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Improved Understanding of Rare Earth Surface Chemistry and Its Application to Froth Flotation

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Improved Understanding of Rare Earth Surface Chemistry and Its Application to Froth Flotation IMPROVED UNDERSTANDING OF RARE EARTH SURFACE CHEMISTRY AND ITS APPLICATION TO FROTH FLOTATION by Caelen D. Anderson i A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Metallurgical and Materials Engineering). Golden, Colorado Date ______________ Signed: ____________________________ Caelen D. Anderson Signed: ____________________________ Dr. Patrick Taylor Thesis Advisor Golden, Colorado Date ______________ Signed: ____________________________ Dr. Ivar Reimanis Department Head Department of Metallurgical and Materials Engineering ii ABSTRACT The United States contains a variety of domestic rare earth element mineral sources containing bastnasite, monazite, and xenotime. This research was performed to investigate the fundamental surface chemistry involved in the froth flotation of these minerals, and apply the knowledge gained to substantiate the results through experimental testwork on a domestically sourced rare earth ore. Electrokinetic studies were performed to establish the potential determining ions for all minerals. An extensive study on the effect of various activators (Co2+, W6+, Cr3+, Mo6+, Fe2+, Al3+, Cu2+, Pb2+, and Mn2+) was performed. Adsorption studies suggest that the mechanism of hydroxamate adsorption is chemisorption, as hydroxamate adsorption increased on all minerals with an increase in temperature. In addition, adsorption occurred at equilibrium pH values where the zeta potential was negative for all minerals. Fundamental thermodynamic calculations (Stern-Grahame free energy, enthalpy, and entropy of adsorption) were performed. Results indicate that the adsorption of hydroxamate is thermodynamically spontaneous in the temperature range tested, and endothermic in nature for all minerals tested. Microflotation studies were performed on a bastnasite ore, to determine the appropriate operating parameters for bench flotation. The flotability of the bastnasite ore increased with an increase in collector concentration. Bench flotation results illustrated that temperature has an effect on the grade and recovery of bastnasite ore when using hydroxamate as a collector. With the addition of the depressant lignin sulfonate at elevated temperature, it is possible to produce a rougher flotation concentrate containing 37% REO at 80% recovery. The dosage rates required to produce this concentrate are likely too high for industrial use. An economic analysis comparing the reagent costs of a typical rare earth flotation plant to the reagent costs used experimentally was performed to determine industrial feasibility. iii TABLE OF CONTENTS ABSTRACT .......................................................................................................................III TABLE OF CONTENTS .................................................................................................. IV LIST OF FIGURES ........................................................................................................ VIII LIST OF TABLES ........................................................................................................ XVII ACKNOWLEDGEMENTS .......................................................................................... XXV DEDICATION .............................................................................................................XXVI CHAPTER 1 INTRODUCTION ......................................................................................1 1.1 Background ............................................................................................................1 1.2 Motivation .............................................................................................................1 1.3 Approach ...............................................................................................................1 CHAPTER 2 LITERATURE REVIEW ...........................................................................3 2.1 The Rare Earth Elements .......................................................................................3 2.1.1 History of the Rare Earths ..........................................................................4 2.1.2 Properties of the Rare Earths ......................................................................6 2.1.3 Applications of Rare Earths ......................................................................10 2.2 Worldwide Rare Earth Reserves and Production ................................................11 2.2.1 Domestic Rare Earth Reserves and Production ........................................13 2.3 Rare Earth Prices .................................................................................................16 2.4 Rare Earth Minerals .............................................................................................16 2.4.1 Bastnasite ..................................................................................................18 2.4.2 Monazite ...................................................................................................20 2.4.3 Xenotime ...................................................................................................21 2.5 Rare Earth Gangue Minerals ...............................................................................21 2.5.1 Calcite .......................................................................................................23 2.5.2 Barite .........................................................................................................23 2.6 Rare Earth Flotation Review ...............................................................................25 2.6.1 Bastnasite Flotation ..................................................................................25 2.6.2 Monazite/Xenotime Flotation ...................................................................32 iv CHAPTER 3 FLOTATION SURFACE CHEMISTRY FUNDAMENTALS ...............35 3.1 Electrical Double Layer Model ...........................................................................35 3.2 Zeta Potential .......................................................................................................35 3.3 Adsorption Density ..............................................................................................37 3.4 Microflotation ......................................................................................................39 CHAPTER 4 EXPERIMENTAL MATERIALS AND METHODS ..............................41 4.1 Minerals ...............................................................................................................41 4.2 Bastnasite Ore ......................................................................................................41 4.3 Reagents ..............................................................................................................43 4.4 Zeta Potential .......................................................................................................43 4.5 Adsorption Density ..............................................................................................44 4.6 Microflotation ......................................................................................................46 4.7 Bench Flotation ...................................................................................................47 CHAPTER 5 ZETA POTENTIAL RESULTS AND ANALYSIS ................................48 5.1 Calcite ..................................................................................................................48 5.2 Barite 52 5.3 Bastnasite .............................................................................................................55 5.4 Monazite ..............................................................................................................59 5.5 Xenotime .............................................................................................................63 5.6 Bastnasite Ore ......................................................................................................66 5.7 Hydroxamate .......................................................................................................69 5.8 Potential Use of an Activator ..............................................................................69 5.8.1 Effect of Cobalt on Rare Earth/Gangue Minerals Zeta Potential .............71 CHAPTER 6 ADSORPTION PHENOMENA ...............................................................77 6.1 Adsorption Isotherms ..........................................................................................77 6.2 Physical versus Chemical Adsorption .................................................................78 6.3 Hydroxamate Adsorption ....................................................................................79 6.4 Kinetics of Adsorption ........................................................................................82 6.5 Calcite ..................................................................................................................82 6.6 Barite... ................................................................................................................85 6.7 Bastnasite .............................................................................................................87 6.8 Monazite ..............................................................................................................88 v 6.9 Xenotime .............................................................................................................90
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