Inclusion removal is critical for the production of clean steel. A better understanding of removal processes require knowledge of the effect of process parameters on dissolution kinetics. The present research focuses on the kinetics of calcium aluminate inclusion dissolution in relevant steelmaking slags that contain CaO, Al2O3 and SiO2. In-situ observation of inclusion dissolution in slag is conducted using a high temperature, confocal scanning laser microscope (HT-CSLM). The particles used in this experimental work are produced in the laboratory and the production technique is explained in detail. The change in particle size is recorded with time and the effects of temperature, slag composition and inclusion morphology are investigated. The images are extracted from video and they are analysed to record the change in equivalent radius of a single particle during the dissolution process. The original and normalized dissolution data is used to determine the dissolution mechanism and to improve existing dissolution models. It has been found that an increase in temperature increases the dissolution rate. At 1550°C and 1600°C, there is no product layer formation at the slag-inclusion interface and so, the dissolution process is faster. Slag composition shows a significant influence on the dissolution kinetics due to differences in the dissolution driving force and viscosity. Additionally, the dissolution rate depends on the morphology of inclusion as available reaction sites vary significantly. Rate limiting steps are discussed based on the shrinking core model and diffusion in stagnant fluid model. It is shown that the rate limiting step for dissolution is the diffusion v through a product layer at 1500°C whereas it is mass transfer in slag at 1550°C and 1600°C. The diffusion coefficient of alumina is obtained by applying a one-dimension diffusion model. The calculated results varied between 5.5×10-11 and 2.6×10-10 m2/s depending on experimental conditions. Slag viscosity was found to be an important parameter for the modelling of the dissolution process. A modification to the correlation between the correction coefficient and slag viscosity was proposed. This modification improved the prediction of the dissolution path for calcium aluminate and alumina inclusions in steelmaking slags. This novel study provides an understanding of dissolution mechanisms and it offers data on the dissolution rate of CA2 inclusions in the slags related to the process of steelmaking. The results from this work can be used by steelmakers to aid in process design. / Thesis / Master of Applied Science (MASc) / The present work is a pioneer study on the dissolution of calcium aluminate particles in liquid oxide mixtures using the unique real-time observation approach. Experiments were conducted to provide a better understanding of the effects of various steelmaking conditions on inclusion removal during the refinement of liquid steel. An existing dissolution model is further refined by introducing an additional parameter that is correlated to the properties of oxide mixtures. It has been found that the dissolution model can be applied not only to calcium aluminate inclusions but also to alumina inclusions. Hence, the approach proves the potential university nature of the dissolution model. A clear understanding of the dissolution kinetics of inclusions helps to optimize the current steelmaking routes and enhance the removability of inclusions. Steel with a minimum amount of inclusions has better properties from all aspects, which improves its applicability in all fields.

The grades of steel used for automotive bodies are interstitial free steel grades and titanium stabilized ultra-low carbon steel grades. During the manufacturing of these grades, the addition of titanium in liquid steel is achieved in the steel refining units and may cause processing problems. Titanium reacts with the dissolved aluminum and oxygen to form complex solid aluminum titanate type micro-particles (inclusions). During the flow of titanium alloyed steel grades containing solid inclusions (such as aluminum titanate and alumina type inclusions), the clog accompanied by steel skull can be formed at the submerged entry nozzle between the tundish and the mold. To reduce the effects of aluminum titanate type inclusions, they can be either modified or removed. The current study focused on the removal of Al2O3, TiO2, and Al2TiO5 inclusions by dissolving them in slag in the temperature range of 1430 – 1600 °C using a high-temperature confocal scanning laser microscope. In this technique, a single particle (inclusion) is placed on the surface of a solid slag, and the inclusion-slag system is heated to steelmaking temperatures. The dynamic changes in inclusion size are measured to determine dissolution kinetics and mechanism. This work has developed a complex oxide particle synthesis technique and provides the first-ever kinetic data for removal of aluminum titanate inclusions into steelmaking slags. It is found that Al2O3 inclusions have a slower dissolution rate than that of Al2TiO5 inclusions followed by TiO2 inclusions. The rate-controlling steps are investigated using a shrinking core model. It is shown that the rate-controlling step for dissolution of both Al2O3 and Al2TiO5 inclusion types is the mass transfer of alumina. Evidence in support of this finding is the particle-slag interface characterization by line scan analysis and calculated diffusivity values being inversely proportional to the viscosity of slag. The dissolution path of aluminum titanate is proposed in the following steps. First, aluminum titanate dissociates into alumina, titanium oxide and oxygen while slag penetrates through the particle. In the next step, the alumina and titanium oxide dissolves in slag, and the oxygen leaves the system. The existence of gas bubbles enhances the overall rate of Al2TiO5 dissolution. The current work establishes a detailed understanding of the dissolution of Al-Ti-O type inclusions in steelmaking slags. This knowledge will inform steelmakers on which inclusions of different chemistry can be removed preferably and develop strategies on better slag design to produce superior quality steel with reduced operational downtime. / Thesis / Doctor of Philosophy (PhD)

The acidic, non-oxidative dissolution of galena nanocrystals has been studied using both microscopic and wet-chemical methods. The effects of particle size, shape, aggregation state, and grain proximity on dissolution rates were investigated. Nearly monodisperse galena nanocrystals with an average diameter of 14.4 nm and a truncated cubic shape were synthesized. In the dissolution experiments of dispersed nanocrystals, galena nanocrystals attached on the surface of a TEM grid were exposed to deoxygenated HCl solutions (pH 3) at 25 °C. Capping groups on nanocrystals were removed via a washing process, and chemistry of nanocrystals was examined using X-ray photoelectron spectroscopy (XPS). The evolution of the size and shape of the pre- and post-dissolution nanocrystals were studied using transmission electron microscopy (TEM), and the dissolution rate was calculated directly according to the size shrinking of galena nanocrystals. To assess the size effect, galena microcrystals (~ 3 μm) were synthesized and dissolved under similar conditions to the dispersed nanocrystals. The results showed that the nanocrystals dissolved at a surface area normalized rate of one order of magnitude faster than the microcrystals. In addition, dissolution rate is orientationdependent on a single nanocrystal. High-resolution TEM (HRTEM) images indicated the {111} and {110} faces dissolve faster than {100} faces on galena nanocrystals, rationalized by the average coordination number of ions on each of these faces. To assess the aggregation effect, dissolution experiments of aggregated galena nanocrystals were conducted using a wet-chemical method, and the results were compared with the rates of microcrystals and dispersed nanocrystals. These experiments showed that the rate of aggregated nanocrystals is in the same order of magnitude as the rate of microcrystals, but one order of magnitude smaller than that of dispersed nanocrystals. Finally, the effect of the close proximity between nanocrystals on dissolution was observed by HRTEM. Dissolution was greatly inhibited on nanocrystal surfaces that were closely adjacent (1-2nm, or less) to other nanocrystals, which is probably relevant to the slow dissolution of aggregated nanocrystals. The dissolution phenomena of galena nanocrystals observed in this study is likely important for understanding the environmental fate and behavior of nanoparticles in aquatic systems. / Ph. D.

Living systems possess seemingly bottomless complexity. Attempts to parse the details of one cellular process from all other concurrent processes are challenging, if not daunting undertakings. The apparent depth of this problem, as it pertains to biomineralization, is related to the small number of existing studies focused on the development of a mechanism-based understanding of intracellular mineralization processes. Molecular biologists and geneticists have only begun to turn their attention towards identification and characterization of molecules involved in regulating and controlling biomineral formation. With this new knowledge, a number of new and exciting research opportunities are currently awaiting development upon a barren landscape. Silica biomineralization is one of these emerging frontiers. As new information about the chemical and structural nature of the macromolecules involved in biosilicification is revealed, the means these species employ to control the temporal and spatial onset of silica deposition in vivo become available for exploration. The first chapter of this dissertation outlines those aspects of silicate metabolism that are directly relevant to the controlled biomineralization of silica in eukaryotic organisms and identifies pervasive and unanswered questions surrounding biosilica formation. Particular attention is paid to the diatoms, which are the most abundant, and extensively investigated silica-mineralizing organisms in modern seas. The extent, and mechanism through which specific organic moieties work individually or in concert to direct mineral formation at biological interfaces is a central concern of modern biomineralization research. Chapter two addresses this forefront issue for silica mineralizing systems, and reports the results of an experimental investigation designed to measure the effects of individual surface-bound organic functional groups on the rate of surface-directed silica nucleation. Chapter three discusses an additional aspect of this research aimed at investigating the reactivity of nanoparticulate biogenic silica produced by marine phytoplankton and terrestrial plants in natural environments. Density Functional Theory and ab initio molecular orbital calculations are employed to explore potential mechanisms underlying the catalytic activity of divalent metal cations during the hydrolysis of Si – O bonded networks. / Ph. D.

The fundamental understanding of the dynamic characteristics of metal dissolution and redeposition behavior at the electrode-electrolyte interface is essential, which provides the basis for the development of advanced energy and conversion devices (such as electrochromic devices, electrocatalysts, and batteries) with superior electrochemical performances. We firstly demonstrate the feasibility of resynthesizing the electrode surface chemistry and tuning the electrochemical reactions at the solid-liquid interface by selectively changing the electrolyte composition and electrochemical cycling conditions. Amorphous TiO2 surface layers can be formed on WO3 electrodes by adding exotic Ti cations to the electrolyte, and slow electrochemical cycling. The dissolution and redeposition of electrodes and surface coatings are intertwined, helping to establish a dissolution-redeposition equilibrium at the interface, which can inhibit metal dissolution, stabilize electrode morphology, and promote electrochemical performance. Since the diffusion layer generated by the dissolution of transition metals is ubiquitous at the electrochemical solid-liquid interface, by combining in situ three-electrode electrochemical reaction cell with advanced spatially resolved synchrotron X-ray fluorescence microscopy and micro-X-ray absorption spectroscopy, we then successfully demonstrate the formation and chemical identification of the diffusion layer. By studying the evolution of diffusion layers(tens of micrometers thick) when using WO3 electrodes in acidic electrolytes, we find that with increasing distance of the dissolved species from the electrode surface, the oxidation state remains largely unchanged, but the local electronic environment of the dissolved W species becomes more distorted. We subsequently report a systematic experimental approach by collecting a series of twodimensional fluorescence images at the electrodes to study electrode dissolution and redeposition under different electrochemical conditions. The results show that (1) metal dissolution and redeposition behaviors greatly evolve under different electrode polarization and electrolyte compositions; (2) metal dissolution and redeposition behaviors are independent of bulk electrolyte pH but depend on interfacial pH; and (3) the accumulation of interfacial dissolved species promotes the formation of polytungstate interfacial networks, which ultimately manifest as temporal heterogeneity of redeposition. Lastly, we provide an in-depth study of the underlying mechanism of electrochemicalcycling induced crystallization at the electrode-electrolyte interface through a combination of advanced synchrotron radiation characterization techniques and an in situ electrochemical reaction setup. We have discovered that (1) foreign cations from the electrolyte engender both tensile and compressive strains inside the crystal; (2) repeated electrode dissolution and redeposition promote crystal growth through a non-classical crystallization pathway of particle attachment, but the initial growth of crystals is inhibited by internal strains; and (3) as the strain accumulates, the crystal rotates or moves, which is the fundamental reason for the dynamic structure evolution of the crystal during electrochemical cycling. To our knowledge, this is the first study of electrochemical-cycling-induced crystallization and its strain evolution. These new findings reveal a previously unknown relationship between crystal growth and its internal strain at the electrode-electrolyte interface. / Doctor of Philosophy / Energy drives the entire economy and human civilization. Energy is needed in every aspect of everyday life, and energy is an essential raw material for making and delivering all the products and services that modern society needs, even though it is invisible to us. Since 2000, the global energy demand has increased tenfold and economic growth has spawned a large number of new energy industries, but billions of people are still in urgent need of clean water, sanitation, nutrition, and medical care. Energy is a key factor in meeting these basic requirements for all of humanity. The increasing global energy demand and the increasing impact of climate change have put enormous pressure on the energy market. Therefore, it is necessary to accelerate the relevant actions of energy transition in the world. Among them, the research and innovation of electrochemical energy storage and conversion technology is a major direction. The electrochemical energy storage and conversion technology heavily relies on the various electrochemical reactions in practical devices such as rechargeable batteries, water electrocatalysts, and energy-saving electrochromic smart windows. Within numerous electrochemical reactions under the application, the solid (electrode)-liquid (electrolyte) interface dominates the most important electrochemical reactions. How to understand thephysicochemical reactions at the interface under electrochemical conditions is of great significance. As a major component of research innovations, this research contributes to the design of rational electrode materials, electrolyte compositions, and more efficient and durable electrochemical performance. From a fundamental perspective, my research enriches the understanding of solid-liquid interface reactions under electrochemical conditions, pointing out that electrode dissolution and redeposition and dynamic structural evolution of solid-liquid interfaces are important for further optimizing electrode material design and improving electrochemical performance.

Selective dissolution also known as dealloying is a corrosion process in which one component of a binary alloy system is selectively removed through an electrochemically controlled process which leads to the formation of a porous metal "sponge" with a porosity that is completely interconnected and random in direction. Nanoporous metals are desirable since they have larger surface areas than an equal volume of non-porous material. Because of their enormous surface area per volume, these highly porous metal electrodes are superior materials for high surface area applications such as in biomedical devices, microfilters and catalysts. Understanding the kinetic processes governing the development of porosity during dealloying and having ability to change the electrochemical conditions will allow us to better control over the average ligament size and distribution in porosity. The basic kinetic processes involved in the formation of these structures are related to such issues as environmental effects and electrochemical conditions on diffusion, microscopic coarsening phenomenon at room temperature and elevated temperatures, alloy passivation, and Gibbs-Thomson effects. The average pore size and distribution was found to depend on the electrolyte composition, dealloying rate, applied potential and time. The porosity was found to significantly coarsen at room temperature during the dealloying process and this coarsening was highly dependent on the applied potential. It is showed that the commonly accepted measurement of the critical potential for alloy dissolution calculated based on extrapolation of anodic polarization data results in an overestimation of this quantity. A series of constant applied potential experiments prove to be a more accurate method for critical potential determination. / Ph. D.

One of the most significant challenges faced when modeling mass transfer from contaminant source zones is uncertainty regarding parameter estimates. These rate parameters are of particular importance because they control the connectivity between a simulated contaminant source zone and the aqueous phase. Where direct observation has fallen short, this study attempts to interpret field data using an inverse modeling technique for the purpose of constraining mass transfer processes which are poorly understood at the field scale. Inverse modeling was applied to evaluate parameters in rate-limited models for mass transfer. Two processes were analyzed: (i) desorption of hydrophobic contaminants and (ii) multicomponent Non-Aqueous Phase Liquid (NAPL) dissolution. Desorption was investigated using data obtained from elution experiments conducted with weathered sediment contaminated with 2,4,6 trinitrotoluene (TNT) (Sellm and Iskandar, 1994). Transport modeling was performed with four alternative source models, but predictive error was minimized by two first-order models which represented sorption/desorption using a Freundlich isotherm. The results suggest that first-order/Freundlich models can reproduce dynamic desorption attributed to high-and-low relative energy sorption sites. However, additional experimentation with the inversion method suggests that mass constraints are required in order to appropriately determine mass transfer coefficients and sorption parameters. The final portion of this research focused on rate-limited mass transfer from multicomponent NAPLs to the aqueous phase. Previous work has been limited to bench and intermediate scale findings which have been shown to inadequately translate to field conditions. Two studies were conducted in which numerical modeling was used to reproduce dissolution from multicomponent NAPL sources. In the first study, a model was generated to reproduce dissolution of chloroform (TCM), trichloroethylene (TCE) and tetrachloroethylene (PCE) observed during an emplaced-source field experiment conducted within a flow cell (Broholm et al., 1999). In the second study, a methodology was developed for analyzing benzene, toluene, ethylbenzene and xylene (BTEX) data during a field-scale mass transfer test conducted within a vertically-smeared source zone (Kavanaugh, 2010). The findings suggest that the inversion technique, when provided appropriate characterization of site and source parameters and when given appropriate dataset resolution, represents a viable method for parameter determination. Furthermore, the findings of this research suggest that inversion-based modeling provides an innovative predictive method for determining mass transfer parameters for multicomponent mixtures at the field scale. / Ph. D.

Incorporation of silver nanoparticles (AgNPs) into an increasing number of consumer products has led to concern over the potential ecological impacts of their unintended release to the environment. Dissolution is an important environmental transformation that affects the form and concentration of AgNPs in natural waters; however, studies on AgNP dissolution kinetics are complicated by nanoparticle aggregation. Herein, nanosphere lithography (NSL) was used to fabricate uniform arrays of AgNPs immobilized on glass substrates. Nanoparticle immobilization enabled controlled evaluation of AgNP dissolution in an air-saturated phosphate buffer (pH 7, 25 °C) under variable NaCl concentrations in the absence of aggregation. Atomic force microscopy (AFM) was used to monitor changes in particle morphology and dissolution. Over the first day of exposure to ≥10 mM NaCl, the in-plane AgNP shape changed from triangular to circular, the sidewalls steepened, and the height increased by 6-12 nm. Subsequently, particle height and in-plane radius decreased at a constant rate over a 2-week period. Dissolution rates varied linearly from 0.4 to 2.2 nm/d over the 10-550 mM NaCl concentration range tested. NaCl-catalyzed dissolution of AgNPs may play an important role in AgNP fate in saline waters and biological media. This study demonstrates the utility of NSL and AFM for the direct investigation of un-aggregated AgNP dissolution. / Master of Science

This thesis presents a multiscale model of an industrial nylon-6 leacher. We develop several models at various spatial scales and implement them together in a simplistic, efficient way to develop an overall leacher model. We solve dynamic transport differential equations using the finite-volume method and method of lines in an in-house-developed FORTRAN program. We use the ODEPACK package of ordinary differential equation (ODE) solvers to solve our system of coupled ODEs. Our multiscale model performs transport, thermodynamic, physical property, and mass-transfer calculations at a finite-volume scale. We introduce two additional scales: a mesoscale, in which we perform computational fluid dynamic (CFD) simulations, and a molecular scale. Our CFD simulations solve for turbulent properties of fluid flowing over a packed bed. We incorporate the turbulent diffusivity of the fluid into our finite-volume leacher model. We perform molecular simulations and use the conductor-like screening model-segment activity coefficient (COSMO-SAC) model to generate solubility predictions of small, cyclic oligomers in water and ε-caprolactam. Additionally, we develop an extension of COSMO-SAC to model polymer species, which we refer to as Polymer-COSMO-SAC, and apply it to solve liquid-liquid equilibrium equations. We present a unique methodology to apply COSMO-based models to polymer species, which shows reasonable results for nylon-6. Because of the computational intensity of our Polymer-COSMO-SAC liquid-liquid equilibrium algorithm, we generate pre-computed tables of equilibrium predictions that we may import into our leacher model. Our integration of multiscale models maximizes efficiency and feasibility with accuracy. We are able to use our multiscale models to estimate necessary parameters, but we need to fit two mass-transfer related parameters to industrial data. We validate our model against the plant data and find average-absolute errors in the final mass percent of ε-caprolactam and cyclic dimer in polymer chips of 25.0% and 54.7%, respectively. Several plant data sets are suspected outliers and we believe an unforeseen equilibrium limitation may cause this discrepancy. If we remove these outlying data sets, we then find average-absolute errors of 7.5% and 19.3% for ε-caprolactam and cyclic dimer, respectively. We then use our validated model to perform application and sensitivity studies to gain critical insight into the leacher's operating conditions. / Master of Science

No / This study assesses experimentally the role of gas dissolution in gas entrainment which hitherto has been speculated on but not measured. In this paper, we used dip coating as the model experimental flow and performed the experiments with a dip coater encased in a vacuum chamber in which we admitted various gases. An appropriate choice of gases (air, carbon dioxide and helium) coupled with low pressure conditions from atmospheric down to 75 mbar enables us to test whether gas solubility is a key determinant in gas entrainment. The data presented here track the evolution in time of the size of bubbles of gas entrained in the liquid (silicone oil) which we observed to always occur at a critical speed, immediately after the dynamic wetting line breaks from a straight line into a serrated line with tiny vees the downstream apices of which are the locations from which the bubbles stream out. The results suggest that permeability combining solubility and diffusivity as a single parameter dictates the rate of dissolution when at atmospheric pressure. Helium, despite its comparatively sluggish rate of dissolution/growth into silicone oil was observed to have a more enhanced gas entrainment speed than air and carbon dioxide. Thus, the hypothetical contention from previous work (Miyamoto and Scriven, 1982) that gas can be entrained as a thin film which breaks into bubbles before dynamic wetting failure occurs is not realised, at least not in dip coating. The data presented here reinforce recent work by Benkreira and Ikin (2010) that thin film gas viscosity is the critical factor, over-riding dissolution during gas entrainment. This finding is fundamentally important and new and provides the experimental basis needed to develop and underpin new models for gas entrainment in coating flows.