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Leaching of chalcopyrite concentrate using extreme thermophilic bacteria Timmins, Michael Glenn 2001

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LEACHING OF CHALCOPYRITE CONCENTRATE USING EXTREME THERMOPHILIC BACTERIA by MICHAEL GLENN TIMMINS B.Sc.(Hons.), Bishop's University, 1997 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of Metals and Materials Engineering We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA November 2000 ©Michael Glenn Timmins, 2 00 0 In p resen t i ng this thesis in partial fu l f i lment of the requ i remen ts fo r an a d v a n c e d d e g r e e at the Univers i ty of Brit ish C o l u m b i a , I agree that the Library shal l m a k e it f reely avai lable fo r re fe rence and s tudy. I fur ther agree that p e r m i s s i o n fo r ex tens ive c o p y i n g of this thesis for scholar ly p u r p o s e s may b e g ran ted by the h e a d o f m y depa r tmen t or by his o r her representa t ives . It is u n d e r s t o o d that c o p y i n g or pub l i ca t i on of this thesis for f inancia l ga in shal l no t b e a l l o w e d w i t hou t m y wr i t t en p e r m i s s i o n . D e p a r t m e n t of T h e Un ivers i ty of Brit ish C o l u m b i a V a n c o u v e r , C a n a d a D E - 6 (2/88) A B S T R A C T The application of thermophilic bacteria to the leaching of a chalcopyrite (CuFeS2) concentrate from the Gibraltar Mine has been investigated in both shake flask and stirred tank reactor experiments. The conventional hydrometallurgical treatment of chalcopyrite is difficult due to a surface passivation phenomenon of the leaching particles. This passivation can be overcome through biological leaching at temperatures exceeding 50°C. The thermophilic leaching system is characterized by low temperatures (with respect to autoclave processes), high acidity and most importantly, a low redox potential. Due to the thermodynamic, chemical and biological properties inherent to high temperature bioleaching, the redox potential of the leaching slurry is held within the narrow potential range of 500 - 600 mV SHE. It is thought that in this range, chalcopyrite can be leached successfully. Of the various thermophilic species selected, a mixed thermophilic culture (growing at 70°C) outperformed all other thermophiles as well as the standard mesophile (growing at 35°C). It was observed that the thermophilic system is very sensitive to changes in temperature and consequently, leaching at the highest possible temperature (80-85°C) is advantageous. Inhibitions due to copper revealed that bacterial activity and therefore the leaching rate begins to be inhibited at copper concentrations above 10 g/L. Stirred tank reactor (STR) experiments resulted in copper extractions in excess of 90 % in 15 days. Approximately 75 to 99 % of the sulfide oxidized reported as elemental sulfur. It was found that the relative rates of leaching and the extent of sulfide oxidation (copper extraction) decreased with increasing pulp density over the range 2 to 10 %. Thermophilic bioleaching of the Gibraltar concentrate is thought to proceed under a biologically assisted indirect chemical leaching mechanism with the avoidance of chalcopyrite passivation as the result of low solution ORP. T A B L E OF CONTENTS ABSTRACT ii LIST OF TABLES vi LIST OF FIGURES viii N O M E N C L A T U R E xii A C K N O W L E D G E M E N T S xiv INTRODUCTION 1 1.1 Background 1 1.2 Obj ectives of the Study 1 1.3 Bacterial Leaching of Chalcopyrite 2 1.4 Outline of the Thesis 3 A REVIEW OF THE LITERATURE 4 2.1 Introduction 4 2.2 Chalcopyrite Chemistry 4 2.3 Electrochemical Nature of Leaching 5 2.4 Bacteria Morphology and Physiology 10 2.5 Bacterial Leaching of Chalcopyrite 11 2.5.1 Mesophilic Bioleaching of Chalcopyrite 13 2.5.2 Thermophilic Bioleaching of Chalcopyrite 17 2.6 Summary of the Literature 20 THEORETICAL AND EXPERIMENTAL METHODOLOGIES 22 3.1 Leaching Reactions 22 3.2 Bacterial Growth 25 iii 3.3 Bioleaching Mechanisms 28 3.4 Experimental Methods 28 3.4.1 Chalcopyrite Concentrate. 28 3.4.2 Bacterial Methods 29 3.4.3 Bacterial Adaptation 30 3.4.4 Cell Enumeration 31 3.5 Shake Flask Experiments 31 3.5.1 Apparatus & Procedures 31 3.5.2 Control Flasks 32 3.6 Stirred Tank Experiments 32 3.6.1 Apparatus & Procedures 33 3.6.2 TlOBioreactor 33 3.7 Analytical Methods 34 3.7.1 Determination of Ferrous Iron in Solution 35 3.7.2 Determination of Solution ORP 35 3.7.3 Determination of Solution pH 36 RESULTS A N D DISCUSSION 37 4.1 Shake Flask Experiments 37 4.1.1 Typical Bioleach Results 38 4.1.2 Mesophiles vs. Thermophiles 47 iv 4.1.3 Different Thermophiles at 60°C 50 4.1.4 Effect of Pulp Density 52 4.1.5 Effect of Temperature 57 4.1.6 Copper Inhibitions 63 4.1.7 Summary 69 4.2 Stirred Tank Experiments 71 4.2.1 T10 Experiment 71 4.2.2 Summary 77 SURVEY OF THE CONCLUSIONS 78 REFERENCES 82 Appendix 90 v LIST OF T A B L E S Table 2-1 Lattice energy of chalcopyrite compared to other sulfide minerals (after Habashi, 1978)...... .. 5 Table 2-2 Volume Change Associated with the Reaction (after Peters, 1986): 7 M S X > M z + + xS° + ze.... : 7 Table 2-3 Examples of the varying rest potentials of chalcopyrite. 9 Table 2-4 Classifications of common bacterial groups 10 Table.2-5 Factors that may be taken into consideration when assessing the biological leaching of mineral sulfides 11 Table 2-6 Comparative results for silver catalyzed and test without silver (Blancharte-•Zuritaefa/, 1987) : 15 Table 2-7 Typical chalcopyrite bioleaching conditions and kinetic results with mesophiles as derived from the literature 16 Table 2-8 Typical chalcopyrite bioleaching conditions and kinetic results with thermophiles as derived from the literature. 20 Table 3-1 Mineralogy of Gibraltar Concentrate (Hackl, 1995) 28 Table 3-2 Composition of Gibraltar Concentrate 29 Table 3-3 Mesophilic and Thermophilic Nutrient Media 30 Table 4-1 Statistical Analysis of Typical Test 41 Table 4-2 Average Copper Leaching Rates.. 49 Table 4-3 Initial^ Rates of Copper and. Iron Leaching for N M and S. acidocaldarius Experiments : 54 Table 4-4 Summary of Results for Varying Temperatures 63 Table 4-5 Copper Inhibition Experiments : .64 Table 4-6 Lag Times and Initial Copper Leaching Rates for Figure 4-23 64 Table 4-7 Sulfur Speciation in Leach Solids for T10 Experiment..:........... 76 vii LIST OF FIGURES Figure 3-1 Bacterial growth and copper extraction curves 25 Figure 3-2 Experimental and theoretical bacterial growth curves. The y-axis is X (cell concentration) for the experimental curve and In X for the theoretical curve. Letters A through D associated with the theoretical curve indicate the four phases of bacterial growth: lag, exponential, stationary and death phases 26 Figure 3-3 Schematics of the T10 Apparatus .....33 Figure 3-4 H G Hydrofoil used in the T10 STR Experiments 34 Figure 4-1 Typical copper and iron leaching curves for the standard thermophilic shake flask leaching test compared to an abiotic control. Standard conditions: 70°C, pH controlled at 2, using the N M culture, pulp density of 2 % and P90 38 um 38 Figure 4-2 Typical ORP and pH curves for the standard thermophilic shake flask leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M culture, pulp density of 2 % and P90 38 um. . . . . 39 Figure 4-3 Iron speciation curves for the standard thermophilic shake flask leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M culture, pulp density of 2 % and Pop 38 um 41 Figure 4-4 Copper leaching curves for the thermophilic shake flask leaching test with and : without pH control. Standard conditions: 70°C, pH controlled at 2, using the N M culture, pulp density of 2 % and P% 38 (am 43 Figure 4-5 ORP and pH curves for the thermophilic shake flask leaching test without pH control. Standard conditions: 70°C, pH controlled at 2, using the N M culture, pulp density of 2 % and P90 38 \im 44 Figure 4-6 Copper leaching curves for the thermophilic shake flask leaching test at different particle size distributions of P90 38 um being the fine grind and P90 105 um representing the coarse grind. Standard conditions: 70°C, pH controlled at 2, using the N M culture, pulp density of 2 % and P% 38 urn „ 46 Figure 4-7 ORP curves for mesophile (35°C) and thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture^  and T. ferrooxidans mesophile culture, pulp density of 2 % and P90 38 |am. 47 Figure 4-8 Copper leaching curves for mesophile (35°C) and thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture and T. ferrooxidans mesophile culture, pulp density of 2 % and P 9 0 38 um. : : 48 Figure 4-9 pH curves for mesophile (35°C) and thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture and T. ferrooxidans mesophile culture, pulp density of 2 % and P90 38 )j.m............ 49 Figure 4-10 Copper leaching curves for A. brierleyi and M. sedula shake flask leaching tests. Conditions: 60°C, pH controlled at 2, pulp density of 2 % and P90 38 um 50 Figure 4-1,1 ORP and pH curves for A. brierleyi and M. sedula shake flask leaching tests. Conditions: 60°C,pH controlled at 2, pulp density of 2 % and P90 38 um. 52 Figure 4-12 Copper leaching curves for thermophilic shake flask leaching tests at increasing pulp densities. Standard conditions: 70°C, pH controlled at 2, using the , N M thermophile culture, pulp density of 2 % and P90 38 um.................................. 53 Figure 4-13 ORP curves for thermophilic shake flask leaching tests at increasing pulp densities. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 um. 55 . Figure 4-14 pH curves for thermophilic shake flask leaching tests at increasing pulp densities. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 um. 56 ix Figure 4-15 Copper leaching curves for thermophilic shake flask leaching tests at varying temperatures. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 um... 57 Figure 4-16 Iron leaching curves for thermophilic shake flask leaching tests at varying temperatures. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 um 58 Figure 4-17 ORP curves for thermophilic shake flask leaching tests at varying temperatures. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 um..... 60 Figure, 4-18 pH curves for thermophilic shake flask leaching tests at varying temperatures. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 3 8 um. 62 Figure 4-19 Effect of initial added copper on copper leaching for thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 um.............. : 65 Figure 4-20 Effect of initial added copper on ORP for thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 um 66 Figure 4-21 Effect of 5g/L added copper on copper leaching for thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 um........ ..„ 68 Figure 4-22 Effect of 5g/L added copper on pH for thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 um : ; 69 x Figure 4-23 Copper and iron leaching curves for the thermophilic T10 leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 5 % and P90 38 um.. : 72 Figure 4-24 ORP and pH curves for the thermophilic T10 leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 5 %andP 9 0 3 8 um.. : 73 Figure 4-25 Acid Consumption for the thermophilic T10 leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 5 % and P90 38 um 74 Figure 4-26 Relationship between bacterial growth and copper extraction for the thermophilic T10 leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 5 % and P90 38 um ..75 Figure 4-27 Iron speciation curves for the thermophilic T10 leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 5 %andP 9 0 3 8 um..;... .76 xi NOMENCLATURE A C abiotic control A L aFe3+ &Fe2+ activity of ferric iron activity of ferrous iron air-lift reactor C, cer concentration of cerium (IV) sulfate solution, M DO dissolved oxygen dt change in time dX change in cell concentration E redox potential, mV E° standard redox potential, mV exp. exponential F Faraday constant, 96 485 Cmol"1 (g) gas phase i initial LPZ low potential zone n number of electrons in redox reaction, charge number ORP oxidation reduction potential P90 90% passing R gas constant, 8.3145 JrT'mol"1 r x volume rate of biomass production, kgm"3h"' (s) . solid phase SF shake flask reactor SHE standard hydrogen electrode ; sp. species STR stirred tank reactor . T temperature, °C t time, s t0 time zero, s V S O i volume of sample, mL xii Vr titration volume, mL Wt% weight percent . a sigma bond, covalent u specific growth rate, h"1 X viable cell concentration, kgm" X 0 viable cell concentration at time zero, kgm"3 xiii A C K N O W L E D G E M E N T S The Biohydrometallurgy Chair provided the funding for this study. Special thanks to my supervisors, Ralph Hackl and David Dixon for their guidance and understanding. My sincere thanks are also extended to Hongguang Zhang and Lyn Jones for donating much of their time and expertise to this project. The assistance of Masoud Aftaita, Berend Wassink and Christopher Pasetka in the lab is gratefully acknowledged. The excellent and thorough editing skills of Sylvie Bouffard and Jesus Munoz are also greatly appreciated. I would finally like to thank my mother and father for their love and encouragement. xiv FOR CHER ANN XV CHAPTER 1 INTRODUCTION 1.1 Background The success of copper dump and heap leaching has been attributed in part to the natural ability of microorganisms to catalyze the oxidation of sulfide minerals. Their application arises from the ability to derive energy for cellular growth from the oxidation of reduced iron and sulfur species. This leaching technique coupled with proven solvent extraction and electrowinning technologies has resulted in a popular and economic method for the recovery of copper from sulfide minerals. Many mining companies and research groups have studied the chemical and biological leaching of chalcopyrite. To date, these ores and concentrates have not been efficiently or economically bioleached using conventional methods and current reactor technologies. Heaps that contain chalcopyrite, an abundant copper sulfide are not leached to completion due to the passivation of the mineral surface. Therefore, attention must be directed towards the understanding of processes that treat refractory copper heaps and flotation concentrates created from these deposits. 1.2 Objectives of the Study The primary objective of this work is to revisit the bacterial leaching of chalcopyrite by introducing thermophilic bacteria to the leaching of chalcopyrite concentrates. The underlying theme of the research is the endeavor to gain some insight into the thermophilic bioleaching system. Preliminary work by other researchers (Marsh, 1983; Ballester, 1993; LeRoux & Wakerley, 1987; Norris, 1990; Norris & Parrott, 1986; Duarte et al, 1993) suggest that thermophiles may be superior to the mesophiles in leaching chalcopyrite. These promising observations and results raise the notion that the impedance of leaching by classical chalcopyrite passivation may be overcome by the use of thermophiles with increased rates and conversions possible. This preliminary research ushers in many unanswered questions. Will the current problems associated with increasing pulp densities still be observed? Should copper toxicity and the effect of temperature be studied? Will the presence of other minerals issue the possibility of a galvanically enhanced system? To address these questions and to test the merits of this new application, shake flask as well as batch experiments were performed. 1.3 Bacterial Leaching of Chalcopyrite Microorganisms have been employed to catalyze the dissolution of metals from ores and concentrates for hundreds of years. Their application has led to several large-scale mesophilic bioleaching plants and many interesting bioprocesses tailored to a specific ore body or concentrate (Lawrence & Poulin, 1996; Jordan et al, 1996; Bailey & Hansford, 1992). y-[-r_ ,,;;v. ' ;•- ,;. Hydrometallurgical treatment of chalcopyrite concentrates at high redox potentials and at temperatures below the melting point of sulfur almost invariably results in the preferential dissolution of iron (Linge, 1976; Baur et al, 1974; Natarajan, 1992; Ammou Chokroum et al, 1981). In conventional bioleaching (at 35°C with mesophiles) chalcopyrite is either oxidized directly by the bacteria (sulfide oxidized to sulfate) or indirectly by biologically generated ferric iron. Elemental sulfur that is produced by the indirect ferric leach is then oxidized to sulfate by the mesophilic culture resulting in an elemental sulfur yield of essentially zero. Due to the very high redox potentials of the mesophilic culture, the initial rates of leaching chalcopyrite are high. However, the cessation of leaching may be due to the passivation of the mineral surface caused by the high redox potential (Kametani & Aoki, 1985). This premature cessation of leaching has been attributed to the formation by an increasingly insulating passive film on the mineral surface obstructing the transfer of electrons from the mineral to the oxidant in solution. Mesophilic leaching is carried out at pH values < 2 and always results in low copper extractions. Typically, only 40-50 % of the copper can be extracted from chalcopyrite using the current bioleaching technology. 2 The behaviour of the redox potential in bioleaching systems is paramount to successful biological growth and in general can be used as a diagnostic tool to evaluate the status of the leach. The redox potential is a measure of the ferric to ferrous iron concentration ratio. A high ratio of ferric to ferrous iron results in high solution potentials (> 700 mV SHE) that are traditionally interpreted as being beneficial. The high redox potentials created by the mesophiles seem to have a detrimental effect on the leaching of chalcopyrite. 1.4 Outline of the Thesis This paper consists of 5 chapters. Chapter 2 is an overview of the current literature on the various aspects of treating chalcopyrite biologically. Leaching reactions, experimental procedures, characterization of materials and equipment can be found in chapter 3. Chapter 4 hosts a discussion on the results of the thermophilic bioleaching of chalcopyrite, while chapter 5 carries into a summary of the conclusions: 3 CHAPTER 2 A REVIEW OF THE LITERATURE 2.1 Introduction A review of the literature provides background information on the use of thermophilic bacteria and the experimental evidence to show the recalcitrance of chalcopyrite. This chapter presents a discussion of the inorganic chemistry of chalcopyrite as well as its role in the electrochemical nature of leaching. Different researchers have performed many mesophilic and thermophilic experiments (LeRoux & Wakerley, 1987; Jordan et al, 1996; Boon, 1996; Bailey & Hansford, 1992; and references therein). Their results and observations are discussed in this chapter. Conclusions based on the literature can be found in the summary. 2.2 Chalcopyrite Chemistry Chalcopyrite, the most abundant copper sulfide, is a semiconducting mineral and has the empirical formula CuFeS2. The pure mineral has a density of 4.1 - 4.3 g/cm3 and a hardness of 3.5 -4.0 on the Moh scale (Habashi, 1978). The lattice energy of a crystal solid is defined as the energy released when the gaseous ions combine to form one mole of the solid. In the case of chalcopyrite: Fe2+(g) + cV+(g) + 25%,) -» CuFeS2(s) [2-1] . The fact that a crystal resists attempts to separate it into its components indicates that there is an energy holding the lattice together. Table 2-1 illustrates the extreme crystal stability of chalcopyrite. 4 Table 2-1 Lattice energy of chalcopyrite compared to other sulfide minerals (after Habashi, 1978) Sulfide Mineral Lattice Energy kJ CuFeS2 17500 FeS2 4260 CuS 3785 Cu2S 2935 However, the lattice energy is not a measure of the difficulty in removing electrons from the crystal. The covalent bonding in the metal sulfide crystal facilitates the derealization of charge and thus would display conductive properties. These facts simply indicate that there may be special implications in the leaching of chalcopyrite. 2.3 Electrochemical Nature of Leaching Electrochemistry of metal sulfides is best described by the earliest literature (Peters, 1973 & 1977; Linge, 1976; Biegler, 1976; Biegler & Swift, 1974). The oxidation of chalcopyrite is governed by the thermodynamic properties of the mineral, the leaching properties of the solution as well as the interactions at the mineral/electrolyte interface. More specifically, in an electrochemical leaching mechanism, the rates are heavily influenced by the electrical configuration and chemical structure of the mineral surface (Peters, 1973). The overall chemistry on the chalcopyrite surface can be described as the following electrochemical process (corrosion cell): Anode: CuFeS2 Cu2++ Fe2++ 2S? + Ae [2-2] Cathode: Ae+AFe^ -> 4Fe2~T [2-3] Researchers have found it difficult to reproduce and obtain reliable anodic and cathodic polarization curves for this mineral (Peters, 1977). This has been attributed to the small differences in stoichiometry and/or the presence of impurities in the crystal structure (Norris & Owen, 1993). In other fields of engineering it is beneficial to have impurities in a semiconducting mineral to change the material's conductive properties. However, the presence of impurities may, of course have detrimental effects on the conduction of electrons through the crystal. It should come as no surprise that the leaching of semiconducting minerals (since leaching is largely electrochemical) may not be easily reproduced. The measured redox potential of the leach solution may also be greatly affected by galvanic interactions (the result of having two semiconducting minerals in contact). The more electro-noble metal will be cathodically protected while the other mineral is preferentially and anodically dissolved. Researchers have observed the enhanced leaching properties of chalcopyrite concentrates that contain varying percentages of pyrite (Berry et al, 1978; Mehta & Murr, 1983). They found that the rate of copper extraction from chalcopyrite increased with increasing pyrite to chalcopyrite ratio to a maximum enhancement of 1:1. At this ratio, the rate of leaching was double that of a concentrate containing no pyrite. The redox potential of the leaching slurry has an important effect on the leaching of CuFeSjIFeSi concentrates. On the pyrite surface, coulometric measurements taken from the cathodic sites have shown that between 0 and 0.62 V SHE (Peters, 1977) the only reaction proceeding to any appreciable extent (and thus providing must of the current) is the reduction of oxygen, viz: 02 + 4rf + 4e~ -» 2H20 - " [2-4] Pyrite can transfer the electrons gathered from the less noble minerals to molecular oxygen faster than any other sulfide mineral (Peters, 1986). However, kinetic results have shown that the oxidation of pyrite with oxygen is an order of magnitude slower than with ferric iron. Ferric ion binds chemically (a bond) to the surface while O2 is only chemisorbed (Goldhaber, 1983; McKibben & Barnes, 1986). Oxygen is a very insoluble gas and the cathodic reaction [2-4] is limited by the mass transfer rates from the bulk solution to the mineral surface, leading to concentration polarization assuming the reacting surface is active. Biegler (1976), measured the reduction of oxygen on a number 6 of metal sulfides and concluded that the reduction can also be limited by the activation polarization (a physical property of the surface). -Pyrite exhibits a high rest potential and leaches at a very slow rate between 0.25 and 0.62 V SHE (Peters, 1977). However, it does allow for a significant rate of oxygen reduction in this potential range. In this case the electrons may reduce ferric or molecular oxygen. Elemental sulfur is the sulfur moiety formed at almost 100 % yield in the oxidative leaching of CuFeSilFeSi systems in acidic media and at low redox potentials and temperatures of 70 - 80°C. Table 2-2 Volume Change Associated with the Reaction (after Peters, 1986): M S X ^ M z + + xS° + ze : - .' • Mineral Formula Molar Volume cm 3 Sulfur Volume cm 3 Volume Change % Pyrite FeS2 24.0 31 +29 Chalcocite Cu2S 28.4 15.5 -45 Covellite CuS 20.8 15.5 -25 Chalcopyrite CuFeS2 42.65 31 -27 Table 2-2 demonstrates how pyrite exhibits the only positive volume change during leach as compared to the other copper sulfides. A high negative shrinkage indicates that the anodically formed elemental sulfur will be present in the cracks and fissures of the particle and thus would not cover and protect the mineral surface from oxidation (Barriga Mateos, 1987). In many instances, the elemental sulfur formed may flake and fall off the mineral surface (Hirato, 1987; Munoz et al, 1979; Dutrizac, 1989). Wan et al (1984) noticed that when the elemental sulfur was removed from the reactive surface, the original leaching kinetics could be established. During leaching, pyrite particles undergo a net positive volume change and the elemental sulfur produced may cover and protect the mineral surface. In order to leach the pyrite, the elemental sulfur would have to be oxidized to sulfate. When the leaching of pyrite is successful, sulfate is the main sulfur entity formed. Once a galvanic interaction between chalcopyrite and pyrite can be established, the leaching of chalcopyrite is accelerated with a high yield of sulfur, while the pyrite in the sample is cathodically protected. Chalcopyrite will therefore undergo the anodic dissolution reaction equation [2-2]. Berry (1978) provided evidence that showed an enhancement of the galvanic leaching of chalcopyrite in the presence of iron and sulfur oxidizing bacteria. He attributed the enhancement to the bacterial ability to convert ferrous to ferric iron and to oxidize elemental sulfur. A better explanation may be the rendering of the CuFeSilFeSi surface more catalytic for the reduction of oxygen. More elegantly described, the bacteria may depolarize the anodic reactions (Munoz et al, 1995). It is believed that microorganisms migrate to the cathodic areas of the mineral characterized by sharp edges and points where the current density is most concentrated. The thermophile may act as an oxidation-catalyzing mediator operating at high temperatures. In this role these organisms would act as conductors in the electron transfer from the crystal lattice to dissolved oxygen by reducing the activation energy of the reduction of oxygen on the CuFeSz/FeSi surface. The rest potential of the mineral is the electrical potential where no net cathodic or anodic current flow. This surface potential must not be higher in value than the reversible potential of the oxidant that accepts the electrons from the mineral. It also cannot be low enough to fall into the stability region for the mineral. In either case, no leaching of any kind will occur. Table 2-3 presents examples of pyrite and chalcopyrite rest potentials from various researchers. 8 Table 2-3 Examples of the varying rest potentials of chalcopyrite. Rest Potential Conditions Reference mV SHE 500-600 1M H 2 S0 4 , 25°C Jones & Peters, 1976 400 1M H2SO4, 85°C 450-550 1M H2SO4, 25°C Warren etal, 1982 610 IMH2SO4, 25°C 520 1M H2SO4, 25°C Mehta&Murr, 1982 430 Barriga Mateos et al, 1993 There has been a large interest in the bioleaching of chalcopyrite under controlled redox or applied potential. Ahonen and Tuovinen (1993) bioleached chalcopyrite with mesophiles at a low redox potential range (500 - 550 mV SHE) and at a high potential range (600 - 650 mV SHE). Under low redox conditions the oxidation of pyrite was suppressed and the oxidation of chalcopyrite was only slightly enhanced. Natarajan (1992) showed that the leaching of copper from chalcopyrite was optimum at an applied voltage of 600 mV SHE. It was also found that iron is preferentially leached at an optimum applied potential of 650 mV SHE. If the iron has been depleted from the reacting surface, passivation may be the result. To avoid this passivation it may be beneficial to leach CuFeS2 concentrates at low ORP values. Mesophilic and thermophilic bioleaching may be a bacterially mediated electrochemical process. Electrons inside the sulfide move freely and it can be assumed there is a uniform potential throughout the mineral. The mineral/electrolyte interface behaves like a short-circuited electrochemical cell. The mineral assumes a mixed potential determined by the spatial separation and areas of the anodic and cathodic processes. The double electrode may derive either from anodic and cathodic reactions occurring on its surface that results in a corrosion cell or from the electrical contact between two or more solid phases resulting in a galvanic couple. Both of the above events may occur in the thermophilic bioleaching of chalcopyrite. 9 2.4 Bacteria Morphology and Physiology Prokaryote cells lack a true nucleus and are unlike eukaryote cells (e.g. mammalian) which do have a nucleus to house their genetic information. Prokaryotes are geologically active bacteria and include all the members of the archaea and eubacteria. The extreme thermophiles are members of the archaea while the mesophiles are eubacteria. Archaea and eubacteria differ in cell wall components, structure as well as in some enzymatic details. In bioleaching systems, temperature regulation is key to a successful operation. Above a critical temperature, bacterial will begin to die from the denaturing of cellular components. This irreversible damage renders the cell inactive. For most bacteria there is a temperature minimum below which growth does not occur. There is an optimum temperature at which the growth of the bacteria is maximized and a temperature maximum above which growth is not possible. Table 2-4 compares the four classes of bacteria relative to their optimum growth temperatures. Table 2-4 Classifications of common bacterial groups Group Temperature Class Growth Range °C Environment Psychrophiles Low < 30 Arctic regions Mesophiles Medium 30-45 Mammals Moderate Thermophiles Medium to High 35-50 Extreme Thermophiles High 50-90 Hot springs Hyperthermophiles Very high 100+ Deep sea hot vents It is assumed that the predominant bacterial species in the November Mix culture is Sulfolobus acidocaldarius. This species of Archaebacteria proliferates in hot acidic environments. They are largely spherical, but can be an irregular shape. On average a S. acidocaldarius cell is 0.8 - 1.0 um in size (Brock, 1978). S. acidocaldarius are non-motile bacteria having no flagella. They are known to form mucous capsules that insulate the bacteria and aid in the attachment to a solid substrate. Their cell wall structure is 10 atypical in comparison to bacteria in general. The cell wall is extremely rigid and able to withstand great physical stress due to a large percentage of structural protein (Brock, 1978). However, Bailey & Hansford (1992) believe that the S. acidocaldarius: cell wall is not as rigid as T. ferrooxidans. Pili have been detected on the outside of the cell wall that aid in attachment and mobility. On the other hand, Barr et al (1992) concluded that the cell surface of S. acidocaldarius is highly hydrophilic and therefore attachment to mineral sulfides is thermodynamically unfavourable. In contrast, this genus is non-motile by cellular design, having ten times more bacteria attached to the mineral than in solution (Rossi, 1990). 2.5 Bacterial Leaching of Chalcopyrite Table 2-5 depicts the gamut of physiochemical, mineralogical, electronic and biological parameters associated with bioleaching. Table 2-5 Factors that may be taken into consideration when assessing the biological leaching of mineral sulfides. Factors to consider Discipline Rest potential of mineral Electro-mineralogical Redox potential of slurry Electro-mineralogical Semiconducting properties Electro-mineralogical Temperature and pH Thermochemical O2 and CO2 mass transfer Thermochemical Solubility of leach products Thermochemical Particle size and pulp density Operational Bacterial species and mixed cultures Biological Cells to solids ratio Operational Shear stress and mechanical damage Bio-operational Metal cation toxicity Chemical Inhibition of attachment Physical Leaching mechanisms Biological Galvanic interactions Electro-mineralogical Addition of catalysts Thermochemical Reproducibility Mineralogical 11 Bioleaching at high solids concentrations have traditionally resulted in lower oxidation rates and affects the bioleach in threes ways: increasing lag times, decreasing leaching rates and diminishing the extent of sulfide oxidation (Clark & Norris, 1996; Norris, 1988; Bailey & Hansford, 1992). The bacteria do not seem to grow optimally resulting in decaying bioleaching kinetics at pulp densities > 20% (w/w). This may be due to the poor oxygen and carbon dioxide mass transfer (Sakagushi & Silver, 1976; Bailey & Hansford, 1992). The stoichiometric oxygen demand is dependent on sulfide content, particle size and solids concentration. An oxygen limitation would be the result of the chemistry demanding a terminal oxidant, a process that is ultimately governed by the sulfide grade of the substrate. Solids interfere with the mass transfer properties of oxygen and carbon dioxide to the organisms. The effect of increasing solids concentration on the kia (s"1) has been reviewed by many researchers (Joosten, 1977; Mills, 1987; Oguz, 1987; Schumpe, 1987). The mass transfer coefficient kLa is altered by a change in the interfacial turbulence. The a in the kLa is the specific interfacial area. It is affected by an increase in the apparent viscosity of the slurry and by altered bubble coalescence rates. A decrease in the gas hold-up was also an affect of increased solids concentration (Mills et al, 1987). However, Boon et al (1992) observed that bacterial growth (and therefore leaching) would be limited by carbon dioxide before oxygen if the aeration was not supplemented with CO2. On the same note, they concluded that shake flask (SF) and air lift reactors were unsuitable for competent kinetic study at high solids concentration. The oxygen and carbon dioxide mass transfer characteristics are far more favourable in a STR (Silverman & Lundgren, 1959; Liu et al, 1988). An increase in the lag phase and retention time for these processes can be attributed to the low bacteria to solids ratio at the higher pulp densities. Mechanical damage to the bacteria or the prevention of bacterial attachment due to attrition events (Mills et al, 1986) and/or high shear may also be a factor (Toma et at, 1991). The increase in agitation and aeration generally enhance bioleaching by improving the oxygen and carbon dioxide mass transfer. At some impeller speeds, the excess 12 turbulence inhibits growth. The problems are two fold: mechanical damage and the inhibition of cellular attachment to the mineral surface. High turbulence and the dislodging of bacteria from the surface can further specify inhibition of attachment. It would especially be difficult if the bacteria have to migrate to specific regions of high electrical potential such as the edges and points of cracks and fissures. The bacteria that are attached may also detach in search of oxygen. The question must be raised about the possible inhibition due to higher sulfide or copper concentrations as well as the various biological flotsam produced. Direct contact of the bacteria with mineral particles may assist the efficiency of mineral oxidation due to a direct mechanism and no diffusion limitations between the bacteria and the reacting surface. 2.5.1 Mesophilic Bioleaching of Chalcopyrite Mesophiles are well suited to treat many metal sulfides in order to recover their metal values. Mesophiles have been known to grow in very harsh and oxidizing environments. Some researchers have determined that the mesophilic bioleaching of chalcopyrite proceeds in two stages (Barriga Mateos et al, 1993; Almendras et al, 1987). The rapid release of copper from the mineral, a high acid consumption and a high rate of bacterial growth characterize the first stage. The rate in. the first stage is controlled by the dissolution of chalcopyrite (indirect leaching). Stage two is characterized by a slowing of the leaching rate to an eventual plateau. An iron depleted product layer (changed semiconducting properties) becomes thick enough to become rate controlling through diffusion limitations of the reactants and/or products. The rate of reaction does eventually cease due to the complete passivation of the mineral. However, at an applied potential of 600 mV SHE, the copper leaching from chalcopyrite increased by a factor of four with respect to the control. This phenomenon is called the "active potential effect" where the rate and selectivity of the dissolution of a mineral at a specific impressed potential in the absence and in the presence of galvanic contacts is enhanced (Nataraj an, 1992). 13 Natarajan (1988) reports that both biological and electrochemical events play an important role in the bacterial oxidation of bimetallic sulfides. The presence of T.ferrooxidans catalyzed the galvanic interactions in the leach and led to an increased copper extraction. He found that the addition of the bacteria to CuFeS2/FeS2 mixture caused a factor of ten increase in the copper leaching with respect to the sterile control. The precipitation of ferric iron as jaroske follows the chemistry below. 3Fe2(S04)i + \4H20 '-> 2(H30)Fe3(S0^2(OH)6 + 5H2S04 [2-5] It is commonly known that a low pH (<2) will theoretically prevent jarosite formation. However, at extreme thermophilic temperatures (60 to 90°C) the kinetics of jarosite are quite fast even at very low pH. Kingma & Silver (1980) bioleached a chalcopyrite concentrate containing 8% pyrite. The leaching proceeded at a fast rate until day 10 when the copper extraction had reached a terminal value of only 60%. These researchers attributed the passivation of the mineral to the deposition of jarosites on the mineral surface. Jarosite was not seen on day 10 however jarosites constituted approximately 21 % of the leach residue on day 22. Blancarte-Zurita et al (1986) studied the effect of particle size on the leaching of chalcopyrite. They demonstrated an increased leaching rate with decreasing particle size. In 1987, the same researchers studied the silver catalyzed bioleaching of chalcopyrite. They achieved a 10-fold increase in the leaching rate. Table 2-6 helps to explain that their success was attributed to the prevention of jarosite formation in the presence of silver. 14 Table 2-6 Comparative results for silver catalyzed and test without silver (Blancharte-Zurita etal, 1987) Experiment Fate of Sulfide Copper extraction % Bioleach with Silver Bioleach without Silver Elemental sulfur Sulfate 70 30 Kingma and Silver (1980) found that the development of jarosites coincided with the decrease of copper leaching and the eventually passivation of the mineral surface. Blancarte-Zurita et al (1987) detected a large amount of jarosite in bioleaching tests that were: not silver catalyzed. These researchers seemed to believe that jarosites form a passive film on chalcopyrite and the difference in the precipitate formed was attributed to the inherent difference in the redox potential between silver catalyzed and non-catalyzed tests. Silver catalyzed systems exhibited a low redox potential while the non-catalyzed tests showed a high redox potential. Boon (1996) has another interpretation of these findings. The silver that is added inhibits the bioxidation of elemental sulfur. The sulfur forms a porous layer on the surface which jarosites cannot adhere to. Therefore, in the non-catalyzed tests the bacteria oxidize the elemental sulfur and thus jarosite can form on the chalcopyrite surface. With these results, the researchers assumed that the elemental sulfur is the result of chemical leaching and that the role of bacteria in the system is simply to regenerate ferric iron. Khinvasara & Agate (1987) leached chalcopyrite in shake flasks as well as in a bioreactor. These researchers observed that the bioreactor was more efficient in the leach due to an increase in the rate of oxygen and carbon dioxide mass transfer. Acevedo et al (1989) did a comparison test between leaching of chalcopyrite in a Pachuca airlift reactor versus leaching in a STR. The STR performed the most efficiently in terms of amount copper extracted per unit energy. Espejo & Ruiz (1987) and Bhattachuya et al (1990) used the mesophile t.ferrooxidans to leach a low-grade chalcopyrite ore. It was observed that only 1 - 10 % of the biomass in 15 the experiments was attached to the mineral. It was also shown that the free-range bacteria primarily use ferrous iron in solution for growth. Elzeky & Altia (1989) studied the effect of bacterial adaptation on the leaching of chalcopyrite. They found that bacteria adapted prior to the leach exhibited rates two to four times greater than non-adapted bacteria. The rate of bioleaching chalcopyrite can be increased by countering the oxygen and carbon dioxide mass transfer limitations with better aeration (Avecedo et al, 1987; Khinvasara et al, 1987; Le Roux et al, 1987). Bioleaching tests were performed on a mixture of pure chalcopyrite, sphalerite and pyrite. Sphalerite with the lowest rest potential leached first followed by chalcopyrite and finally by pyrite (Rossi et al, 1974; Dave et al, 1979). Table 2-7 Typical chalcopyrite bioleaching conditions and kinetic results with mesophiles as derived from the literature. Reference Conditions Maximum Leach Rate mg Cu/L/hr Copper Extraction % or g/L Jordan, 1996 30 °C, 3%, SF 2.44 na Mehta & Murr, 1982 30 °C, SF 83 38 Jordanetal, 1993 30 °C, 5%, AL 17-22 65-70 Kingma & Silver, 1980 27 °C, 60 g/L, STR 100 na Brierley & Brierley, 1978 25 °C, 1%, SF na 36.8-81.5 Third etal, 2000 37 °C,4%, SF 13-35 70 Khinvasara, 1987 25 °C, 400 g/L, STR 23 na Khinvasara, 1987 25 °C,400 g/L, AL 110 na 16 Table 2-7 displays some of the results obtain from a number of important researchers. In practice, some researchers report unusually high leaching rates that are several order of magnitude higher than those in the general literature. The literature in general shows the leaching of chalcopyrite using mesophiles has not been completely successful. Mesophiles operate at very high redox potentials (+750 mV SHE) which are significantly higher than the optimum potential range for chalcopyrite dissolution. A high ferric iron concentration should promote fast leaching of chalcopyrite. Instead, the fast initial leaching rates recede as passivation occurs. It can be concluded that the high redox conditions sustained by mesophile may result in the passivation of chalcopyrite. 2.5.2 Thermophilic Bioleaching of Chalcopyrite Jordan (1996) found that extreme thermophiles are more sensitive to increased solids and metal cation concentrations. In bioleaching a CuFeS^FeSj concentrate, the thermophilic leaching rates were 15.99 mg/L/hr and 12.40 mg/L/hr for Sulfolobus BC and A.brierleyi respectively. Thermophiles have exhibited low pH and high cation tolerances that make them suitable for application to bioleaching (Duarte et al, 1993). Norris & Parrott (1986) observed that thermophiles performed better than mesophiles in the leaching of chalcopyrite and that the most important feature of the thermophile group is their potential use in the rapid and high temperature leaching of low grade chalcopyrite ores and flotation concentrates. They achieved 90 % copper extraction at pH 2.5 in a medium supplemented with 0.02 % yeast extract and 0.2 % ferrous iron. March et al (1983) discovered noted differences in copper extraction when bioleaching of chalcopyrite concentrates from one thermophile strain to the other. Groudev (1986) also found this to be true as he achieved poor results when leaching chalcopyrite with a Sulfolobus sp. Barr et al (1992) reported that ferrous iron in solution always seemed to reach a maximum value followed by a sudden drop in the ferrous concentration around day 16 of 17 the experiment as it oxidized and precipitated. They concluded that the leaching mechanism was galvanic at high temperature. Gomez et al (1996) found that the attack on chalcopyrite first occurred in cracks and fissures and crystal defects where the electrical potential is greatest. They added ferrous iron to the bioleach and found an increase in the bacterial activity and bacterial population, but no positive change in the leach. Dutrizac et al (1987) and Hirato et al (1987) both observed that ferrous iron addition suppresses the oxidation of chalcopyrite in the chemical leaching system. An addition of ferrous would decrease the redox potential. If the redox was in a region unfavourable to the oxidation of chalcopyrite, its leaching would be suppressed. Hiroyoshi (1997) added ferrous and observed an increase in the consumption of oxygen on the sample surface. They concluded that ferrous iron enhances the chemical leaching of chalcopyrite. CuFeS2 + ()2 + 4 / T -> CM 2 " + Fe2+ + 25* + 2H2() [2-6] The added ferrous iron simply lowers the ORP into a region where the chalcopyrite will not passivate (Kametani & Aoki, 1985). The amount of ferrous needed to create a favourable Fe3+/Fe2+ ratio will vary with the test characteristics and most importantly the rest potential of the mineral (which can vary due to different quantities and types of impurities). The redox is now favourable due to the added ferrous, the leaching of chalcopyrite proceeds and therefore the consumption of oxygen as well as acid will increase. An increase in the bacterial population should have no effect on chalcopyrite leaching, but may affect pyrite oxidation. Also, if the bacteria are utilizing the ferrous iron, they are clearly not utilizing the electrons from the galvanic leach or a direct leach of chalcopyrite. This would result in no observable effect on the leach. The ferric produced, depending on the pH may precipitate. Le Roux et al (1987) leached chalcopyrite concentrates in batch and continuous experiments. The pH was controlled at 1.5, the pulp density was 5 % and the particle size range was < 90 um. Using the thermophile S. acidocaldarius at 70°C, these researchers 18 achieved complete solubilization of copper. They measured the rate of thermophilic bioleaching to be eight times that of the mesophile T. ferrooxidans. They attributed the faster leaching due to the faster kinetics at the higher temperatures. Boon (1996) seems to believe that the observed increase in the leaching rate is simply due to the chemical oxidation and therefore to an indirect mechanism. At the higher temperature there is very little ferric iron in solution. The only alternate oxidant is molecular oxygen and even at 70°C, the leaching is still quite slow. Konishi (1998) leached an 86 % pure chalcopyrite concentrate to 80 % extraction with no elemental sulfur formation. The tests were performed at 68°C and at pH 1.2 using a chalcopyrite concentrate at 1 % pulp density and a particle size range of 38 - 53 jam. They concluded through the testwork that the mechanism of leaching the pure mineral was the classic direct mechanism with the formation of sulfate rather than elemental sulfur. The redox potential remained quite high during these tests (Konishi, 1999) although indirect attack by ferric iron was found to be insignificant. Konishi describes the overall chemistry as: 4CuFeS2+ 1702 + 2H2S04 ^ 4CuS04 + 2Fe2(S04)3 + 2H20 [2-7] A brief summary of the selected thermophile leach results can be reviewed in Table 2-8. Table 2-8 Typical chalcopyrite bioleaching conditions and kinetic results with thermophiles as derived from the literature. Reference Conditions Estimated Rate Copper mg Cu/L/hr Extraction % or g/L Jordan eta l , 1996 50 °C, 3%, S F 15.64-23.75 5g /L 70 °C, 3%, S F 12.40-15.99 na Mehta & Murr, 1982 55 °C, S F 167 57 Norris & Owen, 1993 84 °C, 4%, AL 94 na Clark & Norris, 1996 70°C ,2%,AL 28 na 80°C ,2%,AL 72 na Norris & Parrott, 1985 70 °C,4%, STR 41 8 g/L Jordan eta l , 1993 68 °C, 5%,AL 36-44 65-70 Gomez et al, 1999 69 °C, 3%, S F . na 85 Compared to the leaching of the same concentrates but under mesophilic conditions, thermophilic leaching rates, in general, are much faster. Thermophilic bioleaching is successful with the one caveat that there are major problems with leaching at increased solids concentrations. 2.6 Summary of the Literature Bioleaching is performed with mixed cultures that have a high level of bacterial diversity. The literature brings to light the lack of understanding of the role of bacteria in the bioleaching system as well as outlines the fact that every chalcopyrite sample will leach differently. Different concentrates will exhibit different intrinsic leaching properties based on their mineralogy (percentage of pyrite and its spatial arrangement in relation to chalcopyrite) and chemistry (mixed potentials, galvanic interactions and inorganic 20 impurities). Thermophilic bacteria, for the most part, are unable to cope with higher concentrations of solids in agitated systems due to a number of possible factors; detrimental shear forces, physical attrition, chemical toxicity (sulfide and copper) as well as unfavourable mass transfer phenomena (Clark, 1996). Chalcopyrite in the presence of pyrite will leach faster through a galvanic interaction, Pyrite will leach very slowly between 0 and 620 mV SHE. However, there can be considerable current generated by the reduction of oxygen on its surface (Peters, 1977). Sulfur produced shrinks and can be found in cracks and fissures if it has not already fallen off the mineral surface. Bacteria may simply have an electrochemical role by depolarizing the anodic reactions on the chalcopyrite surface. Chalcopyrite can be leached optimally at an applied potential of +600 mV SHE. Leaching of pure chalcopyrite results in the formation of sulfate rather than elemental sulfur. However, in the presence of pyrite, the sulfide sulfur is totally converted to elemental sulfur. The literature indicates that thermophilic bacteria have a special role in the oxidation of chalcopyrite, the leaching being heavily electrochemically influenced. It also shows that researchers are well aware that there is a need for progress in the leaching of high-density slurries both under mesophilic and thermophilic conditions. 21 CHAPTER 3 THEORETICAL AND EXPERIMENTAL METHODOLOGIES The experimental methods and theory used to guide this study are described in this chapter. Theoretical issues such as an evaluation of the leaching chemistry will be covered with mechanisms and bacterial growth introduced. Common chemical measurements employed are pH, redox potential and temperature. Titrations and atomic absorption spectroscopy (AAS) represent chemical analysis. Elements of biotechnology introduced in this study include bacterial culture maintenance, cell enumeration and medium selection. Several series of shake flask tests were conducted to obtain comparative baseline data on the thermophilic system and also to test the selected parameters and characteristics of the leach. Secondly, these results and methodologies were applied to batch testing using a STR to verify the results and observations made in the previous phase of the experimentation. Batch bioleach experiments were conducted in conventional aerated STR, using suitable conditions previously determined. The STR was aerated with 1% CO2 supplemented compressed air and temperature maintained with an immersion heater and controlled by a thermocouple. The STR was run at 600 rpm so as to provide uniform solids suspension and to maintain a dissolved oxygen concentration greater than 2 ppm ensuring no kinetic limitations. Experiments were monitored for pH, ORP, soluble copper and iron, ferrous iron concentration, dissolved oxygen and solution cell populations. Leach residues were assayed for total copper, total iron, total sulfur, sulfide sulfur, sulfate sulfur and elemental sulfur. Copper and iron mass balances were performed. 3.1 Leaching Reactions One can appreciate the many chemical and biochemical reactions possible in biological leaching systems. The oxidation of elemental sulfur and ferrous iron shown in equations 22' [3-1] and [3-2] are the most industrially important reactions. The oxidation of elemental sulfur produce large volumes of sulfate which result in great financial expenditures in terms of reactor space and heat control. The oxidation of ferrous iron is industrially important because it is the primary oxidant in the leaching system. By themselves, these reactions contribute greatly to the thermodynamics and kinetics of the leaching system. S°+ I.5O2 + //2O H2SO4 [3-11 2FeSO4 + 0.5O2 + H2SO4 Fe2(S04)3 + H20 [3-2] As will be seen in this work, the sulfur yields for thermophilic bioleaching are quite high as a result of the slow kinetics of reaction [3-1]. Some elemental sulfur produced leaves the mineral surface and tends to deposit on the reactor lid or in condensing equipment. The chemical leaching of chalcopyrite is described by equation [3-3]. 4CuFeS2 + 10//2SO4 + 50 2 "> 4CuS04 + 2Fe2(S04)3 + \0H2O + 8S3 [3-3] This reaction is kinetically unfavourable and does not proceed to an appreciable extent at 70°C. • Reaction [3-4] is the chemical oxidation of pyrite and as will be shown, this reaction does proceed during the leaching of CuFeS2/FeS2 concentrates at 70°C only after the associated chalcopyrite in the sample has been leached. The oxidation of pyrite is acid generating and also increases the solution ORP. FeS2 + 3J5O2 + 0.5H2O -> Fe3++ 2S04~2 + FT [3-4] Reactions [3-5], [3-6] and [3-7] illustrate the electrochemical mechanism of leaching of chalcopyrite. Chalcopyrite undergoes anodic dissolution forming elemental sulfur while ferric iron or oxygen is reduced at a cathodic site. The concentration of ferric iron is very low at 70°C and at low pH due to ferric hydrolysis. The reduction of oxygen on a chalcopyrite surface is kinetically slow (Biegler, 1976). Bacteria may catalyze the reduction of oxygen of the chalcopyrite surface. Therefore, both cathodic reactions may contribute to the overall current. Anode: CuFeS2 -> • Cu2++ Fe2++2? + 4e [3-5] Cathode (1): 4e+4Fe3+ 4Fe2+ [3-6] Cathode (2): 02 + 4lf + 4e -> 2H20 (chalcopyrite surface) [3-7] The galvanic interaction between chalcopyrite and pyrite during leaching is demonstrated in reactions [3-8] and [3-9]. Chalcopyrite serving as the anode breaks down to form elemental sulfur. The pyrite surface is known to be a catalytic for the reduction of molecular oxygen. Pyrite therefore, serves as the cathode completing the galvanic cell. Anode: CuFeS2 Cu2+ + Fe2+ +2S° + 4e [3-8] Cathode: 02 + 4lf + 4e ->' 2H20 (pyrite surface) [3-9] An alternate explanation for leaching chemistry has been described (Dutrizac, 1989) since concentrations of hydrogen sulfide have been detected in the leaching of chalcopyrite. CuFeS2 + 2H2S04 -> CuS04 + FeS04 + H2S(aq) [3-10] 2H2S + 2Fe2(S04)3 -> 4FeS04 + 2H2S04 + 2S° [3-11] The addition of reactions [3-10] and [3-11] is the net reaction [3-12] which is the ferric leaching of chalcopyrite and akin to reaction [3-3]. CuFeS2 + 2Fe2(S04)3 -» CuS04 + 5FeS04 + 2S° [3-12] Reaction [3-10] is not a redox equation. It shows how the leaching rate would increase with increasing acidity ([H+]). It is commonly known that the acid leaching of chalcopyrite does not occur. Although hydrogen sulfide has been found in the leaching system, this mechanism seems to play an unlikely role in the bioleaching of chalcopyrite. As will be shown later, thermophilic leaching of chalcopyrite proceeds under low redox conditions and often below 600 mV SHE. Another possible low redox leaching chemistry could be oxidation of chalcopyrite using the cupric / cuprous couple (Cu 2 +/Cu +) at a 24 standard potential of 560 mV SHE. This option would be consistent with the low redox behaviour as well as result in the production of elerhental sulfur. Reactions [3-13] and [3-14] outline this possible and competitive chemistry. CuFeS2 + Cu2+ 2CuS + Fe2+ . [3-13] CuS + Fe2(S04h •» CuS04 + S° + 2FeS04 [3-14] 3.2 Bacterial Growth Bacterial leaching (metal extraction) curves are inherently linked to the growth curves of a batch population of bacteria. A graphical representation of this relationship is shown in Figure 3-1. . - " * " " " " * . * • s * y * / i f ' ' f * f • ' i f i f i f " . * / * -* / w ' g / / --- Bacterial Population * • * >* •* •* y^ / Copper Extraction * ^^ ^^  0^^^^^^^^ TIMF > Figure 3-1 Bacterial growth and copper extraction curves Figure 3-2 below shows the experimental and theoretical bacterial growth curves. : 25 X o X c Time Figure 3-2 Experimental and theoretical bacterial growth curves. The y-axis is X (cell concentration) for the experimental curve and ln X for the theoretical curve. Letters A through D associated with the theoretical curve indicate the four phases of bacterial growth: lag, exponential, stationary and death phases. We can break down the bacterial growth curve into four distinct phases. These phases are lag, exponential growth, stationary and death. Each phase of the growth curve has a corresponding phase on the metal leaching curve as shown in Figure 3-1. The initial phase is the lag phase. This phase is attributed to the bacteria adjusting to their new environment and is characterized by a specific growth rate of zero (p. = 0). The duration of the lag phase is dependent on many things such as the extent of adaptation to the substrate and the presence of inhibiting substances. On the corresponding leaching curve, the initial rates of leaching are low. Only acid soluble metals are being dissolved at this point of the leach. 26 Once adapted to the leaching conditions, an exponential growth phase follows the lag phase as the bacteria begin to utilize a fresh source of electrons. As a result, the population increases exponentially by cellular division. The rate of population growth is given by: r x = dX/dt = u.X [3-15] where: r x is the volume rate of biomass production (kg m"3h_1), p is the specific growth rate (h"), and X is the viable cell concentration (kg m") at time t. If the specific growth rate p is constant, [3-15] can be integrated under the following conditions, (X = X 0 at t = t0) to give: fl ' X = X 0 e M t [3-16] where: X is the viable cell concentration at (kg m"3) at time t, X 0 is the viable cell concentration at (kg m"3) at time t0 and e^1 is the exponential growth term By taking the natural log of both sides of equation [3-16]. lnX = lnX 0 + ut [3-17] A plot of lnX versus t gives a straight line with a slope p. This exponential growth phase corresponds with high metal leaching rates. The growth phase is followed by a third phase or stationary phase where the growth rate is equivalent to the death rate. A change in the leaching environment prompts the stationary phase with a sharp decrease in the leaching rates as the once thriving population runs out of substrate. More specifically, it can be attributed to the depletion of the energy source and nutrients. Alternatively, a toxic effect may be produced by a build-up of metabolic products or an increase in the ionic strength of the solution. The stationary phase manifests itself on the leaching curve by a steady to total decline in the leaching rate. 27 The final phase of the batch growth cycle is the death phase that occurs when the death rate begins to predominate over the decreasing growth rate. The leaching medium can no longer sustain life and an exponential decline in the population is possible. With no active bacteria, metal leaching will cease and the leaching rates will remain at zero. 3.3 Bioleaching Mechanisms There is no fixed agreement upon which mechanism, direct or indirect is responsible for the leaching of metal sulfides. In the case of this study, a number of mechanisms may play a role in the leaching of the chalcopyrite/pyrite substrate. The many mechanistic and hybrid combinations are as diverse as the bacterial species which perform them. Zhang and Yang (1997) feel that the bioleaching of sulfide minerals must accede to at least one of the following mechanisms; direct with sulfate production, indirect, indirect with transfer through sulfur, indirect with transfer through jarosite, galvanic leaching or a galvanic interaction. 3.4 Experimental Methods 3.4.1 Chalcopyrite Concentrate The substrate used in this study was a copper concentrate from the Gibraltar mine in central British Columbia, Canada. Table 3-1 shows the mineralogical chemistry of the Gibraltar concentrate and Table 3-2 outlines the chemical composition of the same concentrate. Table 3-1 Mineralogy of Gibraltar Concentrate (Hackl, 1995) Mineral Wt % Chalcopyrite 60 Pyrite 17 Chalcocite 8 Copper & Iron Oxides 5 Siliceous Gangue 10 28 Table 3-2 Composition of Gibraltar Concentrate Cone. C u % Fe % Stot % Ss042- % S ° % S 2 ' % No.1 No.2 25.36 25.15 25.23 27.61 28.85 30.30 2.53 0.80 0.02 0.01 26.15 29.33 The Gibraltar No.l concentrate was received dry and having a particle size of approximately 90 % passing 38 um. The Gibraltar No.2 concentrate was received dry but was only 52 % passing 38 um. This concentrate was reground at an external facility to a new particle size of 98 % passing 38 um. The reground No.2 concentrate arrived back in slurry form at a density of 60 %. The concentrate was filtered and dried at room temperature for immediate experimental use. 3.4.2 Bacterial Methods Bacteria that oxidize reduced species of iron and sulfur are suitable for the application of bioleaching. Three thermophile cultures were obtained for use in the testwork. Pure cultures of Sulfolobus acidocaldarius (S. acidocaldarius) growing at 70°C and Metallosphaera sedula (M. sedula) species growing at 60°C were obtained from the American Type Culture Collection (ATCC). A culture containing primarily Acidianus brierleyi (A. brierleyi) and growing at 60°C was provided by J.A. Brierley. For leaching on the 10L scale, a second mix culture growing at 70°C and called the November Mix (NM) was created using equal volumes of S. acidocaldarius and A. brierleyi. The N M bacteria were quite well adapted to. the Gibraltar concentrate having over 7 months of exposure. All shake flask tests were run against the standard mesophile culture (T. ferrooxidans) at 35°C containing Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirillum ferrooxidans which have been growing on the Gibraltar concentrate in the standard 0 K nutrient medium for some time. All bacterial cultures had been adapted to the substrate prior to testing through the serial transfer method. 29 Bacteria were grown in 250 mL conical flasks with foam stoppers. The flasks were rotated at 200 rpm at the optimum growth temperatures of the respective cultures. The bacterial nutrient medium employed was a simple basalt solution having the compositions below in Table 3-3. Table 3-3 Mesophilic and Thermophilic Nutrient Media Nutrient Mesophilic, 1 / 2 0 K* Thermophilic, basalt g/L g/L (NH 4) 2S0 4 1.5 0.4 KH 2 P0 4 0.25 0.04 MgS0 4 7H 2 0 0.25 0.4 KCI 0.05 -Ca(N0 3) 24H 20 0.005 -Addition 6M H 2 S0 4 pH 1.8 pH1.8 0 K indicates no iron added. The pH was adjusted to 1.8 with 6 M F^SCv When mineral sulfides were used as substrates, the overall concentration was typically 2-10 % w/w or approximately 20-100 g/L solids. Each flask was inoculated with 10 % v/v of culture. If control flasks are required, 5 % v/v of 2 % thymol solution was added in place of the inoculum (Jordan, 1996). In the case of shake flask sampling, 1 mL of supernatant was filtered and placed in 9 mL of 2 % HC1. Metal analysis was performed by AAS. 3.4.3 Bacterial Adaptation There was one facet of bioleaching that did not remain constant throughout the course of the research. Bacterial adaptation, a function of natural selection versus time was not taken into consideration from the experimental point of view. It was expected that through serial transfer, the bacterial cultures would adapt to the chalcopyrite concentrate, increased pulp densities and thus as increased copper 30 concentration. However, there was no experimental design put in place to directly address this biological phenomenon. 3.4.4 Cell Enumeration Bacterial counts with an Askania RME 5 X40 power light microscope and following the procedure outlined in the Appendix. 3.5 Shake Flask Experiments This section will outline the apparatus used and the procedures followed during the shake flask testing of the Gibraltar concentrate. 3.5.1 Apparatus & Procedures The shake flask is a stand-alone leaching system. A New Brunswick incubator and a Lab-line Orbit Environ-shaker provided the rotary mixing (200 rpm) and the constant temperature control needed. Using a clean, pre-weighed 250 mL conical flask (with baffled bottom) and foam plug, the desired amount of the concentrate was weighed out and added to the flask. 90 mL of nutrient medium at pH 1.8 was also added to the flask. The shake flask was placed in the shaker at the required temperature for 2 hours to bring the system to temperature and put the acid soluble minerals into solution. For metal mass balance purposes, the inoculum source was sampled. Approximately 10 mL of the inoculum was added to the flask. The final weight of the entire shake flask system was taken. Deionized water was used to compensate for evaporative losses. Shake flask evaporative losses overnight averaged 5 mL at 60°C and 6 mL at 70°C. Diagnostic readings were taken daily while solution samples were taken every three days for a duration of thirty days. The flask was allowed to settle and 2 mL sample was removed, left to stand for 5 minutes and 1 mL drawn from the surface and diluted 10 times in 2 % HC1 and subsequently sent for analysis to International Plasma Laboratory (IPL) in Vancouver. Daily pH readings were taken as a function of temperature and 6M 31 sulfuric acid to maintain the pH at 2. The ORP of the system was taken daily and also corrected for temperature. On test termination, a final sample was taken. The slurry was then filtered and the solids washed with a minimum volume of distilled water. The amounts, volumes and/br specific gravities of the filtrate, wash solution and solid residues were recorded. All samples are analyzed and a final mass balance verified. 3.5.2 Control Flasks Control flasks were used at different temperatures throughout the testing for comparative purposes. It is important to run identical tests with no biological presence in the flask. The control flasks are identical to the normal leaching system with 5 mL of 2 g/L thymol in methanol used instead of the bacterial inoculum. 3.6 Stirred Tank Experiments This section will outline the apparatus used and the procedures followed during the stirred tank testing of the Gibraltar concentrate. 32 3.6.1 Apparatus & Procedures A. 10L baffled acrylic tank B. immersion heater C. glass thermometer D. thermocouple E. hydrofoil impeller F. air/C0 2 sparger G. pH probe H. ORP electrode Figure 3-3 Schematics of the T10 Apparatus 3.6.2 T10 Bioreactor The STR shown in Figure 3-3 was a baffled 10 L acrylic tank with an associated aeration system. Manual temperature measurements were taken with a standard glass/mercury thermometer fitted with 316SS armor to prevent the impeller blades from rupturing the bulb. This 10 L acrylic tank was heated with a CP Ulanet immersion-heating element. A PMC Dataplate 520 Temperature Controller with Timer using a thermocouple controlled the heating power. 33 £ 1 33cm B D G H 23cm Figure 3 - 4 HG Hydrofoil used in the T 1 0 STR Experiments The special feature of the T10 bioreactor was the low shear impeller selected for the testing (Figure 3-4). A lab size (9.5 cm diameter) A H high solidity hydrofoil model 4AH39 provided by Hayward Gordon Ltd. was used in the T10 experiments. The material of construction for this hydrofoil is titanium. The shaft was 316SS. Four very wide blades at a 45° mount to the hub characterize this type of impeller. This impeller was chosen for coarse gas dispersion application where bottom flow velocities are required. In the case of bioleaching, good gas dispersion and solids suspension are required. The start-up, maintenance and termination procedures for the T10 bioreactor experiment were the same as for the shake flask testing. 3.7 Analytical Methods pH and ORP parameters of the bioleach experiment are paramount in determining the characteristics of the leach and therefore provide a means to evaluate its progressive status. In this capacity, pH and ORP measurements for all experiments were taken on a daily basis. pH values not only provide information on the leach; changing pH values also show that acidity is either being consumed or produced. Most importantly however is the precipitation behavior of metal cations with respect to temperature and pH. An attempt can be made to minimize the metal hydrolysis by maintaining the pH at a low value. Growth of many microorganisms is carried out within a specific pH range. It is 34 therefore important to accurately measure the pH conditions in the culture as well as in the leaching tests to evaluate the leaching environment. ORP readings provide some insight on the electrical potential or potential driving forces of the leach environment. These readings are commonly described as a measure of the ferric to ferrous ratio in solution. However, other sources such as mineral couples and dissolved oxygen (DO) may have an effect on the ORP reading. Also, ORP measurements fall short in describing the redox conditions at the mineral surface that can be effected by attached bacteria and/or galvanic couples. The determination of dissolved oxygen in a leaching slurry does not characterize the redox potential conditions of the medium. Oxidation arises from the loss of an electron from the atoms in the mineral irrespective of whether molecular oxygen takes part in this reaction or not. In describing the redox conditions, it is not only necessary to account for the concentration of the oxidant and DO but also the intensity with which the redox reactions take place in solution. The value of the redox potential serves as an indicator of the intensity and the driving force of the oxidation process. Microbial processes can only take place in a finite potential range. 3.7.1 Determination of Ferrous Iron in Solution Ferrous iron in solution was determined through titrimetric method for the using Cerium (IV) Sulfate. This protocol can been found in the appendix. 3.7.2 Determination of Solution ORP All ORP measurements were taken versus the Ag°/AgCl reference electrode . and converted to the hydrogen scale (mV SHE). The electrodes were filled with a standard solution of 4M KCI which was replaced biweekly. Inorganic scale deposits were dissolved by immersing the electrode in O.lM HC1 (5 min.), in 0.1M NaOH (5 min.) and again in the HC1 for 5 minutes. The Zobell solution was used to calibrate the redox electrode. As a check, another reference solution was used to verify the calibration of the electrode. The Light's solution (Light, 1972) was used for this purpose. 35 3.7.3 Determination of Solution pH A Ross Sure Flow pH probe was used to measure the leach solution pH. This probe was chosen because it was a glass electrode designed to provide stable and accurate readings at high temperatures and in dirty solutions. 36 CHAPTER 4 RESULTS AND DISCUSSION Following the experimental design, batch leaching experiments consisting of shake flask and 10 L STR tests were performed. This chapter presents the leaching results and attempts to provide an explanation for the behaviour of the leach. The first section deals with the results obtained through shake flask testing. Typical shake flask results will be described first followed by a discussion of the specific experiments conducted. Section 4.2 endeavors to present and explain the results achieved through 10 L STR experimentation. At the end of each section the results will be reviewed in the form of a summary. 4.1 Shake Flask Experiments This section will present and discuss the results obtained and observed during the shake flask testing of the Gibraltar concentrate. Presented first will be typical results followed by the results gained from the abiotic tests. Further, there will be discussion on the comparative results between particle size distributions, mesophiles vs. thermophiles and different thermophiles. Also presented in this section will be the results obtained from tests at different pulp densities and temperatures as well as the results from copper toxicity experiments. Figures that will be seen include metal concentration vs. time and ORP/pH vs. time curves for each experiment. The standard leaching conditions for a typical thermophile leaching test were 70°C, pH controlled at 2, using the N M culture and a pulp density of 2 %. 37 4.1.1 Typical Bioleach Results 6 0 5 10 15 20 25 30 . Time, days B io leach So lub le C o p p e r B io leach S o l u b l e Iron - _ - Ab io t i c So lub le C o p p e r - x - A b i o t i c So lub le Iron Figure 4 - 1 Typical copper and iron leaching curves for the standard thermophilic shake flask leaching test compared to an abiotic control. Standard conditions: 70°C, pH controlled at 2, using the N M culture, pulp density of 2 % and P90 38 pm. The typical test was performed at 70°C and under pH control. With the pH fixed at 2 the reground concentrate was leached with the N M culture at a pulp density of 2 %. These are the standard thermophile leaching conditions. Figure 4-1 illustrates the typical copper and iron leaching curves resulting from the typical thermophilic bioleaching of chalcopyrite with respect to an abiotic control at 70°C. Copper extractions from the concentrate in excess of 90 % are commonly achieved at this pulp density with average copper and iron leaching rates of 29 mg Cu/L/hr and 19 mg Fe/L/hr respectively. These rates are comparable to the literature values found in Table 2-8. The concentration of iron in solution (and therefore the calculated iron extraction) is subject to changes in pH and temperature of the leach. A large amount of the leached iron re-precipitated as basic ferric sulfate compounds such as jarosite which definitely became evident by day 15 of 38 Figure 4-1. Also found on Figure 4-1 are the copper and iron curves for the abiotic control (AC) under the same conditions. The copper and iron extractions of the A C test were next to negligible and due to the presence of acid soluble copper and iron compounds in the concentrate. However, after day 17 copper did begin to slowly leach in the A C tests. This must be attributed to weak bacterial activity brought about by the volatilization of the antibacterial agent thymol from the leach slurry. 600 0 10 20 Time, days 30 Redox Potential Figure 4-2 Typical ORP and pH curves for the standard thermophilic shake flask leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M culture, pulp density of 2 % and P90 38 urn. At 2 % density, the total iron in solution always approached 50 % extraction before declining. Under the standard conditions, sulfide oxidation varied from 50 to 55 % with an associated average solid weight loss of 38 %. Acid was consumed throughout most of the leach following the chemistry depicted in reactions [3-3] and [3-5] and therefore the pH of the typical test was maintained at 2. 39 Figure 4-2 is a ORP/pH vs. time curve for the typical leach under the standard conditions. Acid addition stopped once the copper leaching curve had reached its apex (marking the near complete dissolution of chalcopyrite), leading to a sharp drop in pH, due to the possible oxidation of elemental sulfur and pyrite or the hydrolysis of ferric iron. It would be instructive to define a term that will be referred to at many points during this discussion. The redox potential range from 500 to 600 mV SHE will be called the "Low Potential Zone" or LPZ. The redox values for the typical test fall in this zone for the entire experiment. The characteristic redox potential curve is also depicted in Figure 4-2. This curve shows how the redox potential rises slowly during the leach. The ferrous iron concentration increases as the mineral leaches and then decreases somewhat as the leaching stops and the oxidation of ferrous and precipitation of ferric iron predominate. In some instances elemental sulfur formed on the surface and then sloughed off. When the elemental sulfur flakes off the mineral a clean leaching surface is available. In a mesophilic leach at high redox potentials, elemental sulfur is oxidized to sulfate and therefore the reacting surface (while undergoing its own classic passivation) may fall victim to jarosite passivation. Examination of the residue assays as well as through physical inspection, revealed that the pyrite in the concentrate was only slightly leached. The redox potential typically measures about 540 to 560 mV SHE at the start of a shake flask test. Figure 4-3 is an example of the iron speciation during a typical test. The Eh does however stay within the LPZ. The maximum ferric iron concentration encountered during a typical test was 0.46 to 0.5 g/L which equates to approximately 0.01 M . The initial ferric iron concentration for a common chemical leaching test is also 0.1 M . The concentration of ferric iron in the thermophile system is in equilibrium with a number of different chemistries and therefore its contribution to the chemical leaching of chalcopyrite is not known. 40 0 10 Fer rous Iron 15 Time, days • Fer r i c Iron 20 25 Total Iron 30 Figure 4-3 Iron speciation curves for the standard thermophilic shake flask leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M culture, pulp density of 2 % and P90 38 um. A statistical analysis of the many trials at the standard conditions can be found in Table 4-1 and can attest to the reproducibility of the typical chalcopyrite bioleaching using thermophiles. Table 4-1 Statistical Analysis of Typical Test Rate, mgCu/L/hr or Extraction, % Initial Exponential Extraction Copper Iron • 28.5 15.3 95.3 1.3 1.3 i.5 • 18.5 9.5 na 2.1 2.1 D denotes average and S denotes standard deviation. 41 As will be demonstrated, thermophile tests produce very large amounts of elemental sulfur (with respect to sulfate) and in several cases, the quantitative conversion of sulfide to elemental sulfur was observed. Formation of elemental sulfur froth on the surface of the solution as well as elemental sulfur deposits on the walls of the shake flask were noted. This was also seen by Hirato et al (1987) who observed the two-dimensional flaking of elemental sulfur from the mineral surface as well as floating flakes of sulfur during chemical leaching of chalcopyrite. Dutrizac (1989) concluded that the elemental sulfur formed during chemical ferric leaching was not significantly oxidized by ferric iron and that the elemental sulfur produced is relatively inert with respect to the reacting mineral surface (provided the temperature is below the m.p. of elemental sulfur). The high production of elemental sulfur occurred well before any appreciable sulfate (elemental sulfur oxidation) was formed resulting in the large production. The direct mechanism implies that the bacteria are attached to the mineral surface. The localized area around the bacteria will be eventually converted to an elemental sulfur product layer. This product layer subsequently falls off the particle. The questions remain if the bacteria are attached firmly to the flakes of elemental sulfur or not. In this situation, bacteria would constantly be migrating to and breaking away from the mineral surface. This would result in a slow leaching rate as at any one instance the bacteria (attached) to solids ratio would be very low. DO readings varied between approximately 3.0 - 4.0 mg/L (ppm) of oxygen for the typical test (as well as the specific tests). The concentration of carbon dioxide in solution unfortunately could not be determined. Measures were taken to supplement the aeration in an attempt to maintain a constant level of carbon dioxide in the slurry. Previously in Figure 3-1, the relationship between cell growth and copper extraction can be seen quite clearly. It may be interpreted from this plot that the rate and extent of copper leaching was limited by the cell population growth. Cellular growth is limited by carbon dioxide, a value that could not be quantified in these tests. A series of shake flask tests (at the standard conditions) were performed and supplemented with 0.2 g/L yeast extract. This addition (as well as earlier additions of 0.1 g/L) proved foo toxic for the bacteria as no 42 leaching occurred and bacterial cell counts diminished to nil. The maximum bacterial count in a thermophile shake flask test was 48 Mcells/mL. Figures 4-4 and 4-5 present the results obtained from testing under standard conditions without the addition of acid. With the low production of sulfate in these systems, the primary source of sulfate is from the additions of 6 M sulfuric acid. Under no pH control chalcopyrite is not leached to any great extent with respect to the typical test. Extractions of only 30 to 40 % can be attributed to acid leaching of copper oxides and chalcocite oxidation shown in reactions [4-1] to [4-3]. With slight chalcopyrite leaching occurring, the relationship between redox potential and pH can be seen quite clearly in Figure 4-5. The redox potential remains in the LPZ as the pH responds correspondingly to the changes in slurry potential. 0 5 10 15 20 25 30 Time, days -0_ Soluble Copper No Acid Addition _fj_ Soluble Iron No Acid Addition - ± - Soluble Copper Standard Conditions Soluble Iron Standard Conditions Figure 4-4 Copper leaching curves for the thermophilic shake flask leaching test with and without pH control. Standard conditions: 70°C, pH controlled at 2, using the N M culture, pulp density of 2 % and P90 38 um. 43 620 10 15 20 Time, days Redox Potential -_-pH Figure 4-5 ORP and pH curves for the thermophilic shake flask leaching test without pH control. Standard conditions: 70°C, pH controlled at 2, using the N M culture, pulp density of 2 % and P90 38 pm. 4.1.1.1 Abiotic Controls Several abiotic control (AC) experiments were performed throughout the course of the research. Sterile tests were conducted at 35°, 50°, 60°, 70° and 80°C. The leaching environment was kept sterile by the addition of 5 mL of 2 g/L thymol in methanol rather than a bacterial inoculum. Verification of the abiotic environment was achieved through a nominal bacterial count. In the absence of bacteria, the leaching of chalcopyrite was not significant (9%) at all temperatures (even in the presence of pyrite). Copper extractions were in the range of 10 - 20 % with 12 % being the maximum possible contribution to extraction from acid soluble copper minerals and the oxidation of chalcocite. The oxidation of chalcocite is likely to proceed via the following chemistry. CU2S+O.5O2 + H2SO4 -> CuS04 + CuS + H20 44 [4-1] CuS + 202 ->CuS04 [4-2] CuS + Fe2(S04)3 •> CuS04 + S° + 2FeS04 [4-3] Reaction [4-3] is the most probable reaction for the oxidation of covellite as elemental sulfur was the most predominate sulfur entity formed. At low pulp densities, it was assumed that sufficient oxygen, carbon dioxide and sulfate were present to warrant the possible electrochemical mechanisms. However, the oxidation of chalcopyrite in the absence of bacteria was insignificant. Moreover, the redox potential of all the sterile tests also fell within the LPZ. There may have been a kinetic limitation such as the reduction of oxygen on the chalcopyrite and pyrite surface. Alternatively, the pyrite may have been locked within the host chalcopyrite and therefore a galvanic or electrochemical mechanism will not proceed. 4.1.1.2Particle Size Figure 4-6 demonstrates the identical leaching behaviour of two concentrates ground to different particle size distributions. The coarse grind was the concentrate at a P90 of 105 um and the fine grind is the reground concentrate at a P90 of 38 um. It can be seen that particle size has no significant effect on the leaching of copper as the ultimate copper extraction in both cases was greater than 90 %. 45 0 5 10 15 20 25 30 Time, days _«) -F ine Gr ind -m- C o a r s e Gr ind Figure 4-6 Copper leaching curves for the thermophilic shake flask leaching test at different particle size distributions of P90 38 um being the fine grind and P90 105 um representing the coarse grind. Standard conditions: 70°C, p H controlled at 2, using the N M culture, pulp density of 2 % and P90 38 um. There is a large difference in the mineral surface area between the two size distributions. If there were no oxygen mass transfer limitations the leaching of chalcopyrite would be chemically controlled and thus we would observe faster rates of leaching for the smaller size distribution. This was not observed and may suggest that the leaching reactions are limited by the mass transfer of oxygen. Carbon dioxide limitations and therefore bacterial growth may have also been limiting as both tests had similar bacterial counts. The surface area of pyrite available may also govern the leaching. Both distributions may have had similar levels of oxygen reduction on pyrite contributing to the cathodic current and therefore, completing the electrochemical leaching of chalcopyrite. 46 4.1.2 Mesophiles vs. Thermophiles 850 yj 800 X o T3 co 750 c/) > E •• 1- 7 0 0 -I—• £ 650 o 600 550 500 0 10 15 20 25 Time, days Mesophile -_-Thermophile 30 Figure 4-7 ORP curves for mesophile (35°C) and thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture and T. ferrooxidans mesophile culture, pulp density of 2 % and P90 38 pm. Another way to evaluate thermophilic bioleaching is to compare the leaching behaviour and observations to the mesophile culture. In the following comparison, the thermophile is represented by the N M culture and the mesophile culture is represented by T. ferrooxidans. Due to the significant difference in temperature a comparison of iron solublization will not be done. The mesophile tests commonly operate at potentials in excess of 700 mV SHE as can be seen in Figure 4-7. Despite the mesophiles well known ability for extensive sulfur and iron oxidation, these bacteria did not leach chalcopyrite significantly. The high potential reflects the mesophile's strong ability to oxidize ferrous iron. By contrast, the thermophiles operate in the LPZ; fixed by the thermodynamics of the leaching chemistry. 47 0 5 10 15 20 25 30 Time, days Mesophile thermophile Figure 4-8 Copper leaching curves for mesophile (35°C) and thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture and T. ferrooxidans mesophile culture, pulp density of 2 % and P90 38 jim. Figure 4-8 shows the poor copper leaching for the mesophile culture. These experiments suggest that as a result of the high redox potential, the mineral passivates (via classical or jarosite) and total copper extractions are commonly in the range 30 - 40 %. The copper extractions from chalcopyrite can be reported at 18 - 28 % with corrections made for acid soluble minerals and the oxidation of chalcocite. As a result of this passivation, acid was not consumed. This is realized in Figure 4-9 where the pH of the mesophile leach decreases throughout the leach and the acid consumption was zero. The copper extractions attainable in the thermophilic system are quite high and in excess of 90 % for low density systems as shown in Figure 4-8. 48 2.2 1.2 J — , _ i r- , : , 0 5 10 15 20 25 30 Time, days ^ M e s o p h i l e -_-Thermophile Figure 4-9 pH curves for mesophile (35°C) and thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture and T. ferrooxidans mesophile culture, pulp density of 2 % and P90 38 pm. Table 4-2 Average Copper Leaching Rates Bacteria 3 Day Rate Type mg Cu/L/hr Thermophile 29 Mesophile 5.75 Table 4-2 shows the extreme difference in the average copper leaching rates for the mesophile and the thermophile culture. The rates were calculated from day 0 to day 3 in each case and then averaged over 4 shake flask experiments. Contrary to what was thought, the initial high redox potential did not (in any mesophile test) result in a fast leaching rate. 49 4.1.3 Different Thermophiles at 60°C Figure 4-10 shows the copper extraction curves for the tests involving the two thermophiles A. brierleyi and M. sedula. The A. brierleyi culture contains some M. sedula. The pure M. sedula culture performed less effectively than the A. brierleyi. Through shake flask testing and observations, M. sedula was found to be less effective at oxidizing the chalcopyrite concentrate. Hansford (1998) corroborated this observation. Therefore, we can say that the enhanced leaching at 60 °C was probably due to the A. brierleyiV: The M. sedula copper extraction curve is characterized by three zones of leaching. A slow zone (and also inhibited zone), a rapid zone and a final slower zone resulting in extractions ranging from 50 to 70 %. 0 5 10 15 20 25 30 Time, days A. brierleyi -m- M. sedula Figure 4-10 Copper leaching curves for A brierleyi and M. sedula shake flask leaching tests. Conditions: 60°C, pH controlled at 2, pulp density of 2 % and P90 38 um. 50 The redox values, found in Figure 4-11 fall within the LPZ for both tests. After an initial dip, the potential steadily rises throughout each test. The pH curves for these thermophile tests can also be found in Figure 4-11. The very low initial pH was due to the high acidity of the inoculum used. After a quick rise in pH these tests consumed acid throughout the experiment. One peculiar observation was that the inoculum for the A. brierleyi and M. sedula (as well as T. ferrooxidans) were very acidic and often less than pH 1 (in comparison to the pure S. acidocaldarius or N M which ranged from 1.6 to 1.8). The initial leaching solution is mostly distilled water and has very little buffering ability. This may have been the result of using an older inoculum where some oxidation of elemental sulfur or pyrite had occurred. The older the inoculum, the lower the pH (but not always). The thermophiles that grow at 60°C are not as efficient as Sulfolobus sp. or the November mix culture at the leaching of copper from this particular concentrate. This may have been a function of the lower temperature. Since the A. brierleyi arrived growing on pyrite at 60°C, we may have also seen the effects of copper toxicity as the culture clearly had to adapt to this concentrate and a new higher growth temperature. However, the A. brierleyi originally constituted half of the November mix culture. This species, with time may still prove to be effective in the leaching of copper. 51 570 565 LU X CO > 560 £ '•»-» c 555 CD o X 550 o CJ CO a: 545 540 0 10 20 25 30 15 T i m e , d a y s Acidianus brierleyi Eh -a- Metallosphaera sedula Eh Acidianus brierleyi pH Metallosphaera sedula pH Figure 4-11 ORP and pH curves for A brierleyi and M . sedula shake flask leaching tests. Conditions: 60°C, pH controlled at 2, pulp density of 2 % and P90 38 um. 4.1.4 Effect of Pulp Density The most important shortcoming in the use of thermophiles is the negative effects of increasing pulp density. These effects were studied using the N M culture and in some instances compared to the pure S. acidocaldarius culture. 52 100 c g _ x LU 0 5 10 15 20 25 Time, days ^ NM at 2% -_- NM at 5% -__ NM at 10% 30 Figure 4-12 Copper leaching curves for thermophilic shake flask leaching tests at increasing pulp densities. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 pm. Figure 4-12 and Table 4-3 demonstrate how a decrease in the extent of extraction was observed with increasing pulp density. Also, the curves become progressively linear as the pulp density is increased. This effect may be due to a chemical limitation on the rate of leaching and in fact an absolute rate is achieved and is similar for all test densities as is shown in Table 4-3. This hypothesis is illustrated in Figure 4-12 as the leaching curves at increasing pulp density become more and more linear. Poor CO2 mass transfer may be the likely candidate for the limitations observed with increasing pulp densities. We may also be seeing the effects of physical attrition that would prohibit cell attachment. Pinches et al (1991) was limited to pulp densities below 15 % when using Sulfolobus sp. 53 If we assume the direct mechanism, increasing the number of particles in the system will have a two-fold effect: prevention of bacterial attachment and cellular damage through attrition. Table 4-3 Initial Rates of Copper and Iron Leaching for N M and S. acidocaldarius Experiments Pulp Density November Mix S. acidocaldarius % mg Cu/L/hr mg Fe/L/hr* mg Cu/L/hr mg Fe/L/hr* 2 15.1 8.6 10.3 10.2 5 16.5 17.5 15.1 9.8 10 16.5 24.2 15.3 2.7 T Initial indicates a rate calculated during the first 3 days of the experiment. * Assuming the rate of ferric precipitation is insignificant in the first 3 days of the leach. Table 4-3 displays the copper and iron leaching rates for tests of varying pulp densities for the N M and S. acidocaldarius cultures. The S. acidocaldarius experiments were performed three months prior to the N M experiments. We can see the effect of adaptation in the copper extraction from one culture to the other. The A. brierleyi culture, which originally constituted one half of the N M arrived to the lab growing on pyrite. The iron leaching rates increase from S. acidocaldarius to N M cultures. In light of this, the dominant species in the N M culture may be A. brierleyi: The effects of leaching at higher solids concentrations may be due to the inhibition of cellular attachment (as the inoculum contains free-swimming bacteria) as well as a very low bacteria to solids ratio (Roy & Mishra, 1981). A small bacterial population would be inadequate for the oxidation of significant amounts of chalcopyrite or ferrous iron. The redox behaviour for the 2 % test is typical of the low pulp density thermophile tests. The redox increases steadily and stays in the LPZ. The 5 and 10 % tests did exhibit some very interesting redox behaviour. Like the typical leach, the potential does stay in the LPZ however it does not increase smoothly or at the same rate as the typical test. Both the 5 and 10 % redox curves follow the same general trend throughout the leach. Much like the redox curves, the pH curves for the 5 and 10 % are closely related. 54 600 590 580 LU X CO > E .1 5 7 0 £ 560 o Q. X o TJ CU 550 540 530 15 20 Time, days 25 30 NM at 2% -m- NM at 5% NM at 10% Figure 4-13 ORP curves for thermophilic shake flask leaching tests at increasing pulp densities. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 pm. 55 2.05 0 5 10 15 20 25 30 Time, days ^ N M a t 2 % NM at 5% -_-NMat10% Figure 4-14 pH curves for thermophilic shake flask leaching tests at increasing pulp densities. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 pm. It is interesting to note the drops in pH for the 5 % test (day 17) and the 10 % (day 15). The ORP in these tests remained in the LPZ and was not high enough to cause the oxidation of elemental sulfur or significant amounts of pyrite. A simpler explanation for the drops in pH lies in the behaviour of iron in solution. The hydrolysis of iron in solution is "acid producing. Considerably more iron was being leached and subsequently hydrolyzed at 70°C and at increasing pulp density. The pH of the 10 % test drop occurred earlier in the experiment due to the critical concentration of iron needed for hydrolyzation being established first. 56 4.1.5 Effect of Temperature It would be interesting to see if there are any observable differences or changes in the leaching performance of the bacteria by varying the temperature. To study the effect of temperature, test were performed at 50, 60, 70 and 80°C at 2 % pulp density. Figures 4-15 and 4-16 show the copper and iron leaching curves for tests run at the varying temperatures respectively. 0 5 10 15 20 25 30 Time, days NM at 50°C -m- NM at 60°C + NM at 70°C NM at 80°C Figure 4-15 Copper leaching curves for thermophilic shake flask leaching tests at varying temperatures. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 urn. There seems to be little difference in leaching behaviour between the 60 and 70°C tests. The 80°C test produced a dramatic leaching curve and more indicative of a theoretical type curve. After a short adaptation period the increased temperature resulted in faster leaching rates in the exponential phase of growth. 57 The iron leaching behaviour, found in Figure 4-16 also differed between the 60°, 70°, 80°C grouping and the 50°C leaching curve. Being almost linear, it is not difficult to speculate that some phenomenon is forcing this curve to flatten. 0 5 10 15 20 25 30 Time, days + NM at 50°C -m- NM at 60°C NM at 70°C NM at 80°C Figure 4-16 Iron leaching curves for thermophilic shake flask leaching tests at varying temperatures. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 pm. There is a significant difference in the soluble copper profile for the 50°C test with respect to the 60, 70 and 80°C grouping in Figure 4-15. The 50°C extraction curve is very close to being linear and not representative of the typical thermophile leaching curve. At 50°C, the culture is 15°C below the optimum mixed growth temperature and if we assume that the leaching of chalcopyrite is related to cellular growth, the leaching behavior would have to be negatively effected. Bacteria that are not growing are therefore not utilizing electrons (regardless of the leaching mechanism). 58 The electrons are removed from the mineral by whatever oxidant is available, biologically generated ferric ions or molecular oxygen. From an electrochemical viewpoint, the system is operating well below the maximum attainable current from the corrosion cell and may be the reason why the leaching curve for the 50°C test is linear. This system is being electronically limited by the poor performance and efficiency of the bacteria at 50°C. The low temperature slows the bacterial activity and therefore bacteria oxidize ferrous iron or sulfide at a low rate resulting in a low extraction and a low ORP. At the higher temperatures (60, 70 and 80°C) the chalcopyrite began to leach in the first half-hour after start-up. However, the acid consuming reactions proceed much slower at 50°C. This accounts for the low initial pH. Figure 4-17 is a plot of the redox potential with time for each test temperature. The redox behaviour is opposite of what is expected of the thermophile leach. The redox decreases over the duration of the leach. The rate of ferrous oxidation is slower than the leaching reactions and thus there is a net generation of ferrous iron. 59 0 5 10 15 20 25 30 Time, days NMat50°C -m- NM at 60°C -__NMat70°C ^ N M a t 8 0 ° C Figure 4-17 ORP curves for thermophilic shake flask leaching tests at varying temperatures. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P% 38 pm. Since the mixed culture contains bacteria that grow optimally at both 60 and 70°C, it is no surprise that the copper leaching curves for these two temperatures are very similar. Taking into account experimental and statistical errors, their curves are very close to being identical. These tests were acid consuming throughout the duration of the leach while having the pH maintained at 2. At 80°C, the bacteria are operating 15° higher than the optimum growth temperature and therefore the leaching curve followed a different path than the typical curves for 60 and 70°C. The initial leaching rate was a third of that of the 70°C experiment. However, around day 10 the rate of copper leaching is significantly faster at 80°C than at any other temperature. Day 15 is the point where the extraction curves seems to become increasingly linear until the final extraction value is reached. Pinches (1998) described how the leaching of chalcopyrite with thermophiles at temperatures above 70°C was less 60 effective. There was an initial period of inhibition of the bacteria, however the copper leaching rate was faster. The 80°C test required no external acid addition. Somewhere between 70° and 80°C the chemistry changes from acid consuming to net acid producing. The redox curve rises steadily throughout the test and to a greater value than the 70°C. The biooxidation of ferrous iron seems to proceed faster at the higher temperatures. E = E° + (RT/nF)ln(aFe3+/aFe2+) [4-4] Where E is the redox potential in mV, E° is the standard redox potential in mV, R is the gas constant 8.314 JK^mol"1, T is temperature in K, n is the charge number, F is Faraday's constant 96485 Cmol"1, aFe3+ and aFe2+ are the activities of ferric and ferrous iron respectively. The Nernst equation, above, relates slurry temperature to slurry ORP. In fact, they are inversely proportional and therefore a decrease in temperature will lead to a decrease in ORP. It follows that the same relationship holds for increasing temperatures and therefore increasing ORP values. Figure 4-17 illustrates the effect of the Nernst relation on thermophilic bioleaching at various temperatures. 61 0 5 10 15 20 25 30 Time, days NM at 50°C -_-NMat60°C -_- NM at 70°C ' NM at 80°C Figure 4-18 p H curves for thermophilic shake flask leaching tests at varying temperatures. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 pm. The pH curve was also greatly affected by the redox behaviour of the system. It can be said that the pH of the thermophile system is affected to a greater extent by the redox potential at lower temperatures. Table 4 - 4 summarizes the results of thermophilic leaching of chalcopyrite at varying temperatures. 62 Table 4-4 Summary of Results for Varying Temperatures EXP TEMP EXTR Initial Final Final Residue s° Name °C % Leach pH ORP S° yield Rate* mV %* g mg/L/hr NM-S5-50 50 67 4.7 2 510 13.5 0.15 NM-S5-60 60 94 15.1 1.72 571 20.6 0.27 NM-S5-70 70 94 15.0 1.76 592 22 0.29 NM-S5-80 80 98 5.1 1.69 622 9.6 0.11 AC70 70 32 0 1.77 628 3.7 0.05 * The sulfur assay of the head sample was 0.26%. T 3 day initial rate. 4.1.6 Copper Inhibitions The final parameter investigated at the shake flask level was the limitation of copper toxicity. Copper cations, being one of the most toxic species to biological systems may prove to be a problem when bioleaching chalcopyrite slurries at high pulp densities. Cultures that are maintained on the Gibraltar concentrate grow at 2 % pulp density and are adapted to a maximum copper concentration of about 5 g/L. Shake flask tests performed at 2 % pulp density and assuming an inoculum concentration of 5 g/L were used as a baseline. Experiments were designed (Table 4-5) to test the initial inhibitions due to copper on the thermophiles. These experiments were not performed with the intention of achieving good bacterial adaptation. 63 Table 4-5 Copper Inhibition Experiments Experiment Maximum* Copper in Cupric Added Solution g/L g/L NM-S7-0 0 NM-S7-5a 10 5 NM-S7-1 15 10 NM-S7-1.5 20 15 NM-S7-2.0 25 20 * Indicates the maximum possible copper concentration, total leachable copper from the concentrate and the copper added. Data on the effect of increasing copper concentration at a fixed pulp density of 2 % are shown in Table 4-6 and Figures 4-20 to 4-22. Table 4-6 Lag Times and Initial Copper Leaching Rates for Figure 4-23 Initial Added Lag Time Leaching Final Extraction Leaching Rate1" Copper days Rate1" o / o After Lag [g/L] mg/L/hr mg/L/hr 0 na* 15.1 97 na 5 na 6.5 97 na 10 3 na 85 12.5 15 6 na 62 9.4 20 9 na 54 7.2 * na indicates not available. * 3 day leaching rate The increasing lag times indicate the culture's attempt at growing in an inhibitory environment. Norris & Owen (1990) found that the TC culture could not grow on an initial copper concentration of 10 g/L. However, when using Sulfolobus BC they achieved growth with an initial copper of 40 g/L (Norris & Owen, 1990, unreported data). 64 10 0 10 15 20 Time, days 25 30 Og/LCu -B-5g/LCu -^-10g/LCu ^ 1 5 g / L C u -^20g /LCu Figure 4-19 Effect of initial added copper on copper leaching for thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 um. 65 750 0 5 10 15 20 25 30 Time, days ^-Og/L -_-5g/L -_-10g/L -*-15g/L ^ 2 0 g/L Figure 4-20 Effect of initial added copper on ORP for thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 pm. Figure 4-19 displays the leaching curves for the tests performed with 0, 5, 10, 15 and 20 g/L added copper respectively. Figure 4-19 illustrates the inhibitory effect of increasing copper concentrations. j 1 With no copper added, the thermophiles leached this concentrate in the usual fashion and achieved greater than 90 % copper extraction. Successive additions of increasing copper resulted in a decrease in the initial leaching rates, increasing lag times and a decrease in the final copper extraction. The redox behaviour, (for the most part) was typical regardless of the initial copper concentration. However, the additional 5 g/L as well as the 10 g/L had a significant effect on the ORP. The additional 5g/L curve breaks the upper limits of the LPZ and increases past 725 mV. The additional 10 g/L also behaved in a similar manner, breaking the LPZ 66 latter in the test and only increasing to approx. 625 mV. This effect was reproduced with the 5 g/L addition. It seems that the addition of 5 g/L of copper had little effect of the leaching of the concentrate. It did however have a significant effect on the redox potential after day 15. The redox potential rapidly rose almost 150 mV in 3 days; an event that has not been seen ever in this work on thermophile bioleaching of chalcopyrite. It must be noted that the leaching of copper from chalcopyrite was near complete as the redox potential dramatically increased and therefore there was probably no chalcopyrite in the slurry to be passivated. The dramatic increase in redox potential may be attributed to the surprising and unexpected behavior in the iron extraction and precipitation. Figure 4-21 illustrates the marked difference in iron extraction and precipitation between the no copper addition test and the 5 g/L test. There was a 76.5 % weight loss during the leaching of the concentrate with the copper addition. This was quite uncharacteristic compared to the average weight loss of 38 % which is usually observed with this concentrate to achieve a very high extraction of copper. In the standard test, pyrite is not leached to a great extent and constitutes a large portion of the leach residue. However, in the copper addition test the pyrite as well as the chalcopyrite were leached resulting in both very high copper and iron concentrations in solution seen in Figures 4-21 and 4-22. 67 0 5 10 15 20 25 30 Time, days No Copper Addition 5g/L Copper Addition Figure 4-21 Effect of 5g/L added copper on copper leaching for thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 pm. 68 2.2 2.1 -0 5 10 15 20 25 30 Time, days No Copper Addition • 5 g/L Copper Addition Figure 4-22 Effect of 5g/L added copper on pH for thermophilic shake flask leaching tests. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 2 % and P90 38 urn. The addition of copper resulted in an increase in ORP due to the increased concentration of iron in solution. Iron precipitation did not seem to be a problem in the copper addition test. Moreover, Figure 4-22 relays the fact that the pH was well below the set point of 2 and therefore no acid (and therefore sulfate) was added to the copper addition test. The sulfate required by the leaching chemistry must have been provided by the leaching of elemental sulfur or pyrite. 4.1.7 Summary Thermophile bioleaching of the Gibraltar chalcopyrite concentrate at 2 % pulp density in shake flasks yielded greater than 90 % copper extraction. All tests stayed in the LPZ. Acid was consumed throughout the test until day 15. After day 15, the pH decreased and 69 the redox potential increased as the bacteria reverted to oxidizing ferrous iron and elemental sulfur. Large amounts of sulfur were produced and then flaked off the mineral surface. In some cases, elemental sulfur formed on the walls of the shake flask. In the typical test, the oxidation of elemental sulfur was not observed to any great extent. However, it was observed in the test were the addition of 5 g/L of copper proved catalytic for the oxidation of pyrite in the concentrate. The increased iron in solution drove up the ORP and in turn promoted the oxidation of elemental sulfur. Particle size had no appreciable effect on the leaching of the Gibraltar concentrate. Mesophilic bioleaching of the concentrate resulted in very low copper extraction due to the passivation of the chalcopyrite. By contrast, the thermophiles were able to leach chalcopyrite completely. During the A C tests there was no biological leaching. Increasing the pulp densities of the thermophile test above 2 % resulted in a decrease in the rate and extent of copper leaching. The bacterial population was stricken by problems of copper inhibition and/or increasing pulp densities. These shortcomings tend to dampen the success of thermophiles. An increase in temperature led to an increase in the extent of copper leaching, an increase in iron precipitation and an increase in elemental sulfur production. An increase in the initial copper concentration resulted in a decline in the rate and extent of copper leaching. The addition of 5 g/L seemed to have a catalytic effect on the leaching of the pyrite in the concentrate. Iron in solution increased dramatically and correspondingly the ORP increased. This was the result of the dissolution of pyrite which until now was not leached to any great extent in the thermophilic system. There seemed to be no chemical or physical barrier that could stop the transmission of electrons from the crystal to the bacteria or oxidants in solution. 70 After a strict regime of serial transfers, the N M culture may be adapted to very high concentrations of copper. 4.2 Stirred Tank Experiments This section will present and discuss the results obtained and observed during the stirred tank bioleaching of the Gibraltar concentrate. 4.2.1 T10 Experiment This 10 L test was performed at the standard test conditions, 70°C and pH control. The pH was fixed at 2 and the reground concentrate was used with the N M culture at a pulp density of 2 %. These are the standard thermophile leaching conditions, with the exception of the pulp density, which was 5 % and an agitation speed of 600 rpm (the minimum required for solids suspension using a low shear impeller). Figure 4-23 illustrate the soluble copper and iron curves for the T10 experiments. Much like the shake flask tests, the fastest rate of copper leaching commenced on day 10 and the extent of copper, extraction was also quite comparable. The behaviour of the soluble iron was very similar to that of the earlier small scale experiments as well. The precipitation of ferric iron can be definitely noted by day 15 when the total iron in solution starts to decrease. A copper extraction of 91 % was achieved with a corresponding solids weight loss of approximately 25 %. The average weight loss for a shake flask test under the same conditions was also around 25 %. 71 16 0 5 10 15 20 25 30 Time, days -•-Copper _ Iron Figure 4-23 Copper and iron leaching curves for the thermophilic T10 leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 5 % and P90 38 pm. Like the shake flask tests, Figure 4-25 shows how acid was consumed throughout the entire leach until day 15 when the leaching rate of chalcopyrite began to diminish and the pH dropped below the set point of 2. The decreasing pH was accompanied by the associated increase in redox potential. The redox potential did stay in the LPZ until day 24 when it traveled to a final potential of 613 mV SHE. There was an outstanding elemental sulfur production of 99.6 % of the maximum possible yield. Very interesting to note that on dismantling the apparatus, a yellow-white crystalline material was encountered in the 316SS condenser. Analysis of this solid (0.283 g) revealed that it contained 95.95 % elemental sulfur. On production, the sulfur floated to the surface and was carried away from the slurry surface and deposited in the condenser. The initial three day copper leaching rate was 7.5 mg Cu/L/hr and the fastest leaching rate around day 10 was 50.1 mg Cu/L/hr. Acid additions and therefore the acid consumption are depicted in 72 Figure 4-25. The total acid consumption was 629 kg H2SO4 / tonne of concentrate. It is clear from this plot where the leaching chemistry switches from net acid consuming to net acid generating. Before day 15, the rate of chalcopyrite leaching is much greater than the ferrous oxidation and therefore acid is consumed. After day 15 the rate of pyrite and sulfur oxidation is much less than that of the oxidation of ferrous iron and subsequent re-precipitation as ferric iron. Pyrite and sulfur oxidation are acid producing events. 0 5 10 15 20 25 30 Time, days • Redox Potential • pH Figure 4-24 ORP and pH curves for the thermophilic T10 leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 5 % and P90 38 um. 73 700 0 2 4 6 8 10 12 14 16 Time, days Cumulative Acid Consumption Figure 4-25 Cumulative acid consumption for the thermophilic T10 leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 5 % and P90 38 pm. The maximum bacterial count in the T10 experimentation was 2.4X10 cells/mL as can be determined from Figure 4-26 which shows the growth of the experiments bacterial population. 74 5 10 15 20 25 Time, days Bacterial Population Copper Extraction 30 Figure 4-26 Relationship between bacterial growth and copper extraction for the thermophilic T10 leaching test. Standard conditions: 70°C, p H controlled at 2, using the N M thermophile culture, pulp density of 5 % and P90 38 um. The oxidation of sulfide is more thermodynamically probable than ferrous (Barr et al, 1992), sulfide being the only solid source of electrons (besides sulfur which is not attacked). As Figure 4-27 shows, the thermophilic leaching of chalcopyrite results in the release of large amounts of ferrous iron. It would seem that this ferrous would be a potential substrate for the bacterial population. The concentration of ferric in solution was very low at a maximum value of approximately 1.0 g/L. There was an apparent ferrous oxidation limitation up to day 15 (preferred sulfide oxidation). This may have been the result of the low bacteria to solids ratio brought about by carbon dioxide limitations as explained earlier. The rate of ferrous oxidation becomes dominant around day 15. Eventually, a punitive switch to the oxidation ferrous iron at the expense of the diminishing reactive surfaces available on chalcopyrite. This supports the disappearance 75 of ferrous iron after day 15. It also illustrates this cultures ability to rapidly switch and oxidize the next available substrate. 0 5 10 15 20 25 30 Time, days Ferrous Iron -m- Ferric Iron Total Iron Figure 4-27 Iron speciation curves for the thermophilic T10 leaching test. Standard conditions: 70°C, pH controlled at 2, using the N M thermophile culture, pulp density of 5 % and P90 38 pm. Table 4-7 Sulfur Speciation in Leach Solids for T10 Experiment Day Total Sulfur % Sulfide Sulfur % Elemental Sulfur % Sulfate Sulfur % 0 30.3 29.3 0 0.8 15 40.9 19.9 19.5 1.3 30 39.6 10.9 26.4 2.1 76 The production of elemental sulfur was a very significant event in this thermophile leach. Table 4-7 helps to relay this significance and reports the experiments solids sulfur speciation for a selected T10 bioleaching test. 4.2.2 Summary The observation and results obtained in the 10 L experimentation were very comparable to those of the previous shake flask testing. Copper and iron extractions were similar as was the average weight loss of 25 %. The copper leaching rate was maximum on day 10 when it reached 50.1 mgCu/L/hr. This is conceivably not the fastest rate achievable as the optimization of this particular test and its test conditions were not done. Acid was consumed until day 15. ORP behaved correspondingly and remained in the LPZ until day 24. The many possible chemical and physical limitations also apply to the 10L tests; carbon dioxide being the most probable. 77 CHAPTER 5 SURVEY OF THE CONCLUSIONS This study investigated the application of thermophilic bacteria to the bioleaching of chalcopyrite flotation concentrates. Based on the experimental results and observations the following conclusions have been drawn on the use of these bacteria. The N M mixed culture leached copper from chalcopyrite at an initial rate of 29.8 mg Cu/L/hr for shake flask tests and 50.1 mg Cu/L/hr during the 10L STR experimentation resulting in greater than 90 % extractions of copper. Leaching results from the shake flask and STR testing were very similar. In the typical leaching of the concentrate, pyrite was not leached. In the A C tests chalcopyrite was not leached. The redox potential may have been too low for the leaching of pyrite. Even if some pyrite grains were available for oxidation, they would most likely be cathodically protected while in the presence of chalcopyrite. By contrast the mesophiles exhibited an initial rate of only 5.75 mg Cu/L/hr followed by mineral passivation. Despite their well documented reputation for iron and sulfur oxidation, mesophiles are not the bacterium of choice for the treatment of chalcopyrite. However, if the redox potential is held in the LPZ (perhaps by the addition of ferrous iron), the leaching of chalcopyrite with mesophiles may be successful. There may have been growth limitations encountered during this testwork. Lower than average cell counts were observed in the shake flask tests. A low bacterial to solids ratio would result in a slow rate of sulfide oxidation, ferrous oxidation as well as elemental sulfur oxidation. It would also verify the problems of increasing pulp density as well as restrict the redox potential to relatively low values (as the rate of ferrous oxidation is much less than the leaching of chalcopyrite). The low rate of ferrous oxidation and therefore the low redox potential would also account for the large production of elemental sulfur. The use of thermophilic bacteria may have been inhibited by a number of chemical limitations. The various inhibitions resulted in increasing lag times and a decrease in the leaching rate as well as the extent of sulfide oxidation. Copper cation toxicity became a problem at concentrations of 10 g/L and greater. Copper extractions and their respective leaching rates decayed seriously at pulp densities higher than 10 %. 78 A 5 g/L addition of copper to the typical leach seemed to have a catalytic effect. The addition of copper promoted the leaching of both chalcopyrite and pyrite. Therefore, a large amount of iron was in solution increasing the ORP and further accelerating the leaching of pyrite as well as the oxidation of elemental sulfur. The problem of high pulp densities may be due to attrition damage, lack of O2 or CO2. The low copper extraction would not be due to cellular destruction but simply due to the lack of bacteria successfully attached to the mineral. Shear stress and increasing solids concentration prove to be a poor environment for thermophilic bacteria in a stirred system. Acid was consumed throughout the tests. Due to the low redox potential, the elemental sulfur formed was not oxidized to sulfate. The flaking of elemental sulfur occurred in most of the successful thermophile experiments. In this case, elemental sulfur would not have played a passivating role. The sulfur produced in mesophile test is not passivating. Mesophiles operate at very high redox potentials and therefore the elemental sulfur that is produced will be quickly oxidized to sulfate. The flaking of sulfur product would reveal a clean leachable surface. It seems that the rate of leaching (elemental sulfur formation) is much faster than the rate of jarosite precipitation as the driving force for precipitation is nonexistent with such a small concentration of ferric in solution. In the mesophile experiments there is no elemental sulfur to protect the mineral surface. The classical passivation of chalcopyrite (the formation of a mixed copper and iron sulfide) was not observed in the thermophile tests. This may have been due to the low redox potential maintained. By comparison, the mesophile tests may have exhibited some of the classical chalcopyrite passivation due to the high redox potentials making the mineral surface unsuitable for the retrieval of electrons by ferric iron or the assorted biological depolarizers in solution. Notwithstanding the many experimental and observational evaluations of the thermophile system, there is still considerable dispute on the mechanisms of leaching. The mineralogy plays a critical role in the understanding and prediction of the predominate leaching mechanism. However, there is no firm agreement on the mechanisms of bioleaching any sulfide mineral, as the bacteria's tactical approach to leaching seems to be different in every case. In the presence of pyrite, chalcopyrite may be leached through a galvanic 79 mechanism with the formation of elemental sulfur. The thermophilic population in solution contribute to the overall leaching of the mineral through the slow oxidation of ferrous iron. It was found (Konishi, 1999) that in the absence of pyrite, chalcopyrite is leached by thermophiles resulting in the formation of sulfate through the classical direct mechanism. They stated that the bacteria in solution keep the redox potential very high. This may have also contributed to the total oxidation of sulfide to sulfate in the absence of pyrite. However, it is difficult to draw any certain conclusions on a possible leaching mechanism based on the work presented. Seemed to be a chemical leach from biologically produced ferric iron. Some type of limitation in cellular growth would ultimately result in a slow rate of ferrous iron oxidation resulting in a low ORP. The leaching of chalcopyrite is quite temperature dependent. The extent of sulfide oxidation to elemental sulfur and copper extraction increase with increasing temperature from 50 to 70°C. However, initial bioleaching at 80°C using the N M culture was not significantly better than the 70°C leach. However, during the exponential phase of bacterial growth the rate of copper leaching at 80°C was significantly faster than at 70°C. This observation is contrary to what was encountered by others (Pinches, 1998). The oxidation of chalcopyrite in the absence of bacteria was insignificant at all temperatures. The redox potential plays an important role in the thermophilic bioleaching of CuFeS^FeSi concentrates. When testing the Gibraltar concentrate the redox potential remained in the range 500 - 600 mV SHE. This zone has been coined the Low Potential Zone (LPZ). The LPZ will vary according to the chalcopyrite sample (percentage of pyrite and other impurities). The redox potential is largely determined by the mixed potential of the corrosion cell between chalcopyrite and pyrite and is also effected by ions in solution which are governed by temperature. The potential does rise slowly during the test as the rate of ferrous oxidation begins to predominate over the dissolution reactions. 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Zhang, H. , personal communication, UBC, March 3,1998. 89 Appendix Bacterial Cell Counting Take a liquid sample with Pasteur pipette and put it on the counting surface. Cover the counting surface with a glass cover slip. Focus on sample to a maximum magnification of X40. Choose the best plane of view with the most bacteria. Count the number of bacteria in 10 of the 16 squares. If the number of bacteria is too high, the sample must be diluted until each square has approx. 5 bacteria. The total number of bacteria is given by the following equation: Cells/mL = (SCells x 2 x 107 x dilution)/SCounted squares (usually 10) Titrimetric Method for the determination of Fe(II) using Cerium (IV) Sulfate Reagents Cerium (IV) Sulfate Solution (0.1 M) is available from the majority of the chemical supply companies. Spekker Acid Solution Measure out 1.6L of distilled water into a 5L beaker and agitate. Slowly add 600mL of concentrated (98%) sulfuric acid followed by 600mL of concentrated (85%) phosphoric acid. Allow solution to cool before transferring to storage bottle. Indicator Ferroin (1,10 phenanthroline-ferrous complex solution), which is available from most chemical supply companies. Procedure Obtain lmL of filtered sample from the experiment and bulk to lOOrhL in a volumetric flask. Transfer the entire amount into a 200mL beaker. Add lOmL of Spekker acid solution and 1-3 drops of ferroin indicator. Titrate with the standard cerium (IV) sulfate solution until the first permanent colour change from orange/red to colourless/blue is obtained. Calculation Fe(II) g/L = C C E R x V T x 55.84 / V s o l Where C C E R = concentration of cerium (IV) sulfate solution (M) V j = titration volume (mL) 90 Vsoi = volume of sample (mL) Determination of Solution O R P Zobell solution. Solution A 1/300 M K 4Fe(CN) 6 in 0.1 M KCI Solution B 1/300 M K 3 Fe(CN) 6 in 0.1M KCI Mix 10 mL A and 10 mL B. Immerse elecrode. The reading should read 230 ± 1 0 mV. The electrode was calibrated daily. A constant value of 199 mV was added and then adjusted for temperature by subtracting 1 mV for every degree above 25 °C. Light's solution. The Light's solution (Light, 1972). Standard Ferrous-Ferric Solution for Oxidation-Reduciton Potential (ORP) Measurements (Ag°/AgCl, 4M K C I : 475 ± lOmV) Composition: 39.21 g/L Fe(NH 4 ) 2 (S0 4 ) 2 6H 2 0 0.1M 48.22 g/L FeNH 4 (S0 4 ) 2 T2H 2 0 0.1M 56.20 mL/L= 103.24 g/L H 2 S 0 4 1.0M 91 

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