Chemical and Biomolecular Engineering - Theses

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    A new paradigm in near-net-shape advanced ceramic components processing: development of the novel processing technique
    TANURDJAJA, STEPHEN ( 2011)
    There are two general ways of processing ceramic materials namely dry and colloidal (wet) processing technique. The colloidal processing route has been widely studied and known to produce better ceramic articles. However, the process of removing the solvents through drying by heating them at high temperature is highly energy intensive (especially if the solvent is water). This thesis will discuss an alternative route using high vapour pressure organic solvents to speed up the solvent removal process. This route involves the reduction in pressure (by applying vacuum) to promote drying as oppose to heating. The surface of alumina which is inherently hydrophilic was made hydrophobic through alkylation reaction with alcohols (chem-adsorbed suspensions) or addition of polymers (phys-adsorbed suspensions) such as Hypermer A70 or Solsperse 3000. The purpose of manipulating the surface chemistry was to enable the dispersion of alumina in the organic solvents such as dodecane, cyclohexane or pentane. The viscosity of suspensions was found to decrease with the increase in the length of the stabilising molecules. This was consistent with the calculated reduction in the inter-particles attraction as the extent of the stabilising molecules increases. A high solid loading of alumina (50%vol) in dodecane using Hypermer A70 (3%wt) was achieved while maintaining a low viscosity (0.25 Pa.s at 100s-1). The observed rheological behaviour fitted the Quemada viscosity model quite well when the effective volume of the polymer was accounted for, which indicated the absence of depletion attraction due to the unadsorbed polymer. The optimum concentration of Hypermer A70 was determined using UV-Visible Spectroscopy technique and was found to be 2.8%wt. The vacuum moulding process produced an interesting and intricate formation of macroscopic voids or channels instead of cracks inside the green body upon the removal of pentane. The fraction of the void space was found to decrease linearly with the increase in the initial solids concentration. By extrapolating this relationship, it was shown that the void space should disappear when the initial solid concentration of the suspension was 62.3% vol. Density measurements performed on the denser sections of the green body showed that the density values were very close to the maximum close packing of spheres (on average 60.7% of the theoretical density). This result is also consistent with the data from the air-driven filtration where the volume fraction of the cake (in essence, the green body) was found to be 64.1%, when the suspension was consolidated by the maximum capillary pressure. The close proximity of these values suggests that they are self-consistent and the dense part is consolidated by the capillary pressure at the same time as the formation of channels. The formation of these voids or channels was thought to be a drying-rate dependent process. It was possible to achieve the green and sintered densities of up to 65.7% and 97.1% theoretical density, respectively, when the rate of removal was slowed down.
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    Rheological characterisation of nickel laterite slurry in processing environments
    Fisher, Daniel Thomas ( 2006)
    With China’s continuing economic boom, the demand for nickel has seen unprecedented growth over the past 10 years. Most of the world’s nickel is present in nickel laterite deposits. These high volume, low grade deposits are now being exploited and processed. An understanding of nickel laterite rheology and the ability to obtain meaningful rheological data is essential to process intensification and stability. The properties and physical characteristics of 8 industrial nickel laterite slurries as well as two alumina slurries were examined using various rheological techniques. The samples chosen covered a wide range of physical conditions such as differing pH, particle size distributions, solids densities and mineralogy as well as country and deposit of origin. The rheological parameters investigated were the yield stress and shear stress vs. shear rate of the particulate slurries. Considerable attention was focused on the techniques used in shear stress vs. shear rate characterisation, including capillary rheometry, smooth and roughened cup and bob rheometry and the vane in infinite medium technique. This work confirmed the finding of previous works, showing nickel laterite slurry rheological behaviour ranging from time independent to thixotropic to rheopectic. It found the vane in infinite medium technique highly suitable for testing nickel laterites at process relevant yield stresses. This technique gave data that correlated well with vane yield stresses and capillary rheometry data. Cup and bob tests showed significant slip at lower shear rates. In a number of cases, the cup and bob techniques also showed erroneously high stresses at higher shear rates. The vane yield stress was found to be a fast and accurate method for monitoring nickel laterite sample aging and the samples tested exhibited 100 Pa yield stresses at solids fractions ranging from 0.389 to 0.524. Blending of nickel laterites was found to be nonlinear, and confirmed that characterisation at various blend ratios is necessary if blending is to be utilised during production.