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The role of mineral surface composition and hydrophobicity in polysaccharide/mineral interactions Liu, Qi 1988

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THE ROLE OF MINERAL SURFACE COMPOSITION AND HYDROPHOBICITY IN POLYSACCHARIDE/MINERAL INTERACTIONS by Ql LIU B.A.Sc, Wuhan Inst. I & S Tech. (PRC), 1982 M.A.Sc., The Univ. of British Columbia, 1985 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Department of Mining and Mineral Process Engineering We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April 1988 © Qi Liu, 1988 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 M i n i n g and M i n e r a l Process E n g i n e e r i n g The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date A p r i l 27, 1988 A B S T R A C T The i n t e r a c t i o n s of polysaccharides (dextrin, amylopectin and carboxymethyl c e l l u l o s e (CMC)) with v a r i o u s l y modified quartz samples were investigated using f l o a t a b i l i t y , w e t t a b i l i t y , e l e c t r o k i n e t i c and adsorption t e s t s , supplemented by conventional t i t r a t i o n and i n f r a r e d spectroscopic studies. The quartz samples were treated either by methylation (rendered hydrophobic), lead coating (introduction of m e t a l l i c adsorption centres), or both forms of surface modification. The presence of metal i o n i c s i t e s on a quartz surface played a d e c i s i v e r o l e i n polysaccharide adsorption. The adsorption d e n s i t i e s of both d e x t r i n and CMC on lead-coated quartz were both much higher and much more pH-dependent than those on uncoated quartz. The "hydrophobic bonding" of dextrin with mineral surfaces as reported i n the l i t e r a t u r e was not observed with hydrophobic (methylated) quartz. However, i f the quartz contained surface lead i o n i c s i t e s and was al s o methylated, i t adsorbed more dextrin than unmethylated, lead-coated quartz. This was also true f o r the adsorption of CMC onto s i m i l a r l y modified quartz samples. To obtain a r a t i o n a l understanding of the importance of metal ions i n polysaccharide adsorption, studies of the sol u t i o n chemistry of polysaccharides and metal ions were conducted. CMC co-precipitated with both metal cations and metal hydroxy complexes, (including metal hydroxides), whereas dex t r i n co-precipitated only with metal hydroxides. C o - p r e c i p i t a t i o n i n v o l v i n g either polysaccharide caused a ii decrease i n the sol u t i o n pH. Dextrin-metal c o - p r e c i p i t a t i o n occurred at pH optima of 7.5, 8, 9, 11, and 12 for f e r r i c , aluminum, cupric, lead and magnesium ions, r e s p e c t i v e l y . Infrared spectroscopic studies of the p r e c i p i t a t e s revealed the elimination of glucose r i n g deformation, suggesting a chemical basis for the i n t e r a c t i o n between de x t r i n and metal hydroxides. The surfaces of sulphide minerals behaved l i k e hydroxide during d e x t r i n adsorption. Since copper and lead hydroxides form over d i f f e r e n t pH ranges, the pH ranges f or optimum adsorption of dextrin on copper sulphides and lead sulphides were d i f f e r e n t . The r e s u l t s of preliminary f l o t a t i o n t e s t s i n d i c a t e d that d e x t r i n could be u t i l i z e d i n the d i f f e r e n t i a l f l o t a t i o n of Cu-Pb sulphides. Small scale f l o t a t i o n t e s t s conducted on synthetic mixtures of chalcopyrite and galena confirmed t h i s point. iii TABLE OF CONTENTS Abstract i i Table of Contents i v L i s t of Tables v i L i s t of Figures v i i Acknowledgement x i v Chapter 1 INTRODUCTION 1 Chapter 2 SCOPE AND APPROACH 3 Chapter 3 LITERATURE REVIEW 5 3.1 Chemistry of Starch and Ce l l u l o s e 5 3.1.1 Structure 5 3.1.2 Molecular Weight 7 3.1.3 Chemical R e a c t i v i t y 10 3.2 Depressant Function 13 3.2.1 Starches and Starch Derivatives 13 3.2.2 C e l l u l o s e Derivatives 16 3.3 A c t i v a t i o n Behaviour 18 3.4 Adsorption 20 3.4.1 Mechanisms of Adsorption 20 3.4.2 Adsorption Isotherms 26 3.4.3 Conformation of Adsorbed Polysaccharides 28 3.4.4 E f f e c t of Surface Hydrophobicity on Polysaccharide Adsorption 31 3.5 Surface Properties of Quartz 35 3.5.1 Modifications of the Quartz Surfaces .... 37 3.5.2 A p p l i c a b i l i t y i n the Present System 46 3.6 Summary 48 Chapter 4 MATERIALS 50 4.1 Mineral Samples 50 4.1.1 Quartz Samples 50 4.1.2 Other Mineral Samples 57 4.2 Chemical Reagents 58 4.2.1 Polysaccharides 58 4.2.2 Other Chemicals 69 Chapter 5 EXPERIMENTAL METHODS 71 5.1 Hallimond Tube F l o t a t i o n Tests 71 5.2 Contact Angle Measurements 72 5.3 Co-P r e c i p i t a t i o n Tests 73 5.4 T i t r a t i o n Analysis 74 5.5 Infrared Spectroscopic Studies 74 iv 5.6 Adsorption tests 75 5.7 Electrophoretic Measurements 76 Chapter 6 RESULTS AND DISCUSSION ' 77 6.1 Depressive E f f e c t of the Polysaccharides 77 6.1.1 F l o t a t i o n Experiments 77 6.1.2 Contact Angle Measurements 87 6.1.3 Summary 94 6.2 Aggregation i n Polysaccharide-Metal Aqueous Solutions 97 6.2.1 Co - p r e c i p i t a t i o n Tests 97 6.2.2 Summary 108 6.3 The Nature of Polysaccharide/Metal Hydroxy Complex Interactions 108 6.3.1 Dextrin/Metal Hydroxide Interactions ... 109 6.3.2 Amylopectin/Lead Hydroxide Interactions 128 6.3.3 CMC/Metal Interactions 131 6.3.4 Summary 135 6.4 Adsorption of Dextrin and CMC 137 6.4.1 Adsorption of Dextrin 138 6.4.2 Adsorption of CMC 147 6.4.3 Discussion of the Adsorption Behaviour . 151 6.4.4 Summary 162 6.5 Examples of an A p p l i c a t i o n : Sulphide F l o t a t i o n . 163 Chapter 7 CONCLUSIONS 172 Chapter 8 SUGGESTIONS FOR FURTHER WORK 177 REFERENCES 179 APPENDIX I. Determination of Polysaccharide Concentration and Adsorption 192 APPENDIX I I . Logarithmic Concentration Diagrams for Lead Nitrate-Water Systems 203 APPENDIX I I I . O p t i c a l Rotatory Dispersion Measurements 207 APPENDIX IV. Thermodynamic Calculations of Dextrin-Pb + + and Dextrin-Lead Hydroxide Interactions 212 APPENDIX V. C a l c u l a t i o n of the pH Drop i n the T i t r a t i o n of Lead N i t r a t e with CMC 216 APPENDIX VI. The E f f e c t of Dextrin on the Hydrolysis of Litharge 218 APPENDIX VII. The Intensity of IR absorption Bands of Dextrin i n Dextrin-Lead hydroxide P r e c i p i t a t e s 220 v L I S T O F T A B L E S Table 2.1 - D i f f e r e n t modifications of quartz samples 4 Table 3.1 - E q u i l i b r a t i o n times used for polymer adsorption 29 Table 4.1 - Impurity l e v e l s i n the polysaccharides, % 65 Table 4.2 - Chemicals used i n the experiments 70 Table 6.1 - Dextrin-lead p r e c i p i t a t e s , method a 117 Table 6.2 - Dextrin-lead p r e c i p i t a t e s , method b 117 vi LIST OF FIGURES F i g . 3.1 - Structure of D-glucose (from Leja, 1982) 6 F i g . 3.2 - Schematic representation of the reaction between adjacent hydroxyl groups and cuprammonium (from Davidson, 1967). 12 F i g . 3.3 - O r i g i n of the e l e c t r i c a l charge at the quartz/aqueous s o l u t i o n i n t e r f a c e (from Gaudin and Fuerstenau, 1955) 36 F i g . 3.4 - End-groups on s i l i c a surfaces (from Boehm, 1966) 37 F i g . 3.5 - Schematic i l l u s t r a t i o n of the general electrophoretic m o b i l i t y behaviour of c o l l o i d a l systems i n the presence or absence of hydrolyzable metal ions (from James and Healy, 1972). 41 F i g . 3.6 - Mechanisms of quartz a c t i v a t i o n by metal ions (from - Fuerstenau and Palmer, 1976) 43 F i g . 4.1 - Infrared spectrum of a t y p i c a l quartz sample 52 F i g . 4.2 - E f f e c t of pH on the zeta p o t e n t i a l s of t y p i c a l quartz samples 54 F i g . 4.3 - E f f e c t of pH on the Hallimond tube f l o t a t i o n of t y p i c a l quartz samples. Conditions: 2 grams of -150+400# quartz i n a 200 ml solution 55 F i g . 4.4 - E f f e c t of sodium oleate concentration on the Hallimond tube f l o t a t i o n of PbQ. Conditions: 2 grams of -150+400* quartz i n a 200 ml solu t i o n 56 F i g . 4.5 - V i s c o s i t y of aqueous NaCMC solutions 59 F i g . 4.6 - V i s c o s i t y of aqueous dex t r i n solutions 60 F i g . 4.7 - V i s c o s i t y of aqueous amylopectin solutions 61 F i g . 4.8 - T i t r a t i o n of aqueous NaCMC and de x t r i n solutions with sodium hydroxide 63 F i g . 4.9 - T i t r a t i o n of aqueous NaCMC and de x t r i n solutions with hydrochloric a c i d 64 F i g . 4.10 - Infrared spectra of the polysaccharides 66 F i g . 4.11 - Schematics of the v i b r a t i o n s of the glucose r i n g . A). asymmetric and B). symmetric (from Barker et a l , 1954) 67 vii F i g . 6.1 - E f f e c t of pH on the f l o t a t i o n of PbQ i n the presence or absence of NaCMC. Conditions: 2 grams of -150+400* quartz i n a 200 ml so l u t i o n . Sodium oleate: 2x10- 5 M 78 F i g . 6.2 - E f f e c t s of pH and NaCMC on the Hallimond tube f l o t a t i o n of MQ and MPbQ. Conditions: 2 grams of -150+400# quartz i n a 200 ml so l u t i o n 79 F i g . 6.3 - E f f e c t of NaCMC concentration on the Hallimond tube f l o t a t i o n of quartz samples. Conditions: 2 grams of -150+400# quartz i n a 200 ml solution 80 F i g . 6.4 - E f f e c t s of pH and various reagents on the Hallimond tube f l o t a t i o n of PbQ. Conditions: 2 grams of -150+400# quartz i n a 200 ml solu t i o n 82 F i g . 6.5 - E f f e c t of de x t r i n concentration on the Hallimond tube f l o t a t i o n of PbQ. Conditions: 2 grams of -150+400# quartz i n a 200 ml solu t i o n 83 F i g . 6.6 - E f f e c t s of pH and 100 ppm de x t r i n on the Hallimond tube f l o t a t i o n of MQ and MPbQ. Conditions: 2 grams of -150+400* quartz i n a 200 ml solu t i o n 84 F i g . 6.7 - E f f e c t of de x t r i n concentration on the Hallimond tube f l o t a t i o n of MQ and MPbQ. Conditions: 2 grams of -150+400* quartz i n a 200 ml so l u t i o n 85 F i g . 6.8 - E f f e c t s of pH and amylopectin on the Hallimond tube f l o t a t i o n of PbQ. Conditions: 2 grams of -150+400* quartz i n a 200 ml solution 88 F i g . 6.9 - E f f e c t of amylopectin concentration on the Hallimond tube f l o t a t i o n of PbQ at pH 11. Conditions: 2 grams of -150+400* quartz i n a 200 ml solu t i o n 89 F i g . 6.10 - E f f e c t s of pH and 100 ppm amylopectin on the Hallimond tube f l o t a t i o n of MQ and MPbQ. Conditions: 2 grams of -150+400* quartz i n a 200 ml solu t i o n 90 F i g . 6.11 - E f f e c t of amylopectin concentration on the Hallimond tube f l o t a t i o n of MQ and MPbQ. Conditions: 2 grams of -150+400* quartz i n a 200 ml solu t i o n 91 F i g . 6.12 - E f f e c t s of pH and depressants on the w e t t a b i l i t y of MQ plates 92 F i g . 6.13 - Ef f e c t s of pH and depressants on the w e t t a b i l i t y of MPbQ plates 93 viii F i g . 6.14 - E f f e c t s of pH and various reagents on the w e t t a b i l i t y of PbQ p l a t e s . The plates were f i r s t treated with 2x10- 5 M sodium oleate, then with 100 ppm polymer 95 Fi g . 6.15 - E f f e c t s of pH and various reagents on the w e t t a b i l i t y of PbQ pl a t e s . The plates were f i r s t treated with 100 ppm polymer, then with 2x10- 5 M sodium oleate. 96 Fi g . 6.16 - E f f e c t of pH on the r e s i d u a l concentrations of NaCMC and lead, a f t e r c e n t r i f u g i n g at 8000 rpm for 3 minutes. Residual concentration of NaCMC: 1, NaCMC sol u t i o n ; 2, NaCMC-lead n i t r a t e s o l u t i o n . Residual concentration of lead: 3, lead n i t r a t e s o l u t i o n ; 4, lead nitrate-NaCMC s o l u t i o n 99 Fi g . 6.17 - E f f e c t of pH on the r e s i d u a l concentrations of amylopectin and lead, a f t e r c e n t r i f u g i n g at 8000 rpm for 3 minutes. Residual concentration of amylopectin: 1, amylopectin solution; 2, amylopectin-lead n i t r a t e s o l u t i o n . Residual concentration of lead: 3, lead n i t r a t e solution; 4, lead nitrate-amylopectin s o l u t i o n 101 Fi g . 6.18 - E f f e c t of pH on the r e s i d u a l concentrations of dex t r i n and lead, a f t e r c e n t r i f u g i n g at 8000 rpm for 3 minutes. Residual concentration of dextrin: 1, de x t r i n s o l u t i o n ; 2, dextrin-lead n i t r a t e s o l u t i o n . Residual . concentration of lead: 3, lead n i t r a t e s o l u t i o n ; 4, lead n i t r a t e - d e x t r i n s o l u t i o n 102 Fi g . 6.19 - E f f e c t of pH on the r e s i d u a l concentrations of de x t r i n and aluminum, a f t e r c e n t r i f u g i n g at 8000 rpm for 3 minutes. Residual concentration of dextrin: 1, dext r i n s o l u t i o n ; 2, dextrin-aluminum n i t r a t e s o l u t i o n . Residual concentration of aluminum: 3, aluminum n i t r a t e s o l u t i o n ; 4, aluminum n i t r a t e - d e x t r i n s o l u t i o n 103 6.20 - E f f e c t of pH on the r e s i d u a l concentrations of dex t r i n and magnesium, a f t e r c e n t r i f u g i n g at 8000 rpm for 3 minutes. Residual concentration of dextrin: 1, dext r i n s o l u t i o n ; 2, dextrin-magnesium c h l o r i d e s o l u t i o n . Residual concentration of magnesium; 3, magnesium ch l o r i d e solution; 4, magnesium ch l o r i d e - d e x t r i n s o l u t i o n 104 Fi g . 6.21 - E f f e c t of pH on the r e s i d u a l concentrations of de x t r i n and ir o n , a f t e r c e n t r i f u g i n g at 8000 rpm for 3 minutes. Residual concentration of dextrin: 1, dext r i n s o l u t i o n ; 2, d e x t r i n - f e r r i c c h l o r i d e s o l u t i o n . Residual concentration of i r o n : 3, f e r r i c chloride s o l u t i o n ; 4, f e r r i c c h l o r i d e - d e x t r i n s o l u t i o n 105 Fi g . ix F i g . 6.22 - E f f e c t of pH on the r e s i d u a l concentrations of d e x t r i n and copper, a f t e r c e n t r i f u g i n g at 8000 rpm for 3 minutes. Residual concentration of de x t r i n : 1, d e x t r i n s o l u t i o n ; 2, dextrin-cupric c h l o r i d e solution. Residual concentration of copper: 3, cupric chloride s o l u t i o n ; 4, cupric c h l o r i d e - d e x t r i n solution 106 F i g . 6.23 - C o r r e l a t i o n between the i.e.p. of metal hydroxides and the pH of maximum c o - p r e c i p i t a t i o n i n metal-dextrin solutions 107 F i g . 6.24 - E f f e c t of dex t r i n addition on e l e c t r i c a l conductivity and pH changes i n 10-3M lead n i t r a t e solutions. 200 ml d e x t r i n solutions of d i f f e r e n t concentrations were prepared and kept at the same pH, then 66.25 mg lead n i t r a t e was added and the f i n a l pH and conductivity were recorded. Series one involved 200 ml of d e x t r i n solution plus 0.5 ml of 1 N NaOH: 1, i n i t i a l pH; 2, f i n a l pH a f t e r a d d i t i o n of lead n i t r a t e ; 3, f i n a l conductivity a f t e r a d d i t i o n of lead n i t r a t e . Series two involved 200 ml of dex t r i n s o l u t i o n at a natural pH of 6: 4, f i n a l c onductivity a f t e r a d d i t i o n of lead n i t r a t e 110 F i g . 6.25 - The e f f e c t of dextrin concentration on the e l e c t r i c a l c onductivity of dextrin-lead n i t r a t e solutions. 0.5 ml of 1 N NaOH was added to 200 ml d e x t r i n solution, followed by the addition of 66.25 mg lead n i t r a t e . Cf: f i n a l c onductivity a f t e r the addition of lead n i t r a t e ; C i : i n i t i a l conductivity before the add i t i o n of lead n i t r a t e .112 F i g . 6.26 - T i t r a t i o n of lead n i t r a t e solutions with 10 g/1 d e x t r i n . T i t r a t e volume: 100 ml 113 F i g . 6.27 - T i t r a t i o n of 0.1 M lead n i t r a t e with 10 g/1 d e x t r i n at pH 11.2. T i t r a t e volume: 100 ml 114 F i g . 6.28 - Infrared spectra of lead hydroxide 116 F i g . 6.29 - Infrared spectra of PX1, PX3 and PX5 (for sample designation see Table 6.1). 118 F i g . 6.30 - Infrared spectra of PX2, PX4 and PX6 (for sample designation see Table 6.1) 119 F i g . 6.31 - Infrared spectra of PXS1, PXS3 and PXS5 (for sample designation see Table 6.2) 120 F i g . 6.32 - Infrared spectra of PXS2, PXS4 and PXS6 (for sample designation see Table 6.2) 121 x F i g . 6.33 - T i t r a t i o n of 0.1 M f e r r i c c h l o r i d e with 10 g/1 dex t r i n at pH 8. T i t r a t e volume: 100 ml 126 F i g . 6.34 - T i t r a t i o n of 0.1 M cupric chloride with 10 g/1 dex t r i n at pH 8. T i t r a t e volume: 100 ml 127 F i g . 6.35 - T i t r a t i o n of 0.1 M lead n i t r a t e with 5 g/1 amylopectin at pH 11.2. T i t r a t e volume: 100 ml 129 F i g . 6.36 - Infrared spectra of PAMY1 and PAMY2. Amylopectin/lead r a t i o s (w/w): PAMY1, 1:10 and PAMY2, 2.5:1 130 F i g . 6.37 - T i t r a t i o n of 0.1 M lead n i t r a t e with 10 g/1 NaCMC at pH 4.3. T i t r a t e volume: 100 ml 132 F i g . 6.38 - Infrared spectra of PCTL and PCTH. PCTL: p r e c i p i t a t e from test #1, F i g . 6.37; PCTH: p r e c i p i t a t e from the t i t r a t i o n of 0.1 M lead n i t r a t e with CMC at pH 11.2 134 F i g . 6.39 - T i t r a t i o n of 0.1 M lead n i t r a t e with 10 g/1 amylose and 0.01 M sodium laurate at pH 11.2. T i t r a t e volume: 100 ml. .136 F i g . 6.40 - E f f e c t of pH on the adsorption of d e x t r i n on quartz and PbQ. I n i t i a l concentration of dextrin was 50 ppm. 1, quartz; 2, PbQ; 3, PbQ reacted f i r s t with 25 ml 2xl0- 4 M sodium oleate 139 F i g . 6.41 - E f f e c t of pH on the adsorption of d e x t r i n on MQ and MPbQ. I n i t i a l concentration of dext r i n was 50 ppm. 1, MQ; 2, MPbQ; 3, MPbQ reacted f i r s t with 25 ml 2x10-* M sodium oleate 141 F i g . 6.42 - Adsorption isotherms of dext r i n on d i f f e r e n t quartz samples. 1, quartz, pH 10.56; 2, MQ, pH 10.56; 3, PbQ, pH 10.88; 4, MPbQ, pH 10.90; 5, PbQ reacted f i r s t with 2xl0-* M sodium oleate, pH 10.72; 6, MPbQ reacted f i r s t with 2x10- 4 M sodium oleate, pH 10.41 142 F i g . 6.43 - E f f e c t of pH on the adsorption of d e x t r i n on d i f f e r e n t oxides. Conditions: i n i t i a l concentration of dextrin, 100 ppm; 1 gram of -400# oxide i n a 50 ml solut i o n . ..144 F i g . 6.44 - E f f e c t of pH on the adsorption of d e x t r i n on d i f f e r e n t sulphides. Conditions: i n i t i a l concentration of dextrin, 50 ppm; 1 gram of -400# sulphide i n a 50 ml so l u t i o n . 145 F i g . 6.45 - E f f e c t of pH on the adsorption of NaCMC on d i f f e r e n t quartz samples. Conditions: 1 gram of -400# quartz i n a 50 ml solu t i o n 148 x i F i g . 6.46 - E f f e c t of pH on the adsorption of NaCMC on PbQ. Conditions: 1 gram of -400# quartz i n a 50 ml so l u t i o n 149 F i g . 6.47 - Adsorption isotherms of NaCMC on quartz samples. Conditions: 1 gram of -400# quartz i n a 50 ml solu t i o n 150 F i g . 6.48 - E f f e c t of pH on the adsorption of NaCMC on hematite and graphite. Conditions: 1 gram of -400# hematite or 0.5 gram of -400# graphite i n a 50 ml solution 152 F i g . 6.49 - E f f e c t of pH on the zeta p o t e n t i a l of d e x t r i n c o l l o i d s dispersed i n aqueous solutions 154 F i g . 6.50 - E f f e c t of pH on the zeta p o t e n t i a l of l i t h a r g e i n the presence or absence of either NaCMC or d e x t r i n . Conditions: 50-mg of -400# l i t h a r g e i n a 50 ml s o l u t i o n . ...156 F i g . 6.51 - E f f e c t of pH on the zeta p o t e n t i a l of PbQ i n the presence or absence of either NaCMC or d e x t r i n . Conditions: 50 mg of -400# PbQ i n a 50 ml solu t i o n 158 F i g . 6.52 - E f f e c t of pH on the zeta p o t e n t i a l of quartz i n the presence or absence of either NaCMC or d e x t r i n . Conditions: 50 mg of -400# quartz i n a 50 ml solu t i o n 159 F i g . 6.53 - E f f e c t of dex t r i n concentration on the zeta p o t e n t i a l s of d i f f e r e n t s o l i d samples 160 F i g . 6.54 - E f f e c t of NaCMC concentration on the zeta p o t e n t i a l s of d i f f e r e n t s o l i d samples 161 F i g . 6.55 - E f f e c t of pH on the Hallimond tube f l o t a t i o n of galena i n the presence or absence of de x t r i n . Conditions: 2 grams of -150+400# galena i n a 200 ml solu t i o n 164 F i g . 6.56 - E f f e c t of pH on the Hallimond tube f l o t a t i o n of chalcopyrite i n the presence or absence of d e x t r i n . Conditions: 2 grams of -150+400# chalcopyrite i n a 200 ml sol u t i o n 166 F i g . 6.57 - E f f e c t of dextrin concentration on the f l o t a t i o n of sulphides. Dextrin was added p r i o r to xanthate. 2 grams of -150+400# sulphide i n a 200 ml solu t i o n were u t i l i z e d f o r each experiment 167 F i g . 6.58 - E f f e c t of pH on the d i f f e r e n t i a l f l o t a t i o n of a 1:1 mixture of galena and chalcopyrite. Dextrin was added p r i o r to xanthate. Conditions: 2 grams of -150+400# sulphide mixture i n a 200 ml solution 169 xii F i g . 6.59 - E f f e c t of pH on the d i f f e r e n t i a l f l o t a t i o n of a 1:1 mixture of galena and chalcopyrite. Dextrin was added a f t e r xanthate. Conditions: 2 grams of -150+400# sulphide mixture i n a 200 ml solution 170 F i g . A l - E f f e c t of phenol add i t i o n on the absorbance of d e x t r i n and CMC solutions using the sulphuric acid-phenol method ....193 F i g . A2 - E f f e c t of the wavelength of the l i g h t source on the absorbance of CMC solutions 194 F i g . A3 - C a l i b r a t i o n curves of dextrin and CMC 195 F i g . A4 - K i n e t i c s of dext r i n adsorption 199 F i g . A5 - K i n e t i c s of CMC adsorption 200 F i g . A6 - Logarithmic concentration diagram of 10- 3 M lead n i t r a t e aqueous solution 204 F i g . A7 - Logarithmic concentration diagram of 10- 2 M lead n i t r a t e aqueous solution 205 F i g . A8 - Logarithmic concentration diagram of 10- 1 M lead n i t r a t e aqueous solution 206 F i g . A9 - O p t i c a l rotatory d i s p e r s i o n of dext r i n aqueous s o l u t i o n .208 F i g . A10 - O p t i c a l rotatory d i s p e r s i o n of dextrin-cupric c h l o r i d e aqueous s o l u t i o n 209 F i g . A l l - O p t i c a l rotatory d i s p e r s i o n of dextrin-lead n i t r a t e aqueous sol u t i o n 210 F i g . A12 - O p t i c a l rotatory d i s p e r s i o n of d e x t r i n - f e r r i c c h l o r i d e aqueous sol u t i o n 211 F i g . A13 - The e f f e c t of de x t r i n on the hydrolysis of l i t h a r g e (PbO). Conditions: one gram of -400# l i t h a r g e i n a 100 ml solut i o n 219 xiii ACKNOWLEDGEMENT F i r s t of a l l , I acknowledge my deep gratitude and l a s t i n g indebtedness to my supervisor, Dr. J . S. Laskowski, for h i s a l l round guidance and encouragement during the course of t h i s work. My gratitude extends to the members of my supervisory committee, Dr. G. G. S. Dutton (Chemistry), Dr. G. W. Poling and Professor A. L. Mular (Mining and Mineral Process Engineering), for t h e i r h e l p f u l c r i t i c i s m and suggestions concerning t h i s research. I appreciate the useful discussions with Drs. G. D. Senior (now with CSIRO, A u s t r a l i a ) and A. C. de Araujo (now with UFMG, B r a z i l ) , who were my fellow graduate students during my stay at UBC. I would a l s o l i k e to thank the tec h n i c a l s t a f f i n the Department of Mining and Mineral Process Engineering (UBC), e s p e c i a l l y to Mrs. S a l l y Finora, Mr. Frank Schmidiger and Mr. Pius Lo, for t h e i r assistance at various stages of t h i s work. I am g r a t e f u l to Mr. Robert Boissy (Biochemistry, UBC) for h i s valuable help i n e d i t i n g and proof reading the t h e s i s . I must not f a i l to mention my devoted wife X i a o l i , who kept on l o v i n g l y waiting f o r me during my long absence from home. Without her enduring patience, understanding and support t h i s work would never have been f i n i s h e d . To my wife and our lovel y daughter I dedicate t h i s d i s s e r t a t i o n and express my profoundest debts. xiv CHAPTER 1 INTRODUCTION Starch, c e l l u l o s e , and r e l a t e d polysaccharides - are organic polymers that have found widespread a p p l i c a t i o n i n the mineral process industry. The i n i t i a l use of polysaccharides i n mineral f l o t a t i o n dates back to 1931, when Lange patented starch as a s e l e c t i v e depressant i n the c a t i o n i c f l o t a t i o n of quartz from phosphate.t Starch has since been applied i n the f l o t a t i o n separation of quartz/hematite, salt-type minerals and sulphide bulk concentrate. It has also been used to depress the f l o t a t i o n of a number of inherently hydrophobic minerals. A starch d e r i v a t i v e , d extrin, has been found even more e f f e c t i v e as a f l o t a t i o n depressant, e s p e c i a l l y for the depression of inherently hydrophobic minerals. Carboxymethyl c e l l u l o s e (CMC), a commonly used d e r i v a t i v e of c e l l u l o s e , has been applied i n the f l o t a t i o n separation of s u l p h i d e / s i l i c a t e , c o a l / p y r i t e and sulphide bulk concentrates. Despite the widespread a p p l i c a t i o n s , the polysaccharide depressants are s t i l l i n a r e l a t i v e l y e arly stage of serious research and developement. The nature of the i n t e r a c t i o n between fReferences have been omitted in this Chapter for s impl ic i ty . They appear in the Chapter "Literature Review" 1 INTRODUCTION / 2 polysaccharides and mineral surfaces remains unclear. A general b e l i e f i s that the adsorption of such a polymer i s through non-selective hydrogen bonding, although chemical i n t e r a c t i o n has o c c a s i o n a l l y been suggested. However, recent evidence indicates that the adsorption of polysaccharides can r e s u l t from quite a d i f f e r e n t mechanism. For example, d e x t r i n has been reported to adsorb e x c l u s i v e l y on hydrophobic minerals, whether inherently hydrophobic or rendered hydrophobic with surfactants. Such observations led to the "hydrophobic bonding" theory. Dextrin has b a s i c a l l y the same chemical composition as starch. The d i f f e r e n c e i s that the d e x t r i n chain i s smaller and more highly branched than that of starch. It i s therefore s u r p r i s i n g that dextrin's adsorption properties are so d i f f e r e n t from those of starch. In f a c t , the "hydrophobic bonding" theory was only i n f e r r e d from the observation that d e x t r i n adsorbs on hydrophobic minerals. The importance of other surface properties, such as the presence of metal i o n i c species, had not been considered. The main objective of t h i s project i s to i n v e s t i g a t e the r o l e of the mineral surface hydrophobicity and m e t a l l i c adsorption centres i n the adsorption of polysaccharides onto mineral surfaces. This study was deemed worthwhile because polysaccharides function as both f l o t a t i o n depressants and flocculants/dispersants i n f i n e p a r t i c l e processing, and because they are generally cheaper than other synthetic polymers. CHAPTER 2 SCOPE AND APPROACH The main objective of the proposed research, as mentioned in Chapter 1, is to investigate the effects of the hydrophobicity and metal ionic sites of mineral surfaces on the adsorption of polysaccharides. Quartz, a common silicate, was chosen as a model solid for this study. It was either coated with lead ions, methylated, or methylated following lead coating, yielding solid samples with different surface properties (Table 2.1). The adsorption densities of dextrin and carboxymethyl cellulose on a l l such modified quartz samples were measured as a function of both solution pH and residual polymer concentration. The resulting floatability, surface wettability and electrophoretic mobility of the samples were also determined. Infrared spectroscopic studies, titration and co-precipitation tests were conducted to characterize the bonding that occurred. Some tests were also conducted with amylopectin. Natural minerals, such as galena and chalcopyrite, were subsequently employed to confirm the conclusions drawn from the quartz-polysaccharide interactions. 3 SCOPE AND APPROACH / 4 Table 2.1 - D i f f e r e n t modifications of quartz samples Hydrophilic Hydrophobic No m e t a l l i c s i t e s Quartz (Q) Methylation (MQ) With m e t a l l i c s i t e s Pb-coating (PbQ) Methylation a f t e r Pb-coating (MPbQ) CHAPTER 3 LITERATURE REVIEW 3.1 CHEMISTRY OF STARCH AND CELLULOSE In carbohydrate chemistry, "polysaccharides" represent a v a r i e t y of organic polymers containing many monosaccharide units per molecule. Starch and c e l l u l o s e are the most abundant and most important natural polysaccharides (Morrison and Boyd, 1977), and are most often used i n mineral processing. These two types of polysaccharides were thus the main subjects of t h i s i n v e s t i g a t i o n . 3.1.1 Structure The basic s t r u c t u r a l unit f or both starch and c e l l u l o s e i s D-glucose ( C 6 H 1 2 0 6 ) . The chemical structure of D-glucose i s shown schematically i n F i g . 3.1. The f i v e carbon atoms and one oxygen atom form a c y c l i c molecule. In the most favourable conformation, the hydroxyl groups on C-2, C-3 and C-4 are a l l i n equatorial p o s i t i o n s ; and the hydroxyl groups on C - l can be either i n the equatorial p o s i t i o n ( i n t h i s case i t i s c a l l e d 0-D-glucose), or i n the a x i a l p o s i t i o n (a-D-glucose). 5 L I T E R A T U R E R E V I E W / 6 Haworth projection Conformational formula formula a-D-glucose Haworth projection Conformational formula formula 0-D-glucose F i g . 3 .1 - S t r u c t u r e of D-glucose (from L e j a , 1982). LITERATURE REVIEW / 7 Starch contains a water-soluble f r a c t i o n (amylose) and a water-insoluble f r a c t i o n (amylopectin). End-group analysis indicates that amylose i s a s t r a i g h t chain polymer composed of an average of 250 to 500 D-glucose residues connected by a-1,4 linkages. Amylopectin i s a branched polymer. Branching (a-1,6 linkages) occurs approximately every 25 glucose units ( A s p i n a l l , 1970; Wolfrom and Khadem, 1965). C e l l u l o s e i s a l i n e a r polymer which consists of approximately 1000 D-glucose residues connected by 0-1,4 linkages (Davidson, 1967). The average number of glucose units per polysaccharide molecule i s c a l l e d the "Degree of Polymerization (DP)". In an aqueous solu t i o n , amylose behaves as a random c o i l or h e l i x with six glucose residues per turn. The h e l i x i s hydrophobic i n the i n t e r i o r and hy d r o p h i l i c on the exterior (Whistler et a l , 1965). Branched polymers, such as amylopectin, do not seem to have such a structure. Charged polysaccharides (obtained through the introduction of i o n i z a b l e groups) can behave as extended rods i n aqueous s o l u t i o n due to e l e c t r o s t a t i c repulsion. 3.1.2 Molecular Weight Polysaccharides show a s i g n i f i c a n t degree of p o l y d i s p e r s i t y i n t h e i r molecular s i z e s . Thus a meaningful representation of t h e i r molecular weight i s an average: LITERATURE REVIEW / 8 Number average: Mn = Z(n,M.)/Zn. (1) Weight average: Mw = Zdi.M.M/Kn.M^ (2) where n. i s the number of molecules of molecular weight M.. i i In the number-average formula, small molecules a f f e c t the denominator to the same extent as large molecules. The resultant average molecular weights are therefore strongly influenced by low molecular weight impurities. In the weight-average formula, molecular weight i s p a r t i c u l a r l y s e n s i t i v e to traces of large contaminants, such as dust or aggregates (Foster, 1965). A number-average molecular weight i s obtained experimentally through chemical end-group an a l y s i s or osmotic pressure measurement, whereas a weight-average molecular weight can be obtained by l i g h t s c a t t e r i n g or u l t r a c e n t r i f u g e techniques (Foster, 1965). One of the simplest p h y s i c a l measurements capable of providing useful information about a polysaccharide so l u t i o n i s the determination of i n t r i n s i c v i s c o s i t y , [rj], expressed as: [TJ] = lim ( ( ( 7}/rj 0)-l)/c) c •* 0 (3a) or [ 7 7 ] = lim ( l n ( 7 7 / j 7 0 ) / c ) c •+ 0 (3b) where T?0 and rj are the v i s c o s i t i e s of the pure solvent and of the polymer so l u t i o n at concentration c, r e s p e c t i v e l y . 7 7 / 7 7 0 i s re f e r r e d to LITERATURE REVIEW / 9 as r e l a t i v e v i s c o s i t y , n^, and ( ( T J / T J 0 ) - 1 ) i s r e f e r r e d to as s p e c i f i c v i s c o s i t y , 77 . Generally, a plo t of ln(7i ) vs. - c should give a Sp IT s t r a i g h t l i n e with a small negative slope, whereas that of 77 vs. c gives a str a i g h t l i n e with a p o s i t i v e slope. The two l i n e s , when extrapolated, should intercept at zero concentration (c=0), and the intercept i s the i n t r i n s i c v i s c o s i t y . For a given polysaccharide i n a p a r t i c u l a r solvent, the i n t r i n s i c v i s c o s i t y bears a simple r e l a t i o n s h i p to the molecular weight, as expressed by the Mark-Houwink-Sakurada equation (Kurata et a l , 1975): [7?] = k M a (4) where k and a are empirical parameters. In a ^-solvent ( i n which polymer-polymer i n t e r a c t i o n s are exactly balanced by polymer-solvent i n t e r a c t i o n s ) , a i s equal to 0.5. In a better than 0-solvent ( i n which polymer-solvent i n t e r a c t i o n s are preferred to polymer-polymer i n t e r a c t i o n s ) , a i s greater than 0.5. Otherwise a i s l e s s than 0.5. For example, the r e l a t i o n f o r carboxymethyl c e l l u l o s e i n a 0.1% NaCl s o l u t i o n at 25°C i s (Stelzer and Klug, 1980): [TJ] = 2.9 x 10-*M 0 , 7 8 (5) where M i s the weight-average molecular weight. LITERATURE REVIEW / 10 3.1.3 Chemical Reactivity Starch and c e l l u l o s e , as polyhydroxyl compounds, undergo many reactions c h a r a c t e r i s t i c of a l c o h o l s . These include hydrolysis, oxidation, e t h e r i f i c a t i o n , e s t e r i f i c a t i o n and complexation. The p o s i t i o n s of hydroxyl groups a f f e c t t h e i r r e a c t i v i t y . Generally, hydroxyl groups i n equatorial p o s i t i o n s are more r e a c t i v e than those i n a x i a l p o s i t i o n s . Hydroxyl groups on C-6 are the most rea c t i v e (Davidson, 1967). 3.1.3.1 Hydrolysis Hydrolysis of starch and c e l l u l o s e i s accomplished by either a c i d or enzymes, or both. Acid i s capable of breaking 1-4 and 1-6 linkages, whereas some enzymes can break only 1-4 linkages (whistler et a l , 1983). Dextrin i s produced by the thermal degradation of starch under a c i d i c conditions. This type of r e a c t i o n i s c a l l e d " p y r o l y s i s " . White dextrins, which are only s l i g h t l y converted and have l i m i t e d s o l u b i l i t y i n cold water, are produced at low temperatures (79 -121°C), short conversion times ( 3 - 8 hours) and with high concentrations of a c i d . The yellow or canary dextrins are the products of longer conversion times (6 - 18 hours) at higher temperatures (150 - 180°C) and with lower concentrations of a c i d . Canary dextrins have a higher degree of conversion and are 90 to 100% soluble i n cold water (Horton, 1965). Heating starch to the range of 170 - 195°C LITERATURE REVIEW / 11 without the addition of mineral a c i d produces B r i t i s h gums. Both de x t r i n and B r i t i s h gums have highly branched structures and low molecular weights (Whistler et a l , 1983). 3.1.3.2 Oxidation Oxidation of the hydroxyl groups of polysaccharides, e.g. with hypochlorite, gives aldehydes, ketones or carboxylic a c i d s . Other oxidants include nitrogen dioxide, c h l o r i n e , permanganate, dichromate and ozone (Whistler et a l , 1983). 3.1.3.3 Etherif ication and Esterification A v a r i e t y of starch and c e l l u l o s e d e r i v a t i v e s are obtained by e t h e r i f i c a t i o n and e s t e r i f i c a t i o n reactions. For example, carboxymethyl c e l l u l o s e (CMC) i s produced by reacting a l k a l i c e l l u l o s e with chloroacetate ( e t h e r i f i c a t i o n ) (Stelzer and Klug, 1980). C e l l u l o s e xanthate i s obtained by f i r s t steeping c e l l u l o s e i n a strong sodium hydroxide sol u t i o n , then removing the excess a l k a l i and reacting the c e l l u l o s e with carbon disulphide ( e s t e r i f i c a t i o n ) (Muller and Purves, 1963). The average number of foreign groups introduced per glucose monomer i s c a l l e d the "Degree of Substitu t i o n (DS)", which depends on the i n t e n s i t y of the treatment. It can be seen that the maximum DS for polysaccharides i s 3. LITERATURE REVIEW / 12 3.1.3.4 Complexation reactions Many hydrolyzable multivalent metal ions form complexes with polysaccharides, even when the polysaccharides are not chemically modified. Cuprammonium reacts with the hydroxyl groups on C-2 and C-3 of D-glucose units, forming the complexes shown i n F i g . 3.2 (Davidson, 1967). Cobalt forms complexes with starch (at pH 11) and de x t r i n (Klotz, 1961). Williams and A t a l l a (1981) studied the i n t e r a c t i o n of group II cations with nonionic saccharides. They suggested that adjacent hydroxyl groups i n an a x i a l - e q u a t o r i a l - a x i a l sequence favour complex formation. F i g . 3.2 - Schematic representation of the reaction between adjacent hydroxyl groups and cuprammonium (from Davidson, 1967). The so l u t i o n pH plays an important r o l e i n the complexation re a c t i o n between polysaccharide and metal ions. BeMiller (1965) l i s t e d a number of polysaccharide-metal complexes and concluded that i n almost a l l cases the complexes are formed i n a l k a l i n e s o l u t i o n s . Indeed, i n an a l k a l i n e solution, the hydroxyl groups of polysaccharide can, a f t e r l o s i n g a proton, form much stronger complexes (Angyal, LITERATURE REVIEW / 13 1973). Polysaccharides a l s o form complexes with anions such as borate, non-polymeric organic compounds such as o l e i c a c i d , and other organic polymers (BeMiller, 1965). The formation of complexes between cuprammonium and polysaccharides can be r e a d i l y detected by conductivity and o p t i c a l r o t a t i o n measurements (Davidson, 1967). The c o n d u c t i v i t i e s of the solutions decrease and the o p t i c a l rotations of the polysaccharides change when complexes form. 3.2 DEPRESSANT FUNCTION 3.2.1 Starches and Starch Derivatives Starches are used i n the depression of a v a r i e t y of minerals, i n c l u d i n g oxides, sulphides, s a l t - t y p e minerals and inherently hydrophobic minerals. Starch was f i r s t discovered to improve s e l e c t i v i t y i n the c a t i o n i c f l o t a t i o n of quartz i n 1931 (Lange, 1931). It i s believed to function i n t h i s system by depressing phosphate. Starch was subsequently studied i n the f l o t a t i o n separation of quartz from hematite by depressing the l a t t e r (Cooke et a l , 1952; Chang et a l , 1953; Chang, 1954; Iwasaki and L a i , 1965; Iwasaki et a l , 1969; Balajee and Iwasaki, 1969a, 1969b; Partridge and Smith, 1971). Most of the above mentioned work was conducted on a laboratory scale. LITERATURE REVIEW / 14 Iwasaki and Lai (1965) found i n t h e i r study on quartz/hematite f l o t a t i o n that starch becomes an e f f e c t i v e depressant for hematite when a c r i t i c a l amount of starch i s added, beyond which excess starch appears i n measurable qu a n t i t i e s i n the aqueous phase. They a l s o found that corn starch induces the maximum s e t t l i n g r a t e at the point where an excess of starch begins to appear i n the supernatant. This was l a t e r confirmed by Colombo (1972), who used Tapioca f l o u r i n a p i l o t - p l a n t t e s t . Iwasaki and L a i (1965) concluded that a c e r t a i n saturation coverage by starch i s e s s e n t i a l f o r hematite depression during f l o t a t i o n . Another major l i n e of research i s the use of starch i n the f l o t a t i o n of salt-type minerals. Starch has been used on a pla n t - s c a l e for the separation of fluorspar from s i l i c a t e s and calcium carbonate gangue (West and Walden, 1954). Somasundaran (1969) found that starch strongly depresses c a l c i t e f l o t a t i o n when o l e i c a c i d i s used as a c o l l e c t o r . Hanna (1973) reported adsorption and f l o t a t i o n r e s u l t s of an S-starch i n the f l u o r i t e - b a r i t e - c a l c i t e system. His r e s u l t s on singl e mineral f l o t a t i o n showed that s e l e c t i v e separation of f l u o r i t e from the other two minerals i s po s s i b l e . Recently, Parsonage et a l (1984) reported that gum arabic e f f e c t i v e l y depresses c a l c i t e and dolomite, provided that a c e r t a i n l e v e l of calcium ion i s present. It has been reported that water-soluble starch i n a neutral medium depresses chalcopyrite and sphalerite but not galena, with xanthate as a c o l l e c t o r . Thus the separation of a r t i f i c i a l PbS-ZnS, LITERATURE REVIEW / 15 PbS-CuFeS2 mixtures with the use of starch has been t r i e d (Dolivo-Dobrovosky and Rogachevskaia, 1957). On the other hand, de x t r i n has been used to depress galena i n the separation of Cu-Pb bulk sulphide concentrate by the Brunswick Mines and Concentrator i n Canada (Schnarr, 1978). The pH of the pulp was not reported, but since t h i s process employs sulphur dioxide the pH i s expected to have been r e l a t i v e l y low. Matabi Mines Ltd. (Canada) has used guar gum to depress galena i n Cu-Pb separation at a pH of 4 by adding sulphur dioxide ( A l l a n and Bourke, 1978). These two examples of the successful i n d u s t r i a l a p p l i c a t i o n of dextrin are inconsistent with the laboratory r e s u l t s of Dolivo-Dobrovosky and Rogachevskaia (1957). The discrepancy might be a t t r i b u t a b l e to the use of sulphur dioxide. The depressive action of starch and starch d e r i v a t i v e s on inherently hydrophobic minerals i s well known. Steenberg and Harris (1981) reported that gum arabic and guar gum depress the natural f l o a t a b i l i t y of t a l c . Afenya (1982) reported the depressive action of a maize starch on synthetic graphite. Dextrin was reported to depress both molybdenite and coal very strongly (Wie and Fuerstenau, 1974; Haung et a l , 1978; M i l l e r et a l , 1984). In 1966, Klassen reported the depressive action of starch and d e x t r i n on coal f l o t a t i o n (Klassen, 1966). Later, Im and Apian (1981) evaluated 55 starch and starch d e r i v a t i v e s as f l o t a t i o n depressants for c o a l . They observed that branched polysaccharides are superior depressants compared to l i n e a r ones. They also found that a l l dextrins LITERATURE REVIEW / 16 are good coal depressants, regardless of t h e i r degree of conversion or -source. However, t h e i r parent compounds are much poorer depressants and show a wide degree of v a r i a b i l i t y based on source, structure and molecular weight. Perry and Apian (1985) studied the f l o t a t i o n depression of p y r i t e with d i f f e r e n t polysaccharides. Contrary to the findings mentioned i n the l a s t paragraph, they observed that branched polysaccharides are not as e f f e c t i v e as l i n e a r ones i n either the adsorption or depression i n t h i s system. 3.2.2 Cellulose Derivatives C e l l u l o s e d e r i v a t i v e s , such as c e l l u l o s e sulphuric ethers, copper-ammonia complexes of c e l l u l o s e , and hydroxyethyl c e l l u l o s e , have found use i n the upgrading of sulphide concentrate and coal by depressing s i l i c a t e gangues and p y r i t e , r e s p e c t i v e l y (Bakinov et a l 1964; Gorlovski, 1965; Rhodes, 1979; Perry and Apian, 1985). However, the most commonly used c e l l u l o s e d e r i v a t i v e i n mineral f l o t a t i o n i s carboxymethyl c e l l u l o s e (CMC). CMC has been tested as a depressant for s i l i c a t e s (Vaneev, 1957; Bakinov et a l , 1964), magnesia-bearing minerals (e.g. t a l c , c h l o r i t e , amphoiboles, etc.) (Rhodes, 1979), t a l c (Steenberg and Harris, 1981; Steenberg, 1982), c o a l - p y r i t e (Laskowski et a l , 1985; Perry and Apian, 1985), graphite ( S o l a r i et a l , 1986) and c e r t a i n sulphide minerals (Gorlovski, 1965; J i n et a l , 1987). In 1957, Vaneev reported the use of CMC instead of sodium LITERATURE REVIEW / 17 s i l i c a t e i n a Russian Cu-Ni sulphide processing plant. The contents of both copper and n i c k e l i n the r e s u l t i n g concentrate were increased by 20-30% whereas those of MgO and S i 0 2 decreased. Use of CMC instead of sodium s i l i c a t e also improves thickening e f f i c i e n c y (Vaneev, 1957). Bakinov et a l (1964) reported the r e s u l t s of p i l o t - p l a n t tests using CMC instead of sodium s i l i c a t e for the cleaning f l o t a t i o n of Cu-Ni bulk rougher concentrate. CMC displays superior s e l e c t i v e depression compared to sodium s i l i c a t e . Bakinov et a l (1964) a l s o conducted some i n t e r e s t i n g tests showing the e f f e c t s of chemical modifications on the depressive a c t i o n of CMC. They found that when the DS of CMC i s reduced from 100 to 32, the depressive action of CMC upon s i l i c a t e s increases, and the grade of concentrate i s improved (note that the DS was for the whole molecule instead of for a monomer). A further decrease i n the DS below 30-35 sharply reduces the depressive e f f e c t of CMC. Bakinov et a l (1964) concluded that the optimum value for the DS i s 40-50, whereas the optimum DP l i e s between 450 to 550. Rhodes (1979) studied i n d e t a i l the physical v a r i a b l e s a f f e c t i n g the depressant function of CMC. In the depression of r e a d i l y f l o a t a b l e magnesia-bearing minerals, both laboratory and plant scale t e s t s showed that the depressive e f f e c t of CMC i s unchanged when the DP varies from 250 to 800. When t h i s value increases to around 1000 depression improves and the dosage of CMC can be reduced without a f f e c t i n g the MgO content i n the concentrate. When carboxymethyl groups are introduced as c l u s t e r s rather than d i s t r i b u t e d evenly along the chain, the depressive e f f e c t of CMC i s further improved and t h i s LITERATURE REVIEW / 18 also reduces the dosage required (Rhodes, 1979). Some of the most recent work on CMC-mineral i n t e r a c t i o n has been conducted i n the Department of Mining and Mineral Process Engineering at the U n i v e r s i t y of B r i t i s h Columbia. S o l a r i , de Araujo and Laskowski (1986) c a r r i e d out a d e t a i l e d study on the influence of CMC on the f l o t a t i o n and surface properties of graphite. The graphite .samples were e i t h e r unpurified or p u r i f i e d by a c i d leaching. They found that i n an a c i d i c solution, CMC depresses unpurified graphite whereas i n an a l k a l i n e solution, CMC promotes i t s f l o t a t i o n . P u r i f i e d graphite appears to be less a f f e c t e d by CMC. Laskowski et a l (1985) have also used CMC to upgrade Eastern Canadian high sulphur c o a l . They found that i n an a l k a l i n e solution CMC reduces p y r i t i c sulphur content i n coal concentrate without s i g n i f i c a n t l y a f f e c t i n g the recovery of c o a l . J i n et a l (1987) observed that under a l k a l i n e conditions, galena i s depressed by sodium carboxymethyl c e l l u l o s e , while Cu(II)-activated s p h a l e r i t e exhibits a high f l o a t a b i l i t y so that s e l e c t i v e separation can be achieved. 3.3 ACTIVATION BEHAVIOUR Although mostly used as depressants, polysaccharides have o c c a s i o n a l l y been reported to a c t i v a t e mineral f l o t a t i o n . Shneerson (1940) found that at very low concentrations (10- 5 to 10- 3 percent s o l u t i o n ) , potato starch renders the surfaces of some minerals hydrophobic. It was used for enhancing the f l o t a t i o n of lead oxide LITERATURE REVIEW / 19 minerals with shale o i l and sodium sulphide. Keck et a l (1937) reported that gum arabic and soluble starch s l i g h t l y a c t i v a t e the f l o t a t i o n of hematite. Klassen and Mokrousov (1963) reported that starch enhances the f l o t a t i o n of hydroboracite and depresses gypsum, r e s u l t i n g i n improved separation. They considered t h i s a c t i v a t i o n behaviour of starch i n the presence of an anionic c o l l e c t o r as not r e l i a b l y established, and the example that was mentioned was only treated as an "unusual" case (Klassen and Mokrousov, 1963). The flotation-promoting behaviour of CMC was f i r s t reported by Gorlovski (1965). He observed that when CMC i s added before xanthate, the f l o t a t i o n of c h a l c o c i t e and bornite i s enhanced at low CMC concentration; when i t i s added a f t e r xanthate, i t has l i t t l e e f f e c t on the f l o t a t i o n of the minerals. Twenty years l a t e r , S o l a r i et a l (1986) observed that CMC promotes graphite f l o t a t i o n i n a l k a l i n e solutions of low i o n i c strength. C l e a r l y t h i s i s an important f i e l d of research. The activation/depression properties of these polysaccharides toward d i f f e r e n t minerals need to be c a r e f u l l y characterized. A great deal of work seems necessary to c l a r i f y the mechanisms involved. LITERATURE REVIEW / 20 3.4 ADSORPTION The adsorption of polysaccharide depressants on mineral surfaces i s not well understood, perhaps because of the complicated nature of the system and a lack of appropriate experimental techniques. 3.4.1 Mechanisms of Adsorption Polysaccharides are long-chain molecules with a large number of polar groups and a f l e x i b l e structure. Thus during t h e i r adsorption on mineral surfaces some weak types of bonds may become important because of the large number of such bonds which are capable of being formed per molecule. Polysaccharides are believed to adsorb on mineral surfaces by the following mechanisms: a. Hydrogen bonding. Even though the strength of hydrogen bonds are only of the order of 2x10* joule/mole, the cumulative e f f e c t of t o t a l adsorption energy becomes quite s i g n i f i c a n t for polysaccharides of medium to high molecular mass (Steenberg, 1982). The bonds e x i s t between hydrogen atoms on the polymer and oxygen atoms on the mineral surface. This type of i n t e r a c t i o n i s thought to occur i n CMC-sulphide-silicate (Bakinov et a l , 1964), starch-hematite-quartz (Balajee and Iwasaki, 1969), and starch-graphite systems (Afenya, 1982). In a l l these cases LITERATURE REVIEW / 21 hydrogen bonding i s believed to be p r i m a r i l y responsible for adsorption, although no d i r e c t evidence has been provided. Rhodes (1979) found that increasing the DP of CMC above 1000 improves the depressive e f f e c t of CMC on magnesia-bearing minerals. However, t h i s r e s u l t alone appears to be inadequate proof of hydrogen bonding as the mechanism underlying adsorption. Evidence that hydrogen bonding i s not involved i n polysaccharide adsorption has also been reported. Somasundaran mentioned (1969) that f l u o r a p a t i t e (Ca 5(P0 4) 3(F,OH)), which should form hydrogen bonds with starch more r e a d i l y than c a l c i t e , a c t u a l l y adsorbs less starch than c a l c i t e . Bakinov et a l (1964) stated that there i s no e s s e n t i a l d i f f e r e n c e between the adsorption of CMC by t a l c or by p y r r h o t i t e ; s e l e c t i v e separation i s achieved because the l a t t e r adsorbs more xanthate. Talc exposes both basal hydrophobic planes and hydrophilic edges when cleaved. Hydrogen bonding to these basal planes should be r e l a t i v e l y l i m i t e d and could be made with ease only to the edges. The adsorption should thus be much less than on the p y r r h o t i t e surface. Bakinov et a l (1964) d i d not present any adsorption data. I t i s therefore not possible to decide whether t h e i r conclusion was on a unit weight basis, or on a unit surface area ba s i s . If the former was the case, then the larger surface area of t a l c might compensate for i t s lower LITERATURE REVIEW / 22 adsorption density. It i s , however, to be noted that i n aqueous solutions, i f the polysaccharides can form hydrogen bonds with mineral surfaces, then both the polysaccharides and the mineral surfaces must have already been ei t h e r hydrogen-bonded with the water molecules, or i n t e r n a l l y hydrogen-bonded. Thus formation of a polysaccharide-mineral hydrogen bond must involve the breakage of the two e x i s t i n g hydrogen bonds. The energetics of such a process cannot be e a s i l y j u s t i f i e d . Therefore, unless other factors contribute to the s t a b i l i t y of the polysaccharide-mineral hydrogen bonding, t h i s adsorption mechanism should be applied with caution. b. 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 . This i n t e r a c t i o n can be either a t t r a c t i o n or repulsion depending on the charges that mineral surfaces and polymer chains develop. T h e o r e t i c a l l y , starch molecules are non-ionic. However, they can be modified by the introduction of e i t h e r anionic or c a t i o n i c i o n i z a b l e s i t e s (Cook et a l , 1952; Chang, 1954; Balajee and Iwasaki, 1969; Iwasaki et a l , 1969). Natural starches can a l s o be charged due to the presence of minor i m p u r i t i e s . Schulz and Cooke (1953) found that a B r a z i l i a n hematite adsorbs more starch than quartz; when aminoethyl groups are introduced i n t o starch, the adsorption on quartz i s increased considerably. These r e s u l t s seem quite LITERATURE REVIEW / 23 understandable since quartz i s generally h i g h l y negatively charged. S i m i l a r l y , Balajee and Iwasaki (1969) found that a negatively charged corn starch i s adsorbed more r e a d i l y on hematite than on quartz, and that adsorption decreases with increasing pH. However, for Q-TACt c a t i o n i c starch No. 3891, the adsorption behaviour i s reversed. They then postulated a model showing that adsorption of starch on oxide surfaces i s governed mainly by 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 s , with hydrogen bonding being the same i n a l l cases 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 s l i m i t i n g the extent of adsorption. This model seems to explain the observed adsorption behaviour quite s a t i s f a c t o r i l y . It has been reported that CMC i s adsorbed much less than starch under i d e n t i c a l conditions, owing to i t s strong e l e c t r o l y t e character (Steenberg and Harris, 1981, 1984). c. Chemisorption. Complexation between ions i n a mineral l a t t i c e and polar groups i n polysaccharides r e s u l t s i n chemisorption of the polymer. This i s e s p e c i a l l y true when foreign f u n c t i o n a l groups are introduced i n t o the polymer. Somasundaran (1969) a t t r i b u t e d adsorption of a corn starch on a c a l c i t e surface to such a chemisorption. He mentioned that i f complex formation between starch and calcium i s responsible for adsorption, then ad d i t i o n of starch should f Q - T A C is a trade name. LITERATURE REVIEW / 24 a l s o cause an increase i n the t o t a l amount of calcium ions i n bulk s o l u t i o n . His r e s u l t s agreed with t h i s p r e d i c t i o n . The same phenomenon was observed by de Araujo (1988). de Araujo (1988) found that the amount of C a + + released from f l u o r a p a t i t e i s dependent upon the amount of starch adsorbed. The adsorption isotherms i n d i c a t e that starch p r e f e r e n t i a l l y adsorbs onto f l u o r a p a t i t e over quartz. de Araujo (1988) s p e c i f i c a l l y a t t r i b u t e d the importance of calcium to the formation of complexes between starch impurity-related i o n i c groups and calcium i o n i c species. Somasundaran (1969) also found that adsorption of starch onto c a l c i t e decreases s i g n i f i c a n t l y with increasing pH, approaching zero at a pH above 12. However, de Araujo (1988) observed that a l k a l i n e pH favours the depression of f l u o r a p a t i t e by starch. S o l a r i et a l (1986) concluded that metal i o n i c impurities i n a graphite sample play an important r o l e i n CMC adsorption. They suggested that these m e t a l l i c s i t e s are adsorption centres for CMC through chemical i n t e r a c t i o n s with carboxymethyl groups. However, Bakinov et a l (1964) reported that adsorption of CMC i s not dependent on the presence of metal ion i m p u r i t i e s . They claimed that t a l c adsorbs the same amount of CMC whether or not copper ions are present on the surface. They d i d not provide any adsorption r e s u l t s . Talc i t s e l f contains magnesium ions which might influence LITERATURE REVIEW / 25 the adsorption phenomenon. Salt linkage. This can be achieved by multivalent metal ions acting as bridges for the adsorption of polysaccharides. A c t u a l l y , a number of authors, (Schulz and Cooke, 1953; Iwasaki and L a i , 1965; Brien and Kar, 1968; Hanna, 1973; Parsonage et a l , 1984), have reported that the a d d i t i o n of calcium or barium ions s i g n i f i c a n t l y increases the adsorption of starch and starch d e r i v a t i v e s . Hydrophobic bonding. Dextrin appears to adsorb e x c l u s i v e l y on hydrophobic minerals. This observation led to the theory of "hydrophobic bonding" between dex t r i n and mineral surfaces, (Wie and Fuerstenau, 1974; Haung et a l , 1978; M i l l e r et a l , 1983, 1984). Dextrin has the same adsorption density on various hydrophobic minerals ( t a l c , coal and molybdenite), but not on h y d r o p h i l i c minerals ( p y r i t e and quartz). Moreover, when the h y d r o p h i l i c mineral p y r i t e i s i n a s s o c i a t i o n with coal ( i n t h i s case a small amount of locked c o a l , which confers a s l i g h t l y hydrophobic character to the p y r i t e p a r t i c l e s ) , " p a r t i a l " hydrophobic bonding takes place and the adsorption of d e x t r i n i s increased; when the ore p y r i t e i s made hydrophobic by amyl xanthate, the adsorption of d e x t r i n i s also increased ( M i l l e r et a l , 1984). The e f f e c t of hydrophobicity on polysaccharide adsorption w i l l be LITERATURE REVIEW / 26 discussed more f u l l y l a t e r i n t h i s chapter. 3.4.2 Adsorption Isotherms Three types of isotherms can r e s u l t from polymer adsorption: a. Langmuir-type isotherm. This isotherm exhibits a sharply defined saturation value. It suggests monolayer adsorption with no intermolecular i n t e r a c t i o n and in d i c a t e s the involvement of a sing l e type of adsorption s i t e . One of the versions of the isotherm i s (Smellie and La Mer, 1958): c / r = 1/(KT 0) + c /r„ (6) where c i s the equilibrium polymer concentration i n mg/1; r„ and T are the maximum adsorption density and the adsorption density at concentration c, r e s p e c t i v e l y ; K i s the e q u i l i b r a t i o n constant, where K = exp(-AE /(RT)) (Wie and cL Fuerstenau, 1974; Haung et a l , 1978). Thus a p l o t of c / r vs. c gives a s t r a i g h t l i n e . The free energy change for the adsorption process can be calculated from the slope and intercept of t h i s l i n e (Wie and Fuerstenau, 1974; Haung et a l , 1978). The r e s u l t s of most of the adsorption studies with polysaccharides agree with t h i s type of isotherm, e.g., Hanna (1973), Wie and LITERATURE REVIEW / 27 Fuerstenau (1974), Haung et a l (1978), and Afenya (1982). Ostwald-Freundlich isotherm. Polymer adsorption has sometimes been described by the following empirical isotherm: x and c are the adsorbed amount and polymer concentration, r e s p e c t i v e l y ; k and n are empirical constants. A p l o t of log x vs. log c gives a s t r a i g h t l i n e with a slope of 1/n. This equation i s generally only applicable i n an intermediate concentration range because the adsorption density of a polymer usually reaches a l i m i t i n g value with increasing polymer concentration. Simha-Frisch-Eirich isotherm. This model was proposed by Simha, F r i s c h and E i r i c h based on a s t a t i s t i c a l a n alysis of the adsorption process. Assuming that the adsorbed polymer chains are characterized by a Gaussian d i s t r i b u t i o n of end-to-end distances, and that a segment touching the surface has a f i x e d p r o b a b i l i t y of being adsorbed, the following equation was derived (Simha et a l , 1952): x = kc 1/n C7> e = kc 1/<V> /(1+kc l/<v> (8) LITERATURE REVIEW / 28 where <v> i s the average number of adhesion points per molecule; 6 i s the f r a c t i o n of the surface covered by adsorbed segments at concentration c. Balajee and Iwasaki (1969) reported that the adsorption of corn starch, B r i t i s h gum 9084 and an anionic starch on hematite agrees with t h i s isotherm. The e q u i l i b r a t i o n time required for polymer adsorption depends on the i n t e r a c t i o n forces between polymers and surfaces, and on the d i f f u s i o n rates of polymers. The l a t t e r i s determined by the chemical nature of the polymer, i t s molecular weight, the properties of the solvent and the type of adsorbent. In the case of nonporous adsorbents, equilibrium i s usually reached i n a few seconds or minutes. For porous adsorbents, equilibrium i s established much less r a p i d l y because d i f f u s i o n of polymers i n t o the pores i s slow. Various e q u i l i b r a t i o n times have been reported (Table 3.1). 3.4.3 Conformation of Adsorbed Polysaccharides Here the term "conformation" denotes the p h y s i c a l o r i e n t a t i o n of an adsorbed polymer on a s o l i d surface. Unlike simple i o n i c or monomeric species, polymers have numerous adsorbable segments which can occupy many adsorption s i t e s on s o l i d surfaces. The conformation of polymers on s o l i d surfaces i s thus complicated because i t i s extremely u n l i k e l y that each segment of the polymer i s attached to the surface. For example, polymer chains i n the form of s t i f f rods LITERATURE REVIEW / 29 Table 3.1 - E q u i l i b r a t i o n times used for polymer adsorption Author E q u i l i b r a t i o n time System Somasundaran, (1969) 12 : minutes S t a r c h / c a l c i t e / o l e a t e Hanna, (1974) 1 hour Starch/spar minerals Schulz, Cooke, (1953) 1 hour Starch/quartz/hematite Wie and Fuerstenau, 1 hour Dext r in/molybdenite (1974) M i l l e r et a l , 1984 1 hour Dextrin/pyrite Chong and Curthoys, 2 hours Polyacrylamide/TiO 2 (1979) Rubio and Kitchener, 4 hours Poly(ethylene oxide) 1976 / s i l i c a Partridge and Smith, 6 hours Starch/hematite (1971) Fontana and Thomas, 6 - 10 hours Poly-alkylmethacrylat e (1961) / S i 0 2 S o l a r i et a l , (1986) 16 - 18 hours CMC/graphite Edwards and Rutter, 18 hours Dextran/Agl/Si0 2/etc. (1980) Tadros, (1980) 20 - 24 hours Ligninsulphonate /polymer latex could occupy a surface s i t e i n either a f l a t or an end-on o r i e n t a t i o n . However, polymers may be adsorbed with only some segments being attached to the surface, and the rest extending out i n t o the surrounding solution i n the form of "loops" or " t a i l s " ( E i r i c h , 1976; Steenberg, 1982; Fleer and Lyklema, 1984). The "bound f r a c t i o n " ( f r a c t i o n of segments a c t u a l l y i n contact with the s o l i d surface) and the "adsorbed layer thickness" give some clues as to the conformation of an adsorbed polymer. LITERATURE REVIEW / 30 E i r i c h (1976) discussed t h i s subject i n d e t a i l and described a number of the variables involved. These include the nature of the adsorbent, the polymer's structure and the properties of the solvent. For example, at low i o n i c strength a polymer molecule with a high charge density would be i n an extended form, whereas i n the absence of charged groups the polymer would have a random c o i l conformation. The solvent a l s o has a pronounced influence on the conformation of polymers. In a better than ^-solvent (see page 9) the polymer w i l l p refer an extended conformation, whereas i n a worse than 0-solvent the random polymer c o i l w i l l change to a compressed one. The number of studies on the conformation of polysaccharides on mineral surfaces i s rather l i m i t e d . This may be due to the u n s u i t a b i l i t y of the techniques used for conformational studies, which require either o p t i c a l grade materials or sophisticated instruments. They also require q u a n t i t a t i v e l y characterizable i n t e r a c t i o n bonds between s o l i d surfaces and f u n c t i o n a l groups i n the polymer (Lipatov and Sergeeva, 1974), which are d i f f i c u l t to apply i n a mineral-polysaccharide system. However, the conformations of polysaccharides on mineral surfaces play an extremely important r o l e i n t h e i r depression/activation behaviour, and i t i s worthwhile to d i r e c t more at t e n t i o n to t h i s subject. LITERATURE REVIEW / 31 3.4.4 Effect of Surface Hydrophobicity on Polysaccharide Adsorption The ways i n which polysaccharides i n t e r a c t with hydrophobic or hy d r o p h i l i c minerals are poorly understood at the present time. Recently Steenberg and Harris (1984) suggested that polysaccharides can adsorb on hydrophilic minerals as well as on the hydrophobic portions of the same mineral. They concluded that the extent of adsorption i s determined by the polymer rather than the composition of the s o l i d surface. Futhermore, hydrogen bonding i s the main adsorption mechanism and substituent groups i n the polymer presumably influence i t s conformation rather than i t s bonding onto the surface (Steenberg and Harris, 1984). However, these conclusions do not seem to be j u s t i f i e d by the experimental evidence. 3.4.4.1 Inherently Hydrophobic Minerals Polysaccharides exhibit d i f f e r e n t adsorption properties toward hydrophobic and hy d r o p h i l i c minerals. Since de x t r i n only adsorbs onto hydrophobic minerals, the i n t e r a c t i o n of de x t r i n with mineral surfaces i s believed to r e s u l t from "hydrophobic bonding" (Wie and Fuerstenau, 1974; Haung et a l , 1978; M i l l e r et a l , 1983, 1984). In contrast, starch seems to adsorb on both hydrophobic and h y d r o p h i l i c mineral surfaces. Afenya (1982) has reported that maize starch adsorbs on synthetic graphite, and the adsorption density decreases s l i g h t l y with increasing pH. Haung et a l (1978) found that amylose (at concentrations of up to 100 ppm), adsorbs on hydrophobic coal samples LITERATURE REVIEW / 32 to the same extent as d e x t r i n . At concentrations over 100 ppm, amylose adsorption i s higher than that of d e x t r i n . The adsorption of starch on quartz, hematite and c a l c i t e , etc., as reported by Schulz and Cooke (1953), Balajee and Iwasaki (1969) and Somasundaran (1969), proves that starch can a l s o adsorb on hydrophilic minerals. There i s no postulate i n the l i t e r a t u r e that starch adsorbs on hydrophobic minerals through "hydrophobic bonding". Dextrin i s simply a low molecular weight d e r i v a t i v e of starch . with a hi g h l y branched structure. If hydrophobic bonding plays so important a r o l e i n the adsorption of dextrin, then why does the same mechanism not apply to starch? An obvious explanation would be that branching i s a c r u c i a l f a c t o r . For example, amylose (a component of starch) i s a l i n e a r molecule that forms a h e l i c a l structure with a hydrophobic i n t e r i o r and h y d r o p h i l i c exterior (Whistler et a l , 1965). Here "hydrophobic bonding" with mineral surfaces i s impossible; t h i s kind of bonding i s only p o s s i b l e with branched starch. However, i t has been reported that amylose adsorbs more r e a d i l y than de x t r i n onto hydrophobic coal surfaces (Haung et a l , 1978). Futhermore, a branched starch, amylopectin, strongly depresses the f l o a t a b i l i t y of hematite, quartz and f l u o r a p a t i t e (which are h y d r o p h i l i c i n nature) (Chang et a l , 1953; de Araujo, 1988). These r e s u l t s are inconsistent with the suggestion that branching i s a c r u c i a l factor f o r the adsorption of d e x t r i n . According to Chander and Fuerstenau (1972, 1974) and Chander, Wie and Fuerstenau (1975), the inherently hydrophobic minerals such as LITERATURE REVIEW / 33 molybdenite and t a l c are very a n i s o t r o p i c , i . e . , the mineral surface has both hydrophobic faces and hy d r o p h i l i c edges. The r a t i o of these two d i f f e r e n t types of surfaces exposed a f t e r sample preparation varies considerably depending on the method of comminution. This has also been shown by A r b i t e r et a l (1975) and Kelebek (1987). Inclusion of other minerals (hydrophilic ones) i n t o the inherently hydrophobic mineral i s a common case. This i s e s p e c i a l l y true for coal and graphite ( S o l a r i et a l , 1986). Hence adsorption of a polysaccharide onto a hydrophobic mineral does not n e c e s s a r i l y i n d i c a t e that the polymer i s bonded to hydrophobic s i t e s . The r o l e of polar hydrophilic s i t e s cannot be completely ruled out without further experimental i n v e s t i g a t i o n . Steenberg and Harris.(1981) concluded that there are three steps i n the adsorption of CMC on t a l c . They boi l e d a t a l c suspension i n the presence of oleate to l e t the oleate adsorb on charged t a l c edges. They found that only the f i r s t CMC adsorption step i s unaffected by such an oleate treatment; the l a s t two steps do not occur. These r e s u l t s suggest that adsorption of CMC occurs f i r s t on the basal hydrophobic planes of t a l c , and thereafter on the hyd r o p h i l i c edges. The adsorption mechanism was not discussed by the authors. It i s unclear why CMC does not adsorb on oleate treated t a l c edges i f i t can adsorb on the hydrophobic basal planes. LITERATURE REVIEW / 34 3.4.4.2 Hydrophobic Surfaces Created by Surfactants There are a number of observations which show that the adsorption of polymer depressants i s enhanced by c o l l e c t o r s . The e a r l i e s t one was reported by Gorlovski (1965), who observed an increase i n CMC adsorption i n the presence of xanthate. According to Somasundaran (1969) the adsorption of starch on a c a l c i t e surface begins to increase when the concentration of oleate i n solution exceeds 10- 5 m o l e / l i t e r at a l l pH values studied (from 7.2 to 12.1). The adsorption of oleate i s promoted by the p r i o r a d d i t i o n of starch. The mutual enhancement of the adsorption of c o l l e c t o r and polymer depressant as observed by Somasundaran was a l s o reported by a number of other researchers. Schulz and Cooke (1953) reported that there i s a s l i g h t increase i n amine adsorption on hematite surfaces at low amine concentrations (up to 300 ppm) i n the presence of Gum 3502. A small amount of amine (approximately 50 ppm for hematite and up to 150 ppm for quartz) increases the adsorption of Gum 3502 on the respective minerals, although at higher concentrations of amine the adsorption of Gum 3502 decreases. Partridge and Smith (1973) reported s i m i l a r r e s u l t s . Balajee and Iwasaki (1969) found that dodecylammonium ch l o r i d e (DAC) has very l i t t l e e f f e c t on the adsorption of B r i t i s h Gum on hematite but appreciably enhances the adsorption of B r i t i s h Gum on quartz. LITERATURE REVIEW / 35 3.5 SURFACE PROPERTIES OF QUARTZ The chemical composition of quartz i s S i 0 2 . The surface chemistry of t h i s mineral, as well as i t s amorphous counterpart ( s i l i c a ) , has been more thoroughly investigated than that of almost any other s o l i d . It seems to be well established that there i s e s s e n t i a l l y no d i f f e r e n c e i n the surface groups on quartz and on amorphous s i l i c a . Most in v e s t i g a t o r s have reported that a disturbed layer of amorphous character i s present on the surface of quartz (Boehm, 1966). Thus much of the following d i s c u s s i o n i n v o l v i n g s i l i c a can be considered a p p l i c a b l e to quartz. When crushed, the -Si-O-Si- bonds i n quartz c r y s t a l are broken and a polar surface i s created. In aqueous media H + ions react with the negative oxygen s i t e s and OH- ions react with the p o s i t i v e s i l i c o n s i t e s to form s i l a n o l groups on the quartz surface. The surface becomes charged when d i s s o c i a t i o n of surface s i l a n o l groups occurs ( F i g . 3.3), with H + and OH- serving as the p o t e n t i a l determining ions (Gaudin and Fuerstenau, 1955). The quartz surface i s normally h i g h l y negatively charged. The point of zero charge (p.z.c.) has been reported to l i e within pH 1.3 - 3.7 (Gaudin and Fuerstenau, 1955; Parks, 1965; Manser, 1975). Two types of end-groups have been postulated to occur on s i l i c a surfaces, i . e . , s i l a n o l and siloxane groups ( F i g . 3.4). The presence of s i l a n o l groups has been p o s i t i v e l y i d e n t i f i e d by a number of i n v e s t i g a t o r s using d i f f e r e n t techniques (e.g., as summarized by LITERATURE REVIEW / 36 i 0 SiC 0 —»• S! ond 0 FRACTURE >-{ \ / P - \ / 0 H \ 0-A + 2 H + — x \ — / S t x + 2H + ^0 ' 0 - ^0 OH 0 0 -ADSORPTION DISSOCIATION ^ S i + ^S i -OH ^ S i - O --t20H~ *- 0^ *- 0^ + 2H + .SJ+ Si-OH S i -O-' \ ^ \ ' \ F i g . 3.3 O r i g i n of the e l e c t r i c a l charge at the quartz/aqueous s o l u t i o n i n t e r f a c e (from Gaudin and Fuers tenau, 1955). LITERATURE REVIEW / 37 Snoeyink and Weber, 1972), and the surface d e n s i t y has been estimated t o be 5 s i l a n o l groups per nm2 (Boehm, 1966). The presence of s i l o x a n e groups, on the other hand, i s only i n f e r r e d from the f a c t t h a t the number of observed s i l a n o l groups i s not s u f f i c i e n t f o r complete surface coverage (Boehm, 1966). H e r (1955) c a l c u l a t e d that t h e r e should be 8 s i l a n o l groups per nm2 of quartz s u r f a c e . S i l a n o l groups on the surface can p a r t i c i p a t e i n a number of chemical r e a c t i o n s , as w e l l as hydrogen bonding. However, the surface s i l o x a n e groups are u n r e a c t i v e and have shown almost no propensity f o r hydrogen bonding. SILANOL SILOXANE OH n F i g . 3.4 - End-groups on s i l i c a surfaces (from Boehm, 1966). 3.5.1 Modifications of the Quartz Surfaces Quartz i s normally n o n - f l o a t a b l e with a n i o n i c c o l l e c t o r s and i s considered to be a t y p i c a l h y d r o p h i l i c mineral, due t o i t s high s u r f a c e charge and the presence of a high d e n s i t y of r e a c t i v e s i l a n o l groups capable of hydrogen bonding with water. I t has a l s o been found th a t t h i s h y d r o p h i l i c character can be a l t e r e d d r a s t i c a l l y by pretreatment. 3 . 5 . 7 . ; Heat Treatment LITERATURE REVIEW / 38 According to Young (1958), heat "treatment of a quartz sample causes the following phenomena: 1) condensation of surface s i l a n o l groups when the temperature exceeds 170°C; 2) rehydration by water vapour of s i t e s previously occupied by s i l a n o l groups ( r e v e r s i b l e at temperatures up to 400°C); 3) above 400°C the number of dehydrated s i t e s that can be r e v e r s i b l y rehydrated by water vapour decreases (aft e r heating at 850°C, no chemisorption of water vapour occurs so that a hydrophobic surface i s created); 4) not a l l of the surface s i l a n o l groups are removed even at the s i n t e r i n g temperature. The s i l a n o l groups remaining at t h i s high temperature are r e f e r r e d to as " i s o l a t e d " s i l a n o l groups. The removal of s i l a n o l groups by heat treatment has been studied by i n f r a r e d (IR) spectroscopic methods. The IR spectrum of high surface area amorphous s i l i c a reveals strong absorption bands at 1300, 1100 and 800 cm-1. These are a t t r i b u t e d to silicon-oxygen v i b r a t i o n s and are observed i n a l l forms of s i l i c a (Snoeyink and Weber, 1972). An absorption peak at 3650 cm-1 i s assigned to -OH within the bulk structure of s i l i c a . On a f u l l y hydrated surface, a band at 3550 cm-1 i s a t t r i b u t e d to paired (or hydrogen-bonded) -OH groups. After heating s i l i c a up to 600°C, the free (isolated) -OH groups which remain on the surface give r i s e to an absorption peak at 3750 cm-1 (Snoeyink and Weber, 1972). Griot and Kitchener (1965) reported a value of 3650 cm-1 for t h i s type of absorption by i s o l a t e d -OH groups. LITERATURE REVIEW / 39 The behaviour of s i l i c a upon heat treatment has been explained by the presence of strained siloxane groups. Heat treatment r e s u l t s i n the condensation of two neighbouring s i l a n o l groups, leading to a Si-O-Si linkage that i s under considerable s t r a i n i f the two s i l i c o n atoms are not the appropriate distance apart. This type of strained siloxane group can adsorb water, and can thus be r e a d i l y rehydrated. However, on heating to higher temperatures, surface m o b i l i t y might be s u f f i c i e n t to r e l i e v e the s t r a i n , so that the s i t e s are no longer susceptible to rehydration (Young, 1958). In f a c t , i t i s known that common a-quartz w i l l rearrange i t s structure to /3-quartz when heated to 573°C at atmospheric pressure. This transformation involves only a s l i g h t displacement of atoms and readjustment of bond angles - no breakage of bonds occurs (Hulburt and K l e i n , 1977). Further evidence supporting Young's model (1958) was reported by Pashley and Kitchener (1979). They found that the surface charge of quartz i s reduced upon heat treatment due to the removal of surface s i l a n o l groups. 3.5.1.2 Metal Ion Activation The a c t i v a t i o n of quartz by multivalent metal ions i n f l o t a t i o n i s well known and has been studied by many workers. In a soap f l o t a t i o n of quartz, these cations seem to have two a c t i v a t i n g functions: 1) they reduce the high negative surface charge to allow the adsorption of an anionic type c o l l e c t o r and 2) they act LITERATURE REVIEW / 40 chemically as a linkage between the quartz surface and the c o l l e c t o r . The r o l e of metal ions i n the f l o t a t i o n with f a t t y acids-type c o l l e c t o r s was demonstrated i n the pioneering s o l i d i f i c a t i o n studies of Wolstenholme and Schulman (1950). They observed that the o r i g i n a l l i q u i d - t y p e m y r i s t i c a c i d f i l m s o l i d i f i e d when the sol u t i o n substrate was i n the pH range where metal hydroxy complexes, such as Fe(OH) + +, Cu(OH) +, etc., were formed. Outside t h i s pH range the formation of s o l i d f i l m ceased. Since i r o n hydroxy complexes and copper hydroxy complexes form i n d i f f e r e n t pH ranges, Wolstenholme and Schulman (1950) s u c c e s s f u l l y separated p y r i t e and c h a l c o c i t e mixtures using caproic a c i d , i . e . , at pH 3, p y r i t e was f l o a t e d leaving c h a l c o c i t e behind, whereas at pH 6.5, c h a l c o c i t e was f l o a t e d leaving p y r i t e i n the residue. The adsorption of metal ions on oxide surfaces i s s t i l l a matter of controversy. Simple monovalent e l e c t r o l y t e s are adsorbed onto oxide surfaces merely as "counter ions". However, i n the case of s p e c i f i c a l l y adsorbing or hydrolyzable multivalent cations, the s i t u a t i o n i s much more complicated. These multivalent metal ions do not adsorb r e a d i l y onto a negatively charged surface u n t i l a pH i s reached at which some of the hydroxylated species are formed. The s o l i d substrate usually exhibits two or three charge reversal points i n the presence of such multivalent metal ions ( F i g . 3.5). Fuerstenau and Palmer (1976) have reviewed the f l o t a t i o n of oxides and s i l i c a t e s and concluded that the adsorption of multivalent LITERATURE REVIEW / 41 P H F i g . 3.5 - Schematic i l l u s t r a t i o n of the general e l e c t r o -phoretic mobility behaviour of c o l l o i d a l system i n the presence or absence of hydrolyzable metal ions (from James and Healy, 1972). LITERATURE REVIEW / 42 metal ions i s through t h e i r f i r s t order hydroxylation products, such as Cu(OH) +, Ca(OH) +, etc. They suggested that these species can adsorb on the oxide surface through hydrogen bonding or through a water release reaction ( F i g . 3.6). However, t h i s adsorption mechanism cannot explain the presence of multiple charge reve r s a l p o i n t s . Some consider, that there i s always at l e a s t one layer of water molecules present on the substrate so that the adsorbed species are a c t u a l l y separated from the surface (James and Healy, 1972). James and Healy (1972) conducted a series of adsorption studies of Co(II), La(III) and Th(lV) on T i 0 2 and S i 0 2 surfaces. They observed the c h a r a c t e r i s t i c zeta p o t e n t i a l curves of oxides i n the presence of hydrolyzable metal ions, which show three charge r e v e r s a l points (see F i g . 3.5). They a t t r i b u t e d CR1 to the i.e.p. (or p.z.c.) of the underlying oxide. CR3 normally occurs at pH values below the i.e.p. of the metal hydroxide but moves up as the thickness of the p r e c i p i t a t e d f i l m increases, c o i n c i d i n g with the behaviour of bulk hydroxide a f t e r about f i v e layers have been established. They proposed that CR2 r e s u l t s from the p r e c i p i t a t i o n of metal hydroxide i n the i n t e r f a c i a l region at a pH below that at which bulk p r e c i p i t a t i o n can occur. Because the high e l e c t r i c f i e l d (10 6 V cm-1) lowers the d i e l e c t r i c constant of the i n t e r f a c i a l medium well below that of the bulk aqueous solution, the s o l u b i l i t y product for the metal hydroxide i n the layer adjacent to the s o l i d i s lower (James and Healy, 1972). Fuerstenau and Manmohan (1979) observed that the order of adsorption of a l k a l i earth metal ions onto T i 0 2 i s Mg + +<Ca + +<Sr + +<Ba + +. Hence the LITERATURE REVIEW / 43 X n O H 0—C u U \ / H St / \ 0 0-H / 0 o 0|HH0|cu+ 0 X / O - C U X S i — Si + H 20 0-H '0-H F i g . 3.6 - Mechanisms of quartz a c t i v a t i o n by meta l ions (from Fuerstenau and Palmer , 1976). LITERATURE REVIEW / 44 degree of hydration i s an important factor during the adsorption of hydrolyzable metal ions onto oxide surfaces, because the hydration energies of the metal ions decrease i n the order of Mg + +>Ca + +>Sr + +>Ba + +. Mat i j e v i c et a l (1961) a t t r i b u t e d t h i s type of adsorption to the occurrence of polynuclear i o n i c species, e s p e c i a l l y f o r A l 3 + adsorption. James and Healy (1972) i n i t i a l l y believed that "adsorption cannot be due i n any general way to the appearance of s p e c i f i c polynuclear complexes". Wiese and Healy (1975), i n studying the adsorption of A l ( I I I ) on T i 0 2 , found that " i t i s not pos s i b l e to determine whether the abrupt increase i n ze t a - p o t e n t i a l , analogous to CR2, i s due to the adsorption of polynuclear species or to the nucleation of a surface p r e c i p i t a t e " . Some have suggested that there i s no fundamental co n t r a d i c t i o n between M a t i j e v i c et a l ' s model and that of James and Healy (Hunter, 1981). Somasundaran (1984), on the other hand, concluded that there i s no need to assume a d i f f e r e n t equilibrium constant for the i n t e r f a c e or to assume formation of various hydroxy complexes. He suggested that mineral-solution e q u i l i b r i a , along with i n t e r f a c i a l p o t e n t i a l s , play important r o l e s i n determining the adsorption behaviour of mineral systems. For the purpose of t h i s p r oject, i t seems reasonable to assume that: a. Simple monovalent metal ions adsorb as counter-ions at the LITERATURE REVIEW / 45 surface and only reduce the magnitude of the surface charge; b. Hydrolyzable multivalent metal ions can attach to the surface and are capable of reducing the magnitude or even reversing the sign of the surface charge. The metal ions are shielded by various H 20 and OH- species as coordination complexes. 3.5.1.3 Methylation When s i l i c a i s reacted with t r i m e t h y l c h l o r o s i l a n e ( ( C H 3 ) 3 S i C l ) , the following reaction occurs at the surface (Boehm, 1966): SILICA-OH + ( C H 3 ) 3 S i C l = SILICA-0-Si(CH 3) 3 + HC1 (9) The s i l i c a surface i s then coated with trimethyl groups and becomes hydrophobic. Preheating the s i l i c a at 800°C r e s u l t s i n very l i t t l e r e a ction between s i l i c a and trimethylchlorosilane, presumably because of the removal of s i l a n o l groups. S t e r i c hindrance l i m i t s the s u b s t i t u t i o n of surface s i l a n o l groups by t r i m e t h y l c h l o r o s i l a n e to about 40% (Snoeyink and Weber, 1972). Although 60% of the s i l a n o l groups s t i l l remain on the surface, many of these groups are not a c c e s s i b l e to water vapour. The surface coverage of trimethyl groups can be determined using a pH measurement (Blake and Ralston, 1985). Laskowski and Kitchener (1969) studied the contact angle and LITERATURE REVIEW / 46 zeta p o t e n t i a l s of methylated s i l i c a . They found that the methylated surface rehydrates slowly, f i n a l l y becoming hyd r o p h i l i c when immersed i n water ( e s p e c i a l l y i n a l k a l i n e s o l u t i o n s ) . This process i s r e v e r s i b l e , i . e . , by drying at 110°C the hydrophobicity of the methylated s i l i c a can be restored. They concluded that the f i n a l h y d r o p h i l i c state of methylated s i l i c a i s due to water molecules that are only p h y s i c a l l y associated with the surface (removal of surface s i l a n o l groups does not occur at t h i s temperature). They a l s o observed that the zeta p o t e n t i a l s of methylated and unmethylated s i l i c a are very s i m i l a r . Lamb and Furlong (1982) measured the contact angle on quartz c r y s t a l surfaces which had undergone either heat treatment at d i f f e r e n t temperatures or methylation, following t h i s heat treatment. These researchers obtained quartz surfaces with c o n t r o l l e d hydrophobicity by a combination of heat treatment and methylation. 3.5.2 Applicability in the Present System Kitchener and co-workers have conducted a number of i n t e r e s t i n g experiments i n v o l v i n g surfactant-(polymer)-quartz i n t e r a c t i o n s . Griot and Kitchener (1965) found that polyacrylamide adsorbs to the "free " s i l a n o l groups on the surface of s i l i c a , p o ssibly through hydrogen bonding. Thus f r e s h l y ground quartz (or t a l c ) i s s e n s i t i v e to f l o c c u l a t i o n i n aqueous solutions by the addition of polyacrylamide, but loses t h i s a b i l i t y upon extensive ageing. This i s presumably due LITERATURE REVIEW / 47 to i n t e r n a l hydrogen bonding among the surface s i l a n o l groups, which reduces the number of free s i l a n o l groups a v a i l a b l e f o r the adsorption of polyacrylamide. Later, Rubio and Kitchener (1976) studied the adsorption of poly(ethylene oxide) (PEO) on s i l i c a surfaces with d i f f e r e n t hydrophobicities. Again, they found that " i s o l a t e d " s i l a n o l groups- are probably the main adsorption s i t e s f o r PEO, po s s i b l y through hydrogen bonding. Such adsorption i s favoured when the region between the i s o l a t e d s i l a n o l groups i s hydrophobic (siloxane or methylated). This i s p o s s i b l y due to hydrophobic a s s o c i a t i o n between the -CH 2CH 2- chains i n ethylene groups and the hydrophobic regions of the s i l i c a surface. Polysaccharides are cur r e n t l y believed to i n t e r a c t with mineral surfaces through four types of forces, i . e . , hydrogen bonding, hydrophobic "bonding", and e l e c t r o s t a t i c and chemical i n t e r a c t i o n s . A quartz surface contains a large number of s i l a n o l groups which are capable of forming hydrogen bonds. These groups can either be removed by heat treatment or replaced or shielded by treatment with t r i m e t h y l c h l o r o s i l a n e . "Impurities" such as those capable of s p e c i f i c a l l y adsorbing multivalent metal ions can a l s o be introduced to the surface. Measurement of the adsorption d e n s i t i e s of polysaccharides on quartz samples with the above mentioned differences i n surface components should reveal those properties i n f l u e n c i n g the adsorption process and provide some in s i g h t concerning the mechani sm(s) involved. LITERATURE REVIEW / 48 3.6 SUMMARY 1. Starch and c e l l u l o s e are organic polymers composed of D-glucose residues. As such they can p a r t i c i p a t e i n a series of chemical reactions c h a r a c t e r i s t i c of polyhydroxyl compounds. Important reactions include h y d r o l y s i s , e t h e r i f i c a t i o n , e s t e r i f i c a t i o n and chemical complexation with metal cations. 2 . Starch, c e l l u l o s e and t h e i r d e r i v a t i v e s are useful as f l o t a t i o n depressants i n p r a c t i c a l l y a l l mineral systems. These include oxides, s i l i c a t e s , sulphides, salt-type minerals and inherently hydrophobic minerals. Although most of them are on a laboratory scale, some are used i n i n d u s t r i a l operations. 3. The depressant functions of these polysaccharides depend on t h e i r molecular weight and structure. 4. Polysaccharides have a l s o been reported to promote the f l o t a t i o n of c e r t a i n minerals, although t h i s behaviour has not been well established. 5. The most puzzling aspect of the topic under study i s the disparate adsorption properties of rela t e d polysaccharides onto mineral surfaces. Starch i s believed to adsorb through hydrogen bonding, although no experimental evidence has been provided to support such a conclusion. Hydrophobic bonding i s proposed to account for the adsorption of dextrin, but t h i s i s only i n f e r r e d from the observation that d e x t r i n e a s i l y adsorbs on hydrophobic minerals. In a very few cases researchers have proposed that chemical complexation of unmodified polysaccharides with mineral LITERATURE REVIEW / 49 surfaces occurs. In general, researchers seem to believe that chemical complexation with mineral surfaces i s only a r e s u l t of chemical s u b s t i t u t i o n or the presence of impurities i n the polysaccharides. 6. The properties of quartz have been well studied. Quartz surfaces can be r e a d i l y modified. This can be u t i l i z e d to study the adsorption of polysaccharides as a function of the substrate surface properties. CHAPTER 4 MATERIALS 4.1 MINERAL SAMPLES 4.1.1 Quartz Samples C r y s t a l quartz from Minas Gerais, B r a z i l was purchased from Ward's S c i e n t i f i c Establishment, Inc.. It was crushed i n laboratory jaw and cone crushers to a p a r t i c l e s i z e of about 5 mm. Crushings f i n e r than 2 mm were discarded. Coloured pieces were then removed manually. The remaining sample was leached with 0.1 M hydrochloric a c i d f o r a few days, then washed repeatedly with d i s t i l l e d water u n t i l no trace of c h l o r i d e ion could be detected by the AgN03 t e s t . Quartz samples were then dri e d at 60°C for several days and dry-ground i n a Retsch RM-0 agate mortar p u l v e r i z e r for 30 minutes. The -0.105 mm f r a c t i o n was screened out with a Ro-tap® f o r 30 minutes. This material was further separated i n t o -0.105+0.038 mm and -0.038 mm s i z e f r a c t i o n s using an Alpine Screen Analyser. The former f r a c t i o n was used for f l o t a t i o n tests and the l a t t e r for adsorption and electrophoretic measurements. 50 M A T E R I A L S / 51 X-ray d i f f r a c t i o n of the -0.038 mm f r a c t i o n showed that i t was pure quartz. No other minerals were detected. Elemental analysis d i d not show the presence of impurities, such as Fe, Ca, P and S. The surface areas of both f r a c t i o n s were measured by nitrogen adsorption using a Quantasorb Sorption system and was found to be 0.277 m2/g for the -0.105+0.038 mm f r a c t i o n and 1.153 m2/g for the -0.038 mm f r a c t i o n . An i n f r a r e d spectrum of a t y p i c a l quartz sample i s shown i n F i g . 4.1. Quartz has several strong absorption peaks below 1200 cm-1 which prevent the i d e n t i f i c a t i o n of adsorption products of polysaccharides, as w i l l be discussed l a t e r . Fused quartz plates used i n contact angle measurements wer-e purchased from VERTEQ-U.S. Quartz D i v i s i o n ( C a l i f o r n i a ) . The plates were leached with 1 N hydrochloric a c i d and then placed i n HN0 3-ethanol fumes for a few minutes. They were then washed repeatedly with d i s t i l l e d water and d r i e d at 110°C. These cleaned plates showed perfect hydrophilic character (contact angle zero). They were stored i n well-sealed sample b o t t l e s . 4.1.1.1 Preparation of Lead-coated Quartz The lead-coated quartz (PbQ) was prepared by conditioning a c e r t a i n amount of quartz (16 g of the -0.038 mm f r a c t i o n and 64 g of the -0.105+0.038 mm f r a c t i o n ) i n 400 ml of 0.0025 M lead n i t r a t e s o l u t i o n f o r 20 minutes at pH 10.5. A preliminary t e s t indicated that MATERIALS / 53 10.5 was the optimum pH value for the coating r e a c t i o n . After the reaction was completed the quartz sample was f i l t e r e d through a 934-AH microglass f i b r e f i l t e r , followed by drying at 110°C for 12 hours. The uptake of lead by the sample was determined by acid-leaching the quartz and then measuring the lead concentration i n the solution by atomic absorption spectroscopy. This was found to be 2.1x10-5 mole lead per m2 of quartz surface. Quartz samples prepared i n d i f f e r e n t batches were mixed well and stored i n sealed sample j a r s . Fused quartz plates were coated by lead n i t r a t e i n the same manner as described above. The drying procedure appeared to change the zeta p o t e n t i a l of the PbQ to negative over the e n t i r e pH range ( F i g . 4.2). Nevertheless, the PbQ was found to be r e a d i l y f l o a t a b l e i n sodium oleate solutions ( F i g . 4.3 and 4.4). This indicates that the lead was s t i l l present on the quartz surface. The PbQ f l o a t e d with oleate i n the pH range of 7 to 12, which i s i n good agreement with published r e s u l t s ( i . e . Fuerstenau and Fuerstenau, 1982). 4.1.1.2 Preparation of Methylated Quartz The quartz samples to be methylated (including both quartz and lead-coated quartz) were f i r s t d r i e d at 110°C for 24 hours. Twenty grams of the -0.038 mm f r a c t i o n (or 80 grams of the -0.105+0.038 mm f r a c t i o n ) was placed i n 100 ml of a benzene solution containing 0.04 m o l e / l i t e r t r i m e t h y l c h l o r o s i l a n e . In order to break p a r t i c l e MATERIALS / 54 O quar tz 9 MQ • PbQ (dr ied ) A PbQ ( f i l t e r e d ) -4 O quartz i n presence of 5x10 M Pb(N0„) O +100 +80 -80 F i g . 4 .2 - E f f e c t of pH on the ze ta p o t e n t i a l s of t y p i c a l quar tz samples. MATERIALS / F i g . 4 .3 - E f f e c t of pH on the Hal l imond tube f l o t a t i o n of t y p i c a l quartz samples. C o n d i t i o n s : 2 grams of -150+400// quar tz i n a 200 ml s o l u t i o n . MATERIALS / 56 F i g . 4.4 - E f f e c t of sodium o l e a t e c o n c e n t r a t i o n on the Ha l l imond tube f l o t a t i o n of PbQ. C o n d i t i o n s : 2 grams of -150+400# quar tz i n a 200 ml s o l u t i o n . MATERIALS / 57 aggregates i t was found necessary to place the -0.038 mm quartz samples together with the benzene solution i n an u l t r a s o n i c bath for about 60 seconds. The reaction tube was sealed and then rotated end-over-end for 15 minutes. A f t e r the reaction was completed the quartz was f i l t e r e d through a 934-AH microglass f i b r e f i l t e r , washed with benzene and then d r i e d at 110°C for 24 hours. Samples methylated i n d i f f e r e n t batches were mixed together and stored i n an oven at 110°C. Fused quartz plates were methylated i n the same manner. The methylated quartz (MQ) and quartz methylated a f t e r lead coating (MPbQ) both f l o a t e d e a s i l y (see F i g . 4.3). Coverage of methyl groups on the quartz surface was determined using a C0 2 Coulometer manufactured by Coulometrics Inc.. The t o t a l carbon contents of both methylated and unmethylated quartz samples were measured. The d i f f e r e n c e was taken to represent the amount of carbon associated with the methyl groups adsorbed on the surface. This was found to be 5.65x10-* mole of methyl groups per m2 of quartz surface and 1.83x10-' mole of methyl groups per m2 of PbQ surface. These data indicated that the methyl groups were bonded to quartz surfaces which had not been occupied by lead. F i g . 4.3 a l s o seems to show t h i s point. 4.1.2 Other Mineral Samples Chalcopyrite samples from S h i r a t a k i , Japan, and galena samples from Kansas, USA were a l l purchased from Ward's S c i e n t i f i c MATERIALS / 58 Establishment, Inc.. The chalcopyrite and galena were both leached i n 0.5 N hydrochloric a c i d solution for 60 minutes. After r i n s i n g and drying, they were crushed and ground i n a Brinkmann laboratory jaw crusher and agate mortar p u l v e r i z e r . The -0.105+0.038 mm and -0.038 mm f r a c t i o n s were then sieved and retained f o r f l o t a t i o n and adsorption t e s t s . The s p e c i f i c surface area of the -0.038 mm siz e f r a c t i o n was found to be 0.3742 m2/g for galena and 0.5562 m2/g for chalcopyrite. Chemical analyses showed that PbS content i n the galena samples was 95% and CuFeS 2 content i n the chalcopyrite samples was 87%. X-ray d i f f r a c t i o n t e s t s d i d not show any diagnostic peaks to i d e n t i f y the remaining impurities. 4.2 CHEMICAL REAGENTS 4.2.1 Polysaccharides The de x t r i n was a J . T. Baker product, G193-7, purchased from Johns S c i e n t i f i c , Inc.. The same reagent was used by Wie and Fuerstenau (1974); i t s molecular weight was reported to be 7,900. Carboxymethyl c e l l u l o s e with a DS of 0.7 and a molecular weight of 250,000 (as s p e c i f i e d by the manufacturer) was purchased from Polysciences, Inc.. Amylopectin was purchased from BDH Chemicals. The i n t r i n s i c v i s c o s i t i e s of the dextrin, CMC and amylopectin used i n t h i s study were measured with an Ostwald viscometer and found to be 9.3, 655 and 129 cm3/g, r e s p e c t i v e l y (Figs. 4.5 to 4.7). The MATERIALS / 59 Fig. 4 .5 - Viscosity of aqueous NaCMC solutions. M A T E R I A L S / 6 0 0.15 IX CO a i H i-l O ^ 0.10 o 0.05 0 1 2 3 4 Concentra t ion of D e x t r i n , g/100 ml F i g . 4.6 - V i s c o s i t y of aqueous d e x t r i n s o l u t i o n s . MATERIALS / 61 0 1 2 Concentra t ion of A m y l o p e c t i n , g/100 ml F i g . 4.7 - V i s c o s i t y of aqueous amylopect in s o l u t i o n s . MATERIALS / 62 c a l c u l a t e d molecular weight of CMC ([77] = 655) obtained using equation (5) i s 380,000, which i s i n reasonably good agreement with the manufacturer's s p e c i f i c a t i o n . The amylopectin used i n t h i s work had a s l i g h t l y lower i n t r i n s i c v i s c o s i t y than that of potato amylopectin, which was reported by A s p i n a l l (1970) to be i n the range of 150-200 cm3/g. The use of l i g h t s c attering y i e l d e d molecular weights of 4x10 s, 2xl0 7 and 2x10 s for CMC, amylopectin and dextrin, r e s p e c t i v e l y . The exceedingly high molecular weight obtained f o r de x t r i n (200,000) cannot be considered r e l i a b l e . The impurity l e v e l s i n the three polysaccharides were analysed by Canadian M i c r o a n a l y t i c a l Inc. and are l i s t e d i n Table 4.1 ("ND" stands for "not determined"). I t i s seen that the contents of these elements (N, P and S) are very low. The sodium content of the NaCMC was 7.04 per cent, i n d i c a t i n g 0.65 sodium carboxylate groups per glucose monomer. This i s i n a good agreement with the DS of 0.7 sp e c i f i e d by the manufacturer. T i t r a t i o n of d e x t r i n and CMC solutions with NaOH and HC1 (Figs. 4.8 and 4.9) showed that d e x t r i n does not contain any detectable f u n c t i o n a l groups, whereas NaCMC has a basic (anionic carboxylate) groups. Infrared spectra of the three polysaccharides are shown i n F i g . 4.10. A l l three spectra show common absorption peaks of -C-C or -C-0 MATERIALS / 65 Table 4.1 - Impurity l e v e l s i n the polysaccharides, % N P S Na NaCMC <0.05 ND ND 7.04 Amylopectin <0.05 ND ND ND Dextrin 0.087 <0.05 <0.05 ND groups (1200 to 1000 cm- 1). NaCMC exhibits one diagnostic absorption peak at 1610 cm-1, and probably one at 1420 cm-1 as w e l l . The undissociated -C00H group has a t y p i c a l IR absorption peak at around 1700 cm-1 due to the carbonyl double bond. However, when disso c i a t e d , the electrons are equally d i s t r i b u t e d between the carbon and the two oxygen atoms so that a double bond does not e x i s t . The consequence i s the s p l i t t i n g of the o r i g i n a l absorption peak at around 1700 cm-1 i n t o two absorption peaks at lower frequencies, i . e . , one around 1600 cm-1 (asymmetric stretching of the carboxylate group), and one around 1400 cm-1 (symmetric stretching of the carboxylate group). Several absorption peaks i n the region between 1000 and 700 cm-1 should also be noted. NaCMC shows a peak at 890 cm-1, and d e x t r i n and amylopectin have three consecutive absorption peaks i n t h i s region: 925, 855 and 760 cm-1. According to Barker et a l (1954), the peaks at 890 cm-1 i n the NaCMC and at 855 cm-1 i n the d e x t r i n and amylopectin spectra are the r e s u l t s of the C-H deformation on C - l , which i s diagnostic for the 0- and a- anomers of glucose. The peaks at 925 and 760 cm-1 i n the d e x t r i n and amylopectin spectra are the r e s u l t s of asymmetric and symmetric v i b r a t i o n s of the glucose r i n g , r e s p e c t i v e l y ( F i g . 4.11). MATERIALS / 67 A B O Oxygen atom below-xy plane C' Carbon atom below xy plane O Carbon atom above xy plane F i g . 4 .11 - Schematics of the v i b r a t i o n s of the g lucose r i n g . A ) , asymmetric and B ) . symmetric (from Barker et a l , 1954). MATERIALS / 68 4.2.1.1 Preparation of CMC Stock Solutions A 200 ml aliquot of 0.01 M NaCl was heated to 50°C and then 1 gram of CMC powder was slowly added with s t i r r i n g . A f t e r s t i r r i n g f o r approximately 30 minutes, the solu t i o n was transferred to a o n e - l i t e r volumetric f l a s k and d i l u t e d with 0.01 M NaCl to the mark. 4.2.1.2 Preparation of Dextrin Stock Solutions One gram of de x t r i n powder was weighed out and then s t i r r e d with a small amount of cold water to form a thick, homogeneous paste. Approximately 200 ml of b o i l i n g 0.01 M NaCl was then q u i c k l y added to dis s o l v e the paste. Af t e r cooling, the suspension was transferred to a o n e - l i t e r volumetric f l a s k and d i l u t e d with 0.01 M NaCl to the mark. 4.2.1.3 Preparation of Amylopectin Stock Solutions A homogeneous amylopectin paste was made up by digest i n g 1 gram of amylopectin with 2 ml of 0.5 N NaOH. Approximately 200 ml of b o i l i n g 0.01 M NaCl was then added to di s s o l v e the paste. A f t e r cooling, the solu t i o n was transferred to a o n e - l i t e r volumetric f l a s k , n e u t r a l i z e d with 2 ml 0.5 N HC1, and then d i l u t e d with 0.01 M NaCl to the mark. The amylopectin, d e x t r i n and CMC stock solutions were a l l prepared fresh every 3 to 4 days. MATERIALS / 69 4.2.2 Other Chemicals Table 4.2 l i s t s other reagents used i n t h i s work. MATERIALS / 70 Table 4.2 - Chemicals used i n the experiments Chemical Grade Supplier Usage Pb(H0.,)2 A.C.S. reagent A l l i e d Chemicals Quartz coating, t i t r a t i o n , IR, co - p r e c i p i t a t i o n MgCl 2-6H 20 "Baker analysed" J . T. Baker c o - p r e c i p i t a t i o n reagent CuCl 2 Fisher c e r t i f i e d Fisher T i t r a t i o n , reagent c o - p r e c i p i t a t i o n F e C l 3 «6H 20 A.C.S. reagent A l l i e d Chemicals same as above A1(N0 3) 3 Fisher c e r t i f i e d Fisher C o - p r e c i p i t a t i o n reagent Phenol A.C.S. reagent MCB Determination of Manufacturing polysaccharide Chemists, Inc. concentration H 2S0 4 A n a l y t i c a l BDH Chemicals same as above reagent NaCl A n a l y t i c a l BDH Chemicals Maintain i o n i c reagent strength Sodium oleate Fisher p u r i f i e d Fisher PbQ F l o t a t i o n , adsorption KEX p u r i f i e d - Sulphide f l o t a t i o n Benzene A n a l y t i c a l BDH Chemicals Quartz reagent methylation ( C H 3 ) 3 S i C l A n a l y t i c a l BDH Chemicals same as above reagent HC1 A.C.S. reagent American pH regulator S c i e n t i f i c and Chemical NaOH A.C.S. reagent same as above pH regulator Ethanol A.C.S. reagent Commercial Glassware Alcohols Ltd. cleaning HN03 A n a l y t i c a l A l l i e d Chemicals same as above reagent CHAPTER 5 EXPERIMENTAL METHODS 5.1 HALLIMOND TUBE FLOTATION TESTS Hallimond tube f l o t a t i o n tests were used to evaluate the f l o a t a b i l i t i e s of quartz samples and other minerals. A Hallimond tube modified by Siwek et a l (1981) was employed. In a t y p i c a l t e s t , 2 grams of a mineral (-0.105+0.038 mm) was mixed with 200 ml of sol u t i o n at a given pH. The mixture was conditioned for 10 minutes a f t e r the addition of each reagent using a mechanical s t i r r e r . The pH of the solu t i o n was maintained at the same value during conditioning. Af t e r conditioning the mineral sample was transferred to the Hallimond tube and f l o a t e d at 30 standard ml/minute nitrogen. The f l o t a t i o n time was 2 minutes for methylated quartz and sulphides and 5 minutes for other quartz samples and synthetic sulphide mineral mixtures. The Hallimond tube displayed very low mechanical carryover. For example, with the -0.105+0.038 mm quartz the y i e l d of f l o a t e d material i n d i s t i l l e d water a f t e r a 2 minute f l o t a t i o n was les s than 2%. It was found necessary to trea t the glass f r i t with concentrated n i t r i c a c i d r e g u l a r l y to "regenerate" i t , e s p e c i a l l y i n the f l o t a t i o n 71 EXPERIMENTAL METHODS / 72 of sulphide minerals. Generally t h i s Hallimond tube worked well and the recoveries were reproducible within ±5%. Senior and de Araujo (1984) reported an accuracy of ±4% with the same tube. 5.2 CONTACT ANGLE MEASUREMENTS Contact angles were measured with a Rame-Hart 100-00 Goniometer using the captive bubble technique. The procedures adopted for quartz and lead-coated quartz d i f f e r e d s l i g h t l y . For quartz and MQ, the p l a t e was placed i n a 100 ml s o l u t i o n without adjusting the pH. A f t e r s t i r r i n g for 10 minutes, 40 ml of the sol u t i o n together with the p l a t e was transferred to a measuring c e l l and the contact angle was determined. The quartz p l a t e was then re-used to make successive contact angle measurements at d i f f e r e n t pH values. For PbQ and MPbQ, the 100 ml solution was f i r s t adjusted to the appropriate pH value. The quartz p l a t e was then placed i n t o the sol u t i o n and e q u i l i b r a t e d for 10 minutes. Then 40 ml of the so l u t i o n together with the plate was transferred to the contact angle c e l l f o r measurement. The plate was then discarded and a new p l a t e was used for a d i f f e r e n t pH. The procedure for determining the contact angle was as follows: an a i r bubble formed at the t i p of a blunt-ended #16 hypodermic needle was pressed against the quartz p l a t e with gentle tapping of EXPERIMENTAL METHODS / 73 the supporting platform. After at l e a s t a two minute e q u i l i b r a t i o n , the needle was r a i s e d u n t i l the area of contact between the a i r bubble and quartz p l a t e just began to contract. The angles of contact through the aqueous phase on both sides of the bubble were then measured. At l e a s t three bubbles were tested at d i f f e r e n t locations on the plate, and an average of these measurements was taken as the contact angle. The standard deviation of these measurements varied but never exceeded 5.5 degrees. 5.3 CO-PRECIPITATION TESTS These te s t s were conducted to v i s u a l i z e the i n t e r a c t i o n between polysaccharides and metal i o n i c species i n aqueous solutions. A c e r t a i n amount of metal c h l o r i d e or metal n i t r a t e s o l u t i o n was mixed with a polysaccharide s o l u t i o n so that the f i n a l concentrations of metal ion and polymer were 10- 3 M and 100 ppm, r e s p e c t i v e l y . The s o l u t i o n pH was then adjusted, accompanied by gentle s t i r r i n g . After standing for a few minutes, the so l u t i o n was centrifuged at 8,000 rpm f o r 3 minutes i n an IEC model HT centrifuge. The supernatant was assayed for the r e s i d u a l metal ion and polysaccharide concentrations. Blank te s t s containing either polysaccharide or metal i o n i c species were also c a r r i e d out. 5.4 TITRATION ANALYSIS EXPERIMENTAL METHODS / 74 These te s t s were conducted with a Metrohm automatic t i t r a t i o n u n i t , a highly precise instrument. A metal ch l o r i d e or metal n i t r a t e s o l u t i o n of a given concentration was adjusted to a f i x e d pH value. This s o l u t i o n was then t i t r a t e d slowly by the addition of a polysaccharide solution of the same pH. The pH of the r e s u l t i n g s o l u t i o n was recorded as a function of the amount of polysaccharide added. Blank te s t s were a l s o c a r r i e d out. 5.5 INFRARED SPECTROSCOPIC STUDIES The p r e c i p i t a t e s from the c o - p r e c i p i t a t i o n or from the t i t r a t i o n t e s t s were examined by i n f r a r e d (IR) spectroscopy. The s o l i d samples to be studied were d r i e d at 110°C for at l e a s t 24 hours. Two mg of s o l i d sample was well mixed with 298 mg C s l powder i n an agate mortar. The mixture was then transferred to a d i e , evacuated for approximately 2 minutes and then pressed i n t o a transparent p e l l e t at 4,800 p s i . The p e l l e t was scanned from 4,000 cm-1 to 200 cm-1 with a Perkin-Elmer model 283B double beam i n f r a r e d spectrophotometer. The humidity i n the IR spectrophotometer sample compartment was maintained at 16% using a dehumidifier. A l l instrument settings were kept at the normal p o s i t i o n s . 5.6 A D S O R P T I O N T E S T S EXPERIMENTAL METHODS / 75 The adsorption d e n s i t i e s of d i f f e r e n t polysaccharides on mineral samples were determined as functions of both pH and r e s i d u a l polysaccharide concentration. Polysaccharide concentrations were determined by the phenol-sulphuric a c i d method as described by Dubois et a l (1956). In the adsorption t e s t , 1 gram of the -0.038 mm mineral sample was mixed with 25 ml 0.01 M NaCl solution at the appropriate pH. After the s o l i d was completely wetted, 25 ml of a two-fold concentrated polysaccharide solution of i d e n t i c a l pH was added. After mixing, the pH of the 50 ml suspension was checked and adjusted i f necessary. The suspension was then placed i n a Lab-line Orbit Environ Shaker and conditioned for 60 minutes at 300 rpm, 25°C. K i n e t i c a n a l y s i s revealed that the e q u i l i b r a t i o n time was long enough for equilibrium to be reached (see Appendix I ) . After e q u i l i b r a t i o n , the s o l u t i o n pH was rechecked and recorded, and the solution was centrifuged at 8,000 rpm for 3 minutes. The supernatant was assayed for the r e s i d u a l concentration of polysaccharide. For t h i s purpose, 4 ml of the supernatant was taken and mixed with 1.5 ml of a 5% phenol s o l u t i o n . A f t e r shaking, 10 ml of concentrated sulphuric acid was added r a p i d l y . This phenol-sulphuric a c i d solution was allowed to cool f o r 10 minutes i n a i r , then placed i n a waterbath and e q u i l i b r a t e d at 25°C for 15 minutes. The absorbance of the solution was measured at 490 nm against d i s t i l l e d water with a Perkin-Elmer Lambda 3 UV-VIS spectrophotometer. Polysaccharide concentrations were determined using a c a l i b r a t i o n curve. The d i f f e r e n c e between the o r i g i n a l and r e s i d u a l EXPERIMENTAL METHODS / 76 polysaccharide concentration was taken as the amount adsorbed (Appendix I shows an example of t h i s c a l c u l a t i o n ) . The adsorption r e s u l t s were found to be very reproducible (Appendix I ) . Generally the accuracy of the adsorption density was within ±0.1 mg/m2. 5.7 ELECTROPHORETIC MEASUREMENTS The electrophoretic m o b i l i t i e s of various mineral samples were measured with a Zeta Meter. F i f t y mg of the -0.038 mm mineral powder was mixed with a 50 ml a l i q u o t of a polysaccharide solution of a p a r t i c u l a r concentration and pH. The suspension was then shaken for 30 minutes i n a Lab-line Orbit shaker at 25°C, 300 rpm, followed by the transfer of the suspension to an electrophoresis c e l l and the measurement of p a r t i c l e e l e c trophoretic m o b i l i t y . At least eight p a r t i c l e s were tracked i n each d i r e c t i o n and the average value was used for c a l c u l a t i o n of the zeta p o t e n t i a l . The i o n i c strength was maintained at 10- 2 M NaCl for the electrophoretic measurements as well as the f l o t a t i o n , contact angle and adsorption experiments. CHAPTER 6 RESULTS AND DISCUSSION 6.1 DEPRESSIVE EFFECT OF THE POLYSACCHARIDES 6.1.1 Flotation Experiments Methy la ted quar tz (MQ), l e a d coated then methyla ted quar tz (MPbQ) and lead-coa ted quar tz (PbQ) were t e s t ed i n the f l o t a t i o n exper iments . Sodium o l e a t e was used as a c o l l e c t o r fo r PbQ. The f l o t a t i o n experiments were conducted i n the presence or absence of CMC, d e x t r i n or amy lopec t in . Both pH and polymer concen t r a t i ons were v a r i e d . In the f l o t a t i o n of PbQ, the a d d i t i o n sequence of sodium o l e a t e and p o l y s a c c h a r i d e was a l s o r e v e r s e d . F i g s . 6.1 t o 6.3 show the d e p r e s s i v e e f f ec t of CMC on the f l o t a t i o n of quar tz samples. CMC d i d not depress the f l o a t a b i l i t y of MQ, except i n s t r o n g l y a c i d i c s o l u t i o n s ( F i g s . 6.2 and 6 . 3 ) . F i g . 6.2 shows tha t below pH 3 the f l o a t a b i l i t y of MQ began to decrease i n the presence of CMC. In c o n t r a s t , the f l o t a t i o n of MPbQ appeared to be a f f e c t e d a t h igh pH va lues i n the presence of CMC. Al though the c r i t i c a l pH fo r the f l o t a t i o n of MPbQ was unchanged, the f l o a t a b i l i t y 77 RESULTS AND DISCUSSION / 78 F i g . 6 .1 - E f f e c t of pH on the f l o t a t i o n of PbQ i n the presence or absence of NaCMC. C o n d i t i o n s : 2 grams of -150+400^ quar tz i n a 200 ml s o l u t i o n . Sodium o l e a t e : 2x10 M. F i g . 6 - E f f e c t s of pH and NaCMC on the Hal l imond tube f l o t a t i o n of MQ and MPbQ. C o n d i t i o n s : 2 grams of -150+400// quar tz i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION / 80 Concentra t ion of CMC, ppm F i g . 6.3 - E f f e c t of NaCMC c o n c e n t r a t i o n on the Ha l l imond tube f l o t a t i o n of quartz samples. C o n d i t i o n s : 2 grams of -150+400// quartz i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION / 81 of MPbQ was depressed even further i n the presence of CMC (Figs. 6.2 and 6.3). The f l o t a t i o n of PbQ by sodium oleate was i n h i b i t e d i f CMC was added p r i o r to oleate ( F i g . 6.1). However, i f CMC was added a f t e r oleate, the depressive e f f e c t was i n s i g n i f i c a n t ( F i g . 6.3). A depressant cannot function i f (a) i t does not adsorb onto the surface of the s o l i d which i s to be depressed, or (b) upon adsorption i t s molecular o r i e n t a t i o n does not confer a strongly h y d r o p h i l i c character. Inadequate adsorption of CMC may have been the primary reason for the observed decrease i n the depressive e f f e c t of CMC upon rev e r s a l of the order of oleate and CMC a d d i t i o n ( F i g . 6.3). Mutually exclusive occupancy of PbQ surface adsorption centres by either sodium oleate or CMC appeared l i k e l y . PbQ was negatively charged over the e n t i r e pH range studied ( F i g . 4.2). Its f l o t a t i o n by sodium oleate (an anionic c o l l e c t o r ) r e s u l t s from the chemisorption of oleate groups on the Pb s i t e s of the PbQ surface (e.g. Leja, 1982). The competitive e f f e c t of CMC was a t t r i b u t e d to the presence of carboxymethyl groups i n t h i s polymer. Fig s . 6.4 to 6.7 show the depressive e f f e c t of d e x t r i n . Dextrin had only a s l i g h t depressive e f f e c t on PbQ-flotation i f added a f t e r oleate. However, i t caused a strong depression of PbQ i f added before oleate (Figs. 6.4 and 6.5). This depression was s i g n i f i c a n t even at d e x t r i n concentrations as low as 1 ppm. The d i f f e r e n c e i n the f l o a t a b i l i t y of PbQ obtained by reversing the order of d e x t r i n and RESULTS AND DISCUSSION / 82 100 ' ' 1 ' 1 1 ! 2 4 6 8 10 12 14 PH 6.4 - E f f e c t s of pH and v a r i o u s reagents on the Ha l l imond tube f l o t a t i o n o f PbQ. C o n d i t i o n s : 2 grams of -150+400# quar tz i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION A 83 Concentra t ion of D e x t r i n , ppm F i g . 6.5 - E f f e c t of d e x t r i n c o n c e n t r a t i o n on the Ha l l imond tube f l o t a t i o n of PbQ. C o n d i t i o n s : 2 grams of -150+400// quar tz i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION / 84 Fig. . 6.6 - E f f e c t s of pH and 100 ppm d e x t r i n on the Hal l imond tube f l o t a t i o n of MQ and MPbQ. C o n d i t i o n s : 2 grams of -150+400// quar tz i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION / 85 F i g . 6.7 - E f f e c t of d e x t r i n c o n c e n t r a t i o n on the Hal l imond tube f l o t a t i o n of MQ and MPbQ. C o n d i t i o n s : 2 grams of -150+400# quar tz i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION / 86 oleate a d d i t i o n was 50% at pH 11, a r e s u l t obtained over a dex t r i n concentration range from 1 to 100 ppm ( F i g . 6.5). Futhermore, F i g . 6.5 a l s o shows that depression of PbQ by dex t r i n was strongly pH-dependent. At pH 9.2, there was no depression of PbQ by 100 ppm de x t r i n , even i f i t was added before oleate. However, at pH 11, PbQ was completely depressed by only 1 ppm dex t r i n (added p r i o r to o l e a t e ) . These r e s u l t s confirmed the a b i l i t y of d e x t r i n to compete with sodium oleate. Unlike CMC, dex t r i n contains no carboxylate (or other i o n i z a b l e ) functional groups. Hence the observed competition could only be a t t r i b u t e d to the large number of hydroxyl groups i n the de x t r i n molecule, as these are the only polar groups present i n the polymer. The strong pH dependence of the depressant function of de x t r i n toward PbQ has important mechanistic implications which w i l l be discussed l a t e r . Dextrin d i d not show any depressive e f f e c t on MQ, i r r e s p e c t i v e of the pH. However, when the methylated quartz a l s o contained lead i o n i c species on i t s surface (MPbQ), strong depression occurred at a pH higher than about 7, a two pH units decrease from i n the absence of d e x t r i n ( F i g . 6.6). F i g . 6.7 shows more c l e a r l y the e f f e c t of lead ions on the depressive action of d e x t r i n i n the f l o t a t i o n of methylated quartz. F i n a l l y , i t should be noted that d e x t r i n acted as a stronger depressant than CMC for a l l of the quartz samples studied. The depressive action of amylopectin i s shown i n F i g s . 6.8 to RESULTS AND DISCUSSION / 87 6.11. The behaviour of amylopectin was s i m i l a r to that of de x t r i n . Thus amylopectin a l s o appeared to be a stronger depressant than CMC for the quartz samples studied. In addition, the introduction of lead ions to the quartz surface played a major r o l e i n the depressive ac t i o n of amylopectin. 6.1.2 Contact Angle Measurements The hydrophobicity of the quartz plates as determined by , contact angle measurements correlated well with the Hallimond tube f l o t a t i o n behaviour. In F i g . 6.12, the contact angles measured on MQ plates are pl o t t e d as a function of pH i n the presence or absence of 100 ppm of CMC or d e x t r i n . The presence of 100 ppm of dext r i n d i d not change the contact angle at any pH value. In the presence of 100 ppm CMC, the contact angle decreased i f the pH was less than 4. This decrease i n contact angle i s i n agreement with the Hallimond tube f l o t a t i o n test r e s u l t s obtained under the same conditions ( F i g . 6.2). Contact angles measured on MPbQ plates decreased continuously with increasing pH ( F i g . 6.13). The presence of de x t r i n or CMC reduced the contact angle s l i g h t l y . Again, d e x t r i n seemed to be more e f f e c t i v e i n reducing the contact angle than CMC. However, there was no sharp decrease i n the contact angle as the pH was increased, i . e . , the " c r i t i c a l pH" observed i n the f l o t a t i o n tests was not r e f l e c t e d i n the contact angle measurements. Perhaps the MPbQ surface was a c t u a l l y s l i g h t l y hydrophobic, but the time required to e s t a b l i s h RESULTS AND DISCUSSION / 88 F i g . 6.8 - E f f e c t s of pH and amylopect in on the Hal l imond tube f l o t a t i o n of PbQ. C o n d i t i o n s : 2 grams of -150+400# quar tz i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION / 89 F i g . 6.9 - E f f e c t of amylopect in c o n c e n t r a t i o n on the Ha l l imond tube f l o t a t i o n of PbQ at pH 11. C o n d i t i o n s : 2 grams of -150+400# quar tz i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION / 90 F i g . 6.10 - E f f e c t s of pH and 100 ppm amylopect in on the Hal l imond tube f l o t a t i o n of MQ and MPbQ. C o n d i t i o n s : 2 grams of -150+400# quar tz i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION / 91 F i g . 6.11 - E f f e c t of amylopect in c o n c e n t r a t i o n on the Ha l l imond tube f l o t a t i o n of MO and MPbQ. C o n d i t i o n s : 2 grams of -150+400// quar tz i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION / 100 6.12 - E f f e c t s of pH and depressants on the w e t t a b i l i t y of MQ p l a t e s . RESULTS AND DISCUSSION / 93 100 Fig . 6.13 - Effects of pH and depressants on the wettability of MPbQ plates. RESULTS AND DISCUSSION / 94 contact between a i r bubbles and quartz p a r t i c l e s (induction time) i n f l o t a t i o n was longer at high pH. This k i n e t i c b a r r i e r was a c t u a l l y observed i n the experiments. At high pH, i t usually took several seconds to get a bubble attached to the MPbQ p l a t e , whereas t h i s period was usually less than two seconds for the MQ p l a t e . Contact angles measured on PbQ plates are shown i n F i g s . 6.14 and 6.15. The maximum contact angle on PbQ i n the presence of 2x10- 5 M sodium oleate occurred around pH 10, which agreed well with the f l o t a t i o n r e s u l t s . When introduced a f t e r sodium oleate, d e x t r i n reduced the contact angle on PbQ s l i g h t l y at a pH above 7, whereas CMC had no e f f e c t ( F i g . 6.14). If either polymer was introduced p r i o r to oleate, the contact angle was reduced. In these cases, d e x t r i n was somewhat more e f f e c t i v e at high pH values, whereas CMC was more e f f e c t i v e at lower pH values ( F i g . 6.15). For example, at pH 10 the contact angle was 40 degrees i n 100 ppm CMC, but only 20 degrees i n 100 ppm d e x t r i n . At pH 6.2, the contact angle was zero i n 100 ppm CMC and 22 degrees i n 100 ppm de x t r i n . 6.1.3 Summary Both Hallimond tube f l o t a t i o n t e s t s and contact angle measurements showed that none of the polysaccharides studied depressed the f l o a t a b i l i t y of hydrophobic methylated quartz. However, i f the methylated quartz also contained lead ions on the surface (MPbQ), strong depression of quartz was observed above a c e r t a i n pH value. RESULTS AND DISCUSSION / 95 F i g . 6.14 - E f f e c t s of pH and var ious reagents on the w e t t a b i l i t y ..of PbQ p l a t e s . The p la te s were f i r s t t rea ted w i t h 2x10 M sodium o l e a t e , then w i t h 100 ppm polymer. RESULTS AND DISCUSSION / 96 100 1. t r ea ted w i t h 2x10 ^ M sodium o l e a t e 2 . t r ea ted w i t h 100 ppm d e x t r i n , then w i t h 2x10 M sodium o l e a t e 80 3 . t r e a t e d w i t h 100 ppm NaCMC, then w i t h 2x10 M sodium o l e a t e cn cu cu )-> 60 cu T3 cu 60 c < o u c o o 60 40 20 10 2 M NaCl PH F i g . 6.15 - E f f e c t s of pH and var ious reagents on the w e t t a b i l i t y of PbQ p l a t e s . The p l a t e s were f i r s t t r e a t e d w i t h 100 ppm polymer , then w i t h 2x10 M sodium o l e a t e . RESULTS AND DISCUSSION / 97 F l o t a t i o n of PbQ w i t h sodium o l e a t e was a l s o depressed by the p o l y s a c c h a r i d e s above a c e r t a i n pH. The dep res s ive a c t i o n was g rea te r i f the polymer was in t roduced p r i o r to o l e a t e . Both d e x t r i n and amylopec t in seemed to be most e f f e c t i v e a t pH 11 as depressants for PbQ and MPbQ; CMC ac ted as a much weaker depressant under the same c o n d i t i o n s . 6.2 AGGREGATION IN POLYSACCHARIDE-METAL AQUEOUS SOLUTIONS The l i t e r a t u r e on d e x t r i n a d s o r p t i o n and dep re s s ion con ta ins abundant re fe rences to the theory of "hydrophobic bonding" (Sec t ions 3 .4 .1 and 3 . 4 . 4 ) . However, there i s no exper imenta l evidence fo r such an i n t e r a c t i o n . The r e s u l t s of t h i s work suggest a v e r y important r o l e fo r meta l i o n a d s o r p t i o n cen t res i n the a p p l i c a t i o n of a l l th ree p o l y s a c c h a r i d e s ( d e x t r i n , CMC and a m y l o p e c t i n ) . In m i n e r a l process e n g i n e e r i n g , the importance of metal i o n s i n the behaviour of CMC has been r epo r t ed ( S o l a r i et a l , 1986) and i s b e l i e v e d to be due t o the presence of carboxymethyl groups i n t h i s p o l y s a c c h a r i d e . The na ture of the i n t e r a c t i o n between d e x t r i n (or amylopect in) and metal i o n i c spec ies i s p o o r l y understood and needs fu r the r i n v e s t i g a t i o n . 6.2.1 Co-precipitation Tests In the absence of p o l y s a c c h a r i d e , the l ead n i t r a t e s o l u t i o n turned t u r b i d upon i n c r e a s i n g the s o l u t i o n pH. When p o l y s a c c h a r i d e and l ead n i t r a t e s o l u t i o n s were mixed, l a r g e f l o e s appeared i n c e r t a i n pH RESULTS AND DISCUSSION / 98 ranges, i n d i c a t i n g i n t e r a c t i o n s between these polymers and the lead i o n i c species. To f i n d out what types of i o n i c species were i n t e r a c t i n g with the polysaccharides, the r e s i d u a l concentrations of polysaccharide and lead ions were determined using the procedure described i n Section 5.3. The r e s u l t s are shown i n F i g s . 6.16 through 6.22. CMC was p r e c i p i t a t e d completely over the pH range from 3 to 11 when present at 100 ppm i n a 10- 3 M lead n i t r a t e s o l u t i o n (Fig 6. 16). In t h i s s o l u t i o n lead p r e c i p i t a t i o n increased l i n e a r l y with increasing pH u n t i l a maximum was reached at pH 11. The major species present i n an aqueous lead n i t r a t e s o l u t i o n i n the pH range from 3 to 11 are P b + +, PbOH-, Pb(OH) 2(aq.), Pb(OH) 2(s), P b 3 ( C 0 3 ) 2 ( O H ) 2 t and HPbOj. It appeared that CMC was able to i n t e r a c t with a l l such species. The DS of CMC was 0.7, thus 100 ml of 100 ppm CMC contained 3.2x10-5 mole of carboxymethyl groups. On a stoichiometric basis these should have reacted with 1.6x10-5 mole of lead ions, which represented only 16% of the t o t a l lead present i n the s o l u t i o n . At pH 11, 90% of the lead p r e c i p i t a t e d from the lead nitrate-CMC s o l u t i o n ( F i g . 6.16). Afte r c o r r e c t i n g for the f r a c t i o n (25%) of lead p r e c i p i t a t e d i n the absence of CMC, i t was determined that CMC p r e c i p i t a t e d 65% of the t o t a l lead present. This discrepancy between the expected (16%) and obtained (65%) values for the p r e c i p i t a t i o n of lead by CMC suggested f Atmospheric C0 2 can dissolve into the lead nitrate solution and form such complexes with lead above pH 8. However, since the "surface" of this precipitate is dominated by lead hydroxy complexes (e.g., Valdivieso et al, 1986), only the lead hydroxy complexes will be considered in this discussion. RESULTS AND DISCUSSION / 99 P H F i g . 6.16 - E f f e c t of pH on the r e s i d u a l c o n c e n t r a t i o n of NaCMC and l e a d , a f t e r c e n t r i f u g i n g at 8,000 rpm f o r 3 minutes . R e s i d u a l c o n c e n t r a t i o n of NaCMC: 1, NaCMC s o l u t i o n ; 2, NaCMC-lead n i t r a t e s o l u t i o n . R e s i d u a l c o n c e n t r a t i o n of l e a d : 3, lead n i t r a t e s o l u t i o n ; 4, l ead ni t ra te-NaCMC s o l u t i o n . RESULTS AND DISCUSSION / 100 that: (1) i n addition to the reac t i o n between lead and carboxymethyl groups, lead hydroxy complexes a l s o interacted with the carbohydrate backbone of CMC and/or (2) CMC adsorbed on the lead hydroxide p r e c i p i t a t e and f l o c c u l a t e d i t . The exact mechanism w i l l be discussed i n the next section. It i s i n t e r e s t i n g to note that s i m i l a r r e s u l t s have been reported. During the p r e c i p i t a t i o n of pe c t i n by aluminum hydroxide, the aluminum present i n the product exceeds, several times over, the amount needed for t h e o r e t i c a l combination with the carboxylate groups (Jameson et a l , 1924; De Lucca and Joslyn, 1957; BeMiller, 1965). Fi g s . 6.17 and 6.18 show that i n a mixture of either lead n i t r a t e - d e x t r i n or lead nitrate-amylopectin, p r e c i p i t a t i o n began at about pH 7 and reached a maximum around pH 11. The logarithmic concentration diagrams (see Appendix II) i n d i c a t e that the major species present i n t h i s pH range (7 - 11) are Pb(0H) 2(aq.) and Pb(0H) 2(s). Thus dextrin and amylopectin appeared to have only i n t e r a c t e d with such lead hydroxide species. C o - p r e c i p i t a t i o n experiments with dextrin and other metal ions ( A l + + + , Mg + +, F e + + + and Cu + +) were c a r r i e d out to test the above hypothesis. The r e s u l t s are shown i n Figs. 6.19 to 6.22. Maximum p r e c i p i t a t i o n of both de x t r i n and metal ions always occurred over the pH range where the respective metal-hydroxide p r e c i p i t a t e s were formed. F i g . 6.23 shows the c o r r e l a t i o n between the pH of maximum p r e c i p i t a t i o n and the i.e.p. of the metal-hydroxides. RESULTS AND DISCUSSION / 101 100 6 Cu Cu a <u Cu o I o c o • H 4 J ca SH • U CU u c o C J •H CO 3 -a • H CO p i - 210 180 150 - 120 pH F i g . 6.17 - E f f e c t of pH on the r e s i d u a l c o n c e n t r a t i o n of amylopec t in and l e a d , a f t e r c e n t r i f u g i n g at 8,000 rpm f o r 3 minute s . R e s i d u a l c o n c e n t r a t i o n of a m y l o p e c t i n : 1, amylopect in s o l u t i o n ; 2, amylopect in- lead n i t r a t e s o l u t i o n . R e s i d u a l c o n c e n t r a t i o n of l e a d : 3, l e ad n i t r a t e s o l u t i o n ; 4 , l e a d n i t r a t e - a m y l o p e c t i n s o l u t i o n . RESULTS AND DISCUSSION / 102 F i g . 6.18 - E f f e c t of pH on the r e s i d u a l concentration of dextrin and lead, a f t e r centrifuging at 8,000 rpm for 3 minutes. Residual concentration of dextrin: 1, dextrin s o l u t i o n ; 2 , dextrin-lead n i t r a t e s o l u t i o n . Residual concentration of lead: 3, lead n i t r a t e solution; 4, lead n i t r a t e - d e x t r i n s o l u t i o n . RESULTS A N D D I S C U S S I O N / 103 PH F ig . 6.19 - Effect of pH on the residual concentration of dextrin and aluminum, after centrifuging at 8,000 rpm for 3 minutes. Residual concentration of dextrin: 1, dextrin solution; 2, dextrin-aluminum nitrate solution. Residual concentration of aluminum: 3 , aluminum nitrate solution; 4, aluminum nitrate-dextrin solution. R E S U L T S A N D D I S C U S S I O N / 1 0 4 F i g . 6.20 - E f f e c t of pH on the r e s i d u a l c o n c e n t r a t i o n of d e x t r i n and magnesium, a f t e r c e n t r i f u g i n g at 8,000 rpm f o r 3 minutes . R e s i d u a l c o n c e n t r a t i o n of d e x t r i n : 1, d e x t r i n s o l u t i o n ; 2, dextrin-magnesium c h l o r i d e s o l u t i o n . R e s i d u a l concen-t r a t i o n of magnesium: 3, magnesium c h l o r i d e s o l u t i o n ; 4, magnesium c h l o r i d e - d e x t r i n s o l u t i o n . RESULTS AND DISCUSSION / 105 pH F i g . 6.21 - E f f e c t of pH on the r e s i d u a l c o n c e n t r a t i o n of d e x t r i n and i r o n , a f t e r c e n t r i f u g i n g at 8,000 rpm f o r 3 minutes. Resid u a l concentration of d e x t r i n : 1, d e x t r i n s o l u t i o n ; 2, d e x t r i n - f e r r i c c h l o r i d e s o l u t i o n . R e s i d u a l concentra-t i o n of i r o n : 3, f e r r i c c h l o r i d e s o l u t i o n ; 4, f e r r i c c h l o r i d e - d e x t r i n s o l u t i o n . RESULTS AND DISCUSSION / 106 F i g . 6.22 - E f f e c t of pH on the r e s i d u a l c o n c e n t r a t i o n s of d e x t r i n and copper, a f t e r c e n t r i f u g i n g at 8,000 rpm f o r 3 minutes . R e s i d u a l c o n c e n t r a t i o n of d e x t r i n : 1, d e x t r i n s o l u t i o n ; 2 , d e x t r i n - c u p r i c c h l o r i d e s o l u t i o n . R e s i d u a l c o n c e n t r a -t i o n of copper : 3 , c u p r i c c h l o r i d e s o l u t i o n ; 4 , c u p r i c c h l o r i d e - d e x t r i n s o l u t i o n . RESULTS AND DISCUSSION / 107 i ep o f Meta l Hydroxide F i g . 6.23 - C o r r e l a t i o n between the i . e . p . of meta l hydrox ides and the pH of maximum c o - p r e c i p i t a t i o n i n m e t a l - d e x t r i n s o l u t i o n s . R E S U L T S A N D D I S C U S S I O N / 108 6.2.2 Summary The c o - p r e c i p i t a t i o n tests conducted with polysaccharide-metal s o l u t i o n mixtures revealed that: a. CMC co-precipitated with both Pb + + ions and lead hydroxy complexes (including lead hydroxide); b. P r e c i p i t a t i o n of lead by CMC was maximal at around pH 11. At t h i s pH the lead p r e c i p i t a t e d was i n great excess of the stoichiometric requirement for the i n t e r a c t i o n between lead and carboxymethyl groups i n CMC; c. Dextrin and amylopectin co-precipitated only with lead hydroxide; d. The maximum c o - p r e c i p i t a t i o n for dextrin-metal systems occurred around pH 7.5 for f e r r i c , 8 for aluminum, 9 for cupric, 11 for lead and 12 for magnesium ions. These are the pH ranges where the respective metal hydroxides are formed. There was a good c o r r e l a t i o n between the pH of maximum co - p r e c i p i t a t i o n and the i.e.p. of the metal hydroxide. 6.3 THE NATURE OF POLYSACCHARIDE/METAL HYDROXY COMPLEX INTERACTIONS As discussed i n the previous section, the polysaccharides studied behaved d i f f e r e n t l y i n the reaction with metal hydroxy complex species. CMC interacted with both metal cations and metal hydroxy complexes, whereas dextrin and amylopectin only interacted with metal RESULTS AND DISCUSSION / 109 hydroxides. To investigate the nature of such i n t e r a c t i o n s , the changes i n solu t i o n pH r e s u l t i n g from the polysaccharide/metal i n t e r a c t i o n s were measured by t i t r a t i o n . The product formed i n the react i o n was subjected to i n f r a r e d spectroscopic a n a l y s i s . Some o p t i c a l r o t a t i o n tests were a l s o conducted, and the r e s u l t s are shown i n Appendix I I I . 6.3.1 Dextrin/Metal Hydroxide Interactions Dextrin reacted only with metal hydroxides, an observation which would suggest the involvement of a hydrogen bonding mechanism. It i s known that metal hydroxide p r e c i p i t a t e s are polynuclear s o l i d species (e.g., Butler, 1964; Stumm, 1967); they accumulate large numbers of hydroxyl groups and thus hydrogen bonding with de x t r i n could e a s i l y have occurred. The experimental evidence indicated, however, that some other i n t e r a c t i o n s must also be considered. F i g . 6.24 shows the r e s u l t s of e l e c t r i c a l conductivity measurements. In these experiments, a serie s of de x t r i n solutions with d i f f e r e n t concentrations were prepared at two pH values, pH 6 and pH 11.4. A f i x e d amount of lead n i t r a t e powder was added to each solu t i o n to give a f i n a l concentration of Pb(N0 3) 2 of 10- 3 M, and the r e s u l t i n g e l e c t r i c a l conductivity and pH of the so l u t i o n were recorded. Both the solu t i o n conductivity and pH decreased with increasing d e x t r i n concentration at pH 11.4 ( F i g . 6.24). However, at pH 6 there was no s i g n i f i c a n t change i n solu t i o n conductivity. These RESULTS AND DISCUSSION / 110 400 0 200 400 600 800 1000 Concent ra t ion of D e x t r i n , ppm F i g . 6.24 - E f f e c t of d e x t r i n a d d i t i o n on the e l e c t r i c a l c o n d u c t i v i t y and pH changes i n 0.001 M lead n i t r a t e s o l u t i o n s . 200 ml of d e x t r i n s o l u t i o n s of d i f f e r e n t concent ra t ions were prepared and kept at the same pH, then 66.25 mg l e a d n i t r a t e was added and the f i n a l pH and c o n d u c t i v i t y were r e c o r d e d . S e r i e s one i n v o l v e d 200 ml of d e x t r i n s o l u t i o n p lu s 0.5 ml of 1 N NaOH: 1, i n i t i a l pH; 2, f i n a l pH a f t e r a d d i t i o n of l e a d n i t r a t e ; 3, f i n a l c o n d u c t i v i t y a f t e r a u d i t i o n of lead n i t r a t e . Ser ie s two i n v o l v e d 200 ml of d e x t r i n s o l u t i o n at a n a t u r a l pH of 6: 4, f i n a l c o n d u c t i v i t y a f t e r a d d i t i o n of l ead n i t r a t e . R E S U L T S A N D D I S C U S S I O N / 111 r e s u l t s confirmed that d e x t r i n i n t e r a c t s with lead hydroxide. At pH 11, when Pb(0H) 2(aq.) i s predominant (see Appendix I I ) , both the solution pH and e l e c t r i c a l c onductivity decreased i n the Pb(N0 3) 2-dextrin mixture. This decrease i n so l u t i o n conductivity at pH 11.4 i s more c l e a r l y seen from F i g . 6.25, i n which the d i f f e r e n c e between the f i n a l conductivity a f t e r adding lead n i t r a t e and the i n i t i a l c onductivity of the de x t r i n s o l u t i o n i s p l o t t e d as a function of dextrin concentration. In strongly a l k a l i n e solutions the hydroxyl groups of polysaccharides are ionized, and i f complexed with m e t a l l i c species would therefore cause a decrease i n conductivity (Davidson, 1967). The e l e c t r i c a l conductivity measurements at pH 11 were thus i n d i c a t i v e of chemical i n t e r a c t i o n s between d e x t r i n and lead hydroxide. This point was also supported by the r e s u l t s of the t i t r a t i o n and i n f r a r e d spectroscopic studies. F i g . 6.26 shows the r e s u l t s of the t i t r a t i o n of lead n i t r a t e solutions at pH 11.2 with a 10 g / l i t e r d e x t r i n s o l u t i o n (also at pH 11.2). The so l u t i o n pH decreased with increasing d e x t r i n addition, and the most s i g n i f i c a n t decrease was observed at the highest lead n i t r a t e concentration. F i g . 6.27 shows the r e s u l t s of such a t i t r a t i o n using a 0.1 M lead n i t r a t e s o l u t i o n together with blank t e s t s . The above t i t r a t i o n r e s u l t s suggested that at pH 11.2 the dextrin/lead hydroxide i n t e r a c t i o n was accompanied by the release of H + ions i n t o the so l u t i o n . This behaviour was not s t r i c t l y pH-dependent. Addition of RESULTS AND DISCUSSION / 1 1 2 -200 -240 0 200 400 600 800 1000 Concentrat ion of d e x t r i n , ppm F i g . 6.25 - E f f e c t of d e x t r i n c o n c e n t r a t i o n on the e l e c t r i c a l c o n d u c t i v i t y of d e x t r i n - l e a d n i t r a t e s o l u t i o n s . 0.5 ml of 1 N NaOH was added to 200 ml of d e x t r i n s o l u t i o n , f o l lowed by the a d d i t i o n of 66.25 mg lead n i t r a t e . C^: f i n a l c o n d u c t i v i t y a f t e r the a d d i t i o n of l ead n i t r a t e ; C . : i n i t i a l c o n d u c t i v i t y before the a d d i t i o n of l ead n i t r a t e . r 11.5 11.5 ta 10.5 2, 3 t i t r a t e t i t rant ^ " ^ ~ - ^ l 1. 0.1 M P b ( N 0 3 ) 2 > pH 11.2 10 g/1 d e x t r i n , pH 1 1 . 2 ^ - - ^ 2 . d i s t i l l e d water , pH 11.2 10 g/1 d e x t r i n , pH 11.2 3. 0.1 M P b ( N 0 3 ) 2 , pH 11.2 l d i s t i l l e d water , 1 pH 11.2 0 10 20 30 Volume o f T i t r a n t , ml F i g . 6.27 - T i t r a t i o n of 0 .1 M lead n i t r a t e w i t h 10 g/1 d e x t r i n at pH 1 1 . 2 . T i t r a t e volume: 100 m l . RESULTS AND DISCUSSION / 115 de x t r i n to a solution containing lead (or other metal) hydroxide always res u l t e d i n a decrease i n solution pH, regardless of the o r i g i n a l pH of the so l u t i o n . Infrared spectroscopic studies were subsequently conducted to characterize the p r e c i p i t a t e s formed by the dextrin-Pb(N0 3) 2 s o l u t i o n s . F i g . 6.28 shows the spectra of lead hydroxide p r e c i p i t a t e s (blanks) at pH 8 and pH 11. Both spectra are e s s e n t i a l l y i d e n t i c a l , with the three successive bands at 438, 340 and 260 cm-1 i n d i c a t i n g the presence of lead hydroxide. In preparing the p r e c i p i t a t e from the dextrin-Pb mixture, the dextrin/Pb r a t i o was v a r i e d . On a weight to weight basis, t h i s r a t i o was 5:1, 1:1 and 1:5, r e s p e c t i v e l y . This was achieved by mixing 50 ml of 10 g/1 d e x t r i n with 50 ml of 0.01 M lead n i t r a t e (5:1), or 50 ml of 10 g/1 d e x t r i n with 50 ml of 0.05 M lead n i t r a t e (1:1), or 50 ml of 4 g/1 dex t r i n with 50 ml of 0.1 M lead n i t r a t e (1:5). Two pH values (8 and 11) were used f or each r a t i o . Two d i f f e r e n t methods of preparing the p r e c i p i t a t e were employed: a. The d e x t r i n and lead n i t r a t e solutions were mixed and the pH of the mixture was adjusted to the desired value; b. The d e x t r i n and lead n i t r a t e solutions were prepared separately, and the pH values of the solutions were both adjusted to the desired value. A f t e r standing for 20 minutes, the solutions were mixed and the f i n a l pH was 911 / N o i s s n o s i a Q N V S l l f l S B H RESULTS AND DISCUSSION / 117 recorded (see Table 6.2). The solutions were then either f i l t e r e d through #42 Whatman f i l t e r paper or centrifuged at 10,000 rpm for 10 minutes, and the p r e c i p i t a t e s were drie d at 110°C. Table 6.1 and 6.2 l i s t the names of the IR samples obtained from method "a" and "b", r e s p e c t i v e l y . Table 6.1 - Dextrin-lead p r e c i p i t a t e s , method a Dextrin:lead Mixture pH IR sample 5:1 8.06 PX1 5:1 11.05 PX2 1:1 8.03 PX3 1:1 11.03 PX4 1:5 8.02 PX5 1:5 11.02 PX6 Table 6.2 - Dextrin-lead p r e c i p i t a t e s , method b Dextrin :lead Dextrin pH Lead pH Mixture pH IR sample 5:1 8.18 8.19 7.22 PXS1 5:1 11.00 11.02 10.54 PXS2 1:1 8.16 8.19 7.57 PXS3 1:1 11.09 11.12 9.23 PXS4 1:5 8.05 8.11 7.82 PXS5 1:5 11.07 11.10 10.56 PXS6 Infrared spectra of these samples are shown i n F i g s . 6.29 to 6.32. The spectra shown i n F i g . 6.29 c l o s e l y resemble those displayed i n F i g . 6.30. This i n d i c a t e s that s i m i l a r products were formed at pH 8 and pH 11. The PX1, PX2, PX3 and PX4 spectra are a l l i d e n t i c a l with that of d e x t r i n (see F i g . 4.10); they show no sign of the presence of lead hydroxide. However, i n the PX5 and PX6 spectra, the three successive peaks below 500 cm-1, and the double peaks between 1400 and 1300 cm-1 point to the presence of lead hydroxide. Note that the F i g . 6.29 - I n f r a r e d spectra of PX1, PX3 and PX5 ( f o r sample d e s i g n a t i o n see Table 6 . 1 ) . RESULTS AND DISCUSSION / 119 uOTss -ra isuEJix % RESULTS AND DISCUSSION / 122 absorption peaks of de x t r i n i n the range of 1000 to 700 cm-1 are "absent i n those spectra (PX5 and PX6, F i g . 6.30) where the lead hydroxide peaks are present. The i n f r a r e d spectroscopic r e s u l t s suggest that a chemical complex was formed as a r e s u l t of dextrin-lead hydroxide i n t e r a c t i o n s . In such a complex, the hydroxyl groups on C-2 and C-3 of glucose may be bonded with Pb to form a five-membered r i n g (whose members are the Pb atom, the two oxygen atoms and the two carbon atoms), as shown below: The formation of such a complex requires that these f i v e members be co-planar or almost co-planar (Davidson, 1967). This conformation exerts tremendous s t r a i n on the glucose r i n g since the projected valence angle of the hydroxyl groups on C-2 and C-3 i s 60 degrees. This s t r a i n l i m i t s the r i n g v i b r a t i o n i l l u s t r a t e d i n F i g . 4.11. Such a phenomenon would be expected to cause a reduction i n i n t e n s i t y or even disappearance of the r i n g deformation peaks i n the region from RESULTS AND DISCUSSION / 123 1000 to 700 cm-1 i n the IR spectrum of d e x t r i n . t In samples PX1, PX3 and PX5, the r a t i o of d e x t r i n to lead was 5:1, 1:1 and 1:5, r e s p e c t i v e l y . At dextrin/lead r a t i o s of 5:1 and 1:1 there was r e s i d u a l d e x t r i n l e f t a f t e r complexation. The PX1 and PX3 spectra ( F i g . 6.29) show the presence of t h i s r e s i d u a l dextrins. If one glucose monomer reacted with one atom of lead, then one gram of dext r i n would have been equivalent to 1.28 grams of lead. Therefore at dextrin/lead r a t i o s of 5:1 and 1:1, one would expect r e s i d u a l d e x t r i n to have been present. Conversely, at a dextrin/lead r a t i o of 1:5, r e s i d u a l lead hydroxide was present a f t e r complexation. In t h i s case a l l of the dex t r i n was complexed, and consequently the dex t r i n peaks from 1000 to 700 cm-1 do not appear i n the PX5 and PX6 spectra (Figs. 6.29 and 6.30). In these experiments both de x t r i n and lead were present i n the so l u t i o n . When the pH was increased, lead hydroxide may have been formed with the concomitant formation of lead-dextrin complexes. Further evidence was therefore required to support the claim that d e x t r i n reacts with the "lead hydroxide" species. Such evidence i s provided i n F i g s . 6.31 and 6.32. These spectra (PXS1 to PXS6) were taken using samples obtained by mixing de x t r i n and lead solutions that were prepared separately. Hence any lead hydroxide i n the sol u t i o n was present before the dex t r i n was introduced. When the dextrin/lead r a t i o fThese bands wi l l not shift. According to Barker et al (1954), the intensity of this type of vibration is more sensitive than the frequency to the ring conformation and to the introduction of substituents at the hydroxyl groups. RESULTS AND DISCUSSION / 124 was 5:1, a l l of the lead hydroxide was disrupted. When the r a t i o was 1:1, some lead hydroxide was l e f t over at pH 8 but none at pH 11. It seems that the tendency for complexation was favoured by higher pH, an observation which has been reported by Angyal (1973). Lead hydroxide can be viewed as a polynuclear s o l i d species i n aqueous s o l u t i o n (Stumm, 1967), and thus a complexation scheme can be i l l u s t r a t e d as follows: (10) This complexation scheme explains the i n f r a r e d absorption spectra obtained, and a l s o explains why the pH decreased upon mixing de x t r i n with lead n i t r a t e solutions when lead hydroxide was present. (The e f f e c t of d e x t r i n on the hydrolysis of l i t h a r g e seems to confirm such an i n t e r a c t i o n mechanism, see Appendix VI.) Due to the p r i o r formation of lead hydroxide, dextrin could only react with the "surface" of the lead hydroxide p r e c i p i t a t e , and hence the rea c t i o n d i d not s t r i c t l y follow the stoichiometric r a t i o . This can be seen from spectrum PXS3, F i g . 6.31. However, i n solutions containing both d e x t r i n and lead n i t r a t e , lead hydroxide could be formed as a metastable species when the pH was increased, and then react with d e x t r i n to form the dextrin-Pb complex. RESULTS AND DISCUSSION / 125 Addition of d e x t r i n to a lead n i t r a t e solution containing predominantly Pb + + metal ions (e.g., at pH 3.7) d i d not cause any decrease i n pH. Furthermore, the o p t i c a l r o t a t i o n of d e x t r i n d i d not change i n lead-dextrin mixtures at such low pH (see also Appendix I I I ) . Hence, the d i r e c t r e a c t i o n of d e x t r i n with P b + + ions to form a complex could not have occurred, since formation of such a complex would have been accompanied by both H + release and a change i n o p t i c a l r o t a t i o n . The reason why d e x t r i n reacted only with lead hydroxide to form such complexes but not with Pb* + ions i s not quite c l e a r . It i s believed that the release of a hydrated proton (or the water-release mechanism) shown i n equation (10) i s the major d r i v i n g force for the reaction (see Appendix IV). The reaction product, H 30 +, i s very s t a b l e . For example, the energy required to separate H 30 + into a H 20 molecule and a proton (H +) i s about three times the energy required to break most covalent bonds (Butler, 1964). The r e s u l t s of the t i t r a t i o n of f e r r i c c h l o r i d e and cupric chloride solutions with d e x t r i n are shown i n F i g s . 6.33 and 6.34. At the optimum p r e c i p i t a t i o n pH (pH 8), the pH of the s o l u t i o n decreased with in c r e a s i n g addition of d e x t r i n (Figs. 6.33 and 6.34). This may also have r e f l e c t e d the occurrence of a chemical reacti o n . However, no change i n pH was observed when F e C l 3 or CuCl 2 was t i t r a t e d by d e x t r i n at pH 1.8 or 3.5, r e s p e c t i v e l y . In these pH ranges, f e r r i c or cupric cations are predominant i n the solutions. No change i n the o p t i c a l r o t a t i o n of the d e x t r i n was observed at these low pH's, either (Appendix I I I ) . In summary, dextrin was found to i n t e r a c t only with t 8.5 2, 3 t i t r a t e 0.1 M F e C l 3 , pH 8.0 d i s t i l l e d water, pH 8.0 0.1 M FeCl , pH 8.0 t i t r a n t 10 g/1 dextrin, pH 8.0 10 g/1 d e x t r i n , pH 8.0 d i s t i l l e d water, pll 8.0 12 24 36 Volume of T i t r a n t , ml 48 Fig . 6.33.- T i t r a t i o n of 0.1 M f e r r i c chloride with 10 g/1 dextrin at pH 8.0. T i t r a t e volume: 100 ml. 60 o c (J) c/> o z 7.5 ta P. 6.5 2,3 1 t i t r a t e t i t r a n t 1. 0.1 M C u C l 2 , pH 8.0 10 g/1 d e x t r i n , pH 8.0 2. d i s t i l l e w water , pH 8.0 10 g/1 d e x t r i n , pH 8.0 3. I 0.1 M C u C l 2 , pH 8.0 i d i s t i l l e d water i , pH 8.0 i 0 12 24 Volume of T i t r a n t , ml 36 48 60 F i g . 6.34 - T i t r a t i o n of 0.1 M c u p r i c c h l o r i d e w i t h 10 g/1 d e x t r i n at pH 8 .0 . T i t r a t e volume: 100 m l . 33 m to c CO > z D D CO o c CO CO RESULTS AND DISCUSSION / 128 metal hydroxides. The mechanism of t h i s i n t e r a c t i o n seems to be chemical complexation, not conventional "hydrogen bonding". 6.3.2 Amylopectin/Lead Hydroxide Interactions The maximum p r e c i p i t a t i o n i n amylopectin-lead n i t r a t e s o l u t i o n also occurred at pH 11. T i t r a t i o n of lead n i t r a t e with amylopectin conducted at t h i s pH led to a s i g n i f i c a n t decrease i n pH ( F i g . 6.35). IR spectra of the p r e c i p i t a t e s show that i n the presence of an excess of lead hydroxide, a l l of the amylopectin p a r t i c i p a t e d i n the reaction, as the peaks a t t r i b u t a b l e to uncomplexed amylopectin from 1000 to 700 cm-1 disappeared (PAMY1, F i g . 6.36). However, at higher amylopectin/lead r a t i o , r e s i d u a l uncomplexed amylopectin was present, and gave r i s e to the diagnostic IR peaks (1000 to 700 cm-1) i n spectrum PAMY2 ( F i g . 6.36). In general, the behaviour of amylopectin resembled c l o s e l y that of d e x t r i n , and presumably involved a chemical complexation mechanism as w e l l . Analogous r e s u l t s have already been reported. For example, the i.e.p. of cobalt hydroxide i s around pH 11 (Parks, 1965), and cobalt forms complexes with soluble starch around pH 11 (Klotz, 1961). In f a c t , there are a number of reports on the formation of polysaccharide-metal complexes i n the reaction between polysaccharide and metal hydroxides. Barium hydroxide and calcium hydroxide react with many polysaccharides (BeMiller, 1965). Z a i d i and h i s co-workers were able to form various complexes by reacting cadmium hydroxide, t 11.5 10.5 9 . 5 2, 3 1 t i t r a t e t i t r a n t 1. 0.1 M Pb (N0 3 - ) 2 , pH 11.2 5 g/1 a m y l o p e c t i n , pH 1 1 . 2 2. d i s t i l l e d water , pH 11.2 5 g/1 a m y l o p e c t i n , pH 1 1 . 2 3. 0 .1 M P b ( N 0 3 ) 2 , pH 11.2 I 1 ...... d i s t i l l e d water , pH 11.2 l 1 12 24 Volume of T i t r a n t , ml 36 48 F i g . 6.35 - T i t r a t i o n of 0.1 M lead n i t r a t e w i t h 5 g/1 amylopect in a t pH 11 .2 . T i t r a t e volume: 100 m l . 60 CO o c CO CO o 2 CD OSL / Noissnosia Q N V s i i n s a a RESULTS AND DISCUSSION / 131 zinc hydroxide and cobalt hydroxide with many saccharides and polysaccharides (Zaidi et a l , 1962, 1963; Z a i d i and Mahdihassan, 1963). Synowiedzki (1962) obtained starch-Fe(III) complex by reacting Fe(OH) 3 with starch. Bourne et a l (1949) and Cantor and Wimmer (1957) used aluminum hydroxide and a l k a l i n e earth metal hydroxide, r e s p e c t i v e l y i n the f r a c t i o n a t i o n of starch. However, none of the above proposed a complexation mechanism. In Section 3.5.1.2, i t i s mentioned that Wolstenholme and Schulman (1950) observed that only the metal hydroxy complexes s o l i d i f y f a t t y acids; M. C. Fuerstenau and Palmer (1976) reported, with abundant examples, that while metal cations do not adsorb on oxide mineral surfaces, metal hydroxy complexes do adsorb. It i s i n t e r e s t i n g to observe that starch-type polysaccharides d i d not react with metal cations, but with metal hydroxy complexes. 6.3.3 CMC/Metal Interactions The presence of carboxymethyl groups i n CMC apparently appeared to enhance CMC/metal i n t e r a c t i o n s . As a r e s u l t , CMC was able to in t e r a c t with Pb + + ions. This can be seen from the t i t r a t i o n r e s u l t s shown i n F i g . 6.37. At pH 4.3, the 0.1 M lead n i t r a t e s o l u t i o n contained predominantly Pb + + cations, and about half of the carboxyl groups i n CMC were protonated (the pK of the carboxyl groups i n CMC i s 4.4 (Stelzer and Klug, 1980)). F i g . 6.37 shows that the pH of the s t i r r e d s o l u t i o n continued to decrease with increasing CMC RESULTS AND DISCUSSION / 133 concentration. Hence the following reaction may have occurred: HOOC (11) The s o l u t i o n pH should decrease as a consequence of t h i s r e a c t i o n . This r e s u l t was observed experimentally (see a l s o Appendix V). Furthermore, a change i n the carboxyl stretching frequency was expected i f t h i s reaction took place. Spectrum PCTL ( F i g . 6.38) shows that the peak previously occurring at 1610 cm-1 for carboxyl groups was s h i f t e d to 1575 cm - 1 due to the complexation of lead ions. Note that no absorption bands a t t r i b u t a b l e to lead hydroxide were detected at t h i s low pH (PCTL, F i g . 6.38). When a lead n i t r a t e solution was t i t r a t e d with CMC at pH 11 . 2 , there was no change i n pH with increasing CMC concentration. Thus both the hydroxyl and carboxyl groups i n CMC appeared capable of forming complexes with lead hydroxide. Equation (10) p r e d i c t s that the r e a c t i o n between glucose hydroxyl groups and lead hydroxide should release H + ions, whereas the reaction between carboxyl groups and lead hydroxide should release OH- ions: + OH (12) F i g . 6.38 - I n f r a r e d spec t ra of PCTL and PCTH. PCTL: p r e c i p i t a t e from te s t #1, F i g . 6 .37 ; PCTH: p r e c i p i t a t e from the t i t r a t i o n of 0 .1 M lead n i t r a t e w i t h NaCMC at pH 1 1 . 2 . RESULTS AND DISCUSSION / 135 The simultaneous occur rence of these two m u t u a l l y n e u t r a l i z i n g processes perhaps e x p l a i n s the absence of any pH change i n t h i s system. In f a c t , i t i s noted that the s o l u t i o n pH i n c r e a s e d when l ead n i t r a t e was t i t r a t e d w i t h sodium l a u r a t e a t pH 11.2 ( F i g . 6 . 3 9 ) . Th i s f i g u r e a l s o shows tha t s o l u t i o n pH decreased when l e a d n i t r a t e was t i t r a t e d w i t h the unbranched p o l y s a c c h a r i d e amylose a t pH 11 .2 . Equat ion (12) shows b a s i c a l l y an ion-exchange r e a c t i o n between the s o l i d surface h y d r o x y l groups and the c a r b o x y l a t e groups i n the CMC. The exchange r e a c t i o n i t s e l f does not a l t e r the sur face charge of the s o l i d . However, the " f ree" ca rboxy la t e groups a long the adsorbed CMC c h a i n w i l l change s i g n i f i c a n t l y the e l e c t r o k i n e t i c behaviour of the s o l i d . Th i s w i l l be d i s cus sed l a t e r i n S e c t i o n 6 . 4 . 3 . 6.3.4 Summary The t i t r a t i o n and i n f r a r e d spec t ro scop ic s tud i e s of the p o l y s a c c h a r i d e - m e t a l systems have r e v e a l e d t h a t : a . The i n t e r a c t i o n s between d e x t r i n (or amylopec t in) and l ead hydroxide caused a decrease i n s o l u t i o n pH, i n d i c a t i n g r e l e a s e of H + i o n s i n t o the s o l u t i o n as a r e s u l t of such i n t e r a c t i o n s ; b . The i n f r a r e d abso rp t ion bands a r i s i n g from the asymmetric and symmetric r i n g deformations of g l u c o s e , the r e p e a t i n g u n i t of both d e x t r i n and amylopec t in , were e l i m i n a t e d as a 11.5 3, 4, 5 10.5 " P. t i t r a t e t i t r a n t 1. 0.1 M P b ( N 0 3 ) 2 , pH 11.2 10 g/1 amylose, pH 11.2 2. 0.1 M P b ( N 0 3 ) 2 , pH 11.2 _2 10 M sodium l a u r a t e , pH 11.2 3. d i s t i l l e d water , pH 11.2 10 g/1 amylose, pH 11.2 4. d i s t i l l e d water , pH 11.2 _2 10 M sodium l a u r a t e , pH 11.2 5. 0.1 M P b ( N 0 3 ) 2 , pH 11.2 d i s t i l l e d water , pH 11.2 31 m CV) c 9.5 CO > z o Q O O c CT) cn 10 20 Volume of T i t r a n t , ml 30 F i g . 6.39 - T i t r a t i o n of 0.1 M lead n i t r a t e w i t h 10 g/1 amylose and 0.01 M sodium l a u r a t e at pH 11 .2 . T i t r a t e volume: 100 m l . 40 CO RESULTS AND DISCUSSION / 137 r e s u l t of the dextrin-lead hydroxide and amylopectin-lead hydroxide i n t e r a c t i o n s ; c. Based on the above observations i t was concluded that dextrin-lead hydroxide and amylopectin-lead hydroxide i n t e r a c t i o n s r e s u l t from chemical complexation; d. The i n t e r a c t i o n s between d e x t r i n and f e r r i c (or cupric) hydroxide also caused a decrease i n solution pH. This may have r e f l e c t e d the formation of chemical complexes as well; e. Interactions between CMC and P b + + ions caused a s i g n i f i c a n t decrease i n solution pH and a s h i f t of the carboxyl group's IR stretching band, which apparently re s u l t e d from the i n t e r a c t i o n between Pb + + ions and the carboxyl group. f . CMC-lead in t e r a c t i o n s at pH 11 d i d not cause any change i n s o l u t i o n pH. This may have been due to the n e u t r a l i z a t i o n of H + ions ( r e s u l t i n g from CMC hydroxyl-lead hydroxide i n t e r a c t i o n s ) by OH- ions ( r e s u l t i n g from CMC carboxyl-lead hydroxide i n t e r a c t i o n s ) . 6.4 ADSORPTION OF DEXTRIN AND CMC The i n t e r a c t i o n s of polysaccharides with metal ions or metal hydroxy complexes i n solution have been described i n Sections 6.2 and 6.3. Although these r e s u l t s do not provide d i r e c t evidence of i n t e r a c t i o n s with mineral surfaces, they do r e f l e c t the chemical a f f i n i t y of polysaccharides toward c e r t a i n metal hydroxy complexes. Furthermore, the i n t e r a c t i o n s of polysaccharides with metal hydroxy RESULTS AND DISCUSSION / 138 complexes described i n Sections 6.2 and 6.3 d i d not n e c e s s a r i l y take place i n "bulk s o l u t i o n " . Metal ions i n aqueous solutions undergo a sequence of h y d r o l y t i c and condensation reactions ( o l a t i o n and oxolation) when solu t i o n pH increases. Under conditions of oversaturation with respect to metal hydroxides, these reactions lead to the formation of c o l l o i d a l hydroxo polymers and u l t i m a t e l y to the formation of p r e c i p i t a t e (Stumm, 1967; Butler, 1964). Thus, metal hydroxide p r e c i p i t a t e i s a macromolecule, and polysaccharides may only i n t e r a c t with t h i s "metal-hydroxide-solid" surface (equations (10) and (12)). The adsorption properties of d e x t r i n and CMC on quartz samples were determined as functions of both the s o l u t i o n pH and polysaccharide r e s i d u a l concentration. Unfortunately, IR spectroscopy cannot be u t i l i z e d to analyse polysaccharide-treated quartz samples, because quartz i t s e l f absorbs strongly i n the wavelength region of i n t e r e s t ( F i g . 4.1). Hence only the adsorption t e s t s were c a r r i e d out. 6.4.1 Adsorption of Dextrin The adsorption of d e x t r i n on quartz samples was examined using the experimental procedure described i n Section 5.6. F i g . 6.40 shows the e f f e c t of pH on the adsorption of d e x t r i n on unmodified quartz (Q) and on lead-coated quartz (PbQ). There was only a very small amount of d e x t r i n adsorbed on quartz i n the e n t i r e pH range tested. However, when the quartz was coated by lead, the amount of d e x t r i n RESULTS AND DISCUSSION / 139 RESULTS AND DISCUSSION / 140 adsorbed increased s u b s t a n t i a l l y . Adsorption on lead-coated quartz increased with increasing pH, reached a maximum i n the pH range from 10 to 11, and then decreased sharply. The pH of maximum adsorption cor r e l a t e d well with the r e s u l t s of the c o - p r e c i p i t a t i o n and Hallimond tube f l o t a t i o n t e s t s . The increased adsorption of de x t r i n on PbQ was at t r i b u t e d to the presence of lead species on the quartz surface: i f the PbQ was f i r s t treated with sodium oleate, d e x t r i n adsorption was markedly reduced ( F i g . 6.40). The adsorption behaviour of de x t r i n on MQ and MPbQ c l o s e l y resembled that on quartz and PbQ. MPbQ adsorbed much more d e x t r i n than MQ, and the maximum adsorption on MPbQ occurred between pH 10 to 11. Note that when MPbQ was f i r s t treated with sodium oleate, the adsorption of dextrin was again markedly reduced ( F i g . 6.41). Comparison of F i g . 6.40 with F i g . 6.41 shows that the adsorption behaviour of dextrin on quartz and methylated quartz (MQ) was unchanged. However, quartz methylated a f t e r lead coating (MPbQ) adsorbed more dextrin than the merely lead-coated quartz (PbQ) d i d . The adsorption isotherms of d e x t r i n on the quartz samples at around pH 10.5 are shown i n F i g . 6.42. Dextrin showed a strong a f f i n i t y f o r both PbQ and MPbQ, with adsorption reaching a plateau at very low d e x t r i n concentrations. Again, adsorption on MPbQ was higher than on PbQ. If either PbQ or MPbQ was f i r s t exposed to sodium oleate, d e x t r i n adsorption was markedly reduced. The adsorption isotherms were of the Langmuir-type (equation RESULTS AND DISCUSSION / 141 1.6 2 4 6 8 10 12 14 P H F i g . 6 .41 - E f f e c t of pH on the a d s o r p t i o n of d e x t r i n on MQ and MPbQ. I n i t i a l c o n c e n t r a t i o n of d e x t r i n was 50 ppm. 1^ MQ; 2, MPbQ; 3, MPbQ reacted f i r s t w i t h 25 ml 2x10 M sodium o l e a t e . RESULTS AND DISCUSSION / 142 10 M NaCl 0 40 80 120 160 200 E q u i l i b r i u m Concentrat ion o f D e x t r i n , ppm F i g . 6.42 - A d s o r p t i o n isotherms of d e x t r i n on d i f f e r e n t quar tz samples. 1, q u a r t z , pH 10.56; 2, MQ, pH 10 .56 ; 3 , PbQ, pH 10.88: 4, MPbQ, pH 10.90; 5 , PbQ reac ted f i r s t w i t h 2x10 M sodium o l e a t e , pH 1 0 . 7 2 ; 5 , MPbQ reac ted f i r s t w i t h 2x10 M sodium o l e a t e , pH 10 .41 . RESULTS AND DISCUSSION / 143 (6), Section 3.4.2): Dextrin on quartz ( f i r s t three p o i n t s ) : c/T 31.33c + 47.48 (13) r = 0.9997 Dextrin on PbQ: c/T 0.951c + 5.351 (14) r = 0.9963 Dextrin on MPbQ: c /r 0.775c + 5.765 (15) r = 0.9932 Assuming that the meaning of the parameters i n the Langmuir equation i s v a l i d , from these equations i t was ca l c u l a t e d that the maximum adsorption d e n s i t i e s of de x t r i n on quartz, PbQ and MPbQ were 0.032, 1.051 and 1.29 mg/m2, re s p e c t i v e l y . The adsorption free energy changes could a l s o be calculated from these equations (Section 3.4.2). However, t h i s c a l c ulated free energy change i s only an " o v e r a l l " value: i t i s the sum of a l l the fr e e energy changes f or the various types of i n t e r a c t i o n s . Unless a l l such i n t e r a c t i o n s are well characterized, i t i s not possible to pr e d i c t , with the use of such a value, whether the i n t e r a c t i o n i s chemical or p h y s i c a l . The adsorption behaviour of de x t r i n on c e r t a i n natural minerals was studied and the r e s u l t s are presented i n F i g s . 6.43 and 6.44. F i g . 6.43 shows the adsorption behaviour of de x t r i n on two oxides, l i t h a r g e (PbO) and hematite ( F e 2 0 3 ) , as a function of pH. The RESULTS AND DISCUSSION / 144 pH F i g . 6.43 - E f f e c t of pH on the a d s o r p t i o n of d e x t r i n on d i f f e r e n t o x i d e s . C o n d i t i o n s : i n i t i a l c o n c e n t r a t i o n of d e x t r i n , 100 ppm; 1 gram of -400# ox ide i n a 50 ml s o l u t i o n . RESULTS AND DISCUSSION / 145 F i g . 6.44 - E f f e c t of pH on the a d s o r p t i o n of d e x t r i n on d i f f e r e n t s u l p h i d e s . C o n d i t i o n s : i n i t i a l c o n c e n t r a t i o n of d e x t r i n , 50 ppm; 1 gram of -400# s u l p h i d e i n a 50 ml s o l u t i o n . RESULTS AND DISCUSSION / 146 adsorption maxima were between pH 10 to 12 for l i t h a r g e , and around pH 7 for hematite. Under these conditions the adsorption of d e x t r i n on l i t h a r g e was much higher than on hematite. These r e s u l t s correlated very well with the optimum c o - p r e c i p i t a t i o n pH values for dextrin-lead and d e x t r i n - f e r r i c systems. Oxide surfaces are hydrolyzed i n aqueous solutions and they apparently behave l i k e hydroxides. F i g . 6.44 shows the adsorption behaviour of de x t r i n on two sulphides, galena (PbS) and chalcopyrite (CuFeS 2), as a function of pH. The adsorption maximum was around pH 11 f o r galena. For chalcopyrite, the adsorption density was almost constant i n the pH range from 4 to 10, with a small maximum around pH 9. Again, the pH values for maximum adsorption correlated well with the r e s u l t s of the c o - p r e c i p i t a t i o n t e s t s . This suggests that metal ions on the tested sulphide surfaces behaved more l i k e hydroxide than sulphide. This i s reasonable since i n neutral and a l k a l i n e solutions the oxidation of sulphide minerals produces metal hydroxides on the mineral surfaces (e.g. Guy and Trahar, 1985). Note that the adsorption of de x t r i n onto galena was much higher than onto c h a l c o p y r i t e . Perry and Apian (1985) found that the maximum adsorption of a series of polysaccharides on p y r i t e occurs around pH 7. Khosla et a l (1984) measured the adsorption of starch and amylopectin on F e 2 0 3 , and found that adsorption i s higher at pH 6 than at pH 8. Furthermore, Iwasaki and Lai (1965) observed that the adsorption of a corn starch on hematite i s much higher at pH 6.8 than at pH 11.3. The increased RESULTS AND DISCUSSION / 147 adsorption of polysaccharides on iron-bearing minerals at around neutral pH, as observed i n t h i s work and i n the c i t e d examples, strongly suggests that the polysaccharides i n t e r a c t with the i r o n hydroxide species on these minerals. In f a c t , several researchers have observed that i t i s the i r o n hydroxide on the surface that i s responsible for the chemisorption of oleate and hydroxamate (e.g., Peck et a l , 1966; Fuerstenau et a l , 1967). 6.4.2 Adsorption of CMC The adsorption of CMC on quartz samples was a l s o found to increase i n the presence of lead surface adsorption centres. The adsorption d e n s i t i e s of CMC on quartz and MQ were s i m i l a r ( F i g . 6.45), but was much higher on PbQ. Unlike d e x t r i n , the maximum adsorption of CMC on PbQ occurred around pH 7 (instead of pH 11), and then decreased s l i g h t l y as the pH was increased. If the lead s i t e s were blocked by p r i o r reaction with oleate, CMC adsorption was markedly reduced i n the same pH region where CMC was previously found to adsorb onto the quartz surface ( F i g . 6.46). Again, CMC seemed to be adsorbed s l i g h t l y more by MPbQ than by PbQ ( F i g . 6.47). A multilayer build-up at higher r e s i d u a l NaCMC concentrations was apparent ( F i g . 6.47). Below 100 ppm, a Langmuir-type isotherm could be obtained: for CMC on PbQ: c/T = 3.039c + 11.336 (16) RESULTS AND DISCUSSION / 148 A q u a r t z , = 100 ppm O MQ, C Q = 100 ppm • PbQ, C = 100 ppm 10 M NaCl f 0.8 -o tn 2 4 6 8 10 12 14 p H 1.0 h 60 F i g . 6.45 - E f f e c t of pH on the a d s o r p t i o n of NaCMC on d i f f e r e n t quar tz samples. C o n d i t i o n s : 1 gram of -400# quar tz i n a 50 ml s o l u t i o n . RESULTS AND DISCUSSION / 149 oo CU U o cn i d cd C D O 1.0 0.8 0.6 0.4 0.2 O PbQ, CQ = 100 ppm A PbQ reacted w i t h o l e a t e (2xl0~%l) f i r s t , CQ = 100 ppm O MPbQ, CQ = 100 ppm -2 10 M NaCl 10 12 14 pH F i g . 6.46 - E f f e c t of pH on the adsorp t ion of NaCMC on PbQ. C o n d i t i o n s : 1 gram of -400// quartz i n a 50 ml s o l u t i o n . RESULTS AND DISCUSSION / 150 1.4 1.2 1.0 0.8 0.6 0.4 0.2 • quartz, pH 9.5(0.1) a MQ, pH 9.7(0.1) O PbQ, pH 10.68(0.03) e MPbQ, pH 9.8(0.1) A PbQ, reacted with oleate f i r s t , pH 9.92(0.06) -2 10 M NaCl 40 80 120 160 Equilibrium CMC concentration, ppm 200 F i g . 6.47 - A d s o r p t i o n isotherms of NaCMC on quartz samples. C o n d i t i o n s : 1 gram of -400# quartz i n a 50 ml s o l u t i o n . RESULTS AND DISCUSSION / 151 r = 0.9966 for CMC on MPbQ: c/T = 2.086c + 8.89 (17) r = 0.9957 Unlike dextrin, CMC adsorption on hematite was not maximal at neutral pH. Adsorption of CMC on hematite increased when the pH was decreased, with a rapid r i s e i n adsorption density below pH 6 ( F i g . 6.48). 6.4.3 Discussion of the Adsorption Behaviour The r e s u l t s i n d i c a t e that the adsorption of de x t r i n on mineral surfaces occurred through i n t e r a c t i o n s with metal hydroxide species present on mineral surfaces. The pH of maximum adsorption c o r r e l a t e d well with the pH of maximum p r e c i p i t a t i o n i n the dextrin-metal systems. The adsorption of dextrin on lead-containing substrates, whether they were oxides or sulphides, was always higher than on the other metal-bearing substrates tested. F i g . 6.23 shows a good c o r r e l a t i o n between the pH of maximum c o - p r e c i p i t a t i o n i n metal-dextrin mixtures and the i.e.p. of the metal hydroxide p r e c i p i t a t e s . Closer examination of the f i g u r e reveals that the pH of maximum c o - p r e c i p i t a t i o n was always s l i g h t l y lower than the i.e.p.. This d i f f e r e n c e increased with increasing pH. Perhaps de x t r i n exhibited an i n c r e a s i n g l y anionic character with increasing pH, and therefore i t s adsorption on a p o s i t i v e or neutral surface was RESULTS AND DISCUSSION / 152 F i g . 6.48 - E f f e c t of pH on the a d s o r p t i o n o f NaCMC on hemati te and g r a p h i t e . C o n d i t i o n s : 1 gram of -400// hemati te or 0.5 g of -400// g raphi te i n a 50 ml s o l u t i o n . RESULTS AND DISCUSSION / 153 favoured. Indeed, F i g . 6.49 shows that the dex t r i n c o l l o i d s e x h i b it a s l i g h t negative charge i n neutral and a l k a l i n e solutions. However, the PbQ surfaces were negatively charged i n the e n t i r e pH range tested ( F i g . 4.2). The fac t that d e x t r i n adsorption on PbQ was nonetheless found to be very high at high pH suggested that a no n - e l e c t r o s t a t i c e f f e c t (or e f f e c t s ) , e.g. chemical complexation, played an important r o l e . In f a c t , i t i s the appearance of metal hydroxides on mineral surfaces that determines the dex t r i n adsorption. The optimum pH for the adsorption of dex t r i n on a s o l i d surface c o r r e l a t e s well only with the pH i n which the metal hydroxides predominate,t i r r e s p e c t i v e of the zeta p o t e n t i a l s of the s o l i d s . Therefore, a chemical i n t e r a c t i o n mechanism between d e x t r i n and surface metal hydroxide species as depicted i n equation (10) i s very p l a u s i b l e . It i s noted, however, that outside t h i s optimum pH range, the metal hydroxide species do not completely disappear from the s o l i d surface, although the proportion of such species w i l l be much less (Fuerstenau and Fuerstenau, 1982). Therefore, dextrin adsorption on the s o l i d surface i n these pH ranges can s t i l l take place, but to a lesser extent. f Fo r an oxide mineral in which the metal ions are the lattice constituents, the pH in which metal hydroxides predominate is close to the i.e.p. of the mineral (e.g. hematite); if the metal ions are not a part of the crystal lattice, the pH of the metal hydroxide predominance does not necessarily coincide with the i.e.p. of the mineral (e.g., quartz coated by lead ions (PbQ)). RESULTS AND DISCUSSION / 154 F i g . 6.49 - E f f e c t of pH on the zeta p o t e n t i a l of d e x t r i n c o l l o i d s d i s p e r s e d i n aqueous s o l u t i o n s . RESULTS AND DISCUSSION / 155 CMC, on the other hand, interacted with any metal i o n i c species on the surface i r r e s p e c t i v e of t h e i r nature (metal ions, or metal hydroxy complexes). Adsorption was a f f e c t e d by the strong negative charge of CMC. Thus CMC adsorption was lower than that of dextrin under comparable conditions. F i g . 6.45 shows that the maximum adsorption of CMC on PbQ occurred around pH 7, where the zeta p o t e n t i a l of PbQ was at i t s lowest absolute value ( F i g . 4.2). The subsequent s l i g h t decrease i n adsorption density with increasing pH probably re s u l t e d from increasing e l e c t r o s t a t i c repulsion between CMC and the PbQ surface. F i g . 6.50 shows the e f f e c t of pH and d e x t r i n (or CMC) on the zeta p o t e n t i a l of l i t h a r g e . Litharge o r i g i n a l l y had an i.e.p. at pH 9.5. However, i n the presence of 100 ppm of CMC, l i t h a r g e remained strongly negatively charged over the whole pH range tested (7 to 12). Dextrin, on the other hand, d i d not seem to s h i f t the i.e.p. of l i t h a r g e s i g n i f i c a n t l y . It only reduced the magnitude of the zeta p o t e n t i a l of l i t h a r g e . CMC i s a strongly anionic polymer and i t s adsorption increases the negative charge of a s o l i d (see equation (12) and the discussion following that equation). According to equation (10), the adsorption of d e x t r i n should a l s o induce a negative surface charge. However, only the adsorbed segments of d e x t r i n w i l l give a negative charge, and the number of such adsorbed segments i s generally small, with the rest of the segments of the adsorbed de x t r i n extending i n t o solution as "loops" and " t a i l s " . Furthermore, the adsorption of such polymers w i l l s h i f t the shearing plane of the RESULTS AND DISCUSSION / 156 F i g . 6.50 - E f f e c t of pH on the zeta p o t e n t i a l of l i t h a r g e i n the presence or absence of e i t h e r NaCMC or d e x t r i n . C o n d i t i o n s : 50 mg of -400# l i t h a r g e i n a 50 ml s o l u t i o n . RESULTS AND DISCUSSION / 157 e l e c t r i c a l double layer farther away from the i n t e r f a c e , reducing s i g n i f i c a n t l y the magnitude of the zeta p o t e n t i a l . Therefore the observed s h i f t of i.e.p. w i l l not be pronounced. F i g . 6.51 to 6.54 show the e f f e c t s of pH and d e x t r i n (or CMC) on the zeta p o t e n t i a l s of quartz and PbQ. Consistent with previous r e s u l t s , CMC adsorption drove the zeta p o t e n t i a l s i n the negative d i r e c t i o n , whereas d e x t r i n adsorption only reduced th e i r magnitude. Both CMC and d e x t r i n were adsorbed more on MPbQ than on PbQ. D-glucose has both h y d r o p h i l i c groups (-0H) as well as hydrophobic e n t i t i e s (-CH). Therefore a substrate with a surface having hydrophobic s i t e s i n a d d i t i o n to m e t a l l i c adsorption centres should favour the adsorption of polysaccharides. These hydrophobic s i t e s should not be a long-chain surfactant such as oleate group, but rather a short-chain surfactant, or the o r i g i n a l hydrophobic mineral surface. Long chain surfactants w i l l stretch far out i n t o the bulk phase whereas the short chain surfactants can be wrapped up i n the "loops" of the adsorbing polysaccharides. In the l a t t e r case, adsorption of the polysaccharide i s a s s i s t e d by a d d i t i o n a l hydrophobic i n t e r a c t i o n s between the polysaccharide and the s o l i d surface. For surfaces which are only hydrophobic or have only m e t a l l i c adsorption centres, adsorption should be much le s s , or even n i l . This probably explains why i n some previous publications ( M i l l e r et a l , 1984), a h y d r o p h i l i c mineral was reported not to adsorb dextrin, whereas hydrophobicity markedly enhanced the adsorption of dextrin. Similar RESULTS AND DISCUSSION / 158 F i g . 6.51 - E f f e c t of pH on the zeta p o t e n t i a l of PbQ i n the presence or absence of e i t h e r NaCMC or d e x t r i n . C o n d i t i o n s : 50 mg of -400# PbQ i n a 50 ml s o l u t i o n . RESULTS AND DISCUSSION / 159 F i g . 6 . 5 2 - E f f e c t of pH on the ze ta p o t e n t i a l of quar tz i n the presence or absence of e i t h e r NaCMC or d e x t r i n . C o n d i t i o n s : 50 mg of -400# quar tz i n a 50 ml s o l u t i o n . RESULTS AND DISCUSSION / 1 10 2 M NaCl pH 11 l i t h a r g e (PbO) -cr • PbQ quartz Q 20 40 60 80 100 Concentra t ion o f D e x t r i n , pp m 6.53 - E f f e c t of d e x t r i n c o n c e n t r a t i o n on the z e t a p o t e n t i a l s d i f f e r e n t s o l i d samples. RESULTS AND DISCUSSION / 161 +40 +20 4J c CU 4-1 o cd 4-1 cu N -80 10 2 M NaCl rtAlitharge (PbO), pH 10.2 A • • PbQ, pH 10.2 -5 £ -Q O q u a r t z , pH 7.25 _L _L _L 20 40 60 80 100 Concent ra t ion o f NaCMC, ppm F i g . 6.54 - E f f e c t of NaCMC c o n c e n t r a t i o n on the ze ta p o t e n t i a l s of d i f f e r e n t s o l i d samples. RESULTS AND DISCUSSION / 162 observations have been reported by Rubio and Kitchener (1976) (see Section 3.5.2). Groszek (1975) also observed higher adsorption of p o l y c y c l i c aromatic hydrocarbons on graphite samples containing a higher proportion of polar edge s i t e s . 6.4.4 Summary The adsorption and e l e c t r o k i n e t i c measurements have shown that: a. The adsorption d e n s i t i e s of dex t r i n or CMC on both quartz and methylated quartz (MQ) were very low and not affe c t e d by changing pH; b. The adsorption d e n s i t i e s of d e x t r i n or CMC on quartz were s i g n i f i c a n t l y increased i f the quartz contained lead i o n i c adsorption centres. The adsorption was strongly pH dependent. The adsorption of either d e x t r i n or CMC was increased further i f the quartz contained lead i o n i c adsorption centres and was methylated (MPbQ); c. The adsorption of dextrin or CMC on quartz containing lead i o n i c adsorption centres was of a h i g h - a f f i n i t y type, reaching a plateau at low polysaccharide concentrations. The adsorption isotherms f i t t e d the Langmuir-type equation; d. The adsorption of dextrin on several minerals showed that i n a l l cases the pH of maximum adsorption agreed very well with the maximum c o - p r e c i p i t a t i o n pH. Adsorption maxima on hematite, galena, l i t h a r g e and chalcopyrite occurred at pH values of 7, 11, 11 and 9, r e s p e c t i v e l y . RESULTS AND DISCUSSION / 163 e. CMC i s a strong anionic p o l y e l e c t r o l y t e which drove the zeta p o t e n t i a l s of the adsorbents toward negative d i r e c t i o n upon adsorption. Dextrin adsorption, on the other hand, only seemed to reduce the magnitude of the zeta p o t e n t i a l s . f. Dextrin only i n t e r a c t e d with the metal hydroxide species on the mineral surfaces, whereas CMC intera c t e d with a l l surface metal i o n i c species, i r r e s p e c t i v e of t h e i r nature (metal ions, or metal hydroxy complexes). 6.5 EXAMPLES OF AN APPLICATION: SULPHIDE FLOTATION Chalcopyrite and galena behaved l i k e hydroxide i n dextrin adsorption. Since the respective metal hydroxides occur i n d i f f e r e n t pH ranges on the surfaces of these two minerals, d e x t r i n adsorption maxima were observed i n d i f f e r e n t pH ranges. The separation of copper and lead sulphides i s a major objective i n the d i f f e r e n t i a l f l o t a t i o n of p o l y m e t a l l i c sulphide ores. F l o t a t i o n t e s t s on the a b i l i t y of dex t r i n to f a c i l i t a t e t h i s separation were therefore conducted as a model system. Small scale f l o t a t i o n tests were conducted i n the modified Hallimond tube using -0.105+0.038 mm si z e f r a c t i o n s of chalcopyrite and galena. Alternate sequences of dext r i n and potassium ethyl xanthate (KEX) addit i o n were investigated. F i g . 6.55 shows the f l o t a t i o n r e s u l t s f o r galena. The f l o a t a b i l i t y of galena decreased sharply above pH 9 . If d e x t r i n was added p r i o r to xanthate, as low a RESULTS AND DISCUSSION / 164 lOOf 4 6 8 10 12 14 pH F i g . 6.55 E f f e c t of pH on the Hallimond tube f l o t a t i o n of galena i n the presence or absence of dextrin. Conditions: 2 grams of -150+400# galena i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION / 165 concentration as 1 ppm dext r i n caused the complete depression of galena. Reversing t h i s a d d i t i o n sequence changed the f l o t a t i o n behaviour s l i g h t l y , but the f l o a t a b i l i t y of galena remained depressed above pH 9. Increasing the concentration of de x t r i n to 50 ppm d i d not make any d i f f e r e n c e . These r e s u l t s were consistent with the adsorption behaviour, i . e . , maximum adsorption of de x t r i n on galena was found around pH 10 to 12. The f l o t a t i o n of chalcopyrite with potassium ethyl xanthate and de x t r i n displayed i n t e r e s t i n g features. Addition of 50 ppm dext r i n caused the complete depression of chalcopyrite above pH 6 ( F i g . 6.56). If 1 ppm dext r i n was added p r i o r to xanthate, the f l o a t a b i l i t y was al s o depressed above pH 6. If 1 ppm dext r i n was added a f t e r xanthate, the f l o a t a b i l i t y was s l i g h t l y higher. Here f l o a t a b i l i t y was at a minimum at pH 7, then increased s i g n i f i c a n t l y with increasing pH. Comparison of F i g s . 6.55 with 6.56 reveals that Cu-Pb sulphide separation i s poss i b l e i n two pH ranges: from pH 6 to 8 and from pH 11 to 12. In the pH range of 6 to 8, dext r i n can be added p r i o r to xanthate to depress chalcopyrite and to f l o a t galena. In the pH range of 11 to 12, xanthate can be added p r i o r to dextrin to f l o a t c h a l c o p y r i t e while depressing galena. The optimum d e x t r i n concentration for the d i f f e r e n t i a l f l o t a t i o n of Cu-Pb sulphides i s shown i n F i g . 6.57. At pH 7.4, f l o t a t i o n of galena was not af f e c t e d by de x t r i n concentrations up to 10 ppm (added p r i o r to xanthate), while f l o t a t i o n of chalcopyrite was depressed at RESULTS A N D D I SCUSS ION / 166 100 4 6 8 10 12 14 pH Fig. 6 . 5 6 - Effect of pH on the Hallimond tube flotation of chalcopyrite in the presence or absence of dextrin. Conditions: 2 grams of -150+400// chalcopyrite in a 200 ml solution. RESULTS AND DISCUSSION / 167 10Q-V/-c h a l c o p y r i t e , pH 7.4 ! ^ Vi CU > o o cu Pi ena, pH 7.4 10 2 M NaCl 1.25x10 M KEX Hh 0 0.01 33_ 0.1 1 10 100 Concentra t ion of D e x t r i n , ppm 1000 F i g . 6.57 _ E f f e c t of d e x t r i n c o n c e n t r a t i o n on the f l o t a t i o n of s u l p h i d e s . D e x t r i n was added p r i o r to x a n t h a t e . 2 grams of -150+400// su lph ide i n a 200 ml s o l u t i o n were u t i l i z e d f o r each experiment . RESULTS AND DISCUSSION / 168 above 1 ppm. Selective separation may therefore be p o s s i b l e i n the range of d e x t r i n concentrations from 1 to 50 ppm. An a r t i f i c i a l mixture of 1:1 chalcopyrite and galena was then prepared and subjected to f l o t a t i o n separation i n the same Hallimond tube. The mixture was conditioned with 10 ppm de x t r i n f or 10 minutes at a given pH and then with 1.25x10-4 M potassium e t h y l xanthate, followed by f l o t a t i o n for 5 minutes at a nitrogen flowrate of 30 standard ml/minute. F i g . 6.58 indicates c l e a r l y that galena was s a t i s f a c t o r i l y separated from chalcopyrite between pH 6 and 7. As was mentioned e a r l i e r , the separation of galena and chalcopyrite should a l s o be possible i n strongly a l k a l i n e s o l u t i o n (above pH 11). F i g . 6.59 shows that the 1:1 mixture of galena and chalcopyrite was separated by adding 1.25x10-* M KEX followed 10 minutes l a t e r by 1 ppm d e x t r i n , then f l o a t i n g f o r 5 minutes at a nitrogen flowrate of 30 standard ml/minute. It was observed that at pH 13, very f i n e N 2 bubbles were formed i n the Hallimond tube, which aided the f l o t a t i o n of chalcopyrite. The d i f f e r e n t i a l f l o t a t i o n schemes for the separation of Cu-Pb sulphides were not the major objectives of t h i s p r o j e c t . However, these f l o t a t i o n r e s u l t s do confirm the v a l i d i t y of the adsorption r e s u l t s obtained with the model s o l i d s . Conversely, the experiments c a r r i e d out with the polysaccharides and the quartz samples with c o n t r o l l e d surface properties are c l e a r l y relevant to the f l o t a t i o n systems described above. It i s worthy of mention that the use of R E S U L T S A N D D I S C U S S I O N / 169 100 0 I I 1 1 1 5 6 7 8 9 pH F i g . 6.58 - E f f e c t of pH on the d i f f e r e n t i a l f l o t a t i o n of a 1:1 mixture of galena and c h a l c o p y r i t e . D e x t r i n was added p r i o r to xantha te . C o n d i t i o n s : 2 grams of -150+400// s u l p h i d e mixture i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION / 170 100 80 10 2 M NaCl 1 . 2 5 x l 0 ~ 4 M KEX / c h a l c o p y r i t e 60 1 ppm d e x t r i n / A 40 20 -Q galena 0 I i 9 10 11 12 13 14 PH F i g . 6.59 - E f f e c t of pH on the d i f f e r e n t i a l f l o t a t i o n of a 1:1 mix ture of galena and c h a l c o p y r i t e . D e x t r i n was added a f t e r xantha te . C o n d i t i o n s : 2 grams of -150+400// s u l p h i d e mixture i n a 200 ml s o l u t i o n . RESULTS AND DISCUSSION / 171 de x t r i n i n Cu-Pb sulphide separation has been adopted by several i n d u s t r i a l operations i n order to depress galena while simultaneously f l o a t i n g c h a l c o p y r i t e . However, the mechanism by which dextrin f a c i l i t a t e d t h i s separation was unknown. This research provides some in s i g h t s i n t o the mechanistic basis f o r t h i s and po s s i b l y other ap p l i c a t i o n s of polysaccharide chemistry by the mineral processing industry. CHAPTER 7 CONCLUSIONS Adsorption measurements of dextrin and carboxymethyl cellulose (CMC) on variously modified quartz samples and on natural minerals indicate that: a) Dextrin and CMC have very low adsorption densities on both the unmodified hydrophilic quartz and the hydrophobic methylated quartz; b) The presence of lead ionic sites on a quartz surface significantly increases the adsorption densities of both dextrin and CMC; c) Dextrin adsorption reaches a maximum at pH values where the metal ions that constitute the adsorbent form metal hydroxides. Thus the maximum adsorption of dextrin occurs at pH 7 on hematite, pH 9 on chalcopyrite, and pH 11 on litharge, galena and lead-coated quartz. Contact angles measured on fused quartz plates using the captive bubble technique indicate that: 172 CONCLUSIONS / 173 a) Contact angles on methylated quartz do not change whether or not CMC or de x t r i n i s present; b) Contact angles on lead-coated then methylated quartz plates decrease i n the presence of either d e x t r i n or CMC; c) Lead-coated quartz plates show d e f i n i t e contact angles i n sodium oleate so l u t i o n . However, these angles are s i g n i f i c a n t l y reduced when CMC or dex t r i n i s added. This i s e s p e c i a l l y true when CMC or dextrin i s added p r i o r to sodium oleate. 3. The f l o a t a b i l i t i e s of quartz samples were determined i n the presence or absence of either dextrin, CMC or amylopectin. The r e s u l t s are consistent with the adsorption measurements. The major findings are: a) None of the tested polysaccharides a f f e c t the f l o a t a b i l i t y of hydrophobic methylated quartz (MQ). If the quartz i s methylated a f t e r lead-coating (MPbQ), i t s f l o a t a b i l i t y i s depressed by polysaccharides i n the pH range i n which the polymers are adsorbed. Thus depression of MPbQ f l o a t a b i l i t y occurs above pH 9 i n the presence of 100 ppm CMC, above pH 8 i n 100 ppm de x t r i n and above pH 7 i n 100 ppm amylopectin. C O N C L U S I O N S / 1 7 4 b) The f l o t a t i o n of lead-coated quartz (PbQ) by sodium oleate i s depressed by polysaccharides i n the pH range i n which the polymers are adsorbed. The depressive e f f e c t i s more pronounced i f the polysaccharides are added before sodium oleate. It seems, therefore, that the hydrophobicity of a mineral surface alone does not contribute s i g n i f i c a n t l y to the adsorption of polysaccharides. However, the presence of metal i o n i c species on a mineral surface plays a d e c i s i v e r o l e i n adsorption. The following observations and t h e i r mechanistic implications characterize polysaccharide-metal i n t e r a c t i o n s : a) Dextrin and amylopectin i n t e r a c t only with lead hydroxide, and they do not i n t e r a c t with lead cations; b) The optimum i n t e r a c t i o n between d e x t r i n and metal i o n i c species always occurs i n the pH ranges where metal hydroxides are formed. Thus the optimum pH i s 7.5 for f e r r i c , 8 f or aluminum, 9 f o r cupric, 11 for lead and 12 for magnesium ions. These pH values are very close to the i.e.p.'s of the respective metal hydroxides; c) The i n t e r a c t i o n between dextrin (or amylopectin) C O N C L U S I O N S / 175 and lead hydroxide causes a decrease i n sol u t i o n pH, i n d i c a t i n g the release of H* ions into the solut i o n . Infrared spectroscopic studies on the i n t e r a c t i o n products reveal the loss of IR absorption bands a r i s i n g from deformation of the glucose r i n g (the constituent monomer of the polysaccharides d e x t r i n and amylopectin). Based on such information i t appears that the dextrin-lead hydroxide and amylopectin-lead hydroxide i n t e r a c t i o n s r e s u l t from chemical complexation and not from hydrogen bonding; d) The i n t e r a c t i o n between f e r r i c (or cupric) hydroxide and d e x t r i n also causes a decrease i n solution pH. These i n t e r a c t i o n s may r e s u l t from chemical complexation as well; e) CMC i n t e r a c t s with lead ions and lead hydroxy complexes (including lead hydroxide). The i n t e r a c t i o n i s chemical i n nature, as shown by the t i t r a t i o n and i n f r a r e d spectroscopic r e s u l t s . 6. The adsorption maxima of de x t r i n on galena and chalcopyrite occur i n d i f f e r e n t pH ranges. Therefore the small scale f l o t a t i o n separation of synthetic mixtures of galena and chalcopyrite using d e x t r i n and potassium ethyl xanthate was attempted at various pH l e v e l s . At a pH near 6, chalcopyrite i s depressed while galena CONCLUSIONS / 176 can be floated off; at a pH near 12, galena is depressed while chalcopyrite can be floated from the mixture. CHAPTER 8 SUGGESTIONS FOR FURTHER WORK Since de x t r i n only i n t e r a c t s with metal hydroxides, i t could be useful as a p o t e n t i a l l y s e l e c t i v e depressant, as long as the hydroxides of the metal ions i n the minerals to be separated do not occur i n the same pH region. The a p p l i c a t i o n of dex t r i n i n Cu-Pb sulphide d i f f e r e n t i a l f l o t a t i o n deserves further study. Dextrin should also be tested i n Cu-Ni sulphide separation. Since the i.e.p. of n i c k e l hydroxide i s around pH 11 to 12 (Parks, 1965), the i n t e r a c t i o n of d e x t r i n with nickel-bearing minerals should be pronounced i n t h i s pH range. This work has not dealt with the e f f e c t s of polysaccharide s t r u c t u r a l v a r i a t i o n s on the i n t e r a c t i o n s between mineral surfaces and polysaccharides. The pos s i b l e e f f e c t s of such s t r u c t u r a l v a r i a t i o n s as chain configuration, p o s i t i o n of substituents, etc. are very c o n t r o v e r s i a l and should be further studied. The a c t i v a t i o n behaviour of polysaccharides has oc c a s i o n a l l y been reported but has not been well characterized. It may be worthwhile to study t h i s e f f e c t further. This behaviour seems to depend on the o r i e n t a t i o n (conformation) of the adsorbed 177 SUGGESTIONS FOR FURTHER WORK / 178 polysaccharides on the mineral surface. Conformational studies of polymers on mineral surfaces are d i f f i c u l t . However, one may begin by analysing the structure of metal-polysaccharide complexes. Such complexes can be extracted from aqueous solutions (e.g. Klotz, 1961). 4. "As discussed i n Section 6.1.2., the c o r r e l a t i o n between the Hallimond tube f l o t a t i o n , and the contact angle measurements on the quartz plates, i n the presence or absence of dex t r i n or CMC i s consistent. However, i n the contact angle measurements there was no c r i t i c a l pH as was observed i n the Hallimond tube f l o t a t i o n . This has been a t t r i b u t e d q u a l i t a t i v e l y to dynamic e f f e c t s which p r e v a i l i n a r e a l f l o t a t i o n process. To confirm such a statement, however, i t may be necessary to conduct a series of induction time measurements. REFERENCES Adamson, A. W., 1967, Physical chemistry of surfaces, 2nd ed., John Wiley and Sons, New York Afenya, P. 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H., Husain, S. A., Mahdihassan, S., 1962, "Cobalt-carbohydrate complexes", Pakistan J . S c i . Ind. Res., 5, 193-5 Z a i d i , S. A. H., Husain, S. A., Mahdihassan, S., 1963, "Carbohydrate complexes of cadmium", Pakistan J . S c i . Ind. Res., 6, 105-7 Z a i d i , S. A. H., Mahdihassan, S., 1963, "Carbohydrate complexes of zi n c " , Pakistan J . S c i . Ind. Res., 6, 103-4 APPENDIX I. DETERMINATION OF POLYSACCHARIDE CONCENTRATION AND ADSORPTION 1. Determination of polysaccharide concentration The concentrations of both dextrin and CMC i n aqueous solutions were determined by the c o l o r i m e t r i c method of Dubois et a l (1956). In t h i s method, a c e r t a i n amount of phenol i s added to the polysaccharide solution, followed by the rapid addition of concentrated sulphuric a c i d . This produces a stable orange colour whose i n t e n s i t y i s proportional to the concentration of the polysacchar i d e : According to Dubois et a l , the amount of phenol and the wavelength to be used to measure the i n t e n s i t y of the orange colour are c r i t i c a l parameters that must be determined e m p i r i c a l l y . A preliminary t e s t showed that addition of 1.5 ml of 5% phenol plus 10 ml of concentrated sulphuric a c i d to 4 ml of polysaccharide solution provided the optimum conditions. The absorbance of the coloured complex was determined at 490 nm (Figs. A l and A2). Under these conditions d e x t r i n (or CMC) concentration was a l i n e a r function of A 4, 0 ( F i g . A3). POLYSACCHARIDE 5-HYDROXY ME 'FURFURAL (Al) jphenol COLOURED COMPLEX 192 / 193 1.6 1.4 dex t r i n 1.2 sample s o l u t i o n : 4 ml concentrated H_SO.: 10 ml 2 4 1.0 J3 O 0.8 0.6 0.4 0.2 I I I 2 3 4 Volume of 5 % Phenol, ml F i g . A l Effect of Phenol addition on the absorbance of d e x t r i n and CMC solutions using the sulphuric acid-phenol method 200 ppm CMC blank i i l i I I I i I I I I i 1 i I I I I I I I I I I i i i 1 l 450 550 450 550 450 550 450 550 450 550 Wavelength of. the l i g h t source, nm Fi g . A2 E f f e c t of the wavelength of the l i g h t source on the absorbance of CMC solutions CD / 196 2. Determination of adsorption d e n s i t y The r e g r e s s i o n equations shown i n F i g . A3 can be transformed t o : In the c a l c u l a t i o n of polysaccharide concentration, the slope of the r e g r e s s i o n l i n e s ( k x ) was assumed constant, and the second parameter, k 2, was determined i n every batch of t e s t s by one or s e v e r a l blank p o i n t . For example, f o r d e x t r i n : When the ad s o r p t i o n t e s t was conducted with the -400# c h a l c o p y r i t e at d i f f e r e n t pH ( o r i g i n a l concentration of d e x t r i n was 50 ppm; 1 gram of -400# c h a l c o p y r i t e i n a 50 ml s o l u t i o n ) , the f o l l o w i n g r e s u l t s were obtained: c = k i - a b s . + k 2 (A2) c = 70.274 • abs. + k 2 (A3) F i n a l pH abs. 2.04 0.609 3.25 0.592 3.15 0.602 5.52 0.587 blank 0.759 (50 ppm d e x t r i n ) / 197 Subst i t u t i n g c 0 = 50 ppm i n equation (A3) leads to k 2 = -3.338 and t h i s allows the r e s i d u a l concentration of dextrin to be calcul a t e d : F i n a l pH abs. c, ppm 2.04 0.609 39.46 • 3."25 0.592 38.26 3.15 0.602 38.97 5.52 0.587 37.91 blank 0.759 50.00 Once the r e s i d u a l concentration was found, the amount of dextr i n which had been adsorbed on the chalcopyrite was calculated according to the following equation: T = ( ( c 0 - c)-V)/(W-S) (A4) Where T i s the amount adsorbed (mg/m2); c„ and c are the o r i g i n a l and re s i d u a l concentration of dex t r i n (ppm), respectively; V i s the volume of the suspension ( l i t e r ) ; W i s the sample weight (g) and S i s the s p e c i f i c surface area of the sample (m 2/g). Thus, at pH 2.04, T = ((50 - 39.46)-0.05)/(l-0.556) = 0.95 mg/m2. The res t of the adsorption density can be calculated accordingly: F i n a l pH abs. c, ppm T, mg/m2 2.04 0.609 39.46 0.95 3.25 0.592 38.26 1.06 3.15 0.602 38.97 0.99 5.52 0.587 37.91 1.09 / 198 3. K i n e t i c s of dextrin and CMC adsorption K i n e t i c analysis on the adsorption of de x t r i n and CMC revealed that i n both cases the process was f a i r l y f a s t , reaching equilibrium within 30 minutes (Figs. A4 and A5). An e q u i l i b r a t i o n time of 60 minutes was therefore used i n the adsorption studies. 4. The r e p r o d u c i b i l i t y of the adsorption t e s t s The adsorption r e s u l t s were very reproducible. This was shown by repeating some t e s t s : a. Adsorption of dext r i n on PbQ vs. pH, c 0 = 50 ppm Nov. 14, 1986 F i n a l pH abs. c, ppm T, mg/m2 2.10 0.739 47.40 0.113 5.22 0.675 42.90 0.309 7.04 0.603 37.84 0.529 7.47 0.589 36.84 0.572 7.90 0.574 35.80 0.617 8.64 0.506 31.02 0.825 10.74 0.489 29.83 0.877 12.36 0.614 38.62 0.495 blank 0.776 (50 ppm d e x t r i n ) 5 on Pb-activated quartz, "Cn = 20 ppm, pll = 9.4 1 2 3 Reaction time, hour Fi g . A5 K i n e t i c s of CMC adsorption / 201 In the above t e s t s , the 25 ml of 0.01 M NaCl was adjusted to a ce r t a i n pH then mixed with the PbQ. After shaking, 25 ml of 100 ppm dex t r i n s o l u t i o n of the same pH was introduced. On A p r i l 30th, 1987, t h i s set of test s was repeated. The procedure was the same except that the pH of the NaCl-PbQ solu t i o n was checked and adjusted i f necessary. A p r i l 30th, 1987 F i n a l pH abs. c, ppm T, mg/m2 2.05 0.768 48.95 0.04 2.78 0.733 46.49 0.15 5.91 0.710 44.87 0.22 6.19 0.594 36.72 0.58 9.66 0.485 29.06 0.91 10.92 0.493 29.62 0.89 12.20 0.576 35.45 0.63 blank 0.783 (50 ppm de x t r i n ) I t i s seen that the adsorption d e n s i t i e s at comparable pH values do not d i f f e r by more than 0.1 mg/m2. / 202 b. Adsorption of dextrin on chalcopyrite vs. pH A p r i l 9th, 1987 F i n a l pH abs. c, ppm T, mg/m2 2.32 0.638 39,10 0.98 5.60 0.611 37.21 1.15 6.84 0.618 37.70 1.10 7.9.0 0.606 36.86 1.18 8.38 0.590 35.73 1.28 11.36 0.625 38.19 1.06 13.00 0.663 40.86 0.82 blank 0.792 (50 ppm d e x t r i n ) May 3rd, 1987 F i n a l pH abs. c, ppm T, mg/m2 2.04 0.609 39.46 0.95 3.25 0.592 38.26 1.06 3.15 0.602 38.97 0.99 5.52 0.587 37.91 1.09 blank 0.759 (50 ppm d e x t r i n ) Again the adsorption de n s i t i e s do not d i f f e r by more than 0.1 mg/m2 at comparable pH values. APPENDIX II. LOGARITHMIC CONCENTRATION DIAGRAMS FOR LEAD NITRATE-WATER SYSTEMS 1. Equilibrium Constants:t Pb(OH) 2 (aq.) = Pb + 2 OH" P b ( 0 H ) + = Pb*"*" + OH" K=4.2xl0 K=1.5xl0 -15 Pb(0H) 2 (aq.) = H + HPb0~ K=1.2xl0 -11 Pb(OH) 2 (s) = Pb(OH) 2 (aq.) K=6.4xl0" 2 . Logarithmic Concentration Diagrams Logarithmic concentration diagrams were constructed for three d i f f e r e n t lead n i t r a t e concentrations, using the above thermodynamic constants ( F i g s . A6, A7 and A8). f D a t a taken from Latimer (1952) and CRC Handbook of Chemistry and Physics 203 / 204 aqueous s o l u t i o n / 205 2 4 6 8 10 12 14 pH -2 . A7 L o g a r i t h m i c concent ra t ion diagram of 10 M Pb(N0.j)2 aqueous s o l u t i o n / 206 aqueous s o l u t i o n APPENDIX III. OPTICAL ROTATORY DISPERSION MEASUREMENTS Dextrin i s composed of a-D(+)-glucose, which i s an o p t i c a l l y a c t i v e substance. When a plane-polarized l i g h t beam passes through a dextrin s o l u t i o n , the transmitted l i g h t w i l l be rotated by a c e r t a i n angle. Changes i n the glucose r i n g (for example, by s u b s t i t u t i o n on the hydroxyl groups) w i l l change t h i s angle of r o t a t i o n . This property has been used to detect the presence of metal-saccharide complexes (Davidson, 1967). O p t i c a l rotatory d i s p e r s i o n measurements were conducted f o r dextrin-Pb, dextrin-Fe and dextrin-Cu systems at t h e i r unadjusted pH values. At these pH values p r i m a r i l y metal cations were present i n the solutions. As shown i n Fig s . A9 to A12, there was no change i n the o p t i c a l r o t a t i o n of dext r i n under such conditions. Hence no metal-dextrin complexes were formed at these low pH's. The tests were conducted with a JASCO ORD/UV-5 Op t i c a l Rotatory Dispersion Recorder i n the Department of Chemistry, The Univ e r s i t y of B r i t i s h Columbia. 207 +1.0 +0.8 - \ a. 0.01 M CuCl 2 aqueous s o l u t i o n , pH 3.5 +0.6 b. mixture of 1.23 g/1 dextrin and 0.01 M CuCl,, pH 3.5 * +0.4 -+0.2 - " b 0 a -0.2 i l l 250 350 450 550 650 Wavelength of the Incident l i g h t , nm Fig . A10 Optical r o t a t a r y dispersion of dextrin-CuCl„ aqueous solution t +0.6 +0.2 0 a. 0.01 H FeCl^ aqueous s o l u t i o n , pH 2.0 - b. mixture of 1.23 g/1 d e x t r i n and 0.01 M F e C l 3 , pH 2.0 - " ~~~~ — — — — b a -,. 1 ... . ... 1 -. 1 1 . . . 400 450. 500 550 600 650 Wavelength of the incident l i g h t , nm Fig. A12 O p t i c a l rotatary dispersion of dextrin-FeCl aqueous s o l u t i o n A P P E N D I X I V . T H E R M O D Y N A M I C C A L C U L A T I O N S O F D E X T R I N - P B * + A N D D E X T R I N - L E A D H Y D R O X I D E I N T E R A C T I O N S OH In the f o l l o w i n g equa t ions , represents a g lucose monomer Un. i n the d e x t r i n m o l e c u l e s ; and the two h y d r o x y l groups are the ones t h a t are i n the C-2 and C-3 p o s i t i o n s . R ^ * + P b ^ = Kpb + 2 H + (A5) K A A5 [ R C ^ P b ] [ H + ] 2 I f a h y d r o x y l group i s added on both s ides o f equa t ion (A5) , t h e n : K^OH + Pb" 1^ + OH" = R C ° > b + 2 H + + OH (A6) S i n c e : [ R ^ b ] [ H + ] 2 [0H~] K = = K [RC°|] [ P b * ] [OH"] PbOH + = P b * * + OH (A7) K s = 1.5 x 1 0 " 8 A7 212 and / 213 OH" + H + = H 2 0 (A8) > 14 K = 1 x 10 A8 (A6) + (A7) + (A8) , get R ^ J + PbOH + = K ^ P b + H + + H 2 0 (A9) [RC°;Pb] [ H + ] K. _ = .———-- -•-" = K, K. K. A9 „ A6 A 7 As [PbOH +] = 1.5 x 1 0 6 K p A5 I t i s thus seen tha t the w a t e r - r e l e a s e - r e a c t i o n as shown i n equa t ion (A9) between d e x t r i n and Pb0H + i s more favoured than the one between d e x t r i n and Pb ( i e . , equat ion (A5)) . F u r t h e r , i f two h y d r o x y l groups are added on both s i d e s o f e q u a t i o n (A5) , then R C ° U + P b " ^ + 2 OH = R C ^ P b + 2 H + + 2 OH (A10) Un. X) [ R ; ° > b ] [ H + ] 2 [ O H " ] 2 K A 1 0 = - 77 - = K A 5 [ R ^ ] [ P b ^ ] [ O H " ] 2 Since / 214 Pb(0H) 2 (aq.) = Pb""" + 2 OH" ( A l l ) K = 4.2 x 1 0 " 1 5 A l l (A10) + ( A l l ) + 2 (A8) , get R ^ J + Pb (0H) 2 (aq.) = < ° > b + 2 H 2 0 (A12) 2 K A 1 2 " ~ 1 " K A 8 K A H F A 1 1 [ R ^ ] [Pb(0H) 2 ( aq . ) ] = 4 .2 x I O 1 3 K A 5 I t i s seen tha t r e a c t i o n (A12) i s more f e a s i b l e than e i t h e r (A9) o r (A5) . However the r e a c t i o n mechanism as shown i n equa t ion (A12) does not cause a decrease i n s o l u t i o n pH. Based on the exper imen ta l obse rva t ions i n t h i s work, i t may be concluded tha t both r e a c t i o n (A9) and (A12)are p o s s i b l e ! I f the concen t r a t i on o f aqueous Pb(0H) 2 i s h i g h enough that Pb(OH) 2 p r e c i p i t a t e s appear i n the s o l u t i o n , and R^jJ + Pb(0H) 2 (s) = K°Q^b + 2 H 2 0 (A13) Since / 215 Pb(OH) 2 (s) = PB(OH) 2 (aq.) (A14) K . . . = 6.4 x I O - 4 A14 and '(A12) = (A13) - (A14) , so tha t K a 1 2 = KA13 KA14 K A . = K . . . K . . . = 2.69 x 1 0 1 0 K • • A13 A12 A14 A5. Thus , d i r e c t r e a c t i o n o f d e x t r i n w i t h p r e c i p i t a t e d l e a d hydrox ide i s s t i l l more favoured than the d i r e c t r e a c t i o n o f d e x t r i n I | w i t h Pb i o n s . In the above c a l c u l a t i o n s , the d e x t r i n - l e a d complexes are assumed to be s o l u b l e . I t i s very l i k e l y tha t the complexes are p r e c i p i t a t e s . In the l a t t e r case, the c o n c e n t r a t i o n s o f such complexes can be s u b s t i t u t e d by the s a t u r a t i o n c o n c e n t r a t i o n s and the r e a c t i o n s (A13), (A12) and (A9) w i l l be even more f avoured ! APPENDIX V. CALCULATION OF THE PH DROP IN THE TITRATION OF LEAD NITRATE WITH CMC It i s shown i n F i g . 6.37 that when an aqueous lead n i t r a t e solution was t i t r a t e d by CMC at pH 4.3, the r e s u l t i n g s o l u t i o n pH decreased continuously with increasing CMC a d d i t i o n . This i s believed to r e s u l t from the following chemical reaction: -COOH + P b + + •* -COOPb+ + H + (A15) The following c a l c u l a t i o n seems to support the above rea c t i o n mechanism. After the addition of 40 ml of 10 g/1 CMC, the f i n a l s o l u t i o n pH was 2.7 (see F i g . 6.37). Since 40 ml of 10 g/1 CMC = 0.4 g CMC Average M.W. of a glucose residue i n CMC (DS 0.7) = 217.7 Thus the number of carboxymethyl groups present i n the CMC s o l u t i o n was: 0.4 x 0.7 /217.7 = 0.001286 mole 216 / 217 The carboxymethyl group concentration was therefore: 0.001286/0.14 = 0.00918 M. Since the pK of the carboxymethyl groups i n CMC i s 4.4 Si (Stelzer and Klug, 1980), the maximum concentration of carboxymethyl groups that could have contributed to H + release was: t-COOH] = 0.00918 x 0.5 = 0.00459 M According to equation (A15), there should have been 0.00459 M H + ions released, which would give a f i n a l pH of 2.34. As seen from F i g . A8, at pH 4.3, 0.001 M of the lead species was i n the form of PbOH+ i n the 0.1 M lead n i t r a t e s o l u t i o n . At a pH of 2.3, t h i s PbOH+ d i s s o c i a t e s i n t o P b + + : PbOH- + H + •+ Pb + + + H 20 (A16) Thus 0.001 M H + produced during the t i t r a t i o n would have been consumed by t h i s d i s s o c i a t i o n and the actual f i n a l H + concentration should have been 0.00359 M. This corresponds to a f i n a l pH of 2.44, which i s very close to the experimental value (pH 2.7). APPENDIX VI. THE EFFECT OF DEXTRIN ON THE HYDROLYSIS OF LITHARGE When l i t h a r g e (PbO) powder was placed i n d i s t i l l e d water, the pH of the suspension increased as a r e s u l t of the hydrolysis of t h i s oxide. However, the presence of d e x t r i n i n d i s t i l l e d water reduced the extent of such an increase i n pH. This was shown by the following experiment: 100 ml solutions containing varying amount of dext r i n (from 0 to 1000 ppm) were prepared at natural pH of 5.6; then 1 gram of -400# l i t h a r g e was added to the solution and the pH change was recorded as a function of time. F i g . A13 shows that the presence of dextr i n reduces the rate of pH increase and r e s u l t s i n a lower f i n a l pH. This seems to confirm the i n t e r a c t i o n mechanism shown i n equation (10), page 124. 218 0 1 2 3 4 Time, minute A13 The e f f e c t of d e x t r i n on the h y d r o l y s i s o f l i t h a r g e (PbO) . C o n d i t i o n s : one gram o f -400// l i t h a r g e i n a 100 ml s o l u t i o n . APPENDIX VII. THE INTENSITY OF IR ABSORPTION BANDS OF DEXTRIN IN DEXTRIN-LEAD HYDROXIDE PRECIPITATES The following c a l c u l a t i o n shows that the i n t e n s i t y of" absorption at 925 cm-1 decreases (compared with that at 1010 cm-1) with inc r e a s i n g lead/dextrin r a t i o . THE INTENSITY OF IR ABSORPTION BANDS OF DEXTRIN Spectrum 9 P e a k ^ cm V % I , % A (abs.) V A2 PX1 PX3 PX5 1 2 1 2 1 2 1020 930 1015 930 1010 920 63.0 69.0 63.0. 66.5 79.0 80.0 15.0 53.0 13.0 58.0 53.0 78.0 1.435 0.260 1.578 0.136 0.399 0.025 5.44 11.60 15.75 PX2 PX4 PX6 1 2 1 2 1 2 1020 930 1020 935 1005 920 65.0 68.0 70.0 74.0 67.0 68.5 28.5 62.0 25.0 65.0 42.0 67.0 0.824 0.092 1.030 0.129 1.380 0.022 8.92 7.94 62.73 PXS1 PXS3 PXS5 1 2 1 2 1 2 1020 925 1010 925 1010 920 65.0 70.0 57.5 59.0 75.0 75.5 13.0 57.0 30.0 55.0 51.0 73.0 1.609 0.205 0.650 0.070 0.380 0.034 7.84 9.29 11.34 PXS2 PXS4 PXS6 1 2 1 2 1 2 1010 930 1010 925 1000 920 64.5 69.0 40.0 41.0 78.0 79.0 18.0 55.0 15.0 38.0 57.5 77.0 1.276 0.226 0.980 0.076 1.356 0.025 5.63 12.89 54.26 220 

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