The results of the measurements show that H_C increases with the increasing hydrophobicity of a solid and varies inversely with the work of adhesion (W_A) of water on the solid. The relationship between H_C and W_A has, therefore, been used to estimate the values of H_C for the systems in which direct measurement is not possible. Thermodynamic calculations have been carried out for the bubble-particle adhesion process using the DLVO theory modified to include interaction energies due to structural forces. The calculations required knowledge of the values of various parameters such as, H_C, zeta potentials, and Hamaker constants for both bubbles and particles. The results of the calculations conducted on methylated silica and coal show that the thin film ruptures mainly due to the attractive structural force (i.e., hydrophobic interaction). For silica and mica immersed in dodecylamine hydrochloride solutions, the major driving force for film rupture is either the attractive electrostatic force or the hydrophobic interaction force, depending on the pH and the amine concentration. / Ph. D.

This research investigates which theory -- an optimal, irrelevance, or modified pecking order -- best explains a firm's capital structure. A sample of 457 debt and equity utility offerings made from 1973-1982 is used in logit regression analysis to test the predictions of the different theories and the relevance of flotation costs to the financing decision. Target leverage ratios are constructed as averages from industry and firm-specific data. These ratios change over time suggesting that leverage targets are moving in response to general economic conditions. Miller's irrelevance and the modified pecking order theories (if utilities operate well below their debt capacity) are supported. In spite of using leading and lagging targets, no support is found for an optimal capital structure theory. Also, there is no support for flotation costs when measured as the savings from issuing debt rather than equity. An anomalous finding that overlevered firms continue to lever with their next financing decision seems to be robust to the different measures of a target leverage ratio. This finding is inconsistent with the three capital structures theories tested. / Ph. D.

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2010. / Thesis submitted in partial fulfilment of the requirements for the Degree of MASTER OF SCIENCE IN ENGINEERING (MINERAL PROCESSING) in the Department of Process Engineering at the University of Stellenbosch / ENGLISH ABSTRACT: The use of dimensional analysis to scale-up mechanically agitated flotation processes and to identify deficiencies in froth flotation plants was explored. The full range of operating variables was considered, such as particle size distribution, reagent suite, conditioning time, retention time, machine geometry, aeration, solids suspension, power requirements and turbulence. Dimensional analysis offers a methodology to combine variables into dimensionless groups to guide the scale-up process based on the notion of similarity. Ten dimensionless groups were developed and combined with metallurgical variables, such as liberation, reagents dosage and flow diagrams to produce a scale-up and evaluation tool, applicable to any mechanically agitated flotation process. In many hydrodynamic studies, the researchers considered hydrodynamic variables based on rotor diameter. In this case the hydrodynamic variables based on rotor diameter represent mechanism “ability”, while parameters based on cell diameter are considered “requirement”. Dimensionless groups have also been applied to the definition of basic parameters of the kinetic constant, such as floatability, bubble surface area flux and froth recovery factor. It also showed that the bubble surface area flux has a maximum with increased aeration, where similar models do not show this dependence. Analysis by computational fluid dynamics and Perspex modelling revealed valuable insight into the inner working of the Wemco flotation machine, such as air dispersion, turbulence levels, separation zones and solids concentration. Design changes to the rotor, disperser, hood and geometrical lay-out produced a marked improvement in flotation conditions. It also supported certain dimensionless numbers measured in full scale plants. Case studies confirmed that almost all flotation plants, irrespective of the minerals floated, suffer from the same deficiencies. Dimensional similitude offers a unique tool to identify these deficiencies and to predict the effect of recommended improvements. In almost every case where the fundamental requirement of similarity was applied, an improvement in performance was observed. Finally a new algorithm is proposed for the scale-up of flotation plants and the application is demonstrated in the design and testing of a pilot plant. / AFRIKAANSE OPSOMMING: Die gebruik van dimensionele analise in die opskaal van flottasieprosesse en die identifisering van flottasieaanlegprobleme is ondersoek. Die volle bereik van bedryfsveranderlikes is ondersoek, soos partikelgrootte, kondisioneringstyd, retensietyd, geometrie, lugvloei, suspensie van vastestowe, turbulensie en drywingsvereistes. Dimensielose analise is die proses waardeur veranderlikes deur wiskundige manipulasie gekombineer word in dimensielose groepe. Tien dimensielose groepe is ontwikkel en is tesame met metallurgiese veranderlikes soos vrystelling, reagensdosering en vloeidiagramme gekombineer om gebruik te word om gelykvormigheid te bewerkstellig. Hierdie proses is van toepassing op enige flottasieproses gebaseer op meganies geagiteerde toerusting. Dimensielose groepe is ook gebruik in die definisie en kwantifisering van turbulensie, agitasie, geometrie, suspensie van vastestowe, verspreiding van lug en drywingsvereistes. Daarbenewens is die groepe gebruik in die definisie van die basiese veranderlikes van die kinetiese konstante soos lugborreloppervlakvloed, suspensie, en herwinning in die skuimfase. Die groepe is ook gebruik in die bewys dat die lugborreloppervlakvloed ´n maksimum het met toename in lugvloei. In baie gevalle word hidrodinamiese veranderlikes uitgedruk in terme van die rotordiameter en in hierdie studie word dit beskou as meganisme “vermoë”. Die hidrodinamiese veranderlikes gebaseer op sel-diameter word beskou as “behoefte”. Berekeningsvloeidinamika en Perspex modellering het waardevolle insig verskaf in die binne-werking van die Wemco flottasiemasjien soos lugverspreiding, turbulensie en partikelkonsentrasie en is ook gebruik om sekere dimensielose getalle wat in volskaalse aanlegte gemeet is, te verifieer. Gevallestudies het bevestig dat feitlik alle flottasieaanlegte, ongeag die soort mineraal, gebuk gaan onder dieselfde afwykings. Dimensionele analise bied ‘n eenvoudige benadering om hierdie afwykings te identifiseer en om die effek van veranderings te voorspel. In alle gevalle waar die beginsels van gelykvormigheid slaafs gevolg is, het n merkbare verbetering in prestasie voorgekom. Ten slotte is ´n nuwe opskaleringsalgoritme ontwikkel en is die toepassing daarvan gedemonstreer deur die ontwerp en toets van ´n loodsaanleg, gebaseer op die Wemco geometrie.

Froth flotation is the most commonly used process to recover and upgrade the portion of the coal preparation plant feed that has a particle size smaller than 150 microns. Problems that occur when employing froth flotation in the coal industry include i) coal surfaces that are weakly-to-moderately hydrophobic, and ii) flotation systems that are overloaded and limited by insufficient retention time. Research was performed to evaluate techniques that could be implemented to improve flotation performance under the aforementioned scenarios. Pre-aeration of flotation feed using a cavitation system was extensively evaluated in laboratory and full-scale test programs. The benefits of adding hydrophobic, magnetic plastic particles were also investigated to improve froth stability and increase bubble surface area. Laboratory tests revealed that pre-aeration through a cavitation tube improved coal recovery by as much as 20 absolute percentage points in both conventional cells and flotation columns when treating difficult-to-float coals. Carrying capacity increased by 32% which was projected to provide a 4 t/h increase in flotation recovery for a typical 4-m diameter flotation column. Product size analyses suggest that the improved particle recovery was more pronounced for the finest coal fractions as a result of particle agglomeration, resulting from the use of the nucleated air bubbles on the coal surfaces as a bridging medium. In-plant testing of a commercial-scale cavitation system found that feed pre-aeration could reduce collector dosage by 50% when no additional air is added and by 67% when a small amount of air is added to the feed to the cavitation system. At a constant collector dosage, recovery increased by 10 absolute percentage points with cavitation without additional air and 17 absolute points when additional air is provided. The addition of hydrophobic plastic particles to the flotation feed at a 10% concentration by weight was found to substantially improve froth stability thereby elevating the recovery and enhancing carrying-capacity. Test results showed that the primary flotation improvements were directly linked to the coarsest particle size fractions in the plastic material which supports the froth stability hypothesis. Combustible recovery was increased up to 10 percentage points while producing the desired concentrate quality.