Surface chemistry characteristics associated with bastnaesite flotation with Lauryl Phosphate as a collector

Bastnaesite is a major mineral resource of importance in the production of rare earth materials. Fatty acids and hydroxamate are typically reported as collectors for bastnaesite flotation. In this research, it was found that lauryl phosphate is an appropriate collector for bastnaesite flotation. Results from contact angle and microflotation experiments for bastnaesite are reported using lauryl phosphate as a collector. Almost 90% bastnaesite flotation recovery is achieved at a low level of 5×10-6 M lauryl phosphate at pH 5.1, when compared to the use of 1×10-4 M octyl hydroxamate at pH 9.3. Furthermore, wetting characteristics and microflotation responses were examined as a function of pH and at different levels of lauryl phosphate adsorption. The wetting characteristics of bastnaesite with the adsorbed collectors under a vertical monolayer of 8.46 μmol/m2 were examined using both contact angle measurements and molecular dynamics simulations (MDS). The adsorption isotherm at low levels of lauryl phosphate adsorption was established, and microcalorimetry results suggested chemical adsorption of lauryl phosphate on bastnaesite. Typically calcite and quartz are associated with bastnaesite as gangue minerals. Results from the contact angle, zeta potential, and microflotation experiments for bastnaesite, calcite, and quartz are reported using lauryl phosphate as a collector. With 90% bastnaesite flotation recovery and less than 5% calcite and quartz flotation recoveries were achieved at 1×10-5 M lauryl phosphate, and the results compared to the iv results of 1×10-4 M octyl hydroxamate. Finally, initial evaluation indicates that the branched chain 2-ethylhexyl phosphate increased the bastnaesite grade from 52% to 95% without sacrificing bastnaesite recovery at a concentration of 3×10-6 M and pH 5.0, when compared to lauryl phosphate. The zeta potential, single mineral microflotation, and molecular dynamics simulation results indicate that lauryl phosphate has stronger adsorption when compared with the 2-ethylhexyl phosphate results. The adsorption difference between bastnaesite and calcite/quartz for 2-ethylhexyl phosphate is greater than for lauryl phosphate. Therefore, it is expected that better selectivity can be achieved with 2-ethylhexyl phosphate. The results of this research will enable us to understand bastnaesite/calcite/quartz flotation chemistry using alkyl phosphate collectors with consideration of chemical structure, solution chemistry, and the hydrophobic surface state.

SURFACE CHEMISTRY CHARACTERISTICS ASSOCIATED WITH BASTNAESITE FLOTATION WITH LAURYL PHOSPHATE AS A COLLECTOR by Weiping Liu A dissertation submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Metallurgical Engineering The University of Utah May 2019 Copyright © Weiping Liu 2019 All Rights Reserved The University of Utah Graduate School STATEMENT OF DISSERTATION APPROVAL The dissertation of Weiping Liu has been approved by the following supervisory committee members: , Chair Jan D. Miller Jan. 4, 2019 Date Approved , Member Xuming Wang Jan. 4, 2019 Date Approved , Member Luther W. McDonald, IV Jan. 4, 2019 Date Approved , Member Michael L. Free Jan. 4, 2019 Date Approved , Member York R. Smith Jan. 4, 2019 Date Approved and by Michael Simpson the Department/College/School of , Chair/Dean of Metallurgical Engineering and by David B. Kieda, Dean of The Graduate School. ABSTRACT Bastnaesite is a major mineral resource of importance in the production of rare earth materials. Fatty acids and hydroxamate are typically reported as collectors for bastnaesite flotation. In this research, it was found that lauryl phosphate is an appropriate collector for bastnaesite flotation. Results from contact angle and microflotation experiments for bastnaesite are reported using lauryl phosphate as a collector. Almost 90% bastnaesite flotation recovery is achieved at a low level of 5×10-6 M lauryl phosphate at pH 5.1, when compared to the use of 1×10-4 M octyl hydroxamate at pH 9.3. Furthermore, wetting characteristics and microflotation responses were examined as a function of pH and at different levels of lauryl phosphate adsorption. The wetting characteristics of bastnaesite with the adsorbed collectors under a vertical monolayer of 8.46 µmol/m2 were examined using both contact angle measurements and molecular dynamics simulations (MDS). The adsorption isotherm at low levels of lauryl phosphate adsorption was established, and microcalorimetry results suggested chemical adsorption of lauryl phosphate on bastnaesite. Typically calcite and quartz are associated with bastnaesite as gangue minerals. Results from the contact angle, zeta potential, and microflotation experiments for bastnaesite, calcite, and quartz are reported using lauryl phosphate as a collector. With 90% bastnaesite flotation recovery and less than 5% calcite and quartz flotation recoveries were achieved at 1×10-5 M lauryl phosphate, and the results compared to the results of 1×10-4 M octyl hydroxamate. Finally, initial evaluation indicates that the branched chain 2-ethylhexyl phosphate increased the bastnaesite grade from 52% to 95% without sacrificing bastnaesite recovery at a concentration of 3×10-6 M and pH 5.0, when compared to lauryl phosphate. The zeta potential, single mineral microflotation, and molecular dynamics simulation results indicate that lauryl phosphate has stronger adsorption when compared with the 2-ethylhexyl phosphate results. The adsorption difference between bastnaesite and calcite/quartz for 2-ethylhexyl phosphate is greater than for lauryl phosphate. Therefore, it is expected that better selectivity can be achieved with 2-ethylhexyl phosphate. The results of this research will enable us to understand bastnaesite/calcite/quartz flotation chemistry using alkyl phosphate collectors with consideration of chemical structure, solution chemistry, and the hydrophobic surface state. iv TABLE OF CONTENTS ABSTRACT ....................................................................................................................... iii LIST OF TABLES ............................................................................................................ vii LIST OF FIGURES ........................................................................................................... ix ACKNOWLEDGMENTS ................................................................................................ xii Chapters 1. INTRODUCTION .......................................................................................................... 1 1.1 Bastnaesite flotation ............................................................................................. 1 1.2 Phosphorous-containing anionic collectors ......................................................... 2 1.2.1 Phosphate collector .................................................................................. 3 1.2.2 Phosphonate collector .............................................................................. 4 1.3 Physicochemical properties of phosphate collectors ........................................... 5 1.3.1 Distribution of collector species in water ................................................ 5 1.3.2 Critical micelle concentration (CMC) ..................................................... 6 1.4 Surface chemistry features of bastnaesite ............................................................ 6 1.4.1 Zeta potential of bastnaesite..................................................................... 6 1.4.2 Solution equilibrium of the bastnaesite-CO2-H2O ................................... 8 1.5 The interaction between the metal ions and phosphate groups............................ 8 1.6 Microflotation results ......................................................................................... 10 1.7 Research objectives and dissertation organization ............................................ 10 2. MATERIALS AND METHODS .................................................................................. 24 2.1 Materials and sample preparation ...................................................................... 24 2.2 Turbidity and pKa measurements ...................................................................... 24 2.3 Contact angle measurements.............................................................................. 25 2.4 Electrophoresis measurements ........................................................................... 26 2.5 Microflotation tests ............................................................................................ 26 2.6 Adsorption density determinations .................................................................... 27 2.7 Interaction energy calculations .......................................................................... 28 2.8 Molecular dynamics simulations (MDS) ........................................................... 29 2.9 Microcalorimetry ............................................................................................... 31 3. BASTNAESITE FLOTATION WITH LAURYL PHOSPHATE .............................. 38 3.1 Introduction ........................................................................................................ 38 3.2 Results ................................................................................................................ 39 3.2.1 Turbidity measurements......................................................................... 39 3.2.2 Distribution of species in solution ......................................................... 39 3.2.3 Contact angle and surface tension measurements .................................. 40 3.2.4 Zeta potential of bastnaesite................................................................... 41 3.2.5 Microflotation results ............................................................................. 42 3.3 Discussion .......................................................................................................... 43 3.4 Summary ............................................................................................................ 44 4. LAURYL PHOSPHATE ADSORPTION BY BASTNAESITE ................................ 54 4.1 Introduction ........................................................................................................ 54 4.2 Results and discussion ....................................................................................... 55 4.2.1 Interaction energy calculation ................................................................ 55 4.2.2 Lauryl phosphate adsorption isotherm ................................................... 57 4.2.3 Contact angle at low levels of lauryl phosphate .................................... 59 4.2.4 MDS wetting characteristics .................................................................. 60 4.3 Summary ............................................................................................................ 61 5. SELECTIVE FLOTATION OF BASTNAESITE WITH LAURYL PHOSPHATE .. 74 5.1 Introduction ........................................................................................................ 74 5.2 Results ................................................................................................................ 75 5.2.1 Captive bubble contact angle experiments ............................................ 75 5.2.2 Zeta potential experiments ..................................................................... 76 5.2.3 Microflotation experiments .................................................................... 77 5.2.4 Interaction energy calculations .............................................................. 78 5.3 Summary ............................................................................................................ 80 6. COMPARISON OF LAURYL PHOSPHATE WITH 2-ETHYLHEXYL PHOSPHATE ................................................................................................................... 97 6.1 Introduction ........................................................................................................ 97 6.2 Results ................................................................................................................ 98 6.2.1 Contact angle experiments ..................................................................... 98 6.2.2 Microflotation and zeta potential experiments .................................... 100 6.2.3 Microcalorimetry and MDS ................................................................. 102 6.3 Discussion ........................................................................................................ 107 6.4 Summary .......................................................................................................... 108 7. CONCLUSIONS......................................................................................................... 124 8. REFERENCES ........................................................................................................... 127 vi LIST OF TABLES Tables 1.1 Type of collector used in plant practice for the flotation of rare earth minerals. ....... 15 1.2 Names of common phosphorous organic compounds. 8 ............................................. 16 1.3 Summary of chemical properties for phosphonate and phosphate collectors in the flotation of bastnaesite (Ce,La)FCO3. ............................................................................... 17 1.4 Summary of pKa values of phosphoric acid and phosphate........................................ 18 1.5 Critical micelle concentration of alkyl phosphate. ..................................................... 19 1.6 IEP values and zeta potential for bastnaesite (Ce,La)FCO3. ...................................... 20 1.7 Speciation of Ce-bastnaesite in aqueous solution (0.1%wt; 10-3.5 atm CO2). 36b........ 21 1.8 Concentration of surface atoms before and after flotation. 9b ..................................... 22 1.9 Bastnaesite recovery using different collectors. ......................................................... 23 2.1 Number of atoms at the bastnaesite (100) surface for water drop contact angle measurements with lauryl phosphate adsorption. ............................................................. 35 2.2 Parameters for water interactions at the bastnaesite surface with absorbed lauryl phosphate. ......................................................................................................................... 36 2.3 Gaussian calculated charge parameters for lauryl phosphate. .................................... 37 4.1 Dominant lauryl phosphate species as a function of pH. ............................................ 69 4.2 UFF optimized structures [θopt (in degrees) and ropt (in angstrom units)] for lauryl phosphate adsorbed at the (100) bastnaesite surface. ....................................................... 70 4.3 UFF/ PM6 interaction energies (kJ/mol) for adsorption of lauryl phosphate species at the surface of bastnaesite. ................................................................................................. 71 4.4 Energy difference (eV) between frontier orbitals of bastnaesite, ROPO3H-, ROPO3H2, ROPO32-, and water. .......................................................................................................... 72 4.5 Contact angle results from MDS................................................................................. 73 5.1 UFF optimized structures [θopt (in degrees) and ropt (in angstroms)] for lauryl phosphate at the surface of bastnaesite, calcite, and quartz. ............................................. 95 5.2 UFF interaction energies (kJ/mol) of lauryl phosphate and octyl hydroxamate adsorption at bastnaesite, calcite, and quartz surfaces. ..................................................... 96 6.1 Captive bubble contact angle comparison between lauryl phosphate and 2-ethylhexyl phosphate ........................................................................................................................ 119 6.2 Zeta potential comparison between lauryl phosphate and 2-ethylhexyl phosphate, mV................................................................................................................................... 120 6.3 Microflotation recovery with lauryl phosphate......................................................... 121 6.4 Selectivity comparison between lauryl phosphate and 2-ethylhexyl phosphate for mixed mineral flotation. .................................................................................................. 122 6.5 Theoretical HOMO, LUMO, ΔE|HOMO-LUMO|, and ESP charge of the polar functional group for alkyl phosphate collectors. .............................................................................. 123 viii LIST OF FIGURES Figures 2.1 Schematic representation of the geometry of the adsorbed alkyl phosphate complex. ............................................................................................................................ 33 2.2 Simplified schematic diagram of microcalorimetry experiments. .............................. 34 3.1 The turbidity of potassium lauryl phosphate solution as a function of concentration. .................................................................................................................... 46 3.2 Distribution of potassium lauryl phosphate species, H2A, HA-, and A2- in pure water. ................................................................................................................................. 47 3.3 The pH value of potassium lauryl phosphate solution as a function of concentration. .................................................................................................................... 48 3.4 The intermediate contact angle of bastnaesite and surface tension as a function of pH for 1×10-4 M potassium lauryl phosphate solution; 22 ºC. ............................................... 49 3.5 The intermediate contact angle comparison between potassium lauryl phosphate and octyl hydroxamic acid at a bastnaesite surface. Also included is the surface tension of potassium lauryl phosphate as a function of concentration. ............................................. 50 3.6 Zeta potential of bastnaesite in10 mM KCl solution and in 10 mM KCl with 5×10-6 M potassium lauryl phosphate solution as a function of pH. ................................................ 51 3.7 Bastnaesite flotation with potassium lauryl phosphate collector as a function of pH...................................................................................................................................... 52 3.8 Microflotation recovery comparison between potassium lauryl phosphate solution and hydroxamic acid solution as a function of concentration. ................................................ 53 4.1 UFF optimized complex of lauryl phosphate species at the bastnaesite (100) surface. White: hydrogen; red: oxygen; cyan: carbon; yellow: cerium; tan: phosphorus; pink: fluorine. (A) ROPO3H2, (B) ROPO3H-, and (C) ROPO32-................................................ 63 4.2 Captive bubble contact angle and flotation results for bastnaesite with potassium lauryl phosphate collector as a function of pH. ................................................................ 64 4.3 Interaction energy comparison, (A) UFF interaction energy and (B) PM6 interaction energy of lauryl phosphate species on a bastnaesite (100) surface. .................................. 65 4.4 Lauryl phosphate isotherm at the bastnaesite surface (natural pH 4.7-6.5). ............... 66 4.5 Contact angle and flotation results for bastnaesite with potassium lauryl phosphate at natural pH 4.7-6.0. ............................................................................................................ 67 4.6 Visualization of the two-dimensional water density analysis for a water droplet after 1ns at the bastnaesite surface with different levels of lauryl phosphate coverage: (A) 8.3%, (B) 25%, and (C) 50%. ........................................................................................... 68 5.1 The contact angle of bastnaesite with potassium lauryl phosphate as a function of pH...................................................................................................................................... 83 5.2 The contact angle of calcite with potassium lauryl phosphate as a function of pH. ... 84 5.3 The contact angle of quartz with potassium lauryl phosphate as a function of pH. ... 85 5.4 Zeta potential of calcite in 10 mM KCl solution, with and without 5×10-6 mol/L potassium lauryl phosphate solution as a function of pH. ................................................ 86 5.5 Zeta potential of quartz in 10 mM KCl solution, with and without 5×10-6 mol/L potassium lauryl phosphate solution as a function of pH. ................................................ 87 5.6 Bastnaesite flotation with potassium lauryl phosphate as a function of pH. .............. 88 5.7 Calcite flotation with potassium lauryl phosphate as a function of pH. ..................... 89 5.8 Quartz flotation with potassium lauryl phosphate as a function of pH....................... 90 5.9 Structures of lauryl phosphate and octyl hydroxamate, White: hydrogen; red: oxygen; cyan: carbon; tan: phosphorus; blue: nitrogen. (A) lauryl phosphate, (B) octyl hydroxamate. ..................................................................................................................... 91 5.10 UFF optimized complex of lauryl phosphate on bastnaesite (100), calcite (104) and quartz (101) surface. White: hydrogen; red: oxygen; cyan: carbon; yellow: cerium; tan: phosphorus; pink: fluorine; ochre: silicon; green: calcium. (A) bastnaesite, (B) calcite and (C) quartz. ......................................................................................................................... 92 5.11 Flotation and interaction energy comparison using lauryl phosphate, (A) Flotation of bastnaesite, calcite, quartz as a function of potassium lauryl phosphate concentration. (B) UFF interaction energies of potassium lauryl phosphate at bastnaesite (100), calcite (104), and quartz (101) surfaces. ...................................................................................... 93 5.12 Flotation and interaction energy comparison using octyl hydroxamate, (A) Flotation of bastnaesite, calcite, quartz as a function of octyl hydroxamate concentration. (B) UFF x interaction energies of octyl hydroxamate at bastnaesite (100), calcite (104), and quartz (101) surfaces. ................................................................................................................... 94 6.1 Captive bubble contact angles of bastnaesite (A), calcite (B) and quartz (C) with 2ethylhexyl phosphate as a function of pH. ...................................................................... 110 6.2 Zeta potential of bastnaesite (A), calcite (B) and quartz (C) in 10 mM KCl solution, with and without 5×10−6 mol/L 2-ethylhexyl phosphate solution, as a function of pH.. 111 6.3 Single mineral microflotation results for bastnaesite, calcite, and quartz using 2ethylhexyl phosphate as a function of concentration (A) and pH (B). ........................... 112 6.4 Chemical structures of 2-ethylhexyl phosphate (A) and lauryl phosphate (B). ........ 113 6.5 The first four injections of 2-ethylhexyl phosphate (A) and lauryl phosphate (B) for microcalorimetry. ............................................................................................................ 114 6.6 The heat of adsorption for the interaction of 2-ethylhexyl phosphate and lauryl phosphate with bastnaesite (pH 5.0). .............................................................................. 115 6.7 Interfacial behavior of 2-ethylhexyl phosphate (A) and lauryl phosphate (B) at the bastnaesite surface. ......................................................................................................... 116 6.8 Relative density distribution of selected atoms along the normal to the bastnaesite basal plane surfaces. Water (A), 2-ethylhexyl phosphate (B) and lauryl phosphate (C). 117 6.9 Mean square displacement of water, (A) collector (B) in 2-ethylhexyl phosphate and lauryl phosphate solution on bastnaesite (100) surface. ................................................. 118 xi ACKNOWLEDGMENTS I would like to express my sincere appreciation to my advisors, Dr. Jan D. Miller, and Dr. Xuming Wang, for their mentorship, guidance, encouragement, and patience during my dissertation research, which are paramount and precious for my long term career goals. My appreciation is extended to my committee members: Dr. Luther McDonald, Dr. Michael Free, and Dr. York Smith, for their precious time reviewing my dissertation and providing valuable comments. Particularly, I want to thank Prof. Hui Xu and Dr. Zhixing Wang for their kindly help, and Dr. Luther McDonald for the support with the microcalorimetry experiments. Thanks are extended to Dr. Anita M. Orendt for assisting with the Gaussian calculation, Dr. Thomas E. Cheatham, III, Dr. Xia Zhang, Dr. Jiaqi Jin, and Daniel Roe for their assistance with Amber molecular dynamics simulations, Dr. Keith A. Prisbrey for assistance with the MOPAC 2012 calculations, and Ms. Dorrie Spurlock for her assistance in the preparation of the manuscript. Thanks are also extended to all the faculty and staff in the Department of Metallurgical Engineering at the University of Utah for providing me this great opportunity working with researchers from all over the world. I would also like to thank all my colleagues and friends for their support and encouragement during my dissertation research. I am grateful to the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, of the U.S. Department of Energy through Grant Number DE-FG03-93ER14315 for their financial support of this research project. Last, but not least, I would like to express my love and appreciation to my family and my wife for their support and encouragement all these years. xiii CHAPTER 1 INTRODUCTION Bastnaesite is one of the major mineral resources used in the production of rare earth elements. Present flotation practice typically uses fatty acid or hydroxamate as a collector. In addition, phosphorous-containing anionic collectors, including phosphate and phosphonate collectors, also have drawn the attention of researchers. However, there is no systematic research and discussion of bastnaesite flotation using phosphorouscontaining anionic collectors. In this regard, the nomenclature of phosphorous-containing anionic collectors has been discussed and summarized in order to clarify the different nomenclature systems used in the past. The distribution of collector species, critical micelle concentration, and zeta potential have been compared and discussed for improved understanding. Finally, at the end of the introduction, the nature of the reaction and flotation behavior have been reviewed. 1.1 Bastnaesite flotation The rare earth fluorine carbonate mineral bastnaesite ((Ce,La)CO3F) is a semisoluble salt mineral with ionic bonding and limited solubility in water, which distinguishes bastnaesite from soluble salt minerals. The mineral bastnaesite, consisting of the cerium subgroup or the lighter rare earth elements, 1 is found mainly at Mountain 2 2 Pass California in the United States and at Boyan Obo Mongolia in China. Calcite and/or quartz are the major gangue minerals recovered during bastnaesite flotation. 3 Fatty acids, together with soda ash and lignin sulfonate, are used as collectors and depressants at high temperatures in the flotation of the Mountain Pass ore. 4 Alkyl hydroxamic, phosphate, and fatty acids were used in the flotation of the Bayan Obo ore. 2b Collectors used in the plant operations for the flotation of rare earth semisoluble salt minerals are summarized in Table 1.1. 5 The problem in bastnaesite flotation using fatty acid as a collector is low selectivity. 4 Alkyl hydroxamate is a promising collector due to the stronger adsorption affinity to the rare earth cations when compared to the alkaline earth cations. 4, 6 In this regard, the octyl hydroxamate has better selectivity when compared to fatty acids in bastnaesite flotation at concentrations greater than 1×10-4 mol/l. 3-4, 7 However, the application of hydroxamate acid is limited by its cost. It is generally true that the more selective collectors are more costly to produce. In this regard, more efforts are necessary in the search to find an economical and selective collector. In addition to fatty acid and hydroxamate, phosphorous-containing anionic collectors, which include phosphate and phosphonate, are potential collectors in the flotation of rare earth minerals. 1.2 Phosphorous-containing anionic collectors Phosphorous ions in organophosphorus compounds exist mainly in two valence states (3 and 5). A systematic investigation of nomenclature problems by a joint AngloAmerican subcommittee is mentioned in the literature. 8 A number of phosphorus hydrides and acids were used as parent structures, which even included hypothetical 3 chemical structures. Table 1.2 lists the principal methods for naming common organophosphorus compounds. 8 Esters are named by prefixing the hydrocarbon radical name to the remainder of the name and replacing the ending -ic or -ous with -ate or -ite, respectively. For example, CH3P(O)(OC2H5)2 is called diethyl methyl phosphonate or diethyl methane phosphonate. The phosphorous-containing anionic collectors that have been utilized in the flotation of the rare earth minerals include phosphate and phosphonate. Alkyl phosphate has other common names, for example, mono-alkyl ester phosphoric acid (P538) phosphoric acid esters (Flotinor SM15). 10 9 and Meanwhile, phosphonate also has different names such as α-styryl phosphonic acid 11 and α-hydroxyl-benzyl phosphonic acid. 12 The issues of nomenclature and synthesis of the phosphorus-containing collectors can be found in the literature. 8, 13 1.2.1 Phosphate collector Phosphate has the general structure of P(=O)(OR)3 with a phosphorus valence state of 5. However, phosphates are esters of phosphoric acid instead of organophosphorus compounds, in the technical sense, due to lacking a P−C bond. The phosphorous atom with low electronegativity has a positive effect on the electron donor capacity of the oxygen atoms, thereby facilitating the complex formation between the oxygen atoms and metal cations. 6 The phosphate collectors have been used in the flotation of calcite, ilmenite and rutile, chromite, 15 wolframite, 16 magnesite, 17 14 perovskite, and smithsonite. 18 Trade names of some phosphate collectors are the following: (I)"Arlatone MAP" series of 4 monoalkyl phosphates (Uniqema), "Emcol CS/PS/TS" (Witco), "Empiphos TM" (Albright & Wilson), "Hostaphat F" series and "Flotinor SM15"(Clariant), and "Marlophor" series (Condea); (II) "Celanol PS," "Soprophor PA/PS" series and "Rhodafac PC-100" (all of Rhodia), "Crodafos N-3/10" (Croda), "Empiphos DF" series (Albright & Wilson), "Forlanit P," "Forlanon" and "Crafol AP 261" (all of Henkel), and "Phosfetal" series (Zschimmer & Schwarz). 13 Alkyl phosphate P538 has been used as a collector for the flotation of bastnaesite and monazite due to its good performance as an extraction agent for rare earth elements. The phosphoric acid based collector also played an important role in the flotation of rare earth minerals in the Nechalacho rare earth deposit. 19 Flotinor SM15 was found to chemically adsorb at the bastnaesite surface. 10 1.2.2 Phosphonate collector Phosphonate includes CPO(OH)2 or CPO(OR)2 groups (where R represents alkyl or aryl). The α-styryl phosphonate collector has shown good performance for the flotation of cassiterite, titanium ore, niobium-tantalum mineral, and wolframite. 12, 20 In this regard, α-styryl phosphonate was used in the flotation of Weishan bastnaesite in China. Organic phosphonate such as α-hydroxyl-benzyl phosphonate, styryl phosphonate, (α-hydroxy-1,3 dimethyl) butyl phosphonate, tolyl phosphonate, and benzyl phosphonate were also used as collectors for the flotation of Weishan rare earth ores. 12 Table 1.3 summarizes the chemical properties of phosphate and phosphonate collectors used in the flotation of bastnaesite. 5a, 9, 11-12, 21 5 1.3 Physicochemical properties of phosphate collectors 1.3.1 Distribution of collector species in water Phosphate collectors are expected to have better flotation performance when compared to the phosphonate collectors. 9, 12 In this regard, the characteristics of phosphate, such as species solution distribution and critical micelle concentration, are discussed in this section. The acid dissociation constant (pKa) measures the solution behavior of the acid and each acid has a different value of pKa. The pKa values of phosphoric acid and phosphate are presented in Table 1.4. Small variations of pKa values were observed among the phosphates, which might be due to the purity of samples, experimental procedures, etc. In order to understand the distribution of phosphate species in solution, the dissociation of ROPO3Na2 22 must be considered since ROPO3Na2, where R represents the alkyl hydrocarbon chain group, is originally a strong electrolyte. ROPO3Na2 is partially hydrolyzed in solution as shown in Equation (1.1). With an increase in the concentration or a decrease in pH, species II is involved in further hydrolysis as shown in Equation (1.2): ROPO32- + 2Na+ + H2O ⇄ ROPO(OH)O- + 2Na+ + OH(I) (1.1) (II) ROPO(OH)O- + H2O ⇄ ROPO(OH)2 + OH(II) (1.2) (III) Species III has higher hydrophobicity and lower solubility when compared to species I and II. In this regard, species III prefers to absorb at the air/water interface or form an aggregate in bulk solution. 23 6 1.3.2 Critical micelle concentration (CMC) Micellization is an important characteristic for the study of flotation chemistry. Micelles are formed by aggregation of collector ions into a colloidal size by Van der Waal's bonding between the hydrophobic hydrocarbon chains of the collectors. The critical micelle concentration (CMC) is defined as the concentration at which molecular aggregation occurs. These aggregates can be detected by light scattering and often experimental techniques. The system free energy decreases with the aggregate formation at a rate of 2.5 kJ per CH2 group per mole. 22, 24 The critical micelle concentrations for alkyl phosphates are listed in Table 1.5. The CMC values of the phosphates (ROP(O)(OM)2) remained the same values of 30×10-3 mol/l from 8 carbons to 12 carbons and then decreased to 2.9×10-3 mol/l at 20 carbons as shown in Table 1.5. 8b 25 26 27 Furthermore, it seems that the potassium and sodium ions have a limited effect on dodecyl phosphate with the CMC values of 63×10-3 mol/l and 57×10-3 mol/l, respectively. 25 1.4 Surface chemistry features of bastnaesite 1.4.1 Zeta potential of bastnaesite The surface charge of solid mineral particles in aqueous solution is determined by the ionization of the surface groups, preferential dissolution of the surface ions, specific ion adsorption toward the mineral surface, and the mineral lattice isomorphous substitution. A layer of counter ions is attracted to the charged mineral surface and then by the diffuse layer, which together are called the electrical double layer. The direct measurement of the surface charge is difficult. In this regard, the interface on the mineral 7 surface is defined as the slipping plane when the counterions on the interface are moving with the mineral particle in solution. Since the electrochemical potential decreases as a function of distance from the mineral surface, the distinct electrochemical potential occurring at the shear plane was the zeta potential of the mineral particle. 10, 22, 24 The isoelectric point (IEP), importance constant in the evaluation of the electrical double layer, 28 is the pH value at a zeta potential value of zero. In this regard, the IEP value for different minerals is critical in the understanding of the selective flotation of valuable mineral from gangue minerals. 29 As shown in Table 1.6, the IEP values for the bastnaesite from Mountain Pass vary from pH 4.6 to pH 9.3. 10, 30 31 The synthetic bastnaesite has an IEP value of 8.2, 32 while the bastnaesite from China, Vietnam, Brazil, Pakistan, and Madagascar have the IEP values of 4.5-8.0, 7, 9, 33 4.7, 34 4.9, 35 8.1, and 6.2, respectively. 10, 36 The deviation between the reported IEP values, especially the IEP values from Weishan, might be due to origins of the samples, experimental procedures, etc. The zeta potential of bastnaesite decreased significantly in the presence of anionic Flotinor SM15. In this regard, anionic Flotinor SM15 is thought to adsorb at the bastnaesite surface by chemisorption in alkaline pH and a combination of physisorption and chemisorption in acid pH. The bastnaesite from Weishan is positively charged below pH 4.5 and negatively charged above pH 4.5. Meanwhile, phosphate anions are the dominant species in the solution from pH 4.5, according to the pKa of P538. In this regard, P538 is at the bastnaesite surface by chemical adsorption due to its good flotation performance at pH 7-11. 9b 8 1.4.2 Solution equilibrium of the bastnaesite-CO2-H2O The distribution of species diagram for bastnaesite aqueous solution in a closed system was obtained based on theoretical computations of mineral-solution equilibration. 30b The speciation distribution diagram with consideration of CO2 was calculated by Stabcal software, 32b as shown in Table 1.7. The cerium fluoride and bastnaesite are the stable solid phase below pH 6.2 and above pH 6.6, respectively. There is a small area between pH 6.2 to pH 6.6, where cerium fluoride and bastnaesite coexist. 36b Bastnaesite is transformed into cerium fluoride and cerium hydroxide in acid and alkaline environments, respectively, according to Equations 1.3-1.4. HCO3- and CO32- species appear in the equilibration as shown in Equations 1.5 to 1.7, and thereby the pH range of the solid phase is extended or shrunk. For example, cerium hydroxide reacted with CO2 and thereby dissolved as carbonate anions in Equation (1.8). 3CeFCO3 + 3H+ ⇄ CeF3 + 2Ce3+ + 3HCO3- (1.3) CeFCO3 + 3OH- ⇄ Ce(OH)3 + F- + CO32- (1.4) 2CO2 + H2O ⇄ H2CO3 (1.5) H2CO3 ⇄ HCO3- + H+ (1.6) HCO3- ⇄ CO32- + H+ (1.7) 2Ce(OH)3 + 2CO2 + OH- ⇄ Ce(CO3)2- + 2H2O (1.8) 1.5 The interaction between the metal ions and phosphate groups The phosphate affinity toward cations, such as sodium and copper ions, was examined by surface pressure determinations and infrared spectroscopy. The copper ion, with higher polarizing power, has higher adsorption affinity toward the phosphate ion 9 compared to the sodium ion. The ionization and protolysis of the phosphate groups are the dominant factors influencing the adsorption affinity of phosphate group toward the sodium and copper, respectively. 37 The P538, P538-La, bastnaesite, and P538-bastnaesite were examined by infrared spectroscopy. 9b Compared with the infrared spectrum of P538, the peak of -P=O moved to the low wave value of 1150 cm-1 in the infrared spectrums for both P538-La and P538bastnaesite, which indicates that -P=O has a complexation reaction with a metal ion. Consequently, presumably, P538 chemisorbed at the bastnaesite surface through a chelating reaction. 9b The result of ESCA (Xray photoelectron spectroscopy) measurements of bastnaesite and P538-bastnaesite 9b in Table 1.8 show that La bonding energy increased 0.4 eV, and the concentration of C and P also increased at the same time, which means a chemical reaction existed between P538 and the rare earth ions, and P538 absorbed on the bastnaesite surface through chemical adsorption or a chemical reaction. 9b The behavior of monolayers of monophosphate on the surface of the solution with the rare earth cations was examined by pressure-area relationships and radiometrically analysis of the films. 38 The limited molecular area of the monoalkyl phosphate decreased in the presence of the rare earth ions. Some phosphate ester molecules may enter into the bulk solution from the surface film due to the formation of three dimensional complexes. In this regard, a sandwich layer with the metal ion in the middle and monophosphate on both sides was formed, thereby decreased the limited molecular area of the monophosphate. The compound of Eu[ROP(O)(OH)O]3 was formed based on the analysis result of the films, and only one acidic hydrogen is exchangeable at pH 2. The 10 compound of EuK[ROP(O)O2]2 was formed at pH 3.8. 1.6 Microflotation results The optimum recovery for the flotation of bastnaesite from Weishan rare earth ore with 6.10% grade was achieved using α-styryl phosphonate as a collector, which resulted in a 48.36% recovery of bastnaesite with a grade of 60.13%. 11 Organic phosphoric acids such as α-hydroxyl-benzyl phosphonate, styryl phosphonate, (α-hydroxy-1,3 dimethyl) butyl phosphonate, tolyl phosphonate, and benzyl phosphonate were also used as collectors for the flotation of Weishan rare earth ores, and their flotation performance declined in the same sequence. α-hydroxyl-benzyl phosphonate with a recovery of 70% at pH 7 12 is a promising collector compared to the four other collectors. Bastnaesite flotation using phosphorous-containing anionic collectors is given in Table 1.9. Bastnaesite flotation recovery reaches 70-80% at alkaline pH, which is better than the performance of phosphonic acid. Smith flotation cell 10 9 Previous results with a modified Partridge also indicate that SM15 (phosphate) is a promising collector for bastnaesite. In this regard, phosphate collectors were selected to be studied in detail as a bastnaesite collector in the present research. 1.7 Research objectives and dissertation organization The overall objective of this research has been to understand the surface chemistry issues associated with bastnaesite flotation using selected alkyl phosphate collectors, such as lauryl phosphate and 2-ethylhexyl phosphate for the first time. This understanding will help in the design and selection of collectors in order to improve 11 flotation grade and recovery in the processing of bastnaesite ores. Furthermore, such a phosphate collector may be important for more efficient rare earth recovery from domestic resources and thus contribute to a sustained rare earth supply for the United States. Specific research objectives included the following: 1. Evaluate surface chemistry features in the lauryl phosphate flotation of bastnaesite from preliminary experiments. 2. Establish the adsorption isotherm at low levels of lauryl phosphate adsorption. Furthermore, examine the relationship between hydrophobicity and adsorption density by MDS, and compare to the results with octyl hydroxamate at low collector concentrations. 3. Identify the selectivity of lauryl phosphate in the flotation of bastnaesite from calcite/quartz, including a comparison of lauryl phosphate selectivity to that obtained with octyl hydroxamate. 4. Understand the contribution of the phosphate chemical structure to the bastnaesite flotation response and hydrophobicity. 5. Determine the adsorption reaction occurring at the bastnaesite surface by microcalorimetry. The dissertation research is presented in seven parts. The materials and methods are introduced in Chapter 2. A preliminary evaluation of the surface chemistry features of potassium lauryl phosphate flotation of bastnaesite is reported in Chapter 3. The fundamental features of potassium lauryl phosphate adsorption at a bastnaesite surface are reported in Chapter 4. Bastnaesite flotation selectivity using a lauryl phosphate collector with respect to calcite/quartz is reported in Chapter 5. The effects of the phosphate chemical structure on the bastnaesite flotation response are reported in Chapter 12 6. The dissertation continues with Chapter 7, the Conclusions. In Chapter 3, it was found that certain alkyl phosphates are potential collectors for the flotation of bastnaesite. Results from contact angle, zeta potential, and microflotation experiments for bastnaesite indicate that better flotation is achieved at a lower concentration of alkyl phosphate of 5×10-6 M when compared to octyl hydroxamate as the collector. Initial evaluation indicates that alkyl phosphates should be promising collectors for bastnaesite flotation. These results, published in Minerals Engineering, 39 suggest that the alkyl phosphate collectors may be important to improve rare earth recovery from bastnaesite and might help sustain the supply of rare earth minerals, which are critical for US industries. In Chapter 4, wetting characteristics and microflotation responses of bastnaesite are examined as a function of pH and at different levels of lauryl phosphate adsorption. Theoretical computations for the bastnaesite-lauryl phosphate system were accomplished using the universal force field (UFF) and semiempirical quantum chemical methods. The interaction energy and frontier orbital results correlate remarkably well with the experimental contact angle and microflotation test results. The wetting characteristics of bastnaesite with adsorbed collector were examined using both contact angle measurements and molecular dynamics simulations (MDS). The adsorption isotherm at low levels of lauryl phosphate adsorption was established, together with corresponding contact angle measurements. Finally, the relationship between hydrophobicity and adsorption density are examined by MDS and compared to the results for octyl hydroxamate at low collector concentrations. The results, published in the Journal of Colloid and Interface Science, 40 improved the understanding of lauryl phosphate 13 adsorption at the bastnaesite surface and confirmed that lauryl phosphate might be a better collector for bastnaesite when compared with octyl hydroxamate at low concentration (≤5×10-5 M). In Chapter 5, it was expected that potassium lauryl phosphate would provide a stronger bastnaesite flotation response when compared to the flotation of calcite and quartz gangue minerals. Results from captive bubble contact angle, zeta potential, and microflotation experiments for bastnaesite, calcite, and quartz are reported using potassium lauryl phosphate as the collector. Better selectivity for bastnaesite was achieved using potassium lauryl phosphate when compared with the selectivity using octyl hydroxamate as the collector. These results indicate that potassium lauryl phosphate might be a promising collector in the flotation of bastnaesite ores. Finally, the computation results of UFF interaction energies for the interactions between mineral and reagent are reported and were found to correlate remarkably well with the experimental microflotation results. The results, published in Minerals & Metallurgical Processing, 41 improve the understanding of bastnaesite flotation selectivity using lauryl phosphate collector with respect to calcite/quartz. In Chapter 6, microflotation results with mineral mixtures indicate that 2ethylhexyl phosphate has even better selectivity compared with lauryl phosphate. Zeta potential and microcalorimetry, single mineral microflotation, and molecular dynamics simulations (MDS) were used to examine the effect of phosphate chemical structure on hydrophobicity and the flotation response. The zeta potential, single mineral microflotation, and microcalorimetry results indicate that lauryl phosphate has stronger adsorption on minerals when compared with the results for 2-ethylhexyl phosphate. 14 Molecular dynamics simulations further revealed that more water is accommodated at the bastnaesite surface with 2-ethylhexyl phosphate when compared with lauryl phosphate. However, the adsorption difference between bastnaesite and calcite/quartz for 2ethylhexyl phosphate is greater than the case for lauryl phosphate. Therefore, it is expected that better selectivity can be achieved with 2-ethylhexyl phosphate. These results, published in Minerals Engineering, 42 enable us to understand bastnaesite/calcite/quartz flotation chemistry using alkyl phosphate collectors by consideration of chemical structure, which includes the hydrophobic surface state, the adsorption mechanism, the criteria for the phosphate collector design, etc. 15 Table 1.1 Type of collector used in plant practice for the flotation of rare earth minerals. Rare earth mineral Bastnaesite ((Ce, La)CO3F) or monazite ((Ce,La,Y,Th)PO4) Bastnaesite ((Ce, La)CO3F) Bastnaesite ((Ce, La)CO3F) Bastnaesite ((Ce, La)CO3F) Bastnaesite ((Ce, La)CO3F) Type of collector Example Condition Reference Alkyl carboxylic acid Fatty acid, Oleate, tall oil pH 6-9 5 Hydroxamic acid Aromatic carboxylic acid Hydroxamate H205 L108 DH pH 8-8.5 5 O-phthalic acid pH 5 5 pH 5-6 5 Aromatic amide Combination reagent A mixture of fatty acid and hydroxamate 5 16 Table 1.2 Names of common phosphorous organic compounds. 8 Adapted from O'Brien, R. D., Toxic Phosphorus Esters: Chemistry, Metabolism, and Biological Effects. Academic Press: London, 1960; Wasow, G. W., Phosphoruscontaining anionic surfactants. In Anionic Surfactants: Organic Chemistry, Stache, H. W., Ed. CRC Press: New York, 1995; Vol. 56, p 553. Formula Trivalent acids and derivative P(OH)3 RO-P(OH)2 (RO)2P-OH (RO)3P Pentavalent acids and derivative (HO)3P=O Recommended name Other common names Phosphorous acid Alkyl dihydrogen phosphite Dialkyl hydrogen phosphite Trialkyl phosphite Orthophosphorous acid Alkyl phosphite Dialkyl phosphite Trialkoxy phosphite Phosphoric acid Orthophosphoric acid Alkyl phosphoric acid ester Alkyl phosphoric ester Alkyl phosphate Dialkyl phosphate (HO)2P(O)OR Alkyl dihydrogen phosphate HO-P(O)(OR)2 (RO)3P=O H-P(O)(OH)2 Dialkyl hydrogen phosphate Trialkyl phosphate Phosphonic acid Alkyl dihydrogen phosphonate R-P(O)(OH)2 Alkyl phosphonic acid Alkanephosphonic acid 17 Table 1.3 Summary of chemical properties for phosphonate and phosphate collectors in the flotation of bastnaesite (Ce,La)FCO3. Surfactant P538 (Alkyl dihydrogen phosphate) α-styryl dihydrogen phosphonate Tolyl dihydrogen phosphonate α-hydroxylbenzyl dihydrogen phosphonate Benzyl dihydrogen phosphonate (α-hydroxy-1,3 dimethyl) bytyl dihydrogen phosphonate Formula Structure pKa1 pKa2 Reference C12-18H26-38PO4 3.80 9.23 5a, 9, 21 C8H9PO3 2 7.10 11-12 C9H12PO3 12 C7H9PO4 12 C7H9PO3 12 C6H13PO3 12 18 Table 1.4 Summary of pKa values of phosphoric acid and phosphate. Formula pKa1 pKa2 H3PO4 2.1 7.2 *ROPO3Na2 2.5 7.01 C12H25-OPO3Na2 2.8 7.2 *R is the abbreviation for an alkyl chain, such as CH3(CH2)n. Reference 43 22 23 19 Table 1.5 Critical micelle concentration of alkyl phosphate. Number of Carbon atoms 8 8 8 10 10 10 12 12 12 12 12 13 16 20 R M Cation n-Octyl 2-Ethylhexyl Dimethylhexyl n-Decyl Dimethyloctyl Trimethylheptyl Trimethylnonyl Dodecyl Dodecyl Dodecyl Dodecyl Tridecyl Hexadecyl Eicosyl Na Na Na Na Na Na Na Na K Na K Na Na Na CMC×10-3 mol/l ROP(O)(OM)2 Reference 30 8b 8b 30 30 8b 8b 8b 30 30 57 63 40 8b 8b 25 25 26 27 30 8 2.9 8b 8b 8b 20 Table 1.6 IEP values and zeta potential for bastnaesite (Ce,La)FCO3. Bastnaesite Zagi Mountain, Pakistan Synthesis Maoniuping, China Weishan, China Mountain Pass, California Purity of samples Condition IEP - Pure water 8.1 36b 100% pure 96.5% pure - Pure water 32 Pure water 8.2 7.8 8.0 4.5 - Pure water 4.6 30a 9.3 31 4.9 35 6.2 4.7 10 Pure water With or without NaNO3 1×10-3M KCl solution Pure water 0.08 g/l Flotinor SM15; pH 3-10 Mountain Pass, California 57.4% REO Pocos de Caldas, MG, Brazil Madagascar Vietnam 45.3% RE2O3 - Madagascar - Mountain Pass, California - Pure water - 0.08 g/l Flotinor SM15; pH 3-10 Mountain Pass, California Zeta potential(mV) Reference 7 33 9 34 -65～-55 10 10 6.3 -40～-35 10 21 Table 1.7 Speciation of Ce-bastnaesite in aqueous solution (0.1%wt; 10-3.5 atm CO2). 36b Adapted from Zhang, X. Surface Chemistry Aspects of Flourite and Bastnaesite Flotation Systems. Ph.D. Thesis, University of Utah, Salt lake city, UT, USA, 2014. pH range <6.2 6.2-6.6 >6.6 Solid phases present CeF3 CeFCO3,CeF3 CeFCO3 22 Table 1.8 Concentration of surface atoms before and after flotation. 9b Adapted from Zhou, G.; Luo, J., Mechanism of flotation using mono-alkyl ester phosphoric acid for bastnaesite. J. Chin. Rare Earths Soc. 1990, 8 (3), 261-264. Sample Bastnaesite P538-Bastnaesite C 34.82 52.00 La 2.86 1.97 Ce 3.40 2.02 O 53.51 38.52 F 5.40 2.89 P 2.60 La+Ce 6.26 3.99 23 Table 1.9 Bastnaesite recovery using different collectors. Condition Optimum pH Recovery efficiency (%) Reference 10 mg/l P538; MIBC 10 mg/l 7-11 70-80 9 2 kg/ton SM15 7 95 10 2.5 kg/ton α-styryl phosphonic acid 5.6-7 41.67 50.50 12 3.5 kg/ton Tolyl phosphonic acid; 7 43.23 37.29 12 1.5 kg/ton α-hydroxyl-benzyl phosphonic acid 7 55.55 66.35 12 2.5 kg/ton Benzyl phosphonic acid; 9.5 34.39 44.05 12 3.0 kg/ton (α-hydroxy-1,3 dimethyl) bytyl phosphonic acid 7 37.77 53.69 12 Grade (%) CHAPTER 2 MATERIALS AND METHODS 2.1 Materials and sample preparation Cola®Fax PME (Potassium lauryl phosphate, C12H26O4PK), 2-ethylhexyl phosphate (C8H19O4P), and octyl hydroxamate (CH3(CH2)6CONHOH) were provided by Colonial Chemical Incorporated Company (South Pittsburg, Tennessee, USA), Sigma Aldrich Company (Missouri, USA), and Cytec (Woodland Park, New Jersey, USA), respectively. Bastnaesite (Zagi Mountains, Pakistan), calcite, and quartz (University of Utah) were used for contact angle, zeta potential, and flotation experiments. Acetone, methanol, and Diwater were used to clean the Glassware. The Diwater from a Milli-Q system (Billerica, Massachusetts, USA), having a resistivity of larger than 18 MΩ, was used in all experiments. 2.2 Turbidity and pKa measurements In order to assure that no precipitation occurred within the potassium lauryl phosphate solution, the turbidity of aqueous solutions of potassium lauryl phosphate was measured as a function of concentration using a DR/850 Portable Colorimeter (Hach Company, Loveland, Colorado, USA). The instrument was adjusted to measure turbidity by using the "program 95" of the instrument. The meter was zeroed with 10 ml of 25 deionized water (blank), and the turbidity of collector solutions read directly from the instrument. The turbidity of the deionized water is 0 FAU (Formazin Attenuation Unit). If the turbidity of the collector solution is 0 FAU (Formazin Attenuation Unit), the collector solution should be transparent. If precipitation occurs, then the turbidity value will increase. More applications and details of turbidity measurement can be found in the literature. 44 The pKa was measured by the potentiometric titration method using a pH electrode to monitor the course of titration. The pKa value was calculated from the change in shape of the titration curve compared with that of a blank titration without lauryl phosphate present. Excellent introductions to the method for determination of pKa, can be found in the literature. 45 2.3 Contact angle measurements For contact angle measurements, the mineral surface was polished and cleaned by rinsing with acetone, methanol, and copious amounts of deionized (DI) water, followed by blow drying with high purity nitrogen. The samples were then treated with plasma and again dried with high purity nitrogen gas. Contact angles were measured with a RameHart goniometer (Rame-Hart, Succasunna, NJ, USA) using the captive bubble technique. The measurement of an intermediate contact angle was accomplished by the release of an air bubble from the needle tip after formation with a syringe, the bubble was then captured beneath the mineral surface, followed by film rupture and bubble attachment. The equilibrium contact angle was measured for all cases of attachment. For each specific measurement, at least five bubbles were generated and measured at different locations on 26 the surface. The reported values are the average values from these measurements. The maximum experimental variation in contact angle measurements was found to be ±1°. 46 2.4 Electrophoresis measurements Zeta potential is the electric potential in the interfacial double layer at the slipping plane and can be affected by pH and solution composition. 47 It is widely used for quantifying of the magnitude of the surface charge. In the present study, the mineral sample was dry ground to -45 μm. Before measurement, mineral suspensions of 0.1% were prepared and centrifuged for 10 min. Zeta potentials of bastnaesite (Zagi Mountain, Pakistan) were measured using a ZetaPALS instrument, Brockhaven Instruments Corporation (Holtsville, NY, USA), based on the Doppler Effect combined with a phase shift of the reflected light. Mineral particle mobilities were measured at different pH values and then converted to zeta potentials (ξ) using Smoluchowski’s equation as follows: 𝑈= 𝜀ξ 4𝜋η 𝐸∞ (2.1) where U is the particle mobility, E∞ is the applied electric field, and ε and η are the dielectric constant and viscosity of the solvent. 2.5 Microflotation tests The microflotation tests were performed using a 112 ml column cell with a porous sintered glass bottom for gas dispersion and a magnetic stirrer. In each test, the mineral sample (100×200 mesh) of 1 g was added to the collector solution and then conditioned for 5 min by magnetic stirring. After that, the sample, together with a solution, was 27 transferred to the flotation cell. The flotation tests were conducted for 2 min using nitrogen at a flow rate of 50 ml/min. Each microflotation test was repeated three times, and the mean value of recovery was reported. The maximum experimental variation in microflotation tests was found to be ±1%. 2.6 Adsorption density determinations Bastnaesite samples were dry ground to −45 μm for the adsorption isotherm measurements. The Brunauer–Emmett–Teller (BET) adsorption method was used to evaluate the surface area of the ground bastnaesite sample, using nitrogen gas with a Micromeritics ASAP 2020 analyzer (University of Utah, Salt Lake City, Utah, USA). 36a The surface area of the ground bastnaesite was 0.76 m2/g. The total organic carbon analyzer (model: TOC-VCPH/CPN) from Shimadzu Corporation (Kyoto, Japan) was used to measure the adsorption density of lauryl phosphate by the bastnaesite surface. The adsorption experiment was done at natural pH 4.7-6.5, where maximum flotation recovery was found. 39 The following concentrations of potassium lauryl phosphate were prepared for the adsorption experiment, 5×10-6 M, 1×10-5 M, 2×10-5 M, 3×10-5 M, 5×10-5 M, and 1×10-4 M. With a solid to liquid ratio of 1:100, bastnaesite powder samples of 0.5 g (particle size −45 μm) were mixed with 50 ml of potassium lauryl phosphate solution. Each time a blank test was also made using ultrapure water. Then the solutions were shaken simultaneously for 4 h at a speed of 200 rpm on a Barnstead-Lab Line Max Q2000 Orbital Shaker (University of Utah, Salt Lake City, Utah, USA). After filtration, the residual potassium lauryl phosphate was measured. The concentration of potassium lauryl phosphate solution before adsorption was also measured. After that, the adsorption 28 density was calculated by solution depletion according to the amount of adsorbed potassium lauryl phosphate and the surface area of bastnaesite. 2.7 Interaction energy calculations The universal force field 48 from Avogadro 49 and the semiempirical quantum chemistry program, MOPAC2012 (Molecular Orbital Package) at the PM6 level 50 were used to model the interactions between minerals and collectors. The collector structures were obtained from geometry optimization using the Gaussian 09 program 51 at the HF/6-31G(D) level for the following theoretical computations. The lattice parameters from the American Mineralogist Crystal Structure Database were used for building the structures of bastnaesite, 52 calcite, 53 and quartz. 54 The mineral surface was created from the unit cell of the mineral at a given Miller plane (usually the cleavage plane), such as the bastnaesite cleavage (100) surface, 36a, 55 calcite cleavage (104) surface, 56 and quartz (101) surface. 57 The collector was placed on the bastnaesite surfaces using the visual molecular dynamics (VMD) molecular graphics tool. 58 The UFF 48 and the semiempirical quantum chemistry program, MOPAC 2012 (Molecular Orbital Package) level PM6, 50c were used to optimize the mineral-collector complex by varying r and θ, as shown in Figure 2.1, where r indicates the shortest distance between the collector molecule and the mineral surface, and θ indicates the angle between the alkyl chain in the collector molecule and the mineral surface plane. Thus, the optimized configuration with the optimum distance ropt and optimum angle θopt were obtained. 55 The interaction energy (△E) of mineral-lauryl phosphate, for both the UFF and 29 PM6 methods, was computed using the following equation: △E=Ecomplex - (Emineral surface + Ecollector) Ecomplex, Emineral surface, (2.2) and Ecollector are the energies of the optimized mineral- collector complex, mineral, and collector (lauryl phosphate), separately. It should be noticed that smaller values of the interaction energy (△E) indicate stronger interactions between the mineral surface and the collector. 2.8 Molecular dynamics simulations (MDS) The MDS program Amber 14 59 was used for the simulation. The total energy in MDS is based on the evaluation of the appropriate energy terms for the interaction between every atom in the system. As shown in Eq. (2.3), this total energy in MDS includes the Coulombic (electrostatic) energy, the short range energy (van der Waals energy term), the bond stretch energy, and the angle bend energy. 60 𝐸𝑡𝑜𝑡𝑎𝑙 = 𝐸𝐶𝑜𝑢𝑙𝑜𝑚𝑏𝑖𝑐 + 𝐸𝑉𝐷𝑊 + 𝐸𝑏𝑜𝑛𝑑 𝑠𝑡𝑟𝑒𝑡𝑐ℎ + 𝐸𝑎𝑛𝑔𝑙𝑒 𝑏𝑒𝑛𝑑 (2.3) Many molecular water models have been developed. From those models, the rigid extended simple point charge (SPC/E) water model was chosen. Configurationally this model has the closest average energy to the experimental value (-41.6 kJ/mol). 61 The SPC/E water model was used for measuring the simulated contact angles in this study. 62 As shown in Eq. (2.4), the Coulombic energy is inversely proportional to the distance of separation 𝑟𝑖𝑗 . The terms 𝑞𝑖 and 𝑞𝑗 are partial charges for atoms 𝑖 and 𝑗. The term 𝑒 and 𝜀0 are the charge of an electron and the dielectric permittivity of a vacuum (8.85419 × 10-12 F/m), respectively. 𝐸𝐶𝑜𝑢𝑙𝑜𝑚𝑏𝑖𝑐 = 𝑒2 4𝜋𝜀0 ∑𝑖≠𝑗 𝑞𝑖 𝑞𝑗 𝑟𝑖𝑗 (2.4) 30 The van der Waals energy term is represented by the conventional Lennard Jones (12-6) function in Eq. (2.5). 63 The terms εij and rm,ij are the depth of the potential well and the distance of the minimum potential, respectively. 𝑟𝑚,𝑖𝑗 𝐸𝑉𝐷𝑊 = ∑𝑖≠𝑗 𝜀𝑖𝑗 [( 𝑟𝑖𝑗 12 ) − 2( 𝑟𝑚,𝑖𝑗 𝑟𝑖𝑗 6 ) ] (2.5) Concerning the unlike atoms, the distance parameter rm,ij, and the energy parameter εij were calculated by the arithmetic mean rule and the geometric mean rule, respectively. 48 1 𝑟𝑚,𝑖𝑗 = 2 (𝑟𝑚,𝑖 + 𝑟𝑚,𝑗 ) (2.6) 𝜀𝑖𝑗 = √𝜀𝑖 𝜀𝑗 (2.7) As for the simulation, a simple cubic cell was constructed. This cubic cell included water molecules using the extended simple point charge (SPC/E) model, 64 and bastnaesite based on the lattice parameters provided by the American Mineralogist Crystal Structure Database. 52 The bastnaesite cleavage (100) surface was used for simulation, 36b, 55 and the periodic boundary condition was introduced. Tables 2.1 and 2.2 list the number of atoms in MDS and the intermolecular potential parameters. Table 2.3 shows the charge parameters for the lauryl phosphate collector. The charges and force field parameters for lauryl phosphate molecules were calculated using the Gaussian 09 program 51 at the HF/6-31G(D) level. Three levels of lauryl phosphate coverage, 8.3%, 25%, and 50%, were investigated. A comparison between lauryl phosphate and 2ethylhexyl phosphate at the same concentration by molecular dynamics simulation was also reported. An NVT [moles (N), volume (V), and temperature (T) are conserved] ensemble 31 using Hoover's thermostat was introduced in this simulation. 65 The Leap frog method with a time step of 2 fs (femtoseconds) was used to integrate the particle motion. The Ewald sum was used to represent the electrostatic interactions. A final simulation time of 1 ns (nanosecond) (5 × 105 steps) was performed after a 500 ps (picosecond) equilibration period. The diffusion coefficient (D) was derived using the following equation. 42 1 𝑁𝑎 ⟨[𝑟𝑖 (𝑡) − 𝑟𝑖 (0)]2 ⟩ 𝐷 = 6𝑁 lim ∑𝑖=1 𝑎 (2.8) 𝑡→∞ where Na is the number of diffusive atoms in the simulation cell; ri(0) and ri(t) are the mass center positions of the solutes at the time of origin ri(0) and time t ri(t), respectively. 2.9 Microcalorimetry The heat of adsorption was measured by a TAM III isothermal titration microcalorimeter (TA Instruments, Salt Lake City, Utah, USA) at 25 °C using the heat flow model. The results appear as power peaks as a function of time, and each peak represents the injection of collector solution. The collector adsorption heat at the mineral surface was calculated by the subtraction of the dilution heat in the blank experiment from the total heat in the standard experiment as shown in Figure 2.2. The bastnaesite sample with 0.0062 ±0.0005 g (-45 um) was added to the sample and reference ampoules in the standard experiment. Water (0.7 Ml, pH 5.0) was added to the sample and reference ampoules in both standard and blank experiments after 30 s of sonication. The collector solution at pH 5.0 was added into the titration syringe. A volume of 8.8 μL (8.8×10-9 mol), equivalent to 25% pseudomonolayer coverage, was injected into 0.7 ml of water at pH 5 every 60 mins using the titration syringe. The heat of adsorption corresponding to 50% pseudomonolayer coverage is reported. 66 Collector dosages were 32 calculated according to the bastnaesite surface area and the crosssectional area of 19.63 A2 for the phosphate headgroup. 40 33 Figure 2.1 Schematic representation of the geometry of the adsorbed alkyl phosphate complex. 34 Figure 2.2 Simplified schematic diagram of microcalorimetry experiments. 35 Table 2.1 Number of atoms at the bastnaesite (100) surface for water drop contact angle measurements with lauryl phosphate adsorption. Species Cerium Fluorine Carbon Oxygen 8.3% lauryl phosphate coverage 25% lauryl phosphate coverage 50% lauryl phosphate coverage Number of atoms 5100 5100 5100 15300 36 molecules 106 molecules 212 molecules 36 Table 2.2 Parameters for water interactions at the bastnaesite surface with absorbed lauryl phosphate. Species Cerium Fluorine Bastnaesite carbon Bastnaesite oxygen Lauryl phosphate carbon Lauryl phosphate oxygen Lauryl phosphate phosphorous Lauryl phosphate hydrogen Water oxygen Water hydrogen Charge [e] 3 -1 0.883 -0.961 -0.8476 0.4238 ε[kcal/mol] 0.007 0.1673 0.0403 0.1554 0.1094 0.2104 0.2000 0.0157 0.1554 0 r [Å] 4.4470 3.5279 3.4879 3.5536 3.8160 3.442 4.2000 2.2112 3.1659 0 Reference 32b 67 68 64 Calculated from Gaussian 64 64 37 Table 2.3 Gaussian calculated charge parameters for lauryl phosphate. Atom name C C C C C C C C C C C C H H H H H H H H H H Charge 0.298849 0.009229 -0.100366 0.012063 0.045456 -0.071550 0.007257 0.098564 -0.157994 -0.032325 0.188546 -0.366161 0.425456 -0.011516 -0.004198 0.000221 0.020706 0.014384 0.013003 -0.013175 0.000258 0.005655 Atom name H H H H H H H H H H H H H H H H O O O O P Charge -0.000320 0.010489 0.002642 -0.009370 0.004752 -0.027899 -0.007144 0.033687 0.037460 0.015216 0.014054 -0.020431 -0.015202 0.080477 0.079537 0.082171 -0.853735 -0.804467 -0.735303 -0.583075 1.314099 CHAPTER 3 BASTNAESITE FLOTATION WITH LAURYL PHOSPHATE  3.1 Introduction The typical collectors for bastnaesite flotation are hydroxamate and fatty acid. Fatty acid is cheap but limited by its low selectivity. In contrast, hydroxamate has high selectivity but is limited by its cost. Such selective collectors are generally more expensive to produce than traditional fatty acid collectors; the fatty acid is $0.70-1.00/lb compared with $4.00-6.00/lb for octyl hydroxamic acid. Consequently, research efforts are appropriate to identify economical and selective collectors for the flotation of rare earth minerals, especially bastnaesite. In addition to fatty acid and hydroxamic acid, phosphorous-containing anionic collectors for bastnaesite include phosphonate collectors 11-12 and phosphate collectors. 9-10, 16, 69 Some researchers reported that alkyl phosphates have better performance as collectors for bastnaesite than alkyl phosphonates. 9 However, the fundamental surface chemistry associated with alkyl phosphate adsorption at the bastnaesite surface is still unknown. Therefore, a preliminary evaluation of the surface chemistry features of potassium lauryl phosphate in the flotation of bastnaesite has been conducted. The results for microflotation of bastnaesite with potassium lauryl phosphate and octyl hydroxamic acid are compared. The discussion  Weiping Liu, Xuming Wang, Zhixing Wang, J.D. Miller. Flotation Chemistry Features in Bastnaesite Flotation with Potassium Lauryl Phosphate [J]. Minerals Engineering, 2016, 85, 17–22. 39 includes wetting characteristics, surface tension results, microflotation results, and the results from electrophoretic measurements. 3.2 Results 3.2.1 Turbidity measurements The turbidity of potassium lauryl phosphate solution as a function of concentration is shown in Figure 3.1. It is evident that the collector is soluble at concentrations of less than 1×10-4 M. Flotation experiments were conducted below this concentration. 3.2.2 Distribution of species in solution The acid dissociation constant (pKa) is a quantitative measure of the strength of an acid in solution. Each acid has a different pKa, knowledge of which is essential for understanding solution behavior. Further understanding of the acid dissociation constant (pKa) with respect to solution pH and its method of determination can be found in the literature.45b Potassium lauryl phosphate (C12H26O4PK) is a salt comprised of one potassium ion and hydrogen ion, and the acid monoalkyl phosphate (denoted as ROPO32-, where R corresponds to C12H25). When C12H26O4PK is dissolved in pure water, there are two equations involved; the measured pKa values from this current research are 2.85 and 7.35. ROPO(OH)2 + OH-⇄ROPO(OH)O- + H2O pK1= 2.85 (3.1) ROPO(OH)O- + OH- ⇄ROPO32- + H2O pK2= 7.35 (3.2) The ROPO(OH)2 species is more hydrophobic and lower in solubility than the 40 ROPO(OH)O- and ROPO32- species, suggesting that this species may be quickly adsorbed at the air/water interface and can form an aggregate in solution more easily. 23 The distribution of potassium lauryl phosphate (C12H26O4PK) species in solution is shown in Figure 3.2, where the ROPO32- species is designated by A. It is evident that HA- is the dominant species within the pH range from 4 to 6. The first dissociation of the acid form of lauryl phosphate, according to Eq. (3.1), begins at a solution pH of about 1 and is almost complete at pH 5. As the concentration is raised to a higher pH, the second dissociation according to Eq. (3.2) begins at a pH of about 5 and is almost complete at a pH of 10. 3.2.3 Contact angle and surface tension measurements From Figure 3.3, it can be concluded that the measured pH value of the potassium lauryl phosphate solution rises steadily in the acidic region from pH 4.7 to pH 6.5 as the concentration increased from 1×10-7 M to 1×10-4 M. It is evident from Figure 3.4 that when the pH is greater than pH 6.5, the contact angle of bastnaesite is reduced, which indicates that the bastnaesite surface changes from a hydrophobic surface state to a hydrophilic surface state as the pH is increased. Since the pK1 of lauryl phosphate acid is 2.85, the formation of molecular aggregates is expected in the acidic pH region, but less so in the alkaline pH region. Therefore, the contact angle of bastnaesite might be expected to be larger in the acid region than in the alkaline region. Further research is needed to examine surface spectroscopic features as a function of pH. An intermediate contact angle of about 80°for bastnaesite in 1×10-4 M potassium lauryl phosphate was found from pH 4.0 - 6.5, and the surface tension fluctuated between 41 40-50 dynes/cm as a function of pH, which showed that pH did not affect the surface tension of the potassium lauryl phosphate solution. We know that at a potassium lauryl phosphate concentration of less than 1×10-4 M the natural pH was less than pH 7, as shown in Figure 3.3. Therefore the natural pH was selected for subsequent experiments. In Figure 3.5, a contact angle comparison between potassium lauryl phosphate at pH 4.7-6.5 and octyl hydroxamic acid 36a at pH 9.3 is presented. The surface tension of potassium lauryl phosphate is also shown as a function of concentration. The surface tension of potassium lauryl phosphate solution decreased from 72 dynes/cm at 1×10-7 M to about 47 dynes/cm at 1×10-4 M, which was the same trend as reported for the surface tension of SM15 (an ester of phosphoric acid) as a function of concentration. 16 The results also indicate a substantial reduction in the air/water interfacial tension for solutions having a higher concentration (≥1×10-5 M) of potassium lauryl phosphate. This could be one of the contributing factors for improved contact angle results obtained at higher concentrations since a favorable air/water interfacial tension is required for good flotation. 16 The intermediate contact angle increased from a mere 33° to >60° when the potassium lauryl phosphate concentration was ≥1×10-5 M. Furthermore, and most importantly, it is evident that the potassium lauryl phosphate has a high contact angle of more than 60°at a low concentration when compared to octyl hydroxamic acid. 3.2.4 Zeta potential of bastnaesite Zeta potential is the electric potential for the interfacial double layer at the location of the slipping plane relative to a point in the bulk solution away from the 42 interface. 10, 24b The pH value where the zeta potential is zero, the isoelectric point (IEP), is an important mineral property that can be used to characterize charging of the mineral surface. 28 The IEP values for bastnaesite reported in the literature vary from pH 4.6 to pH 9.25, 7, 10, 30a, 31-32, 33-36, 70 which might be due to different levels of purity, experimental procedures, solution, etc. As shown in Figure 3.6, the IEP value of bastnaesite in this study was found to be pH 9.0, and the addition of 5×10-6 M potassium lauryl phosphate reduced the IEP value to pH 3.7. This fact suggests that the lauryl phosphate anion is strongly adsorbed at the bastnaesite surface which accounts for the charge reversal and the more negative zeta potential, even at the low lauryl phosphate concentration of 5×10-6 M. 3.2.5 Microflotation results Excellent flotation was achieved at low potassium lauryl phosphate concentrations in the acidic pH region, see Figure 3.7. These results were in accord with the contact angle results presented in Figure 3.4. The dominant collector species from pH 4-6 is the protonated lauryl phosphate anion (HA-), which suggests that HA- is the effective species reacting with the bastnaesite surface. Further research to determine the adsorption characteristics and to examine surface spectroscopic features needed. The microflotation results for bastnaesite with both potassium lauryl phosphate and octyl hydroxamic acid are presented in Figure 3.8. The data illustrate important results showing that excellent flotation of bastnaesite is achieved at low lauryl phosphate concentration (5×10-6 M), when compared to octyl hydroxamic acid. The results clearly 43 indicate that flotation with octyl hydroxamate at pH 9.3 requires ten times the concentration needed for lauryl phosphate flotation in accordance with the captive bubble contact angle results in the literature. 32b 3.3 Discussion Limited results from fundamental research are reported in the literature. For example, analysis of the infrared spectra indicates that adsorption of the di-2-EHPA (di2-ethyl-hexyl phosphonic acid) acid dimmer (1235 cm-1) occurs at pH 4.0 20a on the surface of cassiterite. In addition, a molecular form of the alkyl phosphate collector precipitated as aggregates at the wolframite surface and might be the surface-active species responsible for flotation. 16 Further research is needed to establish the adsorption state of lauryl phosphate at the surface of bastnaesite. The flotation results for bastnaesite with hydroxamic acid do not agree exactly with previous Hallimond tube flotation tests for bastnaesite 31 in which case complete bastnaesite recovery was only possible at a higher octyl hydroxamate concentration (＞4×10-4 M). The difference between these results may be due to the apparatus used for the microflotation tests, the octyl hydroxamate, and/or the bastnaesite sample. The potassium lauryl phosphate collector provided a better flotation response when compared with octyl hydroxamate, especially at low concentration (＜5×10-5 M). The flotation results for bastnaesite with potassium lauryl phosphate agree very well with previous results on bastnaesite flotation, 10 which results also indicate that SM15 (an ester of phosphoric acid) is a strong collector for bastnaesite. However, it should be noted that octyl hydroxamate and lauryl phosphate have a different carbon chain. Hydroxamate with 44 the same carbon chain may or may not have better performance than lauryl phosphate. A further consideration is needed in future research. The heat of adsorption measures the strength of the bond formation between the collector and the mineral surface. The wetting heat reflects the adsorption strength between the collector and the minerals. The higher absolute value of heat indicates stronger adsorption affinity of the collector on the mineral surfaces. 71 An absolute exothermic enthalpy heat value of 17.19 kJ/mol was observed between lauryl phosphate and bastnaesite, and the entropy change was positive based on the molecular dynamics simulation examination. In this regard, the adsorption free energy change is negative for the lauryl phosphate adsorption on the bastnaesite surface, and chemical adsorption should occur. 42 A detailed explanation can be found in Chapter 6. As demonstrated in the literature, octyl hydroxamate chemisorbs at the bastnaesite surface due to the stability of the rare earth hydroxamate salts such as Ce and La hydroxamate. In this regard, it is expected that the good flotation of bastnaesite at low levels of lauryl phosphate concentration reflects an even stronger chemisorption potential, and preliminary thermodynamic considerations confirm this expectation. The high stability of Ce, La lauryl phosphate, and the corresponding chemisorption reaction at the bastnaesite surface seem to account for flotation at such low levels of collector addition. 3.4 Summary Potassium lauryl phosphate has been used as a collector for bastnaesite for the first time. Experimental results from captive bubble contact angle measurements and 45 microflotation experiments showed that potassium lauryl phosphate is a promising collector for bastnaesite with complete recovery for a collector addition of 5×10-6 M at pH 5.1. These results were compared to results for octyl hydroxamate collector as reported in the literature and suggest that potassium lauryl phosphate may be better for bastnaesite flotation. Further research is in progress. Conclusions at this time include the following points: 1. Captive bubble contact angle measurements of more than 60°for bastnaesite at pH 4.7-6.5 with potassium lauryl phosphate at a low concentration (≤5×10-5 M) suggest that potassium lauryl phosphate might be a better collector for bastnaesite than octyl hydroxamate. Under these conditions, the lauryl phosphate anion is protonated, and the bastnaesite is positively charged at pH 4.7-6.5 before collector adsorption. 2. Results from microflotation experiments with a high quality bastnaesite sample confirmed the expectation established from contact angle measurements. Complete flotation recovery of bastnaesite was achieved with a potassium lauryl phosphate concentration of 5×10-6 M at pH 5.1. 3. Electrophoretic mobility measurements suggest strong adsorption of lauryl phosphate at the bastnaesite surface with the zeta potential at pH 6 changing from about +40 mv to -50mv with the addition of 5×10-6 M lauryl phosphate. 46 4 Natural pH 4.7-6.5 ; 22 ℃ Turbidity (FAU) 3 2 1 0 1×10-5 1×10-4 1×10-3 Potassium Lauryl Phosphate Concentration (mol/l) Figure 3.1 The turbidity of potassium lauryl phosphate solution as a function of concentration. 47 Distribution of collector species 1.0 0.8 H2A HA- A2- 0.6 0.4 0.2 0.0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH Figure 3.2 Distribution of potassium lauryl phosphate species, H2A, HA-, and A2- in pure water. pH value 48 6.6 6.4 6.2 6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 22 ℃ 1×10-7 1×10-6 1×10-5 1×10-4 Potassium lauryl phosphate concentration (mol/l) Figure 3.3 The pH value of potassium lauryl phosphate solution as a function of concentration. 49 Surface tension (Dynes/cm) 60 80 50 60 40 30 40 20 20 10 0 2 Intermediate contact angle Surface tension 3 4 5 6 7 8 9 10 Intermediate contact angle 100 70 0 11 pH Figure 3.4 The intermediate contact angle of bastnaesite and surface tension as a function of pH for 1×10-4 M potassium lauryl phosphate solution; 22 ºC. 50 100 80 60 70 Potassium lauryl phosphate; Natural pH 4.7-6.5 Octyl hydroxamic acid; pH 9.3 Surface tension of potassium lauryl phosphate; Natural pH 4.7-6.5 65 60 40 55 20 50 0 1×10-7 1×10-6 1×10-5 Collector concentration (mol/l) Surface tension (Dynes/cm) Intermediate contact angle (°) 75 45 1×10-4 Figure 3.5 The intermediate contact angle comparison between potassium lauryl phosphate and octyl hydroxamic acid at a bastnaesite surface. Also included is the surface tension of potassium lauryl phosphate as a function of concentration. 51 80 Bastnaesite-10mM KCl Bastnaesite-10mM KCl, -5×10-6 M potassium lauryl phosphate Zeta potention (mv) 60 40 20 0 -20 -40 -60 2 4 6 8 10 12 pH Figure 3.6 Zeta potential of bastnaesite in10 mM KCl solution and in 10 mM KCl with 5×10-6 M potassium lauryl phosphate solution as a function of pH. 52 100 Flotation recovery (%) 90 80 70 60 50 40 30 1×10-5 M potassium lauryl phosphate 5×10-6 M potassium lauryl phosphate 20 10 2 4 6 8 10 12 pH Figure 3.7 Bastnaesite flotation with potassium lauryl phosphate collector as a function of pH. 53 100 Flotation recovery (%) 90 80 70 60 50 40 30 20 10 0 1×10-6 Potassium lauryl phosphate; Natural pH; 22℃ Octyl hydroxamic acid; pH 9.3 1×10-5 Collector concentration (mol/l) 1×10-4 Figure 3.8 Microflotation recovery comparison between potassium lauryl phosphate solution and hydroxamic acid solution as a function of concentration. CHAPTER 4 LAURYL PHOSPHATE ADSORPTION BY BASTNAESITE  4.1 Introduction Potassium lauryl phosphate might be a better collector in the flotation of bastnaesite when compared to octyl hydroxamate at a low concentration (≤5×10-5 M). 39 However, the fundamental features of potassium lauryl phosphate adsorption are still unknown and are the subject of this chapter. Potassium lauryl phosphate (C 12 H26 O4 PK) is a weak acid salt. Monoalkyl phosphate is denoted as ROPO32- in which R corresponds to C12H25. The distribution of the potassium lauryl phosphate species as a function of pH is summarized in Table 4.1. 39 In order to have a fundamental understanding of the competitive adsorption behavior of lauryl phosphate and water at the bastnaesite surface, the semiempirical quantum chemical method was used to calculate the frontier orbital for water and for the bastnaesite surface. Also, the interaction energies between different phosphate species with the bastnaesite surface were calculated using the universal force field (UFF) and semiempirical quantum chemical methods, thereby giving an improved understanding of the fundamental aspects of potassium lauryl phosphate adsorption as a function of pH. The results were compared to the contact angle and microflotation results.  Weiping Liu, Xuming Wang, Hui Xu, J.D. Miller. Lauryl Phosphate Adsorption in the Flotation of Bastnaesite, (Ce,La)FCO3 [J]. Journal of Colloid and Interface Science, 2017, 490, 825-833. 55 Good flotation of bastnaesite with potassium lauryl phosphate has been shown to occur at a low concentration (≤5×10-5 M). 39 In this regard, the adsorption of potassium lauryl phosphate at low concentrations was studied, and the results are discussed, together with data reported in the literature. Contact angle measurements and MDS were used to investigate the hydrophobic surface state of bastnaesite at different levels of lauryl phosphate adsorption below monolayer coverage. The relationship between the contact angle and the adsorption density was also established. It is intended that these results will improve the fundamental understanding of the hydrophobic surface state associated with bastnaesite flotation chemistry. The effective collector species also can be examined in more detail based on UFF and semiempirical quantum chemical interaction energy calculations. 4.2 Results and discussion 4.2.1 Interaction energy calculation As shown in Table 4.2, the UFF optimized distances (ropt) and optimized angles (θopt) for lauryl phosphate absorbed at the (100) bastnaesite surface are listed. It should be further noticed that in Table 4.2, r is defined as the distance of phosphorus (P in the – PO4– functional group) from a reference cerium atom in the lauryl phosphate molecule at the bastnaesite surface. Thus, the optimized distances (ropt) presented in Table 4.2 appear to be higher than the optimized distances in Figure 4.1. The UFF optimized structures of the bastnaesite-lauryl phosphate complexes are shown in Figure 4.1(A), 1(B), and 1(C) for ROPO3H2, ROPO3H-, and ROPO32-, separately. As shown in Figure 4.1, the O–Ce distance between the phosphate functional 56 group and surface Ce atoms is around 3A˚, and ROPO3H- has the shortest distance (3.04Å) compared with the two other collector species. The interaction energies computed by the PM6 method were of the same order as the UFF interaction energies, and the results are summarized together with the UFF method results in Table 4.3. Based on the relative values of the corresponding interaction energies between the phosphate species and bastnaesite, it is suggested that the order of the bastnaesite flotation response with the lauryl phosphate species should be ROPO3H>ROPO3H2>ROPO32-. The frontier orbital of bastnaesite, the phosphate species, and water were compared using MOPAC2012 at the accuracy level of PM6. The reactivity is calculated by comparing the difference between bastnaesite HOMO (the highest occupied molecular orbital)/LUMO(the lowest unoccupied molecular orbital) and the phosphate species/water LUMO/HOMO. The smaller value of the absolute difference between these frontier orbitals indicates stronger reactivity of the phosphate species/water toward the bastnaesite surface. As shown in Table 4.4, these results indicate that ROPO3H- has greater reactivity with bastnaesite (the smallest difference of 2.79 eV) when compared to the others. Water, however, has less reactivity than all the phosphate species, which indicates that the phosphate species can replace water at the bastnaesite surface, and in this way, a hydrophobic surface state is formed. Captive bubble contact angle and microflotation results of bastnaesite with three collector species ROPO3H2, ROPO3H-, and ROPO32- are presented in Figure 4.2. ROPO3H- responds most favorably, and ROPO32- the least favorably. The order of the captive bubble contact angles and flotation responses for bastnaesite among the three 57 collector species were thus observed to be ROPO3H->ROPO3H2>ROPO32-. It should be mentioned that ROPO3H2 is shown to have better reactivity with bastnaesite than ROPO32-, according to the interaction energy results of ROPO3H2 and ROPO32-from UFF (-787 and -786 kJ/mol), and especially from the PM6 level (15 and 188 kJ/mol). However, these results are not in accord with the frontier orbital results in Table 4.4, which suggest that ROPO32- has better reactivity (difference of 3.02 eV) than ROPO3H2 (difference of 6.14 eV). The reason is still unknown; however, it is still significant given that the interaction energy results have such good agreement with the contact angle and microflotation results in Figure 4.2 and Figure 4.3. The frontier orbital results in Table 4.4 also confirmed that the lauryl phosphate species can replace water on the bastnaesite surface. The interaction energies (computed by both UFF and PM6) of the lauryl phosphate species on a bastnaesite (100) surface are plotted in Figures 4.3(A) and 4.3(B), respectively. The relative order of interaction energy of the lauryl phosphate species as predicted from theoretical consideration agrees well with the experimental contact angle and microflotation results (Figure 4.2). Also, the results confirm the use of UFF and semiempirical quantum analyses as advanced tools to explain surface chemistry experimental results, and even to predict the potentially reactive collector species for flotation, which has been observed by other researchers. 55, 72 4.2.2 Lauryl phosphate adsorption isotherm As shown in Figure 4.4, the vertical monolayer adsorption density of 8.46 μmol/m2 occurs at a potassium lauryl phosphate concentration of about 2.5 ×10−5 M, 58 which is based on the phosphate group crosssectional area of 19.63 Å2. Below this concentration, the bastnaesite surface is not fully covered by phosphate molecules. Considering the natural hydrophobicity of bastnaesite, 36a the uncovered mineral surface can interact with water molecules and this accounts for the moderate hydrophobicity under vertical monolayer concentration. The contact angle is expected to increase with increased lauryl phosphate adsorption and less uncovered mineral surface. Below monolayer coverage, the adsorption of lauryl phosphate at the bastnaesite surface is expected to be chemisorption. IR spectra, ESCA (XPS), 9 and zeta potential data 10 suggest that chemisorption occurs between the phosphate collector and the bastnaesite surface. The bastnaesite surface under vertical monolayer concentration still maintains its hydrophobic state after washing with acetone, alcohol, and DI water. Direct evidence for chemisorption has not yet been established. The behavior of monolayers of monophosphates in solutions containing rare earth cations has been studied through pressure-area relationships and radiometric analysis of films. 38 Monoalkyl phosphate appeared to have a remarkably limited molecular area in the presence of rare earth ions. This phenomenon suggested that some esters were pulled out of the surface to form a "sandwich layer" with the metal ion in the middle of the solution. Dimerized quarter-potassium salt also formed under certain conditions. Therefore, it is expected that one cerium ion (or La3+) can interact with as many as three phosphate molecules based on the reaction of phosphate with rare earth cations. However, the adsorption is limited by the phosphate crosssectional area, together with the different coexisting phosphate species; not every cerium atom at the surface can accommodate three phosphate molecules. 59 It should be noted that pH 4.7-6.5 is the region in which ROPO3H- is the dominant species. 39 The cerium surface site density is calculated to be 4.74 μmol/m2. 73 Thus, an adsorption density of about 8.46 μmol/m2 for a vertical monolayer of lauryl phosphate corresponds to a ratio of about 2 lauryl phosphate molecules for each cerium at the bastnaesite surface. Above the monolayer coverage, multilayer physisorption of lauryl phosphate occurs at the bastnaesite surface. The physisorption may happen through the hydrophobic interaction of hydrocarbon chains or hydrogen bonding as the driving force. From bastnaesite speciation distribution diagrams, 36b aqueous Ce3+, CeF2+, CeF2+ and CeCO3+ are available in the system at pH 4.7-6.5. An excess of lauryl phosphate is present in the high potassium lauryl phosphate concentration solution, and thus forms cerium (or other rare earth cations) lauryl phosphate complexes. Consequently, surface precipitation of anionic lauryl phosphate species, as well as cerium lauryl phosphate, may occur with nucleation and growth of multilayers of the cerium lauryl phosphate. 4.2.3 Contact angle at low levels of lauryl phosphate Contact angle measurements were made on the bastnaesite surface with adsorbed lauryl phosphate at low levels (below vertical monolayer coverage) by the sessile drop method. The results are shown together with the microflotation results in Figure 4.5. It can be noted that the contact angle and flotation results increase with an increase in potassium lauryl phosphate concentration, which indicates stronger hydrophobicity of the bastnaesite surface. Also, the contact angle and flotation results increase with an increase of the adsorption density from Figure 4.5, as expected. Based on the sessile drop contact angle, microflotation, and adsorption density results, it is expected that full monolayer 60 coverage is achieved at a lauryl phosphate concentration of about 2.5×10-5 M (contact angle = 50°, adsorption density for monolayer coverage = 8.46 μmol/m2). Above this point, multilayer adsorption occurs. 4.2.4 MDS wetting characteristics In order to compare with octyl hydroxamate acid at the bastnaesite surface with the same adsorption coverage, 36b one water drop on the three levels of lauryl phosphate coverage at the bastnaesite surface, 8.3%, 25%, and 50%, were examined for MD contact angle simulations. Figure 4.6 shows the two-dimensional water density distribution maps of the bastnaesite surface with different levels of lauryl phosphate coverage (lauryl phosphate molecules are transparent). The contact angles (θ) were determined to be 41°, 43°, 45°at the lauryl phosphate coverages of 8.3%, 25%, 50%, respectively. From Table 4.5, it can be seen that the 8.3%, 25% and 50% lauryl phosphate coverages correspond to adsorption densities of 1.05, 2.11, 4.23 μmol/m2, respectively. These results are in good agreement with the experimental sessile drop contact angle values in Figure 4.5. Perhaps unexpectedly, it is noticed that the MDS contact angle in Table 4.5 and the sessile drop contact angle in Figure 4.5 increased slightly after the concentration was increased beyond 2×10-6 mol/l (8.3% coverage). This indicates the substantial hydrophobicity of potassium lauryl phosphate at low concentration/adsorption coverage. It is somewhat surprising that such a strong flotation response was observed for a contact angle of less than 50°. Compared with the MDS results of octyl hydroxamate acid at the same adsorption coverage (8.3% coverage), 36a which is 0°, the MDS result of 41° at 61 8.3% coverage of lauryl phosphate in this study supports the suggestion that potassium lauryl phosphate might be a better collector for bastnaesite than octyl hydroxamate at low concentration (≤5×10-5 M). 39 4.3 Summary Captive bubble contact angle measurements, microflotation results, and theoretical computations indicated that the C12H25OPO3H- species of lauryl phosphate may adsorb and be responsible for the bastnaesite hydrophobic surface state. Subsequently, the adsorption density of potassium lauryl phosphate at low concentration was established, and surface chemistry characteristics were analyzed by experimental sessile drop contact angle measurements, microflotation, and MDS contact angle measurements. The correlation between the experimental results and molecular dynamics simulated contact angles, together with the molecular dynamics simulated contact angle comparison with the octyl hydroxamate acid results reported in Zhang et al. (2014), 36a suggest that potassium lauryl phosphate might be a better collector for bastnaesite when compared with octyl hydroxamate at low concentration (≤5×10-5 M). Conclusions at this time include the following points: 1. Interaction energy results from both UFF and semiempirical quantum chemical methods agree with the order of contact angle measurements and microflotation results for bastnaesite with the three collector species, which were observed to be C12H25OPO3H>C12H25OPO3H2>C12H25OPO32-. The frontier orbital absolute difference of the bastnaesite, phosphate species and water, confirm that the phosphate species can replace the water on the bastnaesite surface. The results also confirm that the UFF and 62 semiempirical quantum chemical methods are advanced tools that can explain and even predict potential reagents, which can be used as collectors in flotation. 2. Adsorption density measurements of lauryl phosphate have been done using high sensitivity TOC analysis, which shows that the adsorption density increases with increased lauryl phosphate concentration. Monolayer adsorption occurs at about 2.5×10− 5 M of potassium lauryl phosphate, considering the phosphate crosssectional area to define the monolayer condition. Below monolayer coverage, the chemisorption of lauryl phosphate is expected to occur at the bastnaesite surface. 3. Sessile drop contact angle measurements and microflotation results increase with an increase in lauryl phosphate concentration/adsorption density. It is expected that full monolayer coverage is achieved at 8.46μmol/m2, with a sessile drop contact angle of 50°and flotation recovery of 80%. 4. Contact angle results from MDS with different levels of lauryl phosphate coverage show that the bastnaesite surface becomes hydrophobic even at a low lauryl phosphate concentration of 2×10-6 M (8.3% coverage). These results are compared with an MDS contact angle of 0°for octyl hydroxamate at a coverage of 8.3% as reported in the literature. 36a The MDS contact angle result of 41° at 8.3% coverage of lauryl phosphate explains the improved performance of potassium lauryl phosphate as a collector for bastnaesite when compared to octyl hydroxamate at a low concentration (≤5×10-5 M). 39 63 3.14 (A) 3.04 3.14 4 (B) 3.08 (C) 3.14 4 Figure 4.1 UFF optimized complex of lauryl phosphate species at the bastnaesite (100) surface. White: hydrogen; red: oxygen; cyan: carbon; yellow: cerium; tan: phosphorus; pink: fluorine. (A) ROPO3H2, (B) ROPO3H-, and (C) ROPO32-. 100 100 80 80 60 60 ROPO3H2 - ROPO3 ROPO3H 2- 40 40 20 20 1×10-4 M Potassium lauryl phosphate 1×10-5 M Potassium lauryl phosphate 0 0 2 4 pH 6 8 Captive bubble contact angle Flotation recovery(%) 64 0 10 Figure 4.2 Captive bubble contact angle and flotation results for bastnaesite with potassium lauryl phosphate collector as a function of pH. 65 UFF interaction energy (kcal/mol) 0 -500 -785.882 -787.206 -1000 -1500 -1373.05 ROPO3H2 ROPO3H- ROPO23 PM6 interaction energy (kcal/mol) (a) 188.246 200 100 14.524 0 -100 -200 -300 -400 -500 -514.996 -600 ROPO3H2 ROPO3H- ROPO23 (B) Figure 4.3 Interaction energy comparison, (A) UFF interaction energy and (B) PM6 interaction energy of lauryl phosphate species on a bastnaesite (100) surface. 66 Adsorption density, μmol/m2 30 25 20 15 10 vertical monolayer, 8.46 5 0 3×10-6 1×10-5 3×10-5 1×10-4 Potassium lauryl phosphate concentration, M Figure 4.4 Lauryl phosphate isotherm at the bastnaesite surface (natural pH 4.7-6.5). 67 100 Flotation recovery (%) Sessile drop contact angle (°) 80 80 60 60 40 40 20 0 Sessile drop contact angle (°) Flotation recovery (%) 100 20 1×10 -6 2×10 -6 5×10 -6 1×10 -5 -5 2×10 3×10 -5 Potassium lauryl phosphate concentration (M) Figure 4.5 Contact angle and flotation results for bastnaesite with potassium lauryl phosphate at natural pH 4.7-6.0. 68 θ θ θ (A) (B) (C) Figure 4.6 Visualization of the two-dimensional water density analysis for a water droplet after 1ns at the bastnaesite surface with different levels of lauryl phosphate coverage: (A) 8.3%, (B) 25%, and (C) 50%. 69 Table 4.1 Dominant lauryl phosphate species as a function of pH. pH range <2.85 2.85-7.35 >7.35 Dominant species ROPO3H2 ROPO3HROPO32- 70 Table 4.2 UFF optimized structures [θopt (in degrees) and ropt (in angstrom units)] for lauryl phosphate adsorbed at the (100) bastnaesite surface. Method UFF ROPO3H2 θopt 109.49 ropt 3.16 ROPO3Hθopt 101.57 ropt 3.17 ROPO32θopt 100.56 ropt 3.22 71 Table 4.3 UFF/ PM6 interaction energies (kJ/mol) for adsorption of lauryl phosphate species at the surface of bastnaesite. Method UFF PM6 ROPO3H2 -787 15 ROPO3H-1373 -515 ROPO32-786 188 72 Table 4.4 Energy difference (eV) between frontier orbitals of bastnaesite, ROPO3H-, ROPO3H2, ROPO32-, and water. Bastnaesite ROPO3H2 ROPO3HROPO32Water HOMO -5.52 -10.92 -6.11 -0.30 -11.90 LUMO -3.32 0.62 5.41 6.42 4.07 Difference 6.14 2.79 3.02 8.58 73 Table 4.5 Contact angle results from MDS. Surface coverage (%) 8.3 25 50 Concentration (mol/l) 2×10-6 6×10-6 1.2×10-5 Adsorption density (μmol/m2) 1.05 2.11 4.23 MDS contact angle (θ) 41° 43° 45° CHAPTER 5 SELECTIVE FLOTATION OF BASTNAESITE WITH LAURYL PHOSPHATE  5.1 Introduction An initial evaluation of the surface chemistry features for the potassium lauryl phosphate/bastnaesite system 39 and the adsorption features of lauryl phosphate on bastnaesite 40 were reported recently. The results suggest that potassium lauryl phosphate might be a better collector for bastnaesite when compared with octyl hydroxamate at a low concentration of less than 5×10-5 M. However, bastnaesite flotation selectivity using a lauryl phosphate collector has not been reported concerning calcite/quartz. Most existing literature 10, 74 on bastnaesite flotation focuses on selectivity concerning the calcite gangue mineral. However, quartz is also a common gangue mineral in many rare earth deposits being developed, which includes the Thor Lake deposit in Canada. 2a, 10, 19 Therefore, this chapter reports on investigations of the interactions of lauryl phosphate with bastnaesite, calcite, and quartz, and are based on contact angle measurements, microflotation experiments, zeta potential measurements, and theoretical molecular modeling calculations using the universal force field (UFF). Also, comparison and discussion regarding the use of potassium lauryl phosphate and  Weiping Liu, Xuming Wang, Hui Xu, J.D. Miller. Physical Chemistry Considerations in the Selective Flotation of Bastnaesite with Lauryl Phosphate [J]. Minerals & Metallurgical Processing Journal, 2017, 34 (3), 116-124. 75 octyl hydroxamate are presented, which include microflotation results and the results of theoretical calculations. It is expected that the results of this current research will improve the fundamental understanding of bastnaesite/calcite/quartz flotation chemistry, which include the hydrophobic state of the mineral surface, the principle of lauryl phosphate adsorption, etc. Also, the comparison between lauryl phosphate and octyl hydroxamate is examined in more detail with molecular modeling results from the universal force field (UFF) interaction energy calculations. 5.2 Results 5.2.1 Captive bubble contact angle experiments For various minerals, the flotation performance is affected by the solution pH and dosage of the collector. 75 Figures 5.1-5.3 show the effects of pH and collector dosage on captive bubble contact angle measurements for bastnaesite, calcite, and quartz using potassium lauryl phosphate as a collector. It is clear that with potassium lauryl phosphate as a collector both bastnaesite and calcite have the same contact angle of less than 40°in the acidic pH range, from pH 4 to 8, with the lauryl phosphate concentration below 5×105 M. When the concentration of potassium lauryl phosphate was increased to 1×10-4 M, the contact angle of calcite was still 30-40°(see Figure 5.2), compared to a contact angle of about 80°for bastnaesite in 1×10-4 M potassium lauryl phosphate, pH 4.0-6.5, 41 as shown in Figure 5.1. On this basis, selective flotation of bastnaesite from calcite can be expected. As for quartz, there was no bubble attachment, and a 0° contact angle was observed even at a high potassium lauryl phosphate concentration of 5×10-4 M (152.2 76 mg/l). However, quartz is reported to be floated with SM15, a kind of phosphoric acid ester, a chemical structure not disclosed. 10, 16 A significant recovery of 50-80% was achieved in a 3.3 mg/l SM15 solution (200 g/ton). 10 These results are not easily explained since the chemical structure of SM15 is unknown. Nevertheless, the significant contact angle difference between bastnaesite and quartz indicates that potassium lauryl phosphate should be a promising collector in the flotation of bastnaesite from quartz. 5.2.2 Zeta potential experiments Zeta potential can affect the adsorption of reagents and thus the floatability of minerals. As shown in Figures 5.4 and 5.5, zeta potentials of calcite and quartz minerals were measured as a function of pH in 10 mM KCl and in the presence of 5×10 -6 M potassium lauryl phosphate. The point of zero charge of the calcite mineral was at pH 7.5, which is in accord with the IEP value of pH 8.0, determined by the electrophoretic mobility method 76 and pH 8.2 from streaming potential measurements. 77 The zeta potential of quartz is negative at pH 2.5, which agrees well with the reference IEP of pH 2.3. 10 In the presence of 5×10-6 M potassium lauryl phosphate, the zeta potential of calcite is decreased by about 15-40 mv; however, the zeta potential of quartz remains essentially unchanged. Previously the zeta potential for bastnaesite in the presence of 5×10-6 M potassium lauryl phosphate was found to decrease from 60-100 mv. 39 Based on these results, it is expected that the lauryl phosphate anion is more strongly adsorbed at the bastnaesite surface than at the calcite and quartz surfaces. 77 5.2.3 Microflotation experiments Single mineral flotation results involving bastnaesite, calcite, and quartz with potassium lauryl phosphate as a collector are shown in Figures 5.6-5.8. It is clear from Figure 5.6 that a high bastnaesite flotation recovery of 90% is achieved at a low potassium lauryl phosphate concentration of 3×10-6 M from pH 4.0-7.0. The flotation recovery was reduced to 50-70% when the concentration of potassium lauryl phosphate was reduced to 2×10-6 M. In contrast, calcite flotation recovery of less than 20% was realized at the same lauryl phosphate concentration as shown in Figure 5.7. It seems that potassium lauryl phosphate is a potential collector for the selective flotation of bastnaesite from calcite at low collector concentration for pH 4.0-7.0. This analysis agrees with the conclusion made from contact angle experiment results, which indicate that there is a flotation possibility at pH 4.0-6.5 with a potassium lauryl phosphate concentration of 1×10-4 M. However, it should be noticed that there is no good correspondence between the contact angle and flotation results at the same concentration. For example, bastnaesite has a flotation recovery of 90% at a potassium lauryl phosphate concentration of 3×10-6 M from pH 4.0-7.0. In contrast, no contact angle could be found at the same concentration and pH. The reason is not clear, and the same phenomenon is also found in other research 18 ; further consideration is needed in future research. See Figures 5.1-5.3, at higher concentrations, the microflotation results of calcite are in accord with the direct flotation of calcite from phosphate ores in a slightly acidic pulp, pH ranging from about 4.5 to 6.5, using ethoxylated phosphoric ester surfactants. 78 When the concentration of potassium lauryl phosphate is increased to 1×10-5 M (Figure 78 5.7), the flotation recovery of calcite is about 90% from pH 5.0-11.0, which means potassium lauryl phosphate can also serve as a collector for calcite at this higher concentration of 1×10-5 M. Compared with bastnaesite's flotation recovery of 70-95% at the same concentration of potassium lauryl phosphate from pH 4.0-9.0, 39 one can expect that a depressant will be necessary for the flotation of bastnaesite from calcite at higher concentrations of potassium lauryl phosphate, such as 1×10-5 M. As for quartz (Figure 5.8), flotation was not observed with potassium lauryl phosphate at a concentration of 1×10-5 M. Also, the flotation recovery was less than 20% at 1×10-4 M. Compared with the high bastnaesite flotation recovery of 90% at a very low potassium lauryl phosphate concentration of 3×10-6 M (pH 4.0-7.0), it is evident that potassium lauryl phosphate is a promising collector in the flotation of bastnaesite from quartz. These results are in accord with the contact angle results from Figures 5.1-5.3 and the zeta potential results in Figures 5.4-5.5. 5.2.4 Interaction energy calculations The universal force field (UFF) was used to examine the interaction between the collector and the mineral surface. The molecular structures of lauryl phosphate and octyl hydroxamate are shown in Figure 5.9. The UFF optimized distances (ropt) and optimized angles (θopt) for lauryl phosphate at the surface of bastnaesite, calcite, and quartz are presented in Table 5.1. One important thing to note about the optimized distance (ropt) in Table 5.1 is that we have defined r as the distance of the phosphorus (P in the –PO4H- functional group) of lauryl phosphate from a reference cerium/calcium/silicon atom at the respected surfaces 79 in order to keep the reference point the same. While the definition of r in Figure 5.10 is that of the distance between oxygen (O in the –PO4H- functional group) and a reference cerium/calcium/silicon atom at the respected surfaces. Therefore, the optimized distance (ropt) shown in Table 5.1 appears to be higher compared with the distances presented in Figure 5.10. The UFF optimized structures of lauryl phosphate at the mineral surfaces are shown in Figure 5.10(A), 10(B), and 10(C) for bastnaesite, calcite, and quartz, respectively. With regard to the exact optimized distances shown in Figure 5.10, the O– Ce distance for bastnaesite has the shortest distance (3.04 Å) compared to the O-Ca and O-Si distances for calcite and quartz. These results indicate that stronger adsorption of lauryl phosphate occurs at the surface of bastnaesite than at the surfaces of calcite and quartz. The interaction energies for octyl hydroxamate at the same mineral surfaces, computed by the UFF method following the same procedures as those calculated for lauryl phosphate, results are summarized in Table 5.2. Based on the UFF interaction energies from Table 5.2, it is expected that the order of the flotation response with either lauryl phosphate or octyl hydroxamate should be bastnaesite > calcite > quartz. However, it should be further noted that the difference of interaction energy between bastnaesite and calcite/quartz in the presence of lauryl phosphate is larger than for octyl hydroxamate. These results indicate a stronger bastnaesite flotation response might be expected for lauryl phosphate than for octyl hydroxamate when compared with calcite and quartz. The interaction energies of lauryl phosphate at the bastnaesite, calcite, and quartz 80 cleavage surfaces are compared with the flotation response of the minerals in Figure 5.11. In the same way, the comparison is made for octyl hydroxamate in Figure 5.12. It is really interesting to notice that the order of the UFF interaction energy, based totally on force field calculation, is the same order observed for the flotation response of the true minerals. Potassium lauryl phosphate provided better bastnaesite flotation selectivity at low collector concentrations of less than 1×10-5 M when compared with octyl hydroxamate as shown from the data presented in Figures 5.11(A) and 5.12(B). The UFF interaction energy differences between bastnaesite and calcite/quartz in Figures 5.11(B) and 5.12(B) also confirm the selectivity. The excellent correlation between theoretical and experimental results thus indicates that molecular modeling computations can be used to evaluate and even predict the strength of the interactions between various collector molecules and mineral surfaces. Also, the results indicate that the interaction between the collectors and the mineral surface is the main factor controlling the selectivity in flotation separations, as expected. 5.3 Summary Contact angle measurements, zeta potential determinations, and microflotation experiments for bastnaesite, calcite, and quartz using potassium lauryl phosphate as the collector, suggest that potassium lauryl phosphate will provide selective flotation of bastnaesite from both calcite and quartz at low collector concentrations. Now, the use of potassium lauryl phosphate as a collector in the flotation of bastnaesite and other rare earth minerals must be examined in ore flotation experiment, and such experiments are in 81 progress. Use of potassium lauryl phosphate as a collector resulted in the improved flotation of bastnaesite when compared with octyl hydroxamate, especially at concentrations of 5×10-5 M. 39 However, selectivity with respect to quartz was not satisfactory with SM15, which float both bastnaesite and quartz. 10 This result indicates that the chemical structure of the phosphate ester (ester of phosphoric acid) may play an important role in the flotation separation, and further research concerning the relationship between the chemical structure of phosphate esters and the flotation response is necessary. The UFF interaction energies for lauryl phosphate and octyl hydroxamate at the bastnaesite, calcite, and quartz surfaces, as predicted by molecular modeling, are in agreement with the experimental microflotation results. The results are consistent with similar correlations between theory and experiment for several other systems as reported by other researchers. 72c, 79 Based on the experimental results for bastnaesite, calcite, and quartz using potassium lauryl phosphate as a collector, one can expect that potassium lauryl phosphate will be a promising collector for the selective flotation of bastnaesite from calcite and quartz, at a low concentration of 3×10-6 M and pH 4.0-7.0. This conclusion is based on captive bubble contact angle measurements, zeta potential determinations, and microflotation experiments. Further, UFF interaction energy calculations for potassium lauryl phosphate and octyl hydroxamate at bastnaesite, calcite, and quartz surfaces confirmed that potassium lauryl phosphate should be a more selective collector when compared to octyl hydroxamate. The UFF interaction energy calculations agree with the microflotation results. Research concerning the application of potassium lauryl phosphate 82 as a collector in the flotation of bastnaesite from the Mountain Pass deposit (California, USA) is in progress. Conclusions at this time include the following points: 1. Captive bubble contact angle results demonstrate that bastnaesite hydrophobicity in the presence of potassium lauryl phosphate is greater than the hydrophobicity of calcite and quartz at 1×10-4 M and pH 4.0-6.5. 2. From the zeta potential determinations, it is expected that the lauryl phosphate anion is more strongly adsorbed at the surface of bastnaesite than at the calcite and quartz surfaces. 3. A high bastnaesite flotation recovery of 90% at a low potassium lauryl phosphate concentration of 3×10-6 M from pH 4.0-7.0 was observed. Calcite flotation recovery of less than 20% and no quartz flotation were observed under the same conditions. It is evident that potassium lauryl phosphate is a potential collector for the selective flotation of bastnaesite from calcite/quartz at a low concentration of 3×10-6 M and pH 4.0-7.0. 4. The excellent correlation between theoretical calculations and microflotation results indicate that the UFF method is an advanced tool that can explain, and even predict the potential for collector effectiveness in flotation. The calculations confirm that potassium lauryl phosphate should be a promising selective collector in the flotation of bastnaesite from calcite/quartz. 83 Intermediate contact angle 100 Bastnaesite;Potassium lauryl phosphate 1×10-4M 5×10-5M 1×10-5M 6.75×10-6 M 80 60 40 20 0 4 6 8 10 12 pH Figure 5.1 The contact angle of bastnaesite with potassium lauryl phosphate as a function of pH. 84 Intermediate contact angle 100 Calcite; Potassium lauryl phosphate 1×10-4 M 5×10-5 M 1×10-5 M 6.75×10-6 M 80 60 40 20 0 4 6 8 10 12 pH Figure 5.2 The contact angle of calcite with potassium lauryl phosphate as a function of pH. 85 Intermediate contact angle 100 Quartz; Potassium lauryl phosphate 5×10-4 M 1×10-4 M 80 60 40 20 0 4 6 8 10 12 pH Figure 5.3 The contact angle of quartz with potassium lauryl phosphate as a function of pH. 86 80 Calcite 10mM KCl 10mM KCl, 5×10-6 M potassium lauryl phosphate Zeta potential (mv) 60 40 20 0 -20 -40 -60 2 4 6 8 10 12 pH Figure 5.4 Zeta potential of calcite in 10 mM KCl solution, with and without 5×10-6 mol/L potassium lauryl phosphate solution as a function of pH. 87 80 Quartz 10mM KCl 10mM KCl, 5×10-6M potassium lauryl phosphate 60 Zeta potential (mv) 40 20 0 -20 -40 -60 -80 2 4 6 8 10 12 pH Figure 5.5 Zeta potential of quartz in 10 mM KCl solution, with and without 5×10-6 mol/L potassium lauryl phosphate solution as a function of pH. 88 Flotation efficiency (%) 100 80 60 40 Bastnaesite Potassium lauryl phosphate 3×10-6 M 2×10-6 M 20 2 4 6 8 10 12 pH Figure 5.6 Bastnaesite flotation with potassium lauryl phosphate as a function of pH. 89 Flotation efficiency (%) 100 80 Calcite Potassium lauryl phosphate 1×10-5 M 5×10-6 M 3×10-6 M 2×10-6 M 60 40 20 0 2 4 6 8 10 12 pH Figure 5.7 Calcite flotation with potassium lauryl phosphate as a function of pH. 90 Flotation efficiency (%) 100 Quartz Potassium lauryl phosphate 1×10-4 M 1×10-5 M 80 60 40 20 0 2 4 6 8 10 12 pH Figure 5.8 Quartz flotation with potassium lauryl phosphate as a function of pH. 91 (A) (B) Figure 5.9 Structures of lauryl phosphate and octyl hydroxamate, White: hydrogen; red: oxygen; cyan: carbon; tan: phosphorus; blue: nitrogen. (A) lauryl phosphate, (B) octyl hydroxamate. 92 3.04 (A) 3.14 3.14 4 (B) 3.84 (C) 3.14 4 Figure 5.10 UFF optimized complex of lauryl phosphate on bastnaesite (100), calcite (104) and quartz (101) surface. White: hydrogen; red: oxygen; cyan: carbon; yellow: cerium; tan: phosphorus; pink: fluorine; ochre: silicon; green: calcium. (A) bastnaesite, (B) calcite and (C) quartz. 93 Bastnaesite Flotation efficiency (%) 100 Calcite Quartz 80 60 40 20 0 (A) -6 -5 -4 1×10 1×10 1×10 1×10-3 Potassium lauryl phosphate(mol/l); pH 4.7-6.5 UFF interaction energy (kcal/mol) 0 -20 -238 -500 -1000 -1500 (B) -1373 Bastnaesite Calcite Quartz Potassium lauryl phosphate Figure 5.11 Flotation and interaction energy comparison using lauryl phosphate, (A) Flotation of bastnaesite, calcite, quartz as a function of potassium lauryl phosphate concentration. (B) UFF interaction energies of potassium lauryl phosphate at bastnaesite (100), calcite (104), and quartz (101) surfaces. 94 Flotation efficiency (%) 100 Bastnaesite Calcite Quartz 80 60 40 20 (A) 0 1×10-5 1×10-4 1×10-3 Octyl hydroxamate (mol/l); pH 9.3 UFF interaction energy (kcal/mol) 0 -33 -284 -500 -717 -1000 (B) -1500 Bastnaesite Calcite Quartz Octyl hydroxamate Figure 5.12 Flotation and interaction energy comparison using octyl hydroxamate, (A) Flotation of bastnaesite, calcite, quartz as a function of octyl hydroxamate concentration. (B) UFF interaction energies of octyl hydroxamate at bastnaesite (100), calcite (104), and quartz (101) surfaces. 95 Table 5.1 UFF optimized structures [θopt (in degrees) and ropt (in angstroms)] for lauryl phosphate at the surface of bastnaesite, calcite, and quartz. Lauryl phosphate Bastnaesite θopt ropt 101.57 3.17 Calcite θopt ropt 96.64 3.50 Quartz θopt ropt 86.56 4.24 96 Table 5.2 UFF interaction energies (kJ/mol) of lauryl phosphate and octyl hydroxamate adsorption at bastnaesite, calcite, and quartz surfaces. Collector Lauryl phosphate Octyl hydroxamate Bastnaesite -1373 -717 Calcite -238 -284 Quartz -20 -33 CHAPTER 6 COMPARISON OF LAURYL PHOSPHATE WITH 2-ETHYLHEXYL PHOSPHATE 6.1 Introduction Recent initial evaluation indicates that the branched chain alkyl phosphate, 2ethylhexyl phosphate, has even better selectivity compared with lauryl phosphate. The effect of the phosphate collector chemical structure on bastnaesite flotation, such as hydrophobicity, selectivity, and collector adsorption states, is the subject of this paper. The intent is to understand the effect of the alkyl phosphate chemical structure on the bastnaesite flotation response, which includes, but is not limited to, the interactions of the branched chain phosphate with bastnaesite, calcite, and quartz. The evaluation is based on contact angle and zeta potential measurements, microflotation, and isothermal titration calorimetry experiments, as well as density functional theory calculations and molecular dynamics simulations. Also, comparison and discussion regarding the use of branched chain phosphate and lauryl phosphate are included. It is expected that the results of this research will enable us to further understand bastnaesite/calcite/quartz flotation chemistry using alkyl phosphate collectors, with consideration of chemical structure, which includes the hydrophobic surface state, selectivity in flotation, and adsorption phenomena for the  Weiping Liu, Luther W McDonald IV, Xuming Wang, J.D. Miller. Flotation Chemistry Issues Association with the Phosphate Chemical Structure [J]. Minerals Engineering, 2018. 127, 286-295. 98 design of alkyl phosphate collectors. 6.2 Results The experimental results from contact angle and zeta potential measurements, microflotation and microcalorimetry experiments, as well as density functional theory calculations and molecular dynamics simulations, were used to examine bastnaesite flotation issues using alkyl phosphates, such as hydrophobicity, selectivity, and adsorption state. 6.2.1 Contact angle experiments The contact angle measurements (sessile drop, captive bubble) are an indicator for the hydrophobicity of a mineral surface. 80 Hydrophobicity is the physical property describing the rejection or repulsion of water, and it is the fundamental property that accounts for the flotation of mineral particles. In this regard, the captive bubble contact angle measurements for bastnaesite, calcite, and quartz are shown in Figure 6.1 as a function of pH and 2-ethylhexyl phosphate concentration. It is obvious that 2-ethylhexyl phosphate has better adsorption at an acid pH compared to an alkaline pH for both bastnaesite and calcite. Furthermore, the contact angle of bastnaesite is higher than for calcite at both 2-ethylhexyl phosphate concentrations of 1×10-4 M and 5×10-5 M, which indicates greater hydrophobicity at the bastnaesite surface when compared to calcite. However, compared to the contact angle of bastnaesite and calcite at the same concentration of lauryl phosphate, as shown in Table 6.1, 39, 41 Section 6.2.2, the contact angle for 2-ethylhexyl phosphate has a relatively lower value, which suggests a lower 99 hydrophobicity/flotation response with 2-ethylhexyl phosphate compared to lauryl phosphate. Of course, it is known that longer hydrocarbon chains impart greater hydrophobicity when adsorbed at mineral surfaces, thereby resulting in an increased hydrophobicity/contact angle. 81 As for quartz, there is almost no response even at higher concentrations, such as 1×10-4 M. Similar results were reported for the case of lauryl phosphate. 39, 41 Zeta potentials of the mineral particles are influenced by the adsorption of a collector and may be indicative of the hydrophobicity/flotation response. The zeta potentials of bastnaesite, calcite, and quartz minerals in 10 mM potassium chloride (KCl) with and without 5×10−6 M 2-ethylhexyl phosphate as a function of pH are shown in Figure 6.2. The decrease in zeta potential for bastnaesite is greater than for calcite and quartz, which is indicative of the preferred collector adsorption by bastnaesite, and a higher degree of hydrophobicity. Single mineral flotation results for bastnaesite, calcite, and quartz using 2ethylhexyl phosphate as a function of concentration and pH are shown in Figure 6.3. A bastnaesite recovery of 80-90% is achieved from 3×10-6 M to 1×10-5 M 2-ethylhexyl phosphate as shown in Figure 6.3(A). In contrast, calcite and quartz recoveries are around 10% and 1%, respectively. Furthermore, the bastnaesite flotation response decreased from 83% to 50% with an increase in pH using 2-ethylhexyl phosphate, as shown in Figure 6.3(B), which is in accord with the bastnaesite flotation response for the same lauryl phosphate concentration of 5×10-6 M. 39 100 6.2.2 Microflotation and zeta potential experiments The contact angle and zeta potential measurements and the microflotation experiments were used to examine the selectivity of lauryl phosphate and 2-ethylhexyl phosphate, which describes the preferential recovery of a valuable mineral when compared to the recovery of gangue minerals. The contact angle difference between bastnaesite and calcite at a high concentration of 1×10-4 M of 2-ethylhexyl phosphate is 15° or less, compared to almost 50° in the case of lauryl phosphate from pH 4-6, as shown in Table 6.1. When the concentration is decreased to 5×10-5 M, the contact angle difference between bastnaesite and calcite for 2-ethylhexyl phosphate increased to 15°at pH 4-6 compared to 5°in the case of lauryl phosphate in Table 6.1. In this regard, the higher selectivity of 2-ethylhexyl phosphate compared to lauryl phosphate is expected at a lower concentration of 5×10-5 M. The 0°contact angle at the quartz surface compared to the relatively high contact angle of 40°-85°for bastnaesite using lauryl phosphate and 2-ethylhexyl phosphate indicates that these alkyl phosphate collectors should be promising collectors in the flotation of bastnaesite from quartz. Compared with the decreased zeta potential of 10-50 mv and 0-20 mv for bastnaesite and calcite at 5×10-6 M 2-ethylhexyl phosphate in Figure 6.2, the zeta potential of bastnaesite and calcite decreased 30-80 mv and 15-40 mv, respectively, at the same concentration of lauryl phosphate, as shown in Table 6.2, which may be indicative of the stronger adsorption of the lauryl phosphate. Furthermore, the zeta potential difference between bastnaesite and calcite in the presence of 2-ethylhexyl phosphate is around 25-52 mv, which is slightly larger than 1-50 mv for the case of lauryl phosphate, 39, 41 as shown in Table 6.2, which further confirms the higher selectivity of 2-ethylhexyl 101 phosphate compared to lauryl phosphate at a collector concentration of 5×10-6 M. The flotation difference between bastnaesite and calcite/quartz using 2-ethylhexyl phosphate at 3×10-6 M and pH 5 is around 82% in Figure 6.3(A), while with lauryl phosphate the difference is only 75% at the same concentration and pH, as shown in Table 6.3. 39, 41 As the collector concentration is increased to 1×10-5 M at pH 5, the flotation difference between bastnaesite and calcite/quartz using 2-ethylhexyl phosphate is still around 80%, compared to almost no flotation difference in the case of lauryl phosphate in Table 6.3. 39, 41 Furthermore, the flotation difference between bastnaesite and calcite/quartz at a concentration of 3×10-6 M 2-ethylhexyl phosphate as a function of pH is around 60-80%, which is still higher than the value of 7-75% in the case of lauryl phosphate, as shown in Table 6.3. 39, 41 The flotation results are in accord with contact angle and zeta potential results, as shown in Figure 6.1 and Figure 6.2, which further indicates that 2-ethylhexyl phosphate should be a promising collector for the flotation of bastnaesite from calcite and quartz. Since quartz was not sensitive to alkyl phosphate addition in contact angle, zeta potential, and single mineral flotation experiments, mixed flotation experiments only involved bastnaesite and the typical gangue mineral, calcite. As shown in Table 6.4, the bastnaesite recovery and grade are 80% and 52%, respectively, at 3×10-6 M of lauryl phosphate and pH 5.0. When 2-ethylhexyl phosphate was used, the bastnaesite recovery decreased to 75% whereas the bastnaesite grade increased to 95% at the same concentration and pH. This phenomenon was also observed in the flotation of zinc oxide, as a decrease in the hydrocarbon chain length resulted in a decreased zinc recovery and an increased zinc oxide grade. 82 102 The hydrocarbon chain length and the polar group are the main factors that affect the performance of the collector. 83 In the computational chemistry analysis, the electrostatic potential (ESP) charges were derived using semiempirical methods. 85 84 or ab initio In this regard, the optimized structures and ESP charges for 2-ethylhexyl phosphate and lauryl phosphate are shown in Figure 6.4. The 2-ethylhexyl phosphate anion with lower HOMO (highest occupied molecular orbital) and higher LUMO (lowest unoccupied molecular orbital) indicates the weaker electron-donating and accepting ability compared to lauryl phosphate, as shown in Table 6.5. However, the reactive atoms of O1 and O2 in 2-ethylhexyl phosphate with a total value of -1.656 have better electron-donating ability compared to lauryl phosphate with a total value of -1.634, as shown in Figure 6.4. In this regard, 2-ethylhexyl phosphate shows good collecting power toward bastnaesite and excellent selectivity against calcite and quartz. In a similar analysis reported in the literature, a monothiophosphate collector 86 or a short chain amine collector 82 with a lower collecting ability were also observed to have better selectivity due to the stronger electron-donating ability of the reactive atoms in the polar functional group. 6.2.3 Microcalorimetry and MDS Chemical, electrostatic, hydrogen bonding, and hydrophobic interactions contribute to collector adsorption at mineral surfaces. 87 In this regard, microcalorimetry, collector analysis, and molecular dynamics simulations have been used to examine the alkyl phosphate adsorption states at the bastnaesite surface. The microcalorimetry results for the first four injections of 2-ethylhexyl phosphate and lauryl phosphate are shown in 103 Figure 6.5. The adsorption enthalpy measured as a function of time and the heat evolved from each addition is calculated from the area under the peak. It is obvious that lauryl phosphate has a slightly higher exothermic heat of -17.19 KJ/mol compared to the case of -14.36 KJ/mol for 2-ethylhexyl phosphate, as shown in Figure 6.6. The higher absolute value of adsorption heat suggests stronger adsorption affinity of the collector for the mineral surfaces. 71 In this regard, lauryl phosphate is expected to have slightly stronger adsorption at the bastnaesite surface when compared to 2-ethylhexyl phosphate. The heat of adsorption of 10-20 KJ/mol by bastnaesite for alkyl phosphates is significantly less than recently reported heat of 20-60 KJ/mol for collector chemisorption at the surface of sulfide minerals. 66a However, it is still indicative of a strong interaction, which together with other results, suggest the chemisorption of alkyl phosphate at the surface of bastnaesite. The partial charge of -OPO3H for lauryl phosphate is -1.279 compared to -1.265 for 2-ethylhexyl phosphate, as shown in Table 6.5. The longer alkyl hydrocarbon chain length in lauryl phosphate imparted a stronger positive inductive effect, which increased the functional group’s electron density and rendered the higher reactivity of the collector. 88 In this regard, lauryl phosphate has higher adsorption affinity on the mineral surface compared to 2-ethylhexyl phosphate, which is in accord with the contact angle, zeta potential, single mineral flotation, and microcalorimetry results, as shown in Figures 6.16.3 and 6.5-6.6. The energy difference between HOMO and LUMO represented by ΔEHOMO-LUMO indicates the stability and reactivity of the collector in the chemical reaction. A larger value of ΔEHOMO-LUMO for the collector represents greater stability and lower reactivity of 104 the collector. In contrast, the smaller value of the ΔEHOMO-LUMO represents lower stability and the higher reactivity of the collector. 82 The ΔEHOMO-LUMO values of lauryl phosphate and 2-ethylhexyl phosphate are 0.50061 a.u. and 0.56077 a.u., respectively, as shown in Table 6.5, which indicate that the reactivity of lauryl phosphate is higher than 2ethylhexyl phosphate. In this regard, the increase of the alkyl hydrocarbon chain length increased the reactivity of the collector, which is accord with zinc oxide flotation using amine collector. 82, 89 The phosphate collectors were adsorbed at the bastnaesite (100) surface after 1 ns of molecular dynamics simulation, specifically, with the phosphate groups in preferred positions with respect to cerium atoms in the crystal structure, as shown in Figure 6.7. This phenomenon agrees with the thermodynamic analysis and density functional theory calculation results. 40 Furthermore, the 2-ethylhexyl phosphate is adsorbed on the bastnaesite surface totally by the polar headgroup, while lauryl phosphate is adsorbed on the bastnaesite surface by the combination of the headgroup and hydrophobic attraction between adjacent hydrocarbon chains. However, lauryl phosphate excludes more water due to its longer hydrocarbon chain, thereby imparting higher hydrophobicity compared to 2-ethylhexyl phosphate, as shown in Figure 6.8 (A). As shown in Figure 6.8 (A), the relative density of water along the normal to the bastnaesite surface was compared with and without phosphate collectors. The water density decreased in the presence of phosphate collector for the first three water peaks located at 32.7 Å, 36 Å, and 37.5 Å from the bastnaesite surface. Meanwhile, the relative density of water at the same concentration of lauryl phosphate and 2-ethylhexyl phosphate for the first three water peaks is almost the same. These results agree with the 105 adsorption state of the phosphate collector in Figure 6.7, in which almost all the 2ethylhexyl phosphate and lauryl phosphate collectors adsorbed at the bastnaesite surface. However, the relative density of water in the presence of 2-ethylhexyl phosphate is almost the same as that of pure water about 40 Å from the bastnaesite surface, while the relative density of water decreased in the case of lauryl phosphate, as shown in Figure 6.8 (A). This is in accord with the phosphate adsorption state in Figure 6.7 and further proves that the longer hydrocarbon chain of lauryl phosphate excluded more water from the surface when compared to 2-ethylhexyl phosphate. Thus, the increased hydrophobicity of bastnaesite with adsorbed lauryl phosphate is observed. The relative densities of O, P, and C from the phosphate collectors are also presented in Figure 6.8(B) and Figure 6.8(C). The location of the peak of O for both 2ethylhexyl phosphate and lauryl phosphate is found at 33.3 Å from the surface compared to the first peak of water at 32.7 Å in Figure 6.8(A). This analysis indicates that the phosphate collectors adsorbed at the bastnaesite surface through the oxygen atoms in the phosphate headgroup, which further replaced the water molecules on the mineral surface. This observation agrees with the molecular orbital analysis of the adsorption of lauryl phosphate on bastnaesite. 40 Furthermore, lauryl phosphate has carbon relative density at 35 Å to 45 Å from the bastnaesite surface while 2-ethylhexyl phosphate only from 35 Å to 40 Å, as expected, indicating the chain length effect at the bastnaesite surface; in the case of lauryl phosphate excluding more water from the mineral surface. The adsorption free energy change should be negative (ΔGads ) for spontaneous collector adsorption at mineral surfaces. In this regard, the adsorption should be sufficiently exothermic (ΔHads ) and/or the entropy change sufficiently positive (ΔSads ) 106 as shown in Eq. (6.1). ΔGads = ΔHads − 𝑇ΔSads (6.1) The microcalorimetry results in Figure 6.5 and Figure 6.6 showed that lauryl phosphate has higher exothermic heat/enthalpy change compared to 2-ethylhexyl phosphate. In order to examine the contribution of enthalpy and entropy to the collector adsorption free energy, the entropy change was also examined by mean square displacement and diffusion coefficients (D), which indicate the displacement of the water/collector as a function of time. In this regard, mean square displacement of the water and collectors in the 2-ethylhexyl phosphate/lauryl phosphate solutions and pure water at the bastnaesite (100) surface are shown in Figure 6.9. It is obvious that the diffusion coefficients of water and collector in lauryl phosphate solution are slightly higher than in the case of the 2-ethylhexyl phosphate solution. Furthermore, the diffusion coefficient of water increased from 3.61×10-3 cm2/s to 5.21×10-3 cm2/s and 5.74×10-3 cm2/s in the presence of lauryl phosphate and 2-ethylhexyl phosphate, respectively, as shown in Figure 6.9 (A). Since the diffusion coefficient indicates the displacement of molecules, which relates to the entropy, the increase of the entropy for water in the presence of phosphate collector might be due to the replacement of the water by the phosphate collector at the mineral surface. 90 In this regard, water is transformed to a less structured and disordered state from a more structured and ordered state at the mineral surface in the presence of the alkyl phosphate collectors. Therefore, entropy is also a driving force for the spontaneous adsorption process, which is also observed for oleate chemisorption at calcite and fluorite surfaces. 91 In this regard, lauryl phosphate has more negative adsorption free energy compared to 2-ethylhexyl phosphate, based on the 107 microcalorimetry and mean square displacement results from molecular dynamics simulations. 6.3 Discussion Contact angle and zeta potential measurements, single mineral microflotation, and microcalorimetry experiments, as well as, density functional theory calculations and molecular dynamics simulations were used to examine the effect of alkyl phosphate chemical structure on hydrophobicity, selectivity, and collector adsorption states. The contact angle, zeta potential, single mineral microflotation, and microcalorimetry results indicate that lauryl phosphate has a stronger adsorption potential compared to 2ethylhexyl phosphate. However, the reactivity difference between bastnaesite and calcite/quartz for 2-ethylhexyl phosphate is greater than for lauryl phosphate. In this regard, the microflotation results for a mineral mixture of bastnaesite and calcite show that 2-ethylhexyl phosphate has better selectivity when compared to the results with lauryl phosphate. The molecular dynamics simulations revealed that more water is accommodated at the bastnaesite surface with 2-ethylhexyl phosphate compared to lauryl phosphate. The microcalorimetry and molecular dynamics simulations mean square displacement results further indicate that lauryl phosphate has a more negative adsorption free energy when compared to 2-ethylhexyl phosphate. The polar functional headgroup and nonpolar hydrocarbon chain length of the collector affect the extent of hydrophobicity and the bond strength of the collector at the mineral surface. 66a Both the absolute adsorption heat and microflotation recoveries increase with an increase in the collector chain length, which is in accord with the 108 observation that adsorption heat is a strong predictor of the hydrophobicity for collectors with the same ligand type and different alkyl hydrocarbon chain lengths. The lower flotation recovery of 2-ethylhexyl phosphate also might be due to greater steric hindrance caused by the branched structure at the mineral surface, thereby leading to the lower surface coverage and consequently lower hydrophobicity. 66a 6.4 Summary Bastnaesite flotation chemistry issues with alkyl phosphate collectors, including hydrophobicity, selectivity, and adsorption states were examined and discussed based on the results of captive bubble contact angle and zeta potential measurements, as well as microflotation and microcalorimetry experiments. 2-ethylhexyl phosphate has lower adsorption affinity/hydrophobicity and higher selectivity toward bastnaesite compared to lauryl phosphate. The microcalorimetry experiments, collector structure analysis, and molecular dynamics simulation result further prove that 2-ethylhexyl phosphate has a lower adsorption affinity but higher flotation selectivity with respect to calcite when compared to lauryl phosphate. Further research is in progress. Conclusions at this time include the following points: 1. The contact angle, zeta potential, and single microflotation results indicate 2ethylhexyl phosphate has lower adsorption affinity and hydrophobicity for the bastnaesite surface compared to lauryl phosphate. 2. The flotation of the mineral mixture results (bastnaesite and calcite) indicate that 2-ethylhexyl phosphate provides for higher flotation selectivity when compared to lauryl phosphate. Collector analysis showed that 2-ethylhexyl phosphate has a lower 109 headgroup charge and less reactivity. However, the reactive atoms of O1 and O2 in 2ethylhexyl phosphate have better electron-donating ability compared to lauryl phosphate. In this regard, 2-ethylhexyl phosphate possesses effective collecting power for bastnaesite and excellent selectivity against calcite and quartz. 3. The microcalorimetry results indicate that 2-ethylhexyl phosphate has lower exothermic heat of adsorption on bastnaesite when compared to lauryl phosphate. Together with the molecular dynamics simulations mean square displacement results, lauryl phosphate was found to have lower free energy adsorption compared to 2ethylhexyl phosphate. Furthermore, the adsorption heat is a strong predictor of the hydrophobicity for collectors with the same ligand type and different alkyl chain lengths. 4. Molecular dynamics simulations showed that both 2-ethylhexyl phosphate and lauryl phosphate adsorb at the bastnaesite surface. However, lauryl phosphate excluded more water from the bastnaesite surface, thereby contributing to a stronger hydrophobicity/flotation response when compared to 2-ethylhexyl phosphate. 110 80 100 Bastnaesite; 2-ethylhexyl phosphate 1x10-4 M (A) Intermediate contact angle Intermediate contact angle 100 5x10-5 M 60 40 20 0 4 6 8 10 80 (B) Calcite; 2-ethylhexyl phosphate 1X10-4 M 5X10-5 M 60 40 20 0 12 4 6 pH 8 10 12 pH Intermediate contact angle 100 80 (C) Quartz; 2-ethylhexyl phosphate 1x10-4 M 5X10-5 M 60 40 20 0 4 6 8 10 12 pH Figure 6.1 Captive bubble contact angles of bastnaesite (A), calcite (B) and quartz (C) with 2-ethylhexyl phosphate as a function of pH. 111 80 40 Calcite (B) 10 mM KCl -6 10 mM KCl; 5x10 M 2-ethylhexyl phosphate 60 Zeta potential (mv) 60 Zeta potential (mv) 80 Bastnaesite (A) 10 mM KCl -6 10 mM KCl; 5x10 M 2-ethylhexyl phosphate 20 0 -20 -40 -60 40 20 0 -20 -40 -60 2 4 6 8 10 12 2 4 6 pH 8 10 12 pH 80 Quartz (C) 10 mM KCl -6 10 mM KCl; 5x10 M 2-ethylhexyl phosphate Zeta potential (mv) 60 40 20 0 -20 -40 -60 2 4 6 8 10 12 pH Figure 6.2 Zeta potential of bastnaesite (A), calcite (B) and quartz (C) in 10 mM KCl solution, with and without 5×10−6 mol/L 2-ethylhexyl phosphate solution, as a function of pH. 112 100 100 (B) (A) 80 Bastnaesite Calcite Quartz pH 5.0 60 40 20 Recovery % Recovery % 80 60 40 20 0 Bastnaesite Calcite Quartz 5X10-6 M 2-ethylhexyl phosphate 0 3X10-6 1X10-5 2-ethylhexyl phosphate, M 5 7 9 pH Figure 6.3 Single mineral microflotation results for bastnaesite, calcite, and quartz using 2-ethylhexyl phosphate as a function of concentration (A) and pH (B). 113 (A) (B) Figure 6.4 Chemical structures of 2-ethylhexyl phosphate (A) and lauryl phosphate (B). 114 4 4 (A) (B) 3 Heat flow (uJ/s) Heat flow (uJ/s) 3 2 1 0 2 1 0 -1 -1 0 5000 Time (s) 10000 15000 0 5000 10000 15000 Time (s) Figure 6.5 The first four injections of 2-ethylhexyl phosphate (A) and lauryl phosphate (B) for microcalorimetry. 115 Heat of adsorption (kJ/mol) -30 -25 -20 -15 -17.19 -14.36 -10 -5 0 2-ethylhexyl phosphate Lauryl phosphate Figure 6.6 The heat of adsorption for the interaction of 2-ethylhexyl phosphate and lauryl phosphate with bastnaesite (pH 5.0). 116 (A) (B) Figure 6.7 Interfacial behavior of 2-ethylhexyl phosphate (A) and lauryl phosphate (B) at the bastnaesite surface. 117 Pure water (A) 2-ethylhexyl phosphate Lauryl phosphate 2-ethylhexyl phosphate P C O 200 Relative density Relative density of water 6 4 2 (B) 100 0 0 30 35 40 45 30 50 35 45 50 (C) Lauryl phosphate P C O 200 Relative density 40 Distance, Angstrom Distance, Angstrom 100 0 30 35 40 45 50 Distance, Angstrom Figure 6.8 Relative density distribution of selected atoms along the normal to the bastnaesite basal plane surfaces. Water (A), 2-ethylhexyl phosphate (B) and lauryl phosphate (C). Mean square displacement of water, Å2 118 2000 Lauryl phosphate solution D= 5.7410-3 cm2/s 2-ethylhexyl phosphate solution D= 5.2110-3cm2/s water D= 3.6110-3 cm2/s 1500 1000 500 (A) 0 0 100 200 300 400 500 Mean square displacement of collector, Å2 Time, ps Lauryl phosphate solution D= 3.6010-7 cm2/s 2-ethylhexyl phosphate solution D= 2.6210-7 cm2/s 20 15 10 5 (B) 0 0 100 200 300 400 500 Time, ps Figure 6.9 Mean square displacement of water, (A) collector (B) in 2-ethylhexyl phosphate and lauryl phosphate solution on bastnaesite (100) surface. 119 Table 6.1 Captive bubble contact angle comparison between lauryl phosphate and 2ethylhexyl phosphate 1×10-4 M Lauryl phosphate 1×10-4 M 2-ethylhexyl phosphate 5×10-5 M Lauryl phosphate 5×10-5 M 2-ethylhexyl phosphate Bastnaesite Calcite Bastnaesite Calcite Bastnaesite Calcite Bastnaesite Calcite pH 4 80° 45° 35° 36° 30° 15° pH 6 85° 32° 40° 25° 35° 30° 20° 11° pH 8 40° 27° 30° 15° 30° 26° 15° 5° pH 10 8° 0° 0° 0° - 120 Table 6.2 Zeta potential comparison between lauryl phosphate and 2-ethylhexyl phosphate, mV. 10 mM KCl 5×10-6 M Lauryl phosphate; 10 mM KCl 5×10-6 M 2-ethylhexyl phosphate; 10 mM KCl Bastnaesite Calcite Quartz Bastnaesite Calcite Quartz Bastnaesite Calcite Quartz pH 4 58 5 -50 -20 -21 -45 10 -10 -51 pH 6 35 5 -65 -45 -25 -65 -10 -15 -60 pH 8 20 -3 -66 -51 -24 -68 -30 -16 -55 pH 10 -20 -8 -65 -58 -26 -75 -35 -11 -65 121 Table 6.3 Microflotation recovery with lauryl phosphate. -5 1×10 M Lauryl phosphate 3×10-6 M Lauryl phosphate pH 5 Difference pH 7 Bastnaesite 85% 76% Calcite 89% Bastnaesite 90% Calcite 15% 4% 75% 87% 82% 10% Difference 11% 72% pH 9 70% 79% 15% 8% Difference 9% 7% 122 Table 6.4 Selectivity comparison between lauryl phosphate and 2-ethylhexyl phosphate for mixed mineral flotation. 3×10-6 M, lauryl phosphate, pH 5.0 3×10-6 M, 2-ethylhexyl phosphate, pH 5.0 Bastnaesite recovery, % 80% Bastnaesite grade, % 52% 75% 95% 123 Table 6.5 Theoretical HOMO, LUMO, ΔE|HOMO-LUMO|, and ESP charge of the polar functional group for alkyl phosphate collectors. Species Lauryl phosphate anion 2-ethylhexyl phosphate anion HOMO, a.u. -0.22521 LUMO, a.u. 0.27540 ΔE|HOMO-LUMO|, a.u. 0.50061 ESP charge of – O(P=O)(OH)O -1.279 -0.23115 0.32962 0.56077 -1.265 CHAPTER 7 CONCLUSIONS Bastnaesite, (Ce,La)FCO3, is one of the critical mineral resources recovered by flotation for the supply of rare earth elements. Typical flotation collectors include fatty acid and hydroxamic acid. In this research, certain alkyl phosphates not studied previously were found to be potential collectors for the flotation of bastnaesite. Results from the contact angle, zeta potential, and microflotation experiments for bastnaesite indicate that better flotation is achieved at a lower concentration of lauryl phosphate when compared to octyl hydroxamate as a collector. Compared to octyl hydroxamic acid, excellent flotation of bastnaesite is achieved at a low concentration (5×10-6 M) of lauryl phosphate. Initial evaluation indicates that lauryl phosphate should be a promising collector for bastnaesite flotation due to its low price and the low dosage in the flotation. These results suggest that the lauryl phosphate collectors may be important to improve rare earth recovery from bastnaesite, and might help sustain the supply of rare earth elements, which is a critical commodity for US industries. The fundamental feature of potassium lauryl phosphate adsorption is crucial in the understanding of excellent flotation performance of lauryl phosphate when compared to octyl hydroxamate. In this regard, wetting characteristics and microflotation responses of bastnaesite were examined as a function of pH and at different levels of lauryl phosphate 125 adsorption. Theoretical computations for the bastnaesite-lauryl phosphate system were accomplished using the universal force field (UFF) and semiempirical quantum chemical methods. The interaction energy and frontier orbital results correlate remarkably well with the experimental contact angle and microflotation test results. The wetting characteristics of bastnaesite with adsorbed collector were examined using both contact angle measurements and molecular dynamics simulations (MDS). The adsorption isotherm at low levels of lauryl phosphate adsorption was established together with corresponding contact angle measurements. Finally, the relationship between hydrophobicity and adsorption density was examined by MDS, and compared to the results with octyl hydroxamate at low collector concentrations. The results improved the understanding of lauryl phosphate adsorption at the bastnaesite surface and confirmed that lauryl phosphate should be a better collector for bastnaesite when compared with octyl hydroxamate at low concentration (≤5×10-5 M). The selectivity of lauryl phosphate is critical in the flotation of bastnaesite from typical gangue minerals such as calcite and quartz. In this regard, contact angle, zeta potential, and microflotation experiments were used to examine the differential lauryl phosphate adsorption on the surface of bastnaesite, calcite, and quartz. The results suggest that lauryl phosphate is a promising collector in the selective flotation of bastnaesite from calcite and quartz. Furthermore, UFF interaction energy agrees with the experimental results, which further confirmed that lauryl phosphate has greater selectivity when compared to hydroxamate in the flotation of bastnaesite from calcite and quartz. The branched chain 2-ethylhexyl phosphate has even better selectivity when compared to lauryl phosphate. In this regard, flotation chemistry issues such as 126 hydrophobicity, selectivity, and adsorption state using alkyl phosphate in the flotation of bastnaesite were examined by contact angle and zeta potential measurements, microflotation and microcalorimetry experiments, density functional theory calculations, and molecular dynamics simulation. The contact angle and microflotation results show that bastnaesite flotation with alkyl phosphates (lauryl phosphate and 2-ethylhexyl phosphate) is accomplished at a remarkably lower collector concentration. Selective flotation of bastnaesite from calcite with 2-ethylhexyl phosphate increased the bastnaesite grade from 52% to 95% without sacrifice of recovery when compared to the case of lauryl phosphate. 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