DISSOLUTION OF IRON OXIDES BY OXALIC ACID A thesis submitted for the degree of Doctor of Philosophy by SUNG OH LEE M.Sc (Resources Recycling Engineering) B.Sc (Mineral and Resources Engineering) Chonnam National University, KOREA School of Chemical Engineering and Industrial chemistry Faculty of Engineering Sydney, Australia September, 2005 LEE SUNG OH Ph.D Chemical Engineering & Industrial Chemistry Engineering DISSOLUTION OF IRON OXIDES BY OXALIC ACID The iron content of industrial minerals can be reduced by physical and chemical processing. Chemical processing is very efficient to achieve a high degree of iron removal at minimum operating cost. Both inorganic acids and organic acids have been used for clay refining. However, due to environmental 2- - pollution and contamination of products with the SO4 and Cl , inorganic acids should be avoided as much as possible. This research investigated the use of oxalic acid to dissolve iron oxides and the dissolution characteristics of natural iron oxides. The dissolution of iron oxides in oxalic acid was found to be very slow at temperatures ranging from 25℃ to 60℃, but increased rapidly at a temperature above 90oC with increasing oxalic acid concentration, whereas the pH caused the reaction rate to decrease at pH>2.5 and improved the rate from pH 1 to pH 2.5. The iron oxides such as goethite (α-FeOOH), lepidocrocite (γ-FeOOH) and iron hydroxide (Fe(OH)3) can be dissolved faster at the presence of magnetite which exhibits an induction period at the initial stage and showed the bell-shaped curves for the dissolution. In titration tests, however, the increase of temperature causes an increase in solubility of the oxalate complexes, resulting in an increased stability of ionized species in solution. During the addition of NaOH, NaHC2O4⋅H2O was precipitated without forming Na2C2O4⋅H2O, but it was re-dissolved at pH>4.0. On the other hand with NH4OH, NH4HC2O4⋅H2O and (NH4) 2C2O4⋅H2O co-precipitated at pH 0.93, but also re-dissolved at over pH 2.03. The reaction temperature was found not to affect the removal of iron from the ferric oxalate complex solution using lime. Iron is removed as iron hydroxide and calcium oxalate is then precipitated during the iron removal step. The formation of Fe(OH)3 in the solution was affected by the dissociation of Ca(OH)2. The thermodynamics of sodium, ammonium and iron oxalate complexes were investigated and the standard free energy, ∆Go was calculated using thermodynamic data and solubility products. The dissolution of pure hematite by oxalate was found to follow a shrinking core model of which the kinetic step of the reaction is the controlled mechanism. b Sticker c ACKNOWLEDGEMENTS I am deeply grateful to my supervisor, Associate Professor Tam Tran, for his guidance, advice, support and invaluable help over the duration of my research. I would also like to sincerely thank my co-supervisor, Dr. Frank Lucien of the School of Chemical Engineering and Industrial Chemistry, and Associate Professor Myong Jun Kim of Chonnam National University, for their guidance, advice, support and invaluable help over the duration of my research. I gratefully thank Mr. Jin Kon Song of the School of Chemical Engineering and Industrial Chemistry, for his encouragement, assistance in experimental work and invaluable help. I am also grateful to the following: Young Jun Hong, Il Joon Bae, Him Chan Cho, Sang Yoon Lee, Eung Yeul Park and Eun Chul Cho, for their friendship and offers of help for my research and overseas life. Finally, my thanks also go to my wife, Seo Jin, Seo Kyoung and all my brothers for their moral support. I would like to offer thanks to God for seeing me through my study and stay in Australia. d LIST OF PUBLICATIONS 1. Sung Oh LEE, Tam TRAN, Yi Yong PARK and Myong Jun KIM, Dissolution of iron oxide using oxalic acid, paper submitted to J. of Int. Min. Proc. (October 2005). 2. Sung Oh LEE, Tam TRAN, Yi Yong PARK and Myong Jun KIM, Study on the kinetics of dissolution of iron oxide, paper submitted to Hydrometallurgy (October, 2005). 3. Sung Oh LEE, Jong Kee OH and Bang Sup SHIN, Dissolution Of Iron Oxide Rust Materials Using Oxalic Acid, Journal of MMI of Japan, Vol.115, No.11, (1999). 4. Sung Oh LEE, Myong Jun KIM, Jong Kee OH and Bang Sup SHIN, Removal of Ferric Ions from Iron (III) Oxalato Complexes Reacted with Calcium Hydroxide in Solution, Journal of MMI of Japan, Vol.115, No.11, (1999). 5. Sung Oh LEE, Sung Kyu KIM, Jong Kee OH and Bang Sup SHIN, Dissolution Characteristics of Hematite and Magnetite with Oxalic acid, Journal of the Korean Inst. of Mineral and Energy Resources Engineering, Vol.35, No.6, (1998)-(in Korean). 6. Sung Oh LEE, Wan Tae KIM, Jong Kee OH and Bang Sup SHIN, Iron-removal of Clay Mineral with Oxalic Acid, Journal of MMI of Japan, Vol.113, No.11, (1997). e ABSTRACT The iron content of industrial minerals can be reduced by physical and chemical processing. Chemical processing is very efficient to achieve a high degree of iron removal at minimum operating cost. Both inorganic acids and organic acids have been used for clay refining. However, due to environmental pollution and contamination of 2- - products with the SO4 and Cl , inorganic acids should be avoided as much as possible. This research investigated the use of oxalic acid to dissolve iron oxides and the dissolution characteristics of natural iron oxides. The dissolution of iron oxides in oxalic acid was found to be very slow at temperatures ranging from 25℃ to 60℃, but increased rapidly at a temperature above 90oC with increasing oxalic acid concentration, whereas the pH caused the reaction rate to decrease at pH>2.5 and improved the rate from pH 1 to pH 2.5. The iron oxides such as goethite (α-FeOOH), lepidocrocite (γ-FeOOH) and iron hydroxide (Fe(OH)3) can be dissolved faster at the presence of magnetite which exhibits an induction period at the initial stage and showed the bell-shaped curves for the dissolution. In titration tests, however, the increase of temperature causes an increase in solubility of the oxalate complexes, resulting in an increased stability of ionized species in solution. During the addition of NaOH, NaHC2O4⋅H2O was precipitated without forming Na2C2O4⋅H2O, but it was re-dissolved at pH>4.0. On the other hand with NH4OH, NH4HC2O4⋅H2O and (NH4) 2C2O4⋅H2O co-precipitated at pH 0.93, but also re-dissolved at over pH 2.03. The reaction temperature was found not to affect the removal of iron from the ferric oxalate complex solution using lime. Iron is removed as iron hydroxide and calcium oxalate is then precipitated during the iron removal step. The formation of Fe(OH)3 in the solution was affected by the dissociation of Ca(OH)2. f The thermodynamics of sodium, ammonium and iron oxalate complexes were investigated and the standard free energy, ∆Go was calculated using thermodynamic data and solubility products. The dissolution of pure hematite by oxalate was found to follow a shrinking core model of which the kinetic step of the reaction is the controlled mechanism. g TABLE OF CONTENTS STATEMENT b ACKNOWLEDGMENTS d LIST OF PUBLICATIONS e ABSTRACT f TABLE OF CONTENTS h LIST OF FIGURES n LIST OF TABLES t CHAPTER ONE INTRODUCTION 1 1.1 BACKGROUND 1 1.2 OBJECTIVES OF THE STUDY 2 CHAPTER TWO LITERATURE REVIEW 4 2.1 INTRODUCTION 4 2.2 DISSOLUTION OF IRON OXIDES 5 2.2.1 Dissolution mechanisms 7 2.2.1.1 Protonation reaction 8 2.2.1.2 Complexation reaction 10 2.2.1.3 Reductive dissolution 13 2.2.1.4 Comparison of the three different types of dissolution reactions 18 2.2.2 Dissolution kinetics 20 2.3 THERMODYNAMIC ANALYSIS OF THE Fe-H2C2O4⋅H2O h SYSTEM 23 2.3.1 Oxalic acid and oxalates system 23 2.3.1.1 Speciation of oxalic acid as a function of pH 23 2.3.1.2 Stability of oxalic acid and the oxalates 25 2.3.2 Iron oxalato complex 25 2.3.2.1 Equilibrium diagram of iron (III) oxalato complexes 27 2.3.2.2 Equilibrium diagrams of iron (II) oxalato complexes 33 2.4 PASSIVITY OF IRON IN OXALATE SOLUTIONS 36 2.5 SUMMARY 40 CHAPTER THREE THERMODYNAMIC ANALYSIS OF THE REACTIONS OF SODIUM, AMMONIUM AND IRON OXALATE COMPLEXES AND THEIR SOLUBILITIES 43 3.1 INTRODUCTION 43 3.2 EXPERIMENTAL 44 3.2.1 Materials 44 3.2.2 Procedures 44 3.3. RESULTS AND DISCUSSION 45 3.3.1 Speciation of oxalic acid 45 3.3.2 Formation of sodium, ammonium and iron oxalate complexes 46 3.3.3 Calculations of standard free energy, ∆Go 47 3.3.3.1 Sodium hydroxide – oxalic acid system 47 3.3.3.2 Ammonium hydroxide – oxalic acid system 49 i 3.3.3.3 Iron oxides (Hematite and Goethite)-oxalic acid system 50 3.3.4 Reactions of oxalic acid with the sodium and ammonium hydroxide 53 3.3.4.1 Effect of concentration 53 3.3.4.2 Effect of temperature 56 3.3.4.3 Properties of precipitated oxalate complexes 58 3.4 CONCLUSIONS 59 CHAPTER FOUR DISSOLUTION OF IRON OXIDE RUST MATERIALS 61 4.1 INTRODUCTION 61 4.2. EXPERIMENTAL 62 4.2.1 Materials and reagents 62 4.2.2 Experimental methods 63 4.3. RESULTS AND DISCUSSION 63 4.3.1 Effect of oxalic acid concentration 64 4.3.2 Effect of pH 67 4.3.3 Effect of temperature 69 4.3.4 Morphology study 71 4.3.5 Comparison of the dissolution reaction of hematite with magnetite ore 72 4.4.