Recovery of Desilication Product in Alumina Industry Hasitha Indrajith de Silva B.Sc. Engineering (Honours) A thesis submitted for the degree of Doctor of Philosophy at The University of Queensland in 2013 School of Chemical Engineering Abstract Bayer process is used for the production of alumina from bauxite which contains siliceous minerals known as reactive silica. Reactive silica is also digested during the Bayer process, forming desilication product (DSP) which traps significant amount of sodium from the caustic soda used. DSP containing red mud is discarded to the bauxite residue storage areas causing alarming economic and environmental concerns to the alumina industry. Separation of DSP from red mud is a critical step to subsequent sodium recovery. Therefore, methods of separating DSP from red mud originated in a Western Australian alumina refinery were investigated in the present study. Red mud classified into five size classes was characterised using particle size measurement, material density, X-ray diffraction, X-ray photon spectroscopy, electron microscopy fitted with energy dispersive spectroscopy. Size separation alone resulted in an increase of DSP from ~7% to 12-14% in finer size classes of red mud. Since red mud contains more than 50% iron oxides, dissolution of these oxides would invariably result in an increase of DSP, therefore, several dissolution mechanisms of these oxides were investigated. Sodium Citrate-Bicarbonate-Dithionite (CBD) method resulted in an increase of DSP content from ~12 to 22%, while a comparative study was also carried out using deferroxiamine B (DFO-B) as the complexing agent. The use of ultrasonically assisted-CBD resulted in a significant improvement of iron dissolution from red mud. Floatation behaviour of three key minerals (DSP, goethite and haematite) in red mud was also investigated through the use of ionic surfactants (CTAB and potassium Oleate) in neutral and basic pH value ranges (7.5-11.5). Electrostatic attraction was recognised as the mechanism of ionic collector attachment during floatation, which was explained through zeta potential and PZC measurements. Actual red mud was also used for the floatation studies with the ionic surfactants as collectors. CTAB provided superior performance compared with potassium oleate during floatation of red mud in terms of DSP recovery after ultrasonic pre- treatment. ii Declaration by author This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis. I have clearly stated the contribution of others to my thesis as a whole, including statistical assistance, survey design, data analysis, significant technical procedures, professional editorial advice, and any other original research work used or reported in my thesis. The content of my thesis is the result of work I have carried out since the commencement of my research higher degree candidature and does not include a substantial part of work that has been submitted to qualify for the award of any other degree or diploma in any university or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been submitted to qualify for another award. I acknowledge that an electronic copy of my thesis must be lodged with the University Library and, subject to the General Award Rules of The University of Queensland, immediately made available for research and study in accordance with the Copyright Act 1968. I acknowledge that copyright of all material contained in my thesis resides with the copyright holder(s) of that material. Where appropriate I have obtained copyright permission from the copyright holder to reproduce material in this thesis. iii Publications during candidature No publications Publications included in this thesis No publications included Contributions by others to the thesis I was responsible for the experimental designs, performing the experiments, data analysis, establishing figures and tables and drafting the manuscript. Prof. John Zhu and Dr. Steve Rosenburg guided me with research directions and ideas, while Dr. Fu-Yang Wang guided and helped me in drafting the manuscript. Mr. David Appleton performed the inductively couple plasma spectroscopy and Dr. Barry Wood performed the X-ray photoelectron spectroscopy analyses for all samples. The transmission electron microscopy and X-ray analysis for Bruker diffractometer were carried out by Ms. Li Wang and Ms. Anya Yago, respectively. For these works, I prepared the samples, provided the analysis conditions, interpreted the data and prepared the figures and tables. Statement of parts of the thesis submitted to qualify for the award of another degree None iv Acknowledgements This thesis was made possible due to the immense support that I received from a large number of people during my PhD candidature at the School of Chemical Engineering in the University of Queensland. First and foremost, I would like to thank my principal supervisor Prof. John Zhu and co-supervisor Dr. Steven Rosenberg for their sustained support, encouragement and feedback throughout my candidature. Although not in my main supervisory team, the guidance given by Dr. Fu-Yang Wang in many ways throughout my candidature should also be greatly appreciated. Examiners and various scholars helped me by raising research related questions, assisted in understanding the research thoroughly. Next, I would like to thank Dr. Jiuling Chen, Dr. Thomas Rufford, Dr. Pradeep Shukla and Dr. Wei Zhou for technical and administrative support. I would also like to thank our group members who proof read my manuscript, especially Mr. Taiwo Odedairo. My sincere thanks go to all present and past members of Environmental and Catalyst group during my candidature; Dr. Archana, Dr. Ge Lei, Dr. Feng Li, Taiwo, Rapidah, Li Wang, Yang Ying, Meng Rai, Rija and Xiaoyong for their friendship. Mr. David Page’s initial support on training me on master sizer and XRD was invaluable. All other staff and facilities that assisted me at the University of Queensland should also be recognised including the workshop, UQ glass, AIBN, CMM, Food and Agricultural department, School of Mining and Minerals department, IT department, administrative staff at school of chemical engineering, physical sciences library and graduate school. I am grateful to Australian Research Council for the financial support through APAI scholarship and BHP Billiton Worsley Alumina for their sponsorship of the project. All staff at Worsley Alumina who helped me in many ways should be thanked, especially Dr. Leon Munro. Special thanks to my wife Priyasha de Silva, for her unconditional love, support, encouragement and patience. My son Dineth for providing me a relaxed time when I got home. My parents’ and siblings’ moral support was vital in staying focused in difficult times. All my formal and informal teachers who helped me throughout my life should also be thanked. Finally to all who helped in any way is acknowledged. v Keywords Red mud, desilication product, separation, floatation, iron dissolution, size classification, CBD method Australian and New Zealand Standard Research Classifications (ANZSRC) ANZSRC code: 091404, Mineral Processing/Beneficiation, 50% ANZSRC code: 090406, Powder and Particle Technology, 30% ANZSRC code: 050304, Soil Chemistry (excl. Carbon Sequestration Science), 20% Fields of Research (FoR) Classification FoR code: 0904, Chemical Engineering, 50% FoR code: 0914, Resources Engineering and Extractive Metallurgy, 50% vi Table of Contents Abstract .......................................................................................................................... ii Declaration by author ................................................................................................... iii Acknowledgements ........................................................................................................ v List of Figures ........................................................................................................... xi List of Tables ........................................................................................................... xvi List of Abbreviations ................................................................................................. xvii Chapter 1 Introduction ................................................................................................... 1 1.1 Background ...................................................................................................... 1 1.2 Objectives ......................................................................................................... 2 1.3 Significance ...................................................................................................... 2 1.3.1 Economic Benefits ...................................................................................... 3 1.3.2 Other Benefits ............................................................................................. 4 1.4 Thesis outline ................................................................................................... 5 Chapter 2 Literature Review .......................................................................................... 6 2.1 Bayer process and alumina production in Australia......................................... 6 2.1.1 Bayer process .............................................................................................