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SELECTIVE OF MINERALS

A thesis submitted for the degree of Doctor of Philosophy in the University of London

by

YOSPY ABDELHADY ISMAIL ATTIA

Imperial College of Science and Technology,

London, S.W.7. July, 1974. 1

ABSTRACT

This thesis can be considered to consist of three parts. The first refers to the surface chemistry of copper minerals; the solubility, surface energy, oxidation and surface electrical properties of copper minerals in water under atmospheric conditions have been studied and the inter-relation between these phenomena has been illustrated. Thus the solubility is affected by the surface energy and oxidation (or reduction), and the electrical charge is influenced by the solubility. The correspondence of the zero point of charge (z.p.c.) to the pH of minimum solubility of the mineral has also been discussed.

The zeta-potential of malachite at different pH values has been measured by the micro-electrophoresis method; the z.p.c. is between pH 9-9.5. The effects of copper sulphate, carbonate, polyphosphate and poly- acrylate on the zeta-potential of malachite have been determined at various pH values. The second part is concerned with the chemistry of polymeric flocculants and the origins of their selectivity. A series of flocculants containing thiol (-SH) and other metal-complexing groups has been developed and tested. The anticipated selectivity for copper minerals has been confirmed by flocculation tests on separate minerals and synthetic mixtures. The various aspects of the preparation, purification, analysis, physical and chemical properties of the polymeric flocculants have been established, and a number of techniques for characterization of these polymers have been attempted. 2

The third part is concerned with the technical application of the previous two parts to the processing of a real copper . For this purpose, conditions have been worked out for beneficiating a particular dolomitic copper ore (from Zaire) containing a variety of finely disseminated copper minerals. In the development of this process, attention has been given to finding a suitable dispersion medium which would give full liberation and enhance the differential response of the various minerals to floccula- tion. Finally, substantially selective flocculation of copper minerals from the ore, at high solid suspensions (up to 31% by weight) in laboratory tap-water has been achieved, with up-grading from 4.5% to 18.2% copper. The process appears to be economical. 3

ACKNOWLEDGEMENTS

I wish to express my sincere gratitude to

Dr. J.A. Kitchener for his guidance and encouragement.

I am also grateful .to B.T.I. Chemical Limited, Bradford, for their support and interest in this project, especially Dr. P.F. Wilde and Dr. R.W. Dexter who prepared polyacrylamide-dithiocarbamate for me.

My thanks are due to Dr. R. Gochin who has kindly supplied me with information and invaluable assistance in appraising the economics of the selective flocculation process. It is a pleasure to recognize the assistance received from the technical staff, in particular Mr. C. Emmitt the glass blower and Mr. J.D. Sullivan of the workshop. I am also indebted to the Higher Institute of Petroleum and Mining Engineering, Suez, Egypt and the Egyptian Ministry of Higher Education for the scholarship and leave

of absence which enabled me to carry out this research. ° CONTENTS

Page ABSTRACT 1

ACKNOWLEDGEMENTS 3

Chapter 1. GENERAL INTRODUCTION 11

1.1. The application of selective flocculation 11 to mineral processing

1.2. The concept of flocculation 12 of polymers on solid surfaces : factors controlling adsorption

1.3. Selective flocculation 17 previous work : selective adsorption of flocculants 1.4. Aims of this work 20

Chapter 2. SURFACE CHEMISTRY OF COPPER. MINERALS 23 2.1. Introduction; types of mineral 23 2.2. Hydrolysis of cupric in distilled 25 water in equilibrium with atmospheric carbon dioxide 2.3. Solubility of copper minerals 26 2.3.1. effect of pH on solubility of: 27 cupric hydroxide, tenorite, cuprite, malachite, azurite, covellite and chalcocite.

2.3.2. effect of particle size 44 2.3.3. effect of "inert" electrolytes 46 2.4. Solubility of chrysocolla 47 2.4.1. effect of pH, experimental 148

2.4.2. effect of sodium chloride, 50_ experimental 2.5. Surface oxidation of copper minerals : 51 equilibria-

2.5.1. control of oxidation potential of 51 aqueous suspensions by atmos- pheric oxygen : oxidation of Cu+ 2+ to Cu . 2.5.2. surface oxidation of chalcocite 52 2.5.3. surface oxidation of covellite 53 2.5.4. surface oxidation of cuprite and 54 other copper minerals 2.6. Surface electrical properties of copper 55 minerals

2.6.1. introduction; origins of surface 56 charge; preferential adsorption of hydrolyzed metal ions 2.6.2. Zeta-potential of malachite 58 experimental procedure 2.6.3. pH of zero point of charge and pH 61 of minimum solubility 2.6.4. pH of copper minerals 62 zpc Chapter 3. THE CHEMICAL ORIGIN OF SELECTIVITY OF POLYMERIC 64 FLOCCULANTS 3.1. Formation and stability of complex com- 65 pounds; the stability constants 3.2. Classification of ligands; the chelate 67 effect 3.3. Classification of metal cations 70 3.4. Factors affecting the formation and 71 stability of complexes 3.5. Selectivity and specificity of complex 75 formation

3.6. Synthesis of selective polymeric floceulants 80 6

Chapter 4. CELLULOSE XANTHATE 84 4.1. Introduction; formation, structure and 84 selectivity of cellulose xanthate

4.2. Preparation of cellulose xanthate 88 4.2.1. laboratory preparation; standard 92 method, preparation of cellulose xanthate from different cellulose sources

4.2.2. preparation of NaCX of high 93 molecular weight 4.2.3. preparation of NaCX of different 94 degrees of xanthation; effect of CS ratio; effect of temperature 2 4.2.4. preparation of uniformly distributed 96 xanthate groups

4.2.5. preparation of dry NaCX powder of 98 uniform reactivity; emulsion xanthation 4.2.6. xanthation of various derivatives 99 cawc/se preparation of: methyl/xanthate, sodium carboxy methyl cellulose xanthate, hydroxypropyl methyl cellulose xanthate 4.3. Purification of cellulose xanthate 100 4.3.1. precipitation with alcohol 100 4.3.2. dialysis of cellulose xanthate 102 4.3.3. -exchange method 103 4.4. Analysis of cellulose xanthate 107 4.4.1. detection and measurement of 107 xanthate groups 4.4.2. detection and measurement of 111 cellulose in dilute 7

4.5. Physical and chemical properties of 113 cellulose xanthate:physical character- istics, chemical reactions with heavy metal ions; decomposition of xanthate groups, oxidative degradation, of cellulose chain

4.6. Flocculation properties of cellulose 121 xanthate:selective flocculation of sulphides, flocculation of chrysocolla, flocculation of sulphidized chrysocolla, selective flocculation of chrysocolla from quartz

4.7. Conclusions 129

Chapter 5. OTHER SELECTIVE FLOCCULANTS CONTAINING SULPHER 131 5.1. Polyvinyl alcohol xanthate

5.1.1. introduction, formation 131 5.1.2. preparation of polyvinyl alcohol 133 xanthate (PVAX),purification

5.1.3. flocculation properties of PVAX 134

5.2. Polyacrylamide-Dithiocarbamate (PAD) 138

5.2.1. composition and properties 138

5.2.2. flocculation effects on mineral 139 suspensions, selectivity of PAD

5.2.3. selective flocculation of copper 141 minerals from mixed suspensions

5.2.4. inhibition of flocculation of 143 galena in mixtures with copper minerals, effect of Na2S and NaF, effect of K2Cr207

5.2.5. Discussion and conclusions 145 8

Chapter 6. POLYACRYLAMIDE-GLYOXAL-BIS-(2-HYDROXYANIL) 147

6.1 Introduction: choice of GBHA: formation 147 of PAMG polymers 6.2 Preparation of PAMG polymers- experimental 154

6.2.1. preparation of PAMG 2.1, PAMG 2.2 155

6.2.2. preparation of PAMG 6 157

6.2.3. preparation of PAMG 7 157 6.3. Purification of PAMG 2 polymers 158 6.3.1. the alcohol precipitation method 158 6.3.2. Gel method 161 6.4. Preparation of dry powder of pure PAMG 2.3 166 polymer: preparation; purification and ; grinding; 'grinding dry solids; solubility in water

6.5. Characterization of PANG polymers 168 6.5.1. The alcohol precipitation technique 168

6.5.2. The dialysis technique 169 6.5.3. Membrane technique: 169 size distribution of PAMG 2.1 polymer segments 6.5.4. ultra-violet and infra-red spectra 179

6.5.5. degree of substitution 189

6.6. Selective flocculation properties of 192 PAMG 2.1

6.6.1. flocculation effects on mineral 193 suspensions

6.6.2. selective flocculation of copper 194 minerals from mixed suspensions: chryscocolla from calcite; chrysocolla and malachite from mixtures with feldspar, calcite and quartz; chalcocite, malachite and chrysocolla from mixtures with feld- spar, calcite and quartz; malachite from dolomite. 9

6.6.3. the comparative selectivity of 195 PAMG 2.1 6.6.4. the role of unattached GBHA 197 groups on the flocculation behavior of methylolated PAM 6.7. Conclusions 198

Chapter 7. PROCESSING OF COPPER BY SELECTIVE 200 FLOCCULATION 7.1. Introduction, criteria of selectivity, 200 materials and equipment 7.2. Preliminary investigations 207 7.3. Design of flow-sheets for selective 212 flocculation process

7.3.1. flow-sheet 1 213 7.3.2. bulk flocculation procedure: 217 flow-sheet 2 7.3.3. multi-stage flocculation: flow- 220 sheet 3 7.3.4. "starvation" addition of flocculant: 223 flow-sheet 4 7.3.5. multi-stage addition of flocculant: 225 semi-cyclic flow-sheet 5, effect of "ageing" on selective flocculation 7.3.6. the standard flow-sheet; flow-sheet 7 229

7.4. Studies to improve grade and recovery 232

7.4.1. experiment with PAMG 7 232 7.4.2. dispersion of the ore suspensions: 238 effect of ultrasonic vibrations, effect of reducing the interfacial energy 7.5. Effect of solids content on selective 246 flocculation 7.6. ECfect of tap water on selective flocculation 249 10

7.7. Use of tap water and high solids content 251 7.8. Discussion 253 7.9. Economic assessment of selective flocculation 256 process

7.10.Conclusions 260

Chapter 8. CONCLUSIONS 261

REFERENCES 26.6

APPENDIX 1: Calorimetric determination of copper with 276 bis-cyclohexane oxalyldihydrazone APPENDIX 2: Determination of copper content by atomic 278 absorption spectrometry APPENDIX 3: Adsorption of polyacrylamide on hydrophobic 279 surfaces

11

CHAPTER 1 GENERAL INTRODUCTION

1.1 The a . .lication of selective flocculation to mineral

processing (1'2,3,4,5) Several authors have emphasized the need to develop a new technology to cope with the of fine grained minerals. At present, the fine size particles ( <101.1m), known as "slimes" in the mineral technology field, are often discarded after crushing of the rock and before applying such as to the ores, thus resulting in losses of up to 30% of the total ore values in some plants. Also, there exist some large ore bodies in which the valuable minerals are already too finely disseminated in the original ore to be extracted satisfactorily by con- ventional methods. Typical examples are some copper ore ( 6) deposits in Mount Isa (Australia) , and some cassiterite ores in Cornwall (England)( 7). The authors quoted consider that the most promising method proposed to deal with these problems is "selective flocculation". The various aspects of the slimes problems and the possible ways of . recovering fine grained valuable minerals, including selective flocculation, have been well reviewed in the literature by Collins and Read( 8) It is well known that chrysocolla (a copper silicate) presents great difficulties to extraction on a commerical scale by any process at present available, except acid leaching. In the ores where chrysocolla is associated with other acid-soluble minerals (e.g. calcite, dolomite) the 12

acid leaching process becomes economically unattractive. (9,10,11) Ammoniacal leaching or extraction with EDTA is feasible, but costly. This problem might be solved by selective flocculation, as will be shown in later Chapters. Although malachite and azurite can be extracted by froth flotation after sulphidization, their extraction is often not very efficient and it becomes even less efficient when the particles size is very small. The recovery of copper sulphide minerals by froth flotation is well developed; however, when the particle size becomes very small (<5 pm), the process becomes less efficient, apparently because of hydrodynamic problems. It is for these reasons it appeared that the develop- ment of a process of selective flocculation would be timely so, as a contribution to the better use of the worlds limited resources in non-ferrous metals. The case of the copper minerals was chosen for detailed study firstly because of the high (and rising) value of copper (which would justify expenditure on chemical reagents) and secondly, because copper should be particularly amenable to selection by "complexing" flocculants, as explained later.

1.2 212222.222 aatl_on Flocculation by polymers is a process of aggregation brought about by means of a bonding agent (flocculant) (12,13) that ties the particles together .In order to achieve this result, the flocculant must first be adsorbed on the particles and be capable of briding the gap between 13

(12 14 adjoining particles ' Thus the effective flocculant must have an extended and flexible (or elastic) configuration in the ; the first is to achieve bridging and the latter is to produce strong flocs capable of withstanding (1205) moderate shear forces without rupturing It is there- fore understandable that the most effective flocculants known are water soluble compounds of high molecular weight 6, ( .?-10 ). The earliest flocculants were natural products such as proteins and gums; but linear polymers are more

efficient because of their greater lengths. The development of the "bridging mechanism" concept has been recently (14,16) reviewed in the literature

Adsorption of polymers on solid surfaces

A polymer cannot flocculate unless it is adsorbed on the particles. Therefore the surface chemistry of the minerals is a basic question. The chief factors involved in the surface reactions of minerals (in aqueous suspensions) are (i) dissolution, (ii) hydration (e.g. of ) and (iii) surface ionization with formation of an electrical double layer. (17) It is known that the surfaces of natural minerals (in water) are to some extent heterogeneous with regard to electrical charge. Thus there may be some areas of higher or lower local zeta-potential or even with a zeta- potential opposite in sign to the overall zeta-potential of the surface (as with kaolinite). However, the surfaces may be made more uniform by treatment with electrolytes(17)

The anchoring of the polymer on the surface is thought to be by the multiple-point attachment(12). This attachment 14 arises from the interaction of particular functional groups on the polymer with sites on the solid with the formation of "bonds". This bonding may be due to either chemical type forces (which operate over very short distances), or to electrical type forces (which extend over longer distances) (18/ 19) or a combination of both types. The electrical forces can briefly be summarized as follows: 1. ElectrostaticlEoulombial_forces result in the adsorption of polyelectrolytes on to any surface of opposite • charge, irrespective of their chemical nature. Typical examples(17) are: (a) the adsorption of ,anionic polyacrylamide on positively charged fluorite, barite, calcite and synthetic corundum; (b) the flocculation of clays by cationic derivatives of polyacrylamide. The interaction energy of this bonding -1 (39) may be much greater than 10 kcal mole and, therefore, practically irreversible. (12,20) 2. Di ole attraction forces were suggested to explain the flocculation of ionic-type crystal (e.g. fluorite) by non-ionic polyacrylamide. This type of bonding has a weak interaction energy (<2kcal mole -1)(39)

3. London-van der Waals attraction forces: Although these forces are fundamentally electromagnetic in character they are considered electric forces(39), because their origin can be explained in terms of temporary dipole interactions. The neutral molecules or atoms cor4tute systems of oscillating charges producing synchronized dipoles that attract each other(39). The energy of inter- action resulting from these forces ranges between 2 - 10 kcal -1 mole . It is established that London forces are approx- 15

±mately additive, and this explains the mutual attraction of all disperse particles(12) 4. Hydrophobic association The tendency of non-polar molecular groups (or substances) to escape from an aqueous environment leads to the well known property of (physical) "Surface-activity": for example, alcohols are adsorbed at an air/water or an oil/water interface, irrespective of the chemistry of the phases. It is believed that association between non-polar ("hydrophobic") molecular groups occurs within protein molecules (as in micelles of soaps) and also explains the adsorption of proteins and amphipathic polymers (such as incompletely hydrolyzed polyvinyl acetate) at oil/water interfaces and on to hydrophobic solids such as . The important chemical forces which result in a covalent type bondscan be classified to:

5. Chemical bonding: Reactions of the polymer groups with ionic sites on the solid surface with the formation of insoluble compounds. For example, adsorption of polyacrylic acid on calcium-containing minerals (e.g. limestone, calcite, sheelite), with the formation of insoluble deposits of calcium acrylate on the solid surfaces(17) The interaction energy of -1 (39) the chemical bonding is generally greater than 10 kcal mole 6. "Coordination" bonding: (i.e., chelation and complex formation): Examples of this type of bonding are:

(1) flocculation of copper carbonate by polyethylene imine(12), (2) flocculation of the various copper minerals by the special chelating polymers PAMG to be described in

Chapters 6 and 7 in this thesis. The principles of specific 16

chelating (and complexing) bonding are discussed in Chapter 3. 7. Hydrogen bonding: In the organic compounds where the hydrogen atom is combined with a strongly electronegative atom (0,S,N), the hydrogen atom is able to accept electrons (17) from atoms on the solid surface , mainly from the -OH groups of the hydrated surface of an mineral(111° , with the formation of the hydrogen bond. The proton resonates between two electronegative atoms. The interaction energy for this bonding ranges between 2 - 10 kcal mole-1 (39) An example is the adsorption of polyacrylamide onto the WO hydroxyl groups of oxide surfaces

The adsorption of polymers on solid surfaces by chemical forces against electrostatic repulsion can only happen when the polymer approaches the surface closely by means of other mechanisms. These may be either due to London-van der Waals forces or to strong collisions between the polymer molecules and the solid surfaces. The contribution of the London-van der Waals forces to specific adsorption by chemical forces is illustrated in the case where a n- is polystyrene sulphonate [CH2-CH-CH-SO] n adsorbed readily on to a negatively charged silica surface(39) Adsorption of anionic polymers on negative surfaces is common. In this work, another example is provided by the anionic cellulose xanthate polymer which readily adsorbs on the negatively charged chryscolla (in Chapter 4). 17

1.3 Selective flocculation Previous work: It has been found empirically by several 0,2,3,22-32) authors that under certain conditions, various polymeric flocculants can exhibit,some degree of selectivity when applied to mixed dispersions. A number of successful separations by selective flocculation on binary and ternary systems have been reported in the literature. For example, selective flocculation of calcite and galena from mixtures with quartz by strongly hydrolysed polyacrylamides (A 130, A150 13.T.I.) and that of galena from mixtures with calcite by a weakly hydrolysed polyacrylamide (A100) were performed (2) (1) by Yarar and Kitchener . Usoni and co-workers reported selective flocculation of pyrite, sphalerite and smithonite; each from mixtures with quartz. Similarly,

Read(3) achieved selective flocculation of hematite from a mixture with silicate and silicate from mixtures with hematite by strongly and weakly hydrolysed (anionic) polyacrylamide, respectively. Similar results were achieved 2) earlier on similar systems by Frommer and Iwasaki and co-workers 2: , using various natural starches. Selective flocculation of a phosphate ore from clays was patented (24) by Haseman and selective flocculation of manganese ) slimes during the flotation of quartz or dioxide (Mn02 prior to its flotation with carboxy methyl cellulose and polyacrylamide type flocculants was achieved by Yousef et al (25,26,27) and Gogitidze et al and Temchenko et al Selective flocculation of coal slimes from clays, as quoted (28) by Schulz have been performed in some east European (29) countries . The use of selective flocculation in the 18 flotation of sylvinite ores has been described by Aleksand- al(31). rovich et al( 30) and Brogoitti et A review on various selective flocculation systems has been presented (32) by Mosina . Except for the American work with iron ores, all these are laboratory scale experiments. It is believed that Frommeros iron ore process is being put on a pilot scale. However, these examples did not illustrate the basic principles governing selectivity of the flocculants. In fact, most of these examples were discovered after considerable (4) work of trial and error . What is needed for the proper design of the selective flocculation processing is a better understanding of the factors controlling adsorption of polymeric substances by minerals. Selective adsor tion of flocculants Selective flocculation can only be operated if the flocculant is selectively adsorbed on certain components in the mixed suspension. Adsorption due to electrical attraction and hydrogen bonding are generally unselective (though the II-bonding is more specific than electrical forces), although, the electrostatic forces could be used to improve selectivity by employing adjustments in the (4 5) surface potentials of the various minerals ' . The chemical and "coordination" bonding, therefore seem to offer most promise for obtaining selective adsorption of flocculants by certain minerals.

It has been suggested(14) that more selective flocculants than those at present available could be prepared by incorporating in the polymer chains chemical groupings 19

having a strong affinity for ions in the mineral to be flocculated (i.e., complexing or chelating groups). This

possibility stemmed from the well-known selectivity of the various types of flotation "collectors". A familiar example is the use of various thiol ("sulphydryl") compounds such as xanthates, dithiocarbamates and dithio- phosphates, as flotation reagents for the "heavy metal" minerals. Such reagents selectively form insoluble compounds

with (e.g., Pb and Cu) and not with earth alkalineLmetals. If such groups were grafted onto water- soluble polymers, they would be expected to act as selective flocculants, ideally giving a case of adsorption or no adsorption. Examples of this type of flocculants are given in Chapter 4 and 5. Gutzeit(33) pointed out that a great many organic reagents which form insoluble (often coloured) chelates with metal ions can act as flotation collectors. Whether these chelating collectors react directly with the metallic atoms of the crystal lattice or

with identical ions adsorbed by the mineral surface from

the solution has not been proven, though, according to Gutzeit, certain experimental facts seem to favour the latter theory. Gutzeit also pointed out that the same

principle could be used to find selective depressants for preventing flotation of particular minerals by having chelating groups attached to other hydrophilic groups. Since the adsorption of polymeric molecules is the result of multiple linkings of some of the individual molecular

groups in the surface, it is clear that adsorption could

be enhanced by having many chelating units in the chain. 20

Selectivity of the flocculant could also arise from its differential adsorption strength on the various minerals.

For example, the polymer may strongly adsorb onto copper minerals but be weakly held on calcite or feldspar and in both cases it would cause flocculation. But by introducing another ligand (i.e. depressant), strong enough to "compete"

with the polymer for the calcite or feldspar, the flocculant would then be free to adsorb predominantly on the copper

minerals. Examples of this type of selective flocculants are given in Chapers 5 and 6. Whatever the chemical type, all copper minerals release 2+ minute traces of Cu ions into aqueous media, as shown in Chapter 2. Even if other complexing agents - such as OH, 2 CO , etc., are present and anionic Cu- species may be the 3 dominant ones at high pH - nevertheless there are low 2+ concentrations of Cu ions in equilibrium, capable of being complexed by strong enough complexing agents. Fo-r, example, malachite is virtually "insoluble" in weakly \2- alkaline media only traces of. Cu(OH)3, Cu(OH)4 , etc., go into solution, but malachite can be dissolved by ammonia (9,10) or EDTA The choice of complexing groups and the factors controlling their selectivity are discussed in detail in Chapter 3.

1.4 The aims of this work were to illustrate the chemical origin of selectivity; to explore the possibilities of developing new, more selective flocculants and to establish

the fundamental principles for the proper design of the 21

selective flocculant process, as a method of separation for a copper ore. The findings of this research should also apply to other types of ore. However, to arrive to these aims, basic studies must first be made on individual minerals and on artificial mixtures of minerals. As in froth flotation, selective flocculation exploits the differences of the surface-chemical properties of the minerals. In order to select a region for selectivity, three aspects have to be carefullyconsidered.

a) The flocculant - the type of the functional groups, physical and electro-chemical properties.

b) The mineral(s) - the electro-chemical state of the surface and other physical properties.

c) The aqueous medium of flocculation - types and con- centrations of the various electrolytes present, influence of pH and the hydrodynamic factors. The aqueous medium affects the electro-chemical and physical properties of the polymer. Coiling up and uncoiling (or even decomposition) of the polymer are certainly affected

by the types and content of the electrolytes, as well as pH of the medium. The electro-chemical state of the mineral surface is controlled by the medium, Thus adsorption of the polymer on the mineral surface can be strengthened or prevented by proper control of the medium. Selective flocculation, although it depends primarily on the functional groups of the polymer and the surface chemical groups on the minerals, is the result of the differential effects of

the aqueous medium on the different minerals. 22

This thesis can conveniently be divided into three parts. The first part deals with the "aquatic" chemistry of copper minerals (Chapter 2). The second part (Chapters 3 - 6) deals with the chemical origin of selectivity and the development of new selective flocculants. The third part is concerned with the processing of one particular copper ore by selective flocculation (Chapter 7). 23

Chapter 2 SURFACE CHEMISTRY OF COPPER MINERALS

2.1 Introduction; minerals: In an ore, copper minerals are generally found in small proportions and in various forms of different composition and structure, in association with other minerals. The practical methods of extracting the copper minerals from (or "upgrading") the ore depend on their composition and structure; for instance, copper sulphides are easily separable by froth flotation and not by acid leaching, while extraction of copper carbonates and silicates is easier by acid leaching than by froth flotation. The types of copper minerals most commonly treated are often divided into "sulphides" and "oxides"; the term oxide often implies all non-sulphide minerals. It is relevant to consider both classes as potential subjects for selective flocculation, as they often occur together. The types of the main groups of minerals, both copper-containing and commonly associated non-copper minerals, are summarized in (34) Table 2.1 below following the literature In this thesis, selective flocculation experiments were made on the various types of copper minerals except the halide and the sulphate minerals, as they are less commonly found in copper ores. The surface chemical properties (namely; solubility, surface oxidation reduction (redox), electrical properties and surface energy) of some common copper minerals are investigated in this Chapter. 24

Table 2.1: Types of copper and associated gangue minerals

Copper minerals Associated non-copper minerals Chemical class name formula name formula

covellite CuS pyrite FeS2 chalcocite Cu S galena 2 PbS Sulphides bornite Cu FeS sphalerite ZnS 5 4 chalcopyrite CuFeS2 neodigenite Cu S 9 5

Oxides, tenorite CuO quartz SiO 2 hydroxides, cuprite Cu 0 hematite Fe 0 2 2 3 and cassiterite Sn0 2 multiple goethite Fe0(OH) oxides ilmenite FeTiO 3

atacamite u ci(oH) fluorite CaF2 Halides 3 sylvite KC1

malachite dolomite CaMg(CO (OH)2CO3 3)2 Carbonates calcite CaCO 3 azurite (OH) (CO rhodochriocite MnC0 3 2 3 3

chalcanthite cu50 5H 0 barite BaS0 4' 2 4 Sulphates gypsum CaS .2H °4 2° brochantite u4SO4(OH)6 anglesite PbSO4

crypto- chrysocolla CuSiO .2H 0 crystalline 3 2 (approx.) microcline KA1Si 0 framework 3 8 (triclinic

s feldspars)

te sheet- Kaolinite Al2Si205(OH)4 a structure talc Mg3S14010(OH)2 muscovite KA1 [AlSi o )(co)

Silic 2 3 10 2 25

2.2 Hydrolysis of cupric ions in distilled water in equilibrium with atmosRlheric carbon dioxide Traces of Cu can be detected in the supernatant when finely-divided copper minerals are dispersed in water, but 2+ simple Cu ions are not the only form present. From the solubility equilibria, the theoretical concentrations of soluble species such as Cu00, Cu2(OH)2+ , CuCO3, Cu(OH)3, N2- N2 Cu(011) , Cu(CO3)2' , HCO , 0 and OH- in equilibrium 3 2+ with Cu and CO2(g) in the atmosphere can be calculated. If the concentration of Cu2+ is equal to or larger than that needed to precipitate cupric hydroxide, then more soluble species will be controlled by the solubility

product of Cu(OH)2 (see section 2.3.1). At this stage, it is assumed that the concentration of Cu2+ is less than is needed for precipitating Cu(OH)2. The following equations hold for 25°C and ionic strength I = 0(35,36,37,61)

2+ 1. Cu + H2O Cu OH + H ; pK = 8 . . log [Cu0H+] = pH -8 + log [Cu2+] (1)

2+ = 2+ 2. 2Gu + 2H20 Cu2(OH)2 + Pe; pK = 10.6 ] ] . . log [Gu2(OH) 21 = 2pH - 10.6 + 2 log [0u2-1 (2) 2+ 3. Cu + 30H = Cu(OH),3 pK = -15.3 .• . log [Cu(OH)3 ]= 3 pH - 26.8 + log [Cu2+] (3) 4. Cu(OH)3 + H2O = Cu(OH)2- + 111- ; pK 13.1 . . [cu(oH) ] = 4 pH - 39.9 + log [2+] 4 (4) 26

If the solutions are allowed to come to equilibrium with air, the CO2-H20 equilibria must be taken into account.

+; pK = 7.9 5. CO2 (g) H2O = HCO3 + 11 for pure air, log PCO2 = -3.52 log [HCO3] = pH - 11.42 (5)

6. HOC = CO2- + H+; pK = 10.3 3 . . log [CO3 -] = 2 pH -21.72 (6)

7. cu2+ + CO3- = CaCO 3 (aq.) ; pK = -6.77 . . log [CuCO3 ] 2pH= - 14.95 log [Cu21 (7)

8. Cu2+ + 2CO2--- Cu(CO3) 2 ; pK = -10 3 2 ] . . log [Cu(CO3) 2- ] = 4 pH -33.44 log [Cu2+ (8)

From these equations, the concentration of any cupric species can be calculated at any pH provided that the 2+ concentration of Cu is known. In solving the solubility equilibria of copper minerals these equations will be used 2+ whenever applicable. The source of Cu ions will be from the dissolution of the mineral in water.

2.3 Solubility of copper minerals In considering the theoretical ("thermodynamic") properties of minerals in water it is important to realise that dissolution (and oxidation) processes are often slow, so that during a practical period of "conditioning", full equilibrium will rarely be reached. The thermochemical data therefore indicate the direction of change and the limits that would be reached if sufficient time were available. 27

2.3.1. Effect of pH In calculating the theoretical solubilities, it is assumed that copper minerals are in equilibrium with the ionic species in solutions and the atmospheric carbon dioxide. The solubility of a given copper mineral is defined as the sum of the stoichlometric concentrations (38,39,40) of all dissolved species containing copper . In order to study the effect of pH on solubility, hydrolysis reactions of the mineral involving hydrogen ions or hydroxyl ions must be considered. The solubility of a given mineral obtained by the thermodynamic calculations is only approximate, since the value for the solubility constant (e.g., solubility product) given by different authors often differsmarkedly. Yet it is possible in many cases to calculate the pH of minimum solubility. The importance of this pH will be discussed later in this Chapter, since it was found in some cases to correspond to the zero point of charge (z.p.c.) of the mineral. In this work the pH of minimum solubility was obtained from the solubility diagram of the mineral. The solubility diagram is constructed by considering all the known possible reactions for the solid phase including the formation of polynuclear complexes as a function of pH. The sum of all the soluble copper species gives the line that surrounds the shaded area, i.e., the solubility at any pH. The data for solubility constants and the hydrolysis reactions were obtained from the literature(35- . The solubility diagrams for a few copper minerals were constructed in the following manner.

28

SolubilitxEfauurichydroxide (Cu(OH)21 Although cupric hydroxide is not commonly found as a mineral, its solubility was calculated because of its importance as a metastable phase in the equilibriUm system 2+ Cu -H2O-0O2 . The solubility of Cu(OH)2 in equilibrium with various ionic species and the atmospheric CO2 gas was calculated from the following equations (at 25°C and I = 0). 9. Cu(OH)2(0= Cu.24- 20e ; pK = 18.8 . . log LCr u. 24-1j = log K -2 log [0H] From the solubility product of water, log OH- ] = pH - 14. r 2+1 . . log LCu j = 9.2 - 2 pH (9)

A- 10.Cu(OH).... e (aq.) = liCuO2 + H ; pK = 10 . . log [HC13.02] = pH -10 (io)

11.Cu(OH) 2 (s) 20H = Cu(OH)42 ; pK = 2.7

. . log [cu(OH)4 J. 2pH - 30.7

12.Cu(OH) 2 (s) + OH- = Cu(OH) ; pK = 3.7 3 . . log [Cu(OH)3 ] = pH -17.7

13.Cu(OH) 2 (s) + 3C0 - = Cu(CO3)3- + 20H ; pK = 7.2 from equation 6 and the solubility product of water ... log [Cu(CO3) ] = 4 pH - 44.36 2- -1- 14.HC -a072 = Cu02 + H ; pK = 13.1 2- , . .log [Cu02 j = 2pH - 23.1 (((111234:) Comparison of the data in the classical works (e.g., refs. 36,42,43) with that in the modern ones (e.g., refs: 35, 39,4o,41)'revealed that the Cu(OH) (aa ) in eqn.10, is

equivalent to the "cupric acid" (H2CuO2) in the old texts. Consequently, the following two species known as "bicuprate" (Heu0 2- 2) and "cuprate" (CuO2 ) are equivalent in the modern x2- texts to Cu(OH) and Cu(OH) • respectively. (This is due 3 4 29

to the water molecules in the hydration shells of cupric ions being considered in the modern interpretations of these reactions). The stability constant for the aqueous cupric hydroxide in equilibrium with the solid cupric hydroxide has not yet been determined experimentally. Nonetheless, it is believed that the activity of the aqueous hydroxide is negligible and eqns. 10 and 14 will not therefore,be considered in the construction of the solubility diagram for the solid cupric hydroxide.

In the metastable equilibrium of Cu(OH)2' eqns. 11 and 4 should produce the same result for Cu(OH)‘2- 4 and eqns. 12 and 3 should also produce the same results for

Cu(OH) * (Minor differences, however, occurred due to 3 the differences in the values of the constants chosen.) The cupric ions from eqn.(9) can be used in eqns. 1 - 4, 7 and 8 to give the concentrations of the various cupric complexes in solution at a given pH. Table 2.1 summarizes the calculated concentrations of these species in equilibrium with cupric hydroxide in pH range 2 - 12. From Table 2.1 the solubility diagram of cupric hydroxide was constructed (Fig. 2.1). The pH of minimum solubility is seen to be between 8-9. The solubility of Cu(OH)2 in the alkaline range was 43) observed to diminish with time (42,43 and the solid phase is simultaneously slowly converted to probably tenorite or aqueous oxide. According to the same authors (42,43) the oxide solubility does not change with time and the solubility equilibrium of Cu(OH)2 is therefore a metastable one. Table 2.2 - log [ JCu 2+ soluble species in equilibrium with Cu(OH)2 solid and the atmospheric CO2•

+ ( )2+ 2 pH CuOH CU OB 2 CUCO~ CU(C03)~- Cu + CU(OH)~- CU(OH); CU(COJ)~- Approx. total - 2 -- f-. 2 0.8 I -3.8 5.75 20.24 -5.2 26.7 15.7 36.36 -5.22

3 1 .8 -1.8 5.75 18.24 -3.2 24.7 14.7 32.36 -3.22

4 2.8 0.2 5.75 16.24 -1.2 22.7 13.7 28.36 -1.22

5 3.8 2.2 5.75 14.24 0.8 20.7 12.7 24.36 0.8

(1 4.8 4.2 5.75 12.24 2.8 18.7 11 .7 20.36 2.75

7 5.8 6.2 5.75 10.24 4.8 16.7 10.7 16.36 4.76

8 6.8 8.2 5.75 8.24 6.8 14.7 9.7 12.36 5.68

9 7.8 10 .. 2 5.75 6.24 8.8 12.7 8.7 8.36 5 .. 63

_. 10 8 .. 8 12.2 5.75 4.24 10.8 10.7 7.7 4.36 3.99

11 9.8 14.2 5.75 2.24 12.8 8 .. 7 6.7 0.36 0.36

1t: r) • 10.8 16.2 5.75 0.24 14.8 6.7 5.7 -3.64 -3.64 ... W· o 31

pH

Fig.2.1 Solubility diagram of Cupric hydroxide 32

Solubility of tenorite CuO The following equations at 25°C and I = 0 were used:

15.CuO (s) H2O = Gu2+ + 20H-; pK = 20.5 . . log [0u21 = 7.5 - 2pH (15)

- 16.Ou0 + H2O + 20H- = 0u(OH) (s) 4 • pK = 4.4 . . log [Cu(OH) = 2 pH - 3 (16) 4 2.4

17.Ou0( s) + OH- = H0u02 K = 1.03 x 10-5

(17) . . log [H0u07.21- = pH - 18.99 2- 18.CuO( s) + 20H = Cu02 + H20; K = 8.1 x 10 5 r 2-1 (18) . . log Lu02 j = 2 pH - 32.1 2 As explained earlier with Cu(OH)2, HCuO2 and Cu02 in eqns. 17 and 18 correspond to Cu(OH)3 and Cu(OH)x 2- calculated by eqns. 3 and 4 respectively with the aid of eqn. 15. The Cu2+ released in equation 15 may undergo the hydrolysis reactions 1-4, 7 and 8. The concentrations of all the soluble cupric species in equilibrium with tenorite are summarized in Table 2.3 overleaf. From this table the solubility diagram for tenorite was constructed in Fig. 2.2. The pH of minimum solubility is about 8.5. It is seen that CuO is much less soluble than Cu(OH)2, (which, as mentioned previously, is a metastable phase). Table 2.3: - log[ ]Cu soluble species in equilibrium with tenorite and (g) in the atmosphere CO2

2+ / .2 + / .2+ / .2- pH Cu Cu(OH)4 CuOH kOH)C 2u CO 3 Cu CO3)2 Cu(OH)3 Total

2 -3.5 28.4 2.5 -0.4 7.45 21.94 17.3 -3.5

3 -1.5 26.4 3.5 1.6 7.45 19.94 16.3 -1.5

4 0.5 24.4 4.5 3.6 7.45 17.94 15.3 0.5

5 2.5 22.4 5.5 5.6 7.45 15.94 14.3 2.5

6 4.5 20.4 6.5 7.6 7.45 13.94 13.3 4.5

7 6.5 18.4 7.5 9.6 7.45 11.94 12.3 6.42

8 8.5 16.4 8.5 11.6 9.94 11.3 7.38

9 10.5 14.4 9.5 13.6 7.94 10.3 7.32

10 12.5 12.4 10.5 15.6 7.45 5.94 9.3 5.93

11 14.5 10.4 11.5 17.6 7.45 3.94 8.3 3.94

12 1 16.5 8.4 12.5 19.6 7.45 1.94 7.3 1.94 ____J 3't

TENORITE

4, o Cu CO3 0 E n 10- 0

15-

20 2 4 12

Fig.2.2 Solubility diagram of Tenorite 35

• Solubilit of cuLite_LC12.21 At 25 C and I = 0, this reaction holds:

19. 2 cu20 (s) .1. ,.11120 = Cu+ + 0H-; pK = 14.7 from this reaction . . log [Cu+] = pH - 0.7 (19)

In the pH range 2 - 11.85, the Cu+ ions will be 2+ oxidized quickly to Cu and the cuprite surface will be slowly changing to tenorite due to the atmospheric oxygen.

These oxidations are discussed in more detail in 2.5.1. 2+ The Cu ions thus obtained may undergo hydrolysis to the various complexes as in eqns. 1 - 4 and 7 - 14. For the surface chemical purposes, in the presence of dissolved oxygen. (02) in the solution, the cuprite surface may be considered as that of tenorite. Hence the solubility of cuprite would be expected to be controlled by that of

tenorite. The equilibrium in eqn. 19 therefore, cannot be satisfied under atmospheric conditions. No actual data was available on the kinetics of the oxidation of cuprite to tenorite at room temperature, although it is known to be slow

SolUIDilitofn 2(OH)2 003,1 At 25°C and I = 0, the following equation holds: 20. CU2(OH)2 003 (s) + = Ca2+ 002 (g) + H20; pK = - 49

For air, log p002 = -3.52

. . log [Cu2+] .8.25- 2 pH (20)

The cupric ions thus released from malachite, may undergo hydrolysis according to eqns. 1 - 4, 7 and 8. The concentrations of the hydroxy and carbonato complexes

In equilibrium with malachite under atmospheric conditions 36

(i.e., constantPC0 ) were calculated using eqn. 20. Accordingly the solubility diagram for malachite was constructed in Fig. 2.3. The pH of minimum solubility is between 8 - 9. In nature, malachite is the commonest "oxidized" copper mineral. Solubility of azurite (21.3(OH)21223121

The following equation holds at 25°C and I = O.

Cu3 (OH)2 (003)2 (s) + Cu2+ +liCO2 (g) +i H20; (21)

(under atmospheric conditions, log p = -3.52) pK = -6.47 CO2 . . log LCur 2+1 j = 8.82 - 2 pH.

From this equation the concentrations of the hydroxy and carbonato complexes in equilibrium with azurite were calculated from eqns. 1 - 4, 7 and 8. Accordingly the solubility diagram for azurite in equilibrium with the atmospheric CO2 was constructed in Fig. 2.4. The pH of minimum solubility is between 8 - 9. Solubility of covellite (CuS) The following equation holds for 25°C and I = 0:

22. CuS (s) + 2H+ = Cu2+ + H2S (g); PK =14.2 (22)

The solubility of covellite in equilibrium with H2S gas was calculated for three hypothetical cases: Case 1: The mineral suspension is closed to the

atmosphere and the partial pressure of H2S gas = 1 atm. From eqn. 22; log [Cu2+] -14.2 - 2 pH (23) The cupric ions released may only form hydroxy complexes. The theoretical concentrations of the soluble species in equilibrium with covellite are calculated in

Table 2.4, from which the solubility diagram was constructed in Fig. 2.5. According to the diagram, the pH of minimum solubility is around pH 9.5, but in practical termsIthe 37

I I I I I 2 8 10 12 pH

Fig.2-3 Solubility diagram of Malachite in equilibrium with atmospheric CO2 38

0

1 -111111 2 4 6 8 10 12 pH

Fig. 24 Solubility diagram of Azurite in equilibrium with atmospheric CO2 39

18K-=~ ---===_. ===--~

20 ~-=~=_-=~~~~~-~~

'---COVELLITE-- '---

~------25 1~ ~------; 0) q ~ ~------~J .... 0 ~x --Q) ,------"'-,-- 0 E. n u 0") 0 30 I

35

2 4 6 8 10 12 pH

Fifj.2 a 5 Solubility diagram of CovelHte

at pH.,s=1t;,. atfn., in a ciosed sysh~rn 40

• mineral is virtually completely "insoluble" to all ordinary analytical techniques.

Table 2.4: - log[ ]soluble species in equilibrium with covellite 2+ + pH Cu CuOH Cu(OH); Cu(OH) - total

2 18.2 24.2 39 50.1 18.2 3 20.2 25.2 38 48.1 20.2 4 22.2 26.2 37 46.1 22.2 5 24.2 27.2 36 44.1 24.2 6 26.2 28.2 35 42.1 26.196 7 28.2 29.2 34 40.1 28.159 8 30.2 30.2 33 38.1 29.9 9 32.2 31.2 32 36.1 31.159 10 34.2 32.2 31 34.1 31.0 11 36.2 33.2 30 32.1 30.0 12 38.2 34.2 29 30.1 29.0

The equilibria of the system H2S (g) - H2O are characterized by the following equations. 24.H 2S (g) = H2S (aq.); log K = -0.99 at pH 2s = 1 . . log [F[2S] = -0.99 (24)

25.H + HS- = H2S (aq.); log K = 6.99 . . log [HS-] = pH -7.98 (25)

26.H + + 82- = HS ; log K = 12.9 r 2-1 . . log LS j = 2 pH - 20.88 (26) 41

2+ Since no complexation reactions between Cu and

HS or S2 were reported in the literature, these equations will not affect the solubility diagram of covellite.

Case 2: The system (covellite suspension) is open =3.52 -4 atm. (p = 10 and to atmosphere and pH2S is 10 CO2 p0 2 = 0.21). But no oxidation is supposed to occur. 2+ From reaction 22, the concentration of Cu in equilibrium with covellite under these conditions can be obtained from equation 27, .• . log [Cu21= -10.2 - 2 pH (27) The Cu2+ ions released may undergo hydrolysis according to eqns. 1 - 4, 7 and 8. The theoretical concentrations of these complexes were calculated and the solubility diagram was constructed in Fig. 2.6 accordingly. The pH of minimum solubility is between 8 - 9. Because of the lower S the solubility is postulated partial pressure of H2 ' greatly increased; but it is still analytically insignificant.

Case 3: The system is open to the atmosphere and -10 -3.52 = 0.21). p = 10 atm = 10 ' p H2S ' (p00 02 But no oxidation is supposed to occur. Under these conditions, equation 27 can be rewritten r as follows: log [Cu2+]= -4.2 - 2 pH (28) The solubility diagram for covellite under these conditions was constructed in Fig. 2.7, in a similar way as in case 2 above. According to the diagram, the pH of minimum solubility is between 8 - 9. It is interesting to note that the pH of minimum solubility for the two open systems is the same regardless of the partial pressure of L12

T. 4J a) 0 E

U C4 0

pH

Fig.2•6 Solubility diagram of CoveHite at pH2s=10-4 atm., with access of atmospheric CO2 _ log C ,mole/ litre at pH Fig. with access 2.7 2 s =10 Solubility diagram ofCoveHite -1 ° atm., of atmospheric CO2 411

S in the atmosphere. The solubility, however, is higher H2 at the lower ID It is clear from eqns. 23, 27 and 28, -H2 will not be precipitated at any pH range, that Cu(OH)2 2+ since the concentration of Cu released from covellite in the three cases is always less than that required to form the hydroxide in eqn. 9. Dissolution of sulphide minerals is, however, greatly increased by oxidation, which is discussed later. Whatever the mechanism, this is thermodynamically equivalent to.maintainence of a very low vo - H2S• Solubilit of chalcocite Cu2SJ The following reaction holds at 25°C and I = 0; 29. 2 Gu2S (s) H+ = cu+ (g) ; pK = 13.5 However, in the pH range 2 - 11.85, both the Cu+ ions and the chalcite surface will presumably be oxidized 2+ rapidly to Cu and covellite, due to the atmospheric oxygen as shown later in section 2.5. Hence the equilibrium in eqn. 29 cannot be satisfied in this pH range. For surface chemical purposes, in the presence of dissolved air in the solution, the chalcocite surface can be considered as that of covellite, though the solubility of chalcOcite may be higher than that of covellite due to the initial stages of oxidation as shown later by eqn. 39 in section 2,5. 2.3.2. Effect of article size on solubility (39,) It has been established that very finely divided solids have a greater solubility than large crystals. In general, for particles smaller than 1 far;, or of specific surface area greater than a few square metres per gram, surface energy may become sufficiently large to 45

influence surface properties. The change in the free energy AG involved in grinding a coarse solid suspended in water ( S = 0 d =co ) into a finely powdered one of molar surface S and particle size d is given by

eqn. 30 (39,AG401 fiS. where 7■3 is the mean free surface energy (interfacial tension) of the solid- liquid interface.

The molar surface S = NS (31 ) where N is the number of particles per mole, S = surface area of a single particle = Kd2

The volume of a single particle, v = ld3

and the molar volume V = = Nv (32) v = M« from 31 and 32 S - (33) d Pd

where m = 1 = the shape factor, and M = formula weight of solid.

From the relation, AG = RT in [ K so (fines)/kso (coarse)] (40 ) and equations 31 and 33, Shindler obtained the following equations 9-!2.1.21j5 log so (s) = log Kso (s=o) + RT (34) or log K50 (d 2:211L na2 d-1 log K50 (d) = = 00) + RT (35)

Although values of for solid/solution interfaces are not reliably known, the effect of the particle size(or

molar surface) on the solubilities of CuO, Cu(OH)2 and (40,41) ZnO has been confirmed by Shindler and coworkers In their studies, it was assumed that V remains independ- ent of the surface area. The increase in solubility of a given copper mineral due to the fine particle size is Li 6

not likely to change the pH of minimum solubility signi-

ficantly, since the concentrations of almost all the

complexes of cupric or cuprous ions are dependent on the 2+ concentration of Cu .

2.3.3. Effect of "inert" electrolytes on solubility

The effect of adding "inert electrolytes" (i.e., those

of different chemical composition that do not have common ions with the solid crystal) on the solubility of a solid

can be explained in terms of the activities of the ionic

species. In dilute solutions the charged ions exert long- range electrostatic forces upon one another which generally result in reducing the activity coefficients('g )(38) . On further dilution, the effects of these electrostatic forces

diminish and the activity coefficients increase. As the

solution approaches infinite dilution, the interaction

between cations and anions vanishes and the activity

coefficients are taken as unity, by definition. The

electrostatic effects depend primarily on the ionic strength

of the solution, I, the charge and diameter of the hydrated

ionic species according to the Debye-Huckel theory. The 2 is the ionic strength is defined as I =M1 Zi' where Mi molality and Z. is the charge of the ith ion in the solution.

The activity coefficient 0) was shown(38) to decrease

as the ionic strength is increased up to I = 1. At values

of I >1, the activity coefficient may increase again. The

relationship between 1.) and I given by the Debye-Huckel

theory is I— - A Zi tNI I log/ ai BIT. where A and B are constants 47 characteristic of the at known temperature and pressure. The quantity a. has a value dependent on

the "effective diameter" of the hydrated ion. The solubility of a given copper mineral (Mt),is the sum of all dissolved species containing copper Ms: M = . The activity of the ionic species a= Ms V s. t s • as as • • M and M = Vs t V s The last equation means that the mineral solubility is inversely proportional to the activity coefficient of the solute species. Thus it can be concluded that in dilute solutions (1 <1), the solubility of the mineral is generally enhanced in the presence of inert electrolytes. In more concentrated solutions (I >1), the activity coefficients may become greater than unity and the solubility of the mineral may become smaller again. The increase in solubility will not, however, change the pH of minimum solubility significantly so long as no new species are formed as a result. Activity coefficient effects are generally negligible in the dilute solutions encountered in mineral processing operations.

2.4 Solubilit of chr socolla Chrysocolla is a copper silicate, commonly present in oxidized zones of ores. Because of a complete absence of published data on the solubility constants of chrysocolia, its solubility was investigated experimentally. The lack of data may be due to the widely varying composition and structure of the mineral. Chrysocolla is a heterogeneous /48 mineral and the copper content varies over a distance of (4) a few {441 According to sdme author the structure of the mineral was found to be a mixture of a crystalline copper silicate phase dispersed in an amorphous silica hydrogel with a composition that can be expressed in terms of CuO, Si02 and H20. The structure of the crystallites was proposed(45) to be a distorted chain silicate structure of the general formula (Si4010)2 Cu8(OH)12. 8H20 0 or (Si, 010)2 Cu8(OH)12, in which there may be minor Cu substitution by di- and trivalent ions. This proposition is largly in agreement with an earlier work(46) where the structure was stated to be of a porous aggregate of approxi- mately 100 R diameter crystallites. The copper is present in the cupric state and both hydration water and bound OH- are present in the structure. Therefore the varying copper content in different mineral samples depends on the variations in the abundance of this crystalline copper silicate phase within each grain of chrysocolla. The surface of chrysocolla should consist of alternating crystalline copper silicate phase and amorphous silica hydrogel(47) 2.4.1. Effect of H on solubility of chrysocolla Experimental: In a series of experiments, 0.5 g samples of finely ground chrysocolla (obtained from Arizona, U.S.A.) were shaken in 50 ml double-distilled water of pre-adjusted pH (2, 4, 5.9, 8 and 10) for 5, 15 and 30 minutes. When the suspensions were filtered on filter paper, colloidal suspensions were obtained. 10 ml of the colloidal filtrate were mixed with 10 ml of 2.55% 49

NaCI to effect immediate coagulation and precipitatioll of

the colloidal particles leaving clear supernatants which

were filtered easily. The clear filtrates were analysed 2+ . f or CU lons by the colorimetric method using b~s-cyclo- hexane oxalyl-dihydrazone. This method is described in

Appendix I. The resul ts are shown in Table 2.5 below'.

2 Table 2.5: Concentrations of Cu + in p.p.m. released from chrysocolla at different pH.values.

4 10 T~(min. ) 2' 5.9 9

5 130 2.3 2.0 0.8 0.7

15 330 1 .0 2.0 1 .7 1 .0

30 430 1 • 1 1.2 2. 1 1 .0

2 From Table 2.5 it appears that [Cu +] sharply 2 increased b~low pH 4 and the [Cu +] in the pH range 4 -10

was more or less constant. It is clear from this table

that raising the leaching time from 15 to 30 min. did

not increase the ~u2+] significantly in this range. 2 There may be some exaggeration in the amount of Cu +

released due to the use of NaCl in the coagulation stage

because of cation exchange from colloidal chrysocolla

particles. Thus no particular significance can be attached

to the low values in the pH range 4 - 10, except to note

that they never reached the theoretical solubility

expected for CU(OH)2 (Fig. 2.1), and only below pH 4 solubility

became large .. One important consequence of these results 50

is that in practice chrysocolla is found to be sufficiently soluble at pH 2 to produce precipitation of Cu(OH)2 if the pH is subsequently raised into the region 6 - 8. 2.4.2. Effect of sodium chloride on solubility of of chrysocolla

Experimental: In a series of experiments, 0.5 g samples of finely ground chryscolla were shaken in 50 ml distilled water containing preadjusted amounts of NaCl. The pH of the distilled water was 5.9. The shaking was performed for 15 min. and 30 min. at each level of NaC1 (except at 2.55 % NaC1). The suspensions were filtered and 10 ml of the filtrates were taken for the determination of [Cul by the calorimetric method described in Appendix 1. Some experiments were duplicated. The results are shown in Table 2.6 below.

Table 2.6: Concentrations of Cu in p.p.m. released from chrysocolla at different NaC1 % in solution.

NaC1 % Time 0.0255 0.255 1.835 2.55 (min.)

15 0.62 0.8 2.7 _ ___ 30 1.2 *1.5 *2.8 6.0

(* these values are the average of two measurements

(duplicates) ).

The results show that [Cul[Cul released increases with increasing NaC1 content. The mechanism was probably ion- exchange. 51

2.5. Surface oxidation of copperjniz2=122 iLadox equilibria 2.5.1. Control of oxidation .otential of aqueous suspensions br atmospheric oxygen

All sulphide minerals are subject to atmospheric oxidation to greater or lessuextent. Oxidation reactions of substances by molecular 02 at room temperature are often much slower than the reactions of other oxidizing agents having theoretically less favourable potentials(53). This sluggishness of oxygen (02) is due in part to the initial difficulty in breaking the bond between the two oxygen atoms. (According to Sato(5‘i, the faster reactions of

02 are those in which the bond remains unbroken). It is established(36) that hydrogen peroxide (H202) or its basic equivalent peroxyl ion (H02 ) is an intermediate product + of 02 reduction in the presence of H . Thus the oxidation potential of an aqueous system in contact with free 02 is controlled primarily by the potential of H202-02 couple. The oxidation potential of the aqueous system, therefore, should be at or above the potential of the H202-02 couple so long as a detectable amount of free oxygen is present in the system. If the oxidation potential of the aqueous system is less than the potential (Eh) of H202-02, then

0 will immediately be consumed in the oxidation of sub- 2 stances in the system, being itself reduced to H202 and further to H O. 2 When the oxidation potential of the solution exceeds couple, the peroxide is catalytically that of H202-02 decomposed by the heavy metal species, and oxygen becomes 52

no longer available for further oxidation. At this state,

a metastable equilibrium is reached between 0 and H 0 2 2 2 (53) and the Eh value of the system is determined as follows • p0 + 36. H 01. 0 + 2H P = 0.682 - 0.0591 pH + 0.0295 log B--(57 2 2 +.2e -h 2 where the round bracket represents the activity. The ratio PO 6 was chosen by Sato(53) to be equal to 10 , at I. (11202)

PO2 = 0.21 atm, which appeared to agree with his results. Thus the oxidation potential, Fh , of the aqueous solution at equilibrium will be: E = 0.859 - 0.0591 pH. h (37) This equation will be used in this. work to calculate the oxidation potential of the aqueous systems at the various pH values. + 2+ Oxidation of Cu to Cu • Cuprous ions are oxidized to cupric in aqueous solutions according to the equation (39): 2+ 38. CV + e = Cut; log K = 2.7, Eh° (standard potential) = 0.16. 2 i.e. Eh = 0.16 + 0.0591 log (38) According to equations 37 and 38, substantial oxidation Cu2+ of cuprous ions into cupric ions, i.e., when = 1.0, should proceed up to pH 11.85, where the standard potential 2+ + of Cu - Cu system is equal to the oxidation potential of

H202-02 couple. 2.5.2. Surface oxidation of chalcocite According to Sato (S5), chalcociteis fairly rapidly oxidized to covellite, which in turn is slowly oxidized to cupric ions and sulphur. 2+ 39. Cu S = CuS + Cu + 2e 2

40. CuS = Ou24- + S + 2e

The electrode potential of copper sulphides should be

dependent only. on the activity of rsuprits ions since fale 53 other oxidation products are solids. For reaction 39; 2+ = 0.530 + 0.0295 log (Cu ). (39). This reaction will be taken in this work as the oxidation determining step for chalcocite, due to its rapidity. Comparison of equations 39 and 37, reveals that this oxidation is possible in the pH range considered here (2 - 11.5). (55) to be independent The Eh for chalcocite was stated of the sulphate ion SOT or sulphur activities. Thus it can be concluded that the surface of chalcocite is generally coated with the covellite mineral as an oxidation product.

The electrode potential of chalcocite- in alkaline (5) solutions was measured by Lekki and Laskowski . They concluded that the chalcocite electrode in alkaline solutions can be treated as an oxide electrode, for which

the potential is determined by the 11+ and OH ions. In arriving to this conclusion they assumed that solid cupric hydroxide forms under these conditions. The same

assumption was made by Sato(55). However, precipitation 2+1 of Cu(OH) can only be possible when the [ Cu jreleased 2 from the mineral is equal to or greater than that is in equation 9. It may be required for forming Cu(OH)2 possible that the surface oxidation of chalcocite to S ) covellite, proceeds with formation of digenite (Cu9 5 as an intermediate(55) 2.5.3. Surface oxidation of covellite The oxidation reaction for covellite is (55) 2+ 2+ CuS = Cu + S + 2e., Eh = 0.591 + 0.0295 log (eu ) (40) 54

The activity of cupric ions will be taken as approximately 2+ equal to the calculated concentration of Cu in section 2.3. Comparison of equations 40 and 37 reveals that the oxidation of covellite is possible in the pH range 2 - 12, 10_10. S gas However, even at a partial pressure of H2 (55) this reaction is said to be slower than reaction 39. In the presence of free 02, sulphur thus produced should eventually be oxidized to sulphate ions (S00 through various stages of oxidation. The Eh of

covellite oxidation, however, was found to be independent (55) of the SO activity showing that the oxidation chain of reactions was not in thermodynamic equilibrium. 2.5.4. Surface Oxidation of cuprite and other copper minerals According to Garrels and Christ(38), the oxidation of cuprite surfaces may be represented by the following

equations:

1 41. cup(s) 02 (g) = 2 CuO(s) ; K = PoZ

42. 3 cup(s) + 4 002 (g) + 2 02 (g) + 21120 = 2 Cu3 (010 2(CO3)2 (s) ; 1 K = p4 p/2 02 2

43 . 01120(s) + 2 02 (g) 002 (g) + H2O = Cu2(011)2 CO3 (s). _ 1 P 02 PCO2

Reaction 41 explains the darkening of red-colour

cuprite powder on long storage. However, at ordinary temperatures these reactions are slow because of the

sluggishness of reactions with molecular oxygen, as 55

explained earlier. 'Therefore for the purpose of investigating the effect of surface oxidation on selective flocculation or flotation processes during the experimental periods

usually encountered, these reactions can possibly be ignored. Similarly, the effects of the surface alteration of tenorite to malachite and other solid state reactions: 2 CuO CO2 (g) H2O = Cu2 (OH)2 CO3; and of azurite to

malachite; 3 Cu2 (OH)2 CO3 002 (g) = 2 Cu3(OH)2(CO3)2 H20; or vice versa could only be considered relevant if their reaction rates were fast enough. Apparently nothing is known on the kinetics of these reactions at room temperature.

2.6. Surface electrical propeELLtsofamatnzinerals

2.6.1. Introduction: Knowledge of the electrical charge of the solid surface is often necessary to interpret and predict behaviour of the mineral suspensions e.g., the stability and state of dispersion, rheology of suspensions, adsorption properties, flotation, flocculation and coagulation (47,49, 52,58, 5,21). Since the potential difference between the charged mineral surface (or its "plane of closest approach) and the bulk of the solution ( yo or yo cannot be measured directly(39) zeta-potential ( ), althoughLis y8(59,60) smaller in magnitude than yo or offers useful information. The zeta-potential or the charge at the plane of shear'' is usually computed from electrophoretic

mobility or other electrokinetic measurements. These

measurements concern only the diffuse part of the electrical

double layer.

56

Origins of surface charge: These were briefly summarized as follows:(39) 1. Chemical reactions on the surface: which can be classified

into: a. Acid-base ionization of the surface functional zamaas: The charge of the surface is dependent on the

degree of ionization and consequently on the pH of the medium, e.g., silica surface (ionization of silanol

groups).

b. Coordination of solutes to the solid surface°

s,ecific adsorption.: The alteration in surface charge results from chemical reactions. 2- 2- Examples: /c-/ s) + S / S / / 3

Cu(s) + 2H S = ICu(SH)2 + 2 2 2 111- . and Fe0OH(s) + HP02 Fe0HP0 + OH / 4 4 •

2. Lattice imperfections and isomorphous substitutions: For example, in clays replacement of silicon ions by Al structure results in a negatively charged ions in the SiO2 frame-work.

3. 22-La.a.192- aa: Preferential adsorption of one type of ion on the surface can arise from London-van der Waais

interactions and from hydrogen bonding. Examples are (a) adsorption of surfactants on clays or humic acid on

silica surface,(b) adsorption of multinuclear hydroxy-

complexes and polymers on a calcite surface( °9 57

Preferential adsorption of hydrolyzed metal ions: It (39,48,49,51,52,57) has been observed that the hydrolysis products of multivalent ions, cationic as well as anionic (even polysilicates and polyphosphates) are adsorbed more readily at the particle-water interfaces, than non- hydrolyzed metal ions. This tendency to be adsorbed even against electrostatic repulsion is especially pronounced for polynuclear polyhydroxy species. This may be because(39): (1) the hydrolyzed species are larger and less hydrated than non-hydrolyzed species; (2) or the presence of a coordinated hydroxyl group, (the anchoring of the metal hydroxy complexes may be due to the formation of a covalent bond between the central metal atom in the hydroxy species and specific sites on the solid surface); (3) more than one hydroxyl group per "molecule" can become attached at the interface. Likewise polymers have a strong tendency to accumulate at interfaces because of multiple-link attach- ment. From the foregoing discussion it may be supposed that the metal hydroxy complexes can be considered as potential- (48,49,52,57,51,47,39) determining ions for metallic minerals, all of which are controlled by the pH of the medium. Accordingly, the potential-determining ions of copper minerals, may be considered to be: Cu 2+ N -F 2(OH)2 , Cu(OH) , 2+ 4- 2- CuCO33,, Cu , Cu(CO )2- , Cu(CO ) , Cu(OH) 3 2 3 V -.. + Cu(OH)3 , OH and H , all of which are, of course controlled by the pH of the medium. 58

2.6.2. Zeta-•Ote tial of malachite Experimental: The mobility of malachite particles was measured at various pH values with Rank Bros.Particle Electrophoresis Apparatus (Mark II) (Cambridge, England), using a quartz flat cell (1 x 10 mm). The interelectrode distance (or the effective length of the cell "L") and the microscope eye piece graticule (X = 97.5 vu) were calibrated. The first was determined by measuring the

resistance (R) of the cell containing electrolyte solution (0.1 M KC1) of known specific conductance (K = 1.289 x 10-2 -1 -1 cm at 25°C) by a conductance bridge (Portland conductivity meter). The effective cell length (L) was calculated from the equation: R = KA , where A is the cross-sectional area of the cell (0.1 x 1 cm2.). The stationary levels were taken at 19.4% and 80.6% of the observed thickness of the cell (approximately 0.75 mm in water). The applied voltage across the cell was 100 volts and the time required for the particles to move one square in the graticule in seconds (t) was measured at the station- ary levels at a constant temperature of 25°C. The zeta- potential (C) was calculated from the Smoluchowski equation at 25°C. 4 = 12.9 u, where u is the mobility, x/t - 12.9 v/L i.e., y (my) = 12.9 velocitpotential gradient Procedure: A sample of malachite was finely ground in an agate mill to minus 325 mesh. The powder was dispersed in distilled water (at the natural pH 5.9) in an open beaker at high shear with a magnetic stirrer for a few minutes.

The dilute suspension was allowed to stand for a few minutes

and the settled particles were discarded, only the suspension 59 being used. Small samples of this suspension were taken for measurements. The pH of each sample was adjusted, while stirring, by HCl or NaOH solutions. The average time of adjusting the pH was about 25 minutes, or until the change in the particular pH reading was very slow or had apparently stopped. The times taken for the particles to travel one square in the graticule were measured at both the stationary levels in the electrophoretic cell at the pH values 6 - 11. The zeta-potential of malachite was calculated for the various pH values from the above equation. The effects of cupric sulphate t 00-4 M CuSO4), f 00 4M Na2CO3),Dispex N 40 (50 p.p.m.) and Calgon (400 p.p.m.) on the zeta-potential of malachite at various pH values were measured in the same way as before. The results are shown on Table 2.7 and Fig. 2.8.

Table 2.7: Zeta-potential (in mV) of malachite at 25°C.

PH 7 8 9 10 11 Solution medium dist. water +26,1 +24.7 +21.5 + 9.2 -12.3 -24.4 CuS0 4 +22.8 +17.1 +10.2 -11.6 -22.0 4' 10 m +20.4 Na2 -4 - -19.5 -21.9 -26.7 CO3 10 M - - Dispex, 50 p.p.m. - - - -43.8 -44.7 -45.9 Calgon,400 p.p.m. - - - -49.9 -53.2 -53.5

From Fig. 2.,8 the pH of zero point of charge of malachite in distilled water is seen to be between 9 and 9.5 and is _4 2+ probably at pH 9.3. The addition of 10 M Cu ions did not change the z.p.c., while the carbonate changed the Zeta-potential,(mV ) 40 50 25 45 35 55 9 20 10 30 10 15 o 6 indistilledwater(+HCIorNaOH) A o in in 50p.p.m.Dispexsolution in 10 400 p.p.m.Calgon 7 -4 M Na M CuSO t CO3solution 4 solution pH 9 solution

10 47

11 (" ,o N co 041 4 4:3 IT; 0T; 0 Rs 6 0 42.) C th 61

z.p.c. to a lower value. The non-effect of CuSO4 on z.p.c. 4 can be explained by the fact that the 10 m CuSO4 would be precipitated as Cu(OH)2 in the pH range 7 - 10. The figure also shows the sharp increase in negative zeta-potential on addition of Calgon and Dispex, indicating their adsorption. These are clear examples of "specific" adsorption because the already negative surface is rendered more strongly negative. It seems certain that the binding of these polymers must arise from close interaction of some of the anionic groups with Cu2+ ions from the solid, the net strength of adsorption being enhanced by multiplicity of linkages.

2.6.3. kliatzallaP2Int of c12221aLa2112Ellalmiaimam solubility It is interesting to note that the pH of malachite zpc in distilled water nearly corresponds to its pH of minimum solubility ( "/ pH 9) obtained from the solubility diagram in Fig. 2.3. Similar observations were made in the case of calcite by Somasundaran and Agar(49) who found that the of Calcite (8 71/9.5) in equilibrium with carbon PH zpc dioxide in the air closely corresponds to pH of minimum solubility (8 - 10). Earlier, Parks and DeBruyn(51) showed that the pHzpc or i.e.p. of cc-hematite corresponds to its pH of minimum solubility. The concept that pH of minimum solubility for a solid (48) phase in equilibrium with the solution was shown by Beck to correspond to the solids isoelectric point (i.e.p.). This may be true only in the absence of complications such as those caused by structural or adsorbed impurities and the equilibrium conditions specified°9). Bearing this in 62

mind, the z.p.c. of the solid should correspond to the pH of charge balance (electroneutrality) of the potential- determining ions(39,/43) which corresponds under the conditions specified to the pH of minimum solubility. The concept corresponds to the pH of minimum solubility that pHzpc should help to predict at least qualitatively the effects of certain variables on the z.p.c.. Measurements of z.p.c. by electrokinetic methods could lead to varying results, depending on the time allowed for the mineral to equilibrate with the solution at each pH. The correspondence of pHzpc to the pH of minimum solubility, thus depends in part on the kinetics of attaining equilibrium between the mineral and its hydrolysis products in the solution at each pH value. The theoretical identification of these two points implies that the acid-base character of groups exposed on the surface of the solid is exactly the same as for isolated units in solution. This assumption may be a useful approximation, but is not expected to be exactly correct.

2.6.4. pH zpc of copper miae of CuO (tenorite) has been given as 9.5(39) The pHzpc which corresponds to its pH of minimum solubility (pH ms) before reaching an equilibrium with (or in the absence of)

CO in the atmosphere in Fig. 2.2. If the mineral is 2 -3.52x allowed to equilibrate with CO, in the air (P = 10 ), CO 2 the pHms should lie between pH 8 - 9. of cupric Assuming this concept holds, the pHzpc hydroxide, azurite, covellite, chalcocite and cuprite in equilibrium with their solutions under atmospheric conditions should correspond to pH 8-9, If the zop,ce of these minerals 63

are measured before reaching equilibrium with the atmospheric CO2' their values will tend to be higher than the corresponding pHms. The of Cu(OH) was given as 7.7 by Yoon and pHzpc 2 (56) Salman • Chrysocolla did not exhibit a zero point of charge at any pH value(50,47) and was always negative. Partially leached chrysocolla on the other hand had two zero points of charge at pH 6.4 and 9.4(47). This behaviour was the result of initially conditioning the chrysocolla at pH 4 before determining the mobility of the particles as the pH was raised. The lower pH value of z.p.c- (6.4) was attrib- uted to the precipitation of Cu(OH)2 on the surface of the particles. The behaviour of the mineral at higher pH was similar to that of CuO (tenorite) with a z.p.c. at pH 9.5. It is interesting to note that in these measure- ments(47s50) the mineral suspensions were not allowed to reach equilibrium with CO2 in the atmosphere. The negative charge below this was attributed to the silicate part of the structure(50) 64

CHAPTER 3 THE CHEMICAL ORIGIN OF SELECTIVITY OF POLYMERIC FLOCCULANTS

(12,14,15) It has been established that an effective polymeric flocculant consists of a water-soluble macro- molecular substance which could be adsorbed through its functional groups by mineral particles in a suspension, and acts as a molecular bridge between them. A fully selective flocculant would adsorb only on to certain minerals in a mixed mineral suspension. The selectivity of adsorption could be due to the affinity of the functional groups of the polymer towards certain metal ions, supplied by the mineral, and indifference to other types of metal ions; e.g., xanthates and dithiocarbamates tend to form strong complexes with heavy metals, but are largely 2+ 2+ indifferent to cations such as Ca , Al3+ and Mg . On the other hand, the functional groups may complex with many metal ions, but with varying degrees of strength; the stability of some (or one) metal complexes being much higher than the rest. In this case, the presence of another ligand in the system may help to suppress complex formation of the ions, leaving only those of strongest affinity to the functional groups of the polymer, thus resulting in selective adsorption.

Although many commercial flocculants at present available are found to exhibit some degree of selectivity, their use in selective flocculation processes is rather (14) limited. It hasas been suggested that selectivity of flocculants could be enhanced by incorporating in the 65

polymer chemical groupings which have strong affinity for ions in the mineral to be flocculated. The choice of these chemical groupings, however, requires better understanding of the chemical origin of their selectivity, which is the main concern of this chapter. An attempt is made in this chapter to summarize current ideas on the fundamental aspects of selective reactions and the possible methods of synthe5izing selective flocculants.

3.1 Formation and stabilit of complex compounds;

the stabilit constants

It is well established that metallic ions occur as hydrated complexes in aqueous solutions; consequently, formation of complex compounds usually involves replacement of water molecules in the hydration shell by other ligands.

Generally, complex formation is therefore accompanied by reduction of hydration. The complex may become oil-soluble.

The complex may form a precipitate if the ligand links two or several metallic ions together (bridging ligands = e.g.,CO3 , OH ). For most complexes, reactions between:Jigands and cations proceed stepwise; that is if the concentration of a ligand "L" in a solution with a metal ion "M" is successively increased, a whole series of complexes, ML1, with value of i from zero to maximum of n, is formed one after the other. The processes generally come almost (62) immediately to equilibrium. According to Schwarzenbach there are almost a1ways more than two complexes present simultaneously in solution. The equilibrium constants,

K. and B. therefore provide suitable criteria for assessing quantitatively the coordinative behaviour of 66

various cations and their general reactivity. The values

of K and J3 which are the formation constants of individual complexes and for the over-all process respectively, are obtained from the equations: K. [M.]

[M.] [111 [L.] fi - [1,1 However, not all complexes form new chemical bonds, many are formed simply as an ion association because of the electrostatic forces between the cations and anions

or a polar ligand. In such reactions, the change in enthalpy, AH, is numerically small and may even be positive (endothermic); nevertheless the formation constants of the complexes can still be large. This is because of the large and positive entropy change, AS, accompanying the reaction, owing to the liberation of many water molecules from the hydration shell following the partial neutralization of charges. It is because of this fact, together with the lack of information on AH for many complexing reactions, that the change in the free energy, AG, is used as a measure for the overall tendency of formation and stability of complexes, the quantitative relationship being AG° = -RT ln K (where AG isi the standard free energy for the reaction). In this chapter, the stability constant

K/ , was used as an indicator for the change in the free energy of formation of the first complex. The formation of precipitates can also be taken into consideration in judging the tendency to coordination. Mononuclear species are almost always present in the solution in equilibrium with the precipitate; therefore, their formation constants, K1 , when available, could be 67

used for the comparison with other soluble complexes.

If ne such values for K1 are available, the solubility products, K , of the slightly soluble precipitates may so be compared with one another. The smaller the solubility products, of course, the more stable are the "bonds" between metal cations and ligand anions. To illustrate this point, the following example was given(63,64,65)in a solution in which AgCl is being precipitated by an chrok alkali/, the complex species AgC1, Agel + 2 and Ag are present in equilibrium with the precipitate. The total concentration of all species in solution could be measured at different alkali chloride concentration and the values of K1 , K 2 for the formation of the simple chloro-complexes of silver could thus be obtained. 3.2 Classification of ligands The ligandsjor the complexing groups, can be classified according to the type of the "donor" atom, the number of donor atoms in a ligand and the structure of the complex formed. The ligand behaviour is largely determined by the nature of the donor atom which is responsible for binding the ligand to the metal ion. Some "common" ligand atoms are briefly considered here: 1. Halogens donors; F , Cl , Br and I coordinate as simple anions, producing mononuclear complexes or sparingly soluble halides. 3— 2. Oxygen donors; e.g., OH , NO2 , NO3 , C0,1 , P014 , polyphosphates, SO -- -- and various organic complexing , 5 `-SO 4 / agents such as ethers, R-O-R, alcohols phenols and 68 carboxyl groups, form mono- and polynuclear species or precipitates with many metal ions.

3. §2.221arciaaam;- such as HS , S , SO3 S203 , SCN, thioethers R-S-R, mercaptans and their anions

(R,SH, R.S-), thiocarboxylates, dithiocarbamates and dithiocarbonates (xanthates); these show selective tendency to coordinate with group B and transition metal cations and not with A-cations (see below).

4. NitroP-en donors; like NO2 , SCN often add on to metal ions through nitrogen. Organic ligands are exemplified in primary, secondary and tertiary amines

(RNH R NH R N)* Schiff's bases and carboxylic amides 2' 2 3 R.CO. NH2.

5. Carbon donors; the only known ligand that has been studies in aqueous solution is CN.

Ligands can also be classified according to the number of donor atoms they contain, as unidentate and multidentate (e.g., ammonia NH and EDTA respectively) 3 depending whether the ligand contains one or more donor atoms, suitably situated to bond simultaneously to the one acceptor atom. In general the multidentate ligands tend to form much more stable complexes than unidentate ligands. Another classification of ligands is according to the structure of the complexes they form. (a) Those which form mononuclear complexes and essentially unidentate are considered "simple" ligands e.g., NH 13 , F-, Cl, (b) Those which form polynuclear complexes and precipitates by linking two or several metallic ions __ together are called "br:Idging" ligands e.g,, OH-, CO, 3 _ 11 ( 0-c-0- ) . 69

(c) And those which form closed rings of atoms by coordination links are termed "chelates", e.g., ethylene- diamine (CH2 NH2)2, EDTA [(CH2.N(CH7„CO2)2)2 1, anions -- tartarates and citrates. of iminodiacetic acid NH(CH2-COO)2 , The chelate complexes are usually much more stable (66) than complexes with simple ligands. It has been shown that five-membered chelate rings are more stable than six- membered rings and the increase in stability in forming chelates with seven-and eight-membered rings is insignificant

(67,68) On the other hand, ring strain arises for rings with less than five-members; four..-membered rings as a result do not increase the stability much over simple complexes, and the formation of three-membered rings is still less likely(62). Apparently some ring strain can also occur with five-and six-membered rings, depending on the coor- + 4.4.(( ,69) dination positions of the metal ion, as for Ag and Hg ;48 where the coordination positions are diametrically opposite • and therefore, they form linear arrangements rather than chelate rings. The chelate effect. The reason for the higher stability of chelate complexes in comparison with those of simple ligands can be explained as follows: It is well known that metal cations occur as hydrated complexes in aqueous solutions; thus when a ligand combines with a metal cation, it replaces water molecule from the hydration shell, and the water-molecule as a result becomes free. With simple ligands, the number of liberated water molecules will be the same as the number of ligands which disappear from the solution during complex formation, 70

whereas a multidentate ligand (e.g.,chelate) replaces several water molecules from the hydration shell. There- fore, the association of a chelate ligand leads to a greater entropy increase than the association of simple ligands, although the change in enthalpy, AH, may be practically the same, because the nature of the bonds which are broken and newly formed is identical in both cases. The large entropy rise for the chelate formation results in a greater decrease in the free energy, AG, or a larger equilibrium constant, than for the association of simple ligands. Thus, chelate K1 complexes have greater probability of formation than the

corresponding complexes of unidentate ligands. 3.3 Classification of metal cations The metal cations are classified according to the number of electrons in their outer shell to A-cations and B-cations, with the transition metal cations ranges between them.

(a) Class A-cations have the electron configuration of inert gases (d°-cations) and are of low polarizability(62,39,94,95) i.e., their electron sheaths are not readily deformed under

the influence of an electric field. Such cations include 2+ 2+ 2+ 2+ 4+ 4+ Mg , Ca , Be , Ba , Al3+, Ti and Zr . These cations have a tendency to form complexes only with F- and oxygen as donor atoms in aqueous solutions. They do not coordinate with sulphur and nitrogen atoms to form complexes of appreciable stability. There is no reaction with sulphur . donors such as dithiocarbamates and xanthat e s (61 in addition, no sulphides (precipitates or complexes) are

formed. Addition of ammonia, alkali sulphides or alkali

cyanides produces solid hydroxides, because the ligandstake protons from the solvent and leave OH to react with the 71

metal ions. A-cations tend to form difficultly - soluble 2- precipitates with OH , CO and PO 3- 3 4 (b) Class B-cations. These have an outer shell of / 10 12\ eighteen electrons, kd - d ), and high pblarizability; + + + 2+ 2+ 2+ such cations include Au , Ag , Cu $ Zn $ Cd , Hg , and 4+ Sn. . In general they form more stable complexes with N,S, Cl and I donors than with oxygen and fluoride. B-cations react with sulphur donors to produce both insoluble and soluble compounds. (c) Transition metal cations are intermediate between class A- and B-cations; that is, they have between zero and 0 10\ ten d-electrons (d0kd - d ). The divalentcations in this ) class follow the well-known Irving-Williams order (70,71 in which the stability of complexes increase in the series 2+ Mn21- Zn . This rule is recognized to be valid for almost every ligand except for certain compounds such as some polydentate chelate ligands whose structure do not fit sterically into the quasi-square 2+ coordination of Cu (in which case, the octahedral N. 2+ (62)). forms more stable complexes than Cu Cations of this class, like B-cations, tend to form stronger complexes than class A-cations. 3.4 Factors affectin the formation and stabilit of

complexes In the complexes of A-cations, electrostatic forces are mainly responsible for binding the cations with the (72) ligands ; this type of interaction is referred to as "electrovalent". Therefore, the formation and stability of these complexes depend on the strength of these forces. 72

These binding forces are known to depend on the following factors : (a) The electric charge on both the metal ions and the ligands, so that, the stability increases markedly with increase in charge. (b) The cation-radius; .thus for a series of cations of the same valency, those with the smallest radius form the most stable complexes, except when the chelating species possesses more than three to four ligand atoms; e.g., EDTA. (c) The ligand basIallz; i.e., affinity for protons; for a series of anionic ligands, the stability of the complex increases sharply with the basicity of the ligand oxygen, so that the following stability sequence 01I > phenolate > carboxylate > F is valid. It must be remembered that the complex stability also increases with the number of donor atoms in the ligand; this is generally true for all three classes of cation. On the other hand, in the complexes of B and transition metal cations, non-electrovalent forces (i.e., all other types of forces apart from electrostatic, including the possibility of crystal field stabilization (73) ), in addition to electrovalent forces are operative. The bond formed between the central atom and the ligand is essentially covalent. The tendency toward formation and stability of complexes depends on the following factors: (a) The ionization .otential of the metal, which determines the capability of the cation to take up electrons; (b) The electronegativity of the-li and donor atom; i.e., the affinity for electrons, which controls the tendency of the ligand atom to donate electrons into a covalent bond.

The stability of the complex is found to increase with the 73

ionization potential of the metal and with decreasing electronegativity of the ligand atom. In the electro-

negativity series F>a›N>C1>Br>it,iCrvS, the complex stability increase from left to right. Electrovalent interaction, however, increases with increasing charge and decreasing radius of the cation. There are other factors such as geometry of the ligand structure and entropy effects. Thus the stability sequence with B-cations are often irregular.

The stability sequency of Irving-Williams can be explained

in terms of the difference between electrovalent and non- electrovalent interaction. The radii of transition metal 2+ 2+ cations decrease from Mn to Cu but increase again for 2+ - Zn , while the ionization potential of the metal increases 2+ from Mn to Cu and falls again with Zn . However, since the large changes in ionization potentials are much more effective than the smaller changes in ionic radii, non-electrovalent

interaction is considered responsible for the marked changes

in complex stability. The above factors were arranged in Table 3.1 so that

electronegativity of the ligand atoms is in decreasing

order from left to right, whereas the charge,basicity and number of donor atoms are in an approximately increasing order from left to right. The metal cations are arranged according to their respective classes; within each class, the cations are arranged according to their charge (or valency), in increasing order from top to bottom. Further- more, within the same valency group, radii of cations decrease

from top to bottom. The equilibrium constant for mononuclear

1 was used as a quantitative index for the complexes, K , tendency of formation or stability of complexes. Values of donor F OXYGEN NITROGEN Cl SULPHUR Class atoms of OH CO= SO= P0 P 04- Phenol Cit Tart EDTA PAA NO- SCI- NH tren Penten Cl Et X- Et Cb- 8= SO= S 0= Ligand Cations F- 3 4 4 4 2 2 3 2 2 3 Ca tion 0..I.5 0.64 -8.3 *- 9.96 4.99 2.59 2.5 8 <0 < 0 -0.13 2.2 Ea++ -5.98 1.35 N 1.04 1.5 5.2 2.28 2.7 5.4 3.55 2.8 10.6 2.0 -0.1 <0 < 0 o '26.5 1.98 Ca++ 0 -10.6* -5.26 1'0.35 -5.0* -26.0* com 0.99A 9

5.17 3.96 1.3 8.7 1.8 -0.1 pl 1.8 Mg++ n 1.82 2.58 3.4 2.21 2.5 P 10.9* -5.0* ex f 0.65 X el- 1-, Sli 0 f 0 ormati ormati Al3+ h M 7.0 33.5* -18.2* 16.5 Al(OH 3 PP U yd ght X 0.5 o P I c+ roxi m l Zr4+ b vi 9.3 14.3 3.7 29.5 2.0 y 0.3 0 on on 0.8 X ---1 Ci a sol

54.0* d es 12.5 6.5 ubl Ti 4+ fl,

e 4.68 d9 2 1.25 6.47 6.77 2.32 -36.9* 3.18 5.9 3.2 18.8 4.8 1.23 2.5 4.0 18.8 22.4 0.4 14.9 1'35.4 6.1 oil++ -15.2* 9.6* Org 0.96X

88 cati 0.66 4.7 10.9 2.4 -30.3* 4.95 5.4 18.6 1.76 2.8 14.8 19.3 0.23 112.0' -* 20.7 2.06 ++ T

Ni ra -15.2* -6.87* 0 * 87 ons (do nsiti `0 ,-,.4 -r--, 8 2.4 -34.7* 5.0 16.5 2.6 1.51 1.99 12.8 15.8 0.69 -* 13.0 -22.5 2.05 CO++ -14.8* o° g * * on 3.9 10.3 2.3 4.4 14.2 1.31 1.4 8.8 11.2 0.3 - 8.0 -17.3 2.0 d6

m

Fe - d10 -15.1* r10.7* et a * * 85 al 4.5 10.9 2.3 2.6 5.74 2.7 14.5 3.36 1.23 0.8 5.8 9.4 0.0 + 2.0 -12.6 1.95 ++ a

mn s

-12.8* -10.7* 0.91 X ) * 810 1.26 4.36 10.3 2.3 -32.0* 4.98 2.68 16.2 3.32 1.19 2.59 14.7 16.2 -0.19 - 9.0 -* 23.8 2.29 Zn++ -15.7* -10.7* 0.74 * -30.0 Au+ 1.37 % oa0 :I 1 * * 0.56 2.3 -11.4* 1.3 -* 19.9 0.34 -12.2 7.3 1.15 4.75 3.31 10.3 3.23 8.3 -51.2 5.6 8.8 A-g+ o n -7.8* -3.2* -12.0* _9.5* Org 1.26 A M -4.5* 1-, * 0 L1 2.34 5.93 : 6.7 :20.0 -48.0 10.35 cu+ 0 4.7* 0.96 A m

11.5 -16.0* 10.9 21.7 6.77 8.8 25.8 29.6 6.7 -* 51.8 12.0 14.7 Hg++ o ill 25.5* 1.10 A n ++ 0.46 5.5 -12.o* 2.29 :32.6 3.75 16.9 1.7 2.51 2.57 12.3 16.8 2.0 -14.0 14.9 -* 27.9 2.1 3.9 Cd Org 0.97 A M..... -14.2* A.) 1.19 14.7 2.29 Zn++ V 0.74 A 1

* Logarithm of the solubility product, Org = in organic medium, K [Mil 1 [Ml[Ll 75 K were taken from "The Stability Constants" b5). 1 3.5 Selectivit and s ecificitv of com lex formation The selectivity of complex formation and even specificity (i.e., capability of a ligand to attach to only one metal ion) are influenced mainly by two important factors amongst others mentioned earlier: I) The nature of the ligand atoms As shown in the previous sections and on Table 3.1

F and 0 donors form complexes with practically all polyvalent metal cations, while N, S and C donors do not form complexes of appreciable stability with A-cations and therefore are more selectiVe. The selecttivity is noted to increase with decreasing electronegativity of the donor atom. Nitrogen donors, notably, polyamines (e.g. "tren." and "penten." on Figures 3.1 and 3.2 respectively) are much stronger and selective complex formers for B-cations and transition metals than polycarboxylates, e.g. citrates and EDTA, Figures 343 and 3.4 . Sulphur donors show even more marked selectivity where insoluble compounds are formed only with B- and transition cations; such sulphur ligands are dithio- carbonates (xanthate or R-X-) in Figure 3.5 and dithio- - Cb) in Figure 3.6 carbamate (P.2 II) Geometr of the 11 and structure Increased selectivity could be produced when the metal ion and the chelate ligand are geometrically as compatible as possible. The configuration of the chelate donor atoms should have a steric arrangement to fit especially well to the coordination sphere of one of another meta]. cation. Thus the enclave of the diaminocyclohexanetetra-acetate, CH2.CH2.NH2 CH2-000- N CH2-CH2.NH2 HOC COO- CH2-CH2.N H2 CH2.000- Fig.3.1 iren:c6 H18 N4: .3.3 Cit:c6t1507,citrate Triaminotriethylarnine

,--CH2.CH2.NH2 _---N _--N CH2 ------CH2CH2.N H2 CH2 --"*"--C1.42.000" 1 CH2 CH 2'CH2'N H2 CH2 _--CH2.000- ----N -----N CHTCH2'N H2 Fig.:3.2 Penten:cio H28 N6; Fig.3-4 ED1A:c10 H16 08 N2 Tetra-(2-aminoethyl )- ethylenediamine

'S C2H5 S c2H5—o—c N—C Ns- C----2H5 NS- Fig.3.5 Et-X:Ethyl xanthate Fig.3- 6 Et 2rb: Diethyldithiocarbarnate

CH2 _--N CH2 CH -----CH2-000-

CH2 CH CH2C00- CH2 Fig.3.7Diaminocyciohexanetetraacetate 77

Fig. 3."7k fits especially well for the dimensions of the calcium ion but is too small for the barium ion and a little too large for the ion, and, as a resit, it forms a calcium complex of higher stability than those (66) of magnesium and barium . Specificity for a metal ion would best be attained if the steric arrangement of the ligand were rigid. Many examples of the effect of the structural geometry on selective formation of 94, 95, 96) complexes can be found in the literature Although perfect specificity for a single metallic element has not yet been achieved with synthetic chelating agents, there are certain living organisms which are capable of achieving this result. Bielig and Bayer (74, 75, 76 ) have found that the concentration of vanadium in the blood cells of the tunicate "Phallusia mamillata" is a million times greater than that in sea- water, and the concentration of copper in the blood of the cuttlefish "Octopus vulgaris" is about a hundred thousand times. The phenomena of enrichment of metals (77) in the marine organisms have been treated by Goldberg , who found a qualitative correspondence between the enrich- ments and the Irving-Williams order. Another example of "specific" complexing agents in nature is a group of compounds known as "ferrichromes", which strongly bind 3+ Fe ions (39). Ferrichromes appear to be widely distributed in micro-organisms, and fungi have been used for routine preparation of ferrichromes in the laboratory(78) 78

Although ideal specificity of this order cannot yet be attained with synthetic chelates, any attempt to prepare chelating groups with high affinity even to few metals would represent a step forward toward specificity.

With this aim in mind, Bayer and co-worders (79 synthesized chelating resins possessing high selectivity for rare metal ions in the hope of extracting these metals from sea-water. The work of these authors will be discussed in more detail in chapter 6. A synthetic chelating resin was prepared from poly- (80) methacroylacetone by Teyssie and co-worker who used the resin as a selective ion-exchanger in the separation of several cation-systems e.g., Cu/Ni; Cu/Co; Cu/Zn; (81) Be/Mg, Ca; and Zn/Mg.. Fanta and co-w orkers have grafted mixture of acrylamide and the nitrate of dimethylaminoethyl methacrylate onto unmodified wheat starch, with ferrous ammonium sulphate hydrogen peroxide initiation. The polymer produced was difficultly soluble in water. After many trials, they finally dissolved it by a steam-jet cooker, which unfortunately reduced the molecular 1,7ight, and, when the polymer was tested for flocculation on a 3% suspension of diatomaceous silica (celite), it was found inefficient as a flocculant. ( Packham s2) has prepared cross-linked co-polymers of 4-acetoxystyrene and divinylbenzene which on hydrolysis yielded insoluble poly-4-hydroxystyrene with different degrees of cross-linking, and according to Packham, most of these polymers exhibited ion-exchange and chelating 79 properties in aqueous solution. Audsley and co-workers(83) prepared anion-exchange resins by the reaction of poly- vinyl chloride beads with concentrated aqueous solution of aliphatic di- and polyamines; the resin was suitable for the recovery of from siliceous liquors as the amine-groups were more selective to uranium than to (84) silica. Shepherd and Kitchener prepared transparent colourless rods of anion-exchange resin based on poly- ethyleneimine (cross-linked by reaction with ethylene dibromide); the resin had a high affinity for copper and to a smaller extent and by ammine formation. The possibility of using the chelating resin Dowex A-1 for the separation of metals was ruled out by Van Willigen and his colleagues (85) because the differences between stability constants of metal-resin complexes were too small to promote efficient separation. Stamberg et al(86) investigated an ion-exchange resin selective for nickel, which contained dioxime and oxime groups. Green and La-w(87) investigated a commercial ion-exchange resin with high selectivity for gold. The resin was found to collect only gold from acid solutions, while the "common" metals were not collected nor did they interfer. The authors did not (88) reveal the nature of their resin. Parrish prepared a number of ion-exchange resins based on polystyrene. which showed selective behaviour, e.g. polythiolmethylstyrene took up mercury but not magnesium, and polystyrene - (4-azo-5)-8- hydroxyquinoline strongly adsorbed copper, nickel, and cobalt in the pH range 2-3, whereas , mapganese, , magnesium and calcium were either 80 weakly adsorbed or not taken up at all, depending on pH and ionic strength of solutions. Gregor and co-workers( 89) claimed that the m-phenylene diglycine-formaldhyde resin, which they synthesized, had a high degree of specificity (90 at certain -pH levels. In a review, Hale cited numerous examples of attempts to produce selective ion- exchange resins for the separation of various metal cations. From the above examples, it can be safely concluded that selectivity of one resin or another was primarily due to the presence of either nitrogen or sulphur as donor atoms in the corresponding chelating groups. Consequently, water-soluble polymers of the same chemical types (i.e. not cross-linrd). 44 could confidently be expected to bind the metal ions in the same way. 3.6 Synthesis of selective of meric flocculants There are four general methods by which polymers could be prepared. These methods are briefly defined in this section, but more detailed descriptions are to be found in the literature (91,92,93) 3.6.1 Polymer modification In this method, an existing polymer is subjected to certain chemical reactions so that a distinctly different polymer is obtained. For example, cellulose may be modified through reaction of some of its hydroxyl groups to give polymers like cellulose xanthate (Chapter 4) , cellulose acetate, methyl cellulose, etc. Polyvinyl acetate when treated with methanol (alcoholysis), changes to polyvinyl alcohol, which can be further modified to polyvinyl alcohol xanthate (Chapter 5), or treated with aldehyde to give polyvinyl acetal. Similarly, polyacrylamido may be modified to produce many new polymers some of which 81

are already known in industry (e.g. anionic and cationic flocculants). Two new modifications are described in Chapters 5 and 6, where polyacrylamide is linked with copper-selective groups namely; dithiocarbamates and glyoxal-bis-hydroxyanil. The number of possibilities is obviously very great, though the ease of preparation.: of derivatives can vary enormously. 3.6.2 Polrmerization throu h functional groups

In this method, polymers are prepared from relatively simple starting materials (monomers), where interaction proceeds through their functional groups. The polymers produced may contain new groups which are not found in the monomers, e.g. polyamides (nylon 11, nylon 6,6, and nylon 6.10), polycarbonate, polysulphide, etc. These polymerizations almost always proceed in a stepwise manner and the polymer chain is built up by a sequence of discreet interactions between pairs of functional groups.

3.6.3 Polm=iaallanthrgouhmultilep bonds This method is regarded as the joining together of unsaturated molecules through the opening of multiple bonds and linking as a chain. The polymer composition and structural units are otherwise essentially unchanged. This type of polymerization may be divided into 3-main categories:

1. Linylolymerization, where the vinyl compounds, i.e., those containing the CH2=c11- group, polymerizes to R produce long-chair polymers with -C-C-C- backbone.

For example, ethylene, propylene, styrene, vinylchloride and methyl methacrylate, can be reacted to give the 82

corresponding polymers. The process of vinyl polymerization consists of 3 parts; namely; initiation, in which an active species capable of starting polymerization is formed, 21222L2112a, in which the molecular weight of the polymer is determined and finally, termination, in which the polymerization process is deactivated and the final stable polymer is produced. Vinyl polymerization is further, divided into 3-different methods, according to the type of the active species, namely; free, radical, anionic and cationic polymerizations. 2. 211912211.2117219.11aLtion, where the unsaturated diene compounds (i.e., organic compounds containing two carbon to carbon double bonds) undergo polymerization through their multiple bonds, e.g. butadiene [CH2 = CH - CH = CH2]; chloroprene [CH2 = y - CH = CH2], Cl and isoprene [CH = 9- CH = CH2] . As in vinyl 2 CH polymerization, diene 3polymerization could be accomplished by the three methods namely; free radicals, anionic and cationic processes. 3. TknIIA.IT1.2E1Raila=iapli2a, unlike vinyl and diene processes, where the carbon-carbon double bond is the active cite, this method untilizes other elements besides carbon to form polymers of hetero- atomic chains. An example is found in formaldhyde polymerization:/1 H_ C= 0 [- CH - O -1 2 2 n • 3.6.11 Polymerization through ring-opening In this method, polymer formation results from polymerization of cyclic compounds undergoing ring-opening reactions, e.g., the formation of polyether (polyethylene 83

oxide) from ethylene oxide. Polymers prepared in this way usually have the same chemical composition as the monomers.

The polymerization reactions are generally classified into two types; namely, condensation polymerization and addition polymerization. The condensation method leads to a polymer having a structural unit containing fewer atoms than the monomers, whereas the addition polymerization results in a polymer having a structural unit with the same molecular formula as the monomer. Polymerization reactions are also classified as stepwise and chain polymerization. The main difference between the two types is the rate of polymerization; thus in a stepwise process the polymer is built up relatively slowly by a sequence of discrete reactions; while in the chain polymer- ization the polymer molecule grows extremely rapidly once the initiation has occurred.

Of all these methods, polymer modification probably offers the most convenient technique for the synthesis of selective flocculants. It is important to obtain a very inng molecule in the final product, but many polymerization reactions are difficult to control, and so it is simpler to start with a pre-formed polymer of high molecular weight and modify it. By incorporating certain chemical groupings having strong affinity to particular metal ions the polymer could be made to selectively complex with such ions and since the complexes are less water soluble, it seems likely that the polymer would selectively adsorb on the corresponding minerals. Examples of modified polymers are given in chapters 4, 5 and 6 where attempts to produce selective flocculants are described

111

84

CHAPTER 4 CELLULOSE XANTHATE

4.1 INTRODUCTION

4.1.1 Formation o cellulose xanthate Cellulose xanthate in the sodium form is an inter- mediate in the manufacture of rayon, the concentrated viscous solution in aqueous sodium hydroxide solutions being technically called "viscose". It is formed by reacting alkali cellulose with carbon disulphide, CS2, according to the equationM: Cell-ONa + CS 2 = Cell-OC(S)SNa, where "Cell" means the cellulose molecule (C6H7(OH)3)n , "n" ranges between 50-5000. In this reaction some of the many OH-groups are substituted by xanthate groups. The degree of substitution .T”, "the number of xanthate groups per 100 glucose units", usually ranges between 50-70 on average. Higher degrees of substitution can, however, be obtained. Apparently, the substitution of xanthate groups is not the same for all the OH-groups of the glucose ring(98) It has been established that substitution onto position 6- (fig.4.1), "1?6", may amount to 60-75/0 of in freshly prepared xanthate .4,ttal ()) and to about 75-100% of nytotal" in newly prepared viscose The distribution of xanthate groups along the polymer chain in fresh viscose has been found to be random, depending however, on the conditions of xanthation. This distribution may be modified in the subsequent stages of "ripening" of viscose due to the de-xanthation and re-xanthation processes 6 0O ) 85

4.1.2 Structure of cellulose xanthate

The basic unit of organization of native cellulose,

whether fibre, membrane or unorganized products, is a 0 fibril about 100A in diameter and of varied length

These fibrils bundle together to form the fibre with a 0 diameter of about 100,000A, while the fibrils themselves

consist of many cellulose molecules with a diameter of 0 .less than 10A. Figure 4.1 shows a diagram of the

structure of cellulose.

The structure of sodium cellulose xanthate, NaCX,

has been investigated by infra-red spectrophotometry0o1)

It was found that the C=S group is perpendicular to the

plane of the pyranose ring, while the C-S group is parallel

to it. Figure 4.2 shows the possible structure of

sodium cellulose xanthate accordingly.

4.1.3 Choice of cellulose xanthate as a selective

flocculant

The choice of cellulose xanthate in selective floccul-

ation stemmed from the fact that xanthates of low moleulcar

weights are used as selective flotation collectors of

sulphides and some sulphidized oxides and carbonates. This

selectivity, as shown in Chapter 3, is due to the fact that some ligands containing sulphur as the donor atom do not

react with minerals of cations with rare gas configuration, 2+ 2+ 2+ like Ca , Mg , Ba , Al3+, nor with silica. It is also

known that xanthates react with transition metal cations . 2+ Zn2+ 2+ 2+ such as Cu2+ , , Co and Fe to form water- 043). insoluble compounds Therefore, when xanthates are

attached to a long-chain polymer such as cellulose, they

are expected to act as selective flocculants.

Fig.4.1 Structure of Cellulose

6CH2OH ICI GH 5 I OH H 0 C( )C4 1C )C C( 0 H H OH OH H 1-1 2 i 3C C C I H OH CH2OH n-2 n=50-5000

Fig.4.2 Structure of Cellulose Xanthate

9H2o-c-s- c

H 0 \ / C C ( \ 0 H \ OH H \ i 1 C C I 1 H 0—c---s- g 87

4.1.4 Evidence of selectivity_

Sodium cellulose xanthate ("NaCX") in its unpurified

form, "viscose", was prepared and tested for flocculation

on some minerals at pH 10. It is believed that most

minerals acquire a negative charge at this pH; therefore

any flocculation effect due to charge neutralization

would be eliminated as the polymer acquires negative,

charge too. It was found that it has some flocculation

effects on minerals like galena, pyrite, chalcopyrite

and later chrysocolla. The "NaCX" was added at a dose

of 1 p.p.m. Flocculation of sphalerite was noticed at

pH 6.8, as the sphalerite suspension could not be

brought to pH 10 without coagulation. Furthermore, the

flocculation became more rapid, when the concentrations

of "NaCX" in suspensions were increased to 5 p.p.m.

On the other hand, it has no flocculation effects

on minerals such as quartz, calcite, feldspar and clays even at a dose of 100 p.p.m. in the case of clays (6) The clay minerals tested were bentonite, kaolinite and

illite.

4.1.5 The aim of the present work was to explore

the flocculation properties further and the possibility

of recovering the wasted chrysocolla fines from the various copper processing plants. In order to achieve that aim, various preparation methods had to be examined or developed, so that the desirable properties of the flocculant such as molecular weight, degree and uniformity of xanthation could be obtained. The study of the physical 88

and chemical properties should help to utilize the polymer properly and to establish a method for obtaining a dry, pure polymer which could be kept for long periods and withstand long transportation.

4.2 PREPARATION OF CELLULOSE XANTHATE

The standard procedure of preparation in the textile industry can be summarized in 5 steps (A - E) as follows:-

A. Treatment of the cellulose raw, material with 18%

sodium hydroxide, this process being known as

mercerization or steeping). The objectives of this

step are to induce swelling of the cellulose fibres

and distortion of the crystalline cellulose so that

the penetration of CS2 into the fibrils becomes easier,

also absorption of NaOH,with uniform and complete

formation of "alkali cellulose- I". This reaction is

exothermic, and a rise in temperature of 2-3°C

usually occurs(". At the same time, hemicellulose

and other low molecular weight impurities are removed

from the cellulose.

The success of this step depends on temperature,

concentration of NaOH and time. It is also

influenced by the nature of cellulose starting

material. Apparently swelling is not enhanced at

high temperatures and high NaOH concentrations. This

step is a relatively rapid reaction. The time of

steeping is determined largely by the time necessary

to solubilize and remove hemicellulose and other 89

impurities from the pulp. The minimum time is 15 minutes depending on the nature of cellulose. Wetting agents are sometimes used to improve the penetration of NaOH and shredding, thus giving more uniform xanthation.

B. Removal of the excess NaOH by pressing and filtering the cellulose pulp to a press ratio of 3-4 of the

cellulose weight. Excess of NaOH will react waste-

fully with CS2 and increase the by-products; on the other hand, too much pressing will lower the NaOH content, which affects the ageing process and the uniformity of CS2 uptake.

C. Shredding the "alkali cellulose I" by mechanical disintegration to increase the surface area of the

alkali cellulose and hence the reactivity and uniformity of CS 2 uptake. This process is sensitive to oxygen and temperature.

D. Ageing the shredded "alkali cellulose I" by storing in a closed container for 65 hours at 25-30°C, during which time oxidative degradation of the cellulose

chains by oxygen takes place(102)and results in a lower molecular weight. This process is controlled to give a suitable molecular weight for the purpose of the textile industry.

E. Xanthation i.e. treatment of the aged "alkali

cellulose I" with CS2 at a temperature between o 20-35C for 1-3 hours, forming sodium cellulose 90

xanthate and other by-products of yellow-red colour.

The reacted crumbs are then dissolved in water and NaOH (18%), producing the viscose solution. The viscose is left to ripen, during which period re- xanthation and de-xanthation occur as well as

oxidative degradation of the polymer chain to a lower molecular weight.

The xanthation of alkali cellulose causes an additional swelling of the micelles but the native fibre morphology

of cellulose is largely retained, as observed by optical (103} microscopy Subsequently, the xanthated fibre is dispersed in dilute NaOH as a soluble polyelectrolyte. Figure 4.3 shows the swelling of partially reacted micelles.

The formation of cellulose xanthate is always accompanied by formation of low molecular weight poly- sulphides which may interfere in the flocculation process OW+) of minerals. Oprits and Rassolov have derived mathe- matical equations to give optimum operating conditions which Five the shortest reaction time and the lowest yields of sodium polysulphide by-products. Sintola(105) has patented a quick method with no side reactions. Von Horstig(10 prepared cellulose xanthate at a temperature of 50-60o C from CS24!-H,0 vapour mixtures and the alkali cellulose, but this process, besides being unsafe, also increased the production of polysulphides. Andreason al(107) et have studied the formation of cellulose xanthate in homogeneous medium; they concluded that xanthate 91

Fig.4-3 Reactivity of Cellulose

• 0 0 0 0 • • • • 0 • • e • • • • • • • • • • • • 0 0 • • • ••

• • • • 0 0 0 • • • • • 0 •

• 0 • 0 • • • • • • • • • • • • ••

O • • • • • • • • • • • • 0 • • •

0 • • • • • • • • • • 0 • • 0 • • 0 0 0

0 • 0 • • • • • • 0 0 0 0 0 0 • 0 0 0 • • • • • 0

• • • • • • • O 0 • 0 • 0

• • • • 0 • • • 0 • 0 • •

0 • • • • • • • • • • • • • • • • •

ORIGINAL MICELLE PARTIALLY REACTED MICELLE

0 Glucose ring • OH-group s- Xanthate - group

92

formation is a second order reaction determined by the

concentrations of CS2 and alcoholate. CZajlik and Treiber0100 measured the total heat of reaction of xanthation as 12 kcal/mole CS2. The heat of the real xanthation reaction is estimated to be less than 10 kcal/mole CS2.

4.2.1 Laboratory preparation of sodium cellulose xanthate:

Standard Method:

The source of cellulose used in the preparation of sodium cellulose xanthate in a highly viscous form was

ordinary filter paper. Approximately 10g of shredded filter paper were soaked in 18% sodium hydroxide in a sealed container at 21°C for 1 hour, thus forming the "alkali cellulose I". The excess sodium hydroxide was removed by filtration and the "alkali cellulose I" was stored in the sealed container for 65 hr. The aged alkali cellulose was shaken with 4g CS2 in the sealed container at 21°C for 3 hours; after which it was diluted with 17 ml of 18% sodium hydroxide and 76 ml distilled water. The shaking was continued for another 2 hours with a laboratory mechanical shaker.

The "viscose" thus produced normally contains about 6% cellulose and 7% sodium hydroxide, and the CS2 content is 40%, based on the cellulose weight. The colour of

"viscose" is orange to deep carrot red. Preparation of cellulose xanthate from different cellulose sources. The "viscose" prepared from filter paper contained some undissolved crumbs; therefore it was thought that less fibrous cellulose sources would 93

yield more homogeneous solutions. Therefore three samples of 10 g each from 3 different cellulose sources namely; "cellophane", cellulose powder and filter paper, were treated with 18% sodium hydroxide to form the "alkali cellulose I", as described in the standard method. The alkali celluloseswere stored in sealed containers for

17 hours and thereafter shaken with 4 g CS2 at 21°C for 3 hours. The three samples were diluted with 17 ml NaOH

and 76 ml distilled water and shaking was continued for a further.2 hours. The cellulose powder and cellophane gave the most uniform solutions, but on the other hand less viscous products than the filter paper. The three products were tested for flocculation on galena, chalcopyrite, pyrite and sphalerite. They all showed flocculation effects; the filter paper product, however, was more effective (i.e. gave better flocculation) than the cellophane and the cellulose powder products, which induced weak flocculation at a concentration of 1 p.p.m. and 5 p.p.m. in the mineral suspensions. When the three products were tested for flocculation

on quartz, calcite and feldspar, no flocculation effects were noticed even at a dose of 10 p.p.m. or more. 4.2.2 Preparation of cellulose xanthateallhiah

molecular weight For the purpose of using cellulose xanthate in flocculating mineral particles, it is necessary to obtain a high molecular weight polymer. Therefore, it seemed logical that ageing and ripening time must be reduced or eliminated. This was supported by the findings of Das and Choudhury(135), that by reducing the ageing time, higher molecular weight polymers were obtained. According to the literature, products of molecular weight up to 106 can be obtained. This is the order of magnitude used in modern synthetic flocculants.

Therefore, the cellulose xanthate was next prepared from filter paper as in the standard method, in three experiments of varying ageing times namely; 3 days, 65 hours and 17 hours respectively. A difference of viscosity of the three products was detected visually, and was found to increase with decreasing the ageing time. On the other hand, the longer ageing times yielded more uniform solutions. In another experiment, two batches of cellulose content of 400%, were prepared according xanthate with CS2 to the standard method but the ageing times were 3 days and zero (i.e. no ageing), respectively. Uniform products were obtained but the non-aged product was more viscous than the aged product. 4.2.3 Pre aration of cellulose xanthate of different

degrees of xanthation Sufficiently high degree of xanthation must be attained to produce solubility and in order to increase the effective- ness of the polymer in flocculation. The highest degree of xanthation is when the three hydroxide groups of each cellulose molecule are substituted with xanthate groups.

However, this can only be achieved when the cellulose raw material is in the molecular state and not in the fibrous state. 95

Effect of CS 2------ratio. To investigate the effect of CS 2 ratio, three samples, of cellophane of 10 g each were shredded and treated with 18% sodium hydroxide as described in the standard procedure. The alkali cellulose were stored in sealed containers for 17 hours and thereafter' shaken with different amounts of CS2 namely; 2g, 4g and 8g at 21°C for 3 hours. The three samples were diluted as before and the shaking was continued for a further 2 hours. It was found that the solutions became more uniform at higher CS2 contents.- The xanthate content was markedly higher for the 80% CS2 than for the 20% or 40%, as measured by ultra-violet spectroscopy at wave lengths X= 300 - 303 mp. In another experiment, two batches of filter paper of lOg each were treated as before, but the CS2 ratios were 40% and 400%, and the ageing time was 3 days. The xanthate content of the 400% product was higher than the

40% products; it was also more uniform. Effect oflEmerature. With the objective of reducing degradation of cellulose, an experiment was carried out on 10g filter paper in the same manner as the standard procedure except that the alkali cellulose was stored in the refrigerator at about 5 - 6°C for 12 hours. It was to any found that the crumbs did not react with CS2 significant extent and consequently did not di4olve in the dilute sodium hydroxide solution. 96

4.2.4 a-‘earationz Ia distributed

xanthate groups

The problem of uniformity of xanthation has taken the (97-138). attention of several authors They all agree that the reaction of carbon disulphide with alkali cellulose is non-uniform. This is mainly because of the incomplete exposure of the hydroxyl groups of the cellulose melecules for reaction with CS2. As the CS2 reagent transfers to the fibrils by , and the diffusion rate depends on the degree of swelling, the incomplete exposure of the hydroxyl groups is due to incomplete swelling of the cellulose fibres.

Even in the swollen alkali cellulose, the reactivities of the three OH-groups of the cellulose unit are not equal.

It has been stated (97)that the reactivity of the primary hydroxyl group is tenfold greater than any of the two secondary hydroxyls. If the nature of substitution is determined by the rates of reactions of the three hydroxyls, the proportion of substitution will be as the ratio 1:1:10.

This ratio, however, may be modified according to the medium surrounding the individual hydroxyl groups.

The incomplete exposure of the hydroxyl groups to

CS can also be attributed to the incomplete coverage of 2 the swollen alkali cellulose because of the small volume of carbon disulphide. Therefore, the ways to solve the problem of uniformity should be along the following lines: a) improving the swelling properties of cellulose fibres, b) introducing CS2 in large volume, or diluted in an

organic solvent, 97

c) using water-soluble cellulose derivatives or non- crystalline cellulose sources.

The study of the problem of uniformity in this work was pursued because of its importance to improve the flocculation characteristics of the polymer.

In an attempt to improve uniformity of xanthation

by adding CS2 in large volume, an experiment was carried out on a 5g sample of cotton wool. The sample was shredded

and treated with 18% NaOH while shaking at 30°C for 1 hour. The alkali cellulose was recovered, after filtering the excess NaOH, and 4g of carbon disulphide was introduced as a 1.0% solution in diethyl ether. The shaking was

continued at 30°C for 2 hours, but the colour of the crumbs was only pale yellow. The shaking was then continued at

30°C for 12 hours, but the orange colour did not form and the crumbs were still pale yellow and did not dissolve to a great extent in 4% NaOH aqueous solution. When the solution was analyzed by ultraviolet spectroscopy, the xanthate content was very low.

Improving the swelling properties of cellulose fibres with the aid of a wetting agent was attempted(108? which improved the rate of xanthation through reducing the surface tension of the liquids, thus-promoting the penetration of NaOH into the cellulose fibres. "Emulsion xanthation" was found satisfactory. This process is described in section 4.2.5

The use of water-soluble cellulose derivatives to produce uniform cellulose xanthate polymer is described in section 4.2.5. 98

4.2. 512r owd22:of2-21form reactivity. A sample of 5g of cotton wool was shredded and steeped in

18% NaOH while shaking at 30°C for 60 minutes. The excess sodium hydroxide was filtered and the alkali cellulose was

shaken with Itg carbon disulphide at 30°C for 2 hours. In this process, the alkali cellulose had taken up all the

added CS2. The solids were then shaken with dry diethyl ether (dried by sodium) at 0°C for about 10 minutes. The

diethyl ether was filtered off and the solids were dried under vacuum desiccator over phosphorous pentoxide. The dry solids were shredded to fine size particles by a mechanical shredder.

Tests for solubility in 4% sodium hydroxide aqueous solutions indicated that about 85% of the solids were dissolved in 60 minutes of moderate stirring. The undissolved white portion, when dried in the oven at 120°C, developed a yellow-orange colour. This indicates that these solids may require a longer period to dissolve.

Emulsion xanthation. In this procedure(109), 2.5g of cellulose raw material (cotton wool) is shaken mechanically for six hours with 15 .ml of CS and 30m1 of 18% aqueous 2 NaOH at 23°C. At the end of xanthation, the product is vigorously stirred with an equal volume of water until homogeneity is attained. In the experiment, the cellulose raw material used was cotton wool (medical type), which was disintegrated before treatment. It was observed that

the by-products of carrot-red colour were formed after 1 hour, and the cotton was yellow. At the end of xanthation, homogeneity of the viscose solution was attained after 99 about 60 minutes of vigorous shaking. It was not, however, completely uniform, but it was definitely more uniform than that obtained by the standard procedure. 4.2.6 Xanthation of various cellulose derivatives

prepalationofri se,canthate. The methyl cellulose used is technically known as "TYLOSE" H4000p, from Kalle Aktiengesllschaft, Wiesbaden-Biebrick, West

Germany. A sample of "TYLOSE", 2.5g was shaken in 30 ml 18% sodium hydroxide and 15 ml carbon disulphide at 30-

35°C in a water-bath for 2.5 hours. At the end of xanthation, 50 ml distilled water was added and the product was shaken at room temperature for 1.5 hours. The product was homogeneous and free from solids. The presence of the xanthate groups was confirmed by the u.v. spectroscopy. The sodium methyl cellulose xanthate was later purified and tested for flocculation, as described in section 4.6. Preparation of sodium carboxy methyl cellulose xanthate. A sample of sodium carboxy methyl cellulose from W. & R. Balston Ltd., England, commercially coded CM70, was used in this experiment. Thus 2.5g of CM7O was treated in the same manner as methyl cellulose. Uniformity of the product was attained after about 60 minutes of mechanical shaking with distilled water. The colour of the product was orange-carrot like. The xanthate groups were detected by the u.v. spectroscopy in both the unpurified and the purified forms. It was purified by ion exchange.

The purified sodium carboxymethyl cellulose xanthate

(NaCMCX) of 1.0% aqueous solution was noticed to decompose to orange-colour products after 12 days storage at 5-6°C 100

The dilute solution, 0.1%, decomposed after 6 - 7 days storage. During this time, the purified methyl cellulose xanthate and the ordinary cellulose xanthate 1.0% solutions, did not decompose. The flocculation properties are described in section 4.6. Preparation of hydroxylpropyl methyl cellulose xanthate. Approximately 2.5g of hydroxypropyl methyl cellulose xanthate supplied by British Celanese Ltd.,

Spondon, Derby, England, was emulsion xanthated in the same way as methyl cellulose and carboxy methyl cellulose in the previous sections. The hydroxypropyl methyl cellulose xanthate dissolved to a uniform solution when shaken with distilled water for less than 60 minutes. The solution was more viscous than any other cellulose xanthate prepared by the emulsion xanthation method. The presence of the xanthate groups was also detected by the u.v. spectroscopy.

4.3. PURIFICATION OF CELLULOSE XANTHATE

4.3.1. Precipitation with alcohol In this method, the high molecular weight cellulose xanthate is precipitated with methyl or ethyl alcohol, while the low molecular weight polysulphides remain in solution. Thus on filtering, the low molecular weight polysulphides are removed. This process is known in the (97, 109) texts as "coagulation" of cellulose xanthate with alcohol. The "viscose" solution is poured into a 4 litre beaker and 3 litres of ice-cold methanol is added while o stirring continuously. The stirring is continued at 0 C for 1.5 hours or until enough gel is formed. The solution 101 is then filtered and the precipitate is washed with 1i00 ml

of cold methanol. The solids are transferred to a beaker and kept in 1 litre of 5% acetic acid in methanol for 20 minutes at 0°C with occasional stirring. The solution is again filtered and the solids are washed several times with cold methanol until the filtrate fails to turn moist litmus paper red. The solids are then shaken with dry

ether for sometime, after which, the ether is filtered

off and the solids are dried in a vacuum desiccator over phosphorus pent oxide. This method was slightly modified and performed on various cellulose xanthates prepared by emulsion xanthation.

Thus, the viscose of emulsion xanthated cotton wool was dropped in small portions into a tall beaker containing methanol at 0°C, in order to obtain fairly uniform particles. Instead, it formed a pale-yellow bulky precipitate. At the end of the process, the vacuum dried cellulose xanthate was too tough to be ground in the agate mortar, or shredded by a mechanical shredder. When dissolved in water, the product did not give a uniform solution. Some of the undissolved solids were in the colloidal state, which gave rise to difficulties in

filtration. The emulsion xanthated sodium carboxy methyl cellulose

solution was dropped into ethylalcohol contained in a 1

litre beaker which was surrounded by 2 litre beaker with ice particles in between. The solution was stirred by a

magnetic stirrer and a glass rod. The precipitates were noticed to stick on the bottom of the beaker, the magnetic

stirrer and the glass rod. Attempts to separate the 102

polymer precipitates were not very successful. The method was therefore modified. The 1 litre beaker was replaced by a 1-litre measuring cylinder and the solution was stirred intensely by the magnetic stirrer. The "viscose" was injected from a 20 ml syringe to produce fine drops, which precipitated to small, uniform size particles. Then the process was continued as before.

The colour of the vacuum-dried particles was pale yellow. The solids were dissolved in water to a uniform solution. But the 1% solution decomposed to an orange- coloured solution and white sediment, after about 12 days storage at 5-600. It was concluded that this method of purification was not convenient and an ion-exchange column was used instead.

4.3.2. Dialysis of cellulose xanthate

The dialysis method was established in industry to (111). remove mainly sodium hydroxide from the viscose The method can be summarized as follows: About 600m1 of the viscose solution of about 4% cellulose is kept in a regenerated cellulose bag at 15°C for 1 hour, or until dialysis is complete. The solution is diluted to 2% cellulose and the pH is adjusted to pH 11. The dilute viscose solution is then fed to a spray dryer, where heated air at 130°C is introduced at high velocity. According to the author (111) ,this method will give a powder of 12.5% xanthate sulphur. This powder has the trade name "Sup-R-S".

In the laboratory, a small sample of viscose was 103

put in a dialysis bag and kept in a beaker, where a current of water was flowing continuously. After many hours, the low molecular weight sulphides were still colouring the flowing water. This method was not pursued because it was very slow.

4.2.3. Ion-exchange column Removal of low molecular weight polysulphides by an ion-exchange column was established by Samuelson and Gartner(112) In this method, the alkaline-saturated ion- exchange resin retains the polysulphides and not the cellulose xanthate because of its large molecular size. Dux and Phifer(113) have modified the ion-exchange column to purify the cellulose xanthate in a shorter time; they introduced air pressure to a steel column to force the viscose through. In the present work, the ion exchange multi-column apparatus was designed to work at 0°C, in order to avoid decomposition of xanthate. The reason for the multi- columns was that the cellulose xanthate was not completely separated in only one column. The three consecutive columns provided long contact time between the viscose and the resin, thus enhancing the fractionation process. Vacuum was applied to the system to accelerate the flow. The ion-exchange operation of the viscose solution takes about 3-5 minutes, depending on the concentration and the viscosity of the solution. The strong basic resin

"Deacidite FF-IP" was used. Design of the vacuum multi-column ion-exchange

Apparatus

The apparatus, Fig. 4.4, consists of glass tubes each 5cm in diameter and 37-40cm long, surrounded by larger 04

t or p up

d s co be a lass d g

tere co U Sin C O co 0 X Lk]

0 C E

0

747a -5

000 00000 00 a 0

te ha

C t n Xa

E e

cn s lo llu Ce d fie i r Pu 105

ones to provide enough space for the ice granules. The connecting tubes were made as short as possible to avoid unnecessary complications. The tubing was mainly of soft polyvinyl chloride, so that dismantling of any part of the system was easy. The end tube was connected to three vacuum flasks for receiving the purified cellulose xanthate, sampling during ion-exchange operation, and washing the resins. The viscose solution was fed from a beaker contained in a larger one, with ice granules between them. The resin bed supports were sintered glass and metal sieves of 200 mesh. The various components of the apparatus were mounted on metal stands.

In the design of the apparatus, attention was given to simplicity and flexibility. Any part of the apparatus could be easily dismantled for cleaning, repairs, changing the resin, etc. The design consists of the minimum number of pieces required.

Preparation of the resin. The anion exchange resin used was "Deacidite FF-IP" SRA 61, manufactured by The Permutit Company Ltd., (supplied by BDH Chemicals Ltd.,). The functional groups are quarternary ammonium type I. The resin was supplied in the chloride form and bead size range 14-52 mesh. It has an exchange capacity of 1.2 meq./m1 The beads were elutriated many times to discard the fine particles, to avoid clogging of the bed support. In packing the beds, the columns were half filled with distilled water and the resin beads were allowed to settle freely down the columns. This process was continued until the height of the beds were about 30cm, The resin was allowed to swell for about 30 minutes, then vacuum as applied 106

for a short time to Make the beds more firm. The resin beds were washed with distilled water for sometime. The resin beds represented low and uniform resistance to the eluant flow and were free from air bubbles. It was difficult to calculate the equivalent amount of resin needed (in terms of equivalents) to ion-exchange with the polysulphides because the concentration of polysulphides was not known, and it was considered unnecessary if excess of resin were provided. Conversion of the resin to the hydroxide form. The resin beds were treated with an excess of 4% sodium hydroxide, the flow-rate being kept very low for 30 minutes until the colour of the beds became darker. The beds were then washed with 2-3 litres of distilled water until the pH of the effluent became neutral. By this treatment, the resin was transformed to the hydroxide form. Regeneration of anion exchanged resin. The resin was transferred to a beaker and covered with excess hydrochloric acid of moderate strength. This process had to be carried out in the fume cupboard because of the unpleasant effects of the sulphur compounds generated from the reaction. The reaction was left to proceed until no more hydrogen sulphide could be detected. The excess hydrochloric acid was then removed and the resin washed with distilled water. This process should produce the resin in the chloride form, which can be re-used. Operating _the ion-exchangeapparatus. For optimum results, the operating procedure must be carried out as follows: The resin beds are first washed with excess 107

distilled water, the effluent is received in the washing flask. The viscose solution is then run through. The flow of viscose should be watched carefully through the

columns. When it reaches the end tube, the washing flask is switched off and the solution is diverted to the xanthate flask until the whole viscose solution is run through. Washing of the beds with distilled water is then resumed and the effluent is diverted again to the washing flask.

By this procedure unnecessary dilution was avoided. The concentration of xanthate in the purified form should be more or less equal to its concentration in the viscose form. When sampling is needed, the xanthate solution can be diverted either wholly or partly to the sampling flask. Some viscose solutions prepared earlier were run through the ion-exchange columns as 1.0% solutions under moderate vacuum. The xanthate groups were detected in both the viscose and the purified form by u.v. spectra-

scopy. Careful operation was needed in order to minimize the difference in xanthate concentration of the viscose and the purified form.

4.4 ANALYSIS OF CELLULOSE XANTHATE 4.4.1. Detection and measurement of xanthate Following many investigators (113,114,115,116,117)

the concentration of xanthate groups in dilute solutions of purified sodium cellulose xanthate was measured by u.v. spectrophotometry. The xanthate group has two absorption bands, a primary peak at 300-3 mi and secondary peak at 226 ma.. The molar extinction coefficient (e) ) 108

113) -1 reported by Dux and Phifer( was 15,900 (mole-1 cm , measured on cellulose xanthate which was purified by ion exchange.

In the present work, measurements were made on the purified "NaCX" solutions, In an experiment, 5g of cellulose powder was emulsion xanthated to form 5% viscose solution. A sample of viscose was diluted to 0.5% in distilled water - based on the cellulose content - and ion-exchanged. The cellulose xanthate was further purified by the alcohol precipitation method and dried under vacuum. The pure sodium cellulose xanthate powder was dissolved in distilled water to make up 0.1% and 0.01% solutions. The absorbances of the xanthate groups were measured by a double-beam u.v. spectrophotometer in 1cm quartz cells. The absorption spectra are shown in Fig. 4.5.

There are alternative methods of determining the xanthate content directly from the viscose i.e. without S113,114,116). purification of "NaCX" solution In one method, the absorption wave bands and the molar extinction coefficients of the various components of viscose were predetermined in separate experiments. The absorbances of the viscose sample at the various wave bands were measured and a series of simultaneous equations were formulated according to Beer-Lambert law of light absorption. Solutions of these equations were done with the aid of computer and the concentrations of the various components of the viscose, including xanthate, were established. Table 4.1 shows the viscose components together with their absorption wave bands and their molar extinction 109

Fig .4.5 Absorpilon Spectrum of Cellulose Xanthat,,

2.0

—1.9

0-17,NaCX1 1.8

1.7

1.6

1.5

1.4

1.3

0.01X, NaCX

I 1 I I F I I- 1 I 00 180 200 220 240 260 280 300 320 340 360 380 400 Wavelength mu Table 4.1 : Molar Extinction Coefficients of Viscose Compounds

------,,..___zavelength —,______336 my 303 IN 272 my 250 IN 226 TN 206 TN Compounds

Sod, cellulose xanthate 17508.2 10928.3

Sod.cellulose dixanthogen 6861.64 12396.2

Sod. dithiocarbonate 600. 10500. 3250.

1--Sod. trithiocarbonate 18200. 3440. 12230.

Sod. sulphide 1690. 7730.

Carbon disulphide I 60,000-70,000

0 1 1 1 •

coefficients according to the literature(114)

This method of computation was not used in this work because the purification of the viscose was made possible by the vacuum ion-exchange apparatus. 4.4.2. Detection and measurement of cellulose in dilute solutions For the purpose of estimating the degree of sub- stitution, the cellulose content must be known. The importance of the degree of substitution is in the study of the chemical stability of the sodium cellulose xanthate solutions. Because of the decomposition of cellulose xanthate in 'dilute solutions and some inevitable dilution during the ion-exchange process, the cellulose con- centration is expected to change. Unfortunately, there was no quick method for deter- mining the cellulose content in dilute solutions of

"NaCX". Elmgren(117) determined the cellulose content by a dichromate titration but this method is not suitable- for quick determinations. A simple method was attempted in the laboratory. It was based on the fact that cellulose itself is not soluble in water; therefore decomposition of xanthate groups by dilute acid will result in precipitating the cellulose. The acid, however, should not be concentrated in order to avoid the degradation of cellulose to small segments and the excess should be titrated by sodium hydroxide. In this method, a known volume of cellulose xanthate solution of certain strength (i.e. percent NaCX) is 112 treated with an aqueous solution containing 10% sulphuric acid and 2% magnesium sulphate. When precipitation of cellulose has taken place, the excess acid is titrated with 10% sodium hydroxide, using phenolphaleiln as an indicator. The end-point is noted when the colour changes to pink. The precipitate is filtered on a filter paper of known weight and washed several times with, distilled water and dilute acetic acid (5%). The solids are then dried at 100°C and weighed. The amount of dry cellulose obtained should be equal to that in the starting solution.

An experiment was carried out to check this method. In this experiment, 0.5g of dry sodium cellulose xanthate powder was dissolved in 50m1 distilled water for 30 minutes. The solution was filtered off on a filter paper of known weight. The undissolved portion when dried; it weighed 0.234g, hence the dissolved cellulose xanthate was 0.266g, i.e. 0.532% of the solution. The solution was treated as described above. It was noticed that precipitation of cellulose took a long time; it was left to settle for 12 hours. The dried cellulose, when weighed, was only 0.018g, therefore about 0.249g were not precipitated in this method. A modification of this method was attempted. The dilute solution of "NaCX" was evaporated by an infra-red lamp in a known weightevaporating dish. The dry solids were treated with 100m1 of the 10% sulphuric acid, 2% magnesium sulphate solution. The settling of the precipi- tate was accelerated by decantation, and the decanted 1 1 3

solution was treated twice with excess of the acid solution as before in order to recover more cellulose. The cellulose precipitate was mixed with 50m1 of 0.2% sodium sulphide warm solution (about 60-65 C) for a few minutes. The mixture was centrifuged and the cellulose solids were separated, washed with distilled water and dried by infra-red lamp. It was noticed that the cellulose material was burned in the drying process. That was perhaps due to some acid left with the cellulose precipitate which became too concentrated upon evaporation. Although the idea of this method:seems sound, the method itself failed to give accurate results. It was not pursued further in this work, but future consideration of this method would be desirable.

4.5 PHYSICAL AND CHEMICAL PROPERTIES OF CELLULOSE

XANTHATE Physical characteristics Sodium cellulose xanthate is a polyelectrolyte, the functional groups being negatively charged xanthates 134,136,138) (-0C(S)S-) . According to the literature(791 electrolytes in aqueous medium affect the polymer configuration (or "coiling up") through a screening effect on the functional groups, i.e. reducing the repulsion between charged xanthate groups. Sodium hydroxide, however, has most effect in reducing the viscosity, perhaps due to interaction with hydroxyl groups on the cellulose chain. Thus in 0.2% NaOH solutions, the polymer uncoils and assumes a fairly rod-like configuration, while it coils up to a considerable 114

extent in 6% NaOH solution. This was confirmed from the viscosity measurements(134) on cellulose xanthate samples of different degrees of polymerization (D.P.), i.e. of different numbers of cellulose monomers in the chain segments. The extended length of cellulose xanthate chain of D.P. of 790 in 0.2% NaOH was 22808, and in 6% NaOH was 14408; the total length was 4040.. The total length was computed by multiplying D.P. by the length of the monomer unit (5.150. At a D.P. of 264, the extended length in 0.2% NaOH was 870R and in 6% NaOH was 670, the total length of the polymer chain was 13508. These results demonstrate that the molecules were not completely extended in 0.2% NaOH, indicating that further uncoiling of the cellulose xanthate molecules could occur in very dilute solutions.

The molecular weight of NaCX in dilute NaOH solutions has been estimated from viscosity, light scattering and sedimentation velocity measurements. Das and co-workers (135,137)measured the viscosity of 12 samples of purified cellulose xanthate in 1N sodium hydroxide solutions. The average molecular weights of these samples were found to 4 6 range between 5 x 10 and 1 x 10 . Their studies also showed that the CX chain is substantially stiffer than synthetic polymers but comparable to other cellulose derivatives and that the viscosity of cellulose xanthate in dilute NaOH solutions increases with increasing degree of substitution. Chemical reactions with heavy metal ions

Literature on the reactions of heavy metal cations 115

with cellulose xanthate is rather meagre. In industry, zinc cellulose xanthate is an intermediate in the pre- (141) paration of cellulose thiourethan In this method, viscose is reacted with a Zinc salt to produce a pale yellow precipitate of zinc cellulose xanthate. Cellulose xanthate is reacted with copper salt in the manufacture 142) of copper xanthate fibre( . It has been reported that insoluble metal xanthates were converted into "cellulose

IV" on decomposition by dilute sulphuric acid in hot water (6°c) (140), while sodium cellulose xanthate was converted into "cellulose II". Xanthates of lower molecular weight, e.g. ethyl xanthate, butyl xanthate, etc., are known to form water- insoluble compounds with heavy metal cations(143) such + + ++ ++ ++ ++ as: Au , Ag , Cu{ Pb , Ni Zn , Fe , etc. Cupric sulphate reacts with potas'sium xanthate to form a dark S ++ + S brown cupric xanthate: 2R0-C-SK + Cu + Cu[-S-C-OR]2' but cupric xanthate, being unstable, rapidly decomposes into cuprous xanthate of intense yellow colour and dixanthogen: S S S S [-S-C-OR] + ROCSSCOR . 2 Cu [ -s-C-oR1 2 —0 Cu2 2 In the laboratory, 20m1 of 1% CuSO4 solution was added to 15m1 cellulose xanthate 1% solution; the mixture was shaken for 1 minute, and a large pale yellow precipitate was obtained, the pH being 4.8. This experiment was repeated at much lower concentrations of CuSO4 (10 p.p.m. 0.1%) and NaCx (1-10 p.p.m,), with the aid of . The pale yellow precipitates were obtained in each case.

These precipitates are believed to be cuprous cellulose 116 xanthate.

Cellulose xanthate is susceptible to oxidation, producing cellulose dixanthogen,(143) Oxidation of cellulose xanthate for 24 hours results in a 50-60% conversion to dixanthogen. Cellulose dixanthogen has also been prepared by treating an aqueous solution of sodium cellulose xanthate with diethyl dixanthogen. In technical viscose, however, oxidation is not of great significance because the other sulphur compounds present are more susceptible to oxidation, and therefore, consume a large proportion of oxygen and hinder oxidation of cellulose xanthate.

Decomposition of xanthate groups in NaCX solutions

In order to understand the decomposition of Cell- xanthate, it was necessary to study the reactions during xanthation and ripening of viscose. The following (79,1231 reactions are believed to occur many of them are reversible. Rcell denotes the cellulose macromolecule

A - During xanthation = NaOH = TR ONa (sod. cellulose) 1 - Rcell cell 2 - R ONa+C52 = Rcell-O-C(=S)-SNa (cellulose xanthate) cell CO +3H 0 3 - 3CS2 + 6NaOH = 2Na2CS3(sod. trithiocarbonate)+Na2 3 2 S (sod.monothiocarbonate) 4 - Rcell-0-C(=S)-SNa + 2NaOH = Naj 2CO2 R + Na2CS3 cell - NaSH 5 NaOH = Na2CO2S = Na2CO3 B - During ripening of viscose (deomposition) cell -0-C(=S)-SH + NaOH 6 - Rcell-0-C(=S)-SNa + H2O = R 2O = HOCSSH R 7 - Rcell-0-C(=S)-SH H cell [r H2O CS2 117

8 - -0-C(=S)-SNa + 2H Rcell 20 = NaOH + CS2 + H2O + Rcell 9 - 5CS + 12NaOH = Na S + 2Na CO + 2NaCS + 6H 0 2 2 2 3 3 2 10- CS2 + Na2S = Na2CS3 11- C$2 + H2O = H2S + COS 12- CS + 2NaHS = Na CS + H S 2 2 3 2 13- 3NaOH + Na 2CS3 = 3NaHS + Na2CO3 14- NaCS + 3H 0 = Na CO + 3H S 3 2 2 3 2 15- NaCS + 2H 0 = H CS + 2NaOH 3 2 2 3 H2S + CS2 From these equations the following comments can be made: a) Reaction (5), by consuming Na2CO2S (which is a product of reaction (4) ) increases the tendency forreaction (4) to proceed to the right i.e., decomposition of NaCX by NaOH, while reaction (3) decreases the tendency of both reaction (4) and (5) to proceed to the right.

Therefore it helps to limit the decomposition of NaCX during xanthation. b) The main causes of decomposition of cellulose xanthate are hydrolysis and rise of temperature. From (8), NaCX decomposes to CS2, NaOH, H2O and solid cellulose. This suggests increasing [NaOH] and [CS2] in order to restrict the decomposition of NaCX. However, it is more logical to remove the H2O altogether when storing NaCX for long periods, i.e., by storing dry NaCX al(118) powder. Yamada et stored NaCX dry powder

treated with CaO for six months without losing its

solubility in water. 1 1 8

c) The idea of increasing [C52] and [NaOH] in order to

decrease the tendency of decomposition (reaction 8) must take into consideration the results of reaction (9), for increasing these reactants will help to

increase the amounts of sodium sulphides and thio- carbonates. The effect of temperature should also be taken into account. Elmgren(117) studied the dexanthation rate of

ripened viscose at 25, 18, 0 and -14°C and found a relation between the gross dexanthation rate constant k' (in units 1N hr ) and the absolute temperature T as follows: log k' = 14.6-

5000/T. At -14°C, 0.05% of the xanthate groups decomposed per day or 25% decomposed over a one year period. He found no decomposition of viscose at -65°C. Lyselius and Samuelson (11 ) 9- investigated the actual rate of decomposition by ripening the viscose in the presence of anion exchange resin which eliminates the rexanthation. They concluded that the decomposition rate is the sum of two first-order reactions occurring simultaneously, a fast one which is ascribed to the decomposition of 2,3-xanthate, and a slower one which is ascribed to the decomposition of 6-xanthate. They also studied the influence of NaOH concentration upon the rate of decomposition and concluded that the net rate of ripening is only slightly affected by the NaOH (122) , concentration (see also ). Theycalculated the activation energies for the actual dexanthation rates of 6- and 2,3-xanthates as 19 kcal/mole and 20 kcal/mole respectively. Their work has been confirmed by Dunbrant (120) and Samuelson . The rate of dexanthation of viscose

119

(121), increases in dilute solutions probably because of a lower rate of rexanthation; at very low cellulose con- centration, the rexanthation can be eliminated completely. The rate of dexanthation was found to be independent of the cellulose concentration. The role of pH has also (124,125,126,127,128,130,131) been studied • As the de- composition of cellulose xanthate passes through the r 1 formation of xanthic acid (reactions 6 8c7 ), [H]be+ comes important. In general, the decomposition rate increases with increasing pir +,j. Lissfelt (129)suggested a formula for the de-composition rate of xanthate in aqueous buffer solutions, namely:- -d[X]idt = k[X] [H+] + k2 [X] [1120]. He claims that in the pH range 2-6 the first term dominates, while above pH 7 the second term dominates and the reaction is substantially independent of pH. He calculated the activation energies for the decomposition of cellulose xanthate at pH 4 and 30°C as 19 kcal per (119,132) mole, which is in agreement with previous work He also believes that cellulose xanthate does not decompose via molecular xanthic acid. The decomposition and oxidation of xanthate was found to be enhanced in the presence of ions of those metals that form insoluble (133). complexes with xanthates Therefore the rate of dexanthation in a xanthate solution containing a suspension of solid metal xanthate is much greater than its normal rate of reaction in a homogeneous solution.

Conclusions. From the above studies, it can be concluded that storing purified or unpurified NaCX solutions is possible under conditions of low temperature, 120

and high [NaOH] - about 6N. Air must be excluded as well as heavy metallic ions. Dilute solutions will decompose fairly quickly: therefore it is best to prepare them

freshly when needed from a stock of concentrated NaCX solution stored at low temperatures. Storing dry NaCX powder is more convenient still.

Oxidative degradation of the cellulose chain Chain degradation which accompanies some types of cellulose oxidation is not the result of direct scission

of the molecular chain(79), but is the result of the formation of chemically labile groups in the molecule, which are sensitive to alkaline cleavage. Periodic acid oxidizes the glycol groups in the 2,3-position to the corresponding aldehydes with a carbon-carbon bond cleavage. In this case the hydrogen on the« -carbon is removed by the base; this is followed by an electron shift to form a double bond between cc - and p- carbon atoms with simultaneous carbon-oxygen scission. The cleavage which occurs results

in a chain break in the molecule and a corresponding increase in fluidity of the solution. Oxidation of alkali cellulose by oxygen is initiated according to the following reactions: R CHO + 02 Rcell CO. + H00. cell ---* R CO.+ R CO(00.) cell 02 cell R cell CO(00) + Rcell H —40 Rcell CO(00H) +.Rcell

The net results of these oxidations are the degradation of chains to shorter segments. This has been confirmed by

electron microscopy and viscosity studies. In the present work, the fall in viscosity of dilute solutions of purified 121

NaCX was observed. Furthermore, such aged solutions formed very weak flocs or none at all. 4.6 FLOCCULATION PROPERTIES OF CELLULOSE XANTHATE Selective flocculation of sulphide minerals

The flocculation properties of cellulose xanthate were investigated on two classes of mineral; the "common" valuable minerals e.g., galena, chalcopyrite, sphalerite and pyrite, and the "common" gangue minerals like quartz, calcite and feldspar. The minerals were dry4-ground separately in an agate mill to particle size below 400 mesh

(B.S). They were stored in sealed polythene bags during the period of investigations. No purification processes were attempted on these minerals. The cellulose xanthate flocculants tested included sodium cellulose xanthate (NaCX), sodium carboxy methyl cellulose xanthate (NaCMCX), and sodium methyl cellulose xanthate (NaMCX). They were prepared by the emulsion xanthation method as dry powders. Fresh solutions of 1% of these polymers were ion-exchanged at 0°C to remove low molecular weight polysulphides. Solutions of 0.1% of these polymers were used in the flocculation experiments. The purified polymer solutions were stored at low temperature to avoid decomposition of xanthate.

The procedure of the flocculation experiments was as follows: A 250m1 suspension of the mineral was made as 1.0% solids at pH 10. The suspension was transferred to a 250m1 measuring cylinder and kept standing for 6 minutes. The suspension was decanted into a beaker and the coarse settled particles were discarded. The pH was readjusted 122

and the flocculant was added while stirring by a magnetic

stirrer at high rate for 30 seconds. The shear rate was dropped to low intensity and continued for 1.5 minutes. The suspension was transferred again to the 250m1 measuring

cylinder and the flocs were helped to grow by slow rotation of the cylinder on an angle for a few minutes. Distilled water was used throughout the experiments. The qualitative results of these experiments are

shown on Table 4.2 • The following notations were used to discribe the flocculation effects: = good flocculation, leaving clear supernatant = partial flocculation, leaving turbid supernatant = slight flocculation no flocculation Description of the flocculation process and the

size of the flocs were expressed as follows: f = fast flocculation process in which the flocs were formed and settled rapidly, i.e., within

1 minute of adding the flocculant. sl = slow flocculation where the flocs were formed and settled in a relatively longer period.

1 = large floc size small floc size. Discussion of the results The experiments were carried out at pH 10 ± 0.1 in order to avoid the effect of the electrostatic forces on flocculation (particles and polymer all being negatively charged). The cellulose xanthate flocculant

is known to alquire negative charge in the alkaline region 123

Table 4.2

Dose Minerals 1 p.p.m. 5 p.p.m. 10 p.p.m. Flocculants NaCX - + s ± f.s galena NaMCX - - NaCMCX - -

NaCX 7 s ± f.s. 2' f.l. chalcopyrite NaMCX + s + s ±+ f.s. NaCMCX

NaCX + f -+ f.l. + f.l. pyrite NaMCX + f -+ f - f.l. NaCMCX 7 s 7 s —-1- s

NaCX - + s ± f.s. sphalerite NaMCX - + s -F s NaCMCX

NaCX - - - quartz NaMCX - - - NaCMCX - - -

NaCX - - - calcite NaMCX - - - NaCMCX - - -

NaCX - - - feldspar NaMCX - - - NaCMCX - - - 1214

of pH. As most of•the minerals also acquire electrical charge at pH 10, any flocculation effect due to electro- static attraction between the negative polymer and the positive mineral surface is eliminated. The flocculation effects at pH 10 should therefore be due only to chemical bonding between the polymer and the mineral surface; however, the flocculation of the sphalerite suspension was carried out at pH 6.7 because of the instability of this suspension at pH 9-10, where coagulation of the particles took place. This instability phenomenon was noticed only on suspensions freshly prepared from the sphalerite powder and not on the 'aged' suspensions. The phenomenon was dependent on pH and reversible.

The results on Table 4.2 indicate definitely that cellulose xanthate polymers produce selective flocculation effects on the sulphides minerals, while no flocculation was detected. on the common gangue minerals. The results also suggest that plain sodium cellulose xanthate (NaCX) was more effective than sodium carboxymethyl cellulose xanthate (NaCMCX) and sodium methyl cellulose xanthate (NaMCX). Its superiority was probably due to higher molecular weight, rather than different chemical composition. Floceulation of chrysocolla with cellulose xanthate 221ining the pH of flocculation. When the pH of a chrysocolla suspension was raised from 4 to 9 or 10, an immediate coagulation of the particles took place.

The coagulation disappeared when the pH was lowered to pH4, but reappeared on raising the pH to 9 again. This coagulation was reversible and believed to be due to the 125

formation of cupric hydroxide. It has been shown earlier that chrysocolla releases curpric ions in acidic solutions in amounts depending on the concentration of electrolytes. These ions form cupric hydroxide at high pH. The zero point of charge (z.p.c.) of cupric hydroxide, as mentioned in Chapter 2 , lies in the pH range 7.7- 10, depending on the concentration of electrolytes in solution. It has also been stated that chrysocolla particles acquire a negative charge over the pH range 4-12 in distilled water. Therefore, to separate the actions of coagulation and possible effects of charge neutralization from the real effects of cellulose xanthate, the pH of the suspension must not be more than 7 (or alternatively must be above 10). The pH chosen for the flocculation experiments was pH 7. Flocculation experiments and results

Because of the high negative charge on both the mineral surface and the sodium cellulose xanthate, sodium chloride was used to lower the repulsion forces so that the polymer can reach the particles surfaces. This was found necessary. About lg of chrysocolla particles of size below 30im was dispersed in 250m1 distilled water containing 1% sodium chloride in a 400m1 beaker at pH7. The suspension was transferred to a 250m1 measuring cylinder and kept standing for 5 minutes. The suspension was decanted back into the beaker and the settled particles were rejected. The pH of the suspension was readjusted carefully to pH 7, and the flocculant was added while stiring at high shear 126

rate by a magnetic stirrer for 30 seconds. The flocculation process was continued at low shear rate for another 1.5 minutes; thereafter the suspension was transferred to the 250m1 cylinder and slowly rotated at an angle for

3-4 minutes. Small flocs of chrysocolla particles were noticed when the concentration of cellulose xanthate in the suspension was raised from 1 p.p.m. to 5 p.p.m. More flocculation and bigger flocs were obtained at 10 p.p.m. CX; much of the suspension was flocculated in about 12 minutes. At a concentration of 20 p.p.m. CX, most of the suspension was flocculated with big flocs in about 10 minutes, and the supernatant was practically clear. The above experiment was repeated on chrysocolla suspension of particles size below 18im, and the cellulose xanthate was added at a dose of 20 p.p.m. in one addition. Flocculation of the suspension took place and the super- natant was clear after 14 minutes. The difference between this experiment and that on coarser size above was that the formation of the flocs was initially slow. Effect of sodium chloride concentration on flocculation. In this experiment, the concentration of (NaC1) was raised from 1% to 2.6% (; 1%N .1. ) in the aqueous medium. The flocculation experiment was repeated on the minus 18im particles and the CX flocculant was run in, with incremental additions. Flocculation was detected at 5 p.p.m. CX dose and was found to increase with the CX concentration. At 20 p.p.m. CX almost all the particles were flocculated and the supernatant was nearly clear after 14 minutes. 127

When the CX concentration was raised to 30 p.p.m. the rate of flocculation was increased and flocculation was complete after 14 minutes. The results of this experiment were not significantly different from the previous experiments where the NaC1 content was 1%. This experiment suggests that the high content of NaC1 did not enhance flocculation. Experiments without sodium chloride were performed at different pH values, namely, 4, 7, 9 and 10 on fresh suspensions of chrysocolla. Different cellulose xanthates were used in the purified and the unpurified states at varying concentrations from 1-10 p.p.m. No flocculation was produced in any of these experiments, probably because of repulsion between the negatively charged surfaces of the particles and the polymer. Therefore the presence of sodium chloride (or similar electrolyte) was essential for flocculation. When attempted on a different sample of chrysocolla of poor quality, these flocculation experiments -could hot be easily reproduced. Flocculation of sul hidized chr socolla A 250m1 dilute suspension of fine particles of chrysocolla containing 1% NaC1 at pH7 was prepared. The suspension was transferred to a measuring cylinder and left for 10 minutes. The settled coarse particles were rejected after decanting off the suspension into a 400m1 beaker. Sodium sulphide (Na2S) was added to the suspension at a concentration of 80 p.p.m. while stirring at high 128

shear-rate for 5 minutes. Purified cellulose xanthate was mixed with the suspension at concentrations of 5, 10

and 20 p.p.m. The pH was kept at 7 throughout the

experiment. Strong flocculation took place. The floc size increased with the concentration of CX. At 20 p.p.m., the flocculation was complete, leaving a clear supernatant. This experiment was repeated four times, and the results were confirmed.

Again presence of NaC1 was found to be essential; without it flocculation was not obtained. Thus the adsorption of negatively charged CX molecules was enhanced, resulting in strong flocs. S NaC1 and NaCX on flocculation of The effects of Na2 chrysocolla were investigated qualitatively at different

doses of Na2S (40-200 p.p.m.), NaC1 (0.3 - 1.0%), and NaCX (5-20 p.p.m.) In general, some flocculation was obtained at low doses, but more complete and rapid flocculation at higher doses. Selective flocculation of chr socolla from uartz.

When dilute quartz suspensions were treated with

80 p.p.m. Na9S, and 1% NaC1 at pH 7, no flocculation was detected with CX even at 25 p.p.m. These were the conditions where chrysocolla was strongly flocculated. A 250 ml dilute suspension (,\/ 2% solids) of a mixture of quartz and chrysocolla containing 1% NaC1 was treated with 80 p.p.m., and the flocculation procedure

was continued as before. Only chrysocolla flocculated, while the quartz

remained suspended. The experiment was reproduced

several times. 129

At high concentration of NaC1 t >1%) some quartz

particles were noted with the sediment, but this was probably due to partial coagulation of some quartz particles. Therefore, NaC1 should be used at concentrations lower than or equal to 1%.

4.7. CONCLUSIONS Sodium cellulose xanthate has proved to be a selective flocculant. It has flocculation effects on heavy metal sulphides such as galena, sphalerite, chalcopyrite and pyrite. Flocculation of these minerals can be improved by finding the optimum conditions e.g., pH, electrolyte concentrations and adequate mixing and dispersion. Cellulose xanthate does not flocculate quartz, clays, calcite and feldspar. Therefore, selective flocculation of heavy metal sulphides from these common gangue minerals is possible in principle. When chrysocolla was flocculated by cellulose xanthate, sodium. chloride was found necessary to reduce the repulsion between the negatively charged xanthate and chrysocolla particles. Flocculation of sulphidized chrysocolla by cellulose xanthate was also achieved. Selective flocculation of sulphidized chrysocolla from quartz was shown possible.

Although cellulose xanthate has shown selective flocculation properties and is simple to prepare cheaply, it has two disadvantages: a) the molecular weight is normally lower than that desirable for formation of strong flocs, b) there are difficulties in preparing a stable 130

form which could survive long storage and transportation. The lowering of the molecular weight of cellulose xanthate was shown to result from degradation of the cellulose chain by oxidation. Therefore, air should be excluded from the polymer during preparation and storage. To minimize decomposition of xanthate, the polymer is best kept in dry form at low temperature; fresh solutions could be prepared when needed.

Xanthation of methyl cellulose, carboxymethyl cellulose and hydroxy propyl methyl cellulose was established and uniform solutions were achieved. The flocculation properties were similar to ordinary sodium cellulose xanthate, though the molecular weights of methyl cellulose xanthate and sodium carboxy methyl cellulose xanthates were rather low, which resulted in weak flocculation. 131

CHAPTER 5 OTHER SELECTIVE FLOCCULANTS CONTAINING SULPHUR

5.1 POLYVINYL ALCOHOL XANTHATE 5.1.1 Introduction

Polyvinyl alcohol xanthate (PVAX) may be formed by reacting polyvinyl alcohol (PVA) with sodium hydroxide and carbon disulphide according to the equation:

-'CH -CH-CH -CH" + NaOH + CS ---4----p e■CH -CH-CH -CH,-.." . 2 1 2 1 2 t 2 1 OH OH 0 OH + C-SNa H S In this reaction some or most of the secondary hydroxyl

groups in the polyvinyl alcohol are replaced by xanthate groups. The reaction is allowed to proceed in the emulsion xanthation process as described in detail in section 5.1.2. It is expected that PVAX thus produced would be stronger and more selective flocculant for heavy metal minerals than the unxanthated PVA. The xanthate group,as shown in chapter 3 and 4, tends to selectively form strong compounds with heavy metals as against earth alkaline metals, whereas the alcoholic OH-groups of PVA are only weakly acidic and not particularly selective. Although PVA was found to adsorb strongly on clay minerals,

(namely, montmorillonite, illite and kaolinite,) the bond between the polymer and solid surfaces is not a strong

one. In addition to clay minerals, Greenland(144) examined

the adsorption of polyvinyl alcohol on a range of alumin- ium oxides and hydroxides as well as silica. He found

that apart from clay minerals, none of the other minerals 1:32 adsorbed any measurable amount of PVA. Ignited silica (heated at 80000 for 4hrs.) however, adsorbed the polymer strongly whereas hydrated silica did not. This adsorption phenomenon was explained in.terms of hydrophobic associa- tion and hydrogen bonding. Thus the adsorption of PVA on clays and ignited silica results in a large positive entropy change owing to the displacement of a large number of water-molecules by each polymer molecule adsorbed (similar to the chelate effect described in chapter 3). The failure of PVA to adsorb at the silanol (Si-OH) and aluminol (Al-OH) surfaces was probably due to the fact that water is strongly hydrogen bonded to these surfaces, while it is weakly held at the siloxane surfaces (Si-0) of the ignited silica and clay minerals. Polyvinyl alcohol has been found to have flocculation (145) properties and examples of its use as a flocculant (17,145-150) were reported by several authors . For instance Fleer (14'6 )achieved an efficient flocculation. of silver iodide with PVA in the presence of a salt, but the flocs were small in size. Kuzkin and Nebera(17) tested the action of PVA, amongst other polymers, for flocculation; they tacitly concluded that although flocculation was not strong in some cases, PVA could serve as a good flocculant in neutral and alkaline media. The small size of these flocs could be explained in terms of the molecular weight of the polymer. For most commercial polyvinylalcohols, the average molecular weight varies between 25,000 and

300,000, depending on the initial polyvinylacetate(91) from which the PVA is prepared. 133

Polyvinylalcohol is a non-ionic, water-soluble polymer. It is prepared commercially from polyvinyl- acetate (PVAc) because vinylalcohol monomer transforms into acetaldehyde(151). Thus polyvinylacetate is hydrolyzed by treating an alcoholic solution with aqueous acid or alkali. The polyvinylalcohol thus obtained usually contains some acetate groups if prepared by alkaline hydrolysis, or traces of acid which are difficult

to remove and may lead to instability of the polymer, if

prepared by acid hydrolysis. A more efficient method for preparing PVA is by the alcoholysis of polyvinylacetate. In this method, PVAc is treated with methanol in the presence of sodium methoxide as catalyst. Detailed description of preparation, properties, and applications (91,151-154) of polyvinylalcohol are to be found in the literature Because of the reactivity of the secondary hydroxyl (91) groups of PVA, many derivatives have been prepared Among other commercial derivatives are the polyvinylacetals which form by treating PVA with aldehydes or ketones, the acid sulphates (suggested as ion-exchange resins), the hydroxylethyl ethers and the thiols, which find use in the isolation of metals such as silver, mercury and platinum by the formation of insoluble mercaptides. 5.1.2 Preparation of221/zial lathatt LA2frimental: Polyvinylalcohol used in this work was supplied by BDH Chemicals Ltd., (Poole, England) and was stated by them to have an average molecular weight of 125,000. Polyvinylalcohol xanthate (PVAX) was prepared in the laboratory by the emulsion xanthation 134

method. In this procedure, 2g of PVA was dissolved in 40 ml of 18% aqueous NaOH. The solution was treated with 10 ml of CS 2 and the mixture was shaken for 6 hrs. at 27°C. At the end of xanthation, the product was diluted with 50 ml distilled water, and shaking was continued until homogeneity was attained. Excess CS2 was removed by applying a vacuum. The colour of the product was orange to deep carrot-red. Purification of PVAX: During the xanthation process, low molecular weight polysulphides were produced as by- products (indicated by the carrot-red colour). These by-products were removed from the PVAX solution by the ion-exchange method, using the vacuum ion-exchange multi- column apparatus described in Chapter 4. Thus the 2% xanthation product was run through the apparatus, following the same procedure in section 4.3.3, to avoid unnecessary dilution. The ion-exchanged PVAX solution was clear, and slightly pale yellow in colour. Some frothing was noticed inside the tubes of the apparatus, indicating the partially hydrophobic character of the polymer. The presence of xanthate groups in the ion-exchanged solution was detected by ultra-violet spectroscopy at wave length band = 300 - 303 my. 5.1.3 Flocculation ro erties of PVAX The xanthate groups were shown earlier not to react 2+ 2+ with metals like Ca , Mg , Al3+ and silica; therefore the PVAX is not expected to adsorb onto (and subsequently flocculate) minerals such as calcite, dolomite, feldspar, 135

clays, quartz and aluminium oxides. It has been already (14-4)t shown by Greenland hat PVA does not adsorb to any appreciable extent onto hydrated silica and various aluminium oxides and hydroxides; therefore PVAX is not likely to adsorb on these minerals either. Galena, chalcopyrite and other heavy metal sulphides, on the other hand, are expected to react with xanthate leading to the adsorption of the polymer and the subsequent

flocculation. Thus PVAX should act as a selective flocculant for heavy metal bearing minerals and act in a similar way to cellulose xanthate (described in Chapter

Li). Another factor affecting flocculation is the con- figuration of the polymer. It has been reported(17) that X-ray investigation of PVA confirmed its crystalline structure and plane zig-zag configuration of carbon chain. Because of the negative charge of xanthate groups, their presence in the polymer structure should extend its linear configuration, thus improving bridging and

flocculation effectiveness of the polymer. Flocculation_procedure: A250 ml suspension of fine galena particles was made using a magnetic stirrer. It

was transferred and kept standing in a 250 ml measuring cylinder for 5 min., then the suspension was decanted back into a beaker and the settled particles were rejected. The suspension was treated with the polymer while stirring at a moderate shear-rate for about 1 min., and at low shear-rate for another 1 min. The suspension was then transferred to the 250 ml cylinder, where gentle rotation of the inclined cyclinder was applied for 136 a few minutes. The pH was maintained throughout the experiment at 7. Results: Flocculation of galena particles was noted visually at 5 p.p.m., increasing at higher doses of the polymer. The flocs were small in size and some particles were noted to adhere to the water surface; this was probably due to the residual acetate groups in the PVAX polymer which makes it (like the PVA) weakly surface- active.

In a similar experiment, the effect of polyvinyl- alcohol (PVA) on galena was studied, following the same procedure and conditions as above. When the concentration of PVA in the galena suspension was 1 p.p.m., no flocculation occurred at all. Then the dosage of PVA was progressively increased by increments of 1 p.p.m. up to 5 p.p.m., thereafter it was adjusted to 10, 15 and 20 p.p.m., consecutively. The polymer was added to the cylinder, where dispersion was carried out by shaking and turning the cylinder end over end and after each addition followed by rotation for a few minutes.

Results: There was no flocculation up to 5 p.p.m.; however, only slight flocculation occurred at 10 p.p.m., which was marginally increased at 15 and 20 p.p.m. On the other hand, the rate of settling of the fine particles was roughly inversely proportional to PVA concentration, so that the particles remained suspended for a much.. longer period at 20 p.p.m., than that at p.p.m.,or 1 p.p.m. This experiment clearly indicated that PVA had adsorbed on galena particles, yet it did not effect 1 37

strong flocculation, whilst PVAX flocculated galena at

5 p.p.m. Thus the introduction of xanthate groups to

PVA, has markedly improved its flocculation effectiveness, which might be due to the stronger bonding between xanthate and galena and the extended configuration of PVAX as a result of the negative charge of xanthates. Conclusions: In spite of the encouraging features

of PVAX, no more flocculation experiments were carried

out on other minerals to study its. selectivity. It was foreseen that this polymer, like cellulose xanthate, would suffer from two main disadvantages: (a) Decomposition of

xanthate groups, (b) The relatively low molecular weight of the polymer. Most efficient, modern flocculants have an average molecular weight of 106. However, if polyvinylalcohol of high molecular weight could be obtained, and the PVAX could be stored dry without losing the xanthate groups, polyvinylalcohol xanthate would prove a potentially selective flocculant. Polyvinylalcohol has an advantage over cellulose raw materials in producing readily uniform xanthation, and having a -C-C-C- chain, which is much less susceptible to hydrolytic scission than is the carbohydrate chain of cellulose. 138

5.2 POLYACRYLAMIDE-DITHIOCARBAMATE 5.2.1 Comp1tiLLITLLLLTEMEIEILU. By polyacrylamide-dithiocarbamate (PAD) is meant ordinary polyacrylamide with some dithiocarbamate groups introduced into it. The dithiocarbamate group, as mentioned in Chapter 3, is a sulphur donor ligand which does not react with class A-cations i.e., mainly earth alkaline cations. On the other hand, dithiocarbamate

tends to form strong compounds with heavy metal ions, and especially with copper ions. (Sodium diethyldithio- carbamate is a well known colorimetric reagent for copper). Thus by incorporating these groups into the polyacrylamide chain, a markedly improved selectivity should be attained. PAD was kindly prepared and supplied by B.T.I. Chemicals Co. Ltd., (Bradford, England).

PAD was anticipated to be an anionic flocculant, because of dithiocarbamate groups. The polymer was, of

course, water-soluble and of high molecular weight, having been prepared from ordinary PAM flocculant. It was slightly yellow in colour. Its dilute aqueous solutions (e.g., 0.1%) decomposed more slowly than cellulose xanthate; some of the solutions decomposed extensively only after 5-6 weeks. No measurements were made on the decomposition products nor the decomposition rate. The decomposition was indicated by formation of precipitates.

No purification was attempted on this polymer, and it was used in flocculation as supplied. 139

5.2.2 Flocculation effects on mineral sus ensions

Exploratory experiments: The unpurified PAD polymer was tested on suspensions of approximately 0.4% solids of the following minerals: malachite, chrysocolla, chalcopyrite, galena, calcite, feldspar and quartz. The experiments were

run in the same way as those described earlier in section

5.1.3, where the pH was kept constant at 7 and the polymer concentration in the suspensions was 1 p.p.m. All but

quartz were found to be flocculated by this polymer. In order to ascertain the difference between PAD and unmodified polyacrylamide, two partially hydrolysed polyacrylamides, namely, A130 and A80 manufactured by B.T.I. Co., were used. These polymers, like PAD, are characterized by their anionic functional groups. Thus, flocculants A130 and A80 were tested on suspensions of

the following minerals: chrysocolla, chalcopyrite, galena, calcite, feldspar and quartz. The experiments were run in the same manner as before at pH 7 and a polymer dose of 1 p.p.m. Except for quartz, flocculation was noted in all suspensions. In comparison with PAD flocculent, the only noticeable difference was that PAD gave stronger flocs (larger size) than A130 and A80.

Selectivity of PAD: Selectivity of the flocculent could arise from its differential adsorption strength on the various minerals. For example, the polymer may strongly adsorb on to copper minerals but be weakly held on calcite or feldspar and in both cases it would cause flocculation. But by introducing another ligand strong enough to compete with the polymer for calcite and feldspar, 1240

the flocculant would then be free to adsorb on copper minerals only. Thus, the inhibition effect of this ligand on gangue minerals together with the strong affinity of the polymer groups to bind with copper minerals, could result in selective flocculation of the latter. In this work both 'Calgon' (sodium hexa- metaphosphate) and "Dispex N40", a sodium salt of a synthetic polycarboxylic acid supplied by Allied

Colloids Co. Ltd., were used as flocculation inhibitors for gangue minerals. The main advantage of PAD over A130 and A80 was found to be its capability of forming strong linkages with copper minerals in the presence of Dispex N40 and Calgon. Thus in a series of experiments, A130 failed to flocculate chrysocolla at pH 7 in the presence of 0.2% and 1.0% Dispex in suspension, while PAD caused flocculation at 1 p.p.m. Similarly malachite was flocculated by PAD in the presence of 0.5% Dispex, while A130 had no effect; and a mixture of galena, calcite, feldspar and quartz could be inhibited by 1 % Dispex from flocculation with A130 but not with PAD. Another set of experiments was carried out to explore the selective action of PAD on dilute suspensions, following the same procedure as before at pH 7 using Calgon and

Dispex as inhibitors for gangue minerals. Thus calcite was inhibited from flocculation with PAD by addition of

50 p.p.m. Calgon and feldspar was not flocculated in the presence of 100 p.p.m. Calgon and 1% Dispex. Galena was also depressed by 100 p.p.m. Calgon, while chrysocolla and malachite were flocculated in the presence of 100 p.p.m. Calgon and 1% Dispex. The qualitative results of these experiments are summarized in Table 5.1. 5.2.3 Selective flocculation of copper minerals ...... ftTIampts2.211raa:E The selectivity of PAD to copper minerals (i.e. chrysocolla and malachite) was further investigated at pH 10.5. It was assumed that at this pH both the minerals and the polymer groups acquire a negative charge.

Adsorption of PAD would then be due to chemical bonding with Cu-sites on the mineral surfaces. In one experiment, a mixed suspension of calcite, feldspar and quartz was treated with 100 p.p.m. Calgon at pH 10.5. The suspension was stirred by a magnetic stirrer for 5 min. and Dispex solution was added at a dose of 1% while stirring was continued for further 5 min. The treated suspension was transferred into a 250 ml cylinder and was kept for about 5 min. in order to reject coarse particles. Thereafter, the suspension was decanted back into a beaker where 2 p.p.m., PAD polymer was added during stirring at high shear-rate for 1 min., followed by low shearing for 1 min. and gentle rotation in a cylinder fora fewminutes. No flocculation occuried even after 60 minutes. In another experiment, a mixed suspension of chryso- colla malachite, calcite, feldspar and quartz was treated in the same way as in the preceding experiment, that is in the presence of 100 p.p.m. Calgon and 1% Dispex at pH 10.5.

The green flocs of chrysocolia and malachite were noticed 142

Table 5.1 : Flocculation effects of PAD compared with A130 at H7 on mineral sus ions of > 0.4° solids

Exp. Reagents A130 PAD No. Minerals 1 p.p.m. 1 p.p.m.

1 chrysocolla + + - « +a; -+ b,c

2 malachite + + - b -+ b,c

3' chalcopyrite + +

4 quartz - -

5 feldspar + + - c * - c,p

6 calcite + + - c * - «

7 galena + + - c * - p

8 mixture ,of + + galena, quartz, - c -+ c calcite and feldspar

Notations: + good flocculation no flocculation partial flocculation deduced from exp. No. 8 Dispex NLIO a = 0.2%, b = 0.5%, c = 1% Calgon « = 50 p,p.m., p = 100 p.p.m. 143

at the bottom of the cylinder, at a PAD dose of 1 p.p.m., and complete flocculation was formed at higher doses. No

white flocs were noticed even at 15 p.p.m. PAD and the white suspension remained stable. Thus copper minerals were selectively flocculated, while calcite, feldspar and

quartz particles remained suspended. The experiment was reproducible.

Selective flocculation of malachite and chrysocolla

from mixed suspensions containing galena was carried out. The aim of this attempt was to see whether PAD is especially selective to copper minerals. Thus, a mixture of galena, feldspar, calcite and quartz was inhibited from flocculation with PAD in the presence of 100 p.p.m.,•Calgon and 1% Dispex at pH 10.5, following the same procedure as above. When another suspension containing chrysocolla and malachite, in addition to galena, feldspar, calcite and quartz, was treated with 2 p.p.m. PAD under the same conditions of the preceding experiment, galena was noticed to flocculate with the copper minerals. It was decided to investigate the possibility of depressing flocculation of galena from mixtures with copper minerals.

5.2.4 Inhibition of flocculation of galena in mixtures with copper minerals

Effect of Na2S and NaF: These two compounds were used so that the sulphide ion would precipitate any mobile 2+ 2+ Cu or Pb released by copper minerals and galena, and 2+ q+ the fluoride ion would complex Ca , Al' and K from feldspar and calcite, thereby avoiding random activation.

Thus a mixed suspension containing galena, chrysocolla 144

malachite, calcitelfeldspar and quartz was treated with 100 p.p.m. Na2S at pH 10.5 and stirred at high shearing for 5 min. An addition of 100 p.p.m. NaF was made and stirring was continued for 5 min. The suspension was transferred to a cylinder and was kept for 10 min. in order to get rid of coarse particles. 1 p.p.m. of PAD solution was then added to the decanted suspension, while stirring at high shear rate for 1 min. follwed by 1 min. low shearing and a few minutes gentle rotation in a cylinder. The experiment was also repeated, using 200 S and NaF. In both experiments galena p.p.m. of each Na2 was noticed to flocculate with chrysocolla and malachite. Thus Na S and NaF failed to inhibit flocculation of 2 galena. Effect of potassium dichromate: A series of experiments were made on galena suspensions, using potassium dichromate at doses of 10, 20, 50, 100 and 200 p.p.m. Thus 50 ml suspensions were shaken by hand (including turning the tubes end over end) for a few at minutes before and after the addition of K2Cr207 pH 10.5. The suspensions were then treated with 1 p.p.m. PAD flocculant and shaking was continued for 1 min. followed by rotation of tubes at an angle for a few minutes. Flocculation took place at all doses.

In another experiment, a suspension containing galena, chrysocolla, and malachite was treated with

200 p.p.m. K2Cr207 at pH 10.5, while stirring for 3 min. by magnetic stirrer. The coarse particles were rejected after keeping the suspension in a cylinder for about 3 145 min. PAD solution was added to the suspension at a dose of 1 p.p.m. while stirring at high shear rate for 1 min. and at low shearing for 1 min. further. Flocculation of the mixture took place but was apparently unselective. Thus K2 2O Cr 7 also failed to depress galena from floccula- tion. This matter was not pursued any further. 5.2.5 Discussion and conclusions According to the literature mentioned in Chapter 3 dithiocarbamate groups do not add on earth alkaline metals. Therefore PAD should adsorb strongly on minerals containing transition and B-cations, especially copper 2+ minerals, because Cu ions form very strong compounds with dithiocarbamate groups, but also lead minerals. Flocculation of calcite and feldspar was somewhat unexpected with PAD, but this might have been due to some sites on the polymer which were not converted to dithiocarbamate. Since not much information was available about the analysis and structure of the polymer this explanation remains speculative. In spite of the flocculation effects of PAD on calcite and feldspar, selective flocculation of copper minerals, namely malachite and chrysocolla, from mixed suspensions was achieved. PAD could there- fore be considered a selective flocculant. The experimental procedure adopted in this work was mostly arbitrary and results were qualitative since the main concern was the study of the basic principles of the chemistry of the flocculation process. 1116

Little knowledge is available about the decomposition behaviour of the polymer groups. The polymer was noted to decompose and lose its strong flocculation effect after a period of 6 weeks. This problem could be overcome if the polymer could be stored dry and fresh solutions may then be made. PAD flocculant may prove useful in the future and therefore deserves further detailed studies on the various aspects of preparation, purification, analysis, physical and chemical properties, so that it can be used to best advantage. 147

CHAPTER 6 EalET2111121EzEIELLL.JIL27.:1271111222g2EEL/

6.1 Introduction

The selectivity of glyoxal-bis-(2-hydroxyanil) (or GBHA) to copper ions was first discovered by Bayer (155,156) who found that GBHA groups were particularly suitable for sequestering copper and uranyl ions. Only copper, uranyl and nickel ions were strongly bound to GBHA in weakly acidic media; there was no complex formation with either alkaline earth ions or with other heavy metals below pH7(157) This selective behaviour towards a few metals is said to be mainly due to the special steric structure and the iso- merism of GBHA(79) In alkaline media, the ring form of GBHA Fig. 6,1a, re-arranges to form the open-chain structure Fig. 6.1b, which is the true complexing agent and as a result, less stable complexes are formed with 2+ 2+ 2+ Co Zn Cd and even with alkaline earth ions. The stability of the metal-GBHA chelates has been shown to depend essentially on the covalent bonding between the metal atom and the two nitrogen atoms. The phenolic oxygens cannot approach the metal atom sufficiently closely to form either a covalent oxygen-metal bond or a firm electrostatic bond. Therefore, the five-membered chelate rings, N-M-N-C-C, are obtained(79). The stability order of the coordinative complexes of heavy metal ions is:

Cu>IJO2>Ni > Co >Mn> Zn >Cd, whereas vanadium and iron form weak complexes in alkaline media. Bayer 056050 was able to prepare water insoluble resins containing GBHA groups by condensing di- and 1148

tri-aminophenols with excess of glyoxal. This resin was

used (79,159) successfully to selectively extract copper and uranium from sea-water. GBHA groupings were also used in the micro-determination and selective separation of uranium, with the aid of,light absorption spectroscopy (160) Bayer's findings are in agreement with the principles mentioned in Chapter 3, where the nitrogen donor ligands have no strong tendency to form stable coordinative com- pounds with class A-cations. Accordingly the selectivity of GBHA could be utilized in the separation of metallic ions by one of the following alternatives:

A - In acidic media (pH 2-7), where GBHA reacts only with copper, nickel and uranyl ions but not with alkaline earth and other heavy metal ions, or B In alkaline media (pH 7-11), where GBHA forms complexes with many cations but with different degrees of strength, in which case, by introducing another competing ligand in the medium to suppress those com- plexes of lower strength, selectivity to those cations

forming strong complexes with GBHA, especially copper, could be achieved. It was anticipated that if GBHA groups were introduced into a long-chain water-soluble polymer such as poly- acrylamide, a selective flocculant for copper minerals

could thus be obtained. Polyacrylamide-glyoxal-bis-(2-hydroxyanil) polymer was the reaction product of a non-ionic polyacrylamide (PAM) with formaidhyde and glyoxal -bis-(2-hydroxyanil)

groups (GBHA). These chelating groups were also known as

149

di-(2-hydroxy phenyl-imino)-ethane, [C6H4(OH).N:CH] 2. For simplicity, this polymer will be referred to as PAMG. The techniques of introducing GBHA into polyacrylamide through the reaction with formaldhyde, probably involved the following reactions: A. -CH -CH- +HCHO 2 1 -CH 2 -CH-/ C=0 C=0 NH2 NH (PAM) (formaldhyde) CH2OH

B. -CH2-H- C +GBHA------* -CH2-CH- 1 1 +H2O. C=0 C=0 I I NH NH I I CH OH 2 CH1 2 GBHA

C. GBHA + HCHO CH GBHA - 2OH •

D. GBHA - CH OH + -CH -CH- -CH -CH - +H 0 • 2 2 1 2 1 2 C=0 C=0 I I NH2 NH CH2 GBHA

The structure of GBHA, according to Bayer(? 9) in acidic and the alkaline pH is shown in Fig. 6.1 (a,b), and the possible structure of PAM-GBHA polymer is illustrated in Fig. 6.2. The average molecular weight of this chelating polymer was expected to be about one

million, since it was prepared from a polyacrylamide

150

o"0 H C — CH I I H H / (a) In acidic media HC—CH (b) In alkaline media Fig.6-1 Structure of GBHA

CH2 CHi — CH— C H—CH — CH— CH—CH—CH —CH—1 CH—CH- i C=--- 0 c =0 C=----0 C=0 C=0 C=0 I I I I I I NH NH NH2 NH NH NH I I I I CH2 CH2 CH2 CH2 O .0 0 HC —CH / P1 N'i \N Ck‘ 0, ,-G'-:,‘,‘ H H ■ "A*4 N\ \A' - \A \*4 Vt‘ \A

Fig.6.2 Possible structure of PAMG polymers 151 sample of that average molecular weight. It has been established (91,161,162,163)that polyacrylamide reacts with formaldhyde under alkaline conditions to produce methylolated polyacrylamide, owing to the reactivity of the amide groups. The reaction proceeds to equilibrium (equation A) and as a result some free formaldhyde remain (161). in solution Schiller and Suen(163)prepared anionic derivatives of polyacrylamide through sulphomethylation de with formalhyde and sodium bisulphite, and cationic derivatives via the aminomethylation (Mannich reaction) with formaldhyde and amines. In the sulphomethylation de method, they found that the rate of formalhyde uptake on PAM depended on the pH and temperature. The reaction did not proceed to any significant extent below pH 10 even at 70-75°C in 2 hours. However, the rate of uptake increased sharply upon raising the pH to 10.5; about 60% of the formaldpyde had already been combined with PAM before the temperature reached 50°C. When the reaction was carried out at 70°C, the initial rate of uptake was greater than that at 50°, yet the same extent of reaction was reached at both temperatures over a period of 4 hours. Schrieber and Reinwald(164) modified polyacrylamide by the reaction with paraformaldhyde and piperidine at room temperature in acidic pH (Mannich reactior065Vhey used the product for flocculation of enzymes formed in Bacillus subtilis cultures. The modified form gave a clear supernatant as against turbid supernatant with unmodified polyacrylamide.

Polyacrylamide can also react with glyoxal through the amide groups under alkaline conditions to produce a 152

water-insoluble polymer, provided the molecular weight is (161) high .The amide groups of PAM undergo hydrolysis under alkaline conditions, which results in the formation of carboxylate groups on the polymer. Apparently, the amide groups of PAM could be converted to yet more active sites for covalent coupling of biomolecules( 160_ for example, the conversions of "Bio-Gel p" (polyacrylamide gel) to the aminoethyl and hydrazide forms(166). The first was formed by reacting PAM with ethylene diamine: -CH -CH- + H NCH CH NH 90 C + NH3 2 2 2 2 2 o, -CH2 -CH- . C=0 C-0 I I NH 2 NH CH 2-CH 2 NH 2 and the hydi!zide form was prepared by reacting PAM with hydrazine (N2H4):

-CH -CH- + H NNH -CH2-CH- + NH3 . 2 1 2 2 C=0 C=0 NH NH2 NH2 The hydrazide derivative could be used directly to couple protein amino groups. Detailed properties and reactions (167) of polyacrylamide were reviewed in the literature The reactions between GBHA and formaldhyde can be best explained in terms of the condensation of phenol and formaldhyde, since GBHA consists of two substituted e phenols. Phenol and formaldhyde normally condense to form polymers consisting of aromatic rings linked together (91,92,168,169) mainly by methylene bridges The bridging is primarily in the ortho or para positions to the phenolic 153

hydroxyls. In these condensations, phenol is a tri- functional molecule and formaldhyde a difunctional one. The ortho or para substituted phenols, however, can only yield linear. polymers (9 . The reactions are usually carried out in the presence of catalysts, with the aid of heating; the catalysts can be either bases or acids. The reactions of phenol and formaldhyde in alkaline conditions result in the formation of 0-, p-methylol OH OH OH groups (51,9); CH OH 2OH, Z 2 CH CH2OH, CH2OH CH OH,

0-120H which are more reactive towards formaldhyde than the original phenol. If the reactions were allowed to proceed to a higher degree of condensation, these phenol alcohols would undergo self-condensation with the formation of (170) ethylene- and ether-bridges .The products of these base-catalysed condensations, known as tresoll resins, tend to folat water-insoluble net-work polymers upon prolonged heating, due to the branching effects of the reactive phenoxide ions. The reactions between phenol and formaldhyde under acidic conditions (usually below pH 3.6), i.e. acid- catalysed condensations, proceed through the protonation f-Ns + of formaldhyde to give carbonium ions: CH2=0 + H CH?-OH, which react with phenol to form 0-$ p-methylol groups (91) Acidic catalysts seem to favour the formation of p, p' 0(6) linkages, although 0, p' and 0, 0' linkages may also form

In the presence of more acid, the methylol groups react with free phenol to form dihydroxydiphenyl methanes(91).

Unlike the base-catalysed condensations, in the acid- 154

catalysed reactions, the first substitution in the phenolic nucleus substantially deactivates the ring against further substitution(91) and as a result, the polymer has a linear structure. The polymer would remain water-soluble so long as the ratio of phenol to formaldhyde was less than equimolar(92). As the condensation products are unable to polymerize to a great extent (the average molecular weight of a typical commercial novolak is about 600 which corresponds to about 6 phenolic nuclei per chain(91)), there is no danger of gelation during production (171'172)

6.2 Preparation of PAMG polymers

In this work, three main techniques were attempted; in two techniques the condensation of GBHA on PAM was carried out by direct reactions with formaldhyde. In the third technique, formation and reaction of GBHA with

PAM was conducted through the condensation of a diamino- phenol (in the hydrochloride form) with glyoxal and formaldhyde. The reactions of GBHA with PAM and formaldhyde were conducted at pH 10.5, where PAM has been shown (161,163) to react with formaldhyde at an appreciable rate. The intention of the dissolution of GBHA in either aqueous or alcoholic media containing formaldhyde at pH 2.5 in the methods described in 6.2.1 and 6.2.3 was to avoid the possibilities of cross-linking of GBHA groups, which might have resulted in forming water insoluble resins.

It was believed that formaldhyde would form methylol groups on GBHA, catalysed by the acidic medium, without much polymerization of GBHA, thus leaving them ready to 155

condense on PAM in alkaline media. However, condensation

of formaldhyde on GBHA would also be expected in the alkaline medium, where the phenolic rings of GBHA, by analogy with (91,91 other phenols , would have more active sites for reaction.

The chemicals used in these preparations were from BDH chemicals Ltd., Poole, England, and the polyacrylamide

was NlOOS from B.T.I.Chemicals Ltd., Bradford, England.

According to information supplied by B.T.I.Ltd., the poly- acrylamide N100S was of high grade i.e. of low content of carboxylate groups (iv .5%) and was of an average molecular weight of 106. EXPERIMENTAL 6.2.1 Preparation of PAMG2 polymers

In this method, an ethanol solution of GBHA was used in the reaction because of its limited solubility in cold water. Three PAMG2 polymers namely; PAMG 2.1, PAMG 2.2

and PAMG 2.3, were prepared. PAMG 2.1 and PAMG 2.2 are described in this section and PAMG 2.3 is described in 6.4. Preparation of PAMG 2.1: 0.2 g GBHA powder was dissolved in 25 ml cold ethyl alcohol. At the end of dissolution, 25 ml double distilled water was added together with 0.02 g formaldhyde (0.05 ml formalin) and the pH was adjusted to 2.5. The solution became turbid on addition of water. It was kept in a water-bath at 75-80°C under reflux for 30 min. The colour of solution was yellow. After cooling, the product solution was

mixed with 50 ml of 0.2% aqueous solution of PAM (i.e.

0.1 g) and the final pH was adjusted to 10.5. The 156

• o mixture was kept at 75-80 C in a water-bath under reflux for 30 min. The product was dark brown in colour and contained some solids. The concentration of this solution was taken as 0.1%, based on the weight of PAM. The product had flocculation effects on suspensions of chrysocolla,

feldspar and calcite at pH 5, and no effect on quartz. The effect of excessively high temperature on the polymer structure was demonstrated in an experiment where

the amounts of reactants and conditions were the same as

those of PAMG 2.1, except that the mixture (i.e., after adding PAM solution) was allowed to boil for a few seconds on a hot-plate, without refluxing. The product contained much of a dark brown, insoluble gel precipitate. Never-

theless it produced good flocculation on suspensions of chrysocolla, feldspar and calcite at pH 5 and had no effect on quartz suspension. The product was named PANG 3. Preparation of PAMG 2.2: 0.1 g GBHA was dissolved in 25 ml ethyl alcohol (with the aid of warming and shaking) to give bright-yellow colour solution. 25 ml double

distilled water and 0.02 g formaldhyde were added and the pH was adjusted to 2.5 (with 3 drops of conc. H01). The solution was maintained at an average temperature of 73 (68-76°) in a water-bath under reflux for 30 min. The

colour of the solution changed to red-wine colour but there was no solids. After cooling, the solution was mixed with 50 ml of 0.1% aq. solution PAM (i.e. 0.05 g)

and the pH was adjusted to 10.5, usinp. NaOH aq. solution. The mixture solution was kept at 75°C for 30 min. under reflux. The product was dark brown in colour and did not 157

contain solids. The polymer solution (0.05% PAM), produced good flocculation on chrysocolla suspension. It was used in the alcohol precipitation purification processes described

in sections 6.3.1 and 6.5.1. 6.2.2 22:222Ea-Li2ILaiLyA_,,

In this technique, the polymer was formed by the condensation of a diaminophenol with glyoxal and formaldhyde on polyacrylamide. Thus 3.15 g of 2,4- diaminophenol

hydrochloride(amidol) [ (NH2)2 C6H3.0H.3HCL], 0.5 g glyoxal [(CH0)21 , 0.25 g formaldhyde (CH20) and 1.0 g polyacrylamide PAM ( N100S), were dissolved in one litre of double distilled water. The pH of the mixture was raised to 10.5. The solution was then refluxed at an average temperature of 55°C (50-60o.) for 2 hours, in a 2 litre, flat bottom flask, while stirring in a nitrogen atmosphere. The product was very dark brown in colour and contained insoluble gel particles. It had good flocculation

effects at pH 6 on suspensions of chrysocolla and feldspar but not on a quartz suspension; it was used in the flocculation experiments in Chapter 7. 6.2.3 Preparation of PAMG 7 polymer In this method, an aqueous solution of GBHA was used instead of the ethanol solutions used for PAMG 2 polymers. Thus 0.3 g GBHA powder was dispersed in 100 ml distilled water at pH 10.7. The dispersion was kept at 70°C in a water-bath under reflux for 3-days. On filtration, however, it appeared that only 0.1 g GBHA had dissolved.

11 ml of 1.0% aq. solution of PAM and 0.163 g formaldhyde

(0.41 ml formalin) were added and the pH was adjusted to 158

10.7. The mixture was heated at 5000 for 1.5 hour. The product solution (0.1% based on PAM) was yellow to pale- brown in colour with only a little solids suspended. The theoretical molar ratios of reactants used in the preparations of PAMG polymers namely; PAMG 2.1, PAMG 2.2, PAMG 2.3 (Section 6.4), PAMG 6 and PAMG 7, are shown on Table 6.1.

Table 6.1 The molar ratios of reactaILUUIIRE221=Ina PAMG polymers

reactants PAM GBHA formaldhyde glyoxal diaminophenol (A.M=71 (240) (30) (58) hydrochloride polymers (197) ---, PAMG 2.1 1 0.589 0.475 - - PAMG 2.2 1 0.589 0.95 - - PAMG 2.3 1 0.298 0.473 - - PAMG 6 1 - 0.59 0.61 1.13 PAMG 7 1 0.298 3.8 - -

6.3 Purification o£ 6.3.1 Thea22212. .method Excess ethyl alcohol was found to precipitate these polymers from their aqueous solutions to give dark brown precipitates. Similarly, ethyl and methyl alcohols also precipitated (unreacted) polyacrylamide from its aqueous solutions to give white amorphous precipitate (ppt.). The alcohol content must, however, be equal to or more than 50% of the total volume of the solution. PAM 159 granules were found experimentally to dissolve in 30% ethanol or methanol solutions, whereas in 50% ethanol solutions there was no dissolution or even swelling of PAM.

On the other hand, GBHA powder was, of course, soluble in ethyl and methyl alcohols. Thus by treating the PAMG 2 polymer solutions with equal volumes or excess of ethyl alcohol, the polymer would precipitate in a solid form while any free GBHA and other by-products of lower molecular weight would remain in solution. Separation of the two phases by filtration or decantation and washing with alcohol several times should therefore yield a purified polymer. The rapidity and completeness of precipitation would, of course, .depend on the concentration of the polymer in solution.

Experimental: This method was applied to PAMG 2.2, prepared as in 6.2.1. 100 ml of polymer solution was mixed with 200 ml of absolute ethyl alcohol and left for 30 minutes at room temperature. In order to aid settling of the precipitate, part of the polymer suspension was centrifuged and the ppt. was washed several times with ethyl alcohol and was recovered. The other part was left overnight in a covered beaker, where the polymer precipitated in a large volume at the bottom of the beaker. It was filtered on an ordinary filter-paper and washed 4 times with ethyl alcohol until there was no more colour released into the solution above the precipitate. The colour of the ppt. was brown; when the ppt. was dissolved in distilled water, it gave a yellow solution. When 100 ml of this 160 yellow solution was mixed with 20 ml ethyl alcohol, the brown ppt. formed immediately leaving no colour in the alcohol solution.

An experiment was carried out to study whether any by- products of the formation reactions of PAMG 2.2 had also precipitated in alcohol. Thus 0.1 g GBHA was dissolved in

25 ml of warm ethylalcohol (warming at 50-60° and shaking for few minutes); the colour of the solution was bright yellow. Then 75 ml of cold distilled water was added and the pH was adjusted to 10.5; the colour changed to pink. On addition of water some precipitation of GBHA took place, the dispersion was kept at an average temperature of 73°C in a water-bath, under reflux for 40 min. When the dispersion was allowed to cool, the colour was a brownish- green. After cooling, the precipitate was filtered and dried and weighed; about 40% of GBHA remained in solution.

To the clear GBHA solution, 0.05 ml formalin (0.02 g formaldhyde) was added at pH 10.5 and the mixture was kept at 75°C for 30 min. The colour became dark brown and there was no solids formed. 20 ml of this solution "Product I" was mixed with 30 ml ethyl alcohol and was left for 20 minutes. There was no precipitation of solids at all, even after many hours.

Another experiment was carried out with PAM. Thus 0.1 g PAM was dissolved in 100 ml dist. water containing 25% ethyl alcohol, the pH was raised to 10.5 and 0.05 ml formalin (0.02 g formaldhyde) was added. The mixture was kept at 75 C iin a water-bath under reflux for 30 min.

When it was allowed to cool, there was no formation of 161 solids or gel i.e. no cross-linking of PAM. 20 ml of this solution "Product II" (methyolated PAM) was mixed with 30 ml ethyl alcohol. A white precipitate was formed immediately, which re-dissolved readily in 20 ml distilled water. A mixture of 10 ml of product II was mixed with 5 ml of product I and 30 ml ethyl alcohol, but no precipitation took place immediately, probably due to dilution of the polymer solution;

Instead, 10 ml of 1.0% PAM (ordinary) aq. solution was mixed with 6 ml of product I plus 30 ml ethyl alcohol. Only a white precipitate was formed, leaving the yellow solution alone. The ppt. was separated and washed with alcohol several times but the alcohol solution remained uncoloured. This experiment was repeated several times with different amounts of product I and even with GBHA ethanol solution, and the same results were obtained.

These tests proved that there was no entrapment of colouring substances in the precipitate of PAM, nor was there precipitation of any reaction by-products in to alcohol. Therefore, the brown colour of PAMG 2.2 product precipitated by alcohol was due to a truly new polymer. 6.3.2 Gel chromatography method This technique was tried in this work in an attempt to isolate the PAMG polymers from other lower-molecular weight substances. This method works on the principle that different size molecules will be eluted at different speeds from a column of swollen gel particles. Molecules larger in size than the largest pores of the swollen gel beads i.e. above "the exclusion limit", cannot penetrate 162

the gel particles; instead they move outside the particles through the bed and thus are eluted first. On the other hand, smaller molecules penetrate the gel particles to a varying extent depending on their size and shape. Molecules are therefore eluted from the gel bed in order of decreasing molecular size or molecular weight. Dasii ....I192221211E: The glass column used was 40 cm x 1.7 cm with a bed support of 400 mesh nylon cloth; the gel bed height was 30 cm. The tubing was mainly glass and soft polyvinylchloride of diameter 2.0 mm. The length of the tubing segments was kept as short as possible to minimize the dead space volume. The dead space volume beneath the bed support was also minimal, to avoid unnecessary dilution of effluent fractions and to enhance the chromatographic resolution since large dead-space volume could act as an effluent mixing chamber(173) The eluant was de-aerated double-distilled water and the hydrostatic pressure was regulated simply by calibrating a measuring tape so that zero pressure corresponded to the bottom of the air vent in the eluant container. The design was simple and the column was easy to dismantle for cleaning, repacking and repairs. The gel medium used was "Bio-Gel P-300" (BIORAD Laboratories) U.S.A. According to the manufacturers(17 substances of molecular size larger than the exclusion limit would be eluted from the bed at a volume equal to the void volume (i.e. the volume of the space outside the gel beads), which they estimated as 38-42% of the total bed volume. The maximum separation between the different

1'63

molecular weight substances could be estimated in advance' provided that the ratio of elution volume to void volume lEv ‘Evo) for a particular molecular weight was known. According to the manufacturers (174) theEv/Evo for sub- stances of molecular weight 106 on Bio-Gel P-300 was approximately equal to 1.225. The separation (in terms of elution volume) between different molecular size, fractions could be estimated as follows: Total gel bed = 11* x (1.7)2 x 30 = 68.06 ml (or 68 ml) 7-- • The void volume = 68 x 38% = 25.8 ml or = 68 x 42% = 28.6 mi.

Hence, the"peak heigheof 106 polymer = 25.8 x 1.225 = 31.6 ml or = 28.6 x 1.225 = 33.2 ml

This should be the difference in elution volume between 6 the 10 substances and other substances. Similarly the "peak height"of 105 polymer would be = 25.8 x 2.0 = 51.6 ml or 28.6 x 2.0 = 57.2 ml, where Ev/Evo = 2.0. This volume should be the difference in elution volume between 105 fraction and the 104 fraction which was the minimum separable on this gel P-300. Therefore it was expected that the 106 molecular weight fraction would be completely eluted in an effluent volume of 30-33 ml and the 105 fraction within 50-57 ml and the rest of the substances within perhaps 70 ml. LIEtrialLEILl: The gel used was of bead size range 100-200 mesh. About 10 g of the dry gel beads were hydrated

in 500 ml double-distilled water for 8 days. The gel 164

slurry was elutriafed to get rid of the fines and was de-aerated before packing into the column. The gel-bed was carefully packed, avoiding entrapped air bubbles in the system. Thus the column and the side tube were filled with the de-aerated eluant by vacuum the column was immersed in the eluant ) so that no air bubbles were present. The hydrated gel beads were poured through a funnel into the column and allowed to settle for a few

centimeters, a mild pressure of 10 cm -15 cm was gradually applied to pack the bed more tightly. The bed was covered with filter-paper on top to retain solid matter. After washing the bed with the eluant for at least 3 days, the

samples (10 ml of 0.01% PAMG 2.1, PAMG 6, PAMG 7 at pH 8.3) were applied gently over the bed and were allowed to drain down to the bed level before addition of the eluant and connecting to the eluant reservoir. In practice the effluent flow-rate was slow, about 1.7 ml/hr. at a hydrostatic pressure of 15 cm (which was

the maximum recommended). When the gel bed was repacked carefully, an initial flow-rate of 4.0 ml/hr could be obtained but it eventually became 1.7 ml/hr. at 15 cm pressure.

In an experiment, 10 ml of 0.01% PAMG 2.1 flocculant was run through and samples were taken every 4 hours (i.e. 6.8 ml), a total of 10 samples being taken. The PAMG 2.1 polymer was detected in the effluent solution by u.v. absorption spectroscopy at 558 my.. The results are shown in Figure 6.3. It appeared that a fraction of the

polymer was eluted within 17-20 ml, presumably the 106

0.20

0.16—

) t

8 mi 0.12— 55 ( . •

bance • 0.08 — ••

Absor • • • •

0.04— . . • • • . • Sample • • 1 2 3 4 5 6 7 0.00 1 I I I 1 O 0 a a O 0 (-) nr to

Fig.6.3 Fractionation PA M G 2.1 by gel chromatography 166 fraction,and another fraction was separated at the peak height 44-50 ml; the separation in elution volume, was therefore between 27-30 ml which was roughly in agreement with predicted estimation. Conclusions: Unfortunately the flow-rate became very slow due to partial blockage of the bed. When examined, the gel bed was found to contain bacterial growth.

Attempts to sterilize the bed were not successful and repacking the bed did not improve the flow-rate. Also, this process resulted in dilution of the polymer solution to an only roughly known extent; therefore the doses of the polymer to be used subsequently in the flocculation experiments would be very inaccurate.. This process was not therefore considered suitable for a routine purification process, but might be useful as an analytical aid. Possibly a column of porous glass beads would be more convenient than "Bio-Gel p".

6.4 Preparation of dry powder of purePAMG 2.3 polymer, Experimental: 3 g of GBHA powder was dissolved in 250 ml absolute alcohol in a 2-litre flask, with the aid of warming at 50°C for 2.5-3 hours under reflux. 250 ml dist. water was added together with 1.5 ml formalin (0.6 g forrnaldhyde) and the pH was adjusted to 2.5. The mixture was kept at an average temperature of 65°C

(60-70°) for 35 min. The solution was brown to wine in colour and contained solids. 3 g of PAM 1005, dissolved in 500 ml double distilled water, was added to the GBHA suspension and the final pH was adjusted to 10.5. The 167 mixture was kept at 73°C for 35 min. and at 75°C for a further 35 min. The product was a brown, viscous solution, and contained colloidal solids. It was kept in two 1-litre beakers in a refrigerator over-night.

Purification aal slExiaa af PAMG 2.2: The contents of both beakers were added to a total of 2.5-3 litres ethyl alcohol in small portions and stirred with glass rods. The precipitated PAMG 2.3 polymer was collected on these glass rods. Washing the polymer was carried out by stirring the glass rods in consecutive beakers of ethyl alcohol, until no colour was noticed in the last beaker. The precipitated polymer was skimmed off the glass rods into a weighed evaporating dish, and washed with alcohol again. The solid product was dried in a vacuum desiccator, under vacuum over P205 for about 2 days. An attempt to filter some of the product (before mixing with alcohol) on a 1.0 v. membrane, under 2 kg/cm2 of nitrogen was not successful and some of the product was lost during handling. The product solution was very viscous, and the idea of filtration was rejected. Another attempt to produce granules of PANG 2.3 by dropping the solution into a long cylinder (2-litre) containing about 1-litre of alcohol, while stirring by a magnetic stirrer, was also not successful. The droplets became relatively large in size under the impact on the alcohol surface. However, this technique could be improved by reducing both the volume of the droplet (e.g. by using a fine needle) and the travelling distance to the surface of alcohol. 168

Grinding of the dry solids was done with a mortar

and pestle by hand. The colour of the powder was light brown and it was light in weight (i.e., larger volume

per unit weight) compFed0 with PAM granules. About 2.7 g of the PAMG 2.3 powder was recovered. Solubility in water: At least, 0.05 g PAMG 2.3 was dissolved in cold water out of 0.1 g powder, in 2 days

stirring (i.e., 50 %) by a magnetic stirrer at a moderate

shear-rate. It was easier to disperse 0.05 g PAMG 2.3 in 100 ml dist. water (or 0.1 g in 200 ml), but some solids remained undissolved and could be removed by filtration. The 0.05% solution of PAMG 2.3 was tested for flocculation on suspensions of dolomite and malachite,

both separately and in mixture. For each of the 3 different suspensions, 2 different depressant conditions were used; 50 p.p.m. Dispex N40 and 350 p.p.m. Calgon, both at pH 10.5. Selective flocculation of malachite was achieved, though

the flocculation rate was rather slow at 4 p.p.m. PAMG 2.3.

6.5 Characterization of PAMG polymers

6 - 5.1 - Ilt2122222121722i211211sIIfuluELlaa In section 6.3.1, it was shown that PAMG 2 polymers could be precipitated from their aqueous solutions by alcohol. This method was also used here to establish whether any modification of PAM took place as a result of the reaction with GBHA. In this technique, the criterion was the colour of the precipitate. It has already been shown that any low-molecular weight by-products and free 169

GBHA remained in solution in alcohol; therefore the ppt. would only be the PAMG 2 polymer. PAM precipitated by ethyl alcohol is white, while PAMG 2 was brown. This difference in colour indicated that modification of PAM and formation of a new polymer PAMG 2 took place. This technique was performed on many samples of PAMG 2 solutions and the brown ppt. was produced in every case, and remained after repeated dissolution and reprecipitation.

6.5.2 The dialysis technique Experimental: 10 ml of 0.1% solution of unpurified PAMG 2.1 was placed in a regenerated cellulose dialysis bag. The bag was immersed in 800 ml double distilled water and stirred gently by a magnetic stirrer. After 24 hours the solution outside the dialysis bag became yellow; it was replaced by another 800 ml of double distilled water. The replacements of the effluent solution was continued until no more coloured substances diffused into the solution i.e. colourless effluent. The process was then stopped after approximately a week and the polymer inside was recovered. It was brown to dark brown in colour. This finding was a further confirmation of the result of the alcohol precipitation technique and was another indication for the definite change of polyacrylamide due to the reaction with GBHA. 6.5.3 Membrane filtratiortechre In this technique, a membrane filter would retain the polymer segments of molecular size larger than the membrane pores and let through the smaller size fractions.

If the optical absorption density was measured before and 170

after filtration, then the difference-of absorbance would be due to the polymer retained on the membrane. If there were no absorbing groups on the polymer (i.e. no reaction) there should be no significant difference in the absorption density at 400 mkt, since PAM does not absorb at this wavelength (see fig. 6.5). Therefore the difference in the absorbance would indicate that PAM had reacted with GBHA to produce a new polymer. The ratio of the absorb- ance(difference before and after filtration, in relation to the 'absorbance of solution before filtration, that is, the absorbance would be due to the GBHA groups on the polymer plus the other by-products and including free GBHA) could indicate the extent of reaction of PAM with GBHA. Thus the greater this ratio, the greater the number of GBHA groups reacted with PAM. However, this simple ratio did not include the proportion of the undissolved polymer (probably due to cross-linking), nor the proportion of reacted polymer of sizes smaller than the pores of the finest membrane used (0.01 kim) (which might be due to the configuration of the polymer segments). Nevertheless, this ratio could be an indication of the extent of reaction of the soluble polymer in the large size fraction, where the effective flocculation properties were to be expected. LaaLanalLp.nd materials: 1. Pressure filter: The filter used was from "Sartorius Membrane filteeGMBH D-34 Gottingen, Germany. 171

2. Filter membranes: The 1.0 ym and 0.01 ym membrane filters were also from Sartorius-Membran filter;

the 0.3, 0.5, 0.22 ym membranes were from Millipore Filter Corp. Bedford, U.S.A. These membranes are made of cellulose derivatives.

3. Light absorptionsuestr2m2Ier_ : The Perkin-Elmer double beam spectrometer was used. The tungsten

lamp was used for the visible range and the deuterium lamp from 380-180 my.. When the deuterium lamp was used for determining the spectra of PAMG 2.1 solution in the range 600-360 my, an absorption peak appeared at about 560 my which was sensitive to the change in concentration of the polymer. This peak also appeared with GBHA ethanol solutionsbut not with ethanol alone nor with water. However, when the tungsten lamp was used in the range 600-

370 my this peak disappeared from GBHA and PAMG 2.1 solutions. After several measurements on GBHA and PAMG 2.1 solutions of different concentrations using the tungsten lamp from 600-380 my and the deuterium

lamp from 380-180 my, it appeared that the range 440-400 my was very sensitive to change in con- centration. In this section, measurements at 400 my were considered best.

Experimental rocedure: 20 ml of 0.01% solutions of PAM 2.1, PAMG 6 and PAMG 7 were filtered on 1.0 dam membranes under nitrogen pressure of about 15 p.s.i..

Some frothing was noted during the filtration of PAMG 2.1 172 and PAMG 6. The membranes were placed in 20 ml dist. water to disperse the retained portions of the polymers. 5 ml of these dispersions were tested for flocculation. The minus 1.0 im filtrates were also tested for flocculation on 50 ml chrysocolla suspensions at natural pH using 2 ml of the filtrates,and their absorption spectra were recorded from 500-200 my. Filtration of the minus 1.0 iim solutions on 0.01 im membranes was carried out under nitrogen pressure of about 25 p.s.i. and the absorption spectra of the filtrates were recorded. The were repeated in order to check that the absorption spectra were constant. The minus 0.01 tm solutions were also tested for flocculation on suspensions of chrysocolla at natural pH, using 5 ml of filtrate solutions. The 0.01 ym membranes were kept in 18 ml dist. water to recover the retained polymers. 5 ml of these plus 0.01 ym solutions were tested for flocculation. The results are summarized in Table 6.2, and the u.v. spectra of the polymers fractions PAMG 2.1, PAMG 6 and PAMG 7 are shown in Fig. 6.4 (a, b and c). This procedure was repeated twice on different solutions of these polymers and the above results were confirmed. NOTE: Because the pressure filter was made of stainless steel (i.e. it might contain nickel), polymer solutions were run through the filter several times to cover the the contact surfaces of the filter with a polymer coating before commencing the actual testing procedure described above. This should eliminate possible errors due to specific absorption of the polymers on the filter contact surfaces. Table 6.2

No. Properties PAMG 2.1 PAMG 6 PAMG 7

flocculation effect of minus 1.0 pm filtrate good weak good

2 flocculation effect of minus 0.01 pm filtrate no flocn. no flocn. weak 3 flocculation effect of plus 0.01 pm dispersion good weak good 4 flocculation effect of plus 1.0 pm dispersion good good weak 5 absorbance of minus 1.0 pm filtrate at 400 mu 0.48 0.27 0.127 6 absorbance of minus 0.01 pm filtrate 0.25 0.21 0.097

7 absorbance difference of minus 10 pm plus 0.01 pm 0.23 0.06 0.03 8 ratio of the absorbance difference to minus 47.9% 22.2% 23.6% 1.0 pm filtrate absorbance __J.ILILIJIII1111

—1.2

e nc ba r Abso

—0.6 0.01 filterate

i-Ou filterate

(a) PAMG 2.1 I I I 1 1TIII11111 0.0 200 300 400 500

Wavelength

Fig.6.4.1Nspectra of (a) PAMG 2.1 (b)PAMG 6 (c)PAMG 7 solutions fractioned by the membrane filteration technique 200 I l

I

11_11111 300 Wavelength mi 400

_I 0-01u filterate 1.0 filterate

500 Absorbance I 1:2 r5 00 0. 6 176

L I

—1-2

bance Absor

0.01v filterate

10 filterate

•• ...... (c) PAMG 7 •••••• ...... 0.0 I I I I I I I I I 1 I I 200 300 400 5005

Wavelength mil 177

Size distribution of PAMG 2.1 polymer segments: Experimental: A dilute solution of PAMG 2.1 was filtered on 1.0, 0.3, 0,2, 0.05 and 0.01 lm membranes and the absorptions of the filtrates were measured at Ltoo mn. The filtrates were also tested for flocculation on 50 ml suspensions of chrysocolla at natural pH. All fractions except the minus 0.01 tm effected good floccula- tion, though to varying degrees. The percent absorbance differ-

ences were taken as a measure of the size distribution of the polymer molecules. The results are shown in Table 6.3. It must be remembered that the minus 0.01 -vm fraction contained also other by-products of the reaction. The effective flocculation range (E.F.R.) was found in the plus 0.01 kxm fractions. Therefore the size distri- bution of polymer molecules in the effective flocculation range could be calculated on the basis of the absorbance differences, excluding the absorbance of the minus 0.01 km fraction, as shown in Table 6.3. Conclusions: The results on Fig. 6.4 (a, b and c) and table 6.2, had clearly indicated that PAM had reacted with GBMA and formed new polymers. Table 6.2 also shows that PAMG 2.1 had the highest extent of reaction while table 6.3 indicated that PAMG 2.1 consisted of molecules of wide size-range. The larger size fractions were usually more effective flocculants than the smaller fractions. From the u.v spectra, a new absorption peak or "hump" had appeared between 280-265 mu, which did not appear in GBHA nor PAM nor methylolated PAM in

Fig. 6.5. It is interesting to note that the dispersion Membrane absorbance Size fraction absorbance absorbance size distribution size, v.m im difference difference % of E.F.R.

1.0 0.47 - 1.0 + 0.3 0.05 10.6 20.4 0.3 0.42 - 0.3 + 0.2 0.045 9.6 18.4 0.2 0.375 - 0.2 + 0.05 0.03 6.4 12.3 0.05 0.345 - 0.05 + 0.01 0.12 25.5 48.0 0.01 0.225 - 0.01 0.225 47.9 -- Total 0.47 100.0 100.0

Table 6.3 Size distribution of PAMG 2.1 molecules in solutions, in terms of the absorbance difference percent at 400 mkt. 179

of the solids residue of PAMG 2.1, PAMG 6 and PAMG 7 on 1.0 tam membranes also had effective flocculation properties, which was probably due to some soluble polymer molecules, glued to the solid particles. 6.5.4 Ultraviolet and infra-red s ectra

A. Ultra violet spectroscopy The u.v. spectra of the dilute solutions of GBHA, PAM, methylated PAM and pure PAMG 2.3 are recorded in

Fig. 6.5 (a-d), measured on the Perkin-Elmer double- beam spectrometer. The tungsten lamp was used from 500-380 mkt and the deuterium lamp for 380-180 mkt. 10 mm

quartz cells were used. The GBHA spectrum in Fig. 6.5 a has the characteristic absorption peaks at wave-length 442-420, 417.5, 290 and 240 mkt, which are in agreement (157). with published work The concentration of GBHA used for this spectrum was 0.01% in methylalcohol; the solution was heated to 70°C for 30 min. under reflux at pH 10.9, using NaOH aq. solution,before recording the u.v. spectrum. The 0.01% methanol solution was made from 0.1% methanol solution of GBHA, previously heated at 70°C for 60 minutes at pH 10.7. This treatment was carried out in order to simulate the GBHA reacted in the PAMG 2.3 polymer. Since it was noticed experimentally that the absorption of GBHA increased at high pH and temperature especially in the region 500-320 mkt, the effect of temperature was more pronounced than that of pH. In Fig. 6.5 b, the u.v. spectrum of PAM (0.01 % aq- solution), there is an absorption peak at ni 195 mkt.; however, there is also some absorption gradually increasing from 320-220 mki. The 180

2•

GBHA, pH 10.9 heated 2-0 30 min. at 70°C (0-01% soln. in methanol

U 1.5 c 0 L. 0 < 1.0

0.5

0 200 250 300 350 400 450 500 Wavelength (mpl

Fig 6.5a: Ultra violet spectrum of glyoxal-bis --- (2-hydroxyanill 1 8 1

1.0' U C 0 1 0 (4 0.5

0 r 200 250 300 350 400 450 500 Wavelength (mp)

FIG 6.5b: Ultra violet spectrum of polyacrylamide. 1 82

1-5

PAM-1-1CHO Produc (0.01% solni co 1-0

<0•5

200 250 300 350 466i5.) 500 Wavelength (m),J)

,FIG 6-5c: Ultra violet spectrum of methyloltated polyacrylamide. 183

urified PAM 2.3 (--0.05%) 1.5

bance 1.0 or Abs

0.5

op 200 250 300 350 400 450 500 Wavelength (mp)

FIG 6.5 d: Ultra -violet spectrum of polyacrylamide -glyoxal -( 2 -hydroxyanil .) 184

spectrum of the methylolated PAM 0.01% aq. soln.) in Fig. 6.5 c is rather similar to that of PAM except for one extra absorption hump between 290-280 my. On the other hand the spectrum of PAMG 2.3 ( 0.05% aq. soln.) in Fig. 6.5 d, shows an absorption hump between 442-420 my, 280-260 mi and a peak at 204 my. By comparing these spectra, the absorption at 442-420 my is the same as in the GBHA spectrum; however, the 417.5 my in GBHA has disappeard in the PAMG 2.3 spectrum. Also, the absorption at 290 my in GBHA has disappeared in the PAMG 2.3 spectrum; instead a new absorption between 280-260 my has appeared, which is characteristic to the new polymers. For example, in Fig. 6.4, the absorption at 280-270 mi for PAMG 2.1, PAMG 6 and PAMG 7 appears to varying extents. This is an indication of the reaction between PAM and GBHA with formaldhyde.

B. Infra-red spectroscopy The infra-red spectra of pure PAMG 2.3, PAM and GBHA were recorded in Fig. 6.6 (a-c).. Samples of the dry powder of these compounds were dispersed in potassium bromide discs. The samples discs were prepared as follows: 5 mg of each of the dry powder samples was mixed and ground with 400 mg KBr (infra-red grade) in a small vibratory mill (lined with tungsten carbide) for 1 min. Then 202.5 mg samples of each of the ground mixtures (containing 2.5 mg sample and 200 mg KBr) were placed in a die, where 6.8 ton/sq. inch pressure was applied for 1-2 minutes, in a manual hydraulic press. The discs were then carefullyextracted from the die and evacuated in a vacuum desiccator over 185

P 0 for 2-3 days before recording their infra-red 2 5 spectra against a 200 mg KBr disc as a reference. The discs were stored in a desiccator over silica gel for re-investigation purposes. Details of preparing sample 075) discs with alkali halide are described in the literature The infra-red spectrometer used was the double beam spectromaster made by Grubb, Parsons and Co. Ltd.,

Newcastle upon Tyne, (G.B.). Results and discussion, Inspection of Fig. 6.6 c shows that the spectrum of GBHA against KBr is in good agreement with published work (157) and that the sample used in this work was probably a mixture of GBHA and its sodium salt. According to Bayer (157) the absorption band at 2.94 kam (3400/cm) is due to the N-Hbond, whereas the absorption band at 6.1 km is characteristic'for the

C = N double bond. However, it seemed from the literature 076,177,178) that the N-Hstretching bond invariably absorbs in the region 2.86-3.24 dam and the absorption region of 5.95-6.1 was assigned by some authors(177) to C = 0 stretching bond in the amide structure. The absorption at 6.2-6.3 was suggested to be due to N-H deformation. The C - N stretching bond absorbs in the regions 6.45-6.7 -km and 7.15-7.8 tam. These absorption regions were noticed in PAM spectrum (Fig. 6.6 b). In the spectrum of pure PAMG 2.3 polymer, a new absorption peak appears between 9.65-9.85 vm, namely at 9.7 um which also appears in the GBHA spectrum. The spectrum of PAMG 2.3 against PAM was also recorded (Fig. 6.6 d). It was

expected that the difference spectrum would be due to 100 3-3.15 rn :4 -4 — (3)

W X 60

0 40 z FIG 6.6a: Infra-red ' • spectrum of PAMG 2:3 using KBr as a reference.

i i 0 co ti

WAVE LENGTH ( pm) 100 2. 5-3.17 ,J — Crl 8 0 03 Ob lJ

60 -0 u.' r N.) cr) 0 40 z FIG 6.61D:infra-red spectrum of PAM 2 using KBr as a reference I 1 0 ti (4) 100 - ;-l ~\ . 0'1 ~) l~ w I~ Ul ~f I ..... 0"1 ~ I /" Ii --" 1/ W W » 80 m CD 11 -:-: ~ Y i--. to Ul (J1 '" CP N.r-.. o 1\ ~OlN ~ "¥.) I ""fJ~ r\ ,I I I \ :;u 60 I I ~ ~ ! v -0 ! I V\ v~ -1 i~ Ii II <:p' ~ V ~ I ~ ~ 040 / V ~\ L --- FIG 6·6c: Infra-red spectrum Ul 'v .r-.. ~ U1 of GBHA using KBr as a 20 ~} I reference.

o J I Ir~ \ <0 ...... I\) t\) ...... (Q ...... tv

WAY E LENGTH ( }Jm 100

J> 80 f------(0 I tn o ::0 60 w U I ~ FIG 6·6d : tnfra~red spectrum -I I of PAMG_2·3 using P~.M as a Ih to .-- 040 ()) .- 0'1 co -- Z --. 0 refer'ence. ~ -::::J ~ "- f\ ~ I '" "-- i 1\ / ""I'.... '-~!I VI u .r-.. " ~ I I f\ I I ~ II 7 1.1 - ...... , ...... -...... I\) t\) <0 <0 -... tGO "'-J m CPW, lD ~ ... ::: n w Ul "'I~:'>"'~11'111 Ul f )ill/i:~ --~ . W ·W no I /i\ ~I ~ I, f~ I L\ /'\ t\ I \ ~ »OJ0 .-- IIj ;V"-..1 I I ' -....J L.n Ui I v:':" ~;\j ~r \A CP V ~I '-' I -t-r o I I I ~ :rJ 60 I ~ U "'-I--- --( J ( ~ ----v~ 040 Ul . Ul Z Ul . . FIG 6·70 :Infra-red spectrum I Ul'-J I of a 2:1 mixture of PAM and ~20 f\ .- ! G B HA using KBr as a reference. . I : I a-... I ...... " vVAVE LENGTH (}Jm) 100 0:> 0:> 0 Ww c» lD W U1 u,~ 0:> en W ~N ..: -- 80 Ul » U1 ::;l? CD '-JlD (j') ":'1 IlD ~ ~ - ~ ~60 iLJ rJ U -0 FIG 6'7b: Infra -red I ..... ~ spectrum of a mixture -i -- U1 2:1 -40 (I ~ IV W ~ vV o w of PAM &GBHA usingPAM ~ v U Z ( "- as a reference. I 'CR. 20 I \~ I~ 1\ \ ~ ~ lD w W ~ .,IV'- j I "1\ ( Ul ~ - ...... ~ -- ..... " 189

the new groups in PAMG 2.3. It was not possible, however, to determine the exact structure of the PANG 2.3 by examination of the 1.r. spectrum. In order to check that PAMG 2.3 was not just a mixture of PAM and free GBHA, a mixture of GBHA and PAM dry powder of the ratio 1:2 was dispersed in KBr and made into a disc following the same method mentioned earlier. The spectra of the mixture against KBr and PAM as references were recorded in Fig. 6.7 (a,b). Comparison of these spectra with those of Fig. 6.6 (a-d) reveals at once that PAMG 2.3 was a reaction product between PAM and GBHA and not a mixture. It should be remembered that the initial molar ratio of GBHA/AM (PAM) used in preparing PANG 2.3 was about 0.3:1, whereas the initial molar ratio used in preparing PAMG 2.1 was about 0.6 : 1. PAMG 2.1 was used throughout the flocculation experiments on the copper ore reported in Chapter 7. 6.5.5 Degree of substitution In principle, the number of GBHA groups per 100 acrylamide monomers i.e. the degree of substitution in the PAMG polymers, could be estimated from the change in the atomic ratio of nitrogen to carbon WO. In the unmodified polyacrylamide, the acrylamide monomer theore- tically contains 3 carbon atoms (total atomic weight 36) and one nitrogen atom (at. wt. 1!t); hence the ratio N/C is 1 4 equal to -5-6- i.e. 0.389. If the molar ratio of GBHA/AN in the PANG polymers was 1/1, that is one GBHA group per one AM monomer, the PANG monomer would contain 18 carbon (at. wt. 216) and 3 nitrogen atoms (42); therefore the N/C 190

2 ratio would be = 0.194. Thus for 100% substitution, the change of N/C ratio would be = 0.38888 - 0.19444 = 0.194, and for 10% substitution, the change of N/C ratio would be 0.0194, and for 1.0% substitution the change in N/C ratio would be equal to 0.00194 and so on. On the other hand if the molar ratio of GBHA/AM was 1/2, that is one GBHA was attached to two amide groups of two acrylamide monomers, the PANG monomer would contain 11 carbon atoms (at wt. 132) and 2 nitrogen atoms (at wt. 28); hence the N/C ratio would be 28/132 = 0.212. Thus for a 100% substitution, the change of N/C would be 0.3888 - 0.21212 = 0.17676, and for 10% substitution, the change of N/C would be 0.017676, and for 1.0% substitution, N/C change would be 0.00177, and so on. Thus depending on the structure of PAMG polymers, that is on whether each GBHA group had reacted with one or two amide groups, the N/C ratio could be used to estimate the degree of substitution. Experimental: Three samples of PAMG 2.3, PAM and alkali-treated PAM were analysed for the carbon and nitrogen contents by the Microanalytical Laboratory at Imperial College, London. The purpose of analyzing the alkali- treated PAM was to establish whether there was any change in N/C ratio prior to the reaction with GBHA during the preparation of PAMG 2.3 polymer. Thus 50 ml of 1.0% PAM aqueous solution was kept at pH 10.5 for 1 hour at an average temperature of 65°C under reflux. After cooling to room temperature, the polymer was precipitated in 100 ml ethyl alcohol and the precipitate was dried at 80°C overnight. 191

The dry solid was ground to powder in an agate mortar and pestle. Results: The analysis of the three samples for carbon and nitrogen contents were shown in Table 6.4. The accuracy was stated to be on each element ± 0.3%. The change of N/C ratio of PAMG 2.3 from PAM was = 0.0131. Therefore the degree of substitution of PAMG 2.3 • calculated on these figures would probably be 6.6% or 7.4% depending on whether the imolai ratio of GBHA/AM was 1/1 or 1/2.

Table 6.4 Microanalysis of PAM, PAMG 2.3 and alkali-treated PAM.

Polymer PAM PAMG 2.3 Alkali-treated PAM carbon% 42.41 44.01 46.03 nitrogen% 16.69 16.80 17.26 N/C 0.3948 0.3817 0.3753 N/C, change in relation to - 0.0131 0.0195 PAM.

However, when the limits of error of the analysis are taken into account, the result must be treated with caution, as the change of N/C is within the experimental error and the analysis was made only once. 192

6.6. Selective flocculation pro e ties of PAMG 2.1 The selective flocculation properties of PANG 2.1 were tested in suspension media containing competing ligands at pH 10.5, following the principles laid down in Chapter 5. The competing ligands used were "Calgon" a sodium hexametaphosphate, an oxygen donor ligand and "Dispex N40", believed to be a polyacrylamide and sodium acrylate copolymer of average molecular weight between

2000 and 4000; (Allied Colloid Chemicals Ltd). The experimental procedure followed throughout the tests can be summarized as follows. About 1% suspension of fine particles of the minerals or mixtures of minerals were made in distilled water. The required amounts of Calgon or Dispex N40 (or both) were added in dilute solutions and the suspensions were dispersed with a magnetic stirrer at high shear rate at pH 10 for approx- imately 10 min. The suspensions were transferred to a cylinder where they were kept still for 5 minutes; then the suspensions were decanted back into the beaker and settled particles were rejected. The pH of the suspensions was checked before adding the flocculant PANG 2.1, while stirring at high shear rate for lmdnute in order to disperse the flocculant,and at low shear- rate for a further 1 min; then the flocculation sus- pensions were transferred again into the cylinder, where slow rotations were performed for a few minutes and flocculation was observed visually in the cylinder. 193

Results:. 6.6.1 Flocculation effects on minerals sus ensions at pH 10.

1. Calcite was not flocculated by 6 p.p.m. PAMG 2.1 in the presence of 300 or 200 p.p.m. Calgon; however, in the presence of only 50 p.p.m. Calgon, calcite was flocculated.

2. Quartz was not flocculated by PANG 2.1 and even in the absence of Calgon at pH 5.

3. Feldspar was inhibited from flocculation with 5. p.p.m. PAMG 2.1 by 300 p.p.m. Calgon but not with 200 or 100 p.p.m.

4. Dolomite was inhibited from flocculation with 5 p.p.m. PANG 2.1 by 350 p.p.m. Calgon at pH 10.5; also 20 and 50 p.p.m. Dispex N40 inhibited its flocculation with 10 p.p.m. PANG 2.1.

5. Chrysocolla and malachite were flocculated in the presence of 350 p.p.m. Calgon at pH 10 and 10.5 by 2-3 p.p.m. PANG 2.1.

6. Chalcocite was flocculated with 1 p.p.m. PANG 2.1 In the presence of 350 p.p.m. Calgon.

7. Chalcopyrite was flocculated with 1-2 p.p.m. PANG 2.1, in the presence of 350 p.p.m. Calgon, leaving slightly turbid supernatant which did not flocculate even at high doses of PANG 2.1 and PAM. The flocculated portion, 1914

however formed strong and large flocs. The turbidity was probably due to impurity minerals.

8. Cuprite was partly flocculated with 1-2 p.p.m. PAMG 2.1 in the presence of 350 p.p.m. Calgon. The flocculated portion formed strong flocs, while the turbid supernatant remained stable even at 6 p.p.m. PANG 2.1.

Thus in the presence of a competing ligand, namely Calgon, the selectivity of PAMG 2.1 to copper minerals against common gangue minerals in the form of feldspar, calcite, quartz and dolomite was proved.

6.6.2 Selective flocculation of copper minerals from mixed suspensions at pH 10.

1. Chrysocolla from calcite: A mixture of chrysocolla and calcite of ratio 1:1 by weight was treated with 300 p.p.m. Calgon. Chrysocolla was strongly flocculated with 2.5 p.p.m. PANG 2.1, while calcite remained suspended. The green flocs of chrysocolla re-formed readily after re-dispersing the suspension while no white flocs were noticed to form. The same results were achieved in the presence of 200 and 1000 p.p.m. Calgon.

2. Mixed suspension of calcite, feldsp_a tz was inhibited from flocculation with 7 p.p.m. PANG 2.1 by 350 p.p.m. Calgon.

3. Selective flocculation of chrysocolla and malachite from mixtures with feldspar, calcite and quartz was achieved with 2-3 p.p.m. PAMG 2.1 and 350 p.p.m. Calgon. 195

The green flocs re-formed readily after redispersing the

suspension, but they became smaller in size after 2 days.

4. Selective flocculation of chalcocite t. malachite and chrysocolla from mixtures with feldspar, calcite and quartz was repeatedly achieved with 2-3 p.p.m. PANG 2.1 in the

presence of 35❑ p.p.m. Calgon or 50 p.p.m. Dispex N40 or both. The flocculation was more rapid at higher doses of PANG 2.1 ( ?„. 5 p.p.m.) and the floes re-formed again after re-dispersing the suspension even after a period of few weeks.

5. Selective flocculation of malachite from mixtures with dolomite was achieved with 3-4 p.p.m. PANG 2.1 (and PANG 2.3) in the presence of 50 p.p.m. Dispex at pH 10.5.

The settling rate of some malachite floccules was rather

slow, but was improved by increasing PAMG 2 dose up to 6 p.p.m.; meanwhile the dolomite particles remained suspended.

From these examples the PAMG 2.1 polymer was con- sidered a selective flocculant for copper minerals and

was later used to separate copper minerals from a dolomitic

ore; the details and results of the process are described in Chapter 7. Flocculants PAMG 6 and PAMG 7 were also used in the separation of copper minerals from the dolomitic

ore by selective flocculation.

6.6.3 The comparative selectivity of PAMG 2.1 A series of experiments was run following the same procedure as_in 6.6.1, at pH 10 in the presence of 350 p.p.m. Calgon to study (1) the selectivity of PAM to 196 copper minerals, and (2) the improvement in selectivity due to reaction with GBHA, by comparing the flocculation effects of PAM.with those of PAMG 2.1. Results: PAM flocculated a mixture of chrysocolla and malachite with 2 p.p.m., but partly flocculated suspensions of feldspar, calcite and quartz with 1-2 p.p.m.; with PAMG 2.1, however, there was no flocculation of suspensions of feldspar, calcite and quartz even at 7-10 p.p.m. In a mixed suspension with feldspar, calcite and quartz, PAM flocculated chrysocolla and malachite (forming green flocs) but with also some (separate) white flocs at 1-2 p.p.m., whereas with PAMG 2.1 there were no white flocs. From these experiments, it can be concluded that PAM had shown some selectivity to copper minerals, but the modified PAM, namely PAMG 2.1, was more selective. PAM, presumably being a strong hydrogen bonding agent (as shown in appendix 3 ), is more capable of binding to minerals like calcite, feldspar and dolomite than PAMG 2.1. Possible improvement of PAM selectivity would therefore need the expense of using vast quantities of depressants. This was confirmed when PAM and PAMG 2.1 were later tested under identical conditions on two separate samples of the dolomitic copper ore (cf. Chapter 7). In those experiments, PAM flocculated 48% of the total weight of the ore sample and upgraded the copper content from 5.8% to 9.1%. In contrast, PAMG 2.1 flocculated only 18% of the total sample weight and upgraded the copper 197

content from 5.18% to 16.0%. The "enrichment ratio" concentrate (CuGo in )\ obtained with PAM was about 1.56, and C140 in feed with PAMG 2.1 was 3.1. The "selectivity ratio" (Cu°0 in concentrate \ 1.459.1 - 6.28 for PAM, compared 011°0 in tailings ) was 16 0 with " 8 = 8.9 for PAMG 2.1. 6.6.4 The role of unattached GBHA groups on the flocculation behaviour of meth lolated PAM A series of experiments was carried out in order to establish. whether the selectivity shown by PAMG 2.1 might

have been due to the methylolated PAM, perhaps also activated by unattached GBHA groups or their by-products. Thus samples of the reaction product of GBHA and formald- hyde "Product I" in 6.3.1, and methylolated PAM "Product II" (also in 6.3.1), were tested for flocculation on suspensions of malachite, chrysocolla, calcite and feldspar. The flocculation procedure was essentially the same as

described in 6.6.1; i.e. using 350 p.p.m. Calgon at pH 10. The doses of GBHA "Product I" used were twice that of methylolated PAM "Product II" to correspond to the proportion used in the preparation of PAM 2.1. Results: Malachite and chrysocolla mixed suspension was flocculated with a mixture of 2 p.p.m. "Product II" and 4 p.p.m. GBHA "Product I", leaving slightly turbid supernatant compared with clear supernatants obtained with PAM in section 6.7.3. Feldspar and calcite suspensions were noticed to start flocculation when the mixture con- centration was 2 p.p.m. PAM "Product II" and 4 p.p.m. GBHA ."Product I". Flocculation was increased gradually as the doses of the two products were increased gradually 198

up to 6 p.p.m. PAM "product II" and 12 p.p.m. GBHA "Productl", where partial flocculation of the suspensions occurred. 6.7. Conclusions: PANG is a less powerful flocculant for feldspar and calcite than a simple mixture of methylated PAM and GBHA- formaldhyde product. Increasing the dosage of PAMG 2.1 up to 10 p.p.m. still did not flocculate feldspar and calcite. The reaction conditions in the preparation of PAMG polymers were not optimized and higher degrees of substitution could possibly be obtained for PAMG polymers. However, very high degrees of substitution may not be essential for obtaining the desired properties; for example, most of the commercially produced cellulose xanthates have a:degree of substitution of 50, which corresponds to about 16.7% of the number of reactive OH-groups, yet it is sufficient to change the cellulose polymer properties completely. It should be possible to graft GBHA onto water- soluble polymers such as polyvinylalcohol (PVA), starches (e.g. amylose) and polyvinyl pyrrolidone (PVP), since they (161) too react with formaldhyde If GBHA groups are attached to lower molecular weight polymers, selective dispersants and depressants for copper minerals (or ions) could be obtained. Similarly, there are many chelating groups selective to copper ions which could be grafted onto various long- chain polymers to obtain a large number of selective 199

flocculants. These principles can also be applied to many other cations and their corresponding minerals. 200

CHAPTER 7 PROCESSING OF COPPER ORES BY SELECTIVE FLOCCULATION

7.1 Introduction In treating a poly-disperse suspension by selective flocculation, all minerals should have similar electrical charge (either negative or positive) to avoid the possibility of hetero-coagulation. For example, in a mixture

of calcite (z.p.c. pH 9-10)(17 , malachite (z.p.c. pH 9-9.5) and quartz (z.p.c. pH 2-3), hetero-coagulation would be expected in the pH region 2-9 or 10. In order to avoid this happening, the pH must be 10 or E 2. It is obviously impracticable to maintain the mixture at the low pH (problem of dissolution); therefore the appropriate pH should be 10. One of the essential requirements for the selective flocculation process is that the suspension must be well dispersed and comparatively stable. However, the stability period of the suspension need not be longer than that needed for selective flocculation to be carried out. To induce stability, the zeta--potentialof the minerals must be strongly negative and the ionic strength low. Since the zeta-potential acquired by the minerals, even at high pH, is not usually high enough to keep the suspension stable, the use of low molecular weight dispersants is necessary. In Chapters 1 and 2, it has been shown that natural minerals, suspended in water, usually release some ionic species common to their lattice structure and these ions behave differently at different pH values, forming various 201

hydroxy complexes. These ionic species may adsorb specifically or randomly onto other minerals in the suspension. These effects could become more complicated in the presence of contaminating species, originally adsorbed on the minerals and the possible interaction of the various ionic species. To prevent the interference of these ionic species with the selective adsorption of the flocculant, the use of masking agents would be necessary.

In the flocculation experiments, "Calgon" and "Dispex N40" were used as multi-function reagents. As well as masking the various soluble cationic species, they can also adsorb on the solids surfaces at high pH values, thus increasing the zeta-potential. This effect was recorded on malachite suspension in Chapter 2. The adsorption of these reagents can also result in inhibition of flocculation by competing with the flocculant groups for the surface sites on the minerals. Criteria of selectiva. Assessment of the performance of the flocculation processes on a plant can be described by the "grade" and "recovery" of copper in concentrates.

In developing the present experiments, the absolute values of grade and recovery were not adequate description of the various facets of the performance. Therefore, two more terms were found useful, besides the copper grade in the tailing, to assess the selectivity of the process. These terms are:

a) "Selectivit ratio": which is the ratio of the copper grade in the concentrate to that of the tailings; 202

b) "Enrichment ratio": which is the ratio of the

copper grade in the concentrate to that of the untreated

ore (feed).

Thus in judging the performance of the selective flocculation process, the following criteria were employed: the weight of each of the flocculated concentrates and tailings, copper grade and recovery in both the concentrates

and tailings, enrichment and selectivity ratios.

Materials and equipment:

1. The eepperare: A sample of a dolomitic copper ore from the Congo, kindly supplied by Charter Consolidated Co.,

was investigated. The ore was stated by the company to

consist of malachite, chalcocite, chalcopyrite, neodigenite,

covellite and chrysocolla as copper minerals, with dolomite,

quartz, calcite, pyrite and rhodocryiocite as gangue minerals 080 as well as heterogenite and traces of some unidentified

cobalt minerals. A preliminary mineralogical examination

supplied by the company is given in Table 7.1. According

to a report from the company, the copper sulphides are

not associated with gangue minerals to a significant

extent and their liberation is good, although chalcocite

and neodigenite are often intergrown in composite grains.

Malachite, on the other hand is associated with the

carbonates and mainly with dolomite. The liberation

size of malachite and the size distribution of the various

copper minerals were not reported. The determination of

the size distributions and the liberation size for copper

minerals was not performed in this work either, since

the main concern at that stage was to establish the funda- 203

Table 7.1 Approximate mineralogical of the dolc2jLLLcsa2ptrIEtE

Mineral 12r....9.221..i Malachite 3

chalcocite 3

neodigenite 3 chalcopyrite < 1

covellite < 1

pyrite < 1

rhodocriocite <1

dolomite 20

calcite 1

quartz 70 204

mental aspects of the selective flocculation process. However, it is firmly believed that knowledge of the liberation size will certainly help to improve the results. The typical particle size of a dry ground sample of the ore, shown in Fig. 7.1, was generally below 10 microns and mainly in the 1-2vm range. There was an appreciable number of particles in the sub-micron range. The copper content in the ore sample was about 5.0%.

2. The flocculants: PAMG 2.1, PAMG 6 and PAMG 7 were used "as prepared" in Chapter 6,1.e., they were not purified. The concentration of the polymers in solutions was 0.1% based on the original weights of PAM used in their preparation.

3. The flocculation apparatus: This apparatus consisted of a glass cylinder either 5 cm or 10 cm in diameter and 42 cm total length, including a conical discharge outlet, as shown in Fig. 7.2. In designing this cylinder, it is arranged that the angle of joining the conical part with the cylindrical part was high so that the flocs could slide along smoothly without hinderance. The rotating system consisted of two steel rollers about 3 cm x 9 cm each (total length 14 cm), connected with a variable speed motor. The distance between the rollers was about 3 cm. The whole system was mounted on a pivoted metal support about 11.5 x 32.5 cm so that the inclination angle could be adjusted. The total length of base was 40.5 cm. Separation of suspensions from the flocs was 0 5 4 • • '7) •

G. a

S • •

4 0

10 tim

FIG. 7.1: PHOTOMICROGRAPH OF ZAIRE OXIDIZED COPPER ORE, AFTER FINE GRINDING (AS USED IN SELECTIVE FLOCCULATION TESTS).

FIG. 7.2: LABORATORY CONDITIONING APPARATUS FOR IMPROVING SELECTIVE FLOCCULATION SEPARATIONS. 206

'done by either vacuum suction or introducing water jets, as shown in Fig. 7.2. However, to avoid dilution in the flocculation experiments, the vacuum suction was preferred The purpose of this apparatus was to favour the growth of the floes and to aid their separation as well as releasing some of the entrapped particles. At the start of flocculation, the inclination angle of the cylinder should be small to minimize the travel of the flocs. By increasing the angle at a later stage, the flocs could be made to move to the discharge outlet, leaving the suspension above. The role of the slow rotation of the inclined cylinder in increasing the floc growth and the release of entrapped particles was first observed and described by Yarar and Kitchener( 2' 179). 4. Micronizin mill: A laboratory vibratory mill from McCrone Research Associates Ltd., London) was used for the wet grinding of the ore samples up to 10 g in 40 ml of water. The grinding elements in this mill are stated by the supplier to be of fine-grained non-porous sintered corundum (alumina) and are contained in a 125 ml polythene jar. Some abrasion of the polythene jar due to grinding was noticed. The abraded particles appeared floating on the suspension surface and were skimmed before commencing the flocculation experiments.

5. Agate mortar and porcelain mill: The agate mortar was used in the dry grinding of small samples, and the porcelain mill in the wet grinding of larger samples

(up to 41 g). The grinding medium was porcelain balls, 207 cleaned with conc. HC1 and washed with distilled water several times before use. 6. 2ihprapR22 An ultrasonic probet ma.gxietic stirrer, light absorption spectrometer, glass beakers and cylinders were also used. An atomic absorption spectrometer was used for the determination of copper content in solutions prepared from the minerals. Dissolution of minerals for use with atomic absorption is described in Appendix 2.

7.2 Preliminary investigations A small sample of the ore was dry ground in an agate mortar and made into approximately 1% suspension with dist. water containing 350 p.p.m. Calgon at pH 10.5. When this suspension was treated in the same manner as that in Chapter 6.6 - much of the ore sample was flocculated unselectively by 1 p.p.m. PAMG 2.1 - contrary to expectations based on the previous tests with separate minerals or synthetic mineral mixtures. This observation led to a study of the difference between an ore and a synthetic mixture. In an attempt to understand the difference, the possible differences were rationalized as overleaf: 208

Ore Synthetic mineral mixture

1 The minerals have been exposed The minerals are usually from to the same geological con- different origins, and were not ditions i.e., etc., exposed to the same geological which may result in altering conditions as the ore; the the minerals to different phases degrees and kinds of alter- ations may be very different.

2 The minerals may contain The minerals may also be con- traces of different ionic taminated but it may be species and possibly colloids entirely different kinds and as contamination, degrees of contamination.

3 The minerals are ground to- The minerals are essentially . gether; problem of slimes ground separately. coating and smearing of minerals are familiar in mineral processing.

4 The proportion of copper The proportion may be higher. minerals may be low. 209

The following steps were taken, to investigate factors 1, 2 and 4 by simulating the ore and checking selective flocculation. Thus a mixture of (a) uartz dolomite calciLl_aaallpariLa of the .same proportions to those in the ore (i.e. approximately 7 : 2 : 0.1 : 0.1) was made into a 1% suspension with distilled water containing 350 p.p.m. Calgon at pH 10.5. When the suspension was treated with the flocculant in the same manner as before, no flocculation was noticed even at 7 p.p.m. PAMG 2.1. A mixture (b) of chalcocite, malachite, chrysocolla

2.0. 22.2129- 1pyriteof the proportions 6 : 3 : 2 : 1 was made into a 1% suspension and treated as before. Flocculation was noticed to start at 1 p.p.m. PAMG 2.1 and increased at 2-3 p.p.m. Then a mixture (c) of all minerals of mixtures (al ana_021, also of a similar proportion to those in the ore, was treated as in mixtures (a) and (b). However, the flocs did not form immediately as in the previous synthetic mixtures in Chapter 6, where copper minerals were in higher proportions. Instead, they formed in appreciable amount only after about 10 minutes of slow rotation; no white or black flocs were seen to form, and only grey-to-green flocs of the copper minerals were formed. This indicated that selective flocculation of copper minerals from the ore was feasible. The slowness of the rate of flocculation might be explained in terms of the rate of collision of the copper minerals particles, which depend on. the probability of 210

collision between these particles. The probability of collision is a function of the following factors: (a)the rate and time of shearing (i.e. intensity), (b)the hydrodynamic pattern of mixing, (c) solids content in suspension, (d) proportion of copper minerals particles in the suspension, and (e) the particle size distribution. With an ordinary magnetic stirrer at a constant moderate shear-rate, there can be a situation of non-collision between two copper mineral particles moving at a distance with the rotating mass of suspension. The higher solids content and particles size distribution (i.e. amount of fine particles) would increase the viscosity of the suspension which might present physical hindrance to collision. It can be concluded that with high solids content suspension containing fine particles and a small proportion of copper minerals, good mixing techniques and high shear intensity would be necessary to obtain a high rate of collisions and hence a high rate of flocculation. To investigate the significance of factor (3), that is the effect of co-grinding, again a model of the ore was simulated by a synthetic mixture of the major minerals in the same proportions as the ore. A small sample of this mixture was dry ground in an agate mortar for 1 hour, then tested for flocculation in a similar fashion to that of mixture (c) earlier. Much of it was Immediately flocculated unselectively by 1 p.p.m. PAMG 2.1. In another experiment, the co-ground mixture suspension was dispersed with an ultrasonic probe at high energy for 211

3 min., and with magentic stirring at high shear-rate for 2 min. before adding the flocculant. Again much of it was flocculated unselectively by 1 p.p.m. PAMG 2.1. From these experiments the important problems were seen to be (1) the effects of dry-grinding, (2) the effect of the proportion of copper minerals in the ore, and (3)the effect of contaminations naturally found with an ore. These problems were overcome as follows:

(1) wet grinding in the presence of dispersants at high pH, (2) improving the collision rate by increasing the shear-rate during flocculation, and (3) use of greater amounts of Calgon and Dispex N40. Experimental:

In three experiments, 3 samples of 5 g each were ground in the McCrone mill with 40 ml dist. water at pH 10.5 containing (1) 50 p.p.m. Dispex, for 35 min., (2)350 p.p.m. Calgon for 30 min, and (3) 350 p.p.m. Calgon and 50 p.p.m. Dispex for 30 min. Small parts of these suspensions were diluted to 50 ml to make the following solids content: 1.4%, 2.0% and 1.3% respectively. The reagents were adjusted accordingly by the same ratio.

1 p.p.m. of PAMG 2.1 was added to each suspension, while stirring at high shear-rate for 1 min. and at low shear- rate for a further 1 min. The suspensions were then transferred to 250 ml cylinders where they were diluted to 200 ml and rotated slowly for about 5 min. The flocculated portions ('boncentratd) were separated from the suspensions ("tails") by decantation, then dried, weighed and assayed for copper. 212

From the results shown in Table 7.2, experiment (3) (using both Calgon and Dispex) gave the highest copper grade in the flocculated fractions. It was decided, therefore, to use both Calgon and Dispex in the grinding and flocculation circuits at pH 10.5-11.

Table 7.2 Results of experiments 1-3

' gio.p Products Wt. % Cu % Cu-distribution % 1 conc. 22.5 8.5 43.5 tails 77.5 3.2 56.5 calc. heads 100.0 (4.39) 100.0

2 conc. 17.0 8.5 32.2 tails 83.0 3.5 66.8 talc. heads 100.0 (4.35) 100.0

3 conc. 5.1 15.0 14.4 tails 94.9 4.8 85.6 talc. heads 100.0 (5.32 100.0

In all the following experiments, grinding was carried out in the McCrone mill except in experiment 25. Also, 400 ml beakers were used for suspensions volumes between 200-300 ml, and 250 ml glass beakers for volumes between 50-150 ml.

7.3 Design of flow-sheets for selective flocculation arocess The flow-sheets attempted in this work were somewhat

analogous to those of froth flotation, in that they 213 included rougher (R), cleaner (C1), recleaner (Reel) and scavenger (Sc) stages. Each stage was preceeded by a dispersion (D) process. Several flow-sheets were attempted, with the combined variations in the levels of "conditioning" reagents, flocculent dose, mode of addition of reagents and the shear-rate. 7.3.1. Flow-sheet This flow-sheet, shown in Fig. 7.3, consisted of 3 stages of flocculation namely; rougher, cleaner and scavenger, each preceeded by a dispersion process. This flow-sheet was attempted in two experiments, 4 and 5. Experiment 4: 5 g of the ore sample was ground in 40 ml dist. water containing 350 p.p.m. Calgon and 50 p.p.m. Dispex at pH 10.5 for 1 hr. The suspension was diluted to 250 ml (i.e., 2% solids) and the concentrations of reagents were readjusted accordingly. The suspension was stirred at high shear-rate by a magnetic stirrer for 5 min. and was left still in the beaker for further 5 min. The purpose of this step was to estimate the amount of coarse particles present in the suspension which might interfere with the flocculation process. The coarse particles were separated from the suspension by decantation; when dried, this fraction was about 8% of the total sample weight. This fraction was not used in the experiment. The suspension was treated with 1 p.p.m. PAMG 2.1 while stirring at high shear rate for 1,5 min. then at low shear-rate for 2 min., followed by 7 min. of slow rotation in a 250 ml cylinder. The flocculated portion was recovered by decantation and washed with 50 ml dist. water while 214 stirring for 5 min. The flocculated fraction (Cl.conc.) was recovered, dried and weighed. The concentrations of Calgon and Dixpex were readjusted to 350 p.p.m. and 50 p.p.m. respectively at pH 10.5 in the suspension (300 ml). 0.5 p.p.m. PAMG 2.1 was added during stirring at high shear for 2 min. and at low shear for further 2 min. followed by slow rotation in a cylinder (by hand) for 7 min. The flocculated portion (Sc. conc.) was separated from the stable suspension (Tails). The products were dried, weighed and assayed for copper. The results are shown in Table 7.3. The total consumption of PAMG 2.1, Dispex and Calgon in kg tonne ore treated was: 0.087, 3.0 and 21.0 respectively.

Table 7.3: results of experiment 4

Products Wt. % Cu % Cu-distribution %

Cl. conc. 6.9 18.0 25.3 Sc. conc. 26.1 8.4 44.7 Tails 67.0 2.2 30.0 calc. heads 100.0 (4.9) 100.0 selectivity ratio = 8.2 enrichment ratio = 3.67

The results indicated that selective flocculation had occurred but the recovery of copper in the concentrate was rather low. Therefore an attempt to increase the recovery, as described in experiment 5. 215

Experiment 5: A 5 g sample of the ore was ground in 40 ml dist. water containing 350 p.p.m. Calgon and 50 p.p.m. Dixpex at p11 10.5 for 2 hrs. Part of the suspension was diluted for 250 ml (to make roughly 3.6% solids) and the doses of both Calgon and Dixpex were readjusted accordingly (i.e. to 350 p.p.m. Calgon and 50 p.p.m. Dispex) at pH 10.5. After stirring at high shear for 5 min., the suspension was mixed with 1.5 p.p.m. PAMG 2.1, while stirring was continued for 1.5 min. at high shear. The suspension was left still in the beaker for 2 minutes, then the flocculated fraction was recovered and washed with 50 ml dist. water during stirring for 5 min., at high shear. The flocculated concentrate (Cl.conc.) was recovered, dried and weighed. The concentrations of Calgon and Dispex in the suspension (300 ml) were adjusted to 350 p.p.m. and 50 p.p.m. respectively at pH 10.5. 3 p.p.m. PAMG 2.1 was then administered while stirring for 2 min. at high shear. The suspension was left still in a cylinder for 2 min. followed by slow rotation by hand for 10 min. The flocs (Sc. cone.) were separated from suspension by decantation and were dried and weighed. The total consumption in kg/tonne of PAMG 2.1, Dixpex and Calgon was: 0.354, 4.16 and 29.1 respectively. The results are shown in Table 7.4. 216

Ore suspension

Tails

Cl.conc.

Fig.7.3 Flow-sheet 1

Ore suspension

Flocculant

R Tails

Cl Sc Sc.tail

Cl.conc. Sc.conc.

Fig.7-4 Flow-sheet 2:Bulk flocculation 217

Table 7.4: Results of experiment 5

Products wt. Cu % Cu-distribution %

Cl. conc. 63.4 7.4 92.0 Sc. conc. 0.8 0.75 0.1 Tails 35.8 1.10 7.9 calc. heads 100.0 (4.99) 100.0 selectivity ratio = 6.6 enrichment ratio = 1.48

The results showed an increased recovery of copper in the Cl. conc. but the grade was rather low. Much of the ore was flocculated (63.4%), and the selectivity ratio (6.6) and the enrichment ratio (1.48) were not high. An attempt to improve the performance of the process was made in flow-sheet II. 7.3.2. Bulk flocculation rocedure• flow-sheet II

Experiments 6 and 7 were carried out following flow- sheet II, illustrated in Fig. 7.4, which consists of a rougher, a cleaner and a scavenger stage.

Experimental; A 5 g sample was ground in 40 ml dist. water containing 400 p.p.m. Calgon and 100 p.p.m. Dispex at pH 10.5 for 2 hrs. The suspension was divided into 2 parts, each of which was diluted to 100 ml suspension and the concentrations of Calgon and Dispex were adjusted accordingly at 400 p.p.m. and 100 p.p.m. respectively, at pH 10.5. It appeared 218

later that the suspension used in experiment 6 contained 2.2% solids and that in experiment 7 contained 2.8% solids. Experiment 6: After stirring for 5 min. at high shear, the 100 ml (2.2% solids) suspension was treated with 3 p.p.m. PAMG 2.1 and stirring was continued for 1 min. at high shear rate, then at low shear for 2 min. followed by 10 min. of slow rotation in a cylinder. The flocculated concentrate was separated by decantation and redispersed in 100 ml dist. water containing 400 p.p.m. Calgon and 100 p.p.m. Dispex at pH 10.5 by stirring for 5 min. at high shear rate, followed by slow rotation in a cylinder for 10 min. The flocculated fraction (Cl. cone.) was recovered, while the decanted suspension- was allowed to flocculate for further 10 min. under slow rotation in the cylinder. The flocs,(Sc. conc.) were separated from the suspension (Sc. tail). All samples were dried, weighed and assayed for copper. The results are shown in Table 7.5. The total consumption of PAMG 2.1, Dispex and Calgon in kg/tonne was: 0.136, 9.1 and 36.4 respectively.

Table 7.5: results of experiment 6

Products ELt_i, Cu % La=c1LtLELlaIlaa%.

Cl. conc. 17.8 16.0 55.0

Sc.conc. 3.8 7.8 5.7 Sc. tail 28.2 4.0 21.8 fail 50.2 1.8 17.5 calc. heads 100.0 5.18) 100.0 selectivity ratio = 8.9

'enrichment ratio 3.1

219

The results indicated an overall improvement in the performance. The grade in the concentrate was higher than in experiment 5. Also, the enrichment ratio and selectivity ratio were higher in experiment 6 than those of experiment 5. Experiment 7.: In this experiment, ordinary poly- acrylamide (PAM) was tested, in order to establish the improvement in performance due to its modification to PAMG 2.1. Thus the 100 ml suspension (2.8% solids) was

treated with 3 p.p.m. PAM, following the same procedure and conditions in experiment 6. The results are shown in Table 7.6. The total consumption of PAM, Dispex and Calgon in kg/tonne was: 0.107, 7.15 and 28.5 respectively.

Table 7.6: results of experiment 7

Products Wt. Cu- distribution O

1. cone. 48.1 9.1 74.5 :c. cone. 6.3 9.1 9.8 1. tail 34.0 2.2 12.8

ail 11.6 1.45 2.9 calc. heads 100.0 (5.86) 100.0

selectivity ratio = 6.28

nrichment ratio = 1.56

irrlir'at,=,d that a large proportion of the ore sample was flocculated compared with experiment 6. The

enrichment ratio (1.56) and selectivity ratio (6.28) were low compared to 3.1 and 8.9 with PAMG 2.1 in experiment 6. 220

Although the reagents consumption in experiment 7 was slightly less than in experiment 6, it is not likely that the performance of PAM would be better or even similar to that of PAMG 2.1 at the same or even higher reagents consumption, as these flocculation experiments were proved later to be not very sensitive to small changes in reagents concentrations. Therefore it can be concluded that PAMG 2.1 was more selective flocculant than PAM.

7.3.3. Multi-stage flocculation; flow-sheet This flow-sheet, as shown in Fig. 7.5, consisted of one rougher and two cleaning stages, followed by two scavenging stages of the two cleaners tailings. Experimental: An ore sample of 5 g was ground in 40 ml dist. water containing 400 p.p.m. Calgon and 100 p.p.m. Dispex at pH 10.5 for 2 hrs. The ground ore suspension was diluted to 200 ml and divided into two 100 ml volumes and the levels of Calgon and Dispex were readjusted accordingly. One volume was used in experiment 8 and the other was used in experiment 9 in 7.3.4. Experiment 8: The 100 ml suspension (2.2% solids) containing 400 p.p.m. Calgon and 100 p.p.m. Dispex at pH 10.5, was dispersed by stirring at high shear for 5 min. The flocculant PAMG 2.1 was added while stirring was continued for 1 min. at high shearing, and for 2 min. at low shearing, followed by slow rotation in a cylinder by hand for 10 min. This dispersion procedure was adopted in all the dispersion stages in this experiment. Then 6 p.p.m. PAMG 2.1 221

(•v0.273 kg/tonne) was added and the flocculated fraction was recovered from the suspension (Tail) by decantation. The flocs were redispersed twice in 100 ml dist. water containing 400 p.p.m. Calgon and 100 p.p.m. Dispex at pH 10.5 to produce recleaned concentrate(Recl.conc.). The tailings of each cleaning stage were treated with 1 p.p.m. PAMG 2.1 to produce the corresponding scavenging concentrates (Sc. conc. 1 and Sc. conc. 2). The total consumption of PAMG 2.1, Dispex and Calgon in kg/tonne ore was : 0.364, 13.6 and 54.5 respectively. The results are shown in table 7.7. below.

Table 7.7: results of experiment 8

Products Wt. -_-/Cu % Cu-distribution %

reel. conc. 20.3 20.0 68.8

c. conc. 1 4.5 4.6 3.5 .c. conc. 2 3.6 10.0 6.1 all 40.5 1.88 12.9 1. tail 1 22.1 1.56 5.85 '1. tail 2 9.0 1.89 2.88 alc. heads 100.0 (5.9) 100.03

-electivity ratio = 10.65

-nrichment ratio = 3.39

The results indicated improvements in the grade and recovery in the flocculated concentrate (Reel. conc.) over those of experiment 6. Also, both the selectivity ratio and the entrichment ratio are higher than those in 027 Ore suspension

— Flocculant

Tails

Cl. tail 1

Hoc culant Sc. conc.1

Recl Cl.tail 2

Recl conc. Sc.conc.2

Fig.7.5 Flow-sheet 3:Multi-stage flocculation

Ore suspension

Flocculant

Tails

Flocculant

Cl. tail 3

CI. tail 1 Cl. tail 2 Cl.conc.1 Cl.conc.2 Cl. Sc. conc.

Fig.7.6 Flow-sheet 4: Starvation flocculation 223

experiments 6 and 7. The copper content in the feed (5.9%) was higher than the stated average content, perhaps due to inadequate sample selection. However, when experiment 9 is also considered, as both experiments were from one ore sample, the average copper content in that sample was about 5.55% which was not far off the bulk value. In this work, no special attention was given to sampling and each ore sample in each experiment was taken independently.

An attempt to increase the copper grade in the concentrate and thence the selectivity and enrichment ratios was made in experiment 9. 7.3.4 "Starvation" addition of flocculant; flow-sheet IV This flow-sheet, as shown in Fig. 7.6, consisted of 6 flocculation stages.

Experiment 9: The 100 ml suspension of 2.8% solids, containing 400 p.p.m. Calgon and 100 p.p.m. Dispex at pH 10.5 was stirred for 3 min. at high shear. The suspension was treated with 0.5 p.p.m. PAMG 2.1 while stirring was continued for 2 min. at high shear (to disperse the flocculant), followed by 2 min. at low-shear and 10 min. of slow rotation in a cylinder (250 ml). This dispersion procedure was used wherever there was an addition of the flocculant; otherwise, it was only stirred for 5 min. at high shear-rate. The flocculated portion was recovered and then washed (i.e. redispersed) in 100 ml dist. water containing 400 p.p.m. Calgon and 100 p.p.m. Dispex at pH 10.5. The products of this cleaning stage (C1- conc. 1

and Cl, tail 1) were recovered. The tailings of the 224

first flocculation stage (Rougher 1) were treated with another 0.5 p.p.m. PAMG 2.1, (Rougher 2) and the flocculated fraction was similarly cleaned in 100 ml dist. water containing 400 p.p.m. Calgon and 100 p.p.m. Dispex at pH 10.5 to produce a second concentrate (Cl. conc. 2), and tailing (Cl. tail 2). The tailings of the Rougher 2 were treated with 6 p.p.m. PAMG 2.1 to recover more copper (scavenging) as shown in Fig. 7.6, the product of which was similarly redispersed in 100 ml dist. water containing 400 p.p.m. Calgon and 100 p.p.m. Dispex at pH 10.5 to produce scavenger conc. (Cl. sc. conc.) and tailings (final tail and Cl. tail 3). The results are shown in Table 7.8. The total consumption of PAMG 2.1, Dispex and Calgon in kg/tonne ore treated was 0.267, 14.3 and 57.2 respectively.

Table 7.8: results of experiment 9

Products Wt. Cu % Cu-distribution °o

Cl. conc. 1 1.13 28.0 6.03 Cl. tail 1 6.59 8.5 10.64 Cl. conc. 2 10.82 16.5 33.92 Cl. tail 2 7.82 1.07 1.59 Cl. sc. conc. 30.76 6.45 37.70 CL. tail 3 19.31 1.45 5.32 final tail 23.57 1.07 4.80 calc. heads 100.00 (5.26) 100.00 selectivity ratios = 28/1.07 = 26.2 & 16.5/1.07 = 15.2 enrichment ratios = 28/5.26 = 5.32 & 16.5/5;26 = 3.14 225

The results shdwed a sharp rise in the 'copper grade

in the concentrate (Cl. conc. 1) due to the starvation additions of the flocculant, indicating a high selectivity But unfortunately the recovery in that concentrate was

very low. (However, a combined concentrate of Cl. conc. 1 and Cl. conc. 2, could make a mixture of average 17.5% copper and 40% recovery). The selectivity ratio (26.2) and the enrichment ratio (5.32) in the first concentrate

were the highest achieved so far. The reagents consumption was rather high; but because the economic factor was not the main concern at this stage, no attempt to reduce the consumption was made. The economic factor was considered

in later experiments (21-25). 7.3.5. Multi-sta e addition of flocculent• semi-c clic flow-sheet V. In this more complicated arrangement in Fig. 7.7, the flocculant was added in small increments, followed by short periods of dispersion, following the results of experiment 9. Experiment 10: In this experiment, the dispersion

procedure was as follows. The suspension was stirred at high shear for 5 min. before adding the polymer and for 1 min. after each addition, then at low shear for further 1 min. The suspension was transferred to the flocculation

apparatus in Fig. 7.2, and was kept rotating slowly (between about 20-40 r.p.m.) for 10 min., except that in

the rougher stage it was kept for 50 min. Thus a 100 ml suspension of /%., 2.8% solids containing

400 p.p.m. Calgon and 100 p.p.m. Dispex at pH 10.5 was

treated with 1 p.p.m. PANG 2.1 according to the flow-sheet. 226

Ore suspension Fig:7-7 Flow-sheet 5 (Semi-cyclic)

Tail 1

CI Sc 2 —Tail 2

Reel Sc 3 Tail 3

Red. conc. Sc. conc.

Ore suspension

R Fig.7.8 Flow-sheet 6

Cl Tail 1

Red Tail 2

Red. conc. Sc. conc. 227

The polymer was added in 5 increments of 0.2 p.p.m. each. The suspensions from the rougher and cleaning stages were treated with 1 p.p.m. PAMG 2.1 in the scavenger. The flocculated concentrates of the rougher and scavenger were cleaned twice in dist. water containing 400 p.p.m. Calgon and 100 p.p.m. Dispex using 0.2 p.p.m. PAMG 2.1 in each stage, to produce recleaned concentrate (Reel. conc.), scavenger concentrate (Sc. conc.) and three tailings. The results are shown in Table 7.9. The total consumption of PAMG 2.1, Dispex and Calgon in kg/tonne ore was: 0.171, 17.85 and 71.5 respectively.

Table 7.9: Results of experiment 10.

Products 21.L2Z Cu % Cu-distribution 2

Reel. conc. 11.6 22.0 50.4 Sc. conc. 16.7 7.5 24.7 Tail 1 51.5 1.75 17.8 Tail 2 11.4 1.4 3.2 Tail 3 8.8 2.25 3.9 calc. heads 100.0 (5.06) 100.0 selectivity ratio = between 22.0/1.4 = 15.75 & 22/2.25 = 9.8 enrichment ratio = 22.0/5.06 = 4.35

The results show an increase in copper grade in concentrate (Recl. conc.) over experiment 8, but the recovery was lower. There is also an increase in the selectivity ratio and the enrichment ratio. However, 228

it can be concluded that the effect of addition of the polymer in many small increments did not increase the performance significantly. It seems that the total concentration of the polymer in each flocculation stage is the important factor and not the mode of addition. Effect 2ffsatil on selective flocculation was investigated in the following experiment, following flow- sheet VI in Fig. 7.8.

Eu2rimentili A 100 ml suspension of 2.7% solids containing 450 p.p.m. Calgon and 200 p.p.m. Dispex at pH 10.5 was stored for 7 days at room temperature. The suspension was dispersed following the same procedure of experiment 10, but the flocculation time in the apparatus was increased to 20 min, and for the rougher stage to 30 min. 1.5 p.p.m. PAMG 2.1 was added in small increments of 0.25 p.p.m. in the rougher, and the flocculated fraction was washed twice in 100 ml dist. water containing 450 p.p.m. Calgon and 200 p.p.m. Dispex at pH 10.5. Only 0.1 p.p.m. flocculant was added to each cleaning stage. The two suspensions from the rougher and the first cleaner were treated in the scavenger with 1.5 p.p.m. PAMG 2.1, added in small increments of 0.25 p.p.m. The flocculated fraction was cleaned in the 100 ml suspension (Tail) of the second cleaner as shown in Fig. 7.8 and treated with

0.1 p.p.m. flocculant to produce scavenger concentrate (Sc. conc.) and Tail 2, The results are shown in Table 7.10. The total consumption of PAMG 2.1, Dispex and Calgon in kg/tonne ore was 0.178, 22.2 and 50.0 respectively.

229

Table 7.10; Rdsults of experiment 11.

Products 12/2 Cu % Cu-distribution

Reel. conc. 22.0 14.0 64.3 • Sc. cone. 2.5 6.0 3.1 Tail 1 65.9 2.15 29.5

'Mal 2 9.6 1.55 3.1 calc. heads 100.0 (4.8) 100.0

14. = 9.0 & 14/2.15 = 6.5 selectivity ratio = between -----1 .55 enrichment ratio = 2.92

The results clearly indicate a sharp drop in the copper grade in the concentrate (Reel. conc.) compared with experiment 10. Also, the enrichment and the selectivity ratios were lower than in experiment 10. Therefore, it can be concluded that ageing of suspension can have a deleterious effect on selective flocculation. The com- parison with experiment10 is valid despite the difference in reagents levels which might make it less accurate. 2 This conclusions is in agreement with published literature

7.3.6. The standard flow-sheet; flow-sheet VII In this design, there were four flocculation stages, as shown in Fig. 7.9; one of each of the rougher and the scavenger, and two for cleaning the concentrate. This flow-sheet was adopted in all of the flocculation experiments in the following sections in this Chapter (unless other- wise stated). 230

Ore suspension

R --Tail

Sc. conc.

CI Cl.tail

L Red Recl.tail

Rect. conc.

Fig.7-9 Flow-sheet 7: Standard flow-sheet

231

Experiment 12: A 5 g sample was ground in 40 ml dist. water containing 300 p.p.m. Dispex and 200 p.p.m. Calgon at pH 11 for 2.5 hrs. The suspension was diluted to 200 ml ( i.e. 2.5% solids) and the levels of reagents were adjusted accordingly. After stirring at high shear for 5 min, 3 p.p.m. PAMG 6 was added in two portions of 1.5 p.p.m. each while stirring was continued for a further 5 min. after each addition. The flocculating suspension was slowly rotated in the flocculation apparatus for about 15 min., except that in the rougher it was 36 min- The flocculated concentrate was cleaned twice in 100 ml dist. water con- taining 300 p.p.m. Dispex and 200 p.p.m. Calgon at pH 11. It was treated with 1.5 p.p.m. PAMG 6 in the first cleaner, and with 0.5 p.p.m. in the second cleaner. The tailings of the rougher were treated with 1.5 p.p.m. flocculant in the scavenger, to produce scavenger concentrate as a middling and a final tail. The results are shown in Table 7.11. The total consumption of PANG 6, Dispex and Calgon in kg/tonne ore was: 0.22, 24.0 and 16.0 respectively.

Table 7.11: Results of experiment 12

Products EIILZ2-m Wt. % Cu % units Cu-distribution

ecl. conc. 0.804 15.9 22.5 357.8 68.8 Sc. conc. 0.605 12.0 7.25 87.0 16.7 Reel tail 0.288 5.7 0.725 4.13 0.8 Cl. tail 0.541 10.7 1.4 14.98 2.9 Final tail 2.812 55.7 1.01 56.26 10.8 calc. heads 5.05 100.0 (5.202)520.17 100.0

selectivity ratio 22.5/1.01 = 22.3 r nrichment ratio 22.5/5.2 4.32 232

The results indicate good overall performance of the process. The selectivity ratio and the enrichment ratio were high and 'the copper grade and recovery in the con-

centrate were higher than in most of the previous experiments. The results were encouraging but more development work was needed to improve them further. This is described in the following sections.

7.4 Studies to improve grade and recovery In the previous experiments, copper concentrates of copper grade over 20% (up to 28%) had been achieved, while the recovery in most cases was about 60-70%. To improve the recovery, it was first thought that the effectiveness of the polymer should be enhanced further. Thus PAMG 7 was developed with this aim in mind. Preparation of PAMG 7 was described in Chapter 6. 7.4.1. E)Jatmentsae aT. Experiment 13: A 5 g sample was ground in 40 ml dist. water containing 410 p.p.m. Dispex and 140 p.p.m. Calgon at pH 11 for 2,5 hrs. It was diluted to 200 ml (1.e. 2.5% solids) and the Calgon and Dispex concentrations were readjusted accordingly at pH 11. The suspension was treated with 4 p.p.m. polymer, added in two portions of 3 and 1 p.p.m., following the standard flow-sheet in Fig. 7.9. The dispersion procedure was as follows: stirring at high shear for 1 min. after the polymer addition and at low shear for 2 min. after the last addition of the polymer, followed by 20 min. of slow rotation in the flocculation apparatus. The initial dispersion of suspensions 233

was at high shear rate for 5 min. The flocs were washed twice in 200 ml and 100 ml dist. water containing 410 p.p.m. Dispex and 140 p.p.m. Calgon each, at pH 11. The polymer additions were 2, 0.7 and 1.5 p.p.m. in the cleaner, recleaner and the scavenger respectively. The suspensions were separated from the flocculated con- centrate by vacuum suction. The results are shown in Table 7.12. The total consumption of PAMG 7, Dispex and Calgon in kg/tonne ore was : 0.314, 41.0 and 14.0 respectively.

Table 7.12: Results of experiment 13

Products Wt. dram Wt. Cu al-dis... t._7_9...22.2a.1

Reel. conc. 2.22 43.9 14.0 97.4 Recl. tail 0.485 9.6 0.335 0.5 Cl. tail 0.805 15.9 0.20 0.5 O'inal tail 1.544 30.6 0.335 1.6 $c. conc. 0.00 0.0 calc. heads 5.054 100.0 (6.312) 100.0 selectivity ratio = 14/0.335 = 41.7 and 14/0.2 = 70.0 enrichment ratio = 14.0/6.3 = 2.22

The results show very high recovery of copper in the concentrate. Also, the selectivity ratio was very high and the copper loss in tailings was minimal but the enrichment ratio was only 2.22 and the grade was rather low. 2311-

There was no flocculation in the scavenging stage (i.e., weight of Sc. conc. = 0). The weight of flocculated concentrate was more than in experiment 12 which accounts for the high recovery in experiment 13.

Experiment 14: A 2.5 % solids suspension was treated in the same way as experiment 13, except that the polymer addition was 1 p.p.m. more in the two cleaning stages (i.e., 2 p.p.m. instead of 1 p.p.m.). A third cleaning stage was attempted in this experiment, where 1 p.p.m. of the polymer was added. The total reagents consumption of PAMG 7, Dispex and Calgon in kg/tonne ore was : 0.334, 49.3 and 16.8 respectively.

Table 7.13: Results of experiment 14

Products Wt. % -Cu % Cu-distribution

Reel. conc. 31.98 12.2 85.8 Sc. conc. 0.35 3.364 0.25 Recl. tail 2 4.86 1.75 1.85 Reel. tail 1 8.79 1.30 2.5 Cl. tail 15.96 0.826 2.9 Final tail 38.06 0.80 6.7 cab. heads 100.00 (4.55) 100.0 selectivity ratio = 15.2 enrichment ratio = 2.68 ---- 235

From these results it was concluded that the extra stage did not improve the copper grade, although the selectivity ratio was 15.2. Experiment 15: A 5 g sample was ground in 40 ml dist. water containing 410 p.p.m. Dispex and 140 p.p.m. Calgon at pH 11 for 3 hrs. The suspension was diluted to 200 ml (i.e. 2.5% solids) and the concentrations of Calgon and Dispex were readjusted accordingly at pH 11. The dispersion procedure was the same as in experiment 13. 2 p.p.m. of the polymer was added in small portions of 0.5 p.p.m. each, in the rougher stage and 1 p.p.m. of two increments of 0.5 p.p.m. in each of the two cleaning stages. 1.5 p.p.m. polymer was used in the scavenger stage in one addition. Each of the two cleaning stages was carried out in 100 ml dist. water containing 410 p.p.m. Dispex and 140 p.p.m. Calgon at pH 11. The total consumption of PAMG 7, Dispex and Calgon in kg/tonne ore was: 0.32, 32.8 and 11.2 respectively. The results are shown in Table 7.14 below: Table 7.14: Results of experiment 15

Products Wt. gram W . °,4 Cu °/0 units - -distribution V tecl. conc. 1.805 35.9 3.5 484.65 94.6 Recl. tail 0.445 8.85 0.595 5.27 1.0 Cl. tail 0.896 17.8 0.315 5.61 1.1 Sc. conc. 0.048 0.95 1.40 1.33 0.3 inal tail 1.832 36.5 0.425 15.51 3.0 calc. heads 5.026 100.0 5.124) 512.37 100.0 ..0m...... electivity ratio = 31.8 enrichment ratio = 2.65 236

The results show high recovery and selectivity ratio but the grade and the enrichment ratio were rather low.

It seems that PAMG 7 is a strong flocculant, but the copper grade was always low in experiments 13-15. An experiment was made with PAMG 6 in order to check whether the lower grade in these experiments was due to the effect of the concentration of Dispex and Calgon. The experiment is described below. Experiment 16: A sample of 5 g was ground 40 ml dist. water containing 410 p.p.m. Dispex and 140 p.p.m. Calgon at pH 11 for 2.5 hrs. It was diluted to 200 ml (i.e. 2.5% solids) and the levels of reagents were readjusted accordingly at the same pH. The suspension was stirred for 5 min. at high shear rate, then 3 p.p.m. PAMG 6 was added followed by 2 min. stirring at high shear. This was followed by another addition of 1 p.p.m. PAMG 6 and the dispersion was continued for 5 min. at high shear, then at low shear for 2 min. followed by 30 min. slow rotation in the floccu- lation apparatus. The flocculated concentrate was cleaned (re-dispersed) twice each in 100 ml dist. water containing

410 p.p.m. Dispex and 140 p.p.m. Calgon at pH 11. The dispersion was carried out at high shear for 5 min. The polymer additions in the cleaner, precleaner and scavenger stages were 1, 0.5 and 1.5 p.p.m. respectively. The polymer additions were followed by 2 minutes of high shear,

2 min. of low shear and 20 min. of slow rotation in the flocculation apparatus. The total consumption of PAMG 6,

Dispex and Calgon in kg/tonne ore was: 0.25, 32.8 and

11.2 respectively. The results are shown in Table 7.15 237

below.

Table 7.15: Results of experiment 16

Products W-tti° 922Z Cu-distribution °°

Reel. conc. 15.0 17.82 55.3 Reel. tail 4.3 0.955 0.85 Cl. tail 6.6 0.85 1.15

Sc. conc. 176 6.55 23.8

Final tail 56.5 1.62 18.9 calc. heads 100.0 (4.84) 100.0 selectivity ratio = 11.0 enrichment ratio = 3.7

By comparison with experiment 12, the performance of the process was rather inferior. But the copper grade in the concentrate was higher than that of experiments 13-15. Thus the change of reagents levels and modes of addition was partly responsible for the lower grade in experiments 13-15. The levels of reagents in these experiments 13-16 were determined from a computer programming of the previous experiments. The results of the flocculation experiments recorded in this Chapter so far, showed in one extreme that when the copper grade was high (28%), the recovery was very low

(6%), and on the other extreme, when the copper recovery in the concentrate was high (97.4 %), the grade WU5 rather low (14%). Aruiattempts to improve the grade were invariably 238

accompanied by losses in the recovery and vica versa. This is the familiar dilemma in mineral processing of grade versus recovery. In an attempt to understand the "grade versus recovery"

dilemma, the following viewpoint was put forward. If the copper minerals are surrounded by gangue minerals, either by "slimes coating" or embedding in a matrix, then the copper particles will behave as gangue minerals, which results

in low recovery. On the other hand, if the copper minerals are surrounding the gangue minerals, the ore particles will

behave as copper minerals, thus resulting in low-grade concentrate. The phenomenon of slimes coating can be overcome by good dispersion whereas embedding in matrix requires further grinding to liberation sizes. 7.4.2 Dis ersion of the ore suspensions Good dispersion can be effected by (1) mechanical

"de-aggregation" of slimes (e.g. use of high-shear mixers, ultra-sonic vibrations or even grinding) and (2) chemical modulation of interfaces. The latter can be implemented by (a) regulating the electrical potential to induce repulsion between interfaces, or (b) reducing the inter- facial energy of the particles by addition of a surface

active agent. In all the previous experiments, factor (2-a) was fulfilled by the use of Calgon and Dispex. Since both reagents were found to increase the zeta- potential of

malachite as described in Chapter 2. It can be supposed that electrostatic repulsion alone was not sufficient to

produce good dispersion (i.e. dispersion of the individual 239

particles) in concentrated suspensions, seven though the suspensions were apparently sufficiently stable. Therefore extra aids of mechanical and chemical means were possibly needed. A comprehensive account on the various aspects of dispersion is given in the literature(21) Effect of ultrasonic vibrations on dispersion of the ore suspensions Exerimemt17:p A 5 g sample of the ore was ground in 40 ml dist. water containing 400 p.p.m. and 150 p.p.m.

Calgon at pH 11 for 2.5 hrs. The suspension was divided into approximately two equal volumes, and each was diluted to 100 ml (i.e. about 2.5% solids). The levels of Calgon and Dispex were readjusted accordingly at pH 11. One volume was subjected to the maximum shear-rate by a magnetic stirrer and the other to irradiation of an ultra- sonic probe at intermediate energy, for 3 minutes each. To test the effect, two one-stage flocculation experiments were performed using 3 p.p.m. of PAMG 7 in each experiment. The additions of the polymer were followed by 1 min. dispersion at the same level in each system, followed by 20 min. of slow rotation in the flocculation apparatus. The results arc shown 4n Table 7.16 below. Table 7.16: Results of experiment 17 dispersion Products t 222E2 CuC -distribution 'c techRiana Et

conc. 19.8 14.0 57.1 ultrasonic tail 80.2 2.6 42.9 probe calc. heads 100.0 (/4.86) 100.0

conc. 51.1 6.95 85.3 magnetic tail 48.9 1.25 14,7 stirrer calc. heads 100.0 (4.16) 100.0 24o

It is evident that ultrasonic dispersion had a superior effect in dispersing the ore particles, which subsequently raised the copper grade in the concentrate to about twice that provided by the magnetic stirrer. The effect of ultrasonic dispersion on the stability of the ore suspension was studied in the following experiment.

appriment 18: Two suspensions of 50 ml each (2.5%

solids) of the ground ore sample (for 2.5 hrs.), containing 400 p.p.m. Dispex and 150 p.p.m. Calgon at pH 11 were tested. One suspension was dispersed by a magnetic stirrer at high shear-rate (its maximum r.p.m.) and the other with

the ultrasonic probe at an intermediate energy level (i.e., at 0.6 of its maximum) each for 3 min. The suspensions were transferred into 50 ml cylinders and were left standing for a few days while small samples were taken from the top of the suspensions for measuring the percent ransmission at the wavelength 602 my in a spectrophotometer. The results are shown in Fig. 7.10. The results indicate that the ultrasonic dispersion improved the stability of the suspension, even though the suspension obtained by stirring only by magnetic stirrer was sufficiently stable for selective flocculation purposes. The stability was probably due mainly to the effects of Calgon and Dispex in both cases. The effect of ultrasonic dispersion was believed to be in liberating the finest particles of copper minerals from the gangue slimes. To confirm these conclusions the following experiment was carried out. 25

1 I I t 15 30 45 60 75 Time( hours)

Fig.7.10 Stability of ore suspensions dispersed by; magnetic stirrer® , ultrasonic vibrations®

242

Eperiment 1 . A 100 ml suspension of 2.8% solids of the ground ore sample (for 2.5 hrs.), containing 400 p.p.m. Dispex and 150 p.p.m. Calgon at pH 11 was used. The suspension was dispersed by a magnetic stirrer at high shear-rate (max. r.p.m.) for 3 min. followed by ultra- sonic dispersion at high energy (0.6 max.) for further 3 min. The suspension was stirred at high shear by the magnetic stirrer for 1 min. before and after adding 2 p.p.m. PANG 7, followed by 1 min. of low shear and 20 min. of slow rotation in the flocculation apparatus. The flocculated fraction was re-dispersed twice each in 100 ml dist. water containing 400 p.p.m. Dispex and 150 p.p.m. Calgon at pH 11, following the same dispersion procedure above, except that the slow rotation in the apparatus was only 15 min. 1 p.p.m. PANG 7 was added to each of the cleaning stages. The rougher tailing was dispersed by a magnetic stirrer for 3 min. at high shear, then 1.5 p.p.m. of the polymer was added while stirring was continued for 1 min. followed by 1 min. of low shear and 15 min. of slow rotation. The result are shown in Table 7.17. The total consumption of PANG 7, Dispex and Calgon in kg/tonne ore was 0.196, 42.8 and 16.1 respectively. Table 7.17: Results of experiment 19

18.7 19.3 75.12 8.5 1.56 2.76 13.7 1.50 4.28 12.9 3.44 9.24 46.2 0,895 8.6 100.0 (4.8) 100.0 selectivity ratio = 21.5 enrichment ratio 243

These results show a marked improvement in the

performance of the process with PAMG 7 compared with experiments 13-15, especially in the enrichment ratio, the weight of concentrate and the copper grade. The

higher copper grade and the relatively small weight of

concentrate, in particular, appeared to be the direct

results of the effect of the improved dispersion techniques.

Effect of reducing the interfacial energy on dispersion

The effect of adsorption of surface-active substances by a solid on its structural strength has been investigated by Rehbinder and others(182-187),In theory, if the inter- facial energy (surface tension) could be decreased to very low values, the solid would become very weak and eventually breakdown spontaneously even in the absence of external forces (if the discrete particles were small enough units). In practice, however, spontaneous peptization is very rare, and virtually unknown for dried solids, though swelling clays almost achieve this phenomenon. To weaken the strength of solids by reducing the interfacial energy, three basic requirements have to be fulfilled: (1) Tht surface-active agent must bring the interfacial energy of solids in to low values; (2) It must, therefore, be absorbed on the solids; (3) For the purpose of dispersion of slimes, the surface-active agent must have a lbw molecular weight (small size) in order to penetrate between the fine particles. The effect of three surface-active agents, namely; n butanol , sodium lignosulphonate and "Dispersol T"

(which is believed to be a naphthalene-sulphonic acid / formaldehyde condensate) was tested. It is known that the 244 interfacial tension of n-butanolfwater at 250 is 1.6 ,(-U) dyne/cm It is also known that the other two reagents help dispersion by reducing the interfacial energy of solids (1$9). The testing procedure is described in experiment 20. Experiment 20: One-stage flocculation experiments were carried out on 100 ml suspensions of 2.5% solids (of the finely ground ore samples) to test the effect of these surface-active agents in the presence of Dispex at pH 11. The level of reagents used in each experiment were 400 p.p.m. surface-active agent and 100 p.p.m. Dispex. Each suspension was subjected to the ultrasonic probe at high energy (0.6 max.) for 3 min., followed by 1 min. of high shear by magnetic stirrer before and after adding 3 p.p.m. PAMG 2.1 flocculant. The suspension was then stirred at low shear-rate for 1 min. and slowly rotated in the flocculation apparatus for about 20-25 min. The results are shown in Table 7.18 below.

Table 7.18: Results of experiment 20 ,_._.. Dispersant Products Wt. % Cu °o Cu-distribution % ------r--- - - 1B-conc. 53.85 8.1 84.0 "n-Butanol" B-tail 46.15 1.8 16.0 calc. heads 100.00 (5.19) 100.0

S-conc. 37.32 9.6 65.6 Sodium ligno- S-tail 62.68 3,0 34.4 sulphonate calc. heads 100.00 (5.46 100.0

D-conc. 47.63 9.4 84.2 "Dispersol T" D-tail 52.37 1.6 15.8 calc. heads 100.00 (5.32) 100.0 245

The results of these experiments were not encouraging. Presumably, this was because the surface-active agents did not satisfy the three basic requirements stated earlier. The butanol, although possessing very low surface tension against water and being of low molecular weight; does not have special affinity to adsorb on the various ore particles. The other two reagents are known to adsorb on various minerals, but their high molecular weight may hinder their migration in between the slimes aggregates. They may also interfer in the subsequent conditioning. The investigations were not pursued any further, though the possibility of utilizing the Rehbinder effect for dispersing heterogeneous ores deserves more study. The late Academician Rehbinder remarked privately to Dr. Kitchener that water is already a powerful reducer of surface energy for hydrophilic minerals: therefore, to get any substantial further improvement would require some rather special surfactant). There are possibilities of developing new dispersants to help to improve the dispersion process. Two new reagents were designed and prepared in the laboratory and proved to be good dispersants as well as flocculation inhibitors. They were named "Acrisol I" and "Acrisol II". It is believed that they consist of ordinary Dispex with some phenolic groups attached to it. It was found that Acrisol I depresses the common gangue minerals from flocculation at very low levels of additions. If it is used at high levels, it depresses the copper minerals also from flocculation and keeps them dispersed for long periods. It has been used in some of the flocculation experiments. 246

7.5 Effects of solids content on selective , flocculation Two experiments were carried out on suspensions of 5% and 10 % solids content. The flow-sheets used were the same as shown in Fig. 7.9, (i.e., the standard flow- sheet). Experiment 21: A 10 g sample of the ore was ground in 40 ml dist. water containing 100 p.p.m. Acrisol I and

400 p.p.m. Calgon at pH 11 for 3 hrs. in the McCrone mill. It was diluted to 100 ml (1.e.,10 % solids) and the levels of reagents were readjusted accordingly at pH 11. The dispersion processes were carried out with a magnetic stirrer at high shear for 2 min., then by ultrasonic probe for 3 min. at high energy, followed by 1 min. high shear by magnetic stirrer during which the polymer PAMG 2.1 was added. Dispersion of the flocculant was continued at high shear for 1 min. and at low shear for further 1 min, both stages by magnetic stirrer. This was followed by 20 min. of slow rotation in the flocculation apparatus. The flocculated concentrate was redispersed twice each in 100 ml dist. water containing 50 p.p.m. Acrisol I and 200 p.p.m Calgon at pH 10.8. The flocculant additions were 6, 4, 3 and 2 p.p.m. in the following stages: rougher, cleaner, recleaner and scavenger. The total. consumption of PAMG 2.1, Acrisol I and Calgon in kg/tonne ore was : 0.17, 2.0 and 8.0 respectively. The results are shown in Table 7.19 overleaf. 247

Table 7.19: Results of experiment 21

Products Wt.gram Wt. % units Cu-distribution 00

Reel. cone. 1.673 17.1 28.2 482.22 68.2

Reel. tail 0.702 7.2 3.18 22.896 3.2 Cl. tail 1.35 13.8 3.05 42.09 6.0 Sc. cone. 1.217 12.4 3.95 48.98 6.9 Final tail 4.858 49.5 2.25 111.375 15.7

calc. heads 9.8 100.0 (7.076) 707.561 100.0

selectivity ratio .n.,, 12.5

enrichment ratio /..v 4.0

The results indicate that there was no fundamental

change in the performance of the process. The enrichment and the selectivity ratios are comparable with those of the previous experiments at lower solids content. The estimated copper grade in the feed was rather high in relation to the previous experiments, but that might be due to segregation of copper minerals in that ore sample. (No special attention was given to statistical sampling, since the main concern was to establish the fundamental

aspects of the process). Experiment 22: A sample of 10 g was ground in 40 ml

dist. water containing 200 p.p.m. Dispex and 400 p.p.m. Calgon at pH 11 for 3 hrs. It was diluted to 200 ml

(i.e., 5 % solids) and the levels of reagents were readjusted accordingly. The dispersion procedure was the same as in

experiment 21. The suspension was treated with 3 p.p.m. 248

PAMG 2.1 flocculant and the flocculated fraction was redispersed (cleaned) twice: first (i.e., cleaner) in 100 ml dist. water containing 100 p.p.m. Dispex and 200 p.p.m. Calgon, and in the second (i.e. recleaner) in 100 ml dist. water containing 100 p.p.m. Dispex and 100 p.p.m. Calgon; both at pH 10.8. The levels of additions of PAMG 2.1 in each of the cleaner, recleaner and the scavenger were about 4 p.p.m. The total consumption of PAMG 2.1, Dispex and Calgon in kg/tonne ore was: 0.24, 6.0 and 11.0 respectively. The results are shown in Table 7.20 below.

Table 7.20: Results of experiment 22.

Products E-12 Cu % 2.12112.1.11.1-1,211-an_L.

Reel. conc. 15.7 23.5 64.4 Reel. tail 3.5 2.23 1.4 Cl tail 6.4 2.1 2.3 Sc. conc. 34.2 3.8 22.7 Final tail 40.3 1.3 9.2 calc. heads 100.1 (5.72) 100.0 selectivity ratio = 18.1 enrichment ratio = 4.13

The results confirm the previous conclusions, that there were no fundamental changes in the performance of the process due to increasing the solids content. The overall performance of this experiment was quite good, but the recovery needs to be improved. 249

7.6 Effect of t -water on selective flocculation Experiment 23: A 10 g sample of ore was ground in 40 ml of London tap-water containing 100 p.p.m. Acrisol I and 400 p.p.m. Calgon at pH 11.5 for 4 hrs. in the McCrone mill. It was diluted to 100 ml with tap water (i.e., 10 % solids) and the levels of reagents were adjusted accordingly. The dispersion procedure was the same as in experiment 22. The suspension was treated with 6 p.p.m. PAMG 2.1 in the rougher stage at pH 10.8. The flocculated concentrate was redispersed (cleaned) twice; (i.e., in the cleaner and recleaner stages) in 100 ml tap-water containing 50 p.p.m. Acrisol I and 100 p.p.m. Calgon at pH 10.5-10.8. respectively. The polymer was added to the cleaner, recleaner and the scavenger in the respective amounts; 4, 3 and 2 p.p.m. The suspension was diluted in the scavenger to 200 ml, in order to facilitate recovering more copper in the scavenger middling (Sc. conc.) The total consumption of PAMG 2.1, Acrisol I and Calgon in kg/tonne ore was: 0.17, 2.0 and 6.0 respectively. The results are shown in Table 7.21 below. Table 7.21: Results of experiment 23

Products wt. % Ea26 Cu-distribution % Reel. conc. 17.2 19.6 62.4 Reel. tail 6.7 1.85 2.3 Cl. tail 6.9 1.4 1.8 Sc. conc. 12.2 5.3 12.0 Final tail 57.0 2.05 21.5 calc. heads 100.0 (5.41) 100.0 selectivity ratio = 9.55 enrichment ratio 3.63

250

The result suggest a drop in both the copper grade

and recovery in the concentrate compared with experiment

22, though these differences are not of great significance. The tap-water, thus, had a deleterious effect on the performance of the process, which could be minimized by raising the level of reagents. The reagents consumption was still economical.

Experiment2L1 A 10 g sample was ground in tap-water

containing 150 p.p.m. Acrisol I and 500 p.p.m. Calgon at pH 11 for 4 hrs. It was diluted to 140 ml (i.e., 7 % solids) and the level reagents were regulated accordingly. The

dispersion procedure was the same as in experiment 23. The suspension was mixed with 8 p.p.m. PAMG 2.1 in the rougher at pH 10.8. The flocculated concentrate was redispersed twice; in 100 ml tap-water containing 150 p,p.m. Acrisol I and 200 p.p.m. Calgon in the cleaner, and 50 p.p.m.

Acrisol I and 150 p.p.m. Calgon in the recleaner, both at pH 10.8. The concentration of PAMG 2.1 used in the cleaner, recleaner and scavenger were: 5,6 and 2 p.p.m. respectively. The total consumption of PAMG 2.1, Acrisol I and Calgon in kg/tonne ore was: 0.252, 4.1 and 10.5 respectively. The results are shown in table 7.22 below:

Table 7.22: Results of experiment 24

Products Wt. 1

Recl. conc. 22.23 16.7 Reel. tail 17,3 3.9 Cl. tail 15.2 1.65 Sc. conc. 13.4 1.7 Final tail 31.87 1.25 calc, heads loo.00 .26) selectivity ratio = 13,4

enrichment rate 0 1-7I 251

The results suggest another decreaSe in enrichment ratio and the copper grade in concentrate compared with experiment 23. On the other hand the selectivity ratio and the recovery were higher. It was noticed, however, that some white coarse particles of quartz were settling with the flocs, which implied insufficient grinding. (The McCrone mill becomes inefficient when used to grind quantities of about 10g).Thus the drop in grade was mainly due to the settling of the coarse particles rather than the effect of tap-water.

7.7 Use of tap-water and high solids content Experiment 25: About 40 g sample was ground in 100 ml tap-water containing 150 p.p.m. Dispex and 500 p.p.m. Calgon at pH 11 for 2 hrs. in a porcelain mill. After washing the grinding media with tap-water, the volume of suspension became 650 ml; it was centrifuged and the clear aqueous medium was rejected. The solids were redispersed in 130 ml (i.e., 30.8 % solids) tap-water containing 150 p.p.m. Dispex and 500 p.p.m. Calgon at pH 11. The dispersion procedure was the same as experiment 24. The suspension was treated with 8 p.p.m. PAMG 2.1 at pH 10.8 and the flocculated concentrate was redispersed (cleaned) twice; in 130 ml tap-water containing 75 p.p.m. Dispex and 200 p.p.m. Calgon in the cleaner, and in 100 ml tap-water containing 50 p.p.m. Dispex, 200 p.p.m. Calgon in the recleaner, both at pH 10.8. The amounts of

PAMG 2.1 used in the cleaner, recleaner and scavenger stages were: 4, 4 and 3 p.p.m. respectively. The total 252 consumption of PAMG.2.1, Dispex and Calgon in kg/tonne ore was: 0.06, 0.855 and 2.77 respectively. . If, however, the amounts of. Calgon and Dispex lost due to centrifugation were taken into account, then the total consumption of PAMG 2.1, Dispex and Calgon would be : 0.06, 1.23 and 4.025 kg/tonne. The results are shown in Table 7.23 below.

Table 7.23: Results of experiment 25

Products III4 Elam Wt. % cyt. units Cu-distribution

Reel. cone 6.19 15.4 18.2 280.28 62.3 Reel, tail 2.99 7.5 3.15 23.625 5.3 Cl. tail 5.117 12.7 2.0 25.4 5.6 Sc. conc. 1.269 3.2 3.0 9.6 2.1 Final tail 24.558 61.2 1.82 111.384 24.7 calc. heads 40.124 100.0 (4.503) 450.289 100.0 selectivity ratio = 10.0

enrichment ratio = 4.05

The results show that selective flocculation was successfully achieved and the combined effects of the high solids and tap-water on the selectivity criteria were not of fundamental importance. Both the enrichment and the selectivity ratios are comparable with those in earlier experiments. During flocculation, however, some coarse white particles of quartz were seen to settle with the flocs. This was confirmed by microscopic examination of 253 of the concentrate. These coarse particles were, of course, due to insufficient grinding. (In treating an ore containing soft copper minerals and some hard gangue grains such as quartz, it would obviously be advantageous to reject the latter in a hydrocyclone after selective grinding). The reagents consumption in this experiment was considered to be very economical, considering the rising price of copper in the world market.

7.8 Discussion In developing the selective flocculation process in this Chapter, the parameters investigated can be grouped, for simplicity, as follows: A - Dispersion of the ore suspension: To study the mechanical factors of the dispersion process, the effects of the following parameters were investigated. The shear-rate, shear time, both together with the volume of suspension and size of the container for defining the hydrodynamic pattern. Solids content and technique of dispersion were also investigated. The effects of the chemical parameters such as: zeta-potential, surface energy and ageing time as well as the quality of water, on dispersion and stability of suspensions were studied.

B - Dispersion of the flocculant: The effects of the following parameters were established: shear-rate, shear- time and multi-stage additions of the polymer.

C Conditionin of flocculation after addition of the polymer: The effects of the shear-rate, flocculation time and slow rotation in the flocculation apparatus were illustrated. 254

D - Flocculation reagents: Types, amounts, and mode of additions of the flocculants, depressors and dispersants.

E - Grinding time and technique, F - pH of flocculation, and G Flow-sheet arran events: multi-stage flocculation, including redispersion processes to help release the entraped particles. The effects of these variables on the processing of this ore by selective flocculation have been established from the experiments in this Chapter and also in Chapter 6. For further development work, these variables can be reduced to only three, namely, the flocculant dose, the concentrations of the modifying reagents (e.g. Calgon and

Dispex or Acrisol I) and grinding to liberation size. The use of ultrasonic dispersion could probably be replaced by the use of a good mixer capable of producing high shear- rate mixing. The roles of modifying agents, solids content, dispersion of the flocculant and conditioning of flocculation at low shear in the selective flocculation process have also been previously studied by Yarar and Kitchener ( 2). The stable state of dispersion was found necessary and redispersion was suggested as a means for releasing entrapted particles. The rheological properties of flocculated suspensions treated with polymeric flocculants as against coagulation were investigated by Friend and

Kitchener(5). The authors studied the possibilities of employing zeta-potential as a means of controlling selective flocculation. The effectof low molecular weight polyacrylates on the dispersion and depression of 255

of dolomite gangue during the sulphidization flotation of (181 malachite was investigated by Van Lierde ) who found that lower molecular weight polymers (5,000 - 10, 000) were more efficient than those of higher molecular weight. More recently, Schulz (28) studied the effects of solids content, agitation time, agitation rate, sedimentation time and multi-stage flocculation with inserted redispersion on selective flocculation. The author deduced the theore- tically and practically obtainable parameters, proceeding • from the statement of Smoluchowski for quick coagulation. For operating selective flocculation on a large scale,

Read and Whitehead(191) suggested the use of large elutriation columns, while the possibility of using classifier cyclones in the separation of flocs in the clarification of industrial effluents has been studied by Visman and Hamza(191 Another possible technique for separating the flocs from 092) suspensions was described by Friend, Iskra and Kitchener Thus, the engineering of selective flocculation does not seem to present insuP'erable problems. 256

7.9. Economic assessment of the selective flocculation process applied to an oxilizedsozpeLrore To be practical a new separation process must be not only technically feasible but also economical in comparison with alternative procedures for obtaining the same result. In the case of the fine-grained oxidized copper ore investi- gated in this Chapter selective flocculation appears to be a feasible method of up-grading the ore from 5.0 % Cu to about 23.0 % Cu. Several routes might subsequently be taken to extract copper metal from the concentrate. The simplest might involve leaching with acid, followed by solvent extraction with a chelating agent dissolved in kerosene). Therefore, in this case, the question is whether selective flocculation would be cheaper than direct leaching of the ore. Unfortunately, however, a break-down of the costs of this route are not yet published, as the solvent extraction process for copper metallurgy is too recent. As a second best, one can estimate the saving in sulphuric acid and size of the leaching plant, though this is not the whole advantage, since the leach liquor from the concentrate will be very superior to that obtained from the raw ore. Alternatively, the cost of traditional of a copper ore can be crudely compared with the cost of the selective flocculation step. It is recognized, of course, that with-

out pilot scale tests only a very approximate estimate of

the cost can be obtained. Experiment 25 described in 7.7 was chosen for the economic evaluation of the selective flocculation process,

although this was not fully. optimized. In this experiment 257 the process employs hard water and about 31% solids content. The results in Table 7.23 show three "middlings" products namely; "Recleaned tail" "Cleaned tail" and "Scavenger concentrate", besides the main two products, the "Recleaned concentrate" and "Final tailings". In a continuous process, however, these middlings would eventually split up to the main two products (i.e., Recl.conc. and Final tail). Assuming the middling units (wt.% x Cu%) would split into Z (wt.%) of 18.2% Cu as concentrate, and 23.4 - Z(wt.%) of 1.82% Cu as the tailing (where 23.4 is the total wt.% of the three middlings). Thus from Table 7.23 . . 68.63 . Z x 18.2% + (23.4 - z) x 1.82% . . Z = 1.55 % ( This increase in weight of concentrate would increase the recovery to 67.3%. Therefore, in order to produce one tonne of dry con- centrate containing 18.2% Cu and with 67.3% recovery from an ore containing 4.5% Cu (in experiment 25), approximately (18.2 x 100 . ) o tonnes of ore would have to be treated. 4.5 67 . 3 1. Cost of chemical rea•ents in selective flocculation

a. Cost of sodium hydroxide: In order to raise the pH of one litre of water from pH 7 to pH 10.8 (i.e., p0H 3.2), approximately 25.2 mg of sodium hydroxide will be needed. Therefore, in experiment 25, where a total of 360 ml suspensions were used, the consumption of NaOH can be taken as 0.23 kg/tonne ore. However, it was found in an experiment that 4 ml of 0.1 N sodium hydroxide was needed to bring the pH of a 100 ml ore suspension (3% solids) from the natural pH 7.8 (in tap water) to pH 10.8. That is, the consumption of NaOH is about 5.3 kg/tonne ore. 258

Although the presence of Calgon and Dispex in the suspension medium will reduce the consumption of NaOH, this rate (5.3 kg/tonne) was used in this calculation. The price of

NaOH (U.K. price) is about £4.56 /100 kg (98% solid)(193). . . cost of NaOH per tonne ore = 5.3 x 0.046 = £0.24.

b. Cost of Calgon: The price of Calgon was taken as approximately the same as that of a tripolyphosphate"3), namely, £11.5 /100 kg (U.K. price). The consumption of Calgon in this process is 2.77 kg /tonne.

. . Cost of Calgon per tonne ore = £0.33. c. Cost of Dispex: The price of Dispex delivered in Congo (Zaire) would probably be between £1000- 1200/dry

tonne (1910. In this calculation, the price of 1 kg is taken as £1.2. The consumption of Dispex is 0.855 kg/tonne. . . Cost of Dispex per tonne ore = £1.03. d. Cost of PAMG 2.1. flocculant: The price of this polymer may be estimated as roughly twice of that of

polyacrylamide, (which is between £1100-1400 /tonne, delivered in Zaire(194)).\ In this calculation the price of PAMG 2.1 is estimated at £2.5/kg. . Cost of flocculant per tonne ore = £0.15. Total cost of reagents per tonne of ore treated = 0.15 + 1.03 + 0.33 + 0.24 = £1.75. . . Total cost of reagents per 6 tonnes of ore treated by selective flocculation = £10.5 cv$25). 1. Other costs: Values for costs of the following processes are based on practical experience in processing copper

ores with flotation (195)

259

2. Mining cost: at an average of $5/tonne'ore mined. . . cost of mining 6 tonnes of ore = $30.

3. Other treatments: (Such as crushing, grinding, thickening and drying) cost on average $4/tonne ore. . . Cost of 6 tonnes of ore = $24. The total processing costs to produce one dry tonne concentrate of 18.2% Cu = 24 + 30 + 25 = $79, say $80.

4. Smelting costs: The price of copper metal was taken as announed by the London Metal Exchange at an average

value of £1000/tonne (i.e., £1/kg or $2.4/kg). Smelting costs may include the following items:

a. The total cost of freight (plus assay, etc.) = $10/tonne dry concentrate.

b. Smelter deduction of 1% of concentrate weight (in copper metal) that is the value of 10 kg Cu for each tonne concentrate.

. . Smelter deduction = 10 x 2.4 = $24 . . The total copper metal in one tonne of concentrate after smelter deduction = 182 - 10 = 172 kg.

c. Smelter charges = $30 /tonne concentrate d. Pefinin charges = $0.10 /kg copper metal . cost of refining = 172 x 0.1 = $17.2. e. Provision for smelter participation in high copper prices (about 5%) = 5% x M000 = ( $120). Total smelting cost = 10 + 24 + 30 + 17.2 + 120 = $201.2 . . The total value of copper from one tonne of

concentrate = 172 x 2.4 = $412.8 • Revenue from the smelter = 412.8 - 201.2 = $211.6,

(smelter return) TV 212 The net'profit after treatments costs (items 1-3) = $132 260

Profit ratio net profit x 100 . . - total treatments cost (items 1-3) - 132 - 80 x 100 = 162%

Even if another $20 was allowed for machinery, labour and development work in the selective flocculation process, the profit ratio would be 112%. The relative cost of selective flocculation reagents is about 8.9% of the overall cost of producing copper metal. It is anticipated that the capital cost of a selective flocculation plant will be low.

7.10 Conclusions The flocculation experiments reported in this Chapter are considered sufficient evidence for the selectivity of the new flocculant when used on a real copper ore, and the feasibility of selective flocculation as a unit process. Further improvements with a given ore could certainly be achieved by further attention to details of liberation, dispersion, optimization of reagent consumptions and processing routes, but such development work for commercial purposes is considered outside the scope of the present investigation. The reagents consumption was low (especially in experiments 21-25) which makes the selective flocculation process very economical. The reagents consumption could be reduced further by removing most of the clays (by scrubbing and desliming) and quartz (by employing "stepwise" grinding and desliming) in the early stages of the processing. 261

CHAPTER 8: CONCLUSIONS

The main conclusions reached as a result of the present research can be briefly summarized as follows:

1. The zero point of charge (z.p.c.) of malachite in distilled water is between pH 9-9.5 (about pH 9.3) and closely corresponds to its calculated pH of minimum solubility (pH 8-9). The zeta-potential of malachite becomes more negative and consequently the z.p.c. shifts in to the acidic range) in the presence of "Dispex N40", "Calgon" and sodium carbonate, but no significant effect was detected on the z.p.c. in the presence of cupric sulphate (10-4 M). This is probably due to the precipi- tation of CuSO as cupric hydroxide in the range pH 7-10. 4

2. Chrysocolla releases some copper ions on shaking with distilled water, but leaching of Cu by acid sets in only below pH 4. In the presence of sodium chloride, the concentration of copper ions released from chrysocolla increases with increasing NaC1 content. The mechanism is probably ion-exchange. 2+ Although the concentrations of Cu ions released from other copper minerals (oxide, carbonate and sulphide) are very much lower than that from chrysocolla, all such minerals release some copper ions, which could react with 2+ compounds that strongly bind Cu . In the case of the copper sulphide minerals, surface oxidation plays an important part. 2+ It would be valuable to study the release of Cu ions from copper minerals with the aid of an ion-selective 262

electrode (though this was not available in the present work).

3. The selectivity of the chemical bonding between the polymer's "functional" groups and the mineral surface is the principal factor controlling the selectivity of

adsorption of flocculants. This may be enhanced or hindered by the electrical forces.

4. Sodium cellulose xanthate (NaCX) has shown selective

flocculation properties on heavy metal minerals and especially copper minerals, but has no flocculation effects on minerals like quartz, clays, calcite and feldspar. Selective flocculation of chrysocolla from mixtures with quartz can be achieved with or without sulphidization of chrysocolla. Although the polymer is simple to prepare cheaply, it has two disadvantages; (a) the molecular weight is normally lower than that desirable for formation of strong flocs, (b) there are difficulties in preparing a stable form which could survive long storage and trans- portation. However, decomposition of xanthate can be minimized if the polymer is stored in the dry form at low temperature and fresh solutions could then be made up when

needed. To avoid degradation of the cellulose chain,

oxygen should be excluded from the polymer during prepara- tion and storage.

5. Xanthation of methyl cellulose, carboxymethyl cellulose and hydroxypropylmethyl cellulose was proved possible and uniform solutions can be achieved. Their selective flocc-

ulation properties were similar to ordinary cellulose 263 but xanthate,Ithe molecular weights of methyl and carboxymethyl cellulose xanthate were rather low which resulted in weak flocculation.

6. The introduction of xanthate groups to polyvinyl- alcohol (PVA) produced a marked improvement on its floccu- lation effectiveness on galena. This might be due to the stronger bonding between the polymer's xanthate groups and galena surface, and the extended configuration of polyvinyl- alcohol xanthate (PVAX) as a result of the negative charge of xanthate. The PVAX polymer could prove a more potentially selective flocculant if prepared from a polyvinylalcohol of higher molecular weight (i.e., of the \ order of 106 . The problem of decomposition of xanthate can be overcome by storing dry.

7. Selective flocculation of malachite and chrysocolla from mixtures with calcite, feldspar and quartz can be achieved with polyacrylamide-dithiocarbamate (PAD) flocculant. The polymer was noted to slowly decompose but this can be avoided by storing the polymer dry and fresh solutions may then be made up. Little knowledge is available at present about PAD, and therefore, it deserves further detailed study on the various aspects of preparation, purification, analysis, physical and chemical properties so that it could be used to best

advantage.

8. The chelating polymers, PAMG, have proved to be selective flocculants for copper minerals from gangue

minerals (e.g., calcite, dolomite, feldspar and quartz) in both artificial mixtures and natural ores. Their 264 improved selectivity over ordinary polyacrylamide is due to presence of the copper chelating groups (GBHA), Glyoxal- bis-(2-hydroxyanil), in the polymer structure. Further improvements in PAMG polymer's selectivity could certainly be achieved by an extended investigation of the organic chemical reactions which might be employed to "graft" the GBHA complexing group on to polyacrylamides.

9. Selective flocculation of copper minerals from a dolomitic ore has proved both technically and economically feasible as a unit process, even under conditions of high solids content ( ^i 31%) and of hard water (London tap- water). Further improvements with a given ore could certainly be achieved by further study of the liberation size, dispersion, optimization of reagents consumption and processing routes; but such development work for commercial purposes is considered outside the scope of the present thesis.

10. Following the same principles of preparation of PAMG polymers, it should be possible to graft GBHA groups onto other water soluble polymers such as polyvinyl- alcohol (PVA), starches (e.g., amylose) and polyvinyl pyrrolidone (PVP), since they, too, react with formallhyde. Similarly, a selective cdepressant for copper minerals could be obtained by grafting GBHA groups onto lower molecular weight polymers ( <

11. Similarly, there are many other chelating groups with a high affinity for copper ions, which could be grafted onto various long-chain polymers to obtain a large number of 265 selective flocculant•s, or onto short-chain polymers to obtain selective depressants.

12. These principles should also be applicable to many other cations and their corresponding minerals. Thus an almost unlimited series of chelating polymers can be envisaged and could no doubt be prepared, based on the principles illustrated by the present research. 266

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APPENDIX 1. Colorimetric determination Of copper with o h This method is said to be very sensitive; the complex obeys Beer-Lambert's law from 0-1 p.p.m. Cu(196) (i.e., it gives a linear plot of optical density against copper (196) concentration). According to the same authors 7 it is not affected by the presence of 100 kig of the following 3+ 2+ 2+ 2+ 2+ 2+ 2+ ions: Fe , Ni , Co , Mn , Mg , Ca , Zn , A13+, 2- Cr4 and PO43- . Reagents 1. EiszcyclohexanemLLELILLIae: 0.5% solution in equal volumes of ethyl alcohol and distilled water; it dissolves by heating. 2. Ammonium citrate; 10% solution 3. Borate buffer: 400 ml of 0.5 M boric acid + 60 ml of 0.5 M (2%)NaOH to give pH 8.1. 4. Saliarnide: 3N (i.e., 12%) solution. 5. Neutral red; 0.05% solution.

Construction of the calibration curve: Procedure: 1. Transfer 10 ml of copper solution of different concentrations (5, 2.5, 1, 0.5, 0.3, 0.1, 0.05 and 0.0 p.p.m.) in eight flasks of 50 ml capacity each. 2. To each flask, add 5 ml of ammonium citrate (10% soln.) followed by one drop of neutral red (0.05% soln.).

3. Neutralize the solutions with about 6 drops NaCl (12% and 2% soln.) until the colour becomes yellow, then add 5 ml of the borate buffer (pH 8.1). 277

4. 0.5 ml of bis-cyclohexane oxalyldihydraZone (0.5% soin.) is then added to each flask and their volumes are made to 50 ml with dist. water. The final conc- centrations would be: 1, 0.5, 0.2, 0.1, 0.06, 0.02, 0.01 and 0.00 p.p.m. Cu respectively.

5. The optical density of the solutions are then measured by a spectrophotometer at wave length, X= 595 mit, in a 5 cm cylindrical cell. A linear plot should be obtained of the optical density/copper concentration. The calibration curve thus obtained, was used to determine the copper content in solutions in contact with chrysocolla in Chapter 2, following the same procedure. The blue copper complex develops within 10 minutes (i96), and it is said to be stable for 3 hours thereafter it fades away. However, it was found in the construction of the calibration curve, that measurements after 2.5 h gave abnormal readings and the measurements made after 30 minutes did not give a straight line plot. But measurements made within only 20 minutes after adding the complexing agent (bis-cyclohexane oxalyldihydrazone) gave a straight line graph. All glass ware used in these determinations were cleaned by a mixture of nitric acid and sulphudric acid, then by washing 3-times with distilled water. 278

APPENDIX 2: Determination of copper content-by atomic absorption spectrometry The copper content of the various samples solutions was determined by measuring the atomic absorption density.

Before each determination, a calibration curve was established by measuring the atomic absorption density of known concentration (1-10 p,p.m.) copper solutions (cupric chloride at pH 0.5 - 1.0). The solid samples were dissolved as follows:

1. To 0.1 g (or less) solid sample, 20 ml of cold concentrated nitric acid was added. The suspension was gradually heated on a hot-plate until its volume became small (or just before dryness). 2. After cooling 20 ml of cold concentrated hydrochloric acid was added and heating was resumed until the suspension was reduced to small volume. The suspension was allowed to cool before adding distilled water and then filtered. The copper chloride solutions thus obtained were made up to 100m1 volumes and the atomic absorption densities were measured on a Atom-Spek (Hilger and Watts) apparatus.

279

APPENDIX 3. The action 012111Elmata=paugImjall_ on the flocculation of talc by polyacrylamide

A series of flocculation experiments were made in order to establish the mechanism of attachment of poly- acrylamide (PAM) on the naturally hydrophobic surface of talc. If adsorption is due to hydrogen bonding between PAM and the talc surface, then this should be inhibited by the presence of "competing H-bonding agents. Griot and Kitchener(197) reported that flocculation of pyrogenic silica ("Aerosil") was prevented by addition of various simple compounds such as ethers, phenols etc, which are thought to be powerful hydrogen-bonding substances. The failure of these H-bonding agents to inhibit flocculation of talc with PAM would suggest that other mechanism was operating. This mechanism was suggested by Dr. J.A. Kitchener to be due to the h32",c :2213.0.s...22211-11212. of the hydrophobic part of PAM and the hydrophobic surface of talc. Procedure: 1.0% suspensions of finely ground talc were prepared by stirring the powders in distilled water containing different amounts of the various H-bonding agents at pH 5 for 5 min. at high shear rate. The suspensions were then treated with 1 p.p.m. non-ionic polyacrylamide (N100S, D.T.I.), and flocculation was observed visually. The results are shown in the following

Table. The following notations were used:

▪= strong flocculation; = no flocculation;

•- = partial flocculation. 280

, . content no. 10% 20% reagent type ---"--"------, 1 urea (carbamide) + + 2 bis [2-(2-methyoxyethoxy)ethyllether + + + 3 methoxy polyethylene glycol laurate - _* -* 4 phenol + 5 aniline + 6 polyethyleneglycol 400 + 7 formdimethylamide + 8 diethyl digol +

even at 8 p.p.m. PAM

From these results, it seems that adsorption of PAM. or talc is not likely to be due to H-bonding; but more probably with the hydrophobic association. However, further tests should be made to confirm these results.