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Starch modification of the flocculation and flotation of apatite Correa de Araujo, Armando 1988

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STARCH MODIFICATION OF THE FLOCCULATION AND FLOTATION OF APATITE by ARMANDO CORREA DE ARAUJO Mining Engineer,Universidade Federal de Minas Gerais,Hons.,1979 M.Sc., Universidade Federal de Minas Gerais,1982 A th e s i s submitted i n p a r t i a l f u l f i l m e n t of the requirements for the degree of DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES (Department of Mining and Mineral Process Engineering) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA February,1988 Vancouver,B.C. © Armando Correa de Araujo, 1988 3 9 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Mining and Mineral Process Engineering The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date February 24th. , 1988 DE-6G/81) i i ABSTRACT Although the technical l i t e r a t u r e contains abundant references on applications of starch i n mineral processing, the majority i s not concerned with phosphate mineral systems. Nevertheless, the i n t e r a c t i o n between starch and apatite surfaces i s relevant to both s e l e c t i v e f l o c c u l a t i o n and f l o t a t i o n of phosphate ores. The main objective of t h i s thesis i s to investigate i n d e t a i l such i n t e r a c t i o n i n order to provide a more c l e a r understanding on the behaviour of apatite/starch systems. Considerable research e f f o r t was dedicated to a thorough characterization of the starch samples used, e s p e c i a l l y i n those aspects most pertinent to the a p p l i c a t i o n of starches as flocculants and depressants. Presence of i o n i c impurities i n the starch samples tested was i d e n t i f i e d by i n f r a r e d spectroscopy and microelectrophoresis. These impurities(proteins, carboxylic groups and, possibly, phosphate esters)were found to play an important r o l e i n the mechanisms governing the i n t e r a c t i o n of starch macromolecules and mineral surfaces. In a f i r s t stage of t h i s research, the i n t e r a c t i o n between aqueous solutions of starches (and starch f r a c t i o n s - amylose and amylopectin) with calcium i o n i c species and s u r f a c t a n t s ( f l o t a t i o n c o l l e c t o r s ) was investigated. Depression of solution e l e c t r i c a l conductivity, experienced i n Ca-starch systems, was i n d i c a t i v e of chemical reactions taking place(complexation). For surfactants, evidence for t h e i r i n t e r a c t i o n with starch f r a c t i o n s was obtained by UV-Vis. spectroscopy. The spectra of starch/surfactant solutions i n the presence of iodine were alt e r e d i n d i c a t i n g the sub s t i t u t i o n of iodine species by surfactant molecules at the h e l i c a l s i t e s of starch macromolecules. The next step involved the study of the adsorption of starch onto both apatite and s i l i c a mineral surfaces. Preliminary t e s t s pointed out that a much stronger i n t e r a c t i o n took place i n the case of apatite. Starch adsorption isotherms obtained f o r f l u o r a p a t i t e and quartz confirmed the p r e f e r e n t i a l adsorption of starch onto the phosphate mineral surface. Both amylose and amylopectin were strongly adsorbed on f l u o r a p a t i t e but the l a t t e r polymer displayed the largest extent of i n t e r a c t i o n on a weight per area basis. The shape of the adsorption isotherms for the two starch f r a c t i o n s on f l u o r a p a t i t e also corroborates the idea of a stronger i n t e r a c t i o n by amylopectin. In turn, whole starches displayed adsorption isotherms resembling more c l o s e l y that obtained for amylopectin. Adsorption of starches on f l u o r a p a t i t e was increased considerably i n the presence of Ca i o n i c species. i v In the absence of externally added Ca i o n i c species, the amount of Ca released by the mineral surface was dependent upon the amount of starch adsorbed. These two phenomena can be interpreted as i n d i c a t i n g the importance of Ca s i t e s and presence of Ca species for the adsorption of starches,hence j u s t i f y i n g the p r e f e r e n t i a l adsorption displayed for apatite.Adsorption of starch on quartz surfaces was also enhanced i n the presence of Ca i o n i c species, once more confirming the important r o l e played by calcium on the adsorption of starches. Fl o c c u l a t i o n studies were also conducted with f l u o r a p a t i t e , k a o l i n i t e and quartz suspensions i n the presence of d i f f e r e n t starches.Under the conditions tested, a l l starches samples f a i l e d to promote aggregation of the two non Ca-bearing minerals.In turn, f l u o r a p a t i t e suspensions were flo c c u l a t e d rather strongly by a l l starches. Maximum f l o c c u l a t i o n of f l u o r a p a t i t e was achieved at p a r t i a l polymer coverage conditions. With one exception(amylose), increasing the concentration of the polymers above an optimum l e v e l , generated p a r t i a l r e - s t a b i l i z a t i o n of the suspensions,probably v i a a s t e r i c e f f e c t . A l l starches depressed both anionic and c a t i o n i c f l o t a t i o n of fluorapatite.Amylose was the l e a s t e f f e c t i v e depressant among a l l starches,especially for the c a t i o n i c V f l o t a t i o n system.The depressant action was a function of pH and of the r e l a t i v e amounts of polymer and surfactant. A l k a l i n e pH favoured depression,whereas as the c o l l e c t o r l e v e l was increased, the depressant action was diminished and eventually eliminated. The experimental evidence gathered i n the present research supports a chemical mechanism fo r the i n t e r a c t i o n between starch and apatite surfaces. Calcium plays a dominant r o l e , and i t s importance f o r the adsorption of starches onto mineral surfaces i s most probably related to the formation of complexes between starch impurity-related i o n i c groups and Ca i o n i c species.Hydrogen bonding and to l e s s e r extent e l e c t r o s t a t i c forces are also important for the o v e r a l l i n t e r a c t i o n between starch and apatite surfaces. The larger extent of i n t e r a c t i o n for the amylopectin fraction(highest molecular wt.) as compared to that of amylose(lowest molecular wt. fraction) gives support for the accessory r o l e of hydrogen bonding. T A B L E O F C O N T E N T S v i CHAPTER PAGE ABSTRACT i i TABLE OF CONTENTS v i LIST OF TABLES x i LIST OF FIGURES x i i i LIST OF PLATES x i x ACKNOWLEDGMENTS xx 1 - INTRODUCTION 01 2 - OBJECTIVE 03 3 - SCOPE AND IMPORTANCE OF THE PRESENT WORK 04 4 - REVIEW OF THE LITERATURE 06 4.1 - Phosphate Minerals,Rock and Ores 06 4.1.1 - Terminology 06 4.1.2 - Production and Reserves 08 4.1.3 - Uses 09 4.1.4 - Mineralogy and Geology 11 4.1.5 - Infrared Spectroscopy and the Structure of Apatites 19 4.2 - Mining of Phosphate Ores 25 4.3 - F l o t a t i o n Beneficiation of Phosphate Ores 26 4.3.1 - Apatite/Water Interface 2 6 4.3.2 - Interaction Between Apatite and F l o t a t i o n Reagents 34 4.3.2.1 - Apatite/Collectors 35 2 M difiers 4 0v i i 4.4 - Starches 47 4.4.1 - Composition and Structure 47 4.4.2 - Molecular Weight 52 4.4.3 - Minor Constituents 55 4.4.4 - Solution Preparation and Retrogradation .... 56 4.5 - Selective F l o c c u l a t i o n and the Use of Starches i n Mineral Processing 57 4.5.1 - Basic P r i n c i p l e s 59 4.5.2 - S t a b i l i t y of Mineral Suspensions 61 4.5.3 - Dispersants 65 4.5.4 - (Selective) D e s t a b i l i z a t i o n of Mineral Suspensions 69 4.5.5 - Flocculant Applications 78 4.5.6 - E f f e c t of Dissolved Species on Adsorption and F l o c c u l a t i o n 80 4.5.7 - Selective Flocculation of Phosphates 81 5 - EXPERIMENTAL 89 5.1 - Preparation of Mineral Samples 89 5.2 - Chemicals 93 5.3 - Equipment and Techniques 98 5.3.1 - Si z i n g 98 5.3.2 - Surface Area Determination 100 5.3.3 - Spectroscopic and Photometric Techniques ... 100 5.3.3.1 - Infrared Spectroscopy 100 5.3.3.2 - U.V.-Visible Spectroscopy 101 5.3.3.3 - Light Scattering Photometry 103 5.3.4 - Electrophoresis and Related Experiments .... 106 v i i i 5.3.5 - Viscometry 108 5.3.6 - Conductivity Testwork 110 5.3.7 - Qua l i t a t i v e Dispersion Testwork 112 5.3.8 - Adsorption Testwork 113 5.3.9 - Single Mineral F l o c c u l a t i o n Testwork 116 5.3.10- M i c r o f l o t a t i o n Testwork 119 5.3.11- Miscellaneous 121 5.3.11.1 - Mixing and Shaking 121 5.3.11.2 - pH Measurement 122 5.3.11.3 - F i l t r a t i o n and Drying 122 5.3.11.4 - Centrifugation 12 3 5.3.11.5 - Weighing 123 5.3.11.6 - Time Measurement 123 5.3.11.7 - Microscopy 12 3 6 - RESULTS ANS DISCUSSION 12 5 6.1 - Characterization of Mineral Samples 125 6.1.1 - Infrared Spectroscopy and Other Techniques . 125 6.1.2 - S i z i n g and Surface Area Measurement 141 6.1.3 - Electrophoresis 147 6.1.4 - Dispersion Studies 159 6.2 - Characterization of Starches 175 6.2.1 - Infrared Spectroscopy 175 6.2.2 - Electrophoresis 192 6.2.3 - Viscometry 19 6 6.2.4 - Light Scattering and Solution Preparation .. 199 ix 6.3 - Interaction Between Starches and Solution Species 212 6.3.1 - Starches/Surfactants 212 6.3.2 - Starches/Calcium Aqueous Species 232 6.4 - Adsorption Studies 240 6.4.1 - Preliminary Work 240 6.4.2 - Selection of A n a l y t i c a l Procedure 247 6.4.3 - Adsorption K i n e t i c s 248 6.4.4 - Equilibrium Adsorption Tests 251 6.4.4.1 - Adsorption of Starch on the Absence of Other Solutes 251 6.4.4.2 - E f f e c t of Calcium Ionic Species 260 6.5 - Fl o c c u l a t i o n Studies:Starch as a Flocculant .... 2 67 6.5.1 - Preliminary Tests 267 6.5.2 - Graduated Cylinder Tests 271 6.5.3 - E f f e c t of Calcium Ions 279 6.6 - M i c r o f l o t a t i o n Studies:Starch as a Depressant .. 282 6.6.1 - F l o a t a b i l i t y of Fluorapatite i n the Absence of Starch 282 6.6.1.1 - Na Oleate 285 6.6.1.2 - Dodecylamine Hydrochloride 288 6.6.2 - Action of Starches 296 7 - GENERAL DISCUSSION 3 08 7.1 - The Importance of the Preparation of Starches Solutions 308 7.2 - S e l e c t i v i t y of Adsorption 310 7.3 - The Role of Starches i n Apatite F l o c c u l a t i o n Systems 314 X 7.4 - The Role of Starches i n Apatite F l o t a t i o n Systems 317 8 - CONCLUSIONS 321 9 - SUGGESTIONS FOR FURTHER WORK 326 10- REFERENCES 328 APPENDICES 345 I - FURTHER CHARACTERIZATION OF MINERAL SAMPLES 346 II - INFRARED SPECTRA OF COLLECTORS 348 I I I - CALIBRATION CURVES FOR STARCH ANALYSIS AND STARCH SOLUTION PREPARATION 351 IV - STATISTICAL TREATMENT OF DATA 357 x i LIST OF TABLES 4.1 - World Phosphate Production(1980-1984) and Reserves 9 4.II - Some Examples of Minerals i n the Apatite Series (Nriagu, 1984) 12 4 . I l l - Chemical Analyses of Apatite (major elements only) 13 4.IV - Some Possible Substituions i n the Apatite Structure (after Nathan,1984) 15 4.V - Experimentally Determined iep(pH) and pzc(pH) of Apatites 31 4.VI - Amylose Content of Various Starches 51 4.VII - Molecular Weight of Starch F r a c t i o n ( a f t e r Young, 1984) 53 4.VIII- Selective Flocculation of Phosphate Ores and Minerals 8 3 4.IX - Selective Flocculation(Corn Starch,1 kg/t)/ F l o t a t i o n of Sedimentary Phosphate Ores (de Araujo et al.,1986) 88 5.1 - Chemical Characterization of Mineral Samples ... 91 5.II - Chemicals 94 6.1 - Vibra t i o n Spectra of Apatites.PO Assignments .. 134 6.II - Assignments of I.R. Vibra t i o n Modes of Apatites (spectra shown i n figures 6.1 and 6.2) 137 6 . I l l - Assignements of I.R. Vibra t i o n Modes to Quartz and Kaolinite(spectra shown i n figures 6.3 and 6.4) 139 6.IV - BET Surface Area Determinations 146 6.V - Determination of pzc(pH) of Fluorapatite by the Ahmed's Method 161 6.VI - Action of Selected Dispersants on the S t a b i l i t y of Mineral Suspensions 168 x i i 6.VII - E.M. of Quartz and Fluorapatite i n the Presence of Selected Dispersants 171 6.VIII- Infrared Absorption Bands of aD(+)-Glucose 177 6.IX - Major C-H Vibrations i n the I.R. Spectra of Starches (figures 6.20 to 6.23) 185 6.X - Major O-H vibrations i n the I.R. Spectra of Starches (figures 6.20 to 6.23) 187 6.XI - C-0, C-OH and Ring Vibrations i n the I.R. Spectra of Starches(figures 6.20 to 6.23) 188 6.XII - Tentative Assignments of Impurity Related Bands i n the I.R. Spectra of Starches (figures 6.20 to 6.23) 191 6.XIII- Microelectrophoretic M o b i l i t i e s of Starch Grains i n D i s t i l l e d Water 195 6.XIV - E l e c t r i c a l Conductivity Depression f o r Corn and Tapioca Starch Solutions i n the Presence of 2 X 10 _ 3mol/l CaCl 2at pH 10.2 239 6.XV - Comparison of A n a l y t i c a l Procedures Commonly Used f o r the Determination of Starch Concentration i n Aqueous Solutions 249 6.XVI - E f f e c t of pH on the Depressant Action of Starches 303 6.XVII- E f f e c t of Col l e c t o r Concentration on the Depression Caused by Starches 303 6.XVIII-Effect of Col l e c t o r Concentration on the Depression of Fluorapatite by Tapioca Starch ... 306 I I . A - Assignments of Infrared Bands of Col l e c t o r s (Spectra shown i n Figure II.1) 348 I I I . A - Preliminary Results Obtained with Potato Starch. 352 x i i i LIST OF FIGURES 4.1 - Structure of Apatite(Young,1980 and Montel et. al.,1977) 23 4.2 - The Posi t i o n of the Calculated Line of Zero Charge(lzc) Shown by the Heavy Dashed Line.Also Shown Are Some Experimental pzc Values on the S o l u b i l i t y Surface of Hydroxyapatite(for d e t a i l s see o r i g i n a l work by Chander and Fuerstenau, 1979) 29 4.3 - Hallimond Tube M i c r o f l o t a t i o n Results of Apatites i n the Presence of 5 E(-5)mol/l Na Oleate as a Function of pH( 1 - S i l v a et a l . , 1985; 2 - Smani et al.,1975) 37 4.4 - Schematic View of (a) Glucose(monomer), (b) Amylose and (c) Amylopectin (polymers) (Leja, 1982) 49 4.5 - Fundamental Stages i n the Selective F l o c c u l a t i o n of a Two-Phase Ore(Read and Hollick,1976) 60 4.6 - F l o c c u l a t i o n by Polymers under Complete and P a r t i a l Coverage Conditions(Somasundaran,1980) .. 70 4.7 - Adsorption of Corn Starch (a) and Cationic Starch (b) on Hematite (H) and Quartz (Q) (Balajee and Iwasaki,1969) 7 3 4.8 - A Model Showing Mode of Adsorption by Various Starches on Negatively Charged Oxide Mineral Surfaces (Balajee and Iwasaki,1969) 76 5.1 - Schematic View of Tube Used f o r Preliminary Dispersion/Flocculation Tests 117 6.1 - Infrared Spectra of Apatites 12 7 6.2 - Infrared Spectrum of Synthetic Hydroxyapatite ... 12 8 6.3 - Infrared Spectrum of Quartz 129 6.4 - Infrared Spectrum of K a o l i n i t e 130 6.5 - The PO^Group: (I) Regular Tetrahedron with Symmetry T D (II) Distorted Tetrahedron with Symmetry C s (Klein et a l . ,1970) 132 x i v 6.6 - Size D i s t r i b u t i o n of Fluorapatite 142 6.7 - Size D i s t r i b u t i o n of Quartz 143 6.8 - Size D i s t r i b u t i o n of Fluorapatite(Beaker Decantation "Overflow") 145 6.9 - Electrophoretic M o b i l i t y of Fluorapatite as a Function of pH 148 6.10- Electrophoretic M o b i l i t y of Hydroxyapatite as a Function of pH 150 6.11- E f f e c t of A l Aqueous Species on the Electrophoretic M o b i l i t y of Fluorapatite 151 6.12- E f f e c t of Ca and Mg Ionic Species on the Electrophoretic M o b i l i t y of Fluorapatite 153 6.13- E f f e c t of Al,Ca and Mg Species on the Electrophoretic M o b i l i t y of Quartz 157 6.14- Electrophoretic M o b i l i t y of K a o l i n i t e as a Function of pH 158 6.15- Determination of the pzc(pH) of Fluorapatite by the Mular and Roberts Method 160 6.16- S t a b i l i t y of Fluorapatite Suspensions as a Function of pH 164 6.17- S t a b i l i t y of Quartz Suspensions as Function of pH 165 6.18- S t a b i l i t y of K a o l i n i t e Suspensions as Function of pH 167 6.19- Infrared Spectrum of a D(+)-Glucose 179 6.20- Infrared Spectra o f : ( l ) - Potato Amylopectin and (II) - Potato Amylose 181 6.21- Infrared Spectra o f : ( l ) - Tapioca Starch and (II) - Potato Starch 182 6.22- Infrared Spectra o f : ( l ) - Collamil(Commercial Grade Corn Starch) and (II) - Corn Starch 183 6.23- Infrared Spectra of Corn Starch:(1) - As i n F i g . 6.22 and (2) - New 184 XV 6.24- Determination of the Limiting V i s c o s i t y Number of Potato Amylose 198 6.25- Tu r b i d i t y of Potato Amylopectin, Potato Starch and Tapioca Starch Solutions as a Function of Solution Concentration 2 02 6.26- Turbi d i t y of Corn Starch Solutions 203 6.27- E f f e c t of Aging on the Turbidity of Tapioca Starch Solutions 2 04 6.28- E f f e c t of Temperature on the Turbi d i t y of Aged Tapioca Starch Solution 206 6.29- E f f e c t of pH on the Turbidity of Potato Amylopectin Solution 2 08 6.3 0- E f f e c t of Shearing on the Turbi d i t y of Corn Starch Solution 2 09 6.31- E f f e c t of Sonic Treatment on the Turbi d i t y of Potato Amylopectin Solution 210 6.32- U l t r a v i o l e t ( U V ) - V i s i b l e ( V i s . ) Spectrum of K l / Iodine Aqueous Solution 216 6.33- UV-Vis. Spectra of (a) Potato Amylose and (b) Potato Amylopectin 218 6.34- Determination of Absorption Maxima i n the UV-Vis. Spectra of Potato Amylose and Potato Amylopectin i n the Presence of Iodine 219 6.35- E f f e c t of pH on the UV-Vis. Spectra of (a) Potato Amylose and (b) Potato Amylopectin 220 6.36- UV-Vis. Spectrum of Potato Amylose at pH 1.5 i n the Presence of Iodine 221 6.37- UV-Vis. Spectra of Na Oleate and Dodecylamine Hydrochloride(DDAHCl) i n the Presence of Iodine . 22 3 6.38- E f f e c t of pH on the UV-Vis. Spectra of Potato Amylose i n the Presence of Iodine and Na Oleate (a) - pH6 and (b) pHIO 224 6.39- E f f e c t of Na Oleate on the UV-Vis. Spectrum of Potato Amylose at pH3.5 i n the Presence of Iodine 225 xv i 6.40- E f f e c t of DDAHC1 on the UV-Vis. Spectra of Potato Amylose i n the Presence of Iodine 226 6.41- E f f e c t of DDAHC1 on the UV-Vis. Spectrum of Potato Amylose at pH3 i n the Presence of Iodine . 228 6.42- E f f e c t of (a) Na Oleate and (b) DDAHC1 on the UV-Vis. Spectra of Potato Amylopectin i n the Presence of Iodine 229 6.43- Solution Conductivity of Potato Amylopectin i n the Presence and Absence of Calcim Chloride 2 34 6.44- Solution Conductivity of Potato Amylose i n the Presence and Absence of Calcium Chloride 2 35 6.45- Solution Conductivity of Potato Starch i n the Presence and Absence of Calcium Chloride 236 6.46- Adsorption Kin e t i c s of Potato Starch on Fluorapatite 250 6.47- Adsorption Isotherms of Potato Starch, Amylose and Amylopectin on Fluorapatite 252 6.48- Adsorption Isotherm of Tapioca Starch on Fluorapatite 255 6.49- Residual Calcium Concentration upon Adsorption of Potato and Tapioca Starches on Fluorapatite 256 6.50- E f f e c t of pH on the Adsorption of Tapioca Starch on Fluorapatite 259 6.51- E f f e c t of Ca Ionic Species on the Adsorption of Potato Amylopectin on Fluorapatite 2 61 6.52- E f f e c t of Ca Ionic Species on the Adsorption of Tapioca Starch on Fluorapatite 2 62 6.53- Adsorption Isotherms of Tapioca and Potato Starches on Quartz 2 63 6.54- E f f e c t of Ca Ionic Species on the Adsorption of Tapioca Starch on Quartz 2 65 6.55- Degree of Flo c c u l a t i o n of Fluorapatite and Ka o l i n i t e i n the Presence of Potato Starch, Amylose and Amylopectin 269 x v i i 6.56- Degree of Floc c u l a t i o n of Fluorapatite and Ka o l i n i t e i n the Presence of Tapioca Starch 270 6.57- Degree of Floc c u l a t i o n of Fluorapatite and Ka o l i n i t e as a Function of pH 272 6.58- S e t t l i n g Behaviour of Fluorapatite Suspensions .. 273 6.59- S e t t l i n g Rate of Fluorapatite Suspensions i n the Presence of Potato Starch, Amylose and Amylopectin 275 6.60- S e t t l i n g Rate of Fluorapatite Suspensions i n the Presence of Corn and Tapioca Starches 276 6.61- E f f e c t of Ca Ionic Species on the S e t t l i n g of Fluorapatite Suspensions i n the Presence of Tapioca Starch 280 6.62- Hallimond Tube Entrainment Related F l o a t a b i l i t y . 284 6.63- M i c r o f l o t a t i o n of Fluorapatite i n the Presence of Na Oleate as a Function of pH 286 6.64- M i c r o f l o t a t i o n of Fluorapatite i n the Presence of DDAHC1 as a Function of pH 289 6.64a-Detail of the Mi c r o f l o t a t i o n Response of Fluorapatite i n the Presence of DDAHC1 as a Function of pH 290 6.65- M i c r o f l o t a t i o n of Apatite from I t a t a i a i n the Presence of DDAHC1 as a Function of pH 293 6.66- E f f e c t of Potato Starch,Amylose and Amylopectin on the Mi c r o f l o t a t i o n of Fluorapatite with Na Oleate 297 6.67- E f f e c t of Tapioca Starch on the M i c r o f l o t a t i o n of Fluorapatite with Na Oleate 299 6.68- Depression of Fluorapatite by Potato Starch at a Higher Na Oleate Level 301 6.69- E f f e c t of Potato Starch, Amylose and Amylopectin on the M i c r o f l o t a t i o n of Fluorapatite with DDAHC1 304 6.70- E f f e c t of Tapioca and Corn Starches on the Mi c r o f l o t a t i o n of Fluorapatite with DDAHC1 3 05 x v i i i I I . 1- Infrared Spectra of:(1) - Na Oleate and (2) -Dodecylamine Hydrochloride 350 I I I . 1 - I n i t i a l C a l i b r a t i o n for Potato Starch 353 I I I . 2 - F i n a l C a l i b r a t i o n f o r Potato Starch 354 x i x LIST OF PLATES A - Reaction of Starch with Iodine 214 B - Interaction of Potato Starch,Amylose and Amylopectin with S i l i c a and Hydroxyapatite 24 3 C - Details of Sediments i n Test Tubes "a" and "b" Shown i n Plate B 244 D - Deta i l s of Sediments i n Test Tubes "c" and "d" Shown i n Plate B 245 E - Det a i l s of Sediments i n Test Tubes "g" and "h" Shown i n Plate B 246 ACKNOWLEDGMENTS xx The author wishes to express h i s gratitude to Dr. George W. Poling, f o r h i s valuable guidance and assistance during the course of the present work. Sincere thanks should also be extended to a l l committee members.Many useful c r i t i c i s m s and comments provided by Dr.G.D. Senior (UBC now with CSIRO,Australia),Dr.J.S.Laskowski(UBC), Dr.J.Leja(UBC) and Dr.A.E.C.Peres (Universidade Federal de Minas Gerais,Brazil)are also acknowledged with appreciation. Technical assistance was provided by a number of people at various stages of t h i s research.For t h e i r considerable help, e s p e c i a l l y to Mrs. S. Finora and Mr. F. Schmidiger,my sincere thanks. The author also wishes to express h i s gratitude to a l l fellow graduate students at the Dept. of Mining and Mineral Process Engineering, e s p e c i a l l y to Mr. Li u Qi,Mr. Bernhard K l e i n and Mr. Marty Hudyma. Drafting and typing assistance was given by Miss Charlotte H a l l and Mrs. Marilda Brandao. To them my sincere appreciation. Help i n a l l aspects was contantly provided by my wife Mary. Without her support, understanding and caring, t h i s work would never have been fi n i s h e d . To Mary I dedicate t h i s thesis and express my most deep gratitude. F i n a n c i a l support was mainly provided by CNPq(Conselho Nacional de Desenvolvimento C i e n t i f i c o e Tecnologico,Brazil) and also by the William A. MacKenzie Scholarship and Cy and Emerald Keyes Fellowship. The author i s deeply indebted to h i s sponsors and also to the Universidade Federal de Minas Gerais f o r the concession of a leave of absence. 1 1 - INTRODUCTION Starches are extensively used i n the processing of many ores. They can act as depressants i n both anionic and c a t i o n i c f l o t a t i o n c i r c u i t s . In the past they have also been used as flo c c u l a n t s f o r f i n e t a i l i n g s , and today they are s t i l l used as s e l e c t i v e f l o c c u l a n t s i n the s e l e c t i v e f l o c c u l a t i o n - f l o t a t i o n of low grade i r o n ores. F l o t a t i o n of phosphate ores i s a r e l a t i v e l y well established p r a c t i c e . Although t h i s process has been i n use for more than 40 years, many problems are s t i l l encountered. Among them, high losses to fine s , poor s e l e c t i v i t y f o r c a l c i t i c and / or dolomitic ores and low enrichment r a t i o s are most frequent. In the f l o t a t i o n of some phosphate ores, starches act as depressants of c a l c i t e and dolomite i n the anionic f l o t a t i o n of apatite. They have been tested as s e l e c t i v e f l o c c u l a n t s f o r phosphate fines i n many instances. One of the f i r s t attempts i n the f i e l d of s e l e c t i v e f l o c c u l a t i o n was r e l a t e d to the b e n e f i c i a t i o n of phosphate fines discarded i n the processing of sedimentary phosphate ores from F l o r i d a , USA. Vast amounts of r e l a t i v e l y r i c h phosphatic "slimes" are l o s t i n the b e n e f i c i a t i o n of these ores. Haseman (1953) patented a process to increase the o v e r a l l recovery by s e l e c t i v e l y f l o c c u l a t i n g these slimes with starch. Although the r e s u l t s were encouraging, due to some tec h n i c a l d i f f i c u l t i e s a l l i e d to economic considerations, the studies 2 were not further pusued. Since then, several other attempts were made i n t h i s f i e l d . They w i l l be discussed l a t e r on i n t h i s t h e s i s . The l i t e r a t u r e review section of t h i s t h e s i s gives an o v e r a l l picture of the phosphate mineral industry, the state-of-the-art of the b e n e f i c i a t i o n of phosphate ores and the chemistry and physico-chemistry of starches and t h e i r applications i n mineral processing. 3 2 - OBJECTIVE The objective of t h i s thesis i s to investigate the fundamental aspects of the f l o t a t i o n and f l o c c u l a t i o n of apatite i n the presence of d i f f e r e n t starches. To achieve t h i s objective the following research was conducted: (i) - characterization of the starch samples used: t h e i r composition and aqueous so l u t i o n chemistry. ( i i ) - study of the adsorption behaviour of starches onto apatite surfaces. ( i i i ) - study of f l o c c u l a t i o n of apatite with starches. (iv) - study of the depressant action of starches i n the f l o t a t i o n of apatite with sodium oleate and dodecylamine hydrochloride. 4 3 - SCOPE AND IMPORTANCE OF THE PRESENT WORK The scope of t h i s thesis i s to investigate i n d e t a i l the fundamental aspects of the i n t e r a c t i o n between starch and apatite surfaces, as applied to the f l o t a t i o n and f l o c c u l a t i o n of t h i s mineral. The model system chosen includes pure mineral samples of apatite and d i f f e r e n t starches (reagent and commercial grade). Many other chemicals were u t i l i z e d , including pH modifiers, s a l t s of commonly occuring ions such as calcium and magnesium, and d i f f e r e n t dispersants (other than pH modifaction). The s e l e c t i o n of these reagents was based on published information on t h i s subject (current p r a c t i c e and research papers), and also on some preliminary testwork. Although previous research work on s i m i l a r systems i s a v a i l a b l e i n the l i t e r a t u r e , fundamental studies such as the adsorption behaviour of starches onto apatite surfaces are very scanty. In contrast f o r example, the i n d u s t r i a l a p p l i c a t i o n of the s e l e c t i v e f l o c c c u l a t i o n process f o r the treatment of low grade i r o n ores was pre-dated by many studies of fundamental character (Iwasaki, 1965, Iwasaki and co-workers, 1969; Balajee and Iwasaki 1969; j u s t to c i t e a few). One of the main contributions of the present work i s to f i l l t h i s void i n the f i e l d of the b e n e f i c i a t i o n of phosphate ores. 5 This work also aims to provide a more complete understanding of the mechanisms p r e v a i l i n g i n the i n t e r a c t i o n between starches and apatites. For instance, the r o l e of calcium aqueous i o n i c species i n both adsorption and f l o c c u l a t i o n of apatite with starch was found to be very important. The very high a f f i n i t y of starch f o r the apatite surface i s probably a d i r e c t consequence of the presence of Ca s i t e s at the interface. The importance of Ca i n the adsorption of starches on apatite suggests that monitoring i t s concentration i n f l o t a t i o n and f l o c c u l a t i o n systems involving apatite and other Ca bearing minerals i s , at least, strongly advised. 6 4 - REVIEW OF THE LITERATURE 4.1 - Phosphate Minerals, Rock and Ores 4.1.1 - Terminology (Anonymous, 1981 and Notholt and Hartley, 1983) The term "phosphate rock" (or "commercial phosphate rock"), as normally used i n the phosphate industry, re f e r s to a sedimentary phosphatic material, concentrated or only washed and c l a s s i f i e d , containing a minimum saleable grade. In some cases, phosphate rock has a more general meaning, being used to describe any phosphate product that i s traded i n the market, i r r e s p e c t i v e of the the o r i g i n of the material. In t h i s report phosphate rock^*) w i l l be used i n i t s more general sense, describing ei t h e r a treated ore (an igneous or a sedimentary phosphate concentrate) or a c l a s s i f i e d , washed and/or calcined high grade ore. Apatite i s also a term used i n the industry to describe the phosphate concentrate produced from an igneous ore. Due to the adopted meaning of phosphate rock, apatite w i l l not be used here i n t h i s meaning. Wherever apatite os referre d to i t w i l l have only i t s mineralogical meaning. (*) Phosphorite i s the geological term used to describe a sedimentary phosphate rock. Although most phosphate ores mined today are phosphorites, a phosphorite i s e s s e n t i a l l y a geological term, without any r e l a t i o n s h i p to the e x p l o i t a b i l i t y of the rock as a phosphate ore. 7 A phosphate ore i s defined f o r the purpose of t h i s report as any rock (sedimentary, metamorphic or igneous) containing enough phosphorus to make i t an economically exploitable source of phosphate. Phosphorus pentoxide (%P205) i s one of the conventional ways to express value content i n the phosphate industry. Also very common are %B.P.L. The conversion figures are as follows: %P 20 5 = 0.4576% B.P.L. or %T.P.L. %P 20 5 = 2.2915% P Marketable phosphate rock entering i n t e r n a t i o n a l trade usually contains more than 30% P 2 0 5 (65% B.P.L.). Some r e s t r i c t i o n s concerning other elements apply. For example, the so c a l l e d "R 20 3 grade" (A1 20 3 + Fe 20 3) may not exceed 3 - 5% i n most cases, and a chloride content greater than 600 ppm can cause serious corrosion problems. Lower grade phosphate rock (approx. 24 - 2 6% P 20 5) i s used i n some cases f o r d i r e c t a p p l i c a t i o n to impoverished s o i l s . Details on the d i f f e r e n t s p e c i f i c a t i o n s f o r phosphate rock are given by Lehr and McClellan, 1973 and Gremillion and McClellan, 1975. 8 4.1.2 - P r o d u c t i o n and Reserves T a b l e 4.I shows the world phosphate rock p r o d u c t i o n by c o u n t r y f o r t h e p e r i o d from 1980 t o 1984. In t h e same t a b l e measured r e s e r v e s of phosphate ores a r e a l s o i n c l u d e d . Data f o r t h e 1984 f e r t i l i z e r year were r e c e n t l y p u b l i s h e d by Weston (1985). They show an i n c r e a s e o f approximately 6.5% over the 1983 f i g u r e (143 m i l l i o n tonnes i n 1984 compared t o 134 m i l l i o n tonnes i n 1983). World p r o d u c t i o n i s expected t o double by the ye a r 2000 (Lawver e t a l . , 1978). Such a dramatic i n c r e a s e r e f l e c t s the need t o i n c r e a s e a g r i c u l t u r a l y i e l d s through the use of f e r t i l i z e r s . D i s c u s s i o n on the p r o d u c t i o n and demand t r e n d s and more d e t a i l e d i n f o r m a t i o n on t h i s s u b j e c t , can be found on the l i t e r a t u r e (see f o r example: Emigh, 1978 and 1983; Barry, 1980; Stowasser, 1980; Manderson, 1978; Anonymous, 1981). World r e s e r v e s shown i n Table 4.I r e p r e s e n t o n l y e x p l o i t a b l e m a t e r i a l t h a t i s reasonably w e l l known. The p r i n c i p a l r e s e r v e s are i n Morocco and the U.S.S.R. (approximately 54% and 13% of the world t o t a l , r e s p e c t i v e l y ) . R e l a t i v e l y w e l l known re s o u r c e s ( p r e s e n t l y uneconomic d e p o s i t s , due t o e i t h e r c u r r e n t p r i c e s o r l a c k of technology) o f phosphate rock, amount t o the o r d e r of a t l e a s t 200-300 b i l l i o n tonnes (Notholt and H a r t l e y , 1983). Emigh (1972) estimated the p o t e n t i a l r e s o u r s e s of phosphate at 1,177,000 m i l l i o n tonnes. TABLE 4.1 WORLD PHOSPHATE PRODUCTION (1980-1984) AND RESERVES(*) 1,000 tonnes C o u n t r y 1980 1981 1962 1983 19.84 v 'Reserves N o r t h A m e r i c a 54,745 53,745 37,829 41,890 48,820 U n i t e d S t a t e s 54,415 53,624 37 ,414 41,890 48,820 1,800,000 Mexico 330 252 415 — — 500,000 TOTAL 2,300,000 South America 2 ,940 2,791 2,732 3 ,208 3,400 B r a z i l 2,921 2,764 2,732 3 ,208 3 ,400 Colombia 5 15 18 - -P e r u 14 12 29 -TOTAL 2,500,000 Europe 24 ,876 25 ,549 26,963 27 ,700 28,890 E a s t e r n Eur.) 24,668 25,220 26,600 27,700 28,890 4,500,000 and U.S.S.R.) Other 208 329 363 - - 15,000 TOTAL 4,515,000 A f r i c a 32 ,893 33,200 29 ,906 32 ,103 33,622 Morocco 18 ,824 19 ,696 17,753 20 ,106 21 ,13318,400,000 S e n e g a l 1,459 1,927 975 1,250 1,874 130,000 South A f r i c a 3,282 3,034 3,173 2 ,742 2,593 3,000,000 T u n i s i a 4,582 4,596 4,196 • 5,924 5,346 6,000,000 Other 4,748 3,947 3 ,809 2,081 2,696 3,100,000 TOTAL 23,430,000 A s i a 20,247 20,910 23,038 22,646 25 ,139 I s r a e l 2,611 2,373 2,711 2,969 3,312 100,000 J o r d a n 4,243 4 ,244 4,431 4,749 6,263 100,000 C h i n a 10,726 11,500 12 ,500 12,500 13,200 1,000,000 S y r i a 1,319 1,321 1,455 1,229 1,514 200,000 Vietnam 400 500 500 - - 100,000 Other 948 972 1,441 1,199 850 100,000 TOTAL 1,600,000 O c e a n i a 3 ,800 2,918 2,922 2,779 2,611 Aus t r a l i a / 1,713 1,438 1,563 1,095 1,252 100,000 C h r i s t m a s I s . Nauru 2,087 1,480 1,359 1,684 1,359 TOTAL 100 ,000 Not S p e c i f i e d 3,995 4 ,095 WORLD TOTAL 139 ,501 139,244 123,437 134 ,321 142 ,977 34,445,000 (*) Data i n t h i s t a b l e are from: (a) M i n i n g Annual Review (1982, 1983, 1984 and 1985, Phosphate by M.C. Mew and M.R. Freeman) (b) U.S. Bureau of Mines, M i n e r a l F a c t s and P r o b l e m s , 1980 and 1981. (**) P r e l i m i n a r y . At present rates of increase, known reserves should , l a s t f o r at l e a s t 200 years. 4.1.3 - Uses More than 90% of world phosphate rock i s u t i l i z e d f o r a g r i c u t u r a l purposes, mainly i n the manufacture of phosphatic f e r t i l i z e r s (Notholt and Hartley, 1983). Low-grade " s t r a i g h t " phosphatic f e r t i l i z e r s , such as superphosphate are now being superceded by highly concentrated, multi-nutrient (NPK) formulations, based on ammonium phosphate and t r i p l e superphosphate. These high-grade f e r t i l i z e r s now account f o r more than one-half of world consumption of phosphate rock. De t a i l s of the production of phosphoric acid (intermediate product) and NPK f e r t i l i z e r s can be found i n the l i t e r a t u r e (de Lima, 1976; McLellan, 1983; Anonymous, 1981). Other uses include the manufacture of animal feed supplements, d i r e c t a p p l i c a t i o n to a c i d i c s o i l s , detergents, soaps, f i r e retardants, elemental phosphorus, i n s e c t i c i d e s , and pharmaceutical chemicals (de Lima, 1976 and Notholt and Hartley, 1983). 11 4.1.4 - Mineralogy and Geology Although there are at le a s t 350 recognized phosphate minerals (Nriagu, 1984), very few of them are important i n terms of phosphate ores. A c t u a l l y only those i n the apatite se r i e s can be regarded as ore minerals f o r phosphate. Table 4.II presents a summary of some properties of members of the apatite s e r i e s . Table 4 . I l l gives a compilation of chemical analyses of various apatites, some of which are present i n the phosphate ores. There i s no simple way to represent these minerals by a unique chemical formula. For apatites of igneous or metamorphic o r i g i n , one possible generalized formula i s as follows: Me l f J(X0 4) 6Z 2 where Me = Ca, Sr, Ba, Mn, Y X = P (mainly) Z = OH, F, C l , Br. Sedimentary apatites are much more complex and the degree of su b s t i t u t i o n of l a t t i c e ions i s much higher. Nathan (1984) gives some of the p o s s i b i l i t i e s (see table 4.IV). The existence of so many substitutions i s rela t e d to the type of l a t t i c e of apatite. An "open l a t t i c e " , allows TABLE 4 . I I - Some Examples o f M i n e r a l s i n t h e A p a t i t e S e r i e s ( f r o m N r i a g u , 1 9 8 4 ) ( * ) Compound Fo r m u l a D e n s i t y S t r o n g e s t D i f f r a c t i o n D i s c r e d i t e d L i n e s Names C a 1 0 ( P 0 4 ) 6 F 2 C a 1 0 ( P O 4 ) 6 C 1 2 F l u o r a p a t i t e C h l o r a p a t i t e H y d r o x y a . p a t i t e C a 1 Q (PO^) fi (OH) 2 S t r o n t i u m - a p a t i te ( S r , C a ) i n ( P O ^ ) 6 ( O H , F ) 2 C a r b o n a t e - a p a t i t e C a 5 (PO^, , C 0 3) 3 (OH , F) ^ **^ WiIke i te 3.1-3.2 3.1-3.2 3.08 3.84 2.9-3. 1 C a c ( S i O . ,P0. ,S0.) 0(0,OH,F) 3.12 5 4 4 A 3 2.800(100) , 2.7.02 (60) 2.78(100) , 2 . 86 (55) 2 . 8 1 4 ( 1 0 0 ) , 2.778(60) 2 . 8 9 ( 1 0 0 ) , 3.167(70) 2 . 822 (100) , 2. 722 (90) 2.80(100) , 2. 70(90) Saami t e C o l l o p h a n e , Moni t e ; N a u r i t e , K u r s k i t e , G r o d n o l i t e (*) Most m i n e r a l s i n t h i s s e r i e s b e l o n g to t h e H a x a g o n a l System, c l a s s 6/m. (**) A t l e a s t two w e l l d e f i n e d v a r i e t i e s a r e known: F r a n c o l i t e ( C a 3 F ) 2 ( P , C ) g ( 0 , O H , F ) 2 4 ( C a , C ) 4 , and D a h l l i t e C a g ( O H ) 2 ( P , C ) 6 0 2 4 ( C a , C ) 4 ro (*) / TABLE A . I l l - C h e m i c a l A n a l y s e s of A p a t i t e s ( m a j o r e l e m e n t s o n l y ) 1 2 3 A 5 6 7 8 9 10 P2°5 42. 2 41. 3 40. 98 42. 19 42. 20 36. 3 37.1 40. 1 34. 7 38.2 CaO 55. 6 55 . 16 52 . 40 55 . 47 50.0 55. 4 55.5 55 . 6 55. 2 54. 7 F 3. 77 3 . 67 1. 15 1. 01 3.5 4. 68 4.56 4.09 4. 93 2.8 C l 0 0. 09 3. 74 - - - - - - n. a H 20 0 0 . 01 0 . 06 1. 73 - - - - n. a c o 2 0 0 . 05 - 4. 53 3.95 1. 59 5. 70 -O t h e r 0 1. 41 2. 27 0. 66 4. 40 1. 28 1.08 . 0 . 39 1. 80 0. 1 (*) r e f e r e n c e s f o r t h i s t a b l e a r e : P a l a c h e e t a l . , 1951; M c C l e l l a n and C l a y t o n , 1982; N r i a g u , 1984. 1 - T h e o r e t i c a l f l u o r a p a t i t e 2 - F l u o r a p a t i t e , O n t a r i o , Canada 3 - C h l o r l a n f l u o r a p a t i t e , K u r o h u r a , J a p a n 4 - F l u o r i a n h y d r o x y l a p a t i t e , H o s p e n t h a l , S w i t z e r l a n d 5 - F l u o r a p a t i t e , J a c u p i r a n g a , B r a z i l 6 - F r a n c o l i t e , Morocco 7 - F r a n c o l i t e , F l o r i d a , U.S.A. 8 - F r a n c o l i t e , Western U.S.A. 9 - F r a n c o l i t e , T u n i s i a 10 - F l u o r a p a t i t e , Durango, M e x i c o 14 d i f f e r e n t i o n i c species to replace one or a l l of the o r i g i n a l components of pure (end) members of the apatite s e r i e s . Carbonate-apatite ( i . e . apatite containing Co 3 ions p a r t i a l l y s u b s t i t u t i n g for P 0 3 4~ ions) i s the most common type of apatite found i n sedimentary deposits. F r a n c o l i t e (or carbonate-fluorapatite) probably has the chemical composition most nearly t y p i c a l of apatites u t i l i z e d as sources of phosphorus. McClellan and Clayton (1982) proposed a model for f r a n c o l i t e , incorporating the hydroxyl v a r i e t y , as follows: C a 1 0 - x - y N a x M 5 y ( p o 4 ) 6 - z ( c o 3 ) z F 0 . 4 z ( F ' 0 H ) 2 with y =0.4x and x/(6-z) =0.30. This model emphasizes the complexity of the chemical composition of d i f f e r e n t apatites. This complexity points to the need for s i t e s p e c i f i t y of mineral processing studies with phosphate ores. Although some properties might not change s i g n i f i c a n t l y with the o r i g i n of, and substituents i n the apatites, surface properties can be dominated even by small chemical changes. TABLE 4.IV - Some P o s s i b l e S u b s t i t u t i o n s ( a f t e r N a t h a n , 19 84) . i n t h e A p a t i t e S t r u c t u r e C o n s t i t u e n t i o n S u b s t i t u t i n g i o n 2+ + +' + • Ca Na , K , Ag 2+ 2+ 2+ 2+ 2+ 2 + Sr , Mg , Mn , Zn , Cd , Ba „ 3+ „3+ „ ^ 3+ „.3+ Sc , Y , R.E. , B l 4 + U 3- 2- 2- 2-PO^ > C 0 3 , S 0 4 , C r 0 4 A s 0 4 " , V 0 4 ~ , C 0 3 . E 3 _ , C 0 3 . O H 3 _ . 4-4 F~ 0H~, C l " , B r " o 2 -VJ1 16 Today's phosphate ores are b a s i c a l l y of two types: igneous and sedimentary. Sedimentary phosphates (mostly phosphorites) are the most important, accounting f o r more than 82% of the world production and over 95% of the known resources and reserves (Cook, 1984). In the case of igneous deposits there are again two major types: primary and secondary (or r e s i d u a l ) . Primary deposits are normally related to a l k a l i n e and c a r b o n a t i t i c intrusions (plutons). They generally have small areas (1 to 50 km ). These rocks are characterized by t h e i r high degree of c r y s t a l l i n i t y . In terms of mineralogy, the phosphate mineral i s usually apatite (predominantly f l u o r a p a t i t e ) . Non-phosphatic minerals include s i l i c a t e s such as feldspars and pyroxenes for a l k a l i n e intrusions (e.g. Kola Peninsula complex, U.S.S.R.), and c a l c i t e and dolomite f o r carbonatitic deposits (e.g. Jacupiranga, B r a z i l ) . Residual (secondary) igneous deposits are formed by the weathering of the o r i g i n a l c a r b o n a t i t i c i n t r u s i o n s . Many deposits exploited today belong to t h i s type (e.g., Catalao, Araxa, Tapira and Ipanema i n B r a z i l ; Sukulu and Tororo i n Uganda; Lueshe i n Zaire). Several residual c a r b o n a t i t i c deposits are known i n Ontario, Canada, such as C a r g i l l (19.6% "P2°5' estimated 56.7 m i l l i o n tonnes), Martison Lake, Schryburt Lake and Goldray (Anon., 1981). De t a i l s on the world carbonatites can be found i n Tu t t l e and G i t t i n s , 1966. Emigh (1983) also describes some of the igneous deposits u t i l i z e d as sources of phosphate. 17 Sedimentary phosphate d e p o s i t s a re known t o occur on every c o n t i n e n t except A n t a r c t i c a (Cook, 1976). T h e i r ages range from the Precambrian t o Recent. They can be c l a s s i f i e d as f o l l o w s : ( i ) - G e o s y n c l i n a l ( i i ) - P l a t f o r m ( i i i ) - Weathered or R e s i d u a l . In the f i r s t category, two types a re d i s t i n g u i s h e d : e u g e o s y n c l i n a l and m i o g e o s y n c l i n a l . In economic terms o n l y m i o g e o s y n c l i n a l d e p o s i t s are important. Some examples are the Kara t a u d e p o s i t s i n the U.S.S.R. and the Western Phosphate F i e l d o f t h e U.S.A. (Phosphoria Formation). These d e p o s i t s are c h a r a c t e r i z e d by a common a s s o c i a t i o n w i t h c h e r t , f i n e o r g a n i c r i c h a r g i l l a c e o u s sediments, and carbonates ( p a r t i c u l a r l y d o l o m i t e ) . These p h o s p h o r i t e s tend t o be p e l l e t a l and commonly extend over hundreds t o thousands of square k i l o m e t r e s . P2°5 c o n t e n t i s h i g h throughout. P l a t f o r m phosphorites are g e n e r a l l y found on, or b o r d e r i n g c r a t o n s . Deposits occur as r i c h d i s c r e t e pockets tens t o hundreds of square k i l o m e t r e s i n area, and s e v e r a l metres t h i c k . Examples are the Pamlico Sound d e p o s i t o f North C a r o l i n a , U.S.A. and the Mishash d e p o s i t s o f I s r a e l . A l t e r n a t i v e l y they occur as t h i n low-grade no d u l a r d e p o s i t s o f the Moscow b a s i n of U.S.S.R. They are commonly a s s o c i a t e d w i t h t e r r i g e n o u s sediments such as quartz s i l t s t o n e and sandstone o r w i t h carbonates. G l a u c o n i t e i s abundant i n some deposits. Weathered phosphorites are t y p i f i e d by the Pliocene Bone Va l l e y Formation of F l o r i d a , U.S.A. Concentration of phosphate can occur as a r e s u l t of chemical or mechanical weathering, the f i r s t being responsible f o r producing a high-grade phosphorite by the leaching-out of a more soluble (generally calcareous) nonphosphatic matrix. Reworked deposits such as the Bone Valley phosphorite are commonly "pebbly" and nodular. Quartz, chert, d i f f e r e n t clays and , i n some cases, dolomite are generally present. Some secondary phosphate minerals can also be present, such as c r a n d a l l i t e (Ca A l 3 ( P 0 4 ) 2 (OH) 5.H 20), v i v i a n i t e (Fe 3 (P0 4) 2.8H 20) and brushite (Ca H P0 4.2H 20). A review of the s p a t i a l and temporal controls on the formation of sedimentary marine phosphate deposits was published recently by Cook (1984). An i n t e r e s t i n g account f o r the o r i g i n of marine phosphorites i s given by B l a t t and co-workers (1972). Guano phosphates, once very important, account for l e s s than 2 - 3% of today's world phosphate rock. They are formed d i r e c t l y or i n d i r e c t l y by the accumulation of b i r d droppings. Insular guano phosphate deposits are found i n several islands i n the d i f f e r e n t oceans. The l a r g e s t known deposits are situated i n Makatea (French Polynesia), Curacao (Dutch A n t i l l e s ) , Christmas Island, Nauru and Ocean Island (completely worked out). Po°5 grades can be as high as 1 9 4.1.5 - I n f r a r e d Spectroscopy and the S t r u c t u r e o f A p a t i t e s a) - T h e o r e t i c a l C o n s i d e r a t i o n s (Estep-Barnes,1977): The i n f r a r e d a b s o r p t i o n spectrum of a m i n e r a l i s a p l o t o f t h e amount o f absorbed (or t r a n s m i t t e d ) r a d i a t i o n by t h e sample v e r s u s the wavelength (or frequency o r "wavenumber") o f the r a d i a t i o n employed. A b s o r p t i o n bands (peaks and shoulders) are produced i n an i n f r a r e d spectrum from t h e i n t e r a c t i o n s of the r a d i a t i o n w i t h the v i b r a t i o n s of t h e atoms o r r o t a t i o n s of the m o l e c u l a r system (the l a t t e r i s not p o s s i b l e i n m i n e r a l s ) . There are i n f r a r e d a c t i v e and i n a c t i v e (or forbidden) v i b r a t i o n s . The a c t i v e ones (which produce bands i n the spectrum) occur when a d i p o l e moment change happens d u r i n g the v i b r a t i o n . The s p e c i f i c band p o s i t i o n s , i n t e n s i t e s and shapes of a b s o r p t i o n bands i n the i n f r a r e d spectrum o f a m i n e r a l are determined by the r e l a t i v e masses, r a d i i and c o - o r d i n a t i o n numbers o f i t s c o n s t i t u e n t atoms, and by the d i s t a n c e s , a n g l e s and bond f o r c e s between them. Thus, t h e r e are no two m i n e r a l s t h a t g i v e e x a c t l y the same spectrum. The i n f r a r e d spectrum o f a m i n e r a l y i e l d s i n f o r m a t i o n r e g a r d i n g the c r y s t a l s t r u c t u r e and bonding. I t i s sometimes as unique as an X-ray d i f f r a c t i o n p a t t e r n . 20 The most predominant absorption bands i n the i n f r a r e d spectrum of a mineral are generally produced by the fundamental i n t e r n a l vibrations of i s o l a t e d molecular groups 2- 3- 2-or complex anions. For example,C03, P0 4 and S0 4 give strong fundamental infrared bands at wavenumbers of approximately 1400-1599 cm - 1, 1000-1100 cm"1, and 1100-1200 cm - 1, respectively. Weaker bands are due to combinations and overtones of the fundamentals, and to l a t t i c e (external) vibrations. I t i s possible to derive group frequency correlations which allow the immediate assignment of a mineral to a s p e c i f i c s t r u c t u r a l c l a s s , s i m i l a r to the s p e c t r a l - s t r u c t u r a l c o r r e l a t i o n s extensively developed f o r functional groups i n organic compounds. The application of group-theoretical methods makes possible the assignment of absorption bands i n the spectrum i s a mineral to s p e c i f i c modes of v i b r a t i o n of i t s constituent atoms. The discussion of these and other methods i s outside the scope of t h i s work. They are discussed by Cotton, 1963 and Ferraro and Ziomek, 1969. The site-group analysis (Halford, 1946) which considers only molecular symmetry and disregards intermolecular interactions, can also be applied to molecular c r y s t a l s such as carbonates, sulphates and phosphates. The number of fundamental vibra t i o n s expected for a given molecular configuration of atoms i s given by the formula 3N - 6 where N i s the number of atoms i n the configuration (3N - 5 for a l i n e a r group). For example, f o r a phosphate group, which contains 5 atoms, the number of fundamental vibrat i o n s should be 9. Vibrations of atoms i n various atomic configurations can be e i t h e r stretching motions along i r bending motions between bond axes. Stretching vibrations usually occur at much higher wavenumber than bending v i b r a t i o n s . I t i s common to use a v n simbolism to l a b e l the d i f f e r e n t v i b r a t i o n s . By convention the symmetric vi b r a t i o n s (stretching or bending) are l a b e l l e d f i r s t i n order of decreasing frequencies (wavenumbers). A f t e r assigning a l l symmetric vibr a t i o n s , the asymmetric vi b r a t i o n s are l a b e l l e d again also i n order of decreasing frequencies. Generally, asymmetric stretching v i b r a t i o n s occur at higher frequencies than symmetric stretching v i b r a t i o n s . Further d e t a i l s can be found i n Farmer ( 1 9 7 4 ) . b) - Structure of Apatites (Montel et a l . , 1 9 7 7 ; Young, 1980): As previously seen (section 4.1.4), apatites possess an "open l a t t i c e " . This type of l a t t i c e os responsible for the large number of substituents (impurities) found i n most natural apatites. The three most common Ca-apatites,viz. f l u o r a p a t i t e , hydroxyapatite and chlorapatite, c r y s t a l l i z e i n the hexagonal system.Apatites have a complex structure. They belong to t h e d i p y r a m i d a l hexagonal c l a s s , h a v ing a symmetry r e p r e s e n t e d by the space group P 6/m ( i n t e r n a t i o n a l symbolism). P r e f e r s t o the p r i m i t i v e c e l l . 6/m means one hexad r o t a t i o n symmetry a x i s and one r e f l e c t i o n symmetry p l a n e . The u n i t c e l l c o n t a i n s 10 c a t i o n s , 6 anions X0 4 and 2 anions Y. F i g u r e 4.1a and b shows the p o s i t i o n of t h e s e i o n s i n the u n i t c e l l . In f i g u r e 4.1a ( c o n s i d e r i n g 3-f l u o r a p a t i t e as an example), m the t e t r a h e d r a l P 0 4 i o n s , two of the oxygen atoms are i n a h o r i z o n t a l p l a n e , and the o t h e r two l i e i n the same v e r t i c a l , thus superimposed i n f i g u r e 4.1a. These ions are d i v i d e d i n t o two "groups", where the y c o n s t i t u t e hexagonal arrangements s i t u a t e d i n terms of h e i g h t a t 1/4 and 3/4 of the u n i t c e l l . C o n s i d e r i n g the s t r u c t u r e p a r a l l e l t o the " c " a x i s 3-( v e r t i c a l ) , f i g u r e 4.1b shows the P0 4 i o n s i n t h e form o f " c o l o n i e s " , l i n k e d t o each other i n such a way t h a t each phosphorus i s bound t o t h r e e oxygens i n one " c o l o n y " and t o one oxygen i n a neighbor "colony". In t h i s s t r u c t u r e one can f i n d two types o f " t u n n e l s " . One type of t u n n e l has a mean diameter of 2.5 A . I t i s o c c u p i e d i n the case of C a - a p a t i t e s , by C a 2 + i o n s , c a l l e d C a ( l ) , approximately a t l e v e l s of 0 and 1/2 o f the u n i t c e l l (these i o n s are not shown i n f i g u r e 4.1b). The second type of t u n n e l i s wider, w i t h a mean diameter between 3 and 3.5 A . I t s c e n t r e i s l o c a t e d a t the hexad symmetry a x i s . I t i s v e r y important i n terms of p h y s i c o - c h e m i c a l 23 F i g u r e - Structure of Apatite(Young, 1 980 and Montel et al.,1977) (a) - Atomic arrangement i n f l u o r a p a t i t e as seen i n p r o j e c t i o n along "c"; (b) - Schematic r e p r e s e n t a t i o n of a tunnel of second type. b e h a v i o u r o f a p a t i t e s . I t i s surrounded by Ca^ i o n s , c a l l e d Ca(2), arranged as e q u i l a t e r a l t r i a n g l e s , s i t u a t e d a t l e v e l s of 1/4 and 3/4 of the u n i t c e l l . T h i s second type of t u n n e l i s a l s o surrounded by 4 o t h e r t r i a n g l e s formed by 3-. oxygens from neighbour P0 4 i o n s . The Y i o n s ( C l , F , 0H~) a r e s i t u a t e d i n s i d e the t u n n e l s o f the second type. T h e i r p o s i t i o n s are i n d i c a t e d i n f i g u r e s 4.1b. F~ l i e s a t t h e c e n t r e o f a t r i a n g l e of Ca(2) i o n s , C l ~ n e a r l y a t the c e n t r e of t h e t r i a n g l e of oxygen i o n s , the oxygen 0(H) o f the h y d r o x y l i o n l i e s about 0 .4A out of the p l a n e o f the Ca(2) t r i a n g l e and the hydrogen l i e s l A f u r t h e r away, n e a r l y i n the p l a n e o f the n e a r e s t oxygen t r i a n g l e . In terms of the d i f f e r e n t s u b s t i t u e n t s found i n n a t u r a l a p a t i t e s , i n c l u d i n g d e f i c i e n t s t r u c t u r e s w i t h h o l e s , t h e y are g e n e r a l l y l o c a t e d i n the t u n n e l s . C a t i o n i c s u b s t i t u e n t s are p r e s e n t i n the s m a l l e r t u n n e l s , i n p l a c e of C a ( l ) i o n s . The a n i o n i c s u b s t i t u e n t s however, can be found i n s i d e t h e t u n n e l s of the second type. D e f i c i e n t a p a t i t e s can a l s o p r e s e n t h o l e s ( l a c k of s t o i c h i o m e t r y ) , and t h e s e e n t i t i e s can a l s o occur p r e f e r e n t i a l l y i n t u n n e l s o f the second t y p e . Other a n i o n i c s u b s t i t u e n t s , e s p e c i a l l y those of the X0 4 type can r e p l a c e the P0 4 groups. N e v e r t h e l e s s , 2-C0 3 can a l s o r e p l a c e the phosphate anion. Carbonate-a p a t i t e s a r e c l a s s i f i e d as type A or B depending upon the p o s i t i o n o f the carbonate i o n , whether i n s i d e the t u n n e l of t h e second type or r e p l a c i n g the phosphate group, r e s p e c t i v e l y . 25 The p e c u l i a r i t i e s of the structure of apatites can promote s i g n i f i c a n t changes i n the physico-chemical behaviour of these minerals. The structure re l a t e d e f f e c t s i n apatites have received considerable research e f f o r t i n geological and b i o l o g i c a l sciences. The same cannot be said i n the case of mineral processing. Further development i n the area of f l o t a t i o n and other physico-chemical methods of mineral processing can probably be achieved i f these s t r u c t u r a l e f f e c t s are taken into consideration. 4.2 - Mining of Phosphate Ores (Notholt, 1973; McLellan, 1983; and Notholt and Hartley, 1983). Both open p i t and underground methods are used to mine phosphate ores. Most of the world's production comes from the open p i t mining of sedimentary deposits. These deposits have varying thicknesses that may be l e s s than lm. In open p i t s a c h a r a c t e r i s t i c feature i s the use of large, c a p i t a l - i n t e n s i v e mining equipment such as e l e c t r i c walking draglines (25 - 3 0 m3 buckets) and bucket-wheel excavators. The use of t h i s type of equipment enables low-grade phosphate beds to be worked economically. Conventional s t r i p p i n g , d r i l l i n g , b l a s t i n g , and extraction by bucket excavators and shovels i s used i n the majority of igneous deposits. Another c h a r a c t e r i s t i c feature of open p i t operations i s t h e i r large c a p a c i t i e s , of the order of 2 to 3 m i l l i o n tonnes per year or more. Stripping of the overburden i s done ei t h e r by the draglines themselves (e.g. Florida) or power shovels (e.g. Kola Peninsula, U.S.S.R.). A combined dredge/dragline operation i s encountered i n one case i n North Carolina. Borehole mining has been tested i n Northeastern F l o r i d a where the phosphate "matrix" i s more than 76 m below the surface i n waterbearing s t r a t a . More convential open p i t mines are found i n B r a z i l , i n the case of the car b o n a t i t i c ores. Higher grade deposits i n Morocco, U.S.S.R., Western U.S.A., Jordan and Tunisia are mined by d i f f e r e n t underground methods such as block caving (U.S.S.R.), continuous r e t r e a t room-and-pillar (Morocco), and sublevel stoping (U.S.A.). 4.3 - F l o t a t i o n Beneficiation of Phosphate Ores F l o t a t i o n of phosphate minerals i s affected by surface properties of these minerals, including surface e l e c t r i c a l properties. The int e r a c t i o n between apatite and f l o t a t i o n reagents i s dependent on the knowledge of the reactions taking place at the mineral/water i n t e r f a c e . 4.3.1 - Apatite/Water Interface Apatites can be considered as weak aci d s a l t s and have a very complex mechanism of charge generation (Parks, 1975). L a t t i c e ions, e.g. Ca(II), P047 F~, and CO^-, undergo hydrolysis controlled by pH. In a sense, H + and 0H~, although not present i n the l a t t i c e of these minerals (except OH" i n the case of hydroxyapatite), can be considered together with the l a t t i c e ions as p o t e n t i a l determining ions (pdi) (Smani et a l . , 1975) or at l e a s t as pdi of second order under Parks'(1975) concepts. Somasundaran (1968) and Somasundaran and Agar (1972) showed that a l l l a t t i c e ions of f l u o r a p a t i t e g r e a t l y a f f e c t the zeta p o t e n t i a l of t h i s mineral. Somasundaran (1968) obtained an i s o e l e c t r i c point (iep) at pH 6 f o r a Canadian f l u o r a p a t i t e sample; t h i s value was achieved only a f t e r weeks of e q u i l i b r a t i o n . The method used was streaming p o t e n t i a l . Non-equilibrium iep values were found to vary from pH 4 to the "equilibrium" iep (pH) of 6. The l a t t i c e ions added exter n a l l y modified the zeta p o t e n t i a l curves at pH values above and below the iep. Saleeb and De Bruyn (1972) performed an exhaustive experimental programme involving the i n v e s t i g a t i o n of the e f f e c t s of the d i f f e r e n t pdi for many synthetic apatite samples. They were the f i r s t to recognize e x p l i c i t l y the existence of a l i n e of zero p o t e n t i a l (or charge) i n the s o l u b i l i t y surfaces of these minerals (see discussion below). They also concluded that f l u o r i d e ions play a s p e c i a l r o l e i n c o n t r o l l i n g the electrophoretic behaviour of apatites, even though i t i s not a l a t t i c e ion (e.g.,fluoride fo r hydroxyapatite). To account for t h i s e f f e c t they propsed that ion exchange reactions take place, involving at l e a s t a monolayer of the s o l i d surface. 28 Smani et a l . (1975) measured iep (pH) f o r d i f f e r e n t natural apatite samples (sedimentary and igneous) and found that the values varied from 3.8 to 4.9, depending on the sample. They also showed that H +, OH", Ca (II) and -HP04 ions are pdi for t h i s system. The surface charge generation mechanism was considered to be r e l a t e d to the hydrolysis of some l a t t i c e species and the adsorption of 2-. Ca(II) and HP0 4 ions. In recent investigations by Chander and Fuerstenau (1979 and 1984), equilibrium diagrams for the hydroxyapatite -water system were calculated. In these diagrams, they were able to define the t h e o r e t i c a l l i n e of zero charge ( l z c ) . This l i n e i s p l o t t e d i n the diagram shown i n figure 4.2, which involves the concentration of a l l pdi as coordinates (actually the diagram i s a bidimensional representation of a tridimensional construction). This mathematical treatment introduces the important f a c t that f o r minerals with such a complex structure as apatite ( " t r i - i o n i c " ) , the determination of iep and pzc values must always be accompanied by the proper experimental conditions of the t e s t s . Also shown i n t h i s diagram are some of the published experimental iep and pzc values for hydroxyapatite, which are i n f a i r agreement with the t h e o r e t i c a l l i n e of zero charge. - l o g (ACTIVITY OF PREDOMINANT CALCIUM SPECIES) F i g u r e 4.2 - The P o s i t i o n of the C a l c u l a t e d L i n e of Zero Charge(lzc) Shown by the Heavy Dashed Line.Also Shown Are Some Experimental pzc Values on the S o l u -b i l i t y Surface of H y d r o x y a p a t i t e ( f o r d e t a i l s see o r i g i n a l work by Chander and Fuerstenau ,1979) • 30 Table 4.V shows a c o l l e c t i o n of the published values of iep (pH) and pzc (pH) for d i f f e r e n t types of apatites. The r e s u l t s obtained i n the present i n v e s t i g a t i o n are included i n the table for completeness but they w i l l be discussed l a t e r . One important trend can be found i n t h i s t a b l e : a l l iep (pH) values are lower than the pzc (pH) values for the same type of apatite. In the case of hydroxyapatite, the average pzc (pH) i s 8.3 and the average iep (pH) i s 6.9. For f l u o r a p a t i t e the s i t u a t i o n i s s i m i l a r . The average pzc (pH) i s 6.8 and the average iep (pH) i s 5.0. These discrepancies can not be attributed only to experimental errors, d i f f e r e n t procedures and d i f f e r e n t samples (e.g. impurities and pre-treatment schemes). The theory i s c l e a r i n s t a t i n g (for example see Parks, 1975 and the IUPAC d e f i n i t i o n s given by Everett, 1972) that the pzc i s defined as the negative logarithm of the pdi a c t i v i t y corresponding to zero true surface charge, oQ = 0. I t i s best determined by d i r e c t measurement of the adsorption d e n s i t i e s of pdi (via a potentiometric t i t r a t i o n technique, f o r example). E l e c t r o k i n e t i c methods y i e l d only the net charge, a z , or the Zeta p o t e n t i a l , r, , e f f e c t i v e at the s l i p p i n g or shear plane of the e l e c t r i c a l double layer, and not the true surface charge of the p o t e n t i a l . I f a Q i s zero but a s p e c i f i c a l l y adsorbed ion i s present, 5 w i l l not be zero. Even i n the absence of s p e c i f i c a l l y adsorbed i o n i c species there may be source of p o t e n t i a l , such as ion exclusion or oriented water molecules between the surface and the shear 31 TABLE 4.V - Experimentally Determined iep(pH) and pzc(pH) f o r Apatites Sample Molar pH No. D e s c r i p t i o n Ca/P Method E l e c t r o l y t e iep or pzc E q u i l . time 1 CaHA s y n t h e t i c 1.59 m.e. K N 0 3 iep 7.4 3 0 minutes 2 CaHA . . . n. a. m. e. KNO3 iep 7.0 24 hours 3 CaHA . . . 1.636 jn. e. KC1 iep 6.8 24 hours 4 CaHA . . . 1.636 m.e. NaF iep 6.8 24 hours • 5 CaHA . . . 1.67 m.e. K N 0 3 iep 7.0 overnight 6 CaHA . . . 1.61 m. e. NaN0 3 iep 6.5 n. a. 7 CaHA . . . n. a. m.e. - iep 7.0 3 0 minutes 8 CaHA . . . . 1.66 p.t. NaCl pzc 7.6 n. a. 9 CaHA . . . 1. 66 p. t . KNO3 pzc 8.5 16 hours 10 CaHA . . . 1.66 p.t. K C 1 0 4 pzc 8.5 16 hours 11 CaHA . . . 1.66 p.t. NaCl pzc 7.6 16 hours 12 CaHA . . . 1. 66 p.t. KC1 pzc 8.6 16 hours 13 CaHA . . . 1.66 p.t. KC1 pzc 8.6 144 hours 14 CaHA . . . 1.66 p.t. KC1 pzc 8.5 504 hours 15 CaHA . . . 1.67 p.t. KCl pzc 8.3 16 hours 16 CaFA sy n t h e t i c n. a. m. e. KNO3 iep 6.5 24 hours 17 CaFA . . . 1. 66 p.t. KCl pzc 6.7 16 hours 18 CaFA . . . 1. 66 p.t. K C 1 0 4 pzc 6.8 16 hours 19 CaFA . . . 1. 66 p.t. KCl pzc 6.9 504 hours 20 CaFa nat.Canada n. a. s.p. KNO3 iep 4 .0 hours 21 CaFA • • • n. a. s .p. KNO3 iep 6 .0 weeks 32 TABLE 4.V - Continued Sample Molar pH No. D e s c r i p t i o n Ca/P Method E l e c t r o l y t e iep or pzc E q u i l . time 22 CaFA nat.Canada n.a. 23 CaFA . . . n.a. 2 4 CaFA nat.Mexico n.a. 25 CaFA . . . n.a. 2 6 CaFA . . . 27 CaFA . . . 28 CaFA n a t . B r a z i l 1.688 m.e. 29 CaFA nat.Sweden 1.68 30 CaFA n a t . B r a z i l 1.688 m.e. 31 CaFA . . . 1.688 M&R 32 CaFA . . . 1.688 M&R 33 CaFA . . . 1.688 pH ch. S.p. pH ch. m.e. m. e. 1.681 m.e. 1.688 pH ch. m. m.e. K N 0 3 KNO3 N a C 1 0 4 KCI KNO-, KCI KCI KNO; KNO, iep 5.6 pzc 7.0 iep 6.4 iep 5.6 iep 4.0 pzc 6.0 iep 4.0 iep 3.0 iep 4.1 pzc 7.0 pzc 7.0 pzc 7.0 hours up to 600 hr. n.a. n.a. n.a. n.a. n.a. 15 minutes 3 0 minutes 3 0 minutes 3 0 minutes up to 72 hr. 34 CaCIA nat.Madag.1.661 s.p. KCI 35 CaCla nat.Aust. n.a. m.e. NaClO-i iep 4.1 iep 6.7 2 hours 3 0 minutes 36 37 38 C O 3-apatite nat.Morocc C 0 3 - a p a t i t e n a t . T u n i s i a C o 3 - a p a t i t e n a t . A l g e r i a n.a. s.p. KCI n.a. s.p. KCI n.a. s.p. KCI iep 4.7 iep 4.9 iep 3.9 2 hours 2 hours 2 hours TABLE 4.V - Continued Sample Molar pH No. D e s c r i p t i o n Ca/P Method E l e c t r o l y t e iep or pzc E q u i l . time 39 C O 3-apatite nat.Christmas n.a. m.e. N a C 1 0 4 iep 3 . 5 3 0 minutes I s l a n d 4 0 SrHA s y n t h e t i c - m. e KNO3 iep 9 .8 24 hours 41 BaA s y n t h e t i c - m.e. KNO3 iep 1 1 . 9 24 hours Notes: CaHA - Calcium hydroxyapatite CaCIA - CAlcium c h l o r o a p a t i t e BaA - Barium a p a t i t e m.e. - microelectrophoresis p.t. - potentiometric t i t r a t i o n M&R - Mular and Roberts method CaFA - Calcium f l u o r a p a t i t e SrHA - Strotium hydroapatite C0 3-apatite - Carbonate-apatite s.p. -streaming p o t e n t i a l pH ch. - pH change versus time n.a. - not a v a i l a b l e References - Sample No.: Amankonah and Somasundaran,1985 - 1 Saleeb and De Bruyn,1972 - 2,3,4,16, Chander and Fuerstenau,1982 - 5 Mishra e t al.,1980 - 6 Present work - 7,3 0-3 3 B e l l et al.,1972 and 1973 - 8-15,17,18,19 Somasundaran,19 68 - 20,21 Somasundaran and Agar,1972 - 22,23, Fuerstenau,M.C. et al.,1968 - 24 Mishra,1978 - 25,35,39 Schulz and Dobias - 26,27 Coelho,1984 - 28 Pugh and Stenius,1985 - 29 Smani et al.,1975 - 34,36-38 Saleeb and De Bruyn,1978 - 40,41 plane. Therefore there i s no guarantee that the true surface charge w i l l be zero when e l e c t r o k i n e t i c p o t e n t i a l i s zero. Values of iep should be defined as the negative logarithm of pdi a c t i v i t y corresponding to zero e l e c t r o k i n e t i c p o t e n t i a l . To explain the difference observed between iep and pzc values presented i n table 4.V, at pH values where the e l e c t r o k i n e t i c p o t e n t i a l i s zero, i . e . at the iep (pH), the surface should s t i l l carry a p o s i t i v e charge. This indicates that negative pdi can be s p e c i f i c a l l y adsorbed i n s i d e the shear plane i n such a way that the surface charge and the charge inside the shear plane compensate each other. Another aspect that should be considered i s the k i n e t i c s of the apatite/water equilibrium. The increase i n iep (pH) reported by Somasundaran (1968) as the e q u i l i b r a t i o n time was increased, may be explained by the change of the concentration of pdi with time (analogous to a movement along the l z c l i n e ) . The e f f e c t of io n i c species other than pdi on apatites i s discussed l a t e r (see section 4.3.2.2). 4.3.2 - Interaction between Apatite and F l o t a t i o n Reagents F l o t a t i o n plays a major r o l e i n the b e n e f i c i a t i o n of phosphate ores. This method i s applied both to sedimentary ores (with s i l i c e o u s gangue) and igneous ores (Huout, 1982; Lawson et a l . , 1978). 35 4.3.2.1 - A p a t i t e / C o l l e c t o r s a) A n i o n i c C o l l e c t o r s F a t t y a c i d s are the most commonly used c o l l e c t o r s i n t h e b e n e f i c i a t i o n o f phosphate ore s . Because o f t h e i r low c o s t and ready a v a i l a b i l i t y , they have been e x t e n s i v e l y used s i n c e t h e f i r s t p l a n t o p e r a t i o n s i n phosphate ore f l o t a t i o n . They c o n f e r , however, r e l a t i v e l y low s e l e c t i v i t y and r e q u i r e m o d i f i e r s t o be used i n a d d i t i o n . Sun e t a l . (1957) showed t h a t the c o l l e c t i n g power of f a t t y a c i d s ( i n r e l a t i o n t o phosphate m i n e r a l s ) i n c r e a s e s as t h e degree of u n s a t u r a t e d bonds i n the hydrocarbon c h a i n i n c r e a s e s up t o a p o i n t . The c o l l e c t o r o r d e r found was l i n o l e i o o l e i o l i n o l e n i o s t e a r i c . T a l l o i l , which i s one of the most common i n d u s t r i a l l y used c o l l e c t o r s , i s a mixture of l i n o l e i c , o l e i c and l i n o l e n i c a c i d s , h a v i n g as i m p u r i t y r o s i n a c i d s . I t i s used i n the form of emulsions (water and o i l ) , s a p o n i f i e d (NaOH) o r neat. Smani e t a l . (1975) s t u d i e d the f l o a t a b i l i t y o f d i f f e r e n t a p a t i t e s w i t h sodium o l e a t e . They observed t h a t most a p a t i t e s when f l o a t e d w i t h t h i s c o l l e c t o r showed two pH ranges o f maximum f l o a t i b i l i t y . The f i r s t , above pH 6, was a s s o c i a t e d w i t h the chemisorption of o l e a t e , r e s u l t i n g i n the f o r m a t i o n o f Ca o l e a t e on the surface.The second f l o a t a b i l i t y maximum o c c u r r e d c l o s e t o the i e p of the samples, and p h y s i c a l adsorption of neutral oleate molecules was proposed. Mishra (1982), also studying the f l o a t a b i l i t y with Na oleate of other d i f f e r e n t apatite samples, found only one pH range f o r maximum f l o a t a b i l i t y . He proposed that the adsorption was due to a combination of chemical and hydrophobic bonds (formation of hemi-micelles). Figure 4 . 3 compares published microflotation r e s u l t s obtained i n the f l o t a t i o n of d i f f e r e n t apatites with Na oleate at the 5 x 10~ 5 m o l / l i t r e concentration l e v e l as a function of pH (for references see figure caption). The analysis of t h i s f i g u r e shows two important factors: (i) - regardless of the mineral sample, high degrees of f l o a t a b i l i t y are obtained i n the medium al k a l i n e pH range (approx. from pH 8 to pH 11); ( i i ) - despite t h i s general trend, the curves also show the importance of the source of the mineral. Concerning the actual mechanisms involved i n the i n t e r a c t i o n between Na oleate and the apatite surfaces, the following points deserve to be highlighted: Figure if.3 - Hallimond Tube Microflotation Results of Apatites i n the Presence of 5 x 10~Sol/litre Na Oleate as a Function of . pH( 1 - S i l v a et al.j1985 ; 2 -Smani et a l . , 1975) 38 ( i ) - t h e r e seems t o be c o n s i d e r a b l e evidence i n favour o f the f o r m a t i o n o f Ca o l e a t e upon a d s o r p t i o n o f o l e a t e s p e c i e s ( i o n s and molecules) as shown by Johnston (1969) and Johnston and L e j a (1978); ( i i ) - the importance o f e l e c t r o s t a t i c f o r c e s i n a d s o r p t i o n i s p r o b a b l y minimal, a l t h o u g h Hanna and Somasundaran (1976) c l a i m t h a t o l e a t e adsorbs on a p a t i t e below t h e i e p (pH) e l e c t r o s t a t i c a l l y , which does not imply i n t h i s author's o p i n i o n t h a t t h e major a d s o r p t i o n d r i v i n g f o r c e i s e l e c t r o s t a t i c ; ( i i i ) - Somasundaran and Ananthapadmanabhan (198 6) c l a i m t h a t the i n t e r f a c i a l a c t i v i t y o f o l e a t e d i m e r i c s p e c i e s i s f i v e o r d e r s o f magnitude g r e a t e r than the monomeric o l e a t e s p e c i e s i n the pH range f o r maximum f l o t a t i o n o f a p a t i t e . T h i s pH range (approx. 8-9) c o i n c i d e s w i t h t h e s u r f a c e t e n s i o n minimum of o l e a t e aqueous s o l u t i o n s . S i m i l a r c o n c l u s i o n s were reached by Pugh and Stenius (1986); 39 (iv) - i n a recent work, Moudgil et a l . (1986) proposed that surface p r e c i p i t a t i o n of Ca oleate i s the predominant mechanism for the observed hydrophobicity of apatite p a r t i c l e s . Simultaneous molecular adsorption (independent of the s o l i d / s o l u t i o n ratio) and surface p r e c i p i t a t i o n occurred i n a region of the adsorption isotherm corresponding to solute concentrations below bulk p r e c i p i t a t i o n conditions at pH 10. The readers are referred to the o r i g i n a l work f o r d e t a i l s ; (v) - the second f l o t a b i l i t y peak observed by Smani et a l . (1975) was interpreted as physical adsorption of neutral o l e i c acid molecules by the authors. However, i t has not been observed i n any other instance and may be related to the impurities ( c a l c i t e ?) i n the samples used by these workers. Somasundaran and Agar (1972) and Smani et a l . (1975) also investigated the adsorption and f l o a t a b i l i t y of apatite with long chain sulphonates. Adsorption through e l e c t r o s t a t i c i n t e r a c t i o n was the proposed mechanism i n t h i s case. Other anionic surfactants such as Na dodecyl sulphate, phosphonic acid, sulphosuccinates and sulphsucciumates have been studied as al t e r n a t i v e c o l l e c t i o n s f o r apatite, e s p e c i a l l y f o r d i f f i c u l t to t r e a t calcite/dolomite bearing phosphate ores (Hanna and Somasundaran, 1976, C o l l i n s et a l . , 1984 and Neves, private communication, 1986). In these cases the work performed so f a r i s not s u f f i c i e n t to draw any conclusions regarding the mechanisms involved i n the adsorption of these c o l l e c t o r s on apatite surfaces. b) Cationic Collectors The problem of be n e f i c i a t i n g carbonate sedimentary phosphate has been the subject of a considerable amount of study without much success (Soto and Iwasaki, 1984 and 1986) . One of the promising methods proposed f o r t h i s type of ore involves the use of a l i p h a t i c primary amines i n a mil d l y a c i d i c pulp, as an apatite c o l l e c t o r (Snow, 1979 c i t e d i n Soto and Iwasaki, 1986) . The method also uses a non-polar hydrocarbon ( a n c i l l a r y collector) and has been su c c e s s f u l l y p i l o t e d (Lawver et a l . , 1978). I n d u s t r i a l l y , c a t i o n i c c o l l e c t o r s are used i n the b e n e f i c i a t i o n of phosphate ores i n reverse c i r c u i t to f l o a t o f f r e s i d u a l s i l i c e o u s gangue, entrained and/or entrapped during the anionic apatite f l o t a t i o n step. This technique has been i n use f o r more than 40 years i n F l o r i d a , U.S.A. ("double f l o t a t i o n " ) . Mishra (1979) investigated the e f f e c t s of dodecylamine hydrochloride i n both the e l e c t r o k i n e t i c and f l o a t i b i l i t y behaviour of apatite. His r e s u l t s could be f i t t e d by the electrostatic/hemi-micelle model of Gaudin and Fuerstenau (1955). Hanna and Somasundaran (1976) also give support to t h i s mechanism. However, more recent investigations of Soto and Iwasaki (1984 and 1986) presented experimental evidence i n favour of a chemical model of adsorption i n which amine ions react with surface phosphate i o n i c s i t e s on apatite, due to the chemical a f f i n i t y between t h i s p a i r of ions. In other words, the chemical i n t e r a c t i o n appears to have an overriding influence on the c o l l e c t o r action, although the e l e c t r o s t a t i c i n t e r a c t i o n (and the formation of hemi-micelles) may play a r o l e i n the t o t a l free energy of adsorption. Thermochemical adsorption measurements associated with studies on the s o l u b i l i t i e s of phosphate/amine s a l t s ( * ) support the chemical model. A cl o s e r look at the r e s u l t s of Smani et a l . (1975) for a c a t i o n i c c o l l e c t i o n would also indicate that the e l e c t r o s t a t i c model could not explain t h e i r data completely. The m i c r o f l o t a t i o n curves presented show a considerable decrease i n f l o a t a b i l i t y of apatite at pH values where the net surface charge i s negative and increasing i n absolute (*) One simple way to appreciate the low s o l u b i l i t y of t h i s type of compound i s to introduce a small amount of an a l k y l amine solut i o n to a phosphate based pH buffer. The p r e c i p i t a t e forms rather quickly and i s quite v i s i b l e . value. Taggart and A r b i t e r (1944) had proposed an ion exchange mechanism (involving the formation of an insoluble s a l t between amine and phosphate) for the i n t e r a c t i o n between amines and apatites. The available evidence now seems to favour t h i s formely d i s c r e d i t e d mechanism. c) Amphoteric Collectors Amphoteric c o l l e c t o r s possess simultaneously c a t i o n i c and anionic functions, according to the working pH. This type of c o l l e c t o r i s used i n d u s t r i a l l y i n Finland to f l o a t an igneous phosphate ore (Kiukkola, 1980). In the Finnish a p p l i c a t i o n , the amphoteric c o l l e c t o r i s an N-sarcosin and only the anionic function of the surfactant i s used i n the d i r e c t f l o t a t i o n of apatite. Houot et a l . (1985) used a se r i e s of amphoteric c o l l e c t o r s ( a l k y l amino propionic acid, a l k y l propylene diamino propionic acid, a l k y l dipropylene triamino propionic acid and a l k y l dipropylene triamino dipropionic acid) i n t h e i r studies of the f l o t a t i o n of sedimentary phosphate ores (dolomitic and argillaceous gangue). In t h e i r testwork, the amphoteric c o l l e c t o r s s u c c e s s f u l l y played the roles of c a t i o n i c and anionic c o l l e c t o r s i n c i r c u i t s resembling the conventional "double f l o t a t i o n " c i r c u i t s already mentioned. The major advantage of the amphoteric c o l l e c t o r s i n t h i s case i s the elimination of the wash/scrubbing step, necessary to remove f a t t y acid 43 c o a t i n g on a p a t i t e . 4.3.2.2 - A p a t i t e / M o d i f i e r s A l a r g e v a r i e t y of m o d i f i e r s , b o t h o r g a n i c and i n o r g a n i c are used i n the f l o t a t i o n o f phosphate o r e s . They can a c t as pH m o d i f i e r s , a c t i v a t o r s , d e p r e s s a n t s , f r o t h c o n t r o l l e r s , complexants, e t c . but t h e i r r o l e s and mechanisms of i n t e r a c t i o n are not c l e a r l y e s t a b l i s h e d i n most cases. The need f o r such reagents i s v e r y s i g n i f i c a n t , and without them, s e l e c t i v e s e p a r a t i o n would not be p o s s i b l e . The e f f e c t s of p d i s p e c i e s have been a l r e a d y d i s c u s s e d ( s e c t i o n 4.3.1). The e f f e c t o f d i f f e r e n t i n o r g a n i c m o d i f i e r s such as Na s i l i c a t e s and N a - t r i p o l y p h o s p h a t e was i n v e s t i g a t e d i n more d e t a i l i n a few papers. M i s h r a (1978) u s i n g Na m e t a s i l i c a t e w i t h S i 0 2 / N a 2 0 r a t i o 1 : 1 , showed t h a t t h e z e t a p o t e n t i a l of a Christmas I s l a n d a p a t i t e i n c r e a s e d from about -3 0mV t o about -4 0mV a t pH 11 i n the presence of 1.2g/l of the reagent. Parsonage e t a l . (1984) used a d i f f e r e n t Na s i l i c a t e w i t h a 1.35 : 1 S i 0 2 / N a 2 0 r a t i o , but a l s o r e p o r t e d a change i n the z e t a p o t e n t i a l of the f l u o r a p a t i t e a t pH 9.5 from -18mV t o -45mV i n the presence of . 5 g / l of the reagent. They concluded t h a t Na s i l i c a t e d i s p e r s e d f l u o r a p a t i t e by forming a p r o t e c t i v e l a y e r of h y d r o p h i l i c s i l i c a t e . The mechanisms o f a d s o r p t i o n are not w e l l e x p l a i n e d y e t . A c c o r d i n g t o M a r i n a k i s and Shergold (1985) Na s i l i c a t e monomeric s p e c i e s are chemisorbed on c a l c i t e . The i o n i c monomers would i n t e r a c t w i t h Ca s i t e s a t the s u r f a c e and a s i m i l a r mechanism c o u l d be expected f o r a p a t i t e s . M i s h r a (1978) s t u d y i n g the system a p a t i t e / c a l c i t e / o l e a t e / m e t a s i l i c a t e concluded t h a t t h e r e was p r e f e r e n t i a l a d s o r p t i o n of s i l i c a t e on c a l c i t e a t pH 10. However, Parsonage e t . a l (1984) showed t h a t i f a p a t i t e i s c o n d i t i o n e d w i t h s i l i c a t e i n the presence o f c a l c i t e , f l o t a t i o n o f both m i n e r a l s w i t h o l e a t e i s depressed by the s i l i c a t e s p e c i e s . M ishra (1978) a l s o proposed t h a t hydrogen bonding between s u r f a c e atoms and OH groups o f s i l i c a t e aqueous s p e c i e s would p l a y an important r o l e i n the o v e r a l l a d s o r p t i o n d r i v i n g f o r c e . Parsonage e t a l . (1984) a l s o o b t a i n e d a l a r g e i n c r e a s e i n the n e g a t i v e v a l u e of the z e t a p o t e n t i a l o f a p a t i t e on the presence of Na t r i p o l y p h o s p h a t e (-80mV a t pH 9.5 i n t h e presence of l g / 1 of the r e a g e n t ) . Rao e t a l . (1985) observed s e l e c t i v e d e p r e s s i o n o f a p a t i t e by potassium orthophosphate i n a carbonated ore a t a c i d i c c o n d i t i o n s . The a d s o r p t i o n o f orthophosphate would take p l a c e v i a r e a c t i o n between Ca s i t e s and phosphate ions,CaHP0 4.Johnston and L e j a (1978) suggested t h a t the s e l e c t i v e d e p r e s s i o n o f a p a t i t e by orthophosphate/water occurs by hydrogen bonding on a p a t i t e s u r f a c e s . At the same pH, dolomite and c a l c i t e s u r f a c e s would generate C0 2 v i a d i s s o l u t i o n , d i s t u r b i n g the f o r m a t i o n o f the l a y e r , thus p e r m i t t i n g the a d s o r p t i o n o f o l e a t e s p e c i e s . 45 Aquino e t a l . (1987) obtained improved s e l e c t i v i t y i n t h e s e p a r a t i o n of a p a t i t e / c a r b o n a t e u s i n g p h o s p h o r i c a c i d a t pH 5.5 as compared t o s u l p h u r i c a c i d a t the same pH, i n the r e v e r s e carbonate f l o t a t i o n w i t h o l e i c a c i d . The depressant e f f e c t o f c a t i o n s (other than pdi) i s a l s o of i n t e r e s t . Coelho (1984) r e p o r t e d t h a t A l aqueous s p e c i e s depressed the Na o l e a t e f l o a t a b i l i t y of both c a l c i t e and a p a t i t e . The same e f f e c t i s r e p o r t e d i n the l i t e r a t u r e f o r o t h e r c a t i o n s (Hanna and Somasundaran, 1976). The cause f o r t h e depressant a c t i o n i s normally r e l a t e d t o the f o r m a t i o n of complexes between c a t i o n and c o l l e c t o r s p e c i e s . However, t h e i r a c t u a l mechanisms s t i l l need t o be d e s c r i b e d . The most accepted model f o r the i n t e r a c t i o n between m e t a l l i c i o n i c s p e c i e s and m i n e r a l s u r f a c e s i s t h a t o f James and Healy (1972). T h i s model p r e d i c t s t h a t upon r e a c t i n g these m e t a l l i c s p e c i e s w i t h m i n e r a l s , charge r e v e r s a l s w i l l take p l a c e . The f i r s t charge r e v e r s a l (CRI) i s the m i n e r a l i e p (pH). The second charge r e v e r s a l (CR2) i s r e l a t e d t o the a d s o r p t i o n of t h e m e t a l l i c i o n i c s p e c i e s , more s p e c i f i c a l l y i t s f i r t hydroxo-complex. Charge r e v e r s a l 3 (CR3) i s a s s o c i a t e d w i t h the r e t u r n t o n e g a t i v e s u r f a c e charge a t pH v a l u e s where the m e t a l l i c i o n i c s p e c i e s are a n i o n i c i n c h a r a c t e r ( h y d r o l y z e d ) . D e t a i l s of t h i s model are a l s o g i v e n by S e n i o r (1987). 46 The combined e f f e c t of sodium s i l i c a t e / c a t i o n s i s even l e s s w e l l understood. Klassen and Mokrousov (1963) p r e s e n t e d experimental r e s u l t s of the combined e f f e c t of c a t i o n / s o d i u m s i l i c a t e on the r e c o v e r y of Ca-minerals ( f l u o r i t e , c a l c i t e , s c h e e l i t e and a p a t i t e ) w i t h o l e i c a c i d a t pH 9.1 - 9.5. In the absence of c a t i o n s but i n t h e presence o f 1.5 k g / t N a - s i l i c a t e , a l l m i n e r a l s d i s p l a y e d r e c o v e r i e s above 95%. In t h e presence of 300 g/t A 1 ( S 0 4 ) 3 s c h e e l i t e and a p a t i t e were p r e f e r e n t i a l l y depressed, w h i l e i n the presence of MgS0 4 s c h e e l i t e and c a l c i t e were the two m i n e r a l s most a f f e c t e d ( a p a t i t e r e c o v e r y remained unchanged i n t h i s c a s e ) . I n the case of c a t i o n i c f l o t a t i o n , anions such as phosphate and s u l p h a t e are found t o produce a c t i v a t i o n e f f e c t s . C a t i o n s promote d e p r e s s i o n i n most c a s e s . A g a i n the mechanisms are not w e l l understood i n t h e s e c a s e s . A p p a r e n t l y phosphate and sulphate i n c r e a s e t h e a d s o r p t i o n of c a t i o n i c c o l l e c t o r s (Hanna and Somasundaran, 1976). The a d s o r p t i o n of polymers and s i m i l a r compounds on a p a t i t e s (except s t a r c h e s t h a t w i l l be d e a l t w i t h l a t e r , see s e c t i o n 4.5.4) was s t u d i e d i n a few cases. P r a d i p and co-workers (1980) i n v e s t i g a t e d the a d s o r p t i o n o f p o l y a c r y l a m i d e s on f l u o r a p a t i t e and h y d r o x y a p a t i t e . For a s l i g h t l y a n i o n i c p o l y a c r y l a m i d e (SEPARAN NF 10) a d s o r p t i o n appears t o r e s u l t mainly from hydrogen bonding between -OH groups on t h e a p a t i t e s u r f a c e and -C0NH 2 groups of the p o l y a c r y l a m i d e macromolecules. E l e c t r o s t a t i c f o r c e s are a l s o i n v o l v e d , s i n c e a d s o r p t i o n decreased as the pH i n c r e a s e d (thus t h e n e g a t i v e v a l u e of the z e t a p o t e n t i a l ) . B e l t o n and Stupp (1983) i n v e s t i g a t e d the a d s o r p t i o n o f i o n i z e d p o l y ( a c r y l i c acid) on t r i b a s i c Ca phosphate s u r f a c e s . They concluded t h a t the a d s o r p t i o n b e haviour was a f f e c t e d s i g n i f i c a n t l y by the s u b s t r a t e ' s e l e c t r o s t a t i c p o t e n t i a l as w e l l as the polymer conformation and i n t e r m o l e c u l a r a s s o c i a t i o n i n s o l u t i o n . 4.4. - S t a r c h e s 4.4.1. - Composition and S t r u c t u r e S t a r c h i s a v e r y important p o l y s a c c h a r i d e . I t i s a n a t u r a l polymer formed by the condensation o f a-D(+) GLUCOSE molecules (see f i g u r e 4.4a). The s t a r c h macromolecule c o n s i s t s o f a-D(+) glucose monomers, po l y m e r i z e d c h i e f l y by 1 :->-4-a and 1 6- a l i n k a g e s . I t occurs as g r a n u l e s whose shape and s i z e are c h a r a c t e r i s t i c of the p l a n t from which the s t a r c h i s d e r i v e d . The c h a i n l e n g t h and m o l e c u l a r weight of the s t a r c h a l s o depend upon i t s source. S e v e r a l books and a r t i c l e s are a v a i l a b l e and cover the s u b j e c t i n g r e a t depth (e.g. W h i s t l e r and Smart, 1953; Radley, 1953; W h i s t l e r and P a s c h a l l , eds., 1965; W h i s t l e r , ed., 1964; Greenwood, 1970; Danishefsky e t a l . , 1970; Be M i l l e r , 1973; Powell,1973; S a t t e r t h w a i t e and I w i n s k i , 197 3; Kennedy and White, 1979; Whistler,1980; 48 and W h i s t l e r e t a l . , eds., 1984). A review on the r e l e v a n t a s p e c t s o f s t a r c h e s and d e r i v a t i v e s i s presented i n the next s e c t i o n s . G e n e r a l i t i e s i n c l u d e d i n t h i s review were obtained m a i n l y from t h e r e f e r e n c e s l i s t e d above. S t a r c h F r a c t i o n s In g e n e r a l , s t a r c h e s c o n t a i n about 2 0% o f a l i n e a r f r a c t i o n c a l l e d AMYLOSE and 80% of a branched f r a c t i o n termed AMYLOPECTIN. A s t r o n g evidence of the branched nature o f s t a r c h comes from the f a c t t h a t end group a n a l y s i s suggests an average c h a i n - l e n g t h o f 24 t o 30 g l u c o s e u n i t s f o r a s t a r c h molecule (Hough and Jones, 1954). An unbranched molecule w i t h 24-30 glucose u n i t s would p r e s e n t a maximum m o l e c u l a r weight (see next s e c t i o n ) o f 4860, w h i l e s t a r c h e s have much l a r g e r m olecular weights. The two s t a r c h f r a c t i o n s , v i z . amylose and amylopectin, a re both made up of °c-D(+) g l u c o s e but d i f f e r i n m o l e c u l a r shape, s i z e and extent of contaminants. Amylose i s the macromolecule r e s u l t i n g c h i e f l y from 1 :-*-4-a l i n k a g e s ( F i g u r e 4.4b). On the other hand, amylopectin i s a l a r g e r , macromolecule r e s u l t i n g from branching, through 1 : + 6 a l i n k a g e s ( F i g u r e 4.4c) of 1 : +4-a l i n e a r c h a i n s . I t s s t r u c t u r e i s t r e e - l i k e , w i t h many of the branches themselves h a v i n g sub-branches. The amylose content o f v a r i o u s s t a r c h e s 6 CH2OH <x-D-( + >Glucose"(m.p. 146°, [*]=+! 12") H (chair conformations assumed) Figure k » k - Schematic View of (a) Glucose(monomer), (b)Amylose and (c) Amylopectin (polymers) (Leja,1 9 8 2 ) . 50 i s p r e s e n t e d i n t a b l e 4.VI Amylose and amylopectin have s u b s t a n t i a l l y d i f f e r e n t p r o p e r t i e s . Many of these d i f f e r e n c e s are d e r i v e d from d i f f e r e n c e s i n m o l e c u l a r weight and m o l e c u l a r shape ( s t r u c t u r e o f the macromolecule). Amylose, u s u a l l y c h a r a c t e r i z e d by i t s s t r a i g h t c h a i n nature g i v e s a deep bl u e c o l o u r when i t s s o l u t i o n s are r e a c t e d w i t h i o d i n e . Hanes (1937) and Freudenberg e t a l . (1939)were the f i r s t t o propose a h e l i c a l s t r u c t u r e f o r amylose i n aqueous s o l u t i o n s . The deep b l u e c o l o r a t i o n with i o d i n e i s a r e s u l t o f the complexation o f i o d i n e i n s i d e the h e l i c a l s t r u c t u r e o f amylose. I o d i n e i s occluded w i t h i n the h e l i x , t o form c l a t h r a t e s t r u c t u r e s . Amylopectin a l s o r e a c t s w i t h i o d i n e , but the i n t e n s i t y and wavelength of the c o l o u r developed by t h i s r e a c t i o n are d i f f e r e n t (much lower i n t e n s i t y a t a lower wavelength). In t h e i r n a t i v e s t a t e , s t a r c h g r a n u l e s are c r y s t a l l i n e and e x i s t i n t h r e e polymorphic forms, namely A-s t a r c h o f c e r e a l s and g r a i n s , B - s t a r c h o f t u b e r s and r o o t s , and C - s t a r c h of v a r i o u s beans, smoothed pea and banana. These polymorphic forms have been i d e n t i f i e d by X-ray d i f f r a c t i o n . Sarko and Wu (1978) c l a i m t h a t the c r y s t a l l i n e s t r u c t u r e s of s t a r c h e s are l a r g e l y due t o the l i n e a r amylose f r a c t i o n . From t h e i r s t u d i e s they found t h a t both A- and B- amylose have v i r t u a l l y i d e n t i c a l conformation: both are right-handed, p a r a l l e l - s t r a n d e d h e l i c e s , d i f f e r i n g i n t he c r y s t a l l i n e packing o f the h e l i c e s and the water 51 TABLE 4.VI - Amylose Content o f V a r i o u s S t a r c h e s (approximate v a l u e s ) ( * ) S t a r c h Amylose Content Alderman pea 65 Amylomaize 52 Apple 19 Arrowroot 21 Babacu nut 22 Banana 16-17 B a r l e y 22 Buckwheat 28 Cashew nut 24 Canna 26 Chick pea 33 Corn (commercial) 26 E a s t e r l i l y 31 Horse c h e s t n u t 26 I r i s t u b e r 27 Mango k e r n e l 24 Oat 27 P a r s n i p 11 Pea (smooth-seeded) 35 Sago 26 Sorghum 27 S t e a d f a s t pea 67 Sugary mutant corn 70 Sweet p o t a t o 20 Ta p i o c a 18(17) T u l i p 26 Waxy c o r n 0-6 Waxy b a r l e y 3 Waxy sorghum 0 Wheat 25 White p o t a t o 23(20) (*) Two important f a c t o r s c o n t r o l the apparent amylose content i n any s t a r c h : p r i m a r i l y g e n e t i c f a c t o r s (e.g. waxy s t a r c h e s which c o n s i s t almost e x c l u s i v e l y o f a m y l o p e c t i n ) ; second, m a t u r i t y o f the p l a n t a t the time o f s t a r c h i s o l a t i o n (amylose c o n t e n t i n c r e a s e s w i t h i n c r e a s e d m a t u r i t y ) (Greenwood, 1970). c o n t e n t . The A-amylose c r y s t a l l i z e s i n an o r t h o g o n a l u n i t c e l l w i t h 8 water molecules per u n i t c e l l . On t h e o t h e r hand, B-amylose c r y s t a l l i z e s i n a hexagonal u n i t w i t h 36 water m o l e c u l e s per u n i t c e l l . The C- s t r u c t u r e i s a mixture of A- and B- u n i t c e l l s , b e i n g i n t e r m e d i a t e between the A-and B- forms i n packing. S t a r c h p r e c i p i t a t e d from s o l u t i o n , or complexed w i t h v a r i o u s o r g a n i c molecules, adopts the s o - c a l l e d V- s t r u c t u r e ( " V e r k l e i s t e r u n g " ) . T h e o r e t i c a l c a l c u l a t i o n s have shown t h a t V-amylose i s more s t a b l e as a l e f t - h a n d e d h e l i x than as a right-handed one (Rao e t a l . , 19 67 and W h i s t l e r and D a n i e l , 1984). The r e l e v a n c e of the s t r u c t u r a l a s p e c t s o f s t a r c h w i l l become c l e a r i n the r e s u l t s and d i s c u s s i o n chapter. 4.4.2 - M o l e c u l a r Weight D i f f i c u l t i e s i n molecular weight d e t e r m i n a t i o n s of p o l y s a c c h a r i d e s are i n c r e a s e d i n the case o f s t a r c h e s not o n l y because s t a r c h e s are u s u a l l y two component mixtures, but a l s o because of the marked tendency of the molecules t o a s s o c i a t e o r r e t r o g r a d e . The m a j o r i t y of molecular weight measurements were made on s e p a r a t e d amylose and amylopectin f r a c t i o n s . Table 4.VII shows a c o m p i l a t i o n f o r some of the a v a i l a b l e data, i n d i c a t i n g the technique used f o r the d e t e r m i n a t i o n . As seen i n t h i s t a b l e , the ranges of molecular weight f o r both amylose and amylopectin are v e r y wide. TABLE 4.VII - Molecular Weight of Starch Fractions (after Young,1984) Parent Starch Method( ) M.W. range Reference (1 x 10"6) [daltons] A)AMYLOSE Potato O.P. 0.082-0.113 Whistler and Smart(1953) Potato L.S. 0.49 Young(1984) Wheat O.P. 0.09-0.16 Whistler and Smart(1953) Wheat L.S. 0.34 Young(1984) Corn U.C. 0.017-0.25 Whistler and Smart(1953) Corn O.P. 0.08-0.3 Whistler and Smart(1953) Tapioca O.P. 0.3 Whistler and Smart(1953) Barley Potato ** Potato( *> V.M. 0.30 Young(1984) L.S. 0.146-2.25 Everret and Foster(1959) L.S. 0.16-2.29 Banks and Greenwood(1963) B)AMYLOPECTIN Potato Potato I<***> Potato u l * * * ) O.P. 0.36 Whistler and Smart(1953) L.S. 440.00 Young(1984) L.S. 6.50 Young(1984) Tapioca L.S. 450.00 Young(1984) Waxy Corn L.S. 400.00 Young(1984) Waxy Corn Sheared L.S. 10.00 Young(1984) Wheat L.S. 400.00 Young(1984) Corn O.P. 0.3 Whistler and Smart(1953) Notes: (*) O.P. = osmotic pressure; L.S. = l i g h t scattering; V.M. = v i s c o s i t y ; U.C. = ult r a c e n t r i f u g a t i o n (**) Fractioned amylose samples (***) Two dif f e r e n t c u l t i v a r s of the same plant 54 Regardless of the technique and the great v a r i a b i l i t y of the molecular weight r e s u l t s shown i n table 4.VII, the molecular weight of amylose i s an order of magnitude smaller than that of amylopectin. The l i n e a r configuration of amylose as compared to the branched amylopectin molecule i s one of the reasons f o r i t s lower molecular weight. Light scattering derived molecular weights are probably closer to the r e a l values as indicated by some of the more recent determinations (Young, 1984). One example of the discrepencies among the methods i s given i n table 4.VII. The osmotic pressure molecular weight measurement quoted by Whistler and Smart (1953) for potato amylopectin i s 360 000 daltons as compared to the 65 and 440 m i l l i o n daltons given by Young (1984), determined by l i g h t s c a t t e r i n g . Potato amylopectin i s one of the largest natural polymers known. Another aspect i s that osmotic pressure and chemical end-group analysis y i e l d number-average molecular weights (Mn). Light s c a t t e r i n g methods y i e l d weight-average molecular weights (Mw). The number-average molecular weight loses s i g n i f i c a n c e for broad molecular weight d i s t r i b u t i o n s (as i n the case of starch f r a c t i o n s ) , since i t i s not s e n s i t i v e to large molecules. In turn, l i g h t s c a t t e r i n g methods are not s e n s i t i v e to small molecules. The r a t i o Mw/Mn gives an i n d i c a t i o n of the extent of the molecular weight d i s t r i b u t i o n (Young, 1984) . V i s c o s i t y r e l a t e d methods are not absolute and are dependent on c a l i b r a t i o n from 55 osmometry or l i g h t scattering (to obtain the constants K and a of the Mark-Howink equation - see section 5.3.5). A curious and important r e s u l t also presented i n table 4.VII deals with the e f f e c t of shearing on molecular weights of starches. Non-sheared waxy corn amylopectin gave a molecular weight (Mw) of 400 m i l l i o n daltons whereas the same sample subjected to shearing (not described i n the o r i g i n a l reference) gave a molecular weight 40 times smaller. Shearing of polymer solutions i s known to e f f e c t t h e i r f l o c c u l a t i o n power (Halverson and Panzer, 1978). 4.4.3 - Minor Constituents Starch i s composed of carbon, hydrogen, and oxygen on the r a t i o of 6 : 10 : 5 (empirical formula ( C 6 H 1 0 O 5 ) x ) . When starch i s subjected to hydrolysis by acids and/or c e r t a i n enzymes, i t y i e l d s glucose. I t i s , therefore, consider a condensation polymer of glucose. The t h e o r e t i c a l values f o r carbon, hydrogen and oxygen i n % by weight are the following: 44.4% C, 6.2% H and 49.4% 0. In addition to carbohydrate, starch grains contain minor consituents which can influence t h e i r properties. The most important are l i p i d s (fatty acids such as s t e a r i c , p a l m i t i c , o l e i c and l i n o l e i c acids and lysophospholipids), phosphorus (bound-esterified or i n the form of adsorbed phosphatides) and nitrogen (generally i n the form of proteins). Starches also contain small amounts of inorganic contaminants (less than 0.5% by wt.). Davies et a l . (1980) presents an extensive l i s t of starches and corresponding contaminant l e v e l s . Proteins are generally associated with the amylose f r a c t i o n whereas phosphorus i s more re l a t e d to amylopectin. Potato starches have the highest phosphorus content among common starches (0.071% P) and tapioca starches the lowest(0.003% P). 4.4.4 - Solution Preparation and Retrogradation As already mentioned d i f f e r e n t solvent systems have been used to prepare aqueous dispersions (solutions) of starches. Two methods of g e l a t i n i z a t i o n a r e commonly used f o r the preparation of starch - thermal and chemical g e l a t i n i z a t i o n . G e l a t i n i z a t i o n temperatures depend on the amylopectin content of the starch. High amylopectin content i s associated with low g e l a t i n i z a t i o n temperatures, f o r example 55 - 66°C for potato, 62 -72°C f o r corn and 67.5 -74°C f o r waxy sorghum. Chemical (or cold) g e l a t i n i z a t i o n can be used. Strong a l k a l i s are the most common reagents used. Sodium hydroxide has the lowest c r i t i c a l g e l a t i n i z a t i o n l e v e l (lowest consumption). The combination of thermal and chemical g e l a t i n i z a t i o n can also be used. (1) G e l a t i n i z a t i o n i s the process by which the strong i n t e r m i c e l l a r network of starch granules i s weakened by disrupting hydrogen bonds. The resultant paste i s water soluble. 57 Retrogradation i s a spontaneous phenomenon that occurs i n aqueous solutions of starches. When they are allowed to stand (under aseptic conditions), they become opalescent, increasingly cloudy and lower i n v i s c o s i t y . F i n a l l y , they undergo p r e c i p i t a t i o n . Amylose retrogrades f a s t e r than amylopectin. Therefore starches with higher amylopectin content form more stable solutions (e.g. waxy corn starch, constituted of almost 100% amylopectin retrogrades l e s s than 10% i n 100 days whereas corn starch retrogrades approximately 60% i n 30 days). Amylose retrogradation takes only a few hours. Retrogradation rate i s increased by lowering the temperature (Whistler and Smart, 1983) . 4.5 - Selective F l o c c u l a t i o n and the Use of Starches i n Mineral Processing The treatment of fin e mineral p a r t i c l e s has received s p e c i a l attention i n the l a s t 10 to 15 years (see f o r example the s p e c i a l volumes edited by Somasundaran and Ar b i t e r , 1979; and Somasundaran, 1980). The reasons for such i n t e r e s t are rel a t e d to two basic f a c t s : (i) - Fine p a r t i c l e s cannot be e f f i c i e n t l y separated by any of the conventional b e n e f i c i a t i o n processes. 58 ( i i ) - e x p l o i t a t i o n of lower grade ores with f i n e l i b e r a t i o n sizes generates large tonnage of d i f f i c u l t to concentrate f i n e p a r t i c l e s . Selective aggregationf 1) of valuable fin e s i s desirable because larger units are more e a s i l y concentrated (Fuerstenau et a l . , 1979). Another advantage i s that a f i n e r impoverished stream, containing mainly the waste p a r t i c l e s , i s produced. Selective f l o c c u l a t i o n by polymers has been a widely studied technique to achieve s e l e c t i v e aggregation. Haseman patented a s e l e c t i v e f l o c c u l a t i o n process f o r phosphate fines i n 1953. His findings are discussed elsewhere i n t h i s thesis (see the section dedicated to the s e l e c t i v e f l o c c u l a t i o n of phosphates). Selective f l o c c u l a t i o n i s based upon the creation of a set of physico-chemical conditions (Yarar and Kitchener, 1970; and Friend and Kitchener, 1973): (i) - to increase the degree of dispersion of waste m i n e r a l s ( 2 ) . ( i i ) - to destroy the dispersion s t a b i l i t y of (1) Aggregation i s used here as a general term. An aggregate of loose structure, formed (generally) by the addition of organic polymers should be referred to as a " f l o e " . An aggregate formed by compression of e l e c t r i c a l double layers (generally more compact than a floe) should be c a l l e d a "coagulum". (2) Reverse s e l e c t i v e f l o c c u l a t i o n i s also possible, f o r example waste clays i n a s y l v i n i t e ore are s e l e c t i v e l y f l o c c u l a t e d and floated (Banks, 1979). 59 valuable minerals through s e l e c t i v e adsorption of a s e l e c t i v e f l o c c u l a n t . ( i i i ) - promote the growth of f l o e s mainly constituted of valuable minerals. 4.5.1 - Basic P r i n c i p l e s In p r i n c i p l e the method i s very simple. Figure 4.5 schematically shows the e s s e n t i a l steps involved i n s e l e c t i v e f l o c c u l a t i o n of a hypothetical two phase ore (Read and H o l l i c k , 1976). F i r s t l y inorganic dispersants (or l e s s frequently low molecular weight organic polymers) are added to the ore pulp to ensure that no heterocoagulation w i l l take place. In the r e a l case dispersants would be added, most probably, to the grinding m i l l where fresh surfaces are created and cross-contamination of mineral p a r t i c l e s can occur. Secondly, a high molecular weight organic polymer i s added which i s s e l e c t i v e l y adsorbed on one of the minerals. T h i r d l y c o l l i s i o n processes r e s u l t i n f l o e formation. F i n a l l y the f l o e s s e t t l e while the p a r t i c l e s of the dispersed mineral remain i n suspension. Flocculated and dispersed phases can then be separated by d i f f e r e n t means. 6 0 » • • o • o • • • ° • o ° o • 1. • o ° $3 • • • • l P # S • • o * • • • 0~0 DISPERSION ADDITION OF FLOCCULANT SELECTIVE ADSORPTION OF FLOCCULANT SELECTIVE FLOCCULATION SEPARATION Fi g u r e Ly.5 Fundamental Stages i n the S e l e c t i v e F l o c c u l a t i o n of a Two-Phase Ore (Read and H o l l i c k , 1 9 7 6 ) . 4.5.2 - S t a b i l i t y of Mineral Suspensions Selective adsorption of polymers (followed by s e l e c t i v e flocculation) i s only achieved i f the d i f f e r e n t mineral p a r t i c l e s i n an ore pulp are properly dispersed. Dispersion i s the f i r s t p re-requisite f o r s e l e c t i v e f l o c c u l a t i o n . The s t a b i l i t y of c o l l o i d a l dispersions i s of s p e c i a l i n t e r e s t i n many f i e l d s . C o l l o i d chemists have been studying the phenomena related to t h i s s t a b i l i t y f o r over a century. In an aqueous dispersion of p a r t i c l e s , a t t r a c t i v e i n t e r a c t i o n s a r i s e from London dispersion forces. These long-range forces are one of the types of the so c a l l e d van der Waals a t t r a c t i v e forces. The other two types, Keesom and Debye interactions, do not contribute to the long-range a t t r a c t i o n between c o l l o i d a l p a r t i c l e s (Napper, 1983). Hamaker (1937) extended the c l a s s i c a l London's treatment of the dispersion forces between atoms to c a l c u l a t e those between c o l l o i d a l p a r t i c l e s . The p o t e n t i a l energy (V A) of i n t e r a c t i o n between two spherical p a r t i c l e s of the same material i s then given by: V A = -A a ^ / 6 (a-i^  + a 2 ) H 0 (EQ. 4.5.1) where A = Hamaker constant a l ' a 2 = r a c * i i °f "the respective p a r t i c l e s H Q = shortest distance between the surfaces 62 of the two p a r t i c l e s . The Hamaker constant depends on t h e p o l a r i z i n g p r o p e r t i e s o f t h e molecules comprising v a r i o u s components of the system. F o r example f o r the quartz/water/quartz system, A=l x 1 0 ~ 1 3 e r g (Healy, 1979). Naked and uncharged c o l l o i d a l p a r t i c l e s undergo v e r y r a p i d c o a g u l a t i o n e f f e c t e d by long-range a t t r a c t i v e f o r c e s . To s t a b i l i z e a c o l l o i d a l d i s p e r s i o n i t i s n e c e s s a r y t o p r o v i d e long-range c o u n t e r a c t i n g r e p u l s i o n f o r c e s between the p a r t i c l e s . At t h e pr e s e n t time (Napper, 1983) t h e r e are two ways t o impart s t a b i l i t y t o c o l l o i d a l d i s p e r s i o n : e l e c t r o s t a t i c s t a b l i l i z a t i o n and polymeric s t a b i l i z a t i o n . The f i r s t type i s d e a l t w i t h by t h e D e r j a g u i n -Landau-Verwey-Overbeek (D.L.V.O.) theory. The i n t e r a c t i o n o f two d i s i m i l a r e l e c t r i c a l double l a y e r s I*) g i v e s r i s e t o an e x p r e s s i o n f o r the r e p u l s i v e energy o f i n t e r a c t i o n (V R) as f o l l o w s (Hogg e t a l . , 1966): 1 +exp(-kHQ) .1 -exp(-kH ). (EQ 4.5.2) + ln(l-exp(-2kH )\ ( a i + a 2 } i V201<2 ^ X } (*) For reviews o f double l a y e r t h e o r i e s and a p p l i c a t i o n s see f o r example Parks(1976),Hiemenz(1977), L e j a ( 1 9 8 2 ) , Lyklema(1982), Usui(1984). where e= d i e l e c t r i c constant of the suspending medium a 1 / 2 = r a d i i of the respective p a r t i c l e s (assumed spherical) H Q = shortest separation distance ^01'^02 = Stern p o t e n t i a l of the respective double layer (generally assumed to be equal to the zeta potential) k = Debye-Huckel r e c i p r o c a l length parameter, given by k 2 = 8 c e z 2 / e K t , where: c = concentration (ions/cm 3) e = e l e c t r o n i c charge K = Boltzmann's constant T = absolute temperature z = i o n i c charge The above rela t i o n s h i p for V R i s general. However, i t holds only for magnitudes of and/or4>02 o f of l e s s than 25mV and for solution conditions such that the double layer "thickness" i s small compared to the p a r t i c l e s i z e . Hogg and co-workers (1966) point out that the given r e l a t i o n s h i p i s a good approximation for and ^ 0 2 l e s s than 50 - 60 mV. The t o t a l energy of i n t e r a c t i o n can then be expressed i n a very s i m p l i f i e d way as follows: V T " VA + VR (EQ. 4.5.3) Using a sign convention such that negative V T leads to coagulation, i n the case of two i n d e n t i c a l p a r t i c l e s , s t a b i l i z a t i o n w i l l occur when V R> V A , since V A i s always negative ( a t t r a c t i v e ) . Homocoagulation occurs when the i d e n t i c a l p a r t i c l e s are uncharged (V R = 0). In a system co n s i s t i n g of p a r t i c l e s of species A and B, the phenomenon of hetercoagulation can take place when the p a r t i c l e s of species A are negatively charged and p a r t i c l e s of species B are p o s i t i v e l y charged or uncharged (and vice-versa). The above discussion demonstrates the importance of surface charge on the s t a b i l i t y of c o l l o i d a l dispersions. The same phenomena occur i n suspensions of f i n e mineral p a r t i c l e s . In ore pulps, where many mineral species are present, the v a l i d i t y of treatments such as the D.L.V.O. theory i s l i m i t e d . The r e a l systems are complicated by a number of factors, f o r example the v a r i e t y of the s i z e and shape of p a r t i c l e s and the presence of many i o n i c species introduced by the p a r t i a l d i s s o l u t i o n of some minerals during m i l l i n g operations. The basic p r i n c i p l e s , however, apply. Heterocoagulation must be avoided by adequate addition of dispersants. Selective coagulation i s possible under very c o n t r o l l e d conditions (see for example a review paper by Laskowski, 1982). Parsonage et a l . , 1982, applied the D.L.V.O. theory to t h e i r studies of slime coating with considerable success. 65 C o l l o i d a l dispersions can also be s t a b i l i z e d by polymeric compounds. S t e r i c s t a b i l i z a t i o n (Napper, 1983) i s imparted by the macromolecules that are attached to the surfaces of the p a r t i c l e s . This type of phenomenon i s also known as the "protective c o l l o i d " s t a b i l i z a t i o n . A combination of e l e c t r o s t a t i c and s t e r i c s t a b i l i z a t i o n ( e l e c t r o s t e r i c s t a b i l i z a t i o n ) can occur. For d e t a i l s on polymeric s t a b i l i z a t i o n see Napper (1983). Examples of the use of acrylate polymers fo r the s t a b i l i z a t i o n (dispersion) of dolomite fi n e s are given by Van Lierde (1974). 4.5.3 - Dispersants The simplest and most widely used dispersants are pH modifiers - most minerals being strongly negatively charged i n an a l k a l i n e media. By pH regulation, the V R term i n the energy equation i s increased because the zeta p o t e n t i a l on the p a r t i c l e s i s increased. A very simple rule-of-thumb (Zeta-Meter manual, 1975) i s that excellent dispersion i s achieved when a l l p a r t i c l e s i n an aqueous environment have zeta p o t e n t i a l s greater than 60mV (calculated by the Helmholtz-Smoluchowski equation). The presence of d i f f e r e n t i o n i c species i n ore pulps i s one of the possible reasons that pH control alone i s sometimes not s u f f i c i e n t to impart a high degree of dispersion. Additional dispersing reagents (inorganic compounds and l e s s frequently organic) have to be used. 66 Sodium s i l i c a t e s with d i f f e r e n t S i 0 2 : Na 20 r a t i o s are frequently used (Rabone, 193 6; Gaudin, 1957; Sollenberg and Geenwalt, 1958; Hines and Vincent, 1962; Smith and Songstad, 1962; Klassen and Mokroussov, 1963; Fuerstenau et a l . , 1968; Falcone J r . , 1982; Leja, 1982; Krishanan and Iwasaki, 1982, to c i t e a few). Most uses of sodium s i l i c a t e s are i n f l o t a t i o n systems, as depressants for oxide and salt-type minerals. Although these reagents are widely used, l i t t l e i s known about the mechanisms of t h e i r action. Commercial sodium s i l i c a t e s can be described by a general formula: N a m H 4 m S i 0 4 , where m denotes the average number of OH- bound per S i atom and varies from 0.5 to 1.6. In aqueous solutions, sodium s i l i c a t e can e x i s t as a v a r i e t y of species: S i ( 0 H ) 4 (uncharged), SiO(OH) 3~ , S i 0 2 ( 0 H ) 4 4", S i 2 0 3 ( 0 H ) 4 2 " and S i 4 0 8 ( 0 H ) 4 4~. The f i r s t 3 species are monomeric, the fourth i s dimeric and the l a s t tetrameric. Polymeric species are also known to form (Leja, 1982 and Falcone, 1982). Some important c h a r a c t e r i s t i c s of the action of sodium s i l i c a t e s are: (i) - they impart highly negative surface charges on d i f f e r e n t minerals. 67 ( i i ) - they can reverse zeta p o t e n t i a l of minerals which have been i n contact with ions such as C a 2 + , Mg 2 + and F e 3 + . ( i i i ) - they can form insoluble surface p r e c i p i t a t e s with the ions mentioned above (Krishhan and Iwasaki, 1982). Given these c h a r a c t e r i s t i c s one can speculate upon the possible mechanisms involved i n the dispersion (and depression) promoted by sodium s i l i c a t e s . One of the p o s s i b i l i t i e s i s a mechanism s i m i l a r to the e l e c t r o s t a t i c s t a b i l i z a t i o n described by Napper (1983). The polymerized sodium s i l i c a t e species would adsorb on the mineral surface through hydrogen bonding or depending on the mineral, through covalent bonding with cations on the surface (forming compounds such as C a - s i l i c a t e s ) . In fact, Krishnan and Iwasaki (1982) have used a scanning transmission electron microscope to show the formation of C a - s i l i c a t e on the surface of quartz, previously treated with Ca ions. Sodium polyphosphates are also common dispersants (Conley, 1974 and Leja, 1982). Sodium pyrophosphate, containing the multivalent ^2G7 anion i s an example of a line a r - c h a i n polyphosphate which can be represented by a general formula ( p n 0 3 n + i ) ( n + 2 ) ~ . Pyrophosphate i s present i n solution as dimeric species. Sodium hexametaphosphate i s a c y c l i c polyphosphate, containing the multivalent Pg0 1 8 anion. Calgon (trade name), one of 68 most common commercial dispersants, i s a mixture of hexametaphosphate and pyrophosphate (Leja, 1982). Polyphosphates are capable of p r e c i p i t a t i n g C a 2 + and Mg 2 + as insoluble phosphates. Their dispersing action may be p a r t i a l l y related to t h i s phenomenon. However they also react with the mineral surfaces imparting high negative charge to them. In t h i s way they act as e l e c t r o s t a t i c s t a b i l i z e r s . Competition with c o l l e c t o r species such as sulphonate and f a t t y acids, i s one of the suggested roles of polyphosphate as depressants for dolomite and c a l c i t e (Hanna and Somasundaran, 1976). More recently, Yongxin and Changgen (1983) and Changgen and Yongxin (1983), investigated the e f f e c t of various polyphosphates on the f l o t a t i o n of sch e e l i t e , c a l c i t e , f l u o r i t e , garnet and quartz with sodium oleate. They found that pyrophosphate and hexametaphosphate were e f f e c t i v e depressants for c a l c i t e and f l u o r i t e , and depressed s c h e e l i t e only weakly. Furthermore they concluded that the s e l e c t i v e d i s s o l u t i o n of c a l c i t e and f l u o r i t e was rel a t e d to the se l e c t i v e depression of Ca ions caused by the polyphosphates. This d i s s o l u t i o n would reduce the a v a i l a b i l i t y of adsorption s i t e s f o r the c o l l e c t o r (sodium oleate) on these minerals. The reason f o r the s e l e c t i v e action was considered to be related p r i m a r i l y to the c r y s t a l structure of the mineral. No detectable adsorption of polyphosphate species on the minerals studied was observed. 69 4.5.4 - (Selective) D e s t a b i l i z a t i o n of Mineral Suspensions When polymers are added to an aqueous suspension of p a r t i c l e s , an extra term i n the t o t a l energy of i n t e r a c t i o n equation i s introduced: V T = V A + V R + V B (EQ. 4.5.4) Where V B i s the i n t e r a c t i o n energy caused by "bridging". For most purposes V B can be considered a component of V A. The bridging mechanism i s shown i n Figure 4.6 f o r the cases of p a r t i a l and complete coverage. I t was f i r s t proposed by Ruehrvein and Ward (1952) and Michaels and Morelos (1955). Medium to high molecular weight polymeric adsorbates with several active s i t e s on them can induce aggregation by attaching themselves to two or more p a r t i c l e s . Such bridging between p a r t i c l e s w i l l p a r t i c u l a r l y occur under conditions where the p a r t i c l e s are not completely coated by the polymeric species (see Figure 4.6b). According to Healy and La Mer (1964), maximum f l o c c u l a t i o n and f i l t r a t i o n rates occur when the f r a c t i o n of p a r t i c l e surface covered by polymer molecules i s close to 0.5. Somasundaran (1980) points out that complete coverage (Figure 4.6a) can also induce f l o c c u l a t i o n . Excess coverage can lead to s t e r i c s t a b i l i z a t i o n . 70 < + (a) COMPLETE COVERAGE. AG - AH - TAS t ^ from polymer increase from released desolvation solvent molecules; decrease from interpene-trating polymers Plocculation under complete coverage i s possible only i f the free energy of interpenetration of adsorbed layers, AGj, i s negative. (b) PARTIAL COVERAGE F i g u r e if.6 - F l o c c u l a t i o n by Polymers under Complete and P a r t i a l Coverage Conditions(Somasundaran,1980) . The second pre-requisite f o r s e l e c t i v e f l o c c u l a t i o n i s the s e l e c t i v e adsorption of polymer molecules on the valuable m i n e r a l s . Very high molecular weight polymeric f l o c c u l a n t s , frequently used for c l a r i f i c a t i o n of process water, are usually nonselective (Laskowski, 1982). The mechanisms of adsorption of f l o c c u l a n t s have been described i n the l i t e r a t u r e (see f o r example Yarar and Kitchener, 1978; and Somasundaran, 1980). In summary, the forces involved i n the adsorption of polymers are: (i) - e l e c t r o s t a t i c bonding ( i i ) - hydrogen bonding ( i i i ) - covalent bonding E l e c t r o s t a t i c forces are predominant i n the adsorption of polymers with a large number of charged units. Both anionic and c a t i o n i c polymers are commonly used i n mineral processing. Hydrogen bonding i s probably present i n almost every case of polymer adsorption on oxygen bearing minerals. This can be the predominant mechanism fo r the adsorption of non-ionic (or s l i g h t l y anionic) polymers. Covalent bonding occurs when active groups of the polymer i n t e r a c t with cations of the mineral surface. Michaels and Morelos (1955) proposed that the adsorption of polyacryla-(1) As already mentioned, "reverse" s e l e c t i v e f l o c c u l a t i o n can also be used (see also Cormode, 1985). mides on k a o l i n was due t o the f o r m a t i o n of s a l t - t y p e compounds by r e a c t i o n between the polymer a n i o n i c groups and t h e C a 2 + i o n s p r e s e n t on k a o l i n s u r f a c e . Another example o f c o v a l e n t bonding i s the a d s o r p t i o n o f c e l l u l o s e xanthate polymers on m i n e r a l s such as c h a l c o p y r i t e ( A t t i a and K i t c h e n e r , 1975; S r e s t y , Raja and Somasundaran, 1978). Of s p e c i a l i n t e r e s t t o t h i s study i s the a d s o r p t i o n o f s t a r c h e s and d e r i v a t i v e s . Iwasaki and co-workers (see f o r example Iwasaki and L a i , 1965: B a l a j e e and Iwasaki, 1969; Iwasaki and L i p p , 1971) e x t e n s i v e l y i n v e s t i g a t e d the a d s o r p t i o n (and f l o c c u l a t i o n ) of m o d i f i e d and non-modified s t a r c h e s on q u a r t z , g o e t h i t e and hematite. F i g u r e 4.7 shows two t y p i c a l a d s o r p t i o n isotherms f o r c o r n s t a r c h and c a t i o n i c s t a r c h on both quartz and hematite. The f i g u r e a l s o shows the pH dependence of a d s o r p t i o n . Assuming t h a t both m i n e r a l s u r f a c e s are capable of hydrogen bonding w i t h a l l types of s t a r c h e s ( n o n - i o n i c , a n i o n i c , and c a t i o n i c ) , and the m o l e c u l a r weight of a l l s t a r c h e s i s e q u i v a l e n t , the c o n t r i b u t i o n o f hydrogen bonding f o r t h e i r a d s o r p t i o n on the m i n e r a l s h o u l d a l s o be e q u i v a l e n t . In F i g u r e 4.7 i t i s r e a d i l y apparent t h a t corn s t a r c h i s more s t r o n g l y adsorbed on hematite than on quartz, but the a d s o r p t i o n d e n s i t y of the c a t i o n i c s t a r c h i s h i g h e r on quartz than on hematite. On c l o s e r examination of the i n d i v i d u a l isotherms, i t w i l l be noted t h a t the a d s o r p t i o n o f corn s t a r c h decreases w i t h i n c r e a s i n g pH, whereas t h a t of c a t i o n i c s t a r c h i n c r e a s e s w i t h i n c r e a s i n g pH. Quartz i s more e l e c t r o n e g a t i v e than hematite (a) 3.0 2.0 < ce 10 5 o.i z o < .05 .01 H; pH 7 H:pH9 -H:pHli O : pH7 _o_ , A . a a O : pH K>6 OS • a 20 40 60 60 100 120 CC UJ a. oi 6 a UJ u < cc f— to m < 2 D O (b) 20 40 60 80 100 RESIDUAL CONCENTRATION ,mg. PER LITER Figure 4.7 - Adsorption of Corn Starch(a) and C a t i o n i c Starch(b) on Heraatite(H) and Quartz(Q) (Balajee and Iwasaki,1969). 74 i n aqueous suspensions i n the pH range 7 to 11. Corn starch i s negatively charged i n t h i s range and the c a t i o n i c starch i s p o s i t i v e l y charged. The d i a m e t r i c a l l y opposite adsorption behaviour of corn starch and c a t i o n i c starch on these two oxide minerals can be understood i f an e l e c t r o s t a t i c i n t e r a c t i o n e x i s t s between the starches and mineral surfaces. Using the experimental facts discussed above, Figure 4.8 schematically shows a model fo r the mode of adsorption of various starches on oxide minerals, incorporating hydrogen bonding and e l e c t r o s t a t i c i n t e r a c t i o n ( a t t r a c t i o n or repulsion). The isotherms shown i n Figure 4.7 e x h i b i t an extra c h a r a c t e r i s t i c , l i k e l y not determined by e l e c t r o s t a t i c i n t e r a c t i o n s . The adsorption densities f o r both starches on hematite (corn and cationic) are much lar g e r than the adsorption densities of corn starch on quartz. Although an e l e c t r o s t a t i c component i s evident from the r e s u l t s shown, the starches c l e a r l y displayed a greater s p e c i f i c i t y f o r hematite, even when the surface of hematite was only s l i g h t l y negative (the isotherm at pH 7 for c a t i o n i c starch on hematite shows a saturation adsorption density of approx. 1 mg per m2 compared to corn starch on quartz, which at the same pH, shows a maximum adsorption density of approx. 0.3mg o , , , per ur).Another important difference i s the shape of the isotherms f o r hematite. The curves are t y p i c a l f o r cases where the polymer has a very high a f f i n i t y f o r the surface, showing a steep r i s e i n the adsorption density at low concentration (Lipatov and Sergeeva, 1971). These isotherms also reach the saturation coverage at low concentrations. Similar adsorption isotherms of other polysaccharides on d i f f e r e n t minerals are reported i n the l i t e r a t u r e . In most cases t h i s high a f f i n i t y type of isotherm was considered to be caused by some sort of covalent bonding occuring between i o n i c groups on the polymer and cations on the mineral surface. In recent research by S o l a r i et a l . (1985), the adsorption of carboxymethylcellulose (CMC) on untreated and acid-leached graphite samples was investigated. The untreated graphite presented adsorption isotherms of the high a f f i n i t y type whereas f o r the acid-leached sample a l o w - a f f i n i t y type was observed and much smaller d e n s i t i e s were obtained. The s t r i k i n g l y d i f f e r e n t behaviour of the two graphite samples (regarding CMC adsorption) was considered to be related to c a t i o n i c impurities (mainly Ca, Mg and Fe) on the surface of the untreated graphite. These impurities would be preferred adsorption s i t e s f or CMC molecules where strong covalent bonding could take place. Somasundaran (1969) investigated the adsorption of corn starch (gelatinized with 0.5M NaOH) on c a l c i t e . The adsorption isotherms were of the high a f f i n i t y type and the adsorption density decreased with the increase of pH (similar to the adsorption behaviour of corn starch on hematite). He also measured the t o t a l calcium concentration i n c a l c i t e suspensions as a function of residual (equilibrium) starch 7 6 V//\ HYDROGEN BONDING N W l E L E C T R O S T A T I C INTERACTION ANIONIC UNMODIFIED CATIONIC y y y y y y y y y y y y y y y y y yy y y y yyy OXIDE MINERAL SURFACE F i g u r e 4 . 8 - A Model Showing Mode of Adsorption "by Various Starches on N e g a t i v e l y Charged Oxide Mi n e r a l Surfaces (Balajee and Iwasaki,1969) concentration, and found that Ca s o l u b i l i t y (at a fix e d pH) increased as the starch concentration increased. Complex formation between starch and surface calcium species can therefore be considered as an adsorption mechanism f o r the c a l c i t e - s t a r c h system. Starch impurities, such as f a t t y acid and phosphate can be involved i n the adsorption mechanism. Ionization of starch hydroxyls (pK 12) could also contribute to the complexation reactions involving Ca surface species. Khosla et a l . (1984) also concluded that the i n t e r a c t i o n between starch and c a l c i t e (as well as between starch and hematite) i s of a chemical nature. Their thermochemical studies for s t a r c h / c a l c i t e and starch/hematite systems indicated that the adsorption of starch macro-molecules on the surfaces of these minerals i s an exothermic process. Desorption testwork also demonstrated the p a r t i a l i r r e v e r s a b i l i t y of adsorption, further i n d i c a t i n g the high a f f i n i t y of the polymers for the surfaces i n question. Steenberg and Harris (1984) also studied the adsorption of a modified potato starch (FLOCGEL 100) on various minerals, including apatite. The adsorption isotherm f o r apatite/FLOCGEL 100 at pH 7 showed a plateau region s t a r t i n g at approximately 12 mg/1. They concluded that hydrogen bonding was the main adsorption mechanism operating i n t h i s system, based only on the shape of the isotherm and on the s i m i l a r behaviour displayed by d i f f e r e n t minerals regarding the adsorption of starch. 78 4.5.5 - F l o c c u l a n t Applications I t i s outside the scope of the present work to discuss f l o c c u l a n t s other than polysaccharides, e s p e c i a l l y starches. An extensive review of f l o c c u l a t i n g agents was published by Halverson and Panzer (1978). Most polymers used as f l o c c u l a n t s by the mineral industry today are synthetic. Polyacrylamides, polyamines and poly(ethylene oxides) are some t y p i c a l examples. Rogers and Poling (1978) discussed the composition and performance of some commercial polyacrylamides. Starches were the most commonly used f l o c c u l a n t s by the mineral industry before the advent of synthetic polymers. Gardner et a l . (1939) tested 25 d i f f e r e n t organic raw materials (most of them starches or containing starches) as flo c c u l a n t s f o r coal t a i l i n g s . They noticed that, depending on the preperation of the flocculant solution, the starches tested were very e f f e c t i v e . Potato starch, c a u s t i c i z e d with NaOH, gave the best r e s u l t s . The i n t e r e s t i n starches as f l o c c u l a n t s and depressants grew considerably a f t e r the early studies of Gardner and co-workers (1939). F l o t a t i o n of ir o n ores received most of the research on starches (Cooke et a l . , 1952; Chang et a l . , 1953; Chang, 1954; Iwasaki and L a i , 1965; Iwasaki, 1965; Colombo and Rule, 1967; Iwasaki et a l . , 1969; Balajee and Iwasaki, 1969; Iwasaki and Lipp, 1971; Colombo 79 and Frommer, 1976; and Jacobs and Colombo, 1981).. In almost a l l o f th e s e s t u d i e s s t a r c h e s (modified and non-modified) were found t o a c t as ( s e l e c t i v e ) d e p ressants f o r hematite and o t h e r i r o n o x i d e s d u r i n g the r e v e r s e f l o t a t i o n o f quartz and o t h e r s i l i c a t e s . The r e v e r s e f l o t a t i o n t e c h n i q u e s used were e i t h e r c a t i o n i c (with amines) or a n i o n i c (Ca a c t i v a t i o n f o l l o w e d by c a r b o x y l a t e f l o t a t i o n ) . S t a r c h e s were a l s o e x t e n s i v e l y used i n s t u d i e s o f the s e l e c t i v e f l o c c u l a t i o n o f i r o n ores (Frommer, 1968; B a l a j e e and Iwasaki, 1969; Iwasaki e t a l . , 1969; Colombo and Frommer, 1976; Colombo, 1977; Colombo, 1979; Iwasaki, 1979; Chen and L e j a , 1980; Colombo, 1980; Jacobs and Colombo, 1980; Paananen and T u r c o t t e , 1980; Abu Rashid and Smith, 1982; Guraraj e t a l . , 1983; Z u l e t a e t a l . , 1984; Coelho, 1984; and Hunamantha Rao and Narasimhan, 1985). The most important i n d u s t r i a l a p p l i c a t i o n of s e l e c t i v e f l o c c u l a t i o n (as a s e l e c t i v e d e s l i m i n g operation) i s encountered i n the b e n e f i c i a t i o n o f i r o n ores ( T i l d e n Mine, U.S.A). As i n the case o f d e p r e s s i o n , s t a r c h e s s e l e c t i v e l y adsorb and f l o c c u l a t e i r o n o x i d e s . S o l u t i o n p r e p a r a t i o n procedures were a l s o found o f c r i t i c a l importance. S t a r c h e s and d e r i v a t i v e s were a l s o used f o r the s e l e c t i v e f l o c c u l a t i o n o f c o a l f i n e s (e.g. Z u l e t a e t a l . , 1985), d e p r e s s i o n o f c o a l (e.g. Im and Apian, 1981; M i l l e r et a l . , 1983), f l o c c u l a t i o n of phosphate s l i m e s (La Mer and S m e l l i e , 1956 a and b) and s e l e c t i v e f l o c c u l a t i o n o f 80 phosphate fin e s and ores (see separate section f o r discussion of t h i s t o p i c ) . The r e s u l t s of La Mer and Smellie (1956 a,b) are p a r t i c u l a r l y i n t e r e s t i n g because of one of t h e i r conclusions. They tested d i f f e r e n t starches and found potato starch was the most e f f e c t i v e . They concluded that the reason f o r the superior performance of potato starch was the presence of e s t e r i f i e d phosphate groups i n the f l o c c u l a n t ( r e c a l l that Gardner et a l . (1939) also found potato starch to be the most e f f e c t i v e starch f l o c c u l a n t ) . Coelho (1985) also reported potato starch as being the most e f f e c t i v e depressant f o r an a p a t i t i c iron ore. 4.5.6 - E f f e c t of Dissolved Species on Adsorption and F l o c c u l a t i o n Adsorption (or "abstraction") of aqueous m e t a l l i c species on minerals i s known to cause major changes i n t h e i r surface properties. In a c l a s s i c a l work, James and Healy (1972) proposed a model for the adsorption of aqueous m e t a l l i c species on mineral substrates. According to t h e i r model, hydrolysable metal ions (such as those derived from A l , F e ( I I I ) , Co(II), Ca(II) and Mg(II) s a l t s ) adsorb on a mineral surface independently of the surface charge, but adsorption i s greatly influenced by pH, concentration of the 81 s a l t and, to a l e s s e r extent, by the nature of the surface. In terms of zeta potentials, most mineral systems present at l e a s t two points of zero potential (the iep and a point of zeta r e v e r s a l ) . For minerals with an iep i n the a c i d i c range of pH, two points of zeta reversal can be i d e n t i f i e d . The i n t e r a c t i o n of starches and other f l o c c u l a n t s with mineral surfaces i s strongly affected by adsorption of hydrolysable metal ions. For example Iwasaki et a l . , 1980; and Heerema et a l . , 1983, investigated a system involving Ca and Mg ions, starch and quartz. They found that both ions increased the adsorption of starch on quartz( and consequently i t s f l o c c u l a t i o n ) . Increased f l o c c u l a t i o n of quartz by starch was maximal at pH values close to that required for p r e c i p i t a t i o n of Ca and Mg hydroxides. Presence of such ions i n hematite-quartz systems caused poor s e l e c t i v i t y i n both s e l e c t i v e f l o c c u l a t i o n and f l o t a t i o n operations. Similar behaviour was found f o r the Al-starch-quartz-hematite system (de Araujo, 1982; de Araujo and Coelho, 1984). The presence of these ubiquitous ions i n ore pulps can be related to poor s e l e c t i v i t y obtained i n some studies with r e a l ores as compared to pure mineral systems. 4.5.7 -Selective Flocculation Of Phosphates Table 4.VIII summarizes the reported attempts to f l o c c u l a t e s e l e c t i v e l y phosphate ores and f i n e s . The following paragraphs w i l l give a more d e t a i l e d account of the 82 major findings i n each case. Haseman (1953) was the f i r s t to patent a process of s e l e c t i v e f l o c c u l a t i o n f o r phosphate f i n e s . In that early time he recognized the need for increasing the o v e r a l l recovery of phosphate ores from F l o r i d a , being treated by the desliming-double f l o t a t i o n technique. Corn starch, s o l u b i l i z e d by heating was used as (selective) f l o c c u l a n t for apatite. Adequate dispersion was achieved by c o n t r o l l i n g the pH with NaOH. The example given i n t h i s patent demonstrates that s e l e c t i v e f l o c c u l a t i o n of f i n e s was able to increase the o v e r a l l P 2 0 5 recovery by at l e a s t 10%. Stated d i f f e r e n t l y , the a p p l i c a t i o n of f l o c c u l a t i o n i n t h i s case was equivalent to recovering fin e s down to lum i n s i z e . Later Davenport and co-workers (1969) also applied s e l e c t i v e f l o c c u l a t i o n to phosphate f i n e s . They used a non-s p e c i f i e d starch to f l o c c u l a t e s e l e c t i v e l y a -53-,um phosphate slime, also from F l o r i d a . The +53 um was f l o a t e d conventionally. Although they were able to recover 75%P 20 5 from the slime, both enrichment r a t i o and s e l e c t i v i t y were poor. They concluded that, because of economic considerations and high R 20 3 of t h e i r s e l e c t i v e f l o c c u l a t i o n concentrate, the use of the technique fo r the sample tested was not a v i a b l e a l t e r n a t i v e to improve recovery. 83 ID t H c« U CD • H a -. > o o — C ns to <D O 0 -P a x ft CO o X PH £H O O • H Ctf rH O o o a. CD > • H O CD vi o 5 3. O " " I O t o —• Z Z I — -3 a.— "o = o -J = — CD CO C m «. O j= r 1 x x 2 'i » v « a —• a — j — o — u vi — -d-•PQ EH a, a o —* • a. n O. M N O e . TABLE if. V I I I - Cont. S e d l a e n l a r y p h o s p h a t e o r . f r o a B r a s l l * S e d i m e n t a r y p h o s p h a t e o r . I torn t r u l l " ' FLOCCUULKT(S) C o r n S t a r c h ( g e l a t l u l l e d w i t h NaOH) 1) -12 p a (1 k g / t ] 2) -12 p a (1 k g / t | 3) -44 p a (1.5 k g / t | 4 ) -105 pn 11.2 k g / t | , C o r n S t a r c h ( g e l a t i n i z e d w i t h NaOH) DisrutsjutfCs) NaOH (1 k g / t I NaOH NaOH NaOH r u n C I I D I coNCENnuTK' C*ADI IP,O, pU XP.O, ZJt.O,' JtP.O, XR.O,' E n c o v r n i i f i m M C l t S ( S ) 10 10 . J 10.2 4.3 4.3 6.1 10.3 N.A. N.A. N.A. N.A. S.O 7.0 11.0 11.5 N.A. N.A. N.A. N.A. 42.7 59.6 75.0 91.2 de O M v . l r a and C o m . i , 1982 I b i d , i b i d , i b i d . P u r * a l n e r a l s ( h y d r o x y a p a t i t e . c a l c l t a and q u a r t ! ) 1) -300 p a groun d t n i 11 k g / t J 2) -106 p a g r o u n d o r . . (1 k j / l | 3) -45 p a groun d u r o i | l k g / l l 1) S u p e r f l o c 16 o r A-100 ( p o l y -a c r y l a a l d e , M.U.- 4 x I0«) 116 g / t l h y d r o x y a p a t i t e . and c a l c l t . o n l y 2) d o., h y d r o x y -a p a l t l a / c a l c l t e / q u a r t z NaOH and Ha n e l a s l l l c a t e ( 0 . 5 k g / t | do. do. C y q u e s t (Na p o l y a c r y l a t a ) (0.615 k g / t I and I l i j i J t O , I I k | / l | do. 10.5 10.5 10.5 10.2 10.2 26.2 1.2 28.0 N.A. 92.0 C o o l h o , 1984 17.6 12.7 21.2 N.A. 94.9 da A r a u j o .1 12.4 13.2 15.3 N.A. 88.2 1 9 8 * 26.2 11.2 29.2 N.A. ' 86.1 I b i d . 17.6 12.7 2 1 . ) N.A. 94.6 12.4 l ) . 2 15.6 N.A. 9 ) . l 26.2 11.2 29.6 N.A. 80.3 I b i d . 17.6 12.7 2 4 . ) N.A. 86.4 12.4 13.2 16.6 N.A. 62.0 14.65 - 29.24 • 84.89 R u b l o and 12.3 2S.5 86.4 H a r a b l n i . 1985 i b i d . NOTL'Si 1. 1 R,0, - X F.,0, + ZA1,0, | maximum a c c e p t a b l e between 4 and 51. 2. C o n c e n t r a t e G r a d . r e f e r s t o a v e r a g e g r a d e o r b e s t g r a d e o b t a i n e d f o r t h e f l o c c u l a t e d p h a s e . ] . N.A. > Not A v a i l a b l e . 4. F l o t a t i o n waa us e d t o I n c r e a s e g r a d e o f f l o c c u l a t e d p h a s e . 5. T h e s e o r e s a r e f r o a t h e same a r e a as l l . u o r e s t u d i e d by de O l l v o l r e and C o a o a . 1982. H i n e r a l s p r e s o n t I n t h e s e o r e s a r e f r a n c o l i t e , q u a r t s . , k a o l l n l t o , a i l r a s ami I r o n l i y . l r u x l i l u s . CO 85 Colombo (1975) investigated the s e l e c t i v e f l o c c u l a t i o n of a scrubbed low grade phosphate slime, again from F l o r i d a . A strongly anionic corn starch was used i n conjunction with sodium s i l i c a t e and NaOH (pH 9.4). Recovery was as high as 75%P 20 5 but the concentrate assayed only 15%P 20 5, from a feed grade of approximately 10%P 2O 5. Baudet and co-workers patented i n 1980 the use of carboxymethyl starch with d i f f e r e n t degrees of sub s t i t u t i o n (D.S.) as a s e l e c t i v e f l o c c u l a n t for phosphate slimes (-45 um).In t h e i r work, sodium s i l i c a t e with a Si0 2/Na 20 r a t i o of 3.5:1 was used i n combination with NaOH (pH 9) as dispersant. They were able to concentrate the slimes, which assayed 23.6 to 26.1%P 20 5, to a grade generally higher than 3 0%P2O5. The %R 20 3 reported i n some te s t s were below 5%. Recoveries varied considerably, from 46.8 to 70%P 2O 5. The best r e s u l t s were achieved by using at le a s t lOkg/t of Na 2 S i 0 3 and 4 - 5kg/t of modified starch with a D.S of 0.40. Despite the fact that Haseman recognized the po t e n t i a l f o r s e l e c t i v e f l o c c u l a t i o n being used as a se l e c t i v e desliming technique, only very recently has t h i s idea been tested for phosphates. In 1974 the f i r s t i n d u s t r i a l a p p l i c a t i o n of s e l e c t i v e f l o c c u l a t i o n (as a desliming operation) went into production. Some pioneering work on phosphate ores from B r a z i l was performed by Coelho (1976 and 1979). In 1982, de O l i v e i r a and Gomes u t i l i z e d , with l i m i t e d success, the combination of s e l e c t i v e f l o c c u l a t i o n - f l o t a t i o n 86 i n t h e i r attempts to concentrate a low grade B r a z i l i a n phosphate ore (10%P 2O 5). After t e s t i n g s e l e c t i v e f l o c c u l a t i o n alone for d i f f e r e n t s i z e f r a c t i o n s , they performed some preliminary anionic f l o t a t i o n t e s t s on the s e l e c t i v e l y deslimed product (or "s e l e c t i v e f l o c c u l a t i o n pre-concentrate"). Although they were able to increase considerably the grade of t h i s product by f l o t a t i o n (up to 2 6.5%P 20 5), f l o t a t i o n recoveries were poor (47 of 54%P 20 5). More recently, Coelho (1984) and de Araujo et al.(1986) reported the use of the s e l e c t i v e f l o c c u l a t i o n -f l o t a t i o n technique i n an attempt to concentrate three d i f f e r e n t sedimentary phosphate ores, also from B r a z i l . Table 4.IX summarizes t h e i r findings. E s p e c i a l l y f o r the lower grade ores (12.4 and 17.6%P 20 5), the use of t h i s b e n e f i c i a t i o n scheme seemed very a t t r a c t i v e . Overall recoveries were as high as 83.4%P 20 5, af t e r the r e j e c t i o n of 23 to 27% of the ore mass as slimes. These slimes for the two lower grade ores tested, assayed less than 4.5%P 20 5 and represented a maximum loss of 8.3%P 20 5. As pointed out i n these works, no attempts to optimize f l o t a t i o n of the s e l e c t i v e l y deslimed product were made. Another recent contribution i n t h i s f i e l d i s the work of Rubio and Marabini (1985). They addressed a more complex subject, involving the sele c t i v e f l o c c u l a t i o n of apatite from c a l c i t e . As previously mentioned, most of the future phosphate ores w i l l have c a l c i t e (or another 87 carbonate) as a major gangue mineral. They were able to f l o c c u l a t e s e l e c t i v e l y hydroxyapatite from c a l c i t e using Na polyacrylate as a (selective) dispersant f o r c a l c i t e and a polyacrylamide (superfloc 16) as f l o c c u l a n t f o r apatite. When quartz was present, Na s i l i c a t e i n conjunction with Na acrylate were used as dispersants. In conclusion, s e l e c t i v e f l o c c u l a t i o n applied to phosphate ores and minerals has given r e l a t i v e l y encouraging r e s u l t s , i n small scale t e s t i n g . Reasons f o r the apparent lack of i n t e r e s t by industry i n attempting to apply the technique, are related to the low value of phosphate rock and to the existence of r e l a t i v e l y large reserves of high grade phosphate ores with low mining and b e n e f i c i a t i o n costs. However, lower grade deposits w i l l have to be u t i l i z e d i n the near future. E x p l o i t a t i o n of lower grade ores i s inherently associated with the need for highly e f f i c i e n t processing techniques. Besides that, as shown i n a recent work by Scheiner and Smelley (1985) for the F l o r i d a phosphate industry, disposal regulations are becoming more severe every year. In F l o r i d a for example, new State regulations r e s t r i c t the current method of disposal of slimes, which i s impoundment behind earthen dams. I f s e l e c t i v e desliming i s able to increase mass recovery for instance by 10%, the reduction i n costs of disposal w i l l probably make s e l e c t i v e f l o c c u l a t i o n an economic al t e r n a t i v e . The best u t i l i z a t i o n of known resources from non-renewable sources such as minerals should also be always considered. SLIMES FLOCCULATED PRODUCT* FLOTATION CONCENTRATE1" FLOTATION TAILINGS SAMPLE/ (Feed Grade) Grade Weight D i s t . % P 2 0 5 % % P 2 0 5 Grade Weight D i s t . % P 2 0 5 % %P2Os Grade Weight D i s t . % P 2 0 5 % % P 2 0 5 Grade Weight D i s t . % P 2 0 5 % % P 2 0 5 HG/ (26.2% P 20 5) 16.8 21.4 13.5 29.4 78.6 86.5 32.3 52.3 63.0 23.7 26.3 23.5 MG/ (17.6% P 20 5) 4.2 23.1 5.6 21.2 76.9 94.4 30.2 47.7 83.4 6.5 29.2 11.0 LG/ (12.4% P 20 5) 3.9 26.8 8.3 15.8 73.2 91.7 27.3 35.9 79.1 4.3 37.3 12.6 * c a l c u l a t e d f i g u r e s t c o n s i d e r i n g losses o f the s e l e c t i v e f l o c c u l a t i o n (recoveries c o n s i d e r i n g f l o t a t i o n alone are: HG = 73.2, MG = 88.4 and LG = 86.3) TABLE Zf.IX - S e l e c t i v e FlocculationCCorn Starch,1 k g / t ) / F l o t a t i o n of Sedimentary Phosphate Ores(de Araujo et al . , 1 9 8 6 ) . 89 5 - EXPERIMENTAL 5.1 - Preparation of Mineral Samples Natural and synthetic mineral samples were used i n the present work. Among the d i f f e r e n t apatite samples ava i l a b l e , a f l u o r a p a t i t e from Monteiro, B r a z i l was chosen. I t was avail a b l e i n enough quantity as hand picked c r y s t a l s . The other a v a i l a b l e apatite samples included: f l u o r a p a t i t e from Durango, Mexico (green c r y s t a l s ) , f l u o r a p a t i t e from Ontario, Canada (blue, massive), and carbonate-apatite containing iron, from I t a t a i a , B r a z i l (brown powder, previously ground). Details on these samples are given i n Appendix I, since they were not used extensively i n the experimental work. Besides these natural apatites, a synthetic hydroxyapatite was also used i n a few experiments. This sample i s a t r i b a s i c calcium phosphate from Fisher S c i e n t i f i c (A.C.S. c e r t i f i e d reagent). The following non-apatitic samples were also u t i l i z e d i n the present work: quartz from Ottawa, Canada (supplied as c r y s t a l s by the Mines Branch); Min-U-Sil-5, a 99.7% S i 0 2 synthetic s i l i c a sample, non-porous;kaolinite, " a i r f l o a t e d kaolin", from Georgia, USA. More d e t a i l s on these samples are also given i n Appendix I. Among the non-apatitic samples, quartz was selected for the majority of the comparative t e s t s . 90 Table 5.1 presents the r e s u l t s f o r the chemical analyses performed on the two natural mineral samples most frequently used. These r e s u l t s were obtained using d i f f e r e n t a n a l y t i c a l techniques, l i s t e d i n the same table. Both minerals have very high degrees of chemical p u r i t y . As expected, the f l u o r a p a t i t e sample has more contaminants than the quartz sample. As previously discussed i n the review section (4.1.4), t h i s i s a consequence of the open l a t t i c e of the apatites. The r e l a t i v e l y high grades f o r Th, La and U were also expected due to the l o c a l geology. The two natural minerals ( f l u o r a p a t i t e from Monteiro and quartz from Ottawa), were crushed i n three stages using conventional laboratory crushers. A f t e r crushing to d 9 5 of about 1 cm, the samples were subjected to a low i n t e n s i t y magnetic separation (with a hand magnet) to remove any wear debris from the crushing operations. A f t e r that they were subjected to wet grinding i n a pebble f i l l e d ceramic m i l l , at 60% s o l i d s , obtained by the addition of d i s t i l l e d water (pH 5.5 - 5.8). The grinding operation was performed i n stages of 30 minutes followed by wet screening on a 38 micron sieve. The oversize of the screen was added to a new batch of unground material and another 30 minute grinding stage was then i n i t i a t e d . A f t e r a number of stages, the grinding was stopped. The plus 38 micron material was dried and screened. The -212 + 150 micron s i z e f r a c t i o n was kept for the m i c r o f l o t a t i o n testwork. The -38 micron f r a c t i o n , a f t e r a 12 hour s e t t l i n g period, was dried overnight at 80°C and kept TABLE 5.1 CHEMICAL CHARACTERIZATION OF MINERAL SAMPLES CONSTITUENTS UNITS METHOD FLUORAPATITE QUARTZ P 2 O 5 CD X wet assay 40.5 0.03 CaO CD % wet as say 54.01 0.04 F e 2 0 3 (1) % wet assay 0.014 0.06 A 1 2 0 3 ( D wet assay 0.057 0.07 S i 0 2 % wet assay 0.27 99.56 F (2) % i . s . e . (*) 2.60 n.a.(* CI (2) % Xray f l u . ( * * *) 0.38 n.a. Mg % ICP (3) 0.03 0.01 T i % ICP 0.01 0.01 Na X ICP 0.08 0.01 Pb ppm • ICP 105 23 Cu ppm ICP 14 32 Mn ppm ICP 123 3 Th ppm ICP 1060 2 U ppm ICP 31 5 La ppm ICP . 290 2 Sr ppm ICP 267 1 Ba ppm ICP 188 9 As ppm ICP 15 6 Zn ppm ICP 11 17 N o t e s : (1) UBC, Department of M i n i n g and M i n e r a l P r o c e s s E n g i n e e r i n g , A n a l y t i c a l L a b o r a t o r y (2) GE0S0L, B e l o H o r i z o n t e , B r a z i l (3) ICP i o n i c c o u p l e d plasma, ACME A n a l . Lab . L t d .., Vane. (*) i e s = i o n s e l e c t i v e e l e c t r o d e (**) n.a. = not a v a i l a b l e . (***)Xray f l u . = Xray f l u o r e s c e n c e . f o r the f l o c c u l a t i o n and adsorption testwork. Part of the -38 micron f r a c t i o n of the f l u o r a p a t i t e sample was subjected to further s i z i n g by beaker decantation to y i e l d a -6 micron material used for part of the f l o c c u l a t i o n t e s t programme. The material not s e t t l e d a f t e r 12 hours, was kept f o r microelectrophoresis. The other mineral samples, either natural or synthetic were used as received. In no circumstance, were any of the samples subjected to any type of chemical pre-treatment. This option was taken because of many factors, among them: (i) - the p u r i t y of the two natural mineral samples was considered high enough f o r the purpose of t h i s work, therefore the use of chemical treatment would not improve s u b s t a n t i a l l y the grade of the pure minerals. ( i i ) - the removal of the low l e v e l of impurities present i n both samples v i a a chemical treatment would probably introduce more impurities or would modify the samples i n a way that such a treatment can not be j u s t i f i e d . 93 ( i i i ) - the other samples were ei t h e r chemically pure (for the synthetic ones) or were employed i n such a way that the characterization performed and described i n Appendix I was considered enough. (iv) - the use of any type of chemical treatment i s , at best debatable. This i s e s p e c i a l l y true for semi-soluble minerals such as apatite. For example, p r e f e r e n t i a l d i s s o l u t i o n of some of the l a t t i c e constituents during chemical treatment may a l t e r the behaviour of t h i s type of mineral. 5.2 - Chemicals Table 5.II l i s t s a l l chemical reagents used i n the present work. The same table also gives t h e i r uses and a few important remarks, when needed. They include simple inorganic chemicals for pH regulation, i o n i c strength control and as sources of i o n i c species. A l l reagents i n t h i s category are of high purity. Dispersants include members of the "polyphosphate" family and sodium s i l i c a t e s . Na tetraborate was also tested as a dispersant. In t h i s category both chemically pure and commercial grade reagents were used. TABLE 5 . I I CHEMICALS COMPOUND FORMULA (M.W.) SUPPLIER (Grade) USES REMARKS Sodium NaCl (58 .44) F i s h e r S u p p o r t i n g e l e c t r o l y t e -C h l o r i d e ( A . C . S . c e r t i f i e d ) P o t a s s ium KCI (74 .56) F i s h e r S u p p o r t i n g e l e c t r o l y t e -C h l o r i d e ( A . C . S . c e r t . ) P o t a s s ium KN0 3(101 .11) F i s h e r S u p p o r t i n g e l e c t r o l y t e -N i t r a t e ( A . C . S . c e r t . ) Sod ium H y d r o x i d e NaOH (40 . 00) Amachem (Reag.Grade) pH m o d i f i e r p r e f e r r e d on the a ' l k a l i n e g e l a t i n i z a t i o n o f s t a r c h P o t a s s ium KOH (56 .11) Amach em pH m o d i f i e r -H y d r o x i d e (Reag.Grade) Ammonium NH AOH(35 .05) A l l i e d C h e m i c a l pH m o d i f i e r NH^ c o n t e n t 28 - 30% H y d r o x i d e ( A . C . S . c e r t . ) H y d r o c h l o r i c HCl (36 .46) Amachem pH m o d i f i e r -A c i d (Reag.Grade) S u l p h u r i c H 2 S 0 4 ( 9 8 .08) Amachem C o l o r i m e t r i c a n a l y s i s may a l s o be used as A c i d (Reag.Grade) of s t a r c h pH m o d i f i e r N i t r i c A c i d HN0 3 (63 .01) A l l i e d C h e m i c a l ( A . C . S . c e r t . ) . C l e a n i n g o f g l a s s w a r e Used i n the HNO3/ e t h a n o l c l e a n i n g p r o c e s s ' C a l c i u m C a C l 2 . 2 H 2° F i s h e r Source of Ca aqueous d i h y d r a t e C h l o r i d e (14 7 .02) ( A . C . S . c e r t . ) s p e c i e s Magnes ium MgCl 2.6H 2° Baker Source of Mg aqueous 6 - h y d r a t e C h l o r i d e (203 .31) ( A . C . S . c e r t . ) s p e c i e s IX) TABLE 5.II - c o n t . COMPOUND FORMULA :(M.W. ) SUPPLIER (GRADE) USES REMARKS Aluminum C h l o r i d e A1C1 3 .6H 20 (241. 43) B aker ( A . C . S . c e r t . ) Source o f A l aqueous s p e c i e s 6 - h y d r a t e Sodium P y r o p h o s p h a t e N a 4 P 2 0 ? . 10H 20 (446. 0.6) F i s h e r ( A . C . S . c e r t . ) d i s p e r s a n t d e c a h y d r a t e Sodium T e t r a b o r a t e N a 2 B 4 ° 7 (201. 22) B.D.H. ( p u r i f i e d ) d i s per s an t. -Sodium Hexametaphosphate ( N a P 0 3 ) 6 (611 . 77) Baker ( p u r i f i e d ) d i s p e r s a n t 65 - 6 8 % P 2 0 5 Sod ium T r i p o l y p h o s p h a t e N a 5 P 3 ° 1 0 (367 . 86) S t a u f e r ( t e c h n i c a l ) d i s p e r s a n t 85% Sod ium M e t a s i l i c a t e N a 2 S i 0 3 . 5 H 2 0 (212. 74) F i s h e r ( t e c h n i c a l ) d i s p e r s a n t -METSO 1048 51% Na 20 47% SiO? N a t i o n a l S i l i c a t e s L t d . ( c o m m e r c i a l ) d i s p e r s a n t sodium s i l i c a t e w i t h Na?0 : SiO? = 1:1 METSO 200 60.8%Na 20, 28. 9 . 5%H?0 8 % S i 0 2 N a t i o n a l S i l i c a t e s L t d . ( c o m m e r c i a l ) d i s p e r s a n t sodium o r t h o s i l i c a t e w i t h Na?0 : S i 0 9 = 2:1 METSO 99 36.7%Na 20, 24. 38.1%H20 l % S i 0 2 N a t i o n a l S i l i c a t e s L t d . ( c o m m e r c i a l ) d i s p e r s a n t h y d r a t e d Na'sesquisilicate w i t h Na20 : S i 0 2 =3:2 EDTA N a 2 C 1 0 H 1 4 ° 8 N 2 -(372 . 2H 20 08) F i s h e r ( A . C . S . c e r t . ) d i s p e r s a n t complexant d i s o d i i j m e t h y l e n e -d i a m i n e t e t r a a c e t a t e Corn S t a r c h ( C 6 H 1 0 V n F i s h e r (U.S.P.) f l o c c u l a n t -P o t a t o S t a r c h ( C 6 H 1 0 ° 5 ) n F i s h e r ( p u r i f i e d ) f l o c c u l a n t -U3 TABLE 5.II - c o n t . COMPOUND FORMULA (M.W.) S u p p l i e r (Grade) Uses Remarks T a p i o c a < C6 H10 - (comme r c i a l ) f l o c c u l a n t from T h a i l a n d S t a r c h C o l l a m i l ( C 6 H 1 0 Vn (343.000) R e f i n a c o e s de M i l h o B f a s i l ( c o m m e r c i a l ) f l o c c u l a n t a non m o d i f i e d c o r n s t a r c h D e x t r an ( C 6 H 1 0 Vn BDH ( 90%) f l o c c u l a n t a b a c t e r i a l grade A (200 . 000 - 275.000) p o l y s a c c h a r i d e Ami l o s e ( C 6 H 1 0 Vn BDH ( p u r i f i e d ) f l o c c u l a n t from p o t a t o s t a r c h Amylopec t i n ( C 6 H 1 0 Vn BDH ( p u r i f i e d ) f l o c c u l a n t f rom p o t a t o s t a r ch D - g l u c o s e C 6 H 1 0 ° 5 - H 2 ° BDH (Reag.Grade) monomer of -( d e x t r o s e ) (180.167) f l o c c u l a n t s Sod ium CH (CH ) CH -CH(CH. ) - F i s h e r ( p u r i f i e d ) c o l l e c t o r -O l e a t e -COO Na (304) Dodecylamine C 1 2 H 2 5 NH 3C1 Eastman Koday Co. c o l l e c t o r -H y d r o c h l o r i d e (221.82) (pure) Carbon CC1. 4 F i s h e r f o r I . R . t e s t w o r k -T e t r a c h l o r i d e (153.82) ( s p e c t r a n a l i z e d ) P o t a s s ium KBr F i s h e r ( I . R . g r a d e ) f o r I . R . t e s t w o r k , -Bromide (119.01) p r e p a r a t i o n of p e l l e t s P h enol C,H c0H O J (94.11) MCB C h e m i c a l s ( A . C . S . c e r t . ) c o l o r i m e t r i c a n a l y s i s of s t a r c h -E t h a n o l CH 30H (36.07) F i s h e r ( A . C . S . c e r t . ) c l e a n i n g of g l a s s w a r e see HN0 3 remark VO TABLE 5.II - c o n t . COMPOUND FORMULA (M.W.) S u p p l i e r (Grade) Uses Remarks B u t a n o l CH 3 ( C H 2 ) 3 O H (74.12) Amach em (Reag.Grade) f r a c t i o n a t i o n o f s t a r c h -M e t h a n o l CH 3CH 2OH (50.07) F i s h e r (A.C. S . c e r t . ) e x t r a c t i o n o f f a t s f r o m s t a r c h -I o d i n e I (At.Wt.126.90) F i s h e r (A.C. S. c e r t . ) i o d o m e t r i c a n a l y s e s of s t a r c h -P o t a s s ium KI F i s h e r (A.C. S . c e r t . ) as above -I o d i n e (166.01) Buf f e r s - F i s h e r (A.C. S . cer t . ) pH meter s t a n d a r d i z a t i o n pH 10. 4.01, 7.41 and 41 98 The polymers used i n the present work include only polysaccharides with monomer a-D- Glucose. Most of them are reagents and a few are reagent grade. They were characterized i n t h i s work by various methods such as viscometry, l i g h t scattering and infrared spectroscopy (see section 6.2). Only two surfactants were selected as c o l l e c t o r s for apatite. They represent two major types of c o l l e c t o r s used i n the b e n e f i c i a t i o n of phosphate ores. The other chemicals l i s t e d i n table 5.II were used for a v a r i e t y of purposes, including for instance analysis of starches, cleaning of glassware and preparation of standards. 5.3 - Equipment and Techniques 5.3.1 - S i z i n g Three d i f f e r e n t s i z i n g procedures were used. Coarser p a r t i c l e s ( >3 8 micron) were sized by screening. A "Ro Tap" screen shaker was used for dry screening and a singl e screen v i b r a t o r (for standard 8" screens) was used for wet screening. Sieves were p e r i o d i c a l l y cleaned i n an u l t r a s o n i c bath. Conventional beaker decantation was used f o r the s p l i t t i n g of s i z e fractions below 38 micron. Stokes law behaviour was assumed i n these cases. Size analyses for sub-sieve s i z e f r a c t i o n s were performed i n Elzone Celloscope ( P a r t i c l e Data Inc.) In the Elzone instrument, an aqueous suspension of le s s than 1% s o l i d s (by wt.) of p a r t i c l e s to be analyzed i s kept from s e t t l i n g by constant agitation. An inorganic dispersant such as Na-hexametaphosphate was used. The aqueous suspension i s prepared with an e l e c t r o l y t e , and has a constant r e s i s t i v i t y . A tube with a ca l i b r a t e d o r i f i c e i s submerged i n the suspension container. Both tube and container have electrodes, and an e l e c t r i c a l c i r c u i t i s created between them. Vacuum i s applied to the tube and the suspension i s drawn through the o r i f i c e , causing a momentary resistance change (AR). The change i n resistance i s r e l a t e d to the p a r t i c l e volume as follows: AR = ( i _ £ }-l (EQ. 5.3.1) A 2 1 ~ P 0 / P A where: P q = e l e c t r o l y t e r e s i s t i v i t y v = p a r t i c l e volume (effective) A = o r f i c e area normal to the axis of the tube p = e f f e c t i v e p a r t i c l e r e s i s t i v i t y a = cross sectional area of p a r t i c l e Each p a r t i c l e passage produces a voltage pulse of magnitude proportional to the p a r t i c l e volume. The resultant s e r i e s of pulses i s e l e c t r o n i c a l l y scaled and counted. 100 These data are then manipulated by a mini-computer. Outputs can be ei t h e r i n the form of a histogram of volume d i s t r i b u t i o n versus log siz e or cumulative per thousand d i s t r i b u t i o n above s i z e versus s i z e . A log-normal d i s t r i b u t i o n i s assumed. Log mean, mode and median are also c a l c u l a t e d automatically. Details on the operation and p r i n c i p l e s of the Elzone size analyser are found i n anon. (1977) . 5.3.2 - Surface Area Determination Surface areas of mineral samples were estimated by applying a multipoint BET technique. Nitrogen and Helium were used as adsorbate and c a r r i e r gases, r e s p e c t i v e l y . A Quantasorb (Quantachrome Corporation ) BET un i t was employed. The technique i s standard for measuring surface area of powders. Details can be found elsewhere (for operational d e t a i l s see anon., 1970; for BET theory and applications i n the determination of surface see for example Hiemenz, 1977 -pp.322-335 and Leja, 1982 - pp. 379-381). 5.3.3 - Spectroscopic Techniques 5.3.3.1 - Infrared Spectroscopy Infrared spectra of minerals and chemicals were recorded on a Perkin Elmer 283-B double beam, o p t i c a l n u l l 101 spectrophotometer. Wavelength c a l i b r a t i o n of the spectrophometer was checked by examining a standard polystyrene f i l m , the spectral band positions of which are accurately known. The spectral range of the u n i t i s from 4000 to 200 cm"1(wavenumbers). The KBr p e l l e t method was used i n a l l cases. A mixture of approximately 1 mg of sample ( f i n e l y ground and oven d r i e d f o r 24 hours at 80-100°C) and 300 mg of KBr powder (inf r a r e d grade) was transferred to a die (Perkin Elmer model 186-00251), evacuated for 2 to 5 minutes and then pressed at approximately 300 kPa for 1 minute. A Perkin Elmer Infarared Data Station i s connected to the spectrophotometer. I t i s capable of performing d i f f e r e n t tasks such as mathematical smoothing, f l a t t e n i n g , peak p o s i t i o n determination and expansion of recorded spectra, which are saved on disc for further treatment and analysis. The same data station unit i s also capable of c o n t r o l l i n g the spectrophotometer's p l o t t e r . 5.3.3.2 - U.V - V i s i b l e Spectroscopy Two U.V. - V i s i b l e spectrophotometers were used i n the present work: (i) Perkin Elmer model Lambda 3 and ( i i ) Baush and Lomb Spectronic 21. The f i r s t u n i t has a double beam design and i s capable of recording U.V. - V i s i b l e spectra l i q u i d samples (5ml o p t i c a l c e l l s with 1.0 cm of path length) from 900 to 180 nm. I t can either p l o t the complete spectrum f o r a given wavelength range or i t can make measurements at a pre-established wavelength. The type of output and scanning can be selected (e.g % transmittance, absorbance or "concentration" are the types of outputs a v a i l a b l e ) . This uni t was used to obtain the spectra of starches a f t e r t h e i r reaction with iodine. I t was also used i n the study of the i n t e r a c t i o n between starches and surfactants. The second unit (Spectronic 21) i s a si n g l e beam spectrophotometer, equipped with 5ml c y l i n d r i c a l glass cuvettes. I t was used f o r most of the quantitative work i n the determination of starch concentrations with the iodine method. The readings of t h i s unit are i n the form of % transmittance at a pre-set wavelength. To adjust the transmittance reading, a black coloured cyli n d e r of the same diameter of the cuvettes i s used to give zero % transmittance,this operation being repeated every time the u n i t i s turned on. The pre c i s i o n of the readings i s 0.5% transmittance units . A f t e r obtaining the transmittance data they were converted to absorbance. I t was also used f o r q u a l i t a t i v e dispersion testwork, where the % transmittance of a suspension to l i g h t of a wavelength of 450 nm a f t e r a determined s e t t l i n g period was used as an i n d i c a t i o n of the s t a b i l i t y of such a suspension. 5.3.3.3 - Light Scattering 103 The molecular weight of a polymer can be determined from the r a t i o s of the i n t e n s i t y of scattered l i g h t (measured at r i g h t angles to the incident beam) to the i n t e n s i t y of the transmitted beam as determined for solutions of several d i f f e r e n t concentrations. This r a t i o can be measured using a l i g h t s c a t t e r i n g photometer. In the present work a Brice-Phoenix Universal Light Scattering Photometer was used. I t comprises, b r i e f l y , a l i g h t source, a set of f i l t e r s and collimators, a turnatable where the sample i s positioned, a photomultiplier tube and a l i g h t trap. A more d e t a i l e d d e s c r i p t i o n can be found i n Hiemenz(1977)- pp.175-178. Theory: I f p o l a r i z e d l i g h t of i n t e n s i t y I Q i s passed through a macromolecular solution, each molecule scatters the l i g h t i n a l l d i r e c t i o n s and becomes e f f e c t i v e l y a source of l i g h t of the same wavelength as the incident beam. The r a t i o of i n t e n s i t y of the scattered beam - i s - (measured at an angle © to the incident beam) to I Q i s defined by the Rayleigh r a t i o R Q given by: i r 2 A.2 R = - i = 1 6 \ a (EQ.5.3.2) 9 I c o s 2 e X 4 o o where: r i s the distance of the observer from the sc a t t e r i n g centre. 104 « i s a constant (the p o l a r i z a b i l i t y of the p a r t i c l e ) X Q i s the wavelength of the l i g h t . I t can be shown (Hiemenz, 1977) that since n 2-n 2 Q=4 TTNCL then 2 2 2 4 TT n (dn/dc) M c _ „ „ . •o = o (EQ.5.3.3) G , 4 N X o where: dn/dc i s the r e f r a c t i v e index gradient with respect to concentration c i s concentration i n g/ml M i s the molecular weight of:the macromolecule N i s Avogadro 1s number n i s the r e f r a c t i v e index of the so l u t i o n n Q i s the r e f r a c t i v e index of the solvent This equation assumes an i n f i n i t e l y d i l u t e i d e a l solution. For r e a l solutions a correction must be applied as follows: k ir 2 n 2 ( d n / d c ) 2 c R = ° (EQ. 5.3.4) 6 N X 4 (1 / M + B* c) o where B* = 2B i s the second v i r i a l c o e f f i c i e n t . 105 A p p l i c a t i o n : With the turntable set at 90° the photomultiplier tube measures only scattered l i g h t . With the di s c of the turntable at 0" the photomultiplier tube measures the transmitted l i g h t . The int e n s i t y of the scattered l i g h t i s several orders of magnitude less than the transmitted beam and i n p r a c t i c e i t i s not feasible to measure such a wide . range with a single photomultiplier tube. Thus the transmitted beam i s attenuated with neutral density f i l t e r s of known transmittance and a cal i b r a t e d working standard f i l t e r which i s i n p o s i t i o n only when t h i s beam i s being measured. The Rayleigh r a t i o (Rg0») when e = 9 0 ° i s given by: R9 0 o = C ) » 2 CF ^ ) (EQ.5.3.5') T r ( l . 0 4 9 ) h Gw where: TD i s the di f f u s e transmittance of the working standard f i l t e r . h i s the width of the diaphragm. F i s the product of the transmittance of the f i l t e r s used i n the determining the scatter r a t i o . G s i s the meter reading on the scattered l i g h t measured at 90° to the incident beam. G w i s the meter reading on the transmitted l i g h t measured d i r e c t l y into the incident beam. 106 n i s the r e f r a c t i v e index of the s o l u t i o n . R g Q o f o r the solute i s determined by subtracting Rg 0.for the solvent from Rgoo f o r the solution. The molecular weight i s determined from a p l o t of H * c / R g 0 . ( s o i u t e ) v e r s u s the solute concentration, c, i n g/ml since from EQ.5.3.4: H ' c / R 9 0 o = l / M + 2Bc (EQ.5.3.5) where: H» = (2 i r V ( n - n / c ) 2 ) / ( X* N) Q O u (EQ.5.3.6) The ap p l i c a t i o n of t h i s l a s t equation to the determination of molecular weights of the starch samples used i n t h i s work i s discussed l a t e r i n t h i s t h e s i s (see section 6 . 2 . 4 ) . 5.3.4 - Electrophoresis and Related Experiments Electrophoretic mobility was determined with a Zeta Meter (Zeta Meter Inc., New York). A c y l i n d r i c a l l u c i t e c e l l , type II U.V.A. (No. S 2 3 4 6 ) , s i z e 0.0, with a constant K=69, was used. I t has an interelectrode distance of 10cm. A Pt cathode ( f l a t ) and Pt or Mo anodes (round),standard for the Zeta Meter,were u t i l i z e d , depending on the conductivity of the sample. 107 Temperature of the sample suspensions was maintained, i n most cases, at 23+0.5°C with a water bath. The electrophoretic c e l l was flushed with d i s t i l l e d water at 23.5°C before the t e s t suspension was added. The expected temperature r i s e i s 1.5°C during electrophoresis, thus the f i n a l t e s t temperature was approximately 25"C (Zeta Meter Manual, Zeta Meter Inc., 1975). F i n e l y divided, pure mineral samples i n very d i l u t e aqueous suspensions were subjected to electrophoresis. An average of at l e a s t 2 0 measurements were made i n each t e s t run, and the mean and standard deviation of the electrophoretic m o b i l i t i e s of the p a r t i c l e s were computed. Only the r e s u l t s showing a standard deviation from the mean of l e s s than or equal to 0.35 were accepted. Electrophoretic mobility/pH curves were obtained under three d i f f e r e n t conditions, f o r the mineral samples: (i) - i n d i s t i l l e d water, ( i i ) - i n the presence of an i n d i f f e r e n t e l e c t r o l y t e , ( i i i ) - i n the presence of s p e c i f i c a l l y adsorbing ions. The starch samples were also subjected to electrophoresis i n order to v e r i f y the presence of i o n i c groups. These experiments were conducted only at one pH value, of 5.5 to 6.0. The point of zero charge of the f l u o r a p a t i t e sample 108 was determined by applying the Mular and Roberts (1966) pH technique. One v a r i a t i o n of t h i s technique, generally refer r e d to as the Ahmed (1969) method was also used. As previously discussed (see section 4.3.1), fo r minerals such as apatite, a s i n g l e value for the point of zero charge (pzc) must be accompanied by the conditions of the experiment, since one can define a l i n e of zero charge f o r the mineral depending on the concentration of the d i f f e r e n t p o t e n t i a l determining ions. The value of zero electrophoretic mobility ( i s o e l e c t r i c point or iep) i s that determined by applying the electrophoretic technique described above. Presentation of the r e s u l t s obtained for the minerals i s given i n section 6.1.3, for starches i n section 6.2.2. 5.3.5 - Viscometry In contrast to starch paste v i s c o s i t y , i n which a rheological property i s the main objective, the measurement of i n t r i n s i c v i s c o s i t y of d i l u t e solutions of starches i s performed to obtain information regarding flow properties. These flow properties for d i l u t e solutions (less than 0.5% by weight) are measured under such conditions that the polymolecules are as free as possible of entanglements. The i n t r i n s i c v i s c o s i t y (or the l i m i t i n g v i s c o s i t y number, according to IUPAC nomenclature) i s generally r e l a t e d to molecular weight by the Mark-Houwink equation as follows (Greenwood, 1964; Seymour and Carraher, 1981): 109 H =KMa (EQ.5.3.7) where: |n| = l i m i t i n g v i s c o s i t y number (ml/g) M = average molecular weight K = e m p i r i c a l c onstant f o r a g i v e n polymer s o l v e n t system, and a = e m p i r i c a l exponent t h a t i s a f u n c t i o n of the shape o f the polymer c o i l i n s o l u t i o n . Furthermore: |n| = l i m ( nsp/c) (EQ.5.3.8) c-0 where: c = grammes of polymer per 100 ml o f s o l u t i o n r ,sp =( n"" no)/ r iO / ^ n w n i c n n Sp i s t h e s p e c i f i c v i s c o s i t y n= i s the v i s c o s i t y of the s o l u t i o n and n Q i s the v i s c o s i t y o f the s o l v e n t . An Ostwald-type c a p i l l a r y v i s c o m e t e r was used f o r the d e t e r m i n a t i o n of l i m i t i n g v i s c o s i t y numbers o f p o t a t o amylose s o l u t i o n s . Although a b s o l u t e v a l u e s of average m o l e c u l a r weight can not be o b t a i n e d by v i s c o s i t y methods wit h o u t c a l i b r a t i o n by an independent measurement, the knowledge of the l i m i t i n g v i s c o s i t y number and the use of 110 t a b u l a t e d c o n s t a n t s f o r t h e system can p r o v i d e some i n f o r m a t i o n on t h e m o l e c u l a r s i z e o f t h e sample u s e d i n t h e p r e s e n t i n v e s t i g a t i o n . D e t a i l s on t h e t e c h n i q u e t o measure l i m i t i n g v i s c o s i t y number f o r s t a r c h e s a r e g i v e n by Greenwood(1964), D a n i s h e f s k y e t a l . ( 1 9 7 0 ) , and Seymour and C a r r a h e r ( 1 9 8 1 ) . The r e s u l t s and d i s c u s s i o n s a r e p r e s e n t e d i n s e c t i o n 6.2.3. The c a p i l l a r y v i s c o m e t e r was c a l i b r a t e d w i t h d i s t i l l e d w a t e r and benzene (A.C.S. c e r t . ) . Each f l o w t i m e measurement was performed a t l e a s t 3 t i m e s . The a v e r a g e f l o w t i m e was used i n t h e c a l c u l a t i o n s . Temperature d i d n o t v a r y more t h a n 0.5°C. 5.3.6 - C o n d u c t i v i t y Testwork The s p e c i f i c c o n d u c t i v i t y o r c o nductance o f a s o l u t i o n can be d e f i n e d as t h e r e c i p r o c a l r e s i s t a n c e a c r o s s two n o n - p o l a r i z e d e l e c t r o d e s w h i c h form t h e o p p o s i t e s i d e s o f a cube o f 1 x 1 x 1 cm. E l e c t r o l y t i c s o l u t i o n s can c o n d u c t e l e c t r i c a l c u r r e n t t h r o u g h t h e movement o f i o n s , b e l o n g i n g t o t h e c a t e g o r y o f c o n d u c t o r s o f t h e second c l a s s ( C r o c k f o r d and K n i g h t , 1964). When two e l e c t r o l y t e s a r e added, t h e n e t c o n d u c t a n c e i s t h e sum o f t h e i n d i v i d u a l c o n t r i b u t i o n s by t h e i o n s p r e s e n t i n b o t h o f them. Thus when two e l e c t r o l y t e s BA and CD a r e added, t h e n e t conductance i s g i v e n by: 1-11 A B A + A C D = A B + + A A - + A C + + A D - ( E Q . 5 . 3 . 9 ) the sum o f the m o b i l i t i e s of a l l i o n s . However, i f t h e r e i s i o n - i o n i n t e r a c t i o n , the above r e l a t i o n s h i p does not h o l d and the net conductance depends upon the s o l u b i l i t y o f t h e new s p e c i e s as w e l l as the m o b i l i t y of i t s i o n s (Khosla, 1983). T h e r e f o r e , upon adding two e l e c t r o l y t e s , i f the conductance can not be e x p l a i n e d by m o b i l i t i e s of the i o n s known t o be p r e s e n t i n the two e l e c t r o l y t e s , an i o n - i o n i n t e r a c t i o n o r complex f o r m a t i o n i s i n d i c a t e d . In the p r e s e n t testwork, known s o l u t i o n s o f s t a r c h a t d i f f e r e n t c o n c e n t r a t i o n s were added t o c a l c i u m c h l o r i d e s o l u t i o n s a l s o o f known c o n c e n t r a t i o n , a t p r e - e s t a b l i s h e d pH v a l u e s . The conductance of these s o l u t i o n s , alone and i n combination, was measured w i t h a Radiometer C o n d u c t i v i t y Meter, Type CDNM2d, equipped w i t h a c o n d u c t i v i t y c e l l t ype CDC 104. T h i s c e l l has t h r e e e l e c t r o d e s i n the form of a p l a t i n u m sheet p l a c e d around a g l a s s tube and e n c l o s e d i n a g l a s s j a c k e t (anon., Radiometer I n s t r u c t i o n Manual, 2nd e d . ) . The top and bottom r i n g s are connected and grounded t o the c h a s s i s through the s h i e l d of the c o a x i a l c a b l e w h i l e the c e n t r e r i n g i s connected t o the c e n t r e conductor o f the c a b l e . T h i s arrangement which i s a s p e c i a l f e a t u r e of the c o n d u c t i v i t y c e l l , p r o v i d e s an e f f e c t i v e e l e c t r i c a l s h i e l d i n g of t h e flows of c u r r e n t s between the e l e c t r o d e s thus e n a b l i n g the c e l l t o be used f o r measurements i n grounded v e s s e l s . 112 When using t h i s c e l l , a l l three rings must be covered with the t e s t s olution, and care should be taken to avoid the formation of a i r bubbles inside the walls of the c e l l . The c e l l i s equipped with small holes where the bubbles can escape. The c e l l constant i s 1.0 cm. The Radiometer Conductivity Meter has two measuring ranges: 0-500 micromho and 0-500 millimho. I t also has two measuring frequencies: 70 Hz for conductance below 150 micromho and 3000 Hz for conductance values above 150 micromho. The measuring accuracy i s 2% of f u l l scale d e f l e c t i o n . The step-by-step operation of t h i s u n i t i s given i n the Instruction Manual (see reference above). 5.3.7 - Q u a l i t a t i v e Dispersion Testwork In a preliminary phase, a series of q u a l i t a t i v e dispersion experiments was performed i n order to characterize the e f f e c t of d i f f e r e n t chemicals on the degree of s t a b i l i t y of mineral suspensions. These tests were performed on three of the mineral samples used i n t h i s work: f l u o r a p a t i t e from Monteiro, quartz and k a o l i n i t e . For t h i s purpose, the % transmittance of the mineral suspension being tested was measured at 450 nm a f t e r a pre-established s e t t l i n g period. The Baush and Lomb U.V. - V i s i b l e spectrophotometer Spectronic 21, previously described, was used. The r e s u l t s obtained should be regarded as a r e l a t i v e measure of the degree of dispersion/aggregation of the suspensions but 113 should not be compared among the three minerals due to t h e i r s i z e and s p e c i f i c gravity differences. The r e s u l t s and discussions are presented i n section 6.1.4. 5.3.8 - Adsorption Testwork To investigate the adsorption of starches on the mineral samples used i n the present work, both a q u a l i t a t i v e , preliminary adsorption t e s t programme and a more comprehensive quantitative t e s t programme were performed. In the f i r s t case, with the help of colour photography, the adsorption of starches on apatite and s i l i c a was followed at one a r b i t r a r i l y set condition (25ppm of polymer and pH of approximately 10). During these t e s t s , an equal amount of I/KI was added to t e s t tubes containing both mineral powder and starch and starch only. The tubes were set side by side i n a t e s t tube rack and colour pictures were taken, a f t e r a period of approximately 24 hours. These photographs were able to show the p r e f e r e n t i a l adsorption of a l l starches tested on the apatite surface as compared to the s i l i c a surface. The colour imparted to the starch solutions by the additio n of iodine was transferred to the mineral, whenever starch was extensively adsorbed. In the case of no adsorption or low a f f i n i t y f or the surface, the colour of the powder ( s i l i c a ) remained unchanged, and the supernatants containing starch stayed strongly coloured. A NIKON 3 5mm camera, equipped with a tr i p o d and standard 200ASA KODAK VR 114 f i l m , was used. Two flood l i g h t s (500 watts each) were set at a 45° angle i n r e l a t i o n to the front of the t e s t tube rack. In the second series of adsorption t e s t s , the amount of starch adsorbed onto the surfaces of f l u o r a p a t i t e and quartz was measured. The following experimental procedure was used: (i) - a pre-weighed amount of mineral powder was transferred to c y l i n d r i c a l glass v i a l s of about 75ml capacity, equipped with p l a s t i c l i d s . ( i i ) - f i f t y ml of the starch so l u t i o n to be tested, at the pre-chosen ( i n i t i a l ) concentration and pH was also added to the v i a l containing the mineral. ( i i i ) - the v i a l was covered with the l i d and hand shaken by turning i t end over end 5 times. (iv) - a set of v i a l s , enough to obtain a complete adsorption isotherm was then transferred to a shaker (Environ Orbit Laboratory Shaker). The shaking speed was 3 00 rpm and the shaking time for most of the t e s t s was 60 minutes (di f f e r e n t only f o r the adsorption k i n e t i c s t e s t s ) . (v) - Afte r the shaking period was completed, the v i a l s were removed from the shaker. Great care was taken not to disturb the s e t t l e d 115 phase i n t h e case o f f l o c c u l a t i o n . I n some t e s t s , t h e s u p e r n a t a n t was removed f o r Ca a n a l y s i s and % t r a n s m i t t a n c e measurements (complementary t e s t s ) . I n most c a s e s ( a d s o r p t i o n t e s t ) t h e s u p e r n a t a n t was f i l t e r e d t h r o u g h a g l a s s f i b e r Whatman A934 paper, i n a c o n v e n t i o n a l vacuum f i l t r a t i o n s e t up. The f i n a l pH was t h e n measured. ( v i ) - t h e f i l t r a t e was t h e n a n a l y s e d f o r r e s i d u a l s t a r c h c o n c e n t r a t i o n by t h e c o l o r i m e t r i c I 2 / K I t e c h n i q u e . Care was t a k e n t o a d j u s t t h e f i n a l pH o f t h e f i l t r a t e t o a known v a l u e . ( v i i ) - t h e amount o f s t a r c h a d s orbed was t h e n c a l c u l a t e d by d i f f e r e n c e ( i . e . t h e amount adsorbed was c o n s i d e r e d t o be e q u a l t o t h e i n t i a l c o n c e n t r a t i o n minus t h e r e s i d u a l c o n c e n t r a t i o n ; a d s o r p t i o n o n t o t h e g l a s s v i a l w a l l s was c o n s i d e r e d t o be e x t r e m e l y s m a l l ) . I n some t e s t s , a s o l u t i o n c o n t a i n i n g Ca i o n s a t a p r e v i o u s l y s e l e c t e d c o n c e n t r a t i o n was a l s o used i n o r d e r t o i n v e s t i g a t e t h e e f f e c t o f t h e s e i o n s i n t h e a d s o r p t i o n b e h a v i o u r o f s t a r c h e s on t h e m i n e r a l s s t u d i e d . B e f o r e p e r f o r m i n g t h e a d s o r p t i o n t e s t programme, an e x t e n s i v e i n v e s t i g a t i o n o f t h e a n a l y t i c a l method chosen f o r t h e d e t e r m i n a t i o n o f r e s i d u a l s t a r c h c o n c e n t r a t i o n s was 116 performed (see section 6.4.2). The e f f e c t of shaking under s i m i l a r conditions of the adsorption tests/and of f i l t r a t i o n were throughly investigated, before the f i n a l s e l e c t i o n of t h i s c o l o r i m e t r i c technique. The Baush and Lomb U.V. -v i s i b l e spectrophotometer Spectronic 21 was used f o r the determination of the residual concentrations and c a l i b r a t i o n curves f o r starch solutions reacted with iodine. 5.3.9 - Single Mineral Flocculation Testwork Although there are many d i f f e r e n t ways to perform t h i s type of experiment, some of them, such as r e f i l t r a t i o n rate, s e t t l e d volume, height of s e t t l e d bed, and height of consolidated f i l t e r cake give only very s p e c i a l i z e d information (see for example Slater and Kitchener, 1966, for d e t a i l s ) . For t h i s reason, the techniques selected i n the present work are " g r a v i t i c " methods where e i t h e r a degree of f l o c c u l a t i o n or a zone s e t t l i n g rate can be d i r e c t l y derived. Some complementary tests were performed using the same technique described e a r l i e r for the q u a l i t a t i v e dispersion t e s t programme. The f i r s t of the two " g r a v i t i c " techniques used a p l e x i g l a s s tube with two outlets, as shown i n figure 5.-1. The type of outlets were designed to avoid suction of material below the set height, which could modify the f i n a l r e s u l t s because of the small volume of sample used i n each t e s t . The tube was "calibrated" with d i s t i l l e d water. After 1 1 7 S o CO cork l e v e l marker top E o CM g o u t l e t ° 1 CM T 1 bottom o u t l e t h .3cm b F i g u r e 5'1 - Schematic View of Tube Used f o r P r e l i m i n a r y D i s p e r s i o n / F l o c c u l a t i o n Tests. 118 removing 20 consecutive 10ml increments, the average value was 10.1 +0.5ml. The other important dimensions of the tube are given i n figure 5.1. A series of t r i a l t e s t s was performed to develop a reproducible procedure with the k a o l i n i t e sample. The r e s u l t s obtained with the tube were checked against a more conventional technique also used for the same purposes-the Andreasen pipette (see Sadowski and Laskowski, 1980). For the k a o l i n i t e sample they were almost i d e n t i c a l (de Araujo and Galery, 1987). The procedure used can be summarized as follows: (i) - 30ml of a suspension containing 0.5% s o l i d s by wt. of the mineral being tested i s transferred to the tube (there i s a 3 0ml mark on the tube wall. ( i i ) - the desired reagents are added to the tube. ( i i i ) - the tube i s stoppered and inverted 5 times. (iv) - a f t e r a pre-established period of time (1,2 or 5 minutes f o r example), one of the two outlets i s opened and 10ml (for the upper outlet) or 20ml (for the bottom outlet) of suspension i s removed. For the majority of tests performed i n the present work, the upper outlet was the one chosen. (v) - the sample removed i s dried and c a r e f u l l y weighed. (vi) - a % degree of f l o c c u l a t i o n i s calculated by the formula: 119 Fh,t = 100 -[lOOWo/Wi] (EQ.5.3.10) where: Fh,t i s the degree of f l o c c u l a t i o n expressed i n % ; WQ i s the wt. i n grams of outlet sample(10 or 2 0ml); i s the wt. i n grams per 10 or 2 0 ml of o r i g i n a l suspension. The degree of f l o c c u l a t i o n calculated by EQ.5.3.10 i s only r e l a t i v e . However, for a set of experiments performed under s i m i l a r conditions, i t can be used as a r e l a t i v e measurement of the state of s t a b i l i t y of the suspension. Most of the tests were capable of s e l e c t i n g the s t a r t i n g conditions for the more extensive t e s t i n g performed by the technique described next. The second " g r a v i t i c " technique used i s a v a r i a t i o n of the conventional graduated cylinder t e s t s used f o r thickener design (discussed for instance by Pearse, 198 0 and 1982) . The basic differences are the siz e of the graduated c y l i n d e r selected and the measurement only of the l i n e a r portion of the s e t t l i n g curve. I t has been used by Iwasaki and co-workers (1980 and 1983) to evaluate f l o c c u l a t i o n . I t consists b a s i c a l l y of measuring the s e t t l i n g of a suspension i n a 100ml graduated cylinder, following only the f i r s t 120 minutes or seconds of zone s e t t l i n g . From the slope of t h i s s e t t l i n g curve (l i n e a r portion) a zone s e t t l i n g rate i n appropriate units i s calculated (e.g. cm/minute). In t h i s t e s t programme a regression analysis of each s e t t l i n g data set was performed. Therefore the zone s e t t l i n g rate reported represents the best straight l i n e obtainable. Very high c o r r e l a t i o n c o e f f i c i e n t s were obtained i n most of the cases. A few 1000ml graduated cylinder tests were performed to compare the order of magnitude of the s e t t l i n g rates obtained with two sizes of sedimentation vessels.These seem to in d i c a t e that both techniques give comparable r e s u l t s f o r the the s o l i d concentration chosen (40 g per l i t r e , i . e . 3.9 s o l i d s by wt.) 5.3.10 - M i c r o f l o t a t i o n Testwork In order to investigate the e f f e c t of starches on the c a t i o n i c and anionic f l o t a t i o n of apatite, m i c r o f l o t a t i o n t e s t s i n a modified Hallimond tube were performed. The tube i s only s l i g h t l y d i f f e r e n t from the one described by Fuerstenau et al.(1957). Medical a i r was used as the gas source, at a flow rate of approximately 40 cm 3per minute. The magnetic s t i r r e r was always set to the same l e v e l . F l o t a t i o n time was also set constant at 1 minute, except for a few experiments i n which the knowledge of k i n e t i c parameters was judged important. Each t e s t used approximately 1 gram of mineral i n the s i z e f r a c t i o n between 121 212 and 150 microns. The mineral samples used were f l u o r a p a t i t e from Monteiro, apatite from I t a t a i a and quartz from Ottawa. The % floated i n 1 minute ( c a l l e d "recovery" by many authors and " f l o a t a b i l i t y " or "hydrophobicity" by others) was calculated from the mass balance i n the case of s i n g l e mineral tests and from the ingredient balance i n the few cases where a mixture of two minerals ( f l u o r a p a t i t e and quartz) was employed. The re s u l t s obtained f o r the c o l l e c t o r systems are presented and discussed i n section 6.6. 5.3.11 - Miscellaneous 5.3.11.1 - Mixing and Shaking Both mechanical and magnetic s t i r r e r s were used to prepare solutions and to condition minerals with reagents. They include the T-Line Laboratory s t i r r e r (mechanical) and Corning hot plate-magnetic s t i r r e r model PC 351. Both have adjustable s t i r r i n g v e l o c i t i e s Shaking of samples for adsorption testwork was done i n a Lab-Line Orbit Environ-Shaker (Lab-Line Instruments), with c o n t r o l l e d temperature (20 to 50°C) and speed (up to 500 rpm). Whenever hand shaking i s referred to i n t h i s t h e s i s , the procedure used i s described. 122 5.3.11.2 - pH Measurement A Fisher Accumet 230 pH/ion meter was used f o r pH measurements. Combination glass electrodes (Beckman) were standardized d a i l y against buffer solutions of pH 4.01 and 10.4 (see table 5.II for d e t a i l s on the b u f f e r s ) . In general, the pH of a t e s t run i s the average pH, i . e . the arithmetic mean obtained from i n i t i a l and f i n a l pH values. In some cases only f i n a l or i n i t i a l pH values were obtained. In these cases the tes t pH i s i d e n t i f i e d as i n i t i a l or f i n a l by the subscripts " i " or " f " . 5.3.11.3 - F i l t r a t i o n and Drying In most cases, f i l t r a t i o n was performed i n a standard laboratory vacuum f i l t r a t i o n apparatus. Buchner f i l t e r s with Whatman 934 AH glass f i b e r f i l t e r paper were used. In a few cases, a micropore f i l t e r apparatus was used, with standard f i l t e r paper of 5 and 1 micron. Drying was accomplished i n laboratory ovens with c o n t r o l l e d temperature. In most cases drying of samples was performed overnight at temperatures always below 100°C. Care was taken to avoid any type of contamination during drying. Whenever necessary aluminum f o i l was used to cover material i n s i d e the ovens. 5.3.11.4 - Centrifugation 123 An IEC International Cetrifuge capable of speeds up to 20,000 rpm was used. The main application was i n the c l a r i f i c a t i o n of solutions and suspensions before analyses. 5.3.11.5 - Weighing A Mettler a n a l y t i c a l balance model H 20 T (160 g maximum charge) was used for high p r e c i s i o n weighing. Two other Mettler balance models Pc440 (400 g maximum charge) and P 5 N (4 kg maximum charge) were used f o r le s s precise work (1/100 and l/10g readings, r e s p e c t i v e l y ) . 5.3.11.6 - Time Measurement A Timex stopwatch with d i g i t a l readings was used for most of the work. The pr e c i s i o n of the readings i s 1/100 of a second. Some pieces of equipment u t i l i z e d i n t h i s work have t h e i r own time measuring devices, and i n such cases, no external c a l i b r a t i o n was performed. 5.3.11.7 - Microscopy Both o p t i c a l and electron microscopy were applied. An ETEC Autoscan scanning electron microscope, equipped with and X-ray energy dispersive spectrophotometer, a JEOL 124 electron microprobe model JXA-3A and d i f f e r e n t models of binocular microscopes were u t i l i z e d f o r performing d i f f e r e n t tasks. 125 6 - RESULTS AND DISCUSSIONS 6.1. - Characterization of Mineral Samples 6.1.1 - Infrared Spectroscopy and Other Techniques In order to characterize f u l l y the mineral specimens used i n the present work, they were subjected to a ser i e s of t e s t s . Infrared spectroscopy was chosen as the main mineral i d e n t i f i c a t i o n and characterization t o o l . E s p e c i a l l y for apatites, there i s an extensive l i t e r a t u r e a v a i l a b l e on the subject. In more general terms, the a p p l i c a t i o n of infrared spectroscopy to pure minerals has some advantages such as (Estep-Barnes, 1977): (i) - i t i s one of the most powerful sing l e techniques available today f o r mineral analysis. ( i i ) - the infrared spectrum of a mineral y i e l d s basic information on interatomic bonding and i t can serve, i n most cases, as a "finger-p r i n t " to give proof of i d e n t i t y without resource to any other a n a l y t i c a l method. ( i i i ) - i t needs of only a few milligrammes of sample for a complete spectrum. A l l i e d to the advantages above, the a v a i l a b i l i t y i n 126 the laboratories of the Department of Mining and Mineral Process Engineering (UBC) of a modern in f r a r e d spectrophotometer (see section 5.3.3.1 f o r d e t a i l s ) , contributed to the selection of infrared spectroscopy as a main characterization t o o l used for mineral specimens. As discussed i n section 5.1, not a l l mineral samples were characterized to a same extent. However, f o r comparison purposes, f i g u r e 6.1 presents the infrared spectra of four d i f f e r e n t apatites, including the one from Monteiro, used for the major part of the testwork done. In figure 6.2, the spectrum of the hydroxyapatite sample i s displayed. In the f i r s t case, only the portion of the inf r a r e d spectrum between wavenumbers of 2000 and 300 cm"1 i s presented. For wavenumbers lower than 300 cm"1, the technique used, v i z . KBr p e l l e t s , i s not applicable due to adsorption by the KBr matrix. Also i n figure 6.1 the ordinate scale does not give absolute values f o r the % transmittance because the spectra were modified by computer to be displayed together i n one fi g u r e . In figure 6.2 the regions of the i n f r a r e d spectrum shown are the ones where major infrared bands were displayed by the mineral. For completeness, figures 6.3 and 6.4 show the i n f r a r e d spectra of quartz and k a o l i n i t e , r e s p e c t i v e l y . They w i l l be b r i e f l y discussed l a t e r i n t h i s section. Interpretation of the I.R. Spectra The i n f r a r e d spectrum of apatites i s dominated by 127 128 F i g u r e 6 . 2 - I n f r a r e d Spectrum of Synthetic Hydroxyapatite 129 2000 1600 1200 800 . 400 WAVENUMBER (cm - 1) Figure 6 .3 - Infrared Spectra of Quartz. 130 3800 3700 3600 3500 1600 1200 800 400 WAVENUM8ER (cm - 1) F i g u r e 6.1+ - I n f r a r e d Spectrum of K a o l i n i t e . 131 v i b r a t i o n s of P0 4 i o n i c groups. In i t s "free" state, P0 4has a group s i t e symmetry T ^ 1 ) ( i . e . a perfect tetrahedron, space group 43m i n the cubic system, having a f o u r - f o l d r o t a t i o n inversion axis(4), a thr e e - f o l d r o t a t i o n axis(3) and a mirror r e f l e c t i o n plane (m)). The s e l e c t i o n rules f o r t h i s space group according to Ross (1974) give an symmetric stretch ( v-^and E bend ( v 2 ) , an F 2 antisymmetric str e t c h (v-|_),and an F 2bend ( v 4 ) . The F 2 modes are both infrared and Raman-active (Figure 6.5). Although there i s some disagreement concerning the assignment f o r the v 2 bandposition( 2) , the following ranges f o r the P-0 fundamental v i b r a t i o n are given by Ross (1974) : 980-930 cm"1 420 or 360 cm"1 (doubly degenerate) V3 1080-1010 cm"1 ( t r i p l y degenerate) v 4 570-515 cm"1 ( t r i p l y degenerate) Considered iso l a t e d and possessing a T d group 3-symmetry, a l l P-0 bonds i n a P0 4 group would be equivalent (1) In molecular spectroscopy, the Schoenflies notation i s often used. Crystallographers prefer to use the international notation. The correspondence between the two can be found i n K i t t e l (1956), p.19. (2) The old l i t e r a t u r e normally assigns v^to a Raman band occurring at 360 cm - 1(e.g. Vratny et a l . , 1961). More recent publications assign a higher value to the same vi b r a t i o n , approximately at 420 cm - 1(e.g. Nakomoto, 1970). 132 F i g u r e 6 .5 - The PO^Group: (I) Regular Tetrahedron with Symmetry T^ ( I I ) D i s t o r t e d Tetrahedron with Symmetry C s A,B,C : d i r e c t i o n s of d i p o l e moments; a : symmetry plane; Phosphorus i s at the c e n t r e , Oxygens at the corners. ( K l e i n et al . ,1970) . and the three fundamental v i b r a t i o n frequencies of v 3 and of v 4 would be equal, and orthonormal to each other ( t r i p l y degenerate bands)(Klein et a l . , 1970). However, the presence of other cations and anions i n the l a t t i c e of apatites, promotes a d i s t o r t i o n i n the P0 4 tetrahedron, lowering i t s symmetry from to C s ( i . e . , containing now only one r e f l e c t i o n mirror symmetry plane "m"). In the case of hydroxyapatite, the P-0 distances are unequal: P-01=1.538; P-02=1.537 ;P-03= P-04= 1.529A5 (Kay et a l . , 1964). The d i s t o r t e d and regular tetrahedra are shown i n figu r e 6.5. The s e l e c t i o n rules predict for C s symmetry the following v i b r a t i o n s , a l l infrared active (Farmer, 1974 and Chakravorty and Ghosh, 1966) : \>1, v 2 a ' v2b' v 3a' v3b' v3c' v4a' v4b' v 4 c i« e« a l i nine v i b r a t i o n s predicted by the formula 3N-6 appear i n the i n f r a r e d spectrum. The assignments of these vi b r a t i o n s are made by taking into consideration the fundamental v i b r a t i o n s of the tetrahedral "free" phosphate group. Table 6.1 i s a compilation of the extensive l i t e r a t u r e a v a i l a b l e i n the case of apatites, presenting only the assignments f o r the fundamental P-0 vibrations. Besides the four fundamental modes, other bands appear i n the infrared spectrum of apatites. In the case of hydroxyapatite the OH group i s responsible for two additional bands. One i s the fundamental v i b r a t i o n of the OH group occurring i n the range 3560-3570 cm - 1(Ross, 1974). The other i s normally assigned to an OH l i b r a t i o n at 630-635 cm - 1. Some difference bands are also found and the most TABLE 6.1 - V i b r a t i o n S p e c t r a o f A p a t i t e s : PO, A s s i g n m e n t s . Compound Ref . V l V 2 " v3 V 4 FA D u r a n g o a 960 315 ,270 1095,1075,1040 603,574,566 FA n a t u r a l b 968 325 1096,1050,1025 605 ,580 ,565 FA D urango c 959 336-305 1100,1071,1058 604,572,570 FA s y n t h e t i c d 966 - 1082,1054 605 .5 ,590.5 FA RAMAN d - - 1060 ,1034 617,582 FA D u r a n g o e 962 472 1090 ,1038 601 ,574 FA s y n t h e t i c e 964 473 1092,1042 601 ,576 CIA s y n t h e t i c f 962 315 1092,1080,1045 605 ,570 CIA n a t u r a l f 960 305 1095,1082,1040 600 ,560 OHA s y n t h e t i c a 962 350 ,270 1092,1065,1028 603,574,564 OHA s y n t h e t i c g 962 255 1095,1055,1035 605 ,568 OHA s y n t h e t i c h 959 - 1090,1025 628 ,602 ,563 C l - F A s y n t h e t i c i 962 340 1095,1090,1045 602 ,575 ,565 N o t e s : FA - f l u o r a p a t i t e ; CIA - c h l o r a p a t i t e ; OHA - h y d r o x y a p a t i t e C l - F A - f l u o r - c h l o r a p a t i t e a l l v a l u e s a r e i n wavenumbers ( cm 1 ) R e f s . a- B a d d i e l and B e r r y ( 1 9 6 6 ) ; b - B h a t n a g a r ( 1 9 6 7 ) ; c - K l e i n e t a l . ( 1 9 7 0 ) ; d - K r a v i t z e t a l . ( 1 9 6 8 ) ; e - K l e e and E n g e l ( 1 9 7 0 ) ; f - B h a t n a g a r ( 1 9 6 8 ) ; g - B h a t n a g a r ( 1 9 6 8 a ) ; h -C h a k r a v o r t y and Ghosh ( 1 9 6 6 ) ; i - B h a t n a g a r ( 1 9 6 7 a ) . common i s v 3 - v 4 occurring at 460-480cm . In the case of carbonate- apatites (either mineral or b i o l o g i c a l . . . 2-specimens), the vibrations of the C0 3 group are also present i n the spectra. According to Le Geros et a l . (1970) and Montel et a l . (1977) the assignments f o r carbonate ions i n the structure of carbonate apatites are the following: v2 860 - 885 cm - 1 (C0 3 i n and out bending mode) v3 14 65 - 1542 cm - 1 (C0 3 stretching, carbonate apatites type A) v3 143 0 - 1460 cm - 1 (same as above, carbonate apatites type B) The other carbonate v i b r a t i o n a l modes are generally absent i n the i n f r a r e d spectra of carbonate-apatites. The i n f r a r e d bands i n the spectra of d i f f e r e n t natural apatites shown i n figures 6.1 and 6.2 can now be assigned. Table 6.II presents the data from the two figures and the assignments f o r t h e i r respective modes of v i b r a t i o n . Three of the four samples are most probably carbonate free. The Durango sample i s a very well characterized example of flu o r a p a t i t e and the r e s u l t s obtained here are i n excellent agreement with previous studies (see references "a" and "e" i n Table 6.1). The Ontario and Monteiro samples are also examples of f l u o r a p a t i t e s (or f l u o r - c h l o r a p a t i t e ) , free of carbonate and and hydroxyl, very well c r y s t a l l i z e d . The chemical analysis p r e s e n t e d i n T a b l e 5.1 f o r the Monteiro sample, d e f i n i t i v e l y c o n f i r m s the i d e n t i f i c a t i o n of t h i s sample as a f l u o r - c h l o r a p a t i t e . The Ca/P molar r a t i o o f t h i s sample i s a l s o c l o s e t o the t h e o r e t i c a l v a l u e , v i z . 1.688 (experimental) v e r s u s 1.667 ( t h e o r e t i c a l ) . L a t e r i n the s e c t i o n , X-ray d i f f r a c t i o n , microprobe and SEM r e s u l t s w i l l f u r t h e r c h a r a c t e r i z e t h i s m i n e r a l . 2-The I t a t a i a sample c l e a r l y shows v 3 C0 3 s t r e t c h i n g v i b r a t i o n s . The p o s i t i o n of t h e s e v i b r a t i o n s i n d i c a t e t h a t t h i s sample i s a carboante a p a t i t e o f type B, 2-. 3-l . e . the C0 3 i o n s r e p l a c e i n p a r t the P0 4. The presence o f carbonate i n the s t r u c t u r e of a p a t i t e s can be v e r y e a s i l y d e t e c t e d by I.R. spectroscopy. Even minute amounts o f carbonate show the c h a r a c t e r i s t i c peaks, as found i n t h i s case. Regarding the spectrum shown i n f i g u r e 6.2, most bands can be a s s i g n e d t o phosphate, carbonate and h y d r o x y l groups o c c u r r i n g i n h y d r o x y a p a t i t e (see T a b l e 6.II) . The v e r y c h a r a c t e r i s t i c h y d r o x y a p a t i t e band (OH s t r e t c h i n g ) i s d e f i n i t e l y p r e s e n t a t 3566 cm - 1. T h i s band can be regarded as an i n d i c a t i o n o f the c r y s t a l l i z a t i o n of the sample. I t i s absent i n amorphous a p a t i t e s . The o t h e r I.R. s i g n a l s a p p e a r i n g i n f i g u r e 6.2 can r e c e i v e d i f f e r e n t assignments. The water peaks f o r i n s t a n c e , o c c u r r i n g a t approximately 3400 c m - 1 (broad) and i n the range from 1620 t o 1680cm" 1 can r e s u l t from the a b s o r p t i o n of atmospheric water vapour d u r i n g the p r e p a r a t i o n of the KBr p e l l e t . They 137 TABLE 6 . I I - A s s i g n m e n t s of the I.R. v i b r a t i o n modes o f a p a t i t e s ( s p e c t r a shown i n f i g u r e s 6.1 and 6 . 2 ) . VIBRATIONS (cm ) K C O 2 OH SAMPLES V I ( v 3 -V *' V 3 V 4 V V V V 2 3 5 L Durango 963 462 1095 ,1045 606 ,577 ,551 _ I t a t a i a 963 456 1095 ,1044 606 ,577 ,550 - 1460,1422 -Monteiro 963 456 1093,1042 605 ,577 ,551 -Ontario 959 460 1097 ,1047 608,574,546 -Hydroxyapatite - 460 1101,1039 608,576,540 827 1451 , 1416 3566 (**) (") a l t h o u g h t h e r e i s some d i s c u s s i o n i n t h e l i t e r a t u r e c o n c e r n i n g t h i s v i b r a t i o n , the a s s i g n m e n t o f a d i f f e r e n c e band i s t h e most a c c e p t e d at t h i s s t a g e ( C a s c i a n i and C o n d r a t e , 1 9 8 0 ) . (**) t h e r e i s a s h o u l d e r i n the r e g i o n where t h e l i b r a t i o n mode i s e x p e c t e d . 138 can a l s o be a consequence of the h i g h s u r f a c e area o f t h i s sample and i t s h y g r o s c o p i c behaviour. The peaks o c c u r r i n g a t 787 c m - 1 and 750 cm" 1 can not be a s s i g n e d t o h y d r o x y a p a t i t e , however. I t i s p o s s i b l e t h a t t h e sample i s contaminated by o t h e r phosphate phases. M o n e t i t e (CaHP0 4) and b r u s h i t e (CaHP0 4.2H 20) pr e s e n t OH o u t - o f - p l a n e bending v i b r a t i o n s r e s p e c t i v e l y i n the ranges o f 700-790cm - 1 and 750-785 cm~ 1(Ross, 1974; C a s c i a n i and Condrate, 1980). Another p o s s i b i l i t y i s t h e i n t e r l a y e r i n g of o c t a c a l c i u m phosphate wi t h h y d r o x y a p a t i t e found i n some s y n t h e t i c samples. T h i s would e x p l a i n the broad water peak and the water bands p r e v i o u s l y mentioned, a l l o f them o c c u r r i n g i n the spectrum of o c t a c a l c i u m phosphate. A l s o p r e s e n t i n t h i s compound are some i n - p l a n e bending o f P-O-P i n t h e range 1200-1295 cm" 1. These bands are p r e s e n t i n the spectrum i n the f i g u r e 6.2 (not marked)(LeGeros, R. and LeGeros, J.,1984). S i n c e t h i s sample was used o n l y i n a few experiments i n t h i s t h e s i s , no f u r t h e r s t u d i e s on the c h a r a c t e r i z a t i o n were pursued. In the s p e c t r a of quartz and k a o l i n i t e ( f i g u r e s 6.3 and 6.4), the assignment of the peaks t o t h e i r r e s p e c t i v e v i b r a t i o n modes i s summarized i n Table 6 . I I I . Both s p e c t r a show the c h a r a c t e r i s t i c peaks accepted f o r the two m i n e r a l s i n q u e s t i o n and are i n good agreement w i t h p u b l i s h e d data (see f o r example Farmer, 1974; Moenke, 1974; de Araujo, 1982) . 139 TABLE 6 . I l l - A s s i g n m e n t s of I.R. v i b r a t i o n modes to q u a r t z and k a o l i n i t e ( s p e c t r a shown i n f i g u r e s 6.3 and 6 . 4 ) . V i b r a t i o n s (cm ^) Q u a r t z (a form) K a o l i n i t e 3691 ,3665 ,3648 ,3615 1171,1143,1087 1114 1099 ,1034 ,1007 936 912 799 , 779 790 753,692 692 516 ,453 ,415 495 ,447 385 ,362 n o t p r e s e n t S i - O - S i a n t i s y m m e t r i c s t r e c t c h i n g OH s t r e c t c h i n g n o t p r e s e n t n o t p r e s e n t S i - O - S i s y m m e t r i c s t r e t c h i n g n o t p r e s e n t "T-O-T" symmetric s t r e c t c h i n g O - S i - 0 b e n d i n g l a t t i c e v i b r a t i o n s S i - 0 p e r p e n d i c u l a r s t r e t c h i n g S i - O - S i a n t i s y m m e t r i c s t r e t c h i n g s u r f a c e OH b e n d i n g i n n e r OH b e n d i n g S i - O - S i s y m m e t r i c s t r e t c h i n g s u r f a c e OH l i b r a t i o n n o t p r e s e n t O-Si-O b e n d i n g 334 l a t t i c e v i b r a t i o n s N o t e : t h e k a o l i n i t e band a t 602 cm can n o t be a s s i g n e d to any f u n d a m e n t a l v i b r a t i o n mode. I t i s p r o b a b l y a c o m b i n a t i o n band. Other Techniques 140 Some m i n e r a l samples were submitted, i n a p r e l i m i n a r y phase, t o c h a r a c t e r i z a t i o n by X-ray d i f f r a c t i o n , e l e c t r o n microprobe (JEOL, model JXA-3A) and scanning e l e c t r o n microscope (ETEC-Autoscan equipped w i t h an EDX X-ray s p e c t r o m e t e r ) . The r e s u l t s d e s c r i b e d below r e f e r t o o n l y the M o n t e i r o a p a t i t e sample. D e t a i l s on the work performed on the o t h e r m i n e r a l samples are g i v e n i n appendix I. a) - X-ray d i f f r a c t i o n The X-ray d i f f r a c t o g r a m of the sample e x h i b i t s a l l the main "d" l i n e s f o r f l u o r a p a t i t e . The t h r e e s t r o n g e s t l i n e s are s i t u a t e d a t 2.83, 2.73 and 2 . 7 9 A . These v a l u e s are r e l a t i v e l y c l o s e t o the t a b u l a t e d l i n e s f o r f l u o r a p a t i t e , v i z , 2.80, 2.702, 2 .772A (see Nriagu, 1984 and T a b l e 4.II).No l i n e s from any other m i n e r a l phase were p r e s e n t i n the d i f f r a c t o g r a m obtained. b) - E l e c t r o n Microprobe Elemental mapping o f p o l i s h e d Monteiro a p a t i t e g r a i n s showed the presence of Ca and P. The elements were e v e n l y d i s t r i b u t e d over the m i n e r a l g r a i n s t e s t e d . No contaminant elements were found i n the g r a i n s examined. c) - EDX/SEM In a d d i t i o n t o the e l e c t r o n microprobe elemental mapping, some Monteiro a p a t i t e g r a i n s were a l s o examined i n a ETEC Autoscan scanning e l e c t r o n microscope, equipped w i t h an e n e r g y - d i s p e r s i v e X-ray spectrometer. The use of energy-141 d i s p e r s i v e spectrometers has some advantages i n comparison t o w a v e l e n g t h - d i s p e r s i v e spectrometers (WDS) as the one i n the JEOL microprobe. Among other advantages, energy- d i s p e r s i v e s p e c t r ometers have a h i g h e r s e n s i t i v i t y . I n f a c t , t h e e n e r g y - d i s p e r s i v e spectrum o f the Monteiro a p a t i t e sample showed t h e presence of c h l o r i n e i n a d d i t i o n t o Ca and P ( f l u o r i n e c o u l d not be d e t e c t e d because o f i t s low atomic number). No o t h e r elements were found. In c o n c l u s i o n , the use of oth e r c h a r a c t e r i z a t i o n t e c h n i q u e s confirmed the i n f r a r e d and chemical a n a l y s e s i d e n t i f i c a t i o n o f the Monteiro a p a t i t e sample as b e i n g an almost pure example of f l u o r - c h l o r a p a t i t e . Throughout t h i s t h e s i s the sample w i l l be r e f e r r e d t o as f l u o r a p a t i t e only, due t o i t s much l a r g e r f l u o r i n e content i n comparison t o c h l o r i n e . 6 . 1 . 2 - S i z i n g and Surface Area Measurement Quartz and F l u o r a p a t i t e (Monteiro) -38um samples have t h e i r s i z e d i s t r i b u t i o n s (as cumulative % above s i z e ) shown i n f i g u r e s 6 . 6 and 6 . 7 . The l o g mean s i z e o f the two s i z e d i s t r i b u t i o n s , a u t o m a t i c a l l y c a l c u l a t e d by the Elzone apparatus, were 1 4 . 4 1 y m f o r f l u o r a p a t i t e and 13.45um f o r q u a r t z . The El z o n e assumes a l o g normal s i z e d i s t r i b u t i o n f o r s i z e d a t a . Below and above s i z e d 95 v a l u e s were found t o be t h e f o l l o w i n g , r e s p e c t i v e l y : 34 wit and 5ym f o r quartz and 38 urn and 3ym f o r f l u o r a p a t i t e . The s i z e d i s t r i b u t i o n s 100 90 80 iii N w70 iu a 60 CD M 50 cc 13 CJ 40 30}-20 10 0 o o p FLUORRPRTITECMontelroj 0 10 20 30 40 50 SIZECfjm) 60 70 80 Figure 6.6 - Size Di s t r i b u t i o n of Fluorapatite. °0T o 10 20 30 40 SIZEC|um) O QUARTZ 50 60 70 Figure 6.7 - Size D i s t r i b u t i o n of Quartz. 144 o b t a i n e d f o r two the m i n e r a l s are v e r y s i m i l a r i n the s i z e range a n a l y s e d by the Elzone. P a r t o f the -38ym f l u o r a p a t i t e sample was s p l i t a t a nominal c u t s i z e o f 6ym by standard beaker d e c a n t a t i o n t e c h n i q u e (assuming s p h e r i c a l p a r t i c l e s w i t h S t o k e s i a n b e h a v i o u r ) . The above s i z e cumulative s i z e d i s t r i b u t i o n of the n o n - s e t t l i n g m a t e r i a l i s shown i n f i g u r e 6.8. The c a l c u l a t e d l o g mean s i z e was 4.88ym and the d g 5 below and above s i z e v a l u e s were r e s p e c t i v e l y 16ym and lym. T h i s sample was o n l y used i n the sedimentation tube t e s t programme. The p r e p a r a t i o n o f t h i s f i n e r f l u o r a p a t i t e sample was conducted t o make the s i z e range compatible f o r comparison w i t h the k a o l i n i t e sample used i n the same t e s t programme. The k a o l i n i t e sample was not s u b j e c t e d t o any s i z i n g p r e p a r a t i o n o r a n a l y s i s . The manufacturer's s i z e s p e c i f i c a t i o n s are the f o l l o w i n g : 55-65% by wt. below 2ym, mean s i z e o f l.lym, and 0.20% maximum of +100ym. S u r f a c e area determinations were made by a p p l y i n g t h e s t a n d a r d m u l t i p o i n t BET technique. Pure n i t r o g e n was used as adsorbent and pure argon as c a r r i e r . A d s o r p t i o n of n i t r o g e n was accomplished a t l i q u i d n i t r o g e n temperature. The r e s u l t s are summarized i n Table 6.IV. They i n d i c a t e t h a t a l t h o u g h t h e Elzo n e s i z e d i s t r i b u t i o n r e s u l t s f o r quartz and f l u o r a p a t i t e a re s i m i l a r i n the s i z e range analysed, the s p e c i f i c s u r f a c e area of the quartz sample i s approximately 51% o f t h a t o f the f l u o r a p a t i t e sample. The s p e c i f i c s u r f a c e a r e a v a l u e s shown i n Table 6.IV were u t i l i z e d i n the 1—4 (0 100 90 80h 70 ui a 60 -m cr w 50 -40 UJ > i—i Y~ •X d30 S 201-10 0 o \ 0 o FLUORRPRTITECMonteiro) BEAKER DECFNTRTI0N 'OVERFLOW o-o-o , 10 15 SIZECpn) 20 25 30 Figure 6.8 - Size D i s t r i b u t i o n of F l u o r a p a t i t e (beaker decantation " o v e r f l o w " ) . TABLE 6.IV - BET Surface Area Determinations M i n e r a l Sample wt. no.of p o i n t s CC* (g) F l u o r a p a t i t e 0.3168 4 0.946 0.329 1.037 Quartz 0.8990 4 0.981 0.478 0.531 K a o l i n i t e 0.3910 4 0.995 6.692 17.115 t o t a l area s p e c i f i c m2 s u r f a c e area m 2/g (*) CC = l i n e a r r e g r e s s i o n c o r r e l a t i o n c o e f f i c i e n t testwork described l a t e r (see section 6.4). 147 6.1.3 - Electrophoresis and Related Work Knowledge of the i n t e r f a c i a l e l e c t r i c a l properties of minerals i s very relevant i n most applied surface chemistry work. In t h i s section, microelectrophoretic tests and r e l a t e d experiments are presented. Figure 6.9 shows the electrophoretic m o b i l i t y ( * ) o f f l u o r a p a t i t e (Monteiro) as a function of pH i n d i s t i l l e d water and i n KCI s o l u t i o n ( i n d i f f e r e n t e l e c t r o l y t e ) . In both solutions an iep (pH) occurs at approximately pH 4.1. Such a value i s r e l a t i v e l y common for natural apatites. This iep(pH) should be considered as a "non-equilibrium" iep since the t o t a l length of time involved i n the t e s t runs was less than 4 hours. Dilute KOH and HCl solutions were used for pH regulation. Both curves i n figure 6.9 show an increase i n the absolute negative value of the electrophoretic mobility (E.M.) as the pH i s increased above the iep(pH). This i s more accentuated i n pH range from 4 to 9. Above pH 9, the absolute value of the E.M.continues to increase but at a much smaller pace. The maximum E.M. absolute value assumed by the (*) - I t i s usual to see microelectrophoretic r e s u l t s f o r mineral system presented as zeta p o t e n t i a l values, generally calculated assuming v a l i d the Helmholtz- Smoluchowski equation (Anon., 1975). This p r a c t i c e w i l l be avoided i n the present t h e s i s because of possible t h e o r e t i c a l objections r e l a t e d to the a p p l i c a b i l i t y of the H.S. treatment. FLUORflPATITECMontelro) LU -g -- 3 -- 4 1 A DISTILLED WATER • t EC-4)mol/lltre KCl 6 7 8 9 AVERAGE pH 10 11 12 13 14 Figure 6.9 E l e c t r o p h o r e t i c M o b i l i t y of F l u o r a p a t i t e as a Function of pH. 149 samples i s approximately 2.7um/s per v o l t / c m i n d i s t i l l e d water at pH 10.4 to 11. Figure 6.10 depicts the E.M./pH r e l a t i o n s h i p f o r the synthetic hydroxyapatite sample i n d i s t i l l e d water. As i n figure 6.9 one can c l e a r l y f i n d an iep(pH) located now at approximately pH7, i n very good agreement with the data presented i n table 4.V. The E.M. values f o r hydroxyapatite are always smaller i n absolute value than those encountered for f l u o r a p a t i t e . The shape of two E.M./pH curves are comparable, however. Many d i f f e r e n t ubiquitous ions occur i n mineral aqueous systems. The important contributions of these ions should not be disregarded whenever aqueous mineral processing separations are involved. Although the presence of such i o n i c species i s generally unavoidable, there are some remedies. For instance, the sometimes unexplained need for d i f f e r e n t "dispersants" and "depressants" i n f l o t a t i o n systems, i s most probably related to the presence of such i o n i c species. Many of the reagents introduced also act as complexing or sequestring agents, c o n t r o l l i n g the concentration of the ubiquitous ions. Figure 6.11 depicts the dramatic e f f e c t of A l aqueous species on the E.M. of fl u o r a p a t i t e as a function of pH. Even minute amounts (5 x 10~ 6 m o l / l i t r e or approx. (*) - the un i t of the E.M. should convey the idea of rate of t r a v e l of the p a r t i c l e s (length/time) based on a standard gradient i n e l e c t r i c a l f i e l d through the electrophoresis c e l l of one v o l t per unit of length. Figure 6.10 - Electrophoretic Mobility of Hydroxyapatite as a Function of pH. Figure 6 .11 - E f f e c t of A l Aqueous Species on the E l e c t r o p h o r e t i c M o b i l i t y of F l u o r a p a t i t e . 152 1.2ppm) of these aqueous species are able to change the E.M. curve of f l u o r a p a t i t e completely. A more d e t a i l e d discussion of the mechanisms involved i n such changes i s outside the scope of t h i s work (see for example James and Healy, 1972 and Senior, 1987). In the s p e c i f i c case of A l species, there i s strong experimental evidence f o r the presence of highly solvated species at the interface. The pH range f o r maximum E.M. i s the same range reported by de Araujo (1982) f o r maximum adsorption of A l species onto a quartz surface. As the surface becomes negative (charge reversal 3 i n the James and Healy model), at the highest A1C1 3 concentration, the reversal occurs at a higher pH value (pH=8.2) than f o r the lowest AICI3 concentration (at 6.5). This i s i n agreement with the James and Healy model. In both cases, under a l k a l i n e conditions, the curves i n the presence of A l species approximate the base curve (which i s the KCI curve, since the te s t s involving A1C1 3 were also performed i n the presence of l x l 0 ~ 4 m o l / l i t r e KCI) . Figure 6.12 shows the e f f e c t of Mg and Ca i o n i c species on the E.M./pH relationship of f l u o r a p a t i t e . Only one concentration l e v e l was tested (1x10 J m o l / l i t r e MgCl 2 and Ca C l 2 ) • The behaviour of these two bivalent ions i s d i f f e r e n t from that of A l ( t r i v a l e n t ) . The i o n i c species d i s t r i b u t i o n pH diagrams calculated by Senior (1987) show the pH for hydroxide p r e c i p i t a t i o n and the mole % of the d i f f e r e n t aqueous A l , Mg and Ca species, at d i f f e r e n t s a l t Figure 6 . 1 2 - E f f e c t of Ca and Mg I o n i c Species on the E l e c t r o p h o r e t i c M o b i l i t y of F l u o r a p a t i t e 154 concentrations. This type of diagram has been used to c o r r e l a t e the changes i n E.M. values with changes i n the m e t a l l i c i o n i c species present i n solutions. The pH value where the f i r s t hydroxo-complex s t a r t s forming i n s o l u t i o n i s s i m i l a r to the pH value where there i s the f i r s t charge rev e r s a l above the substrate's iep(pH) (namely CR2 i n the James and Healy model). This c o r r e l a t i o n holds r e l a t i v e l y well i n the case of Ca and Mg on f l u o r a p a t i t e but i t does not apply to the A l case. One a l t e r n a t i v e l i n e of thinking was introduced by Somasundaran (1984). I t objects to the c o r r e l a t i o n between the diagrams calculated f o r bulk i o n i c species and the observed changes i n the electrophoretic m o b i l i t i e s of minerals i n contact with such ions. According to Somasudaran (1984), at the mineral s o l u t i o n i n t e r f a c e , the a c t i v i t i e s of the i o n i c species are d i f f e r e n t . This s i t u a t i o n would explain some descrepancies between expected and experimental r e s u l t s . One can speculate on the p o s s i b i l i t y f o r the p r e c i p i t a t i o n of the metal hydroxide to occur at lower pH and concentration l e v e l at the i n t e r f a c e . This i s also expected i f the nucleation r o l e of the substrate i s taken into consideration. The Mg-fluorapatite curve shown i n fi g u r e 6.12 presents an extra p e c u l i a r i t y . I n general, the a v a i l a b l e l i t e r a t u r e does not show data points above the pH of maximum p o s i t i v e E.M. In figure 6.12 one can see that the micro-electrophoretic mobility decreases above pH 11 but the curve does not return to i t s o r i g i n a l shape. This aspect addresses 155 another s t i l l not completely answered question: a f t e r adsorption and metal hydroxide p r e c i p i t a t i o n (or "condensation"), does the adsorbed species desorb as the pH i s increased? According to the re s u l t s already mentioned of de Araujo (1982), f o r the Al-quarz system there i s desorption as the pH increases. Similar behaviour was reported by Cooke (1953) f o r the Ca-quartz system. The return to the o r i g i n a l E.M./pH base curve f o r the A l - f l u o r a p a t i t e system shown i n fi g u r e 6.11, seems to suggest that desorption a c t u a l l y takes place. At higher pH conditions, such as i n the Mg-f l u o r a p a t i t e system, the i o n i c strength of the medium may hinder the desorption. The Ca-fluorapatite curve presents another complicating factor: Ca i s a pdi for t h i s system. Li t e r a t u r e r e s u l t s i n the presence of lxl0~ 2mol/dm 3 of Ca(N0 3) 2 on f l u o r a p a t i t e show zeta potential values always p o s i t i v e i n a l l pH ranges tested(Somasundaran and Agar,1972). In figure 6.1 up to pH 13 the E.M. of fluo r a p a t i t e i n the presence of a concentration of Ca i o n i c species 10 times smaller than that of Somasundasan and Agar shows only negative values. Although one can argue that the concentration difference i s s o l e l y responsible f o r the observed behaviour, the techniques u t i l i z e d i n each case are d i f f e r e n t . The c i t e d r e s u l t s were obtained by streaming potential using a plug of coarse (greater than 200ym) fluo r a p a t i t e p a r t i c l e s . In the case of microelectrophoresis, a suspension of very f i n e p a r t i c l e s i s employed. The surface areas available f o r adsorption can be 156 quite d i f f e r e n t . I t i s probably higher i n the case of microelectrophoresis. Of course the concentration e f f e c t plays, l i k e l y , a dominant r o l e but the differences i n the techniques should also be accounted for when comparing r e s u l t s . At such a high pH, compression of the double layer should also be considered. Figure 6.13 shows E.M. re s u l t s f o r quartz i n the presence of A l , Mg, and Ca aqueous species as a function of pH. These r e s u l t s are included for comparison purposes. In the same figure, the E.M./pH curves for quartz i n d i s t i l l e d water and KCl (lxl0~ 4mol/dm 3) are also depicted. This quartz sample shows absolute negative E.M. values greater than those found f o r fl u o r a p a t i t e both i n d i s t i l l e d water and i n KCl solut i o n . The Al-quartz system presents a behaviour s i m i l a r to that of flu o r a p a t i t e , but f o r the lowest metal s a l t concentration there i s no change i n the sign of the E.M. This can be explained by the higher E.M. absolute negative value f o r quartz. The Mg and Ca systems also show a s i m i l a r behaviour to that of fluo r a p a t i t e , e s p e c i a l l y i n the case of Mg. Although there i s a difference i n Mg concentration (twice larger i n the quartz case), the po s i t i o n of CR2 (from James and Healy model) i s only 0.2 pH units smaller and the shape of the curves up to pH 11 i s almost i d e n t i c a l . For completeness, Figure 6.14 shows the E.M./pH curve f o r k a o l i n i t e i n d i s t i l l e d water. Over the entire pH range tested, the sample shows negative E.M. with a minimum E 0 > i_ CO a a LU -8 PDIST.WRTBR |AKCIU EC-4Dmo!/13 • RICI3[6 E(-8)mol/l] +fllCI3[6 E(-4)mol/l] jOMgqi2C2 E(-3)mol/N • CcC! 2t3 EC-3)mol/l] 0 QUARTZ 6 8 AVERAGE pH 10 12 14 Figure 6 . 1 3 - E f f e c t of Al,Ca and Mg Species on the E l e c t r o p h o r e t i c M o b i l i t y of Quartz. Figure 6.14 - E l e c t r o p h o r e t i c M o b i l i t y of K a o l i n i t e as a Function of pH. 159 (in absolute value) occurring at approximately pH 3.5. Clay minerals such as k a o l i n i t e develop e l e c t r i c a l surface charge by two d i s t i n c t mechanisms (Parks, 1975). The charge on the basal planes of k a o l i n i t e i s s t r u c t u r a l and always negative, independently of the pH. At the c r y s t a l edges however, the charge i s pH dependent. The o v e r a l l charge r e s u l t i s the combination of these two charges. More d e t a i l s on the surface charge generation on clays can be found elsewhere (Parks, 1975; van Olphen, 1971). The pzc of the Monteiro f l u o r a p a t i t e sample was determined by two d i f f e r e n t procedures: Mular and Roberts (1966) and Ahmed (1966). Figure 6.15 depicts the r e s u l t s obtained by applying the Mular and Roberts procedure. The & pH/final pH curves were obtained under d i s t i n c t conditions (see figure for d e t a i l s ) . They have a common in t e r s e c t i o n point at ca. pH 7. This value f o r the pzc(pH) of f l u o r a p a t i t e i s i n good agreement with the r e s u l t s presented i n table 4.V. The Ahmed time/pH d r i f t technique confirms the value obtained by the Mular and Roberts procedure. Ahmed's procedure r e s u l t s are presented i n table 6.V. 6.1.4 - Dispersion Studies Qua l i t a t i v e studies on the dispersion of f l u o r a p a t i t e (Monteiro), quartz and k a o l i n i t e suspensions were conducted. These tests were performed for. two main 3 4 5 6 7 8 9 10 FINAL pH Figure 6.15 - Determination of the pzc(pH) of Fluorapat by the Mular and Roberts Method. TABLE 6.V - Determination of the pzc(pH) of Fluorapatite by the Ahmed's Method o Conditions:sample particle size -38um; 1 x 10" mol/l KCI; for long equilibration times samples were kept under N ? MEASURED pH (TIMES) INITIAL pH 1 2 3 k 5 6 7 10 T3 20 23 30 ZJo 60 2if . L8 72~ minutes hours.... if . 1 6.1 6.1 6.1 6.2 6.2 - 6.3 6.k 6 .if 6-if 6.4 6 . i f 6.6 6.6 6.7 6.8 6.9 i f . 5 7.2 8.0 - 8.0 7.8 8.0 8.0 7.8 7-7 7.7 7.2 7.1 - 7.0 5.5 6.9 7.5 - 7.2 7-3 7.k 7-k 7-2 7-2 7.0 6.45 7.0 7.1 7.2 7.1 7.2 7.3 7.2 7.1 7.0 7-0 6.8 7.0 6.9 7.0 7.1 7.1 7-0 7.0 7.4 7.0 7.2 7.1 7.1 7.2 7.0 7.0 7.0 8.i+5 6.7 6.6 6.7 6.8.6.9 6.9 7.1 7.1 7.0 9.3 8.0 8.0 7.8 7.6 7.8 - 7.9 7.8 7-5 7.5 7.2 7.1 7.0 7.0 10.0 9.8 9.7 9.6 9.5 - 9-3 9.2 8.6 7.6 7.2 7.2 7.2 7.1 7.1 7.1 7.1 7.1 1 6 2 reasons: ( i ) - t o l i m i t the pH range f o r a d s o r p t i o n , f l o c c u l a t i o n and f l o t a t i o n s t u d i e s ; ( i i ) - t o i n v e s t i g a t e the s t a b i l i t y o f m i n e r a l suspensions, an important a s p e c t i n almost a l l p h y s i c o - c h e m i c a l m i n e r a l p r o c e s s i n g o p e r a t i o n s . In a l l t e s t s r e p o r t e d i n t h i s s e c t i o n , the % t r a n s m i t t a n c e t o l i g h t a t 450nm ( S p e c t r o n i c 21), measured a f t e r a p r e - e s t a b l i s h e d s e t t l i n g p e r i o d o f t h e m i n e r a l s u s p e n s i o n under i n v e s t i g a t i o n , was employed as an i n d i c a t i o n o f t h e degree of d i s p e r s i o n / a g g r e g a t i o n o f t h e suspension. The m i n e r a l samples used were the same samples used f o r most o f the work r e p o r t e d i n t h i s t h e s i s , namely qu a r t z -38ym, f l u o r a p a t i t e (Monteiro) -38ym and k a o l i n i t e , mean s i z e =l.lum. S i n c e the s i z e ranges of the thes e t h r e e m i n e r a l samples were d i f f e r e n t , and the s e t t l i n g p e r i o d s were s e l e c t e d a r b i t r a r i l y t o g i v e measurable % t r a n s m i t t a n c e v a l u e s , the t e s t s are comparable o n l y f o r each m i n e r a l . The % t r a n s m i t t a n c e i n the case o f k a o l i n i t e , f o r i n s t a n c e , was measured a f t e r l o n g e r s e t t l i n g p e r i o d s than f o r o t h e r m i n e r a l s . I t should be p o i n t e d out t h a t d i f f e r e n c e s i n r e f l e c t i v i t y and shapes of the m i n e r a l p a r t i c l e s i n v o l v e d i n t h i s type o f measurement can a l t e r the r e s u l t s immensely. The t e s t s have t h e r e f o r e a q u a l i t a t i v e c h a r a c t e r , i n d i c a t i n g the pH range f o r maximum/minimum s t a b i l i t y and e f f e c t o f d i s p e r s a n t s i n terms of improving o r worsening the degree of 163 dispersion as compared to tests made i n t h e i r absence. Figure 6.16 shows the e f f e c t of pH on the s t a b i l i t y of a f l u o r a p a t i t e suspension. In the a l k a l i n e region, the e f f e c t of three d i f f e r e n t pH modifiers i s shown (HCl d i l u t e s o l u t i o n was used to reach acid pH values). This figure c l e a r l y shows an increase i n the degree of dispersion of the suspension as the pH i s increased(smaller % transmittance values). KOH and NaOH had the same e f f e c t on the s t a b i l i t y of t h i s mineral. Ammonium hydroxide (NH4 OH) however, showed a more pronounced e f f e c t , notably at pH 11. Ammonium hydroxide i s known to provide stronger dispersion i n c e r t a i n i r o n ore pulps (Bretas, 1980, private communication). At the moment, the mechanisms behind t h i s phenomenon are not known. Figure 6.17 shows the re s u l t s obtained with quartz. The s t a b i l i t y also increased as the pH increases. The magnitude of the increase was smaller than i n the case of f l u o r a p a t i t e . The same e f f e c t of NH4 was also observed i n t h i s case. I f figures 6.16 and 6.17 are compared to the E.M./pH curves already shown (figure 6.97 f o r f l u o r a p a t i t e and 6.13 f o r quartz), i t i s evident that they present the same general trend, i . e . the larger the absolute negative E.M. value the smaller i s the % transmittance value (greater di s p e r s i o n ) . The smaller magnitude of the v a r i a t i o n i n case % transmittance/pH curve for quartz may be r e l a t e d to the f a c t that quartz possesses higher absolute negative values of i t s e l e c t r o k i n e t i c (zeta) potential than f l u o r a p a t i t e f o r a given pH range. One should r e c a l l that absolute values of re 6.16 - S t a b i l i t y of F l u o r a p a t i t e Suspensions as a Function of pH. 100 00 E80 c o !§70 °60 UJ ^50 fx t; 40 CO on 20 10-QUflRTZ o K0H/HC1 A NH OH 2 , 6 h r o " 0 8 1 1 1 l n ° 4 • KOH/HCI 1 hr. eettllrg -D-7 pH 11 13 Figure 6.17 - S t a b i l i t y of Quartz Suspensions as a Function of pH. 166 % transmittance can not be compared between two minerals. The two curves f o r two d i f f e r e n t s e t t l i n g periods shown i n f i g u r e 6.18 demonstrate the importance of t h i s parameter. Figure 6.18 presents the % transmittance/pH curve f o r k a o l i n i t e . Again two d i f f e r e n t s e t t l i n g periods are displayed. The E.M./pH curve for t h i s mineral, shown i n f i g u r e 6.14 was replotted i n figure 6.18. This diagram exh i b i t s a d i r e c t c o r r e l a t i o n between electrophoretic m o b i l i t i e s (or zeta potentials) and % transmittance i n t h i s case. High s t a b i l i t y of a mineral suspension i s associated with high negative values of zeta p o t e n t i a l (repulsion term of the t o t a l i n t e r a c t i o n energy calculated by the DLVO theory). The measurement of % transmittance i s a useful technique when a rough estimate of the s t a b i l i t y of a suspension i s required. As seen, i t can provide the same information as obtained by a more sophisticated technique such as electrophoresis. Table 6.VI shows a compilation of t e s t s made with the three minerals i n the presence of many commonly used dispersants. The pH range selected for these t e s t s i s l i m i t e d to the a l k a l i n e range, from pH 9 to pH 11. This pH range i s the one where maximum dispersion was observed for a l l minerals tested. The values i n table 6.VI should be compared fo r each mineral with the base value obtained with KOH as pH modifier. Among the dispersants selected f o r t e s t i n g (Na s e s q u i s i l i c a t e - METSO 99, Na pyrophosphate, Na tripolyphosphate, Na hexametaphosphate, Na borate - BORAX and Table 6.VT - Action of Selected Dispersants on the Stability of Mineral Suspensions % Transmittance (450nm) Dispersants Mineral (settling time) pH base value NH40H METSO 99 SPP (KDH) SPP (NH40H) STPP (KDH) STPP (NH40H) SHMP (KDH) SHMP (NH4OH) STB (KDH) STB (NH4OH) EDTA (KDH) EDTA (NH40H) Fluorapatite (2 hrs.) 9 10 11 49 49 49 43 26 34 33 31 12 12 12 22 29 30 12 15 10 15 10 17 10 38 21 30 14 Quartz (2.5 hrs.) 9 10 11 85 83 84 82 79 75 - 82 79 76 72 - - 87 78 76 74 84 60 85 - • 57 60 88 48 82 64 Kaolinite (2 hrs.) 9 10 11 4 4 4 2 2 2 1 1 1 0.5 1 1 -1 1 0.5 2 1.5 1 1 1 1 Kaolinite (24 hrs.) 11 20 13 11 11 - - - - - 13 - - -(*) Concentration of dispersants is 100 mg/1 except for NH4OH (measured by the pH value). abbreviations: METSO 99 = hydrated Na sequisilicate SPP = Na pyrophosphate STPP = Na trypolyphosphate SHMP = Na hexametaphosphate STB = Na tetnaborate (borax) EDTA = salt of ethylenediaminotetraacetic acid cr\ CO 169 EDTA). Sodium s i l i c a t e s and phosphates are among the most commonly used dispersants for mineral pulps. The two other dispersants are not conventionally used f o r these purposes. Borax has some cost advantages compared to the phosphate based dispersants. I t may also be less dangerous i n terms of i t s e f f e c t s on fresh water lakes. The organic complexing agent tested-EDTA, was used to make comparisons. A second reason for t e s t i n g EDTA was to shed some l i g h t on the mechanisms involved i n the dispersing action of these reagents. From the re s u l t s presented i n table 6.VI the following points should be emphasized: (i) - at the concentration l e v e l tested a l l dispersants increased the degree of dispersion of f l u o r a p a t i t e and k a o l i n i t e , regardless of pH. For quartz there were three exceptions to t h i s general r u l e , EDTA, STPP and STB, a l l with KOH as the pH modifier. ( i i ) - In te s t s i n which KOH regulated the pH, the following order of increasing dispersing power was found for each mineral at pH 9: -flu o r a p a t i t e EDTA<METSO 99<STPP<SHMP<SPP. -quartz EDTA<STPP<METSO 99<SHMP<SPP. -k a o l i n i t e METSO 99<SPP= STPP= SHMP= EDTA At pH 11: -flu o r a p a t i t e METSO 99<STPP<EDTA SPP<SHMP. 170 -quartz STB< STPP< EDTA< METSO 99< SHMP. -k a o l i n i t e STB< METSO 99= SPP= STPP= EDTA< SHMP. ( i i i ) - as i n ( i i ) but i n NH4 OH pH regulated tests at pH 9: -fl u o r a p a t i t e EDTA= STPP< SHMP. -quartz STPP< EDTA< SHMP. At pH 11: -fl u o r a p a t i t e EDTA< SPP= STPP< STB= SHMP. -quartz EDTA< SPP = SHMP< STB. -k a o l i n i t e STB< EDTA< SPP =SHMP. (iv) - a l l tests using ammonium hydroxide f o r pH regulation showed improved dispersion as compared to KOH, for the three minerals i n a l l pH conditions tested. In some cases, NH4OH was a stronger dispersant than Na s i l i c a t e , EDTA or Na tripolyphosphate. To help i n the understanding of the dispersing mechanisms, some microelectrophoretic tests.were performed i n the presence of some selected dispersants, f o r both quartz and f l u o r a p a t i t e . They are summarized i n table 6.VII. In a l l cases, the absolute values of electrophoretic m o b i l i t i e s were increased. The increase i n E.M. for both minerals are i n almost complete agreement with the r e s u l t s shown i n table TABLE 6.VII - E l e c t r o p h o r e t i c M o b i l i t i e s o f Quartz and F l o u r a p a t i t e i n the Presence o f S e l e c t e d D i s p e r s a n t s 171 E.M. [ ( i J m s " 1 ) / ^ ' 1 ] ^ ) M i n e r a l KOH NH 4 METSO 99 STB/KOH SHMP/KOH base v a l u e (pH) (lg/1) U g/1) (lg/1) Quartz pH 10 -4 -5.4 -4.8 - -4.7 pH 11 -3.8 -7.1 -6.3 -4.8 -6.8 F l o u r a p a t i t e pH 10 -2.6 -2.9 -2.7 -2.8 -3.2 pH 11 -2.7 -3.3 -3.4 -3.25 -3.6 (*) See TABLE 6.VI f o r a b b r e v i a t i o n s . 172 6 .VII. Sodium s i l i c a t e s are dispersants that act through a pr o t e c t i v e c o l l o i d action, covering the p a r t i c l e completely and creating a new e l e c t r i c a l double layer (Klassen and Mokrousov, 1963). In the case of polyphosphates (and probably also for borates), dispersant ions adsorb s p e c i f i c a l l y (chemical forces) onto c e r t a i n minerals, such as hematite (Breeuswma and Lyklema, 1973). Similar reports can be found i n the l i t e r a t u r e f o r other minerals. For apatite, Parsonage et a l . (1984) observed a large increase i n the negative values of the zeta p o t e n t i a l of t h i s mineral i n the presence of Na s i l i c a t e and Na tripolyphosphate (the largest increase was i n the l a t t e r case which agrees with the r e s u l t s reported herein). Parsonage et a l . (1984) agrees on the adsorption of Na s i l i c a t e promoting dispersion through a protective c o l l o i d mechanism, but they r e f r a i n from extending t h i s mechanism to the action of tripolyphosphate. Without any further experimental work (which would be outside the scope of t h i s thesis) the only conclusions that can be drawn at t h i s stage are: (i) - a l k a l i n e pH values, which r e s u l t from additions of NaOH, KOH or NH^OH, increase the s t a b i l i t y of the suspensions of the three minerals tested. This increased s t a b i l i t y seems to be r e l a t e d d i r e c t l y to an increase i n the negative value of zeta 173 p o t e n t i a l of the p a r t i c l e s . The anomalous behaviour of NH4OH defies explanation without further t e s t i n g ; ( i i ) - i n systems where KOH was used as the pH modifier, Na hexametaphosphate shows the strongest dispersion power at pH 11 and Na pyrophosphate at pH 9 (generally c l o s e l y followed by SHMP): ( i i i ) - for tests i n which NH4 was used, SHMP i s the strongest dispersant at the pH l e v e l s tested, except f o r quartz at pH 11, where i t i s second a f t e r Na tetraborate. (iv) - as a general rule, dispersants cause an increase i n E.M. abs. values. The work performed can not,however, ascertain the mechanisms through which the dispersants act. Protective c o l l o i d a l and chelation actions are two of the p o s s i b i l i t i e s . The anomalous behaviour of NH40H deserve further experimental work. As a summary of t h i s mineral characterization section the following points are presented: (i) - i n frared spectroscopy (Section 6.1.1) was 174 successfully used as the main mineral characterization technique. I t proved to be very applicable, e s p e c i a l l y f o r apatites. Infrared spectroscopy of minerals i s a very powerful i d e n t i f i c a t i o n and characterization technique. I t i s not widely used i n the mineralogical f i e l d because i t has to compete with more t r a d i t i o n a l methods such as X-ray d i f f r a c t i o n . Nevertheless, i t can give information accurately and as q u i c l y as any other more conventional techniques; ( i i ) - microelectrophoresis (section 6.1.2), s i z i n g and surface area measurements (section 6.1.3) and dispersion studies (section 6.1.4) provided important information on the i n t e r f a c i a l behaviour of the minerals tested. This information w i l l be frequently referred to i n the next sections; ( i i i ) - although some points w i l l be l e f t unanswered (e.g. dispersant/mineral mechanisms), they are outside of the scope of the work reported here. Nevertheless, experimental facts are useful i n terms of the o v e r a l l knowledge concerning the systems under study. 175 6.2 - Characterization of Starches The characterization studies on starches reported herein should be viewed as complimentary information to that provided i n the review section 4.4. I t i s not an exhaustive c h a r a c t e r i z a t i o n programme. Only important aspects, considered relevant for t h i s thesis, were the object of study. 6.2.1 - Infrared Spectroscopy Infrared spectroscopic techniques are widely employed i n organic chemistry. Extensive l i t e r a t u r e i s ava i l a b l e , covering both t h e o r e t i c a l and p r a c t i c a l aspects of in f r a r e d spectroscopy as applied to the i d e n t i f i c a t i o n and chara c t e r i z a t i o n of organic compounds (e.g. Bellamy, 1958 and 1968; Nakanishi, 1962; Rao, 1963; S i l v e r s t e i n and co-workers, 1979). In the case of starches and rel a t e d compounds, the av a i l a b l e i n f r a r e d l i t e r a t u r e i s more r e s t r i c t e d , although f o r carbohydrates i n general there i s considerable amount of published work (Mathlouthi and Koenig, 1986). In the present work, the b u i l d i n g block of starches, v i z . a-D-Glucose, was characterized f i r s t . A l l infrared v i b r a t i o n s present i n a-D-Glucose molecules are expected to be also present i n the (*) - Infrared spectra of the two surfacants used i n the present work were also recorded. They are discussed separately i n Appendix I I . 176 i n f r a r e d spectra of starches. While i t i s not expected that these bands remain unaffected i n the spectra of the polymers, knowledge of the spectral features of the oxygenated r i n g system of a-D-Glucose would be of great help (Rao, 1962). A survey of published data on the spectra of a-D-Glucose i s given i n Table 6.VIII. An analysis of t h i s table shows the complexity of the infrared spectrum of t h i s compound. Although a-D-Glucose i s formed by three types of atoms only, the structure of the molecule i s such that many i n f r a r e d a c t i v e bands are present i n the spectrum. The heteroatomic r i n g structure i t s e l f i s responsible f o r some important i n f r a r e d bands. According to Rao (1963), the absorption bands occurring at approximately 915cm"1 and 770cm" 1(ring stret c h i n g v i b r a t i o n and symmetrical r i n g breathing v i b r a t i o n , respectively) w i l l vary according to the type of linkages present i n polymers b u i l t from a-D-Glucose u n i t s . For example, f o r a D-Glucose linkages, there i s a gradual increase from 907cm"1 to 930cm - 1for type I v i b r a t i o n (ring stretching) and a decrease for type I I I (ring breathing) from 770cm _ 1to 760cm"1. According to Zhbankov (1966), type II v i b r a t i o n (C-H deformation at 840-850cm-1) can be used, i n some cases, as i n d i c a t i v e of the a-D anomeric form of glucose ( i . e . C 5 hydrogen occupies an equatorial p o s i t i o n i n the chair configuration rather than an a x i a l p o s i t i o n ) . In the case of the fi-D anomer, type II v i b r a t i o n shows a peak at a higher frequency, approximately at 890cm"1. Type III v i b r a t i o n i s also s h i f t e d i n 3-D-Glucose to a lower frequency 177 TABLE 6.VIII - I n f r a r e d A b s o r p t i o n Bands of a-D-Glucose (*) Wavenumber (cm - 1) I n t e n s i t y A s s i g n m e n t 3300-3420 2950 2850 1640 1460 1370 1330 1280 1215 1195 1140 1105 1070 1050,1015 990 915 840-850 760-770 740 400-700 (many) s(broad) s(sharp) s(sharp) w t o m s s m m t o w m m s s w m t o s m m m m w w t o m OH s t r . ( i n t r a / i n t e r m o l e c u l a r H bonding) CH asym.str.CH 2 CH sym.str.CH 2 H 20 adsorbed (H-O-H f l e x i o n ) OH i . p . bending o r CH 2 i n t e r n a l d e f ( s c i s s o r i n g ? ) CH bending OH i . p . bending mixed w i t h CH s t r . CH bending OH i . p . bending o r CH 2 def(wagging?) OH i . p . bending asym.str. C-O-C b r i d g e asym. i n phase r i n g s t r . COH s t r . CO s t r . CO s t r . r i n g s t r . v i b r a t i o n CH def i n anomeric form sym. r i n g b r e a t h i n g v i b r a t i o n CH 2 r o c k i n g def (?) mainly OH o.o.p. bending (*) r e f e r e n c e s : Samec,1957; L i a n g and Marchessault,1959; Rao,1963; Zhbankov,1966; S a d t l e r Standard I n f r a r e d G r a t i n g Spectra,1972; V e r l a g Chemie,Raman/IR Atlas,1977; A l d r i c h , L i b r a r y of I n f r a r e d Spectra,1980. (**) a b b r e v i a t i o n s : s = s t r o n g ; m = medium; w = weak asym. = asymmetric; sym = symmetric; s t r . = s t r e c h i n g ; def = deformation i . p . = i n plane; o.o.p. = out o f p l a n e 178 of approximately 715cm"1. In addition to these three bands, the p o s i t i o n of the OH stretching bands can also be used as i n d i c a t i v e of the anomeric form of glucose (Zhbankov, 1966). The a-D anomer has the Cghydroxyl i n the a x i a l p o s i t i o n . This gives r i s e to intermolecular OH stretching frequencies at approximately 3410cm~1and 3320cm"1. Since t h i s region of the spectrum i s generally treated as one strong and broad peak, the fine structure of the peak i s sometimes overlooked. Figure 6.19 shows the infrared spectrum of a -D(+) Glucose (BDH Chemicals) obtained by the KBr p e l l e t method. A l l main adsorption peaks discussed i n table 6.VIII are present i n the spectrum shown i n f i g . 6.19. The broad OH stret c h i n g band from 3450cm~1to 3200 cm"1, centered approximately at 3300cm"1 i s comprised of many i n d i v i d u a l peaks. Two of them are situated at 3406cm"1 and 3 318cm"1, i n close agreement with the r e s u l t s presented by Zhbankov (1966), in d i c a t i n g the a-D anomeric form. This i s also confirmed by the p o s i t i o n of the type II v i b r a t i o n (Rao, 1963 and Zhbankov, 1966) at 837cm"1. In addition to the bands already discussed i n table 6.VIII, a few other bands appear i n the spectrum shown i n f i g 6.19. The absorption at 2354cm~ 1is a spurious band due to atmospheric C0 2 (caused by an unbalanced spectrophotometer).Two other bands occurring at 1559cm"1 and 1521cm"1 can be associated with a protein type impurity i n the sample (see the discussion on starch spectra). Small peaks (not marked i n figure 6.19) i n the F i g u r e 6 » 1 9 - I n f r a r e d Spectrum of <*D(+)-Glucose. 180 1700-18OOciti-1 wavenumber range can be associated with carboxylic a c i d impurities. Figures 6.20 to 6.23 show the i n f r a r e d spectra (KBr p e l l e t s , best results) of the many starch samples used i n the present work. One common c h a r a c t e r i s t i c of a l l starch spectra i s the much smaller d e f i n i t i o n of most of the peaks as compared to the spectrum of a-D(+) Glucose. To s i m p l i f y the interpretation of starch spectra the various groups responsible for the inf r a r e d bands are discussed separately: (i) - C-H vibrations By analogy with the a-D(+) Glucose spectrum, the in f r a r e d bands deriving from C-H vibrat i o n s f o r a l l starch samples and t h e i r respective assignments are presented i n table 6.IX. In t h i s table one can observe that the asymmetric CH stretching i n CH2(2920-2930cm~1) and the CH deformation (1454-1463cm~1) and the CH deformation (type II v i b r a t i o n , c h a r a c t e r i s t i c of the anomeric form - C5-H, at 850-860cm-1) are present i n a l l spectra. The other CH bands appearing i n the a-D(+)Glucose spectrum are also present i n some of the starch samples. Potato starch shows the most well defined spectra among a l l samples with respect CH v i b r a t i o n s . ( i i ) - OH Vibrations 181 F i g u r e 6 . 2 0 - I n f r a r e d Spectra of: (1) -and (2) - Potato Amylose. Potato Amylopectin 182 \ / ? I 3350 3150 = 5 \ *• •* 2 y s 2 at I at »n ( a « O \ I «> \ A y / * u\ / / a* \ / V V g v s A CM to » a l-TAPIOCA STARCH 1 / V,rt o V. ^ ^ \ . I / n n w s " v — ^ o \ J SJ" 5 5°s s \ \ / N - -? ~ \ 3600 3200 ~ V. H H = J £ 5 i i i i i i i / * \f\ s " ^ s S ^ a» 5 CM \ A i^ -v to V s s 2 -P0TAT0 STARCH 1 1 4000 3000 2000 1600 IZ00 800 4 0 0 WAVENUMBER (cm*1) F i g u r e 6.21 - I n f r a r e d Spectra of: (1) - Tapioca Starch and (2) - Potato S t a r c h . 183 F i g u r e 6.22 - I n f r a r e d Spectra of:(1) - CollamiKCommercial Grade Corn Starch) and (2) - Corn S t a r c h . 184 F i g u r e 6.23 - I n f r a r e d Spectra of Corn S t a r c h : (1) - As i n Fig.6.22 and (2) - New. 1 8 5 TABLE 6.IX - Major C - H Vibrations i n the I.R. Spectra of Starches (Figures 6.20 - 6.23)(*) C - H Vibrations(cm - 1) Starch Sample asym. sym. scissoring bending wagging def s t r . s t r . Potato Amylopectin 2923 2817 1454 1593 absent 853 Potato Amylose 2927 absent 1462 1393 absent 852 Tapioca Starch 2929 2889 1462 absent absent 859 Potato Starch 2923 2895 1462 1385 1290 859 Collamil(Cornstarch) 2930 absent 1460 absent 1300 860 Corn Starch 2929 absent 1463 1381 absent 857 (*) See TABLE 6.VIII for abbreviations. 186 Again by analogy with the assignments made for a -D(+) Glucose (table 6.VIII), the major OH bands f o r a l l starches are given i n the table 6.X. With respect to OH vi b r a t i o n s , the tapioca starch sample i s the one showing most of the peaks found i n th e a - D ( + ) Glucose spectrum. The i n plane OH bending i s assigned at the same frequency of the CH 2 s c i s s o r i n g deformation (superimposed). The lower frequency out of plane OH vibrations are present i n a l l starches. At l e a s t three peaks are well defined. The peaks at 661cm"1 found i n tapioca and potato starch spectra are situate d approx. 20cm"1 above the same peak f o r a-D(+) Glucose. This can be regarded as being caused by the strong intermolecular H-bonding present i n the polymers. ( i i i ) - CO, COH and Ring Vibrations Table 6.XI gives the assignments for these v i b r a t i o n s f o r a l l starches by analogy with a-D(+) Glucose assignments. Some important aspects are presented i n Table 6.XI. A l l starches show type I and type III r i n g v i b r a t i o n s s h i f t e d towards higher and lower wavenumbers, respectively, as compared to a-D(+) Glucose. These s h i f t s have been associated with the type of intermolecular linkages i n d i f f e r e n t polysaccharides (Rao, 1963). In the case of starches, these s h i f t s would be related to<*-D(+) (I'M) linkages, as i n amylose. Literature reported values (Rao, TABLE 6.X - Major 0-H Vibrations i n the I.R. Spectra of Starches (Figures 6.20 to 6.23) O-H vibrations (cm - 1) Starch Sample s t r . H-O-H i.p. bending O.O. P- bending flex i o n (a) (b) (c) Potato Amylopectin 3415- -3425 1644 1459 abs. abs. 614, 551 ,492 Potato Amylose 3415- -3425 1641 1462 abs. abs. 616, 557 ,484 Tapioca Starch 3550--3150 1643 1462 1341 abs. (?)6 61, 609,552,507 Potato Starch 3600--3200 1640 1462 abs. abs. (?)6 61, 619,602,553,508 Collamil (Corn Starch) 3440 1651 1460 abs. abs. 612, 550 ,510 Corn Starch 3431 1645 1463 abs. abs. 614, 551 ,505 (*) See Table 6.VIII for abbreviations. TABLE 6.XI - C-0, C-OH and Ring V i b r a t i o n s i n the I.R. Spectra of Starches (Figures 6.20 to 6.23) St a r c h Samples C-O-G asym. i n COH CO Type I Type I I I asym.str. phase r i n g s t r . s t r . r i n g r i n g s t r . s t r . b r e a t h i n g Potato Amylopectin 1163 abs. shoulder 1043 926 760 Potato Amylose 1160 abs. 1080 1043 930 760 Tapioca S t a r c h 1157 abs. 1084 1023, 989 949 736 Potato S t a r c h 1164,1154 abs. 1098 1038, 1014 985 930 761 C o l l a m i l ( C o r n Starch) 1158 abs. 1082 1011 929 764 Corn Starch 1159 abs. <* 1078 1016, 999 928 761 (*) See t a b l e 6.VIII f o r a b b r e v i a t i o n s . 189 1963) f o r the s h i f t e d peaks are 93 0cm - 1 f o r type I v i b r a t i o n and 7 60cm"1 f o r type I I I , i n very good agreement with the values given i n table 6.XI. The absence of the asymmetric in-phase r i n g stretching at approx. 1100cm~ 1is i n agreement with the res u l t s of de Araujo (1982) and Khosla (1983). I t i s i n p a r t i a l agreement with the r e s u l t s of Colombo and Rule (1967). In the l a t t e r case, very weak signals were found for some starch samples at 1100cm"1, although t h i s peak was absent i n tapioca starch and amylopectin. Being a weak signal located near a strong band (at 1080cm"1), i t may be superimposed on t h i s stronger band. (iv) - Impurity Related Vibrations Starches are known to possess d i f f e r e n t impurities. The presence of such impurities can sometimes be detected by in f r a r e d spectoscopy. Colombo and Rule (1967),e.g., reported the presence of protein impurities i n starches through a ca r e f u l analysis of infrared (ATR) spectra. Protein impurities are known to be p r e f e r e n t i a l l y associated with the amylose f r a c t i o n . Other commonly occurring impurities are f a t t y acids and e s t e r i f i e d phosphate groups. In the f i r s t case, the presence of COOH group w i l l give r i s e to very c h a r a c t e r i s t i c i n f r a r e d absorption bands. In the second case, the d i f f e r e n t i a t i o n i s more d i f f i c u l t or even impossible because the frequency range for major P.O.C 190 v i b r a t i o n s coincides with some of strong starch bands. Table 6.XII presents the tentative assignments of the v i b r a t i o n s previously l e f t unassigned. An analysis of table 6.XII demonstrates that of the s i x starch samples investigated only " c o l l a m i l " corn starch did not show impurity r e l a t e d bands. E a r l i e r i n f r a r e d studies of de Araujo (1982) also indicated the absence of impurities i n t h i s same reagent. This starch sample was not used i n the experimental work and i t s infrared spectrum was recorded for comparison purposes only (in view of the author's previous experience with t h i s sample). As expected, potato amylose gave definable protein r e l a t e d peaks at 1551cm"1 and 1500cm"1. These bands were absent i n potato amylopectin. However, both potato amylose and amylopectin presented an i n f r a r e d peak at approx. 173 0cm"1 which can be attributed to a carbonyl v i b r a t i o n (C=0 s t r e t c h i n g ) . Protein impurities were also found i n the spectra of potato, corn and tapioca starches. Apparently the most impure starch i s the tapioca sample (commercial grade, from Thailand). The presence of phosphate groups could not be ascertained from the recorded spectra. The frequencies where P-0 v i b r a t i o n s are expected to appear coincide with major v i b r a t i o n s of starch bonds ,hence precluding any conclusive i n t e r p r e t a t i o n on t h i s regard. The peak at 1244cm"1 displayed by tapioca starch could have been assigned to a P = 0 stretching v i b r a t i o n . However i t could also be assigned to C - 0 stretching or C - N stretching plus NH i n plane bending. The v i b r a t i o n assigned to adsorbed TABLE 6 . X I I - T e n t a t i v e Assignments of Impurity R e l a t e d Bands i n the I.R. Spectra o f Starches ( f i g u r e s 6 . 2 0 to 6 . 2 3 ) ^ S t a r c h Samples Wavenumber(cm - 1) Assignment Source(s) Potato Amylopectin 2 3 5 0 C 0 2 s t r . ( g a s ) Spurious C0 2 band 1 7 3 1 C = 0 s t r . F a t t y A c i d or A l k y l E s t e r Potato Amylose 1 7 3 0 C = 0 s t r . it ti I I I I I I 1 5 5 1 N-H bending P r o t e i n 1 5 0 0 NH 2 def. Protein(Amide II) Potato Starch 1 7 6 9 ( s h ) C = 0 asym.str. F a t t y A c l d ( ? ) ' 1 5 6 2 NH 2 def. Protein(Amide I I ) U f O O C00~asym.str. Carb. A c i d Dimer C o l l a m l K C o r n Starch) 7 0 7 N-H o.o.p. bend, .Prot e i n Corn Starch 1 5 1 Ir- NH 2 def. Protein(A.mlde II) Tapioca Starch 1 7 9 0 C = 0 asym. def. A l i p h a t i c Carb.Acid 1 7 3 0 C = 0 s t r . F a t t y A c i d 1 5 7 0 N-H bending P r o t e i n 1 4 1 6 C00~asym.str. Carb . A c i d Dimer 1 2 M , P=0 s t r / C - O s t r . E s t e r i f i e d Phos./ /C-N s t r . + N-H F a t t y A c i d / i . p . bending P r o t e i n (*) see TABLE 6.VIII f o r a b b r e v i a t i o n s 192 H 20 at ca. 1650cm~1can also be related to C = 0 stretching i n protein impurities. Although the assignment of t h i s v i b r a t i o n to adsorbed H 20 i s the most probable, one should consider the superimposition of t h i s band by a protein r e l a t e d band. El-Hinnawy et a l . (1982) assigned a band at 1650cm"1 to phosphate groups i n the i n f r a r e d spectrum of potato starch. Although t h i s assignment can also be correct, i t i s t h i s author's opinion that i t can not be conclusive. In summary, inf r a r e d spectroscopy applied as a characterization technique f o r starches was able to provide useful information on the following aspects: (i) - a l l samples studied are starches, presenting a l l major and unique i n f r a r e d bands of the monomer a-D(+)Glucose; ( i i ) - as expected, d i f f e r e n t impurities are present i n the starches tested. Later i n t h i s t h e sis, t h i s information w i l l be used as support for mechanistic i n t e r p r e t a t i o n of the behaviour of apatite/starch systems; ( i i i ) - the type of the impurities detected i n the i n f r a r e d spectra of the samples i s i n agreement with data a v a i l a b l e i n the l i t e r a t u r e . 6.2.2 - Electrophoresis Although e l e c t r o k i n e t i c methods are not widely used 193 for the characterization of starches, they can provide information regarding the state of the starch grain/water in t e r f a c e . The studies described i n t h i s section were c a r r i e d out to show the presence of i o n i c groups on the surface of starch grains. Starch grains are known to carry negative surface charge i n sea water (Parks, 1975). This charge has been shown to be a function of pH, grain pre-treatment (e.g. l i p i d extraction) and starch type (Marsh and Waight, 1982). Water-extracted wheat starch (B type) was found to present an iep at pH 3.7. In turn, potato starch was shown to carry a negative surface charge from pH 1.8 to pH 10, regardless of the pre-treatment routine used (Marsh and Waight, 1982). The nature of the surface charge displayed by non-modified starch ( i . e . not transformed into an anionic or c a t i o n i c polymer by reaction with s p e c i f i c chemical groups), should be r e l a t e d to the presence of i o n i c impurities. Marsh and Waight (1982) explained the surface charge on wheat B starch as due to the presence of both l i p i d s (phosphate ester, quaternary ammonium groups and free f a t t y acids) and protein impurities. The p o s i t i v e charge below the iep was r e l a t e d to amino groups on surface l i p i d s and protein. This p o s i t i v e charge could be removed by an EDTA treatment. The negative charge was correlated to both phosphate and carboxylic groups. In the case of potato starch (which did not display an iep i n the pH range tested), the negative 194 charge was considered to be present i n a phosphate diester, with pKa<2.5. Table 6.XIII summarizes the r e s u l t s obtained i n the present work. From the results shown i n t h i s table, i t i s c l e a r that a l l samples displayed a negative surface charge at pH 5.7 i n d i s t i l l e d water. Potato amylose and amylopectin had to be ground i n a agate mortar and pestle before the measurements (performed i n a Zeta Meter). The other samples were submitted to electrophoresis as received, except i n the case of ethanol treatment (at room temperature f o r 60 minutes). Potato amylose presented the smallest negative absolute E.M. value among the samples. The highest negative absolute value was reached by corn starch. The r e l a t i v e l y small negative E.M obtained for amylose and amylopectin as compared to the other samples might be re l a t e d to the grinding of these samples j u s t p r i o r to electrophoresis. This could have p a r t i a l y altered the p o s i t i o n i n g of surface impurities of the grains. The other three samples (viz. potato, tapioca and corn starches) presented s i m i l a r E.M. values. Corn and potato starches when subjected to the ethanol treatment had t h e i r absolute E.M. values reduced by approx. 30% I f the Helmholtz-Smoluchowski equation i s used to convert the obtained E.M. to zeta p o t e n t i a l values (see table 6.XIII), these r e s u l t s can then be d i r e c t l y compared to those of Marsh and Waight (1982). These workers also used 195 TABLE 6.XIII - Microelectrophoretic M o b i l i t i e s of Starch Grains i n D i s t i l l e d Water (pH 5.7) Starch Sample Electrophoretic Zeta Po t e n t i a l M o b i l i t y Helmholtz-Smoluchowski equation, 23°C (um/s per v/cm) (mV) Potato Amylose -1.20 -16.0 Potato Amylopectin -1.15 -15.5 Potato Starch -2.10 -28.0 Tapioca Starch -2.20 -29.5 Corn Starch -2.30 -30.8 Potato Starch treated with ethanol -1.50 -20.1 Corn Starch treated with ethanol -1.50 -20.1 (*) - ZETA POTENTIAL(mV)= 13.4 (E.M.) 196 microelectrophoresis i n t h e i r studies ( t h e i r i n v e s t i g a t i o n covered the pH range from 2 to 10; t h i s author attempted to make measurements also at pH 10 but the grains of starch used i n the present work started to swell r a p i d l y at that pH). In the case of potato starch, Marsh and Waight's r e s u l t at pH 5.7 was approx. - 38mV, ten mV more negative than the r e s u l t obtained here. Marsh and Waight treated t h e i r potato starch sample by ethanol followed by re f l u x i n g with CHCI3/CH3OH to remove surface l i p i d s . After t h i s treatment t h e i r potato starch sample showed a zeta pot e n t i a l of approx. -llmV at pH 5.7. The milder treatment used i n the present work reduced the potato starch zeta potential value at the same pH from -28mV to -20.1mV. Although the modification obtained i n t h i s work was smaller, i t confirms that part of the i o n i c impurities associated with starches can be removed from i t s grains. The testwork performed here demonstrates the presence of impurities of io n i c nature associated with the starch samples used. These impurities were also detected by the i n f r a r e d spectroscopy work already discussed. 6.2.3 - Viscometry C a p i l l a r y viscometers can be used f o r the determination of the l i m i t i n g v i s c o s i t y number ( i n t r i n s i c v i s c o s i t y ) of potato amylose solutions. In the present i n v e s t i g a t i o n measurements of t h i s property were made i n order to provide an insight to the molecular s i z e of the 197 sample used. Tabulated constants for the Mark Houwink equation f o r s i m i l a r systems were used to c a l c u l a t e the molecular weight. There are no available data f o r amylopectin since the technique i s not recommended fo r measurements on branched polymers (Greenwood, 1964). A few measurements were attempted on starch solutions, but the r e s u l t s obtained were not r e l i a b l e . Figure 6.24 depicts the l i m i t i n g v i s c o s i t y number (ml/g) obtained from p l o t t i n g the s p e c i f i c v i s c o s i t y / concentration r a t i o against potato amylose s o l u t i o n concentration. This same figure also shows a second a l t e r n a t i v e way to obtain the l i m i t i n g v i s c o s i t y number. The t e s t s were performed at pH 12.3 (KOH) on potato amylose solutions f r e s h l y prepared and centrifuged f o r 10 minutes at 12,000 rpm. The average l i m i t i n g v i s c o s i t y number (taking into consideration the two alternative ways to c a l c u l a t e the intercept f o r zero concentration) i s 139ml/g. This value i s i n f a i r agreement with published values ranging from 140 to 590 ml/g f o r d i f f e r e n t amyloses (Greenwood, 1970) at pH 13 and 25"C, and i s i n very good agreement with the data presented by Khairy et al.(1966) who give 139ml/g also at 25°C and pH 13. Using the constants compiled by Kurata et a l . (1975) f o r potato amylose (25°C) at 0.15 m o l / l i t r e KOH (pH = 13.18) which are: K = 8.36 x 10" 3ml/g and 1-1 0) \ 140 E U \ 130 a (0 ""•A. A. P0TRT0 AMYLOSE PH-I2.3CK0H) '•A. 1 2 3 4 6 CONCENTRATION-C-Cg/m!)X 1000 -160 -140 Figure 6,2k - Determination of the L i m i t i n g V i s c o s i t y Number of Potato Amylose. 199 o= 0.77 the molecular weight of t h i s sample i s then (from the Mark Houwink equation 5.3.7): M.W = 303,000 daltons. This value compares r e l a t i v e l y well with the molecular weight of potato amylose presented i n table 4.VIII of 490,000, determined by l i g h t scattering (Young, 1984) and with the r e s u l t s of Khosla (1983) by u l t r a c e n t r i f u g a t i o n (320,000 daltons). 6.2.4. - Light Scattering Photometry and Starch Solution Preparation. The l i g h t scattering r e s u l t s presented herein are more concerned with the state of starch aqueous solutions and how these solutions are affected by pH, temperature, ageing and shearing than the actual determination of molecular weight. The molecular weight of starch f r a c t i o n s has been already discussed i n a previous section. I t should be clear at t h i s point that molecular weight determinations of starch as a whole have l i m i t e d s i g n i f i c a n c e . Only f o r the starch f r a c t i o n s (amylose and amylopectin) can one f i n d meaningful data. 200 The l i g h t scattering r e s u l t s to be presented as absolute s o l u t i o n t u r b i d i t y values were gathered at the following experimental conditions (see Anon. 1970a f o r d e t a i l s ) : (i) - Brice Phoenix Light Scattering photometer equipped with a rectangular quartz c e l l (Rw/Rc - 1), operating at a wavelength of 546nm (green f i l t e r ) ; ( i i ) - temperature of 25°C; ( i i i ) - at lea s t three measurements f o r each determination of the sc a t t e r r a t i o (Gs/Gw); (iv) - solutions were centrifuged f o r 10 minutes at 12,000 rpm p r i o r to the measurements. To give a " b a l l park" idea on the r e l a t i o n s h i p between absolute tu r b i d i d t y and molecular weight of starch solutions, a 2.5g/l solution of Dextran A (a b a c t e r i a l l i n e a r polysaccharide) was subjected to study i n the photometer. This polysaccharide has a known molecular weight (as given by i t s supplier) i n the range of 2 00,000 to 250,000 daltons. I t s s o l u t i o n at pH 5.5 showed an absolute t u r b i d i t y of 1.12 x 10 . Potato amylose, which, according to the r e s u l t s presented i n the previous section, has a molecular weight of approximately 3 00,000 daltons, displayed a t u r b i d i t y of 6.51 x 10~ 3for a solut i o n under s i m i l a r conditions. Potato amylopectin, which i s known to possess a much higher molecular weight, at s i m i l a r conditions gave an absolute 201 t u r b i d i t y 6 times higher than that of potato amylose. According to Forster and Sterman (1956) an absolute turbidity/concentration r a t i o i n the range of 2-4 x 10~ 2 corresponds to a molecular weight of approx. 1 m i l l i o n daltons f o r amylose.If the Forster and Sterman r e s u l t s are considered v a l i d , the potato amylose solut i o n described above would possess a molecular weight i n the neighbourhood of 1 m i l l i o n daltons, considerably larger than the molecular weight obtained by the v i s c o s i t y measurements previously described.Molecular weigth discrepancies of t h i s order, as discussed e a r l i e r i n the l i t e r a t u r e review section, are not s u r p r i s i n g . Applying equation 5.3.5 to the l i g h t s c a t t e r i n g r e s u l t s obtained for potato amylopectin solutions (up to 1.25g/litre) and assuming that R 9 0 ° = absolute t u r b i d i t y and that H 1 can be evaluated from tabulated data ( ( n - n Q y c ) 2 = 0.142ml/g, Huglin, 1975); one finds a M.W. of approx. 10 m i l l i o n daltons. This value i s probably underestimated i n view of other reported values. Even so, i t demonstrates that amylose has a much lower molecular weight than amylopectin. The r e s u l t s used for the c a l c u l a t i o n described and s i m i l a r data f o r tapioca and potato starches are depicted i n f i g u r e 6.25. Tapioca starch solutions at pH 5.5 show a l i n e a r r e l a t i o n s h i p to solution concentration, which i s not the case f o r potato amylopectin and potato starch. Figure 6.26 shows a s i m i l a r relationship f o r corn starch at pH 10. Figure 6.27 shows the e f f e c t of ageing f o r 60 hours Figure 6.25 - T u r b i d i t y of Potato Amylopectin,Potato Starch and Tapioca Starch S o l u t i o n s as a Function of S o l u t i o n Concentration. 0.101 I CORN STRRCH/pH-IOj 0.08| >-H §0.06| UJ §0.04| 0.02 h 0.001 0 1 2 CONCENTRflTIONC g/1 Itre) F i g u r e 6.26 - T u r b i d i t y of Corn S t a r c h S o l u t i o n s . 0.02 >-i — i a CD H0.0I UJ d tn S 0.00 TfiPIOCfl STflRCH/pH-5.6 + AGED 80 HOURS AT 35*C • RGB] t 2 CONCENTRRTIONCg/lItre) Figure 6.27 - Effect of Aging on the Turbidity of Tapioca Starch Solutions. to o 205 on the t u r b i d i t i e s of tapioca starch solutions stored at 2 d i f f e r e n t temperatures (10 and 35°C). In both cases, t u r b i d i t y decreased s i g n i f i c a n t l y , p a r t i c u l a r l y f o r the so l u t i o n stored at the lower temperature. This f i n d i n g confirms that the rate of retrogradation i s increased by lowering the temperature. I t also confirms that the de t e r i o r a t i o n of starch aqueous solutions under normal mineral processing laboratory conditions (not completely aseptic) takes place very rapidly. Figure 6.28 depicts the e f f e c t of temperature on the t u r b i d i t y of tapioca starch aged for 60 hours at 35°C. Although a small increase of t u r b i d i t y was achieved by lowering the temperature to 22°C, changing temperatures from 15°C to 35°C had l i t t l e e f f e c t on t u r b i d i t y . At higher temperatures(close to the g e l a t i n i z a t i o n temperature of tapioca starch) t u r b i d i t y decreased i n d i c a t i n g that the thermal r e g e l a t i n i z a t i o n of a degraded starch s o l u t i o n does not take place. Retrogradation of starch solutions i s generally associated with the amylose f r a c t i o n which can e a s i l y p r e c i p i t a t e from solution due to i t s l i n e a r configuration. In fact, amylopectin solutions are much more stable than amylose solutions, (Young, 1984). Young (1984) also states that retrograded amylose i s insoluble i n water unless i t i s heated above 124°C. The s t a b i l i t y of amylose solutions prepared by alkaline g e l a t i n i z a t i o n i s also decreased when the solutions are neutralized (as i n the procedure used herein). 0.02 >-h-a ca •0.01 m 0.00 ATRPICCfi STARCH STORED AT 36°C/2.5g/l I tre/pH-6.6 SOLUTION AGS] FOR 60 HOURS 10 20 30 40 50 TEMPERRTUREC deg rees °C) 60 70 Figure 6.28 - Eff e c t of Temperature on the Turbidity o Aged Tapioca Starch Solution. 207 The next figure (6.29) shows the e f f e c t of pH on t u r b i d i t y of potato amylopectin solutions. Except for the pH range from 11 to 12, the t u r b i d i t y was independent of pH. In the pH range aforementioned, a peak i n t u r b i d i t y appeared. This can be interpreted as the amylopectin macromolecules increasing t h e i r r e l a t i v e s i z e by being more f u l l y extended. The next two figures (6.30 and 6.31) show the e f f e c t of p h y s i c a l modification on the t u r b i d i t i e s of corn starch and potato amylopectin, respectively. In the f i r s t case, shearing of polymer solution was effected i n a Waring Blender for d i f f e r e n t time spans. For a f a i r l y short shearing time of 0.5 min. there was already a very considerable reduction i n the t u r b i d i t y (54% reduction). For longer shearing times (up to 7 minutes) the t u r b i d i t y was reduced to 44% of the o r i g i n a l value. Shearing of polymer solutions promotes degradation of molecular aggregates by breaking them into smaller pieces. This phenomenon i s of extreme importance for both f l o c c u l a t i o n and f l o t a t i o n systems using polymers. In a very recent inves t i g a t i o n by Henderson and Wheatley (1987) sheared polyacrylamide solutions showed reduced performance as f l o c c u l a n t s (sheared polymers displayed a c t i v i t i e s as % of the o r i g i n a l s e t t l i n g rate i n the range from 19 to 37%, greater values were achieved by more anionic polymers). In another recent investigation by Reis (1987) the depressant e f f e c t of an impure corn starch ("gritz") used i n i r o n ore reverse f l o t a t i o n systems was shown to be extremely dependent on the extent of physical (mechanical) modification by 2.6g/l (ORIGINALLY RT pH-6.63 A POTATO AMYLOPECTIN 0 . 0 3 1 1 3 5 7 0 11 13 pH F i g u r e 6.29 - E f f e c t of pH on the T u r b i d i t y of Potato Amylopectin S o l u t i o n . to o 03 4. 6 TINE OF SHEfiRINBCmlnutes) 10 Figure 6 .30 - E f f e c t of Shearing on the T u r b i d i t y of Corn Starch S o l u t i o n . (shearing performed i n a Waring Blender) o 0.04 >• t-i—i a i—i m •-'0.03 U J _ J 0.02 TIME IN SONIC BflTH-lmln. • POTATO AMYLOPECTIN pH-5.5/2.6g/litre 10 20 30 40 60 80 70 80 SONIC DISMENBRATOR SETTING Figure 6.31 - Effect of Sonic Treatment on the Turbidity of Potato Amylopectin Solution. H O 211 shearing. Under c e r t a i n optimized shearing conditions both recovery and concentrate grade were found to increase i n comparison to non-sheared starch. Similar conclusions were also reached by Khosla (1983). Figure 6.31 shows another type of physical modification. The solution of potato amylopectin was submitted f o r 1 minute to d i f f e r e n t set l e v e l s i n a sonic dismembrator (these l e v e l s are p l o t t e d i n figure 6.31 as a r b i t r a r y sonic l e v e l settings of the equipment used). Contrary to shearing, there was a small increase i n t u r b i d i t y f o r low l e v e l s of the sonic e f f e c t . T u r b i d i t y however, decreased abruptly above a c e r t a i n l e v e l . The small increase i n t u r b i d i t y might be associated with an improved homogenization of the s o l u t i o n when submitted to the low i n t e n s i t y sonic f i e l d . The abrupt decrease i n t u r b i d i t y (to 56% of the o r i g i n a l value) probably r e f l e c t s a shearing-like e f f e c t . In summary, testwork characterizing the starch samples used herein, provided the following relevant points: (i) - i n f r a r e d spectroscopy was s u c c e s s f u l l y used to characterize a l l starch samples. I t provided information regarding the impurities associated with the starch grains. ( i i ) - electrophoretic measurements on aqueous 2 1 2 starch suspensions confirmed the presence of anionic groups on the surface of starch grains; ( i i i ) - v i s c o s i t y related experiments were able to provide information on the molecular s i z e of amylose; (iv) - l i g h t s c attering photometry was successfully used to y i e l d information on the r e l a t i v e molecular sizes of amylose and amylopectin as well as the e f f e c t s of pH, temperature, aging and shearing on the properties of aqueous solutions of starches. 6.3. - Interaction Between Starches and Solution Species 6.3.1. - Starches/Surfactants The reaction between starch and iodine has interested chemists since 1812 (Rundle et a l . , 1944). Amylose aqueous solutions reacted with iodine display a blue colour, derived from a complex formed between amylose and iodine species. Amylopectin also interacts with iodine i n aqueous solutions but the complex formed i s l e s s stable, d i f f e r e n t l y coloured (red to purple) and the i n t e n s i t y of the reaction i s much smaller (Rundle et a l . , 1944). Plate A shows the colours developed by reacting potato amylopectin, potato starch and potato amylose solutions with Kl/Iodine solutions at neutral 213 PLATE fl - RERCTION OF STRRCH WITH IODINE R - RMYL.OPECT IN B - PDTRTO STRRCH C - RMYLOSE 214 pH and room temperature (test tubes A, B and C, r e s p e c t i v e l y ) . Iodine reaction with starch i s a function of pH, temperature, concentration and presence of foreign solutes. I t i s well accepted that iodine molecules and ions become situated i n the core of h e l i c a l l y oriented amylose molecules (Greenwood, 1970). In turn, amylopectin binds much less iodine than amylose. I t has been suggested that the low binding power of amylopectin i s re l a t e d to the large number of branching points i n the amylopectin molecules. This would simply disrupt possible h e l i x formation (Greenwood, 1970). In both cases however, a complete understanding of the mechanisms involved i s s t i l l lacking. The conformation of these two polymeric molecules i n so l u t i o n has considerable i n t e r e s t regarding t h e i r i n t e r a c t i o n with minerals. For instance, random c o i l and extended c o i l conformations w i l l probably increase the f l o c c u l a t i n g power of the polymers, while the h e l i x conformation should decrease i t . S p e c i f i c a l l y i n the case of amylose there are 3 d i f f e r e n t proposed models f o r the conformation of t h i s macromolecule i n aqueous solutions (Senior and Hamori, 1973). They are: (i) the t i g h t - h e l i x model of H o l l o - S z e j t l i ; ( i i ) the random c o i l model of Banks- Greenwood; and ( i i i ) the extended c o i l model of Senior- Hamori. There i s one fundamental aspect concerning the models and i n t e r a c t i o n of amylose with iodine that should be emphasized. I f the f i r s t model above i s correct, iodine 215 aqueous species react with amylose by occupying an already e x i s t i n g space inside the h e l i c a l structure. I f ei t h e r of the other two models i s correct, iodine species create the h e l i c a l structure by reacting with amylose. Senior and Hamori (1973) presented i n t r i n s i c v i s c o s i t y studies that contradict p a r t i a l l y H o l l o - S z e j t l i 1 s model. Their extended h e l i x model incorporates aspects of the other two proposed models. Amylose macromolecules would possess loose h e l i c a l portions (not t i g h t l y packed) joined together by random c o i l sections. The formation of the amylose iodine complex according to the Senior-Hamori model can be envisaged as r e s u l t i n g from the entrapment of iodine species by the contraction of h e l i c a l regions of the macromolecule into t i g h t h e l i c e s ("V" amylose). This l a t t e r aspect i s important f o r the understanding of how surfactants i n t e r a c t with starch molecules as w i l l become cle a r l a t e r i n t h i s section. The i n t e r a c t i o n between surfactants and starches has been an object of study for some years, e s p e c i a l l y i n the food industry (Takagi and Isemura, 1960; Kim and Robinson, 1979; Bhide et a l . , 1981; and Bulpin et a l . 1982). Most researchers agree that the loss of the c h a r a c t e r i s i t i c blue colour of the amylose/iodine complex i n surfactant solutions r e s u l t s from surfactant species occupying part of the h e l i c a l c a v i t y that otherwise would be f i l l e d by iodine. Figure 6.32 shows the u l t r a v i o l e t (UV) - v i s i b l e (vis.) absorption spectrum for an Iodine/KI aqueous solut i o n at pH 6, containing 1.92 x 10~ 3 mol/dm3KI and 4 x 10" 5 [.3 1.4 -1.3 i . a k 1.1 -l .9 .9 .7 .S A u <J c r CD tt o cn to cr -A A A A * * * A A .3 - V . 4 A .3 -.2 - A. . 1 B -1 3D 3 0 0 4BD s e a W H V E L E N G T H ( n m ) 6 3 0 7S£» 8D.B F i g u r e 6.32 - U l t r a v i o l e t ( U V ) - V i s i b l e ( V i s . ) Spectrum of K l / I o d i n e Aqueous S o l u t i o n ( 1 . 9 2 x 1(T 3mol/l KI and k x ICT^mol/l I o d i n e ) . 217 mo1/dnr Iodine. The spectrum was obtained i n a quartz c e l l with 1.0cm of path length. The two absorption peaks at approx. 250 and 350nm are both related to the yellow iodine/ KI sol u t i o n . Figure 6.33 (a and b) depicts the u v - v i s . spectra of potato amylose and amylopectin solutions (25mg/litre, pH6) i n the presence of Kl/Iodine. This figure c l e a r l y shows the d i f f e r e n t i n t e r a c t i o n between amylose and amylopectin with iodine. The maximum absorbance f o r amylose i s situated at approx. 600nm. In turn, the absorbance maximum f o r amylopectin i s located at 550nm and has a smaller intensity.The positions of the peaks are i n good agreement with the data of Bhide et al.(1981). Another feature i n figure 6.3 3 a and b i s the absorbance values f o r residual iodine. I t i s c l e a r from these values that amylose binds much more iodine than amylopectin. In figure 6.34, the p o s i t i o n of the two absorption maxima i s more c l o s e l y compared. In t h i s case the amount of iodine/KI was doubled f o r amylopectin. This fig u r e shows more c l e a r l y the s h i f t towards lower wavelength occurring for the amylopectin solution. Figures 6.35 (a and b) and 6.3 6 depict the e f f e c t of pH on amylose and amylopectin. At pH 10, the conformation of the macromolecules i s probably d i f f e r e n t from that at neutral pH. This i s r e f l e c t e d by the decrease i n i n t e n s i t y of the absorption peaks i n both cases.Under a c i d i c conditions, how u <J E cc o tn tn (b) 4D0 . 5 8 S 6B.B WnVELENGTH(nm) F i g u r e 6.33 - UV-Vis. Spectra of (a) Potato Amylose and (b) Potato Amylopectin u u CE CO QL O cn CD cr .9 -RMYLOPECTIN AMYLOSE 530 680 W f l V E L E N G T H ( n m ) F i g u r e 6.34 - Determination of Abso r p t i o n Maxima i n the UV-Vis. Spectra of Potato Amylose and Potato Amylopectin i n the Presence of Iodine. (a) RMYL05E/DW-pH6 U U 2 cr CO Of o cn tn • DW-0HI8 e £•0 3 0 0 4BQ 5SQ W A V E L E N G T H ( n m ) SDO (b) RMYLOPECTIN/DW-pHe U (J S m O tn CP cc • • A RMYLOPECTIN/DW-pHIG 4HB see ene H f i V E L E N G T H ( n m ) F i g u r e 6.35 E f f e c t of pH on the UV-Vis. Spectra of (a) Potato Amylose and (b) Potato Amylopectin. 1 .a -. a A* A .7 - A * A u ~y . E CC * A CD A M Y L O S E x K I / i a - p H l . 5 ce ' A o . 3 tn A CQ OC .4 . 3 * A A A A t 2 V . 1 - ' A A I I I I t Ad 308 4QB 5Q© 630 799 8Q0 W H V E L E N G T H C n m ) F i g u r e 6.36 - UV-Vis. Spectrum of Potato Amylos at pH 1.5 i n the Presence of Iodine. 222 t i g h t e r h e l i x conformation, as shown by the increase i n i n t e n s i t y of the absorption maximum at 600nm. An a l t e r n a t i v e explanation f o r the phenomena described resides i n the iodine s o l u t i o n species, which themselves are a function of pH. Blanks that were run at pH 10 did not indicate, however, any considerable decrease i n absorption maxima re l a t e d to iodine. Figure 6.37 shows the absorption spectra f o r Na Oleate and Dodecylamine Hydrochloride (DDAHC1) solutions both at concentrations of 5 x 10~ 5 m o l / l i t r e (pH6) i n the presence of iodine/KI. There i s no peak f o r e i t h e r surfactant i n the wavelength range covered by the spectra (800-250nm). Nevertheless, t h i s figure shows that the c a t i o n i c surfactant interacted with the iodine solution, reducing the i n t e n s i t y of the f i r s t absorption maximum re l a t e d to iodine. Figure 6.38 (a and b) depicts the e f f e c t of Na Oleate (5 x 10~5mol/dm3) on the spectra of 25 mg/litre amylose/iodine solutions at pH 6 and pH 10. In the f i r s t case ( f i g . 6.38a) there i s a considerable reduction i n peak i n t e n s i t y accompanied by a s h i f t towards lower wavelengths. At pH 10 (fig.6.38b) the long wavelength peak disappears completely. Figure 6.39 shows a s i m i l a r s i t u a t i o n at pH 3.5. The next figure (6.40) shows the e f f e c t of DDAHC1 (5 x 10~ 5mol/dm 3). Curves 1 and 3 represent amylose solutions at pH 6 and 10, respectively i n the absence of the surfactant. Curves 2 and 4 resulted from the addition of DDAHC1 to the system, also at pH 6 and pH 10, respectively. u o 2 CC CQ ce o cn CD cc A A AA A If* A 0 O NaO1eite-5xl0(-5)M V D D f i H C l - 5 x l 0 < - S ) H B 290 320 350 330 413 443 473 393 33a 369 339 S29 B59 539 719 WAVELENGTH(nm) '•49 7"9 3D3 F i g u r e 6.37 - UV-Vis. Spectra of Na Oleate and Dodecylamine Hydrochloride(DDAHCl) i n the Presence of Iodine. LJ O tr co ce o tn m tr 3Q0 224 (a) HHYLOSE/DW-pHS A A^MYLDSE/'NaOL. -pH6 4OB sea HRVELENGTHCnm) y tr CO ct o tn CO cr B 300 (b) t RMYLOSE/NaOL.-pHll3 4DD see W R V E L E N G T H ( n m ) F i g u r e 6 . 3 8 - E f f e c t of pH on the UV-Vis. Spectra of Potato Amylose i n the Presence of Iodine and Na Oleate (a)-pH6 and (b)-pHIO. F i g u r e 6.39 - E f f e c t of Na Oleate on the UV-Vis. Spectrum of Potato Amylose at pH3«5 i n the Presence of Iodine. .3 r A * * * " " * * A A A u cc ca ce o cn 03 cc • • « 350 603 W A V E L E N G T H ( n m ) F i g u r e 6 .40 - E f f e c t of DDAHC1 on the UV-Vis. Spectra of Potato Amylose i n the Presence of Iodine: 1 - Potato Amylose at pH6 2 - Potato Amylose/DDAHCl at pH6 3 - Potato Amylose at pH10 4 - Potato Amylose/DDAHCl at pHlO. Figure 6.41 depicts the same s i t u a t i o n at pH 3. As i n the case of Na Oleate, the e f f e c t of DDAHC1 was to decrease the i n t e n s i t y of the amylose/iodine peaks, accompanied by a blue s h i f t . According to Bhide and co-workers (1981), the order of addition of the three reagents (iodine/KI, amylose and surfactant) i s important to the f i n a l outcome. In t h e i r testwork (as i n the present testwork) the surfactant solutions were added to the starch solutions p r i o r to the addition of the iodine/KI solution. Bhide et a l . found that t h i s sequence of mixing promoted the large s t e f f e c t i n terms of decreasing the in t e n s i t y of the amylose/iodine peaks. I t seems that i f iodine i s allowed to i n t e r a c t f i r s t , only a small portion of i t s molecules are exchanged by surfactants species. In contrast to the re s u l t s obtained i n the present testwork and those of Bhide et al.(1981), Kim and Robinson (1979) d i d not observe a large blue s h i f t i n t h e i r absorption spectra of amylose/iodine i n the presence of polyoxethlene sorbitan monostearate. They did observe however, s i m i l a r decreases i n peak i n t e n s i t i e s to those obtained here. Bhide et al.(1981) found that the e f f e c t of surfactants on the amylopectin/iodine UV - v i s . spectrum i s les s pronounced than i n the case of amylose. Figure 6.42 (a and b) shows the e f f e c t of Na Oleate (at pH 6 and pH 10) and DDAHC1 (at pH 6) on the absorption spectra of amylopectin/ iodine. The concentration of Kl/Iodine i s twice as much as 228 .3 .a .? CJ .6 cc cn Ct o . 3 tn CO cc .4 .3 # 2 . 1 0 RMYL0SE/DW-pH3 •4^ fiMYLOSE/DDHHCl-pH3 4QB see HRVELENGTHCnm) 6DD F i g u r e 6 . 4 1 - E f f e c t of DDAHC1 on the UV-Vis. Spectrum of Potato Amylose at pH 3 i n the Presence of Iod i n e . 1 . 3 . B .7 Ul CJ 2 .5 CC CQ s i n CQ = .4 . 3 . 3 . 1 B 2E (a) • • • • * A *£ • RMfLOPECTIM/NaOL.-pH6 • JFv^'m m • in • • r in *rZ"+ • -A B RMYLOPECTIN/'NaOL.-pHlQ *• • A • *' f 1 t t t 1 * Al io 3oe 4DD see eao 7eo sao HRVELENGTHCnm) 1 . 3 . 3 .7 Ul CJ •Z. . 6 CC tn g • = tn CQ C .4 . 3 • 3 . 1 0 r (b) * A * A -«. A flMYLOPECTIN/DDRHCl-pHS A •» A A A * * A A A A A A A A OD 303 '400 5Q0 600 78© 33D WflVELENGTHCnm) F i g u r e 6.^2 - E f f e c t of (a) Na Oleate and (b) DDAHC1 on the UV-Vis. Spectra of.Potato Amylopectin i n the Presence of Iodine. 230 that used i n the amylose spectra. The spectra obtained present a r e l a t i v e l y small decrease i n peak i n t e n s i t y i n a l l cases, i f these spectra are compared to those of amylose. No s h i f t i n the peak p o s i t i o n was detected, again i s agreement with the r e s u l t s of Bhide et a l . (1981). The same reasoning used e a r l i e r to j u s t i f y the low iodine binding capacity for amylopectin, can be also used to explain the smaller changes encountered i n amylopectin/iodine spectra. The absence of s i g n i f i c a n t changes i n the spectra can not be used as proof or evidence i n d i c a t i n g low l e v e l s of i n t e r a c t i o n however. A more detailed i n v e s t i g a t i o n would be necessary to c l a r i f y the possible ways of i n t e r a c t i o n between giant amylopectin molecules and surfactants. Such an i n v e s t i g a t i o n i s outside the scope of the present work. Interpretation of the available data and l i t e r a t u r e l e d to the following conclusions: (i) - amylose interacts with anionic surfactants by entrapping t h e i r aqueous species into the h e l i c a l cavity of the macromolecules; ( i i ) - reduction of i n t r i n s i c v i s c o s i t y reported by Kim and Robinson (1979) for amylose-surfactant solutions i s r e l a t e d to conformational changes that the macromolecules undergo i n the presence of surfactants; ( i i i ) - the s h i f t towards lower wavelengths might be 231 associated with the breakage of polyiodine resonating chains inside the h e l i c a l cavity of amylose, caused by the presence of surfactant species; (iv) - the studies of Takagi and Isemura (1961) indicated that i f the surfactant concentration was increased above i t s CMC, no more changes were observed i n the UV -v i s . spectrum of the amylose/iodine complex. This suggests that i n d i v i d u a l surfactant species are concerned with the transformation of the h e l i c a l complex; (v) - Davies et a l . (1980) presented evidence f o r the active p a r t i c i p a t i o n of starch f a t t y acid i n the high temperature (>60°C) retrogradation of amylose. They showed that many surfactants (mostly anionic) yielded amylose c r y s t a l s of d i f f e r e n t shapes upon retrogradation. (vi) - Robb (1981) c i t i n g o r i g i n a l work by Fisherman and M i l l e r (1980) states that c a t i o n i c surfactants also i n t e r a c t with starch f r a c t i o n s . A quaternary ammonium s a l t (C 1 6H 3 3N Methyl 3Br) p r e c i p i t a t e d both amylose and amylopectin, the l a t t e r being more e f f e c t i v e l y p r e c i p i t a t e d at high pH. While anionic surfactants are reported 232 to f i t into the h e l i c a l structure of amylose, c a t i o n i c surfactants might not be able to be accomodated due to the bulkiness of t h e i r head groups, ( v i i ) - i n conclusion, starch f r a c t i o n s i n t e r a c t with surfactants i n the absence of mineral surfaces. In a f l o t a t i o n system, addition of modifiers (starches f o r example) generally precede the addition of c o l l e c t o r s (surfactants). I t i s then probable that the interactions such as those ju s t discussed w i l l predominantly take place at the mineral surfaces. Although the place where these interactions occur should not i n t e r f e r e with t h e i r existence, one should always keep i n mind the d i f f e r e n t energetic and entropic environment of i n t e r f a c e s . In t h i s way, the interactions can be e i t h e r enhanced or diminished i n t h e i r degree. 6.3.2 - Starches/Ca Aqueous Species To investigate the i n t e r a c t i o n between starches and Ca aqueous species, a d i f f e r e n t experimental approach was pursued. Attempts to u t i l i z e UV - v i s spectra of starch f r a c t i o n s i n the presence of iodine and Ca species were unsuccessful. An approach si m i l a r to that of Khosla (1983) , 233 Khosla and Biswas (1984) and Cross et a l . (1985), which involves the measurement of the depression of s o l u t i o n e l e c t r i c a l c o n d u c t i v i t i e s (after mixing starch and Ca solutions) was found appropriate. The basic p r i n c i p l e s of t h i s approach have already been discussed (section 5.3.6). Figure 6.43 shows the r e l a t i o n s h i p between solution conductivity and potato amylopectin concentration i n the presence and absence of a Ca chloride s o l u t i o n (2 x 10~ 3 m o l / l i t r e ) . Figures 6.44 and 6.45 show s i m i l a r r e s u l t s for potato amylose and potato starch, respectively. The lowest curve represents the experimentally measured conductivity of the starch s o l u t i o n i n question. The intermediate curve represents the experimentally measured conductivity obtained by adding 2 x 10~ 3mol/litre of CaCl 2to the starch sol u t i o n . The top curve represents the arithmetic sum of the i n d i v i d u a l s o l u t i o n c o n d u c t i v i t i e s . In the absence of i n t e r a c t i o n the intermediate and top curves sould be coincident. A c l o s e r look at the data reveals that the % decrease i n the measured Conductivity i n r e l a t i o n to that expected by the addition rule presented the following general trends: (i) - i n general, the % decrease diminishes as the concentration i s increased (e.g. 19% at 25 mg/litre of potato amylose i n contrast to 14% at 228 mg/litre; 17% at 25mg/litre of O a i_ o 100 E >-CJ a 10 0 A — ^ POT. AMYLOPECTIN/pH10 A DIST. WATER • 2 EC-3)mol/l CaCI 2 + NO INTERACTION sum) CCMXJCTIVITY DF DIST.WRTER AT pHB 100 200 CONCENTRATIONCmg/1 I t r e ) 300 Figure 6.43 - S o l u t i o n C o n d u c t i v i t y of Potato Amylopectin i n the Presence and Absence of Calcium C h l o r i d e . POTATO RMYLOSE/pHlO A DIST. WATER n 2 EC-3)mol/l CaCI 2 + NO INTERACTION sum) COMUCTIVITY OF DIST. WATER AT pH6 ; i i 0 100 200 300 CONCENTRATION mg/1 I tre) Figure 6.44 - S o l u t i o n C o n d u c t i v i t y of Potato Amylose i n the Presence and Absence of Calcium C h l o r i d e . ONO INTERflCTION pHIO 100 200 300 OjNCENTRFITIONCmg/l I t r e ) Figure 6 . 4 5 - S o l u t i o n C o n d u c t i v i t y of Potato Starch i n the Presence and Absence of Calcium Ch l o r i d e . 237 potato amylopectin i n contrast to 11% at 228 mg/litre; 18% at 61 mg/litre of potato starch i n contrast to 11% at 268 mg/litre; a l l data for pH 10.2) ( i i ) - lowering the pH from 10.2 to 6 i n the case of potato starch decreased the difference obtained between the experimental and the arithmetic sum value f o r no i n t e r a c t i o n (maximum % decrease occurred at 217 mg/litre of potato starch and measured 6.3%) ( i i i ) - i n a l l cases the experimental value obtained for the mixture of starch and Ca solutions was smaller than the no i n t e r a c t i o n arithmetic sum value. This p a r t i c u l a r l y contradicts the r e s u l t s obtained by Khosla (1983). Khosla's experimental data show a small decrease only i n the case amylose/Ca at 150 mg/litre of amylose and 1 x 10~ 3 m o l / l i t r e of amylose and 1 x 10~ 3 mol/ l i t r e Ca(II). For lower amylose concentrations as well as f o r amylopectin, the percentage discrepancy was ei t h e r n i l or a small p o s i t i v e value (maximum of 2.3%). Khosla did not mention the pH conditions of hi s t e s t s . Nevertheless, Cross and co-workers 1 (1985) experimental data show only depression i n the con d u c t i v i t i e s f o r various 238 carbohydrate systems. Table 6.XIV summarizes the r e s u l t s obtained for tapioca and corn starches also at pH 10.2. In both cases a large % decrease was observed (approx. 25% f o r tapioca starch and 3 0% f o r corn starch). Most studies on the a b i l i t y of starches to bind m e t a l l i c ions are related to the f i e l d of food technology (Cross et a l . , 1985; K r o l l , 1984; Sukan et a l . , 1979; Hood and O'Shea, 1977). According to Cross and co-workers the complexing of m e t a l l i c ions by both low and high molecular weight carbohydrates i s well established and can be demonstrated by depression of conductivity among other techniques. Interesting r e s u l t s were obtained by Sulkan et al.(1979) i n t h e i r work on the Ca binding capacity of potato and corn starches. The more relevant conclusions from t h e i r work were: (i) - Ca binding capacity decreased as the pH was decreased from 7.0 to 4.0 f o r both starches. ( i i ) - Maximum Ca binding values (as %) were 33% for non-treated potato starch followed by 24% f o r non-treated corn starch. ( i i i ) - For 1, 4 dioxan extracted starches (used to remove l i p i d s and e s t e r i f i e d phosphorus) the amount of Ca bound was reduced from 16% to 18%. • 239 TABLE 6.XIV - E l e c t r i c a l C o n d u c t i v i t y Depression f o r Corn and T a p i o c a S t a r c h e s i n the Presence of 2 x l . O - 3 m o l / l i t r e C a C l 2 a t pH 10.2 S t a r c h Type Starch/Cone. m g / l i t r e C o n d u c t i v i t y (micromho) S t a r c h alone mixture % o f decrease T a p i o c a 50 75 460 600 875 975 24.57 25.00 Corn 50 950 1150 30.30 75 1050 1125 30.00 240 Hood and 0'Shea's (1977) r e s u l t s are also relevant. They also found that the Ca binding capacity of tapioca, corn and waxymaize starches were also affected by pH i n a s i m i l a r way as the r e s u l t s of Sukan et a l . The order of r e l a t i v e decrease i n binding capacity among the 3 samples used was as follows: tapioca> corn> waxymaize starches. A l l studies available seem to indicate that the binding of Ca (and other m e t a l l i c ions) by starches has both nonionic and i o n i c components (Sukan et a l . , 1979; Hood and O'Shea, 1977). The presence of e s t e r i f i e d phosphorus and other i o n i c impurities appears to be important to the Ca binding a b i l i t y of starches. 6.4 - Adsorption of Starches onto Mineral Surfaces 6.4.1 - Preliminary Work The preliminary adsorption work was designed to show q u a l i t a t i v e l y (and p i c t o r i a l l y ) the i n t e r a c t i o n between starches and mineral surfaces. This t e s t programme was based upon the well established reaction known to occur between starch and iodine (already discussed). Using the colour developed by t h i s reaction, i t was possible to show photographically conditions f o r adsorption and non-adsorption of starches onto two d i f f e r e n t mineral surfaces, namely s i l i c a (Min-U-Sil 5) and hydroxyapatite (Tribasic Calcium Phosphate C-127, Fis h e r ) . These two powders were used instead of quartz and f l u o r a p a t i t e because of t h e i r high s p e c i f i c surface areas which made the v i s u a l impact of the colour photographs greater. These photographs are shown i n plates B, C, D and E. Plate B shows 10 t e s t tubes (a to ± ) . The caption i n p late B describes i n d e t a i l the t e s t conditions and the compositions of each solution. Test tubes a and b show s i l i c a and hydroxyapatite, respectively, subjected to a sol u t i o n of potato starch. No iodine was added i n these two t e s t tubes. Plate C shows a close-up view of the s e t t l e d s o l i d s . I t i s c l e a r that the two s o l i d s s e t t l e d under d i f f e r e n t conditions. The height of the sediment i s much smaller f o r s i l i c a than f o r hydroxyapatite, i n d i c a t i n g that the f i r s t s o l i d s e t t l e d to a compacted bed ( i . e . i t was dispersed), and the l a t t e r s e t t l e d to much l i g h t e r bed, t y p i c a l of fl o c c u l a t e d sediment. In the absence of starch both s o l i d s s e t t l e d to compact sediments, s i m i l a r to the s i l i c a sediment i n the presence of starch. Test tubes c and d i n plate B show exactly the same s i t u a t i o n encountered for t e s t tubes a and b but i n the presence of iodine. Plate D shows again close-up d e t a i l of these sediments. Adsorption of potato starch onto hydroxyapatite i n the presence of iodine caused a remarkable change i n the colour of the s e t t l e d powder (test tube d). Under the conditions of t h i s t e s t , as indicated by the t o t a l absence of any colouration i n the supernatant sol u t i o n i n t e s t tube d, there was 100% adsorption of starch on the powder. Test tube c, containing s i l i c a , depicts an opposite 2i+2 PLRTE B - INTERACTION OF POTATO STARCH,AMYLOSE AND AMYLOPECTIN WITH SILICA ANO HYDROXYAPATITE. CONDITIONS:SETTLING TIME-24HRS/pH-tO POLYMB*-«»ro/l 4ml CF KI/IODINE > lKU-.Z4m\/\ CI 2 3-.0O5mol/l TEST TUBES A- SILICA + POTATO STARCHCno Iodine) B- HYDROXYflPATITE + POT.STARCHCno Iodine) C- AS IN *R"C iodine addedJ •- AS IN -B-Ciodine added) E- SILICA + AMYLOPECTINC Iodine added) F- HYDROXYAPATITE + AMYLOPECTINCladIne added) G- SILICA + AMYLOSECiodine added) H- HYDROXYAPATITE + AMYLOSECiad Ine added) I- POTATO AMYLOSEC iodine added) J - POTATO AMYLDPECTINC iodine added) PLRTE C- DETAILS OF THE SEDIMENTS IN TEST TUBES *FT RND "B" SHOWN IN PLRTE B R - SILICR + PQTRTQ STRRCH B - HYDRQXYRPRTITE + POTRTO STRRCH 244 PLATE DETAILS OF THE SEDIMENTS IN TEST TUBES " C " FIND "D" SHOWN IN Ft_RTE 8 C THIRD TUBE CONTRIVE POT. SI A R C H + I C O I N E D C - S I L I C R + POTRTO S T R R C H C I o d I n e a d d e d 5 • - HYDROXYRPRTITE + POT .STRRCHC I o d I n e a d d e d :> 2 4 5 P L R T E E - D E T A I L S OF THE SEDIMENTS IN T E S T TUBES " G " RND I H " SHOWN IN P L R T E B Q - S I L I C R + H M Y L O S E C I o d i n e o d d e c l D H - H Y D R O X Y R P P T I T E + A M Y L O S E C I o d I n e a d d e d 5 246 s i t u a t i o n . There i s no v i s i b l e change i n the colour of the sediment but, i n turn, the supernatant solu t i o n i s blue, hence i n d i c a t i n g a non-adsorption condition. Test tubes e and f i n plate B show the same s i t u a t i o n f o r potato amylopectin. For potato amylose, t e s t tubes g and h again show s i m i l a r r e s u l t s . Plate E also shows a close-up view of the sediments i n t e s t tubes g and h from plate B. This introductory section to the adsorption t e s t programme discussed l a t e r , demonstrates some important features concerning the adsorption of starches onto mineral surfaces: (i) - starch adsorbs p r e f e r e n t i a l l y onto hydroxyapatite as compared to s i l i c a surfaces (under the conditions tested); ( i i ) - the adsorption of starch on hydroxyapatite promotes i t s f l o c c u l a t i o n . Although the photographs show only the f i n a l r e s u l t a f t e r 24 hours, the complete s e t t l i n g of hydroxyapatite p a r t i c l e s i n the t e s t tube took only a few minutes; 247 ( i i i ) - both amylose and amylopectin adsorb on hydroxyapatite and promote f l o c c u l a t i o n . 6.4.2 - Selection of the A n a l y t i c a l Procedure For the quantitative estimation of the adsorption of starches, two d i f f e r e n t a n a l y t i c a l procedures are ava i l a b l e : (i) - starch-iodine method (Cooke et a l . , 1952, Hanna, 1973; Whistler and Daniel, 1983) ( i i ) - phenol-sulphuric acid method f o r carbohydrates (Dubois et a l . , 1956; Khosla, 1983; Steenberg and Harris, 1984; S o l a r i et a l . , 1986) Both methods r e l y on colorimetric measurements. The f i r s t method has some advantages regarding handling and speed. The second method i s probably more precise since i t i s free from interferences caused by l i p i d s . Since the adsorption t e s t programme envisaged i n the present work did not involve the co-adsorption of surfactants 248 and i n view of the r e s u l t s obtained i n preliminary t e s t s described i n the next paragraph, the f i r s t method was chosen. Details on the c a l i b r a t i o n curves used f o r each starch sample tested are given i n Appendix I I I . Comparative adsorption tests involving both a n a l y t i c a l procedures are presented i n table 6.XV. These r e s u l t s demonstrated that, within the experimental error, both methods gave equivalent r e s u l t s . 6.4.3 - Adsorption Kin e t i c s Adsorption of macromolecules i s a slower process than the adsorption of low molecular weight substances. This i s p a r t i c u l a r l y true i f the molecular weight d i s t r i b u t i o n i s wide (Fleer and Lyklema, 1983), as i n the case of starches. Most research work avail a b l e i n the l i t e r a t u r e on the adsorption of starch on d i f f e r e n t mineral systems points to 1 hour e q u i l i b r a t i o n times to ensure complete adsorption (Balajee and Iwasaki, 1969; Hanna, 1973; Steenberg, 1982; Khosla, 1983; Khosla and Biswas, 1984; and Steenberg and Harris, 1984). Figure 6.46 shows the e f f e c t of time on the adsorption of potato starch on f l u o r a p a t i t e at pH 10.1. The i n i t i a l concentration of starch was 50mg/litre and a l l data points were obtained independently. Equilibrium (or TABLE 6.XV - Comparison of A n a l y t i c a l Prcoedures Commonly Used f o r the Determination of Starc h Concentrations i n Aqueous S o l u t i o n s (adsorbate-hydroxapatite^*) a t pH 10, 1 hour e q u i l i b r a t i o n time). S t a r c h Sample I n i t i a l C oncentration R e s i d u a l C o n c e n t r a t i o n (mg/1) (as % of the i n i t i a l cone.) Phenol-Sulphuric A c i d Iodine Potato Amylose 20 5.2 6.1 Potato Amylopectin 20 0.8 1.1 50 3.7 4.0 Potato S t a r c h 50 5.3 4.0 (*) The s p e c i f i c s u r f a c e area of t h i s sample i s much h i g h e r i s much higher than t h a t of the -38ym f l u o r a p a t i t e sample used f o r most of the ad s o r p t i o n testwork. CD 1.0 0.8 tr •5 0.8 W «_> a I 0.4 s a cr 0.2 FLUORflPrlTITEC Monteiro) POTATO STARCH/50mg/l A A 0.0 70 8 0 § t-4 60 jjj 40 30 ARDSORPTION • EQUILIBRIUM CONCBNTRflTION pH-10.1 20 4 io 3 10 20 30 40 60 TIMECmlnutes) 60 70 Figure 6.46 - Adsorption K i n e t i c s of Potato Starch on F l u o r a p a t i t e . 251 residual) starch concentrations are also shown i n the same diagram. From the analysis of the data presented, an e q u i l i b r a t i o n time of approx. 10 minutes i s s u f f i c i e n t to a t t a i n maximum adsorption. Without any further t e s t i n g , an e q u i l i b r a t i o n time of 1 hour was considered long enough to represent adsorption at equilibrium. 6.4.4 - Equilibrium Adsorption Tests 6.4.4.1 - Adsorption of Starches i n the Absence of Other Solutes. Figure 6.47 shows the a d s o r p t i o n i s o t h e r m s (23°C) obtained for the adsorption of potato starch, amylose and amylopectin on f l u o r a p a t i t e at pH 10.1. A l l three isotherms present a s i m i l a r shape, and a plateau region i s achieved i n a l l cases. The isotherm shape encountered, e s p e c i a l l y f o r the case of potato amylopectin, i s what i s known i n the technical l i t e r a t u r e as a h i g h - a f f i n i t y isotherm (Lipatov and Sergeeva, 1974; Fleer and Lyklema, 1983). Most polymers adsorbed onto s o l i d s display such a h i g h - a f f i n i t y isotherm, characterized by the f a c t that i n the i n i t i a l part of the isotherm, at (*) - Adsorption i s expressed i n mg per m2 of a v a i l a b l e surface. In systems such as the ones studied here where the adsorbent has a wide molecular weight d i s t r i b u t i o n these adsorption isotherms should be viewed more as "abstraction" curves, as suggested by Balajee and Iwasaki (1969). Figure 6.i+7 - Adsorption Isotherms of Potato S t a r c h , Amylose and Amylopectin on F l u o r a p a t i t e . undetectably low concentration, the adsorbed amount r i s e s steeply, whereas at higher concentrations i t reaches a (pseudo) plateau (Fleer and Lyklema, 1983). S i m i l a r l y shaped isotherms can be found i n the l i t e r a t u r e f o r the adsorption of starches onto c e r t a i n mineral surfaces (Balajee and Iwasaki, 1969; Hanna, 1973). Figure 6.47 also shows the potato amylopectin plateau i s higher than that observed f o r potato starch or amylose (the l a t t e r being the lowest). For the concentration range covered, there was no sign of an S-shaped isotherm s i m i l a r to that found by Steenberg (1982) and Steenberg and Harris (1984) fo r the adsorption of a modified potato starch on apatite at pH 7.0. These researchers obtained an isotherm characterized by a plateau region situated between 10 and 40 mg/1 (equilibrium concentration), followed by a second steep r i s e i n the amount adsorbed above 50mg/l and up to 75mg/l (equilibrium concentrations). They did not o f f e r any explanation f o r t h e i r findings. Khosla (1983) found d i f f e r e n t l y shaped isotherms i n h i s adsorption studies for starch, amylopectin and amylose on both hematite and c a l c i t e . In some instances the isotherms encountered by Khosla resemble those shown i n figure 6.47. For amylopectin and amylose adsorbed on c a l c i t e , he obtained isotherms of a very unusual shape, not c h a r a c t e r i s t i c of the adsorption of polymers on mineral surfaces. In an e a r l i e r work, Somasundaran (1969) found isotherms without a plateau region f o r h i s studies on the 254 adsorption of starch on c a l c i t e . His isotherms however bear more resemblance to the ones shown i n figure 6.47 than to those found by Khosla (1983) and Steenberg (1982). Figure 6.48 shows the adsoption isotherm for tapioca starch on flu o r a p a t i t e , also at pH 10.1. I t i s s i m i l a r i n shape to the amylopectin isotherm shown i n figure 6.47 but the maximum adsorption density (0.91 mg/m2) i s smaller then that achieved by amylopectin (0.99 mg/m2). I t i s however higher than the 0.85 mg/m2 reached by potato starch ( f i g . 6.47) . The differences both i n shape and adsorption maxima can be rela t e d to the r e l a t i v e molecular weights of the polymers. I t can also be related however to a higher a f f i n i t y displayed by tapioca starch and amylopectin f o r the surface. Somasundaran (1969) was the f i r s t to indicate that Ca i o n i c species can play a r o l e i n the adsorption of starches on Ca-bearing surfaces. The a f f i n i t y between starch and Ca has already been subject to experimental work i n the present t h e s i s . The depression i n s o l u t i o n c o n d u c t i v i t i e s observed when Ca and starch solutions are mixed together has been interpreted as evidence f o r the formation of Ca/starch complexes. Figure 6.49 shows an extra piece of evidence i n favour of Ca playing a major r o l e i n the adsorption of starch on apatite. In t h i s figure the Ca residual concentration i s p l o t t e d as a function of starch equilibrium concentration Figure 6.48 - Adsorption Isotherm of Tapioca Starch on F l u o r a p a t i t e . ) I I I I I I I I I 0 10 20 30 40 50 80 70 80 STRRCH EQ. CONCENTRATION m g / l i t r e ) Figure 6 . 4 9 - Residual Calcium Concentration upon Adsorption of Potato and Tapioca Starches on F l u o r a p a t i t e . 257 fo r t e s t s involving the adsorption of potato and tapioca starches. At low equilibrium concentrations, where the plateau regions i n both adsorption isotherms had not yet been reached, there was a decrease i n the amounts of Ca l e f t i n sol u t i o n . As the r e s i dual concentration approached plateau values i n the isotherms, the amount of Ca i n s o l u t i o n increased. Data of Somasundaran (1969) fo r the s t a r c h / c a l c i t e system are s i m i l a r to those shown i n F i g . 6.49 although h i s do not show the i n i t i a l decrease i n Ca concentration. In any case, the p a r t i c i p a t i o n of Ca i o n i c species i n the adsorption mechanism i s evident. One possible i n t e r p r e t a t i o n for the data presented i n figure 6.49 i s that at low equilibrium concentrations starch complexes Ca ions at the interface. As the concentration i s increased, the equilibrium condition i s re-established. At pH 10.1 the surface of f l u o r a p a t i t e i s negatively charged, i n d i c a t i n g that the majority of s i t e s on the surface are anionic i n character. Calcium i o n i c species at t h i s pH are present only i n the form of Ca(II) cations. They probably populate the e l e c t r i c a l double layer together with H +ions, as counter-ions. At low equilibrium (residual) starch concentrations, most of the polymer i s adsorbed. This might be the reason f o r the lowering of Ca residual concentration i n the bulk s o l u t i o n . Starch macromolecules can decrease the surface s o l u b i l i t y by blocking many of the Ca(II) surface s i t e s a v a i l a b l e . As the 258 adsorption proceeds to higher amounts adsorbed, hydrogen bonding among the many adsorbed macromolecules can provide an extra d r i v i n g force. Equilibrium conditions would then be re-established f o r Ca ions i n solutions. Figure 6.50 shows the pH dependence fo r the adsorption of tapioca starch on f l u o r a p a t i t e . Maximum adsorption i s found i n the pH range from 10 to 11. Most studies on the adsorption of starches on minerals show a s l i g h t decrease i n adsorption as the pH i s increased from 7 to 10, followed by a more d r a s t i c decrease at pH values above 11 (Somasundaran, 1969; Balajee and Iwasaki, 1969). Khosla (1983) observed decreased adsorption f o r starch on hematite as the pH was increased from 7 to 10, but d i d not observe any e f f e c t i n the case of c a l c i t e (contrary to the data of Somasundaran, 1969). The experimental facts depicted i n figure 6.50, although contrary to most accepted theories of a diminishing adsorption as the pH i s increased (as a r e s u l t of e l e c t r o s t a t i c repulsion), provide new evidence for the proposed adsorption mechanism involving Ca ions. At pH values close to the samples' pzc, minimum s o l u b i l i t y i s expected. Therefore, i f more Ca ions become avail a b l e as the pH i s raised above the pzc(pH) adsorption can indeed increase as w e l l . The eventual decrease i n adsorption i s experienced at a much higher pH. E l e c t r o s t a t i c repulsion might then be considered relevant to the o v e r a l l adsorption mechanism. Steenberg (1982) also found increased adsorption as pH was Figure 6.50 - E f f e c t of pH on the Adsorption of Tapio Starch on F l u o r a p a t i t e . ca 260 raised from 7 to 11. She d i d not comment on t h i s r e s u l t . 6.4.4.2 - E f f e c t of Ca Ions(*) As demonstrated above, Ca species play an important r o l e i n the adsorption of starches on f l u o r a p a t i t e . Tests involving the external addition of Ca ions were thereby considered important for the understanding of the r o l e of such ions i n the adsorption of starches. Figure 6.51 shows the adsorption of potato amylopectin on f l u o r a p a t i t e i n the presence of Ca ions (the adsorption isotherm i n the absence of Ca ions was replotted from Figure 6.47). The isotherms shown i n figure 6.51 demonstrate that Ca ions were capable of increasing the l e v e l of the adsorption plateau, by approximately 40%. Figure 6.52 shows a s i m i l a r s i t u a t i o n for the adsorption of tapioca starch (now i n the presence of a concentration of Ca ions ten times higher). Maximum adsorption was increased by more than 60%. The data presented i n both figures confirm the a c t i v a t i n g e f f e c t of Ca ions on the adsorption of starches. To investigate further the involvement and the role of Ca ions i n the adsorption of starches onto mineral surfaces, some adsorption isotherms were obtained f o r quartz. F i r s t l y , figure 6.53 shows the adsorption of tapioca and (*) - Care was exerted during the analyses of starch residual concentrations i n the presence of Ca ions (see appendix I I I ) . FLUORflPATITECMonteiro) POTATO RMYLOPECTIN A J EC-43mol/l CoClg • DIST.WRTB* pH-10.1 10 20 30 40 50 80 70 EQUILIBRIUM CQNCENTRflTIONCmg/1Itre) 80 Figure 6.51 - E f f e c t of Ca Ionic Species on the Adsorptic of Potato Amylopectin on F l u o r a p a t i t e . FLUORfiPflTITEC Monte Iro) 0.0' 1 1 1 1 1 1 1 1 0 10 20 30 40 50 60 70 80 EQUILIBRIUM CONCENTRRTIONCmg/1 11re) Figure 6.52 - E f f e c t of Ca Ionic Species on the Adsorption of Tapioca Starch on F l u o r a p a t i t e . 0.5 o 0.4 CD 1_ 8*0.3 2 0.2r h-& o a a: 0.1 0.0 QUARTZ A POTATO STRRCH OTAPIOCA STRRCH pH-10.1  —A, 10 20 30 40 50 80 70 EQUILIBRIUM CONCENTRRTIONCmg/ilire) 80 Figure 6.53 - Adsorption Isotherms of Tapioca and Potato Starches on Quartz. 264 potato starches on quartz at pH 10.1. The adsorption isotherms f o r f l u o r a p a t i t e obtained e a r l i e r are also replotted i n f i g . 6.53 f o r comparison purposes. The shape of the isotherms obtained for quartz i s d i f f e r e n t from that of f l u o r a p a t i t e . Also, the amount adsorbed on quartz i s very much smaller. This r e s u l t i s not s u r p r i s i n g i f one r e c a l l s the preliminary adsorption testwork (discussed i n section 6.4.1). Figure 6.54 shows the adsorption of tapioca starch on quartz with two l e v e l s of Ca ion additions. In both cases, there i s an increase i n adsorption on quartz i n the presence of Ca ions. For the highest l e v e l of additon, the isotherm displays high adsorption d e n s i t i e s , p a r t i c u l a r l y at high r e s i d u a l concentrations of starch. The data i n figures 6.53 and 6.54 manifest the importance of Ca ions i n the adsorption of starches. While a diminished e l e c t r o s t a t i c repulsion, caused by the presence of Ca ions, might be l a r g e l y responsible f o r the increased adsorption, one should also consider the considerable evidence pointing to Ca / starch s p e c i f i c i n t e r a c t i o n s . Quartz p a r t i c l e s i n the presence of 1 x 1 0 ~ 3 m o l / l i t r e Ca ions possess a surface charge le s s negative than that of f l u o r a p a t i t e i n the absence of such ions at pH 10.1 (see figures 6.9 and 6.13). Considering both surfaces equally capable of hydrogen bonding, i f e l e c t r o s t a t i c forces were the only other component to the o v e r a l l adsorption mechanism, quartz would display higher adsorption than f l u o r a p a t i t e . 2.0 © 1.6 & 1.2 s . Q 0.8 8? a 03 g0.4h 0.0 QUARTZ TRPIOCfl STRRCH CaCl2 pH-10.1 A2 EC-4)mol/l • 1 EC-3)mol/l FLUDRflPRTITE l"EC-3)CcC]2" R-UDRflPRTrrE 2 EC-45caCl2 10 20 30 40 50 80 70 80 EQUILIBRIUM CONCENTRBTIDNCmg/I!treD 00 Figure 6.54 - E f f e c t of Ca Ionic Species on the Adsorption of Tapioca Starch on Quartz. to Ul 266 The shape of the isotherms does not confirm such a postulate since adsorption isotherms on f l u o r a p a t i t e a l l depicted a very sharp increase i n the amount adsorbed f o r very low r e s i d u a l concentrations. That i s not the case f o r quartz, even i n the presence of Ca ions. Hence, there must e x i s t forces other than e l e c t r o s t a t i c and hydrogen bonding, contributing to the o v e r a l l adsorption mechanism. Most probably, the presence of Ca s i t e s i n the surface together with Ca ions at the interface, are necessary to promote high-a f f i n i t y adsorption isotherms for starch on minerals. This would explain the very noticeable p r e f e r e n t i a l adsorption of starch on f l u o r a p a t i t e as compared to quartz. This observation i s i n complete agreement with the well established r o l e of starch as a s e l e c t i v e f l o c c u l a n t f o r phosphate ores with s i l i c e o u s gangue (Coelho, 1984 and de Araujo et a l . , 1986). Data showing the increased adsorption of starch on quartz i n the presence of Ca species should also be emphasized. In a mineral pulp, the presence of ubiquitous ions i s a constant. Ca i o n i c species present i n such systems, eith e r from more soluble minerals or externally added (hard water source) can cause serious s e l e c t i v i t y problems. The presence of Ca (and other ions) might represent one of the reasons f o r the necessity of using dispersants. They would also control the t o t a l amount of dissolved cations, which could promote a loss i n s e l e c t i v i t y i n the adsorption of starch i n r e a l mineral pulps. 267 In summary, the adsorption of starches on f l u o r a p a t i t e i s dependent on the pH, starch molecular weight and presence of Ca species. The o v e r a l l adsorption mechanism i s , most probably, of a chemical nature, involving the formation of Ca/starch complexes. The formation of these complexes i s l i k e l y to be dependent on the presence of i o n i c groups on starch macromolecules. Examples are the well known impurities l i k e e s t e r i f i e d phosphate and carboxylic groups. I t i s also possible however that complexes are formed through a non-ionic i n t e r a c t i o n between starch and Ca, as proposed by some researchers (Sukan et a l . , 1979). The extent of adsorption of starch onto quartz surfaces i n the absence of foreign Ca species i s much smaller than on f l u o r a p a t i t e . Nevertheless, i n the presence of Ca ions, starch also adsorbs on the modified quartz surfaces. This can cause considerable problems i n the action of starch as a s e l e c t i v e f l o c c u l a n t for fluorapatite/quartz systems. I t i s one reason for the need for dispersants to promote a s e l e c t i v e action of starches. 6.5 - F l o c c u l a t i o n Studies: Starch as a Flocculant. 6.5.1 - Preliminary Tests Preliminary f l o c c u l a t i o n tests were set up to obtain conditions (pH and starch dosages) for the more quantitative work described i n sections 6.5.2 and 6.5.3. 268 This t e s t programme was c a r r i e d out using the sedimentation tube described e a r l i e r (section 5.3.9). Two mineral samples were used, namely k a o l i n i t e and f l u o r a p a t i t e (overflow from the beaker decantation t e s t s ) . The degree of f l o c c u l a t i o n (Fh,t) was calculated according to the equation 5.3.10. Figure 6.55 shows the degree of f l o c c u l a t i o n f o r fl u o r a p a t i t e and k a o l i n i t e (single mineral tests) i n the presence of potato starch, amylopectin and amylose at pH 10. The f i r s t s t r i k i n g f a c t shown i n Figure 6.55 i s that the degrees of f l o c c u l a t i o n of k a o l i n i t e are much smaller than those obtained for fl u o r a p a t i t e , i r r e s p e c t i v e of the starch used. The second observation deals with the r e l a t i v e f l o c c u l a t i o n power of the three starch solutions. Amylopectin exhibits the highest f l o c c u l a t i o n a c t i v i t y among the three starches, c l o s e l y followed by potato starch. Potato amylose was also able to f l o c c u l a t e f l u o r a p a t i t e but higher degrees of f l o c c u l a t i o n were reached only at higher polymer dosages. Figure 6.56 shows the e f f e c t of tapioca starch on the f l o c c u l a t i o n of f l u o r a p a t i t e and k a o l i n i t e at pH 10. The degrees of f l o c c u l a t i o n were again much higher f o r fl u o r a p a t i t e than for k a o l i n i t e . At approximately 10mg/l of tapioca starch, there was a peak i n the f l o c c u l a t i o n degree. Tapioca starch also presented a more noticeable r e - s t a b i l i z a t i o n of the suspension at high dosages as compared to potato starch, amylopectin and amylose. 0 10 20 30 40 50 60 70 LTjtCENTTTRFiTIONCmg/1 I tra) Figure 6.55 - Degree of F l o c c u l a t i o n of F l u o r a p a t i t e and K a o l i n i t e i n the Presence of Potato Starch Amylose and Amylopectin. 100-00 -80 -70 -80 ->—» l~\ CM 50 -CO >_/ l i . 40 f 30 -A 20 h 10 -o -0 Figure TAPIOCA STARCH A FLUORAPATITE + KAOLINITE pH-10 10 20 30 40 60 60 CONCEMTRATIONCmg/1 Itre) 70 80 6.^6 - Degree of F l o c c u l a t i o n of F l u o r a p a t i t e and K a o l i n i t e i n the Presence of Tapioca Starch, ro O Figure 6.57 shows the e f f e c t of pH on the f l o c c u l a t i o n of f l u o r a p a t i t e and k a o l i n i t e with 10mg/l of tapioca starch. The pH dependence of the degree of f l o c c u l a t i o n c l o s e l y follows that of the adsorption. Similar r e s u l t s were recently reported by da Luz (1987) for an apatite/starch system. An analysis of the data presented i n figures 6.55 to 6.57 provides the following points: (i) - adsorption of starch on f l u o r a p a t i t e promotes i t s f l o c c u l a t i o n ; ( i i ) - f l o c c u l a t i o n of k a o l i n i t e i s small under the conditions tested; ( i i i ) - starch concentration ranges needed to promote f l o c c u l a t i o n are s i m i l a r to those used i n the adsorption t e s t programme; (iv) - amylose (lowest molecular weight) causes les s f l o c c u l a t i o n than the other starches fo r f l u o r a p a t i t e . 6.5.2 - Graduated Cylinder Tests. Figure 6.58 shows a comparison between the technique used i n the present work (100cm3 graduated cylinders) and a more standard s e t t l i n g t e s t , generally used for thickener design (1000cm3 graduated c y l i n d e r ) . Both s e t t l i n g curves were obtained f o r f l u o r a p a t i t e pulps containing 3.9% s o l i d s Figure 6.57 - Degree of F l o c c u l a t i o n of F l u o r a p a t i t e and K a o l i n i t e as a Function of pH. 50 FLUQRfPnTITE CMontalro) ^40-I I 1 I I L 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 SETTLING TIMECmln.) Figure, 6 . 5 8 - S e t t l i n g Behaviour of F l u o r a p a t i t e Suspensions. by wt. at pH 10 i n the presence of 5 mg/1 tapioca starch. The two curves present the c h a r a c t e r i s t i c l i n e a r section for zone s e t t l i n g rates. In both cases, s e t t l i n g rates were calculated as the slope of the two l i n e a r sections and t h e i r values are also shown i n the figure. This comparison supports the v a l i d i t y of the te s t s performed, using the smaller graduated cylinders. Figure 6.59 shows a s i t u a t i o n s i m i l a r to that encountered i n F i g . 6.55. The s e t t l i n g rates f o r fl u o r a p a t i t e i n the presence of potato starch, amylopectin and amylose are shown as a function of the polymer solu t i o n concentration (in mg/1), at pH 10. The experimental curves presented i n figs.6.55 and 6.59 show impressive s i m i l a r i t y , although the techniques used were d i f f e r e n t . Under conditions as those i n f i g . 6.59, quartz and k a o l i n i t e suspensions remained t o t a l l y dispersed. Figure 6.60 shows the s e t t l i n g rates of f l u o r a p a t i t e i n the presence of tapioca and corn starches. I t also depicts the e f f e c t s of aging (in the case of tapioca starch) and shearing (in the case of corn starch). Both corn and tapioca starch additions promoted s i m i l a r l y shaped s e t t l i n g responses. Maximum s e t t l i n g rates were obtained at 15mg/l i n both cases. High l e v e l additions of the polymer, however, decreased the s e t t l i n g rates extensively. The aged tapioca starch s o l u t i o n and the sheared corn starch s o l u t i o n displayed reduced f l o c c u l a t i n g a c t i v i t i e s , a s expected. The phenomena shown i n figures 6.59 and 6.60 deserve 40 80 80 100 120 140 160 STRRCH CONCENTRATIONmg/1) 180 200 Figure 6.59 - S e t t l i n g Rate of F l u o r a p a t i t e Suspensions i n the Presence of Potato Starch,Amylose and Amylopectin. 35 FLUORRPRTITE CMonte Iro) • SHEARED CORN STRRCH • CORN STARCH + AGED TAPIOCA STRRCH OTAPIOCA STRRCH 3.0Z SOLIDS BY VT. pH-10 40 80 80 100 120 STRRCH CONCENTRRTIONCmg/l) 140 180 Figure 6 . 6 0 - Se t t l i n g Rate of Fluorapatite Suspensions i n the Presence of Corn and Tapioca Starches. ro CTv 277 close attention. During the adsorption testwork, the release of Ca upon starch adsorption was also investigated. The same supernatant samples from the adsorption t e s t s , used f o r Ca determinations, were also submitted to tranmittance measurements. Transmittance values provided a rough i n d i c a t i o n of the state of dispersion/aggregation. According to transmittance measurements(*),the best f l o c c u l a t i o n was achieved at an i n i t i a l potato starch concentration of 17.5mg/l. This corresponded to the lowest Ca l e v e l measured i n solution (24ppm). The same procedure applied to tapioca starch demonstrated that the best f l o c c u l a t i o n was again reached at an i n i t i a l concentration very close to the minimum Ca l e v e l (20mg/l with minimum Ca of 13ppm at 25mg/l). Assuming that the molecular weight of potato starch i s of the same order as that of potato amylopectin (10 m i l l i o n daltons), and that the area occupied by one starch molecule on apatite i s approx. 1.6 x 10 6A 2 (Steenberg 1982), the f r a c t i o n of the mineral surface covered by potato starch at the f l o c c u l a t i o n maximum i s estimated to be 0.391. Making s i m i l a r assumptions f o r tapioca starch, the f r a c t i o n i s then 0.577. A d i f f e r e n t value f o r the uni t area of starch i s given by Hanna (1973) at 6 x 10 4A 2 per molecule. I f t h i s (*) transmittance measurements should be more se n s i t i v e to the o v e r a l l s i z e d i s t r i b u t i o n of the f l o e s whereas s e t t l i n g rates are d i r e c t l y affected by the average f l o e s i z e . 278 value i s used i n the above cal c u l a t i o n s , r i d i c u l o u s l y small surface coverages are encountered, v i z . 0.016 for potato starch and 0.022 for tapioca starch. These l a t t e r r e s u l t s can not be considered r e a l i s t i c . In fact, i f saturation adsorption i s assumed fo r the plateau region of the adsorption isotherms of these two starches, the calculated areas per starch molecule resemble c l o s e l y that obtained by Steenberg (1982). For instance, i n the case of potato starch, at a saturation adsorption of approx. 0.84 mg/m2, the calculated u n i t area i s 1.97 x 10 6A 2 per molecule (as compared to 1.6 x 10 6 A 2 found by Steenberg). According to many researchers (see Steenberg, 1982 for a c r i t i c a l review of the l i t e r a t u r e on t h i s subject), the f l o c c u l a t i o n maximum should occur when the f r a c t i o n of the surface covered i s approx. 0.5. This value compares well with the r e s u l t s obtained above. More extensive surface coverage by polymers might lead to s t e r i c (or e l e c t r o s t e r i c ) s t a b i l i z a t i o n (Napper, 1983). In the cases of corn and tapioca starches (Fig. 6.60) there was a considerable reduction i n the s e t t l i n g rate at large starch additions. This reduction was also associated with an increase i n the v i s u a l l y assessed t u r b i d i t y of the supernatants. S i m i l a r l y shaped s e t t l i n g rate/polymer dosage curves can be found i n the l i t e r a t u r e (La Mer and Smellie, 1956 and Somasundaran et a l . , 1984). 279 6.5.3 - E f f e c t of Ca Ions. As indicated i n the pioneering work of La Mer and Smellie (1956) the presence of cations, such as Ca(II), i s necessary to produce complete f l o c c u l a t i o n of phosphate slimes. Their i n t e r p r e t a t i o n was centred on the formation of insoluble phosphate s a l t s (such as Ca phosphate) during the adsorption of a phosphorus r i c h modified potato starch (FLOCGEL). In t h e i r own words, "the effectiveness of potato starch ... was attributed to the c r o s s - l i n k i n g made possible by cations (such as Ca + +) producing insoluble phosphates with the production of a three-dimensional network and hence large f l o e s " . Inorganic e l e c t r o l y t e s can act as coagulants, promoting aggregation by reducing the e l e c t r o s t a t i c repulsion (according to the DLVO treatment). Calcium chloride indeed decreases the absolute negative value of f l u o r a p a t i t e 1 s surface charge at pH 10. Therefore, i t should not be a surprise to observe an increase i n the s e t t l i n g rate of fl u o r a p a t i t e i n the presence of Ca ions. Figure 6.61 shows exactly t h i s contribution. S e t t l i n g rates were almost doubled i n the presence of 1 x 10 J C a C l 2 . I f the assessmentt made e a r l i e r that surface coverage plays a major r o l e i n polymer f l o c c u l a t i o n , the observed increase i n s e t t l i n g rate i s probably more related to an increase i n the average f l o e (aggregate) s i z e than to increased adsorption. The curves presented i n F i g . 6.61 i n 50 FLUORAPRTITEC Monte iro) pH-10 + NO CfLCIUH RLTJED A l EC-3)mol/l CoCI 2 3.91 SOLIDS BY VT. 0 20 40 60 80 100 120 140 TRPIOCfl STRRCH CONCENTRflTIONC mg/1 3 160 F i g u r e 6.61 - E f f e c t of Ca I o n i c S p e c i e s on the S e t t l i n g of F l u o r a p a t i t e S u s p e n s i o n s i n the Presence of T a p i o c a S t a r c h . 2 8 1 the absence and presence of CaCl 2 also show the same o v e r a l l shape with diminished s e t t l i n g rates at high polymer additions. This s i m i l a r behaviour i n both systems corroborates the proposed explanation. Although adsorption of tapioca starch i s known to increase s i g n i f i c a n t l y i n the presence of Ca ions, i t does not lead to a d i f f e r e n t s e t t l i n g behaviour. The s h i f t towards higher s e t t l i n g rates with s i m i l a r l y shaped s e t t l i n g curves, shows c l e a r l y that maximum f l o c c u l a t i o n i s a function of the surface coverage. In summary, the major aspects regarding the f l o c c u l a t i o n of apatite with starches are as follows: (i) - the f l o c c u l a t i o n caused by amylose i s smaller ,for the concentration range investigated , than that imparted to f l u o r a p a t i t e by a l l other starch samples tested; ( i i ) - f l o c c u l a t i o n i s dependent on pH, presence of Ca ions and surface coverage of the polymer: ( i i i ) - Ca i o n i c species probably p a r t i c i p a t e i n the f l o c c u l a t i o n process by providing anchoring s i t e s f o r bridging of starch molecules and also by increasing the o v e r a l l i o n i c strength of the medium. (iv) - maximum f l o c c u l a t i o n i s found at surface coverage r a t i o s smaller than 0.6. 282 6.6 - M i c r o f l o t a t i o n Studies: Starch as a Depressant. 6.6.1 - F l o a t a b i l i t y of Fluorapatite i n the Absence of Starch Hallimond Tube mic r o f l o t a t i o n has been widely used when the f l o t a t i o n behaviour of pure minerals i s under inv e s t i g a t i o n . A modified version of the re-designed Hallimond tube of Fuerstenau et a l . (1957), was used i n a l l t e s t s reported herein. This technique i s very simple and can give very reproducible r e s u l t s with r e l a t i v e ease. Operational and experimental d e t a i l s are given elsewhere (Fuerstenau et a l . , 1957 and i n the Experimental chapter of t h i s t h e s i s ) . I t i s very important to recognize c l e a r l y the l i m i t a t i o n s of t h i s technique. Hallimond tube t e s t s provide the mineral processing researcher information on a r e l a t i v e l y l i m i t e d basis. I t does have advantages over other "micro" s t a t i c t e s t s such as vacuum f l o t a t i o n and bubble pick-up. However, Hallimond tube m i c r o f l o t a t i o n t e s t s give r e s u l t s that represent only a r e l a t i v e measure of the " f l o a t a b i l i t y " of a mineral under ce r t a i n pre-established and co n t r o l l e d conditions. I t i s the opinion of t h i s author that indiscriminate use of the term "recovery" i n Hallimond tube t e s t r e s u l t s may convey the wrong idea that a mineral can or can not be recovered i n a f l o t a t i o n operation under s i m i l a r conditions to the ones u t i l i z e d i n Hallimond tube t e s t s . The 283 use of a le s s compromised term such as " % F l o a t a b i l i t y " ("or % weight floated") i s preferred and should always be accompanied by information regarding the time of the m i c r o f l o t a t i o n t e s t run (and other operational d e t a i l s ) . Sixty seconds has been the most frequently used time span for Hallimond tube t e s t s . Another aspect of mi c r o f l o t a t i o n t e s t s i s f l o t a t i o n by mechanical entrainment of p a r t i c l e s , caused by a combination of factors such as a g i t a t i o n l e v e l , gas flowrate, height of the f l o t a t i o n column i n the Hallimond tube, and of course the s i z e range and s p e c i f i c gravity of the mineral p a r t i c l e s being tested. In the present work, p a r t i c l e s i n the s i z e range of -212+150 m were selected. The entrainment c h a r a c t e r i s t i c s of the tube used are shown i n Figure 6.62, for the following operational parameters: a i r flowrate = 40cm 3min - 1, room temperature (in the range of 19 to 23 degrees C), and approximately l g of mineral sample. In figure 6.62 the cumulative % f l o t a b i l i t y of f l u o r a p a t i t e (Monteiro) i n d i s t i l l e d water (pH 6) i s plo t t e d against time, up to 15 minutes. For these t e s t s the Hallimond tube was cleaned with extreme care (three times) by the n i t r i c acid/ethanol p r o c e d u r e . This procedure was normally only used once when changing reagent systems. Since most t e s t runs i n the present work lasted 60 seconds, an (*) This procedure consists of adding concentrated n i t r i c a cid to the vessel to be cleaned, followed by dropwise addition of ethanol u n t i l a strong oxidation reaction i s i n i t i a t e d (the vessel i s wetted by the reacting l i q u i d and throughly washed with a large volume of d.w.) 100 -QO -80-FLUORRPflTITEC Mon to Iro) £ 70 tr g 80- -212+150 pn o c r x L e r r a R L E S s 8 10 12 TIMECminutes) 16 18 20 Hallimond Tube Entrainment Related F l o a t a b i l i t y . to CO 285 entrainment related % f l o a t a b i l i t y of approx. 1% was considered small enough and no adjustments i n the operational conditions were made. 6.6.1.1 - Sodium Oleate Figure 6.63 shows Hallimond Tube f l o a t a b i l i t y r e s u l t s f o r f l u o r a p a t i t e (Monteiro) i n the presence of s i x d i f f e r e n t Na Oleate concentrations as a function of pH. The pH range from pH 3.5 to pH 11.5 was investigated. The concentration range for Na Oleate was from 1 x 10~ 5 m o l / l i t r e to 1 x 10~ 4 m o l / l i t r e . For the lowest concentration tested, a maximum i n the % f l o a t a b i l i t y was found at pH 7.5. The same picture was repeated f o r the two concentration l e v e l s above. At 5 x 10""5 m o l / l i t r e Na Oleate, the maximum was very broad, occupying the pH range from 7.5 to 10.5. For the highest concentration, % f l o a t a b i l i t y was almost pH-independent above pH 7.5 up to pH 11.5 (95% f l o a t a b i l i t y ) . Comparing the r e s u l t s i n Figure 6.63 with the r e s u l t s found i n the l i t e r a t u r e (see section 4.3.2), the pH range f o r maximum f l o a t i b i l i t y at the lowest c o l l e c t o r concentration was i n good agreement with most of the published data f o r the apatite/oleate system. Some researchers (e.g. Smani et a l . , 1973), found two maxima i n the microfloation t e s t s on apatites with Na Oleate. One of the two maxima was situated always at pH < 4.5, and was RVERR6E pH Figure 6.63 - M i c r o f l o t a t i o n of F l u o r a p a t i t e i n the Presence of Na Oleate as a Function of pH. 287 interpreted as r e s u l t i n g from the adsorption of o l e i c acid molecules. On the other hand, the stronger f l o t a t i o n response at neutral pH conditions i s associated with the formation of calcium oleate upon adsorption of oleate ions. This l a t t e r mechanism i s s t i l l the most accepted (Leja and Johnston, 1978; Johnston, 1969; Somasundaran et a l . , 1985). Very recently, Moudgil and co-workers (1986) presented a de t a i l e d account of the adsorption mechanism of oleate on apatite, including the i d e n t i f i c a t i o n of rate l i m i t i n g steps. Another aspect that should be pointed out i s the importance of sample h i s t o r y ( o r i g i n and preparation), e s p e c i a l l y i n the case of minerals such as apatite. This aspect can be responsible f o r s i g n i f i c a n t differences i n mi c r o f l o t a t i o n behaviour (see examples i n S i l v a et a l . , 1985). An al t e r n a t i v e explanation for the f l o t a t i o n behaviour of minerals submitted to weak e l e c t r o l y t e c o l l e c t o r s , such as Na Oleate has been recently advanced by Castro et al.,(1986),Vurdela and Laskowski (1987), and Laskowski (1987). These researchers propose that the low pH f l o a t a b i l i t y behaviour displayed by many minerals i n the presence of Na Oleate and other s i m i l a r c o l l e c t o r s , and always associated with physical adsorption of neutral o l e i c a c i d molecular species, i s better explained by the p r e c i p i t a t i o n of c o l l i d a l c o l l e c t o r species on the surfaces of the minerals i n question (also see next section f o r d e t a i l s ) . 288 6.6.1.2 - Dodecylamine Hydrochloride Cationic c o l l e c t o r s used i n phosphate f l o t a t i o n usually play the r o l e of gangue c o l l e c t o r s . They have the p o t e n t i a l f o r use as apatite c o l l e c t o r s i n some instances. In both cases, a more complete understanding of t h e i r action i n apatite systems i s important. Figures 6.64 and 6.64a (for a more de t a i l e d look at the pH range of interest) show the m i c r o f l o t a t i o n r e s u l t s obtained at s i x d i f f e r e n t concentrations of dodecylamine hydrochloride (DDAHC1), again as a function of pH. F l o t a t i o n pH i s a very important var i a b l e . I t acts on f l o t a t i o n pulps i n many ways, such as: (i) - c o n t r o l l i n g surface charge of minerals; ( i i ) - c o n t r o l l i n g the hydrolysis of reagents and ubiquitous ions; and ( i i i ) - c o n t r o l l i n g the adsorption of reagents onto mineral surfaces, thus c o n t r o l l i n g s e l e c t i v i t y . The e f f e c t of pH on the m i c r o f l o t a t i o n response of f l u o r a p a t i t e with DDAHC1 was, i n many aspects, d i f f e r e n t than that shown by Na Oleate. The ranges of concentration and pH covered i n both cases were s i m i l a r . At the lowest DDAHC1 concentration (1 x 10~ 5mol/l) tested, the f l o a t a b i l i t y was >-100 00 80 70 60 m fE 60 cc o 30 H 20 10 0 FLUORfiPRTITE(Montelro) DDFHCI Cmol/I] + I EC-6) * 3 EC-5) A 5 EC-B) D7 EC-B) 0 7.5 EC-5) • I EC-4) 0 6 AVERAGE pH 10 12 F i g u r e 6.64 - M i c r o f l o t a t i o n o f F l u o r a p a t i t e i n the Presence of DDAHC1 as a F u n c t i o n of pH. ru 00 vO 8 9 10 It 12 AVERAGE pH Figure 6.6^a - D e t a i l of the M i c r o f l o t a t i o n Response of F l u o r a p a t i t e i n the Presence of DDAHC1 as a Function of pH. 291 very small and p r a c t i c a l l y independent of pH. As the concentration was increased, f l o a t a b i l i t y also increased. At 5 x 10"* 5mol/litre c o l l e c t o r concentration f l o a t a b i l i t i e s of 70% and higher were achieved i n the pH range from 5.5 to 9.5. At the highest concentration, a 90% f l o a t a b i l i t y plateau was present from pH 4.5 to pH 10. Above pH 10, the curves for the three highest concentrations l e v e l s displayed an unusual pattern, showing i n a very narrow pH range both a maximum and a minimum. Because of p e c u l i a r i t i e s i n m i c r o f l o t a t i o n responses above pH 9, the discussion w i l l emphasize the pH range from 9 to 11.5. One of the most accepted hypotheses concerning adsorption of anionic and c a t i o n i c c o l l e c t o r s (non-thio ionizable surfactants) i s the hemi-micelle/electrostatic i n t e r a c t i o n "theory", advanced by Gaudin and Fuerstenau (1955). Since then, an enormous amount of research work has been c a r r i e d out i n many d i f f e r e n t f l o t a t i o n systems. The hypothesis assumes that the i n t e r a c t i o n between c o l l e c t o r and mineral surface occurs primarily v i a an e l e c t r o s t a t i c a t t r a c t i o n between the i o n i c polar head of a surfactant ion and oppositely charged s i t e s on the mineral surface (ions of the c o l l e c t o r would act as counter-ions i n the double l a y e r ) . Upon adsorption, a second mechanism takes place, involving interactions among the hydrocarbon chains of adsorbed c o l l e c t o r species. This i n t e r a c t i o n would lead to the formation of aggregates c a l l e d hemi-micelles at the s o l i d / l i q u i d interface. For many mineral f l o t a t i o n systems t h i s p i c t u r e f i t s well the avail a b l e experimental data (adsorption, contact angle, microelectrophoresis and mic r o f l o t a t i o n t e s t s ) . One of the best arguments i n favour of the hemi-micelle hypothesis i s the very good c o r r e l a t i o n among the experimental data (Fuerstenau, and Urbina, 1987). Later, Finch and Smith (1973) and Somasundaran (1976) advanced a new hypothesis, incorporating further experimental and t h e o r e t i c a l information on the increased surface a c t i v i t y f o r amines i n the pH range from 8 to 11. In t h i s pH range, ion-molecular complexes were supposed to form. These complexes would then explain rapid f l o t a t i o n f o r many primary amine/mineral systems under s l i g h t l y a l k a l i n e conditions. Primary amines are weak e l e c t r o l y t e s . The pKa for dodecylamine i s 10.63, i t s s o l u b i l i t y l i m i t i s 2 x 10~ 5 m o l / l i t r e and i t s CMC i s 1.3 x 10" 2 m o l / l i t r e (Somasundaran, 1976). Below the s o l u b i l i t y l i m i t no association of the aqueous species i s expected, regardless of the pH. However i n the concentration range between the s o l u b i l i t y l i m i t and the CMC, depending upon the pH, d i f f e r e n t pre-micellar associations can occur f o r dodecylamine solutions. In recent investigations by Laskowski and co-workers (Castro et a l . , 1986; Vurdela and Laskowski, 1987, Laskowski, 1987) these associative interactions among ionizable and molecular species r e s u l t i n g from hydrolysis of dodecylamine and other surfactants were extensively investigated. Some of t h e i r major findings were as follows: 100 90 80 r 70 >-3 60r i — i m £ 50 tr a DL 40 X -20 -10 8 FPRTITECItataia) } M S EC-•5)mol/l DDflHC 9 AVERAGE pH 10 11 12 Figure 6.65 - M i c r o f l o t a t i o n of A p a t i t e from I t a t a i a i n the Presence of DDAHC1 as a Function of pH. 294 (i) - dodecylamine c o l l o i d a l p r e c i p i t a t e s exhibited an iep at approx. pH 11 ( i i ) - mic r o f l o t a t i o n of quartz i n the presence of DDAHC1 at d i f f e r e n t concentrations above the s o l u b i l i t y l i m i t displayed a depression (which was concentration dependent) i n the pH range from 10 to 12. As pH was increased f l o a t a b i l i t y increased again to decrease eventually to almost n i l at very high pH values. Smith (1987) also presented s i m i l a r m i c r o f l o t a t i o n r e s u l t s f o r quartz i n the presence of dodecylamine HC1. His re s u l t s showed depression i n the pH range from 9 to 11, followed by a secondary maximum at approx. pH 12. Although these r e s u l t s are s i m i l a r to the r e s u l t s shown i n Figure 6.64, the pH range where both depression and the secondary f l o a t a b i l i t y maximum are observed i s d i f f e r e n t fo r the fl u o r a p a t i t e system. To confirm that the r e s u l t s obtained were not caused by a sample rel a t e d a r t i f a c t , another apatite sample was submitted to micr o f l o t a t i o n with DDAHC1 (5 x l O ~ 5 m o l / l i t r e ) . The r e s u l t s , shown i n Figure 6.65, confirmed the previous findings. The apatite/amine system presents an extra complicating factor: the presence of phosphate i o n i c species. 295 Soto and Iwasaki (1984 and 1986) proposed that chemisorption takes place i n primary amine/apatite systems. Their claim was supported by thermochemical measurements of heats of adsorption and s o l u b i l i t y testwork. The heats of adsorption f o r primary amines on f r a n c o l i t e surfaces at low adsorption den s i t i e s were found to be exothermic and very close to the heats of reaction of amine with phosphate ions. One possible explanation for the observed depression followed by a secondary f l o a t a b i l i t y peak at a d i f f e r e n t pH range from that reported i n the l i t e r a t u r e would c a l l for a combination of the newest in t e r p r e t a t i o n given by Laskowski and co-workers with the very pl a u s i b l e chemisorption mechanism of Soto and Iwasaki. Both the depression and the secondary f l o a t a b i l i t y peak have been interpreted as being caused by the p r e c i p i t a t i o n and redispersion of the c o l l o i d a l c o l l e c t o r species. The pH range f o r such reactions can, however, be altered. In the amine/apatite system p a r a l l e l reactions involving chemical bonding between ammonium and phosphate ions are taking place. At 5 x 10~ 5mol/litre of DDAHC1 the onset of depression i n the f l o t a t i o n of f l u o r a p a t i t e occurs at pH 9.3, whereas at the same concentration the onset of depression for quartz s t a r t s at pH 10 (Laskowski, 1987). Maximum depression for f l u o r a p a t i t e was found at pH 10.4 while f o r quartz i t occurred at pH 12.0. The secondary f l o a t a b i l i t y peak occurred at pH 10.8 f o r f l u o r a p a t i t e and at pH 12.5 for quartz. I t seems that the same phenomenon i s occurring i n both cases, 296 however at d i f f e r e n t pH values. This difference i n pH could be caused by amine chemisorption. In the presence of phosphate s i t e s the r e l a t i v e concentration of i o n i c surfactant species would decrease more r a p i d l y at a lower pH, hence the whole process was s h i f t e d towards a lower pH range. 6.6.2 - Action of Starches. As already mentioned, starches are employed to perform a v a r i e t y of d i f f e r e n t tasks i n the b e n e f i c i a t i o n of phosphate ores. One of the most common functions i s as a depressant f o r carbonates ( c a l c i t e and dolomite) and f e r r i f e r o u s (hematite, limonite, goethite, i r o n s i l i c a t e s ) gangue minerals. In t h i s section, the depressant action of starches on f l u o r a p a t i t e was tested f o r both anionic and c a t i o n i c c o l l e c t o r systems. Figure 6.66 depicts the e f f e c t of amylose, amylopectin and potato starch on the f l o a t a b i l i t y of f l u o r a p a t i t e i n the presence of 5 x 10"* 5mol/litre Na Oleate at pH 10.5. Of the two starch f r a c t i o n s , amylopectin caused stronger depression than amylose. This can be p a r t i a l l y a t t r i b u t e d to the more extensive adsorption of amylopectin on f l u o r a p a t i t e . Both potato starch and amylopectin showed s i m i l a r behaviour. The onset of constant depression occured at 10mg/l fo r amylose and at 5mg/l for amylopectin and potato starch. Experimental conditions for these t e s t s included a Figure 6.66 - E f f e c t of Potato Starch,Amylose and Amylopectin on the M i c r o f l o t a t i o n of F l u o r a p a t i t e with Na Oleate. 2 9 8 five-minute conditioning f o r starches and a one-minute fo r c o l l e c t o r s . Under such conditions (which were chosen a f t e r some preliminary r e p r o d u c i b i l i t y tests) starch adsorption might not have reached equilibrium even though the available surface area f o r the much coarser f l u o r a p a t i t e p a r t i c l e s used i n the m i c r o f l o t a t i o n t e s t s , was much smaller than i n the adsorption t e s t s . Hence, a d i r e c t comparison between the concentration f o r maximum depression obtained and that for saturation adsorption should not be attempted. Figure 6.67 shows s i m i l a r r e s u l t s f o r tapioca starch. Again strong depression occurred at very low starch additions. The lowest % f l o a t a b i l i t y i s the same as was achieved before f o r potato starch and amylopectin. At pH 7.5 the depression caused by a l l starches tested was smaller than at pH 10.5. These r e s u l t s are presented i n table 6.XVI. The observed pH dependence of the depressant action of starches i s i n l i n e with the adsorption r e s u l t s . This smaller depressant e f f e c t can also r e f l e c t the larger a f f i n i t y of Na Oleate for f l u o r a p a t i t e surfaces at pH 7.5 as compared to pH 10.5. As previously shown, both amylose and amylopectin i n t e r a c t with Na Oleate. U l t r a v i o l e t - v i s i b l e spectra of amylose and amylopectin i n the presence of iodine and Na Oleate showed increased i n t e r a c t i o n at more basic pH conditions. These findings also support the stronger depressant action at high pH values. 100 oo I 80 >. 70 h-!J 60 m £ 50 cr 3 40 * 30 20 10 0 FLUORRPHTITECMonte 1ro) IN ABSENCE OF STRRCH 1 i Na0leate-5 X lO"6mol/l • TRPIOCR STRRCH . i pH-t0.5 • . i i i • 10 20 30 STRRCH CONCENTRATIONmg/1D 40 50 Figure 6.67 - E f f e c t of Tapioca Starch on the M i c r o f l o t a t i o n of F l u o r a p a t i t e with Na Oleate. 300 With an increase i n the concentration of Na Oleate, the depression i s reduced, as shown i n Figure 6.68 for potato starch. I f one compares the r e s u l t s i n terms of collector/depressant ratios,the effectiveness of the depression caused at the higher c o l l e c t o r concentration i s many times lower. This s i t u a t i o n probably indicates that starches act as depressants not by eliminating (or decreasing) c o l l e c t o r adsorption. In fact under c e r t a i n conditions starch enhances c o l l e c t o r adsorption (Khosla, 1983) . This researcher proposes three d i s t i n c t p o s s i b i l i t i e s to explain the adsorption of starch i n the presence of surfactants: (i) - decreased c o l l e c t o r adsorption through the c l a s s i c a l competition between adsorbing species; ( i i ) - enhanced c o l l e c t o r adsorption by accomodating c o l l e c t o r within the amylose h e l i c e s ; ( i i i ) - induced co-adsorption through binary interactions. Strong depression occurs at low c o l l e c t o r concentration but the much smaller depressant action as the c o l l e c t o r concentration i s increased seems to favour ( i i ) and/or ( i i i ) above. Table 6.XVII presents the r e s u l t s for other starches at a higher c o l l e c t o r concentration (1 x 10"" 4mol/litre) . These r e s u l t s support the idea that Figure 6.68 - Depression of F l u o r a p a t i t e by Potato Starch at a Higher Na Oleate L e v e l . 302 starches depress the f l o a t a b i l i t y of f l u o r a p a t i t e not by suppressing the adsorption of the c o l l e c t o r s . Figure 6.69 shows the e f f e c t of potato starch, amylopectin and amylose on the f l o a t a b i l i t y of fl u o r a p a t i t e with dodecylamine hydrochloride (5 x 10~ 5mol/litre, pH 9.2). These r e s u l t s are s i m i l a r to those presented for Na Oleate (Fig. 6.66). However, the depression promoted by amylose i s smaller for the DDAHC1 case. This might r e f l e c t the smaller a b i l i t y of c a t i o n i c surfactants to react with amylose by being entrapped inside the h e l i c e s . Figure 6.70 shows the e f f e c t of tapioca and corn starches, which again i s si m i l a r to that caused eith e r by potato starch or potato amylopectin. The e f f e c t of increasing the c o l l e c t o r concentration i s presented i n table 6.XVIII for tapioca starch. The re s u l t s showed that at increasing DDAHC1 concentrations, the depressant action was diminished and eventually supressed completely. The r e s u l t s obtained f o r the DDAHC1 system give further support to the idea that starch does not impede c o l l e c t o r adsorption. In summary, the roles of amylose and amylopectin i n starch depressant action are probably d i f f e r e n t . Amylose reacted more r e a d i l y with surfactants, but the larger and more extensively adsorbed amylopectin macromolecule dominated the depression. Most of the starches used i n mineral processing are amylopectin-rich (potato, corn and tapioca). 303 TABLE .6.XVI E f f e c t of pH on the Depressant A c t i o n of Starches (Na Oleate - 5 x 1O"^raol/litre; Starch - 10.5mg/l) Starch % F l o a t a b i l i t y pH 7.5 PH 10.5 Potato Amylose 61 21 Potato Amylopectin 45 14 Potato Starch 53 16 Tapioca Starch 47 14 TABLE 6 . X V I I - E f f e c t of C o l l e c t o r Concentration on the Depression Caused by Starch ( s t a r c h =21 mg/1) % F l o a t a b i l i t y Starch N a 0 1 e a t e 5x10" 5mol/l 1x10~\nol/l Potato Starch 10 62 Potato Amylose 18 74 Potato Amylopectin 14 45 Tapioca Starch 10 52 pH = 10.5 80 FLUORflPflTITECMonte Iro) A -n-IN RBSBEE OF STARCH DDRHCI-5 X I0 uitol/l OP0TRT0 RMYLOSE A POTATO AMYLOPECTINI • POTATO STARCH pH-9.2 20 30 40 50 80 70 80 STARCH CONCENTRATIONCmg/l) 00 100 F i g u r e 6.69 - E f f e c t o f Pota to S t a r c h , A m y l o s e and A m y l o p e c t i n on the M i c r o f l o t a t i o n of F l u o r a p a t i t e wi th DDAHC1. 100 90 80 70 60 50 m tr tr o al * 30 -FLUORRPflTITECMonts1ro3 IN ABSENCE DF STARCH i \ COAHCI-6 X lo"bmol/l OCORN STARCH A TAPIOCA STRRCH pH-8.2 - V P ° A A A i i i i i i i i 0 10 20 30 40 50 80 70 STARCH CONCENTRATIQNCmg/lD 80 90 Figure 6.70 - Eff e c t of Tapioca and Corn Starches on the Microflotation of Fluorapatite with DDAHC1. 306 TABLE 6.XVIII E f f e c t of the C o l l e c t o r Concentration on the Depression A c t i o n of Tapioca Starch (Tapioca Starch-42mg/l; pH 9.2) DDAHCKmol/litre) % F l o a t a b i l i t y 5 x 1 0 " 5 8 1 x 1 0 - Z f 30 3 x 1 0 _ Z f 55 5 x 10"^ 100 307 The supression of hydrophobicity imparted by starches i s l i k e l y due to the a b i l i t y of adsorbed polymer molecules to i n t e r a c t with the adsorbing surfactant species i n such a manner that the hydrophobic hydrocarbon chains are occluded within the polymer molecules. Increased c o l l e c t o r addition leads to the return of hydrophobicity. 308 7 - GENERAL DISCUSSION 7.1 - The Importance of Starch Solution Preparation Research performed i n t h i s thesis gave evidence of the great importance of the preparation (and storage) of aqueous starch solutions. This aspect has been the subject of attention by many researchers. In a l l cases, three major points should be emphasized: (i) aging of the solutions, independently of the mechanisms responsible f o r i t , a f f e c t s the r e s u l t s considerably; ( i i ) - degradation of the polymer solutions (molecular weight decrease) takes place due to e i t h e r excessive a g i t a t i o n ("shearing") or sonic i r r a d i a t i o n ; ( i i i ) - cold g e l a t i n i z a t i o n i s s u f f i c i e n t to obtain good d i s s o l u t i o n of the starch samples used, however heating at 45°C of the water u t i l i z e d f or d i l u t i o n of the gel obtained by adding NaOH to the i n i t i a l starch suspension improves the qu a l i t y of the f i n a l solution. 309 In the case of aging, minute amounts of v i s i b l e p r e c i p i t a t e s are a good i n d i c a t i o n of the process taking place. The solution also becomes more tu r b i d and the t u r b i d i t y measured by l i g h t s c attering decreases. The sometimes suggested (and used) storage at low temperature has no t e c h n i c a l basis whatsoever. Retrogradation rates increase as the temperature i s decreased. Laboratory work using starch aqueous solutions must always be performed with f r e s h l y prepared (same day) solutions. This problem i s not always detected because most i n d u s t r i a l applications of starch i n mineral processing occur i n plants that run continuously. During long plant shut downs, starch solution storage tanks should be emptied and fresh solutions prepared j u s t before the plant i s re-started. Degradation by physical means i s a phenomenon that never should be overlooked. Sheared polymer solutions have t h e i r f l o c c u l a t i o n power diminished. Suitable preparative procedures are an important prerequisite i n the use of starch solutions. Cold (caustic) g e l a t i n i z a t i o n i s the preferred method fo r starch solution preparation i n mineral processing applications. One of the reasons i s simply that the processing stages are generally performed i n a l k a l i n e c i r c u i t s ( v i z . reverse c a t i o n i c f l o t a t i o n of i r o n ores and d i r e c t anionic f l o t a t i o n of phosphate ores). In t h i s way, the starch s o l u t i o n also plays ( t o t a l l y or p a r t i a l l y ) a r o l e 310 as a pH modifier. In the l a b o r a t o r y , i f short preparation times are required (or desired), heating of the d i l u t i o n water to a temperature below the g e l a t i n i z a t i o n temperature for the starch i n question, decreases the time required to obtain reproducible and well prepared solutions. In summary, the procedure employed i n the preparation and storage of starch solutions play a major role i n the r e s u l t s obtained. Although t h i s concern has been r e a l i z e d by many researchers, one s t i l l can f i n d i n the modern l i t e r a t u r e technical papers describing the use of starch solutions kept i n a r e f r i g e r a t o r f o r over a week (Somasundaran, 1968). Another even more basic f a c t concerning starches i s the type of starch used. Some authors seem to consider starch as a well defined chemical compound, with unique properties reagardless of i t s source. This, of course, i s not true. Again, the de t a i l e d description of the starch(es) employed i n any research programme i s of paramount importance. 7.2- S e l e c t i v i t y of Adsorption The adsorption t e s t programme c a r r i e d out i n the present work indicated that starches possess higher a f f i n i t y f o r apatite surfaces than f o r quartz. Although t h i s r e s u l t can not be considered as unexpected i n view of the available l i t e r a t u r e , the mechanisms responsible for t h i s behaviour deserve more attention. The r e s u l t s also showed that Ca 311 species increase considerably the adsorption of starch onto mineral surfaces. Even for quartz, on which starch was adsorbed only i n very small quantities, the presence of Ca ions increased many times the amount adsorbed. Assuming f i r s t that e l e c t r o s t a t i c forces are of primary importance, the much larger extent of adsorption of starch on apatite at pH 10 as compared to that on quartz at the same pH should not have occurred. Both minerals displayed negatively charged surfaces at t h i s pH (electrophoretic m o b i l i t i e s of -4 and -3 micron/s per cm/V f o r quartz and apatite, r e s p e c t i v e l y ) . I f only e l e c t r o s t a t i c repulsion i s considered as the reason f o r the smaller adsorption on quartz, assuming that both surfaces are equally capable of performing hydrogen bonding, then at l e a s t the shapes of the adsorption isotherms would be s i m i l a r . As demonstrated e a r l i e r , they are not s i m i l a r . The adsorption at low starch residual (equilibrium) concentrations i s very high f o r apatite. Also, the isotherms on quartz do not show a plateau region i n the range of concentration tested. The e l e c t r i c a l double layer on both surfaces should be populated by cations as counter-ions at pH 10. However, i n the case of apatite, C a 2 + ions d i s s o l v i n g from the surface should also be situated at the interface. In turn, being much less soluble, counter-ions at the quartz/ water interface should be predominantly H +. The a f f i n i t y between starch and Ca ions was demonstrated by the conductivity t e s t programme. Starch 312 molecules should possess an extra force d r i v i n g them to the apatite/water interface. This extra force i s necessary to overcome the presence of e l e c t r o s t a t i c repulsion since starch molecules carry negative charge. Thus, one of the reasons for the p r e f e r e n t i a l adsorption of starches on apatite as compared to quartz, i s probably the presence of Ca ions. This explains p a r t i a l l y why, i n the presence of Ca i o n i c species, the amount of adsorption on quartz also increases. Nevertheless, the adsorption on apatite i s also greater than that on quartz i n the presence of Ca i o n i c species because the starch macromolecules attracted to the apatite/water interface have on the surface of t h i s mineral, Ca s i t e s f o r a more stable anchoring. Therefore the extent of adsorption i s much higher i n t h i s case. Another aspect favouring the small relevance of e l e c t r o s t a t i c forces, i s that the adsorption of starches on apatite d i d not decrease as the pH was increased ( i . e . the negative value of the surface charge also increases). The increase i n pH should also be accompanied by an increase i n the concentration of Ca ions at the interface. The shape of the Ca release curves upon adsorption of starch on apatite i s another important r e s u l t . At equilibrium concentrations smaller than the concentrations required f o r the plateauing of the isotherms, there i s a considerable decrease i n the amount of Ca ions i n solution. This phenomenon might be related to the complexation of these 313 ions by starch macromolecules. One can even speculate on a mechanism of adsorption involving the p r e c i p i t a t i o n of starch/calcium complexes on the surface of apatite. The complexation properties of starches are a well known fact, e s p e c i a l l y i n the food technology l i t e r a t u r e , as already mentioned. In summary, the adsorption mechanisms of starch on apatite are dependent on: (i) - presence of Ca species both at the interface and at the surface; ( i i ) - pH; ( i i i ) - molecular weight of the starch i n question. (iv) - polymer conformation i n solu t i o n . In view of r e s u l t s obtained and discussed, the following mechanisms are proposed: a - the strongest d r i v i n g force operating i s due to the high a f f i n i t y between Ca i o n i c species and starch. The reasons for t h i s a f f i n i t y can be found i n the presence of io n i c impurities present i n the starch macromolecules, most probably those containing carboxylic and phosphate groups; b - the importance of hydrogen bonding can not be ruled out. Since the polymers are 314 contructed b a s i c a l l y from intramolecular H bonds , these forces w i l l be present also i n the o v e r a l l adsorption forces; c - the e l e c t r o s t a t i c component of the adsorption forces should be of minimal sig n i f i c a n c e , e s p e c i a l l y i f t h i s type of force i s used to explain the p r e f e r e n t i a l adsorption on apatite. d - higher adsorption at more a l k a l i n e conditions may r e f l e c t the presence of more extended macromolecules i n such solutions. 7.3 - The Role of Starches i n Apatite F l o c c u l a t i o n Systems The most i n t e r e s t i n g use f o r starches i n apatite systems i s as a s e l e c t i v e f l o c c u l a n t i n s e l e c t i v e desliming applications. A l l of the l i t e r a t u r e a v a i l a b l e on t h i s subject presents data on the successful use of starches (modified or non-modified) playing the r o l e of s e l e c t i v e f l o c c u l a n t . The systems studied involve generally phosphate ores with predominant s i l i c e o u s gangue minerals. Another constant i n such studies i s the use of an a l k a l i n e pH for maximum s e l e c t i v i t y . The r e s u l t s obtained i n the present work f u l l y support the s e l e c t i v e action of starch i n the systems described above. The adsorption of these macromolecules on 3 1 5 apatite surfaces i s much higher than that obtained on quartz and k a o l i n i t e . The pH range for maximum adsorption i s also i n l i n e with the use of a l k a l i n e pH values f o r the se l e c t i v e f l o c c u l a t i o n studies. One can then ask the question: Why hasn't s e l e c t i v e f l o c c u l a t i o n (or s e l e c t i v e desliming) encountered any i n d u s t r i a l a p p l i c a t i o n i n the treatment of phosphate ore with predominant s i l i c e o u s gangue? There are many answers to that question. F i r s t , there i s an economic reason. The implementation of a s e l e c t i v e desliming stage i n operating systems w i l l have a considerable cost. Second, and more important, s e l e c t i v e f l o c c u l a t i o n as a way to concentrate phosphate ores w i l l not work i n most cases. Although there i s considerably more adsorption on apatite, the small amount of starch adsorbing on s i l i c e o u s minerals, associated with the complicating factor of the presence of dissolved i o n i c species, hinder the achievement of high enrichment r a t i o s i n such operations. One can also r e c a l l the entrapment of dispersed phase p a r t i c l e s within the structure of the flo e s formed. A l l of these problems make the use of s e l e c t i v e f l o c c u l a t i o n , as proposed by many authors as a concentration uni t operation f o r f i n e phosphate ores, p r a c t i c a l l y impossible, unless new means are devised to separate f l o c c u l a t e d and dispersed phases. These new processes would have to minimize the physical entrapment of dispersed p a r t i c l e s into the f l o e s . The use of s e l e c t i v e desliming with starch also acting as a s e l e c t i v e f l o c c u l a n t (but without the compromise 316 of requiring high enrichment ratios) might be one possible s o l u t i o n to increase the o v e r a l l recovery of phosphate. This route was described i n d e t a i l by de Araujo et a l . (1986), who used successfully corn starch as a s e l e c t i v e f l o c c u l a n t for sedimentary phosphate ores. The process, tested only at bench scale, involved the s e l e c t i v e desliming of the ores followed by f l o t a t i o n of apatite with an anionic c o l l e c t o r . Along the same l i n e , reverse c a t i o n i c f l o t a t i o n of gangue minerals can also be used a f t e r the s e l e c t i v e desliming procedure (this second p o s s i b i l i t y i s b a s i c a l l y the same process scheme used for i r o n ores, the sole true a p p l i c a t i o n i n i n d u s t r i a l scale of s e l e c t i v e d e s l i m i n g / f l o t a t i o n ) . The other possible a p p l i c a t i o n of starches i n apatite systems i s the use of such polymers as "bulk" fl o c c u l a n t s , i n s o l i d / l i q u i d separation. Although most of the applications of starches as bulk flo c c u l a n t s have ceased since the introduction of synthetic f l o c c u l a n t s (polyacrylamides, polyacrylates, e t c . ) , they are s t i l l encountered, f o r example, i n the B r a z i l i a n i r o n ore industry. One possible d i f f i c u l t y involves the residual starch, introduced back into the c i r c u i t with the reclaimed water. As w i l l be seen i n the next section, starches also play the c r u c i a l r o l e as depressant agents f o r the d i f f i c u l t apatite/ c a l c i t e f l o t a t i o n separation. In such cases, the presence of resi d u a l starch i n the c i r c u i t might a f f e c t f l o t a t i o n performance. 317 7.4 - The Role of Starches i n Apatite F l o t a t i o n Systems One of the most challenging f l o t a t i o n separations i s that of s e l e c t i v e l y f l o a t i n g apatite from c a l c i t e and other carbonates. So f a r , t h i s has been possible on an i n d u s t r i a l scale i n very few cases, a l l of them involving well c r y s t a l l i z e d minerals, from igneous phosphate deposits. In such separations, starches play a very important r o l e - they s e l e c t i v e l y depress c a l c i t e , allowing f o r the anionic f l o t a t i o n of apatite. Plant p r a c t i c e involves an a l k a l i n e c i r c u i t (pH between 10 and 10.5) and conventional anionic c o l l e c t o r s , such as t a l l o i l . The actual r o l e of starch i n t h i s process seems to be of preventing (or diminishing) the adsorption of the c o l l e c t o r on c a l c i t e surfaces, thus leaving apatite surfaces amenable for the adsorption of c o l l e c t o r species. The r e s u l t s obtained i n the present work showed that Ca i o n i c species influenced remarkably the adsorption behaviour of starches on minerals surfaces. The s e l e c t i v e depressant action displayed by starches i n a p a t i t e / c a l c i t e systems i s i n d i r e c t evidence of the very high a f f i n i t y e x i s t i n g between Ca and starches. In the presence of a surface that has even more Ca s i t e s and Ca i o n i c species at the water interface, starch prefers the c a l c i t e surface i n comparison to the apatite surface. There must be an equilibrium condition such that the amount of starch added to the system i s s u f f i c i e n t to cover the c a l c i t e surfaces, 318 while not a f f e c t i n g apatite. Plant practice corroborates t h i s hypothesis, since an increase i n starch dosage above the id e a l l e v e l leads to a decrease i n grade of the concentrate obtained. Concerning the modes of action i n systems involving starch as a depressant, the experimental work described herein indicates that they are dependent on the pH, co l l e c t o r / s t a r c h r a t i o s and starch type. The e f f e c t of pH p a r a l l e l s the adsorption r e s u l t s , i . e . maximum depression of apatite m i c r o f l o t a t i o n i n the presence of Na Oleate occurred at an a l k a l i n e pH, close to the pH of maximum starch adsorption. At pH 7.5 a l l starches depressed apatite to a much smaller extent than at pH 10.5. These r e s u l t s contribute to an hypothesis favouring the complexation of c o l l e c t o r species at the interface, more than i n bulk so l u t i o n . The in t e r a c t i o n of starches with the c o l l e c t o r s i n the absence of the mineral also indicated a stronger i n t e r a c t i o n at a l k a l i n e pH values. The f i r s t hypothesis that maximum starch adsorption leads to increased depression should not be taken as completely satisfactory.The maximum f l o a t a b i l i t y rate for apatite i n the presence of Na Oleate occurs near neutral pH conditions. Thus, the diminished depressant action of starches at t h i s pH can r e f l e c t only the condition of the surface, more hydrophobic at pH 7.5 than at pH 10.5, i n the presence of Na Oleate. A c t u a l l y two d i f f e r e n t mechanisms are probably being superimposed i n t h i s case: maximum adsorption of the depressant does occur at an 319 a l k a l i n e pH and i t i s also at a l k a l i n e conditions where the strongest starch/surfactant i n t e r a c t i o n takes place. Both mechanisms should contribute to the r e s u l t s obtained. The second aspect, involving the r e l a t i v e amounts of starch and c o l l e c t o r species present i n the system, indicate that an increase i n the concentration of the c o l l e c t o r for a fi x e d amount of starch, leads to the r e - e s t a b l i s h i n g of f l o t a t i o n . In such a case, the most probable mechanism i n operation would c a l l for the adsorption of c o l l e c t o r species on a starch coated surface. This means that starch does not impede the adsorption of c o l l e c t o r species. Even more, the s t a r c h / c o l l e c t o r complexes formed e i t h e r i n s o l u t i o n or upon adsorption are themselves dependent on the r e l a t i v e amounts of each constituent. I t i s a reasonable assumption that part of the c o l l e c t o r reacted with the starch macromolecules stays inside the h e l i c a l portions, e s p e c i a l l y of amylose. These portions of the c o l l e c t o r can then be considered as inactive i n terms of making the surface hydrophobic. Upon increasing the amount of c o l l e c t o r , i t might s t i l l i n t e r a c t with starch species, but now, the c o l l e c t o r hydrophobic chains w i l l be a v a i l a b l e for the hydrophobization of the surface, hence f l o t a b i l i t y i s resumed. In summary, starch can act as a depressant i n both anionic and c a t i o n i c f l o t a t i o n of apatite. The depressant action i s a combination (summation) of e f f e c t s , involving the adsorption of the depressant onto the mineral surfaces and the i n t e r a c t i o n occurring between starch and surfactant 320 species. The p r e f e r e n t i a l adsorption of starches onto a s p e c i f i c mineral can be considered as more relevant to the f i n a l depressant action than the i n t e r a c t i o n between starch and c o l l e c t o r species. This idea i s supported by the fact that starch can be and i s used as a s e l e c t i v e depressant i n the f l o t a t i o n of apatite from c a l c i t e . 321 8 - CONCLUSIONS 1 - Starch samples used i n the present work were extensively characterized i n terms of molecular weight, solution preparation and storage, reaction with surfactants, reaction with Ca ions and presence of impurities. The most relevant findings were: a) - As expected, amylose has a much smaller molecular weight than amylopectin. b) - Aqueous solutions of starch can become degraded by: storing, both below and above room temperature; shearing and when subjecting to sonic baths. The degradation of these solutions can be followed by l i g h t s cattering measurements. c) - Both starch f r a c t i o n s were found to react with Na Oleate and dodecylamine hydrochloride, as indicated by UV-visisble spectroscopy i n the presence of iodine. Reaction with surfactants by amylose was explained i n terms of the entrapment of surfactant species inside the h e l i c e s of the polysaccharide. d) - Starch f r a c t i o n s and starch samples were found to react with Ca i o n i c species. This reaction was followed by the depression of 322 solution e l e c t r i c a l conductivity upon mixing Ca and starch solutions. A l k a l i n e solutions favoured a stronger reaction, e) - Impurities i n starch samples were probed by infr a r e d spectroscopy and electrophoresis. Both techniques indicate that a l l samples used contained some impurities such as proteins and f a t t y acids. Apatite samples of d i f f e r e n t o r i g i n s were characterized mainly by i n f r a r e d spectroscopy. Quartz and k a o l i n i t e were also characterized by the same technique. Dispersion and electrophoresis studies on mineral samples indicated the following important findings: a) - Fluorapatite, quartz and k a o l i n i t e displayed t h e i r highest suspension s t a b i l i t y under a l k a l i n e pH conditions. Commonly used dispersants enhanced the s t a b i l i t y of mineral suspensions under most conditions tested. b) - Presence of ubiquituous i o n i c species such as A l , Ca and Mg promoted s i g n i f i c a n t changes i n the electrophoretic mobility/pH curves f o r f l u o r a p a t i t e and quartz. Adsorption of starches onto f l u o r a p a t i t e and quartz surfaces showed the following relevant aspects: a) - Adsorption was influenced by molecular weight. Amylopectin adsorption i n a mg per unit area basis i s greater than amylose adsorption on f l u o r a p a t i t e . b) - Adsorption onto f l u o r a p a t i t e surfaces (as well as on hydroxyapatite) i s many times larger than the adsorption on quartz(as well as on s i l i c a ) . The higher a f f i n i t y of starches f o r apatite surfaces was explained i n terms of Ca/starch reactions, taking place most probably at the i n t e r f a c e . c) - Presence of externally added Ca species increased the adsorption of starches onto both apatite and quartz. d) - Moderately a l k a l i n e pH conditions enhanced starch adsorption on apatite. Fl o c c u l a t i o n of f l u o r a p a t i t e , quartz and k a o l i n i t e by d i f f e r e n t starches gave the following major findings: a) - Amylose has the smallest a b i l i t y to f l o c c u l a t e apatite suspensions as compared to a l l other starches tested. b) - As indicated by the highly p r e f e r e n t i a l adsorption of starches on apatite as compared to quartz, f l o c c u l a t i o n of apatite suspensions was very e f f e c t i v e i n the presence of these polymers. In turn, k a o l i n i t e and quartz suspensions were not affected by starches under the conditons tested. c) - Flo c c u l a t i o n of apatite by starches was enhanced i n the presence of Ca i o n i c species. d) - Maximum f l o c c u l a t i o n p a r a l l e l s adsorption i n terms of i t s pH re l a t i o n s h i p . e) - Maximum f l o c c u l a t i o n occurred under conditions where the surface coverage of apatite by starches was les s than 60%. Fl o t a t i o n of f l u o r a p a t i t e by both anionic and ca t i o n i c c o l l e c t o r s was depressed i n the presence of starches. Amylose depression was smaller than the depression caused by amylopectin and other starches. Depression by starches was a function of pH and the r e l a t i v e amount of starch and c o l l e c t o r . High pH values favoured depression while increased c o l l e c t o r additons diminished (and eventually eliminated) the depressant action of starches. This l a t t e r finding supports the idea that starch-related depression i s not caused by impeding c o l l e c t o r adsorption but rather by reacting with c o l l e c t o r species i n such way that i t no longer makes the surfaces hydrophobic. 326 9 - SUGGESTIONS FOR FURTHER WORK. Many of the findings i n the present t h e s i s warrant further research. Some suggestions are: 1 - Investigate the e f f e c t s of other i o n i c species, commonly occurring i n phosphate ore pulps, such as Mg, Fe ( f e r r i c mainly), Ba, fl u o r i d e , phosphate and sulphate, on the adsorption c h a r a c t e r i s t i c s of starches onto apatite surfaces. 2 - Investigate g r a f t i n g and chemically modifying starch polymers to make t h e i r adsorption even more s e l e c t i v e . 3 - Design ways to investigate adsorption of starches onto mineral surfaces, when two (or more) minerals are present at the same time i n a pulp. Studies of t h i s type would c l e a r l y a s s i s t the understanding of se l e c t i v e f l o c c u l a t i o n and s e l e c t i v e depression processes. Sensible carbon analysis techniques could provide the means of measuring d i r e c t l y adsorbed amounts on the minerals. 4 - Thoroughly investigate the mechanisms involved i n the c a t i o n i c f l o t a t i o n of phosphate minerals. V e r i f y the e f f e c t s of starch b u i l d up i n r e c i r c u l a t e d (recycled) water i n plant operations. Seek an even better understanding of the roles played by each starch f r a c t i o n i n both f l o c c u l a t i o n and depression. Investigate the use of small amounts of organic solvents i n the preparation of starch solutions. 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When s u b m i t t e d t o m i c r o p r o b e a n a l y s i s ( J E O L JXA-3A), e l e m e n t a l mapping o f a few g r a i n s showed t h e p r e s e n c e o f Fe i m p u r i t i e s , e v e n l y d i s t r i b u t e d o v e r t h e s u r f a c e o f each g r a i n ; p r o b a b l y p r e s e n t as a s u b s t i t u t i o n i n t h e a p a t i t e l a t t i c e . S i l i c o n was a l s o d e t e c t e d b u t no d e f i n i t i v e i n f o r m a t i o n c o n c e r n i n g i t s d i s t r i b u t i o n c o u l d be o b t a i n e d . Wet c h e m i c a l analysis(UBC-MMPE l a b o r a t o r i e s ) p r o v i d e d t h e f o l l o w i n g r e s u l t s : P 20s=36.7%; CaO=56.18%; F e ( t ) = 3 . 5 7 3 % ; A 1 2 0 3 =0.265%; S i 0 2 = 6 . 1 5 % and Mn0 2 =0.063%. 2 - APATITE FROM ONTARIO(CANADA) T h i s sample was p r e v i o u s l y u s e d i n J o h n s t o n ' s UBC(MMMPE) M.A.Sc. t h e s i s ( 1 9 6 9 ) . I t c o n s i s t e d o f m a s s i v e b l u e c o l o u r e d hand p i c k e d p i e c e s . I t was used o n l y f o r t h e i n f r a r e d s p e c t r o s c o p y t e s t w o r k . Wet c h e m i c a l a n a l y s i s (UBC MMPE l a b o r a t o r i e s ) i n d i c a t e d : P 20 5=36.20% and CaO=52.5%. 3 - TRIBASIC CALCIUM PHOSPHATE(FISHER SCIENTIFIC C-127) T h i s A.C.S. r e a g e n t has been u s e d i n t h e l i t e r a t u r e as r e p r e s e n t a t i v e o f h y d r o x y a p a t i t e . l t was u s e d f o r t h e i n f r a r e d s p e c t r o s c o p y t e s t w o r k and f o r t h e p r e l i m i n a r y a d s o r p t i o n programme.This l a t e r use was due t o i t s h i g h s p e c i f i c s u r f a c e a r e a ( a p p r o x . 16sq m/g), w h i c h made i t p o s s i b l e t o make t h e a d s o r p t i o n as p r o b e d by t h e i o d i n e c o l o u r r e a c t i o n more c l e a r l y v i s i b l e . 4 - M i n - U - S i l 5 (SILICA) T h i s h i g h p u r i t y c r y s t a l l i n e s i l i c a sample was a l s o u s e d i n t h e . p r e l i m i n a r y a d s o r p t i o n t e s t programmme i n l i e u o f q u a r t z f o r t h e same re a s o n s as s y n t h e t i c h y d r o x y p a t i t e was used. I t s a n a l y s e s and s p e c i f i c a t i o n s g i v e n by t h e s u p p l i e r a r e : S i 0 2 =99.7%; Fe 2 0 3=0.023%,L. I (650 d e g r e e s C) = 0 . 1 4 5 % ; A l 2 0 3 = 0 . 1 0 1 % ; T i O 2 = 0 . 0 1 9 % , s p e c i f i c g r a v i t y = 2 . 6 5 ; s p e c i f i c s u r f a c e area=20.6sq m/g;average p a r t i c l e s i z e = l . 9 m i c r o n ; n o n - p o r o u s . 34 5 - KAOLINITE(PIONEER AIR FLOATED KAOLIN,GEORGIA,USA) This c l a y sample has the f o l l o w i n g s p e c i f i c a t i o n s and c h e m i c a l composition g i v e n by i t s supplier:Si02=45.63% Al2O3=38.51%;Fe 2O 3=0.44%; Ti0 2=1.43%; CaO=0.24%; MgO=0.14% L . I . ( a t 1000 degrees C)=31.51%; average p a r t i c l e s i z e =1.lmicron. This sample was c h a r a c t e r i z e d by s u r f a c e area measurement(BET) and i n f r a r e d s p e ctroscopy as a l r e a d y d e s c r i b e d . APPENDIX I I - INFRARED SPECTRA OF COLLECTORS 348 The i n f r a r e d spectra (KBr p e l l e t s ) of the two c o l l e c t o r s used i n the present t h e s i s are shown i n f i g u r e II.1.The main i n f r a r e d bands are marked on the s p e c t r a . Table I I . A presents the assignments of the various ab s o r p t i o n bands observed. Both sample were found t o be f r e e of any major contaminants. A l l peaks recorded c o u l d be assigned a c c o r d i n g l y to those found i n the l i t e r a t u r e . TABLE II.A - Assignments of I n f r a r e d Bands of C o l l e c t o r s (Spectra shown i n f i g . I I . l ) Compund Peak Wavenumber(cm 1) Assignment SODIUM OLEATE 2925 and 2852 asym.and sym.C-H deformation 1562 asym. COO -vib. 1454 and 1426 sym. COO~vib. superimposed to C-H v i b . 1350-1180 COO~disturbed vib.(wagging, t w i s t i n g , e t c ) 926 out of plane OH angular def. 723 CH 2 r o c k i n g v i b . 696 =CH out of plane bending/cis form DODECYLAMINE HCl 2917 and 2849 asym. and sym. C-H deformation superimposed t o the sym. a x i a l deformation of NH, 2029 and 2010 N-H +overtone def. i n NH3 (very c h a r a c t e r i s t i c f o r primary TABLE I I . A - Continued Compund Peak Wavenumber(cm-1) 349 Assignment amine s a l t s ) asym. def. of NH3 sym. def. of NH3 CH 2 asym. bending CH3 asym.def. C-H sym. def. C-N s t r e t c h i n g v i b r a t i o n N-H +out of plane deformation 1627 1587 1475 and 1462 1402 1372 1162-946 762 Notation:asym.=asymmetric; sym.=symmetric; def.=deformation v i b . = v i b r a t i o n R e f e r e n c e s : S i l v e r s t e i n et al.(1979),Socrates(1980) and Brandao(1982). 350 F i g u r e II.1 - I n f r a r e d Spectra of:(1) - Na Oleate and (2 ) - Dodecylamine Hydrochloride. 351 APPENDIX III - CALIBRATION CURVES FOR STARCH ANALYSIS AND STARCH SOLUTION PREPARATION 1 - GENERAL The development of colour due to the i n t e r a c t i o n of iodine with starches i s well documented..For the adsorption testwork involving starches, the analysis of f i l t r a t e s ( i . e . equilibrium concentrations a f t e r adsorption) was performed using the colour development of starch solutions reacted with KI/Iodine.A Spectronic 21 U.V./VIS. spectrophotometer was used,equipped with c y l i n d r i c a l , o p t i c a l l y c l e a r , glass c e l l s with a constant l i g h t path of 10mm.The wavelength was selected i n d i v i d u a l l y for each sample,so a maximum absorbance was obtained.The colour developed by the reaction between starch and iodine i s stable for many hours,but the i n t e n s i t y i s pH dependent. Therefore the measurements were made a f t e r the f i l t r a t e s from the adsorption t e s t s were brought to a previously fi x e d pH value.In general terms, the i n t e n s i t y of the colour(measured absorbance) increased as the pH decreased. 2 - PRELIMINARY TESTWORK The standard Kl/iodine solution,prepared i n a batch of 4 l i t r e s of d i s t i l l e d water, contained 0.241 m o l / l i t r e KI and 0.005mol/litre Iodine(160g of KI and 2.538g of Iodine i n 4 l i t r e s ) . F o r each te s t , 0.4ml of the Kl/iodine solution was added to the starch solution by using a microburette(maximum capacity of 2ml).The starch t e s t s o l u t i o n had a volume of e i t h e r 25 or 50ml, as measured by a 50ml graduate cylinder. The f i r s t t e s t s involved the analysis of a freshly prepared potato starch solution, d i l u t e d to concentrations varying from 4 to lOOmg/litre. The f i r s t t e s t s o l u t i o n containing 3 0mg/litre was scanned trough the wavelength range from 550 to 660nm.The maximum absorbance was observed at 586nm and a l l other t e s t solutions were then analysed at t h i s wavelength and also at 566nm,correponding to the begining of the absorbance peak.The pH of a l l solutions was 6.0. Starch solutions were d i l u t e d to 25ml t e s t samples.A blank was prepared by adding 0.4ml of Kl/iodine to 25ml of d i s t i l l e d water.The zero absorbance was pre-established as the value obtained with d i s t i l l e d water only.Later i n t h i s t e s t programme, the zero absorbance was set with a blank containing 0.4ml of Kl/iodine solution.Preliminary r e s u l t s are shown i n table III.A and figure III.1.Inclusion of the 100mg/l r e s u l t at t h i s point was considered,but there was a drop i n l i n e a r regression c o r r e l a t i o n c o e f f i c i e n t , a s shown next: 352 a)at 566nm including 7 data points including a l l points r=0.9991 r=0.9981 b)at 586nm 7 data points a l l points r=0.9997 r=0.9988 TABLE III.A - Preliminary Results Obtained with Potato Starch Concentration (mg/1) Absorbance at 566nm at 586nm 0 0.000 0.070 4 0.056 0.119 10 0.108 0.168 16 0.168 0.229 20 0.208 0.268 30 0.297 0.362 40 0.393 0.456 100 0.886 0.939 To confirm further the r e s u l t s shown i n figure III.1,another set of samples was analysed at 586nm only.The new data are plotted i n figure III.2 together with the points from figure I I I . l . L i n e a r i t y was obtained up to 100mg/l of potato starch. The points at 150 and 180mg/l show a drop i n l i n e a r i t y f o r concentrations above 100mg/l. Also shown i n figure III.2 i s the new best straight l i n e and i t s corresponding equation and c o r r e l a t i o n coefficient.Note that the point at 100mg/l can now be safely included i n the best l i n e , s i n c e a regression analysis now indicates an increase i n the c o r r e l a t i o n c o e f f i c i e n t , i f the point at 100mg/l i s included,as follows: a) 14 data points up to 90mg/l r=0.9900 b) a l l data points up to 100mg/l r=0.9920 3 - ROUTINE ANALYSIS Having established that the technique can be used for the analysis of starch solutions up to 100mg/l(except 1.2 1.0 0.8 m CJ I 0.6 D CO m cn n 586nm A 568nm 1 - RBS-.O098CCS)+.0749 2 - fi8S=.0096CCS)T.0t07 POTATO STARCH INITIAL CALIBRATION i I L. _J I L. 0 10 20 30 40 50 60 70 80 00 100 110 120 CONCENTRATIONCmg/1 Itre) F i g u r e III . 1 - I n i t i a l C a l i b r a t i o n f o r Potato S t a r c h . 1.4 1.2 c CD OO So.8 HI CJ 0.6 m cc a in £0.4 • POTRTO STRRCH FINAL CRLIBRRTION pH=7 _ _ _ _ _ _ _ _ ' flaS-.0O88CCSD+.0878 0.2- g O.O1 0 20 40 80 80 100 120 140 180 CONCENTRATIONmg/1I t r e ) 180 200 F i g u r e I I I . 2 - F i n a l C a l i b r a t i o n f o r P o t a t o S t a r c h . 355 fo r potato amylose),a series of c a l i b r a t i o n curves for the d i f f e r e n t starch samples used was obtained. The f i r s t difference i n t h i s new serie s of tests was that blanks f o r zero absorbance consisted of Kl/iodine solutions. The instrument was externally adjusted to read zero absorbance at the desired wavelength.A second difference was that a very c a r e f u l control of pH was observed.The l i n e a r regression equations obtained for a l l starch samples used were as follows(notation=> Abs=absorbance,CS=concentration of starch i n mg/litre, r=linear regression c o r r e l a t i o n c o e f f i c i e n t ) : Potato Starch(at 586nm) Abs= 0.0024(CS) + 0.0071 r=0.999 Tapioca Starch(at 586nm) Abs= 0.0060(CS) + 0.0255 r=0.998 Potato Amylopectin(at 556nm,1ml KI/iodine) Abs= 0.0048(CS) - 0.0057 r=0.997 Potato Amylose(at 600nm up to 30mg/litre) Abs= 0.0227(CS) + 0.0029 r=0.996 More sophisticated s t a t i s t i c a l analysis of the data indicated that i n a l l cases the p a r t i t i o n i n g of the t o t a l v a r i a t i o n was such that a very s i g n i f i c a n t v a r i a t i o n was accounted f o r by the l i n e a r models and an i n s i g n i f i c a n t amount of v a r i a t i o n was due to the lack of f i t . F o r example, i n the case where the lowest c o r r e l a t i o n c o e f f i c i e n t was obtained (potato amylose) , the analysis of variances showed a sum of squares of 0.36449 for the model and 0.0029076 for the err o r , g i v i n g an F-ra t i o of 626.793. During the t e s t runs involving Ca ions, a new c a l i b r a t i o n curve was obtained f o r both potato amylopectin and tapioca starch i n the presence of these ions. In the f i r s t case the best st r a i g h t l i n e was coincident with that already mentioned.For tapioca starch however,the following c a l i b r a t i o n was used: Tapioca Starch(at 586nm and 0.OOlmol/litre Ca chloride) Abs= 0.0062(CS) + 0.0204 r=0.996 356 4 - STARCH SOLUTION PREPARATION Starch solutions were prepared d a i l y by g e l a t i n i z a t i o n with a strong NaOH soluti o n (4 g/1). The procedure u t i l i z e d was as follows: (i) - a previously weighed amount of starch grains was added to a 250ml beaker and wetted with d i s t i l l e d water under a g i t a t i o n by a magnetic s t i r r e r ; ( i i ) - to t h i s wetted suspension of starch grains , 40-50ml of a NaOH solution was added slowly. A g i t a t i o n was maintened to a l e v e l s u f f i c i e n t to promote good homogeneizing conditions; ( i i i ) - a t r a n s l u c i d gel was formed; (iv) - t h i s gel was checked v i s u a l l y for the presence of unreacted starch grains. I f grains were observed the preparation was discarded and a new batch was prepared; (v) - d i s t i l l e d water was then added to d i l u t e the gel to a pre-estabished concentration. The pH was adjusted to 7 with the addition of HCl; (vi) - the r a t i o between starch and NaOH, i n a weight basis was kept close to 4:1. APPENDIX IV - STATISTICAL TREATMENT OF DATA Most data points presented i n t h i s t h e s i s represent an average of at l e a s t two t e s t s . For the adsorption t e s t programme the scatter of the data was estimated at +/-10% f o r the worst case. In turn, f o r the f l o c c u l a t i o n and mi c r o f l o t a t i o n t e s t programmes, the deviation from the average value was situated within the +/- 5% range. The experimental approach taken i n the present th e s i s d i d not involve any s t a t i s c a l design for experiments. Selection of the t e s t conditions was based upon previous r e s u l t s , analyses of trends and of the previously established objectives. 

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