Browsing by Author "Rodriguez-Pascual, Marcos"

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    Effect of antiscalants during eutectic freeze crystallization of a reverse osmosis retentate
    (2015) Peters, Edward Michael; Rodriguez-Pascual, Marcos; Lewis, Alison Emslie
    Eutectic Freeze Crystallization (EFC) is a separation technique which involves simultaneous crystallization of water and solute under eutectic conditions. It can be applied to treatment of various industrial aqueous streams containing dissolved organic and inorganic contaminants, such as reverse osmosis (RO) retentate brine streams. Since antiscalants are dosed in RO feed streams, these become concentrated in the retentate brine stream and could have an undesirable effect on crystallization kinetics of both ice and salt in EFC. In this study, the impact of a phosphonate antiscalant on the kinetic processes of nucleation and growth in EFC was investigated. Firstly, the effect of an antiscalant on the thermodynamic phase equilibria of a binary Na2SO4 aqueous solution was experimentally determined. The effect of the antiscalant on the nucleation and growth rates of both ice and salt in a continuous EFC process was then established for concentrations of 200, 350 and 500 mg/L of antiscalant. Product quality parameters such as the Crystal Size Distribution (CSD), morphology and purity of crystals were also measured since they are directly affected by the kinetic rate processes investigated.
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    Heat and mass transfer effects of ice growth mechanisms in pure water and aqueous solutions
    (American Chemical Society, 2014) Kapembwa, Michael; Rodriguez-Pascual, Marcos; Lewis, Alison Emslie
    Interactions between heat and mass diffusion determine growth mechanisms during ice crystallization. The effects of heat and mass transfer on ice growth in pure water and magnesium sulfate solution were investigated by studying the evolution of the gradient of the refractive index using color Schlieren deflectometry. For pure water, the gradient of the refractive index of water was used to calculate the temperature and therefore the local supersaturation. Its effect on the ice crystal growth rate and morphology was studied. It was found that, for local supersaturations greater than 2.8, the morphology was dendritic ice, with a growth rate 2 orders of magnitude higher than that for layered growth. During dendritic growth, 3−16% of the heat of crystallization diffused to the liquid side, which is counter to current understanding. At the transition (between the time of partial melting of the dendritic ice and the beginning of the layered ice growth), a higher supersaturation than that responsible for layered growth was observed. For ice growth from an aqueous salt solution, a mass and thermal diffusion boundary layer in front of the growing ice was created by diffusion of the solutes from the ice and by the release of heat of crystallization.
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    Improving the settleability of a metal sulphide suspension by the application of a magnetic field
    (2015) Gqebe, Sibongiseni Yamkela; Lewis, Alison; Rodriguez-Pascual, Marcos
    Gravitational sedimentation of suspensions in various precipitation processes is hindered by colloidal stability. More especially in sulphide precipitation, where high levels of supersaturation dominate and nucleation is favoured. This results in a large number of colloidal particles with a highly negative surface charge, which remain suspended in solution. The high surface charge of the suspension results in strong attraction/interaction between the ions on the particle surface and counter-ions in solution. Moreover, this strong interaction between ions on the particle surface and counter-ions in solution results in a charge build-up that renders the suspension stable. In order to induce gravitational sedimentation of these particles, a redistribution of ions close to the particle surface is required. This study therefore seeks to redistribute ions close to the particle surface by applying a magnetic field. This results in the reduction of inter-particle electrostatic repulsive forces and subsequent increase in the zeta potential of a suspension. For the purposes of this study, a copper sulphide suspension was used. A T-mixer was used as the reaction zone for the precipitation of each suspension. Subsequent to this, the suspended copper sulphide particles were exposed to a range of field strengths for set exposure times and their zeta potential was measured before and after exposure to the magnetic field. The effect of magnetic field strength, exposure time and particle speed on the zeta potential were tested. All particles had an initial zeta potential value equal to or less than - 40 mV prior to magnetic field exposure. A significant increase in zeta potential was observed with values reaching a maximum of - 16.5 mV when exposed to a 2 T field strength for 40 minutes. An increase in the zeta potential corresponds to a reduction in repulsive electrostatic forces between suspended particles due to the Lorentz force exerted by a magnetic field on the particle surface. DLVO plots were used to quantify this reduction in repulsive electrostatic forces.