Silica is detrimental to the recovery of zinc from zinc bearing siliceous materials. The difficulties involved in the hydrometallurgical processing of silica are well documented, the major problem being the formation of gels. These are extremely difficult to separate from solution, creating serious problems. This work investigates the mechanism of the formation of granular silica precipitates and how this differs from the mechanism of formation of the gels.Batch precipitation studies of silica from sulfuric acid solutions have revealed valuable information about the processes occurring. A solution of monosilicic acid (Si(OH)4(subscript)) was produced by leaching synthetic zinc orthosilicate (Zn2SiO4) in sulfuric acid. Precipitation was monitored by measuring the turbidity and silica concentration in solution at regular intervals. Electron microscopy was used to examine the precipitate morphology. Variables known to influence silica precipitation are supersaturation, temperature, seeding and the concentrations of sulfuric and hydrofluoric acids and zinc, iron (III) and aluminium sulfates. These were investigated by comparing their effects on precipitation with those of a control experiment. The evaluation criteria used were induction time, precipitation rate, precipitation time, product morphology and filterability. The results of this work define conditions that yield a granular silica precipitate in preference to a gel, low supersaturation being the most important condition. The product morphology is insensitive to precipitation rate over the range of conditions studied.A laboratory scale continuous crystalliser was designed and constructed and used to carry out a series of experiments to determine the operating conditions for suitable processing of natural zinc orthosilicate ore to remove the silica impurity in an easily separable form. The variables examined were ++ / supersaturation as well as mixing and product removal conditions. The relationship between supersaturation and morphology discovered during the batch experiments was confirmed to also occur when employing a continuous basis. This finding has not been previously reported in the literature. The use of classified stirring and classified product removal led to the formation of the highest quality silica product with a reasonable residence time. The most important quality criterion being the filtration rate.Insight into the mechanism of silica precipitation has been gained by analysing information from both batch and continuous experiments. Under the strongly acidic conditions studied, colloidal silica particles are initially formed. At low supersaturation (S </= 2.9) the colloidal particles aggregate and cement together to form solid particles, while at higher supersaturation (S>/= 4.3) the colloidal particles aggregate to form a gel.

Rice hull ash (RHA) is a by-product of the rice industry. RHA is produced when rice hulls are incinerated, for example, when they are used to power steam engines in rice milling plants. Typically, this ash is disposed of in landfill sites, which may cause environmental problems. RHA has a naturally occurring silica content that is very high, ranging from 95 to 98%. This high silica content makes RHA a potential filler for polymer products. The aim of this project was to investigate the application of RHA as a filler in polypropylene. The study used a systematic approach to characterising the RHA physiochemical properties and comparing these to another commercially available grade of silica filler. The processing conditions for mixing RHA with Polypropylene (PP) were optimised to obtain the maximum tensile modulus value. Attempts were made to improve the interaction of RHA and PP by treating the RHA surface with silane coupling agents and adding functionalised polymers to the composite. Mechanical, rheological and morphological properties of the non-silanated, silanated and coupled composites were characterised and compared to determine their structure -property relationships. Rice hull ash (RHA) has a similar chemical structure to other silicas. Like any metals and metalloids, the surface of RHA contains -OH functional groups but these are very limited in quantity. RHA and PP composites have a wide processing window and the optimised processing conditions in a small batch mixer are 12 minutes, 60 rpm and 180°C. Addition of RHA into PP increases the modulus but decreases the tensile strength of the composites, attributed to poor compatibility between RHA and PP, as RHA is hydrophilic and PP is hydrophobic. The optimum loading of RHA is 20wt%. To improve the RHA and PP composites, 2 grades of silane and maleic anhydride grafted PP (MAPP) is used. Silane treated RHA composites have improved mechanical properties, especially tensile strength, attributed to enhanced interfacial interaction. The optimum is 1.5wt% for APS and 2.wt% for MPS in this system. The optimum MAPP concentration in this system is 3wt%. Properties of the RHA / PP composites show modest improvements compared to PP. The properties are not sufficient to make RHA a commercially attractive reinforcing filler for PP for high performance composite. It has potential for a cost reduction filler for low end application composites.

Les concentrations en silice dissoute (DSi) et silice particulaire biogène (BSi) ont été mesurées pendant une année complète (en 2003) dans la zone tidale de la rivière Escaut et dans ses tributaires aux limites tidales. Alors que la DSi est restée, dans les tributaires, à des concentrations élevées toute au long de l’année, et que la BSi s’est maintenue à des concentrations faibles, la DSi a été entièrement consommée pendant l’été dans la rivière tidale et les concentrations en BSi ont augmenté. En comparant ces concentrations avec celles de la biomasse des diatomées et de la matière en suspension, il a pu être estimé que la majeure partie de la BSi en été était associée aux diatomées vivantes. Des bilans de masse de la DSi et de ces deux fractions de BSi ont été effectués sur différentes zones de la rivière tidale pendant la période durant laquelle les diatomées se développent (période productive, Mai à Octobre). Ceci a permis l’estimation de la croissance et de la mortalité des diatomées, ainsi que de la sédimentation nette de la BSi durant cette période : la moitié de la DSi apportée par les rivières a été transformée en BSi dans la rivière tidale, et la rétention de la silice y a atteint un tiers des apports fluviaux en silice “totale” (TSi = DSi + BSi). Les flux annuels de silice ont aussi été calculés pour replacer à une échelle annuelle les résultats obtenus pendant la période productive : les rétentions annuelles de DSi et la de TSi ne s élevèrent respectivement qu’à 14 et 6 %. L’échantillonnage de l’estuaire a été effectué sur l’ensemble du gradient de salinité au cours de 11 campagnes réparties sur trois ans (de 2003 à 2005). Du fait du mélange des eaux douces et marines, les concentrations en DSi diminuèrent toujours de l’amont vers l’aval, mais les profils étaient généralement convexes ou concaves. Ils ont été interprétés en les comparant avec ceux obtenus à l’aide de la modélisation du transport conservatif. Les flux à l’embouchure ont aussi pu être recalculés, ce qui a permis de quantifier la consommation ou le relargage de DSi au sein de l’estuaire : un maximum de consommation a été observé au printemps, mais l’estuaire a été une source nette de DSi d’août à décembre. A l’échelle annuelle, 28 % des apports de DSi à l’estuaire ont été consommés. La comparaison des profils de BSi avec ceux de la biomasse des diatomées et ceux de la matière en suspension indiqua que la plupart de la BSi dans l’estuaire était détritique (c’est-à-dire non associée aux diatomées vivantes). Ces résultats ont été confirmés par des expériences d’incorporation de silice radioactive qui, bien que la méthodologie soit complètement différente, apportèrent des résultats comparables. La dynamique complexe de la BSi a donc pu être interprétée à l’aide de celle déjà bien étudiée de la matière en suspension dans l’estuaire de l’Escaut, et un bilan de masse de la BSi dans l’estuaire a pu être établi à partir d’un bilan pour la matière en suspension obtenu de la littérature. En plus de la production de diatomées, l’estuaire a reçu presque autant de BSi de la rivière tidale que de la zone côtière. Ceci induisit que la rétention de TSi dans l’estuaire (59 %) a été plus importante que celle de la DSi. Au final, le système tidal de l’Escaut apparaît comme un filtre important pour la silice : les rétentions globales de DSi et TSi dans ce système s’élevèrent respectivement à 39 et 61 %. La comparaison des dynamiques de la silice dans la rivière tidale et dans l’estuaire mit en évidence l’importance du rôle de l’estuaire. La consommation de DSi et la déposition de BSi par unité de surface étaient certes plus intenses dans la rivière tidale mais, à l’échelle de l’écosystème, les effets y furent limités du fait de sa faible surface comparée à celle de l’estuaire. L’une des observations les plus importantes de cette étude est celle de l’apport net de BSi à l’estuaire depuis la zone côtière, ce qui induisit une importante rétention estuarienne de la silice. Les différences importantes entre les rétentions de DSi et de TSi mettent ainsi en évidence la nécessité de prendre en compte la dynamique de la BSi dans l’étude de celle de la silice. De plus, l’importance de la BSi détritique implique que la dynamique de la BSi ne peut être étudiée de part l’observation seule de celle des diatomées. Enfin, l’apport net de BSi vers l’estuaire à l’embouchure, ainsi que l’origine en grande partie marine des diatomées se développant dans l’estuaire, soulignent l’importance de prendre en compte l’importance des échanges à l’embouchure pour le fonctionnement biogéochimique de la silice dans l’estuaire ; l’estuaire ne doit pas être vu comme un simple filtre à sens unique des espèces dissoutes et particulaires provenant uniquement des rivières en amont.

Le but général de ce travail fut de quantifier la silice biogène (BSiO2) dans les sédiments du continuum de l'Escaut (estuaire – zone côtière), ainsi que son taux de recyclage - rétention. Le coefficient de diffusion moléculaire de la DSi a été déterminé pour différentes valeurs de température et deux salinités et une relation empirique reliant le coefficient de diffusion à la température et à la viscosité de la solution a été établie. La distribution longitudinale de la BSiO2 dans les sédiments de surface, ainsi que les profils verticaux de BSiO2 et de silice dissoute ont été déterminés durant différentes saisons en 2004 et 2005. Les flux de DSi ont été également estimés via des expériences d’incubations et par modélisation des profiles verticaux de DSi. Des expériences de dissolution des sédiments ont permis de déterminer (via modélisation) les constantes cinétiques de vitesse de la BSiO2 et une première évaluation du taux de recyclage de la BSiO2 (5 – 70%). Sur base des calculs à partir des profiles de DSi dans l’eau interstitielle, le taux de recyclage de la BSiO2 varie entre 8 et 92% dans l’estuaire, ce qui correspond à une rétention moyenne de la BSiO2 supérieure à 60%, et souligne ainsi l'efficacité du filtre estuarien par rapport à la silice. En revanche, le recyclage de BSiO2 dans la zone côtière apparaît comme très intensif, avec des valeurs souvent supérieures à 40%. Ces résultats montrent l'importance d'inclure les estuaires dans le calcul des budgets globaux de Si. / The general objective of this study was to quantify the BSiO2 in the sediments of the Scheldt continuum, together with its recycling and retention in this area. First an empirical relation linking the diffusion coefficient of DSi to temperature and salinity was determined, via diffusion experiments. The longitudinal BSiO2 distribution in surface sediments and the vertical BSiO2 and DSi profiles in sediment cores where then measured during 2 years and different seasons, together with other parameters characterizing pore waters, such as alkalinity, pH, sulphate… DSi fluxes were also estimated either directly through whole core incubations measurements or via modelling of interstitial water profiles. BSiO2 dissolution experiments allowed the evaluation of the kinetic rate constants also through modelling of the results, and gave preliminary information over the BSiO2 recycling rates (5 – 70%). The recycling rate as obtained from pore water profiles vary between 8 and 92%, with an average value of about 32%, which highlights the importance of the estuarine filter for silica. Recycling is much more intense in the coastal area, with values up to 40%, which we explain by higher salinity and higher sediment permeability. The different experiments and results also indicated that secondary mineral precipitation might be and important process affecting BSiO2 in the Scheldt sediments.

The adsorption cooling technology has the potential to replace all vapor compression based chillers in the future. So, in all over the world immense researches are going on in this field. The purpose of this report is to experimentally investigate whether ethanol could serve as a refrigerant in the technology. Compared to water it has freezing point below 0 °C (-114.1 °C) and can therefore in theory be used in refrigeration applications. The report begins with the theory regarding the adsorption cooling process, describing the cycle and parameters that affect the Coefficient of Performance (COP).In the actual experiment, adsorption between the silica gel and the ethanol vapor is studied at various pressures by maintaining isothermal conditions. An experimental apparatus (Constant Volume Variable Pressure apparatus - CVVP) was fabricated, assembled and tested for this project. After the assembly and testing, volume calibration for the apparatus was carried out as it is essential to know in further experimental calculation. All the data related with the fabrication, assembly and testing of the apparatus and the volume calibrations are presented later in this report in detail.Adsorption experiments are conducted at 301.15K, 311.15K, 321.15K and 331.15K with varying inlet pressure condition to the system and then the uptake data is calculated for each and every experiments using ideal gas equation. Subsequently, the validations of the experimental data with the standard adsorption isotherms are done. Dubinin-Astakhov is found to be the most ideal isotherm to simulate the theoretical data. Its RMSE (Root Mean Square Error) value is found to be 0.506%. It is concluded that ethanol valid option for refrigeration, but further research is needed and recommended.

In this study, the molecular dynamics simulation method was used to investigate the mechanical properties of non-crystalline PEEK mixed with SiO2 nanoparticle. It is found the SiO2/PEEK nano-composite has higher mechanical properties in comparison with pure PEEK composite. Therefore, we wish to obtain the reason. The radial distribution function was used to explain the conformation of the change of microstructure and mechanical properties. The parametric study of different SiO2 particle size was discussed, such on the effects on the structure of PEEK and the strength of the structure.

Although concrete is widely considered a very durable material, if conditions are such, it can be vulnerable to deterioration and early distress development. Alkali-Silica Reaction (ASR) is a major durability problem in concrete structures. It is a chemical reaction between the reactive silica existent in some types of rocks and alkali hydroxides in the concrete pore water. The product of this reaction is a gel that is hygroscopic in nature. When the gel absorbs moisture, it swells leading to tensile stresses in concrete. When those stresses exceed the tensile strength of concrete, cracks occur. The main objective of this study was to address a method of testing concrete materials as a combination to assist engineers to effectively mitigate ASR in concrete. The research approach involved capturing the combined effects of concrete materials (water cement ratio, porosity, supplementary cementitious materials, etc.) through a method of testing to allow the formulation of mixture combinations resistant to ASR leading to an increase in the life span of concrete structures. To achieve this objective, a comprehensive study on different types of aggregates of different reactivity was conducted to formulate a robust approach that takes into account the factors affecting ASR; such as, temperature, moisture, calcium concentration and alkalinity. A kinetic model was proposed to determine aggregate ASR characteristics which were calculated using the System Identification Method. Analysis of the results validates that ASR is a thermally activated process and therefore, the reactivity of an aggregate can be characterized in terms of its activation energy (Ea) using the Arrhenius equation. Statistical analysis was conducted to determine that the test protocol is highly repeatable and reliable. To relate the effect of material combinations to field performance, concrete samples with different w/cm?s and fly ash contents using selective aggregates were tested at different alkalinities. To combine aggregate and concrete characteristics, two models were proposed and combined. The first model predicts the Ea of the aggregate at levels of alkalinity similar to field conditions. The second model, generated using the Juarez- Badillo transform, connects the ultimate expansion of the concrete and aggregate, the water cement ratio, and the fly ash content to the Ea of the rock. The proposed models were validated through laboratory tests. To develop concrete mixtures highly resistant to ASR, a sequence of steps to determine threshold total alkali in concrete were presented with examples. It is expected that the knowledge gained through this work will assist government agencies, contractors, and material engineers, to select the optimum mixture combinations that fits best their needs or type of applications, and predict their effects on the concrete performance in the field.

In this study, PEEK/SiO2 nanocomposites were fabricated by means of simple compression molding technique. The performances and properties of the resulting PEEK nanocomposites were examined in terms of tensile loading, hardness, dynamic mechanical analysis (DMA), thermal mechanical analysis (TMA), thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results indicated that the modified nanosilica was seen to disperse more uniformly than the unmodified counterparty. The XRD patterns of the modified-silica filled PEEK composites reveal a systematic shift toward higher angles, suggesting the smaller d-spacing of the PEEK crystallites. As for the thermal properties of the resulting PEEK nanocomposites, there is no significant difference for the melting and crystallization temperatures, as well as the degree of crystallization between the modified and unmodified silica filled PEEK nanocomposites. The TMA results show that the coefficient of thermal expansion (CTE) becomes lowered when the content of the nanosilica increases. Furthermore, the CTE of the modified-silica filled PEEK nanocomposites shows the higher CTE values, as compared with those of the unmodified counterparts. In addition, the inclusion of the nanosilica could improve the microhardness and the stiffness of the resulting PEEK nanocomposites with the sacrifice of the elongation, as evident from the tension and DMA testing.

Thesis (Ph. D.)--Chemical Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Dr. Christopher Jones; Committee Member: Dr. Dennis Hess; Committee Member: Dr. Hang Lu; Committee Member: Dr. Marcus Weck; Committee Member: Dr. Pradeep Agrawal. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Anomalous concentrations of silica in ground water of the eastern San Joaquin Valley originate from several diverse sources and are subsequently modified by recharge or diagenesis. Statistics, geochemical models, and column studies identified potential sources of silica in ground water and those parameters most important in influencing its distribution. Principal components analysis indicated inverse time and potassium as the parameters which best relate to silica. Inverse time relates to ground-water recharge while potassium relates to either the rhyolitic Friant Pumice, the hardpans of the eastern San Joaquin Valley, or diagenetic losses. A lumped parameter model suggests that recharge is responsible for the drop in silica concentrations beneath Fresno since 1971. The Madera County model indicates higher silica concentrations associated with the Friant Pumice, older geologic units with hardpans, finer grained sediments, and areas of ground-water discharge. Lower silica concentrations are associated with unweathered sediments, areas receiving recharge, and areas underlain by the Corcoran Clay. Column leaching studies produced silica concentrations from the Friant Pumice of up to 90 mg/1, Turlock Lake and Riverbank Formation hardpans from 40-50 mg/1, and younger Modesto Formation sediments from 10-40 mg/l. Spatially, volcanogenic sediments impart the highest silica concentrations to the ground water, 70-95 mg/1, and occur along the eastern margin of the valley, north of the San Joaquin River, or in San Joaquin River fluvial deposits. Iron-silica hardpans, found at various depths along the east side of the valley, are associated with silica concentrations of 40-70 mg/l. The typical arkosic sediments of the eastern valley have silica concentrations of 20-40 mg/l. Silica concentrations in ground water are modified by recharge and diagenetic processes. Either natural or artificial recharge having 4-16 mg/1 silica may lower ambient silica concentrations in ground water. Recharge occurs beneath some rivers and in transmissive paleochannel deposits; artificial sources include agricultural overirrigation and basin recharge. The down-gradient loss of silica to the sedimentary column, more prevalent at depth, apparently is by the sorption of silica on clays, the formation of clays, or deposition of amorphous silica.