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Co-Precipitation of Ferrihydrite and Silica from Acidic Hydrometallurgical Solutions and Its Impact on the Paragoethite Process

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Co-Precipitation of Ferrihydrite and Silica from Acidic Hydrometallurgical Solutions and Its Impact on the Paragoethite Process Department of Chemistry Co-precipitation of Ferrihydrite and Silica from Acidic Hydrometallurgical Solutions and its Impact on the Paragoethite Process Laurence G Dyer This thesis is presented for the Degree of Doctor of Philosophy (Chemistry) of Curtin University of Technology August 2010 Acknowledgements The Australian Government, Nyrstar Metals are thanked for the provision of the core research funding via the Australian Research Council, Australian Postgraduate Award (Industry) program. The AJ Parker Collaborative Research Centre for Integrated Hydrometallurgy Solutions and CSIRO Process Science and Engineering are also thanked for additional funding and support. The Nanotechnology Research Institute and its members are thanked for providing much valuable advice and materials for this project, as is Centre for Microscopy, Characterisation and Analysis at the University of Western Australia for the use of instruments and very patient assistance. My most sincere gratitude to my supervisory panel: Dr. Bill Richmond, Dr. Phillip Fawell, Dr. Mike Newman and David Palmer; for providing such an amazing opportunity and fantastic support, and guidance. Thank you for allowing me to flourish while giving me the occasional shove back on track when I needed it. It was obviously not an easy task as it drove Mike to retirement and Bill away from academia. Also thanks to Dr. Mark Ogden, Dr. Franca Jones, Dr. Kate Wright, Peter Chapman and Dave Walton for discussions, advice and technical aid. I would also like to thank Fiona Benn, Andrew Owen and Alton Grabsch from the Alumina group for their assistance with instruments, equipment and generally making me feel welcome at CSIRO. Also thanks to Ian Davies, Milan Chovancek, Rick Hughes, Sophia Morrell, Nancy Hanna and all other members of the analytical and PAS teams. Thanks to Dr. Martin Saunders for many hours spent helping with my TEM work and providing new ideas and access to novel techniques, also Steve Parry and Peter Duncan for their technical assistance. Special thanks to Dr. Geoff Carter, William Rickard and Ross Williams for both technical but more often moral support. To Jean-Pierre Veder, Michael Rutledge and Graeme Clarke for their tireless efforts in providing respite from my research, and dragging me from my work when I really needed it. The deep philosophical discussions we often found ourselves immersed in will be sorely missed. My greatest thanks go to my family, specifically my parents Helen and Trevor, for without them I would not be the man I am today and without their continual love and support completing this thesis would not have been something I would have been able to accomplish. Finally, to Ivana, your constant motivation allowed me to finish my work, you gave me the courage to pursue what I really want and I believe I owe you my sanity. Hopefully now your patience will be duly rewarded, thank you my love. Abstract Ferrihydrite is a common iron oxyhydroxide, produced both naturally and industrially. It is often found in association with silica; an example of this is its occurrence in the Paragoethite process applied in zinc hydrometallurgy for the removal of iron from acidic sulphate solutions. In this process, ferrihydrite is the primary constituent of the precipitation residue, but silica also features heavily and is believed to influence both dewatering and the adsorption of ions from solution. While some information is known regarding the physical interaction of ferrihydrite and silica when they are present as distinct phases, the process by which they simultaneously precipitate remains poorly understood. This thesis details investigations leading to a clearer understanding of this process and provides valuable insights into the factors influencing precipitate behaviour. Ferrihydrite precipitates containing varying proportions of silica were prepared via continuous crystallisation, with the aim of achieving a simplified laboratory-based simulation of the reactions that occur within plant liquors during the Paragoethite process. Multiple anionic environments including sulphate (predominant anion in the Paragoethite process), nitrate (common in natural systems) and chloride were used to monitor the influence of anions on the co-precipitation reaction. Accurate control of the slurry pH and temperature were critical to this simulation, as was maintaining steady-state solution concentrations for iron and silicate. The latter was achieved by continuous addition of iron and silicate, with the corresponding continuous removal of reaction product slurry. Analysis focused on the removed slurry using advanced structural and morphological characterisation of the solid phases formed and quantification of solution species depletion. Reaction conditions were selected to favour the formation of ferrihydrite ensuring it was the dominant phase produced. It was found that the presence of ferrihydrite increased both the rate and extent of silica removed by precipitation from solution. This phenomenon is based on surface adsorption and is hindered by the presence of ions that bind strongly with ferrihydrite. Ions such as sulphate compete with soluble silicate molecules for binding sites on the surface, whereas more weakly bound ions like nitrate do not hinder the process. The level of interference was found to be dependent on available surface area, the affinity for the ion’s adsorption and the concentration of both silicate and the ion in question. Oligomeric and colloidal silica were shown to have a significantly different influence on ferrihydrite to that of monosilicic acid. While the crystallinity of ferrihydrite was seen to decrease when precipitated in the presence of monosilicic acid, no similar effect was observed in the presence of silica in a polymeric state. Although a greater proportion of silica was removed from solution when polymerisation had already progressed to a degree, it still imposed less influence on the product crystallinity. This observation underlines the importance of the adsorption-based process that combines the materials during co-precipitation. X-ray scattering pair distribution function (PDF) analysis of the residue solids was instrumental in displaying correlations between declining ferrihydrite crystallinity and particle size. The data indicated that silicon atoms were not incorporated into the ferrihydrite crystal structure, but by inhibiting the growth of primary particles through surface adsorption, co-precipitated silica produced an apparent decrease in ferrihydrite crystallinity. The analyses presented in this thesis were combined to derive proposed mechanisms of co-precipitate formation based on both the presence of monomeric silica and colloidal particles. When monomeric silica (silicic acid) is present ferrihydrite particles form initially followed closely by the adsorption of silicic acid monomers which restrict further growth. The adsorbed silicic acid molecules condense across the ferrihydrite surface and polymerise outward. Particles aggregate before much, if any, silica polymerisation has occurred, and the silica continues to polymerise of the surface of the aggregates. Where oligomeric or colloidal silica is present ferrihydrite particles are produced; there is little or no immediate silica adsorption and therefore the particles grow uninhibited. During aggregation the silica particles aggregate with the ferrihydrite, being incorporated both within and on the surface of the clusters. The results of this work provide the most detailed description of the reaction mechanism in ferrihydrite/silica co-precipitation, and the most thorough analysis of the structure of co-precipitates thus far reported. i Table of Contents Table of Contents ___________________________________________________ i List of Figures ______________________________________________________ v List of Tables _____________________________________________________ xiii 1. INTRODUCTION _______________________________________________ 1 1.1. Crystallisation _______________________________________________ 3 1.1.1. Saturation ______________________________________________ 3 1.1.2. Nucleation ______________________________________________ 5 1.1.3. Crystal Growth __________________________________________ 7 1.1.4. Aggregation _____________________________________________ 8 1.1.4.1. Particle-Particle Interaction _____________________________ 8 1.1.4.2. Fractals ____________________________________________ 9 1.1.4.3. Cluster-Cluster Aggregation ___________________________ 11 1.2. Iron and Iron Oxides _________________________________________ 13 1.2.1. Occurrence and Applications ______________________________ 13 1.2.2. Formation _____________________________________________ 15 1.2.2.1. Ferrihydrite ________________________________________ 15 1.2.2.2. Schwertmannite _____________________________________ 18 1.2.2.3. Goethite and Hematite________________________________ 19 1.2.3. Structure and Characterisation _____________________________ 21 1.2.3.1. Ferrihydrite ________________________________________ 22 1.2.3.2. Schwertmannite _____________________________________ 24 1.2.4. Properties _____________________________________________ 25 1.2.5. Interaction with Zinc _____________________________________ 26 1.3. Silicon and Silica ___________________________________________ 28 1.3.1. Occurrence and Applications ______________________________ 28 1.3.2. Formation of Silica ______________________________________ 28 1.3.3. Structure of Silica _______________________________________
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