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Extraction of Aluminium from Coal Fly Ash Using a Two-Step Acid Leach Process

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Extraction of Aluminium from Coal Fly Ash Using a Two-Step Acid Leach Process EXTRACTION OF ALUMINIUM FROM COAL FLY ASH USING A TWO-STEP ACID LEACH PROCESS Alan Shemi A dissertation submitted to the faculty of Engineering and the Built Environment, University of Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science in Engineering Extraction of Aluminium from CFA Alan Shemi DECLARATION I declare that this dissertation is my own unaided work. It is being submitted to the degree of Master of Science in Engineering to the University of the Witwatersrand, Johannesburg. It has not been submitted before for any other degree or examination in any other University. ---------------------------------- Alan Shemi 14th Day of May 2013 Page ii Extraction of Aluminium from CFA Alan Shemi ABSTRACT Hydrometallurgical extraction technologies provide a process route for resource recovery of valuable metals from both primary as well as secondary resources. In this study, the possibility of treating coal fly ash (CFA), a residue formed as a result of coal combustion in coal-fired power plants, was investigated. Eskom CFA contains significant amounts of alumina typically, 26-31%, in two dissimilar phases, namely amorphous and crystalline mullite, which may be processed separately. Due to its high silica content, however, CFA cannot be treated through the Bayer process route. Therefore, a leach-sinter-leach process was formulated that employed a two-step acid leach technique to extract alumina from CFA using sulphuric acid. In the preliminary test work, the effect of parameters on CFA leaching characteristics was investigated. From the experimental results, appropriate factor levels were found to be 6M acid concentration, 6 hours leaching time, 75°C temperature and 1:4 solid to liquid ratio. Calcium sulphate precipitate formation was found to inhibit aluminium extraction and activation energy- based kinetic results showed that aluminium extraction from CFA was a product diffusion layer controlled mechanism. By leaching the CFA, and using design of experiments (DOE) and response surface methodology strategy for screening and optimization of significant factors, it was found that temperature and leaching time significantly influence the aluminium extraction process. The theoretical optimum conditions established from the statistically based optimization model, for a maximum aluminium extraction of 23.9%, was found to be a temperature of 82°C and a leaching time of 10.2 hrs. Using the optimum conditions, the first stage leaching was done, followed by sintering at 1150°C for 180 minutes to liberate the mullite phase aluminium and then second stage leaching. An aluminium extraction of 24.8%, representing 89.3% extraction from the CFA amorphous phase, was obtained from first stage leaching. The second stage leaching yielded an aluminium extraction of 84.3%. A combination of the two leaching stages gave a total aluminium extraction of 88.2%. This work has shown that by employing a leach-sinter-leach method based on a two-step acid leach technique, CFA can be optimally leached. Page iii Extraction of Aluminium from CFA Alan Shemi PUBLICATIONS AND PRESENTATIONS This work has produced some publications. Journal Publications 1. Shemi, A., Mpana, R.N., Ndlovu, S., van Dyk, L.D., Sibanda, V., Seepe, L., 2012. Alternative techniques for extracting alumina from coal fly ash. Minerals Engineering 34, pp. 30-37. Conference Proceedings 1. Shemi, A., Ndlovu, S., Sibanda, V., van Dyk, L.D., 2012. Extraction of alumina from coal fly ash: Identification and Optimization of Influential Factors: Anglo American Hydrometallurgy Symposium, University of Cape Town, South Africa, 29th July – 1st August, 2012. 2. Shemi, A., Ndlovu, S., Sibanda, V., van Dyk, L.D., Mpana, R.N., Seepe, L., 2011. Coal fly ash as an alternative source of smelter grade alumina: A comparison of aluminium extraction techniques. The 6th International Conference of the African Materials Research Society, Victoria Falls, Zimbabwe, 11th – 16th December 2011. 3. Sibanda, V., Ndlovu, S., van Dyk, L.D., Shemi, A., Mpana, R.N., 2011. Alternative techniques for extracting alumina from coal fly ash: Conference of metallurgists (COM) 2011, Montreal Canada, 2nd – 5th October 2011. Page iv Extraction of Aluminium from CFA Alan Shemi DEDICATION Dedicated to my lovely wife, Alice and three children, Ettric, Lerato and Elvin for their understanding and support Page v Extraction of Aluminium from CFA Alan Shemi ACKNOWLEDGEMENTS I wish to express my special gratitude to my supervisor Prof. S. Ndlovu, for the many inspirational discussions, insight and technical guidance throughout this work. My sincere gratitude goes to my co-supervisors Dr. V. Sibanda and Dr. L. VanDyk for their many helpful suggestions and technical support. Kendal Power Plant, a division of Eskom (RSA), is gratefully acknowledged for the coal fly ash used in this study. The National Research Foundation of South Africa and Carnegie are gratefully acknowledged for their financial contribution to the research. Birkenmayer (Pty) Ltd is gratefully acknowledged for helping with the pelletization of coal fly ash. While many other persons have contributed either directly or indirectly to this work, I should like to mention some of them by name: Dr. M. Bwalya, Geoffrey Simate, Bruce Mothibedi and Steyn Herman, many thanks for their continued interest and support. Finally, special thanks to the Metals Extraction and Recovery Research Group (MERG) for team work. Page vi Extraction of Aluminium from CFA Alan Shemi Table of Contents DECLARATION........................................................................................................................... ii ABSTRACT .................................................................................................................................. iii PUBLICATIONS AND PRESENTATIONS ................................................................................ iii DEDICATION............................................................................................................................... v ACKNOWLEDGEMENTS ........................................................................................................... vi LIST OF FIGURES ........................................................................................................................ x LIST OF TABLES ......................................................................................................................... xi CHAPTER ONE ............................................................................................................................. 1 INTRODUCTION ........................................................................................................................... 1 1.1 Introduction ...................................................................................................................... 1 1.2 Problem Statement ........................................................................................................... 7 1.3 Objectives ......................................................................................................................... 7 1.4 Research Methodology ..................................................................................................... 7 1.5 Dissertation Lay out ......................................................................................................... 7 1.6 Summary .......................................................................................................................... 8 CHAPTER TWO .......................................................................................................................... 10 LITERATURE REVIEW ................................................................................................................ 10 2.1 General Introduction ...................................................................................................... 10 2.1.1 Aluminium ............................................................................................................... 11 2.2 Coal Fly Ash Source and Mineralogy ............................................................................ 11 2.3 Currently Existing CFA Processing Methods ................................................................ 13 2.3.1 Bioleaching ............................................................................................................. 14 2.3.2 Alkaline Leaching ................................................................................................... 15 2.3.3 Acid Leaching of CFA ............................................................................................. 15 2.4 Sintering Process ............................................................................................................ 21 2.4.1 Pelletization ............................................................................................................ 21 2.4.2 Sintering .................................................................................................................. 21 2.4.3 Post-sinter Leaching ............................................................................................... 22 2.5 The Kinetics of Leaching Processes............................................................................... 24 2.6 Summary ........................................................................................................................ 28 CHAPTER THREE .....................................................................................................................
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